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Unformatted text preview: [(yC)c/(yA)a(yB)b]∙P(c‐a‐b)
Where yi is mole fraction of component i at equilibrium.
Note: Since mole fractions are dimensionless, the units of the equilibrium constant K are: atm(c‐a‐b) Chemical Equilibrium
• For the liquid‐phase reaction aA+bB
on the equilibrium constant, hence: cC. Pressure has no effect K= [(xC) c/(xA) a .(xB) b]
Where xi is mole fraction of liquid component i at equilibrium.
Note: Since mole fractions are dimensionless, the equilibrium constant K is dimensionless. Equilibrium reactions: reversible reactions
CH4 + H2O CO + 3H2 (reversible)
CH4 + H2O CO + 3H2 “at equilibrium”
Equilibrium constant K(T) = [yCOyH23/yCH4yH2O]P2 (thermodynamics)
1.0 fAE 0.8 0.6 High
T 0.4 low
0.0 Page 122… Methodology: Express mole fractions (“y”s) at equilibrium all in terms of the single variable e, the extent of rxn @ equilibrium, then solve for e
using the equilibrium constant K. 23 Page 122… 24 25 Material balance for reacting systems:
(1) Molecular balance General balance equation: input + generation = output + consumption + accum
Degrees of freedom: ndf = nuk + nind reactions – nind mol balances – nother
Simple for single reactions, Complex for multiple reactions (2) Atomic balance
General balance equation: input = output + accumulation
Degrees of freedom: ndf = nuk – nind atom bal – nbal indep, non‐reactive mol ‐ nother Simple for multiple reactions (3) Extent of reaction ( balance (version of molecular balance) ni = nio + i
General balance equation: input + generation = output + consumption + accum
Degrees of freedom: ndf = nuk+ nind reactions– nind reactive species balances – nbal indep, non‐reactive mol – nother
Useful for equilibrium systems, though not very different from Molecular Bal’s!! Example: Dehydrogenation of Ethane to Ethylene Molecular species balance:
2 unknowns + 1 indep chem rxn – 3 indep molec species = 0 Atomic species balance:
2 unknowns – 2 indep atomic species = 0 Extent of Reaction balance: ni = nio + i
2 unknowns + 1 indep chem rxn – 3 indep reactive species = 0
C2H6: C2H4: = 1, 40 = 0 + = ‐1, nC2H6 = 100 – = 1, nC2H4 = 0...
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- Fall '08