Method for steaming comminuted cellulosic fibrous material during

Abstract

A continuous solvent pulping process is practiced with oxygen free gas (e.g. nitrogen) purges of all major treatment vessels during the time when the process is arrested or terminated. The wood chips or other cellulosic fibrous material to be pulped is steamed in a first horizontal steaming zone at pressure of about 10-20 psi, and then in a second horizontal steaming zone at a pressure of about 20-75 psi. The first and second zones are isolated by a low pressure feeder. Steam is introduced into the material in the second steaming zone to flow cocurrently with it. Gases, including vaporized solvent (e.g. ethanol or other alcohol) are vented from the steaming zones, and solvent is added to the steamed material prior to feeding to a high pressure feeder. The high pressure feeder introduces the material into the top of a single digesting vessel, liquid and chips being separated at the top of the digester vessel without mechanical means that could cause a spark. Lignin containing liquid is withdrawn from a central portion of the digester and passed through flash tanks and ultimately for lignin and alcohol recovery. A recirculating screen and system is disposed between the central extraction portion and the top of the digester vessel. A portion of the liquid removed for recirculation is instead passed to lignin recovery, while fresh solvent containing liquid is added to the rest of the recirculated liquid.