H2O-CO2 system

The earliest mention of an ammonium carbonate, salt of hartshorn, appears in English manuscripts of the 14th century. As the name implies, the material was obtained by dry distillation of animal waste such as horn, leather, and hooves. Although many salts have been described in the Hterature for the ternary NH —CO2—H2O system, most, except for ammonium bicarbonate [1066-33-7], NH HCO, ammonium carbonate [506-87-6], (NH 2 02, and ammonium carbamate [1111-78-0], NH4CO2NH2, are mixtures (5,6). 

Ammonium bicarbonate, sp gr 1.586, formula wt 79.06, is the only compound in the NH —CO2—H2O system that dissolves in water without decomposition. SolubiUty in 100 g of H2O ranges from 11.9 g at 0°C to 59.2 g/100 g of H2O at 60°C (8). The heat of formation from gaseous ammonia and carbon dioxide andUquid water is 126.5 kj/mol (30.2 kcal/mol). Ammonium bicarbonate is manufactured by passing carbon dioxide gas... 

In applying this equation to multi-solute systems, the ionic concentrations are of sufficient magnitude that molecule-ion and ion-ion interactions must be considered. Edwards et al. (6) used a method proposed by Bromley (J7) for the estimation of the B parameters. The model was found to be useful for the calculation of multi-solute equilibria in the NH3+H5S+H2O and NH3+CO2+H2O systems. However, because of the assumptions regarding the activity of the water and the use of only two-body interaction parameters, the model is suitable only up to molecular concentrations of about 2 molal. As well the temperature was restricted to the range 0° to 100 oc because of the equations used for the Henry1s constants and the dissociation constants. In a later study, Edwards et al. (8) extended the correlation to higher concentrations (up to 10 - 20 molal) and higher temperatures (0° to 170 °C). In this work the activity coefficients of the electrolytes were calculated from an expression due to Pitzer (9) ... 

To illustrate the power of the general purpose tools we have developed, I will describe the application of our software to two systems. First, since most of the participants in this session are using it as an example, and because it is important, I will give some of our results for the NH3-CO2-H2O system. Secondly, to illustrate the prediction of combined vapor-liquid-solid... 

An important system in industrial pollution control is the NH3-CO2-H2O system. This equilibria represents a good test for the ECES system because data is available on the vapor pressures of NH3 and CO2 over various compositions for a range of temperatures. Furthermore, there is a lack of data on some of the pure salts that are inherent in this system and data estimates are necessary. To complete this test, this system involves both weak and strong electrolytes. 

As can be seen less than 2% error in this multicomponent system occurred when using ECES. This system is quite different than the NH3-CO2-H2O system since we are dealing only with strong electrolytes. For example, the second datum point predicted by ECES give the following results for the concentrations, activity coefficients and water activity in the aqueous phase. 

Of equal importance is the oxygen isotope fractionation in the CO2 - H2O system. Early work concentrated on the oxygen isotope partitioning between gaseous CO2 and water (Brennikmeijer et al. 1983). More recent work by Usdowski and Hoefs (1993), Beck et al. (2005) and Zeebe (2007) have determined the oxygen isotope composition of the individual carbonate species that are isotopically different at room temperature. Table 2.6 summarizes the equations for the temperature dependence between 5 and 40°C (Beck et al. 2005). 

Aside from mineral stabilities, the behavior of the CO2-H2O system with increasing P and T is also important to an understanding of the deep burial diagenesis of carbonate rocks. One reaction of interest, which represents the summation of K0 and Ki (see Chapter 1) for the carbonic acid system, is... 

These reactions undoubtedly verify the special importance of the CO2/H2O system for the efficiency of AOPs. The omnipresent bicarbonate or carbonate ions can compete successfully for OH radicals, especially at low loads of organic matter. The composition of the CO2 (H2C03)/HC0i/C03 system in water is strongly dependent on the pH of the solution (Fig. 6-15). When carbon dioxide is dissolved in water, only a small amount (ca. 0.1%) reacts to form carbonic acid (H2CO3). In fact, most of the undissociated acid is actually present as C02(aq) at pH values lower than 4.3. At a pH of 8.2 the system consists solely of bicarbonate ions and at higher pH values above 12 the carbonate ion is mainly present. For a detailed discussion of the CO2/water system, see Sigg and Stumm (1996). 

A theoretical analysis of oxalate-carbonate transformation The objective of this theoretical approach is to try to compare the equilibrium diagrams of the CaCOs—CaC204—CO2—H2O system with the biogeochemical data available on the oxalate-carbonate transformation. The initial hypothesis is that the oxido-reduction reaction of oxalate-carbonate occurs biochemically, i.e. due to bacterial activity. In other words, activation energy is present and high enough to initiate the reactions. The theoretical system studied is defined by three different phases present ... 

Figure 8 The Eh/pH diagram for the U/O2/CO2/H2O system at 25 °C with a partial CO2 pressure of 10 bar. Solid-solution boundaries are drawn at 10 M dissolved uranium species U02am = amorphous UO2...

A subsequent study of metamorphosed ultra-mafic rocks in the Isua succession (Dymek et al. 1988) extended the earlier work. Olivine-spinel pairs gave equilibration temperatures of 561 18°C, indistinguishable from the results of garnet-biotite thermometry of 541 43°C. Mineral reactions plotted in the CMS-CO2-H2O system indicate high Ycoa conditions. Subsequent retrogression was at T < 450°C and at lower Xcoi One locality, however, contains serpentinites in which orthopyroxene overgrows olivine. Here the... 

Figure 13.9 Eh-pH diagram for aqueous species in the U-O2-CO2-H2O system in pure water at 25°C and 1 bar total pressure for ZU = 10 M and Pqo, = 10" ° bar. UC, UDC, and UTC denote the aqueous complexes U02C0°, U02(C03)2, and U02(C03)3 % respectively. The position of the U02(c) solid/solution boundary for ZU = 10" M is stippled.

Harvie, C. E., Moller, N., Weare, J. H., The prediction of mineral solubilities in natural waters The Na-K-Mg-Ca-H-Cl-S04-OH-HCO3-CO3-CO2-H2O system to high ionic strengths at 25°C, Geo-chim. Cosmochim. Acta, 48, (1984), 723-751. Cited on page 400. 

One of the most useful implementations for geochemical modeling, published by Harvie et al. (1984), is known as the Harvie-Mgeochemical studies. There is, in fact, no published dataset regressed to include virial coefficients for both silica and aluminum. 

This chemical and physico-chemical behavior of the binary H2O-CO2 mixture [38] suggests that water is an attractive liquid to be combined with supercritical carbon dioxide in multiphase catalysis. CO2/H2O systems have adequate mass-transfer properties, especially if emulsions or micro-emulsions can be formed ([39] and refs, therein). The low pH of aqueous phases in the presence of compressed CO2 (pH ca. 3-3.5 [40]) must be considered and the use of buffered solutions can be beneficial in the design of suitable catalytic systems, as demonstrated for colloid-catalyzed arene hydrogenation in water-scC02 [41]. 

The process diagram shown here is used for such gas condensates whose sour gas content is as least as high as their ammonia content. The NH3/CO2/H2O system exhibits different boiling properties at different pressures. The so-called maximum temperature azeotrope lines, which are defined so that the CO2/NH3 ratios are the same in both phases when the liquid and gas phases are in equilibrium, are shifted in the direction of higher ammonia mixes as pressures are increased. 

Due to the lack of experimental data available, alternative methods for properly estimation are needed [3]. Such methods include modelling at both the macroscopic and microscopic scales. At the macroscopic scale, the development of theoretical EoS is needed for accurate prediction of properties and phase equilibria calculations. At the molecular level. Molecular Dynamics (MD) and Metropolis Monte Carlo simulations are used for the elucidation of microscopic structure and the prediction of thermodynamic and transport properties. Liu et al. [7] evaluated a series of force fields in their ability to predict properties of CO2-H2O systems and concluded that different force fields produce optimal results in different ranges of temperatures and pressures. 

An excellent match between the predicted points and the experimental data over the entire range for the CO2-H2O system. An excellent match for NH -H O and SOj-HjO at low concentrations of NHs and SO2. but a methodical worsening of the fit at higher concentrations. 

Equilibrium hydrate formation condition for CO2 has been extensively investigated and available in the literature [32-44], A hydrate phase diagram of the CO2 + H2O system is presented in Fig. 10.5. As can be seen in the figure, equilibrium hydrate formation conditions above and below fi-eezing point of water is presented. 

Figure 2-37. CO2 partial pressure curves for the Diglycolamine agent - H2S - CO2 - H2O system at 180°F, 65/35 amine-water weight ratio. Dingman etal., 19831...

The absorption of carbon dioxide in water has been shown to be almost entirely liquid-film controlled—presumably because of the relatively low solubility of carbon dioxide. Considerable research has, therefore, been conducted on the CO2-H2O system in connection with both absorption and desorption to determine the liquid-frlm resistance to mass transfer when various packings are used. Some of the data obtained are directly applicable to (he design of commercial installations for carbon dioxide absorption and desorption. 

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