(573c) A Conceptual Design Method of the Total Site Energy System in Process Industries

For the production system in the process industries (chemical, petro-chemical, metallurgy), the processing process from raw material to products is always accompanied by energy supply, use, recovery, loss etc. As known, the optimal synthesis technology has played an important role in making improvements in process energy system. So far there are numerous works concerning the optimal synthesis of energy system including heat exchanger networks, utility systems and total site energy system (TSES), but there is lack of a decision support system for determining the TSES performance targets and their corresponding topological structure and main decision variables at the preliminary design stage of the whole plant or enterprise. The TSES is a very complicated system consisted of heat exchanger networks , utility system and other supplementary equipments. There is no need of having a detailed design of the TSES at the preliminary design stage of the whole plant/enterprise because a lot of technological parameters, such as temperatures and pressures of the streams from reactors, separation columns etc., possess some uncertainty,. As known, these technological parameters have great impact on the design of TSES. To obtain a TSES with minimum annual cost and satisfied operation performance, it is required to do some iteration between TSES and a set of technological parameters. Therefore, a more flexible design tool of the TSES is needed at the preliminary stage of the whole plant/enterprise. To tackle this problem this paper proposed a conceptual design method of TSES based on the core decision model proposed earlier by Zhu(1989 ). According to this model, the TSES could be simplified as one consisted of a total heat exchanger network, boiler, turbine and steam pipelines at different pressures and other supplementary equipments. The main decision variables for the core decision model are the DT_min of the total heat exchanger network, boiler pressure, the back pressures of the turbine, the amount of steam produced at pre-specified pressures. Thus, the conceptual design method for determining the performance targets and their corresponding topological structure as well as decision variables could be illustrated as follows:

1. The heat exchange between process streams was carried out at first within each process by using pinch technology. 2. The heat exchange between the process hot and cold streams remained after the heat exchange within different processes could be integrated to one total heat exchanger network by using the pinch technology. 3. A total turbine is integrated with the total heat exchanger network above the pinch, which satisfies the criterion of the appropriate placement of the heat engine to the process heat sink proposed by Linnhoff(1983). 4. A boiler is integrated to the total turbine. 5. A NLP or MINLP mathematical model was formulated and solved for minimizing the total cost of the total site energy system with the above mentioned decision variables. 6. A sensitivity analysis is carried out to find the key technological parameters that affect the performance targets of TSES greatly. 7. Changing the key technological parameters within their feasible regions, return to step 1. Iteration stops when no improvements in the performance targets could be found. The conceptual design of TSES is completed.

This paper concentrates on the essence not the detail. One industrial case study is demonstrated to show the broad application perspective of the proposed method..