(219f) Energy Efficient Control of Distillation Process

Improving the energy efficiency of new and existing processes is an increasingly important issue for consideration by industrialist, academia and policy makers. This is because of the economic, environmental and ecological implications of unutilised energy. Distillation is one of the most widely used energy intensive separation technique in the chemical and petrochemical industries. Distillation unit however poses a great challenge to control engineers because of its complexity, specific operating objectives, significant interactions among the control loops and specialised constraints. These have often led to trade-off between energy usage and product quality in distillation column control and operations. Several methods of control configuration selections are available in the literatures; however, few studies have been reported concerning the energy efficiency of various control configurations.  This study details the choice of an energy efficient control configuration by incorporating thermodynamics second law in the selection criteria. Two binary systems of methanol-water and benzene-toluene were considered. Three control configurations selected for each of the system are LV, LB and DV.  Relative gain array (RGA) and relative exergy array (REA) were used in screening for the best control configuration for each system. REA incorporates minimum energy usage in the control structure selection.  A full detailed analysis of the performance of the controlled structure was conducted in the dynamic mode. Controller parameters were tuned using the biggest log modulus (BLT) method. Set point change in the distillate and bottoms composition were made for each of the configuration and their closed loop responses were obtained. Exergy loss, exergy efficiency and energy usage for the dynamic simulations were then calculated. The results show that the control configuration that was selected as the best from a combination of the RGA and REA for each of the system also has the least energy usage and exergy loss in the dynamic simulation. Thus the process operability analysis results are supported by simulation results.   Incorporating energy efficiency in control structure selection will ensure the control of a distillation process not only in terms of product quality and safety but for energy efficient operations as well.