Colin A. Scholes

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.

Crosslinked PEG and PEBAX Membranes for Concurrent Permeation of Water and Carbon Dioxide

Membrane technology can be used for both post combustion carbon dioxide capture and acidic gas sweetening and dehydration of natural gas. These processes are especially suited for polymeric membranes with polyether functionality, because of the high affinity of this species for both H2O and CO2. Here, both crosslinked polyethylene glycol diacrylate and a polyether-polyamide block copolymer (PEBAX 2533©) are studied for their ability to separate CO2 from CH4 and N2 under single and mixed gas conditions, for both dry and wet feeds, as well as when 500 ppm H2S is present. The solubility of gases within these polymers is shown to be better correlated with the Lennard Jones well depth than with critical temperature. Under dry mixed gas conditions, CO2 permeability is reduced compared to the single gas measurement because of competitive sorption from CH4 or N2. However, selectivity for CO2 is retained in both polymers. The presence of water in the feed is observed to swell the PEG membrane resulting in a significant increase in CO2 permeability relative to the dry gas scenario. Importantly, the selectivity is again retained under wet feed gas conditions. The presence of H2S is observed to only slightly reduce CO2 permeability through both membranes.

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.

Time-resolved evanescent wave-induced fluorescence anisotropy for the determination of molecular conformational changes of proteins at an interface

We have shown that the molecular conformation of a protein at an interface can be probed spatially using time-resolved evanescent wave-induced fluorescence spectroscopic (TREWIFS) techniques. Specifically, by varying the penetration depth of the evanescent field, variable-angle TREWIFS, coupled with variable-angle evanescent wave-induced time-resolved fluorescence anisotropy measurements, allow us to monitor how fluorescence intensity and fluorescence depolarization vary normal to an interface as a function of time after excitation. We have applied this technique to the study of bovine serum albumin (BSA) complexed noncovalently with the fluorophore 1-anilinonaphthalene-8-sulfonic acid. The fluorescence decay varies as a function of the penetration depth of the evanescent wave in a manner that indicates a gradient of hydrophobicity through the adsorbed protein, normal to the interface. Restriction of the fluorescent probe’s motion also occurs as a function of distance normal to the interface. The results are consistent with a model of partial protein denaturation: at the surface, an adsorbed BSA molecule unfolds, thus optimizing protein–silica interactions and the number of points of attachment to the surface. Further away, normal to the surface, the protein molecule maintains its coiled structure.

Electrodialysis in Aqueous-Organic Mixtures

This review investigates the effects of hydro-organic solvents on ion exchange membranes used in conventional electrodialysis. The thermodynamics of electrodialysis is first presented in relation to operation in purely aqueous solutions, where the Donnan potential describes the equilibrium partitioning at the membrane/solvent interface. The mass transfer kinetics through the membrane are described using the Nernst–Planck equation, and concentration polarization describes the mass transfer resistance in the solution boundary layer. Each of these relationships is found to change significantly as the organic concentration in the solvent is increased and the system consequently deviates from ideality. The extent of membrane swelling in these mixed solvents is also critical, as it determines the diffusion coefficient of both ionic and non-ionic species within the membrane structure.

Time-Resolved Evanescent Wave-Induced Fluorescence Anisotropy Measurements

The technique of time-resolved evanescent wave-induced fluorescence spectroscopy (TR-EWIFS) has been extended to incorporate fluorescence anisotropy measurements. We report on the application of these time-resolved evanescent wave-induced fluorescence anisotropy measurements (EW-TRAMs) as a source of information concerning the motion and conformational changes of macromolecules that can occur near a solid/liquid interface. We have applied EW-TRAMs to studies of the adsorption of proteins and polymers onto silica from solution. We also discuss the implications and potential complications of the polarisation properties of the standing evanescent field and how these properties can be used to advantage

Review of Membranes for Helium Separation and Purification

Membrane gas separation has potential for the recovery and purification of helium, because the majority of membranes have selectivity for helium. This review reports on the current state of the research and patent literature for membranes undertaking helium separation. This includes direct recovery from natural gas, as an ancillary stage in natural gas processing, as well as niche applications where helium recycling has potential. A review of the available polymeric and inorganic membranes for helium separation is provided. Commercial gas separation membranes in comparable gas industries are discussed in terms of their potential in helium separation. Also presented are the various membrane process designs patented for the recovery and purification of helium from various sources, as these demonstrate that it is viable to separate helium through currently available polymeric membranes. This review places a particular focus on those processes where membranes are combined in series with another separation technology, commonly pressure swing adsorption. These combined processes have the most potential for membranes to produce a high purity helium product. The review demonstrates that membrane gas separation is technically feasible for helium recovery and purification, though membranes are currently only applied in niche applications focused on reusing helium rather than separation from natural sources.

Simulations of Membrane Gas Separation: Chemical Solvent Absorption Hybrid Plants for Pre- and Post-Combustion Carbon Capture

Solvent absorption and membrane gas separation are two carbon capture technologies that show great potential for reducing emissions from stationary sources such as power plants. Here, plants combining chemical solvent absorption and membrane gas separation are considered for post-combustion capture as well as pre-combustion capture. In all ASPEN HYSYS simulations the membrane stage initially concentrates CO2 into either the permeate or the retentate stream, which is then passed to a monoethanolamine (MEA) based solvent absorption process. In particular, post-combustion capture scenarios examined a membrane that is selective for CO2 against N2, while for the pre-combustion scenario a H2-selective membrane was studied. It was found the energy demand of the combined hybrid plant was always more than that of a stand alone MEA solvent process. This was mainly due to the need to generate a pressure driving force upstream of the membrane in the post-combustion scenario or to recompress downstream gas streams in the pre-combustion scenarios. For both scenarios concentrating the CO2 in the feed to the solvent system reduced the absorber column height and diameter, which could represent a CAPEX saving for the hybrid plant, dependent upon the membrane price. The use of a hydrogen selective membrane downstream of an oxygen fired gasifier was identified as the most prospective scenario, as it led to significant reductions in absorber size, for a relatively small membrane area and energy penalty.

Thermally Rearranged Poly(benzoxazole) Copolymer Membranes for Improved Gas Separation: A Review

Polymeric membranes for gas separation have application in a wide range of industries such as natural gas sweetening and air enrichment. Recently, high-performance gas separation polymeric membranes have been developed based on a novel thermal rearrangement process that produces the resistant poly(benzoxazole) (TR-PBO). This review reports on the current state of the art TR-PBO membranes for gas separation and the underlying chemistry needed to achieve such high separation performance. Particular focus is applied to copolymers based on TR-PBO for membranes as these have attracted considerable research interest recently for their gas separation performance and superior mechanical properties compared with TR-PBO. Also included in this review is a discussion of the future directions of research on TR-PBO-based membranes for gas separation.