Chemically reactive membrane crystallisation reactor for CO₂ separation and ammonia recovery.

This thesis introduces an integrated system comprised of a thermal stripper and a hollow fibre membrane contactor (HFMC) for concentration of ammonia (NH₃ ) from wastewater and control of chemically mediated membrane crystallisation of ammonium bicarbonate (NH₄HCO₃ ) to enable simultaneous ammonia removal, biogas upgrading (through carbon dioxide, CO₂, separation), fertilizer production and harvesting within a single and economical process. In particular, recirculation of a refrigerated aqueous ammonia absorbent within the chemically reactive membrane crystallisation reactor (CR-MCr), demonstrated to reduce free ammonia introduction into the gas phase and convert NH₃ into non-volatile ammonium (NH₄+), thus preventing gas side crystallisation, which leads to process blockage, and promoting liquid side crystallisation of NH₄HCO₃ . The thermodynamic and kinetics of the CO₂-NH₃ -H₂O system have also been investigated to facilitate shell-side (liquid side) crystallisation of the ammonium salt within the CR-MCr. A transition from large (PTFE) to tight (PP) membrane pore size material obviated wetting and enabled consistent and reproducible NH₄HCO₃ crystallisation on the membrane-liquid interface. The X-ray diffraction analysis of the crystals produced with the absorbent recovered from return liquor, indicated the products to be reasonably pure ammonium bicarbonate, which evidenced the reduction in cationic competition through application of pre-treatment. A comparison between batch and membrane crystallisation kinetics demonstrated the hydrophobic fibre to underpin primary heterogeneous nucleation in an unseeded supersaturated solution and laminar regime, decoupled from secondary nucleation and growth, which mainly occur in the bulk downstream, contrarily to batch crystallisation where primary and secondary homogeneous nucleation are followed by growth and agglomeration, promoted by enhanced mixing and CO₂ bubbling, within the same environment. As a result, an increasing population density at raising levels of supersaturation has been observed in the first case, against a declining population density vs. supersaturation in the latter. A slower pH transient in membrane crystallisation, compared with conventional batch operation, could be balanced by raising the membrane-liquid interfacial surface area, which would increase the nucleation rate, whilst the yield of ammonia removal could be maximised up to 99% (ammonium bicarbonate solubility limit) through an increase in absorbent pH, which would eliminate the partial conversion of solute (bicarbonate) into carbonic acid, caused by a dynamic reaction zone, therefore closing the gap to control nucleation and growth in membrane crystallisation of ammonium bicarbonate.