【作者】 刘元法；

【导师】 贺高红；

【作者基本信息】 大连理工大学 ， 化学工程， 2013， 博士

【摘要】 膜污染现象引起微滤膜通量随时间急剧衰减,导致系统能耗显著增加,严重制约了微滤膜技术的应用。湍流促进器可以改善膜组件内的流体动力学条件,显著提高壁面流速或剪切速率,有助于抑制料液中的颗粒在膜表面的沉积,减轻微滤过程的膜污染现象,从而有效提高微滤膜通量。湍流促进器强化传质效果主要依赖于膜表面的流体动力学效应。然而,扰流挡板和螺旋式湍流促进器强化传质的流体动力学机制尚不明确,在微滤过程的强化传质机理尚未完善。本论文使用扰流挡板强化错流微滤碳酸钙悬浮液过程,考察挡板结构参数对强化传质效率的影响,设计了一种新型的具有矩形螺旋截面的螺旋式湍流促进器,CFD (Computational fluid dynamics)模拟管内附加不同湍流促进器的流场,数值分析两种湍流促进器强化传质的流体动力学机制,并以新设计的螺旋式湍流促进器为例,考察了湍流促进器对滤饼参数的影响,完善了湍流促进器在微滤过程的强化传质机理,最后建立可预测湍流促进器强化传质效率的神经网络模型,优化湍流促进器强化微滤过程的操作条件,从而为湍流促进器的应用提供指导。首先,使用扰流挡板强化错流微滤碳酸钙悬浮液过程,考察扰流挡板的类型、结构参数(挡板收缩率和挡板间距)以及挡板排列方式对强化传质效率的影响。研究表明,挡板收缩率(β)对强化传质效率有显著的影响,β值大的扰流挡板可获得较高的强化传质效率。圆形挡板间距的优化依赖于β值,并且要满足旋涡的充分发展,环形挡板间距的优化与β值无关。基于相同的结构参数,圆形挡板可以获得比环形挡板更高的强化传质效率。与单独使用一种类型扰流挡板相比,混合使用两种类型扰流挡板可获得更好的强化传质效果。为了揭示扰流挡板强化传质的流体动力学机制,CFD模拟管内附加扰流挡板的流场。流场数值分析表明,扰流挡板诱发管内流体形成旋涡,引起壁面流速剧烈波动,显著提高流体的湍动强度,可以破坏边界层的发展,抑制料液中的颗粒在膜表面沉积,从而有效提高微滤膜通量。其次,设计了一种新型的矩形截面螺旋式湍流促进器,CFD模拟管内附加新型湍流促进器的流场,分析强化传质的流体动力学机制,并考察湍流促进器结构参数对流场特性的影响。流场分析表明,矩形截面螺旋式湍流促进器引发螺旋流与轴向流的剧烈混合,可显著提高流体的湍动强度和壁面剪切力,壁面附近无滞流区或流动死区,螺旋槽内未形成旋涡或二次流,与文献报道的半圆形截面螺旋式湍流促进器相比,在相同条件下,膜组件轴向压力降减小了25%,壁面剪切力增大了6.7%,可显著降低能耗并获得较高的强化传质效率。螺旋式湍流促进器的结构参数对流场特性有显著的影响：壁面剪切力和膜组件轴向压力降均随着螺纹外径(Dh)和中心杆直径(Dr)的增大而提高,随着螺纹间距(λ)的增大而降低。基于临界剪切力的优化设计依据,确定了矩形截面螺旋式湍流促进器的优化结构参数：Dh=11mm,λ=12mm, Dr=4或5mm。以新设计的矩形截面螺旋式湍流促进器为例,考察了湍流促进器对滤饼参数的影响,通过滤饼阻力理论分析,完善了湍流促进器在微滤膜过程的强化传质机理。研究结果表明,在相同操作条件下(跨膜压力50kPa、入口流速0.5m/s和料液浓度1.0g/L),湍流促进器使滤饼层厚度从0.59mm显著减薄到0.1mm,滤饼孔隙率从0.55增大到0.65,滤饼平均粒径从5.15μm显著减小到1.99μm；滤饼阻力分析表明,滤饼平均粒径的显著减小导致滤饼比阻增大了2.45倍,说明湍流促进器对微滤过程产生了负面效应。由于滤饼层厚度减薄和孔隙率增大的正面效应远大于滤饼比阻增大的负面效应,因此整体滤饼阻力显著降低。微滤过程的操作条件对滤饼参数有显著的影响：随着跨膜压力的升高,滤饼层厚度增大,滤饼孔隙率减小,滤饼平均粒径增大；随时入口流速的升高,滤饼层厚度减小,滤饼孔隙率增大,滤饼平均粒径减小；随着料液浓度的升高,滤饼层厚度增加,滤饼平均粒径减小,滤饼孔隙率在无湍流促进器时增大,在有湍流促进器时保持不变。湍流促进器可以减弱跨膜压力和料液浓度对滤饼参数的影响,增强入口流速对滤饼参数的影响。最后,建立了可预测湍流促进器强化传质效率的神经网络模型,优化了神经网络模型的结构：隐含层神经元数为12,输入层与隐含层之间的传递函数为logsig,隐含层与输出层之间的传递函数为tansig。利用该模型分析了微滤过程的操作条件对湍流促进器强化传质效率的影响：强化传质效率随着跨膜压力的增大先升高后降低,随着入口流速或料液浓度的增大而升高；跨膜压力对强化传质效率的影响最大，料液浓度的影响次之,入口流速的影响最小。利用该模型优化了湍流促进器强化微滤过程的操作条件,从而为湍流促进器的应用提供指导：在较低的料液浓度时,采用较高的入口流速和较低的跨膜压力,湍流促进器可获得较高的强化传质效率；在较高的料液浓度时,采用较高的入口流速和较高的跨膜压力,湍流促进器可获得较高的强化传质效率。更多还原

【Abstract】 The permeate flux of microfiltration sharply declines with filtration time owing to the phenomenon of membrane fouling, which seriously hinders the wide applications of MF due to the increased energy consumption. The use of turbulence promoters can significantly increase the crossflow velocity or wall shear stress under the same operation condition, which can effectively prevent particle deposition on the membrane surface, thereby improving the permeate flux of MF. The flux improvement by turbulence promoter depends on the improved hydrodynamics of fluid flow. However, the hydrodynamics effects and the mechanism of flux enhancement by turbulence promoter have still not been clearly understood. In this thesis, central baffle and wall baffle were used as turbulence promoter to enhance the permeate flux during the crossflow MF of particulate suspension. A novel helical screw insert with a rectangular section was designed. Computational fluid dynamics (CFD) simulations of fluid flow in the tube filled with turbulence promoters were conducted to investigate the hydrodynamics effects responsible for the flux improvement. In order to explore the intrinsic mechanism of flux enhancement, the effects of turbulence promoter on the cake properties were investigated. The turbulence promoter-assisted MF process was successfully modeled by the artificial neural network (ANN) to predict the efficiency of flux enhancement. The operation variables in MF process were optimized by ANN to achieve high flux enhancement efficiency, which provides a useful guide for the applications of turbulence promoter.Firstly, both central baffle and the wall baffle were used to enhance the permeate flux during crossflow MF of calcium carbonate suspension. The effects of baffle configuration, baffle geometric parameters including baffle constriction ratio (β), baffle spacing (L/D) and baffle arrangement on the flux enhancement efficiency were experimentally investigated. It reveals that both β value and (L/D) value of baffle play an important role in the flux enhancement efficiency. The baffle with a larger β value can obtain higher flux improvement efficiency. The optimum (L/D) value of central baffle strongly depends on the β value. While the optimum (L/D) value of central baffle is independent of the β value. The central baffle can achieve higher flux improvement efficiency than wall baffle in terms of same geometric parameters. The combination use of central baffle and wall baffle achieves higher flux enhancement efficiency than the use of central baffle or wall baffle alone. In order to explore the hydrodynamics effects responsible for flux improvement by the baffles, CFD simulations of fluid flow in the baffle-filled tube were conducted. It reveals that the vortex formation due to the presence of baffles induces the intense velocity fluctuation, thereby increasing the turbulence intensity of fluid flow, which greatly disrupts the development of the boundary layer and effectively prevents the particle deposition on the membrane surfaces. Therefore, the permeate flux of MF membrane is significantly improved by the baffles.Then, a novel helical screw insert with a rectangular section was designed. CFD simulations of fluid flow in the tube with the newly designed helical screw insert were conducted to investigate the hydrodynamics effects responsible for flux improvement. The effects of geometric parameters of helical screw insert on the flow hydrodynamics were theoretically studied. Due to the presence of helical screw insert, the fluid flow in the tube is mainly divided into two parts, that is, helical flow within the helical groove and axial flow through the radial clearance gap. The turbulence intensity of flow and wall shear stress is significantly increased owing to the intense mixing of helical flow and axial flow, which is responsible for the flux enhancement. There is no stagnant region or dead zone at the neighborhood of tube wall and no secondary flow or vortex formation within the helical groove. Compared to the helical screw insert with semi-circular section reported in the literature, the pressure drop along the tube can be reduced by25%and wall shear stress is increased by6.7%when using the newly designed turbulence promoter. Simulative results indicate the geometric parameters of helical screw insert have a significant influence on the flow hydrodynamics. Both wall shear stress and pressure drop along the tube increase with the increase in either the helical diameter (Dh) or the central rod diameter (Dr), and decrease with the increase in the width of helical groove (λ). In terms of the critical value of wall shear stress, the geometric parameters of helical screw insert was optimized as following:Dh=11mm, λ=12mm, Dr=4or5mm. The helical screw insert without a central rod produces a smaller pressure drop and a lower wall shear stress.In order to explore the intrinsic mechanism of flux enhancement, the effects of newly designed turbulence promoter on the cake properties were experimentally investigated during the crossflow MF of particulate suspension. And the effects of operation conditions on the cake properties were studied. It reveals that the cake thickness diminishes from0.59to0.1mm, the cake porosity increases from0.55to0.65and the average particle size decreases from5.15to1.99μm due to the presence of helical screw insert under the same operation condition. The specific resistance of cake increases2.45times due to the dimished average particle size, indicating the negative effect of turbulence promoter on the permeate flux. The positive effect of turbulence promoter on the permeate flux due to the remarkable reduction in cake thickness overwhelms its negative effect due to the increased specific resistance. Therefore, the cake resistance is significantly reduced by turbulence promoter. The operation conditions of MF have significant influences on the cake parameters. The cake thickness increases, the cake porosity decreases and the average particle size increases with an increase in transmembrane pressure (TMP). The cake thickness decreases, the cake porosity increases and the average particle size decreases with an increase in the inlet velocity. The cake thickness increases and the average particle size decreases with an increases in the feed concentration. The cake porosity increases with an increase in the feed concentration when without helical screw insert, and almost keeps stable when using a helical screw insert. The effects of both TMP and feed concentration on the cake properties can be weakened to some extent, and the effects of inlet velocity on the cake properties can be strengthened by helical screw insert.At last, the turbulence promoter-assisted MF process was successfully modeled by ANN, which can predict the flux improvement efficiency under various operation conditions. The optimal ANN model architecture is that neuron numbers in the hidden layer is12and transfer functions in the hidden layer and output layer are logsig and tansig, respectively. The effects of operation conditions on the flux enhancement efficiency were analysized using ANN model. It reveals that the flux enhancement efficiency first increases and then decreases with an increase in TMP, and increases with an increase in both the inlet velocity and feed concentration. TMP has more important influence on the flux enhancement efficiency than the feed concentration or the inlet velocity. The optimal operation conditions were optimized to achieve the highest flux improvement efficiency, which provides a useful guide for the applications of turbulence promoter. It suggests that the highest flux enhancement efficiency can be obtained by applying both a high inlet velocity and a low TMP at low feed concentration, and by applying both a high inlet velocity and a high TMP at high feed concentration.更多还原