Identifying the major disturbances is very important in any control problem, and for process
control the production rate (throughput) is often the main disturbance. In addition, the location of where the production rate is actually set (“throughput manipulator”), usually
determines the control structure for the inventory control of the various units. For a plant running at maximum capacity, the location where the production rate is set is usually
somewhere inside the plant, (e.g. caused by maximum capacity of a heat exchanger or a
compressor). Then, downstream of this location the plant has to process whatever comes in (given feed rate), and upstream of this location the plant has to produce the desired quantity
(given product rate). To avoid any “long loops”, it is preferably to use the input flow for inventory control upstream the location where the production rate is set, and to use the output
flow for inventory control downstream this location.
From this it follows that it is critical to know where in the plant the production rate is
set. In practice, the location may vary depending on operating conditions. This may require
reconfiguring of many control loops, but often supervisory control systems, such as model
predictive control, provide a simpler and better solution.