Column for Batch Distillation

First the equations for the condenser will be presented. Then the equations for the accumulator, followed by the equations for the plates in the column and the reboiler are presented. The trays are counted from the top to the bottom. 

The composition of the distillate and of the liquid reflux is same as that of the vapour going into the condenser. Therefore  

The distillate rate to the accumulator or product tank is therefore  

There will be no effect of plate holdup on the steady state mass balance. The component balance on tray j is  

The vapour liquid equilibrium relationship is same as Equation SS.7 with j = N. 

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Abrams et al. (1987), Mujtaba and Macchietto (1994) and Sorensen and Skogestad (1996) used such columns for batch distillation and compared their performances with conventional columns. 

In the mid Eighties, 99 batch processes within 74 UK companies were identified (Parakrama, 1985). In the last decade or more a continuous shift towards batch processes has been noticed and many small-scale companies are using their existing continuous columns for batch distillation (Willet, 1995) without much realising the implications of such practice. Also R D sections of many multi-national chemical companies do the pilot plant study in CBD columns for their continuous distillation columns which are in operation in the plant (Chen, 1995 Jenkins, 2000 Greaves, 2003). Because of confidentiality the results of such studies are not available in public. 

Mujtaba (1997) explored the potentials of using continuous columns for batch distillation in detail. 

Figure 11.2. Continuous Column For Batch Distillation. [Mujtaba, 1997]a...

The features and modelling issues of IBD columns with or without chemical reaction are presented in Chapter 2 and 4 respectively. Simulation of IBD columns without chemical reactions is also presented in Chapter 4. After Robinson and Gilliland (1950) had introduced IBD columns, Abrams et al. (1987), Mujtaba and Macchietto (1994) and Sorensen and Skogestad (1996) used such columns for batch distillation and compared their performances with conventional columns. While Mujtaba and Macchietto (1994) studied simultaneous chemical reaction and separation using IBD columns, Sorensen and Skogestad (1996) presented the most comprehensive study on IBD columns. Some examples from these works are presented below. 

Sorensen (1999) has suggested a cyclic operation policy for batch distillation with repeated filling and dumping of the reflux drum. This configuration achieves the maximum attainable separation and requires minimal control. Furthermore, such a column can be operated very safely. 

While the reduced SQP algorithm is often suitable for parameter optimization problems, it can become inefficient for optimal control problems with many degrees of freedom (the control variables). Logsdon et al. (1990) noted this property in determining optimal reflux policies for batch distillation columns. Here, the reduced SQP method was quite successful in dealing with DAOP problems with state and control profile constraints. However, the degrees of freedom (for control variables) increase linearly with the number of elements. Consequently, if many elements are required, the effectiveness of the reduced SQP algorithm is reduced. This is due to three effects ... 

Note that Sundaram and Evans (1993a,b) used FUG method of continuous distillation directly and developed time explicit model, while Diwekar (1992) developed modified FUG method as described above and time implicit model for batch distillation. Sundaram and Evans used time as an independent variable of the model while Diwekar (1992) used reboiler composition as independent variable. Both models are based on zero column holdup and does not include plate-to-plate calculations. See the original references for further details. 

Attarwala and Abrams (1974) and Mujtaba (1997) used the above model for batch distillation task using a continuous column (see section 2.2.4). 

Over the last few decades many alternative column configurations and operation have been suggested for batch distillation. The features and characteristics of such configurations have been briefly presented in Chapter 2 with process models of different degree of complexity in Chapter 4. The flexibility and operational issues of some of these configurations will be presented in this chapter. 

However, the use of continuous columns Figure 11.2 for batch distillation has several advantages ... 

Separation of multiple mixtures using a single column is a very common feature in CBD columns (Mujtaba and Macchietto, 1996). If a continuous column were to be used for batch distillation then it would be interesting to evaluate the performance of such column undergoing multiple separation duties. 

Truly multicomponent solutions based on continuous distillation shortcut methods have been proposed for batch distillation. The Fenske, Underwood, and Gilliland equations or correlations are commonly used in conjunction with each other to solve continuous distillation problems as described in Section 12.3. Diwekar and Madhavan (1991) describe how these techniques may be modified for the design of batch distillation columns for variable and constant reflux cases. 

The rigorous solution of batch distillation columns carries an extra dimension of complexity over continuous steady-state distillation because it is inherently a transient operation. The basic assumption of steady-state operation in the continuous column model obviously does not apply for batch distillation. The only possible steady-state operation in batch distillation is at total reflux, which is commonly used as the initial condition for the dynamic solution of the column. 

Equation (12.52) for batch distillation is the same as the mass balance equation for continuous distillation except for the term on the left side of the eqnation, which is normally zero for continuous distillation. Thus, it is theoretically possible to employ the same approach for batch distillation as previously presented for continnons distillation, provided an accnmnlation term is introdnced. However, although apparently simple, it is actually very difficult in practice becanse of problems in solving the many simnltaneons differential eqnations involved. In any event, it is erroneons to neglect tray and column holdnp in stage compntations for batch distillation. 

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) ... 

The design methods for batch distillation allowing for liquid holdup in the column are very unsatisfactory, and it is a field that should be actively studied in view of the importance of the operation. Improvements in the design calculations for multicomponent mixtures with no liquid holdup in the column are also needed. 

Batch distillation, which is the process of separating a specific quantity (the charge) of a liquid mixture into products, is used extensively in the laboratory and in small production units that may have to serve for many mixtures. When there are N components in the feed, one batch column will suffice where N — 1 simple continuous-distillatiou columns would be required. 

A batch distillation column, used for distilling nitrotoluene, had not been cleaned for 30 years. A buildup of sludge caused some problems, or... 

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