EKC316 - SEPARATION PROCESS HUMIDIFICATION

DR. AZAM T MOHD DIN

SCHOOL OF CHEMICAL ENGINEERING

UNIVERSITI SAINS MALAYSIA

Semester II, 2014/2015

TEXT BOOKS

OUTLINES

Principles and theory of humidification process

Introduction to industrial humidification &

dehumidification equipment

Design of cooling tower

From left to right: a reinforced concrete tower, a wood

tower, and a hyperbolic tower built with a steel framework

and wood cladding

www.spxcooling.com/

HUMIDITY

The amount of water vapor in air.

How does relative

humidity affects people?

0% humidity 100% humidity

Psychrometry

Moist air

London, 2006-2012

Penang, 2006-2012

TERMINOLOGIES & DEFINITIONS

Temperature : dry-bulb, wet-bulb

Humidity : Relative, Absolute, Percent

Enthalpy

Dry-bulb Temperature

It is true temperature of air measured (or, any

non-condensable and condensable mixture) by a

thermometer whose bulb is dry.

Wet-bulb Temperature

It is the steady-state temperature attained by a

small amount of evaporating water in a manner

such that the sensible heat transferred from the

air to the liquid is equal to the latent heat

required for evaporation.

Dew point

A temperature at which a vapor-gas mixture must be cooled (at

constant humidity) to become saturated.

The dew point of a saturated gas equals the gas temperature.

If a vapor-gas mixture is gradually cooled at a constant pressure,

the temperature at which it just becomes saturated is also called

its dew point.

Relative humidity

It is the ratio of partial pressure of water vapor

(pA) in air at a given temperature to the vapor

pressure of water (pvA) at the same temperature.

%xp

piditylative hum

vA

A 100Re

Relative humidity does not explicitly give the

moisture content of a gas, but gives the degree

of saturation of the gas at a given temperature.

Absolute humidity (simply humidity)

It is the direct measurement of moisture content in

a gas. The mass of water vapor per unit mass of

dry gas is called absolute humidity, Y.

Eq. 1

Percent humidity or percent saturation

It is the relation between absolute humidity to that of saturation humidity at the same temperature

and pressure.

where, Y is absolute humidity of sample of air and Ys is humidity at same temperature and pressure

if saturated with water vapor.

Eq. 2

Eq. 3

and vapor pressure of water can be calculated

by Antoine Equation:

where, pressure is in bar and temperature is in K.

Eq. 4

Humid volume

The humid volume, vH, is defined as the volume of

unit mass of dry air with accompanying water

vapor at a given temperature and pressure.

Assuming ideal gas behaviour.

TG is gas temperature in C. Eq. 5

Humid Heat

The humid heat, cH, is the heat energy required to raise the

temperature of unit mass of dry air with the accompanying

water vapor by one (1) degree.

At ordinary T & P, the heat capacity of dry air = 1.005

kJ/kg.K and that of water vapor as 1.88 kJ/kg.K

air)(K)dry kJ/(kg 88.1005.1 'YcH Eq. 6

Enthalpy

The enthalpy of a vapor-gas mixture, H is the sum of

the relative enthalpies of gas and vapor content.

Enthalpy = Latent heat + Sensible heat

)('' 00 TTcYH GH Eq. 7

At 0 C, 0 = 2500 kj/kg

airdry kJ/kg ))('88.1005.1('2500' 0TTYYH G

Sensible heat

When an object is heated, its temperature rises as heat is added. The increase in heat is called sensible heat. Similarly, when heat is removed from an object and its temperature falls, the heat removed is also called sensible heat. Heat that

causes a change in temperature in an object is called sensible heat.

Latent heat

All pure substances in nature are able to change their state. Solids can become liquids (ice to water) and liquids can become gases (water to vapor) but changes such as these require the addition or removal of heat. The heat that causes

these changes is called latent heat.

Latent heat however, does not affect the temperature of a substance - for example, water remains at 100C while boiling. The heat added to keep the water boiling is latent heat. Heat that causes a change of state with no change in temperature

is called latent heat.

Appreciating this difference is fundamental to understanding why refrigerant is used in cooling systems. It also explains why the terms 'total capacity' (sensible & latent heat) and 'sensible capacity' are used to define a unit's cooling capacity.

During the cooling cycling, condensation forms within the unit due to the removal of latent heat from the air. Sensible

capacity is the capacity required to lower the temperature and latent capacity is the capacity to remove the moisture from

the air.

Ref: https://www.spaceair.co.uk/faqs/what-is-the-difference-between-sensible-and-latent-heat

Adiabatic Saturation Temperature, Ts

The adiabatic saturation temperature is a thermodynamic property of

moist air that is defined as the temperature that the air stream would

achieve if it were allowed to become saturated adiabatically. The

adiabatic saturation temperature is computed by equating the

enthalpy of moist air at a given temperature and relative humidity to

the enthalpy of a saturated air-water mixture at the adiabatic

saturation temperature (ASHRAE, 1996).

The process of adiabatic saturation of air

an adiabatic saturator is a device in which air flows through an infinitely long duct

containing water.

As the air comes in contact with water in the duct, there will be heat and mass transfer

between water and air.

If the duct is infinitely long, then at the exit, there would exist perfect equilibrium

between air and water at steady state. Air at the exit would be fully saturated and its

temperature is equal to that of water temperature.

The device is adiabatic as the walls of the chamber are thermally insulated. In order to

continue the process, makeup water has to be provided to compensate for the amount of

water evaporated into the air. The temperature of the make-up water is controlled so

that it is the same as that in the duct.

Psychrometric Charts

The psychrometric chart characterizes the interdependences of seven properties of water-vapour

mixture:

dry-bulb temperature

wet-bulb temperature,

relative humidity,

absolute humidity,

dew point,

enthalpy and

specific volume

If any two of these quantities are known, the

other five quantities can be readily obtained

from the Psychrometric chart.

If TG is the dry-bulb temperature of air and Y is its humidity, its state is

denoted by point a. It falls on the constant humidity line, A%.

The adiabatic saturation line through a is ab.

c point indicates its humidity, Y.

The adiabatic saturation temperature, Ts is obtained by drawing the

vertical line through b. For air-water system, wet-bulb temperature Tw

is practically same as Ts.

The humidity of the adiabatically saturated air is given by the point e.

The dew point Td is given by the point d that can be reached by moving

horizontally from the point a to 100% humidity line and then moving

vertically down to the temperature axis.

The humid volume of saturated air at TG corresponds to the point f and that

of dry air at TG is given by point g.

The point m gives the humid volume if the humidity is Y and it is reached by

interpolation between g and f.

Enthalpy of a sample of air can also be obtained from humidity chart.

IN-CLASS EXERCISE 1

Determine the following psychrometric properties of a moist air sample having a dry bulb

temperature 27C and a humidity of 0.015 kg/kg dry air using the pyschrometric chart and/or

the vapour pressure equation for water:

a) Relative humidity b) Dew point

c) Adiabatic saturation temperature d) Wet bulb temperature

e) Enthalpy f) Humid volume

g) Humid heat

DEFINITIONS

Humidification involves the transfer of water from the

liquid phase into a gaseous mixture of air and water

vapor (Geankoplis, 2003).

The process of increasing the moisture content of air

is called humidification (Dutta, 2007)

What is dehumidification?

AIR-WATER CONTACTING APPS

Water cooling air-water contacting is done mostly for the purpose of cooling the warm water before it can be reused.

Humidification of gas for drying of solids under controlled condition.

Dehumidification and cooling of gas in air conditioning.

Gas cooling

A cooling tower is a special type of heat exchanger in which the warm water and

the air are brought in direct contact for evaporative cooling.

It provides a very good contact of air and water in terms of the contact area and

mass transfer co-efficient of water vapor while keeping air pressure drop low.

Enthalpy of air is lower than enthalpy of water. Sensible heat and latent heat

transfer take place from water drop to surrounding air.

Thus, cooling is accomplished by sensible heat transfer from water to air and

evaporation of a small portion of water.

Cooling Tower Construction & Operation

General cooling Tower

The hot water which is coming from heat exchanger is sprayed at the top of the cooling tower.

Air enters through the louvers at the two opposite walls of the cooling tower.

During cooling process of water, around 2% water is evaporated. Make water is used to compensate the water loss due to evaporation.

Blowdown is there to drain a part of water containing solid deposit.

The exit cold water from the cooling tower is used in the heat exchanger or other unit operation.

FACTORS GOVERN THE OPERATION OF COOLING TOWER

The dry-bulb and wet bulb temperatures of air

Temperature of warm water

The efficiency of contact between air and water in terms of volume transfer coefficient

Contact time between air and water

The uniformity of the distribution of the phases within the tower

Air pressure drop

Desired temperature of cooled water

Atmospheric Cooling Tower

It is a big rectangular chamber with two opposite louvered walls.

Tower is packed with a suitable tower fill.

Atmospheric air enters the tower through louvers driven by its own velocity.

Direction and velocity of wind greatly influence its performance.

Cooling tower production: https://www.youtube.com/watch?v=qgfQXo6SI4U

Natural draft cooling tower has a large reinforced

concrete shell of hyperbolic shape (also called

hyperbolic tower).

Natural flow of air occurs through the tower; hence it is

called natural draft

Factors responsible for creating natural draft

(a) A rise in temperature and humidity of air in the column reduces its density

(b) Wind velocity at the tower bottom

Fan is used to enhance the air flow rate in fan assisted natural draft tower.

The typical diameter of tower is 100 m, heigh of 150 m and capacity is

5,00,000 gallon/minute.

https://www.youtube.com/watch?v=ggg3C87UVCY

Why hyperbolic shape?

(i) More packing materials can be placed at the bottom

(ii) The entering air gets smoothly directed towards the centre

(iii) Greater structural strength and stability

https://www.youtube.com/watch?v=xKzenFW0ZIg

Mechanical Draft Towers

Forced draft

Induced draft

Fans are used to move air through the tower in mechanical draft cooling towers.

FORDED DRAFT: It has one or more fans located at the tower bottom to push air into tower.

Advantages:

(a) A part of the velocity head of air thrown by the blower is converted to pressure head on entering

into the tower. It makes energy efficient than induced draft.

(b) Less susceptible to vibrations as fans are installed near the ground.

Disadvantages:

(a) Air flow through the packing may not be uniform

(b) Some of the warm and humid air may be recirculated back. Recirculation rate becomes low if the

wind velocity is high. It is not popular except for small capacities.

Forced draft cooling tower

Induced draft towers: One or more fans are installed at the top of the tower. Depending on the air inlet and flow pattern, induced draft towers are of two types, cross-flow and counter flow towers.

Major advantages of countercurrent induced draft cooling tower

(a) Relatively dry air contacts the coldest water at the bottom of the cooling tower

(b) Humid air is in contact with the warm water and hence maximum average driving force prevails for

both heat and mass transfer.

Disadvantage of induced draft towers compared to forced draft towers

(a)It consumes more horse power.

(b) Cross-flow induced draft cooling tower requires less motor horse power than countercurrent induced

draft cooling towers.

Cross-flow induced draft cooling tower

Cross-flow induced draft cooling tower supplies horizontal

air flow along the packed height and requires less motor

horse power than the counter-flow type.

Additional cells may be added to raise the capacity.

Counterflow Cross flow

Structural Components & MOC

The shell, the framework and casing walls wooden, concrete, steel , glass fibre reinforced casing

walls.

The tower fills/packings splash type, film type (counterflow), plastic, wood

Louvers air passage, glass fibre, wood

Drift eliminators - plastic

Fans propeller, centrifugal

Water distribution gravity distribution, spray

Drift Eliminator

In every cooling tower there is a loss of water to the environment due to the evaporative cooling

process.

This evaporation is usually in the form of pure water vapor and presents no harm to the

environment.

Drift, however, is the undesirable loss of liquid water to the environment via small droplets that

become entrained in the leaving air stream.

There, water droplets carry with them chemicals and minerals, thus impacting the surrounding

environment.

Drift eliminators are designed to capture large

water droplets caught in the cooling tower air

stream.

The eliminators prevent the water droplets and

mist from escaping the cooling tower.

Eliminators do this by causing the droplets to

change direction and lose velocity at impact

on the blade walls and fall back into the

tower.

Efficient drift eliminators will keep drift losses

to less than .001% of the re-circulating water

flow rate.

http://www.towercomponentsinc.com/operation-drift-eliminator.php

Tower Problems

Scale inorganic minerals CA2CO3 etc.

Fouling waterborne contaminants

Microbial growth bacteria, algae, fungi, legiollosis

Corrosion

Consequences

Energy losses

Reduced heat transfer efficiency

Increased corrosion and pitting

Loss of tower efficiency

Wood decay and loss of structural integrity of the cooling

tower

Cooling Range & Approach

Cooling range is the difference in the temperature of the inlet hot

water and the outlet cooled water.

If hot water is cooled from 40C to 30C, the range is 10C.

Approach is the temperature difference between what is

being produced and the power source that creates the

product.

In the case of cooling tower, the product is cold water

leaving the tower and ambient wet bulb is the driving force

that creates the cold water.

If a cooling tower produces 85F cold water when the

ambient wet bulb is 78F, then the cooling tower approach is

7F.

If a small approach is targeted, the height of packing

increases rapidly.

Theoretically, the approach is zero if a tower has an infinite

packed height.

What happened if the designed wet bulb temperature is higher than the actual wet bulb

temperature?

Design of Cooling Tower

We need to determine:

The tower cross-section required to take the given load of

warm water.

The height of packing required to achieved the desired

cooling of water.

Basic assumptions for the design of cooling tower are as

follows:

(i) the rate of vaporization of water is much less than the rate

of water input to the tower (about 1% loss of feed water)

(ii) evaporative or adiabatic cooling of water occurs in the

tower

Enthalpy balance diagram of cooling tower

Let, L is the constant water flow rate (kg/m2s) and Gs is the air

rate (kg dry air/m2s). Across a differential thickness dz of the

bed, temperature of water is decreased by dTL and the

enthalpy of air is increased by dH.

Hence, change in enthalpy of water = L.cWL.dTL and,

Change in enthalpy of air = Gs.dH

Differential enthalpy balance over dz is L.cWL.dTL = Gs.dH

Eq. 8

Eq. 9

Eq. 10

Enthalpy balance over envelope I:

LcWL(TL - TL1) = Gs(H - H1) This is the operating line for

air-water contact.

Enthalpy balance over envelope II:

LcWL(TL2 - TL1) = Gs(H2 - H1)

The equilibrium curve for air-water system on TL-H plane is

the plot of enthalpy of saturated air versus liquid temperature

at equilibrium.

Eq. 11

Eq. 12

Rate of transfer of water vapor to air in the differential

volume is

The decrease in temperature of air for sensible heat transfer

to water is

Eq. 13

Eq. 14

Differentiation of Equation 7and multiplication with Gs gives

Eq. 15

The height (z) of the packing in the cooling tower is obtained

by

Number of gas-enthalpy transfer units

Eq. 16

Eq. 17

Height of gas-enthalpy transfer units

Height of gas-enthalpy transfer unit

Hence, height of cooling tower (packing section), z

Eq. 18

Eq. 19

Eq. 20

In Class Example

A cooling tower is to be designed to cool water from 45C to 30C by

countercurrent contact with air of dry bulb temperature 30C and wet bulb

temperature of 25C. The water rate is 5500 kg/m2.h and the air rate is 1.25

times the minimum. Determine the tower height if the individual gas-phase

mass transfer coefficient (kY/) is 5743.5 kg/m3h. The volumetric water side heat

transfer coefficient is given by hL=0.059L0.51Gs, in Kcal/m3hK, where L and Gs are

mass flow rates of water and air (dry basis).

Antoine Equation: ln PVA(bar)=11.96481-3984.923/(T-39.724)

Solutions

TG1 = 30, TW = TS = 25, used a psychrometric chart to read Y.

From chart, Y1 = 0.019

Now, used equation 6 & 7 to calculate H1

TL1 = 30, TL2 = 45

Plot the equilibrium line

Locate point Q(TL1,H1 ) (Lower terminal of operating line) at Q(30, 78.7) on TL-H plane.

Draw a tangent to the equilibrium line through Q. Slope of the tangent is 8.44.

H2 = 180 kJ/kg

locate point P (TL2, H2) (Upper terminal of the operating line) at P (45, 180) on TL-H2 plane.

Randomly choose one upper point within graph (x2, y2).

Choose another one point at lower part of the graph (x1, y1) with x1 is left unknown.

Use the following formula to guess x1.

Solved for x1 and draw the 1st tie line. Subsequently, replicate the line by maintaining the same

slope to get a set of tie lines.

50

70

90

110

130

150

170

190

210

230

250

22 27 32 37 42 47

H' (

kJ/k

g d

ry a

ir)

Temperature (Celcius)

Equilibrium Line

Q point

P Point

Operating Line

Tie line

Tangent

A set of tie lines of this slope is drawn from several points on the operating line. These tie lines

meet the equilibrium line at (TLi,Hi ). Hence, the points (H, Hi ) are obtained.

TL 30 32.5 35 37.5 40 42.5 45

H' 78.7 96.4 112.8 130.3 148.8 165.3 184

TLi 28.7 31.4 33.9 36.6 39 41.4 43.7

H'i 93.0 107.5 123.2 139.9 158.4 177 198.8

1/(H'-H'i) 0.070 0.090 0.096 0.104 0.104 0.085 0.068

Calculate the area under curve

0.000

0.020

0.040

0.060

0.080

0.100

0.120

78.0 98.0 118.0 138.0 158.0 178.0

1(H

'-H

'i)

H'

NtG =Area under the curve= (184-78.7)0.088=9.27

HtG = Gs/kYa = 3410/5743.5 = 0.59 m

z = HtGNtG = 0.59 x 9.27 = 5.47 m

Blowdown

During the cooling process of hot water in cooling tower, around 2% water evaporates.

In the long run, it increases the solid content in the circulating water. Some dust particles also come from the environment and mix with circulating water.

The solid content of the cooled water must be kept under a certain limit to avoid scaling or fouling on the heat exchange equipment.

A part of the circulating water is drained from the bottom of the cooling tower to discard the deposited solids from the cooling tower blowdown.

The losses due to blowdown, evaporation, drift and leakage are compensated by adding make-up water.

Water balance in cooling tower M = B + D + E

Solid balance MC1 = (B + D)C2 + (E)(0)

The water vaporized (E) does not have any solids in it, and the TDS in the

blowdown and in the drift is the same as that in the circulating water.

(B + D + E)C1 = (B + D) C2

Where r = C2/C1

Once the blowdown rate B is known, the makeup rate M may be calculated.

M Makeup rate B Blowdown rate D Rate of looses due to drift and leakages

C1 = dissolved content in the

makeup water

C2 = dissolved content in the

circulating water

E = (water flow rate)(range, F)(0.0008)

IN CLASS EXAMPLE

An induced draft crossflow tower is rated to cool 15000 gpm of

water from 40C to 29C. The total solid concentration must not

exceed 900 ppm. The TDS of the makeup water is 300 ppm. About

0.1% of the water is lost by drift from the tower and leakages in the

circulation system. Calculate the blowdown and makeup rate.

Solutions

The range = 40 29 = 11C = 19.8 F

E = (15000)(19.8)(0.0008) = 237.6 gpm

D = 0.1% x 15000 = 15 gpm

r = C2/C1 = 900/300 = 3

B = [(237.6 15(3-1)]/(3-1) = 104 gpm

M = B + D + E = 104 + 15 + 237.6 = 356.6 gpm