Abstract
Membrane-based processes are considered a promising separation method for many chemical and environmental applications such as pervaporation and gas separation. Numerous polymeric membranes have been used for these processes due to their good transport properties, ease of fabrication, and relatively low fabrication cost per unit membrane area. However, these types of membranes are suffering from the trade-off between permeability and selectivity. Mixed-matrix membranes, comprising a filler phase embedded into a polymer matrix, have emerged in an attempt to partly overcome some of the limitations of conventional polymer and inorganic membranes. Among them, membranes incorporating tubular fillers are new nanomaterials having the potential to transcend Robeson’s upper bound. Aligning nanotubes in the host polymer matrix in the permeation direction could lead to a significant improvement in membrane permeability. However, although much effort has been devoted to experimentally evaluating nanotube mixed-matrix membranes, their modelling is mostly based on early theories for mass transport in composite membranes. In this study, the effective permeability of mixed-matrix membranes with tubular fillers was estimated from the steady-state concentration profile within the membrane, calculated by solving the Fick diffusion equation numerically. Using this approach, the effects of various structural parameters, including the tubular filler volume fraction, orientation, length-to-diameter aspect ratio, and permeability ratio were assessed. Enhanced relative permeability was obtained with vertically aligned nanotubes. The relative permeability increased with the filler-polymer permeability ratio, filler volume fraction, and the length-to-diameter aspect ratio. For water-butanol separation, mixed-matrix membranes using polydimethylsiloxane with nanotubes did not lead to performance enhancement in terms of permeability and selectivity. The results were then compared with analytical prediction models such as the Maxwell, Hamilton-Crosser and Kang-Jones-Nair (KJN) models. Overall, this work presents a useful tool for understanding and designing mixed-matrix membranes with tubular fillers.
Highlights
The use of membranes in separation processes for chemical, petrochemical, and environmental applications has significantly increased in recent years
Four different numerical simulation studies were performed to show unequivocally that the effective permeability of a repeatable unit of an matrix membranes (MMMs) is identical to the effective permeability of the whole mixed-matrix membrane with aligned
Four different numerical simulation studies were performed to show unequivocally that the effective permeability of a repeatable unit of an MMM is identical to the effective permeability of the whole mixed-matrix membrane with aligned and uniformly distributed nanotubes
Summary
The use of membranes in separation processes for chemical, petrochemical, and environmental applications has significantly increased in recent years. The main advantages of using membranes in industrial separation processes are the much lower energy requirements and the smaller plant footprint compared to conventional separation processes. Given their high stability, efficiency, and ease of processing, employing membranes in industrial processes leads to lower environmental impact and cost [1,2]. At about the same time, GENERON introduced the first membrane system to separate nitrogen from air using poly(4-methyl-1-pentene) (TPX) membranes. Many gas processes such as oxygen separation or pervaporation applications such as alcohol separation and dehydration still require better membranes to become more commercially viable [2,6]
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