Steam flash drying system

Flash dryers utilizing superheated steam as the drying medium often have unique quality and energy advantages over air drying systems. More recently, flash dryers consisting of inert media... 

The pressurized-steam flash dryer originally developed at the Chalmers University of Technology, Gothenburg, Sweden, in the early 1970s is ideal for drying peat as well as pulp, bark, and so on. This dryer is a closed, pressurized system in which the peat is exposed to indirectly heated superheated steam. The dryer consists of transport ducts, heat exchangers, a cyclone, and flans. The superheated steam recirculates at a pressure of 2-6 bar. The primary heating steam is condensed (usually 8-15 bar) on the shell side. 

The pulp web was dried exclusively by contact with steam-heated cans until the mid-1950s. However, air drying (air floater dryers) of pulp is predominant today (e.g., 70% of the U.S. paper industry). Although the application of flash drying in the industry is relatively new, it has found an appreciable market (e.g., 15% of the U.S. paper industry). As the steam-dryer system was introduced to the industry only recently, it is not yet an established drying technique for pulp. 

Vacuum Treatment. Milk can be exposed to a vacuum to remove low boiling substances, eg, onions, garlic, and some silage, which may impart off-flavors to the milk, particularly the fat portion. A three-stage vacuum unit, known as a vacreator, produces pressures of 17, 51—68, and 88—95 kPa (127, 381—508, and 660—711 mm Hg). A continuous vacuum unit in the HTST system may consist of one or two chambers and be heated by Hve steam, with an equivalent release of water by evaporation, or flash steam to carry off the volatiles. If Hve steam is used, it must be cuUnary steam which is produced by heating potable water with an indirect heat exchanger. Dry saturated steam is desired for food processing operations. 

Butyl slurry at 25—35 wt % mbber continuously overflows from the reactor through a transferline to an agitated flash dmm operating at 140—160 kPa (1.4—1.6 atm) and 55—70°C. Steam and hot water are mixed with the slurry in a nozzle as it enters the dmm to vaporize methyl chloride and unreacted monomers that pass overhead to a recovery system. The vapor stream is compressed, dried over alumina, and fractionated to yield a recycle stream of methyl chloride and isobutylene. Pure methyl chloride is recovered for the coinitiator (AlCl ) preparation. In the flash dmm, the polymer agglomerates as a coarse cmmb in water. Metal stearate, eg, aluminum, calcium, or zinc stearate, is added to control the cmmb size. Other additives, such as antioxidants, can also be introduced at this point. The polymer cmmb at 8—12 wt % in water flows from the flash dmm to a stripping vessel operated under high vacuum to... 

All of the solution processes require high efficiency in recovering the solvent. The schematic in Fig. 2 shows how a typical solution polymerization plant is arranged. The most widely used process consists of termination of the polymerization and the addition of antioxidant to the polymer solution. The solution may be treated to remove initiator or catalyst residue and then transferred into an agitated steam-stripping vessel in which unreacted monomer and solvent are flashed off, leaving the rubber as particles (crumb) in water. The water/crumb slurry then is dewatered and dried. The recovered monomer/solvent is recirculated to a series of distillation columns to recover monomer and purify the solvent. As both the anionic and the coordination catalyst systems are highly sensitive to impurities such as water, the purification system is very critical for satisfactory process control. 

When steam drying is used in situations where there is no demand for waste energy in the form of low-pressure steam, mechanical vapor recompression (MVR) can be applied as mentioned earlier. Such a plant has been in operation at Harjedalen, Sweden, since 1988, where peat is dried in a flash-steam dryer. The production of briquettes is based on air-dried peat with 60% moisture and amounts to 300,0001 DS per year with 10% moisture. The dryer unit consists of two identical lines with a capacity of 201 of DS. The dryer in each line has five heat exchangers in series with a tube length of 20 m. The total heat-transfer area is 2700 m. Turbo-type compressors are used with a compression ratio of 1 4.7 from 3 to 14 bar the system pressure is 3.6 bar giving a temperature driving force of 60°C. The total electricity consumption is 270kWh/t DS [12]. 

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