Optimization of biodiesel production using heterogenous catalyst in a packed bed reactor

Abstract

Industrial development is associated with an increase in pollution levels and rising fuel prices. Research on clean energy contributes to reduction of fossil fuel dependency, decrease in ozone layer depletion and reduction in emission of toxic gases. The development of renewable energies increases the energy independence and reduces the impact of environmental pollution from fossil fuels.

The biodiesel market is among the fastest growing renewable energy markets and its demand in the energy sector has tremendously increased over the last decade due to its environmental friendly qualities. Biodiesel is considered as a promising diesel fuel substitute based on the similarities of its properties with that of petroleum based diesel fuel. However, the high cost of the feedstock, environmental pollution as a result of wastewater generated from a homogeneous process has limited its full implementation. In addition, other technical challenges encountered during the production such as the immiscibility of the reagents and the reversibility of the transesterification reaction calls for innovative technologies to be developed. One promising solution to these issues is the use of membrane technology to serve as a reaction and separating medium for the production of biodiesel.

This study is aimed at optimizing biodiesel production from vegetable oils using heterogeneous catalysts in a ceramic membrane. The objectives were to evaluate the performance of calcium oxide (CaO) as a catalyst supported on activated carbon in a membrane reactor for biodiesel production. Further still, to evaluate the membrane performance regarding permeate quality and to optimize the process using design of experiment. The final objective was to investigate the influence of operating parameters such as temperature, methanol/oil ratio, catalyst amount and reaction time on biodiesel yield.

The transesterification of soya bean oil with methanol in the presence of a supported catalyst was carried out on a laboratory scale. The membrane reactor was designed and assembled for this purpose. The membrane reactor integrated many procedures such as

combining reaction and separation in a single unit, continuous mixing of raw materials and maintaining high mass transfer between the immiscible phases during the reaction. The effect of the process parameters on the biodiesel production and FAME (fatty acid methyl ester) yields were investigated.

One factor at a time (OFAT) experiments were conducted to identify the optimum range of the yield. The membrane reactor produced a permeate stream which separated at room temperature into a FAME rich non-polar phase and a methanol polar phase. The optimum range was between 90% - 94% within a reaction time of 60 – 180 minutes, methanol to oil ratio 3:1 - 9:1 and temperature range of 60 0C - 70 0C. Methyl ester produced met the ASTM D6751 and SANS 1935 specifications.

The response surface methodology (RSM) based on the central composite design (CCD) was used to optimize the process. The optimization experiments were conducted around the optimum range established by the OFAT method. The optimum condition for transesterification of soya bean oil to fatty acid methyl ester was obtained at 3 g/L catalyst concentration, 65 0C temperature, 4.5:1 methanol to oil molar ratio and 90 minutes reaction time. At these optimum conditions, the FAME yield was 96.9 %, which is well within the yield of 97.7 % as predicted by the model.

In conclusion, this work presents a study of high quality biodiesel production using a ceramic membrane reactor with the advantage of selectively permeating FAME and methanol. This study therefore showed that the use of a membrane for biodiesel production conserved water for other purposes; eliminates the purification step and wastewater generation thereby reducing the cost of biodiesel production.