Humidification And Drying Flashcards
(99 cards)
1
Q
- The wet-bulb temperature is the lowest temperature that can be reached by evaporative cooling of water into the air.
A. True
B. False
A
True
2
Q
- Which of the following best describes the “bound moisture” in a solid?
A. Moisture present on the surface of the material
B. Moisture that exerts vapor pressure equal to free water
C. Moisture that is chemically or physically held within the solid
D. Moisture removed during the constant-rate period
A
Moisture that is chemically or physically held within the solid
3
Q
- Relative humidity is defined as the ratio of the partial pressure of water vapor to the saturation pressure of water vapor at the same temperature.
A. True
B. False
A
True
4
Q
- During which drying phase does the surface of the solid remain saturated with water?
A. Falling-rate period
B. Constant-rate period
C. Equilibrium period
D. Preheating period
A
Constant-rate period
5
Q
- In a drying curve, the critical moisture content marks the end of the constant-rate period.
A. True
B. False
A
True
6
Q
- Humidification processes are typically analyzed using which diagram?
A. Mollier diagram
B. P-x-y diagram
C. Psychrometric chart
D. T-s diagram
A
Psychrometric chart
7
Q
- Absolute humidity is defined as the mass of water vapor per unit mass of dry air.
A. True
B. False
A
True
8
Q
- Which of the following is not a mechanism of moisture movement within solids during drying?
A. Capillary action
B. Liquid diffusion
C. Chemical reaction
D. Vapor diffusion
A
Chemical reaction
9
Q
- Evaporative cooling always leads to an increase in the dry-bulb temperature of air.
A. True
B. False
A
False
10
Q
- In a countercurrent humidification tower, air and liquid:
A. Flow in the same direction
B. Flow in opposite directions
C. Do not mix
D. Are recirculated continuously
A
Flow in opposite directions
11
Q
- The rate of drying during the falling-rate period is influenced primarily by internal moisture movement.
A. True
B. False
A
True
12
Q
- Which factor has the least effect on the rate of drying in the constant-rate period?
A. Air velocity
B. Surface area
C. External heat supply
D. Material porosity
A
Material porosity
13
Q
- Adiabatic saturation temperature is always equal to the wet-bulb temperature for unsaturated air.
A. True
B. False
A
False
14
Q
- Which of the following parameters increases during adiabatic humidification of unsaturated air?
A. Dry-bulb temperature
B. Relative humidity
C. Enthalpy of air
D. Specific gravity
A
Relative humidity
15
Q
- Moisture that remains after a material is dried under ambient conditions is known as free moisture.
A. True
B. False
A
False
16
Q
- Which of the following best defines the “humid volume”?
A. Volume of air per unit mass of dry air
B. Volume of air per unit mass of water vapor
C. Total volume of moist air per unit mass of dry air
D. Volume of saturated air per unit temperature
A
Total volume of moist air per unit mass of dry air
17
Q
- The humidity ratio is also known as the specific humidity.
A. True
B. False
A
True
18
Q
- Which drying method is commonly used for heat-sensitive materials?
A. Spray drying
B. Tunnel drying
C. Fluidized bed drying
D. Freeze drying
A
Freeze drying
19
Q
- During spray drying, the liquid feed is atomized into small droplets before being exposed to hot air.
A. True
B. False
A
True
20
Q
- In which equipment is direct contact between gas and liquid used for humidification?
A. Condenser
B. Tray dryer
C. Packed tower
D. Drum dryer
A
Packed tower
21
Q
- In drying, the critical moisture content always equals the equilibrium moisture content.
A. True
B. False
A
False
22
Q
- Which drying condition results in the highest drying rate?
A. Low temperature, low humidity
B. High temperature, high humidity
C. High temperature, low humidity
D. Low temperature, high humidity
A
High temperature, low humidity
23
Q
- Air with 100% relative humidity cannot absorb more moisture.
A. True
B. False
A
True
24
Q
- Which of the following factors influences the selection of a drying method?
A. Thermal sensitivity of the material
B. Desired product form
C. Energy consumption
D. All of the above
A
All of the above
25
25. Spray dryers are typically used to produce powders from liquid feeds.
A. True
B. False
True
26
26. What is the primary driving force for mass transfer in drying?
A. Heat capacity of air
B. Moisture gradient
C. Surface tension
D. Material thickness
Moisture gradient
27
27. A packed tower in humidification allows for a large contact area between air and liquid.
A. True
B. False
True
28
28. Which device measures both dry-bulb and wet-bulb temperatures?
A. Barometer
B. Thermocouple
C. Psychrometer
D. Manometer
Psychrometer
29
29. Capillary action aids in moving moisture to the surface during the falling-rate period.
A. True
B. False
True
30
30. Which term describes the heat required to raise the temperature of humid air per unit mass of dry air?
A. Sensible heat
B. Humid heat
C. Latent heat
D. Specific humidity
Humid heat
31
31. Desorption and drying are essentially the same processes.
A. True
B. False
False
32
32. The dew point is defined as:
A. The temperature at which air is fully dry
B. The temperature at which condensation begins
C. The temperature of saturation humidity
D. The temperature at which humidity is lowest
The temperature at which condensation begins
33
33. Drying rate is directly proportional to surface area available for mass transfer.
A. True
B. False
True
34
34. Which drying equipment is typically used for slurries and suspensions?
A. Tray dryer
B. Spray dryer
C. Rotary dryer
D. Vacuum shelf dryer
Spray dryer
35
35. Heat transfer in drying can occur through conduction, convection, or radiation.
A. True
B. False
True
36
36. Which variable is held constant in an adiabatic saturation process?
A. Enthalpy
B. Dry-bulb temperature
C. Pressure
D. Wet-bulb temperature
Enthalpy
37
37. Vacuum drying is preferred for thermally stable substances.
A. True
B. False
False
38
38. Which property increases with increased absolute humidity?
A. Dew point
B. Wet-bulb temperature
C. Sensible heat
D. Dry-bulb temperature
Dew point
39
39. A fixed-bed dryer is a type of batch dryer.
A. True
B. False
True
40
40. What is the usual heat source in drum dryers?
A. Superheated steam
B. Hot oil
C. Electric coils
D. Open flame
Superheated steam
41
41. Moisture content is often expressed as a percentage on a wet basis or dry basis.
A. True
B. False
True
42
42. In a rotary dryer, the material moves by:
A. Gravity and rotation
B. Pressure gradients
C. Diffusion
D. Centrifugal force
Gravity and rotation
43
43. Flash drying is typically used for large particles.
A. True
B. False
False
44
44. Which equation is commonly used to estimate drying time?
A. Antoine equation
B. Lewis equation
C. Clapeyron equation
D. Arrhenius equation
Lewis equation
45
45. Higher airflow velocity generally increases the drying rate.
A. True
B. False
True
46
46. During the constant-rate period, moisture is removed by:
A. Surface vaporization
B. Capillary movement
C. Diffusion
D. Adsorption
Surface vaporization
47
47. Equilibrium relative humidity is relevant in storage of dried products.
A. True
B. False
True
48
48. The most energy-efficient region of drying is:
A. Early falling-rate period
B. Late falling-rate period
C. Constant-rate period
D. Preheating stage
Constant-rate period
49
49. Radiation dryers are mainly used for thick, dense materials.
A. True
B. False
False
50
Drying time is influenced by which of the following parameters?
A. Initial moisture content
B. Critical moisture content
C. Drying rate
D. All of the above
All of the above
51
Drying involves the removal of ___________ from a solid, liquid, or gas.
(a) Heat
(b) Moisture
(c) Air
(d) Impurities
Moisture
52
Bound moisture exerts a vapor pressure ___________ than that of pure liquid at the same temperature.
(a) Higher
(b) Lower
(c) Equal
(d) Significantly higher
Lower
53
Unbound moisture exerts a vapor pressure ___________ to that of pure liquid at the same temperature.
(a) Higher
(b) Lower
(c) Approximately equal
(d) Significantly lower
Approximately equal
54
The critical moisture content marks the point when:
(a) The solid becomes completely dry.
(b) The drying rate starts to decrease.
(c) The surface of the solid is no longer saturated with moisture.
(d) Both (b) and (c)
Both (b) and (c)
55
The constant rate period of drying is controlled by:
(a) Diffusion of moisture within the solid.
(b) Heat transfer to the evaporating surface.
(c) Capillary action within the solid.
(d) Chemical reactions within the solid.
Heat transfer to the evaporating surface
56
During the falling rate period, the drying rate is primarily controlled by:
(a) Air velocity over the surface.
(b) Humidity of the drying air.
(c) The rate of moisture movement within the solid.
(d) The temperature of the drying air.
The rate of moisture movement within the solid.
57
Which of the following is NOT a mechanism of moisture movement within a solid during drying?
(a) Liquid diffusion
(b) Vapor diffusion
(c) Capillary flow
(d) Convection in the surrounding air
Convection in the surrounding air
58
Which of the following drying methods involves direct contact between the wet solid and the heating medium?
(a) Freeze drying
(b) Vacuum drying
(c) Convection drying
(d) Drum drying
Convection drying
59
In a tray dryer, heat is transferred to the material primarily by:
(a) Conduction through the trays.
(b) Convection from the hot air circulating in the chamber.
(c) Radiation from heating elements.
(d) A combination of conduction, convection, and radiation.
d
60
A rotary dryer is best suited for drying:
(a) Thin films of liquids.
(b) Large, irregularly shaped solids.
(c) Free-flowing granular solids.
(d) Heat-sensitive materials at low temperatures.
Free-flowing granular solids.
61
In a fluidized bed dryer, the solid particles are suspended in:
(a) A vacuum.
(b) A high-pressure liquid.
(c) An upward stream of hot gas.
(d) A rotating drum.
An upward stream of hot gas.
62
Spray drying is particularly suitable for drying:
(a) Large solid objects.
(b) Heat-sensitive liquids and slurries.
(c) Highly viscous materials.
(d) Solids that need to retain their shape.
Heat-sensitive liquids and slurries.
63
Freeze drying (lyophilization) involves:
(a) Rapid heating of the frozen material.
(b) Sublimation of ice under vacuum.
(c) Evaporation of liquid at high temperatures.
(d) Mechanical removal of moisture.
Sublimation of ice under vacuum.
64
Vacuum drying is employed for materials that are:
(a) Stable at high temperatures.
(b) Prone to oxidation or degradation at high temperatures.
(c) Easily dried by simple air drying.
(d) Non-porous and impermeable.
Prone to oxidation or degradation at high temperatures.
65
Drum dryers are used for drying:
(a) Powders and granules.
(b) Solutions, slurries, and pastes.
(c) Large solid sheets.
(d) Delicate crystalline materials.
Solutions, slurries, and pastes.
66
The rate of drying during the constant rate period is independent of:
(a) The temperature of the drying air.
(b) The humidity of the drying air.
(c) The air velocity over the surface.
(d) The moisture content of the solid.
The moisture content of the solid.
67
The falling rate period occurs when:
(a) The surface of the solid is completely saturated with moisture.
(b) The rate of evaporation from the surface is equal to the rate of moisture diffusion from the interior.
(c) The rate of moisture movement to the surface is less than the rate of evaporation from the surface.
(d) The air temperature drops below the dew point.
The rate of moisture movement to the surface is less than the rate of evaporation from the surface.
68
Fick's law of diffusion is relevant in the ___________ period of drying.
(a) Constant rate
(b) Falling rate
(c) Both constant and falling rate
(d) Neither constant nor falling rate
Falling rate
69
Case hardening during drying refers to:
(a) The solidification of the outer surface, hindering further moisture removal.
(b) The increase in hardness of the material upon drying.
(c) The formation of a protective layer against moisture absorption.
(d) The cracking of the material due to rapid drying.
The solidification of the outer surface, hindering further moisture removal.
70
Shrinkage of a solid during drying typically occurs during the:
(a) Constant rate period.
(b) Falling rate period.
(c) Both periods.
(d) Neither period.
Falling rate period.
71
The equilibrium moisture content is the moisture content at which:
(a) The drying rate is maximum.
(b) The solid is in equilibrium with the surrounding air.
(c) All unbound moisture has been removed.
(d) The critical moisture content is reached.
The solid is in equilibrium with the surrounding air.
72
Hysteresis in sorption isotherms refers to the difference in:
(a) Drying and wetting paths for moisture content versus partial pressure.
(b) Drying rates at different temperatures.
(c) Equilibrium moisture content for different materials.
(d) The effect of particle size on drying rate.
Drying and wetting paths for moisture content versus partial pressure.
73
Which of the following factors affects the drying rate in the constant rate period?
(a) Internal diffusion coefficient of the solid.
(b) Capillary forces within the solid.
(c) Heat transfer coefficient from the air to the surface.
(d) Equilibrium moisture content.
Heat transfer coefficient from the air to the surface.
74
The drying time for a given material is influenced by:
(a) The initial moisture content.
(b) The final desired moisture content.
(c) The properties of the drying medium (temperature, humidity, velocity).
(d) All of the above.
All of the above.
75
Increasing the temperature of the drying air generally:
(a) Decreases the drying rate.
(b) Increases the drying rate.
(c) Has no effect on the drying rate.
(d) May increase or decrease the rate depending on the material.
Increases the drying rate.
76
Increasing the humidity of the drying air generally:
(a) Increases the drying rate.
(b) Decreases the drying rate.
(c) Has no effect on the drying rate.
(d) Stabilizes the drying rate.
b
77
The heat of sorption is the heat released when:
(a) A liquid evaporates.
(b) A solid melts.
(c) A gas is adsorbed onto a solid surface.
(d) Moisture is removed from a solid.
c
78
Adiabatic drying involves:
(a) Heating the air before it contacts the wet material.
(b) Using a vacuum to remove moisture.
(c) No heat exchange with the surroundings; heat for evaporation comes from the air.
(d) Maintaining a constant temperature throughout the process.
c
79
Psychrometric charts can be used in drying calculations for:
(a) Determining the moisture content of solids.
(b) Analyzing air-drying processes.
(c) Calculating heat transfer rates within solids.
(d) Designing vacuum dryers.
b
80
The concept of wet-bulb temperature is important in analyzing:
(a) Freeze drying.
(b) Vacuum drying.
(c) Adiabatic air drying.
(d) Drum drying.
c
81
The rate of drying curves typically plot:
(a) Moisture content versus time.
(b) Drying rate versus time.
(c) Drying rate versus moisture content.
(d) Temperature versus moisture content.
c
82
The shape of the drying rate curve can provide information about:
(a) The type of dryer used.
(b) The mechanism of moisture movement within the solid.
(c) The thermal conductivity of the solid.
(d) The particle size distribution.
b
83
In through-circulation drying, the air flows:
(a) Parallel to the drying surface.
(b) Perpendicular to and through the bed of solids.
(c) In a swirling motion above the material.
(d) Countercurrent to the movement of the solid.
b
84
In tunnel dryers, the material moves through a tunnel where it is contacted by:
(a) Superheated steam.
(b) Hot oil.
(c) Heated air or gas.
(d) Infrared radiation.
c
85
Infrared drying utilizes ___________ to transfer heat to the material.
(a) Convection
(b) Conduction
(c) Electromagnetic radiation
(d) Direct contact with a hot surface
c
86
Microwave drying works by:
(a) Heating the surface of the material.
(b) Generating heat within the material by interaction with microwave energy.
(c) Using high-frequency sound waves to remove moisture.
(d) Creating a strong vacuum to accelerate evaporation.
b
87
Dielectric drying is similar to microwave drying but uses:
(a) Lower frequency electromagnetic waves.
(b) Higher intensity radiation.
(c) A different mechanism of heat generation.
(d) Only for conductive materials.
a
88
The efficiency of a dryer is defined as the ratio of:
(a) Heat input to heat output.
(b) Energy used for evaporation to the total energy input.
(c) Mass of water removed to the mass of dry solid.
(d) Drying time to the ideal drying time.
b
89
Energy losses in a dryer can occur through:
(a) Heat loss to the surroundings.
(b) Sensible heat in the exhaust air.
(c) Incomplete moisture removal.
(d) All of the above.
d
90
The selection of a dryer depends on factors such as:
(a) Properties of the material to be dried.
(b) Required production rate.
(c) Cost and energy efficiency.
(d) All of the above.
d
91
For heat-sensitive materials, ___________ drying methods are preferred.
(a) High-temperature, short-time
(b) Low-temperature, long-time
(c) Direct
(d) Convection
b
92
Materials that tend to case harden should be dried:
(a) Rapidly at high temperatures.
(b) Slowly with controlled temperature and humidity.
(c) Under high vacuum.
(d) In a fluidized bed.
b
93
The final moisture content required for a dried product depends on:
(a) The initial moisture content.
(b) The drying method used.
(c) Storage stability and intended use.
(d) The cost of drying.
c
94
Moisture regain refers to the ability of a dried material to:
(a) Absorb moisture from the surrounding air.
(b) Retain its initial moisture content.
(c) Release any remaining bound moisture.
(d) Undergo further drying.
a
95
The equilibrium relative humidity (ERH) is the relative humidity of the air in equilibrium with a material at a given:
(a) Drying rate.
(b) Moisture content.
(c) Temperature gradient.
(d) Air velocity.
b
96
Water activity (a_w) is related to the equilibrium relative humidity by:
(a) a_w = ERH / 100
(b) a_w = 100 / ERH
(c) a_w = log(ERH)
(d) a_w = 1 - (ERH / 100)
a
97
In biological materials, water activity is an important factor affecting:
(a) The rate of heat transfer.
(b) Microbial growth and enzymatic activity.
(c) The mechanical strength of the material.
(d) The color of the dried product.
b
98
The BET (Brunauer-Emmett-Teller) theory describes:
(a) The rate of drying in the falling rate period.
(b) Multilayer adsorption of gases on solid surfaces, including water vapor.
(c) Heat transfer coefficients in dryers.
(d) The mechanism of freeze drying.
b
99
For efficient drying, it is generally desirable to have a large driving force, which is the difference between:
(a) The temperature of the solid and the air temperature.
(b) The moisture content of the solid and the equilibrium moisture content.
(c) The air velocity at the surface and the internal diffusion rate.
(d) The heat capacity of the solid and the heat capacity of the air.
b
