RDH pulping of southern hardwoods

Southern hardwoods were kraft pulped using laboratory rapid-displacement heating (RDH) and conventional processes. RDH pulping achieved acceptable hardwood kappa numbers at lower H-factors than required by conventional batch pulping. The RDH pulps had lower yields at a given kappa number than a conventional cooks produced the same amount of screen rejects at a given kappa number. Reduction of alkali concentration in the black liquor diminished the rate of delignification, but it did not change the pulp yield. Results for RDH pulping of five hardwood species showed significant interspecies variability with respect to pulping rate and selectivity. This suggests that mills using RDH to pulp hardwoods could experience significant swings in kappa number and yield with a change in the mix of wood species.