Although the cracking of vegetable oils into liquid fuels has been studied in detail, the cracking of triglycerides molecules under realistic FCC conditions is less described in the literature. However, certain number of authors have performed studies about the processing of vegetable oils (Bhatia et al., 2007, 2009; Chew and Bhatia, 2009; Dupain et al., 2007; Li et al., 2009; Melero et al., 2010b; Tamunaidu and Bhatia, 2007; Tian et al., 2008) and animal fats (Lummus, 1988; Melero et al., 2010b; Tamunaidu and Bhatia, 2007; Tian et al., 2008) under conditions that try to simulate operating conditions of the FCC unit. In these studies, the reaction system employed is usually based in a riser reactor and an FCC catalyst. After the catalytic cracking reactions, conversion is usually over 75% (Bhatia et al., 1998; Chew and Bhatia, 2009; Melero et al., 2010b; Tian et al., 2008). Furthermore, there are no remarkable amounts of oxygenated hydrocarbons in the final cracking products, as almost all the oxygen initially present in the triglyceride molecule ends forming water or carboxylic gases (CO and CO2) (Dupain et al., 2007; Melero et al., 2010b; Tian et al., 2008).

Figure 15.5 shows the yields towards different products for the catalytic cracking of crude PO in a fixed bed reactor of short contact time at 565°C and a catalyst-to-PO mass ratio of 4 (Melero et al., 2010b). Besides the oxygenated compounds detected (water and carboxylic gases), main hydrocarbon products are gaseous hydrocarbon products, such as dry gas (H2, methane, ethane, ethylene) and liquid petroleum gases (LPG, propane, propylene, butenes, butanes), and liquid hydrocarbon products such as gasoline (GLN; C5, 221°C), which is divided into light naphtha (LN; C5, 90°C), medium naphtha (MN; 90-140°C) and heavy naphtha (HN; 140-221°C), LCO (221-360°C) and decanted oil (DO; > 360°C). As observed in Fig. 15.5, water is the main oxygenated compound in the cracking of vegetable oils because it involves approximately 70% of the initial oxygen in the triglyceride molecule, which means a yield of water in the final product cracking of ca. 10% when a 100% crude PO feedstock is processed. Similar results have been described in the catalytic cracking experiments performed by different authors (Dupain et al., 2007; Marker, 2007; Ramakrishan, 2004). Water is produced by means of decarboxylation reactions (Idem et al., 1996) as well as catalytic dehydration reactions (Chang and Silvestri, 1977) or condensation processes (Adjaye and Bakhshi, 1995). Carboxylic gases are also important oxygenated compounds with a yield of ca. 5%. Carboxylic gases are formed by CO in 60% mass percentage and CO2 in 40% (Melero et al., 2010b). CO is formed through decarbonylation reactions from different molecules such as ketenes, aldehydes, fatty acids and esters. By-products of this reaction depend on the original oxygenated compound. On the one hand, in the case of ketenes and aldehydes, decarbonylation reactions lead to reactive species such as free radicals and, on the other hand, in the case of fatty acids and esters, they produce alcohols

(Idem et al., 1996). CO2 is formed through fatty acid and ester decarboxylation reactions, producing water and ketenes as by-products (ketene usually loses its oxygen molecule because of molecular decarbonylation reactions to form ethylene). These data mean that around 17% of the initial oxygen ends as CO and 11% as CO2. Hence, 15% of the renewable raw materials that are being fed to the FCC reactor end up as non-valuable products (water and carboxylic gases) under the tested reaction conditions used in this work (Melero et al., 2010b).

Dry gas is mainly a thermal cracking product, although it can be obtained by means of catalytic reactions, especially in the case of ethylene. Dry gas is not an

important cracking product because it is obtained in a small percentage (never higher than 5%) and it has a low commercial value. Ethylene is the main compound, leading to more than 40% of the final dry gas yield, and ethane and methane production is always close to 30% for both compounds. On the other hand, LPG production in the case of PO cracking is a very important fraction with a yield of ca. 25% under the tested reaction conditions (Melero et al., 2010b). High yields of gaseous products have also been achieved by other authors working under the FCC conditions (see Table 15.2). Tamunaidu and Bhatia (2007) achieved yields of gaseous hydrocarbons ranging between 19.9% and 38.1% in the cracking experiments of PO using a riser reactor (temperature = 400-500°C and catalyst-to-oil mass ratio ranging from 5 to 10). Similar experiments were performed by the research group of Chew and Bhatia (2009). These authors obtained yields of gaseous products of 16.2% and 15.9% for crude and used PO, respectively, using a riser reactor at 450°C and a catalyst-to-oil mass ratio of 5. Finally, Li et al. (2009) confirmed these results, reaching yields to gas of 28.8% in their cracking experiments of cottonseed oil in a fluidized bed reactor (temperature = 400-500°C and catalyst-to-oil mass ratio of 6-10).

LPG gases are mainly a catalytic cracking product obtained through dealkylation reactions, in which the hydrocarbon chain bonded to an aromatic ring can be broken to end up as gases (Dupain et al., 2007), or through the initial cracking of higher molecular weight products. LPG hydrocarbons are usually produced by means of b-scission reactions in which a primary carbenium ion and an olefin are formed. Afterwards, it is quite probable that hydride transfer reactions will be produced, transferring the charge from a small carbenium ion onto a large hydrocarbon and, as a consequence, forming new olefins, which can be protonated again by a Bronsted acid site and cracked further or isomerized. Obviously, after hydrogen transfer reactions, paraffins are produced. However, LPG composition is mainly olefinic and much based on propylene (more than 35% of the total LPG), although there are also important amounts of isobutane and, in a less relevant amount, C4 olefins, which are produced in the same quantity between them (Melero et al., 2010b).

The liquid product of a catalytic cracking process is usually composed of cyclic and linear aliphatic hydrocarbons as well as aromatic compounds. The main hydrocarbon liquids considered are GLN, LCO and DO (Melero et al., 2010b; Tian et al., 2008). DO is the heaviest reaction product and, in the case of the renewable raw materials, it is obtained by means of condensation or polymerization reactions (Horne and Williams, 1996; Idem et al., 1996). This fact explains the low yield towards DO of around 2-4.5%, as shown in Fig. 15.5 and Table 15.2, in the results obtained by the research groups of Melero et al. (2010b) and Tian et al. (2008). On the other hand, GLN is the main liquid compound, with a yield that can be close to 40% of the total product distribution (that means more than 75% of the OLP) (Melero et al., 2010b). The LCO presence is less important, and it implies a yield of ca. 10-15% (Melero et al., 2010b; Tian et al., 2008). Both

Research group Experimental conditions Feedstock Product (wt.%) OLP Gases GLN LCO Tamunaidu and Riser reactor Palm oil 19.9-38.1 49.5-59.1 0.1-10.4 Bhatia (2007) T = 400-500°C Catalyst-to-oil ratio = 5-10 Chew et at. (2009) Riser reactor Crude palm oil 16.2 43.5 4.2 T = 450°C Catalyst-to-oil ratio = 5 Used palm oil 15.9 33.0 4.5 Li etal. (2009) Fluidized bed T = 400-500°C Catalyst-to-oil ratio = 6-10 Cottonseed oil 7.5-28.8 25.1-33.7 49.5-64.0 Gases OLP Dry gas LPG GLN LCO DO Coke Tian etal. (2008) Riser reactor Chicken fat 4.48 34.34 32.75 11.40 2.95 2.31 T = 400-500°C Palm oil 6.35 41.56 28.14 8.90 1.97 2.20 Catalyst-to-oil ratio = 6-10 Soybean oil 4.59 29.24 32.27 15.28 4.50 3.98

gasoline and LCO are involved in b-scission, isomerization and hydrogen transfer reactions of the hydrocarbons, which come from decomposition of heavy hydrocarbons. Furthermore, cracking under FCC conditions involves high contents in aromatic hydrocarbons in the organic liquid phase. The high number of dehydrogenation reactions to remove oxygen in the form of water leads to an increase in the olefins formation, which leads to the aromatic compounds formation under the FCC reaction conditions. Concretely, an aromatic content of 30-40% has been reported in the gasoline fraction (Melero et al, 2010b; Tian et al., 2008).

Last reaction product is coke, which is mainly produced by a thermal pathway. Most catalyst deactivation associated with coke formation is produced in the initial reaction period because some of the free radicals formed by thermal processes are not able to go within the catalyst pores and are deposited in the most external part of it (Dupain et al., 2006). Coke can also be obtained from thermal direct polycondensation of either triglyceride molecules or primary heavy oxygenated hydrocarbons (Katikaneni et al., 1997). Furthermore, coke might also be obtained by a catalytic route that involves the formation of polyaromatic compounds coming from a successive hydrogen elimination of aromatic molecules. Nevertheless, coke coming from a catalytic route is always lower than that obtained thermally.