Drying is defined as a mass transfer process that involving the removal of water or another solvent by evaporation process from a solid, semi-solid or liquid. The objectives of this experiment are to remove the water content in the chosen tropical fruit, to dry the fruit in order to reduce its weight prior to its extraction process, to compare the drying process between tray dryer and oven and also to determine the relationship between drying rate and the moisture of content after drying process. In this experiment, the fruit used is carrots. The mass of the trays were taken and then the carrots were spread out onto the trays, and the total weight was recorded. The time taken for the samples to dry, temperature, humidity, mass of samples and flow speed are recorded during the experiment. The moisture content and the drying rate was then calculated using the results obtained and graphs were plotted to give a better understanding of the experiment. The experiment was considered a success.
Drying is defined as a mass transfer process that involving the removal of water or another solvent by evaporation process from a solid, semi-solid or liquid. 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.
The term "drying" is a relative one, and simply means that there is a further reduction in the moisture content from some initial level provided by mechanical dewatering to some acceptable lower level. For example, a moisture content of 10-20% by volume would normally allow particles to flow freely, yet suppress dust formation. The necessity for drying may be to make a product suitable for sale (e.g. paint pigments), or for subsequent processing.
Adiabatic dryers are the type where the solids are dried by direct contact with gases, usually forced air. With these dryers, moisture is on the surface of the solid. Non-adiabatic dryers involve situations where a dryer does not use heated air or other gases to provide the energy required for the drying process. Dryer can be categorised based on the mechanisms of heat transfer as follows:
Direct (convection heat transfer)
Indirect or in contact (conduction heat transfer)
Radiant ( heat transfer through radiation)
Dielectric or microwave drying
There are several type of drying equipment that can be used for drying propose in industry such as vacuum-shelf indirect dryers, continuous tunnel dryers, rotary dryers, drum dryers, spray dryers and tray dryers. In this experiment, the equipment used is tray dryer. Tray dryers are used for removing water from material which can be lumpy solid or a pasty solid. Spreading uniformly solid on a metal tray which has depth 10 mm to 100mm is put in cabinet of dryer. Water is removed by fan which recirculates heated air over the surface of the trays. Air is heated by electrical energy and 10-20 % of air in cabinet is fresh air, remaining is recirculating air.
This dryer type operates by passing hot air over the surface of a wet solid that is spread over trays arranged in racks. Tray dryers are the simplest and least-expensive dryer type. This type is most widely used in the food and pharmaceutical industries. The chief advantage of tray dryers, apart from their low initial cost, is their versatility. With the exception of dusty solids, materials of almost any other physical form may be dried. Drying times are typically long (usually 12 to 48 h).
The tray dryer is designed to dry granules, powders, food material and chemicals. The machine is mostly used by the pharmaceutical industry, chemical industry, food processing industry, paint industry and textile industry. It is available in various sizes as per your requirement and need. There are so many tray dryer manufacturing industries.
To remove the water content in carrot.
To dry the carrot in order to reduce its weight prior to its extraction process.
To compare the drying process between tray dryer and oven.
To determine the relationship between drying rate and the moisture of content after drying process.
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 rehydrated foods will not be affected too much.
Heat transfer and mass transfer are critical aspects in drying processes. Heat is transferred to the product to evaporate liquid, and mass is being transferred as a vapor into the gas. The drying rate is determined by the set of factors that affect heat and mass transfer. Solids drying is generally understood to follow two distinct drying zones, known as the constant-rate period and the falling-rate period. The two zones are demarcated by a break point called the critical moisture content.
In a typical graph of moisture content versus drying rate and moisture content versus time (Figure 2), section AB represents the constant-rate period. In that zone, moisture is
4 considered to be evaporating from a saturated surface at a rate governed by diffusion from the surface through the stationary air film that is in contact with it. This period depends on the air temperature, humidity and speed of moisture to the surface, which in turn determine the temperature of the saturated surface. During the constant rate period, liquid must be transported to the surface at a rate sufficient to maintain saturation.
At the end of the constant rate period, (point B, Figure 2), a break in the drying curve occurs. This point is called the critical moisture content, and a linear fall in the drying rate occurs with further drying. This section, segment BC, is called the first falling-rate period. As drying proceeds, moisture reaches the surface at a decreasing rate and the mechanism that controls its transfer will influence the rate of drying. Since the surface is no longer saturated, it will tend to rise above the wet bulb temperature. This section, represented by segment CD in Figure 2 is called the second falling-rate period, and is controlled by vapor diffusion. Movement of liquid may occur by diffusion under the concentration gradient created by the depletion of water at the surface. The gradient can be caused by evaporation, or as a result of capillary forces, or through a cycle of vaporization and condensation, or by osmotic effects.
In this kind of drying, one of the important parameters is wet bulb temperature. When gas coming from outside and a lot of water contacts each other, saturation is achieved. The adiabatic saturation temperature is the steady state temperature in such situation. If small amount water and a gas inlet is contacted under adiabatic conditions, a temperature which is nonequlibrium and steady state is reached. This temperature is called wet bulb temperature. In adiabatic saturation, temperature and humidity of gas are changed but in this situation, temperature and humidity of gas are not changed because of small amount of water.
In measuring wet bulb temperature, thermometer which is shown in figure 3 is covered with wet cloth which called wick. Air at dry bulb temperature T and humidity H comes and evaporates some water on wick. Due to this evaporation, temperature T is seen as Tw on thermometer. The difference in T and Tw is due to latent heat of evaporation.
5 Figure 3: Wet bulb temperature
If making a heat balance on wick is wanted, amount of heat lost by vaporization is: q=MA NA λw A (1)
Where q is amount of heat lost by vaporization (W, J/s), MA is molecular weight of water, NA is mol of water evaporating/ s.m2, λw is latent heat of vaporization at Tw (J/kg H2O) and A is surface area (m2).
Then the flux is:
NA= ky (yw-y) (2)
Where ky is mass transfer coefficient (mol/s.m2), yw is mole fraction of water vapor in the gas at the surface and y is mole fraction of water vapor in the gas.
There is a relation between humidty H and y which is: y= (H/MA) / (1/MB + H/MA) (3)
Where MA is molecular weight of water and MB is molecular weight of air. Since H is small, then :
y= HMB/MA (4)
Substituting Eq. (4) into Eq. (2) and after substituting into Eq. (1) q= MB ky λw (HW - H) A (5)
6 q= h (T - Tw) A (6)
Substituting Eq. (5) into Eq. (6) gives:
(H – HW) / (T – Tw) = - (h/ MB ky) / λw (7)
(h / MB ky) is called psychrometric ratio. In addition the adiabatic saturation lines can be also used for wet bulb lines with reasonable accuracy but for only water vapor. Therefore, humidity of air water vapor mixture measure can be done by using wet bulb determination
Knowledge about the basic mechanisms of rates of drying, obtaining some experimental measurements of drying rates is necessary in most cases. Therefore, data of loss in weight of moisture during drying at different time intervals should be taken without interrupting the operation. In addition data should be converted to rate of drying curves.
Xt= (W – Ws) / Ws (8)
Where Xt is moisture content (kg total water/kg dry solid), W is weight of the wet solid and Ws is weight of dry solid in kg.
Other important parameter for drying process is the drying rates, v (min-1); 𝑣 =𝑥𝑡1− 𝑥𝑡2
For tray dryer:
1. Drying plates were removed from the support frame. 2. The scale was tared to zero.
3. Drying plates were inserted individually into the support frame. The weight of the individuals drying plates on the digital scale were read and noted.
4. Material to be dried was spread in a thin layer on the drying plates.
5. Drying plates were inserted in the support frame and the total weight was recorded. 6. The fan was switched on and the speed was determined.
7. The flow speed was measured with manual anemometer and the value was recorded. 8. The heating level was switched on and set up.
9. The drying experiment was started and time was measured by using a stopwatch. 10. Temperature and humidity were recorded in 10 minutes interval until total time reached
11. The heater was switched off but fan was let on to cool down the channel. 12. After 15 minutes, the drying plates were removed.
13. The Tray Drier Training Unit was shut down. For microwave:
1. The oven was switched on and left until the temperature reach 50˚C. 2. The initial weight of carrot that will be dried using oven was measured.
3. After the temperature reached 50˚C, the weighted carrot is then placed in the oven for 1 hours.
4. After 1 hour, the carrot is then take out from the oven, and the weight of dried carrots were measured using weighing balance and recorded.
8 APPARATUS AND MATERIALS
208.7 g of carrot Tray dryer Training Unit CE 130
Anemometer Weighing balance Stopwatch
Figure 4: Tray dryer unit
For Tray dryer
Weight of trays: 1389.6 g Initial weight of carrot: 102.9
Time (min) Mass of sample (kg) Moisture content, X Drying rate (min^-1)
0 0.1029 2.1088 0.1547 10 0.0517 0.5619 0.0172 20 0.046 0.3897 0.0142 30 0.0413 0.2477 0.0115 40 0.0375 0.1329 0.0057 50 0.0356 0.0755 0.0076 60 0.0331 0 0 Time (min)
Humidity (%rF) Temperatures (°C) Mass of sample (kg) Anemometer (m/s) I II I II 10 7.5 6.6 47.2 46.8 0.0517 1.15 20 7.5 6.6 47.4 46.9 0.046 1.76 30 7.5 6.6 47.5 46.9 0.0413 1.72 40 7.5 6.6 47.6 47.1 0.0375 1.81 50 7.5 6.6 47.5 47.1 0.0356 1.73 60 7.5 6.6 47.6 47.2 0.0331 1.81
10 For oven
Initial mass of carrots (kg) 0.1058
Final mass of carrots (kg) 0.06808
0 0.5 1 1.5 2 2.5 0 10 20 30 40 50 60 70 Moi stur e C o ntent , x Time (min)
Moisture content versus Time
Figure 6: Graph moisture content versus time
Figure 7: Graph drying rate versus time
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0 10 20 30 40 50 60 70 D ryi ng r ate Time
11 S A M P L E C A L C U L A T I O N S
Moisture content, x calculated using the following formula:
massfinal Example: Moisture Content 1, x1 = 0.1029−0.0331 0.0331 = 2.1088 Moisture Content 2, x2 = 0.057−0.0331 0.0331 = 0.5619
After calculated the moisture content, the drying rate calculated using the following formula:
v =Δx Δt Example: Drying Rate,
v =x1− x2 ∆t = 2.1088 − 0.5619 10 = 0.5147
Drying is a mass transfer process resulting in the removal of water moisture or moisture from another solvent, by evaporation from a solid, semi-solid or liquid to end in a solid state. To achieve this, there must be a source of heat, and a sink of the vapor thus produced. In the most common case, a gas stream, e.g., air, applies the heat by convection and carries away the vapor as humidity.
In this experiment, carrots are used because the moisture contents of carrots is less compared to the other fruit, therefore it is requires short period of drying process. In this experiment there are two equipment that are used in order to learn the drying process which are tray dryer and oven.
Tray dryer is designed to dry granules, powders, food material and chemicals. The machine is mostly used by the pharmaceutical industry, chemical industry, food processing industry, paint industry and textile industry. This dryer type operates by passing hot air over the surface of a wet solid that is spread over trays arranged in racks. Tray dryers are the simplest and least-expensive dryer type.
Hot air oven is electrical devices that are used in sterilization by supplying the dry heat. Generally, they can be operated from 50 to 300 °C, using a thermostat to control the temperature. Their double walled insulation keeps the heat in conserves energy, the inner layer being a poor conductor and outer layer being metallic. The principle of operation is based on a fine gravity air convection in an electrically heated chamber of the unit. A two jacket design of the chamber and an automatic control unit ensure a homogenous temperature profile in the chamber, precise course of processes and short recovery time (return to the preset temperature) after door opening. This line of drying ovens is characterized by its economical operation. It is suitable for simple process of drying and heating of common materials. The units work noiseless.
From the result obtained from the experiment, it show that the mass loss of carrot that are being dried using hot air oven is differ to the mass of carrot that are being dried using tray dryer. The mass loss of carrots that are being dried using tray dryer is greater compared to hot air oven. This is because, tray dryer unit and hot air oven have different working principle. From the result, it show tray dryer is more efficient compared to hot air oven.
13 Based on the graph moisture content versus time, it shows that the moisture content of the carrots are decreasing with time. The longer the time for the drying process, the more moisture content will be evaporated. Based on the graph drying rate versus time, the drying rate is decrease when the time increase. When the time increase, the moisture content will decreases and it will take shorter time to be dried, so the drying rate will become more faster.
Drying occurs in three different periods, or phases, which can be clearly defined. The first phase, or initial period, is where sensible heat is transferred to the product and the contained moisture.
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.
Two processes occur simultaneously. One of them is the transfer of energy, mostly as heat from the surrounding environment to evaporate the surface moisture and the other one is the transfer of internal moisture to the surface of the solid and its subsequent evaporation due to the first process. 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 experiment was successfully conducted and all the objective for this experiment were achieved. From this experiment, it can be concluded that the moisture content and drying rate will decrease when the time increase. A tray dryer is an equipment that are very suitable in order to determine the rate of a wet substance and also to learn the drying process. A tray dryer is more efficient to dry moisture content compared to the hot air oven.
From this experiment, there are some recommendation which can be done to improve the experiment in future:
Set the tray dryer heating until an appropriate temperature which suits the drying substance.
In order to increase the drying rate, the air flow and the air temperature should be increase.
To reduce heat losses and to save energy, the dryer need to be well insulated.
The dryer need a muslin filter over the air inlet in order to reduce the contamination of the product.
1) Solids Drying: Basic and applications. Retrieved from http://www.chemengonline.com/solids-drying-basics-and-applications/?printmode=1
2) Tray dyer. Retrieved from http://www.pharmainfo.net/book/pharmaceutical-machines/tray-dryer
3) Geankoplis, C.J. (2014). Transport Processes & Separation Process Principle (includes unit operations). Pearson Education Limited.
16 Figure 9: Fresh carrots