Gas separation: processes – Selective diffusion of gases – Selective diffusion of gases through substantially solid...
Reexamination Certificate
1999-10-26
2002-10-29
Spitzer, Robert H. (Department: 1724)
Gas separation: processes
Selective diffusion of gases
Selective diffusion of gases through substantially solid...
C096S011000, C055S524000, C055SDIG005, C502S182000, C502S427000
Reexamination Certificate
active
06471745
ABSTRACT:
BACKGROUND OF THE INVENTION
The preparation of nanoporous carbon membranes, previously called carbogenic molecular sieves (CMS) by those skilled in the art, which possess high mechanical strength, simple fabrication procedure and are readily assembled into modules is described by Foley et al. in U.S. patent application Ser. No. 08/671,698 (U.S. Pat. No.5,972,079) which is incorporated in its entirety by reference.
Membranes have gained considerable importance as an inexpensive, low energy alternative to distillation for separation of gases. In particular, sieving of molecules based purely on size differences has emerged as a mechanism for obtaining extremely high selectivities of a particular component.
Currently, inorganic membranes constitute the bulk of separation materials, mostly for their stability at high temperatures. Other potential candidates for use as membrane materials include zeolites, polymers, ceramics and Carbogenic Molecular Sieve materials (hereinafter sometimes referred to as “CMS” or “CMS materials”). CMS materials have the advantage of being relatively inexpensive compared to zeolites, more temperature resistant than polymers and less brittle than ceramics. Numerous studies have shown that a relatively narrow pore size distribution of 4-6 Å can be obtained by controlled pyrolysis of CMS precursor materials. Thus, it would be advantageous to utilize CMS in the form of a membrane to perform molecular sieving.
CMS materials can be derived from natural sources such as wood and coconut shells, as well as synthetic polymer precursors. The basis for their sieving action arises from the complex microstructure, which has been described as consisting of a network of aromatic domains and amorphous carbon. Disclinations between the various domains result in predominantly slit-shaped pores than can exclude certain molecules on the basis of size and shape. However, unlike zeolites, which have a unique pore size, CMS typically has a distribution of pore sizes that can range from 3 to 10 Å. One application of CMS is in the separation of nitrogen and oxygen using the pressure swing adsorption method. The kinetic diameters of the two molecules differ by a mere 0.2 Å—but careful control of the pore size results in very high selectivities for oxygen. This example also demonstrates the difference between a CMS, which performs true molecular sieving, and an activated carbon, whose performance is based on the difference in the adsorption equilibrium of gases. As nitrogen is more strongly adsorbed on activated carbon than oxygen, it would be held back and would have to be desorbed when the separation was complete. In a CMS, however, the equilibrium uptakes of both gases are the same—hence, the time of sieving becomes important to obtain a high selectivity.
CMS materials have been synthesized using a variety of different polymeric precursors. The controlled deposition of pyrolyzed carbon to narrow pores in activated carbons and other supports has also been studied extensively. Established synthesis methods involve pyrolyzing the precursor at a high temperature in an inert gas flow. However, not all polymers can be utilized for CMS production—this depends on whether they undergo cross-linking at high temperatures or not. The thermodynamically preferred structure for carbon at high temperatures is graphite. In the case of “graphitizing” polymers like PVC, graphite-like layers are formed at around 1000° C., which results in a considerable decrease in microporosity of the material. Hence, the resulting carbon is not suitable for gas separations. On the other hand, PAN, PVDC and PFA cross-link at high temperatures to stabilize the structure and prevent the formation of graphite layers. This “non-graphitizing” character of the polymers is due to the presence of heteroatoms such as oxygen and nitrogen, as well as excess hydrogen. The pore sizes obtained are between 4-6 Å, which make them ideal for use as molecular sieves.
CMS materials are globally amorphous and do not exhibit any long range order as evident in zeolites. X-ray diffraction studies, which can resolve features on a length scale of 25 Å, do not reveal a distinct diffraction pattern for the microstructure. HRTEM studies of the structure combined with FFT analysis, can be used to determine the spacing between the graphite layers. The structure of CMS is thought to consist of a tangled network of ribbon-like aromatic regions. The evolution of the microstructure depends on the polymer precursor as well as the pyrolysis parameters of soak time and temperature. Investigations have shown that for most precursors, high temperature sintering leads to shrinkage of pores. There is, however, a collapse of the structure above a certain temperature, leading to a loss in the sieving property. A comprehensive review of CMS materials has been carried out by Foley (see Foley, H. C.,
Carbogenic Molecular Sieves: Synthesis, Properties and Applications
; Microporous Materials, 1995;4; pp. 407-433).
Nanoporous membranes—porous membranes generally having a porosity below 1 nm—have attracted the attention of many researchers because of their potential for technological advances in gas separations and shape selective catalysis. (Saracco et al. 1994, infra.). Permeation experiments often constitute a significant contribution to the characterization of these membranes. As the dimensions of a pore approach that of the molecule, transport generally becomes extremely sensitive to the molecular dimensions of the probe gas and very high separation factors have been reported for ceramic, see Vercauteren, S., Keizer, K., Vansant, E. F., Lutyten, J., and R. Leysen, (1998),
Porous Ceramic Membranes: Preparation, Transport Properties and Applications
, J. of Porous Materials 5, 241, and zeolite, see Bai C, Jia M, Falconer J, et al. (1995),
Preparation and Separation Properties Silicalite Composite Membranes
, J. Membrane Science, 105, 79, and nanoporous carbon membranes of this type described herein.
There are two forms of CMS membranes—the unsupported “hollow fiber” form, and the supported form. The hollow fiber membrane was developed by Koresh and Soffer (see Koresh, J. E. and A. Soffer,
Molecular Sieve Carbon Permselective Membrane Part I. Presentation of a New Device for Gas Mixture Separation
; Separation Science and Technology, 1983; 18(8); pp. 723-734) by pyrolysis of polyacrylonitrile (PAN) fibers. Despite their good sieving properties, the membranes lacked the requisite mechanical strength for use in various applications. A hollow fiber also cannot be converted easily into a module form that would be suitable for industry.
Supported CMS membranes can be synthesized using numerous techniques such as dip coating, spin coating, vapor deposition and sputtering. The ideal structure of such a membrane is shown in FIG.
4
. It consists of a thin CMS layer
5
on top of a macroporous, non-selective support
7
. The support provides mechanical strength to the membrane, which is a considerable improvement over the hollow fiber configuration. It also has the advantage of being available in various geometries such as flat plates, tubes and disks, which can be used depending on the requirements of the particular application. The support should be an inexpensive material and the pores in the support should be much larger than those in the CMS layer. For example, the pores in the support should be at least twice as large as the pores in the CMS material. In a preferred embodiment of the present invention, the pores in the support are from 5-500,000 times as large as the pores in the CMS material. In the most preferred embodiment of the present invention, the pores in the support are from 10 to 2,000 times as large as the pores in the CMS material.
Although the actual size of the pores in the various support materials can be widely varied, the nominal diameter of the pores in the support material should be greater than 100 Å (e.g., typical pore sizes in the support material are from 0.1 to 100 &mgr;m in diameter). The size of
Acharya Madhav
Foley Henry C.
Raich Brenda A.
Strano Michael
Connolly Bove & Lodge & Hutz LLP
Spitzer Robert H.
University of Delaware
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