Provided herein is a process for hydrolyzing a cellulosic feedstock to produce sugar. The process comprises introducing a pretreated cellulosic slurry to an inlet region of a plug flow hydrolysis reactor using a slurry introduction device that reduces the axial momentum of the slurry at the surface of the reactor contents. The cellulosic feedstock slurry is hydrolyzed in the plug flow hydrolysis reactor by contacting the cellulosic feedstock with at least cellulase enzymes to produce glucose. Also provided herein is a vertically-oriented, unmixed downflow hydrolysis reactor for hydrolyzing a pretreated cellulosic feedstock slurry which comprises such a slurry introduction device disposed in a top region thereof.