The undeniable importance of industrial distillation has motivated hundreds, if not thousands of theoretical and applied research projects. A considerable amount of those projects has used a bench or pilot scale distillation columns to have experimental confirmation of the proposed innovations. Nevertheless, the automatic control of bench scale distillation columns is not as straightforward as it appears to be, due to the low magnitude of the involved material flows. To precisely manipulate low magnitude flows, special low flow control valves are necessary; this results in high instrumentation costs which are not desirable at bench nor pilot scale.
This work treats the simulation and control of a low cost, bench scale column with an electrically heated reboiler and water-cooled condenser. It uses a version of an on-off valve as an actuator to control the distillate composition of the desired product. The use of a solenoid valve results in a discontinuous reflux flow, which changes the continuous dynamical model of the distillation column to a piecewise dynamical one. In other words, it can be assumed that the column switches between zero and infinite reflux ratio instantly. The resulting piecewise dynamic model is studied and analyzed, and because of this analysis, the dynamic implications of this strategy in the operation of the column are used to tune a PID-PWM controller and to formulate general recommendations to attain an efficient composition control in the distillate.