WO2007090925A1

WO2007090925A1 - Method for recovering hydrolysis products

Info

Publication number: WO2007090925A1
Application number: PCT/FI2007/050043
Authority: WO
Inventors: Päivi UUSITALO; Panu Tikka
Original assignee: Metso Paper, Inc.
Priority date: 2006-02-10
Filing date: 2007-01-26
Publication date: 2007-08-16
Also published as: FI20065105A0

Abstract

An improved method for treating lignocellulosic material, including a prehydrolysis-mass transfer process, which produces a concentrated hydrolysate volume following steam hydrolysis. The improved process comprises the heating of the digester and chip content by direct steam to the required hydrolysis temperature, the introduction of a volume of washing liquid to the digester and the discharge of said volume of liquid through the opposite end of the digester. Subsequently, the cooking process may start. The volume of washing liquid is sufficiently small to enable the recovery of a hydrolysate solution concentrated enough to make economic utilization of the hydrolysis products possible.

Description

METHOD FOR RECOVERING HYDROLYSIS PRODUCTS

FIELD OF THE INVENTION

The invention relates to a method for production of carbohydrates in connection with pulp production. In particular, the invention relates to the recovery of a prehydrolysate product sufficiently concentrated for economically feasible downstream operations.

BACKGROUND OF THE INVENTON

In general, carbohydrates can be produced from lignocellulosic natural materials by hydrolysis of poly- and oligosaccharides. Apart from a total hydrolysis process only leaving a lignin residue, a combination of hydrolysis and cellulose pulp cooking has been developed, called prehydrolysis pulping. The main emphasis has been on the pulp, reflecting the business incentives. In prior art processes, hemicelluloses are hydrolysed into hydrolysate, and lignin is dissolved by a cooking method for liberating cellulose fibers. The produced pulp has a high content of alpha cellulose and can be used e.g. as dissolving pulp.

From a historical perspective, there are two processes for the production of special pulps having a high content of alpha cellulose: the far-extended acidic bisulfite cooking and the prehydrolysis-sulfate (kraft) cooking. The former was developed at the beginning of the 20th century and the latter in the 1930's, see e.g. Rydholm, S.E., Pulping Processes, p. 649 to 672, Interscience Publishers, New York, 1968. The basic idea in both processes is to remove as much hemicellulose as possible from cellulose fibers in connection with the de- lignification so as to obtain a high content of alpha cellulose. This is essential because the various end uses of such pulps, dissolving pulp for instance, do not tolerate short-chained hemicellulose molecules with a randomly grafted molecular structure. Kraft prehydrolysis pulping processes are disclosed in e.g. Canadian patent application 1,173,602 (Arhippainen et al.) and in U.S. patents 5,589,033 (Tikka and Kovasin), 5,676,795 (Wizani et al.) and 4,436,586 (Elmore).

In the traditional sulfite process, the removal of hemicellulose takes place during the cooking simultaneously with the dissolving of lignin. The cooking conditions are highly acidic and the temperature varies from about 140⁰C to 150⁰C, whereby the hydrolysis is emphasized. The result, however, is always a compromise with delignification. No high content of alpha cellulose is obtained. Another drawback is the decrease in the degree of polymerization of cellulose and yield losses, which also limit the hydrolysis possibilities. In US.Pat. 5,139,617 (Tikka and Virkola), an anthraquinone - neutral sulfite pulping process is disclosed. Various improvements have been suggested, such as modification of the cooking conditions and even a prehydrolysis step followed by an alkaline sulfite cooking stage.

The utilization of the hydrolyzed carbohydrates released in the prehydrolysis has been neglected and no commercial production based on hydrolysis material has been reported in spite of the fact that this option is mentioned in e.g. the above-referred patents. In today's industrial practice, the hydrolysate is neutralized, combined with the spent cooking liquor, evaporated and combusted in the recovery boiler of the pulp mill's energy and chemicals recovery process.

Looking closer into the reasons of neglecting any other reasonable use of the carbohydrate material uncovers practical problems: The prehydrolysis process step is most practically carried out in steam phase, introducing direct steam to the chip column in the digester. Due to the material and energy balance, very little, if any, liquid hydrolysate phase is generated, as all condensate is trapped in the porosity of the wood material. A separate washing stage using a washing liquid within the digester between the prehydrolysis and the cooking steps takes time, lowers production, is very unfavorable to the energy balance and would produce a very dilute carbohydrate solution requiring further expensive evaporation prior to any reasonable use. Another process possibility has been to carry out the prehydrolysis step in liquid phase. In this case, too, the large amount of liquid and the resulting low concentration of carbohydrates have prevented the development of any reasonable production economy. As a result, the lack of an adequate carbohydrate removal process has precluded the utilization of this renewable raw material of natural origin. In US patent application 2005/0065336, a pulping process is disclosed, which involves mechanical treatment of wood chips and subjecting of the resulting mass to prehydrolysis using mineral acid treatment and subsequent steaming. After countercurrent washing, good yields of both alfa-cellulose and hemicellulose are reported. The process requires both further comminution of the chips and special process equipment for the hydrolysis and separation operations.

SUMMARY OF THE INVENTON

An object of the present invention is to provide an improved method for treating lignocellulosic material, including a prehydrolysis-mass transfer process, which produces a concentrated hydrolysate volume following steam hydrolysis. After this low-volume hydrolysate has been discharged from a still hot digester, the process may continue by a neutralization-cooking process known in the art. In accordance with the present invention, the improved process comprises (1) the heating of the digester and chip content by direct steam to the required hydrolysis temperature, (2) the introduction of a volume of washing liquid to the digester, (3) the discharge of said volume of liquid through the opposite end of the digester, (4) subjecting the chips to a cooking process. The volume of washing liquid may be introduced at the top of the digester and discharged through the bottom, or alternatively introduced at the bottom and discharged through the top, the latter option involving introduction of a displacing liquid at the digester bottom. According to the present invention, the volume of washing liquid is sufficiently small to enable the recovery of a hydrolysate solution concentrated enough to make economic utilization of the hydrolysis products possible. Following the passage of the washing liquid through the digester, the cooking process according to the art may start. In the case of hydrolysate displacement from the bottom up, the digester is generally already filled with alkaline liquor; if the hydrolysate has been recovered from the bottom, the next step is conventionally the introduction of alkaline cooking liquor.

Claims

1. A method for recovering carbohydrates in a prehydrolysis pulping process, characterized by comprising the following stages: a) providing a digester containing a column of lignocellulosic material b) heating the digester and its contents by direct steam to a predetermined hydrolysis temperature, c) introducing a volume of washing liquid into the digester d) removing said volume of washing liquid from the end of the digester opposite to the introduction end.