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Experiment No. 3

DRYING

1. Objective:

To determine the critical and equilibrium moisture contents of the wet solid being dried.

2. Intended Learning Outcomes (ILOs):

The students shall be able to: 2.1 understand the operations of tray drier. 2.2 draw the drying curve and drying rate curve for a wet solid being dried with air of fixed humidity and temperature.

3. Discussion:

Drying is the process by which volatile materials usually water are evaporated from a material to yield a solid product. Drying is a heat and mass transfer processes. Heat is necessary to evaporate water. The latent heat of evaporation of water is about 2500 J/g, which means that the driving process requires a significant amount of energy. Simultaneous, the evaporating material must leave the driving material by diffusion and/or convection. Heat transfer and mass transfer is not the only concern when one is designing or operating a dryer. The product quality such as color, particle density, hardness, texture, flavor is also very strongly transformation occurring in the dryer. Batch drying is bringing a whole bulk of solid to dry at the same time. Several trays in the compartment should be identified and weighed before being placed in the drying cabinet. The trays should be weighed at the start and at predetermined intervals. At the same time, wet and dry bulb reading over the product should be taken. Continue these procedures until the product obtains a constant weight or is satisfactory dried in order to establish a totally dry weight. Temperature should be high enough to keep the relative humidity in the surrounding air at 5 percent or less. Drying operations involve the removal of free moisture, hygroscopic moisture, or a combination of both. Free moisture occurs when actual liquid water is used to mix or wash the product prior to drying. Hygroscopic drying is held within the material. This moisture will take up or dispel water in relation to the relative humidity of the air mixture to which it is exposed. When in equilibrium with air at 100% relative humidity, the material will be hygroscopically saturated. Any hygroscopic material containing free moisture must be hygroscopically saturated. The removal of free moisture is a surface evaporation function and follows the calculation shown in the sample computations. The surface water temperature should be assumed to be the wet bulb temperature of the surrounding mixture. Ait velocity is critical to the drying speed. The removal of hygroscopic moisture depends on the relative humidity differences between that of the product equilibrium condition and that of the surrounding air. Velocity of the air over the product has a little or no bearing on the drying speed. Each material has a different physical from that determines how it holds or gives up moisture. Many of the newer materials lack published data on their drying rates, selecting appropriate air drying must be done experimentally.

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4. Resources:

Equipment: Tray Drier Materials: Sand (approximately 500 microns) Sieve Shakers Water 2 - sling Psychrometer 8 Thermometers Analytical Balance Oven Stopwatch Container

5. Procedure:

1. Weigh accurately an empty tray using a top loading balance. 2. Fill the four trays with sand preferably of uniform size to a depth of 10 mm. Make sure that the

surface is even. 3. Weigh and record the mass of the tray and sand. 4. Saturate the sand with water. Avoid any spillage. 5. Load the tray on the weighing scale installed in the equipment. 6. Record the mass of the wet sand before drying commences. 7. Open the inlet valve for steam to preheat the equipment. Simultaneously, the outlet valve should

also be opened even during the experiment to remove all condensed 8. water. 9. Preheat the equipment for 10 minutes. 10. Place thermometers on all the holes seen in the equipment. This is to record the temperatures on

the different sections of the equipment. 11. Measure the moisture content of the sand at a given time. Use 2 minutes as experimental time

interval. Moisture content of the sand at certain time is obtained by:

12. Plot the drying curve relating the moisture content as a function of time. 13. Determine the critical and equlibrium and equilibrium moisture content of the sand by plotting the

rate of drying of the sand as a function of moisture content. The drying rate is computed by taking the value of the moisture content of the sand per unit change in time. The critical moisture content is the point before the falling rate period starts or when there is insufficient water on the surface of the sand to maintain a continuous film of water.

14. Calculate the bound water by taking the difference of the mass of the bone dried sand and mass of the sand after drying.

15. Calculate the unbound water by determining the amount of water in excess and can be removed by drying.

16. Measure the conditions of entering, outgoing and preheated air.

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6. Data and Results:

Course: Expeiment No:

Group No: Section:

Group Members: Date Performed:

Date Submitted:

Instructor:

Weight of Sand as a Function of Time

Time (min)

Wt. of Wet Sand (grams)

Time (min)

Wt. of Wet Sand (grams)

Condition of Entering Air

Time (min)

Dry Bulb Temperature

(0C)

Wet Bulb Temperature

(0C)

Time (min)

Dry Bulb Temperature

(0C)

Wet Bulb Temperature

(0C)

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Condition of Outgoing Air

Time (min)

Dry Bulb Temperature

(0C)

Wet Bulb Temperature

(0C)

Time (min)

Dry Bulb Temperature

(0C)

Wet Bulb Temperature

(0C)

Temperature of Preheated Air (Drying)

Time (min)

Point 1

Point 2

Point 3

Point 4

Point 5

Point 6

Point 7 Point 8

Time (min)

Point 1

Point 2 Point 3

Point 4

Point 5

Point 6

Point 7

Point 8

Weight of Sand (Bone Dry Solid): ___________ Critical Moisture Content: ___________ Equilibrium Moisture Content: ___________ Bound Water: ___________ Unbound Water: ___________

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Drying Curve: Rate of Drying Curve:

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7. Calculations:

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8. Concusion:

9. Questions/Problems:

1. How does particle size influence the equilibrium and critical moisture contents? 2. What is the heat transfer mechanism involves when a granular solid material contained in a metal tray with

insulated edges and bottom is placed inside a batch drier? 3. What is the effect of increasing the absolute humidity of the air on the value of the drying rate constant? 4. Calculate the amount of water removed from 2000 kg/h of feed to be dried from 110% (d.b.) to 5% (w.b.). 5. Wet solid are to be dried from 40% to 10% in 5 hours under constant drying conditions. The critical

moisture content is 20% and the equilibrium moisture content is 7%. All moisture contents are on a dry basis. Determine the time needed to dry from 15% to 5% free moisture under the same drying condition.

10. Answers:

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11. Further Readings:

Cao, E. (2010). Heat transfer in process engineering. Boston: McGraw-Hill Professional. Cussler, E. L. (2009). Diffusion: mass transfer in fluid systems (3rd ed. United Kingdom: Cambridge University

Press. Koenig, D. (2009). Practical control engineering: a guide for engineers, managers and practitioners . New York:

McGraw-Hill Professional. Mann, U. (2009). Principles of chemical reactor analysis and design. New Jersey: John Wiley & Sons. Johnson, W. (2009). Practical heating technology. Australia: Delmar Cengage Learning.

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12. Assessment (Rubric for Laboratory Performance):

CRITERIA

BEGINNER

1

ACCEPTABLE

2

PROFICIENT

3

SCORE

I. Laboratory Skills

Manipulative Skills

Members do not demonstrate needed skills.

Members occasionally demonstrate needed skills.

Members always demonstrate needed skills.

Experimental Set-up

Members are unable to set-up the materials.

Members are able to set-up the materials with supervision.

Members are able to set-up the material with minimum supervision.

Process Skills Members do not demonstrate targeted process skills.

Members occasionally demonstrate targeted process skills.

Members always demonstrate targeted process skills.

Safety Precautions

Members do not follow safety precautions.

Members follow safety precautions most of the time.

Members follow safety precautions at all times.

II. Work Habits

Time Management / Conduct of Experiment

Members do not finish on time with incomplete data.

Members finish on time with incomplete data.

Members finish ahead of time with complete data and time to revise data.

Cooperative and Teamwork

Members do not know their tasks and have no defined responsibilities. Group conflicts have to be settled by the teacher.

Members have defined responsibilities most of the time. Group conflicts are cooperatively managed most of the time.

Members are on tasks and have defined responsibilities at all times. Group conflicts are cooperatively managed at all times.

Neatness and Orderliness

Messy workplace during and after the experiment.

Clean and orderly workplace with occasional mess during and after the experiment.

Clean and orderly workplace at all times during and after the experiment.

Ability to do independent work

Members require supervision by the teacher.

Members require occasional supervision by the teacher.

Members do not need to be supervised by the teacher.

Other Comments / Observations: TOTAL SCORE

RATING = (24

TotalScore ) x 100%