Environ. Sci. Technol. 2000, 34, 3205-3210
Valuable Products from Mineral Waste Oils Containing Heavy Metals M . J . L AÄ Z A R O , † R . M O L I N E R , † I . S U E L V E S , † C . N E R IÄ N , * , ‡ A N D C. DOMEN ˜ O‡ Instituto de Carboquı´mica (Consejo Superior de Investigaciones Cientı´ficas), Maria de Luna, 12, 50015 Zaragoza, Spain, and Departamento Quı´mica Analı´tica, Centro Polite´cnico Superior, Universidad de Zaragoza, 50015, Zaragoza, Spain
The main objective of this paper is to study the conversion of mineral waste oils (MWO) containing heavy metals into valuable products via pyrolysis. The pyrolysis of the MWO has been carried out in a continuous-mode preparative scale pyrolysis unit containing a bed of char obtained from the Samca coal, a low-rank coal. Temperature of 625 °C and 700 °C and pressures of 0.1 and 1 MPa have been used. High yields of valuable light olefins as ethylene and propylene and light aromatics as BTX and naphthalene derivatives have been obtained. Special attention has been paid to the fate of the metals contained in the MWO during pyrolysis and their distribution into the pyrolysis products. In particular, the distribution of six metals (Cd, Ni, Pb, Cu, Cr, V) has been studied. The data show that the Samca coal chars used has the ability to take up some of the metals under study, but the behavior of each metal is different. So, while Pb and Cu are mostly retained in the coal char bed, V is not affected, and Cd, Ni, and Cr are even released from the char. The reduction of the Pb, Cu, and V contents from the MWO to the liquid pyrolysis products is very high, whereas the contents of Cd, Ni, and Cr in the liquids remain almost constant after the pyrolysis process. Variation of the distribution of the metals contained in MWO between the char bed and the liquid products recovered as a function of the pyrolysis conditions are discussed.
Introduction Industrial hydrocarbon wastes such as mineral oils, coating plastics, paints, or petroleum residues have considerable interest as a fuel source or a chemical feedstock. However, serious concern regarding the use of waste oils as a fuel may arise from undesirable emissions of heavy metals commonly found in used oils as an ash constituent (1). The contaminants in mineral waste oils and their fate during distillation/ hydrotreatment have been evaluated in the literature (2, 3). It has been told that this waste can be recovered through conversion into a clean commodity by properly re-refined using vacuum distillation and hydrotreating. Sanjay et al. (4) have claimed that in the coprocessing of coal with waste materials, coal could act as a trap for the metals removed
from the oil during coprocessing. Miller et al. (5) reported that removal of metallic impurities in the oil during coprocessing of coal with a heavy oil has been found to be due to the deposition on the coal residue or pitch. Skala et al. (6) reported that demetalation of used oil during hydrotreatment was also found to be primarily due to the process of physical deposition on the solid catalyst bed. The analysis of the oils derived from coprocessing coals of different rank with an automobile crankcase oil indicated that these oils were lower in overall trace metals compared with the trace metal content of untreated automobile crankcase (7). However, no coal rank dependence was observed for the removal of trace heavy metals (demetalation). Another class of treatment of particular interest for unrecyclable mixed mineral oils involves the production of C1-C12 hydrocarbons by pyrolysis. This treatment has been claimed as an alternative to incineration of unrecyclable mineral waste oils. These residues are, in principle, good materials for obtaining valuable products by pyrolysis, since MWO contains a high amount of organic compounds of long chain, which are susceptible of being transformed in smaller compounds. Nevertheless, the final fractions obtained from pyrolysis critically depend on the pyrolysis conditions applied to the waste oil. Previous studies carried out with these waste oils in a benchtop drop tube pyrolysis reactor without any solid bed inside (8) showed the transformation of long chains, C24 and others, into smaller compounds, BTX, and naphthalene derivatives. However, the pyrolysis products cannot be directly used since they also have heavy metals which act as both contaminants and likely catalyzers, and consequently, a different process is required. The use of a char in a fluidized bed reactor supplies the solid matrix in which heavy metals can be trapped. In the present paper the pyrolysis at preparative scale of a mineral waste oil (MWO) in a fluidized bed reactor filled with coal char has been carried out. The work has two aims: (i) to compare the pyrolysis products from the fluidized bed preparative scale unit (FBPU) used in this work with those obtained from the drop tube bench scale (DTBU) where no coal char was present [Higher heating rate and lower residence time are used at the FBPU. Morever, the presence of coal char can enhance secondary cracking reactions of primary products by cracking on char surface.] and (ii) to study the behavior of coal char as a trap for heavy metals. Pb, Ni, Cd, Cu, V, and Cr have been studied in this case, since these metals are regulated in some countries as potential air pollutants when particulates from combustion processes are emitted. All of them have toxic effects, and consequently, the final products to be considered as valuable products should be free of these metals. Metals can be present in mineral waste oils under different species as a metallic particle, bound to organic compounds, as halides, etc. Each one of these species has a different thermodynamic characteristic and reactivity, so they evolve in a different way depending on pyrolysis conditions. The distribution of the metals between coal, char bed, and the liquid and gaseous pyrolysis products versus the pyrolysis conditions is discussed.
Experimental Section * Corresponding author phone: +34 976 76 1873; fax: +34 976 76 1861; e-mail: cnerin posta.unizar.es. † Instituto de Carboquı ´mica (Consejo Superior de Investigaciones Cientı´ficas). ‡ Universidad de Zaragoza. 10.1021/es9905546 CCC: $19.00 Published on Web 06/23/2000
2000 American Chemical Society
Samca Coal Char. The main characteristics of the Samca coal used have been published in a previous work (9). It is a typical coal from the North Teruel basin, which is presently mined for power generation. The char was obtained by VOL. 34, NO. 15, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
3205
FIGURE 1. Scheme of the pilot plant used for the pyrolysis experiments. pyrolysis of coal at 800 °C in the same fluidized bed reactor used to pyrolyze MWO. Mineral Waste Oil. The sample of MWO used was provided by a local waste oil manager. It was taken from a homogenization tank containing the waste oils recovered in the metropolitan area of Zaragoza (Spain) for 1 month. This oil was a mixture of automotive lubricating oil and other industrial waste oils, all of them considered as a residue. Before pyrolysis, the sample was filtered to < 100 µm, and volatiles and water were eliminated by heating at 110 °C. The FTIR spectrum of MWO corresponds to C-H bonds typically obtained from aliphatic compounds. The band at 720 cm-1 showed that they were long chain (>5C) aliphatic compounds. Moreover, there were no signals indicating the presence of aromatic compounds. GC/MS confirmed that MWO was mostly composed of aliphatic compounds from C16 to C33, most of them being higher than C24. The elemental analysis of the MWO sample used has been published in a previous work (8). Preparative Scale Pyrolysis of MWO. Pyrolysis of MWO were carried out in a fluidized bed preparative scale unit, FBPU. Figure 1 shows a scheme of this unit. The experimental setup consists of a 6.5 cm of Φi × 60 cm stainless steel reactor heated by an electrical oven. The products recovering section has two lines, one of them is used during the not-steadystate period and the other one is switched in when the operative run conditions have been achieved. Pressure is controlled by a back-pressure controller which is allocated just before the gas meter, so that all the recovering section is under pressure. The MWO is fed and dosified by means of a piston pump which supplies to 0.4 g h-1. The MWO is injected from the bottom of the reactor which contains a bed of Samca coal char of around 950 g with particle size of 0.1-0.2 mm. A fresh charge of char is fed before each run at room temperature from the upper part of the reactor. After that, the char bed is pressurized, fluidized, and heated to the reaction temperature. When the reactor is ready, the injection of the MWO starts, and volatiles are conducted to the steady-state line of the recovering products section where liquids are condensed, collected, and separated from gases. Two pyrolysis temperatures, 625 °C and 700 °C and two pressures 0.1 MPa and 1 MPa were used because previous works (8) carried out at bench scale (without any char bed 3206
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 15, 2000
in the reactor) demonstrated that these ranges of temperature and pressure were the best ones to obtain light olefins and BTX. Volatiles escaping from the reactor are first cooled in a water-cooled heat exchanger, and then they go into a coldtrap, cooled by a cooling device, where the temperature goes below -10 °C. The gas stream is sampled every 30 min by collecting a 3 L sample in a gas bag. Liquids collected are removed from the traps every 15 min in order to avoid losses of liquid into the gas stream. The transference of liquids into the gas stream occurs by two ways: by vaporization of lighter compounds and by atomization of the entire liquid as microdroplets. This last way should be mainly avoided since it is the mechanism for transferring heavy metals from the liquids into the gas stream. It was found that microdroplets liquids escaped from the condensed fraction, and they were incorporated into the gazeous fraction. Furthermore, gases contained a high quantity of C5, C6, benzene, and toluene, which should have been kept as liquid fraction. These compounds were analyzed and quantified in the gas samples, but they were accounted for as liquid fraction. At each run, MWO was injected during 2 h in order to accumulate in the char bed a quantity of metals high enough to be accurately determined. After the pyrolysis run, the coal char bed was removed from the reactor, weighed, and analyzed. The pyrolytic char formed from MWO during pyrolysis was calculated by difference from the coal-char bed. The liquids collected in the heat exchanger and the cold trap were mixed, weighed, and analyzed together as one sample. The individual gas compounds yields were calculated from the analysis of each one in the 30 min gas samples, which remained practically unchanged, and the gas volume produced during this period, which is measured by a gas meter at atmospheric pressure. Finally, the total gas yield was calculated as the sum of the individual components. A global mass balances over the mass of MWO injected, the pyrolytic char produced, the over-all liquid fraction recovered, and the total gas yield was calculated for each run. Only those runs whose mass balances were better than 95% for the pyrolysis products were considered as valid. Each figure shown in the tables is the average of the results obtained
from two valid runs. Global results in tables were normalized to 100% in order to facilitate comparison between runs. Analytical Methods. Gases were analyzed by GC using two separate analytical methods: an alumina column with helium as carrier and FID detection for C1-C6 hydrocarbons and two packed columns, molecular sieve 13X and Porapak with helium as carrier and TCD detector for permanent gases. Liquids were analyzed by GC/MS. All compounds present in liquids were identified by using a computerized library of mass spectra. Tri- and tetramethylbenzene were accounted for as alkylbenzenes. All compounds chromatographied between naphthalene and phenanthrene were accounted for as 2,3 rings. Quantitative composition of liquids was determined by a solution of external standards of the main components of the liquids. Metals Determination. Metals were analyzed by GraphiteFurnace-Atomic Absorption Spectrometry (GFAAS), using pyrolitic graphite tubes in all cases except for Ni. Metals were determined in a Thermo-Jarrell As video 11 atomic absorption spectrometer equipped with both a graphite furnace and flame, used for analysis of metals. Sample Preparation. One gram of each sample was accurately weighed and placed in a glass beaker with 5 mL of H2SO4 (96%) and 5 mL of HNO3 (60%) in the case of liquid samples (MWO and pyrolysis liquid products and coal char bed). Five milliliters of HCl (37.5%) and 5 mL of HNO3 (60%) were used in the case of solid samples (chars). A second step with 5 mL of H2SO4 (96%, w/w) and 5 mL of HNO3 (69%, v/v) was necessary for digestion of liquid samples. For chars, 10 mL of HClO4 (70%) was added to the acid solution after the treatment with HCl and HNO3, to get the total dissolution of the solid samples. Appropriate dilutions of the sample solutions were made when necessary. The solutions were directly injected into the graphite furnace of an atomic absorption spectrometer. The experimental conditions used in each case to measure the concentration of metals by GF-AAS are as follows. Dry step: 2 s at 150 °C for all the metals (except Ni which was at 180 °C) and then purge for 1 s. Pyrolysis (first step): 350 °C for Pb (15 s), Cu (25 s), and Cd (25 s); 500 °C for V (15 s); 550 °C for Cr (30 s) and 600 °C for Ni (15 s). Then purge for 2 s in all cases. Pyrolysis (second step): 400 °C for Cd (10 s); 600 °C for Pb (20 s); 750 °C for Cu (25 s) and V (15 s); 800 °C for Ni (15 s) and 900 °C for Cr (30 s). Then purge 1 s for Pb, Cu, Cd and 2 s for Cr, V, and Ni. Atomization: at 1600 °C for Cd, 1800 °C for Pb and Cu, 2100 °C for Cr, 2250 °C for Ni, and 2400 °C for V. Hold 4 s at the mentioned temperature in all cases except for V (1 s). Cleanup: at 2000 °C for Cd, 2100 °C for Pb, 2200 °C for Cu, and 2400 °C for Cr, V, and Ni. Hold at this temperature for 2 s (4 s for V) and then purge 3 s (1 s for V). Smith-Hieftje was used as background corrector in all cases except for Pb where deuterium was used. Wavelengths used were as follows: Pb: 217.0 nm; Cu: 324.7 nm; V: 318.5 nm; Cr: 357.9 nm; Cd: 228.8 nm; and Ni: 232.0 nm.
Results and Discussion Table 1 shows the global yields of char, liquids (>C4), and gases (C4) gases ( 3 rings
625 0.1 4.7 50.6 44.7 0.4 0.3 0.5 6.4 4.3 9.1 1.1 10.7 0.6 8.7 0.2 2.4 3.7 6.0 5.0 6.2 4.5 1.0 10.9 0.3 8.0 0.1 0.9
700 0.1 7.5 43.7 48.8 0.5 0.4 0.3 9.1 4.6 10.4 0.8 12.1 0.2 7.0
700 1 9.5 34.5 56.0 0.7 0.3 0.2 17.7 10.5 9.4 2.4 10.2 0.4 3.2 0.3 0.7 0.1 0.8 11.7 5.5 2.1 0.5 4.3 1.4 5.0 0.7 0.5
3.1 1.9 5.6 9.8 5.4 2.8 0.5 6.2 1.6 5.7 0.4 1.1
a Basic: 100 g of WLO. b C and C are accounted in the liquid yield 5 6 although there are recovered in the gaseous fraction.
Experimental yields of some interesting gas compounds, such as ethylene and propylene, are very high even at the lowest temperatures tested. The increase of the gas yield when temperature increases is due mainly to a higher production of methane, ethylene, and propylene. The increase observed for ethylene and propylene is of special interest, considering the high petrochemical value of these compounds. Methane and hydrogen yields increase in detriment of olefins C2-C4 when pressure increases. The results obtained in the fluidized bed unit agrees with those obtained in the bench scale unit in terms of the total amount of liquid, solid, and char. However, concerning individual gases, it can be seen that more ethylene and propylene is obtained in the fluidized bed and less C6 compounds which suggest that cracking reactions are enhanced in the fluidized bed. Pressure influence is similar to that of temperature, so that, the trend of diminishing the size of produced species is favored by pressure. The concentration of BTX in liquids is very high, so that although the liquid yield decreases as pressure increases, BTX yield increases. The fraction transformed into gases and liquids is similar to that found at a lower temperature and a lower pressure, so that char yield is in all cases lower than 10%. Concerning the valuable compounds obtained from the pyrolysis (10), it should be noticed the high amount of ethylene, propylene, butadiene, benzene, toluene, and xylene obtained since these compounds belong to the basic organic chemical building blocks. Considering just the sum of these compounds it is observed that pyrolysis 2 provides a higher amount of them than pyrolysis 1. However, when benzene derivatives are also considered, the total sum of valuable compounds is similar at pyrolysis 1 and pyrolysis 2. That means that temperature does not affect the total amount of valuable compounds obtained. The influence of pressure is significant and negatively affects the production of the 10 VOL. 34, NO. 15, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
3207
FIGURE 2. Concentration in µg/g of each metal in the original waste oil and the liquid valuable products obtained from the pyrolysis. Run identification as in Table 1.
FIGURE 3. Concentration in µg/g of each metal in chars before and after the pyrolysis process. Run identification as in Table 1. valuable compounds considered. The data show that pyrolysis of waste mineral oil in a char bed is an attractive process that produces valuable products, such as light olefins, BTX, and naphtnalene derivatives. Among the three set of pyrolytic conditions studied, 625 °C and 0.1 MPa seems to be the most promising one because the gas fraction is the lowest and the liquid yield is the highest one. Moreover, the gas fraction contains high quantity of light ethylene and propylene. When the temperature and pressure increase, 3208
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 15, 2000
the yield of methane increases in detriment of liquid yield. Distribution of Metals. All the mentioned fractions (waste oils, chars, and valuable liquid products) were digested as shown in the Experimental Section and analyzed by GFAAS. The results obtained are shown in Figures 2 and 3 where the data from liquid fraction and char respectively are plotted. A different performance can be observed versus pyrolysis conditions as shown in Figure 2. The first conclusion to point
TABLE 2. Total Mass (µG) of Each Metal in the Original Waste Oil and Chars and Final Liquid and Char metals Cd Ni Pb Cu Cr V
waste oil original char waste oil original char waste oil original char waste oil original char waste oil original char waste oil original char
pyr 1
pyr 2
pyr 3
166 2375 1907 21280 33338 902 38217 4370 6300 73245 15840 33630
183 2375 2102 21280 36743 902 42135 4370 6946 73245 17367 33630
163 2375 1875 21280 32767 902 37576 4370 6194 73245 15566 33630
out is that Ni and Cd are slightly affected by the pyrolysis conditions of temperature and pressure, whereas Pb, V, Cr, and Cu are strongly influenced by these conditions. Figure 3 shows the ability of char to trap the heavy metals. Pb and Cu are mainly concentrated on the char, and this concentration depends on temperature. From pyrolysis 1 to pyrolysis 2, the temperature increases at the same pressure, but the char takes up a higher amount of these metals at the lower temperature. This fact could be attributed to an adsorptiondesorption effect which explains the release of metals when the temperature increases. However, if only an adsorption process occurred, the behavior of Pb and Cu should be similar to Ni, Cr, V, and Cd in char, which is not true. Previous studies (7) coprocessing different coals and automotion waste oils under H2 showed that the removal of heavy metals from the oil may involve a chemical reaction with the coal as well as a physical effect to incorporate the heavy metals into the char. The same authors demonstrated, using electron probe microanalysis (EPMA) (11), that the sample areas with high content of heavy metals also had a high concentration of sulfur. This suggests that metals are likely fixed as sulfides, and consequently chars with a high content of sulfur, as is the case of the Samca char, would enhance the trapping ability for metals. According to these studies, the differences of performance between the metals and the pyrolysis conditions above-mentioned could be explained on the basis of the chemical reaction. So, lead and copper sulfides have lower solubility constants (thermodynamic constants) than Ni, whereas Cr and V do not form insoluble sulfides, which agrees with the data. The behavior of V in the char suggests a different chemical reaction. Comparing the data at different pressure (pyrolysis 2 and pyrolysis 3), Pb and Cd are more retained in char at higher pressure, while the behavior of Cr and V is the opposite. The analysis of metals in the final liquid fraction containing the valuable products (Figure 2) agrees with the results of chars, and consequently the liquid fraction has a lower concentration of Pb, Cu, and V, which are mainly trapped on the chars. Again Ni behaves very homogeneous versus pyrolysis conditions as well as Cd. To account for the balance of total mass of metals, the amount of waste oil fed to the pyrolysis unit has to be considered as well as the content of each metal under study in the char together with the concentration of metals in the final fractions. Table 2 shows the data obtained. As can be seen, most of lead and copper are trapped on the char, whereas the other metals are distributed into the liquid, char, and gaseous fractions. Also the losses of lead in the gaseous fraction is very low at 625 °C which agrees with the hypothesis of chemical interaction with the mineral matter of char which was mentioned above. However, when temperature increases
final liquid final char final liquid final char final liquid final char final liquid final char final liquid final char final liquid final char
pyr 1
pyr 2
pyr 3
146 1373 2181 15790 1007 29914 797 31189 2894 34034 1510 34034
132 1455 1039 17767 599 10909 679 15273 1918 23280 839 50495
51 2014 562 17367 1097 20523 1378 16303 1518 15832 309 37593
over 700 °C, considerable losses of Pb in the gaseous fraction were found. On the other hand, a considerable amount of Cr is lost in the balance, probably as gaseous fraction. This can be attributed to the high volatility (low boiling point) of the compound CrO2Cl2 (15), likely present in the waste oil, as showed previously. Also the boiling point of Cd, as metal or even as CdCl2, is lower than 700 °C (15), and consequently the loss of Cd in the gaseous fraction is not surprising. According to the obtained data, most of the lead escapes as gaseous fraction as well as Cu and even a high amount of Cd when the temperature reaches 700 °C (pyr 2). This volatility could be explained by the high content of chlorine in the waste oil, which easily forms volatile chlorine compounds with Cu and others metals (12). These results confirm the theory proposed by Linak (13) and agree with the previous work (12) about the temperature influence on some combustion processes even though combustion and pyrolysis conditions are very different. The volatility of each metal compound is very different, and Pb as PbCl2, Cu as CuCl2, and Cd as Cd or CdCl2 have volatilization temperatures considerably lower than 700 °C (15). Also in this case, the most appropriate experimental conditions are those of pyrolysis 1, since the emission of metals in the vapor phase is lower. Ni is nearly constant versus temperature, as in the chars or in the liquid fractions which agrees with the previous work (14). These results confirm that the most appropriate valuable products would correspond to those obtained applying pyrolysis 1, which means at 625 °C and 0.1 MPa of pressure. In consequence, the recommended pyrolysis conditions are those of pyrolysis 1, since this provides the highest valuable liquid fraction with the least concentration of metals.
Acknowledgments The financial support for this work was obtained from the European Commission, Agreement no. 7220/EC-763 and the Diputacio´n General de Arago´n: CONSI+D PMA-89/97.
Literature Cited (1) Gulyurtlu, Y.; Lopes, H.; Cabrita, Y. Fuel 1996, 75, 940. (2) Brinkman, D. W.; Dickson, J. R. Environ. Sci. Technol. 1995, 29, 81. (3) Brinkman, D. W.; Dickson, J: R.; Wilkinson, D. Environ. Sci. Technol. 1995, 29, 87. (4) Sanjay, H. G.; Tarrer, A. R.; Marks, C. Energy Fuels 1994, 8, 99. (5) Milller, T. J.; Panvelker, S. V.; Wender, Y.; Shah, Y. T.; Huffman, G. P. Fuel Process. Technol. 1989, 23, 23. (6) Skala, D. U.; Saban, M. D. Orlovic, M.; Meyn, V. W., Severin, D. K.; Rahimian, I. G. H.; Marjanovic, M. V. Ind. Eng. Chem. Res. 1991, 30, 0, 2059. (7) Orr, E. C.; Shi Y.; Shao L.; Liang J.; Ding W.; Anderson L. L.; Eyring E. M. Fuel Processing Technol. 1996, 49, 233. (8) Moliner, R.; La´zaro, M.; Suelves, I. Energy Fuels 1997, 11, 1165. VOL. 34, NO. 15, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
3209
(9) Moliner, R.; Suelves I.; La´zaro M. J. Energy Fuels 1998, 12, 963. (10) Song, C.; Schobert, H. H. Fuel 1996, 75, 724. (11) Orr, E. C.; Tuntawizoon, W.; Anderson, L. L.; Eyring, E. M. Prepr. Pap. Am. Chem. Soc., Div. Fuel Chem. 1994, 39(4), 1065. (12) Nerı´n, C.; Domen ˜ o, C.; Garcı´a, J. I.; del Alamo, A. Chemosphere 1999, 38, 1533. (13) Linak, W. P.; Wendt, J. L. Prog. Energy Combust. Sci. 1993, 19, 145.
3210
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 15, 2000
(14) Nerı´n, C.; Domen ˜ o, C.; del Alamo, A.; Echarri, Y. The Analyst 1996, 121, 1731. (15) Lide, D. R. Handbook of Chemistry and Physics; CRC Press: 1992.
Received for review May 13, 1999. Revised manuscript received April 12, 2000. Accepted April 25, 2000. ES9905546
