9 replies to this topic

[Zubair Exclaim]

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I am trying to do hydraulics for a supercritical ethylene line. Undersupercritical conditions it will be exhibiting viscosity of a gas , with density similar to liquid and have no surfface tension. In other words it will be fluid with properties of gas an dliquid both.

Question is how to do do hydraulics calculations for it. Is there any special correlation for it. Conventional pressure drop correlations ddont really cover this type of fluid phase. Data i got is as below


Ethylene polymer grade 99.9 % pure

Plant battery limit conditions for 25 km line sizing connecting to another plant.

Properties provided by client at BL pressure 56.3 barg Temp 47.1 C (Tc=9.209C, Pc=49.3barg)

 

[PingPong]

Supercritical? Who cares in this case.

It is just a compressed gas with a density of 87 kg/m³  at 47 ^oC & 57 bar(abs)

^{http://webbook.nist....cm&RefState=DEF}

 

You can select other properties at Y

[Zubair Exclaim]

Supercritical? Who cares in this case.

It is just a compressed gas with a density of 87 kg/m³  at 47 ^oC & 57 bar(abs)

^{http://webbook.nist....cm&RefState=DEF}

 

You can select other properties at Y

Thanks  alot  ....

 

However i was wondering if you look at this graph below of volume there is almost no change with increased pressure ... meaning the PV = ZRT equation ... VRT is constant and change of P is being absorbed by Z factor.. Shouldnt there be like a supercritical correction factor for compensation of Z ... something feels odd about this fluid phase. You know Z factor is used in pressure drop calculation for gasess and it seems  a bit deviating from normal gas behaviours..

Edited by Zubair Exclaim, 02 September 2015 - 09:03 AM.

[PingPong]

.... if you look at this graph below of volume there is almost no change with increased pressure .....

What graph do you mean?

[Zubair Exclaim]

 

.... if you look at this graph below of volume there is almost no change with increased pressure .....

What graph do you mean?

 

 

Sorry didnt attach last time ...

 

this graph if you see of volume is almost constant in supercritical region ... am just curious if gas laws even apply to thiis phase when it has a constant volume and pressure isnt doing its part ...

Attached Files

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Edited by Zubair Exclaim, 02 September 2015 - 01:18 PM.

[PingPong]

The (specific) volume (m³/kg) is the reciprocal of the density (kg/m³).

Volume is definitely not (almost) constant in the supercritical region, because the density is not (almost) constant.

Volume only looks almost constant because of the chosen range of the scales.

 

The same graph but now not from 1 to 100 bar, but from 50 to 100 bar is shown at:

http://webbook.nist....fState=DEF#Data

 

For the value of Y, select the Volume (m³/kg) and you will see that the volume is not constant at all, but varies as a factor of 3 between 50 and 100 bar.

 

The equation of PV = ZRT always applies, but Z obviously varies with P and T.

Or, more commonly stated: Z varies with Reduced Temperature Tr (= P/Pc) and Reduced Pressure Tr (= T/Tc).

You can find many graphs like the one below in books and on websites:

 

 

[Bobby Strain]

I think you have been distracted a bit. Your task is a simple one. Calculations should be done in segments using average properties for the segment. You only need to know what they are. If you have a process simulator, you can do the calculation with it. And I would check the results with a known model. I think Steve Hall and Harvey Wilson have good software. And, once you complete the calculation, you can post the information and someone might run a check to confirm your results.

 

Bobby

[Zubair Exclaim]

I think you have been distracted a bit. Your task is a simple one. Calculations should be done in segments using average properties for the segment. You only need to know what they are. If you have a process simulator, you can do the calculation with it. And I would check the results with a known model. I think Steve Hall and Harvey Wilson have good software. And, once you complete the calculation, you can post the information and someone might run a check to confirm your results.

 

Bobby

Sure thanks guys for clearing it all up...

[AHT]

I work for a company which licences a supercritical extraction process. The hydraulics for line sizing are done in the way other people in this Forum have described in their previous answer to your post. As long as you don't approach the transition boundary between supercritical and liquid or vapour phases your fluid behaves as a single phase fluid. The only thing worth mentioning is that the liquid and vapour properties are blended using the REFUTAS method for viscosity, and API 14A2.1 procedure for thermal conductivity, Other properties are blended by averaging. I have never compared this approach with doing hydraulics assuming that the liquid is two-phase, but I assume the latest method will result in larger line sizes.

[Zubair Exclaim]

 

I work for a company which licences a supercritical extraction process. The hydraulics for line sizing are done in the way other people in this Forum have described in their previous answer to your post. As long as you don't approach the transition boundary between supercritical and liquid or vapour phases your fluid behaves as a single phase fluid. The only thing worth mentioning is that the liquid and vapour properties are blended using the REFUTAS method for viscosity, and API 14A2.1 procedure for thermal conductivity, Other properties are blended by averaging. I have never compared this approach with doing hydraulics assuming that the liquid is two-phase, but I assume the latest method will result in larger line sizes.

 

hmm Thanks ...

I guess thermal conductivity wont be a big buzz for hydraulics .. however viscosity can be ... i will try some manual calculations and compare with simulators ... This topic seems intresting and explorable