The reaction mechanism may consist of any number of chemical reactions involving the species named in the species data. A reaction may be reversible or irreversible, it may be a three-body reaction with an arbitrary third body and/or enhanced third body efficiencies, and it may have one of several pressure-dependent formulations.
Reaction data must start with the word REACTIONS (or
REAC). On the same line, the user may specify units of the Arrhenius rate
coefficients (see Gas-phase Reaction Rate Formulations) to follow by
including the word CAL/MOLE, KCAL/MOLE,
JOULES/MOLE, KJOULES/MOLE,
KELVIN, or EVOLTS for
Ei, and/or MOLES or
MOLECULES for Ai. If
MOLECULES is specified, then the units for
Ai are cm, molecules, sec, and K.
Tip: If units are not specified, Ai and
Ei must be in cm, mole, sec, K and cal/mole,
respectively. Note that T is always in Kelvin.
Also on the REACTION line, you may indicate that a user-written rate
routine will be used to over-ride the species rates-of-production calculations, by including
the word USRPROD. This would over-ride any other reaction input in the
Gas-phase Kinetics input file. The USRPROD input optionally includes a
slash(/)-delimited integer parameter, which allows you to distinguish one
reaction model from another within their user routine. When the USRPROD
parameter is included, KINetics will call a user-supplied subroutine,
CKUPROD wherever net species production rates are required. A template of
CKUPROG can be obtained from Ansys as well as information about how to compile and link user
routines into KINetics.
Caution: Ansys Chemkin-Pro does not support user-written programming, so you are cautioned to use this feature at your own risk.
The lines following the REACTION line contain reaction descriptions
together with their Arrhenius rate coefficients. The reaction description is composed of
reaction data and optional auxiliary information data.
Each reaction line is divided into two fields. The first contains the symbolic description
of the reaction while the second contains the Arrhenius rate coefficients. Both fields are
format free and blank spaces are ignored. Any line or portion of a line starting with an
exclamation mark (!) is considered a comment and is ignored. Blank lines
are ignored.
The reaction description, given in the first field, must be composed of the species symbols, coefficients, delimiters, and any special symbols defined in Table 3.4: Reaction data criteria.
Table 3.4: Reaction data criteria
| Species Symbols | |
|
Each species in a reaction is described with the unique sequence of characters as
they appear in the species data and the thermodynamic data (for
example, | |
| Coefficients | |
|
A species symbol may be preceded by an integer or real coefficient. The coefficient
has the meaning that there are that many moles of the particular species present as either
reactants or products; for example, | |
| Delimiters | |
| + |
A plus sign is the delimiter between each reactant species and each product species. |
| = |
An equality sign is the delimiter between the last reactant and the first product in a reversible reaction. |
| <=> |
An equality sign enclosed by angle brackets can also be used as the delimiter between the last reactant and the first product in a reversible reaction. |
| => |
An equality sign with an angle bracket on the right is the delimiter between the last reactant and the first product in an irreversible reaction |
| Special Symbols | |
| +M |
An M as a reactant and/or product stands for an arbitrary third body. It should appear as both a reactant and a product. In a reaction containing an M, species can be specified to have enhanced third body efficiencies, in which case auxiliary information data (described below) must follow the reaction line. If no enhanced third body efficiencies are specified, then all species act equally as third bodies and the effective concentration of the third body is the total concentration of the mixture. |
| (+M) |
An M as a reactant and product surrounded by parentheses indicates that the reaction is a pressure-dependent reaction, in which case auxiliary information line(s) (described below) must follow the reaction to identify the fall-off formulation and parameters. A species may also be enclosed in parenthesis. Here, for example, (+H2O) indicates that water is acting as the third body in the fall-off region, not the total concentration M. |
| ! |
An exclamation mark means that all following characters are comments on the reaction line. For example, the comment may be used to give a reference to the source of the reaction and rate data. |
The second field of the reaction line is used to define the Arrhenius rate coefficients
, 
, and 
, in that order, as given by Equation 4–5. At least one blank space must separate the first number and the
last symbol in the reaction. The three numbers must be separated by at least one blank space,
be stated in either integer, floating point, or "E" format (for example,
123, 123.0 or 123E1), and have
units associated with them.
Tip: Unless modified by the REACTION line, the default units for 
are in cgs (cm, sec, K, mole), the exact units depending on the reaction.
The factor 
is dimensionless. The default units for the activation energies are
cal/mole.
Examples of reaction data are shown in Figure 3.1: Examples of reaction data.
Figure 3.1: Examples of reaction data
REACTIONS CAL/MOLE
2 + O2 = 2OH 1.7E13 0 47780. ! Ref. 21
! H2 + O2 = OH + H 1.7E13 0 47780. ! same as previous reaction,
! commented to prevent a duplication error
H + O2 + M = HO2 + M 2.0E15 0.000 -870.
! H + O2 + M = HO2 2.0E15 0.000 -870.
! H + O2 = HO2 + M 2.0E15 0.000 -870.
OH+ + H + E = H2O 1.E19 0 0.0
O + HV = O(*) 1.3A5 0 0
0.5H2 + 0.5O2 = OH ! example of real coefficients
END ! END statement is optional;
! <eof> condition is equivalent
Table 3.5: Summary of the rules for gas-phase reaction data is a summary of the gas-phase reaction data rules.
Table 3.5: Summary of the rules for gas-phase reaction data
| Rule | Description |
|---|---|
| 1 |
The first reaction line must start with the word |
| 2 |
The reaction description can begin anywhere on the line. All blank spaces, except those between Arrhenius coefficients, are ignored. |
| 3 |
Each reaction description must have =, <=> or => between the last reactant and the first product. |
| 4 |
Each reaction description must be contained on one line. |
| 5 |
Three Arrhenius coefficients must appear in order ( |
| 6 |
There cannot be more than six reactants or six products in a reaction. |
| 7 |
Comments are any and all characters following an exclamation mark. |
The format of an auxiliary information line is a character-string keyword followed by a
slash-delimited (/) field containing an appropriate number of parameters
(either integer, floating point, or "E" format).
Neutral Third Body Efficiency
If a reaction contains M as a reactant and/or product, auxiliary
information lines may follow the reaction line to specify enhanced third-body efficiencies of
certain species (that is, 
, Equation 4–12). To define an
enhanced third-body efficiency, the keyword is the species name of the third body, and its one
parameter is the enhanced efficiency factor of that species. A species that acts as an enhanced
third body must be declared as a species. Examples of third body efficiencies are shown in the
first three reactions in Example 3.4: Examples of Gas-phase auxiliary information data.
Pressure Dependent Reaction Parameters
If a pressure-dependent reaction is indicated by a (+M) or by a
species contained in parentheses, such as (+H2O), then one or more
auxiliary information lines must follow to define the pressure-dependence parameters. In this
section we describe formulations where a fixed functional form is described for the pressure
interpolation between a high-pressure limit and low-pressure limit defined by Arrhenius
coefficients. The user has several options for defining the functional form and the parameters
that describe its shape. The user may also want to consider Chebyshev Polynomial Rate
Expressions (see Chebyshev Polynomial Rate Expressions and Pressure
Dependence Through Logarithmic Interpolation (see Pressure Dependence Through Logarithmic Interpolation for alternative methods in describing pressure dependence.
For the formulations described here, the Arrhenius coefficients on the reaction line are
for the high-pressure limit (
, 
, and 
) for unimolecular fall-off reactions and represent the low-pressure limit (
, 
, and 
) for chemically activated bimolecular reactions (see the description of these
reaction types in Chemically Activated Bimolecular Reactions). For all such
pressure-dependent reactions, an auxiliary information line must follow to specify either the
low-pressure limit Arrhenius parameters (for fall-off reactions) or the high-pressure limit
Arrhenius parameters (for chemically activated reactions). For fall-off reactions, the keyword
LOW must appear on the auxiliary information line, with three rate
parameters 
, 
, and 
(Equation 9-14). For chemically activated bimolecular
reactions, the keyword HIGH must appear on the auxiliary information line,
with the three rate parameters 
, 
, and 
(Equation 4–15).
There are then three possible interpretations for the functional form of the pressure dependence for these types of reactions:
To define the Lindemann [6] formulation of a pressure-dependent reaction, no additional parameters are defined.
To define a Troe [7] pressure-dependent reaction, in addition to the
LOWorHIGHparameters, the keywordTROEfollowed by three or four parameters must be included in the following order:
, 
, 
, and 
(Equation 4–22). The fourth
parameter is optional and if omitted, the last term in Equation 4–22 is not used.To define an SRI pressure-dependent reaction [19], in addition to the
LOWorHIGHparameters, the keywordSRIfollowed by three or five parameters must be included in the following order:
, 
, 
, 
, and 
(Equation 4–24). The fourth
and fifth parameters are options. If only the first three are stated, then by default
and 
.
Examples of these options are shown in Example 3.4: Examples of Gas-phase auxiliary information data.
Chebyshev Polynomial Rate Expressions
A Chebyshev reaction is similar to other pressure-dependent reactions described in Pressure Dependent Reaction Parameters, in that it is indicated by a
(+M) as a reactant and product. To specify Chebyshev polynomial
implementation, the keyword CHEB must be included after the reaction line,
and followed by any number of parameters. There may be multiple CHEB lines
required for a given reaction. Following the CHEB line, the first
parameter given must be N, the number of basis functions along the
temperature axis. The second parameter must be M, the number of basis
functions along the pressure axis. The remainder of the parameters must be the
NM
coefficients 
from Equation 4–29, in the order
of 
.
To specify the temperature limits, the keyword TCHEB must be followed
by 
and 
, otherwise the default limits are 300 K and 2500 K, respectively. To specify
pressure limits, the keyword PCHEB must be followed by 
and 
, otherwise the default limits are 0.1 atm and 100 atm, respectively. See
Example 3.4: Examples of Gas-phase auxiliary information data for an example of Chebyshev
polynomial input.
Pressure Dependence Through Logarithmic Interpolation
This generalized way of describing the pressure dependence of a
reaction rate is indicated by the use of the PLOG keyword in auxiliary
lines. In this case, the reaction description should not include (+M) in
it, although this is used to indicate that the reaction is pressure dependent in other cases.
This option for describing pressure dependent reactions can not be combined in any given
reaction with other options for describing pressure dependence. One supplementary line starting
with the PLOG keyword must be supplied for each pressure in the set. The
keyword is followed by slash-delimited values for the pressure (in atmospheres) and the rate
parameters for that pressure. The supplementary lines must be in order of increasing pressure.
If the rate expression at a given pressure cannot be described by a single set of Arrhenius
parameters, more than one set may be provided. See Figure 3.2: Examples of PLOG data using multiples sets of Arrhenius parameters.These
will be treated as DUPLICATE reactions, meaning the sum of the sets of
rates provided for a given pressure will be used. The units of the rate parameters provided
with the PLOG keyword should match the units used for the overall reaction
description. Note that in this case, although rate parameters need to be supplied on the main
reaction line to prevent an error, those values are superseded by the ones provided on the
supplementary lines.
Figure 3.2: Examples of PLOG data using multiples sets of Arrhenius parameters
X + Y = Z A b E PLOG /1.0 A1 b1 E1/ PLOG /1.0 A2 b2 E2/ PLOG /10 A3 b3 E3/ PLOG /10 A4 b4 E4/
Landau-Teller Reactions
To specify Landau-Teller parameters, the keyword LT must be followed
by two parameters: the coefficients 
and 
from Equation 4–31. The
Arrhenius parameters 
, 
, and 
are taken from the numbers specified on the reaction line itself. If reverse
parameters are specified in a Landau-Teller reaction by REV (see
Reverse Rate Parameters ), the reverse
Landau-Teller parameters must also be defined, with the keyword RLT and
two coefficients 
and 
for the reverse rate.
Reverse Rate Parameters
For a reversible reaction, auxiliary information data may follow the reaction to specify
Arrhenius parameters for the reverse-rate expression. Here, the three Arrhenius parameters
(
, 
, and 
) for the reverse rate must follow the keyword REV. This
option overrides the reverse rates that would be normally computed by satisfying microscopic
reversibility through the equilibrium constant, Equation 4–6.
Reaction Order Parameters
Auxiliary data may be included to override the reaction order for a species, using the
auxiliary keywords FORD or RORD, for forward and
reverse reaction descriptions, respectively. Each occurrence of these keywords must be followed
by the species name and the new reaction order. This option overrides the values of
and 
in Equation 4–4, for the species
included in the auxiliary data.
Reaction Units
It is sometimes convenient to specify the units for a particular reaction rate fit that
may differ from the default units specified for other reaction expressions in the chemistry
mechanism. In this case, the user employs the auxiliary keyword UNITS.
This keyword must be followed by one or more of the following unit descriptors:
MOLE(CULE), CAL, KCAL,
JOUL, KJOU, KELV(IN), or
EVOL(TS), where the letters in parentheses are optional. The inclusion of
MOLE(CULE) would indicate that the reaction rate expression is in units of
molecules/cm3 rather than mole/cm3. The
remaining unit descriptors specify the units of the energy 
in the rate expression. Note that the units of 
in the rate expression are always treated as Kelvin.
Duplicate Reaction Descriptions
It may occur that two or more reactions can involve the same set of reactants and
products, but proceed through distinctly different processes. In these cases, it may be
appropriate to state a reaction mechanism that has two or more reactions that are the same, but
have different rate parameters. However, duplicate reactions are normally considered errors by
the Pre-processor. If the user requires duplication (for example, the same reactants and
products with different Arrhenius parameters), an auxiliary information statement containing
the keyword DUP (with no parameters) must follow the reaction line of each
duplicate reaction (including the first occurrence of the reaction that is duplicated). For
example, if the user wishes to specify different rate expressions for each of three identical
reactions, there must be three occurrences of the DUP keyword, one
following each of the reactions.
Any number of auxiliary information lines may follow a reaction line, in any order, and any number of keywords or enhanced third bodies may appear on an auxiliary information line; however, a keyword and its parameter(s) must appear on the same line.
Examples of equivalent ways to state auxiliary information are shown in Example 3.4: Examples of Gas-phase auxiliary information data. The above rules are summarized in Table 8-6.
Optional User Rate Subroutine CKUPROG
The auxiliary keyword USRPROG, optionally appended by a
slash(/)-delimited integer parameter, implies that the net rate of
progress for the reaction will be obtained by calling a user-supplied subroutine,
CKUPROG. The optional parameter allows the user to select from more than
one type of rate formulation. Wherever the net reaction rate is required, it will be
obtained by calling the user-written subroutine. USRPROG applies only to
irreversible reactions, and cannot be used in conjunction with USRPROD
(entered on the REACTION line). A template of
CKUPROG can be obtained from Ansys as well as information on how to
compile and link user routines into KINetics.
Example 3.4: Examples of Gas-phase auxiliary information data
REACTIONS CAL/MOLE HCO+M=H+CO+M 0.250E+15 0.000 16802.000 ! Warnatz CO/1.87/ H2/1.87 CH4/2.81/ CO2/3./ H2O/5./
H+C2H4(+M)=C2H5(+M) 0.221E+14 0.000 2066.000 ! Michael LOW / 6.369E27 -2.76 -54.0 / !Lindemann fall-off reaction H2/2/ CO/2/ CO2/3/ H2O/5/ ! enhanced third-body efficiencies
CH3+CH3(+M)=C2H6(+M)9.03E16 -1.18 654. LOW / 3.18E41 -7.03 2762 / TROE / 0.6041 6927. 132. / ! TROE fall-off reaction, with 3 parameters H2/2/ CO/2/ CO2/3/ H2O/5/ ! enhanced third-body efficiencies
CH3+H(+M)=CH4(+M)6.0E16 -1.0 0.0 LOW / 8.0E26 -3.0 0.0/ SRI / 0.45 797. 979. / ! SRI fall-off reaction H2/2/ CO/2/ CO2/3/ H2O/5/ ! enhanced third-body efficiencies
CH3+CH3(+M)=H + C2H5(+M)4.989E12 0.099 10600.0! Stewart HIGH/ 3.80E-7 4.838 7710. / ! Chemically activated reaction SRI / 1.641 4334 2725 / ! SRI pressure dependence
CH4+H=CH3+H2 1.25E14 0 1.190E4 ! Westbrook REV / 4.80E12 0 1.143E4 /
! The following two reactions are acceptable duplicates:
H2+O2 = 2OH 1.7E13 0 47780 DUPLICATE H2+O2 = 2OH 1.0E13 0 47000. DUPLICATE
H2(1)+H2O(000)=H2(0)+H2O(001) 2.89E15 0 0 LT / -67 62.1/ ! Landau-Teller reaction
! The following is a Chebyshev polynomial rate description
C2H5 + O2 (+M) <=> C2H4E + HO2 (+M) 1.00E+00 .000 0. ! Bozzelli TCHEB/ 300 2500/ PCHEB/1 100/ CHEB/ 7 3 1.0216E+01 -1.1083E+00 -1.9807E-01 7.8325E-01/ CHEB/ 1.1609E+00 1.1762E-01 -9.5707E-02 1.0928E-01 1.1551E-01/ CHEB/ -8.0290E-02 -1.0978E-01 3.7074E-04 -1.4830E-02 -6.0589E-02/ CHEB/ -2.8056E-02 6.9203E-03 -9.7259E-03 -1.3556E-02 7.6648E-03/ CHEB/ 6.6865E-03 -8.8244E-04/
H2O+H = OH+H2 0.117E+10 1.30 3626 FORD /H2O 1.1/
END !END line is optional Table 3.6: Summary of the rules for gas-phase auxiliary information data summarizes the rules for Auxiliary Information Data.
Table 3.6: Summary of the rules for gas-phase auxiliary information data
| Rule | Description |
|---|---|
| 1 |
Auxiliary information lines may follow reaction lines that contain an
|
| 2 |
A species may have only one enhanced third-body efficiency associated with it in any one reaction. |
| 3 |
Only one radiation wavelength may be declared in a reaction. |
| 4 |
The order in which the enhanced third-body declarations are given is the order in which arrays of enhanced third-body information are referenced in the subroutine package. |
| 5 |
There cannot be more than ten enhanced third-bodies in a reaction. |
| 6 |
Keyword declarations may appear anywhere on the line, in any order. |
| 7 |
Any number of keywords may appear on a line and more than one line may be used; however, a keyword and its parameter(s) must appear on the same line. |
| 8 |
Keyword declarations that appear on the same line must be separated by at least one blank space. |
| 9 |
Any blank spaces between a keyword and the first slash are ignored and any blanks between the slashes and parameter(s) are also ignored. However, no blank spaces are allowed within a keyword or a parameter. |
| 10 |
All characters following an exclamation mark are comments. |
In some problems, only information about the elements and species is needed (for example, when surface chemistry is dominant). In such cases, it is not necessary to include reaction data. The KINetics pre-processor will proceed to store only element, species, and thermodynamic data when no reaction data is found in the Gas-phase Kinetics input file.