Effects of baffle length on mass transfer in a parallel plate rectangular electrochemical cell

Global mass transfer measurements in unbaffled and baffled configurations using different baffle lengths and Reynolds numbers have been made in a parallel plate cell of rectangular geometry. The entry jet arrangement and the repeated 180° changes in direction of the flow, followed by the exit, produces extremely complex hydrodynamics in the cell. A plot of mass transfer coefficient against baffle length shows an increase in mass transfer with baffle length. Comparison of data for the present work with those of other workers for similar devices showed higher mass transfer due to the modifications incorporated in the present cell.     

Mass transfer in a laminar-flow parallel plate electrolytic cell with simultaneous development of velocity and concentration boundary layers

Ionic mass transfer in a parallel plate electrochemical cell having a flow entrance representative of practical industrial cells has been studied under laminar flow condition using the ferri- and ferrocyanide redox couple. The working electrode was segmented so that local mass transfer coefficient could be measured. The mass transfer coefficients, measured for the entrance region of the cell, which is defined as the region where both hydrodynamic and concentration boundary layers are developing simultaneously, are correlated and compared with results from other studies. For the electrode section beyond the entrance region, where the flow is fully developed but the concentration boundary layer continues to grow, a correction term is added to the electrode dimension parameter in the Graetz number. In this way the Leveque equation can still be used to correlate the local mass transfer coefficient. The value of the correction term is calculated by matching the downstream mass transfer boundary layer thickness to that of a Leveque-type cell. This scheme improves substantially the convergence of the experimental data correlation and results in good agreement with the Leveque equation.     

Entrance effect on mass transfer in a parallel plate electrochemical reactor

The influence of different fluid inlet types, slits or tubes, on mass transfer in a rectangular reactor was studied. Measurements of mass transfer coefficients were made using the limiting diffusion current technique based on ferricyanide ion reduction at a large nickel electrode located downstream of abrupt expansions. The overall mass transfer coefficients obtained were 3 to 13 times greater than those obtained in fully developed flows. Overall mass transfer coefficients were correlated for Reynolds numbers ranging from 400 to 3500 by a unique equation by introducing a nondimensional expansion factor defined by the ratio of the fluid inlet cross-section to that of the reactor. The correlation equation obtained was compared with literature data.     

Laminar and turbulent mass transfer simulations in a parallel plate reactor

Laminar and turbulent mass transfer in a parallel plate reactor at high Schmidt number obtained from numerical simulation is compared with literature data. In a first step, the fluid flow is determined numerically in the reactor by solving the Navier–Stokes equations. For turbulent flow, a low Reynolds number k— model is used to calculate the turbulent viscosity. Using the obtained flow field and turbulent viscosity, the current density distribution is calculated for different flow velocities by solving the equations describing the transport of multiple ions due to diffusion, convection and migration. For the laminar case, a very good agreement with literature data is obtained. For turbulent flow, different numerical models for turbulent mass transfer are proposed in the literature. A detailed study of the behaviour close to the wall of these different turbulence models is presented, together with a comparison of the calculated results with literature correlations. This allows identification of the benefits and disadvantages of each of the turbulence models for the numerical calculation of mass transfer at high Schmidt numbers in a parallel plate reactor.     

Mixed convection mass transfer studies of opposing and aiding flow in a parallel plate electrochemical flow cell

Studies of combined natural and forced convection in a vertical parallel plate electrochemical cell in laminar conditions in cases of opposing and aiding flow are reported. In an ongoing project it was necessary to identify conditions in which natural convection had no significant influence on mass transfer rates at the cell walls so that data could be validly compared with purely laminar flow computational models. For the different electrode lengths investigated, natural convection dominated at low Reynolds number and there was no Reynolds number dependence. At high Reynolds number the data approached the laminar flow solution. At intermediate Reynolds number, however, there existed a distinct region where free and forced convection were significant. At high electrolyte concentrations data did not merge with laminar flow equations until Re=1000 and low electrolyte concentration data for the large plate could not be compared with numerical predictions below Re of 250. An attempt was made to compare the data with those of other workers on combined forced and natural convection heat and mass transfer.     

The influence of collector and bacterial cell surface properties on the deposition of oral streptococci in a parallel plate flow cell

Abstraet-The deposition of two strains of oral streptococci on collector surfaces with different surface free energies has been studied in a parallel plate flow cell at various buffer concentrations. One of the strains, Streptococcus salivarius HB, was characterized by the presence of proteinaceous surface appendages with lengths between 72 and 178 nm; whereas S. salivarius HB-C12, a spontaneous mutant of S. salivarius HB, was devoid of all surface appendages. Large differences were observed between the deposition rates of the two strains which were not predicted by theoretical calculations based on the convective diffusion equation using a DLVO-type potential function. This difference was attributed to the influence of the surface appendages of S. salivarius HB, which were absent on the mutant strain S. salivarius HB-C12. Furthermore, an additional attractive interaction, not accounted for in the DLVO theory, was needed in order to match the experimental deposition rates on the low surface free energy collector surfaces with theoretical predictions.     

Mass transfer in a reciprocating plate extraction column — effects of mass transfer direction and plate material

The mass transfer performance of 5 cm diameter reciprocating plate column has been measured with the system n-butyric acid/kerosene (dispersed)/water (continuous). In most of the tests, the reciprocating plate stack was made of stainless steel which was preferentially wet by the continuous phase. During the mass transfer process the holdup and in some cases the Sauter mean droplet diameter were measured. The mass transfer effectiveness, expressed as the height of a transfer unit (H_ax) corrected for axial mixing, depended on the phase flow rates, the agitation rate and the direction of mass transfer. For continuous to dispersed phase mass transfer (c → d), the mass transfer was more effective, i.e. H_ax was lower than for d → c mass transfer under the same external conditions. Qualitative and quantitative observations indicated much larger drop sizes in the d → c case due to enhanced coalescence. Although mass transfer was less effective under d → c conditions, the column capacity was increased. The same effects were also observed when the plate stack was modified by inserting some Teflon plates which were partly wet by the dispersed phase.     

折流式旋转床的流体力学与传质性能研究

开发了一种新型的旋转设备——折流式旋转床,其转子由动部件和静部件组合而成.与传统的旋转填料床相比,折流式旋转床易于实现连续精馏过程的中间进料,同时可方便地将多个转子同轴安装在-个壳体内,成倍提高单台设备的分离能力.对折流旋转床的流体力学和传质性能进行了实验研究,结果表明折流式旋转床具有良好的流体力学和传质性能.目前折流式旋转床已经成功应用于化学工业中的气液接触过程,尤其是连续精馏过程中.     

Effect of interfacial instabilities and hydrodynamic interaction on liquid-liquid mass transfer

Mass transfer rate between two liquid phases with a plane interface has been studied in a modified Lewis stirred cell. The time interval between the successive concentration determinations was reduced to thirty seconds by the use of a continuous flow refractometer. The mass transfer coefficients were found to correlate satisfactorily with the surface renewal model unless interfacial motion is opposed by the change in physical properties of the system or interfacial shear stresses. Under these conditions Levich-Davies model could successfully explain the experimental mass transfer coefficients found in this work.Due to the effect of the shear stresses and the changes in the physical properties, bulk Reynolds numbers were not sufficient alone to explain the hydrodynamics at the interface which control the mass transfer rate. Instead, another dimensionless quantity such as the ratio of interfacial stresses to kinetic energies of the eddies should be used as a criterion of the hydrodynamics at the interface.     

Mass transfer in a parallel plate electrochemical cell—the effect of change of flow area and flow cross-section at the cell inlet

Experimental measurements have been made of mass transfer in parallel plate cell which has a circular in-line flow inlet. It has been found that the mass transfer coefficient has a maximum value at a point slightly greater than six equivalent diameters downstream from the inlet. For cell Reynolds numbers between 1630 and 2960 this maximum mass transfer coefficient is directly proportional to the flow rate but the location of the maximum is independent of flow rate. By assuming that the position of the maximum corresponds to the flow reattachment point and considering the mass transfer downstream from that point it has been shown that the local (or average) mass transfer coefficient is proportional to (L^*)^?n where L^* is the distance downstream from the reattachment point. For turbulent flow n = 0.37 and for laminar/transition motion n is in the range 0.50–0.60. A preliminary explanation of these results is proposed.     

Continuous and discrete time response analysis of the backflow cell model for linear interphase mass transfer on a distillation plate

An analysis of the backflow cell model of a distillation plate with linear interphase mass transfer is given. The steady state character is discussed in terms of liquid phase concentration profiles and plate efficiencies. The dynamics due to liquid and vapor phase concentration perturbations are derived for continuous time and discrete times based upon two point time difference methods of numerical solution. The moments of the discrete impulse response are related to those for continuous time. Stability and convergence are discussed. For the liquid on a valve plate, the backflow cell model outlet response due to a measured inlet response is compared to the monitored outlet response resulting from upstream salt tracer injection.     

Control of mass transfer and hydrodynamic holdup in a karr solvent extraction column

This paper reports an experimental application of multiloop control to a Karr solvent extraction column (a reciprocating plate column). The control objective was to keep the extract outlet concentration above some minimum level and to maintain a high value of dispersed phase holdup while keeping away from the flooding point. Transfer function models were identified from step tests about a nominal operating point and various interaction/operability analysis techniques were used to synthesize a control scheme. The control implemented in this study was a model inversion based scheme in which the extract outlet concentration was controlled by manipulating the continuous phase superficial velocity and the dispersed phase holdup was controlled by manipulating the frequency of reciprocation. Both servomechanism and regulatory responses are presented. A variable dead time Dahlin controller was used to control the extract outlet concentration and a Clarke-Gawthrop self tuning regulator was used to control the dispersed phase holdup. This multiloop control scheme is compared to single loop control of the extract outlet concentration.     

离子膜电解槽改膜极距电解槽运行总结

阐述了对离子膜电解槽的改造内容和改造后电解槽的特点.规范了膜极距电槽的开、停车操作,考核了对膜极距电槽运行,提出了运行中的注意事项.     

内循环气升式生物流化床水力学与传质特性研究

通过测定内循环气升式生物流化床的停留时间分布与溶解氧浓度，分析流化床内有效工作体积与氧传质系数的变化，以此研究其水力学与传质特性。结果表明，随着载体含量和气流量的增大，流化床内有效工作体积明显减小；氧传质系数则随着载体含量的增加先增后减，随着气流量和液流量的增大而增大。在本实验范围内，得出最适宜操作参数为载体含量为6％左右、气流量为180～240L／h之间、液流量为90L／h。     

Influence of dust particles on the hydrodynamic behavior and mass transfer in droplet column

Bubble columns are commonly used in industry for polluted gas treatment. Based on the same principle, the droplet column which is not widely known in the literature, uses much higher gas velocities, up to 14 m/s. This study concerns the hydrodynamics and mass transfer in this apparatus, in presence or absence of solid particles. Our results have demonstrated the impact of dusts on mass transfer performance giving rise to an increase in the k_L a_L coefficient with decreasing particle size. However, no influence of dust on the hydrodynamics of the column has been demonstrated within the studied particle size range. A study of liquid holdups obtained by three different methods is also carried out. Our results are comparable, thus validating the methods used.     

气体折流塔板流体力学和传质实验研究

对气体折流塔板的流体力学和传质进行了实验,找出了气液流动对塔板压降和传质的影响规律,通过气体折流塔板与传统筛板在流体力学与传质等方面的比较,找到了气体折流塔板的适用范围及相对于传统筛板的优点,为以后此塔板的应用提供了有效数据。     

Mass transfer to and copper deposition on a round bar in a new type of electrolytic cell: the helix cell

High-rate electrodeposition of copper from CuSO₄-H₂SO₄ baths can be achieved by using crossflow of solution. To obtain copper layers of uniform thickness and quality, a new type of electrolytic cell, the helix cell, has been proposed. An experimental dimensionless relation has been given to describe the mass transfer to a round bar, in crossflow, in a helix cell. Moreover, the current efficiency of copper deposition has been obtained as a function of current density, flow rate of solution, temperature and weight per cent CuSO₄ in the CuSO₄-H₂SO₄ solution.Nomenclature A _e working-electrode surface area (m²) - c concentration of Cu²⁺ (mol m^-3) - c _e c at electrode surface (mol m^-3) - c _b c in bulk of solution (mol m^-3) - C constant factor (-) - d _c inner diameter of central cylinder of helix cell (mm) - d _c volumetric hydraulic diameter of helix cell (mm) - d _h width of helical slots in central cylinder of helix cell (mm) - d _s diameter of working electrode (round bar) (mm) - D _i diffusion coefficient of species i (m² s^-1) - F Faraday constant (C mol^-1) - I current (A) - k _i mass-transfer coefficient for a species i (m s^-1) - i current density (kA m^-2, A m^-2) - L _c length of working-electrode compartment of helix cell (m) - n number of electrons involved in electrode reaction (-) - n₁, n₂, n₃ constants (-) - R gas constant: R=8.31 J K^-1 mol^-1 - Re Reynolds number: Re=v _c d _h/v (-) - Sc Schmidt number: Sc=v/D (-) - Sh Sherwood number: Sh=kd _h/D - t time (s) - T temperature (K) - U _s volumetric rate of solution through the helix cell (m³ s^-1) - v _c flow rate of solution through working-electrode compartment of the helix cell (vc = U/(d _c –dw)Lc) (ms–1) - density of solution (kg m ^-3) - u dynamic viscosityofsolution (kg m-1 s-1) - v kinematic viscosity of solution (m2 s-1) - _i current efficiency for formation of a species i (-)