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THEORY The unit operation drying is a form of simultaneous heat and mass transfer. An important factor in drying is gaining the amount of heat required for vaporization. The heat acquired is transferred to the object causing the mass transfer through evaporation. The moisture from the solid, semi-solid and even liquids is removed through evaporation and transferred to the surroundings as vapors. Drying is observed to have two distinct regions which are the constant rate period and falling rate period. During the constant rate period or the initial period of drying the moisture comes from the surface of the object (McCabe, Smith, & Harriott, 2006). According to Geankoplis (1993), the general definition of drying is the removal of comparatively minute amounts of water as vapor by air from the material. nd the product from a dryer is often ready for packaging (McCabe, Smith, & Harriott, 1993). Geankoplis (1993) adds that the drying process can also be divided into the physical conditions used to add heat and remove water vapor: Addition of heat by direct contact with heated air at atmospheric pressure then removal of water vapor formed using air. Vacuum drying, where evaporation of water proceeds more rapidly at low pressure and the heat is added indirectly by contact with a metal wall or by radiation. Low temperatures can also be used under vacuum for certain materials that may lose their color or decompose at higher temperatures. Freeze drying, where water is sublimed from the frozen material.
Most of the time the air that enters the dryer is not completely dry. It contains moisture and has a relative humidity. When the solid leaves the dryer its moisture content must not be less than the equilibrium moisture. The equilibrium moisture pertains to the humidity of the air entering. The water in the wet solid that cannot be removed by the entering air due to its humidity is referred to as equilibrium moisture. The difference between the total moisture content of the solid and the equilibrium moisture content is defined as the free moisture and is represented Equation 1, (Equation 1) The data for the change in free moisture content () and time () are plotted to generate a curve which represents the rate of drying of the material. The rate is calculated by using Equation 2, (Equation 2) where is the drying rate, is the mass of solid used and A is the exposed surface area (Geankoplis, 1993). In Figures 1 and 2, the rate of drying curve for constant condition is presented. Based on the graphs, point A shows the free moisture content of the material at. At the start of the process, the temperature of the solid is usually colder than its ultimate temperature making the evaporation rate increase. At point B, the surface temperature rises to its equilibrium value. If the solid is quite hot at the start, the rate may start at A’. The slope and rate are constant at point B to C and is represented by a straight line. This is known as the constant-rate period. At point C, the drying rate decreases until it reaches point D. This is known as the falling rate period. Point C is also called the critical moisture content. At this point, there is not enough water on the surface to maintain a continuous film of water. The rate of drying falls quickly until it reaches point E where (Geankoplis, 1993).
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There are a number of dryers available. One of these is the industrial oven dryer. It is a heated chamber which is used for baking, curing, or drying components. The main working principle of an industrial oven dryer is that a pre-determined volume of hot air is continuously transported outwards though the exit fan and replaced with fresh air. It is also known as the dilution principle, thus, preventing the formation of potentially explosive compounds (Bhatia, 2014). Figure 3 shows an example of an industrial oven dryer. In this experiment, the sample, which is potato, will be dried at constant drying conditions using an oven dryer.
OBJECTIVES The main objective of this experiment is to determine the rate of drying for a given material. Specifically, the objectives of this experiment are: (1) to generate drying and drying rate curves; (2) to specify the periods of drying and; (3) to determine the critical and equilibrium moisture content of the sample.
SCOPE AND LIMITATIONS This experiment will be limited to the generation if drying rate curves under constant drying conditions, specification of the periods of drying, and determination of critical and equilibrium moisture content of the sample which is potatoes. The experiment will not include the effect of temperature on the drying rate. The potato will be bought from the local market of Miag-ao, Iloilo and the type of potato that will be used in this experiment is the one used for cooking different kinds of dishes. The experiment will be conducted at the Food Analytical Laboratory of School of Technology for a period of three hours. All of the instruments that will be used will be acquired from the Laboratory Office of the School of Technology.
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METHODOLOGY MATERIALS Potato Kitchen Knife Chopping Board Ruler Tin Foil Tray Desiccator Aluminum foil Tongs Zip lock bag
INSTRUMENTATION Oven Dryer Analytical Balance Weighing scale
METHODOLOGY Preparation of the samples 1. Prepare 72 aluminum foil baskets and label them. Weigh each basket. 2. Preheat the oven dryer to 80 oC. 3. Transfer the samples in zip locks and then store in the desiccator to prevent it from absorbing moisture from the atmosphere 4. Wash and pat dry the potatoes before peeling them 5. Slice potato samples into 2 sets of rectangular cylinder. The first set has dimensions 1.5 in x 1.5 inch with a thickness of 2 mm while the second set has dimensions 1.5 in x 1.5 inch with a thickness of 4 mm. Each set is composed of 36 samples. 6. Transfer each sample into their respective aluminum foil basket. 7. Weigh the samples together with the aluminum foil basket. Calculate for the initial mass of the samples.
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Sample Drying and Determination of Initial Moisture Content 1. After weighing each sample, immediately place it inside the oven dryer 2. Take out three samples every 15-minute interval for a total of 3 hours. (Note: The oven dryer must not be left open for a long period of time for it will affect the data that will be obtained.) 3. Place the removed samples in the desiccator for 5 minutes to let it cool down 4. After 5 minutes, weigh the sample. Record the mass measurement with respect to its particular time interval 5. The last three samples will be used in the determination of the initial moisture content of the sample.
Post Experiment Clean up 1. Dispose properly all the waste generated in the experiment. 2. Clean and return all the instruments and materials used. 3. Clean the workplace before leaving the laboratory.
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REFERENCES Bhatia, R. (2014). Industrial Drying Oven and its Significance. Retrieved February 4, 2017, from http://www.selfgrowth.com/articles/industrial-drying-oven-and-its significance BioBase. (2017). Biobase, the most professional laboratory and medical products supplier. Retrieved February 4, 2017, from http://eyu.biobase.cc/ProductInfo_467.aspx#.WJTaQlN97Dc Geankoplis, C. J. (1993). Transport Processes and Unit Operations. P T R Prentice-Hall, Inc. McCabe, W. L., Smith, J. C., & Harriott, P. (1993). Unit Operations of Chemical Engineering (5th ed.).
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