Several methods of heat transfer are used in the dryers. Where all the heat for vaporizing the solvent is supplied by direct contact with hot gases and heat transfer by conduction from contact with hot boundaries or by radiation from solid walls is negligible, the process is called adiabatic, or direct drying.
In indirect or nonadiabatic drying, the heat is transferred by conduction from a hot surface, first to the material surface and then into the bulk. This chapter discusses only indirect drying.
The problem of equipment selection can be very complex; different factors must be taken into consideration, for example, working capacity, ease of cleaning, hazardous material, dryer location and capital cost (see Fig. 2).
The first step refers to the choice of continuous versus batch drying and depends on the nature of the equipment preceding and following the dryer as well as on the production capacity required. In general, only batch dryers will be considered in the following.
Batch dryers include:
• Fluidized-bed dryers. These may be used when the average particle diameter is < 0.1 mm. (The equipment required to handle smaller particles may be too large to be feasible.) Inert gas may be used if there is the possibility of explosion of either the vapor or dust in the air.
It is easy to carry out tests in a small fluid-bed dryer.
• Shelf dryers. Theys are usually employed ofr small capacities and when the solvent doesn't present particular problems.
• Vacuum dryers. These are the most-used batch dryers.
Vacuum dryers are usually considered when:
• Low solids temperature (< 40°C) must be maintained to prevent heat causing damage to the product or changing its nature
• When toxic or valuable solvent recovery is required
• When air combines with the product, during heating, causing oxidation or an explosive condition
Before starting work on selecting a dryer, it is good practice to collect all the data outlined in Table 1.
In vacuum drying, the objective is to create a temperature difference or "driving force" between the heated jacket and the material to be dried. To accomplish this with a low jacket temperature, it becomes necessary to reduce the internal pressure of the dryer to remove the liquid/solvent at a lower vapor pressure. Decreasing the pressure creates large vapor volumes. Economic considerations arising from concerns of leakage, ability to condense the solvent, size of vapor line and vacuum pump, affect the selection of the operating pressure. Materials handled in vacuum dryers may range from slurries to solid shapes and from granular, crystalline product to fibrous solids. The characteristics of each type of vacuum dryer is discussed below to help make a proper choice.
Vertical Vacuum Pan Dryers. The agitated vertical dryer (Fig. 3.) has been designed for drying many different products which may come from centrifuges or filters. Generally, the body is formed by a vertical cylindrical casing with a flat bottom flanged to the top cover head. The unit is fully heated by an outside half-pipe jacket welded on the cylindrical wall, the bottom and the top head.
Table 1. Data To Be Assessed Before Attempting Drying Selection
- Production capacity (kg/h)
- Initial moisture content
- Particle size distribution
- Drying curve
- Maximum allowable product temperature
- Explosion characteristics (vapor/air and dust/air)
- Toxicological properties
- Experience already gained
- Moisture isotherms
- Contamination by the drying gas
- Corrosion aspects
- Physical data of the relevant materials_
The dished head is provided with the appropriate nozzles for feed inlet, instrumentation, heating or cooling medium, vapor outlet, lamp and rupture disk. The dished head and the cylindrical body are separated by means of a hydraulic system to provide easy access to the vessel for inspection or cleaning. A high powered agitator having two crossed arms located at different heights, is designed for processing products that go through a viscous transition phase (high viscosity). The same dryer can be provided with a different agitator, high speed, which is applicable for low to medium viscous products. The agitator can be totally heated. To eliminate possible agglomerates or lumps formed during the drying process, and discharge problems, a chopper device is supplied. The shaft sealing can be either a stuffing-box or a mechanical seal. A bottom discharge valve for the dry product is hydraulically driven and located in a closed hatch. The geometrical volume of these vertical dryers ranges from a few liters to approximately 500 liters (see Table 3).
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