(62e) Comparison of Batch and Continuous Industrial Distillation

Batch distillation is still widely used in chemical industry. Many smaller chemical plants operate in batch mode, with batch reactors and batch rectifiers. The main reason is production flexibility where different chemicals are being made in the same equipment in short (one- or two-week) campaigns. This is in fact the way how majority of specialty chemicals are made.

Despite great flexibility, batch distillation has many disadvantages when compared with continuous process. Some of them are related to operational difficulties:

• It may be difficult to establish when to switch from one cut to another.
• It may be difficult to establish optimal reflux ratio, since the pot composition continuously changes
• Making significant changes in process parameters may cause operating instabilities.

Other disadvantages are related to inherent inefficiencies of batch distillation. For example, it is impractical and in many cases impossible to switch directly from a “waste cut” to pure cut. There is usually a slop cut (“front cut”) in between, containing off-spec product. This front cut needs to be reprocessed during a separate batch or mixed with the next batch crude (feed). Reprocessing means waste, due to reduced capacity and increased energy consumption. Sometimes, when the product content in the crude is too small, we may not be able to recover it in one batch distillation. That means that the product needs to be concentrated and then reprocessed in a separate batch distillation.

The question arises then: is batch distillation really more economical than continuous distillation, even for small production rate?

Several typical case studies will be presented to compare these two processes.