Steam Cracking and Steam Reforming of Waste Cooking Oil in a

Energy Fuels 2009, 23, 5663–5676 Published on Web 09/01/2009

: DOI:10.1021/ef900529n

Steam Cracking and Steam Reforming of Waste Cooking Oil in a Tubular Stainless Steel Reactor with Wall Effects Julien Gornay, Lucie Coniglio,* Francis Billaud, and Gabriel Wild D
epartement de Chimie Physique des R
eactions, Nancy-University, CNRS, UMR 7630, 1 rue Grandville, BP 20 451, 54001, Nancy, France Received May 25, 2009. Revised Manuscript Received July 22, 2009

Energy production from renewable feedstocks that would simultaneously solve ecological problems related to waste disposals would be very attractive. The present work is aimed at showing that atmospheric pressure thermal cracking of waste cooking oil in the presence of steam would be a potential option, particularly when the operating conditions direct the process either toward steam cracking or toward steam reforming in order to produce specific target bioenergy vectors: hydrogen, synthesis gas, or gaseous fuel. A commercial crude waste cooking oil (VEG) was selected as feed material. Using a bench-scale continuous flow tubular stainless steel reactor, experiments were conducted to study the final product distribution as a function of temperature, residence time of the feed material, extent of dilution, addition of a cracking initiator, and addition of a surface catalytic effect inhibitor. Several operating conditions of the VEG thermal cracking in the presence of steam were identified to meet the above-mentioned objectives. Particularly, when operating steam reforming at 800 °C with a very low steam-to-carbon ratio (less than 1), VEG was totally converted into synthesis gas in a hydrogen-to-carbon monoxide molar ratio close to 2 (favorable for low-temperature Fischer-Tropsch catalysis), with additional hydrogen and light-hydrocarbon (methane, ethylene, propylene) production reaching 40 and 27 mol %, respectively. Further investigations (conducted with the same equipment) confirmed the occurrence of strong reactor wall effects that led to the formation of coke deposits with catalytic activity during the VEG steam cracking and steam reforming.

forefront. The first is the refinery hydroprocessing (involving hydrocracking and hydrogenation) to produce diesel-like fuel (“super cetane” product7). The second is the transesterification by addition of alcohol (methanol or ethanol) to produce biodiesel (fatty acid methyl or ethyl esters).4,5,8-10 Other, lessoften investigated but promising alternatives are catalytic reforming for hydrogen production11 and pyrolysis yielding either synthesis gas, methane, and ethylene when conducted at elevated temperature (800 °C12) or liquid hydrocarbon fuels in the gasoline or diesel range when conducted at low to moderate temperatures (400-600 °C13,14). All these technologies were, however, nearly exclusively devoted to virgin vegetable oils or animal fats. Using waste cooking oils as a new raw material for these processes is a valuable challenge. In the present work, atmospheric pressure thermal cracking in the presence of steam of typical waste cooking oil was conducted with the specific objective to investigate the potential

1. Introduction Due to political, economic, social, and environmental factors, there is a growing need for the development of renewable fuels and chemicals to supplement conventional petroleum-derived products. Waste cooking oils (including restaurant grease and rendered animal fat) contain valuable chemical species (mainly triglycerides and free fatty acids) that make them potential feedstock for the production of bioenergy vectors. In addition, waste cooking oil production is around several hundred of thousands of tons per year in most European countries.1 As a result, using waste cooking oil instead of food-grade vegetable oil as raw material could help to solve the problem of waste disposal,2,3 but also to facilitate the development of bioenergy production technologies as environmentally friendly waste conversion processes with reduced production costs.4,5 Even though in most applications so far, waste cooking oil was used as biofuel or as raw material for lubricant, solvent, and lacquer production,6 more recently other converting processes aroused strong interest. Two of them are currently at the

(7) Stumborg, M.; Wong, A.; Hogan, E. Bioresour. Technol. 1996, 56, 13–18. (8) Leung, D. Y. C.; Guo, Y. Fuel Process. Technol. 2006, 87, 883–890. (9) Issariyakul, T.; Kulkarni, M. G.; Dalai, A. K.; Bakhshi, N. N. Fuel Process. Technol. 2007, 88, 429–436. (10) Van Kasteren, J. M. N.; Nisworo, A. P. Resourc. Conserv. Recycl. 2007, 50, 442–458. (11) Marquevich, M.; Farriol, X.; Medine, F.; Montan
e, D. Ind. Eng. Chem. Res. 2001, 40, 4757–4766. (12) Panigrahi, S.; Chaudhari, S. T.; Bakhshi, N. N.; Dalai, A. K. Energy Fuels 2002, 16, 1392–1397. (13) Dandik, L.; Aksoy, H. A. In Proceedings of the World Conference on Oilseed and Edible Oils Processing, Koseoglu, S. S., Rhee, K. C., Wilson, R. F., Eds.; AOCS Press: Champaign, 1998; Vol. 1, pp 126-129. (14) Adebanjo, A. O.; Dalai, A. K.; Bakhshi, N. N. Energy Fuels 2005, 19, 1735–1741.

*To whom correspondence should be addressed. E-mail: Lucie.Coniglio@ ensic.inpl-nancy.fr. Phone: þ33 383 175 025. Fax: þ33 383 378 120. (1) Canakci, M. Bioresour. Technol. 2007, 98, 183–190. (2) Encinar, J. M.; Gonzalez, J. F.; Rodrı´ guez-Reinares, A. Ind. Eng. Chem. Res. 2005, 44, 5491–5499. (3) Encinar, J. M.; Gonzalez, J. F.; Rodrı´ guez-Reinares, A. Fuel Process. Technol. 2007, 88, 513–522. (4) Zhang, Y.; Dub
e, M. A.; McLean, D. D.; Kates, M. Bioresour. Technol. 2003, 89, 1–16. (5) Zhang, Y.; Dub
e, M. A.; McLean, D. D.; Kates, M. Bioresour. Technol. 2003, 90, 229–240. (6) Maher, K. D.; Bressler, D. C. Bioresour. Technol. 2007, 98, 2351– 2368. r 2009 American Chemical Society

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the water content of food accelerates partial triglyceride hydrolysis that yields formation of free fatty acids, together with di- and monoglycerides. Furthermore, oxygen, temperature, and oil unsaturation degree may promote a lot of other components such as peroxides and dimers. To sum up, waste cooking oils are complex mixtures; their rigorous analytical characterization is a tedious and time-consuming task that only specific laboratories with adequate equipment and experience can achieve successfully. Therefore, no complementary compositional analysis to the technical information delivered by Ecogras Company for their product VEG was conducted in this work. Notwithstanding, with regards to the weight percents (wt %) in fatty acids given in Table 1, VEG is probably an oleic-type oil with 47 wt % of oleic acid, 15 wt % of linoleic acid, and 35 wt % of stearic and palmitic acids as brought together (all fatty acids mainly present as glycerol esters). Also, from the knowledge of the wt % in C, H, O elements (Table 1) and by assuming reasonably that VEG contains mainly triglycerides, an average molecular formula C61H121O6, corresponding to a molecular weight of 949, was deduced for the VEG. 2.2. Bench-Scale Experimental Setup. The experimental setup (Figure 1) can be split into five units: (a) reagent (VEG and water) and gas (nitrogen or air) feed unit; (b) evaporating, mixing, and preheating unit of the reactor feed (mixture of VEG and water with, if necessary, addition of nitrogen); (c) gas phase chemical reaction unit; (d) reaction product trapping unit; and (e) reaction product analysis unit. All units operate at constant, near-atmospheric pressure. 2.2.1. Reagent and Gas Feed Unit. VEG, which is solid at ambient temperature (Table 1), was preheated prior to injecting it into the experimental setup. A heating temperature of 90 °C was chosen in order to maintain VEG in a liquid state of known density value (Table 1). VEG and water (the latter being initially at ambient temperature) were injected separately as fine liquid droplets and with desired flow rates by two piston pumps (respectively, Hamilton Roy and isocratic pump of Spectra Physics type). The isocratic pump used for water feeding yields constant flow rates and was calibrated. Nevertheless, the exact water mass flow rate was determined for each experiment by weighing the water flask at the beginning and at the end of each run. Gases (nitrogen or air) were injected separately by mass flow controllers. Nitrogen was used as inert carrier gas for increasing dilution in several thermal cracking experiments in the presence of steam. Air supply is used at the end of each experiment to quantify the coke formed during the thermal cracking run. 2.2.2. Evaporating, Mixing, and Preheating Unit of the Reactor Feed. All feed materials (VEG, water, and eventually nitrogen or air for quantification of coke deposit) were preheated separately and then mixed together in stainless steel tubes surrounded by thermostatically controlled electrical heaters. The wall temperature of the preheating and mixing section was fixed to 300 °C in order to vaporize the VEG in the steam flow while preventing it from cracking before entering the reactor. 2.2.3. Gas Phase Chemical Reaction Unit. The unit comprises a stainless steel (Incoloy 800) tubular reactor, which is housed in a thermostatically controlled electric resistance furnace (F 79300-type Thermolyne). The furnace ensures a quasi-isothermal section within 150 mm around its middle. The dimensions of the empty tubular reactor are 550 mm in overall length and 14.3 mm in internal diameter. A K-type thermocouple placed in the center of the furnace gave the actual gaseous reactant flow temperature inside the isothermal part of the reactor. Several runs were conducted in order to check the difference between the furnace temperature and the temperature of the gas phase inside the isothermal part of the reactor. The results obtained showed that both temperatures were similar to (2 °C. Nevertheless, it should be mentioned that some cracking may already occur from 500 °C, that is, before reaching the isothermal part of the reactor. Hence, the given temperature of the gas phase (furnace) should be considered here as a “reference

Table 1. Weight Composition and Physicochemical Properties of Vegetamixoila weight composition (%) Class of Fatty Acids and Elements saturated fatty acids 35 monounsaturated fatty acids 47 polyunsaturated fatty acids 15 C content 73.6 O content 9.7 H content 12.2 Physicochemical Properties density (90 °C) 0.9 0.11 kinematic viscosity (cm2/s) (90 °C) minimum melting point (°C) 30 maximum melting point (°C) 45 lower heating value (MJ/kg) 36.4

uncertaintyb (3 (3 (1 (3 (1 (0.5 (0.1 (0.03 (0.8

a

Technical Information Delivered by Ecogras Company, France. Absolute limits in the same units as the mean value given in the former column. b

for producing high value chemicals assimilated to bioenergy vectors. The bioenergy vectors focused on are (i) synthesis gas with a molar ratio H2/CO ≈ 2 favorable for obtaining synthetic diesel fuel by low temperature Fischer-Tropsch catalysis,15,16 (ii) hydrogen for its application in fuel cells, and (iii) light hydrocarbons (methane, ethylene, propylene, etc.) for their use as fuels or intermediate chemicals. The experiments were designed in order to show that it is possible to direct waste cooking oil thermal cracking in the presence of steam to the production of the desired bioenergy vector, by selecting the process operating conditions. 2. Experimental Section 2.1. Feedstock Composition. The feedstock used is marketed under the name Vegetamixoil (VEG) by the French Company Ecogras; it is a concentrate of vegetable oils and animal fats obtained by the melting of used edible oils from the catering and preparing of food, followed by further mechanical treatments (filtration, centrifugation, and decantation) for solid material and water elimination. The main physicochemical properties and mass composition of VEG (technical information delivered by Ecogras Company) are presented in Table 1. The VEG composition is given in terms of carbon, oxygen, and hydrogen elements, on the one hand, in terms of classes of fatty acids (saturated, mono- and poly- unsaturated fatty acids) on the other hand. Detailed molecular composition of the reactant mixture is required to build both realistic chemical mechanisms and consistent result interpretation. Unfortunately, typical cooking processes are very complex and involve a lot of cross-reactions such as oxidation, polymerization, thermal cracking, and hydrolysis occurring between components of air, food, and vegetable oil.17,18 The main components of vegetable oils and animal fats are triglycerides, which consist of three long chain fatty acids (identical or different chains) esterified to a glycerol backbone. The long chain fatty acids have an even number of carbon atoms ranging from 14 to 24 with one, two, or three unsaturations. Concerning cooking oils, the main fatty acid is oleic acid with 18 carbon atoms and one unsaturation. Consequently, the high temperature of typical cooking processes (180 °C) associated with (15) Aasberg-Petersen, K; Bak Hansen, J. H.; Christensen, T. S.; Dybkjaer, I.; Seier Christensen, P.; Stub Nielsen, C.; Winter Madsen, S. E. L.; Rostrup-Nielsen, J. R. Appl. Catalysis, A 2001, 221, 379–387. (16) Song, X.; Guo, Z. Energy Convers. Manage. 2006, 47, 560–569. (17) Dobarganes, C. OCL 1998, 5, 41–47. (18) Graille, J. OCL 1998, 5, 36–40.

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Figure 1. Schematic overview of the bench-scale experimental setup.

offline equipments. Also, standard gas mixtures were used for calibration (and identification concerning the C1-C4 cut). CO and CO2 were analyzed by a nondispersive infrared ray absorption (IR) analyzer (Cosma Crystal 300), and H2 and the C1-C4 cut were analyzed by two gas chromatographs (GC) equipped with different detectors for suitable analysis: a thermal conductivity detector (TCD) for H2 and a flame ionization detector (FID) for the C1-C4 cut. These equipments allow quantification of CO, CO2, and H2 in terms of percent volume fractions in the whole gas product (equivalent to percent molar fractions with regards to the ideal gas law), while quantification of the C1-C4 cut components is obtained in terms of percent mass fractions in the gas product excluding CO, CO2, H2, and the inert gas diluent when used. 2.2.5.2. Liquid Products. Usually, the thermal cracking liquid product contains a wide variety of components with various functional groups. Therefore, a gas chromatography-mass spectrometry (GC-MS) technique was primarily used for component identification (which includes matching of retention times against known standard compounds under identical GC conditions together with analysis of mass spectra against spectral libraries). Nevertheless, a small fraction of components could not be identified using the available GC-MS database and were termed “unidentified” fraction (see Appendix 3). Quantification of the identified liquid components was achieved by a GC-FID in conjunction with the widely used internal standard method that yielded the mass of each component in the whole collected liquid product (the GC-FID calibration was previously conducted by evaluating a carbon response factor for each product relatively to n-octane chosen as internal standard). 2.2.5.3. Coke. Thermal cracking involves coke formation. Coke deposited by successive layers on the tubular reactor walls has to be quantified to account for it in the mass balance. Also, decoking the inside surface of the reactor is necessary to have a good reproducibility of the experiments (constant cross-sectional area of the tubular reactor from one run to

parameter” used for comparing experimental results. This reference parameter was further termed “reactor temperature”. A pressure transducer, located at the inlet of the reactor, gives a measure of the reactor pressure (required for estimating the residence time, eq A2.7, in Appendix 2). 2.2.4. Reaction Product Trapping Unit. When cooled, effluents leave the reactor as aerosols (suspension of very fine liquid droplets in gaseous flow). Hence, complete separation of the condensable (liquid) and the noncondensable (gaseous) products is a very difficult task that nevertheless needs to be conducted to allow the product analysis step. Therefore, efforts were made to separate the gas and liquid product as efficiently as possible. The product stream leaving the reactor was passed through a water-cooled condenser (temperature maintained at 20 °C by a Julabo cryostat). The obtained gas-liquid mixture was sent either to the transient state or to the steady state transfer line, which are both linked together at their extremity by two three-way valves. Each transfer line comprises a glass gas-liquid separator for collecting the liquid product. For trapping the fine liquid droplets that might remain in the gas product, the separator at the outlet was packed with cotton wool. A glass U-tube also partially packed with cotton wool was introduced after the gas-liquid separator as a precaution (but weighing of the cotton pad, before and after each experiment, showed that no product was retained). The gases exiting the gas-liquid separator and U-tube (noncondensable cracking products together with the inert gas when used for extent of dilution) passed through the gas sampler for analysis, and then to the online gas flow meter. 2.2.5. Reaction Product Analysis Unit. The cracking products collected as three phases, that is, gas, liquid, and solid (coke) phases, were analyzed by different techniques. Details relating to the gas chromatography technique are given in Table 2. 2.2.5.1. Gaseous Products. These are hydrogen (H2), carbon monoxide (CO), carbon dioxide (CO2), and hydrocarbons having a carbon number inferior to 4 (C1-C4 cut). All of them were analyzed continuously during the experiment by online or 5665

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Table 2. Conditions of Gas Chromatography Analyses conditions

uncondensate (excluding H2, CO, and CO2)

H2

GC-type detector detector temperature /°C injector temperature /°C carrier gas (N2) volumetric flow rate (NmL/min) or linear velocity (cm/s) split ratio column

oven temperature program

Intersmat IGC 11-type TCD 30 NmL/min

silica-gel filled column/ molecular sieve (5 A˚ thickness) 5 m length, 6 mm inner diameter 60 °C

column pressure program

condensate a

Schimadzu GC 17A FID 310 300 15 cm/s

Schimadzu GC 17A FID 310 300 15 cm/s

1/100 bonded nonpolar (methyl silicone) capillary column (P.O.N.A.-type, Hewlet Packard) 50 m, 0.21 mm, 0.5 μm film thickness 60 °C (4 min), 60-180 °C (10 °C/min, 10 min), 180-300 °C (10 °C/min) 96 kPa (4 min), 96-153 kPa (2.4 kPa/min, 10 min), 153-182 kPa (2.4 kPa/min)

1/100 bonded nonpolar (methyl silicone) capillary column (P.O.N.A.-type, Hewlet Packard) 50 m, 0.21 mm, 0.5 μm film thickness 60 °C (20 min), 60-300 °C (2 °C/min, 40 min) 124 kPa (20 min), 124-182 kPa (0.5 kPa/min, 44 min)

a n-Octane was used as internal standard as it was only formed in negligible amounts during the Vegetamoxoil thermal cracking in the presence of steam conducted under the investigated reaction conditions. The GC system was calibrated by determining a carbon response factor compared with the internal standard (n-octane) for each identified products.

nature and yields of the products formed. Hydrogen peroxide (H2O2) added in small amounts to the reactant mixture should initiate significantly the thermal cracking reactions and thus enhance the reaction rates starting from low temperature (650 °C). As commonly done during industrial pyrolysis process of hydrocarbons, thiophene (C4H4S) added in very small amounts to the reactant mixture (300 ppm) should reduce metallic reactor wall effects that currently occur at high temperature (from 800 °C for hydrocarbon pyrolysis19). 3.1. Experimental Design Features. The operating conditions related to the selected experiments, together with their corresponding material balances and product yields, are given in Table 3. With the aim of producing mainly small molecular species (synthesis gas with a H2/CO molar ratio close to 2, hydrogen, or light hydrocarbons such as methane, ethylene, and propylene), operating conditions of thermal cracking in the presence of steam leading to a total (or quasitotal) conversion of VEG were selected. Therefore, high reaction temperatures (from 650 to 800 °C) with low steam-to-oil weight ratios (1:2 and 1:1), and long residence times of the feed material (0.8 and 1.5 s, eq A2.6) were adopted. This choice is consistent with the work of Idem et al.20 on canola oil thermal cracking in the presence or absence of steam. Indeed, these authors observed that canola oil thermal cracking conducted at 500 °C (the highest temperature investigated by the authors) in the absence of steam led to a 100 wt % conversion, which decreased when operating in the presence of steam (94.5 wt % conversion observed with a 4:1 steam-to-canola oil weight ratio). Among the numerous experiments performed, only experiments with errors in overall material mass and carbon mole balance below 7% were selected. It was unfortunately impossible to run systematic duplicates; however, those made indicate a good reproducibility, as shown in Table 4. Detailed composition of the obtained gas and liquid products are given in Appendix 3 (Table A3).

another) as well as to avoid plugging (and other problems in downstream unit operations). The amount of coke deposit was determined by burning it off (eq 1) and analyzing the resulting CO and CO2 emissions. Details of the procedure are given in Appendix 1. 2C ðcokeÞ þ 3=2 O2 f CO þ CO2

ð1Þ

2.3. Material Balances. Each experiment was validated by calculating balances in terms of mass and carbon mole number between the inlet and the outlet of the reactor. The mass balance was calculated given the volumetric flow rate, the density of the condensable feed materials (VEG and water), the duration of each run, the percent mole fractions of the identified components in the gas product, the total volume of the gas product evolved during each run, the total mass of the liquid product, the mass of the identified organic components in the liquid product representing the further termed organic liquid phase (OLP) product, and the mass of the coke deposit (see Appendix 1). The carbon mole balance was calculated given the previously mentioned parameters in addition of the available elemental carbon composition of VEG (Table 1) and the molecular formulas of the species formed during each run in the gas and liquid streams. The main general equations used to check the material balances for each run are given in Appendix 2 (Table A2).

3. Results and Discussion Distribution of the final products observed during VEG thermal cracking in the presence of steam was investigated as a function of some key reaction variables: temperature, residence time of the feed material (eq A2.6), extent of dilution and nature of diluent (by addition of nitrogen in a 1:1 steamto-nitrogen ratio), addition of a cracking initiator (hydrogen peroxide), and addition of an inhibitor of surface catalytic effects (thiophene). Increase in reaction temperature is expected to favor the reactant mixture conversion and the production of small molecular species (CH4, C2H4, or even H2, CO, CO2, resulting then to higher gas yield). Increase in residence time of the feed material should increase the conversion of the reactant mixture and should involve a significant effect upon the chemical

(19) Billaud, F.; Freund, E. Ind. Eng. Chem. Fundam. 1986, 25 (3), 433–443. (20) Idem, R. O.; Katikaneni, S. P. R.; Bakhshi, N. N. Energy Fuels 1996, 10, 1150–1162.

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Table 3. Experiment Description: Operating Conditions, Overall Material Balance, and Characteristic Molar Ratios for Fischer-Tropsch Catalysis (Run Time: 30 min) reactor temperature/°C reactor pressure/torr residence time/s ((0.1) (eq A2.6) surface catalytic effect inhibitor (thiophene)/ppm cracking initiator (H2O2) a Inlet partial volumetric flow rate/(mL/min) Vegetamixoil (90 °C, 1 atm) b H2O (TA, PA) c N2 (NTP)/103d steam-to-oil weight ratio (S/VEG) Vegetamixoil fed/g gas product/g gas product/(wt %) OLP product e/g OLP product e/(wt %) coke/g coke/(wt %) Vegetamixoil conversion (wt %)

exp 1

exp 2

exp 3

exp 4

exp 5

exp 6

exp 7

exp 8

800 881 1.5

800 852 1.5

700 813 1.5

700 815 0.8

650 807 1.5

650 801 1.5

700 815 1.5

800 874 1.5 yes

yes 0.90 0.40 0.50 1:2

0.90 0.80

1.60 1.45

1.10 0.93

1.10 0.93

0.90 0.88

0.90 0.80

1:1

1:1

1:1

1:1

1:1

29.04 6.36 21.9 20.71 71.3 0.52 1.8 100

23.76 15.35 64.6 6.89 29.0 0.64 2.7 100

23.76 22.14 93.2 N.O. N.O. 1.31 5.5 100

5.4 -6.3

5.0 -3.7

3.7 -1.6

1.3 -0.9

Characteristic Molar Ratios for Fischer-Tropsch Catalysis 2.00 2.15 1.55 0.82 0.64 0.89 1.20 -0.01 0.13 0.09

0.60 -0.32

1.47 0.33

1.52 0.35

1:1

0.90 0.39 0.49 1:2

Mass and Weight Percent of Each Product Phase (wt % of Vegetamixoil Fed) 23.76 23.76 23.76 42.24 29.04 22.93 22.99 9.04 21.58 4.16 96.5 96.7 38.0 51.1 14.3 N.O. N.O. 14.12 18.16 23.14 N.O. N.O. 59.4 43.0 79.7 0.36 0.30 0.19 0.42 0.17 1.5 1.3 0.8 1.0 0.6 100 100 100 100 100 Overall Material and Carbon Balances 2.0 2.0 1.7 4.9 -0.4 0.5 -1.3 -3.6

Material balance (wt%) Carbon balance (%) H2/CO (H2 - CO2)/(CO þ CO2) f

a Solution of H2O2 stabilized in water with a concentration of 2.68 mol L- 1. b Vegetamixoil is solid at ambient temperature and pressure. c TA and PA are ambient temperature and ambient pressure, respectively. d Volumetric flow rate under normal conditions of temperature and pressure (273.15 °C, 1 atm). e N.O.: Not observed. f Key ratio for biomethanol synthesis process. To operate with an optimum synthesis gas composition, this ratio should be close to 2.15,16

Table 4. Result Reproducibility during Vegetamixoil Thermal Cracking in the Presence of Steama exp 2a

exp 2b

exp 2c

Mass and Weight % of Each Product Phase gas product/g 23.06 23.00 22.90 gas product/(wt %) 97.0 96.8 96.4 N.O. N.O. N.O. OLP product b/g or (wt %) coke/g 0.21 0.29 0.40 coke/(wt %) 0.9 1.2 1.7 overall mass balance (wt %) 2.1 2.0 1.9 H2 CO CO2 methane ethylene ethane propylene propaneb 1-butene n-butane benzene toluene b coke

led to the formation of coke layer deposits with various composition and reactivity. Coke formation was suggested to proceed according to three stages in agreement with work developed in the literature for hydrocarbon pyrolysis.22,23 (i) At the initial stage of the reaction, a coke with a dense polyaromatic structure is formed through heterogeneous gas/solid catalytic reactions involving the inner surface of the reactor and the gas phase components. Metal particles incorporated in the reactor construction material such as Ni and Fe catalyze coke formation, whose precursors are part of the gas phase components, mainly alkenes. This stage has a very short duration with respect to the total experiment length. (ii) In a second stage, further coke deposits are formed, still through heterogeneous gas/solid catalytic reactions; these deposits present a less dehydrogenated structure and adsorb on the inner wall of the reactor, thus inhibiting the catalytic activity. This adsorbed coke containing Fe and Ni particles from the reactor construction material (observation made by scanning electron microscopy analysis21) is a metallic coke that still shows a high catalytic activity. (iii) In a third stage, further consecutive coke layers deposited during pyrolysis inhibit in their turn the catalytic activity of the adsorbed metallic coke, to finally behave as a passivator-like agent. The coke metal content decreasing with the distance from the reactor wall, this nonadsorbed coke can be considered as a nonmetallic coke generated through noncatalytic heterogeneous gas/solid reactions. Obviously, the gas phase reaction medium (i.e., feedstock), the nonmetallic coke, the adsorbed metallic coke, and the

average 22.99 ( 0.06 96.7 ( 0.2 N.O. 0.30 ( 0.08 1.3 ( 0.3 2.0 ( 0.1

Product Molar Composition on N2 Free Basis (%) 40.4 40.7 40.5 40.5 ( 0.1 19.1 18.4 19.0 18.8 ( 0.3 8.1 8.3 7.9 8.1 ( 0.2 6.0 5.7 5.8 5.9 ( 0.1 18.2 18.9 18.4 18.5 ( 0.3 1.1 0.8 0.7 0.9 ( 0.2 3.1 3.2 2.8 3.0 ( 0.2 0.2 N.O. 0.1 0.1 ( 0.1 0.5 0.4 0.1 0.3 ( 0.2 1.1 1.6 1.3 1.3 ( 0.2 0.4 0.6 0.8 0.6 ( 0.2 0.1 0.3 N.O. 0.1 ( 0.1 1.7 1.1 2.6 1.8 ( 0.6

a Case of steam reforming; run time: 30 min. Operating conditions related to these runs are those of experiment 2 detailed in Table 3. b N.O.: Not observed.

3.2. Wall Effects and Formation of Catalytically Active Coke Deposit. In previous work,21 octanoic acid pyrolysis was conducted in the same reactor as the one used in this study (stainless-steel tubular reactor made in Incoloy 800); it clearly pointed out the occurrence of strong wall effects that

(22) Wauters, S.; Marin, G. B. Chem. Eng. J. 2001, 82, 267–279. (23) Van Speybroeck, V; Van Neck, D.; Waroquier, M.; Wauters, S.; Saeys, M.; Marin, G. B. Int. J. Quantum Chem. 2002, 91 (3), 384–388.

(21) Gornay, J.; Coniglio, L.; Billaud, F.; Wild, G.; J. Anal. Appl. Pyrolysis 2009, submitted.

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predominate, all activated by the catalytic activity of the Nibased coke layer lining the inner wall of the reactor. Hence, through the steam reforming reactions, the organic molecule CnHmOk used for modeling VEG is adsorbed onto the Ni surface of the coke where it completely converts to CO and H2 according to Table 5, eq 2b (note that according to Table 5, eq 4, identical features can be extended to CO2 reforming reaction of CH4 when this one is formed as product during VEG steam reforming). 3.3. Main Products Observed and Some Formation Route Insights. The main products observed were: (a) H2, CO, CO2; (b) the C1-C4 cut involving methane, ethylene, ethane, propylene, propane, 1-butene, and n-butane; (c) the C4+ cut involving linear 1-olefins and paraffins with 5 to 15 carbon atoms; (d) aromatics such as benzene and toluene; and for some runs (e) C5-C6 cycloolefins and cycloparaffins and (f) oxygen containing hydrocarbons such as acetone and saturated and R-unsaturated carboxylic acids with 2-7 carbon atoms (Table A3, Appendix 3). These products result from the combination of hydrolysis, steam cracking, and steam reforming of VEG, with a more or less important contribution of the two latter competing processes, depending on the operating temperature. Therefore, the mechanism of formation of the observed products should represent this combination and should comprise part of the reaction scheme proposed in literature for the thermal cracking of vegetable oils.14,20 Main features of this mechanism are summarized in Table 6, which illustrates the agreement between experimental observations made in this work and theoretical formation routes proposed in the literature.14,17,20,24,25 3.4. Effects of Temperature and Extent of Dilution (Experiments 1-3 and 7). The effects of operating temperature and extent of dilution on VEG thermal cracking in the presence of steam were investigated at 700 and 800 °C, with or without addition of N2 as inert diluent at a 1:1 steam-toN2 molar ratio. Other operating conditions that may involve significant changes on the final product distribution, that is, the residence time of the feed material (1.5 s) and the (steam þ N2)-to-oil weight ratio (≈ 1) were kept constant (with no agent addition aimed at either surface catalytic effect inhibition or cracking initiation). As can be observed in Table 3, the total amount of gas produced during VEG thermal cracking in the presence of steam considerably increased with reaction temperature: 38 wt % at 700 °C and 96.5 wt % at 800 °C. Conversely, the OLP product sharply decreased with reaction temperature from an amount of 59.4 wt % at 700 °C to a nondetectable value at 800 °C. Analogous trends were observed without addition of N2: 64.6 wt % of gas product and 29.0 wt % of OLP product were obtained at 700 °C, whereas 96.7 wt % of gas product was obtained at 800 °C. These results are commonly expected in classical thermal cracking processes. Indeed, the higher the cracking temperature, the lower the molecular weights of the products formed that actually result in the formation of the bulk of the gas phase. Also, C-C bond cleavage, ethylene elimination from hydrocarbon radicals, as well as reactions such as dehydrogenation, decarboxylation, and decarbonylation (releasing, respectively, in H2, CO2, and CO) are strongly endothermic and are thus favored at high temperature. Nevertheless, the gas product composition as given in Figures 2 and 3 reveals that in addition of thermal cracking, steam cracking, and steam reforming processes compete in the temperature range investigated, with a predominant

Table 5. Synthesis Gas Reactions by Steam and CO2 Reforming15,24,25 process

ΔRH° (298 K) /(kJ mol-1)

Steam Reforming 1. CH4 þ H2O = CO þ 3H2 þ206 þ1175 a 2a. Cn Hm þ nH2 O
 m H2 ¼ nCO þ n þ 2 2b. Cn Hm Ok þ ðn not available
-kÞH2 O  m b -k H2 ¼ nCO þ n þ 2 -41 3. CO þ H2O = CO2 þ H2 CO2 Reforming þ247 4. CH4 þ CO2 = 2CO þ 2H2 a

For n-C7H16. b For vegetable oil.

surface composition of the stainless-steel reactor are highly interdependent. Therefore, nonmetallic cokes generated inside the same reactor (i.e., with the same inner walls) by processing different feedstocks may interfere with the gas phase components differently and may thus show different reactivity. Nevertheless, several arguments lead to the assumption that the difference in feedstock used (octanoic acid and VEG) should not involve significant difference in reactivity of the reactor inner walls within the environment of the present study. First, both feedstocks (octanoic acid and VEG) have some similarities with regards to composition in terms of functional groups. Indeed, as mentioned previously, free fatty acids are components that take part in the VEG composition. Furthermore, carboxylic acids with various carbon chain lengths are intermediate products during vegetable oil steam cracking20 and may also be observed as final products. Second, it is well-known24 that coke formed during steam cracking is partly consumed by water with CO and H2 release according to eq 2. This coke should be related to the nonmetallic coke interfacing with the gas phase components (third and last stage of coke formation previously described). Also, due to its oxidant effects on metal wall tubes, steam inhibits Ni and Fe catalytic activity in coke formation. Concerning VEG thermal cracking in the presence of steam conducted in this work, during the decoking procedure ending each experiment, only small amounts of nonmetallic coke were not consumed by water and were left on the reactor inner wall. C ðcokeÞ þ H2 O ¼ CO þ H2

ð2Þ

Hence, it can be considered that the coke which predominates during VEG thermal cracking in the presence of steam is the adsorbed metallic coke that contains Fe and Ni particles21 and is catalytically active. Particularly, Ni is well-known to catalyze (in addition of coke formation) production of synthesis gas by steam and CO2 reforming, the most important reactions of which are listed in Table 5.15,24,25 Most of these reactions are strongly endothermic. Thus, depending on the operating reaction temperature, either VEG steam cracking (where water plays mainly a physical role and thus acts mainly as a diluent) or VEG steam reforming (where water plays mainly a chemical role and thus acts mainly as a reactant) will (24) Rostrup-Nielsen, J. R.; Sehested, J.; Norskov, J. K. Adv. Catal. 2002, 47, 65–139. (25) Marquevich, M.; Coll, R.; Montan
e, D. Ind. Eng. Chem. Res. 2000, 39, 2140–2147.

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Table 6. Main Features of the Mechanism of Formation of the Main Products Observed during Vegetamoxoil Thermal Cracking in the Presence of Steam14, 17, 20, 24, 25 a observed products • saturated and unsaturated carboxylic acids (with 2-7 carbon atoms) • acetone • CO • CO2 • hydrocarbon radicals • straight chain saturated and unsaturated hydrocarbons (linear paraffins and 1-olefins from C1-C4 and C4+ cuts) • conjugated dienes • C5-cycloolefins • C6-cycloolefins • C5-C6 Cycloparaffins • benzene and toluene • H2

formation route • hydrolysis of triglycerides (occurring from 200 °C17, 25) followed by subsequent thermal cracking with C-C bond cleavage of the released free fatty acids (which add to the free fatty acids initially present in the VEG composition) • thermal decomposition of two molecules of acetic acid with release of CO2 and H2O • decarbonylation of oxygenated hydrocarbons (free fatty acids and acetone with regards to observed products; aldehydes and esters with regards to intermediate species) • decarboxylation of saturated and R-unsaturated carboxylic acids • hydrolysis of triglycerides followed by C-C bond cleavage, decarbonylation, and decarboxylation • hydrocarbon radicals undergoing isomerization, disproportionation and hydrogen transfer • β-scission of unsaturated free fatty acids and hydrocarbons • addition to ethylene of propylene, followed by dehydrocyclization • Diels-Alder addition of ethylene to a conjugated diene • addition of proton to cycloolefins • hydrogen elimination from C6-cycloolefins (as encountered particularly at high temperature) • formation of cycloolefins and aromatics, splitting of hydrocarbons, dehydrogenation of olefins, and coke formation by polycondensation of olefins and aromatics

a Additionally to the mentioned routes, steam reforming reactions should also contribute at high temperature to the production of CO2, CO, and H2.15,24,25 On the other hand, H2 is needed to stabilize the hydrocarbon radicals formed at various stages of the thermal cracking process.20 Thus, the H2 observed among the final products should represent the difference between the amount produced and that which is consumed.

Figure 2. Steam cracking and steam reforming of Vegetamoxoil: gas product composition relative to H2, CO, and CO2 as a function of operating temperature and extent of dilution.

Figure 3. Steam cracking and steam reforming of Vegetamoxoil: gas product composition relative to methane (CH4), ethylene (C2H4), and propylene (C3H6) as a function of operating temperature and extent of dilution.

contribution of the steam cracking process at 700 °C, and conversely, a predominant contribution of the steam reforming process at 800 °C. Indeed, at 800 °C, CO2, CO, and particularly H2 were formed in much higher proportions than at 700 °C at the expense of light hydrocarbons such as methane, ethylene, and propylene. Additionally to the operating temperature, extent of physical dilution on final product distribution also confirms this argument as discussed in further details in the following two subsections. 3.4.1. Effect of Physical Dilution Confirming Predominant Steam Cracking Process at 700 °C (Experiments 3 and 7). As appears in Figures 2 and 3, dilution of steam with N2 at 700 °C had almost no effect on H2, CO, and CO2 production (Figure 2), but significantly disfavored production of ethylene, methane, and propylene, with for each of them a 2-fold decrease (Figure 3). This observation leads to the assumption that dilution plays here exclusively a physical role without chemical contribution. Indeed, one can assume that at 700 °C, water is still playing mainly its physical role

as diluent, with better heat transfer properties than N2 (CoPH2O(T) > CoPN2(T) and λH2O(T) > λN2(T) for temperature ranging between 650 and 800 °C). In such circumstances, the operating temperature would be more homogeneous in the tubular reactor when conducting VEG steam cracking with pure water than with water diluted in N2. In other words, during VEG steam cracking, processing with pure water rather than with water diluted in N2 would allow a lessening of the entrance and outlet effects introduced by the furnace used. This results in a reactor volume more efficiently used, which has the same effect as a longer residence time of the feed material kept at constant mass flow rate. This explains why at 700 °C with pure water (i) larger amounts of gas product and coke were obtained (Table 3); (ii) C2-C7 oxygenated hydrocarbons (mainly acetone and acetic and acrylic acids) were completely converted through subsequent reaction steps into species responsible for the bulk of the gas product (Table A3, Appendix 3); and (iii) production of ethylene, methane, 5669

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and propylene was increased by a factor 2 (Figure 3). This result also confirms that ethylene, methane, and propylene elimination occurs late in the reaction sequence of VEG thermal cracking, as proposed by Idem et al.20 Obviously, the best proof of this thermal explanation would be the measurement of the temperature profile along the tubular reactor, which should change, enlarging/shrinking the intermediate temperature plateau. In the present work, the thermocouple was, however, fixed inside the reactor at the center of the furnace and thus could not offer this kind of measurement. 3.4.2. Effect of Physical Dilution Confirming Predominant Steam Reforming Process at 800 °C (Experiments 1 and 2). As appears in Figures 2 and 3, dilution of steam with N2 at 800 °C enhanced the formation of low-molecular weight species like CO2, CO, and particularly H2 (Figure 2) at the expense of methane, ethylene, and propylene (Figure 3). Indeed, as cracking reactions induce an increase in the total number of moles in the gas phase, addition at constant temperature and pressure of the chemically inert component N2 leads to dilution of the active species of the phase (i.e., both water and VEG components in the case of steam reforming process) and thus to a decrease in their mole fractions (partial pressures). This results in a cracking reaction shift toward formation of low-molecular weight species (CO2, CO, and H2) that induces an increase in the total number of moles in the gas phase. 3.4.3. Coke Formation during Steam Cracking and Steam Reforming Processes. Only small amounts of coke deposits were observed during steam cracking as well as steam reforming of VEG and were on the order of magnitude of experimental uncertainty for some runs. Therefore, further efforts to highlight some trends in that case would be meaningless. 3.5. Effect of Residence Time of the Feed Material. This part concerns experiments 4 and 7. The effect of residence time was investigated in the case of steam cracking experiments conducted at 700 °C. Two values of residence time (1.5 and 0.8 s) were applied by changing steam and VEG flow rates in a S/VEG weight ratio maintained at 1:1 (Table 3). Other operating parameters were kept unchanged (pressure set to the same value with no addition of N2, neither of wall effect inhibition agent nor of cracking reaction initiator). As can be seen in Table 3, the amounts of gas product and coke formed increased with residence time of the feed material at the expense of the produced OLP amount. Furthermore, results given in Figure 4 in terms of product molar fractions show that production of H2 and C2-C4 olefins (i.e., ethylene and 1-butene, Table A3, Appendix 3) were favored at higher residence time while the reverse trend was exhibited by CO, C4þ olefins, C4þ alkanes, and aromatic products (i.e., benzene and toluene, Table A3, Appendix 3). These results are in agreement with the reaction scheme proposed by Idem et al.20 for thermal cracking of vegetable oil and also with the coke formation route discussed in Section 3.2. Indeed, reactions that are responsible for the bulk of the gas product (such as ethylene elimination and dehydrogenation) and for the coke formation (such as aromatic condensation and dehydrogenation, decomposition of long chain hydrocarbon radicals into their elements C and O, VEG polycondensation) are further down the reaction sequence and are thus favored at high residence time. On the other hand, reactions that

Figure 4. Effect of residence time of the feed material: molar fractions of the main products observed during Vegetamoxoil steam cracking (operating temperature: 700 °C, experiments 4 and 7, Table 3).

are responsible for the formation of the OLP product (such as C-C bond cleavage, decarbonylation, and decarboxylation) are earlier in the sequence of reaction steps. Therefore, within the range of residence time used in this study, an increase in residence time showed to be detrimental to OLP production but a positive factor for gas product and coke formation. The drop observed in CO mole fraction with increasing residence time might be explained by CO consumption according to the water gas shift reaction (Table 5, eq 3) which usually occurs at low temperature (650-700 °C26). This CO consumption route might also explain the slight increase with residence time in CO2 production (and part of that in H2). Nevertheless, it is worth noting that the variation of the products with residence time of the feed material was not as drastic as it might have been, because of the complete conversion of the feedstock obtained during the two experiments conducted in this study. This result is in good agreement with the work by Adebanjo et al.14 related to the gas phase composition analysis versus residence time during animal fat pyrolysis conducted from 600 to 800 °C. 3.6. Addition of a Cracking Initiator Agent (Experiments 5 and 6). To enhance the gas product yield while operating at low temperature (650 °C) with higher H2 production, VEG thermal cracking in the presence of steam was conducted with addition of a cracking initiator agent. As thermal cracking is a gas phase process involving elementary reactions with radical species, hydrogen peroxide (H2O2) was selected as cracking initiator. At the operating temperature selected for this test (650 °C), hydrogen peroxide decomposes according to eq 3 (and to a minor extent to eq 4) to form the very reactive radical species hydroxyl HO• (in addition to the radical hydroperoxy HOO• formed in much smaller amounts). H2 O2 f 2HO•

ð3Þ

H2 O2 þ HO• f H2 O þ HOO•

ð4Þ

(26) Visentin, V.; Piva, F.; Canu, P. Ind. Eng. Chem. Res. 2002, 41, 4965–4975.

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Figure 5. Addition of a cracking initiator agent (hydrogen peroxide, H2O2): product molar fractions obtained from Vegetamoxoil steam cracking conducted at 650 °C in the presence or in the absence of H2O2.

Figure 6. Addition of a surface catalytic effect inhibitor agent (thiophene): product molar fractions obtained from Vegetamoxoil steam reforming conducted at 800 °C in the presence or in the absence of thiophene.

In the presence of HO•, the following elementary reactions might then occur:

result from eq 7 with the released alkyl radicals leading to olefinic hydrocarbons. Hence, in the presence of H2O2, H2 production would be the result of eqs 5b and 6; while CO production would result from the difference between the amounts produced (eq 5b) and consumed (eq 6). To sum up, H2O2 addition to water during low temperature VEG steam cracking (650 °C) did not bring any valuable benefit since the main effect observed was a negligible increase of gas-to-OLP products ratio, with additionally enhancement of CO2 production. This result is probably closely linked with the operating conditions used for the VEG steam cracking experiments conducted here, which led to VEG total conversion (no fatty esters or fatty acids were observed among the products formed). 3.7. Addition of an Agent Inhibiting Surface Catalytic Effects (Experiments 2 and 8). As mentioned previously, within the metal tubular reactor used in this study, coke formation with a catalytic activity occurred during VEG thermal cracking in the presence of steam (steam cracking as well as steam reforming); this catalytically active coke, in direct contact with the gas phase, obviously plays an important role in the distribution and nature of the final products formed. To enhance methane and ethylene productions at the expense of synthesis gas, a sulfur-based agent (thiophene) able to inhibit the catalytic activity of the coke formed was added in small amounts (300 ppm) into the VEG feedstock before conducting the thermal cracking in the presence of steam at 800 °C. This procedure of sulfur-based agent addition to the feedstock is often encountered in articles related to hydrocarbon pyrolysis conducted in metal tubular reactors, in order to inhibit surface catalytic effects.19 Indeed, as extensively reported in the literature,24 the Group8 metal catalysts are highly susceptible to sulfur poisoning and furthermore Ni (which is incorporated in the coke formed) is more sensitive to sulfide formation than other Group-8 metals. From Table 3 (exp 8), it appears that exclusively gas and coke are produced (no OLP). Also, as shown in Figure 6, the proportions of the major products formed prove that thiophene addition succeeded to direct the cracking toward the production of the targeted products (i.e., methane and ethylene). Indeed, H2 production was in that case divided by 4 and CO production divided by 3, whereas methane and ethylene molar fractions increased by a factor of 2. Moreover, CO2 formation was also reduced by addition of thiophene (from 8.1 to 5.4 mol %). Hence, steam reforming was

R1 ;CHdCH;R2 þ HO• f R1or2 -CH2 • þ R1or2 ;CHdO

ð5aÞ R1or2 ;CHdO f CO þ R1or2 H with R1or2 dCn H2n þ 1 or H ð5bÞ CO þ HO• f CO2 þ H•

ð6Þ

RH þ HO• f R• þ H2 O with R ¼ CH3 or Cn H2n þ 1 ð7Þ

Equation 5a represents the addition of HO• to an olefinic molecule (unsaturated oxygenated or nonoxygenated hydrocarbons, i.e., fatty acids, fatty esters, or alkenes) with release of an alkyl radical (R1or2-CH2•) and an aldehyde (R1or2CHdO). At the operating temperature (650 °C), this aldehyde can decompose in turn into CO and a hydrocarbon (R1or2H) or H2 according to eq 5b. Equation 6 represents the CO oxidation into CO2 by HO• addition. The released radical H• could then lead to H2 formation or to a minor extent to hydrogenation, which is often favored at low temperature. Results related to the two experiments of VEG steam cracking carried out at 650 °C, either with pure water (exp 5) or with H2O2 in addition to water (at 2.68 mol L -1, exp 6) are shown in Figure 5 in terms of the main products formed. The following features are apparent: in the presence of H2O2, the molar fractions of H2, CO, and particularly CO2 significantly increased: 2-fold for H2 and CO, 5-fold for CO2. However, the H2 mole fraction remained low (10.5 mol %). On the other hand, production in light hydrocarbons, that is, methane, ethylene, and propylene decreased by a factor of 4. These results confirm the occurrence of the elementary reactions (5-7) during the VEG steam cracking process conducted in the presence of H2O2 and suggest some arguments for explaining the observed variations in mole fractions of H2, CO, CO2, ethylene, and propylene. A decrease in ethylene and propylene mole fractions in favor of H2 and CO production might be explained by eqs 5a and 5b, showing ethylene and propylene potential conversion into H2 and CO. According to eq 6, CO2 production enhancement would be due to CO oxidation that also contributes to H2 formation. A decrease in saturated hydrocarbon production would 5671

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successfully inhibited by thiophene addition, causing poisoning of the catalytic activity of the coke adsorbed on the inner reactor walls. The large amounts of methane and ethylene produced resulted from accumulation during their successive elimination from hydrocarbon radicals by secondary cracking. However, coke formation was enhanced with thiophene addition (Table 3) probably because of the increase in ethylene production. Indeed, ethylene is well-known to be a coke precursor that would have led here to a pyrolytic coke.24 3.8. Optimum Operating Parameters for Maximum Gas Product Yields with Target Composition. With reference to the results discussed in the previous sections, process operating conditions could be tuned for orienting VEG conversion toward production of various target bioenergy carriers. All optimum operating conditions isolated in this work led to very high gas product yields and had in common the same residence time (1.5 ( 0.1 s) and reactor temperature (800 °C). When gaseous fuel, methane and ethylene, is the target bioenergy carrier, VEG steam cracking should be conducted at 1:1 steam-to-oil weight ratio (S/VEG), without inert diluent, but with addition of thiophene for inhibiting the surface catalytic effects occurring into the stainless-steel tubular reactor used. In that case, although a large amount of coke might be obtained (around 5 wt %), 93 wt % of gas product could be expected (Table 3) with an overall composition in methane and ethylene of 54 mol %. (Table A3, Appendix 3). On the other hand, when the target bioenergy carrier is synthesis gas with a H2/CO molar ratio close to 2 (favorable to low-temperature FT synthesis of diesel fuel), VEG steam reforming could be conducted at two different sets of operating conditions, depending on the importance attached to the other by-products formed. Indeed, both sets should yield more than 96 wt % of gas product. However, operating at 1:2 S/VEG molar ratio (thanks to dilution with an inert specie) should give an H2-rich gas product, while a 1:1 S/VEG molar ratio should lead to a C1-C3-richer gas product with still high H2 content (1:2 S/VEG: 45 mol % in H2, 14 mol % in C1-C3 hydrocarbons; 1:1 S/VEG: 27 mol % in methane, ethylene, and propylene, 40 mol % in H2, Table A3, Appendix 3, exps 1 and 2, respectively). Since H2 has to be very pure for use in fuel cells, operating with 1:1 S/VEG molar ratio would be all the more recommended because lower amounts of CO, CO2, and coke could be expected (Table A3, Appendix 3).

to direct the waste cooking oil conversion route toward production of the target bioenergy vector: (i) gaseous fuel (methane, ethylene, and propylene) by steam cracking, and (ii) synthesis gas by steam reforming. Particularly, in this last case, it has been possible to isolate operating conditions (800 °C, 1:1 steam-to-oil weight ratio) leading to an interesting compromise: optimum synthesis gas composition for lowtemperature Fischer-Tropsch synthesis of diesel fuel (i.e., a H2/CO molar ratio close to 2),15 with additional production in light hydrocarbons (methane, ethylene, and propylene) reaching 27 mol %. Furthermore, steam cracking as well as steam reforming of waste cooking oil were conducted under low steam-tooil weight ratios, that is, 1:2 and 1:1 (equivalent steam-tocarbon molar ratios ranging from 0.4 to 1 on the basis of the molecular weight estimated for the waste cooking oil used). Designing plants for low steam-to-carbon ratios (typically 2.5 or less) reduces the mass flow through the plant and thus the equipment size. The lowest investment is therefore generally obtained for plants designated for low steam-to-carbon ratios. Furthermore, low steam-to-carbon ratio results in a more energy efficient plant and thus in a lower operating cost.24 Also, using water for its physical and/or chemical properties in a process offers advantages in the product separation step (from incondensable products by quenching and from organic condensable products by decantation). Hence, the present work points out promising routes for upgrading waste cooking oil, because conventional technologies commonly found in petrochemistry could be used with minor modifications. In addition, there would be no obstruction in developing (on the basis of the conventional petrochemical technologies) small scale waste cooking oil converting plants. This option would thus offer the advantage of locating these plants close to the place where waste cooking oil is collected and stocked. Acknowledgment. Ecogras Company is gratefully acknowledged for supplying Vegetamoxoil samples. Also, the authors would express all their grateful acknowledgements to Dr. P.A. Glaude for helpful discussions during the work.

Appendix 1 Experimental Setup Procedure. The tubular reactor was progressively heated in flowing nitrogen gas to the desired reaction temperature. Next, the flow of nitrogen was stopped (when not used as inert diluent) and the desired feed (i.e., either the mixture formed by VEG and water or that formed by VEG, water, and nitrogen with the desired individual volumetric flow rates) was pumped into the reactor. Then, the end of the transient state was determined when all operating conditions (reactor pressure and temperatures of the different units) as well as H2, CO, and CO2 emissions (percent fractions measured every 2 minutes) were stationary (actually, the CO and CO2 IR analyzer as well as the H2-GC were the two key equipments for monitoring the steady-state operation). When the end of the transient state was reached, the reactant flow exiting the condenser was directed through the steady state transfer line, giving the starting time of the experiment. Each run was carried out for a period of 30 minutes, during which the liquid product was collected (in the gas-liquid separator for weighing and GC analysis at the end of the run) while the gas product was continuously analyzed (with the on-line IR analyzer and the off-line GCs)

4. Conclusions Using a bench-scale continuous flow tubular reactor, thermal cracking experiments in the presence of steam of a typical sample of waste cooking oil were conducted to study the final product distribution as function of temperature, residence time of the feed material, extent of dilution by addition of an inert gas, addition of a cracking initiator agent, and addition of a surface catalysis inhibitor agent. A more in-depth study developed in a previous work21 on the state of the walls of the stainless-steel reactor used showed that the coke formed here contained Ni and Fe particles and was thus catalytically active. Within this context, the present work showed that it was possible through selection of the process operating conditions 5672

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at equally spaced time intervals (5 min for CO, CO2, and H2; 15 min for the other noncondensable components of the gas product). Gas product sampling for offline GC analysis was carried out using a gas bulb located after the gas-liquid separation unit. At the end of the experiment, the total volume of the gas product (products formed and inert diluent when used) was read on the gas meter at the measured ambient temperature and pressure, the gas effluent leaving the reactor was directed through the transitory transfer line (thanks to the 3-way valves), and the gas-liquid separator was removed from the setup for further analysis. Then decoking was carried out as follows. After each reaction run, the reactor was heated to very high temperature (furnace temperature: 900 °C) while a mixture of nitrogen

and air (0.3 and 0.7 NL/min, respectively) was passed through it to burn off the coke formed during the run. To optimize decoking, it was decided to proceed by successive oxidation cycles followed by integration on the entire oxidation duration. Therefore, the air volumetric flow rate was increased very slowly by stages at the end of each oxidation cycle while maintaining constant the total volumetric flow rate of the N2 and air mixture. For each decoking cycle, percent volume fractions of CO and CO2 coke (ϑcoke CO,k (%) and ϑCO2,k (%), respectively) were read on the IR analyzer at equally spaced time intervals (Δtk = 2 minutes) while the total volume of the gas product (CO, CO2, N2, and air) was read on the gas meter at ambient temperature (Ta,Pa)). When the decreasing ϑcoke and pressure (Vcoke k CO,k coke (%) and ϑCO2,k (%) parameters reached constant values, the

Appendix 2 Table A2. General Equations Related to the Material Balances Made on the Experimental Setup Used a Equations Related to the Inlet of the Reactor • For the Liquid Feed Materials (Vegetamixoil and Water) in FVEG ¼ FVEG ð90 °CÞQin VEG ð90 °C, 1 atmÞ in

FH2 O ¼ FH2 O ð25 °CÞQin H2 O ð25 °C, 1 atmÞ

ðA2:2Þ

• For the Gas Feed Material (Nitrogen) PNTP Qin in N2 ðT NTP , PNTP Þ F N2 ¼ M N2 RT NTP

ðA2:3Þ

• Total Mass Feeding the Reactor in in in min T ¼ mVEG þ mH2 O þ mN2 in

in

ðA2:1Þ

ðA2:4Þ in

in in where min VEG ¼ F VEG Δt exp , mH2 O ¼ F H2 O Δt exp , and mN2 ¼ F N2 Δt exp

ðA2:5Þ

• Residence Time of the Feed Materialb τ ¼ V R =Qin T ðT R , P R Þ

ðA2:6Þ

with in

in Qin R ðT R , PR Þ ¼ F T

in

RT R PR in

F in T ¼ F VEG M VEG þ F H2 O M H2 O þ F N2 M N2 Equations Related to the Outlet of the Reactor c • Total Mass, Weight and Molar Composition of the Organic Liquid Phase (OLP) Product X Ai out mout mout OLP ¼ i, cond with mi, cond ¼ mE K i=E AE out mout xout i, cond i, cond =M i, cond xout and xout i, cond ¼ P out i, cond ¼ P out mi, cond ðxi, cond =M out i, cond Þ

• Intermediate Normalization of the Detected Incondensable Components (Gaseous Products) X yj = yj where j is either an online or an offline quantified product For an online quantified product: yj = ϑj yj =M j mj Aj For an offline quantified product: yj ¼ P with yj ð%Þ ¼ P 100 and mj ¼ mE K j=E yj =M j AE mj • Mass of the Online Detected Incondensable Products (H2, CO, and CO2) V T ðT A , PA Þ PA F out TG ¼ Δt exp RT A

5673

ðA2:7Þ ðA2:8Þ

ðA2:9Þ

ðA2:10Þ

ðA2:11Þ

ðA2:12Þ

ðA2:13Þ

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Gornay et al. Table A2. Continued

out F out onlineG ¼ F TG

X

out out yout j, onlineG where yj, onlineG ¼ ϑj, onlineG

out

F onlineG ¼ F out TG

X

ðA2:14Þ

out yout j, onlineG M j, onlineG

ðA2:15Þ

mout onlineG ¼ F onlineG Δt exp

out

ðA2:16Þ

• Mass of the Offline Detected Incondensable Products (Other X Gases than Hin2, CO, and CO2) in out out out F out yout offlineG ¼ F TG - F onlineG -F N2 =M N2 ¼ FTG ð1 j, onlineG Þ - F N2 =M N2

ðA2:17Þ

out

F offlineG ¼ F out offlineG

X

out yout j, offlineG M j, offlineG

out

ðA2:18Þ

mout offlineG ¼ F offlineG Δt exp

ðA2:19Þ

• Total Mass of All Incondensable Products out out mout incond ¼ monlineG þ mofflineG

ðA2:20Þ

• Partial Molar Flow Rate for Components Encountered Both in the Gas Phase and in the Organic Liquid Phase out out out ¼ F out F out j offlineG yj, offlineG þ mi, cond =ðM i, cond Δt exp Þ

ðA2:21Þ

• Final Normalization of All Detected Component Molar Fractions As a Whole X F out yk ð%Þ ¼ ðF out k = k Þ100

ðA2:22Þ

out out with F out K ¼ F TG yj, onlineG for online detected gaseous products

ðA2:23Þ

out ¼ F out F out k offlineG yj, offlineG for offline detected gaseous products

ðA2:24Þ

¼ F out k

mout i, cond out M i, cond Δtexp

¼ F out k

for the organic liquid phase ðOLPÞ products

mcoke for the coke deposit ðwith M C ¼ 12:01145Þ M C 3 Δtexp

ðA2:25Þ

ðA2:26Þ

and Fout k given by eq A2.21 for components encountered both in the liquid phase and in the gas phase. Material and Carbon Balance Equations • Overall Material Balance (%) out out out min VEG -ðmVEG þ mincond þ mOLP þ mcoke Þ 100 min VEG • Carbon Balance (%) in out ðF C -F C Þ 100 in FC

ðA2:27Þ

ðA2:28Þ

in out k out with F in C = xCF VEG and F C = νCFk MC, where xC is the weight composition of Vegetamoxoil (Table 1) and MC = 12.01145. a The meaning of each variable is described in the nomenclature section. b With reference to the temperature profile occurring along the tubular reactor used (isothermal section of the reactor restricted to a length of 150 mm), the residence time of the feed material calculated in the present work has no physical meaning and should be considered as a variable taken as reference for comparing experiments between them. c The mass of N2 is invariant between the inlet and the outlet of the reactor. The total mass of coke is given by eq A1.2.

air volumetric flow rate was again increased very slowly at constant total volume flow rate (0.5 NL/min for air and N2) coke for a new oxidation cycle. When the ϑcoke CO,k (%) and ϑCO2,k (%) parameters remained constant in spite of increasing the air input, all the coke deposit was assumed to be removed from the inner surface of the reactor and the decoking ended. Integration, over all the decoking duration, of coke (%) and Vcoke (Ta,Pa) parameters ϑcoke CO,k (%), ϑCO2,k k recorded during each oxidation cycle yields the mass of

coke deposit mcoke according to eq A1.2, where the gas product was assumed to follow the ideal gas law. ( ) Pa X coke coke coke V k ðT a , Pa ÞðyCO, k þ yCO2 , k ÞM C mcoke ¼ RT a k ðA1:2Þ ycoke j,k

is the molar fraction of specie j (CO or CO2) where measured at time k (equal to the volume fraction ϑcoke j,k 5674

Energy Fuels 2009, 23, 5663–5676

: DOI:10.1021/ef900529n

Gornay et al.

Appendix 3 Table A3. Percent Molar Composition on N2 Free Basis of the Products Obtained during Vegetamoxoil Thermal Cracking in the Presence of Steam (Steam Cracking and Steam Reforming) a compound (chemical formula) methane (CH4) ethylene (C2H4) ethane (C2H6) propylene (C3H6) n-propane (C3H8) 1-butene (C4H8) 1,3-butadiene (C4H6) 1-pentene (C5H10) n-pentane (C5H12) 1-hexene (C6H12) n-hexane (C6H14) 1-heptene (C7H14) n-heptane (C7H16) 1-octene (C8H16) 1-nonene (C9H18) 1-decene (C10H20) 1-undecene (C11H22) 1-dodecene (C12H24) 1-tridecene (C13H26) 1-tetradecene (C14H28) n-pentadecane (C15H32) cyclopentene (C5H8) cyclohexene (C6H10) cyclohexane (C6H12) benzene (C6H6) toluene (C7H8) Acetone (C3H6O) acetic acid (C2H4O2) acrylic acid (C3H4O2) peten-4-oic acid (C5H7O2) hexen-5-oic acid (C6H9O2) hepten-6-oic acid (C7H11O2) H2 CO CO2 coke unidentified b total

exp 1

exp 2

exp 3

exp 4

exp 5

exp 6

exp 7

exp 8

2.70 9.35 0.37 1.48 0.04 0.22 0.69

5.91 18.49 0.88 2.97 0.10 0.31 1.28

3.41 15.74 0.83 3.41 0.12

6.93 28.08 2.02 6.72 0.27

6.01 23.93 2.19 5.13 0.26

1.54 6.35 0.53 1.52 0.09

7.42 30.94 1.92 3.76

15.17 38.76 1.87 1.93 0.12 0.19

2.40 0.44 0.36 0.10

1.36 0.27 1.83 0.23 1.20 0.59 0.40 0.39 0.40 0.25 0.16 0.20 0.05 0.21

0.32 0.06

44.84 22.37 13.24 4.20 0.12 100

0.60 0.12

40.49 18.83 8.06 1.80 0.16 100

0.11 0.20 5.41 15.58 13.18 1.84 0.55 0.41 11.48 7.36 11.69 2.97 4.81 100

2.14 0.31 3.84 0.25 2.66 0.31 1.17 0.82 0.81 0.79 0.48 0.31 0.47 0.08

0.10 3.14 1.78 0.21

0.67 1.25 1.23 1.02 0.45

8.32 10.06 6.12 3.70 15.01 100

6.09 9.46 4.83 3.39 19.65 100

1.08 0.22 2.90 0.16 2.45 0.30 1.29 0.89 0.85 0.79 0.50 0.33 0.49 0.12 0.48 0.62 1.10 1.52 1.28

10.53 17.61 23.86 7.50 13.10 100

0.78 0.12 1.01 0.88 0.60 0.47 0.50 0.49 0.29 0.18 0.24 0.10 0.17 0.13 1.96 1.59 0.16

1.30 0.10

11.33 7.71 6.57 8.56 9.72 100

9.61 6.34 5.45 12.28 6.88 100

a Description of the various experiments (operating conditions, overall material balances and yields of products) are given in Table 3. Water was always present at the outlet of the experimental setup but was not quantified in this work. b The components responsible for this fraction could not be identified using the available GC-MS database (see Section 2.2.5.2). First, unidentified components having a retention time in-between the normal alkane with (n - 1) carbon atoms and the normal alkane with n carbon atoms were gathered by Cn-cuts to which an average chemical formula CnH2nþ2 was arbitrarily attributed for estimating the corresponding average molecular weight, and thus, molar fraction. This procedure is a rough approximation when operating at low temperature that yields large amounts of organic liquid phase product with oxygen-containing compounds, but is quite realistic when operating at high temperature that yields mainly short hydrocarbons. All molar fractions were then summed to obtain the molar fraction of the denoted “unidentified species” related to each experiment.

F out C = outlet carbon mass flow rate F out onlineG = mass flow rate of the online detected incondensable products taken as a whole in the outlet gas stream out = molar flow rate of the total outlet gas stream FTG Δtexp = duration of experiment Ki/E = carbon response factor of component i compared with internal standard E for quantification by GC-FID analysis mE = mass of internal standard E in = inlet mass of water mH 2O in = inlet mass of nitrogen mN 2 min T = inlet total mass min VEG = inlet mass of Vegetamixoil mcoke = mass of coke deposited during each experiment mout i,cond = mass of the outlet condensable component i mout incond = total mass of the detected incondensable products taken as a whole in the outlet gas stream mout offlineG = total mass of the off-line detected incondensable products taken as a whole in the outlet gas stream mout OLP = mass of the outlet organic liquid phase product

according to the ideal gas law), and MC is the carbon molecular weight. Finally, the mass of the liquid product collected during each run was determined by weighing of the gas-liquid separator (including cotton) at the beginning and the end of the experiment while composition was obtained by GC analysis. Nomenclature AE = chromatographic surface of internal standard E Ai = chromatographic surface of component i Fin T = inlet total molar flow rate F in C = inlet carbon mass flow rate in FH = inlet water mass flow rate 2O in F N2 = inlet nitrogen mass flow rate F in VEG = inlet Vegetamixoil mass flow rate Fjout = molar flow rate of component j in the outlet gas stream out = molar flow rate of the online detected inFonlineG condensable products taken as a whole in the outlet gas stream 5675

Energy Fuels 2009, 23, 5663–5676

: DOI:10.1021/ef900529n

Gornay et al.

mout onlineG

= mass of the online detected incondensable products taken as a whole in the outlet gas stream mout VEG = mass of the nonconverted Vegetamixoil remaining in the outlet stream Mj = molecular weight of component j Mout i,cond = molecular weight of the outlet condensable component i Mout j,offlineG = molecular weight of the offline detected incondensable product j present in the outlet gas stream Mout j,onlineG = molecular weight of the online detected incondensable product j present in the outlet gas stream PA = ambient pressure PNTP = pressure related to normal temperature and pressure conditions (PNTP = 1 atm) PR = pressure at the inlet of the reactor R = ideal gas constant (R = 8.314411 J mol -1 K -1) Qin H2O (25 °C,1 atm) = inlet volumetric flow rate related to water at 25 °C and 1 atm in (TNTP,PNTP) = inlet volumetric flow rate related to QN 2 nitrogen at TNTP and PNTP Qin T (TR,PR) = inlet total volumetric flow rate of the feed material at TR and PR Qin VEG (90 °C,1 atm) = inlet volumetric flow rate related to Vegetamixoil at 90 °C and 1 atm S/VEG = steam-to-oil weight ratio TA = ambient temperature TNTP = temperature related to normal temperature and pressure conditions (TNTP = 273.15 K) TR = temperature of the reactor along its isothermal part VR = reactor isothermal part volume VT (TA,PA) = total volume of the outlet gas at ambient temperature and pressure xout i,cond = molar fraction related to the outlet condensable component i

xC = mass fraction in carbon element xout i,cond = mass fraction related to the outlet condensable component i yj = molar fraction of component j in the gas phase yj = mass fraction of component j in the gas phase yout j,offlineG = molar fraction of the offline detected incondensable product j in the outlet gas stream yout j,onlineG = molar fraction of the online detected incondensable product j present in the outlet gas stream Greek Symbols ϑout j,onlineG = volumetric fraction of the online detected incondensable product j in the outlet gas stream νkC = stoichiometric coefficient of carbon element in the chemical formula of component k FH2O (25 °C) = density of water at 25 °C FVEG (90 °C) = density of Vegetamixoil at 90 °C τ = residence time of the feed material Abbreviations FID = flame ionization detector FT = Fischer-Tropsch GC = gas chromatography IR = infrared ray absorption MS = mass spectroscopy OLP = organic liquid phase TCD = thermal conductivity detector VEG = Vegetamixoil wt % = weight percent mol % = molar percent

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