Design and Control of Distillation Processes to Separate Azeotropic Mixtures Aided by Liquid-Liquid Separation

This work is divided into two parts. One is the separation of a mixture including methyl methacrylate, methanol, and water. The other is the investigation into hybrid extraction−distillation process with a case study of n-propanol dehydration. In the first part, an energy-saving design for the separation of a mixture including methyl methacrylate, methanol, and water is investigated as compared to a previous two-column design using the same process units but different configurations. In this energy-saving design, top and bottoms products of the regular distillation column are designed to be at unstable and stable nodes, respectively. Thus, results show that significant savings in the steam cost (16.3%) can be realized as opposed to the previous design of placing the bottom product at a saddle point. Another benefit is that the loss of methyl methacrylate product through the water outlet stream is also less than that of the previous design, representing another 9.6% savings of the operating cost for this energy-saving design. Furthermore, an overall control structure for this proposed design is also devised based on a novel way of using open-loop and closed-loop sensitivity tests. By the control of a temperature difference at two trays of the regular distillation column and another single-tray temperature at the stripper, both MMA and water products can be maintained at high purities despite large variations in feed flow rate and feed composition changes. In the second part, hybrid extraction−distillation separation system, which is a process combining the techniques of liquid-liquid separation and also distillation, is well investigated. This hybrid system can be viewed as a derivative from heterogeneous azeotropic distillation method. In this work, the potential for significant energy-saving via this hybrid process is demonstrated with both conceptual illustration and a case study of n-propanol dehydration. Diisopropyl ether (DIPE) is selected as the extraction solvent considering its favorable properties of density, heat of vaporization, and less toxicity. With the optimized solvent flow rate via this process, significant saving on reboiler duty (64.09%) can be realized compared to an extractive distillation system. Furthermore, a novel control strategy is proposed based on closed-loop and open-loop sensitivity tests. Here, an adjustable solvent flow rate during dynamic control allows the operation at the optimal steady-state condition. Dynamic simulation results show that both n-propanol and water products can still be maintained at high-purities despite large variations in feed flow rate and feed composition changes.