A processing system for converting cellulosic material to pulp, the system comprising: a first pressurized reactor vessel operating at a pressure above atmospheric pressure, the first reactor vessel including: a material input receiving cellulosic material and a material discharge for the material, wherein the cellulosic material flows from the material input to the material discharge; a heat energy input port in an upper portion of the first reactor vessel; an extraction screen having an outlet for extracting hydrolysate and liquid from the first reactor vessel; a first region of the first reactor vessel between the material input and the extraction screen, wherein the first region is maintained at a hydrolysis temperature in the cellulosic material; a wash liquid input to the first pressurized reactor vessel and below the extraction screen, wherein the wash liquid input is connected to a source of wash liquid including a source of cool wash water and a source of at least one of sodium hydroxide and essentially sulfur free white liquor; and a second region of the first reactor vessel between the extraction screen and the discharge in which the temperature is below the hydrolysis temperature and the hydrolysis reaction is substantially suppressed, a transport pipe providing a flow conduit from the discharge to a second reactor vessel which is a continuous digesting vessel, and the continuous digesting vessel receiving the cellulosic material discharged from the first reactor vessel. a first pressurized reactor vessel operating at a pressure above atmospheric pressure, the first reactor vessel including: a material input receiving cellulosic material and a material discharge for the material, wherein the cellulosic material flows from the material input to the material discharge; a heat energy input port in an upper portion of the first reactor vessel; an extraction screen having an outlet for extracting hydrolysate and liquid from the first reactor vessel; a first region of the first reactor vessel between the material input and the extraction screen, wherein the first region is maintained at a hydrolysis temperature in the cellulosic material; a wash liquid input to the first pressurized reactor vessel and below the extraction screen, wherein the wash liquid input is connected to a source of wash liquid including a source of cool wash water and a source of at least one of sodium hydroxide and essentially sulfur free white liquor; and a second region of the first reactor vessel between the extraction screen and the discharge in which the temperature is below the hydrolysis temperature and the hydrolysis reaction is substantially suppressed, a transport pipe providing a flow conduit from the discharge to a second reactor vessel which is a continuous digesting vessel, and the continuous digesting vessel receiving the cellulosic material discharged from the first reactor vessel. a material input receiving cellulosic material and a material discharge for the material, wherein the cellulosic material flows from the material input to the material discharge; a heat energy input port in an upper portion of the first reactor vessel; an extraction screen having an outlet for extracting hydrolysate and liquid from the first reactor vessel; a first region of the first reactor vessel between the material input and the extraction screen, wherein the first region is maintained at a hydrolysis temperature in the cellulosic material; a wash liquid input to the first pressurized reactor vessel and below the extraction screen, wherein the wash liquid input is connected to a source of wash liquid including a source of cool wash water and a source of at least one of sodium hydroxide and essentially sulfur free white liquor; and a second region of the first reactor vessel between the extraction screen and the discharge in which the temperature is below the hydrolysis temperature and the hydrolysis reaction is substantially suppressed, a transport pipe providing a flow conduit from the discharge to a second reactor vessel which is a continuous digesting vessel, and the continuous digesting vessel receiving the cellulosic material discharged from the first reactor vessel.