EP2560943A2

EP2560943A2 - Process for preparing carboxylic esters by reactive distillation

Info

Publication number: EP2560943A2
Application number: EP11714782A
Authority: EP
Inventors: Helmut Kronemayer; Ellen Dahlhoff; Andreas Lanver
Original assignee: BASF SE
Current assignee: BASF SE
Priority date: 2010-04-19
Filing date: 2011-04-19
Publication date: 2013-02-27
Also published as: JP2013525325A; WO2011131643A3; CN102844292A; KR20130051452A; WO2011131643A2; CN102844292B

Abstract

In a process for preparing carboxylic esters by transesterification, a first feed stream comprising a first carboxylic ester, e.g. methyl formate, is introduced at the side into a reaction column at a first inlet located between the top and the bottom of the reaction column and a second feed stream comprising a first alcohol, e.g. ethanol, is introduced at the side into the reaction column at a second inlet located above the first inlet and the feed streams are reacted in a reaction zone of the reaction column to form a second carboxylic ester and a second alcohol. The first alcohol has a higher molecular weight than the second alcohol. A product fraction comprising the second carboxylic ester and unreacted first carboxylic ester is taken off at an offtake located above the second inlet. A bottom fraction comprising the second alcohol and unreacted first alcohol is taken off at the bottom of the reaction column. The product fraction is separated by distillation at a pressure which is different from the pressure in the reaction column into second carboxylic ester and a fraction containing unreacted first carboxylic ester and the fraction containing unreacted first carboxylic ester is at least partly recirculated to the reaction zone.

Description

The invention relates to a process for the preparation of carboxylic acid esters, in particular of ethyl formate, by transesterification.

Low molecular weight esters such as formic acid esters find z. B. as fragrances, insecticides, fungicides or in organic synthesis application. Methods for the preparation of low molecular weight esters are described in various ways in the literature. An inexpensive option is the esterification of carboxylic acid and alcohol followed by distillation of the ester. This process is in many cases technically very easy to carry out because the product in the form of the ester is the lowest-boiling compound.

In US-A 5,302,747 a process is described in which an inert gas is passed through an esterification mixture containing an alcohol and a carboxylic acid and maintained at least at the boiling temperature of the alcohol to drive off the ester.

The production of high-purity esters, in particular formic acid esters, with a purity of more than 99.5% by weight, in particular more than 99.8% by weight, is difficult, as will be explained below with reference to the esterification of formic acid with ethanol. In the esterification of formic acid with ethanol, water and ethyl form form. In the distillation of the reaction product, neither ethanol nor water can be completely separated from the ester, since both substances form azeotropes with the ester over wide pressure ranges. As a result, highly pure ethyl formate can not be obtained in this way. JP 10175916 describes the preparation of highly pure formic acid esters. The esterification of formic acid and alcohol is carried out by reactive distillation, wherein the resulting distillate is dehydrated by means of acetic anhydride. In this method, although water can be removed by the use of drying agents. Unreacted alcohol can not be removed in a comparable manner.

WO 2007/099071 describes the preparation of esters by a reactive distillation. In a reaction column, a carboxylic acid, an alcohol and an entraining agent are introduced. The bottom stream comprises the ester formed and not um- put carboxylic acid. The top stream comprises unreacted alcohol, water and entrainers.

The production of an ester from the carboxylic acid and alcohol has the disadvantage that the acid is generally corrosive and acid-resistant materials must be used to handle them.

The invention has for its object to provide a process for the production of high purity ester, which is economical to carry out and provides a lower equipment investment requirements and in particular bypasses the requirement of acid-resistant materials.

According to the invention the object is achieved by a process for the preparation of carboxylic acid esters by transesterification, in which one introduces a first feed comprising a first carboxylic acid ester, and a second feed comprising a first alcohol in a reaction column and in a reaction zone of the reaction column below Forming a second carboxylic acid ester and a second alcohol, wherein the first alcohol has a higher molecular weight than the second alcohol and wherein the second carboxylic acid ester and the second alcohol is continuously removed from the reaction zone.

The process is suitable for the preparation of low molecular weight carboxylic acid esters which can be evaporated without decomposition. The starting material is a first carboxylic acid ester which is the ester of a carboxylic acid with the second alcohol. The first carboxylic acid ester is preferably an ester of a C 1 -C 8 -carboxylic acid, eg. As an ester of formic acid, acetic acid, propionic acid, chloroacetic acid, bromoacetic acid, lactic acid, glycolic acid. In particular, the first carboxylic acid ester is a formic acid ester. The first alcohol has a higher molecular weight than the second alcohol. In suitable embodiments, the first alcohol is a C 2 -C 8 alcohol, preferably ethanol, and the second alcohol is methanol.

A particularly preferred embodiment relates to a process for the preparation of ethyl formate in which the first carboxylic acid ester is methyl formate and the first alcohol is ethanol.

By "reaction zone" is meant a region of the reaction column in which suitable conditions, in particular with respect to temperature, pressure and presence a catalyst, so that the reaction between the first carboxylic ester and the first alcohol proceeds at an appropriate rate. Parallel to the chemical reaction in the reaction zone, a mass transfer takes place. The removal of the second carboxylic acid ester and the second alcohol from the reaction zone on the one hand causes a shift in the reaction equilibrium and on the other hand prevents subsequent reactions, whereby the selectivity of the reaction is greatly increased.

The reaction column contains separating internals, such as separating trays, z. As perforated floors, valve trays or trays which are characterized by a long residence time, ordered packings, eg. B. sheet or tissue packings such as Sulzer Mellapak 250 Y, Sulzer BX, Montz B1 or Montz A3 or Kühni Rhombopak, or random beds of packing, such. Dixon rings, Raschig rings, high flow rings or Raschig super rings. Orderly packages, preferably sheet metal or fabric packages, have a specific surface area of 100 to 750 m ² / m ³ , in particular 250 to 500 m ² / m ³ . They allow high separation performance at low pressure drops. The reaction column used is advantageously a rectification column which has 5 to 100, preferably 20 to 50, actual or theoretical plates.

The bottom of the reaction column is heated with at least one built-in and / or external heater. The external heater can work with forced or natural circulation. Suitably, the operating pressure of the reaction column is 0.5 to 7 bar, preferably 1 to 5 bar and more preferably 1 to 3 bar (absolute). The temperature in the sump depends on the nature of the first carboxylic ester and / or first alcohol and in the reaction of methyl formate with ethanol is usually 50 to 150 ° C, preferably 60 to 100 ° C.

The reaction can be carried out in the presence of a suitable catalyst, for. As an acidic or basic catalyst, preferably a basic catalyst. The catalyst can be both heterogeneous catalysts and homogeneously soluble catalysts. For the purposes of this application, "homogeneously soluble" is intended to mean that the catalyst used is soluble to more than 1 g / 100 ml at 22 ° C., at least in the first alcohol used.

Expediently, heterogeneous basic catalysts are arranged in a stationary manner in the reaction zone. Heterogeneous catalysts are selected, for example, below basic oxides, mixed oxides or hydroxides and ion exchangers in amine or hydroxyl form.

The materials may be used as such or embedded in an oxidic binder matrix, e.g. For example, from alumina, silica, mixtures of fumed silica and alumina, titania, zirconia or clay into moldings such as strands or tablets are formed. Preferably, the heterogeneous basic catalyst is in particulate form having a particle size (maximum elongation) of 1 to 10 mm, preferably 1 to 4 mm.

Likewise suitable are anion exchangers in hydroxyl form, such as styrene and acrylic resins with quaternary ammonium groups, which are bound to an insoluble styrene or acrylic polymer matrix. The heterogeneous catalyst is introduced into the reaction zone in such a way that sufficient gaps remain so that a rectificative mass transfer can take place. The catalyst is preferably used in a concentration of 10 to 60% by volume, based on the void volume of the column. The heterogeneous catalyst can be placed on trays or installed as a catalyst bed in the reaction zone. However, it is also possible to use catalyst-containing packages, e.g. Montz MULTIPAK or Sulzer KATAPAK, or to introduce the catalyst in the form of packing in the column. Furthermore, it is possible, the heterogeneous catalysts between an inert fabric or knitted fabric, for. B. fiberglass, bring in and roll up into bales (so-called Bales). The bales can be arranged next to and above each other so that the bales of one layer cover the interstices of the underlying layer. Furthermore, can be used with catalyst filled tissue bags (so-called Texas Tea Bags).

Alternatively, the heterogeneous catalyst has a suitable particle size and shape such that it can be introduced into the reaction zone as a packing, optionally mixed with inert fillers.

Homogeneously soluble basic catalysts, if used, are metered into the reaction column at any suitable point in the lower to middle column range, expediently together with the first alcohol. The homogeneously soluble catalysts used are, for. B. selected from alkali metal hydroxides and / or alcoholates, such as potassium, sodium methylate. The catalysts are expediently metered in the form of a solution in a suitable solvent. A preferred solvent is the first or second alcohol used in the process of the invention.

The homogeneously soluble catalyst, if used, is usually used in an amount of 0.00001 to 0.2, preferably 0.0001 to 0.1, and more preferably 0.0005 to 0.05, equivalents based on the first carboxylic acid ester.

The first feed is preferably introduced laterally into the reaction column at at least one first feed point located between the top and bottom of the reaction column, and introduces the second feed laterally into the reaction column at a second feed point located above the first feed point. The reaction zone extends at least partially to the column section between the first and second feed points.

Preference is given to introducing the second feed in the central column area in the reaction column, d. H. preferably at the level of a bottom which divides the number of the trays above the column to the underlying soils in the ratio of 3: 1 to 1: 3, preferably 2: 1 to 1: 2.

In general, one introduces 0.5 to 2 equivalents, preferably 0.7 to 1, 2 equivalents and in particular 0.9 to 1, 1 equivalents of the first alcohol, based on the first carboxylic ester in the system.

The ongoing removal of the reaction products from the reaction zone is achieved by the mass transfer processes taking place in the reaction column. From the reaction zone, vapors of a low-boiling fraction which comprises the second carboxylic acid ester formed and the unconverted first carboxylic acid ester and the unreacted first alcohol and the second alcohol are removed. The low-boiling fraction passes into a rectifying section of the reaction column, in which the entrained unreacted first alcohol and the second alcohol are separated and run back into the reaction zone.

At a withdrawal point located above the second feed point, it is possible to remove a product fraction comprising the second carboxylic acid ester and unconverted first carboxylic acid ester, preferably as a side draw. In many cases, the second carboxylic acid ester and the first carboxylic acid ester form azeotropes with the alcohols, so that the composition of the product fraction substantially corresponds to the azeotrope composition. The product fraction may contain minor amounts of first alcohol and second alcohol.

At the top of the reaction column mainly unreacted first carboxylic acid ester is condensed and partly recycled as a head reflux in the reaction column and partially withdrawn as a top fraction. In a preferred embodiment, the top fraction is at least partially returned to the reaction zone, for example by admixing with the first feed. Another part of the top fraction can be removed to prevent accumulation of low boilers.

In addition, runs from the reaction zone, a condensate of a high-boiling fraction comprising in addition to unreacted first alcohol and second alcohol entrained second carboxylic acid ester and unreacted first carboxylic acid ester. The high-boiling fraction passes into a stripping section of the column in which the entrained second carboxylic acid ester and unreacted first carboxylic acid ester are stripped off and returned to the reaction zone. At the bottom of the reaction column, a bottoms fraction comprising the second alcohol and unreacted first alcohol can be withdrawn. In order to remove forming high-boiling components, part of the bottoms fraction can be discarded.

Preferably, the bottom fraction is separated by distillation into the second alcohol and unconverted first alcohol, preferably in a further distillation column. The unreacted first alcohol is advantageously at least partially recycled to the reaction zone by admixing it, for example, the second feed.

The product fraction contains in addition to the second carboxylic acid ester usually unreacted first carboxylic acid ester and small amounts of second alcohol and unreacted first alcohol. The product fraction is therefore preferably separated by distillation into pure second carboxylic acid ester and a fraction containing unreacted first carboxylic acid ester. The distillative separation is preferably carried out continuously in a second column. The fraction containing the unreacted first carboxylic acid ester is preferably at least partly returned to the reaction zone, for example by admixing with the first feed.

Most often, the product fraction comprises azeotropes of the second carboxylic acid ester with second alcohol and unreacted first alcohol and unreacted first Carbonsäureester. Since the azeotrope composition is generally pressure-dependent, the separation of the azeotrope succeeds by distillation at a pressure which is different from the pressure in the reaction column. The person skilled in this phenomenon is known as a two-pressure process or pressure swing rectification or pressure swing distillation. At a different pressure from the pressure in the reaction column, the composition of the product fraction corresponds to another azeotrope composition. In the second column, the second carboxylic acid ester can be withdrawn purely at the bottom or in the lower part of the column, for example near the bottom region of the column, while an azeotropic mixture is formed in the top, albeit in a significantly different composition than in the product fraction. This azeotropic mixture can be fed back into the reaction zone.

The different pressure from the pressure in the reaction column pressure z. B. 1 to 40 bar, preferably 5 to 15 bar (absolute).

In a preferred embodiment of the process in which the first carboxylic acid ester is methyl formate and the first alcohol is ethanol, the stream obtained at the bottom of the second column contains 99.0 to 100% by weight (in particular 99.8 to 100% by weight) Ethyl formate, 0 to 1 wt .-% (in particular 0 to 0.2 wt .-%) of ethanol and 0 to 1 wt .-% (in particular 0 to 0.2 wt .-%) of other compounds.

Alternatively, it is possible to carry out the distillative separation of the product fraction as extractive distillation. The invention is illustrated by the accompanying drawings and the following examples.

Fig. 1 shows schematically a suitable for carrying out the method according to the invention plant.

In the reaction column T1 is introduced via a lateral inlet 2, which is located at the upper end of the reaction zone 1, a first alcohol. In the reaction zone, a heterogeneous catalyst (not shown) is introduced in a stationary manner. Via a lateral inlet 3, which is located at the lower end of the reaction zone 1, introduces a first carboxylic acid ester in the reaction column T1. In the reaction zone 1, the reaction takes place to the second carboxylic acid ester and second alcohol. The second carboxylic acid ester and unconverted first carboxylic acid ester enter the enrichment section 4 of the reaction column T1, where they are largely freed from entrained second alcohol and unreacted first alcohol. About the Ten deduction 6 is subtracted from a second carboxylic acid ester and unreacted first carboxylic acid ester product fraction. Second alcohol and unreacted first alcohol from the reaction zone 1 enter the stripping section 5 of the Reaktionshkolonne T1, where they are stripped off entrained second carboxylic acid ester and unreacted first carboxylic acid ester. The bottoms fraction withdrawn via line 7 consists mainly of second alcohol and unreacted first alcohol.

The vapor 8 obtained at the top of the reaction column is condensed and partly returned to the reaction column via line 9 as a head reflux and partially fed via line 10 as feed into the reaction zone.

The withdrawn at the side offtake 6 product fraction from the reaction column T1 is fed to a distillation column T2 at its upper portion. The column T2 is operated at a different pressure, usually a higher pressure, than the reaction column T1. At the bottom of the distillation column T2 falls to pure second carboxylic acid ester, which is removed via line 1 1. The top hatch of the distillation column T2 is returned via the line 12 in the reaction column T1. The bottoms fraction from the reaction column T1 is withdrawn via line 7 and fed to a distillation column T3. There is a separation into second alcohol, which is discharged at the top of the column T3 via line 13, and first alcohol, which is obtained at the bottom of the column T3 and is returned via line 14 into the reaction column T1.

Example (simulation)

A reaction column operated at 1 bar with 30 theoretical stages were fed at stage 10 about 60 g / h of ethanol. At level 5, 81 g / h of methyl formate was added. It is assumed that on the intervening soils the reaction to ethyl formate and methanol takes place up to chemical equilibrium.

The reflux ratio of the reaction column was about 11. The overhead condensate consisting essentially of methyl formate was partially returned as reflux and partially recycled to the lower part of the reaction column.

At a side draw at stage 25, 305 g / h of a mixture containing about 60.0% by weight of ethyl formate, 27% by weight of methyl formate, 6% by weight of methanol and 2% by weight of ethanol were withdrawn. This mixture was fed to a distillation column with 30 theoretical stages at a pressure of 7 bar at the top. 100 g / h of a mixture containing about 99.9% by weight of ethyl formate and 0.1% by weight of ethanol were obtained at the bottom of this distillation column. The overhead distillate was partially returned as reflux (reflux ratio about 3) and partially recycled to the reaction column.

About 242 g / h of a mixture containing about 19% by weight of methanol and 81% by weight of ethanol were withdrawn from the bottom of the reaction column. This mixture was separated in a further distillation column with 30 theoretical stages at a pressure of 1 bar. At the top of methanol is withdrawn; About 99% by weight of ethanol obtained at the bottoms was recycled to the reaction column.

Claims

claims

Process for the preparation of carboxylic acid esters by transesterification, in which a first feed comprising a first carboxylic acid ester and a second feed comprising a first alcohol is introduced into a reaction column and in a reaction zone of the reaction column to form a second carboxylic acid ester and a second Alcohol is reacted, wherein the first alcohol has a higher molecular weight than the second alcohol and wherein the second carboxylic acid ester and the second alcohol is continuously removed from the reaction zone.

Process according to Claim 1, in which the first feed is introduced laterally into the reaction column at at least one first feed point located between the top and bottom of the reaction column, and the second feed is laterally introduced into the reaction column at a second feed point located above the first feed point.

Process according to Claim 1 or 2, in which a product fraction comprising the second carboxylic acid ester and unconverted first carboxylic acid ester is withdrawn at a point of withdrawal located above the second feed point, and a sump fraction comprising the second alcohol and unreacted first alcohol is withdrawn from the bottom of the reaction column.

A process according to claim 3, further comprising the step of withdrawing at the top of the reaction column a top fraction consisting essentially of unreacted first carboxylic acid ester.

A process according to claim 4, wherein the overhead fraction is recycled at least partially to the reaction zone.

A process as claimed in any of claims 3 to 5 wherein the product fraction is separated by distillation into second carboxylic acid ester and fraction containing unreacted first carboxylic acid ester and the fraction containing unreacted first carboxylic acid ester is at least partially recycled to the reaction zone.

A process according to claim 6 wherein the distillative separation of the product fraction is carried out at a pressure other than the pressure in the reaction column.

8. The method of claim 6, wherein carrying out the distillative separation of the product fraction as extractive distillation.

9. The method according to any one of claims 3 to 8, wherein separating the bottoms fraction by distillation in the second alcohol and unreacted first alcohol and the unreacted first alcohol at least partially recycles into the reaction zone.

10. The method according to any one of the preceding claims, wherein metered into the reaction column, a soluble basic catalyst.

1 1. Process according to any one of claims 1 to 9, wherein a heterogeneous basic catalyst is arranged in the reaction zone.

12. The method according to any one of the preceding claims, wherein the first carboxylic acid ester is a formic acid ester.

13. The method according to any one of the preceding claims, wherein the first alcohol is ethanol and the second alcohol is methanol.

14. The method according to any one of the preceding claims for the preparation of ethyl formate, in which the first carboxylic acid ester is methyl formate and the first alcohol is ethanol.