(587b) On the Geometrical Features of the Optimal Membrane Contactor

Hollow fiber contactors have been successfully used for many decades in a wide range of applications. An important drawback of this contactor format is however the conflicting relation between throughput and kinetics. Fast kinetics can indeed be reached in smaller diameter fibers, but this occurs at the expense of lower axial velocities and a concomitant lower throughput when equal pressures are employed. An approach that is often used to increase the attainable flow rates consists of incorporating an array of fibers, but this is a tricky operation due to difficulties of fabrication, flow distribution, occurrence of fouling, regeneration, etc. An equivalent configuration, omitting the above mentioned problems, can be conceived by a using flat parallel channel plates that sandwich a membrane with spacers. For the application of liquid-liquid extraction, it is desirable that the interface is pinned at the membrane. The stability of this configuration is dictated by the Young-Laplace equation, which can result in stabilizing pressures of up to 5 bar, which is well in line with optimal flow conditions [1].   

Our group has recently provided analytic expressions describing the extraction kinetics in function of operational and geometrical parameters [2] for both co- and counter-flow operation, which were validated using channels with a depth down to 100 µm (width 1 cm). In an attempt to enhance the extraction kinetics lower channel heights are required with a preferential counter-flow operation, for which increasing pressures are required to maintain a given linear velocity, resulting in excessively large pressure gradients across the membrane and a concomitant loss of liquid-liquid interface pinning at the membrane.

A fundamental solution for this stability problem in co-flow mode can in theory be provided by imposing equal in- and outlet pressures at both phases. We observed however that the corresponding flow rate ratio of both phases did not comply with the expected inverse viscosity ratio. The flow rate ratio decreased from 2.8 to 1.6 when inlet pressures increased form 0.1 to 1 bar. By estimating the corresponding dimensions of the channels using Poiseuille’s law, the extent of bending of the flexible membranes (PTFE, polycarbonate) was estimated. The bending was subsequently confirmed and further studied using numerical simulation. Next, a microscopy-based measurement method was developed that enabled to follow bending of the membrane in function of the pressure. It appeared that bending decreased considerably from 95 µm to 10 µm (channel depth: 100 µm) with a decreasing distance between the membrane support structures from 3 mm to 400 µm. Also the shape of these support structures was vital, with radially elongated pillars as the most interesting structures due to a more uniform pillar spacing.  Based on these findings, the optimal geometrical configuration can be conceived depending on the desired specifications of the membrane, the raffinate and the extractant phase. The attainable performance of membrane contactors is compared with emulsion-based approaches in large scale reactors as well as microreactors. 

[1] Hereijgers, J., Breugelmans, T. & De Malsche, W. Breakthrough in a flat channel membrane microcontactor. Chem. Eng. Res. Des. 94, 98–104 (2015).

[2] Hereijgers, J., Callewaert, M., Lin, X., Verelst, H., Breugelmans, T., Ottevaere, H., Desmet, G., De Malsche, W. A high aspect ratio membrane reactor for liquid-liquid extraction. J. Memb. Sci. 436, 154–162 (2013).