Development of a Fluidized-Bed Technique for the Regeneration of Powdered Activated Carbon A. K. Reed, T. L. Tewksbury, and G . R . Smithson, Jr. Battelle Memorial Institute, Columbus Laboratories, Columbus, Ohio 43201
This paper is concerned with the results of research conducted at the Columbus Laboratories of Battelle Memorial Institute o n the regeneration of spent powdered carbon. The study was directed toward the development of a fluidized-bed regeneration technique. Two fluidized-bed systems were considered during the course of the investigation: a system in which the dried spent carbon was regenerated during its passage through a fluidized bed of a n inert material; and a pulsating fluidized-bed system in which the finely divided regenerated carbon served as the bed material. Both techniques were effective in restoring the spent powdered carbon to over 90% of its original adsorptive capacity. Recoveries in excess of 80% of the weight of the dried spent carbon were attained.
T
he use of powdered activated carbon for the tertiary treatment of secondary sewage effluents is being studied on a 10-gram-per-minute scale at Lebanon, Ohio, by the Advanced Waste Treatment Research Activities of the Federal Water Pollution Control Administration. FWPCA's pilotplant study has shown that powdered carbon is as effective as granular activated carbon for removing the organic impurities from the waste water. In addition, the turbidity of the treated effluent is significantly decreased when powdered carbon rather than granular carbon is used. Before powdered carbon can be used commercially for the tertiary treatment of sewage effluents, an economical method of regeneration must be developed. Investigations previously have been conducted o n the thermal regeneration of spent carbon, but neither the optimum process conditions nor operating procedures have been completely delineated. The use of the fluidized bed for regeneration offers the key advantages of excellent temperature and atmosphere control and the ability to process the powdered solids conveniently and continuously. However, the median diameter of the carbon particles is approximately 11 p. which is considerably finer than normally used in fluidized-bed operations. The problems associated with fluidization of very fine powders are the inability to achieve proper fluidization and the high entrainment losses. Thus, the desired control of temperature and retention time is not achieved. The application of fluidization methods for regeneration of powdered carbon therefore requires the development of operating methods for proper fluidization of the fine powder o r the development of an alternative procedure. In considering this approach two techniques appeared to have sufficient merit for study: 432 Environmental Science & Technology
The use of a fluidized bed of coarse, inert particles as a constant-temperature zone. The spent, dried, powdered carbon would be fed into the bottom of the coarse, inert bed and carried through the bed by the action of the fluidizing gas. The inert bed would be expected to retard passage of the fine carbon powder through the reaction zone, thereby increasing retention time and also providing good heat-transfer characteristics. The finely divided carbon would be recovered from the effluent gas stream with cyclone collectors or some other collection device. The use of a fluidized bed of the powdered carbon to which vibration or pulsation is applied. This method would be expected to offer a minimum of fluidizing gas requirements and entrainment losses while achieving the excellent heat-transfer rates and temperature control afforded by the fluidized bed. The objective of the current study was to evaluate these techniques by determining the effects of the process variables on the efficiency of regeneration and on the properties of the regenerated adsorbent. This paper describes the results obtained during the initial phase of the program and recommendations for the continued development of the fluidized-bed technique.
Fluidized Inert Bed System The experimental unit which was used in this phase of the study comprised a 4- l/,-inch-ID by 24-inch long stainless steel vessel h a t e d by an electric resistance furnace. A vibrating-type screw feeder was used to introduce dried spent carbon into the unit by entrainment in the fluidizing gas. Regenerated products were collected in a conventional cyclone dust collector, followed by an absolute filter. A sketch of the fluidized inert bed unit, together with some of the auxiliary equipment. is shown in Figure 1. The general procedure for making an experimental run in this unit was to charge the reactor with bed material and heat the unit to the desired operating temperature. In most cases. the bed comprised about 3300 grams of -35+65 mesh irregular sand or -20+48 mesh flint shot sand. Spent carbon was fed into the unit and the regenerated products were collected in the cyclone and filter devices. After completion of a run, a material balance was made and samples of the various products were analyzed for adsorptive capacity. The technique used for determining the adsorptive capacity of the carbon samples was an empirical method based on ultraviolet absorbance measurement of secondary effluent samples treated with 200 mg. per liter of carbon. The UV absorbance data before and after treatment with the carbon were used to calculate the relative adsorptive capacity of the
TO AT M0 S P H E R E
t
REACTOR TUBE' FLUIDIZED -I N E R T BED
FLU I DIZING GASES
Figure 1. Fluidized inert bed apparatus
regenerated carbon compared to that of the virgin carbon. This provided a measure of the degree of regeneration achieved during the experiments. N o attempt was made, however, to relate these values to the organic loading capacity of the regenerated material. The procedure used in the analyses was as follows. The secondary effluent was treated with 200 mg. per liter of carbon sample; after 30 minutes' contact
time, the suspension was filtered through glass filter paper; the original secondary effluent and the clear filtrate were then measured by a Beckman DU spectrophtometer. The major portion of the experimental study consisted of an evaluation of the fluidized inert bed technique for regeneration of the spent carbon. This phase of the study comprised a total of approximately 40 runs designed to investigate the effects of the operating variables on the degree of regeneration and o n the recovery of regenerated product. Experimental data obtained in the major experiments are summarized in Table I. These data show the operating conditions employed and the recovery and relative adsorptive capacity of the regenerated product. The overall objective of the experimental program was to investigate the effects of the operating variables on the degree of regeneration and recovery of the powdered carbon. The major variables examined during the program and range of study were as follows: Temperature: 500" to 1500" F. Composition of the fluidizing gas: selected mixtures containing NB,02,C o n , and H 2 0 Bed weight: 0 to 6720 grams Bed depth : 0 to 11.5 inches Moisture content of feed: ~ 4 . to 5 50% HzO. Of primary interest during the study was an evaluation of the effects of temperature and gas composition on the efficiency of regeneration. Included in this part of the investiga-
__
Expt
no.
5 7 8 9
10 11 13 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37
Temp., F.
1500 1000 1250 1000 1000 1000 1000 1500 1250 1000 1000 950 1000 1000 1200 1500 1000 1250 1500 1000 1000 1250 1250 1250 1250 1500
Table I. Summary of Regeneration Experiments in Fluidized Inert Bed System Moisture Average Bed Feed Gas ked rate. in feed, weight, weight, flow, g.jmin.h scfm g.ca Gas composition g.h 8Y 14 5 0 8 3000 1 0 90% N!:IOZ Coy -4 5 1 2 127 3000 1 2 90% Ny:lOZ C o r I 8 14 5 46 3000 1 1 90% Ny:lO% Cor -4 5 189 1 6 1000 1 2 Air 22 8 2 3 k4 5 2000 1 2 Air 2 1 -4 5 0 124 1 2 Air 2 8 21 1 -4 5 2000 1 2 N? 2 8 23 1 Y4 5 3282 1 1 80% N2:20% HyO 2 9 231 -4 5 1 1 3300 80% Ny:20% HyO -4 5 2 5 231 3300 1 1 8 0 x N2:20% H?O 55 3 -4 5 315 3300 1 4 Air 360 24 0 -4 5 3300 Air 1 4 19 0 Y4 5 360 3300 1 4 96% N2:4% 0 2 360 18 0 2-4 5 3300 1 4 98% Ny:2% Or 24 0 -4 5 3300 360 1 4 98% Ny:2% Oy ~4 5 360 24 0 3300 1 4 98% N2:2% 0, 22 5 -4 5 3300 360 1 4 Simulated combustion gases,' 25 8 -4 5 3300 360 1 5 Simulated combustion gasesd 17 2 -4 5 3300 360 1 5 Simulated combustion gasesd 360 22 5 -4 5 6720 1 5 Simulated combustion gasesd 71 10 1 14 5 0 1 5 Simulated combustion gasesd 2724 15 6 -4 5 1 5 3300 Simulated combustion gasesd 23 8 596 25 0 3300 1 4 Simulated combustion gases" 632 20 4 33 3 3300 1 4 Simulated combustion gasesd 21 70 31 9 50 7 3300 1 4 Simulated combustion gases" 48 1 921 27 9 6600 1 2 Simulated combustion gasesd
z
Relative adsorptive capacity c 102 68 86 91 93 94 71 110 93 86 100
86 83 78 83 97 85 92 99 81 55 92-99 92 82 71 83
Weight recoiery, "i;;h
59 65 74 73 79 86 54 75 84 86 84 77 87 82 80 86 88 84 57 55 38 38
I n Experiments J through 21, -35+65 mesh sand \vas uszd. In Experiments 22 through 37, -20+38 mesh flint shot w a s used. Values include any moisture present in feed material. c Relative adsorptive capacity = 100 X difference in UV absorbance of seconaary effluent before a n d after treatment with regenerated carbonidiffrrrnce in UV absorbance of secondary effluent before a n d after treatment with virgin carbon. Gas mixture contained approximately 70% N?,10% COS, 2 % 0 2 , and 1 8 % HzO. I n Experiments 34 through 37, however, water vapor was not added because of the high moisture content of feed. I'
Volume 4, Number 5 , May 1970 433
110
>
IO0
0 I-
2 a 0
90
W
> -
kLT % n a
80
C02
B-N,
70
I
i
/
__
C-COMB. GAS D-N2:02
V V
800
1000
1200
1600
1400
1800
TEMPERATURE, F Figure 2. Effect of temperature on regeneration
- 20 s $
v) v)
I-
r 8-N,
IO
0,
C-COMB.GAS
I 0 800
1000
1200
1600
1400
1800
TEMPERATURE, F Figure 3. Effect of temperature on recovery
tion was a series of experiments conducted in the fluidized inert bed system at a fixed gas composition and at temperatures ranging between 1000" and 1500" F. The effect of temperature on the relative adsorptive capacity of the regenerated products during these experiments is shown in Figure 2. Figure 3 shows the effect of temperature on the weight losses during regeneration. As shown, the same trend was noted with various regeneration atmospheres, i.e., both the adsorptive capacity and the weight loss increased with increasing temperatures. Slightly better overall results (higher adsorptive capacity and lower weight loss) were obtained when simulated combustion gases were used for regeneration. These results also show that temperatures of 1500" F. were required to produce a regenerated product equal in adsorptive capacity to that of virgin carbon. For maximum process economy, however, a loner temperature may be desirable to obtain a greater recovery of product with only a slight reduction in adsorptive capacity. The use of the inert sand bed as employed in the initial phase of this study can serve two main functions: it provides initially high heat-transfer rates to the carbon particles ; and it retards passage of the carbon particles through the reactor and thereby increases retention time. It was of interest therefore to examine the importance of these two factors for achieving efficient regeneration in the system. Several experiments were made in the fluidized inert bed system at various bed weights ranging from 100 to 6720 grams. Two experiments also were conducted with no bed material present to evaluate the effectiveness of the inert bed compared to a simple heated tube reactor void of bed material. Experimental data obtained during these experiments are compared in Table 11. In the initial series of experiments, low feed rates were used and the regeneration atmosphere was air. In the second series of experiments, simulated combustion gases were used and feed rates were increased by a factor of 10. The results of the experiments (Table 11) indicate that doubling the bed depth had no significant effect either on the degree of regeneration or on the recovery of the regenerated product when dry spent carbon was used. Thus, no improvement was noted when increasing the retention time. It was anticipated that the significant results obtained during these experiments would be a loss in regeneration effectiveness when a heated reactor void of bed material was used. As can be seen from the data, this effect occurred in Experiment 32, where a value of 55 was obtained for the relative adsorptive capacity compared to a value of 85 obtained in Experiment 28. Thus, the presence of the inert bed resulted in a significant improvement in regeneration effectiveness. The fact that a similar effect was not obtained in the first series of experiments is believed due to the use of
Table 11. Regeneration Data at Various Bed Weights
Expt. no.
Temp., F.
9 10 11 28 31 32
1000 1000 1000 1000 1000 1000
O
434 Environmental Science & Technology
Average
Weight recovery,
g.
Relative adsorptive capacity
1000 2000 0 3300 6720 0
91 93 94 85 81 55
...
Bed weight,
Gas composition
feed rate, g./'min.
Air Air Air Simulated combustion gas Simulated combustion gas Simulated combustion gas
1.6 2.3 2.1 22.5 22.5 10 1
7 i
59 65 87 86 88
much lower feed rates. At low feed rates, the importance of high heat transfer characteristics probably would be minimized ; thus comparable regeneration would be obtained with or without the sand bed. As previously stated, the original scope of the current study was to investigate regenerating characteristics of dry spent carbon feed. Because of the difficulty in drying the spent carbon and the uncertainty concerning the economics of providing dry feed rather than wet feed, the original scope was modified to include a study of regeneration of relatively wet spent carbon containing up to 5 0 z moisture. This level of moisture was considered typical of that which would be obtained from pilot-plant drying equipment to be used in future work. This part of the study comprised four regeneration experiments in the fluidized inert bed system with spent carbon containing from 25 to 50z moisture. During this experimentation, some difficulty was encountered in feeding the relatively wet spent carbon with the screw feeder which was used in previous experiments. A vibrating-type feeder proved satisfactory, however, for feeding materials containing u p to 50 water. The original method of introducing feed into the unit which involved entrainment in the fluidizing gas also was changed to an overhead feed tube arrangement, similar to that used in the pulsed fluidized-bed unit. The experimental conditions which were used in this investigation and the results which were obtained were shown in Table I. These results show that poorer regeneration was obtained when the moisture content of the feed was increased. Although an adverse effect o n regeneration was noted, the results of Experiment 37 indicate that increasing the bed depth and/or the regeneration temperature may offset the effect of increased moisture in the feed. Because of time limitations this effect could not be fully evaluated during the current study. It will be assessed more fully during the future pilot-plant investigation, however. Because of the importance of the efficiency of recovery of the carbon to the economic feasibility of the regeneration process, considerable effort was directed toward the development of basic information on this aspect of the process. The results shown in Figure 2 and Table I on the degree of recovery of regenerated carbon were based on the weights of the various products recovered during the experiments. This method, however, does not account for differences in the moisture and ash contents of the feed and product materials: nor for the adsorbed organics and adsorbed gases present in these materials. To develop additional information o n the actual losses of carbon during regeneration, samples of virgin carbon, spent carbon, and regenerated carbon were analyzed to determine their carbon, hydrogen, and ash contents, as well as the loss in weight when dried at room temperature under a vacuum. The results of these determinations are shown in Table 111. These data show that the actual carbon content of the samples varied considerably and that significant amounts of moisture and/or adsorbed gases were present. Several experiments also were conducted, during which samples of the exhaust gas were analyzed for C o t content by gas chromatography. The gas analyses were used to calculate the loss of carbon by reaction with components of the gas phase. The results obtained from these experiments are shown in 'Table IV. These data show that, with air for regeneration at 1000" F. and at low feed rates, the actual loss of carbon as C 0 2 is relatively high. When the available oxygen was decreased and the feed rate was increased in Experiment 24, however,
z
Table 111. Data on Composition of Virgin, Spent, and Regenerated Carbons _
Loss
_ Composition, _ ~
of weight" Carbon Virgin carbon Spent carbon Regenerated carbon, Expt. No. 5
HY-
drogen
Ash
6 96 4 42
80 9 76 2
0 84 1 48
4 55 8 08
0 34
86 7
0 93
Y 73
Samples were specially dried at room temperature in vacuum.
Table IV. Experimental Data Showing Carbon Losses During Several Regeneration Experiments RelaAver- tive age adsorp- Weight Carfeed tive rebon rate,. capac- covery. losses,a Expt. Temp., Gas 07 composition no. . F. g.jmin. Ity /o
z
22 1000 Air 24 1000 9 6 % N 2 : 4 z O ? 25 1000 9 8 z N z : 2 Z 0 2 26 1200 9 8 Z N 2 : 2 z O ? 27 1500 9 8 z N q : 2 % 0 2 'I
5 3 19 0 18.0 24.0 24.0
100 83 78 83 97
54 84 86 84 77
30.8 3.9 2.5 2.4 2.4
Bawd on CO: content of exhaust gas.
carbon losses decreased significantly. The losses in weight during the latter experiments (14 to 2 5 z ) therefore are believed to be due primarily to the evaporation of moisture (1.4 %), to the volatilization and combustion of adsorbed organic components, and to the combustion of a relatively small portion of the powdered carbon (1.2 %).
Pulsed Fluidized-Bed System The experimental unit which was used for the second phase of experimentation was a specially designed, 4-inch-diameter unit also constructed of stainless steel. This unit differed from the fluidized inert bed unit in that it contained a porous stainless steel plate t o provide proper gas distribution, whereas the former unit contained a conical bottom section. The auxiliary pieces of apparatus, including the cyclone and absolute filter dust collectors, the screw feeder, and the furnace equipment, were identical to those used in the initial work. Figure 4 is a sketch showing the arrangement of major components of apparatus. Preliminary studies in this unit demonstrated that good fluidization of the fine carbon solids could be obtained by applying a pulsating gas flow to the bed. The pulsations were obtained by simply interrupting the flow of fluidizing gas at frequencies in the order of 400 to 500 pulses per minute. Several devices, including a solenoid valve, a diaphragm valve, and rotary pulsator, were evaluated for obtaining the pulsations and found to provide a comparable degree of fluidization. The solenoid valve shown in Figure 4 was used for the major portion of the experimentation. The valve was actuated by two microswitches following a variable speed, motor-driven cam. Experimentation o n the pulsed-bed technique comprised only a brief study of the effects of temperature and retention time o n the degree of regeneration of the spent carbon. Volume 4, Number 5, Mat 1970 435
TO ATMOSPHERE
CYCLONE COLLECTOR
FURN REAC TUBE OVERFLOW DISCHA
FLU I Dl ZE D CARBON BED POROUS GAS DI STRl BUTOR -....-
1--
VARIABLE-SPEED rCAM ACTUATOR
'A I
ROTAMETER
FLUIDIZING GAS
Figure 4. Sketch of pulsed fluidized-bed apparatus
Initial experiments were conducted therefore in a batchwise manner, with no spent carbon being fed into the unit. The procedure used was to charge the unit with a starter bed of 100 to 220 grams of spent carbon. This bed was fluidized and heated to the desired operating temperature. Samples of the bed were withdrawn periodically cia the overflow discharge during both the heat-up period and at various times at a steady temperature. These samples were analyzed for adsorptive capacity and the data used to develop approximate conditions, Le., temperature and retention time, for efficient regeneration.
Expt.
Starter bed,a
no.
g.
2
150
4
6
7
100
50
50
Based on the results obtained during these preliminary studies, two subsequent experiments were made during which spent carbon was continuously fed to the unit cia a n overhead feed tube arrangement, as shown in Figure 4. The results from two preliminary experiments conducted in a batchwise manner are shown in Table V. Also shown are the data generated during two subsequent experiments in which spent carbon was continuously fed into the unit at 10 to 15 grams per minute. The results from the pulsed fluidized-bed experimentation also indicate a similar effect of temperature on regeneration as that shown by the results of the fluidized inert bed experimentation. I n Experiment 2 , for example, it was found that some degree of regeneration occurred at temperatures as low as 500" F., when air was the fluidizing medium; however, a temperature of 1000" F. and a retention time of 15 minutes were required to achieve complete regeneration. It also was determined from these preliminary experiments that a nitrogen-carbon dioxide atmosphere was not as effective as air at the same temperature. I n the two experiments in which both feed introduction and product discharge were continuous, values for adsorptive capacity and weight recovery were comparable to those obtained in the fluidized inert bed system. Thus a similar performance of the two systems was obtained. Conclusion
The primary conclusion drawn from the results of this study is that both fluidization techniques are effective for regeneration of the spent powdered carbon. With the fluidized inert bed system, regenerated carbon products were obtained which had adsorptive capacities of 100% when compared to virgin activated carbon; the weight recoveries were as high as 88 %. Similar regeneration also was obtained with a pulsed
Table V. Summary of Regeneration Experiments in Pulsed Fluidized-Bed System Elapsed Gas Relative Gas time. Temp., flow, adsorptive composition Sample designation 111111. ' F. scfm capacityh Air Spent carbon 0 70 0 25 27 A 10 500 0 21 38 B 13 750 0 21 62 C 21 1000 0 21 83 D 36 1000 0 21 102
90% N? 10% co,
Air
Air
Spent carbon A B C D E Final bed
0 15 22 32 62 92
Discharge Final Bed
30.
Discharge Final bed (f 35 mesh) Final bed ( - 35 mesh) Cyclone dust
47.
70 500 750 1000 1000 1000
1000
1250
Starter bed was dry spent carbon. *See footnote (c) of Table I. e Continuous feeding'period. A minus value indicates UV absorbance decreased after secondary effluent was treated with carbon sample. a
436 Environmental Science & Technology
0 0 0 0 0 0
IO 10 10 IO 10 10
0 08
0.08
-
31 24d 61 82 87 85 82
Weight recovery, %
63
80
80-90
91
82
85-91 96 89 45
79
fluidized-bed system. These results indicate that further development of the process is warranted. Regarding future development activity, emphasis should be placed on evaluating factors such as unit capacity, heat requirements, and material losses which would provide data for economic evaluations. Because of its greater flexibility and higher unit capacity, it was concluded that further research be made with fluidized inert bed system.
Future Work A pilot-scale study of the fluidized inert bed technique is currently being initiated under contract with the FWPCA. This study will involve operation of a 10-inch-diameter fluidizedbed unit for production of 30 pounds of regenerated carbon in an 8-hour period. The unit has been designed with a n integral combustion chamber in which propane will be burned with air to provide the necessary heat input and regeneration atmosphere. Feed to this unit will be partially dried spent
carbon containing up to 50% moisture. The regenerated carbon produced will be recycled to FWPCA's pilot adsorption system for evaluation of the combined system performance. Results from the pilot-scale study are expected t o provide the necessary data for economic evaluation and scale-up of the fluidized-bed process.
Acknowledgment The advice and assistance of the many individuals at FWPCA who participated directly in the program are greatly appreciated. Received for review December 27, 1968. Accepted October 20, 1969. Presented at the 156th National Meeting, ACS, Atlantic City, N . J . , September 11, 1968. The research upon which this unit publication is based was performed pursuant to Contract No. 14-12-113, with the Federal Water Pollution Control Administration, Department of the Interior.
COM MUN ICAT1O N
Ultrafiltration of Aquatic Humus Egil T. Gjessing
Norwegian Institute for Water Research, Blindern, Norway
Filtration of aquatic humus through Diaflo ultrafiltration membranes suggests that approximately 10% of the organic carbon and 1% of the colored matter have molecular weight below 1000. The experiments indicate also that about 50 and 90% of organic carbon and color, respectively, are found in the fraction which is not penetrating the filter which is supposed to retain molecules larger than 20,000 in molecular weight.
T
he brown-yellow color of surface water, originating from soil humus, causes considerable problems for many waterworks. Gel filtration of humus isolated from natural water suggests that it consists of a complex mixture of organic substances ranging in molecular weight between probably less than 1000 to more than 200,000 (Gjessing, 1965, 1966; Gjessing and Lee, 1967; Gassemi and Christman, 1968). In the present work Diaflo ultrafiltration membranes (formerly Diaplex) have been used to separate humus according to molecular size. The Diaflo membranes (Amicon Corp., Lexington, Mass.) are molecular filters manufactured from synthetic polymers. They are maintained to be nonplugging, and may be used over a wide range of pH. The Diaflo membranes were first reported used in 1965, (Blatt, Feinberg, et a[., 1965) and since then have been used by biochemists for concentration and purification of biological fluids (Michaels, 1965 ; Wake and Posner, 1967; Blatt, Hudson, et al., 1967). At present there are eight variants of the Diaflo membrane filters available. In the work presented below the following three have been used :
Diaflo membrane: Code No. UM-2 UM-20 UM-20E
Retaining molecules larger than (M.W.) 1,000 10,000 20,000
The composition of the water sample used in the experiments is given in Table I. Ten milliliter-samples of concentrates of these waters (evaporated at 30" C. under reduced pressure), were filtered through a UM-2 membrane with compressed nitrogen (pressure of 4.5 kg. per using an Amicon filtration cell (Model 50). The fraction retained by the membrane was washed with portions of distilled water until the filtrate contained 5 mg. carbon per liter (or less) and then passed through a UM-10 membrane in the same manner. Finally, after washing, the Table I. Composition of Natural and Concentrated Sample of Aquatic Humus Soec. CalOre. Color cond. Iron cium c a r b h mg. 20°C. mg. mg. mg. Sample Pt/l. pS/cm. Fe/l. Ca/l. C/l. Bogerudmyra 15th Nov. 66 (natural sample) 158 60 0.350 0 . 8 6 28.0 Bogerudmyra conc. 40 times 5.700 960 16.40 36.0 950 Hellerudmyra (st. 21) 6th Febr. 69 (natural sample) 79 25 0.600 1 . 6 2 1 9 . 0 Hellerudmyra conc. 40 times 4.000 555 18.40 56.0 599.0 Volume 4, Number 5, May 1970 437
