Drying Techniques and Applications

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DRYING TECHNIQUE

1. Definition of Drying

Drying is one of the most energy intensive operation units in post-harvest processing. This operating unit is applied to reduce the moisture content of products such as various fruits, vegetables, and other agricultural products after harvest. Drying is the process of transferring heat and water vapor simultaneously which requires heat to vaporize water from the surface of the material without changing the chemical properties of the material. The basis of the drying process is the occurrence of evaporation of water into the air due to differences in moisture content between the air and the dried material. The rate at which water is removed from the material will result in reduced water content in the material.

Video of chemical tools can be viewed at the following link:  https://www.youtube.com/watch?v=vhOpIrUjdw0

Drying is the separation of a small amount of water from a material so as to reduce the residual content of the liquid in the solid to an acceptable low value, using heat. In this drying process water is evaporated using unsaturated air which is blown to the material to be dried. Water (or other liquid) evaporates at a temperature lower than its boiling point due to differences in the moisture content of the interface in the interface of a gas-solid material with the moisture content in the gas phase. Hot gas, called a drying medium, provides the heat needed to evaporate water and simultaneously carries water out. Water can also be separated from solid material, mechanically using presses so that the water comes out, by centrifugal separator, by thermal evaporation or by other methods.

The liquid content in the dried material differs from one ingredient to another. There are ingredients that do not contain liquid at all (bone dry). In general, solids always contain a small fraction of water as bound water. The water content in an ingredient can be stated on a wet basis (% by weight) or dry basis, ie the ratio of the amount of water to the amount of dry matter.

The basis of drying is the occurrence of evaporation of water into the air due to differences in the content of water vapor between the air and the dried material. In this case, the air vapor content is less or air has a low relative humidity resulting in evaporation. The ability of air to carry moisture increases if the difference between the relative humidity of the drying air and the air around the material increases. One factor that speeds up the drying process is the speed of the wind or air flowing. The air that does not flow causes the moisture content around the dried material to become more saturated so that the slower drying.

The purpose of drying is to reduce the moisture content of the material to the limits of the development of organisms and enzyme activities that can cause decay is inhibited or bacteria are completely stopped. Thus the dried material has a longer shelf life.

The drying process is obtained by evaporation of water. This method is done by reducing the relative humidity of the air by flowing hot air around the material, so that the water vapor pressure of the material is greater than the water vapor pressure in the air. The pressure difference causes the flow of water vapor from the material into the air.

In the chemical industry the drying process is one of the important processes. This drying process is usually done as a final step before packing a product or a preliminary process so that the next process is easier, reduces the cost of packaging and transportation of a product and can add value to a material. In the food industry, this drying process is used to preserve a food product. Microorganisms that can cause food spoilage cannot grow on ingredients that do not contain water, therefore to maintain the aroma and nutrition of food so that it can be stored for a longer time, the water content in the food must be reduced by drying (Revitasari, 2010).

2. Factors Affecting Drying

1. Surface Area

The more surface area of ​​the material the faster the material dries. Water evaporates through the surface of the material, while the water in the middle will seep to the surface and then evaporate. To speed up drying, the food that will be dried is cut or sliced ​​first. This happened because:

(1)

cutting or slicing will expand the surface of the material and a broad surface can be in contact with the heating medium so that water easily comes out,

(2)                    small pieces or thin layers reduce the distance where heat must move to the center of food. Small pieces will also reduce the distance through the mass of water from the center of the material which must exit to the surface of the material and then exit the material.

1. Difference in Temperature and Surrounding Air

The greater the temperature difference between the heating medium and food the faster the heat transfer into the material and the faster the removal of water from the material. Water that comes out of the dried material will saturate the air so that its ability to get rid of water is reduced. So the higher the drying temperature, the faster the drying process will be. However, if it does not match the dried material, the result will be an event called “Case Hardening”, which is a condition where the outside of the material is dry while the inside is still wet.

1. Airflow Speed

The higher the air velocity, the more removal of water vapor from the surface of the material so as to prevent the occurrence of saturated air on the surface of the material. Air that moves and has a high movement in addition to taking water vapor will also remove the water vapor from the surface of food, so it will prevent the saturation atmosphere that will slow the removal of water. If the air flow around the drying site is going well, the drying process will be faster, that is, the easier and faster the water vapor is carried and evaporated.

1. Air Pressure

The smaller the air pressure, the greater the air’s ability to transport water during drying, because with smaller pressure means the air density decreases so that water vapor can be more accommodated and removed from food. Conversely, if the air pressure is greater then the air around the drying will be humid, so the ability to accommodate water vapor is limited and inhibits the drying process or rate.

1. Air Humidity

The more humid the air the longer it dries while the more dry the air the faster it dries. Because dry air can absorb and retain moisture. Each material has its own relative humidity balance. humidity at a certain temperature where the material will not lose water (move) into the atmosphere or will not take moisture from the atmosphere (Supriyono, 2003).

3. Basic Principles and Drying Mechanism

The drying process in principle concerns the process of moving heat and moving masses that occur simultaneously (simultaneously). First the heat must be transferred from the heating medium to the material. Furthermore, after the evaporation of water occurs, the water vapor formed must be transferred through the structure of the material to the surrounding medium. This process will involve fluid flow where the liquid must be transferred through the structure of the material during the drying process. So heat must be provided to evaporate water and water must diffuse through various kinds of prisoners so that it can be released from the material and in the form of free water vapor. The duration of the drying process depends on the material being dried and the method of heating used. The higher the temperature and the speed of the drying air flow, the faster the drying process will take place. The higher the temperature of the drying air, the greater the heat energy carried by the air so that more mass of liquid is evaporated from the surface of the dried material. If the drying air flow velocity is higher the faster the mass of water vapor is transferred from the material to the atmosphere. Moisture affects the water vapor removal process. At high air humidity, the difference in water vapor pressure inside and outside the material is small, so that the removal of water vapor from inside the out material becomes impeded. On drying using tools generally consist of propulsion and fans, heating units ( If the drying air flow velocity is higher the faster the mass of water vapor is transferred from the material to the atmosphere. Moisture affects the water vapor removal process. At high air humidity, the difference in water vapor pressure inside and outside the material is small, so that the removal of water vapor from inside the out material becomes impeded. On drying using tools generally consist of propulsion and fans, heating units ( If the drying air flow velocity is higher the faster the mass of water vapor is transferred from the material to the atmosphere. Moisture affects the water vapor removal process. At high air humidity, the difference in water vapor pressure inside and outside the material is small, so that the removal of water vapor from inside the out material becomes impeded. On drying using tools generally consist of propulsion and fans, heating units (heater)  and control devices. As a source of power to drain the air a blower can be used. Energy sources that can be used in heating units are furnaces, gas, petroleum, and electric heating elements.

The main process in drying is the process of evaporation of water it is necessary to first know the characteristics of food hydratation, namely the properties of the material which includes interactions between food ingredients with the water molecules they contain and water molecules in the surrounding air. The role of water in food is expressed in terms of water content and water activity, while the role of water in air is expressed by relative humidity and absolute humidity.

The mechanism for water release from the material during drying is as follows:

1. Water moves through capillary pressure.
2. Withdrawal of water is caused by differences in the concentration of the solution in each part of the material.
3. Withdrawal of water to the surface of the material is caused by absorption of the surface layers of the solid component from the material.
4. The movement of water from material to air is caused by differences in vapor pressure (Dewi, 2010).

4. General Drying Methods

Drying methods and processes can be classified in a variety of different ways. The drying process can be grouped as follows:

(1)   Batch; The material is put into the dryer and the dryer lasts for a certain period of time.

(2)   Continuous; the ingredients are added continuously to the dryer and the dry ingredients are moved continuously (Dewi, 2010).

5. Types of Dryers
6. Tray Dryer

The tray dryer is also called a shelf dryer or cabinet dryer, which can be used to dry lumps or pastes, which are spread on metal trays with a thickness of 10-100 mm. The drying of any type of tray or container is by placing the material to be dried on a straight tray in relation to the drying media. The most commonly used heat transfer method is convection and conduction heat transfer is also possible by heating the residue.

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Gambar 1. Tray Dryer

The advantages of tray dryer as follows:

1. Faster drying rate
2. The possibility of  over drying is smaller
3. Low pressure drying air can pass through layers of dried material (Revitasari, 2010).

The disadvantages of the tray dryer as follows:

1. The tendency of the bottom tray to be hot and the top tray to be less hot
2. Low efficiency.
3. Drum (Rotary) Dryer

Rotary dryer or can be called a drum dryer is a dryer in the form of a drum that rotates continuously which is heated with a furnace or  gasifier . This dryer can work on air flow through the cylinder shaft at a temperature of 1200-1800  ^o F but this dryer is more often used at temperatures of 400-900  ^o F.

Rotary dryers are very well known in the industry because the drying process rarely faces failures in terms of both quality and quantity outputs. However, since the scarcity and high cost of oil and gas fuel, the rotary dryer technology began to be developed to co-exist with substitute fuel technologies such as coal burners, synthesis gas and so on.

Dryer rotary dryer is used to dry the material in the form of powders, granules, lumps of solid particles in large size. Submission and removal of material occurs automatically and continuously due to vibrator motion, feed hole rotation, rotating motion and gravity. The heat source used can come from electric steam, coal, kerosene and gas. The dust produced is collected by  scrubbers  and electrostatic water catchers.

Gambar 2. Rotary Drum Dryer

The benefits of using a rotary / drum dryer as a dryer are:

1. Can dry both the outer or inner layers of a solid
2. Good      material handling so as to avoid attrition
3. A good mixing process, ensuring that the process of drying the material is uniform / evenly distributed
4. High heat efficiency
5. Continuous operation
6. Easy installation
7. Uses little electricity

                        Disadvantages of using a drum dryer include:

1. Can cause reduction in size due to erosion or breakdown
2. Characteristics of inconsistent dry products
3.   Low energy efficiency
4. Maintenance of difficult tools
5. There is no clear dust separation (Heriana, et al., 2012).  
6. Spray drying

Spray drying is a drying process to reduce the moisture content of a material so that the product is in the form of a powder through liquid evaporation. Spray drying uses liquid atomization to form droplets, then droplets that are formed are dried using dry air with high temperature and pressure. Materials used in spry drying can be suspension, dispersion or emulsion. While the final product produced can be powder, granule or agglomerate depending on the physical-chemical properties of the material to be dried, the design of the dryer and the desired final product.

Gambar 3. Spray Drying

Advantages of Spray Drying

Large drying capacity and the drying process takes place very quickly. Drying capacity reaches 100 tons / hour.

2. No loss of large amounts of volatile compounds (aroma)
3. Suitable for products that can’t stand heating (high in protein)
4. Producing dry particles with the size, shape, and water content and other properties that can be controlled as desired
5. Has a large production capacity and is a continuous system that can be controlled manually or automatically.

Disadvantages of Spray Drying

Requires a high enough cost

2. Can only be used on liquid products with a certain degree of thickness
3. Can not be applied to products that have sticky properties because it will cause clumping and sticking to the surface of the tool.
4. Freeze dryer

Freeze Dryer is a drying device that is included in the Conduction Dryer / Indirect Dryer  because the transfer process occurs indirectly, ie between the material to be dried (wet material) and the heating media there is a dividing wall so that the water in the wet / moist material that evaporates does not carry over with the media heating. This shows that heat transfer occurs by conduction, so it is also called Conduction Dryer / Indirect Dryer.

For the freeze dryer process, according to Muchtadi (1992), the dried material is first frozen then followed by drying using low pressure so that the water content that has become ice will immediately become steam, known as sublimation. Drying using a freeze dryer is better than an oven because the water content is lower. Drying using a freeze dryer is safer for the risk of degradation of compounds in the extract. This is probably because the temperature used to dry the extract is quite low.

Freeze drying is a drying method that has the advantage of maintaining the quality of drying results, especially for products that are sensitive to heat.

The advantages of freeze drying, compared to other methods, include:

1. Can maintain product stability (avoid changes in aroma, color, and other organoleptic elements)
2. Can maintain the stability of the material structure (shrinking and deformation after drying is very small)
3. Can increase rehydration power (the result of drying is very hollow and lyophile  so that the rehydration power is very high and can return to physiological, organoleptic and physical form which is almost the same as before drying).

These advantages can of course be obtained if the frozen drying procedures and processes are applied precisely and in accordance with the characteristics of the dried material. Certain operational conditions that correspond to one type of product do not guarantee it will be compatible with other types of products.

Freeze drying

Because the cooling rate is less low and the temperature is relatively unstable so it does not guarantee the durability of frozen food products (Haryani, et al., 2012).

Gambar 4.  freeze drying

1. Fluidized Bed Dryer

             Fluidized Bed Drying is a drying process by utilizing hot air flow with a certain speed that is passed through the material bed so that the material bed has fluid-like properties.

The fluidizing drying method is used to speed up the drying process and maintain the quality of the dry material. This drying is widely used for the drying of particulate or granular materials, both for the chemical, food, ceramic, pharmaceutical, agricultural, polymer and waste industries. The drying process is accelerated by increasing the speed of hot air flow until the material is fluidized. In this condition the material is blowing so that it increases the area of ​​the drying contact, increases the convection heat transfer coefficient, and increases the rate of diffusion of water vapor.

The minimum fluidization velocity is the lowest air flow rate at which the dried material can still be fluidized properly, while the maximum air velocity is the highest speed at which at this rate the material is blown out of the drying chamber

Fluidizing system dryer machine parts:

1. Fan ( Blower )

The fan ( Blower ) works to produce airflow, which will be used in the fluidization process. The fan also acts as a heat exchanger in the dryer as well as lifting the material for the fluidization process to take place.

2. Heating Elements ( heater)     

The heating element ( heater ) functions to heat the air so that the relative humidity of the drying air decreases, where the heat produced is carried by the flow of air that passes through the heating element so that the process of evaporation of water from within the material can take place.

3.      Plenum

       Plenum  in a fluidizing type drying machine is a hot air intake channel which is blown by the fan into the drying room. This part of the air duct can affect the speed of the air flow, which is where the direction of the air flow is deflected into the drying chamber with the help of barriers which also serves to evenly distribute the air flow.

4. Dryer Room.

The drying chamber functions as a place where the material to be dried is placed. The most optimal heat transfer and vapor mass occur in this room.

5. Hopper.

The hopper  serves as the insert for the material to be dried in the dryer.

Advantages of fluidized system dryer:

Fluid-like material flow causes the material to flow continuously so that it automatically facilitates its operation.

2. Mixing or stirring the material causes the condition of the material is almost close to isothermal.
3. Circulation of material between two fluidized beds makes it possible to flow large amounts of heat needed into a large drying chamber.
4. Fluidizing type dryer suitable for large scale.
5. The rate of heat transfer and the rate of mass transfer of water vapor between the drying air and the material is very high compared to the other dryer contact methods.
6. Moving heat using a fluidizing type dryer requires a relatively small surface area.
7. It is ideal for heat sensitive and non-heat sensitive products

Lack of fluidization system dryer:

It is difficult to describe the flow of hot air exhaled into the drying chamber, due to the large deviation from the incoming air flow and the material being passed by air bubbles, making the contact / contact system inefficient.

2. Continuous mixing or mixing of solids on the bed will cause the unevenness of material quiescent time in the drying chamber, because the material is continuously exposed to hot air.
3. Can not process materials that are sticky or high in water and abrasive.

Gambar 5. Fluidized Bed Dryer

1. Vacum dryer

Vacuum comes from the Latin,  vacuus , meaning empty. So vacuum means emptying a room or an absolute under zero pressure. The system of empty space is surrounded by the earth’s atmosphere. To create a vacuum, a pump is needed to get air out of the system. This need is the basic work meaning of vacuum.

Thermodynamic analysis only considers absolute compressive values. However, most pressure measuring devices only show a gauge (gauge) ie the difference in absolute pressure of a system and the absolute pressure of the atmosphere. The bourdon measurements, for example, measure pressure relative to the surrounding atmosphere. Conversion of the absolute pressure measuring pressure obtained with

the following relationship.

P _(absolute)  = P _(ukur)  + P _(atm)

For drying granular or slurry solids, vacuum dryers with various mechanical designs are commercially available. This type of drying is more expensive than atmospheric pressure dryers but is suitable for heat sensitive materials and requires solvent recovery or if there is a fire or explosion ratio. Single or double cone mixing can be applied for drying by heating the blanket of the vessel and placing it to remove moisture. The picture shows the two vacuum dryers available on the market. Pedal type vacuum dryers are suitable for sludge materials while belt-type vacuum dryers are suitable for paste-shaped materials.

Vacum drying machine is a drying machine using vacuum technology. The product drying process is regulated at the desired temperature, accompanied by a vacuum process to speed up drying. The vacuum drying machine is usually used for the dried product must be at a low temperature, so that nutrients are not damaged.

Vacum drying is useful for drying vegetables and other products to your liking. This machine is used for various purposes, including drying vegetables at temperatures not too high, so that the nutritional value is not lost. This machine can also be used for food products.

Advantages of vacum drying

1. Evaporation is faster at low pressure than at high pressure
2. Used for materials which are sensitive to temperature or are easily oxidized
3. Fast drying time
4. Low temperature
5. Little energy is used