Thermodynamic equilibrium programs

As can be seen, conditions in a flowing reactor, even the simplest such as a tube, may be far from the thermodynamic equilibrium conditions predicted by the equilibrium computer programs. However, in the diffusion controlled range, it is possible to use as the driving force for diffusion, the difference between an assumed equi-... 

Direct thermal decomposition of methane was carried out, using a thermal plasma system which is an environmentally favorable process. For comparison, thermodynamic equilibrium compositions were calculated by software program for the steam reforming and thermal decomposition. In case of thermal decomposition, high purity of the hydrogen and solidified carbon can be achieved without any contaminant. 

Such information has been stored within a database in the ECES system. Then, using user input in the form of equation (1), ECES writes the expression for computing the thermodynamic equilibrium constant as a function of temperature to a file where it will eventually become part of a program to solve the many equilibria that might describe a complex system. 

The input of the problem requires total analytically measured concentrations of the selected components. Total concentrations of elements (components) from chemical analysis such as ICP and atomic absorption are preferable to methods that only measure some fraction of the total such as selective colorimetric or electrochemical methods. The user defines how the activity coefficients are to be computed (Davis equation or the extended Debye-Huckel), the temperature of the system and whether pH, Eh and ionic strength are to be imposed or calculated. Once the total concentrations of the selected components are defined, all possible soluble complexes are automatically selected from the database. At this stage the thermodynamic equilibrium constants supplied with the model may be edited or certain species excluded from the calculation (e.g. species that have slow reaction kinetics). In addition, it is possible for the user to supply constants for specific reactions not included in the database, but care must be taken to make sure the formation equation for the newly defined species is written in such a way as to be compatible with the chemical components used by the rest of the program, e.g. if the species A1H2PC>4+ were to be added using the following reaction ... 

The rational design of very short RNA sequences (15 to 40 nucleotides) that are able to adopt alternative folds is one of our major efforts. Part of this program are RNAs in thermodynamic equilibrium with different defined secondary structures. This also includes the development of methods for direct experimental verification of RNA equilibria and, moreover, applications with respect to biologically relevant RNA switches. 

Assumptions made in hydrogeochemical modeling programs complicate the transferability to natural systems, e.g. assuming thermodynamic equilibrium. This assumption is often not true especially for redox reactions being dominated by kinetics and catalyzed by microorganisms, and precipitation of certain minerals. Both processes can maintain disequilibria over a long time period. 

The thermodynamic equilibrium models, including surface complexation models, require the solution of a complex mathematical equation system. For this reason, many computer programs (e.g., CHEAQC, CHEMEQL, CHESS, EQ3/6, F1TEQL, Geochemist s Workbench, H ARPHRQ, JESS, MINTEQ and its versions, NETPATH, PHREEQC, PHRQPITZ, WHAM, etc.) have been developed to calculate the concentration and activity of chemical species, estimate the type and amount of minerals formed or dissolved, and the type and amount of sorbed complexes. 

These programs are able to model the geological systems soil/rock-aqueous solution systems that is the concentration and distribution of the thermodynamically stable species can be determined based on the total concentrations of the components and the parameters just mentioned. In addition, the programs can also be used to estimate thermodynamic equilibrium constants and/or surface parameters from the concentrations of the species determined through experiments. Thermodynamic equilibrium constants can be found in tables (Pourbaix 1966) or databases (e.g., Common Thermodynamic Database Project, CHESS, MINTEQ, Visual MINTEQ, NEA Thermodynamical Data Base Project (TDB), JESS, Thermo-Calc Databases). Some programs (e.g., NETPATH, PHREEQC) also consider the flowing parameters. 

Thermodynamic data, whether determined through calorimetry or solubility studies, are subject to refinement as more exact values for the components in the reaction scheme, or more complete description of the solution phases, become available. Many of the solubility studies on clays were done before digital-computer chemical equilibrium programs were available. One such program, SOLMNEQ, written by one of the authors ( ) solves the mass-action and mass-balance equations for over 200 species simultaneously. SOLMNEQ was employed in this investigation to convert the chemical analytical data into the activities of appropriate ions, ion pairs, and complexes. 

This package provides data on the species of 131 reactants at 298.15 K and programs for calculating various transformed thermodynamic properties. Programs are given for the calculation of apparent equilibrium constants and other transformed thermodynamic properties of enzyme-catalyzed reactions by simply typing in the reaction. 

Having demonstrated that ion exchange was at least a possible reaction, it was necessary to demonstrate the other reactions were unlikely. Fifteen representative analyses from different positions in the flow field were evaluated for thermodynamic mineral equilibrium, using SOLMNEQ, a mineral equilibrium program developed by Kharaka and Barnes (1973). These analyses indicate that all common Na minerals were thermodynamically unsaturated. That is, mineral solution could, if the minerals were present, occur, but that mineral precipitation was unlikely. This demonstrates that kinetically-controlled mineral precipitation is not controlling the concentration but this says nothing about the mineral s solution kinetics. However, it has been the author s observation that minerals equilibrium by solution is... 

Modelling calculations were performed for Crooks Gap and Bonanza to determine how much calcite could dissolve given sufficient time to reach equilibrium between calcite and the added CO2. The purpose of the calculations was to determine how far from equilibrium the natural systems were, and to assess the potential for using a thermodynamic equilibrium modelling program to predict well bore scale (discussed in the next section). 

Thermodynamic equilibrium chemical modelling programs can be used to predict non-equilibrium processes when calibrated with real data. 

A Fortran IV computer program developed by Redifer and Wilson (10) was used to predict thermodynamic equilibrium compositions for 400-700°K and 1 atm total pressure. The calculations are based on a procedure presented by Meissner, Kusik, and Dalzell (11) in which the set of simultaneous reactions is simplified to a set of series-consecutive reactions. Each reaction is carried out in turn on the reactant mixture as though a set of ideal batch equilibrium reactors were aligned in series in which the products from one equilibrium stage become reactants for the next reactor. After all the reactions have been completed, products from the last reactor are recycled to the first reactor, and the reaction sequence is repeated. Equilibrium of all components is complete when the product compositions at the end of two consecutive cycles are identical. The method compares favorably with the free energy minimization technique and is useful for changing conditions or input parameters. 

The two-phase integrated finite difference flow code TOUGH2 was selected to perform the hydro-logical calculations for this I BEX analysis. TOUGH2 is a general-purpose program for the simulation of multi-dimensional, multiphase fluid flow in porous and fractured media. Fluid advec-tion is calculated by a multiphase extension of Darcy s law, and diffusion is included. Local thermodynamic equilibrium is assumed in all... 

It is assumed that the mass tiansfer occurs in the layers on both sides of the phase boundary. According to the model, the mass transfer is caused by molecular diffusion. Therefore, a linear concentration profile is built up in the boundary layers. At the phase boundary itself, thermodynamic equilibrium is reached. More or less empirical correlations for the mass transfer coefficients /I and for the effective phase interface area are usually provided by the simulation program for different column internals (random packings, structured packings, and trays). The necessary diffusion coefficients can again be calculated according to Section 3.3.6. 

Thermodynamic reaction equilibrium for the naphthalene and tetralin hydrogenation to decalins was calculated according to Gibb s free energy change by the FLOWBAT program [12]. The results predict full conversion of both n hthalene and tetralin to decalins under the conditions studied. Moreover, thermodynamics favours the formation of trans-decalin, 93.5-96.6% in the temperature range 85-160°C [12]. The thermodynamic equilibrium of and A -octalin was not calculated since the required thermodynamic properties were not available. Weitkamp [7] has reported that the equilibrium ratio of the octalins varies from 15 to 3.5 at 0-200 C (5.9 and 3.6 at 100 and 177°C, respectively), with A -octalin the major component. 

In order to improve our understanding of the speciation of mercuiy in FGD gypsum, the authors have used thermodynamic equilibrium models in order to predict the composition of the chemical species in gas phase using the HSC-Chemistry 5.0 software. The theoretical study was carried out in the same conditions that the experimental study, at atmospheric pressure and temperatures ranging between 25 and 800 C. HSC-Chemistiy program uses data base with enthalpy, entropy and calorific capacity values for more than 15000 species. This software allows modify the quantity of different species implicated in the reaction and the program determines tlie products formed, theoretically, in the equilibrium. 

The predictions of CO are based on a thermodynamic equilibrium computer program in which the chemical equilibrium compositions for assigned thermodynamic states are calculated. Most often temperature and pressure, calculated from the main CFD code, are used to specify the thermodynamic state. The method for evaluating the equilibrium compositions is based on the minimization of Gibbs energy as mentioned earlier (Wang et al., 2006). 

The reverse reaction rates are determined through the use of the thermodynamic equilibrium constant. The thermochemical data base36 is a modification of that used by Gordon and McBride in the NASA Chemical Equilibrium program. The thermochemical data is stored as polynomial fits for the specific heat Cp, enthalpy H°, and entropy S°, given by... 

Dissolution of a sparingly soluble solid in a multicomponent aqueous solution can be described by the set of the chemically probable heterogeneous and homogeneous reactions and the corresponding thermodynamic equilibrium constants and by the mole balance equations for each component. Usually, the set of coupled algebraic equations is too complicated to solve analytically numerical procedures are required [26]. By now, several computer programs are available that can calculate the equilibrium speciation if thermodynamic equilibrium constants are known (e.g., MICROQL [30], MINTEQA [31]) or the set of experimental activity data can be fitted to obtain the equilibrium constants of probable reactions supposed in an appropriate chemical model (e.g., FFTEQL [32]). 

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