Gas separation: processes – Selective diffusion of gases – Selective diffusion of gases through substantially solid...
Reexamination Certificate
2000-12-05
2002-10-08
Spitzer, Robert H. (Department: 1724)
Gas separation: processes
Selective diffusion of gases
Selective diffusion of gases through substantially solid...
C095S053000, C095S056000, C096S008000, C096S010000, C096S011000, C055S524000
Reexamination Certificate
active
06461408
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an apparatus for the purification of a constituent gas and also to the generation of a constituent gas and the subsequent separation and purification from a mixed gas flow. More specifically, the present invention relates to the generation or purification of hydrogen from a mixture containing hydrogen. The apparatus utilizes one or more gas extraction membrane for removing hydrogen or other extractable gas from a mixed gas flow.
BACKGROUND OF THE INVENTION
A common technology for hydrogen generation involves the following processes in series: high temperature steam reforming at pressures between 150 and 300 psi, a high temperature water-gas shift reactor, a low temperature water-gas shift reactor, and a hydrogen purifier. The purifier is most typically a PSA (pressure swing absorption) system, but can also be a membrane or partial oxidation process. The water-gas shift reactors are employed to generate additional hydrogen from carbon monoxide produced in the high temperature reformer via the following (water-gas shift) reaction:
CO+H
2
O→CO
2
+H
2
.
Cooling is required because the water-gas shift reaction is exothermic and equilibrium limited. Thus, lowering the gas temperature promotes the reaction. There is a limit to the effectiveness of lower temperatures, since the reaction rate generally decreases with decreasing temperature. The high temperature shift reactor is thus designed to operate at an intermediate temperature, lower than that of the reformer but higher than that of the low temperature shift reactor that follows. The high temperature shift reactor generates hydrogen at relatively high catalyst turnover, but gas temperature in this reactor limits the amount of hydrogen that can be produced. Inter-cooling between the two shift reactors reduces this temperature so that additional hydrogen can be produced, albeit at a lower catalyst turnover. A single stage adiabatic reactor (membrane or otherwise) could be used with a sufficiently low feed temperature, but the catalyst effectiveness would suffer because of the low temperature at the entrance. Further, heat generated by the water-gas reaction would raise the temperature in the reactor, limiting the effectiveness of this approach.
A common technology for extracting pure hydrogen from industrial streams, such as for hydrogenation for changing the balance of hydrogen in those streams or to increase reaction selectivity, is to use membranes of palladium or palladium alloys alone or supported structurally by a matrix. Membranes which contain thick enough palladium layers to be made without holes and not break during service tend to be expensive and have relatively high resistance to hydrogen permeation. The membranes are disposed in a housing. A mixed gas flow is conducted to the housing wherein the extraction occurs. Extracted gas (such as hydrogen) is preferentially extracted through the membranes and exits through an outlet port. A second outlet allows for the exhaust of raffinate out of the chamber. Examples of such chambers are shown in U.S. Pat. Nos. 5,205,841 and 4,468,235.
Several membrane variations and module designs have been proposed to minimize this effect. Membranes can include porous ceramics either by themselves or coated with palladium alloys or with silica and palladium coated refractory metals and alloys, especially those based on Nb, V, Ta, Ti, Zr. These have greater strength than palladium and palladium-based alloys, are cheaper per unit volume, and most have greater intrinsic permeabilities to hydrogen. Although the alternatives are less expensive than Pd, they are not less expensive compared to polymers. Thus, with all of these membranes more attention must be directed to module designs that make efficient use of the membrane surface and provide a high recovery percentage without undue gas-phase mass transfer resistance. To date, no commercial module has been described that is particularly efficient for large scale hydrogen extraction using any of these membranes.
An example of an apparatus for hydrogen separation is disclosed in U.S. Pat. No.4,468,235 to Hill (Hill '235). The Hill apparatus for separating hydrogen from fluids and includes, mounted axially in a cylindrical pressure vessel, a plurality of membranes in the form of tubes coated on either the inside or the outside or both sides with coatings having a high permeability to hydrogen. There is also a fluid flow inlet and a raffinate flow outlet and a header to collect hydrogen. No sizes or criticalities are disclosed for the extraction membrane. Additionally, since this design provides no mechanism for flow distribution or turbulence generation, the separation efficiency of this apparatus is not maximized.
Another example of a similar apparatus for hydrogen extraction is disclosed in U.S. Pat. No.5,205,841 to Vaiman (Vaiman '841) issued Apr. 27, 1993. The Vaiman '841 patent discloses an apparatus for separating hydrogen from gas and gas liquid mixtures at low temperature. The Vaiman '841 apparatus includes a plurality of axially mounted tubes coated on both their inside and outside surfaces with palladium/platinum black. There is also a fluid flow inlet and a raffinate flow outlet and a header to collect hydrogen. Vaiman '841 does not teach any sizes or criticalities for the extraction membrane or its arrangement within the structure. Additionally, as similarly stated above regarding the Hill patent, the Vaiman '841 design provides no mechanism for flow distribution or turbulence generation. Separation efficiency of this apparatus is not maximized.
Another typical design for large hydrogen extractors uses tubular membranes of palladium-silver alloy in spiral form. This tubing generally has an outer diameter of 0.0625 to 0.125 inches and wall thickness of approximately 0.003 inches. For the smaller diameter tubes, the source hydrogen flows over the outside of several wound helixes made from 10 to 15 feet of tubing. These hydrogen extractors typically require complex expensive construction that limits heat and mass transport. Also, since pressure drops become excessive when the tube length exceeds about 25 feet, large modules end up with 40 or more nested and stacked helixes that must be hand assembled in a large tubular bundle without damaging any single one of the delicate tubes. This is a delicate construction process by any standard.
Large diameter tubes avoid maldistribution and assembly problems by driving all of the flow through a single tube. The practical limit is reached at about 100 feet. Longer lengths lead to destructive harmonic vibrations, especially during start-up and shut-down. Also, since module size increases with the square of the tube diameter, such units have had to be too big to site comfortably. Further, temperature uniformity is even harder to maintain than with {fraction (1/16)} inch units.
The spiral type designs are particularly difficult to form when dealing with coated refractory metals or with ceramics, as these materials are more brittle than palladium and coated membranes require more gentle handling than homogenous palladium alloys. The spiral type designs inherently have problems with scraping of the membrane surfaces and with kinking of the tubing material during manufacture thereby leading to inherent weaknesses in the tubes which are utilized under pressure. For large scale applications, these spiral-type hydrogen extractors tend to be larger in overall size than the module of the present invention thereby adding to the cost of the structure, sitting, shipping, maintenance, manufacture, and making them unpleasant to the eye.
To date, modules based on tubular ceramics or ceramic-based membranes known are based on a single pair of concentric tubes. The diameter of the ceramic membranes is approximately 0.375 inches. Such designs cannot be readily scaled up for commercial applications.
The present invention provides a hydrogen extraction module which eliminates the spiral-type extraction membranes and is much more
Gifford, Krass, Groh Sprinkle, Anderson & Citkowski, P.C.
Spitzer Robert H.
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