Preparation and characterization of activated carbons from eucalyptus kraft lignin

Carbon. Vol. 31, No. I, pp. 87-95. Printed in Great Britain.

OCO8-6223/93 $6.00 + .Nl Copyright 0 1993 Per!gmon Press Ltd.

1993

PREPARATION AND CHARACTERIZATION OF ACTIVATED CARBONS FROM EUCALYPTUS KRAFl- LIGNIN J. RODRIGUEZ-MIRASOL,T. CORDERO and J. J. RODRIGUEZ Department of Chemical Engineering, University of Mglaga, 2907 I Mblaga, Spain (Received 25 April 1992; accepted in revisedjbrm 2 1 May 1992)

Abstract-Preparation of activated carbons from eucalyptus kraft lignin has been investigated. A pretreatment method has been established to avoid partial fusion and swelling in the carbonization stage. Carbonization has been studied at different temperatures and the structure of the microporous chars has been characterized. Activated carbons have been prepared from CO1 partial gasification of chars obtained at 823 and 1073 K. Both chars show a comparable behavior regarding to the evolution ofporous structure. Activation increases both total and narrow microporosity and develops a substantial mesoporosity. At high burnoff levels, macroporosity becomes also significant. BET surface areas in the vicinity of I ,3001,400 m2/g have been achieved at burnoff levels around 70-7596 which correspond to overall carbon to lignin yields of about IO%- 1 I % (d.a.f. basis). Key Words-Kraft

lignin, carbonization, COz-activation, porous structure.

rous structure development of kraft lignin upon activation. In this paper we study the preparation ofactivated carbons from eucalyptus kraft lignin. Carbonization of the precursor and CO, partial gasification of the resulting chars are investigated. The porous structure of the chars and activated carbons has been characterized.

1. INTRODUCHON conversion of lignocellulosic wastes hasgained attention in the last decade. The use

Thermochemical

of lignocellulosic precursors as raw materials for activated carbons preparation upon carbonization and partial gasification or catalysed pyrolysis has been widely reported in the literature. References [I] and [2], containing extended bibliography, are some representative examples of last decade’s publications on this field. Kraft lignin represents the main waste ofcellulose pulp mills, where it is mostly burned as part of the recovery process of chemical reactants remaining in black liquors after wood pulping. The development of alternative uses for kraft lignin is of potential interest for kraft pulp mills. Among other considerations of chemical and environmental concern, the possibility of alternative ways for black liquors processing gives to the plant the chance of increasing production rate without expensive revamping of the evaporation train. One potential way is precipitation of kraft lignin followed by its further conversion to useful products. Activated carbons are interesting candidates among these potential products. In spite of some references in the patent literature, mainly Russian and Japanese, about the use ofdifferent types of lignins as activated carbon precursors, there is an important lack of scientific information on the carbonization and activation of this material. Otani IZC a1.[3,4] have reported interesting work dealing with carbonization of lignin by-product of ethanol production from eucalyptus wood. Li and Van Heiningen[S] have studied the kinetics of CO* gasification of black liquors and Del Bagno et a1.[6] have investigated char and activated carbon manufacture from these liquors at a pilot-plant scale. Nevertheless, no comprehensive studies have been reported on po-

2. EXPERIMENTAL

Eucalyptus kraft lignin was supplied by the Empresa National de Celulosas (ENCE) as obtained in its acid-precipitation plant. The plant is based on H2S04 precipitation of kraft lignin from eucalyptus black liquors, followed by centrifugation and fluidized bed drying. The starting material used in this work had the typical proximate and ultimate analyses given in Table I. Previous carbonization experiments were performed with this starting material as well as with the resulting from washing up to different final ash contents. Washing was accomplished with 1% aqueous H,SO, and demineralized water at room temperature. When low ash kraft lignin was pyrolysed under Nz atmosphere, it showed a plastic and swelling behavior event at heating rates as low as 1 K/min. This would create serious operating problems in practice and could affect negatively further development of porous structure. This problem showed to disappear gradually as ash content increased and no fusion and swelling were noticed when using kraft lignin with ash content above 8%. Nevertheless, this inorganic percentage in the precursor would lead to a nondesirable high ash content in the final activated carbons. We also observed that washing of chars up to very low ash contents became increasingly difficult as carbon87

88

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RODR~XJEZ-MIRASOL~~

al.

Table 1. Analytical characteristics of the supplied kraft lignin and of the low-temperature char (LTC) Eucalyptus kraft lignin

Low-temperature char (LTC)

35.2 52.4 12.4

64.2 33.7 2.1

64.4 5.0 1.2

12.3 3.2

29.4

2::;

Proximate analysis (% d-b.) Fixed carbon Volatile matter Ash content Ultimate analysis (% d.a.f.) C SH 0 (by difference) Apparent surface area (m’/g) BET (N2 77 K) DR (CO2273 K) Micropore volume (cm3/g) N2 77 K (G) CO2 273 K (DR)

ization temperature raised about 673 K. According to these observations, we designed a pretreatment method to avoid occurrence of plastic phase and swelling allowing at the same time a rapid and easy washing ofthe inorganic matter present after this previous carbonization. As described elsewhere[7], the method consists of precarbonizing high ash content eucalyptus kraft lignin (Table 1) at 623 K followed by 1% H$O., washing. The resulting char is identified as low-temperature char (LTC) and a typical analytical chara~e~zation is also included in Table I. Carbonization and activation were performed in a horizontal furnace consisting of a 16 cm length and 8-cm i.d. stainless-steel reactor and electrical resistance system supplied with temperature and heating rate controls. A thermocouple was placed to register the inside temperature in the reactor. The sample was held in flat ceramic boats and a sample weight of 4050 g was always used. Certified 99.998% N2 and CO2 at 150 ml(STP)/min continuous flow were used in the carbonization and activation runs, respectively, and a 10 K/min heating rate was always used to reach the final temperature. Holding time at final temperature was set at 2 h for ~r~ni~tion. No signifi~nt changes were observed in yield and porosity development in carbonization experiments performed at 1 h and 4 h at 1073 K. For activation, different times ranging from 4 to 50 h were used in order to achieve a wide range of burnoff values. Elemental analyses of chars were performed by means of a 240C model Perkin Elmer analyzer. FTIR spectra of chars were obtained using 2% weight of char in KBr matrix after vacuum drying at 333 K for 48 h. A model 1760X Perkin Elmer spectrophotometer was used. The porous structures of the chars and activated carbons were characterized from 77 K Nz adsorption-desorption and 273 K CO, adsorption isotherms carried out in a Carlo Erba Sorptomatic 1800, which was operated manually. BET and DR surface areas were calculated from N2 and CO, isotherms, respectively. N2 and CO* molecular areas of 16.2181and

-

23 336

-

0.01 0.13

18.7 A*[21 were used. Micropore volumes were obtained from the DR and cy,methods, the second being also used for evaluating the external surface ofthe activated carbons. Elftex 120 was used as standard[9]. Mercury porosimet~ analyses were also carried out using a Carlo Erba porosimeter 4000. SEM micrographs were obtained from selected chars and activated carbons by means of a model JSM840 JEOL apparatus. 3. RESULTS AND DISCUSSION

3.1 Preparation and characterization of chars Different chars were prepared from further carbonization of LTC at temperatures ranging from 723 to 1173 K. Table 2 shows the char yield values together with the C, H, and ash content of the resulting chars. The yield values in Table 2 correspond to the weight of ash-free char referred to the weight of ashfree starting lignin. As can be seen, the char ap proaches an almost constant level close to 40% at 973 K, which is substantially higher than the obtainable from lignocellulosic materials used as activated carbon precursors[2]. The removal of functional groups from lignin upon carbonization can be observed in Fig. 1, which shows the FT-IR spectra of original krafi lignin and increasingly higher temperature chars. Figure 2 shows the 77 K Nz adsorption isotherms of chars obtained at three different tem~~tures. The isotherms correspond to microporous solids and the

Table 2. Yield values and C, H, and ash content of the chars Charring T (K)

Yield (%d.a.f.)

(%zaX)

(96Fa.f.)

723 823 1073 1173

47.2 43.4 39.4 39.0

80.3 87.0 91.1 94. I

2.74 2.89 0.94 0.85

($bAds!& 3.4 2.5 2.9 2.5

Activated carbons from kraft lignin

89

Table 3. Micropore volumes and BET surface areas of chars obtained at different temperatures

Charring T

123 823 1073 1173

823

(cm-‘)

Fig. 1. ET-IR spectra of eucalyptus kraft lignin and chars obtained at different temperatures.

00 0.0

0.2

213 496 463 278

K 0.094 0,224 0.211 0.139

VDR 0.082 0.228 0.210 0.132

VDR 0.082 0.263 0.240 0.238

decrease of volume adsorbed at high charring temperatures can be explained as a result of micropores constriction upon increasingly severe thermal treatment reducing the accessibility of the N2 molecules in

K

Wavcnumbcr

&ET

(m*/s)

W

Micropore volum;O(cn&/$ N2 17 K 2

our experimental conditions. This effect has been observed for other precursors even at temperatures as low as 973 K[ 10,l I]. In the case of the 1173 K isotherm of Fig. 2, nonequilibrium problems derived from narrow microporosity could contribute and partly explain the significant decrease of adsorption values. This is confirmed from Table 3 where we report the micropore volumes of chars obtained at different temperatures as calculated by applying the CY, and DR methods to N2 isotherms and the DR method to CO, isotherms. A substantial microporosity development occurs at increasing carbonization temperatures up to 823 K and no further important changes are observed until 1173 K where the aforementioned constriction effect arises clearly from comparison ofthe CO2 and N2 micropore volumes. For all the chars, the CC,and DR methods lead to very close Nz micropore volume revealing a high contribution of primary filling in narrow micropores. The differences observed between the N2 and CO2 micropore volumes confirm the narrow microporosity of the chars. Restricted diffusion

0.4

0.6

0.8

P/PO Fig. 2. 77 K NZ adsorption isotherms of chars obtained at 823, 1073, and I173 K.

1.0

90

J. KODRiGUEZ-MIRASOL et al.

of Nz at 77 K and/or constrictions in the entry of the micropores would explain the differences observed[ 121. Mercury porosimetry of the chars revealed no significant meso and macroporosity development upon carbonization of kraft lignin. This has been observed also for cellulose chars[ 1 I]. The effect of the ash content of the starting material was investigated from carbonization experiments carried out at 1073 K using the 623 K precarbonized kraft lignin but without further washing (17% ash content) and after mild washing (up to 7.5% ash content). The resulting chars exhibited substantially lower values of BET surface area (7.4 and 23.2 m*/g, respectively) and very small N2 micropore volumes. Nevertheless the DR surface area with CO, of the medium ash char was 508 m2/g showing that it has a high microporosity although within the narrow micropore range. The high ash char yielded a DR surface area of 48 m2/g with CO,. Agglomeration and migration of inorganic matter to the surface seems to be observed from SEM micrographs (Fig. 3), giving rise to pore blockage. Further washing of the high ash content chars with diluted H,SO, and HCl aqueous solutions did not easily decrease ash content below 8%, giving rise to an increase of the BET surface area, but up to a value substantially lower than the reported in Table 3 for the low ash 1073 K char.

c

3.2 Activated carbons Activation was carried out at 1073 K by partial gasification with CO* of the 823 and 1073 K chars described in the preceding section. These will be identified, respectively, as medium temperature char (MTC) and high temperature char (HTC). Table 4 summarizes the yield and bumoff values for the activated carbons obtained, together with the final ash content. Yield and burnoff are expressed in dry-ash-free basis and referred to starting kraft lignin and char, respectively. Identification of the activated carbons is also included in the table. The yield versus activation time values of both series fall fairly close. When looking at the burnoff figures, it has to be considered that the 823 K char is less devolatilized than the 1073 K one and the weight loss includes the corresponding to the remaining volatile

Fig. 3. SEM micrographs of 1073 K chars obtained from precarbonized kraft lignin with different ash contents: (a) 1.5%, (b) 7.5%, (c) 15%.

which takes place essentially within the early activation stages. On the other hand, the specific surface and micropore volume of MTC are somewhat higher than the obtained for HTC (Table 3) and this could be of significance in the early stages of gasification reaction and would explain the lower yield matter

Table 4. Overall yield, burnoff and ash content of activated carbons from MTC and HTC Activation time(h) MLC series 4 12 20 40 HLC series 4 12 20 40

Sample designation

Overall yield (96d.a.f.)

Burnoff (% d.a.f.)

Ash content (% d.b.)

M-4 M-12 M-20 M-40

31.6 26.0 21.2 10.2

27.5 40.5 51.4 76.7

3.8 4.5 5.5 11.1

H-4 H-12 H-20 H-40

36.2 24.9 17.8 11.3

8.1 36.8 54.8 71.3

3.3 5.1 6.5 10.2

Activated carbons from kraft lignin

value of M-4 as comnared with H-4. External surfaces areas determined from the (Y,method yielded 30.1 m2/g for MLC and 10.9 m’/g for HLC. Figures 4 and 5 show the 77-K N2 adsorption-desorption isotherms of activated carbons corresponding to the M and H series, respectively. In both series, an important development of porosity takes place as activation proceeds. Surface area and micropore volume increase substantially with burnoff(Tables 5 and 6); widening of micropores and increasing mesopore contribution is observed and, at high burnoff levels, a substantial development of macroporosity can be noticed. Widening of microporosity at increasing burnoff is confirmed from the comparison of micropore volumes obtained with N2 and CO*, reported in Table 5.

91

Table 5. Effective micronore volumes of activated carbons from the M and H series

Activated carbon

N2 II K

CO2 273 K

M-4 M-12 M-20 M-40 H-4 H-12 H-20 H-40

0.312 0.387 0.413 0.528 0.252 0.356 0.426 0.567

0.267 0.307 0.326 0.380 0.242 0.287 0.354 0.366

1200

1000

-

M-20

-

M-40

c

800

600

0.4

0.6

0.8

1.0

P/PO Fig. 4. 77 K N2 adsorption-desorption isotherms of activated carbons from the M series. Open symbols: adsorption. Closed symbols: desorption.

92

J.

RODRiGUEZ-MIRASOL

-

H-30

-

H-40

-

H-4

et al.

1000

i

800

"1

600

400

r

0.0

0.2

0.4

0.6

0.8

1.0

P/PO Fig. 5. 77 K N2 adsorption-desorption isotherms of activated carbons from the H series. Open symbols: adsorption. Closed symbols: desorption.

Table 6. BET and external surface areas, meso and macropore volumes of activated carbons from the M and H series Activated carbon M-4 M-12 M-20 M-40 H-4 H-12 H-20 H-40

&ET

tm2/g)

760 926 1049 1360 641 888 1123 1343

4

(m*/d

45.7 130.3 254.0 124.5 28.7 158.5 354.4 509.9

V,,

(cm3M

0.099 0.236 0.398 0.992 0.055 0.29 I 0.714 0.740

vm,

(cm3/g)

0.026 0.044 0.08 1 0.510 0.024 0.041 0.189 0.343

Activated carbons from kraft lignin In both series, the micropore volumes derived from the 77 K Nz DR plots are higher than the obtained with CO, at 273 K, the difference being more significant as burnoff increases. Adsorption of COz under relative pressures smaller than 0.03 will allow only primary filling of narrow micropores[ 121, where a high adsorption potential exists. In Nz adsorption, carried out within the entire range of relative pressures, secondary or cooperative filling of wider micropores will also take place. Looking at the micropore volume versus yield values, the evolution of the microporosity of activated carbons shows no significant differences between the two series investigated with both total and narrow microporosity development upon activation. Then, CO2 gasification involves widening of micropores existing in the chars as well

93

as opening of blocked micropores and/or creation of new ones. The development of mesoporosity upon activation can be seen from the values in Table 6 and from Fig. 6, where the mesopore size distribution of activated carbons from the M and H series is reported as determined by the Barret, Joyner, and Halenda method[ 131. In both series, the mesopore volume increases as gasification takes place and an increasingly wide distribution of mesopores size is observed, being more pronounced in the activated carbons obtained from HLC. These carbons show a more developed mesoporosity at intermediate burnoff levels according to the cumulative pore volume curves given in Fig. 7 constructed from mercury porosimetry data. The aforementioned curves also show that the im-

20

15

s S CII

10

5

0

. 15

.... .

M-4

-

M-20

-----

M-40

10

5

0

Fig. 6. Mesopore size distribution of activated carbons from the M and H series.

94

.i. RODRiGUEZ-MIRASOL

et d.

1.0 -\

\.

H-4

-

\

0.8

................... H-12

\ \\ I

_._._._.-

I-I-20

-----

H-40

-.. \

..

\

-.

\

\. \

‘L

\ \

; .*.

\

‘.

\ \

‘. ..

\ \

‘i

1

\

:

‘. \

i

I

0.2

0.0

1.0

0.8

-\

‘\

\\

\

M-l

-

\

\ \

. \ \

\

\

\

\

\.

0.6

\

, \ -5 \ ‘\

... . M_12

-‘-.-.-‘-

M-20

-----

M-U)

.\ ‘\

0.4

0.2

0.0

1

log r (I- in A) Fig. 7. Cumulative pore volume curves from mercury porosimetry for activated carbons of the M and H series.

portant creation of macroporosity occurs at high levels of burnoff. Table 6 summarizes the values of meso and macropore volumes as well as BET and external surface area for the activated carbons of the M and H series. The A, values at the highest burnoff levels could be low confident mainly in the case of M-40 carbon where the shape of the a, plot made difficult and subjective to determine unambiguously the slope of the linear region. The apparent surface area (ABET)shows a continuous increase within the range of burnoff values investigated indicating that whether no significant coalescence of pores as a result of complete g~i~~tion of intermediate walls takes place up to that point or most likely widening of existing micropores and

opening or creation of new ones counterbalances the effect of pores coalescence. The ABETvalues can be satisfactorily compared with the obtained from lignoceliulosic materials used as activated carbons precursors[6,9], mainly when this comparison is made in terms of overall carbon to starting material yield. 4. CONCLUSIONS

Eucalyptus kraft lignin can be an interesting precursor for activated carbons. The presence of inorganic matter in the starting material is an important feature to avoid plastic phase and swelling in the carbonization stage. Carbonization at moderate temperatures gives rise to microporous chars which upon further CO, gasification produce activated carbons

Activated carbons from kraft lignin with a well-developed microporosity and substantial mesoporosity whose relative contribution increases with burnoff. Significant macropore creation takes place at medium to high activation levels. Activation of the 823 and 1073-K chars follow a fairly comparable pattern. The apparent surface area values compare well with the corresponding to commercially available activated carbons. The development of the porous structure as activation proceeds allows, in principle, to obtain a range of products with distinct potential uses in gas and liquid phase operations. Acknowlednements-The authors wish to acknowledge the financial support received from the Empresa National de Celulosas fENCE) and the helpful advice of Prof. K.S.W. Sing and Dr. J. J. Freeman of Brunnel University during the realization of the experimental work.

60 I-642. Martinus Nijhoff Publishers, Dordrecht, The Netherlands (1986). 3. C. Otani, H. A. Polidoro, S. Otani, and A. F. Craievich, J. Chim. Phys. 81,887 (1984). 4. C. Otani, H. A. Polidoro, S. Otani, and A. F. Craievich, Proc. Carbon’88, (Edited by B. McEnaney and T. J. Mays) Newcastle upon Tyne, p. 648, (1988). 5. J. Li and A.R.P. Van Heiningen, Ind. Eng. Chem. Rex 29, 1776 (1990).

6. V. D. Del Bagno, R. L. Miller, and J. J. Watkins, Onsite Production qf Activated Carbon from Kraft Black Liauor. U. S. EPA Renort no 600/2-78-191 (1978). 7. J. *Rodriguez, T. Cordero,

8. 9.

10. REFERENCES

I I.

I. P. V. Roberts, D. M. Mackay, and F. S. Cannon, Prepumtion and Evaluation of Powdered Activated Carbon f+arn Lienocellulosic Materials. U. S. EPA Revert no.

‘600/2-85-123, (1980). 2. F. Rodriguez-Reinoso, in Carbon and Coal Gasificalion (Edited by J. L. Figueiredo and J. A. Moulijn), pp.

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12. 13.

J. Rodriguez-Mirasdl, A. Simon, and A. Bataller, in Pyrolysis and Gasification (Edited by G. Ferrero, K. Maniatis, A. Buekens, and A. V. Bridgwater), pp. 435-438. Elsevier Applied Science, London (1989). S. J. Gregg and K.S.W. Sing, Adsorption, Surface Area and Porosify, Academic Press, London (1982). P.J.M. Carrot, R. A. Roberts, and K.S.W. Sing, Carbon 25, 769 (1987). P. Chiche, S. Durif, and S. Pregermain, Fuel 44, 5 (1965). P. H. Brunner and P. V. Roberts, Carbon 18, 217 (1980). J. Garrido, A. Linares-Solano, J. M. Martin-Martinez, M. Molina-Sabio. F. Rodrkuez-Reinoso. and R. Torregrosa, Langmuir 3,76 (1957). E. P. Barret, L. G. Joyner, and P. H. Halenda, .I. Amer. Chem. Sot. 73, 373 (195 I).