Fractionation: Particle Behavior

Particle separation and fractionation by microfiltration

In the Ph.D. thesis work described here, we explore ways to make use of the full potential of microfiltration for fractionation of milk, which up till now was not possible. Microfiltration (MF) is a process that is used for the separation or fractionation of particles (0.02-20 mm) in a suspension. In MF processes, the suspension is pressed through a membrane; larger particles are retained by the membrane and a size-selective separation is obtained. MF is applied for e.g. water treatment and the manufacture of many products in biotechnological, food and medical applications.

In general, accumulation and deposition of retained particles in and onto the membrane, often referred to as concentration polarisation and cake layer formation respectively, severely limit the flux and selectivity of MF processes and therewith the actual potential of MF is hardly used. This is especially the case for MF of complex suspensions such as milk, which contains a high volume fraction of dispersed matter (±15%) with a broad particle size distribution (1 nm - 20 mm). In this thesis, factors affecting the flux and selectivity of MF processes for milk separation are studied on various levels of detail.

The initial focus was at a microscopic scale by studying suspension flow at conditions relevant to MF. A computer simulation method, based on the Lattice Boltzmann method, was developed for modelling of suspensions in shear flow. This enabled computations on the phenomenon of shear-induced diffusion, a diffusion mechanism with high relevance to MF processes because it induces particle transport away from the membrane and therewith decreases cake layer formation. Knowledge on the suspension flow behaviour at non-zero Reynolds numbers and of polydisperse suspensions was generated this way. An important finding of this research was that shear-induced diffusion is strongly related with the microstructure of the suspension, which on its turn is influenced by the Reynolds number. This implies that structural organisation takes place during suspension flow, and depending on the flow conditions this structure will be more or less pronounced.

Further, a computer simulation model was developed that not only enabled the solution of the flow but also of concentration polarisation and cake layer formation in MF systems. Herewith, a practical tool is obtained to support the design of MF processes of complex suspensions such as milk.

The study then gradually shifted to a macroscopic scale, studying the filtration behaviour of complex suspensions. This resulted in design rules for the separation and fractionation of particles by MF. Combination of the observations on macroscale and the simulations done on microscale led to the surprising finding that size-selective particle separation can already be obtained in the flow of the feed suspension through the membrane channel. Since the membrane filter itself does not play a role as a selective medium, this approach enabled fractionation with high selectivity and without concentration polarisation and cake layer formation. The selectivity of the fractionation process could be controlled with simple process parameters such as the wall shear rate and the permeate flux. With these findings, we are now for the first time able to use the potential of microfiltration for fractionation of milk to the fullest.