Latent heat of vaporication in Pro II 2

[bobbobs]

I am looking at a fire relief case for a vessel. In this case the heat added by the fire will have to increase the liquid temperature to its boiling point and then at that temperature enough heat would need to be added to vaporize the liquid and increase the pressure to the relieving temperature.

I am hoping to use PRO II or maybe HYSYS to determine the latent heat of vaporization of my liquid. I am not very familiar with either simulation model and am hoping that someone could help guide me through this process. I haven't had much luck finding tutorials online. Thanks.

 

[bobbobs]

I meant to clarify that this is a mixture of hydrocarbons. I have come across some discussions on whether the latent heat should be used for the lighter components rather than for the entire mixture. What is the correct method for PSV sizing?

 

[ajs1972]

You can see fluid properties for saturated liquids at relef pressure in PH diagrams for this material. You can see a long database in NIST web page.

 

[ajs1972]

You can assume the fluid properties for mixtures may be based on the predominant component.

 

[bobbobs]

ajs1972,

Will this give a conservative way to size the relief valve? Ir your mixture has a small component of something with a lower boiling point and latent heat of vaporization should the valve be sized based on this component since it will boil off first?

 

[ajs1972]

Yes, is a conservative way. You can consult, for instance, the OSHA 1910.106 standard:

1910.106(a)(5)
Boiling point shall mean the boiling point of a liquid at a pressure of 14.7 pounds per square inch absolute (p.s.i.a.) (760 mm.). Where an accurate boiling point is unavailable for the material in question, or for mixtures which do not have a constant boiling point, for purposes of this section the 10 percent point of a distillation performed in accordance with the Standard Method of Test for Distillation of Petroleum Products, ASTM D-86-62, which is incorporated by reference as specified in Sec. 1910.6, may be used as the boiling point of the liquid.
1910.106(a)(6)
Boilover shall mean the expulsion of crude oil (or certain other liquids) from a burning tank. The light fractions of the crude oil burnoff producing a heat wave in the residue, which on reaching a water strata may result in the expulsion of a portion of the contents of the tank in the form of froth.

"Flashpoint" means the minimum temperature at which a liquid gives off vapor within a test vessel in sufficient concentration to form an ignitable mixture with air near the surface of the liquid, and shall be determined as follows:
1910.106(a)(14)(i)
For a liquid which has a viscosity of less than 45 SUS at 100 deg. F. (37.8 deg. C.), does not contain suspended solids, and does not have a tendency to form a surface film while under test, the procedure specified in the Standard Method of Test for Flashpoint by Tag Closed Tester (ASTM D-56-70), which is incorporated by reference as specified in Sec. 1910.6, shall be used.
1910.106(a)(14)(ii)
For a liquid which has a viscosity of 45 SUS or more at 100 deg. F. (37.8 deg. C.), or contains suspended solids, or has a tendency to form a surface film while under test, the Standard Method of Test for Flashpoint by Pensky-Martens Closed Tester (ASTM D-93-71) shall be used, except that the methods specified in Note 1 to section 1.1 of ASTM D-93-71 may be used for the respective materials specified in the Note. The preceding ASTM standards are incorporated by reference as specified in Sec. 1910.6.
1910.106(a)(14)(iii)
For a liquid that is a mixture of compounds that have different volatilities and flashpoints, its flashpoint shall be determined by using the procedure specified in paragraph (a)(14) (i) or (ii) of this section on the liquid in the form it is shipped. If the flashpoint, as determined by this test, is 100 deg. F. (37.8 deg. C.) or higher, an additional flashpoint determination shall be run on a sample of the liquid evaporated to 90 percent of its original volume, and the lower value of the two tests shall be considered the flashpoint of the material.

You can see this web page too:

http://www.vscht.cz/uchi/procesy/CHISA04EETV.pdf

Regards

 

[ajs1972]

bobbobs:

In the book Chemical Process Principles Part I Material and Energy Balances from Hougen, Watson and Ragatz (John Wiley and Sons Ed.) in the Chapter 8 (Termophysics) there is a graphic that you can use for determinate latent heat of hydrocarbons and petroleum fractions.

 

[CJKruger]

bobbobs, this is an important and suprisingly difficult question to answer.

As far as I know, five approaches are in common use:

1) Psuedo pure component. Assume you mixture is a pure component, and get the heat of vaporization from the Cox chart (API 521).

2) Use reported values from Hysys directly. Problem is that this is the isobaric enthalpy change on vaporizing the entire stream. Includes entire dT effects.

3) Flash the stream from bubble point to say 5% vapour, and calc the heat of vaporization from duty (Q/Wvap).

4) If you simulator allows it, flash the stream to say 25%, than calc heat of vaporization from Hvap-Hliq. You cannot do this in Hysys (as ref H is not 0).

5) Lastly, API suggest we should do the calc at varying amounts of vaporization, but this is really impractical if you have many relief valves.

For most of my life I used approach (4), but because of modern simulators and less conservatism have now moved to approach (3).

 

[sheiko]

Hello,

In order to determine the total latent heat of a mixture, i would follow these steps:

Step 1: Designate a material stream, denoted as Stream 1, to represent the mixture in fully liquid phase at its bubble point; specify its composition and pressure, and assign a vapor fraction of 0 (representing bubble point)

Step 2: Designate a second stream, denoted as Stream 2, having the same composition and pressure as Stream 1 but assign a vapor fraction of 1, representing the dew point.

Step 3: Since the composition of Stream 1 is equal to that of Stream 2, the latent heat, is the difference between the specific enthalpies of the two streams.

Having said that, to ensure calculating relief loads that are neither excessive nor underestimated, the latent heat should be determined for incremental vaporization
points of the system (until vaporization of the entire mixture), as pointed out by CJKruger.

"We don't believe things because they are true, things are true because we believe them."