Parametric and kinetic studies of microwave-assisted pyroligneous acid from oil palm fiber

Abstract

Malaysia is acknowledged as the world’s second largest producer and exporter of palm oil. This in turn has generated large amount of oil palm biomass which is normally left at the plantation to putrefy, burnt or turned into various low-value products. Hence, an alternative approach which is more robust, easier and environmental friendly to process this biomass is required. Microwave-assisted pyrolysis allows the conversion of biomass to various pyrolytic products for example pyroligneous acid (PA), biofuel, biogas, and biochar. However, to date, there are still limited reports available on the application of microwave-assisted pyrolysis for the production of PA in optimized condition, which is the novelty of this study. The objective of this study was to optimize the operating conditions during the microwave-assisted pyrolysis of oil palm fiber (OPF) in order to obtain the highest total phenolic content (TPC) as well as the highest liquid yield of microwave-assisted pyroligneous acid (MWPA). The optimization process was performed using central composite design technique via response surface methodology which focused on the following parameters; final temperature (400-600 °C), holding time (15-30 min) and activated carbon (AC) loading (50-100 g). MWPA was characterized for its chemical and antimicrobial properties. The thermal degradation profiles of OPF were evaluated using the Kissinger-Akahira Sunose (KAS), Ozawa-Flynn wall (OFW) and Coats-Redfern method (CRM) kinetic models based on the thermogravimetric analysis data. The optimum condition to produce the highest TPC and liquid yield of MWPA were as follows; 540 °C, holding time of 23 min and AC loading of 86.74 g. The following profiles were determined for concentrated MWPA extracted with ethyl acetate (C-MWPA); antioxidant properties of 18.52 ± 0.2% with IC50 of 285.66 µg/ml (2,2-diphenyl-1-picryhydrazyl radical scavenging), 0.836 ± 0.003 mM (ferric reducing antioxidant power), 12 ± 0.07% (metal chelating activity), 414.21 ± 4.74 µmol ascorbic acid/g (phosphomolybdenum), 85.42 ± 0.33% (hydrogen peroxide assay), and growth-inhibition of 28.67 ± 0.88 mm towards Escherichia coli American Type Culture Collection 25922 with minimum inhibition concentration value of 0.651 ± 0.13 mg/ml. The kinetics study evaluated for OPF found that the mean value of activation energy (Ea) were 97.01 kJ/mol (KAS) and 101.52 kJ/mol (OFW). The values of Ea calculated for C-MWPA were 28.59 kJ/mol and 33.87 kJ/mol for KAS and OFW, respectively. The validation of kinetic reaction model by using CRM model demonstrated that, OPF thermal degradation was well represented by the three dimension diffusion reaction model with mean value Ea of 84.71 kJ/mol while C-MWPA was well fitted with second order reaction type (Ea of 39.83 kJ/mol). To conclude, this study has successfully demonstrated the application of microwave-assisted pyrolysis to convert oil palm biomass into various useful pyrolytic products. This is considered as a significant finding as this technique can offer a substantial reduction in processing time, able to process large volumes at a given time, easy to operate as well as offering various valuable pyrolytic products.