Drying and gas or vapor contact with solids – Apparatus – With means to treat gas or vapor
Patent
1990-11-13
1993-11-23
Bennett, Henry A.
Drying and gas or vapor contact with solids
Apparatus
With means to treat gas or vapor
34 79, F26B 300
Patent
active
052632660
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a process of low-emission drying of wood chips in which the moist chips are first dried in a first preferably directly heated drying stage and postdried in a second drying stage. The invention further relates to an apparatus for carrying out the process having a first, preferably directly heated dryer for predrying the moist wood chips and a second dryer disposed downstream from the first one.
Dryers disposed in series for the drying of materials are known from e.g. DE-OS 26 40 508 and 35 34 260. The thermal purification of exhaust gases is further known in principle from DE-PS 36 16 333. A process and an apparatus of the type initially mentioned ar further known from DE-OS 28 21 689. No indications are made as to the extent of drying, the type of heating the dryer and the treatment of the exhaust gases.
It is known that large amounts of wood chips are required for the production of particle board, wherein the wood chips are first mixed with a binder, mostly a urea formaldehyde resin, and are then pressed in presses into boards at high pressure and high temperature. If the water content of the wood chips used is too high, an explosion-like destruction of the board occurs on taking it out of the press due to the sudden expansion of the steam present inside of the board. The wood chips must thus be predried to a maximum water content of 6 percent by weight (based on absolute dry weight of the board prior to pressing. But as fresh wood chips, depending on the type of wood, the season of felling and the storage conditions, have a moisture content of 50 to 150 percent by weight, about 100 percent by weight on the average, based on the dry weight of the wood, the drying of the wood chips from this relatively high water content to the required 1 to 6 percent by weight requires a large amount of heat, so that the degree of thermal efficiency is of particular importance in technical drying processes.
The drying of wood chips entails the further problem that in addition to wood, resin-like components such a terpene and mainly .alpha.-pinene are contained in them in addition to cellulose, hemicellulose and ligrin. A portion of these substances has a comparatively low boiling point and is moreover volatile in steam. As a result, the exhaust gases of chip drying plants contain substances of this type in addition to steam. Although these substances are not toxic in the narrow sense, they still impart to the exhaust vapors of the dryer a characteristic unpleasant odor. Although this pollution can be reduced by using the essentially less resinous deciduous wood instead of the highly resinous coniferous wood, resulting in lower terpene emission, this gives rise to another problem: the exhaust gas of dryers also contains small amounts of wood dust in addition to the substances mentioned. It was found that certain types of wood dust, mainly those of beech and oak, might be carcinogenic, and as a result, the admissible limit values for the dust content in the exhaust gas when using these types of wood are drastically reduced. The normal dust collectors such as multicyclones are no longer adequate here and special filters, in particular fibrous filters, must be used, although the high water content of the exhaust gas of the dryer causes frequent operational failures.
Finally, the dryer exhaust gas may contain substances formed by thermal degradation of one or more components of the wood (cellulose, ligrins, resins and the like), such as various aldehydes and acids such as formaldehyde, acetaldehyde, acetic acid and the like. The formation of such substances occurs particuarly if high drying temperatures are used in the dryer, such as they usually prevail in directly heated dryers. Although these substances are normally contained in the exhaust gas in low concentrations (of about 10 to 30 percent of those of terpenes), they are hygienically precarious and some of them have an unpleasant, pungent odor, thus pollute the environment.
Attempts to eliminate the undesirable substances from the exhaust
REFERENCES:
patent: 3875678 (1975-04-01), Vits
patent: 4135309 (1979-01-01), Bosnjak
Bennett Henry A.
M. Kaindl Holzindustrie
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