CO2 retention model

Two geochemical models were used to quantify the exchangeable C reservoir (1) a theoretical model based on calcite equilibrium control (calcite equilibrium model), and (2) an empirical model based on measured losses of CO2 from a surrogate unsaturated zone atmosphere to unsaturated water-sediment mixtures (CO2 retention model). 

Hi4C03 = (St - Od) (2 Keq PC02) / (PI4CO2/PCO2). Aqueous plus Adsorbed Phases (CO2 Retention Model)... 

According to the CO2 retention model the total concentration of aqueous plus adsorbed CO2 per m of unsaturated zone (CO2 ) is ... 

The transport equation for the CO2 retention model is similarly obtained by substitution of Equations 5, 8, and 9 into Equation 12 ... 

Column C lists simulated P C02 values that were calculated using the CO2 retention model. Simulated P C02 values were within 1 S.D. of the onsite data at 9 of 10 sampling locations. 

Solute retention as a function of temperature at constant pressure is seen to be dependent on the partial molar enthalpy of solute transfer between the mobile and stationary phases, the neat capacity of the supercritical fluid mobile phase and the volume expansivity of the fluid. The model was compared to chromatographic retention data for solutes in n-pentane and CO2 as the fluid mobile phase and was seen to fit the data well. 

Experiments have been performed on a preparative SFC system using pure CO2 as the mobile phase under significant pressure drop. The retention times, pressure drop characteristics and the mass transfer behaviour were studied. The trends observed differ from the behaviour of HPLC systems. These trends also emphasize the complexity involved in analyzing the data for SFC measurements, which imply in turn greater complexity of the SFC model as compared to standard liquid chromatography model. 

Measurements of losses of CO2 to unsaturated sediment-water mixtures indicate that diffusion of C02 in the unsaturated zone may be substantially retarded by isotopic exchange of to an adsorbed inorganic C phase. Two geochemical models for calculating C02 retention in the unsaturated zone were compared. 

Laboratory measurements of the losses of CO2 and C02 from a surrogate unsaturated zone atmosphere to unsaturated sediments indicate the presence of an adsorbed C phase that can retard C02 transport in the unsaturated zone. Measured losses of CO2 from the atmosphere were 8 to 17 times greater than those predicted by calcite equilibrium calculations. Modeled predictions of C02 transport in a cross section near buried low-level radioactive waste support the presence of the adsorbed C phase distribution of P C02 was more accurately simulated using a model of C02 retention based on measured CO2 -loss isotherms than with a model based on calcite equilibrium control. Failure to account for the adsorbed C phase can lead to substantial errors when using models to estimate C transport and exchange in the unsaturated zone. 

The obtained data were elaborated with the Knudsen (or free-molecule) diffusion model, according to which the flux of a gaseous species through a membrane is directly proportional to its partial pressure at retentate side and has an inverse dependence on the square root of its molecular weight [83]. Thus, in order to validate the model of Knudsen diffusion through the membrane, the ratios H2/CH4, H2/CO and H2/CO2 were evaluated on the permeate side and compared with the expected ones (Fig. 6.11). 

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