Showing papers on "Mass transfer published in 2003"

Transport processes and separation process principles (includes unit operations) fourth edition

Abstract: The comprehensive, unified, up-to-date guide to transport and separation processes.Today, chemical engineering professionals need a thorough understanding of momentum, heat, and mass transfer processes, as well as separation processes. Transport Processes and Separation Process Principles, Fourth Edition offers a unified and up-to-date treatment of all these topics. Thoroughly updated to reflect the field's latest methods and applications, it covers both fundamental principles and practical applications.Part 1 covers the essential principles underlying transport processes: momentum transfer; steady-state and unsteady-state heat transfer; and mass transfer, including both unsteady-state and convective mass transfer. Part 2 covers key separation processes, including evaporation, drying, humidification, absorption, distillation, adsorption, ion exchange, extraction, leaching, crystallization, dialysis, gas membrane separation, reverse osmosis, filtration, ultrafiltration, microfiltration, settling, centrifugal separation, and more. This edition's extensive updates and enhancements include: A more thorough coverage of momentum, heat, and mass transport processes Detailed new coverage of separation process applications Greatly expanded coverage of momentum transfer, including fluidized beds and non-Newtonian fluids More detailed discussions of mass transfer, absorption, distillation, liquid-liquid extraction, and crystallization Extensive new coverage of membrane separation processes and gas-membrane theoryTransport Processes and Separation Process Principles, Fourth Edition also features more than 240 example problems and over 550 homework problems reflecting the field's current methods and applications.

When good statistical models of aquifer heterogeneity go bad: A comparison of flow, dispersion, and mass transfer in connected and multivariate Gaussian hydraulic conductivity fields

Abstract: [1] We describe the upscaled groundwater flow and solute transport characteristics of two-dimensional hydraulic conductivity fields with three fundamentally different spatial textures and consider the conditions under which physical mobile–immobile domain mass transfer occurs in these fields. All three fields have near-identical lognormal univariate conductivity distributions, as well as near-identical isotropic spatial covariance functions. They differ in the pattern by which high- or low-conductivity regions are connected: the first field has connected high-conductivity structures; the second is multivariate log-Gaussian and, hence, has connected structures of intermediate value; and the third has connected regions of low conductivity. We find substantially different flow and transport behaviors in the three different fields. Flow and transport in the multivariate log-Gaussian field are consistent with stochastic theory. The field with connected high-conductivity paths has an effective conductivity greater than the geometric mean and large variations in fluid velocity. It produces significant mass transfer behavior (i.e., tailing) when the conductivity variance is large and, depending on the system parameters, this mass transfer is driven by either diffusion or advection. In the field with connected low-conductivity regions, the effective conductivity is below the geometric mean and transport is well characterized by the advection–dispersion model with a dispersivity smaller than that in the multivariate log-Gaussian field. Thus, physical mobile–immobile domain mass transfer may occur in smooth hydraulic conductivity fields with univariate log-Gaussian density functions if the variability in conductivity is sufficient and the high values are more connected than modeled by the multivariate log-Gaussian distribution.

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.

Modelling of reactive separation processes: reactive absorption and reactive distillation

Abstract: In the last years chemical process industries have shown permanently increasing interest in the development of reactive separation processes (RSP) combining reaction and separation mechanisms into a single, integrated unit. Such processes bring several important advantages among which are increase of reaction yield and selectivity, overcoming thermodynamic restrictions, e.g. azeotropes, and considerable reduction in energy, water and solvent consumption. Important examples of reactive separations are reactive distillation (RD) and reactive absorption (RA). Due to strong interactions of chemical reaction and heat and mass transfer, the process behaviour of RSP tends to be quite complex. This paper gives an overview of up-to-date reactive separation modelling and design approaches and covers both steady-state and dynamic issues. These approaches have been applied to several different RA and RD processes including the absorption of NOx, coke gas purification, methyl acetate synthesis and methyl tertiary butyl ether (MTBE) synthesis.

Properties of ceramic foam catalyst supports: mass and heat transfer

Abstract: Mass and heat transport properties have been determined for 30 PPI α-Al 2 O 3 ceramic foam containing 6 wt.% γ-Al 2 O 3 washcoat. The foam was loaded with 5 wt.% platinum and the rate of carbon monoxide oxidation measured for a 0.3 cm cylindrical segment of the foam operating with mass transfer controlling at 550 °C. This gave a mass transfer factor versus Reynolds number correlation that was equivalent to a packed bed of particles. A correlation for the radial heat transfer coefficient in a bed of ceramic foam was determined by measuring outlet temperatures achieved when air at varying flow rates and inlet temperatures was passed through a bed of foam pellets. Correlation parameters of a 1D model were fitted from 700 to 1000 °C using a Simplex optimization routine. Radial heat transfer coefficients were two to five times higher than those predicted from packed bed correlations.

Recent developments in numerical modelling of heating and cooling processes in the food industry—a review

Abstract: Numerical modelling technology offers an efficient and powerful tool for simulating the heating/cooling processes in the food industry. The use of numerical methods such as finite difference, finite element and finite volume analysis to describe the heating/cooling processes in the food industry has produced a large number of models. However, the accuracy of numerical models can further be improved by more information about the surface heat and mass transfer coefficients, food properties, volume change during processes and sensitivity analysis for justifying the acceptability of assumptions in modelling. More research should also be stressed on incorporation of numerical heat and mass transfer models with other models for directly evaluating the safety and quality of a food product during heating/cooling processes. It is expected that more research will be carried out on the heat and mass transfer through porous foods, microwave heating and turbulence flow in heating/cooling processes.

Three-phase hydrogenation of d-glucose over a carbon supported ruthenium catalyst—mass transfer and kinetics

Abstract: The catalytic hydrogenation of d -glucose to d -sorbitol over a 5% Ru/C catalyst was studied in a semi-batch slurry autoclave operating at 373–403 K and 4.0–7.5 MPa hydrogen pressure. The d -glucose concentration was varied between 0.56 and 1.39 mol/l. The kinetic experiments were carried out in the absence of mass transport limitations, which was verified by using measured gas–liquid mass transfer coefficients and estimated diffusion and liquid–solid mass transfer coefficients. Many literature reports suffer from transport limitations. In the operating regime studied the reaction rate showed a first order dependency with respect to hydrogen. A shift in the order of d -glucose was observed. At low d -glucose concentrations (up to ca. 0.3 mol/l) the reaction showed a first order dependency, while at higher concentrations this changed to zero order behavior. No inhibition by sorbitol or mannitol was observed. The kinetic data were modeled using three plausible rate models based on Langmuir–Hinshelwood–Hougen–Watson (LHHW) kinetics assuming that the surface reaction is rate-determining. Model 1 involves non-competitive adsorption of hydrogen and d -glucose. Hydrogen adsorption is either molecular or dissociative, but due to the weak adsorption it results in both cases in a linear hydrogen pressure dependency, i.e. the same rate expression; Model 2 is based on competitive adsorption of molecular hydrogen and d -glucose; and Model 3 assumes competitive adsorption of dissociatively chemisorbed hydrogen and d -glucose. All three models described the data satisfactorily and further statistic discrimination between these models was not possible.

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

CFD simulation of coupled heat and mass transfer through porous foods during vacuum cooling process

Abstract: A numerical simulation by using a computational fluid dynamics (CFD) code is carried out to predict heat and mass transfer during vacuum cooling of porous foods on the basis of mathematical models of unsteady heat and mass transfer. The simulations allow the simultaneous prediction of temperature distribution, weight loss and moisture content of the meats at low saturation pressure throughout the chilling process. The simulations are also capable of accounting for the effects of the dependent variables such as pressure, temperature, density and water content, thermal shrinkage, and anisotropy of the food. The model is verified by vacuum cooling of cooked meats with cylindrical shape within an experimental vacuum cooler. A data file for pressure history was created from the experimental pressure values, which were applied in the simulations as the boundary condition of the surface temperature.