3 replies to this topic

[Mugesh]

I have tried to calculate the LPG stock using the Density and molecular Weight given from the Ships documents in our Terminals & also from Gas chromatography. But in Bottling plants there is a lack of such infrastructures. So i tried to find out a way to theoretically find out the Density & Mol Wt. from Vessel Pressure & Temp. using the experimental values obtained from GC. using the Gay-Lussac's Law or The Pressure Temperature Law to calculate te standard pressure & Temp in 15.56C. I have attached the report and want to check if this procedure is right.

 

Attached Files

•  Simple Report for stock calculation.docx   25.2KB   41 downloads

[Bobby Strain]

You can purchase instruments that accurately measure the liquid density. And you can find free software that calculates mixture densities based on composition.

 

Bobby

[Saml]

Please, if you happened to read it, disregard a previous post by me that I've deleted. There is a mistake in it.

 

*******************************************************

 

Update:

 

You  assume that you have 3 componentes (isobutane, n-butane and propane)

Two phases

So for intensive properties you have:

 

F = C-P+2 = 3 degrees of freedom

 

Extensive variables add one more per phase that is, a total of 5 degrees of freedom.

 

You know:

- Pressure

- Temperature

- Total volume of Liquid

- Total volume of Gas.

 

You are missing one variable to be able to solve the system. You may assume a constant ratio of butanes (iso/normal), probably without making a big error (at least better than assuming a constant density).

 

But... you have the following difficulties:

 

- You assume that temperature is uniform. If the tank is not insulated and exposed to sun, this might not be true.

- The the tank is perfectly mixed and in equilibrium.

- There are no other components apart from propane, isobutane and normal butane. Small amoung of volatiles/non condensables will distort your pressure (ethane, methane, nitrogen)

 

If you really need a measurement of density, the best way is to purchase a ASTM D1657 hydrometer. The price is accesible even to a small operation if having an accurate inventory control is an issue.

Edited by Saml, 25 June 2018 - 06:57 PM.

[MrShorty]

Because you are incorporating the "weight in air" correction factor (appears to my untrained eye to be a simple buoyancy correction factor), which amounts to only a fraction of a per cent, I assume you are expecting fairly accurate calculations for this (though it is also possible you are only incorporating this correction factor out of habit and that you really don't expect this calculation to be that accurate). I find that it is important to consider accuracy whenever I am trying to come up with this kind of "simplified" calculation. I need more thought and effort to compute something that I expect to be highly accurate (+/- 0.5% or better) vs. something more rough (+/- 5 to 10%).

 

Your procedure does not seem correct to me -- mostly your use of Gay-Lussac in step III. Gay-Lussac is a GAS law that applies to ideal gases, but you appear to be using it to predict the vapor pressure of a liquid at one temperature based on the vapor pressure at a different temperature. Because much of the remaining calculation depends on the result of step 3, the rest of the calculation could be in error. I expect that if I were to do this, I would probably only use Gay-Lussac to the extent that I was content to use the ideal gas law to determine the density of the vapor.

 

IMO, the important part of this calculation is how to treat the dependency of density on temperature. Depending on what data I had available to me, I would be looking for something based on a known "coefficient of volumetric expansion" (probably depends on composition) or based on the Rackett equation (with a known reference density). Once I have settled on a method for computing density at any temperature based on the reference densities, then I would compute the liquid and vapor densities at T and proceed to compute the overall mass content of the vessel.