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Conversion of olive wastes to volatiles and carbon adsorbents
ARTICLE IN PRESS BIOMASS AND BIOENERGY
32 (2008) 1303 – 1310
Available at www.sciencedirect.com
http://www.elsevier.com/locate/biombioe
Conversion of olive wastes to volatiles and carbon adsorbents N. Petrova,, T. Budinovaa, M. Razvigorovaa, J. Parrab, P. Galiatsatouc a
Institute of Organic Chemistry, Bulgarian Academy of Sciences, Acad. G. Bonchev str. bl. 9, Sofia 1113, Bulgaria Instituto Nacional del Carbon, Apartado de Correos 73, 33080 Oviedo, Spain c Institute of Technology of Agricultural Products, I.S. Venizelow Str., Lykovrissi 141.23, Greece b
art i cle info
ab st rac t
Article history:
Investigations for the utilization of olive stones and solvent-extracted olive pulp are carried
Received 29 February 2008
out. Tar, solid and gas products are obtained by pyrolysis of both precursors under vacuum
Accepted 20 March 2008
and atmospheric pressure. Vacuum pyrolysis causes a decrease in the solid yield and an
Available online 8 May 2008
increase in the liquid and gas yields. The identified compounds of the liquid products are
Keywords: Olive stones Olive pulp Pyrolysis Volatiles Activation Oxidation Activated carbon
1.
predominantly oxygen-containing structures (derivatives of phenol, dihydroxybenzenes, guaiacol, syringol, vanilin, veratrol, furan, acids). Activated carbons with a developed porous structure and alkaline character of the surface are produced by steam activation of the solid product and steam pyrolysis of the raw material. Oxidation treatment with air leads to the formation of a large number of oxygen functional groups with different chemical characters on the carbon surface. Chemical activation with K2CO3 allows the preparing of carbon adsorbents with a high surface area and alkaline character of the surface.
Introduction
At present, agricultural by-products are mainly used as combustion feedstock. There are, however, alternative utilization processes to convert agricultural by-products into solid, liquid and gaseous products for more efficient exploitation of these materials [1]. They were used for the production of activated carbon with a high adsorption capacity, considerable mechanical strength and relatively low ash content [2–4]. The solvent-extracted olive pomace (SEOP) is a waste product from the mechanical processing of olives for the extraction of olive oil. Envisaging the world production of olive oil (2 000 000 ton yr 1) and its average content in the olive fruit (22%) as well as the SEOP quantity after processing (up to 45–55 kg/100 kg olives fruits), it is clear that the utilization of this waste product is of great importance [5]. Several studies are devoted to the preparation of activated carbons from olive
& 2008 Elsevier Ltd. All rights reserved.
waste products: by a two-step physical activation method with steam of SEOP [6,7]; and by a single-step steam pyrolysis process of olive oil mill wastes [8]. The liquid products from the pyrolysis of biomass are suitable precursors for producing carbon adsorbents with a very low ash content [9]. The carbon adsorbents can be applied for the removal of different pollutants from water, purification of waste solutions, separation and concentration of traces of elements and radioactive isotopes, production and analysis of high-purity substances, etc. [10–11]. One of the disadvantages of activated carbon is its relatively high cost. The main reason for this is the high expense of energy indispensable for its production. The development of methods to re-use waste materials is greatly needed and the production of activated carbons from olive wastes is an interesting possibility. There are basically two methods for activated carbons production: physical and chemical activation. Physical activation includes the
Corresponding author. Tel.: +359 2 9606145; fax: +359 2 70 0225.
E-mail addresses: [email protected] (N. Petrov), [email protected] (J. Parra), [email protected] (P. Galiatsatou). 0961-9534/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.biombioe.2008.03.009
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carbonization of the starting material and the activation of the carbonizate using steam and carbon dioxide. In chemical activation and steam pyrolysis, both the carbonization and the activation step proceed simultaneously. This work deals with the influence of the treatment methods of olive stones and SEOP on the structure and surface properties of the produced activated carbons and the yield of the separated liquid products. The aim is evaluation of the quality of the carbon adsorbents produced by different methods, which will indicate their application area as well as study the composition of liquid products, which in turn will give information on the possibilities of their utilization.
2.
Materials and methods
2.1.
Raw material carbonization
Olive stones and SEOP pulp, received from Greece factory, were used as the raw material, SEOP without size reduction over 2 mm; olive stones were crushed and fractions over 2 mm were used. A total of 6 g of the raw materials was heated in a laboratory installation under vacuum (20 kN m 2) and atmospheric pressure with a heating rate of 60 1C min 1 to a final carbonization temperature of 800 1C. The duration of treatment at the final temperature was 10 min. After treatment, the samples were left to cool down.
2.2.
Activation with a water vapor
After carbonization, the obtained carbons were activated in steam at 800 1C. The duration of treatment at the final temperature was 2 h.
2.3.
Oxidation treatment
To obtain carbons with great number of surface oxides, the carbonized material was oxidized with air (8 l h 1) at 400 1C for 1 h.
2.4.
Steam carbonization
A total of 100 g of raw materials was heated in a laboratory installation in a flow of water vapor (120 ml min 1), with a heating rate of 15 1C min 1 to a final carbonization temperature of 750 1C. The duration of treatment at the final temperature was 1 h. After the treatment, the samples were left to cool down. In order to obtain the carbons with a steady content of surface oxides, the steam flow is interrupted at temperature 300 1C. The carbon particles preserve the shapes of the raw materials particles. Thus, the activated carbons have regularly formed particles.
2.5.
32 (2008) 1303 – 1310
110 1C to prepare the impregnated sample. After the same procedure, water was added to the sample with particles 2–3 mm, kept for 12 h and kneaded. This procedure was repeated three times. The impregnated samples were set on a ceramic boat, which was then inserted into a stainless tube. The sample was heated to the carbonization temperature under flow of nitrogen at the rate of 10 1C min 1, and was held for 1 h at the carbonization temperature. The carbonization temperature of samples varied over the temperature range of 500–1100 1C. After carbonization, the sample was cooled down under N2 flow, and the carbonized sample was washed sequentially several times with hot water, and finally with cold distilled water to remove residual chemical. The washed sample was dried at 110 1C to prepare the activated carbon.
2.6.
Liquid products analysis
Volatiles were collected in a vessel, and immediately after measuring their weight acetone was added in proportion 1:9. Silica gel (70–230 mesh ASTM) in a quantity sufficient to adsorb the whole liquid was added to the solution and was dried at ambient temperature. Then a sample prepared in a similar manner was placed in a glass column and separated in paraffinonaphthenic, aromatic and polar compounds by silica gel chromatography using hexane, toluene and methanol/formic acid (9:1) as eluents. The silica gel was activated at 165 1C for 2 h. The eluates were evaporated to dryness under vacuum in a rotary evaporator and were performed on a Pye Unicam series 304 gas chromatograph with ionization detection. A capillary column 25 m 0.2 mm with fused silica film of thickness 0.25 mm was used. Argon was used as a carrier gas. GC–MS analyses were performed on a Jeol JGC-20 K gas chromatograph, directly coupled with a Jeol-D300 mass spectrometer. It is operated in the electron impact mode with an ionization potential of 70 eV. A 30 m 0.25 mm SPBI 0.25 mm film thickness capillary column was used. The temperature program was from 90 to 280 1C, at 5 1C min 1. Samples were introduced via a metal injector operating in the split mode, with helium as the carrier gas. Identification of peaks in the mass chromatograms is based on the combination of mass spectra, retention times and references.
2.6.
Carbon analysis
2.6.1.
Porous structure analyses
The porous structure of carbon adsorbents was studied by N2 adsorption at 77 K. The total pore volume (Vtotal) was derived from the amount of N2 adsorbed at a relative pressure of 0.95, assuming that the pores are then filled with liquid adsorbate. The Dubinin–Radushkevich equation was used to calculate the micropore volume (Vmicro) [12]. The mesopore volume was calculated by subtracting the amount adsorbed at a relative pressure of 0.1 from that at a relative pressure of 0.95.
Chemical activation 2.6.2.
K2CO3 was used as the activating reagent. The precursors were ground and sieved to obtain particles with sizes 2–4 mm and less than 0.5 mm. Samples with particles less than 0.5 mm were mixed with the activating reagent and water, kept for 12 h and kneaded. This mixture was then dried at
Oxygen functional groups content
The oxygen functional groups content with increasing acidity was determined by Boehm’s method [13] of titration with basic solutions of different base strengths (NaHCO3, Na2CO3, NaOH, C2H5ONa). The basic group contents of the oxidized samples were determined with 0.05 N HCl [14].
ARTICLE IN PRESS BIOMASS AND BIOENERGY
2.6.3.
pH determination
3.2.
The procedure is as follows: 4.0 g of carbon (ground, undried) was weighed into a 250 ml beaker and 100 ml of water was added. The beaker was covered with a watch glass and heated to boiling temperature for 5 min. The mixture was set aside and the supernatant liquid was poured off at 60 1C. The decanted portion was cooled to room temperature and was measured to the nearest 0.01 pH.
3.
Results and discussion
3.1. Pyrolysis of olive stones and solvent-extracted olive pulp under vacuum and atmospheric pressure The carbonization treatment of many samples of olive stones and SEOP showed stable result for yield of solid and liquid products. The material balances (Table 1) for both vacuum pyrolysis and pyrolysis under atmospheric pressure of olive stones and SEOP indicate the influence of the vacuum on the yield and properties of the obtained products. The vacuum pyrolysis leads to a decrease in the solid yield and an increase in the liquid yield compared to the products yields obtained after pyrolysis under atmospheric pressure. The vacuum contributes to the formation of the greater extent of fissures and pores in the solid product as a result of the faster evacuation of volatile products, which prevents secondary char formation on the solid surfaces and lead to an increase in the surface area of the obtained solid product (Table 2).
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32 (2008) 1303 – 1310
Liquid products composition
The yields of the three fractions obtained by Silica gel column chromatography of the liquid products are presented in Table 3. Such a composition (higher content of polar compounds) is logical, having in mind the raw material— a residual product of the mechanical processing of fruits of Olea europea. It contains the remains of the basic components of these fruits—skin, pulp, kernel, pit (i.e. polymers, such as cellulose, hemicellulose, lignin, lipids, hydrocarbons, carbohydrates, proteins). Despite the residual oil extracted with hexane from the olive pomace, remains of it can be present as well. Therefore, envisaging the raw material and the pyrolysis conditions we can expect the presence of both products of thermochemical destruction of SEOP and residual olive oil components. Paraffinonaphthenic fraction comprises only a small part of both atmospheric pressure and vacuum pyrolysis products. Its content in vacuum pyrolysis liquids is higher. Saturated hydrocarbons in the range C14–C32 as well as several unsaturated (C14, C16, C18, C20) hydrocarbons are identified in this fraction. n-Alkane distribution is presented in Fig. 1. Larger quantities of hydrocarbons with longer chains evolve during vacuum pyrolysis. This can be explained with the accelerated separation of pyrolysis products as well as with the prevention from secondary cracking of thermally unstable aliphatic structures in vacuum. Even- and oddnumber paraffins, ranging from C11 to C30, are usually reported to be present in olive oil [5]. On the other hand, paraffinic compounds are usually present in the pyrolysis
Table 1 – Material balances of both vacuum and atmospheric pressure pyrolysis Treatment
Solid product (% yield)
Liquid product (% yield)
Gas+losses (% yield)
Olive stones Pyrolysis Vacuum pyrolysis
19.7 17.2
59.0 60.7
21.3 22.1
Olive pulp Pyrolysis Vacuum pyrolysis
22.8 20.2
56.4 58.7
20.8 21.1
Table 2 – Yield and adsorption properties of the carbons obtained from atmospheric pressure and vacuum carbonized olive stones and olive pulp Raw Material
Yield (wt%)
N2 BET surface area (m2/g)
pH
Adsorption (mg/g) Iodine
Methylene blue
Olive stones Carbon Carbon vacuum
19.7 17.2
278 368
7.8 7.6
207 238
32 38
Olive pulp Carbon Carbon vacuum
22.8 20.8
295 396
7.7 7.5
216 294
34 46
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32 (2008) 1303 – 1310
Table 3 – Yields of fractions obtained by column chromatography of pyrolysis products of SEOP Fraction
Solvent
Atm. pressure pyrolysis Yield (wt%)
Vacuum pyrolysis (20 kN m 2) Yield (wt%)
Aliphatic Aromatic Polar
n-Hexane Toluene 90% Methanol+10% Formic acid
1.88 7.25 90.87
2.72 9.38 87.90
atm.pressure vacuum
14 12
%
10 8 6 4 2 0 12
14
16
18
20
22 24 26 C atoms
28
30
32
34
Fig. 1 – n-Alkane distributions in fraction I from column chromatography of liquid pyrolysis products obtained under different pressure.
products of biomass, including olive oil wastes [5]. The range and distribution of paraffinic hydrocarbons in the pyrolysis products of biomass depend on the lipid part of the initial raw materials. A part of polar compounds identified in liquid products obtained by atmospheric pressure and vacuum pyrolysis is presented in Fig. 2. Most of the identified components eluted in this fraction are oxygen derivatives of phenol (alcohol-, aldehyde-, acid-, acetyl-), dihydroxybenzenes, guaiacol, syringol, vanilin, veratrol, furan, acids. These compounds are products of thermal destruction of the basic constituent of the raw material—cellulose, lignin, hemicellulose. Results from previous investigations of pyrolysis products of similar vegetable wastes confirm this [15,16]. The compositions of investigated liquid products indicate an opportunity to involve them in polycondensation and polymerization reactions and characterize them as a promising raw material for the production of synthetic adsorbents. Our earlier investigations display the possibility of producing synthetic carbon adsorbents with insignificant ash and sulfur contents [8] and pitches [17] from mixtures of tars with similar composition.
3.3.
Producing of carbon adsorbents by different methods
Table 4 presents data of proximate and ultimate analyses of olive stones, SEOP and solid products obtained after their carbonization in vacuum and atmospheric pressure. Data
show that olive stones and olive pulp possess relatively low ash and sulfur content, which is a desirable feature for activated carbon production. The higher ash content of SEOP premises the higher ash content of the obtained carbon. Examining the elemental analysis data of raw materials and that obtained at the same conditions activated carbons confirms the influence of the composition and structure of the precursors on the pyrolysis/activation reactions during thermal treatment. Thus the higher carbon and lower hydrogen content of the olive pulp lead to the higher carbon and lower hydrogen content of the produced activated carbon. Table 5 indicates that the use of different methods and various precursors leads to the formation of activated carbon with different adsorption characteristics and chemical character of the surface. Data show that the activated carbon obtained from atmospheric pressure pyrolysis olive pulp is characterized with higher surface area and adsorption capacity towards iodine and methylene blue compared with the same characteristics of the activated carbons obtained from olive stones. The difference in methylene blue adsorption is more considerable and indicates a higher content of pores with a size larger than 1.5 nm in carbon adsorbent obtained from SEOP. More substantial activation of the carbon from vacuum pyrolysis of SEOP can be related to its initial more developed porous structure. The principal feature of the pores of the obtained activated carbon is that they were formed from the existing pores that were opened and enlarged by steam activation. Steam pyrolysis as a one-step method for producing activated carbon allows reducing the energy consumption for the production of carbon adsorbents [18]. The presence of water vapor during the process was suggested to enhance the removal of low molecular weight volatiles, retard the decomposition of high molecular weight products, initiate the controlled gasification at lower temperatures, considerably increase the liquid and gas product yields as well as reduce the sulfur and ash content of the resulting solid carbon. Interaction of steam with the solid carbon and volatile products leads to the activation of solid carbon and changes in the composition of the liquid and gas products. Data in Table 5 show that activated carbons obtained by steam pyrolysis of olive stones and olive pulp are characterized by the prevailing content of micropores. This porous structure is a result of the pyrolysis and activation processes during the pyrolysis of the raw material in the presence of water vapor, as well as of activation of the resulting solid product at the
ARTICLE IN PRESS BIOMASS AND BIOENERGY
HC = CHCH2OH
CHO
MeO
OMe MeO
OMe
OMe MeO
OH I
COOH
OMe
OMe OH IV
H2C - C = CH2 H
OMe
COCH3
OMe
OH
OH
OH
V
VI
VII
OMe OH VIII
COOCH3
HC = CHCHO
HC = CHCH2OH
MeO
III
II
CHO
CH2COOH
OH
OH
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32 (2008) 1303 – 1310
I-
syringol
II-
syringic aldehyde
III-
sinapyl alcohol
IV-
syringyl acetic acid
V-
vanilline
VI-
vanillic acid
VII-
eugenol
VIII-
acetovanillone
IX-
coniferyl alcohol
X-
coniferyl aldehyde
XI-
vanillic acid, methyl ester
OMe
OMe
OMe
XII
XI
X C 2H 5
CH3
OH
OH
OH
OH IX
OMe
OH
CH3
CH3
H 3C
OMe
OMe
OMe OH
OH XIII CH3
XIV
OMe
OH
XV
XVI
OH
OH
OH
CH3 XVII OH
OH
OH
XII-
guaiacol
XIII-
methylguaiacol
XIV-
ethylguaiacol
XV-
veratrol
XVI-
р- cresol
XVII-
2,4,6- trimethylphenol
XVIII-
thymol
XIX -
phenol
XX -
methylpirocatechol
XXI -
hydroquinone
XXII-
pyrocatechol
XXIII-
hroman-2-оne
XXIV-
8- hydroxyhroman-2-
OH
one
C H3C H CH3
CH3 XIX
XVIII
OH
XX
XXI
O
CH3
H2 C-OH
OH O
XXII
O
XXV-
furfuryl alcohol
XXVI-
3- methyl-4- hydroxy2- cyclopenthenon
XXVII-d- glucosa
O OH O
XXIII HO
XXIV
XXV
O XXVI
H CH2 O H
HO HO H
H H OH OH
XXVII Fig. 2 – Part of polar compounds identified in liquid products obtained by atmospheric pressure and vacuum pyrolysis.
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Table 4 – Proximate and elemental analysis of the raw materials and the solid products obtained after pyrolysis of the raw material Raw materials and solid products
Olive stones Carbonization solid product Vacuum carbonization solid product Olive pulp Carbonization solid product Vacuum carbonization solid product
Proximate analysis (wt%)
Elemental analysis (wt%, maf)
W
Ash
Vol
C
H
N
S
Odiff
7.4 3.1 2.7 8.7 1.6 2.0
0.61 2.0 2.4 1.34 6.2 6.7
80.6 3.7 3.2 78.2 10.0 9.4
51.5 89.5 90.8 56.7 90.5 91.7
6.3 2.4 2.0 5.5 2.20 2.00
0.2 1.1 0.9 0.3 1.00 0.80
0.1 0.6 0.6 0.3 0.80 0.90
41.9 6.4 5.7 37.4 5.50 4.60
Table 5 – Yield and adsorption properties of the carbon adsorbents obtained from olive stones and SEOP by different methods Sample
Olive stones Carbon activated with water vapor Carbon oxidized with air Steam pyrolysis carbon Carbon activated with K2CO3 SEOP Carbon activated with water vapor Carbon oxidized with air Steam pyrolysis carbon Carbon activated with K2CO3 Less 0.5 mm 1–2 mm
Carbon yield (%)
N2 BET Surface area (m2 g 1)
pH
Adsorption (mg g 1)
Pore volume (m3 g 1)
Iodine
Methylene blue
Total
Micro
Meso
Macro
12.6
905
9.2
810
236
0.630
0.335
0.098
0.197
11.8 12.2 15.2
420 1090 1610
3.8 9.1 8.9
380 910 1540
158 253 394
0.471 0.860 0.843
0.105 0.430 0.437
0.111 0.110 0.159
0.255 0.320 0.247
13.2
1010
8.8
910
356
0.665
0.355
0.108
0.202
14.2 17.8 16.2
421 998 1850
3.6 8.9 9.0
320 1050 1720
136 377 420
0.485 0.820 0.819
0.125 0.400 0.461
0.115 0.060 0.180
0.245 0.360 0.257
15.8
1510
8.8
1460
386
0.758
0.427
0.110
0.221
final temperature. Furthermore, water vapor also contributes to the formation of more active surface through the oxidation of the carbon and the formation of oxygen-containing groups on its surface. As oxygen is a strong oxidant, the treatment of carbonizates with air leads to the predominant formation of macropores, due to the prevailing widening and collapse of existing pores to the generation of new one. As a result, oxidizing carbon is with a macroporous structure and comparatively low surface area. The aim of this treatment is not to develop a high surface area but to form a large number of oxygen-containing structures on the carbon surface. The data in Table 5 indicate that activated carbon with developed porous structure with the prevailing content of micropores and very high surface area can be prepared by chemical activation with K2CO3 of SEOP and olive stones. Data also show that the size of the sample particles influence the yield and the surface area of the obtained carbon. The yield of activated carbon obtained from the raw material with a size of particles 2–4 mm is considerably higher than the yield of the activated carbon obtained from the raw material with size
of particles less than 0.5 mm. In the same time, the surface area of the latter is higher. Fig. 3 shows the influence of the carbonization temperature on the specific surface areas of the activated carbons prepared on the base of olive stones and solvent-extracted olive pulp. The amount of K2CO3 in the mixture was 40%. The tendency of the change in the surface area with the carbonization temperature resembles each other. It can be shown that the surface areas increase between temperatures of 500 and 950 1C, and then decrease with a further enhancement of temperature from 950 to 1100 1C. This shows that K2CO3 works as activating reagents effectively above 600 1C. Most likely, the pore volumes of activated carbons prepared by alkali metal compound activation generally increase with carbonization temperature up to about 950 1C. This may be due to the fact that K2CO3 evaporates above 900 1C. Fig. 4 shows the influence of the amount of chemical reagent on the specific surface areas of the prepared activated carbons. The carbonization temperature was 950 1C. It can be seen that the surface area of the obtained activated carbon rapidly increases with the increase in the amount of K2CO3 in
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the mixture from 20% to 40%. Further increase in the K2CO3 amount does not significantly change the surface area of the sample. This indicates that the optimum content of chemical reagent in the mixture for obtaining activated carbon with a large surface area is around 40%. The chemical activation does not lead to an appreciable increase in the ash content of the obtained carbons. Different oxygen-containing functional groups are determined on the surface of obtained carbon adsorbents (Table 6). Activated carbons obtained as a result of steam pyrolysis of both raw materials possess mainly oxygen groups with basic character of the surface. On the contrary, groups with acidic character are determined predominantly on the surface of oxidized carbon: the carboxyl groups (NaHCO3 consumption), carboxyl groups in lactone-like binding (difference between Na2CO3 and NaHCO3 consumption), phenolic hydroxyl (difference between NaOH and Na2CO3 consumption) and carbonyl groups. The content of basic groups on the surface of oxidized carbon is lower. It can be summarized that steam carbonization, carbonization under vacuum and atmospheric pressure followed by steam activation lead to the formation of activated carbons
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with developed porous structure with a prevailing content of micropores and alkaline character of the surface. These adsorbents could be used to remove different pollutants from water and air. On the contrary, carbon adsorbent obtained after oxidation treatment with air of the carbonizate is characterized with a macroporous structure and acidic character of the surface due to the great number of oxygen-containing groups on the carbon surface. These oxidized carbons could be used mainly for the removal of metal ions from water. The chemical activation leads to the formation of more developed pore structure, larger surface area, better adsorption characteristics and higher yield of the obtained carbon adsorbents in comparison with the adsorbents obtained by other methods. The physico-chemical properties of obtained activated carbons characterize them as appropriate for the separation and purification processes for gaseous and aqueous solution systems. The results obtained show that using different methods of treatment, olive stones and SEOP can be converted to large-scale carbon adsorbents with different porous structures and surface properties, gas and liquid products. The liquid products possess appropriate composition
2000 1800
Surface area, m2/g
Surface area, m2/g
1800 1600 1400 1200 1000
1600 1400 1200 1000
800
800
600
600 500
600 700 800 900 1000 Carbonization temperature, °C
20
1100
Fig. 3 – The influence of the carbonization temperature on the surface areas of the prepared activated carbons from: ’solvent-extracted olive pulp; K-olive stones.
Table 6 – Acid–base neutralization capacities (m-equiv g
40 50 Amount K2CO3, %
60
Fig. 4 – Influence of the amount of chemical reagent on the specific surface areas of the prepared activated carbons from: ’-solvent-extracted olive pulp;K-olive stones.
1
) of the obtained carbons Base uptake (meq g 1)
Sample
Olive stones Carbonization solid product atm. pressure Solid product activated with water vapor Solid product oxidized with air Chemically activated olive stones SEOP Carbonization solid product atm. pressure Solid product activated with water vapor Solid product oxidized with air Chemically activated SEOP
30
Acid uptake
NaHCO3
Na2CO3
NaOH
EtONa
HCl
0.110 0.025 0.890 0.030
0.272 0.080 1.302 0.096
0.600 0.252 2.110 0.292
1.760 1.301 5.000 1.375
0.545 0.560 0.190 0.590
0.120 0.016 0.720 0.042
0.040 0.023 1.190 1.023
0.090 0.029 2.000 0.305
0.620 0.847 4.220 1.450
0.410 0.575 0.260 0.515
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to be used for production of carbon adsorbents and pitches. Present investigations indicate the possibility to cultivate olive stones and SEOP to high-value materials without external and harmful for environmental products since the gas can be used as energy sources and the liquid products for production of carbon adsorbents and pitches.
4.
Conclusions
The vacuum pyrolysis of the solvent-extracted olive pulp leads to a decrease in the solid yield and an increase in the liquid and gas yield compared to the products yields obtained after pyrolysis under an atmospheric pressure. The vacuum pyrolysis solid product possesses higher surface area and this dependence is retained after the activation step. The composition of the liquid products ( predominantly derivatives of phenol, dihydroxybenzenes, guaiacol, syringol, vanilin, veratrol, furan, acids) characterizes them as promising raw materials for the production of synthetic adsorbents and pitches. The final physico-chemical properties of the activated carbons obtained at common conditions depend on the kind of the raw material. Steam carbonization and activation with water vapor of carbonizates obtained under vacuum and atmospheric pressure lead to the formation of activated carbons with welldeveloped porous structures with a prevailing content of micropores and an alkaline character of the surface. Carbons with the highest surface area with alkaline character are obtained by chemical activation with K2CO3. Carbon oxidized with air is characterized with a great number of oxygencontaining groups with acidic character of the surface.
Acknowledgment The authors acknowledge financial support for this work from MES-NFSI- Bulgaria. R E F E R E N C E S
[1] Larson ED. Advanced technologies for biomass conversion to energy. In: Proceedings of the second Olle Lindstrom symposium on renewable energy-bioenergy. Stochholm, Sweden, Royal Institute of Technology; 1999. p. 1–13.
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[2] Gonzalez MT, Rodriguez-Reinoso F, Garcia AN, Marcilla A. CO2 activation of olive stones carbonized under different experimental conditions. Carbon 1997;35:159–62. [3] Bacaoui A, Yaacoubi A, Dahbi A, Bennouna C, Phan Tan Luu R, Maldonado-Hobar FJ, et al. Optimization of conditions for the preparation of activated carbons from olive-waste cakes. Carbon 2001;39:425–32. [4] Rodriguez-Reinoso F, Molina-Sabio M, Gonzales MT. The use of steam and CO2 as activating agents in the preparation of activated carbons. Carbon 1995;33:15–23. [5] Dimitrius Boskou, editor. Olive oil: chemistry and technology, Library of Congress Catalog- in-Publication Data, ISBN 0935315-73-X. [6] Metaxas M, Kasselouri-Rigopoulou V, Galiatsatou P, Konstantopoulou C, Oikonomou D. Thorium removal by different adsorbents. Journal of Hazardous Materials 2003;97:71–82. [7] Galiatsatou P, Metaxas M, Kasselouri-Rigopoulou V. Adsorption of zinc by activated carbons prepared from solvent extracted olive pulp. Journal of Hazardous Materials 2002;B91:187–203. [8] Girgis B, Khalil LB, Tawfic TAM. Porosity development in carbon derived from olive oil mill residue under steam pyrolysis. Journal of Porous Materials 2002;9:105–13. [9] Savova D, Apak E, Ekinci E, Yardim F, Petrov N, Budinova T, et al. Biomass conversion to carbon adsorbents and gas. Biomass & Bioenergy 2001;21:133–42. [10] Rivera-Utrila J, Ferro-Garcia MA, Mata-Arjona A, GonzalezGomez C. Studies on the adsorption of Cesium, Thallium, Strontium and Cobalt radionuclides on activated Carbons from aqueous solutions. Journal of Chemical Technology & Biotechnology 1984;34A:243–8. [11] Ferro-Garcia MA, Rivera-Utrila J, Rodriguez-Gordillo J, Bautista-Toledo I. Adsorption of zinc, cadmium, and copper on activated carbons obtained from agricultural by-products. Carbon 1988;26:363–73. [12] Gregg SJ, Siny KSW. Adsorption surface area and porosity. London: Academic Press; 1982. p.218. [13] Boehm HP. In: Eley DD, Pines H, Weisz PB, editors. Advances in catalysis, vol. 16. New York: Academic Press; 1966. p. 179. [14] Papirer E, Li S, Donnet J-B. Contribution to the study of basic surface groups on carbons. Carbon 1987;25:243–7. [15] Razvigorova M, Minkova V, Cerni J, Viden I, Zanzi R, Bjornbom E. First world conference and exhibition on biomass for energy and industry, Sevilla, Spain, 2000. [16] Tiscornia E, Fiorina N, Evangelisti F. Riv. Ital Sast Grasse 1982;59:519. [17] Genceli E, Apak E, Razvigorova M, Petrov N, Minkova V, Ekinci E. Preparation, modification, and characterization of pitches from apricot stones. Fuel Processing Technology 2002;75:97–107. [18] Gergova K, Petrov N, Eser S. Adsorption properties and microstructure of activated carbons produced from agricultural by-products by steam pyrolysis. Carbon 1994;32:693–702.
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Available at www.sciencedirect.com
http://www.elsevier.com/locate/biombioe
Conversion of olive wastes to volatiles and carbon adsorbents N. Petrova,, T. Budinovaa, M. Razvigorovaa, J. Parrab, P. Galiatsatouc a
Institute of Organic Chemistry, Bulgarian Academy of Sciences, Acad. G. Bonchev str. bl. 9, Sofia 1113, Bulgaria Instituto Nacional del Carbon, Apartado de Correos 73, 33080 Oviedo, Spain c Institute of Technology of Agricultural Products, I.S. Venizelow Str., Lykovrissi 141.23, Greece b
art i cle info
ab st rac t
Article history:
Investigations for the utilization of olive stones and solvent-extracted olive pulp are carried
Received 29 February 2008
out. Tar, solid and gas products are obtained by pyrolysis of both precursors under vacuum
Accepted 20 March 2008
and atmospheric pressure. Vacuum pyrolysis causes a decrease in the solid yield and an
Available online 8 May 2008
increase in the liquid and gas yields. The identified compounds of the liquid products are
Keywords: Olive stones Olive pulp Pyrolysis Volatiles Activation Oxidation Activated carbon
1.
predominantly oxygen-containing structures (derivatives of phenol, dihydroxybenzenes, guaiacol, syringol, vanilin, veratrol, furan, acids). Activated carbons with a developed porous structure and alkaline character of the surface are produced by steam activation of the solid product and steam pyrolysis of the raw material. Oxidation treatment with air leads to the formation of a large number of oxygen functional groups with different chemical characters on the carbon surface. Chemical activation with K2CO3 allows the preparing of carbon adsorbents with a high surface area and alkaline character of the surface.
Introduction
At present, agricultural by-products are mainly used as combustion feedstock. There are, however, alternative utilization processes to convert agricultural by-products into solid, liquid and gaseous products for more efficient exploitation of these materials [1]. They were used for the production of activated carbon with a high adsorption capacity, considerable mechanical strength and relatively low ash content [2–4]. The solvent-extracted olive pomace (SEOP) is a waste product from the mechanical processing of olives for the extraction of olive oil. Envisaging the world production of olive oil (2 000 000 ton yr 1) and its average content in the olive fruit (22%) as well as the SEOP quantity after processing (up to 45–55 kg/100 kg olives fruits), it is clear that the utilization of this waste product is of great importance [5]. Several studies are devoted to the preparation of activated carbons from olive
& 2008 Elsevier Ltd. All rights reserved.
waste products: by a two-step physical activation method with steam of SEOP [6,7]; and by a single-step steam pyrolysis process of olive oil mill wastes [8]. The liquid products from the pyrolysis of biomass are suitable precursors for producing carbon adsorbents with a very low ash content [9]. The carbon adsorbents can be applied for the removal of different pollutants from water, purification of waste solutions, separation and concentration of traces of elements and radioactive isotopes, production and analysis of high-purity substances, etc. [10–11]. One of the disadvantages of activated carbon is its relatively high cost. The main reason for this is the high expense of energy indispensable for its production. The development of methods to re-use waste materials is greatly needed and the production of activated carbons from olive wastes is an interesting possibility. There are basically two methods for activated carbons production: physical and chemical activation. Physical activation includes the
Corresponding author. Tel.: +359 2 9606145; fax: +359 2 70 0225.
E-mail addresses: [email protected] (N. Petrov), [email protected] (J. Parra), [email protected] (P. Galiatsatou). 0961-9534/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.biombioe.2008.03.009
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carbonization of the starting material and the activation of the carbonizate using steam and carbon dioxide. In chemical activation and steam pyrolysis, both the carbonization and the activation step proceed simultaneously. This work deals with the influence of the treatment methods of olive stones and SEOP on the structure and surface properties of the produced activated carbons and the yield of the separated liquid products. The aim is evaluation of the quality of the carbon adsorbents produced by different methods, which will indicate their application area as well as study the composition of liquid products, which in turn will give information on the possibilities of their utilization.
2.
Materials and methods
2.1.
Raw material carbonization
Olive stones and SEOP pulp, received from Greece factory, were used as the raw material, SEOP without size reduction over 2 mm; olive stones were crushed and fractions over 2 mm were used. A total of 6 g of the raw materials was heated in a laboratory installation under vacuum (20 kN m 2) and atmospheric pressure with a heating rate of 60 1C min 1 to a final carbonization temperature of 800 1C. The duration of treatment at the final temperature was 10 min. After treatment, the samples were left to cool down.
2.2.
Activation with a water vapor
After carbonization, the obtained carbons were activated in steam at 800 1C. The duration of treatment at the final temperature was 2 h.
2.3.
Oxidation treatment
To obtain carbons with great number of surface oxides, the carbonized material was oxidized with air (8 l h 1) at 400 1C for 1 h.
2.4.
Steam carbonization
A total of 100 g of raw materials was heated in a laboratory installation in a flow of water vapor (120 ml min 1), with a heating rate of 15 1C min 1 to a final carbonization temperature of 750 1C. The duration of treatment at the final temperature was 1 h. After the treatment, the samples were left to cool down. In order to obtain the carbons with a steady content of surface oxides, the steam flow is interrupted at temperature 300 1C. The carbon particles preserve the shapes of the raw materials particles. Thus, the activated carbons have regularly formed particles.
2.5.
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110 1C to prepare the impregnated sample. After the same procedure, water was added to the sample with particles 2–3 mm, kept for 12 h and kneaded. This procedure was repeated three times. The impregnated samples were set on a ceramic boat, which was then inserted into a stainless tube. The sample was heated to the carbonization temperature under flow of nitrogen at the rate of 10 1C min 1, and was held for 1 h at the carbonization temperature. The carbonization temperature of samples varied over the temperature range of 500–1100 1C. After carbonization, the sample was cooled down under N2 flow, and the carbonized sample was washed sequentially several times with hot water, and finally with cold distilled water to remove residual chemical. The washed sample was dried at 110 1C to prepare the activated carbon.
2.6.
Liquid products analysis
Volatiles were collected in a vessel, and immediately after measuring their weight acetone was added in proportion 1:9. Silica gel (70–230 mesh ASTM) in a quantity sufficient to adsorb the whole liquid was added to the solution and was dried at ambient temperature. Then a sample prepared in a similar manner was placed in a glass column and separated in paraffinonaphthenic, aromatic and polar compounds by silica gel chromatography using hexane, toluene and methanol/formic acid (9:1) as eluents. The silica gel was activated at 165 1C for 2 h. The eluates were evaporated to dryness under vacuum in a rotary evaporator and were performed on a Pye Unicam series 304 gas chromatograph with ionization detection. A capillary column 25 m 0.2 mm with fused silica film of thickness 0.25 mm was used. Argon was used as a carrier gas. GC–MS analyses were performed on a Jeol JGC-20 K gas chromatograph, directly coupled with a Jeol-D300 mass spectrometer. It is operated in the electron impact mode with an ionization potential of 70 eV. A 30 m 0.25 mm SPBI 0.25 mm film thickness capillary column was used. The temperature program was from 90 to 280 1C, at 5 1C min 1. Samples were introduced via a metal injector operating in the split mode, with helium as the carrier gas. Identification of peaks in the mass chromatograms is based on the combination of mass spectra, retention times and references.
2.6.
Carbon analysis
2.6.1.
Porous structure analyses
The porous structure of carbon adsorbents was studied by N2 adsorption at 77 K. The total pore volume (Vtotal) was derived from the amount of N2 adsorbed at a relative pressure of 0.95, assuming that the pores are then filled with liquid adsorbate. The Dubinin–Radushkevich equation was used to calculate the micropore volume (Vmicro) [12]. The mesopore volume was calculated by subtracting the amount adsorbed at a relative pressure of 0.1 from that at a relative pressure of 0.95.
Chemical activation 2.6.2.
K2CO3 was used as the activating reagent. The precursors were ground and sieved to obtain particles with sizes 2–4 mm and less than 0.5 mm. Samples with particles less than 0.5 mm were mixed with the activating reagent and water, kept for 12 h and kneaded. This mixture was then dried at
Oxygen functional groups content
The oxygen functional groups content with increasing acidity was determined by Boehm’s method [13] of titration with basic solutions of different base strengths (NaHCO3, Na2CO3, NaOH, C2H5ONa). The basic group contents of the oxidized samples were determined with 0.05 N HCl [14].
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2.6.3.
pH determination
3.2.
The procedure is as follows: 4.0 g of carbon (ground, undried) was weighed into a 250 ml beaker and 100 ml of water was added. The beaker was covered with a watch glass and heated to boiling temperature for 5 min. The mixture was set aside and the supernatant liquid was poured off at 60 1C. The decanted portion was cooled to room temperature and was measured to the nearest 0.01 pH.
3.
Results and discussion
3.1. Pyrolysis of olive stones and solvent-extracted olive pulp under vacuum and atmospheric pressure The carbonization treatment of many samples of olive stones and SEOP showed stable result for yield of solid and liquid products. The material balances (Table 1) for both vacuum pyrolysis and pyrolysis under atmospheric pressure of olive stones and SEOP indicate the influence of the vacuum on the yield and properties of the obtained products. The vacuum pyrolysis leads to a decrease in the solid yield and an increase in the liquid yield compared to the products yields obtained after pyrolysis under atmospheric pressure. The vacuum contributes to the formation of the greater extent of fissures and pores in the solid product as a result of the faster evacuation of volatile products, which prevents secondary char formation on the solid surfaces and lead to an increase in the surface area of the obtained solid product (Table 2).
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Liquid products composition
The yields of the three fractions obtained by Silica gel column chromatography of the liquid products are presented in Table 3. Such a composition (higher content of polar compounds) is logical, having in mind the raw material— a residual product of the mechanical processing of fruits of Olea europea. It contains the remains of the basic components of these fruits—skin, pulp, kernel, pit (i.e. polymers, such as cellulose, hemicellulose, lignin, lipids, hydrocarbons, carbohydrates, proteins). Despite the residual oil extracted with hexane from the olive pomace, remains of it can be present as well. Therefore, envisaging the raw material and the pyrolysis conditions we can expect the presence of both products of thermochemical destruction of SEOP and residual olive oil components. Paraffinonaphthenic fraction comprises only a small part of both atmospheric pressure and vacuum pyrolysis products. Its content in vacuum pyrolysis liquids is higher. Saturated hydrocarbons in the range C14–C32 as well as several unsaturated (C14, C16, C18, C20) hydrocarbons are identified in this fraction. n-Alkane distribution is presented in Fig. 1. Larger quantities of hydrocarbons with longer chains evolve during vacuum pyrolysis. This can be explained with the accelerated separation of pyrolysis products as well as with the prevention from secondary cracking of thermally unstable aliphatic structures in vacuum. Even- and oddnumber paraffins, ranging from C11 to C30, are usually reported to be present in olive oil [5]. On the other hand, paraffinic compounds are usually present in the pyrolysis
Table 1 – Material balances of both vacuum and atmospheric pressure pyrolysis Treatment
Solid product (% yield)
Liquid product (% yield)
Gas+losses (% yield)
Olive stones Pyrolysis Vacuum pyrolysis
19.7 17.2
59.0 60.7
21.3 22.1
Olive pulp Pyrolysis Vacuum pyrolysis
22.8 20.2
56.4 58.7
20.8 21.1
Table 2 – Yield and adsorption properties of the carbons obtained from atmospheric pressure and vacuum carbonized olive stones and olive pulp Raw Material
Yield (wt%)
N2 BET surface area (m2/g)
pH
Adsorption (mg/g) Iodine
Methylene blue
Olive stones Carbon Carbon vacuum
19.7 17.2
278 368
7.8 7.6
207 238
32 38
Olive pulp Carbon Carbon vacuum
22.8 20.8
295 396
7.7 7.5
216 294
34 46
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32 (2008) 1303 – 1310
Table 3 – Yields of fractions obtained by column chromatography of pyrolysis products of SEOP Fraction
Solvent
Atm. pressure pyrolysis Yield (wt%)
Vacuum pyrolysis (20 kN m 2) Yield (wt%)
Aliphatic Aromatic Polar
n-Hexane Toluene 90% Methanol+10% Formic acid
1.88 7.25 90.87
2.72 9.38 87.90
atm.pressure vacuum
14 12
%
10 8 6 4 2 0 12
14
16
18
20
22 24 26 C atoms
28
30
32
34
Fig. 1 – n-Alkane distributions in fraction I from column chromatography of liquid pyrolysis products obtained under different pressure.
products of biomass, including olive oil wastes [5]. The range and distribution of paraffinic hydrocarbons in the pyrolysis products of biomass depend on the lipid part of the initial raw materials. A part of polar compounds identified in liquid products obtained by atmospheric pressure and vacuum pyrolysis is presented in Fig. 2. Most of the identified components eluted in this fraction are oxygen derivatives of phenol (alcohol-, aldehyde-, acid-, acetyl-), dihydroxybenzenes, guaiacol, syringol, vanilin, veratrol, furan, acids. These compounds are products of thermal destruction of the basic constituent of the raw material—cellulose, lignin, hemicellulose. Results from previous investigations of pyrolysis products of similar vegetable wastes confirm this [15,16]. The compositions of investigated liquid products indicate an opportunity to involve them in polycondensation and polymerization reactions and characterize them as a promising raw material for the production of synthetic adsorbents. Our earlier investigations display the possibility of producing synthetic carbon adsorbents with insignificant ash and sulfur contents [8] and pitches [17] from mixtures of tars with similar composition.
3.3.
Producing of carbon adsorbents by different methods
Table 4 presents data of proximate and ultimate analyses of olive stones, SEOP and solid products obtained after their carbonization in vacuum and atmospheric pressure. Data
show that olive stones and olive pulp possess relatively low ash and sulfur content, which is a desirable feature for activated carbon production. The higher ash content of SEOP premises the higher ash content of the obtained carbon. Examining the elemental analysis data of raw materials and that obtained at the same conditions activated carbons confirms the influence of the composition and structure of the precursors on the pyrolysis/activation reactions during thermal treatment. Thus the higher carbon and lower hydrogen content of the olive pulp lead to the higher carbon and lower hydrogen content of the produced activated carbon. Table 5 indicates that the use of different methods and various precursors leads to the formation of activated carbon with different adsorption characteristics and chemical character of the surface. Data show that the activated carbon obtained from atmospheric pressure pyrolysis olive pulp is characterized with higher surface area and adsorption capacity towards iodine and methylene blue compared with the same characteristics of the activated carbons obtained from olive stones. The difference in methylene blue adsorption is more considerable and indicates a higher content of pores with a size larger than 1.5 nm in carbon adsorbent obtained from SEOP. More substantial activation of the carbon from vacuum pyrolysis of SEOP can be related to its initial more developed porous structure. The principal feature of the pores of the obtained activated carbon is that they were formed from the existing pores that were opened and enlarged by steam activation. Steam pyrolysis as a one-step method for producing activated carbon allows reducing the energy consumption for the production of carbon adsorbents [18]. The presence of water vapor during the process was suggested to enhance the removal of low molecular weight volatiles, retard the decomposition of high molecular weight products, initiate the controlled gasification at lower temperatures, considerably increase the liquid and gas product yields as well as reduce the sulfur and ash content of the resulting solid carbon. Interaction of steam with the solid carbon and volatile products leads to the activation of solid carbon and changes in the composition of the liquid and gas products. Data in Table 5 show that activated carbons obtained by steam pyrolysis of olive stones and olive pulp are characterized by the prevailing content of micropores. This porous structure is a result of the pyrolysis and activation processes during the pyrolysis of the raw material in the presence of water vapor, as well as of activation of the resulting solid product at the
ARTICLE IN PRESS BIOMASS AND BIOENERGY
HC = CHCH2OH
CHO
MeO
OMe MeO
OMe
OMe MeO
OH I
COOH
OMe
OMe OH IV
H2C - C = CH2 H
OMe
COCH3
OMe
OH
OH
OH
V
VI
VII
OMe OH VIII
COOCH3
HC = CHCHO
HC = CHCH2OH
MeO
III
II
CHO
CH2COOH
OH
OH
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32 (2008) 1303 – 1310
I-
syringol
II-
syringic aldehyde
III-
sinapyl alcohol
IV-
syringyl acetic acid
V-
vanilline
VI-
vanillic acid
VII-
eugenol
VIII-
acetovanillone
IX-
coniferyl alcohol
X-
coniferyl aldehyde
XI-
vanillic acid, methyl ester
OMe
OMe
OMe
XII
XI
X C 2H 5
CH3
OH
OH
OH
OH IX
OMe
OH
CH3
CH3
H 3C
OMe
OMe
OMe OH
OH XIII CH3
XIV
OMe
OH
XV
XVI
OH
OH
OH
CH3 XVII OH
OH
OH
XII-
guaiacol
XIII-
methylguaiacol
XIV-
ethylguaiacol
XV-
veratrol
XVI-
р- cresol
XVII-
2,4,6- trimethylphenol
XVIII-
thymol
XIX -
phenol
XX -
methylpirocatechol
XXI -
hydroquinone
XXII-
pyrocatechol
XXIII-
hroman-2-оne
XXIV-
8- hydroxyhroman-2-
OH
one
C H3C H CH3
CH3 XIX
XVIII
OH
XX
XXI
O
CH3
H2 C-OH
OH O
XXII
O
XXV-
furfuryl alcohol
XXVI-
3- methyl-4- hydroxy2- cyclopenthenon
XXVII-d- glucosa
O OH O
XXIII HO
XXIV
XXV
O XXVI
H CH2 O H
HO HO H
H H OH OH
XXVII Fig. 2 – Part of polar compounds identified in liquid products obtained by atmospheric pressure and vacuum pyrolysis.
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Table 4 – Proximate and elemental analysis of the raw materials and the solid products obtained after pyrolysis of the raw material Raw materials and solid products
Olive stones Carbonization solid product Vacuum carbonization solid product Olive pulp Carbonization solid product Vacuum carbonization solid product
Proximate analysis (wt%)
Elemental analysis (wt%, maf)
W
Ash
Vol
C
H
N
S
Odiff
7.4 3.1 2.7 8.7 1.6 2.0
0.61 2.0 2.4 1.34 6.2 6.7
80.6 3.7 3.2 78.2 10.0 9.4
51.5 89.5 90.8 56.7 90.5 91.7
6.3 2.4 2.0 5.5 2.20 2.00
0.2 1.1 0.9 0.3 1.00 0.80
0.1 0.6 0.6 0.3 0.80 0.90
41.9 6.4 5.7 37.4 5.50 4.60
Table 5 – Yield and adsorption properties of the carbon adsorbents obtained from olive stones and SEOP by different methods Sample
Olive stones Carbon activated with water vapor Carbon oxidized with air Steam pyrolysis carbon Carbon activated with K2CO3 SEOP Carbon activated with water vapor Carbon oxidized with air Steam pyrolysis carbon Carbon activated with K2CO3 Less 0.5 mm 1–2 mm
Carbon yield (%)
N2 BET Surface area (m2 g 1)
pH
Adsorption (mg g 1)
Pore volume (m3 g 1)
Iodine
Methylene blue
Total
Micro
Meso
Macro
12.6
905
9.2
810
236
0.630
0.335
0.098
0.197
11.8 12.2 15.2
420 1090 1610
3.8 9.1 8.9
380 910 1540
158 253 394
0.471 0.860 0.843
0.105 0.430 0.437
0.111 0.110 0.159
0.255 0.320 0.247
13.2
1010
8.8
910
356
0.665
0.355
0.108
0.202
14.2 17.8 16.2
421 998 1850
3.6 8.9 9.0
320 1050 1720
136 377 420
0.485 0.820 0.819
0.125 0.400 0.461
0.115 0.060 0.180
0.245 0.360 0.257
15.8
1510
8.8
1460
386
0.758
0.427
0.110
0.221
final temperature. Furthermore, water vapor also contributes to the formation of more active surface through the oxidation of the carbon and the formation of oxygen-containing groups on its surface. As oxygen is a strong oxidant, the treatment of carbonizates with air leads to the predominant formation of macropores, due to the prevailing widening and collapse of existing pores to the generation of new one. As a result, oxidizing carbon is with a macroporous structure and comparatively low surface area. The aim of this treatment is not to develop a high surface area but to form a large number of oxygen-containing structures on the carbon surface. The data in Table 5 indicate that activated carbon with developed porous structure with the prevailing content of micropores and very high surface area can be prepared by chemical activation with K2CO3 of SEOP and olive stones. Data also show that the size of the sample particles influence the yield and the surface area of the obtained carbon. The yield of activated carbon obtained from the raw material with a size of particles 2–4 mm is considerably higher than the yield of the activated carbon obtained from the raw material with size
of particles less than 0.5 mm. In the same time, the surface area of the latter is higher. Fig. 3 shows the influence of the carbonization temperature on the specific surface areas of the activated carbons prepared on the base of olive stones and solvent-extracted olive pulp. The amount of K2CO3 in the mixture was 40%. The tendency of the change in the surface area with the carbonization temperature resembles each other. It can be shown that the surface areas increase between temperatures of 500 and 950 1C, and then decrease with a further enhancement of temperature from 950 to 1100 1C. This shows that K2CO3 works as activating reagents effectively above 600 1C. Most likely, the pore volumes of activated carbons prepared by alkali metal compound activation generally increase with carbonization temperature up to about 950 1C. This may be due to the fact that K2CO3 evaporates above 900 1C. Fig. 4 shows the influence of the amount of chemical reagent on the specific surface areas of the prepared activated carbons. The carbonization temperature was 950 1C. It can be seen that the surface area of the obtained activated carbon rapidly increases with the increase in the amount of K2CO3 in
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the mixture from 20% to 40%. Further increase in the K2CO3 amount does not significantly change the surface area of the sample. This indicates that the optimum content of chemical reagent in the mixture for obtaining activated carbon with a large surface area is around 40%. The chemical activation does not lead to an appreciable increase in the ash content of the obtained carbons. Different oxygen-containing functional groups are determined on the surface of obtained carbon adsorbents (Table 6). Activated carbons obtained as a result of steam pyrolysis of both raw materials possess mainly oxygen groups with basic character of the surface. On the contrary, groups with acidic character are determined predominantly on the surface of oxidized carbon: the carboxyl groups (NaHCO3 consumption), carboxyl groups in lactone-like binding (difference between Na2CO3 and NaHCO3 consumption), phenolic hydroxyl (difference between NaOH and Na2CO3 consumption) and carbonyl groups. The content of basic groups on the surface of oxidized carbon is lower. It can be summarized that steam carbonization, carbonization under vacuum and atmospheric pressure followed by steam activation lead to the formation of activated carbons
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32 (2008) 1303 – 1310
with developed porous structure with a prevailing content of micropores and alkaline character of the surface. These adsorbents could be used to remove different pollutants from water and air. On the contrary, carbon adsorbent obtained after oxidation treatment with air of the carbonizate is characterized with a macroporous structure and acidic character of the surface due to the great number of oxygen-containing groups on the carbon surface. These oxidized carbons could be used mainly for the removal of metal ions from water. The chemical activation leads to the formation of more developed pore structure, larger surface area, better adsorption characteristics and higher yield of the obtained carbon adsorbents in comparison with the adsorbents obtained by other methods. The physico-chemical properties of obtained activated carbons characterize them as appropriate for the separation and purification processes for gaseous and aqueous solution systems. The results obtained show that using different methods of treatment, olive stones and SEOP can be converted to large-scale carbon adsorbents with different porous structures and surface properties, gas and liquid products. The liquid products possess appropriate composition
2000 1800
Surface area, m2/g
Surface area, m2/g
1800 1600 1400 1200 1000
1600 1400 1200 1000
800
800
600
600 500
600 700 800 900 1000 Carbonization temperature, °C
20
1100
Fig. 3 – The influence of the carbonization temperature on the surface areas of the prepared activated carbons from: ’solvent-extracted olive pulp; K-olive stones.
Table 6 – Acid–base neutralization capacities (m-equiv g
40 50 Amount K2CO3, %
60
Fig. 4 – Influence of the amount of chemical reagent on the specific surface areas of the prepared activated carbons from: ’-solvent-extracted olive pulp;K-olive stones.
1
) of the obtained carbons Base uptake (meq g 1)
Sample
Olive stones Carbonization solid product atm. pressure Solid product activated with water vapor Solid product oxidized with air Chemically activated olive stones SEOP Carbonization solid product atm. pressure Solid product activated with water vapor Solid product oxidized with air Chemically activated SEOP
30
Acid uptake
NaHCO3
Na2CO3
NaOH
EtONa
HCl
0.110 0.025 0.890 0.030
0.272 0.080 1.302 0.096
0.600 0.252 2.110 0.292
1.760 1.301 5.000 1.375
0.545 0.560 0.190 0.590
0.120 0.016 0.720 0.042
0.040 0.023 1.190 1.023
0.090 0.029 2.000 0.305
0.620 0.847 4.220 1.450
0.410 0.575 0.260 0.515
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to be used for production of carbon adsorbents and pitches. Present investigations indicate the possibility to cultivate olive stones and SEOP to high-value materials without external and harmful for environmental products since the gas can be used as energy sources and the liquid products for production of carbon adsorbents and pitches.
4.
Conclusions
The vacuum pyrolysis of the solvent-extracted olive pulp leads to a decrease in the solid yield and an increase in the liquid and gas yield compared to the products yields obtained after pyrolysis under an atmospheric pressure. The vacuum pyrolysis solid product possesses higher surface area and this dependence is retained after the activation step. The composition of the liquid products ( predominantly derivatives of phenol, dihydroxybenzenes, guaiacol, syringol, vanilin, veratrol, furan, acids) characterizes them as promising raw materials for the production of synthetic adsorbents and pitches. The final physico-chemical properties of the activated carbons obtained at common conditions depend on the kind of the raw material. Steam carbonization and activation with water vapor of carbonizates obtained under vacuum and atmospheric pressure lead to the formation of activated carbons with welldeveloped porous structures with a prevailing content of micropores and an alkaline character of the surface. Carbons with the highest surface area with alkaline character are obtained by chemical activation with K2CO3. Carbon oxidized with air is characterized with a great number of oxygencontaining groups with acidic character of the surface.
Acknowledgment The authors acknowledge financial support for this work from MES-NFSI- Bulgaria. R E F E R E N C E S
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