Effect Of Rotation Membrane Distillation Crystallization On Crystal Separation On Membrane Surface

Crystallization is one of the important methods for refining various solid compound products.Membrane crystallization can better regulate the heterogeneous crystallization nucleation and crystal growth of solute molecules on the membrane surface due to the induction effect of the porous membrane.However,during the continuous operation,membrane pore obstruction occurs easily when crystals are deposited on the surface,which makes the membrane performance degraded or even invalid.Therefore,effectively separating the crystals and maintaining the sustainable operation are the development directions.Aiming at the problem of membrane pores blocking,this paper proposes a new method for crystal separation by making membrane surface rotating.Crystals attached to the membrane surface are easily separated from the membrane surface with the drag and centrifugal force.By exploring the laws of crystals nucleation,growth and separation,a new type of rotation membrane distillation crystallization(MDC)technology that can separate the crystals has been developed.Firstly,the process of membrane distillation(MD)on highly-concentration brine was simulated by computational fluid dynamics(CFD).The influence of operating conditions on the membrane crystals nucleation was mainly investigated,and then the operating conditions of crystal nucleation on membrane surface were studied.The concentration,temperature and supersaturation field on the membrane surface were obtained by CFD,and the nucleation work between the membrane surface and the bulk the feed was compared by using the crystallization nucleation kinetic theory,which confirmed that the crystals were easier to nucleate on the membrane surface than in the bulk.In addition,through the comprehensive analysis of the fluid thermodynamics and dynamics,it was found that increasing the feed flow rate would rise the concentration rate of the feed liquid,and then we proposed the critical Reynolds number Re*as a threshold to determine the crystallization.Furthermore,through data regression analysis of the simulation results,the semi-empirical relationship of Re* at different feed temperatures was determined,which showed that the nucleation process of the membrane surface crystallization can be controlled by changing the operating parameters.Secondly,a membrane distillation crystallization experiment system was built,and the growth rate of the membrane surface crystals under different feed concentrations was measured through experiment,and the growth kinetics of the membrane surface crystals was explored.The results showed that the higher the feed concentration,the faster the growth rate.Then combining with the results of the supersaturation distribution of the feed side,the quantitative relationship between the supersaturation and the growth rate was obtained.Finally,in view of the membrane pore obstruction,this paper proposed a rotation membrane distillation crystallization method with rotatable membrane surface.Through the force analysis of the crystals on the rotating membrane surface,the criterion for crystal movement on the membrane surface was obtained.Furthermore,the growth rate,movement state and crystal size distribution changes under different rotation speeds were further explored.The results showed that rotatable membrane surface enabled the crystals which cannot be separated only under the axial flow force to slip on and leave the membrane surface.And the main reason for crystal’s separation action was that the drag force generated by the relative movement of the crystals and the fluid.In the rotating flow field,as the rotation speed increased,the supersaturation on the membrane surface gradually decreased,and the growth rate of crystals decreased.The crystal size distribution of the product was more concentrated by increasing rotation speed.The work in this paper provides theoretical support for the study of the nucleation and growth kinetics of membrane crystallization in the MDC process.The proposed rotation membrane distillation crystallization process provides a new solution to the separation of crystals in the membrane crystallization process.