With McCabe-Thiele distillation method

Combine material-balances information from the McCabe-Thiele graphical method with interphase mass-transfer considerations to determine the packed height of the rectifying and stripping sections of a continuous-contact binary distillation tower. 

Calculation Methods. An often satisfactory approximation is to take the mixture in the presence of the solvent to be a pseudobinary of the keys on a solvent-free basis, and to employ the McCabe-Thiele or other binary distillation method to find tray and reflux demands. Since the relative volatility varies with concentration of the solvent, different equilibrium curves are used for above and below the feed based on average loads in those zones. Figure 13.25 is of such a construction. 

Until about 1930, most distillation calculations were based on Raoult s law in extreme cases, where ideal-mixture behavior was clearly incorrect, distillation calculations required experimental data coupled with graphical methods, as in the McCabe-Thiele diagram. 

Design a distillation column to separate benzene, toluene, and xylene, using (1) the McCabe-Thiele xy diagram and (2) the Fenske-Underwood-Gilliland (FUG) method. Compare the results with each other. Assume that the system is ideal. 

The feed, at a flow rate of 100 kmol/h, is sent as saturated vapor to the distillation column. The column is equipped with a partial condenser with a vapor product, and a reboiler. For a solvent rate of 500 kmol/h, it is required to determine the required number of equilibrium stages and the optimum feed location for a reflux ratio of 1.5 times the minimum. The McCabe-Thiele method may be used on a solvent-free basis. 

The lower part of the column is covered by stepping off stages in a fashion similar to that in the upper part of the column, and the final conni of theoretical stages is then determined. The Ponchon-Savarit method may be used for many situations more complex lhan the simple one just described mixed vapor-liquid distillate product, side draw streams, multiple feeds, and so on. Standard unit operations textbooks should be consulted for more dentils on this methnd. As mentioned, it suffers from a need for enthalpy-concentration data, but even a crude approximation based on linear variation of enthalpy with concentration can be better than the McCabe-Thiele approach if there is a very large difference in the latent heats of vaporization of the iwo components being distillnd. 

If a batch distillation is carried out at variable reflux ratio, it is possible u> maintain a constant purity of distillate and to do this until blocked by material balance limitations, A McCabe-Thiele diagram illustrating this method of operation is shown in Fig. 5.5-11. The value of xu is kept constant, and the slope of the operating line is varied in accordance with the overall material balance, The figure shows the conditions at time 0 and time t. Note that the three equilibrium stages include the stillpot. 

Calculate, by modified McCabe-Thiele methods, residue composition and distillation time for binary batch rectification with constant reflux for a given number of ideal stages, boil-up rate, and specified average distillate composition. 

Figure 16.2 McCabe-Thiele method for mixtures showing equilibrium curves with tangent pinch. Left Position of tangent pinch and critical distillate composition. Right Minimum reflux and feed composition A - feed pinch control, B - tangent pinch control...

If a constant distillate composition is required, this can be achieved by increasing the reflux ratio as the system is depleted in the more volatile material. Calculations are again made with the McCabe-Thiele diagram as described by Bogart and illustrated by the following example. Other methods of operating batch columns are described by Ellerbe. ... 

Total reflux corresponds to a situation where there is no feed, distillate, or bottoms and where the minimum number of stages is required to achieve a desired separation. It will be recalled that in the McCabe-Thiele method this situation corresponded to having the operating line coincide with the y = x line. 

In the calculation of the plate number for batch distillation Zuiderweg [158] takes into account the influence of the total hold-up and the magnitude of the transition fraction. Mixtures having ix-values from 1.07 to 2.42 were used to study the dependence of separation sharpness on relative total hold-up, reflux ratio and plate number. By means of the pole height S the optimum reflux ratio can be determined. The method corresponds to the determination after McCabe-Thiele with a final state. cs = 5 mol%. 

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