Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Composite having voids in a component
Reexamination Certificate
1999-03-04
2003-11-18
Morris, Terrel (Department: 1771)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Composite having voids in a component
C428S312600, C428S312800, C428S315500, C428S316600, C055S523000, C055S524000, C210S500250, C210S500260, C210S510100
Reexamination Certificate
active
06649255
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates to porous inorganic membranes having extremely small pore sizes and a method for producing the fine-pored porous inorganic membranes. These porous inorganic membranes are especially useful in processes for the separation of different size molecules in gases or liquids at high temperatures and in harsh chemical environments, such as are encountered in coal gasification processes and in the petrochemical industry. The United States Government has certain rights to this invention pursuant to Contract No. DE-AC05-84OR21400 with Lockheed Martin Energy Systems, Inc. awarded by the U.S. Department of Energy.
BACKGROUND OF THE INVENTION
Development work has been carried out at a number of international locations with respect to the problem of producing porous inorganic membranes, such as ceramic membranes, having extremely small pore sizes, i.e. pores having pore diameters of a few Angstroms. Such inorganic membranes are needed for use in the separation of gases at high temperatures and in harsh chemical environments, such as are encountered in coal gasification processes and in the petrochemical industry. In particular, the membrane pores must be sufficiently small to separate gas molecules on the basis of molecular size, in a process usually referred to as molecular sieving, in order to achieve high separation factors.
Various prior art techniques have been investigated for preparing inorganic membranes, which may be either porous or nonporous in physical makeup. The latter category is typified by palladium or silver foil metals. See
Ceramic Membranes for Gas Separation
, “Synthesis and Transport Properties” Robert Jan Reinier Uhlhorn, pp 3-5, November 1963. Our invention relates to the porous category of inorganic membranes and in particular to metal oxides, metal carbides, metal nitrides, and cermets.
Membranes may be generally classified by the size of the molecules or particles being separated and generally fall into four broad categories: reverse osmosis (average mean pore diameters—1 Å-10 Å), ultrafiltration (average mean pore diameters—10 Å-1000 Å), microfiltration (1000 Å-10,000 Å), and particle filtration (>10,000 Å). More recently, research has been performed on nanofilters which include the upper molecular weight range of the reverse osmosis domain and the lower molecular weight range of the ultrafiltration domain.
The porous inorganic membranes typically are composed of a porous support or carrier with a thin separation layer. Further, the porous inorganic membranes are housed in modules having various configurations, such as hollow-fibers, spiral wound and plate-and-frame or flat-sheet configurations. See
Emergina Separation and Separative Reaction Technologies for Process Waste Reduction
, Peter P. Radechi et al, pp. 17-18, Center for Waste Reduction Technoloies American Institute of Chemical Engineers, New York, N.Y., 1999.
The prior art for making porous inorganic membranes, which has a market value of in excess of $500 million, is obviously quite extensive. However, new development in the inorganic membrane field is expected to increase the value by a factor by at least 10 fold. Of the prior art methods for preparing inorganic membranes, the one that is the most extensively used is a process commonly called the “sol-gel” process, which has been used to prepare inorganic membranes.
The sol-gel process is basically the use of a colloidal suspension of various metal oxides or other ceramic materials to make ceramic articles, which are either porous or non-porous. Typical materials are alumina, silica, titania, zirconia, or mixtures thereof. The colloidal suspension is formed by various precipitation methods. In general, the colloidal particles are very small, e.g., 1000 Å to reported as small as 30 Å. When a sufficient amount of the liquid (mostly water) is removed, the colloidal suspension (or sol) becomes a gel. To make an article, the sol-gel is formed, further dried, calcined and sintered. Depending on the degree of sintering, the article can be porous to various degrees or can approach full density.
When used to make membranes, in most cases, a porous article is desired. The size of the pores in the membrane is determined by the size and uniformity of the particles. The pores are the interstices between the particles. The effective diameter of the pores is approximately one half the diameter of the particles. If one could make a suspension with 30 Å particles (and that is really difficult), about the smallest expected pore diameters would be 15 Å or larger.
The void fraction (or fraction of the membrane that is pores) of sol-gel membranes is about 50% more or less (but not much). For a membrane with such small pores to have any practical use, it must be very thin, i.e., a few microns or preferably less. Membranes are made by applying a thin layer of the sol to the surface of a porous support material. A simple way would be to pour the sol onto the surface and then allow most of it to drain off (or by other means) to remove most of it. Initially, the sol is pulled into the surface pores of the porous support by capillary action. The sol stays near the surface and the water is pulled into the interior by capillary action. This removes a large fraction of the water from the sol and causes it to gel.
An important factor in achieving thin membranes is to have the pore diameter of the support material to be less than 100 times the expected pore diameter of the membrane. This may require a porous support with one or more intermediate layers.
It is difficult to dry and calcine the membrane layer without having a significant number of cracks (defects) in the membrane layer. The smoother the surface of the support material the fewer the cracks. This problem is frequently solved by applying several layers of the sol so that cracks that do result will be covered by one or more of the layers. See
Emerging Separation and Separative Reaction Technologies for Process Waste Reduction
, above, for additional details of the sol-gel method for producing porous inorganic membranes.
While the class of inorganic membranes commonly called “zelolites” have been prepared with pore sizes in the few Angstrom range, these membranes have a fundamental different physical structure than the typical porous inorganic membrane, such as a metal oxide. The crystallographic structure of a zeolite defines the pore diameters in contrast to a ceramic membrane wherein the pores are the interstices between the particles. Thus, while the zeolites represent an interesting approach to ceramic membranes, the basic problem to use of the zeolites as membranes in industrial applications is that the zeolite particles have to be grown into a membrane; it is difficult to grow them thin enough without defects, which without a major breakthrough limits their commercial or industrial utility.
Currently, no porous inorganic membranes having sufficiently small pores are commercially available for molecular sieving types of gas separation applications.
There is a need to provide porous inorganic membranes that have mean diameter pore sizes on the order of several Angstroms, i.e., below about 20 Å for use in separating molecules based on their size. Also, there is a need to provide an efficient method for preparing extremely small-pored inorganic membranes and in particular to provide a method that lends itself to commercial scale operations.
One objective of this invention is to provide a porous inorganic membrane having a mean pore diameter about 20 Å or less.
Another objective is to provide a process for producing extremely fine-pored inorganic membranes that are suitable for a wide range of industrial uses, including recycle of hydrogen in petroleum refinery, higher yields in olefin production and improved efficiency in a large number of chemical separation processes.
Still a further object is to provide a method for controlling the reduction of the pore diameter of porous
Adcock Kenneth D.
Fain, Sr. Douglas E.
Marshall Bruce B.
Phillips Michael R.
Roettger George E.
Morris Terrel
Poteat Robert M.
Roché Leanna
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