A variable transformation approach to in-line pH control

Current practice and research into pH control principally entails the use of continuous stirred tank reactors (CSTR). Whilst the over-design of this process does indeed damp out disturbances, it is expensive and leads to problems with control. Industry requires systems that are: economical, both in terms of capital and operating costs; robust from a control point of view; operable and flexible from a process point of view. pH control of neutralisation is a difficult nonlinear control problem. This is largely due to the gross nonlinear behaviour of pH measurement, but also due to the variable time delays inherent in the process. Nonlinear processes have traditionally been controlled by a combination of linear control methods and gain scheduling. However in the last few years much progress has been made in the development of nonlinear control systems. This paper outlines a variable transformation approach, that is an anti-logging technique which removes the nonlinearity at source and yields an hydrogen ion concentration which is easier to handle.