Continuous solvent pulping and washing processes and apparatus

Details Continuous solvent pulping and washing processes and apparatus Continuous solvent pulping and washing processes and apparatus

D - Textiles, Paper

21

C

D21C 1/02 (2006.01) D21C 3/20 (2006.01) D21C 3/24 (2006.01) D21C 7/00 (2006.01) D21C 9/02 (2006.01) D21C 9/04 (2006.01)

Patent

CA 2042885

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 a 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 co-currently 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 Lignin containing liquid is withdrawn from a central portion of the digester and passed through flash tanks and ultimately for lignin and alcohol recovery.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. Washing -- which also may be practiced using solvent pulp from a batch system -- is accomplished by first continuously passing the pulp to a pressure diffuser, then to a first multi stage drum displacer washer, and then to a second multi stage drum displacer washer. In the pressure diffuser the lignin is washed out of the pulp, utilizing as wash liquid a mixture of solvent and water having a solvent concentration equal to or higher than that required by the extraction process within the digester, and at a pressure at at least about 350 psi. The pressure in the second and third stages is lower, and water is the wash liquid in the third stage. Alcohol can be recovered from the spent wash liquid of the first wash stage, so that all but about ten gallons or less of alcohol per ton of pulp is recovered.

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