Integration of celite and activated carbon in trickle bed reactor for esterification of free fatty acid: Optimization, kinetics and hydrodynamics studies

Abstract

Industrial oil products such as acidic palm oil (ACPO) are considered renewable raw materials for biodiesel production and should undergo a pretreatment process to reduce their high free fatty acid (FFA) content. Esterification is a typical process for converting FFA into fatty acid methyl esters (FAME). In this study, the pre-treatment of FFA in ACPO was investigated using sulfuric acid (H₂SO₄) supported on a solid porous material in trickle bed reactor (TBR). Catalysts were prepared by impregnating H₂SO₄ onto a hybrid support of celite and activated carbon (AC) in a 1:1 ratio. The hybrid-supported catalysts were evaluated in terms of conversion efficiency, hydrodynamic and kinetic behaviour under optimized conditions, which are 6 g of H₂SO₄ catalyst, 6 g of hybrid support, 3 mL/min of oil flow rate, 50 mL of methanol fed, 65 °C of reaction temperature and 30 min of reaction time. The highest FFA conversion of 92.36% was achieved using H₂SO₄ supported on AC, where the FFA level was reduced from 13.09% to 1.00%. In contrast, H₂SO₄ supported on Celite: AC (1:1) demonstrated a slightly lower conversion of 88.86%, reducing the FFA to 1.46%. Both systems were operated effectively for 120 min and yielded 329.40 g of treated oil. Catalyst deactivation was observed beyond these durations, likely caused by prolonged exposure to reactants and thermal conditions, which limited the active sites. Hydrodynamic parameters were also assessed. Trickle flow regime operation was confirmed, with liquid holdups of 2.46% and 1.45% for the AC and Celite: AC (1:1) supports, respectively. A second-order model best represented reaction kinetics. The rate constant, activation energy and frequency factor of H₂SO₄/Celite: AC (1:1) were 0.0201 L/mol.min, 11.011 kJ/mol and 1.276 min^− 1, respectively. These findings highlight that, while pure AC support exhibited slightly better performance, the hybrid Celite: AC support also offers a viable alternative for the treatment of FFA content in ACPO. The integration of Celite and AC provided an innovative packing system with acceptable catalytic activity. This study provides fundamental insights into the valorization of Celite for the design of packing system in TBR to produce value-added materials such as biodiesel.