Actual Density 5

[MonkeyNuts2479]

Hi all,

Was wondering if someone could help me, I'm trying to calculate the actual density of natural gas under a certain pressure and temperature. I know the actual pressure and the actual temperature however I don't know the actual compressibility factor.

Feedback would be much appreciated.

Thanks

 

[pmover]

fyi, the NIST website provides all the gas properties needed for a specific gas at a Pressure & Temperature.

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

this should get you started . . .

let us know more . . .

good luck!
-pmover

 

[MonkeyNuts2479]

Hi pmover,

Many thanks for your reply however I was hoping for an equation as I would rather calculate it by hand if I can rather than relying on the internet all the time. Keeps the brain working!

I would assume I would use Pv = ZnRT but there are three variables so have been scratching the head??

Cheers again.

 

[IRstuff]

This is confusing. You want to determine the "actual" gas density, but you want to use the "ideal gas" equation. The ideal gas equation will get you the "ideal" gas density, NOT the "actual" gas density.

TTFN

FAQ731-376

 

[zdas04]

IRStuff,
What am I missing? I thought the Ideal gas equation was PV=nRT, and the real gas equation was the one he provided PV=ZnRT which modifies the Ideal equation for compressibility. What is confusing?

David

 

[IRstuff]

And compressibility is a constant over all temperatures and pressures?

TTFN

FAQ731-376

 

[25362]

May be this link is of some help.

http://www.gas.naverex.kiev.ua/soft.htm

 

[SomptingGuy]

Z is a dirty empirical fudge factor used to modify the ideal gas equation. It kind of kills the purity and elegance of the classic relationships between thermodynamic properties.

- Steve

 

[speco]

MonkeyNuts2479,

Two comments:

1. Natural gas is generally a mixture of mostly methane and some heavier components such as ethane, propane, etc. In order to accurately calculate the density you need to know the actual mol weight of your particular gas.

2. The GPSA data book has a set of graphs giving the compressibility of natural gas mixtures at various pressures, temperatures, and mol weights. I don't know if there is an algorithm for calculating compressibility. I always use their graphs.

Regards,

Speco (

http://www.stoneprocess.com)

 

[zdas04]

IRstuff,
Compressibility may be a "dirty empirical fudge factor", but it is definitely a function of thermodynamic conditions. Most empirical equations set it up as a function of pressure and temperature. The correlation that I use (I think it is called the Blasius Correlation, but it has been a while and my notes are not as complete as I wish they were) has it as a function of pressure if the pressure is under 100 psi and both pressure, temperature, and Specific Gravity if pressure is above that. I read through the original paper when I started using this correlation and found the science to be good and the assumptions to be acceptable.

David

 

[MonkeyNuts2479]

My understanding is that the compressibility factor Z is a function of both pressure and temperature which is what prompted my initial question.

speco many thanks for suggesting the GPSA databook, this is exactly the sort of thing I was looking for. I'm always a bit hesitant on doing it online as you don't know what's happening in the back round and whether it is trust worthy, I would rather work it all myself by hand if I can.

The databook costs £150 plus £16 delivery, do you think it's worth it? What else does it have?

Agian many thanks for the advice all, it is much appreciated!

 

[dcasto]

The GPSA data book has all kinds of charts and graphs for physical properties, viscosity, vapour pressure, ect. it also has all the basic design equation for equipment with write ups on them There are sample designs for heat exchangers, air coolers, compressors, amine treaters, separators. the book comes with a cd of all the stuff too. We have been working on excel spread sheets that have the design equations in them. There is a sample of the excel sheet listed in GPA section A on the site.

The GPSA/GPA has not formally decided how we will distribute the excel sheets as they are completed. They be be sold on the website (just like the book is

http://www.gpaglobl.org)

It's better than a Perry's if you want the typical process stuff. There's no oliver rotary vacuum drum driers, or dephleminators (sic), just good ol basic stuff in the book. Look around for other publications and technical papers while you are at it. There are some good papers listed that were given at the annual meetings too.

GPA/GPSA also produces a lot of the VLE data used by the simulation software vendors. The research is paid for by the members and the data sold to users of the lab data. They are doing high pressure VLE data with high CO2 and H2S along with corrosivity data on High CO2 and H2S streams.

Sorry if it sounds like a commercial, but I only spouted off because you asked.

 

[PaoloPemi]

the compressibility factor expresses the nonideality (as deviation from ideal gas law) of a gas, Z=PV/RT , for a ideal gas Z=1 , for real gases Z is generally less than one (except at high P,T), you may read for example The Properties of Gases and Liquids by Reid, Prausnitz and Poling for a detailed discussion. You can estimate Z in different ways, for example with an analytical equation of state, from a generalized compressibility chart, from tables as those compiled by Lee & Kesler etc. etc.
For natural gas mixtures AGA methods give accurate results, I use software tools (Prode Properties and NIST REFPROP) for calculating compressibility factors according AGA and other procedures.

 

[MonkeyNuts2479]

Hi all,

Sorry for the delay in replying but just wanted to say many thanks for the tips. Think I will get the text book, will be worth the investment!

Thanks again.

 

[FeX32]

hmmm. monkey nuts. That's the answer to all of you questions.


Fe