COMSOL 6.2 - Species

) is automatically generated for the participating reactants and products. This feature enables you to review and enter species specific information regarding chemical kinetics, thermodynamics, and transport properties.

It is also possible to add and define an individual node: on the toolbar click or right-click the node and select it from the context menu.

When a node is automatically generated using the text field in the node, the species name is also automatically generated.

For a node added individually, enter a in the field and click . By entering a name consisting of only elements from the periodic table, the chemical formula is added, and molar mass is calculated and added in the section. The species charge, and type, are also deduced from the species name.

	Valid Species Names

Select a species type — , , or .

are solved for volumetric concentrations (SI unit: mol/m3), while are solved for surface concentration (SI unit: mol/m2). The compositions for and (not solved for) use the syntax c_speciesname, while uses csurf_speciesname_surf.

Surface species is referring to species adsorbed on a reactive surface. When is selected, the corresponding reaction formula introduces after the species notation and changes the species’ name to speciesname_surf. Additionally, the Species node name is updated in a similar fashion.

For the reactor only and are allowed.

	Choosing the Species Type

The section contains the species chemical formula, , M, and , z.

If the name entered for the species consists of only elements from the periodic table, and optionally charge and phase indication, then the field will be populated with the name entered in the section, and the molar mass and charge will be added to this section. The species molar mass is computed from the mass of occurring individual elements¹. The rules for writing chemical formulas are the same as those for names.

	Valid Species Names

When the species name contains parts not in the periodic table, for example when a descriptive name such as water is used, the molar mass is set to 0.0 kg/mol. If the species name consists of a valid charge or phase indication, then charge and type will be set automatically. In this case, the molar mass needs to be defined in order to achieve mass balanced reactions and correct definitions of mass basis properties. By entering a chemical formula consisting only of elements from the periodic table (and optionally charge and phase indication), the molar mass, and charge are derived automatically.

When needed, the can be edited in the corresponding text field. Editing the and pressing apply will override the content in the molar mass and charge fields. The field cannot be edited as long as a chemical formula is enabled.

With an enabled it is possible to use the trivial name of a species and balance reactions. Having added the Species water, oxygen, and hydrogen, and filled in H2O, O, and H in their respective chemical formula sections, the oxygen+hydrogen=water is balanced into oxygen+2hydrogen=water by clicking in the node.

It is possible to specify the species density ρ when the fluid is specified as . The default value is that of water at 293 K.

Change the default setting to to use a species reaction rate other than the one set up in the associated Reaction node. For individual species, use the option to set a reaction rate other than zero (that is, nonreactive).

Edit either the (SI unit: mol/(m3s)), the (SI unit: mol/(m2s)), or both. For a bulk species, both expressions appear if surface reactions are present in the reactor since the reaction of the species can depend both on bulk reaction R and surface reaction Rads rates. For a surface species, only the surface reaction rate Rads appears.

The reaction rate is not editable if the species in question participates in an equilibrium reaction and has been selected as a in Equilibrium Species Vector.

To account for nonideality in the fluid mixture adjust the activity coefficient in the text field here. The section is only shown if activity instead of concentration has been chosen in the interface; that is, the check box is selected on the Reaction Engineering interface window

Click to select the check box if the species concentration or activity should be treated as constant.

Here the required parameters to compute various transport properties can be edited. To show this section the check box needs to be selected under the section in the window of the physics interface.

ε/kb (SI unit: K) the potential energy minimum value of the Lennard–Jones/Stockmayer potential, divided by Boltzmann’s constant. Tabulated data in the literature frequently lists values of ε/kb rather than ε

ki, the thermal conductivity of the gas (SI unit: W/(m·K)); and Diffusivity of the species (SI unit: m2/s). The latter two can be set to , when σ and ε/kb have been given.

The parameters utilized for calculation of thermodynamic mixture and reaction properties are set in this section. It is accessed when the Energy Balance is set to for the Reaction Engineering interface, in other words, for nonisothermal conditions.

The are by default computed using data in the . In this case, enter the following to compute the species’ heat capacity, Cp (SI unit: J/(mol⋅K)), the molar enthalpy, h (SI unit: J/mol), and the molar entropy, s (SI unit: J/(mol⋅K)):

Any coefficients for the thermodynamic polynomials entered into the alow,k fields apply to the temperatures in the range Tlo to Tmid; coefficients entered into the ahi,k fields apply to temperatures in the range Tmid to Thi range. The coefficients can also be imported in the section in the Reaction Engineering node.