Impregnation of cellulose-containing material with black liquor

Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – Continuous chemical treatment or continuous charging or...

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Details

162 41, 162 42, D21C 326

Patent

active

060304928

DESCRIPTION:

BRIEF SUMMARY
TECHNICAL FIELD

The present invention relates to a method for continuous cooking of cellulose-containing fiber material, preferably by the kraft process, in a single-vessel system, the fiber material at the start of the cooking process being impregnated with black liquor, primarily for the purpose of achieving good pulp strength, but in a preferred embodiment also for the purpose of achieving relatively low energy consumption. The invention can be used expediently when converting old single-vessel digesters for black liquor impregnation.


STATE OF THE ART AND PROBLEMS

In existing kraft digesters, black liquor is used only in limited quantities, ie. most often in quantities which are substantially less than 50% of the total liquid content in the impregnation zone of the digester. The remaining quantity of externally added liquid normally consists of white liquor in the main. It has been found that this considerable charge of white liquor at such an early stage in the cooking process has a negative effect on the tear strength of the cooked fibers. In order to achieve selective cooking, it has been found that there should be a relatively high content of sulfide ions and a low content of hydroxide ions at the start of cooking.
A related problem concerns difficulties in obtaining a uniform cooking in existing single-vessel digesters, particularly of the hydraulically filled type. In order to achieve a uniform and selective cooking, it is desirable to be able to impregnate the chips in the upper part of the digester with hot black liquor, which has a relatively high content of sulfide ions and a low content of hydroxide ions. However, a high temperature at the top of the digester leads to a high temperature in the transfer circulation which conveys the chips from a so-called high-pressure feeder, that is to say a pressure sluice which transfers the chips together with liquid from a low-pressure system to a high-pressure system. Thereafter, the liquid is in the main separated from the chips with the aid of a top separator at the top of the digester and is returned to the high-pressure feeder. If this returned liquid has a temperature which exceeds the boiling point in the low-pressure system, the liquid will flash in the high-pressure feeder, said flashing manifesting itself in the form of bangs.
U.S. Pat. No. 3,303,088 (Gessner) discloses a method for continuously cooking cellulose-containing fiber material in a single-vessel system, with chips being introduced at a first end of the digester, white liquor being added at a position at said first end, the chips being impregnated in a concurrent impregnation zone, the chips being cooked in a cooking zone downstream of the impregnation zone, hot black liquor being extracted from at least one screen section, black liquor being added to said impregnation zone, and cooked pulp being discharged at the other end of the digester. Gessner further shows that the liquid extracted from the first screen section, which is arranged downstream of the point of addition of the black liquor, is returned to the digester by first being supplied to a container in which white liquor and extracted impregnation and cooking liquids are mixed. As a result of this recirculation, there is a substantial build-up in the content of, inter alia, volatile sulfur and terpene compounds in the impregnation and cooking liquids. In addition to this, the procedure according to Gessner does not offer a sufficiently rapid heating of the cooking liquid for achieving optimal process conditions. Furthermore, it also emerges that Gessner's procedure for continuously cooking cellulose-containing material does not involve process parameters which are necessary for achieving optimal conditions, such as, for example, the correct liquor-to-wood ratio for obtaining the desired movement of the chip column in the digester.
From WO 94/23120 (Collins) it is known to thermally insulate the high-pressure feeder from the digester by means of replacing a first, relatively cold liquid with a second, relatively hot liquid

REFERENCES:
patent: 3303088 (1967-02-01), Gessner
patent: 5658428 (1997-08-01), Oulie

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