Liquid purification or separation – Filter – Material
Reexamination Certificate
1999-06-15
2002-10-15
Fortuna, Ana (Department: 1723)
Liquid purification or separation
Filter
Material
C210S490000, C210S500270, C210S500370, C264S041000, C264S048000, C264S049000
Reexamination Certificate
active
06464873
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to selective, water permeable membranes useful for the separation of fluid mixtures and solutions by reverse osmosis and nanofiltration. In particular, the present invention is directed to bipiperidine-based polyamide, water permeable membranes which are useful for desalination of, or other solute removal from, an aqueous solution.
It is known that dissolved substances can be separated from their solvents by the use of semi-permeable membranes. For example, of great practical interest is the removal of salts from water by reverse osmosis (RO) or by nanofiltration (NF). The efficiency and economy of such removal is of tremendous economic significance in order to provide potable water from brackish or sea water for household or agricultural use. A critical factor in desalination is the performance of the membrane in terms of salt rejection, i.e., the reduction in salt concentration across the membrane, and flux, i.e., the flow rate across the membrane. For practical RO applications, the flux should be on the order of greater than at least 15 gfd (“gallons per square foot per day”) at a pressure of about 15 atmospheres for brackish water. More preferably, commercial RO applications now require fluxes greater than about 25 gfd (about 1.0 m
3/
m
2
-day) at a pressure of about 15 atmospheres for brackish water. NF applications require at least 30 gfd (about 1.2/m
3
m
2
-day) at a pressure of 10 atmospheres. Moreover, salt rejections greater than 99% are required for RO and greater than 50% for NF. The continuing goal of research and development in this area is to develop membranes having increased flux and/or solute rejection, which are useful in desalination and removal of other low molecular weight solutes.
An additional factor which can have a significant economic impact on the desalination performance of RO and NF membranes is the amount of pressure which must be applied to achieve acceptable flux rates as previously discussed. The selectivity of the membranes, i.e., the preferential rejection of a certain ion or ions over a different ion or ions, can have a significant effect on what amount of pressure will be required. Most salt solutions contain several different salts, e.g., sea water contains sodium chloride, magnesium sulfate, calcium sulfate in addition to other salts. RO and/or NF desalination processes, wherein selectivity is low and overall salt rejection is high, can create a high osmotic pressure which in turn requires increased pressure to achieve acceptable water flux rates. The increased concentration of salt ions on one side of a membrane results in an increased osmotic pressure on that side of the membrane. As the osmotic pressure on one side grows, the salt ions increasingly try to pass through the membrane to balance the pressure on each side of the membrane, and thus, greater pressure is required to force desalinated water through the membrane. Selectivity, wherein some ions pass through the membrane while others do not, can alleviate this problem by balancing the osmotic pressure on each side of the membrane.
Among the known membranes used in desalination are included a large number of various types of polyamides which are prepared by a variety of methods. Of particular interest within this broad group of polyamide membranes are crosslinked aromatic polyamide membranes. The crosslinked aromatic polyamide membranes include, for example, those disclosed in the following U.S. Patents.
U.S. Pat. No. 3,904,519, issued to McKinney et al., discloses reverse osmosis membranes of improved flux prepared by crosslinking aromatic polyamide membranes using crosslinking agents and/or irradiation. The polyamides are prepared, for example, by the interfacial polymerization of amine groups and carboxyl groups followed by crosslinking.
U.S. Pat. No. 3,996,318, issued to van Heuven, teaches the production of aromatic polyamide membranes, wherein crosslinking is achieved using a reactant having a functionality of three or greater.
U.S. Pat. No. 4,277, 344, issued to Cadotte, describes a reverse osmosis membrane which is the interfacial reaction product of an aromatic polyamine having at least two primary amine substituents with an aromatic acyl halide having at least three acyl halide substituents. The preferred membrane is made of a poly (phenylenediamine trimesamide) film on a porous polysulfone support.
U.S. Pat. No. 4,761,234, issued to Uemura et al., shows a membrane similar to U.S. Pat. No. 4,277,344 in which aromatic tri- or higher aromatic amines are employed.
U.S. Pat. No. 4,661,254, issued to Zupanic et aL, discloses a reverse osmosis composite membrane formed by the interfacial polymerization of a triaryl triamine with an aromatic carboxylic acid chloride.
U.S. Pat. No. 4,619,767, issued to Kamiyama et al., describes membranes prepared by crosslinking polyvinyl alcohol and secondary di- or higher amines with polyfunctional crosslinking agents. Both aromatic and aliphatic amine components are disclosed.
U.S. Pat. Nos. 4,872, 984 and 4,948,507, issued to the present applicant, describe the interfacial synthesis of reverse osmosis membranes from an essentially monomeric polyamine having at least two amine functional groups and an essentially monomeric polyfunctional acyl halide having at least about 2.2 acyl halide groups per reactant molecule, in the presence of a monomeric amine salt. Both aromatic and aliphatic polyamines and polyfunctional acyl halides are disclosed.
Copending U.S. application Ser. No. 08/944,995, of the present applicant, filed on Oct. 7, 1997 (now allowed), discloses a membrane which is the interfacial polymerization product of an amide-functionalized polyamine with an aromatic acyl halide having at least two acyl halide substituents.
Interesting reviews and comparisons of various composite reverse osmosis membranes are included in J. E. Cadotte, “Evolution of Composite Reverse Osmosis Membranes”,
Materials Science of Synthetic Membranes,
Chapter 12, pp. 273-294, American Chemical Society Symposium Series (1985) and S. D. Arthur, “Structure-Property Relationship in a Thin Film Composite Reverse Osmosis Membrane”,
Journal of Membrane Science,
46:243-260, Elsevier (1989).
While some of the above-referenced membranes are commercially useable, prior art membranes are often not very selective or preferential to one type of ion over another. Therefore, significant pressure is often required to achieve desired flux rates. The goal of the industry continues to be to develop membranes that have better flux and salt rejection characteristics and better resistance to disinfectants such as chlorine, in order to reduce costs and increase efficiency of operation. The development of selective water-permeable membranes wherein selectivity is high is also a goal of the industry. It would be desirable to achieve high flux rates at low pressures wherein sulfate ion rejection is high as compared to chloride ion rejection. Once sulfate has been removed from a salt solution, chloride desalination can be accomplished using a number of distillation techniques without the undesirable effects resulting from the presence of sulfate ions, such as scale formation. Scale formation is known to decrease the efficiency of distillation operations.
The piperazine-derived polyamide membranes disclosed in U.S. Pat. No. 4,619,767 are perhaps the most selective of the prior art membranes, yet still do not provide the requisite selectivity between sulfate ions and chloride ions necessary to reduce operating pressures and increase flux. Thus, there is a need in the art for highly selective membranes which can operate at high flux rates and low pressures with high sulfate rejection.
BRIEF SUMMARY OF THE INVENTION
The present invention includes a water permeable membrane prepared by interfacially polymerizing, on a microporous support: (1) an essentially monomeric bipiperidine reactant of Formula (I):
wherein a and b each independently represent an integer of from 0 to 4 and each R
1
and each R
2
independently represent a no
Akin Gump Strauss Hauer & Feld L.L.P.
Fortuna Ana
Hydranautics
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