M

Vb/olbb

OLdbXd 2,ax

Vc/olcb

Vb/olbb

Vc/olcb

90 100 110 120 Temperature, deg.C. Fig. 9-7.

where all the relative volatilities are with respect to the B component; and in the case of the yB equation, a relative volatility does not appear with xB) since a** is 1. Given the liquid composition on any plate, the values of x are multiplied by the a corresponding to the component in question, and the values of ax are totaled to give 2ax, then the value of y for any component is calculated by dividing ax for the component by Sax. In general, it is desirable to take the volatilities relative to one of the key components; this will cause a to be greater than 1 for the components that are lighter and less than 1 for the heavier components. This method will be most clearly brought out by its application to actual problems. First it will be applied to the benzene, toluene, and xylene problem previously solved. Figure 9-7 shows the volatilities relative to toluene plotted as a function of temperature and also shows the K for toluene as a function of the temperature. It will be noticed that the variation in the relative volatilities with temperature is very small and that for xylene in the lower part of the column a constant value of a equal to 0.43 is well within the design accuracy. The benzene volatility relative to toluene varies more, but even here the variation is small. In the previous example, starting at the still:

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