Bioprocess Engineering Questions and Answers - Heat of Reaction for Processes...

This set of Bioprocess Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Heat of Reaction for Processes with Biomass Production”.

1. What is the process of making biomass energy?
a) Oxidation
b) Combustion
c) Reduction
d) Vaporization
View Answer

Answer: b
Explanation: It is burning (combustion). The combustion process generates heat that’s transformed into energy. In the combustion process biomass is burned and converted into energy.

2. A solid-oxide fuel cell is fed with carbon monoxide and reacts with air to produce CO₂. This reaction will produce 2 electrons which are used to power an electric circuit external to the fuel cell. The reaction equation is shown below:
2CO(g) + O₂(g) -> 2CO₂(g) ΔH_r° = -565.96 kJ/mol
This reaction does not occur for other types of fuel cells which use a catalyst, such as polymerelectrolyte membrane or phosphoric-acid fuel cells. The presence of carbon monoxide on the anode side of these types of fuel cells will cause catalyst poisoning, reducing the efficiency and voltage of the fuel cell.
Determine the rate of enthalpy change for a carbon dioxide production rate of 208 mol/hr.
The extent of the reaction occurring in the fuel cell can be obtained by the following equation:

where:

a) -16.35 kW
b) -15.35 kW
c) -16.45 kW
d) -15.45 kW
View Answer

Answer: a
Explanation:

The extent of reaction & the enthalpy of reaction into the equation yields

Now the rate of change in the enthalpy for the oxidation of carbon monoxide can be calculated as follows:
ΔH = ξ ̇ΔH_r°
Entering the known quantities for the extent of reaction and the enthalpy of reaction into this equation yields:
ΔH ̇= (104 mol/hr) (-565.96 kJ/mol).(1hr/3600 s)
= -16.35 kJ/s
ΔH ̇= -16.35 kW.

3. Assuming a synthesis gas obtained from a coal gasification process is used to produce methanol, which serves as fuel in direct-methanol fuel cells. Determine the rate of methanol production if the reaction shown below releases 21.6 kW of energy:
CO(g) + 2H₂(g) -> CH₃OH(l) ΔH_r° = -128.08 kJ/mol
a) 455.6 mol/hr
b) 457.2 mol/hr
c) 450.9 mol/hr
d) 454.5 mol/hr
View Answer

Answer: b
Explanation: Solving for ξ ̇and substituting the corresponding quantities into this equation yields:

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The rate in enthalpy change was considered to be negative since the problem is stating that the reaction is releasing energy (exothermic reaction).

Now we can enter the calculated extent of reaction into the equation previously solved for the molar production rate of methanol, to get:

4. In this problem we wish to develop the combustion equation and determine the air-fuel ratio for the complete combustion of n-Butane (C₄H₁₀) with theoretical air.

a) 15.4 kg-air/kg-fuel
b) 12.5 kg-air/kg-fuel
c) 12.4 kg-air/kg-fuel
d) 15.5 kg-air/kg-fuel
View Answer

Answer: d
Explanation: i) Theoretical air :

5. Calculate the air-fuel ratio for n-Butane combustion with 50% excess air.

a) 22.2 kg-air/kg-fuel
b) 23.2 kg-air/kg-fuel
c) 20.2 kg-air/kg-fuel
d) 20.3 kg-air/kg-fuel
View Answer

Answer: b
Explanation: 50% Excess air ( 150% Theoretical air):

The 50% excess air 23.2 kg-air/kg-fuel for 150% theoretical air

6. In this problem Propane (C₃H₈) is burned with 61% excess air, which enters a combustion chamber at 25°C. Assuming complete combustion and a total pressure of 1 atm (101.32 kPa), determine the air-fuel ratio [kg-air/kg-fuel].

61% Excess air (161% Theoretical air):
C₃H₈ + (1.61) z (O₂ + 3.76 N₂) => 3 (CO₂) + 4(H₂O) + (0.61)z (O₂) + (1.61) (3.76) z (N₂ )

Equating coefficients, z = 3+2 = 5 (oxygen component balance), thus:

C₃H₈ + 8.05(O₂ + 3.76 N₂) ⇒ 3(CO₂) + 4(H₂O) + 3.05(O₂) + 30.27(N₂)

a) 26.3 kg-air/kg-fuel
b) 25.5 kg-air/kg-fuel
c) 25.3 kg-air/kg-fuel
d) 26.5 kg-air/kg-fuel
View Answer

Answer: c
Explanation:

The air-fuel ratio [kg-air/kg-fuel] 25.3 kg-air/kg-fuel for oxygen component balance

7. In this problem Propane (C₃H₈) is burned with 61% excess air, which enters a combustion chamber at 25°C. and calculate the percentage of carbon dioxide by volume in the products.

a) 7.0%
b) 6.0%
c) 7.4%
d) 6.4%
View Answer

Answer: c
Explanation: N_CO₂ = 3, N_total = 3+4+3.05+ 30.27 = 40.32

8. Determine the dew point temperature of the products for propane combustion with 61% excess air.

a) 45.8°C
b) 40.5°C
c) 30.5°C
d) 35.8°C
View Answer

Answer: a
Explanation: NH₂O = 4, \(\frac{P_{H_{2}O}}{P} = \frac{N_{H_2O}}{N_{total}} = \frac{4}{40.32} => P_{H_2O} = (101.32 kPa)[\frac{4}{40.32}]\) = 10.05 kPa

T_dew-point = T_sat@10kPa = 45.8°C.

9. “Thermal conversion of organic matter with an oxidant (normally oxygen) to produce primarily carbon dioxide and water”, which term is used for this process?
a) Oxidation
b) Pyrolysis
c) Combustion
d) Gasification
View Answer

Answer: c
Explanation: Combustion is defined as thermal conversion of organic matter with an oxidant (normally oxygen) to produce primarily carbon dioxide and water. The oxidant is in stoichiometric excess, i.e., complete oxidation.

10. ”Thermal conversion (destruction) of organics in the absence of oxygen”, which term is used for this process?
a) Reduction
b) Pyrolysis
c) Combustion
d) Gasification
View Answer

Answer: b
Explanation: Pyrolysis is defined as thermal conversion (destruction) of organics in the absence of oxygen. In the biomass community, this commonly refers to lower temperature thermal processes producing liquids as the primary product. Possibility of chemical and food byproducts.

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