Permeate Flux in Ultrafiltration Processes—Understandings and Misunderstandings

Abstract

Concentration polarization refers to the rapid emergence of concentration gradients at a membrane/solution interface resulting from selective transfer through the membrane. It is distinguishable from fouling in at least two ways: (1) the state of the molecules involved (in solution for concentration polarization, although no longer in solution for fouling); and (2) by the timescale, normally less than a minute for concentration polarization, although generally at least two or more orders of magnitude more for fouling. Thus the phenomenon of flux decline occurring over a timescale of tens of minutes should not be attributed to concentration polarization establishing itself. This distinction and a number of questions surrounding modelling are addressed and clarified. There are two paradigmatic approaches for modelling flux, one uses the overall driving force (in which case allowance for osmotic effects are expressed as additional resistances) and the other uses the net driving force across the separating layer or fouled separating layer, although often the two are unfortunately comingled. In the discussion of flux decline models’ robust approaches for the determination of flux-time relationships, including the integral method of fouling analysis, are discussed and various concepts clarified. The final section emphases that for design purposes, pilot plant data are vital.

Membrane-Based Solvent Exchange Process for Purification of API Crystal Suspensions

“…During this, the concentration of the solute near the membrane surface increases rapidly compared to the bulk, resulting in a fast decline of the flux during the first couple of minutes of filtration [ 37 ]. This phenomenon was more dominant for solutions containing HPMC E3 (where an initial flux drop was observed) as compared to those containing only suspended model particles (where no initial flux drop was observed), because it occurs only for molecules that remain dissolved in the feed (HPMC E3 in this case) [ 38 ]. During industrial applications, it can be controlled or reduced with more rigorous mixing close to the membrane.…”

Basic physics of hemodiafiltration

“…There are multiple mechanisms responsible for this effect: (1) the concentration polarization, which increases plasma viscosity in the boundary layers adjacent to the membrane surface, thus hindering solute diffusion, (2) the formation of a relatively impermeable layer of colloid gel, or the so‐called cake, on the membrane surface (also termed “secondary membrane”) as a natural continuation of the concentration polarization process, and (3) the blockage of membrane pores. The latter two processes are termed membrane fouling 30 …”

“…The latter two processes are termed membrane fouling. 30 These processes have been well studied in ultrafiltration systems other than dialysis and can be modeled. 31 In high-flux dialysis, ultrafiltration rates are relatively low, so colloid concentrations near the membrane surface are unlikely to be high enough to cause significant concentration polarization or fouling during the dialysis procedure.…”

Filtration-based technologies for isolation, purification and analysis of extracellular vesicles

“…[35][36][37] The direction of mixture flow relative to the membrane can be controlled in order to improve filtration performance which defines two main categories of filtration operations: normal-flow filtration and cross-flow or tangential-flow filtration. 38,39 Filters can also be chemically modified to change their properties such as surface charge or introduce molecules such as antibodies to capture chosen analytes. 40,41 In recent years, filtration methods gained attention in EV research and development with the general concept summarized in Fig.…”