Analysis And Synthesis For Recovering Effluent Gas From Polyolefin Plant

As the rapid industrialization of developing countries with large population,unbelievable amounts of effluent have been discharged to the environment.This discharge trend has caused serious environmental pollution and waste of resources.To secure a sustainable future,it is im-perative for the industry especially the petrochemical industry to reduce their emissions effectively.However,the multi-source,multi-component,low pressure and low concentration of the effluent gas from the chemical industries brings new challenges to the sustainable development.In this article,taking the effluent gas from gas-phase fluidized bed polyethylene process as the research subject,a few targeted researches have been done,i.e.,a method to select the best recovery sys-tem for certain compositions of effluent gas,a systematic approach to design process networks for recovering effluent gas at subambient temperature,an energy-induced separation network which couples multiple units.These attempts are expected to promote the development of traditional effluent gas recovery technology,and extending their applications in the newly developing fields.The conventional methods for recovering effluent gas in polyethylene processes include com-pression condensing,membrane separation,cryogenic separation systems and pressure swing ad-sorption.They are all not the economical and efficient ways to recover hydrocarbon and purge gas simultaneously,this article presents four kinds of reasonable combination processes for effluent gas recovery.Since the different recovery characteristics of the four processes and the effluent gas content varies during the process,there is no single recovery technic can meet all effluent gas content conditions.This article introduces a method to select the best recovery system for certain compositions of effluent gas.After searching the feasible content region for the recovery systems according to the specified concentration of recycle stream and outlet streams,it shows PSA sys-tem has the largest feasible region while compression condensation system has the smallest one.In comparison of economic and exergetic performance,the cryogenic separation system shows its outstanding peculiarity to recover the effluent gas.The obtained result is effective in guiding the design of recovery system.The combination processes for effluent gas recovery and the selection method have the flexible and widely applications,they can be extended to deal with the separation of other hydrocarbon-containing gas mixtures,such as the effluent gas recovery from butyl octanol unit.The parameter setting of the traditional processes for effluent gas recovery is based on en-gineering experience,this may not obtain the optimal result.This article presents a systematic approach to design process networks for recovering effluent gas at subambient temperature,this approach can be used to optimize the process parameters and design of heat exchange network.Ac-cording to the characteristics of the effluent gas recovery process,an optimization model for heat,power and mass integration is developed.The proposed approach combines HEN operator,pres-sure operator,separation operator and mathematical programming that maximize total annualized revenue.In order to ensure the feasibility of temperature difference inside the HEN,the nonlinear T-H profile of streams are approximated by considering nonisothermal phase changes and piece-wise linearization.The phase changes of streams are identified by disjunctive representation.The kink points of approximated piece-wise straight lines are obtained by cubic correlations between enthalpy and temperature,pressure,composition.Two industrial cases of the polyolefin processes are presented to verify the accuracy and effectiveness of the proposed model.The obtained results demonstrate that appropriately expanding and compressing the process streams could eliminate the cold utilities of ultra-low temperature.Consequently,the investment for additional refrigera-tion cycle can be saved.The total annualized revenue of two cases are 4.63 times and 3.08 times of that of the traditional effluent gas recovery process after optimizing these equation-oriented process models,respectively.For the disadvantages of the process network for recovering effluent gas at subambient tem-perature.such as the certain flowsheet,low nitrogen purity,large consumption of cold utility and so on,in this article,a new separation network which couples heat transfer,pressure operation and multi-separation units is proposed,and it solves all the problems mentioned before.Mean-while,the results show that the purity of nitrogen in the effluent gas can be improved by combining flashing units with gas membrane separation units,and the nitrogen purity can be changed by reg-ulating the membrane area.To improve the products purity of polyethylene gas,the effluent gas preferentially goes through the flashing units,and then using membrane separation units to im-prove nitrogen purity further.While the use of membrane separation unit can improve the purity of nitrogen,the recovery rate of the products will be reduced,therefore,the purity and recovery rate of the products can be optimized as different objectives.