produced during the production of biodiesel contains impurities including catalyst salts, unreacted glycerides, and water depending upon the nature of oil and technology employed. The amount of glycerol in crude glycerol may vary from 45 to 90% depending upon the reaction conditions [5, 6]. Upgrading and refining crude glycerol are necessary to minimize waste production. The industrial-grade glycerol can be obtained by filtration, extraction, and distillation. The glycerol produced from different bio-refineries has a different composition, so it is complicated to outline the properties of crude glycerol. The properties of pure glycerol are outlined in Table 4.1. There are many commercial grades of glycerol; it is named glycerin if the concentration of glycerol is above 95%. The structure of glycerol is shown in Figure 4.4. It is miscible in water due to the presence of hydrophilic hydroxyl groups. It is a colorless, unscented thick fluid having a boiling and melting point of 290 and 17.9 °C, respectively.
Figure 4.1 Transesterification reaction for biodiesel production.
Figure 4.2 Steps involved in the transesterification process.
Figure 4.3 Transesterification reaction for biodiesel production.
Table 4.1 Properties of pure glycerol.
| Molecular formula | C3H8O3 |
| Molar mass | 92.09 g/mol |
| Melting point | 18 °C |
| Boiling point | 290 °C |
| Relative density | 1,260 kg/m3 |
| Viscosity | 1.41 Pa s |
| Flash point | 160 °C |
| Specific heat | 2.43 kJ/kg K |
| Heat of vaporization | 82.12 kJ/kmol |
| Heat of formation | 667.8 kJ/mol |
| Surface tension | 63.4 mN/m |
| Self-ignition | 393 °C |
Figure 4.4 Chemical structure of pure glycerol.
4.3 Refining Process for Crude Glycerol
As discussed earlier, the crude glycerol from bio-diesel industries contains a large number of impurities and cannot be used in this form. Glycerol purification is important for removing unwanted impurities and enhancing its usability for various applications. There are several steps for the purification of crude glycerol that gives an 80–95% purity level. The commonly used steps for removing major impurities are neutralization, vacuum distillation, centrifugation, adsorption, etc. [14].
4.3.1 Neutralization/Acidification
The initial step for the refining of crude glycerol is pre-treatment using strong acid which removes the catalyst and soaps. The acids commonly used for the acidification process include phosphoric acid, sulfuric acid, and hydrochloric acid. The reaction of base catalyst and acid will generate salt and water, and its reaction with soap gives free fatty acids. The pre-treatment of crude glycerol will form three separate layers. The bottom layer is made up of inorganic salts. The glycerol is present in the middle layer with free-floating fatty acids at the top. The glycerol with 93.3% purity was obtained on chemical and physical treatment at pH 1 which is followed by neutralization using 12.5 M NaOH [15].
4.3.2 Methanol Removal
The acidification of crude glycerol is followed by the removal of methanol which was used in excess for high biodiesel yield. The surplus methanol is dispersed between crude glycerol and methyl ester which is having serious health and environmental hazards. This surplus methanol and water are removed by vacuum evaporation at 50–90 °C for 2 h.
4.3.3 Vacuum Distillation
Vacuum distillation is a well-established process of separating the compounds based on their boiling points using thermal energy under reduced pressure. This technique is used for the purification of a broad variety of chemicals. The distillation of glycerol should be carried out in a vacuum to prevent dehydration, polymerization of glycerol into polyglycerol, and glycerol oxidation into glyceraldehydes, glycerose, and di-hydroxylacetone. The temperature, pressure, and pH can be controlled under a vacuum [16]. However, vacuum distillation is energy demanding technique that requires high energy input and may lead to the thermal decomposition of glycerol.
4.3.4 Ion Exchange
Ion exchange is mainly used for the separation of impurities such as fatty acids, free ions, and salt from the glycerol. Isahak et al. [17] have used a vertical column filled with ion exchange resins for the purification of glycerol. It was found by high-performance liquid chromatography (HPLC) that a single peak of glycerol with a smooth baseline was obtained after the ion exchange process.
4.3.5 Adsorption
Adsorption is a process of deposition of ions, atoms, or molecules from solid, gas, or molecule on the surface. The activated carbon is used as an adsorbent for the final step in the purification process which removes the fatty acids, color, and additional components. Manosak et al. [18] have studied a color removal of up to 97.7% by using commercial activated carbon. In addition to color removal, few fatty acids such as myristic acid and lauric acid have been removed.
4.4 Technologies for Glycerol Valorization
After purification, the low-cost glycerol can be used for the preparation of valuable products that are important for the industries. The multifunctional structure of glycerol can be modified with several reaction pathways. Various technologies for glycerol valorization have been categorized into biological conversion and thermochemical conversion (with or without catalyst) [12]. In this respect, catalysis represents an efficient approach for the activation and valorization of glycerol.