Incentives FOR PROCESS CONTROL Ojectives of a Process
Raw materials desired product
This is achieved using certain arrangement of reactors, heat exchangers, pumps, distillation columns, absorbers, evaporators, tanks,…..etc.
The arrangements must satisfy the following requirement:
1- Sefty (of people). Therefore, T, P, C (specially poisoneous or explosive components) should be within allowable range.
2- Product specification. Concentration of products must be maintained at certain level (by means of control).
3- Invironmental Regulation. Regulation requires that T, C and flow rate of certain chemical species be under some limits.
4- Operational constraint. Examples are: net positive suction head, level in the tank, flodding in distillation column, T in reactor of certain material, T and the catalysis for endothermal reactor.
5- Economics: Costs involve raw material, energy, capital and human labor.
Minimum operational cost maximum profit Can be summarized as follows:
- Suppressing the influence of external disturbance - Insuring stability of the process
- Optimization of the performance
Suppressing the influence of external disturbance :
Most common, because it is out of reach of the operator.
Proper controller mechanism makes the suitable changes on the process to cancel out the negative impact on the process.
Example
Fi, Ti
h T
F, T Fst condensate
Objectives:
1- Keep T = Td
2- Keep h = hd Changes: Fi, Ti
1- Because of process requirement. After Td and hd are reached then no need for control.
2- Because of disturbances. Control is needed all times.
Feedback Control
Feed forward meaqsur Ti and close or open the steam valve accordingly.
It does noot wait until the effect is felt by the system (it anticipate the effect)
- Insuring stability of the process Stable process
Y
Return to its initial value
t=t0 t
The process is stable or (self regulating). No need for the controller for stabilization.
Unstable process
Unstable process requires external control for stabilization.
- Optimizing the Performance of a Chemical Process Principal Objective: 1- Safety
2- Product specification
Making the operation of the plant more profitable.
Conditions which effect the operation of the plant (T, P, Concentration, F) should be changed in such away so that profit is maximum. This is done automatically by the controller and operator.
Example:
A 1 B 2 C Endothermic reaction Heat is supplied by steam B is the desired product.
Objective is to maximize the profit (Ф) over the period.
The only variable that we can change in the opartion is steam flow rate (Q) (Q effect T which effect ri which effect distribution ratio).
How Q can be changed with time to maximize Ф.
Q Qmax
Qmin
time
