# Reference conditions for balances on nonreactive processes

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?

• I'd start by looking at Table B8. What does it provide as a value for H of air? – Buck Thorn Dec 8 '18 at 19:36
• I've added a picture of Table B8 – Johan Dec 9 '18 at 12:55
• It seems you are right. I share your confusion. Not sure why H is set to 0. – Buck Thorn Dec 9 '18 at 15:39
• 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. – Chet Miller Jan 11 '19 at 23:39