Optimal Design And Energy Efficiency Analysis Of Membrane-based Water Treatment System

Modeling and energy efficiency analysis of membrane-based treatment system for shale gas fracturing flowback wastewater will help the wastewater reuse and reasonable discharge.The process modeling and sensitivity analysis of industrial fresh water desalination treatment system plays an important role in guiding the optimal design and effective operation of desalination process.Membrane-based desalination systems for shale gas fracturing flowback wastewater treatment normally involves ultrafiltration,reverse osmosis and the use of storage tanks.Note that membrane treatment units can be operated semi-continuously and modeled as partitioning regeneration systems with single inlet and two outlet streams.A membrane treatment unit consists of online washing,operation and offline chemical washing subunits.The operation sub-unit has the same characteristics as batch water-using processes.However,there is only limited research on the modeling and optimal design of membrane-based desalination systems for shale gas fracturing flowback wastewater treatment.Based on the semi-continuous behavior of the desalination treatment system,the models of water treatment operation,online washing and offline chemical washing sub-units and storage tanks are developed.The objective is to achieve the maximum total water production ratio,minimum storage tank capacity and maximum profit.Three nonlinear programming optimization models are presented.Two scenarios – fixed scheduling and fixed operation time length for membrane operation sub-units are investigated.Results of a case study show that increasing the operation time of operation sub-units increases the system water production ratio and the profit.The scheduling of operation sub-units has a significant impact on the water-storage profiles,without adversely affecting the water production ratio.The proposed approach can thus be used to guide the design of membrane-based desalination systems for shale gas fracturing flowback wastewater treatment.This thesis proposes a mathematical model for the fresh water membrane desalination system.The model establishes flow rate balance and contaminant mass balance model for batch partitioning regeneration units and tanks in the desalination water system.A mixed integer linear programming mathematical model(MILP)is proposed,in order to maximize the production ratio of desalination system.The calculated water production ratio is 82.86% and the practical water production ratio is 81.67%.The relative difference is 1.19%.The effectiveness of the mathematical model is verified.Based on this model,we analyze the relationship between the demand for desalted water,the amount of fresh water and the capacity of storage tank,the relationship between peak of water-storage profiles for tank,water production ratio and bed changing time length or operating time length.The results show that the demand for desalted water is positively correlated with the amount of fresh water and the capacity of storage tank.Therefore,when the capacity of storage tank is 270 t,the demand for desalted water in the operating scenario cannot exceed the maximum value of 307 t/h.The time length of bed changing has a great influence on the peak of water-storage profiles for tank,and the time length of bed operating has little influence on the peak of water-storage profiles for tank.The proposed approach can be used for process design and analysis of fresh water desalination system.To achieve the energy efficiency analysis of membrane desalination treatment system for shale gas fracturing flowback wastewater,the mass transfer model of reverse osmosis membrane and membrane module model are proposed.Based on the experimental data,the performance parameters(pure water permeability constant A and salt mass transfer coefficient B)of membrane module are analyzed and calculated.Next,we analyze the energy consumption and economical evaluation of shale gas fracturing flowback wastewater treatment process.The results show that the investment cost of membrane modules accounts for a large proportion of total annual investment cost.In addition,the cleaning and maintenance costs of membrane modules account for the largest proportion of total annual operating costs.Note that,the operting cost of pumps has reached 35.23% of total annual operating cost.The energy consumption of desalination system is high,and an energy recovery system is proposed to recover the energy of residual water.The results show that the total energy consumption and the annual operating cost of pumps can be reduced by 20.45% and 15.29%,respectively.Therefore,the mathematical model proposed in this thesis can not only evaluate the performance of the membrane module,but also evaluate the energy consumption and economics of the membrane desalination system for shale gas fracturing flowback wastewater.