Heat of Reaction Calculation

This example illustrates calculating the heat of reaction of a specific chemical reaction. For this example the heat of reaction of methane (CH4) combustion is calculated. 

This examples requires a process system to be created with the following components and chemical reactions (see Get Started Guide):

Components: CH4(g), CO2(g), H2O(l), O2(g)

Chemical Reactions: CH4(g) + 2 O2(g) →  CO2(g) + 2 H2O(l)

Note, for the purpose of this example, the components are selected all in the gas phase, except H2O in the liquid phase.

 

Process systems view with the process system listed for the current calculation example.

Process systems view with the process system listed for the current calculation example.

Process system view configured with components and reactions for the current calculation example.

Process system view configured with components and reactions for the current calculation example.

This examples illustrates calculating the heat of combustion of methane (CH4) combustion. For combustion reactions, the heat of combustion is typically reported as higher heating value (HHV), being the heat of reaction with products at the pre-combustion temperature and state. For this purpose, H2O in the liquid state is added to the process system and used in the reaction.

The heat of combustion is calculated per 1 kg of methane, at the standard state of 15°C. For this purpose an initial condition is not specified, and the initial and final states will both be calculated at the specified temperature of 15°C.

In the Calculation view, configure the new calculation as follows:

  • Keep Initial Conditions off.
  • Keep the Calculation Target as Temperature.
  • Enter the Temperature as a single value of 15°C.
  • Change the units of the gas components to kg.
  • To the far right of the chemical reaction, tap on the options button to display the component amount options. Select 1 kg CH4(g) for the amounts of reactants to be set stoichiometrically to the basis of 1 kg CH4(g).
  • Keep the reaction extent specified as 1.
Calculation view with temperature configured.

Calculation view with temperature configured.

Options on the calculation view to specify stoichiometric amounts of reactants.

Options on the calculation view to specify stoichiometric amounts of reactants.

Calculation view with component amounts and reactions configured.

Calculation view with component amounts and reactions configured.

After configuring the calculation, tapping on the Calculate button would display the Results view as below.

The heat of reaction of methane is derived from the results as 55.58 MJ/kg. This is indicated as the negative value in the results displayed next to ΔEnthalpy, which itself is the enthalpy of the reaction products minus the enthalpy of reactants, both at 15°C. The negative value of -5.558 x 104 kJ indicates the energy released from this exothermic reaction.

The results further indicate the ΔGibbs Free Energy to be -5.086 x 104 kJ, and being less than zero it indicates the reaction to proceed spontaneously at the calculated temperature. A decrease in the gas volume is calculated, since the H2O is set condense.

The component amounts results indicate that for the 1 kg of CH4, the stoichiometric amount of 3.99 kg of O2 was required in order to produce 2.74 kg CO2 and 2.25 kg H2O(l). The reacting gas mixture would have a stoichiometric composition of 33.3 vol.% CH4 and 66.7% of O2. The molar amounts and mass fractions can also be observed in the results for the initial and final states.

Results view with parameter values.

Results view with parameter values.

Results view with components molar amounts.

Results view with components molar amounts.

Results view with components masses.

Results view with components masses.