Browsing by Author "Brookes, A."

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    The role of solvent temperature and gas pressure on CO2 mass transfer during biogas upgrading within porous and dense-skin hollow fibre membrane contactors
    (Elsevier, 2023-08-02) Luqmani, Benjamin A. ; Brookes, A.; Moore, Andrew; Vale, Peter C. J.; Pidou, Marc; McAdam, Ewan
    Biogas upgrading uniquely requires pressurisation of hollow fibre membrane contactors (HFMC) to be competitive with classical water absorption, and when complemented with an ambient industrial temperature range, these conditions will determine CO2 mass transport phenomena that are distinct dependent upon whether microporous or nonporous membranes are used. This study therefore examines the independent and concomitant role of temperature and pressure in determining CO2 mass transport, and selectivity, within microporous and nonporous HFMC. At low solvent temperatures, higher CO2 flux was achieved which indicates that solvent solubility is more critical than CO2 diffusivity to enhancing mass transport. Low temperatures also favoured mass transfer within the microporous membrane, explained by the reduction in solvent vapour pressure which limited pore wetting by condensation. In contrast, the nonporous membrane exhibited poorer mass transfer at low temperatures due to a decline in dense polymer permeability. Crucially in this study, neither wetting of the microporous membrane or plasticisation of the nonporous membrane were observed following pressurisation. Consequently, CO2 flux increased in proportion to the applied pressure for both membrane types, emphasising the critical role of pressurisation in augmenting process intensification for biogas upgrading which is typically facilitated at pressures of 7–10 bar. Resistance-in-series analysis illustrated how pressurisation reduced gas-phase resistance, and subsequently enhanced selectivity. Consequently, an outlet gas quality of 98% methane could be achieved within a single microporous module at 4.5 bar, meeting the industrial standard for biomethane whilst reducing solvent requirements, separation energy and methane losses. Comparable behaviour was observed during pressurisation of the nonporous membrane, but with a less significant benefit to CO2 mass transfer and selectivity, ostensibly due to the resistance imparted by the dense polymer. When considered collectively, low solvent temperature and high gas pressure enhance process intensification subsequently reducing process size (e.g., membrane area) and separation energy, while also advancing selectivity to deliver a gas product at the composition required for biomethane with minimum methane losses, which are critical factors in demonstrating microporous HFMC as an industrially competitive solution for biogas upgrading.
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    Transitioning through the vapour-liquid equilibrium for low energy thermal stripping of ammonia from wastewater: enabling transformation of NH3 into a zero-carbon fuel
    (Elsevier, 2023-11-17) Luqmani, Benjamin A. ; Brookes, A.; Moore, Andrew; Vale, Peter C. J.; Pidou, Marc; McAdam, Ewan
    Vacuum thermal stripping permits the recovery of ammonia from wastewater in a concentrated form, which is key to its exploitation in the circular economy, but the latent heat demand for thermal separation remains a critical barrier to exploitation. In this study, we investigate the vapor-liquid equilibrium (VLE) for ammonia-water as a mechanism to enhance recovered ammonia quality and minimise the thermal energy required for ammonia separation. Below the dew point (65 °C at 0.25 bar) a two-phase region of the VLE exists where 48 %wt gas-phase ammonia could be produced (61 °C) compared to only 2 %wt within the stripping region adopted widely in the literature. This was complemented by a 98 % reduction in thermal separation energy, since limited water vaporization can occur when the feed is maintained below the activation energy threshold for bulk evaporation. Operation within this practically unexplored region of the ammonia-water VLE fosters a gas-phase product suitable for energy generation in gas turbines or solid oxide fuel cells. Comparable product quality was achieved using concentrated wastewater, which validated the VLE for design in the presence of a broad range of dissolved gases and volatile inorganic compounds. Rapid desorption of CO2 occurred during vacuum stripping, subsequently increasing pH >9 without the requirement for alkali addition to shift the ammonia-ammonium equilibrium in favor of gaseous ammonia. Consequently, the two-phase region of the VLE defined for vacuum thermal stripping provides a synergistic strategy to mitigate chemical demand, minimise separation energy and recover gas-phase ammonia for zero carbon energy generation, constituting a significant advancement toward the net zero ambitions of the water sector.