Distillation Control Variables ◾  9
1. Feed
2. Boilup
3. Reflux
2.2 Distillation System Outlet Streams
There are two main streams and one minor stream leaving a
distillation system:
1. Distillate
2. Bottoms
3. Small vent stream of inert or light vapors leaving the
reflux drum

2.3 Controlled Variables
A controlled process variable is usually a flow rate, pressure,
or temperature signal from a sensor and transmitter that is
used to provide the input to a controller. A controlled variable
is controlled to a setpoint.
The main variables to be controlled in a distillation column
are the following:
1. Distillation column pressure
2. Reflux drum liquid level
3. Column bottom liquid level
4. Separation power base
5. Material balance split
The pressure in a distillation column has an overriding influence on the control of the process. The pressure needs to be
run as steady as possible because a reduction in pressure can