3 replies to this topic

[dhanyamp]

Hi everyone

 

I am simulating steam reforming of biogas using dwsim software. While trying to find out exergy destroyed and exergetic efficiencies, I am stuck at this particular heat exchanger.

 

As far as I could understand, the exergy destroyed in a heat exchanger is the total exergy IN - total exergy OUT. This can be calculated if we know the enthalpies and entropies of the streams in and out (which I can get from the simulation) but it can also be calculated from the thermodynamic average temperatures of the hot and cold streams.

 

But in this case, the exergy OUT is coming greater than the exergy IN. I checked the energy balance, it is fine. The enthalpy change in the hot stream equals the enthalpy change in the cold stream. There is no temperature cross in the counter-current heat exchanger. Water at 40C, 60 bar is heated to 210C, 60 bar (cold stream). on the hot side, syngas at 256C, 20bar is cooled to 150C, 20 bar.  I have not considered the chemical exergies since they would cancel out.

 

I checked the thermodynamic average temperatures of the 2 streams, and the cold stream temperature is higher than the hot stream average, which explains why I am getting a negative exergy destruction. I checked my values several times, each time the exergy destroyed is coming negative.

 

If anyone has any thoughts, please share. Thanks in advance.

[breizh]

Hi,

you may find tutorial using Youtube chanel , keys words  exergy calculation heat exchanger .

 

Good luck

Breizh 

[dhanyamp]

The problem arises when there is phase change in the heat exchanger. I am using the enthalpy and entropy values of the mixture to calculate thermal exergy. Should I use the pure component properties instead?

 

Also is the exergy of a gas mixture the sum of thermo-mechanical and chemical exergy alone, or should we add the exergy of mixing?

[breizh]

Hi,

Consider this link and the paper attached to get hints.

https://www.research...tribution_ratio

 

Good luck

Breizh