Deck 15: Chemical Reactions

Question 1

A fuel is burned with 70 percent theoretical air. This is equivalent to

A)30% excess air
B)70% excess air
C)30% deficiency of air
D)70% deficiency of air
E)stoichiometric amount of air

Answer 1

30% deficiency of air

Question 2

Propane C₃H₈ is burned with 120 percent theoretical air. The air-fuel mass ratio for this combustion process is

A)5.3
B)10.5
C)15.7
D)18.7
E)39.3

Answer 2

18.7

Question 3

One kmol of ethane (C₂H₆) is burned with an unknown amount of air during a combustion process. If the combustion is complete and there are 3 kmol of free O₂ in the products, the air-fuel mass ratio is

A)29.7
B)16.0
C)30.9
D)14.9
E)12.1

Answer 3

29.7

Question 4

One kmol of ethane (C₂H₆) is burned with air during a combustion process. If the combustion is complete and the air-fuel ratio is 22, the number of moles of free O₂ in the products is

A)1.31
B)0.85
C)2.44
D)2.06
E)0

Question 5

An equimolar mixture of carbon dioxide and water vapor at 2 atm and 45°C enter a dehumidifying section where the entire water vapor is condensed and removed from the mixture, and the carbon dioxide leaves at 2 atm and 45°C. The entropy change of carbon dioxide in the dehumidifying section is

A)-3.8 kJ/kg.K
B)-0.13 kJ/kg.K
C)0
D)0.13 kJ/kg.K
E)3.8 kJ/kg.K

Question 6

Liquid ethanol (CH₄) is burned completely with 55% excess air during a steady-flow combustion process. If both the reactants and the products are maintained at 25°C and 1 atm and the water in the products exists in the liquid form, the heat transfer from the combustion chamber per unit mass of fuel is

A)890 MJ/kg
B)27 MJ/kg
C)75 MJ/kg
D)30 MJ/kg
E)16 MJ/kg

Question 7

The higher heating value of a hydrocarbon fuel C_nH_m with m = 8 is given to be 1340 MJ/kmol of fuel. Then its lower heating value is

A)1164 MJ/kmol
B)1466 MJ/kmol
C)1330 MJ/kmol
D)988 MJ/kmol
E)1551 MJ/kmol

Question 8

Methane gas (CH₄) is burned completely during a steady-flow combustion process. The fuel and the air enter the combustion chamber at 25°C, and the products leave at 1500 K. If the enthalpy of the products relative to the standard reference state is -451 MJ/kmol of fuel, the heat transfer from the combustion chamber is

A)177 MJ/kmol
B)227 MJ/kmol
C)526 MJ/kmol
D)376 MJ/kmol
E)451 MJ/kmol

Question 9

Benzene gas (C₆H₆) is burned with 85 percent theoretical air during a steady-flow combustion process. The mole fraction of the CO in the products is

A)1.6%
B)6.8%
C)2.5%
D)18%
E)15%

Question 10

A fuel is burned during a steady-flow combustion process. Heat is lost to the surroundings at 280 K at a rate of 920 kW. The entropy of the reactants entering per unit time is 14 kW/K and that of the products is 12 kW/K. The total rate of exergy destruction during this combustion process is

A)360 kW
B)560 kW
C)918 kW
D)1140 kW
E)1244 kW