Equilibrium Reaction
Use this node to model a reaction where the kinetics is assumed so fast that the equilibrium condition is fulfilled at all times. The node solves for an additional degree of freedom (the reaction rate Req) to fulfill the equilibrium condition at all times in all space coordinates.
This feature is only available in a limited set of add-on products. See www.comsol.com/products/specifications/ for more details on availability.
Equilibrium Condition
The Chemistry list includes all the chemistry interfaces which are coupled to the current mass transfer interface. The conditions for valid coupling are the same as that in the Reaction Sources feature. The Chemistry list can be set to a coupled chemistry or None.
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A coupled chemistry — The Equilibrium reaction formula is available when the Chemistry list is set to a coupled chemistry interface. All the equilibrium reaction formulas in the coupled chemistry will appear in the formula list. If you select one of them, all coefficients (stoichiometric coefficients, equilibrium constant) will automatically be taken from the coupled reaction. There are no source term contributions if the Equilibrium reaction formula list is set to User defined.
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None — If None is selected from the Chemistry list, an equilibrium reaction will be defined by settings from the fields Equilibrium definition, Equilibrium constant, and Unit activity concentration.
Selecting Equilibrium constant in the list, the following equilibrium condition based on the species activities and the law of mass action is used
where νi are the stoichiometric coefficients and the species activities are defined from the concentration, ci, and the unit activity concentration ca0.
Enter a value or expression for the dimensionless Equilibrium constant Keq, and the Unit activity concentration Ca0.
Select User defined from the list to instead enter a manually defined Equilibrium expression Eeq.
Stoichiometric Coefficients
This section is available when the Chemistry list is set to None, and used to set stoichiometric coefficients for the specified equilibrium reaction.
Enter a value for the stoichiometric coefficient ν for all participating species. Use negative values for reactants and positive values for products in the modeled reaction. Species with a stoichiometric coefficient value of 0 are not affected by the Equilibrium Reaction node.
Heat Source
The heat source will be defined as follows,
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An equilibrium reaction selected from the Equilibrium reaction formula list. The heat source is defined as the equilibrium reaction (from the coupled chemistry) heat,
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None selected from the Chemistry list. Enter a value or expression for the heat source (SI unit: W/m3).