Influences On Controllability By Simplified Design Of The Externally Heat-Integrated Double Distillation Columns

Owing to the energy crisis and ecological environment degradation, countries all over the world have reached common consensus on energy conservation and emission reduction. Distillation process, one of the most energy-consuming unit operations in the chemical and petrochemical industries, should certainly be the target for the research of energy saving and environment protection. Externally heat-integrated double distillation columns (EHIDDiC), a novel apparatus with high thermodynamic efficiency, are the resultant outcome of the research activity.The EHDDiC is characterized by external heat integration between rectifying section and stripping section of two distillation columns. These two distillation columns are operated at different pressures in order to provide the necessary temperature driving force for heat exchange between two sections of two columns. Due to the external thermal coupling, a certain amount of heat is transferred from the rectifying section of the HP distillation column to the stripping section of LP distillation column, which creates the downward reflux flow for the LP distillation and upward vapor flow for the HP distillation column. So, the condenser of the HP distillation column and the reboiler of the LP distillation column are not needed, any more.According to the arrangement of heat integration between the rectifying section and the stripping section, the EHIDDiC has various structures, including symmetrical EHIDDiC (S-EHIDDiC) and asymmetrical EHIDDiC (A-EHIDDiC). To facilitate the design and implementation of the A-EHIDDiC, one may use several heat exchangers to simplify external heat integration (SA-EHIDDiC). High energy efficiency and stable operations may not be achieved simultaneously unless the steady operation and control scheme are carefully compromised. The feasibility of the variations from the S-EHIDDiC to the A-EHIDDiC and from the A-EHIDDiC to the SA-EHIDDiC needs to be investigated from both economical and controllability aspects.In this thesis, in terms of the separation of an ethylene/ethane binary mixture, the static model of the EHIDDiC has been established and the three configurations of the EHIDDiC are compared intensively in the aspects of process design and controllability, ranging from static performance to open-loop controllability. Based on the dynamic model and a decentralized quadruple composition control system, the close-loop controllability of three configurations of EHIDDiC is compared. It has been found that the A-EHIDDiC and SA-EHIDDiC are superior to the S-EHIDDiC in not only thermodynamically efficiency but also process dynamics and controllability. As for the comparison between the A-EHIDDiC and SA-EHIDDiC, the latter shows somewhat comparable behaviors in process design and controllability with the former.The obtained outcomes demonstrate that the asymmetrical configuration should be favored over the symmetrical one in the development of the EHIDDiC. It is demonstrated feasible to approximate external heat integration with the employment of three heat exchangers between the HP and LP distillation columns. The prediction of close-loop controllability with process models facilitates the simultaneous consideration of static and dynamic properties of the EHIDDiC.