OCR Text |
Show removal from the material be?~~) dried. In addition, it has been argued that the pressure waves present in the dryer disperse the injected slurries and, thus, increase their surface area and drying rate. The commercial potential of pulse drying resulted in the formation of a number of pulse dryer (~,n~B,cturers in the U.S. in recent years ' . Reported applications include the drying of fish meal, kaolin clay, pou 1 try blood, hatchery waste and so on. A schematic of a cO~~I)cial pulse dryer, developed by Sonodyne is shown in Figure 4. It utilizes a U-shaped valveless pulse combustor wi th a reported capaci ty between 3.2 to 3.5 MBTR/HR. This pulse combustor is housed in a separate compartment which is pressurized with combustion air by an upstream fan. The materi alto be dr i ed is injected into the exhaust section of the pulse combustor which is the upper lej of the U-shaped tube. The injected slurry is entrained and carried by the pulsating flow into the primary cyclone where the drying process is completed. Since th is pu 1 se combustor is valveless, combusti on products are exhausted through the exhaust (i.e., upper) and in 1 et (i. e., lower) sect ions the U-shaped burner. The hot products leaving the inlet section entrain surrounding air and the resulting mixture enters the primary cyclone through an opening in the wall separating the cyclone and the pulse combustor housing. Most of the dried material is collected on the bottom of the primary cyclone where it is removed by an auger. Some of the dried material is removed in the dust collector downstream of the primary cyclone. A downstream exhaust fan assists in moving the gases through the pul se dryi ng system. Manufacturers of pulse dryers claim high overall drying efficiencies in the range 1250-1500 BTU/LB of water evaporated compared with the 1150 BTU/LB theoretically needed for each pound of water removed. Another claimed advantage is that due to the relatively short res i dence type of the dryi ng materi ali n the high temperature gas in the tail pipe of the pulse combustor, the water is evaporated without raisicrg the material temperature to more than 200 F, thus causing little chemical change in the dried material. Since the efficiencies of most chemical processes depend upon the rates of mass, momentum and heat transfer processes, one would expect that the efficiencies of many of these processes could be improved if they took place in a pulsating environment. Candidate processes which have been ~ nvest i gated u,nd,er (~l sat i ng cond it j ~B$ lnclude calclnlng. L~J extraction. \ ) gasl' f,l' ca~1, 0n((3321)) ~nd so on. Other reported appllcatlons lnclude surface cleaning by the . scrubbi ng action of the back-and-forth motlon of the exhaust gaS33 of pul se combustors, fog generation\ ) by the 58 introduction of a liquid into the exhaust flow of a pul se combustor and so on. In a recent trip to Japan, this writer h~s observed applications of pulse combustors ln the development of industrial radiant tubes, 1 a rge baking ovens and custom des i gned industrial pulse heaters for various liquid (e.g., paint) heating applications. It is believed that resistance to change and lack of understanding of the principles and potential advantages of pulse combustion are the main causes for the limited interest of industry in thi s technology to date. To increase industrial usage of pulse combustion techno logy, its advantages wi 11 have to be demonstrated in actual applications or demonstration plants having scales and/or capacities similar to those encountered in industrial processes. Additional research and development programs aimed at the development of practical, industrial scale pulse combustors capable of operating over wide ranges of fuel/air ratios and having large turndown ratios wi 11 have to be undertaken. Also, work must be done on optimizing the integration of pulse combustors into various industrial, chemical and physical processing systems. For example, to optimize a given process efficiency its designers will require knowl edge of the dependence of the process effi c i ency upon the frequency and amp 1 i tude of pulsations and the location of the process within the acoustic field (i .e., should it occur near a pressure node, a pressure antfnode or in between?); i nformati on that wi 11 have to be acqui red through experi ence and separate research and development efforts. Finally, more fundamental investigations of the mechanisms responsible for the enhancement of the various transp'ort processes by flow pulsations are required. In closing, it is believed that the recent successful commercialization of pulse furnaces and water heaters for domestic application, the development of industrial pul se dryers and the increase in DOE's and GRI's interest and support for pulse combustion research and development programs wi 11 acce 1 erate the acceptance of th i s technology by industry. 1. 2. 3. REFERENCES Bartholome, G. L., Hoff, E. J. and Purdy, R. K., Food Technology, March 1969. Borisov, Y. Y. and Gynkina, N. M., Soviet Physics-Acoustics, July-Sept., 1962. Borisov, Soviet 1966. Y. Y. and Dol gopo 1 iv, N. N., Physics-Acoustics, Jan.-Mar., 4. Low, D. I. R. and Hodgins, J. W., The Canadian Journal of Fhem. Eng., 1963. |