Synthesis And Design Of The Externally Heat-Integrated Double Distillation Columns

Heat-integration has been widely used to reduce the utility consumption in the separation operation in the chemical and petrochemical process industries. Compared with the conventional distillation column, the heat-integrated distillation process can lead to high thermodynamic efficiency and low total annual cost (TAC). A novel scheme of external heat-integrated double distillation columns, termed the EHIDDiC, is proposed in this thesis. The major purpose of this study is to explore the potential advantage of the EHIDDiC in the chemical and petrochemical process industries.The EHIDDiC have a high-pressure distillation column and a low-pressure distillation column. The rectifying section of the high-pressure distillation column is heat integrated with the stripping section of the low-pressure distillation column. There is an external heat exchanger between each pair of heat-integrated stages. The external type heat integration can avoid the problems of the internal heat integration and the complexity of heat transfer structure. With the minimum total annual cost as an objective function, a systematic method is proposed for the synthesis and design of the EHIDDiC.As the EHIDDiC needs a lot of external heat exchangers, it makes the EHIDDiC difficult to be designed and implemented in the practical situation. Thus, a simplified scheme of the EHIDDiC, termed as SEHIDDiC, should be derived, using only three external heat exchangers to approximate the external heat integration in the EHIDDiC. One external heat exchanger is placed between the tops of the heat-integrated sections; one is placed between the middles of the heat-integrated sections; the third one is placed between the bottoms of the heat-integrated sections. The locations and sizes of the three external heat exchangers are considered deliberately to maximize thermodynamic efficiency. A systematic method is derived for the SEHIDDiC.Two examples, i.e., the separations of an ethylene/ethane and benzene/toluene binary mixtures are employed to study the EHIDDiC and its simplified schemes. The results obtained indicate that the EHIDDiC is more thermodynamic efficient and needs less TAC in comparison with the conventional distillation column. Furthermore, the SEHIDDiC could be an excellent candidate to approximate the EHIDDiC with even a greater reduction in utility consumption and a less degree of capital investment. The SEHIDDiC offers essentially a much simple way to design and implement the concept of the EHIDDiC in the chemical and petrochemical process industries.