Reference will hereinafter be made to the accompanying drawings, wherein like reference numerals throughout the various FIGURES denote like structural elements, and wherein;
Accompanying
Comminuted cellulosic fibrous material, for example, in the form of hardwood chips 13, are introduced to feed system 12, as described in U.S. Pat. Nos. 5,763,075; 6,106,668; 6,325,890; 6,551,462; 6,336,993; 6,841,042 (the entire content of each patent being expressly incorporated hereinto by reference), and marketed by Andritz Inc. under the trademark TURBOFEED®; or a feed system as described in U.S. Pat. Nos. 5,476,572; 5,700,355; 5,968,314 (the entire content of each patent being expressly incorporated hereinto by reference), and marketed by Andritz Inc. under the trademark LO-LEVEL® which may include forwarding the chips to steam treatment in a vessel 14, which is preferably a vessel as described in U.S. Pat. Nos. 5,500,083; 5,617,975; 5,628,873; 4,958,741; and 5,700,355 (the entire content of each patent being expressly incorporated hereinto by reference), and marketed by Andritz Inc. under the trademark DIAMONDBACK®, though other types of steaming vessels may be used. From the vessel the chips pass through equipment such as a metering device connected to a conduit, which is preferably a Chip Tube provided by Andritz Inc. The slurry of chips and liquid is fed to the impregnation vessel (if used) or to the digester 11 via a pressurizing means.
The slurry pressurized, typically, to a pressure of between about 5 and 15 bar and propels the slurry via conduit 21 to the top of the continuous digester 11. Excess liquor contained in the slurry is removed from the slurry at the inlet of the digester 11 by a separating device 22, typically a conventional Top Separator, and the excess liquid is removed and returned to feed system 12 via conduit 23. During treatment within vessel 14, the slurry of hardwood material may be pretreated at an alkaline charge of between about 20 to about 70% of total effective alkaline (EA) NaOH charge on wood at a temperature between about 80 to about 120° C., preferably between about 100 to about 110° C.
Feed system 12 also typically includes conventional non-illustrated devices such as in-line drainer, level tank, and make-up liquor pump. Cooking liquor, for example, kraft white liquor (WL) is typically provided to the level tank (not shown) as is conventional.
The pretreated hardwood material (which may, but not necessarily, have been treated in an impregnation vessel) fed via line 21 is subjected to a first cooking stage in digester 11. However, in accordance with the present invention, the pretreatment of the hardwood material as described above is not absolutely necessary as the hardwood material may be fed directly into the digester 11. Whether or not the hardwood material is pretreated, the first cook within digester 11 is conducted under low sulfidity conditions (i.e., 0-20% sulfidity) using a beneficial additive such as anthroquinone (AQ) and/or polysulfide. The first cooking stage in digester 11 is conducted at temperatures of between about 120 to about 170° C. using an alkaline charge of between about 2 to about 10% effective alkaline (EA) NaOH charge on wood. The cooked hardwood material exits digester 11 via line 30 having a Kappa number of between about 30 to about 100 (preferably between about 40 to about 60).
The hardwood material after the first cooking stage in digester 11 has a higher proportion of g-lignin relative to s-lignin than at the beginning of the first cooking stage. High concentration of s-lignin in surrounding liquor may interfere with the dissolution of g-lignin remaining in the hardwood material. To reduce this interference, it may be necessary to wash the hardwood material following the first cooking stage in digester 11. Thus, the hardwood material may optionally be transferred to a wash vessel 32 via line 30 and subjected to wash conditions with a wash liquor introduced via line 34. Spent wash liquor is removed from the wash vessel 32 via line 36. Since the wash stage within wash vessel 32 reduces the s-lignin in surrounding liquor, any cooking additives which could later be used would not be detrimentally consumed by the already dissolved s-lignin in the surrounding liquor.
The washed pulp which exits the optional wash vessel 32 via line 38 is directed to a digester 40 where a second cooking stage is conducted so as to further breakdown and reduce the g-lignin content in the hardwood material. Breakdown of g-lignin can be accomplished by use of high sulfidity cooking liquor (i.e., greater than about 20% sulfidity, and preferably between about 25 to about 40% sulfidity) with or without AQ and/or polysulfide. (See for example U.S. Pat. No. 6,576,084, the entire content is expressly incorporated hereinto by reference). Other cooking chemicals that work well under cooking conditions, such as sulfite, could also be used alone or in conjunction with AQ and/or polysulfide as mentioned above.
The second cooking stage in digester 40 is conducted at a temperature between about 130 to about 180° C., preferably between about 140 to about 180° C., with an alkali charge of between about 2 to about 10% EA charge on wood (original wood charge) and a Kappa number at the beginning of the second cooking stage of between about 30 to about 100 (preferably between about 40 to about 60). During this phase, it may be advantageous (but not always necessary) to increase the temperature to above that of the first cooking stage.
If sufficient alkali has not been provided initially at the beginning of the second cooking stage, it may be necessary to introduce additional white liquor to digester 40 during the second cook stage prior to removal from digester 40 in a second location. If this second addition of white liquor is made at the end of the second cook stage, it is preferred that the hardwood material be allowed to continue to cook at a temperature of at least about 130°. At the point where the hardwood cellulosic material exits the digester 40 via line 42, the Kappa number will preferably be between 15 to 30, more preferably between about 17 to about 30.
Important to the present invention is that the hardwood material is divided into two distinct cooking stages. The H-factor achieved at the end of the second cooking stage is as least 50% of the total H-factor that is achieved for the entire cook. Stated another way, the H-factor achieved through the first cooking stage is less than 50% of the H-factor achieved for the entire cook (i.e., at the end of the second cooking stage).
It is also possible that the cooking stages, and washing stage if used, are preformed in a single digester vessel. If a single vessel is used the same operating parameters for each stage as described would be used and end of stage would not require intermediate transfer of material to multiple vessels.
The first and second cooking stages conducted in digesters 11 and 40, respectively, or in a single digester vessel, is preferably conducted in accordance with one or more of the processes described more fully in U.S. Pat. Nos. 5,489,363; 5,536,366; 5,547,012; 5,575,890; 5,620,562; and 5,662,775 (the entire contents of each such patent being expressly incorporated hereinto by reference). The processes and apparatus disclosed in these patents are marketed under the trademark LO-SOLIDS® Cooking by Andritz Inc. of Glens Falls, N.Y.
The processes and apparatus of the present invention produce a pulp low in HexA content as a result of the relatively high temperature of the new treatment and/or cooking in the second cooking stage. This low HexA pulp favors oxygen delignification conditions which in turn provides for a low Kappa number pulp to a subsequent bleaching process. See in this regard, U.S. Pat. Nos. 6,776,876 and 6,475,338, the entire content of each being expressly incorporated hereinto by reference. The low Kappa number pulp may therefore be bleached using less chemicals thereby resulting in lower effluent loading. The following non-limiting example further illustrates the present invention.
A hardwood cooking cycle was performed using a total EA (effective alkaline as NaOH) charge of 18% introduced during the impregnation, first cook and second cook stages. The following conditions were used during each of the stages:
Impregnation:
1st Cook Stage:
2nd Cook Stage:
From this set of cooking conditions, the high temperature in the second stage of the cook gave lower Kappa number and at least 30% lower reject rate than the set of cooking conditions using 140° C. cooking temperature in both cook stages under the same alkali charge and the same H-factor.
A hardwood cooking cycle was performed using a total EA (effective alkaline as NaOH) charge of 17.5% introduced during the impregnation, first (or upper) cook and second (or lower) cook sequences. The following conditions were used during each of the cook sequences:
Impregnation:
1st (Upper) Cook:
2nd (Lower) Cook:
A sample of the cellulosic material (wood chips) was taken after impregnation was complete and tested for s-lignin and g-lignin in the laboratory using the method described in Lin et al, “Methods in Lignin Chemistry”, Springer-Verlag, Berlin (1992) (the entire content of which is expressly incorporated hereinto by reference). In addition, samples of the cellulosic material (wood chips) were taken at the midpoint of cook (e.g., about 90 minutes from the beginning of impregnation and at the end of the 1st (upper) cook (e.g., about 120 minutes from the beginning of impregnation). Each such sample was likewise analyzed for s-lignin and g-lignin content. Finally, a sample was also taken at the mid point of the 2nd (lower) cook (e.g., about 180 minutes from the beginning of impregnation). No analysis for s-lignin and g-lignin in the final pulp produced was made since the lignin content generally is so low that it is difficult to accurately analyze for the s-lignin and g-lignin species.
Accompanying
The data of
While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.
This application is based on and claims domestic priority benefits under 35 USC §119(e) from U.S. Provisional Application Ser. No. 60/791,431 filed on Apr. 13, 2006, the entire content of which is expressly incorporated hereinto by reference.
Number | Date | Country | |
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60791431 | Apr 2006 | US |