1. Kinetic studies of xylan hydrolysis of corn stover in a dilute acid cycle spray flow-through reactor Hongman ZHANG 1 ;Qiang JIN 2 ;Rui XU 2 ;Lishi YAN 2 ;Zengxiang LIN 2 ; 1. College of Science, Nanjing University of Technology, Nanjing 210009, China ; 2. State Key Laboratory of Materials-Oriented Chemical Engineering, College of Biotechnology and Pharmaceutical Engineering, Nanjing University of Technology, Nanjing 210009, China ; Fig.2 A variety of kinetic models for xylan hydrolysis have been reported. The majority of xylan hydrolysis model the biphasic hydrolysis model has been based on the reaction scheme as follows [ 16 ]. Fig.3 The k 1f and k 1s are the reaction rate constants of fast-hydrolysis fraction and slow-hydrolysis fraction of xylan min -1, respectively. The k 2 is the rate constant of xylose degradation. All three reactions k 1 f, k 1 s, k 2 are assumed to be pseudo-homogeneous reaction of first order, and, based upon previous results, when the raw material particle size was up to 40 mesh, flow rate 8 Lmin, the increase of xylose concentration was not visible. So, it was assumed that the internal and external diffusion effects were negligible for the kinetic modeling [ 6 ]. 1 d C H f d t = - k 1 f C H f, 2 d C H s d t = - k 1 s C H s, 3 d C X d t = k 1 f C H f + k 1 s C H s - k 2 C X, 4 d C D d t = k 2 C X. Based on this reaction model and solving differential equations lead to Eq. 5, which expresses xylose concentration C X as a function of time t: 5 C X = C H f 0 k 1 f k 2 - k 1 f [ exp - k 1 f t - exp - k 2 t ] + C H s 0 k 1 s k 2 - k 1 s [ exp - k 1 s t - exp - k 2 t ]. The initial hemicellulose expressed as total xylan and xylose concentrations are defined at time 0 as C H 0 = C X, max C H f 0 = 0.65 C H 0, C H s = 0.35 C H 0 and C X 0 = 0, respectively. The xylan hydrolysis was also modeled by a two-step first-order reaction of Saeman model [ 17 - 19 ]. The k 1 and k 2 are the kinetic coefficients of the reaction of xylose formation and decomposition, respectively. c X y l a n X n → k 1 X y l o s e X → k 2 D e g r a d a t i o n D Solving the differential equations for an isothermal reaction, the following model predicts the concentration of xylose C X 6 C X = C X n 0 k 1 k 1 - k 2 [ exp - k 2 t - exp - k 1 t ]. The reaction rate constants k 1f, k 1s, k 2 and k 1, k 2 are assumed to have an Arrhenius-type temperature dependence, and A: pre-exponential factor min -1 is assumed to be dependent upon acid concentration C : 7 k = A 0 C n exp - E R T. A 0 : pre-exponential factor for xylan hydrolysis min -1, C : acid concentration % w, n : acid concentration exponent for the rate constant k, E : activation energy kJ·mol -1, R : 8.3143 × 10 -3 kJ·mol -1 ·K -1, T : temperature K. The kinetic parameters of biphasic and Saeman model were calculated by nonlinear regression analyses. Estimation of parameters for kinetic constants The typical plots of the effect of different temperature, acid concentration and times on xylan hydrolysis are shown in Fig. 2. Over 90% of the maximum potential xylose monomer was obtained. Xylose concentration increased with the acid concentration and temperature increase. The obtained kinetic data for xylan hydrolysis in corn stover was analyzed with the biphasic and Saeman models described above. The estimated parameters are tabulated in Table 1. The selectivity factor k 1 k 2, the ratio of xylan hydrolysis rate to xylose degradation rate was used to evaluate the hydrolysis efficiency The overall k 1 = 0.65 k 1f + 0.35 k 1s [ 16 ]. The kinetic parameters obtained by the biphasic model were similar to the Saeman model. And the values of k 1f and k 1 s had no difference that did not match with the actual hydrolysis results. The values of k 1f should be higher than those of k 1 s in all of the estimated parameters for fast-hydrolysis xylan part with a faster hydrolysis rate compared to slow part that has been proved by many results reported in the literature [ 12, 16 ]. Furthermore, the biphasic model failed to converge on the other conditions. Therefore, the biphasic model used for the DCF system was not improper. Table 1 also indicates that the selectivity factor is highest with 2% sulfuric acid while at 100°C by the Saeman model. The values of k 1 and k 2 increased with acid concentration. Whereas the values for k 1 were greatly higher than those of k 2 in all of the estimated reaction constants, and it could be estimated from the values that sugar release reactions were close to 2 orders of magnitude higher than the decomposition reactions which implied that all of the experiment conditions favored xylose formation over xylose degradation. Meanwhile, the hydrolysis model resulted in high R 2 - values, indicating a fine fit to the experimental data. Fig.4 Comparison on xylose concentrations of xylan hydrolysis by experiment and model prediction a 100°C, b 2% H 2 SO 4 Frontiers of Chemical Science and Engineering,2011,5(2),252-257. Doi:10.1007/s11705-010-1010-y