Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – Continuous chemical treatment or continuous charging or...
Reexamination Certificate
1998-10-02
2001-08-21
Nguyen, Dean T. (Department: 1731)
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
Continuous chemical treatment or continuous charging or...
C162S025000, C162S037000, C162S041000
Reexamination Certificate
active
06277240
ABSTRACT:
BACKGROUND AND SUMMARY OF THE INVENTION
The pulping system described in U.S. Pat. Nos. 5,489,363; 5,536,366; 5,547,012; 5,575,890; 5,620,562; and 5,662,775 and marketed under the trademark LO-SOLIDS® pulping by Ahistrom Machinery Inc., of Glens Falls, N.Y., has revolutionized the art of chemical pulping. Among other things, the many manifestations of the LO-SOLIDS process has introduced a marked increase in the versatility of operating continuous pulping systems. However, in addition to the versatility that the LO-SOLIDS process provides for the pulping process, this process also introduces versatility to the cooking system and in particular to the vessels and other equipment associated with the pulping process.
It is well established that the counter-current flow of cooking liquors within a continuous digester vessel limits how much cellulose material can be processed in the vessel. In the treatment of comminuted cellulosic fibrous material in a vertical vessel, the driving force for the downward flow of material is gravity. The downward force of gravity is opposed by the buoyant force of the material in the liquid. The typically well-impregnated material is denser than the surrounding liquid and the material “sinks” in the vessel. However, the difference in density of the material and the surrounding liquor is not great; the difference in specific gravity between a typical fully-impregnated, gas-free wood chip and that of the surrounding liquid is only about 0.1 to 0.2. Thus any additional forces that tend to counter-act the force of gravity can interfere with the flow of material through the vessel.
One such force which opposes the downward flow is friction, specifically the friction imposed by the internal surfaces of the vessel. Screen assemblies through which liquid is removed from the material slurry provide a significant source of friction to the downward movement of the cellulose material. The typically lateral flow of liquid toward the annular screen assembly imposes a normal force on the screen assembly which locally increases the force of friction at the screen. If the friction force at the screen exceeds the downward force of the chips, the flow of chips can be hindered, or “hung up” on the screen assembly, with adverse consequences for uniformity of treatment.
Another force that acts upon the chip mass in a continuous digester (the chip mass is often referred to as the “chip column”) is the viscose force of liquid flowing upward or downward in the chip column. Downward, or co-current, flowing liquids aid the force of gravity and promote the downward flow of material. Upward, or counter-current, flowing liquids counter-act the force of gravity. In extreme cases, the force of the upwardly flowing liquid can impede the downward flow of material.
The upward force on the chip column due to the upward flowing liquid is dependent upon the upward flow velocity of the liquid. The higher the flow velocity, the higher the resistance to downward flow and the more the resilient chip column is compacted. This compaction further impedes the upward flow of liquid. The flow velocity is dependent upon the production rate or the amount of cellulose material passing through the vessel per unit time. As the production rate increases the flow required to maintain an upward counter-current displacement of liquid increases. Typically, the resistance and compaction imposed by the counter-current flow of liquid limits how much production can be passed through the vessel. A point can be reached where the resistance and compaction caused by the upward flow of liquid overcomes the downward force of gravity and the downward flow of material is impeded or stopped entirely.
In one embodiment of this invention, this limitation upon the production rate that can be passed through a treatment vessel is substantially eliminated by substantially eliminating or minimizing the need for the counter-current flow of liquors within the chip column. In another embodiment of this invention the size of a continuous digester vessel can be reduced by eliminating the need for one or more countercurrent treatments while still effecting the desired treatment of the pulp.
In conventional continuous digesters, the ratio of length, L, of a vessel to its diameter, D, typically varies between 7 and 9. However, using the present invention the design of continuous digesters is not limited to this conventional range of UD and vessels having larger or smaller L/D ratios may be used. This latitude in vessel dimension provides the designer much more flexibility in designing and constructing digester vessels specifically, and pulping systems in general.
The present invention can be used in situations where space is a limiting factor and a narrower, taller vessel is desired. Also, the present invention is advantageous when expanding the capacity of existing, typically older, narrower digesters where counter-current treatment is not possible or undesirable. The present invention is also advantageous when a wider vessel can be used, for example, when space is not a limiting factor. Again, according to the present invention, L/D ratios larger and smaller than are conventional can be used.
In addition, the present invention is also applicable to zones in a vessel that are production limited due to their geometry. Where the discussion above discusses the dimensions of a vessel as a whole, the present invention is also applicable to cylindrical treatment zones in the digester, for example cooking zones or washing zones, that are production limited or exhibit less than optimum treatment conditions due to their geometry. The production and treatment in these zones can be enhanced by introducing “cross-flow” treatment or by substituting cross-flow treatment for counter-current treatment.
Conventionally the control of temperature and chemical distribution throughout the height of a continuous digester requires that liquors be removed from the vessel by several screen assemblies, augmented with cooking or dilution liquor, and returned to the digester, typically after passing through a heat exchanger. Such typical liquor removal and recirculations are shown in published EP patent application 476,230 which discloses a process marketed by Ahlstrom Machinery under the name EMCC® cooking, or in the US patents identified above for LO-SOLIDS cooking. However, these liquor removal and re-introduction circulations typically require individual pumps and heat exchangers as shown in these patents and the published EP patent application. Typically, the increased process versatility provided by these processes also require additional equipment, in particular additional pumps and heat exchangers. However, the unique features of LO-SOLIDS pulping, in which multiple extractions of cooking liquor having a high concentration of dissolved organic material and replacement of the cooking liquor with fresh cooking liquor and liquor having a lower concentration of dissolved organic material, provides the opportunity of reducing the number of pumps, heat exchangers and other equipment, required to continuously produce chemical pulp.
This is especially true since the liquids introduced to the cooking circulations in the LO-SOLIDS process are relatively uniform in composition. Where in conventional processes the concentration of cooking chemical and the concentration of dissolved material introduced at multiple locations typically varies significantly, the concentration of liquids introduced at two or more locations using the LO-SOLIDS process may be fairly uniform. For example, the liquid introduced at two or more locations in the LO-SOLIDS process may typically have a dissolved solids concentration of 60-80 g/l and an effective alkali (EA) of 12-20 g/l (as NaOH). Thus, using a single pump and associated equipment to distribute these two or more streams of liquid becomes particularly viable when employing the LO-SOLIDS process.
One embodiment of this invention comprises or consists of a method and apparatus for treating comminuted cellulosic fibrous material in
Greenwood Brian F.
Marcoccia Bruno S.
Stromberg C. Bertil
Andritz-Ahlstrom Inc.
Nguyen Dean T.
Nixon & Vanderhye P.C.
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