Binary Batch Distillation with Constant Reflux

1 Binary Batch Distillation with Constant Reflux 

If the reflux ratio R or distillate rate D is fixed, instantaneous distillate and bottoms compositions vary with time. For a total condenser, negligible holdup of vapor and liquid in the condenser and the column, equilibrium stages, and constant molar overflow, the Rayleigh equation can now be written as 

Equation (6-106) cannot be integrated analytically because the relationship between xD and xw depends on the liquid-to-vapor ratio, the number of theoretical stages, and the equilibrium distribution curve. However, it can be integrated graphically with pairs of values for xD and xw obtained from the McCabe-Thiele diagram for a series of operating lines of the same slope. 

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)  

With a constant-reflux policy, the instantaneous distillate purity is above the specification at the beginning of the run and below specification at the end. By an overall material balance, the average mole fraction of the light-key component in the accumulated distillate at time t is given by 

---

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. 

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

Batch with Constant Reflux Ratio, 48 Batch with Variable Reflux Rate Rectification, 50 Example 8-14 Batch Distillation, Constant Reflux Following the Procedure of Block, 51 Example 8-15 Vapor Boil-up Rate for Fixed Trays, 53 Example 8-16 Binary Batch Differential Distillation, 54 Example 8-17 Multicomponent Batch Distillation, 55 Steam Distillation, 57 Example 8-18 Multicomponent Steam Flash, 59 Example 8-18 Continuous Steam Flash Separation Process — Separation of Non-Volatile Component from Organics, 61 Example 8-20 Open Steam Stripping of Heavy Absorber Rich Oil of Light Hydrocarbon Content, 62 Distillation with Heat Balance,... 

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 ... 

A liquid binary mixture with Bo = 10 kmol and xbo = <0.6, 0.4> molefraction is subject to conventional batch distillation shown in Figure 4.3. The relative volatility of the mixture over the operating temperature range is assumed constant with a value of (a=) 2. The total number of plates is, N = 20. The vapour boilup rate is, V = 5.0 kmol/hr and the reflux ratio is, r = 0.75. The condenser and total plate holdups are 0.2 and 0.2 kmol respectively. 

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... 

---