Direct Control of D :
- The most direct scheme makes use of signal from composition controller to adjust distillate flow D.
- The reflux flow is control by an averaging level type controller, and decrease in D will eventually produce an equal increase in R.
- A constant vapor flow is maintained by regulating the steam pressure or by controlling steam flow.
- An upstream pressure regulator is necessary in order to compensate for square root of upstream density.
- Another way for controlling vapor is to control pressure drop across column.
Indirect Control of D :
- Instead of direct control of, composition controller can control R & level controller can adjust D.
- For a very high reflux ratio R/D, this scheme is sensitive to upsets.
- This scheme is more sensitive to changes in reflux temperature.
- The third scheme is making use of ovehead composition controller to control flow of vapor rate.
- The reflux is controlled at fix value.
- This system is slower than first two schemes if sensitive element is at top of column.
- But if sensing element is several plates from top, this scheme is faster than other two.