3 replies to this topic

[ASU]

I need to calculate therotical change in temperature when gas ( natural gas) is expanded from 1100 psig to 70 psig. Gas at 1100 psig is at 250 deg F. What will be temperature of gas at 70 psig? Can we use isentropic relations in this case?

[latexman]

Is the process essentially frictionless (or very short) and adiabatic (or very fast)? If yes, isentropic may be a good model.

[Art Montemayor]

You have not stated HOW the expansion is taking place. That is why Latexman has carefully qualified his correct response. If you are going into the realm of the Second Law of thermodynamics (the realm of Entropy), you have to consider whether a continuous process is “reversible” or not. The explanation of what constitutes “reversibility” is not easy to understand (it always gave me a headache as a student). However, one way to express it is the way Latexman explains it – a frictionless process. Another way to express a method of expansion is to determine if useful work is produced as a result of the expansion. If there is, then the process is an isentropic one.

If you are merely using an expansion valve (with a lot of friction and no useful work produced), then you have an isenthalpic process. The reason for determining the correct type of expansion used is because the isentropic expansion (with creation of useful work) normally produces a very much colder outlet temperature as compared with the isenthalpic version.

[ankur2061]

ASU,

Art Montemayor has given a very nice, fundamental description of the nature of expansion process (isenthalpic and isentropic).

To add to this, a real life example of both types of an expansion would be as follows:

1. Expansion of natural gas through a control valve - or, in other words, pressure drop through a control valve - is an isenthalpic process where the gas' enthalpy remains unchanged upstream or downstream of the control valve and there is no work available.

2. Expansion of natural gas through a turbo-expander is an isentropic (reversible adiabatic) process where the enthaly of the natural gas coming out of the turbo-expander is much lower then that of the inlet gas, causing a much larger temperature drop as compared to that seen by gas expansion across a control valve. Also, in this case, useful energy equivalent to the loss in enthalpy of the outlet gas stream is available as work. An example would be work available to drive an electrical motor or turbine.

Regards,
Ankur.