Читать книгу Biodiesel Production - Группа авторов - Страница 34

Alcoholysis is another term used for the process of transesterification, which involves the molecular breakdown of a glyceride (mono‐, di‐, or tri‐) using an alcohol, which after multiple rounds produces esters and glycerol as a by‐product [17, 18]. Stepwise conversion occurring in a typical triglyceride transesterification with methanol is depicted in Figure 2.1.

Figure 2.1 Catalyzed conversion of triglyceride and FFA into esters using methanol.

Figure 2.2 Dissociation of alkali and saponification with FFA.

Figure 2.3 Hydrolysis of triglyceride leading to FFA formation.

Alcohols used can be polar (such as ethanol and methanol) or nonpolar (such as 2‐propanol or butanol). Although polar alcohols are capable of providing good fuel yields during transesterification, they are not miscible with the oils, and it has been reported by Karmakar et al. in their works with castor–karanja oil blend that 2‐propanol provides better conversion compared to methanol when used simultaneously, due to being miscible [1]. This can lead to the speculation that while activation energy for the nonpolar system would be lower (from reduced diffusion resistance), the reaction rate would be lower since nonpolar compounds are less reactive. Conversions of this nature can be facilitated with the use of alkali or enzyme catalysts, and while alkalis are used more frequently, the system is left vulnerable to saponification due to the presence of FFAs in most feedstock as depicted in Figure 2.2. Triglycerides are also degraded into FFAs at elevated temperatures in the presence of water, through a process termed as “hydrolysis,” depicted in Figure 2.3.

Other modes of transesterification include the use of supercritical fluids heated to 300–400 °C under 10–30 MPa; superheated alcohols, which are heated to above 150 °C and injected to hot oil (above 250 °C); and enzyme catalysts under slightly elevated temperatures [19]. However, these processes are not without drawbacks, discussed in later sections in detail. The formed fatty acid alkyl esters (FAAE) have a much lower viscosity compared to the parent oil due to their smaller molecular sizes, which gives the fuel its ability to be compatible in diesel engines. This process is applicable to all feedstock in which glycerides exist and thus can be used for a wide variety of sources [2].