Is the temperature up or down for liquid after the throttling valve?

[tobebest]

Hi, everyone! while doing hysys simulation, I found that following the J-T valve,the hydrocarbon gas T decreased, but for crude oil(liquid), it increased. why? is the liquid different from the gas on T passing through J-T valve? thanks in advance

 

[MortenA]

When the pressure is let down in a valve - there is work performed. This is true for gasses and for liquids. For liquids there is no expension (almost) so the is no temperature drop from this - but the energy in the work must go somewhere - into the liquid (or gas). So yes the liquid will get warmer. If you expand a gas in a turbine it also get colder than when you throttle in a valve - because the expander "removes" the work. The difference is isentropic vs. isenthalpic expansion.

 

[MortenA]

One more thing - there is no such thing as a J_T valve - a valve is a valve is a valve. It dosnt know what its name is. The name is assigned to describe a function. Its the same wrt. calling a valve a pressure control valve or a flow control lave. It only describes the nature of the contro loop.

 

[moltenmetal]

Puzzled by this a bit. A throttle or choke valve is dissipating P-V energy in the form of heat- or perhaps the downstream pipe is doing so, where the jet of high velocity fluid leaving the valve trim has its kinetic energy dissipated by the fluid. If you put 1 kW of energy into the liquid with the pump, and throttle the discharge across a valve, you need to get back 1 kW of heat. That has to show up as a temperature rise.

 

[25362]

Visit thread124-132825. It may provide answers.

 

[sshep]

Hey Morten,

I would like to make a small technical correction. There are two types of work, PV work and shaft work (such as an expander). In the case of a gas there is real PV work (change in volume) so we get some temperature reduction for a real gas when pressure on the upstream is exchanged for volume on the downstream, but not nearly what we get when shaft work is extracted via an expander- which is why a turbo expander can generate some real cold temperatures.

With a liquid stream there is no PV work if a constant volume assumption is used- this is the technical correction of your statement. All the pressure energy upstream must become heat which shows up as temperature on the downstream side.

Another interesting fact is that for an ideal gas, there is no tmperature change on throttling because enthalpy is only a function of temperature- you can test this in the simulation by choosing an ideal gas model for the properties. The enthalpy of a real gas is usually modeled as the ideal gas enthalpy + an enthalpy departure term which incorporates pressure into the enthalpy calculation. This is how a real gas model calculates the temperature change.

This is my mental model which is very conventional. Others may arrive at the same conclusion via other thought processes.

best wishes,
SShep

 

[tobebest]

thank you everyone! I learned a lot.
MortenA
you are right, i assign the name of J-T valve,describing the function, there is no j-t valve.thank you so much.

Hi,25362
thank you for the thread provided.it includes many useful informations

 

[moltenmetal]

sshep: it's not P-V energy per se in the compressible flow sense, but PxV in an incompressible liquid is still a potential energy term.

 

[MortenA]

@moltenmetal re your pump question, the anser is yes. There is even a very simple type of Wind turbines tha tuses this effekt to generator heat instead of electricity.