Showing papers on "Mass transfer coefficient published in 1998"

Mass-Transfer Properties of Microbubbles. 1. Experimental Studies

Abstract: Synthesis-gas fermentations have typically been gas-to-liquid mass-transfer-limited due to low solubilities of the gaseous substrates. A potential method to enhance mass-transfer rates is to sparge with microbubble dispersions. Mass-transfer coefficients for microbubble dispersions were measured in a bubble column. Oxygen microbubbles were formed in a dilute Tween 20 solution using a spinning disk apparatus. Axial dispersion coefficients measured for the bubble column ranged from 1.5 to 7.2 cm2/s and were essentially independent of flow rate. A laser-diffraction technique was used to determine the interfacial area per unit gas volume, a. The mass-transfer coefficient, KL, was determined by fitting a plug-flow model to the experimental, steady-state, liquid-phase oxygen-concentration profile. The KL values ranged from 2.9 x 10(-5) to 2.2 x 10(-4) m/s. Volumetric mass-transfer coefficients, KLa, for microbubbles with an average initial diameter of 60 microns ranged from 200 to 1800 h-1. Enhancement of mass transfer using microbubbles was demonstrated for a synthesis-gas fermentation. Butyribacterium methylotrophicum was grown in a continuous, stirred-tank reactor using a tangential filter for total cell recycle. The fermentation KLa values were 14 h-1 for conventional gas sparging through a stainless steel frit and 91 h-1 for microbubble sparging. The Power number of the microbubble generator was determined to be 0.036. Using this value, an incremental power-to-volume ratio to produce microbubbles for a B. methylotrophicum fermentation was estimated to be 0.01 kW/m3 of fermentation capacity.

The effect of bubble-mediated gas transfer on purposeful dual-gaseous tracer experiments

Abstract: For air-water gas exchange across unbroken surfaces, the only gas-dependent parameter affecting the transfer velocity is the molecular diffusivity of the transferring species. In contrast, bubble-mediated transfer processes can cause the transfer velocity to depend on both molecular diffusivity and aqueous-phase solubility. This can complicate the analysis of data from dual-gaseous tracer gas transfer experiments. Bubble effects also complicate the estimation of transfer velocities for other gases from the transfer velocity calculated using the dual-tracer data. Herein a method for incorporating the effects of bubble-mediated gas transfer processes on the transfer velocity is presented. This new procedure is used to analyze the data from two recent dual-tracer gas transfer experiments. Transfer velocities that include the effect of bubbles are calculated using the data from two previous oceanic dual-gaseous tracer experiments. Comparing these transfer velocities with transfer velocities calculated by neglecting the effect of bubbles shows that bubble-mediated transfer increased the transfer velocity of helium 3 by 5% at a wind speed of 10.6 m s−1. However, when using the transfer velocities for helium 3 to calculate transfer velocities for carbon dioxide under the same conditions, including the effect of bubbles decreases the transfer velocity of carbon dioxide by 18%. This shows that bubble-mediated transfer does not have a large effect on the analysis of dual-tracer data, but it is important in relating transfer velocities determined using helium 3 and sulfur hexafluoride to transfer velocities of more soluble gases at wind speeds above 10 m s−1.

Microelectrode measurements of local mass transport rates in heterogeneous biofilms.

Abstract: Microelectrodes were used to measure oxygen profiles and local mass transfer coefficient profiles in biofilm clusters and interstitial voids. Both profiles were measured at the same location in the biofilm. From the oxygen profile, the effective diffusive boundary layer thickness (DBL) was determined. The local mass transfer coefficient profiles provided information about the nature of mass transport near and within the biofilm. All profiles were measured at three different average flow velocities, 0.62, 1.53, and 2.60 cm sec-1, to determine the influence of flow velocity on mass transport. Convective mass transport was active near the biofilm/liquid interface and in the upper layers of the biofilm, independent of biofilm thickness and flow velocity. The DBL varied strongly between locations for the same flow velocities. Oxygen and local mass transfer coefficient profiles collected through a 70 micrometer thick cluster revealed that a cluster of that thickness did not present any significant mass transport resistance. In a 350 micrometer thick biofilm cluster, however, the local mass transfer coefficient decreased gradually to very low values near the substratum. This was hypothetically attributed to the decreasing effective diffusivity in deeper layers of biofilms. Interstitial voids between clusters did not seem to influence the local mass transfer coefficients significantly for flow velocities of 1.53 and 2.60 cm sec-1. At a flow velocity of 0.62 cm sec-1, interstitial voids visibly decreased the local mass transfer coefficient near the bottom.

Mass Transfer Coefficients and Correlation for CO2 Absorption into 2-Amino-2-methyl-1-propanol (AMP) Using Structured Packing

Abstract: The volumetric overall mass transfer coefficient (KGav) for CO2 absorption into aqueous solutions of 2-amino-2-methyl-1-propanol (AMP) was investigated with an absorption column packed with laboratory structured packings. The KGav value was evaluated over ranges of main operating variables; that is, up to 10 kPa partial pressure of CO2, 46.2−96.8 kmol/(m2 h) gas molar flux, 6.1−14.6 m3/(m2 h) liquid loading, and 1.1−3.0 kmol/m3 liquid concentration. To allow the mass transfer data to be readily utilized, an empirical KGav correlation for this system was developed. The values of mass transfer coefficient for the CO2−AMP system using a random and the tested structured packings are also compared. For a given system and operating conditions, the structured packing provides, in general, more than eightfold higher overall KGav values compared with those of commercial random packings.

Sonolytic Decomposition of Ozone in Aqueous Solution: Mass Transfer Effects

Abstract: The sonolytic degradation of ozone (O_3) was investigated in both closed and open continuous-flow systems to examine effects of mass transfer on chemical reactivity in the presence of ultrasound Degradation of O_3 followed apparent first-order kinetics at frequencies of both 20 and 500 kHz in all the systems Degassing of O_3 was observed at 20 kHz due to the effects of rectified diffusion and larger resonant radii of the cavitation bubbles than at 500 kHz Increased mass transfer of O_3 diffusing into solution due to ultrasound as measured by the mass transfer coefficient, k_La_2, was observed at both frequencies At 20 kHz, an increase in mass transfer rates in the presence of ultrasound may be partially attributed to turbulence induced by acoustic streaming However, the main process of increased gas−liquid mass transfer in the presence of ultrasonic waves appears to be due to the sonolytic degradation of O_3 creating a larger driving force for gaseous O3 to dissolve into solution From first-order cyclohexene degradation kinetics obtained by sonolysis, ozonolysis, sonolytic ozonolysis, and comparing the large diameter of an O_3 diffusing gas bubble to the size of an active cavitation bubble, it appears that diffusing gas bubbles containing O_3 are not directly influenced by ultrasonic fields

Modeling Mass Transfer Processes in Soil Columns with Pore-Scale Heterogeneity

Abstract: Removal of dissolved organic compounds from natural porous media is rate limited by multiple, simultaneous mass transfer processes, including slow diffusion from immobile zones of varying sizes and shapes, and rate-limited sorption. We examined the variability in mass transfer rates and quantified the effects of multiple, simultaneous mass transfer processes on unsaturated column experiments using a diffusion model with a statistical distribution of diffusion rate coefficients. We examined the validity of conventional first-order mass transfer and diffusion models to represent mass transfer in subsurface systems and compared this with a diffusion model with a lognormal distribution of diffusion rate coefficients. The main conclusions of our work are: (i) even in relatively homogeneous porous media, extreme variability (exceeding four orders of magnitude) in diffusion rate coefficients (D a /a 2 ) must he invoked to represent mass transfer in the experiments we examined; (ii) models using a lognormal distribution of diffusion rate coefficients, while employing only one more mass transfer parameter than conventional models, generally represent mass transfer much better; (iii) single-rate-coefficient models (either first-order or diffusion) very poorly represent mass transfer in all experiments examined, although some of this failure is attributed to our use of a linear isotherm; (iv) models with two diffusion rate coefficients, although containing twice as many estimated parameters, also offer poor representations of mass transfer.

Dissolved oxygen transfer at the sediment-water interface in a turbulent flow

Abstract: Laboratory experiments have been conducted in an effort to elucidate the details of the dissolved oxygen transfer mechanism at the sediment-water interface. Near-bed liquid turbulence and dissolved oxygen concentrations were measured for a range of Reynolds numbers from 2700 to 10,600 over a smooth bed. The results show that the shear stress velocity controls the dissolved oxygen diffusive layer thickness and therefore the local mass transfer coefficient at the sediment-water interface. From a dynamic point of view the results suggest the possibility that streamwise vortices control the interfacial dissolved oxygen transfer process. The research results will provide an opportunity to validate the existing conceptual models as well as enhance the development of new models that can improve our ability to predict dissolved oxygen and transport related parameters in lakes, reservoirs, coastal waters, and rivers.

Application of Surface Diffusion Model to the Adsorption of Dyes on Bagasse Pith

Abstract: A homogeneous solid phase diffusion model (HSDM) has been developed using a computer to predict the performance of a batch adsorber. The computer program utilises a semi-analytical solution for a two resistance model based on external mass transfer and homogeneous solid phase diffusion. The model has been successfully applied to four adsorption systems, namely, the adsorption of AB25, AR114, BB69 and BR22 onto pith. The method produces excellent correlations between experimental and theoretical concentration decay curves in batch adsorbers. The model developed presents a solution using a single solid diffusion coefficient and a single external mass transfer coefficient which are sufficient to characterise the system within a range of initial dye concentration, 25–300 mg · dm3 and solid/liquid ratios (w/v) 0.25–2.

Microporous Hollow Fibres for Carbon Dioxide Absorption: Mass Transfer Model Fitting and the Supplying of Carbon Dioxide to Microalgal Cultures

Abstract: A method of supplying to photosynthetic algal cultures was devel- CO 2 oped based on mass transfer measurements of through microporous hydro- CO 2 phobic hollow -bres for various gas and liquid Now rates. A mathematical model was derived to describe the mass transfer. The designed hollow -bre module led to overall mass transfer coefficient values ranging from 1E26 ) 10~3 to 2E64 ) 10~3 cm s~1. Higher efficiencies of the transmission were obtained CO 2 at high liquid Now rates and low gas Now rates. The use of microporous hydro- phobic hollow -bres enabled an enhancement of the carbon dioxide transfer per area of membrane surface by a factor of 10, in comparison to operation with silicone tubing. The hollow -bre module was operated in an external bypass to a 1d m 3microalgae culture vessel. In this system the algal growth pattern was similar to that obtained with a control culture where was bubbled. CO 2 However, the dissolved oxygen concentration was always lower in the vessel in which was supplied by the module. 1998 SCI. CO 2 ( J. Chem. T echnol. Biotechnol. 71 ,6 1 E70 (1998)

Batch Uptake of Lysozyme: Effect of Solution Viscosity and Mass Transfer on Adsorption

Abstract: In this study, solid-phase adsorption by macroporous and hyper-diffusive resins was investigated in a batch uptake adsorption system to quantify solid-phase diffusion rates as a function of bulk phase viscosity. The performance of chromatographic resins used for adsorption of proteins is dependent on several factors including solid and liquid-phase diffusivity, boundary layer mass transfer, and intraparticle mass transfer effects. Understanding these effects is critical to process development and optimization of both packed and fluidized bed adsorption systems. The macroporous resin used here was Streamline SP, and the hyper-diffusive resin was S-HyperD LS. Both have been frequently used in fluidized bed adsorption of proteins; however, factors that affect uptake rates of these media are not well quantified. Adsorption isotherms were well represented by an empirical fit of a Langmuir isotherm. Solid-phase diffusion coefficients obtained from simulations were in agreement with other models for macroporous and hyper-diffusive particles. S-HyperD LS in the buffer system had the highest uptake rate, but increased bulk phase viscosity decreased the rate by approximately 50%. Increases in bulk phase viscosity increased film mass transfer effects, and uptake was observed to be a strong function of the film mass transfer coefficient. Uptake by Streamline SP particles was slower than S-HyperD in buffer, due to a greater degree of intraparticle mass transfer resistance. The effect of increased film mass transfer resistance coupled with intraparticle mass transfer resistances at an increased bulk phase viscosity resulted in a decrease of 80% in the uptake rate by Streamline SP relative to S-HyperD.

Food and packaging interactions: Determination of the kinetic parameters of olive oil diffusion in polypropylene using concentration profiles

Abstract: The penetration of olive oil into polypropylene was studied in order to allow a complete modellization of food and packaging interactions. Oil concentration profiles through polypropylene food trays were determined by FTIR‐microscopy measurements along the thickness at various times. Calculations of the relevant parameters characterizing Fickian diffusion, namely constant diffusivity, coefficient of convective mass transport on the surface and concentration at equilibrium were carried out. This way of working has proven to be considerably shorter and more accurate than the method consisting of recording the global absorbance of the substance absorbed, especially when the amount of diffusing fat is low. Major conclusions are: that absorption of olive oil is strongly influenced by convection; the diffusion coefficient of olive oil in polypropylene is constant. Possible consequences to simplify global migration testing are discussed.

Film cooling effectiveness and mass/heat transfer coefficient downstream of one row of discrete holes

Abstract: A special naphthalene sublimation technique is used to study the film cooling performance downstream of one row of holes of 35 deg inclination angle with 3d hole spacing and relatively small hole length to diameter ratio (L/d = 6.3). Both film cooling effectiveness and mass/heat transfer coefficient are determined for blowing rates from 0.5 to 2.0 with density ratio of 1.0. The mass transfer coefficient is measured using pure air film injection, while the film cooling effectiveness is derived from comparison of mass transfer coefficients obtained following injection of naphthalene-vapor-saturated air with those from pure air injection. This technique enables one to obtain detailed local information on film cooling performance. The laterally averaged and local film cooling effectiveness agree with previous experiments. The difference between mass/heat transfer coefficients and previous heat transfer results indicates that conduction error may play an important role in the earlier heat transfer measurements.

Characterizing the performance of alternative evaporative cooling pad media in thermal environmental control applications

Abstract: This paper outlines the test procedure and describes how the alternative pad performance is affected by pad thickness and pad materials in the thermal environmental control applications. Many experimental pads were tested including one made of nonwoven fabric perforated pad and one made of coir fiber material. A wind tunnel experiment was performed to obtain equations for heat and mass transfer coefficients for the evaporative process through various thickness of alternative pad media. Heat and mass transfer coefficients are nondimensionalized and curve fitted to yield the working equations: (1) coir fiber pad: hH / hM =, 0.32paCPaLe,2/3 (Les /Le,)1/4, and (2) nonwoven fabric pad: hH / hM , = 1.899paCPaLe,2/3 (Les / Le,)1/4; where hH , is heat transfer coefficient, hM , is mass transfer coefficient, pa , is air density, Cpa , is specific heat of air, Le, is Lewis number, and Les ,, is Lewis number at water temperature. A determination for cooling efficiency in a wind tunnel system is also develop...

A membrane bioreactor for biotransformations of hydrophobic molecules

Abstract: The Membrane Bioreactor for Biotransformations (MBB) is based on the aqueous/organic two-phase system, and uses a tubular silicone rubber membrane to separate the two liquid phases. This avoids the key problem associated with direct contact two-phase processes, specifically, product emulsification. The baker's yeast mediated reduction of geraniol to citronellol was used as a model biotransformation to demonstrate MBB operation. Values for the overall mass transfer coefficient were determined for geraniol, (2.0 x 10(-5) ms-1), and for citronellol, (2.1 x 10(-5) ms-1) diffusion across the silicone rubber membrane. Using these values, and the specific activity of the biocatalyst (5 nmols-1g biomass-1), a suitable membrane surface area: biomass ratio was determined as 2.4 x 10(-3) m2g biomass-1. The bioreactor was operated at this surface area: biomass ratio and achieved a product accumulation rate 90-95% that of a conventional direct contact two-phase system. The slight reduction in product accumulation rate was shown not to be due to mass transfer limitations with respect to reactant delivery or product extraction. Copyright 1998 John Wiley & Sons, Inc.

Determination of pollutant diffusion coefficients in naturally formed biofilms using a single tube extractive membrane bioreactor.

Abstract: A novel technique has been used to determine the effective diffusion coefficients for 1,1,2-trichloroethane (TCE), a nonreacting tracer, in biofilms growing on the external surface of a silicone rubber membrane tube during degradation of 1,2-dichloroethane (DCE) by Xanthobacter autotrophicus GJ10 and monochlorobenzene (MCB) by Pseudomonas JS150. Experiments were carried out in a single tube extractive membrane bioreactor (STEMB), whose configuration makes it possible to measure the transmembrane flux of substrates. A video imaging technique (VIT) was employed for in situ biofilm thickness measurement and recording. Diffusion coefficients of TCE in the biofilms and TCE mass transfer coefficients in the liquid films adjacent to the biofilms were determined simultaneously using a resistances-in-series diffusion model. It was found that the flux and overall mass transfer coefficient of TCE decrease with increasing biofilm thickness, showing the importance of biofilm diffusion on the mass transfer process. Similar fluxes were observed for the nonreacting tracer (TCE) and the reactive substrates (MCB or DCE), suggesting that membrane-attached biofilm systems can be rate controlled primarily by substrate diffusion. The TCE diffusion coefficient in the JS150 biofilm appeared to be dependent on biofilm thickness, decreasing markedly for biofilm thicknesses of >1 mm. The values of the TCE diffusion coefficients in the JS150 biofilms 1-mm thick. The TCE diffusion coefficients in the GJ10 biofilms were apparently constant at about the water value. The change in the diffusion coefficient for the JS150 biofilms is attributed to the influence of eddy diffusion and convective flow on transport in the thinner (<1-mm thick) biofilms.