Process Control Philosophy

Good operating practices and systems are required for the successful operation of the plant. The control philosophy of some of the equipments is described below.

13.1 Reactor R 301

A proper and elegant control system is necessary for the smooth operation of reactor R 301. Ethylene oxide is highly inflammable gas. The other reactant namely CO₂ should always be present in excess amount than ethylene oxide. The molar ratio between EO and CO₂ is maintained using a ratio controller. The other important parameter in reactor is pressure. 15 bar pressure is maintained in the reactor using a feedback PI controller, that in turn adjusts inlet EO flow rate. Temperature in the reactor is controlled by a plate heat exchanger. A cascade control philosophy is provided to the heat exchanger.

Objective	Type of Controller	Variables
		Controlled	Measured	Manipulated
Pressure	Feedback PI	Pressure	Pressure	EO Flowrate
FeedRatio Control	Ratio Controller	Mole Ratio	Flow rate	CO2 Flow rate
Level control	Feedback PI	Level	Level	Outlet flow rate
Temperature control	Cascade
	Primary PID	Temperature	Temperature	Cooling Water Flowrate
	Secondary P	Cooling Water Flowrate	Cooling Water Flowrate	Cooling Water Flowrate

13.2 Distillation Column DC401

Consistent product quality is ensured if distillation column is subjected to a proper process control strategy. Ethylene carbonate is obtained from the top section of the column. Its composition is a function of the distillate temperature. Reflux rate is varied to maintain a constant composition at the top of column. Level of liquid in the column is maintained by adjusting the bottom flowrate. A cascade control around the reboiler manipulates steam flowrate to maintain temperature in the column.

Objective	Type of Controller	Variables
		Controlled	Measured	Manipulated
Composition Control	PID cascade	Composition	Temperature	Reflux flow
Pressure	PID	Pressure	Pressure	Cooling water flow rate
Level control	Feedback PI	Level in the Column	Level in the Column	Bottoms Flowrate
Temperature control	Cascade
	Primary PID	Temperature	Temperature	Steam Flowrate
	Secondary P	Steam Flowrate	Steam Flowrate	Steam  Flowrate

13.3 Heaters H 201, H 202

These are heaters that are used for heating gases EO and CO₂ using steam as heating medium. Theoretically, adjusting a valve in the steam inlet line can control temperature. The condensing pressure is a function of load when the temperature is controlled by throttling the steam inlet. This at low loads and low temperature can result in below atmospheric condensing pressures. But this condensing pressure will not be sufficient to discharge the condensate through the steam trap. As condensate accumulation progresses, more and more of heat transfer area will be covered up resulting in a corresponding pressure. When this pressure rises sufficiently to discharge trap, the condensate is suddenly blown out and effective heat transfer surface area increases several folds. As a result temperature control becomes impossible.

Positioning a control valve in the condensate line is an attractive option. Condensate valve is much cheaper than a valve for steam service. For a precise temperature control a level control loop is introduced. With this instrumentation, it is possible to adjust the size of heater by changing the level set point to match process load. But this is going to have an additional burden on cost.

Objective	Type of Controller	Variables
		Controlled	Measured	Manipulated
Temperature of Process Stream at Exit	Feedback PI	Temperature	Temperature	Steam Flowrate
Reduction of Vacuum Formation	Feedback PI	Heat Transfer Area	Level of Condensate	Condensate Flowrate

13.4 Cooler C 404

This product cooler is a shell and tube type heat exchanger that cools ethylene carbonate using water as coolant. The limits within which process temperature can be controlled are functions of the nature of load changes expected and of the speed of response for the whole unit. In many installations, the process time lag in the heat exchanger is too great to allow for effective control during load changes. In such cases it is possible to circumvent the dynamic characteristics of the exchanger by partially bypassing and blending the warm process fluid with cooled process fluid. The resulting increased system speed of response; together with some cost savings are the main advantages of this control strategy. Cooling water conservation is taken care by this strategy. It tends to maximise the outlet cooling water temperature, thereby minimising rate of water usage. A three-way valve is selected because it is most economical and provides tight shut off.

Objective	Type of Controller	Variables
		Controlled	Measured	Manipulated
Temperature of Process Stream at Exit	Feedback PI	Temperature	Temperature	Flowrate of Bypass Process Fluid
Cooling Water Conservation	Feedback PI	Temperature	Temperature	Cooling Water Outlet Flowrate