Intrinsic kinetics of isothermal xylan degradation in inert atmosphere

Thermal degradation of thin layers of milled, commercial xylan is made to occur under controlled sample temperature, by proper variation in the intensity of the applied radiative heat flux. The process consists of fast sample heating (40-70 K s(-1)) to the desired temperature, which is then maintained until complete conversion. A continuous helium flow avoids extra-particle secondary reactions of primary volatile products and establishes an inert reaction environment. Time-weight loss curves are determined for reaction temperatures in the range 473-613 K, where the ratio between the conversion and the heating times is very large (1000-100) and the weight loss during the heating period is small (0-16%). The isothermal weight loss curves are well interpreted by a two-stage mechanism. The first, fast stage takes into account the competitive formation of volatiles and a solid intermediate. The second, slow stage takes place for temperatures above 523 K and describes the competitive formation of char and volatiles from the degradation of the reaction intermediate. The kinetic data for the two stages are: A(1) = 3.62 x 10(5) s(-1), E-1 = 18.3 kcal mol(-1) and A(2) = 3.83 x 10(2) s(-1), E-2 = 13.1 kcal mol(-1). The final solid char yields vary with the reaction temperature from 68 to 10%, and most of the volatiles are released in the course of the first reaction stage. (C) 1997 Elsevier Science B.V.