step the solvolysis oil is subjected to a catalytic hydrotreatment with CoMoS, ... effected by using the upgraded oil to replace fresh solvent in the solvolysis step.
SOLVOLYSIS AND CATALYTIC HYDROTREATMENT OF WOOD
ELAMIN A.,REZZOUG S., CAPART R., GELUS M. Dept of Chemical Engineering University of Technology of Compiegne BP 649 60206 Compiegne (France)
ABSTRACT The objective of this study is to produce hydrocarbon fuel from wood in a two steps process. In the ftrst step, pine wood powder is treated with acidifted organic solvents such as ethanol, methanol, ethylene glycol, tetralin and mixture of them with phenol at relatively mild conditions. The wood is completely dissolved in a mixture of phenol/ tetralin ( SO/SO by wt.).The yield of the oil obtained, so-called solvolysis oil, is 70 % by wt. of the initial dry wood . It is soluble in aceton and has an oxygen content of 21.S % wt . In the second step the solvolysis oil is subjected to a catalytic hydrotreatment with CoMoS, NiMoS and Iron powder as catalysts, in the presence of tetralin as hydrogen donor solvent. The reaction temperature is ftxed at 3S0 °C . The initial pressure of hydrogen is tested between 30 to 90 bar. The NiMoS has the best catalytic efficiency on oxygen removal. The upgraded oil obtained is rich in aromatic, aliphatic and phenolic compounds. So, some runs have been effected by using the upgraded oil to replace fresh solvent in the solvolysis step. The results of these experiments show that the upgraded oil is very effective in the solubilization of wood.
INTRODUCTION Since the early seventies, several authors have tried to convert lignocellulosic materials into liquid fuels or chemicals by thermochemical conversion. The processes that have been developped in this fteld could be classifted in two (1) groups. Firstly direct liquefaction processes that have been intensively investigated (2) (3) (4) (S) and they combined drastic conditions: high temperatures 300 - 4S0°C, high pressure S - 20 MPa, different types of catalysts and reducing gases (H2, CO, H2 + CO). Even under these drastic conditions, the reported oil is still oxygenated (8 - 12 % by wt), compared with petroleum fuel. Many researchers such as ELLIOT (6) and MEIER (7) concluded that it is hardly possible to eliminate the high oxygen content of the biomass in one step process.The second group is a-two step process in which the ftrst one consists to obtain liquid oiHrom wood by means of thermal treatment (pyrolysis) or by using organic solvents i.e solvolysis and the second step is the upgrading of the oil produced in the preceeding one. Many
1415 A. V. Bridgwater (ed.), Advances in Thermochemical Biomass Conversion © Springer Science+Business Media Dordrecht 1993
pyrolysis processes have been developed to produce bio-oil (8), (9), (10) which is of a low quality compared with the conventional hydrocarbons, so it was subjected to an upgrading treatment to improve its quality, either by thermal cracking with zeolites or by hydrotreatment with different types of catalysts such as noble metals (Pdlcharcoal), bifunctional catalysts (NiMo, CoMo) and metallic catalysts (Fe, Ni). This paper investigates the solubility of wood in some organic solvents and the catalytic hydro treatment of the produced solvolysis oil. Although many studies have been conducted on the solubility of wood in organic solvents (11), (13) few attention has been made till now on the catalytic hydrotreatment of the crude oil, in order to improve its quality as an alternative fuel.
EXPERIMENT AL
All experiments were performed in a batch reactor (Autoclave Engineers) of 0.5 I equiped with a thermocouple, a turbine agitator and a cooling water system.The raw material used through all the experiments is pine wood, mild to powder (200 !lm), dried in an oven to a moisture content of around 4 % wt. Its elemental composition by weight is the following: C=48.5 %
H=6%
0=44.8 %
At the end of solvolysis, the oil obtained is filtered under vacuum to determine the percentage of the converted wood into liquid as measured by the quantity of acetone soluble material. Once a complete conversion is obtained, the produced oil is then separated into different fractions by atmospheric and vacuum distillation (10 mm Hg).The hydrotreatment is effected on the heavy fraction of the solvolysis oil in the presence of tettalin as hydrogen donor solvent, the catalysts employed are iron powder and sulfided form of NiMo and CoMo. The reaction conditions of the treatment are the following : Reaction temperature : 3500C Reaction time : 60 min
Initial hydrogen pressure: 3D, 60, 90 bar Catalyst: 15 % by weight of the charge Tetralin/solvolysis oil : 3/1
RESULTS AND DISCUSSION
Wood solvolysis oil From early studies on the solvolysis of wood using acidified phenol as solvent, YU (11) and MAUGENDRE (12) obtained a complete conversion of wood into liquid, but unfortunately phenol cannot be recovered completely hence around 20 - 30 % of it incorporates with the solvolysis oil. To avoid this loss of solvent, many experiments have been conducted by testing these solvents: methanol, ethanol, ethylene glycol and mixture of phenol - tetralin under the same reaction conditions that lead to complete conversion of wood when using pure phenol, in order to investigate their ability to dissolve wood. The results of these experiments are presented in Figure 1. From Figure I, it can be seen that ethylene-glycol, phenol and a phenoltetralin mixture behave very similarly, they dissolve the wood almost completely, since the conversion obtained is 95.2 %, 99.8 % and 99.5 % respectively. On the other hand methanol and ethanol are not very selective for wood solubilization under these conditions.
1416
Another series of experiments have been effected by varying the temperature between 180 and 300"C, reaction time between 0 and 60 min and sulfwic acid concentration between oand 1 %, in order to determine the optimal conditions that lead to maximum or complete conversion of wood into liquid for each solvent. 100 80
....... ~
'-'
60
....5
5 ~ 8
40
20 0
Ethanol
Methanol
Ethylene glycolPhenol-tettalin
Phenol
Figure l.Conversion of wood in different solvents, reaction conditions: T = 250°C, reaction time = 30 min, acid conc(H2S04) = 0,5% wt on wood basis, solvent/wood = 4/1.
1oo~------------~~~----------~
~ '-'
80
C phenol-tetralin •
methanol
+
ethanol
•
ethylene glycol
20~~~~~~~--~~~--~~~~
150
200
250 300 TEMPERATURE (>C )
350
Figure 2. Conversion of wood versus reaction temperature with different solvents. Reaction conditioris: solvent/wood =4/1, reaction time =30 min Acid conc(H2S04) =0.5% wt on wood basis.
1417
Figure 2 represents the effect of reaction temperature on the solubility of wood. For all solvents tested the solubility of wood increases with temperature up to an optimal value beyond which the conversion or the acetone soluble material decreases due to the recondensation of depolymerized wood material and the formation of coke. The optimal reaction temperature depends on the nature of the solvent, i.e for phenol-tetralin mixture the optimal temperature is 2SOoC, while for ethylene glycol the optimal temperature is 280°C. The solubility of wood in ethanol and methanol is relatively low, it is about 62 % at 240°C and 71.5 % at 280°C respectively and the repolymerization or formation of coke in the case of these last two solvents appear at relatively low reaction temperature and especially with ethanol. The same phenomenon was observed by CHORNET (13). Both influences of reaction time and acid concentration on the solubility of wood have been also studied and the same behaviour that was observed when varying reaction temperature has been reported for these two parameters i.e the solubility of wood increases with reaction time and acid concentration up to an optimal value and then decreases due to coke formation. This parametric study allows us to determine the optimal reaction conditions that lead to maximal or complete conversion of wood into liquid for the solvents tested and they are shown in Table 1. From the Table 1 it can be seen that pure ethanol and methanol are not so effective on wood solubilization, while a mixture of each with phenol (50/50 by weight) leads to complete conversion and this improve the high capacity of phenol to dissolve wood .
TABLE 1 Optimal reaction conditions of wood solvolysis in different organic solvents Solvent
TeC)
Acidity(%)
Reaction time
Conversion
(min)
(%)
0.6
30
98
Ethylene-glycol
280
Ethylene-glycol
250
30
96
Ethanol
240
0.4
30
65
Methanol
240
0.5
30
7l.8
ethylene glycol
250
0.5
30
100
Methanol-phenol
250
0.5
30
100
Ethanol-phenol
250
0.5
30
100
Phenol-tetralin
250
0.5
30
100
Phenol-
Characterization of the solvolysis oil The oil obtained by the solvents that give complete dissolution of wood is characterized first by the yield of its different distillation fractions which are : water
1418
"aqueous fraction", light fraction and heavy fraction. The percentage of each for the tested solvent are shown in Table 2. TABLE 2 Distillation fractions of the solvolysis oil (wt %) and recovering of solvent Phenol methanol
Phenoltetralin
Ethylene glycol
Phenolethanol
Water *
9.35
22.8
24.1
19.5
* Heavy fraction *
6,6
8
8
8.5
84
69.2
67.2
72
82.3
81.3
90
Solvent
Light fraction
Solvent recovered ** 87.3 * % wt. on initial wood basis ** % wt. on the initial solvent
From the Table 2 it can be seen that the percentage of water produced by using phenol as co-solvent appears very high (22 to 24 % of the initial wood), compared with that obtained with pure ethylene glycol (9.5 % of the initial wood). Also, the light fraction is about 8 % with phenol as co-solvent and 6.6 with ethylene glycol. The heavy fractions represent the highest yields (67 % to 84 % wt on the initial wood), it is black, solid at room temperature and free flowing above 80°C. It represents the raw material for the catalytic upgrading and it is so called "solvolysis-oil". Another factor that must be considered in the solvolysis step is the solvent recovering, hence from economic point of view it is interesting to recover the solvent and to use it again for wood dissolution. From Table 2 it is obvious that with phenol-tetralin mixture, up to 90 % of the initial mixture can be recovered by vacuum distillation, which indicates a better recoverability compared with the other tested solvents. The solvolysis oil (heavy fraction) from the different tested solvents is then characterized by its elemental composition which is represented in Table 3. From this table, it can be seen clearly that the solvolysis oil obtained by a mixture of phenol-tetralin has a lower oxygen content (21.5 % wt) and already 50 % wt of the residual oxygen content of the wood was removed, also Table 2 shows that, by using a phenol-tetralin mixture, a high percentage of it can be recovered, so the solvolysis oil produced with this mixture is used as a starting material for the catalytic hydrotreatment . TABLE 3 Elemental composition of the solvolysis oil Solvent system
Elemental composition C%
H%
0%
Ethylene glycol
55.08
7.67
37.30
Phenol-methanol
64.46
5.86
29.65
Phenol-ethanol
66.73
5.85
28.05
Phenol-tetralin
72.56
5.46
21.48
1419
Catalytic: hydrotreatement of the solvolysis oil The oil obtained after the hydrotreatement. is separated by atmospheric and vacuum distillation, the different fractions with their corresponding boiling point are shown in Table 4, they are presented in weight percent based on solvolysis oil. The aqueous fraction is mainly water, the light one is rich in aromatic compounds while the heavy fraction is rich in phenolic derivatives. The hydrogenated oil can be represented by the mixture of the light and the heavy fractions after removing the aqueous fraction (dry oil) and the carrier solvent (tetralin) which can be recovered up to 90% by weight of its initial amount. TABLE 4 Fractionation of the hydrogenated oil Fractions
Yield(wt %)
boiling point range eC)
Distillation pressure
55 -95 95 -100
Atmospheric
15 - 20
Light Aqueous
10 - 15
Tetralin Heav~
Atmospheric
80- 85
90 65 - 35
lOmmHg lOmmHg
85 - 120
The yield of the hydrogenated oil decreases by increasing initial hydrogen pressure, due to more formation of water and gas and hence high rate of oxygen removal as can be seen from Table 5 of the elemental analysis TABLE 5 Elemental analysis of the hydrogenated oil 30 bar Catalyst~
Fe NiMo CoMo·
C% 83.54 88.33 84.7
H% 6.26 7.('1) 6.80
60 bar 0%
C%
10.3 85.2 9.16 87.18 8.50 86.3
H% 8.3 7.53 8.5
90 bar 0%
C%
6.50 4.54 5.20
86.4 89.75 88.24
H%
0%
7.94 8.79 8.89
2.76 1.12 2.39
From Table 5 it can be seen that whatever the type of the catalyst tested, increasing initial hydrogen pressure will increase the rate of oxygen removal. Sulfided NiMo seems to be more effective in oxygen removal and particularly at high pressure (90 bar). Hence the oil obtained under these conditions has an oxygen content between 1.12 and 2.8. The analysis of the hydrogenated oil by coupled GC - MS with NiMoS as catalyst at 90 bar is presented in Table 6. The main identified compounds are benzene, indan, phenolic and naphtalene derivatives. In order to simulate from batch experiments a continuous process self dependant of its solvent. some experiments have been performed by using hydrotreated oil to replace the
1420
fresh solvent for wood solubilization in the solvolysis step. This procedure has been repeated six times and a complete conversion of wood into liquid is obtained with almost the same characteristics for the oil as that produced by using fresh solvent as shown in Table 7. TABLE 6 Products identified in the hydrogenated oil Benzene derivatives; l-isopropyl-3-isopropenyl-benzene 2-methyl-benzofurane 2-vinyl-2,3-dihydrobenzofurane
Benzene Hexamethyl benzene 2-methyl-l-methylene-propyl-benzene Indane derivatives;
2,4,6 trimethyl-indane 2-ethyl-4,6 dimethyl-indane n-butyl-indane
I-methyl-indane 5-indanol Dimethyl-indane Phenolic derivatives;
n-butyl-phenol n-propyl-phenol O-isopropyl-phenol 2,4-Methylene-diphenol
Phenol Methyl-phenol Ethyl-methyl-phenol Di-methyl-phenol NiWhlah,~ derivativ~s;
Naphtalene Methyl-naphtalene
1,2-dihydronaphtalene 3,4-dihydro-l-naphtalenone
TABLE 7 Recycling of the upgraded oil in the solvolysis step Cycle number
% Yield
C%
H%
Initial'" 1 2 3 4 5 6
71.5 71.00 70.00 69.25 69.25 68.5 68.0
72.56 72.72 72.25 74.45 73.36 74.3 73.3
5.46 5.42 5.63 5.68 5.5 5.5 5.68
0% 21.8 21.29 21.58 19.42 20.41 21.4 21.1
Reaction conditions; reaction time =30 ron ; temperature =250°C Recycled oiVwood =4/1 ; acidity =0.5 % by wt on wood basis. '" Starting solvent; mixture of phenoVtetralin (50/50 by wt).
1421
CONCLUSION Simple alcohols such as ethanol and methanol are not so effective in wood solubilization and lead to further char formation with increasing severities, while a mixture of each of them with phenol (50/50 by wt) gives complete solubilization of the wood at relatively mild reaction conditions. Ethylene-glycol behaves very similar as phenol, a better recoverability of the initial solvent was obtained when phenol-tetralin mixture were used. The catalytic hydrotreatment of the solvolysis oil in presence of tetralin allows to obtain a liquid oil with low oxygen content, especially when using NiMoS as catalyst at high intial hydrogen pressure (90 bar). The analysis of the hydrogenated oil shows that it contains aromatic compounds but the abundant compounds are phenolic and naphtalene derivatives.The upgraded oil is very effective in wood solubilization and can be used instead of fresh solvent in the solvolysis step.
REFERENCES 1. Capart, R., Elamin, A. and Gelus, M., A survey of biomass liquefaction process. In Pyrolysis and Gasification, proceeding of an international conference, Luxembourg, eds. G.L. Ferrevo, K. Maniatis, A. Buekens and AV. Bridgwater, Elsevier, Applied Science, London & New-York, May 1989, 158-168. 2. Appel, H.R. and al. Conversion of cellulose waste to oil. US Bureau of Mines. Report of investigation, nO 8013 (1975). 3. Boocok, D.G.B., Mackay D., Lee P., Wood liquefaction: extended batch reactions using Raney-nickel catalysts. Can 1. Chern. En& .. ' 1982,16, 802-808. 4. Rogers D.Z., Trandinh. An examination of alternative catalysts for direct biomass liquefaction. In Proc. of the 14th biomass thermochemical conversion contractors meetin&. Arlington, Virginia, June 1982, pp. 627-645. 5. Baker, E.G. and Elliott, D.C., Catalytic hydrotreating of biomass derived oil liquids to produce. ACS Fuel Chern. 1987,32,2, 257-263 6. Elliot, D.C. and Baker, E.G. Hydrotreating biomass liquids to produce hydrocarbon fuels. In Ener&y from Biomass and Waste, Washington D.e. 7. Nelte, A and Meier zu K6cker, H., Direct liquefaction of wood and agricultural wastes (biomass). In Euroforum New Ener&ies Proc. Int. Con&ress, Saarbrucken, 24,28 Oct. 1988,pp 673-75. 8. Scott, D.S., Piskorz, 1., Grinshpun and Graham, R.G. The effect of temperature on liquid product composition from fast pyrolysis of cellulose. ACS. Production. Analysis and up~adin~ of oils from Biomass, 1987,32,2, 29-35. 9. Diebold, J. and Scahill, 1., Production of primary pyrolysis oils in a vortex reactor. ACS. preprints in production analysis and up~ading of oils from biomass, 1987,32,2, 21-28. 10. Bridgwater A.V. and Bridge SA A review of biomass pyrolysis and pyrolysis technologies in Biomass Pyrolysis Liquids uP&radin~ and utilisation. Edited by A V. Bridgwater and G. Grassi, 1991, 11-92.
1422
11. Yu, S.M., Solvolytic liquefaction of wood under mild conditions. Ph. D. Thesis. University of California, Berkeley, 1982. 12. Bouvier, I.M. G6lus, M. and Maugendre, S. Direct liquefaction of wood by solvolysis in Pyrolysis oils from biomass ; producin~. analysin~ and upgradin~. ed. E.I. Soltes & T.A. Milnes, ACS Symposium series, Washington, DC, 1988, 129-138. 13. Heitz M., Vincent, D., Chornet, E. Overend R.P. Solvent effect on liquefaction: solubilization profiles of a tropical prototype wood ; Eucalyptus in presence of simple alcohols, ethylene glycol, water and phenols. Research in Thermochemical Biomass Conversion. Phoenix, Arizona. Eds. Bridgwater and al. April, 1988, 429-38.
1423
