Simulation of velocity profiles in a laboratory electrolyser using computational fluid dynamics

A commercial CFD code, Fluent, has been used to analyse the design of a filter-press reactor operating with characteristic linear flow velocities between 0.024 and 0.192 m s⁻¹. Electrolyte flow through the reactor channel was numerically calculated using a finite volume approach to solve the Navier-Stokes equations. The length of the channel was divided into 7 sections corresponding to distances of 0, 0.01, 0.04, 0.08, 0.12, 0.14 and 0.15 m from the electrode edge nearest to the inlet. The depth of the channel was divided into three planes parallel to the channel bottom. For each channel section, a velocity profile was obtained at each depth together with the average velocity in each plane. The flow predictions show that the flow development, as the electrolyte passes through the cell, is strongly affected by the manifold causing strong vortex structures at the entrance and exit of the channel. Although the flow disturbances are a function of the flow rate, they gradually disappear downstream along the channel length. Simulated velocity profiles are considered for the typical current density range used in the FM01-LC reactor.     

Liquid-phase mass transfer within KATAPAK-S structures studied using computational fluid dynamics simulations

The liquid-phase mass transfer within the catalyst-packed criss-crossing sandwich structures of KATAPAK-S has been studied with the use of computational fluid dynamics. Due to the “upheaval” caused by the flow splitting at the crossovers, the mass transfer coefficient is about 40% times larger than for fully developed laminar flow in a single, packed tube.     

Using computational fluid dynamics in combating erosion-corrosion

Computational fluid dynamics was used to search for the links between the observed pattern of attack seen in a bauxite refinery's heat exchanger headers and the hydrodynamics inside the header. Validation of the computational fluid dynamics results was done by comparing then with flow parameters measured in a 1:5 scale model of the first pass header in the laboratory. Computational fluid dynamics simulations were used to establish hydrodynamic similarity between the 1:5 scale and full scale models of the first pass header. It was found that the erosion-corrosion damage seen at the tubesheet of the first pass header was a consequence of increased levels of turbulence at the tubesheet caused by a rapidly turning flow. A prismatic flow corrections device introduced in the past helped in rectifying the problem at the tubesheet but exaggerated the erosion-corrosion problem at the first pass header shell. A number of alternative flow correction devices were tested using computational fluid dynamics. Axial ribbing in the first pass header and an inlet flow diffuser have shown the best performance and were recommended for implementation. Computational fluid dynamics simulations have revealed a smooth orderly low turbulence flow pattern in the second, third and fourth pass as well as the exit headers where no erosion-corrosion was seen in practice. This study has confirmed that near-wall turbulence intensity, which can be successfully predicted by using computational fluid dynamics, is a good hydrodynamic predictor of erosion-corrosion damage in complex geometries.     

Optimization of gas-liquid reactor using computational fluid dynamics

The relationship between the geometry and the operating conditions, hydrodynamics and performance of an industrial gas-liquid stirred reactor has been studied with the help of CFD modeling and gamma ray tomography. It is seen that the interfacial area distribution is very wide. This in some areas leads to mass transfer limitations. Strategies for retrofitting the reactor were then developed (e.g. change in the operating speed, change of impeller type, change in the feed introduction, etc.) and tested by CFD for improving the reactor performance. The benefits of the implementation were in terms of the improvement in the product quality, reduction in the by-product formation, increase in the reactor throughput.     

Comparing the energy efficiency of different high rate algal raceway pond designs using computational fluid dynamics

High Rate Algal Ponds with long, shallow, looped channels and powered by paddlewheels have been used since the late 1970s for growing algae to produce nutraceuticals and to remediate wastewater. These high rate or raceway ponds are also being applied to other fields such as CO₂ capture and biodiesel production, where there is an ongoing effort to find ways of minimizing operating and capital costs. One approach for minimizing costs is to improve the energy efficiency of raceway ponds as this would lower operating costs and allow the construction of larger ponds, which should also lower capital costs. A major component of energy loss in a raceway pond is the energy required to circulate the fluid around the raceway, particularly at the hairpin bends.     

Evaluation of carbon electrodes and electrosynthesis of coumestan and catecholamine derivatives in the FM01-LC electrolyser

This work extends the range of electrodes and conditions under which the FM01-LC reactor has been used in a laboratory environment and evaluates the performance of carbon electrodes. Reticulated vitreous carbon (RVC) has been used to provide a stable, inert, three-dimensional electrode surface for organic electrosynthesis; its performance is compared to that of nickel mesh for the oxidation of catechol to o-quinone. This product was then reacted in situ with (i) 4-hydroxycoumarin and (ii) 1,3-dimethylbarbituric acid to produce, respectively, coumestan and catecholamine, products of synthetic interest. In mass transport experiments using hydroquinone oxidation as a model reaction, performance was similar to nickel electrodes, but Sherwood numbers were reduced by about 5–10% when carbon electrodes were used. The best-performing RVC electrode, however, showed poorer behaviour than its nickel counterpart. Yields for the production of coumestan and catecholamine were approximately 45% and 25%, respectively, although this was mostly due to extraction problems, since current efficiencies were both in the region of 65–70%. The electrode material, rather than the fluid flow behaviour, leads to a reduction in overall cell efficiency; this is confirmed by studies which show a film forming on the surface of the electrode.     

计算流体力学在顺流喷雾干燥器模拟中的误差源研究

The paper is focused on identifying error sources in computational fluid dynamics(CFD) predictions of a spray drying process. Seven groups of drying and atomisation parameters were selected for analysis and 13 simulation trials were performed. The theoretical results were compared with experimental data and sensitivity of the simulation results to the analysed factors was determined. The following parameters affecting the accuracy of CFD spray modelling were found: gas turbulence model, particle dispersion, atomising air, initial parameters of atomisation and heat losses to the environment. A major difference in the errors committed during modelling of spray drying process for fine and coarse sprays was observed.     

A computational laboratory on the role of mass transport contributions in electrochemical systems: Copper deposition

This paper presents a computational laboratory that describes the ionic transport of chemical species in an electrochemical process. The system is modeled in 1D using a kinetic model type Butler–Volmer coupled with mass balance equations, i.e. Nernst–Planck formalism. This laboratory is intended to be a practical learning tool to study the deposition of chemical species, e.g. Cu²⁺, subject to the typical mass transfer mechanisms, i.e. diffusion, migration and convection. Sensitivity analyses are used to analyze the effect of each mass transport phenomena over the process reaction rate. The material showed in this paper is a section (laboratory) of two third-year courses in the Nanotechnology and Chemical Engineering undergraduate programs at the University of Waterloo. The pedagogical goals, learning experiences and students’ comments of this laboratory are presented in this work.     

Enabling a commercial computational fluid dynamics code to perform certain nonlinear analysis tasks

In this work we enable the commercial computational fluid dynamics code Fluent, to successfully trace a complete solution branch, even past turning points. Here the so-called Recursive Projection Method (RPM) is implemented as a computational shell “wrapped” around Fluent, in conjunction with a pseudo-arc-length method for convergence on the unstable branch. The case study is a mixed convection flow in a stagnation point chemical vapor deposition (CVD) reactor. Multiple steady states coexist over a range of inlet Reynolds numbers, due to the competition of the two dominant physical mechanisms: forced and free convection. Continuation on the solution branch reveals a curve consisting of a stable branch, dominated by free convection, followed, past the first turning point, by an unstable branch. Past a second turning point, follows another stable branch dominated by forced convection. Taking the problem a step further, it is augmented with a chemical model describing the deposition of silicon (Si) from silane (SiH₄), silylene (SiH₂) and hydrogen (H₂). The solution branch does not alter since the gas mixture is dilute and the carrier gas, in this case nitrogen (N₂), and the precursor, in this case SiH₄, are of similar molar masses; the concentration differences cannot lead to solutal convection. Results for the mass fraction distribution inside the reactor and the film growth rates are reported in all parts of the solution branch.     

焚硫炉内燃烧过程的CFD研究

介绍了焚硫炉计算流体动力学（CFD）模型及模型建立中涉及的一些关键因素。利用CFD模型可以更好地理解焚硫炉、焚硫炉与周围管道的关系、焚硫炉对不同硫原料系统或流量的处理能力,从而为焚硫炉设计和改造提供依据。NORAM公司几项新焚硫炉设计和旧焚硫炉改造项目表明,合理使用CFD技术并选用合适的磺枪,一台与原设备尺寸相同的焚硫炉可以获得更高的产能。     

Analysis of shear-induced coagulation in an emulsion polymerisation reactor using computational fluid dynamics

Shear-dependent coagulation is a costly problem for the latex manufacturing industry, due to product degradation and reactor downtime. In this study, a method for calculating the shear-dependent coagulation rate in emulsion polymerisation is developed. The method combines simple models for coagulation (only binary collisions being considered) with the effects of rheology on the flow field, using computational fluid dynamics (CFD) to solve the detailed flow field in the reaction vessel. By using the local shear rates (LSR), the method developed provides a more detailed and system-specific assessment compared with using an average shear rate (ASR) for calculating the coagulation rate. The difference in the predictions between the ASR and the proposed LSR method was investigated. It was found that the ASR and LSR methods predict different coagulation rates, especially for more sophisticated coagulation models where the coagulation rate is not linearly dependent on the shear rate. The LSR method was also used to study the effect of the rheology of the latex, of the impeller speed and of the reactor design on the coagulation rate. It was found that the LSR method is useful for providing both visual and numerical means to identify regions with elevated coagulation rates in the modelled reaction vessel. The treatment provides estimates of the amounts of coagulum formed on the vessel walls and on the impeller.     

计算流体力学在化学工程中的应用

计算流体力学(CFD)用于求解固定几何形状设备内的流体的动量、热量和质量方程以及相关的其他方程,已成为研究化工领域中流体流动和传质的重要工具。本文概述了CFD的基本原理以及CFD在化学工程领域方面的应用,重点介绍了CFD在搅拌槽、换热器、蒸馏塔、薄膜蒸发器、燃烧等方面的应用。     

Mass transfer enhancement in supercritical fluid extraction by acoustic waves

A computational fluid dynamics model of extraction of a solute (caffeine) from a porous solid matrix (coffee beans) using a supercritical solvent (carbon dioxide) is developed. Supercritical fluid extraction of a solute from a solid matrix is a slow process even when solute free solvent is circulated. The use of acoustic waves represents a potential efficient way of enhancing mass transfer processes. The effect of acoustically excited flows on supercritical fluid extraction from a porous solid matrix is investigated. The mathematical model considers diffusion-controlled regime in the porous solid matrix and convective-diffusive transport in the bulk fluid. Henry's law is used to describe the equilibrium states of the solid and the fluid phases. Accurate representation of the thermo-physical properties of supercritical solvent is considered by using the NIST Standard Reference Database 12. The conservation equations for mass, momentum, energy and species are numerically solved using implicit finite volume method. The effect of process parameters, such as initial state (pressure and temperature) of solvent and acoustic waves on the yield of solute extraction is also investigated numerically.     

A computational fluid dynamics design of a carbon dioxide sorption circulating fluidized bed

A kinetic theory based hydrodynamic model with experimentally determined sorption rates for reaction of CO₂ with K₂CO₃ solid sorbent is used to design a compact circulating fluidized bed sorption‐regeneration system for CO₂ removal from flue gases. Because of high solids fluxes, the sorber does not require internal or external cooling. The output is verified by computing the granular temperatures, particle viscosities, dispersion, and mass transfer coefficients. These properties agree with reported measurement values except the radial dispersion coefficients, which are much higher due to the larger bed diameter. With the solid sorbent prepared according to published information, the CO₂ removal percentage at the riser top is 69.16%. To improve the CO₂ removal, an effort is needed to develop a better sorbent or to simply lower the inlet gas velocity to operate in a denser mode, leading to a larger system. Also, the effect of temperature rise on the removal efficiency is investigated. © 2010 American Institute of Chemical Engineers AIChE J, 2010     

The influence of inclined plates on expansion behaviour of solid suspensions in a liquid fluidised bed — A computational fluid dynamics study

A significant increase in the particle sedimentation rate can be achieved by introducing inclined plates into conventional fluidised beds. In turn, high suspension densities are possible at fluidisation velocities in excess of the particle terminal velocity. The installation of the inclined plates, however, alters the dynamic characteristics of the fluidised bed, in particular, impacting upon the expansion behaviour of the suspension. In the present work a Computational Fluid Dynamics (CFD) approach was employed to investigate the influence of inclined plates on the expansion behaviour of solids suspensions in liquid fluidised beds. The model is based on the solution of the Eulerian multiphase equations for up to two different particle sizes with a continuous phase of water. The momentum equations treat hindered settling behaviour via the inclusion of a volume fraction dependent drag law. The computational model was validated against our experimental data and compared with the predictions of a kinematic model developed in one of our earlier works. In general the predictions made by both the CFD and the kinematic models were found to be in good agreement with the experimental results.     

Single-phase flow model development for macroscopic liquid flow evaluation in gas–liquid reactors, by computational fluid dynamics

This paper proposes a single-phase flow model to simulate the flow induced in a liquid by the injection of gas dispersed in the form of a bubble curtain. It aims at predicting macroscopic liquid flow and mixing time. This single-phase flow model is developed as an alternative to two-phase flow models. The model is based on the assumption that the liquid flow is induced by a density imbalance between the bulk zone and the bubble curtain zone. The density in the bulk is set to the water density while the density in the bubble curtain corresponds to the air–water mixture density and is assessed by numerical simulations, thanks to an iterative procedure. Only the knowledge of the injected air flow rate and the bubble liquid relative velocity is required. The single-phase flow model is applied to assess the liquid flow and the mixing in open quarries having a complex geometry. The liquid velocities and the flow structure in the open quarries simulated with the single-phase flow model are in good agreement with those predicted by numerical simulations based on a two-phase flow model.     

计算流体力学在氧化沟设计中的应用

利用计算流体力学对氧化沟的倒伞曝气机及推流器建立叶轮机械驱动模型,建立氧化沟污水-污泥多相流模型,利用计算流体力学软件Fluent对倒伞曝气机及推流器驱动的氧化沟进行流场模拟,能够较为准确的预判各种情况下的流场,有效地整合表面曝气机与潜水推流器的统一,能够很好地对氧化沟工程设计,设备配置及布置等起优化作用,并对污水厂运营起指导作用。     

计算流体力学在反渗透膜分离中的应用

综述了计算流体力学(CFD)在反渗透膜分离中的应用情况及研究成果.阐述了CFD技术由于精确、效率高、成本低、不受实验条件限制等优势而得到的广泛应用,为研究流体流动提供了新的手段.指出浓差极化和膜污染限制了反渗透技术的进一步广泛应用,而CFD技术则为研究该问题提供了一种强有力的工具.     

Modeling spatial distribution of floc size in turbulent processes using the quadrature method of moment and computational fluid dynamics

A study was performed that utilizes the quadrature method of moments (QMOM) to model the transient spatial evolution of the floc size in a heterogeneous turbulent stirred reactor. The QMOM approach was combined with a commercial computational fluid dynamics (CFD) code (PHOENICS), which was used to simulate the turbulent flow and transport of these aggregates in the reactor. The CFD/QMOM model was applied to a 28 l square reactor containing an axial flow impeller and 100 mg/l concentration of 1 μm nominal clay particles. Simulations were performed for different average characteristic velocity gradients (40,70,90, and 150 s^-1). The average floc size and growth rate were compared with experimental measurements performed in the bulk region and the impeller discharge region. The CFD/QMOM results confirmed the experimentally measured spatial heterogeneity in the floc size and growth rate. In addition, the model predicts spatial variations in the aggregation and breakup rates. Finally, the model also predicts that the transport of flocs into the high shear impeller discharge zone was responsible for the transient evolution of the average floc size curve displaying a maximum before decreasing to a steady-state floc size.     

Using computational fluid dynamics modeling to study the mixing of pseudoplastic fluids with a Scaba 6SRGT impeller

The 3D flow field generated by a Scaba 6SRGT impeller in the agitation of xanthan gum, a pseudoplastic fluid with yield stress, was simulated using the commercial CFD package. The flow was modeled as laminar and a multiple reference frame (MRF) approach was used to solve the discretized equations of motion. The velocity profiles predicted by the simulation agreed well with those measured using ultrasonic Doppler velocimetry, a non-invasive fluid flow measurement technique for opaque systems. Using computed velocity profiles across the impeller, the effect of fluid rheology on the impeller flow number was investigated. The validated CFD model provided useful information regarding the formation of cavern around the impeller in the mixing of yield stress fluids and the size of cavern predicted by the CFD model was in good agreement with that calculated using Elson's model.