Tray Dryer Experiment
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In this experiment, we use tray dryers to dry the wet sample. We choose the tea herbs as our sample because we believe that tea herbs requires short drying time, and the moisture content will decrease faster with time. The measurement of the moisture content is based on the weight of the sample which inversely proportional with the temperature & time. Thus, we can determine the drying rate of the sample. Based on the data from the experiment, the mass and moisture content is decreasing with time, whereas the drying rate is varying rapidly with time.
Drying is the process of thermally removing volatile substances (e.g., moisture) to yield a solid product. Mechanical methods for separating a liquid from a solid are not considered in thermal drying. When a wet solid is subjected to thermal drying, two processes occur simultaneously; first is the transfer of energy, mostly as heat from the surrounding environment to evaporate the surface moisture, and second is the transfer of internal moisture to the surface of the solid and its subsequent evaporation due to the first process.
applications that involve drying operation is tray dryers (or cabinet dryers). The dryers are made of trays held in a cabinet, which is
connected to a source of heated air by gas, diesel or biomass. The food, which is to be dried, is spread out, generally quite thinly, on trays which the drying takes place. Heating may be by an air current sweeping across the trays, by conduction from heated trays or heated shelves on which the trays lie, or by radiation from heated surfaces. Most tray dryers are heated by air, which also removes the moist vapours. The temperature of ambient air is controlled by thermostat which is usually set in the range of 50 to 700°C. The air enters the bottom of the chamber below the trays and then rises, through the trays of food being dried, and exits from an opening in the top of the chamber. Fresh air is allowed to enter the cabinet and moist air is exhausted by using a dehumidistat to control an exhaust fan and air intake shutters. The dryer trays are tight-fitting in the cabinet to prevent air from bypassing the material to be dried. Depending upon the food and the desired final moisture, drying time may be of the order of 10 or even 20 hours. Tray dryers require low capital and maintenance costs but have relatively poor control and produces more variable product quality. However, in recirculation designs, the moisture laden air, after evaporating water from food, would have to be dried before being undergo recirculation, or else it would soon become saturated and further drying of the food would stop. In such a case passing it through a desiccant such as a bed of silica gel, or condensing moisture out by passing the moist air over cold plates or coils could dry the air. Note also that if we are not going to dry the exhaust air for recirculation, then the exhaust vent should not be close to the fresh air intake area, otherwise the moist exhaust air will be drawn back
Thermometers are installed with the sensitive elements directly in the main air current approaching the drying trays, and often also in the air current leaving trays.
One of the most important problems encountered is not to supply the same drying rate at the all position within the tray dryers. The other is the fast drying of food in the position where the air first enters to the system
slow drying of
in the other position.
Therefore, additional heaters and fans may be placed above or alongside the trays to increase the rate of drying. Tray dryers are used for small-scale production or for pilot-scale work.
The purpose of this experiment is to determine the drying rate of the sample by drying method using tray dryer besides learn to use and operate the tray dryer.
Drying occurs with the help of evaporation by supplying heat to the wet feedstock. There are two options for medium of heat transfer that is through convection by direct driers or conduction by contact or indirect driers. The removal of water from drying is to inhibit the growth of microorganisms and therefore the food is well preserved besides reducing the weight and bulk of food for cheaper transport and storage. When carried out correctly, any alteration of nutritional quality, colour, flavour and texture of r ehydrated foods will not be affected too much.
Drying methods and processes can be classified as batch, where the material is inserted into the drying equipment and drying proceeds for a given period of time, or as continuous, where the material
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 formed is removed by the air. In vacuum drying, the evaporation of water proceeds more rapidly at low pressures, and the heat is added indirectly
temperatures can also be used under vacuum for certain materials that may decolourize or decompose at higher temperatures). In freeze drying, water is sublimed from the frozen material. 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. The final choice is then made on the basis of capital and operating costs. Attention must be paid, however, to the costs of the entire isolation system, not just the drying unit alone. Thermally sensitive materials must be dried at low temperature under vacuum, with a low-temperature heating medium, or very rapidly.
economically. Operation and maintenance costs must be excessive; pollution must be controlled; energy consumption must be minimised. As far as the drying operation itself is concerned, adiabatic dryers are generally less expensive than non-adiabatic dryers, in spite of the lower thermal efficiency of adiabatic units.
Unfortunately there is usually a lot of dust carry over from adiabatic dryers,
quantitatively from the drying gas. In herbs and medicinal industry, most medicinal and culinary herbs are sold dried. When drying large quantities of herbs for commercial sale, growers should use a forced-air dryer to preserve their color, flavour, oil content, and medicinal properties. To achieve this, good air circulation within the dryer is important, as it reduces drying time and allows the use of lower temperatures, both of which can prevent the degradation of chemical constituents during the drying process. There-fore, dependable temperature control and the ability to provide high air flow throughout the plant material are important. Drying temperatures and times differ by plants, and plant parts, thus most herbs should be dried at low temperatures, ideally around
however, it may be necessary to raise temperatures to as high as 130°F. The size of the dryer should be based on the herb production area and how much will be harvested at one time. The dryer should have well-spaced racks to ensure that all sides of the plant receive sufficient air flow and the plant material dries evenly.
The shelves should be constructed of food grade
screens or covered with an acceptable food grade material. dryer should be free of dust, dirt, insects, and rodents.
any of these in the final product can lower its value or render it unmarketable. The dryer should be able to hold an entire harvest and dry it fast enough that the crop does not spoil. concerns
must be taken
Efficiency and economic
consideration to ensure
effectiveness of its operation. Likewise, a dryer can be constructed
Additionally, a grower may choose to construct one
from scratch. In this experiment, tea leaves are chosen as herbs that undergo drying process by operating tray dryer. The best method to dry herb will depend largely on the physical characteristics of the material, the volume of herb in the dryer, and the relative humidity of the outside air. Different herbs will require different temperatures. Typically, the higher the ambient humidity, the higher the temperature that is required to dry the material thoroughly. Introducing drier air will reduce the amount of time the material is in the dryer, thus reducing the cost of drying the material. The moisture content of dried products is very important, and if it is too high, moulds and yeasts tend to grow. The moisture content may be checked using scales and an oven. The following is the expression that we may use in order to calculate the moisture content of our drying tea leaves;
Other important parameter for drying process is the drying rates, v (min-1); v = Xt1 – Xt2 T2 – T1
switched on. 2.
Rinse the empty tray using water before start
The mass of the empty tray were measured and tared. The data was recorded.
The dry tea herbs were sprinkled with enough of water to make it wet.
The tea herbs were placed onto the empty tray and put into the tray dryers to be prepared to be dried.
The initial mass of the wet tea herbs were measured and recorded before being dried.
The initial temperature and humidity which shown on the tray dryers taken and recorded. The initial air flow inside the tray dryers were measured using the anemometer and the readings was recorded.
The drying process is started by pressing the ‘HEAT’ button on the tray dryers.
All of the data readed and recorded every 3 minutes in one hour time of the drying process of the tea herbs.
10. The moisture content and the drying rate were calculated. 11. The graph of drying rate against moisture content was plotted.
Tray dryers Analytical balance
Water Anemometer Stopwatch Dry tea herbs
Mass of tray
= 1391.3 g
Mass of dry tea leaves
= 99.4 g
Mass of sample (g)
1. Moisture content,x calculated using the following formula: M o i s t u r e c o n t e n t , x = mass1-massfinalmassfinal
Example: Moisture Content 1, x1
Moisture Content 2, x2
2. After calculated the moisture content, the drying rate calculated using the following formula : D r y i n g R a t e , v = ΔxΔt
Example : For tea herbs sample : D r y i n g R a t e , v = x1- x2∆t = 0.11975 - 0.077795 = 0.008392
Based on graph mass versus time, it shows that the mass of the tea herbs are decreasing with time. This is the same with graph of moisture content versus time, where the moisture content is decreasing with time.
Drying occurs in three different periods, or phases, which can be
phase, or to
moisture. From both graph, the first phase can be seen clearly at time between the first minute until minute 9. The second phase, or constant rate period, is when the free moisture persists on the surfaces and the rate of evaporation alters very little as the moisture content reduces. This is seen from the graph at minute 9 until minute 12. The third phase, or falling rate period, is the phase during which migration of moisture from the inner interstices of each particle to the outer surface becomes the limiting factor that reduces the drying rate. Principles of tray dryer states that during the early stages of drying, the conditions in the dryer, which is at high humidity and moderate temperature, are ideal for the growth of microorganisms. The quicker the drying time the better the final microbial quality of the product. From the graph of drying rate versus time, we can see that the changing pattern is not consistent or in mannered pattern. Hence, it is well said that the drying rates is not depending much upon time.
Based on the data plotted on each graph, we can conclude that the moisture content and mass of the tea herbs are both decreasing with time. Nevertheless, the drying rates of the herbs are varying with time.
In order to obtain better results, there are a few methods or recommendations that may be considered. Firstly, the drying rates can be increased with two different options; by increasing the air flow or increasing the air temperature. However, the temperature must not be set to too high for it may cause damage or change to the products. This is true for herbs and spices, as high temperature will have the risk of losing delicate flavours or colours.
As a matter of fact, fan driven dryers may suck in fine dust particles in dusty areas, causing contamination of the product. In very dusty areas, powered dryers may need a muslin filter over the air inlet. Herbs and spices must be dried to a moisture content that is low enough to prevent the growth of micro-organisms such as moulds and bacteria.
Most commercial dryers are insulated to reduce heat losses, and they recirculate hot air to save energy. Many designs have energy-saving devices, which recover heat from the exhaust air or automatically control the air humidity. Computer control of dryers is increasingly sophisticated and also results in important savings in energy. If the exhaust air is not dried for recirculation, then the exhaust vent should not be close to the fresh air intake area, otherwise the moist exhaust air will be drawn back through the drier and drying efficiency will be lost. Good air circulation within the dryer is important, as it reduces drying time and allows the use of lower temperatures, both of which can prevent the degradation of chemical constituents during the drying process. The dryer should have well-spaced racks to ensure that all sides of the plant receive sufficient air flow and the plant material dries evenly. The dryer should be free of dust, dirt, insects, and rodents. Evidence of any of these in the final product can lower its value or render it unmarketable.
1. h t t p : / / w w w . f a o . o r g / d o c r e p / V 5 3 8 0 E / V 5 3 8 0 E 0 9 . h t m 2. h t t p : / / w w w . e n o t e s . c o m / h o w - p r o d u c t s - e n c y c l o p e d i a / g r e e n - t e a 3. Bird,R.B,Steward.WE,and Lightfoot,EN, Transport Phenomena, John Wiley and Sons, Inc,1960