Modeling and Simulation of Field-Flow ­Fractionation

Asymmetrical Flow Field-Flow Fractionation, AFFFF, and Electrical Field-Flow Fractionation, EFFF, are simple, efficient and robust approaches for separation of nano- and micron-size particles in solutions and dispersions. AFFFF technology is well developed and widely used by industry and academia. It exploits the interplay between diffusion and flow filed for particles with different hydrodynamic radii.

Schematic Representation of the AFFFF Process

The diagram above shows a schematic representation of the AFFFF: Particles having different hydrodynamic radius and thus different diffusion are separated based on the interaction between a horizontal parabolic flow and the interplay between the diffusion and a vertical cross flow.

1. Initial stage: The different size particles are mixed, the particle distribution functions overlap.
2. The distribution functions for the different size particles start to separate with the time because the smaller particles have larger diffusion coefficient which lifts them in the area of higher horizontal velocity.
3. The goal is achieved, when the two types of particles are separated.

The design of the seperation system is based on a careful analytical study of fluid flow and of separation in a microchannel. Further improvement in the design and the performance of the fractionation devices can be achieved via mathematical modeling and computer simulation.

Three Dimensional Flow Simulations

Three dimensional flow simulations allow for obtaining a detailed view on the flow within the spacer, as well as on the particles transport there. Compared to the analytical considerations, the CFD (Computational Fluid Dynamics) simulations provide more detailed information in the case of complicated geometry, or complicated flow control. This information supplements the analytical considerations in optimizing the flow regimes and in further improving the design and the performance of the device.

In particular, simulations with different size and/or location of the injection pipe, with different number and size of the outlet pipes, allow for analyzing the influence of these parameters on the important components of the process.

These components include:

• size and shape of the focusing area
• symmetry, magnitude and separation of the peaks in the fractogram

CFD simulations allow for precise location of the focusing zone for each reasonable flow rate distribution for any selected shape and size of the spacer. It is well known that the size and the shape of the focusing area influences significantly the particles separation during the elution stage, and therefore its study is important in further improving the design of the fractionation device.

Optimization of the Density of the Sample in the Focusing Area

Furthermore, the simulation, combined with properly optimized flow control, allows for optimizing the density of the sample in the focusing zone, what is especially important in the case of hollow fiber where the small focusing area can result in high sample concentration, and thus undesirable agglomeration of particles/moleculae.

EFFF technology is very useful for separating particles with the same size (which can not be therefore separated by AFFFF), but with different electrical properties. A combined EFFF and AFFFF approach allows to separate broader class of particles and to reduce the number of devices used in a Lab. To summarize, CFD simulation is a powerful approach for evaluation and pre-selection of designs and flow control, without building expensive prototypes.