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Tel: 01142 517702

Dryers

Drying: is a mass transfer process resulting in the removal of water moisture or moisture from another solvent, by evaporation from a solid, semi-solid or liquid (hereafter product) to end in a solid state. To achieve this, there must be a source of heat, and a sink of the vapour thus produced.

In the most common case, a gas stream, e.g. air, applies the heat by convection and carries away the vapour as humidity. Vacuum drying, where heat is supplied by contact conduction or radiation (or microwaves) while the produced vapour is removed by the vacuum system. Another indirect technique is drum drying where a heated surface is used to provide the energy and aspirators draw the vapour outside the room.

Freeze drying or lyophilization is a drying method where the solvent is frozen prior to drying and is the sublimed, i.e. passed to the gas phase directly from the solid phase, below the melting point of the solvent. Freeze drying is often carried out under high vacuum to allow drying to proceed at a reasonable rate. This process avoids collapse of the solid structure, leading to a low density, highly porous product, able to regain the solvent quickly. In biological materials or foods, freeze drying is regarded as one of the best if not the best method to retain the initial properties. It was first used industrially to produce dehydrated vaccines, and to bring dehydrated blood to assist war casualties. Now freeze drying is increasingly used to preserve some foods, especially for backpackers going to remote areas. The method may keep protein quality intact, the same as the activity of vitamins and bioactive compounds.

In turn, the mechanical extraction of the solvent, e.g. water, by centrifugation is not considered “drying”. The ubiquitous term dehydration may mean drying of water-containing products as foods, but its meaning is more vague, as it is also applied for water removal by osmotic drive from a salt or sugar solution. In medicine, dehydration is the situation by which a person loses water by respiration, sweating and evaporation and does not incorporate, for whatever reason, the “make-up” water required to keep the normal physiological behaviour of the body. Drying may be either a natural or an international process.

The process of extreme drying is called desiccation.

Methods of Drying:

Application of heated air (convective or direct drying). Air heating reduces air relative humidity, which is the driving force for drying. Beside, higher temperature speed up diffusion of water inside the solids, so drying is faster. However, product quality considerations limit the applicable rise to air temperature. Too hot air almost completely dehydrates the solid surface, so internal pores shrink and almost close, leading to crust formation or “case hardening”.

Indirect or contact drying (heating through a hot wall), as drum drying, vacuum drying.

Dielectric drying (radiofrequency or microwaves being absorbed inside the material). It is the focus of intense research nowadays. It may be used to assist air drying or vacuum drying.

Freeze drying is increasingly applied to dry foods, beyond its already classical pharmaceutical or medical applications. It keeps biological properties of proteins, and retains vitamins and bioactive compounds. Pressure may be reduced by a vacuum pump. If using a vacuum pump, the vapour produced by sublimation is removed from the system by converting it into ice in a condenser, operating at very lower temperatures, outside the freeze drying chamber.

Supercritical drying (superheated steam drying) incolves steam drying of products containing water. Strange as it seems, this is possible because the water in the product is boiled off and joined with the drying medium increasing its flow. It is usually employed in closed circuit and allows a proportion of latent heat to be recovered by recompression, a feature which is not possible with conventional air drying, for instance.

Natural air drying takes place when materials are dried with unheated forced air, taking advantage of its natural drying potential. The process is slow and weather-dependent, so a wise strategy “fan off-fan on” must be devised considering the following conditions: Air temperature, relative humidity and moisture content and temperature of the material being dried. Grains are increasingly dried with this technique and the total time (including fan off and on periods) may last from one week to various months, if a winter rest can be tolerated in cold areas.

Rotary Dryer: The rotary dryer is a type of industrial dryer employed to reduce or minimise the liquid moisture content of the material it is handling by bringing it into direct contact with a heated gas. The dryer is made up of a large, rotating cylindrical tube, usually supported by concrete columns or steel beams. The dryer slopes slightly so that the discharge end is lower than the material feed end in order to convey the material through the dryer under gravity. Material to be dried enters the dryer, and as the dryer rotates, the material is lifted up by a series of internal fins lining the inner wall of the dryer. When the material gets high enough to roll back off the fins. It falls back down to the bottom of the dryer, passing through the hot gas stream as it falls. The gas stream can either be moving toward the discharge end from the feed end (known as co-current flow), or toward the feed end from the discharge end (known as counter-current flow). The gas stream can be made up of mixture of air and combustion gases from a burner, in which case the dryer is called a direct heated dryer. Alternatively, the gas stream may consist of air or another (sometimes inert) gas that is preheated. When the gas stream is preheated by some means where burner combustion gases do not enter the dryer, the dryer known as an indirect-heated type. Often, indirect heated dryers are used when product contamination is a concern.

A thermal screw desorber typically consists of a series of 1-4 auguers. The auger system conveys, mixes, and heats contaminated soils to volatilise moisture and organic contaminants into a purge gas stream. Augers can be arranged in series to increase the soil residence time, or they can be configured in parallel to increase throughput capacity. Most thermal screw systems circulate a hot heat-transfer oil through the hollow flights of the auger and return the hot oil through the shaft to the heat transfer fluid heating system. The heated oil is also circulated through the jacketed trough in which each auger rotates. Thermal screws can also be steam-heated. Systems heated with oil can achieve soil temperatures of up to 500 degrees F, and steam-heated systems can heat soil to approximately 350 degrees F.

Most of the gas generated during heating of the heat-transfer oil does not come into contact the waste material and can be discharge directly to the atmosphere without emission controls. The remainder of the flue gas maintains the thermal screw purge gas exit temperature above 300 degree F. This ensures that volatilised organics and moisture do not condense. In addition, the recycled flue gas has a low oxygen content (less then 2% by volume) which minimizes oxidation of the organics and reduces the explosion hazard. If pre-treatment analytical data indicates a high organic content (greater than 4%), use of a thermal screw is recommended. After the treated soil exits the thermal screw, water is sprayed on the soil for cooling and dust control. Thermal screws are available with soil treatment capacities ranging from 3-15 tons per hour.
Since thermal screws are indirectly heated, the column of purge gas from the primary thermal treatment unit less than one half of the volume from a directly-heated system with a equivalent soil processing capacity. Therefore off gas treatment systems consist of relatively small unit operations that are well suited to mobile applications. Indirect heating also allows thermal screws to process materials with high organic contents since the recycled flue gas is inert, thereby reducing the explosion hazard.

A Conveyor Furnace uses a flexible metal belt to convey soil through the primary heating chamber. A one-inch-deep layer of soil is spread evenly over the belt. As the belt moves through the system, soil agitators lift the belt and turn the soil to enhance heat transfer and volatilisation of organics. The conveyor furnace can heat soils to temperatures from 300 to 800 degrees F. At the higher temperature range, the conveyor furnace is more effective in treating some heavier petroleum hydrocarbons tha n are oil-or-stem heated thermal screws, asphalt plant aggregated dryers, and cardon steel rotary dryers. After the treated soil exits the conveyor furnace, it is sprayed with water for cooling and dust control. The system is mobile and can treat 5 to 10 tons of soil per hour.

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