A study of Two-Stage Transesterification : Base Homogeneous Catalyst and Acid Heterogeneous Catalyst

Abstract

The utilization of base catalyst for two-stage transesterification prompts soap formation and yield loss in biodiesel production. In order to overcome this difficulty, the two-stage transesterification process catalyzed by heterogeneous acid catalyst in the second stage is a good recommendation to reduce the soap formation. The aim of this thesis is to develop the two-stage transesterification process with homogeneous base catalyst in the first stage and heterogeneous acid catalyst in the second stage. A novel chemical method for determining the ester content in biodiesel was demonstrated as an effective method. Moreover, evaluation of two-stage transesterification process as per the determining of the total glycerol content in biodiesel was also a suitable solution in this present study. The first stage transesterification catalyzed by homogeneous base catalyst was studied in a batch reactor. Experimental factors were investigated; including MeOH/RPO molar ratio (5:1-6:1), CHONa catalyst content (0.30- 0.70 wt% to RPO), reaction time (20-60 min) and reaction temperature (45-65 °C). The Composite Central Design (CCD) was applied to investigate the influences of the experimental variables on the ester content and total glycerol content; and to find the optimum conditions for the requested ester content. The requested ester content of 85% was obtained under optimum condition: 5.48 of MeOH/RPO molar ratio, 0.32 wt% of CH,ONa, 40 min and 55 °C. Response surface methodology (RSM) has been applied in modeling and optimization for the first stage transesterification. This model vi investigated that the ChiONa catalyst content is the most significant factor for this stage. Polynomial regression equation for the first stage transesterification was also established as per the analysis of variance (ANOVA). The second stage transesterification catalyzed by heterogeneous acid catalyst was carried out in high pressure apparatus. Experimental runs were changed following to reaction conditions; including MeOH/oil molar ratio (8:1-12:1), Amberlyst-15 catalyst content (4-16 wt%), reaction time (3-12 h) and reaction temperature (115 °C). The factorial design was used to conduct the effects of the experimental factors on the ester content and total glycerol content; and to find the optimum conditions for the requested ester content. The requested ester content of 98% was obtained under optimum condition: 10 of MeOH/oil molar ratio, 12 wt% of Amberlyst-15 catalyst, 9 h and 115 °C. RSM has been applied in modeling and optimizing for the second stage transesterification. In the present study, the polynomial regression equation for the second stage transesterification was also established as per the analysis of variance (ANOVA). As a remarkable point of this thesis, application of the present two- step transesterification technology has led to decrease the soap and the total amount of sodium methoxide. The soap content decreased 50% (by mol%) from one-step transesterification and amount of base catalyst used was 33% (by mol%) as compared to one-step transesterification process. In an effort to enhance the present two-stage transesterification process in this thesis, a process development of two-stage transesterification was also studied. By using the ester phase directly for the second stage, there was a decrease of 20 wt% fresh MeOH used for the second stage transesterification. This decreasing along with not applying the washing and drying process after the first stage transesterification lead to a good solution for the cost reducing of the biodiesel production.