Performance of selected biomass solid catalysts for biodiesel production from vegetable oil in a continuous reactor system

Abstract

The energy security issues and environmental concerns emerging from fossil fuel depletion and increased greenhouse gases emission have led to increased research towards alternative energy sources. Over the last two decades, the interest in biodiesel has grown significantly owing to its eco-friendly characteristic quality properties over fossil fuels. Biodiesel is a combination of mono-alkyl esters of lengthy-chain fatty acids. Biodiesel is viewed as a promising alternative fuel to petroleum diesel, because it exhibits similar properties to petro-diesel. Nonetheless, the cost of production of biodiesel is considerably higher than that of petro-diesel. To address the shortfalls of biodiesel production, the utilization of heterogeneous catalysts derived from biomass wastes represents a pathway. These catalysts have several advantages including renewability, high catalytic activity, non-toxicity, reusability, etc. This study is aimed at investigating the performance of selected solid base catalysts derived from biomass wastes for biodiesel production from vegetable oil in a designed continuous reactor system. The vegetable oil used in this study as feedstock source of triglyceride is waste sunflower oil (WSFO). The experimental approach in this study is divided into two phases. The phase 1 involved the synthesis of four selected biomass-derived solid base catalysts and the investigation of their performance in a batch reactor system for biodiesel production. The phase 2 involved developing an appropriate continuous reactor system to produce biodiesel and studying the kinetics of continuous biodiesel production using the best catalyst identified in phase 1 of the study. A batch reactor system was used for investigating the performance of the biomass-derived solid base catalysts (CBPA [Calcined Banana Peels Ash], CCESP [Calcined Chicken Eggshells Powder], CCPHA [Calcined Cocoa Pod Husks Ash] and CECPHA [Calcined Enterolobium Cyclocarpum Pod Husks Ash]). The set of operating reaction conditions used for the assays were as follows: 4 wt. % catalyst loading; 0.8 (v/v) methanol to oil ratio; 65 °C reaction temperature and 65 minutes reaction time. The best performance catalyst was found to be CECPHA (biodiesel yield= 77.90 wt. % ± 2.43). A supported catalyst was prepared having CECPHA as the active catalytic phase and pumice as the catalyst support. Upon impregnation, a significant reduction in surface area was observed in CECPHA. Nonetheless, the prepared CECPHA-Pumice catalyst was found to be effective in converting WSFO to WSFME (Waste Sunflower Methyl Esters) in the design packed bed reactor (PBR) at the chosen operating reaction conditions. The set of operating reaction conditions for the transesterifications in the PBR were as follows: 253 g catalyst amount corresponding to a catalytic bed height of 8.4 cm; 0.9 (v/v) methanol to oil ratio and 0.5 (w/w) co-solvent (n-hexane) to oil ratio. The continuous reactor system was used for studying the kinetics of the transesterification of WSFO (Waste Sunflower Oil) in the presence of CECPHA-Pumice catalyst. Kinetic experiments to determine the initial rates of reaction were performed at the following volumetric feed flow rates 1.6 mL/min, 2.1 mL/min, 3 mL/min and 4 mL/min while the temperature was maintained constant at 55 °C. The highest conversion (77.90 %) was achieved at 1.6 mL/min feed flow rate. This feed flow rate was used to carry out experiments to estimate the model parameters [reaction rate constant (k), reaction equilibrium constant (K), methanol adsorption equilibrium constant (KM) and glycerol adsorption equilibrium constant (KG)] as well as the activation energy (Ea) which were investigated in the following temperatures 40 °C, 50 °C, 55 °C and 60 °C. The Arrhenius rate law of the reaction was expressed as k=0.149 e−23.25RT⁄, where the activation energy (Ea) was found to be 23.25 kJ/mol. The regression coefficient (R2) of the Arrhenius plot graph in this study was found to be 0.988, indicating that the experimental data fitted the Arrhenius model in the investigated temperatures. Overall the objectives of this study were accomplished. More insight into the kinetics of the transesterification of WSFO in a packed bed reactor in the presence of supported biomass-derived solid base catalyst was provided. The renewability, attractive catalytic qualities and relatively low cost of the supported biomass-derived solid base catalyst (CECPHA-Pumice) developed in this study and used in a continuous reactor arrangement demonstrate its potential for upscaling and possible commercialization.