10 replies to this topic

[Poseilus]

Hi,

I am trying to find for days now how to calculate butane propane mixtures thermodynamic properties.

I have found the therm. properties for butane and propane separately.

More specifically I am interested in vapor pressure and viscosity, so that I can calculate friction losses in pipes etc.

Thanks in advance.

[mbeychok]

Poseilus:

If all you are concerned about is calculating pressure drops in pipes, then you can simply use the arithmetic average of the properties for propane and butane. In fact, for almost all other calculations, that would be sufficient. If you have some specialized calculation that requires extreme accuracy, then you may need to obtain the properties of your exact propane/butane mixture more accurately.

As an aside, I don't understand why you need vapor pressures in order to calculate pressure drops in pipes.

[Poseilus]

Hi,

I have found the mixing rule for gas viscosity:

n_m=(sum(n_i*y_i*sqrt(M_i))/(sum(y_i*sqrt(M_i))

where
n: viscosity
y: Mol fraction
M: Molecular weight

index
i: Each component
m: mix

I dont know whether I can use the same equation for liquid mixture as well.

If I can get the reading of temperature and pressure of the gas/liquid inside the tank then I can find the exact amount of butane/propane mixture only if I have a similar equation for vapor pressure. Also I want to model the whole LPG installation. If I can get the vapor pressure inside the tank I can get the amount of gas that will be available for consumption.

[mbeychok]

Poseilus:

In the discussion thread that you started with this same question in the Eng-Tips Forum, you said that you didn't know what a mol fraction is and that you didn't know how to calculate it.

With that in mind, the best advice I can give you is to locate an experienced chemical engineer that you can talk to face-to-face and ask for his help. It is too difficult and too time-consuming to teach you the very basics of physical chemistry in a forum such as this.

[Poseilus]

Because I am from Greece my problem is mostly the english terms. Plus the fact that after going to military service and working on something that has nothing to do with what I have been studing for 5 years, made me forget many theories.

And now I need my memory freshened up, so I thought internet would be the easier way of reminding me.

[David Southall]

To add a little to what Mr. Beychok has said, you will need the viscosity and density to calculate a Reynolds number, in order to calculate the pressure drop.

You can use a weighted mean to estimate these properties for the mixture. If I remember correctly, you calculate the mean density on a mass basis, and the mean viscosity on a volumetric basis.

But, if I couldn't remember how to do it clearly, I would browse through a related book, attempt the calculation, and ask someone who can remember how to do it to check my calculation for me.

I know it is frustrating to forget things - it disappoints me that I am forgetting things I learnt at University which don't relate to my job, but textbooks are never far away.

[innohex]

Hi Poseilus,

For liquid hydrocarbon mixtures, a mixing rule suggested in "Chemical engineers handbook" 7th edition is shown here:

L(nm) = Sum(xi * Ln(ni))

nm : mixture viscosity.

xi : mole fraction of component i.
ni: liquid viscosity of component i.

Should you require additional information then drop a line

heatxworks@yahoo.com

Good luck.

[avsp]

Hi,

Please get onto the internet and go the NIST Webook website for a quick calculation of properties for your mixture.

http://webbook.nist....hemistry/fluid/


Cheers,

[gvdlans]

AVSP,

It looks like the NIST Webbook calculates properties such as viscosity for pure compounds only, not for mixtures...

[avsp]

gvdlans,

Yes you are right...this website gives properties only for pure components. Actually my intent was to assist Poseilus, who, it appeared to me from this thread that he is looking for an easy way to look for data and this website would help him to get the pure component data and use the same to estimate the properties for his misture of these pure components.
However, there is another website, which belongs to a company called QUEST CONSULTANTS (I forgot the actual web address but it will be available by using a search engine) which I used sometime back for doing a quick estimation of properties of a hydrocarbon mixture though I am not sure how good it is for a rigorous estimation.

Cheers

[gvdlans]

avsp,

The link you gave was indeed useful for finding the properties for pure propane and pure butane.

The Quest Consultants website http://www.questcons...rm/thermot.html can be used to find vapor-liquid equilibria for mixtures, as well as densities and heat capacities. It doesn't seem to provide viscosities.