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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  • $\begingroup$ I'd start by looking at Table B8. What does it provide as a value for H of air? $\endgroup$ – Buck Thorn Dec 8 '18 at 19:36
  • $\begingroup$ I've added a picture of Table B8 $\endgroup$ – Johan Dec 9 '18 at 12:55
  • $\begingroup$ It seems you are right. I share your confusion. Not sure why H is set to 0. $\endgroup$ – Buck Thorn Dec 9 '18 at 15:39
  • $\begingroup$ As long as there is no chemical reaction, you can use a different reference temperature for every species in the mixture. Mathematically, it will still work out OK. $\endgroup$ – Chet Miller Jan 11 '19 at 23:39

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