Tray dryers for drying wet, sticky or granulated products

A range of technologies are used for food drying which include tray and tunnel dryers, spray, roller and freeze dryers. Tray dryers are quite suitable for drying wet chemicals, powders and crude drugs.

A simple tray dryer is a box-shaped cabinet in which a series of trays are inserted on shelves one on top of the other. Unlike an oven which perhaps has a heater in the base of the cabinet or walls, heat is supplied to a tray dryer by circulating air that has been heated either electrically or by some other means. One or more fans circulate the hot air over the trays.

The best tray dryers are those in which the air flow to the trays can be controlled. The materials to be dried are spread out on top of the trays which may be lined with paper. To shorten cleaning time and prevent contamination, disposable papers are used as tray liners. The trays are often supported on tiers made of wire mesh or perforated steel.

In the latest tray dryers, uniform temperature is achieved throughout a well-insulated cabinet by the use of strategically placed deflectors, which direct the air from one tray to pass over the tray above in the opposite direction. Air flowing in alternate directions over the trays ensures uniformity of flow and helps to keep the temperature uniform. The air may also be reheated before it passes from one shelf to the next. Air that passes each shelf loses a certain amount of heat (the latent heat of vaporisation of the water that is expelled from the solid material).

Hot air therefore continuously circulates in a tray dryer, and the heat of the air removes the moisture from the material in the trays by forced convection. At the same time, moist air is expelled from the cabinet to vent.

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. Granulation can increase the consistency and the flowability of material in the tableting process.

Construction of a tray dryer:

There are several variants of tray dryer, ranging from simple manual systems ideal for batch processes through semi-automatic and automatic systems for processing large quantities. The basic principles are the same in each case, as follows.

The dryer consists of a heated box-shaped cabinet into which a number of trays can be loaded. The trays are rectangular or square in shape, typically between 1.2 and 2.4 m² in area and up to 100 mm deep.
A minimum of 3 trays can be often seen used in laboratory dryers; however industrial sized equipment may have up to 20 trays.
The distance between trays and the material being loaded should be 40mm.
Depending on the design of the dryer, several trays may be stacked on a ‘truck’ or trolley that provides the right separation between trays and which can be rolled in and out of the dryer. Alternatively trays can be inserted manually into runners in the dryer one at a time.
Directional vanes are located inside the dryer to direct the air to circulate through the trays. In one design, the hot air passes over the bottom tray first, before passing over the second, and so on up to the top of the cabinet. In simple designs, the lower trays can therefore dry out first.

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 by fans (typically at a rate of 3 to 5 m³/second).
A turbulent flow or air can lower the vapour pressure of water in the air and reduce the air boundary layer thickness, thereby enhancing the drying effect of the hot air.
Water diffuses from the interior of the porous material and evaporates from the surface of the solid matter.
As the contact time is short (air flow rate is large), the amount of water evaporated in one pass through of air is relatively small.
Some 80-90% of the air that has absorbed moisture from the material is recirculated to the inlet of the fans to mix with fresh dry air. Additional moisture is taken up by the air in the second and subsequent passes of air over the material.
The moisture collected in the air is discharged through the outlet of the dryer. The high and uniform air flow ensures that there is a constant temperature within the dryer. These features help to achieve uniform drying of the solid matter.
For wet granules such as tablets or capsules, the desired moisture content may be estimated by measuring the water that is discharged through the air outlet.
The drying trays are then pulled out of the chamber and taken to the dump station.

Applications

Used for drying sticky materials
Can be used for drying of crystalline or granular material
Wet mass, paste, powders, and chemicals can all be dried
A variety of small equipment may also be dried in tray dryers

Advantages

Loading and unloading of materials can be done without any losses.
Batch drying is widely used by pharmaceutical companies.
Each batch is treated as separate entity.
Batch size can be small e.g., 250kg or less per batch (pharmaceutical) and 1000kg (chemical).
Valuable products can be handled efficiently without loss, especially using the truck trays.

Disadvantages

Simple tray dryers can be labour intensive (i.e., the loading and unloading)
For small quantities drying in simple oven may be more cost effective.
Manually loading and unloading can be time consuming

Vacuum Tray Dryer

A vacuum tray dryer is used mainly for drying of high grade, temperature and oxygen sensitive products. It is suitable for drying hygroscopic substances, which are dried to a very low residual moisture content. Vacuum drying cabinets are often the only means of drying lumpy, glutinous products or products of low porosity.

As with the normal tray dryer, a vacuum dryer is box-shaped into which trays are loaded and unloaded via a door that has a vacuum seal around the edges. Inside are several heating plates mounted one above the other on above which the trays are placed.
Since the trays sit directly on the heating plates, the surfaces of each should be as smooth as possible to allow good heat transfer between the heating plates and trays.

Hot water, steam or oil flow inside the heating plates plates to provide the heat required for drying.
The cabinet walls may also be heated by the circulating fluid to prevent condensation of water. are indirectly preheated by the heating plates.

When the dryer is loaded with materials it is first heated at atmospheric pressure. Only after all individual product trays reach the same temperature is the cabinet is evacuated and drying commenced. The initial heating phase is necessary to enable a uniform temperature and drying conditions to be established throughout the cabinet. During the main drying phase the vacuum is in the range of 40 to 80 mbar abs. This may be reduced to a few mbar in the final stages of drying.

Heating temperatures within a vacuum dryer can be much higher than an air dryer, and are often in the range between 800°C and 1100°C. Some systems allow automatic programming of both temperature and vacuum profiles. On completion of the drying, the product can be cooled by circulating of cooling water through the heating plates.