Regeneration of Powdered Active Carbon in Fluidized Bed Luis A. Hernandez and Peter Harriott" School of Chemical Engineering, Cornell University, Ithaca, N.Y. 14850
w Powdered active carbon, loaded with sodium dodecyl benzene sulfonate (ABS), was regenerated with air in a batch fluidized bed. The combustion of ABS was first order to ABS and to oxygen and had an activation energy of 2 1 kcal/mol. With regeneration a t 485 OC, about 95% of the adsorption capacity of the carbon could be recovered with little loss of base carbon. Above 500 OC, oxidation of the carbon was appreciable, and the activation energy for this reaction was 53 kcal/mol. Controlled regeneration with air a t about 500 "C seems a feasible alternative to regeneration with steam a t higher temperatures.
Active carbons in granular and powder forms are widely used in water treatment and in purification of process solutions. Spent granular carbons are regenerated in standard kilns or furnaces, often with only a small loss in adsorption capacity, but procedures for regeneration of powdered carbon are not well developed. Fluidized-bed regeneration seems attractive because good temperature control is possible, and solids are easily added to or removed from the reactor. However, the use of very fine particles can lead to uneven fluidization and excessive entrainment losses. In the study of fluid-bed regeneration by Reed e t al. ( I ) carbon particles with a mean size of 11 bm were diluted with sand to improve the fluidization, or pulsing flow of gas was used. The effects of temperature and gas composition on the adsorption capacity and carbon loss were measured, but the residence time of the carbon in the diluted bed was not known, and the kinetics of regeneration were not determined. The purpose of this work was to study the kinetics of regeneration using a carbon with particles large enough to fluidize smoothly with little entrainment. The carbon chosen was Nuchar CEE-N (Westvaco Co.), which has a mean particle size of 37 bm, and a bulk density of 15 lb/ft3. The minimum fluidization velocity, determined from pressure drop and bed expansion curves, was 0.8 cm/s. The bed expanded about 80% as the velocity was increased to 10 cm/s, and the pressure drop during fluidization was always equal to the weight of the bed. A great variety of adsorbable organic substances are present in wastewaters, but for a preliminary study of regeneration, it seemed best to use one typical compound that would be strongly adsorbed, easily identified, and nonvolatile, so that regeneration couldn't be accomplished simply by heating. Sodium dodecyl benzene sulfonate (ABS) has the desired properties, and on regeneration it leaves an inorganic residue, which occurs in many commercial processes. The adsorption isotherm for ABS on Nuchar CEE-N was determined by batch tests with different carbon loadings. The ABS concentrations in the feed and equilibrium solutions were determined with an ultraviolet spectrophotometer. The adsorption curve was of the Langmuir type and was fitted well by the following equation:
X= where
X
0.0069 C
1
+ 0.0235 C
= g ABS/g carbon
C = mg ABS/l. solution 454
Apparatus and Procedure The fluidized bed reactor was a stainless steel pipe, 3.25 in. in diameter and 16 in. high, topped by a conical section leading to a 6-in. diameter disengaging chamber. The bed was supported on a %e-in. sintered metal plate that ensured good gas distribution. Gas left the reactor through two porous metal filters suspended from the top of the disengaging chamber. The temperature was measured with a movable thermocouple in a central thermowell and controlled by regulating the power to external heating elements. Carbon samples were withdrawn through a Ih-in. tube that extended nearly to the distributor plate. The gases used were nitrogen and air, metered by separate rotameters and joined in a mixing tee. The product gas was analyzed for oxygen using a gas chromatograph with a 12-ft column packed with 5A Molecular Sieves. Helium was the carrier gas, and the column temperature was 40 "C. Carbon to be used for regeneration tests was contacted with enough ABS solution to give about 23 g ABS/100 g carbon. The solid was recovered by filtration and dried a t 130 "C for 20 h. The charge to the bed varied between 110 and 160 g, giving initial bed depths of 4-6 in. The bed was fluidized with air and heated to 350 O C . Nitrogen was then used as the bed was heated to the desired regeneration temperature. Regeneration was started by switching back to air or to an air-nitrogen mixture. The pressure in the reactor was approximately constant a t 1.3 atm. The superficial gas velocity was 10-15 cm/s, 12-20 times the minimum value for fluidization. The bed temperature always increased when regeneration started, and in a few cases significant axial temperature differences (up to 5 O C ) were observed. The progress of the regeneration was followed by product gas analysis to determine oxygen consumption and by removing solid samples. The adsorption capacity of the solid was expressed as the ratio, R , of ABS adsorbed to the amount adsorbed by virgin carbon a t the same solution concentration. The samples were also extracted with water to determine the sodium sulfate content, and in a few cases, the adsorption capacity and ash content of the extracted carbon were measured. Results and Discussion Regeneration of activated carbon should be carried out under conditions where the adsorbed materials are decomposed or oxidized but the base carbon undergoes little or no reaction. Scouting tests made in an oven with air a t 400460 OC showed that samples gradually lost weight over a period of a few hours, and then the weight loss became very small or negligible. At this time, the weight was about equal to the weight of carbon in the original sample, and the adsorption capacity was 94% of virgin carbon. This indicated nearly complete oxidation of the ABS with little damage to the base carbon. In the fluidized reactor, tests were made a t temperatures of 430-580 "C with air and oxygen-nitrogen mixtures. The results of air oxidation in the lower temperature range are shown in Figures 1and 2. The oxygen consumption rate decreased with time in approximately first-order fashion, and the rate was nearly zero after 2 h. The adsorption capacity increased with time, and the initial rate of increase for different temperatures was proportional to the initial rate of
Environmental Science & Technology
.......
5 "C
0.2 k LO2 I min.
\
I
0.4
0.2
0 0
20
40
60 80 time, min.
100
120
Figure 1. Oxygen consumption rate for regeneration at low temperatures 1.0
I
so pyrolysis was neglected in the analysis of regeneration kinetics. Tests at oxygen partial pressures from 0.04-0.27 atm showed that the rate of ABS oxidation was proportional to the oxygen pressure. The oxygen consumption rates for the air runs at 430, 465, and 485 "C were then corrected to Pop = 0.273 atm, and these data were plotted on semilog paper to get the first-order rate constant for ABS oxidation. The oxygen correction was based on the log-mean of the inlet and exit pressures, which is not strictly correct because of gas mixing in the fluid bed, but the correction was generally less than 20%. Other rate constants for ABS oxidation were obtained by brief runs a t higher and lower temperatures, and the results are shown as the upper line in the Arrhenius plot, Figure 3. The activation energy is 21 000 cal/ mol, and the rate of oxidation of adsorbed ABS can be expressed as:
rl = - d X / d t = k l ( X ) P o , I
I
I
I
I
I
0.8
(2)
where k l = 1.54 X lo5 e-1o 600/Tmin-l, atm-l X = g ABS/g carbon The reaction rates for regeneration at the intermediate temperatures of 505 and 525 OC are shown in Figure 4. The
R 0.6
0.4
/ /
A'
time, min. Figure 2. Adsorption capacity ratio, R, for low-temperature regeneration
oxygen consumption. The capacity ratio reached 0.92 after 2 h at 485 OC, but this ratio is based on carbon used directly after regeneration. Analysis showed a gradual increase in water extractables, nearly parallel to the increase in R , and the concentration of sodium sulfate reached 4.4% for the last sample in the 485-OC run. This is close to the expected value of 4.5% for complete oxidation of ABS on a sample containing 0.23 g ABS/g carbon. After water extraction and drying, this sample had an adsorption capacity 94-97% of virgin carbon. In the runs a t 430 and 465 OC, adsorption capacities of 70% and 89% were reached in 2 h, and the last samples had 3.1% and 4.1% NaZS04. Adsorption capacities higher than 92% would be expected with longer treatment a t 430 "C or 465 "C. The kinetics of regeneration were examined by relating oxygen consumption rate to ABS removal. If we assume that none of the base carbon reacted and that all the ABS was oxidized in the 485-OC run, integration of the oxygen consumption curve led to the factor 1.04 1. OZ/g ABS. This is 37% less than the stoichiometric factor for complete combustion to carbon dioxide. Some carbon monoxide is formed, but if the ratio CO/(CO COz) is about 0.24, as found for oxidation of various amorphous carbons ( 2 ) ,carbon monoxide could only partly account for the low oxygen consumption. Pyrolysis of the ABS to produce low-molecular-weight hydrocarbons and a residue low in hydrogen must have occurred as the spent carbon was preheated in nitrogen. Separate tests showed that heating in nitrogen for 3 h a t 480 "C gave only 5% recovery of adsorption capacity,
'
k, min:'
1ci3-
e ;
\
carbon
-
-
\
\ \ \
550°C 1:2
"t5
'
500 'C
1.45
k3
450 "C
'
1.45
114
1-45
+
I 0' 0
505'C
I 1
20
40
60 time, min.
80
100
Figure 4. Oxygen consumption rate for regeneration at intermediate temperatures Volume 10, Number 5, May 1976
455
initial rate is assumed to be due entirely to ABS oxidation, and rate constants based on the initial slope fall on the correlation in Figure 3. However, the reaction rate approaches a constant value often a t about 1 h, showing appreciable reaction of the base carbon. The adsorption capacity, shown on Figure 5, reached a maximum of 8 3 4 4 % in 40-70 min, and then declined significantly as the base carbon oxidation continued. The maximum capacity for the 505 OC run should have been a few percent higher than the value measured, based .on comparison with runs a t other temperatures. Rate constants for carbon burning were calculated from the oxygen consumption rates a t 1.5 h at 505 and 525 "C, assuming that one-fourth of the carbon formed carbon monoxide. Some runs were made using beds which had been completely regenerated at low temperature and then heated to 500-580 O C to study oxidation of the base carbon. Material balances for these runs, based on the initial and final weights of the bed and the calculated amount of carbon reacted, checked within 5%. Other tests showed that the oxidation rate was proportional to oxygen pressure, and all the data were corrected to Po2 = 0.273 atm and plotted in Figure 3. The activation energy is 53 kcal/mol, somewhat larger than the value of 40 kcal/mol reported for coke and carbon deposits on catalysts (2, 3 ) . Calculations indicated negligible internal concentration gradients during oxidation. The large difference in rate constants at 500 O C explains why nearly complete removal of ABS is possible with little loss of base carbon, even if excess oxygen is present. The actual loss of carbon is further reduced because the carbon is initially covered by adsorbate molecules and probably does not start to react until this layer is removed. If the carbon reaction rate is proportional to the fraction of surface uncovered, the rate expression is:
R
I
/ /
time, min. Figure 5. Adsorption capacity ratio, R, for intermediate temperature regenerations 1.o
I
I
I
I
CARBON
465 "C
LOSS
(3) 0
where W = grams of base carbon kp = 5.4 X 10l2e-26 700/Tmin-', atm-' For a constant oxygen pressure, integration leads to:
150 time, min.
200
250
Predicted capacity ratio and carbon loss for batch regeneration at Po2 = 0.25 atm
The effects of time and temperature on the loss of carbon and the adsorption capacity ratio were calculated using Equations 4 and 5. The adsorption capacity was assumed proportional to the fraction of ABS removed and to the fraction of base carbon left. The latter assumption is only an approximation based on the decline in R toward the end of the 505 and 525-OC runs. The predictions shown in Figure 6 match fairly well the experimental results of Figures 2 and 5 , though the measured capacities are a little lower, because they were not corrected for the weight of sodium sulfate on the regenerated carbon. The model indicates that a maximum capacity ratio would have been reached in about 3 h at 465 "C, and this maximum would have been 2% greater than a t 485 OC. Such a slight difference would probably not justify the longer residence time.
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Flgure 6.
(5)
456
50
The best conditions for batch regeneration of this carbon might be about 500 O C , with time adjusted (based on the oxygen concentration) to give a capacity ratio a few percent less than the maximum. There is no evidence that the oxygen must be limited to minimize reaction of the base carbon. Using a low oxygen concentration only increases the reaction time needed for a given degree of regeneration, since both reactions are first order to oxygen. Further work is needed to see if these results apply to regeneration of carbons with different pore structures, which might have different reactivities. Tests should also be made with other adsorbates present in industrial and municipal wastes.
References (1) Reed, A. K., Tewksbury, T. L., Smithson, G. R., Jr., Enuiron.
Sci. Technol., 4,432 (1970). (2) Lewis. W. K.. Gilliland. E. R.. Paxton. R., Znd. Enp. Chern., 46, 1327 (1954). (3) Weisz, P. B., Goodwin, R. D., J . Catal., 2,397 (1963).
Received for review July 3, 1975. Accepted Dec. 16, 1975.
