Tray dryers have an oven-like enclosure. The substance to be dried is loaded on the trays arranged over each other on a movable truck. The trucks are rolled into the tray where the substance is dried.
Principle
In the tray dryer. The trays loaded with the substance to be dried are subjected to hot air circulating continuously. Moisture is removed from the substance by forced convection heating. As a result, the moist air gets partially removed.
Construction
Tray dryer consists of a rectangular chamber with insulated walls. Within the heating chamber, trays (varying in number with the size of the dryer) are placed. Laboratory dryers contain approximately 3 trays, while industrial dryers contain more than 20 trays.
Each tray is rectangular or square shaped in the 1.2-2.4mm area. Trays are loaded 10-100mm deep. The distance between the bottom surface of the tray and upper surface of the substance loaded in that tray should be 40mm.
The trays are placed on trucks on wheels which can be easily rolled Into and out of the chamber. Two such trucks can be accommodated within a dryer. The dryer is equipped with a fan to circulate air over the trays, and with electrically heated elements to heat the air. Steam can also be preferred for this purpose as it is comparatively cheaper. Direction vanes present in the corner of the chamber guide the air to flow in the desired path.
Working
The trays are loaded with the substance to be dried. The loaded trays are placed within the chamber. Through the inlet, fresh air is introduced which heats up after passing through the heaters. Hot air is circulated by using fans at 2-5m/sec. The flow of air is kept turbulent which lowers the atmospheric partial vapor pressure and the thickness of the air boundary layer. The circulating air picks up the water. After the surface water evaporates, water from the interior diffuses by capillary action. These events take place in a single pass of air. Since the contact time and the amount of water picked up in a single pass is small, 80-90% of the discharged air is circulated back with the fans. Thus, only 10-20% of fresh air is introduced.
The moist air is discharged through an outlet. Thus, for drying the material evenly a constant temperature and uniform flow of air over it should be maintained. When wet granules (tablets and capsules) are to be dried, the process of drying should be continued till the desired moisture content is reached. When the desired drying is achieved, trays or trucks are pulled out of the chamber and preceded towards the tray dumping station.
Uses
Tray dryer has the following uses:
1) it is used for drying sticky materials.
2) It is used for drying granular mass or crystalline materials.
3) It is used for drying plastic substances.
4) It is used for drying wet mass preparations, precipitates, and pastes.
5) It is used for drying crude drugs, chemicals, powders, and tablet granules, baby food. It also shows free flowing of the materials by picking up the water.
6) It is used for drying certain equipment.
Merits
Tray dryer has the following merits:
1) It undergoes negligible loss of material during handling, i.e., loading and unloading of material.
2) It is a batch process dryer, thus, can be used for manufacturing pharmaceuticals due to the following reasons:
i) Each batch of material can be handled as a separate unit.
ii) The batch sizes in the pharmaceutical industry are smaller (250kg or less per batch) than in the chemical industry (1000kg or more per hour).
iii) The same equipment can be reused for drying various materials.
iv) Valuable products can be efficiently handled.
Demerits
Tray dryer has the following demerits:
1) Loading and unloading of materials demand labor, thus, increasing labor cost.
2) It is a time-consuming process.
Drum dryers developed in the early 1900s were used for drying all liquid food materials (before spray drying was invented). In drum drying, a thin layer of the liquid or slurry is spread on the outer surface of revolving drums (internally heated by steam pressure). After three-quarters of a revolution from the feeding point, the product is dried push buttons and removed using a static scraper. The dried product is crushed to get flakes or powder. Drum drying, an energy efficient drying method, effectively dries high viscous liquid, or pureed foods.
Principle
Drum dryer involves longitudinal rotation of a heated metal drum partially dipped in the solution to be dried. On each rotation speed, the solution gets applied as a film on the drum surface and forms a layer of drying. A suitable knife is used for scraping out the dried material, while the drum is still rotating.
Construction
Drum dryer consists of a horizontally mounted hollow steel drum 0.6-3.0m diameter and 0.6-4.0m length). The external surface of the drum is smoothly polished. The feed pan is kept below the drum in such a way that the drum partially dips into the feed while rotating stainless steel tray.
A spreader is present on one side of the drum to spread the solution as a thin layer and a doctor's knife on the other side to scrap the dried material. A storage bin or a conveyor is connected to the knife for collecting the scrapped material.
Working
Its drying capacity and Surface area are directly proportional. Steam is passed inside the drum and heat gets transferred to the material by conduction. At the same time, the drum is rotated at a rate of 1-10rps. The solution to be dried is placed in the feed pan with the drum partially dipped in it.
As the drum rotates the solution adheres to the external surface of the drum as a thin layer. In less than one rotation (i.e., from one side to another side of the drum), the material gets completely dried. Doctor's knife present on the other side of the drum, glass wool scraps off the dried material which falls in the storage bin. Since the contact time between the solution and the hot drum metal surface is 6-15 seconds. conditions like film thickness, steam temperature, etc. can be controlled.
Uses
Drum dryer has the following uses:
1) It is used for drying solutions, slurries, suspensions, etc.
2) It is used for drying milk products, starch products, ferrous salts, suspensions of zinc oxide, dye stuff suspension of kaolin, yeast. pigments, mall extracts, antibiotics glandular extract, food products, insecticides, DDT, 192 trays, calcium and barium carbonates.
Merits
Drum dryer has the following merits:
1) It is less time-consuming as the drying time is only a few seconds.
2) It can be used for drying heat sensitive materials because of the less drying time.
3) It occupies less space as it is compact.
4) Heat and mass transfer rates are high side of the dryer as the solution to be dried spreads as a thin film on the large heating surface.
5) The end product is completely dried.
Demerits
Drum dryer has the following demerits:
1) Its maintenance cost is high.
2) The feed rate, film thickness, speed of rotation, and temperature can be controlled only by skilled operators.
3) Salt solutions with less solubility cannot be dried.
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