Batch distillation total boil

The simplest example of batch distillation is a single stage, differential distillation, starting with a still pot, initially full, heated at a constant rate. In this process the vapour formed on boiling the liquid is removed at once from the system. Since this vapour is richer in the more volatile component than the liquid, it follows that the liquid remaining becomes steadily weaker in this component, with the result that the composition of the product progressively alters. Thus, whilst the vapour formed over a short period is in equilibrium with the liquid, the total vapour formed is not in equilibrium with the residual liquid. At the end of the process the liquid which has not been vaporised is removed as the bottom product. The analysis of this process was first proposed by Rayleigh(24). 

The aqueous residue from the three extractions with toluene yields, when the bulk of the water has been distilled off on the steam bath under reduced pressure, a further quantity of catechol by extraction with hot toluene, but the crystals so obtained are contaminated with a red impurity. It is therefore preferable to distil the entire residue, again collecting the fraction boiling at 124—1250/12 mm. and recrystallizing it from fresh toluene. In this way 295-305 g. of pure product is obtained. The forerun contains a little aqueous hydrobromic acid, which may be employed in a subsequent batch. The total yield of pure catechol is 690-705 g. (77-80 per cent of the theoretical amount). 

However, most of the batch distillation models (e.g. Mujtaba and co-workers Sorensen and Skogestad, 1996) relate the amount of distillate collected (Hamodei) with the vapour boil-up rate in the column (Vtomm), the internal reflux ratio (Rmodei) and the total operating time (tdig) by,... 

In a typical batch distillation, the lower bulb of the system is loaded with the solvent and a suitable absorber in the glove box, after which it is connected to the system under flowing argon. The system is purged with argon, the appropriate pressure is adjusted, and the lower bulb is heated to boiling point. The first fraction is taken out, and total reflux is applied. Then the middle fraction, the pure material, is collected in the terminal bulb. As the distillation stops, the terminal bulb is closed hermetically and introduced into the glove box. 

The time t required for batch distillation at constant reflux ratio and negligible holdup in the column and condenser can be computed by a total material balance based on constant boil-up rate V to give the following equation (Seader and Henley, 2006) ... 

Establish the separation capability of a single-stage (differential) batch still processing a mixture of two compounds having a relative volatility of 4.0. At the start of the batch separation, there are 600 mol of the more-volatile compound A and 400 mol of compound B in the kettle. When the remaining charge in the kettle is 80 percent B, how much total material has been boiled off, and what is the composition of the accumulated distillate ... 

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