The external influences, which have the potential to affect the dynamics of the process, are termed as process inputs whereas the entities, which themselves are affected due to the changes in process behaviour, are called process outputs. Flow rate of the feed stream is an example of process input that can influence the process dynamics, whereas the purity of the product stream can be called a process output (as the purity is a direct consequence of changes in process behaviour). In an ideal control configuration, process inputs are manipulated in a calculated manner in order to maintain the process output at a predefined desired set point. Input of a process can be of two types viz. Manipulated input and disturbance. Input, Which can be manipulated as per process requivement, are named manipulated input. And the inputs which are outside the scope of any manipulation but whose changes are difficult to avoid, are termed as disturbance. Feed flow rate and feed concentration are examples of manipulated input and disturbance respectively. Output of a process can be of two types viz, measured and unmeasured. Product flow rate and product concentration are examples of measured & unmeasured outputs respectively.

I.3        Aspects of good control strategy

The development of a good control strategy would ideally consist of identifying the following aspects:

• Control Objectives: The first and foremost aspect of control system design is to identify the control objectives. In other words, the designer should find out the answer for “What needs to be ensured that would lead to best operation of the process?” It is rather common to divide a large process plant into several smaller segments and identify the control objectives of each segment separately. However, it is not unusual to find some conflicting objectives that would promise supposedly best operation. Hence, identifying control objective is not a trivial task.

• Operating Characteristics: One needs to find out whether the process runs on batch mode, semi-batch mode or continuous mode. The operability issues at the time of start-up and shutdown may also be scrutinized.

• Inputs and Outputs: As already discussed before in sec 1.2.

• Constraints: As already discussed before in sec 1.1

• Safety, environmental and economic considerations: As already discussed before in sec 1.1