Gas-phase addition of solvent to ozone bleaching of kraft pulp

Recently, alcohol impregnation of pulp before ozone bleaching has been show n to be very effective for, improving selectivity, However; methanol recove ry is required for this to be economical. In this work, lye added small qua ntities of methanol or acetic acid to the ozone gas stream, allowing them t o diffuse with ozone into the pulp matrix. At the same methanol charge, gas -phase addition was more selective than impregnation but less selective tha n impregnating the pulp with a 50% methanol solution. We found that radical scavenging was the sole protection mechanism involved at low methanol conc entration. At high methanol concentration, both radical depolymerization an d carbonyl group formation were strongly decreased. The cause of the latter is considered to be that high methanol concentration decreases the swellin g of cellulose, retards the penetration of ozone into the cellulose matrix and inhibits the formation of carbonyl groups.