How to dry out wet, sticky or granulated productsPrint
Tray dryers are devices used to dry wet products of chemicals, powders and crude drugs.
The simplest way to explain a tray dryer is a cabinet that has a heater at the bottom or a laboratory oven. The oven has little to no value because control is non-existent for humidity and heat transferred. Fans may be used to force the hot air to circulate thus increasing the heat transfer and reducing its concentration; however, there is still not enough control.
The best type of tray dryers are the ones that have “directed circulation form” as the heated air is directed to the material under a controlled manner. The materials are spread out on top of the tray tiers and dried accordingly. The tiers are usually made of wire mesh or perforated bottoms. The screens are paper lined so that the air circulates across. To shorten cleaning time and prevent contamination, disposable papers are used as tray liners.
Current or new dryers have uniform temperature and the circulating air is maintained by the use of strategically well-placed insulated cabinets with heating coils and fans. Shelves alternate to ensure uniformity of air circulating. The air is reheated before being passed to the next shelf. Air that passes each shelf loses a certain amount of heat thus providing a latent heat of vaporization. This is an excellent way to control heat and humidity.
Drying is commonly used after wet granulation. This is the process in which a tablet mix is turned in to a paste with water and then broken up in to granules. This can increase the consistency and the flowability.
Hot air continuously circulates in tray dryers, forced convection heat removes the moisture from the materials in the tray, and at the same time, moist air is removed partially as well.
- The tray consists of a rectangular chamber and insulated walls.
- The trays are placed inside the heating chamber.
- A minimum of 3 trays can be often seen used in laboratories; however industrial companies may have up to 20 trays.
- Trays can be square or rectangular in shape with 1.2 to 2.4 m in area and loaded from 10 to 100mm deep
- Distance between trays and the material being loaded should be 40mm.
- Trays can be placed on trucks and rolled in and out of chambers.
- Trucks can be arranged in each side of the dryer and fitted with a fan to circulate the air.
- It is cheaper to use electrically heated materials inside than outside.
- Directional vanes are added to direct the air to the right path.
How it Works
- Trays with wet solid materials are placed in the chamber.
- Fresh air is provided through the inlet and passes through the heater to be heated.
- The hot air is circulated through the use of fans (3 to 5 meter/second).
- Turbulent flows can lower the partial vapour pressure in the atmosphere and reduces the air boundary layer thickness.
- The water then evaporates and diffuses from the interior of the solids through capillary action.
- The steps occur in just a single pass of the air. As the contact time is short, the amount of water collected is small.
- The discharged air of about 80-90% will go back through the fans, new air is introduced.
- The moisture collected is discharged through the outlet resulting to a constant temperature and uniform air flow that helps achieve uniform drying of the solid matter.
- For wet granules such as tablets or capsules, the desired moisture content is observed.
- The drying trays are then pulled out of the chamber and taken to the dump station.
- Used for drying sticky matters
- For drying of crystalline or granular matter
- Plastic substances may also be dried
- Wet mass, paste can be dried
- Powders, chemicals can also be dried
- Variety of equipment may also be dried in the tray dryers
- Loading and unloading of materials can be done without any losses.
- Batch drying is used especially for pharmaceutical companies.
- Each batch is treated as separate entity.
- Batch size is small of about 250kg or less per batch (pharmaceutical) and 1000kg (chemical).
- Valuable products are handled more efficiently.
- Labor extensive (loading and unloading)
- Increase in cost
- Time consuming
Variants – tray dryers can be used under the vacuum with direct heating, and done, by using special vacuum trays that are used for heat sensitive products.