A reactor apparatus including: an internal mixing chamber including a first chamber section having a cross-sectional area expanding from a biomass inlet to the internal mixing chamber to the a second chamber section; the second chamber section having a substantially uniform internal cross-sectional area from the opposite end of the first chamber section to a discharge end of the mixing chamber; the biomass inlet is coupled to a source of pre-treated biomass external to the reactor vessel, and a rotating mixing device in the internal mixing chamber and coaxial with an axis of the first chamber section.