Paper making and fiber liberation – Apparatus – Digester
Reexamination Certificate
2001-10-19
2002-04-30
Alvo, Steve (Department: 1731)
Paper making and fiber liberation
Apparatus
Digester
C162S071000, C162S237000, C162S242000, C162S243000, C162S246000, C162S249000, C162S250000
Reexamination Certificate
active
06379504
ABSTRACT:
BACKGROUND AND SUMMARY OF THE INVENTION
Many forms of naturally occurring cellulose are used to produce chemical pulps for the production of paper. The form used depends upon the availability of the material and the capability of the pulping equipment. One of the most common forms is the wood chip, either made from hardwoods or softwoods, but any other form of comminuted cellulose material may be used including grasses or agricultural waste, for example, bagasse and cornstalks.
An additional source of cellulose is the waste from saw mills, namely sawdust. Especially in lumber producing regions there is a plentiful supply of sawdust that can be pulped to produce wood pulp. The pulping of sawdust has both advantages and disadvantages. One advantage for using sawdust as a source of cellulose is that the smaller sawdust particles are relatively easy to impregnate with cooking liquor. For this reason the pretreatment systems for chemical pulping of sawdust are less complex than those used to impregnate wood chips, which are generally more difficult to impregnate than sawdust.
One disadvantage of chemical pulping sawdust is that sawdust can be resistant to the flow of cooking liquors. The finely dividing material tends to form a compact matrix when exposed to a liquid flow and limit flow through the material, if not prevent it altogether. For example, since batch digesters are highly dependent upon the capability of providing a cooking liquor circulation through the medium being pulped, it is difficult—if not impossible—to pulp sawdust in a conventional batch digester. Also, conventional continuous digesters, such as Kamyr® continuous digesters, also have difficulty handling sawdust without incorporating some form of special rotating liquid distribution device.
One common method used to continuously pulp sawdust is by using a drag-chain type digesters, for example, an M&D-type digester as shown in FIG. 138 of Volume 5 of TAPPI's
Pulp and Paper Manufacture
(1989), Grace, ed. These type of digesters consist of an inclined vessel through which sawdust is conveyed through the cooking liquor by means of a conveyor mechanism. However, this conveyor mechanism and its related hardware requires continuous maintenance that makes this type of system unsatisfactory in modern pulp mills.
Another mechanical disadvantage of the M&D-type digester for treating sawdust, and the like, is the rotary feed valve used. A typical device is shown in FIG. 139 of Grace. This rotary valve is a typical star-type feeder that inherently experiences an unbalanced pressure load due to the large pressure difference between the inlet and outlet of the valve. This load imbalance typically causes bearing wear requiring repeated maintenance.
In addition to the mechanical disadvantages, these M&D-type systems also have process disadvantages that make these systems less efficient than desired. One characteristic of the M&D-type process is the relatively short retention times. Two aspects of this type of digester limit the retention time: (a) steam heaving and (2) mechanical conveyance. Since the impermeability of sawdust prevents the sawdust from being heated by liquor displacement, the sawdust is heated by direct exposure to steam. The steam or vapor space required to expose the material to steam consumes some of the space that could be used for cooking retention time and hence limits the retention time.
The mechanical conveyor used in an M&D-type digester, referred to as a “drag conveyor”, also limits the retention time because of the physical limitations of the size of the conveyor. It is simply too costly to manufacture a larger mechanical conveyor to achieve longer retention times.
As a result, the retention times provided by such a digester are limited to less than 1 hour, typically less than 30 minutes. Typically, additional cooking retention time is obtained when treating sawdust by following the M&D-type digester by ne or more retention vessels, or by “piggy-backing” two or more inclined digesters.
These characteristic short retention times also affect the cooking temperatures that are used. In order to obtain the proper degree of cooking, for example, to achieve a desired H factor, a relatively higher temperature must be used because of the shorter retention time. For example, if a typical cook requiring 2 hours retention time is limited to only ½ hour in an M&D-type digester, the cooking temperature must be increased from approximately 325° F. to 360° F. to achieve a comparable cook. this increase in cooking temperature increases the amount of high-pressure steam needed to maintain the higher cooking temperature. Therefore, the M&D-type digester is not as energy efficient as a digester capable of longer retention times.
These higher temperatures also consume more cooking chemicals and can potentially increase fiber damage. The rate of reaction of cooking chemicals with cellulose is highly dependent upon the prevailing temperature. The higher the temperature the faster and more aggressive the reaction. For kraft systems of the M&D type, the higher cooking temperatures, required for the shorter cooking times, result in higher reaction rates. This typically can cause increased chemical consumption and increased cellulose degradation.
The disadvantages of the M&D-type digester for cooking sawdust, and the like, are also seen in the “Pandia”-type digester shown in FIGS. 141 and 143 of Grace.
Another conventional continuous sawdust pulping system, shown in Grace, FIG. 133, and Smook, Handbook for Pulp and Paper Technologists, 1982, page 86 (FIGS.
8
-
17
), comprises a cylindrical vessel fed by two horizontal screw conveyors and a pocket feeder, for example, a Kamyr® asthma feeder. This type of vessel is a steam-phase type in which a liquid level is maintained below the top of the vessel and steam is added to the space above the liquid level. The sawdust fed to this vessel by the pocket feeder is heated to cooking temperature by the added steam. This steam heating avoids the impractical practice of circulating heated liquor to heat to cooking temperature.
As described by Grace, the pulp in this type of digester is cooled by introducing wash filtrate to the bottom of the digester and extracting it by means of a centrally-located rotating cylindrical screen. (See U.S. Pat. No. 3,475,271 of Laakso.) However, due to the impermeability of finely divided material like sawdust, this method of extraction has been shown to be unstable.
This “asthma-feeder” style sawdust cooking system also has the disadvantage that the feed system is located above the digester vessel. This is because the asthma feeder is limited to transporting the sawdust a short distance. This limits the size and flexibility of such installations.
Another sawdust pulping system is shown in Canadian patent 1,242,055. This patent discloses the use of a conventional slurry pump to feed a slurry of sawdust and cooking liquor to a cylindrical digester. This transfer of medium consistency slurry by means of a pump prior to cooking is not energy efficient. Typically, such pumps are limited to medium consistency slurries of between 8 and 16% consistency. In heating such a slurry to cooking temperature the excess liquid volume must also be heated to cooking temperature. For example, a 12% slurry contains 7.33 lbs. of liquid per lb. of fiber. In contrast, a 30% slurry contains 2.33 lbs. of liquid per lb. of fiber, or less than a third of the liquid per lb. of fiber. The lower consistency slurry requires additional energy to heat this excess liquid to cooking temperature.
Furthermore, no effort is made to minimize the mechanical action on the pulp or to recover heat from the cooked pulp slurry. Excessive mechanical action on sawdust slurries can be damaging to fiber properties, and is otherwise undesirable.
The present invention avoids these limitations of prior art continuous cooking systems for sawdust, and other finely divided comminuted fibrous material by first eliminating the need for high pressure mechanical feeders and conveyors; second, by disc
Chamblee J. Wayne
Chasse R. Fred
Marois Marco
Miele Jay J.
Prough J. Robert
Alvo Steve
Andritz Inc.
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