Performance Of Commercial Membranes Research Articles

PH-tunable and pH-responsive polybenzimidazole (PBI) nanofiltration membranes for Li+/Mg2+ separation

Polybenzimidazole (PBI), a polymer known for its superior chemical stability, thermal properties, and mechanical resistance has been regarded as a suitable membrane material for a wide range of applications. Herein, we took advantage of these properties and its amphoteric nature that could endow the formed membranes with capabilities to have different states and responses in different pH conditions for enhanced nanofiltration (NF) performance. Integrally skinned asymmetric flat-sheet PBI membranes were fabricated using the nonsolvent-induced phase separation (NIPS) method under various pH in a coagulant bath. For the first time, we have thoroughly investigated the effects of nonsolvent acidity and alkalinity on PBI membrane formation and their performances. It was revealed that the membrane formed under pH 12 showed a doubled MgCl2 rejection than the one formed under pH 7. Moreover, a green modification method was employed to enhance the membranes’ sieving capabilities by using hyperbranched polyethyleneimine polymers (HPEIs) with different molecular weights (MWs). The PBI membrane formed under pH 12 and then surface modified with a HPEI of MW = 25,000 g/mol (i.e., PBI 12–25K membrane) has a molecular weight cut-off (MWCO) as low as 288 Da and an MgCl2 rejection more than 97%. The membrane also displays pH-responsive characteristics for salt separation with unequal valence ratios of co-ions and counter-ions. It is able to show a competitive SLi,Mg value for Li+/Mg2+ separation surpassing the state-of-the-art performance of commercial membranes.

Synthesis and application of high-permeable zeolite MER membrane for separation of carbon dioxide from methane

In this work, high-permeable zeolite merlinoite (MER) membranes were synthesized for separation of CO2/CH4. The effects of synthesis parameters (temperature, time, and coating number) on permeance and selectivity of synthesized membranes were investigated. The results showed that coating repetition of zeolite layer strongly increased membrane selectivity while decreased permeability for both gasses. Also, it was found that zeolite layers were thickened and larger crystals were formed at higher synthesis temperatures and times. In this research, zeolite MER membranes with reversal tradeoff effect, i.e., simultaneously increased permeability and selectivity, were prepared. The best membrane synthesized in this work showed a CO2/CH4 selectivity of 20.1 and a CO2 permeability of 2.5 × 10−8 mol/m2/s/pa. Due to the upper Robeson’s bound for the separation of CO2/CH4, the selectivities and permeabilities obtained in the current study were related to the points over the upper bounds, while the performance of commercial membranes used to separate CO2 from CH4 reported in the literatures is enclosed in the area below the upper bound. High-permeable zeolite MER membranes with reasonable CO2/CH4 selectivity were successfully synthesized in this work. It found as good choices for the separation of CO2 from CH4 in large-scale separation applications.

Membrane-based technologies for biogas separations

Over the past two decades, membrane processes have gained a lot of attention for the separation of gases. They have been found to be very suitable for wide scale applications owing to their reasonable cost, good selectivity and easily engineered modules. This critical review primarily focuses on the various aspects of membrane processes related to the separation of biogas, more in specific CO(2) and H(2)S removal from CH(4) and H(2) streams. Considering the limitations of inorganic materials for membranes, the present review will only focus on work done with polymeric materials. An overview on the performance of commercial membranes and lab-made membranes highlighting the problems associated with their applications will be given first. The development studies carried out to enhance the performance of membranes for gas separation will be discussed in the subsequent section. This review has been broadly divided into three sections (i) performance of commercial polymeric membranes (ii) performance of lab-made polymeric membranes and (iii) performance of mixed matrix membranes (MMMs) for gas separations. It will include structural modifications at polymer level, polymer blending, as well as synthesis of mixed matrix membranes, for which addition of silane-coupling agents and selection of suitable fillers will receive special attention. Apart from an overview of the different membrane materials, the study will also highlight the effects of different operating conditions that eventually decide the performance and longevity of membrane applications in gas separations. The discussion will be largely restricted to the studies carried out on polyimide (PI), cellulose acetate (CA), polysulfone (PSf) and polydimethyl siloxane (PDMS) membranes, as these membrane materials have been most widely used for commercial applications. Finally, the most important strategies that would ensure new commercial applications will be discussed (156 references).

Relation between membrane characteristics and performance in nanofiltration

The performance of commercial membranes during nanofiltration of aqueous solutions containing dissolved uncharged or charged organic components, was studied on the basis of membrane characteristics by means of multiple linear regression. The membrane characteristics studied were surface hydrophobicity, surface roughness, surface charge, molecular weight cut-off (MWCO), permeability and porosity of the top layer (expressed as the volume fraction of small and large pores, determined by Positron Annihilation Lifetime Spectroscopy). Filtration and adsorption experiments were performed in the presence of various components, which differ in molecular mass, hydrophobicity and (in the case of charged organic components) in charge. It was concluded that in order to minimize fouling, the membrane should have a low volume fraction of small pores in the top layer. When the organic components are charged, a membrane with a large surface charge and a high hydrophilicity is also favourable. Not only the membrane, but also the feed characteristics have an influence on fouling: the best results during nanofiltration of dissolved uncharged or charged components were obtained with hydrophilic or negatively charged components, respectively. Dissolved organic components were the best retained by membranes with a low MWCO. In addition, uncharged organic components should be hydrophilic and small to obtain a high retention and minimal flux decline, while the interplay between membrane and component charge is crucial during filtration of dissolved charged organic components.