7 replies to this topic

[Miche.So]

Hi all,

 

I am wodering about Joule Thomson coefficient,

In Gas dehydration process using TEG, bottom stream of contactor flow through a valve to decrease the pressure. at that time, i found that the temperature of stream increased.

 

I think, joule thomson is the factor which makes this situation (Increase in temperature when pressure decrease)

and also, in case of water it has same tendency. (water Joule thomson coeffcient <0 celcius at 250 celcius)

 

why Joule-thomson has minus sign especially water and TEG ??

Is there any relationship with water chemical structure (hydrogen bonding)?

 

 

Thank you for your time in advance.

 

Regards

 

Soyeon.

[MrShorty]

I'm sure that somewhere in the details is something about chemical structure, we can talk about differences in chemical structure. But I don't think that is the main effect that causes the JT coefficient of liquids to be <0.

 

If you look at the derivation of the JT coefficient (https://en.wikipedia...–Thomson_effect see the derivation section towards the bottom), I think the main thing that controls the sign of the JT coefficient is the coefficient of volumetric expansion. Since alpha tends to be small for liquids, alpha*T tends to be less than 1, so the overall sign of the JT coefficient for liquids tends to be negative.

[Art Montemayor]

Both James Prescott Joule and William Thomson (a.k.a., “Lord Kelvin") worked on and experimented with the change in temperature that accompanies expansion of a real gas (not a liquid!) without the production of work or the transfer of heat - an adiabatic process.

 

The resulting published Joule-Thomson coefficients, I believe, pertain to the expansion of real gases - and not to liquids.  This is clearly taught in thermodynamic courses when comparing the resulting expansion temperatures of real gases such as nitrogen, carbon dioxide, oxygen, etc. with those of hydrogen and helium.  The latter gases yield a temperature INCREASE rather than a decrease.  The Joule-Thomson effect is used extensively in industry in various refrigerating processes - such as the liquefaction of air.  However, note that the expansion employed in a mechanical refrigerating process is NOT a Joule-Thomson step.  It is the vaporization resulting from the free expansion of a LIQUID refrigerant - and not that of a vapor.  There is a big differance between both processes - just look at the process as depicted in a Mollier Diagram.  It tells all.

 

Anyone who has worked in a hydrogen generating plant knows better than to go close to where a hydrogen leak is located in the daytime due to the invisible jet flame it produces.  One can clearly see the jet flame at night.  This is due to the temperature increase upon expansion and the propensity of the hot gas to ignite when touched off by a local ignition source.

[Miche.So]

To. MrShorty

 

First of all, Thank you for your reply.

 

I thouht, the factor which makes joule-thomson coefficient <0 is volum expansion.

In case of water, 0~4celcius has minus volume expansion (because of chemical structure)

 

So that's why i thought like that ["why Joule-thomson has minus sign especially water and TEG ?? Is there any relationship with water chemical structure (hydrogen bonding)?"]

 

As you said, i agree your opinion , ideally liquid is imcompressible fluid (but real liquid is not.)

So liquid tend to be small, But i don't know whether this is main factor or not.

If this is main factor(liquid tend to be small volume expansion), all liquid might be minus joule thomson coefficient.

[Miche.So]

 To Art Montemayor

 

Thank you for your reply.

 

As you said, i was confused when i studied about joule-thomson effect.

In the report, they said, Joule-Thomson experiment is just for gas (vapor)

 

However, why liquid has joule-thomson coefficient value??

 

I think i shoud study more about it, and i'm going to look more Mollier Diagram in detail.

 

Best Regards.

[MrShorty]

Art may be correct that one cannot technically, rigorously apply the term Joule Thomson effect to a liquid phase. However, as you have observed, the phenomenon does occur with liquids as well as gases. We can "expand" a liquid through a throttling valve, the "expansion" occurs isenthalpically, and we can compute a "JT coefficient" using the same derivation to estimate the change in temperature when a liquid expands through a throttling valve.

 

I do not know if we can make the sweeping generalization that all lliquids exhibit a negative JT coefficient. Looking at the derivation for JT coefficient again, the key properties that appear to determine the sign of the JT coefficient are molar volume (1/density) and the slope of the density curve with respect to temperature (dV/dT). At temperatures well below the criticial temperature, liquids tend to have small volumes and dV/dT tends to be small. The overall effect seems to trend towards negative JT coefficients. However, as the temperature approaches the critical point, the volume decreases and the slope dV/dT increases. The overall effect can change the sign of the JT coefficient as one approaches the critical point.

 

A useful tool for exploring this might be NIST's thermophysical properties calculator (http://webbook.nist....hemistry/fluid/ ). You can have the website compute a JT coefficient for several different liquids at several different temperatures and see what happens.

[Neelakantan]

while agreeing with ART (any one disagrees with him?!!) and Shorty on the J/T (ie J/T as cooling effect was essentially studied for expanding the gases without doing a mechanical work, and that the theory why most gases cool internally on expansion through a restriction, the theory of J/T coefficient based on expansion was developed), I believe the OP original question is  pertaining to increase in the temperature of liquid on dropping of pressure in absorber LCV?

[Bobby Strain]

All this discussion about Joule-Thompson has no bearing on your observation. I suspect that your measurement is incorrect. The stream temperature will decrease downstream of the expansion valve. This is due to vapor release when the pressure is reduced. Try the measurement again, using the same calibrated thermometer for both locations.

 

Bobby