Be Careful When Blending Hydrogen and Methane
Hydrogen has recently gained more popularity as an alternative clean fuel source to be used in conjunction with natural gas to reduce greenhouse gas emissions from combustion. More oil and gas companies are looking into using hydrogen as part of their greenhouse gas reduction strategy and therefore, more process engineers are creating a process simulation model of the blending. It is important to understand the peculiar behaviours specifically related to hydrogen.
- At more common operating conditions, hydrogen has a negative Joule-Thomson coefficient. This means unlike many other gases, hydrogen warms up slightly upon expansion (i.e., pressure drop).
- Hydrogen and methane have a non-negligible enthalpy of mixing which results in a non-ideal temperature. Below is a graph showing the excess enthalpy of mixing using the Peng Robinson property package in Aspen HYSYS:
In this case, hydrogen and methane are blended at 25oC, the blended gas has a temperature of 21.6oC instead of the 25oC that one would normally expect in an ideal mixing.
Excess enthalpies of mixing for methane and hydrogen are quantified in the Journal of Chemical Thermodynamics (Wormald et al., 1977).
Peng Robinson in Aspen HYSYS does a good job predicting the excess enthalpy of mixing and matches the experimental data points well.
It is worth noting that excess enthalpy increases with increasing pressure. Below is a chart showing the blend temperature and excess enthalpy of mixing for 80 mol% CH4 - 20 mol% H2 with respect to pressure.
It is important to keep these phenomena in mind and use the properties at the correct temperature for estimation of pressure drop, valve sizing, etc.
This is a great example of why understanding thermodynamics is paramount in process engineering.