| Poly vinyl chloride (PVC), the second most used plastic worldwide, is produced by the polymerization of vinyl chloride monomers (VCMs). The pyrolysis of ethylene dichloride (EDC), is the main commercial route to form VCM. Given the high energy consumption of the EDC cracking process, the EDC cracker is considered as the heart of the entire VCM manufacturing process. Thus, it is very crucial to reduce the energy consumption and improve the quality of VCM production. Due to lack of on-site detecting equipments, only the coil outlet temperature (COT) is measured and controlled to evaluate the EDC cracking performance on site, which is very glib. In order to deepen the understanding of EDC cracking process, it is necessary to build an accurate model of the EDC cracker. This paper aims to establish multi-scale EDC models from different aspects. Besides that, multi-objective optimization strategy of the EDC cracker is developed based on built-up model. The main innovations of this work are summarized as follows:(1) An improved multi-objective optimization algorithm, named SMODE-εCD is proposed which integrates self-adaptive differential evolution multi-objective optimization algorithm with epsilon constrained-domination principle together. In SMODE-εCD, the trial vector generation strategies and the variation and cross-over parameters of DE are adaptively adjusted based on evolutionary progress knowledge. Furthermore, epsilon constrained domination principle is adopted to handle constraints in multi-objective problems. The advantageous performance of SMODE-εCD is tested and validated over nine standard test problems and four engineering design problems with comparisons of other two typical multi-objective problems named NSGA-II and CMODE. The performance indicators show that SMODE-εCD is an effective and feasible approach to solving all kinds of constrained multi-objective problems.Thus, the proposed algorithm is solid foundation and preparation for the multi-objective optimization of EDC cracker.(2)A simplified mechanistic model of EDC cracker at the clean-tube stage is established. Given the intense heat coupling between the furnace and the reactor, the cracker is divided into two parts:the furnace model and the reactor model, with heat flux and flue gas temperature profiles connecting the two models. The heat transfer process in the furnace is described using one-dimensional Lobo-Evans model. A one-dimensional plug flow reactor model and a two-step serried reaction kinetics model are assumed in the reactor. Based on the fist principle model, different fuel gas allocation strategies are investigated to improve performance indices such as selectivity, conversion, and fuel gas consumption (per vinyl chloride monomer production) with the definition of fuel gas allocation factorα. Based on that, the suitable coil outlet temperature (COT) is suggested to make good compromise among the performance indices. (3) Run length model of ethylene dichloride cracker is developed. Radical mechanism with coke formation is adopted to describe the EDC cracking reactions with24reaction equations and31components. Based on the run length model of EDC cracker, the impactions of coke deposition are analyzed with some core information variations with time being discussed. Two important aspects, namely, CCl4concentration and fuel gas allocation, are investigated to understand the overall benefits of the whole operation cycle. A comprehensive analysis shows that the concentration of the CCl4promoter should be controlled at100ppm wt%and the fuel gas allocation factor should be maintained at0.36to guarantee the overall economic benefits of the EDC cracker in the full operation cycle.(4) The computational fluid dynamic (CFD) model of EDC cracker is developed. Reynolds-averaged Navier-Stokes (RANS) model is used to describe the flue gas flow with a standard k-ε turbulence model to close that. A finite-rate/eddy dissipation model is used to model a pre-mixed combustion of the sidewall burners. The discrete ordinate model is applied to simulate the radiative heat transfer of a furnace in CFD simulation. The details of flue gas velocity, temperature, composition information are provided in the CFD model, which is effective supplement and verification for the simplified one dimensional Lobo-Evans model.(5) The multi-obejective optimization of EDC cracker is solved. The EDC cacker operation optimization problems are converted into standard mathematical constrained multi-objective problem with the definition of objective functions, decision variables and constaints based on the simplified EDC cracker model. In order to cut off the computational burden, the multi-objective optimization is divided into three cases named conversion-selectivity, conversion-consumption, selectivity-consumption. The propsed SMODE-sCD is applied to solve these problems. After that, a set of trade-off pareto optimal solutions are figured out, and some typical operational conditions related preferred solutions are selected and presented to help the on-site decision maker to make a more scientific choice of the on-site operation. |
PDF Full Text Request