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Figure 1.1. Continuous Distillation Column

Figure 1.2. Conventional Batch Distillation (CBD)

• A series of product accumulator tanks connected to the product streams to collect the main and or the intermediate distillate fractions.

Operation of such a column involves carrying out the fractionation until a desired amount has been distilled off. The overhead composition varies during the operation and usually a number of cuts are made. Some of the cuts are desired products (main-cuts) while others are intermediate fractions (off-cuts) that can be recycled to subsequent batches to obtain further separation. A residual bottom fraction may or may not be recovered as product (Mujtaba, 1989).

Further details on batch distillation are provided throughout this book.

Figure 1.3. Semi-batch (Semi-continuous) Distillation Column

1.5. Semi-batch (semi-continuous) Distillation

Figure 1.3 shows a typical semi-batch (semi-continuous) distillation column. The operation of such columns is very similar to CBD columns except that a feed is introduced to the column in a continuous or semi-continuous mode. This type of column is suitable for extractive distillation, reactive distillation, etc. (Lang and coworkers, 1994, 1995; Mujtaba, 1999). Further details of semi-batch distillation in extractive mode of operation are provided in Chapter 10.

1.6. Advantages of Batch Distillation

The main advantages of batch distillation over a continuous distillation lie in the use of a single column as opposed to multiple columns and its flexible operation.

For a multicomponent liquid mixture with nc number of components, usually (nc-1) number of continuous columns will be necessary to separate all the components from the mixture. For a mixture with only 4 components and 3 distillation columns there can be 5 alternative sequences of operations to separate all the components (Figure 1.4). For a mixture with only 5 components, 4 distillation columns can be sequenced in 14 different ways. The number of alternative operations grows exponentially with the number of components in the mixture. These alternative operations do not take into account the production of off-specification materials or provision for side streams (this would further increase dramatically the number of columns and or operational sequences).

On the other hand with CBD, only one column is necessary and there is only one sequence of operation (with or without the production of off-specification materials) to separate all the components in a mixture (Figure 1.2). The only requirements here are to divert the distillate products to different product tanks at specified times.

The continuous distillation columns are designed to operate for longer hours (typically 8000 hrs a year) and therefore each column (or a series of columns in case of multicomponent mixture) is dedicated to the separation of a specific mixture.

However, a single mixture (binary or multicomponent) can be separated into several products (single separation duty) and multiple mixtures (binary or multicomponent) can be processed, each producing a number of products {multiple separation duties) using only one CBD column (Logsdon et al., 1990; Mujtaba and Macchietto, 1996; Sharif et al., 1998).

Finally, in pharmaceutical and food industries product tracking is very important in the face of strict quality control and batch wise production provides the batch identity (Low, 2003).

Figure 1.4. Alternative Separation Sequences for Quaternary Mixture Using Continuous Distillation Columns

References

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