Controle de uma coluna de destilação azeotrópica não convencional – um estudo de caso industrial.

Abstract

Distillation columns, in general, constitute a significant fraction of the capital and the cost of producing chemical plants. Therefore, they need to be designed and controlled to maintain optimal operating conditions. In this job, a new proposal for a composition control strategy for a distillation column belonging to a commercial plant producing vinyl Some peculiarities of this column and the requirements of the control system aspects of this case are unusual. This is a distillation high purity multicomponent high reflux ratio, where the unconventional azeotropic behavior. Open-loop responses to perturbations in the feed rate and thermal load of the substantial uprising and reversal. The requirements of the control system are unusual by including two-component composition control at concentration levels of parts per million in the current at the base of the column, one of which is more precisely CCl4 has intermediate volatility in the mixture. The column displays stabilization of this component. The design of a control system for a process with the characteristics cited represents a great challenge. A brief review of the literature on distillation, mathematical modeling of columns and control structures of distillation columns. The stationary simulation of the column was the starting point for the development of the work, because through the same the process model was validated and the composition profiles of the components obtained. It was defined, control the composition of only one of the components, in the in the case of CHCl3, since CCl4 remained within the composition range during the various simulations. The initial proposal to control the composition of the selected component was an inference control, using the temperature of the as a controlled variable and the thermal load of the manipulated, but it was abandoned because of the low sensitivity presented temperature at various points along the column and mainly by the high period of time presented by the column to reach a new steady state. As the flow rate of reflux showed a good sensitivity to of the feed and the thermal load of the reformer, it was chosen for the control of CHCl3 composition at the base of the column. This good sensitivity was proved by a dynamic inference study. The initial problem was how to use this variable to control the composition of the base, since it was used for the control the level of the organic phase in the reflux vessel. It was then thought of a control "Override" with two meshes: the first mesh for inferential control of the composition of the CHCl3 from the reflux rate by manipulating the thermal and the second mesh for controlling the level of the organic phase by manipulating the flow rate. The simulations showed that this control strategy would also have to be abandoned due to the fact that the level of the organic phase presents very that is, tends to a maximum or minimum very quickly. We decided control the level of the organic phase in the reflux vessel from the thermal load leaving the free reflux flow rate can be used for composition control. It was found that the effects of feed flow and reflux were contrary, which led us to conclude that the best strategy control would be a Feedforward control, where for a given change in flow the control system adjusts the reflux flow rate so that the sum of the effects of each of these flows in the CHCl3 composition cancel the controlled composition. The performance of it was excellent mainly because in addition to controlling the composition of CHCl3 in the base, it also stabilized composition of CCl4. It was concluded that in addition to controlling the composition of CHCl3 and stabilize the composition of CCl4 in the base the control system is still capable of these values ​​on an ongoing basis, which may to monitor in real time the possible variations that may occur in the the setpoint.