Oxygen delignification of wood pulp

Claim 1

A method of treating a liquid-solid mixture including the steps of passing said mixture through an elongated pipeline reactor having a plurality of gravitational fall zones disposed therein, introducing oxygen gas under superatmospheric pressure into said reactor to establish a gas space at each fall zone with the turbulence imparted to the mixture falling through the gas space at each fall zone resulting in a dissolution of oxygen in the liquid phase of the mixture, the improvement comprising the steps of: forming said mixture as an essentially thixotropic pulp slurry comprised of at least 2% fibers by introducing pulp, recycle liquor and alkali in a mixing vessel, heating said slurry to a temperature in the range of about 90-160° C; passing said pulp slurry through said reactor in essentially plug flow, whereby oxygen is dissolved in the liquid phase of said slurry at each gravitational fall zone while delignification reactions occur as the slurry is passed through the reactor from one gravitational fall zone to the next; sensing the actual slurry pH at a plurality of said gravitational fall zones of the reactor; adding alkali at said plurality of said gravitational fall zones in response to sensed pH to mix alkali and slurry so that a substantially constant slurry pH between approximately 10.0 and 12 is maintained at each said gravitational fall zone; discharging the delignified pulp slurry from the reactor; separating said slurry discharged from the reactor into pulp and recycle liquor; and returning said recycle liquor to said mixing vessel.

Claim 2

A method of treating a liquid-solid mixture including the steps of passing said mixture through an elongated pipeline reactor having a plurality of gravitational fall zones disposed therein, introducing oxygen gas under superatmospheric pressure into said reactor to establish a gas space at each fall zone with the turbulence imparted to the mixture falling through the gas space at each fall zone resulting in a dissolution of oxygen in the liquid phase of the mixture, the improvement comprising the steps of: forming said mixture as an essentially thixotropic pulp slurry comprised of at least 2% fibers by introducing pulp, recycle liquor and alkali in a mixing vessel, heating said slurry to a temperature in the range of about 90-160° C; passing said pulp slurry through said reactor in essentially plug flow, whereby oxygen is dissolved in the liquid phase of said slurry at each gravitational fall zone while delignification reactions occur as the slurry is passed through the reactor from one gravitational fall zone to the next; sensing the actual slurry pH at a plurality of said gravitational fall zones of the reactor; adding alkali at said plurality of said gravitational fall zones in response to sensed pH to mix alkali and slurry so that a substantially constant slurry pH between approximately 10.0 and 12 is maintained at each said gravitational fall zone; discharging the delignified pulp slurry from the reactor; separating said slurry discharged from the reactor into pulp and recycle liquor; and returning said recycle liquor to said mixing vessel.