Scribd
Upload
60 views3 pages

9.1.2. Reaction Rate Types

This document discusses different types of reaction rates that can be defined in CFX software for modeling chemical reactions: 1) Arrhenius reaction rates depend on temperature and follow standard Arrhenius equations, with options for modified forms and temperature-dependent pre-exponential factors. 2) Arrhenius rates can include a temperature probability density function to describe temperature variation over time. 3) Expressions allow entering custom rate equations by typing them. 4) Equilibrium rates calculate forward and backward reaction rates to model reversible reactions at equilibrium based on concentrations and an equilibrium constant.

Uploaded by

Mohmmed Mahmoud
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
Available Formats
Download as PDF, TXT or read online on Scribd
60 views3 pages

9.1.2. Reaction Rate Types

This document discusses different types of reaction rates that can be defined in CFX software for modeling chemical reactions: 1) Arrhenius reaction rates depend on temperature and follow standard Arrhenius equations, with options for modified forms and temperature-dependent pre-exponential factors. 2) Arrhenius rates can include a temperature probability density function to describe temperature variation over time. 3) Expressions allow entering custom rate equations by typing them. 4) Equilibrium rates calculate forward and backward reaction rates to model reversible reactions at equilibrium based on concentrations and an equilibrium constant.

Uploaded by

Mohmmed Mahmoud
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
Available Formats
Download as PDF, TXT or read online on Scribd
You are on page 1/ 3

9.1.2.

Reaction Rate Types

On the Reaction Rate tab of a reaction definition, the following types of reaction rate may be
available:

Arrhenius

Arrhenius with Temperature PDF

Expression

Equilibrium

9.1.2.1. Arrhenius

Two forms of the Arrhenius dependency are supported in CFX. The first is given by:

(9–2)

where is reaction rate coefficient, is the pre-exponential factor, is the activation energy,
is the universal gas constant, is the temperature exponent and is the temperature. A
modified form:

(9–3)

where is the activation temperature.

The pre-exponential factor may be an expression in space, time, absolute or static pressure,
turbulence quantities ( ), molar mass or mixture composition (species mass fractions, molar
fractions, mass concentrations, molar concentrations).

The units of the pre-exponential factor are:


(9–4)

where n is the sum of the number of reaction orders, (plus 1 for a third body term, if present). As
an example, for a reaction with two reactants, both with a reaction order of 1, and a third body
term, n = 2+1 = 3. The physical dimensions for the pre-exponential factor in this case are:

(9–5)

9.1.2.2. Arrhenius with Temperature PDF

Similar instructions apply here as for Arrhenius, except that a temperature limit list is required to
describe the probability density function. For details, see Arrhenius. The probability density
function is a mathematical model that describes the probability of events occurring over time.
This function is integrated to obtain the probability that the event time takes a value in a given
time interval.

The temperature limit list requires the entry of the upper and lower bounds for temperature
integration range (2 values).

9.1.2.3. Expression

To enter an expression, click the enter expression icon and type the name of the expression into
the box. For details, see Expressions in the CFX-Pre User's Guide.

9.1.2.4. Equilibrium

For any reversible reaction, :

(9–6)

where and represent the number of moles of materials A and B in the reaction. While
quantities [A] and [B] represent the molar concentrations and , the equilibrium constant (which
is dependent on temperature, enthalpy, entropy). The forward and backward reaction rates are
defined as:
(9–7)

and

(9–8)

At equilibrium, , and therefore:

(9–9)

Where is the forward rate and is the backward rate. When applied, the Arrhenius
Reaction Model calculates the rate according to the following:

(9–10)

The equilibrium option is only available if the forward reaction rate is set to Arrhenius.

A reaction is reversible when, for each reactant or production component, the reaction order is
equal to the stoichiometric coefficient.

You might also like