Evaporator-dryers

Table 16.6. Operating costs for a spray dryer evaporating 0.28 kg/s (1 tonne/h) of water140)...

Evaporation is the removal of solvent as a vapor from a solution or slurry. The vapor may or may not be recovered, depending on its value. The end product may be a solid, but the transfer of heat in the evaporator must be to a solution or a suspension of the solid in liquid if the apparatus is not to be classed as a dryer. Evaporators are similar to stills or re-boilers of distillation columns, except that no attempt is made to separate components of the vapor. t 5... 

A few types of dryers evaporate solutions and slurries entirely to dryness by thermal means. Typical examples are spray dryers, thin-film dryers, and drum... 

When the droplets of the spray come into contact with the drying air, evaporation takes place from the droplet surface. Increase in the inlet temperature increases the dryer evaporative capacity at constant air rate. Higher inlet temperatures generally mean a more economic dryer operation. Increased temperature also may produce increased coating thickness due to a more randomized... 

Droplet flow examples absorbers, atomisers, combustors, cryogenic pumping, dryers, evaporation, gas cooling and scrubbers. 

Initial Air Final Air Dryer Evaporation Residence or Unit... 

Aluminium alloys have an excellent resistance to the action of reagents used for the fabrication of cellulose acetate acetic anhydride, concentrated acetic acid, butyric anhydride, and cotton. Many pieces of equipment in various aluminium alloys are used in production units for cellulose esters 3003, 5052, A-S13 (44100), etc., for storage tanks, reaction vessels, dryers, evaporators, and valves. 

As with distillation and absorption, when evaporators and dryers are chosen, no attempt should be made to carry out any optimization at this stage in the design. 

The heat input to diyers is to a gas and as such takes place over a range of temperatures. Moreover, the gas is heated to a temperature higher than the boiling point of the liquid to be evaporated. The exhaust gases from the dryer will be at a lower temperature than the inlet, but again, the heat available in the exhaust will be available over a range of temperatures. The thermal characteristics of dryers tend to be design-specific and quite difierent in nature from both distillation and evaporation. 

It was noted earlier that dryers are quite difierent in character from both distillation and evaporation. However, heat is still taken in at a high temperature to be rejected in the dryer exhaust. The appropriate placement principle as applied to distillation columns and evaporators also applies to dryers. The plus/minus principle from Chap. 12 provides a general tool that can be used to understand the integration of dryers in the overall process context. If the designer has the freedom to manipulate drying temperature and gas flow rates, then these can be changed in accordance with the plus/minus principle in order to reduce overall utility costs. 

Like distillation, the appropriate placement of evaporators and dryers is that they should be above the pinch, below the pinch, but not across the pinch. The grand composite curve can be used to assess appropriate placement quantitatively. 

Dryers are different in characteristic from distillation columns and evaporators in that the heat is added and rejected over a large range of temperature. Changes to dryer design can be directed by the plus/minus principle. 

Distillation columns, evaporators, and dryers should be above the... 

Fig. 3. Typical nitric acid oxidation process. A, reactor B, optional cleanup reactor C, bleacher D, NO absorber E, concentrating stUl F, crude crystallizer G, centrifuge or filter H, refined crystallizer I, centrifuge or filter , dryer K, purge evaporator L, purge crystallizer M, centrifuge or filter N,...

Wet/dry process. Lime slurry absorbs SO2 in vertical spray dryer forming CaSO —CaS, H2O evaporated before droplets reach... 

To produce a spandex fiber by reaction spinning, a 1000—3500 molecular weight polyester or polyether glycol reacts with a diisocyanate at a molar ratio of about 1 2. The viscosity of this isocyanate-terrninated prepolymer may be adjusted by adding small amounts of an inert solvent, and then extmded into a coagulating bath that contains a diamine so that filament and polymer formation occur simultaneously. Reactions are completed as the filaments are cured and solvent evaporated on a belt dryer. After appHcation of a finish, the fibers are wound on tubes or bobbins and rewound if necessary to reduce interfiber cohesion. 

Fig. 3. Discontinuous deoiling of soy lecithin. 1, Acetone storage tank 2, lecithin storage tank 3, mixer 4, separation tank 5, filter/decanter 6, dryer 7, classifier 8, oil misceUa tank 9, evaporator 10, oil extract tank 11, condenser and 12, acetone storage tank.

Pig. 4. Batch process for producing phosphatidylcholine fractions. 1, Ethanol storage tank 2, deoiled lecithin 3, solubiHzer 4, blender 5, film-type evaporator 6, ethanol-insoluble fraction 7, ethanol-soluble fraction 8, aluminum oxide 9, mixer 10, decanter 11, dryer 12, aluminum oxide removal 13, phosphatidylcholine solution 14, circulating evaporator 15, cooler 16, dryer and 17, phosphatidylcholine. 

Fig. 5. Continuous process for producing phosphatidylcholine. 1, Lecithin 2, ethanol 3, blender 4, diffuser 5, thin-type evaporator 6, ethanol-insoluble fraction 7, heat exchanger 8, chromatography column (Si02) 9, prestream 10 and 12, phosphatidylcholine solution 11, circulating evaporator 13, dryer ...

Drum Drying. The dmm or roHer dryers used for milk operate on the same principles as for other products. A thin layer or film of product is dried over an internally steam-heated dmm with steam pressures up to 620 kPa (90 psi) and 149°C. Approximately 1.2—1.3 kg of steam ate requited per kilogram of water evaporated. The dry film produced on the roHer is scraped from the surface, moved from the dryer by conveyor, and pulverized, sized, cooled, and put iato a container. 

The operating variables for a dmm or roHer dryer iaclude coadeasatioa of incoming product ia an evaporator, temperature of incoming product, steam pressure (temperature) ia dmm, speed of dmm, and height of product over dmm. The capacity of the dryer is iacreased by increa sing the steam pressure, the temperature of the milk feed, the height of milk over the dmms, the gap between dmms (double), and the speed of rotation of the dmms. Increasing the capacity is limited by the effect on the product quaHty. 

Sheet Drying. At a water content of ca 1.2—1.9 parts of water per part of fiber, additional water removal by mechanical means is not feasible and evaporative drying must be employed. This is at best an efficient but cosdy process and often is the production botdeneck of papermaking. The dryer section most commonly consists of a series of steam-heated cylinders. Alternate sides of the wet paper are exposed to the hot surface as the sheet passes from cylinder to cylinder. In most cases, except for heavy board, the sheet is held closely against the surface of the dryers by fabrics of carefuUy controUed permeabiHty to steam and air. Heat is transferred from the hot cylinder to the wet sheet, and water evaporates. The water vapor is removed by way of elaborate air systems. Most dryer sections are covered with hoods for coUection and handling of the air, and heat recovery is practiced in cold climates. The final moisture content of the dry sheet usually is 4—10 wt %. 

When the dryer is seen as a heat exchanger, the obvious perspective is to cut down on the enthalpy of the air purged with the evaporated water. Minimum enthalpy is achieved by using the minimum amount of air and cooling as low as possible. A simple heat balance shows that for a given heat input, minimum air means a high inlet temperature. However, this often presents problems with heat-sensitive material and sometimes with materials of constmction, heat source, or other process needs. AH can be countered somewhat by exhaust-air recirculation. 

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