A tray dryer is most widely used in the pharmaceutical industry to dry pharmaceutical products like granules for tablet and capsule manufacturing. Granules prepared by wet granulation contain moisture, which is required to dry the granules to bring the moisture level up to the requirement as mentioned in the batch manufacturing record.
In pharmaceutical industries, different types of dryers are used for drying wet granules for example fluid bed dryers and tray dryers. Fluid bed dryer is an advanced type of dryer and its drying time is less as compared to a tray dryer. A tray dryer is a simple and conventional type of dryer.
Construction of Tray Dryer
Tray dryer used for drying of wet granules is an insolated chamber made off a double layer of stainless steel plates and filled with fibreglass wool. The insolation retains heat inside the tray dryer and facilitates the drying process. Tray dryer is a vertical enclosed chamber in which trays are installed on each other with some space so that hot air can circulate over all trays and can dry the product.
Trays are made of stainless steel. In some dryers, trays are directly installed inside the dryer and in some dryers, a SS trolley is available which carry all trays loaded with product and then this trolley is installed inside the tray dryer.
Principle of tray dryer
Processed and filtered Fresh air from outside is heated by heaters installed inside the dryer and is circulated by a fan in all over the chamber and remove moisture from wet granules by capillary action. The vapours in return air are discharged through return duct to outside environment.
10-20% air is discharged outside to remove moisture and remaining air is recirculated along with fresh air. Fans are used to circulate hot air uniformly with the speed of 2-5 meter per second. This process is repeated until the desired level of LOD according to batch manufacturing record is achieved.
Types of Trays in a Tray dryer
Trays used in tray dryer are of two types,
- Plain bottom trays
- Perforated bottom trays
Plain Bottom trays
Mostly tray dryers have plain bottom trays. In this type of trays, the air passes over the product placed inside the trays so the drying may not be the uniform as top side grains get dry earlier and lower side granules take more time to dry. So one common and conventional method used is that, mix the product in trays after every 2 to 3 hours so it facilitates uniform drying due to shuffling of granules.
The tray dryers are available in different capacities according to no of trays e.g 12,24,96 trays etc.
Perforated bottom trays
Some tray dryers have perforated bottom trays. The perforations in the bottom allow the hot air to pass through the product and drying is uniform in this process.
Working of tray dryer
After wet sieving of granules, wet granules are loaded into trays of tray dryer. Try to add an equal quantity of granules in each tray to ensure uniform drying. After loading granules in a tray, the tray is installed in the trolley of Tray dryer or directly into the dryer. In the same way, all the trays after loading with the product are installed.
In case of separate tray carrying trolley, the trolley is placed inside the dryer. The trolley is fitted with wheels so it’s movement can be easily handled. For continues process, some tray dryers have an extra trolley which can be installed immediately after the removal of the first trolley in case of the same product, drying in 2 or 3 lots.
It saves time because the second trolley can be loaded with the second lot of product before drying of the first lot. After installation of trays inside the dryer, close the door of tray dryer. Some tray dryers have a single door and some have a double door. The gas kit is installed at the door to prevent leakage of hot air. After ensuring sealing of door, switch on the fan and heating system of tray dryer. Air from outside is dried by the heaters and is circulated in the dryer by the help of fan and moisture is removed from the product.
The drying process in tray dryer is lengthy because the contact time of hot air with wet granules is very low. So it takes 7 to 8 hours to dry the product, to monitor the drying time a digital timer is attached with a tray dryer. This drying time of 7-8 hours is very high as compare to drying in FBD or fluid bed dryer which can dry the same lot of granules in 1-2 hours.
After achieving required LOD the product is removed from trays, is stored in containers and is proceeded for the next stage of granulation, which is sieving of dry granules to produce uniform size granules.
For Organic Solvents
If organic solvent like Isopropyl Alcohol or IPA is used for granulation, first of all, fresh air should be circulated for 10-15 minutes to remove fumes of IPA from product to outside because IPA is flammable and direct heating may cause an explosion. The tray dryer material of construction should be flameproof to prevent any explosion.
For Water-based Binder
If water is used during wet granulation then after installation of trays in tray dryer and sealing the door the feed temperature is kept hight at the start of the process. Up to 100-degree centigrade temperature can be achieved in tray dryers. The temperature is controlled by the temperature controller installed at the outside of the dryer.
Quality of granules
The granules dried in tray dryer are hard, as compare to granules produced in FBD, so the disintegration time of product dried in tray dryer may b prolonged as compared to same product dried in FBD or fluid bed dryer.
Drying in tray dryer produce less fine powder as compared to the drying process of FBD.
Mottling or colour migration effect is commonly observed during drying of coloured products in a tray dryer.
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