Gas separation: processes – Selective diffusion of gases – Selective diffusion of gases through substantially solid...
Reexamination Certificate
2001-09-21
2003-09-30
Spitzer, Robert H. (Department: 1724)
Gas separation: processes
Selective diffusion of gases
Selective diffusion of gases through substantially solid...
C095S045000, C095S054000, C096S004000, C096S014000
Reexamination Certificate
active
06626980
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to mixed matrix gas separation membranes incorporating a molecular sieve dispersed in a polymer, and more specifically, to such mixed matrix membranes in which the molecular sieve has a chabazite crystal structure.
BACKGROUND OF THE INVENTION
The use of selectively gas permeable membranes to separate the components of gas mixtures is a well developed and commercially very important art. Such membranes are traditionally composed of a homogeneous, usually polymeric composition through which the components to be separated from the mixture are able to travel at different rates under a given set of driving force conditions, e.g. transmembrane pressure and concentration gradients.
A relatively recent advance in this field utilizes mixed matrix membranes. Such membranes are characterized by a heterogeneous, active gas separation layer comprising a dispersed phase of discrete particles in a continuous phase of a polymeric material. The dispersed phase particles are microporous materials that have discriminating adsorbent properties for certain size molecules. Chemical compounds of suitable size can selectively migrate through the pores of the dispersed phase particles. In a gas separation involving a mixed matrix membrane, the dispersed phase material is selected to provide separation characteristics which improve the permeability and/or selectivity performance relative to that of an exclusively continuous phase polymeric material membrane.
Previous research on mixed matrix membranes with enhanced selectivity has suggested the use of zeolites for the dispersed phase particles. Some types of zeolites identified for use in mixed matrix membranes include: MFI type, such as silicalite, LTA types 4A and 5A, and FAU types X and Y with various substituted cations. Further understanding of conventional use of zeolites in mixed matrix membranes may be understood with reference to the following patents:
U.S. Pat. No. 5,127,925 (Kulprathipanja et al.) describes a process for separating a first gas component from a feed gas mixture calling for contacting the mixture with a mixed matrix membrane of an organic polymer having an adsorbent incorporated therein. The adsorbent material is selected from among zeolites such as crystalline aluminosilicates, silicalite, inorganic oxides, activated carbon or ion exchange resin.
U.S. Pat. No. 4,925,562 (te Hennepe et al.) discloses a pervaporation process which employs a membrane comprising an elastomeric polymer matrix containing zeolite. In a preferred embodiment, the elastomeric polymer matrix is silicone rubber comprising a polysiloxane. Preferred zeolites include hydrophobic aluminosilicates with a high Si/Al ratio. Silicalite is disclosed as a suitable zeolite.
U.S. Pat. No. 4,925,459 (Rojey et al.) discloses a gas separation membrane comprising an active layer including particles of a selective solid dispersed in a continuous non-porous and non-elastomeric polymer phase and a porous support. Rojey et al. teach that zeolites are particularly adapted for use in the dispersed phase.
Although gas separation membrane technology has steadily advanced, it is a persistent general shortcoming of polymeric membranes that usually either high transmembrane flux or high selectivity can be obtained at sacrifice of the other. The advent of mixed matrix membranes provided the ability to better design a membrane to achieve optimum performance by using two permeable materials with different flux and selectivity characteristics. The patents referred to above explain that zeolites may be used for the dispersed phase material of mixed matrix membranes. Despite this knowledge, zeolite-containing mixed matrix gas separation membranes have previously not provided the desirable combination of both high flux and selectivity for many common gas separation applications such as separating oxygen, nitrogen, carbon dioxide, hydrogen, helium or methane from mixtures comprising such gases.
Many traditional zeolite mixed matrix membranes also suffer from the drawback that condensable impurities in the gases being separated, such as water, adversely affect transfer of gases through the zeolite. That is, the flux and/or selectivity of zeolite-containing mixed matrix membranes utilized for gas mixtures containing water vapor usually decrease significantly after such exposure to the condensible-bearing gases. Moreover, the decrease in separation performance parameters persists even after steps are taken to dry the membrane.
It remains highly desirable to provide a mixed matrix gas separation membrane having molecular sieve dispersed in a continuous polymer matrix which can yield a combination of higher flux and selectivity than has heretofore been attainable. It is also desired to provide a molecular sieve-containing mixed matrix membrane which can maintain high flux and selectivity after exposure to gas mixtures containing water vapor.
SUMMARY OF THE INVENTION
Accordingly, the present invention now provides a mixed matrix gas separation membrane comprising particles of CHA type molecular sieve dispersed in a continuous phase consisting essentially of a polymer.
There is also provided a process for separating at least one component gas from a mixture comprising the component gas, the process comprising the steps of
(A) providing a mixed matrix gas separation membrane comprising particles of CHA type molecular sieve dispersed in a continuous phase consisting essentially of a polymer which is permeable to the component gas,
(B) contacting the mixture on one side of the membrane to cause the component gas to permeate the membrane, and
(C) removing from the opposite side of the membrane a permeate gas composition comprising a portion of the component gas which permeated the membrane.
DETAILED DESCRIPTION
This invention pertains to mixed matrix membranes. By “mixed matrix” is meant that the membrane has a selectively gas permeable layer which comprises a continuous phase of a polymeric material and discrete particles of adsorbent material uniformly dispersed throughout the continuous phase. These particles are collectively sometimes referred to herein as the “discrete phase” or the “dispersed phase”. Thus the term “mixed matrix” is used here to designate the composite of discrete phase particles dispersed within the continuous phase.
The adsorbent material utilized in the present invention is a type of molecular sieve. Molecular sieves exist in both natural and synthetic forms. They are well known in the art to encompass an extensive variety of species and are described in
Atlas of Zeolite Structure Types
W. M. Meier, D. H. Olson and Ch. Baerlocher, Zeolites 1996, 17 (A1-A6), 1-230 (hereinafter “IZA”), in
Molecular Sieves: Principles of Synthesis and Identification
, R. Szostak, Van Nostrand Reinhold, (1989), and in
Zeolite Molecular Sieves
, D. Breck, John Wiley and Sons, 1973, (Breck), the complete disclosures of which are hereby incorporated herein by reference. Molecular sieves have framework structures which may be characterized by distinctive wide-angle X-ray diffraction patterns. Zeolites are a subclass of molecular sieves based on an aluminosilicate composition. Non-zeolitic molecular sieves are based on other compositions such as aluminophosphates, silico-aluminophosphates and silica. Molecular sieves of different chemical compositions can have the same framework structure.
Zeolites can be further broadly described as molecular sieves in which complex aluminosilicate molecules assemble to define a three-dimensional framework structure enclosing cavities occupied by ions and water molecules which can move with significant freedom within the zeolite matrix. In commercially useful zeolites, the water molecules can be removed from or replaced without destroying the framework structure. Zeolite composition can be represented by the following formula: M
2
O. Al
2
O
3
. xSiO
2
. yH
2
O, wherein M is a cation of valence n, x is greater than or equal to 2, and y is a number determined by the porosity and the hydration state of the zeolites,
Corbin David R.
Hasse David J.
Kulkarni Sudhir S.
Patel Aspi N.
L'Air Liquide Societe Anonyme pour l'Etude et l'E
Law Jeffrey C.
Russell Linda
Spitzer Robert H.
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