Gas separation: processes – Solid sorption – Including reduction of pressure
Reexamination Certificate
2000-11-09
2003-01-14
Spitzer, Robert H. (Department: 1724)
Gas separation: processes
Solid sorption
Including reduction of pressure
C095S130000, C095S902000, C096S113000, C096S114000, C096S130000, C096S144000
Reexamination Certificate
active
06506234
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to pressure swing adsorption (PSA) and vacuum pressure swing adsorption (VPSA) methods for gas separation, and more particularly to a method for air separation wherein the cost for the O
2
product is reduced by use of a process which employs low pressure ratios and uses adsorbents exhibiting high intrinsic diffusivity.
BACKGROUND OF THE INVENTION
Significant developments of the vacuum swing adsorption (VSA), PSA and VPSA methods for gas separation have taken place over the past thirty years, with major advances occurring during the last decade. Such processes have also been named subatmospheric, superatmospheric, and transatmospheric, respectively. Unless specifically otherwise noted, PSA will be used below to mean any or all of these processes. Commercalization of these processes can be attributed to improvements in the adsorbents, process cycles and advances in adsorber design.
Highly exchanged lithium molecular sieve adsorbents, as illustrated by Chao in U.S. Pat. No. 4,859,217, are representative of advanced adsorbents for O
2
production. Such advanced adsorbents are expensive and represent a significant portion of the capital cost of PSA equipment.
A dominant factor in the total energy requirement of PSA processes is the ratio of adsorption to desorption pressures. Lowering the pressure ratio is a potential method of reducing power consumption. Furthermore, a reduction in PSA cycle time has the potential to reduce the amount of adsorbent required. Unfortunately, the usual consequence of both of these strategies is a reduced product (e.g., O
2
) recovery. Attempts to operate at lower pressure ratios have been accompanied by substantial decreases in adsorbent productivity, e.g. Leavitt, U.S. Pat. No. 5,074,892.
Smolarek, in copending U.S. patent application Ser. No. 08/964,2293, now U.S. Pat. No. 6,010,555, overcomes some of the offsetting effects of low pressure ratio through appropriate selection and operation of vacuum and compression machinery, combined with the improved flow characteristics of radial flow adsorbers. Ackley, et al. in copending U.S. patent application Ser. No. 09/622,961, have taught the maximizing of product recovery and adsorbent productivity through the use of high intrinsic diffusivity absorbents in fast cycles.
While the invention to be described below is applicable to a wide range of gas separations, PSA air separation processes aimed at the production of high purity O
2
(approximately 88% to 95.7% O
2
) are of particular interest. Air separation prior art discussed below reflect this O
2
purity range.
Advanced adsorbents of the types mentioned above are the result of improvements in equilibrium properties. Improved N
2
working capacity and N
2
/O
2
selectivity of adsorbents have been transformed into large gains in process efficiency—such benefits being obtained at the expense of higher adsorbent cost (Smolarek, et al.,
Gas Separation Technology
, 1990). Lithium-exchanged zeolite adsorbents (LiX), in particular, have had a major impact upon the evolution of PSA air separation processes. The higher N
2
capacity and higher N
2
/O
2
selectivity resulting from highly-exchanged LiX zeolites of low SiO
2
/Al
2
O
3
ratio have been more recently exploited for higher performance in air separation PSA processes. Other major improvements to these processes have been the introduction of vacuum for desorption, reduction from three-bed and four-bed processes to two bed cycles, and the use of modified cycle steps such as product pressurization, purge and/or equalization. These and other prior art advances in oxygen production have been summarized by Kumar (“Vacuum Swing Adsorption Process for Oxygen Production—A Historical Perspective,” Sep. Sci. Technology, 31: 877-893, 1996).
Improving process efficiency and reducing the cost of the light component product can be accomplished by decreasing the amount of adsorbent required per unit of product (increasing adsorbent productivity) and by increasing the product recovery. The former is generally expressed in terms of bed size factor (BSF) in lbs adsorbent/TPDO (ton per day of contained O
2
), while the latter is simply the fraction of light component in the feed that is captured as product. Improvement in adsorbents and reduction in cycle time are two primary methods of reducing BSF.
Considerable prior art attention has been focused upon process optimization. Reiss (Chem. Ind. XXXV, p689, 1983) emphasizes the importance of adsorbent qualities and high O
2
product recovery upon energy consumption in vacuum processes (VSA) for the oxygen enrichment of air. Reiss has shown that there is a minimum in the specific vacuum pump power characteristic as the desorption pressure is increased for a fixed adsorption pressure. More specifically, power per unit of O
2
produced initially decreases with decreasing pressure ratio and then increases such that there is an optimum pressure ratio for minimum specific power consumption. Concurrent to the effect of decreasing pressure ratio upon specific pump power, is the uniformly decreasing amount of O
2
product or a decrease in the adsorbent productivity.
Smolarek, et al. (
Gas Separation Technology
, 1990) achieve the objective of lowering unit O
2
product cost by reducing both capital cost and power consumption. Process operating parameters were developed around advanced adsorbent characteristics. Bed size was reduced by using shorter cycles, although the optimum cycle time was selected on the basis of the minimum cost. This minimum cost was established as a compromise between decreasing bed size and decreasing process efficiency as cycle time was shortened. It was also shown that two adsorbent beds was optimum. Lower overall power consumption resulted from the combination of increased O
2
product recovery, reduced adsorbent inventory and reduced equipment size. A reduction in the optimum pressure ratio was attributed to the advanced adsorbent. The type of adsorbent (LiX), the BSF (1000 lb/TPDO), and the pressure ratio (6:1), were not originally provided in the publication.
In the prior art, process optimization takes advantage of improved equilibrium adsorbent properties of higher working N
2
capacity and higher N
2
/O
2
selectivity to achieve higher overall product recovery in processes utilizing vacuum desorption. Desorption pressure was increased (pressure ratio decreased) to reduce power consumption. Cycle time was decreased to keep bed size and adsorbent cost in check. Achieving minimum pressure ratio was not a primary objective of the optimization. Indeed, the reduction in pressure ratio was limited by the accompanying increase in bed size and the reduction in product recovery. The lowest pressure ratios corresponding to “optimum performance” achieved in these prior art were 5:1 or higher.
Much of the prior art attends to the incremental improvement in process efficiency through cycle step modification. A good example of such improvements is given by Baksh et al. (U.S. Pat. No. 5,518,526).
The potential benefits of low pressure ratios in achieving lower power consumption have generally been limited, due to the offsetting effects of higher BSF and lower product recovery. Although adsorbents with improved equilibrium properties allow process improvement at lower pressure ratios, reductions below a critical or limiting pressure ratio have a more severe impact upon processes incorporating advanced, high cost adsorbents. In other words, the increased adsorbent inventory accompanying lower pressure ratio has a significant impact upon the capital investment of the plant. Such critical or limiting pressure ratios were defined theoretically by Kayser and Knaebel (Chem. Eng. Sci. 41,2931, 1986; Chem. Eng. Sci. 44,1, 1989) for
5
A and
13
X adsorbents. The O
2
recovery/pressure ratio characteristics are relatively flat at higher pressure ratios (nearly constant recovery), but show a steep decline in recovery below the critical pressure ratio. The critical pressure ratio depends upon the adsorbent type and upon process ope
Ackley Mark William
Leavitt Frederick Wells
Smolarek James
Follett Robert J.
Praxair Technology Inc.
Spitzer Robert H.
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