CA1059923A - Membrane separation of weak acids from aqueous streams - Google Patents

Abstract

APPLICATION FOR

LETTERS PATENT

FOR

MEMBRANE SEPARATION OF WEAK ACIDS FROM
AQUEOUS STREAMS

Abstract of the Disclosure Weak acids having at least about one percent of the acid molecules in undissociated form are separated from aqueous mixtures containing same by contacting the aqueous feed mixture with a first surface of a nonionic, polymeric membrane which is selectively permeable to unionized acid molecules, maintaining a second and opposite membrane surface at a lower chemical potential than the first surface, and withdrawing from the second membrane surface a mixture having a higher total concentration of acid moieties than the unionized acids concentration in the aqueous feed mixture.

Description

Background of the Invention The invention relates to the nonionic membrane separation of weak acids from aqueous streams. In another aspect the invention relates to the nonionic membrane separation of weak acids from aqueous streams in combination with a solution sink which provides a lower chemical potential on the permeate side of the membrane. Yet in another aspect, the invention relates to a process wherein weak acids can be recovered from aqueous streams. Still another aspect of the invention relates to a process for the removal of environmental contaminants such as weak acids from waste water streams.
The separation of weak acids from aqueous streams has been accomplished by various means, for example, distillation, filtration, solvent extraction, and combination of these and other means. A
major pollution problem associated with industrial waste is the weak acids content of waste water streams. One source of industrial weak acid pollution results from cracking processes or partial oxidation techniques where there is a chance of organic materials combining with oxygen-containing compounds. Another source of weak acids-containing waste water streams results from processes using organic materials as extraction or extractive distillation solvents for the preparation of hydrocarbon compounds. Another source of weak acid contamination flows from the processing of organic polymers, wherein aqueous mixtures of polymers and weak acids result from catalyst residues.
In the synthesis of organic chemicals many of the processes utilize monocarboxylic acids as solvents or as reactants such as in esterification.
Also, in the oxidation of hydrocarbons to oxygenate compounds, low molecular weight and monocarboxylic acids are synthesized directly or are produced as a byproduct of the production of other acids or compounds such as esters, aldehydes, and alcohols. The recovery of the oxidized acids as well as the acids which have been used as solvents or reactants poses a serious problem for the chemical industry.

Known methods for separating the acids either utilize water in the recovery step or, as in oxidation or esterificat;on, water ;s produced as a byproduct. The monocarboxylic acids are then partially recovered by distillation but then inevitably the water present poses a problem in its separation from the acids.
Final removal of the acids from the water must be ach;eved by ;nd;rect methods. Also the corros;ve action of the ac;ds necessitates the use of expensive equipment.
Separation of acids or acidic substances is often required ;n techn;cal processes, either in recovery of acids or in processes of pur;f;cat;on. One way to proceed ;s to separate the ac;ds which are frequently the more volatile compound by distillation.
However, for this purpose complicated corrosion-resistant appa-ratuses are necessary as well as high energy consumption is involved. These factors make the process uneconomical. Another drawback ;s that many materials are decomposed ;n the presence of heat.
According to another method it is possible to separate the acid salts by freezing or crystallizing them out of solution.
This process, however requires special conditions of concentration and highly favorable cond;t;ons for crystall;zation. Moreover, separat;on ;s never quantitative when accomplished by this method. Yet in other methods, membrane separat;on techniques have been ut;lized to separate mixtures of two or more different molecules, for example7 aqueous mixtures, mixed hydrocarbons, azeotropic mixtures and the l;ke. However, known separation techniques ut;l;zed in separat;on of aqueous mixtures, frequently are followed by secondary procedures such as d;stillation and the like. Because of the disadvantages of existing separation methods which presently involve a substantial energy input of a thermal, mechanical or electrical nature, a simple membrane separat;on process for separating weak ac;ds of varying con-centrations from aqueous mixtures is needed. Accordingly, an object of this invention is to provide a method of separation of weak acids from aqueous mixtures. Another object of this invention is to provide a method of nonionic membrane separation of weak acidæ from aqueous streams which is as quantitative as possible.
Su~ary of the Invention It has been discovered in accordance with the present in-vention that weak acids are effectively separated from aqueoùs streams selectively permeable to weak acids having at least about 1 percent of the acid molecules in undissociated form wherein the permeate side of the membrane is maintained at a lower chemical potential than the feed zone of the membrane throug~ chemical or physical means. One essential feature of the nonionic polymeric membrane separation of weak acids from aqueous streams is that the nonionic polymeric membrane be selectively permeable to unionized acid molecules. The process according to the invention separates weak acids from aqueous streams having various concentrations of weak acids which retain at l~ast about 1 percent of the acid molecules in undissociated form through the steps of (a) contacting the weak acids containing aqueous feed streams with the first surface of a-nonionic, organic polymeric membrane which is selectively permeable to unionized acid molecules; ~b) maintaining a second and opposite membrane surface at a lower chemical potential than the first membrane surface through chemical or physical means; (c) permeating a portion of the weak acids into and through the membranes; and (d) withdrawing at the second membrane surface a mixture having a higher ~otal concentration of acid moietieæ than the unionized acids concentration in the aqueous feed mixture.In addition, an op-tional feature of the invention is the utilization of a solution sink as a chemical means for maintaining a lower chemical potential lOS9g23 on the permeate side of the membrane. The solution sink can be selected from potential solvents for weak acids and/or acid complexing solutions.
In a preferred embodiment of the invention there is provided a process for separating weak carboxylic acids sub-stantially in the unionized state from aqueous mixtures characterized by comprising:
contacting an aqueous feed mixture containing weak carboxylic acids having at least 1% of the acid molecules in undissociated form with a first surface of a nonionic, organic-polymer membrane selectively permeable to unionized acid molecules;
maintaining a second and opposite membrane surface at a lower chemical potential than the first membrane surface by contacting the second membrane surface with a solution sink;
permeating a portion of the unionizea acids into and through the membrane; and withdrawing at the second membrane surface a mix-ture having a higher total concentration of acid bodies than the unionized acids concen-tration in the aqueous feed mixture.
The process of the instant invention comprises the utilization of nonionic, organic polymer membranes which are selectively permeable to unionized acid molecules and substan-tially impermeable to other components of an aqueous mixture of weak acids, weak acid solvents, or weak acid complexing solutions which are in contact with the membrane. The process according to the invention can utilize a weak acid solvent com-~ - 5 --- - -plexing solution, or vacuum vapor mode on the permeate side of the membrane for maintaining the lower chemical potential which is an essential feature of the invention. The lower chemical potential provides a force which drives the unionized weak acids permeate through the selective nonionic membrane, and can result from the weak acids solvent, complexing solution or vapor vacuum mode having additional capacity for weak acids permea,te. The multiple stage operations are feasible as scale up utilization of the invention since individual stages permit various concentrations and temperatures in order to achieve optimal driving forces.
Continuous processing according to the invention is achievable wherein a weak acid-containing aqueous stream passes on one side and in contact with a nonionic membrane having selectivity for unionized acid molecules and a solution sink or vapor vacuum is in contact with the other side of the membrane. The lower chemical potential of, for example, the weak acids solution sink together with countercurrent relation-ship of the weak acids-containing aqueous mixture, provides a driving force for permeating weak acids, i.e. unionized por-tions thereof, through - 5a -the selective membrane to enrich the weak acids solution sink.
The weak acid enriched solution sink or vapor can be swept or moved by physical means to suitable processing which promotes the recycling of the solvents or complexing solutions.
For each individual stage the effectiveness of the separation is shown by the separation factor (S.F.). The separation factor (S.F.) is defined as the ratio of the concentration of two substances, A and B, to be separated, divided into the ratio of the concentrations of the corresponding substances in the permeate ; 10 (Ca/Cb)in permeate (Ca~Cb)in permeant where Ca and Cb are the concentration of the preferentially permeable component and any other component of the mixture or the sum of other component~ respectively.
In the pervaporization or vapor vacuum embodiment of the invention, the first or feed side of the membrane is usually under a positive pressure, while the second side is under a negati~e pressure, relative to atmospheric pressure. Another preferred mode of the pervaporization separation is where th- second side of the membrane is maintained at a vacuum of 0.2 mm to about 759 mm of mercury.
The term "chemical potential" is employed herein as des-cribed by Olaf A. Hougen and K. M. Watson ("Chemical Process Principles, Part II," John Wiley, New York, 1947.) The term is related to the escapin~ tendency of a substance from any particular phase. For an ideal vapor or gas, this escaping tendency is equal to the partial pressure so that it varies greatly with changes in the total pressure. For a liquid, change in escaping tendency as a function of total pressure is smaIl. The esoaping tendency of a liquid always depends upon the temperature and concentration. In the present invention, the feed substance is typically a liquid solution and the per-meate side of the membr~le is maintained such that a vapor or liquid phase exists. A vapor feed may be employed when the mixture to be separated is available in that form from an in-dustrial process or when heat economies are to be effected in multi-stage.
In a preferred embodiment of this inventive process, the first or feed surface of the nonionic membrane is contacted with a weak acid-containing aqueous stream in the liquid phase, while the second surface of the membrane is contacted with a weak acids solvent or complexing agent solution. However, the aqueous feed stream can be in the vapor phase wherein it i5 preferable that the feed side of the membrane be under positive pressure in relationship totho permeate side. In order for permeation of the weak acids to occur, there must be a chemical potential gradient between the two zones, i.e. the feed side of the membrane as compared to the permeate side of the membrane. The chemical potential gradient for purpOse of this invention requires that the chemical potential of the feed zone be higher than the chemical potential in the permeate zone. Under such conditions a portion of the weak acids in the aqueous feed stream, that is the unionized portions, will dissolve within the membrane and permeate therethrough since an essential feature of the invention is that the nonionic, organic polymer membrane be selectively permeable to the unionized acid molecules of the weak acids.
The permeation step is conducted by contacting the weak acids aqueous mixture in either the liquid or vapor phase with the non-ionic, organic polymer membrane and recovering a weak acid enriched permeate fraction from the other side of the membrane. The permeate can be either in the form of a weak acids lOS99Z3 vapor, a solution, or a salt or complexing solution of the weak acid. To facilitate rapid permeation of the weak acids, the chemical potential of the permeated weak acids at the surface of the membrane from the permeate side can be kept at a relatively low level through the rapid removal of the permeate fraction, for example, through a continuous process wherein the weak acids enriched vapor, solution, or complex solution is continually removed and replaced by vacuum or non-enriched weak acids solvent and/or complexing agents.
The term "solution sink" for purposes of this disclosure defines a liquid sweep utilized on the permeate side of the mem-brane and is inclusive of both selective solvents for weak acid, and solutions of weak acid complexing agents or both. Suitable selective solvents for weak acids used as solution sink can be selected from solvents which permit the total concentration of the weak acid bodies to be greater on the permeate side than on the feed or permeant side of the membrane. The term weak acids for the purposes of the invention will be defined as those acids having at least about l-percent of the acid molecules in the undissociated form. These weak acids7 that is the unionized acid molecule thereof, are selectively permeated into and through the membrane. With low molecular weight organic and inorganic acids, relatively rapid transfer through the nonionic membranes occurs with an acid having an ionization constant, for example, greater than about 1.0 x 10~3 (pKa, 3.00). Techniques for de-termining ionization constants are well known and for the purpose of the invention a weak acid can be an acid having an ionization constant of less than 1.0 x 10 3 in dilute aqueous solution at 25C7 however ionization constant and the related pKa value of 3.00 are not controlling in defining the term weak acids but rather as an indicator of those acids which will `~\

lOS9923 probably work according to the invention. Water-soluble weak inorganic acids include boric acids (pKa of 9.2), carbonic acid (pKa of 6.4), hypochloric acid (pKa of 7.3) and the like.
The inventive process is most useful in recovery of water-soluble organic acids such as amino acids~ hydroxyacids, mercapto acids and the like of which have pKa values of greater than 3.
For example weak organic acids include acetic acids formic acid, n-octanoic acid, acrylic acid, cyclohexane-carboxylic acid, benzoic acid, phenylacetic acid, methoxyacetic acid, glycolic acid, lactic acid, citric acid, meth;oacetic acid, thioglycolic acid, 2-mercaptopropionic acid, alanine, glycine, leucine acid, and the like as well as fatty acids having up to about 30 to 40 carbon atoms per molecule.
Strong acids which are not operable according to the in-vention generally referred to as low molecular weight inorganic and organic acids having for example ionization constant greater than 1.0 x 10 3 in dilute aqueous solutions at 25C. Examples of such acids include for example, sulfuric, nitric, phosphoric acid, sulfurous acid, periodic acid, and the like as well as strong organic acids for example, such as methane sulfonic acid, trichloroacetic acid, p-toluenesulfonic acid and the like.
Non-ionic permeation membranes used in the inventive process are non porous, that is, free from holes and tears and the like, which destroy the continuity of the membrane surface. Useful nonionic membranes according to the invention are comprised of organic, polymeric materials. The membranes are preferably as thin as possible while permitting sufficient strength and stability for use in the permeation process. Generally separation mem-branes from about 0.1 to about 15 mils or somewhat more are utilized according to the invention. High rates of permeation can be obtained through the use of even thinner membranes which can be supported with structures such as fine mesh wire, screens, porous metals, porous polymers, and ceramic materials. The nonionic membrane may be a simple disc or sheet of the membrane substance which is suitably mounted in a duct or pipe, or mounted in a plate and framed filter pressed. Other forms of membrane may also be employed such as hollow tubes or fibers through which or around which the feed is applied or is recirculated with the permeate being removed from the other side of the tube as a weak acid enriched sweep solution or complex. There are other useful shapes and sizes which are adaptable to commercial insta-llations which are in accordance with the invention. The mem-brane polymeric components may be linear or crosslinked, and vary over a wide range of molecular weights. The nonionic membrane, of course, must be insoluble in the aqueous feed mix-ture and the var;ous sweep liquid solvents and their complexing agents. Membrane insolubility as used herein is taken to in-clude that the membrane material is not substantially soluble or sufficiently weakened by its presence in the sweep solvent or aqueous feed stream to impart rubbery characteristics which can cause creep or rupture resulting from conditions of use, including use pressure. The organic membrane may be polymers which have been polymerized or treated so that specific end groups are present in the polymeric material. The nonionic membrane accord-ing to the inventive process may be prepared by any suitable means such as, for example, the casting of film or spinning of hollow fibers from a "dope" containing organic polymer and sol-vent. Such preparations are well known in the art. An important control of the separation capacity of the particular organic, nonionic membrane is exercised by the method used to form and solidify the membrane, e.g., casting from a melt into control lOS9923 atmosphere or solution at various concentrations and temperatures.
The art of membrane use is known with substantial literature being available on membrane support, flu;d flow and the like.
The present invention is practiced with such conventional apparatus. The membrane must of course, be sufficiently thin to permit permeation as desired but sufficiently thick so as not to rupture under operating conditions. The membrane according to the invention must be selectively permeable to undissociated weak acid molecules in comparison to the other components of the aqueous feed stream such as dissociated acid molecules or ions and the take up solutions and complexing agents on the permeate side of the membrane.
The following exemplary, suitable membranes for the selective permeation of the unionized molecules of the weak acid, aqueous feed mixture: methylsilicone resin, methyl/phenylsili-cone resin, poly(silicone/carbonate), polyvinylfluoride, nylon 6, nylon 66 (cast from solution), nylon 66-extruded, nylon 6 and nylon 9, nylon 6 and nylon 10, nylon 11, nylon 12, polyurethane, - polypropylene, copolymers of ethylene/trimethylvinyl ammonium chloride, copolymers of ethylene/acrylic acid (97/3 mole percent of relationship), polyethylene having a density of about 0.95, polyethylene having a density of 0.92, polybutadiene, poly-silicone carbonate, fluorinated ethylene/propylene copolymer, co-polymers of ethylene/tetrafluoroethylene, polyisoprene, polymers of chlorotriofluoroethylene/vinylidene fluoride, and the like.
Solutions of weak acid complexing agents suitable according to the invention as a solution sink or sweep material might be selected from those complexing agents which in solution form, permit the total concentration of weak acid bodies to be greater on the permeate side of the nonionic membrane than on the feed side. Complexing agents such as aqueous solutions of the hydroxides of alkaline earth and alkali metals in solution which readily form salts and lOS99Z3 complexes of the weak acids can provide a satisfactory solution sink.
The membrane permeation step is preferably operated under conditions of temperature which can vary over a wide range from about -20C to about 200C or more depending upon the selection of the weak acid feed, solution sink, or pervaporization mode and the thermal condition of the aqueous feed mixture. Higher operating temperatures are frequently des;rable because of the increased rates of permeation; however, the present invention is also concerned with energy input efficiency and minimum temperature change for the purpose of separating weak acids from a~ueous streams.
To illustrate further the present invention and the advantage obtained therefrom, the following examples are given without limiting the invention thereto. It is also possible that many changes in the details can be made without departing from the spirit of the invention.
Examples 1 through 15 Weak acids having at least 1~ of the acid molecule in un-dissociated form were removed and concentrated from aqueous streams utiliz;ng suitable membranes. The following Table presents the results achieved according to specific embodiments of the invention including separation of acetic, propionic, butyric, formic, and 2-methylbutyric acids through various mem-branes. Comparative Example 4 is a strong acid (hydrochloric acid), and not in accordance with the invention. Examples 1 through 15 utilize in combination with the nonionic, polymeric membrane a solution sink on the permeate side of the separation membrane which provides a chemical potential gradient. Conditions such as concentration, temperature, time, and permeability constant for the various membranes are presented in the following table.

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lOS99Z3 The degree of dissociation for various organic fatty acids at varying concentrations indicatesthat these acids are ~weak~
and can be considered to be in the undissociated form. For example, the following dissociation-concentration ranges are clearly within the scope of the invention and illustrative of the acids of Examples 1, 3 and 7; (Calculated for acetic acid, - ~ 0.5% dissociated at 4% by weight concentration; for butyric acid, C 0.5% dissociated at 4% and 1.7% dissociated at 0.4%
by weight concentration). For fatty acids, all of the membranes were in excellent condition visually at the end of the experiments.
The strong, completely dissociated acid, the hydrochloric acid of Example 4, yiekds a P close to zero, which is to be expected when only undissociated molecules can be transported or per-meated according to the invention.
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Claims (4)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A process for separating weak carboxylic acids sub-stantially in the unionized state from aqueous mixtures characterized by comprising:
contacting an aqueous feed mixture containing weak carboxylic acids having at least 1% of the acid molecules in undissociated form with a first surface of a nonionic, organic-polymer membrane selectively permeable to unionized acid molecules;
maintaining a second and opposite membrane surface at a lower chemical potential than the first membrane surface by contacting the second membrane surface with a solution sink;
permeating a portion of the unionized acids into and through the membrane; and withdrawing at the second membrane surface a mix-ture having a higher total concentration of acid bodies than the unionized acids concen-tration in the aqueous feed mixture.

2. A process of Claim 1 characterized in that the solution sink is comprised of a selective solvent for the weak acid permeate.

3. A process of Claim 1 characterized in that the solution sink is comprised of a weak acids complexing solution having a total free concentration of weak acids which permits a lower chemical potential on the second membrane surface than on the first membrane surface.

4. A process of Claim 3 characterized in that the complexing solutions are comprised of the hydroxides of alkaline earth and alkali metals.

S. A process of Claim 4 characterized in that the weak acid has a pKa value of at least 3.