Optimization Of Membrane Module Structure In Separation Process Of Saline Water By Direct Contact Membrane Distillation

Membrane distillation for seawater desalination has the advantages of high utilization rate of waste heat and low sensitivity to water quality,but its large-scale commercial application process is limited by low thermal efficiency and membrane flux in the result of existence of temperature polarization and concentration polarization,which can be improved by optimization of operation parameters and the structure of membrane modules.However,There are many limitations because membrane flux is taken by unique evaluation index in most research by the means of experiment.The combination of simulation and experimental validation was used in this paper in term of direct contact membrane distillation(DCMD).The effect of operation parameters and membrane modules optimization were described from two perspectives that the temperature polarization coefficient and concentration polarization coefficient were defined as micro-evaluation indexes,and the thermal efficiency and membrane flux were choosen as macro-evaluation indexes to provide theoretical guidance for optimization process.To investigate the mechanism of operation parameters and flow state on heat and mass transfer process for traditional membrane module(Model A),The literature was quoted to verify simulational model,and then the macroscopic and microscopic comparisons were performed by single factor method and orthogonal method with various working conditions.The results show that the error of the experimental value in the literature and simulated values is less than 6% and 9%,which proves the rationality of the selected model.For all working conditions,the membrane flux of countercurrent is slightly larger than that of downstream,while the thermal efficiency for countercurrent is lower than that for downstream.In addition,the three most significant operating parameters affecting the membrane flux and thermal efficiency are hot side temperature,cold side temperature and hot side inlet flow rate,among of which,hot side temperature and inlet flow rate is positively correlated with membrane flux and thermal efficiency,while cold side temperature is negatively correlated with membrane flux and positively correlated with thermal efficiency.It is found that the inner runner of the membrane module is divided into mainstream zone and reflux zone.The high speed flow in the mainstream zone accelerates exchange between internal and external fluid and weakens polarization phenomenon,which leads to high membrane flux and thermal efficiency.The poor flow condition in the reflux zone and stagnation of the flow at the junction with the mainstream regions results in serious polarization,low membrane flux and thermal efficiency.To eliminate the existence of the reflux zone in Model A,investigate the performance of the new model and make some observation of the optimization effect,the inlet and oulet direction of the membrane module was set perpendicular to the membrane surface,which is called Model B.After obtaining the membrane flux and thermal efficiency of the Model B by numerical simulation with different working conditions,the internal flow pattern and temperature concentration distribution were analyzed.The results show that the effect of hot side temperature on membrane flux enhancement is more significant with higher flow rate,and there is no interaction among other factors.The effect of other operating parameters is consistent with that of Model A.What reaveals from streamline diagram is the disappearance of zoning phenomenon and uniform flow,but the internal velocity is much lower than that of Model A.In addition,the "bullhorn" region with serious polarization phenomena is found near X=13 and X=-13,and its position gradually shifts to the edge with the increase of hot side inlet flow rate.Polarization is inhibited by the collision between the inflow and membrane surface and the confluence formed at the outletTo remove bullhorn boundary layer and accelerate flow speed in Model B,the Model C membrane module was obtained by adding three diversion plates.The internal temperature and concentration distribution were simulated and analyzed,and contrast analysis of three model is made by evaluation indexes.It has been shown that the temperature polarization of Model A was the most serious,and the concentration polarization of Model B was the most serious.Polarization existing in Model C was effectively relieved due to diversion plates.In addition,the membrane flux of Model C is at least 90% higher than that of Model A.For thermal efficiency,Model C is the highest,In addition,membrane flux of Model C is most sensitive to the hot side temperature and less sensitive to hot side inle flow rate than other models,while the membrane flux of Model B is most sensitive to the concentration.The model C membrane module was designed and processed according to the size of 1:1,and the DCMD experimental system was built.The experimental results show that the trend of membrane flux increasing with operation parameters is basically consistent with the simulation values,while the error between experiment and simulation is within 14%,of this,the error of few parts is over 10%.The conclusion is drawn that the simulation values with all working condition are in good agreement with the experimental values.In this way,the rationality of the optimized membrane distillation process based on Model C was proved from both simulation and experiment perspectives.