Reactive transport modeling

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Under the reactive transport modeling means the calculation of processes in porous or fractured solids , by transport processes in the pores or cavities and related chemical reactions are determined. The terms reactive mass transport modeling , reactive transport or transport-reaction simulation are also used in some cases . The simulations are used to forecast time-dependent changes in the solid and the substance in the pores (liquid or gas). With reactive transport modeling, processes can be better understood if they are either difficult or inaccessible in laboratory experiments, run on a large scale in the field or are of so long duration that they cannot be observed. Typical fields of application are the simulation of the groundwater flow with reactive components, e.g. the breakdown of nitrate, the spread of radionuclides in the area of ​​a repository, or the leakage of CO 2 into near-surface aquifers during CO 2 storage. Reactive modeling is also used in materials science.

Simulation of chemical reactions

The computer simulation of a chemical reaction begins with assigning a volume (the chemical system) an initial chemical composition, a pressure and a temperature. Then the thermodynamically stable chemical composition (stable phase inventory ) is calculated . The difference between the initial composition and the thermodynamically stable composition is the chemical reaction that takes place. A more general approach does not calculate the chemical reaction for a thermodynamic equilibrium, but rather on the basis of reaction rates over time, i.e. taking into account the reaction kinetics .

Two calculation methods are available for calculating the thermodynamically stable phases: On the one hand, a system of equations can be set up and solved with the aid of the equilibrium constants of the reactions using the law of mass action ("law-of-mass-action" approach, LMA). This method is used e.g. Used by codes EQ3 / 6, PHREEQC and The Geochemist's Workbench . The other way is based on an optimization of the Gibbs energy of the chemical system ("Gibbs Energy Minimization" - GEM). This method uses z. B. the program GEM-Selektor, FactSage and the thermodynamics module of the program Transreac . For the GEM method, no chemical reaction equations have to be set up, only the substances involved in possible reactions need to be known. Constraints of both methods are the conservation of mass and charge in the chemical system.

The simulation result of a chemical reaction depends on the thermodynamic data used, e.g. B. equilibrium constants , parameters for temperature correction and for calculating activity coefficients . Due to the experimental boundary conditions used to determine the data, the thermodynamic data show a certain range, which means that the results calculated with these data can also show a range if different databases are used. The uncertainty caused by these parameters is known as database uncertainty and affects the forecasts that are simulated using the geochemical codes.

The simulation of chemical systems with highly concentrated aqueous solutions (high ionic strengths) depends heavily on the activity models (equations and parameters) used. A model that shows very good agreement with experimental data at high ionic strengths is the Pitzer model

Reaction models can often also be used to consider reaction pathways by successively adding or removing components to a system. An example of this is the simulation of a titration . In this way, one can follow the course of reactions without being able to apply a time scale to the process.

Simulation of transport processes

Transport processes in porous bodies are described using differential equations that are solved numerically, e.g. B. via a difference method or the finite element method . The finite element method has advantages when the body to be considered has a complicated geometry. Often, different transport processes are linked and cannot be viewed independently of one another. An example from building physics : The heat transfer through external building components depends heavily on their moisture content, since the thermal conductivity of the building materials is moisture-dependent. As a rule, heat and moisture transport cannot therefore be viewed independently of one another. The following transport processes and effects can occur:

Change in transport parameters due to the chemical reactions taking place

Solid phases are dissolved or newly formed by the chemical reactions taking place. This can change the pore system and the transport parameters such as porosity and permeability. Transport reaction simulations therefore require a module with which this change in transport parameters can be described.

Influence of chemical kinetics

The chemical kinetics can be traced back to the transport processes of the reacting ions from the surface of the reacting substance into the solution. In reactive transport modeling, the speed of the reaction processes is essentially described via the simulation of the transport processes in the pores, that is, via the transport of starting materials to the reaction site or from the reaction products to the reaction site. When considering a time dependency, it is necessary to take into account kinetic reactions that occur e.g. B. due to the limited speed of the reaction between the substances in the pore and the pore wall in contact with it.

Reactive transport models

Reactive transport models combine the modules described above so that a numerical calculation enables a location- and time-dependent simulation of the chemical, hydraulic, thermal and mechanical processes. Reactive transport models come from different areas. The PHREEQC program, for example, comes from hydrogeochemistry, but only calculates 1-D transport and geochemical reactions. The OpenGeoSys program can link different codes and thereby calculate different processes. The Transreac program, for example, comes from building materials research. Transreac was later extended to a probabilistic method by embedding it in a Monte Carlo simulation , with which the scattering of the results can also be calculated. In addition, an extension to the adaptive model was carried out, in which the inclusion of measurement data from the building enables an improvement in the prognosis of future component behavior. In some cases, transport-response models are also coupled with mechanical models, as e.g. B. the onset of cracking due to corrosion processes has an impact on the transport processes. The following abbreviations are used to outline the capabilities of corresponding models:

  • C - simulation of chemical processes
  • T - simulation of thermal processes
  • H - Simulation of hydraulic processes, whereby it is better to speak of mass transfer here in general
  • M - simulation of mechanical processes

Applications

The main applications of transport-reaction simulations are in the field of geochemistry and building material technology . Geochemistry uses the process to investigate the interactions between rocks or soils and the solutions they contain. Building material technology uses the process to investigate corrosion processes in components made of porous building materials. Corresponding simulations are also used in process engineering and other areas.

Web links

supporting documents

  1. ↑ a b c d F. Schmidt-Döhl : Material testing in construction . Fraunhofer irb-Verlag, Stuttgart 2013, ISBN 978-3-8167-8747-1 .
  2. ↑ EQ3 / 6 overview. (PDF; 422 kB) Lawrence Livermore National Laboratory, USA, accessed November 3, 2012 .
  3. ↑ a b PHREEQC overview. US Geological Survey, accessed November 3, 2012 .
  4. ^ Documentation GWB. Aqueous Solutions LLC, accessed May 13, 2016 .
  5. ↑ GEM selector overview. Paul Scherrer Institute, Switzerland, accessed November 3, 2012 .
  6. ^ CW Bale, P. Chartrand, SA Degterov, G. Eriksson, K. Hack: FactSage thermochemical software and databases . In: Calphad . tape 26 , no. 2 , June 1, 2002, p. 189-228 , doi : 10.1016 / S0364-5916 (02) 00035-4 ( sciencedirect.com [accessed November 17, 2016]).
  7. ↑ a b c d e F. Schmidt-Döhl: A model for the calculation of combined chemical reaction and transport processes and its application to the corrosion of mineral building materials. (= Series of publications by the Institute for Building Materials, Solid Construction and Fire Protection at the TU Braunschweig. Issue 125). 1996, ISBN 3-89288-104-9 . (also: Braunschweig. TU, dissertation, 1996)
  8. ↑ a b E. Rigo: A probabilistic concept for assessing the corrosion of cement-bound building materials through dissolving and driving attack. In: Series of publications by the Institute for Building Materials, Solid Construction and Fire Protection, Issue 186, 2005, ISBN 3-89288-169-3 , also: Braunschweig. TU, dissertation. 2005.
  9. ↑ Craig M. Bethke: Geochemical and Biogeochemical Reaction Modeling . Cambridge University Press, 2007, ISBN 978-1-139-46832-9 ( google.de [accessed November 17, 2016]).
  10. ↑ Irina Gaus, Pascal Audigane, Laurent André, Julie Lions, Nicolas Jacquemet: Geochemical and solute transport modeling for CO2 storage, what to expect from it? In: International Journal of Greenhouse Gas Control (=  TCCS-4: The 4th Trondheim Conference on CO2 Capture, Transport and Storage ). tape 2 , no. 4 , October 1, 2008, p. 605-625 , doi : 10.1016 / j.ijggc.2008.02.011 ( sciencedirect.com [accessed November 17, 2016]).
  11. ↑ a b Christoph Haase, Frank Dethlefsen, Markus Ebert, Andreas Dahmke: Uncertainty in geochemical modeling of CO2 and calcite dissolution in NaCl solutions due to different modeling codes and thermodynamic databases . In: Applied Geochemistry . tape 33 , June 1, 2013, p. 306-317 , doi : 10.1016 / j.apgeochem.2013.03.001 ( sciencedirect.com [accessed November 17, 2016]).
  12. ↑ Christoph Haase: Hydrogeochemical modeling of CO2 storage and leakage in geological formations - uncertainties caused by thermodynamic databases and numerical codes . May 24, 2016 ( uni-kiel.de [accessed November 17, 2016]).
  13. ↑ Kenneth S. Pitzer: Thermodynamics of electrolytes. I. Theoretical basis and general equations . In: The Journal of Physical Chemistry . tape 77 , no. 2 , 1973, ISSN  0022-3654 , pp. 268-277 , doi : 10.1021 / j100621a026 .
  14. ↑ Kenneth S. Pitzer, Guillermo Mayorga: Thermodynamics of electrolytes. II. Activity and osmotic coefficients for strong electrolytes with one or both ions univalent . In: The Journal of Physical Chemistry . tape 77 , no. 19 , September 1, 1973, ISSN  0022-3654 , pp. 2300-2308 , doi : 10.1021 / j100638a009 .
  15. ↑ S. Bruder: Adaptive model of durability in the course of monitoring concrete structures. (= Series of publications by the Institute for Building Materials, Solid Construction and Fire Protection. Issue 196). 2007, ISBN 978-3-89288-178-0 . (also: Braunschweig. TU, dissertation, 2006)