Dynamics and Control of Thermally Heat-Integrated Systems

The most crucial objective in a chemical process is to synthesize/design a robust control structure, which can ensure a safe, smooth, and profitable operation within the process, even in case of disturbances. In this work, a plantwide control structure has been synthesized for an example process where tetrahydrofuran (THF)–water mixture is separated using an extractive distillation technique. The process design flow sheet consists of two distillation columns from where THF and water are being recovered at high purities while the entrainer is recovered and recycled as a material recycle stream within the process. Further, two feed-effluent heat exchangers (FEHEs) are used to exchange the heats between the hot and the cold streams within the process to provide the heat integration, thus making the process more energy efficient. An improvement of 20.79% in the overall energy requirements has been observed upon applying the heat integration circuit. A plantwide control structure is later synthesized on the base case design, and its performance is evaluated for ±10% change in throughput change and ±5% change in composition change. The significant feature of this work is the synthesis of control strategy when the control degree of freedom is lost during the implementation of heat integration circuits. To counter this issue, Hi-Select control loop is used along with supervisory composition controllers.