Dryers fluidized-bed

Closed-system dryers, spray dryers, fluidized bed dryers Batch or continuous tray, vibrated bed dryers Use dielectric heating (microwave or radio frequency) over part of drying... 

Dryers suitable for drying of the biomass include batch through-circulation dryer (perforated floor bin dryer), PMB or continuous through-circulation dryer, direct rotary dryer (rotary cascade dryer), indirect rotary dryer (steam-tube rotary dryer), fluidized bed dryer (pressurized steam fluid bed dryer), and pneumatic conveying dryer (flash dryer) (Amos 1998 Brammer and Bridgwater, 1999 Wimmerstedt, 2006 Roos, 2008). Drying media can be flue gas, hot air, or superheated steam. 

Chapter 9 summarizes key information about equipment to increase or change the size of drops, bubbles and particles demisters, coalescers, flocculators, spray dryers, fluidized beds, agglomeration, pelletizing, extrusion, flakers, prilling and coating. 

Another important class of dryer is the fluidized-bed dryers. Some designs combine spray and fluidized-bed dryers. Choice between dryers is usually based on practicalities such as the materials handling characteristics, product decomposition, product physical form (e.g., if a porous granular material is required), etc. Also, dryer efficiency can be used to compare the performance of different dryer designs. This is usually defined as follows -. 

FIOR Process. In the FIOR process, shown in Figure 5, sized iron ore fines (0.04—12 mm) are dried in a gas-fired rotary dryer. A skip hoist dehvers the dry fines to lock hoppers for pressurizing. The fines pass through four fluidized-bed reactors in series. Reactor 1 preheats the ore to 760°C in a nonreducing atmosphere. Reactors 2, 3, and 4 reduce the ore at 690—780°C. At higher (ca 810°C) temperatures there is a tendency for the beds to defluidize as a result of sticking or hogging of the reduced material. 

The common types of dryers are rotary, hearth, flash (spray), and fluidized beds (10). Hot gases are used invariably to remove moisture. The gas flow can be either cocurrent or countercurrent to the flow of soHds, the former tends to be more efficient. In the hearths, the gas flow is countercurrent as the soHds are raked down from one hearth to the next below. Flash dryers are very rapid because the soHds are exposed only briefly to the hot gases. Fluidized-bed dryers, which use hot gases to suspend the soHds, are rapid and efficient, but require elaborate dust coHection systems. These are preferred when fine soHds are involved, and are used commonly for drying fine coal. Indirect-fired dryers are used when the soHds are heat sensitive or combustible. 

Suspended Particle Techniques. In these methods of size enlargement, granular soHds are produced direcdy from a Hquid or semiliquid phase by dispersion in a gas to allow solidification through heat and/or mass transfer. The feed Hquid, which may be a solution, gel, paste, emulsion, slurry, or melt, must be pumpable and dispersible. Equipment used includes spray dryers, prilling towers, spouted and fluidized beds, and pneumatic conveying dryers, all of which are amenable to continuous, automated, large-scale operation. Because attrition and fines carryover are common problems with this technique, provision must be made for recovery and recycling. 

Significant improvements over the PPG process involve reaction of lime with concentrated HOCl (50%) and drying of the resultant slurry in a spray grainer or fluidized-bed spray dryer to produce hydrated product with av CI2 as high as 82% (126,127,129,210,211). 

Containment (Explosion-Pressure-Resistant Design for Maximum Explosion Overpressure) An explosion-resistant construction is understood to mean the possibihty of designing vessels and equipment for the full maximum explosion ove (pressure, which is generally of the order P = 9 bar. The explosion-resistant vessel can then be designed as explosion pressure resistant or explosion pressure shock resistant. This protective measure is generally employed when small vessel volumes need to be protected, such as small filter units, fluidized-bed dryers, cyclones, rotaiy valves, or mill housings. 

Ammonium sulfate is produced as a caprolactam by-product from the petrochemical industry, as a coke by-product, and synthetically through reaction of ammonia with sulfuric acid. Only the third process is covered in our discussion. The reaction between anunonia and sulfuric acid produces an ammonium sulfate solution that is continuously circulated through an evaporator to thicken the solution and to produce ammonium sulfate crystals. The crystals are separated from the liquor in a centrifuge, and the liquor is returned to the evaporator. The crystals are fed either to a fluidized bed or to a rotary drum dryer and are screened before bagging or bulk loading. 

Particulate matter is the principal air pollutant emitted from ammonium sulfate plants. Most of the particulates are found in the gaseous exhaust of the dryers. Uncontrolled discharges of particulates may be of the order of 23 kg/t from rotary dryers and 109 kg/t from fluidized bed dryers. Ammonia storage tanks can release ammonia, and there may be fugitive losses of ammonia from process equipment. 

Fluidized bed dryers work best on particles of a few tenths of a mm dia, but up to 4 mm dia have been processed. Gas velocities of twice the minimum fluidization velocity are a safe prescription. In continuous operation, drying times of 1-2 min are enough, but batch drying of some pharmaceutical products employs drying times of 2-3 hr. 

The Subpart O standards apply to units that treat or destroy hazardous waste and which meet the definition of an incinerator. An incinerator is any enclosed device that uses controlled flame combustion and does not meet the criteria for classification as a boiler, sludge dryer, carbon regeneration unit, or industrial furnace. Typical incinerators1 2 3 include rotary kilns, liquid injectors, fixed hearth units, and fluidized bed incinerators (Table 23.1). The definition of an incinerator also includes units that meet the definition of an infrared incinerator or plasma arc incinerator. An infrared incinerator is any enclosed device that uses electric-powered resistance as a source of heat and which is not listed as an industrial furnace. A plasma arc incinerator is any enclosed device that uses a high-intensity electrical discharge as a source of heat and which is not listed as an industrial furnace. 

Another important class of dryers is the fluidized-bed dryer. 

Some designs combine spray and fluidized-bed dryers. 

The output of such high-speed dryers is limited by the increasing density of the water vapor flow. The grains of the product are floating in the vapor stream as in a fluidized bed, and the smallest particles are carried along with the vapor to the condenser. Even if only 1 % of the dried product is carried away, it sumps up to 10 kg per day if the throughput is 1000 kg per day. In 4 weeks, this totals to 280 kg or 1 m3 of coffee powder. To remove this out from the vapor stream very large filters have to be used in order to minimize the pressure drop in the filters. 

See also Fluidized-bed entries Fluid-bed direct oxidation process, 10 656 Fluid-bed dryers, 9 122-123, 130-131 two-stage, 9 125 Fluid-bed roasters, 16 141 Fluid catalytic cracking (FCC), 11 678-699, 700-734 18 651, 653 20 777 24 257, 271. See also FCC entries Fluidized-bed catalytic cracking (FCC) clean fuels production and, 11 686-689 defined, 11 700... 

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