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Unformatted text preview: nA,fed – nA,stoich)/ nA,stoich
Where nA,stoich is the amount of A required for complete stoichiometric conversion of A with the limiting reactant. Fractional conversion: fraction of initial limiting reactant that reacted.
fCH4 (or xCH4) = moles of CH4 reacted/initial moles of CH4
fA (or xA) = moles of A reacted/initial moles of A (nA0)
CH4 + H2O = CO + 3H2
Initial: 1 5 0 0
Final: 0.5 4.5 0.5 1.5 fCH4 = 0.5/1.0 = 0.5 or 50%
Extent of reaction: moles of limiting reactant reacting, ξ (“ksai”)
ξA = nAo ‐ nA
What is the relation
between fA and ξA?
ξA = fAnAo e.g. for the example above, ξCH4 = 0.5 moles, and
nCH4 = nCH4,o – ξCH4 = 1 – 0.5 = 0.5 moles
nH2O = nH2O,o – ξCH4 = 5 – 0.5 = 4.5 moles
nCO = nCO,o + ξCH4 = 0 + 0.5 = 0.5 moles
nH2 = nH2,o+ 3ξCH4 = 0 + 1.5 = 1.5 moles In general :
ni nio i , ni nio i Generation of product = ni – nio
Consumption of reactant = nio – ni is always positive = (ni – nio)/i is the same for all components in a reaction For rxn: 3A+2B 4C+D assuming A is limiting, & no C or D in feed: Example 4.6‐1 15 16 Multiple reactions:
(1) C2H6 = C2H4 + H2 desired (2) C2H6 + H2 = 2CH4 undesired fractional conversion, fC2H6, includes both Reaction (1) and (2)
Selectivity: Yield (3 ways) S = moles of desired product
moles of undesired product
Y(1) = moles of desired product
initial moles of reactant
moles of desired product
moles of reactant consumed
Y(3) = Use this unless otherwise stated moles of desired product moles of product formed if complete conversion of limiting reactant and no side reactions Example 4.6‐3 18 Chemical Equilibrium
• In reversible reactions, an equilibrium state is formed at which all reactants and products co‐exist in some proportions. At equilibrium the rates of the forward and the reverse reactions are equal. The composition of the mixture is determined by the equilibrium constant K.
• The equilibrium constant is a function of a reaction temperature and is defined as follows:
• For the gas‐phase reaction aA+bB cC, at P atm. Total pressure: K=...
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- Fall '08