A reactor for hydrolytic splitting of cellulose under high pressure and heat has an inlet and outlet each with a cross-sectional area smaller than the cross-sectional area of the reactor. The pressure and heat in the reactor forms a cellulose plug within the inlet and the outlet. The inlet plug stops cellulose from escaping out the inlet. Cellulose begins to breakdown under heat, pressure, and if required an acid or a lubricant. The outlet plug slows downstream movement of cooking cellulose. Cellulose may be pre-treated by the addition of water, a weak acid, a lubricant, or a combination of the foregoing. The outer surface of the plug cooks faster than the inner core of the plug, becomes a liquefied slurry, and slides faster towards the outlet than the inner core.