and T. F. Rutledge Atlas Chemical Industries, Inc. Wilmington, Delaware
Harry E. Weir
A Small Fluidized-Solid Constant Temperature Bath
This note describes the construction and operation of a small (2 in. X 16 in.) constant temperature bath in which a fluidized solid is the heat-transfer medium. Most laboratories are equipped with constant temperature baths in which a liquid or vapor is the heat transfer medium. The flexibility of such heating baths is obviously limited by boilimg point and by thermal and oxidative stability of the liquids. Wood's metal, Dowtherm, or molten salt baths are used to attain temperatures above 300°C, but these materials are inconvenient for small baths and present considerable safety hazards. Fluidized solids beds have been used as constant temperature baths. Adams, Gernand, and Icimberlin' have described a relatively large apparatus (about 6 in. X 36 in.) in which a fluidizedbed of inert solid was used as a coustant temperature bath. Adams, et al., included in their publication an excellent review of the literature. They also described the principles and mechanics of operatioo of fluidized beds of solids. Manes and Clralmers2 used a 2-in. X 30-in. glass apparatus in which 8&200 mesh silica gel was fluidized a t .70O0C (*2") by passage of 360 l/hr of laboratory air. Our apparatus is much smaller and more compact than the equipment already described.'J In addition, the apparatus does not require an external source of compressed air. The small size of the equipment makes it more useful for laboratory work in which small vessels must be heated a t a constant temperature. The fluidizer was constructed from standard wall borosilicate glass tubing. A frit,ted glass disc (51 mrn diameter, porosity "C" (40-60 micron), Corning) was sealed 1 in. from the bottom of a piece of 2'/,-in. od glass tubing, 11 in. long. The bottom was rounded and a hose connection sealed to the center of the bottom. The heater consists of a piece of 2a/l-in. glass tubing, 9'/% in. long, fire polished on both ends, wrapped with 141/1ft of Tmphet A (W. B. Driver Co.) resistance wire, size in. X 0.0063 in. (1.370 ohms per ft). The heater was rated at approximately 650 watts at 110 v. A protertive jacket of 31/r-in. od glass tubing 10'/, in. long was placed around both the heating jacket and fluidizer. The whole assen~bly(the fluidizer, heating jarket, and outside jacket) was mounted on a 6-in. X %in. X 4-in. aluminum chassis. In the chassis two aquarium type aerat,ors were mounted. The aerators were connected in parallel and the output connected to the hose connection on the bottom of the fluidizer. Figure 1 shows I AIXMS,C. E., GERXAND, M. O., A N D KIXRERLIY, C. N., JR. Ind. Eng. ('hem., 46,2458-2460 (1054). * .MANIIS,M., A Y D CITAI~MERS, D. G, J. CHEM.EDUC., 38,. I02 (1961 ).
construction details. Figure 2 is a photograph of the apparatus. The bottom panel has been removed to show arrangement of the pumps in the bottom part of .the unit.
HEATER-.
CHASSIS
d
Figure 1.
Construction details of the opporotur
Figure 2.
Photograph of the apparatus.
Volume 40, Number 8, August 1963
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425
The pumps produce air flows of 12-22 l/hr through the empty apparatus when operated a t 60-80 v. When a 311rin. bed of silica-alumina microspheres (Davison F-1, 25 silica-alumina, average particle size 65 microns) is placed in the apparatus, flows of 2-20 l/hr a t 25'C are obtained when the pumps are operated a t 60-110 v. Optimum fluidization and mixing of solid are obtained a t 90-100 v. (12.5-16.5 l/hr). Fluidized bed height is4 in. The bed can be heated easily to 250°C in 15 minutes. Rate of heating obviously depends on wattage of the heater. Temperature was controlled by pluggiug a Powemtat into a West JS-1 Gardsmau controller. Temperature control is excellent; a variation of *3'C is easily achieved. By careful setting of the Powerstat, equally good control can be realized without the Gardsman controller. Temperature within the bed is essentially constant a t any given time, regardless of the position of the temperature sensing thermocouple. With a 31/2- X 4-in. bed of solids, the pumps produce 12-19 l/hr flow a t 200-350°C. This flow is optimum for fluidization and mixing. The flow pattern of solid can be seen easily if a little dark colored solid is added.
426
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Journal of Chemkol Education
Flow of solid is upward in the center of the bed, and downward a t the sides, much like convection currents. The h e a r rate of flow of solid particles is 8-10 in. per minute. Flow rates were measured as follows: A rubber stopper was placed in the top of the bath. A T-tube led to a filter and then to an orifice flow meter. Very little solid was carried into the filter. When a vessel, such as a test tube, was heated in the bath, the thermocouple was placed near the bottom of the test tube. A piece of aluminum foil was crimped around the top of the bath and around the test tube. This simple closure prevented significant losses of solid to the air. Several other solids were evaluated because they have higher specific heats than silica-alumina. Carbon, 80 mesh, was satisfactory, but 200 mesh was not. Carbon would obviously be useful only a t moderate temperatures. Fine sand, 4&80 mesh, was not fluidized. A mixture of sand and Davison F-1 microspheres was mediocre. Carborundum, 90 mesh, mixed with the microspheres, was poor; 320 mesh Carborundum and the microspheres was mediocre.
