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From: (Bruce Hamilton)
Newsgroups: sci.chem,
Date: Sat, 28 Jun 1997 20:13:27 GMT (Bruce Hamilton) wrote:

>Consider LPG, it's a liquid mixture of propane, iso-butane, and
>normal butane with vapour above it.
>If you pump LPG vapour into a container, once the pressure exceeds
>~30psi at ambient temperature then it will condense with the higher
>boiling point compounds condensing first as they reach saturation

Just answering a post that hasn't appeared here about the
~30 psi number. Liquified Petroleum Gases are a mixture of
propane, iso-butane and n-butane. The ratios vary according to
economically-available feeds, and the national specifications
- with some having up to 33% propane, but others having
virtually no propane, and thus a much lower vapour pressure.

Some larger markets have two LPGs, for example, the UK and
US have "propane" which can vary from 90-120psi at 15C, or
"butane" which can vary from 10-30psi at 15C. The ~30 psi
just happened to be the pressure at 15C of the last tank
I encountered, but it could be anywhere between 10-120psi
at 15C, depending on the national specifications, and
intended end use. Note that the names don't mean the LPGs
are pure fractions, they aren't - just that n-propane is
the largest component in "propane" and that butanes are
the dominant component in "butane".

LPGs usually have maximum pressure limits measured at
37.8C for safety reasons ( US "propane" max. is 208psi,
which equates to approx 105psi at 15C, and "butane" is 70psi
which equates to approx 30psi at 15C.). They may specify
limits on the less volatile fraction, eg 95% of US "butane"
must have evaporated at near-ambient pressure by the
time it reaches 2 degrees C or, alternatively, must not
contain more than 2% of pentanes and larger molecules.
These limits ensure the fuel will fully evaporate in the
vaporisers designed for it.

      Bruce Hamilton

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