Method and System for Semi-Chemical Pulping

Abstract

Method and apparatus for minimizing adherence of lignin to an extraction screen in a semi-chemical pulping process. The method and apparatus generally relate to supplying white liquor and NaOH to as to increase the pH of cooking liquor at or near an extraction screen.

Description

CROSS RELATED APPLICATION[0001]This application claims the benefit of U.S. Provisional Patent Application Ser. No. 60 / 954,446 filed Aug. 7, 2007, the entirety of which is incorporated by reference.BACKGROUND AND SUMMARY OF THE INVENTION[0002]The present invention generally relates to a method and system for making a semi-chemical pulp from lignocellulosic material.[0003]Semi-chemical pulping processes are known. See, e.g., U.S. Pat. No. 4,229,251 to Temler; U.S. Pat. No. 4,073,678 to Hammond et al. Semi-chemical pulping processes have different requirements and different operating conditions than pulping processes involving Lo-Solids® Cooking, such as those disclosed in U.S. Pat. No. 5,489,363 to Marcoccia et al.[0004]When making semi-chemical pulps (pulps with yields in the 65-85% yield range, typically used for production of corrugated medium used in box making), several different chemicals can be used, such as NaOH, Na2CO3, Na2SO3, kraft white liquor or kraft green liquor. In som...

AI Technical Summary

Benefits of technology

[0010]In an aspect, certain embodiments of the present invention generally relate to a method for inhibiting fouling of a black liquor extraction screen in a cooking vessel for semi-chemical pulping. The method may comprise the steps of: feeding lignocellulosic material and green liquor to the cooking vessel for semi-chemical pulping; cooking the lignocellulosic material in the cooking vessel for semi-chemical pulping such that some or all lignin is separated from the lignocellulosic material, and wherein lignin is dissolved into a cooking liquor; extracting via an extraction screen black liquor comprising dissolved lignin, wherein the black liquor is formed during the semi-chemical pulping process from the cooking vessel; and adding white liquor or a base to the cooking vessel at or near the extraction screen, such that a pH at or near the extraction screen is elevated and inhibits lignin from adhering to the extraction screen.

Problems solved by technology

The simple process and relatively short retention time may make it impractical to try to perform multi-stage cooking processes.

This clogging or coating may be caused by lignin that has precipitated out of solution due to the lower pH at the end of the cook.

As the solubility of lignin, which as been exposed to alkaline cooking conditions, decreases with lower pH, it may form a sticky tar-like substance, which coats the digester screens and may make them non-functional to the point that the digester has to be shut down and cleaned.

A higher pH at the beginning of the cook, however, may have a negative impact on the pulp yield and may make the operation less economical, as white liquor is more expensive to produce than green liquor.

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