Flocculation on a chip: a novel screening approach to determine floc growth rates and select flocculating agents

Flocculation is a key purification step in cell-based processes for the food and pharmaceutical industry where the removal of cells and cellular debris is aided by adding flocculating agents. However, finding the best suited flocculating agent and optimal conditions to achieve rapid and effective flocculation is a nontrivial task. In conventional analytical systems, turbulent mixing creates a dynamic equilibrium between floc growth and breakage, constraining the determination of floc formation rates. Furthermore, these systems typically rely on end-point measurements only. We have successfully developed for the first time a microfluidic system for the study of flocculation under well controlled conditions. In our microfluidic device (μFLOC), floc sizes and growth rates were monitored in real time using high-speed imaging and computational image analysis. The on-line and in situ detection allowed quantification of floc sizes and their growth kinetics. This eliminated the issues of sample handling, sample dispersion, and end-point measurements. We demonstrated the power of this approach by quantifying the growth rates of floc formation under forty different growth conditions by varying industrially relevant flocculating agents (pDADMAC, PEI, PEG), their concentration and dosage. Growth rates between 12.2 μm s−1 for a strongly cationic flocculant (pDADMAC) and 0.6 μm s−1 for a non-ionic flocculant (PEG) were observed, demonstrating the potential to rank flocculating conditions in a quantitative way. We have therefore created a screening tool to efficiently compare flocculating agents and rapidly find the best flocculating condition, which will significantly accelerate early bioprocess development

Elucidation of Flocculation Growth Kinetics Using a Microfluidic Approach

The inter-disciplinary work in this thesis entails the development of a microfluidic device with bespoke imaging methodology to study flocculation growth kinetics dynamically in real-time. Flocculation is an advantageous downstream operation that increases the product-separation efficiency by selectively removing impurities. Yet, there is no unifying model defining the effect of different physico-chemical parameters on the rates of flocculation. Conventional setups for said analyses require large experimental space that are tedious to perform, and are limited by their dependence on end-point analysis, requiring sample-handling and further dispersion into typical particle-sizing instruments. In spite of the counter-intuitiveness of implementing microfluidics to study flocculation due to the anticipated channel-clogging issue, it is hypothesised that the growth kinetics can be measured by achieving a continuous, steady-state flocculation under a lower-shear environment. Flocculation within a spiral microfluidic device (~151.8 µl volume) is evaluated against a bench-scale setup (~50 ml volume) through the comparison of floc size and zeta potential. The fluid hydrodynamics in the microchannel is assessed by an experimental mixing-time analysis (tmix = 7.5 s) and a residence time distribution study (tm = ca. 70 s). In situ measurement of floc size and morphology is facilitated through high-speed imaging, with an image-processing script for robust analysis. Different flocculants are tested and growth rates calculated (~ 8 and ~12 µm s-1 for PEI and pDADMAC). Flocs grew linearly up to 250 µm for cationic polymers, while no growth was observed with a non-ionic PEG. Using an improved parameter-fitting step, the growth rates are compared to a simplified model for monodisperse perikinetic flocculation. The work presented should thus, enable an experimental estimation of flocculation growth kinetics and pave way for the development of accurate flocculation models for polydisperse particles. The developed system also facilitates a rapid screening of new flocculants useful for quicker process development