A chemical plant is an arrangement of processing units (reactors, heat exchanger, pumps, distillation column, absorber, evaporators, tanks, etc), integrated with one another in a systematic manner. The plant’s overall objective is to convert certain raw materials into desired products using available sources of energy, in the most economical way.
During its operation, a chemical plant must satisfy several requirement imposed by its designers and general technical, economic, and social conditions in the presence of ever-changing external influences (disturbances). Among such requirements are the following:-
The safe operation of a chemical process is a primary requirements foe the well-being of the people in the plant. Thus the operating pressures, temperatures, concentrations of chemicals, and so on, should always be within allowable limits.
- Production specifications:
A plant should produce the desired amounts and quality of the final products. For example, we may require the production of 2 million pounds of ethylene per day, of 99% purity. Therefore, a control system is needed to ensure that the production level (2 million pounds per day) and purity specifications (99.5% ethylene) are satisfied.
(3) Environmental regulations:
Various federal and state laws may specify that the temperatures, concentrations of chemicals, and flow rates of the effluent from a plant be within certain limits. Such regulations exist, for example, on the amounts of SO2 that a plant can reject to the atmosphere, and on the quality of water returned to a river or a lake.
(4) Operational constraints:
The various types of equipment used in a chemical plant have constraints inherent to their operation. Such constraints should be satisfied throughout the operation of plant. Foe example, pumps should maintain a certain net positive suction heads; distillation columns should not flooded; the temperature in a catalytic reactor should not exceed an upper limit since the catalyst will be destroyed.
The operation of plant must confirm with the market conditions, that is, the availability of raw material and the demand of final products. Thus it is required that the operating conditions are controlled at given optimum levels of minimum operating cost, maximum profit, and so on.
All the requirements listed above dictate the need for the continuous monitoring of the operation of chemical plant and external intervention (control) to guarantee the satisfaction of operational objectives. This is accomplished of a rational arrangement of equipment (measuring devices, valves, controllers, computers) and human intervention (plant designers, plant operators), which together constitute control system.
Control Over Continuous Stir Tank Reactor:
For temperature control we employed cascade control configuration. In a cascade control configuration we have one manipulated variable and more than one measurement.
The reaction is endothermic and heat is supplied by dowtherm, which flows in the jacket around the tank. The control objective is to keep the temperature of the reacting mixture, T, constant at the desired value. Possible disturbances to the reactor include the feed temperature Tf. and the dowtherm temperature Th. The only manipulated variable is the dowtherm flow rate Fh.
We control the reaction temperature by measuring Th and taking control action before its effect has been felt by the reacting mixture. Thus if Th goes down, increase the flow rate of dowtherm to give the same amount of heat. Decrease the flow rate when Th increases. Disturbances arising within the secondary loop are corrected by the secondary controller before they can affect the value of the primary controlled output.
Control Over Heat Exchanger:
The control objective is to keep the exit temperature at 110 oC for our shell and tube heat exchanger.
The possible disturbances are:
- Offset of temperature value from its desired value of 110o
- Variation in temperature of dowtherm used as a coolant media.
- Control over Distillation Column:
Cascade control is usually employed to regulate the temperature (and consequently the concentration) at the bottom or top of a distillation column.