Drying, growth towards a unit operation

Within the field of separations drying is one of the unit operations with a very wide variety of materials, equipment type, capacity, product requirements and added value. The scope and depth of published papers about the subject shows a similar variety. At the present time the systematic approach to design and operation, as appears to exist for other unit operations, for drying is still largely incomplete. In the paper some aspects of unification are considered. After some thoughts about types of drying research, examples are given from thermodynamics, transport phenomena and mathematics to show perspectives of using common concepts and theory towards fruitful integration of knowledge from other areas into drying knowledge

Countercurrent plug flow mass exchange with internal particle diffusion

The model for countercurrent steady state mass transfer between uniform dispersed phase particles and a continuous phase, both assumed to move in plug flow, with diffusion resistance inside the particles, and film resistance around the particles, is solved anal. for three particle geometries: infinite plates, infinite cylinders and spheres. In the soln., the direct relation between the local continuous phase concn., and the av. dispersed phase concn. is accounted for in the boundary condition. Concn. profiles in the mass exchanger are derived from the anal. soln., as well as asymptotic Sherwood nos. The anal. soln. requires the detn. of roots of the characteristic equations, which is cumbersome for spherical particles. Correlations are provided for the asymptotic Sherwood nos. for easy computation. The model can easily be used for the analogous direct heat exchange problem

Analysis and extension of the theory of multicomponent fluid diffusion

Recently we have presented an extended theory for molecular mass transport (Kerkhof and Geboers, 2005). We have indicated the possibility, and the need, to define a new framework for studying and teaching the theory of multicomponent molecular transport in fluids (Kerkhof and Geboers, 2004). Here a new, structured, overview of the subject is presented. An analysis is given of complicated issues in textbooks and review papers

Determination of diffusion coefficients from experimental moisture concentration profiles in clay

In order to measure moisture-concn. profiles, a method based on neutron transmission was applied. Drying curves and diffusion coeffs. were calcd. from these profiles. The diffusion coeffs. appeared to be similar to the diffusion coeffs. detd. by comparing the numerical soln. of the diffusion equation with exptl. drying curves. The accuracy of both methods is about the same. Because the moisture concn. profiles are flat at high moisture concns., the accuracy of the calcd. diffusion coeffs. at high moisture concns. is not as high as is obtained at lower moisture concns. The neutron-transmission-based method is reproducible and has the advantage that it is not necessary to assume a certain relation between the diffusion coeff. and the moisture content. [on SciFinder (R)

Estimation of effective diffusivity in drying of heterogeneous porous media

This work presents a new method to evaluate the effective diffusivity in porous solids, considering the mass-transfer mechanisms occurring in the pores and on the surface. The binary friction model was used to evaluate the effective gas-phase diffusivity and the Maxwell-Stefan surface diffusion equation to the transport of bound water. Different expressions for the effective diffusivity for porous solids were derived considering parallel diffusion, series diffusion, and the Clausius-Mossotti model. The numerical results were compared to experimental data obtained for Son clay. In the pore diffusivity, when both are present, capillary flow showed dominance over gas diffusion. For the entire moisture content range, only a combination of the models presented good agreement with experimental data, showing changes in the interdependence of mechanisms during the drying process

Solute rejection in the presence of a deposited layer during ultrafiltration

During ultrafiltration deposited layers are often formed on the membrane surface. These layers not only reduce the volumetric flux through the membrane, but also may influence the rejection of other solutes in the feed. In the present paper we will show that besides an increase in the rejection, a decrease in rejection may also occur, which can completely alter the aimed selectivity of the separation process. The influence of deposited layers has been studied experimentally by two types of depositing components: silica sol and the protein BSA. In the presence of a relatively open silica deposit a strong drop in the rejection of PEG and dextran was found compared to the rejection on a clean membrane. For thick deposit layers the rejection even decreased to zero, thus resulting in a total permeation of a normally partially rejected solute. On the other hand an increase in PEG rejection occurred in the presence of a BSA deposit. Due to the compressibility of the protein deposit the highest rejections were measured at the highest pressures. The effects were the most pronounced at the isoelectric point of BSA. A model is presented to describe the underlying phenomena