Activated Carbons from Sugar Cane Bagasse Max Ruiz and Carlos Rolz' Applied Research Division, Central American Research Institute jor Industry ( I C A I T I ) ,Guatemala City, Guatemala, C.A.
Results are given for laboratory production of activated carbons from sugar cane bagasse, coconut husk, and filter mud. The technique employed consisted of low temperature carbonization with simultaneous chemical activation. Values for some of the physical characteristics were compared with those of two commercial activated carbons recommended for adsorption from solution operations. These tests were complemented with kinetic and equilibrium experiments. The carbon obtained from sugar cane bagasse activated with zinc chloride showed a kinetic behavior similar to the two commercial carbons. However, all of the carbons from agricultural wastes showed a lower capacity for color removal as compared to the two commercial types tested. The difference was higher at lower concentrations.
A c t i v a t e d carbon is utilized as a n adsorbent t o remove color from the clarified juice in sugar refineries. According t o Dietz and Carpenter (1964), this treatment efficiently removes several colored impurities such as colloidal matter, pigments (natural and formed during the process), and inorganic constituents (ash). Powdered and granular types of carbons are employed, depeiiding mainly on the refineries' capacity and mode of operation. I n batch systems (low capacities and pressure filtrat,ion), the former is preferred. I n semi or continuous systems (high capacity and packed or moving beds), the granular form is being employed more frequently owing to certain economic advantages (Schuliger, 1969). Bone charcoal and ion exchange resins are also used for the same purpose. Carbon (and charcoal) and the resins are often used in series steps in the refining process. Liquid phase act'ivated carbons have been obtained mainly from bones, wood, lignin, coconut, natural carbons, and petroleum residues (Fornwalt e t al., 1963). Many alternatives have been proposed in the general literature for the economical utilization of t'his agricultural byproduct (Srinivasan and Pathak, 1961; Paturau, 19694. With respect to the product'ion of activated carbon from bagasse, the amount of experimental data published is limited. Erdos (1960) reports the product yields obtained under different carbonization conditions, but he does not present experimental results of the resulting products' effect'iveness as liquid phase adsorbents Susrez et al. (1968) and Gayol (1968) have presented detailed experimental data on the carboiiization, high temperature activation, and adsorption characteristics of carbons from sugar bagasse and filter mud (cachaza) from the sugar mill. I n the present work, results are given on the color removal from cane juice by various carbons activated b y different chemical methods from bagasse. Also, experiment,al data are reported for two commercial products and carbons obtained from coconut and filter mud from the sugar mill. Experimental
The experiments consisted of three steps: producing t,he carbon, determining its physical propert'ies, and evaluat,ing its effectiveness as a color adsorbent. I n the first step the bagasse was passed through a Riley mill Model 3, 16-mesh. The resulting powder was placed in a t'ubular container To whom correspondence should be addressed.
adapted to a laboratory furnace (Heavy D u t y Electric Co. Type 056-PT) for 60-75 min at 500-550°C. The carbon obtained was washed with distilled water and dried in a laboratory oven a t 100°C for 12 h r (Precision Scientific Model 625-A); finally, i t was ground with the Wiley mill, 60-mesh. Four types of carbons were obtained by use of this method, three activated with the addition of various chemicals and the other without activant. The chemicals used were those recommended by Fornwalt et al. (1963) : sulfuric and phosphoric acids and zinc chloride, in the proportion of 65y0 by weight of the original bagasse. The physical properties tested u ere moisture content, ash, and p H value according to standard techniques (de Whalley, 1964), surface area by the method of Gilesand Trivedi (1969), and equivalent particle diameter by analysis of sieving data (Zenz and Othmer, 1960). I n the last step two methods were tested: kinetic and equilibrium runs b y employment of a sugar solution of known concentration and color value. I n the kinetic experiments the proportion of carbon to sugar solution was fixed, and the color decrease was followed with time. I n the equilibrium runs after an appropriate amount of time had elapsed, the color removed was determined. This was done for various experiments with differences in the proportion of carbon t o sugar solution employed. However, in both cases the following experimental procedure was employed. Crude export sugar was dissolved in tTater to make a 50" Brix solution. This was clarified by following the phosphoric acid-soda ash technique (Meade, 1964). The p H and color were determined. To measure color of the sugar solutions their absorbance a t 460 and 720 nm were taken with a Gilford spectrophotometer &[ode1242. The values a e r e combined in the usual manner as the absorbance index (Gillet, 1953; Meade, 1964). Samples of 250 ml were taken for every run, the corresponding amount of carbon was added, and the Erlenmeyer was placed in a gyratory water bath shaker (New Brunswick Scientific Co. Model G76) set a t 82°C. Samples of 25 ml for color analysis were then taken every 5 min u p to 30 min, and two additional ones a t 45 and 60 min. The samples were cooled to 10°C for 2 h r and adjusted to pH 7 with 1N HCl or NaOH as required; their color was measured as described. For the kinetic runs the proportion of carbon to sugar solution was coilstant and equal to 0.057 g/ml, and the agitator revolutions Ind. Eng. Chem. Prod. Res. Develop., Vol. 10, No. 4, 1971
429
12
x n
10
Zz
oa
W
I.2
1.0 O
r
z
w
g 3a
a
B2 a
0.a
v) 0
06
m 0.6
a
04 10
0
20
30
40
50
60
0.4
TIME, MINUTES
Figure 1 . 1
Kinetic runs for carbons
0 Bagasse 0 Bagasse
+
Table 1. Physical Properties
+ + + +
+
+
Z,Clz
of Activated Carbons
Ash,
%
3.39 4.62 15.11 7.04 15.28 14.36
11.39 18.34 12.49 11.58 9.82 26.68
%
pH value
Surface area, rn2/g
Equiv particle diam, cma
6.90 5.30 2.50 3.30 2.85 2.90
24.6 27.4 112.8 498.0 603.0 277,O
0.0025 0.0029 0.0026 0,0030 0.0020 0.0038
l b 7 . 7 2 11.57 4.60 171.0 0.0030 Commercial carbon 2. 7 . 2 8 5.49 5.90 362.0 0.0030 a Equivalent, particle diameter values for the two commercial carbons were assumed. Darco S-51, Atlas Chemical Industries, Inc. c Carboraffin, Lurgi Gesells Chaft fur Chemotechnick.
(200 rpm) were set a t a range where they did not influence color adsorption. KO experiments were done with different particle sizes. The equilibrium runs were done a t 0.05, 0.1, 0.3, 0.5, 0.7, 0.9, and 1.0 X IO+ g/ml. Results and Discussion
The values obtained for the physical properties of the various carbons are shown in Table I. All of the properties vary to an extent for the carbons listed. With respect to the pH value, the carbon from bagasse without activant is almost neutral. This indicates the absence or the relatively low quantity of acidic surface oxides which give carbon its acid characteristics (Boehm et a]., 1964; Coughlin, 1969; Snoeyink and Weber, 1967; Studebaker and Huffman, 1956). The carbons from bagasse obtained with phosphoric acid and with zinc chloride are quite acid, as are those obtained from coconut and filter mud with zinc chloride. Coughlin and Ezra (1968) have shown that the surface acidity influences largely the amount of organic substances absorbed from solution. Besides this consideration, and specifically referring to its use as adsorbent for colored sugar solutions, an acidic surface can catalyze the inversion of sucrose. This reaction is undesirable in the overall economy of the process. K i t h respect to ash content, compared with the commercial types, the values are generally on the high side, especially that obtained from filter mud. T o a certain extent this was 430
Kinetic runs for carbons
1 0 Coconut Z,Clz 2 0 Filter mud f Z,Clz 3 @ Commercial carbon 1 4 c) Commercial carbon 2
H3P04
Moisture, Carbon
Figure 2.
+ HpSOa
2 3 @Bagasse f 4 c ) Bagasse
Bagasse Bagasse H2SOd Bagasse H3P04 Bagasse Z,ClZ Cocollut 2,Clz Filter mud Z,Cls Commercial carbon
TIME, MINUTES
Ind. Eng. Chern. Prod. Res. Develop., Vol. 10, No. 4, 1971
expected owing to the initial high content of inorganic salts present in the mud (Paturau, 196913). Liquid phase carbons have a surface area on the order of 1600-1800 sq m/g (Fornwalt et al., 1963). The values reported in Table I are far below these figures. There are two possible explanations for this difference. The method of Giles and Trivedi used in the present work determines surface area by the adsorption of certain dyes from solution, differing from the classical gas adsorption methods. These authors compare both methods for various solid materials, and from their figures as solid porosity increases, the values obtained with their method are less than the corresponding figures for gas adsorption. On the other hand, perhaps the carbons from bagasse do have the low values noted in Table I, characteristic of the type of chemical activat'ion employed, temperature used, and raw material. The results for the kinet'ic runs and the equilibrium experiments are shown in Figures 1-3. The curves shown for the color changes with time (Figures 1 and 2), although different for the various carbons, demonstrate that most' of the color removed from solution is accomplished during t'he first 15 min of solid-liquid contact. After this time has elapsed, the experimental points tend to a constant value although the curve for bagasse without chemical activation shows a gradual color increase. This type of behavior was also present for most of the carbons obtained without chemical activation, but from a different raw material, coconut and filter mud (Ruiz, 1970). No clear explanation can be offered except t h a t desorption of t'he color compounds from these carbons seems to be favored after approximately 30 min of contact, time. illass transfer in adsorption from solut'ion can be visualized as a three-step mechanism (Vermeulen, 1958) : bulk liquid diffusion, intraparticle pore transport, and surface adsorption. The latter is practically instantaneous (de Boer, 1953), and the resistance associated with the first step can easily be made negligible in small experimental batch systems by proper external agitation (Brecher et' al., 1967b). This then leaves as the limiting step in the process, the diffusion of solute in the pores of the solid phase. This type of transport can occur in various ways according to the solute-solid system and the pore size distribution and arrangement within the solid (Satterfield and Sherwood, 1963). Applications to liquidsolid adsorption have been published by Brecher et' al. (1967a, 196713) and Dedrick and Beckmann (1967). The
1.8
I
I /
I
I
I
I
Figure 3. Equilibrium data for different carbons 1 0 Bagasse 4-H I P O ~ 2 0 Bagasse Z,Cln 3 0 Coconut Z,Clz 4 @ Filter mud Z,Clz 5 @ Commercial carbon 1 6 0 Commercial carbon 2
+-+ +
C,
Beckman, 1967). D was then evaluated by Equation 2. Results have been arranged in Table I1 and Figure 4. The values for the diffusion coefficients are within the same order of magnitude as those obtained by Dedrick and Beckman (1967) for adsorption from solution in activated carbon of 2,4dichlorophenoxyacetic acid. The results (Figure 4) agree well with the homogeneous diffusion model. The equilibrium data in Figure 3 could be fitted t o the simple Freundlich type of adsorption isotherm:
simplest model for the case of radial diffusion into a sphere with a constant effective diffusivity is the “homogeneous diffusion model,” from which the following approximation can be derived (Dedrick and Beckman, 1967) :
$e;&
-
E
=
6
E = 1---exp 772
[
-T’
Dt
~
5 0.25
1, $ *
5 0.25
Color of Residual Solution
(2)
-
J,
-
where D = effective particle diffusivity, sq cm/sec; a = radius of sphere for intraparticle effects, em; t = time, see; and E = fractional approach to equilibrium. Instead of 0.25 as the boundary value, some workers prefer t o use 0.4. As shown in Figure 4, this difference did not matter in our work. For a certain time f , E can be calculated from Figures 1 and 2, The corresponding value of a can be approximated by the diameter of the sphere equal to six times the average particle volume divided b y the external surface area (Dedrick and
m
=
Kc“
(3)
The results obtained are shown in Figure 5 , and the value for the constants iq shown in Table 111. The data of the carbons obtained from bagasse, coconut husk, and filter mud !%ere grouped together owing to their similarity, although as observed in Table 111, the correlation coefficient is lower for the experimental carbons than the corresponding ones for the commercial carbons. By comparison of the three curves in
I O
oa
Figure 4. Kinetic summary according to homogeneous diffusion model correlation - M e a n of data from Dedrick and Beckman (1 9 6 7 )
0 Experimental points 1 Bagasse f HzSOa 2 Coconut f Z,CIz and filter mud 3 Bogasre 4 Bagasse H3POa 5 Bagasse Z,CIz 6 Commercial carbons 1 and 2
++
0.6 E 04
+ Z,CIz 02
0 0
01
02
03
04
05
06
07
08
Ind. Eng. Chem. Prod. Res. Develop., Val. 10,
09
No, 4, 1971
IO
431
carbonization process with simultaneous chemical activation, The carbons obtained have different physical characteristics depending on the type of chemical added. The sugar cane bagasse carbon activated with zinc chloride shows a kinetic behavior similar to the two commercial carbons tested for the removal of color from sugar solutions. However, these same carbons do have a lower color removal capacity than the commercial carbons. Before final word is given in this respect, more data are needed, both of the chemical and physical characterization type, such as pilot batch and column adsorption testing.
2.0
C
1.0 0
u 0.8
.-C
0.6
L
n
-b
8
0.4
Acknowledgment
The authors acknowledge Atlas Chemical Industries, Inc., and Lurgi Gesellschaftt fur Chemotechnik for the commercial carbon samples. Nomenclature
a = radius of sphere for intraparticle effects, cm
0.1
0.I
0.2
0.4
0.6
0.8 1.0
C, Color of Residual Solutlon
Figure 5. Equilibrium correlation with Freundlich type of adsorption isotherm Nomenclature same as Figure 3
~~
Table II. Diffusion Coefficients D X loB, Carbon E” cmZ/sec
Bagasse 0 74 0 90 Bagasse H804 0 41 0 20 Bagasse 0 75 1 02 0 86 2 24 Bagasse Z,Clz Coconut Z,Cls 0 69 0 46 Filter mud Z,Cl, 0 69 1 65 Commercial carbon 1 0 91 2 91 Commercial carbon 2 0 91 2 91 a Obtained from Figures 1 and 2 a t 10 min.
+ + + +
c = color of residual solution D = effective intraparticle diffusivity, sq cm/sec E = fractional approach to equilibrium K = coefficient in Freundlich equation m = carbon mass, grams n = exponent in Freundlich equation T = correlation coefficient t = time, see 2 = color adsorbed Literature Cited
(Dt/az)
0.293 0.121 0,300 0.386 0,261 0.261 0.440 0.440
Boehm, H. P., Diehl, D., Heck, W., Sappock, W., Angew. Chem., Znt. Ed., 3 (lo), 669 (1964). Brecher, L. E., Frantz, D. C., Kostecki, J. A., Chem. Eng. Progr. Symp. Ser., 63 (74), 24, Part I1 (1967a). Brecher, L. E., Kostecki, J. A., Camp, D. T., ibid., p 18, Part I (1967h-, i. \ - - -
Conclusions
Coughlin, R. W., Znd. Eng. Chem. Prod. Res. Develop., 8 (I), 12 (1969). Coughlin, R. W., Ezra, F. S., Environ. Sci. Technol., 2 (4), 291 (1968). de Boer, J. H., “The Dynamical Character of Adsorption,” Oxford, London, England, 1953, p 40. Dedrick, R. L., Beckmann, R . B., Chem. Eng. Progr. Symp. Ser., 63 (74), 68 (1967). de Whalley, H. C. S., “ICUMSA Methods of Sugar Analysis,” Elsevier, London, England, 1964, p 141. Dietz, V. R., Carpenter, F. G., in “Cane Sugar Handbook,” G. P. Meade, Ed., Wiley, New York, N.Y., 1964, p 342. Erdos, J., Rev. Quim. Mer., IV (4), 112 (1960). Fornwalt, H. J., Helbig, W. A., Scheffler, G. H., Brit.Chem. Eng., 8 (S), 546 (1963). Gayol, J. P., ICIDCA (Cuba), 2 (2), 28 (1968). Giles, C. H., Trivedi, A. S., Chem. Znd., 40, 1426 (1969). Gillet, T. R., in “Principles of Sugar Technology,” P. Honing, Ed., Vol. I, Elsevier, London, England, 1953, p 214. Meade, G. P., “Cane Sugar Handbook,” Wiley, New York, N.Y., 1964, pp 327,505. Paturau, J. AI., “By-Products of the Sugar Cane Industry,” Elsevier, New York, N.Y., 1969a, pp 23-116. Paturau, J. &I., zbid., 1969b, p 119. Ruiz, J. M.,Ing. Tesis, Universidad de San Carlos, Guatemala City, Guatemala, 1970. Satterfield, C. N., Scherwood, T. K., “The Role of Diffusion in Catalysis,” Addison-Wesley, Reading, Mass., 1963, p 12. Schuliger, W. G., presented at the 64th National Meeting, AIChE, New Orleans, La., 1969. Snoeyink, V. L., Weber, W. J., Envaron. Sci. Technol., 1 ( 3 ) , 228 (1967). Srinivasan, V. R., Pathak, S. R., Znt. Sugar J . , 6 3 , 208 (1961). Studebaker. 31.L.. Huffman. E. W. D.. Ind. Ena. Chem.. 48 (1). . ,, 162 (1956). Sukrez, A,, Kudriachov, B., Gayol, J. P., presentado en la IV Conferencia de Quimica, Universidad de Oriente, Cuba, 1968. Vermeulen, T., Advan. Chem. Eng., 11, 148 (1958). Zenz, F. A., Othmer, D. F., “Fluidization and Fluid-Particle Systems,” Reinhold, New York, N.Y., 1960, pp 94-104. RECEIVED for review December 24, 1970 ACCEPTEDAugust 23, 1971
It is possible to obtain activated carbons from sugar cane bagasse, filter mud, and coconut husk b y a low temperature
This work was supported by El Salto, S.A., Guatemala City, Guatemala.
+
Table Ill. Freundlich Constants Carbon
+ + +
k
Bagasse &Po4 Bagasse Z,Clz Coconut Z,C12 1.1589 Filter mud Z,Cli 0.4615 Commercial carbon 1 Commercial carbon 2 0.3852 Correlation coefficient.
+
n
ra
2,4494
0.77
0.5465 0.9056
0.96 0.99
Q
Figure 5, both commercial carbons show a higher capacity than any of the carbons from agricultural by-products. Gayol (1968) shows similar results for carbon from sugar cane bagasse activated by temperature. His data indicate, however, that carbon from agricultural wastes has a higher capacity than commercial carbons a t higher values of the residual solution color. This tendency is also shown in Figure 5; however, experimental data that could confirm the proposed extrapolations are needed. Gayol (1968) attributes this to the larger pore size distribution of the carbons from bagasse, but he does not give experimental results that prove the statement.
432
Ind. Eng. Chem. Prod. Res. Develop., Vol. 10,
No. 4, 1971
I
