In one of the examples in my book (Elementary Priniciples of Chemical Processes) where I am asked to calculate the rate of heat input required to heat a stream of methane and air from 20°C to 300°C, two references conditions are chosen (see picture).

In the text below the box it says "The reference condition for methane was chosen so that $H_{in}$ could be set equal to zero, and that for air was chosen so that $H_{in}$ and $H_{out}$ could be determined from Table B.8."

Why isn't it that $H_{in}$ for air is set equal to zero since the specific enthaply for air at 25°C and 1 atm is 0? How did they get that $H_{in}$ for methane should be zero?

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  • I'd start by looking at Table B8. What does it provide as a value for H of air? – Try Hard Dec 8 at 19:36
  • I've added a picture of Table B8 – Johan Dec 9 at 12:55
  • It seems you are right. I share your confusion. Not sure why H is set to 0. – Try Hard Dec 9 at 15:39

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