CHEMICAL ADDA: HOW PROCESS CONTROL

Thursday, 14 July 2016

Consider the tank heater system shown in Figure

Assumption

Fi, Ti = flow rate (ft3/min) and temperature(°F) of entering liquid into the tank

F = flow rate of steam which heated liquid (lb/mm).

F, T = the flow rate and temperature of the stream leaving the tank.

The tank is considered to be well stirred, which implies that the temperature of the effluent is equal to the temperature of the liquid in the tank.

Objectives

The operation of the heater is disturbed by external factors such as changes in the feed flow rate and temperature (Fiand Ti).
If nothing changed, then after attaining T = Ts and V = Vs, we could leave the system alone without any supervision and control.
Consequently, some form of control action is needed to alleviate the impact of the changing disturbances and keep T and V at the desired values.
In Figure we see such a control action to keep T = Ts when Ti or Fi, changes.
A thermocouple measures the temperature T of the liquid in the tank.
Then T is compared with the desired value Ts, yielding a deviation ε = Ts - T.
The value of the deviation ε is sent to a control mechanism which decides what must be done in order for the temperature T to return back to the desired value T.
If ε > 0, which implies that T < Ts, the controller opens the steam valve so that more heat can be supplied.
On the contrary, the controller closes the steam valve when ε < 0 or T> Ts.
It is clear that when T = Ts (i.e., ε = 0), the controller does nothing.
This control system, which measures the variable of direct importance (T in this case) after a disturbance had its effect on it, is called the feedback control system.
The desired value Ts is called the set point and is supplied externally by the person in charge of production.

Returning to the tank heater example, we realize that we can use a different control arrangement to maintain T= Ts when Ti, changes.
Measure the temperature of the inlet stream T, and open or close the steam valve to provide more or less steam.
Such a control configuration is called feedforward control.
The feedforward control does not wait until the effect of the disturbances has been felt by the system, but acts appropriately before the external disturbance affects the system, anticipating what its effect will be.

In Figure we see a control action to keep h = hs when Ti or Fi, changes. So that tank will not overflow or go dry
A level measuring device measures the height h of the liquid in the tank.
Then h is compared with the desired value hs, yielding a deviation ε = h - hs.
The value of the deviation ε is sent to a control mechanism which decides what must be done in order for the height h to return back to the desired value hs.
It may open or close the valve that affects the effluent flow rate F
It is also feedback control systems act post facto (after the fact), that is, after the effect of the disturbances has been felt by the process.
If ε > 0, which implies that h < hs, the controller opens the steam valve so that more heat can be supplied.
On the contrary, the controller closes the steam valve when ε < 0 or T>Ts.
It is clear that when T = Ts (i.e., ε = 0), the controller does nothing.
This control system, which measures the variable of direct importance (T in this case) after a disturbance had its effect on it, is called the feedback control system.
The desired value Ts is called the set point and is supplied externally by the person in charge of production.

CHEMICAL ADDA