MX2008010536A - Process for the preparation of an alkanediol and a dialkyl carbonate - Google Patents

Abstract

A process for the preparation of an alkanediol and a dialkyl carbonate is provided, which process comprises:(a) contacting an alkylene carbonate with an alkanol feedstock under transesterification conditions in a reactive distillation column to obtain an upwardly moving stream comprising dialkyl carbonate and the alkanol and a downwardly moving stream comprising the alkanediol;(b) recovering the alkanediol at the bottom of the column;(c) withdrawing a dialkyl carbonate- and alkanol-containing product stream at the upper part of the column, which upper part is below the top of the column;and (d) removing lower-boiling compounds at the top of the column.

Description

PROCESS OF PREPARATION OF AN ALCANODIOL AND A CARBONATE DIALQUILO DESCRIPTION OF THE INVENTION The present invention describes a process for preparing an alkanediol and a dialkyl carbonate. More particularly, the invention describes a process for preparing said compounds from alkylene carbonate and alkanol. These processes are known from US-A 5,231,212. This document describes a process for the continuous preparation of dialkyl carbonates by transesterification of an alkylene carbonate with alcohols in the presence of a catalyst in the reaction column. The countercurrent reagents are passed in such a way that the alkylene carbonate in the upper part of the column and the alcohol in the lower part of the column are graduated. The catalyst is arranged as a fixed bed or is graduated in the upper part of the column in solution or suspension. The dialkyl carbonate formed, if it is suitable to mix it with alcohol, is removed at the top of the column and the alkane diol removed at the foot of the column. If the catalyst is provided as a solution or suspension, the catalyst is also removed at the bottom of the column. In the known process it is indicated that the columns REF. : 194658 inferiors that are removed at the foot of the column may contain some contaminants, for example, alcohol and alkylene carbonate. The known process does not address the problem of forming low-boiling byproducts. These by-products may be, for example, carbon dioxide which may be formed due to the hydrolysis of alkylene carbonate by small amounts of water which may be present in the alkanol or any other starting material. Other secondary products that can be formed include acetaldehyde, propionaldehyde and acetone. While the alkylene carbonate is generally produced from alkylene oxide and carbon dioxide, the alkylene carbonate source may contain alkylene oxide. Reagents can suspend other gases such as nitrogen. At an industrial level, it is very important that the content of the secondary products in the reaction products be as low as possible. Although known processes provide appreciably pure products, it has also been found that the purest product can be obtained if the dialkyl carbonate product is not removed at the upper end of the column, but at the top of the column. The present invention describes a process for preparing an alkanediol and a dialkyl carbonate which includes: (a) Contacting the alkylene carbonate with an alkanol source under transesterification conditions in a reagent distillation column to obtain an upstream stream which includes dialkyl carbonate and an alkanol and a downstream stream including alkanediol; (b) Recover alkanediol at the bottom of the column; (c) Remove the product stream containing dialkyl carbonate and the alkanol at the top of the column, which is below the top of the column; and (d) Remove the low-boiling compounds at the top of the column. The process of the present invention includes the transesterification of an alkylene carbonate with alkanol. This transesterification reaction is known, as is apparent from US-A-5,231,212 and US-A-5,359,118. The starting materials of the transesterification are preferably selected from C2-C6 alkylene carbonate and C1-C4 alkanols. More preferably, the starting materials are ethylene carbonate of propylene carbonate and methanol, ethanol or isopropanol. The most preferred alkanols are methanol and ethanol. Suitably, the transesterification step is carried out in a column in which alkylene carbonate is poured in the upper part, such that the alkylene carbonate flows downstream against the current., with respect to the alkanol of ascending direction. The product of the reaction is a dialkyl carbonate and an alkanediol. The dialkyl carbonate is recovered in the upper part of the column. The alkanediol is recovered in the final stream. It is suitable to perform the transesterification in the presence of a catalyst. Suitable catalysts have been described in US-A 5 patent. 359 118 and include hydrides, oxides, hydroxides, alcoholates, amides, or alkali metal salts, namely, lithium, sodium, potassium, rubidium and cesium. Preferred catalysts are potassium or sodium hydroxides or alcoholates. It is advantageous to use the alkanol alcoholate which is used as a source. Said alcoholate may be added as such, or may be formed in situ. Other suitable catalysts are alkali metal salts, such as acetates, propionates, butyrates, or carbonates. Other catalysts are described in US-A 5, 359, 118 and the references mentioned therein, for example, EP-A 274 953, US-A 3, 803, 201, EP-A 1082, and EP-A 180 387. The transesterification conditions are known in the field and suitably include temperatures from 4 to 2002C, and pressures from 50 to 400 kPa. Preferably, the pressure is close to atmospheric pressure. The temperature depends on the source of alkanol and the pressure used. The temperature is maintained so that it is close to the boiling point of the alkanol, for example, up to 52C above the boiling point. In the case of methanol and atmospheric pressure, the temperature is close to 65 eC or above that value, for example, between 65 and 702C. The transesterification reaction is advantageously carried out in a column with internal sections, such as a distillation column. Therefore, it can contain trays with bubble caps, sieve trays or Raschig rings. The skilled person will see that various arrangements and tray configurations can be used. The expert person should know, according to their knowledge, the theoretical trays for the columns. The alkylene carbonate is poured into the top of said column and flows downwards. Unexpectedly, it has been found that a purer dialkyl carbonate product stream can be obtained when the alkylene carbonate is poured into the column at positions above the position from which the dialkyl carbonate product stream is removed. The distance between the position at which the alkylene carbonate is poured into the column and the position at which the product stream is suitably removed is comprised between 1 and 10 theoretical trays.

Generally, alguylene carbonate has a higher boiling point than alkanol. In the case of ethylene carbonate and propylene, the atmospheric boiling points are above 2402C. The alkylene carbonate flows downwards on the trays or rings and is brought into contact with the alkanol which flows upwards. When the transesterification catalyst is homogeneous, such as for example the alkali metal alcoholate, it is also introduced into the upper part of the column. The alkanol source is introduced at a lower point. The source can be completely vaporous. However, it is possible to enter the source in the column, partially in the liquid phase. It is believed that the liquid phase guarantees a higher concentration of alkanol in the lower part of the column with an advantageous effect on the overall transesterification. It is distributed over the width of the column by the internal sections of the column and the entrance. The ratio between the vapor phase and the liquid phase of the alkanol source can be varied between wide ranges. The vapor / liquid weight ratio is suitably 1: 1 to 10: 1 p / P- The person skilled in the art will know that transesterification is an equilibrium reaction. Therefore, in these circumstances you should use excess alkanol. The molar ratio of the alkanol and the alkylene carbonate suitably is from 5: 1 to 25: 1, preferably from 6: 1 to 15: 1, more preferably from 7: 1 to 9: 1. The catalyst concentration can obviously be much higher. Suitable amounts include from 1 to 5.0% p, based on alkylene carbonate, preferably 0.2 to 2% p. The results of reactive distillation result in an updraft containing the dialkyl carbonate and any excess of unreacted alkanol, and a downflow containing the alkane diol and the catalyst recovered to the bottom of the column. Due to the fact that there may be water in the alkanol, there may be hydrolysis of alkylene carbonate, forming alkanediol and carbon dioxide. There may be other by-products or low-boiling point contaminants suspended with reagents, such as nitrogen, such as aldehydes, ketones, and alkylene oxides, and gases. In this description, low-boiling compounds are understood to be those with a lower boiling point than alkanol. The alkanediol stream recovered to the bottom is suitably subjected to the separation of the alkanediol. From there, the bottom stream is suitably separated, in a fractionating column towards the catalyst-rich stream and the stream that includes the alkanediol, and, optionally, part of the alkanol. After the optional purification, for example, by subsequent distillation, the alkanediol is recovered as an eventual product. The catalyst rich stream is recycled to the reactive distillation zone. The alkanol that separates from the bottom stream can also be recycled. The upstream current is removed at the position below the top of the column. Due to the distillation action that takes place in the column, a significant portion of the low-boiling byproducts is separated between the top of the column and the position below which the dialkyl carbonate containing stream is removed. and alkanol. The low-boiling byproducts are removed at the top of the column. The distance between the upper part and the position at which the product is removed is in the range of 1 to 10 theoretical trays. The product stream with dialkyl carbonate and alkanol is subsequently removed in a stream rich in alkanol and a dialkyl carbonate rich stream. Suitably, this is done by distillation. However, as indicated in US-A-5,359,118, many alkanols and their corresponding dialkyl carbonates form azeotropes. Therefore, simple distillation may not be sufficient to achieve adequate separation. Therefore, it is preferable to use an extractor to simplify the separation between dialkyl carbonate and alkanol. The extractor can be selected from various compounds, in particular alcohols such as phenol, or anisole. However, it is preferred to employ an alkylene carbonate as an extractant. It is even more advantageous to obtain the separation with the alkylene carbonate which is being used as starting material for the eventual alkanediol. The distillation by extraction is preferably carried out in two columns. In the first column, the separation between the alkanol and the dialkyl carbonate / alkylene mixture is carried out. In the second column a separation is made between the dialkyl carbonate and the alkylene carbonate. Suitably, the alkylene carbonate is recycled to the first column for reuse as an extractor. The ratio between the alkylene carbonate and the alkanol and the alkylene carbonate and the dialkyl carbonate can vary between wide ranges. Suitable ranges include 0.2 to 2 moles of alkylene carbonate per mole of the sum of alkanol and dialkyl carbonate, preferably 0.4 to 1.0 mole per mole. The distillation conditions for this separation can be selected within wide ranges, as the person skilled in the art knows. The pressures are suitably in the range of 5 to 400 kPa, and temperatures of 40 to 2002C. In view of the stability of the alkylene carbonate, the temperature is below 180 eC, while the lower temperatures are determined with the boiling point of the alkanol. When the two distillation columns are used, it is preferable to carry out the separation between alkanol and carbonate / alkylene dialkyl at higher pressures, such as 60 to 120 KPa, and the second separation between dialkyl carbonate and alkylene carbonate at lower pressures, such as 5 to 50 kPa. This allows operating at temperatures low enough to maintain the necessary stability for the alkylene carbonate and efficiently separate the carbonate compounds from each other. The obtained dialkyl carbonate is recovered as a product, optionally after a subsequent purification. This subsequent purification may include a subsequent distillation step or an ion exchange step, as described in US-A 5,455,368. The alkanol-rich stream obtained from the distillation of the product from the reactive distillation column is suitably recycled to the reactive distillation zone. Therefore, the alkanol source includes purely pure alkanol produced and at least part of this alkanol-rich stream. This current can be liquid and / or gaseous. The recycle stream can be mixed with the pure alkanol produced and then introduced into the reactive distillation zone as a source of alkanol. However, it is preferable to introduce the alkanol produced in the reactive distillation zone below the level at which the recycle stream is introduced. This allows to enjoy the advantages described in US-A 5,359,118. The process of the present invention can be employed for a variety of sources. The process is suitably used to prepare ethylene glycol, propylene glycol, dimethyl carbonate and / or diethyl carbonate. The process is advantageously used to produce propylene glycol (1,2-propane diol) and dimethyl carbonate from propylene carbonate and methanol. Next, examples are presented to describe the invention. EXAMPLES Comparative example A The reactive distillation column has 40 theoretical trays. Propylene carbonate (5897 kg / h) is poured into tray 2. A homogeneous catalyst solution is poured into tray 2. The reaction occurs in all trays below the pouring level of the catalyst. Steam methanol (16834 kg / h) is poured into tray 35. Monopropylene glycol is removed from the bottom of the column. The mixture of dimethyl carbonate and methanol is removed from the top of the column. A reflux of 0.75 mol / mol is applied. The low-boiling compounds present in the sources or formed in the column are the following: Nitrogen 5 kg / h, C02 50 kg / h, propylene oxide 22 kg / h.

- The following table shows the composition of the product stream. Example 1 The reactive distillation column has 45 theoretical trays. Propylene carbonate (5897 kg / h) is poured into tray 7. A homogeneous catalyst solution is poured into tray 7. The reaction occurs in all trays below the pouring level of the catalyst. Steam methanol (16834 kg / h) is poured into tray 40. Monopropylene glycol is removed from the bottom of the column. The mixture of dimethyl carbonate and methanol is removed as side steam extraction in tray 6. The capacitor cycle is that described in comparative example 1. A small stream of steam is removed at the top of the column by the which remove compounds of low boiling point of 32 kg / h. The steam stream contains approximately 5 kg / h of dimethyl carbonate and 10 kg / h of methanol. The low-boiling compounds present in the sources or formed in the column are the following: Nitrogen 5 kg / h, C (¾ 50 kg / h, propylene oxide 22 kg / h The table shows that the product stream contains substantially lower concentration of low boiling compounds in this example if compared to example A. Example 2 The same column as used in Example 1 is used. The process of Example 1 is repeated, except that Propylene carbonate (5897 kg / h) is poured into tray 2 and not into tray 7.

- - The low-boiling compounds present in the sources or formed in the column are the following: Nitrogen 5 kg / h, CO2 50 kg / h, propylene oxide 22 kg / h. The small vapor stream that is removed at the top of the column contains about 31 kg / h of low-boiling compounds, about 2 kg / h of dimethyl carbonate and about 12 kg / h of methanol. The table below shows that the product stream contains substantially less light ends in Comparative Example A. Compared to Example 1, less desired dimethyl carbonate product is lost in the upper vapor stream containing the majority of the compounds of low boiling point.

Table Composition of the product stream It is noted that in relation to this date, the best method known to the applicant to carry out the said invention, is that which is clear from the present description of the invention.

Claims (6)

1. CLAIMS Having described the invention as above, the content of the following claims is claimed as property: 1. Process for preparing an alkanediol and a dialkyl carbonate, characterized in that it includes: (a) Contacting the alkylene carbonate with a source of alkanol under conditions of transesterification in a reagent distillation column to obtain an upstream stream including dialkyl carbonate and an alkanol and a downstream stream including alkanediol; (b) Recover alkanediol at the bottom of the column; (c) Remove the product stream containing dialkyl carbonate and the alkanol at the top of the column, which is below the top of the column; and (d) Remove the low-boiling compounds at the top of the column.
2. 2. The process according to claim 1, characterized in that the distance between the upper part and the position to which the product is withdrawn is in the range of 1 to 10 theoretical trays.
3. 3. The process according to claim 1 or 2, characterized in that the stream containing dialkyl carbonate and alkanol are separated in a stream rich in alkanol and a carbonate-rich stream.
4. The process according to any of claims 1 to 3, characterized in that the alkylene carbonate is poured into the column in the position above the position at which the product stream containing dialkyl carbonate and alkanol is removed.
5. 5. The process according to claim 4, characterized in that the distance between the position at which the alkylene carbonate is poured into the column and the position at which the product stream is suitably removed is comprised between 1 and 10 trays theoretical
6. 6. The process according to any of claims 1 to 5, characterized in that the alkylene carbonate is propylene carbonate and the alkanol is methanol.