Membrane Filtration with Liquids: A Global Approach with Prior Successes, New Developments and Unresolved Challenges

Abstract

After 70 years, modern pressure‐driven polymer membrane processes with liquids are mature and accepted in many industries due to their good performance, ease of scale‐up, low energy consumption, modular compact construction, and low operating costs compared with thermal systems. Successful isothermal operation of synthetic membranes with liquids requires consideration of three critical aspects or “legs” in order of relevance: selectivity, capacity (i.e. permeation flow rate per unit area) and transport of mass and momentum comprising concentration polarization (CP) and fouling (F). Major challenges remain with respect to increasing selectivity and controlling mass transport in, to and away from membranes. Thus, prediction and control of membrane morphology and a deep understanding of the mechanism of dissolved and suspended solute transport near and in the membrane (i.e. diffusional and convective mass transport) is essential. Here, we focus on materials development to address the relatively poor selectivity of liquid membrane filtration with polymers and discuss the critical aspects of transport limitations. Machine learning could help optimize membrane structure design and transport conditions for improved membrane filtration performance.

Membrane fouling by lysozyme: Effect of local interaction

“…It has become the mainstay in water treatment, 4,5 and is also a popular choice for food processing, 6,7 petrochemical refining, 8 and pharmaceutical separations 9,10 . Unfortunately, the efficiency of membrane processes is adversely affected by fouling, which refers to the phenomenon whereby foulants present in the feed deposit into or onto the membrane and thus provide an additional resistance to the permeate flow across the membrane 11‐13 . Consequently, the separation performance becomes significantly deteriorated in terms of diminished recovery and permeate quality, elevated feed pressure requirement, increased operating cost, as well as shortened unit lifespan 14‐16 …”

Can Self-Assembly Address the Permeability/Selectivity Trade-Offs in Polymer Membranes?

“…Ultrafiltration membranes are subject to fouling due to pore blocking, and isoporous structures introduce an additional challenge for fouling mitigation. If the foulant is nearly the same size as the pores, this could lead to rapid failure of the membrane as many of the nearly identical pores quickly become clogged with the foulant, which would occur less readily for membranes with broader pore size distributions . Provided this fouling issue can be overcome, developing techniques to produce isoporous membranes with controlled pore sizes could enable access to new separations previously inaccessible by conventional UF membranes, just as development of materials like PIMs and TR polymers with narrow free volume distributions led to advances in the capabilities of gas separation membranes .…”

“…While there is a strong case to be made for highly uniform UF membranes, the use of modern self-assembly techniques to create isoporous structures is still an emerging field. Pore size distributions in UF membranes can be optimized by changing conditions of the nonsolvent-induced phase separation (NIPS) process ,− but without direct design control over pore size and shape. , NIPS is a highly dynamic, nonequilibrium process that sensitively depends on processing conditions, such as polymer solution, solvent, and nonsolvent compositions, among other variables. , Indeed, it has been argued that is not possible to design isoporous polymer membranes for filtration or even membranes with specific, narrow pore size distributions and morphologies by using conventional phase inversion processes …”

“…If the foulant is nearly the same size as the pores, this could lead to rapid failure of the membrane as many of the nearly identical pores quickly become clogged with the foulant, which would occur less readily for membranes with broader pore size distributions. 30 Provided this fouling issue can be overcome, developing techniques to produce isoporous membranes with controlled pore sizes could enable access to new separations previously inaccessible by conventional UF membranes, just as development of materials like PIMs and TR polymers with narrow free volume distributions led to advances in the capabilities of gas separation membranes. 4 Conventional UF membranes are relatively application-agnostic, and many commercial producers make UF membranes with only a few broad molecular weight cutoff values (e.g., 100 or 300 kDa).…”

“…32,37 Indeed, it has been argued that is not possible to design isoporous polymer membranes for filtration or even membranes with specific, narrow pore size distributions and morphologies by using conventional phase inversion processes. 30 In addressing this limitation, inspiration is found in the block copolymer literature, specifically the directed self-assembly (DSA) of block copolymers as lithographic resists for microelectronic and storage applications. 38−43 Block copolymer DSA is frequently employed for nanolithography due to the ability to form highly controlled patterns at length scales between 5 and 100 nm while exhibiting long-range order.…”