Gas separation: apparatus – Apparatus for selective diffusion of gases
Reexamination Certificate
2000-11-21
2003-04-15
Spitzer, Robert H. (Department: 1724)
Gas separation: apparatus
Apparatus for selective diffusion of gases
C095S051000, C095S054000, C096S010000, C055S524000, C055SDIG005
Reexamination Certificate
active
06547859
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a process for making microporous membranes that incorporate selected gas selective sites. Examples of selected gas selective sites include selective transition element complexes (TEC's) for oxygen, amines and/or exposed cations for carbon dioxide, and copper(I) and/or silver(I) sites for carbon monoxide. The selected gas selective sites are incorporated onto internal surfaces of porous membranes in a controlled spacing and orientation manner and such resulting microporous membranes being suitable for separating and purifying selected gas from the selected gas containing gas mixtures. The invention also relates to the microporous selective membranes so made.
2. Description of the Prior Art
For many years, air has been separated and purified by cryogenic distillation, for which operating temperatures are set by the vapor-liquid equilibria of the liquefied mixture. Cryogenic separation is capital intensive, particularly for production rates below several hundred tons per day. Alternative technologies compete in the marketplace particularly for air separations to produce oxygen and nitrogen at lower purities and lower production rates than cryogenic systems. Pressure swing adsorption (PSA) has been applied to air separation and purification at near ambient temperature using either gas-solid equilibria or differences in uptake and release rates between adsorbates for a given adsorbent. Gas separation using membrane systems takes advantage of differences in permeation rates for the different feed components. Process aspects of membrane gas separation are simpler than either cryogenic or PSA systems.
Conventional air separation using membrane systems employs thin polymeric coatings which exhibit permselectivity for one or more components of the feed. For air separation, the primary concern is the separation of oxygen and nitrogen with the selectivity for other feed components such as water and carbon dioxide playing a lesser role. Advances in membrane materials have relied on the identification and utilization of polymers showing increased selectivity and permeability, combined with improvements in coating technology to give thinner separation layers.
Certain transition element complexes possess sites which show high selectivities for oxygen over nitrogen, argon, and other air components. Several attempts have been made to incorporate oxygen selective TEC sites into membrane systems to give rise to facilitated oxygen transport. Alternative approaches have been utilized such as those which are supported liquid membranes where the TEC sites function as mobile carriers or dense polymer membranes or microporous membranes where the TEC's serve as fixed sites. For example, a membrane containing Co(3-MeOsaltmen) in &ggr;-butyrolactone containing 4-(N,N-dimethylamino)pyridine (DMAP) at −10° C. showed a selectivity &agr;(O
2
/N
2
) >20 with an oxygen permeability of 260 Barrer. Several problems have been identified which restrict or limit the application of supported liquid membranes to air separation. These include practical restrictions such as solubility limits for the TEC, membrane thickness, and low carrier mobility. In addition, decline in membrane performance can occur by irreversible TEC oxidation, evaporation loss of the liquid medium, and contamination of the liquid membrane with minor atmospheric components.
Permeation within polymeric materials can be described by a combination of diffusional and solubility effects. The incorporation of TEC's into polymeric substrates has been disclosed to improve oxygen transport through solubility enhancement. Several alternative configurations have been examined such as examples where the TEC/axial base is physically incorporated in the polymer, or where the TEC and/or axial donor are covalently linked to the polymer.
Permeation studies for dense polymer membranes containing TEC's have indicated that the increasing oxygen permeability with decreasing upstream oxygen pressure is consistent with facilitation. For example, permeation studies at 35° C. for polybutylmethacrylate containing 4.5 wt. % 1-Melm/Co(T
piv
PP), where 1-Melm refers to 1-methylimidazole and Co(T
piv
PP) refers to meso-tetra (&agr;,&agr;,&agr;,&agr;-o-privalamidophenyl) porphyrinato-cobalt(II), indicated a P(O
2
) of 23 Barrer with selectivity &agr;(O
2
/N
2
) of 12 at 5 mmHg upstream pressure and lifetimes on the order of months.
Oxygen facilitation in styrene-butadiene-vinylpyridine graft copolymers, and epoxidized styrene-butadiene block copolymers containing TEC's have been disclosed in the prior art. Pressure dependent oxygen permeation with &agr;(O
2
/N
2
) up to 6.2 and P(O
2
) 29.4 Barrer with an upstream pressure of 50 mmHg have been reported. Permeation studies for polyhexylmethacrylate-co-vinylpyridine containing N,N
1
-bis(salicylidene)ethylenediaminocobalt(II) (abbreviated as Co(salen)) prepared by an interfacial reaction between a polymer solution and a TEC impregnated in a porous membrane have been reported. Selectivities over 20 were reported with an oxygen permeation rate of 3.12×10
−8
cm
3
/cm
2
/s cmHg at 100 torr upstream pressure. In an alternative approach, a rigid porous support for butylacrylate-co-vinylimidazole membranes containing Co(T
piv
PP) have provided permeation measurements indicating &agr;(O
2
/N
2
) of 16.
Gas separation membranes consisting of TEC's related to Co(salen) and DMAP physically incorporated in polysulfone which exhibit facilitated oxygen transport have been disclosed. A dry/wet phase inversion process was utilized to give thin separation layers and to prevent TEC crystallization. The stability of the separation membranes have been found to be satisfactory over 3 months using either a synthetic air or a compressed air feed. The highest selectivity reported corresponds to asymmetric membranes containing 15.1 wt. % Co(5-NO
2
-salen) which gives &agr;(O
2
/N
2
) of 23.46.
Carbon molecular sieve (CMS) membranes have been prepared by controlled carbonization of polymeric substrates, and permit gas separations on the basis of molecular dimensions. For the separation of components with similar kinetic diameters such as oxygen and nitrogen, fine control of the pore size distribution of the CMS membranes is generally required.
There are several examples of microporous membranes where surface diffusion plays a significant role in transport including the transport of organic vapor mixtures through porous glass, and gas permeation for porous glass modified using tetraethoxysilane. The transport of pure gases and binary gas mixtures through microporous composite membranes based on alumina have shown surface diffusion of carbon dioxide with a surface diffusion coefficient estimated to lie in the range 2×10
−5
to 5×10
−5
cm
2
/sec.
U.S. Pat. No. 5,104,425 describes porous membranes incorporating a adsorbent which separate gas mixtures by virtue of the selective adsorption of at least one primary component. The adsorbed component diffuses by surface flow in the adsorbed phase due to concentration gradients created by a pressure difference across the membrane. Pore diameters, for the substrate described in the patent should be between 0.1 and 50 &mgr;m, and the thickness of the active layer containing porous adsorptive material should be <201 &mgr;m. The incorporation of porous adsorbent coatings involves introduction of an adsorbent precursor, heating to convert the precursor to the porous adsorbent, and cooling. Methods for the modification of the porosity of the adsorptive layer are disclosed, together with post-treatments for the sealing of defects using a thin layer (<1 &mgr;m) of high permeability, low selectivity polymer such as a silicon containing polymer. Permeation measurements, for example of an adsorbent membrane at 25° C. indicate a(CO
2
/H
2
) of 7.1 and a CO
2
permeability of 3360 Barrer.
Oxygen separating porous membranes have been disclosed in the prior art where oxy
Mullhaupt Joseph Timothy
Notaro Frank
Prasad Ravi
Stephenson Neil Andrew
Follett Robert J.
Praxair Technology Inc.
Spitzer Robert H.
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