Skarstrom process

The age of pressure swing adsorption (PSA) began so far as air separation is concerned in the late 1950s. The first process patent was that of Skarstrom [12]. The scientists and engineers of the Linde Laboratories were very active in this field. It is worth mentioning that the discovery of synthetic zeolite molecular sieves was... 

Cyclic adsorption processes such as parametric pumping, cycling zone adsorption and heatless adsorption have received much attention both theoretically and experimentally in the past several years (3j. Skarstrom (4J has reviewed applications of heatless adsorption (synonymous with pressure swing adsorption) to air drying, hydrogen purification and air fractionation. 

The two-column process (Figure 2) is the heatless adsorption process of Skarstrom (5 ) This process can perform both recovery and purification of a light component Local equilibrium models of this process have been presented by Shendalman and Mitchell (6), Chan et al (7), and Knaebel and Hill (8) In each case the method of characteristics was used. Models with finite mass transfer rates have been published by Kawazoe and Kawai (9 ), Mitchell and Shendalman (10), Chihara and Suzuki (11), and Richter et al (12). In these models the method of characteristics and direct integration of partial differential equations were employed ... 

Figure 13 is a schematic flow diagram for a two column Skarstrom type [36] PSA drier. The process can be used to obtain very dry product gas (< —60 C dew point). The product purity depends on the type of alumina used, feed gas pressure and dry purge gas quantity. Typically a practical PSA drier uses 15-30% of product gas as purge. The typical total cycle time for a PSA process is 2-4 minutes. 

C.W. Skarstrom and J. Kertzman, Process for separating fluids and apparatus, USPat 3,735,558. 

The simple Skarstrom cycle for PSA shown in Figure 18-llA has constant pressure (isobaric) periods and periods when pressure is changing. We will assume that a very dilute gas stream containing trace amounts of adsorbate A in an weakly adsorbed carrier gas is being processed and that over the concentration range of interest the linear isotherm, Eq. fl8-5bl. is accurate. If mass transfer is very rapid, then the solute movement theory can be applied. Since the system is very dilute, the gas velocity is constant and the system is assumed to be isothermal. In more concentrated PSA systems neither of these assunptions are true, and a more conplicated theory must be used fRuthvenetal.. 19941. 

Co-Current Depressurization. The first major process improvement after the invention of the Skarstrom and Guerin-Domine cycles was the introduction of the co-current depressurization step. To incorporate this step into the Skarstrom cycle, the adsorption step is cut short well before the breakthrough point, that is, the concentration front is far from reaching the outlet of the bed. The adsorption step is immediately followed by co-current depressurization before the bed is desorbed by further blowdown and purge, as required in the Skarstrom cycle. 

The idea of pressure equalization was first suggested in a patent granted to Marsh et al. in 1964 (Marsh et al., 1964). The process described in the patent required an empty tank in addition to the two beds used in the Skarstrom cycle. The tank was used to store a portion of the compressed gas from a saturated bed, and the gas was used to later purge the same bed. The primary objective was to recover the components contained in the compressed gas. The pressure equalization step, as it is currently used in commercial processes, was disclosed in the patents to Berlin (1966) and Wagner (1969). Four- to five-bed arrangements were given by Wagner. No empty tanks are required in this process. 

Consider the two columns or two beds used in Skarstrom s scheme for air drying. A simple PSA cycle for this process involves four steps, shown schematically in Figure 7.1.13(a), where two beds, bed 1 and bed 2, are identified... 

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