Production of biodiesel is relied on either strong base or strong acid homoge­neous catalysts in the transesterification reaction. The examples of base catalysts are potassium hydroxide (KOH) and sodium hydroxide (NaOH), while sulfuric acid (H2SO4) is commonly used as an acid catalyst (Freedman et al. 1984). The alkali — catalyzed transesterification is economically feasible process as both NaOH and KOH catalysts are cheap. The alkali-catalyzed transesterification pro­cess is carried out under a low temperature (methanol boiling reflux) and atmo­spheric pressure environment, and the conversion rate is high with no intermediate steps (Leung and Guo 2006). However, the alkali homogeneous catalysts are highly hygroscopic. They also form water when dissolved in the alcohol reactant and this will affect biodiesel yield [Eq. (10.1)]. Therefore, they should be prop­erly handled.

KOH or NaOH + CH3OH ^ CH3O — K or CH3O — Na + H2O (10.1)

Today, the alkali-catalyzed process produces most of the biodiesel. The conven­tional transesterification process for biodiesel manufacturing process consists of four main principal steps (Cheng 2009):

Table 10.2 Advantages and disadvantages of catalysis for transesterification reaction Type Advantages Disadvantages

1. Pretreatment of the crude feedstock to remove component (water or FFAs) that is unfavorable to subsequent processing steps.

2. Transesterification reaction: the pretreated oils or fats are reacted with alcohol (normally methanol) to form mono-alkyl esters and by-product glycerol.

3. Alkyl ester (biodiesel) purification: the excess methanol, catalyst, and glycerol are removed by water washing step and chemical treatment. The methanol will recycle for the next reaction.

4. Glycerol purification: methanol and catalyst in glycerol phase are recycled and removed by water washing and chemical treatment step to produce higher grade glycerol for commercialization.

Figure 10.1 show the industrial process for homogeneous catalyzed transesterifi­cation reaction.