Dryers solids handling

Due to the nature of batch operations, transferring and charging of process materials is a common activity. This can entail gas, liquids, and/or solids handling via open equipment. This may include pumping of liquids from drums or dumping of solids from other containers into an open vessel, shoveling material into a dryer, or making temporary connections such as at hose stations. 

Solid materials are often dried or heated using combustion gas exhaust from a fired heater as the material is conveyed through a hot combustion gas zone. Solids handling dryers may take a number of forms, e.g., a rotary kiln. Losses involving dryers usually involve internal fires or explosions. 

There are seven drying steps in the process. This is energy intensive and requires solids to be handled (charging and discharging the dryer), the exposure potential for operators increasing with each solids handling operation. 

CLASSIFICATION OF DRYERS. There is no simple way of classifying drying equipment. Some dryers are continuous, and some operate batchwise some agitate the solids, and some are essentially unagitated. Operation under vacuum may be used to reduce the drying temperature. Some dryers can handle almost any kind of material, while others are severely limited in the type of feed they can accept. 

A preliminary necessity to the selection of a suitable type of dryer and design and sizing there of is the determination of the drying characteristics. Information also required are the solid-handling characteristics, solid moisture equilibrium, and material sensitivity to temperature, together with the limits of temperature attainable with the particular heat source. These will be considered later and in other sections of this book. 

Generally we need different dryers to handle these physically different feed materials. In many cases it is possible to modify the feed (e.g., dilute a thick paste (B) to make it a pumpable slurry (A), or mix a thin paste (B) with dry product to make it a wet particulate solid (C)). 

Drum dryers are shown in Fig. 3.15c. his consists of a heated metal roll. As the roll rotates, a layer of liquid or slurry is dried. The final dry solid is scraped off the roll. The product comes ofiF in flaked form. Drum dryers are suitable for handling slurries or pastes of solids in fine suspension and are limited to low and moderate throughput. 

Agitoted-pon dryers. These may operote otmotpher/coily or under vocuum, ond con handle smell production of neoriy ony form of wet solid, i.e., liquids, slurries, pastes, or gronuior solids... 

Uniform depth of loading in dryers and furnaces handling particulate solids is essential to consistent operation, minimum heating cycles, or control of final moisture. After a tray has been loaded, the bed should be leveled to a uniform depth. Special preform devices, noodle extruders, pelletizers, etc., are employed occasionally for preparing pastes and filter cakes so that screen bottom trays can be used and the advantages of through circulation approached. 

Drying Fhiidized-bed units for drying solids, particularly coal, cement, rock, and limestone, are in general acceptance. Economic-considerations make these units particularly attrac tive when large tonnages of solids are to be handled. Fuel requirements are 3.3 to 4.2 MJ/kg (1500 to 1900 Btu/lb of water removed), and total power for blowers, feeders, etc., is about 0.08 kWh/kg of water removed. The maximum-sized feed is 6 cm (IV2 in) X 0 coal. One of the major advantages of this type of dryer is the close control of conditions so that a predeterminea amount of free moisture may be left with the solids to... 

Dust explosions usually occur in two stages a primary explosion which disturbs deposited dust followed by the second, severe, explosion of the dust thrown into the atmosphere. Any finely divided combustible solid is a potential explosion hazard. Particular care must be taken in the design of dryers, conveyors, cyclones, and storage hoppers for polymers and other combustible products or intermediates. The extensive literature on the hazard and control of dust explosions should be consulted before designing powder handling systems Field (1982), Cross and Farrer (1982), Barton (2001), and Eckhoff (2003). 

Unless employed by one of the specialist equipment manufacturers, the chemical engineer is not normally involved in the detailed design of proprietary equipment. His job will be to select and specify the equipment needed for a particular duty consulting with the vendors to ensure that the equipment supplied is suitable. He may be involved with the vendor s designers in modifying standard equipment for particular applications for example, a standard tunnel dryer designed to handle particulate solids may be adapted to dry synthetic fibres. 

The handling of sticky materials can present difficulties, and one type of dryer which is useful for this type of material is the turbo-dryer. As shown in Figure 16.30, wet solid is fed in a thin layer to the top member of a series of annular shelves each made of a number of segmental plates with slots between them. These shelves rotate and, by means of suitably placed arms, the material is pushed through a slot on to a shelf below. After repeated movements, the solid leaves at the bottom of the dryer. The shelves are heated by a row of steam pipes, and in the centre there are three or more fans which suck the hot air over the material and remove it at the top. 

A safe estimate of power requirement for double drum dryers is approx 0.67 HP/(rpm)(100 sqft of surface). Maintenance can be as high as 10%/yr of the installed cost. Knives last from 1 to 6 months depending on abrasiveness of the slurry. Competitors for drum dryers are solid belt conveyors that can can handle greater... 

A detergent drying plant handles 86,722 lb/day of a slurry containing 52% solids and makes 45,923 lb/day of product containing 2% water. The dryers are two sets of steam-heated double drums, each 3.5ftdia by 10 ft, with a total surface of 440 sqft. Each drum is driven with a 10 HP motor with a variable... 

Frequently, filtration, washing, and drying operations are integrated especially where noxious substances are being handled or when a crystal slurry of an API is being processed to a dry solid in a controlled environment room (CER). In the latter case, all types of combinations of filters and dryers are used (Figure 13). 

For foods and pharmaceutical products, the handling of slurries and solids at industrial scale presents problems where the product is exposed on filters and in dryers, rather than being enclosed within a pipe or vessel as a liquid. Contamination of the crystalline material can occur, and the product itself or the solvent phase it must be removed from might be toxic. Various types of equipment have therefore been developed to enable wet crystalline cakes and dry powders to be handled in a contained manner, using specially designed booths, isolators, and pack off systems to protect the product from its environment (and vice versa for highly potent materials). 

Drying equipment may be classified in several ways. The two most useful classifications are based on (1) the method of transferring heat to the wet solids or (2) the handling characteristics and physical properties of the wet material. The first method of classification reveals differences in dryer design and operation, while the second method is... 

The physical nature of the material to be handled is the primary item for consideration. A slurry will demand a different type of dryer from that required by a coarse crystalline solid, which, in turn, will be different from that required by a sheet material (Table 12-9). 

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