Unconventional Batch Distillation

11.1. Introduction

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.

11.2. Use of Continuous Columns for Batch Distillation

11.2.1. Introduction

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.

In CBD operation (Figure 11.1) each cut (one pass) produces either a desired product (main-cut) or an off-specification product (off-cut). The recovery of key component in each cut can be maximised by controlling reflux ratio optimally and by having a number of time sequenced reflux ratio (Chapter 5, 6). The process model for CBD operation may result in a simple or a complex set of differential and algebraic equations (DAEs) depending on the underlying assumptions and the accuracy of calculations required (Chapter 4). The optimisation problem using such model is however always dynamic. And the computation time for the solution of such problem is always high but varies depending on the type of model used and on the total number of variables to be optimised (Chapter 5, 6).

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

(a) modelling task becomes similar to that of steady state continuous distillation,

(b) model results in a set of non-linear algebraic equations rather than a set of non-linear DAEs,

(c) optimisation task becomes easier which results in a steady state optimisation problem as opposed to a dynamic optimisation problem and therefore the computation becomes cheaper.

Figure 11.1. Conventional Batch Distillation (CBD)



Intermediate Products

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

Intermediate Products

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

During each pass of a continuous column operation, the feed flow rate, feed location, vapour boilup rate, etc. will influence the recovery of component, time of operation, energy consumption, etc. Also the number of equilibrium stages and the relative volatility of the components in the feed mixture, feed location will dictate the number of passes which will be required to achieve a high recovery and high purity of a particular component. However, very little information regarding these was available in the literature until the work of Mujtaba (1997). Whether the CBD operation can be replaced by the continuous column operation without compromising the efficiency in terms of recovery of component, total time of operation and energy consumption is an open area. Mujtaba (1997) has addressed some of these issues using optimisation techniques.

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.

Mujtaba (1997) considered single separation duty where a single column is used for separating only one mixture during its total operation period in a year and compared the performances of CBD with continuous column operation. The author also considered multiple mixtures and multiple separation duties in continuous columns and evaluated the optimum performance of such columns. Some of these works will be presented in the next few sections.

11.2.2. SPSS, SPSSS, MPSSS Operations

Mujtaba (1997) provided the following definitions of the terms SPSS, SPSSS, MPSSS frequently used for continuous column operation. Single Pass Steady State Operation (SPSS)

When only one product (distillate) is produced in a continuous column using only one pass by processing a binary or a multicomponent mixture, the operation is defined as SPSS operation. In this type of operation the bottom product after the first pass is not processed further. Figure 11.3 shows the operation for a ternary mixture. Only one product (rich in component A) is obtained using one pass.

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