In this work,the impact of internal heat integration upon process dynamics and controllability by superposing reactive section onto stripping section,relocating feed locations,and redistributing catalyst within the reactive section is explored based on a hypothetical ideal reactive distillation system containing an exothermic reaction:A + BC + D.Steady state operation analysis and closed-loop controllability evaluation are carried out by comparing the process designs with and without the consideration of internal heat integration.For superposing reactive section onto stripping section,favorable effect is aroused due to its low sensitivities to the changes in operating condition.For ascending the lower feed stage,somewhat detrimental effect occurs because of the accompanied adverse internal heat integration and strong sensitivity to the changes in operating condition.For descending the upper feed stage,serious detrimental effect happens because of the introduced adverse internal heat integration and strong sensitivity to the changes in operating condition.For redistributing catalyst in the reactive section,fairly small negative influence is aroused by the sensitivity to the changes in operating condition.When reinforcing internal heat integration with a combinatorial use of these three strategies,the decent of the upper feed stage should be avoided in process development.Although the conclusions are derived based on the hypothetical ideal reactive distillation column studied,they are considered to be of general significance to the design and operation of other reactive distillation columns.
Yang YuanLiang ZhangHaisheng ChenShaofeng WangKejin HuangHuan Shao
In this work,the dynamics and operation of the totally reboiled reactive distillation columns are visualized in terms of transfer function based process models.This kind of processes is found to be characterized by underdamped step responses due to the special topological configuration and the intricate interplay between the reaction operation and the separation operation involved.The under-dampness can be substantially alleviated through the tight inventory control of bottom reboiler and this presents beneficial effects to process dynamics and operation.Two totally reboiled reactive distillation columns,separating,respectively,a hypothetical synthesis reaction from reactants A and B to product C,and a real decomposition reaction from 1,4-butanediol to tetrahydrofuran and water,are employed to demonstrate these uncommon behaviors.The results obtained give full support to the above qualitative interpretation.Despite the strong influences of reaction kinetics and thermodynamic properties of the reacting mixtures,the totally reboiled reactive distillation columns are generally considered to present such unique behaviors and require tight inventory control of bottom reboiler to facilitate their control system development.
Kejin HuangYang YuanLiang ZhangHaisheng ChenShaofeng WangNian Liu
Because of the complicated interplay between the prefractionator and main distillation column involved,the black-hole problem might occur and prohibit the assignment of four specifications to dividing-wall distillation columns(DWDCs)(e.g., the three main product compositions plus an impurity ratio in the intermediate product), which lowers terribly process flexibility and operability. In this paper, a feed thermal condition adjustment strategy, achieved by the installation of a pre-heater in feed pipeline, is employed to eliminate the black-hole problem and serve to enhance process flexibility and operability. Through the strong influence to the overall mass and energy balance of the DWDC, the feed thermal condition adjustment can alter the interlinking flows between the thermally coupled prefractionator and main distillation column and work effectively to coordinate their relationship. A DWDC separating a benzene, toluene, and o-xylene mixture is chosen to ascertain the feasibility of the philosophy proposed. The static and dynamic studies demonstrate that the feed thermal condition adjustment is an effective way to refine process design and can completely eliminate the black-hole problem and elevate consequently process flexibility and operability.
Yang YuanKejin HuangLiang ZhangHaisheng ChenShaofeng WangYingjie Jiao
Temperature inferential control (TIC) is studied for a reactive distillation column with double reactive sections (RDC-DRSs) processing a hypothetical two-stage consecutive reversible reaction (A + B■C + D, C + B■E + D with αD > αB > αC > αA > αE). Because of the complicated dynamic behaviors, the controlled stages by sensitivity analysis lead to great steady-state deviations (SSDs) in top and bottom product purities. Since TIC involves considerably reduced settling times in comparison with direct composition control, small SSDs in product qualities correspond generally to small transient deviations (TDs) in product qualities. An objective function that measures SSDs in product qualities is formulated to represent the performance of a TIC system and an iterative procedure is devised to search for the best control configuration. The application of the procedure to the RDC-DRS gives considerably suppressed TDs and SSDs in top and bottom product qualities as compared with the one by sensitivity analysis. The method is simpler in principle and less computationally intensive than the current practice. These striking outcomes show the effectiveness of the proposed principle for the development of TIC systems for complicated reactive distillation columns.
Lijing ZangKejin HuangTing GuoYang YuanHaisheng ChenLiang ZhangXing QianShaofeng Wang
