Using bleaching stage models for benchmarking hardwood ECF bleach plants

Steady-state models estimated the performance of the D-0(EOP)D-1 bleach sequence at two mixed brightens the pulp to similar to 84% ISO was monitored for two weeks. Mill 2 considered the partial sequence that brightens the pulp to similar to 86% ISO for nearly four weeks. Elevated levels of chlorine dioxide were linked to increased washer carryover in brownstock and extraction areas. For Mills 1 and 2, an extra 0.24% and 0.33% chlorine dioxide was consumed in the D-0 stage. This extra bleach demand was equivalent to an additional 4.8 and 5.5 kappa load to the brownstock, respectively. Some differences were observed for the D-1 stage. Mill 1 had extraction carryover that averaged 1.1 units higher than was measured, contributing to use of an extra 0.22% of chlorine dioxide. Mill 2 had extraction carryover that averaged between 0 and 0.7 kappa units and consumed up to 0.13% more chlorine dioxide. Another data set from Mill 2 showed high brownstock and extraction carryover, leading to similar to 0.90% more total chlorine dioxide usage to brighten to 84% ISO. Overall, this investigation illustrated that the models could be employed as benchmarks.