Low Purity Oxygen for Oxygen Delignification, 1996 International Pulp Bleaching Conference Proceedings

As mills strive toward lower effluent levels in the quest for Totally Effluent Free bleaching, their reliance on oxygen for bleaching increases. Since many mills are remote from industrialized areas where oxygen is readily available at attractive pricing, there is a need for on-site generation of oxygen. Recent advances in Vacuum Pressure Swing Adsorption plant design have made on-site supply of oxygen practical at levels as low as 5 tpd, thereby bringing this economical supply mode available to all bleached pulp mills. The production of gaseous oxygen from VPSA plants results in oxygen of 90% purity. Large mills have used this purity successfully for years in both medium and high consistency oxygen delignification stages. Low purity oxygen has been used since the mid-70’s on high consistency softwood pulp’ and used at increasing number of mills worldwide in recent years successfully. In the past, there have been questions about the effect of the lower partial pressure of oxygen on delignification. While previous laboratory work has suggested some decrease in delignification performance at lower purity, actual mill experience shows no difference.’ Table 1 shows data from both a hardwood and softwood mill confirming the suitability of low purity oxygen in the bleach plant. Laboratory data are shown in Figure 1 indicating no impact on purity at low pressure typical of atmospheric oxygen delignification stages as well. Additional laboratory work suggests that above 80% purity, oxygen purity has no effect on oxygen delignification performance. Eucalyptus Kraft pulp of Kappa number 16.7 and brightness of 33.5 ISO was delignified in 300 g batches in a Quantum Mixer. The experiments were repeated three times to check for consistency. Figure 2 shows data for constant total pressure with variable oxygen partial pressure. Figure 3 shows data from another set of experiments in which the oxygen partial pressure was held constant while the system total pressure was varied. While there is no doubt that oxygen purity impacts delignification performance up to about 75 %, there is little evidence of an effect above 80%. Indeed, there is no evidence of an impact on oxygen purity at constant partial pressure and a weak indication of delignification degradation from 80 to 90% purity.