Process for the energy-efficient production of alkali metal alkoxides

Abstract

A process produces sodium and/or potassium alkoxides in countercurrent by reactive rectification. Alcohol is reacted in countercurrent with the respective alkali metal hydroxide. The vapours containing alcohol and water are separated in at least two serially arranged rectification columns. The energy of the vapour obtained in the second rectification is utilized for operating the first rectification. This specific energy integration coupled with establishing a certain pressure difference in the two rectification stages makes it possible to cover a particularly large proportion of the energy required for the rectification through heating steam and minimizes the use of electricity.

Claims

1. A process for producing at least one alkali metal alkoxide of formula M.sub.AOR, wherein R is a C.sub.1 to C.sub.6 hydrocarbon radical, and wherein M.sub.A is sodium or potassium, the process comprising: (a1) reacting a reactant stream S.sub.AE1 comprising ROH with a reactant stream S.sub.AE2 comprising M.sub.AOH, in countercurrent at a pressure p.sub.3A and a temperature T.sub.3A in a first reactive rectification column RR.sub.A, to afford a crude product RP.sub.A comprising M.sub.AOR, water, the ROH, and the M.sub.AOH, wherein R is a C.sub.1 to C.sub.5 hydrocarbon radical, and M.sub.A is sodium or potassium, and withdrawing a bottoms product stream S.sub.AP comprising the ROH and the M.sub.AOR at a lower end of the first reactive rectification column RR.sub.A, and withdrawing a vapour stream S.sub.AB comprising the water and the ROH at an upper end of the first reactive rectification column RR.sub.A, and (a2) optionally, simultaneously with and spatially separate from (a1), reacting a reactant stream S.sub.BE1 comprising ROH with a reactant stream S.sub.BE2 comprising M.sub.BOH, in countercurrent at a pressure pam and a temperature T.sub.3B in a second reactive rectification column RR.sub.B, to afford a crude product RP.sub.B comprising M.sub.BOR, water, the ROH, and the M.sub.BOH, wherein M.sub.B is sodium or potassium, and withdrawing a bottoms product stream S.sub.BP comprising the ROH and the M.sub.BOR at a lower end of the second reactive rectification column RR.sub.B, and withdrawing a vapour stream S.sub.BB comprising the water and the ROH at an upper end of the second reactive rectification column RR.sub.B, (b) passing the vapour stream S.sub.AB into a first rectification column RD.sub.1, and, if (a2) is performed, the vapour stream S.sub.BB is also passed into the first rectification column RD.sub.1 in admixture with the vapour stream S.sub.AB or separately from the vapour stream S.sub.AB, to obtain a mixture G.sub.RD1 comprising water and ROH in the first rectification column RD.sub.1, (c) separating the mixture G.sub.RD1 in the first rectification column RD.sub.1 at a pressure p.sub.1 and a temperature T.sub.1, into an ROH-comprising vapour stream S.sub.RDB1 at an upper end of the first rectification column RD.sub.1, and a bottoms stream S.sub.RDS1 comprising water and ROH at a lower end of the first rectification column RD.sub.1, (d) passing the bottoms stream S.sub.RDS1 completely or partially into a second rectification column RD.sub.2, to obtain a mixture G.sub.RD2 comprising water and ROH in the second rectification column RD.sub.2, (e) separating the mixture G.sub.RD2 at a pressure p.sub.2 and a temperature T.sub.2, into an ROH-comprising vapour stream S.sub.RDB2 at a top of the second rectification column RD.sub.2, and a bottoms stream S.sub.RDS2 comprising water at a lower end of the second rectification column RD.sub.2, and (f) transferring energy from the vapour stream S.sub.RDB2 to the mixture G.sub.RD1 in the first rectification column RD.sub.1, wherein p.sub.2>p.sub.1, p.sub.2>p.sub.3A, and wherein if (a2) is performed, p.sub.2>p.sub.3B.

2. The process according to claim 1, wherein in (f), energy is directly transferred from the vapour stream S.sub.RDB2 to the mixture G.sub.RD1.

3. The process according to claim 2, wherein at least one of (α-i), (α-ii), and/or (α-iii) is performed: (α-i) a first portion S.sub.RDS11 of the bottoms stream S.sub.RDS1 discharged from the first rectification column RD.sub.1 is passed into the second rectification column RD.sub.2, and energy is transferred from the vapour stream S.sub.RDB2 to a second portion S.sub.RDS12 of the bottoms stream S.sub.RDS1 discharged from the first rectification column RD.sub.1, and the second portion S.sub.RDS12 is then recycled into the first rectification column RD.sub.1; (α-ii) at least one stream S.sub.RDX1, distinct from the vapour stream S.sub.RDB1 and the bottoms stream S.sub.RDS1, comprising ROH and water is discharged from the first rectification column RD.sub.1, and energy is then transferred from the vapour stream S.sub.RDB2 to the at least one stream S.sub.RDX1, and the at least one stream S.sub.RDX1 is recycled into the first rectification column RD.sub.1; and/or (α-iii) the vapour stream S.sub.RDB2 is passed through the first rectification column RD.sub.1, thus transferring energy from the vapour stream S.sub.RDB2 to the mixture G.sub.RD1.

4. The process according to claim 1, wherein in (f), energy is indirectly transferred from the vapour stream S.sub.RDB2 to the mixture G.sub.RD1.

5. The process according to claim 4, wherein at least one of (β-i), (β-ii), and/or (β-iii) is performed: (β-i) a first portion S.sub.RDS11 of the bottoms stream S.sub.RDB1 discharged from the first rectification column RD.sub.1 is passed into the second rectification column RD.sub.2, and a second portion S.sub.RDS12 of the bottoms stream S.sub.RDS1 discharged from the first rectification column RD.sub.1 is recycled into the first rectification column RD.sub.1, wherein energy is transferred from the vapour stream S.sub.RDB2 to at least one heat transfer medium W.sub.i1, distinct from the second portion S.sub.RDS12, and is then transferred from the at least one heat transfer medium W.sub.i1 to the second portion S.sub.RDS12, and the second portion S.sub.RDS12 is then recycled into the first rectification column RD.sub.1; (β-ii) at least one stream S.sub.RDX1, distinct from the vapour stream S.sub.RDB1 and the bottoms stream S.sub.RDS1, comprising ROH and water is discharged from the first rectification column RD.sub.1, and energy is transferred from the vapour stream S.sub.RDB2 to at least one heat transfer medium W.sub.ii1, distinct from the at least one stream S.sub.RDX1, and then transferred from the at least one heat transfer medium W.sub.ii1 to the at least one stream S.sub.RDX1, and the at least one stream S.sub.RDX1 is then recycled into the first rectification column RD.sub.1; and/or (β-iii) energy is transferred from the vapour stream S.sub.RDB2 to at least one heat transfer medium W.sub.iii1, distinct from the mixture G.sub.RD1, and the at least one heat transfer medium Win, is then passed through the first rectification column RD.sub.1, thus transferring energy from the at least one heat transfer medium W.sub.iii1 to the mixture G.sub.RD1.

6. The process according to claim 5, wherein each of the at least one heat transfer medium W.sub.i1, the at least one heat transfer medium W.sub.ii1, and the at least one heat transfer medium W.sub.iii1 is water.

7. The process according to claim 5, wherein the at least one stream S.sub.RDX1 is withdrawn below the vapour stream S.sub.RDB1 on the first rectification column RD.sub.1.

8. The process according to claim 1, wherein the vapour stream S.sub.RDB2 is at least partially employed as the reactant stream S.sub.AE1 in the first reactive rectification column RR.sub.A, and, if (a2) Is performed, the vapour stream S.sub.RDB2 is alternatively or in addition employed as the reactant stream S.sub.BE1 in the second reactive rectification column RR.sub.B.

9. The process according to claim 1, wherein the vapour stream S.sub.RDB1 is at least partially employed as the reactant stream S.sub.AE1 in the first reactive rectification column RR.sub.A, and, if (a2) is performed, the vapour stream S.sub.RDB1 is alternatively or in addition employed as the reactant stream S.sub.BE1 in the second reactive rectification column RR.sub.B.

10. The process according to claim 1, wherein a stream S.sub.XE1, distinct from the reactant stream S.sub.AE1 and the reactant stream S.sub.BE1 comprising ROH is added to at least one of the columns selected from the group consisting of the first rectification column RD.sub.1, the second rectification column RD.sub.2, and the first reactive rectification column RR.sub.A, and, if (a2) is performed, the stream S.sub.XE1 is alternatively or in addition added to the second reactive rectification column RR.sub.B.

11. The process according to claim 1, wherein R is methyl or ethyl.

12. The process according to claim 1, wherein (a2) is performed.

13. The process according to claim 1, wherein p.sub.3A>p.sub.1, and wherein if (a2) is performed, p.sub.3A>p.sub.1.

14. The process according to claim 1, wherein the bottoms stream S.sub.RDS2 comprises water and ROH.

15. The process according to claim 1, wherein the process is carried out continuously.

16. The process according to claim 3, wherein the at least one stream S.sub.RDX1 is withdrawn below the vapour stream S.sub.RDB1 on the first rectification column RD.sub.1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0060] FIG. 1 shows a process not according to the invention for producing alkali metal alkoxides with a corresponding interconnection of the rectification columns.

[0061] FIG. 2 shows a further process not according to the Invention for producing alkali metal alkoxides.

[0062] FIG. 3 shows one embodiment of the process according to the invention for producing alkali metal alkoxides with a corresponding interconnection of the reactive rectification and rectification columns.

[0063] FIG. 4 shows a further embodiment of the process according to the invention for producing alkali metal alkoxides with a corresponding interconnection of the rectification columns.

[0064] FIG. 5 shows a further embodiment of the process according to the invention.

[0065] FIG. 6 shows a further embodiment of the process according to the invention for producing alkali metal alkoxides with a corresponding interconnection of the rectification columns.

[0066] FIG. 7 shows an embodiment of the process according to the invention which corresponds to that described in FIG. 5.

[0067] FIG. 8 shows an embodiment of the process according to the invention which corresponds to that described in FIG. 6.

[0068] FIG. 9 shows an embodiment of the process according to the invention which corresponds to that described in FIG. 5.

[0069] FIG. 10 shows an embodiment of the process according to the invention which corresponds to that described in FIG. 6.

[0070] FIG. 11 shows an embodiment of the process according to the invention which shows a combination of the embodiments according to FIGS. 7 and 9.

[0071] FIG. 12 shows an embodiment of the process according to the invention which corresponds to a combination of the embodiments of FIGS. 8 and 10.

DETAILED DESCRIPTION OF THE INVENTION

4.1 Step (a1) of the Process According to the Invention

[0072] In step (a1) of the process according to the invention for producing at least one alkali metal alkoxide of formula M.sub.AOR a reactant stream S.sub.AE1 comprising ROH is reacted with a reactant stream S.sub.AE2 comprising M.sub.AOH in countercurrent at a pressure p.sub.3A and a temperature T.sub.3A in a reactive rectification column RR.sub.A to afford a crude product RP.sub.A comprising M.sub.AOR, water, ROH, M.sub.AOH.

[0073] According to the invention, a “reactive rectification column” is a rectification column in which the reaction according to step (a1) or step (a2) of the process according to the invention proceeds at least in some parts. It may also be abbreviated to “reaction column”.

[0074] In step (a1) of the process according to the invention a bottoms product stream S.sub.AP comprising ROH and M.sub.AOR is withdrawn at the lower end of RR.sub.A. A vapour stream S.sub.AB comprising water and ROH is withdrawn at the upper end of RR.sub.A.

[0075] “Vapour stream” means that the respective stream is a gaseous stream.

[0076] In the process according to the invention, R is a C.sub.1-C.sub.6 hydrocarbon radical, preferably selected from the group consisting of methyl, ethyl, n-propyl, iso-propyl, n-butyl, sec-butyl, iso-butyl, tert-butyl, isomers of pentyl, such as n-pentyl, more preferably selected from the group consisting of methyl, ethyl, n-propyl, iso-propyl, yet more preferably selected from the group consisting of methyl, ethyl, R is particularly preferably methyl and ROH is accordingly methanol.

[0077] M.sub.A is selected from sodium, potassium, and preferably sodium.

[0078] The reactant stream S.sub.AE1 comprises ROH. In a preferred embodiment the mass fraction of ROH In S.sub.AE1, based on the total mass of the reactant stream S.sub.AE1, is ≥95% by weight, yet more preferably ≥99% by weight, wherein S.sub.AE1 otherwise comprises especially water.

[0079] The alcohol ROH used as reactant stream S.sub.AE1 in step (a1) of the process according to the invention can also be a commercially available alcohol having a mass fraction of alcohol, based on the total mass of the reactant stream S.sub.AE1, of more than 99.8% by weight and a mass fraction of water, based on the total mass of the reactant stream S.sub.AE1, of up to 0.2% by weight.

[0080] The reactant stream S.sub.AE1 is preferably introduced in vapour form.

[0081] The reactant stream S.sub.AE2 comprises M.sub.AOH. In a preferred embodiment S.sub.AE2 comprises not only M.sub.AOH but also at least one further compound selected from water, ROH. S.sub.AE2 more preferably also comprises water in addition to M.sub.AOH, in which case S.sub.An is an aqueous solution of M.sub.AOH.

[0082] When the reactant stream S.sub.AE comprises M.sub.AOH and water, the mass fraction of M.sub.AOH, based on the total weight of the reactant stream S.sub.AE2, is especially in the range from 10% to 55% by weight, preferably from 15% to 54% by weight, more preferably from 30% to 53% by weight and particularly preferably from 45% to 52% by weight, most preferably 50% by weight.

[0083] When the reactant stream S.sub.AE2 comprises M.sub.AOH and ROH, the mass fraction of M.sub.AOH, based on the total weight of the reactant stream S.sub.AE2, is especially in the range from 10% to 55% by weight, preferably from 15% to 54% by weight, more preferably from 30% to 53% by weight and particularly preferably from 45% to 52% by weight.

[0084] In the particular case in which the reactant stream S.sub.AE2 comprises both water and ROH in addition to M.sub.AOH, it is particularly preferable when the mass fraction of M.sub.AOH, based on the total weight of the reactant stream S.sub.AE2, is especially in the range from 10% to 55% by weight, preferably from 15% to 54% by weight, more preferably from 30% to 53% by weight and particularly preferably from 45% to 52% by weight.

[0085] Step (a1) of the process according to the invention is performed in a reactive rectification column (or “reaction column”) RR.sub.A.

[0086] Step (a2) of the process according to the invention is performed in a reactive rectification column (or “reaction column”) RR.sub.B.

[0087] The reaction column RR.sub.A/RR.sub.B preferably contains internals. Suitable internals are, for example, trays, structured packings or unstructured packings. When the reaction column RR.sub.A/RR.sub.B contains trays, bubble cap trays, valve trays, tunnel trays, Thormann trays, cross-slit bubble cap trays or sieve trays are suitable. When the reaction column RR.sub.A/RR.sub.B contains trays, it is preferable to choose trays where not more than 5% by weight, more preferably less than 1% by weight, of the liquid trickles through the respective trays. The constructional measures required to minimize trickle-through of the liquid are familiar to those skilled in the art. In the case of valve trays, particularly tightly closing valve designs are selected for example. Reducing the number of valves also makes it possible to increase the vapour velocity in the tray openings to twice the value typically established. When using sieve trays it is particularly advantageous to reduce the diameter of the tray openings while maintaining or even increasing the number of openings.

[0088] When using structured or unstructured packings, structured packings are preferred in terms of uniform distribution of the liquid. In this embodiment it is further preferable when in all parts of the column cross section corresponding to more than 2% of the total column cross section the average ratio of liquid stream to vapour stream is not exceeded by more than 15%, more preferably by more than 3%. In respect of the liquid. This minimized liquid amount makes it possible for the capillary effect at the wire meshes to eliminate local peaks of liquid trickling density.

[0089] For columns comprising unstructured packings, especially comprising random packings, and for columns comprising structured packings, the desired characteristics of the liquid distribution may be achieved when the liquid trickling density in the edge region of the column cross section adjacent to the column shell, corresponding to about 2% to 5% of the total column cross section, is reduced compared to the other cross-sectional regions by up to 100%, preferably by 5% to 15%. This can easily be achieved by, for example, targeted distribution of the drip points of the liquid distributors or the holes thereof.

[0090] The process according to the Invention may be carried out either continuously or discontinuously. It is preferably carried out continuously.

[0091] According to the Invention “reaction of a reactant stream S.sub.AE1 comprising ROH with a reactant stream S.sub.AE1 comprising M.sub.AOH in countercurrent” is achieved, in particular, as a result of the feed point for at least a portion of the reactant stream S.sub.AE1 comprising ROH in step (a1) being located on the reaction column RR.sub.A below the feed point of the reactant stream S.sub.AE1 comprising M.sub.AOH.

[0092] The reaction column RR.sub.A preferably comprises at least 2, in particular 15 to 40, theoretical plates between the feed point of the reactant stream S.sub.AE1 and the feed point of the reactant stream S.sub.AE2.

[0093] The reaction column RR.sub.A is preferably operated as a pure stripping column. Accordingly the reactant stream S.sub.AE1 comprising ROH is especially supplied in vaporous form in the lower region of the reaction column RR.sub.A. Step (a1) of the process according to the Invention also comprises the case where a portion of the reactant stream S.sub.AE1 comprising ROH is added in vapour form below the feed point of the reactant stream S.sub.AE2 comprising M.sub.AOH but nevertheless at the upper end or in the region of the upper end of the reaction column RR.sub.A. This makes it possible to reduce the dimensions in the lower region of the reaction column RR.sub.A. When a portion of the reactant stream S.sub.AE1 comprising ROH, in particular methanol, is added especially in vaporous form at the upper end or in the region of the upper end of the reaction column RR.sub.A, only a fraction of 10% to 70% by weight, preferably of 30% to 50% by weight (in each case based on the total amount of the alcohol ROH employed in step (a1) as S.sub.AE1) is supplied at the lower end of the reaction column RR.sub.A and the remaining fraction is added in vaporous form in a single stream or divided into a plurality of substreams preferably 1 to 10 theoretical trays, particularly preferably 1 to 3 theoretical trays, below the feed point of the reactant stream S.sub.AE2 comprising M.sub.AOH.

[0094] In the reaction column RR.sub.A, the reactant stream S.sub.AE1 comprising ROH is then reacted with the reactant stream S.sub.AE1 comprising M.sub.AOH according to the reaction <1> described hereinabove to afford M.sub.AOR and H.sub.2O, where these products are present in admixture with the reactants ROH and M.sub.AOH since an equilibrium reaction is concerned. Accordingly a crude product RP.sub.A which contains not only the products M.sub.AOR and water but also ROH and M.sub.AOH is obtained in the reaction column RR.sub.B in step (a1) of the process according to the invention.

[0095] The bottoms product stream SA comprising ROH and M.sub.AOR is then obtained and withdrawn at the lower end of RR.sub.A.

[0096] A water-containing alcohol stream, presently referred to as “vapour stream S.sub.AB comprising water and ROH”, is withdrawn at the upper end of RR.sub.A, preferably at the column top of RR.sub.A.

[0097] The amount of the alcohol ROH comprised by the reactant stream S.sub.AE1 is preferably chosen such that said alcohol simultaneously serves as a solvent for the alkali metal alkoxide M.sub.AOR obtained in the bottoms product stream S.sub.AP. The amount of the alcohol ROH in the reactant stream S.sub.AE1 Is preferably chosen to achieve in the bottom of the reaction column RR.sub.A the desired concentration of the alkali metal alkoxide solution which is withdrawn as a bottoms product stream SAP comprising ROH and M.sub.AOR.

[0098] In a preferred embodiment of the process according to the invention, and especially in cases in which S.sub.AE2 also comprises water in addition to M.sub.AOH, the ratio of the total weight (masses; unit:kg) of alcohol ROH used as reactant stream S.sub.AE1 in step (a1) to the total weight (masses; unit:kg) of M.sub.AOH used as reactant stream S.sub.AE2 in step (a1) is from 1:1 to 50:1, more preferably 5:1 to 48:1, yet more preferably 9:1 to 35:1, yet still more preferably 10:1 to 30:1, yet still more preferably 13:1 to 22:1, most preferably 14:1.

[0099] The reaction column RR.sub.A is operated with or without, preferably without, reflux.

[0100] “Without reflux” means that the vapour stream S.sub.AB withdrawn at the upper end of RR.sub.A comprising water and ROH is completely supplied to the first rectification column RD.sub.1 according to step (b). The vapour stream S.sub.AB comprising water and ROH is preferably supplied to the first rectification column RD.sub.1 in vaporous form.

[0101] “With reflux” means that the vapour stream S.sub.AB withdrawn at the upper end of the respective column, the reaction column RR.sub.A in step (a1), comprising water and ROH is not completely discharged, i.e. is not completely supplied to the first rectification column RD.sub.1 in step (b), but rather Is at least partially, preferably partially, recycled to the respective column, i.e. the reaction column RR.sub.A in step (a1), as reflux. In the cases where such a reflux is established, the reflux ratio is preferably 0.05 to 0.99, more preferably 0.1 to 0.9, yet more preferably 0.11 to 0.34, particularly preferably 0.14 to 0.27 and very particularly preferably 0.17 to 0.24. A reflux may be established by attaching to the top of the respective column, the reaction column RR.sub.A in step (a1), a condenser K.sub.RRA in which the vapour stream S.sub.AB is at least partially condensed and sent back to the respective column, the reaction column RR.sub.A in step (a1). Generally and in the context of the present invention, a reflux ratio is to be understood as meaning the ratio of the mass flow (kg/h) recycled to the respective column in liquid form (reflux) to the mass flow (kg/h) discharged from the respective column in liquid form (distillate) or gaseous form (vapour).

[0102] In the embodiment in which a reflux is established on the reaction column RR.sub.A, the alcohol M.sub.AOH employed in step (a1) as reactant stream S.sub.AE2 may also be at least partially, preferably partially, mixed with the reflux stream and the resulting mixture thus be supplied to step (a1).

[0103] Step (a1) of the process according to the Invention is in particular performed at a temperature T.sub.3A in the range from 25° C. to 200° C., preferably in the range from 45° C. to 150° C., more preferably in the range from 47° C. to 120° C., more preferably in the range from 80° C. to 110° C.

[0104] Step (a1) of the process according to the invention is in particular performed at a pressure p.sub.3A of 0.5 bar to 40 bar, preferably in the range from 0.75 bar to 5 bar, more preferably in the range from 1 bar to 2 bar, more preferably in the range from 1 bar to 1.8 bar, yet more preferably at 1.1 bar to 1.6 bar. It is an essential feature of the invention that when establishing the pressure p.sub.3A:p.sub.2>p.sub.3A. It is especially also the case that p.sub.3A>p.sub.1.

[0105] The reaction column RR.sub.A comprises in a preferred embodiment at least one evaporator which is in particular selected from intermediate evaporators VZ.sub.3A and bottoms evaporators VS.sub.3A. The reaction column RR.sub.A particularly preferably comprises at least one bottoms evaporator VS.sub.3A. Evaporators are special embodiments of heat exchangers WT.

[0106] Condensers K are likewise special embodiments of heat exchangers WT. Typical condensers are known to those skilled in the art. These are preferably employed as liquefiers at the top of rectification columns and reaction columns. In the direct energy transfer from the top stream of one column to the bottoms or intermediate stream of another column a condenser of one column may simultaneously be employed as an evaporator of the other column (as shown in the examples).

[0107] According to the Invention “intermediate evaporators” VZ (for example VZ.sub.3A in RR.sub.A, VZ.sub.3B in RR.sub.B, VZ.sub.RD1 in RD.sub.1, VZ.sub.RD2 in RD.sub.2) are to be understood as meaning evaporators arranged above the bottom of the respective column, in particular above the bottom of the reaction column RR.sub.A/RR.sub.B or above the bottom of the rectification column RD.sub.1 or RD.sub.2. They in particular evaporate crude product RP.sub.A/RP.sub.B or S.sub.RDX1Z as sidestream.

[0108] According to the invention “bottoms evaporators” VS (for example VS.sub.3A in RR.sub.A, VS.sub.3B in RR.sub.B, VS.sub.RD1 in RD.sub.1, VS.sub.RD2 in RD.sub.2) are to be understood as meaning evaporators which heat the bottom of the respective column, in particular the bottom of the reaction column RR.sub.A/RR.sub.B or rectification column RD.sub.1 or RD.sub.2. They evaporate bottoms product stream (for example S.sub.AP/S.sub.BP or S.sub.RDX1S).

[0109] An evaporator is typically arranged outside the respective reaction column or rectification column. The mixture to be evaporated in the evaporator is withdrawn from the column via a takeoff or “withdrawal point” and supplied to the at least one evaporator.

[0110] The evaporated mixture is recycled back into the respective column, optionally with a residual proportion of liquid, via a feed or “feed point”. When the evaporator is an intermediate evaporator, the takeoff by means of which the respective mixture is withdrawn and supplied to the evaporator is a sidestream takeoff and the feed by means of which the evaporated respective mixture is sent back to the column is a sidestream feed. When the evaporator is a bottoms evaporator, i.e. heats the column bottom, at least a portion of the bottom takeoff stream is evaporated and recycled back into the respective column in the region of the column bottom.

[0111] However, it is alternatively also possible for example on a suitable tray when using an intermediate evaporator or in the bottom of the respective column to provide tubes which are traversed by the relevant heating medium. In this case, the evaporation occurs on the tray or in the bottom of the column. However, it is preferable to arrange the evaporator outside the respective column. Suitable evaporators employable as intermediate evaporators and bottoms evaporators include for example natural circulation evaporators, forced circulation evaporators, forced circulation evaporators with decompression, steam boilers, falling film evaporators or thin film evaporators. Heat exchangers for evaporation typically employed in the case of natural circulation evaporators and forced circulation evaporators are shell and tube or plate apparatuses. When using a shell and tube exchanger the heating medium may flow either through the tubes with the mixture to be evaporated flowing around the tubes or else the heating medium may flow around the tubes with the mixture to be evaporated flowing through the tubes. In the case of a falling-film evaporator, the mixture to be evaporated is typically introduced as a thin film on the inside of a tube and the tube is heated externally. In contrast to a falling-film evaporator, a thin-film evaporator additionally comprises a rotor with wipers which distributes the liquid to be evaporated on the inner wall of the tube to form a thin film.

[0112] In addition to the recited evaporator types it is however also possible to employ any desired further evaporator type known to those skilled in the art and suitable for use on a rectification column.

[0113] When the reaction column RR.sub.A/reaction column RR.sub.B has an intermediate evaporator VZ.sub.3A or VZ.sub.3B it is preferable when the respective intermediate evaporator is arranged in the stripping region of the reaction column RR.sub.A in the region of the feed point of the reactant stream S.sub.AE1 or in the case of the reaction column RR.sub.A in the region of the feed point of the reactant stream S.sub.AE1. This makes it possible to introduce a predominant portion of the heat energy via the intermediate evaporator VZ.sub.3A/VZ.sub.3B. It is thus possible for example to introduce more than 80% of the energy via the intermediate evaporator. According to the invention the intermediate evaporator VZ.sub.3A/VZ.sub.3B is preferably arranged and/or configured such that it introduces more than 50%, in particular more than 75%, of the total energy required for the reactive rectification.

[0114] When the reaction column RR.sub.A/reaction column RR.sub.B has an intermediate evaporator VZ.sub.3A or VZ.sub.3B it is additionally advantageous when the intermediate evaporator is arranged such that the reaction column RR.sub.A/RR.sub.B has 1 to 50 theoretical trays below the intermediate evaporator and has 1 to 200 theoretical trays above the intermediate evaporator. It is especially preferred when the reaction column RR.sub.A/RR.sub.B then has 2 to 10 theoretical trays below the intermediate evaporator and has 20 to 50 theoretical trays above the intermediate evaporator.

[0115] When the reaction column RR.sub.A/reaction column RR.sub.B has an intermediate evaporator VZ.sub.3A/VZ.sub.3B it is also advantageous when the sidestream takeoff (i.e. the “withdrawal point E.sub.RRA” on the reaction column RR.sub.A/the “withdrawal point E.sub.RRB” on the reaction column RR.sub.B) by means of which the crude product RP.sub.A/RP.sub.B is supplied to the intermediate evaporator VZ.sub.3A/VZ.sub.3B and the sidestream feed (i.e. the “withdrawal point Z.sub.RRA” on the reaction column RR.sub.A/the “withdrawal point Z.sub.RRB” on the reaction column RR.sub.B) by means of which the evaporated crude product RP.sub.A/RP.sub.B from the intermediate evaporator VZ.sub.3A/VZ.sub.3B is sent back to the respective reaction column RR.sub.A/RR.sub.B are positioned between the same plates of the reaction column RR.sub.A/reaction column RR.sub.B. However, it is also possible for the sidestream takeoff and sidestream feed to be arranged at different heights.

[0116] In a preferred embodiment when using an intermediate evaporator VZ.sub.3A/VZ.sub.3B in RR.sub.A/RR.sub.B the diameter of the reaction column RR.sub.A/RR.sub.B above the intermediate evaporator RR.sub.A/RR.sub.B is greater than the diameter of the reaction column RR.sub.A/RR.sub.B below the intermediate evaporator VZ.sub.3A/VZ.sub.3B. This has the advantage of allowing capital expenditure savings.

[0117] In such an intermediate evaporator VZ.sub.3A/VZ.sub.3B liquid crude product RP.sub.A comprising M.sub.AOR, water, ROH, M.sub.AOH present in the reaction column RR.sub.A or liquid crude product RP.sub.B comprising M.sub.BOR, water. ROH, M.sub.BOH present in the reaction column RR.sub.B may be converted into the gaseous state or, if already in the gaseous state, heated further, thus Improving the efficiency or the reaction according to step (a1)/(a2) in the process according to the invention.

[0118] Arranging one or more intermediate evaporators VZ.sub.3A in the upper region of the reaction column RR.sub.A or one or more intermediate evaporators VZ.sub.3B in the upper region of the reaction column RR.sub.B makes it possible to reduce the dimensions in the lower region of the reaction column RR.sub.A/RR.sub.B. In the embodiment having at least one, preferably two or more, intermediate evaporators VZ.sub.3A/VZ.sub.3B, it is also possible to supply substreams of the ROH in liquid form in the upper region of the reaction column RR.sub.A/RR.sub.B.

[0119] According to the invention bottoms evaporators are arranged at the bottom of the reaction column RR.sub.A/RR.sub.B and are then referred to as “VS.sub.3A” and “VS.sub.3B”. Bottoms product stream S.sub.AP/S.sub.BP present in the reaction column RR.sub.A/RR.sub.B may be passed into such a bottoms evaporator and ROH at least partially removed therefrom to obtain a bottoms product stream S.sub.AP′ having an elevated mass fraction of M.sub.AOR compared to S.sub.AP/to obtain a bottoms product stream S.sub.BP′ having an elevated mass fraction of M.sub.BOR compared to S.sub.BP.

[0120] In step (a1) of the process according to the invention a bottoms product stream S.sub.AP comprising ROH and M.sub.AOR is withdrawn at the lower end of the reaction column RR.sub.A.

[0121] It is preferable when the reaction column RR.sub.A comprises at least one bottoms evaporator VS.sub.3A through which the bottoms product stream S.sub.AP is then at least partially passed to at least partially remove ROH, thus affording a bottoms product stream S.sub.AP′ having an elevated mass fraction of M.sub.AOR compared to S.sub.AP.

[0122] The mass fraction of M.sub.AOR in the bottoms product stream S.sub.AP is especially elevated compared to the mass fraction of M.sub.AOR in the bottoms product stream S.sub.AP by at least 1%, preferably by ≥2%, more preferably by ≥5%, yet more preferably by ≥10%, yet still more preferably by ≥20%, yet still more preferably by ≥30%, yet still more preferably by ≥40%, yet still more preferably by ≥50%, yet still more preferably by ≥100%, yet still more preferably by ≥150%.

[0123] It is preferable when S.sub.AP or, if at least one bottoms evaporator VS.sub.3A through which the bottoms product stream S.sub.AP is at least partially passed to at least partially remove ROH is used, S.sub.AP′ has a mass fraction of M.sub.AOR in ROH in the range from 1% to 50% by weight, preferably 5% to 32% by weight, more preferably 15% to 32% by weight, most preferably 30% to 32% by weight, in each case based on the total mass of S.sub.AP/S.sub.AP′.

[0124] The mass fraction of residual water in S.sub.AP/S.sub.AP is preferably <1% by weight, preferably <0.1% by weight, more preferably <0.01% by weight, based on the total mass of SP/S.sub.AP′.

[0125] The mass fraction of reactant M.sub.AOH in S.sub.AP/S.sub.AP′ is preferably <1% by weight, preferably <0.1% by weight, more preferably <0.01% by weight, based on the total mass or S.sub.AP/S.sub.AP′.

4.2 Step (a2) of the Process According to the Invention (Optional)

[0126] According to the invention step (a2) is performed or else not performed. In the optional step (a2), which proceeds simultaneously with and spatially separately from step (a1) of the process according to the invention, a reactant stream S.sub.BE1 comprising ROH is reacted with a reactant stream S.sub.BE2 comprising M.sub.BOH in countercurrent at a pressure pau and a temperature T.sub.3B in a reactive rectification column RR.sub.B to afford a crude product RPs comprising M.sub.BOR, water. ROH, M.sub.BOH.

[0127] In the optional step (a2) of the process according to the invention a bottoms product stream S.sub.BP comprising ROH and M.sub.BOR is withdrawn at the lower end of RR.sub.B. A vapour stream S.sub.BB comprising water and ROH is withdrawn at the top end of RR.sub.B.

[0128] M.sub.B is selected from sodium, potassium, and preferably potassium.

[0129] The reactant stream S.sub.BE1 comprises ROH. In a preferred embodiment the mass fraction of ROH in S.sub.BE1, based on the total mass of the reactant stream S.sub.BE1, is ≥95% by weight, yet more preferably ≥99% by weight, wherein S.sub.BE1 otherwise comprises especially water.

[0130] The alcohol ROH used as reactant stream S.sub.BE1 in the optional step (a2) of the process according to the invention can also be a commercial alcohol having a mass fraction of alcohol, based on the total mass of the reactant stream S.sub.BE1, of more than 99.8% by weight and a mass fraction of water, based on the total mass of the reactant stream S.sub.BE1, of up to 0.2% by weight.

[0131] The reactant stream S.sub.BE1 is preferably introduced in vapour form.

[0132] The reactant stream S.sub.BE2 comprises M.sub.BOH. In a preferred embodiment S.sub.BE2 comprises not only M.sub.BOH but also at least one further compound selected from water. ROH. It is yet more preferable when S.sub.BE2 comprises water in addition to M.sub.BOH, thus rendering S.sub.BE2 an aqueous solution of M.sub.BOH.

[0133] When the reactant stream S.sub.BE2 comprises M.sub.BOH and water, the mass fraction of M.sub.BOH, based on the total weight of the reactant stream S.sub.BE2, is especially in the range from 10% to 55% by weight, preferably from 15% to 54% by weight, more preferably from 30% to 53% by weight and particularly preferably from 45% to 52% by weight, most preferably 50% by weight.

[0134] When the reactant stream S.sub.BE2 comprises M.sub.BOH and ROH, the mass fraction of M.sub.BOH, based on the total weight of the reactant stream S.sub.BE2, is especially in the range from 10% to 55% by weight, preferably from 15% to 54% by weight, more preferably from 30% to 53% by weight and particularly preferably from 45% to 52% by weight.

[0135] In the particular case in which the reactant stream S.sub.BE2 comprises both water and ROH in addition to M.sub.BOH, it is particularly preferable when the mass fraction of M.sub.BOH, based on the total weight of the reactant stream S.sub.BE2, is especially in the range from 10% to 55% by weight, preferably from 15% to 54% by weight, more preferably from 30% to 53% by weight and particularly preferably from 45% to 52% by weight.

[0136] Step (a2) of the process according to the invention is performed in a reactive rectification column (or “reaction column”) RR.sub.B. Preferred embodiments of the reaction column RR.sub.B are described in section 4.1.

[0137] According to the invention “reaction of a reactant stream S.sub.BE1 comprising ROH with a reactant stream S.sub.BE2 comprising M.sub.BOH in countercurrent” is especially achieved as a result of the feed point for at least a portion of the reactant stream S.sub.BE1 comprising ROH in the optional step (a2) being arranged below the feed point for the reactant stream S.sub.BE2 comprising M.sub.BOH on the reaction column RR.sub.B.

[0138] The reaction column RR.sub.B preferably comprises at least 2, in particular 15 to 40, theoretical plates between the feed point of the reactant stream S.sub.BE1 and the feed point of the reactant stream S.sub.BE2.

[0139] The reaction column RR.sub.B is preferably operated as a pure stripping column. Accordingly the reactant stream S.sub.BE1 comprising ROH is especially supplied in vaporous form in the lower region of the reaction column RR.sub.B. The optional step (a2) of the process according to the invention also encompasses the case of part of the reactant stream S.sub.BE1 comprising ROH being introduced in vapour form below the feed point of the reactant stream S.sub.BE1 comprising aqueous sodium hydroxide solution M.sub.BOH but nevertheless at the upper end or in the region of the upper end of the reaction column RR.sub.B. This makes it possible to reduce the dimensions in the lower region of the reaction column RR.sub.B. When a portion of the reactant stream S.sub.BE1 comprising ROH, in particular methanol, is added especially in vaporous form at the upper end or in the region of the upper end of the reaction column RR.sub.B only a fraction of in particular 10% to 70% by weight, preferably of 30% to 50% by weight (in each case based on the total amount of the alcohol ROH employed in the optional step (a2)) is supplied at the lower end of the reaction column RR.sub.B and the remaining fraction is added in vaporous form in a single stream or divided into a plurality of substreams, preferably 1 to 10 theoretical trays, particularly preferably 1 to 3 theoretical trays, below the feed point of the reactant stream S.sub.BE2 comprising M.sub.BOH.

[0140] In the reaction column RR.sub.B the reactant stream S.sub.BE1 comprising ROH is then reacted with the reactant stream S.sub.BE2 comprising M.sub.BOH according to the reaction <1> described hereinabove to afford M.sub.BOR and H.sub.2O, where these products are present in admixture with the reactants ROH and M.sub.BOH since an equilibrium reaction is concerned. Accordingly a crude product RP.sub.B which contains not only the products M.sub.BOR and water but also ROH and M.sub.BOH is obtained in the reaction column RR.sub.B in optional step (a2) of the process according to the invention.

[0141] The bottoms product stream S.sub.BP comprising ROH and M.sub.BOR is then obtained and withdrawn at the lower end of RR.sub.B.

[0142] A water-containing alcohol stream, presently referred to as “vapour stream S.sub.BB comprising water and ROH”, is withdrawn at the upper end of RR.sub.B, preferably at the top or RR.sub.B.

[0143] This vapour stream S.sub.BB comprising water and ROH is supplied to step (b) of the process according to the invention. Said stream is mixed with S.sub.AB before being supplied to step (b) of the process according to the invention or is not, i.e. is supplied to step (b) of the process according to the invention separately from S.sub.AB. Vapour stream S.sub.BB is preferably mixed with S.sub.AB and the resulting mixed vapour stream is then introduced into step (b) of the process according to the invention.

[0144] The amount of the alcohol ROH comprised by the reactant stream S.sub.BE1 is preferably selected so that it simultaneously serves as solvent for the alkali metal alkoxide M.sub.BOR present in the bottoms product stream S.sub.BP. The amount of the alcohol ROH in the reactant stream S.sub.BE1 is preferably chosen to achieve in the bottom of the reaction column the desired concentration of the alkali metal alkoxide solution which is withdrawn as a bottoms product stream S.sub.BP comprising ROH and M.sub.BOR.

[0145] In a preferred embodiment of optional step (a2) of the process according to the invention, and especially in the cases where S.sub.BE2 also comprises water in addition to M.sub.BOH, the ratio of the total weight (masses; units:kg) of alcohol ROH employed in step (a2) as reactant stream S.sub.BE2 to the total weight (masses; unit:kg) of M.sub.BOH employed in step (a2) as reactant stream S.sub.BE2 is 1:1 to 50:1, more preferably 5:1 to 48:1, yet more preferably 9:1 to 35:1, yet still more preferably 10:1 to 30:1, yet still more preferably 13:1 to 22:1, most preferably 14:1.

[0146] The reaction column RR.sub.B is operated with or without, preferably without, reflux.

[0147] “Without reflux” means that the vapour stream S.sub.BB withdrawn at the upper end of RR.sub.B comprising water and ROH is completely supplied to the rectification column RD.sub.1 according to step (b). The vapour stream S.sub.BB comprising water and ROH is preferably supplied to the rectification column RD.sub.1 in vaporous form.

[0148] “With reflux” means that the vapour stream S.sub.BB withdrawn at the upper end of the respective column, the reaction column RR.sub.B in step (a2), comprising water and ROH is not completely discharged, i.e. is not completely suppled to the first rectification column RD.sub.1 in step (b), but rather is at least partially, preferably partially, recycled to the respective column, i.e. the reaction column RR.sub.B in step (a2), as reflux. In the cases where such a reflux is established the reflux ratio Is preferably 0.05 to 0.99, more preferably 0.1 to 0.9, yet more preferably 0.11 to 0.34, particularly preferably 0.14 to 0.27 and very particularly preferably 0.17 to 024. A reflux may be established by attaching at the top of the respective column, the reaction column RR.sub.B in step (a2), a condenser K.sub.RRB in which the vapour stream S.sub.BB is at least partially condensed and sent back to the respective column, the reaction column RR.sub.B in step (a2).

[0149] In the embodiment in which a reflux is established on the reaction column RR.sub.B the alcohol M.sub.BOH employed in optional step (a2) as reactant stream S.sub.BE2 may also be at least partially, preferably partially, mixed with the reflux stream and the resulting mixture thus be supplied to step (a2).

[0150] Optional step (a2) of the process according to the invention is in particular performed at a temperature T.sub.3B in the range from 25° C. to 200° C., preferably in the range from 45° C. to 150° C., more preferably in the range from 47° C. to 120° C., more preferably in the range from 60° C. to 110° C.

[0151] Optional step (a2) of the process according to the invention is in particular performed at a pressure pau of 0.5 bar to 40 bar, preferably in the range from 0.75 bar to 5 bar, more preferably in the range from 1 bar to 2 bar, more preferably in the range from 1 bar to 1.8 bar, yet more preferably at 1.1 bar to 1.8 bar. It is an essential feature of the invention that when establishing the pressure p.sub.3B:p.sub.2>p.sub.3B. It is especially also the case that p.sub.3B>p.sub.1.

[0152] In a preferred embodiment the reaction column RR.sub.B comprises at least one evaporator which is in particular selected from intermediate evaporators VZ.sub.B and bottoms evaporators VS.sub.B. The reaction column RR.sub.B particularly preferably comprises at least one bottoms evaporator VS.sub.3B.

[0153] In optional step (a2) of the process according to the invention a bottoms product stream S.sub.BP comprising ROH and M.sub.BOR is withdrawn at the lower end of the reaction column RR.sub.B.

[0154] It is preferable when the reaction column RR.sub.B comprises at least one bottoms evaporator VS.sub.3B through which the bottoms product stream S.sub.BP is then at least partially passed to at least partially remove ROH, thus affording a bottoms product stream S.sub.BP′ having an elevated mass fraction of M.sub.BOR compared to S.sub.BP.

[0155] The mass fraction of M.sub.BOR in the bottoms product stream S.sub.BP′ is especially elevated compared to the mass fraction of M.sub.BOR in the bottoms product stream S.sub.BP by at least 1%, preferably by ≥2%, more preferably by ≥5%, yet more preferably by ≥10%, yet still more preferably by ≥20%, yet still more preferably by ≥30%, yet still more preferably by ≥40%, yet still more preferably by ≥50%, yet still more preferably by ≥100%, yet still more preferably by ≥150%.

[0156] It is preferable when S.sub.BP or, if at least one bottoms evaporator VS.sub.3B through which the bottoms product stream S.sub.BP is at least partially passed to at least partially remove ROH, S.sub.BP′ has a mass fraction of M.sub.BOR in ROH in the range from 1% to 50% by weight, preferably 5% to 32% by weight, more preferably 10% to 32% by weight, most preferably 15% to 30% by weight, in each case based on the total mass of S.sub.BP/S.sub.BP′.

[0157] The mass fraction of residual water in S.sub.BP/S.sub.BP′ is preferably <1% by weight, preferably <0.1% by weight, more preferably <0.01% by weight, based on the total mass of S.sub.BP/S.sub.BP′.

[0158] The mass fraction of reactant M.sub.BOH in S.sub.BP/S.sub.BP is preferably <1% by weight, preferably <0.1% by weight, more preferably <0.01% by weight, based on the total mass of S.sub.BP/S.sub.BP′.

[0159] In the embodiments of the present process in which step (a2) is also performed, it is preferable when the bottoms product stream S.sub.AP is at least partially passed through a bottoms evaporator VS.sub.3A and ROH is at least partially removed from S.sub.AP to afford a bottoms product stream S.sub.AP′ having an elevated mass fraction of M.sub.AOR compared to S.sub.AP and/or, preferably and, the bottoms product stream S.sub.BP is at least partially passed through a bottoms evaporator VS.sub.3B and ROH is at least partially removed from S.sub.BP to afford a bottoms product stream S.sub.BP′ having an elevated mass fraction of M.sub.BOR compared to S.sub.BP.

[0160] In the embodiments of the present invention in which it is performed, step (a2) of the process according to the invention is performed simultaneously with and spatially separate from step (a1). Spatial separation is ensured by performing steps (a1) and (a2) in the two reaction columns RR.sub.A and RR.sub.B.

[0161] In an advantageous embodiment of the invention the reaction columns RR.sub.A and RR.sub.B are accommodated in one column shell, where the column is at least partially subdivided by at least one dividing wall. Such a column having at least one dividing wall will according to the invention be referred to as “DWC”. Such dividing wall columns are familiar to those skilled in the art and are described for example in U.S. Pat. No. 2,295,256, EP 0 122 367 A2. EP 0 126 288 A2, WO 2010/097318 A1 and I. Dejanović, L J. Matijašvić, Ž. Olujić, Chemical Engineering and Processing 2010, 49, 559-580. In the dividing wall columns suitable for the process according to the invention, the dividing walls preferably extend to the column floor and, in particular, preferably span at least a quarter, more preferably at least a third, yet more preferably at least half, yet more preferably at least two thirds, yet still more preferably at least three quarters, of the column by height. They divide the columns into at least two reaction spaces in which spatially separate reactions may be carried out. The reaction spaces provided by the at least one dividing wall may be of identical or different sizes.

[0162] In this embodiment the bottoms product streams S.sub.AP and S.sub.BP may be separately withdrawn in the respective regions separated by the dividing wall and preferably passed through the bottoms evaporator VS.sub.3A/VS.sub.3B attached for each reaction space formed by the at least one reaction wall in which ROH is at least partially removed from S.sub.AP/S.sub.BP to afford S.sub.AP′/S.sub.BP′.

4.3 Step (b) of the Process According to the Invention

[0163] In step (b) of the process according to the invention the vapour stream S.sub.AB and, if step (a2) is performed, the vapour stream S.sub.BB, in admixture with S.sub.AB or separately from S.sub.AB, is passed into a first rectification column RD.sub.1,

[0164] to obtain a mixture G.sub.RD1 comprising water and ROH in the rectification column RD.sub.1.

[0165] In the optional embodiment of the process according to the invention in which step (a2) is performed, the vapour stream S.sub.BB is preferably mixed with S.sub.AB and the obtained mixed vapour S.sub.ABB then introduced into a rectification column RD.sub.1.

[0166] In one embodiment of the present invention (when p.sub.3A<p.sub.1/p.sub.3B<p.sub.1) the vapour stream S.sub.AB and, in the cases where the optional step (a2) is performed, the vapour stream Se may be compressed before they are passed into the rectification column RD.sub.1. This may be effected via a compressor VD.sub.31. However, in the embodiments of the present invention in which p.sub.3A>p.sub.1 and p.sub.3B>p.sub.1 the provision of a compressor VD.sub.31 is not necessary, and it is then possible to save on the provision thereof and thus the electrical energy required therefor.

[0167] It will be appreciated that even in the embodiments in which the optional step (a2) is performed, and S.sub.BB is introduced into the rectification column RD.sub.1 separately from S.sub.AB, S.sub.AB and S.sub.BB undergo mixing in the rectification column RD.sub.1 with the result that a mixture G.sub.RD1 comprising water and ROH is always obtained in the first rectification column RD.sub.1 after performance of step (b).

[0168] Any desired rectification column known to those skilled in the art may be employed as rectification column RD.sub.1 in step (b) of the process according to the invention. The rectification column RD.sub.1 preferably contains internals. Suitable internals are, for example, trays, unstructured packings or structured packings. As trays, use is normally made of bubble cap trays, sieve trays, valve trays, tunnel trays or slit trays. Unstructured packings are generally beds of random packing elements. Random packing elements normally used are Raschig rings, Pall rings, Berl saddles or Intalox® saddles. Structured packings are for example marketed under the trade name Mellapack® from Sulzer. Apart from the internals mentioned, further suitable internals are known to a person skilled in the art and can likewise be used.

[0169] Preferred internals have a low specific pressure drop per theoretical plate. Structured packings and random packing elements have, for example, a significantly lower pressure drop per theoretical plate than trays. This has the advantage that the pressure drop in the rectification column remains as low as possible and thus the mechanical power of the compressor and the temperature of the alcohol/water mixture G.sub.RD1 to be evaporated remains low.

[0170] When the rectification column RD.sub.1 contains structured packings or unstructured packings these may be divided or in the form of an uninterrupted packing. However, typically at least two packings are provided, one packing above the feed point of the vapour stream S.sub.AB/the feed points of the two vapour streams S.sub.AB and S.sub.BB and one packing below the feed point of the vapour stream S.sub.AB/the feed points of the two vapour streams S.sub.AB and S.sub.BB/the feed point of the mixed vapour S.sub.ABB. If an unstructured packing is used, for example a random packing, the random packing elements are typically disposed on a suitable sieve tray or mesh tray.

[0171] At the end of step (b) of the process according to the invention, a mixture G.sub.RD1 Comprising water and ROH is finally obtained in the rectification column RD.sub.1. The composition of the mixture G.sub.RD1 results in particular from the composition of the vapour stream S.sub.AB and, if step (a2) is performed, partly from the composition of the two vapour streams S.sub.AB and S.sub.BB in particular.

4.4 Step (c) of the Process According to the Invention

[0172] In step (c) of the process according to the invention the mixture G.sub.RD1 comprising water and ROH is in the first rectification column RD.sub.1 at a pressure p.sub.1 and a temperature T.sub.1 separated into an ROH-comprising vapour stream S.sub.RDB1 at the upper end (=top) of RD.sub.1 and a bottoms stream S.sub.RDS1 comprising water and ROH at the lower end (=bottom) of RD.sub.1.

[0173] With the exception of the proviso that p.sub.2>p.sub.1, the pressure p.sub.1 in RD.sub.1 may be chosen by those skilled in the art according to their knowledge of the art. It is preferably in the range between 1 bar and 20 bar, preferably 1 bar and 15 bar, more preferably 1 to 10 bar, yet more preferably 1.00 to 3.00 bar, yet more preferably 1.00 to 2.00 bar, yet more preferably 1.10 to 1.50 bar, wherein, simultaneously, p.sub.2>p.sub.1.

[0174] The temperature T.sub.1 in RD.sub.1 may be chosen by those skilled in the art according to their knowledge of the art. It is preferably in the range from 40° C. to 220° C., preferably from 60° C. to 190° C.

[0175] In a preferred embodiment, p.sub.3A>p.sub.1 and, in the cases where step (a2) is performed, in addition p.sub.3B>p.sub.1. As a result of this established pressure the total energy demand of the process is surprisingly minimized compared to the embodiments where p.sub.3A<p.sub.1/p.sub.3B<p.sub.1.

[0176] The separation according to step (c) of the process according to the invention is a distillative separation of the alcohol/water mixture G.sub.RD1 as is known to those skilled in the art.

[0177] At the lower end (also: “bottom”) of the rectification column RD.sub.1 a bottoms stream S.sub.RDS1 still comprising alcohol ROH is obtained. S.sub.RDS1 comprises ROH in a mass fraction of in particular 0.005% to 95% by weight, preferably 25% to 95% by weight, based on the total mass of S.sub.RDS1. S.sub.RDS1 preferably comprises essentially water in addition to the alcohol ROH.

[0178] In a preferred embodiment of the invention S.sub.RDB1 is at least partially employed as reactant stream S.sub.AE1 in the reactive rectification column RR.sub.A and, if step (a2) is performed, alternatively or in addition employed as reactant stream S.sub.BE1 in the reactive rectification column RR.sub.B.

[0179] Also obtained at the top of the rectification column RD.sub.1 is the vapour stream S.sub.RDB1 comprising ROH. The preferred mass fraction of ROH in this vapour stream S.sub.RDB1 is ≥99% by weight, preferably ≥99.6% by weight, more preferably ≥99.9% by weight, in each case based on the total mass of S.sub.RDB1, wherein the remainder is especially water.

[0180] In step (c) the vapour S.sub.AB or S.sub.AB and S.sub.BB obtained in step (a1) or step (a1) and (a2) is subjected to distillative separation. These vapours comprise essentially the alcohol ROH and water. In particular, S.sub.AB or S.sub.AB and S.sub.BB are each a water/alcohol mixture in which the mass fraction of ROH is preferably in the range >80% by weight, more preferably >85% by weight, yet more preferably >90% by weight (based on the total mass of S.sub.AB or S.sub.AB and S.sub.BB). Thus in particular G.sub.RD1 too is an alcohol/water mixture in which the mass fraction of ROH is preferably in the range >80% by weight, more preferably >95% by weight, yet more preferably >90% by weight (based on the total mass of G.sub.RD1).

4.5 Step (d) of the Process According to the Invention

[0181] In step (d) of the process according to the invention the bottoms stream S.sub.RDS1 is completely or partially, preferably partially, passed into a second rectification column RD.sub.2.

[0182] This affords a mixture G.sub.RD2 comprising water and ROH in the second rectification column RD.sub.2.

[0183] In the embodiment of the present Invention in which S.sub.RDS1 is partially passed into RD.sub.2, this is especially performed such that a first portion S.sub.RDS11 of the bottoms stream S.sub.RDS1 discharged from the first rectification column RD.sub.1 is passed Into a second rectification column RD.sub.2, and a second portion S.sub.RDS12 of the bottoms stream S.sub.RDS1 discharged from the first rectification column RD.sub.1 is recycled into the first rectification column RD.sub.1. It is yet more preferable when energy is transferred to S.sub.RDS12, yet still more preferable when S.sub.RDS12 is heated. Once S.sub.RDS12 has been recycled to RD.sub.1 it undergoes mixing in RD.sub.1 with G.sub.RD1 and thus provides energy for separating G.sub.RD1 according to step (c).

[0184] In this preferred embodiment of step (d) of the process according to the invention it is yet more preferable when the ratio of the masses (in kg) of S.sub.RDS11 to S.sub.RDS12 are in the range 9:1 to 1:9, yet more preferably 4:1 to 1:4, yet more preferably 7:3 to 3:7, yet more preferably 3:2 to 2:3, yet more preferably 1:1.

[0185] In this preferred embodiment of step (d) of the process according to the invention it is possible to supply energy to the stream S.sub.RDS12. In a preferred embodiment this is effected when the stream S.sub.RDS12 is passed through a bottoms evaporator VS.sub.RD1 in which energy is transferred from S.sub.RDB2 or another heat transfer medium to S.sub.RDS12. This energy transfer may advantageously be undertaken when S.sub.RDS12 and S.sub.RDB2/S.sub.RDS12 and the heat transfer medium are passed through a bottoms evaporator VS.sub.RD1. After the recycling of S.sub.RDS12 into the reaction column RR.sub.A, S.sub.RDS12 then transfers the energy to G.sub.RD1.

[0186] Any desired rectification column known to those skilled in the art may be employed as rectification column RD.sub.2 in step (d) of the process according to the Invention. The rectification column RD.sub.2 preferably contains internals. Suitable internals are, for example, trays, unstructured packings or structured packings. As trays, use is normally made of bubble cap trays, sieve trays, valve trays, tunnel trays or slit trays. Unstructured packings are generally beds of random packing elements. Random packing elements normally used are Raschig rings, Pall rings, Berl saddles or Intalox® saddles. Structured packings are for example marketed under the trade name Mellapack® from Sulzer. Apart from the internals mentioned, further suitable internals are known to a person skilled in the art and can likewise be used.

[0187] Preferred internals have a low specific pressure drop per theoretical plate. Structured packings and random packing elements have, for example, a significantly lower pressure drop per theoretical plate than trays. This has the advantage that the pressure drop in the rectification column RD.sub.2 remains as low as possible and thus the mechanical power of the compressor and the temperature of the alcohol/water mixture G.sub.RD2 to be evaporated remains low.

[0188] When the rectification column RD.sub.2 contains structured packings or unstructured packings, these may be divided or in the form of an uninterrupted packing. However, typically at least two packings are provided, one packing above the feed point of the stream S.sub.RDS1/the portion of S.sub.RDS1, in particular S.sub.RDS12, and one packing below the relevant feed point. If an unstructured packing is used, for example a random packing, the random packing elements typically rest on a suitable sieve tray or mesh tray.

[0189] S.sub.RDS1/the portion of S.sub.RDS1 which is passed into RD.sub.2 and which is preferably S.sub.RDS12 is at least partially liquid.

[0190] It is thus further preferable to pass this stream into RD.sub.2 via a liquid compressor or a pump P since according to the invention RD.sub.2 is at the highest pressure (p.sub.2>p.sub.1; p.sub.2>p.sub.3A; p.sub.2>p.sub.3B).

[0191] At the end of step (d) of the process of according to the invention, a mixture G.sub.RD2 comprising water and ROH is finally obtained in the rectification column RD.sub.2. The composition of the mixture G.sub.RD2 results especially from the composition of the stream S.sub.RDS1/the portion of the stream S.sub.RDS1, preferably S.sub.RDS12, which is passed into RD.sub.2.

4.6 Step (e) of the Process According to the Invention

[0192] In step (e) of the process according to the invention the mixture G.sub.RD2 comprising water and ROH is at a pressure p.sub.2 and a temperature T.sub.2 separated into an ROH-comprising vapour stream S.sub.RDB2 at the top of RD.sub.2 and a bottoms stream S.sub.RDS2 comprising water and optionally ROH at the bottom of RD.sub.2.

[0193] With the exception of the proviso that p.sub.2>p.sub.1, the pressure p.sub.2 in RD.sub.2 may be chosen by those skilled in the art according to their knowledge of the art. It is preferably in the range between 1 bar and 20 bar, preferably 1 bar and 15 bar, more preferably 1 to 10 bar, yet more preferably 3.00 bar to 9.00 bar, yet more preferably 3.20 to 8.90 bar, wherein, simultaneously, p.sub.2>p.sub.1.

[0194] The temperature T.sub.2 in RD.sub.2 may be chosen by those skilled in the art according to their knowledge of the art. It is preferably in the range from 40° C. to 220° C., preferably from 60° C. to 190° C.

[0195] The separation according to step (e) of the process according to the invention is a distillative separation of the alcohol/water mixture G.sub.RD2 as is known to a person skilled in the art.

[0196] Obtained at the bottom of the rectification column RD.sub.2 is a stream S.sub.RDS2 which may comprise <1% by weight of alcohol, based on the total mass of S.sub.RDS2.

[0197] Also obtained at the top of the rectification column RD.sub.2 is the vapour stream S.sub.RDB2 comprising ROH. The preferred mass fraction of ROH in this vapour stream S.sub.RDB2 is ≥99% by weight, preferably ≥99.8% by weight, more preferably ≥99.9% by weight, in each case based on the total mass of S.sub.RDB2, wherein the remainder is especially water.

[0198] In a preferred embodiment of the present invention S.sub.RDB2 is at least partially employed as reactant stream S.sub.AE1 in the reactive rectification column RR.sub.A and, if step (a2) is performed, alternatively or in addition employed as reactant stream S.sub.BE1 in the reactive rectification column RR.sub.B.

[0199] In step (e) the stream S.sub.RDS1, preferably the portion S.sub.RDS12, completely or partially passed into the second rectification column RD.sub.2 in step (d) is subjected to distillative separation.

4.7 Pressure Management as a Characterizing Feature

[0200] The process according to the invention is characterized in that during operation of the rectification columns RD.sub.1 (step (c)) and RD.sub.2 (step (e)) a certain pressure ratio is established.

[0201] Accordingly, p.sub.2>p.sub.1, p.sub.2>p.sub.3A and, in the cases where step (a2) is performed, p.sub.2>p.sub.3B.

[0202] It has surprisingly been found that maintaining these pressures allows the demand for externally supplied electrical energy to be minimized and the majority of the energy required for the process to be covered by heating steam.

[0203] It is yet more advantageous when in addition the pressures are established such that p.sub.3A>p.sub.1 and, in the cases where step (a2) is performed, p.sub.3B>p.sub.1. Establishing the pressures p.sub.3A and p.sub.3B in such a way reduces the altogether required energy demand compared to the case where p.sub.3A<p.sub.1/p.sub.3B<p.sub.1.

4.8 Characterizing Step (f): Energy Transfer from S.sub.RDB2 to G.sub.RD1

[0204] The step (f) of the process according to the invention which is characterizing in addition to the pressure regime is that energy is transferred from S.sub.RDB2 to the mixture G.sub.RD1 in the first rectification column RD.sub.1.

[0205] According to the invention “energy transfer” is in particular to be understood as meaning “heat transfer”.

[0206] This step (f) and the pressure regime according to the invention allow a particularly advantageous integration of the energy which would otherwise dissipate, which makes it possible to cover a particularly large portion of the energy demand of the process through the use of low-energy heating steam instead or through electrical current. This makes the process according to the invention particularly energy-efficient.

[0207] According to the invention the transfer of energy from S.sub.RDB2 to G.sub.RD1 in RD.sub.1 may be effected in various ways familiar to those skilled in the art and preferably comprises heating G.sub.RD1 in RD.sub.1 with S.sub.RDB2, for example via a heat exchanger WT.

[0208] According to the invention in step (f) the energy is especially transferred from S.sub.RDB2 to G.sub.RD1 in RD.sub.1 directly or indirectly, preferably directly.

4.8.1 Direct Energy Transfer Tom S.sub.RDB2 to G.sub.RD1 in RD.sub.1

[0209] According to the invention “direct energy transfer from S.sub.RDB2 to G.sub.RD1 in RD.sub.1” means that an energy transfer, preferably heating, of G.sub.RD1 in RD.sub.1 with S.sub.RDB2 is effected such that G.sub.RD1 is contacted with S.sub.RDB2 without G.sub.RD1 undergoing mixing with S.sub.RDB2, thus transferring energy from S.sub.RDB2 to G.sub.RD1. However, according to the invention, cases of direct energy transfer are to be understood as also including cases where an energy transfer, preferably heating, of a stream S.sub.X discharged from RD.sub.1 with S.sub.RDB2 effected without S.sub.RDB2 undergoing mixing with S.sub.X, thus transferring energy from S.sub.RDB2 to S.sub.X, and S.sub.X is then passed back into RD.sub.1 where it undergoes mixing with G.sub.RD1 in RD.sub.1 and thus transfers the energy absorbed from S.sub.RDB2 to G.sub.RD1 in RD.sub.1.

[0210] In a particular embodiment of the present invention, S.sub.X is selected from the group consisting of S.sub.RDS1, S.sub.RDX1.

[0211] Contacting without mixing is achieved by processes known to those skilled in the art, for example by contacting via a dividing wall made of metal, plastic etc., in particular in the heat exchanger WT, preferably a condenser K or evaporator V which is in particular selected from bottoms evaporators VS and intermediate evaporators VZ.

[0212] According to the invention it is preferable when direct energy transfer from S.sub.RDB2 to the mixture G.sub.RD1 in the first rectification column RD.sub.1 is performed according to at least one of the steps (α-i), (α-ii), (α-iii), more preferably performed according to at least one of the steps (α-i), (α-ii).

[0213] (α-i) A first portion S.sub.RDS11 of the bottoms stream S.sub.RDS1 discharged from RD.sub.1 is passed into the second rectification column RD.sub.2, and energy is transferred from S.sub.RDB2 to a second portion S.sub.RDS12 of the bottoms stream S.sub.RDS1 discharged from RD.sub.1, preferably via a heat exchanger WT, and S.sub.RDS12 is then recycled into RD.sub.1. This step (α-i) also comprises embodiments in which energy is initially transferred from S.sub.RDB2, preferably via a heat exchanger WT, to the overall bottoms stream S.sub.RDS1, and the bottoms stream S.sub.RDS1 is only then divided into S.sub.RDS11 and S.sub.RDS12, and S.sub.RDS11 is then passed into RD.sub.2 and S.sub.RDS12 is recycled into RD.sub.1.

[0214] (α-ii) At least one stream S.sub.RDX1 distinct from S.sub.RDS1 and S.sub.RDS1 comprising ROH and water is discharged from RD.sub.1, energy is then transferred from S.sub.RDB2 to S.sub.RDX1, preferably via a heat exchanger WT, and S.sub.RDX1 is recycled into RD.sub.1.

[0215] It is preferable when S.sub.RDX1 is withdrawn below the vapour stream S.sub.RDS1 on RD.sub.1. S.sub.RDX1 is then especially selected from bottoms stream S.sub.RDX1S, intermediate stream S.sub.RDX1Z.

[0216] A bottoms stream S.sub.RDX1S is a stream whose withdrawal point on RD.sub.1 is at the same height or below the withdrawal point of S.sub.RDS1. S.sub.RDX1S may then be passed through a heat exchanger WT, in particular a bottoms evaporator VS, and energy may be transferred from S.sub.RDB2 to S.sub.RDX1S therein.

[0217] An intermediate stream S.sub.RDX1Z is a stream whose withdrawal point on RD.sub.1 is between the withdrawal points of S.sub.RDB1 and S.sub.RDS1. S.sub.RDX1Z may then be discharged from RD.sub.1 and passed through a heat exchanger WT, in particular an intermediate evaporator VZ, and energy may be transferred from S.sub.RDB2 to S.sub.RDX1Z therein.

[0218] (α-iii) S.sub.RDB2 is passed through RD.sub.1, thus transferring energy from S.sub.RDB2 to G.sub.RD1, preferably via a heat exchanger WT. Such an embodiment may be realized for example when S.sub.RDB2 is passed through the rectification column RD.sub.1 through a conduit whose surface S.sub.RDB2 transfers energy to G.sub.RD1 in RD.sub.1.

4.8.1 Indirect Energy transfer from S.sub.RDB2 to G.sub.RD1 in RD.sub.1

[0219] According to the invention “indirect energy transfer from S.sub.RDB2 to G.sub.RD1 in RD.sub.1” means that an energy transfer, preferably heating, of G.sub.RD1 with S.sub.RDB2 is effected in RD.sub.1 such that G.sub.RD1 is not directly contacted with S.sub.RDB2 but rather at least one additional, preferably precisely one additional, heat transfer medium W.sub.1 distinct from G.sub.RD1 und S.sub.RDB2 is employed which during energy transfer from S.sub.RDB2 to G.sub.RD1 in RD.sub.1 undergoes mixing neither with S.sub.RDB2 nor with G.sub.RD1 in RD.sub.1. Energy is transferred from S.sub.RDB2 to the at least one heat transfer medium W.sub.1 without S.sub.RDB2 and the at least one heat transfer medium W.sub.1 undergoing mixing. and then transferred from the at least one heat transfer medium W.sub.1 to G.sub.RD1 in RD.sub.1 without the at least one heat transfer medium W.sub.1 and G.sub.RD1 undergoing mixing.

[0220] According to the invention, cases of indirect energy transfer are to be understood as also including cases where energy is transferred from S.sub.RDB2 to the at least one, preferably precisely one, heat transfer medium W.sub.1 without S.sub.RDS2 and the at least one heat transfer medium W.sub.1 undergoing mixing, and subsequently an energy transfer, preferably heating, of a stream S.sub.X discharged from RD.sub.1 with the at least one heat transfer medium W.sub.1 is effected without the at least one heat transfer medium W.sub.1 undergoing mixing with S.sub.X, thus transferring energy from the at least one heat transfer medium W.sub.1 to S.sub.X, and S.sub.X is then recycled into RD.sub.1 where it undergoes mixing with G.sub.RD1 in RD.sub.1 and thus transfers the energy absorbed by S.sub.RDB2 via the at least one heat transfer medium W.sub.1 to G.sub.RD1 in RD.sub.1.

[0221] In a particular embodiment of the present invention, S.sub.X is selected from the group consisting of S.sub.RDS12, S.sub.RDX1.

[0222] “At least one heat transfer medium W.sub.1” comprises the cases where the energy of W.sub.1 is first transferred to one or more further heat transfer media W.sub.2, W.sub.3, W.sub.4, W.sub.5 etc. distinct from G.sub.RD1 and S.sub.RDB2 and the last of these heat transfer media, referred to as “W.sub.Y” is contacted with G.sub.RD1 in RD.sub.1, thus transferring energy, preferably heat, from W.sub.Y to G.sub.RD1 but without W.sub.Y and G.sub.RD1 undergoing mixing. Energy, preferably heat, may likewise be transferred from W.sub.Y to a stream S.sub.X discharged from RD.sub.1 without W.sub.Y and S.sub.X undergoing mixing, and S.sub.X subsequently recycled into RD.sub.1 where it undergoes mixing with G.sub.RD1 in RD.sub.1, thus transferring the energy absorbed by W.sub.Y to G.sub.RD1 in RD.sub.1.

[0223] The described contacting is in each case preferably performed in a heat exchanger WT, preferably a condenser K or evaporator V, which is especially selected from bottoms evaporators VS and intermediate evaporators VZ.

[0224] According to the invention it is preferable when indirect energy transfer from S.sub.RDB2 to the mixture G.sub.RD1 in the first rectification column RD.sub.1 is performed according to at least one of the steps (β-i), (β-ii), (β-iii), more preferably performed according to at least one of the steps (β-i), (β-ii).

[0225] (β-i) A first portion S.sub.RDS11 of the bottoms stream S.sub.RDS1 discharged from RD.sub.1 is passed into the second rectification column RD.sub.2, and a second portion S.sub.RDS12 of the bottoms stream S.sub.RDS1 discharged from RD.sub.1 is recycled into RD.sub.1. Energy is transferred from S.sub.RDB2 to at least one, preferably precisely one, heat transfer medium W.sub.i1 distinct from S.sub.RDS12 and then transferred from the at least one heat transfer medium W.sub.i1 to S.sub.RDS12, and then S.sub.RDS12 is recycled into RD.sub.1. This step (β-i) also comprises embodiments in which energy is initially transferred from the at least one, preferably precisely one, heat transfer medium W.sub.i1 distinct from S.sub.RDS12, preferably via a heat exchanger WT, to the overall bottoms stream S.sub.RDS1, and the bottoms stream S.sub.RDS1 is only then divided into S.sub.RDS11 and S.sub.RDS12, and S.sub.RDS11 is then passed into RD.sub.2 and S.sub.RDS12 is recycled into RD.sub.1.

[0226] (β-ii) At least one stream S.sub.RDX1 distinct from S.sub.RDB1 and S.sub.RDS1 comprising ROH and water is discharged from RD.sub.1. Energy is transferred from S.sub.RDB2 to at least one, preferably precisely one, heat transfer medium W.sub.ii1 distinct from S.sub.RDX1, preferably via a heat exchanger WT, and then transferred from the at least one heat transfer medium W.sub.ii1 to S.sub.RDX1, and S.sub.RDX1 is then recycled into RD.sub.1.

[0227] It is preferable when S.sub.RDX1 is withdrawn below the vapour stream S.sub.RDB1 on RD.sub.1. S.sub.RDX1 is then especially selected from bottoms stream S.sub.RDX1S, intermediate stream S.sub.RDX1Z.

[0228] A bottoms stream S.sub.RDX1S is a stream whose withdrawal point on RD.sub.1 is at the same height or below the withdrawal point of S.sub.RDS1. S.sub.RDX1S may then be passed through a heat exchanger WT, in particular a bottoms evaporator VS, and energy may be transferred from S.sub.RDB2 to S.sub.RDX1S therein.

[0229] An intermediate stream S.sub.RDX1Z is a stream whose withdrawal point on RD.sub.1 is between the withdrawal points of S.sub.RDB1 and S.sub.RDS1. S.sub.RDX1Z may then be discharged from RD.sub.1 and passed through a heat exchanger WT, in particular an intermediate evaporator VZ, and energy may be transferred from S.sub.RDB2 to S.sub.RDX1Z therein.

[0230] (β-iii) Energy is transferred from S.sub.RDB2 to at least one heat transfer medium W.sub.iii1 distinct from G.sub.RD1 and the at least one heat transfer medium W.sub.iii1 is then passed through RD.sub.1, thus transferring energy from the at least one heat transfer medium W.sub.iii1 to G.sub.RD1.

[0231] Such an embodiment may be realized for example when the at least one heat transfer medium W.sub.iii1 is passed through the rectification column RD.sub.1 through a conduit whose surface transfers energy from the at least one heat transfer medium W.sub.iii1 to G.sub.RD1 in RD.sub.1.

[0232] Employable heat transfer media W.sub.1, W.sub.2, W.sub.3, W.sub.4, W.sub.5/at least one heat transfer medium W.sub.iii1 at least one heat transfer medium W.sub.ii1/at least one heat transfer medium W.sub.iii1 include any heat transfer media known to those skilled in the art. Such heat transfer media are preferably selected from the group consisting of water; alcohol-water solutions; salt-water solutions, also including Ionic liquids, such as for example LiBr solutions, dialkylimidazolium salts, such as especially dialkylimidazolium dialkylphosphates; mineral oils, such as for example diesel oils; thermal oils such as for example silicone oils; biological oils such as for example limonene; aromatic hydrocarbons such as for example dibenzyltoluene. The most preferred heat transfer medium is water.

[0233] Salt-water solutions that may be used are also described for example in DE 10 2005 028 451 A1 and WO 2006/134015 A1.

4.9 Addition of Fresh Alcohol

[0234] The alcohol ROH is consumed in the process according to the invention, and especially in a continuous process mode therefore requires replacement with fresh alcohol ROH.

[0235] Fresh alcohol is in particular added to at least one of the columns selected from rectification column RD.sub.1, rectification column RD.sub.2, reactive rectification column RR.sub.A and, if step (a2) is performed, alternatively or in addition added to the reactive rectification column RR.sub.B.

[0236] In a preferred embodiment of the present invention a stream S.sub.XE1 distinct from S.sub.AE1 and S.sub.BE1 comprising ROH is accordingly added to at least one of the columns selected from rectification column RD.sub.1, rectification column RD.sub.2, reactive rectification column RR.sub.A and, if step (a2) is performed, alternatively or in addition added to reactive rectification column RR.sub.B.

[0237] The supply of the fresh alcohol ROH is especially effected directly as reactant stream S.sub.AE1 comprising ROH into the reaction column RR.sub.A or in the embodiments in which step (a2) is performed into the reaction columns RR.sub.A and RR.sub.B.

[0238] In the process according to the invention it is further preferable to employ the ROH-comprising vapour stream S.sub.RDB1 at least partially as reactant stream S.sub.AE1 in step (a1) and optionally as reactant stream S.sub.BE1 in step (a2). The vapour stream S.sub.RDB2 may alternatively or in addition be employed at least partially as reactant stream S.sub.AE1 in step (a1) and optionally as reactant stream S.sub.BE1 in step (a2).

[0239] In the particularly preferred embodiment in which S.sub.RDB1 and S.sub.RDB2 are employed at least partially as reactant stream S.sub.AE1 in step (a1) and optionally as reactant stream S.sub.BE1 in step (a2), S.sub.RDB1 and S.sub.RDB2 may be supplied to the respective reactive rectification column RR.sub.A/RR.sub.B separately from one another or first mixed with one another and then supplied to the respective reactive rectification column RR.sub.A/RR.sub.B. S.sub.RDB1 and S.sub.RDB2 are preferably firstly mixed with one another and then supplied to the respective reactive rectification column RR.sub.A/RR.sub.B.

[0240] In this preferred embodiment it is yet more preferable when the fresh alcohol ROH is added to one of the rectification columns RD.sub.1, RD.sub.2, preferably RD.sub.1.

[0241] When the fresh alcohol ROH is added to the rectification column RD.sub.1 or RD.sub.2 it is preferably supplied either in the rectifying section of the respective rectification column or directly at the top of the respective rectification column. The optimal feed point depends on the water content of the employed fresh alcohol and also on the desired residual water content in the vapour stream S.sub.RDB1/S.sub.RDB2. The higher the proportion of water in the employed alcohol and the higher the purity requirement in the vapour stream S.sub.RDB1/S.sub.RDB2, the more advantageous is a feed a number of theoretical trays below the top of the rectification column RD.sub.1/RD.sub.2. Up to 20 theoretical trays below the top of the rectification column RD.sub.1/RD.sub.2 and in particular 1 to 5 theoretical trays are preferred.

[0242] When the fresh alcohol ROH is added to the rectification column RD.sub.1/RD.sub.2 it is added at the top of the rectification column RD.sub.1/RD.sub.2 at temperatures up to boiling point, preferably at room temperature. The fresh alcohol may have a dedicated feed provided for it or else when a portion of the alcohol withdrawn at the top of the rectification column RD.sub.1/RD.sub.2 is recycled may be mixed therewith after condensation and supplied to the rectification column RD.sub.1/RD.sub.2 together. In this case it is particularly preferable when the fresh alcohol is added to a condensate container in which the alcohol condensed from the vapour stream S.sub.RDB1/S.sub.RDB2 is collected.

FIGURES

FIG. 1

[0243] FIG. 1 shows a process not according to the invention for producing alkali metal alkoxides with a corresponding interconnection of the rectification columns. Employed are a reactive rectification column (“reactive rectification column” is hereinbelow abbreviated to “reaction column”) RR.sub.A <3A> at a pressure p.sub.3A and two rectification columns RD.sub.1 <1> and RD.sub.2 <2> at the pressures p.sub.1 and p.sub.2 respectively. Here, p.sub.1>p.sub.3A>p.sub.2.

[0244] In RR.sub.A <3A> NaOH (stream S.sub.AE2 <3A02>) is reacted with methanol (stream S.sub.AE1 <3A01>) to afford a crude product RP.sub.A <3A07> comprising water, methanol, NaOH and sodium methoxide. At the lower end of RR.sub.A <3A> a methanol-sodium methoxide mixture S.sub.AP <3A04> is withdrawn. The bottoms evaporator VS.sub.3A <3A06> at the lower end of the reaction column RR.sub.A <3A> is used to adjust the concentration of the methoxide solution to the desired value in the resulting mixture S.sub.AP′ <3A08>. There may additionally be attached at the bottom of the reaction column RR.sub.A <3A> a further evaporator, especially for startup of the reaction column RR.sub.A <3A> (not shown).

[0245] At the top of RR.sub.A <3A> a methanol-water mixture is withdrawn as vapour stream S.sub.AE1 <3A03>. S.sub.AB <3A03> is supplied to the first water/methanol column RD.sub.1 <1>, wherein optionally S.sub.AE1<3A03> is at the top of the reaction column RR.sub.A <3A> partially condensed in the condenser K.sub.RRA <3A05> and recycled in liquid form as reflux to the top of RR.sub.A <3A>. At least a portion of the vapour S.sub.AB <3A03> is then passed through a compressor VD.sub.31 <10>, thus increasing the pressure of the vapour S.sub.AB <3A03> from p.sub.3A to the pressure p.sub.1.

[0246] A methanol/water mixture G.sub.RD1 <108> is thus obtained in the first rectification column RD.sub.1 <1>. Methanol is distillatively recovered as vapour S.sub.RDB1 <101> in this first water/methanol column RD.sub.1 <1>. The methanol recovered as vapour stream S.sub.RDB1 <101> is at the withdrawal point <109> at the top of RD.sub.1 <1> discharged therefrom and partially at the top of the rectification column RD.sub.1 <1> condensed in the condenser K.sub.RD1 <102> and recycled in liquid form as reflux to the top of RD.sub.1 <1>. The remaining portion of the methanol recovered as vapour S.sub.RDB1 <101> is for example via a throttle D.sub.13 <11> decompressed to the pressure p.sub.3 and introduced into RR.sub.A <3A> as methanol stream S.sub.AE1 <3A01>.

[0247] At the lower end (another term for “lower end of a rectification column” is “bottom of a rectification column”) of RD.sub.1 <1> a bottoms stream S.sub.RDS1 <103> comprising water and methanol is discharged at the withdrawal point <110>. A first portion S.sub.RDS11 <104> of the stream S.sub.RDS1 <103> is supplied to a second water/methanol column RD.sub.2 <2>, a second portion S.sub.RDS1 <105> of the stream S.sub.RDS1 <103> is via a bottoms evaporator VS.sub.RD1 <106> recycled to RD.sub.1 <1>. S.sub.RDS11 <104> is for example via a throttle D.sub.12 <12> decompressed to the pressure p.sub.3 before it is introduced into RD.sub.2 <2>.

[0248] A methanol/water mixture G.sub.RD2 <206> is thus obtained in the second rectification column RD.sub.3 <2>. In the rectification column RD.sub.2 <2> residues of methanol from S.sub.RDS11 <104> are separated from the water and distillatively recovered as vapour stream S.sub.RDB2 <201> at the top of RD.sub.2 <2>. The methanol recovered as vapour stream S.sub.RDB2 <201> is at the withdrawal point <208> at the top of RD.sub.2 <2> discharged therefrom and partially at the top of the rectification column RD.sub.2 <2> condensed in the condenser K.sub.RDB2 <203> and recycled in liquid form as reflux to the top of RD.sub.3 <2>. The remaining portion of the methanol recovered as vapour S.sub.RDB2 <201> is passed through a compressor VD.sub.23 <13>, thus compressed to the pressure p.sub.3 and, together with the vapour S.sub.RDB1 <101> from RD.sub.1 <1> decompressed to the pressure pa, introduced as methanol stream S.sub.AE1 <3A01> into RR.sub.A <3A>.

[0249] At the lower end of RD.sub.2 <2> a bottoms stream S.sub.RDS2 <202> comprising water and optionally methanol is discharged at the withdrawal point <207>. S.sub.RDS2 <202> is partially heated via a bottoms evaporator VS.sub.RD2 <204> and recycled into RD.sub.2 <2>.

[0250] For the heating of the portion of the bottoms stream S.sub.RDS2 <202> which is recycled via VS.sub.RD2 <204> into RD.sub.2 <2>, the energy liberated upon condensation or S.sub.RDB1 <101> in the condenser K.sub.RD1 <102> at the top of the rectification column RD.sub.1 <1> is utilized. Said energy is supplied to VS.sub.RD2 <204>, as indicated by the dashed arrow <4>. The heat transfer may be effected Indirectly, i.e. using a heat transfer medium distinct from S.sub.RDB1 <101> and S.sub.RDS2 <202>, or else directly, i.e. through contacting of S.sub.RDB1 <101> with S.sub.RDB2 <202> in the condenser K.sub.RD1 <102> or bottoms evaporator VS.sub.RD2 <204>. In the case of direct contacting it is sufficient to employ only the condenser K.sub.RD1 <102> and omit the bottoms evaporator VS.sub.RD2 <204> or to employ only the bottoms evaporator VS.sub.RD2 <204> and omit the condenser K.sub.RD1 <102>, and then in each case to pass both streams S.sub.RDS1 <101> with S.sub.RDS2 <202> through the condenser K.sub.RD1 <102> or the bottoms evaporator VS.sub.RD2 <204> such that energy, preferably heat, is transferred from S.sub.RDS1 <101> to S.sub.RDS2 <202>.

FIG. 2

[0251] FIG. 2 shows a further process not according to the invention for producing alkali metal alkoxides. This differs from the process shown in FIG. 1 in terms of the pressures in the respective column. In the embodiment shown in FIG. 1 p.sub.1>p.sub.3A>p.sub.2, while in the embodiment shown in FIG. 2 p.sub.1>p.sub.2>p.sub.3A. This different pressure regime makes the compressor VD.sub.23 <13> unnecessary, and a throttle D.sub.23 <14> for example is attached. The throttle D.sub.23 <14> decompresses the vapour stream S.sub.RDB2 <201> from p.sub.2 to the pressure p.sub.3A, while in the embodiment according to FIG. 1 the compressor VD.sub.23 <13> increases it from p.sub.3 to the pressure p.sub.3A.

FIG. 3

[0252] FIG. 3 shows one embodiment of the process according to the invention for producing alkali metal alkoxides with a corresponding interconnection of the reactive rectification and rectification columns. Employed here, similarly to the embodiments described in FIGS. 1 and 2, are a reactive rectification column RR.sub.A <3A> at a pressure p.sub.3A and two rectification columns RD.sub.1 <1> and RD.sub.2 <2> having the pressures p.sub.1 and p.sub.2 respectively. Here, p.sub.2>p.sub.1>p.sub.3A. The setup shown in FIG. 3 corresponds to the setup shown in FIG. 2 with the following exceptions:

[0253] 1. Arranged at the rectification column RD.sub.1 <1> next to the bottoms evaporator VS.sub.RD1 <106> is an intermediate evaporator VZ.sub.RD1 <107> which may be used to supply energy to the mixture G.sub.RD1 <108> in RD.sub.1 <1>. To this end the mixture G.sub.RD1 <108> is at a withdrawal point <111> discharged from the rectification column RD.sub.1 <1> as stream S.sub.RDX1 <112>. S.sub.RDX1 <112> is heated in VZ.sub.RD1 <107> and recycled into the rectification column RD.sub.1 <1>.

[0254] 2. The throttle D.sub.12 <12> is on account of the different pressures in the rectification columns RD.sub.1 <1> and RD.sub.3 <2> (p.sub.2>p.sub.1) replaced by a pump P <15>. The reason for this difference is that the pressure of S.sub.RDB2 <104>, when this stream is passed into RD.sub.2 <2>, is according to the invention increased to p.sub.2.

[0255] 3. In an optional embodiment additional methanol as stream S.sub.XE1 <205> is via the reflux at the rectification column RD.sub.2 <2> added thereto.

[0256] 4. The energy liberated upon condensation of the vapour S.sub.RDB2 <201> at the top of RD.sub.2 <2> is via the intermediate evaporator VZ.sub.RD1 <107> transferred to S.sub.RDX1 <112> and, after reintroduction of S.sub.RDX1 <112> into RD.sub.1 <1>, transferred from S.sub.RDX1 <112> to the mixture G.sub.RD1 <108> present in RD.sub.1 <1>. Alternatively or in addition, energy liberated upon condensation of the vapour S.sub.RDB2 <201> at the top of RD.sub.2 <2> is via the bottoms evaporator VS.sub.RD1 <106> transferred to the portion S.sub.RDS12 <105> or the stream S.sub.RDS1 <103>. Once S.sub.RDS12 <105> is recycled into RD.sub.1 <1>, it transfers the energy to the mixture G.sub.RD1 <108> present in RD.sub.1 <1>. The energy flow is shown by the dashed arrow <4>.

[0257] In the case of direct contacting it is sufficient to employ only the condenser K.sub.RD2 <203> and omit the bottoms evaporator VS.sub.RD1 <106> or to employ only the bottoms evaporator VS.sub.RD1 <106> and omit the condenser K.sub.RD2 <203>, and then in each case to pass both streams S.sub.RDB2 <201> with S.sub.RDS12 <105> through the condenser K.sub.RD2 <203> or the bottoms evaporator VS.sub.RD1 <106> such that energy, preferably heat, is transferred from S.sub.RDB2 <201> to S.sub.RDS12 <105>.

FIG. 4

[0258] FIG. 4 shows a further embodiment of the process according to the invention for producing alkali metal alkoxides with a corresponding interconnection of the rectification columns. Employed here, similarly to the embodiments described in FIGS. 1 and 2, are a reactive rectification column RR.sub.A <3A> at a pressure p.sub.3A and two rectification columns RD.sub.1 <1> and RD.sub.2 <2> having the pressures p.sub.1 and p.sub.2 respectively. Here, p.sub.2>p.sub.3A>p.sub.1. The setup shown in FIG. 4 corresponds to the setup shown in FIG. 3 with the exception that the pressure p.sub.3A>p.sub.1. This allows the compressor VD.sub.31 <10> to be omitted, while the throttle D.sub.13 <11> is replaced by the compressor VD.sub.13 <16>.

FIG. 5

[0259] FIG. 5 shows a further embodiment of the process according to the Invention. Said embodiment employs two reactive rectification columns (RR.sub.A <3A> at a pressure p.sub.3A and RR.sub.B <3B> at a pressure p.sub.3B) and two rectification columns (RD.sub.1 <1> and RD.sub.2 <2> having the pressures p.sub.1 and p.sub.2 respectively). Here, p.sub.2>p.sub.1>p.sub.3A and p.sub.1>p.sub.3B.

[0260] In RR.sub.A <3A> NaOH (stream S.sub.AE2 <3A02>) is reacted with methanol (stream S.sub.AE1 <3A01>) to afford a crude product RP.sub.A <3A07> comprising water, methanol, NaOH and sodium methoxide.

[0261] At the lower end or RR.sub.A <3A> a methanol-sodium methoxide mixture S.sub.AP <3A04> is withdrawn. The bottoms evaporator VS.sub.3A <3A06> at the bottom of the column RR.sub.A <3A> is used to adjust the concentration of the methoxide solution to the desired value in the resulting mixture S.sub.AP′<3A08>. There may additionally be attached at the bottom of the column RR.sub.A <3A> a further evaporator, especially for startup of the column RR.sub.A <3A> (not shown).

[0262] At the top of RR.sub.B <3A> a methanol-water mixture is withdrawn as vapour S.sub.AB <3A03>. S.sub.AB <3A03> is supplied to the first water/methanol column RD.sub.1 <1>, wherein optionally S.sub.AB <3A03> is at the top of the reaction column RR.sub.A <3A> partially condensed in the condenser K.sub.RRA <3A05> and recycled in liquid form as reflux to the top or RR.sub.A <3A>.

[0263] Simultaneously with the reaction in RR.sub.B <3A>, in RR.sub.B <3B> KOH (stream S.sub.BE2 <3B02>) is reacted with methanol (stream S.sub.BE1 <3B01>) to afford a crude product RP.sub.B <3B07> comprising water, methanol, KOH and potassium methoxide.

[0264] At the lower end of RR.sub.B <38> a methanol-potassium methoxide mixture SOP <3804> is withdrawn. The bottoms evaporator VS.sub.3B <3B06> at the lower end of the column RR.sub.B <3B> is used to adjust the concentration of the methoxide solution to the desired value in the resulting mixture S.sub.BP′ <3B08>. There may additionally be attached at the lower end of the column RR.sub.B <3B> a further evaporator, especially for startup of the reaction column RR.sub.B <3B> (not shown).

[0265] At the top of RR.sub.B <3B> a methanol-water mixture is removed as vapour S.sub.BB <3B03>, wherein optionally S.sub.BB <3B03> is at the top of the reaction column RR.sub.B <3B> partially condensed in the condenser K.sub.RRB <3B05> and recycled in liquid form as reflux to the top of RR.sub.B <3B>.

[0266] At least a portion of the vapours S.sub.AB <3A03> and S.sub.BB <3B03> is combined and the combined vapour is then passed through a compressor VD.sub.31 <10> which Increases the pressure of the combined vapours to the pressure p.sub.1. The combined vapours are then passed Into the first rectification column RD.sub.1 <1> which then affords a methanol/water mixture G.sub.RD1 <108>.

[0267] In RD.sub.1 <1> methanol is distillatively recovered overhead as vapour S.sub.RDB1 <101>. The methanol recovered as vapour stream S.sub.RDB1 <101> is at the withdrawal point <109> at the top of RD.sub.1 <1> discharged therefrom and partially at the top of the rectification column RD.sub.1 <1> condensed in the condenser K.sub.RD1 <102> and recycled in liquid form as reflux to the top of RD.sub.1 <1>. The remaining portion of the methanol recovered as vapour S.sub.RDB1 <101> is for example via a throttle D.sub.13 <11> decompressed to the pressure p.sub.3A/p.sub.3B and introduced into the two reaction columns RR.sub.A <3A> and RR.sub.B <3B> as methanol stream S.sub.AE1 <3A01> and S.sub.BE1 <3B01> respectively.

[0268] Arranged at the rectification column RD.sub.1 <1> next to the bottoms evaporator VS.sub.RD1 <106> is an intermediate evaporator VZ.sub.RD1 <107> which may be used to supply energy to the mixture G.sub.RD1 <108> in RD.sub.1 <1>. To this end the mixture G.sub.RD1 <108> is at a withdrawal point <111> discharged from the rectification column RD.sub.1 <1> as stream S.sub.RDX1 <112>. S.sub.RDX1 <112> is heated in VZ.sub.RD1 <107> and recycled into the rectification column RD.sub.1 <1>.

[0269] At the lower end of RD.sub.1 <1> a bottoms stream S.sub.RDS1 <103> comprising water and methanol is discharged at the withdrawal point <110>. A first portion S.sub.RDS11 <104> of the stream S.sub.RDS1 <103> is supplied to a second water/methanol column RD.sub.2 <2>, a second portion S.sub.RDS12 <105> of the stream S.sub.RDS1 <103> is via a bottoms evaporator VS.sub.RD1 <106> recycled to RD.sub.1 <1>. The pressure of S.sub.RDS11 <104> is via a pump P <15> increased to the pressure p.sub.2 before this stream is Introduced Into RD.sub.2 <2>.

[0270] A methanol/water mixture G.sub.RD2 <206> is thus obtained in the second rectification column RD.sub.2 <2>. In the rectification column RD.sub.2 <2> residues of methanol from S.sub.RDS11 <104> are separated from the water and distillatively recovered as vapour stream S.sub.RDB2 <201> at the top of RD.sub.2 <2>. The methanol recovered as vapour stream S.sub.RDB2 <201> is at the withdrawal point <208> at the top of RD.sub.2 <2> discharged therefrom and partially at the top of the rectification column RD.sub.2 <2> condensed in the condenser K.sub.RD2 <203> and recycled in liquid form as reflux to the top of RD.sub.2 <2>. Optionally, additional methanol as stream S.sub.XE1 <205> is via the reflux at the rectification column RD.sub.3 <2> added thereto.

[0271] The remaining portion of the methanol recovered as vapour S.sub.RDB2 <201> not supplied to the condenser K.sub.RD2 <203> is passed through throttle D.sub.23 <14>, thus decompressed to the pressure pa or p.sub.3B and, together with the vapour S.sub.RDB1 <101> from RD.sub.1 <1> decompressed to the pressure pa or pas, introduced into RR.sub.A <3A> and RR.sub.B <3B> as methanol stream S.sub.AE1 <3A01> and S.sub.1 <3B01> respectively.

[0272] At the lower end or RD.sub.3 <2> a bottoms stream S.sub.RDS2 <202> comprising water and optionally methanol is discharged at the withdrawal point <207>. S.sub.RDS2 <202> is partially heated via a bottoms evaporator VS.sub.RD2 <204> and recycled into RD.sub.2 <2>.

[0273] The energy liberated upon condensation of the vapour S.sub.RDS2 <201> at the top of RD.sub.2 <2> is via the intermediate evaporator VZ.sub.RD1 <107> transferred to S.sub.RDX1 <112> and, after reintroduction of S.sub.RDX1 <112> into RD.sub.1 <1>, transferred from S.sub.RDX1 <112> to the mixture RD.sub.1 <108> present in RD.sub.1 <1>. Alternatively or in addition, energy liberated upon condensation of the vapour S.sub.RDB2 <201> at the top of RD.sub.2 <2> is via the bottoms evaporator VS.sub.RD1 <106> transferred to the portion S.sub.RDS12 <105> of the stream S.sub.RDS1 <103>. Once S.sub.RDS12 <105> is recycled into RD.sub.1 <1>, it transfers the energy to the mixture G.sub.RD1 <108> present in RD.sub.1 <1>. The energy flow is shown by the dashed arrow <4>.

FIG. 6

[0274] FIG. 6 shows a further embodiment of the process according to the Invention for producing alkali metal alkoxides with a corresponding Interconnection of the rectification columns. Employed here, similarly to the embodiment described in FIG. 5, are two reactive rectification columns RR.sub.A <3A> and RR.sub.B <3B> at a pressure p.sub.3A and p.sub.3B respectively and two rectification columns RD.sub.1 <1> and RD.sub.2 <2> having the pressures p.sub.1 and p.sub.2 respectively. In contrast to the embodiment according to FIG. 5, p.sub.2>p.sub.3A>p.sub.1 and p.sub.2>p.sub.3B>p.sub.1. The setup shown in FIG. 6 corresponds to the setup shown in FIG. 5 with the exception that the pressure p.sub.3A>p.sub.1 and the pressure p.sub.3B>p.sub.1, thus allowing the compressor VD.sub.31 <10> to be omitted and the throttle D.sub.13 <11> to be replaced by the compressor VD.sub.13 <16>.

FIG. 7

[0275] FIG. 7 shows an embodiment of the process according to the Invention which corresponds to that described in FIG. 5. Here, p.sub.2>p.sub.1>p.sub.3A and p.sub.1>p.sub.3B. It shows the embodiment in which the energy liberated upon condensation of the vapour S.sub.RDB2 <201> at the top of RD.sub.2 <2> is directly via the bottoms evaporator VS.sub.RD1 <106> transferred to the portion S.sub.RDS1 <105> of the stream S.sub.RDS1 <103> which is recycled into RD.sub.1 <1>. The transfer is effected directly since the vapour S.sub.RDS1 <201> is contacted with S.sub.RDS12 <105> in VS.sub.RD1 <106>, thus allowing energy transfer from S.sub.RDS2 <201> to S.sub.RDS12 <105> without for example interposing a further heat transfer medium. The condenser K.sub.RD2 <203> shown in FIG. 5 may be omitted, and the stream of fresh methanol S.sub.XE1 <205> is here introduced directly into the column RD.sub.2 <2>.

FIG. 8

[0276] FIG. 8 shows an embodiment of the process according to the invention which corresponds to that described in FIG. 6. Here, p.sub.2>p.sub.3A>p.sub.1 and p.sub.2>p.sub.3B>p.sub.1. It shows the embodiment in which the energy liberated upon condensation of the vapour S.sub.RDB2 <201> at the top of RD.sub.2 <2> is directly via the bottoms evaporator VS.sub.RD1 <106> transferred to the portion S.sub.RDS12 <105> of the stream S.sub.RDS1 <103> which is recycled into RD.sub.1 <1>. The transfer is effected directly since the vapour S.sub.RDB2 <201> is contacted with S.sub.RDS12 <105> in VS.sub.RD1 <106>, thus allowing energy transfer from SR.sub.RDB2 <201> to S.sub.RDS12 <105> without for example interposing a further heat transfer medium. The condenser K.sub.RD2 <203> shown in FIG. 6 may be omitted, and the stream of fresh methanol S.sub.XE1 <205> is here Introduced directly into the rectification column RD.sub.3 <2>.

FIG. 9

[0277] FIG. 9 shows an embodiment of the process according to the invention which corresponds to that described in FIG. 5. Here, p.sub.2>p.sub.1>p.sub.3A and p.sub.1>p.sub.3B. It shows the embodiment in which the energy liberated upon condensation of the vapour S.sub.RDB2 <201> at the top of RD.sub.2 <2> is via the intermediate evaporator VZ.sub.RD1 <107> transferred to the water/methanol mixture G.sub.RD1 <108> from RD.sub.1 <1>. G.sub.RD1 <108> is withdrawn from the rectification column RD.sub.1 <1> at a withdrawal point <111> as stream S.sub.RDX1 <112>. S.sub.RDB2 <201> transfers energy to S.sub.RDX1 <112> in VZ.sub.RD1 <107>, especially by heating or S.sub.RDX1 <112>. S.sub.RDX1 <112> is then recycled into the rectification column RD.sub.1 <1>.

[0278] In RD.sub.1 <1> the stream S.sub.RDX1 <112> then transfers the energy to the mixture G.sub.RD1 <108> present in RD.sub.1 <1>. The transfer of energy from S.sub.RDX1 <201> to S.sub.RDX1 <112> is effected directly since the vapour S.sub.RDB2 <201> is contacted with S.sub.RDX1 <112> from RD.sub.1 <1> in VZ.sub.RD1 <107>, thus allowing energy transfer from S.sub.RDB2 <201> to S.sub.RDX1 <112> from RD.sub.1 <1> without for example interposing a further heat transfer medium. The condenser K.sub.RD2 <203> shown in FIG. 5 may be omitted, and the stream of fresh methanol S.sub.XE1 <205> is here introduced directly into the rectification column RD.sub.2 <2>.

FIG. 10

[0279] FIG. 10 shows an embodiment of the process according to the invention which corresponds to that described in FIG. 6. Here, p.sub.2>p.sub.3A>p.sub.1 and p.sub.2>p.sub.3B>p.sub.1. It shows the embodiment in which the energy liberated upon condensation of the vapour S.sub.RDS2 <201> at the top of RD.sub.2 <2> is via the intermediate evaporator VZ.sub.RD1 <107> transferred to the water/methanol mixture G.sub.RD1 <108> from RD.sub.1 <1>. G.sub.RD1 <108> is withdrawn from the rectification column RD.sub.1 <1> at a withdrawal point <111> as stream S.sub.RDX1 <112>. S.sub.RDB2 <201> transfers energy to S.sub.RDX1 <112> in VZ.sub.RD1 <107>, especially by heating of S.sub.RDX1 <112>. S.sub.RDX1 <112> is then recycled into the rectification column RD.sub.1 <1>.

[0280] In RD.sub.1 <1> the stream S.sub.RDX1 <112> then transfers the energy to the mixture G.sub.RD1 <108> present in RD.sub.1 <1>. The transfer of energy from S.sub.RDB2 <201> to S.sub.RDX1 <112> is effected directly since the vapour S.sub.RDB2 <201> is contacted with S.sub.RDX1 <112> from RD.sub.1 <1> in VZ.sub.RD1 <107>, thus allowing energy transfer from S.sub.RDB2 <201> to S.sub.RDX1 <112> from RD.sub.1 <1> without for example interposing a further heat transfer medium. The condenser K.sub.RD2 <203> shown in FIG. 6 may be omitted, and the stream of fresh methanol S.sub.XE1 <205> is here Introduced directly into the rectification column RD.sub.2 <2>.

FIG. 11

[0281] FIG. 11 shows an embodiment of the process according to the invention which shows a combination of the embodiments according to FIGS. 7 and 9. Here, p.sub.2>p.sub.1>p.sub.3A and p.sub.1>p.sub.3B. It shows the embodiment in which the energy liberated upon condensation of the vapour S.sub.RDB2 <201> at the top of RD.sub.2 <2> is directly via the bottoms evaporator VS.sub.RD1 <106> transferred to the portion S.sub.RDS12 <105> of the stream S.sub.RDS1 <103> which is recycled into RD.sub.1 <1>. The transfer is effected directly since the vapour S.sub.RDB2 <201> is contacted with S.sub.RDS12 <105> in VS.sub.RD1 <106>, thus allowing energy transfer from S.sub.RDB2 <201> to S.sub.RDS12 <105> without for example interposing a further heat transfer medium.

[0282] After this heat transfer further energy from the vapour S.sub.RDB2 <201> is transferred via the intermediate evaporator VZ.sub.RD1 to the water/methanol mixture G.sub.RD1 <108> from RD.sub.1 <1>. G.sub.RD1 <108> is withdrawn from the rectification column RD.sub.1 <1> at a withdrawal point <111> as stream S.sub.RDX1 <112>. SRO <201> transfers energy to S.sub.RDX1 <112> in VZ.sub.RD1 <107>, especially by heating of S.sub.RDX1 <112>. S.sub.RDX1 <112> is then recycled into the rectification column RD.sub.1 <1>.

[0283] The transfer of energy from S.sub.RDB2 <201> to S.sub.RDX1 <112> is effected directly since the vapour S.sub.RDB2 <201> is contacted with S.sub.RDX1 <112> from RD.sub.1 <1> in VZ.sub.RD1 <107>, thus allowing energy transfer from S.sub.RDB2 <201> to S.sub.RDX1 <112> from RD.sub.1 <1> without for example Interposing a further heat transfer medium.

[0284] In RD.sub.1 <1> the streams S.sub.RDX1 <112> and S.sub.RDS12 <105> then transfer the energy absorbed by S.sub.RDB2 <201> to the mixture G.sub.RD1 <108> present in RD.sub.1.

FIG. 12

[0285] FIG. 12 shows an embodiment of the process according to the invention which corresponds to a combination of the embodiments of FIGS. 8 and 10. Here, p.sub.2>p.sub.3A>p.sub.1 and p.sub.2>p.sub.3B>p.sub.1. It shows an embodiment in which the energy liberated upon condensation of the vapour S.sub.RDB2 <201> at the top of RD.sub.2 <2> is directly via the bottoms evaporator VS.sub.RD1 <106> transferred to the portion S.sub.RDS12 <105> of the stream S.sub.RDS1 <103> which is recycled into RD.sub.1 <1>. The transfer is effected directly since the vapour S.sub.RDB2 <201> is contacted with S.sub.RDS12 <105> in VS.sub.RD1 <106>, thus allowing energy transfer from S.sub.RDB2 <201> to S.sub.RDS12 <105> without for example interposing a further heat transfer medium.

[0286] After this heat transfer further energy from the vapour S.sub.RDB2 <201> is transferred via the intermediate evaporator VZ.sub.RD1 to the water/methanol mixture G.sub.RD1 <108> from RD.sub.1 <1>. G.sub.RD1 <108> is withdrawn from the rectification column RD.sub.1 <1> after withdrawal point <111> as stream S.sub.RDX1 <112>. S.sub.RDS2 <201> transfers energy to S.sub.RDX1 <112> in VZ.sub.RD1 <107>, especially by heating of S.sub.RDX1 <112>. S.sub.RDX1 <112> is then recycled into the rectification column RD.sub.1 <1>.

[0287] The transfer of energy from S.sub.RDB2 <201> to S.sub.RDX1 <112> is effected directly since the vapour S.sub.RDB2 <201> is contacted with S.sub.RDX1 <112> from RD.sub.1 <1> in VZ.sub.RD1 <107>, thus allowing energy transfer from S.sub.RDB2 <201> to S.sub.RDX1 <112> from RD.sub.1 <1> without for example interposing a further heat transfer medium.

[0288] In RD.sub.1 <1> the streams S.sub.RDX1 <112> and S.sub.RDS12 <105> then transfer the energy absorbed by S.sub.RDB2 <201> to the mixture G.sub.RDB1 <108> present in RD.sub.1 <1>.

5. EXAMPLES

5.1 Example 1 (Noninventive)

[0289] The setup according to example 1 corresponds to the two-column interconnection according to FIG. 1, wherein p.sub.1>p.sub.3A>p.sub.3.

[0290] A stream S.sub.AE2 <3A02> of aqueous NaOH (50% by weight) of 5 t/h is supplied to the top of a reaction column RR.sub.A <3A> at 25° C. A vaporous methanol stream S.sub.AE1 <3A01> of 70.2 t/h is supplied IN countercurrent above the bottom of the reaction column RR.sub.A <3A>. The reaction column RR.sub.A <3A> is operated at a pressure p.sub.3A of 1.6 bar. At the bottom of the column RR.sub.A <3A> a virtually water-free product stream S.sub.AP′ <3A08> of 10.8 t/h is withdrawn (30% by weight sodium methoxide in methanol). At the evaporator VS.sub.3A <3A06> of the reaction column RR.sub.A <3A> about 0.7 MW of heating power are introduced using heating steam. A vaporous methanol-water stream S.sub.AB <3A03> is withdrawn at the top of the reaction column RR.sub.A <3A>. A portion of this stream is recycled via a condenser K.sub.RRA <3A05> to the reaction column RR.sub.A <3A>, and the remaining portion (64.4 t/h) compressed in a compressor VD.sub.31 <10> to 7.1 bar, wherein about 4 MW of compressor power are necessary, and supplied to a first rectification column RD.sub.1 <1>. The rectification column RD.sub.1 <1> is operated at p.sub.1=˜7 bar. At the top of the rectification column RD.sub.1 <1> a liquid fresh methanol stream of 9.5 t/h is supplied (not shown in FIG. 1) and vaporous methanol stream S.sub.RDB1 <101> is withdrawn. A portion of S.sub.RDS1 <101> is recycled via the condenser K.sub.RD1 <102> into column RD.sub.1 <1>. The remaining portion of S.sub.RDB1 <101> (42.9 t/h) is supplied to the reaction column RR.sub.A <3A>. The condenser K.sub.RD1 <102> of column RD.sub.1 <1> which is simultaneously the evaporator VS.sub.RD2 of the second rectification column RD.sub.2 <2> provides the heating power for the column RD.sub.2 <2>. The embodiment according to example 1 utilized direct contacting where the condenser K.sub.RD1 <102> is simultaneously employed as bottoms evaporator VS.sub.RD2 <204>.

[0291] Discharged at the bottom of the rectification column RD.sub.1 <1> is a liquid stream of a water-methanol mixture S.sub.RDS1 <103> of which a portion S.sub.RDS12 <104> of 30.9 t/h is passed into the rectification column RD.sub.2 <2> and the remaining portion of the stream S.sub.RDS1 <103> is recycled as S.sub.RDS1 <105> into RD.sub.1 <1>. At the evaporator VS.sub.RD1 <106> of the rectification column RD.sub.1 <1> about 5.4 MW of heating power are introduced via heating steam.

[0292] The rectification column RD.sub.2 <2> is operated at a pressure p.sub.2 of 1.1 bar. Withdrawn at the top of the rectification column RD.sub.3 <2> is a vaporous methanol stream S.sub.RDS2 <201>. A portion of S.sub.RDB2 <201> is recycled via the condenser K.sub.RD2 <203> into column RD.sub.2 <2>. The remaining portion of S.sub.RDB2 <201> (27.3 t/h) is supplied to the reaction column RR.sub.A <3A>. This portion or the vaporous stream S.sub.RDB2 <201> is compressed to 2 bar in a compressor VD.sub.23 <13>, wherein about 0.6 MW of compressor power are necessary. Discharged at the bottom of the rectification column RD.sub.2 <2> is a liquid stream of water S.sub.RDS2 <202> (contaminated with 500 ppmw of methanol) of 3.7 t/h. For evaporation at the rectification column RD.sub.2 <2> (since direct heat integration is effected, the function of the bottoms evaporator VS.sub.RD2 <204> shown in FIG. 1 is co-assumed by the condenser K.sub.RD1 <102>) about 14.8 MW of heating power are introduced via heat integration with the column RD.sub.1 <1>.

[0293] The respective, non-recycled portions of the vaporous methanol streams S.sub.RDS1 <101> and S.sub.RDB2 <201> withdrawn at the tops of RD.sub.1 <1> and RD.sub.2 <2> are mixed and recycled to the bottom of the reaction column RR.sub.A <3A>.

[0294] Altogether in this example about 6.1 MW of heating power via heating steam and about 4.6 MW of electrical power (compressor power) are required and must be externally provided.

5.2 Example 2 (Noninventive)

[0295] The setup according to example 2 corresponds to the two-column interconnection according to FIG. 2, wherein p.sub.1>p.sub.2>p.sub.3A.

[0296] A stream S.sub.AE2 <3A02> of aqueous NaOH (50% by weight) of 5 t/h is supplied to the top of a reaction column RR.sub.A <3A> at 25° C. A vaporous methanol stream S.sub.AE1 <3A01> of 70.2 t/h is supplied in countercurrent above the bottom of the reaction column RR.sub.A <3A>.

[0297] The reaction column RR.sub.A <3A> is operated at a pressure p.sub.3A of 1.1 bar. At the bottom of the column RR.sub.A <3A> a virtually water-free product stream S.sub.AP <3A08> of 10.8 t/h is withdrawn (30% by weight sodium methoxide in methanol). At the evaporator VS.sub.3A <3A06> of the reaction column RR.sub.A <3A> about 0.8 MW of heating power are introduced using heating steam. A vaporous methanol-water stream S.sub.AB <3A03> is withdrawn at the top of the reaction column RR.sub.A <3A>. A portion of this stream is recycled via a condenser K.sub.RRA <3A05> to the reaction column RR.sub.A <3A> and the remaining portion (64.4 t/h) compressed in a compressor VD.sub.31 <10> to 9 bar, wherein about 5.8 MW of compressor power are necessary, and supplied to a first rectification column RD.sub.1 <1>. The rectification column RD.sub.1 <1> is operated at p.sub.1=˜8.9 bar. At the top of the rectification column RD.sub.1 <1> a liquid fresh methanol stream of 9.5 t/h is supplied (not shown in FIG. 2) and vaporous methanol stream S.sub.RDB1 <101> is withdrawn. A portion of S.sub.RDB1 <101> is recycled via the condenser K.sub.RD1 <102> into column RD.sub.1 <1>. The remaining portion of S.sub.RDB1 <101> (42.9 t/h) is supplied to the reaction column RR.sub.A <3A>. The condenser K.sub.RD1 <102> of column RD.sub.1 <1> which is simultaneously the evaporator VS.sub.RD2 <204> of the second rectification column RD.sub.2 <2> provides the heating power for the column RD.sub.2 <2>. The embodiment according to example 2 utilized direct contacting where the condenser K.sub.RD1 <102> is simultaneously employed as bottoms evaporator VS.sub.RD2 <204>.

[0298] Discharged at the bottom of the rectification column RD.sub.1 <1> is a liquid stream of a water-methanol mixture S.sub.RDS1 <103> of which a portion S.sub.RDS1 <104> of 31.9 t/h is passed into the rectification column RD.sub.2 <2> and the remaining portion of the stream S.sub.RDS1 <103> is recycled as S.sub.RDS11 <105>into RD.sub.1 <1>. At the evaporator VS.sub.RD1 <106> of the rectification column RD.sub.1 <1> about 5.2 MW of heating power are Introduced via heating steam.

[0299] The rectification column RD.sub.2 <2> is operated at a pressure p.sub.2 of 1.5 bar. Withdrawn at the top of the rectification column RD.sub.2 <2> is a vaporous methanol stream S.sub.RDB2 <201>. A portion of S.sub.RDB2 <201> is recycled via the condenser K.sub.RD2 <203> into column RD.sub.2 <2>. The remaining portion of S.sub.RDB2 <201> (28.2 t/h) is supplied to the reaction column RR.sub.A <3A>. Discharged at the bottom of the rectification column RD.sub.2 <2> is a liquid stream of water S.sub.RDS2 <202> (contaminated with 500 ppmw of methanol) of 3.7 t/h. For evaporation at the rectification column RD.sub.2 <2> (since direct heat integration is effected, the function of the bottoms evaporator VS.sub.RD2 <204> shown in FIG. 1 is co-assumed by the condenser K.sub.RD1 <102>) about 15.9 MW of heating power are Introduced via heat integration with the column RD.sub.1 <1>.

[0300] The respective, non-recycled portions of the vaporous methanol streams S.sub.RDS1 <101> and S.sub.RDB2 <201> withdrawn at the tops of RD.sub.1 <1> and RD.sub.2 <2> are mixed and recycled to the bottom of the reaction column RR.sub.A <3A>.

[0301] Altogether in this example about 6.0 MW of heating power via heating steam and about 5.8 MW of electrical power (compressor power) are required and must be externally provided.

5.3 Example 3 (Inventive)

[0302] The setup according to example 3 corresponds to the two-column interconnection according to FIG. 3, wherein p.sub.2>p.sub.1>p.sub.3A. The intermediate evaporator VZ.sub.RD1 <107> with the stream S.sub.RDX1 <112> withdrawn at the withdrawal point <111> shown in FIG. 3 is likewise omitted in the setup according to example 3. The condenser K.sub.RD2 <203> is also simultaneously the bottoms evaporator VS.sub.RD1 <106>. The fresh methanol stream S.sub.XE1 <205> is in FIG. 3 supplied to the rectification column RD.sub.2 <2> but in the setup according to example 3 supplied to RD.sub.1 <1>.

[0303] A stream S.sub.AE2 <3A02> of aqueous NaOH (50% by weight) of 5 t/h is supplied to the top of a reaction column RR.sub.A <3A> at 25° C. A vaporous methanol stream S.sub.AE1 <3A01> of 70.2 t/h is supplied in countercurrent above the bottom of the reaction column RR.sub.A <3A>. The reaction column RR.sub.A <3A> is operated at a pressure p.sub.3A of 1.1 bar. At the bottom of the column RR.sub.A <3A> a virtually water-free product stream S.sub.AP′ <3A08> of 10.8 t/h is withdrawn (30% by weight sodium methoxide in methanol). At the evaporator VS.sub.3A <3A06> of the reaction column RR.sub.A <3A> about 1.4 MW of heating power are Introduced using heating steam. A vaporous methanol-water stream S.sub.AB <3A03> is withdrawn at the top of the reaction column RR.sub.A <3A>. A portion of this stream is recycled via a condenser K.sub.RRA <3A05> to the reaction column RR.sub.A <3A> and the remaining portion (64.4 t/h) compressed in a compressor VD.sub.31 <10> to 1.7 bar, wherein about 1.1 MW of compressor power are necessary, and supplied to a first rectification column RD.sub.1 <1>. The rectification column RD.sub.1 <1> is operated at p.sub.1=˜1.5 bar. At the top of the rectification column RD.sub.1 <1> a liquid fresh methanol stream S.sub.XE1 <205> of 9.5 t/h is supplied (shown at the top of the rectification column RD.sub.2 <2> in FIG. 3) and vaporous methanol stream S.sub.RDB1 <101> is withdrawn. A portion of S.sub.RDS1 <101> is recycled via the condenser K.sub.RD1 <102> into column RD.sub.1 <1>. The remaining portion of S.sub.RDS1 <101> (58.8 t/h) is supplied to the reaction column RR.sub.A <3A>. The embodiment according to example 3 utilized direct contacting where the condenser K.sub.RD2 <203> simultaneously serves as bottoms evaporator VS.sub.RD1 <106>.

[0304] Discharged at the bottom of the rectification column RD.sub.1 <1> is a liquid stream of a water-methanol mixture S.sub.RDS1 <103> of which a portion S.sub.RDS12 <104> or 17 t/h is passed into the rectification column RD.sub.2 <2> and the remaining portion of the stream S.sub.RDS1 <103> is recycled as S.sub.RDS11 <105> into RD.sub.1 <1>.

[0305] The pressure of the discharged stream S.sub.RDS12 <104> is increased in a pump P <15> to 9 bar and the stream S.sub.RDS12 <104> supplied to the second rectification column RD.sub.2 <2>. The rectification column RD.sub.2 <2> is operated at a pressure p.sub.3A of 8.9 bar. In the condenser K.sub.RD2 <203> of column RD.sub.2 <2> which is simultaneously the evaporator of the column RD.sub.1 <1> about 8.2 MW of heating power are provided for the column RD.sub.1 <1>. Withdrawn at the top of the rectification column RD.sub.2 <2> is a vaporous methanol stream S.sub.RDB2 <201>. A portion of S.sub.RDB2 <201> is recycled via the condenser K.sub.RD2 <203> into column RD.sub.2 <2>. The remaining portion of S.sub.RDB2 <201> (13.4 t/h) is supplied to the reaction column RR.sub.A <3A>. Discharged at the bottom of the rectification column RD.sub.2 <2> is a liquid stream of water (contaminated with 500 ppmw of methanol) of 3.7 t/h. At the evaporator VS.sub.RD2 <204> of the rectification column RD.sub.2 <2> about 12.9 MW of heating power are Introduced using heating steam.

[0306] The respective, non-recycled portions of the vaporous methanol streams S.sub.RDB1 <101> and S.sub.RDB2 <201> withdrawn at the tops of RD.sub.1 <1> and RD.sub.2 <2> are mixed, decompressed and recycled to the bottom of the reaction column RR.sub.A <3A>.

[0307] Altogether in this example about 14.3 MW of heating power via heating steam and about 1.1 MW of electrical power (compressor power) are required and must be externally provided.

[0308] Compared to the embodiment according to the noninventive example 1 this variant thus saves about 75% of the required compressor power to be externally provided (electrical energy).

5.4 Example 4 (Inventive)

[0309] The setup according to example 4 corresponds to the two-column interconnection according to FIG. 4, wherein p.sub.3>p.sub.3A>p.sub.1. The intermediate evaporator VZ.sub.RD1 <107> with the stream S.sub.RDX1 <112> withdrawn at the withdrawal point <111> shown in FIG. 4 is likewise omitted in the setup according to example 4. The condenser K.sub.RD2 <203> is also simultaneously the bottoms evaporator VS.sub.RD1 <106>. The fresh methanol stream S.sub.XE1 <205> is in FIG. 4 supplied to the rectification column RD.sub.2 <2> but in the setup according to example 4 supplied to RD.sub.1 <1>.

[0310] A stream S.sub.AE2 <3A02> of aqueous NaOH (50% by weight) of 5 t/h is supplied to the top of a reaction column RR.sub.A <3A> at 25° C. A vaporous methanol stream S.sub.AE1 <3A01> of 70.2 t/h Is supplied in countercurrent above the bottom of the reaction column RR.sub.A <3A>. The reaction column RR.sub.A <3A> is operated at a pressure p.sub.3A of 1.6 bar. At the bottom of the column RR.sub.A <3A> a virtually water-free product stream S.sub.AP′ <3A08> of 10.8 t/h is withdrawn (30% by weight sodium methoxide in methanol). At the evaporator VS.sub.3A <3A06> of the reaction column RR.sub.A <3A> about 0.8 MW of heating power are introduced using heating steam. A vaporous methanol-water stream S.sub.AB <3A03> is withdrawn at the top of the reaction column RR.sub.A <3A>. A portion of this stream Is recycled via a condenser K.sub.RRA <3A05> to the reaction column RR.sub.A <3A> and the remaining portion (64.4 t/h) is supplied to a first rectification column RD.sub.1 <1>. The rectification column RD.sub.1 <1> is operated at p.sub.1=˜1.1 bar. At the top of the rectification column RD.sub.1 <1> a liquid fresh methanol stream S.sub.XE1 <205> of 9.5 t/h is supplied (shown at the top of the rectification column RD.sub.2 <2> in FIG. 4) and vaporous methanol stream S.sub.RDB1 <101> is withdrawn.

[0311] A portion of S.sub.RDS1 <101> is recycled via the condenser K.sub.RD1 <102> into column RD.sub.1 <1>. The remaining portion of S.sub.RDB1 <101> (55 t/h) is compressed to 2 bar in a compressor VD.sub.13 <16>, wherein about 1.2 MW of compressor power are required, and supplied to the reaction column RR.sub.A <3A>. Discharged at the bottom of the rectification column RD.sub.1 <1> is a liquid stream of a water-methanol mixture S.sub.RDS1 <103> of which a portion S.sub.RDS12 <104> of 18.9 t/h is passed into the rectification column RD.sub.2 <2> and the remaining portion of the stream S.sub.RDS1 <103> is recycled as S.sub.RDS11 <105> into RD.sub.1 <1>.

[0312] The pressure of the discharged stream S.sub.RDS12 <104> is increased in a pump P <15> to 3.4 bar and the stream supplied to the second rectification column RD.sub.2 <2>. The rectification column RD.sub.2 <2> is operated at a pressure p.sub.2 of 3.2 bar. In the condenser K.sub.RD2 <203> of column RD.sub.2 <2> which is simultaneously the evaporator of the column RD.sub.1 <1> about 6.3 MW of heating power are provided for the column RD.sub.1 <1>. Withdrawn at the top of the rectification column RD.sub.2 <2> is a vaporous methanol stream S.sub.RDS2 <201>. A portion of S.sub.RDB2 <201> is recycled via the condenser K.sub.RD2 <203> into column RD.sub.2 <2>. The remaining portion of S.sub.RD2 <201> (15.2 t/h) is supplied to the reaction column RR.sub.A <3A>. Discharged at the bottom of the rectification column RD.sub.2 <2> is a liquid stream of water (contaminated with 500 ppmw of methanol) of 3.7 t/h. At the evaporator VS.sub.RD2 <204> of the rectification column RD.sub.2 <2> about 11.4 MW of heating power are introduced using heating steam.

[0313] The respective, non-recycled portions of the vaporous methanol streams S.sub.RDB1 <101> and S.sub.RDB2 <201> withdrawn at the tops of RD.sub.1 <1> and RD.sub.2 <2> are mixed and recycled to the bottom of the reaction column RR.sub.A <3A>.

[0314] Altogether in this example about 12.2 MW of heating power via heating steam and about 1.2 MW of electrical power (compressor power) are required and must be externally provided.

[0315] Compared to the embodiment according to the noninventive example 2 this variant thus saves about 79% of the required compressor power to be externally provided (electrical energy).

5.5 Result

[0316] Comparison of the proportion of heating steam and electrical current required to cover the energy demand in the inventive and noninventive examples reveals that the inventive process surprisingly makes it possible to cover a large proportion of the energy demand through heating steam and to minimize the proportion of the power to be provided through electrical energy.