Bioprocess Engineering Questions and Answers for Aptitude test focuses on “Solving Unsteady – State Energy Balances”.
1. “Chemical reactions or phase changes occur” is this statement applicable for unsteady state energy balance?
A. True
B. False
Answer: B
Explanation: No chemical reactions or phase changes occur as the system is well mixed with uniform temperature and composition. Properties of the outlet stream are therefore the same as within the system and mixtures and solutions are ideal.
2. A boat has a thermo-server attached to it. While floating with the fluid, it measures the temperature of the fluid. Both flow and temperature are under unsteady-state condition:
T (x, y, z, t) = x2 + yz + t
(vec{V}(x, y, z, t) = 2xi + 2 t^2yhat{j} + hat{k})
Determine the rate of change of the temperature recorded by the sensor at t = 1, when the boat flows past the location, whose spatial co-ordinates are 2i + j + k.
DT/ dt = (frac{partial T}{partial t} + (V.nablA. T)
(frac{partial T}{partial t} = 1)
(frac{partial T}{partial x} = 2x; frac{partial T}{partial y} = z; frac{partial T}{partial z} = y)
Here, (x, y, z) = (2, 1, 1)
A. 30
B. 25
C. 20
D. 35
Answer: C
4. Refer to Q3, and estimate what is the total amount of heat (kcal) that must be removed by the cooling coil when this conversion is reached?
Using the following equation:
Q ̇= ΔHR r VR
A. -2.3 × 104 kcal
B. -2.2 × 104 kcal
C. -2.4 × 104 kcal
D. -2.6 × 104 kcal
Answer: A
Explanation: We assume the incompressible-fluid energy balance is accurate for this liquidphase reactor. If the heat removal is manipulated to maintain constant reactor temperature, the time derivative in Equation vanishes leaving
Q ̇ = ΔHR r VR
Substituting dcA/ dt = -r and multiplying through by dt gives
dQ = – ΔHR VR dcA
Integrating both sides gives
Q = – ΔHR VR (cA– cAO) = -2.3 × 104 kcal.
5. Refer to Q3 and Q4 and estimate what is the maximum rate at which heat must be removed by the cooling coil (kcal/min) and at what time does this maximum occur?
Using the following equation:
Q ̇ = ΔHR kc2Ao VR
A. −828 kcal/min
B. –880 kcal/min
C. -825 kcal/min
D. – 850 kcal/min
Answer: A
Explanation: The right-hand side is a maximum in absolute value (note it is a negative quantity) when cA is a maximum, which occurs for cA = cAo, giving
Qmax = ΔHR kc2Ao VR = (−10 kcal/mol A. (0.01725 L/mol•min) (2.0 mol/L )2 (1200 L)
= −828 kcal/min.
7. “All of the coolant is at a uniform temperature, Tc”, is this statement correct according to the assumptions for the Unsteady-state energy balance?
A. True
B. False
Answer: A
Explanation: All of the coolant is at a uniform temperature, Tc as the increase in coolant temperature as the coolant passes through the coil is neglected.
8. Fruit juice is fed to a heat exchanges at the rate of 15 kg/h. Saturated steam at 145 kPa pressure is used to heat the juice from 70C to 900C. Assuming the heat capacity of juice 5kJ/kg°C, find out the quantity of steam required for the operation.
Using the equation:
FHF + SHS = PHP + CHC
λs (Latent heat of condensation) = 2229 at 145 kPa.
A. 2.90 kg/h
B. 2.97 kg/h
C. 2.99 kg/h
D. 2.95 kg/h
9. The evaporator economy is dependent on the ________
A. Heat transfer rate
B. Mass transfer rate
C. Energy balance considerations
D. Mass balance considerations
Answer: A
Explanation: There are three main measures of evaporator performance:
Capacity (kg vaporized/time), Economy (kg vaporized/ kg steam input), Steam consumption (kg/hr).
The performance of a evaporator is evaluated by the capacity and the economy.
The rate of heat transfer q through the heating surface of an evaporator, by the definition of overall heat transfer coefficient, is product of three factors
i) The area of heat transfer surface A
ii) The overall heat transfer coefficient U
iii) The overall temperature drop ΔT
Q = U * A * ΔT.
10. The fouling factor is _________
A. Is a safety factor
B. Accounts for all resistances due to mass transfer
C. Is a dimensionless number
D. Accounts for all resistances due to heat transfer
Answer: D
Explanation: The fouling factor represents the theoretical resistance to heat flow due to a build-up of a layer of dirt or other fouling substance on the tube surfaces of the heat exchanger, but they are often overstated by the end user in an attempt to minimise the frequency of cleaning. In reality, if the wrong fouling factor is used, cleaning may actually be required more frequently.
11. The units for the log mean temperature difference are __________
A. 1 / °C
B. °C
C. 1/2 °C
D. Dimensionless
Answer: B