Constructing Equilibrium Stages

Class 3 Here, one end of the operating line is fixed and the other end is bounded 

Class 4 This class of specifications fixes one end of the operating line and its slope. The other end is bonnded by one coordinate. The required number of stages is found by constructing steps starting at the fixed point and ending at the intersection of the specified coordinate with the operating line. If the point of intersection does not coincide with the completion of a step, the last step should extend past the intersection. 

In Classes 3 and 4, the unknown flow rate, L or V, is calculated from the slope L/V as determined from the graphical construction, and the specified flow rate V or L. 

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The system with A = heptane, B = tetramethylene sulfone, and C = toluene at 50°C [Triparthi, Ram, and Bhimeshwara, J. Chem. Eng. Data 20, 261 (1975)] The feed contains 40% C, the extract 70% C on a TMS-free basis or 60% overall, and raffinate 5% C. The construction shows that slightly more than two equilibrium stages are needed for this separation. The compositions of the streams are read off the diagram ... 

The methods developed for graphical construction of equilibrium stages on the Y-X... 

Applied to the bottom stage, j-1 = N,j = N+l,B = L, Y= = 0, and X = Xg. That is, the operating line intersects the X-axis at X = Xg. This intersection point represents passing streams below the column. With open steam, the operating line crosses the diagonal and ends at the X-axis. Equilibrium stage steps are constructed in the usual manner, between the operating line and the equilibrium curve. 

The operating line shown in Figure 8.5 is constructed such as to meet the absorption specifications with three equilibrium stages. This is accomplished by trial and error, with the lower end of the operating line fixed at X g = 0 and Y = 0.01, and its upper end moved along the Y = 0.03 line to a point where the number of stepped stages is three. The liquid to vapor ratio is calculated from the slope of the operating line ... 

FIGURE 11.4 Graphical construction of extractor equilibrium stages. 

The graphical construction of operating lines for the equilibrium stages proceeds in the usual manner, using difference point A for the upper column section and A" for the lower column section (Figure 11.6). The procedure is continued from both ends of the column until the external feed F point is reached on the diagram. This determines... 

This graphical construction is shown in Fig. 10.1-10. From this figure, we see as before that the desired separation can be achieved in a total of four equilibrium stages, one of which is the reboiler, and that the feed should be on the second stage from the top of the column. The temperature on each tray, the condenser, and the reboiler can now be determined from the T-.v-y diagram for this mixture using the liquid composition in each of these locations. 

An absorption column for laboratory use has been carefully constructed so that it has exactly 4 equilibrium stages, and is being used to measure equilibrium data. Water is used as the solvent to absorb ammonia from air. The system operates isothermally at 300 K and 1 atm. The inlet water is pure distilled water. The ratio LtV - 1.2, the inlet gas concentration is 0.01 mole fraction ammonia, and the measured outlet gas concentration is 0.0027 mole fraction ammonia. Assuming that Henry s law applies, calculate the slope of the equilibrium line. 

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