Gas Compressibility factor spreadsheet

[layloban]

Hi,

I'm working on a project to predict pressure drop in a pipe for various gases. One piece of information that I need is the compressibility factor for the working fluid. Does anybody have any suggestion on software or excel spreadsheet that I could use or purchase? Thanks in advance.

Chan

 

[Montemayor]

Chan:

It's not enough information to state you're working on a project. What is the level of accuracy that you want or need? What gases are you dealing with? There are many sources for compressibility factors. The best and possibly the most accurate would be the top simulation programs like Aspen, SimSci, HySim, etc. How much are you prepared to spend for the software?

Why not just purchase an available, on-the-shelf, and proven pressure drop program instead of re-inventing the egg? That would be my suggestion if I were your supervisor. I doubt if anyone is going to seriously use your software to design a pipework without prior knowledge of there being no bugs and that it is recognized and proven. If this is an academic exercise, fine; but professional engineers don't try to out-program existing proven programs. It isn't practical nor cost-effective.

Art Montemayor
Spring, TX

 

[dcasto]

Try these equations
This is for hydrocarbon based gases.
CO2 and N2 cannot exceed 2%.
Pressure is less than 1500 psig

ts = flowing Temp F
ps = flowing pressure psia
CTC = Criticle Temp R
CPC = Criticle press psia
TRG = Reduced Temp
PRG = reduced pressure
SPGR = specfic gravity air=1.0


CPC = 708.75 - 57.5 * SPGR
CTC = 169.01 + 314.001 * SPGR

or use actual CPC and CTC

TRG = (ts + 460) / CTC
PRG = ps / CPC
Zfac = 1.0 + (.0703 * PRG / TRG) * (1.0 - 6.0 / TRG ^ 2)




Another BWR based Zfac that is iterative
just use above Zfac as beginning



A1 = 1.29024E-03
A2 = .381903
A3 = .0221993
A4 = .1221548
A5 = -.0156748
A6 = .023834219
A7 = 2.383422E-02
A8 = .4361778


PPR = .27 * PRG * Zfac (from above) / TRG

Zfac = 1 + (A1 + A2 / TRG + A3 / TRG ^ 3) * PPR +
(A4 + A5 / TRG) * (PPR) ^ 2 +
A5 * A6 * (PPR) ^ 5 / TRG +
A7 * (PPR) ^ 2 / TRG ^ 3 * (1 + A8 * (PPR) ^ 2) * EXP(-A8 * (PPR) ^ 2)

 

[katmar]

If you apply cold logic then Art is correct. Buy proven software. But engineers always distrust software written by others. I know I always prefer my own cobbled together programs than some fancy off-the-shelf package. But you must be aware of the professional risks you are taking and cross check your calcs again and again and again.

If you want to estimate gas compressibilities by the Peng-Robinson EOS you can try one of my free programs. It has critical properties built in for a few common gases, or you can enter them for other gases. Make sure you understand the relevance and applicability of Peng-Robinson to your application. I give no guarantees.

The program is called Uconeer and you can download it (0.5MB) for free from

http://www.katmarsoftware.com

 

[israelkk]

Try

http://webbook.nist.gov/chemistry/fluid/

 

[Montemayor]

israelkk:

This is a great site for:

1) Density;
2) Cp;
3) Enthalpy;
4) Internal energy;
5) Viscosity;
6) Joule-Thomson coefficient;
7) Specific volume;
8) Cv;
9) Entropy;
10) Speed of Sound;
11) Thermal conductivity; and,
12) Surface tension (saturation curve only).

For 34 compounds. But I'm afraid you won't find any compressibility factors or their derivation there. However, it's still an excellent site for Thermodynamic data.



Art Montemayor
Spring, TX

 

[zdas04]

Montmayer has a very good point. If you need compressibility for custody-transfer gas measurement, AGA Report No. 8 is a 203 page book that gives the official US Gas Measurement industry calculation for compressibility. It is truly an epic work by some truly committed and talented people. I find it quite irrelevant outside of custody-transfer gas measurement.

If you need to tell the difference between .98 and .92, then the GPSA Engineering Data book has an almost trivial equation that is OK for many things. Below 100 psig the equation is simply a function of pressure. Above 100 psig it calls for pressure, temperature, and specific gravity. "Accuracy" is probably within 10% of AGA 8 or any of the formal EOS functions.

By the way, I often wonder how I ever lived without katmar's Uconeer. It is a superb program that I have start up when I boot my computer.

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[israelkk]

I think you do not need the compressibility. You can calculate the real gas constant R=Cp/Cv and from the data in NIST and from that all other gas properties and the flow properties. In the ideal P*V=Ro*R*T the compressibility Z is added to compensate for the difference between the real gas and ideal gas. But from NIST you get the real gas constant R therefore you do not need the compressibility.

 

[israelkk]

Montemayor

Just want to add that in the last 25 years I am calculating gas flow (Nitrogen, Hellium, Air etc) and weight of gas inside a pressure vessel for low and high pressures (0 to 700 atm) and never used the compressibility factor Z. Giving the real gas constant R and specific volume you can do all your calculations. From the NIST web page you can get the specific volume (weight per volume) of the gas vs pressure and temperature. From that you can calculate the gas weight inside a pressure vessel. The gas constant you can calculate as I wrote in my previous post.

 

[katmar]

Hi israelkk,

The difference in approach seems to be that mainly for historical reasons chemical engineers measure the quantity of matter in moles (actually kmols) while mechanical engineers use mass units (kg or pound).

If the amount of matter is expressed in moles then you use the Universal Gas Constant which remains the same for all gases, and the variation for the actual gas and its conditions is taken up in the compressibility factor. If you use mass units then all the variables are rolled into one i.e. the Specific Gas Constant.

In practice it makes very little difference because you have to account for the actual gas and its conditions one way or the other. So really it just comes down to your personal preference.

 

[layloban]

All,

Thank you very much for your advice. I really appreciate it.

Katmar,
Is uconeer a unit convertion program? I can't find anything that is related to gas compressibility.

Thanks again!

 

[katmar]

Layloban,

Sorry, I didn't explain it well. Yes, Uconeer is a units conversion program. One of the conversions it can do is to convert gas flowrates between mass and volume units. To do this you obviously need the gas density and to get this you need the compressibility (if you are a chemical engineer!!!). Uconeer estimates the compressibility using the Peng Robinson EOS.

Click the fan on the tool bar and the flowrate calculator will pop up.

Sorry for the confusion.

regards
Katmar

 

[athomas236]

There is a simple Excel add-in that is based on a program developed by a professor from stanford university for HP programmable calculators. It can still be down loaded, try

http://www.cgrpttc.lsu.edu/products/pvt/pvtdoc.pdf

or use google to search for fluids pack


regards

athomas236