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Testing and evaluation of air classifier performance
Resource Recovery and Conservation, 4 (1979) 247-259 o Elsevier Scientific Publishing Company, Amsterdam - Printed
in The Netherlands
TESTING
PERFORMANCE
AND EVALUATION
OF AIR CLASSIFIER
247
WILLIAM A. WORRELL Brown and Caldwell, Engineers, Atlanta, Georgia (U.S.A.) and P. AARNE
VESILIND
Duke University, Durham, North Carolina 27706 (Received
10th October
1978; accepted
(U.S.A.)
in revised form 4th June 1979)
ABSTRACT This paper introduces a new parameter for describing air classifier performance, “Total Efficiency”, found by first calculating the fractions recovered of both light and heavy fractions at specific air speeds and then multiplying the two fractions. Total Efficiency can be plotted against air speed, and such plots yield information about peak efficiencies and sensitivity to air flow rate variations. This method allows the comparison of different air classifiers and also leads to a technique in which the data can graphically express contamination of the light or heavy fraction at different air speeds. These curves can then be used to determine the optimum air speed in terms of market value of the product produced. Data on three different throat designs - the zigzag throat and two variations of the zigzag, are used to illustrate the method.
INTRODUCTION
Municipal Solid Waste (MSW) is a heterogeneous material, and any fraction recovered must first be separated from the whole MSW. One technique used to separate materials is air classification which introduces the MSW to an air stream that allows the heavy, mostly inorganic fraction to drop out, fluidizes the light, mostly organic fraction, and transports it by the air stream to a cyclone where the light fraction is deposited. Air classifier manufacturers and researchers do not agree on .IOW to compare the results of air classification experiments. Some will report the results in terms of “percent overall recovery”, whereas others will use “purity of product” or “percent lights recovery” [l-7] . This disagreement is compounded by the fact that it is difficult to adequately describe the performance of a materials separation unit operation such as an air classifier by a single parameter, as demonstrated below. A separate but related problem in air classification is that the terms describing the operation of classifiers have not been properly defined. Some authors use “lights “, 6‘organics” and “combustibles” synonymously to describe the fraction in an air classifier feed that is fluidized and transported up by the air stream. The other fraction correspondingly has been referred to as “heavies”, “inorganics” and “non-combustibles”.
248
In this paper, the term “lights” is used to describe the fraction of the feed which it is desired to recover with an air stream, and “heavies” is defined as those which should drop out of the air stream. The material exiting with the air stream is called the “overflow”, whereas the material dropping through and exiting at the bottom is called the “underflow”. Unless 100% separation is achieved, therefore, the overflow consists of mostly lights, with a few heavies, while the underflow consists of mostly heavies and a few lights. There are four common methods used to evaluate the performance of air classifiers. Each method relies on a mass balance between the incoming fraction and exiting fraction. The four methods used are: Overall Recovery Purity of Overflow
Weight of Overflow %= Weight of Feed %=
x loo
Weight of Lights in Overflow Weight of Overflow
Recovery
of Light Fraction
Recovery
of Heavy Fraction
‘%=
x 1oo
Weight of Lights in Overflow Weight of Lights in Feed
5%=
(2) x 100 (3)
Weight of Heavies in Underflow Weight of Heavies in Feed
x loo (4)
The ideal air classifier separates 100% of the lights in the feed, so that they would report to the overflow and conversely, 100% of the heavies would report to the underflow. None of the above equations can by itself adequately express the performance of air classifiers. For example, 100% overall recovery occurs when all the feed material reports to the overflow. If this feed, however, contains heavies (otherwise there would be no point in air classifying the waste), ideal separation is obviously not achieved. Another parameter, such as the percent lights in the overflow, is required in conjunction with overall recovery in order to adequately describe performance. The objective of this paper is to develop a method by which the efficiency of separation of the light fraction from the heavy fraction by an air classifier can be evaluated by means of a single value parameter and thus to develop a method by which the performance of any number of air classifiers can be compared. A second objective of this study is to compare the efficiency of three different throat designs: the zigzag design, a modified zigzag, and a newly developed design. The total efficiency and the sensitivity of the efficiency to air velocity is to be determined for each throat, and this will provide the basis for comparison.
249 EQUIPMENT
The Duke University Resource air classifier, ferrous magnet, and designed to operate as a complete operate each piece of equipment
Recovery Laboratory consists of a shredder, trommel screen. While the laboratory is processing facility, it is also possible to [ 81. independently
CYCLONE
Fig. 1. Air classification
u&
used in these studies.
The air classification unit, shown in Fig. 1, consists of a throat section through which air is drawn by a 0.75 kW (1 h.p.) fan. The feed hopper introduces the refuse into the 5 cm X 15 cm throat, and the light fraction of the refuse is fluidized and transported by the air into the cyclone separator where it loses its velocity and falls into the collection hopper while the air continues to be drawn to the fan. The fraction of the refuse that is too heavy to be fluidized falls against the flow of air and exists at the bottom of the throat. A sliding gate valve and a venturi orifice placed in the pipe between the cyclone and the blower is used to regulate and measure the air flow. Zigzag throat The first throat design tested was a zigzag throat consisting of 12 stages, with the refuse feed introduced at the 8th stage from the bottom, described in Fig. 2 [3]. Each stage is 10 cm long, and inclined 60” from the horizontal. In order to observe the turbulence within the throat, two sides of the throat section are constructed of clear plastic. The advantage of the zigzag throat is that it produces vortices that cause turbulence within the throat and thus helps to break apart clumps of refuse, facilitating separation [9, lo].
250 ZIGZAG
THROAT
UTAH
THROAT
DUKE
THROAT
Fig. 2. Air classifier throat designs used.
Utah throat The Utah throat, developed at the University of Utah, has the bottom half of the zigzag removed (Fig. 2) [ll] . This modification allows impaction of agglomerated particles on both the horizontal and vertical surfaces. The design also increases the intensity of the vortices. The unit tested in this study contained 12 stages, with the feed introduced at the 8th stage from the bottom. All surfaces of the throat were constructed from clear plastic so that the turbulence occurring in the throat could be observed. Unlike the throat developed at Utah, the unit used in this study had a constant distance between the vertical walls. Duke throat The Duke throat was developed in an attempt to further modify the initial zigzag concept. While the Utah throat produces increased turbulence, it also perpetuates a problem that occurs in the zigzag throat, which is that after conglomerated refuse is broken into lights and heavies, these particles must either pass up through the throat or down through the throat at which time interference from particles moving in the opposite direction might cause loss of momentum or a reconglomeration of the particles. In an attempt to alleviate this problem, the Duke throat, illustrated in Fig. 2, provides a free path for the light particles and a separate chute for the heavies which can now fall down the outside without encountering any upflowing lights. The Duke throat was constructed completely out of clear plastic so that the separation and performance of the throat could be observed.
251 MATERIAL
CLASSIFIED
The feed material to be used in the testing of the performance of air classifiers must be carefully selected. There are four basic options: (1) using actual shredded municipal solid waste (MSW): (2) using MSW that has been seeded with specially marked items; (3) using a feed that simulates the composition of actual MSW but reducing the number of the actual components that constitute refuse; and (4) using artificial materials that symbolically represent MSW. Each method has advantages and disadvantages. Since air classifiers are usually designed to separate MSW, using actual MSW should be the first option considered. There are several advantages to testing a new throat with genuine MSW. When doing so, there is little concern that the characteristics of the test materials will be significantly different from the material the throat is designed to separate. Another advantage in using actual MSW is that there is usually a plentiful supply of the actual material which a classifier will be expected to separate. There are also several disadvantages to using MSW. There are the obvious difficulties with using a putrescible waste, and problems occur when attempts are made to quantify the composition of the refuse. Not only is this a dirty and time-consuming job, but it is also potentially dangerous. Another disadvantage to using MSW is that its composition varies from sample to sample, and little experimental control is possible. One way to alleviate some of the disadvantages of working with MSW while retaining most of the advantages is to seed the MSW. Seeding of MSW is accomplished by marking various components of waste with bright paint, and after the waste has been classified, the coded material can be separated. The advantage of this method is that the amount of handsorting is greatly reduced. There are, however, several disadvantages to using seeded MSW. The same problem of relatively short storage life applies, as does the problem of being exposed to possible pathogenic organisms, etc. A third type of feed that could be used is simulated MSW. By choosing only a few typical materials, it is possible to standardize the quantity of each component of the simulated MSW and thus obtain a “standardized bag” of feed material. Another advantage of simulated MSW is that putrescible organic wastes can be excluded. Hand sorting the clean material is also much easier than sorting MSW. The major disadvantage to using simulated MSW is that the characteristics of the waste could differ from MSW. In the case of this experiment it was decided to use simulated municipal solid waste. Since different throat designs are being compared it is of the utmost importance that the refuse input into the different throats always be identical. The simulated MSW used is shown in Table 1. Newspaper, plastics (mostly sheet polystyrene and polyethylene), aluminum (beverage cans) and ferrous metal (turnings from a machine shop) were shredded to a nominal size of 2 cm. Next, the material from each
252 TABLE 1 Simulated feed material composition -___-... Material
Percent by weight
Lights Newspaper Plastics
72 9
Heavies Aluminum Steel
12 7 100
____
category was weighed out and mixed in a large plastic bag. The paper and plastics are organic and are here considered to be the “lights” which should report to the overflow (carried up in the air stream). The aluminum and ferrous metal turnings are the “heavies” and it is desired that they drop out in the underflow. PROCEDURE
AND RESULTS
The material was fed into the feed hopper which contains a rotary air lock feeder that introduces the material into the throat section at a constant feed rate. The underflow and overflow were collected, separated by hand into various components, and weighed without drying. Zigzag throat The zigzag throat
test results are shown in Table 2.
Utah throat The Utah throat test results are shown in Table 3. Run #6 at the lowest air speed presented a problem because the paper tended to clog the throat as it traveled downward in the throat. At other air speeds no problems were encountered. Duke throat Of the three, this throat had the highest tendency to clog. This is directly related to the increased number of potential areas where low velocities exist and particles can build up and eventually clog the throat. At low air speeds (below 2.5 m/s) the paper tended to accumulate on the flat surface
2.3 4.6 7.3 10.3 13.5 15.3
1 2 3 4 5 6
16.0 727.8 744.2 799.1 808.3 809.8
Paper and plastics reporting as overflow (g)
Air speed (m/s)
2.3 4.6 7.3 10.3 13.5 15.3
Run number
1 2 3 4 5 6
Paper reporting as overflow (g)
0.0 720.0 720.0 720.0 720.0 720.0
190.0 188.9 181.7 150.8 91.5 53.6
0.0 721.7 787.7 806.3 810.0 810.0
0.0 1.7 67.7 86.3 90.0 90.0
Plastic reporting as overflow (g)
0.0 10.8 24.2 79.1 88.3 89.8
16.0 717.0 720.0 720.0 720.0 720.0
_~
Plastic reporting as overflow (g)
Paper reporting as overflow (g)
Aluminum and ferrous reporting as underflow (g)
190.0 188.0 187.3 173.3 123.4 86.5
Aluminum and ferrous reporting as underflow (g)
Paper and plastics reporting as overflow (g)
Results of test runs on Utah throat
TABLE 3
Air speed (m/s)
Run number
Results of test runs of zigzag throat
TABLE 2
120.0 119.7 116.2 96.1 60.3 34.0
Aluminum reporting as underflow (g)
120.0 118.1 117.4 105.7 69.4 52.6
Aluminum reporting as underflow (g)
70.0 69.2 65.5 54.7 31.2 19.6
Ferrous reporting as underflow (g)
70.0 69.9 69.9 67.6 54.0 33.9
Ferrous reporting as underflow (g)
2.3 4.6 7.3 10.3 13.5 15.3
1 2 3 4 5 6
2.3 4.6 7.3 10.3 13.5 15.3
Air speed (m/s)
*Paper and plastics. **Aluminum and ferrous
1 2 3 4 5 6
__-
Run number
Percent inorganics* * reporting as underflow 100.0 98.9 98.8 91.2 64.9 45.5
Percent organics* reporting as overflow
0.0 89.6 91.9 98.7 99.8 100.0
_.
_
0.0 99.6 100.0 100.0 100.0 100.0
Percent paper reporting as overflow
0.0 718.9 720.0 720.0 720.0 720.0
190.0 189.0 188.0 180.0 135.4 124.3
0.0 718.9 723.5 768.4 799.9 803.4 ~.
Paper reporting as overflow (g)
Aluminum and ferrous reporting as underflow (g)
Paper and plastics reporting as overflow (g)
Percent recovery of organic6 and inorganics from zigzag throat
TABLE 5
Air speed (m/s)
Run number
Results of test runs of Duke throat
TABLE 4
Percent aluminum reporting as underflow 100.0 98.4 97.8 88.1 57.8 43.8
0.0 12.0 26.9 87.9 98.1 99.9
120.0 120.0 119.1 110.0 81.9 77.4
Aluminum reporting as underflow (g)
Percent plastic reporting as overflow
3.5 48.4 79.9 83.4
0.0
0.0
Plastic reporting as overflow (g)
~______
100.0 99.9 99.9 96.6 77.1 48.4
Percent ferrous reporting as underflow
70.0 69.0 68.9 70.0 53.5 46.9
Ferrous reporting as underflow (g)
0.0 88.6 90.8 90.0 64.8 45.5
Total efficiency (%)
255
above the feed throat. As air speeds were increased, the turbulence increased and alleviated this problem. The results from the Duke throat runs are presented in Table 4. ANALYSIS
OF RESULTS
One objective of this study was to develop a method of evaluating air classifier performance which would use a single parameter to describe the performance of an air classifier. The parameter developed is termed “Total Efficiency”, and is defined as the product of the fractional recovery of light in the overflow and fractional recovery of heavies in the underflow (eqns. (3) and (4)). These values for the three classifiers tested are shown in Tables 5, 6 and 7. This parameter satisfies the requirement that in an ideal air classifier 100% Total Efficiency would be achieved only when 100% of the lights reported as the overflow, and 100% of the heavies reported as the underflow. The Total Efficiency thus calculated can be plotted versus air speed for each throat tested as in Fig. 3. The peak of each of the curves is the point of maximum efficiency for the specific throat design, and defines the air speed that should be used to achieve this maximum efficiency, for the given feed rate. The shape of the curves represents the overall separation character-
YO-
BO-
70-
F
k/
L
co-
g
E w
50 ~
_J 2
4O-
3O-
ZO-
IO -
0 0
25
I
I
I
5.0
7.5
IO.0
AIR
SPEED
I 12.5
I 15.0
1
(m/SW)
Fig. 3. Total Efficiency for three different throat designs.
2.3 4.6 7.3 10.3 13.5 15.3
1 2 3 4 5 6
0.0 89.1 97.2 99.5 100.0 100.0
Percent organics* reporting as overflow 100.0 99.4 95.6 79.4 48.2 28.2
Percent inorganics** reporting as underflow
2.3 4.6 7.3 10.3 13.5 15.3
1 2 3 4 5 6
*Paper and plastics. **Aluminum and ferrous.
Air speed (m/s)
Run number
0.0 88.8 89.3 94.9 98.8 99.2
Percent organics* reporting as overflow 100.0 99.5 98.9 94.7 71.3 65.4
Percent inorganics** reporting as underflow
Percent recovery of organics and inorganics from Duke throat
TABLE 7
*Paper and plastics. **Aluminum and ferrous.
Air speed (m/s)
Run number
Percent recovery of organics and inorganics from Utah throat
TABLE 6
0.0 99.8 100.0 100.0 100.0 100.0
Percent paper reporting as overflow
0.0 100.0 100.0 100.0 100.0 100.0
Percent paper reporting as overflow
0.0 0.0 3.9 53.8 88.8 92.7
Percent plastic reporting as overflow
0.0 1.9 75.2 95.9 100.0 100.0
Percent plastic reporting as overflow
Percent ferrous reporting as underflow 100.0 98.6 98.4 100.0 76.4 67.0
100.0 100.0 99.2 91.7 68.2 64.5
_
100.0 98.9 93.6 78.1 44.6 28.0
Percent ferrous reporting as underflow
aluminum reporting as underflow
100.0 99.8 96.8 80.1 50.2 28.3
Percent aluminum reporting as underflow
0.0 86.3 88.3 89.9 70.4 64.9
Total efficiency (%)
0.0 88.6 93.0 79.0 48.2 28.2
Total efficiency @)
257
istics of the throat and defines its sensitivity to fluctuations in air speed. A further step, not attempted in this study, is to develop a family of such curves for each classifier, with each curve corresponding to a different materials feed rate. Obviously, at high feed rates the total efficiency can be expected to drop. It is apparent that although the Utah throat achieves the higher peak efficiency of separation, it also seems to be sensitive to variations in air speed. The zigzag and Duke throats exhibit similar characteristics, with both curves reaching a flat peak of approximately 89% Total Efficiency. This peak remains constant between 5 and 10 m/s, but above an air speed of about 10 m/s the zigzag throat efficiency drops off more rapidly than the Duke throat. Another method of presenting air classifier data is to plot the percent of the various incoming materials reporting to the overflow (Fig. 4). As an example of how such plots might be used, consider a case where 8% of the incoming aluminum and 5% of the incoming ferrous metal might be allowed as the maximum level of contaminants in the overflow. If the zigzag air classifier was used to separate the incoming material, then according to Fig. 4 the maximum allowable aluminum contamination is reached at an air speed of 9.7 m/s and the maximum allowable ferrous contamination is reached at 10.2 m/s. By using an air speed of 9.7 m/s, the air classifier produces an overflow that contains 100% of the incoming paper, 70% of the incoming plastic, 8% of the incoming aluminum, and 4% of the incoming ferrous material. 2 : g 100
2 90 Yz c 80 B ; 70 a ‘r 60 Z 5 50 2 = 40 CJ 3: 30 ?I z 20 s 2s E
IO 0 0
25
5.0
7.5 AIR
Fig. 4. Percent (zigzag throat).
of incoming
paper,
SPEED
plastic,
10.0
12.5
15.0
I7.5
~mpcc)
aluminum,
and ferrous
reporting
to overflow
258 APPLICATION
OF RESULTS
In order to compare the efficiencies of the three throats tested in this study it was necessary to develop the new method of determining efficiency. The “Total Efficiency” parameter allows for a more acceptable comparison of performance data and thus facilitates the evaluation and specification of air classifiers. The percent contamination by heavies of the overflow fraction for a specific throat and air speed can be used to develop the value of the products. For example, with reference to Fig. 4, using any given air speed, the composition of the overflow can be determined. The market value, or the acceptability of the product to prospective purchasers can thus be evaluated. Using prevailing market values and purchaser specifications for recovered material, it is possible to construct curves such as Fig. 5, which show the total potential value of the two fractions given the performance curve. This curve was developed assuming, for example, that at higher air speeds, aluminum was lost into the overflow and was no longer recoverable, thus decreasing the overall value of the products. Similarly at low air speeds, the total lights reporting as overflow was lower, thus decreasing the income from the sale of refuse-derived fuel. The obvious operating air speed would be at the peak of the curve, the point of maximum product value.
01 0
I 2.5
I 5.0
I 7.5 AIR
SPEED
I IO.0 (m/m)
Fig. 5. Value of recovered material (Utah throat).
I 12.5
I 15.0
I 17.5
259
ACKNOWLEDGEMENT
The data and methodology for the production of the market value of air classified refuse was developed by Bruce Polkowsky, a graduate student of the Department of Civil Engineering, Duke University. This research was funded in part by the Richard C. Leach Endowment Research Fund.
REFERENCES 1 Taggart, A.F., 1974. Handbook of Mineral Dressing, Wiley, New York. 2 Fan, D.N., 1975. Resource Recovery and Conservation, 1: 141-150. 3 Boettcher, R.A., 1972. Air Classification of Solid Waste, US EPA SW-3Oc, Washington, D.C. 4 Murray, D.L. and Liddell, C.L., 1977. Waste Age, 8 (3). 5 Sweeney, P., 1978. Air Classification, presented at Engineering Foundation Conference, Rindge, N.H. 6 Midwest Research Institute, 1979. Study of processing equipment for resource recovery systems, EPA Contract No. 68-03-2387, Washington, D.C. 7 Chrismon, R.L., 1977. Air classification in resource recovery, National Center for Res. Rec., RM 78-1, Washington, D.C. 8 Vesilind, P.A., 1977. ASCE, J. Envir. Eng. Div., 103 (EES): 761-765. 9 Senden, M.M.G. and Tels, M., 1978. J. Powder & Bulk Sol. Tech., 2(l). 10 Senden, M.M.G., 1979. Ph. D. Dissertation, Techn. Hogeschool Eindhoven, The Netherlands. 11 Eckhoff, D., 1974. University of Utah, personal communication and unpublished report.
in The Netherlands
TESTING
PERFORMANCE
AND EVALUATION
OF AIR CLASSIFIER
247
WILLIAM A. WORRELL Brown and Caldwell, Engineers, Atlanta, Georgia (U.S.A.) and P. AARNE
VESILIND
Duke University, Durham, North Carolina 27706 (Received
10th October
1978; accepted
(U.S.A.)
in revised form 4th June 1979)
ABSTRACT This paper introduces a new parameter for describing air classifier performance, “Total Efficiency”, found by first calculating the fractions recovered of both light and heavy fractions at specific air speeds and then multiplying the two fractions. Total Efficiency can be plotted against air speed, and such plots yield information about peak efficiencies and sensitivity to air flow rate variations. This method allows the comparison of different air classifiers and also leads to a technique in which the data can graphically express contamination of the light or heavy fraction at different air speeds. These curves can then be used to determine the optimum air speed in terms of market value of the product produced. Data on three different throat designs - the zigzag throat and two variations of the zigzag, are used to illustrate the method.
INTRODUCTION
Municipal Solid Waste (MSW) is a heterogeneous material, and any fraction recovered must first be separated from the whole MSW. One technique used to separate materials is air classification which introduces the MSW to an air stream that allows the heavy, mostly inorganic fraction to drop out, fluidizes the light, mostly organic fraction, and transports it by the air stream to a cyclone where the light fraction is deposited. Air classifier manufacturers and researchers do not agree on .IOW to compare the results of air classification experiments. Some will report the results in terms of “percent overall recovery”, whereas others will use “purity of product” or “percent lights recovery” [l-7] . This disagreement is compounded by the fact that it is difficult to adequately describe the performance of a materials separation unit operation such as an air classifier by a single parameter, as demonstrated below. A separate but related problem in air classification is that the terms describing the operation of classifiers have not been properly defined. Some authors use “lights “, 6‘organics” and “combustibles” synonymously to describe the fraction in an air classifier feed that is fluidized and transported up by the air stream. The other fraction correspondingly has been referred to as “heavies”, “inorganics” and “non-combustibles”.
248
In this paper, the term “lights” is used to describe the fraction of the feed which it is desired to recover with an air stream, and “heavies” is defined as those which should drop out of the air stream. The material exiting with the air stream is called the “overflow”, whereas the material dropping through and exiting at the bottom is called the “underflow”. Unless 100% separation is achieved, therefore, the overflow consists of mostly lights, with a few heavies, while the underflow consists of mostly heavies and a few lights. There are four common methods used to evaluate the performance of air classifiers. Each method relies on a mass balance between the incoming fraction and exiting fraction. The four methods used are: Overall Recovery Purity of Overflow
Weight of Overflow %= Weight of Feed %=
x loo
Weight of Lights in Overflow Weight of Overflow
Recovery
of Light Fraction
Recovery
of Heavy Fraction
‘%=
x 1oo
Weight of Lights in Overflow Weight of Lights in Feed
5%=
(2) x 100 (3)
Weight of Heavies in Underflow Weight of Heavies in Feed
x loo (4)
The ideal air classifier separates 100% of the lights in the feed, so that they would report to the overflow and conversely, 100% of the heavies would report to the underflow. None of the above equations can by itself adequately express the performance of air classifiers. For example, 100% overall recovery occurs when all the feed material reports to the overflow. If this feed, however, contains heavies (otherwise there would be no point in air classifying the waste), ideal separation is obviously not achieved. Another parameter, such as the percent lights in the overflow, is required in conjunction with overall recovery in order to adequately describe performance. The objective of this paper is to develop a method by which the efficiency of separation of the light fraction from the heavy fraction by an air classifier can be evaluated by means of a single value parameter and thus to develop a method by which the performance of any number of air classifiers can be compared. A second objective of this study is to compare the efficiency of three different throat designs: the zigzag design, a modified zigzag, and a newly developed design. The total efficiency and the sensitivity of the efficiency to air velocity is to be determined for each throat, and this will provide the basis for comparison.
249 EQUIPMENT
The Duke University Resource air classifier, ferrous magnet, and designed to operate as a complete operate each piece of equipment
Recovery Laboratory consists of a shredder, trommel screen. While the laboratory is processing facility, it is also possible to [ 81. independently
CYCLONE
Fig. 1. Air classification
u&
used in these studies.
The air classification unit, shown in Fig. 1, consists of a throat section through which air is drawn by a 0.75 kW (1 h.p.) fan. The feed hopper introduces the refuse into the 5 cm X 15 cm throat, and the light fraction of the refuse is fluidized and transported by the air into the cyclone separator where it loses its velocity and falls into the collection hopper while the air continues to be drawn to the fan. The fraction of the refuse that is too heavy to be fluidized falls against the flow of air and exists at the bottom of the throat. A sliding gate valve and a venturi orifice placed in the pipe between the cyclone and the blower is used to regulate and measure the air flow. Zigzag throat The first throat design tested was a zigzag throat consisting of 12 stages, with the refuse feed introduced at the 8th stage from the bottom, described in Fig. 2 [3]. Each stage is 10 cm long, and inclined 60” from the horizontal. In order to observe the turbulence within the throat, two sides of the throat section are constructed of clear plastic. The advantage of the zigzag throat is that it produces vortices that cause turbulence within the throat and thus helps to break apart clumps of refuse, facilitating separation [9, lo].
250 ZIGZAG
THROAT
UTAH
THROAT
DUKE
THROAT
Fig. 2. Air classifier throat designs used.
Utah throat The Utah throat, developed at the University of Utah, has the bottom half of the zigzag removed (Fig. 2) [ll] . This modification allows impaction of agglomerated particles on both the horizontal and vertical surfaces. The design also increases the intensity of the vortices. The unit tested in this study contained 12 stages, with the feed introduced at the 8th stage from the bottom. All surfaces of the throat were constructed from clear plastic so that the turbulence occurring in the throat could be observed. Unlike the throat developed at Utah, the unit used in this study had a constant distance between the vertical walls. Duke throat The Duke throat was developed in an attempt to further modify the initial zigzag concept. While the Utah throat produces increased turbulence, it also perpetuates a problem that occurs in the zigzag throat, which is that after conglomerated refuse is broken into lights and heavies, these particles must either pass up through the throat or down through the throat at which time interference from particles moving in the opposite direction might cause loss of momentum or a reconglomeration of the particles. In an attempt to alleviate this problem, the Duke throat, illustrated in Fig. 2, provides a free path for the light particles and a separate chute for the heavies which can now fall down the outside without encountering any upflowing lights. The Duke throat was constructed completely out of clear plastic so that the separation and performance of the throat could be observed.
251 MATERIAL
CLASSIFIED
The feed material to be used in the testing of the performance of air classifiers must be carefully selected. There are four basic options: (1) using actual shredded municipal solid waste (MSW): (2) using MSW that has been seeded with specially marked items; (3) using a feed that simulates the composition of actual MSW but reducing the number of the actual components that constitute refuse; and (4) using artificial materials that symbolically represent MSW. Each method has advantages and disadvantages. Since air classifiers are usually designed to separate MSW, using actual MSW should be the first option considered. There are several advantages to testing a new throat with genuine MSW. When doing so, there is little concern that the characteristics of the test materials will be significantly different from the material the throat is designed to separate. Another advantage in using actual MSW is that there is usually a plentiful supply of the actual material which a classifier will be expected to separate. There are also several disadvantages to using MSW. There are the obvious difficulties with using a putrescible waste, and problems occur when attempts are made to quantify the composition of the refuse. Not only is this a dirty and time-consuming job, but it is also potentially dangerous. Another disadvantage to using MSW is that its composition varies from sample to sample, and little experimental control is possible. One way to alleviate some of the disadvantages of working with MSW while retaining most of the advantages is to seed the MSW. Seeding of MSW is accomplished by marking various components of waste with bright paint, and after the waste has been classified, the coded material can be separated. The advantage of this method is that the amount of handsorting is greatly reduced. There are, however, several disadvantages to using seeded MSW. The same problem of relatively short storage life applies, as does the problem of being exposed to possible pathogenic organisms, etc. A third type of feed that could be used is simulated MSW. By choosing only a few typical materials, it is possible to standardize the quantity of each component of the simulated MSW and thus obtain a “standardized bag” of feed material. Another advantage of simulated MSW is that putrescible organic wastes can be excluded. Hand sorting the clean material is also much easier than sorting MSW. The major disadvantage to using simulated MSW is that the characteristics of the waste could differ from MSW. In the case of this experiment it was decided to use simulated municipal solid waste. Since different throat designs are being compared it is of the utmost importance that the refuse input into the different throats always be identical. The simulated MSW used is shown in Table 1. Newspaper, plastics (mostly sheet polystyrene and polyethylene), aluminum (beverage cans) and ferrous metal (turnings from a machine shop) were shredded to a nominal size of 2 cm. Next, the material from each
252 TABLE 1 Simulated feed material composition -___-... Material
Percent by weight
Lights Newspaper Plastics
72 9
Heavies Aluminum Steel
12 7 100
____
category was weighed out and mixed in a large plastic bag. The paper and plastics are organic and are here considered to be the “lights” which should report to the overflow (carried up in the air stream). The aluminum and ferrous metal turnings are the “heavies” and it is desired that they drop out in the underflow. PROCEDURE
AND RESULTS
The material was fed into the feed hopper which contains a rotary air lock feeder that introduces the material into the throat section at a constant feed rate. The underflow and overflow were collected, separated by hand into various components, and weighed without drying. Zigzag throat The zigzag throat
test results are shown in Table 2.
Utah throat The Utah throat test results are shown in Table 3. Run #6 at the lowest air speed presented a problem because the paper tended to clog the throat as it traveled downward in the throat. At other air speeds no problems were encountered. Duke throat Of the three, this throat had the highest tendency to clog. This is directly related to the increased number of potential areas where low velocities exist and particles can build up and eventually clog the throat. At low air speeds (below 2.5 m/s) the paper tended to accumulate on the flat surface
2.3 4.6 7.3 10.3 13.5 15.3
1 2 3 4 5 6
16.0 727.8 744.2 799.1 808.3 809.8
Paper and plastics reporting as overflow (g)
Air speed (m/s)
2.3 4.6 7.3 10.3 13.5 15.3
Run number
1 2 3 4 5 6
Paper reporting as overflow (g)
0.0 720.0 720.0 720.0 720.0 720.0
190.0 188.9 181.7 150.8 91.5 53.6
0.0 721.7 787.7 806.3 810.0 810.0
0.0 1.7 67.7 86.3 90.0 90.0
Plastic reporting as overflow (g)
0.0 10.8 24.2 79.1 88.3 89.8
16.0 717.0 720.0 720.0 720.0 720.0
_~
Plastic reporting as overflow (g)
Paper reporting as overflow (g)
Aluminum and ferrous reporting as underflow (g)
190.0 188.0 187.3 173.3 123.4 86.5
Aluminum and ferrous reporting as underflow (g)
Paper and plastics reporting as overflow (g)
Results of test runs on Utah throat
TABLE 3
Air speed (m/s)
Run number
Results of test runs of zigzag throat
TABLE 2
120.0 119.7 116.2 96.1 60.3 34.0
Aluminum reporting as underflow (g)
120.0 118.1 117.4 105.7 69.4 52.6
Aluminum reporting as underflow (g)
70.0 69.2 65.5 54.7 31.2 19.6
Ferrous reporting as underflow (g)
70.0 69.9 69.9 67.6 54.0 33.9
Ferrous reporting as underflow (g)
2.3 4.6 7.3 10.3 13.5 15.3
1 2 3 4 5 6
2.3 4.6 7.3 10.3 13.5 15.3
Air speed (m/s)
*Paper and plastics. **Aluminum and ferrous
1 2 3 4 5 6
__-
Run number
Percent inorganics* * reporting as underflow 100.0 98.9 98.8 91.2 64.9 45.5
Percent organics* reporting as overflow
0.0 89.6 91.9 98.7 99.8 100.0
_.
_
0.0 99.6 100.0 100.0 100.0 100.0
Percent paper reporting as overflow
0.0 718.9 720.0 720.0 720.0 720.0
190.0 189.0 188.0 180.0 135.4 124.3
0.0 718.9 723.5 768.4 799.9 803.4 ~.
Paper reporting as overflow (g)
Aluminum and ferrous reporting as underflow (g)
Paper and plastics reporting as overflow (g)
Percent recovery of organic6 and inorganics from zigzag throat
TABLE 5
Air speed (m/s)
Run number
Results of test runs of Duke throat
TABLE 4
Percent aluminum reporting as underflow 100.0 98.4 97.8 88.1 57.8 43.8
0.0 12.0 26.9 87.9 98.1 99.9
120.0 120.0 119.1 110.0 81.9 77.4
Aluminum reporting as underflow (g)
Percent plastic reporting as overflow
3.5 48.4 79.9 83.4
0.0
0.0
Plastic reporting as overflow (g)
~______
100.0 99.9 99.9 96.6 77.1 48.4
Percent ferrous reporting as underflow
70.0 69.0 68.9 70.0 53.5 46.9
Ferrous reporting as underflow (g)
0.0 88.6 90.8 90.0 64.8 45.5
Total efficiency (%)
255
above the feed throat. As air speeds were increased, the turbulence increased and alleviated this problem. The results from the Duke throat runs are presented in Table 4. ANALYSIS
OF RESULTS
One objective of this study was to develop a method of evaluating air classifier performance which would use a single parameter to describe the performance of an air classifier. The parameter developed is termed “Total Efficiency”, and is defined as the product of the fractional recovery of light in the overflow and fractional recovery of heavies in the underflow (eqns. (3) and (4)). These values for the three classifiers tested are shown in Tables 5, 6 and 7. This parameter satisfies the requirement that in an ideal air classifier 100% Total Efficiency would be achieved only when 100% of the lights reported as the overflow, and 100% of the heavies reported as the underflow. The Total Efficiency thus calculated can be plotted versus air speed for each throat tested as in Fig. 3. The peak of each of the curves is the point of maximum efficiency for the specific throat design, and defines the air speed that should be used to achieve this maximum efficiency, for the given feed rate. The shape of the curves represents the overall separation character-
YO-
BO-
70-
F
k/
L
co-
g
E w
50 ~
_J 2
4O-
3O-
ZO-
IO -
0 0
25
I
I
I
5.0
7.5
IO.0
AIR
SPEED
I 12.5
I 15.0
1
(m/SW)
Fig. 3. Total Efficiency for three different throat designs.
2.3 4.6 7.3 10.3 13.5 15.3
1 2 3 4 5 6
0.0 89.1 97.2 99.5 100.0 100.0
Percent organics* reporting as overflow 100.0 99.4 95.6 79.4 48.2 28.2
Percent inorganics** reporting as underflow
2.3 4.6 7.3 10.3 13.5 15.3
1 2 3 4 5 6
*Paper and plastics. **Aluminum and ferrous.
Air speed (m/s)
Run number
0.0 88.8 89.3 94.9 98.8 99.2
Percent organics* reporting as overflow 100.0 99.5 98.9 94.7 71.3 65.4
Percent inorganics** reporting as underflow
Percent recovery of organics and inorganics from Duke throat
TABLE 7
*Paper and plastics. **Aluminum and ferrous.
Air speed (m/s)
Run number
Percent recovery of organics and inorganics from Utah throat
TABLE 6
0.0 99.8 100.0 100.0 100.0 100.0
Percent paper reporting as overflow
0.0 100.0 100.0 100.0 100.0 100.0
Percent paper reporting as overflow
0.0 0.0 3.9 53.8 88.8 92.7
Percent plastic reporting as overflow
0.0 1.9 75.2 95.9 100.0 100.0
Percent plastic reporting as overflow
Percent ferrous reporting as underflow 100.0 98.6 98.4 100.0 76.4 67.0
100.0 100.0 99.2 91.7 68.2 64.5
_
100.0 98.9 93.6 78.1 44.6 28.0
Percent ferrous reporting as underflow
aluminum reporting as underflow
100.0 99.8 96.8 80.1 50.2 28.3
Percent aluminum reporting as underflow
0.0 86.3 88.3 89.9 70.4 64.9
Total efficiency (%)
0.0 88.6 93.0 79.0 48.2 28.2
Total efficiency @)
257
istics of the throat and defines its sensitivity to fluctuations in air speed. A further step, not attempted in this study, is to develop a family of such curves for each classifier, with each curve corresponding to a different materials feed rate. Obviously, at high feed rates the total efficiency can be expected to drop. It is apparent that although the Utah throat achieves the higher peak efficiency of separation, it also seems to be sensitive to variations in air speed. The zigzag and Duke throats exhibit similar characteristics, with both curves reaching a flat peak of approximately 89% Total Efficiency. This peak remains constant between 5 and 10 m/s, but above an air speed of about 10 m/s the zigzag throat efficiency drops off more rapidly than the Duke throat. Another method of presenting air classifier data is to plot the percent of the various incoming materials reporting to the overflow (Fig. 4). As an example of how such plots might be used, consider a case where 8% of the incoming aluminum and 5% of the incoming ferrous metal might be allowed as the maximum level of contaminants in the overflow. If the zigzag air classifier was used to separate the incoming material, then according to Fig. 4 the maximum allowable aluminum contamination is reached at an air speed of 9.7 m/s and the maximum allowable ferrous contamination is reached at 10.2 m/s. By using an air speed of 9.7 m/s, the air classifier produces an overflow that contains 100% of the incoming paper, 70% of the incoming plastic, 8% of the incoming aluminum, and 4% of the incoming ferrous material. 2 : g 100
2 90 Yz c 80 B ; 70 a ‘r 60 Z 5 50 2 = 40 CJ 3: 30 ?I z 20 s 2s E
IO 0 0
25
5.0
7.5 AIR
Fig. 4. Percent (zigzag throat).
of incoming
paper,
SPEED
plastic,
10.0
12.5
15.0
I7.5
~mpcc)
aluminum,
and ferrous
reporting
to overflow
258 APPLICATION
OF RESULTS
In order to compare the efficiencies of the three throats tested in this study it was necessary to develop the new method of determining efficiency. The “Total Efficiency” parameter allows for a more acceptable comparison of performance data and thus facilitates the evaluation and specification of air classifiers. The percent contamination by heavies of the overflow fraction for a specific throat and air speed can be used to develop the value of the products. For example, with reference to Fig. 4, using any given air speed, the composition of the overflow can be determined. The market value, or the acceptability of the product to prospective purchasers can thus be evaluated. Using prevailing market values and purchaser specifications for recovered material, it is possible to construct curves such as Fig. 5, which show the total potential value of the two fractions given the performance curve. This curve was developed assuming, for example, that at higher air speeds, aluminum was lost into the overflow and was no longer recoverable, thus decreasing the overall value of the products. Similarly at low air speeds, the total lights reporting as overflow was lower, thus decreasing the income from the sale of refuse-derived fuel. The obvious operating air speed would be at the peak of the curve, the point of maximum product value.
01 0
I 2.5
I 5.0
I 7.5 AIR
SPEED
I IO.0 (m/m)
Fig. 5. Value of recovered material (Utah throat).
I 12.5
I 15.0
I 17.5
259
ACKNOWLEDGEMENT
The data and methodology for the production of the market value of air classified refuse was developed by Bruce Polkowsky, a graduate student of the Department of Civil Engineering, Duke University. This research was funded in part by the Richard C. Leach Endowment Research Fund.
REFERENCES 1 Taggart, A.F., 1974. Handbook of Mineral Dressing, Wiley, New York. 2 Fan, D.N., 1975. Resource Recovery and Conservation, 1: 141-150. 3 Boettcher, R.A., 1972. Air Classification of Solid Waste, US EPA SW-3Oc, Washington, D.C. 4 Murray, D.L. and Liddell, C.L., 1977. Waste Age, 8 (3). 5 Sweeney, P., 1978. Air Classification, presented at Engineering Foundation Conference, Rindge, N.H. 6 Midwest Research Institute, 1979. Study of processing equipment for resource recovery systems, EPA Contract No. 68-03-2387, Washington, D.C. 7 Chrismon, R.L., 1977. Air classification in resource recovery, National Center for Res. Rec., RM 78-1, Washington, D.C. 8 Vesilind, P.A., 1977. ASCE, J. Envir. Eng. Div., 103 (EES): 761-765. 9 Senden, M.M.G. and Tels, M., 1978. J. Powder & Bulk Sol. Tech., 2(l). 10 Senden, M.M.G., 1979. Ph. D. Dissertation, Techn. Hogeschool Eindhoven, The Netherlands. 11 Eckhoff, D., 1974. University of Utah, personal communication and unpublished report.