Abstract
Over the past three decades, mixed-matrix membranes (MMMs), comprising an inorganic filler phase embedded in a polymer matrix, have emerged as a promising alternative to overcome limitations of conventional polymer and inorganic membranes. However, while much effort has been devoted to MMMs in practice, their modeling is largely based on early theories for transport in composites. These theories consider uniform transport properties and driving force, and thus models for the permeability in MMMs often perform unsatisfactorily when compared to experimental permeation data. In this work, we review existing theories for permeation in MMMs and discuss their fundamental assumptions and limitations with the aim of providing future directions permitting new models to consider realistic MMM operating conditions. Furthermore, we compare predictions of popular permeation models against available experimental and simulation-based permeation data, and discuss the suitability of these models for predicting MMM permeability under typical operating conditions.
Highlights
Different alternatives have been explored to enhance polymer membranes to perform beyond Robeson’s upper bound, including surface modification [29], facilitated transport [30,31], polymer blends [32], and mixed-matrix membranes (MMMs) [33]. Amongst these alternatives, devoted attention to the synthesis of MMMs has been intensified over the last three-decades [21,34,35], with a myriad of studies focusing on novel materials to increase efficiency of CO2 capture [18,35,36], natural gas purification [37,38,39], water purification [40,41], and olefin/paraffin separation [42,43,44]
The MMM permeability is based on both volume fractions and permeabilities of the MMM constituent phases while assuming uniformity of the field across the MMM, and negligible filler phase particle sizes compared to membrane thickness
These considerations lead to concentration-independent permeabilities across the MMM, which mask effects of isotherm nonlinearity and finite filler size in the effective permeability calculation
Summary
In the last few decades, membrane technologies have attracted increasing attention to be used in a variety of industrial applications, which include gas separation [1,2,3], water desalination [4,5,6], food processing [7], pervaporation [8,9], membrane contactors [10,11], and membrane reactors [12,13]. A mixed-matrix membrane (MMM) consists of a selective inorganic filler phase embedded to continuous polymer matrix [21,35] In this way, an MMM combines high intrinsic permeability and separation efficiency of advanced molecular sieving materials (e.g., zeolites, carbons, metal-organic frameworks) or nanoscale materials (e.g., carbon nanosheets or nanotubes) with robust processing capabilities and mechanical properties of glassy polymers [23,52].
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