The fractionation and concentration of algae using shear induced diffusion
Algae have not only become an interesting source for the production of biofuels but also in the food sector algae (including components present in algae, i.e. fatty acids and proteins) are seen as promising nutritional components. Therefore, a lot of research is done on growing and harvesting algae. During my PhD I will contribute to this research by investigating the fractionation and concentration of algae using membrane filtration.
An important issue during microfiltration processes as currently applied is fouling. The particles present in the suspension can form a cake layer on top of the membrane which reduces the flux and therewith the efficiency of the membrane. A solution would be to increase the pore to particle diameter ratio to a value that is higher than 1 , although particles could then permeate. So only if particles are kept away from the membrane, this approach may work. Obviously, the particles will then not block the membrane and the flux will not be reduced. But as mentioned this is only possible if mechanisms can be used to influence particle movement.
In a suspension particles will be displaced mainly due to the following three transport mechanisms :
Shear induced diffusion
The type of moveme nt that will be most important is amongst others dependent on the particle diameter (figure 1). In this project shear induced diffusion is targeted as the key transport mechanism for fractionation and concentration.
Figure 1: membrane flux plotted against the particle diameter for different transport mechanisms .
During shear induced diffusion a shear field is generated by velocity gradients in the flow . Particles will therefore interact and transport towards the centre of a channel where the shear is low . Since the rate of migration differs between small and big particles fractionation can be achieved in a closed channel [1,3] after which membrane filtration can be used for separation (figure 2).
Figure 2: fractionation due to shear induced diffusion .
The project is a continuation of the projects by A.M.C. van Dinther* and H.M. Vollebregt*. They already looked into the principle of shear induced diffusion in combination with membrane filtration by doing experimental work and by setting up an elaborated model. Although they have made significant advances in this field many questions remain. Their results are based on spherical particles, on a flow through the membrane that is perpendicular to the flow through the channel (boundary condition) and on mono- and bidisperse suspensions. During my project I will therefore focus on and continue with these latter three aspects: particles of different shapes will be considered, the flow around a pore will be modelled and polydisperse systems will be taken into account.
The aim of this project is to fractionate and concentrate algal suspensions. The focus will be on shear induced diffusion in combination with membrane design and filtration conditions.
March 2012- March 2016
* Food Process Engineering Department, Wageningen University, the Netherlands
 Dinther, A.M.C. van; Schroën, C.G.P.H.; Boom, R.M. 2013. Membrane separation of very concentrated emulsions and suspensions for application in the food industry. Innovative Food Science and Emerging Technologies, published on line, 28 January 2013.
 R.H. Davis, Modeling of fouling of cross-flow microfiltration membranes, Separation and purification methods, 21, 1992.
This work is supported by NanoNextNL, a micro and nanotechnology consortium of the Government of the Netherlands and 130 partners.