Operating Policies Batch Distillation by Haydn I. Furlonge

The manner in which a column is operated, i. e. the choice of values for the operating decision variables ( e. g. reflux ratio), defines the operating policy. Conventional operating policies for batch distillation include constant reflux ratio, constant distillate composition and optimal reflux ratio operation. A less conventional policy is cyclic peration.
These policies are described below:

Constant reflux ratio

The reflux ratio is fixed at a pre-defined value which results in a variable distillate composition. The reflux ratio may also be varied in a piecewise-constant manner, whereby a different (constant) reflux ratio is used for each product cut.

Constant distillate composition
The distillate composition is held constant during the operation by varying the reflux ratio using a controller.

Optimal reflux ratio

The reflux ratio profile used is optimal with respect to a particular objective function ( e. g. profit) and set of constraints (e. g. product purity).

Cyclic operation
Comprises repetition of the following steps which make up one cycle (see Figure 1.2):

1. Flling-up

• low reflux to the column
• no distillate withdrawal
• holdup in the reflux drum increases to a pre-specified level

2. total reflux

• reflux flowrate is equal to the vapour flowrate through the condenser, so that the reflux drum holdup remains constant
• no distillate withdrawal

3. dumping

• low reflux to the column
• distillate is withdrawn
• reflux drum holdup decreases

The above operating policies apply primarily to a regular batch distillation column.

However, in principle, all column configurations may be operated under any one of these policies.It should be noted that it may be possible to combine two or more of these operating policies for a given separation. From an operational point of view, the simplest operating policy is the constant reflux ratio policy.

Apart from the reflux ratio, there are other operating decision variables such as the reboiler heat duty and condenser pressure. The choice of these depends on the properties of the mixture being separated as well as on economic considerations.