Gas separation: processes – Selective diffusion of gases – Selective diffusion of gases through substantially solid...
Reexamination Certificate
2000-03-29
2001-08-14
Spitzer, Robert H. (Department: 1724)
Gas separation: processes
Selective diffusion of gases
Selective diffusion of gases through substantially solid...
C095S050000, C210S640000
Reexamination Certificate
active
06273937
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for the separation of a multicomponent feed stream into a permeate stream rich in one or more components of the feed stream and a retentate stream lean in those same components.
BACKGROUND OF THE INVENTION
Pervaporation is a commercially-practiced membrane separation process in which a non-porous membrane is contacted with a multicomponent liquid feed, resulting in the selective absorption of one or more of the species from the feed. These sorbed species permeate across the membrane under the influence of a concentration gradient that is produced by evaporating the sorbed molecules from the product side of the membrane using a vacuum or sweep gas. Permeate vapor is then condensed and recovered as a liquid. Vapor permeation differs from pervaporation in that the feed is already in the vapor phase.
The concept of pervaporation has been recognized for a long time (Mitchell, J. V.;
J. Roy. Inst
., 2 (101), 1831, 307); however, only in the past 40 years has development of this technology as a process taken place. Binning and coworkers at American Oil Co. (Binning, R. C.; R. J. Lee; J. F. Jennings and E. C. Martin;
Ind. and Eng. Chem
., 53(1), 1961, 45-50; Binning, R. C et al., U.S. Pat. No. 2,970,106, January 1961) were the first to suggest, based on their experiments with organic liquid mixtures, that pervaporation had commercial potential. During the next two decades, however, most of the work on pervaporation focused on alcohol (ethanol)-water separation, since this demonstrated that pervaporation could achieve something not possible by ordinary distillation-breaking of the ethanol-water azeotrope. SETEC and GFT (now part of Le Carbone-Lorraine) in Germany commercialized pervaporation membrane systems based on composite polyvinyl alcohol membranes for this application in the 1980's as reported by Bruschke and coworkers (Bruschke, H. E. A.; G. F. Tusel and R. Rautenbach;
ACS Symposium Series
, 281, 1985, 467-478); and there are numerous commercial facilities around the world with capacities as large as 150,000 liters/day using this technology. The current invention relates to an improvement in both pervaporation systems and vapor permeation systems and specifically an improved method of maintaining a low partial pressure of permeate on the permeate side of the membrane. U.S. Pat. NO. 5,753,008, which is incorporated herein by reference, which discloses a vapor permeation process, teaches that there are three known ways used for maintaining a sufficiently low partial pressure of permeate on the downstream side of a membrane: (a) the vacuum method, (b) the dilution method and (c) the countercurrent sweep method. Most commercial pervaporation systems use the vacuum method. Small systems use vacuum pumps while larger systems, of necessity, use multi-stage steam ejectors; and the cost of operation with the latter becomes a significant part of the total package cost.
There are problems with both methods of maintaining vacuums. Vacuum pumps achieve excellent vacuums (<10 torr), but are expensive to maintain. Unfortunately, the better the vacuum, the harder it is to condense the permeate; and many commercial pervaporation systems have expensive refrigeration systems as part of their condensers. Furthermore, because it is impossible even at very low temperatures to totally condense the permeate (especially at <10 torr), vacuum pump oil may become contaminated with permeate, thus requiring more maintenance. Vacuum pumps cannot be used for large systems, because their displacements are generally too low.
Steam ejectors are simpler, but are limited in the level of vacuum they can achieve at reasonable steam flow rates. Three-stage ejectors are normally required for pervaporation systems, thus adding to their complexity. Furthermore, a small amount of permeate is mixed with the steam condensate; and, while for most organic systems, the water phase and the organic phase separate, there is always a finite concentration of permeate dissolved in the water condensate that still needs to be removed prior to discharge into natural waterways.
It would be desirable to have an inexpensive process for the selective pervaporative separation of either aqueous or organic feeds that is applicable over a wide range of feed rates. Such a process would represent a significant advance in the art.
It is therefore an object of the present invention to provide a highly efficient method and apparatus for pervaporation or vapor permeation that can be used in both small and large process applications.
It is a further object of the present invention to provide a highly efficient method and apparatus for treatment of either aqueous or organic feeds. It is still a further object of the present invention to provide a highly efficient method and apparatus that provides superior performance using different module designs (hollow fiber, spiral wound or stacked flat).
These and other objects, which will become apparent to one of ordinary skill, are summarized and described in detail below.
SUMMARY OF THE INVENTION
In accordance with the present invention there is provided a process for the separation of multicomponent feed streams into a permeate stream rich in one or more components of the feed and a retentate stream lean in those same components comprising: a) passing said feed stream to a permeation unit containing a membrane having a first surface and a second opposing surface; b) contacting said first surface with said feed stream wherein one or more components of said multicomponent feed stream selectively dissolves into said membrane at said first surface and permeates through said membrane from said first surface to said opposing second surface under the influence of a concentration gradient across said membrane, said concentration gradient being maintained by the use of a vacuum on said second opposing surface, which vacuum is produced by a fluid passing through a Venturi-type nozzle, said permeate being drawn from said second opposing surface into said Venturi-type nozzle and into a working fluid passing through said nozzle; c) separating said permeate from said working fluid.
In preferred embodiments of the present invention the feedstream is selected from those comprising liquids, vapors and permanent gases, such as inert gases.
In another preferred embodiment of the present invention the feedstream comprises a naphtha stream, more preferably a gasoline stream.
In yet another preferred embodiment of the present invention the feedstream comprises a mixed olefin/paraffin stream.
In still another preferred embodiment of the present invention the relative volatility of said permeate versus said working fluid is at least about 1.05, preferably at least about 1.2.
In other preferred embodiments of the present invention the working fluid is comprised of one or more solvents that have an affinity for the permeate molecules, preferably from those selected from the group consisting of propylene carbonate, ethylene carbonate, N-methyl pyrrolidone, tetramethylene sulfone, tetraethylene glycol, N-formyl morpholine, furfural, nitrobenzene, dipropylene glycol, glycerol, diethylene glycol, ethylene glycol and n-butyl-methyl immidazolium hexafluorophosphate.
In yet other preferred embodiments of the present invention the fluid leaving said Venturi-type nozzle is at a temperature range of about 0° C. to 150° C.
REFERENCES:
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patent: 4929357 (1990-05-01), Schucker
patent: 4929358 (1990-05-01), Koenitzer
patent: 4952751 (1990-08-01), Blume et al.
patent: 5030356 (1991-07-01), Blume et al.
patent: 5039417 (1991-08-01), Schucker
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patent: 5226932 (1993-07-01), Prasad
patent: 5290452 (1994-03-01), Schucker
patent: 5753008 (1998-05-01), Friesen et al.
Mitchell, J.V.; “J. Roy. Inst. 2”, (1010), 1831, 307.
Binning, R.C.; R.J. Le
Naylor Henry E.
Spitzer Robert H.
Trans Ionics Corporation
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