## 1 Introduction
### Mass Transport in Processes That Use Polymers in Particle Form  
At present, porous particles with different size, porosity and chemical composition are used as column packing and applied to different processes, such as affinity chromatography. This technique involves the separation, isolation and/or purification of biological compounds and is based on biospecific interactions between the biomolecule to be purified and a ligand, which is bound to a polymeric matrix.  
This type of support presents advantages in chromatography, in spite of two inherent limitations: the great interparticle volume, and the slow diffusion of high-molecular-weight solutes into the pores of the particles [8]. Concerning the latter, the movement or flow of the solutes (dissolved in a solvent) into the pores is due to their capacity to diffuse. While solutes of low molecular weight are capable of moving relatively fast, solutes of high molecular weight (proteins, polysaccharides or synthetic polymers) move more slowly, since they present lower diffusion coefficients. Thus, this problem becomes important in processes in which the rate of mass transfer is the determining factor in chromatographic separations, catalysis, _etc._[9].  
The resistance of large molecules to mass transfer is mainly due to the discontinuity found in packed columns with particulate system due to the empty spaces between the particles. Theoretical studies reveal that the void volume corresponding to the empty spaces between the particles (considering spherical particles of the same size) as a result of the particulate character of the packingrepresents at least 30-40% of the total volume of the column. Therefore, the mobile phase flows mostly through the interstitial voids between the particles in which the resistance at the flux is less. On the other hand, the liquid in the pores practically does not flux, remaining stagnated [8, 9, 10]. When the molecules of the solute are in the mobile phase, they begin to migrate mainly in the inter particle voids. Then, as a consequence of the difference in concentration between the solute in the mobile phase and the liquid in the pores, the molecules begin to diffuse toward the interior of the pores of the particles until reaching the concentration equilibrium between them. Then, the concentration of the solutes in the mobile phase decreases, the gradient in concentration is reversed, and the molecules of the solute flux, in this case from the interior of the pores toward the liquid that fluxes in the inter particle spaces [8, 9].  
In the case of low-molecular-weight solutes (small organic molecules and ions), the molecules diffuse fast and the concentration equilibrium in the pores is quickly reached. Nevertheless, in the case of high-molecular-weight solutes (such as proteins, nucleic acids and synthetic polymers), the macromolecule diffuses slowly toward the interior of the pores, interacts with the polymer active site, and then diffuses toward the mobile phase that fluxes in the inter particle spaces. Now then, if the velocity of diffusion is smaller than the time necessary for the interaction between the biological macromolecule and the ligand bound to the support to occur, the first is the determining step in the process. In these cases, the molecules of solute interact only with the few and more accessible active sites into the pores, producing a less effectiveness in the separation medium. By all reasons previously mentioned, large columns or slow fluxes must be used [9].