Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – Charging and/or discharging fibrous material
BACKGROUND AND SUMMARY OF THE INVENTION
This invention relates to a method and system for feeding comminuted cellulosic fibrous material to a treatment vessel, such as a continuous digester. The invention simplifies and dramatically reduces the number of components needed when compared to the existing art.
U.S. Pat. Nos. 5,476,572, 5,622,598 and 5,635,025 and 5,766,418 introduced the first real breakthroughs in the art of feeding comminuted cellulosic fibrous material to a treatment vessel in over forty years. These patents and the application disclose several embodiments, collectively marketed under the trademark Lo-Level® feed system by Ahlstrom Machinery Inc. of Glens Falls, N.Y., for feeding a digester using a slurry pump, among other components. As described in these patents and application, using such a pump to feed a slurry to a high-pressure transfer device dramatically reduces the complexity and physical size of the system needed, and increases the ease of operability and maintainability. The prior art systems employing a high-pressure transfer device, for example a High-Pressure Feeder as sold by Ahlstrom Machinery Inc., but without such a pump, are essentially unchanged from the systems sold and built since the 1940s and 1950s.
The present invention relates to an even more dramatic improvement to the methods and systems disclosed in the above-mentioned patent and applications. The present invention actually eliminates the need for transfer devices, such as a High-Pressure Feeder, by using high-pressure pumping devices to transfer a slurry of comminuted cellulosic fibrous material directly to a digester.
The reaction of pulping chemicals with comminuted cellulosic fibrous material to produce a chemical pulp requires temperatures ranging between 140-180° C. Since the aqueous chemicals used to treat the material would boil at such temperatures, commercial chemical pulping is typically performed in a pressure-resistant vessel under pressures of at least about 10 bars gauge (approximately 150 psi gauge). In order to maintain this pressure, especially when performing a continuous pulping process, special accommodations must be made to ensure that the pressure is not lost when introducing material to the pressure vessel. In the prior art this was accommodated by what is known in the art as a “High-Pressure Feeder”. This feeder is a specially-designed device containing a pocketed rotor which acts as a means for transferring a slurry of material from a low pressure to a high pressure while also acting as a valve for preventing loss of pressure. This complicated and expensive device has long been recognized as an essential component for introducing slurries of comminuted cellulosic material to pressurized vessels, typically at elevated temperatures, especially to continuous digesters.
According to the invention a system which replaces the High-Pressure Feeder—which has been recognized for over forty years as being essential to continuous digesting—is provided, greatly simplifying construction of a pulp mill.
According to one aspect, a system for producing chemical cellulose pulp from comminuted fibrous cellulose material, such as wood chips, comprises the following components: A steaming vessel in which comminuted fibrous cellulose material is steamed to remove the air therefrom. A superatmospheric pressure vertical treatment vessel having an inlet for a slurry of comminuted cellulose fibrous material at a top portion thereof and an outlet at a bottom portion thereof. And, pressurizing transfer means for pressurizing a slurry of material from the steaming vessel and transferring it to the treatment vessel inlet, the pressurizing transfer means consisting of one or more high pressure slurry pumps located below the top portion of the treatment vessel.
The one or more pumps preferably comprises first and second high pressure slurry pumps connected in series and each having a pressure rating, an inlet and an outlet, the first pump inlet operatively connected to the steaming vessel, the first pump outlet operatively connected to the second pump inlet, and the second pump having a higher pressure rating than the first pump. The slurry pumps may be helical screw centrifugal pumps, double-piston solids pumps, or other similar conventional pumping devices that are capable of pressurizing a slurry having a relatively high percentage of solids to (in one or more stages) a pressure of at least about 5 bar gauge. The pressurizing and transferring may also be effected by an one or more eductors, of conventional construction, driven by a pressurized fluid supply, such as supplied by conventional centrifugal pump.
One typical unit of measure that indicates the relative amount of solids in a slurry containing solids and liquid is the “liquid-to-solids ratio”. In this application, this ratio is the ratio of the volume of liquid being transferred to the volume of cellulose, or wood, material being transferred. Typical conventional centrifugal liquid pumps are limited to pumping liquid having a solids content of at most 3%. This 3% solids content corresponds to a liquid-to-solids ratio of about 33. In the slurry pumps of this invention, the liquid-to-solids ratio of the slurry being pumped is typically between 2 and 10, preferably between 3 and 7, and most preferably between 3 and 6. In other words, the slurry pumps of this invention transfer slurries having a much greater solids content than can be handled by a conventional pump.
A liquid return line may be provided from the top portion of the treatment vessel, containing liquid separated from the slurry at the top of the treatment vessel (preferably a continuous digester). The return line may be operatively connected to an inlet or outlet of one of the slurry pumps, either directly or indirectly. Preferably the liquid return line is connected to a pressure reduction means for reducing the pressure of liquid in the return line before the liquid passes to the inlet or outlet of the slurry pump. The pressure reduction means may take a variety of forms, such as a flash tank and/or a pressure control valve in the return line, or other conventional structures for effectively reducing the pressure of liquid in a line while not adversely affecting the liquid. Where a flash tank is utilized the liquid outlet from the flash tank is connected to the inlet to the first slurry pump, and the steam produced by the flash tank may be used in the steaming vessel.
Alternatively, the pressure reduction may be effected, or even avoided, by using an eductor which uses the pressurized return line liquor as its source of pressurized fluid. An eductor may be used in place of or in conjunction with one or more of the slurry pumps, or other devices, to transfer slurry to the digester.
A conventional chute, as well as other optional components, is preferably connected between the steaming vessel and the at least one slurry pump, the steaming vessel being located above the chute and the chute above the at least one slurry pump. The at least one slurry pump is typically located a distance at least 30 feet (about 10 meters) below the top of the digester, and typically more than about 50 feet (about 15 meters) below.
When the high pressure transfer device is eliminated it is desirable to utilize other mechanisms to retain one of the functions of the high pressure transfer device, namely providing pressure relief prevention should an aberrant condition occur, the high pressure transfer device typically preventing backflow of liquid from the digester into the feed system. Pressure relief preventing means according to the present invention are preferably distinct from the at least one slurry pump, although under some circumstances the inlets to or outlets from the slurry pumps may be constructed in a manner so as to provide pressure relief prevention. The pressure relief preventing means may comprise an automatic isolation valve in each of the slurry conduits transferring slurry from the pumps to the top of the treatment vessel and the return line from the treatment vessel, a
Pease Tim S.
Prough J. Robert
Stromberg C. Bertil
Fiorilla Christopher A.
Nixon & Vanderhye P.C.
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