Gas: heating and illuminating – Processes – Fuel mixtures
Reexamination Certificate
1999-12-23
2003-07-08
Johnson, Jerry D. (Department: 1764)
Gas: heating and illuminating
Processes
Fuel mixtures
C048S128000, C048S198200, C048S198300, C048SDIG005, C423S650000, C423S652000, C423S655000
Reexamination Certificate
active
06589303
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to production of high-purity hydrogen by a combination of membrane gas separation, steam reforming and pressure swing adsorption.
BACKGROUND OF THE INVENTION
A number of off-gas streams containing hydrogen and hydrocarbons are generated during refinery and petrochemical plant operations. These streams include overheads from phase separators; fractionation columns; stabilization columns; demethanizers; debutanizers; absorption, stripping and scrubbing units; and so on. In some cases, the composition of the stream renders it suitable for reintroduction into the train of operations upstream or downstream of its generation point. Frequently, however, the stream composition is such that it is not cost-effective to treat it further and it is passed to the plant fuel header.
Streams passed to the fuel header represent lost products, both hydrogen and hydrocarbons; in addition, only a finite quantity of fuel gas is needed, so plants can become bottlenecked by over supply of fuel gas. Meanwhile, most refineries operate with a hydrogen deficit, and the demands of the refining and chemical industries for high-purity hydrogen continue to grow year by year. Improved processes for hydrogen manufacture and/or recovery from such light hydrocarbon/hydrogen off-gases would clearly be useful to industry.
For hydrogen separation from light hydrocarbons, techniques that have been employed in refineries and petrochemical plants include pressure swing adsorption (PSA) and membrane separation. Representative references that teach the use of PSA to treat off-gases from petrochemical processes include U.S. Pat. Nos. 5,332,492 and 5,457,256, to UOP, and U.S. Pat. No. 5,675,052, to BOC. Representative references describing membrane separation processes include U.S. Pat. Nos. 4,362,613 and 4,367,135, to Monsanto, U.S. Pat. No. 4,548,619, to UOP, U.S. Pat. No. 5,053,067, to L'Air Liquide, U.S. Pat. No. 5,082,481, to Lummus Crest, U.S. Pat. No. 5,157,200, to Institut Francais du Petrole, and U.S. Pat. No. 5,689,032, to Krause/Pasadyn. Other references that describe membrane-based separation of hydrogen from gas streams in a general way include U.S. Pat. No. 4,654,063, to Air Products, and U.S. Pat. No. 4,892,564, to Cooley.
The use of polymeric membranes to treat off-gas streams in refineries is also described in the following papers: “Hydrogen Purification with Cellulose Acetate Membranes”, by H. Yamashiro et al., presented at the Europe-Japan Congress on Membranes and Membrane Processes, June 1984; “Prisms™ Separators Optimize Hydrocracker Hydrogen”, by W. A. Bollinger et al., presented at the AIChE 1983 Summer National Meeting, August 1983; “Plant Uses Membrane Separation”, by H. Yamashiro et al., in Hydrocarbon Processing, February 1985; and “Optimizing Hydrocracker Hydrogen”, by W. A. Bollinger et al., in Chemical Engineering Progress, May 1984. These papers describe system designs using cellulose acetate or similar membranes that permeate hydrogen and reject hydrocarbons. The use of membranes in refinery separations is also mentioned in “Hydrogen Technologies to Meet Refiners' Future Needs”, by J. M. Abrardo et al. in Hydrocarbon Processing, February 1995. A chapter in “Polymeric Gas Separation Membranes”, D. R. Paul et al. (Eds.) entitled “Commercial and Practical Aspects of Gas Separation Membranes”, by Jay Henis describes various membrane-based hydrogen separations.
Besides individual treatment by PSA or membranes, numerous processes are known in which membrane separation and PSA are combined in a complementary way to carry out an integrated process. These include the following U.S. Pat. Nos. 4,229,188; 4,238,204; 4,398,926; 4,690,695; 4,701,187; 4,783,203; 4,836,833; 4,863,492, and 5,411,721.
In all of the above-cited references, the membranes used to perform the hydrogen/hydrocarbon separation are hydrogen-selective, that is, they permeate hydrogen preferentially over hydrocarbons and all other gases in the mix. A difficulty that hampers the use of both PSA systems and membrane separation systems of this type is the presence in off-gases of the C
5
and heavier hydrocarbons, water vapor and hydrogen sulfide. In the case of membrane systems, the presence of these materials can cause catastrophic collapse of the membranes, as discussed in detail in co-owned U.S. Pat. No. 6,011,192, entitled “Membrane-Based Conditioning for Adsorption System Feed Gas”, which is incorporated herein by reference in its entirety. In the case of PSA systems, the C
5+
hydrocarbons and other contaminants may sorb preferentially onto the bed, both reducing the capacity of the beds to sorb the light hydrocarbons that they are intended to remove, and giving rise to serious regeneration difficulties.
Instead of using hydrogen-selective membranes, it is possible to carry out membrane separations in which hydrocarbons permeate selectively and hydrogen is rejected in the residue stream. Processes that rely on selective permeation of hydrocarbons to separate at least some hydrocarbons from at least some other less condensable gases are taught, for example, in U.S. Pat. Nos. 4,857,078; 4,963,165; 5,032,148; 5,089,033; 5,199,962, 5,281,255; 5,401,300; 5,407,466; 5,407,467; and 5,501,722, all to Membrane Technology and Research (MTR). Literature from Membrane Associates Ltd., of Reading, England, shows and describes a design for pooling and downstream treating various refinery off-gases, including passing of the membrane permeate stream to subsequent treatment for LPG recovery. A report by Membrane Technology and Research, Inc. to the U.S. Department of Energy entitled “Low Cost Hydrogen/Novel Membrane Technology for Hydrogen Separation from Synthesis Gas” (October 1990) lists permeation data for polyamide copolymer membranes and shows diagrams indicating potential positions for membrane separation units in a coal gasifier train.
U.S. Pat. No. 4,857,078, to Watler/MTR, mentions that, in natural gas liquids recovery, streams that are enriched in hydrogen can be produced as retentate by a rubbery membrane.
An alternative approach, also using membranes that reject hydrogen and preferentially permeate hydrocarbons, is to use not a polymeric membrane but a carbon membrane, such as those taught in U.S. Pat. No. 5,104,425, to Air Products and Chemicals. These membranes are made up of a microporous adsorbent material on a porous substrate, and can separate gas mixtures based on selective adsorption onto the pore walls, rather than by the solution/diffusion mechanism of conventional polymeric membranes. Thus, the mechanism of separation is akin to the separation mechanism in PSA. This allows separation between various hydrocarbon fractions to be made, and hydrogen tends to be retained in the membrane residue stream.
It is known to combine these membranes with PSA to carry out integrated separations of light hydrocarbons from hydrogen. U.S. Pat. No. 5,332,424 describes fractionation of a gas stream containing C
1
-C
4
hydrocarbons and hydrogen using a bank of membrane modules followed by a PSA unit. Other Air Products patents that show processes involving separation by carbon adsorbent membranes followed by PSA include U.S. Pat. No. 5,507,856 and 5,753,011. U.S. Pat. No. 5,435,836 teaches PSA followed by adsorbent carbon membranes for a similar separation. U.S. Pat. No. 5,634,354 teaches combinations of adsorbent membranes and PSA to treat gases containing hydrogen and olefins.
Adsorbent membrane systems similar to those disclosed in the above patents are described in papers by M. B. Rao and S. Sirkar in Journal of Membrane Science (Vol. 85, 253-264 (1993)) and Gas Separation and Purification (Vol.7, No.4, 279-284 (1993)). Adsorbent membrane/PSA hybrid systems are described in some detail in reports by M. Anand and K. A. Ludwig to the U.S. Department of Energy (“Novel Selective Surface Flow Membranes for the Recovery of Hydrogen from Waste Gas Streams”, Phase I (1995) and Phase II (1996) Final Reports under contract number DE-FC
04-93
AL94461), and in materials distributed at a U.
Baker Richard W.
Lokhandwala Kaaeid A.
Farrant J.
Johnson Jerry D.
Membrane Technology and Research, Inc.
Ridley Basia
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