Open and Closed Carbonate Systems

Although a huntite stability field is not shown, for 10 bar we compute that huntite is 

A closed carbonate system is defined as one in which the carbon dioxide (carbonic acid) initially present in the water is not replenished as it is consumed in carbonate mineral dissolution. This may simply reflect that soil moisture/infiltration is charged with CO2 chiefly in the A horizon of the soil, whereas carbonate mineral dissolution by H2CO3 takes place at greater depths in the soil C horizon or below the water table in the absence of further sources of carbon dioxide. (There may be, however, sources of additional CO2 at depth, including pollution. See Chap. 5.) 

The equation describing the change in bicarbonate concentration with pH during the closed-system dissolution of calcite may be derived as follows (cf. Langmuir 1971c). First, the dissolution reaction is written 

This equation indicates that for every mole of carbonic acid consumed, two moles of bicarbonate are produced. In differential terms this statement corresponds to 

The same equations and general approach have been used to constnict Fig. 6.14 in which the no CO2 added curves show the pathways of carbonate mineral dissolution for closed-system conditions, and the fixed CO2 pressure lines indicate the trend of open-system dissolution of the carbonates. 

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Sec. 6.6 Open and Closed Carbonate Systems This can be integrated with the result... 

TABLE 4-8 Comparison of Composition and pH of Open and Closed Carbonate Systems... 

Controlled cutting and opening of closed carbon systems Direct applications of CNTs (requires 20-100 nm in length) Inner filling and impregnation of CNTs with metal nanoparticles and complexes... 

Several reaction pathways are built into the FREZCHEM model including (1) temperature change, (2) evaporation, (3) pressure change, (4) equilibrium or fractional crystallization and, for gas hydrates, (5) open or closed carbon systems, and (6) pure or mixed gas hydrates. Under the temperature change option, the user can specify the upper and lower temperature range and a decremental temperature interval (AT) at which equilibrium at a fixed pressure is calculated (e.g., 298.15 to 253.15K with AT = 5 would result in... 

Last but not least, one should check the inertness of the auxiliary electrodes in single-pellet arrangements, both under open and closed circuit conditions and, also, via the closure of the carbon balance, the appearance of coke deposition. This is especially important in systems with a variety of products (e.g. selective oxidations), where the exact value of selectivity towards specific products is of key interest. This in turn points out the importance of the use of a good analytical system and of its careful calibration. 

Dissolution of carbonates can only occur if the solution is thermodynamically undersaturated, pH is an important variable affecting the saturation ratio (Appendix 8.1 gives a brief review of the CaC03 solubility characteristics in open and closed systems). 

Metathesis of alkenes has been reviewed in terms of cross-metathesis, ring opening and closing, disproportionation, transmutation, and self-metathesis.34 A review on catalytic processes involving ft -carbon elimination has summarized recent progress on palladium-catalysed C-C bond cleavage in various cyclic and acyclic systems.35... 

This use of hydrogen as a furnace fuel can dramatically change the relative economics of the OPEN and CLOSED systems as the cost of carbon emissions increases. This is demonstrated in Figure 7.10. 

As is demonstrated by this figure, the use of hydrogen as a fuel can close the relative gap between the OPEN and CLOSED system, but this requires a carbon cost in excess of 80/tonne when the ethylene production cost is forced over 900/tonne. 

Table 9.5 gives the statistics for cracking naphtha (1 million tonnes of ethylene/year) for the OPEN and CLOSED systems. The first coluimi gives the mass of fuel and feed contributing to on-site emissions. The second column, the energy (HHV basis) for fuel and feed used. The third column gives the carbon dioxide emissions and when calculated on a unit basis this emission is attributed to the principal olefin products (ethylene and propylene). The fourth column gives the pertinent unit emissions when the carbon dioxide emission is distributed across all of the products. 

Figure 9.14 Sensitivity of ethylene production cost from naphtha to carbon emission cost - OPEN and CLOSED systems...

In Figure 3, the stoichiometric difference between the calcium ion concentration in the rock runoff and the hydrogen ion concentration in the incident rain is plotted versus the rain pH. Lines are drawn for the two calculated stoichiometric differences for both open and closed systems. The representative points from the onsite experiment fall within these two limiting cases. These results illustrate that the onsite stoichiometry is dependent on the initial pH of the incident rainfall and on whether the reacting solution is open or closed to exchange of carbon dioxide in the atmosphere. 

As in the case of calcite, the activities of various dissolved species have been calculated at different pH values for other minerals using the relevant chemical equilibria and stoichiometric restrictions. The distribution of the activities of dissolved species of hydroxy apatite is given in Figs. 3.5a and 3.5b for open and closed systems, respectively. If the system is brought to equilibrium with atmospheric carbon dioxide, the distribution will follow that shown in Fig. 3.5a. For the closed system, Ca species are the most predominant... 

Calculation of Carbonate Species Concentrations in Open and Closed Systems... 

In an open system the CO2 partial pressure is kept constant and in a closed system the amount of CO2 is constant. Thus, in open system CO2 gas is allowed to leave the system and partial pressure of CO2 equals atmospheric pressure (380 ppm). In closed system the total amount of CO2 is constant and partial pressure of carbon dioxide piCO ) will change. The calculated curves are comparable to the results obtained by Pilson [27]. In a closed system, even a small increase in sulphuric acid decreases the pH, which lowers the liquid s ability to form dissolved carbon and calcite. In an open system, the change of pH is smaller but important in natural systems. At higher acid concentrations, the pH of the open and closed systems becomes the same. The actual measured sulphuric acid concentration in natural waters is less than 2 X 10 molkg(H2O) In terms of aquatic ecosystems the CO2 partial pressure is relatively constant despite the observed increase of atmospheric CO2 concentration during the last 100 years and the ecosystems thus resemble the open system. 

An interesting and novel use of a soHd desiccant, the reduction of cold condensate corrosion in automotive exhaust systems, illustrates a hybrid closed—open system. Internal corrosion occurs in mufflers when the water vapor in the exhaust condenses after the engine is turned off and the muffler cools. Carbon dioxide dissolves in the condensate to form an acidic soup. In an essentially closed static drying step, an acid- and heat-resistant desiccant located in the muffler adsorbs water vapor from the exhaust gas as it cools to prevent formation of corrosive acidic condensate. When the engine is restarted, the system becomes open, and the desiccant is regenerated by the hot exhaust gas to be ready for the next cooldown step (19). 

In Figure 2 the solubility and speciation of plutonium have been calculated, using stability data for the hydroxy and carbonate complexes in Table III and standard potentials from Table IV, for the waters indicted in Figure 2. Here, the various carbonate concentrations would correspond to an open system in equilibrium with air (b) and closed systems with a total carbonate concentration of 30 mg/liter (c,e) and 485 mg/liter (d,f), respectively. The two redox potentials would roughly correspond to water in equilibrium wit air (a-d cf 50) and systems buffered by an Fe(III)(s)/Fe(II)(s)-equilibrium (e,f), respectively. Thus, the natural span of carbonate concentrations and redox conditions is illustrated. 

Another source of divergence is the use of different models for the aqueous carbonate systems. Precipitation and dissolution experiments can be carried out in closed or open systems and various ways of pH-adjustments (see 8.2). 

The chemical structure of vitamin D3 is closely related to its precursor, 7-dehydrocholesterol, from which it is produced by a photochemical reaction. Therefore, vitamin Do is closely related structurally to the four-ring nucleus of steroids derived from the cyclopentanoperhydro-phenanthrene ring system. No vitamin D activity is noticed until the B ring of 7-dehydrocholesterol is opened between C-9 and C-10. Thus, vitamin D3 is a 9,10-seco steroid and its carbon skeleton is numbered accordingly (Scheme I). The important aspects of its chemistry center about the 5,6,7-cis-triene structure. The formula for vitamin D3 is C27H44O and its formula weight is 384.64. 

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