Method of dewatering organic liquids

Details Method of dewatering organic liquids Method of dewatering organic liquids

2001-06-05

2003-06-24

Drodge, Joseph (Department: 1723)

Liquid purification or separation

Processes

Liquid/liquid solvent or colloidal extraction or diffusing...

C210S644000, C210S689000, C210S770000, C502S085000, C568S916000

Reexamination Certificate

active

06582607

ABSTRACT:

The present invention relates to a method of dewatering organic liquids, especially alcohols, using a molecular sieve which has been pretreated to absorb the water therefrom.
Alcohols and esters are usually produced in an environment containing water or moisture, be it as a reactant during hydration of olefins to form the alcohol or as a by product of a condensation reaction between a carboxylic acid and an alcohol to form the ester. The product alcohol and ester are usually contaminated inter alia with water. More, specifically where a synthetic route is used to produce an alcohol such as ethanol or isopropanol by the hydration of an olefin such as ethylene or propylene respectively, water is a reactant and hence it is inevitable that the product is contaminated with water. Again, alcohols produced by the biofermentation routes from agricultural feedstocks such as corn, beet and sugarcane, and by processing of biomass such as agricultural residues, herbaceous crops, waste paper and pulp, or municipal wastes are also contaminated with water. More importantly, methods of removing water from such products are complicated by the fact that in the case of ethanol, for instance, it forms an azeotrope with water thereby making the dewatering thereof difficult. Cumbersome and expensive methods have to be used. Of the various methods suggested for dewatering aqueous alcohols, the following processes may be considered typical: use of ion-exchange resins (DE-A-4118156), pervaporation using membranes (JP-A-04308543); treatment with an ortho-ester and followed by passing through a set of catalyst beds (DD-A-278336); by reaction with 2,2-dialkoxy-propane on a catalyst bed comprising acid ion-exchange resin and an acid zeolite (DD-265139); selective extraction of ethanol in the mixture into liquid carbon dioxide (EP-A-231072); using a combination of extraction with liquid carbon dioxide and a molecular sieve and then fractional distillation (EP-A-233692); azeotropic distillation in the presence of an entrainer such as eg cyclohexane; and, of course, the use of various types of molecular sieves or zeolites (EP-A-205582, GB-A-2151501, EP-A-142157, EP-A-158754, U.S. Pat. No. 4,407,662, U.S. Pat. No. 4,372,857, GB-A-2088739 and FR-A-2719039). The use of molecular sieves is an attractive method because of its relatively simplicity and low cost. In the last-named FR-A-2719039, the principle feature is the use of a super-heated, partially dried alcohol to regenerate the used molecular sieve.
One of the problems associated with the use of conventional molecular sieves is that by-products are usually formed due, e.g. to the reversal of the olefin hydration reaction, i.e. back conversion of isopropanol to propylene and water, or the hydrolysis of an ester back to the reactant alcohol and carboxylic acid, thereby resulting in the loss not only of the valuable product but also the chemicals, effort and energy expended in the first place in the hydration and esterification reactions respectively.
It has now been found that the cause of this reversal and the consequent loss of purity can be avoided if the molecular sieves are pretreated according to the invention prior to contact with the aqueous organic liquids.
Accordingly, the present invention is a process for dewatering organic liquids admixed with water, said process comprising bringing the admixture into contact with a molecular sieve, characterised in that the molecular sieve is pretreated so as to reduce its acid site concentration and attain an ammonia TPD value of 18 &mgr;mol/g or less prior to contact with the admixture.
By “molecular sieve” is meant here and throughout the specification the sieve as such or when such sieve is bound in or with a binder.
By “ammonia TPD value” is meant here and throughout the specification, an ammonia temperature desorption value which is the amount of ammonia desorbed from a molecular sieve after said sieve has been fully saturated with ammonia and then subjected to a thermal desorption until no more ammonia is evolved. As such the “ammonia TPD value” represents the concentration of acid sites in the molecular sieve accessible to ammonia. The acid site concentration can of course be defined by other well known characterisation techniques such as infrared spectroscopy and microcalorimetry. The ammonia TPD value of the molecular sieves used in the present invention for dewatering is suitably determined by initially heating a preweighed amount of a commercial sample of a molecular sieve to an elevated temperature e.g. about 150° C., at the rate of about 10° C. per minute in an inert atmosphere, then reducing the temperature of the heated sieve to about 100° C. in an inert atmosphere over an extended period, eg overnight at that temperature, and then re-heating the ammonia saturated sieve to about 700° C. at the rate of 10° C. per minute and measuring the amount of ammonia desorbed from the molecular sieve. Determination of the desorbed ammonia can be carried out by titration of the desorbed gases using a dilute mineral acid solution such as e.g. 0.02N hydrochloric acid.
In the case of commercially available molecular sieves which are in the so called “potassium cation form”, the ammonia TPD value is generally greater than 19 &mgr;mol/g and is typically in the range from 19 to 25 &mgr;mol/g. However, after pretreatment, the ammonia TPD value of the treated molecular sieve is ≦18 &mgr;mol/g, suitably less than 15 &mgr;mol/g and preferably less than 12 &mgr;mol/g, eg from 1-11.5 &mgr;mol/g.
Molecular sieves which are capable of adsorbing the water from an admixture thereof with an alcohol are well known. Typically, such molecular sieves are crystalline although the particular sieve employed is not critical. Such sieves should, however, be capable of adsorbing at least 2% by weight of water, e.g. from 2-30% w/w, preferably from about 5-25% w/w under the adsorption conditions. The sieve is suitably a zeolitic molecular sieve having an average pore diameter of about 3 Angstroms (Å). Typical examples of such molecular sieves are the A type zeolites, especially 3A, although others having different pore diameters such as eg 4A and 5A may also be used. Almost all commercially available molecular sieves which have hitherto been used in the dewatering process especially of alcohols though sold as a “potassium cation form” invariably have an ammonia TPD value of greater than 19 &mgr;mol/g. Typical examples of such commercially available zeolitic molecular sieves are those sold as UOP AS-5078 and Ceca Siliporite® NK30 although such molecular sieves are also available from other sources. These, so-called “potassium cation forms” as described e.g. in EP-A-0 142 157, when used as such for dewatering( aqueous alcohols result in a significant amount of by products formation such as e.g. olefins, ethers and/or aldehydes. This is unacceptable for the by-products may not only contaminate the solvent alcohol being treated but may also undergo further degradation or polymerisation in the presence of the untreated molecular sieve thereby further adversely affecting the quality of the dewatered alcohol and the consequent loss of alcohol purity. That this is the case can be seen e.g. from the description at column 2, lines 50-60 of U.S. Pat. No. 4,460,476 referred to above and also from the examples and comparative tests shown below.
The feature of the present invention is that such so-called “potassium cation form” of zeolitic molecular sieves can be further treated to reduce the ammonia TPD value thereof to the levels now claimed prior to contact with the organic liquid-water admixture in order to carry out the dewatering process. The further treatment is suitably carried out by bringing the commercially available molecular sieve into contact with a solution of an ammonium or an alkali metal salt, such as e.g. a salt of sodium or potassium, especially e.g., the nitrate salt to enable any residual H
+
cations in the commercial sieve to be exchanged with the additional alkali metal cations. A final washing procedure is then

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