Chemical Reactions

Example 15.2 dew point temperature of combustion products
Ethane (C 2 H 6 ) is burned with 20% excess air during
combustion process. Assuming complete combustion
and a total pressure of 100 kPa, determine (a) the
air fuel ratio and (b) the dew point temperature of
the products.
Solution
(a) write the chemical reaction
equation at 120% theoretical air
C 2 H 6 +1.2a (O 2 + 3.76 N 2 )2CO 2 + 3H 2 O + 0.2aO 2 +(1.2x3.76)aN 2
From O 2 : 1.2a = 2+ 3/2+0.2a then a = 3.5
C 2 H 6 +4.2(O 2 + 3.76 N 2 ) 2CO 2 + 3H 2 O + 0.7O 2 + 15.79N 2
Air Fuel Ratio, AF = m air /m fuel = (NM) air /(NM) fuel
= (4.2x4.76 kmol)(29 kg/kmol)/[(2kmolx12 kg/kmol)x(3 kmol x 2 kg/kmol)]
= 19.3 kg air/kg fuel
C 2 H 6 +4.2(O 2 + 3.76 N 2 ) 2CO 2 + 3H 2 O + 0.7O 2 + 15.79N 2
(b) T dp = T sat @ P v (H 2 O)
Ideal gas mixture P i /P m = N i /N m
P v = (3 kmol/21.49 kmol)(100 kPa)
P v = 13.96 kPa T dp = 52.3 o C
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