McCabe-Thiele diagram varying reflux

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. 

A batch column with plates above a still pot functions as a rectifier, which can provide a sharper separation than differential distillation. If the reflux ratio is fixed, distillate and still bottoms compositions will vary with time. Equation (9-2) applies with yo = Xo- Its use is facilitated with the McCabe-Thiele diagram as described by Smoker and Rose. ... 

The batch distillation column can also be operated with variable reflux ratio to keep x constant. The operating Eq. 19-261 is still valid. Now the slope will vary, but the intersection with the y=x line will be constant at Xp,. The McCabe-Thiele diagram for this case is shown in Figure 9-7. This diagram relates Xs to Xp). Since Xp, is kept constant, the calculation procedure is somewhat different. 

Scheme J. This scheme directly adjusts the column material balance by manipulation of the distillate flow. The main advantage of this scheme is that it has the least interaction with the eneigy balance. In terms of a McCabe-Thiele diagram, this means that the slopes of the column operating lines can be held constant in spite of energy balance upsets. This independence ftom energy balance upsets is achieved by the scheme s ability to maintain a constant internal reflux even for variations in external reflux subcooling. When the temperature of the external reflux varies, the external reflux adjustment to maintain accumulator level offsets temporary internal reflux variations. If the accumulator level loop responds rapidly, the dis-tuibanoe will not propagate down the column, and the column s overall material balance remains undisturbed.

Note The varying value of and matching reflux ratios have to be read off the McCabe-Thiele diagram. 

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