5 replies to this topic

[newzeadan]

I am trying to understand a mass balance sheet for an ethanol plant.  Looking at the beer column overhead vapor here's the original conditions:

     flow   80,064 lb/hr

    vapor mole frac. 1

    mass fractions:

       -water 0.44125

       -ethanol  0.595875

    Enthalpy   -287.43 MMBtu/h 

   Temp   229.3166F

    Press   30.2286 psia

 

I'm not a chemical engineer, so having a little trouble understanding the enthalpy value.  

 

I'm using NIST Refprop software, and for that same mixture at that temperature and pressure their enthalpy value for vapor phase is 696.99 btu/lbm

multiply that by the flowrate and i'm getting 55.8MMBtu/h.

 

I'm sure I'm missing some fundamental principle, but i'm stumped as to why it's a negative value and so much more than what i'm coming up with. 

 

Any help is greatly appreciated.  Thanks!    

[PingPong]

The enthalpy of a stream has no real meaning as it depends on the basis (reference state) that was used to calculate that enthalpy. Different sources or software use a different basis and therefor come with different enthalpy values for streams.

 

What matters in enthalpy calculations of a distillation column is always the enthalpy difference at different temperatures or phases and that is not affected by the basis.

For example: the duty of the column condenser is equal to the difference between the enthalpy of the overhead vapour into and enthalpy of the vapour plus liquid out of the condensor.

[newzeadan]

Oh ok, so it is calling out the enthalpy of the fluid, however it's off of a reference point, not for the specific stream. 

 

I attached a few pictures of the pfd and the mass balance to show what I was talking about.  

 

Can I ask why there is the basis in the first place? What does that tell the user by the reference point? Edit: googled reference states and it makes sense from knowing where the system is i suppose.  So how would I go about calculating it for this scenario?  say the reference point is air at STP. 

 

also, in your example the enthalpy of vapor + liquid out is the enthalpy of evaporation (latent heat) correct?  Just want to make sure I have it right. 

 

Thanks for the clarification.   

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Edited by newzeadan, 12 January 2018 - 02:36 PM.

[Bobby Strain]

Use your favorite search enegine for enthalpy. This should clear your understanding.

 

Bobby

Edited by Bobby Strain, 12 January 2018 - 07:42 PM.

[PingPong]

Enthalpy is like, for example, an elevation. You get a different number for the elevation of an object if you quote that elevation relative to another reference. Ask several people: what is the elevation of a lamp above a table on the third floor of a building on a mountain? Their answers will depend on what each of them considers as the reference level: the table, or the floor the table stands on, on the ground floor of the building, or sea level, or ........

 

Unfortunately different sources use a different reference level for determining the "elevation" of enthalpy.

Your printout seems from some sort of process simulator. The documentation of that software should somewhere state what the reference state for calculating the enthalpy is. If not don't worry about it because it is arbitrary and irrelevant. The only thing that matters is the change (difference) in enthalpy that occurs when a stream is heated or cooled or condensed or .....

That enthalpy change (difference) is not affected by the chosen reference, provided that all enthalpy data are based on the same reference.

 

Just like the elevation difference between lamp and table is not affected by whether you measure the elevations of both the lamp and the table relative to this, or that reference level, as long as it is the same reference level for both.

Edited by PingPong, 12 January 2018 - 04:17 PM.

[newzeadan]

I get the reference now.  Thanks.   I appreciate the help.  I understand enthalpy, just was getting really caught up on the values as I thought they were stating that was the total enthalpy of the stream, as I never really used the reference points when doing any calculations.  

 

Anyway that helped out tremendously! so thanks!   As for the reference, I'm not sure what program created it, but as you said, it's not really relevant! 

 

Thanks again!