Browsing by Author "Hermassi, Mehrez"

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    Integrating crystallisation into transmembrane chemical absorption: Process intensification for ammonia separation from anaerobic digestate
    (Elsevier, 2020-05-15) Davey, Christopher J.; Hermassi, Mehrez; Allard, E.; Amine, M.; Sweet, N.; Schmieder Gaite, T.; McLeod, A.; McAdam, Ewan J.
    In this study, reactive crystallisation is introduced into a liquid-liquid membrane contactor for the selective separation, purification and recovery of ammonia from concentrated waste. Whilst liquid-liquid membrane contactor technology has been previously demonstrated for ammonia absorption, further process intensification can be achieved by incorporating crystallisation into transmembrane chemisorption to recover the ammonia as crystalline ammonium sulphate. Reactive crystallisation occurred in the draw solution (sulphuric acid) which was supplied to the lumen-side of the polypropylene hollow-fibre. The ammonium sulphate concentration in the draw solution increased through ammonia mass transfer to supersaturation, at which time induction (the onset of nucleation) commenced. Ammonia mass transfer at draw concentrations above the solubility limit was not limited provided sufficient ‘free’ sulphate was available. This resulted in nucleation which occurred at a low level of supersaturation (C/C*, 1.03) to produce small crystals of around 2.5 μm, which indicated that nucleation was favoured. The nucleation rate was found to be proportional to the ammonia flux in the draw solution. As the solution became more saturated, crystal number increased but crystal growth was comparatively small; this is symptomatic of reactive crystallisation, where the rate of reaction exceeds the rate of mass transfer. Due to the large difference in the ratio between the lumen internal diameter and the mean crystal diameter (dfibre/dmean,CSD, ∼180), no fibre clogging was observed despite facilitating crystallisation on the lumen-side of the membrane. Transmembrane chemisorption crystallisation presents a feasible process intensification for the selective separation of ammonia from environmental applications. For its integration into environmental applications, solutions to wetting and fouling remain due to associative interactions with the complex organic matrix that are practically achievable through engineering intervention. Subsequent transformation of ammonia into a crystalline phase of ammonium sulphate presents a new product which is of commercial interest.
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    Is chemically reactive membrane crystallisation faciliated by heterogeneous primary nucleation? Comparison with conventional gas-liquid crystallisation for ammonium bicarbonate precipitation in a CO2-NH3-H2O system
    (American Chemical Society, 2020-01-27) Bavarella, Salvatore; Hermassi, Mehrez; Brookes, Adam; Moore, Andrew; Vale, Peter C. J.; Di Profio, Gianluca; Curcio, Efrem; Hart, Phil; Pidou, Marc; McAdam, Ewan
    In this study, membrane crystallisation is compared to conventional gas-liquid crystallisation for the precipitation of ammonium bicarbonate, to demonstrate the distinction in kinetic trajectory and illustrate the inherent advantage of phase separation introduced by the membrane to crystallising in gas-liquid systems. Through complete mixing of gas and liquid phases in conventional crystallisation, high particle numbers were confirmed at low levels of supersaturation. This was best described by secondary nucleation effects in analogy to mixed suspension mixed product removal (MSMPR) crystallisation, for which a decline in population density was observed with an increase in crystal size. In contrast, for membrane crystallisation, fewer nuclei were produced at an equivalent level of supersaturation. This supported growth of fewer, larger crystals which is preferred to simplify product recovery and limit occlusions. Whilst continued crystal growth was identified with the membrane, this was accompanied by an increase in nucleation rate which would indicate the segregation of heterogeneous primary nucleation from crystal growth, and was confirmed by experimental derivation of the interfacial energy for ammonium bicarbonate (σ, 6.6 mJ m-2), which is in agreement to that estimated for inorganic salts. The distinction in kinetic trajectory can be ascribed to the unique phase separation provided by the membrane which promotes a counter diffusional chemical reaction to develop, introducing a region of concentration adjacent to the membrane. The membrane also lowers the activation energy required to initiate nucleation in an unseeded solution. In conventional crystallisation, the high nucleation rate was due to the higher probability for collision, and the gas stripping of ammonia (around 40% loss) through direct contact between phases which lowered pH and increased bicarbonate availability for the earlier onset of nucleation. It is this high nucleation rate which has restricted the implementation of gas-liquid crystallisation in direct contact packed columns for carbon capture and storage. Importantly, this study evidences the significance of the membrane to governing crystallisation for gas-liquid chemical reactions through providing controlled phase separation.
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    Recovery and concentration of ammonia from return liquor to promote enhanced CO2 absorption and simultaneous ammonium bicarbonate crystallisation during biogas upgrading in a hollow fibre membrane contactor
    (Elsevier, 2020-01-27) Bavarella, Salvatore; Hermassi, Mehrez; Brookes, Adam; Moore, Andrew; Vale, Peter C. J.; Moon, I. S.; Pidou, Marc; McAdam, Ewan
    In this study, thermal desorption was developed to separate and concentrate ammonia from return liquor, for use as a chemical absorbent in biogas upgrading, providing process intensification and the production of crystalline ammonium bicarbonate as the final reaction product. Applying modest temperature (50°C) in thermal desorption suppressed water vapour pressure and increased selective transport for ammonia from return liquor (0.11MNH3) yielding a concentrated condensate (up to 1.7MNH3). Rectification was modelled through second-stage thermal processing, where higher initial ammonia concentration from the first stage increased mass transfer and delivered a saturated ammonia solution (6.4MNH3), which was sufficient to provide chemically enhanced CO2 separation and the simultaneous initiation of ammonium bicarbonate crystallisation, in a hollow fibre membrane contactor. Condensate recovered from return liquor exhibited a reduction in surface tension. We propose this is due to the stratification of surface active agents at the air-liquid interface during primary-stage thermal desorption which carried over into the condensate, ‘salting’ out CO2 and lowering the kinetic trajectory of absorption. However, crystal induction (the onset of nucleation) was comparable in both synthetic and thermally recovered condensates, indicating the thermodynamics of crystallisation to be unaffected by the recovered condensate. The membrane was evidenced to promote heterogeneous primary nucleation, and the reduction in the recovered condensate surface tension was shown to exacerbate nucleation rate, due to the reduction in activation energy. X-ray diffraction of the crystals formed, showed the product to be ammonium bicarbonate, demonstrating that thermal desorption eliminates cation competition (e.g. Ca2+) to guarantee the formation of the preferred crystalline reaction product. This study identifies an important synergy between thermal desorption and membrane contactor technology that delivers biogas upgrading, ammonia removal from wastewater and resource recovery in a complimentary process.