Composition Control Systems

Composition Control Systems

The control of composition is probably the most important objective in the chemical industry due to the requirement for specification on products. It is thus a strategic rather than an operational control problem and can only be considered sensibly in the context of whole process control. This will be discussed fully later but it is relevent to include a short example here.

Simple Composition Control Problem

To illustrate composition control consider the simplest process in which composition can be changed, namely blending . Here two streams of different compositions are mixed together e.g. a concentrate and a diluent as shown in the diagram below.

Either of the above schemes could be used although the first is preferred. The reasons why are discussed in a later section.

It is worth mentioning that the composition of a stream is rarely measured directly.

Typical composition analysers include

  • Gas chromatagraphs
  • Spectroscopic analysers

Features of this type of hardware which make them ineffective for control purposes are

  • Large time delay in their response
  • Low operational reliability
  • Relatively high cost

Thus an alternative method has to be sought to control the composition. This could be via the

  • Temperature of the mixture
  • Pressure of the mixture
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