Batch Distillation with Rectifying Column
Batch distillation is an unsteady state operation. It is usually carried out in a batch still to which a column equivalent to a number of equilibrium stages is attached. Alternatively, packings may be used. The Figure below showed a typical set-up for batch distillation with rectifying column.
As in the case of simple distillation, a fixed quantity of liquid is originally charged to the batch still. During distillation, the vapour passes upward through the column. The whole column is an enriching section. The vapour is condensed into liquid at the top of the column. Part of the liquid is returned to the column as reflux, and the remainder withdrawn as distillate. Nothing is added or withdrawn from the still until the run is completed. As distillation progress, the MVC in the batch still decreases.
In principle, the alternative setup as shown in the Figure is also feasible. Here the vessel is located at the top of the column that is operated as a stripping column. Such a set-up is often known as the inverted batch distillation or the batch stripper. During operation, the LVC are primarily separated from the system. The liquid is the still depletes in LVC and enriches in MVC.
This mode of operation is seldom applied in industrial processes,
but it is necessary for the separation of mixtures that form minimum-boiling
azeotropes. This will covered in the section on azeotropic distillation.
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In a batch process, the main steps are operated discontinuously. In contrast with a continuous process, a batch process does not deliver its product continuously but in discrete manner. This means that mass, temperature, concentration, and other properties vary with time. In practice, most batch processes are made up of a series of batch and semi-continuous steps. A semi-continuous step runs continuously with periodic start-ups and shutdowns.
are economical for small volumes
are flexible in accommodating changes in product formulation
are flexible in changing production rate
allow the use of standardized multi-purpose equipment for the production of a variety of products from the same plantare best if equipment needs regular cleaning because of fouling or needs regular sterilization
allow better product integrity: each batch of product can be clearly identified in terms of the feeds involved and conditions of processing. This is particularly important in industries such as pharmaceuticals and foodstuffs.
The following sections discuss batch distillation in greater details:
Advantages and Disadvantages (elaboration of the point above)
Operation and Control
Other Batch Distillations (e.g. Steam, Azeotropic, Extractive)
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