Anders Rasmuson

The influence of particle shape on the dynamics of fixed beds

Abstract Transient mass transfer in a fixed-bed is studied mathematically. A previous analytical solution for beds of spherical particles is generalized to include ”infinite” slabs and cylinders. Processes that are accounted for are advection and longitudinal dispersion in the interparticle void space, fluid-to-particle diffusion, intraparticle diffusion, and reversible first-order adsorption for a first-order reaction on the interior particle surface. The same generalization of a solution is also obtained for the conversion in continuous-slurry and batch reactors. Calculations show that if the area-to-volume ratio of the slabs, cylinders and spheres is the same, identical breakthrough curves are produced for short and long contact times. In the intermediate range the first breakthrough times are in the order of spheres In systems with no chemical reaction, using a shape factor, based on the matching of second central moments, gives better agreement for intermediate bed lengths and longer times. At early times a poor agreement is obtained.

Modeling of solute transport in aggregated/fractured media including diffusion into the bulk matrix

Abstract In this paper the influence of mass-transfer resistance on solute transport and uptake in soils is stressed. The resistance is generally divided into three distinct stages: (1) diffusion of the component from the flowing water to the solid interface (external or film diffusion); (2) diffusion through the porous network of the aggregates (internal diffusion); and (3) the sorption process itself, when the component is bound to some sorption site in the micropores (ion-exchange, adsorption, etc). In the general case, all three steps can contribute to the overall sorption rate. The sorption process is usually rapid (third mass transfer resistance). Film diffusion (first mass transfer resistance) may be of importance at low velocities. The bulk of the present paper discusses the impact of internal diffusion (second mass transfer resistance) on ion-transport in soils. This effect can be very important since large internal surfaces become available for sorption. Mathematical models are formulated and solved analytically. In these models chemicals are transported by flow and dispersion in larger pores or cracks. In addition, diffusion-type equations are used to describe transfer of solute from the larger pores into the bulk soil matrix. The effects of various dimensionless parameters, taking into account transport distance, water velocity, flow porosity, film diffusion, diffusivity, sorption constant, reaction rate, longitudinal dispersion coefficient and aggregate radius are explored. The influences of aggregate shape and a particle size distribution are treated. It is shown that if the area-to-volume ratio of slabs, cylinders and spheres is the same, identical breakthrough curves are produced for short and long contact times. In the intermediate range the first breakthrough times are in the order of spheres

Chemical Transport in Fractured Rock

Transport of dissolved species in fractured rock has become an area of special interest in recent years when deep lying crystalline rocks have become potential sites for repositories for nuclear waste. In Sweden, research was started in 1977 to investigate the flow and transport in low permeability crystalline rocks such as granites and gneisses.

Model for Near Field Migration

A model is proposed which describes the transport to and from a waste canister in a repository. The model includes flow and diffusion in the fractures in the bedrock as well as diffusion in the backfill. Calculations have been made on the inward transport of corrosive agents to a buried canister from the surrounding bedrock. The oxidants are transported from the flowing groundwater through the backfill to the surface of the canister, were they are assumed to react instantaneously with the canister material. The rock is modeled as a discrete fracture system. The fractures are assumed to be evenly spaced and the groundwater movement is described by potential flow. The model is three dimensional.

Transport processes and conversion in isothermal slurry reactors

Abstract Transient mass transfer and chemical reaction in isothermal continuous-flow and batch agitated reactors is studied mathematically. Analytical solutions are derived in the form of infinite integrals. The solutions include the effects of fluid-to-particle diffusion, intraparticle diffusion and reversible first-order adsorption for a first-order reaction on the particle surface. The conversion at steady-state is explored in some detail. An efficient numerical scheme for evaluating the infinite integral in the transient solution is given. A large number of calculations are presented, for both the continuous feed and the batch reactor, for the case with negligible adsorption rate resistance. The influences of the different mass transfer resistances on the conversion are clearly demonstrated.