Particle migration in laminar shear fields: A new basis for large scale separation technology?

Abstract

Abstract Particles and droplets of micrometre scale are present in many industrial products and processes, where they can be either part of the product (e.g. emulsion droplets), need to be separated in order to be further utilised in product formulations (e.g. starch particles of specific size or oil from enhanced recovery emulsions) or discarded as waste (such as cleaning liquids that contain small oil droplets or other particulates). In large scale operation the attention is primarily directed toward the throughput of a system, and dimensions of production lines are generally orders of magnitude different from that of the particles or droplets. Whereas the overall flow behaviour in large scale operation is well-understood, that of particles/droplets on micrometre scale is just coming of age, with many new developments in microfluidics and membrane separation adding to the knowledge base. In this review, we give an overview of the current advances that in our opinion underpin the development of novel as well as improvement of existing separation technologies. For this we highlight the micrometre, and sometimes nanometre scale as is customary in interface and colloid science, but is rather of the beaten path in engineering. We will show that when small scales are taken as a starting point, drastically new approaches may become feasible and offer solutions for large scale separation. First, we discuss particle behaviour in general terms; the computational models and experimental data from literature both indicate that in defined flow fields, particles segregate based on their size, but the effect of operational parameters on this segregation is not well understood. Next, we relate these findings to membrane and microfluidic separation processes and illustrate with concrete examples how these processes may benefit from this knowledge. In addition to the behaviour of solid particles as discussed for experiments and simulations, we also touch upon the expected effects of deformability of particles and geometry variations on the separation performance. In a concluding section we estimate dimensions and energy requirements of processes based on these new insights, and show that the amount of energy saved (up to 70% compared to conventional microfiltration) is an important aspect of processes that are designed around particle behaviour in laminar flow fields; therewith highlighting the importance of investigations at the relevant scale.