I
.
Rotary Air Dryer Operation
i
J. ROBERT SPRAUL General American Transportation Corp., Easf Chicago, lnd.
I
NVESTIGATORS of the problems of fillage, time of passage, and scale up of rotary air dryers usually have made extensive studies with one or a t most a limited number of different materials. This article subjects the equations and concepts developed from studies of a few materials to the test of application t o a wide variety of materials. The work reported is the outgrowth of studies made in the pilot plant of a rotary dryer manufacturer. Experimental unit i s designed for study of wide range of operating variables
The pilot plant dryer used in this work is 1 foot in diameter by 6 feet long and is similar to t h a t used by Friedman and Marshall (3), except t h a t the feed and discharge arrangements are different.
The dryer is shown in Figure 1.
T h e unit can operate with either parallel or countercurrent air flow, The dust can be collected by exhausting the air into a filter bag, 4 feet in diameter by 8 feet long, connected to the outlet duct. This bag is easily detachable for weighing the quantity of dust collected. Air is pulled through the dryer by a fan; the fan velocity is controlled by a damper. Air velocities t o 300 feet per minute through the dryer shell can be obtained. Retention time in this unit can be varied by the introduction of different sized dam rings a t the discharge end of the dryer. Heated air is introduced into the dryer either through a gasfired combustion chamber or through a steam coil heated radiator. Each heating unit is movable so that both parallel and countercurrent operations are available. The combustion chamber is 2 feet in length and is insulated. The gas burner is a ring type which gives a very short flame length. This is particularly important when organic or animal feed materials which might be charred by long flame propagation are tested. At the feed end of the dryer are six spiral flights, 1 inch high and extending 7 inches into the dryer. These are followed by an 18-inch section of straight flights which extend 11/z inches directly into the dryer, without lips or bends. All flights in the dryer are arranged in rows 6 inches long, and each row is staggered 1 inch from the preceding one. There are three rows of straight flights. These are followed by three rows of flights set a t a 45" angle. They also extend l l / zinches into the dryer with the angle beginning 3/4 inch from the dryer circumference. Toward the dlscharge end of the dryer are foul rows of 90"-angle flights which again extend l l / zinches into the dryer wlth a lip '/z inch high. The last 5-inch length of the dryer shell is without flights, and the space allows insertion of discharge dam rings. This flight system has been used throughout the tests reported in this paper, for it has been found to be the most suitable for the widest variety of materials. I n conducting pilot plant tests for commercial dryer design, limitations of time and, particularly, availability of material usually do not permit changes of flight arrangement. Therefore, the most versatile flight design must be used. The staggering of the flights leads to a better curtain of material and a better heat transfer rate. Many of the tests reported in this paper were conducted with essentially zero slope and with the insertion of dam rings. The
368
use of such conditions leads to higher fillages than generally used by previous investigators and, further, permits the use of longer retention time without excessive lengthening of the dryer. AB Friedman and hlarshall (l+jpointed out, the increase in fillage offers only a slight increase in heat transfer rate (they found u p to 20%); the fact that a shorter dryer can be employed justifies the use of higher fillages. Test Procedure. The test procedure was essentially that described by Friedman and Marshall ( S ) , except that the material was fed periodically by weight instead of volume. The time interval between introduction of material usually was 1 minute. During the course of the test or a t the completion of the study the following information was obtained, essentially as prescribed by the American Society of Mechanical Engineers (10): Air velocity or volume through the dryer (by Pitot tube measurements) Dryer rotation, r.p.m. Dryer slope Dryer holdup, pounds Dust loss (by periodic product recovery or actual dust collection) Screen analysis, if indicated Bulk density of feed, product, and dryer holdup Time of passage and holdup studies are made on extensive variety of materials
The essential data on the t'ime of passage and holdup obtained with 25 different materials are given in Table I. The holdup in pounds, H , was determined by actually discharging and weighing the material remaining in the dryer after the test,. This figure can be accurately determined except, in cases where there is unusual sticking to the dryer shell. The determination of the t,ime of passage, 7 , by dividing the holdup in pounds, H , by the feed rate in pounds per hour, R, is based on two weight measurements which can be made precisely; from an experimental standpoint this method is t,he most reliable. In Table I, the holdup in percentage of dryer volurnc, X , was obtained by two different methods, depending on the material, and no record of the method used was made at the time of the test. One procedure consisted of discharging the fillage of the dryer, det,ermining its density, and calculating t,he volume percentage from the density and weight, €€. In a number of cases, however, it was almost, impossible to obtain a representative sample of the fillage. For example, the materia1 may be fed to the dryer as a thick sludge or mud, arid it subsequently may pass through stages in the form of large balls, then smaller pellets, and may finally be discharged as a fine powder. In such cases, sampling or act,ual density determination of the fillage material is difficult. The other method of determining percentage of dryer holdup, X,consisted of using the average of the densities of the feed and dryer products as the density
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 47, No. 3
of the fillage material and calculating X from this figure and the weight, H. Table I shows that the agreement between the time of passage, 7 , determined by two weights, H / R , and from densities, LX/IOO F , are generally satisfactory, but in certain cases there is a variation that reflects both the difficulties of msking density determinations and material variations during passage of the material through the dryer. The visual time of passage was the time a t which the operator believed the dryer had reached equilibrium. In certain cases this visual time of passage varied widely from that determined from physical measurements. The data in Table I were used to test the reliability of various equations and theories that have been proposed concerning the relationship among such dryer conditions as feed rate, time of pamage, slope, speed of rotation, diameter, length, and number of flights. None of the tests recorded in Table I were made on dry materials simply for the purpose of studying time of passage or holdup. The primary purpose of these tests was to obtain a properly dried product and to collect data for dryer design. A considerable amount of the previous work reported in the literature on holdup-time of passage relationships and transport properties has been conducted on dry materials. Friedman and Marshall ( 4 ) reported a considerable difference in the fillage when wet sand was used in place of dry sand. Many of the tests were run a t zero slope, which was satisfactory for pilot plant tests. On wet materials the effect of slope, rate of rotation, and number of flights is not as pronounced as would be indicated by studies on dry materials. Actual slopes used on commercial dryers are of the order of 0 to 0.5 inch per foot. A strictly dimensional relationship between the time of passage and holdup, based on dryer length and solid feed rate, can be expressed by the equation
different feed rates. With the following materials, only the feed rate was altered, and operation in accordance with Equation 1 was observed-amine hydrochloride (countercurrent only), aspirin, o-chlorobeneoic acid, reclaimed rubber, plastic material, granulated cork, and clothespins. Varying Operating Conditions. With other materials, variations in feed rate and sometimes in other operating conditions were introduced that led to results a t variance with Equation 1. AMINEHYDROCHLORIDE. For parallel flow only, in run 2 the feed rate of amine hydrochloride was one half that in run 1 ; however, a somewhat lower air velocity was used. This combination of lower feed rate and lower air velocity produced a drier product, and this product was more dusty even though the air velocity was less. I n this case a slight change in the moisture content of the product nullified the relationship expected between the feed rate and time of passage. The time of passage was less than would be expected from the changes in feed rate and air velocity. BENZOIC ACID. The two runs reported with benzoic acid conform to Equation 1. The air velocity in run 3 was 160 feet per minute through the dryer, compared t o 200 feet per minute for run 2. Unlike the amine hydrochloride, this change in air velocity did not alter the expected relationship since the products of both tests were very similar in moisture content and physical characteristics. CALCIUM PHOSPHATE.All tests on calcium phosphate were made under the same conditions except for increased feed rates. The time of passage did not decrease as rapidly as would be expected from Equation 1. The variation can be explained on the basis of differences in product moisture-0, 4.2, and 9.3% for runs 2, 3 and 4, respectively. MOLDING POWDER.The time of passage of molding powder in run 5 was decreased considerably because of a n increase in air LX T = velocity from 210 to 300 feet per minute. Thus, the dust loss in 100 F run 5 was higher than in run 4,even though similar product moistures were obtained. Equation 1 expresses a relationship for a particular dryer and INORGANIC WAPITE.All conditions of the two tests on inorpredicts, for example, a reduction in time of passage with an inganic waste were the same, including product moistures, except crease in feed rate. The equation implies identical operating for feed rates and the installation of one perforated baffle for conditions such as air velocity, slope, rate of rotation, discharge run 2. This baffle was placed 2 feet from the discharge end and rings, etc. lengthened the expected retention time by decreasing the dusting Constant Operating Conditions. Many of the tests reported in of this finely divided material. SYNTHETIC RUBBER. Both the feed and the wroduct moisTable I were conducted under very similar conditions but with tures were different in the two runs reported for synthetic rubber. Although the physical appearance and handling characteristics of the two rubbers seemed to be the same, the expected relationship was not obtained. PEANUTMEAL WASTE. The air velocity in run 7 for peanut meal waste was higher than run 6, decreasing the retention time. A higher inlet air temperature was used that produced a much drier product. P H O T O G R AIPcHCHEMICAL.The smaller percentage holdup of photographic chemical in run 7 is due to the use of a higher air velocity in the run, compared to run 1. A higher inlet air temperature also was maintained in run 7. PHTHALICACID. The p r i n c i p a l difference between the two runs with phthalic acid was a change in feed material to wet-dry mix with a somewhat lower moisture content. The holdup percentage and time of passage Figure 1 . Rotary air dryer for fillage and retention time studies ’ were appreciably changed.
March 1955
INDUSTRIAL AND ENGINEERING CHEMISTRY
369
ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT
Table 1.
Operating Data for Pilot Plant Rotary Dryer (Discharge ring height, 2.5 inches)
Run NO.
Exhaust Air Volume, Cu. Ft./Min. C = Countercurrent P = Parallel
Moisture Content (Wet Basis), % Feed Product
13 1 13.1 13.1 13.1 4.0
1 2 3 4 5
4.05 2.70 1.8 0.4
0.3
Rate, Feed (E) Lb./Hr. 20 10 15 5
30
Dryer Holdup Dryer Speed Slope X ,% of dryer, (N), (Sd) t H, vol. R.P.M. Ft./Ft. lb. Amine Hydrochloride 5 0 11 6 7.4 5 0 8 7 5.7 5 ’ 0 00695 20 17 5 0.00695 26 17 5 0 00695 26 li
Solid Feed Rate ( F ) , Cu. Ft./ (Hr.)(Sq. Ft.) 0.67 0.34
Time of Passage ( r ) , Hr.
H/R
0.505 0 169
1.02
0 0 1 5 0
58 87 73 2 80
- -_ L. d __ 100 F
0 68
Visual
0.99 2.01 f j 03 .0
’.
0 06 1.0 2.0 0 2.5 1 0
Aspirin 4a 75
235 (P) 235 ( P )
2
3.4 3.4
0.02 0.01
50 75
B 6
157 (C) 118 (C)
39.2 34.5
0 . 58 0.3
’?
5
8
9b 10
204 (C) 212 ( C ) 212 ( C )
26.3 28.4 24.8
0 6 8 2 3.2
105 120 90
6 6 G
2 3 4
157 (PI 157 (P) 157 ( P j
32.3 30.8 30.8
0
6
9.31
75 120 150
19.8 18.23
7.04 0.13
30 15
6
3
5
0 0
9
8
4 2 3.8
1.46 2.18
0 18 0 10
0 174 0.104
0 183 0 083
Benzoic Acid 0 14 0 14
9 .5 9.5
0 39 0.195
1 4 2 8
2 92
1 . 4fi
2.5 6.0
10.6 7.07 10.6
2.23 2.55 1.91
0 28 0 158 0 32
0,285 0 166 0.33
O.-lf56 0 133 0 33
12.3 15.3 17.1
1,73 2.70 3 4G
0 13 0 33 0 297
0.126 0 33 0.298
0,50 0 4:2 0.33
8.2 7.9
1.91 0,955
0 22 0 43
0 257 0,:
0.66
4
10 2 10.2 9
2.32 3 09 1.55
0 206 0 2 0 35
0 261 0.198 0 348
0 20 0 33
Cocoa Sludge 0.00695 14
8.2
2 G3
0 1s7
0 183
0 33
21.2 21 2
0 106 0 123
0.167 0.124
0.083
9.23 10.0
0.703 0.94
0 43
0 53
0 79 0.69
0.5 0.83
11 16.4
0.827 1.27
0 8 0 77
0.79 0.77
...
Calcium Chloride
‘4 2
6 6
0 0.00695 0.01733
29 3 19 20
Calcium Phosphate 0 32 0 40 0 4.4.; o-Chlorobenz3ic Acid
1
2
78.5 (P) 78.5 ( P )
6
275 (PI 275 (P) 275 ( P )
37.5 38.5 31.1
19.0 9.2
30 40 20
6 6
4
5
104 (PI
40.5
0.4
75
6
1
3
2
137 (PI 137 (P)
23.0 23.45
B
20
157 (P) 157 ( P )
62.6 81.7
3
157 (P) 137 (P)
23.6 20.5
1
0.41
6
0 0
6.7 6 5
Clothespins 0 0
8
30 40
6
6
Granulated Corh 0 5 0 5
0.33 0.6
45 BO
6 6
Inorganic Waste 0 24 0 26
0.42 4.3
30 45
6
22.1 6.6
7.6 10.2
1.23
0 23
0.110
Insecticide 4
2 3
157 ( P ) 157 (P)
4
165 (P) 235 ( P )
5
2.7 3.75 29.9 29.9
0
0
4.0 3.6
240 360
6 6
Mineral Slag 0 102 0.0139 85
20.5 17.1
2.91 4.37
0 425 0 236
0.422 0.238
0.417 0.25
25 30
6 6
Molding Powder 0 15 0 10
11.4 7.6
1.14 1.36
0 6 0 3
0.605 0.335
0 45 0.25
SODIUMMETABISULFITE.Runs 1 to 3 with sodium metabisulfite gave the expected holdup-time of passage relationship despite wide feed rate changes and a higher air velocity in run 3. A comparison of runs 4 and 6, Rhich were made under similar conditions, shows that the relationship was nearly normal except that an increase in air velocity in run 6 somewhat increased the time of passage. ISSECTICIDE.I n the tests of insecticide the reduction of feed moisture by 3% changed the handling characteristics of the material sufficiently t o produce different results in the two runs reported. SomuIlr SULFATE. For sodium sulfate, the major difference in the two runs, besides the feed rate, was the use of an air velocity of 250 feet per minute in run 6. Despite this variation, the percentage holdup and time of passage were similar. This is a heavy material and is not affected markedly by air velocity changes. MINERAL SLAG. Despite the use of a definite slope in run 3, the time of passage of mineral slag was only slightly less than would be expected because of the increased feed rate. UREA. The percentage holdup and time of passage of urea were decreased in run 4 over that obtained in run 3 by the use of
370
24 31 5
6
0 0
...
increased air velocity. The use of the slope in run 1 further decreased the time of passage. PHARMACEUTICAL (TYPENo. 1). I n run 4 both the feed rate of pharmaceutical S o . 1 and inlet air temperature was increased considerably. This produced an entirely different percentage holdup. PHARRIACEUTICAL (TYPENo. 2). The two runs for pharmaceutical KO. 2 reflect the same results obtained with pharmaceutical S o . 1. POLYVINYL ACETATE. ,411 conditions were essentially the same in both tests of polyvinyl acetate, except for the difference in discharge ring heights. The increased ring height in run 4 was necessary to lengthen the retention time in order to remove an additional 0.4% moisture, required by the product specifications. Limitations. From an examination of the data presented on these 25 different materials, certain additional limitations on the basic relationship expressed in Equation 1 become apparent: The equation is not valid if the change in feed rate, holdup, or time of passage produces any appreciable change in the handling characteristic of the material in the dryer, such as a netter or drier product.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 47, No. 3
PILOT PLANT
Table I
Run
No. 6 7 Id
3d 4d
Exhaust Air Volume, cu. Ft,/Min. Moisture Content (Wet Basis), % C = Countercurrent P = Parallel Feed Product 173 (P) 216 (P)
56.0 56.0
10.7 0.7
:;i;;
10.0 10.0 5.6
1.6 I .2 0.6
90 (P)
(Continued)
(Discharage ring height, 2.5 inches) Dryer Dryer Holdup Feed X,% of' Speed Slope Rate, dryer, (Ar), ( S d ) , vol. (€2) Ft./Ft. Lb /Hr. R.P.M. Peanut Mea1 Waste 6 0 29 15 4 60 t i 0 18 9.55 75
2.
Solid Feed Rate (F), Cu. Ft.) Ft./ (Hr.)(Sq.
Time of Paasage H/R LX
(T)&
rF Visual
1.91 2.38
0.483 0.24
0.48 0.24
0.73 0.25
10 15 60
6 6 6
Pharmaceutical (Type 1) 0 8 0 9 0 17
4 2 4 8 9 1
0.303 0.455 1.82
0.8 0.6 0.283
0.83 0.63 0.3
1.0 0.75
5 1 8.4
0,934 1.86
0.3 0.275
0.33 0.27
0 33
19.5 11.7
0.44 1.17
2.66 0.6
2.66 0.6
1.16 1.35
10.0 12.7
0 97 0.97
0.6 0.8
0,657 0.787
0.5 0.5
14.4 15.2
0.694 0.555
1.13 1 5
1.24 1.64
... ...
...
3d
90 (P) 90 (P)
6.1 6.4
0.32 0.27
30 60
Pharmaceutical (Type 2) 6 0 10 6 0 16.6
1 7
7 8 . 5 (C) 177 (C)
9.8 8.6,
2.5 0 05
15 40
6 6
1 2
157 (PI 138 (P)
45.4 34.3
2.5 0.07
30 30
6 G
Phthalic Acid 20 0 24 O
5 6
98 (P) 98 (P)
36.6 36.0
2.9 0.7
15 12
6 6
Plastic Material 0 17 0 18
3d 4
98 (0 98 (C)
11.8 12.6
1.1
10
10
6 6
Polyvinyl Acetate 0.0138 12.5 24.5 0.0138
6.1 11.9
0.303 0.303
1 25 2.45
1.60 2.36
1.75 2.91
4 6
196 (PI 196 (P)
36.2 36.1
0 10.9
30 60
6 6
Reclaimed Rubber 10.75 0 11.25 0
10.1 10.7
1.53 3.05
0.358 0.187
0.396 0.21
0.5 0.33
1 2 3 4=
4.1 4.0 4.4 4.4 4.2
0.03 0.04 0.4 0.12 0.17
50 80 100
tic
118 (C) 118 ( C ) 196 (C) 118 (C) 188 !C)
120
4 4 4 4 4
Sodium Metabisulfate 74.7 0,00971 74.7 0,00971 74.7 0,00971 37.3 0.00971 37.3 0,00971
26 26 26 13 13
1.16 1.88 2.32 2.32 2.77
1.49 0.93 0.747 0.373 0.31
1.34 0.83 0 674 0.336 0.28
1.16 0 75 0.5 0.33 0.282
4 6
196 (P) 118 (P)
7.95 8.95
0.01 0.03
420 360
6 6
Sodium Sulfate 80 0 78 0
20.6 20.2
6.53 5.6
0.19 0.216
0.19 0.216
0 21 0.18
2 3
I96 (P) 196 (P)
49.1 28.5
2.75 0.56
10
Synthetio Rubber 12 0 IO 0
9.6
8
6 6
8.0
0.462 0.37
1.2 1.25
1.25 1.3
1 5 1.75
1
118 (P) 118 (P) 177 (P)
4.3 3.4 3.4
0.12 0.25 0.07
30 20 20
6 6 G
3.1
0.89 0.59 0.59
0.233 0.55 0.45
0,209 0.49 0.398
0.25 0.33 0.33
Id
3 4
0.75
100
Photographic Chemical 40 0.0104 24 0,0104
Urea 0.0208 0 O
a No discharge ring. b Discharge ring height = 1 inch.
C
Correlations based on physical properties are nof reliable from practical standpoint
T~~ investigations have correlated the time of passage with the dynamic angle of repose of the material. Sullivan, Maier, and Ralston (9) proposed the following relationship: o.000517 L w
(2)
SdDN
BaYard (1) elaborated on their work and proposed the modified equation 7 =
March 1955
4.8 3.9
Perforated baffle.
d Discharge ring height = 1.5 inches.
The equation is not necessarily valid over a wide range of feed rates. Large variations in feed rates may radically change the percentage holdup, even though the inlet air temperature is adjusted to produce a material with the same moisture content, This situation was encountered with both types of phafmaceuticals. This same result was obtained by Prutton, Miller, and Schuette (7) in their studies on the action of sand and fullers earth in a rotary dryer.
7 =
7 11 9
...
o.oooos~s(s+ 24)L SdDN
(3)
The investigations that led to the development of Equations 2 and 3 were in rotarykilns without any flights. Prutton, Miller, and Schuette ( 7 ) have reported that their experiments did not agree with Equation 2. The present author, on the basis of experience with a wide variety of materials, obrelying on the dynamic angle of repose. jects to any A relatively high percentage of materials handled in rotary dryers will exhibit several widely different angles of repose as they proceed through the dryer because of changes in physical characteristics or particle size. This leaves the engineer with the much more difficult problem of selecting or obtaining an average value for this property. Further, the author believes that Equations 2 and 3 are much too sensitive to changes in dryer slope, diameter, and speed of rotation, and not sensitive enough to changes in dynamic angle of repose. Prutton, Miller, and Schuette ( 7 ) were the first to propose a formula for the time of passage that incorporates an air velocity factor,as shown by Equation 4. Their use of two constants that are subject to experimental determination is an important step forward, although the fact that they determined k with dry materials limits the accuracy and versatility of Equation 4.
INDUSTRIAL AND ENGINEERING CHEMISTRY
371
ENGINEERING. DESIGN. AND PROCESS DEVELOPMENT (4) where k is a constant that is characteristic of the dryer design, particularly of the number and type of flights, and m is a constant that depends on the nature of the material and the direction of the air flow. However, Prutton, Miller, and Schuette found onlya small variation in the value of k from 0.0046 for six flights to 0.0053 for 12 5ights (with retention time in hours and slope in feet per feet). For the sake of calculation a value of k of 0.005 was selected for the dryer used in this work, and values of m n-ere determined for individual runs where dryer slope was used. These values are presented in Table 11, and they show a much wider variation among materials than previously reported. For countercurrent air flow, values of m varied from +0.0313 to -0.0021. Unexpected negative values were obtained with calcium chloride and sodium metabisulfate. Since k was selected and not determined experimentally, it is difficult to ascertain the cause for the negative values. For parallel current air flow,values of m varying from -0.0044 t o -0.00022 were obtained. The data of Table I1 are not sufficient t o permit any relationships between the values of m and any physical propertieq or characteristics of the materials t o be established. The data for countercurrent flow do point up one definite limitation of Equation 4-that the value of ?n varies with solid feed rate and in every case becomes smaller as the feed rate is increased. FT-ith calcium chloride and sodium metabisulfite the values of m have even become negative with high feed rates. These limited results show that the dryer constant, k , is subject to more variation than indicated by the original work of Prutton, Miller, and Schuette ( 7 ) . The development of an equation involving the constants, k and m, ie important because the characteristics of the material itself may be evaluated and incorporated into the relationship. From an experimental standpoint, however, the determination of the value of k would be rather difficult with a great many of the materials presently handled in rotary air dryers. If the material in the dry state acts the same in the dryer as the same material during an actual drying operation, then the value of k has meaning. Otherwise, the value of k rrould be determined on a material that is essentially different, from a handling characteristic, from the material actually being dried. Relationships based on pilot plant studies take into account changes in material during drying
reliability of the equation when pilot plant conditions are evaluated is indicated, since dryer lengths of the order of five to seven times the diameter were obtained. This is the range of length to diameter ratios commonly used commercially. The most extensive work on holdup and retention time in rotary air dryers has been conducted by Friedman and Marshall (3). Unfortunately, much of their work was done on dry materials with no air flow. They proposed the following equation relating holdup to air velocity:
X , = Xo
+ KG
(6)
From an experimental standpoint, Equation 6 has the ~ a m e disadvantage as Equation 4 in that one of the conditions of the equation requires tests to be conducted a t zero air velocity. U7ith many materials the handling characteristics a t zero air velocity are different from those with air flow in that the material may change considerably during drying. This author prefers to use an adaptation of the Friedman and hlarshall equation in which a romparison of holdup a t tx1-o different air mass velocities is used. Xa
Xa
+ K(Ga - Ga)
(7)
This relationship permits an evaluation of the effect of air velocity on holdup in the range \There proper drying characteristics are encountered. Since, with the materials listed in Table I, other factors besides air velocity were changed, values of K for different materials cannot be reported. If Equation 7 is used, care should be taken that extrapolation is not attempted over a wide range of air velocities. Entirely different handling characteristics may result from slight variations in air velocity because of changes in product moistures or temperatures, and thus in action in the dryer. This author feels that the relationship between the constant, K , and particle size found by Friedman and Marshall may have limited application. Since much of the work reported in this paper was conducted a t zero slope and with dam rings, insufficient data are available to test the relationship proposed by Friedman and Marshall relating holdup and the factor, F / S d N o 9D. It does seem more reasonable to use the 0.9 power of the rate of dryer rotation, X , than the full value of A+. If relationship in Equation 1 is used, and it is remembered that even this relationship holds only for very limited conditions with many materials, Equation 7 can be converted t o a relationship incorporating solid feed rate, retention time, and dryer length,
Smith (8) proposed the following equation for the time of passage in a rotary dryer:
He obtained values of k' varying from 0.0042 t o 0.017 for countercurrent air flow and 0.0017 to 0.0058 for parallel air flow. This equation has an advantage over Equation 4 in that the constant, k', can be determined under actual pilot plant operating conditions and it thus reflects the action of the material during drying. As shown in Table 111, values of k' Tere obtained varying from 0.035 to 0.00115 for countercurrent air flow and from 0.00434 to 0.0013 for parallel air flow. These are somewhat wider ranges but of the same order as those reported by Smith. Some of these values were obtained a t slopes less than 1/8 inch per foot (0.0104 foot per foot), and Smith has reported that his equation may be unreliable a t slope values in this range. The applicability of Equation 5 has been evaluated from the data of Table 111. For each test it was amumed that air velocity and volume requirements dictated a 6-foot-diameter unit for a hypothetical commercial dryer. With the same Conditions of slope, retention time, and speed of rotation obtained in t h e pilot plant test, Equation 5 was used to calculate a dryer length. The
372
Table II. Values of Material Constant in Equation 4, Developed by Prutton, Miller, and Schuette (7) Run NO.
Material
Material Constant (m)
Parallel .4ir Floiv Mineral slag Urea Cocoa sludge
3 1 5
Countercurrent Air Flow Amine hydrochloride 3 Calcium chloride
4 5 9
10 Sodium metsbisulfite
1 2 3 4 G
Polyvinyl acetate
3
INDUSTRIAL AND ENGINEERING CHEMISTRY
4
-0 00066 -0 00022 - 0.0044 0.006
0.0313 0.00065 -0.0021 0.00015 0.0036 0.00027 - 0.00038 -0.000301 -0.0021 0.0067 0.0159
Vol. 47, No. 3
PILOT PLANT
Table 111.
Values of Constant in Equation 5, Developed Smith (8) Run
No.
Material
Constant
(k')
Calcd. Length of 6-Ft.-Diam. Dryer, F t .
Parallel Air Flow 3 0 0032
Mineral slag Urea Cocoa sludge
1
5
selves and not be entirely a property of the material being tested. This further emphasizes that the range of application of any of the equations is very limited and varies widely with different materials. For example, between runs 3 and 4 with urea (Table I ) a value of K from Equation 10 of -73.75 was calculated. The only difference in dryer conditions was the use of 150-feet-per minute air velocity for run 3 and 225 feet per minute in run 4. This changed the product moistures from 0.25% in run 3 to 0.07% in run 4. The latter product was considered dry, and its final condition was reflected in different dusting, holdup, and handling characteristics. The calculation based on these two runs produced a meaningless value of K despite very similar operating conditions. If the air velocities or air mass velocities in Equation 10 are the same in two different runs, then the product of the solid feed rate times the retention time for one run should be the same as the second test. Comparison of these calculated products are given in Table IV. The results shown in Table I V emphasize that the effect of change of feed rate on retention time can be calculated within an accuracy that is generally less than 10% unless the altered feed rate produces a change in dusting, product moisture, or physical handling. Van Krevelen and Hoftijzer (6) studied the time of passage of three different materials in a small dryer, 10 em. in diameter and 76 cm. long. The materials studied were nitrochalk fertilizer granules, sand, and marl powder. They proposed the relationship given in Equation 11
by
36 9 40.2 36.0
0,00434 0.0013
Countercurrent Air Flow Amine hydrochloride Calcium chloride Sodium metabisulfite
3 4 5 9 10 1
2 3 4 6 Polyvinyl acetate
3 4
0.0117 0.035 0,00498 0 00115 0 00577 0,0087 0 00573 0 0043 0 00217 0.00182
30.8 31.0 36.0 34 3 34 7 40.0 37 8 40 5 40 0 39 6 37.4 37.4
0.0166
0.0325
or air velocities, V , may be used. (9)
Thus, either Equation 8 or 9 may be used to express the effect of air velocities or air mass velocities on the retention time at the same feed rate. Since most of the data reported in Table I includes runs in which more than one variable was changed between runs, no values of K as expressed by Equation 8 or 9 have been calculated. Differences in the feed rates can be evaluated by the following equation:
r N D t'an
Product of Solid Feed Rate and Retention Time with Same Air Velocity (r =
Material Granulated cork
Run No. 1
Plastic material
2 5
Pharmaceutical (Type 1)
6 1
Pharmaceutical (Type 2)
3 1 3
Sodium metabisulfate
1
Inorganic waste Clothespins Calcium phosphate Amine hydrochloride
2 1 2 3 4 2 3 4 3 4
Aapirin
4
Reclaimed rubber
7 4
Synthetic rubber
6 2
3
March 1955
=
where 01 is equal to the angle of inclination of the dryer. These authors initially proposed that c is a constant for each dryer and is dependent upon dryer construction, with particular emphasis on the flights and internal structure. Later they found that the constant, c, also reflects the nature of the material being dried, for they found values of c of 0.13 to 0.165 for sand and the nitrochalk fertilizer granules, but values of 0.27 to 0.285 for the marl powder. In Table V are given some of the values of c obtained from the data in Table I. Only those cases when the dryer was operated with a slope and 2.5-inch dam ring, and a t a speed of 6 r.p.m. were used for comparison. These results show that the value of c depends as much on the material as on the
Again, no values of K (Equation 10) were calculated. There were several materials for which such values could have been calculated but because of the experimental conditions other variations such as slight changes in product moisture would cause the calculated value of K to reflect the difference in the tests them-
Table IV.
O(
L
Calculated
i:;;;
F r Values Difference
H/R)
Increase,
(1.014
1.1
0.049
6,2
70
Speoial Conditions
Knocker used on run 5,not on run 6; dust loss somewhat higher on run 6 and drier product obtained
0.2424 0.2730 0.2802 0.5115 1.728 1.748
0.0306
12.6
Feed material for run 3 riddled through 1/*-inch screen
0.2313
82.5
Run 1 inlet air temp. = 180' F., run 1 air temp. = 300' F.
0.02
1.2
~:~~~
0.031
8.3
;:I;!: ,0276
::Ti
0.076
14.0
o,1668 (runs 2, o, 1168 (runs 3,4)
22,4 11 ,3
9.005
Product moisture of run 2 was 07' with product temp. of 390' F ' run 3 product moiBture was 4.19% with product temp. of 200' F:: run 4 product moisture and temp. were 9.34% and 185O F.
0.57
0.043
19.7
;:g;O"
0.022
4.0
0.554 0.462
::::;
Higher feed rate in run 7 reduced dust loss Product moisture was 0% in run 4, 10.9% in run 6
19.9
INDUSTRIAL AND ENGINEERING CHEMISTRY
373
ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT Table V. Values of Constant in Equation 1 1, Developed b y van Krevelen and Hoftijzer (6)
Nomenclature
(1 = H / R )
Material Amine hydrochloride
Run No. 3
Constant 0 ‘72
4 5
Calcium chloride Sodium metabisulfate Mineral slag Urea Cocoa sludge Photographic chemical Polyvinyl acetate
characterist,ice, an assignment, of certain equat,ion constants to these groups, is n o t feasihle.
10 1 2 3 3 1 B
1 ’7 3 4
(c)
O(
c
2 1’7 0 369 0.333
D P G
0.868 0.541 0.435
H K
0 197 0.290 0.077 1 66
k
0.374
k‘ L ?n .V I?
I ,033
2 029
dryer construction and that Equation 11 has applicability only in very limited ranges for most materials. Conclusions
As earlier workers have stated (a,5 ) there really is no substitute for a pilot plant test on a material to be dried. The studies reported in this paper have indicated the critical importance of the material characteristics and also the profound effect of sometimes small changes in dryer product characteristics upon retention time and fillage. Friedman and Marshall ( 4 ) found that the heat transfer rate vas only slightly effected by changes of dryer slope and speed of rotation and, therefore, these factors are not critical from a thermal dandpoint. Any equation discussFd in this paprr relating time of passage and retention time or holdup to dryer Characteristics has a limited range of application, and this range varies n-ith thp material being dried. From the standpoint of practical application, upe of the equation should not require the determination of material handling characteristics with dry material with no air flow. For semiquantitative purposes, this author prefers adaptations of the Friedman and Marehall relationship (Equations 7 to lo), the Smith equation, Equation 5, or the van Krevelen and Hoftijzer equation, Equation 11. Estimation of dri er size based on classification of materials into different groups or types according to drying or handling
sd T e
V X Xo
angle of inclination of dryer, degrees constant of Equation 11, devcloped by van Krevelin and Hoftijaer (6) = dryer diameter, it. = solid feed rate, cu. ft./(hr.) (sq. ft. of dryer cross section) = air mass velocity through dryer, lb.l(hr.) (sq. ft. of dryer cross section) = dryer holdup, lb. = constant in Equation8 7-10> adaptations of Friedman and Xlarshall cquatioiis ( 3 ) = dryer constant in Equation 4, developed by Prutton, Miller, and Schuet,te ( 7 ) . = constant in Equation 5, developed by Smith (8) = dryer length, f t . = material constant in Equation 4 = rate of dryer rotat,ion, r.p.m. = feed rate to dryer, lb./hr. = dryer slope, ft./ft. = time of passage, hr. = dynamic angle of repose of material, degrees = air velocity, ft./min. = dryer holdup, % of dryFr volume = dryer holdup with no air flow, yo of dryer volume = =
Subscripts a, b denote different operating conditions
Literature Cited (1) Bayard, K. A , , C h e m . h M e t . Eng., 52, 100 (1945). (2) Friedman, S. J., Heating and Ventzlating, Reference Sect., 95 (February 1951). (3) Friedman, S.J., and Marshall, W. R., Jr., Chem. Eng. Proor., 45, 482 (1949). I b i d , p 573. Rarrigan, H. IT-.,and Boyd, J. A , Chem. & Met. Eng., 4 6 , 214
(1939). Krevelen, D. Vi. van, and Hoftijzer, P. J., J . Soc. Chem. Ind. ( L o n d o n ) , 68, 91 (1949).
Prutton, C. F . , hIillei, C. O., and Sohuette, W. H., Trans. Am. Inst Chem. Engrs., 38, 123 (1942). Smith, B. A , , Trans. A m . Inst. Chern. Engrs., 38, 251 (1942). Sullivan, J. D., Maier, C. G., and Ralston, 0. C., U. S. Bur. Nines, Tech. Paper 384, 1927. Thomas, E. W., and Weisselberg, A., “Suggested Recornmended Practice for Testing Drying Equipment,” S m . Sac. Llech. Engrs., 29 West 39th St.,New York. RECEIVED for review April 23, 1954. ACCEPTEDKovetnber 29, 1954.
Continuous Ion Exchange with an Endless Belt of Phosphorylated C. H. MUENDEL AND W. A. SELKE Deparfmenf o f Chemical Engineering, C o l u m b i a Universify,
IN
New
A continuous ion exchange process the exchanger should be circulated countercurrent to the feed streams, passing alternately through an exhaustion and a regeneration section. The advantages of this method, which are of special importance when the regenerant is fed to subsequent processes, are well known. Although the problem of continuous ion exchange has received considerable attention, the results of relatively few investigations have been published. Patents have been granted to Kordell (11) and Wilcox (1.6) for continuous water softeners. Stanton ( 1 8 ) and Hiester and coTYorkers ( 5 ) have studied the separation of ions by continuous means. Keister also demonstrated the use of a mixer-settler apparatus that consists of a consecutive series of batch separations. Selke and Bliss ( l a ) ,
374
York,
N. Y.
\Torking on the reclamation of copper ion from very dilute solutions, used count,ercurrent moving beds. Their work was continued by Crits (2). Recently, other methods of continuous ion exchange using commercial resin have been proposed by Higgins and Roberts ( 6 ) , Koenig, Babb, and McCarthy (S), and McCormack and Howard (10). McCormack and Howard employed an endless tube of cloth filled with resin and tied off at intervals. On the basis of this esperience it is possible to dralx- up a Bet of conditions to which a successful continuous ion exchange apparatus must conform: 1. The apparatus must give positive and predictable displacement of the exchanger relative to the solution. The im-
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 47, No. 3
