Introduction

A chemical process plant is an integration of various processing units (eg. reactor, columns, exchangers, pumps etc.) placed in a systematic manner whose overall objective is to convert a certain raw material into useful product in a safe and economical manner.

I.1        Necessity for Process Control Applications

During its operation, a chemical process is required to satisfy several operational criteria imposed by the design engineer. These criteria are set in order to meet various technical, economic and social demands. A brief account of such criteria is given below in the order of their relative importance.

      • Safety: In a plant operation, particularly in hazardous condition, safety has the top most priority. Human lives are most important and a control engineer has to ensure maximum security for the people working inside the plant while designing its control system. For an example, if a furnace has been designed to operate with maximum allowable temperature of 1000°C, one needs to ensure that the controlled temperature is maintained much below this value. Alarm switches of different warning levels are integral parts of a control system.

      • Environmental regulations: Effluent management is a critical issue for any plant operation. Government regulations are set in order to limit the release of toxic gas (in atmosphere) and wastewater (into river). A control system is required to ensure that such regulations are strictly followed.

      • Product specifications: Maintaining the quality of the final product is important. A poor quality product would not be salable in the market and thus very purpose of the process plant would be defeated, unless this important criteria is taken care of by the designer of the control system.

      • Operational constraints: The operational constraints include various types of limitation that the associated equipment may have. For example, a valve cannot open more than 100% even if the best operability demands so; a pump has to maintain the NPSH; flooding of plates in a distillation column should be avoided; etc.