Equilibrium Reaction
Use this node to model a reaction where the kinetics is 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.
If the Apply equilibrium condition on inflow boundaries checkbox is selected, the specified inflow concentration values in all active Inflow boundary nodes for the physics interface are modified to comply with the equilibrium condition.
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A necessary requirement for this is feature to be available is that two or more species are solved for by the interface.
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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 shows two ways to define an equilibrium reaction.
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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. Select one of them, then all coefficients (stoichiometric coefficients, equilibrium constant) will be taken from the coupled reaction automatically. There would be no any source term contributions if the Equilibrium reaction formula list is set to None.
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 None — If None is selected from the Chemistry list, an equilibrium reaction will be defined by settings from fields of Equilibrium definition, Equilibrium constant, and Unit activity concentration.
The default value for Equilibrium definition list is Equilibrium constant or select User defined. For either option, the Apply equilibrium condition on inflow boundaries checkbox is selected by default.
For Equilibrium constant, enter an Equilibrium constant Keq (dimensionless). Also enter a value or expression for the Unit activity concentration Ca0 (SI unit: mol/m3). Selecting Equilibrium constant defines an equilibrium condition based on the stoichiometric coefficients, the species activities, and the law of mass action.
For User defined, enter an Equilibrium expression Eeq (dimensionless).
Stoichiometric Coefficients
This section is available when the Chemistry list is set to None.
Enter a value for the stoichiometric coefficientνc (dimensionless). The default is 0. 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 from reaction heat from this equilibrium reaction.
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An equilibrium reaction defined from a Chemistry (from the Equilibrium reaction formula list). The heat source is defined as the same as that of equilibrium reaction in Chemistry.
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None selected in the Chemistry list. Enter a value or expression for the heat source (SI unit: W/m3).
Further Reading
See
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Equilibrium Reaction Theory