A design method for the integration of heat and control in a process of toluene hydrodealkylation

Abstract The integration of heat and process control, strategies for designing a process that consumes minimum energy and improves controllability has been received an increasing interest by process engineers. The techniques of heat integration to improve efficiency of energy consumption normally consume necessary degree of freedom and raise difficulties in control design. This study aims to develop a method that compromise between heat recovery of a heat exchanger network and control performance of a process by balancing the duties of utility streams. A process without heat recovery should transfer all required energy (Q-Total) using utilities. This study suggested to leave a part of Q-Total as utility streams (Q-Utility) to serve as manipulated variables of a control system. This method can be described in a four-step procedure. In the first step, the Q-Utility of a process is determined. The pinch technology is applied to develop a heat exchanger network for heat recovery in the second step. The streams which relates to the Q-Utility should not be included in the pinch analysis. In the third step, a control system is properly designed by using the Q-Utility streams as manipulated variables. In the fourth step, some disturbances are introduced to the system in the form of pulse inputs to test the controllability and resiliency of the control system. This method was applied successfully to an industrial scale case study which was hydrodealkylation of toluene process. The control system was considered with three levels of Q-Utility which were 5%, 10% and 100% of Q-Total. The dynamic performance results revealed that when disturbances occurred, the process with 100%-Utility was very stable at its designed parameters. However, the alternative with 100%-Utility is not a good option because of its economic waste. Either 5% case or 10% case involved its own benefits and drawbacks. If a plant is well equipped with a good emergency response system and fast troubleshooting, then the 5% case should be chosen. It means the utility percentage for control design can be decreased and economic benefits of heat recovery can be increased. If the 10% case is chosen, the controllability can be improved but it should be compensated by increasing the utility cost.