Page 334 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

336 Chapter 11 Distillation

3. At this time, the coolant flow to the condenser is started and the liquid condensed starts
accumulating in the reflux drum. Some product may also be collected during this period. The
reflux valve is opened once the liquid level in the reflux drum reaches the normal operating level.
4. If no product is withdrawn in step 3, the column is run under total reflux operation, i.e., the entire
vapour reaching the condenser is condensed and refluxed back. This is continued till a steady
state, evidenced by steady temperatures in the column top and the still, is reached. Alternatively
the operation can be continued till the distillate composition reaches the desired product purity.
Duration of the first step is usually small compared to the overall batch time, whereas the duration
of steps 2e4 is important and in some cases it may take a long time to reach a steady state or the
desired initial distillate composition.
Production phase
Production period starts with withdrawal of distillate product from the reflux drum. Operation in
this period and its duration depends on the required specifications of the product or on the economics
of the process.
Operation in the production phase can be under the following conditions:
(i) Constant distillate composition operation (variable reflux operation): On reaching the required
distillate purity, the start-up period ends. Product take off is started and a constant composition
product is collected by steadily increasing the reflux ratio until a specified amount of distillate
has been collected. During this period the column top temperature is kept constant by
continuously increasing the reflux flow. The operation ends when the reflux ratio has attained
some high value considered to be uneconomic.
(ii) Constant reflux operation: The total reflux start-up period ends with the column reaching its
steady state, i.e., the still and the column top temperatures become steady. During this production
phase, the column operates with a suitably preset fixed reflux ratio so that the distillate purity
from the initial to the end reduces from a high value to a lower value and the collected distillate
with average purity matches the target.
In general, the constant reflux operation is preferred. The pros and cons of each operation is dis-
cussed in Section 11.8.1.

(iii) Optimal operation: A third mode of operation is a trade-off between the above two modes. An
optimal reflux policy is chosen so that some objective function is optimised (minimum batch
time, maximum product yield, maximum profit, etc.), subject to constraints (e.g., on product
amount and purity) at the end of the process.

Shutdown
At the end of the production phase, a batch distillation column is shut down in the following
sequence:

1. Heat supply to the column is cut off.
2. Holdup in the column is collected in the still.
3. Condenser holdup may be mixed with the top product or with the still material.