A case study of Frontrol System application and ethanol distillation columns' efficiency boost.
Distillation columns are a good example of a multi-variable nonlinear system. They have at least five control and process variables that are completely interdependent. Therefore, the use of simple PID (proportional, integral, and derivative) controllers is usually not successful. This is the reason why a great number of distillation columns are controlled manually. In this case, the application of fuzzy controllers is described for a set of columns.
Usually, a set of three columns are used to obtain ethanol at 99.5% from the third column (C). The second column offers ethanol at 93.8% which is utilized as a fuel source for Brazilian cars (fig1).
Column B uses strategies to mantain/control ethanol's density at 93.8 %. The most used strategy is the change of ethanol flow at the output valve of column B. Increasing the output flow would decrease the ethanol density because it causes a small residence time inside the column.
A simple control loop having the output density as a process variable and the output flow as a control variable is not effective because of several disturbances involving the whole process. The most important are the input wine flow, the bottom level of column B, the top column temperature, the steam flow, and the steam enthalpy.
Considering that all those variables are strongly interconnected, the use of an advanced control strategy is very useful, specially if it considers the nonlinearity of the dynamic system, and the interdependence of them. Frontrol System's fuzzy controller can keep the ethanol density as close as possible to 93.8 % and reducing the steam use. In some plants which are using Frontrol System, the variability of the ethanol density was strongly reduced. This could be seen by the reduction of the standard deviation error in more the 20%, when compared to the system running without Frontrol System's technology. Also, due to the variabilyty reduction, a sensible reduction of as much as 10% was obtained in the steam usage.