Controlling shell-side crystal nucleation in a gas–liquid membrane contactor for simultaneous ammonium bicarbonate recovery and biogas upgrading

Abstract

A gas-liquid hollow fibre membrane contactor (HFMC) process has been introduced for carbon dioxide (CO 2) separation from biogas where aqueous ammonia (NH 3) is used to chemically enhance CO 2 absorption and initiate heterogeneous nucleation of the reaction product ammonium bicarbonate at the membrane-solvent interface. Aqueous ammonia absorbents (2 to 7 M) were initially used in single pass for CO 2 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 NH 3 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 NH 3 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 NH 3 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 CO 2 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.

Is Chemically Reactive Membrane Crystallization Faciliated by Heterogeneous Primary Nucleation? Comparison with Conventional Gas–Liquid Crystallization for Ammonium Bicarbonate Precipitation in a CO2–NH3–H2O System

“…To support this hypothesis, the interfacial energy was estimated for membrane crystallisation from experimental nucleation data (Table 3) 21,22 as 6.6 mJ m -2 which corresponds to the estimated interfacial energy for the primary nucleation of ammonium bicarbonate (Equation 15). Evidence for the formation of nuclei local to the membrane pore structure has been previously presented 9 . To illustrate, at an equivalent supersaturation level (C/C* 1.7), the free energy required for critical nucleus formation is around V !…”

“…Therefore the CO 2 flux reduces due to physical rather than chemical absorption dominating mass transfer in the second stage, the extent that CO 2 flux is sustained was then dependent upon the initial ammonia concentration (Figure 2). 9 Whilst physical absorption was the dominant mechanism for CO 2 mass transport during induction for both the MCr and batch crystalliser, induction was identified at an earlier supersaturation ratio for the batch crystalliser (Figure 10) which also achieved a considerably higher yield at an equivalent supersaturation ratio (Figure 11). We propose that this occurred due to the direct gas injection used, which induced complete mixing, subsequently lowering metastable zone width (MSZW) providing crystallisation at a lower supersaturation ratio as was evidenced by the production of a comparatively high number of small crystals in this study (Figure 9).…”

“…Through approximation of the interfacial surface area of the CO 2 bubbles, an analogous CO 2 flux to the membrane crystallisation reactor is reached, which indicates that raising absolute CO 2 mass transfer may be best accommodated through an increase in membrane surface area 10 . Importantly, the controlled addition of CO 2 at the membrane-solution interface, introduces a counter-diffusional CO 2 -NH 3 concentration gradient, to support the creation of a localised supersaturated region 9 . This is evidenced by a comparatively small loss of ammonia from the MCr (Figure 7) which can be ascribed to the laminar hydraulic characteristics of the MCr which minimises the transport of NH 3 to the gas-liquid interface as NH 3 transport is limited to radial diffusion 29 , in addition to the effective mediation of the chemical reaction within the boundary layer which confines ammonia through a shift in the ammonia-ammonium equilibrium toward non-volatile ammonium 5 .…”

Renewable absorbents for CO2 capture: from biomass to nature

“…McLeod et al . reported a method to recover the reaction product, ammonium bicarbonate in crystalline form, which not only benefits ammonium bicarbonate application, but also increases the profit of wastewater treatment works …”

Novel percrystallisation process by inorganic carbon membranes

“…Membrane crystallisation offers several advantages over traditional crystallisation technologies by being less energy intensive [61,83,94]. This process also provides effective means for crystal size control [79,[95][96][97], in addition to attaining heterogeneous nucleation, thus reducing the levels of supersaturation needed for crystallisation by providing a nucleation site [67,70,91,98]. Another important advantage found with membrane crystallisation is the scalability of the systems [99].…”