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Homework Help: How to calculate number of moles and gas formation volume factor?

  1. Jul 29, 2010 #1
    1. The problem statement, all variables and given/known data
    I am trying to calculate the gas formation volume factor for a reservior, however I am stuck when trying to find the value n - number of moles for a gas to plug into the equation to find Bg, this is what I know;

    Bg (gas FVF)= volume of gas at reservoir conditions / volume of gas at standard conditions, I know I have to apply the real gas law here i.e PV=znrt = V=znrt/p, I have the following knowns;

    R=10.73 psia ft3 / lb mol Deg R
    T=580 deg R
    P=2195 psia

    R= as above
    Tsc= 520 deg R
    Psc = 14.7 psia

    Volume %
    methane 83.4
    Ethane 9.8
    Propane 4.5
    isobutane 1.4

    I am completely stuck when trying to find n, as I do not know the gas volume (only in %)or the gas weight, I know n=total gas weight / molecular weight, I have tried to work this out by calculating the gas density but I can not progress further than this, I am completely stuck!

    Is there any guidance you can offer me on this?

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Oct 11, 2012 #2
    To calculate Number of moles for a gas mixture, is like finding the proportional behaviour of each gas constituent in the mixture. You have a %proportion of each constituent in the mixture, you will use this to get a proportional molecular weight for the mixture
    i.e. ƩYi*Mwi=Mw Yi is the proportional percentages for each member gas in the mixture, while, Mwi is there corresponding Molecular weights which you know.
    From the real gas eqn:
    also recall that density, ρ=m/V
    therefore, ρ=mP/nZRT
    note that m/n = Mw and in this case it will be Mw for the mixture which you have gotten from ƩYi*Mwi=Mw.
    therefore, ρ=Mw*P/ZRT
    With the above calculation, you can use it to get the density,ρ, at reservoir condition and the density,ρ, at standard conditions.
    formation value factor Bg = ρr/ρsc.
    Best of luck
  4. Oct 11, 2012 #3
    Assume you have 1 mole of gas. You have the value of z under reservoir conditions, so you can calculate the volume under reservoir conditions. You know the value of z under standard conditions (520?, 14.7 psi), so you can calculate the volume under standard conditions.
    Last edited: Oct 11, 2012
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