Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – Continuous chemical treatment or continuous charging or...
Reexamination Certificate
2001-09-28
2003-08-12
Alvo, Steve (Department: 1731)
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
Continuous chemical treatment or continuous charging or...
C162S019000, C162S029000, C162S037000, C162S040000
Reexamination Certificate
active
06605180
ABSTRACT:
PRIOR APPLICATION
This application claims priority from Swedish Application No. 0004050-1, filed Nov. 3, 2000.
TECHNICAL FIELD
The invention relates to the continuous cooking of pulp.
BACKGROUND AND SUMMARY OF THE INVENTION
To increase the productivity in existing continuous pulp digesters, different modifications have successively been made to the cooking technique. When the production capacity is increased in the digester, the flow of pulp through the digester increases, whereupon the dwell time is reduced at the cooking temperature, which temperature is necessary to maintain sufficient release of the lignin and dissolution of the pulp chips.
A natural step has been to take the impregnation step from the digester itself and arrange it in a pretreatment vessel prior to the actual digester. In this way it is possible to maintain the dwell time for the pulp chips in the digester and the cooking temperature despite the speed of the flow of pulp through the digester increasing.
As production increases, it is also desirable that the main extraction screen for used cooking liquor, called black liquor, is moved down in the digester, so that the length of the cooking zone is extended. The main extraction screen for consumed cooking liquor draws off warm and pressurized black liquor, and steam is generated by the pressure of the black liquor first being released in a flash tank. The black liquor is then taken for evaporation after which it is conveyed onwards to the recovery arrangement (recovery boiler).
This involves a conflict with the demands on achieving an effective wash zone at the bottom of the digester, which wash zone is intended on the one hand to wash out residual lignin but also to have the effect of lowering the temperature of the pulp.
Lowering the temperature to below about 100 degrees has been considered necessary so that the strength of the pulp is not reduced. If the pulp at a temperature of over 100° C. is exposed to atmospheric pressure from the digester through a pressure-release delivery system, this results in blowing-off of heat, so-called flashing. If the temperature is substantially above 100° C. (near the cooking temperature of ~140-160° C.) and the pulp pressure is released to 1 bar, this results in very powerful flashing on account of the cooking liquor's conversion from liquid phase to steam phase, which greatly reduces the strength of the pulp.
To ensure a sufficient washing effect in the wash zone and a sufficiently low temperature in pulp blowing, the reduced length of the wash zone demands ever more powerful countercurrent flows of wash liquid in the wash zone. Particularly with increasing production in a given digester and with a constant dilution factor, the relative speed between liquor and chips increases, which results in increasing lift forces. This has a detrimental effect on the plug flow of pulp through the digester and tends to lift the whole pulp column in the digester, which effects both reduce the operability of the digester, with production shutdowns as a consequence.
U.S. Pat. No. 4,123,318 discloses a cooking system for pulp in which a specially adapted digester vessel is followed by two series-connected vessels for conventional countercurrent washing, i.e. the same type of washing as essentially always applies at the bottom of the digester.
EP-A-476,230 discloses a system in which a limited quantity of white liquor is added in the countercurrent zones during the extraction of consumed cooking liquor. Here, a heat exchanger is used for heating, in a recirculation loop above the bottom of the digester, the wash liquid delivered through the dilution nozzles. The pulp is fed to a diffuser which in normal circumstances is assumed to be an atmospheric diffuser, and where the wash liquid is assumed to be collected in a conventional manner from a downstream position in the fibre line. EP-A-476,230 states that the temperature in the countercurrent zones is increased to 140-175° C., in sample tests 165° C., and for a dwell time of 180 minutes. Here, full use has not been made of the fact that the dilution liquid/wash liquid added at the bottom of the digester will also already have this high temperature at the time of addition.
U.S. Pat. No. 5,066,362 discloses a digester and pressure diffuser system in which the pulp is taken from the bottom of the digester at temperatures of around 148-160° C. (300-320° F. in the text) and where the first stage of the pressure diffuser is provided with heated white liquor, expediently at the level of the blow temperature for the pulp. The aim here is to obtain an extended delignification of kraft pulp.
In a variant in said U.S. Pat. No. 5,066,362, only wash liquid from a subsequent drum wash is used, and at temperatures of around 74° C. (166 F. in the text) of the wash filtrate from the drum wash. Here, a countercurrent wash is established in a conventional manner at the bottom of the digester, where filtrate from the pressure diffuser is fed as wash liquid at the bottom of the digester and extracted via a screen arranged at a distance from the bottom of the digester. Thus, the wash liquid moves counter to the descending movement of the wood chips. The cooking liquor extracted from the screen is then led to a flash tank.
This document also includes extraction of some of the pressure diffuser filtrate to the flash tank, which sub-quantity only represents the excess which is not needed for the necessary amount of wash liquid in the wash zone. This system does not fully use the establishment of a co-current flow of cooking liquor and wood chips down through the whole digester, which impairs the operability particularly if production is to be increased as the flow speed of the wood chips has to be increased.
SE-C-501,848 (=EP 670,924; U.S. Pat. No. 5,919,337) has proposed a system in which a higher temperature can be maintained across substantially the whole of the digester, in so-called ITC cooking. This document has discussed the advantage of having the same pressure in the pulp flow's transfer to a so-called pressurized diffuser, which was at the bottom of the digester. The wash filtrate from the pressure diffuser is recirculated in full back to the bottom of the digester and has, upon recirculation, a temperature of 100° C., maximum 110° C., resulting in a wash zone/temperature-reducing zone at the bottom of the digester. Cooking liquor/wash liquid is extracted in a screen immediately above the bottom of the digester and is recirculated to this level via a heat exchanger so that the cooking temperature can be maintained over the lowest placed screen. The pulp issuing from the digester has a temperature of 105-115° C. Using the innovative solution of a pressure diffuser directly after the digester, which pressure diffuser is capable of working at digester pressure levels of 10-20 bar, there is no flashing directly after the digester. This eliminates the problems of blowing to atmospheric pressure from 105-115° C., which would cause an explosion-like disintegration of the pulp fibres.
In connection with special digesters for handling branch wood chips/sawmill chips, special problems arise when a very high degree of packing is obtained, which normally makes effective extraction of cooking liquor from the whole pulp column impossible using screens in the wall of the digester. The branch wood chips and sawmill chips represent raw materials with most of their content in fine fractions well below the normally well-defined wood chips for cooking.
Normal wood chips for cooking are obtained using chippers which give wood chips with lengths of about 20-25 mm.
The sawmill chip fraction is often defined as the fine fraction, or the material which passes through a sieve with round holes of about 3 mm.
The branch wood chip fraction is often defined as the intermediate fraction, or the material which passes through sieves with holes exceeding 3 mm but below 8 mm (where sawmill chips have already been sieved out) Thus, wood chips normally contain long slivers which can be allowed to pass through such a siev
Gustavsson Lennart
Olsson Sven-Erik
Snekkenes Vidar
Alvo Steve
Fasth Rolf
Fasth Law Offices
LandOfFree
Process for continuous cooking of pulp does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Process for continuous cooking of pulp, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Process for continuous cooking of pulp will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3087650