Ind. Eng. Chem. Res. 2010, 49, 10845–10851
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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 Waste car engine oil (WO) is an environmentally hazardous, high-volume waste that has become a major concern for modern society. WO is difficult to dispose of due to the presence of undesirable species such as soot, metals from the wear of engine blocks, polycyclic aromatic hydrocarbons (PAHs), and impurities from additives such as chlorinated paraffins. On a global basis, nearly 24 million metric tonnes (Mt) of WO are generated each year.1 In particular, nearly 7.6 Mt are produced in the United States, in addition to the approximately 2.2 Mt produced in the European Union.2 Incineration and combustion are currently the most common existing disposal processes for WO, though vacuum distillation and hydro-treatment have been investigated as methods to recycle this waste.3,4 These existing treatment processes are becoming increasingly impracticable as concerns over environmental pollution and additional sludge disposal are recognized due to contaminants present in WO.5 Alternatively, pyrolysis techniques have been considered as a treatment process for waste oil, with the resulting products capable of being used as industrial feedstock and the char produced capable of being used as a substitute for activated carbon. Much literature has been reported on the pyrolysis of wastes of a hydrocarbon nature, such as used tires,6–8 plastic waste,9–11 and municipal solid waste,12 demonstrating pyrolysis as a promising process to treat various types of waste. Although several studies have revealed the potential of pyrolysis as a disposal method for waste oil,1,2,13–15 the use of this technology is not widespread at the present time. Pyrolysis using microwave heating is a relatively new process that was initially developed by Tech-En Ltd. in Hainault, United Kingdom.9 In this process, waste hydrocarbons are mixed with a highly microwave-absorbent material such as particulate carbon; as a result of microwave heating, they are then thermally * To whom correspondence should be addressed. Tel.: +44(0)1223(3) 30132. Fax: +44(0)1223334796. E-mail:
[email protected]. † University of Cambridge. ‡ University Malaysia Terengganu.
cracked in the absence of oxygen into shorter hydrocarbon chains. The resulting gaseous products are subsequently recondensed into liquid oils of different compositions depending on the characteristics of the input substances and reaction conditions. It is well-known that microwave heating offers a reliable, low cost, powerful heat source, with modern equipment operating at over 90% efficiency. The diffuse nature of the electromagnetic field allows microwave heating to evenly heat many substances in bulk, without relying on slower and less efficient conductive or convective techniques. Thus, the use of microwave radiation as a heat source offers an even heat distribution and excellent heat transfer and allows exceptional control over the heating process. Furthermore, energy is targeted only to microwave receptive materials and not to air or containers within the heating chamber. In addition, the use of carbon as the microwave receptor offers a number of advantages over conventional pyrolysis techniques. Its high microwave absorbency allows for rapid heatingsthe thermal energy is then transferred to the target material via conduction. Short heat transfer distances, the enveloping nature of the well-mixed carbon bed, and small particle size (with corresponding large surface area) make this an efficient method of heat transfer. Using carbon as a reaction bed also provides a highly reducing environment, which decreases the formation of undesirable oxidized species. These factors have the potential to increase yields of desirable pyrolysis products, which can be further treated and used as valuable industrial feedstock. This pyrolysis using microwave heating shows considerable potential for recycling otherwise difficult to dispose of waste. While studies have been conducted using microwave heating to pyrolyse other wastes,16–19 we believe that this is the first application of this technology to the treatment and recycling of WO; this paper seeks to demonstrate the technical feasibility and applicability of this process. Experimental Section Apparatus. The experimental apparatus developed and used 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
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Figure 1. Schematic layout of bench-scale microwave-induced pyrolysis system.
modified catering microwave oven [1] operating at a frequency of 2.45 GHz with a maximum power output of 5 kW. The oven has four magnetrons, each of which is controlled by a separate switch such that the power output can be controlled to 25, 50, or 75% of the maximum, with a continuous generation of microwaves rather than with on/off cycles. The reactor [2] is a bell-shaped quartz vessel measuring 180 × 180 × 180 mm, which is seated inside the microwave oven. This vessel is placed in a molded base made of a microwave-transparent heat insulating material (VF1500AK prefired, M.H. Detrick, Mokena, IL). The reactor has an agitation system that consists of an impeller with two 45° pitched blades, a 11-mm-diameter stainless steel shaft, and a motor [3] operating at 6 rpm. The physical mixing resulting from this agitation system ensures a uniform temperature throughout the reactor. The temperature of the carbon load in the system is monitored using two thermocouples: one is ducted into the carbon through the center of the shaft that protrudes from the bottom of the stainless steel stirrer shaft; the other enters the reaction chamber through a side port on the top of the reactor and stays at the top layer of the carbon bed. Both thermocouples are in direct contact with the carbon inside the reactor. The thermocouples are connected via a data acquisition card (DT302, Data Translation, Marlboro, MA) to a computer that runs a control program developed in the VEE package (Agilent Technologies, Palo Alto, CA). This software reads the temperature at a rate of 100 Hz, averages the readings, and sends on/off commands back to the magnetrons to maintain the desired temperature. The reactor temperature is logged for subsequent analysis. The reactor is continuously fed with waste oil using the injection vessel [4] connected with a variable-speed peristaltic pump (Masterflex 07518-00, Cole-Parmer, Vernon Hills, Illinois) and flexible fuel-resistant tubing. Valves permit inert nitrogen gas (N2) to purge the incoming material of oxygen to avoid any combustion occurring in the reactor. The flow rate of the purging gas is monitored using a rotameter. The pyrolysis products leave the reactor and pass through a system of three water-cooled Liebig condensers [5, 6, 7], which collect condensed hydrocarbons in main and secondary collection flasks [8, 9]. The pyrolysis gases then flow through a cold trap [10] comprising a collection flask maintained at -78 °C using a slurry of dry ice/acetone; the remaining noncondensable gases are passed through a cotton wool filter [11] to collect any aerosols present before being vented from the system.
Materials and Methods. Shell 10W/40 highly refined base oil was used throughout the experiments. The WO was collected from the engine of an MG-ZT diesel car driven for approximately 23 000 km. Before pyrolysis, the oil samples were filtered such that the size of any remaining particulates was less than 100 µm. Volatiles and water were eliminated by heating at 110 °C; samples were examined for hydrocarbon and metal composition by gas chromatography-mass spectrometry (GCMS) and atomic absorption spectrometry (AAS), respectively. Scanning electron microscopy/energy dispersive X-ray scans (SEM/EDX) were also performed on samples to investigate the size, morphology, and presence of metals on particles present in WO. Particulate carbon (TIMREX FC250 Coke, TIMCAL Ltd., Bodio, Switzerland) was used as a microwave absorbent to heat the WO; this was preheated to 800 °C for 50 min to remove any water and sulfur-containing compounds. The specifications of the carbon are presented in Table S1 of the Supporting Information. Experimental Procedure. A total of 1 kg of carbon was placed into the quartz reactor. The apparatus was assembled as in Figure 1, and N2 gas was vented through the apparatus at a flow rate of 0.2 L/min. A complete purge of all air within the apparatus was ensured by flushing the system with N2 for at least 10 min before heating commenced. The bed of carbon particles was stirred by the agitator at 6 rpm. The carbon was heated to temperatures ranging from 250 to 700 °C and maintained within 1% of the target temperature by computer control. Once the target temperature was attained, the reactor was left for 5 min to ensure complete temperature equilibration. The oil sample was then injected into the reactor at a constant feeding rate of about 6 g/min (i.e., 7 mL/min) over a period of about 1 h; it was previously ascertained that the magnetron system of the microwave oven was able to generate sufficient heat to maintain the target temperature at this flow rate. The noncondensable gas stream was sampled after the cotton wool filter (Figure 1) into 10 L gas collection bags for later analysis. When the accumulation of liquid product had stopped and further evolution of vapor phase products was no longer observed in the system, the reactor was visually inspected to ensure that the reaction was fully completed. This was deemed to be the case if no oil sample remained in the reactor and the carbon appeared dry with no remaining “sticky” texture. Once the reaction had finished, the microwave generator was switched
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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 measurement 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 composition. Analytical Methods. Samples of the liquid products were transferred into 1 mL vials without the use of solvents and analyzed using a 6890/5973 GC-MS instrument (Agilent Technologies, Palo Alto, CA). Before injection, the 1 mL samples were heated to 80 °C to ensure complete liquification; 2 µL injections were performed using a 10 µL syringe that was also heated to 80 °C. The GC-MS was operated in nonisothermal mode, ramping from 30 to 325 °C using a HP-5MS 30 m fused silica capillary column (cross-linked 5% PH ME Siloxane, I.D. 0.25 mm, film thickness 0.25 µm). Gas samples were analyzed in isothermal mode at 40 °C using an HP-5 60 m column (crosslinked 5% PH ME Siloxane, I.D. 0.25 mm, film thickness 1.0 µm). The carrier gas used was helium with a constant flow rate of 1.1 mL/min. The total ion chromatogram produced for each sample was analyzed using Agilent ChemStation analysis software and the Wiley library of mass spectra (sixth edition). The chromatograph integrator was programmed in two different modes, allowing the quantification of compounds by both species and size. In this way, a single GC-MS analysis permitted the identification of the products and the classification of the sample by chain length. The GC-MS was not calibrated for the individual compounds in the samples; hence, the compounds are quantified as total ion content percentage (TIC%)san integration of the peaks present within the chromatogram. While this should not be confused with a true weight percentage (wt %), this figure still gives a good approximation of the composition of the sample.21 Metal Analysis. Samples of untreated waste oil, liquid products, and carbon (before and after pyrolysis) were analyzed using a Varian Spectr AAS instrument (Agilent Technologies, Palo Alto, CA). In sample preparation, 1 g samples were weighed and heated, then 5 mL of HNO3 (60% v/v) and 5 mL of H2SO4 (96% w/v) were added twice to completely digest and extract metal ions from the samples. Solid samples were digested first with 5 mL of HCl (37.5% w/v) and 5 mL of HNO3 (60% v/v), followed by 10 mL of HClO4 (70% w/v). The final colorless solutions were then diluted accordingly and injected into the furnace of the AAS for determination of the metal content. Experimental analysis of each metal was performed according to La´zaro et al.;20 the samples were analyzed for their content of Cd, Cr, Cu, Ni, Pb, Zn, and Fe. Results and Discussion Characteristics of Waste Car Engine Oil. WO was characterized by GC-MS before being subjected to pyrolysis (see Figure S1, Supporting Information). Linear and branched
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Figure 2. Product yields (wt %) from microwave pyrolysis of waste oil as a function of the temperature.
paraffins with carbon chain lengths higher than C24 (i.e., 85.8%) are the predominant hydrocarbon structures in the composition of WO; similar results were obtained by Go´mez-Rico et al.1 It is thought that some of the heavier hydrocarbons originally present in unused engine oil were converted to lighter hydrocarbons (
