Showing papers on "Mass transfer published in 1998"

Principles of gas-solid flows

Abstract: Part I Basic Relationships: 1 Size and properties of particles 2 Collision mechanics of solids 3 Momentum transfer and charge transfer 4 Basic heat and mass transfer 5 Basic equations 6 Intrinsic phenomena in a gas-solid flow Part II System Characteristics: 7 Gas-solid separation 8 Hopper and stand pipe flows 9 Dense-phase fluidized beds 10 Circulating fluidized beds 11 Pneumatic conveying of solids 12 Heat and mass transfer phenomena in fluidization systems Appendix Summary of scalar, vector, and tensor notations

Modeling of Simultaneous Heat and Water Transport in the Baking Process

Abstract: In the recent past, several experimental and mathematical studies have been conducted to understand the basic mechanism of heat and water transport involved in the baking process. The experimental studies have helped in developing phenomenological hypotheses for the baking of bread and biscuit. The experimental studies on baking of bread showed that the major transport mechanism involved was evaporation-condensation of water and not heat conduction. Studies on the baking of biscuit assumed a uniform temperature and water content within the product during the baking process. The hypotheses have been translated into mathematical models which simulate experimental results with good accuracy. Much of the research has focused on the heat and water transport in the product. Limited results have been reported for heat and water transfer within the baking chamber and kinetics of physico-chemical changes. This article reviews the studies published on heat and moisture transport in the bakery products during the baking process.

Time domain reflectometry : a seminal technique for measuring mass and energy in soil

Abstract: Soil water exerts a strong influence on the transfer and storage of solutes, heat, air, and even water itself, within the soil profile. Soil water also dominates the mass and energy balance of the soil–atmosphere interface. Over the last decade or so, the development and continuing refinement of the time-domain reflectometry (TDR) technique for in situ, nondestructive measurement of water, ionic solutes and air has revolutionized the study and management of the transfer and storage of mass and energy within the soil profile. TDR-measured water content has been applied successfully to water balance studies ranging from the km scales of small watersheds to the mm scale of the root–soil interface. TDR-measured ionic solute status, which applies to the same sample volume as the water content measurement, has been used successfully on soil column, field plot and whole field scales for in situ determination of solute transport parameters, such as pore water velocity and dispersivity. TDR-measurement of air-filled porosity in space and time has given new insights into the mechanisms controlling aeration and gaseous exchange in the crop root zone. The combined water content – solute mass measurement capability of TDR has made this technique a very powerful tool for characterizing solute leaching characteristics, as well as for evaluating solute transport theories and solute transport models. The portability of TDR instrumentation coupled with the simplicity and flexibility of TDR soil probes has allowed the separation of water and solute content measurement error from soil variability, resulting in the capability for determining the mechanisms behind the spatial and temporal variability in field-based soil water content distributions and solute leaching patterns. The usefulness and power of the TDR technique for characterizing mass and energy in soil is increasing rapidly through continuing improvements in operating range, probe design, multiplexing and automated data collection.

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.

Theory and Experiment on the Measurement of Kinetic Rate Constants for Surfactant Exchange at an Air/Water Interface.

Abstract: The paper focuses on the measurement of the rate constants for the kinetic steps of adsorption and desorption of surfactant between an air/water surface and the aqueous bulk sublayer adjacent to the surface. Kinetic constants are determined in nonequilibrium experiments in which either a clean surface is contacted with a bulk solution and surfactant diffuses toward and adsorbs onto the interface, or the area of an established monolayer in equilibrium with an underlying solution is changed, and surfactant exchanges between the surface and bulk. The dynamic tension change due to the surfactant exchange is measured, and compared to predictions of kinetic-diffusive transport models in order to infer the kinetic coefficients as well the diffusion coefficients. Model comparisons for highly surface active surfactants have resolved only the diffusion coefficient as the transport was found to be diffusion controlled; kinetic constants have only been established for less active materials such as alcohols or bolaform surfactants. In this study, we demonstrate that kinetics can be differentiated from diffusion in clean interface adsorption and re-equilibration if high bulk concentrations of the surfactant are used, or in re-equilibration, if the surface is compressed sufficiently. We first establish theoretically that mass transfer shifts from diffusion-limited to mixed as the bulk concentration increases in clean interface adsorption, or the surface compression is increased in re-equilibration. We then experimentally verify this idea by using the polyethoxylated surfactant C 12 E 6 (C 12 H 25 (OCH 2 CH 2 ) 6 -OH) and by measuring dynamic surface tensions in clean interface adsorption and re-equilibration, respectively by the shape analysis of pendant bubbles. We find values of 6 × 10 −10 m 2 /s for the diffusion coefficient, and 1.4 × 10 −5 m/sec and 1.4 × 10 −4 s −1 for the adsorption and desorption rate constants, respectively, in a Frumkin kinetic formulation. While the adsorption constant is comparable to previously measured values for the less surface active surfactants, the desorption rate constant is a few orders of magnitude smaller. This indicates that the more surface active materials may have much smaller desorption rate constants than had been previously anticipated based on the studies of the less surface active materials.

Interpretation of two‐region model parameters

Abstract: Apart from advection and diffusion/dispersion, other physical and chemical processes can affect the movement of solute through a porous medium. The classical advection-dispersion equation does not usually model adequately the breakthrough curves resulting from such effects. The two-region model (TRM) is an attempt to model succinctly and easily the effects of physical and/or chemical nonequilibrium. Physical nonequilibrium is the focus of this paper. The additional parameters appearing in the TRM are the ratio of mobile to total pore fluid, β, and the apparent transfer rate of the solute between the mobile and the immobile regions, α. Meaning is ascribed to these parameters by identifying the various ways in which physical nonequilibrium can arise. An examination of published data shows that the dominant trend is a linear variation of the transfer rate α with mobile fluid velocity Vm . In order to develop an approach incorporating all porous media types, timescales of solute transport are identified and compared to the mass transfer timescale (1/α). It was found that the local advection timescale best characterizes the mass transfer timescale. Two trends were observed for the mobile water fraction β. For aggregated/saturated porous media, β was found to be constant or decreased with increasing pore water velocity, while for partially saturated soils, β was constant or increased with increasing moisture content.

Droplet–turbulence interactions in low-Mach-number homogeneous shear two-phase flows

Abstract: Several important issues pertaining to dispersion and polydispersity of droplets in turbulent flows are investigated via direct numerical simulation (DNS). The carrier phase is considered in the Eulerian context, the dispersed phase is tracked in the Lagrangian frame and the interactions between the phases are taken into account in a realistic two-way (coupled) formulation. The resulting scheme is applied for extensive DNS of low-Mach-number, homogeneous shear turbulent flows laden with droplets. Several cases with one- and two-way couplings are considered for both non-evaporating and evaporating droplets. The effects of the mass loading ratio, the droplet time constant, and thermodynamic parameters, such as the droplet specific heat, the droplet latent heat of evaporation, and the boiling temperature, on the turbulence and the droplets are investigated. The effects of the initial droplet temperature and the initial vapour mass fraction in the carrier phase are also studied. The gravity effects are not considered as the numerical methodology is only applicable in the absence of gravity. The evolution of the turbulence kinetic energy and the mean internal energy of both phases is studied by analysing various terms in their transport equations. The results for the non-evaporating droplets show that the presence of the droplets decreases the turbulence kinetic energy of the carrier phase while increasing the level of anisotropy of the flow. The droplet streamwise velocity variance is larger than that of the fluid, and the ratio of the two increases with the increase of the droplet time constant. Evaporation increases both the turbulence kinetic energy and the mean internal energy of the carrier phase by mass transfer. In general, evaporation is controlled by the vapour mass fraction gradient around the droplet when the initial temperature difference between the phases is negligible. In cases with small initial droplet temperature, on the other hand, the convective heat transfer is more important in the evaporation process. At long times, the evaporation rate approaches asymptotic values depending on the values of various parameters. It is shown that the evaporation rate is larger for droplets residing in high-strain-rate regions of the flow, mainly due to larger droplet Reynolds numbers in these regions. For both the evaporating and the non-evaporating droplets, the root mean square (r.m.s.) of the temperature fluctuations of both phases becomes independent of the initial droplet temperature at long times. Some issues relevant to modelling of turbulent flows laden with droplets are also discussed.

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.

Theoretical and experimental studies on air gap membrane distillation

Abstract: Air gap membrane distillation (AGMD) is an innovative membrane separation technique for pure water extraction from aqueous solutions. In this study, both theoretical and experimental investigations are carried out on AGMD of different aqueous solutions, namely, tap water, salted water, dyed solutions, acid solutions, and alkali solutions. A simple mechanistic model of heat and mass transfer associated with AGMD is developed. Simple relationships of permeate flux, total heating or cooling load and thermal efficiency of AGMD with respect to the membrane distillation temperature difference are obtained. Effects of solution concentration and the width of the air gap in AGMD are analyzed. In the experimental study, the experiments were conducted using 1m PTFE membrane with a membrane distillation temperature difference up to 55∘C. The AGMD system yields a permeate flux of pure water of up to 28kg/m2h. Direct comparison of the experimental results with the proposed modeling predictions shows a fairly good match.