Constant-level batch distillation

Gentilcore [Chem. Eng. Progr, 98(1), 56 (Jan. 2002)] describes an alternative strategy of constant-level batch distillation where the replacement solvent is added at a rate to keep the volume of liquid in the pot constant. For simple distillation without rectification the analog of Eq. (13-126) is... 

Equipment can be arranged in a variety of configurations. In sinple batch distillation (Figure 9-11. the vapor is withdrawn continuously from the reboiler. The system differs from flash distillation in that there is no continuous feed input and the liquid is drained only at the end of the batch. An alternative to sinple batch distillation is constant-level batch distillation where solvent is fed continually to the still pot to keep the liquid level constant tGentilcore. 20021. 

For a constant-level batch distillation the general mole balance is... 

For the total mole balance this is In - Out = 0, since the moles in the still pot are constant. Thus, if dS moles of the second solvent are added, the overall mole balance for a constant-level batch distillation is... 

Since vapor and liquid are assumed to be in equilibrium, y is related to by the equilibrium relationship. Equation (9-17) can be integrated graphically or numerically in a procedure that is quite similar to that used for simple batch distillation. Problem 9.D17 will lead you through these calculations for constant-level batch distillation. 

Gentilcore 2002 presents a constant relative volatility sanple calculation that illustrates the advantage of constant-level batch distillation when it is used to exchange solvents. 

Solve problems for constant-level batch distillation... 

D17. A nonvolatile solute is dissolved in 1.0 kmol of methanol. We wish to switch the solvent to water. Because the solution is already concentrated, a first batch distillation to concentrate the solution is not required. We desire to have the solute in 1.0 kmol of solution that is 99.0 mol% water and 1.0 mol% methanol. This can be done either with a constant-level batch distillation or by diluting the mixture with water and then doing a sinple batch distillation. VLE data (ignore the effect of the solute) are in Table 2-7. Do a constant-level batch distillation fromxjyf jj i = 1.0 (pure methanol) to Xj = 0.01. Find the moles of water added during the constant-level batch... 

This process is analogous to constant level batch distillation fsection 9.3.1 and the analysis for immiscible extraction is very similar to the analysis in that section. If we assume that the level in the mixed tank is constant, then the overall mass balance becomes in = out, and for dS kg of entering solvent,... 

Batch Distillation Chapter 4 is devoted to batch distillation. This is one of the most important and one of the most studied unit operations in batch industries. Separation is based on vapor-liquid equilibria. There are a number of configurations possible in conventional batch colmnn, namely, the constant reflux mode, the variable reflux mode, and the optimal reflux mode. There are a number of new configurations that have emerged in the literature for batch distillation. This chapter describes aU these operating modes and configurations. Various levels of models are available for different analysis. Different numerical integration techniques are needed to solve equations of these different models. Optimization and optimal control are well studied for this unit operation. 

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