Variability and repeatability of laboratory-scale kraft cooks of European spruce and pine

Abstract The variability of laboratory Kraft pulping experiments is often overlooked, despite its importance for accurate data interpretation (e.g., pulp yield or Kappa number). Neglecting variability can lead to incorrect conclusions, especially when differences between cooking procedures are minor. This study assessed the repeatability and variability of a fully automated state-of-the-art laboratory scale recirculation digester and a common laboratory rotary digester, cooking spruce and pine mixtures under identical conditions. The baseline configuration utilized fractionated chips pulped with white liquor and water. To explore increased complexity, subsequent experiments incorporated unfractionated chips, segregated chips, and the addition of black liquor. Seven experiments were conducted per setup, using a liquor-to-wood ratio of 7:1, a final H-factor of 1005, and an effective alkali concentration of 50 g/L. The resulting average total yield ranged from 47.19% to 48.56%, while the average Kappa number varied between 33.1 and 36.1. The coefficients of variation for Kappa (CV Kappa ) ranged from 2.21% to 3.25% in the recirculation digester, versus 3.60% in a rotary batch digester. Total yield variability was lower, with a coefficient of variation for yield (CV yield ) between 0.70% and 0.88%, compared to 1.63% in the rotary digester, highlighting the recirculation system’s precision. Dry matter determination was identified as a significant contributor to yield variability. In order to achieve ± 1 Kappa point with 95% confidence level, seven cooking trials are required, while four trials suffice for ± 0.5 pp yield precision. This research highlights the importance of addressing variability and provides a reference for laboratory-scale precision and necessary repetitions.