Process for preparing polyoxymethylene dimethyl ethers from formaldehyde and methanol in aqueous solutions

Abstract

Process for preparing polyoxymethylene dimethyl ethers having 3 oxymethylene units (OME.sub.n3), comprising the steps: (i) introduction of formaldehyde, methanol and water into a reactor R and reaction to give a reaction mixture containing formaldehyde, water, methylene glycol, polyoxymethylene glycols, methanol, hemiformals, methylal (OME.sub.n=1) and polyoxymethylene dimethyl ethers (OME.sub.n>1);
(ii) introduction of the reaction mixture into a reactive distillation column K1 and separation into a low boiler fraction F1 containing formaldehyde, water, methylene glycol, polyoxymethylene glycols, methanol, hemiformals, methylal (OME.sub.n=1) and polyoxymethylene dimethyl ethers having from 2 to 3 oxymethylene units (OME.sub.n=2-3) and a high boiler fraction F2 containing polyoxymethylene dimethyl ethers having more than two oxymethylene units (OME.sub.n3).

Claims

1. Process for preparing polyoxymethylene dimethyl ethers having 3 oxymethylene units (OME.sub.n3), comprising the steps: (i) introduction of formaldehyde, methanol and water into a reactor R and reaction to give a reaction mixture containing formaldehyde, water, methylene glycol, polyoxymethylene glycols, methanol, hemiformals, methylal (OME.sub.n=1) and polyoxymethylene dimethyl ethers (OME.sub.n>1); and (ii) introduction of the reaction mixture into a reactive distillation column K1 having a feedpoint, a bottom offtake, and an overhead offtake and separation into a low boiler fraction F1 containing formaldehyde, water, methylene glycol, polyoxymethylene glycols, methanol, hemiformals, methylal (OME.sub.n=1) and polyoxymethylene dimethyl ethers having from 2 to 3 oxymethylene units (OME.sub.n=2-3) and a high boiler fraction F2 containing essentially polyoxymethylene dimethyl ethers having more than two oxymethylene units (OME.sub.n3), wherein the high boiler fraction F2 contains at least 90% by weight of polyoxymethylene di-methyl ethers having more than 2 oxymethylene units (OME.sub.n3), wherein the ratio of a volume of the liquid in the region between feedpoint and bottom offtake including any side reactors present and the bottom to the volume flow of the bottom offtake stream in the reactive distillation column K1 is at least 10 min, wherein the low boiler fraction F1 and the high boiler fraction F2 are withdrawn from the reactive distillation column K1.

2. Process according to claim 1 for preparing polyoxymethylene dimethyl ethers having 3 to 6 oxymethylene units (OME.sub.n=3-6), comprising the steps: (i) introduction of formaldehyde, methanol and water into a reactor R and reaction to give a reaction mixture containing formaldehyde, water, methylene glycol, polyoxymethylene glycols, methanol, hemiformals, methylal (OME.sub.n=1) and polyoxymethylene dimethyl ethers (OME.sub.n>1); (ii) introduction of the reaction mixture into a reactive distillation column K1 and separation into a low boiler fraction F1 containing formaldehyde, water, methylene glycol, polyoxymethylene glycols, methanol, hemiformals, methylal (OME.sub.n=1) and polyoxymethylene dimethyl ethers having from 2 to 3 oxymethylene units (OME.sub.n=2-3) and a high boiler fraction F2 containing essentially polyoxymethylene dimethyl ethers having more than two oxymethylene units (OME.sub.n3); (iii) introduction of the high boiler fraction F2 into a distillation column K2 and separation into a product fraction F3 containing polyoxymethylene dimethyl ethers having from 3 to 6 oxymethylene units (OME.sub.n=3-6) and a high boiler fraction F4 containing polyoxymethylene dimethyl ethers having more than 6 oxymethylene units (OME.sub.n>6); (iv) optionally recirculation of the high boiler fraction F4 into the reactor R.

3. Process according to claim 1, characterized in that the high boiler fraction F2 contains at least 95% by weight of polyoxymethylene dimethyl ethers having more than two oxymethylene units (OME.sub.n3).

4. Process according to claim 2, characterized in that the product fraction F3 contains more than 95% by weight of polyoxymethylene dimethyl ethers having from 3 to 6 oxymethylene units (OME.sub.n=3-6).

5. Process according to claim 1 comprising the additional steps: (v) introduction of the low boiler fraction F1 into an apparatus for water separation and removal of water or a water-rich stream F6, giving a low-water stream F5; (vi) optionally recirculation of the low-water stream F5 into the reactor.

6. Process according to claim 5 comprising the additional step: (vii) distillation of the water-rich stream F6 to give a stream F7 which contains methanol and methylal and is recirculated into the reactor R and a stream F8 containing water, formaldehyde and polyoxymethylene dimethyl ethers having from 2 to 3 oxymethylene units (OME.sub.n=2-3).

7. Process according to claim 2, characterized in that the columns K1 and K2 are configured as a single reactive distillation dividing wall column, with the low boiler fraction F1 being obtained as overhead offtake stream, the product fraction F3 being obtained as side offtake stream and the high boiler fraction F4 being obtained as bottom offtake stream from the dividing wall column.

8. Process according to claim 1, characterized in that formaldehyde is introduced as aqueous formaldehyde solution having a formaldehyde content of from 20 to 95% by weight into the reactor R.

9. Process according to claim 1, characterized in that formaldehyde is introduced as paraformaldehyde as melt or suspension into the reactor R.

10. Process according to claim 1, characterized in that the reaction in step (i) is carried out in the presence of a solid catalyst selected from the group consisting of ion-exchange resins, zeolites, aluminosilicates, aluminium dioxide and titanium dioxide.

11. Process according to claim 1, characterized in that the pH in the reactive distillation column K1 is from 4 to 14.

12. Process according to claim 5, characterized in that the apparatus used for water separation in step (v) is a pervaporation, vapour permeation or adsorption plant.

13. Process according to claim 1, characterized in that the reactive distillation column has, in operation, the following features: the pressure in the reactive distillation column K1 is from 0.2 bar to 5 bar; the temperature at the top of the reactive distillation column K1 is from 25 C. to 125 C.; the temperature at the bottom of the reactive distillation column K1 is from 110 C. to 255 C.; the reactive distillation column K1 has from 10 to 40 theoretical plates; below the feedpoint, the reactive distillation column K1 has from 3 to 20 theoretical plates; the reflux ratio in the reactive distillation column K1 is at least 0.20 g/g; the pH in the reactive distillation column K1 in the region between feedpoint and bottom offtake including any side reactors present and the bottom is greater than 4.

14. Process according to claim 13, characterized in that the pressure in the reactive distillation column K1 is from 0.5 to 4 bar; the temperature at the top of the reactive distillation column K1 is from 45 to 115 C.; the temperature at the bottom of the reactive distillation column K1 is from 140 to 240 C.; the reactive distillation column K1 has from 15 to 30 theoretical plates; below the feedpoint, the reactive distillation column K1 has from 4 to 15 theoretical plates; the pH in the reactive distillation column K1 in the region between feedpoint and bottom offtake is greater than 5.

15. Process according to claim 14, characterized in that the pressure in the reactive distillation column K1 is from 1 to 3 bar; the temperature at the top of the reactive distillation column K1 is from 65 to 105 C.; the temperature at the bottom of the reactive distillation column K1 is from 170 to 225 C.; the reactive distillation column K1 has from 20 to 25 theoretical plates; below the feedpoint, the reactive distillation column K1 has from 5 to 10 theoretical plates; the pH in the reactive distillation column K1 in the region between feedpoint and bottom offtake is in the range from 7 to 14.

Description

(1) A detailed sketch of a preferred embodiment of the process claimed is shown in FIG. 1.

(2) FIG. 1: Sketch of the process claimed. The FIGURE shows a preferred variant of the process of the invention.

(3) In detail, the process comprises the steps: (i) introduction of formaldehyde and of methanol, generally in the form of one or more aqueous solutions, (stream 1) and also the recycle streams 7 and 9 into a reactor and reaction to give a mixture (stream 3) containing formaldehyde, water, methylene glycol (MG), polyoxymethylene glycols (MG.sub.n), methanol, hemiformals (HF), methylal (OME.sub.n=1) and polyoxymethylene dimethyl ethers (OME.sub.n>1); here, stream 2 is a mixture of streams 1, 7 and 9. The feedstream 1 can contain small amounts of further components (e.g. formic acid, methyl formate and further impurities known to those skilled in the art); in general, formaldehyde is introduced as aqueous formaldehyde solution having a formaldehyde content of from 20 to 95% by weight into the reactor R. Formaldehyde can also be introduced in highly concentrated, liquid or gaseous form or as paraformaldehyde in the form of a melt or as aqueous suspension into the OME synthesis reactor; (ii) introduction of the reaction mixture (stream 3) into the specific reactive distillation column K1 and separation into a low boiler fraction containing essentially formaldehyde, water, methylene glycol (MG), polyoxymethylene glycols (MG.sub.n), methanol, hemiformals (HF), methylal (OME.sub.n=1) and polyoxymethylene dimethyl ethers (OME.sub.n=2) (stream 4) and a high boiler fraction consisting essentially of polyoxymethylene dimethyl ethers (OME.sub.n3) (stream 5); (iii) introduction of the high boiler fraction (stream 5) into the distillation column K2 and separation into a low boiler fraction (product fraction) containing the desired polyoxymethylene dimethyl ether OME.sub.n=3-6 product (stream 6) and a high boiler fraction containing polyoxymethylene dimethyl ethers (OME.sub.n>6) (stream 7); (iv) recirculation of the high boiler fraction (stream 7) into the reactor; (v) introduction of the low boiler fraction (stream 5) into an apparatus for water separation (preferably pervaporation, vapour permeation and adsorption) and separation into a low-water stream 9 and a water-rich stream 8; (vi) recirculation of the low-water stream 9 into the reactor.

(4) The water-rich stream 8 can be discharged from the process and optionally be recirculated into a formaldehyde plant.

(5) The term low-boiler fraction is used for the mixture taken off in the upper part of the column, and the term high boiler fraction is used for the mixture taken off in the lower part. In general, the low boiler fraction is taken off at the top of the column, and the high boiler fraction is taken off at the bottom of the column. However, this is not absolutely necessary. Taking off the fractions via side offtakes in the stripping section and enrichment section of the column is likewise possible.

(6) Here and in the following, consisting essentially of means that the fraction concerned comprises at least 90% by weight, preferably at least 95% by weight, of the components mentioned.

(7) The reaction of formaldehyde with methanol to form OME occurs according to the net reaction equation (11).
nCH.sub.2O+2CH.sub.3OH.Math.CH.sub.3O(CH.sub.2O)n-CH.sub.3+H.sub.2O(Reaction 11)

(8) As solid catalysts, use is made of, for example, ion-exchange resins, zeolites, aluminosilicates, aluminium dioxide, titanium dioxide. Preference is given to ion-exchange resins, and particular preference is given to ion-exchange resins whose skeleton consists of sulfonated polystyrene (e.g. Amberlyst 15, Amberlyst 46). However, all solid catalysts which have an acidic centre generally come into question. The apparent residence time of the reaction mixture over the catalyst is in the range from 1 s to 7200 s, preferably from 5 s to 3600 s, particularly preferably from 8 s to 1800 s. The apparent residence time is defined here as the mass of the catalyst divided by the mass flow of the fluid which flows through the reactor. The reaction can be carried out in any apparatus which is suitable for carrying out reactions of fluid media over a fixed-bed catalyst: it can, for example, be carried out in the suspension mode in a continuous stirred tank reactor (CSTR), a tube reactor or a loop reactor. In a less preferred case, a reactive distillation can also be used. Preference is given to a fixed-bed reactor, e.g. a catalyst bed through which a single pass occurs, as for the OME synthesis from trioxane and methylal, described in Burger J., Strfer E., Hasse H.: Production process for diesel fuel components poly(oxymethylene) dimethyl ethers from methane-based products by hierarchical optimization with varying model depth. Chemical Engineering Research and Design. 2013, 91(12), 2648-2662, a tray reactor or a shell-and-tube reactor or other apparatuses known to those skilled in the art. The temperature conditions in the reactor follow rules known to those skilled in the art. Owing to the comparatively small evolution of heat, the temperature can be regulated in a standard manner. The term reactor can encompass one or more apparatuses arranged in parallel or in series.

(9) The product mixture can subsequently be brought into contact with an anion-exchange resin in order to obtain a substantially acid-free product mixture.

(10) The reaction is generally carried out at a temperature of from 0 to 200 C., preferably from 30 to 150 C., particularly preferably from 40 to 120 C., and a pressure of from 0.3 to 30 bar, preferably from 1 to 20 bar, very particularly preferably from 1.2 to 10 bar. It is also possible to introduce the starting materials in gaseous form into a fixed-bed reactor. In this case, liquid starting materials have to be vaporized before entry into the reactor. As an alternative, gaseous feedstreams from an upstream production unit can be fed directly without intermediate condensation and after possible intermediate heating/cooling into the reactor. Such gaseous streams can, for example, be a gaseous reactor output stream containing formaldehyde and water from a formaldehyde plant. If the starting materials are fed in gaseous form into the fixed-bed reactor, condensation or partial condensation of the reactor output occurs before or in the work-up by distillation. This condensation or partial condensation can also occur even in the reactor itself when the temperature in the reactor goes below the dew point of oligomers formed due to the temperature conditions in the reactor.

(11) The liquid reaction mixture forms polyoxymethylene glycols and polyoxymethylene hemiformals as coproducts even without addition of a catalyst. The condensation and chain-buildup reactions involved in the formation of the polyoxymethylene glycols, polyoxymethylene hemiformals and OMEs are equilibrium reactions and therefore also proceed (depending on the position of the chemical equilibrium) in the reverse direction as dissociation and chain-degradation reactions. The particular requirements which the reactive distillation column K1 has to meet are determined thereby.

(12) Since the chemical reactions in the column K1 proceed at a finite reaction rate, the choice of suitable column internals is of great importance. Owing to the finite reaction rate of the chemical reactions, preference is given to using internals which make a high residence time and a high liquid content of the mixture in the column possible. If structured packings are used, specific banking-up packings as are described in EP 1 074 296 are particularly useful. If trays are used, specific residence time trays (e.g. Thormann trays) are particularly useful. To increase the residence time, it is likewise conceivable to equip the column K1 with a side reactor or a plurality of side reactors. In this case, a side offtake stream is taken from the column for each side reactor and is introduced into a side vessel in which the chemical reactions can additionally proceed. The discharge stream from the side vessel is returned to the column.

(13) The higher the concentration of formaldehyde in the feed to the column, the higher is the pressure in the column preferably selected in order to avoid precipitation of solids. Up to about 5% by weight of formaldehyde, the column is preferably operated at 1 bar. Above about 5% by weight of formaldehyde, the column is preferably operated as a pressure column.

(14) Apart from the column K1, the further distillation columns are generally columns of conventional design. Columns containing random packing elements, tray columns and columns containing ordered packing and combinations thereof are possible, with preference being given to tray columns and columns containing ordered packing.

(15) The distillation column K2 is operated at a pressure of from 0.01 to 1 bar, preferably at a pressure of from 0.05 to 0.9 bar and particularly preferably at a pressure of from 0.1 to 0.5 bar. The operating temperatures are generally in the range from 60 to 300 C. The column has from 5 to 40, preferably from 8 to 20, particularly preferably from 10 to 18, theoretical plates. The design of the column K2 is carried out in a standard manner according to rules known to those skilled in the art. The separation in the column K2 is described in the literature (Burger J., Strfer E., Hasse H. Production process for diesel fuel components poly(oxymethylene) dimethyl ethers from methane-based products by hierarchical optimization with varying model depth. Chemical Engineering Research and Design. 2013, 91(12), 2648-2662).

(16) Since the desired OMEs having a chain length of n=3-6 represent a middle-boiling fraction of the reactor output, it is likewise conceivable to integrate the two columns K1 and K2 in a single reactive dividing wall column in order to increase the efficiency of the overall process. Both the capital costs and the operating costs of the plant are decreased thereby. Such dividing wall columns are, for example, described in U.S. Pat. No. 5,914,012.

(17) In this case, a dividing wall which extends continuously over the stripping section, the region of the feedstream and at least part of the enrichment section of the column is installed in the column. However, the dividing wall can also extend over, inter alia, the entire enrichment section. In this reactive dividing wall column, a mixture of formaldehyde, methanol, water, methylal and OME.sub.n=2 is obtained as overhead product. The side product which is taken off from the column at the height of the feedpoint or between the feedpoint and the bottom on the side of the dividing wall opposite the feedpoint is the desired product stream containing OMEs having a chain length of n=3-6. The OMEs having chain lengths of n6 are obtained at the bottom of the column. This reactive dividing wall column is operated at a pressure of from 0.01 to 5 bar, preferably at a pressure of from 0.05 to 4 bar and particularly preferably at a pressure of from 0.01 to 3 bar. The operating temperatures are essentially analogous to the temperatures when the columns K1 and K2 are operated individually. The column has from 15 to 80, preferably from 30 to 50, particularly preferably from 35 to 40, theoretical plates. Here, from 10 to 30, preferably from 15 to 25, theoretical plates are in the region having the installed dividing wall.

(18) To increase the energy efficiency of the process, it is possible to couple the distillation columns thermally. Here, for example, the vapour from one column is used for heating another column as a result of suitable temperature and pressure conditions in the columns. It is likewise conceivable to compress the vapours from the columns and utilize them as heating medium for various vaporizers in the process. It is likewise conceivable to couple the plant in terms of energy with other plants in the integrated facility, preferably with a formaldehyde plant. The heat of reaction removed from the formaldehyde reactor can, for example, be used for generating heating steam for the OME plant.

(19) Stream 6 represents the product of value from the process of the invention. It contains more than 95% by weight of OME.sub.n=3-6, preferably more than 98% by weight of OME.sub.n=3-6, very particularly preferably more than 99% of OME.sub.n=3-6.

(20) In the case of the water separation being configured as a pervaporation or vapour permeation plant, it is possible to use inorganic and polymeric membranes. Possible polymeric membranes are, for example: polyamide, polyamidimide, polyacrylonitrile, polybenzimidazole, polyester, polycarbonate, polyetherimide, polyethylenimine, polyimide, polymethyl methacrylate, polypropylene, polysulfone, polyether sulfone, polyphenyl sulfone, polytetrafluoroethylene, poly-vinylidene fluoride, polyvinylpyrrolidone, polyvinyl alcohol, polydimethylsiloxane. The degree of crosslinking of the polymeric membrane can be set freely. The mechanical stability of the membranes can be increased by a support layer. Possible inorganic membranes are, for example: zeolites, metal oxides such as aluminium dioxide, zirconium dioxide, silicon dioxide, titanium oxide and glass. The pore structure of the membranes can be either symmetric or asymmetric. Preference is given to using inorganic membranes, with particular preference being given to using inorganic zeolites as membrane. The arrangement of the membrane modules is preferably, but not necessarily, a Christmas tree structure. As a result, all membrane modules used are equally loaded hydrodynamically. Possible membrane modules are all module forms known to those skilled in the art, for example tube modules, plate modules, capillary modules and hollow fibre modules. If polymeric membranes are used, plate modules are preferred. If inorganic membranes are used, tube modules are preferred.

(21) The operating pressure on the feed side of the membrane is in the range from 0.1 to 200 bar, preferably from 1 to 150 bar. The pressure on the permeate side is in the range from 0.001 to 10 bar, preferably from 0.01 to 5 bar. The temperature is in the range from 30 to 200 C. A person skilled in the art will know that, particularly in the case of pervaporation, heating of the retentate streams by means of intermediate heat exchangers is generally carried out between the membrane modules. In particular, it is possible to use the retentate stream from the last membrane module for preheating the feed to the first membrane module.

(22) Depending on the pressure selected on the permeate side of the membrane, condensation of the permeate stream is effected by means of cooling water or cooling brine. Other process streams from the OME plant or a formaldehyde plant can likewise be used. Cooling brine is preferably used in the case of very low pressures on the permeate side. If a vacuum is applied on the permeate side, it is possible to use either a mechanical vacuum pump or a thermally operating vacuum pump. In the latter case, a jet compressor is preferably used.

(23) If a vapour permeation plant is used, it is possible to operate the column K1 with only a partial condenser, so that the uncondensed vapour from the column K1 is conveyed directly into the vapour permeation plant. However, it is furthermore also possible to operate the column K1 with a total condenser and vaporize the feedstream to the vapour permeation plant beforehand by means of an additional heat exchanger.

(24) In the case of the water separation being configured as an adsorption plant, all conventional adsorbents can be used. Examples are zeolites, activated carbon, adsorber resins, metal oxides such as aluminium oxide, zirconium dioxide, silicon dioxide (silica gels) and titanium oxide, salts such as magnesium sulfate, sodium sulfate, calcium hydride, calcium oxide, calcium sulfate, potassium carbonate, potassium hydroxide, copper sulfate, lithium aluminium hydride, sodium hydroxide, and elemental alkali metals and alkaline earth metals. Preference is given to using zeolites and metal oxides, with very particular preference being given to using zeolites. The preferred adsorbents can be regenerated particularly simply by means of suitable temperature and pressure conditions. If zeolites are used as adsorbents, preference is given to using zeolites of the type NaA, particularly preferably zeolites of the type NaA whose pore width is in the range from 2 to 10 Angstrm. In discontinuous operation, the water-rich stream F6 is obtained in the regeneration of the adsorber. Thus, water and the additionally adsorbed components desorb.

(25) The adsorption plant is preferably configured as a simple fixed-bed plant. However, all further types of adsorbers known to those skilled in the art are possible. In this case, regeneration of the adsorbent is preferably effected by means of a pressure and/or temperature change in discontinuous operation. For this reason, two adsorber fixed beds are generally required, so that during operation of one fixed bed, the other fixed bed can be regenerated. An arrangement having 3 adsorber beds operated in parallel is usual. The feedstream to the adsorber can be either gaseous or liquid.

(26) It is also conceivable to recirculate the stream 4 directly, without water separation, to a formaldehyde plant and carry out the water separation in the formaldehyde plant. Here, not only methanol but also full acetals from stream 4 react in the formaldehyde synthesis reactor to form formaldehyde and water (Masamoto J., Matsuzaki K., Development of methylal synthesis by reactive distillation; Journal of Chemical Engineering of Japan, 1994, 27(1), 1-5). The water separation can be effected by subsequent concentration of the formaldehyde solution obtained, as is described, for example, in U.S. Pat. Nos. 7,342,139; 7,345,207; 7,273,955; 7,193,115; 6,610,888; 7,414,159. The solution which has been concentrated is again provided for the OME synthesis in the reactor of the OME synthesis. It is also possible, as described in U.S. Pat. No. 7,390,932, to prepare highly concentrated formaldehyde in the gas phase and feed this to the reactor. Furthermore, the aqueous formaldehyde solution can be converted by known methods into paraformaldehyde, which is introduced as melt or suspension of solid into the reactor. Regardless of the recirculation of the stream 4, highly concentrated liquid or gaseous formaldehyde or paraformaldehyde as melt or suspension can also be used in the OME synthesis reactor.

(27) Apart from the preferred starting material methanol, derivatives of methanol, e.g. methylal, dimethyl ether, OME.sub.2 and/or mixtures of these with one another and/or with methanol, can also be used as reaction partners for formaldehyde in the OME synthesis.

(28) Apart from methanol as simplest alcohol and derivatives thereof, it is in principle also possible to use other alcohols, e.g. ethanol, propanol and butanol and/or mixtures of these and/or with methanol, as reaction partners for the formaldehyde. What has been said above about the derivatives of methanol applies analogously to the derivatives of other alcohols, with mixed derivatives such as C.sub.2H.sub.5OCH.sub.2OCH.sub.3 or C.sub.2H.sub.5OCH.sub.3 also being possible. In the case of miscibility gaps, emulsions are used.

(29) The process of the invention can be operated entirely or partly continuously or batchwise. This also applies to the reactor and the columns. Preference is given to continuous operation of the process. Units such as the water separation by adsorption, which are generally preferably operated batchwise in the industry, are configured in a plurality of apparatuses connected in parallel so that a virtually continuous flow of all streams through this unit is possible.

(30) The invention further provides the products obtainable by the process of the invention and also the blends of these products with fuel and oil fractions from a refinery or an integrated refinery site. These blends can additionally contain fuel auxiliaries and additives as are known to a person skilled in the field of fuel.

(31) The composition of a mixture of OME oligomers is, when the reactor of the process is operated close to the point of equilibrium conversion: 0.35 g/gx.sub.OME30.79 g/g, 0.17 g/gx.sub.OME40.36 g/g, 0.04 g/gx.sub.OME50.31 g/g, x.sub.OME60.06 g/g, preferably 0.37 g/gx.sub.OME30.70 g/g; 0.23 g/gx.sub.OME40.35 g/g, 0.07 g/gx.sub.OME50.26 g/g, x.sub.OME60.08 g/g, very particularly preferably 0.40 g/gx.sub.OME30.62 g/g, 0.26 g/g, x.sub.OME40.35 g/g, 0.11 g/gx.sub.OME50.26 g/g, x.sub.OME60.12 g/g. Here, x.sub.OMEn is the mass fraction of OME oligomers having n oxymethylene units, based on the mass of the mixture.

(32) Here, close to equilibrium means that no concentration of the abovementioned individual OME species is more than 40% away from the value corresponding to the composition of a reaction mixture which is in thermodynamic equilibrium.

(33) The composition of this mixture of OME oligomers is a consequence of the above-described process conditions.

(34) However, depending on customer demand, other mixtures of OME oligomers can also be prepared. This preparation is carried out by varying the process conditions in the reactor and in particular the position of the separation cuts in the columns. The recycle streams then have to be adapted appropriately and the process no longer operates at the economic and ecological optimum.

(35) A further separation of the OME.sub.n=3-6 product according to chain length can be effected by process engineering operations, for example distillation. The design of a distillation sequence for the separation of OME.sub.n=3-6 according to chain length can in this case be carried out in a standard manner. Preference is given to OME.sub.3 being obtained as overhead product from a first distillation column, OME.sub.4 being obtained as overhead product from a second distillation column, OME.sub.5 being obtained as overhead product from a third distillation column and OME.sub.6 being obtained as bottom product from the third distillation column (but this is not absolutely necessary). However, the optional separation of the OME.sub.n=3-6 product does not change the above-described distribution of the OME oligomers relative to one another, since the ratios of the OME chain lengths are influenced only insignificantly by the separation due to the separation steps which are in reality not perfectly sharp.

(36) The invention is illustrated by the following examples.

EXAMPLES

Example 1

Overall Process

(37) Table 1 to Table 4 show four typical stream series of a preferred variant of the claimed process within measurement accuracy. The numbers have been appropriately rounded. Here, the mass fraction 0.00 g/g means a mass fraction of <0.005 g/g. In Table 1 to Table 3, the mass ratio of formaldehyde to methanol in stream 1 is 1.65. The proportion of water in stream 1 was varied between 0.05 g/g (Table 1), 0.10 g/g (Table 2) and 0.20 g/g (Table 3). In Table 4, the mass ratio of formaldehyde to methanol in stream 1 is 1.60. The proportion of water in stream 1 is 0.10 g/g.

(38) The water separation was in all cases dimensioned so that the proportion of water in stream 9 is about 0.02 g/g. Since the proportion of water in stream 1 was increased from Table 1 to Table 3 so that the proportion by mass of water in stream 9 is constant, the installed membrane area in the case of the water separation being configured as vapour permeation or pervaporation plant was generally increased and in the case of the water separation being configured as adsorption plant, the mass of adsorbent was increased.

(39) However, since the experiments were carried out on the micro scale, the increase in the membrane area is negligibly small in this example.

(40) The mass flow and the composition of stream 8 (discharged water stream) can vary as a function of the configuration of the water separation. In the tables, stream 8 (not shown) is given by the difference between streams 4 and 9.

(41) TABLE-US-00001 TABLE 1 Example (case 1) for a stream series of the process of the invention; feed composition (stream 1):proportion of water = 0.05 g/g, mass ratio of formaldehyde to methanol = 1.65 Stream 1 2 3 4 5 6 7 9 Mass 1.2 6.1 6.1 5.0 1.1 1.0 0.1 4.8 flow/(kg/h) Mass fractions/(g/g) Formaldehyde 0.59 0.36 0.25 0.30 0.00 0.00 0.00 0.32 Methanol 0.36 0.20 0.13 0.16 0.00 0.00 0.00 0.17 Water 0.05 0.03 0.04 0.05 0.00 0.00 0.00 0.02 Methylal 0.00 0.24 0.24 0.29 0.00 0.00 0.00 0.30 OME.sub.2 0.00 0.15 0.15 0.19 0.00 <0.02 0.00 0.19 OME.sub.3 0.00 0.00 0.09 <0.01 0.47 0.54 0.00 0.00 OME.sub.4 0.00 0.00 0.05 0.00 0.26 0.30 0.00 0.00 OME.sub.5 0.00 0.00 0.03 0.00 0.14 0.16 0.00 0.00 OME.sub.6 0.00 0.01 0.01 0.00 0.07 <0.02 0.56 0.00 OME.sub.7 0.00 0.01 0.01 0.00 0.04 0.00 0.29 0.00 OME.sub.8 0.00 0.00 0.00 0.00 0.02 0.00 0.15 0.00

(42) For the stream series of Table 1 (case 1), an experimental plant was operated on the micro scale. A fixed-bed reactor containing 200 g of the heterogeneous catalyst Amberlyst 46 which had previously been flushed with methanol was used as reactor. The reactor was operated at 60 C. The reactive distillation column K1 was equipped with the structured packing Sulzer CY having 20 theoretical plates. The feed was introduced at the level of plate 10. The pressure in the column K1 was 2 bar. The temperature at the top of the column K1 was 85 C., and the temperature at the bottom was 200 C. The column K1 was operated with a reflux ratio of 1.0 kg/kg. The temperature of the feed to the column K1 corresponded to the reactor temperature of 60 C. The column K2 was equipped with the structured laboratory packing Sulzer DX having 15 theoretical plates. The feed was introduced at plate 8 with a temperature of likewise 60 C. The pressure in the column K2 was 0.1 bar. The temperature at the top of the column K2 was 97 C., and the temperature at the bottom was 203 C. The column K2 was operated with a reflux ratio of 0.8 kg/kg. The water separation was carried out by means of pervaporation. As membrane, a membrane composed of NaA zeolite having a pore width of 4.2 was used. The effective membrane area was about 2 m.sup.2. The pervaporation was carried out at 70 C. and a feed pressure of 2 bar. A vacuum of 50 mbar was established on the permeate side. The experimental plant was operated for 8 hours in continuous operation.

(43) TABLE-US-00002 TABLE 2 Example (case 2) of a stream series of the process claimed; feed composition (stream 1):proportion of water = 0.10 g/g; mass ratio of formaldehyde to methanol = 1.65 Stream 1 2 3 4 5 6 7 9 Mass 1.2 6.5 6.5 5.4 1.1 1.0 0.1 5.1 flow/(kg/h) Mass fractions/(g/g) Formaldehyde 0.56 0.37 0.27 0.32 0.00 0.00 0.00 0.34 Methanol 0.34 0.20 0.14 0.17 0.00 0.00 0.00 0.18 Water 0.10 0.03 0.05 0.06 0.00 0.00 0.00 0.02 Methylal 0.00 0.22 0.22 0.27 0.00 0.00 0.00 0.29 OME.sub.2 0.00 0.14 0.14 0.17 0.00 <0.02 0.00 0.18 OME.sub.3 0.00 0.00 0.08 <0.01 0.47 0.54 0.00 0.00 OME.sub.4 0.00 0.00 0.05 0.00 0.26 0.30 0.00 0.00 OME.sub.5 0.00 0.00 0.02 0.00 0.14 0.16 0.00 0.00 OME.sub.6 0.00 0.01 0.01 0.00 0.07 <0.02 0.56 0.00 OME.sub.7 0.00 0.01 0.01 0.00 0.04 0.00 0.29 0.00 OME.sub.8 0.00 0.00 0.00 0.00 0.02 0.00 0.15 0.00

(44) For the stream series of Table 2 (case 2), an experimental plant was operated on the micro scale. The temperature at the top of the reactive distillation column K1 was in this case 89 C., and the temperature at the bottom was 202 C. The operating parameters of all other apparatuses correspond to those of case 1 within the limits of measurement accuracy. In particular, the temperatures at the top and bottom of the column K2 are the same because of the similar composition of the feedstream 5.

(45) TABLE-US-00003 TABLE 3 Example (case 3) of a stream series of the process claimed; Feed composition (stream 1):proportion of water = 0.20 g/g; mass ratio of formaldehyde to methanol = 1.65 Stream 1 2 3 4 5 6 7 9 Mass 1.4 7.3 7.3 6.2 1.1 1.0 0.1 5.8 flow/(kg/h) Mass fractions/(g/g) Formaldehyde 0.50 0.39 0.30 0.35 0.00 0.00 0.00 0.38 Methanol 0.30 0.21 0.15 0.18 0.00 0.00 0.00 0.19 Water 0.20 0.05 0.07 0.08 0.00 0.00 0.00 0.02 Methylal 0.00 0.20 0.20 0.24 0.00 0.00 0.00 0.25 OME.sub.2 0.00 0.13 0.13 0.15 0.00 <0.02 0.00 0.16 OME.sub.3 0.00 0.00 0.07 <0.01 0.47 0.54 0.00 0.00 OME.sub.4 0.00 0.00 0.04 0.00 0.26 0.30 0.00 0.00 OME.sub.5 0.00 0.00 0.02 0.00 0.14 0.16 0.00 0.00 OME.sub.6 0.00 0.01 0.01 0.00 0.07 <0.02 0.56 0.00 OME.sub.7 0.00 0.01 0.01 0.00 0.04 0.00 0.29 0.00 OME.sub.8 0.00 0.00 0.00 0.00 0.02 0.00 0.15 0.00

(46) For the stream series of Table 3 (case 3), an experimental plant was operated on the micro scale. The temperature at the top of the reactive distillation column K1 was in this case 91 C., the temperature at the bottom was 202 C. The operating parameters of all other apparatuses correspond to those of case 1 within the limits of measurement accuracy. In particular, the temperatures at the top and bottom of the column K2 are the same because of the similar composition of the feedstream 5.

(47) TABLE-US-00004 TABLE 4 Example (case 4) of a stream series of the process claimed; feed composition (stream 1):proportion of water = 0.1 g/g; mass ratio of formaldehyde to methanol = 1.60. Stream 1 2 3 4 5 6 7 9 Mass 1.2 8.0 8.0 6.9 1.1 1.0 0.1 6.7 flow/(kg/h) Mass fractions/(g/g) Formaldehyde 0.55 0.27 0.19 0.22 0.00 0.00 0.00 0.22 Methanol 0.35 0.18 0.13 0.15 0.00 0.00 0.00 0.16 Water 0.10 0.03 0.05 0.05 0.00 0.00 0.00 0.02 Methylal 0.00 0.33 0.33 0.39 0.00 0.00 0.00 0.40 OME.sub.2 0.00 0.17 0.17 0.19 0.00 <0.02 0.00 0.20 OME.sub.3 0.00 0.00 0.08 <0.01 0.57 0.62 0.00 0.00 OME.sub.4 0.00 0.00 0.03 0.00 0.25 0.27 0.00 0.00 OME.sub.5 0.00 0.00 0.01 0.00 0.11 0.11 0.00 0.00 OME.sub.6 0.00 0.01 0.01 0.00 0.04 <0.02 0.64 0.00 OME.sub.7 0.00 0.00 0.00 0.00 0.02 0.00 0.26 0.00 OME.sub.8 0.00 0.00 0.00 0.00 0.01 0.00 0.10 0.00

(48) For the stream series of Table 4 (case 4), an experimental plant was operated on the micro scale. The temperature at the top of the column K1 was in this case 82 C., the temperature at the bottom was 195 C. The temperature at the top of the column K2 was in this case 94 C., and the temperature at the bottom was 200 C. The operating parameters of all other apparatuses correspond to those of case 1 within the limits of measurement accuracy.

Example 2

Operation of the Special Reactive Distillation Column K1

(49) The operating parameters of an embodiment of the special reactive distillation column K1 are summarized in Table 5. Here, the mass fraction 0.00 g/g means a mass fraction of <0.005 g/g. The column was here supplied with a feed having the composition of stream 3 in Table 2 at plate 12. For this feed, the minimum number of theoretical plates required is in the range from 10 to 18, depending on the desired yield of OME.sub.n3 at the bottom of the column. The column was operated at 2 bar and a reflux ratio of 1.0 kg/kg, and the number of theoretical plates is 30 (including vaporizer and condenser). The structured packing Sulzer CY having a specific surface area of 700 m.sup.2/m.sup.3 was used as separation-active internal. The diameter of the column was 53 mm and the liquid content at each theoretical plate was in the range from 10 to 80 ml of liquid. This corresponds to from 1% to 8% of the empty tube volume. In addition, special collection trays and liquid distributors were installed between each section in order to make a long residence time of the mixture in the column possible.

(50) TABLE-US-00005 TABLE 5 Operating parameters for an embodiment of the special reactive distillation column K1 at p = 2 bar. Feed Bottom product Overhead product Temperature/( C.) 60 202 89 Mass flow/(kg/h) 1.10 0.19 0.91 Mass fractions/(g/g) Formaldehyde 0.27 <0.01 0.33 Methanol 0.14 <0.01 0.17 Water 0.05 <0.01 0.06 Methylal 0.22 <0.01 0.27 OME.sub.2 0.14 0.00 0.17 OME.sub.3 0.08 0.43 <0.01 OME.sub.4 0.05 0.28 0 OME.sub.5 0.02 0.11 0 OME.sub.6 0.01 0.06 0 OME.sub.7 0.01 0.06 0 OME.sub.8 0.01 0.06 0

(51) In the same column, an additional distillation experiment was carried out with a feed having the composition of stream 3 of Table 4 being fed in at plate 12. This represents a further embodiment of the special reactive distillation column K1. For this feed, the minimum number of theoretical plates required is in the range from 5 to 13, depending on the desired yield of OME.sub.n3 at the bottom of the column. The column was operated at 1.6 bar and a reflux ratio of 0.9 kg/kg, and the number of theoretical plates was once again 30 (including vaporizer and condenser). The liquid content at each theoretical plate was in the range from 10 to 80 ml of liquid. This corresponds to from 1% to 8% of the empty tube volume. The operating parameters of this column are summarized in Table 6.

(52) In both embodiments of the specific reactive distillation column, OME.sub.n3 can be obtained in a very high yield at the bottom of the column.

(53) TABLE-US-00006 TABLE 6 Operating parameters for an embodiment of special reactive distillation column K1 at p = 1.6 bar Feed Bottom product Overhead product Temperature/( C.) 60 185 75 Mass flow/(kg/h) 1.20 0.16 1.04 Mass fractions/(g/g) Formaldehyde 0.19 <0.01 0.22 Methanol 0.13 <0.01 0.15 Water 0.05 <0.01 0.05 Methylal 0.33 <0.01 0.38 OME.sub.2 0.17 0.00 0.19 OME.sub.3 0.08 0.56 <0.01 OME.sub.4 0.03 0.26 0 OME.sub.5 0.01 0.11 0 OME.sub.6 0.01 0.05 0 OME.sub.7 0.00 0.02 0 OME.sub.8 0.00 0.01 0

Example 3

Water Separation by Means of Adsorption

(54) In a batch shaken flask experiment, four different liquid mixtures consisting of formaldehyde, water, methanol, methylal, OME.sub.2 and OME.sub.3 were brought into contact with the adsorbent zeolite 3A at a temperature of 25 C. Here, the proportion by weight of water was systematically varied, while the ratios of all other components remained constant. The mass ratio of liquid to adsorbent is 2:1. The composition was measured before and after the adsorption procedure. The results are shown in Table 7 (before adsorption) and Table 8 (after adsorption). It can be seen here that the proportion by mass of water has been significantly reduced by addition of the adsorbent. The adsorbent was subsequently regenerated at a temperature of >50 C. and a pressure of <500 mbar and could be reused.

(55) TABLE-US-00007 TABLE 7 Compositions of the liquid mixtures before adsorption. Mixture 1 2 3 4 Mass fractions/(g/g) Formaldehyde 0.21 0.19 0.18 0.17 Methanol 0.29 0.27 0.25 0.24 Water 0.01 0.07 0.12 0.16 Methylal 0.30 0.28 0.26 0.25 OME.sub.2 0.20 0.19 0.18 0.17

(56) TABLE-US-00008 TABLE 8 Compositions of the liquid mixtures after adsorption Mixture 1 2 3 4 Mass fractions/(g/g) Formaldehyde 0.21 0.20 0.19 0.19 Methanol 0.27 0.28 0.27 0.26 Water 0.00 0.02 0.05 0.09 Methylal 0.30 0.28 0.29 0.27 OME.sub.2 0.21 0.21 0.20 0.19

Example 4

Water Separation by Means of a Membrane Process

(57) In a laboratory pervaporation plant, a membrane composed of NaA zeolite having a pore width of 4.2 was supplied with a feedstream corresponding to the composition of stream 4 of Table 4. The feed pressure was 2 bar, and a vacuum of 50 mbar was set on the permeate side. The pervaporation was carried out at 80 C. and the effective membrane area is 70 cm.sup.2. The permeate stream was cooled by means of a cold trap using liquid nitrogen and subsequently analysed. The proportion by mass of water in the permeate stream was >0.95 g/g. Methanol, formaldehyde and methylal were also detected in addition to water in the permeate stream.