Until the 1940s of the last century, manual operation required a very large number of operators. Large tanks were also employed among various units in the plant, to cushion disturbances.
The digitalization of control occurred in the 1970s with the advent of digital computers. Currently, operators, controllers, indicators, registers and alarms are in the control room, from which it is possible to monitor and control the entire process of the field.
“About 80 percent of the time spent in a plant is spent in the control room,” and “the control room is the nerve center of the plant.”
The industrial processes are very varied and encompass different equipment that demand the precise and exact control of the variables that determine the performance of the unit operations.
Process control refers to the maintenance of process variables (temperatures, pressures, flow rates, compositions, etc.) at some desired operating value.
The chemical processes are, by nature, DYNAMIC, that is, they vary continuously with time, either by external interferences or by changes in operational policy.
Thus, the chemical engineer must be able to: monitor certain process variables and induce changes in suitable variables, both of which are primary functions of control systems.
Process control aims to:
- Safety of personnel – maximum pressure must not be exceeded;
- Environmental protection – material not to be disposed of into the atmosphere;
- Equipment protection – the flow through the pump must be greater than or equal to the minimum;
- Smooth operation – the load should have little variability;
- Product quality – desirable values of the liquid product;
- Efficiency and optimization – reduction of heat exchange costs;
- Monitoring and diagnosis – sensors, displays, variables calculated to inform the operator about normal and abnormal operating conditions.
There are two forms of control used, feedback control and feedforward control.
Control feedback (FB): The traditional way of controlling a process is to measure the variable to be controlled, compare its value with the desired value (the controller set-point) and feed the difference (the error) in a feedback controller that will modify a manipulated variable to bring the controlled variable to the desired value.
Feedforward control (FF): the disturbance is detected when it enters the process and an appropriate change is made in the manipulated variable in order to keep the variable controlled in the set-point.
The implementation of control systems is of paramount importance for industrial processes, given all the economic benefits and justifications, such as: operating the plant safely, according to environmental restrictions and operating limits, reducing production stops and risk of accidents.
In a competitive market, one must: increase productivity, reduce the use of raw materials and maintain the final quality of the product, with control application that becomes easier.
This text belongs to the author and should not be reproduced without permission from BetaEQ and the same.
Patricia Rodrigues Gonring
Chemical Engineering Student of Federal University of Espírito Santo – Alegre – ES