Parameters ยป
In the Parameters Node, you may enter the kinetic constants. PreExponentials, Activation Energies and Reaction Orders can be entered for all reactions. LHHW Sites and their parameters can also be defined here. These sites can later be associated to the reactions in the Kinetics Sites node.
We have two formulations for rates:
- Mass Action (also known as Power-Law): For reactions that have no inhibition (LHHW) site assigned. The rate equation is proportional to the product of the concentration of reactants raised to some exponent.
Using Molarity basis, it has the general form:
The proportionality constant kc is the kinetic or rate constant, Ci is the molar concentration of reactant i, and the exponent ni is the order for this reactant.
When using Mass Density, the Ci in the equation above refers to mass concentration of compound i.
When using a Partial Pressure basis for the rate equation, the rate expression becomes:
where the rate constant is kp, and Pi is the partial pressure of compound i. The unit of the overall rate(r) is independent of whether partial pressure or molarity is chosen for the concentration basis. The units of the rate constant will change depending on this choice.
- LHHW (Langmuir-Hinshelwood-Hougen-Watson): When a reaction has an LHHW Site assigned to it in the Kinetics Sites node, its equation has the following general form:
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Where the numerator is equivalent to the Mass Action formulation, and the denominator represents the inhibition defined by the sites associated to the reaction. The LHHW example illustrates the setup of such rate equations.
Tabbed Views
Actions
Tabbed Views
Mass Action
LHHW Sites
Mass Action
For Mass Action Reactions, the information entered in this tab describes all their kinetics. For LHHW Reactions, the information entered here is for the numerator of the LHHW Kinetics.
Reaction Parameters
Reaction Name Orders
LHHW Sites
In the LHHW Sites tab, you may add information about the adsorption denominators, which are organized as sites and terms. Each site corresponds to an adsorption site and each term corresponds to a compound adsorbing on this site. While this organization is meaningful for solid-catalysed reactions, you may use the LHHW Equation type to express complex reactions by artificially creating sites and terms to create the desired rate expression. Each reaction may have one or more sites, and each site may have one or more term. The best way to understand this organization is to see the example of an LHHW expression.
Sites
Terms
Orders
ActionsFor Mass Action Reactions, the information entered in this tab describes all their kinetics. For LHHW Reactions, the information entered here is for the numerator of the LHHW Kinetics.
Grid Views
Reaction Parameters
Reaction Name Orders
Reaction Parameters
This grid is used to select options for the following items:
The rate constant (kc) for the mass action kinetics is expressed as:
In this grid, you may enter these rate parameters. This columns in this grid are organized as follows:
Reaction Name Orders
This grid depends on the Reaction Parameters grid and it is used to add the components and their orders.
To add components and the respective orders:
Top of Topic
This grid is used to select options for the following items:
The rate constant (kc) for the mass action kinetics is expressed as:
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where A is the PreExponential factor, E is the Activation Energy. |
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In this grid, you may enter these rate parameters. This columns in this grid are organized as follows:
- The Reaction column denotes the name of each reaction. This column is read-only.
- The PreExponential column is used to enter the values of the PreExponential (A) of the kinetic constant (Mass Action). Since the reaction orders are not known a priori, REX does not show the units of the rate parameters. Instead, the units of the overall rate [ for example: Moles/(Time-Volume) for homogeneous systems] are shown in the caption of this grid title. Once the reaction orders are known, the preexponential units can be inferred from the overall rate units.
- The Activation Energy column is used to enter the values of the Activation Energy (E) of the kinetic constant (Mass Action). Its units must be always kJ/mol.
Reaction Name Orders
This grid depends on the Reaction Parameters grid and it is used to add the components and their orders.
To add components and the respective orders:
- Select the reaction in the Reaction Parameters grid.
- Select the compound or pseudo-compound to be added for this reaction equation in the Component column.
- Enter the order in the Order column.
- Press <ENTER>; or click on the pencil in the row header
LHHW Sites
In the LHHW Sites tab, you may add information about the adsorption denominators, which are organized as sites and terms. Each site corresponds to an adsorption site and each term corresponds to a compound adsorbing on this site. While this organization is meaningful for solid-catalysed reactions, you may use the LHHW Equation type to express complex reactions by artificially creating sites and terms to create the desired rate expression. Each reaction may have one or more sites, and each site may have one or more term. The best way to understand this organization is to see the example of an LHHW expression.
Grid Views
Sites
Terms
Orders
Sites
The Sites grid is used to enter the sites with their constants. You may add as many sites as you need. Their names must meet a Special Syntax.
A Site has the following form:
Terms
The Terms grid is used to enter the adsorption constant of each term. You may enter the adsorption constant in the same way as the Reaction Parameters grid. A term caption Term1 is automatically created for first term. If you need to create additional terms for this site, please enter the PreExponential or Activation Energy and a new term caption will be automatically created.
A Term has the following form:
where Kij is the corresponding adsorption constant, Ck are the adsorbed components over the catalyst, and nijk are the component order for compound k on term j of site i.
Orders
The Orders grid is used to enter the orders of the absorbed components. You may enter the components and their order in the same way as the Reaction Name Orders grid.
Example: Kinetics - LHHW
Top of Topic
The Sites grid is used to enter the sites with their constants. You may add as many sites as you need. Their names must meet a Special Syntax.
A Site has the following form:
Terms
The Terms grid is used to enter the adsorption constant of each term. You may enter the adsorption constant in the same way as the Reaction Parameters grid. A term caption Term1 is automatically created for first term. If you need to create additional terms for this site, please enter the PreExponential or Activation Energy and a new term caption will be automatically created.
A Term has the following form:
where Kij is the corresponding adsorption constant, Ck are the adsorbed components over the catalyst, and nijk are the component order for compound k on term j of site i.
Orders
The Orders grid is used to enter the orders of the absorbed components. You may enter the components and their order in the same way as the Reaction Name Orders grid.
Example: Kinetics - LHHW
Initialize Orders
Quick Run
Open Solver
Initialize Orders
This action is used to enter the components and orders automatically. When this action is executed, the order of each compound is set equal to the molecularity of the compound in the reaction stoichiometry. The action is performed only for the reactions that are shown in this node. For reactions unincluded either in Reactions or in Kinetics nodes, this action is not executed.
Top of Topic This action is used to enter the components and orders automatically. When this action is executed, the order of each compound is set equal to the molecularity of the compound in the reaction stoichiometry. The action is performed only for the reactions that are shown in this node. For reactions unincluded either in Reactions or in Kinetics nodes, this action is not executed.
See Also: