Thermal decomposition of vegetable oil was performed to prove the theory of the origin of mineral oil from organic matter [14] as early as 1888. Literature up to 1983 has been reviewed by Schwab et al. [15]. In many cases, inadequate characterization of products formed in pyrol­ysis of vegetable oils was found. Therefore, analytical data obtained by gas chromatography—mass spectrometry (GC-MS) from thermally decomposed soybean oil and high oleic safflower oil in the presence of air or nitrogen were reported [15].

The ASTM standard method for distillation of petroleum products D86-82 has been used for decomposition experiments. Catalytic systems were excluded in this destructive distillation. The actual temperature of the oil in the feeder flask was about 100oC higher than the vapor temper­ature throughout the distillation. Under these conditions, GC-MS analy­sis showed that approximately 75% of the products were made up of alkanes, alkenes, aromatics, and carboxylic acids with carbon numbers ranging from 4 to more than 20 (see Table 8.1).

A comparison of fuel properties is given in Table 8.2. The carbon — hydrogen ratio shows 79% C and 11.88% H for the pyrolyzate of soybean

oil. This indicates considerable amounts of oxygenated compounds in the distillate. Consequently, methylation of these oils has revealed 9.6-12.2% of carboxylic acids ranging from C-3 to over C-18. This is reflected in the higher viscosity compared to diesel.

Mass-spectral fingerprints of the entire pyrolysis product slate from tripalmitin, different vegetable oils, and extracted oils from microalgae confirm that the decomposition of ester bonds in the absence of external catalysts is extensive [16-18]. However, a great variability in primary pyrolysis/vaporization product slates was observed [18].

Thermodynamic calculation in the degradation process shows that the cleavage of C-O bond takes place at 288°C and fatty acids are the main product [19]. The actual pyrolysis temperature should be higher than 400°C to obtain maximum diesel yield [20]. The mechanism of pyrolysis of vegetable oil has been discussed by various authors [9, 15, 19]. Generally, thermal decomposition proceeds through either a free-radical or carbonium ion mechanism. The primary R-COO splits off carbon dioxide. The alkyl radicals (R), upon disproportionation and elimination of ethene, give rise to alkanes and alkenes. The formation of aromatics is facilitated by a Diels-Alder addition of ethene to a conjugated diene formed in the pyrol­ysis reactions. However, the product mix and product quality are influenced by many factors such as feed pretreatment, heating rate, and temperature. As vegetable oils may contain trace elements, catalytic effects cannot be completely excluded from any thermal degradation process [21].

8.1.1 Catalytic cracking (CC)

In 1979, a paper [22] from the petrochemical industry reported for the first time that high-molecular-weight triglycerides such as corn oil (C57H104O6) and castor oil (C57H104O9) were convertible to a high-grade gasoline when passed over H-ZSM-5, a catalyst. The latter is a synthetic, medium-pore, shape-selective acid catalyst. Lipids were fed with a piston displacement pump at a rate of 2 mL/h with flowing hydrogen (300 mL/h) over 2 mL of H-ZSM-5 catalyst (0.77 g, 14-30 mesh) contained in a ver­tical Pyrex reactor at atmospheric pressure and T = 400-450°C. Paraffins, olefins, aromatics, and nonaromatics could be detected in the product mixture. The distribution of hydrocarbons is similar to selective conversion of methanol into hydrocarbon units with up to 10 carbon atoms per molecule. In all cases, a high degree of BTX aromatics (ben­zene, toluene, and xylene) was achieved. The precondition for the catalytic conversion is that the molecule penetrate the cavities of microporous zeolite.

This new catalytic approach has paved the way for a variety of appli­cations. A schematic diagram of experimental arrangements for pyrol­ysis and catalytic conversion is given in Fig. 8.1.

Conversion of different kinds of vegetable oils over medium-pore H- ZSM-5 have been investigated in detail [23-26]. Catalytic cracking of by-products from palm oil mills with a selectivity of 51wt.% toward aro­matic hydrocarbon formation has been reported [27]. To achieve higher yields, this type of work was extended to pyrolysis and zeolite conver­sion of both whole algae and their major components as well as whole seeds and selected vegetable oils [18, 28-31]. Hot vapors from solid organic material (microalgae, seeds, etc.) or vaporized vegetable oils were passed directly over the H-ZSM-5 catalyst. Products of different

algae, seeds, or vegetable oils emerging from the passage showed a uni­form, high-octane, aromatic gasoline product. Obviously, the molecular pattern of products is insensitive to the nature of lipids used. This is in contrast to pyrolysis without a catalyst [18].

Upgrading of crude tall oil to fuels and chemicals has been studied at atmospheric pressure and in the temperature range of 370—440oC, in a fixed-bed microreactor containing H-ZSM-5 [32]. The oil was co-fed with diluents such as tetralin, methanol, and steam. High oil conversions, in the range of 80-90 wt.%, were obtained using tetralin and methanol as diluents. Conversions under steam were reduced to 36-70 wt.%. The maximum concentration of gasoline-range aromatic hydrocarbons was 52-57 wt.% with tetralin and steam, but only 39% with methanol. The amount of gas product in most runs was 1-4 wt.% [32].