Examples Of Distillation

So, by returning reflux to the column at a constant rate, the mole fraction of benzene was increased to 0.84 in the receiver and decreased to 0.13 in the still pot. (Remember, the simple still only concentrated the product to 0.58 mole fraction while reducing still concentration to 0.10 mole fraction.)

Other useful information can be developed by further consideration of Fig. 5.2a through e.

1. Because the column has only a rectifying section, the concentration of the light component in the still can never be reduced to zero (except in the trivial extreme of evaporating all the liquid). Note that as overhead composition regularly reduces by increments of 0.100 mole fraction, the residual still composition changes by decreasing increments.

2. Once the initial overhead composition has been determined in the trial-and-error process, both minimum and actual reflux ratios can be determined from the relationship.

Minimum reflux ratio is obtained by extending a line connecting overhead composition with that of the vapor composition that is in equilibrium with the feed (in this case, y = 0.744). From the dashed line in Fig. 5.2a yinl = 0.52 bo 0.83

Also from Fig. 5.2a, the value ofyint corresponding to the use of four theoretical separation stages is 0.36, so Ract = 1.64. Therefore, for this particular separation,

In the event that some specific distillate composition is desired rather than the reduction of residual composition to some low acceptable value), the procedure outlined above is followed, but trial-and-error is required to determine when the distillation must be stopped. The result of stopping the distillation process shown in Example 5.3 before the benzene concentration in the still reached 13 mole percent, for example, would be a distillate with a benzene concentration greater than 84 mole percent. Continuing the distillation further than shown in the example would produce a more dilute distillate ("overhead") concentration.

5.1.3 Varying reflux ratio

By continuously varying the reflux ratio during the course of the distillation, an essentially constant overhead concentration can be obtained. The boil-up rate is constant in this case, too, but as reflux ratio increases, the amount of liquid returned to the column increases.

Therefore, the LIV ratio increases as the distillation proceeds.

The varying-reflux-ratio case is analyzed mathematically in exactly the same manner as was the constant-reflux case.

Example 5.4 A mixture containing equal molar quantities of benzene and toluene is to be distilled under conditions of varying reflux ratio to maintain a constant overhead composition of 95 mole percent, using the column specified in Example 5.3.

Calculate the material balance for this case if the final reflux ratio is 13:1.

solution Beginning with an overhead concentration of 95 mole percent, a reflux ratio is selected such that the four theoretical separation stages will produce a bottoms composition which essentially conforms to that of the feed. Then, different reflux ratios are selected, and four theoretical stages are stepped off, with the corresponding still compositions noted. See Figs. 5.3a to e.

This process can be simplified by recalling the previously discussed relationship:

Inserting various reflux ratios into this relationship produces values of y^, which simplify plotting of the corresponding component balance lines.

Mccabe Thiele Minimum Reflux Ratio

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Û.9 1.0 Mole fraction, liquid benzene

Figure 5.3 Batch distillation of benzene-toluene with variable reflux ratio, Example 5.4. (a-e) McCabe-Thiele diagram for progressively increasing reflux ratio to 13:1.

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Û.9 1.0 Mole fraction, liquid benzene

Figure 5.3 Batch distillation of benzene-toluene with variable reflux ratio, Example 5.4. (a-e) McCabe-Thiele diagram for progressively increasing reflux ratio to 13:1.

Mccabe Thile Toluene
Mole fraction, liquid benzene (b)
Mole Fraction Examples
Mole Iraclion, liquid benzene (c)

Rgw« 5.3 (Continued) Batch distillation of benzene-toluene with »¿iable reflux ratio, Example 5.4. la-e) McCabe-Thiele diagram fcr progressively increasing reflux ratio to 13:1.

Mccabe Thiele Batch Distillation
Mole fraction, liquid benzene
Benzene Distillation

Male fraction, liquid benzene

Figure 5.3 (Continued) Batch distillation of benzene-toluene with variable reflux ratio, Example 5.4. (a-el McCabe-Thiele diagram for progressively increasing reflux ratio to 13:1.

Male fraction, liquid benzene

Figure 5.3 (Continued) Batch distillation of benzene-toluene with variable reflux ratio, Example 5.4. (a-el McCabe-Thiele diagram for progressively increasing reflux ratio to 13:1.

The values of the still composition from Figs. 5.3a to e are plotted in Fig. 5.3if as follows:

Fractionating Reflux Still
0 0

Responses

  • cecilia
    What is the composition of benzene and toulene at different reflux ratios?
    7 years ago
  • theodoric
    Do batch distillation columns mccabe thiele?
    7 years ago

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