Development And Performance Research Of Three-Cylinder Valve-Controlled And Swash Plate-Plunger Energy Recovery Devices

As an important energy-saving equipment,the energy recovery device can greatly reduce the energy consumption and water production cost of SWRO system.In this paper,a three-cylinder valve-controlled energy recovery device（TC-ERD）is developed to improve the operation stability of the traditional two-cylinder energy recovery device.However,the structural characteristics of valve-controlled device determine that it is more suitable for large-scale SWRO systems,so a compact swash plate-plunger energy recovery device（SP-ERD）is innovatively developed to meet the energy saving requirements of small and medium-sized SWRO systems.In order to solve the limitation that the traditional two-cylinder device cannot realize the continuity of pressurization stroke and depressurization stroke at the same time and the resulting problem of poor operation stability,a valve-controlled pressure exchange single-cylinder module is developed,and a TC-ERD is constructed by paralleling three modules.TC-ERD supplemented by intelligent control strategy can simultaneously realize the continuity of pressurization stroke and depressurization stroke,thus breaking through the structural bottleneck of traditional two-cylinder valve-controlled device.Under the design conditions（30 m³/h,5.0 MPa）of TC-ERD,the pressure pulsation amplitudes of high-pressure（HP）brine and low-pressure（LP）seawater are only 0.08 MPa and 0.05 MPa,respectively,but their flow pulsation amplitudes are as high as 2.75 m³/h（pulsation rate 9.2%）and 1.82 m³/h（pulsation rate 5.2%）.The analysis results show that the larger flow pulsation of the TC-ERD is mainly due to the flow resistance loss change caused by the alternation of pressurization/depressurization stroke and the flow resistance loss difference when the three parallel modules perform pressurization（depressurization）strokes alone.In order to solve the flow pulsation problem of TC-ERD,the overlapping degree ofthe two single-cylinder modules that simultaneously perform the pressurization（or depressurization）stroke is regulated by adjusting the time parameter（t₁）in the control strategy,the flow pulsation caused by the alternation of pressurization/depressurization stroke is significantly reduced.The results show that the flow pulsation amplitudes of HP brine and LP seawater decrease with the decrease of t₁.When t₁ decreases from 3.0 s to 0.5 s,the flow pulsation amplitude of HP brine decreases from 2.75 m³/h to 1.03 m³/h,that of LP seawater decreases from 1.82 m³/h to 0.64 m³/h.On this basis,the flow resistance loss uniformity of the three parallel modules is improved by changing the opening degree of switcher in single-cylinder module,so that the flow pulsation amplitudes of HP brine and LP seawater are reduced to 0.16 m³/h（pulsation rate 0.5%）and 0.31 m³/h（pulsation rate 0.9%）,respectively.The TC-ERD replaces the physical piston with a"liquid piston",which improves operational reliability,but also causes mixing between brine and seawater.The numerical model of the brine/seawater mixing process of the single-cylinder module is constructed,and the formation and development law of the"liquid piston"is explored.The results show that the"liquid piston"accumulates and develops with the increase of the alternating times of pressurization/depressurization stroke,and gradually tends to be stable.Moreover,the transient maximum volumetric mixing degree(V_m-max)is much greater than the average volumetric mixing degree.The V_m-maxunder the same volume utilization rate（R_L）is almost the same regardless of the changes in capacity and stroke time,so an empirical formula between V_m-max and R_L is deduced,which has guiding significance for the design of valve-controlled devices.When the V_m-max of TC-ERD is 6%,the R_L is 57.5%,the cycle period and drive leakage flow under design capacity are 16.5 s and 0.098 m³/h,the efficiency is 97.5%.In order to better meet the energy saving requirements of small and medium-sized SWRO systems,a compact SP-ERD is innovatively developed.The SP-ERD converts the rotary motion of swash plate into the reciprocating motion of hollow plunger in the plunger sleeve,and makes the lateral flow distribution ports on the plunger alternately communicate with that on the plunger sleeve,thereby realizing the rapid switching of pressurization/depressurization stroke in the plunger hollow cavity.The CFD simulation study of SP-ERD shows that its unique lateral symmetrical impingement inflow mode can make the fluid flow in the plunger hollow cavity close to the plug flow,which lays a foundation for its low mixing characteristics.The volumetric mixing degree of SP-ERD under the design conditions（20 m³/h,6.0 MPa）and 436 rpm swash plate speed is only 1%,while the energy recovery efficiency can reach 96.4%.