V T is also generally considered an input variable and is mainly used to control the column pressure. Since this is usually a self-regulating loop, we will ignore it in this discussion. Obviously, it is desirable to maximise the purity of the top and bottom products. Normally in industry, the control variables are set manually by human operators. Some plants use automatic control for only one product, and manual control for the other.
Manual control can result in overpurification or loss of valuable product.
These difficulties can be overcome by using dual composition control , i.e. automatic control for both products. There are some difficulties involved in designing a dual composition controller. In particular, the system is both strongly nonlinear and strongly interactive. Choosing the control structure is the most important step in designing a controller for the system. The column pressure and reboiler and condenser levels must be controlled very carefully to maintain stable operation.