EXPERIMENT 4: EVAPORATIVE MASS TRANSFER UNIT

7/28/2019 EXPERIMENT 4: EVAPORATIVE MASS TRANSFER UNIT

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EXPERIMENT 4: EVAPORATIVE MASS TRANSFER UNIT

1.0 OBJECTIVE

- Demonstration of evaporation process

- Understanding of mass and energy balance through evaporation process

- Humidity measurement with wet and dry bulb temperature

2.0 INTRODUCTION

Evaporation is a type of vaporization of a liquid that only occurs on the surface of a liquid.

The other type of vaporization is boiling, which, instead, occurs within the entire mass of the

liquid. On average, the molecules in a glass of water do not have enough heat energy to

escape from the liquid. With sufficient heat, the liquid would turn into vapor quickly (see

boiling point). When the molecules collide, they transfer energy to each other in varying

degrees, based on how they collide. Sometimes the transfer is so one-sided for a molecule

near the surface that it ends up with enough energy to 'escape'. Evaporation is an essential

part of the water cycle. The sun (solar energy) drives evaporation of water from oceans,

lakes, moisture in the soil, and other sources of water. In hydrology, evaporation and

transpiration (which involves evaporation within plant stomata) are collectively termed

evapotranspiration.

Evaporation of water occurs when the surface of the liquid is exposed, allowing molecules to

escape and form water vapor; this vapor can then rise up and form clouds. For molecules of

a liquid to evaporate, they must be located near the surface, be moving in the proper

direction, and have sufficient kinetic energy to overcome liquid-phase intermolecular

forces.[1] When only a small proportion of the molecules meet these criteria, the rate ofevaporation is low. Since the kinetic energy of a molecule is proportional to its temperature,

evaporation proceeds more quickly at higher temperatures. As the faster-moving molecules

escape, the remaining molecules have lower average kinetic energy, and the temperature of

the liquid decreases. This phenomenon is also called evaporative cooling. This is why

evaporating sweat cools the human body.

Evaporation also tends to proceed more quickly with higher flow rates between the gaseous

and liquid phase and in liquids with higher vapor pressure. For example, laundry on a

clothes line will dry (by evaporation) more rapidly on a windy day than on a still day. Three


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key parts to evaporation are heat, atmospheric pressure (determines the percent humidity)

and air movement. On a molecular level, there is no strict boundary between the liquid state

and the vapor state. Instead, there is a Knudsen layer, where the phase is undetermined.

Because this layer is only a few molecules thick, at a macroscopic scale a clear phase

transition interface can be seen. Liquids that do not evaporate visibly at a given temperature

in a given gas (e.g., cooking oil at room temperature) have molecules that do not tend to

transfer energy to each other in a pattern

Factors influencing the rate of evaporation:

a. Concentration of the substance evaporating in the air

- If the air already has a high concentration of the substance evaporating, then the given

substance will evaporate more slowly.

b. Concentration of other substances in the air

- If the air is already saturated with other substances, it can have a lower capacity for the

substance evaporating [citation needed].

c. Flow rate of air

- This is in part related to the concentration points above. If fresh air is moving over the

substance all the time, then the concentration of the substance in the air is less likely to

go up with time, thus encouraging faster evaporation. This is the result of the boundary

layer at the evaporation surface decreasing with flow velocity, decreasing the diffusion

distance in the stagnant layer.

d. Inter-molecular forces

- The stronger the forces keeping the molecules together in the liquid state, the moreenergy one must get to escape. This is characterized by the enthalpy of vaporization.

e. Pressure

- Evaporation happens faster if there is less exertion on the surface keeping the

molecules from launching themselves.

f. Surface area

- A substance that has a larger surface area will evaporate faster, as there are more

surface molecules that are able to escape.


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g. Temperature of the substance

- If the substance is hotter, then its molecules have a higher average kinetic energy, and

evaporation will be faster.

h. Density

- The higher the density the slower a liquid evaporates.

3.0 EQUIPMENT

A = Fan

B = Air duct

C = Digital weighing scale

D = Observation door

E = Humidifier

F = Control panel

G = Dust temperature controller with heater ON/OFF

H = Sample pre-heating controller with heater ON/OFF

I = Wet bulb temperature indicator with selector

J = Fan ON/OFF switch

K = Humidifier ON/OFF switch

L = Main ON/OFF switch

M = Frequency inverter

N = Dry bulb temperature indicator with selector

4.0 PROCEDURE

a. Read the safety instructions before the experiment conducted

b. Theories related to the experiments will be done carefully read and understand

accessories required when experiment conducted previously provided

c. all parts and component checked first so that everything is in good condition

d. water filled into containers humidifier


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4.1 Experiment procedure (Experiment A without humidifier)

a. Main power supply device (L) in the open

b. Observers door (D) ON .The area stainless steel cross section measured

c. Prepared the sample solution and poured it to the stainless steel tray provided. Placed

the tray on the tray holder. Submerge the thermocouple into the sample solution.

d. Filled the wet bulb thermocouple's trays with water, wet the wet bulb thermocouple's

cloth with water and place the trays under the thermocouple. Ensure the cloth is

submerging to the water.

e. Closed the observation door.

f. Checked the four hanging bars of the tray holder. Ensured they are not touching the air

duct (B).

g. Switched ON the axial fan (J). Set the frequency to 7 Hz by adjusting the frequency

inverter (M).

h. Switched ON the digital weighing scale (C).

i. Switched ON the air ducted heating temperature heater (G). Set the temperature to

desired heating temperature.

j. Switched ON the sample preheating heater (H) to desired temperature.

k. Allowed the system to reach steady state where by the air duct temperature and sample

heating temperature reached set value.

l. Recorded the all the reading displayed on the digital meters. Recorded these readings

to the table provided.

m. Checked the reading of the digital scale; ensured the initial reading is zero. Pressed the

"O" button to tare the reading.

n. Started a stop watch, for every five minutes elapsed, take the readings and recorded to

the table.

o. Run the experiment for at least 30 minutes to obtained set of data.

p. The graph of mass against time is a plot.


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4.2 Experiment B (with humidifier)

a. Switched ON the apparatus main power supply (L).

b. Opened the observation door (D). Measured the stainless steel tray cross section area"

c. Prepared the sample solution and poured it to the stainless steel tray provided. Placed

the tray on the tray holder. Submerge the thermocouple into the sample solution.

d. Filled the wet bulb thermocouple's trays with water; wet the wet bulb thermocouple's

cloth with water and placed the trays under the thermocouple. Ensured the cloth is

submerging to the water.

e. Closed the observation door.

f. Checked the four hanging bars of the tray holder. Ensured they are not touching the air

duct (B).

g. Switches ON the axial fan (J) Set the frequency to 7 Hz by adjusting the frequency

inverter (M).

h. Switches ON the digital weighing scale (C).

i. Switches ON the air duct heating temperature heater (G). Set the temperature to your

desired heating temperature.

j. Switches ON the sample preheating heater (H) to desired temperature.

k. Switches ON the humidifier (K). Pressed on the power button, set the setting humidity to

"cn" by pressing the humidity button Control the mist output by pressing the mist button

l. Allowed the system to reach steady state where by the air duct temperature and sample

heating temperature reached your set value.

m. Recorded the all the reading displayed on the digital meters. Record these readings to

the table provided.

n. Started a stop watch, for every five minutes elapsed, take the readings and recorded to

the table.

o. Run the experiment for at least 30 minutes to obtain set of data


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5.0 RESULT

5.1 Experiment A (without humidifier)

Weight

(kg)

Time

(minute)

Before After

Dry Wet Dry Wet

0.163 0 58.9 42.1 52.7 38.8

0.159 5 58.8 43.4 54.7 38.9

0.095 10 58.7 44.1 55.8 39.5

0.090 15 58.5 44.5 56.0 39.5

0.079 20 58.5 44.9 55.9 39.6

0.043 25 58.5 44.6 56.5 39.8

0.031 30 58.4 44.6 56.5 39.9

5.2 Experiment B (with humidifier)

Weight

(kg)

Time

(minute)

Before After

Dry Wet Dry Wet

0.066 0 50.1 45.1 56.5 37.0

0.061 5 50.1 43.6 55.6 37.0

0.051 10 49.8 44.6 56.1 37.3

0.046 15 49.8 44.6 55.5 37.4

0.033 20 49.0 44.4 55.8 37.6

0.023 25 48.7 44.7 56.0 37.7

0.018 30 48.0 44.6 55.8 37.7


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6.0 DISCUSSION

Based on the result, it shows that there is a two type of experiment table is determined,

which is Experiment A and Experiment B. In Experiment A, it is conducted without using a

humidifier while in Experiment B, it is conducted by using a humidifier. Compared to the both

of the result table, there is a comparison between Experiments A and Experiments B. It

shows that the reading for dry and wet (before and after) in Experiment B is higher

compared to the readings for the Experiment A. From both experiment, it can be conclude

that the rate of evaporation in experiment B is much higher compared to the rate of

evaporation in experiment A which is can be determined from the value difference for both of

the readings (before and after). In this case, it shows that the rate of evaporation can be

influenced by humidity, where the humidifier usage in experiment B affected both of the dry

and wet readings.

7.0 CONCLUSION

Evaporation is the process that changes a liquid into a gas. It can occur at any temperature,

and is due to the movement of molecules in the liquid. The water molecules escape into the

air as a gas that is water vapor. The amount of water vapor in the air is called its humidity.

Relative humidity is the actual amount of water vapor in the air compared with the amount

the air would hold if saturated. Relative humidity is usually expressed as a percentage, and

in many regions is of major importance in weather reports. Evaporation also tends to

proceed more quickly with higher flow rates between the gaseous and liquid phase and in

liquids with highervapor pressure. For example, laundry on a clothes line will dry (by

evaporation) more rapidly on a windy day than on a still day. This problem can be proving by

conducting this experiment where refer from above discussion, it can be conclude that the

experiment B is using a humidifier, where a humidifier can be assumed as a windy dayweather in the world real life.
http://en.wikipedia.org/wiki/Vapor_pressurehttp://en.wikipedia.org/wiki/Vapor_pressure


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8.0 QUESTIONS

1. Give three (3) examples of drying equipment in the industrial

a. Pharmaceutical and biotechnology

- Pharmaceutical companies often use freeze-drying to increase the shelf life of products,

such as vaccines and other injectable. By removing the water from the material and

sealing the material in a vial, the material can be easily stored, shipped, and later

reconstituted to its original form for injection.

b. Food industry

- Freeze dried bacon bars and Freeze-dried coffee, a form of instant coffeeFreeze-drying

is used to preserve food, the resulting product being very lightweight. The process has

been popularized in the forms of freeze-dried ice cream, an example of astronaut food.

It is also widely used to produce essences or flavourings to add to food.

c. Technological industry

- In chemical synthesis, products are often freeze-dried to make them more stable, or

easier to dissolve in water for subsequent use. In bio separations, freeze-drying can be

used also as a late-stage purification procedure, because it can effectively remove

solvents.

2. Describe why drying process is important in some industrial application

Supercritical drying (superheated steam drying) involves steam drying of products

containing water. This process is feasible because water in the product is boiled off, andjoined with the drying medium, increasing its flow. It is usually employed in closed circuit

and allows a proportion of latent heat to be recovered by recompression, a feature

which is not possible with conventional air drying, for instance.


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3. Discuss the effect of air speed, air humidity, air operationg, pressure and air

temperature on drying performance

The air speed, air humidity, air operating pressure and air temperature volumes are

influenced the drying performance. Its depending on the temperature surrounded.

Temperature is important in several ways. Heat of course contributes to drying. If the

temperature is higher, then the drying process more rapid. Among the four affecting

factors, the air temperature and relative velocity play a significant role on the drying

performance. The particle evaporation rates are always controlled by the drying medium

due to its small diameter.

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EXPERIMENT 4: EVAPORATIVE MASS TRANSFER UNIT