Feeding of comminuted fibrous material to a pulping process

Details Feeding of comminuted fibrous material to a pulping process Feeding of comminuted fibrous material to a pulping process

1998-04-06

2001-03-13

Doerrler, William (Department: 3744)

Drying and gas or vapor contact with solids

Process

With fluid current conveying or suspension of treated material

C034S367000, C034S384000, C034S138000, C034S139000, C034S141000, C034S166000, C034S182000

Reexamination Certificate

active

06199299

ABSTRACT:

BACKGROUND AND SUMMARY OF THE INVENTION
U.S. Pat. Nos. 5,500,083; 5,617,975; and 5,628,873 (the disclosures of which are incorporated by reference herein) disclose assorted methods and devices for storing, treating, and discharging comminuted cellulosic fibrous material prior to treatment in a chemical pulping process. Such devices, typically chip bins, are marketed under the trademark DIAMONDBACK® by Ahistrom Machinery of Glens Falls, N.Y. The DIAMONDBACK® chips bins have been a remarkably successful innovation that has received widespread acceptance throughout the pulping industry. The DIAMONDBACK® bins are characterized by uniform movement and treatment without the need for mechanical agitation or vibration that is characteristic of the prior art bins and of the other offerings in the field.
In addition to the single-convergence DIAMONDBACK® chip bin technology disclosed in these patents, U.S. Pat. No. 5,617,975 also discloses an alternative geometry employing “chisel-type” convergences. In particular FIGS. 2 and 3 of U.S. Pat. No. 5,617,975 disclose a bin discharging arrangement which includes a substantially cylindrical bin having a discharge transition with gradually converging opposite side walls which converge to a substantially rectangular outlet. The substantially rectangular outlet includes a metering screw that transfers the material to a discharge. The chisel-type convergence device has also proven to be effective for uniformly treating and discharging material from the bin while insuring uniform movement of the material through the bin. Vessels having this geometry are marketed under the trademark CHISELBACK™ by Ahistrom Machinery.
Although, the CHISELBACK™ bin has also been proven to be effective for treating and handling wood chips, research has shown that certain improvements can be made to the CHISELBACK™ bin to enhance its performance. For example, due to the uniform movement of material through the upper cylindrical portion and the lower transition portion, it has been found that non-uniform agitation of the material in the outlet below the bin can affect the movement of the material above. Where the DIAMONDBACK® bin is typically not as sensitive to such non-uniformities (since the DIAMONDBACK® bin converges to a generally localized, circular or square-shaped discharge) the CHISELBACK™ bin, having a relatively elongated discharge, is more susceptible to non-uniformities. For example, when the metering screw of a CHISELBACK™ bin transfers material to one side of the bin the horizontal thrust of the screw produces a build up of material on one side of the bin which hampers the movement of material on that side. In addition, the proximity of a screw or other conveyor to the outlet of the bin transition can also hinder the uniform flow of material out of the discharge and thus affect the movement of the material above. These and other deficiencies of the chisel-type bin disclosed in U.S. Pat. No. 5,617,975 are addressed by the present invention
In accordance with the invention a method and apparatus are provided such that: the chips are substantially continuously and uniformly withdrawn from the “chisel”; the upward or lateral thrust of the discharge device into the chip column above is minimized, if not eliminated entirely; and the discharge of the metering device to the subsequent device/conduit is substantially uniform and continuous.
Substantially continuous and uniform withdrawal from the transition heretofore was not typically required when treating comminuted cellulosic fibrous material, nor was the reason for desiring such a uniform withdrawal recognized in this art. In applications dealing with the uniform column movement in digesters, it has only been recently recognized how critical it is to establish uniform flow and treatment throughout the height of a vessel transferring comminuted cellulosic fibrous material. It has recently been learned that this is particularly important in the lower portions of a vessel since conditions there can affect the movement throughout the height of the vessel. Vessels in which the treatment in the vessel is highly dependent upon this uniform movement, such as the vessels in which steam is introduced to heat and displace the air, or other gases or liquids are introduced to treat the material, are particularly sensitive to the effects of non-uniform chip column movement.
The prior art vessels have been characterized by non-uniform withdrawal, as evidenced by vibrating discharges in the chip bins and rotating agitators in the chip bins, impregnation vessels and digesters. Except for the DIAMONDBACK® bins, substantially uniform discharge was heretofore unknown.
According to an embodiment of the present invention, the chips are preferably continuously withdrawn so that little or no back-up of chips into the chisel transition occurs and the flow across the outlet of the chisel is continuous and as uniform as possible so that the treatment is as uniform and efficient as possible. For example, this substantially uniformity may be exemplified by a method and apparatus in which the retention time in the vessel does not vary more than +/−5 minutes for any individual volume of chips compared to any other individual volume, preferably +/−4 minutes, or less, for a throughput of at least 30 tons of chips per day (e.g. at least about 50 tons per day).
The minimization of upward thrust, or lateral thrust having an upward component is discussed below. Several ways of perfecting this are disclosed, including a height H of a third transition at least as great as the width of the open bottom of a second transition; an adjustable outlet width; and a “step-in” of the outlet to a width narrower than the transition conduit. These options may be used alone or in combination. For example, the lower transition having a height H may be unnecessary if the step-in or adjustable outlet is sufficient to ensure that little or no upward thrust is imposed on the chip column. However, there are many other ways that this could be perfected, in addition to a multi-star-type metering device and a downward-sloping screw. For example, the screw flights could be re-designed so that they are leaning or canted forward instead of backward (as is conventional). Having the flights leaning forward can impose a load on the chips having a horizontal and downward force component and little or no upward component.
Therefore, in one embodiment of the invention, the mechanism for metering chips from the vessel imposes little or no force on the downflowing chip mass that impedes the movement of material in the chip mass.
In addition, if horizontal or upward forces cannot be eliminated, it is preferred that the force be directed against a generally vertical wall of the transition or a wall that is tapering inward and not against a wall tapering outward. The generally vertical wall or a wall tapering inward is less likely to transfer a thrusting force upward into the chip column A wall that is tapering outward undesirably allows the force to be transferred upward along the taper and into the chip column and thus interferes with the column movement.
The continuous discharge is preferred to prevent the discharge of “slugs” of chips to the downstream conduit or device. There are also various ways of achieving this. One way is to modify a multi-chip meter, or other multi-star-type feeder, design so that the pockets of the feeder are offset, or out of phase, so that as the pockets of one star-type feeder are being filled the pockets of another feeder are emptying. Thus a fairly uniform discharge of the metering device is obtained instead of having a “slug” discharged that would occur if the star-feeders were not off-set, that is, synchronous. In addition, instead of the pockets being oriented parallel to the axis of the shaft, the pockets, or their paddles, may also be angled relative to the axis of rotation. The pockets of one angled rotor may also be off-set, or out of phase, with the pockets of the adjacent rotor.
In addition, in order to improve the uni

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