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# Mollier Diagram - Propane

## Mollier Diagram - Propane

(OP)
Greetings,
From the Mollier Diagram for Propane, I am seeking to confirm my suspicion that, as a result of a somewhat extraordinary installation, the temperature of the vapour on the outlet of a valve installed shortly after the first stage regulator manifold is below Propane's Dew Point.  Two scenarios are considered.

Condition 1: Regulator inlet and outlet pressures of 750 kPa and 140 kPa resp., vapour inlet temperature is 30 C.  What is the temperature of the regulated outlet pressure?
Condition 1a:  Vapour at reduced temperature then passes through a control valve exhibiting a throttling effect such as to result in a 30 kPa pressure drop.  In other words, vapour inlet and outlet pressures of 140 kPa and 110 kPa.  What is the temperature of the vapour as it exits this valve?

Condition 2:  Regulator inlet and outlet pressures of 750 kPa and 200 kPa resp., vapour inlet temperature is 30 C.  What is the temperature of this regulated outlet prrssure?
Condition 2a:  Vapour at this reduced temperature then passes through the same control valve resulting in a similar pressure drop, i.e., 30 kPa.  Vapour inlet and outlet pressures of 200 kPa and 170 kPa.  What is the temperature of the vapour as it exits this valve?

Regards,

Peninsular

Replies continue below

### RE: Mollier Diagram - Propane

From what I could get from the NIST webbook, isenthalpic expansions would cool the gas at the given conditions, but it still would remain above its dew point. Apparently the irreversible throttling path across valves is composed of two quick steps:

First. The gas gains kinetic energy (a higher velocity) o/a of its enthalpy in a quasi-isentropic step that, due to the little original superheat, may involve a transitory production of "wet" propane.

Second. An almost complete recovery of the enthalpy o/a of the following isobaric reduction in kinetic energy at the lower pressure.

The throttling process is almost isenthalpic (J-T free expansion), since there is a small enthalpy loss by friction.

### RE: Mollier Diagram - Propane

Nice to have a copy of the GPSA data book in SI units.

In condition 1 the inlet vapor is slightly superheated at 750 kPa (you don't specific if gauge or absolute but it's not that significant). When you expand it to 140 kPa, the temperature cools to about 18C.  When you subsequently expand it to 110 kPa, there would be a very slight cooling, perhaps a degree (assuming no heat losses from the system)

For the second question, expanding it to 200 kPa, I get an outlet temperature of about 20C.  For the second expansion, the same minor temperature loss of about a degree.

If you can find a copy of the Mollier curve for propane, it's easy to see you are not saturated at the inlet to the regulator and as you reduce the pressure through the regulators (remembering it's a constant enthalphy expansion) you move away from the dewpoint temperature.

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