Attempts to explain differences between laboratory and mill trial data have led to the understanding that the performance of AQ and AQ/penetrant varies depending upon the time of year in which the mill trials are employed. Mill data have shown that increased amounts of white liquor are required during the late winter through the early spring months for mills employing standard kraft cooking, and for mills using digester additives in the cooking process. Seasonal white liquor variation was found to be 0.7% EA on wood for the standard kraft process. Seasonal liquor variation was found to be 0.8% and 0.4% EA on wood for mills employing AQ and AQ/Penetrants respectively. These seasonal changes may account for as much as 95% of the expected benefit for employing a digester additive. Thus, when summer or fall mill baseline conditions are employed in a laboratory study or as a baseline for a comparative mill study, the performance of the digester additive would be reported as substandard or nonfunctional for trials conducted in the late winter or early fall.
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