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.PhD in Water including Desig

Chemically reactive membrane crystallisation reactor for CO2–NH3 absorption and ammonium bicarbonate crystallisation: Kinetics of heterogeneous crystal growth

The feasibility of gas-liquid hollow fibre membrane contactors for the chemical absorption of carbon dioxide (CO2) into ammonia (NH3), coupled with the crystallisation of ammonium bicarbonate has been demonstrated. In this study, the mechanism of chemically facilitated heterogeneous membrane crystallisation is described, and the solution chemistry required to initiate nucleation elucidated. Induction time for nucleation was dependent on the rate of CO2 absorption, as this governed solution bicarbonate concentration. However, for low NH3 solution concentrations, a reduction in pH was observed with progressive CO2 absorption which shifted equilibria toward ammonium and carbonic acid, inhibiting both absorption and nucleation. An excess of free NH3 buffered pH suitably to balance equilibria to the onset of supersaturation, which ensured sufficient bicarbonate availability to initiate nucleation. Following induction at a supersaturation level of 1.7 (3.3 M NH3), an increase in crystal population density and crystal size was observed at progressive levels of supersaturation which contradicts the trend ordinarily observed for homogeneous nucleation in classical crystallisation technology, and demonstrates the role of the membrane as a physical substrate for heterogeneous nucleation during chemically reactive crystallisation. Both nucleation rate and crystal growth rate increased with increasing levels of supersaturation. This can be ascribed to the relatively low chemical driving force imposed by the shift in equilibrium toward ammonium which suppressed solution reactivity, together with the role of the membrane in promoting counter-current diffusion of CO2 and NH3 into the concentration boundary layer developed at the membrane wall, which permitted replenishment of reactants at the site of nucleation, and is a unique facet specific to this method of membrane facilitated crystallisation. Free ammonia concentration was shown to govern nucleation rate where a limiting NH3 concentration was identified above which crystallisation induced membrane scaling was observed. Provided the chemically reactive membrane crystallisation reactor was operated below this threshold, a consistent (size and number) and reproducible crystallised reaction product was collected downstream of the membrane, which evidenced that sustained membrane operation should be achievable with minimum reactive maintenance intervention

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

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

CO2 absorption into aqueous ammonia using membrane contactors: Role of solvent chemistry and pore size on solids formation for low energy solvent regeneration

Solids formation can substanitally reduce the energy penalty for ammonia solvent regeneration in carbon capture and storage (CCS), but has been demonstrated in the literature to be difficult to control. This study examines the use of hollow fibre membrane contactors, as this indirect contact mediated between liquid and gas phases in this geometry could improve the regulation of solids formation. Under conditions comparable to existing literature, NH4HCO3 was evidenced to primarily crystallise in the gas-phase (lumen-side of the membrane) due to the high vapour pressure of ammonia, which promotes gaseous transmission from the solvent. Investigation of solvent reactivity demonstrated how equilibria dependent reactions controlled the onset of NH4HCO3 nucleation in the solvent, and limited ‘slip’ through transfomation of ammonia into its protonated form which occurs prior to the phase change. Crystallisation in the solvent was also dependent upon ammonia concentration, where sufficient supersaturation must develop to overcome the activation energy for nucleation. However, this has to be complemented with a reduction in solvent temperature to offset vapour pressure and limit the risk of gas-phase crystallisation. While changes to the solvent chemistry were sufficient to shift from gas-phase to liquid phase crystallisation, wetting was observed immediately after nucleation in the solvent. This was explained by a local region of supersaturation within the coarse membrane pores that promoted a high nucleation rate, altering the material contact angle of the membrane sufficient for solvent to breakthrough into the gas phase. Adoption of a narrower pore size membrane was shown to dissipate wetting after crystallisation in the solvent, illustrating membrane contactors as a stable platform for the sustained separation of CO2 coupled with its simultaneous transformation into a solid. Through resolving previous challenges experienced with solids formation in multiple reactor configurations, the cost benefit of using ammonia as a solvent can be realised, which is critical to enabling economically viable CCS for the transition to net zero, and can be exploited within hollow fibre membrane contactors, eliciting considerable process intensification over existing reactor designs for CCS

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

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