In-line drainer enhancements

Details In-line drainer enhancements In-line drainer enhancements

2000-05-18

2002-09-17

Griffin, Steven P. (Department: 1731)

Paper making and fiber liberation

Apparatus

Digester

C162S043000, C162S251000, C210S498000

Reexamination Certificate

active

06451172

ABSTRACT:

BACKGROUND AND SUMMARY OF THE INVENTION
U.S. Pat. Nos. 5,476,572; 5,622,598; 5,635,025; 5,766,418; and 5,968,314 disclose methods and devices for feeding a slurry of comminuted cellulosic fibrous material to a treatment vessel that have revolutionized the art of treating comminuted cellulosic fibrous material to produce cellulose pulp. The disclosed inventions, sold under the trademark LO-LEVEL® by Ahlstrom Machinery Inc., of Glens Falls, N.Y. employ one or more slurry-type pumps for treating and transferring comminuted cellulosic material to one or more treatment vessels. Not since the initial development of the continuous cooking process in the 1940s and 1950s have such dramatic improvements been made to equipment used to transfer material to a treatment vessel, for example, a continuous or batch digester. This is confirmed by the broad acceptance of this technology by the Pulping Industry.
The present invention introduces improvements to the systems and methods described in the above patents which further simplify and enhance the effectiveness of the methods and devices disclosed therein.
The prior art systems for introducing a slurry of comminuted cellulosic fibrous material, for example, as exemplified by the system disclosed in U.S. Pat. No. 5,476,572, use a two-stage pressurization and transfer of slurry. In the first stage, the slurry is pressurized to a first pressure and transferred to a high-pressure transfer device, such as, a High Pressure feeder designed and marketed by Ahlstrom Machinery. The first stage pressurization and transfer is typically performed using a specially-designed slurry pump which handles slurries of material and liquid. In the second stage the High Pressure Feeder pressurizes the slurry to a second pressure, higher than the first pressure, by exposing the material to a high pressure liquid stream, and transports the slurry to a treatment vessel, for example, a continuous or batch cellulose pulp digester. However, according to this prior art, the amount of cellulose material, such as, wood chips, that can be transferred to the High Pressure Feeder by the slurry pump, per unit volume of liquid, is limited by the capacity of the pump to transfer solid material.
Typically, the relative amount of liquid present in slurry is indicated by a “liquid-to-solids” ratio, or, in the case of transferring slurries of wood chips, a “liquid-to-chip” ratio, or, more specifically, a “liquor-to-wood” (L/W) ratio. The liquid-to-wood ratio is a dimensionless ratio of the volume of the liquid present in the slurry to the volume of the wood present in the slurry. Conventional High Pressure Feeders can accept slurries having L/W of below 3.0:1, typically even below 2.5:1. The lower limit of the L/W ratio of a slurry being introduced to a High Pressure Feeder is about 2.0:1. Note that a reduction in L/W ratio from 3.0:1 to 2.0:1 corresponds to a 25% reduction in the volume of liquid that must be accepted by the High Pressure Feeder, or a corresponding 25% increase in the volume of chips that can be processed by the High Pressure Feeder.
Desirably the volume of liquid that is transferred to the High-pressure Feeder is reduced so that more wood chips can be introduced and processed in the digester system being fed per revolution of the High Pressure Feeder. This has the further advantage of allowing for the reduction in size of the High-pressure Feeder for a given project, or allowing for an increase in the capacity of a production-limited facility.
After introducing the slurry of chips to a high-pressure transfer device, for example, a High-Pressure Feeder sold by Ahlstrom Machinery, the slurry is displaced from the feeder by a flow of high-pressure liquid, typically at a pressure between about 5 and 15 bar gage, provided by a high-pressure pump. Typically this flushing of the slurry from the feeder by the liquid results in the slurry being propelled to a treatment vessel having a L/W ratio of between about 4.0:1 to 10.0:1, and is typically greater than 5:1, often greater than 7:1, sometimes greater than 9:1. For example, for a L/W ratio of 9:1, the volume of liquid present in the conduit transferring the slurry from the feeder to the treatment vessel, for example, to a pulping digester, the volume of liquid is 9 times the volume of the cellulose material, such as, wood chips. Typically, this volume of liquid is required in order to flush the chips from the pockets of the feeder. This relatively large volume of liquid requires a relatively large conduit in which to pass the slurry from the feeder to the digester and sufficient energy to propel the relatively large volume of liquid up to the top of the pressurized digester.
The L/W ratio of the slurry exiting the High Presser Feeder is also a function of the equipment which feeds the slurry to the feeder. In conventional, “suck through” systems typically having a pressurized chip chute the L/W ratio of the slurry introduced to the High Pressure Feeder is about 2.0-2.5:1. In “pump through” systems, such as Lo-Level Feed Systems sold by Ahlstrom Machinery, the L/W ratio of the slurry introduced to the High Pressure Feeder is about 3.0-3.5:1.
Desirably the liquid volume in the slurry transferred from the feeder to the treatment vessel is minimized by removing at least some of the liquid from the slurry after the slurry has been discharged from the feeder and before the slurry is introduced to the treatment vessel. One advantage of this embodiment of the invention is that, with reduced liquid volume, the diameter of the transfer conduit to the treatment vessel can be reduced. Reducing the size of this conduit has the further advantage of reducing the sizes, and hence the cost, of the associated valves and instruments that are located in this conduit.
The above embodiment is particularly effective in limiting the amount of heat returned to the feed system from the treatment vessel, for example, via what is known as the “Top Circulation” or “TC” line. As recognized in the art, exposing the feed system, for example, the High-pressure Feeder, to liquids having temperatures at or above 100° C. can cause flash-evaporation of this liquid (known as “flashing”) when the liquids are exposed to the atmospheric pressures present in the vicinity of the high-pressure feeder. However, when excess liquid is removed from the slurry when introducing the slurry to the treatment vessel, for example, by using of a Top Separator, heat present in the treatment vessel can migrate, for example, by convection, to the vicinity of the Top Separator and be drawn out of the vessel with the removal of liquid from the Top Separator. This heat can raise the temperature of the liquid returned to the feed system via the TC line. This increased TC line temperature can cause flashing and vibration in the feed system and interfere with the normal operation of the feed system.
One way of reducing the potential of returning undesirable heat to the feed system is by limiting the flow of liquid removed from the slurry as the slurry is introduced to the treatment vessel. That is, a liquor removal device is located in the conduit which feeds the slurry to the treatment vessel, preferably, near to or adjacent the inlet of the treatment vessel. At least some liquid is removed from the slurry using this device and returned to the feed system such that less liquid needs to be removed from the slurry as the slurry is introduced to the vessel. This reduced removal of liquid from the vessel reduces the potential for heat in the vessel to be withdrawn with the removed liquor and returned to the feed system.
One liquid separating device that is novel according to the invention, and that is particularly useful in the system and practice of the method of the present invention, is a cylindrical device having a cylindrical screen through which the slurry passes and from which liquid is removed, for example, an In-line Drainer, as sold by Ahlstrom Machinery Inc. of Glens Falls, N.Y. An In-line Drainer is typically used to isolate a stream of liquid from a stream of liquid that typ

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