Efficient CFD-based method for designing cross-flow nanofiltration small devices

The aim of the present work is to study the transfer processes in a cross-flow filtration cell, in order to determine the conditions for stable and efficient operation of a side-stream filtration module, integrated with a bioreactor. The current interest in membrane integrated bioreactors is connected with the pursuit of energy and cost efficiency in a wide area of industrial applications, including wastewater treatment, food industry, pharmaceutical industry, and fuel production. A numerical CFD model is employed, based on previous experience with experimental concentration of antioxidants, such as polyphenols and flavonoids from extracts of natural products by nanofiltration. The geometry under investigation is a 3D model of the experimental flat-sheet cell with tangential orientation of the feed inlet. The swirling turbulent flow in the feed channel is favourable for reducing the concentration polarization layer on the membrane surface and preventing fouling. The main factors, affecting the filtration process, are the shear stress distribution and the concentration profiles in the vicinity of the membrane surface. The CFD models of mass transfer in cross-flow nanofiltration are scarce and there are none for the reference experimental filtration cell. The present CFD simulation reveals the concentration distribution in the feed channel. It complements previous data for the flow pattern with new knowledge on the mass transfer there, directed to understanding and control of the concentration polarization phenomenon. The numerical study uses the tools of ANSYS Fluent R13, based on the finite volume method for solving the Reynolds-Averaged Navier-Stokes (RANS) equations. The obtained results are analysed in rapport with available experimental data.

Mass transfer characteristics of the liquid film flow in a rotating packed bed for CO2 capture: A micro-scale CFD analysis

A critical comparison between local heat and mass transfer coefficients of horizontal cylinders immersed in bubbling fluidised beds

It is difficult to determine the deposition levels of gaseous polycyclic aromatic hydrocarbons (PAHs), as even atmospheric PAHs mainly exist in the gas phase. Gaseous PAH fluxes across the air-water interface were measured in Merinos, Bursa, Turkey. Direct gaseous fluxes of PAHs were measured intermittently for the period August 2004 through May 2005 when there was no precipitation. A modified water surface sampler (WSS), including a filter holder and an XAD-2 resin column, was employed to collect the flux samples. Average gaseous PAH flux into the WSS, determined by analyzing the XAD-2 resin column, was 31,068 +/- 16,035 ng m(-2) d(-1). This flux value was about 90% of the total (gaseous + particulate) PAH flux, which may indicate that gas exchange is a major removal mechanism for atmospheric PAHs in our sampling site. Variations in gas exchange fluxes likely resulted from changes in both meteorological conditions and concentrations whose level depends on many parameters including fuel characteristics and strength, combustion types, and their efficiencies. Vapor phase PAH fluxes were divided by ambient air concentrations measured with a high volume sampler to calculate overall gas phase PAH mass transfer coefficients (MTCs) (K(G)). The calculated average PAH MTC was 0.38 +/- 0.17 cm s(-1). The average MTC is comparable with ones calculated using a similar configuration of a WSS.

Power consumption, local and average volumetric mass transfer coefficient in multiple-impeller stirred bioreactors for xanthan gum solutions

The growth rate of a crystal in a supersaturated solution is limited by both reaction kinetics and the local concentration of solute. If the local mass transfer coefficient is too low, concentration of solute at the crystal-solution interface will drop below saturation, leading to a defect in the growing crystal. Here, mass transfer coefficients are calculated for a rotating crystal growing in a supersaturated solution of potassium diphosphate (KDP) in water. Since mass transfer is difficult to measure directly, the heat transfer coefficient of a scale model crystal in water is measured using temperature-sensitive paint (TSP). To the authors' knowledge this is the first use of TSP to measure temperatures in water. The corresponding mass transfer coefficient is then calculated using the Chilton- Colburn analogy. Measurements were made for three crystal sizes at two running conditions each. Running conditions include periodic reversals of rotation direction. Heat transfer coefficients were found to vary significantly both across the crystal faces and over the course of a rotation cycle, but not from one face to another. Mean heat transfer coefficients increased with both crystal size and rotation rate. Computed mass transfer coefficients were broadly in line with expectations from the full-scale crystal growth experiments. Additional experiments show that continuous rotation of the crystal results in about a 30% lower heat transfer compared to rotation with periodic reversals. The continuous rotation case also shows a periodic variation in heat transfer coefficient of about 15%, with a period about 1/20th of the rotation rate.

The paper deals with the experimental determination of the spatial distribution of local mass transfer coefficients at a solid surface receiving unsubmerged liquid multijets issued from several categories of distributors. The experimental technique uses microelectrodes imbedded in the solid surface and allows the local mass transfer coefficients and the local shear stress to be determined at the same places. Very porous distributors lead to distributions much more uniform than those obtained with poorly porous distributors. A special interest is given to distributors made of metallic foam. The experimental mass transfer coefficients measured at the stagnation point are compared with correlations from the literature.

The Navier-Stokes equations for a laminar flow of a compressible multispecies gas have been used to model numerically the heat and mass transfer processes in high-temperature chemical reactions of methane in water vapor with activation of reactions on the microchannel walls and external heat supply. The temperature and concentration fields are obtained, as well as the distributions of heat fluxes, reacting species, and local coefficients of heat and mass transfer along the channel. It is shown that a high degree of chemical conversion leads to nonmonotonical changes in reaction rates and velocities of transverse heat flows, and species along the microchannel, considerably affecting the local coefficients of heat and mass transfer.

In view of the large number of urban inhabitants, ventilation and pollutant transport in urban areas are major research problems for several decades. This paper focuses on the studies of the correlation between the ventilation and pollutant removal over idealised two-dimensional urban street canyons using wind tunnel experiments in the Department of Mechanical Engineering, The University of Hong Kong. To construct street canyon models of different building-height-to-street-width (aspect) ratios, movable rectangular aluminium blocks were placed in the test section of the wind tunnel. Water evaporation method was used throughout the experiments, in which soaked filter papers were applied on the model, simulating a uniform mass concentration on the building facades and ground surface. The variations of the average mass transfer coefficients over a wider range of aspect ratios were examined. Scaling effect, which occurs even if the kinematic similarity is fully complied with, is observed so a more in-depth data analysis and another extensive comparison with our mathematical modelling results are required.

Mass-transfer coefficient in impingement flow from slotted nozzles

Study of an analytical model for hollow fiber reverse osmosis module systems

In this paper, the results of an experimental investigation of the distribution of local coefficients of mass transfer on a pipe wall for a flow of a two-phase gas-liquid mixture with a turn of 90° are presented. The mass transfer coefficients were measured by using an electrodiffusion method. It has been shown that the distribution of local mass transfer coefficients on the inner surface of a curvilinear channel is nonuniform both in length and along the perimeter and depends on the content of gas in the two-phase flow.

Quantitative evaluation of concentration polarization under different operating conditions for forward osmosis process

Mammalian cells have been widely used to produce therapeutic proteins in stirred bioreactors in suspension culture. Local hydrodynamics can have a great impact on cell proliferation and protein synthesis, but there are few reports on spatial heterogeneity of nutrients, gas bubbles, and mass transfer coefficients. We have employed computational fluid dynamics (CFD) coupled with population balance equations to study local hydrodynamics in a 20 L stirred bioreactor. The flow patterns, energy dissipation rates, gas volume fraction, gas bubble size distribution and local mass transfer coefficient have been displayed throughout the whole bioreactor. Their implications for mammalian cell culture have been discussed. This study provides an insight into rational design and optimum operation conditions in a stirred bioreactor for mammalian cell cultivation.

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.