Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – With regeneration – reclamation – reuse – recycling or...
Reexamination Certificate
2001-01-19
2002-02-12
Nguyen, Dean T. (Department: 1731)
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
With regeneration, reclamation, reuse, recycling or...
C162S043000, C162S061000
Reexamination Certificate
active
06346167
ABSTRACT:
BACKGROUND AND SUMMARY OF THE INVENTION
According to conventional knowledge in the art of kraft pulping of cellulose, the level of dissolved organic materials (DOM)—which mainly comprise dissolved hemi-cellulose, and lignin, but also dissolved cellulose, extractives, and other materials extracted from wood by the cooking process—is known to have a detrimental affect in the later stages of the cooking process by impeding the delignification process due to consumption of active cooking chemical in the liquor before it can react with the residual or native lignin in wood. The effect of DOM concentration at other parts of cooking, besides the later stages, is according to conventional knowledge believed insignificant. The impeding action of DOM during the later stages of the cook is minimized in some state-of-the-art continuous cooking processes, particularly utilizing an EMCC® digester from Kamyr, Inc. of Glens Falls, N.Y., since the counter-current flow of liquor (including white liquor) at the end of the cook reduces the concentration of DOM both at the end of the “bulk delignification” phase, and throughout the so-called “residual delignification” phase.
According to the present invention, it has been found that not only does DOM have an adverse affect on cooking at the end of the cooking phase, but that the presence of DOM adversely affects the strength of the pulp produced during any part of the cooking process, that is at the beginning, middle, or end of the bulk delignification stage. The mechanism by which DOM affects pulp fibers and thereby adversely affects pulp strength has not been positively identified, but is hypothesized that it is due to a reduced mass transfer rate of alkali extractable organics through fiber walls induced by DOM surrounding the fibers, and differential extractability of crystalline regions in the fibers compared to amorphous regions (i.e. nodes). In any event, it has been demonstrated according to the invention that if the DOM level (concentration) is minimized throughout the cook, pulp strength is increased significantly.
It has been found, according to the present invention, that if the level of DOM is close to zero throughout a kraft cook, tear strength of the puml is greatly increased, i.e. increased up to about 25% (e.g. 27%) at 11 km tensile compared to conventionally produced kraft pulp. Even reductions of the DOM level to one-half or one-quarter of their normal levels also significantly increase pulp strength.
In state-of-the-art kraft cooks, it is not unusual for the DOM concentration at some points during the kraft cook to be 130 grams per liter (g/l) or more, and at 100 g/l or more at numerous points during the kraft cook (for example in the bottom circulation, trim circulation, upper and main extractions and MC circulation in Kamyr, Inc. MCC® continuous digesters), even if the DOM level is maintained between about 30-90 g/l in the wash circulation (at later cook stages, according to conventional wisdom). In such conventional situations it is also not unusual for the lignin component of the DOM level to be over 60 g/l and in fact even over 100 g/l, and for the hemi-cellulose component of the DOM level to be well over 20 g/l. It is not known if the dissolved hemi-cellulose component has a stronger adverse affect on pulp strength (e.g. by adversely affecting mass transfer of organics out of the fibers) than lignin, or vice versa, or if the effect is synergistic, although the dissolved hemi-celluloses are suspected to have a significant influence.
According to the present invention it has been recognized for the first time that the DOM concentration throughout a kraft cook should be minimized in order to positively affect bleachability of the pulp, reduce chemical consumption, and perhaps most significantly increase pulp strength. By minimizing DOM levels, one may be able to design smaller continuous digesters while obtaining the same throughput, and may be able to obtain some benefits of continuous digesters with batch systems. A number of these beneficial results can be anticipated by keeping the DOM concentration at 100 g/l or less throughout substantially the entire kraft cook (i.e., beginning, middle and end of bulk delignification), and preferably about 50 g/l or less (the closer to zero DOM one goes, the more positive the results). It is particularly desirable to keep the lignin component at 50 g/l or less (preferably about 25 g/l or less), and the hemi-cellulose level at 15 g/l or less (preferably about 10 g/l or less).
According to the present invention it has also been found that it is possible to passivate the adverse affects on pulp strength of the DOM concentration, at least to a large extent. According to this aspect of the invention it has been found that if black liquor is removed and subjected to pressure heat treatment according to U.S. Pat. No. 4,929,307 (the disclosure of which is hereby incorporated by reference herein), e.g. at a temperature of about 170-350° C. (preferably 240° C.) for about 5-90 minutes (preferably about 30-60 minutes) and then reintroduced, an increase in tear strength of up to about 15% can be effected. The mechanism by which passivation of the DOM by heat treatment occurs also is not fully understood, but is consistent with the hypothesis described above, and its results are real and dramatic on pulp strength.
According to the present invention various methods are provided for increasing kraft pulp strength taking into account the adverse affects of DOM thereon, as set forth above, for both continuous and batch systems. Also according to the present invention increased strength kraft pulp is also provided, as well as apparatus for achieving the desired results according to the invention. Further, according to the invention, the H factor can be significantly reduced, e.g., at least about a 5% drop in H factor to achieve a given Kappa number. Also, the amount of effective alkali consumed can be significantly reduced, e.g., by at least about 0.5% on wood (e.g. about 4%) to achieve a particular Kappa number. Still further, enhanced bleachability can be achieved, for example, increasing ISO brightness at least one unit at a particular full sequence Kappa factor.
According to one aspect of the present invention, a method of producing kraft pulp by cooking comminuted cellulosic fibrous material is provided. The method comprises the steps of continuously, at a plurality of different stages during kraft cooking of the material to produce pulp: (a) Extracting liquor containing a level of DOM substantial enough to adversely affect pulp strength. And, (b) replacing some or all of the extracted liquor with liquor containing a substantially lower effective DOM level than the extracted liquor, so as to positively affect pulp strength. Step (b) is typically practiced by replacing the withdrawn liquor with liquor selected from the group consisting essentially of water, substantially DOM free white liquor, pressure-heat treated black liquor, washer filtrate, cold blow filtrate, and combinations thereof. For example for at least one stage during cooking, black liquor may be withdrawn, and treated under pressure and temperature conditions (e.g. superatmospheric pressure at a temperature of about 170-350° C. for about 5-90 minutes, and at least 20° C. over the cooking temperature) to significantly passivate the adverse affects of DOM. The term “effective DOM” as used in the specification and claims means that portion of the DOM that affects pulp strength, H factor, effective alkali consumption and/or bleachability. A low effective DOM may be obtained by passivation (except for effect on bleachability), or by an originally low DOM concentration.
The method according to the invention can be practiced in a continuous vertical digester, in which case steps (a) and (b) may be practiced at at least two different levels of the digester. There is also typically the further step (c) of heating the replacement liquor from step (b) to substantially the same temperature as the withdrawn liquor prior to the replacement liquor
Chasse R. Fred
Laakso Richard O.
Marcoccia Bruno S.
Phillips Joseph R.
Prough J. Robert
Andritz-Ahlstrom Inc.
Nguyen Dean T.
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