Absorption equilibrium curve

Figure 8-62. Absorption equilibrium curve. Used by permission, Hutchison, A. J. L., Petroleum Refiner, V. 29 (1950), p. 100, Gulf Pub. Co., all rights reserved.

Determination of the number of theoretical stages for desorption using countercurrent solvent flow stripping, by means of a stripping gas is analogous. In this case, the fact that Xg is the entry concentration and Xg the exit concentration of the solvent to the stripper must be considered. According to the operating conditions, the course of the desorption equilibrium curve is different from that for the absorption equilibrium curve. The balance line is now below the equilibrium curve and so the reciprocal value of the absorption coefficient is required. 

Fig. 21. Single absorption equilibrium-stage diagram where the equiUbrium curve is for 8% SO2, 12.9% the diagonal lines represent the adiabatic temperature rise of the process gas within each converter pass the horizontal lines represent gas cooling between passes, where no appreciable conversion...

For dilute systems in countercurrent absorption towers in which the equilibrium curve is a straight line (i.e., yj = mXi) the differential relation of Eq. (14-60) is formulated as... 

Figure 2. Operating and equilibrium curves showing the total S02 conversion for a 4-bed S02 converter (3+1 layout) with intermediate absorption of S03 downstream bed 3 and conventional catalyst in bed 4. The feed gas contains 11% S02 and 10% 02.

In some absorption processes, especially where a chemical reaction occurs, there is a liberation of heat. This generally gives rise to an increase in the temperature of the liquid, with the result that the position of the equilibrium curve is adversely affected. 

If the equilibrium curve can be represented by the relation ye = mi, then the number of plates required for a given degree of absorption can conveniently be found by a method due to Kremser(56) and Souders and Brown(57). The same treatment is applicable for concentrated solutions provided concentrations are expressed as mole ratios, and if the equilibrium curve can be represented approximately by Ye = mX. 

The curve in Figure 21 represents S02 equilibrium conversions vs temperature for the initial S02 and 02 gas concentrations. Each initial S02 gas concentration has its own characteristic equilibrium curve. For a given gas composition, the adiabatic temperature rise lines can approach the equilibrium curve but never cross it. The equilibrium curve limits conversion in a single absorption plant to slightly over 98% using a conventional catalyst. The double absorption process removes this limitation by removing the S03 from the gas stream, thereby altering the equilibrium curve. 

Fig. 24. Relationship between partial pressure of oxygen and oxygen absorption rate by poly(L-lysine)-heme complex (a) and equilibrium curve for CO binding by poly(L-lysine)-heme complex (b)107,108)...

Figure 14-8 illustrates the graphical method for a three theoretical stage system. Note that in gas absorption the operating line is above the equilibrium curve, whereas in distillation this does not happen. In gas stripping, the operating line will be below the equilibrium curve. 

Absorption factor Ratio of liquid to gas flow rate divided by the slope of the equilibrium curve. 

This absorption factor is the ratio of slope of the operating line to that of the equilibrium curve. When the absorption factor is lower than unity, the pinch is located near the bottom of the column (Fig. 11a) when it is higher than unity, the pinch is located near the top of the column (Fig. lib). 

For a dilute gas, and when the equilibrium curve can be approximated by a linear relationship passing through the origin, Eq. (25) is applicable, and an average absorption factor A can be applied to describe the contactor. Under these conditions, an analytical solution of the material balance equation and the equilibrium relationship is possible, giving the Kremser equation ... 

PACKED HEIGHT FOR ABSORPTION WITH CURVED EQUILIBRIUM AND OPERATING LINES 11.16... 

A representation of the various concentrations and driving forces in a y—x diagram is shown in Figure 4. The point representing the interfacial concentrations 0, x must he on the equilibrium curve since these concentrations are at equilibrium. The point representing the bulk concentrations (yA,. A) may be anywhere above the equQibrium line for absorption or below it for desorption. The slope of the tie line connecting the two points is given by equations 4 and 5 ... 

Example 9.12 A packed absorption tower is designed to removed SO2 from a coke oven stack. The stack gas flow rate measured at one atmosphere and 30°C is lOmVs, and the SO2 content is 3.0%. Using an initially pure water, 90% removal is desired. The equilibrium curve of SO2 in water may be approximated by y = 30ji . Determine the water requiranent if 150% of the minimum flow rate is deemed adequate. Calculate the height of the tower. Assume KyfU =0.024 kgmols/m s mol fraction. Assume the total cross-sectional area of tower equals 11.0 m. ... 

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