NOVELTY - Reactor vessel (10) comprises: an upper inlet (12) and a bottom discharge; a generally vertically oriented sidewall (18) between the upper inlet and bottom discharge, where the sidewall defines a perimeter of an interior flow passage in the vessel; and a first or second pairs of support plates arranged on opposite sides of the sidewall, where the first pair or second pair of support plates reduces a cross-sectional flow area of the flow passage in a first or second single direction of convergences and the second single direction is angularly offset to the first single direction.    USE - The reactor vessel is useful for processing biomass (claimed) e.g. annual plant biomass.    ADVANTAGE - The reactor vessels: provides the biomass and facilitate the downward movement of the biomass through the reactor; reduces the tendency of compacted biomass to form channels, stagnant arches and pockets, and to have intermittent or permanent flow stoppages; and allows for a smaller and less expensive gearing and motor to drive the stirring device and reduces the energy needed to drive the stirring device.    DETAILED DESCRIPTION - Reactor vessel (10) comprises: an upper inlet (12) and a bottom discharge; a generally vertically oriented sidewall (18) between the upper inlet and bottom discharge, where the sidewall defines a perimeter of an interior flow passage in the vessel; and a first or second pairs of support plates arranged on opposite sides of the sidewall, where the first pair or second pair of support plates reduces a cross-sectional flow area of the flow passage in a first or second single direction of convergences, the second single direction is angularly offset to the first single direction, and the second pair of support plates is at a different elevation of the vessel than the first pair of support plates. An INDEPENDENT CLAIM is also included for processing biomass in a reactor vessel, comprising: continuously feeding comminuted biomass to an upper inlet of the reactor vessel; continuously adding water or other liquor to the biomass so that a liquid level (35) in the reactor vessel is below an upper level of the biomass in the reactor vessel; treating the biomass in the reactor vessel; continuously discharging the treated biomass from a bottom outlet of the reactor vessel; moving the biomass between many pairs of support plates as the biomass flows gradually and continuously downward in the reactor vessel from the upper level of the biomass to the bottom outlet, where (i) the pairs of support plates are arranged at different elevations in the vessel, (ii) the plates of each pair are mounted to sidewall of the vessel at a slope with respect to the sidewall that converges the plate inwardly towards an opposite plate along a downward direction, and (iii) the convergence of each pair of plates is along a single direction of convergence; gradually increasing compaction of the biomass as the biomass flows down between each pair of support plates; and rapidly releasing some of the compressive forces on the biomass as the biomass flows pass lower edges on each pair of support plates.    DESCRIPTION OF DRAWING(S) - The figure shows a cross-sectional view of the reactor vessel.    Reactor vessel (10)    Vertical axis (13)    Sidewall (18)    Trailing edge (22)    Apex (24)