a question about enthalpy and heat release

[pan....@gmail.com]

Hello, 

I am doing a ideal gas reaction: CH4+2O2+3.77*2N2 = CO2+2H2O+3.77*2N2, inside a closed, adiabatic, constant-volume reactor (no mass change).  

The initial conditions are: 1400K, 5.2MPa,  volume= 0.5 m3, and  mole scale: CH4:1, O2:2, N2: 7.54.  Reaction step: 1e-6 s until equilibrium.  

I am a little bit confused about the enthalpy curve.  

As we know in this system, the total internal energy is constant (without doubt), and the pressure & temperature will increase.   However, the enthalpy curve (enthalpy=enthalpy_mass*total_mass) is also increased.  Based on my understand,  CH4 reaction is exothermic reaction, which means the product enthalpy will be smaller than the reactant enthalpy (so the reaction can release heat to the system, and increase pressure and temperature, right?).  Thus, enthalpy curve should be decreased but not increased in this case, right?  However, I always feel there is something wrong, but cannot tell where and what? 

Could anyone explain this enthalpy curve?  

Also, in Cantera, how to get the reaction heat release or total heat release from reactions?? what function can do that?  And does enthalpy change equal to the total heat release??  

Thanks a lot. 

Regards,

Eric

[Ray Speth]

Eric,

Both the enthalpy and internal energy values reported by Cantera include sensible and chemical components. Your notion of why the enthalpy should decrease only takes into account the chemical enthalpy, but not the corresponding increase in enthalpy due to increase in temperature and pressure. The reason why the enthalpy goes up can be seen directly from the definition of enthalpy:

The internal energy and volume are constant, and the pressure increases, so the enthalpy must go up.

The term “heat release” is a bit imprecise, due to the differences between enthalpy and internal energy that you’re grappling with. There are multiple ways of calculating this, two of which are given in the Cantera Python Tutorial.

Regards,
Ray