Toward gas-phase controlled mass transfer in micro-porous membrane contactors for recovery and concentration of dissolved methane in the gas phase

A micro-porous hollow fibre membrane contactor (HFMC) operated in sweep-gas mode has been studied to enable the recovery of dissolved methane from water in concentrated form. At high sweep-gas flow rates, up to 97% dissolved methane removal efficiency is achievable which is sufficient to achieve carbon neutrality (around 88%). An increase in methane composition of the recovered sweep-gas was achievable through two primary mechanisms: (i) an increase in liquid velocity which improved dissolved methane mass transfer into the gas phase; and (ii) a reduction in gas flow which lowered dilution from the receiving gas phase. It was posited that further refinement of the methane content was provided through counter-diffusion of the nitrogen sweep-gas into the liquid phase. Within the boundary conditions studied, the methane composition of the recovered gas phase exceeded the threshold for use in micro-turbines for electricity production. However, reducing the gas-to-liquid ratio to enhance gas phase methane purity introduced gas-phase controlled mass transfer which constrained removal efficiency. Whilst this reduction in removal efficiency can be compensated for by extending path length (i.e. more than one module in series), it is suggested that the gas-phase controlled conditions encountered were also a product of poor shell-side dispersion rather than an approach toward the limiting theoretical gas-to-liquid ratio. This implies that further optimisation can be ascertained through improved membrane contactor design. Importantly, this study demonstrates that micro-porous hollow fibre membrane contactors provide a compact process for recovery of dissolved methane in sufficient concentration for re-use

Biogas upgrading by chemical absorption using ammonia rich absorbents derived from wastewater

The use of ammonia (NH3) rich wastewaters as an ecological chemical absorption solvent for the selective extraction of carbon dioxide (CO2) during biogas upgrading to ‘biomethane’ has been studied. Aqueous ammonia absorbents of up to 10,000 gNH3 m−3 demonstrated CO2 absorption rates higher than recorded in the literature for packed columns using 20,000–80,000 g NH3 m−3 which can be ascribed to the process intensification provided by the hollow fibre membrane contactor used in this study to support absorption. Centrifuge return liquors (2325 g m−3 ionised ammonium, NH4+) and a regenerant (477 gNH4+ m−3) produced from a cationic ion exchanger used to harvest NH4+ from crude wastewater were also tested. Carbon dioxide fluxes measured for both wastewaters compared reasonably with analogue ammonia absorption solvents of equivalent NH3 concentration. Importantly, this demonstrates that ammonia rich wastewaters can facilitate chemically enhanced CO2 separation which eliminates the need for costly exogenic chemicals or complex chemical handling which are critical barriers to implementation of chemical absorption. When testing NH3 analogues, the potential to recover the reaction product ammonium bicarbonate (NH4HCO3) in crystalline form was also illustrated. This is significant as it suggests a new pathway for ammonia separation which avoids biological nitrification and produces ammonia stabilised into a commercially viable fertiliser (NH4HCO3). However, in real ammonia rich wastewaters, sodium bicarbonate and calcium carbonate were preferentially formed over NH4HCO3 although it is proposed that NH4HCO3 can be preferentially formed by manipulating both ion exchange and absorbent chemistry

Quantifying the loss of methane through secondary gas mass transport (or 'slip') from a micro-porous membrane contactor applied to biogas upgrading

Secondary gas transport during the separation of a binary gas with a micro-porous hollow fibre membrane contactor (HMFC) has been studied for biogas upgrading. In this application, the loss or ‘slip' of the secondary gas (methane) during separation is a known concern, specifically since methane possesses the intrinsic calorific value. Deionised (DI) water was initially used as the physical solvent. Under these conditions, carbon dioxide (CO2) and methane (CH4) absorption were dependent upon liquid velocity (VL). Whilst the highest CO2 flux was recorded at high VL, selectivity towards CO2 declined due to low residence times and a diminished gas-side partial pressure, and resulted in slip of approximately 5.2% of the inlet methane. Sodium hydroxide was subsequently used as a comparative chemical absorption solvent. Under these conditions, CO2 mass transfer increased by increasing gas velocity (VG) which is attributed to the excess of reactive hydroxide ions present in the solvent, and the fast conversion of dissolved CO2 to carbonate species reinitiating the concentration gradient at the gas-liquid interface. At high gas velocities, CH4 slip was reduced to 0.1% under chemical conditions. Methane slip is therefore dependent upon whether the process is gas phase or liquid phase controlled, since methane mass transport can be adequately described by Henry's law within both physical and chemical solvents. The addition of an electrolyte was found to further retard CH4 absorption via the salting out effect. However, their applicability to physical solvents is limited since electrolytic concentration similarly impinges upon the solvents' capacity for CO2. This study illustrates the significance of secondary gas mass transport, and furthermore demonstrates that gas-phase controlled systems are recommended where greater selectivity is required

Fate and impact of organics in an immersed membrane bioreactor applied to brine denitrification and ion exchange regeneration

The application of membrane bioreactors (MBRs) to brine denitrification for ion exchange regeneration has been studied. The developed culture was capable of complete brine denitrification at 50 gNaCl.l−1. Denitrification reduced to c.60% and c.70% when salinity was respectively increased to 75 and 100 g.l−1, presumed to be due to reduced growth rate and the low imposed solids retention time (10 days). Polysaccharide secretion was not induced by stressed cells following salt shocking, implying that cell lysis did not occur. Fouling propensity, monitored by critical flux, was steady at 12–15 l.m−2.h−1 during salinity shocking and after brine recirculation, indicating that the system was stable following perturbation. Low molecular weight polysaccharide physically adsorbed onto the nitrate selective anion exchange resin during regeneration reducing exchange capacity by c.6.5% when operating up to complete exhaustion. However, based on a breakthrough threshold of 10 mgNO3−-N.l−1 the exchange capacity was comparative to that determined when using freshly produced brine for regeneration. It was concluded that a denitrification MBR was an appropriate technology for IEX spent brine reco

Geomagnetic extreme statistics for Europe

Rapidly changing geomagnetic field variations constitute a natural hazard, for example to grounded power grids and pipeline networks. To understand this hazard we have continuous magnetic measurements across the world for typically less than 100 years. Much of the older data is also in analogue form, or is only available digitally as hourly or daily magnetic indices or mean levels. So it may not yet be clear what the true extremes in geomagnetic variation are, particularly on time scales - seconds to minutes - that are relevant for estimating the hazard to technological systems. We therefore use a number of decades of one minute samples of magnetic data from observatories in Europe, together with the technique of 'extreme value statistics', to explore estimated maxima in field variations in the horizontal strength and in the declination of the field. These maxima are expressed, for example, in terms of the variations that might be observed on time scales of 100 and 200 years. We also examine the extremes in the one-minute rate of change of these field components on similar time scales. The results should find application in hazard assessment and navigation applications

Investigating the significance of coagulation kinetics on maintaining membrane permeability in an MBR following reactive coagulant dosing

In this study, the impact of kinetically controlled floc growth on sustaining membrane permeability following reactive coagulant dosing was determined using a model particle system. Floc formation was indicated to comprise of two stages following coagulant addition: (i) an initial destabilisation phase which encouraged complexation of protein and polysaccharide; and (ii) entrapment of the coarse model particles (3 µm Firefli™ microspheres) in the polymeric complex during the floc growth phase. Floc growth was characterised by an expected time lag as with conventional flocculation systems and biopolymer aggregation was kinetically favoured. When coagulant was dosed during the filtration cycle, the intermediate biopolymer aggregates (comprised of protein and polysaccharide) were preferentially transported toward the membrane increasing fouling. However, when coagulant was dosed at the onset of filtration, membrane fouling was constrained. It is asserted that by dosing at the onset of filtration: (i) early development of biopolymer aggregation is initiated which inhibits transport of the individual biopolymers to the membrane; and (ii) by dosing coagulant in the absence of a developed polarised layer, formation of biopolymer complexes local to the membrane is obviated. However, when dosing coagulant at the onset of filtration, only limited floc growth occurred which can be explained by the low applied wall shear rate and the absence of a ‘polarised’ region which ostensibly promoted floc growth when coagulant was dosed mid-filtration. Based on results from the model particle system studied, it is proposed that reactive coagulant dosing is best undertaken when: (i) filtration is stopped; (ii) modest shear is applied within the bioreactor to promote coagulant dispersion; and (iii) sufficient contact time is allowed to promote floc growth before commencement of filtration

Transgender Healthcare Teaching in the Undergraduate Medical School Curriculum

With increasing recognition of the diverse and specific needs of transgender individuals in a health care setting, lack of knowledge, poor attitudes and prejudice towards transgender patients can result in this population being afraid to access medical care. Educating medical students early in their career in a sensitive and inclusive manner could help change these attitudes. It has been shown that medical undergraduates and post-graduates often feel unprepared or uncomfortable in caring for transgender patients due to lack of training and experience2-4. The aim of this study was to address this through introduction of basic transgender healthcare education into the University of Glasgow undergraduate medical curriculum, with the goal of implementing further interactive and fully inclusive teaching

Controlling shell-side crystal nucleation in a gas-liquid membrane contactor for simultaneous ammonium bicarbonate recovery and biogas upgrading

A gas–liquid hollow fibre membrane contactor (HFMC) process has been introduced for carbon dioxide (CO2) separation from biogas where aqueous ammonia (NH3) is used to chemically enhance CO2 absorption and initiate heterogeneous nucleation of the reaction product ammonium bicarbonate at the membrane–solvent interface. Aqueous ammonia absorbents (2–7 M) were initially used in single pass for CO2 separation from a synthetic biogas where nucleation of ammonium bicarbonate crystals was observed at the perimeter of the micropores. Recirculation of the aqueous ammonia absorbent encouraged the growth of ammonium bicarbonate crystals on the shell-side of the membrane that measured several microns in diameter. However, at high aqueous NH3 concentrations (3–7 M), lumen side crystallisation occurred and obstructed gas flow through the lumen of the HFMC. The suggested mechanism for lumen-side crystallisation was absorbent breakthrough into the lumen due to pore wetting which was promoted by low absorbent surface tension at high NH3 concentration. Preferential shell-side nucleation can therefore be promoted by (1) raising surface tension of the absorbent and (2) selection of a membrane with a more regulated pore shape than the PTFE membrane used (d/L 0.065) as both actions can diminish solvent ingress into the pore. This was evidenced using 2 M NH3 absorbent where shell-side crystallisation was evidenced without the onset of lumen side crystallisation. Raising surface tension through the inclusion of salt into the chemical absorbent also promoted greater CO2 flux stability. Importantly, this study demonstrates that chemically enhanced HFMC are an attractive prospect for gas–liquid separation applications where reaction product recovery offers further economic value

A theoretical model of a new electrostatic transducer incorporating fluidic amplification

This article concerns the design of a new electrostatic transducer whose backplate consists of a series of drilled pipes. A new one-dimensional model is derived which considers the interaction of the membrane with the air load, the air cavities, and the drilled pipes in the backplate. Dynamic equations for the impedance in each component of the device are calculated analytically and connected using interface conditions of continuity of pressure and radiation conditions into the air load. The model is able to produce solutions to the mechanical impedance of the device and the displacement of the membrane as a function of the device's design parameters. Model results for the output pressure compare well with previous experimental data. The inverse problem of retrieving the design parameters for a desired output is discussed