The feed is pumped into the column whose flow rate and composition are controlled. Feed mainly comes through a major supply line where the feed composition may vary due to various factors which are beyond the scope of the operator of the distillation column. An auxiliary supply line with guaranteed purity of material can be mixed with the main supply through a regulated dose in order to maintain the composition of the feed entering the column. A flow controller at the entry point of the column maintains the feed flow rate. The pressure of the top section of the column is maintained by regulating the flow of vapour going out of the column. The vapour is condensed and collected in a tank. The outlet temperature of distillate coming out the condenser (heat exchanger) is controlled by regulating the flow rate of cooling water used in the heat exchanger. The liquid level inside the storage tank in the downstream of condenser is controlled with a level controller by regulating the tank outlet. The L/D ratio of a distillation column is very important and it needs to be controlled by a ratio controller. The ratio transmitter is essentially a flow measuring device that sends the ratio of the two flow rates rather than the flow rates themselves. In all the above examples, only one type of final control element has been demonstrated viz . control valve. Nevertheless, there are various other final control elements available such as variable speed metering pump, heating regulators etc .

Note : The composition transmitter/controller is usually a high value delicate instrument which is not used in a rugged process plant environment. On-line gas chromatograph is an example of such sophisticated instrument. They are used only in a few critical operations where tighter regulation of composition is mandatory, such as automated drug delivery.

The above example indicates that the basic hardware components of a feedback control configuration are the following:

•  Process

•  Measuring devices

•  Transmission lines

•  Controllers

•  Final Control Elements

In the following sections, we shall discuss each and every hardware item in an elaborate manner.