文献翻译-干扰床分选.doc

1、英文翻译TeeteringAbstractBefore 1940, when the term “fluidization” was coined to describe a gassolids contacting process,liquid-fluidized beds of granular solids had long been used for classification of minerals by size (sizing) or density (sorting), and were referred to in the ore dressing literature a

2、s “teeter columns” or as being in the “teeter condition”. Liquid fluidization or “teetering” has subsequently achieved several other applications,in recent years most notably as extended electrodes for electrowinning metals from dilute solutions, as self-cleaning heat exchangers and as bio-catalytic

3、 reactors. Since several important applications, in addition to classification per se, involve particle stratification by size and/or density, the main focus of this paper is on the current understanding of particle segregation and mixing in liquid-fluidized beds.1. IntroductionThe term “fluidizatio

4、n” was coined in the U.S.A.around 1940 to describe the hydrodynamics of the newly developed technique for gassolids contacting, anticipated two decades earlier in Germany by the Winkler process for coal gasification 1. However, even before that, mineral engineers were actually using fluidization wit

5、h a liquid to classify solid particles, usually unpurified ores,by size (“sizing”) and/or by density (“sorting”). The process of levitating solids with a liquid was referred toin the ore dressing literature as “teetering”, an elegantly descriptive term that was rapidly replaced by “liquid fluidizati

6、on” after 1940. The older term is occasionally revived 2.Applications of liquid-fluidized beds are discussed in a recent Handbook 3 and, somewhat less summarily forsome applications, in a recent on-line publication 4. Beloware eight of the more important uses to which liquidfluidized beds have been

7、put, listed roughly in the chronological order in which they appeared on the industrial scene:1. Particle classification2. Backwashing of granular filters3. Crystal growth4. Leaching and washing5. Adsorption and ion exchange6. Electrowinning7. Self-cleaning heat exchangers8. Bioreactors.Traditionall

8、y, continuous liquid-fluidized or teeter bed classifiers were devoted mainly to mineral separations, but they have now extended their domain to other materials such as plastic particles, recently modelled for sizing in a pilot plant using multi-sized glass beads 5. Sorting, whichlike sizing, can be

9、performed in a pure liquid medium, can also be performed, especially for a bidisperse-density particle mixture, by the equivalent of sink-and-float separation, using as the medium a homogeneously fluidized bed of narrowly sized fine particles with the liquid velocity adjusted to give the bed density

10、 required to sink the heavier and float the lighter particles.Periodic backwashing of downflow granular filters by upward water fluidization is a well entrenched procedure which, as will be explained later, leads to possible mixing and segregation problems when dual or even triple filtration media a

11、re used. Crystal growth in a bed of seed crystals in Krystal or Oslo “classifying crystallizers” is another longstanding,well-established industrial process. Less entrenched is the use of liquid fluidization for leaching and washing of solids as well as for adsorption and ionexchange, the principal

12、advantage over the more conventional use of agitators for the former being operation in a single column with no mechanical parts, and over the use of fixed beds for the latter being the continuous addition and removal of solids. Somewhat analogous to crystal growth in a classifying crystallizer is t

13、he growth of small metallic particles that act as extensions to the cathode in a fluidized bed electrowinning cell, using a dilute electrolyte as the fluidizing liquid, the velocity of which must be close to minimum fluidization (in order to maintain electrical contact) in the case of electrowinning

14、 but not in crystal growth. The use of a liquid-fluidized bed of non-friable particles to scour the walls of a vertical tubular heat exchanger, thus minimizing fouling, has the additionaladvantage of increasing the wall-to-liquid heat transfer coefficient by a factor up to seven over that for a part

15、iclefree liquid flowing at the same rate, depending on the system and the liquid velocity.Bioreactors in the liquidsolid fluidization mode are commonly used to immobilize enzymes or, as illustrated in Fig. 1, biofilms, e.g. for aerobic wastewater treatment. The biofilms, with a density similar to th

16、at of the fluidizing water, tend to grow selectively on some of their higher density carrier particles. The size increase would tend to send those particles to the bottom of the fluidized bed,whereas their average density decrease would tend to drive them upward. The question of which effect would tend to win out under different circumstances will be addressed in what follows.Segregation or stratification of particles by size and/or dens