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These values are plotted on Fig. 5.5g. Using Simpson's rule:

= 15 moles vapor

Having determined that three theoretical stages are required for the separation of P from R, attention can be redirected to the problem of C. As has been stated, there is no assurance that C will be present in any still charge, but any C which is present must be effectively removed (to at least fairly low levels).

Considering that the freezing point of commercial-grade R is 109.6°C and those of pure R and C are 110.7 and 105.0°C, the composition of a mixture of pure R and pure C which has a freezing point of 109.6°C can be calculated by assuming a linear relationship. Let i = moles C and 1 - x = moles R

Therefore, any C present must be removed down to a mole fraction of 0.193 if the linear assumption is appropriate. To be conservative, it is assumed that a residual mole fraction of 0.100 is an acceptable concentration of C. The relative volatility of C to R can be calculated from vapor pressure values.