1. Introduction
In general, drying is the removal of relatively small amounts of water and other liquid from the solid material, to reduce the content of residual liquid to an acceptably low value. Drying is one of the several situations where there is both heat and mass transfer. Usually, dying is applied in the final stages in a series of operations in a manufacturing process because operations involving thermal means of liquid removal are more expensive and more difficult to use than mechanical methods.
In a drying drying process, the heat can be transported to the the wet material through conduction, convection, or radiation. Convective heat transfer takes place in a dryer where there is circulating air. On the other hand, Other methods of drying which are quite commonly encountered are drying by contact with a hot surface or transmitted through an empty space by thermal radiation. From the heat source, heat must pass to the wet material’s surface and through the material to wherever evaporation of liquid occurs. The vapour generated must then travel to the material’s surface and proceeds to the moisture sink which can be a condenser or an exhaust of humid air. The mass transfer involved in the internal part of the material can be a combination of diffusion, capillary movement, and evaporation-condensation.
There are many types of dryers and they may be classified according to the type of the wet material inputted, the modes of heat transfer involved, or whether it is batch or continuous. Nevertheless, in this experiment a vacuum tray dryer was used. In a vacuum tray dryer, the heat transfer is largely by conduction or by radiation. The trays are enclosed in a large capsule, which is evacuated.
The moisture content of a wet material is commonly expressed as the mass of water per mass of the dry material. In the case of this experiment, ethanol was used as the wetting agent. Eventually, after exposure of the wet material sufficiently long for equilibrium to be reached, the solid will attain definite moisture content or in our case ethanol content, This is known as the equilibrium moisture/ethanol content. This equilibrium moisture content varies widely with the type of material, the humidity of the surrounding air and the temperature of the air. Moisture in excess of the equilibrium 2
moisture content is called as a free moisture. The wetting agent can also be in the form of a bound moisture. The bound moisture in the solid exerts an equilibrium vapor pressure lower than that of pure water at the same temperature. Water retained in small capillaries in the solid absorbed at solid surfaces as solutions in cells or fibers, falls in to the category of bound moisture.The driving force for mass transfer in a wet solid is the free moisture content which is the difference between the total moisture content and the equilibrium moisture content [McCabe and Smith, 1956].
If the change in moisture content for a material is determined as a function of time, a smooth curve is obtained from which the rate of drying at any given moisture content may be evaluated. There are typically two main drying periods that can be observed in the process of drying wet materials: The constant rate drying period and the falling rate drying period. This portion of drying is characterized by a rate of drying independent of moisture content. It is manifested as a horizontal line in the plot. In this period, a continuous film of liquid exists at the top of the solid slab and evaporates at the same rate as a free liquid is supplied from the interior. The point terminating the constant rate is the critical point. This marks the start of the falling rate drying period. It is at this time where the surface is no longer capable of supplying sufficient free moisture to be evaporated. Figure 1 shows a plot of the drying rate curve.
Fig. 1 Rate of drying vs. moisture content
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2. Objectives
2.1 Extract design data from experimental drying curve. 2.2 Predict drying time at constant rate and falling rate phases, and compare predicted values with the actual time of drying. 3. Methodology 3.1 Materials
Sawdust
Ethanol
Ruler
Stopwatch
Weighing scale
Pan
Spatula
3.2 Equipment Vacuum Tray Dryer (Heraeus GMBH Hanau)
The vacuum tray dryer is composed of a chamber capable of accommodating pressures less than atmospheric condition. It also has a shelf or a tray in which it serves as the heating medium. It is also equipped with a pump that sucks out vapors from the wet solids. The vacuum dryer is capable of temperatures ranging from 0-400°C and a pressure of 0-760 torr. The vacuum pump accompanying the dryer is a “Westinghouse” model. Figure 2 illustrates the different parts of the vacuum tray dryer.
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Figure 2. Vacuum Tray Dryer Legend: 1. Pump Switch
10. Dryer Shelf (Tray)
2.
11. Temperature Gauge
(4-way) Dryer Switch
3. Light Indicator (Dryer)
12. Pressure Gauge
4. Light Indicator (Pump)
13. Air Vent (and Valve)
5. Light Indicator (Thermostat)
14. Connection to Vacuum Pump
6. Temperature Controller 7. Dryer Door 8. Air Vent 9. Screw Locks
3.3 Procedure
Sawdust was wetted with ethanol and was subjected to the dying operation at constant temperature of 50°C and a pressure of 300 torr. The initial ethanol content was 0.60 grams ethanol per gram dry sawdust. The Drying Rate curve was obtained from the experimental drying curve. In order to achieve this, the mass of the wet material in the pan was monitored with respect to time until no further changes in the weight of the wet material were observed. Then, the experimental drying curve was determined by plotting the amount of ethanol removed versus time. The amount of ethanol evaporated from the material was obtained knowing the mass of the dry sawdust and the pan. The rate-of5
drying curve was then made by determining the values of the slopes of the drying curve and plotted it against the free ethanol content. The plot then showed two distinct periods in the curve corresponding to the two main types of rates in a drying process. From the rate-of-drying curve, the total time of drying can be predicted. Moreover, the drying time for the falling rate and the constant rate period can be obtained from the rate-of-drying curve. 4. Results and Discussions 4.1 Experimental Drying Curve
In a drying process, relatively small amounts of solvents are removed from a material which is usually solid. In this experiment, ethanol is mixed with sawdust and then undergoes drying in a vacuum tray dryer. Drying characteristics of a wet material under specific conditions can be characterized by its equilibrium ethanol content, critical ethanol content, and its constant and falling rate drying period. The vacuum tray dryer is operated at 50 C and 300 torr. °
The weight of the wet sawdust in the pan was monitored, every time a change of 5g in the reading of the scale,to determine the amount of ethanol that is removed through time under the set conditions. Figure 3 shows the relationship of ethanol content against time. Moreover, the equilibrium ethanol content, xe, was obtained using the plot below.
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0.70
0.60
0.50
0.40 t
x
0.30
0.20
xe
0.10
0.00 0.00
50.00
100.00
150.00
200.00
250.00
time
Figure 3 Drying Profile for ethanol wetted Sawdust at 50 C and 300 torr
The curve shown in the plot implies the decreasing amount of ethanol on the wet material as the drying operation proceeds. The curve reaches a point at which the ethanol content is constant and this is known as the equilibrium ethanol content of the wet material. The lowest ethanol content is achieved when the drying process reach its equilibrium, wherein the remaining ethanol content of the sawdust can no longer be removed even when drying is continued for a long period of time. As shown in Figure 3, the ethanol content stopped decreasing at 198 minutes. The value of the equilibrium ethanol content is found to be 0.15 g ethanol/g dry sawdust.
To study the mechanism of drying under constant conditions, a plot of the rate of drying, in grams ethanol per drying area in square meter per minute, as a function of free ethanol content is useful. The free ethanol content of the material under specified conditions is the ethanol content before the wet material reaches its equilibrium ethanol content. Figure 4 shows the rate of drying of the ethanol wetted sawdust at 50°C and 300 torr as a function of the free ethanol content.
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14.0
C
12.0 ) 2
10.0 m * n i m / l 8.0 o n a h t E 6.0 g ( R 4.0
2.0
0.0 0
0.1
0.2
0.3
0.4
0.5
xF (g Ethanol/g dry saw dust)
Figure 4 Rate of drying as a function of free ethanol content
The differences in the shapes of the curves are the result of how drying occurred in the wet material or how ethanol flow through the material. It is apparent in the drying rate curve that there are two distinct segments. The horizontal segment in the graph pertains to the first major drying period- the constant-rate period. During this period, the solid is so wet that a continuous film of ethanol is exists over the entire drying surface. The constant drying rate of the wet material was determined to be 11.47 g 2
ethanol/m ·min. In a porous solid, like the sawdust, the ethanol removed in this period is supplied from the interior of the material. As the ethanol content decreases, the constantrate period ends at definite ethanol content. The point terminating the constant-rate period is the critical point, shown by point C in the plot. The corresponding ethanol content on this point is called the critical ethanol content which was found to be 0.45 g ethanol/g dry sawdust. The critical point is reached when the rate of moisture flow to the surface no longer equals the rate of evaporation. The critical moisture content varies with the thickness of the material and with the rate of drying. It is therefore not a property of a material itself [McCabe, 2008]. The period subsequent to the critical point is called the falling-rate period. In this period, the rate is a function of the free ethanol content. The 5
entire surface is no longer wetted, and the wetted area continually decreases until the surface is completely dry. At ethanol contents lower than this point, all evaporation occurs from the interior of the solid. The drying period continues to fall and the equilibrium ethanol content is obtained where further drying does no t occur anymore.
4.2 Drying Time
Two portions of the drying rate curve are often present under different constant conditions-constant rate period and falling rate period [McCabe, 2008]. It is expected that the ethanol that is loosely held will be removed most easily. Thus, it would be expected that drying rates would decrease as ethanol content decreases, with the remaining ethanol being bound more and more strongly as its quantity decreases. The behaviour, in which the drying behaves as though the ethanol were at free surface, is called constant-rate drying. Then as the drying proceeds, the ethanol content falls and the access of ethanol from the interior of the sawdust to the surface affects the rate and decreases it. And this called the falling-rate period of drying. The total drying time corresponds to the summation of the time of constant rate period and falling rate pe riod. Table 2.2.1 shows the difference between the actual time of drying, time when the ethanol content reaches its equilibrium observed in the experiment, and the drying time at constant rate and falling rate periods.
Table 1. Drying Time
Constant Rate Period (min)
28.68
Falling Rate Period (min)
161.73
Total Time of Drying (min)
190.41
Actual Time of Drying (min) %Difference
198.6 5.8
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Table 1 shows that at the falling rate period, the drying time is longer compared at constant rate period. The drying time becomes long at the falling rate period since the remaining ethanol is bound more strongly at the surface of the sawdust. It also shows that the actual drying time is longer compared to the predicted drying time at constant and falling rate periods with a 5.8%difference. The difference may be caused by the errors in the data processing. The predicted drying time may not be accurate since in the calculation of the drying time, it assumed that the temperature and the relative humidity of the drying air are constant. Even though the vacuum tray drier is regulated at 50°C and 300 torr, the equipment operates for sometime before maintaining the desired temperature and pressure. The predicted time did not account for surface properties of the sawdust and the chemical interaction of ethanol in the sawdust which contributed in the difference in the time of drying. It was also assumed that the points in between two data points collected constitute a straight line. The area under a 1/R vs. x F is important in determining the drying time.
5. Conclusion
From the monitored masses of the wet material (sawdust plus ethanol) with time, one can determine the equilibrium ethanol content as well as the critical point of the process. The results showed that the equilibrium ethanol content was 0.15 grams ethanol per grams of dry sawdust. By obtaining the rate-of-drying curve, one can estimate the critical point of the drying process where it marks the start of the falling rate period. The corresponding critical ethanol content was 0.45 g ethanol per grams dry sawdust. Lastly, the predicted time of drying or the time calculated using the different equations for the different periods is just 5.8% different from what was recorded during the actual experiment. .
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6. References
Geankoplis, C. J. (2003). Principles of Transport Processes and Separation Processes, 4th Edition, Prentice Hall, New Jersey. nd
McCabe, W.L. et. Al. (1956). Unit Operations of Chemical Engineering, 2
Edition,
McGraw-Hill, Inc., Singapore.
Coulson, J.M. and Richardson, J. F. (2002) Coulson’s and Richardson’s Chemical th
Engineering, Volume 1: Fluid Flow, Heat Transfer and Mass Transfer, 6
Edition,
Butterworth-Heinemann Ltd., Oxford.
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