Ask Greg McMillan

Greg's Response:

Ratio control is much more effective than flow feedforward control if there are good flow measurements. Ratio control enables the operator to see and if necessary, adjust a flow ratio. Many unit operations startup on ratio control without process control feedback until the process reaches equilibrium conditions. It is particularly important that extensive ratio control (e.g., distillate to feed, steam to feed, and bottoms to feed ratios) be used in the startup of distillation columns until the temperatures and concentrations reach normal operating conditions. Dynamic simulations are used in the same way as was done for feedforward control to provide the dynamic compensation needed. Ratio and bias-gain blocks are used in the PID configuration where the operator can see the actual ratio and manipulate a desired ratio to handle startup and abnormal operating conditions. Dynamic Simulation is critical to provide the guidance for the operators needed in startup and for equipment and instrumentation problems. Dynamic simulation can be used to automate the startup by procedure automation or automate solutions by state-based control.

When the process is ready for PID control, the process PID manipulates the ratio block setpoint (equivalent to a feedforward multiplier used in dead time dominant processes) with an adaptive PID manipulating the bias/gain block bias or the process PID manipulates the bias/gain block bias (equivalent to the more prevalent feedforward summer described in last week’s post) with an adaptive PID manipulating the ratio block setpoint. Simulation can find how the dynamics change with operating conditions and the adaptive PID tuning to provide a gradual approach to the goal of negligible feedback correction of feedforward by the process PID with minimal interaction between the process and adaptive PID controllers.