Читать книгу Handbook of Biomass Valorization for Industrial Applications - Группа авторов - Страница 69

Worldwide biodiesel production has been rising extensively in the last few years. In 2018, the top five biodiesel producing nations were the United States (US), Argentina, Brazil, Germany, and China [10]. Both the US and Brazil have accounted for a combined share of nearly 87% of the total production in 2018. It is predicted that in the near future, the annual production of biodiesel will rise by approximately 4.5% [10]. Biodiesel is mainly produced from a broad diversity of materials that contains triglycerides as a key component. This can be produced by vegetable oils from jatropha, sunflower, babassu, soybean, macauba, palm, oilseed radish, canola, crambe, castor bean, peanut, and lupine [4]. Alternatively, it can also be produced from photosynthetic algae, animal fat, and waste cooking oil. The transesterification or esterification of triglycerides with alcohol (methanol) produces methyl esters of fatty acids (biodiesel) and glycerol as depicted in Figure 4.1. The catalysts used for transesterification are mainly alkalis, acids, or enzymes.

The transesterification reaction takes place in three steps as shown in Figure 4.2 in which triglycerides react with methanol. First, the reaction involves the conversion of triglycerides into diglycerides which then get converted into monoglycerides. Finally, the monoglycerides transformed into glycerol. The overall reaction is reversible, so, plenty of alcohol is required to favor the reaction in the forward direction. The theoretical stoichiometric ratio for the conversion is 1:3 (triglycerides: alcohol), but for practical applications surplus amount of alcohol is required to shift the equilibrium in the direction of the product. Overall, for the formation of 9 kg of biodiesel, approximately 1 kg of glycerol is generated [5].

The flow chart for the formation of biodiesel and byproduct glycerol is shown in Figure 4.3.

The commonly used catalyst during biodiesel production is NaOH which leads to the generation of faulty smell, dark-colored glycerol with high pH. The crude glycerol 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.