22.4.1 Introduction

For the recovery of minor components a lot of research has been reported. However, industrial applications of the described processes are not much published, except for in the patent literature.

In the production of biodiesel/biofuel from DD, its heterogeneous composition (FFA, acylglycerols, sterols, squalene and tocopherols) should be considered, not only for the selection of a good conversion process but also for the valorization of all the compounds which would make the process economically more interesting.

For their purification, different aspects should be considered. Since bioactive compounds, such as tocopherols, phytosterols and squalene, are minor components in DD, their enrichment is vital before they can be effectively fractionated and separated into an individual compound. The main challenge is to separate them from each other, especially in the case of the following pairs of components: tocopherol-squalene, tocopherol-fatty acids, tocopherol-sterol and sterol-squalene.

Numerous methods have been proposed for treating DD to isolate one or more compounds. In general, the selective separation of compounds in DD is based on differences in their chemical and physical properties such as solubility, polarity, molecular weight or differences in volatility.

Molecular distillation or short-path distillation is by far the preferred method for isolating both thermosensitive and high molecular weight compounds (Top et al., 1993; Shimada et al, 2000; Watanabe et al., 2004; Martins et al., 2005; Nagao et al., 2005). However, the disadvantages of this method are that the equipment is expensive and the operation cost is high. Furthermore, the sterols are one of the major components of DD and cannot be removed by molecular distillation because their molecular weights and vapor pressure are similar to those of tocopherols. A conventional method to concentrate tocopherols using molecular and vacuum distillation after removing the sterols via alcohol recrystalization requires several steps such as solvent recovery and purification, and further requires high amounts of solvents and energy.

Other separation processes are based on differences in their polarity like solvent extraction (Brown and Smith, 1964; Chu et al., 2002; Gunawan et al., 2008; Leng et al., 2008). The advantage of solvent extraction over molecular distillation is that it operates under atmospheric pressure and lower temperature, and requires simpler equipment. Although modified solvent extraction is capable of separating tocopherols, free phytosterols, FFA and acylglycerols from DD on laboratory scale, it is difficult to apply this method in large-scale operations.

Other processes involve supercritical fluid extraction (Lee et al., 1991; Bondioli et al., 1993; Stoldt and Brunner, 1998; Stoldt and Brunner, 1999; Chia-Cheng et al., 2000; King and Dunford, 2002; Mendes et al., 2002; Mojca et al., 2003; Nagesha et al., 2003; Pereira et al., 2004; Liu et al., 2006; Vazquez et al., 2006; Fang et al., 2007; Vazquez et al., 2007; Fornari et al., 2009; Torres et al., 2009; Martinez-Correa et al., 2010), crystallization (De Areal Rothes and Verhe, 2005; Pan et al., 2005), crystallization and/or membrane separation (Lin et al., 2007), treatment with urea (Sampathkumar, 1986; Maza, 1992) and batch adsorption (Chu et al., 2004; Fabian et al., 2009).

Solvent extraction and crystallization are mainly used to recover sterols over tocopherols. This process has the advantage of not causing tocopherol oxidation

and does not use high pressure, but the amount of solvent required is still very high and the use of such quantities does not lead to an environmentally friendly process. It is also noted that extraction with solvent requires laborious manipulations. Research in this area is not very extensive due to the low recovery and low purity of sterols and tocopherols (Lin and Koseoglu, 2003; Moreira and Baltanas, 2004).

Besides the target compounds (sterols, tocopherols and squalene), the DD contains acylglycerols and FFA which make the purification more difficult. Consequently, it is necessary to modify DD with esterification and/or alcoholysis reactions using chemical or enzymatic means, which convert most of the fatty acids, free sterols and acylglycerols to FAME and esterified sterols.

A schematic representation of the most used pathway to separate the minor components is shown in Fig. 22.4.