This study investigates the supercritical transesterification of waste beef tallow using methanol, with carbon dioxide employed as a cosolvent to reduce the operating pressure. The experimental design, guided by response surface methodology, evaluated the effects of reaction temperature (250–360 °C), carbon dioxide/methanol molar ratio (0.000–0.421), and methanol/beef tallow molar ratio (40–50) on conversion and quality during 60 min in a batch reactor. Bulk properties and spectroscopic analyses confirmed that the fatty acid methyl esters (FAMEs) met reference standard values. The results showed that temperature was the most significant factor affecting conversion, while carbon dioxide addition proved beneficial at high methanol-to-fat ratios. The process proved to be a feasible method for producing high-quality biodiesel while lowering required pressure compared with non-cosolvent supercritical systems.

Understanding the relationships between operating conditions and product formation pathways in biomass pyrolysis remains challenging due to the complex interactions among temperature, catalytic effects, and feedstock composition. In this work, principal component analysis (PCA) was applied to investigate the combined influence of temperature and catalyst-to-biomass ratio on the pyrolysis of sugarcane bagasse and Salicornia. To preserve mechanistic interpretability, two complementary analyses were performed: one considering only catalytic experiments and a second integrating both thermal and catalytic conditions. Separate PCA were conducted for product yields, gas and liquid compositions, and solid-phase FTIR features. The results reveal that thermal conditions promote severe cracking and solid carbonization, whereas catalytic operation favors secondary pathways associated with controlled dehydration and partial stabilization of liquid products. Distinct patterns between the two feedstocks were also identified, reflecting their intrinsic compositional differences. Based on the multivariate trends identified, a conceptual reaction scheme is proposed to describe the interplay between thermal and catalytic pathways.

This study presents the experimental characterization of the volumetric and transport properties of pseudo-binary mixtures of commercial diesel and residual chicken fat methyl ester biodiesel over the temperature range of 293.15–353.15 K at 0.078 MPa. Density measurements were performed using a U-shaped vibrating-tube densimeter; kinematic viscosities were obtained using Cannon–Fenske capillary viscometers. The results show that density decreased with increasing temperature and diesel content. The excess molar volume (V^E) was negative for all mixtures; the strongest volumetric contraction took place at around x₁ ≈ 0.4–0.6. The Redlich–Kister equation and the Prigogine–Flory–Patterson (PFP) model were applied to represent excess molar volumes, with an absolute average deviation (AAD) lower than 14.92%. The thermal expansion coefficient (${\alpha }_P$) and its excess property (${\alpha }_P^E$) further confirmed the existence of non-ideal mixing driven by polar–apolar interactions. The kinematic viscosity ($\nu$) was confirmed to be temperature-dependent and increased with the amount of FAMEs; this effect can be associated with the greater polarity and structural rigidity of esters. The McAllister model also adequately reproduced the dynamic viscosity ($\eta$) with an AAD < 4.2%. Furthermore, an increase in the activation enthalpy (${\Delta H}^{\ne }$) was observed at higher FAME fractions, indicating a high energy demand is required to overcome the internal energy barrier for the initial displacement of the molecules.