Kinetics of delignification and hexenuronic acid decomposition during kraft pulp bleaching with peroxyacetic acid

This paper presents kinetic equations for the removal of lignin and hexenur onic acid from an oxygen-delignified kraft pulp. Lignin removal is described as a single reaction. The rate of this reaction increases with increasing temperature, pH and lignin and peroxyacetic acid concentrations. The reaction order with respect to lignin is high (4.1), p robably because the single reaction is actually a combination of several pa rallel and consecutive reactions. The reaction orders with respect to perox yacetic acid and hydronium ion are 1.37 and -0.29, respectively. The activa tion energy of delignification is 80 kJ/mol. The assumption that hexenuronic acid decomposition takes place through mio reactions very satisfactorily explains the experimental results. The first reaction, between hexenuronate anion and peroxyacetic acid is faster than t he second, between undissociated hexenuronic acid and peroxyacetic acid. Th e reaction orders of the first reaction art 0.91 and 1.71 with respect to h exenuronate anion and peroxyacetic acid, respectively. The reaction orders of the second reaction 1.12 and 0.92 with respect to hexenuronic acid and p eroxyacetic acid, respectively. The activation energies are 60 and 63 kJ/mo l for decomposition of hexenuronate anion and undissociated hexenuronic aci d, respectively. The rate of hexenuronic acid decomposition is much faster than that of deli gnification. All hexenuronic acid groups decompose within 90 min under almo st all delignification conditions, while 50% lignin removal is nor achieved during the full treatment time (180 min) under any conditions. The activat ion energies of delignification and hexenuronic acid decomposition are simi lar, indicating that both treatments are equally temperature sensitive. The kinetic behaviour should be applicable when modelling and simulating pu lp bleaching processes and for predicting bleaching results when experiment al data are not available.