Batch Distillation with Constant Distillate Composition

2 Batch Distillation with Constant Distillate Composition 

An alternative to the constant-reflux-ratio policy described above is to maintain a constant-molar-vapor rate, but continuously vary the reflux ratio to achieve a constant distillate composition that meets the specified purity. This policy requires a more complex control system, which may be justified only for large batch distillation systems. 

Calculations for the policy of constant distillate composition can also be made with the McCabe-Thiele diagram for binary mixtures. An overall material balance for the light-key component at any time, t, is given by rearrangement of equation (6-108) at constant xD for W as a function of 

Differentiating equation (6-111) with respect to t for varying W and xw gives 

For constant molar overflow and negligible liquid holdup, the rate of distillation is given by the rate of loss of charge, or 

---

FIGURE S.5-11 Batch distillation with constant distillate composition and varying reflux (three theoretical stages). 

Calculate, by modified McCabe-Thiele methods, residue composition and distillation time for binary batch rectification with constant reflux for a given number of ideal stages, boil-up rate, and specified average distillate composition. 

Example 6.19 Binary Batch Rectification with Constant Distillate Composition... 

FIG. 13-98 Typical variation in distillate and reboiler compositions with amount distilled in binary batch distillation at a constant-reflux ratio. 

Figure 5.22. (e) Batch distillation, reflux flow cascaded with temperature to maintain constant top composition... 

Operation at constant reflux ratio is better than operation with constant distillate composition for high-yield batch separations. However, operation with constant distillate composition might be necessary if high product purity is required. In fact, it is not necessary to operate in one of these two special cases of constant reflux ratio or constant distillate composition. Given the appropriate control scheme, the reflux ratio can be varied through the batch... 

Other synonyms for steady state are time-invariant, static, or stationary. These terms refer to a process in which the values of the dependent variables remain constant with respect to time. Unsteady state processes are also called nonsteady state, transient, or dynamic and represent the situation when the process-dependent variables change with time. A typical example of an unsteady state process is the operation of a batch distillation column, which would exhibit a time-varying product composition. A transient model reduces to a steady state model when d/dt = 0. Most optimization problems treated in this book are based on steady state models. Optimization problems involving dynamic models usually pertain to optimal control or real-time optimization problems (see Chapter 16)... 

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). 

Figure 13.10. Batch distillation McCabe-Thiele constructions and control modes, (a) Construction for constant overhead composition with continuously adjusted reflux rate, (b) Construction at constant reflux at a series of overhead compositions with an objective of specified average overhead composition, (c) Instrumentation for constant vaporization rate and constant overhead composition. For constant reflux rate, the temperature or composition controller is replaced by a flow controller.

Robinson (1969) considered the following example problem. A binary feed mixture with an initial amount of charge, B0 = 100 kmol and composition xB0 = <0.50, 0.50> molefraction, having constant relative volatility of 2.0 was to be processed in a batch distillation column with 8 theoretical stages. The aim was to produce 40 kmol of distillate product (D) with composition (xd) of 0.5 molefraction for component 1 in minimum time (tF) using optimal reflux ratio (/ ). 

Constant reflux, varying overhead composition. Reflux is set at a predetermined value at which it is maintained for the run. Since pot liquid composition is changing, instantaneous composition of the distillate also changes. The progress of a binary separation is illustrated in Fig. 13-98. Variation with time of instantaneous distillate composition for a typical multicomponent batch distillation is shown in Fig. 13-99. The shapes of the curves are functions of volatility, reflux ratio, and number of theoretical plates. Distillation is continued until the average distillate composition is at the desired value. In the case of a binary, the overhead is then diverted to another receiver, and an intermediate cut is withdrawn until the remaining pot hquor meets the required specification. The intermediate cut is usually added to... 

Batch Distillation With Fractionation li-ays—Constant Overhead Product Composition, Multicomponent and Binary... 

In a binary batch distillation process with no reflux (differential distillation), constant relative volatility is assumed throughout the process, a = 2. If the initial liquid composition X° = 0.4, what is the initial distillate composition What is the composition of the liquid remaining in the boiler when 50% of the original liquid has been distilled When 99% has been distilled ... 

Batch distillation columns may be operated uader either of two reflux policies, constant reflux with varying distillate composition, and variable reflux, with constant distillate composition. These policies have the usual limitations of total reflux and minimum stages,... 

A batch distillation column with three theoretical stages (the first stage is the still pot) is charged with 100 kmol of a 20 mol% n-hexane in n-octane mixture. At a constant reflux ratio R - 1.0, how many moles of the charge must be distilled if an average product composition of 70 mol% n-hexane is required If the boilup ratio is 10 kmol/h, calculate the distillation time. The equilibrium distribution curve at column pressure is given in Figure 6.27. 

A batch distillation column with eight theoretical stages (the first stage is the still pot) is charged with 500 kmol of a 48.8 mol% A in B mixture at atmospheric pressure (relative volatility aA B = 2.0). The boil-up ratio is constant at 213.5 kmol/h. If the distillate composition is to be maintained con-... 

Example 9.1. A batch still is loaded with lOOkgmole of a liquid containing a binary mixture of 50 mole% benzene in toluene. As a function of time, make plots of (a) still temperature, (b) instantaneous vapor composition, (c) still pot composition, and (d) average total distillate composition. Assume a constant boilup rate of lOkgmole/hr and a constant relative volatility of 2.41 at a pressure of 101.3 kPa (1 atm). 

If a constant distillate composition is required, this can be achieved by increasing the reflux ratio as the system is depleted in the more volatile material. Calculations are again made with the McCabe-Thiele diagram as described by Bogart and illustrated by the following example. Other methods of operating batch columns are described by Ellerbe. ... 

---