Gas separation: processes – Selective diffusion of gases – Selective diffusion of gases through substantially solid...
Reexamination Certificate
2000-12-19
2002-05-07
Spitzer, Robert H. (Department: 1724)
Gas separation: processes
Selective diffusion of gases
Selective diffusion of gases through substantially solid...
C095S051000, C096S014000, C210S500390
Reexamination Certificate
active
06383258
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to gas separation membranes comprising a copolyimide and the process for separating one or more gases from a gaseous mixture using such membranes.
BACKGROUND OF THE INVENTION
Many industrial gas separation processes utilize selectively gas permeable membranes. Aromatic copolyimides have been suggested for use as the membrane material in some gas separations. Certain aromatic copolyimide membranes have been developed to provide high relative selectivities for one gas over another gas permeating through the membrane. Such membranes, however, suffer from having low gas permeation rates. On the other hand, different copolyimide gas separation membranes have much higher gas permeation rates, but they exhibit correspondingly lower relative gas selectivities.
In addition to good gas separation characteristics, commercially important processes often impose other formidable demands on the membrane material. For example, the purification of natural gas involves the separation of carbon dioxide from methane and/or nitrogen in the presence of liquid and gaseous hydrocarbons that contaminate the mixtures to be separated. The membrane material in this use should be highly resistant to the solvent effect of the hydrocarbon contaminants. Another important consideration is that the material should be easily fabricated into an appropriate membrane structure.
It is desirable to have a copolyimide gas permeation membrane which has both high selectivity and high gas permeation rates for gases being separated. It is also desirable that such a copolyimide membrane additionally has a strong resistance to hydrocarbon solvent activity. It is still further desired to have a copolyimide material that is readily fabricable to form membrane structures while preserving its combination of high selectivity, high gas flux and resistance to hydrocarbons
U.S. Pat. No. 4,690,873 discloses a gas separating material of copolyimide formed from recurring units of tetracarboxylic acids and moieties derived from diaminodimethyldiphenylene sulfone (“AMPS”) isomers, which include the compound o-tolidine sulfone (“TSN”). A glossary of selected chemical compounds referenced in this application is found in Table III, below. The compositions do not include 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (“6FDA”). The product membranes exhibit very high selectivity of carbon dioxide relative to methane but the carbon dioxide permeability is quite low.
U.S. Pat. No. 4,705,540 discloses polyimide gas separation membranes in which the membrane composition is polymerized from mixtures containing aromatic diamines and 4,4′-(hexafluoroisopropylidene)-bis(phthalic anhydride). This produces polyimides with extremely rigid chains. The monomer mixtures do not include blends of o-tolidine sulfone with other hydrophilic diamines. The membranes demonstrate very high carbon dioxide permeability but rarely carbon dioxide/methane selectivity above 25.
U.S. Pat. No. 5,042,992 discloses a gas separation material which is a polyimide formed by reacting 4,4′-(hexafluoroisopropylidene)-bis(phthalic anhydride) (“6FDA”) with diamino-dialkyldiphenylenesulfone. The material can form into membranes that have both high carbon dioxide permeability and high carbon dioxide/methane selectivity. The patent does not disclose a monomer mixture also containing a second, hydrophilic diamine, and further, it does not mention whether the polyimides are resistant to hydrocarbon solvents.
U.S. Pat. No. 5,591,250 discloses a process for separating carbon dioxide from a methane using a membrane of polyimide formed by reacting monomers of the single dianhydride 6FDA and one or more diamines. The use of o-tolidine sulfone as one of the diamines is not disclosed. Only rarely among the many examples are both high carbon dioxide permeability and high carbon dioxide/methane selectivity demonstrated. The resistance of the membranes to hydrocarbon solvents is not reported.
SUMMARY OF THE INVENTION
The present invention now provides a gas separation membrane of a copolyimide composition which advantageously provides a favorable combination of high selectivity and high transmembrane flux for commercially important gas mixtures while remaining resistant to attack from hydrocarbon chemicals. The copolyimide of the gas separation membrane is formed by copolymerization of monomers comprising o-tolidine sulfone, a hydrophilic diamine other than o-tolidine sulfone, and 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride. Optionally, one or more aromatic dianhydride can be included in the monomers utilized to form the membrane. The copolyimide is readily fabricable to a membrane form suitable for gas separation.
There is also provided a process for separating component gases of a gas mixture comprising the steps of
(a) providing a gas separation membrane of a polyimide formed by copolymerization of diamine monomers and dianhydride monomers in which the diamine monomers comprise o-tolidine sulfone and a hydrophilic diamine other than o-tolidine sulfone, and the dianhydride monomers comprise 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride,
(b) contacting the gas mixture on one side of the membrane to cause the component gases to selectively permeate the membrane, and
(c) removing from the opposite side of the membrane a permeate gas composition enriched in concentration of the component gases which are more preferentially permeable through the membrane.
The process of this invention is well suited to separating commercially important gases from gas mixtures and is especially valuable for separating carbon dioxide from methane and/or nitrogen in the purification of natural gas. The novel copolyimide gas separation membrane is advantageously resistant to hydrocarbon contaminants likely to be present in crude natural gas such as cycloalkanes, represented by cyclohexane, cycloheptane, cyclooctane, methylcyclohexane, methylcyclopentane and 1,2-dimethylcyclopentane, and aromatic hydrocarbons represented by benzene, toluene and xylene.
DETAILED DESCRIPTION
The present invention involves a gas separation membrane formed from a copolyimide composition. Generally stated, the copolyimide is produced by conventional process steps in which firstly a diamine and a dianhydride undergo polycondensation reaction to form a polyamic acid. Subsequently, the polyamic acid is dehydrated to obtain a polyimide. It has been discovered that production of the copolyimide from a particular selection of diamine and dianhydride monomers provides a gas separation membrane which exhibits superior gas separation, flux and hydrocarbon resistance properties.
At least two diamine monomers are utilized. One is 3,7-diamino-2,8-dimethyl diphenylsulfone, commonly known as “o-tolidine sulfone” and, as mentioned, is sometimes referred to as “TSN”. At least one other diamine monomer is a hydrophilic diamine. The hydrophilic diamine can be aromatic, aliphatic or a combination of both. Preferably, the hydrophilic diamine has structure of formula I, as follows:
in which R is an aromatic hydrocarbon radical of 6-24 carbon atoms, an aliphatic hydrocarbon radical of 3-12 carbon atoms or a mixture thereof, and X is a hydrophilic radical. By “hydrophilic radical” is meant that the pendant X group is highly polar. Great preference is given to hydrophilic diamine of formula I in which X is —OH, —SO
3
H, —CO
2
H, —NHR
1
, —NR
2
R
3
, or a mixture thereof, in which each of R
1
, R
2
, and R
3
is an alkyl or aryl group. Combinations of such hydrophilic diamines are also contemplated and preferred. Representative hydrophilic diamines include 1,3-diamino-2-hydroxypropane (“DAHP”), 1,3-diaminobenzene-4-sulfonic acid (“HSMPD”), 2,2-bis(3-amino-4-hydroxyphenoxy)hexafluoropropane (“bisAPAF”), 3,3′-dihydroxybenzidine (“HAB”), L-lysine, 1,3-diamino-5-benzoic acid (“DABA”), and mixtures thereof.
TSN is an ingredient in the dianine monomers utilized in all copolyimides of this invention. While not wishing to be bound by a particular theory, it is believed tha
L'Air Liquide Societe Anonyme pour l'Etude et l'E
Lew Jeffrey C.
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