偏氟乙烯乳液聚合的气液传质效果影响因素的研究

首先推导出表征气液传质效果的K值计算方法,借此研究了聚合釜装填量、搅拌桨型和转速等因素对气液传质效果的影响,结果表明:随着聚合釜装填量和搅拌转速的增大,K值增大,气液传质效果提高;在搅拌转速大于500r/min情况下,桨型采用上层二叶平桨、下层二叶斜桨的方式明显比单层的二叶斜桨或二叶平桨的K值大,气液传质效果好。确定了5L釜偏氟乙烯(VDF)乳液聚合必需的良好搅拌效果的条件:装填量为3L、搅拌转速700r/min、桨型采用上层二叶平桨和下层二叶斜桨;VDF聚合速度达到了115gPVDF/L·h,合成出含固体质量分数为20%、粒径分布窄、稳定性好的聚偏氟乙烯(PVDF)乳液。     

偏氟乙烯乳液聚合的气液传质效果影响因素研究

推导了表征气液传质效果的K值计算方法,并研究了聚合釜装填量、搅拌桨型和转速等因素对气液传质效果的影响。结果表明,随着聚合釜装填量和搅拌转速的增大,K值增大,气液传质效果提高;在搅拌转速大于500r/min的情况下,桨型采用上层二叶平桨、下层二叶斜桨的方式明显比单层的二叶斜桨或二叶平桨的K值大,气液传质效果好。确定了5L釜偏氟乙烯(VDF)乳液聚合必需的良好搅拌效果的条件:装填量3L,搅拌转速700r/min,桨型采用上层二叶平桨和下层二叶斜桨;VDF聚合速率达到115g/(L·h),合成出固体质量分数20%、粒径分布窄、稳定性好的聚偏氟乙烯(PVDF)乳液。     

湍流状态下单组分气液相平衡体系压力的变化

以水、乙醇、正丙醇、异丙醇、乙酸和丙酮为研究目标,使纯液体与其气相在密闭容器中达到静态气液相平衡。用恒温水浴振荡床使气液相平衡体系以一定速度做回旋运动,研究流动对单组分气液相平衡体系压力变化的影响。实验共选取8个温度,从10～80 ℃每隔10 ℃选取一个;14个实验振荡速度,从80～210 r/min每隔10 r/min选取一个。在实验条件下,体系动态平衡压力与相平衡压力的偏移率（R）与实验温度和湍动程度成正比,这一数值最大可达74.20%。在实验数据的基础上,拟合出了R与雷诺数和对比温度的关联式。通过该关联式,可以预测当湍动程度足够剧烈时,液相的动态饱和蒸汽压可达静态数值的两倍以上。     

偏氟乙烯乳液聚合的气液传质效果影响因素的研究

首先推导出表征气液传质效果的K值计算方法,借此研究了聚合釜装填量、搅拌桨型和转速等因素对气液传质效果的影响,结果表明:随着聚合釜装填量和搅拌转速的增大,K值增大,气液传质效果提高;在搅拌转速大于500r·min-1情况下,桨型采用上层二叶平桨、下层二叶斜桨的方式明显比单层的二叶斜桨或二叶平桨的K值大,气液传质效果好。确定了5L釜偏氟乙烯(VDF)乳液聚合必需的良好搅拌效果的条件:装填量3L、搅拌转速700r·min-1、桨型采用上层二叶平桨和下层二叶斜桨;VDF聚合速度达到了115gPVDF·(L·h)-1,合成出固含量20wt%、粒径分布窄、稳定性好的聚偏氟乙烯(PVDF)乳液。     

基于颗粒动力学理论的搅拌器中固液流动的数值模拟

在欧拉双流体模型基础上引入颗粒动力学理论(KTGF),对带挡板圆盘涡桨式搅拌器内的固液两相流动进行数值模拟。结果表明,搅拌器底部颗粒温度分布与固相浓度分布趋势吻合,转速低于600 r/min时,槽底会形成明显的颗粒沉积,转速从600 r/min增至1500 r/min,堆积区向轴中心收缩,基于颗粒动力学理论可以合理解释挡板及叶轮转速对固相浓度分布的影响。随叶轮转速增大,搅拌器内固液两相湍流运动加剧,颗粒温度、湍动能及轴向速度增加,颗粒分布更均匀,但达到完全悬浮状态后颗粒温度趋于稳定。搅拌器底部和挡板处颗粒堆积导致了局部颗粒浓度增加及颗粒平均自由行程减少,颗粒温度反而降低;同时挡板布置使搅拌器内形成了双循环回路,加强了流体的湍流程度,增强了湍动能,但导致颗粒在挡板处积聚,不利于固相在挡板处均匀分布。     

带稳定翼四斜叶-Rushton组合桨釜内转速对流场的影响

利用计算流体力学（CFD）对带稳定翼四斜叶-Rushton组合桨釜内流场进行数值模拟，分析了不同转速下釜内流型、速度场、湍动能的变化规律。研究结果表明：搅拌器转速显著影响釜内流场和湍动能的分布，转速N=110 r/min时，釜内流型为径向流，稳定翼的切割推流作用表现明显，在釜内组合桨中间区域形成了一个高速涡旋，釜内流型为最佳流型；在该转速条件下釜内低速区趋近消失，速度分布合理；此时釜内上部和釜底的湍动能分层消失，釜内流体整体湍动程度较高，混合效果明显增强。     

基于能量最小多尺度曳力模型的搅拌槽内气液两相流计算液体力学模拟及实验研究

采用双电导探针和欧拉-欧拉双流体模型对涡轮桨搅拌槽内局部气液分散特性分别进行了实验和三维计算流体力学(CFD)数值模拟研究。重点研究了转速对搅拌槽上下循环区局部气含率分布、全槽液相流场和湍动动能的影响。实验表明,转速对上循环区气含率分布的影响大于下循环区,且上循环区气含率随转速的增大而增大。CFD模拟比较了TOMIYAMA曳力模型和基于能量最小多尺度理论(EMMS)的DBS-Local曳力模型对局部气含率的预测结果。结果显示DBS-Local曳力模型能够较好地预测出不同搅拌转速下搅拌槽循环区气含率径向分布;TOMIYAMA曳力模型只能定量预测出低搅拌转速下(140r/min,280r/min)循环区的气含率分布,高转速下(420r/min,560r/min)该曳力模型不能模拟出下循环区壁面附近的气体,且低估了上循环区气含率。     

含气液两相的搅拌釜内盘管外侧的对流传热特性研究

许多强放热反应,若温度控制不当,易引发物质分解产生气体,进而恶化相关反应过程,甚至引发事故,因此对浸没在含气液两相搅拌釜内的盘管外侧对流传热系数进行研究尤为重要。以空气和水分别作为气相和液相,在直径D=0.300m的标准椭圆形底搅拌釜内进行非稳态传热实验,研究了桨型、桨径、转速和表观气速对盘管外侧对流传热的影响。结果表明,在低转速下,通入气体能够显著增加釜内的湍动程度,使管外对流传热系数明显增大,在高转速下,对于大搅拌桨,其气体的表观气速过小时,通入气体会使对流传热系数减小。通气量一定时,在较高通气速率和高搅拌转速下,随着转速增大,对流传热系数反而减小。引入通气数对实验数据进行拟合关联。研究结果对反应釜的优化设计及反应过程的控制具有参考价值。     

淤浆聚合釜中三种桨型固液分散性能的比较

为考察淤浆聚合釜桨叶形式及转速对釜内液固分散性能的影响,建立了锥形釜体内锚式桨、螺带桨和斜叶桨的固液分散计算流体力学(CFD)模型和冷模实验。3种桨型的模拟扭矩与实验值平均误差在10%以内,模拟所得浓度场与摄像实验所得的固含率分布规律一致,模拟结果较为可靠。研究结果表明:搅拌功率随转速增大而增加,相同转速下三者功耗从大到小的顺序为锚式桨、螺带桨和斜叶桨;相同雷诺数(Re)下三者固液功率准数从大到小的顺序为斜叶桨、螺带桨和锚式桨;达到相同的浓度场方差的功耗从大到小的顺序为锚式桨、螺带桨和斜叶桨。以特征线上浓度分布行为判定,3种桨型均存在混合程度的临界转速;以浓度场方差为量化指标,3种桨型均存在最优固液分散转速。结合两者,锚式桨的实际操作转速推荐范围为120～480 r/min,螺带桨为240～600 r/min,斜叶桨为120～600 r/min。     

同心双轴搅拌器气液分散性能的实验研究

针对气液搅拌操作形式单一的现状,将框式桨和Rushton桨组成的同心双轴搅拌器与黏稠体系中的气液分散操作相结合,实验考察了转动模式、内桨转速、通气量和体系黏度等因素对其气液分散性能的影响。结果表明,与单相黏稠体系中同向旋转模式下的功率与混合性能均占优不同,同心双轴搅拌器在反向旋转模式下的气液分散性能相对更好;外桨转速不变时,内桨转速从100增加到300 r·min~(-1),整体气含率提高94.3%,局部气含率也均有增大;通气量从0.4提高到1.2 m3·h~(-1),整体气含率提高了72.5%,但局部气含率和气泡尺寸的增大不显著;体系黏度增加,气泡在釜内的停留时间加长,整体气含率单调增加。作为影响搅拌釜气液分散性能的重要参数,转动模式、内桨转速和通气量的确定还必须兼顾系统的功耗与混合效率,并避免发生气泛。     

水冷却塔传质系数计算模型初探

冷却塔填料表面传热传质过程的理论计算都与一个经验参数——传质系数有关。因为过去人们都将这个参数表示成水气流量的函数,未能正确地反映传质系数对水冷却塔传热传质过程重要影响的本质。采用液膜蒸发理论,将传质系数表示成湿空气温差及压差的函数,新的计算方法将改善冷却塔的设计计算。     

气液传质理论研究进展

文内对气液传质的理论研究进行了评述，并预测了未来的发展方向     

Mass transfer between solid particles and liquid in a three phase fluidized bed

Particle-liquid mass transfer in a co-current three-phase fluidized bed of glass beads, water and air was studied with two measurement techniques. Both techniques measured the weight loss of a few particles coated with benzoic acid in a bed of inert glass beads. The effects of liquid and gas velocities, gas distribution and surface active agents on particle-liquid mass transfer in a three-phase fluidized bed were thus determined. In the absence of surface active agents in the liquid, particle-liquid mass transfer rates in a three-phase fluidized bed were up to 30% higher than in the corresponding liquid fluidized bed. When surface active agents were added to the liquid, the particle-liquid mass transfer rate was increased by up to about 100% in the three-phase fluidized bed, relative to the liquid fluidized bed. The particle-liquid mass transfer coefficient was found to be inversely proportional to the liquid hold-up in the three-phase fluidized bed.     

Particle-liquid mass transfer in three phase sparged reactor

Particle-liquid mass transfer in a three phase sparged reactor has been studied over a wide range of particle sizes, for the chum-turbulent regime. The particle-liquid Sherwood number has been correlated in the usual form with the Reynolds and Schmidt numbers. Use of the hindered particle settling velocity in the Reynolds number yields good agreement (±20%) with the present as well as most of the literature data. The proposed correlation also holds for power law non-Newtonian liquids when the effective viscosity is used.     

Enhancement of oxygen mass transfer rate in the presence of nanosized particles

Absorption of gases into a liquid is essentially important for two- or three-phase reactions, because the diffusion of a sparingly soluble gas, like oxygen, across a gas-liquid interface generally limits the reaction rates. Using a third, dispersed phase, the mass transfer rate could be significantly increased. The question arises how the absorption rate can be described in the presence of very fine, nanometer size particles or droplets. Its mathematical model should take into account the specific properties of the nanoparticles, e.g. the Brownian motion of particles, its effect on the diffusion of the bulk phase molecules, the mass transfer rate into the nanoparticles, its dependency on the particle size, etc. The mass transfer rate of oxygen, in the presence of nanometer size, organic droplets, has been investigated both experimentally, using organic submicron n-hexadecane droplets, and theoretically. The effect of the Brownian motion of the nanoparticles as well as its effect on the diffusivity in the nanofluid has been discussed. Accordingly, the enhanced diffusion coefficient, due to the convective motion of the continuous liquid phase induced by the moving particles, has been predicted and its effect on the mass transfer enhancement has been calculated using both homogeneous and heterogeneous mathematical models. The predicted data were compared to the measured ones.     

Mass transfer from cylindrical pellets submerged in parallel flowing fluid streams

A model is proposed wherein overall mass transfer from single cylinders of benzoic acid placed in parallel flowing water streams depends on different hydrodynamic regimes prevailing along the surfaces of the cylinder. The results for laminar and turbulent conditions are correlated in terms of Sherwood and Reynolds numbers. The mass transfer in the wake region of the cylinder is observed to be significantly different from the boundary layer mass transfer through the lateral surface or the stagnation and boundary layer mass transfer through the leading frontal area.     

Ozone Mass Transfer in Stirred Vessel

A method is proposed and a model is developed which are capable of providing a correlation of the mass transfer coefficient k_La, with stirrer speed and gas superficial velocity. The method can be adopted for deriving a correlation which can be profitably used for ozone gas–liquid reaction both for assessing the absorption regime and for the simulation of oxidation processes which evolve according to slow reaction regime.     

Importance of sample intraparticle diffusivity in investigations of the mass transfer mechanism in liquid chromatography

The effective diffusivity of a nonretained (thiourea) and of a strongly retained (phenol) compounds were measured with the peak parking method in two different columns (both 150 × 4.6 mm) packed with two types of porous particles having different mesopore sizes [5 μm Jupiter‐C₁₈, 320 Å and Luna(2)‐C₁₈, 100 Å]. The eluent was a methanol–water mixture (10/90 v/v) and the temperature 294 K. The effective diffusivity data acquired were used to determine the intraparticle diffusivity, D_p, based on two different diffusion models. The first one assumes that the diffusion fluxes across the particles and in the interparticle volume are additive (parallel diffusion model). The second model was rigorously derived on the basis of the effective medium theory of diffusion (diffusion model) in a binary composite medium (particles + interparticle volume). In both models, it was assumed that the rate of equilibrium between the liquid and the solid phases was infinitely faster than the rate of axial diffusion along the column at zero flow rate. Both models provide physically meaningful intraparticle diffusivity coefficients that take into account the average mesopore size of the particles, their specific surface area, and the retention factor of the analyte. Although the actual effective intraparticle diffusivity remains unknown, these result confirm that the mass transfer resistance due to diffusion through the porous particles has almost negligible effects in reversed phase liquid chromatography due to the importance of surface diffusion. Combining the results of the peak parking method with the h data measured at high linear velocities allows the unambiguous measurement of the film mass transfer and the surface diffusion coefficients. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Mass transfer in tube bundles with liquid fluidized particles

Experimental studies were conducted of overall and local tube-to-liquid mass transfer for bundles of horizontal tubes in staggered and in-line arrays in a liquid fluidized bed. The mass transfer coefficients are compared with those obtained using a fixed packing and those obtained without particles at the shell-side of the bundle.     

超声内环流气升式反应器传质性能的研究

研究了超声内环流气升式反应器中空气-水-离子交换树脂体系的气液传质与液固传质性能.分别采用电极动态法和离子交换树脂法实验测定了气液传质和液固传质系数.重点考察了表观气速、超声电功率对于气液传质系数以及液固传质系数的影响.实验表明,内环流气升式反应器中表观气速对于气液传质性能影响较大,而超声作用对气液传质影响不大.该反应器中液固传质系数随表观气速的增加而增大;无超声作用情况下,当表观气速达到一定程度时,液固传质系数保持一恒定值.超声作用时,气速增加到4 cm·s-1左右,传质系数达到最大值,随后液固传质系数随着表观气速的增加而逐渐减小,传质系数仍然大于无超声作用时的值.超声对液固传质有较强的促进作用,电功率在150 W左右时,超声对液固传质的促进作用最佳,是不加超声作用时传质系数的两倍左右;但随着电功率的进一步增大,液固传质系数呈现下降趋势.