Mass transfer coefficient

About: Mass transfer coefficient is a research topic. Over the lifetime, 7827 publications have been published within this topic receiving 168354 citations.

The Adsorption of Pollutants by Peat, Lignite and Activated Chars

Abstract: The ability of peat, lignite and activated chars made from peat and lignite to adsorb dyes and metals from wastewater and NO 2 from air was investigated. Equilibrium isotherms were determined to assess the maximum adsorption capacity of the adsorbents for the pollutants. Kinetic studies for the adsorption of dyes and metal ions onto the adsorbents were undertaken in agitated batch adsorbers. Mass transport models were tested to predict the concentration decay curves in batch adsorbers. The models tested were single resistance models based on the assumption of a single external mass transfer coefficient and two resistance models which included an internal diffusion coefficient and an external mass transfer coefficient. The surface phenomena which influence the extent and the rate of uptake have been studied. The equilibrium capacity data conform to Langmuir plots. A previously proposed model was used to evaluate the external single resistance mass transfer model and was successfully applied to predict the adsorption of metal ions in single component systems under batch conditions. It has been shown that the assumption of negligible intraparticle diffusion is valid and that external film diffusion is the rate limiting step in describing the adsorption processes at high sorbent loadings. The same type of result is not observed for the adsorption of coloured organic matter onto peat where the sorption processes cannot be successfully modelled by use of a single resistance model and a two resistance model incorporating internal diffusion is required. The surface phenomena which influence the extent and the rate of uptake of NO 2 have been studied. The type of chars produced and the activation processes affect the adsorption. As activation increases, micropore volume and surface area increase and the maximum capacity of the adsorbent increases. Surface area alone is not the only parameter which affects equilibrium uptake.

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.

General HETP equation for the study of mass-transfer mechanisms in RPLC.

Abstract: Classical HETP equations including the Van Deemter and the Knox equations, are semiempirical, approximate equations that provide apparent mass-transfer coefficients with little sound physical justifications. The conventional A and B coefficients are revisited, the former through the use of the fundamental theory of eddy diffusion due to Giddings, the latter by taking into account the intraparticle diffusion (pore and surface diffusion). Our work confirms that eddy diffusion originated from three different sources in RPLC: trans-channel, short-range interchannel, and long-range interchannel velocity biases. Accordingly, the eddy diffusion term is given by the ratio of two third-degree polynomials. Finally, the C term is the sum of two terms corresponding to the resistance to mass transfer due to diffusion through the external stationary film of liquid phase surrounding the silica particles and to the classical resistances to mass transfer due to diffusion through the silica particles. It is easily related...

Effects of transpiration and changing diameter on heat and mass transfer to spheres

Abstract: Mass transfer to spheres suspended in an agitated liquid has been studied both experimentally and theoretically. Finite-difference solutions are obtained for mass transfer from a sphere to a fluid flowing past it in steady viscous flow. The effects of a transpiration velocity at the surface of the sphere and of a continuously changing sphere diameter are included. A normalized presentation of these effects is quite insensitive to the bulk flow Peclet number. When these theoretical corrections for transpiring and shrinking spheres are applied to the mass transfer data for ice spheres that are melting in an agitated brine bath, the corrected mass transfer coefficients are brought into agreement with a generalized correlation published elsewhere. This agreement suggests that the theoretical results apply, with reasonable accuracy, to a shrinking and transpiring sphere that is suspended in a turbulent liquid.