Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – With heat recovery
Reexamination Certificate
1998-09-22
2001-04-24
Alvo, Steve (Department: 1731)
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
With heat recovery
C162S065000, C162S068000, C162S078000
Reexamination Certificate
active
06221207
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to oxygen delignification of chemical pulp and, more precisely, to such oxygen delignification in two stages.
BACKGROUND OF THE INVENTION
Swedish developments in the bleaching of chemical pulp have increased the interest in extending delignification with oxygen prior to final bleaching, in order to improve the pulp quality of totally chemical free bleached pulp, i.e. TCF-pulp (Totally Chlorine Free), and in order to facilitate the closing of both TCF-systems and ECF-systems (Elementary Chlorine Free).
Several variations of two-stage systems for oxygen delignification are currently in operation. These systems use, for example, extended retention time, re-mixing, and intermediate treatment for chemical distribution, all of which have become possible by use of a two-stage system. Both of these stages are thus carried out in upstream systems, and are pressurized to various degrees.
The following conditions have been found to be particularly advantageous for such delignification processes:
in the first stage the chemical concentration (oxygen and liquor) should be high, i.e. it has been found to be advantageous to charge all of the chemicals to the first stage. In other words, it is important to maintain an initially high pressure in order to support the oxygen reactions.
for extended delignification in the first phase of the oxygen system it is extremely important to maintain a high temperature. If the alkalinity is sufficiently high for a good reaction to take place, no additional chemicals need to be charged to the second stage (the final phase).
These two-stage systems have the most significant disadvantage of requiring much too high investment costs, but also that only high-quality intermediate pressure steam can be used for heating the pulp between the two stages.
SUMMARY OF THE INVENTION
In accordance with the present invention, these and other disadvantages have been overcome by the discovery of a method for the oxygen delignification of a chemical pulp which comprises contacting the chemical pulp with oxygen in an upstream reactor vessel having a top and a bottom by supplying the chemical pulp to the bottom of the upstream vessel and withdrawing a partially oxygen delignified chemical pulp from the top of the upstream vessel, maintaining a pressure of greater than about 3 bar in the upstream reactor vessel, contacting the partially oxygen delignified chemical pulp with oxygen in the downstream reactor having a top and a bottom by supplying the partially oxygen delignified chemical pulp to the top of the downstream reactor vessel and withdrawing oxygen delignified chemical pulp from the bottom of the downstream reactor vessel, maintaining the downstream reactor vessel at a temperature between about 90 and 120° C. and a corresponding predetermined pressure, and heating the partially delignified chemical pulp between the upstream and the downstream reactor vessels by contacting the partially delignified chemical pulp with low pressure steam. A preferred embodiment of the method of the present invention includes adding delignification chemicals to the chemical pulp prior to the upstream reactor vessel.
In accordance with one embodiment of the method of the present invention, the method includes maintaining a gas space at the top of the downstream reactor vessel and the method includes degassing the downstream reactor vessel from the top of the downstream reactor vessel.
In accordance with another embodiment of the method of the present invention, the method includes recovering the heat content from the oxygen delignified pulp. Preferably, such recovery comprises flashing or heating dilution liquid therewith.
In accordance with another embodiment of the method of the present invention, the method includes adding hydrogen peroxide to the partially oxygen delignified chemical pulp prior to the downstream reactor vessel. In a preferred embodiment, the hydrogen peroxide is added in an amount of up to about 5 kg/ton of pulp. Most preferably, the hydrogen peroxide is added by means of a mixer.
A primary object of the present invention is to solve the aforesaid problems by carrying out the two oxygen delignification stages under pressure, in that the first stage is carried out in an upstream reactor vessel and the second stage in a downstream reactor vessel. Further extended delignification, as well as an increase in pulp brightness, are obtained by supplying hydrogen peroxide in amounts of up to 5 kg per ton of pulp, to a mixer located prior to the downstream reactor. A minor adjustment for the increased alkali demand of the peroxide reactions can thus be made in this mixer. By means of this peroxide supply to the downstream reactor the pulp can be delignified to a greater extent and the brightness of the pulp can be increased prior to the final bleaching step. In this manner, closing of the bleaching plane is further facilitated.
The method according to the present invention additionally offers several other advantages, as follows:
no separate pump is required between the reactors;
the pulp is heated at the lowest pressure level in the system, so that low pressure
steam can now be used for heating the pulp;
possible admixture of additional chemicals, such as hydrogen peroxide, and adjustment of the alkali level, can take place before the second stage in a mixer located at the top of the second reactor vessel;
due to the fact that the downstream reactor is not filled with pulp, the system can easily be de-aired (degassed) from the top of the downstream reactor vessel;
the steam recovered by flashing the pulp downstream of the second stage can be used for heating purposes, such as the heating of water;
the temperature of the pulp at the bottom of the downstream reactor can also be lowered by dilution with colder liquid, thereby making it possible to recover the heat which is not flashed off; and
the downstream reactor constitutes a buffer by acting both as a reactor and as a storage tower.
REFERENCES:
patent: 3668063 (1972-06-01), Engstrom
patent: 3719552 (1973-03-01), Farley et al.
patent: 3944463 (1976-03-01), Samuelson et al.
patent: 3963561 (1976-06-01), Richter
patent: 4198266 (1980-04-01), Kirk et al.
patent: 5217575 (1993-06-01), Backlund
patent: 5306391 (1994-04-01), Cirucci et al.
patent: 5460696 (1995-10-01), Kirschner et al.
patent: 5607545 (1997-03-01), Henricson et al.
patent: 5690786 (1997-11-01), Cirucci et al.
patent: 2132056 (1995-03-01), None
patent: 9403074 (1996-03-01), None
patent: 93/00470 (1993-01-01), None
patent: 94/25669 (1994-11-01), None
“Alkaline Pulping”,Pulp and Paper Manufacture, Third Edition, vol. 5, 1989, pp. 411-414.
“Current State of the Art of EO, EP and EPO Technologies”. Non Chlorine Bleaching Conference, Hilton Head, S.C.USA, Mar. 1992.
Bokstrom Monica
Forslund Kjell
Larsson Rune
Sundin Matts
Alvo Steve
Lerner David Littenberg Krumholz & Mentlik LLP
Valmet Fibertech Aktiebolag
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