Page 281 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

11.2 Conceptual design 283

Table 11.1 General Features of different Distillation arrangements.
Number of Vapour
Distillation Nature of contacting Feed entry Feed Top Reflux at
type operation stages location phase(s) Reflux bottom

Fractionator Continuous Multiple Between top Vor L or Yes Yes
and the V þ L
bottom stage
Reboiled Continuous Multiple Top stage L No Yes
stripper
Flash Continuous One Above the LorV or No No
distillation drum liquid V þ L
level
Multistage Batch Multiple At the still L Yes No
batch before start
distillation of distillation
Single-stage Batch One (see At the still L No No
batch Section 11.6) before start
distillation of distillation

to accommodate the entire feed per batch and the investment for a moderate to higher capacity plant is
higher as compared to continuous distillation for the same average daily processing capacity. In
addition, the long time of exposure of the mixture to high temperature increases the risk of thermal
degradation or decomposition of substances. However, in small scale, batch distillation plant may be
cheaper than continuous distillation due to its simplicity. It is therefore resorted to only for low ca-
3
pacity plants; say processing about 1e2m per day of feed produced in batches in any upstream unit or
in pilot plant operations.
In spite of being thermally less efficient and requiring relatively higher capital investment, batch
distillation provides much more flexibility in operation; a single column can produce any number of
products. To achieve the same result using a continuous plant, (Ne1) columns would typically be
required for N product streams. It is an attractive option when the purity of product is a major concern
and any off specification product is undesirable especially when the cost of energy is not prohibitive
compared to value addition in this separation step. This is often the case for speciality, high value
products in small volume, as in pharmaceutical industry and in manufacture of speciality chemicals. It
is also used when the batch size as well as the feed quality (composition) may vary over wide ranges or
the product quality (composition) requirements may vary from time to time and possibly from batch to
batch. When the feed has a tendency to leave deposits in the equipment that requires frequent cleaning
and maintenance, a batch process is often opted.
Continuous processes make almost full utilisation of the plant facility except short phases of start-
up, shutdown and plant emergencies and therefore have high productivity. Large plants like refineries,
petrochemicals, fertilisers and other medium to large chemical industries go for continuous distillation
processes.