[Lab Report Operation Unit] Experiment 5 : INTRODUCTION TO DRYING PROCESS: DRYING A SOLID

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Title: Experiment 5

INTRODUCTION TO DRYING PROCESS: DRYING A SOLID

Objectives:

To investigate the change of moisture content of a solid being dried at a constant temperature.

Apparatus & Materials:

Carrot, drying oven, glass plate/tray and knife.

Theory:

In industries, drying is normally the final step in a series of operations; after which, the product from a dryer is often ready for final packaging. Drying a solid generally refers to the removal from a dryer is often ready for final

packaging. Drying a solid generally refers to the removal by thermal vaporization of relatively small quantity of water or other liquid from the solid material to reduce the content of residual liquid to an acceptable low value. Drying is normally performed by either direct heating or indirect heating. In many

commercial drying processes, drying are typically done by direct heating where heated gas such as heated air or steam are brought into direct contact with wet solids to be dried.

A wet solid is composed of a bone-dry solid and moisture. The moisture content in a wet solid is present as free-moisture content and

equilibrium-moisture content. Free equilibrium-moisture content refers to the maximum possible amount of moisture in a wet solid that can be removed by drying. It is not possible to completely remove all of the moisture content in wet solid by drying, because there will be always a small amount of liquid that coexist in equilibrium with the solid. This liquid is called the equilibrium moisture content. Thus in a drying, this phase equilibrium is the limit to the extent of the removal of liquid.

When a wet solid is heated in a dryer, its moisture content will decrease with time until it remains constant at the equilibrium-moisture content after all the free moisture has been removed. The rate at which the moisture content

decreases is called the rate of drying or drying rate. Rate of drying depends on how many factors such as air humidity, air velocity and temperature of drying. In drying, two types of drying rates are typically observed namely the constant rate and the falling rate. The constant rate is observed when moisture content

decreases linearly with time. At this constant-rate period, an equal amount of liquid is being vaporized per unit time. The falling rate occurs when the moisture

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content is observed to decrease non-linearly with time. At this falling-rate period, the quantity of liquid was being vaporized per unit times no longer equal.

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Procedure:

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Results:

Mass of empty tray = 56.605g Time

(minutes) Mass of tray + solid (g) Mass of solid (g)

0 63.030 6.425 5 61.600 4.995 10 60.300 3.695 15 59.520 2.595 20 58.500 1.895 25 57.950 1.345 30 57.720 0.915 35 57.200 0.595 40 57.150 0.545 45 57.100 0.495

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Calculations:

Mass of dry solid = mass of final product – mass of equilibrium-moisture* *assume that the equilibrium-moisture content is 0.05g water/g dry solid

% Moisture content at time t = [(mass of solid at time t – mass of dry solid)/mass of dry solid] X 100

Rate of drying = - (moisture content at time t2 – moisture content at time t1)/(t2 – t1) min

Given;

Mass of final product = 0.495g, Mass of equilibrium-moisture = 0.05g Mass of dry solid = 0.495g – 0.050g = 0.445g

Time (minute s) Mass of tray + solid (g) Mass of solid (g) % Moisture

content Drying rate

0 63.030 6.425 1343.82 64.27 5 61.600 4.995 1022.47 61.64 10 60.300 3.695 714.29 46.23 15 59.520 2.595 483.15 31.46 20 58.500 1.895 325.84 24.72 25 57.950 1.345 202.25 19.33 30 57.720 0.915 105.62 14.38 35 57.200 0.595 33.71 2.25 40 57.150 0.545 22.47 2.25 45 57.100 0.495 11.24 0

At time 0 min, %moisture content = [(6.425 – 0.445)/ (0.445)] X 100 = 1343.82 At time 5 min, %moisture content = [(4.995 – 0.445)/ (0.445)] X 100 = 1022.47 At time 10 min, %moisture content = [(3.695 – 0.445)/ (0.445)] X 100 = 714.29 At time 15 min, %moisture content = [(2.595 – 0.445)/ (0.445)] X 100 = 483.15 At time 20 min, %moisture content = [(1.895 – 0.445)/ (0.445)] X 100 = 325.84 At time 25 min, %moisture content = [(1.345 – 0.445)/ (0.445)] X 100 = 202.25 At time 30 min, %moisture content = [(0.915 – 0.445)/ (0.445)] X 100 = 105.62 At time 35 min, %moisture content = [(0.595 – 0.445)/ (0.445)] X 100 = 33.71 At time 40 min, %moisture content = [(0.545 – 0.445)/ (0.445)] X 100 = 22.47

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Rate of drying between t0 & t5 = - (1022.47 – 1343.82) / (5-0) = 64.27 Rate of drying between t5 & t10 = - (714.29 – 1022.47) / (10-5) = 61.64 Rate of drying between t10 & t15 = - (483.15 – 714.29) / (15-10) = 46.23 Rate of drying between t15 & t20 = - (325.84 – 483.15) / (20-15) = 31.46 Rate of drying between t20 & t25 = - (202.25 – 325.84) / (25-20) = 24.72 Rate of drying between t25 & t30 = - (105.62 – 202.25) / (30-25) = 19.33 Rate of drying between t30 & t35 = - (33.71 – 105.62) / (35-30) = 14.38 Rate of drying between t35 & t40 = - (22.47– 33.71) / (40-35) = 2.25 Rate of drying between t40 & t45 = - (11.24 – 22.47) / (45-40) = 2.25 Rate of drying between t45 & t50 = - (11.24 – 11.24) / (50-45) = 0

Graphs:

: Change in % moisture content vs. time

0 200 400 600 800 1000 1200 1400 1600

Change in % moisture content vs. time

% Moisture content

Time (minutes) % Change in moisture content

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: Drying rate vs. % moisture content 0 10 20 30 40 50 60 70

Drying rate vs. % moisture content

Drying rate

% Moisture content Drying rate

: Drying rate vs. time

0 5 10 15 20 25 30 35 40 45 0 10 20 30 40 50 60 70

Drying rate vs. time

Drying rate

Time (minutes) Drying rate

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Discussion:

Based on the experiment, it shows that the water was evaporated from the sliced carrot as time goes by with the constant temperature. Based on the graph, the change in moisture content and drying rate decreased as the time goes by. After 45 minutes of the experiment, there was no changes in mass as the water in the sliced carrot cannot be remove anymore because it has

achieved equilibrium with the solid.

The error that might be occur or made by students is that during the experiment the carrot is not sliced properly. Apart from that, during the reweigh of sliced carrot at intervals, the drying oven was being left open for a long period of time thus the temperature is not constant. Another, the sliced carrot was exposed to the temperature surrounding in the laboratory during the reweigh at each intervals.

As a precaution, make sure that the carrot is slice properly with a sharp knife. Make sure to use hand gloves to prevent any burn as the temperature inside the oven is approximately 90˚C. On the other hand, do not open the drying oven to long because the temperature will not be constant. And then, do not expose the sliced carrot to the surrounding of the laboratory as it can affect the drying rate and it moisture content.

Conclusion:

The change of moisture content of a solid being dried at a constant temperature can be investigated by drying process.

References:

Chemistry, 10

th

edition by Raymond Chang 2010

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