Ind. Eng. Chem. Res.

2010, 49, 10845–10851 10845

Pyrolysis Using Microwave Heating: A Sustainable Process for Recycling Used

Car Engine Oil
Su Shiung Lam,*,†,‡ Alan D. Russell,† and Howard A. Chase†
Department of Chemical Engineering and Biotechnology, UniVersity of Cambridge, New Museums Site,
Pembroke Street, Cambridge CB2 3RA, United Kingdom, and Department of Engineering Science, UniVersity
Malaysia Terengganu, 21030 Kuala Terengganu, Terengganu, Malaysia

We demonstrate the applicability of pyrolysis using microwave heating to recycle used car engine oil. Waste
oil was thermally cracked in an inert microwave-heated bed of particulate carbon from which oxygen was
excluded, and the relationship between temperature, the chemical composition of the hydrocarbons, and the
metal fraction produced was determined. A reaction temperature of 600 °C provided the greatest yield of
commercially valuable products: the recovered liquid oils were composed of light paraffins and aromatic
hydrocarbons that could be used as industrial feedstock; the remaining incondensable gases comprised light
hydrocarbons that could potentially be used as a fuel source to power the process. In addition, the recovered
liquid oils showed a significant reduction in the metal contaminants accumulated throughout their use cycle:
a 93-97% reduction in Cu, Ni, Pb, Zn, Fe; a 46% reduction in Cd; and a 32% reduction in Cr. Our results
indicate that microwave pyrolysis shows exceptional promise as a means for recycling and treating problematic
waste oil.

Introduction cracked in the absence of oxygen into shorter hydrocarbon

chains. The resulting gaseous products are subsequently recon-
Waste car engine oil (WO) is an environmentally hazardous, densed into liquid oils of different compositions depending on
high-volume waste that has become a major concern for modern the characteristics of the input substances and reaction conditions.
society. WO is difficult to dispose of due to the presence of It is well-known that microwave heating offers a reliable,
undesirable species such as soot, metals from the wear of engine low cost, powerful heat source, with modern equipment operat-
blocks, polycyclic aromatic hydrocarbons (PAHs), and impuri- ing at over 90% efficiency. The diffuse nature of the electro-
ties from additives such as chlorinated paraffins. On a global magnetic field allows microwave heating to evenly heat many
basis, nearly 24 million metric tonnes (Mt) of WO are generated substances in bulk, without relying on slower and less efficient
each year.1 In particular, nearly 7.6 Mt are produced in the conductive or convective techniques. Thus, the use of microwave
United States, in addition to the approximately 2.2 Mt produced radiation as a heat source offers an even heat distribution and
in the European Union.2 excellent heat transfer and allows exceptional control over the
Incineration and combustion are currently the most common heating process. Furthermore, energy is targeted only to
existing disposal processes for WO, though vacuum distillation microwave receptive materials and not to air or containers within
and hydro-treatment have been investigated as methods to the heating chamber. In addition, the use of carbon as the
recycle this waste.3,4 These existing treatment processes are microwave receptor offers a number of advantages over
becoming increasingly impracticable as concerns over environ- conventional pyrolysis techniques. Its high microwave absor-
mental pollution and additional sludge disposal are recognized bency allows for rapid heatingsthe thermal energy is then
due to contaminants present in WO.5 Alternatively, pyrolysis transferred to the target material via conduction. Short heat
techniques have been considered as a treatment process for waste transfer distances, the enveloping nature of the well-mixed
oil, with the resulting products capable of being used as carbon bed, and small particle size (with corresponding large
industrial feedstock and the char produced capable of being used surface area) make this an efficient method of heat transfer.
as a substitute for activated carbon. Much literature has been Using carbon as a reaction bed also provides a highly reducing
reported on the pyrolysis of wastes of a hydrocarbon nature, environment, which decreases the formation of undesirable
such as used tires,6–8 plastic waste,9–11 and municipal solid oxidized species. These factors have the potential to increase
waste,12 demonstrating pyrolysis as a promising process to treat yields of desirable pyrolysis products, which can be further
various types of waste. Although several studies have revealed treated and used as valuable industrial feedstock. This pyrolysis
the potential of pyrolysis as a disposal method for waste using microwave heating shows considerable potential for
oil,1,2,13–15 the use of this technology is not widespread at the recycling otherwise difficult to dispose of waste.
present time. While studies have been conducted using microwave heating
Pyrolysis using microwave heating is a relatively new process to pyrolyse other wastes,16–19 we believe that this is the first
that was initially developed by Tech-En Ltd. in Hainault, United application of this technology to the treatment and recycling of
Kingdom.9 In this process, waste hydrocarbons are mixed with WO; this paper seeks to demonstrate the technical feasibility
a highly microwave-absorbent material such as particulate and applicability of this process.
carbon; as a result of microwave heating, they are then thermally

* To whom correspondence should be addressed. Tel.: +44(0)1223(3) Experimental Section

30132. Fax: +44(0)1223334796. E-mail: ssl28@cam.ac.uk.
†
University of Cambridge. Apparatus. The experimental apparatus developed and used
‡
University Malaysia Terengganu. during this investigation is shown in Figure 1. It consists of a
10.1021/ie100458f  2010 American Chemical Society
Published on Web 06/18/2010
10846 Ind. Eng. Chem. Res., Vol. 49, No. 21, 2010

Figure 1. Schematic layout of bench-scale microwave-induced pyrolysis system.

modified catering microwave oven [1] operating at a frequency Materials and Methods. Shell 10W/40 highly refined base
of 2.45 GHz with a maximum power output of 5 kW. The oven oil was used throughout the experiments. The WO was collected
has four magnetrons, each of which is controlled by a separate from the engine of an MG-ZT diesel car driven for ap-
switch such that the power output can be controlled to 25, 50, proximately 23 000 km. Before pyrolysis, the oil samples were
or 75% of the maximum, with a continuous generation of filtered such that the size of any remaining particulates was less
microwaves rather than with on/off cycles. The reactor [2] is a than 100 µm. Volatiles and water were eliminated by heating
bell-shaped quartz vessel measuring 180 × 180 × 180 mm, at 110 °C; samples were examined for hydrocarbon and metal
which is seated inside the microwave oven. This vessel is placed composition by gas chromatography-mass spectrometry (GC-
in a molded base made of a microwave-transparent heat MS) and atomic absorption spectrometry (AAS), respectively.
insulating material (VF1500AK prefired, M.H. Detrick, Mokena, Scanning electron microscopy/energy dispersive X-ray scans
IL). The reactor has an agitation system that consists of an (SEM/EDX) were also performed on samples to investigate the
impeller with two 45° pitched blades, a 11-mm-diameter stain- size, morphology, and presence of metals on particles present
less steel shaft, and a motor [3] operating at 6 rpm. The physical in WO.
mixing resulting from this agitation system ensures a uniform Particulate carbon (TIMREX FC250 Coke, TIMCAL Ltd.,
temperature throughout the reactor. Bodio, Switzerland) was used as a microwave absorbent to heat
The temperature of the carbon load in the system is monitored the WO; this was preheated to 800 °C for 50 min to remove
using two thermocouples: one is ducted into the carbon through any water and sulfur-containing compounds. The specifications
the center of the shaft that protrudes from the bottom of the of the carbon are presented in Table S1 of the Supporting
stainless steel stirrer shaft; the other enters the reaction chamber Information.
through a side port on the top of the reactor and stays at the top Experimental Procedure. A total of 1 kg of carbon was
layer of the carbon bed. Both thermocouples are in direct contact placed into the quartz reactor. The apparatus was assembled as
with the carbon inside the reactor. The thermocouples are in Figure 1, and N2 gas was vented through the apparatus at a
connected via a data acquisition card (DT302, Data Translation, flow rate of 0.2 L/min. A complete purge of all air within the
Marlboro, MA) to a computer that runs a control program apparatus was ensured by flushing the system with N2 for at
developed in the VEE package (Agilent Technologies, Palo Alto, least 10 min before heating commenced. The bed of carbon
CA). This software reads the temperature at a rate of 100 Hz, particles was stirred by the agitator at 6 rpm. The carbon was
averages the readings, and sends on/off commands back to the heated to temperatures ranging from 250 to 700 °C and
magnetrons to maintain the desired temperature. The reactor maintained within 1% of the target temperature by computer
temperature is logged for subsequent analysis. control. Once the target temperature was attained, the reactor
The reactor is continuously fed with waste oil using the was left for 5 min to ensure complete temperature equilibration.
injection vessel [4] connected with a variable-speed peristaltic The oil sample was then injected into the reactor at a constant
pump (Masterflex 07518-00, Cole-Parmer, Vernon Hills, Illinois) feeding rate of about 6 g/min (i.e., 7 mL/min) over a period of
and flexible fuel-resistant tubing. Valves permit inert nitrogen about 1 h; it was previously ascertained that the magnetron
gas (N2) to purge the incoming material of oxygen to avoid system of the microwave oven was able to generate sufficient
any combustion occurring in the reactor. The flow rate of the heat to maintain the target temperature at this flow rate. The
purging gas is monitored using a rotameter. The pyrolysis noncondensable gas stream was sampled after the cotton wool
products leave the reactor and pass through a system of three filter (Figure 1) into 10 L gas collection bags for later analysis.
water-cooled Liebig condensers [5, 6, 7], which collect con- When the accumulation of liquid product had stopped and
densed hydrocarbons in main and secondary collection flasks further evolution of vapor phase products was no longer
[8, 9]. The pyrolysis gases then flow through a cold trap [10] observed in the system, the reactor was visually inspected to
comprising a collection flask maintained at -78 °C using a ensure that the reaction was fully completed. This was deemed
slurry of dry ice/acetone; the remaining noncondensable gases to be the case if no oil sample remained in the reactor and the
are passed through a cotton wool filter [11] to collect any carbon appeared dry with no remaining “sticky” texture. Once
aerosols present before being vented from the system. the reaction had finished, the microwave generator was switched
 Ind. Eng. Chem. Res., Vol. 49, No. 21, 2010 10847
off and the reactor cooled with the aid of a fan. The N2 flow
was continued until the temperature of the reactor had fallen to
80 °C. The reactor was then disconnected from the condensation
system and sealed to prevent contact of the carbon bed with
air.
The amount of residue material was determined by measure-
ment of the weight change in the reactor and its contents before
and after the reaction. The yield of liquid product was
determined by measuring the weight increase in the collecting
vessels and filter. The gas yield was determined by mass balance
and assumes that whatever mass of added sample that is not
accounted for by the residue and liquid product measurements
left the system in gaseous form. The data recorded is the average
of the results obtained from three valid repeated runs performed
under identical conditions. The pyrolysis products were analyzed
by GC-MS, AAS, and SEM/EDX to identify their chemical Figure 2. Product yields (wt %) from microwave pyrolysis of waste oil as
a function of the temperature.
composition.
Analytical Methods. Samples of the liquid products were
transferred into 1 mL vials without the use of solvents and paraffins with carbon chain lengths higher than C24 (i.e., 85.8%)
analyzed using a 6890/5973 GC-MS instrument (Agilent are the predominant hydrocarbon structures in the composition
Technologies, Palo Alto, CA). Before injection, the 1 mL of WO; similar results were obtained by Gómez-Rico et al.1 It
samples were heated to 80 °C to ensure complete liquification; is thought that some of the heavier hydrocarbons originally
2 µL injections were performed using a 10 µL syringe that was present in unused engine oil were converted to lighter hydro-
also heated to 80 °C. The GC-MS was operated in nonisothermal carbons (<C24) due to the high temperature and pressures
mode, ramping from 30 to 325 °C using a HP-5MS 30 m fused experienced while acting as an engine lubricant. The aromatic
silica capillary column (cross-linked 5% PH ME Siloxane, I.D. compounds, representing 1.3% of the hydrocarbons present in
0.25 mm, film thickness 0.25 µm). Gas samples were analyzed WO, consist of benzene and naphthalene derivatives.
in isothermal mode at 40 °C using an HP-5 60 m column (cross- SEM/EDX scans of the particulate matter present in the WO
linked 5% PH ME Siloxane, I.D. 0.25 mm, film thickness 1.0 revealed that most of the particles in the oil had a nearly
µm). The carrier gas used was helium with a constant flow rate spherical shape and porous structure with a size less than 10
of 1.1 mL/min. µm, while some particles were present in a smaller size (e1
The total ion chromatogram produced for each sample was µm) and much less regular in shape (see Figure S2, Supporting
analyzed using Agilent ChemStation analysis software and the Information). The EDX spectrum showed the presence of Si,
Wiley library of mass spectra (sixth edition). The chromatograph P, S, Cl, Ca, Cr, Fe, Ni, Cu, Zn, and Pb in the particulate matter
integrator was programmed in two different modes, allowing in the WO (see Figure S2, Supporting Information); similar
the quantification of compounds by both species and size. In findings have been reported by several authors.21–23 These
this way, a single GC-MS analysis permitted the identification elements are likely to arise from the additives (e.g., dissolved
of the products and the classification of the sample by chain organo-metallic compounds, chlorines, metallic chlorides) in the
length. The GC-MS was not calibrated for the individual original car engine oil, and from the wear and abrasion of the
compounds in the samples; hence, the compounds are quantified engine block during the lubrication process. Cd, an element
as total ion content percentage (TIC%)san integration of the thought likely to be present in WO, was not detected by EDX.
peaks present within the chromatogram. While this should not Overall, the SEM/EDX analysis demonstrated that WO contains
be confused with a true weight percentage (wt %), this figure a fine dispersion of particulate matter (<10 µm) that contains
still gives a good approximation of the composition of the metalloid, nonmetals, halogen, and metals.
sample.21 Product Yield. Figure 2 shows the product yields of WO
Metal Analysis. Samples of untreated waste oil, liquid pyrolysis obtained at different pyrolysis temperatures. Data are
products, and carbon (before and after pyrolysis) were analyzed not recorded for temperatures of 350 °C and below, as only a
using a Varian Spectr AAS instrument (Agilent Technologies, small amount of gas was produced and no liquid products were
Palo Alto, CA). In sample preparation, 1 g samples were observed after 1 h of reaction time. Reaction times varied
weighed and heated, then 5 mL of HNO3 (60% v/v) and 5 mL between 3 and 22 min at the other temperatures considered.
of H2SO4 (96% w/v) were added twice to completely digest At temperatures between 400 and 600 °C, the liquid yield
and extract metal ions from the samples. Solid samples were was found to increase with increasing temperature. The high
digested first with 5 mL of HCl (37.5% w/v) and 5 mL of HNO3 amount of residue observed at temperatures e500 °C indicates
(60% v/v), followed by 10 mL of HClO4 (70% w/v). The final that incomplete pyrolytic decomposition occurred at these
colorless solutions were then diluted accordingly and injected temperatures; this material is a mixture of residual unpyrolyzed
into the furnace of the AAS for determination of the metal WO and char. At higher pyrolysis temperatures (g500 °C), the
content. Experimental analysis of each metal was performed amount of residual material remaining within the reactor
according to Lázaro et al.;20 the samples were analyzed for their decreased dramatically as more complete pyrolysis of the WO
content of Cd, Cr, Cu, Ni, Pb, Zn, and Fe. occurred, allowing the pyrolysis products from the WO to
vaporize, leave the reactor, and recondense into a liquid product,
accounting for the greater quantities of liquid product collected.
Results and Discussion
A higher temperature should enhance the cracking of
Characteristics of Waste Car Engine Oil. WO was char- hydrocarbons into smaller molecules, resulting in a higher yield
acterized by GC-MS before being subjected to pyrolysis (see of the gaseous product. However, the opposite was observed in
Figure S1, Supporting Information). Linear and branched the gaseous product between 500 and 600 °C. Whether or not
10848 Ind. Eng. Chem. Res., Vol. 49, No. 21, 2010

Table 1. Yield Comparison between Conventional and Microwave on the surface of the carbon particles as a result of the
Heating microwave induction, which may influence the reaction pathway.
temperature residuea gasesb liquid: Product Chemical Composition. Table 2 outlines the effect
(°C) (wt %) (wt %) oils (wt %) reference of temperature on the chemical compounds obtained in the
600 7(1 17 ( 3 76 ( 3 microwave oven (this study) pyrolysis products. Temperature has a significant effect on both
5 35 60 electric oven24 the size of the molecules produced and the fraction of original
3 50 46 electric furnace27 waste oil converted to gas and liquid products.
700 18 ( 2 33 ( 1 49 ( 1 microwave oven (this study)
8 49 44 electric oven21 In the gaseous products, C2-C4 olefins were detected:
ethylene, propylene, butenes, and 1,3 butadiene. The yields of
a
Residue: Between 400 and 500 °C, they are considered unpyrolyzed
ethylene and propylene were found to be proportionally high
oils and char. At 550 °C and above, only chars are considered. b Gas
yields were calculated by mass balance of the product yields (i.e., gas compared to other gaseous compounds, even at the lowest
yield ) 100% of WO - (liquid yield + residue yield)). pyrolysis temperature; this observation is particularly interesting
considering the high chemical value of these two compounds.
the hydrocarbons simply evaporate or undergo cracking into The yields of these compounds together with methane were
shorter molecules, the volume increase accompanying the phase found to increase with increasing temperature; these three
change from liquid to gas creates a pressure build-up that drives compounds are believed to comprise the bulk of the increased
gas out of the reactor into the product collection system. At amount of gas produced at higher pyrolysis temperatures (see
higher temperatures, this process occurs more rapidly, and Figure 2). Overall, increasing pyrolysis temperature leads to a
molecules enter the gaseous state earlier, causing a more rapid higher production of light hydrocarbons in the gaseous products.
flow of gases out of the reaction “hot zone” where there is These light hydrocarbons represent a potentially high-value
sufficient thermal energy for the cracking process to occur. The chemical feedstock or fuel source. For example, the gaseous
decrease in residence time has a greater effect than the increase products from the microwave pyrolysis of WO could be added
in cracking that is expected at higher temperatures and helps to to a petroleum refinery feedstock for further processing and
explain the greater liquid yield and smaller gaseous yield. This upgrading. If refinement to higher value products were not
agrees with Kim and Kim,5 who noticed in their WO pyrolysis possible, these gases could be used as a fuel sourceseither to
study that higher yields of gaseous products were achieved when power the generation of electricity for the microwave pyrolysis
the primary volatiles remained in the high temperature reaction process itself or to be sold into the wider electrical grid. The
zone for a longer period of time, and has also been observed study also revealed that higher amounts of light olefins (i.e.,
during the pyrolysis of other materials such as plastic wastes.17 ethylene and propylene) and fewer C5 and C6 compounds were
Above 600 °C, greater quantities of gaseous products were obtained with our microwave pyrolysis process compared with
observed, indicating that at these temperatures, cracking resulting conventional pyrolysis (see Table 2), suggesting that cracking
from the greater thermal energy dominates over the shorter reactions are enhanced in microwave pyrolysis.
residence time of the oil in the reactor. The increase in thermal
energy enhances secondary cracking of primary volatiles, thus The liquid products were found to contain large quantities
enhancing the cleavage of larger chains present in the pyrolyzed of aliphatic and aromatic compounds, comprising alkanes,
vapors, leading to a higher corresponding yield of gaseous alkenes, and aromatics with carbon numbers ranging between
product. These results are in agreement with the findings of C2 and around C20. In particular, valuable light aromatics such
other workers.7,24 At these high temperatures, the residue as BTX and benzene derivatives comprised a significant portion
retained within the reactor is likely to be char, the amount of of the liquid products even at the lowest reaction temperature.
which increases with temperature as reported by Lázaro et al.21 In this study, the change in temperature from 400 to 700 °C
and is the result of increased rates of tertiary cracking reactions. resulted in a dramatic increase in the aromaticity of the liquid
The highest liquid yield (76 wt %) was observed at 600 °C. product from 31% to 52%; similar findings were reported by
It is thought that this represents the optimum balance between other workers.21,24 This was accompanied by a large decrease
a sufficiently high reaction temperature to produce condensable in aliphatic content from 56% to 29%. This disagrees with the
vapor and not being so high as to promote secondary or tertiary results reported in Sinağ et al.,15 who observed that the increase
cracking. Our results have shown that, except for the experi- in temperature has a different and less significant effect on the
ments conducted at e400 °C, a recovery of liquid product of product distribution: a reduction in aromatic content was
approximately 41-76% of the initial mass of WO is possible. reported, along with an increase in aliphatic content. Our
Table 1 compares our results to those of other researchers microwave heated pyrolysis produced many benzene derivatives,
conducted using conventional heating. Our microwave heated such as hexyl-benzene, propyl-benzene, and 1-methyl-2-propyl-
pyrolysis method shows an increased yield of desirable liquid benzene, as well as benzene rings with short alkyd groupssmainly
products compared to pyrolysis using conventional heating, toluene, ethyl-benzene, and xylene. The subsistent chains
especially at 600 °C. Possible explanations accounting for this attached to the benzene rings ranged from C1 to C6 groups and
difference include the presence of the carbon bed used in our tended to be nonbranched saturated compounds. Our results have
setup (in which the sample is totally immersed, providing shown that particulate carbon promotes the cracking of WO
excellent heat transfer, and also acts as a reducing reaction during microwave-induced pyrolysis, forming a unique product
environment) and the microwave heating process itself, which distribution of low carbon numbers and predominately alkenes,
has been shown to produce different products from conventional aromatics, and linear alkanes.
heating when all other factors are held equal.16,25,26 Mechanisms It is thought that the WO hydrocarbons used in this study
underlying this difference are beyond the scope of this paper undergo degradation by a free-radical-induced random scission
but include possibilities such as different heat distributions process,28 which would account for the alkane, alkene, and
(microwave heating volume heats only the carbon, creating a dialkenes observed for each carbon number across all collected
localized reaction zone as opposed to electric heating which is samples. Aromatics and other molecules are likely to be formed
externally applied and heats all substances including the by more complex reaction mechanisms and/or through secondary
surrounding gas) or the creation of an excess of free electrons cracking.29 It has been reported that aromatic formation in
 Ind. Eng. Chem. Res., Vol. 49, No. 21, 2010 10849
Table 2. Chemical Yields (TIC%) of Product Obtained in Microwave Pyrolysis of Waste Oil
product compound 400 °C 500 °C 600 °C 700 °C
gases methane 5.8 4.7 13.3 20.1
ethane n.d.a 2.3 2.4 6.0
ethylene (C2H4)b 11.5 14.7 33.9 36.3
propane 0.1 0.3 0.5 0.6
propylene (C3H6)b 14.6 16.9 28.5 29.6
butanes (C4H10) 5.5 3.9 0.3 n.d.
butenes (C3H8)b 15.9 15.0 11.2 2.1
1,3 butadiene (C4H6)b 10.1 7.7 5.6 0.7
literatureg
C2H4: e 31.5% for g600 °C21,24,27
C3H6: e 25.2% for g600 °C21,24,27
C5 and C6: g 15.4% for g600 °C21,24,27
liquids C5c 6.8 3.9 2.1 n.d.
C6c 9.9 7.5 4.7 0.4
linear hydrocarbonsd 55.7 53.2 37.5 29.1
benzene 4.6 5.1 11.5 17.8
toluene 5.8 6.1 12.4 18.6
xylene 7.1 7.2 9.3 4.2
alkylbenzenes e 13.5 17.9 18.6 9.3
BTX f 17.5 18.4 33.2 40.6
PAHs naphthalene n.d. 0.6 1.7 2.1
acenaphthene n.d. n.d. 0.2 0.3
phenanthrene n.d. 1 1.1 1.3
fluoranthene n.d. n.d. 0.1 0.1
pyrene n.d. 0.7 1.3 1.4
a
n.d.: Not detectable. b Olefins: alkenes, dialkenes. c C5 and C6 were detected in the gas fraction; however, they are included in the liquid yield, as
those compounds can be condensed. d Linear hydrocarbons: aliphatic hydrocarbons such as alkanes, alkenes, dialkenes, trialkenes. e Alkylbenzenes:
ethylbenzene, propylbenzene, methylethybenzene, and other single ring aromatics. f BTX: the sum of benzene, toluene, and xylene. g The data from
literature were recalculated as TIC% from wt %.

thermal pyrolysis is due to Diels-Alder type secondary reac- Table 3. Concentrations (µg/g)a of Each Metal in the Original
tions, which involve post pyrolysis cracking of alkenes and, Waste Oil and Liquid Products
less commonly, alkanes.30,31 original liquid pyrolysis product
It is widely accepted that higher pyrolysis temperatures are waste
metals oil, Mwo 400 °C 500 °C 600 °C 700 °C
associated with higher aromatic yields (which were also
observed during this study); however, not much literature has Cd 0.28 ( 0.03 0.15 ( 0.02 0.17 ( 0.01 0.18 ( 0.01 0.21 ( 0.03
Cr 2.2 ( 0.1 1.6 ( 0.1 1.5 ( 0.1 1.7 ( 0.1 1.7 ( 0.1
been published regarding other factors affecting the aromatic Cu 13.0 ( 2.0 0.4 ( 0.01 0.4 ( 0.01 1.3 ( 0.03 2.2 ( 0.02
compositions from pyrolysis. Sinağ et al.15 reported recently Ni 56.0 ( 2.0 2.5 ( 0.1 2.6 ( 0.2 2.8 ( 0.1 2.9 ( 0.1
that catalysts hydrogenate and decrease the aromatics produced Pb 75.5 ( 0.8 2.6 ( 0.2 3.3 ( 0.1 4.4 ( 0.1 8.9 ( 0.4
from waste oil pyrolysis. Ludlow-Palafox and Chase10 proposed Zn 780 ( 10 58 ( 3 103 ( 5 167 ( 2 241 ( 7
Fe 88.0 ( 3.0 5.4 ( 0.5 5.8 ( 0.4 7.2 ( 0.3 9.9 ( 0.2
that an increased residence time may promote secondary
reactions such as aromatic formation. The presence of particulate
a
Values are means ( SD of three valid runs (N ) 3).
carbon is thought to increase the product residence time in the
reaction “hot zone”; this may be a factor contributing to the to be relatively high compared to other metals. It is thought
large increase in aromatic formation observed during this study. that lead and iron are mostly produced from the wear of the
Overall, our results indicate that increasing temperature engine parts made of lead alloy and steel or cast iron (e.g., solder
decreases yields of alkyl derivatives while increasing the yields joints, engine cylinders, pistons), whereas zinc is from the
of aromatics in the liquid product. The main implication is that antiwear and antioxidant additives (i.e., 12-15 wt %) present
a very high temperature is not required to produce valuable in the engine oil.23 On the other hand, cadmium was detected
products with the microwave pyrolysis apparatus. The benefit in very low concentrations, which corroborates its absence in
of pyrolyzing WO at very high temperatures to increase the the EDX spectrum of the WO (see Figure S2, Supporting
yield of valuable aromatic compounds is negated by the Information). These results are in agreement with the findings
significant decrease in total liquid product under these conditions. of other workers.22,33
Some PAHs were found in the pyrolysis products, namely, A significant decrease in metal concentration was observed
naphthalene, acenaphthene, phenanthrene, fluoranthene, and in the liquid products compared with those in the original waste
pyrene; however, they only existed in minor quantities (i.e., oil. As shown in Table 3, Cu, Ni, Pb, Zn, and Fe were highly
e2.1%). The highly carcinogenic PAHs such as benzo(a)pyrene retained within the carbon bed of the pyrolysis process, while
were not detected. This is to be expected as they require their levels in the liquid products were significantly reduced.
pyrolysis temperatures of 850 °C and above before they are The levels of Cd and Cr only showed a small reduction due to
formed.2 On the whole, the PAHs detected in this study were the small initial quantities in the WO, and the possible formation
composed of alkylated two- and three-ring compounds, with of highly volatile Cd and Cr chlorides (see Table S2, Supporting
naphthalene the most dominant; similar results have been Information), which would be able to vaporize and distill over
obtained by other authors.2,32 into the liquid products at the pyrolysis temperatures used in
Metal Distribution. Table 3 presents the concentrations of this study (i.e., >400 °C).27
heavy metals both in the waste oil and the liquid pyrolysis It is believed that some metals (i.e., Cu, Fe, Ni, Pb) are present
products obtained at different pyrolysis temperatures. The as fine metallic particles in addition to metallic chlorides in the
concentrations of lead, iron, and zinc in the waste oil were found WO as a result of the wear of engine parts made of metal. These
10850 Ind. Eng. Chem. Res., Vol. 49, No. 21, 2010

Table 4. Concentrations (µg/g)a of Each Metal in the Carbon before and after Pyrolysis
carbon after pyrolysis, mf
metals original carbon, Mi 400 °C 500 °C 600 °C 700 °C
Cd 0.53 ( 0.01 0.56 ( 0.01 0.55 ( 0.01 0.53 ( 0.01 0.53 ( 0.01
Cr 3.7 ( 0.1 4.0 ( 0.1 4.0 ( 0.1 3.9 ( 0.1 3.9 ( 0.1
Cu 2.7 ( 0.3 12.0 ( 0.1 11.7 ( 0.1 10.3 ( 0.1 7.9 ( 0.1
Ni 9.9 ( 0.9 54.9 ( 0.3 49.4 ( 0.2 44.9 ( 0.1 43.0 ( 0.2
Pb 1.2 ( 0.1 69.9 ( 0.6 66.7 ( 0.4 62.8 ( 0.2 55.9 ( 0.3
Zn 4.5 ( 0.6 512 ( 4 461 ( 8 401 ( 6 366 ( 5
Fe 7.6 ( 0.2 84 ( 1 80 ( 1.0 67 ( 1.0 59 ( 1

retention in carbon (%)b

metals 400 °C 500 °C 600 °C 700 °C
Cd 11 7 0 0
Cr 14 14 9 9
Cu 72 69 58 40
Ni 80 71 63 59
Pb 91 87 82 72
Zn 65 59 51 46
Fe 87 82 68 58
a
Values are means ( SD of three valid runs (N ) 3). b Retentions of each metal on the carbon bed were calculated on the basis of Mi, Mf, and the
Mwo in Table 3, i.e. (Mf - Mi) × 100%/Mwo.

fine metallic particles originally present in the WO (see Figure atures, which leads to a lower metal concentration in the liquid
S2, Supporting Information) mix with the other small solid product; similar findings were reported by Lázaro et al.21 This
particles (e.g., carbon) produced during the pyrolysis process can be explained by adsorption-desorption effects that result
and form a fine dispersion of particulate matter inside the in a greater release of metals to the liquid product at higher
pyrolysis reactor; the small particles could be formed by three pyrolysis temperatures, as well as greater vaporization energy
mechanisms: coagulation, condensation, and nucleation.27,34 at higher temperatures. The higher retention efficiency at lower
These fine particles are carried from the pyrolysis chamber with temperatures (<550 °C) may also be due to residual unpyrolyzed
other gaseous products during the pyrolysis process. SEM/EDX waste oil remaining in the carbon bed.
scans of the particulate matter present in the liquid products The reduction of metal content in the liquid product is also
and in the cotton wool filter (i.e., the aerosol trap for noncon- very much dependent on the types of metal and the chemical
densable gases shown in Figure 1) revealed the presence of these form of the metals in the WO, i.e., the chemical nature of the
fine particles (with a size less than 5 µm) as well as the metals metals and any metallic compounds formed in the pyrolysis
that escaped from the pyrolysis reactor (see Figures S3 and S4, process, and their interaction with the carbon bedseither
Supporting Information), suggesting that metals originally chemically adsorbing onto the carbon or becoming vaporized
present in the WO could be transferred and distributed into gas and escaping from the reactor. At higher pyrolysis temperatures,
and liquid products by two pathways during the pyrolysis a higher fraction of metals is likely to form volatile metal
process: a vaporization of the metals as metallic chlorides chlorides and volatile metallic particles according to their
depending on their volatility, as previously mentioned, and a associated lower volatilization temperatures (see Table S2,
convection of very fine metallic particles in the vapor stream Supporting Information) and distill over into the liquid products;
leaving the pyrolysis reactor. this explains the higher yields of metals in the liquid product
It should be noted that the majority of metals remain in the and lower metal retention efficiency of carbon as the temperature
particulate carbon bed in the reactor. is increased.
Table 4 demonstrates the concentration of heavy metals These results indicate that metals are mainly present in varied
contained in the carbon bed and its associated retention amounts as metallic particles, halides (i.e., chlorides), and
efficiency during pyrolysis over the temperature range of organo-metallic compounds depending on the composition of
400-700 °C. The carbon bed, acting as a sorbent, retains a WO. Owing to their different physical and chemical natures,
significant amount of Cu, Ni, Pb, Zn, and Fe (g65% at 550 these compounds evolve in different ways depending on
°C) and minimal amounts of Cd and Cr. The results suggest pyrolysis conditions (e.g., temperature): they may undergo
that the removal of metals can be explained by two mechanisms: chemical transformation into volatile metal chlorides, which then
first, a physical and chemical adsorption of these metals onto exit the pyrolysis reactor as vapor; they may leave the reactor
the carbon bed and, second, the retention of the metals in the as metallic particles in the very fine particulate matter that is
carbon bed due to there being insufficient vaporization or convected out of the reactor with the exiting gases; or they may
convective energy for them to leave the reaction chamber. The remain trapped in the particulate carbon in the reactor. The range
metal polluted carbon can be disposed of as solid waste or of different metals present could explain why not all the metals
regenerated using chemicals (i.e., acids). This metal contami- in the WO are retained in the reactor, recovered in the liquid
nated waste represents a 93% reduction in weight compared to phase, or released in the vapor phase. Nevertheless, the results
the original waste oil that would need to be disposed of. in this study indicate that only a small fraction of metals are
Overall, the results shown in Tables 3 and 4 demonstrate that distilled over into the liquid products. If complete decontamina-
the pyrolysis temperature has some influence on the quantity tion of the liquid products were required, either a hot cleaning
of metallic compounds retained in the carbon bed and the of the pyrolysis gases before liquid condensation or a demet-
fraction of each metal present in the original waste oil that is alization of the WO before addition to the reactor could be
transferred to the gas and liquid products. In general, the carbon performed. Future work will be undertaken to determine the
bed retains a higher amount of these metals at lower temper- applicability of low-cost catalysts or sorbents to ensure efficient
 Ind. Eng. Chem. Res., Vol. 49, No. 21, 2010 10851
removal of metals in the WO (e.g., installation of a unit (11) Conesa, J. A.; Font, R.; Marcilla, A.; Garcia, A. N. Pyrolysis of
containing a filter and solid catalyst sorbents to remove the polyethylene in a fluidized bed reactor. Energy Fuels 1994, 8, 1238–1246.
(12) Jiao, Z.; Jin, Y. Q.; Chi, Y.; Wen, J. M.; Jiang, X. G.; Jiang, N. M.
metals in pyrolysis gases before they pass through the condensa- Pyrolysis characteristics of organic components of municipal solid waste
tion system). at high heating rates. Waste Manage. 2009, 29, 1089–1094.
We have demonstrated the technical feasibility of using (13) Kim, S. S.; Chun, B. H.; Kim, S. H. Non-isothermal pyrolysis of
microwave-induced pyrolysis to produce significant quantities waste automobile lubricating oil in a stirred batch reactor. Chem. Eng. J.
of commercially valuable products from waste car engine oil, 2003, 93, 225–231.
(14) Balat, M. Diesel-like fuel obtained by catalytic pyrolysis of waste
such as light olefins and gaseous hydrocarbon and liquid engine oil. Energy Explor. Exploit. 2008, 26, 197–208.
hydrocarbon oils containing BTX and benzene derivatives; these (15) Sinağ, A.; Gülbay, S.; Uskan, B.; Uçar, S.; Özgürler, S. B.
products can be treated and used as either an energy source or Production and characterization of pyrolytic oils by pyrolysis of waste
industrial feedstock. In addition, our process concentrates the machinery oil. J. Hazard. Mater. 2010, 173, 420–426.
(16) Domı́nguez, A.; Fernández, Y.; Fidalgo, B.; Pis, J. J.; Menéndez,
toxic metal contaminants within waste engine oil into a solid J. A. Bio-syngas production with low concentrations of CO2 and CH4 from
carbonaceous char with 93% smaller mass than the original microwave-induced pyrolysis of wet and dried sewage sludge. Chemosphere
waste oil. 2008, 70, 397–403.
(17) Ludlow-Palafox, C.; Chase, H. A. Microwave pyrolysis of plastic
wastes. In Feedstock Recycling and Pyrolysis of Waste Plastics; Scheirs,
Conclusion J., Kaminsky, W., Eds.; John Wiley and Sons Ltd.: Chichester, U. K., 2006;
pp 569-594.
Our methodology indicated an optimum process temperature (18) Monsef-Mirzai, P.; Ravindran, M.; McWhinnie, W. R.; Burchill,
of 600 °C for waste oil pyrolysis based on the highest yield of P. Rapid microwave pyrolysis of coal. Methodology and examination of
valuable compounds, good reduction of metals in the liquid the residual and volatile phases. Fuel 1995, 74, 20–27.
product, and lower heat energy requirements at this temperature. (19) Wan, Y.; Chen, P.; Zhang, B.; Yang, C.; Liu, Y.; Lin, X.; Ruan,
It is evident that microwave-induced pyrolysis has huge potential R. Microwave-assisted pyrolysis of biomass: Catalysts to improve product
selectivity. J. Anal. Appl. Pyrolysis 2009, 86, 161–167.
as a means of recovering commercially valuable products from (20) Lázaro, M. J.; Moliner, R.; Domeño, C.; Nerı́n, C. Low-cost
problematic waste oil, in addition to diverting waste streams sorbents for demetalisation of waste oils via pyrolysis. J. Anal. Appl.
from current yet environmentally harmful disposal techniques Pyrolysis 2001, 57, 119–131.
such as landfilling and incineration. (21) Lázaro, M. J.; Moliner, R.; Suelves, I.; Nerı́n, C.; Domeño, C.
Valuable products from mineral waste oils containing heavy metals. EnViron.
Sci. Technol. 2000, 34, 3205–3210.
Acknowledgment (22) Nerin, C.; Domeno, C.; del Alamo, A.; Echarri, I. Fate of metals
in the combustion of industrial waste oils. Analyst 1996, 121, 1731–1736.
We thank BP plc (U.K.) and Enval Ltd. for the advice, (23) Audibert, F. Waste engine oils: rerefining and energy recoVery,
materials, and financial support of this project. Su Shiung Lam 1st ed.; Elsevier: Amsterdam, 2006; p 340.
acknowledges the prestigious King Scholarship offered by Public (24) Moliner, R.; Lázaro, M.; Suelves, I. Valorization of lube oil waste
by pyrolysis. Energy Fuels 1997, 11, 1165–1170.
Service Department of Malaysia Government and University
(25) Menéndez, J. A.; Domı́nguez, A.; Inguanzo, M.; Pis, J. J.
Malaysia Terengganu for the financial assistance provided. Microwave pyrolysis of sewage sludge: analysis of the gas fraction. J. Anal.
Appl. Pyrolysis 2004, 71, 657–667.
Supporting Information Available: Details of some methods (26) Fernández, Y.; Arenillas, A.; Bermúdez, J. M.; Menéndez, J. A.
and results. This material is available free of charge via the Comparative study of conventional and microwave-assisted pyrolysis, steam
Internet at http://pubs.acs.org. and dry reforming of glycerol for syngas production, using a carbonaceous
catalyst. J. Anal. Appl. Pyrolysis 2010, 88, 155–159.
(27) Nerı́n, C.; Domeño, C.; Moliner, R.; Lázaro, M. J.; Suelves, I.;
Literature Cited Valderrama, J. Behaviour of different industrial waste oils in a pyrolysis
process: metals distribution and valuable products. J. Anal. Appl. Pyrolysis
(1) Gómez-Rico, M. a. F.; Martı́n-Gullón, I.; Fullana, A.; Conesa, J. A.; 2000, 55, 171–183.
Font, R. Pyrolysis and combustion kinetics and emissions of waste lube (28) Wampler, T. Applied Pyrolysis Handbook, 1st ed.; CRC Press: Boca
oils. J. Anal. Appl. Pyrolysis 2003, 68-69, 527–546. Raton, FL, 1995; p 376.
(2) Fuentes, M. J.; Font, R.; Gómez-Rico, M. F.; Martı́n-Gullón, I. (29) Richter, H. Formation of polycyclic aromatic hydrocarbons and their
Pyrolysis and combustion of waste lubricant oil from diesel cars: decom- growth to soot s a review of chemical reaction pathways. Prog. Energy
position and pollutants. J. Anal. Appl. Pyrolysis 2007, 79, 215–226. Combust. Sci. 2000, 26, 565–60.
(3) Brinkman, D. W.; Dickson, J. R.; Wilkinson, D. Full-Scale Hy- (30) Depeyre, D.; Flicoteaux, C.; Chardaire, C. Pure n-hexedecane
drotreatment of Polychlorinated Biphenyls in the Presence of Used thermal steam cracking. Ind. Eng. Chem. Process Des. DeV. 1985, 24, 1251–
Lubricating Oils. EnViron. Sci. Technol. 1995, 29, 87–91. 1258.
(4) Brinkman, D. W.; Dickson, J. R. Contaminants in used lubricating (31) Williams, P. T.; Williams, E. A. Fluidised bed pyrolysis of low
oils and their fate during distillation/hydrotreatment re-refining. EnViron. density polyethylene to produce petrochemical feedstocks. J. Anal. Appl.
Sci. Technol. 1995, 29, 81–86. Pyrolysis 1999, 107–126.
(5) Kim, S. S.; Kim, S. H. Pyrolysis kinetics of waste automobile (32) Wong, P. K.; Wang, J. The accumulation of polycyclic aromatic
lubricating oil. Fuel 2000, 79, 1943–1949. hydrocarbons in lubricating oil over time - a comparison of supercritical
(6) Islam, M. R.; Haniu, H.; Beg, M. R. A. Liquid fuels and chemicals fluid and liquid-liquid extraction methods. EnViron. Pollut. 2001, 112, 407–
from pyrolysis of motorcycle tire waste: product yields, compositions and 415.
related properties. Fuel 2008, 87, 3112–3122. (33) Used oil analysis and waste oil furnace emissions study EPA-456/
(7) Uçar, S.; Karagöz, S.; Yanik, J.; Saglam, M.; Yuksel, M. Copyrolysis R-95-001; Office of Air Quality Planning and Standards, U.S. EPA Control
of scrap tires with waste lubricant oil. Fuel Process. Technol. 2005, 87, Technology Center: Waterbury, VT, 1995.
53–58. (34) Linak, W. P.; Wendt, J. O. L. Toxic metal emissions from
(8) Yatsun, A. V.; Konovalov, P. N.; Konovalov, N. P. Gaseous products incineration: Mechanisms and control. Prog. Energy Combust. Sci. 1993,
of microwave pyrolysis of scrap tires. Solid Fuel Chem. 2008, 42, 187– 19, 145–185.
191.
(9) Scheirs, J.; Kaminsky, W. Feedstock recycling and pyrolysis of waste ReceiVed for reView March 1, 2010
plastics: conVerting waste plastics into diesel and other fuels; Wiley: ReVised manuscript receiVed May 29, 2010
Chichester, U. K., 2006; Vol. xxvii, pp 2-785. Accepted June 2, 2010
(10) Ludlow-Palafox, C.; Chase, H. A. Microwave-induced pyrolysis
of plastic wastes. Ind. Eng. Chem. Res. 2001, 40, 4749–4756. IE100458F