Beginners > PHREEQC manual examples

Example 7, Gas volume

(1/1)

Jeonghwan Hwang:
Hello,
I have a question at Example 7.
At the example code, the volume of gas was calculated by using ideal gas equation as below;
"  40 vol = moles * 0.08207 * TK / 1.1"

When I saw the simulation results, I found the 'Gas volume'
Can anyone advice me what these results mean?
I'm confused because the 'Volume, in liters' at fixed_pressure results in USER_Graph2 was not matched with 'Gas volume'

-----------------------------------Gas phase-----------------------------------
Total pressure:  1.10      atmospheres          (Peng-Robinson calculation)
Gas volume:   1.13e+00 liters
Molar volume:   2.22e+01 liters/mole
P * Vm / RT:  0.99636  (Compressibility Factor Z)
Moles in gas
----------------------------------
Component         log P           P    phi     Initial       Final       Delta
CH4(g)            -0.17   6.818e-01  0.998   0.000e+00   3.152e-02   3.152e-02
CO2(g)            -0.43   3.697e-01  0.994   0.000e+00   1.709e-02   1.709e-02
H2O(g)            -1.50   3.162e-02  0.993   0.000e+00   1.462e-03   1.462e-03
N2(g)             -1.77   1.682e-02  1.000   0.000e+00   7.776e-04   7.776e-04

Thank you,
Sincerely,

Jeonghwan Hwang
Modify message

dlparkhurst:
The gas volume is consistent with the moles in the gas phase.

--- Code: ---3.152e-2+1.709e-2+1.462e-3+7.776e-4 = 0.0508496 moles

--- End code ---
and

--- Code: ---.0508*0.082*298.15/1.1 = 1.1290 L

--- End code ---

Note that example 7 is a Peng-Robinson calculation, which corresponds closely to the ideal gas law at low pressures, but will differ at higher temperature and pressure because it uses the Peng-Robinson Equation of State.