Bleaching and dyeing; fluid treatment and chemical modification – Fluid treatment – Special forms and forming
Reexamination Certificate
1999-05-06
2001-08-14
Stinson, Frankie L. (Department: 1734)
Bleaching and dyeing; fluid treatment and chemical modification
Fluid treatment
Special forms and forming
C068S18100D, C068S0180FA, C162S060000, C162S251000
Reexamination Certificate
active
06272710
ABSTRACT:
BACKGROUND AND SUMMARY OF THE INVENTION
Diffusion washing of comminuted cellulosic fibrous material has been practiced since the 1960s in large cylindrical vessels containing reciprocating screen assemblies. One prominent design is built and marketed by Ahistrom Machinery Inc. of Glens Falls, N.Y. under the name Atmospheric Diffusion Washer. These screen assemblies, referred to as “diffusers”, typically comprise spaced concentric rings with perforated screen plates on the internal and external surfaces of the rings. Treatment liquids are typically distributed by rotating arms with upward or downward pointing distribution nozzles. Typical conventional designs are shown in U.S. Pat. Nos. 5,183,536 and 5,116,476.
This concentric ring design is not new, but was a characteristic of the very first diffuser designs. Examples of these early designs are shown n U.S. Pat. Nos. 3,348,390; 3,524,551; 3,563,891; and 3,760,948. Ostensibly, this circular ring design appears to be a preferred geometry for the diffusion of wash or bleaching liquids through a bed of medium consistency (i.e. about 8-15%) wood pulp. The circular rings provide not only an aesthetically pleasing, symmetric appearance but also appear to provide the optimum diffusion of treatment fluid through an upwardly flowing medium consistency bed of cellulosic material, e.g., wood pulp. The annular spaces between the rings form uniform pulp beds to which treatment medium can be applied and then extracted through the adjacent screens. The efficacy of this design has been confirmed by the hundreds of diffusers sold, and still being sold, since the 1970s. The Ahlstrom Atmospheric Diffuison washer is recognized today as one of the leading technologies in medium consistency pulp treatment.
However, regardless of the technological and commercial success of these devices there are some shortcomings of the concentric ring design that have shown this design to be less than optimum. For example, the concentric ring design produces a non-uniform resistance to the upward flow of pulp. The vertical screen plates of a typical diffuser design produce friction between the screen plates and the pulp flowing past it. This resistance to flow is directly proportional to the area of the screen over which the pulp flows. Since the area of screen to which each annular pulp bed is exposed increases as the diameter of each ring increases, the resistance to pulp flow is greater in the outer pulp annuli than in the inner annuli. In practice, this phenomenon is seen as a faster pulp flow in the inner annuli than in the outer annuli. This gradient in pulp velocity produces non-uniform treatment of pulp, or, in extreme cases, “channeling” of faster flowing pulp passed regions of slower moving pulp.
Another drawback of the present circular diffuser designs is the disruption of the pulp bed by the support structure of the rings. In all conventional diffuser designs the concentric rings are supported by some form of radial arms or beams which support the rings and provide part of the means for reciprocating the assembly. (Conventional diffusers are reciprocated to wipe the screen plates clean and prevent pluggage of the screen plates.) Several different non-circular configurations are illustrated in U.S. Pat. No. 4,276,167, but these designs all employ some form of radial or transverse support arms which traverse the pulp bed. However, these radial arms traverse the pulp flow path and disrupt the bed of pulp either entering or leaving the annulus between the screens. Not only do these arms interfere with the uniform flow of pulp, but the cracks and crevices produced also provide flow paths for treatment liquids. These flow paths can produce non-uniform concentration or “channeling”, of treatment liquids in the pulp bed. The combined effect of the disruption of the pulp bed and channeling of the treatment medium produce inefficiencies that are manifest as non-uniform treatment of pulp and increased chemical consumption.
The rotating liquid distribution arms and nozzles also disrupt the pulp bed and promote non-uniform treatment. Elimination of these elements can not only improve treatment efficiency but also eliminate the drive mechanism and assorted hardware associated with these distribution arms.
The present invention avoids all of these shortcomings of the concentric ring design and provides a much more efficient treatment of the pulp.
A new approach, primarily for an atmospheric diffuser for washing or bleaching cellulose pulp is proposed, although some aspects of the invention may also be applicable to pressure diffusers (at a pressure of more than about 2 atmospheres). The objective is to get high efficiency with no moving parts (in-line), a modular design for improved expandability, and with ready accessibility for maintenance.
As described in U.S. Pat. No. 5,183,536 conventional atmospheric diffusion washer design includes an inherent inefficiency, namely, the dirty “backflush” liquid is undesirably re-introduced to the pulp bed. The actual data from tests performed on conventional atmospheric diffusion washers to determine the extent of this inefficiency appear in Table 1 below. Table 1 contains the average Norden efficiency numbers, often expressed as “E
k
” numbers, for several atmospheric diffuser operating conditions. The Norden efficiency number provides a relative indication of the washing efficiency of a pulp washing device or stage of a device. The higher the E
k
number the higher the washing efficiency of the device or stage. The efficiencies in Table 1 are based upon the removal of both the dissolved sodium and dissolved wood solids from the pulp.
TABLE 1
AVERAGE NORDEN NUMBER
1st
2nd
(Sodium and Dissolved Solids)
OVERALL
STAGE
STAGE
With Backflush on Both Stages
7.7 ±
4.7 ±
3.0 ±
0..5
0.3
0.4
Without 1st Stage Backflush
8.5 ±
5.5 ±
3.1 ±
1.5
0.9
0.7
Without 1st and 2nd Stage Backflush
9.5 ±
6.4 ±
3.1 ±
1.4
1.4
0.2
In a conventional two-stage atmospheric diffusion washer, the stages comprise concentric annular screen assemblies mounted for reciprocation within a circular vessel. The pulp flow through these stages is vertically upward and the first stage is mounted beneath the second stage as shown in FIG. 1 of U.S. Pat. No. 5,203,045. During operation, wash water is introduced to the screen assemblies by rotating wash distribution nozzles and then extracted by the annular screens, for example, as clearly shown in FIG. 2 of U.S. Pat. No. 5,203,045. At the same time, the screen assembly is raised by hydraulic cylinders at the approximate speed of the upflowing pulp and then rapidly lowered, or “downstroked”. Prior to downstroke, some of the flow of extracted liquid is momentarily reversed, or “backflushed”, to dislodge the pulp bed from the screen prior to the downstroking. This very desirable, if not essential, displacement of the pulp bed from the screen assembly prior to downstroking releases the pulp bed from the screen such that the downstroking of the diffuser is neither hindered by the pulp bed, nor is the pulp bed disrupted by the stroking action of the diffuser. However, it is this backflushing of previously extracted liquid that is associated with reducing the washing efficiency of these devices. The data in Table 1 correspond to the overall efficiency and the individual efficiency of each stage of two-stage atmospheric diffusion washer.
The first line of data in Table 1 is associated with normal operation of the two-stage diffuser, that is, with backflushing during both the first and second stages, which is the baseline efficiency for this study. Though the overall efficiency for such a device corresponds to an average E
k
of 7.7, most of this efficiency is attributed to the first stage, E
k
of 4.7, and less to the second stage, E
k
of 3.0. The second line of data corresponds to operation of the two-stage device without backflushing in the first stage. As shown, the overall efficiency increases to 8.5; however, this increase largely occurs in the first stage which c
Stinson Frankie L.
Vanderhye Robert A.
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