Most chemical processes are nonlinear in nature. When large set point changes or load disturbances occur frequently, nonlinear control systems are required. Instead of using the differential geometric method or nonlinear model predictive method, simple gain scheduling may be sufficient for many nonlinear single input-single output (SISO) processes. To design such simple control systems systematically, a nonlinear first-order plus time delay model is proposed for model-based control. A logarithmic transformation which is very useful for control of high purity distillation columns is shown to be effective in general. Several chemical process examples are also given.