Modelling the Molecular Permeation through Mixed-Matrix Membranes Incorporating Tubular Fillers

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

Adsorption Separation of Methyl Chloride from Nitrogen Using ZSM-5 and Mesoporous SBA-15

The adsorption capacities of zeolite ZSM-5 and periodic mesoporous molecular sieve SBA-15 towards methyl chloride (chloromethane) and nitrogen were examined for the separation of these gases. Adsorption isotherms were obtained by using a constant volume technique up to 1.6 atm over the temperature range 40–80°C. The Langmuir, Freundlich, Sips and Toth adsorption models were fitted to the isotherms and validations of these models were discussed. Adsorption isosteres were obtained by using the parameters of the Toth equation. Binary adsorption isotherm predictions were undertaken using the Extended Langmuir (EL) and Ideal Adsorbed Solution Theory (IAST) in order to obtain a better understanding of the separation of methyl chloride from air. Henry's law constants, heats of adsorption and separation factors for both adsorbents were also obtained