IMPROVED PROCESS FOR PREPARING METAL ALKOXIDE COMPOUNDS

Abstract

The present invention relates to a process for preparing metal alkoxide compounds MOR.sup.2 from the metal hydroxides MOH and the compounds of the formulae R.sup.1OH and R.sup.2OH, where the boiling point of R.sup.1OH is lower than that of R.sup.2OH. R.sup.1 and R.sup.2 here are alkyl radicals or haloalkyl radicals, the carbon chain of which may be interrupted by ether groups, and which may have hydroxy groups. M here is a metal, preferably an alkali metal.

The process, by contrast with the conventional processes for transalcoholization, which require at least two reaction steps in two different reactive distillation columns, is conducted as a multiple reactive distillation in a reactive distillation column. This results in a decrease in apparatus complexity and a reduction in the need for power and heating steam. The process is especially suitable for preparation of compounds MOR.sup.2 for which the corresponding compound R.sup.2OH forms an azeotrope with water and/or for which the boiling point of R.sup.2OH is close to the boiling point of water.

Claims

1-13. (canceled)

14. A process for preparing a compound of formula MOR.sup.2, wherein: (a) a reactant stream S.sub.1 comprising a compound of formula R.sup.1OH, is fed via a lateral feed into a reactive distillation column RR at a feed point, and wherein RR optionally has a bottoms circuit S.sub.U1, a column section B above the feed point and a column section A below the feed point; (b) a reactant stream S.sub.0 comprising a compound of formula MOH is fed into column section B; (c) reactant stream S.sub.1 is reacted with reactant stream S.sub.0 in countercurrent in column section B to give a crude product RP.sub.B comprising MOR.sup.1, water, and R.sup.1OH; (d) a reactant stream S.sub.2 comprising a compound of the formula R.sup.2OH is fed into column section A; (e) reactant stream S.sub.2 is reacted with the compound MOR.sup.1 obtained in step (c) in countercurrent in column section A to give a crude product RPA comprising MOR.sup.2, R.sup.2OH, and R.sup.1OH; (f) a bottom product stream S.sub.U comprising MOR.sup.2 and R.sup.2OH is withdrawn from the bottom of RR and, if column RR has a bottoms circuit S.sub.U1, alternatively or additionally from the bottoms circuit S.sub.U1 of RR; and (g) a vapour stream S.sub.0 comprising water and R.sup.1OH is withdrawn at the upper end of RR; wherein: M is a metal; R.sup.1 is an alkyl radical optionally having one or more hydroxy groups, or a haloalkyl radical optionally having one or more hydroxy groups, R.sup.2 is an alkyl radical optionally having one or more hydroxy groups, or a haloalkyl radical optionally having one or more hydroxy groups, and wherein, for R.sup.1 and R.sup.2, the carbon chain of the alkyl radical or haloalkyl radical may be interrupted by one or more oxygen atoms, wherein there are at least two carbon atoms between interrupting oxygen atoms and any hydroxy group included in R.sup.1 or R.sup.2, and R.sup.1 and R.sup.2 are different.

15. The process of claim 14, wherein reactant streams S.sub.0, S.sub.1, S.sub.2 are fed simultaneously into reactive distillation column RR.

16. The process of claim 14, wherein M is an alkali metal.

17. The process of claim 14, wherein R.sup.1 is methyl.

18. The process of claim 17, wherein R.sup.2OH is a compound that forms an azeotropic mixture with water.

19. The process of claim 17, wherein R.sup.2 is selected from the group consisting of C.sub.2 to C.sub.10-alkyl, (CH.sub.2).sub.2OH, (CH.sub.2).sub.2O(CH.sub.2).sub.2OH, (CH.sub.2).sub.3OH, (CH.sub.2).sub.4OH, and 1-methoxypropan-2-yl.

20. The process of claim 19, wherein R.sup.2 is selected from the group consisting of ethyl, iso-propyl, sec-butyl, 2-methyl-2-butyl, tert-butyl, 2-methyl-2-pentyl, 3-methyl-3-pentyl, 3-ethyl-3-pentyl, 2-methyl-2-hexyl, and 3-methyl-3-hexyl.

21. The process of claim 17, wherein at least a portion of vapour stream S.sub.O is directed into a rectification column RD.sub.A and is separated in RD.sub.A into at least one vapour stream S.sub.OA comprising methanol which is withdrawn at the upper end of RD.sub.A, and at least one stream S.sub.UA comprising water which is withdrawn at the lower end of RD.sub.A.

22. The process of claim 21, wherein at least a portion of stream S.sub.OA is used as reactant stream S.sub.1 in step (a).

23. The process of claim 14, wherein stream S.sub.2 in step (d) is fed in liquid form into column section A.

24. The process of claim 14, wherein stream S.sub.2 in step (d) is fed into the bottom of column RR, and, if column RR has a bottoms circuit S.sub.U1, alternatively or additionally, stream S.sub.2 in step (d) is fed into the bottoms circuit S.sub.U1 of column RR.

25. The process of claim 24, wherein column RR has a bottoms circuit S.sub.U1.

26. The process of claim 25, wherein the bottoms circuit S.sub.U1 of column RR comprises a forced circulation evaporator, and stream S.sub.2 is fed in liquid form into the feed to the forced circulation evaporator.

27. The process of claim 15, wherein R.sup.1 is methyl.

28. The process of claim 27, wherein R.sup.2OH is a compound that forms an azeotropic mixture with water.

29. The process of claim 27, wherein R.sup.2 is selected from the group consisting of C.sub.2 to C.sub.10-alkyl, (CH.sub.2).sub.2OH, (CH.sub.2).sub.2O(CH.sub.2).sub.2OH, (CH.sub.2).sub.3OH, (CH.sub.2).sub.4OH, and 1-methoxypropan-2-yl.

30. The process of claim 29, wherein R.sup.2 is selected from the group consisting of ethyl, iso-propyl, sec-butyl, 2-methyl-2-butyl, tert-butyl, 2-methyl-2-pentyl, 3-methyl-3-pentyl, 3-ethyl-3-pentyl, 2-methyl-2-hexyl, and 3-methyl-3-hexyl.

31. The process of claim 30, wherein at least a portion of vapour stream S.sub.O is directed into a rectification column RD.sub.A and is separated in RD.sub.A into at least one vapour stream S.sub.OA comprising methanol which is withdrawn at the upper end of RD.sub.A, and at least one stream S.sub.UA comprising water which is withdrawn at the lower end of RD.sub.A.

32. The process of claim 31, wherein at least a portion of stream S.sub.OA is used as reactant stream S.sub.1 in step (a).

33. The process of claim 32, wherein stream S.sub.2 in step (d) is fed in liquid form into column section A.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0049] The present invention relates to a process for preparing a compound of the formula MOR.sup.2 by reactive distillation.

[0050] The process according to the invention is based on two reactions.

[0051] In a first reaction, according to the following reaction <C1>, the compound MOR.sup.1 is obtained:

##STR00003##

[0052] Compound MOR.sup.1 is then reacted in the subsequent reaction <C2>, corresponding to a transalcoholization, with compound R.sup.2OH to give compound MOR.sup.2:

##STR00004##

[0053] R.sup.1 is an alkyl radical or haloalkyl radical that optionally has one or more hydroxy groups, where, for R.sup.1, the carbon chain of the alkyl radical or haloalkyl radical may be interrupted by one or more oxygen atoms, where there are at least two carbon atoms between interrupting oxygen atoms and any hydroxy group included in R.sup.1.

[0054] R.sup.2 is an alkyl radical or haloalkyl radical that optionally has one or more hydroxy groups, where, for R.sup.2, the carbon chain of the alkyl radical or haloalkyl radical may be interrupted by one or more oxygen atoms, where there are at least two carbon atoms between interrupting oxygen atoms and any hydroxy group included in R.sup.2.

[0055] R.sup.1 and R.sup.2 are different. The person skilled in the art will also appreciate that the compound R.sup.2OH will have a higher boiling point than R.sup.1OH since this is a necessary prerequisite for R.sup.1OH to be obtained at the top and R.sup.2OH at the bottom of column RR. This prerequisite is met automatically when R.sup.1=methyl.

[0056] The compound of the formula MOR.sup.2 is referred to in the context of the invention as metal alkoxide compound. Metal alkoxide compound in the context of the invention is especially understood to mean metal alkoxides and metal ether alkoxides. According to the invention, the metal alkoxide compound is preferably a metal alkoxide.

[0057] When the compound MOR.sup.2 is a metal ether alkoxide, R.sup.2 is an alkyl radical optionally having one or more hydroxy groups, and optionally interrupted by one or more oxygen atoms, where there are at least two carbon atoms between interrupting oxygen atoms and any hydroxy group encompassed by R.sup.2.

[0058] An oxygen atom interrupting an alkyl radical, where there are at least two carbon atoms between the latter and any further oxygen atoms interrupting the alkyl radical and any hydroxy group encompassed by the alkyl radical, is referred to as ether group.

[0059] When the compound MOR.sup.2 is a metal alkoxide, R.sup.2 is an alkyl radical optionally having one or more hydroxy groups.

[0060] Alkyl in the context of the invention includes cycloalkyl.

[0061] Haloalkyl is preferably an alkyl radical in which at least one hydrogen atom has been exchanged for a halogen atom, where the halogen atom is more preferably selected from fluorine, chlorine.

[0062] In a preferred embodiment, R.sup.1 is alkyl, more preferably C.sub.1 to C.sub.4-alkyl, even more preferably methyl or ethyl, and most preferably methyl.

[0063] More preferably, R.sup.1 is methyl, and R.sup.2 is selected from the group consisting of C.sub.2 to C.sub.10-alkyl, (CH.sub.2).sub.2OH, (CH.sub.2).sub.2O(CH.sub.2).sub.2OH, (CH.sub.2).sub.3OH, (CH.sub.2).sub.4OH, 1-methoxypropan-2-yl.

[0064] Yet more preferably, R.sup.1 is methyl, and R.sup.2 is C.sub.2 to C.sub.10-alkyl. Yet more preferably, R.sup.1 is methyl, and R.sup.2 is selected from the group consisting of ethyl, iso-propyl, sec-butyl, 2-methyl-2-butyl, tert-butyl, 2-methyl-2-pentyl, 3-methyl-3-pentyl, 3-ethyl-3-pentyl, 2-methyl-2-hexyl, 3-methyl-3-hexyl.

[0065] Yet more preferably, R.sup.1 is methyl, and R.sup.2 is selected from the group consisting of ethyl, iso-propyl, sec-butyl, 2-methyl-2-butyl, 3-methyl-3-pentyl, 3-ethyl-3-pentyl.

[0066] Yet more preferably, R.sup.1 is methyl, and R.sup.2 is selected from the group consisting of ethyl, iso-propyl, 2-methyl-2-butyl.

[0067] Most preferably, R.sup.1=methyl and R.sup.2=ethyl.

[0068] In another preferred embodiment, R.sup.1=methyl and R.sup.2OH is a compound that forms an azeotropic mixture with water. These especially include ethanol, n-propanol, iso-propanol, n-butanol, sec-butanol, iso-butanol, tert-butanol, cyclohexanol, preferably ethanol, n-propanol, iso-propanol. Most preferably, the compound R.sup.2OH that forms an azeotropic mixture with water is ethanol.

[0069] In a further preferred embodiment, the boiling point of compound R.sup.2OH at standard pressure (1 bar) is in the range from 70 C. to 130 C., preferably in the range of 78 C. to 120 C.

[0070] M is a metal, especially an alkali metal, preferably selected from lithium, sodium, potassium, more preferably from sodium, potassium.

[0071] Most preferably, M=sodium.

[0072] The process according to the invention can be performed either continuously or batchwise. The process according to the invention is preferably performed continuously. Within the reactive distillation column RR, constant mass transfer and the changing concentrations within the gas phase and the liquid phase result in constant readjustment of the reaction equilibrium, which enables a high conversion.

[0073] Preferably, reactant streams S.sub.0, S.sub.1, S.sub.2 are fed simultaneously into reactive distillation column RR.

[0074] The process according to the invention is conducted especially at a temperature within a range from 45 C. to 150 C., preferably within a range from 47 C. to 120 C., more preferably within a range from 60 C. to 110 C., and at a pressure within a range from 0.2 bar abs. to 40 bar abs., preferably 0.5 bar abs. to 10 bar abs., preferably within a range from 0.7 bar abs. to 5 bar abs., more preferably within a range from 0.8 bar abs. to 4 bar abs., more preferably within a range from 0.9 bar abs. to 3.5 bar abs., yet more preferably within a range from 1.0 bar abs. to 3 bar abs., yet more preferably at 1.25 bar abs.

[0075] The equilibrium position of the reactions underlying the process according to the invention is temperature-dependent in the preparation of some compounds MOR.sup.2, especially the alkoxides. In these cases, high temperatures may advantageously result in a higher conversion. It may also be advantageous to conduct the process according to the invention under elevated pressure, for example at least 1.5 bar absolute, at least 2.5 bar absolute, or at least 5.0 bar absolute.

[0076] In other embodiments, it is advantageous to conduct the process according to the invention under reduced pressure, for example within a range of 0.1 to 0.9 bar abs., especially 0.3 to 0.75 bar abs.

Step (a)

[0077] In step (a) of the process according to the invention, a reactant stream S.sub.1 comprising a compound of the formula R.sup.1OH is fed via a lateral feed into a reactive distillation column RR optionally having a bottoms circuit S.sub.U1, and with a column section B above the feed point and a column section A below the feed point.

[0078] The word lateral is understood to mean that the feed is below the top of the column and above the bottom of the reactive distillation column RR. The feed point of stream S.sub.1 divides the column into a column section B (above the feed point) and a column section A (below the feed point).

[0079] The reaction corresponding to the aforementioned reaction <C1> takes place in the reaction column RR above the feed point for stream S.sub.1 (step (c) of the process according to the invention). The reaction corresponding to the aforementioned reaction <C2> takes place in the reaction column RR below the feed point for stream S.sub.1 (step (e) of the process according to the invention). If the stream S.sub.1 is directed into the reaction column RR via multiple feed points, column section B is above the feed point closest to the top of the column RR, and column section A is below the feed point closest to the top of the column RR.

[0080] According to the invention, reactive distillation column (synonym: reactive rectification column) defines a distillation column in which the reaction according to the invention as per the above reaction equations <C1> and <C2> proceeds at least in some parts. It can also be abbreviated to reaction column or, in the context of the present invention, to column.

[0081] According to the invention, column section A (as reaction section for reaction <C2>) and column section B (as reaction section for reaction <C1>) are arranged one on top of another in a single column RR. Since two reactions thus proceed in column RR in the present process, column RR may also be referred to as a multiple reaction column, and the process according to the invention as a multiple reactive distillation.

[0082] The reactive distillation column RR used may be a standard reactive distillation column. Column RR is selected, for example, from columns with random packing, columns with structured packing and tray columns, more preferably tray columns and columns with structured packing.

[0083] Installed in suitable tray columns are sieve trays, bubble-cap trays or valve trays, through which the liquid phase flows. The reactive distillation column in the process according to the invention preferably has trays as internals, for example selected from bubble-cap trays, valve trays, bubble-cap trays, cross-slit bubble-cap trays, sieve trays, Thormann trays.

[0084] Columns with random packing may be filled with different random packings. Heat and mass transfer are improved by the increase in the surface area on account of the shaped bodies that are usually about 25 to 80 mm in size. Examples are the Raschig ring (a hollow cylinder), Pall ring, Hiflow ring, Intalox saddle and the like. The random packings may be introduced into the column in an ordered manner or else randomly (as a bed). Useful materials include glass, ceramic, metal and plastics.

[0085] Structured packings are a further development of the ordered random packings. They have a regular-shaped structure. It is thus possible in the case of structured packings to reduce pressure drops in the flow of gas. There are various designs of structured packings, for example fabric packings or sheet metal packings. Column section B preferably comprises structured packings, while column section A comprises trays.

[0086] The top of the column refers to the region free of internals above the uppermost tray, or above the uppermost layer of structured packing. It is generally formed by a curved plate (hood, e.g. dished end or torispherical head), which forms the concluding element of the reactive distillation column. According to the invention, the top of the column RR is also part of column section B.

[0087] The bottom of the column refers to the region free of internals below the lowermost tray, or below the lowermost layer of structured packing. According to the invention, the bottom of column RR is part of column section A. According to the invention, if column RR has a bottoms circuit S.sub.U1, the bottoms circuit S.sub.U1 is also part of column section A.

[0088] The suitable number of theoretical plates in column RR depends on the difference in the vapour pressures of R.sup.1OH and R.sup.2OH, with a greater number of theoretical plates being advantageous in the case of a smaller difference. It also depends on the equilibrium position of reactions <C1> and <C2>, where the advantage of a greater number of theoretical plates increases with the extent to which the equilibrium is to the reactant side. The suitable number of theoretical plates in the two column sections also depends on the purity of bottom product and top product which is to be achieved, andif a reflux is establishedthe reflux rate used, where a higher number of theoretical plates is required to achieve a higher purity for a given reflux rate.

[0089] The reactant stream S.sub.1 comprises a compound of the formula R.sup.1OH. In a preferred embodiment, the proportion by mass of all compounds of the formula R.sup.1OH in S.sub.1 is >95% by weight, yet more preferably >99% by weight, and S.sub.1 otherwise comprises especially water.

[0090] The reactant stream S.sub.1 preferably comprises methanol. In a more preferred embodiment, the proportion by mass of methanol in S.sub.1 is then >95% by weight, yet more preferably 99% by weight, and S.sub.1 otherwise comprises especially water.

[0091] The methanol used in step (a) as reactant stream S.sub.1 in this preferred embodiment may also be commercially available methanol having a proportion by mass of methanol of more than 99.8% by weight and a proportion by mass of water of up to 0.2% by weight.

[0092] The reactant stream S.sub.1 is preferably introduced into step (a) in vapour form.

Step (b)

[0093] In step (b) of the process according to the invention, a reactant stream S.sub.0 comprising a compound of the formula MOH is fed into column section B.

[0094] The reactant stream S.sub.0 comprises MOH. In a preferred embodiment, especially when R.sup.1OH=methanol, S.sub.0 comprises not only MOH but also at least one further compound selected from water, methanol. It is yet more preferable when S.sub.0 also comprises water in addition to MOH; in that case, S.sub.0 is an aqueous solution of MOH.

[0095] When the reactant stream S.sub.0 comprises MOH and water, the proportion by mass of MOH, based on the total weight of the aqueous solution forming S.sub.0, is especially within a range from 10% to 75% by weight, preferably in the range from 15% to 54% by weight, more preferably in the range from 30% to 53% by weight and especially preferably in the range from 40% to 52% by weight.

[0096] When the reactant stream S.sub.0 comprises MOH and methanol, the proportion by mass of MOH in methanol, based on the total weight of the solution forming S.sub.0, is especially within a range from 10% to 75% by weight, preferably in the range from 15% to 54% by weight, more preferably in the range from 30% to 53% by weight, and especially preferably in the range from 40% to 52% by weight.

[0097] In the particular case in which the reactant stream S.sub.0 comprises both water and methanol in addition to MOH, it is particularly preferable that the proportion by mass of MOH in methanol and water, based on the total weight of the solution forming S.sub.0, is especially within a range from 10% to 75% by weight, preferably in the range from 15% to 54% by weight, more preferably in the range from 30% to 53% by weight, and especially preferably in the range from 40% to 52% by weight.

Step (c)

[0098] In step (c) of the process according to the invention, reactant stream S.sub.1 is reacted with reactant stream S.sub.0 in countercurrent in column section B to give a crude product RP.sub.B comprising MOR.sup.1, water and R.sup.1OH, with or without MOH.

[0099] In step (c), the reaction accordingly takes place according to the aforementioned reaction equation <C1>.

[0100] The reaction of the reactant stream S.sub.1 comprising a compound of the formula R.sup.1OH with reactant stream S.sub.0 comprising a compound of the formula MOH in countercurrent is achieved in accordance with the invention in that the feed point for reactant stream S.sub.1 in step (a) in reaction column RR is below the feed point for reactant stream S.sub.0 in step (b). In particular, S.sub.1 is added in vaporous form, and S.sub.0 as a solution, such that the two streams S.sub.0 and S.sub.1 meet and are reacted with one another in column section B.

[0101] The reaction column RR preferably comprises at least 1, in particular at least 2, preferably 15 to 40, theoretical plates between the feed point of the reactant stream S.sub.1 and the feed point of the reactant stream S.sub.0.

[0102] In column section B of the reaction column RR, the reactant stream S.sub.1 comprising a compound of the formula R.sup.1OH is then reacted with the reactant stream S.sub.0 comprising a compound of the formula MOH in the above-described reaction <C1> to give MOR.sup.1 and H.sub.2O, where these products, since the reaction is an equilibrium reaction, are present in a mixture with the reactant R.sup.1OH and possibly (since R.sup.1OH is especially added in molar excess to MOH) the reactant MOH. Accordingly, a crude product RP.sub.B comprising not only the products MOR.sup.1 and water but also R.sup.1OH and possibly MOH is obtained in step (c) in column section B of the reaction column RR.

[0103] At the same time, the more volatile components become enriched in the gas phase in the direction of the top of column RR, and the less volatile components in the liquid phase in the direction of the bottom of the column RR.

[0104] A portion of compound R.sup.1OH is thus in gaseous form in the reactive distillation column RR and ascends as vapour in the direction of the top of column RR. As a result, a vapour comprising R.sup.1OH is obtained in column section B. This is drawn off at the top of column RR in step (d) of the process according to the invention as vapour stream S.sub.O comprising R.sup.1OH. At the same time, at the upper end of column section B of RR (and hence at the upper end of RR), water is also driven out as a vapour together with R.sup.1OH and withdrawn in step (g) as vapour stream S.sub.O. Even if the boiling point of compound R.sup.1OH is below the boiling point of water, the feed of reactant stream S.sub.1 can be adjusted such that the water of reaction and the water added with stream S.sub.0 are driven out as a vapour.

[0105] At the lower end of column section B of RR, compound MOR.sup.1 is then obtained, which is converted further in column section A in step (e) and in accordance with the above reaction equation <C2>.

[0106] In a preferred embodiment of the process according to the invention, and especially in the cases in which S.sub.0 comprises not only MOH but also water, the ratio of the total weight (mass; unit: kg) of all compounds R.sup.1OH that are used as reactant stream S.sub.1 in step (a) to the total weight (mass; unit: kg) of all compounds MOH used as reactant stream S.sub.0 in step (b) is in the range from 4:1 to 50:1, more preferably in the range from 8:1 to 48:1, even more preferably in the range from 10:1 to 45:1, more preferably in the range from 20:1 to 40:1, even more preferably 22:1.

Step (d)

[0107] In step (d) of the process according to the invention, a reactant stream S.sub.2 comprising a compound of the formula R.sup.2OH is fed into column section A.

[0108] Feeding reactant stream S.sub.2 into column section A in step (d) comprises feeding below the feed point of reactant stream S.sub.1, especially into the bottom of column RR, and, if column RR has a bottoms circuit S.sub.U1, alternatively or additionally into the bottoms circuit S.sub.U1 of column RR. Feeding into column section A is effected in the cases in which RR has a bottoms circuit S.sub.U1, preferably in such a way that S.sub.2 is fed into the bottoms circuit S.sub.U1.

[0109] Stream S.sub.2 in step (d) can be fed in liquid form or gaseous form into column section A, especially the bottom, or in the cases in which RR has a bottoms circuit S.sub.U1, alternatively or additionally into the bottoms circuit S.sub.U1 of column RR. Preferably, stream S.sub.2 in step (d) is fed in liquid form into column section A, especially the bottom, or in the cases in which RR has a bottoms circuit S.sub.U1, alternatively or additionally into the bottoms circuit S.sub.U1 of column RR.

[0110] More preferably, column RR has a bottoms circuit S.sub.U1 into which stream S.sub.2 is fed in liquid form in step (d).

[0111] Bottoms circuit S.sub.U1 is understood to mean the portion of the bottom stream withdrawn from column RR which is fed back into column RR. In FIG. 1, the bottoms circuit S.sub.U1 is thus formed by stream <106>. This recycled portion of the bottom stream (in FIG. 1: <106>) is preferably heated by means of a forced circulation evaporator (in FIG. 1: <12>) before being fed back into column RR.

[0112] The feeding of S.sub.2 into the bottoms circuit S.sub.U1 can accordingly be effected into the bottom stream before stream S.sub.U (in FIG. 1: <104>) is withdrawn from the bottoms circuit S.sub.U1 (in FIG. 1: <106>).

[0113] Alternatively, S.sub.2 can be fed into the bottoms circuit S.sub.U1 after stream S.sub.U has been removed from the bottoms circuit Sur. If a forced circulation evaporator is used, S.sub.2 can also be fed directly into the forced circulation evaporator.

[0114] In a preferred embodiment of the process according to the invention, the molar ratio of the molar amount (unit: mol) of all compounds R.sup.1OH that are used as reactant stream S.sub.1 in step (a) to the molar amount (unit: mol) of all compounds R.sup.2OH used as reactant stream S.sub.2 in step (d) is in the range from 9:1 to 1:9, more preferably in the range from 8:1 to 1:2, even more preferably in the range from 7:1 to 1:1, more preferably in the range from 5:1 to 2:1, even more preferably 4.3:1.

Step (e)

[0115] In step (e) of the process according to the invention, stream S.sub.2 which is fed in in step (d) and the compound MOR.sup.1 which is obtained in step (c), in the context of the crude product RP.sub.B, are reacted with one another in countercurrent in column section A. This affords a crude product RPA comprising MOR.sup.2, R.sup.2OH, R.sup.1OH and possibly MOR.sup.1.

[0116] In step (e), the reaction accordingly takes place according to the aforementioned reaction equation <C2>.

[0117] As already described similarly in connection with step (c), the constituents of the crude product RPA will accumulate in column section A according to their volatility. The more volatile components such as R.sup.1OH accumulate in the gas phase in the direction of the top of column RR, while the less volatile components MOR.sup.2 and R.sup.2OH accumulate in the liquid phase in the direction of the bottom of the column.

[0118] At least a portion of the compound R.sup.1OH obtained as crude product RPA is thus in gaseous form in column section A of the reactive distillation column RR after step (e) has been performed, and rises in the form of vapour in the direction of the top of column RR, where it mixes with the R.sup.1OH in the crude product RP.sub.B in column section B, and the vapour accumulates in column section B. This vapour is drawn off at the top of column RR in step (g) of the process according to the invention as vapour stream S.sub.O comprising R.sup.1OH and water.

[0119] At least a portion of the compounds MOR.sup.2, R.sup.2OH obtained as crude product RPA is thus in liquid form in column section A of the reactive distillation column RR and accumulates in the bottom of column RR after performance of step (e). In step (f) of the process according to the invention, it is then withdrawn from the bottom of RR as a bottom product stream S.sub.U comprising MOR.sup.2 and R.sup.2OH, and, if column RR has a bottoms circuit S.sub.U1, alternatively or additionally from the bottoms circuit S.sub.U1 of RR.

Step (f)

[0120] In step (f) of the process according to the invention, a bottom product stream S.sub.U comprising MOR.sup.2 and R.sup.2OH is withdrawn from the bottom of RR and, if column RR has a bottoms circuit S.sub.U1, alternatively or additionally from the bottoms circuit S.sub.U1 of RR.

[0121] A bottom product stream S.sub.U comprising MOR.sup.2 and R.sup.2OH is withdrawn from the bottom of RR and, if column RR has a bottoms circuit S.sub.U1, alternatively or additionally from the bottoms circuit of RR means in accordance with the invention that, in the cases in which column RR has a bottoms circuit S.sub.U1, the bottom product stream S.sub.U, alternatively or additionally to direct withdrawal from the bottom, can be withdrawn from the bottoms circuit S.sub.U1. This is the case, for example, in an embodiment in which a bottom product stream comprising MOR.sup.2 and R.sup.2OH is withdrawn from the bottom of column RR, then this is recycled into column RR as bottoms circulation stream S.sub.U1, optionally via a forced circulation evaporator, and S.sub.U is withdrawn from the bottoms circuit S.sub.U1 as bottom product stream S.sub.U (for example discharged from the process).

[0122] The stream S.sub.U drawn off at the bottom of the column RR and/or from the bottoms circuit S.sub.U1 typically consists essentially of R.sup.2OH and the product MOR.sup.2. Stream S.sub.U can therefore be used further as it is, optionally after cooling in a heat exchanger, or else stored.

[0123] It is optionally possible to separate R.sup.2OH from MOR.sup.2 in stream S.sub.U in order to increase the concentration of MOR.sup.2. Alternatively, further R.sup.2OH may be added to stream S.sub.U in order to decrease the concentration of MOR.sup.2 in stream S.sub.U.

[0124] The solution of MOR.sup.2 in R.sup.2OH drawn off as stream S.sub.U advantageously includes only a small amount of compound R.sup.1OH, which permits an efficient process. Preferably, the proportion of all compounds R.sup.1OH in the solution drawn off as stream S.sub.U is not more than 1.0% by weight, preferably not more than 0.5% by weight, more preferably not more than 0.3% by weight, even more preferably <0.01% by weight, yet more preferably <0.001% by weight, for example 0.001% to 0.20% by weight or 0.01% to 0.10% by weight, based on the total weight of the solution drawn off as stream S.sub.U.

[0125] The solution of MOR.sup.2 in R.sup.2OH drawn off as stream S.sub.U likewise advantageously includes only small amounts of water, which permits an efficient process. Preferably, the proportion of water in the solution drawn off as stream S.sub.U is not more than 1.0% by weight, preferably not more than 0.5% by weight, more preferably not more than 0.3% by weight, even more preferably <0.01% by weight, yet more preferably <0.001% by weight, for example 0.001% to 0.20% by weight or 0.01% to 0.10% by weight, based on the total weight of the solution drawn off as stream S.sub.U.

[0126] If R.sup.1=methyl, the methanol concentration in the solution can be determined, for example, by headspace analysis or by gas chromatography, as described in WO 2021/122702 A1.

[0127] The proportion of all compounds of the formula MOR.sup.2 in the solution drawn off as stream S.sub.U is especially in the range of 3% to 60% by weight, preferably 5% to 55% by weight and more preferably 7% to 50% by weight, for example 7% to 30% by weight, 15% to 25% by weight, 19% to 25% by weight or 21% to 24% by weight, based on the total weight of the solution drawn off as stream S.sub.U.

[0128] According to the concentration of compounds MOR.sup.2 and R.sup.2OH in stream S.sub.U, stream S.sub.2 can also be used to dilute stream S.sub.U. Then stream S.sub.2 is fed into the bottom stream in the bottoms circuit S.sub.U1 of the column in step (d) before bottom stream S.sub.U (labelled <104> in FIG. 1) is withdrawn from the bottoms circuit S.sub.U1 (labelled <106> in FIG. 1).

[0129] The concentration of the compounds of the formula MOR.sup.2 in the solution drawn off as stream S.sub.U can be determined, for example, by titration, as described in WO 2021/122702 A1.

[0130] The reactive distillation column RR typically has an evaporator, preferably a forced circulation evaporator. This evaporator may be integrated in the column bottom. But it is preferably an evaporator accommodated in the bottoms circuit S.sub.U (=circulation evaporator). In this case, in particular, a substream of the stream drawn off at the bottom of the column can be fed to the bottoms circuit S.sub.U1 and then returned to the column as a heated, possibly biphasic fluid stream.

[0131] Alternatively or additionally, the bottoms are heated directly.

[0132] Suitable evaporators are, for example, boilers, natural-circulation evaporators, forced circulation evaporators and forced circulation flash evaporators.

[0133] In the case of forced circulation evaporators, a pump is used to conduct the liquid to be evaporated through the heater. The resultant vapour/liquid mixture is then returned to the column RR.

[0134] In the case of forced circulation flash evaporators, which is a particular embodiment of forced circulation evaporators, a pump is likewise used to conduct the liquid to be evaporated through the heater. A superheated liquid recycle stream is obtained, which is expanded into the bottom of the column. The pressure on the solution drawn off from column RR, which is returned to the column, is increased by superheating. The superheated recycle stream is expanded through a flow limiter. This results in superheating of the liquid above its boiling point in relation to the pressure within the column.

[0135] On passage of the superheated liquid through the flow limiter and reentry into the column, the liquid is evaporated abruptly. This abrupt evaporation proceeds with a considerable increase in volume and leads to acceleration of the fluid flow entering the column. Advantageously, the flow limiter is disposed immediately upstream of the reentry of the superheated liquid into the column, or even within it. The flow limiter used is preferably a diaphragm, a valve, a throttle, a perforated plate, a nozzle, a capillary or combinations thereof, especially a valve. For example, it is possible to use a rotary plug valve. It is particularly preferable when the opening characteristics of the flow limiter are adjustable. In this way, it is possible always to keep the pressure in the evaporator above the boiling pressure of the liquid, based on the pressure within the column, even in the case of changed flow rates, as can occur, for example, in startup and shutdown operations. It is advantageous that operating the evaporator by forced circulation or by forced circulation flashing achieves an elevated flow rate of the liquid in the heating apparatus compared to operation with natural circulation, for example in the tube bundle of the heat exchanger. The elevated flow rate results in improved heat transfer between heat exchanger and heated liquid, which in turn contributes to avoidance of local superheating.

[0136] The pump to be used in the case of forced circulation or forced circulation flash evaporators is preferably disposed between the withdrawal conduit and the evaporator.

[0137] In a preferred embodiment of the process according to the invention, the reactive distillation column RR has a forced circulation evaporator and stream S.sub.2 is fed in liquid form into the feed to the forced circulation evaporator.

[0138] Alternatively or additionally to the forced circulation evaporator, the bottoms may be heated directly, for example by means of a boiler.

[0139] The bottom temperature of the reactive distillation column RR at a given pressure determines the concentration of the compound MOR.sup.2 in the solution drawn off as stream S.sub.U at the bottom of column RR or from the bottoms circuit S.sub.U1. The temperature and hence the concentration are appropriately chosen such that compound MOR.sup.2 always remains in solution in the bottoms. The bottom temperature is adjusted, for example, by means of an evaporator and/or direct heating of the bottoms.

[0140] In a further embodiment, the reactive distillation column RR is filled with R.sup.2OH prior to startup, and R.sup.2OH is at first also used as reflux. On attainment of the operating temperature, streams S.sub.0 and S.sub.1 are then fed in.

Step (g)

[0141] In step (g) of the process according to the invention, a vapour stream S.sub.0 comprising water and R.sup.1OH is withdrawn at the upper end of RR.

[0142] This vapour stream S.sub.0 comprising water and R.sup.1OH is preferably directed at least partly into a rectification column RD.sub.A, where it is separated by distillation at least partly into water and R.sup.1OH. According to the boiling point of R.sup.1OH relative to the boiling point of H.sub.2O, water or R.sup.1OH is withdrawn at the bottom or top of RD.sub.A.

[0143] If R.sup.1OH=methanol, the separation in RD.sub.A is into at least one vapour stream S.sub.OA comprising R.sup.1OH which is withdrawn at the upper end of RD.sub.A, and at least one stream S.sub.UA comprising water which is withdrawn at the lower end of RD.sub.A.

[0144] At least a portion of the R.sup.1OH, especially methanol, obtained in the distillation in RD.sub.A can be used as reactant stream S.sub.1 in step (a).

[0145] When R.sup.1=methyl, it is accordingly preferable that at least a portion of vapour stream S.sub.0 is directed into a rectification column RD.sub.A and is separated in RD.sub.A into at least one vapour stream S.sub.OA comprising methanol which is withdrawn at the upper end of RD.sub.A, and at least one stream S.sub.UA comprising water which is withdrawn at the lower end of RD.sub.A.

[0146] Even more preferably, in that case, at least a portion of stream S.sub.OA is used as reactant stream S.sub.1 in step (a).

[0147] The reaction column RR is operated with or without, preferably with, reflux.

[0148] With reflux means that the vapour stream S.sub.0 withdrawn at the upper end of the respective column, in step (g) that withdrawn from the reaction column RR, is not conducted away completely. In step (g), the vapour stream S.sub.0 in question is then fed at least partly, preferably partly, back to the reaction column RR as reflux. In the cases where such a reflux is established, the reflux ratio is preferably 0.01 to 1, more preferably 0.02 to 0.9, yet more preferably 0.03 to 0.34, yet more preferably 0.04 to 0.27, yet more preferably 0.05 to 0.24, yet more preferably 0.06 to 0.10, yet more preferably 0.07 to 0.09. Generally and in the context of the present invention, a reflux ratio is understood to mean the ratio of the proportion of the mass flow withdrawn from the column (kg/h) that is recycled into the column in liquid form (reflux) to the proportion of this mass flow (kg/h) that is discharged from the respective column in liquid form or gaseous form.

[0149] A reflux can be established by mounting a condenser at the top of the respective column. A condenser K.sub.RR may be mounted, for example, atop the reaction column RR. In the condenser K.sub.RR, the vapour stream S.sub.0 is condensed at least partly, preferably partly, and the condensate is fed back to the reaction column RR.

[0150] In the embodiment in which a reflux is established in the reaction column RR, the MOH used as reactant stream S.sub.0 in step (b) may also be at least partly mixed with the reflux stream, and the resulting mixture may be supplied as such to the reaction column RR.

Advantages

[0151] The process according to the invention accordingly permits the advantages described for transalcoholization (flexibility in the process regime, which is important particularly in the case of alcohols having a similar boiling point to water, or alcohols that form azeotropes with water). Compared to the prior art transalcoholization processes, there is additionally a distinct saving of energy and minimization of apparatus complexity.

EXAMPLE

Example 1 (Inventive)

[0152] In the apparatus according to FIG. 1, a gaseous methanol stream S.sub.1 <101 > of 5500 kg/h is run into a multiple reactive distillation column RR <10>. In the upper section B <14> of column RR <10>, a 50% sodium hydroxide solution S.sub.0 <100> of 550 kg/h is run in countercurrent.

[0153] In the upper section B <14> of column RR <10>, NaOH and methanol are converted to sodium methoxide in the first forty trays.

[0154] In the lower section A <15> of column RR <10>, the transalcoholization of sodium methoxide to sodium ethoxide is effected. For this purpose, 1800 kg/h of S.sub.2 <102 > ethanol is run into the bottoms circuit S.sub.U1 <106> of the multiple reactive distillation column RR <10>. In the lower section A <15> of column RR <10>, the transalcoholization takes place.

[0155] At the bottom of column RR, 1900 kg/h of sodium ethoxide (solution in ethanol) is separated off, which is recycled as bottoms circulation stream S.sub.U1 <106> into column RR <10>, and stream S.sub.U <104> is led off from the bottoms circuit S.sub.U1<106>.

[0156] A 99% methanol stream S.sub.0<103> is drawn off overhead, which can be fed partly to a further rectification column RD.sub.A for workup and partly recycled to column RR <10> as reflux <107>.

Example 2 (Non-Inventive)

[0157] The amount of sodium ethoxide corresponding to Example 1 is prepared according to the prior art as outlined in FIG. 2, i.e. methanolic sodium methoxide solution is first obtained (step I) from sodium hydroxide solution and methanol in column <20> (cf. Example 2.3 of EP 1 997 794 A1). Thereafter (step II), this methanolic solution of NaOCH.sub.3 is reacted with ethanol in a further reaction column <30> in a transalcoholization to give ethanolic sodium ethoxide solution (as described in WO 2021/122702 A1).

Example 3 (Non-Inventive)

[0158] Ethanolic sodium ethoxide solution is obtained from aqueous sodium hydroxide solution and ethanol according to Example 2.3 of EP 1 997 794 A1, using the corresponding amount of ethanol (1035 g) rather than the amount of 720 g of methanol (22.5 mol) specified therein.

Results

[0159] The advantages of the process according to the invention are directly apparent from comparison of Example 1 with the conventional methods shown in Examples 2 and 3:

[0160] 1. In the comparative process according to Example 2, for preparation of ethoxide by transalcoholization from the corresponding methoxide (step II), the prior preparation of the methoxide from methanol and aqueous sodium hydroxide solution (step I) is necessary. This entails the operation of two reactive distillation columns <20> and <30>. By contrast, in the process according to the invention, only one reactive distillation column <10> is required. Thus, the apparatus complexity and the heating output to be provided are halved.

[0161] 2. In non-inventive Example 3, only one reaction column is used, but this is not a transalcoholization. This has the above-described disadvantages of the conventional process regime. Thus, in the preparation of corresponding ethoxides, vapours are obtained in which water is present in a mixture with ethanol. Ethanol, which forms azeotropes with water, can then be separated from the water in the vapour only in a complex manner. This problem is observed for all alkoxides for which the alcohols form azeotropes with water (for example including iso-propanol and n-propanol) or which have boiling points close to those of water.

[0162] By contrast, in the case of the procedure according to the invention, it is possible to avoid such azeotropic mixtures, since the alcohol present in a mixture with water in the vapour is different regardless of the alcohol for which the alkoxide is obtained. In the reaction column RR in Example 1, contact of ethanol and water is minimized, since ethanol is formed in the lower section A <15> and water in the upper section B <14> of the reaction column RR, and they are largely separated from one another by virtue of the vaporous methanol fed in as stream S.sub.1 <101>. Thus, the process according to the invention permits preparation of sodium ethoxide by transalcoholization from sodium methoxide. It thus provides the advantages of a transalcoholization, but without having the disadvantages thereof with regard to high energy demand.