A low alcohol‐to‐oil ratio is needed, with deemulsification conferring reusability of enzymes.
Low product inhibition and reacting temperature, with easy separation in case of heterogeneous immobilized enzymes.
Single‐step conversion with appreciable yields and are insensitive to moisture exposure.
However, they do require far longer durations to complete conversion, and since enzymes are very temperature sensitive, the reaction must be closely monitored. Another problem associated with them is their high costs as well as limited reusability due to structural denaturation and moderate conversion efficiencies compared to acid‐ or base‐catalyzed systems [30, 31]. Lipase is the most common enzyme used and is obtained from animals, plants, or microbes, and must not be stereospecific for maximum conversion efficiency. Bacterial and fungal lipases (example being Novozym 435 obtained from Candida antarctica or other enzymes extracted from sources such as Penicillium spp., Rhizopus spp., and Aspergillus niger) used can show maximum yield up to 90%, when operated between 30 and 50 °C for anywhere between 8 and 90 h depending on feedstock [2]. The variety of studies reported by researchers are numerous; a select few of which have been summarily presented in Table 2.2. However, Nelson et al. reported that polar alcohols tend to inactivate enzymes much faster than nonpolar alcohols [28].
2.5.1.4 Other Novel Heterogeneous Catalysts
There exist quite a number of reports by researchers on successful conversion of oil into biodiesel using catalysts that do not fall under the general spectrum of acids, bases, or enzymes. Mostly heterogeneous in nature, they are usually insensitive to the presence of FFAs and can convert them as well into esters (Table 2.2). The preparation strategies for each catalyst, therefore, vary greatly as they can be the source material itself (albeit modified to a certain extent) [32], a chemical compound that exists naturally as a salt [33], or other inert supports (carbonaceous or siliceous) that have been doped with transition metals, which are able to catalyze the transition much more efficiently [15, 26]. The form of doping in the last category is usually by the use of analytical grade salts containing the metal ion, which gets impregnated, leaving the anion to be washed off. Natural waste materials containing such elements can also be processed and used as a cost‐efficient alternative (such as cow bones for calcium doping).
2.5.1.5 Two‐Step Catalyzed Process
Many researchers opt for this method, in which acid esterification is used for pretreating the oil in order to make it suitable for base‐catalyzed conversion before performing alkali‐catalyzed transesterification, which can completely convert the glycerides into esters, since bases are sensitive to high FFAs (owing to saponification) as well as moisture (owing to hydrolysis) [27]. The process can comprise of either esterification–transesterification steps or hydrolysis and esterification steps (Table 2.2) [34]. Hydrolysis combined with esterification is comparatively more wasteful as generation of FFAs is an energy‐intensive process since high temperatures (exceeding 300 °C) and pressure (exceeding 10 MPa) are required. In the two‐step catalyzed process involving esterification and transesterification, acid catalysts remove almost all of the FFAs through conversion to esters and water, which can be then purified and dried prior to using base catalysts, which convert the glycerides into esters and glycerol [22]. The glycerol and excess alcohol can be removed through washing or by ultracentrifugation before being tested for suitability as fuel. As mentioned in Section 2.4, nonpolar alcohols result in better biodiesel yield compared with polar alcohols, and, thus, they hold great potential for use in biodiesel production [1, 6].
2.5.2 Modern Conversion Approaches
2.5.2.1 Supercritical Fluids
Critical point of a fluid designates the temperature and pressure at which compounds exit the liquid–vapor phase equilibrium. Beyond this stage (under supercritical conditions), the formed vapor cannot return to liquid state even under high pressure. For pressure‐driven chemical conversions, this is a huge benefit that ensures spontaneous product removal after reaction (for continuous production). Hence the use of supercritical fluids is another PI approach, which has been successfully used for complete conversion of various feedstock (fed‐batch or continuous). First proposed by Saka and Dadan [35], supercritical transesterification uses alcohols (polar or nonpolar) that have been preheated at 250–400 °C under 10–65 MPa pressure, during which the alcohol shows an increase in viscosity, resulting in decreased dielectric constant. This ensures that even polar alcohols become completely miscible, forming a single phase with superheated oil. The mixture is then pumped into the supercritical reactor (small capacity stainless steel reactor in a bath‐type heater). After the desired duration, the reactor is withdrawn from the bath, depressurized, and cooled for product collection. The supercritical state ensures that diffusional resistances among the reactants are drastically reduced; with E a no longer a hindrance, the reaction occurs spontaneously with complete conversion in a few minutes. However, the supercritical reactor must have high durability against such extreme pressure and temperature while requiring frequent maintenance, which are setbacks regarding process economics and energy efficiency. Formed glycerol is also not usable due to high impurity and exposure to such extreme conditions [36]. Consequently, few modifications were proposed and tested by researchers to minimize these issues, making the process more lucrative for small‐scale commercial production (Table 2.3).
Using cosolvents such as CO2, n‐hexane, tetrahydrofuran, propane, etc. facilitates solubilization, achieving homogeneous phase under much benign conditions, thereby increasing energy efficiency [46]. Metal oxides (ZnO, SrO2, TiO2, etc.) were successfully used for facilitating conversions under lowered pressure and temperature; however, catalyst separation is a hurdle that impedes smooth operation [47]. As a better alternative, uncatalyzed subcritical hydrolysis followed by supercritical esterification was proposed and tested by Kusdiana and Saka [48]. Also glycerol‐free processes (alcohol‐free) have been developed that use compounds such as methyl acetate to yield triacetin (used in leavening of bread and flavor enhancing of beverages), dimethyl carbonate to yield glycerol carbonate (used as fuel additive) and citramalic acid (used in cosmetics for skin toning), and methyl tert‐butyl ether, which yields glycerol tert‐butyl ether, used as biodiesel additive to lower cloud point and enhance cetane number.
Table 2.3 Modern approaches in biodiesel production from nonedible/waste feedstock.
| Supercritical fluid‐assisted biodiesel production | ||||||||
|---|---|---|---|---|---|---|---|---|
| Feedstock | Solvent (+ catalyst/cosolvent) | Reaction temperature (°C) | Reaction pressure (MPa) | Alcohol:oil ratio | Residence time (min) | Yield (%) | References | |
| Waste frying oil | Methanol | 300 | 20 |
| ||||