Pyrolysis (thermal cracking) of triglycerides for fuels and chemicals.
Pyrolysis or thermal cracking of triglyceride materials represents an alternative method of roducing renewable bio-based products suitable for use in fuel and chemical applications. This option is especially promising in areas where the hydroprocessing industry is well established because the technology is very similar tothat of conventional petroleum refining. There are significant advantages of this type of technology over transesterification including lower processing costs, compatibility with infrastructure, engines and fuel standards, and feed stock flexibility (Stumborg et al., 1996). More importantly, the final products are similar to diesel fuel in composition.
There are several studies that haveinvestigated the thermal cracking of triglycerides. These studies fall into two categories. One focuses on pyrolysis of model triglycerides for food science research (Crossley et al., 1962; Higman et al., 1973; Kitamura, 1971; Nawar, 1969; Nichols and
Holman, 1972) while the other is devoted to cracking vegetable oils and fats for fuel applications (Adebanjo et al.,2005; Alencar et al., 1983; Changand Wan, 1947; Dandik and Aksoy, 1998a; Egloff and Morrell, 1932; Egloff and Nelson, 1933; Fortes and Baugh, 1999, 2004; Idem et al.,
1996; Lima et al., 2004; Niehaus et al., 1986; Schwab et al., 1988).
There have been studies conducted on the decomposition of both saturated and unsaturated triglycerides during the applications of heat (Crossley et al., 1962; Higman etal., 1973; Kitamura, 1971; Lipinsky et al., 1985; Nawar,1969). It is well recognized that at 300 _C the gross pyrolysis of fats results in the formation of fatty acids and acrolein. At higher temperatures, (400–500 _C) cracking occurs, producing short chain hydrocarbons (Crossley et al., 1962). The mechanisms involved thermal cracking of saturated triglycerides have been studied (Alencar et al.,1983; Chang and Wan, 1947). Chang and Wan (1947) proposed a reaction scheme for the pyrolysis of saturated triglycerides, which includes 16 types of reactions and is shown in Fig. 1. It is believed that the larger part of the acids, acrolein, ketenes formed in Eq. (1) are rapidly decomposed according to Eqs. (2) and (3) and that Eqs. (6) and (11) are chiefly responsible for the formation ofhydrocarbons constituting liquid fuels, especially in the gasoline fraction.
Based on the scheme proposed by Chang and Wan (1947) and on results by Greensfelder et al. (1949), Alencar et al. (1983) also proposed a scheme for the cracking of saturated triglycerides. The scheme is presented in Fig. 2. The cracking of the triglyceride produces free radicals (A) RCOO_ and (B) RCH2O_. The odd n-alkanes and1-alkenes are formed by decarboxylation of Radical (A) and then by subsequent disproportionation and ethylene elimination. The even series of alkanes and alkenes are produced by the loss of a ketene from radical (B) and followed again by disproportionation and ethylene elimination.
Catalytic cracking of triglycerides for fuels and chemicals
In most studies involving theconversion of triglycerides into hydrocarbons at high temperatures, a catalyst is used
and there is a rich body of literature in this area. A comprehensive list of early studies using a variety of different catalysts is listed in US Patent 4,102,938 (Rao, 1978). Over the past several decades a variety of conventional hydrotreating catalysts have been studied in the conversion of triglyceridematerials to fuels. The two major groups of catalysts used include transition metal catalysts which are common in the hydroprocessing industry and molecular sieve type catalysts. The use of transition metal catalysts under high hydrogen partial pressures result in diesel like products which molecular sieve catalysts result in highly aromatic, gasoline type products. Pure insulator oxides, most...
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