Latent Heat 4

[npf]

All:

How would you determine latent heat in hysys?I know its (enthalpy Hv- enthalpy liquid Hl).I am trying to do a relief valve size for fire case and I need to know the latent heat for that

Montemayor: I am sorry, I did not use a new thread and I should have rephrased my question. My concern is for supercritical fluids, I am trying to find vapor properties -MW, compressibilty,Cp/Cv(for fire case to determine the relief valve size)

Help would be appreciated?

Regards,
npf

 

[MortenA]

I usually do it this way:

Copy the stream (e.g. by use of the extention: Virtual Stream)

Set the P to the SP+21% (fire case) and the vapour fraction to 0 (calculates teh bupl point.

Then i run the tream through a heater unit with a know small heat unput. The heat input and mol flow must "match" so that the downstream has a low vapour fraction (around 0.01) Then i use the know heat put and the mass flow rate to calculate the latent heat.

I actually think that you can find an extention that does this procedure automatically on the support web.

Best regards

Morten

 

[SeanB]

Correct me if I am wrong, but isn't Latent Heat the same as Mass Heat of Vaporization which you can get directly from "Properties" in Hysys?

 

[dcasto]

From the original post, there is no latent heat for a supercriticle fluid. However, MortenA kinda knows what npf is after.
Cp/Cv = k which is used in the equations for calculating the capacity of the relief valve. W know that Cv = R- Cp, so we only need to find Cp. This ratio is basically a value that assumes the process is isentropic and the energy flow is such. That is why it is part of the equations. (i'm not pulling out thermal books to get all the calculus out).
Cp can be found by finding the change in enthalpy between to temperature at a constant pressure, then divide by the change in temperature. The values are available in lists or can be found from a PH diagram.

 

[npf]

Thanks everybody for their inputs

Morten: In some cases when I am trying to set the vapor fraction to 0 and pressure to relieving pressure, hysys would not calculate the temperature.Here is how I proceeded then, I took the stream passed it thru heater, set the vapor fraction of the outlet stream from the heater to 0.05 (set heat exchanger pressure drop to O) and downstream pressure to relieving pressure. Does that makes sense.

Any help would be appreciated

Regards,

npf

 

[MortenA]

dcasto: I think he _is_ looking for the latent heat (and i dont think its for super critical fluid). Latent heat (both for molar and mass) is actually in the property list (its a new item i believe) but i dont really know if its "for the first fraction" or if you want to evaporate the "whole lot".

If you egt what i mean its a big difference. The latent heat tends to be much higher if you want to evaporate the whole mix when comparing to just boiling of a small lighter fration. An important difference when considering PSV sizing since this leads to much higher flow rates.

Best regards

Morten

 

[Montemayor]

This thread is going to the dogs mainly because of a lack of basic data and a determination to obtain a concrete answer without furnishing any data or specifics for the case. It can't be resolved the way its going.

I advised npf in his prior attempt to hijack another thread to start this one and to be specific about what he is talking about. Apparently it didn't get through. Until an engineer has the ability to describe his/her problem accurately and with details, he/she will never be capable of resolving it. If you can't describe it, you don't understand it. So how can you resolve it?

Let me cite some specifics that are missing:
1) What's the fluid in question?
2) what are the stored conditions of the fluid during relief?
3) What's the stored physical state of the fluid? (before the fire)
4) Is the scenario an API pool fire case?

Until we get a poster, who seems to be a Chemical Engineer, to describe his/her problem as a Chemical Engineer, this forum can't help because we don't know what he/she has -- since he/she hasn't addressed the specific data requirement.

The responses can only be qualitatively as good as the input.

 

[ifreeman]

According to API 521 page 18,

"The recommended practice of finding a relief vapor flow
rate from the heat input to the vessel and from the latent
heat of liquid contained in the vessel becomes invalid near
the critical point of the fluid, where the latent heat
approaches zero and the sensible heat dominates.

When no accurate latent-heat value is available for these
hydrocarbons near the critical point, a minimum value of 50
British thermal units per pound is sometimes acceptable as
an approximation."

Regards,