Once the feed and effluent specifications are established, the design engineer can calculate the minimum solvent rate. The minimum rate is the rate below which separation is impossible even with a column of infinite height. The engineer calculates flowrates as illustrated in Figure 5.20.2. The mole fraction of the pollutant in the solvent (XT) and effluent gas (YT) at the top of the column as well as that in the feed gas (Yb) are known. A straight line is drawn from (Xt,Yt) to intercept the equilibrium line at YB. The slope of this line represents the ratio of the minimum solvent rate to the feed gas rate.
The normal operating solvent rate should be 30 to 70% above the minimum rate.
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