Response surface optimization of Entandrophragma cylindricum (Sapelli) pulping/delignification using a hybrid organosolv ethanol/soda process

Abstract The pulping potential of high lignin-containing tropical hardwoods is not well-reported in the literature. The forestry exploitation of these woods generates high amounts of residue each year of first and second transformations of the woods. This work studied the pulping of Sapelli (Entandrophragma cylindricum) wood species using a hybrid caustic soda solution/ethanol process using the Response Surface Methodology with Central Composite Design. The water/ethanol ratio was fixed at 40/60 (v/v) following preliminary experiments. The three pulping parameters namely temperature, time and alkali charge were chosen as independent variables. The pulp response characteristics were yield, kappa number and cellulose content, as well as the interactions between these pulping parameters. The results of the study and the quadratic equations models equations obtained provided the basis for selecting the optimal conditions of pulping according to the desired pulp characteristics. The generated mathematical relationship showed that the optimal pulping conditions were time 120 min, temperature 180°C and alkali charge 29.95%. The experimental yield, 45.01%, kappa number, 9.6 and cellulose content, 0.4028 g/g of initial wood obtained by applying the optimal conditions were close to the theoretical values generated by the experimental designs. The amount of cellulose in the pulp was also close to the amount of cellulose in the raw wood, showing that the hybrid pulping method caused only a slight degradation of cellulose. The cellulose in optimal pulp was type I cellulose as shown by X-ray diffraction. The optimal pulp was also characterized by Fourier Transformed Infrared spectroscopy and thermogravimetric analyses. Experimental designs can be used to optimize the caustic soda/ethanol pulping of wood to produce only slightly degraded cellulose as an intermediate for various industrial applications.