Why do we need control?

Although, as chemical engineers, we are used to making design calculations assuming steady-state models, and having a complete knowledge of the values of all the variables in a problem, in a real plant things are not quite as simple. Many things can disturb the smooth operation of a process, for example: raw material variations, cooling water temperature variations, environmental temperature, rainfall ( most distillation columns are outside, and a shower of rain can cause big changes in the temperature profile), and many more. Also, periodically, processes have to change their operating conditions to switch from one product to another, or one grade of raw material to another.

Disturbances, and desired changes in process conditions, mean that continuous active intervention is required to keep the process at a desired state. In the old days, this used to be done by large numbers of operators who kept an eye on the process and went around changing valve positions. Full manual control quickly became unrealistic as processes became bigger and more complex, and the trend today is toward complete automation of all control functions.

Without process control process plants couldn't be operated. With poor control plants may be dangerous, uneconomic, wasteful, polluting and generally pretty awful. Good control is vital for chemical plant operation and this is reflected by the increasing number of chemical engineers who are being recruited into control functions.