Geoff W. Stevens

Carbon Dioxide Separation through Polymeric Membrane Systems for Flue Gas Applications

The capture and storage of carbon dioxide has been identified as one potential solution to greenhouse gas driven climate change. Efficient separation technologies are required for removal of carbon dioxide from flue gas streams to allow this solution to be widely implemented. A developing technology is membrane gas separation, which is more compact, energy efficient and possibly more economical than mature technologies, such as solvent absorption. This review examines the recent patented developments in polymeric based membranes designed for carbon dioxide separation from mixed-gas systems. Initially, the background to polymeric membrane separation is provided, with an overview of past polymeric designs. This is followed by a discussion on the current state of the art; in particular developments in mixed matrix polymeric membranes and facilitated transport polymeric membranes for improved carbon dioxide permeation and selectivity. Recent developments in other membrane types, carbon and inorganic, are reviewed for comparison purposes with polymeric developments. Finally, a brief comment on the future directions of polymeric membrane gas separation technologies is provided.

Effects of Minor Components in Carbon Dioxide Capture Using Polymeric Gas Separation Membranes

Abstract: The capture of carbon dioxide by membrane gas separation has been identified as one potential solution to reduce greenhouse gas emissions. In particular, the application of membranes to CO2 capture from both pre‐ and post‐combustion strategies is of interest. For membrane technology to become commercially viable in CO2 capture, a number of factors need to be overcome, one being the role of minor components in the process on membrane performance. This review considers the effects of minor components in both pre‐ and post‐combustion use of polymeric membranes for CO2 capture. In particular, gases such as SOx, NOx, CO, H2S, NH3, as well as condensable water and hydrocarbons are reviewed in terms of their permeability through polymeric membranes relative to CO2, as well as their plasticization and aging effects on membrane separation performance. A major conclusion of the review is that while many minor components can affect performance both through competitive sorption and plasticization, much remains unknown. This limits the selection process for membranes in this application.

Ultrathin chitosan-poly(ethylene glycol) hydrogel films for corneal tissue engineering

Due to the high demand for donor corneas and their low supply, autologous corneal endothelial cell (CEC) culture and transplantation for treatment of corneal endothelial dysfunction would be highly desirable. Many studies have shown the possibility of culturing CECs in vitro, but lack potential robust substrates for transplantation into the cornea. In this study, we investigate the properties of novel ultrathin chitosan-poly(ethylene glycol) (PEG) hydrogel films (CPHFs) for corneal tissue engineering applications. Cross-linking of chitosan films with diepoxy-PEG and cystamine was employed to prepare ~50 μm (hydrated) hydrogel films. Through variation of the PEG content (1.5-5.9 wt.%) it was possible to tailor the CPHFs to have tensile strains and ultimate stresses identical to or greater than those of human corneal tissue while retaining similar tensile moduli. Light transmission measurements in the visible spectrum (400-700 nm) revealed that the films were >95% optically transparent, above that of the human cornea (maximum ~90%), whilst in vitro degradation studies with lysozyme revealed that the CPHFs maintained the biodegradable characteristics of chitosan. Cell culture studies demonstrated the ability of the CPHFs to support the attachment and proliferation of sheep CECs. Ex vivo surgical trials on ovine eyes demonstrated that the CPHFs displayed excellent characteristics for physical manipulation and implantation purposes. The ultrathin CPHFs display desirable mechanical, optical and degradation properties whilst allowing attachment and proliferation of ovine CECs, and as such are attractive candidates for the regeneration and transplantation of CECs, as well as other corneal tissue engineering applications.

Facile Pretreatment of Bacillus circulans β-Galactosidase Increases the Yield of Galactosyl Oligosaccharides in Milk and Lactose Reaction Systems

The commercially available preparation of beta-galactosidase from Bacillus circulans , known as Biolacta FN5, has been extensively used in the production of prebiotic galactooligosaccharides (GOS). This study focuses on characterizing the production of GOS in two reaction systems: 10% lactose (w/v) in buffer and skim milk. Analysis of the temperature dependence of the GOS yield along with the relative rates of GOS synthesis and degradation leads to the finding that GOS degradation activity was selectively decreased in Biolacta FN5 above 40 degrees C. Facile heat treatment of Biolacta FN5 solution prior to use allowed for GOS yields to be significantly increased in both reaction systems.

Effect of the Substrate Concentration and Water Activity on the Yield and Rate of the Transfer Reaction of β-Galactosidase from Bacillus circulans

Prebiotic galactosyl oligosaccharides (GOS) are produced from lactose by the enzyme β-galactosidase. It is widely reported that the highest GOS levels are achieved when the initial lactose concentration is as high as possible; however, little evidence has been presented to explain this phenomenon. Using a system composed of the commercial β-galactosidase derived from Bacillus circulans known as Biolacta FN5, lactose and sucrose, the relative contribution of water activity, and substrate availability were assessed. Oligosaccharide levels did not appear to be affected by changes in water activity between 1.0 and 0.77 at a constant lactose concentration. The maximum oligosaccharide concentration increased at higher initial concentrations of lactose and sucrose, while initial reaction rates for transfer increased but remained constant for hydrolysis. This suggests that the high oligosaccharide levels achieved at the raised initial saccharide concentration are due to increases in reactions that form oligosaccharides rather than decreases in concurrent reactions, which degrade oligosaccharides. There were different effects from changing the initial concentration of lactose compared to sucrose, suggesting that the ability of lactose to act as a donor saccharide may be more important for increasing maximum oligosaccharide concentrations than the combined ability of both saccharides to act as galactosyl acceptors.

Shell-Side Mass-Transfer Performance in Hollow-Fiber Membrane Contactors

Abstract The accurate prediction of hollow-fiber membrane performance is limited by models of the shell-side mass-transfer coefficient. A review of the literature suggests that there have been in excess of 30 correlations for this coefficient. This article investigates these correlations in parallel-flow randomly-packed modules and cross-flow hollow-fiber membrane contactors. In particular, the variation of the Sherwood number on the shell side against the Reynolds number and packing density is considered. Shell-side mass-transfer performance is also investigated experimentally in a 10% (v/v) tributyl phosphate (TBP) in Shellsol2046/phenol/water system in four membrane modules. Data are compared with existing model predictions. The predicted values from four correlations agreed well with the observed data in randomly-packed hollow-fiber modules. In the commercial modules, experimental results varied greatly as a function of the Reynolds number. The use of a more consistent approach for the calculation of the linear fluid velocity on the shell side allowed the development of a generalized correlation, Shs = 0.055Re0.72 Sc0.33 , (R 2 = 0.77), that can predict the shell-side mass-transfer coefficient for both handmade and commercial membrane modules.

A Study of the Mass Transfer of CO2 through Different Membrane Materials in the Membrane Gas Absorption Process

Abstract The mass transfer of carbon dioxide through hydrophobic membrane materials into aqueous solutions of monoethanolamine has been studied. Microporous polypropylene, polytetrafluoroethylene and polyvinylidene fluoride hollow fiber membranes were compared. Membranes were characterized before and after use and wetting studies showed that the mass transfer resistance increased by 15% for polypropylene after 45 hours. Wetting may be due to membrane degradation as a result of contact with the solvent. This study highlights the need to choose membrane‐solvent systems that utilize a low cost membrane that remains unwetted by the solvent over long periods and when subjected to reasonable solvent‐side pressures.

Recovery of sulfuric acid from copper tank house electrolyte bleeds

Many hydrometallurgical processes produce large amounts of acid waste. For example, copper SX/EW process plants typically bleed a concentrated sulphuric acid stream (electrolyte) from their tank house in order to limit the build up of impurities in the electrowinning stage. A process based on solvent extraction, to selectively recover up to 90% of the sulphuric acid from electrolyte bleed streams, has been developed. This is recovered as a pure aqueous acid stream at a concentration of up to 130 g/L that can be recycled back into the tank house circuit thus reducing both neutralisation and acid make up costs. The acid concentration of the electrolyte is reduced from 180 g/L to as low as 18 g/L. The extraction system involves the use of a branched long chain aliphatic tertiary amine tris(2-ethylhexyl)amine (TEHA) as the extractant, Shellsol 2046 as the diluent and octanol as a modifier. The equilibrium data and simulation results were also compared with an alternative extractant, CYANEX 923.

The use of pulsed perforated plate extraction column for recovery of sulphuric acid from copper tank house electrolyte bleeds

In this investigation the use of a pulsed perforated plate extraction column is considered for the recovery of sulphuric acid from copper tank house electrolyte bleed using a solvent extraction process based on tris(2-ethylhexyl)amine, TEHA. The hydrodynamic and mass transfer performance of the extraction column is presented and forms the basis for scale up. The results of the pilot plant experiments have demonstrated the viability of operating the acid recovery process using a pulsed perforated plate column.

Membrane Gas-Solvent Contactor Pilot Plant Trials of CO2 Absorption from Flue Gas

Membrane gas-solvent contactors have received much attention for CO2 absorption, as the approach incorporates advantages from both solvent absorption and membrane gas separation. This study reports on pilot plant trials of three membrane contactors for the separation of CO2 from flue gas. The contactors were porous polypropylene (PP), porous polytetrafluoroethylene (PTFE), and non-porous polydimethylsiloxane (PDMS), with the solvent PuraTreatTM FTM. To enable performance comparison, laboratory measurements based on a gas mixture of 10% CO2 in N2 were also undertaken on the same contactor–solvent systems. It was found that the PP contactor experienced significant pore wetting in both laboratory and pilot plant studies. In contrast, the PTFE contactor experienced only minor pore wetting in the laboratory. However, in the pilot plant trial of the PTFE contactor extensive pore wetting was observed, and the overall mass transfer coefficient measured was comparable with the PP contactor. The non-porous PDMS contactor had an overall mass transfer coefficient two orders of magnitude less than the PP contactor, due to the greater mass transfer resistance of the polymeric film. However, the non-porous membrane does not experience pore wetting, which resulted in the overall mass transfer coefficient being similar for both laboratory and pilot plant measurements.