Temperature reduces solvent vapour pressure to reduce gas phase crystallisation

To examine the role of vapour pressure on gas phase crystallisation (independent of solvent concentration), the temperature of a 3 M NH3 solvent was reduced from 20 to 5 °C, which reduced the vapour pressure from 6.1 to 2.9 kPa [30]. This also has implications for shell-side crystallisation, since the reduction in temperature lowers the solubility limit for NH4HCO3 by 30%, which favours the onset of nucleation at lower CO2 concentrations (Appendix B, Fig. B1). Initial CO2 flux was slightly lower at 5 °C than at 20 °C which can be explained by the reduced reaction kinetics (Fig. 6[45]. However, when operating at the lower solvent temperature, a more consistent CO2 flux was sustained over a longer duration, resulting in a supersaturated solvent. Crystallisation was first observed on the shell-side in the 5 °C solvent at a theoretical C/C*of 1.34, corresponding to a CO2/NH3 loading of 0.78. In addition to crystals collected from the solvent, nucleation was coincident with crystal growth on the fibre which were identified through direct observation (Fig. 7). Crystals formed on the fibre section closest to the CO2 inlet were of larger diameter and greater in number, indicating that the concentration boundary layer at the membrane-solution interface plays a role in mediating nucleation.

Fig. 6. Effect of absorbent temperature (3 M L−1 NH3 in recirculation) on reactive membrane crystallisation. Cumulative carbonate ratio compares CO2 absorbed to HCO3 concentration at saturation. Conditions: PTFE membrane (µm pore size). Error bars indicate standard deviation. Horizontal lines indicates ammonium bicarbonate solubility at 20 °C (red) and 5 °C (blue).

Fig. 7. Evidence for shell-side crystal nucleation using PTFE membrane using direct observation technique: (a) gas inlet (lumen), and aquaous ammonia outlet (shellside); (b) centre point of fibre; and (c) gas outlet (lumen) and aqueous ammonia inlet (shellside). Pore size, dmax 3.4 µm; initial ammonia concentration, 3 M; liquid temperature, 5 °C.

Wetting occurred soon after shell-side crystallisation in the 5 °C solvent, as evidenced by a reduction of gas flow at the outlet of the lumen, and the bubbling of CO2 into the liquid phase. This is in contradiction to observations at 20 °C where wetting was ostensibly followed by lumen-side crystallisation (Fig. 1). The membrane has been hypothesised to lower the activation energy for nucleation due to the interfacial energy of PTFE (contact angle, ∼135°; [6], where the heterogeneous substrate can lower the level of supersaturation required for induction (ΔGhet/ΔGhom, ∼90%; [10]:(16)ΔGhetΔGhom=0.252+cosθ1-cosθ21-ε1+cosθ21-cosθ23

This expression has been adjusted to account for the membrane porosity (ε) which can also mediate local supersaturation in the region where the membrane (solid phase), liquid and gas phases intersect [26]. We propose that nucleation within the pores on the shell-side of the membrane for the 5 °C solvent subsequently alters the material contact angle, reducing the breakthrough pressure of the solvent which promotes pore wetting following shell-side crystallisation, leading to the transmission of solvent into the gas phase [34].

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