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 equilibriummoisture content. Free 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
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.
R f1 b .S p W o d lP v s u n m 5 c y tra h ig w e k 0 '6 Procedure:
Results: Mass of empty tray = 56.605g Time (minutes) 0
Mass of tray + solid (g)
Mass of solid (g)
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
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) 0
Mass of tray + solid (g)
Mass of solid (g)
% Moisture content
Drying rate
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
At time 45 min, %moisture content = [(0.495 – 0.445)/ (0.445)] X 100 = 11.24
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
Change in % moisture content vs. time 1600 1400 1200 1000 % Change in moisture content
800 600 400 200 0
Time (minutes)
% Moisture content
: Drying rate vs. % moisture content
Drying rate vs. % moisture content 70 60 50 40 30 20 Drying rate
Drying rate
10 0
% Moisture content
: Drying rate vs. time
Drying rate vs. time 70 60 50 40 Drying rate
Drying rate
30 20 10 0 0
5
10
15
20
25
30
Time (minutes)
35
40
45
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, 10th edition by Raymond Chang 2010 General Chemistry, 3rd edition by Daniel D. Ebbing
