APV Dryer Handbook - Umbc

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18 whether there is any need to re-slurry or dilute, in order to make the feed suitable for spray drying. As a rule of thumb, the limiting viscosity for good atomization is 250 – 300 Centipoise. The economic viability of a drying process ultimately depends on the cost per pound of the dried product. The spray dryer usually has a greater amount of water to remove by thermal methods than other types. For example, to dry 20,000 lbs/hr (9,000 kg/h) of a 30% solids/water slurry to 0.5% moisture, the spray dryer would have an approximate diameter of 30 ft for the evaporation of 14,000 lbs/hr (6,360 Kg/h). If, however, the feed solids were increased to 50% by evaporation, the hourly evaporation rate would decrease to 6,000 lbs/h (2,700 Kg/h) and the chamber diameter would be about 20 ft (6 m), with a corresponding decrease in thermal input and air volume. The former system would, as a result, also require larger fans and product collection systems. The overall thermal efficiency would remain substantially constant at 76% with increasing feed solids. However, the cost per pound of dry product is significantly lower when drying from the higher feed solids. Spray drying does have many advantages, particularly with regard to the final product form. This is especially so where pressing grade materials are required, i.e., in the production of ceramics and dust-free products such as dyestuffs. With the introduction of new geometries and techniques, there has been further development into areas such as foods, and in the production of powders which may be easily reconstituted. These spray dryers usually incorporate one or two fluid beds – static and vibrating – for the final drying and cooling of the agglomerated powder. ROTARY DRYERS Another type of dryer, popular in the chemical and process industries, is the continuous rotary dryer. This machine is generally associated with tonnage product, and as a result of its ability to handle products having a considerable size variation, can be used to dry a wide range of materials. The principle sources of thermal energy are oil, gas and coal. While typical inlet temperatures for direct-fired dryers using these fuels is in the order of 1200°F (650°C), they
DIRECT FIRING PARALLEL FLOW COUNTER-CURRENT FLOW INDIRECT/DIRECT FIRING may be as high as 1600°F (850°C), depending largely on the nature of the product handled. Where feed materials are thermo-sensitive, steam heating from an indirect heat exchanger is also used extensively. These dryers are available in a variety of designs, but in general, can be divided into two main types: those arranged for direct heating and those designed for indirect heating. As seen in Figure 8, certain variants do exist. For example, the direct/indirect dryer simultaneously uses both systems. INDIRECT FIRING CONDUCTION TYPE Where direct heating is used, the products of combustion are in intimate contact with the material to be dried. MATERIALS HEAT FLOW In the case of indirect systems, Figure 8. Typical rotary dryer arrangements the hot gases are arranged to circulate around the dryer shell. Heat transfer is then conducted and radiated through the shell. With the indirect-direct system, hot gases first pass down a central tube, coaxial with the dryer shell, and return through the annular space between the tube and shell. The material being cascaded in the annulus picks up heat from the gases, as well as by conduction from direct contact with the central tube. This design is thermally, highly efficient. While there are a number of proprietary designs employing different systems of air flow, the two main types most commonly used are parallel and counter-current flow. With parallel flow, only high moisture content 19
Page 1: Improving Process Profitability...C
Page 5: I N T R O D U C T I O N The drying
Page 8 and 9: 8 POWDER TYPE D R Y E R S Fine Free
Page 10 and 11: 10 Direct heating is used extensive
Page 12 and 13: 12 corresponding number of conventi
Page 14 and 15: 14 Two particular types are the dou
Page 16 and 17: 16 particular application and the m
Page 20 and 21: 20 material comes into contact with
Page 22 and 23: 22 together since both share simila
Page 24 and 25: 24 circulation. Alternatively, and
Page 26 and 27: 26 Each of the types available are
Page 28 and 29: 28 At this time Pulse Drying techno
Page 30 and 31: 30 E F F I C I E N T E N E R G Y U
Page 32 and 33: 32 DRYER FEED MOISTURE CONTENT (% W
Page 34 and 35: 34 DRYING COLUMN SCREW FEEDER PARAM
Page 36 and 37: 36 way a dryer can be turned down i
Page 38 and 39: 38 3. The wetted particles sink int
Page 40 and 41: 40 TEMPERATURE (°C) 150 100 50 0 4
Page 42 and 43: 42 flow patterns, residence time di
Page 44 and 45: 44 As indicated by this partial lis
Page 46 and 47: 46 FEED HP PUMP COMPRESSED AIR AGGL
Page 48 and 49: 48 S P I N F L A S H D R Y E R S fo
Page 50 and 51: 50 PROCESS DESCRIPTION The Spin Fla
Page 52 and 53: 52 Figure 24 illustrates the very r
Page 54 and 55: 54 The drying chamber would be main
Page 56 and 57: 56 COST BENEFITS In comparison to a
Page 58 and 59: 58 S P R A Y D R Y E R S SPRAY DRYE
Page 60 and 61: 60 inwards towards the centerline o
Page 62 and 63: 62 a high speed by a spinning disc.
Page 64 and 65: 64 internal bowl diameter, radial d
Page 66 and 67: 66 variations, some plants utilize
Page 68 and 69:
68 Condensation deposits can freque
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70 HIGH FAT PRODUCTS If the powder
Page 72 and 73:
72 INLET TEMPERATURE The powder coo
Page 74 and 75:
74 POWDER DENSITY The density of sp
Page 76 and 77:
76 chamber wall, tending to scour a
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Local Office APV Separations Produc
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