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 hexenuronic 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), probably because the single reaction is actually a combination of several parallel and consecutive reactions. The reaction orders with respect to peroxyacetic acid and hydronium ion are 1.37 and -0.29, respectively. The activation 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 the second, between undissociated hexenuronic acid and peroxyacetic acid. The reaction orders of the first reaction art 0.91 and 1.71 with respect to hexenuronate anion and peroxyacetic acid, respectively. The reaction orders of the second reaction 1.12 and 0.92 with respect to hexenuronic acid and peroxyacetic acid, respectively. The activation energies are 60 and 63 kJ/mol for decomposition of hexenuronate anion and undissociated hexenuronic acid, respectively. The rate of hexenuronic acid decomposition is much faster than that of delignification. All hexenuronic acid groups decompose within 90 min under almost all delignification conditions, while 50% lignin removal is nor achieved during the full treatment time (180 min) under any conditions. The activation energies of delignification and hexenuronic acid decomposition are similar, indicating that both treatments are equally temperature sensitive. The kinetic behaviour should be applicable when modelling and simulating pulp bleaching processes and for predicting bleaching results when experimental data are not available.