Drying, Humidification and Membrane Separation - Mass Transfer - Chemical Engineering Previous Year Questions

1

2007 · Chemical Engineering · Mass Transfer · Drying, Humidification and Membrane Separation

Chemical Engineering (CH) 2007

If the percent humidity of air (30 °C, total pressure 100 kPa) is 24 % and the saturation pressure of water vapor at that temperature is 4 kPa, the percent relative humidity and the absolute humidity of air are

A

25.2, 0.0062

B

25, 0.0035

C

20.7, 0.0055

D

18.2, 0.0035

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2

2007 · Chemical Engineering · Mass Transfer · Drying, Humidification and Membrane Separation

Chemical Engineering (CH) 2007

A 50cm X 50 cm X 1 cm flat wet sheet weighing 2 kg initially, was dried from both the sides under constant drying rate period. It took 1000 secs for the weight of the sheet to reduce to 1.75 kg. Another 1m X 1m X 1cm flat sheet is to be dried from one side only. Under the same drying rate and other conditions, time required for drying (in secs) from initial weight of 4 kg to 3 kg is

A

1000

B

1500

C

2000

D

2500

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3

2009 · Chemical Engineering · Mass Transfer · Drying, Humidification and Membrane Separation

Chemical Engineering (CH) 2009

The equilibrium moisture curve for a solid is shown below :
The total moisture content of the solid is X and it is exposed to air of relative humidity H. In the table below, Group I lists the types of moisture, and Group II represents the region in the graph above.

GROUP I	GROUP II
P. Equilibrium moisture	1
Q. Bound moisture	2
R. Unbound moisture	3
S. Free moisture	4

Which ONE of the following is the correct match ?

A

P-1, Q-2, R-3, S-4

B

P-1, Q-3, R-4, S-2

C

P-1, Q-4, R-2, S-3

D

P-1, Q-2, R-4, S-3

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4

2012 · Chemical Engineering · Mass Transfer · Drying, Humidification and Membrane Separation

Chemical Engineering (CH) 2012

Consider the drying operation shown in the figure below for a solid loading (dry basis) of 50 kg/m² with a constant drying rate of 5 kg/m².h. The falling rate of drying is linear with moisture content. The drying time (in hrs) required to reduce an initial moisture content of 25% to a final moisture content of 2% is

A

1.55

B

1.75

C

3.25

D

4.55

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5

2013 · Chemical Engineering · Mass Transfer · Drying, Humidification and Membrane Separation

Chemical Engineering (CH) 2013

A wet solid is dried over a long period of time by unsaturated air of nonzero constant relative humidity. The moisture content eventually attained by the solid is termed as the

A

unbound moisture content

B

bound moisture content

C

free moisture content

D

equilibrium moisture content

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6

2014 · Chemical Engineering · Mass Transfer · Drying, Humidification and Membrane Separation

Chemical Engineering (CH) 2014

A wet solid of 100 kg is dried from a moisture content of 40 wt% to 10 wt%. The critical moisture content is 15 wt% and the equilibrium moisture content is negligible. All moisture contents are on dry basis. The falling rate is considered to be linear. It takes 5 hours to dry the material in the constant rate period. The duration (in hours) of the falling rate period is __________

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7

2018 · Chemical Engineering · Mass Transfer · Drying, Humidification and Membrane Separation

Chemical Engineering (CH) 2018

A fiberboard sheet (1.5 m × 2.0 m × 15 mm) is being dried by suspending it horizontally in a current of hot, dry air. The edges are insulated so that drying takes place only from the top and bottom surfaces. The wet sheet weighing 16 kg with initial moisture content of 60% loses moisture at a constant rate of 1.25 × 10^-5 kg m^-2 s^-1 until the moisture content falls to 30%. All moisture contents are on dry basis. The time required for drying during constant rate period (in hour) is __________ (rounded off to third decimal place).

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8

2018 · Chemical Engineering · Mass Transfer · Drying, Humidification and Membrane Separation

Chemical Engineering (CH) 2018

The humidity of air at a dry-bulb temperature of 65 °C is 0.025 kg water/kg dry air. The latent heat of vaporization of water at 0 °C is 2500 kJ/kg. The psychrometric ratio of air is 0.95 kJ (kg dry air)^-1 K^-1. Considering 0 °C as reference temperature, the enthalpy of air (in kJ/kg) at its adiabatic saturation temperature of 35 °C is __________ (rounded off to two decimal places).

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9

2019 · Chemical Engineering · Mass Transfer · Drying, Humidification and Membrane Separation

Chemical Engineering (CH) 2019

In the drying of non-dissolving solids at constant drying conditions, the internal movement of moisture in the solid has a dominant effect on the drying rate during

A

the initial adjustment period only

B

the constant rate period only

C

the falling rate period only

D

both the initial adjustment and constant rate periods

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10

2021 · Chemical Engineering · Mass Transfer · Drying, Humidification and Membrane Separation

Chemical Engineering (CH) 2021

In a batch drying experiment, a solid with a critical moisture content of 0.2 kg H₂O/kg dry solid is dried from an initial moisture content of 0.35 kg H₂O/kg dry solid to a final moisture content of 0.1 kg H₂O/kg dry solid in 5 h. In the constant rate regime, the rate of drying is 2 kg H₂O/(m²·h). The entire falling rate regime is assumed to be uniformly linear. The equilibrium moisture content is assumed to be zero.
The mass of the dry solid per unit area is _______ kg/m² (round off to nearest integer).

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11

2022 · Chemical Engineering · Mass Transfer · Drying, Humidification and Membrane Separation

Chemical Engineering (CH) 2022

A wet solid containing 20% (w/w) moisture (based on mass of bone-dry solid) is dried in a tray-dryer. The critical moisture content of the solid is 10% (w/w). The drying rate (kg m⁻² s⁻¹) is constant for the first 4 hours, and then decreases linearly to half the initial value in the next 1 hour. At the end of 5 hours of drying, the percentage moisture content of the solid is __________ % (w/w) (rounded off to one decimal place).

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12

2022 · Chemical Engineering · Mass Transfer · Drying, Humidification and Membrane Separation

Chemical Engineering (CH) 2022

The dry-bulb temperature of air in a room is 30 °C. The Antoine equation for water is given as
ln P^sat = 12.00 – 4000/(T – 40)
where T is the temperature in K and P^sat is the saturation vapor pressure in bar. The latent heat of vaporization of water is 2000 kJ kg^-1, the humid heat is 1.0 kJ kg^-1 K^-1, and the molecular weights of air and water are 28 kg kmol^-1 and 18 kg kmol^-1, respectively. If the absolute humidity of air is Y′ kg moisture per kg dry air, then for a wet-bulb depression of 9 °C, 1000 × Y′ = ______ (rounded off to one decimal place).

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13

2023 · Chemical Engineering · Mass Transfer · Drying, Humidification and Membrane Separation

Chemical Engineering (CH) 2023

Spray dryers have many advantages. Which one of the following is NOT an advantage of a typical spray dryer?

A

Has short drying time

B

Produces hollow spherical particles

C

Has high heat efficiency

D

Is suitable for heat sensitive materials

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14

2023 · Chemical Engineering · Mass Transfer · Drying, Humidification and Membrane Separation

Chemical Engineering (CH) 2023

Partially saturated air at 1 bar and \( 50 \text{ }^{\circ}\text{C} \) is contacted with water in an adiabatic saturator. The air is cooled and humidified to saturation, and exits at \( 25 \text{ }^{\circ}\text{C} \) with an absolute humidity of \( 0.02 \text{ kg water per kg dry air} \). Use latent heat of vaporization of water as \( 2450 \text{ kJ.kg}^{-1} \), and average specific heat capacity for dry air and water, respectively as \( 1.01 \text{ kJ.kg}^{-1}\text{K}^{-1} \) and \( 4.18 \text{ kJ.kg}^{-1}\text{K}^{-1} \). If the absolute humidity of air entering the adiabatic saturator is \( \mathcal{H} \times 10^{-3} \text{ kg water per kg dry air} \), the value of \( \mathcal{H} \) is ______ (rounded off to two decimal places).

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15

2024 · Chemical Engineering · Mass Transfer · Drying, Humidification and Membrane Separation

Chemical Engineering (CH) 2024

Alumina particles with an initial moisture content of 5 kg per kg dry solid are dried in a batch dryer. For the first two hours, the measured drying rate is constant at 2 kg m^{-2} h^{-1}. Thereafter, in the falling-rate period, the rate decreases linearly with the moisture content. The equilibrium moisture content is 0.05 kg per kg dry solid and the drying area of the particles is 0.5 m^2 per kg dry solid. The total drying time, in h, to reduce the moisture content to half its initial value is

A

4.13

B

2.55

C

3.22

D

5.13

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16

2025 · Chemical Engineering · Mass Transfer · Drying, Humidification and Membrane Separation

Chemical Engineering (CH) 2025

Consider moist air with absolute humidity of 0.02 (kg moisture)/(kg dry air) at 1 bar pressure. The vapour pressure of water is given by the equation
\[ \ln P^{sat} = 12 - \frac{4000}{T - 40} \]
where \(P^{sat}\) is in bar and \(T\) is in K. The molecular weight of water and dry air are 18 kg/kmol and 29 kg/kmol, respectively. The dew temperature of the moist air is _____ °C (rounded off to the nearest integer).

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17

2025 · Chemical Engineering · Mass Transfer · Drying, Humidification and Membrane Separation

Chemical Engineering (CH) 2025

A wet solid of 100 kg containing 30 wt% moisture is to be dried to 2 wt% moisture in a tray dryer. The critical moisture content is 10 wt% and the equilibrium moisture content is 1 wt%. The drying rate during the constant rate period is 10 kg/(h·m²). The drying curve in the falling rate period is linear. If the drying area is 5 m², the time required for drying is ____ h (rounded off to 1 decimal place).
Note: All the moisture values given are on dry-solid basis.

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18

2026 · Chemical Engineering · Mass Transfer · Drying, Humidification and Membrane Separation

Chemical Engineering (CH) 2026

Vapor pressure of water at various temperature is given in the table.

Temperature (in K)	284	289	294	299	310
Vapor pressure of water (in kPa)	1.28	1.80	2.50	3.40	6.40

An air and water vapor mixture at 100 kPa with a relative humidity of 20% has a dry-bulb temperature of 310 K. Assume latent heat of vaporization for water is 44 kJ mol^-1 and specific heat capacity of the air and water vapor mixture is 0.035 kJ mol^-1K^-1. Which one of the following is the closest to its wet-bulb temperature (in K)?

A

284

B

294

C

310

D

321

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