Measurement of delignification diversity within kraft pulping processes

The objective of this research was to measure delignification diversity within kraft pulping processes.Pulps were prepared from loblolly pine using 2.5-mm-thick chips and 10.0mm-thick chips to produce controlled pulps with narrow and broad lignin content distributions.In addition, pulps were obtained from a conventional s'mgle vessel continuous digester before and after conversion to Lo-solids TM pulping and Lo-level TM feed.Interfiber variability in density was measured and inferred to lignin content in each mill process condition and related to laboratory pulp interfiber uniformity.In addition, the application of infrared microspectroscpy in quantitative single fiber analyses was investigated.The new density gradient column designed for this project provided improved structural integrity and a near linear optical path for imaging.Cross-polarization techniques provided high contrast images of the cellulose fibers through digital image averaging.Experimental techniques were modified to increase measurement accuracy and precision.The standard error of prediction was 6.5 kappa number for single fiber kappa number analyses, and measurement error was insignificant.Density gradient column Valonia cellulose density measurements were not statistically different than X-ray diffraction and electron diffraction estimates of Valonia cellulose density.The average kraft holocellulose density was 1.5458 +0.0015 g/ml, and the calculated lignin density was !.2719 +0.0090 g/ml.Lignin content distributions were modeled using a composite of two normal distributions termed distribution A and distribution B. Distribution A was limited to a delignification minimum of approximately 20 kappa number, whereas distribution B was limited to a delignification minimum of approximately 35 kappa number.The quantity of distribution B contributed a maximum of 35-40%, regar_ess of chip thickness or process condition.The overall standard deviation in kappa number ranged from 11.5 to 17.7 in 10.0-mm-thick chip pulps and from 2.2 to 9.0 in 2.5-mm-thick chip pulps.Standard deviations for the component distributions were statistically different for each pulp sample, and distribution B disappeared with extended delignification in 2.5-mm-thick chip pulps.Efforts to use infrared microspectroscopy were unsuccessful in determining interfiber lignin content diversity.However, difference spectra taken at each stage of delignification suggest that lignin becomes highly substituted when pulping continues beyond 30 kappa number.