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Plate diffuser for treating comminuted cellulosic fibrous material
| Details |
Inventors: Prough, James R.;
Assignee:
Primary Examiner: Stinson; Frankie L.
Assistant Examiner:
Attorney, Agent or Firm: Vanderhye; Robert
The problems of non-uniform treatment of paper pulp that may be caused by differential flow velocities within a vessel, channeling, and/or disruption of the pulp flow, are substantially eliminated by providing a non-circular cross-section substantially atmospheric pressure vessel with at least one screen surface, and typically a liquid introducing device. The vessel, screen surface, and liquid introducing device are constructed and positioned with respect to each other so that during the treatment of a pulp slurry the slurry has substantially uniform width in a treatment zone, and there is substantially uniform resistance to the flow of slurry over the screen surface over any particular cross-section of the vessel in the treatment zone, and its consistency varies less than 6% in the treatment zone (e.g. between 8-14%). The screen surface may comprise substantially vertical screening portions vertically spaced by non-screening portions, and the vessel wall may bulge out at the location of the non-screening portions so as to provide a slurry width at the bulges at least five percent wider than at the screening portions. The vessel may have a substantially race track shape in cross-section at the treatment zone, and the slurry width may form an annulus in the vessel, the annulus being substantially uninterrupted in the treatment zone (there being no arms or like structures which disrupt the flow). The screen surface may be rotated 5-30.degree., or oscillated or vibrated horizontally (transverse to pulp movement) to minimize plugging. |
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DETAILED DESCRIPTION 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 Ahlstrom 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 in 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 Diffusion 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
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