Multi-objective Optimization Of Shale Gas Processing Based On Kinetic Model

Shale gas is a sort of unconventional natural gas resource with enormous reserves.Since raw shale gas contains various contaminants,it needs to go through a series of pretreatments to meet certain quality specifications before utilization.Recent developments in mining technology have made the largescale commercial extraction of shale gas available and lead to a significant decline of shale gas price.Consequently,in addition to the direct combustion for energy supply,the utilization of shale gas as a cheap feedstock for valueable chemical production is a more promising option.Featuring the potential of profit improvement,energy saving and emission reduction,the downstream production of shale gas provides new opportunities and challenges for shale gas industry.With pretreatment and downstream production included,shale gas processing is a complex reaction-spearation system.During production of different downstream chemicals,significant differences are presented in the process performances of annual profit and environmental impacts.Therefore,this thesis investigates multi-objective optimization of shale gas processing based on kinetic modeling.First,mechanistic and semi-mechanistic kinetics-based reaction models are developed on the basis of segmentation method.Then,process performances are appraised from the perspectives of economics and environment,according to the economic evaluation based on equipment purchased cost curve model and the environmental assessment based on hybrid Life Cycle Inventory model.Ultimately,downstream production routes based on different objectives is optimized via multi-objective optimization.The main contents of this thesis are as follow:(1)In an effort to remove the impurities,process simulation and parameter analysis of shale gas pretreatment are conducted.First,on the basis of physical dissolution and chemical reaction,shale gas sweetening model in which diethanolamine serves as absorbent is established to remove H2S and CO2.The gas dissolution model and sweetening reaction model are introduced to improve accuracy.Then,Claus sulfur recovery process is simulated to recover element sulfur and alleviate pollutant emissions.The optimization method of operating condition in each catalytic reactor is proposed.In order to eliminate moisture in shale gas,two dehydration approaches,namely,conventional and stripping gas dehydration processes are developed.The performances of purification effectiveness and alkane loss are compared.Finally,NGLs recovery process is simulated and the parameter of demethanizer is optimized with the goal of minimum cryogenic utility usage.By virtue of simulation and parameter analysis of pretreatment,shale gas meets the quality specifications with contaminants removed.(2)In allusion to drawbacks of investigating chemical reaction via conventional thermodynamic method,mechanistic and semi-mechanistic kinetics-based reaction models which are adequate for isothermal and adiabatic processes are developed on the basis of segmentation method.In addition to predicting the equilibrium status of reacting system,kinetics-based reaction model can also accurately calculate rate constant,driving force,composition and temperature at any position in the reactor.The model explains complicated appearance phenomena on the basis of microscopic parameters such as reaction rate,and reveals the mechanism between thermodynamic factors,kinetic factors and reaction performance.Based on the proposed modeling method,methane reforming process,methanol synthesis process,dimethyl carbonate synthesis process and dimethyl ether synthesis process are simulated.The effects of feed capacity and catalyst usage on conversion and selectivity are analyzed,providing the basis of mass flow and energy flow for process evaluation and parameter optimization.(3)Framework of process evaluation for shale gas pretreatment and downstream production is developed.In allusion to the limitation of "six-tenths rule",economic evaluation method based on equipment purchased cost curve model is adopted to quantify the effect of each process parameters on total annual cost.In order to correct the flaws of simplex evaluation indicator and insufficient pollutant source investigation in current environmental assessment methods,a hybrid Life Cycle Inventory model based on Input-Output model and process model is established to thoroughly appraise various pollutant emissions derived from material products,equipment manufacture and process operation.Then,the process evaluation method is applied to methanol production from shale gas as a case study.The operating condition of reactor is optimized aiming at the lowest production cost and pollutant emissions.The results show that operating temperature of POX reactor is the key factor which effects process performance extensively.The example verifies the method proposed in this thesis can achieve the correlation between operating parameter and process performance,and provide theoretical foundation for selection of optimal downstream production route.(4)Based on process simulation and evaluation method,process performance of shale gas pretreatment and downstream production is appraised from economic and environmental points of view.In terms of pretreatment,composition of total annual cost and distribution of pollutant sources are mainly investigated.The results present that total annual cost of shale gas pretreatment is 15.2M$/y.Claus process governs the emissions of CO2 and SO2,while the emissions of NOx and soot are primarily derived from NGLs recovery process.The results of shale gas downstream production illustrate that different processes for the same product and processes for different products present significant discrepancy in annual profits and pollutant emissions.The downstream production route with optimal process performance varies with different economic and environmental objectives.The route of methanol synthesis from shale gas via steam methane reforming and dimethyl carbonate synthesis via urea alcoholysis has the highest annual profit,while the route of methanol synthesis from shale gas via dry methane reforming shows the lowest CO2 emission.Based on data normalization,a multi-objective optimization method which investigates economic performance and environmental impacts simultaneously is proposed for the purpose of comparison in comprehensive performance.The results demonstrate that the route of methanol synthesis from shale gas via partial oxidation process and dimethyl carbonate synthesis via urea alcoholysis features the optimal comprehensive performance.