Batch distillation 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... 

Although this problem is one of multicomponent batch distillation, the product remains of constant composition so that normal methods can be used for plate-to-plate calculations at a point value of the varying reflux ratio. 

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 11.35. Batch distillation-constant product composition...

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.

Hence the reflux ratio, the amount of distillate, and the bottoms composition can be related to the fractional distillation time. This is done in Example 13.4, which studies batch distillations at constant overhead composition and also finds the suitable constant reflux ratio that enables meeting required overhead and residue specifications. Although the variable reflux operation is slightly more difficult to control, this example shows that it is substantially more efficient thermally—the average reflux ratio is much lower—than the other type of operation. 

Batch distillation (Fig. 16) is often preferable to continuous distillation when small quantities of feed material are processed. A liquid feed is charged to a still pot and heated until vaporization occurs. Vapor leaves the top of the column, and after condensation, part is removed as product and the rest returned to the column as reflux. As distillation proceeds, the contents of the still pot and the overhead product become richer in less volatile components. When operated at a fixed reflux ratio, an overhead product cut is collected until the product composition becomes unaccceptable. As an alternative, the reflux ratio can be gradually increased to hold the product composition constant as the cut is taken. For a fixed rate of heat addition to the still pot, the latter option results in a steadily declining product flow rate. After the first cut, subsequent... 

Using the constant reflux operation strategy outlined in section 3.3.2 Greaves et al. (2001) carried out few experiments using different values of Rexp and different batch time. The accumulated distillate composition profiles as functions of batch time and distillate holdup are shown in Figures 3.13 and 3.14 respectively. Figure 3.15 shows the instant distillate composition profile for Rexp = 2. 

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 (/ ). 

FIG. 13-121 Distillate composition for a batch distillation of a four-component mixture at a constant 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... 

The graphical procedure is applicable to any binary batch distillation process and is not limited to operations at constant reflux ratio or constant distillate composition. 

A mixture of n-butane and n-pentane is loaded into a batch distillation reboiler to produce a distillate at constant rate and composition. The column pressure is constant at 500 kPa, the initial charge is 100 kmol, and the distillate rate is 10 kmol/h. The column has three theoretical trays plus the reboiler, giving it four theoretical stages. The initial charge composition, the required distillate composition, and the component relative volatilities, assumed constant, are as follows ... 

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 hydrocarbon mixture is to be separated by batch distillation at 520 kPa to produce a constant-composition distillate, the key components being propane and n-butane. The starting charge composition, the relative volatilities, and the desired distillate composition are as follows ... 

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