Question: It is desired to design a simple linear feedforward controller to compensate for the effect of changes in feed rate to a distillation column on the overhead product composition. The reflux flow is to be manipulated to control the overhead composition. Two steps tests are applied to the column, one on feed rate and one on reflux flow set point; in each case, the response of the overhead composition is recorded and analyzed. Results of the tests are summarized in the following table.
The feed flow transmitter has a calibrated range of 0 to 50 klb/h, and the reflux flow transmitter has a range of 0 to 100 klb/h. Both transmitters generate signals that are linear with flow. Draw the block diagram that will show the effect of feed rate and reflux flow on the overhead composition. Approximate the transfer functions with firstorder-plus-dead-time models, and show the numerical values of all parameters. Using the block diagram, design a linear feedforward controller to compensate for the feed disturbance on the control objective. Draw the instrumentation diagram for your feedforward controller. Include feedback trim, assuming that an on-line analyzer is installed on the overhead product stream.