Thermal Cracking of Canola Oil: Reaction Products in the Presence

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Energy & Fuels 1996, 10, 1150-1162

Thermal Cracking of Canola Oil: Reaction Products in the Presence and Absence of Steam Raphael O. Idem, Sai P. R. Katikaneni, and Narendra N. Bakhshi* Catalysis and Chemical Reaction Engineering Laboratory, Department of Chemical Engineering, 110 Science Place, University of Saskatchewan, Saskatoon, Saskatchewan, Canada S7N 5C9 Received February 20, 1996X

The product distribution obtained from the thermal cracking of canola oil was studied at atmospheric pressure in a fixed-bed reactor in the temperature range 300-500 °C and gas hourly space velocity (GHSV) in the range 3.3-640 h-1 over inert materials and in the presence and absence of steam. Results showed that canola oil conversions were high (54-100 wt %) and depended strongly on the operating variables. Products essentially consisted of C4 and C5 hydrocarbons, aromatic and C6+ aliphatic hydrocarbons, and C2-C4 olefins, as well as a diesellike fuel fraction and hydrogen. GC-MS analyses showed that product distribution as well as the lengths of the carbon chain of hydrocarbons and oxygenated hydrocarbons depended strongly not only on the cracking temperature and space velocity but also on whether cracking was conducted in the presence or absence of steam. On the other hand, cracking over inert materials showed that both conversion and product distribution were completely independent of morphology of the cracking surface. A reaction scheme has been proposed to account for the product distribution obtained from the thermal cracking of canola oil. The observed changes in both canola oil conversion and product distribution with changes in the operating variables were found to be consistent with the reaction scheme.

Introduction Hydrocarbons are used in large quantities in a wide range of industrial applications. Traditionally, hydrocarbons are produced from petroleum sources.1 However, there is a growing interest in studies involving their production from other sources such as from plant oils and animal fats.2-18 Most of these studies are * Author to whom correspondence should be addressed [fax (306) 966-4777]. X Abstract published in Advance ACS Abstracts, September 1, 1996. (1) Waddams, A. L. Chemicals from Petroleum; Gulf Publishing: Houston, 1980. (2) Weisz, P. B.; Haag, W. O.; Rodewald, P. G. Science 1979, 206, 57-58. (3) Milne, T. A.; Evans, R. J.; Filly, J. In Research in Thermochemical Biomass Conversions; Bridgewater, A. V., Kuester, J. L., Eds.; Elsevier Applied Science: London, 1988; pp 910-926. (4) Campbell, I. Biomass, Catalysis and Liquid Fuels; Holt, Rienhart and Winston: London, 1983. (5) Baker, E. G.; Elliott, D. C. In Research in Thermochemical Biomass Conversion; Bridgewater, A. V., Ed.; Elsevier Applied Science: London, 1987; pp 883-895. (6) Craig, W.; Coxworth, E.; Saddler, J. In Proceedings of the Fifth Canadian Biomass R & D Seminar; Hagnain, S., Ed.; Elsevier Applied Science: London, 1984; pp 143-146. (7) Boocock, D. G. B.; Konar, S. K.; Mackay, A.; Cheung, P. T. C.; Liu, J. Fuel 1992, 71, 1291-1297. (8) Prasad, Y. S.; Bakhshi, N. N.; Mathews, J. F.; Eager, R. L. Can. J. Chem. Eng. 1986, 64, 278-284. (9) Prasad, Y. S.; Bakhshi, N. N.; Mathews, J. F.; Eager, R. L. Can. J. Chem. Eng. 1986, 64, 285-292. (10) Craig, W.; Coxworth, E. In Proceedings of the Sixth Canadian Bioenergy R & D Seminar; Ganger, C., Ed.; Elsevier Applied Science: London, 1987; pp 407-411. (11) Chantal, P. S.; Kaliaguine, S.; Grandmaison, J. L.; Mahay, A. Appl. Catal. 1984, 10, 317-332. (12) Katikaneni, S. P. K.; Adjaye, J. D.; Bakhshi, N. N. Can. J. Chem. Eng. 1995, 73, 484-497. (13) Katikaneni, S. P. K.; Adjaye, J. D.; Bakhshi, N. N. Energy Fuels 1995, 9, 599-609. (14) Adjaye, J. D.; Bakhshi, N. N. Biomass Bioenergy 1995, 8, 131149.

aimed at augmenting the anticipated future shortfall in hydrocarbons obtained from petroleum sources as well as minimizing environmental pollution that arises from the release of large amounts of SO2, NOx, and CO2 from petroleum processes. In the recent literature,2-15 studies on the production of hydrocarbons from plant oils and animal fats have been conducted using cracking catalysts such as HZSM5, Pt/HZSM-5, silica-alumina, H-Y, H-modernite, aluminum pillared clays, alumina, silicalite, and physical mixtures of these catalysts. In most of these studies, hydrocarbons in the gasoline boiling range were predominant in the organic liquid product (OLP) fraction, whereas the gaseous fraction contained mostly paraffinic hydrocarbon. Also, oil conversion and type of products obtained over these catalysts were attributed mostly to the acidity and shape selectivity of the catalysts used.2,8,9,12,13 For example, it has been shown8,9,12,13 that the higher the acidity (especially the Brønsted acidity) of the catalyst, the greater the feed conversion. Also, the formation of aromatic hydrocarbons was ascribed specifically to the presence of highstrength Brønsted acid sites on the catalysts.3,8,9,12,13 These results are in contrast to those of Chang and Wan,18 Egloff and Morrel,19 Egloff and Nelson,20 Lipinsky et al.,21 Schwab et al.,22 and Crossley et al.,23 as well as studies using silicalite catalyst by Katikaneni et al.12 (15) Sharma, R. K.; Bakhshi, N. N. Fuel Process. Technol. 1991, 27, 113-130. (16) Nawar, W. W. J. Agric. Food Chem. 1969, 17, 18-21. (17) Alencar, J. W.; Alves, P. B.; Craveiro, A. A. J. Agric. Food Chem. 1983, 31, 1268-1270. (18) Chang, C. C.; Wan, S. W. Ind. Eng. Chem. 1947, 39, 15431548. (19) Egloff, G.; Morrell, J. C. Ind. Eng. Chem. 1932, 24, 1426-1427. (20) Egloff, G.; Nelson, E. F. Ind. Eng. Chem. 1933, 25, 386-387.

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Thermal Cracking of Canola Oil

which showed that the formation of aromatic hydrocarbons or any other type of hydrocarbons did not necessarily require the presence of any acid centers on the catalyst. Proper design of catalyst for any reaction requires knowledge of the mechanism of the reaction. A number of workers2,7-9,12,13,24 have postulated reaction schemes showing various reaction pathways for the conversion of plant oils to different hydrocarbon and oxygenated hydrocarbon products over acid catalysts. Although most of these schemes are similar, there are conflicting views concerning the actual reaction intermediates as well as the correct sequence of intermediate reaction steps. On the other hand, there is information in the literature on the mechanism(s) for plant oil conversion by thermal cracking.17,18,22 However, these are restricted to either saturated17,18,21,23 or unsaturated21-23 triglyceride molecules. These do not represent the composition of a typical natural oil such as canola oil which contains a mixture of both saturated and unsaturated fatty acid moieties in the triglyceride molecule. The mechanism for the thermal cracking of canola oil is not available in the literature. Studies involving the thermal cracking of plant oils and animal fats have been reported in the literature.18-23 In most of these studies, the major objective was to provide an alternative source for conventional fuels and chemicals. Their results showed that products of industrial importance such as aromatic and aliphatic hydrocarbons as well as C2-C4 olefins and a diesel-like fuel were produced. Also, in almost all of these studies, batch processing involving the destructive distillation of the oil was employed. However, since batch processes require cleanup and charging of feed after each run, interruptions are frequent and throughputs are generally low. They are therefore unsuitable for the continuity and high throughputs required for industrial operations. Thus, to improve throughputs, flow reactors are preferred for industrial operations. The use of flow reactors for studies involving the thermal cracking of plant oils or animal fats has not been reported before in the literature. It is therefore highly desirable to conduct an in-depth study on thermal cracking in a flow reactor setup. In the literature,8,9,12 studies on plant oil conversions involving cofeeding with steam have been conducted mostly using catalysts containing strong acid catalysts. Cofeeding with steam resulted in an increase in olefins production. According to these workers, this increase was attributed mainly to poisoning of the acid sites by steam. However, according to Nawar,16 Billaud et al.,25 and Gillet-Dominguez,26 cofeeding steam also results in a decrease in residence time of the oil in the reactor and facilitates the hydrolysis of the triglyceride molecules. The occurrence of these latter two processes in addition to acid site poisoning is certain to introduce drastic (21) Lipinsky, E. S.; Anson, D.; Longanbach, J. R.; Murphy, M. J. Am. Oil Chem. Soc. 1985, 62, 940-942. (22) Schwab, A. W.; Dykstra, G. J.; Selke, E.; Sorenson, S. C.; Pryde, E. H. J. Am. Oil Chem. Soc. 1988, 65, 1781-1786. (23) Crossley, A.; Heyes, T. D.; Hudson, B. J. F. J. Am. Oil Chem. Soc. 1962, 3, 9-14. (24) Vonghia, E.; Boocock, D. G. B.; Konar, S. K.; Leung, A. Energy Fuels 1995, 9, 1090-1096. (25) Billaud, F.; Dominguez, V.; Broutin, P.; Busson, C. J. Am. Oil Chem. Soc. 1995, 72, 1149-1154. (26) Gillet-Dominguez, V. Ph.D. Thesis, INPL-ENSIC, University of Nancy, 1994.

Energy & Fuels, Vol. 10, No. 6, 1996 1151 Table 1. Chemical Composition of Canola Oil acid oleic linoleic linolenic stearic palmitic

formula

wt %

60 CH3(CH2)7CHdCH(CH2)7COOH 20 CH3(CH2)4CHdCHCH2CHd CH(CH2)7COOH CH3CH2CHdCHCH2CHdCHCH2CHd 10 (CH2)7COOH 2 CH3(CH2)16COOH 4 CH3(CH2)14COOH

no. of C atoms 18 18 18 18 16

modifications in the product distribution and plant oil conversion. However, to determine the actual effect of steam on conversion and product distribution in a flow reactor, it is necessary to conduct cofeeding steam studies over catalysts that do not contain acid sites or in the absence of a catalyst. In this work, we have studied the type of products and product yields obtained from the thermal cracking of a typical plant oil (canola oil) in a fixed-bed microreactor as a function of reaction temperature and canola oil gas hourly space velocity (GHSV) in the presence and absence of steam. We have also studied the thermal cracking of canola oil over inert materials such as ceramic chips, quartz glass chips, and glass wool to determine the effect of surface area and morphology of material on conversion and product distribution. The results have been used to postulate the reaction pathways involved in the conversion of a mixed saturated and unsaturated triglyceride molecule (as in canola oil) in the absence of a catalyst to various products. These results are presented in this paper. Experimental Section Materials. (a) Canola Oil. Canola oil was obtained from CSP Foods, Saskatoon, Canada, and was of the degummed and refined variety. It consisted mainly of unsaturated triglycerides having an average molecular formula of C59H94O5. The fatty acid moieties present in canola oil and their distributions are as follows: 60 wt % oleic acid (18:1), 20 wt % linoleic acid (18:2), 10 wt % linolenic acid (18:3), 2 wt % stearic acid (18:0), 4 wt % palmitic acid (16:0), and small quantities of eicosenic acid (20:0) and erucic acid (22:1) totaling close to 4 wt %. The first and second numbers in parentheses refer, respectively, to the numbers of carbon atoms and CdC bonds per molecule of the acid. Details are given in Table 1. (b) Inert Materials. The inert materials used for thermal cracking reaction studies were ceramic chips, quartz glass chips, and glass wool. The ceramic and quartz glass chips were obtained by crushing and sieving Berle saddles and quartz glass, respectively, to obtain materials of 1 mm average particle size. Glass wool was obtained from Supelco Inc., Mississauga, ON, Canada. Thermal Cracking Studies. (a) Equipment. All reaction runs were carried out at atmospheric pressure in a continuous down-flow fixed-bed stainless steel (SS-316) microreactor (10 mm i.d. and 508 mm overall length). The reactor was equipped with a stainless steel grid positioned centrally within the reactor on which the inert materials were supported during reaction. The reactor was heated by a furnace with temperature controlled by a series SR22 microprocessor-based autotuning PID temperature controllers (Shimaden Co. Ltd., Tokyo, Japan) using a K-type thermocouple placed on the furnace side of the annulus between the furnace and the reactor. Two other thermocouples were used to monitor the temperature along the length of the inside of the reactor. These were contained in thermowells positioned from the top and bottom of the reactor as shown in Figure 1. Thermocouple 1 was used to monitor temperatures from the stainless steel grid to the top of the reactor, while thermocouple 2 was used

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1152 Energy & Fuels, Vol. 10, No. 6, 1996

Figure 1. Experimental setup for the thermal cracking of canola oil. G, stainless steel grid. to monitor temperatures from the grid to the bottom of the reactor. This arrangement was capable of ensuring an accuracy of (2 °C for the temperature of the inside of the reactor at any point along the length of the reactor. (b) Empty Reactor Studies. Two types of reactions were conducted in the empty reactor. These were reactions in the presence and absence of steam. Reaction runs in the absence of steam were carried out at temperatures in the range 300-500 °C and canola oil feed rate in the range 5.11-24.2 g/h [i.e., canola oil gas hourly space velocity (GHSV) in the range 3.3-15.4 h-1 at standard temperature and pressure (STP)]. In the presence of steam, reactions were carried out at temperatures in the range 400500 °C at a canola oil feed rate of 5.11 g/h. The steam/canola oil ratios used were 0:1, 1:1, and 4:1, and the corresponding water feed rates were 0, 5.11, and 20.44 g/h. These resulted in canola oil GHSV of 3.3, 162.6, and 640 h-1 (STP), respectively. The experimental procedure for each run for both cases was as follows: The empty reactor was heated in flowing argon gas to the desired reaction temperature. After this temperature was reached, the flow of argon was stopped and the desired feed (i.e., either canola oil alone or both canola oil and water) was then pumped into the reactor using a single pump (Eldex, Model A-60-S) in the case of feeding canola oil alone and two separate pumps (Eldex, Model A-60-S) in the case of cofeeding canola oil with steam. The reaction was carried out for a duration of 30 min starting from when the first bubble of gas product was observed to enter the gas collector (i.e., about 1 min from the start of the canola oil feed pump). (c) Conversion over Inert Materials. Reaction runs were carried out at atmospheric pressure in the same continuous down-flow fixed-bed microreactor as in the case of empty reactor studies (Figure 1) over ceramic chips, quartz glass chips, and glass wool at 500 °C. In each case, 2 g of the desired inert material (1 mm particle size except in the case of glass wool) was used. Canola oil was fed at the rate of 5.11 g/h [i.e., canola oil GHSV of 3.3 h-1 (STP) with respect to the total volume of reactor]. The inert material was loaded onto the stainless steel grid. The experimental procedure was the same as in the empty reactor studies. (d) Collection of Reaction Products. The product mixture leaving the reactor was condensed in a water-cooled heat exchanger, followed by an ice-cooled condenser, to sepa-

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Idem et al. rate the gaseous and liquid products. The gas product was collected over saturated brine while the liquid product was collected in a glass trap positioned after the condenser. The scheme for collection and analysis of various products is given in Figure 2. A detailed description is given elsewhere.12,13 (e) Analysis of Products. (1) Gaseous Products. The gaseous product was analyzed with a Carle 500 gas chromatograph (GC) using a combination of packed and capillary columns and both flame ionization and thermal conductivity detectors. The sample was injected into a 2 m long precolumn, packed with OV-101 silicone oil. All components lighter than ethane (C2H6) passed rapidly through this column and were separated at 70 °C in Porapak Q and molecular sieve 13X columns, each 2.7 m in length, into CO, CO2, C1, and C2, which were measured by a thermal conductivity detector (TCD). The components having a carbon number of three and above (i.e., C3+) were back-flushed onto the fused silica capillary for which the oven temperature was programmed from 40 to 200 °C. The C3+ components were measured by flame ionization detector (FID). Analysis took approximately 30 min. The identities of the individual GC peaks had earlier been established using a gas chromatography-mass spectrometry (GC-MS) technique. After normalization of the components, an average molecular weight for the gas product was calculated. The overall weight of the gas product was then determined from the average molecular weight and the total volume of gas evolved during the run. All of the columns were purchased from Supelco. (2) Organic Liquid Product (OLP). The OLP was also analyzed with a Carle 500 GC, which was equipped with a bonded nonpolar (methyl silicone) 50 m × 0.2 mm i.d. capillary column and a FID. Temperature in the GC oven was programmed from 40 to 200 °C. Usually, the OLP contains a wide variety of components (hydrocarbons with isomers and various functional groups). Therefore, a GC-MS analysis in conjunction with the use of known pure compounds and calibration mixtures (PolyScience Corp.) was necessary to establish the identity of the components in the OLP. GC-MS analysis was performed using a VG Analytical 70VS extended magnet mass spectrometer. The attached GC was a GC 800 series that was equipped with the same set of columns and operated under the same oven temperature program as the Carle GC. Details concerning the analysis of both gas and liquid products are given elsewhere.12,13 (3) Residual Oil. The nondistilled (i.e., total) liquid product was also analyzed using both GC and GC-MS techniques. These in conjunction with OLP analysis were used to qualitatively determine the composition of residual oil. However, to obtain a direct qualitative evaluation, it was necessary to perform a direct GC-MS analysis of residual oil itself. Essentially, residual oil (obtained from vacuum distillation of the total liquid product) was diluted in acetone prior to GC-MS analysis, and the components were identified by their mass spectra. For each complete set of analyses (i.e., for gas, OLP, and residual oil fractions), the individual compounds identified were placed under similar chemical groups. These included light paraffin gases (i.e., methane, ethane, and propane), C4 and C5 hydrocarbon gases, C2-C4 olefins, alcohols, ketones, aldehydes, C6+ aliphatic hydrocarbons, benzene, toluene, xylenes, ethylbenzene, C9+ aromatic hydrocarbons, total aromatic hydrocarbons, and residual oil. Those peaks that could not be identified using the available data base were termed “the unidentified fraction”. (4) Coke. Coke was estimated as indicated in Figure 2. Essentially, after each reaction run, the inside surface of the reactor/accessories was first washed with acetone to remove residual oil and then dried at 110 °C to remove the acetone. The dried reactor was then heated at 500 °C while air was passed through it to burn off the coke formed during the thermal reaction run. The difference between the weight of

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Energy & Fuels, Vol. 10, No. 6, 1996 1153

Figure 2. Scheme for collection and analysis of various products. the dried reactor and the weight of the heated reactor after the coke burn-off was taken as an estimate of the coke formed.

Table 2. Mass Balance and Yields of Products as a Function of Cracking Temperaturea temp, °C product

Results and Discussion Products from Empty Reactor Runs. The products from the thermal cracking of canola oil are shown in Tables 2 and 3. Table 2 shows the conversion and yields of various products, while Table 3 shows the concentrations of components in the gas and OLP fractions as a function of reaction temperature. As shown in these tables, the gross products were coke, residual oil, gas, OLP fraction, and those products which could not be identified. It was possible to determine CO, CO2, and most of the C1-C4 hydrocarbon components (such as methane, ethylene, propane, propylene, isobutane, isobutylene, and 1-butene) in the gas product from GC analysis alone. GC-MS analysis was, however, required for identification of diene, cyclic, and all C5+ hydrocarbon components. Typical diene, cyclic, and C5+ hydrocarbon components in the gas phase were butadiene, pentane, cyclopentane, 2-pentene, 2-methyl-1butene, 1,2-dimethylcyclopropane, 1,3-pentadiene, and cyclopentene. Typical products in the OLP (identified by their mass spectra) were benzene, 1-pentyl-2-propylcyclopropane, 3-methylbutanol, naphthalene, 2,4-octadiene, 1-dodecene, 1-heptene, and indene. On the other hand, both the direct and indirect analyses showed that the components in the residue obtained from vacuum distillation of the total liquid product were high molecular weight materials with GC retention times >30 min and boiling points higher than the temperature used for vacuum distillation (200 °C at 172 Pa). In this study, these constituted “residual oil”. Typical components of residual oil were 1-(1,5-dimethylhexyl)-4-(4-methylpentyl)cyclohexane, palmitic acid, fluoranthene, and the ethyl ester of 2-cyclopentene-1-undecanoic acid.

500

450

400

370

Overall Mass Balance, wt % of Canola Oil Fed gas 75.0 71.0 55.8 38.0 OLP 14.8 17.2 34.4 45.9 coke 3.9 3.9 3.9 3.9 residual oil 0 1.2 1.6 6.1 unaccounted fraction 6.3 6.0 4.3 6.1 total

100

% canola oil convrn

100

100 98.8

100 98.4

100 93.9

300 15.0 38.1 0 41.9 5.0 100 58.1

Yields of Products Gas Phase Components, wt % of Canola Oil Fed methane 10.6 9.9 7.1 4.1 ethylene 23.6 22.1 17.0 10.8 ethane 7.0 6.7 5.4 4.1 propylene 13.5 12.2 10.3 7.0 propane 1.1 1.1 0.9 1.0 isobutane 0.02 0.01 0.01 0.01 n-butane 6.5 6.3 5.5 4.1 isobutylene 1.0 1.0 0.8 0.6 1-butene 0.5 0.6 0.4 0.2 C5+ CxHy gases 4.0 5.5 4.5 3.7 DME 0.1 0.1 0.1 0 CO + CO2 3.8 3.7 2.9 2.0 hydrogen 1.6 1.3 0.9 0.5 C2-C3 olefins 37.1 34.3 27.3 17.8

1.4 4.1 1.7 2.7 0.5 0 1.6 0.3 0.1 1.6 0 0.8 0.1 6.8

OLP Components, wt % of Canola Oil Fed alcohols 0.9 1.0 1.2 1.9 acetone 0.1 0 0 0.2 ketones 0 0 0 0.1 benzene 4.0 4.1 5.0 4.6 toluene 2.8 2.8 3.9 3.5 xylenes 0.6 0.7 1.1 1.2 ethylbenzene 0.4 0.5 0.8 0.9 C9+ aromatics 1.2 1.4 2.7 3.4 aliphatics 0.6 0.7 1.5 1.9 unidentified 4.3 6.6 18.1 28.1

1.9 1.5 0.1 2.5 1.6 0.5 0.4 2.7 1.0 25.7

total aromatics aRuns

9.0

9.5

conducted at GHSV of 3.3 h-1.

13.5

13.6

7.7

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Idem et al.

Table 3. Composition of Gas and OLP Fractionsa temp , °C product

500

450

400

370

Composition of Gas Fraction, wt % of Gas methane 14.2 13.9 12.7 10.7 ethylene 31.4 31.1 30.5 28.3 ethane 9.3 9.4 9.7 10.7 propylene 18.0 18.2 18.5 18.4 propane 1.4 1.5 1.7 2.7 isobutane 0.02 0.02 0.02 0.02 n-butane 8.7 8.9 9.8 10.7 isobutylene 1.3 1.4 1.4 1.6 1-butene 0.7 0.8 0.7 0.6 C5+ CxHy gases 0.7 0.8 0.7 0.6 dimethyl ether 0.1 0.1 0.1 0.1 CO + CO2 5.0 5.2 5.2 5.3 hydrogen 2.1 1.8 1.6 1.2 total

100

C2-C4 olefins C4 CxHy av mol wt of gas alcohols acetone ketones benzene toluene xylenes ethylbenzene C9+ aromatics aliphatics unidentified

5.0 3.8 0.1 6.5 4.3 1.4 1.1 7.1 2.6 67.8

total aromatics a

100

Composition of OLP, wt % of OLP 5.9 5.6 3.5 4.2 0.5 0.2 0 0.4 0 0 0 0.3 27.0 23.3 14.7 10.0 18.6 16.2 11.3 7.6 4.1 3.8 3.4 2.7 2.8 2.7 2.2 2.0 8.0 8.0 7.9 7.4 4.2 4.8 4.3 4.2 29.1 34.4 52.7 61.2

60.5

100 54.7

51.1 11.9 37.1

100

47.7 13.3 39.3

100

51.5 11.1 36.4

100

9.6 27.0 11.5 18.3 3.6 0.03 10.8 1.8 0.7 0.7 0.1 5.3 0.6

48.3 12.9 38.5

total

51.3 10.7 35.6

100

300

100 36.5

100 29.7

100 20.4

Runs conducted at GHSV of 3.3 h-1.

Mechanism of Formation of Various Products. In our earlier work12,13 involving the catalytic conversion of canola oil over acid catalysts, various products such as aromatics, aliphatics, oxygenated hydrocarbons, and gas were obtained. Reaction schemes were postulated to account for the production of these products. The nature of the reaction scheme was a strong function of the type of catalyst used. On the other hand, information is scant concerning the reaction scheme for canola oil cracking in the absence of a catalyst, as was previously mentioned. It was therefore of interest to obtain an understanding of the processes that resulted in the formation of each product during the thermal cracking of canola oil to realistically assess the effects of process variables such as reaction temperature, canola oil GHSV, cofeeding steam, and steam/canola oil feed ratio on the yields of these products. This is discussed below using the reaction scheme shown in Figure 3. This scheme represents a further modification of the reaction mechanism earlier proposed by Chang and Wan18 for saturated triglyceride molecules and modified by various workers to account for unsaturated triglyceride molecules17,22 and thermal hydrolysis of oil due to the presence of moisture16,21,23 as well as oxidative cracking of oil due to the presence of air or oxygen.16 The modifications introduced in the scheme given in the present work (Figure 3) are intended to account for the presence of a mixture of different types of both saturated and unsaturated fatty acid moieties in the triglyceride molecule of canola oil. (a) Heavy Oxygenated Hydrocarbons. In the present work, heavy oxygenated hydrocarbons refer to

such product components as long-chain fatty acids (RCOOH), ketones (RCOR′), aldehydes (RCHO), and esters (RCOOR′). These products result from the initial decomposition of the triglyceride molecule of canola oil (step 1 in Figure 3). According to the literature.16,18 typical reactions involved are as given in eqs 1-3. CH2OCOR

CH2

CHOCOCH2R

CH + RCOOH + RCOOH + RCH

CH2OCOR

CHO

CO

(1)

canola oil

2RCOOH f CO2 + H2O + RCOR′

(2)

RCOOH f CO + RCHO

(3)

The initial decomposition of oils to heavy oxygenated hydrocarbons begins at temperatures in the range 240300 °C.23 Some of the saturated free fatty acids obtained (such as stearic and palmitic acids) are those released from the parent triglyceride molecule without any subsequent decomposition of the fatty acid. The R and R′ in ketone and ester molecules indicate that each molecule may contain nonidentical saturated and/or unsaturated hydrocarbon radicals of the form CnH(2n(1), CnH(2n-1), CnH(2n-3), and CnH(2n-5). (b) Carbon Monoxide and Carbon Dioxide. Carbon monoxide and carbon dioxide were obtained as products from the thermal cracking of canola oil (see Table 2). According to Chang and Wan,18 carbon monoxide is formed from the decarbonylation of oxygenated hydrocarbons such as ketones, aldehydes, fatty acids, and esters as in steps 3 and 6 of the reaction scheme (Figure 3). While the byproducts of decarbonylation of ketones and aldehydes are hydrocarbon radicals, R or R′′, those of fatty acids and esters are mainly alcohols, ROH or R′′OH, where R′′ ) CnH2n+1, X g n > 1 and X is the number of carbon atoms in the original saturated or unsaturated fatty acid. On the other hand, the literature16,18 indicates that carbon dioxide is produced by decarboxylation of saturated acids such as stearic and palmitic acids (step 4) as well as unsaturated acids (R′′COOH) as in step 5 in Figure 3. Oxygen atoms present in radicals of the type RCHCO (see eq 1) are eliminated by the loss of a ketene molecule (CHdCO) which eventually loses the oxygen by bimolecular decarbonylation to form ethylene. Table 2 shows that the most abundant oxygenated compounds are carbon dioxide and carbon monoxide, which are obtained in the gas phase product. The absence of significant amounts of other oxygenated compounds in the gas phase product implies that decarboxylation, decarbonylation, and ketene elimination are dominant steps in the cracking of a triglyceride molecule in the absence of a catalyst17 and they result in the formation of hydrocarbon radicals, R and R′′. These radicals undergo various reactions for the production of C1-C5 hydrocarbons as follows. (c) C1-C5 Straight- and Branched-Chain Hydrocarbon Gases. The presence of C1-C5 hydrocarbon gases in the product (Table 2) can be attributed to decomposition by C-C bond cleavage of both saturated

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Figure 3. Proposed reaction scheme for the thermal cracking of canola oil to various products: 1, initial cracking of canola oil; 2, C-C bond cleavage of unsaturated oxygenated hydrocarbons; 3, decarbonylation of saturated oxygenated hydrocarbons; 4, decarboxylation of saturated oxygenated hydrocarbons; 5, decarboxylation of short-chain oxygenated hydrocarbons; 6, decarbonylation of short-chain oxygenated hydrocarbons; 7, bimolecular dehydration of alcohols; 8, dehydration of ether; 9, bimolecular dehydration of short-chain alcohols to form short-chain ether; 10, dehydration of short-chain ether; 11-14, ethylene elimination, isomerization, and hydrogen transfer reactions; 15, cyclization to form C3-C5 cycloolefins; 16, cyclization to form C3-C5 cycloparaffins; 17, Diels-Alder addition of dienes to olefins to form C6+ cycloolefins; 18, dehydrogenation of C3-C5 cycloparaffins to form C3-C5 cycloolefins; 19, hydrogenation of C3-C5 cycloolefins to form C3-C5 cycloparaffins; 20, dehydrogenation of C6+ cycloparaffins to form C6+ cycloolefins; 21, hydrogenation of C6+ cycloolefins to form C6+ cycloparaffins; 22, polymerization/ dehydrogenation of olefins to form dienes, acetylenes, and poly-olefins; 23, aromatization of C6+ cycloolefins to form C6+ aromatics; 24, polymerization of aromatics to from polyaromatics; 25, coking from polyaromatics; 26, coking by polycondensation of oxygenated hydrocarbons; 27, coking by polycondensation of canola oil; 28, splitting of long-chain hydrocarbons into its elements and ultimately to coke; 29, polymerization of olefins to form coke; 30, direct route for C1-C5 hydrocarbon formation from the triglyceride molecule; I, residual oil; II, OLP.

and unsaturated oxygenated hydrocarbon molecules as well as the long-chain hydrocarbon radicals (R) and the undecomposed triglyceride molecule of canola oil itself (if still present). This last route is shown as step 30 of Figure 3. In all three cases for the oxygen-containing molecules, there is the implication that it is possible for heavy oxygenated hydrocarbons to decompose according to two competing routes: (route i) decarboxylation and decarbonylation reactions followed by C-C bond cleavage of the resulting hydrocarbon radicals (i.e., steps 3 and 4 followed by steps 11 and 12 in Figure 3) or (route ii) C-C bond cleavage within the hydrocarbon section of the oxygenated hydrocarbon molecule followed by decarboxylation and decarbonylation of the resulting short-chain molecule (i.e., step 2 followed by steps 5 and 6 in Figure 3). Since C-C bond cleavage is greatly facilitated by the presence of unsaturation in the molecule,22 the reaction routes for these heavy oxygenated hydrocarbons will depend on whether the molecule is saturated or unsaturated. Thus, the presence of straight- or branched-chain paraffins with carbon num-

bers higher than C7 (i.e., C7+) as well as palmitic and stearic acid in the product shows that decaboxylation and decarbonylation reactions take place before C-C bond cleavage for saturated oxygenated hydrocarbons (i.e., route i). On the other hand, the absence of C7+ straight- or branched-chain olefins in the product [for the runs conducted at low GHSV (i.e., 3.3 h-1) in the absence of steam] shows that the alternate route (ii) is more probable for unsaturated oxygenated hydrocarbons. It has been shown16 for unsaturated oxygenated hydrocarbons that CsC bond cleavage occurs mainly at positions β to the CdC bond. Also, it has been shown22 that CsC bond cleavage for unsaturated molecules (step 2 in Figure 3) results in the formation of C4+ dienes, hydrocarbon radicals, Ru, and short-chain fatty acids, RuCOOH (where Ru ) CnH(2n-1), CnH(2n-3), or CnH(2n-5) and n e 7). On the other hand, any CsC bond cleavage of saturated oxygenated hydrocarbons results in the formation of hydrocarbon radicals, Rs, and short-chain fatty acids, RsCOOH, where Rs ) CnH2n+1, Z g n > 1

+

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Idem et al.

and Z is the number of carbon atoms in the original saturated fatty acid. Secondary cracking (steps 11-14 in Figure 3) then proceeds by elimination of ethylene molecules from the hydrocarbon radicals, Ru or Rs. The successive elimination of ethylene molecules from both the saturated and unsaturated hydrocarbon radicals followed by disproportionation, isomerization, and subsequent hydrogen transfer reactions yields straight- and branched-chain hydrocarbons of which those in the C1C5 range are in the gas phase product. Tables 2 and 3 show that there is a preponderance of ethylene product. The large amount of ethylene produced results from accumulation during its successive elimination from hydrocarbon radicals during secondary cracking. (d) Alcohols and Dimethyl Ether. Both methanol (CH3OH) and dimethyl ether (CH3OCH3) were obtained only in small quantities (see Tables 2 and 3) as products from the thermal cracking of canola oil. However, the presence of methanol product can be attributed specifically to decarbonylation of methyl formate ester (HCOOCH3) or acetic acid (CH3COOH) as in step 6 in Figure 3, whereas the presence of dimethyl ether can be attributed principally to bimolecular dehydration of methanol as in step 9 in Figure 3. The presence of only small quantities of dimethyl ether suggests that either methyl formate, acetic acid, and methanol were formed only in small quantities during the thermal cracking reaction or the dimethyl ether formed was used up for subsequent reactions such as methane, carbon monoxide, and hydrogen formation:

CH3OCH3 T CH4 + CO + H2

(4)

(e) Diolefinic, Cyclic, and Acetylenic Hydrocarbon Gases. Diolefins (or dienes) are formed from CsC bond cleavage of unsaturated fatty acid (step 2) or hydrocarbon chains (steps 11 and 12) at positions β to the CdC bond. In the case of fatty acids, it is clear that the type of acid determines the number of carbon atoms and the positions of the CdC bonds in the diene. Thus, the presence of fatty acids such as oleic acid, linoleic acid, and linolenic acid moieties in the triglyceride molecule of canola oil implies that the formation of a wide variety of dienes is possible. Dienes can also be formed by dehydrogenation of olefins (step 22 in Figure 3). Dehydrogenation in this case does not involve the hydrogen atoms bonded to the CdC bond. The processes mentioned above account for the presence of such dienes as 2,4-octadiene, butadiene, 1,4-pentadiene, and 2,3-pentadiene as thermal cracking reaction products of canola oil. GC-MS results showed the presence of cycloparaffins and cyco-olefins as reaction products. Those containing e5 carbon atoms per molecule were present in the gas phase products, while those containing >5 carbon atoms per molecule were present in the liquid phase products. According to Morrison and Boyd,27 C4 and C5 cyclohydrocarbons are formed by addition to ethylene of ethylene and propylene, respectively, followed by dehydrocyclization and double-bond conjugation. The C3 cyclohydrocarbons are formed from dehydrocyclization and double-bond conjugation involving either C3 paraffins or olefins. Although a large number of workers16-23 (27) Morrison, R. T.; Boyd, R. E. Organic Chemistry; Allyl and Bacon: Boston, 1974.

have studied the thermal cracking of plant oils, there is no report in the literature concerning the production of C3-C5 cyclohydrocarbons. We have used both GC and GC-MS analysis techniques to show that these species are formed during the thermal cracking of a plant oil. Thus, this represents the first report of both the presence and the determination of the mechanisms of formation of C3-C5 cyclohydrocarbons. GC-MS analysis also showed the presence of acetylenic hydrocarbons and polyolefins. Acetylenic hydrocarbons are formed by dehydrogenation of olefins (step 22) at high temperatures. However, unlike in the case of diene formation, the hydrogen atoms removed from the olefin molecule are those attached to the CdC bond. Also, another reaction that takes place at high temperatures is the polymerization of olefins to form polyolefins. All of these processes are grouped under step 22 in Figure 3. (f) C6+ Aliphatic Hydrocarbons. It is known that aliphatic hydrocarbons are composed of cyclic and straight- and branched-chain alkanes, olefins, alkynes, and diolefins as well as combinations of these classes of hydrocarbons. The term “C6+ aliphatic hydrocarbons” used in this work refers to aliphatic hydrocarbons composed of components with six or more carbon atoms per molecule. These compounds were obtained only in the liquid phase product. The C6+ cycloolefins can be accounted for by cyclization reactions in which the CdC bonds present in unsaturated fatty acids such as oleic,17 linoleic, and linolenic acids play an important function. This function involves CsC bond cleavage at positions β to the CdC bond to produce a conjugated diene in the first instance. Cycloolefins are then formed by Diels-Alder addition (as shown in step 17 in Figure 3) of ethylene or other olefins to the conjugated diene formed in step 2.22 This involves proton extraction followed by ring closure.28 Cycloparaffins are then formed by subsequent addition of a proton to cycloolefins.27 The mechanisms of formation of straight- and branched-chain aliphatic hydrocarbons are as discussed previously for corresponding classes of aliphatic hydrocarbons in the gas phase products. (g) Aromatic Hydrocarbons. It was interesting that aromatic hydrocarbons were also produced from the cracking of canola oil in the absence of a catalyst. The formation of aromatic hydrocarbons involves hydrogen elimination from C6+ cycloolefins at high temperatures (step 23 in Figure 3). The thermal reaction of olefins and diolefins to form aromatic hydrocarbons has been illustrated experimentally by Sarkai et al.29 Also, aromatic hydrocarbons can be derived directly from linolenic acid (which contains three double bonds per molecule) after it has undergone decarboxylation. On the other hand, it can be seen in Table 3 that there is a preponderance of benzene and toluene in the liquid phase products. This implies that the dienes and olefins involved in purely thermal aromatization processes are in the C4-C5 and C2-C3 ranges, respectively. (h) Heavy Hydrocarbons. Heavy hydrocarbons consist of high carbon number cyclic or straight- or (28) Michael, W. R.; Alexander, J. C.; Artman, N. R. Lipids 1966, 5, 353-357. (29) Sarkai, T.; Nehara, D.; Kunugi, T. In Industrial and Laboratory Pyrolysis; Albright, L. F., Ceryness, B. L., Eds.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976; p 32.

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branched-chain aliphatic hydrocarbons such as 1-(1, 5-dimethylhexyl)-4-(4-methylpentyl)cyclohexane as well as polyaromatic hydrocarbons such as fluoranthene. The elimination of the carboxyl group from fatty acids containing more than one CdC bond to form a longchain conjugated diene followed by Diels-Alder addition of the diene to ethylene or other olefins and then cyclization is responsible for the presence of long-chain cyclic hydrocarbons.22 Further elimination of hydrogen is responsible for the presence of long-chain aromatic hydrocarbons. On the other hand, the presence of heavy aromatic hydrocarbons containing more than one benzene ring implies the occurrence of reactions such as polymerization of aromatics and polycondensation of oxygenated hydrocarbons. In this work, the mixture of heavy oxygenated hydrocarbons, heavy hydrocarbons, and the unconverted oil is referred to as “residual oil”. This mixture is the residue remaining after vacuum distillation of the total liquid product at 200 °C at 170 Pa. On the other hand, the total liquid itself has been reported18, 21, 22 to exhibit characteristics similar to those for diesel fuel obtained from petroleum sources. In this work, it is referred to as “diesel-like fuel”. (i) Coke. Tables 2 and 3 show that coke is obtained as one of the reaction products in the thermal cracking of canola oil. The routes for its formation are (i) decomposition of long-chain hydrocarbon radicals into its elements [carbon and hydrogen18 (step 28)], (ii) polymerization of olefins (step 29) and aromatic hydrocarbons (step 25), and (iii) polycondensation of canola oil12,13 (step 27) and heavy oxygenated hydrocarbon molecules (step 26). Evidence for route ii is given by the presence of polyaromatic materials such as fluoranthene in the residual oil. Polyaromatic hydrocarbons are the intermediates in the formation of coke from aromatic hydrocarbons. As was discussed previously, these materials are formed by successive elimination of hydrogen from aromatics. Extensive removal of hydrogen from hydrocarbons results in coke. (j) Hydrogen. Hydrogen was observed in significant quantities in the gas phase product of the thermal cracking of canola oil. It is shown in the reaction scheme (Figure 3) that hydrogen is produced by proton extraction from processes such as the formation of cycloolefins (step 17) and aromatic hydrocarbons (step 23) as well as coke formation by polymerization of olefins and aromatics, polycondensation of canola oil, and splitting of hydrocarbon molecules into its elements. It is also produced by dehydrogenation of olefins to diolefins and acetylenic hydrocarbons. On the other hand, hydrogen is needed to stabilize the hydrocarbon radicals, Ru, Rs, or R′′, at various stages of the cracking process. Thus, the hydrogen observed in the product represents the difference between the amount produced and that which is consumed. Table 1 shows that canola oil consists of mixed triglyceride molecules containing various proportions of saturated and unsaturated fatty acid moieties. The mechanism(s) of formation of various products from the thermal cracking of canola oil have not been presented so far to our knowledge. Thus, the scheme provided in this work (Figure 3) represents an attempt to derive the mechanism(s) responsible for the formation of each product or product group from the thermal cracking of a complex material such as canola oil.

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Effect of Operating Conditions. It is well-known in the literature that for any chemical process the changes in product distribution resulting from changes in operating variables are due to the effects of these variables on the reaction mechanism. Thus, for any mechanism to be plausible, such changes in product distribution must be consistent with the effects of operating variables on the mechanism. The variables considered in the present study are reaction temperature and space velocity as well as cofeeding canola oil with steam and steam/canola oil ratio. (a) Reaction Temperature. Table 2 shows that canola oil conversion increased with reaction temperature. This is expected and is typical of all cracking processes.12,13 Also, Table 2 shows that the amount of total gas produced during the thermal cracking of canola oil increased with temperature. This implies that reactions such as decarboxylation, decarbonylation, C-C bond cleavage, and ethylene elimination that resulted in the formation of the bulk of the gas phase product are strongly endothermic. Table 2 also shows that the yield of individual gas phase products increased with cracking temperature. This also implies that reactions that led to their formation are endothermic. However, within the gas phase product itself, Table 3 shows that there were drastic variations in the concentrations of some components with reaction temperature. For example, the concentrations of methane, ethylene, and dimethyl ether increased with reaction temperature, while a reverse trend was observed for ethane, propane, n-butane, isobutylene, and C5+ hydrocarbons. Propylene concentration was not affected by changes in cracking temperature. The variation in concentration of these components with reaction temperature is explained below. It is well-known30 that the severity of any cracking reaction increases with temperature. Also, the higher the cracking temperature, the lower the molecular weights of the products formed. This implies that hightemperature cracking is needed for the formation of large amounts of methane. Conversely, low-temperature cracking is needed for the formation of large amounts of combined C4 and C5+ hydrocarbons in the gas phase. On the other hand, the opposing trends exhibited by ethane and ethylene show that hydrogen transfer reactions such as hydrogenation (as in the formation of ethane from ethylene or ethyl radical) are favored at low temperatures. In contrast, ethylene elimination from hydrocarbon radicals as well as dehydrogenation reactions for ethylene formation from ethane or ethyl radical are favored at high temperatures. It is seen in Table 3 that the average molecular weight of the gas product increased as the reaction temperature decreased. This can be explained on the basis of the presence in the gas phase of increasing concentrations of components of high molecular weights (i.e., combined C4 and C5+ hydrocarbons) with decreasing temperature. Coke was formed at temperatures higher than 300 °C, which suggests that high temperature is required for coke formation. It appears from Table 2 that beyond 300 °C the amount of coke formed was independent of the cracking temperature. In an earlier work involving the catalytic conversion of canola oil,12,13 the amount of (30) Gary, J. H.; Handwerk, G. E. Petroleum Refining Technology and Economics; Dekker: New York, 1984.

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coke formed decreased with increasing temperature. This was attributed to increased gasification of coke as the temperature increased. It thus appears from the present results that in the absence of a catalyst, coke gasification at high temperatures is not significant. The amount of hydrogen produced (Table 2) as well as its concentration in the gas phase product (Table 3) both increased with reaction temperature. This can be explained as follows: It was shown earlier that the extents of processes such as cycloolefin and aromatic hydrocarbon formation, dehydrogenation of olefins, coke formation by polymerization of olefins and aromatics, polycondensation of canola oil, and splitting of hydrocarbons into elements that result in the formation of hydrogen increase as the temperature increases. In contrast, the extents of processes that lead to hydrogen consumption (for example, stabilization of hydrocarbon radicals as well as hydrogenation of alkenes to alkanes and cycloolefins to cycloparaffins) decrease with an increase in temperature. The result is a net increase in hydrogen with cracking temperature. Table 2 shows that a maximum exists at 370 °C in the relationship between the yield of OLP and reaction temperature. On the other hand, the sum of the yields of OLP and residual oil (i.e., diesel-like fuel or total liquid product obtained) increased as the temperature decreased. As was shown earlier, low-temperature cracking results in the formation of relatively long-chain compounds. This leads to increases in yields of both OLP alone and total liquid product (i.e., OLP + residual oil) with decreasing temperature at the expense of gas phase product and coke. However, cracking at a low temperature also implies that heavy oxygenated hydrocarbons present in residual oil as well as canola oil itself will undergo only limited decomposition and will consequently yield a small amount of the OLP. Thus, there is a minimum temperature below which thermal cracking becomes unfavorable for OLP production. On the basis of only Table 2, it appears the maximum yield for the OLP occurred at 370 °C. The variation of the concentrations of aromatic hydrocarbons in the OLP with reaction temperature is shown in Table 3. It is seen that the concentration of each group of aromatic hydrocarbons increased with cracking temperature. This shows that the extent of aromatization reactions increased as the cracking temperature increased. This is expected and is consistent with the observed trend of an increase in the extent of dehydrogenation with cracking temperature. As was shown earlier, dehydrogenation is a key reaction in aromatization processes, and it is well-known that the latter increases as dehydrogenation increases. It is also seen in Table 3 that the concentrations of aromatic hydrocarbons increased in the order benzene (C6) > toluene (C7) > xylene and ethylbenzene (C8). This shows that in the absence of a catalyst, the extent of Diels-Alder addition of ethylene to a conjugated diene decreases as the total number of carbon atoms involved in the process increases. Table 2 shows the yields of benzene, toluene, xylene, ethylbenzene, and total aromatics. In general, the yield of aromatic hydrocarbons increased with temperature up to 370 °C. Beyond this temperature aromatic hydrocarbon yield decreased. This is explained below. It was mentioned earlier that the amount of aromatic

+

Idem et al.

hydrocarbons in the OLP does not include the polyaromatics present in residual oil. It is also known that the higher the temperature, the greater the extent of polyaromatic hydrocarbon formation. This was evidenced by the presence in both the OLP and the residual oil of an increasing fraction of polyaromatic hydrocarbon as the cracking temperature increased. Also, it is known that the higher the number of aromatic rings in the polyaromatic hydrocarbon, the higher its boiling point and, as such, the greater the chances of the material ending up in residual oil. Thus, even though an increase in cracking temperature results in the formation of increased amount of total aromatics, the fraction of these that end up in the OLP decreases as the cracking temperature increases. Furthermore, it was shown earlier that the OLP, of which aromatics constitute a substantial part (see Tables 2 and 3), had a maximum yield at 370 °C. The combination of all these factors is responsible for the maximum at 370 °C. The amounts of C6+ aliphatic hydrocarbons obtained from canola oil cracking reactions are shown in Table 2 as a function of temperature. In general, the yield of these aliphatic hydrocarbons increased with temperature up to 370 °C. Beyond this temperature, their yields decreased. This decrease is explained below. The OLP is composed mainly of aromatic and C6+ aliphatic hydrocarbons. It was shown earlier that aromatic hydrocarbon formation increases with cracking temperature. This implies that the amount of total aliphatic hydrocarbons produced will decrease with increasing cracking temperature. As such, increased aliphatic hydrocarbon yields should be obtained as the temperature decreases. However, at low temperatures, some of these aliphatic hydrocarbons end up in residual oil, thus decreasing the amount of the aliphatic hydrocarbons in the OLP. Thus, there is a minimum thermal cracking temperature below which cracking becomes unfavorable for C6+ aliphatic hydrocarbon formation. As in the case of OLP product, the minimum cracking temperature favorable for C6+ aliphatic hydrocarbon production is 370 °C (b) Space Velocity. Table 4 shows the yields of various products from the thermal cracking of canola oil for runs conducted at low and high GHSVs (i.e., 3.3 and 15.4 h-1 at STP, respectively) at cracking temperatures of 400 and 500 °C. The corresponding compositions of the gas and OLP fractions are given in Table 5. Table 4 shows that canola oil conversion was higher for runs conducted at low space velocity than for those at high space velocity for the two cracking temperatures used. This is typical of cracking reactions and is consistent with the observations of Katikaneni et al.12,13 On the other hand, while the gas yield decreased with an increase in space velocity, the OLP yield followed the reverse trend. These results can be explained on the basis of the reaction scheme given in Figure 3. This shows that ethylene elimination (steps 11-14), which is responsible for the formation of the bulk of the gas phase products, is further down the sequence of reaction steps than those reaction steps responsible for the formation of oxygenated and aliphatic hydrocarbons in the OLP (steps 2, 5, and 6). These reactions are C-C bond cleavage, decarboxylation, and decarbonylation. Thus, within the range of space velocities used in this study, an increase in space velocity is detrimental to

+

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Table 4. Mass Balances and Yields of Products as a Function of Space Velocities 400 °C product

500 °C

% canola oil convrn

100 98.4

100 70.0

100 100

70.0 19.3 1.0 6.7 3.0 100 93.3

7.9 23.3 5.3 11.7 1.0 0.0 7.8 0.9 0.4 7.6 0.1 3.4 0.8 35.0

OLP Components, wt % of Canola Oil Fed alcohols 1.2 2.1 0.9 acetone 0 2.0 0.1 ketones 0 0 0 benzene 5.0 2.5 4.0 toluene 3.9 1.6 2.8 xylenes 1.1 0.5 0.6 ethylbenzene 0.8 0.2 0.4 2.7 2.0 1.2 C9+ aromatics aliphatics 1.5 1.1 0.6 unidentified 18.1 41.9 4.3

0.3 0 0 2.6 1.4 0.6 0.3 1.5 0.2 12.0

total aromatics

100 13.5

100 6.8

100 9.0

100 6.4

gas formation, whereas such an increase in space velocity is favorable for OLP formation. In the case of residual oil, it was observed not only that the amount produced was larger but also that it contained concentrations of oxygenated hydrocarbons which were larger for the high space velocity runs than for those at low space velocity. For example, GC-MS analysis showed qualitatively that the amounts of stearic and palmitic acids were larger for runs at 15.4 h-1 than for those at 3.3 h-1. Long-chain oxygenated hydrocarbons are products of the initial decomposition of canola oil (step 1 in Figure 3). As was discussed previously, subsequent reactions are the C-C bond cleavage or decarboxylation and decarbonylation of these oxygenated hydrocarbon products. This means that the amount of long-chain oxygenated hydrocarbons produced depends on the residence time used for the reaction. As is well-known, a low space velocity, which implies a long residence time, is favorable for subsequent decomposition of an intermediate product (heavy oxygenated hydrocarbons in this case), whereas a high space velocity, which implies a short residence time, is not, thus resulting in the presence of large amounts of heavy oxygenated hydrocarbons in the latter case. Table 5 shows that in the gas phase products the amounts of ethylene, n-butane, and C5+ hydrocarbon gases (high molecular weight gases) were larger in the runs conducted at 15.4 h-1 than in those conducted at

500 °C

3.3 GHSV, 15.4 GHSV, 3.3 GHSV, 15.4 GHSV, h-1 h-1 h-1 h-1

product

Yields of Products Gas Phase Components, wt % of Canola Oil Fed methane 7.1 2.1 10.6 ethylene 17.0 16.3 23.6 ethane 5.4 1.8 7.0 propylene 10.3 3.5 13.5 propane 0.9 0.3 1.1 isobutane 0.01 0.0 0.02 n-butane 5.5 2.1 6.5 isobutylene 0.8 0.3 1.0 1-butene 0.4 0.1 0.5 4.5 2.5 4.0 C5+ CxHy gases dimethyl ether 0.1 0 0.1 2.9 0.9 3.8 CO + CO2 hydrogen 0.9 0.2 1.6 27.3 9.8 37.1 C2-C3 olefins

total

400 °C

3.3 GHSV, 15.4 GHSV, 3.3 GHSV, 15.4 GHSV, h-1 h-1 h-1 h-1

Overall Mass Balance, wt % of Canola Oil Fed gas 55.8 20.1 75.0 OLP 34.4 47.9 14.8 coke 3.9 0.0 3.9 residual oil 1.6 30.0 0.0 unacctd fraction 4.3 2.0 6.3 total

Table 5. Composition of Gas and OLP Fractions as a Function of Space Velocity

Composition of Gas Fraction, wt % of Gas methane 12.7 10.5 14.2 ethylene 30.5 31.5 31.4 ethane 9.7 9.1 9.3 propylene 18.5 17.5 18.0 propane 1.7 1.3 1.4 isobutane 0.02 0.02 0.02 n-butane 9.8 10.2 8.7 isobutylene 1.4 1.3 1.3 1-butene 0.7 0.5 0.7 C5+ CxHy gases 8.1 12.2 7.7 dimethyl ether 0.1 0.1 0.1 CO + CO2 5.2 4.6 5.0 hydrogen 1.6 1.1 2.1 total

100

C2-C4 olefins C4 CxHy av mol wt of gas alcohols acetone ketones benzene toluene xylenes ethylbenzene C9+ aromatics aliphatics unidentified total total aromatics

51.1 11.92 37.1

100 50.8 12.02 38.8

100 51.4 10.72 35.6

Composition of OLP, wt % of OLP 3.5 3.7 5.9 0 3.5 0.5 0 0 0 14.7 4.4 27.0 11.3 2.9 18.6 3.4 0.9 4.1 2.2 0.3 2.8 7.9 3.6 8.0 4.3 1.9 4.2 52.7 75.5 29.1 100 36.5

100 12.1

100 60.5

11.3 33.3 7.6 16.7 1.4 0.02 11.1 1.3 0.5 10.8 0.1 4.8 1.2 100 51.8 12.92 38.2 1.5 0.2 0.5 13.4 7.4 3.2 1.5 7.7 1.2 62.4 100 33.2

3.3 h-1 for the two cracking temperatures. Conversely, it was observed that the amount of methane (low molecular weight gas) produced was larger for runs at 3.3 h-1 than for those at 15.4 h-1. As was discussed earlier, the gas phase hydrocarbon products are formed mainly by ethylene elimination from hydrocarbon radicals (i.e., secondary cracking). Thus, it can be seen that a high space velocity is detrimental to large extents of ethylene elimination from each individual hydrocarbon molecule because this reaction step occurs later in the reaction sequence (steps 11-14 in Figure 3). This results in the presence of hydrocarbon components with high molecular weights in the gas phase. The converse argument is true for methane. The increase in the concentration of ethylene with space velocity can be attributed to its successive elimination from long hydrocarbon molecules without a corresponding formation of other gas phase hydrocarbons except the C4 and C5 hydrocarbons. In the OLP, Table 5 shows that the concentrations of total aromatics, benzene, toluene, xylenes, and C9+ aromatics were larger for the low space velocity runs than for those conducted at high space velocity. Also, C6+ aliphatics decreased with space velocity. As shown in Figure 3, the reaction steps required for the formation of both C6+ aliphatics and aromatic hydrocarbons appear late in the reaction sequence. Thus, our results show that the initial decomposition of the triglyceride molecule into oxygenated hydrocarbons is not affected significantly by high canola oil space velocity (i.e., low canola oil residence time). However, subsequent de-

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Idem et al.

Table 6. Mass Balances and Yields of Products as a Function of Steam/Canola Oil Weight Ratio (S/C) 500 °C product gas OLP coke residual oil unacctd fraction

0:1 S/C 75.0 14.8 3.9 0 6.3

total

100

% canola oil convrn

100

methane ethylene ethane propylene propane isobutane n-butane isobutylene 1-butene C5+ CxHy gases dimethyl ether CO + CO2 hydrogen C2-C3 olefins

10.6 23.6 7.0 13.5 1.1 0.02 6.5 1.0 0.5 4.0 0.1 3.8 1.6 37.1

alcohols acetone ketones benzene toluene xylenes ethylbenzene C9+ aromatics aliphatics unidentified

0.87 0.07 0 4.00 2.75 0.61 0.41 1.19 0.62 4.3

total total aromatics

100 8.65

1:1 S/C

450 °C 4:1 S/C

0:1 S/C

1:1 S/C

400 °C 4:1 S/C

Overall Mass Balance, wt % of Canola Oil Fed 71.1 61.1 71.0 44.9 30.5 21.1 27.5 17.2 46.5 57.0 3.9 3.9 3.9 3.9 0 2.1 5.5 1.2 3.0 8.0 1.8 2.0 6.7 1.7 4.5 100 97.9

100

100

94.5

98.8

100 97.0

100 92.0

Yields of Products Gas Phase Components, wt % of Canola Oil Fed 8.1 5.4 9.9 6.6 2.7 26.3 26.5 22.1 16.4 12.9 5.3 2.1 6.7 3.5 1.3 12.7 9.4 12.2 7.3 4.8 0.6 0.5 1.1 0.5 0.3 0.01 0 0.01 0 0 6.7 8.6 6.3 4.3 4.1 0.6 0.3 1.0 0.4 0.2 0.3 0.2 0.6 0.2 0.1 4.5 4.6 5.5 2.9 2.2 0.1 0.2 0.1 0 0.1 3.5 3.1 3.7 2.3 1.6 1.2 0.8 1.3 0.7 0.4 39.0 35.9 34.3 23.7 17.7 0.44 0.12 0 5.27 3.68 1.65 0.49 1.31 0.62 7.26 100 11.31

OLP Components, wt % of Canola Oil Fed 0.25 0.97 1.09 0.74 0.06 0.04 0.09 0.06 0 0 0 0 2.89 3.69 5.38 5.25 2.16 2.54 4.76 4.19 2.49 0.66 2.70 2.05 1.20 0.44 1.15 1.68 4.59 2.04 4.09 2.52 0.64 0.13 0.31 0.77 11.83 6.63 25.05 36.36 100

100

13.33

composition or secondary cracking of these oxygenated hydrocarbons to the OLP is affected. That is why an increase in space velocity (i.e., decrease in residence time) is detrimental to both aromatic and aliphatic hydrocarbon formation. The foregoing discussion shows that, like in the case of the effects of cracking temperature, results concerning the effects of canola oil space velocity on product distribution are consistent with the sequence of reaction steps given in Figure 3 for the thermal cracking of canola oil to various products . (c) Cofeeding Steam with Canola Oil and Steam/ Canola Oil Feed Ratio. Tables 6 and 7 show, respectively, the yields of various products and compositions of the OLP and gas phase products as a function of steam/canola oil feed ratio. In terms of canola oil conversion, OLP, residual oil, and gas yields as well as concentrations of methane, normal hydrocarbons, and isohydrocarbons (in the gas phase), aliphatics and aromatics (in the OLP), and heavy oxygenated hydrocarbons (in residual oil), these tables show that an increase in steam/canola oil feed ratio produced effects similar to those obtained for an increase in space velocity (Tables 4 and 5). These results can be explained on the basis that an increase in steam/canola oil feed ratio results in an increase in canola oil space velocity. Thus, reasons given in the case of space velocity are applicable in this case as well. However, GC-MS

9.37

100 18.08

100 15.69

0:1 S/C

1:1 S/C

4:1 S/C

55.8 34.4 3.9 1.6 4.3

30.1 46.1 0 20.0 3.8

9.9 41.3 0 46.0 2.8

100

100

100

98.4

80.0

54.0

7.1 17.0 5.4 10.3 0.9 0.01 5.5 0.8 0.4 4.5 0.1 2.9 0.9 27.3

3.7 11.0 2.4 5.6 0.4 0 3.2 0.3 0.1 1.9 0 1.6 0.4 16.6

1.0 4.1 0.6 1.5 0.1 0 1.1 0.1 0 0.6 0 0.5 0.1 5.6

1.22 0 0 5.04 3.89 1.17 0.75 1.70 0.95 20.14

3.57 0.03 0 2.18 2.11 1.54 0.77 4.46 0.77 29.36

2.04 0.87 0 1.04 1.48 1.26 0.86 4.52 0.86 26.38

100 12.55

100 11.06

100 9.16

results for the runs in the presence of steam showed that it was possible to obtain even unsaturated fatty acids with the same carbon length as the ones in the parent triglyceride molecule. This shows that the presence of steam during cracking facilitated hydrolysis of the triglyceride molecule but did not facilitate the subsequent decomposition of the released fatty acids. On the other hand, Table 7 shows that while an increase in steam/canola oil ratio resulted in an increase in the concentration of ethylene, the reverse trend was observed for propane and propylene. This is explained below. It was shown earlier under Experimental Section that the increase in steam/canola oil feed ratio produced a more dramatic increase in space velocity (3.3-640 h-1) compared to that produced by an increase in canola oil feed rate (3.3-15.4 h-1). The use of an extremely high space velocity (as in the case of cofeeding with steam) implies that there is limited ethylene elimination from each long hydrocarbon molecule. Consequently, the probability of secondary cracking to yield hydrocarbon molecules containing fewer than 5 carbon atoms (for odd-numbered carbon hydrocarbons) or 4 carbon atoms (for even-numbered carbon hydrocarbons) per molecule decreases as the space velocity increases, hence the decrease in the C3 hydrocarbons and the consequent increase in the C4 hydrocarbons with steam/ canola oil ratio (i.e., space velocity). In addition, the use of an extremely high space velocity also implies that

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Table 7. Composition of Gas and OLP Fractions as a Function of Steam/Canola Oil Weight Ratio (S/C) 500 °C product methane ethylene ethane propylene propane isobutane n-butane isobutylene 1-butene C5+ CxHy gases dimethyl ether CO + CO2 hydrogen total

0:1 S/C 14.2 31.4 9.3 18.0 1.4 0.02 8.7 1.3 0.7 7.7 0.1 5.0 2.1 100

1:1 S/C 11.4 37.1 7.5 17.9 0.9 0.01 9.4 0.8 0.4 6.3 0.1 4.9 1.7 100

450 °C 4:1 S/C

0:1 S/C

1:1 S/C

Composition of Gas Fraction, wt % of Gas 8.8 13.9 14.6 8.9 43.0 31.1 36.5 42.4 3.4 9.4 7.7 4.3 15.3 18.2 16.2 15.7 0.8 1.5 1.1 0.9 0 0.02 0.01 0 14.0 8.9 9.5 13.5 0.5 1.4 0.9 0.5 0.3 0.8 0.5 0.2 7.4 7.7 6.5 7.1 0.3 0.1 0.1 0.2 5.0 5.2 5.1 5.1 1.3 1.8 1.5 1.2 100

100

100

C4 CxHy C2-C4 olefins av mol wt of gas

10.7 51.3 35.6

10.6 53.1 35.7

14.8 59.1 37.4

alcohols acetone ketones benzene toluene xylenes ethylbenzene C9+ aromatics aliphatics unidentified

5.9 0.5 0 27.0 18.6 4.1 2.8 8.0 4/2 29.1

2.1 0.6 0 25.0 17.4 7.8 2.3 6.2 2/9 34.4

Composition of OLP, wt % of OLP 0.9 5.6 2.4 0.2 0.2 0.2 0 0 0 10.5 23.8 12.0 7.9 16.2 10.6 9.1 3.8 6.0 4.4 2.7 2.6 16.7 8.0 9.1 2/3 4/8 0.7 43.0 34.9 55.7

total

100

100

100

total aromatics

60.5

58.7

400 °C 4:1 S/C

48.6

11.1 51.5 36.4

100 54.7

reactions steps (such as cyclization and aromatization) that are further down the sequence will have a much smaller probability of taking place than those that appear relatively early in the sequence (such as ethylene elimination). Thus, for these high space velocity runs, while ethylene production still occurred to a reasonable extent, those involving its reactions with dienes to produce cycloolefins and then aromatics occurred only to limited extents, hence the apparent dramatic increase in ethylene concentration with steam/canola oil ratio. This also accounts for the decrease in the concentration of such aromatic hydrocarbons as benzene and toluene with increasing steam/canola oil ratio. It was shown earlier that benzene and toluene are the aromatic hydrocarbons that are likely to be formed by the addition of ethylene to a C4 or C5 diene. Thermal Cracking over Inert Materials. Inert materials are used in reaction engineering studies for a number of reasons. In general, these inert materials are not expected to contribute to the activity of the catalyst. As was discussed earlier, a large extent of reaction was observed for an empty reactor (see Tables 2 and 4). Thus, it becomes necessary to conduct studies on the use of inert materials for the thermal cracking of canola oil to determine whether changes in the morphology and surface area of the material (compared with those of the empty reactor) had any effect on canola oil conversion and product distribution. The overall gas yields as well as the compositions of the OLP and gas product fractions are given in Table 8, while the yields of the individual products are given in Table 9. Table 8 shows that the yields of gas and the various products in the gas phase were almost

11.0 54.2 35.3

100 40.3

100

0:1 S/C

1:1 S/C

4:1 S/C

12.7 30.5 9.7 18.5 1.7 0.02 9.8 1.4 0.7 8.1 0.1 5.2 1.6

12.4 36.4 8.0 17.0 1.4 0.01 10.5 0.9 0.4 6.4 0.1 5.2 1.3

10.4 41.5 6.1 15.5 1.1 0.01 11.5 0.6 0.3 6.4 0.1 5.1 1.0

100

100

100

14.2 58.8 37.2

11.9 51.1 37.1

11.8 54.7 36.0

12.3 58.2 36.1

1.3 1.1 0 9.2 7.4 3.6 2.9 4.4 1.4 63.8

3.5 0 0 14.7 11.3 3.4 2.2 7.9 4.3 52.7

7.7 0.7 0 4.7 4.6 3.3 1.7 9.7 1.7 63.6

4.9 2.1 0 2.5 3.6 3.1 2.1 11.0 2.1 64.0

100 27.7

100 36.5

100 24.0

100 22.3

identical to those obtained from the empty reactor run at the same temperature. Also, the yields of the individual products (see Table 8) were identical to those obtained in the empty reactor run. Thus, it can be concluded that none of the inert materials used contributed to either the conversion or product distribution obtained from the thermal cracking of canola oil. Economic and Environmental Importance of Products and the Process. The yields of ethylene and propylene obtained from the thermal cracking of canola oil were extremely high, with a combined yield of 36 wt % in some cases. This is particularly important because this process thus provides an attractive set of materials the transformation products [such as polyethylene, poly(vinyl chloride), polypropylene, ethylene glycol, and acrylonitrile] of which are well established in their end use markets. Also, substantial amounts of C4 and C5 hydrocarbons were produced. These are all desirable products commercially. Aromatics such as benzene, ethylbenzene, xylenes, toluene, propylbenzene, and naphthalene were also produced in substantial quantities. Their use in a wide range of chemical and petrochemical applications such as the production of styrene and terephthalic acid is well established. There are tremendous advantages that can be derived from this type of process involving the production of fuels and chemicals from plant oils. These advantages are given below. It is known that plant oils are a renewable resource, and the plant itself provides ground level ozone layer protection and hence helps reduce the greenhouse effect. Also, quite unlike petroleum oils, plant oils do not possess any heteroatoms such as

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Idem et al.

Table 8. Mass Balances and Yields of Products for Thermal Cracking over Inert Materials

Table 9. Composition of Gas and OLP Fractions for Thermal Cracking over Inert Materials

500 °C product

quartz chips

glass wool

500 °C

ceramic chips

Overall Mass Balance, wt % of Canola Oil gas 75 79 78 OLP 16 15 14 coke 3.9 3.9 3.9 residual oil 0 0 0 unacctd fraction 5.1 2.1 4.1

empty reactor 75 14.8 3.9 0 6.3

total

100

100

100

100

% canola oil convrn

100

100

100

100

Yields of Products Gas Phase Components, wt % of Canola Oil Fed methanol 9.8 11.6 10.6 ethylene 22.3 25.1 24.0 ethane 7.2 6.8 7.3 propylene 14.3 13.4 14.7 propane 1.2 0.9 1.2 isobutane 0.02 0.02 0.02 n-butane 6.9 7.0 6.9 isobutylene 1.1 0.9 1.2 1-butene 0.6 0.6 0.6 C5+ CxHy gases 6.5 7.1 5.9 dimethyl ether 0.1 0.1 0.1 CO + CO2 3.6 3.9 4.1 hydrogen 1.5 1.7 1.8 C2-C4 olefins 36.6 40.1 40.6 OLP Components, wt % of Canola Oil Fed alcohols 0.8 1.0 0.78 acetone 0.2 0.03 0.31 ketones 0 0 0 benzene 3.41 3.7 3.65 toluene 3.1 2.7 3.24 xylenes 0.8 0.7 0.7 ethylbenzene 0.5 0.4 0.55 C9+ aromatics 1.4 1.6 1.14 aliphatics 0.7 0.3 0.7 unidentified 5.1 2.1 4.1 total total aromatics

100 9.21

100 9.1

100 9.28

10.6 23.6 7.0 13.5 1.1 0.02 6.5 1.0 0.5 4.0 0.1 3.8 1.6 37.1 0.87 0.07 0 4.0 2.75 0.61 0.41 1.9 0.62 6.3 100 9.36

nitrogen and, as such, do not result in the production of NOx during processing. In addition, metal particle emission during processing is negligible. Furthermore, the production and use of fuels and chemicals from plant oils are CO2 neutral. Conclusions 1. Canola oil conversions from the thermal cracking of canola oil increased with an increase in cracking temperature and a decrease in both canola oil space velocity and steam/canola oil ratio. On the other hand, conversion was completely independent of the morphology of the cracking surface. 2. Products essentially consisted of C4 and C5 hydrocarbons, aromatic and C6+ aliphatic hydrocarbons, and C2-C4 olefins as well as a diesel-like fuel and hydrogen. These are all highly desirable products commercially. The lengths of the carbon chain of individual groups of hydrocarbon and oxygenated hydrocarbon products increased with an increase in space velocity and steam/ canola oil ratio as well as a decrease in temperature.

quartz chips

product

glass wool

ceramic chips

Composition of Gas Fraction, wt % of Gas methane 13.1 14.5 13.6 ethylene 29.8 31.8 30.8 ethane 9.6 8.6 9.3 propylene 19.0 17.0 18.9 propane 1.6 1.2 1.5 isobutane 0.02 0.02 0.03 n-butane 9.2 8.8 8.8 isobutylene 1.5 1.2 1.5 1-butene 0.8 0.7 0.8 C5+ CxHy gases 8.6 9.0 7.6 dimethyl ether 0.1 0.1 0.1 CO + CO2 4.8 4.9 5.3 hydrogen 2.0 2.2 2.3 total C2-C4 olefins C4 CxHy av mol wt of gas alcohols acetone ketones benzene toluene xylenes ethylbenzene C9+ aromatics aliphatics unidentified

100 51.3 10.7 35.6

Composition of OLP, wt % of OLP 5.0 6.7 5.6 1.3 0.2 2.2 0 0 0 21.3 24.7 26.1 19.4 18.0 23.1 5.0 4.7 5.0 3.1 2.7 3.9 8.8 10.7 8.1 4.4 2.0 5.0 32.5 14.7 30.2

5.9 0.5 0 27.0 18.6 4.1 2.8 8.0 4.2 29.1

total aromatics

100 57.6

100 50.7 10.7 36.6

14.2 31.4 9.3 18.0 1.4 0.02 8.7 1.3 0.7 7.7 0.1 5.0 2.1

100 52.0 11.1 36.1

total

100 51.1 11.5 37.2

empty reactor

100 60.8

100 66.2

100 60.5

3. The selectivity for C2-C4 olefins in the gas phase product increased with an increase in both temperature and steam/canola oil ratio. 4. Aromatic hydrocarbon formation and gas yield were favored at high cracking temperature and low canola oil space velocity, whereas aliphatic hydrocarbon and OLP formation were favored at low cracking temperature and high canola oil space velocity. There was a minimum temperature (370 °C) below which cracking became detrimental to both OLP and aliphatic hydrocarbon formation. 5. A reaction scheme has been proposed for the thermal cracking of an oil composed of both saturated and unsaturated fatty acids (such as canola oil) to various products. The observed changes in both canola oil conversion and product distribution with changes in operating variables were consistent with the reaction scheme. Acknowledgment. The financial support provided by the Saskatchewan Canola Development Commission (SCDC) and the Agricultural Development Fund (ADF) both of Saskatchewan, Canada, is gratefully acknowledged. EF960029H

Thermal Cracking of Canola Oil: Reaction Products in the Presence

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