Modelling very large pressure pressure drops in liquids 2

[ColourfulFigsnDiags]

I have a system where there is a near 100Barg pressure drop in a liquid stream over a valve. According to HYSYS this pressure drop results in an almost 1% increase in the absolute temperature of the liquid.

Is this correct? If so, what is the fundamental reason why this happens?

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[UmeshMathur]

The Joule Thompson coefficient, (dt/dp)|h, is all that matters when discussing isenthalpic processes. For water, and almost all normal fluids, this is always positive so there is cooling as the pressure goes down at constant enthalpy.

If there is flashing across the valve, the valve pressure drop will go up. That simply increases the total DP to be overcome to maintain the measured flow, assuming there is a pump. (Of course, the flow is not measurable in the two phase region after the valve - it must be measured upstream while you are in a single phase condition.) However, we are not discussing the effect of DP on flow here, I believe.

 

[DickRussell]

Umesh, in your reply of April 28 you said "The Joule Thompson coefficient, (dt/dp)|h, is all that matters when discussing isenthalpic processes. For water, and almost all normal fluids, this is always positive so there is cooling as the pressure goes down at constant enthalpy."

But back in your reply of April 22 you said: "The Joule-Thompson coefficient is negative only for real gases above reduced temperatures of about 6, and also for hydrogen and helium gases. For liquids, it is always negative."

Which is it for water and other liquids, positive or negative?

 

[sailoday28]

UmeshMathur (Chemical)/DickRussell (Chemical
Please refer to my posting of property of water at atmos press and 4C where water has a maximum density/

At the condition of atmos press
J-T coef (?T/?P)|H = 1/Cp*[-T*(?V/?T)|p + V] becomes
@ 4C (?T/?P)|H=v/Cp which is positive.
At temp below 4C
(?V/?T)|p will be negative and J-T positive.

Regards

 

[rbcoulter]

UmeshMathur:

Your post on 22 Apr 06 19:09 appears to be misleading in stating that "maintaining fluid enthalpy after pressure is reduced drastically cannot be done without a temperature increase for most fluids". You may be referring to only liquids but "fluid" generally means gases and/or liquids.

As noted in earlier posts most real gases, except for some few exceptions like H2, will cool on expansion through a throttling valve.

 

[UmeshMathur]

DickRussell and rbcoulter:

I apologize profusely for the confusion created by the contradiction re. the J-T coefficient in my earlier posts.

Unfortunately, there is a “Joule-Thompson inversion curve” for every substance: On a P-T diagram, lines of constant enthalpy above this curve have a negative slope, and a positive slope below this curve. Further, this phenomenon can be exhibited both below and above the critical temperature. At very low temperatures and above the saturation pressure, the slope becomes consistently negative.

However, for sub-critical temperatures and at pressures below the saturation curve, the slope is positive (“normal” condition). Thus, T goes down as P is reduced at constant enthalpy for such “normal” vapors. For compressed liquids above the saturation line, at very low temperatures, the J-T slope is consistently negative. However, closer to the critical point, it can become positive for liquids and then reverse sign at higher pressures.

A good description of these phenomena can be found in K.E. Bett, J.S. Rowlinson, and G. Saville: “Thermodynamics for Chemical Engineers” (MIT Press, 1975).


To check the points made above, using the SRK EOS, I ran a simple series of isenthalpic flashes for compressed liquid methane, starting each case at 500 kg/cm2 and -125 C before the valve:

END P [kg/cm2] = 450. 400. 350. 300. 250. 200. 150. 100. 50. 40. 30. 20. 10. 5. 2. 1.
END T [C] = -122.9 –120.8 –118.9 –117.2 –115.6 -114.4 -113.4 –113.0 –113.3 –113.5 –113.8 –114.1 –124.6 –138.0 –152.5 –164.4

It is clear that temperature increases until the pressure drops to about 100 kg/cm2. Hence, the J-T coefficient is negative up to this pressure. After this point, however, the temperature begins to decrease, at first slowly and then far more rapidly; the J-T coefficient has clearly switched sign to become positive. This behavior is entirely consistent with Bett et al’s description.

This pattern of behavior has been exhibited by all hydrocarbons and organic liquids that I have tested. Hydrogen and helium are the main exceptions (based on the special equations of state on the NIST website).

If anyone has found a case where the description provided above has NOT been observed, it would be extremely interesting to study it.