Note: Descriptions are shown in the official language in which they were submitted.
1 339~54
CHLORINE-RESISTANT SEMIPERMEABLE MEMBRANES
BACKGROUND OF THE INVENTION
The separation of various components found in liquids or
gases may be effected in a multitude of processes, the techniques
for effecting the separation utilizing asymmetric or composite
membranes including selective permeation, ultrafiltration or
reverse osmosis. A particular example of the latter type of
separation involves a desalination process in which water which is
rendered potable or suitable for other purposes is obtained from
sea water, contaminated water, brackish water or brine. This
process is of especial value in areas of the world where the water
found in the area is brackish or is saline in nature. The
desalination of this water is necessary in order to provide large
amounts of potable or relatively nonsalty water for industrial,
agricultural or home use. The desalination of the water is
effected by forcing the water through a reverse osmosis membrane
whereby the purified water is passed through the membrane and
recovered, while the contaminants or salts do not pass through the
membrane, thus, in effect, being rejected by the membrane and
recovered as the retentate.
A reverse osmosis membrane, in order to be utilized for such
a purpose, must possess certain characteristics applicable to the
process. For example, the membrane must have a very high salt
rejection coefficient. In addition, another important charac-
teristic and a problem which must be addressed when utilizing the
membrane, is the ability of the membrane to be resistant to
chlorine attack. Another important factor which is present in the
use of a reverse osmosis membrane is that said membrane also
possess a high flux characteristic, that is, the ability to pass a
relatively large amount of water through the membrane at
relatively low pressures. If a membrane possesses these desirable
characteristics, it will be commercially feasible in its
applicability to the desalination process.
1 ~39054
Reverse osmosis membranes have been prepared and used from a
wide variety of known polymeric materials. While many of these
polymeric materials possess the ability of reducing the concen-
tration of a solute to where the salt rejection capability is in
excess of 98%, some do not possess the necessary flux rate whereby
the volume of water which is required to be produced by the
membrane per unit of membrane surface is sufficient for the
application of the technology.
As was hereinbefore set forth, many prior U.S. patents
describe various membranes which are useful in desalination
processes. For example, U.S. Patents 3,567,632, 3,600,350,
3,710,945, 3,878,109, 3,904,519, 3,920,612, 3,951,815, 3,993,625
and 4,048,144 illustrate various semipermeable membranes prepared
from polyamides. Likewise, U.S. Patents 3,260,691 and 3,480,588
disclose coating compositions which are obtained from the
condensation products of aromatic primary diamines and aromatic
tricarboxylic acid derivatives.
Inasmuch as the semipermeable membrane which is used for the
desalination process should be relatively thin in nature in order
to provide a desirable flux rate, it is necessary, in many
instances, that the reverse osmosis membrane be composited or
laminated on a porous backing support material. This porous
support backing material should in itself possess certain
characteristics which make it desirable for such a use. For
example, the porous support material should possess pore sizes
which are sufficiently large enough so that the water or permeate
can pass through the support without affecting or lessening the
flux rate of the entire composite. Conversely speaking, the pore
size should not be large enough so that the thin composite
semipermeable membrane will tend to fill up or penetrate too far
into the pores, thus distorting the shape of the thin film
membrane with the attendant possibility of rupturing the membrane
when operated under high pressure, thus causing said membrane to
lose its effectiveness in the reverse osmosis process.
In addition to the aforementioned U.S. patents, another U.S.
Patent, namely 4,277,344, discloses an interfacial synthesized
1 339n54
reverse osmosis membrane. This membrane is prepared from an
interfacially polymerized aromatic polyamine which has been
prepared from an essentially monomeric polyacyl halide and an
essentially monomeric arylene polyamine. The composite membrane
is prepared by coating a support material with a liquid layer
comprising an aqueous solution containing the polyamine reactant,
contacting the liquid layer with essentially monomeric
volatilizable polyfunctional acyl halide dissolved in a liquid
aliphatic or liquid halogenated aliphatic solvent and drying the
product formed thereby to form the desired membrane. In addition,
the membrane may then be treated with an oxidizing agent and
chlorine or a chlorine releasing agent to improve its chlorine
resistance. The patent teaches that the membrane contains a
plurality of sites having the formula:
Ar(CONH-)2COOH
in which Ar represents the aromatic nucleus residue of the
polyfunctional aryl halide. In addition, the membrane is
described as being lightly cross-linked in nature. The reaction
is effected in the absence of any surface active agents and acid
acceptors, the patentee stating that these compounds do not appear
to provide any advantages in the context of the invention and that
it is preferred to carry out the interfacial polymerization
without the presence of surface active agents or acid acceptors.
Furthermore, the structure of the membrane will be dependent
upon the water provided for in the aqueous solution to serve as a
reactant and states that the aryl halide groups on the
polyfunctional aryl halide are in a competitive state during the
reaction with the aqueous solution of the polyamine. The patentee
theorizes that the acyl halide groups can react either with water
or with the primary amine groups or conversely that a sequential
reaction occurs in which hydrolysis precedes condensation with an
amine group.
1 ;~3~054
SUMMARY OF THE INVENTION
This invention relates to composite membranes comprising of a
permselective barrier on a porous support. More specifically, the
invention is concerned with membranes which exhibit an extensive
resistance to chlorine and oxidants degradation, a superior degree
of solvent permeation rate and a superior degree of solute
rejection.
As was previously discussed, the use of membranes for the
separation of gas from a gaseous mixture, liquid from liquid
mixture, gas from liquid mixture or solids from liquids are
important articles of commerce. This is especially true in the
area of separation whereby water which is brackish or saline in
nature or having other solid and/or organic materials dissolved
therein may be rendered potable or suitable for use in other
industrial or agricultural regions by passing the water through
separation (permselective) membranes. The particular membranes
which constitute the inventive feature of the present application
will comprise the reaction product resulting from the reaction of
an aromatic polyamine and an aromatic polycarboxylic acid
chloride, said membrane being composited or coated on a porous
support backing material. By utilizing these membranes in a
separating (desalination) process, it is possible to treat water
source over a relatively long period of time without replacement
of the membrane, the long life of the membrane being, in part, due
to the resistance to degradation resulting from exposure to
chlorine or other oxidizing agents are present in the water
source.
It is therefore an object of this invention to provide a
composite membrane, suitable for use in separation processes,
which possesses desirable characteristics.
A further object of this invention is to provide a process
for preparing a semipermeable membrane which is resistant to
chlorine and other oxidizing agents, thus rendering the membrane
suitable for use in separation processes such as desalination of
water where chlorine or other oxidizing agents are present in an
amount sufficient to degrade other types of membranes.
1 3~05~
In one aspect, an embodiment of this invention resides in a
chlorine-resistant semipermeable membrane prepared by casting an
aqueous solution of an aromatic polyamine which contains a
polyhydric compound on a porous support backing material, removing
excess solution, contacting the coated porous support material
with an organic solvent solution of an aromatic polycarboxylic
acid halide to form an interfacial condensation reaction product
on the surface of said porous support material, curing the
resultant composite at curing conditions to form said chlorine-
resistant semipermeable membrane.
A further embodiment of this invention is found in a process
for the preparation of a chlorine-resistant semipermeable membrane
which comprises casting an aqueous solution of an aromatic
polyamine which contains a polyhydric compound and an acid
acceptor on a porous support backing material, removing excess
solution, contacting the coated porous support material with an
organic solvent solution of an aromatic polycarboxylic acid halide
to form an interfacial condensation reaction product on the
surface of said porous support material, curing the resultant
composite at curing conditions, washing the cured membrane with an
alkaline compound at an elevated temperature and pH, leaching the
washed composite at an elevated temperature with sodium bisulfite,
treating the leached composite with a polyhydric compound, and
recovering the resultant chlorine-resistant semipermeable
membrane.
A specific embodiment of this invention is found in a
chlorine-resistant semipermeable membrane prepared by casting an
aqueous solution of m-phenylenediamine, said aqueous solution
containing ethylene glycol and sodium carbonate on a polysulfone
30 backing material, removing excess solution, contacting the coated
polysulfone with a naphtha solution of trimesoyl chloride, curing
the resultant composite at a temperature in the range of from
about 20~ to about 150~C for a period of time in the range of from
about 10 minutes to about 2 hours, subjecting the composite to
35 treatment with sodium carbonate at a temperature in the range of
from about 20~ to about 100~C at a pH in the range of from about 9
1 33~()54
to about 11, leaching the treated membrane with sodium bisulfite
at a temperature in the range of from about 20~ to about 100~C.
The leached membrane can be further treated with glycerine, or
heat at 20~-100~C.
Other objects and embodiments will be found in the following
further detailed description of the invention.
DETAILED DESCRIPTION OF THE INVENTION
As hereinbefore set forth, the present invention is concerned
with semipermeable membranes which are resistant to chlorine and
to a method for preparing these membranes. The membranes are
prepared by casting an aqueous solution of an aromatic polyamine
on a porous backing support material, removing excess solution by
drawing, rolling, sponging, air knifing or other suitable
techniques, and thereafter contacting the coated support material
with an organic solvent solution of an aromatic polycarboxylic
acid halide. The organic solvent which is used to prepare this
solution of the aromatic polycarboxylic acid halide is immiscible
or sparingly miscible with the aqueous solution, thus permitting
20 the formation of an interfacial polymerized condensation reaction
product onto the surface of the support material. The resulting
composite is then cured to provide a semipermeable membrane which
exhibits favorable characteristics with respect to salt rejection
and flux as well as resistance to chlorine.
As was previously discussed, a membrane of this type is set
forth in U.S. Patent 4,277,344. This membrane was described as
containing a plurality of sites having the formula:
Ar(CONH-)2COOH
in which Ar represents the aromatic nucleus residue of the
polyfunctional aryl halide utilized as one component thereof. It
has now been discovered that by utilizing an aromatic polyamine in
an aqueous solvent solution which contains a polyhydric compound
35 and an acid acceptor in which the pH of the aqueous solution is
kept in a range of from about 9 to about 11 wi 11, when contacted
1 339054
with an organic solvent solution of an aromatic polycarboxylic
acid halide, result in the formation of a membrane which exhibits
superior performances in term of permeation rate and separation
characteristics over the membranes made according to the teaching
of U.S. Patent 4,277,344 which did not utilize polyhydric
compounds. An additional difference between the membrane of the
present invention and the membrane of U.S. Patent 4,277,344, is
that due to the high pH of the reaction medium and use of an acid
acceptor, any COOH units will be present in the salt form as
lO carboxylates rather than undissociated carboxylic acid forms.
In one embodiment, the chlorine resistant semipermeable
membranes of the present invention may be prepared by coating a
porous support backing material with an aqueous solution of the
aromatic polyamine, said aqueous solution being of a composition
15 hereafter set forth in greater detail. The porous support backing
material comprises a polymeric material containing pore sizes
which are of sufficient size to permit the passage of permeate
therethrough, but are not large enough so as to interfere with the
bridging over of the resulting ultrathin reverse osmosis
20 membrane. In the preferred embodiment of the invention, the pore
size of the porous support backing material will range from about
1 to about 5,000 millimicrons inasmuch as pores which are larger
in diameter than 5,000 millimicrons will permit the ultrathin
reverse osmosis membrane to sag into the pore, thus disrupting the
25 flat sheet configuration which is a desirable characteristic of
the membrane. Examples of porous support backing materials which
may be used to prepare the desired membrane composite of the
present invention will include such polymers as polysulfone,
polycarbonate, microporous polypropylene, the various polyamides,
30 polyimines, polyphenylene ether, various halogenated polymers such
as polyvinylidine fluoride, etc.
The porous support backing material may be coated utilizing
either a hand coating or continuous operation with an aqueous
solution of monomeric polyamines or to render the resulting
35 membrane more resistant to environmental attacks, of monomeric
substituted polyamines. These monomeric polyamines may comprise
1 339054
cyclic polyamines such as piperazine, etc.; substituted cyclic
polyamines such as methyl piperazine, dimethyl piperazine, etc.;
aromatic polyamines such as m-phenylenediamine, o-
phenylenediamine, p-phenylenediamine, etc.; substituted aromatic
polyamines such as chlorophenylenediamine, N,N'dimethy-1,3
phenylenediamine, etc.; multi-aromatic ring polyamines such as
benzidine, etc.; substituted multi-aromatic ring polyamines such
as 3,3'dimethylbenzidine, 3,3'dichlorobenzidine, etc.; or a
mixture thereof depending on the separation requirements as well
as the environmental stability requirements of the resulting
membranes. The solution which is utilized as the carrier for the
aromatic polyamine will comprise water in which the aromatic
polyamine will be present in the solution in an amount in the
range of from about 0.1 to about 5% by weight of the solution.
Another component of the aqueous solution will include polyhydric
compounds such as ethylene glycol, propylene glycol, glycerine,
other longer carbon atom backbone glycols, i.e. (C4-C12),
polyethylene glycol, polypropylene glycol, copolymers of ethylene
glycol and propylene glycol, etc. used singly or mixed with each
other. The aqueous solution may also contain basic acid acceptors
such as sodium hydroxide, potassium hydroxide, sodium carbonate,
potassium carbonate, etc. The polyhydric compound may be present
in the aqueous solution in amounts ranging from about 5 to about
90% while the acid acceptor may be present in a relatively small
amount ranging from about 5 to about 500 parts per million.
Furthermore, the pH of the aqueous solution is maintained in a
relatively high range of from about 9 to about 11.
After coating the porous support backing material with the
aqueous solution of the aromatic polyamine, the excess solution is
removed by suitable techniques previously discussed and the coated
support material is then contacted with an organic solvent
solution of the aromatic polycarboxylic acid halide. Examples of
aromatic polycarboxylic acid halides which may be employed will
include di- or tricarboxylic acid halides such as trimesoyl
chloride (1,3,5-benzene tricarboxylic acid chloride), isophthaloyl
chloride, terephthaloyl chloride, trimesoyl bromide (1,3,5-benzene
1 33q()54
tricarboxylic acid bromide), isophthaloyl bromide, terephthaloyl
bromide, trimesoyl iodide (1,3,5-benzene tricarboxylic acid
iodide), isophthaloyl iodide, terephthaloyl iodide, as well as
mixtures of di-tri, tri-tri carboxylic acid halides, that is,
trimesoyl halide and the isomeric phthaloyl halides. The di- or
tricarboxylic acid halides may be substituted to render them more
resistant to further environmental attack. Again, in the
preferred embodiment of the invention, the aromatic polycarboxylic
acid halide is present in the organic solvent solution in a range
of from about 0.01 to about 5% by weight of the solution. The
organic solvents which are employed in the process of this
invention will comprise those which are immiscible with water,
immiscible or sparingly miscible with polyhydric compounds and may
comprise paraffins such as n-pentane, n-hexane, n-heptane,
cyclopentane, cyclohexane, methylcyclopentane, naphtha, etc. or
halogenated hydrocarbon such as the freon series or class of
halogenated solvents. It is to be understood that the above
listing of polyhydric compounds, acid acceptors, aromatic
substituted and unsubstituted polyamines, aromatic polycarboxylic
acid halides and organic solvents are only representative of the
class of compounds which may be employed, and that the present
invention is not necessarily limited thereto.
Inasmuch as the organic solvent and the aqueous solvent
mixture for the aromatic polyamine are substantially immiscible or
incompatible, the polymerization of the two components of the
membrane will occur substantially only at the interface between
the solvent phases and thus an interfacially polymerized reaction
product comprising a thin film membrane will be formed thereat.
The contact time required for the formation of the thin film
membrane will fluctuate over a relatively wide range of from about
1 second to about 60 seconds. Following the formation of the
interfacially polymerized reaction product on the surface of the
porous support backing material, the resultant composite may be
cured to remove any remaining solvent and reactants and firmly
affix the thin film membrane on the surface of the support. The
curing of the composite membrane may be effected over a wide
1 339~S4
temperature range, said temperature being from ambient (20~-25~C)
up to about 150~C for a period of time ranging from about 1 minute
to about 2 hours or more in duration. The operating parameters of
time and temperature will be interdependent, the primary criteria
for the curing of the membrane being that said curing time is
sufficient to provide the desired membrane but being insufficient
to affect the desired characteristics of the thin film membrane
and the porous backing support material. For example, excessive
heat or curing time may affect the pore size of the backing
material, thus resulting in a decrease of the desired flux rate of
the membrane.
The composite of chlorine-resistant membrane is then
subjected to a post treatment in which the membrane is subjected
to a wash utilizing an aqueous solution of a basic material at a
pH in the range of from about 9 to about 11. The basicity of the
solution is afforded by the presence of a basic compound such as
sodium hydroxide, potassium hydroxide, lithium hydroxide, sodium
carbonate, potassium carbonate, lithium carbonate, etc. The wash
is effected at a temperature which may be in the range of from
2~ about 20~ to about 100~C for a period of time in the range of from
about 1 to about 15 minutes.
Following the wash of the membrane, it is then subjected to a
leaching step in which any unreacted aromatic polyamine which may
still be present on the membrane will be removed. The removal of
the polyamine is effected inasmuch as the amine may tend to
oxidize and discolor the membrane as well as causing a subsequent
handling problem of the membrane downstream in the equipment. The
leaching of the unreacted aromatic polyamine is effected by
treating said membrane with a substance such as sodium bisulfite
at a temperature in the range of from about 20~ to about 100~C
again for a period of time in the range of from about 1 to about 5
minutes. In the preferred embodiment of the invention the sodium
bisulfite is present in the leached solution in a range of from
about 0.5 to about 1%. Additional leaching substances which may
3~ be utilized will include organic acids such as sulfamic acid or
mineral acids such as nitric acid, etc.
1 339054
Following the leaching treatment, the chlorine-resistant
semipermeable membrane is then further treated with a polyhydric
compound such as glycerine which may be present in a range of from
about 10 to about 50% V/V in order to protect the thin film
membrane from damage in handling as well as preventing the
membrane from drying out, the latter leading to a loss of
performance of the membrane when used in a separation process.
It is contemplated within the scope of this invention that
chlorine-resistant semipermeable membranes may be prepared in a
continuous manner of operation. When this type of operation is
employed, a porous support backing material of the type
hereinbefore set forth in greater detail is continuously passed
through a bath of an aqueous solution of the aromatic polyamine
which contains the polyhydric compound and acid acceptor. After
passage through the bath, the backing material is continuously
withdrawn and the excess solution which may be present is removed
by suitable techniques. The coated support material is then also
continuously passed through the organic solvent solution of the
aromatic polycarboxylic acid halide. The interfacial
polymerization reaction will occur during the contact time by the
solutions following which the composite comprising the interfacial
polymerized reaction product in the form of a thin film
semipermeable membrane on the surface of the porous support
backing material will then be cured as, for example, by passage
through an oven which is maintained at the desired curing
temperature, the passage through said oven being at a
predetermined rate so as to avoid any possible damage to the
composite membrane. Thereafter, the membrane is continuously
withdrawn from the curing oven and continuously passed through the
30 washing, leaching and coating zones for the post treatment, and
the desired membrane is subsequently recovered.
The resultant chlorine-resistant semipermeable membrane may
then be employed for the separation process desired such as the
desalination of sea water or brackish water, other treatments of
water such as softening of hard water whereby salts are removed,
boiling said water treatment, concentration of whey or fruit
12 ~ 319n54
juices, etc. The membranes which are in the form of flat sheets
are particularly applicable for use in modules either in single
sheet or multiple sheet units whereby the sheet or sheets are
wound in a spiral type configuration.
The following examples are given for purposes of illustrating
the novel chlorine-resistant semipermeable membranes which have
been prepared according to the process hereinbefore set forth and
to the use thereof as separation agents. However, it is to be
understood that these examples are provided merely for purposes of
illustration and that the present invention is not necessarily
limited thereto.
EXAMPLE I
A chlorine-resistant semipermeable membrane was prepared by
passing a sheet of porous polysulfone through an aqueous solution
containing 2% by weight of _-phenylenediamine, 70 ppm of sodium
carbonate and 5% by volume of ethylene glycol. The polysulfone
sheet was in contact with the solution which had a pH of 9.85 for
a period of 30 seconds. The membrane now loaded with aqueous
solution mixture was air dried at room temperature for a period of
15 minutes and thereafter was passed through a naphtha solution
containing 0.15% by weight of trimesoyl chloride for a period of
15 seconds. The membrane composite was then dried at a
temperature of 21.7~C in air for a period of 23 minutes.
The post treatment of the membrane was effected by washing
the membrane with a sodium carbonate solution which had a pH of
10.0 for a period of 5 minutes at room temperature. The leaching
step of the post treatment was effected by treating the membrane
with a sodium bisulfite solution for a period of 5 minutes at a
temperature of 40~C. Finally the membrane was coated with a 20%
solution of glycerine. This membrane was designated as membrane
A.
A
13 l 339054
EXAMPLE II
A second membrane was prepared in a manner similar to that
set forth in Example I above by passing a sheet of polysulfone
through an aqueous solution of _-phenylenediamine which was
present in an amount of 2.1% by weight, said solution containing
70 ppm of sodium carbonate and 20% by volume of ethylene glycol,
the pH of the solution being maintained at 9.4. The polysulfone
was contacted with this solution for a period of 30 seconds
following which it was withdrawn and the excess solution was
removed in a manner similar to that set forth above. The coated
polysulfone was then passed through a solution of 0.1% by weight
of trimesoyl chloride in naphtha for a period of 15 seconds and
thereafter dried at a temperature of 21.7~C in a forced air
atmosphere for a period of 20 minutes.
The resultant membrane was post treated by rinsing with a
sodium carbonate solution at room temperature for a period of 5
minutes, the pH of the solution being 10Ø Following this, the
residual m-phenylenediamine was removed by treatment with a sodium
bisulfite solution at a temperature of 40~C for a period of 5
minutes and thereafter coated with a glycerine solution. This
membrane was designated as membrane B.
EXAMPLE III
A third membrane was also prepared by passing a sheet of
polysulfone through an aqueous solution of m-phenylenediamine,
said polyamine being present in an amount of 2.1% by weight, the
aqueous solution containing 70 ppm of sodium carbonate and 50% by
volume of ethylene glycol. The coated polysulfone was treated in
a manner similar to that set forth above, that is, by being passed
through a naphtha solution containing 0.15% by weight of trimesoyl
chloride under identical conditions. The post treatment of the
resultant membrane was also similar to that described above and
this membrane was designated as membrane C.
, .
~ 339054
14
EXAMPLE IV
For comparative results, a like membrane was prepared with
the exception that the aqueous solution did not contain any
ethylene glycol, being 100% aqueous by nature. The remainder of
the process was identical to that described above and the
resultant membrane after post treatment was designated a membrane
D.
EXAMPLE V
Again for comparative results, two membranes were prepared
according to the method set forth in U.S. Patent 4,277,344. The
first membrane was prepared by passing a sheet of polysulfone
through an aqueous solution containing 2.0~ by weight of m-
phenylenediamine for a period of 36 seconds. The coated
polysulfone sheet was removed, the excess solution was also
removed and the coated sheet passed through an organic solution of
trichlorotrifluoroethane containing 0.1% weight per volume of
trimesoyl chloride for a period of 10 seconds. The membrane was
dried in air at a temperature of 23.3~C.
A second membrane was also prepared according to the above
paragraph and after drying with air was treated with a solution
containing 100 ppm of hypochlorite for a period of 20 hours at
room temperature and recovered. The first membrane in this
example was designated membrane E and the second as membrane F.
EXAMPLE VI
The membranes which were prepared according to the above
examples were placed in a cell and a synthetic brackish water feed
containing 2 g/liter of sodium chloride was passed across the
surface of a membrane at a feed flow rate of 5.68 liters/minute.
The test conditions which were employed during the experiments
included a pressure of 1516.85 kPa gauge on one side of the
membrane while the other side of the membrane was maintained at
~ 33qO54
atmospheric pressure. A temperature of 25~C was maintained
throughout the experiment while keeping the pH of the feed at
5.5. The permeate, which was collected from the flowthrough, was
measured and the rejection of sodium chloride and flux rate were
determined. The results of these tests are set forth in the table
below.
TABLE
Membrane 0 A B C E F
Flux (l/m2/d) 9291087.9 1344.6 1320.2 884.2 1052.9
Rejection (~) 98.598.6 98.7 95.3 96.3 95.99
It is to be noted from the above table that the membranes
which were prepared according to the present invention utilizing
ethylene glycol and sodium carbonate in the aqueous solution as
well as a post treatment of the type set forth in greater detail
in the above examples exhibited higher flux rates with comparable
or greater salt rejections than were found when utilizing
membranes which had been prepared according to U.S. Patent
4,277,344 or without the presence of ethylene glycol in the
aqueous solution.
'~
