A controlled precipitation method was also used by Hong et al. [ 36 ]. The process of precipitating zinc oxide was carried out using zinc acetate (Zn(CH 3 COO) 2 ·H 2 O) and ammonium carbonate (NH 4 ) 2 CO 3 . These solutions were dosed into a vigorously mixed aqueous solution of poly(ethylene glycol) with an average molecular mass of 10,000. The resulting precipitate was calcined by two different methods. In the first, calcination at 450 °C for 3 h produced ZnO labelled as “powder A”. In the second process, calcination took place following heterogeneous azeotropic distillation of the precursor; the resulting zinc oxide was labelled as “powder B”. Structural testing (XRD) and morphological analysis (TEM) showed that powder A contained particles with a diameter of 40 nm, while powder B contained particles with a diameter of 30 nm. Heterogeneous azeotropic distillation completely reduces the occurrence of agglomerates and decreases the ZnO particle size.
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Catalyst concentration has a significant role on conversion of FFA to methyl ester. Increase of catalyst concentration increases the percentage of FFA conversion. At a certain catalyst concentration the conversion was higher. HCl was used as catalyst. The effect of catalyst concentration on conversion of FFA was investigated the results are represented in Figure 3. From the Figure 3, it can be seen that the conversion was 98% at the catalyst concentration of 5 wt% of FFA. Further increasing the catalyst concentration conversion does not increase. The optimum catalyst concentration was 5 wt% of FFA.