Reactive transport modeling in porous media

Bottom feed -response simulation refers to the calculation of processes in porous solids, which are determined by transport processes in the pores and associated chemical reactions. Partly also the term reactive transport used. The simulations are used to predict the time-dependent changes of the solid and the present in the pores of the substance ( liquid or gas). They are also a tool to better understand these processes if they are difficult or impossible to access in a laboratory.

Simulation of chemical reactions

Computer simulation of a chemical reaction begins to assign a volume ( the chemical system ) a chemical starting composition, a pressure and temperature. Then we can calculate the thermodynamically stable chemical composition (stable phase composition ). The difference between the starting composition and the thermodynamically stable composition is the elapsed reaction.

To calculate the thermodynamically stable phase content are two methods: One is to build and solve a system of equations using the equilibrium constants of the reactions. This method is used for example by the program EQ3 / 6 and the thermodynamics module of the program PHREEQC. The other way is based on an optimization of the Gibbs energy of the chemical system ( Gibbs Energy Minimization - GEM). This method uses such as the GEM program selector and the thermodynamics of the module Transreac program. The GEM method is more flexible. So no chemical reaction equations must be placed, only the substances involved in the possible reactions to be known. Constraints of both methods are the conservation of mass and charge in the chemical system.

The quality of the simulation of chemical reactions depends strongly on the quality of the underlying thermodynamic data. For mixed-phase mixture models and methods must be available to determine the activity coefficients. Chemical systems with highly concentrated aqueous solutions require, for example, the so-called Pitzer model for determining the activity coefficients of the solutes. Otherwise, for example, is already such a simple question such as the solubility of sodium chloride in water is not predictable. Often with reaction models can also be reaction path considerations perform, be in the one system successively added or removed components, for example in the simulation of a titration. One can thus follow the course of reactions, without being able to add to the process time scale.

Simulation of transport processes

Transport processes in porous bodies are described by the differential equations to be solved numerically, for example, a finite difference method or the finite element method. The finite element method has advantages when the body has to be considered a complicated geometry. Frequently various transport processes are interconnected and can not be considered independently. An example from physics: The heat transfer through building envelope depends strongly on its moisture content, since the thermal conductivity of the materials is moisture-dependent. Heat and moisture transport can not be considered independently of one another, therefore, as a rule. The following transport processes and effects can occur:

  • Diffusion of gases, such as water vapor diffusion
  • Capillary transport of water and other liquids
  • Flow processes (permeation, convection) of liquids and gases, including the associated dispersion
  • Multiphase flow
  • Heat conduction
  • Heat transfer by thermal radiation
  • Heat transport by flow processes in liquids and gases
  • Moisture transfer and heat transfer at interfaces of solids to gases or liquids
  • Entrainment of solutes in the capillary and in the flow of fluids ( piggyback transport)
  • Diffusion of solutes
  • Solute transport due to a diffusion potential which is built up due to the different diffusion rate of the solute
  • Solute transport in an electric field ( electrophoresis)

Changes in transport characteristics through the chemical reactions

Due to the chemical reactions occurring solid phases are dissolved or reformed. Thus, the pore system and the transport parameters change. Therefore transport -response simulations need a module with which this change can be described by transport parameters.

Influence of the chemical kinetics

Chemical kinetics is due to transport processes. In a transport response simulation, the speed of the reaction processes will now be described mainly about the simulation of transport processes in the pores, that is, on the subsequent transport of feedstock to the reaction site and the transporting away of the reaction products from the reaction site. For slow reactions, it may be necessary to consider additional reaction kinetic effects, for example, go back to the limited speed of the reaction between substances in the pore and the pore wall in contact therewith.

Transport -response models

Transport - reaction models add together the modules described above, so that a numerical calculation of a site- and time-dependent simulation of the process is possible. Transport - reaction models come from very different areas. The PHREEQC program comes for example from geochemistry. The Transreac program originates, for example, the building materials research. Transreac was later expanded on the embedding in a Monte Carlo simulation to a probabilistic method by which the scattering of the results can be calculated. Furthermore, an extension to the adaptive model, in which the inclusion of data from the building to improve the forecast of future device performance is possible. In part transport -response models are also coupled with mechanical models, as for example an incipient cracking due to corrosion processes has an effect on the transport processes. In order to outline the performance of these models, one uses the following abbreviations:

  • C - Simulation of Chemical Processes
  • T - Simulation of thermal processes
  • Here to refer generally to simulate hydraulic processes, and it is better mass transport - H
  • M - Simulation of mechanical processes

Applications

Main applications of transport -response simulations are in the range of geochemistry and Building Material Technology. The geochemistry uses the method to study the interactions between rocks and soils and the solutions therein. The building material technology uses the method of investigation of corrosion processes of components made of porous building materials. Corresponding simulations are, however, also used in process engineering and other fields.

Phase (matter)
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