1 Chemical Equilibrium In general the combustion products consist of more than just CO 2, H 2 O, O 2, and N 2 For rich mixtures CO also exists in the products.

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Presentation transcript:

1 Chemical Equilibrium In general the combustion products consist of more than just CO 2, H 2 O, O 2, and N 2 For rich mixtures CO also exists in the products and at high temperatures the molecules dissociate to form H, O, OH, NO via the following reactions: At equilibrium the rate of the forward reaction equals the rate of the backward reaction. The opposite direction reactions are also possible

2 Chemical Equilibrium At equilibrium the relative proportion of the species mole fraction is fixed For the general equilibrium reaction The equilibrium composition for species  C  D is given by: where K is the equilibrium constant which is tabulated as a function of temperature for different equilibrium reactions, P ref is 1 atm and P is in units of atmospheres.

Chemical Equilibrium Recall that for a rich mixture (  <1) the reaction equation could not be balanced (5 unknowns a, b, d, e, f and only 4 atom balance equations for C,H,O,N ) even if we neglect dissociation (i.e., low product temperature) If the product species CO 2, H 2 O, CO and H 2 are at equilibrium and described by the water-gas reaction: The equilibrium constant for this reaction provides the fifth equation : Note K is tabulated as a function of T

Chemical Equilibrium (example) 1 kmol of CO 2, ½ kmol of O 2 and ½ kmol of N 2 reacts to form a mixture consisting of CO 2, CO, O 2, N 2 and NO at 3000K and 1 atm. Determine the equilibrium composition of the product mixture. 3000K and 1 atm C 1 = a+c c = 1 - a O 3 = a+b+2c+2d d = 1/2(1 + a - b) N 1 = b+2e e = 1/2(1 - b) Have 2 unknowns a, b so need 2 equilibrium equations

n tot = a+b+c+d+e = a+b+(1-a)+1/2(1+a-b)+1/2(1-b) = (4+a)/2 Chemical Equilibrium (example) From the equilibrium constant expression Substituting yields:

Solving equations 1 and 2 yields: a= b= From the atom balance equations get: c= d= e= Chemical Equilibrium (example) Similarly for the second equilibrium reaction

If the products are at high temperature (>2000K) minor species will be present due to the dissociation of the major species CO 2, H 2 O, N 2 and O 2. Hand calculations are not practical when many species are considered, one uses a computer program to calculate product equilibrium composition. Role of Equilibrium Solvers

Mole fraction of each species Temperature (K) Composition of Octane-air Mixtures at Equilibrium (lean)(rich) (stoich)