Study On The Identification And Construction Of Multi-stream Heat Exchanger Networks

The heat exchanger network (HEN) is used to recover the energy of process system. Since 1960's, the research to the synthesis of HEN is unceasing. The primary kind of heat exchanger is the single-hot and single-cold heat exchanger and this kind of heat exchanger is used in process industry such as petrochemical industry. The multi-stream heat exchanger is a new type of heat exchanger; it is currently used in gas processing. Compared with traditional single-hot and single-cold heat exchangers, multi-stream heat exchangers possess many advantages such as high heat transfer efficiency, compact structure and lower cost.Much work during the last half of century is focused on the conventional single-hot and single-cold heat exchanger networks; the research to the multi-stream heat exchanger networks is few. The way to the synthesis of multi-stream heat exchanger either synthesizes all streams to one multi-stream heat exchanger or merges some single-hot and single-cold heat exchangers to construct one multi-stream heat exchanger on the basis of optimized single-hot and single-cold heat exchanger network. The research aim for the multi-stream heat exchanger is to make them applied better in industry. This paper proposes a way of identification and construction for the multi-stream heat exchanger network on the basis of optimized single-hot and single-cold heat exchanger network, and forms the appropriate mathematical model considering the need of industry. The primary work is narrated as follows:1) Proposing the rule of identification and construction for multi-stream heat exchanger networksAt first, combining with the super-structure model, a genetic and simulated annealing algorithm (GA/SA) is used to get the optimized single-hot and single-cold heat exchanger network. Then the rules of identification and construction for multi-stream heat exchangers are proposed on the basis of this optimized network. The multi-stream heat exchanger is divided into two kinds, one is "one-multi" multi-stream heat exchanger in which the heat is exchanged between one stream and more streams, and the other is "multi-multi" multi-stream heat exchanger in which heat is exchanged between several streams and several streams. Of course, such annual network cost is the lowest according to the proposed rules in this paper.2) Modifying the original superstructure modelThe original superstructure model is modified by considering the model's practicality. Inthe model, the heat load of single-hot and single-cold heat exchangers is limited and the number of stages is increased properly. Some examples prove the original way to determine the number of stages is inapplicable if the heat load of heat exchanger is limited, so a new way to determine the number of stages is established. There are many large-area single-hot and single-cold heat exchangers burdening a large of heat in original optimized HEN, they will be divided into many small heat exchanger in industry. It is no doubt that the cost of heat transfer equipment increases. This paper looks the allowable maximum heat load as one constraint in superstructure model, the optimized HEN by this way is better than the former in the annual network cost, especially when the equipment cost has a big ratio in total cost.3) Analyzing the bag problemThe selections for proper streams and heat load are faced because of the limit of multi-stream heat exchangers' scale when the optimized single-hot and single-cold heat exchanger network according to the evolutional superstructure is synthesized to multi-stream heat exchanger network. This selection is the same to bag problem in data structure. This paper describes the synthesis process detailedly.The research to multi-stream heat exchanger network synthesis is significant for industry application. Much work should be done in designing the proper multi-stream heat exchanger and enlarging its scope in use.