Chemical equilibrium models, computer-based

Several workers have intended to estimate the chemical compositions of Kuroko ore fluids based on the chemical equilibrium model (Sato, 1973 Kajiwara, 1973 Ichikuni, 1975 Shikazono, 1976 Ohmoto et al., 1983) and computer simulation of the changes in mineralogy and chemical composition of hydrothermal solution during seawater-rock interaction. Although the calculated results (Tables 1.5 and 1.6) are different, they all show that the Kuroko ore fluids have the chemical features (1 )-(4) mentioned above. 

An alternative way of relating concentrations (mass fractions) of individual species to/ is the assumption of chemical equilibrium. An algorithm based on minimization of Gibbs free energy to compute mole fractions of individual species from / has been discussed by Kuo (1986). The equilibrium model is useful for predicting the formation of intermediate species. If such knowledge of intermediate species is not needed, the much simpler approximation of mixed-is-burnt can be used to relate individual species concentrations with/. In order to calculate the time-averaged values of species concentrations the probability density function (PDF) approach is used. 

Dissolved Organic Matter-Solution Speciation Few studies report actual measurement of Cd + speciation in soil solutions. Many authors report Cd speciation based on measured total dissolved Cd concentration and computation of free Cd2+ using chemical equilibrium models. The accuracy of the... 

The difficulties of experimentally determining the speciation of actinides present at very low concentrations in natural waters have encouraged the use of computer simulations, based on thermodynamic data, as a means of predicting their speciation and hence their environmental behaviour. The use of modelling techniques to describe the speciation, sorption, solubility and kinetics of inorganic systems in aqueous media has been reviewed in the papers given at an international conference in 1978. Both chemical equilibrium models, exemplified by computer programs such as MINEQL and SOLMNQ, and dynamic reaction path models, exemplified by EQ6, have been developed. Application of the equilibrium models to radioactive waste disposal... 

Few studies report actual measurement of Cd + speciation in soil solutions. Many authors report Cd speciation based on the measured total dissolved Cd concentration and computation of free Cd using chemical equilibrium models. The accuracy of the speciation depends on the correctness of Cd-complex stability constants (Turner, 1995). For example, the proportion of free Cd (as Cd ) in solution varies from 16 to 82% depending on the log K values for Cd-(DOM) dissolved organic matter complexes (Table 4). The stability constants of Cd-humics are particularly uncertain. The reported high values of pCd + ranged... 

Whatever the aim of a particular titration, the computation of the position of a chemical equilibrium for a set of initial conditions (e.g. total concentrations) and equilibrium constants, is the crucial part. The complexity ranges from simple 1 1 interactions to the analysis of solution equilibria between several components (usually Lewis acids and bases) to form any number of species (complexes). A titration is nothing but a preparation of a series of solutions with different total concentrations. This chapter covers all the requirements for the modelling of titrations of any complexity. Model-based analysis of titration curves is discussed in the next chapter. The equilibrium computations introduced here are the innermost functions required by the fitting algorithms. 

Closed models of total chemical equilibrium are base in hydrochemical modeling as they are used in all other hydrogeochemical models. They are used at the construction of Bjerrum and Pourbaix diagrams and for the determination of mineral solubility, for computation of equilibrium composition of the water solution or the entire geological medium as a whole by available analytical data. 

The development of computer-based algorithms from surface complexation models is discussed in J. Westall, Chemical equilibrium including adsorption on charged surfaces, in Particulates in Water (M. C. Kavanaugh and J. O. Leckie, eds.). American Chemical Society Washington, D.C., 1980. 

The concept of partial and local chemical equilibrium in natural system has been proposed and developed by Thompson (1959) and Helgeson (1979). After that many application and computer simulations based on partial chemical equilibrium have been conducted (e.g., Wolery 1983 Reed 1983). The principle of this model will be briefly described and application of this model to hydrothermal system accompanied by hydrothermal alteration and formation of hydrothermal ore deposits will be given below. 

ABSTRACT. We describe an apparatus by which the detonation products of an explosive can be identified and whose relative concentrations can be determined quantitatively. These measurements can be made on products that have been formed in less than one microsecond after the passage of the detonation wave. The technique is based on the rapid quenching of chemical reactions by virtue of the free expansion of the products into vacuum. Of course, products that have been formed over a longer period of time and under different pressure/temperature conditions can also be studied. Time resolved molecular-beam mass spectrometry is used, so that whether detonation occurred or not in forming the products can be determined. We describe optical techniques, principally Schlieren photographs, that also confirm detonation. We report measurements made on six standard explosives, PETN, RDX, HMX, HNS, TNT and TATB, and one research explosive, nitric oxide. For none of the standard explosives do we measure product distributions that agree with model predictions based on equilibrium assumptions. A computer model of the free expansion is described briefly and its importance to the interpretation of the data is emphasized. 

Subsequently, the model has been extended [37, 38] to the case of associated electrolytes by using a recent model for associating electrolytes[39]. Unlike the classic chemical model of the ion pair the effect of the pairing association is included in the computation of the MSA screening parameter F. Simple formulas for the thermodynamic excess properties have been obtained in terms of this parameter when a new EXP approximation is used. The new formalism based on closures of the Wertheim-Ornstein-Zernike equation (WOZ)[40, 41 does accommodate all association mechanisms (coulombic, covalent and solvation) in one single association parameter, the association constant. The treatment now includes the fraction of particles that are bonded, which is obtained by imposing the chemical equilibrium mass action law. This formalism was shown to be very successful for ionic systems, both in the HNC approximation and MSA [42, 43, 44, 45, 46, 47]. 

There are in general several steps of refinement to model a gasification system. Zero-dimensional models show the lowest complexity, and rely on empirical correlations or thermodynamic equilibrium calculations. The next step is a onedimensional model that usually requires kinetic expressions either to resolve the space or time coordinate using idealized chemical reactor models. Approaching two- or three-dimensional calculations provokes the use of computational fluid dynamics (CFD) that may incorporate either equiUbriiun or kinetics-based turbulence chemistry interactions. Each step of modeling adds significant complexity and calculation time. 

Such a system would include a program similar to that of Storer and Comish-Bowden to do equilibrium calculations. A communication-control subprogram would be Hiked to an expert model by using the EXPERT knowledge-base shall (or system-builder) which is advantageous here because it can interact with procedures such as those written in FORTRAN for numerical computation. Additional programs and a small data base, which EXPERT can handle, would keep track of which chemical was what array element, and other requirements mentioned above. 

The various methods used in quantum chemistry make it possible to compute equilibrium intermolecular distances, to describe intermolecular forces and chemical reactions too. The usual way to calculate these properties is based on the independent particle model this is the Hartree-Fock method. The expansion of one-electron wave-functions (molecular orbitals) in practice requires technical work on computers. It was believed for years and years that ab initio computations will become a routine task even for large molecules. In spite of the enormous increase and development in computer technique, however, this expectation has not been fulfilled. The treatment of large, extended molecular systems still needs special theoretical background. In other words, some approximations should be used in the methods which describe the properties of molecules of large size and/or interacting systems. The further approximations are to be chosen carefully this caution is especially important when going beyond the HF level. The inclusion of the electron correlation in the calculations in a convenient way is still one of the most significant tasks of quantum chemistry. 

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