Energy & Fuels 1997, 11, 1165-1170
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Valorization of Lube Oil Waste by Pyrolysis Rafael Moliner,*,† Marı´a La´zaro, and Isabel Suelves Instituto de Carboquı´mica, CSIC, Luciano Gracia 5, 50015 Zaragoza, Spain Received February 7, 1997X
Valorization of selected wastes by pyrolysis has been claimed as an alternative to incineration. Lube oil waste, LOW, coming from automotion and industry is a good candidate for valorization via pyrolysis and subsequent utilization of gases and liquids produced. In this paper, the results obtained from pyrolysis of a LOW at temperature from 600 to 700 °C and pressure from 0.1 to 1 MPa are presented. High yields of valuable light olefins as ethylene and propylene and light aromatics, BTX, are obtained. Distribution of pyrolysis yields significantly varies as function of temperature and pressure. The optimum conditions for pyrolysis according to two different approaches to LOW valorization are discussed. The first one, called petrochemical, assumes that pyrolysis is carried out near a refinery so that the pyrolysis products are added to the refinery feedstocks. Production of light olefins is favored by this approach. The best results are obtained at a temperature of 650 °C and 0.1 MPa. The second approach, called hybrid, assumes that pyrolysis is carried out in a specific installation and a gas fuel to be used in situ and liquids to be upgraded in a separate installation are produced. Production of light alkanes which are preferred as fuel and BTX is favored by this approach. Both yields are enhanced at high temperature and pressure.
Introduction The treatment of wastes has become one of the most important concerns of modern society. The diversity of the nature of wastes makes it difficult to find a universal treatment for them. Recycling is the best alternative because it is the most environmentally friendly way: Most urban wastes such as glass, plastics, or papers can be recycled, whereas organic rubbish can be converted into compost, provided it is not contaminated by strange wastes. However, other types of wastes are hardly recyclable because the parent product contains additives that are degraded during use, or the changes suffered by the base product are so dramatic that recycling is not possible. In these cases a more severe treatment of wastes is needed to reprocess them into valuable products. For wastes of a hydrocarbon nature, incineration is the most widespread valorization process. In this use, wastes are valorized on the basis of their calorific value. However, combustion of wastes is often difficult and cleaning of flue gases is complex and expensive because they contain important quantities of contaminants. As a result, the actual cost of the waste as a fuel is much higher than the worst of the commercial grade fuels. Pyrolysis of unrecyclable wastes has been claimed as an alternative to incineration for several reasons: 1. Valuable products, such as gases and liquids that may be added to petrochemical feedstocks and char useful as substitute carbon black or activated carbon, are obtained. 2. Energy and products obtained from wastes do not have to be used at the incineration plant but can be transported and consumed in industrial installations. Apart from biomass, which should not be considered as a waste, most of the research works reported in † X
E-mail:
[email protected]. Abstract published in Advance ACS Abstracts, October 1, 1997.
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literature on pyrolysis of wastes have been focused on solid wastes from different sources,1-3 used tyres,4-7 and plastic waste.8,9 Among the suitable wastes to be pyrolyzed, lube oil waste (LOW) is one of the most attractive candidates, since its base oil consists of intermediate-chain paraffinic compounds. Pyrolysis of LOW has not been reported in the literature but the pyrolysis of lighter paraffinic compounds such as jet fuel components has been studied by several authors.10 Pyrolytic degradation of paraffinic model compounds such as decane has been intensively studied by Braekman et al.11 The cracking of liquid hydrocarbons to produce light olefins and BTX is a common practice in refineries: For instance, the Kellog Co.12 licenses a commercial process to produce polymergrade ethylene and propylene, a butadiene-rich C4 product, an aromatic-rich raw pyrolysis gasoline, a hydrogen product, and fuels by steam pyrolysis of hydrocarbons, ranging from ethane to vacuum gas oils. So, although there is a lack of knowledge on the (1) Avenell, C. S.; Sainz-Diaz, C. I.; Griffiths, A. J. Fuel 1996, 75, 1167-1174. (2) Fjellerup, J.; Gjernes, E.; Hansen, L. K. Energy Fuels 1996, 10, 649-651. (3) Garcı´a, A. N.; Font, R.; Marcilla, A. Energy Fuels 1995, 9, 648658. (4) Cypre`s, R.; Bettens, B. Presented at the Conference Internationale sur la Pyrolyse et la Gazeification des de´che´ts. Luxembourg, May 23-25, 1989. (5) Conesa, J. A.; Font, R.; Marcilla, A. Energy Fuels 1996, 10, 134140. (6) Willians, P. T.; Besler, S. Fuel 1995, 74, 1277-1283. (7) Teng, H.; Serio, M. A.; Wo´jtowicz, M. A.; Bassilakis, R.; Solomon, P. R. Ind. Eng. Chem. Res. 1995, 34, 3102-311. (8) Conesa, J. A.; Font, R.; Marcilla, A.; Garcı´a, A. N. Energy Fuels 1994, 8, 1238-1246. (9) Ng, S. H.; Seoud, H.; Stanciulescu, M.; Sugimoto, Y. Energy Fuels 1995, 9, 735-742. (10) Song, Ch.; Eser, S.; Schobert, H. H.; Hatcher, P. G. Energy Fuels 1993, 7, 234-243. (11) Braekman-Danheux, C.; Fontana, A. CECA Agreement 7220/ EC/209. Final Report. December 1994. (12) Hydrocarbon Process. 1987 Petrochemical Handbook, 1987, 74.
© 1997 American Chemical Society
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Table 1. Characteristics of LOW gross heating value (J/g)
density (mL)
C
H
O
S
44329 J/g
0.88
83.2
13.0
1.2
1.2
pyrolysis behavior of LOW, it can be assumed from the results reported for materials with a similar chemical nature that a very wide spectrum of pyrolysis products ranging from methane to PAH can be expected. In the present paper, the pyrolysis behavior of a LOW under different temperatures and pressures is studied. Pyrolysis runs were carried out at two scales. The first one, called bench scale, was run on 10 g samples, whereas the second one, called preparative scale, was run on 1 kg samples. A higher number of runs were carried out at bench scale in order to determine the influence of temperature and pressure on the pyrolysis yields and only some selected runs were scaled up to preparative scale. The results presented here are the ones obtained at bench scale. The results obtained at preparative scale will be presented in a forthcoming paper. Experimental Section Lubricating Oil Wastes. The sample of LOW used was provided by a local waste manager. It was taken from a homogenization tank containing the waste oils recovered in the metropolitan area of Zaragoza (Spain) for 1 month. Before pyrolysis the sample was filtered to 5C) aliphatic compounds. Moreover, there were no signals indicating the presence of aromatic compounds. GC/MS confirmed that LOW was mostly composed of aliphatic compounds from C16 to C33, most of them being higher than C24. Table 1 shows the elemental analysis of the LOW sample used. Bench Scale Pyrolysis. Bench scale pyrolysis runs were carried out at temperature from 600 to 700 °C and pressure from 0.1 to 1 MPa. Figure 1 shows a scheme of the bench scale pyrolysis unit, BPU. The pyrolysis reactor is heated by an electrical oven split into two steps. The upper one is set at 500 °C in order to evaporate the LOW droplets entering the reactor. The lower one is set at the pyrolysis temperature. The reactor is a 2.5 cm diameter, 50 cm long tube and was completely cleaned up after each run. The upper part is empty, letting the droplets of the feed evaporate with any contact with hot surfaces. The lower part, where the oil vapor is pyrolyzed, is filled with ceramic rigs. The feed is forced to flow up to the top of the reactor and falls inside by gravity. Feeding of LOW could have been carried out by a pump. However, the target of the project was to pyrolyze mixtures of LOW with coal and none of the commercial pumps tested showed a good performance with these mixtures. For this reason a feeding device able to be used with both LOW and LOW/Coal mixtures was designed. It consists of a heated (55 °C) and stirred vessel which is overpressurized in relation to the reactor. The feed is forced to flow through the feeding pipe toward the top of the reactor by a controlled overpressure. By controlling the differential pressure between the feeding vessel and the reactor, the feeding flow can be regulated. This feeding system has proved to be very efficient for pumping both LOW and LOW/coal mixtures and it presents some advantages versus the conventional pumping systems: First, troubleshooting derived from the blocking of moving parts, especially valves, is avoided. Second, a large rank of flows and pressures
can be regulated with the differential pressure controller as the unique control element. Using this feeding device, LOW was fed to the reactor at a flow of 0.5 g‚min-1 for 20 min. The actual mass of LOW introduced into the reactor was evaluated by the weight loss of the feeding vessel after the run. The pyrolysis products were swept out from the reactor by a nitrogen flow of 10 mL/min. When the pyrolysis reaction starts this flow is around 150 mL/min due to the contribution of the pyrolysis products. The carrier gas with the pyrolysis products leaves the reactor by the bottom side and it enters a trap cooled by Peltier effect where liquids are condensed. Liquids were recovered at two points: the Peltier trap and the deposition trap which is just a recipient with a larger section where the linear velocity of the gas stream decreases and the liquid droplets are depleted. Gases leaving from the deposition trap are filtered to 3 rings”. Quantitative composition of liquids was determined by the internal standard method using octane as the internal standard. Standard solutions of the compounds to be analyzed were used for calibration.
Results Variation of Pyrolysis Yields with Temperature. Table 2 shows the global yields and the individual yields of the gas compounds obtained at different temperatures. Char yield does not significantly vary with temperature whereas liquid yield decreases and gas yield increases by increasing the temperature. Yields of some interesting gas compounds, such as ethylene and propylene, are very high even at the lowest temperature tested. The most important yields correspond in all cases to C2-C4 olefins: ethylene, propylene, butenes, and butadiene, and to hexenes. The increase of gas yield as temperature increases from 600 to 650 °C is mainly due to the increase of methane,
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Energy & Fuels, Vol. 11, No. 6, 1997 1167
Figure 1. Bench scale pyrolysis unit.
ethylene, and propylene. The increase observed for ethylene and propylene is really interesting, considering the high petrochemical value of these compounds. The increase of temperature up to 700 °C does not significantly change the gas compounds yields. On a molar basis, the increase of temperature leads to a higher production of the smallest molecules, methane and ethylene, to the detriment of the biggest ones.
Regarding liquid compounds, Table 3 shows the yields obtained at different temperatures. Yield of benzene significantly increases as temperature increases up to 700 °C whereas the increase of the toluene yield is less important and yields of xylene and ethylbenzene decrease as temperature increases. Yield of alkylbenzenes does not vary with temperature, so that the ratio of alkyl benzenes to BTX decreases as temperature increases.
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Table 2. Variation with Temperature of Product Yield by Pyrolysis of LOW at 0.1 MPa
Table 4. Variation with Pressure of Product Yield by Pyrolysis of LOW at 650 °C
yield (g/100 g of LOW) char liquids gases H2 CO CO2 H2S CH4 C2H6 C2H4 C3H8 C3H6 i-butane n-butane 1-butene i-butene c-butene t-butene i-pentane 1,3-butadiene pentenes hexenes
yield (g/100 g of LOW)
600 °C
650 °C
700 °C
5.0 60.4 34.6 0.1 0.3 0.4 0.3 3.5 2.5 5.0 1.1 5.4 0.2 0.2 0.6 3.8 2.3 0.4
4.5 35.7 59.7 0.3 0.6 0.6 0.3 9.0 5.2 9.8 1.4 10.7 0.2 0.1 0.9 4.0 3.4 0.8
6.0 36.0 59.7 0.2 0.4 0.8 0.4 9.3 4.7 9.9 1.1 10.6 0.1 0.1 0.8 4.2 3.2 0.7
1.2
2.4 0.4 9.3
3.4 0.5 9.0
6.8
Table 3. Variation with Temperature of Liquid Product Yield by Pyrolysis of LOW at 0.1 MPa
char liquids gases H2 CO CO2 H2S CH4 C2H6 C2H4 C3H8 C3H6 i-butane n-butane 1-butene i-butene c-butene t-butene i-pentane 1,3-butadiene pentenes hexenes
0.1 MPa
0.5 MPa
1.0 MPa
4.5 35.7 59.7 0.3 0.6 0.6 0.3 9.0 5.2 9.8 1.4 10.7 0.2 0.1 0.9 4.0 3.4 0.8
8.1 33.9 58.0 0.7 0.7 0.7 0.3 13.8 10.8 8.9 2.7 8.3 0.4 0.3 0.5 1.2 1.9 1.4
12.6 37.3 50.1 0.4 0.6 0.6 0.2 14.4 10.8 5.9 3.1 6.8 0.4 0.3 0.4 0.8 1.5 1.1
2.4 0.4 9.3
0.6 0.3 2.6
0.4 0.2 1.6
Table 5. Variation with Pressure of Liquid Product Yield by Pyrolysis of LOW at 650 °C
yield (g/100 g of LOW) benzene toluene xylene ethylbenzene alkylbenzenes naphthalene phenanthrene 2-3 rings >3 rings
yield (g/100 g of LOW)
600 °C
650 °C
700 °C
4.0 6.3 4.6 1.7 6.3 0.6
5.6 6.1 2.8 0.8 5.8 1.8 0.2 5.1 0.2
9.4 7.1 2.6 0.7 6.4 3.4 0.8 0.7 0.2
2.8
Yield of 2-3 ring compounds increases from 600 to 650 °C, which is probably due to a higher conversion of LOW, but dramatically decreases at 700 °C. It is noticeable that yields of BTX increase from 650 °C, although the global liquid yield decreases. This is due to the presence of unconverted oil in the liquids obtained at 600 °C which leads to a high liquid yield. At 650 °C the transformation of oil is nearly total; in fact, the results obtained at 650 °C are very similar to those obtained at 700 °C, except for benzene production. The main implication is that a severe temperature is not needed to achieve almost total transformation of LOW into valuable products. In general terms, it is observed that the increase of temperature forces alkyl derivative yields to decrease whereas yields of nonsubstituted aromatic compounds increase; i.e., the aromatic character of the liquids produced increases. Variation of Pyrolysis Yields with Pressure. Table 4 shows the yields obtained at different pressures from pyrolysis of LOW at 650 °C. The increase in char yield produced by increasing pressure is noticeable, since the yield at 0.1 MPa is tripled at 1 MPa. Yield of liquids does not vary significantly with pressure, whereas the yield of gas decreases at high pressure. This highlights the fact that pressure promotes secondary reactions converting gas compounds into char. The compounds mostly affected by increasing pressure are olefins: yields of ethylene, propylene, butenes, butadiene, and hexenes decrease whereas yields of C1-C3 alkanes increase.
benzene toluene xylene ethylbenzene alkylbenzene naphthalene phenanthrene 2-3 rings >3 rings
0.1 MPa
0.5 MPa
1 MPa
5.6 6.1 2.8 0.8 5.8 1.8 0.2 5.1 0.2
7.6 6.2 3.4 1.1 2.6 2.0 0.5 2.7 0.4
14.0 6.2 1.9 0.6 2.5 0.7 0.4 2.9 0.6
Table 5 shows the variation of yields of valuable products in liquids with pressure. Variation is different for each of them: Benzene yield significantly increases with pressure whereas toluene yield does not vary and that of xylene decreases at 1 MPa. Yields of alkylbenzenes and 2-3 rings compounds decrease as pressure increases. Results obtained show that the increase of pressure promotes the formation of nonsubstituted aromatic compounds, and as a consequence, decreases production of substituted aromatic compounds and increases the production of C1-C3 alkanes. Discussion Interest in LOW pyrolysis is based on the technological value of the products obtained. In order to achieve a higher valorization of LOW pyrolysis products, raw products should not be used directly; i.e., they should be separated into components and these upgraded before using. To ease the subsequent treatment of the pyrolysis products, pyrolysis of LOW should be carried out at or near a petroleum refinery which has the facilities to benefit from the technological value of the pyrolysis products. In fact, the figures obtained in this work for the different gas compounds are lower than those reported for low-pressure high-temperature gasoil cracking and hydrocracking carried out at refineries.12,13 However, it should be pointed out that the processes referred to are optimized for the production
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Energy & Fuels, Vol. 11, No. 6, 1997 1169
Figure 2. Variation with temperature of the fraction of LOW hydrogen transferred to the pyrolysis products.
Figure 3. Variation with pressure of the fraction of LOW hydrogen transferred to the pyrolysis products.
of light olefins and this way higher yields for these products can be obtained. The composition of liquids obtained from LOW pyrolysis compares to those reported for severe thermal cracking of naphthas and crudes carried out at refineries to produce aromatics.14 Consequently, in the “petrochemical approach” of LOW pyrolysis, production of compounds of petrochemical interest such as light olefins and BTX should be preferred against others such as light alkanes, which can only be valorized as gas fuel. Results presented in Tables 2-5 show that experimental conditions have a great influence on pyrolysis yield. In particular, it is of interest to investigate into how temperature and pressure exert an influence on the distribution of hydrogen among the pyrolysis products, since hydrogen contained in LOW is, in some way, the most valuable component. Figure 2 shows the variation with temperature of the fraction of LOW hydrogen contained in each of the pyrolysis products. Most of the figures obtained at temperatures of 650 °C are similar to those obtained at 700 °C. This confirms that conversion of LOW is nearly completed at 650 °C. In contrast, the results obtained at 600 °C are significantly lower. Hydrogen contained in the compounds in Figure 2 accounts for near 90% of the LOW hydrogen at 650 and 700 °C, but only for 50% at 600 °C. The most important differences are observed for H2 and C1 to C3 compounds, since the fraction of LOW hydrogen contained in C4 to C6 olefins and BTX slightly varies with temperature. This behavior of hydrogen distribution shows that increase in temperature mainly affects the formation of the lightest compounds, in particular the formation of methane, while the fraction of LOW hydrogen converted into BTX and C4-C6 olefins is only slightly affected. Pressure also has an important influence on LOW hydrogen distribution. Figure 3 shows the variation with pressure of the fraction of LOW hydrogen contained in each of the pyrolysis products. The increase of pressure transforms nearly one-third of the hydrogen in LOW to methane and more than a half is turned into C1-C4 alkanes. In contrast, the fraction of hydrogen
converted into BTX only slightly increases on increasing pressure from 0.1 to 1 MPa. The conclusion to which this analysis leads is that the “petrochemical approach” to LOW valorization has to be carried out within a temperature window between 650 and 700 °C and at atmospheric pressure. In this way, hydrogen contained in LOW is mostly converted into petrochemical feedstocks. If the petrochemical approach were not feasible, the alternative may well be a “hybrid approach” consisting of obtaining a high calorific value gas fuel and petrochemical-feedstock grade liquids. The undertaking of this approach does not need to be linked to a petrochemical plant because a fuel gas can be used in situ in domestic and industrial installations and liquids obtained can be transported to a separate plant for upgrading and commercialization. In this case, the goal would be to transfer to the pyrolysis gas most of the calorific value of LOW and to produce high yields of valuable products such as BTX and some selected aromatics such as naphthalene and phenanthrene. The qualitative composition of liquids obtained from LOW pyrolysis is similar to that of the liquids obtained from coal pyrolysis, except for the presence of phenols which are not present in LOW pyrolysis liquids. So, plants processing coal pyrolysis liquids could also be users of LOW pyrolysis liquids. Recently, Chunsan and Schobert15 have reported a good analysis and discussion of possible new routes for developing chemicals and materials from coal pyrolysis liquids, which may well be applied to LOW pyrolysis liquids. Regarding the pyrolysis gas as a fuel, Table 6 shows the fraction of the calorific value contained in LOW transferred to the gas compounds as a function of the pyrolysis conditions. Figures in Table 6 were calculated as follows:
(13) Sebor, G.; Blazek, J; Lederer, J.; Bajus, M. Fuel Process. Technol. 1994, 40, 49-59. (14) Encyclopedia of Chemical Technology; Kirk, R. E., Othmer, D. F., Eds.; The Interscience Encyclopedia, Inc.: New York, 1950; Vol. 5, p 889.
fraction of LOW calorific value contained in compound i ) HiYi/Hl × 100 where Hi is the ideal gross heating value16 of compound i expressed as J‚g-1, Yi is the yield of the compound (15) Song, Ch.; Schobert, H. H. Fuel 1996, 75, 724-736 (16) ASTM D 3588-91. Annual Book of ASTM Standard; ASTM: Philadelphia, PA, 1994; Section 5, Vol. 05.05, p 81.
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Table 6. Variation with Temperature and Pressure of the Fraction of the LOW Calorific Value Transferred to the Gas Products product
600 °Ca
650 °Ca
700 °Ca
0.5 MPab
1 MPb
hydrogen methane ethane ethylene propane propylene isobutane n-butane butenes pentenes hexenes total
0.3 4.4 2.9 5.7 1.2 6.0 0.2 0.2 9.0 7.4 37.3
1.0 11.3 6.1 11.1 1.6 11.8 0.2 0.1 12.5 0.4 10.1 66.2
0.6 11.6 5.5 11.2 1.2 11.7 0.1 0.1 13.4 0.5 9.8 65.7
2.2 17.3 12.6 10.1 3.1 9.2 0.4 0.3 6.1 0.3 2.8 64.4
1.3 18.0 12.6 6.7 3.5 7.5 0.4 0.3 4.6 0.2 1.7 56.8
a
Pressure: 0.1 MPa. b Temperature: 650 °C.
taken from Tables 2-4 expressed as wt % (undimensional), and Hl is the gross heating value of LOW from Table 1. It can be observed that increasing the temperature from 600 to 650 °C dramatically increases the total fraction transferred whereas increasing it up to 700 °C does not significantly modify either the total fraction or the fraction transferred to each of the gas components. It can be observed that an important part of the LOW calorific value is transferred to the C2-C6 olefins, which is not a desirable effect. Increasing the pressure from 0.1 to 1 MPa at 650 °C does not modify the total fraction of the LOW calorific value transferred to the gas compounds but it does change the distribution between the different compounds produced: The higher the pressure, the higher the fraction transferred to C1C3 alkanes which thus increases the quality of the
pyrolysis gas as a fuel. The conclusion is that pyrolysis under pressure could be preferred in the hybrid approach to LOW valorization since a higher quality of fuel gas and a higher BTX yield are obtained. Conclusions Pyrolysis of lube oil waste yields important quantities of valuable products such as C1-C3 alkanes, C2-C4 olefins, and BTX. Total conversion of LOW can be achieved at low severe conditions of pyrolysis. The relative distribution of product yields can be changed as a function of the pyrolysis conditions. The selection of pyrolysis conditions depends on the final use of the pyrolysis products. In the petrochemical approach, which considers the products as petrochemical feedstocks, a temperature window of 650-700 °C and a pressure of 0.1 MPa should be used. At these conditions most of the hydrogen contained in LOW is transferred to C2-C4 olefins and BTX. In the hybrid approach which produces gas fuel and liquid feedstocks, a temperature of 700 °C and a pressure of 0.5-1 MPa should be used. In this case, a higher fraction of LOW hydrogen and LOW calorific value is transferred to C1C3 alkanes, this maintaining almost unvaried the hydrogen transferred to BTX. Acknowledgment. The financial support for this work was obtained from the European Commission, Agreement no. 7220/EC-763, and the Diputacion General de Arago´n. EF970025S
