Note: Descriptions are shown in the official language in which they were submitted.
10599Z3
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.
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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-
1059923
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-
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1059923
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
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10599Z3
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
10599Z3
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
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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
1059923
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
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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
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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.
1059923
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-- 13 -
1059923
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-- 14 --
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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