intermediate and internal passageways 40 and 42 are provided by coaxial outer and inner tubular members 41 and 43 outer and inert tubular members 41 and 43 are housed within body portion 28 such that external passageway 38 surrounds intermediate passageway 40 and intermediate passageway 40 surrounds internal passageway 42 the charge of wood pulp enters reactor 28 through a wood pulp inlet 44 of body portion 28 and passes between internal and external passageways 40 and 42 wood pulp is discharged from a wood pulp outlet 46 to mixer 24 it is understood that although tubular members 41 and 43 are of cylindrical configuration for ease of fabrication they could also be of other shapes for instance tubes having a square transverse cross section and etc outer and inner tubular members 41 and 43 and therefore intermediate and internal passageways 40 and 42 are provided with perforations the perforations are sized to retain the wood pulp between intermediate and internal passageways 40 and 42 while admitting oxygen into the wood pulp the oxygen is introduced as an oxygen containing gas into gas inlet 48 of body portion 36 the oxygen containing gas passes into external passageway 38 through the perforations of intermediate passageway 40 and then into the wood pulp the oxygen containing gas travels in an inward radial direction of the.