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JDJACKSON (Mechanical) (OP)
26 Sep 03 19:57
I need the formula to convert lb/hr of 20.07 mole weight gas to scfm.

J. D. Jackson B.S. M.E. & B.S. E.E.
TransContinental Engineering

Latexman (Chemical)
26 Sep 03 20:11
1 lb/hr x (1 hr/60 min) x (1 lb-mole/20.07 lb) x
(359.05 std cu ft/1 lb-mole) = 0.2982 std cu ft/min

1 lb/hr = 0.2982 std cu ft/min

STP = 492 degrees R and 14.696 psia

Good luck,
Latexman

JDJACKSON (Mechanical) (OP)
26 Sep 03 22:13
Latexman, thanks for your help.  Isn't there 379 lb/moles per standard cubic foot?

I am trying to convert 19,640 lbs/hr of 20.07 mole weight gas at a temperature of 43 degrees F to SCFM.

Do I need to incorporate the temperature into the equation for a more accurate SCFM equation?  Somehow logic tells me the temperature was used in a previous equation to determine the lbs/ft3 thus resulting in the flowrate of the lbs/hr.  Am I correct about the temperature?

J. D. Jackson B.S. M.E. & B.S. E.E.
TransContinental Engineering

Latexman (Chemical)
26 Sep 03 23:45
It depends on the definition of STP (standard temperature and pressure).  Most conventions agree on the P part, i.e. 1 atm or 14.696 psia.  The T part is usually where differences come in.  At the University, it was usually 0 C (273 K) or 32 F (492 R).  Different industries use different T's.  60 F (520 R) is quite common in North America.  So, it depends on where you are and what you use it for.  It all comes from the ideal gas law, PV = nRT.  The number I used above is derived as follows:

V = nRT/P

V = 1 x 10.732 x 491.67 / 14.696 = 359.05 (I hope you can figure the units)

Now, once you know the definition of the STP you need, you can figure out the V.

Good luck,
Latexman

Helpful Member!  Montemayor (Chemical)
27 Sep 03 20:28
The problem that one always encounters with other engineers is the failure to state the basis for "standard" conditions up front.  That is why I always have used the GPSA's definition:

1.0 lbmole of gas occupies 379.49 cu.ft. @ 14.696 psia and 60 oF

This definition, I believe, is where JDJACKSON is getting his number of 379.  I never discuss Scfm or other standard gas conditions without knowing the basis for it: the LB MOLES FLOW RATE.  You can't go wrong with moles because they are the real "stuff" of which gas volume is made.

So the equation should be:

Scf/time @ 14.696 psia & 60 oF = [(lb/time)/MW]*379.49

You can conver to another pressure and temperature base by using the gas law, as Latexman  points out.  This is probably why they call Chemical Engineers "Mole Chasers".

Art Montemayor
Spring, TX

JDJACKSON (Mechanical) (OP)
28 Sep 03 2:22
Thanks Art, As you can see I am an M.E. and E.E. and got stuck with a problem while I am out of town rebuilding a gas processing liquid separation terminal in Louisiana.  I guess I am going to have to go back to school when I get back to Deer Park, Texas and take some ChemE courses.  I appreciate the help this forum has given me with this problem.

J. D. Jackson B.S. M.E. & B.S. E.E.
TransContinental Engineering

25362 (Chemical)
28 Sep 03 3:43
To convert mass to moles, via molecular mass, you don't need to know the temperature or the pressure.
However, to convert moles to volume units you indeed need determine both, as cleary explained by Montemayor and Latexman.

As a small digression:  ideal gas molecules are considered by the kinetic theory to be of zero volume non-interacting point particles, and the formula brought by Latexman applies.

Real gas molecules take up space and collide, and when they are close a weak electrical attractive force named the van der Waals force, plays a role. When molecules move apart they do work to overcome this force, and as a result the molecular kinetic energy drops. Thus, a rarified real gas, i.e, with low particle densities n/V, approaches ideality.

But this is another issue for another thread.

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