Fractionating Still

Thus, even when starting with the relatively dilute concentration of 5 mole percent benzene, 60 percent of the mixture has to be boiled off in order to reduce the residual benzene concentration to 1 mole percent. Furthermore, the concentration of the recovered benzene is not a dramatic improvement over the starting mixture.

As shown by the two examples above, simple distillation produces only poor separations, with either high or low concentrations of the light component, even with large energy expenditures. This performance led to the development in which a fractionating column, with a reflux stream from the condenser fed back to it, was incorporated into the distillation system.

5.1.2 Constant reflux ratio

Batch distillation under conditions of constant reflux is similar to simple distillation inasmuch as the distillate and still compositions continuously vary with time. Again, the initial distillate contains the highest concentration of the light component, and distillate composition continuously gets heavier as the distillation proceeds. However, because of the interaction between the liquid reflux falling down through the column and the vapor rising up through the column, the rate of change of the distillate composition is much slower.

Batch distillation under constant reflux ratio is analyzed mathematically by considering that the moles lost from the still represent moles of distillate collected in the product receiver. Thus,

where W = moles in still

D = moles of distillate

Taking a material balance on the light component,

where xw - mole fraction in still xD = mole fraction in receiver

Differentiating,

- (*, dW + Wdxw) = xDdD (5.16) and substituting Eq. (5.14) into Eq. (5.16), xwdW+ Wdx„ = xB dW (5.17)

Rearranging,

Integrating,

and, again, it is typical to evaluate the integral graphically. [Note the similarity between Eqs. (5.19) and (5.12).]

Example 5.3 A mixture containing 50 moles each of benzene and toluene is to be distilled under conditions of constant reflux ratio until mole fraction of the residual benzene is less than 0.20. The column contains three theoretical stages. Calculate the material balance for this separation.

solution By trial and error, a reflux ratio is determined which allows the construction of four theoretical stages (three in the column and one for the still) between the composition in the still and the x - y line. See Fig. 5.2a. Then, other lines are drawn parallel to this component balance (see Chap. 2) line, and four theoretical stages are stepped off (Figs. 5.26 to e). This process is continued until the residual benzene concentration reaches the desired value.

Fractionating Still
(aï
Batch Distillation Mccabe Thiele Diagram

Mole fraction, liquid benzene <t>)

Figure S.2 Batch distillation of benzene-toluene at constant reflux ratio, Example 5.3. ta-e) McCabe-Thiele diagram for progressively reducing still concentration to 0.13 mole fraction benzene.

Mole fraction, liquid benzene <t>)

Figure S.2 Batch distillation of benzene-toluene at constant reflux ratio, Example 5.3. ta-e) McCabe-Thiele diagram for progressively reducing still concentration to 0.13 mole fraction benzene.

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