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 ZnC12activation 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 654 708 664 720
S, mz/g
13.6 18.6 17.6 16.4
567 703 695 742
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
400 450 450 500 500 550 550 550 550 600 600 600 700 700
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 1.1 1.2 1.6 1.2 1.6 1.2 1.1 1.4 1.4 1.3 1.4 1.1
S, mg/g mg/g ma/g 645 651 676 732 697 634 637 652 654 655 670 678 695 685
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
300 400 500 600 700 800
42.9 52.5 56.4 58.5 58.8 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
270
Ind. Eng. Chem. Prod. Res. Dev., Vol. 23, No. 2, 1984
Table IV. Temperature Influence (R = 25%; t = 2 h ;
D = 1.41-2.00 m m ) T, L.W., R.R., A.C., I.N., expt
"C
%
%
%
AT25 AT26 AT27 AT28 AT29
300 400 500 600 700
43.8 49.3 53.4 55.2 56.7
79.9 80.6 75.8 47.5 23.4
1.4 1.7 1.3 1.0 0.8
P.N., S, mg/g mg/g mz/g 256 514 589 469 536
7.6 10.2 14.8 13.0 15.0
112 507 570 520 526
Table V. Estimated Optimum Conditions (D= 0.21-0.50mm; t = 2 h; R = 100%;T = 500 "C)
L.W., expt OP1 OP2
R.R., A.C., I.N., % % mg/g 81.5 0.6 986 83.2 0.5 997
%
53.8 53.5
S, mZ/g 1648 1654
P.N., mg/g 25.8 27.2
200
300
400
500
600
700
800
TOC
Figure 3. Temperature influence on iodine adsorption capacity of samples obtained in the following conditions: D = 1.41-2.00 mm; t = 2 h and R = 25%. V cm3/9 800
i
500
6oo
400
400
300
400
600
500
700
IN mg/g
1
800
800
T
OC
1
b
0
Figure 4. The 77 K N2adsorption isotherms from samples OP1 (R = loo%, T = 500 "C), AR4 (R = 100%, T = 600 "C), AT20 (R = 25%, T = 400 "C), and AT21 ( R = 25%, T = 500 "C). Table VI. Relative Micro- and Mesoporosity Contribution from 77 K N, Adsorption Data
400 1 300
400
500
600
700
k O
TOC
Figure 1. Temperature influence on iodine adsorption capacity of samples obtained in following conditions: D = 0.50.84 mm; t = 2 h; R = 25%. (a) Data from Table I; (b) data from Tables I and I1 globally.
500
.
6oo 400
I!'
1
300
400
500
6W
700
800
oc
Figure 2. Temperature influence on iodine adsorption capacity of samples obtained in the following conditions: D = 0.21-0.50 mm; t = 2 h; and R = 25%.
oration, and only 22.3% of the initial amount remains when operating at 700 OC. Note, however, that ash content does not seem to depend directly on operating temperature, since it is always between 1.2 and 2.1%. All determined adsorption indexes show the same trends. Figure l a shows, as an example, I.N. vs. T varia-
expt OP1 AR4b AT20 AT21
a, b, cm3/g cm3/la
450 395 220 240
605 540 240 265
m.C.,
M.C.,
%
%
S,a mz/e
74.4 73.1 91.7 90.4
25.6 26.9 8.3 9.6
1825 1602 852 967
Nominal specific surface determined by the complete BET method. Experiment reported in the present work part 1.
tion. An unexpected behavior is observed, with sigmoidal dependence of iodine adsorption capacity with successive temperatures. In order to reproduce this behavior, some experiments were repeated up to three more times using exactly the same conditions; 450 and 550 "C temperatures were added to the experimental planning in order to mark out the observed 500 "C relative adsorption maximum. Results obtained and conditions used are shown in Table I1 and they confirmed earlier findings. Loss in weight increases markedly with temperature up to 500 "C, and this increase is smaller at higher temperatures. Activating reagent recovery goes down with increasing temperatures as a result of ZnClz evaporation. All results from adsorptive capacity measurements pointed to a relative adsorption maximum capacity at 500 "C operating temperature. Figure l b shows I.N. vs. T,and all determined adsorption indexes follow similar trends. Furthermore, two new seta of experiments were planned changing raw material particle size, using D = 0.21-0.50 mm and D = 1.41-2.00 mm fractions, respectively. Activating time and impregnation ratio were kept constant at 2 h and 25 % , respectively. Results obtained and nomen-
Ind. Eng. Chem. Rod. Res. Dev.. VOI. 23.
/I
....
w. b.:,.. 1,
"
I , ,
..._ L c
,. .
Figure 5. Micrographs showing clear texture differences: (A) sample ATZO. 1 4 W x (T= 400 "C. R = 25%): (B) sample AT21.14OOx (T= 500 O C , R = 25%), and (C) sample OP1 1400x (T= 500 "C.
R = 1W46). clature are presented in Tables 111and IV. Figures 2 and 3 show LN. vs. T variation, all determined indexes follow the same tendencies as can be seen from corresponding tables. An adsorption capacity maximum appeared again when using 500 "C as operating temperature. Loss in weight, activating reagent recovery, and ash content followed the same pointed trends. Raw material particles size influencecan be noticed as decreasing adsorption capacity indexes as it was increased. All results are in accordance with those obtained in earlier studies (Ruiz et al., 1984). A maximum adsorptive capacity with varying temperature in this type of processes has been proved elsewhere (Le., Nacco and Aquarone, 1978; Yamada, 1959). Hence, using an operation temperatwe of 500 "C would represent very favorable operating conditions in the utilization of almond shells as raw material to obtain activated carbons, producing yields of approximately 48%, and some 78% of the used activating reagent would be recoverable from washing liquors. To complement previous work, raw material particle size of 0.214.50 mm, 2 h activation time, 100% impregnation ratio, and 500 "C operating temperature were used. Results obtained are presented in Table v , which show good yields (46%). activating reagent recovery up to 82%. a very
NO. 2. 1984
271
low ash content, and highly developed adsorption power as compared with characterized commercial activated carbons (Ruiz et al., 1984). The 77 K N2 adsorption isotherms from OP1, AR4 (reported in part 1 of this work), AT20, and AT21 were determined; they are shown in Figure 4. It can be observed that the isotherm corresponding to 500 OC appears a t higher adsorbed N, amounts than the one corresponding to 400 "C when R = 25% (samples AT21 and AT20, respectively). When R = 100%, the OP1 isotherm corresponding to 500 "C appears at higher adsorbed N, amounts than the AR4 isotherm, a t 600 "C. These results are in accordance with observed maximum at 500 OC from I.N., P.N., and S determinations. Table VI presents the relative meso- (M.C.) and micropores (m.C.) contributions (M.C. = (a - b ) / 6 ; m.C. = a / b ; a = amount of adsorbed N, a t 0.95 relative pressure; and 6 = amount of adsorbed N, at 0.1 relative pressure). It is shown that temperature affects both of them while relative contributions remain practically unchanged. Table VI also shows nominal surfaces of these samples obtained by using the complete BET method and exhibiting the same trends as those obtained by the simplified method. Scanning electron micrographs (Figure 5) show clear differences in porous texture of samples obtained a t different activation levels, especially between OP1 on the one hand and AT20 and AT21 on the other. These differences are more sensitive to impregnation ratio than to temperature. Conclusions 1. A maximum in adsorption capacity a t 500 OC has been observed and sufficiently proved. This fact represents very favorable activated carbon obtention conditions when using almond shells as raw material in ZnCl, chemical activation process. 2. In general, at low temperature (500 "C) and low impregnation ratio (25%) equivalent adsorption capacities to commercial products were obtained. Nomenclature LI = amount of adsorbed N2a t 0.95 relative pressure, cm3/g A.C. = ash content, % 6 = amount of adsorbed N, 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. mglg P / P . = 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 N,, cm3/g Registry No. ZnCI,, 7646-85-7; carbon, 7440-44-0.
Literature Cited BNMUr. S.; Emmet. P. H.;TsLn. E. J . Am. Unn. SOC. 1931. 60. 309. Klplhg. J. J.: Shmwocd. J. N.; yoota. P. V.: Thompson. N. R. C a l m 1984. 1 . 321.
Nacco. R.; A q u s m . E. CarLm 1978. 6 , 31. Rulz Bsv!4. F.; RaU Rim. D.: MarcIIh Oamb. A. F. I d . Eng chsm. Rod. Res. Dsv. 1984. ~re.cadln(iartkle In m b hue. Yamads. 0. Bull. Fac. E n g . . ~ Y o k ~ m Nan. a . IMv. 1959.8.
Received (or reuiew May 31, 1983 Revised manuscript received September 30, 1983 Accepted October 21, 1983
