ChE or chemist (a) Estimate the lower heating value of liquid n-butane in kJ/gmo
ID: 518682 • Letter: C
Question
ChE or chemist
(a) Estimate the lower heating value of liquid n-butane in kJ/gmol and in Btu/lb.
(b) Estimate the adiabatic flame temperature if liquid ethanol is burned with 100 % excess air.
Assume that the ethanol and air are initially at 25 oC.
For simplicity use the following average specific heats.
CO2 51.3 J/gmol-K H2O 39.8 J/gmol-K
O2 33.8 J/gmol-K N2 31.9 J/gmol-K
Explanation / Answer
a)
Butane combustion is
C4H10 + 6.5O2 = 4CO2 + 5H2O
Gross heating value of C4H10 is 21640 btu/lb
Hvap = 970.4 btu/lb
Lower heating value of C4H10 = gross heating value - xHvap
where x is mole fraction of water in product i.e 4/9
Lower heating value of C4H10 = gross heating value - xHvap
Lower heating value of C4H10 = 21640 - 4/9*970.4
Lower heating value of C4H10 = 21208.71 Btu/lb
1 Btu/lb = 2.326 kJ/kg
Lower heating value of C4H10 = 21208.71 Btu/lb = 49331.46 kJ/kg
49331.46 kJ/kg = 2269.25 kJ/mol
b)
Liquid ethanol is burnt with 100% excess air at 250C i.e. 298K
The air contains 21% O2 and 79% N2 by moles
C2H5OH + 6O2 + 22.57N2 ------> 2CO2 + 3H2O + 3O2 + 22.57N2
Basis: for 1 mole of ethanol at 298K
Standard enthalpy of formation of ethanol is -278 kJ/mol
Standard enthalpy of formation of CO2 is -393.5 kJ/mol
Standard enthalpy of formation of H2O is -242 kJ/mol
At constant pressure, we have,
Hreactants = Hproducts
Hreactants = Enthalpy of formation of reactants
Hreactants = H(C2H5OH) + H(O2) + H(N2)
Hreactants = -278000 + 0 + 0
Hreactants = -278000 J/mol
HProducts = ni[(Hf ) + Cp (Tad - 298)]
HProducts = ni[(Hf ) + Cp (Tad - 298)]CO2 + ni[(Hf ) + Cp (Tad - 298)]H2O + ni[(Hf ) + Cp (Tad - 298)]O2 + ni[(Hf ) + Cp (Tad - 298)]N2
HProducts = 2[-393500+51.3(Tad - 298)] + 3[-242000+39.8(Tad - 298)] + 3[0+33.8(Tad - 298)] + 22.57[0+31.9(Tad - 298)]
HProducts = -1513000+1043.383(Tad - 298)
Hreactants = Hproducts
-278000 = -1513000+1043.383(Tad - 298)
1235000 = 1043.383(Tad - 298)
(Tad - 298) = 1183.65
Tad = 1481.65 K (Adiabatic temperature)