Biodiesel production from coconut oil and methanol using heterogeneous catalyst in a packed bed reactor

• Main Author: Co, Charles Edric T.
• Format: text
• Language: English
• Published: Animo Repository 2009
• Subjects: Biomass energy; Coconut oil as fuel; Transesterification; Catalysts; Chemical Engineering
• Online Access: https://animorepository.dlsu.edu.ph/etd_masteral/6918
• Institution: De La Salle University

The most common method used in the production of biodiesel uses homogeneous sodium hydroxide catalyst which gives high reaction rate and conversion; nevertheless, problems such as saponification and catalyst consumption often come with this method. Hence, studies were conducted using heterogeneous catalysts like ion-exchange resins for transesterification. In this study, the anion-exchange resin catalyst provided by the sponsoring company was used in the transesterification of coconut oil and methanol under different conditions using a glass reactor designed and fabricated for this research. The actual flow experiments conducted had a net conversion in the range of 68.46% to 74.61%. The perceived reaction rate had a correlation of 0.6736 with the square root of the velocity and a correlation of only 0.1707 with the exponential of temperature; hence, we can say that mass transfer is the rate- determining step. The net conversion also had a high correlation with the amount of catalyst, showing the effect on this of the number of available pores through which the reactants could diffuse. Multiple linear regression studies had also been conducted, and it was seen that the amount of catalyst is the most significant parameter on the reaction rate with a p-value of 0.0017 while the amount of excess methanol follows with a p-value of 0.0078. The fact that the amount of catalyst is significant on the reaction rate in the 99.8% confidence interval verifies the fact that diffusion is the rate-controlling mechanism in this study. The Wagner-Weisz-Wheeler Modulus was calculated by estimating the diffusivity of the reactants through the pores using the correlation proposed by Wilke and Chang for infinitely dilute solutions and that by Leffler and Cullinan for concentrated solutions. The modulus obtained was 12.91 which, being above 4, showed that transesterification proceeded in the strong pore resistance regime. In the continuous flow experiments conducted, the net conversion remained above 70% even if 383 bed void volumes of biodiesel had been produced. The relative standard deviation of the net conversion of coconut oil to biodiesel during the production of 2,285 grams of biodiesel without regenerating the resins was only 3.27%, showing no noticeable decline in the catalyst activity. Nevertheless, the catalysts were observed to become darker as the reaction progressed, owing perhaps to the fact that glycerine and other substances had been adsorbed on the catalyst surface. Because of this, citric acid wash is necessary to remove the adsorbed substances prior to the regeneration of the catalyst with sodium methylate dissolved in methanol. Analysis of biodiesel produced showed that the standards set for the acid value and the percent free glycerol were met. Nevertheless, the moisture content and the percent total glycerol values exceeded the limits set which are 500 ppm and 0.24% respectively. The presence of moisture, in particular, could be attributed to the moisture in the coconut oil, methanol and catalyst, and that produced during the reaction itself. Noting that the acid value is negatively correlated to the moisture content, it could be hypothesized that some of the moisture present in the final product came from the esterification of free fatty acids which has water as its unwanted product in addition to the methyl esters formed. For the purposes of scaling-up and pilot plant studies, a Levenspiel plot was made which could be used to determine the amount of catalyst needed for certain reaction conditions. This, together with an equation derived from the Ergun equation, could be used to optimize the design of the reactor as shown in the example provided.