Comparison between primary and secondary current distributions in bipolar electrochemical reactors

The primary and secondary current distributions in bipolar electrochemical reactors with recessed electrodes are compared. When the electrolyte of the different reactors of the stack is connected, and thus a leakage current is possible, the secondary current distribution is more pronounced than the primary one for the cases of industrial importance. In the absence of a leakage current the usual behavior of a monopolar system is observed.     

Simplified model to predict the effect of the leakage current on primary and secondary current distributions in electrochemical reactors with a bipolar electrode

A simplified mathematical model to calculate the current distributions in bipolar electrochemical reactors is proposed. The current distributions are deduced from a combination of the voltage balance in the reactor with a voltage balance including the electrolyte inlet and outlet. Thus, equations to predict the effect of geometric and operational variables on the current distributions at the electrodes are reported. The parameters acting upon the current distributions were lumped into two dimensionless variables and their effects on the current distributions are discussed. The primary current distributions are obtained as a limiting case. Comparisons between calculated and experimental primary current distributions are reported.     

Mass transfer and pressure distributions in electrochemical pump cells

Macroscopic and microscopic measurements of mass transfer rate have been made in a pump cell operating at up 6000 rpm. The dependence of the average Sherwood number is given by

where G = h/r_o is the gap ratio, Re is the channel Reynolds number and R_ø is the rotational Reynolds number. The mass transfer rate is so high, and the diffusion layer thickness consequently so small (～2 × 10^?2mm), that the rotor behaves essentially as a free disc even when the gap is less than 0.1 mm. Local measurements using microelectrodes confirm this behaviour, and show that the current density increases monotonically towards exit (in contrast to the behaviour of a non-rotational, capillary gap cell).Agreement between measured and theoretically derived pressure distributions is fair, and both show that pressures below atmospheric are easily attained at entry. Under almost all operating conditions there is then a net rise in pressure across the disc so the cell may be used as its own pump; an estimate of pumping power is derived.     

A new technique for two-dimensional current distribution measurements in electrochemical cells

A new technique has been developed using a magnetic loop array to measure current distribution in electrochemical cells. The main advantage of this approach is the combination of high spatial and time resolution and stack integration with an easily handled measurement carried out independently of the cell operation. A polymer electrolyte fuel cell (PEFC) of technically relevant dimensions (about 600 cm² electrode area) with 5 × 8 current sensors has been constructed and operated, thus confirming the feasibility of the measurement technique.     

A new method for calculating and correcting molecular weight distributions from gel permeation chromatography

A new method for calculating and correcting molecular weight distributions of polymer samples from GPC chromatograms is presented. The integral equation which relates the true molecular weight distribution of polymer sample to the chromatogram is reformulated into an equivalent variational problem of quadratic functional. The method of steepest descent in the function space is then applied to the minimization problem to obtain the true molecular weight distribution. This method is efficient and reduces some of the oscillation problems encountered in the previous methods. Examples are given.     

Analysis of primary and secondary current distributions in a wedge-type aluminum-air cell

The primary and secondary current distributions near the leading edges of the cathode and anode of a wedge-type aluminum-air cell design were analyzed. Numerical calculations were accomplished by using a finite difference method and introducing an overlapping two-grid system technique. The calculations indicate that the current distributions on the cathode and anode at distances from the edges greater than 2 times the cell gap are uniform. In the edge region, the wedge angle between 0 and 10° has a negligible effect on the current distribution. High current densities at the cathode edge, which are detrimental to cathode life, are reduced by kinetic effects and by oversizing the cathode itself. The latter also favors cell performance but adds to the cell costs. An effectiveness factor is introduced which demonstrates the effectiveness of cathode oversize and the sensitivity to kinetics as represented by the Wagner number. The calculations indicate that only marginal performance gains can be expected when the cathode extends beyond the anode a distance greater than that of 1.5 times the amode-cathode gap.Nomenclature A1, A2, A3 anode curves 1, 2, 3 - b slope ofi vs curve at mean value of _mean (A cm^–2V^–1) - C1, C2, C3 cathode curves 1, 2, 3 - D distance between electrodes (cm) - i _s local current density on electrode (A cm^–2) - I ^* dimensionless local current value defined by Equation 9 - N dimensionless effectiveness factor defined by Equation 10 - V _met constant potential of electrode (V) - W dimensionless Wagner number defined by Equation 7 - X dimensionless cathode oversize defined asx/D - x position parallel to anode with origin at the anode apex (cm) - y position perpendicular to electrode surface (cm) - K conductivity of electrolyte (ohm cm^–1) - surface overpotential (V) - potential in solution phase (V) - ₀ potential in solution adjacent to electrode surface (V)     

A segment probability method for calculating the molecular weight distributions of linear polycondensates in a continuous reactor

The molecular weight distribution (MWD) of polymers is a target of the optimization and control of industrial polymerization processes, as it dictates the processability and properties of polymers. A method, named as segment probability method, is developed to calculate the MWD of polycondensates produced by monomers of types A₂ and B₂ in a continuous reactor. It considers a growing chain as being composed of A and B segments in the middle of the chain and two terminal segments at the chain ends. It calculates the propagation probabilities of these different types of segments upon taking into account both the polycondensation and side reaction kinetics as well as the residence time distribution of the continuous reactor. The method is validated by poly(butylene terephthalate) (PBT) obtained from an industrial polymerization process composed of a continuous esterification reactor. The MWDs of the PBT calculated by this method are in agreement with those measured by size exclusion chromatography with mean square errors less than 10%.     

Effect of leakage currents on the secondary current distribution in bipolar electrochemical reactors

The secondary current distribution in an electrochemical stack with one bipolar electrode was experimentally determined and compared with the theoretical prediction according to the Laplace equation. A close agreement between both results is reported. The parameters acting upon the current distribution were lumped into a dimensionless variable, called the bipolar Wagner number, and its effect on the current distribution and predictive suitability of the theoretical treatment is discussed.     

A method of calculating densities of polymers

Molar volumes of substantially amorphous polymers may be calculated by using additive constants derived from constitutive atomic and structural parachor contributions. For 34 polymers, ranging in density from 0.83 to 2.03 g/cm³, the greatest error in the calculated molar volume compared with the measured value was 13.9%, while for 30 of the polymers the error was less than ±7%, with a mean error of ±3.9%.     

A method of calculating copolymerization reactivity ratios

An improved linearization method is proposed for determining the monomer reactivity ratios of free-radical copolymerizations by fitting the data of cumulative copolymer compositions and overall weight fractional conversions, or cumulative copolymer compositions and residual monomer compositions of the reaction mixture. The reactivity ratios of monomers for four different copolymerization reactions are determined. The results are very close to those of literature reports and account well for the experimental results.     

基于单电流阶跃法的淀粉间接电化学氧化研究

以钛板作为阴极,石墨烯为阳极,NaCl为支持电解质兼氧化源,进行淀粉的间接电氧化。以电流密度、可溶性淀粉(SS)质量分数、NaCl质量分数、电解时间4个因素进行L_9(3~4)正交实验。正交实验结果表明,最佳的SS和NaCl质量分数分别为10%、2%。采用单电流阶跃法对电流密度、电解时间进行了单因素考察,结果表明,最佳的电流密度是20mA/cm~2。根据图像中电势的跃迁特征取样进行淀粉氧化过程醛基分析,获得间接氧化实验体系最佳时间为电势的转折点。分析发现,转折点上的氧化淀粉含醛基质量分数已接近最大平衡。FTIR及XRD的辅助分析证明,在转折点的氧化淀粉结构与原淀粉之间已发生了明显氧化变化,确定最佳电解时间是1200 s。在此条件下,得到氧化淀粉含醛基质量分数为0.432%。     

A numerical procedure for calculating droplet deformation in dispersing flows and experimental verification

A three-step numerical procedure for studying droplet deformation in mixed, dispersing-type, flow fields is described. Finite element and numerical particle tracking techniques are used to obtain the history of shear and elongation rates along a particle trajectory in the flow field, and from this history, boundary integral techniques are used to determine the deformation a drop would experience along this path. This approach is then used to investigate the effect of a small change in geometry on the breakup behavior of drops in the annular gap flow between two eccentric cylinders. This flow geometry serves as an idealization of a rotor-stator dispersing device used for highly viscous fluid systems. It is found that an increase in eccentricity produces an increase in dispersing capability. Experiments in an eccentric cylinder geometry were performed to verify the simulation procedure. Under the experimental conditions considered, it is found that the simulations perform well, correctly predicting whether or not drop breakup occurs and the qualitative drop evolution behavior. The simulation procedure outlined in this paper can serve as an effective tool to determine drop breakup in dispersing geometries and hence to optimize dispersing procedures.     

Applying the boundary element method to electrochemical calculations of primary current distribution

The computation of primary current distribution using the boundary element method has been analysed. A numerical example has been included. Values obtained numerically have been compared with those obtained experimentally. A fairly good accuracy has been observed. Some computational problems occurring when using the boundary element method for electrochemical problems have been discussed.     

In-situ methods for the determination of current distributions in PEM fuel cells

This article describes three different techniques for the determination of the current density distribution in operating fuel cells and compares their relative benefits with respect to ease of implementation and information gain. Real-time current density distribution data under steady state as well as transient conditions are presented and it is shown that they can contribute to an improved understanding of water management and reactant distribution over the active fuel cell area. The importance of these factors for the optimisation of fuel cell performance is discussed.     

一种计算含盐有机溶液气液相平衡的方法

提出了一种计算含盐混合溶剂气液相平衡的新模型,即把活度系数表示为盐的质量摩尔浓度ms和溶剂组成摩尔分数xi的函数,其中盐质量摩尔浓度项表示为盐质量摩尔浓度的二次函数,溶剂组成项由UNIQUAC模型计算。以文献中水-醇-盐和醇-醇-盐的11个体系为例,将新模型用于其气液平衡的计算,关联得到了各体系的模型参数。结果表明:将活度系数表示为ms和xi的函数是合适的;每种溶剂组分气相分压的计算值与实验值吻合良好,最大误差为0.928 k Pa。因此,新模型是一种可广泛用于含盐有机溶液体系气液相平衡计算的方法。     

The current distribution and shape change of zinc electrodes in secondary silver-zinc cells

The current and potential distributions of zinc electrodes in secondary zinc-silver oxide cells during cycling were studied using a sectioned electrode technique. The shape change occurring in zinc electrodes resulting from cell cycling was examined. The zinc electrodes used for this study were of a conventional type of design and were fabricated by a slurry paste method. The positive electrodes used were sectioned silver oxide electrodes, each of which comprised nine sections of sintered silver plates. The results indicate that, by adding a layer of non-woven fabric treated with Fe₂O₃ to the edge sections of the electrode separator system, the uniformity of the current and potential distributions on the zinc electrodes during cell cycling can be improved, and the zinc electrode shape change resulting from cell cycling can be effectively suppressed.