Drying of PHB - biodegradable polymer obtained from sugar cane - in a rotating pulsed fluidized bed with application of microwaves

Author(s)

Gabriela Silveira da Rosa

• Document type: Doctoral Thesis
• Press: Campinas, SP.
• Institution: Universidade Estadual de Campinas (UNICAMP). Faculdade de Engenharia Química
• Defense date: 2012-11-23

Examining board members

Sandra Cristina dos Santos Rocha; Michele Nehemy Berteli; Melissa Gurgel Adeodato Vieira; Nádia Rosa Pereira; Marcello Nitz da Costa

• Advisor: Antonio Marsaioli Junior; Sandra Cristina dos Santos Rocha

Abstract

Poly-hydroxybutyrate (PHB) is one of the most well-characterized homopolymers of the polyhydroxyalkanoates family, which are renowned for their desirable biocompatibility and biodegradability. Due to being biodegradable and to the possibility of its integrated production in a sugar and ethanol mill, the PHB has aroused technological, scientific and commercial interests. The drying process is an important step of the PHB production because an adequate post-processing can be achieved only at low moisture contents. The aim of this project was to investigate the drying of PHB in a combined microwave/rotating pulsed fluidized bed (RPFB) dryer. The experimental design technique and statistical analysis of the results were performed to analyze the influence of hot air and power level conditions on the final moisture content of PHB, for 90 min of process. Two experimental designs were proposed and executed. For the experimental design 1, a first drying stage of 20 min was identical for all experiments, which involved convective drying with air at 90 oC and no microwave power applied, and in a second stage (20 to 90 min of drying), microwave power was supplied. For the experimental design 2, in the first stage of 30 min of drying, a higher microwave power was used to permit faster evaporation of the superficial moisture content. After that stage the microwave power was decreased to lower levels for the remaining 60 min of drying. The results obtained for the final moisture content were in the range of 0.38 to 1.62 % (w.b.). The drying kinetics of PHB showed a constant drying rate period followed by a decreasing drying rate period. Comparing the results with the ones presented in Godoi (2009) for optimal conditions of convective drying in RPFB, a reduction of least 25 % in drying time is verified. It was observed that the constant drying rate for the experimental design 2 comprised 35 min, while the constant drying rate time observed for experimental design 1 was 35 to 45 min. Page's mathematical model for drying kinetics resulted in the best fit to the experimental data. Through the desorption isotherms of PHB at temperatures of 40, 50 and 60 oC, the isosteric heat of desorption (Qst) was estimated as a function of equilibrium moisture content for a 60 oC temperature. The adsorption isotherms for dry PHB showed extremely low values for the equilibrium moisture content and revealed that the dried PHB will hardly tend to adsorb ambient moisture even at high relative humidity, being adequate for post-processing. No differences on the properties of PHB before and after the drying process were verified through the characterization analyzes performed (DSC, TGA, molar mass distribution, particle size distribution and scanning electronic microscopy). Through evaluating the dielectric properties it was possible to establish that the drying potential of PHB by applying microwaves is closely related to the water content of the material. Drying energy efficiencies were in the range of 10.8 to 30.5 % for the convective-microwave drier, and the estimated energy efficiency for convective drying of PHB in RPFB was 10.5 %. For the experiments that resulted in the specified final moisture content for PHB (0.5% w.b.), energy efficiencies up to 19 % higher than the result without microwaves were obtained (AU)