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东北大学毕业设计（论文） 附录（外文翻译）
Table 2 Comparison of conventional versus emerging drying technologies
Energy (heat source) Conventional Natural gas, Oil Biomass, Solar/wind Electricity (MW/RF) Waste heat Fossil fuel combustion Mode of heat transfer Conventional Convection (>85%) Conduction Hybrid Radiation Microwave/radio frequency Emerging trends No change yet. Renewal energy sources when fossil fuel becomes very expensive Pulse combustion modes Non-adiabatic (<1%) dryers Periodic or on/off heat input Superheated steam Hot air + superheated steam mixture or two-stage Drying dryers) medium (convective Hot air Flue gases Number of stages One—most common Two or Multi-staging with different dryer three—same dryer type types Fuzzy logic, Model based control, Artificial neural nets Dryer control Manual Automatic The development of in-place cleanable bag filters makes it possible to retain the particulates within the dryer chamber; this is achieved by mounting the filter elements in the roof of the spray dryer chamber. No external cyclones are needed in this case. This technology, coupled with the popular fluidized-spray dryer featuring a fluid bed dryer integrated into the base of the spray dryer chamber, allows efficient production of dust-free agglomerated or granulated products at substantially lower product temperatures than those found in conventional spray dryers.
For drying granular or particulate solids, the most common dryers in use today are cascading rotary dryers with or without internal steam tubes, conveyor dryers, and continuous tray dryers (e.g., turbo or plate dryers), which must compete with fluidized-bed dryers (with or without internal exchangers) and vibrated bed dryers, among others. At least 20 variants exist of the fluidized-bed dryer alone. For larger particles, a spouted bed dryer is preferable to the conventional fluidized-bed dryer. For difficult-to-fluidize, sticky particles or feed stocks with a wide particle size distribution, the vibrated bed dryer offers advantages over the conventional fluid bed because it allows use of low drying air velocities while mechanical
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东北大学毕业设计（论文） 附录（外文翻译）
vibration assists in pseudo-fluidizing the solids. Recently, the pulsed fluidization technique has found some interesting applications similar to those for vibrated fluid beds.
Fluidized-bed dryers have been operated successfully using superheated steam as the drying medium both at near atmospheric pressures (e.g., drying of pulverized lignite) and at elevated pressures (3–5 bar for drying of beet pulp). In addition to eliminating potential fire and explosion hazards, use of superheated steam permits utilization of the exhaust steam by condensation, reheating, or compression. Of course, such steam is often contaminated and must be cleaned before re-utilization, depending on the application. Net energy consumption in superheated steam dryers may be as low as 700–1000 kJ/kg water evaporated, which is 5–10 times lower than many conventional convective air drying systems consume. Mujumdar (1990) discusses the principles, practice and potential of the rapidly emerging superheated steam drying technologies in various industrial sectors, principally for drying paper, wood, some processed foods, sludge, etc. Numerous relevant references are cited as well. It is commonly recognized as the drying technology of the future. It is known that low-temperature superheated steam dryers are feasible for drying very heat-sensitive materials like fruits and vegetables, silk cocoons, etc. SEM pictures of steam-dried products clearly show the quality advantages such products can offer. Easy to re-hydrate dried fruits such as strawberries, pears, apple, have been produced by low-temperature steam drying in Argentina. Vacuum steam drying of wood is probably the best technology for drying of wood. It is more commonly found in the developing countries (e.g., Malaysia, India, Thailand, etc.) rather than in developed countries of North America which have invested heavily in the hot air kiln dryers for wood, which cannot easily be switched to steam operation.
The use of volumetric heating by microwave (MW) and radio frequency (RF) fields has yet to make major inroads in the chemical industry. It is well established that MW/RF-assisted drying is faster, but the energy costs are also significantly higher and scale-up for large production capacities is much more difficult. Such dryers are expected to find some niche applications. RF drying under vacuum has been applied successfully on a commercial scale by a Canadian company for drying of timber and thick veneer with future applications anticipated for drying of chemicals, polymers, and foods. An RF dryer, in conjunction with impingement with hot air jets, is already a commercial process for drying of coated paper. The efficiency of conversion of line power into electromagnetic energy and the cost of electricity are major impediments in commercializing this technology. Hybrid drying concepts will certainly make rapid inroads in industrial drying in the coming decade. These consist of intelligent combinations of well-established drying technologies and hence involve
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东北大学毕业设计（论文） 附录（外文翻译）
less risk. They can combine advantages as well as limitations of each individual technology. Hence, care must be exercised in designing hybrid dryers.
Some of the newer drying technologies utilize newly developed gas–solid contactors as dryers for particulates, e.g., impinging streams, rotating spouted beds, pulsed-fluid beds, etc. In batch drying, one can take advantage of intermittent multi-mode heat input to optimize both the energy consumption and product quality. Intermittencies may be in the form of changes in flow regimes (packed versus fluidized), drying air conditions (air velocity, temperature, humidity, etc), operating pressure (atmospheric, above or below atmospheric), and/or changes in modes of heat input (convection, conduction, radiation or microwave/RF applied sequentially or jointly). It has been shown by many investigators that intermittent operation enhances energy efficiency as well as product quality for heat-sensitive materials. When several parameters of the process are altered with time then, we have multiple intermittencies. This is a field that has not been explored in depth yet, although this can be done quite easily. Many of the traditional dryers, which we assume to be operating under continuous heat input, in fact operate intermittently, e.g., the multi-cylinder paper machine. On the other hand, all batch dryers requiring very long-dwell times in the dryer are often dried under time-varying drying schedules, e.g., freeze dryers, wood drying kilns, etc.
The list in Table 3 is very short and included only for illustrative purposes. The proceedings volumes of IDS series and several other drying conferences (e.g.ADC,NDC, IADC, etc.) and the archival journal Drying Technology (www.dekker.com) provide rich sources for new ideas for further R&D. Searches on the Internet are also invaluable in identifying new technologies and new research challenges. The whole field is far from maturity; it is still in a state of rapid flux.
Table 3 Some R&D problems in selected drying equipment Dryer type Rotary (direct/indirect) Nature of R&D problems Prediction of particle motion/residence time distribution for poly-disperse solids including effect of cohesion, heat/mass transfer rates; effect of flight design, internal heat exchangers; effect of hold-up; effect of hot air injection into particle bed, noncircular shape of dryer shell, etc. - 56 -
东北大学毕业设计（论文） 附录（外文翻译）
Fluid bed (direct/indirect)dryers Effect of particle wetness/poly-dispersity on hydrodynamics, agglomeration, heat/mass transfer rates, etc. Design of internal heat exchangers. Effects of agitation, vibration, etc. Classification of particle types according to fluidization regime including effect of particle wetness/stickiness. Math models. Effect of agitation, vibration, pulsation, acoustic radiation, etc. Flash dryers Detailed discrete particle modeling including effects of agglomeration and attrition, effects of geometry, use of pulse combustion exhaust, superheated steam, internal heat exchanger surfaces, variable cross section ducts, hot air injection at various axial locations, CFD models. Drum dryers Batch dryers Heat transfer to thin films of suspensions including effects of crystallization, boiling, etc. enhancement of drying rate by radiant heat or jet impingement Effects of intermittent heat input using different modes of heat transfer; cyclic pressure swings; variable gas flow; use of heat pumps (including chemical heat pumps), etc Spray dryers Effects of various types of atomizers on flow patterns, product properties, agglomeration, size distribution; chamber geometry effects; injection of supplementary air; superheated steam operation; CFD models to investigate novel dryer designs, e.g. horizontal spray dryers; multi-stage horizontal spray/fluid bed dryers Impingement dryers Effects of high temperatures (tissue drying); non-circular multiple jets; variable spacing arrays of round or non-circular jets
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东北大学毕业设计（论文） 附录（外文翻译）
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