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# Latent heat of vaporization

## Latent heat of vaporization

(OP)
Hello,

Is it possible to have latent heat of vaporization of Nitrogen at 1 atm and 25 deg.C, despite that nitrogen in vapor phase?
If the answer is no than why ASPEN, during simulation, shows latent heat of vaporization for Nitrogen.

Need help regarding in this case.

Many Thanks.
shiplu48

### RE: Latent heat of vaporization

No, it's above the critical temperature and below the critical pressure.

Be sure the thermodynamic and physical property models are set up correctly.

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.

### RE: Latent heat of vaporization

(OP)
Thanks Latexman,

I have Propylene Oxide and Nitrogen and other chemicals in vessel in overpressure case scenario. I have to size PSV for Fire Case (Propylene Oxide and Nitrogen will pass through PSV).
For that I have to determine Required Relieving Capacity by the equation w=q/Hv, where,
w = Required Relieving Capacity
q= heat absorption
Hv= latent heat of vaporization
Hv has to be determined at relieving condition (Relieving T and P). But both Propylene Oxide and Nitrogen are in vapor phase at Relieving condition.
Then how am I supposed to get Hv at vapor phase?

Any suggestion is highly appreciated.

Many Thanks.
shiplu48

### RE: Latent heat of vaporization

As you begin venting both PO and N will vaporize at pressures higher than ambient, so you have to solve this in steps.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

### RE: Latent heat of vaporization

There are equations where you do not need the latent heat of vaporization to size a PSV. Refer to section 5 of the Engineering Data Book of the GPSA.

### RE: Latent heat of vaporization

(OP)
Dear srfish,

As per your suggestion, section 5 of the Engineering Data Book of the GPSA, I have given look at that section.
I didn't found any equation to calculate W(Required Capacity)without latent heat of vaporization. But it mentions that
"When no better information is available, a conservative minimum value of 50 Btu/lb is typically used".

Will it be appropriate to use 50 Btu/lb for latent heat of vaporization (for any vapor or liquid: specially polymers
that does not have data like latent heat of vaporization ) if no information could be achievable?

Many Thanks.
shiplu48

### RE: Latent heat of vaporization

Can you explain why there is no PO vaporization during the fire, and when the PSV opens and the pressure starts reducing?

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.

### RE: Latent heat of vaporization

(OP)
Hi Latexman,

The set pressure of the PSV 20 barg and relief T is 170 deg C.
PO will be vapor at relieving case.
The problem is that Latent Heat of vaporization have to be determined
at relieving condition. At relieving condition PO and N2 both are vapor.
Then how could we get latent heat of vaporization of vapor phase?!!

Many Thanks

### RE: Latent heat of vaporization

I suspect someone's given you the wrong relieving conditions. If you're starting with liquid propylene oxide under a nitrogen blanket, boiling PO will hit relieving pressure at 155 deg C. Assuming your RV lifts at this point, it won't get any hotter until it has boiled dry, at which point you lose structural integrity rather rapidly. If it's vapour phase to start with, that is the wrong sizing equation (and an RV is probably the wrong way to protect the vessel).

Fire below vessel vaporises the liquid in the reactor, pressurising the vessel. The heat of vaporisation is for the liquid, not the gas phase above it or the whole mixture.

If you've got enough vapour space for the differences in composition to be significant, you'll need to look at the equilibrium composition at relieving pressure to get the gas phase composition, use your sizing equation to determine the number of moles of vapour being generated (or volume), and displace the equivalent from the vapour space. If you really have to sharpen your pencil, this could even be done dynamically.

I can't vouch for how compliant this is with your code of choice and regulator.

Matt

### RE: Latent heat of vaporization

(OP)
Hi,

For fluid properties calculation of PSV, would the calculation be performed at the relieving pressure
(corresponding of the saturation temperature) regardless of the relieving temperature and fluid phase?

At section 3.3.2.1.4 of "Guidelines for Pressure Relief and Effluent Handling Systems", it has been
mentioned that all properties should be determined at maximum venting pressure.

Many Thanks.
shiplu48

### RE: Latent heat of vaporization

Temperature, not pressure.

### RE: Latent heat of vaporization

In Pro II ( Simsci), this can be easily simulated using the firecase sub option in the Depressure main routine. Cannot recall if a similar feature is available in Hysis. Results obtained using this routine indicate smaller relief loads than with conventional API manual calcs for multicomponent mixes in the liquid phase.

### RE: Latent heat of vaporization

(OP)
Hi,

In reply to mbt22, the mentioned reference book recommend to use relieving pressure for determining all fluid properties.

### RE: Latent heat of vaporization

Yes, the relieving pressure and the temperature the mixture boils at at that pressure.

Talking through what happens inside the vessel to cause this relief case with a colleague may help, starting from a pool fire ignition underneath it at normal operating conditions.

Matt

### RE: Latent heat of vaporization

for a multicomponent mixture there are several things to consider,
as said by others, vaporization (latent heat contribute) applies to liquid fractions,
for calculating temperatures inside a vessel protected by a PSV you can solve (in sequence) a series of flash operations (solving at each step heat and mass balance),
there are several posts discussing these aspects,
a software can help (many simulators include specific procedures),
for the initial step see (as example)
http://prodesoftwareapplications.blogspot.com/

### RE: Latent heat of vaporization

To shiplu48

If you do not have the latent heat of vaporisation input you can take the minimum value 50btu/lb or 27kcal/kg. You will get the maximum relief value. If your design capacity is more then its ok, but if you designed less capacity then it will be a problem. Don't worry do your calculation

Navneet (Process)

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