Browsing by Author "McLeod, Andrew J."

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    Biogas enhancement with membranes
    (Cranfield University, 2014-04) McLeod, Andrew J.; McAdam, Ewan; Jefferson, Bruce
    Biogas is generated during anaerobic digestion (AD) of sewage sludge at wastewater treatment works (WWTW) and consists of approximately 50-70 % methane (CH4) balanced primarily by carbon dioxide (CO2). It is commonly used directly as a fuel gas for the renewable generation of electricity on-site by combined heat and power (CHP) engines. However, as a result of governmental incentivisation, biogas possesses a greater value when applied to the national gas grid as a natural gas substitute. However, this requires enhancement of the CH4 content to that comparable to natural gas by selective removal of CO2; a process known as biogas upgrading. This thesis explores the potential of hydrophobic micro-porous hollow fibre membrane contactors (HFMCs) to biogas upgrading. HFMCs allow non-dispersive contact between the biogas and a liquid solvent for the preferential absorption of CO2, which is conventionally facilitated by packed-column gas scrubbing technology. However, recent gas absorption literature has demonstrated many practical and operational advantages of HFMCs, which suggests they may be effective for biogas upgrading at WWTW. In this thesis, HFMCs were used to explore the mechanism and controllability of the undesirable co-absorption of CH4, known as methane slip. This was found to be attributable to the phase limiting mass transfer, with liquid-limited physical absorption in water exhibited 5.2 % slip whereas gas-limited chemical absorption displayed just 0.1 %. Ammonia-rich wastewaters were investigated as sustainable chemical absorbents using HFMCs and exhibited comparable chemically enhanced absorption to analogue synthetic ammonia solutions. The recovery of the subsequent reaction product (ammonium bicarbonate) by crystallisation facilitated by the membrane was also examined. The potential of this approach was summarised within two hypothetical wastewater flowsheets, where upgrading using a return liquor absorbent acts as a return liquor treatment and where ion exchange allows 100 % application of wastewater derived ammonia to biogas upgrading. These both offered potential economic advantages versus conventional flowsheets with 100 % biogas application to CHP.
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    Biogas upgrading by chemical absorption using ammonia rich absorbents derived from wastewater
    (IWA Publishing, 2014-09-18) McLeod, Andrew J.; Jefferson, Bruce; McAdam, Ewan J.
    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.
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    Controlling shell-side crystal nucleation in a gas-liquid membrane contactor for simultaneous ammonium bicarbonate recovery and biogas upgrading
    (Elsevier, 2014-09-17) McLeod, Andrew J.; Autin, Olivier; Jefferson, Bruce; McAdam, Ewan J.
    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.
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    Dissolved methane recovery from anaerobic effluents using hollow fibre membrane contactors
    (Elsevier, 2015-12) Cookney, Joanna; McLeod, Andrew J.; Mathioudakis, Vasileios; Ncube, Philani; Soares, Ana; Jefferson, Bruce; McAdam, Ewan J.
    Hollow fibre membrane contactor (HFMC) systems have been studied for the desorption of dissolved methane from both analogue and real anaerobic effluents to ascertain process boundary conditions for separation. When using analogue effluents to establish baseline conditions, up to 98.9% methane removal was demonstrated. Elevated organic concentrations have been previously shown to promote micropore wetting. Consequently, for anaerobic effluent from an upflow anaerobic sludge blanket reactor, which was characterised by a high organic concentration, a nonporous HFMC was selected. Interestingly, mass transfer data from real effluent exceeded that produced with the analogue effluent and was ostensibly due to methane supersaturation of the anaerobic effluent which increased the concentration gradient yielding enhanced mass transfer. However, at high liquid velocities a palpable decline in removal efficiency was noted for the nonporous HFMC which was ascribed to the low permeability of the nonporous polymer provoking membrane controlled mass transfer. For anaerobic effluent from an anaerobic membrane bioreactor (MBR), a microporous HFMC was used as the permeate comprised only a low organic solute concentration. Mass transfer data compared similarly to that of an analogue which suggests that the low organic concentration in anaerobic MBR permeate does not promote pore wetting in microporous HFMC. Importantly, scale-up modelling of the mass transfer data evidenced that whilst dissolved methane is in dilute form, the revenue generated from the recovered methane is sufficient to offset operational and investment costs of a single stage recovery process, however, the economic return is diminished if discharge is to a closed conduit as this requires a multi-stage array to achieve the required dissolved methane consent of 0.14 mg l−1.
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    The effect of maintenance on the performance of sand-filled synthetic turf surfaces
    (2010-09-27T00:00:00Z) James, Iain T.; McLeod, Andrew J.
    The effect of infill quantity and contamination on the performance of second generation sand-filled synthetic turf sports surfaces was investigated in a laboratory study. Three 1m2 test surfaces were constructed by placing synthetic turf over a stone-tar-macadam-rubber shockpad sub-base. Ball rebound, ball roll, surface rebound hardness and rotational resistance of a dimpled rubber sole were measured for a range of infill quantities (0-35 kg/m2) and infill contamination concentrations (0, 10 and 20%). Increasing infill quantity increased hardness, reduced ball rebound and reduced rotational resistance linearly (p , 0.01). Ball deceleration increased up to 10 kg/m2 after which there was no further significant increase in the range tested. An optimum infill quantity of 25-30 kg/m2, based on performance characteristics and the length of fibre above the infill, was identified for the synthetic turf surface tested. Increasing contamination also increased ball deceleration and reduced infiltration rate and kept surfaces wetter for longer during drying (p , 0.001), resulting in conditions suitable for moss and algae formation. Maintenance, including regular brushing and monitoring of infill quantity, is required to ensure even distribution of the correct quantity of infill and the minimization of infill contamination in all infilled synthetic turf surfaces.
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    The management and maintenance of second generation sand-filled synthetic sports pitches
    (Cranfield University, 2008-01) McLeod, Andrew J.; James, Iain
    Synthetic sports surfaces have increased in popularity since their introduction into the United Kingdom in the early 1970's. In many sports, such as hockey and athletics, they have become the standard for play. The benefit of synthetic turf is commonly judged to be lower in maintenance requirements and operating costs, and having an increased quantity of play, when compared to natural turf. Synthetic turf has, historically, been perceived to be 'maintenance free' and there has been little or no research into the effect that maintenance has on its performance and physical characteristics. The aim of this thesis was to develop a fundamental understanding of the mechanical wear and decline in hydraulic performance of second generation synthetic turf surfaces, its impact on technical performance characteristics, and economic costs in relation to maintenance and usage.
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    Quantifying the loss of methane through secondary gas mass transport (or 'slip') from a micro-porous membrane contactor applied to biogas upgrading
    (IWA Publishing, 2013-04-29) McLeod, Andrew J.; Jefferson, Bruce; McAdam, Ewan J.
    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,
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    Toward gas-phase controlled mass transfer in micro-porous membrane contactors for recovery and concentration of dissolved methane in the gas phase
    (Elsevier, 2016-03-18) McLeod, Andrew J.; Jefferson, Bruce; McAdam, Ewan J.
    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.