I.4        Classification of Process Variables

Not all the inputs are within the scope of measurement/manipulation. For an example, the specification of raw feed (viz. concentration) does have the potential to affect the reaction dynamics of a chemical reactor however usually it is neither desirable nor practical to manipulate concentration of raw feed. Nevertheless, quality of raw material is likely to alter due to various facts which are beyond the control of a plant engineer. Thus, any change of this process input is regarded as a disturbance to the process. Hence, process inputs are broadly categorized into two types (1) Manipulated Input (2) Disturbance Input. Similarly, not all the outputs are within the scope of measurement/control. For an example, temperature or flow rate of the outlet stream can be measured on-line through appropriate measuring instruments whereas quality (concentration) of product is usually not measured on-line unless criticality of process (such as drug delivery) demands stricter monitoring of quality (of medicine). Hence, process outputs are broadly classified into two types (1) Measured Output (2) Unmeasured Output.

Let us take the example of a continuous stirred tank reactor (CSTR) as shown in the Fig. I.3. Reactant A is entering into the CSTR at concentration C_Ai , flow rate F_i and temperature T_i. Product is coming out of the CSTR at concentration C_A, flow rate F and temperature T. The tank volume is V. The reaction is exothermic and hence coolant is passed through the cooling jacket at a flow rate F_c. Coolant is entering the jacket at temperature T_ci and exiting at T_co .