Showing papers on "Mass transfer coefficient published in 2003"

Characterization of gas–liquid mass transfer phenomena in microtiter plates

Abstract: Gas-liquid mass transfer properties of shaken 96-well microtiter plates were characterized using a recently described method. The maximum oxygen transfer capacity (OTR(max)), the specific mass transfer area (a), and the mass transfer coefficient (k(L)) in a single well were determined at different shaking intensities (different shaking frequencies and shaking diameters at constant filling volume) and different filling volumes by means of sulfite oxidation as a chemical model system. The shape (round and square cross-sections) and the size (up to 2 mL maximum filling volume) of a microtiter plate well were also considered as influencing parameters. To get an indication of the hydrodynamic behavior of the liquid phase in a well, images were taken during shaking and the liquid height derived as a characteristic parameter. The investigations revealed that the OTR(max) is predominantly dependent on the specific mass transfer area (a) for the considered conditions in round-shaped wells. The mass transfer coefficient (k(L)) in round-shaped wells remains at a nearly constant value of about 0.2 m/h for all shaking intensities, thus within the range reported in the literature for surface-aerated bioreactors. The OTR(max) in round-shaped wells is strongly influenced by the interfacial tension, determined by the surface tension of the medium used and the surface properties of the well material. Up to a specific shaking intensity the liquid surface in the wells remains horizontal and no liquid movement can be observed. This critical shaking intensity must be exceeded to overcome the surface tension and, thus, to increase the liquid height and enlarge the specific mass transfer area. This behavior is solely specific to microtiter plates and has not yet been observed for larger shaking bioreactors such as shaking flasks. In square-shaped microtiter plate wells the corners act as baffles and cause a significant increase of OTR(max), a, and k(L). An OTR(max) of up to 0.15 mol/L/h can be reached in square-shaped wells.

Characterization of polymer coated glass as a passive air sampler for persistent organic pollutants.

Abstract: The use of thin-film polymer-coated glass surfaces or POGs as passive air samplers was investigated during an uptake experiment in an indoor environment with high levels of gas-phase polychlorinated biphenyls (PCBs). POGs consisted of a micron thick layer of ethylene vinyl acetate (EVA) coated onto glass cylinders. The uptake was initially linear with time and governed by the air-side mass transfer coefficient and surface area of the sampler. This was followed by a curvilinear region and finally a constant phase when equilibrium was established between air and EVA. The high surface area-to-volume ratio of the POGs allowed rapid equilibrium with gas-phase PCBs; equilibration times were on the order of hours for the low molecular weight congeners. The equilibrium concentration was dependent on the EVA-air partition coefficient, KEVA-A, which was shown to be very well correlated to the octanol−air partition coefficient, KOA. When POGs of varying thickness were equilibrated with air, the amount of PCB accumul...

Mass transfer enhancement of gas absorption in oil-in-water systems: a review

Abstract: A review is presented on the gas–liquid mass transfer enhancement due to the presence of a second dispersed liquid phase An attempt has been made to describe the mass transfer characteristics in a gas–liquid–liquid system The ability of an immiscible oil phase to influence the possible pathway for gas transfer from the gas phase to the aqueous phase and to affect the gas–liquid interface and the volumetric mass transfer coefficient kLa is considered Though the mass transfer in series looks the most logical explanation, there are many gaps and contradictions in the reported results of kLa, preventing any definite conclusion being reached An enhancement factor (E), which quantifies the effect of the oil addition on the gas–liquid mass transfer, is defined Experimental enhancement factors are reported and compared to the theoretical maximum attainable enhancement factor (Emax) Possible mechanisms (“bubble covering”, “shuttle effect” and “permeability effect”) involved in mass transfer enhancement are assessed in detail The commonly used “shuttle effect” mechanism, whose model proposes a direct usable expression of the enhancement factor, underestimates the reported experimental enhancement factors by about 20% However, to date, it is not possible to satisfactorily propose a unique theory explaining the influence of the presence of an immiscible oil on mass transfer enhancement Moreover, the development of sophisticated models has not yet reached satisfactory levels Recommendations have been made for future research

Dehydration kinetics of red pepper (Capsicum annuum L var Jaranda)

Abstract: Shredded and whole red pepper samples were dehydrated in a laboratory drier with a through-flow air velocity of 0.5 m s−1 at 50, 55, 60 and 70 °C. Shredded peppers dried faster than whole peppers. The drying behaviour of whole samples was characterised by a constant- and a falling-rate drying period, whilst that of shredded samples was characterised by a falling-rate drying period only. The mass transfer coefficient for whole samples during the constant-rate period was computed experimentally. The effect of temperature on the mass transfer coefficient was described by the Arrhenius model. The activation energy was 58 kJ mol−1. In the falling-rate period the mass transfer was described by a diffusional model, and the effective diffusion coefficient at each temperature was determined. Diffusion coefficients were estimated to lie between 4.38 × 10−11 and 10.99 × 10−11 m2 s−1 for whole peppers and between 37.23 × 10−11 and 99.61 × 10−11 m2 s−1 for shredded peppers. The effect of temperature on the effective diffusion coefficient was described by the Arrhenius equation, with an activation energy of 44 kJ mol−1 for whole peppers and 56 kJ mol−1 for shredded peppers. © 2003 Society of Chemical Industry

A novel method of simulating oxygen mass transfer in two‐phase partitioning bioreactors

Abstract: An empirical correlation, based on conven- tional forms, has been developed to represent the oxy- gen mass transfer coefficient as a function of operating conditions and organic fraction in two-phase, aqueous- organic dispersions. Such dispersions are characteristic of two-phase partitioning bioreactors, which have found increasing application for the biodegradation of toxic substrates. In this work, a critical distinction is made be- tween the oxygen mass transfer coefficient, kLa, and the oxygen mass transfer rate. With an increasing organic fraction, the mass transfer coefficient decreases, whereas the oxygen transfer rate is predicted to increase to an optimal value. Use of the correlation assumes that the two-phase dispersion behaves as a single homogeneous phase with physical properties equivalent to the weighted volume-averaged values of the phases. The addition of a second, immiscible liquid phase with a high solubility of oxygen to an aqueous medium increases the oxygen solubility of the system. It is the increase in oxygen solu- bility that provides the potential for oxygen mass trans- fer rate enhancement. For the case studied in which n- hexadecane is selected as the second liquid phase, addi- tions of up to 33% organic volume lead to significant increases in oxygen mass transfer rate, with an optimal increase of 58.5% predicted using a 27% organic phase volume. For this system, the predicted oxygen mass transfer enhancements due to organic-phase addition are found to be insensitive to the other operating vari- ables, suggesting that organic-phase addition is always a viable option for oxygen mass transfer rate enhance- ment. © 2003 Wiley Periodicals, Inc. Biotechnol Bioeng 83: 735- 742, 2003.

Determination of Mass Transfer Rates in PVDF and PTFE Hollow Fiber Membranes for CO2 Absorption

Abstract: The gas–liquid mass transfer accompanied by chemical reaction was studied in a membrane absorber for the separation of CO2 from mixture gases. The membranes used were made of polytetrafluoroethylene (PTFE) and polyvinylidenefluoride (PVDF) and the aqueous MEA solution was used as an absorbent possessing a high chemical reaction with carbon dioxide. The numerical model for the CO2 concentration profile in a fiber was developed, and the influence of its flux on the external mass transfer resistances, including gas and membrane, was simulated with this numerical model and compared with the experimental results. Experimentally, it is found that absorption rate per surface area was higher in PVDF membrane than that in PTFE membrane because of the non-wetted condition of membrane pore. The membrane pore wetted with an absorbent showed the low absorption performance by high membrane resistance. We could predict the liquid resistance and membrane–gas resistance (external resistance) from experimental and numerica...

Gas Dispersion and Bubble-to-Emulsion Phase Mass Exchange in a Gas-Solid Bubbling Fluidized Bed: A Computational and Experimental Study

Abstract: Knowledge of gas dispersion and mass exchange between the bubble and the emulsion phases is essential for a correct prediction of the performance of fluidized beds, particularly when catalytic reactions take place. Test cases of single rising bubble and a bubbling fluidized bed operated with a jet without a chemical reaction were studied in order to obtain fundamental insights in the prevailing mass transfer phenomena. Numerical simulations were carried out to predict the dispersion of tracer gas using a two-fluid model based on Kinetic Theory of Granular Flow (KTGF). The simulations of a single-bubble rising through an incipiently fluidized bed revealed that the assumptions often made in phenomenological models in the derivation of correlations for the mass transfer coefficient, mainly that the bubble diameter remains constant and that the tracer concentration is uniform in the bubble, are not valid. The predicted bubble-to-emulsion phase mass transfer coefficient showed good agreement with the estimated values from the literature correlations assuming additive convection-diffusion transport for different bubble sizes and different particle sizes, indicating the importance of the convective distribution even for relatively small particles. Experiments were carried out to measure the steady state concentration profiles of a tracer gas in a pseudo two-dimensional bubbling fluidized bed operated with a jet. The simulated steady state concentration profiles of the tracer gas agreed well the experimental measurements. The radial convection of the gas is significantly influenced by the bubble `throughflow? and therefore depends upon the particle and bubble size. The experimental comparison of theoretical results was extended to study the influence of the jet velocity and the particle diameter on the radial dispersion of the tracer gas in the bed.

VOF-Simulations of Mass Transfer From Single Bubbles and Bubble Chains Rising in Aqueous Solutions

Abstract: This paper presents numerical simulations of two-phase flow with high-density ratio, taking into account mass transport of a soluble component and its interfacial mass transfer. The mathematical model and the numerical method allow for different solubility of the species in the respective fluid phases, while volume changes due to mass transfer are neglected. The discontinuous changes in species concentrations at the interface are modeled by means of Henry’s law. Simulations are carried out with an extended version of the highly parallelized code FS3D, which employs an advanced Volume-Of-Fluid (VOF) method. Transfer and transport of oxygen is examined in case of single bubbles as well as bubble chains rising in aqueous solutions. Numerical simulations show good qualitative agreement with experimental data and render the observed mass transfer phenomena correctly.Copyright © 2003 by ASME

Shell Side Mass Transfer Characteristics in a Parallel Flow Hollow Fiber Membrane Module

Abstract: A cell model was introduced and an analytical solution was obtained to describe the shell side mass transfer problem in a parallel flow hollow fiber membrane module. To develop the analytical model, a uniform fiber arrangement was assumed, and the shell side flow was described by Happel's free surface model, and the boundary condition of a uniform fiber wall concentration was taken into account. The analytical solution showed that the shell side Sherwood number (Sh) was the function of Graetz number (Gz) and the module packing density (Φ). Experiments involving the absorption of CO2 into water with polypropylene hollow fiber membrane module were performed. It was found that the experimental results could be described bycitationrefSh=(0.163+0.27φ)Gz0.6. The experimental mass transfer coefficient was higher than that predicted by the model when the packing density of the module was 20%, whereas it was lower than the predicted value when increasing the packing density exceeded 30%. This experimental correlat...

The effects of geometry and operational conditions on gas holdup, liquid circulation and mass transfer in an airlift reactor

Abstract: In airlift reactors transport phenomena are achieved by pneumatic agitation and circulation occurs in a defined cyclic pattern through a loop. In the present work, the effect of geometrical relations on gas holdup and liquid velocity, and consequently on the gas-liquid mass transfer coefficient, was studied in a 6-liter airlift bioreactor with AD/AR = 0.63; AD, downcomer cross-sectional area, and AR, riser cross-sectional area. Measurements of the volumetric oxygen transfer coefficient (kLa) were taken in a water-air system using a modified sulfite oxidation method. Different conditions were examined by varying parameters such as superficial air velocity in the riser (UGR), bottom clearance (d1) and top clearance (d2). It was observed from the experimental results that d1 and d2 have a remarkable effect on kLa values. The effect is due to their influence on gas holdup and liquid velocity, consequently affecting kLa. Superficial air velocity in the riser (UGR) ranged from 0.0126 to 0.0440 m.s-1 and kLa varied between 40 to 250 h-1, whereas gas holdup (e) reached values up to 0.2. The volumetric oxygen transfer coefficient (kLa), gas holdup in the riser (eR) and downcomer (eD) and superficial liquid velocity in the riser (ULR) for all the geometrical relations were successfully correlated with dimensionless numbers, namely, the Sherwood number (Sh) and the Froude number (Fr) as well as with geometrical relations such as the bottom space ratio (B = d1/DD) and top space ratio (T = (d2 + DD)/DD).

Transport Phenomena for Chemical Reactor Design

Abstract: Preface.PART I: ELEMENTARY TOPICS IN CHEMICAL REACTOR DESIGN.Multiple Chemical Reactions in Plug Flow Tubular Reactors and Continuous Stirred Tank Reactors.Start Up Behaviour of a Series Configuration of Continuous Stirred Tank Reactors.Adiabatic Plug-Flow Tubular Reactor That Produces Methanol Reversibly in the Gas Phase from Carbon Monoxide and Hydrogen.Coupled Heat and Mass Transfer in Nonisothermal Liquid-Phase Tubular Reactors with Strongly Exothermic Chemical Reactions.Multiple Stationary States in Continuous Stirred Tank Reactors.Coupled Heat and Mass Transfer with Chemical Reaction in Batch Reactors.Total Pressure Method of Reaction-Rate Data Analysis.PART II: TRANSPORT PHENOMENA: FUNDAMENTALS AND APPLICATIONS.Applications of the Equations of Change in Fluid Dynamics.Derivation of the Mass Transfer Equation.Dimensional Analysis of the Mass Transfer Equation.Laminar Boundary Layer Mass Transfer around Solid Spheres, Gas Bubbles, and Other Submerged Objects.Dimensional Analysis of the Equations of Change for Fluid Dynamic s Within the Mass Transfer Boundary Layer.Diffusion and Chemical Reaction Across Spherical Gas-Liquid Interfaces.PART III: KINETICS AND ELEMENTARY SURFACE SCIENCE.Kinetic Mechanisms and Rate Expressions for Heterogeneous Surface-Catalyzed Chemical Reactions.PART IV: MASS TRANSFER AND CHEMICAL REACTION IN ISOTHERMAL CATALYTIC PELLETS.Diffusion and Heterogeneous Chemical Reaction in Isothermal Catalytic Pellets.Complete Analytical Solutions for Diffusion and Zeroth-Order Chemical Reactions in Isothermal Catalytic Pellets.Complete Analytical Solutions for Diffusion and First-Order Chemical Reactions in Isothermal Catalytic Pellets.Numerical Solutions for Diffusion and nth-Order Chemical Reactions in Isothermal Catalytic Pellets.Numerical Solutions for Diffusion and Hougen-Watson Chemical Kinetics in Isothermal Catalytic Pellets.Internal Mass Transfer Limitations in Isothermal Catalytic Pellets.Diffusion Coefficients and Damkohler Numbers Within the Internal Pores of Catalytic Pellets.PART V: ISOTHERMAL CHEMICAL REACTOR DESIGN.Isothermal Design of Heterogeneous Packed Catalytic Reactors.Heterogeneous Catalytic Reactors with Metal Catalyst Coated on the Inner Walls of the Flow Channels.Designing a Multicomponent Isothermal Gas-Liquid CSTR for the Chlorination of Benzene to Produce Monochlorobenzene.PART VI: THERMODYNAMICS AND NONISOTHERMAL REACTOR DESIGN.Classical Irreversible Therodynamics of Multicomponent Mixtures.Molecular Flux of Thermal Energy in Binary and Multicomponent Mixtures Via the Formalism of Nonequilibrium Thermodynamics.Thermal Energy Balance in Multicomponent Mixtures and Nonisothermal Effectiveness Factors Via Coupled Heat and Mass Transfer in Porous Catalysts.Statistical Thermodynamics of Ideal Gases.Thermodynamic Stability Criteria for Single-Phase Homogeneous Mixtures.Coupled Heat and Mass Transfer in Packed Catalytic Tubular Reactors That Account for External Transport Limitations.References.Index.

Prediction of in-tube condensation heat transfer characteristics of binary refrigerant mixtures

Abstract: This study presents a prediction model for the condensation heat transfer characteristics of binary zeotropic refrigerant mixtures inside horizontal smooth tubes. In this model, both the vapor-side and liquid-side mass transfers are considered, and the high flux mass transfer correction factor is used to evaluate mass transfer coefficients. The model was applied to the binary zeotropic refrigerant mixture R134a/R123, which has a large temperature glide. Calculation results showed that the heat transfer degradation of R134a/R123 due to gradients in the mass fraction and temperature is considerable, and depends on the mass fraction of the more volatile component and the vapor mass quality of the refrigerant mixture. By comparison with experimental data, incorporating the present finite mass transfer model for the liquid film side into the calculation algorithm was shown to reasonably well predict the condensation heat transfer coefficients of binary refrigerant mixtures with the mean deviation of about 10.3%. In the present calculations, however, it was also found that the high flux mass transfer correction factor had only a slight effect on the condensation heat transfer.