Skarstrom cycle

FIGURE 11 Sequence of steps in a two-bed pressure swing adsorption system, (a) Skarstrom cycle, (b) modified cycle for production of nitrogen using a carbon molecular sieve adsorbent. 

We started out to analyze the PSR operated according to the classical four step Skarstrom cycle cis depicted in figure 1 for various cycle times. The selectivity decreases compared to the PFR. The highest conversion is reached in the steady state limit f oo). In... 

Figure 1. Single bed version of the Skarstrom cycle. Cycle steps I pressurization, II adsorption, III depressurization, IV purge, S stirred storage tank. The PSR is filled with a mixture of catalyst (o) and adsorbent ( ).

Figure 2. Selectivity towards B versus conversion of -A. in the PSR operated under the Skarstrom cycle. Line PFR, A PSR, Parameters table 2, Upurge= 5 and from left to right t /=t/v= 0, 5, 10, 30, 60, 120, 240, 500, oos...

Figure 3. The trajectory of component B under the Skarstrom cycle in the gas phase. Parameters table 2, tu—tiv = 30 s, Upurge= 5 10 m s Legend + Start cycled end of A pressurization, o adsorption, O depressurization, purge.

Figure 11. Productivity of the PSR versus PFR. Legend Line PFR Markers PSR, 0 Skarstrom cycle (figure 2), Alternative cycle A series A, O series B, + series C, O series E from figure 5, series F, V series G from figure 6...

When a PSR unit is operated according to a Skarstrom cycle, the feed and product end of the reactor are periodically cleaned as they are purged with feed and high pressure product gas respectively. As a result the intermediate component is concentrated in the middle part of the reactor and its residence time is increased which in turn decreases the selectivity of the overall unit towards this product. 

Figure 18-11. Pressure swing adsorption (A) steps for single column in Skarstrom cycle, (B) use of two columns in parallel for continuous feed and product. Period of feed step = period of blowdown +...

Following the feed step at the higher pressure, the pressure in the column is reduced to the lower pressure by counterflow blowdown. At this reduced pressure the column is purged (counterflow to the feed) using part of the pure product gas. Since pure product gas is used as a purge, the purge product (or waste) gas contains both adsorbate and carrier gas. The volume of purge gas required for the Skarstrom cycle is. 

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. 

A 0.50 m. long column is used to remove methane (M) from hydrogen using Calgon Carbon PCB activated carbom The feed gas is 0.002 mole fraction methane. Superficial velocity is 0.0465 m/s during the feed step. The high pressure is 3.0 atm while the low pressure is 0.5 atm. A standard 2-column Skarstrom cycle is used. The symmetric cycle is ... 

All. Many variations of the basic Skarstrom cycle shown in Figure 18-11 have been developed. One variation is to include a partial co-flow blowdown step before the counterflow blowdown shown in Figure 18-11. What are the likely advantages and disadvantages of doing this ... 

Figure 3.1. The Skarstrom cycle. Two steps are involved in the cycle for each bed ...

The Skarstrom cycle has been widely used for air-drying. For oxygen production from air and other bulk separations, however, the product recovery was too low, and hence the energy requirement too high, to be economical. 

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 net result of incorporating the CD step in the Skarstrom cycle is the product purity enhancement of the strong adsorptive, which in turn increases the product recovery of the weak adsorptive. 

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. 

Analytical Solution for Skarstmnt Cycle. It is desirable to have a simple, analytical solution for PSA that can be used for estimation purposes, e.g., for sizing the beds and comparing different sorbents. As Knaebel quoted Chilton, The simpler things become in a piece of research or development, the closer one has come to the truth. (Ruthven et al., 1994). The following is the solution by Chan et al. (1981) which has proved to be quite usefiil. The PSA cycle is a simple two-step Skarstrom cycle with linear isotherms. 

Neither of these parameters has been tested. The main commercial kinetic separation is nitrogen production from air by using molecular sieve carbon. The cycle used is a simple Skarstrom cycle. This parameter may be tested for this system. 

Although the Guerin-Domine cycle proved to be more effective for the separation of air than the Skarstrom cycle, the former cycle was not economic. The main improvements over the basic two-bed cycle which have occurred in the last three decades are the introduction to the cycle of cocurrent depressurization and pressure equalization. Extension of the number of beds in series and the sequence of operational steps in the cycle have led to major process improvements. 

Incorporation of a cocurrent depressurization step immediately following the pressurization and feed steps into the basic Skarstrom cycle increases the... 

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