Single Fiber Lignin Distributions Based on the Density Gradient Column Method, 2007 Engineering, Pulping & Environmental Conference

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The density gradient column method was used to determine the effects of uniform and non-uniform pulping processes on variation in individual fiber lignin concentrations of the resulting pulps. A density gradient column uses solvents of different densities and a mixing process to produce a column of liquid with a smooth transition from higher density at the bottom to lower density at the top. Properly prepared pulp fibers float in the column, stabilizing at the level where the mixed solvent density equals the density of the fiber. Because lignin is the lowest density component of pulp fibers and has the largest influence on fiber density, the column effectively separates fibers by lignin concentration and allows them to be counted and the distribution of lignin concentrations determined. Ten experimental kraft pulps and three commercial pulps were evaluated. The laboratory pulps were produced from a single loblolly pine tree using 2.5-mm and 10-mm chips. All cooks used a 24% effective alkali (EA) charge on wood, 6-to-1 liquor-to-wood ratio, and 30% sulfidity. The cooking schedule was constant at 60 min rise to temperature and 240 min at temperature. The maximum cooking temperature was varied from 150°C to 170°C to provide a kappa number variation from about 60 to approximately 20. Pulps produced from 2.5-mm-thick chips gave uniform density distributions that were accurately modeled with a standard normal distribution. However, pulps produced using 10-mm chips contained an extended low density (high kappa number) tail that can be modeled as a second, high-standard-deviation distribution. The two distributions can be explained as that portion of the wood chips with ready access to pulping chemicals and rate controlled by pulping kinetics, and the interior portion of the chip where chemical penetration is slow and the pulping rate is controlled by diffusion. The commercial pulps show variations in the lignin concentration standard deviation that demonstrate improved uniformity when using modified continuous cooking processes.