Distillation with Reflux and McCabe-Thiele Method

4 DISTILLATION WITH REFLUX AND McCABE-THIELE METHOD 

For the countercurrent contact with multiple stages in Fig. 10.3-2, the material-balance or operating-line equation (10.3-13) was derived which relates the concentrations of the vapor and liquid streams passing each other in each stage. In a distillation column the stages (referred to as sieve plates or trays) in a distillation tower are arranged vertically, as shown schematically in Fig. 11.4-1. 

The feed enters the column in Fig. 11.4-1 somewhere in the middle of the column. If the feed is liquid, it flows down to a sieve tray or stage. Vapor enters the tray and bubbles 

4 Distillation With Reflux and McCabe-Thiele Method 

The main assumption made in the McCabe-Thiele method is that there must be equimolar overflow through the tower between the feed inlet and the top tray and the feed inlet and bottom tray. This can be shown in Fig. 11.4-2, where liquid and vapor streams enter a tray, are equilibrated, and leave. A total material balance gives 

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Sec. 11.4 Distillation With Reflux and McCabe-Thiele Method... 

FIGURE 11.4-17. Processflow for a rectification tower with a liquid side stream. Sec. 11.4 Distillation With Reflux and McCabe-Thiele Method... 

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. 

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...

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. 

A modified McCabe-Thiele method employed in extractive distillation has been described by Nagel and Sinn [78]. Kortiim and Faltusz [79] have dealt with a variety of problems involved in selective separating processes ranging from the design of an automatic apparatus of special steel for continuous operation to the calculation of the minimum reflux ratio and the required amount of additive. 

The operation of a binary distillation has two important limiting cases minimum and maximum stages for a given separation. Minimum stages occur when the column is operated with total reflux (i.e., no top product is withdrawn). This situation, when applied to the McCabe-Thiele method, uses the y = x line as the mass balance for both rectifying and stripping. The situation is depicted in Figure 12-11. 

The elegance and simplicity of the McCabe-Thiele method make it tempting for the beginning student to view it as the centerpiece of binary distillation column design. There are, in fact, a number of key questions that need to be addressed before proceeding with plate calculations, setting reflux rations, and the like. 

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. 

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. 

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. ... 

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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