GB2127011A - Treatment of mixtures of inorganic liquids containing ethanol - Google Patents

Abstract

Ethanol can be recovered from mixtures, e.g. azeotropes, containing ethanol with organic compounds having a cross-sectional diameter larger than that of ethanol, by passing the mixture through a column containing a suitable molecular sieve. Using specified superficial velocities, improved product quality and/or sieve capacity can be obtained in comparison with static batch-type treatments.

Description

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

In many industrial chemical processes ethanol, ether alone or in combination with other organic liquids, is used or is produced. A by-product of these processes is a mixture of ethanol and other organic liquids in a waste solvent stream. The efficiency of the process may be increased if the ethanol is removed from the mixture to leave relatively pure solvent and to allow recovery of the ethanol.

This can be achieved to a certain extent by fractional distillation where there is a sufficient difference between the boiling points of the organic liquids and ethanol. However, fractional distillation of many mixtures of organic liquids containing ethanol leads to the formation of one or more azeotropes from which it is not possible to recover any further pure components by further distillation.

It has been proposed to remove ethanol 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 ethanol from a mixture of organic liquids containing it which mixtures does not contain water or methanol, and which therefore avoids the problem associated with fraction distillation, using a dynamic molecular sieve separation method.

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

Preferably the mixture contains up to 20% w/w ethanol, and the present invention is of particular use when it is applied to an azeotrope derived from the fractional distillation of a waste or other solvent stream.

The mixture will comprise ethanol and one or more other organic liquids, for instance aliphatic or aromatic ketones, aldehydes, higher branched alcohols, esters, glycols or halogenated compounds, branched-chain or cyclo alkanes, or aromatic hydrocarbons, or ethers which are miscible with the ethanol in the proportions in which they are present in the mixture, and must have a molecular cross-sectional diameter greater than 5A, and in particular should not include water or methanol. 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 exclusiveiy, useful in the removal of ethanol 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 5A. In the case of the separation of ethanol from a higher linear alcohol or n-alkane, it would be possible to use a 4A type sieve but both the efficiency of separation and the sieve capacity would be very much lower than when using a type 5A sieve for other separations.

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 ambient temperature but may be carried out at 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 ethanol 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 ethanol from the gas. The gas may be, for example, an inert gas such as nitrogen, or air, or flue gase, at a temperature in the range from 60 to 2000C but is preferably about 1 000C.

The gas is preferably passed through the column at a superficial velocity of 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. Handling charges for batch-type operations make these currently uneconomic. Preferably, a plant for carrying out the process of the present invention has at least two coiumns 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 ethanol 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 quantity 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 tetrahydrofuran (THF) (45 g) and ethanol (5 g) was added to a flask containing a type 5A 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 contairied 0. 1% w/w ethanol, and was therefore 99.9% w/w pure THF.

The capacity of the sieve for ethanol was 10%.

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

Example A mixture containing 10% ethanol and 90% THF, 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 5A 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 THF containing less than 0.1% ethanol by wt. 2200 ml of mixture were treated before there was breakthrough of ethanol, and the usable capacity of the sieve was 1 5%.

The ethanol 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 THF entrained on the outside of the molecular sieve beads. Quickly, however, an ethanol fraction is recovered. If the purity of the ethanol is not adequate for the intended use or market, it is easily distilled and any THF/ethanol azeotope produced may be easily recycled.

Claims (7)

Claims

1. A process for the removal of up to 50% of ethanol from a mixture thereof with an organic liquid which liquid contains only molecules having a larger cross-sectional diameter than ethanol, comprising passing the mixture at a superficial velocity of less than 300 cm/min through a column containing a molecular sieve capable of retaining ethanol to yield a purified liquid and recovering the ethanol from the molecular sieve using a temperature, pressure or vacuum swing techique.

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

3. A process according to claims 1 or 2, wherein the sieve is a type 5A sieve.

4. A process according to claim 1,2 or 3, wherein the mixture contains up to 20% of ethanol by wt.

5. A process according to any one of the preceding claims, wherein the mixture contains an aliphatic or aromatic ketone, aldehyde, higher branched alcohol, ester, glycoi, halogenated compound, branched or cyclo alkane, aromatic hydrocarbon or ether, which is mixable with ethanol.

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

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