Kinetic Modelling of CO2 Degassing from Magnesium Bicarbonate Solutions and Resultant Precipitation
International Conference on Accelerated Carbonation for Environmental and Material Engineering (ACEME)
2018
International Conference on Accelerated Carbonation for Environmental and Material Engineering (ACEME)
General Submissions
Principles II
Monday, March 12, 2018 - 11:00am to 11:20am
Degassing of carbon dioxide (CO2) from aqueous solutions can be utilised as a process for aqueous mineral carbonation, specifically the production of magnesium (Mg) carbonates. Our research involved experimentation to examine degassing and factors which affect this physicochemical process. Batch experiments involved sparging Mg sulfate (MgSO4) and sodium bicarbonate (NaHCO3) solutions with nitrogen (N2) and measurement of the concentration of CO2 in the vented gas stream. Experiments were conducted using fine and coarse bubble generating sparging devices. Very similar trends were observed for all experiments with CO2 degassing rapidly producing Mg carbonates. Measured pH trends and quantitation of CO2 loss for the solutions were interpreted in terms of the thermodynamic transformation of the system. Batch degassing experiments conducted at varying temperature showed that the time needed to induce precipitation was far less, and the rate at which the carbonates formed was far greater, for solutions at 50oC compared to those at 30oC. Nesquehonite (MgCO3·3H2O) was the precipitated mineral phase for both the 30oC and 50oC experiments. Degassing and precipitation experiments were also conducted with carbonic anhydrase (CA) as a supplement to reagents and these experiments showed greater rates of degassing and resultant precipitation.
A mathematical model describing the kinetic response of the bulk solution to CO2 degassing, and the resulting precipitation of Mg carbonate, was formulated by considering the system of chemical reactions describing homogeneous liquid phase kinetics accounting for species activities, the liquid-solid heterogeneous reaction in the bulk solution, and gas-liquid phase kinetics incorporating an open system CO2 flowrate for the loss of CO2 from the bulk solution through the gas-liquid interface. The coupled set of ordinary differential equations (ODEs) describing the system was solved and the kinetic model depicting the batch system under the experimental conditions was found to closely follow that calculated through thermodynamics.
The kinetic model that was developed and fitted to the results of batch experiments showed that precipitation was dependent on the level of mineral supersaturation achieved at the end of the induction phase. After this point, mineral supersaturation rapidly declined with concomitant precipitation and thereafter the rate of N2 sparging (that is, the rate of CO2 removal) was of progressively increasing control to the rate of precipitation or growth of the carbonate but subject to the threshold mineral supersaturation required for precipitation. The kinetic modelling was also used to show that rapid bulk rates of precipitation at elevated temperature related to the temperature dependency of the precipitation reaction but also in part to the growth in reactive area stemming from homogeneous nucleation rather than heterogeneous nucleation shown for experiments at lesser temperature. The model allowed the performance of batch, semibatch and continuous degassing reactors or crystallisers to be assessed. Extension of the kinetic model to semibatch and continuous mode of operation also showed that rate of degassing was controlling to steady state rate of precipitation. Semibatch experiments demonstrated that it is possible to continuously precipitate Mg carbonates through degassing.
A mathematical model describing the kinetic response of the bulk solution to CO2 degassing, and the resulting precipitation of Mg carbonate, was formulated by considering the system of chemical reactions describing homogeneous liquid phase kinetics accounting for species activities, the liquid-solid heterogeneous reaction in the bulk solution, and gas-liquid phase kinetics incorporating an open system CO2 flowrate for the loss of CO2 from the bulk solution through the gas-liquid interface. The coupled set of ordinary differential equations (ODEs) describing the system was solved and the kinetic model depicting the batch system under the experimental conditions was found to closely follow that calculated through thermodynamics.
The kinetic model that was developed and fitted to the results of batch experiments showed that precipitation was dependent on the level of mineral supersaturation achieved at the end of the induction phase. After this point, mineral supersaturation rapidly declined with concomitant precipitation and thereafter the rate of N2 sparging (that is, the rate of CO2 removal) was of progressively increasing control to the rate of precipitation or growth of the carbonate but subject to the threshold mineral supersaturation required for precipitation. The kinetic modelling was also used to show that rapid bulk rates of precipitation at elevated temperature related to the temperature dependency of the precipitation reaction but also in part to the growth in reactive area stemming from homogeneous nucleation rather than heterogeneous nucleation shown for experiments at lesser temperature. The model allowed the performance of batch, semibatch and continuous degassing reactors or crystallisers to be assessed. Extension of the kinetic model to semibatch and continuous mode of operation also showed that rate of degassing was controlling to steady state rate of precipitation. Semibatch experiments demonstrated that it is possible to continuously precipitate Mg carbonates through degassing.