TECHNOLOGICAL INSTITUTE OF THE PHILIPPINES363 P. CASAL ST., QUIAPO, MANILACOLLEGE OF ENGINEERING AND ARCHITECTURECHEMICAL ENGINEERING DEPARTMENT

EXPERIMENT NO. 2: DRYING

SUBMITTED BY:

SUBMITTED TO:ENGR. CARLA TAMAYO

NOVEMBER 27, 2014EXPERIMENT NO. 2DRYING

Discussion:

Drying is used to refer to removal of other organic liquids, such as benzene or organic solvents, from solids. In general, drying means removal of relatively small amounts of water from material. Evaporation refers to removal of relatively large amounts of water from material. In evaporation, the water is removed as vapour at its boiling point. In drying, the water is usually removed as a vapour by air.

Drying processes can also be categorized according to the physical conditions used to add heat and remove water vapour. In the first category, heat is added by direct contact with heated air at atmospheric pressure, and the water vapour is removed by the air. In vacuum drying, the evaporation of water proceeds more rapidly at low pressure and the heat is added indirectly by contact with a metal wall or by radiation. In freeze drying, water is sublimed from the frozen material.

There are many types of equipment for drying. In tray dyers, the material is spread uniformly on a metal tray to a depth of 10 to 100 mm. Steam-heated air is recirculated by a fan over and parallel to the surface of the trays. Vacuum-Shelf dryers are indirectly heated batch dryers similar to tray dryers. Such a dryer consists of a cabinet made of cast-iron or steel plates fitted with tightly fitted doors so that it can be operated under vacuum. These dryers are used to dry expensive, or temperature-sensitive, or easily oxidizable materials. Continuous tunnel dryers are often batch truck or tray compartments operated in series. A rotary dryer consists of a hollow cylinder which is rotated and usually slightly inclined toward the outlet. A drum dryer consists of a heated metal roll on the outside of which a thin layer of liquid or slurry is evaporated to dryness. The final dry solid is scraped off the roll, which is revolving slowly. In spray dryer, a liquid or slurry is sprayed into a hot gas steam in the form of a mist of fine droplets.The first consideration in selecting a dryer is its operability. Above all else, the equipment must produce the desired product in the form at the desired rate. The quality required in a finished product, and its necessary physical characteristics, are determined by its end use. However, attention must be paid to the costs of the ebtire isolation system, not just te drying unit alone.

Materials:

Tray Drier 2-sling Pychrometer Analytical balance Stopwatch Beaker Zeolite Water

Procedure:

1. Weigh accurately an empty 250 mL beaker using an analytical balance.2. Fill the four beakers with zeolite preferably of uniform size to a depth of 10 mm. Make sure that the surface is even.3. Weight and record the mass of the beaker and zeolite.4. Saturate the zeolite with water. Avoid any spillage.5. Load the beaker on the weighing scale installed in the equipment.6. Record the mass of the wet zeolite 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 water.8. Preheat the equipment for 10 minutes.9. Place thermometers on all the holes seen in the equipment. This is to record the temperatures on the different sections of the equipment.10. Measure the moisture content of the zeolite at a given time. Use 5 minutes as experimental time interval. Moisture content of the zeolite at certain time is obtained by:

11. Plot the drying curve relating the moisture content as a function of time.12. Determine the critical and equilibrium moisture content of the zeolite by plotting the rate of drying of the zeolite as a function of moisture content. The drying rate is computed by taking the value of the moisture content of the zeolite per unit change in time. The critical moisture content is the point before the falling rate period starts or hen there is insufficient water on the surface of the zeolite to maintain a continuous film of water.13. Calculate the bound water by taking the difference of the mass of the bone dried zeolite and mass of the zeolite after drying.14. Calculate the unbound water by determining the amount of water in excess and can be removed by drying.15. Measure the conditions of entering, outgoing and preheated air.

Experminental Set-Up:

Data and Results:Time (min)Wt of Wet Zeolite (g)Time (min)Wt of Wet Zeolite (g)

0173.7435165.84

5172.9840164.52

10172.1245162.4

15171.0650160.9

20169.7555159.84

25168.8060156.99

30167.4565155.45

Condition of Entering AirTime (min)Dry Bulb Temperature (C)Wet Bulb Temperature (C)Time (min)Dry Bulb Temperature (C)Wet Bulb Temperature (C)

23125230.824.9

2312522924.5

230.525230.0524.9

230.524.9231.524.9

2312523125

23125230.924.5

231.52523124.5

Condition of Outgoing AirTime (min)Dry Bulb Temperature (C)Wet Bulb Temperature (C)Time (min)Dry Bulb Temperature (C)Wet Bulb Temperature (C)

230.524.923124.5

23124.823024.9

2312523125

230.924.923124.9

23125231.525

231.52523125

Time(min)Wt of wet zeolite (g)Moisture Content, XRate of Drying, RdX/dt

0173.740.10840.0015-0.0023

5172.980.09710.0011-0.0017

10172.120.08860.0017-0.0025

15171.060.07610.0016-0.0024

20169.750.06400.0016-0.0024

25168.800.05220.0015-0.0023

30167.450.04080.0010-0.0015

35165.840.03340.0009-0.0013

40164.520.02680.0011-0.0016

45162.40.01860.0009-0.0013

50160.90.01220.0007-0.0010

55159.840.00710.0001-0.0001

60156.990.00640.0009-0.0013

65155.45-0.00010.00000.0000

Analysis and Interpretation:

Based on the graph of moisture content versus time, it shows that the moisture contet of the zeolote decreases with time. From the graph of drying rate versus time, it is shown that the changing pattern is not consistent or in mannered pattern. Thus, it can be said that the drying rates is not dependent upon time.

Drying occurs in three different periods. The initial period is where sensible heat is transferred to the product and the contained moisture. Based on the moisture content vs time graph, the first period is between the first minute to 55 minutes. The constant rate period is when the free moisture persists on the surfaces and the rate of evaporation alters very little as the moisture reduces. It is betwwen 55 minutes to 60 minutes, based on the graph. The falling rate period is the period during which the drying rate slowly decreases until it approaches zero.

Conclusions and Recommendations:

Based on the data, the experiment can be concluded that the moisture content is decresing with time. It can be said that moisture content is directly proportional with time. The data on drying rates, however, are with varying time. It can be concluded that drying rates do not depend on time.

In order to obtain better results, there are few reccomendations that may be considered. Clean apparatus is a must. Dirty apparatus may cause contamination on the chemicals, therefore affecting the results. The same can be said on the equipments. Good air circulation within the dryer is imporatnt, as it reduces drying time and allows the use of lower temperatues, both of which can prevent the degradation of chemical constituents during the drying process.

Questions:1. How does particle size influence the equilibrium and critical moisture contents?2. What is the heat transfer mechanism involved when a granular solid material contained in a metal tray with insulated edges and bottom is placed inside the batch dryer?3. What is the effect of the increasing the absolute humidity of the air on the value of the drying rate constant?4. Calculate the amount of water removed from 2000kg/hr 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.

Answers to Questions:1. The particle size determines the distance internal moisture must travel to reach the drying surface. It affects the equilibrium moisture content, which is obtained under steady-state conditions, by gaining or losing moisture at specific temperature and humidity of the materials. The critical moisture content varies with the thickness of the material. Large particles exhibit higher critical moisture contents under specific drying conditions than fine particles of the same material.2. Convection is the heat transfer mechanism involved.3. The rate of drying is determined by the moisture content and the temperature of the grain and the temperature, the (relative) humidity and the velocity of the air in contact with the grain. In general the drying rate decreases with moisture content, increases with increase in air temperature or decreases with increase in air humidity.5.