2015 Sem 1 CN3132

Separation Processes (II)

Lecture 10:

Definitions in Humidification

Dr. ZHAO Dan

Department of Chemical and Biomolecular Engineering

4 Engineering Drive 4, Blk E5, #02-16

Tel: (65) 6516 4679

chezhao@nus.edu.sg

Treybal: Chapter 7

2

Review: Concept

Packed Column Internals

Packing Materials

Specific Pressure Drop & Specific Liquid Holdup

Flooding and Pressure Drop Correlation

Calculation of Packed Column Diameter

3

Review: Equation

'0.5

'abscissa ( )G

L

L

G

'2 0.2

ordinateG L c

G F

g

2

10Flooding curve: log 1.6678 1.085log 0.29655(log )Y X X

''2

(10 )( )L

G

Gp

'

2

lbmol lb( )(M.W. vapor )

s lbmolArea lb

s ft

c

V

A

G

4Diameter c

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Learning Outcomes of Lecture 10

Describe the mass transfer in evaporation and humidification

Understand the basic working mechanism of cooling tower and evaporative cooler

Explain the common definitions in humidification

Apply those definitions in calculations

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5

Evaporation & Humidification

Evaporation is a type of vaporization of a liquid that occurs from the surface of a liquid into a gaseous phase that is not saturated with the evaporating substance.

The enthalpy of vaporization is the energy required to transform a given quantity of a substance from a liquid into a gas at a given pressure (often atmospheric pressure).

Humidification refers to the process of raising the water vapor content of a gas.

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Case Study (1): Nuclear Cooling Tower

Beginning with the Three Mile Island accident in 1979, the gigantic, hyperboloid cooling tower became the primary visual symbol used by the news media when covering nuclear power plants.

As a result, two understandable misconceptions were generated. First, that cooling towers are a nuclear device. Second, the power plants reactor is located inside the cooling tower.

There is yet another misconception common to cooling towers attached to nuclear power plants; that the cloud pouring out of the tower is radioactive.

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How does a cooling tower work?

A cooling tower intimately contacts a flow of warm water with a flow of ambient air which is not saturated with water vapor.

That causes part of the warm water to evaporate and the air absorbs that evaporated water.

The heat required to evaporate part of the water is derived from the water itself and thus causes the water to cool.

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Case Study (2): Evaporative Cooler

An evaporative cooler (also swamp cooler, desert cooler and wet air cooler) is a device that cools air through the evaporation of water.

The temperature of dry air can be dropped significantly through the phase transition of liquid water to water vapor (evaporation), which can cool air using much less energy than refrigeration.

In extremely dry climates, evaporative cooling of air has the added benefit of conditioning the air with more moisture for the comfort of building occupants.

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Definitions (1)

Absolute Humidity The ratio (mass of vapor)/(mass of gas) is the absolute humidity Y.

If the quantities are expressed in moles, the ratio is the molal absolute humidity Y.

Under conditions where the ideal-gas law applies,

For air-water system at pressure of 1.0133105 Pa,

Dry-bulb Temperature This is the temperature of a vapor-gas mixture as ordinarily determined by

immersion of a thermometer in the mixture.

Percentage Saturation (Percentage Absolute Humidity) Defined as 100Y/Ys and 100Y/Ys, where the saturated values Ys and Ys are

computed at the dry-bulb temperature of the mixture.

moles A

moles B

A A A

B B t A

y p pY

y p p p

mass A'

mass B

A A A A A

B B B t A B

M p M p MY Y

M p M p p M

2

2

2

5

0.622 kg H O'

1.0133 10 kg air

H O

H O

pY

p

2

2

2

5

0.622 kg H O'

1.0133 10 kg air

H O

s

H O

PY

P

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Definitions (2)

Relative Saturation (Relative Humidity) A percentage defined as 100pA/PA, where PA is the vapor pressure at

the dry-bulb temperature of the mixture.

Normally, relative saturation (100pA/PA) percentage saturation (100Y/Ys).

Humid Volume The volume of unit mass (1 kg) of dry gas and its accompanying vapor

at the prevailing temperature and pressure.

For air-water system at pressure of 1.0133105 Pa,

' ', , 1,

' '

A A

t A B t A t AA AA A

A As A t A t A s A

t A B

p M

p p M p P p Pp pY YP p

P MY P p p p p Y P

p P M

5273 2731 ' 1.013 10 1 '22.41 8315

273

G GH

B A t B A t

t tY Y

M M p M M p

(0.00283 0.00456 ')( 273)H GY t

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Definitions (3)

Humid Heat The heat required to raise the temperature of unit mass of gas and its

accompanying vapor one degree at constant pressure.

For a mixture of absolute humidity Y,

For air-water system at pressure of 1.0133105 Pa,

Enthalpy The sum of the relative enthalpies of the gas and of the vapor content.

For air-water system at pressure of 1.0133105 Pa and t0 = 0 C,

'S B AC C Y C

1005 1884 ' J for mixture/(kg air) CSC Y

0 0 0 0 0' ( ) '[ ( ) ] ( ) 'B G A G S GH C t t Y C t t C t t Y

' (1005 1884 ') 2502300 ' J for mixture/kg airGH Y t Y

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Summary

Treybal 3rd p. 234

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Example Question (1)

In a mixture of benzene vapor (A) and nitrogen gas (B) at a total pressure of 800 mmHg and a temperature of 60 C, the partial pressure of benzene is 100 mmHg. Express the benzene concentration in mole fraction, volume fraction, and absolute humidity.

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Example Question (2)

A gas (B)-benzene (A) mixture is saturated at 1 std atm, 50 C. The equilibrium vapor pressure of benzene is 275 mmHg at 50 C. Calculate the absolute humidity if B is (a) nitrogen and (b) carbon dioxide.