Numerical Simulation Of Multiphase Extraction And Remediation Options For Sites Contaminated With LNAPLs

The petrochemical industry is one of the major industries in national economic development,and the associated site pollution problems are serious,endangering the living environment and human health.Light non-aqueous phase liquids(LNAPLs)are often highly toxic and volatile,and can cause contamination of the gas pocket and groundwater after a spill.Multi-phase extraction(MPE)technology is the main means of remediating LNAPLs and can be used to remediate sites quickly and efficiently.However,the MPE remediation process is complex and the remediation cost is high.Therefore,it is of practical importance to develop a reasonable and efficient remediation plan by combining numerical modelling and intelligent analysis methods to improve the remediation effect and reduce the remediation cost at the same time.In this paper,based on the national key research and development program "Northeast Heavy Industrial Area Site Comprehensive Pollution Treatment Technology and Integrated Engineering Demonstration",we firstly applied Petrasim software to establish a numerical simulation model of multiphase flow and multiphase extraction of LNAPLs pollution in porous media of a hypothetical polluted site;then we conducted sensitivity analysis on the multiphase extraction system and process parameters in the simulation model.The most accurate support vector regression machine model was selected for the multi-objective optimization study;and using the simulationoptimization method,the more sensitive multiphase extraction system process parameters were used as the objective function with remediation efficiency and operation cost as the decision variables to build a multi-objective optimization model.A multi-objective optimization model was established using the simulationoptimization method,with restoration efficiency and operating cost as the objective functions and multiple sensitive extraction system process parameters as the decision variables,and the improved NSGA-II multi-objective genetic algorithm was combined to obtain the Pareto optimal solution set.The best restoration solution is obtained.(1)Benzene is rapidly transported to the capillary zone by gravity after the spill,forming a contaminated area with NAPL phase as the main form of existence under the combined effect of vertical migration and lateral diffusion;the remediation efficiency of benzene in the upper part of the envelope zone is much higher than that in the lower part of the envelope zone and the saturated zone in the early stage of MPE remediation;in the middle and late stage of MPE remediation,the remediation efficiency gradually decreases and tends to zero,and the trailing phenomenon of LNAPLs type pollutants becomes prominent.The phenomenon of tailing of LNAPLs was prominent.Therefore,the end point of extraction should be judged reasonably and a more efficient remediation plan should be implemented.(2)Different extraction system parameters have different effects on the remediation effect.Extraction time has the greatest influence on remediation effectiveness,extraction rate has a greater influence,extraction vacuum sensitivity is intermediate,and filter position and productivity index have the least influence on remediation effectiveness.(3)The alternative model constructed by applying the support vector regression machine method has a higher approximation accuracy and can be embedded in the optimization model for calculating the remediation effect of different optimization scenarios.(4)A multi-objective optimization model is constructed with minimum operating cost and maximum rehabilitation efficiency as the objective functions,and the Pareto optimal solution set obtained by applying the improved NSGA-II multi-objective optimization algorithm is solved to provide decision makers with diversified solutions for optimizing the system process parameters.(5)The optimal solution was obtained after decision optimization of the optimal solution set,which improved the restoration effect and reduced the operational and time costs while satisfying the constraints,verifying the feasibility and effectiveness of the simulation model-alternative model-optimization model-solution optimization framework.