Jizhong Ren

Development of asymmetric 6FDA-2,6 DAT hollow fiber membranes for CO2/CH4 separation: 1. The influence of dope composition and rheology on membrane morphology and separation performance

We have determined the effects of dope composition, rheology and process conditions on membrane morphology and gas separation performance of asymmetric poly(2,6-toluene-2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane diimide) (6FDA-2,6 DAT) hollow fiber membranes for air and CO2/CH4 applications. Two spinning dope systems were prepared; one is a binary system in N-methyl-2-pyrrolidone (NMP), the other is a ternary system in a mixture of NMP and ethanol (EtOH). Both dopes exhibit non-Newtonian and shear-thinning power-law characteristics. Experimental data show that the shear stress within the spinneret significantly changes the skin and cross-section morphology and separation performance. The degree of changes is more severe in a highly viscous binary system than in an EtOH containing ternary system. For the 6FDA-2,6 DAT/NMP system, both permeances of O2, N2, CO2 and CH4 and selectivities of O2/N2 and CO2/CH4 decrease slightly with an increase in shear rate. While for the 6FDA-2,6 DAT/NMP/EtOH system, similar to the previous case, permeances of all testing gases decrease with increasing shear rate because of shear-induced orientation. However, very interestingly and contradictory to the binary dope system, the CO2/CH4 selectivity increases with increasing shear rate. The difference in selectivity trend is probably due to the fact that the ternary system has EtOH as a non-solvent additive which shortens the precipitation path and reduces the degree of vigorousness during the phase inversion process. Scanning electron microscopy (SEM) pictures show fibers spun from the viscous binary system tend to have more shear-induced defects in high shear rates. In addition, the elongational stress induced by the take up unit has a tendency to induce molecular orientation as well as to create skin surface defects.

Poly(amide-6-b-ethylene oxide)/SAPO-34 mixed matrix membrane for CO2 separation

Abstract In this paper, poly(amide-6-b-ethylene oxide) (Pebax1657)/SAPO-34 mixed matrix membranes (MMMs) were prepared by solvent-evaporation method with acetic acid as a novel solvent. CO2, N2, CH4 and H2 permeation properties were investigated, and the physical properties of Pebax/SAPO-34 MMMs were characterized by XRD and SEM. At low SAPO-34 content, it was homogeneously distributed in the Pebax matrix, and then precipitated and agglomerated at high SAPO-34 content. The crystallinity of Pebax phase in Pebax/SAPO-34 MMMs decreased initially and then rebounded as a result of phase separation. With the increase of transmembrane pressure difference, CO2 permeability was enhanced due to the effect of pressure-induced plasticization. Owing to the happening of stratification, the CO2 permeability of Pebax/SAPO-34 MMMs (50 wt% SAPO-34) increased to 338 Barrer from 111 Barrer of pristine Pebax, while the selectivities of CO2/CH4 and CO2/N2 were almost unchanged. Compared with the pristine Pebax, the gas separation performances of Pebax/SAPO-34 MMMs were remarkably enhanced.

The coupling effect of the thermodynamic swelling process in pervaporation

In order to analyze the coupling effect in thermodynamic swelling process, the relationship between the activity and volume fraction of each component within membrane was defined as αmi=γmiφmi. The concept of reference state in thermodynamic swelling process which only considered the interaction between individual component and membrane matrix was also set up. The difference of activity coefficient of each component within the membrane from reference state reflected the contribution of coupling effect to thermodynamic swelling process. For chitosan and H2SO4 crosslinking chitosan membrane the influence of coupling effect on selectivity of H2O in thermodynamic swelling process could be described from distribution coefficients and competition distribution coefficients of H2O and EtOH successfully. The interaction between H2O and the membrane matrix dominated the activity coefficient of H2O within the membrane, and the coupling effect between H2O and EtOH within the membrane only had a little influence on the activity coefficient of H2O. However, the activity coefficient of EtOH was dominated strongly by the coupling effect between H2O and EtOH within the membrane. This was attributed to the large amount of H2O present in the membrane. The distribution coefficient of EtOH, km2 and competition distribution coefficient of membrane km12 directly determined the selectivity of H2O in thermodynamic swelling process and the coupling effect between H2O and EtOH within the membrane had a negative influence on the selectivity of H2O within the membrane.

Preparation of Polymeric Membranes

This chapter mainly describes the principles of membrane formation process for polymeric membranes. With a brief introduction of relevant background information such as various membranes and membrane processes, a comprehensive list of polymer materials, which are suitable for making membranes, has been given. The most common technique used to prepare polymeric membranes – phase inversion process, including thermally induced phase separation (TIPS) and diffusion induced phase separation (DIPS), is discussed in detail. The thermodynamic behavior of the casting polymer solution, the process of membrane formation, and the fabrication of hollow fiber and flat sheet membranes are involved.

Polyvinyl acetate/poly(amide-12-b-ethylene oxide) blend membranes for carbon dioxide separation

Abstract In this paper, blend membranes from polyvinyl acetate (PVAc) and block copolymer poly(amide-12-b-ethylene oxide) (Pebax1074) are prepared by solution casting and solvent evaporation method. Although they are homogeneous on a macro-scale, the observations from DSC and SEM indicate micro-phase separation for PVAc/Pebax1074 blend membranes. With the increase of Pebax1074 content, gas permeabilities of CO 2 , H 2 , N 2 and CH 4 all increase greatly. PVAc/Pebax1074 blend membranes with high PVAc content are appropriate for CO 2 /CH 4 separation. The temperature dependence of gas permeability is divided into rubbery region and glassy region. The activation energies of permeation in rubbery region are smaller than those in glassy region, and they all decrease with increasing Pebax1074 content. For N 2 , H 2 and CH 4 , their gas permeation properties are mainly influenced by the dual-mode sorption and hydrostatic pressure effect. But for CO 2 , its permeability increases with the increase of pressure due to CO 2 -induced plasticization effect, which is more obvious for PVAc/Pebax1074 blend membranes with high PVAc content.

Permeation Characteristics of Light Hydrocarbons Through Poly(amide-6-β-ethylene oxide) Multilayer Composite Membranes

Abstract In this paper, poly(amide-6-β-ethylene oxide) (PEBA1657) copolymer was used to prepare multilayer polyetherimide (PEI)/polydimethylsiloxane (PDMS)/PEBA1657/PDMS composite membranes by dip-coating method. Permeation behaviors of ethylene, ethane, propylene, propane, n -butane, methane and nitrogen through the multilayer composite membranes were investigated over a range of operating temperature and pressure. The permeances of light hydrocarbons through PEI/PDMS/PEBA1657/PDMS composite membranes increase with their increasing condensability, and the olefins are more permeable than their corresponding paraffins. For light hydrocarbons, the gas permeances increase significantly as temperature increasing. When the transmembrane pressure difference increases, the gas permeance increases moderately due to plasticization effect, while their apparent activation energies for permeation decrease.

Effect of Hydrogen Reduction of Silver Ions on the Performance and Structure of New Solid Polymer Electrolyte PEI/Pebax2533/AgBF4 Composite Membranes

Abstract In this paper, the effect of hydrogen reduction of silver ions on the performance and structure of new solid polymer electrolyte polyetherimide (PEI)/Pebax2533 (Polynylon12/tetramethylene oxide block copolymer, PA12-PTMO)/AgBF 4 composite membranes is investigated. For PEI/Pebax2533/AgBF 4 composite membranes prepared with different AgBF 4 concentration, the permeances of propylene and ethylene increase with the increase of AgBF 4 concentration due to the carrier-facilitated transport, resulting in a high selectivity. But for propylene/propane mixture, the mixed-gas selectivity is lower than its ideal selectivity. The hydrogen reduction strongly influences the membrane performance, which causes the decrease of propylene permeance and the increase of propane permeance. With the increase of hydrogen reduction time, the membranes show a clearly color change from white to brown, yielding a great selectivity loss. The data of X-ray diffraction and FT-IR prove that silver ions are reduced to Ag 0 after hydrogen reduction, and aggregated on the surface of PEI/Pebax2533/AgBF 4 composite membranes.