Ultrafiltration of non-spherical molecules

Information about the sizes of the solute molecules and membrane pores is needed to estimate solute rejection in filtration processes. Molecules are normally regarded as spheres, and the Stokes radius is commonly used to represent their molecular size. However, many molecules used in food and pharma processes are oligomers or polymers which are strongly elongated; therefore, considering them spherical affects the accuracy of the model predictions. We here adapt the so-called Steric Pore Model to a more realistic representation of the transfer of rigid elongated molecules into and through ultrafiltration membrane pores. To do so, sugars with different degree of polymerization were used as model molecules. They were considered to be capsule-shaped to facilitate their size estimation. In order to represent the system as accurately as possible, the effect of hydration on the sugars size was included, and the membrane pore size distribution was estimated based on rejection data. It was demonstrated that considering these molecules to be capsule-shaped instead of spherical generates better predictions over the entire rejection spectrum using a unique pore size distribution. Additionally, this capsular geometry lets us simplify the calculations, making the estimation of the rejection straightforward.