Step 2: Determine control degrees of
freedom
Count the number of control valves available.
This is the number of degrees of freedom for control, i.e., the number
of variables that can be controlled to setpoint. The valves must be
legitimate (flow through a liquid-filled line can be regulated by only
One control valve). The placement ofthese control valves can sometimes
be made to improve dynamic performance, but often there is no choice
in their location.
Most of these valves will be used to achieve basic regulatory control
of the process: (1) set production rate, (2) maintain gas and liquid
inventories, (3) control product qualities, and (4) avoid safety and environmental
constraints. Any valves that remain after these vital tasks
have been accomplished can be utilized to enhance steady-state economic
objectives or dynamic controllability (e.g., minimize energy consumption,
maximize yield, or reject disturbances).
During the course of the subsequent steps, we may find that we
lack suitable manipulators to achieve the desired economic control
objectives. Then we must change the process desigu to obtain additional
handles. For example, we may need to add bypass lines around heat
exchangers and include auxiliary heat exchangers.