Gas separation: processes – Selective diffusion of gases – Selective diffusion of gases through substantially solid...
Reexamination Certificate
2000-07-27
2002-04-09
Spitzer, Robert H. (Department: 1724)
Gas separation: processes
Selective diffusion of gases
Selective diffusion of gases through substantially solid...
C095S051000, C095S052000, C096S013000, C096S014000, C055S524000, C055SDIG005
Reexamination Certificate
active
06368382
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to semipermeable asymmetric membranes useful in selective separations of gases and liquids
BACKGROUND OF THE INVENTION
Gas permeation may be defined as a physical phenomenon in which certain components selectively pass through a substance such as a membrane. Basically, a gas permeation process involves introducing a gas into one side of a module which is separated into two compartments by a permeable membrane. The gas stream flows along the surface of the membrane and the more permeable components of the gas pass through the membrane barrier at a higher rate than those components of lower permeability. After contacting the membrane, the depleted feed gas residue stream is removed from contact with the membrane via a suitable outlet on the feed compartment side of the vessel. The other side of the membrane, the permeate side, is provided with a suitable outlet through which the permeated gaseous components can be removed from contact with the membrane. The purpose of a membrane in a gas permeation process is to act as a selective barrier, that is, to permit passage of some but not all components of a gaseous feed stream. Generally, in gaseous membrane separation processes, the separation is due to molecular interaction between gaseous components of the feed stream and the membrane. Because different components interact differently with the membrane, the transmission rates (permeation fluxes) are different for each component. Hence, separation of different components can be effected. U.S. Pat. No. 4,130,403, which is hereby incorporated by reference, discloses the use of cellulosic membranes in processes for the separation and removal of acid components from hydrocarbon gases.
Semipermeable asymmetric cellulosic “skinned” separation membranes formed by phase inversion and solvent exchange methods are known (see U.S. Pat. No. 3,133,132 which is hereby incorporated by reference). Such membranes are characterized by a thin, dense, selectively semipermeable surface “skin” and a less dense void-containing, non-selective support region, with pore sizes ranging from large in the support region to very small proximate to the “skin”. Such membranes have a serious shortcoming in that, in operation, fluxes decrease substantially over time. This decrease has been attributed to a collapse of some of the pores near the skinned surface of the membrane, resulting in an undue densification of the surface skin. One attempt at overcoming this problem has been the development of thin film composite or “TFC” membranes, comprising a thin selective skin deposited on a resilient porous support. See, for example, “Thin-Film Composite Membrane for Single-Stage Seawater Desalination by Reverse Osmosis” by R. L. Riley et al.,
Applied Polymer Symposium
No. 22, pages 255-267 (1973). While TFC membranes are less susceptible to flux decline than phase inversion-type membranes, fabrication of TFC membranes that are free from leaks is difficult, and fabrication requires multiple steps and so is generally more complex and costly.
Asymmetric membranes may be formed from other polymers such as polysulfone, polyethersulfone, polyamide, polyimide, polyetherimlide, cellulose nitrate, polyurethane, polycarbonate, polystyrene, etc. are also susceptible to flux decline and fabrication of asymmetric membranes from such other polymeric materials which are free of leaks is similar in difficulty and expense to producing TFC membranes.
One approach to overcoming the problem of leaks in asymmetric membranes has been the fabrication of an asymmetric gas separation membrane comprising a relatively porous and substantial void-containing selective “parent” membrane such as polysulfone or cellulose acetate that would have permselectivity were it not porous, wherein the parent membrane is coated with a material such as a polysiloxane or a silicone rubber in occluding contact with the porous parent membrane, the coating filling surface pores and other imperfections comprising voids (see U.S. Pat. No. 4,230,463). However, the coatings of such coated membranes, especially where the coatings are polysiloxane, are subject to degradation by solvents inherently present in the gaseous feed streams of common acid gas separations, and otherwise tend to leach out, thus permitting either flux decline or low selectivity that is more characteristic of the uncoated parent membrane. For example, U.S. Pat. No. 4,877,528 attempts to overcome the flux decline characteristic of coated cellulosic membranes by grafting or bonding an asymmetric cellulosic semipermeable membrane and a polysiloxane. U.S. Pat. No. 4,877,528 discloses that the grafting of the polysiloxane can be accomplished by either a polycondensation reaction whereby a polysiloxane containing one or more hydroxyl-reactive functional groups is reacted with the hydroxyl groups of the cellulose polymer of the cellulosic membrane, or by a polyaddition reaction whereby a polysiloxane containing one or more vinyl alkynyl, or free radical-forming groups is reacted with the cellulosic substrate. In both cases, the result is the bonding of the grafting of the polysiloxane to the cellulosic membrane by non-crosslinked, covalent bonds. Examples presented indicate that the grafting did reduce some of the loss of the siloxane content and loss in performance during operation, but such grafted membrane still exhibited more than a 50 percent loss in siloxane content and a 50 percent loss in performance.
These coatings are usually applied to substrates as dispersions in a solvent system in order to reduce the viscosity sufficiently so that the coating composition is easily coatable. Such prior art post-treatments may provide gas separation membranes which exhibit improved selectivity; however, unless the solvent is highly inert toward the membrane polymer, the treating solution may also cause or induce change or damage to the morphology of the membrane, which may cause loss of trans-membrane flux. The presence of the solvent, either water or some suitable low boiling organic solvent, necessitates evaporation. Thus, heat is typically applied to substrates coated with silicones for the removal of solvent and to thermally induced curing. This heating step may result in damage to the underlying substrate.
Fluorosilicones are generally well suited to use as conformal coatings because of their resistance to solvent swelling and degradation. Photo-curable silicones that are intended for low modulus cured coatings typically consist of linear silicone molecules that have photo-reactive centers widely separated by non-functional polysiloxane segments so that a low cross-link density results when the silicone is cured. Because they consist almost entirely of polydimethylsiloxane segments, such photo-curable silicone polymers are incompatible with onium ionic photo-catalysts. This incompatibility results in an inefficient and/or slow photo-cure. The fluorosilicone polymers suffer from these same drawbacks.
Therefore, a membrane post-treatment is needed which improves selectivity but does not change or damage the membrane, or cause the membrane to lose performance with time. In addition, gas separation membranes desirably have a high permeability to gases. This means that the effective portion of the membrane should be as thin as possible. Making the membrane as thin as possible necessarily leads to the membrane containing imperfections or defects. These defects pass gases indiscriminately thus reducing the selectivity of the membrane. In the past, these membrane defects have been sealed or reduced to improve gas separation membrane performance.
Other methods of improving membrane performance have relied on the chemical treatment of the membrane substrate with a swelling agent which are believed to improve the selectivity of a membrane.
What is needed, therefore, is an asymmetric membrane that can be inexpensively made by conventional single casting techniques, yet has a high selectivity and a stable flux rate which essentially do not decline in use. These needs and othe
Goldberg Mark
Molinaro Frank S.
Spitzer Robert H.
Tolomei John G.
UOP LLC
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