GB2098611A - Treatment of mixtures of organic liquids containing methanol - Google Patents

Abstract

A method of removing methanol from a mixture of organic liquids containing up to 50% w/w methanol, wherein the mixture, for instance a mixture of 88% acetone and 12% methanol, is treated in a dynamic process with a molecular sieve capable of retaining methanol, for instance an A-type zeolite, which removes the methanol from the mixture. In particular a type 4A zeolite molecular sieve is used. Using specified superficial velocities, improvements in usable capacity of the molecular sieve are obtained compared to static batch-type absorption processes. Methanol is recovered from the sieve using temperature, pressure or vacuum swing techniques.

Description

SPECIFICATION Treatment of mixtures of organic liquids containing methanol This invention relates to the treatment of mixtures of organic liquids containing methanol, and in particular to the removal of methanol from such mixtures.

In many industrial chemical processes methanol, either alone or in combination with other organic liquids, is used as a solvent. Also some processes produce methanol which is entrained in a solvent stream. A by-product of these processes is a mixture of methanol and other organic liquids in a waste solvent stream.

The efficiency of the process may be increased if the methanol is removed from the mixture to leave relatively pure solvent and to allow recovery of the methanol.

This can be achieved to a certain extent by fractional distillation. However, fractional distillation of mixtures of organic liquids containing methanol leads to the formation of one or more azeotropes from which it is possible to recover any further pure components by further distillation.

It has been proposed to remove methanol from mixtures with some organic liquids by contacting the mixture with a molecular sieve. However, such proposals were exclusively concerned with a "static" batch-type process in which good product quality can only be achieved with a poor capacity of the sieve.

It is therefore an object of the present invention to provide a method of removing substantially all methanol from a mixture of organic liquids containing it, and which therefore avoids the problem associated with fractional distillation, using a dynamic molecular sieve separation method.

The present invention provides a process for the removal of up to 50% w/w of methanol from a mixture thereof with an organic liquid, comprising passing the mixture at a superficial velocity of less than 300 cm/min through a column containing a molecular sieve capable of retaining methanol to yield a purified organic liquid and recovering the methanol from the molecular sieve using a temperature, pressure or vacuum swing technique.

Preferably the mixture contains up to 20% w/w methanol, and is advantageously an azeotrope derived from the fractional distillation of a waste or other solvent stream.

The mixture will comprise methanol and one or more other organic liquids, for instance aliphatic or aromatic ketones, aldehydes, alcohols, esters, glycols or halogenated compounds, straight- or branched-chain or cyclo alkanes, or aromatic hydrocarbons, or ethers which are miscible with methanol. The other components must be fully miscible with the methanol in the proportions in which they are present in the mixture. The liquids may also contain dissolved solids provided that these materials do not impair the efficiency of the molecular sieve. It is envisaged that the invention will be particularly but not exclusively, useful in the removal of methanol from ketones and esters, for instance acetone or ethyl acetate.

Preferably, the molecular sieve used in the present invention is a zeolite having an A-type crystal structure, for example those marketed under the designations 4A and 5A. Most preferably the type 4A sieve is used. If the molecules of the liquids in the mixture are of only slightly larger cross-sectional diameter than the methanol molecules the type 4A should be used to ensure efficient removal of methanol.

The mixture is treated with the molecular sieve in continuous flow through a column. Preferably, the superficial velocity of the mixture through the column is less than 30 cm/min. The sieve is suitably used in the form of beads of 1 to 3 mm diameter; these are commercially available.

Normally, the absorption stage of the present invention will be carried out at elevated or reduced temperature or pressure in any combination, depending upon the quality of the product required. Although this stage would normally be carried out in the liquid phase it may also be carried out in the vapour phase.

Although the recovery of the methanol from the sieve, which is also the regeneration of the sieve for a further cycle of operation, may be done by temperature, pressure or vacuum swing techniques, it is preferably carried out by the passage of a hot gas through the column and recovering the methanol from the gas. The gas may be, for example, an inert gas such as nitrogen, or air, of flue gas, at a temperature in the range from 60 to 2000C but is preferably about 1000C.

The gas is preferably passed through the column at 30 to 90 cm/sec, although it may be more or less.

The use of a dynamic process in the present invention permits the construction of an economically sound and practical plant, in which the molecular sieve is retained in the column for absorption and recovery/regeneration. Preferably, a plant for carrying out the process of the present invention has at least two columns in parallel, to enable one column to undergo the recovery/regeneration stage while the other is treating the mixture.

The present invention also, surprisingly, offers a better compromise between product quality and capacity of the sieve for methanol than can be achieved with a static batch-type treatment, even when the overall contact time is the same. In static treatments, good product quality is only attainable with a relatively poor capacity, necessitating the use of a greater volume of molecular sieve, or good capacity can be achieved only by accepting poorer product quality, for a given amount of molecular sieve.

The present invention will now be illustrated with reference to the following Examples.

Comparative Example A batch of waste solvent (50 g) comprising acetone (44 g) and methanol (6 g) was added to a flask containing a type 4A zeolite molecular sieve (50 g). The flask was shaken gently for one hour to ensure that the mixture was fully treated by the molecular sieve. The supernatant liquid was separated from the molecular sieve by filtration and was analysed. The supernatant liquid contained 0.1% w/w methanol, and was therefore 99.9% w/w pure acetone. The capacity of the sieve for methanol was 12%.

The molecular sieve was regenerated by use of a temperature swing technique. The sieve was heated slowly under vacuum to 2000 C. In a first stage acetone adsorbed on the sieve was evaporated and collected. In a second stage the methanol absorbed by the sieve was driven off and collected. By careful control of the rate of heating of the sieve it is possible to produce two fractions containing substantially pure acetone or methanol respectively. If necessary, the methanol fraction may be further purified by fractional distillation.

EXAMPLE A mixture containing 12% methanol and 88% acetone, by wt., was passed through a column of 2.54 cm i.d. and 3.05 m length, packed with 1175 g of a type 4A molecular sieve in the form of beads of 1.4-2.0 mm diameter. The mixture was passed through the column from bottom to top at a superficial velocity of 5.1 cm/min and at ambient temperature and pressure. The product liquid was acetone containing less than 0.1% methanol by wt. 1940 ml of mixture were treated before there was breakthrough of methanol, and the usable capacity of the sieve was 16%.

The methanol was recovered from the sieve by a regeneration stage using hot nitrogen at about 1000C passed from the top to bottom through the column at a superficial velocity of 30 to 90 cm/sec. with a condensor on the end of the column. Initially, the condensate is substantially pure acetone entrained on the outside of the molecular sieve beads. Quickly, however, a methanol fraction is recovered. If the purity of the methanol is not adequate for the intended use or market, it is easily distilled using a recycle of the methanol/acetone azeotrope.

Claims (9)

1. A process for the removal of up to 50% w/w of methanol from a mixture thereof with an organic liquid, comprising passing the mixture at a superficial velocity of less than 300 cm/min through a column containing a molecular sieve capable of retaining methanol to yield a purified organic liquid and recovering the methanol from the molecular sieve using a temperature, pressure or vacuum swing technique.

2. A process according to claim 1, wherein the superficial velocity of the mixture through the column is less than 30 cm/min.

3. A process according to claim 1 or 2, wherein the molecular sieve is an A-type zeolite.

4. A process according to claim 3, wherein the zeolite is a type 4A zeolite.

5. A process according to any one of claims 1 to 4, wherein the mixture contains up to 20% w/w of methanol.

6. A process according to any one of the preceding claims, wherein the mixture includes an aromatic or aliphatic ketone or ester.

7. A process according to claim 6, wherein the mixture is a mixture of methanol and acetone.

8. A process according to any one of the preceding claims, wherein methanol is recovered from the molecular sieve by passing a hot gas through the column and recovering methanol from the gas.

9. A process according to claim 1, substantially as hereinbefore described.