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RECOVERY PROCESSING

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1980-National-Waste-Processing-Conference-Disc-30-00011980-National-Waste-Processing-Conference-Disc-30-00021980-National-Waste-Processing-Conference-Disc-30-00031980-National-Waste-Processing-Conference-Disc-30-0004DESIGN MODELS OF TROMMELS FOR RESOURCE RECOVERY PROCESSING HARVEY ALTER Chamber of Commerce of the United States Washington, D.C. JEROME GAVIS Department of Geography and Environmental Engineering Johns Hopkins University Baltimore, Maryland MARC L. RENARD National Center for Resource Recovery, Inc. Washington, D.C. Discussion by Rodgers M. Hill Triple/S Dynamics, Inc. Dallas, Texas This paper marks a significant step forward in determining the design parameters to be considered in sizing trommels for municipal solid waste processing. To date, information concerning sizing has been limited to observation of existing units, and the use of methods adapted from the standard sizing formulae used for vibrating screens and applying to materials that are generally granular and free-flowing. As we all know, garbage is not granular, nor is it free-flowing, and screen area calculations that apply to vibratory units are not applicable. Consequently, estimations based on the knowledge obtained from those areas is at best, sketchy, and at worst can result in grossly over or undersized units. The paper is excellent in deriving the number of impingments necessary for a given separation problem at a given efficiency, and relating that number as a function of the trommel dimensions, its rotational speed, and angle of inclination. However, particle to particle interaction is not considered, since the equations are based on an infmite dilute loading. The effect of interparticle reaction 99 can best be deterrnifled by experimental tests of the model to verify it. I understand that such a test is planned and that the results will be made available in a form similar to this initial information. It is interesting to note that in the example given, rotational speed according to Eq. (9) is 18. This is Significantly greater than the 11 rpm at which that particular unit runs. Other factors that affect the efficiency of screening municipal solid waste in a trommel should also be determined and will provide invaluable design parameters for use in sizing this type of equipment for future installations. Discussion by Floyd Hasselriis Combustion Equipment Associates New York, New York At the EPA Workshop in New Orleans on February 8-10, 1977, the needs for research on Preprocessing Equipment for Waste-to-Energy Systems were delineated. At this meeting dissatisfaction was expressed with the performance of air classifiers, and attendees asked for both theoretical and practical investigations of air classifiers and trommels. This paper and the paper by Savage etal.on the performance characterization of air classifiers, represent the consequences of this expression of research needs, published three years later. It is fortunate that private industry does not have to suffer such time lags. The papers presented in this session illustrate a theoretical mathematical model, presented without test validation, a theoretical mathematical model validated with bench-scale testing under ideal (light load) conditions of single materials, and testing of full-scale classifiers with real RDF, but without theoretical models against which to compare the results. Ideally, the mathematical model, the bench test and the full-scale test would be integrated into a total understanding of the behavior of the device and the materials processed. Alter, Gavis and Renard have done an excellent job of bringing together the mathematical relationships which describe screening of poly size materials and the dynamics of rotary devices such as the trommel screen. Based on equations describing the fall path of non-aerodynamic particles, they have presented a method for determining how many times such a particle would drop, the point at which it would impact, and the axial advance per drop, given only the trommel diameter, rotational speed, and angle of incline. Knowing the number of such impingements and the probability of passage through a given hole, the number of particles passing a trommel of given length can be determined. For a feed material with a distribution of sizes the process can be integrated, given the number of particles in each size range, and the overall efficiency calculated. As the authors point out, few data on the number distributions of MSW are available. It is fortunate that the number distributions seem to be log-normal. This paper is indeed a firm step toward a rational method for prediction of size separation. However, as the authors clearly point out, there is a need to determine experimentally the effect of departures from the assumptions made, so as to answer questions such as: 1. What happens when the particles are light flakes which take twice as long to fall as dense objects? 2. What affect does the thickness of the bed of material have on screening efficiency? 3. What is the effect of interference between one type of material and another? 4. What is the best speed at which to operate the trommel with real materials? 5. How does actual throughput compare with calculated throughput? 6. How much and what kinds of materials fall through the holes at points other than the point of impingement? 7. What is the effect of varying hole size? 8. How much blinding of the holes takes place, with what effects? The authors applied the method of calculation to the New Orleans Trommel, fmding that about 60 percent of the minus 4.75 in. (120 mm) material would pass through these holes, which is close to the observed performance which is reported to range from 60/40 to 40/60. They estimated the efficiency in passing cans to be 67 percent with a 45 ft (13.7 m) trommel, although higher efficiencies have been reported. The calculations seem to be conservative; a considerable amount of material is discharged through holes other than those directly impinged upon. There is no substitute for testing of trommels with real materials under actual running conditions, due to the variations in material characteristics and the degree of degradation which inevitably takes place due to hole blinding and other factors. Theory is especially useful for extrapolation away from known conditions and


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