master luiz paulo

Mathematical modeling of mass transfer in the supercritical fluid extraction process from red pepper

Author: Luiz Paulo Sales Silva (2013)

Abstract: This work used the supercritical technology in the process of extraction, using carbon dioxide as solvent. This technology is based on concepts of sustainable development and respects the principals of green chemistry. It appears as an alternative to processes that use toxic organic solvents. Mathematical modeling is an interesting tool to understand better all phenomenological mechanisms involved in this process and to be able to control and optimize them. Capsaicinoids, which are responsible for the pungent sensation caused by peppers, have well-known beneficial properties for human organism. These substances are present in large quantities in several pepper species. Capsaicinoids were chosen as target substances for the study of mass transfer phenomena. Capsaicinoid contents were analyzed for three pepper species: Capsicum frutescens, Capsicum chinense, Capsicum boccatum. The species Capsicum frutescens showed higher concentration of these substances and was chosen as raw material for further steps. An experimental design of supercritical extraction from this pepper species was carried out varying pressure and temperature. These extractions showed that the extraction condition of 15 MPa and 313 K gave the best combination of yield and capsaicin concentration. Therefore, extraction kinetics was studied under this condition, varying solvent flow rate, particle diameter and extraction bed volume. The highest extraction rates were obtained for high solvent flow rates, low particle diameters and low extraction bed volume. This can be explained by the greater importance of the convective phenomenon under these conditions. The Sovová’s model (1994) for intact and broken particles was used to fit experimental data to curves and obtain model parameters. Three types of mathematical modeling were established: (1) fitting of each individual curve, (2) simultaneous fitting creating a set of parameters for pairs of duplicates, (3) multiple fitting that adjsuts a single value for the parameter XK for each set of curves with the same particle diameter. These fits allowed calculating the convective mass transfer coefficient for each condition and the respective values of the experimental Sherwood number. Experimental data was used to calculate dimensionless numbers of Reynolds and Schmidt for of each condition. Other mathematical modelings were performed using these new data sets of dimensionless numbers, which allowed proposing new correlations. These new equations were based on the existence of forced and free convection, even though the importance of the second phenomenon was considered small. The efficiency of these new models was assessed with a comparison of calculated convective mass transfer coefficients to those fitted from experimental curves. A good coherence was found between both. Finally, a pilot scale extraction was performed and the results obtained using the proposed correlations suggested that free convection cannot be neglected at such scales.