Pyrolysis of vegetable oil soaps—Palm, olive, rapeseed and castor oils

Abstract

Saponified, palm, olive, rapeseed and castor oils were pyrolysed (at 750 °C for 20 s) by pyrolysis gas chromatography with mass selective and flame ionisation detection (Py-GC/MSD and FID) to clarify their thermochemical behaviours. The liquefiable compounds recovered from palm, olive and rapeseed oils mainly contained linear alkenes (up to C₁₉) and alkanes (up to C₁₇), both similar to those found in gasoline (C₄–C₁₀) and diesel fuel (C₁₁–C₂₂) boiling range fractions of petroleum, whereas in the case of castor oil a significant amount of undesired oxygen-containing products (e.g., ketones and phenols) were formed. The obtained data on reaction mechanisms can also be utilised in applications where various biofuels are produced, for example, from the extractive-derived by-product (tall oil) of kraft pulping.

Introduction

Biofuels which can be used as fuel or fuel additives have recently become more attractive because of their environmental benefits and the uncertainties concerning petroleum availability [1], [2], [3], [4], [5]. Among the different possible renewable raw materials, extractive-based glycerides (i.e., esters of fatty acids with glycerol) in plants present one source for producing hydrocarbon-rich products resembling petroleum [6], [7].

Vegetable oils are primarily composed of triglycerides (about 90%) with lesser amounts of mono- and diglycerides, as well as free fatty acids [8], [9], [10]. In addition, some other compounds such as phospholipids, sterols and their esters, tocols, hydrocarbons and fat-soluble vitamins are present. The most common fatty acid compounds comprise saturated palmitic (C_16:0) and stearic (C_18:0) acids together with unsaturated oleic (C_18:1) and linoleic (C_18:2) acids [8], [11]. Many vegetable oils also contain, for example, lauric (C_12:0), myristic (C_14:0) and erucic (C_22:1) acids in significant amounts. For example, the content of unsaturated oleic acid (C_18:1) is typically 55–85% in olive oil, about 60% in rapeseed oil and about 40% in palm oil [4], [5], [6]. On the other hand, saturated palmitic acid (C₁₆) is also a prominent acid component in palm oil (about 45%), whereas clearly lower amounts (<5%) of this acid can be found in rapeseed and olive oils (8–20%). In contrast, castor oil is practically comprised of monounsaturated ricinoleic acid (12-hydroxy-9-octadecenoic acid) (about 90%) with minor amounts of oleic and palmitic acids [12], [13], [14]. Due to the hydroxyl group of this acid, its pyrolytic behaviour may differ characteristically from other vegetable oils, making castor oil one of the most interesting raw materials.

Pyrolysis reactions offer a potential way of producing fuels and chemicals from vegetable oils [15]. This option is especially promising in areas where the hydroprocessing industry is well established because the technology is rather similar to that of conventional petroleum refining. Pyrolysis, which is the thermal decomposition of materials in the absence of oxygen, is the simplest and oldest method of processing one fuel to produce a better one [11].

The pyrolysis of triglycerides has been investigated for over more than 100 years, especially in areas of the world that lack deposits of petroleum [16]. Earlier studies were also conducted on the thermal cracking of soaps of various oils. In 1974 Chang and Wan [17] reported a large-scale thermal cracking of tung oil calcium soap which yielded diesel-like fuel and small amounts of gasoline and kerosene. Fortes and Baugh [18] made a product with a similar composition to diesel fuel, besides the presence of some ketones, when pyrolysing the calcium soap of Macauba fruit vegetable oil.

In general, free radical pathways dominate the mechanisms for various pyrolysis reactions of organic materials [19], [20]. Due to the complexity of possible radical reactions taking place, a wide range of products is also to be expected. According to Chang and Wan [17], Alencar et al. [21] and Idem et al. [22], the elimination of heavily-oxygenated hydrocarbons such as carboxylic acids, ketones, aldehydes and esters is a dominant step in the thermochemical behaviour of triglycerides. After these initial decomposition reactions, possible reaction routes include (i) the decarboxylation and decarbonylation of saturated oxygenated hydrocarbons followed by the C–C bond cleavage of the resulting hydrocarbon radicals or (ii) the C–C bond cleavage of unsaturated oxygenated hydrocarbons followed by decarboxylation and decarbonylation of the resulting short-chain molecules. Radicals formed in these reactions then undergo various other reactions such as disproportionation, β-scission, isomerisation, hydrogen abstraction and aromatisation. Heavy hydrocarbons are produced by polymerisation and polycondensation reactions [23].

This investigation, dealing with the pyrolysis of vegetable oil soaps, continues our earlier studies [24] on the thermochemical behaviour of fatty acid sodium salts during pyrolysis. The selected raw materials were palm oil from the fruits of the palm tree (Elaeis guineensis), castor oil from the castor beans of the castor plant (Ricinus communis), olive oil from the olive tree (Olea europaea) and rapeseed oil from the rapeseed plant (Brassica napus). Based on this approach, the main aims of the present study were to clarify the pyrolysis chemistry of fatty acid soaps in natural matrices and to verify earlier findings obtained from model substances. Furthermore, fatty acid sodium salts form a significant fraction (20–40% of the total extractives) of the tall oil soap recovered from kraft pulping [25]. For this reason, it can be assumed that a better understanding of pyrolytic fatty acid reactions will also be of benefit to the development of the tall oil-based production of biofuels. This topic will be discussed in detail in forthcoming papers.