NOVELTY - A reactor vessel (10) comprises a mixing chamber (2) having a vertical length, an upper inlet, and a lower outlet; and at least one paddle (3) having a vertical orientation within the mixing chamber and having a cross-sectional shape of a hydrofoil, where the paddle moves with respect to the mixing chamber while maintaining the vertical orientation.    USE - As reactor vessel for mixing biomass and enzyme (claimed); for enzymatic conversion of biomass to monomeric sugars.    ADVANTAGE - The vertical paddles having a varying hydrofoil cross-section or angle of attack can be used to keep the rotation of the biomass and enzyme material substantially constant over the entire length of the mixing chamber. This design can promote plug flow and reduce rotation deltas. When using the vertical paddle design it may not be necessary to use any horizontal mixing arms above or below the vertical paddles. The paddles can rotate in a slow, constant, unidirectional manner, e.g. less than 10 revolutions per minute, to prompt mixing of biomass and enzyme material at the same elevation. The paddle's slow, constant, unidirectional movement reduces tendency of the mixing action to change the elevation of the biomass and enzyme material. This movement can also allow the biomass and enzyme material to move downward in a uniform, slow, and continuous manner while the biomass and enzyme material is mixed. Within the mixing chamber of the reactor vessel, the movement of the biomass and enzyme material is desirably uniform and slow to avoid stagnant regions of biomass and enzyme material and to allow sufficient time for the reactions to occur to the biomass. The horizontal blending motion of the biomass and enzyme material within the mixing chamber can help to establish and maintain the desired reaction environment. The slow horizontal blending can create plug flow, and plug flow is generally desirable for a more controllable reaction pattern. The vertical paddles with a hydrofoil shape used in the mixing chamber of the reactor vessel, provides desired horizontal blending movement or motion.    DETAILED DESCRIPTION - A reactor vessel (10) comprises a mixing chamber (2) having a vertical length, an upper inlet, and a lower outlet; and at least one paddle (3) having a vertical orientation within the mixing chamber and having a cross-sectional shape of a hydrofoil, where the paddle moves with respect to the mixing chamber while maintaining the vertical orientation. The reactor vessel further comprises a center shaft (5) coupled to the paddle; upper support bar and a lower support bar coupled, respectively, to opposite end regions of the paddle; and center shaft coaxial with a center axis of the mixing chamber. An INDEPENDENT CLAIM is included for mixing biomass and an enzyme in a reactor vessel comprising a vertically-oriented mixing chamber and at least one paddle blade extending vertically through the mixing chamber involving feeding a mixture of biomass and enzyme continuously into an upper inlet of the reactor vessel and adding the mixture of biomass and enzyme into the mixing chamber; moving at least one paddle blade through the mixture of biomass and enzyme in the mixing chamber while maintaining the paddle blade in a vertical orientation; mixing the mixture of biomass and enzyme by the movement of the paddle blade as the mixture of biomass and enzyme moves down through the mixing chamber; and discharging the mixture of biomass and enzyme from a lower outlet of the reactor vessel.    DESCRIPTION OF DRAWING(S) - The figures shows a side view of the vertical cylindrical reactor vessel with vertical paddles.    Enzymes (1)    Mixing chamber (2)    Paddle (3)    Support bar (4)    Center shaft (5)    Motor (6)    Reacted biomass slurry (7)    Reactor vessel (10)