Chemical User kinetics

Smith, P.A. and Edwards, R.B., 1993, Proc. 6th RC User Forum Europe, Zermatt, Sept. 29-30. Smith. J.M., 1981 Chemical Engineering Kinetics , 3rd ed., McGraw-Hill, New York. 

The present chapter will focus on the practical, nuts and bolts aspects of this particular CA approach to modeling. In later chapters we will describe a variety of applications of these CA models to chemical systems, emphasizing applications involving solution phenomena, phase transitions, and chemical kinetics. In order to prepare readers for the use of CA models in teaching and research, we have attempted to present a user-friendly description. This description is accompanied by examples and hands-on calculations, available on the compact disk that comes with this book. The reader is encouraged to use this means to assimilate the basic aspects of the CA approach described in this chapter. More details on the operation of the CA programs, when needed, can be found in Chapter 10 of this book. 

WASP/TOXIWASP/WASTOX. The Water Quality Analysis Simulation Program (WASP, 3)is a generalized finite-difference code designed to accept user-specified kinetic models as subroutines. It can be applied to one, two, and three-dimensional descriptions of water bodies, and process models can be structured to include linear and non-linear kinetics. Two versions of WASP designed specifically for synthetic organic chemicals exist at this time. TOXIWASP (54) was developed at the Athens Environmental Research Laboratory of U.S. E.P.A. WASTOX (55) was developed at HydroQual, with participation from the group responsible for WASP. Both codes include process models for hydrolysis, biolysis, oxidations, volatilization, and photolysis. Both treat sorption/desorption as local equilibria. These codes allow the user to specify either constant or time-variable transport and reaction processes. 

CHEMRev The Comparison of Detailed Chemical Kinetic Mechanisms Forward Versus Reverse Rates with CHEMRev, Rolland, S. and Simmie, J. M. Int. J. Chem. Kinet. 37(3), 119-125 (2005). This program makes use of CHEMKIN input files and computes the reverse rate constant, kit), from the forward rate constant and the equilibrium constant at a specific temperature and the corresponding Arrhenius equation is statistically fitted, either over a user-supplied temperature range or, else over temperatures defined by the range of temperatures in the thermodynamic database for the relevant species. Refer to the website http //www.nuigalway.ie/chem/c3/software.htm for more information. 

Solvent extraction Database (SXD) software has been developed by A. Varnek et al.51 Each record of SXD corresponds to one extraction equilibrium and contains 90 fields to store bibliographic information, system descriptions, chemical structures of extractants, and thermodynamic and kinetic data in textual, numerical, and graphical forms. A search can be performed by any field including 2D structure. SXD tools allow the user to compare plots from different records and to select a subset of data according to user-defined constraints (identical metal, content of aqueous or organic phases, etc.). This database, containing about 3,500 records, is available on the INTERNET (http //infochim.u-strasbg.fr/sxd). 

In conclusion, pharmacokinetics is a study of the time course of absorption, distribution, and elimination of a chemical. We use pharmacokinetics as a tool to analyze plasma concentration time profiles after chemical exposure, and it is the derived rates and other parameters that reflect the underlying physiological processes that determine the fate of the chemical. There are numerous software packages available today to accomplish these analyses. The user should, however, be aware of the experimental conditions, the time frame over which the data were collected, and many of the assumptions embedded in the analyses. For example, many of the transport processes described in this chapter may not obey first-order kinetics, and thus may be nonlinear especially at toxicological doses. The reader is advised to consult other texts for more detailed descriptions of these nonlinear interactions and data analyses. 

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 computational scheme presented here can be used for any reaction scheme with only a user-supplied subroutine for the kinetic rate form for the various reactions and the physico-chemical parameters of the system. It can also be used as rate... 

This book is intended for users of techniques for the physico-chemical analysis of industrial catalysts and those who potentially require this analysis. The authors are research engineers at the departments of Physics and Analysis and of Kinetics and Catalyst of the Institut Francis du Petrole (IFP). Their role is to develop characterisation techniques and satisfy the requirements of research projects which involve the characterisation of solids. The book is derived from an internal IFP training course, prepared by the authors and intended for new technical staffin laboratories preparing catalytic solids. The purpose of this course is twofold. On the one hand, we arc clearly aiming to show what can be done. On the other hand, and perhaps more importantly, it is necessary to establish what cannot be done, by specifying the limits in terms of quantification and range of application of the techniques. 

Another class of models relevant to chemical weathering is based on the reaction path approach originally developed by Helgeson et al. (1970). EQ3/EQ6 (Wolery et al. (1990), PHREEQC (Parkhurst and Plummer, 1993), and PATHARC.94 (Gunter et al., 2000) are some codes currently used to describe the progressive reaction of primary silicates and the precipitation of secondary phases as a function of time and mass. These codes are discussed in Nordstrom (see Chapter 5.02). They commonly permit the introduction of user-defined silicate reaction rates. Such models also commonly consider solubility controls on reaction kinetics as defined by... 

Nevertheless, a number of problems remain in modelling both the physical processes (fluid dynamics, heat transfer), the complex chemistry and the coupling between them. There are major limitations on the applicability of computer models and in the accuracy which simulations can achieve. It is of the utmost importance for model users to be aware of the main sources of error blind belief in the output from models can be dangerous and expensive. In this chapter we consider the major source of uncertainty in chemical simulations, whether full or reduced mechanisms are used, namely the quality and quantity of the available kinetics data. 

The screen for each chemical kinetic calculation simultaneously displays a variety of characterizations in multiple windows and allows analysis of time/temperature-dependent species and reaction information including species concentrations, species steady-state analysis, individual reaction rates, net production/destruction rates, reaction equilibrium analysis and the temperature/time history of the system. The interactive user-sorting of the species and reaction information from the numerical simulations is mouse/cursor driven. An additional feature also allows interactive analysis and identification of dependent and independent species and reaction pathways, on-line reaction network analysis and pathway/flowchart construe-... 

Takeuchi et al. 7 reported a membrane reactor as a reaction system that provides higher productivity and lower separation cost in chemical reaction processes. In this paper, packed bed catalytic membrane reactor with palladium membrane for SMR reaction has been discussed. The numerical model consists of a full set of partial differential equations derived from conservation of mass, momentum, heat, and chemical species, respectively, with chemical kinetics and appropriate boundary conditions for the problem. The solution of this system was obtained by computational fluid dynamics (CFD). To perform CFD calculations, a commercial solver FLUENT has been used, and the selective permeation through the membrane has been modeled by user-defined functions. The CFD simulation results exhibited the flow distribution in the reactor by inserting a membrane protection tube, in addition to the temperature and concentration distribution in the axial and radial directions in the reactor, as reported in the membrane reactor numerical simulation. On the basis of the simulation results, effects of the flow distribution, concentration polarization, and mass transfer in the packed bed have been evaluated to design a membrane reactor system. 

Specification of necessary information/data related to flow process under consideration. Once a suitable grid is generated, the user has to specify the necessary information concerning the physicochemical properties of fluids such as molecular viscosity, density, conductivity etc. for the solution of model equations. If the process under consideration involves chemical reactions, all the other necessary data about reaction kinetics (and stoichiometry, heat of reaction etc.) need to be supplied. In addition to system-specific data, specification of boundary conditions on the edges/external surfaces of the solution domain is a further crucial aspect of the solution process. It is also necessary to provide all the information related to the numerical method selected... 

However, the popular List-of-Reactions data model has several serious deficiencies. First of all, it is difficult in this data model to adequately document the origins of all the numerical parameters used in the simulation, and the associated (usually highly correlated) uncertainties in these parameters. The format is also not very user-friendly it is almost impossible for a human to check that all the numbers and reactions in a large kinetic model expressed in this format are consistent and reasonable, much less correct. The conventional List format does not allow for graphical representation of chemical structures, leading to sometimes serious inconsistencies in naming conventions. 

There are many ways one can try to reduce the computational burden. Ideally, one would find numerical methods which are guaranteed to retain accuracy while speeding the calculations, and it would be best if the procedure were completely automatic i.e. it did not rely on the user to provide any special information to the numerical routine. Unfortunately, often one is driven to make physical approximations in order to make it feasible to reach a solution. Common approximations of this type are the quasi-steady-state approximation (QSSA), the use of reduced chemical kinetic models, and interpolation between tabulated solutions of the differential equations (Chen, 1988 Peters and Rogg, 1993 Pope, 1997 Tonse et al., 1999). All of these methods were used effectively in the 20th century for particular cases, but all of these approximated-chemistry methods share a serious problem it is hard to know how much error is... 

Nowadays, improved computing facilities and, more importantly, the availability of the Chemkin package (Kee and Rupley, 1990) and similar kinetic compilers and processors have made these complex kinetic schemes more user-friendly and allows the study of process alternatives as well as the design and optimization of pyrolysis coils and furnaces. In spite of their rigorous, theoretical approach, these kinetic models of pyrolysis have always been designed and used for practical applications, such as process simulation, feedstock evaluations, process alternative analysis, reactor design and optimization, process control and so on. Despite criticisms raised recently by Miller et al. (2005), these detailed chemical kinetic models constitute an excellent tool for the analysis and understanding of the chemistry of such systems. 

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