Batch distillation material balances

The system material balance from Treybal [129] using a heated kettle and distillation column following a McCabe-Thiele diagram, using reflux, but having only a batch (kettle) charge ... 

For a batch differential distillation where no reflux is used, there is only boilup of a mixture of the desired lighter component, which leaves the kettle, and a desired residual bottoms composition is left in the kettle. This type of distillation follows the Raleigh equation to express the material balance. However, while simple, not having tower packing or trays or reflux does not offer many industrial applications due to the low purities and low yields involved. Repeated charges of the distillate back to the kettle and redistilling w411 improve overhead purity. 

The general approach to the solution of unsteady-state problems is illustrated in Example 2.15. Batch distillation is a further example of an unsteady-state material balance (see Volume 2, Chapter 11). 

A mixture of chlorinated phenols can be represented as an equivalent binary with 90% 2,4-dichlorphenol (DCP) and the balance 2,4,6-trichlorphenol with a relative volatility of 3.268. Product purity is required to be 97.5% of the lighter material, and the residue must be below 20% of 2,4-DCP. It is proposed to use a batch distillation with 10 theoretical stages. Vaporization rate will be maintained constant. 

The definition of, and the necessary conditions for, the existence of an arheotrope are based on the material balances of a batch distillation process in the presence of a stagnant or flowing sweep gas (Fig. 4.13). Let us first consider a nonreactive liquid mixture. In this case, the component and total mass balances are given by ... 

This chapter addresses each of these concerns in order. Before passing on to that discussion, however, the distillation nomenclature will be briefly restated, both because batch stills are analyzed after the same fashion as continuous stills, and for completeness within this chapter, The overall material balance is... 

If a batch distillation is carried out at variable reflux ratio, it is possible u> maintain a constant purity of distillate and to do this until blocked by material balance limitations, A McCabe-Thiele diagram illustrating this method of operation is shown in Fig. 5.5-11. The value of xu is kept constant, and the slope of the operating line is varied in accordance with the overall material balance, The figure shows the conditions at time 0 and time t. Note that the three equilibrium stages include the stillpot. 

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) ... 

A reflux splitter is difficult to incorporate in continuous distillation MB control schemes because it couples variations of the material balance stream (i.e., the distillate) with reflux rate changes. This is especially troublesome when reflux ratio is high, and a rise in distillate rate will effect a severalfold increase in reflux rate. The author experienced one case of poor continuous distillation control which was attributed to the above shortcoming. The author recommends restricting application of reflux splitters to batch distillation, where they work well. In continuous distillation, Sutro weir dividers are far more suitable. 

Table 4.2 lists the equipment models (or procedures) and operations in each of the two simulators. Some of the models carry out simple material balances given specifications for the feed stream(s) and the batch (or vessel) size or batch time. Others, like the batch distillation... 

Equilibration time is of interest in continuous system startups as welt as in batch distillation. Cieariy, the amount of liquid holdup in the condenser, cohimn, and stillpot is a major factor in the time requirement. For a stage n (see Fig. 5. 1), the basic material balance equation is... 

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