Life Cycle Assessment For Polypropylene Off-gas Membrane Separation System

The polypropylene off-gas produced by petrochemicals contains a large amount of light hydrocarbon gas,which has high recycling value.As a low energy consumption and easy to operate separation method,membrane separation technology is more and more used in polypropylene off-gas recovery and related light hydrocarbon recovery processes.With the proposition of relevant national energy conservation and emission reduction policies,it is increasingly important to realize industrial membrane separation process with high economic benefits,low energy consumption and environmental load.Life cycle assessment is a method used to evaluate the environmental impact and resource utilization in the life cycle of the production process.It can be used to evaluate the environmental load,resource consumption and economic cost of the entire process from raw material mining,transportation to product production and use,so as to achieve multi-angle assessment of the production process.Therefore,based on the industrial polypropylene off-gas recovery system,this paper establishes a life cycle assessment model of the membrane separation process that combines a process simulation model and a life cycle assessment model to achieve process optimization from a life cycle perspective.The research content and results are as follows:(1)Process simulation model and life cycle assessment model were established.The process simulation of polypropylene off-gas recovery process was carried out and compared with the original design,which proved the reliability of the simulation results.Based on the process simulation model and the life cycle assessment model,the life cycle assessment model was established from three perspectives,and the process life cycle assessment was carried out:the life cycle 3E assessment method of the membrane separation process was proposed,and five individual index models were established to calculate the specific performance of the process in terms of economy,energy,environment;based on the requirements of the national energy-saving and emission-reduction policy,a"energy-saving and emission-reduction index"model containing pollutant emissions(for example,CO2)and primary energy consumption was proposed;the life cycle assessment indexes were coupled by objective weighting method,and a"coupling assessment index"model was established to calculate the comprehensive performance of the process in the aspects of economy,energy and environment.This formed the"5+1+1"assessment model of process life cycle assessment from three major angles.(2)Life cycle 3E assessment and influencing factors analysis of polypropylene off-gas membrane separation system.Using the five single index models for the life cycle 3E assessment of the chemical process proposed above,we explore the utility consumption and equipment manufacturing contribution of the polypropylene off-gas recovery process from the perspectives of economy,energy and environment,the link that contributes the most to economic benefits,energy consumption and environmental load are found out;on this basis,the influence of the membrane area,compressor outlet pressure,and condensation temperature on the assessment index is analyzed in order to provide relevant guidance and suggestions for process optimization,for example,when the three-step membrane area are 120,120,40m~2 and condensing temperature is-13?,the compressor outlet pressure can be selected at 2.5MPa,at this time,"exergy efficiency"reaches the maximum value.(3)Process optimization of polypropylene off-gas membrane separation system.Process optimization is carried out from three perspectives of life cycle.Firstly,take the life cycle 3E assessment of five single indexes as the objective function,and optimize the off-gas recovery process from the perspectives of life cycle economy,energy,and environment by changing the corresponding process parameters and processes;secondly,by changing the corresponding parameters and process,the“Energy Conservation and Emission Reduction Index”is optimized,and the recycling process considers economic benefits that minimizes environmental impact of and energy use is the original three-step process.The three-step membrane area is 89.6,22.2,and 60m~2,respectively.The outlet pressure is 2.39 MPa and the condensation temperature is-40°C;then,the"coupling assessment index"is used as the objective function,so that the optimal process is two-stage three-step membrane process and the three-step membrane area are 116.6,57.8,38.3m~2,the compressor outlet pressure is 2.26MPa,and the condensing temperature is-40?;finally,the optimization results of the above assessment methods are compared,and the optimized results of the"coupling assessment index"are the optimal process and parameters.