266
Ind. Eng. Chem. Prod. Res. Dev. 1904, 2 3 , 266-269
Activated Carbon from Almond Shells. Chemical Activation. 1. Activating Reagent Selection and Variables Influence F. Rulz Bevlb,' D. Prats Rlco, and A. F. Marcllla Gomls Departamento de Qdmica TGcnica, Universidad de Alicante, Apartado 99, Alicante, Spain
This paper reports the chemical preparation of activated carbon with almond shells as raw material. Of the different activating reagents studied, ZnCI, gave the best results. Final products obtained had higher surface area than commercially tested ones. Furthermore, raw material particle size, activation time, and impregnation ratio influence on adsorptive propertles are reported. Of these, impregnation ratio was the most critical parameter, giving excellent products at higher than 100% ratio.
Introduction Almond shell is a very abundant byproduct in Spain, especially in Mediterranean areas. A lot of work has been done with this raw material showing that it is adequate for physical activation processes (Linares-Solano, 1980; Berenguer, 1980), producing activated carbons with an excellent set of adsorptive properties. Results obtained in the study of a "chemical" activation process of almond shells are reported in this paper. The influence of some activating chemicals, H3P04, ZnC12, K2C03,and Na2C03,as well as material particle size, activation time, and activating reagent/raw material ratio (impregnation ratio, R),once the activating agent has been selected, are described. A complete study of the temperature effect will be reported in a later paper. Experimental System Apparatus. Activation was carried out by the chemical activation method (Smicek and Cerny, 1970). Raw material is impregnated with an activating reagent solution and heated up afterward to different temperatures in an inert atmosphere. Experiments were carried out in an 18/8 stainless steel reactor (100 mm internal diameter, 700 mm high). Heating was achieved by means of a cylindrical refractory oven 400 mm high, consisting of two nicrome wires with resistances of 2 and 0.5 kW, respectively, independently controlled by two regulators. Nitrogen from an industrial source was used to ensure oxygen absence inside the reactor, and distilled vapors were collected. Experimental arrangement is shown in Figure 1. Raw Material Preparation. Almond shells were water washed and, once dried, they were crushed in a roll mill and sieved. Afterward, the collected fractions were stored; 0.50-0.84-mm particles were selected to study the effect of the different activating reagent tested. Impregnation-Activation. The particles were then dried a t 110 "C for 24 h; 50 g ( W l )of this material was mixed with 25 mL of distilled water solution containing 12.5 g (W2)of the activating reagent under study, shaken to homogenize the mixture (5 min, approximately), and dried again a t 110 "C for 24 h. Activation was run at 600 and 700 "C, respectively, and for 2 h for every activating reagent (H3P04,ZnClz, K2C03, and NazC03). The activated products obtained were removed from the reactor, weighed (W3), washed with a 500-mL solution of 0.1 N HC1, stirred for 2 h, and then re-washed with tap and distilled water until there was complete C1- removal. Finally, the solutions were dried at 110 "C for 24 h, weighed again (W4), and stored for their characterization. From weights cited (Wi), measured at 0196-4321/84/ 1223-0266$01.50/0
Table I. Commercial Activated Carbons A.C.,
trade name Hidrafyn-Ls Hidrafyn-LS-Extra Hidrafy n-LS-Supra Panreac Polvo Panreac Granulado 2 Panreac Granulado 3
% 4.2 2.5 2.8
1.3 6.6 7.4
I.N., mg/g
P.N., mg/g
561 547 761 901 461 706
17.5 18.4 20.3 29.9 12.3
17.1
S,
m'/g 920 955 980 1915 87 0 1163
different process steps, loss in weight percentage (L.W.) is obtained as follows (dry basis) L.W. = ((Wl - W4)/W1) X 100 The yield of the process is obviously given by 100 - L.W. Activating reagent recovery (R.R.) represents the amount (percentage) of activating reagent remaining with the product before washing and thus recoverable from washing solutions. This yield is given by R.R. = (( W3 - W4)/ W2) X 100
Characterization. (a) Ash Content (A.C.). It is obtained using 0.5-g activated carbon samples and heating at 800 "C in an oven until constant weight is reached. (b) Iodine Adsorption Capacity (Iodine Number, I.N.). This index represents the number of mg of adsorbed iodine per gram of dry activated carbon. Procedure: approximately 0.5 g of dried activated carbon was accurately weighed and 50 mL of iodine-iodide solution (0.1 N) was added. After 45 min stirring at 25 "C, the residual iodine concentration was determined by 0.1 N Na2S203titration. (c) Phenol Adsorption Capacity (Phenol Number, P.N.), Milligrams of Phenol Adsorbed per Gram of Dry Activated Carbon. Procedure: approximately 0.3 g of dried activated carbon was accurately weighed, and 100 mL of phenol solution (0.1 g/L) was added. After 24 h a t 25 "C with occasional shaking, the residual concentration was determined with a 610-10s Perkin-Elmer Fluorescence Spectrometer, a t 302 nm maximum emissivity and 292 nm maximum excitation wavelengths. (d) Nominal Specific Surface (Kipling et al., 1964) ( S ,m2/g). This was determined by the simplified BET method (i.e. Gregg and Wing, 1967) with Micromeritics 2200 equipment. On some samples the nominal surface area was also determined by the complete BET method (Brunauer et a1.,1938) and the complete 77 K nitrogen adsorption isotherms were obtained with a Micromeritics 2100 instrument. Results Characterization of a Series of Commercial Activated Carbons. A series of six commercially available C 1984 American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 23, No. 2, 1984 267
Table 11. Activating Reagent Selection (Impregnation Ratio ( R ) = 25%, Activation Time ( t ) = 2 h, Raw Material Particle Size (D) = 0.50-0.84 mm) expt
act. reagent
AS 1 AS 2 AS3 AS4 AS5 AS6 AS7 AS8
H3PO4 HPO, ZnC1, ZnC1, K7.C03
KZ
co
3
Na2C03 Na,C03
T,"C
L.W., %
A.C., %
I.N., mg/g
600 700 600 700 600 700 600 700
44.1 44.3 57.2 58.5 70.4 73.7 48.2 50.0
0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5
176 134 678 685 391 709 188 376
7.3 5.8 17.6 16.9 19.1 21.3 11.2 10.7
711 742 367 746
P.N., mg/g
S, m2/g
Table 111. Raw Material Particle Size Influence (R = 25%, t = 2 h; T = 600 "C) expt
D, mm
L.W., %
R.R.,%
A.C., %
I.N.,mg/g
P.N., mg/g
S,m2/g
AD1 AD2 AD3 AD4 AD5
0.00-0.21 0.21-0.50 0.50-0.84 0.84-1.41 1.41-2.00
60.3 58.0 57.2 56.1 55.4
61.2 61.6 58.8 49.5 45.7
2.0 1.3 2.1 2.1 1.0
736 710 664 537 487
22.8 20.3 17.6 14.5 13.2
745 716 695 525 504
I.N., mg/g
P.N.,mg/g
S, m2/g
653 664 607 651 693
15.0 17.6 14.7 16.6 18.6
664 695 645 667 796
Table IV. Activation Time Influence (R = 25%, T = 600 'C, D = 0.50-0.84 m m ) expt
t, min
L.W., %
R.R., %
At1 At2 (= AD3) At3 At4 At5
80 120 240 360 480
56.2 57.2 57.4 57.7 58.1
65.7 58.8 32.6 21.1 19.4
A.C.,% 1.1 2.1 0.9 0.8 0.6
Table V. Impregnation Ratio Influence ( T = 600 'C, t = 2 h, D = 0.50-0.84 mm)
R, % 0 25 50 100 150 200
expt AR 1 AR2 (= AD3) AR3 AR4 AR5 AR6
L.W., %
R.R., %
A.C., %
I.N., mg/g
76.2 57.2 57.6 59.2 59.3 59.2
58.8 65.2 69.0 71.1 73.8
2.1 2.1 0.9 0.2 0.5 0.2
104 664 957 1054 1079 1088
P.N., mg/g
S, m'/g
3.7 17.6 20.5 24.3 25.2 25.4
104 695 998 1644 2018 2111
IN
CONTROLLERS 1 1
mg 19 1200
600
0 0
100
R %
200
Figure 2. Impregnation ratio influence on iodine adsorption capacity of samples prepared in following conditions: D = 0.50-0.84 mm, t = 2 h, and T = 600 'C.
REACToRT I 1 ISOLATION
L
J
1
CONDE NSATED
Figure 1. Experimental system.
activated carbons were subjected to the same testa in order t o provide a comparison basis for activated carbons obtained in the present work. Trade names and resulta from these measurements are shown in Table I. Activating Reagent Selection. Four activating reagents, H3P04,ZnC12, K2C03, and Na2C03, were tested. Experimental planning, nomenclature as well as results obtained are shown in Table 11.
Raw Material Particle Size and Activation Time. Tables I11 a n d IV show experimental planning, nomenclature, a n d results for some experiments indicating the
800
I
AR5
I
400
P -
0
0
0.2
0.4
0.6
0.8
l.OP/P,
Figure 3. The 77 K N2adsorption isotherms frdm samples prepared with R = 150% (AR5), 100% (AR4), 50% (AR3), and 25% (AR2).
268
Ind. Eng. Chem. Prod. Res. Dev., Vol. 23, No. 2, 1984
Relative Micro- and Mesoporosity Contribution from 7 7 K N, Adsorption Data
Table VI.
a, cm3/g
b, cm3/g
m.C.,
M.C.,
expt
%
%
S: m*/g
AR2 AR3 AR4 AR5
220 330 395 450
240 370 540 605
91.7 89.2 73.1 55.9
8.3 10.8 26.9 44.1
888 1280 1602 1881
Nominal specific surfaces obtained by the complete BET method.
influence of these two process variables. Impregnation ratio influence. Table V lists experimental planning, nomenclature, and results obtained in the study of the influence of this variable on adsorptive characteristics of the activated carbons prepared. Figure 2 shows (as example) the I.N. vs. R variation. Figure 3 shows the 77 K nitrogen adsorption isotherms on some of these samples. Table VI presents the effect of this variable on the micro and mesoporosity of the samples, and the specific surface by the complete BET method as well.
Discussion Activating Reagnet Selection. From Table I1 it is apparent that activated carbons obtained when using K2C03and ZnC1, as activating reagents develop the best adsorption properties under test conditions. However, ZnC1, gives much better yields (Le., lower loss in weight). These results are in good accordance with the ones obtained by other authors (Nacco and Aquarone, 1978) in their studies of these chemicals on other raw materials. Carbons activated by K2C03gave very low yields, and final products had a highly foamed appearance with a complete lack of mechanical resistance. Moreover, K2C03showed, as compared with activated carbons obtained by use of Na2CO3,the potassium effect on carbon gasification, already observed by other authors (Croft, 1960; Durie and Schafer, 1979). Thus, ZnC1, was selected as the most adequate activating reagent when using almond shells as raw material to obtain activated carbon. Raw Material Particle Size and Activation Time Influence. From Table I11 it can be observed that L.W. slightly decreases as particle size increases, probably due to better use of activating reagent when increasing the interfacial surface between it and the raw material. Obtained yields were about 40-45%, which is normal in this kind of rocesses (Yamada, 1959; Nacco and Aquarone, 1978). Activating reagent recovery decreases as the raw material particle size increases. Adsorptive capacity increases as particle size decreases and this effect is smaller from D. = 0.50-0.84 mm to D = 0. Ash content was lower than 2% in most of the cases, which is quite low considering that the raw material was only prepared with water. From Table IV it is apparent that with 2 h of activation time a very good adsorption capacity is developed in the samples. Yields vary very slightly with activation time, but activating reagent recovery markedly decreases as time increases, showing an appreciable ZnC1, evaporation effect. Ash content decreases to 0.8% with 8 h activation time. Impregnation Ratio Influence. Table V shows results obtained in this step. Loss in weight presented a remarkable decrease between R = 0 and R = 2570, being from this point on practically constant. On the other hand, collected tars were much lower in any case when R # 0, in accordance with probable mechanisms involved in these reactions (Yamada, 1959). Ash content markedly decreases when increasing R, reaching 0.5% from R = 100% to R = 200%. This ash content is remarkably low and similar to that obtained by other authors (i.e., Linares-Solano, 1980)
when they physically activated this raw material with COP, after washing the almond shells with H2S04solutions, a step which is lacking in the present work. All other determined indexes presented similar behavior, a great increase up to R = 100% while this increment is less marked for higher R values. Figure 2 shows I.N. vs. R variation as an example of what has been said above. In comparison with commercial products, very high adsorption capacities (up to 2111 m2/g) have been reached, with higher indexes than any of the commexial activated carbons (up to 1915 m2/g) and with lower ash contents. The 77 K N2 adsorption isotherms corresponding to samples AR2, AR3, AR4, and AR5 have been measured. They are shown in Figure 3, where it can be clearly seen that the effect of the activating chemical is to increase, as R increases, the amount of adsorbed N2 in all the relative pressure range, changing the shape of the curves. The isotherm corresponding to R = 25% presented a completely flat plateau at relative pressures higher than 0.2. This limit moves up to higher relative pressures when R = 50% and continues doing so when R increases. That is to say, the impregnation ratio clearly affects pore size distribution, increasing relative mesopores contribution (M.C. = ( a - b ) / a , where a = amount of adsorbed N2 at 0.95 relative pressure, and b = amount of adsorbed Nzat 0.1 relative pressure) by decreasing micropores contribution (m.C. = b / a ) . These facts can be observed in Table VI where these relative contributions as well as nominal surfaces obtained by the complete BET method are presented. These specific surfaces showed the same tendences as observed from the simplified method. It can be concluded that impregnation ratio is the most effective variable on the adsorptive properties of activated carbons obtained.
Conclusions 1. Almond shells are an appropiate raw material to obtain activated carbon by the chemical method. 2. ZnC1, was selected as the best activating reagent for almond shells. 3. Raw material particle size, activation time, and impregnation ratio were studied, showing the impregnation ratio to be the most effective variable in increasing the adsorptive power of the products (Le., up to 2111 m2/g). The 77 K N2 adsorption isotherms revealed an increase in the mesoporosity effect of this variable on the porous structure of the activated carbons obtained. Nomenclature a = amount of adsorbed Nz at 0.95 relative pressure, cm3/g A.C. = ash content, % b = amount of adsorbed N2 at 0.1 relative pressure, cm3/g D = raw material particle size, mm I.N. = iodine number, mg/g L.W. = loss in weight, % M.C. = relative mesopores contribution, % m.C. = relative micropores contribution, % P.N. = phenol number, mg/g P/Ps = relative pressure R = impregnation ratio, % R.R. = activating reagent recovery, % S = nominal specific surface, m2/g T = activation temperature, OC t = activation time, min V = amount of adsorbed Nz, cm3/g Registry No. ZnClz, 7646-85-7; K z C O ~ 584-08-7; , carbon, 7440-44-0.
Literature Cited Berenguer, M. C. Doctoral Thesis, University of Granada, Spain, 1980. Bruanuer, S.; Emmett, P. H.; Teller, E. J. Am. Chem. Soc. 1938, 6 0 , 309 Croft, R. C. Quart. Rev. Chem. SOC.1960, 74(1), 1.
269
Ind. Eng. Chem. Prod. Res. Dev. 1904, 2 3 , 269-271
Durie, R. A.; Schafer, H. N. S. fuel 1079, 58, 472. Gregg, S. J.; Wing, K. S. W. "Adsorption, Surface Area and Porosity"; Academic Press: London-New York, 1967. Kipling, J. J.; Sherwood, J. N.; Shooter, P. V.; Thompson, N. R. Carbon 1964, 1, 321. Linares-Soiano, A.; Lbpez-GonzBlez, J. D.; Mollna-Sabio, M.; RodriguezReionoso, F. Chem. Tech. Elotechnol. 1060, 30, 65. Nacco, R.; Aquaron, E. Carbon 1978, 6 , 31.
Smicek, M.; Cerny, S. "Active Carbon"; Elsevier: Amsterdam-London-New York, 1970. Yamada, D. Bull. Rac. Eng., Yokohama, Natl. Unlv. 1950, 8 .
Received for review May 31, 1983 Revised manuscript received September 30, 1983 Accepted October 21, 1983
Activated Carbon from Almond Shells. Chemical Activation. 2. ZnCI, Activation Temperature Influence F. Rulz BevlB,' D. Prats Rlco, and A. F. Marcllla Gomls Departamento de OGmica TGcnica, Unlversldad de Allcante, Apartado 99, Alicante, Spain
Activation temperature influence on adsorptive properties of activated carbons obtained from almond shells by chemical activation with ZnCi, is reported. A maximum in adsorption capacity is found at 500 O C , a temperature lower than commercially used ones. The 77 K N, adsorption data correlate acceptably well with solution adsorption measurements.
Introduction In an earlier paper, Ruiz et al. (1984) reported results obtained in the study of four activating reagents on almond shells as raw material for activated carbon preparation. ZnC12 reagent was the most appropiate among the four. The above-mentioned paper also reported the influence of raw materigl particle size, activation time, and impregnation ratio on adsorptive properties developed in products obtained; impregnation ratio proved to have the biggest effect. The present report completes earlier work with the study of temperature influence on the ZnC12 activation process. Experimental Section Experimental procedures to determine loss in weight (L.W), activating reagent recovery (R.R.), ash content (A.C.), iodine adsorption capacity (I.N.), phenol adsorption capacity (P.N.), nominal specific surface (Kipling et al., 1964) (S), and 77 K N2 adsorption isotherms were described in an earlier report (Ruiz et al., 1984). In addition, scanning electron micrographs were made from samples a t different activation levels with a Phillips PSEM 55 electromicroscope at 25 kV accelerating potential. Results Experimental planning, nomenclature and results obtained are shown in Tables I to IV. For example, Figures 1-3 describe the dependence of I.N. on temperature (T). The same behavior can be shown for P.N. and S from Tables I-IV. On the other hand, Table V presents results obtained when using a set of operating conditions estimated as quite favorable for this process. In addition, 77 K N2adsorption isotherms, corresponding to four samples as indicated, are shown in Figure 4. Moreover, Table VI presents the relative micro- (m.C.) and mesopores (M.C.) contribution to porous structure of the samples and specific surfaces obtained by using the complete BET method (Brunauer et al.,1938). Figure 5 shows electromicrographs obtained from three samples at different activation levels. Discussion Table I shows experimental planning, nomenclature, and results obtained in a first set of experiments a t 400,500, 0196-4327f841 1223-Q269$01.5OfO
Table I. Temperature Influence ( R = 25%; t = 2 h; D = 0.50-0.84 mm)
T, L.W., R.R., A.C., I.N., P.N.,
expt
"C
%
%
%
AT1 AT2 AT3 AT4
400 500 600 700
51.4 55.0 57.2 58.2
83.8 80.4 58.8 22.3
1.5 1.6 2.1 1.2
mg/g mg/g
S, mz/g
654 708 664 720
567 703 695 742
13.6 18.6 17.6 16.4
Table 11. Temperature Influence ( R = 25%; t = 2 h ; D = 0.50-0.84 mm)
T, L.W., R.R., A.C., I.N., P.N.,
expt
"C
%
%
%
AT5 AT6 AT7 AT8 AT9 AT10 AT11 AT12 AT13 AT14 AT15 AT16 AT17 AT18
400 450 450
51.0 53.5 53.0 55.3 55.3 56.1 55.9 55.7 55.7 57.2 57.4 57.0 58.1 58.5
83.1 85.0 81.4 80.4 76.5 70.2 73.2 67.2 71.2 54.4 56.1 52.1 24.3 27.1
1.4 0.7
500 500
550 550 550 550 600 600 600 700 700
S, mg/g mg/g ma/g
645 651 1.1 676 1.2 732 1.6 697 1.2 634 1.6 637 1.2 652 1.1 654 1.4 655 1.4 670 1.3 678 1.4 695 1.1 685
13.6 15.0 14.3 19.3 18.6 16.4 16.0 16.0 15.7 16.9 16.6 16.2 17.2 16.9
621
-
657
-
711
~
Table 111. Temperature Influence ( R = 25%; t = 2 h ; D = 0.21-0.50 mm) T, L.W., R.R., A.C., I.N., P.N., S, expt
"C
AT19 AT20 AT21 AT22 AT23 AT24
300 42.9 400 52.5 500 56.4 600 58.5 700 58.8 800 60.2
%
%
%
85.3 83.8 80.3 60.0 27.1 11.9
0.7 0.5 1.0 0.7 1.4 0.2
mg/g mg/g m2/g 415 735 775 694 767 869
14.2 20.8 23.4 20.7 20.8 22.3
255 664 757 722 745 762
600, and 700 "C, respectively. It can be noticed from these figures that loss in weight increases with increasing temperature, and this increase is much more marked up to 500 "C. Activating reagent recovery, logically, goes down with increasing temperature, as a consequence of ZnClz evap0 1984 American Chemical Society
