McCabe-Thiele diagrams optimum feed stage

The McCabe-Thiele diagram for this design, showing that the feed is to the optimum stage, is shown in Fig. 13-44. The flow profiles are shown in Fig. 13-45 note the step changes due to both the feed and the sidestream. As was the case in Example 3, the curvature in the flow profiles is due to enthalpy changes. 

Figure 4-8. McCabe-Thiele diagram for entire column (A) optimum feed stage (stage 3) (B) feed...

This problem was also run on the Aspen Plus process simulator (see Problem 4.G1 and chapter appendix). Aspen Plus does not assume CMO and with an appropriate vapor-liquid equilibrium (VLE) correlation (the nonrandom two-liquid model was used) should be more accurate than the McCabe-Thiele diagram, which assumes CMO. With 5 equilibrium stages and feed on stage 4 (the optimum location), = 0.9335 and Xg = 0.08365, which doesn t meet the specifications. With 6 equilibrium stages and feed on stage 5 (the optimum), Xq = 0.9646 and Xg = 0.0768, which is slightly better than the specifications. The differences in the McCabe-Thiele and process simulation results are due to the error involved in assuming CMO and, to a lesser extent, differences in equilibrium. 

Since the column shown in Figure 4-23A has four sections, there will be four operating lines. This is illustrated in the McCabe-Thiele diagram of Figure 4-23B. One would specify that the liquid be withdrawn at flow rate S at either a specified concentration Xg or a given stage locatiom The saturated vapor is at concentration yg = Xg. Thus there is a horizontal feed line at yg. If the optimum location for... 

D12. Solve Problem 3.E2 for the optimum feed location and the total number of stages. Assume CMO and use a McCabe-Thiele diagram Expand the portions of the diagram near the distillate and bottoms to be accurate. 

Report the number of stages and the optimum feed stages in each column, the reflux ratios, the tenperatures at the top and bottom of each column (in K), the heat duties in the reboilers and condensers (kJ/s), the conpositions (in mole fractions) and flow rates of the three products and of the interconnecting stream (kmol/h), the column diameters, and any other information you consider to be relevant. If you use DSTWU or the Fenske-Underwood-Gilliland approach or a McCabe-Thiele diagram for the second column, conpare these results with RADFRAC. 

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