Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – Continuous chemical treatment or continuous charging or...
Reexamination Certificate
2000-12-01
2002-04-23
Nguyen, Dean T. (Department: 1731)
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
Continuous chemical treatment or continuous charging or...
C162S047000, C162S052000, C162S246000, C162S248000
Reexamination Certificate
active
06375795
ABSTRACT:
BACKGROUND AND SUMMARY OF THE INVENTION
The term “chemical pulping” applies to the process of treating comminuted cellulosic fibrous material, for example, hardwood or softwood chips, with an aqueous solution of chemicals which dissolve the non-cellulose components of the material, and some of the cellulose components, to produce a slurry of cellulose fibers that can be used to produce cellulose paper products. The commercially significant chemical pulping process in the late twentieth century is the alkaline process, a process more commonly referred to as the “kraft” process. In the kraft process, the active chemicals with which the wood is treated are sodium hydroxide [NaOH] and sodium sulfide [Na
2
S]. The aqueous solution of sodium hydroxide and sodium sulfide is referred to as “kraft white liquor”.
Kraft pulping is typically performed at a temperature of over 100° C., and the process is typically performed under superatmospheric pressure, preferably 5-10 bar, in a sealed pressure-resistant vessel known in the art as a digester. Typically, the cellulose material is sequentially raised to this treatment temperature and pressure, and cooking chemical is introduced to the material, in a series of steps that take place in what is known in the art as the “feed system”.
In the case of a continuous digester in which material is continuously introduced at one end and discharged at the other, the feed system typically comprises or consists of several vessels for heating the material, raising its pressure, and introducing cooking liquid. For instance, continuous cooking feed systems typically include some form of chip bin into which the comminuted cellulosic fibrous material, referred to hereafter as “wood chips”(the most common form), are first introduced. This chip bin typically includes some form of isolation device at its inlet to prevent the escape of gases from the bin. The bin may also include an exhaust outlet for releasing the gases that may accumulate in the bin. Typically, treatment of the chips begins in the chip bin when the chips are exposed to high temperature steam. The steam begins the heating process, but, more importantly, the steam displaces the air in the chips so that the air content of chips is minimized. This removal of air and other gases from the chips promotes the “sinking” of the chips during subsequent aqueous treatment.
After steaming in the chip bin, the de-aerated chips are discharged from the chip bin by some form of metering device, for example, a Chip Meter sold by Ahlstrom Machinery Inc., of Glens Falls, N.Y. or a metering screw or any other form of conventional metering device. After discharge from the chip bin and metering device, the pressure of the chip mass is increased from approximately atmospheric pressure to a pressure of about 18 psi. This is typically achieved by a pressure isolation device, for example, a Low Pressure Feeder [LPF] as sold by Ahistrom Machinery. The LPF is a device having a rotating star-type rotor within a stationary housing having an inlet and an outlet. Typically, as the rotor turns in the housing, chips drop through the inlet into the pockets of the rotor. As the rotor turns toward the outlet, the chips are exposed to a higher pressure and the chips fall through the outlet of the LPF to further treatment below. The clearance between the times of the rotor and the inside surface of the housing is closely toleranced so that the higher pressure typically below the LPF does not escape to the area of lower, atmospheric pressure above and around the LPF.
The LPF typically includes some form of steam purge to purge the rotor cavities of chips during and after the chips are discharged from the outlet of the feeder. This purge usually comprises or consists of low-pressure steam introduced to a port in the housing of the feeder. The LPF also typically includes some form of exhaust gas relief port to release any gases that may accumulate in the feeder such that these typically pressurized gases are not introduced to the inlet of the feeder where they can interfere with the flow of chips into the feeder or interfere with the flow of chips through the metering device or chip bin above.
In conventional feed systems, the LPF discharges chips to the pressurized atmosphere of another treatment vessel. Conventionally, this vessel typically performs a further treatment of the chips with steam under a pressure of about 18 psi. This conventional pressurized steaming typically removes any further air that may be present and also increases the temperature of the chips to about 120 ° C. prior to being immersed in cooking liquor. One preferred treatment vessel for performing this pressurized steam treatment is a Steaming Vessel as sold by Ahlstrom Machinery. The Steaming Vessel is most often a horizontally-oriented vessel having a cylindrical housing and horizontal screw conveyor. Steam is added to the housing through one or more ports typically located on the bottom of the housing. The source of this steam is typically flashed spent cooking liquor. That is, hot cooking liquor removed from the cooking process in the digester is expanded under controlled conditions by exposing the liquor to a pressure lower than its boiling point. In addition to generating steam from the flashed liquor, other volatile, typically malodorous, gases are also generated in the flashing process, such as hydrogen sulfide [H
2
S], methyl mercaptan [CH
3
SH], dimethyl sulfide [CH
3
SCH
3
], and dimethyl disulfide [CH
3
SSCH
3
], as well as other often malodorous gases. These gases, which are referred to collectively as Total Reduced Sulfur gases or TRS gases, are typically also introduced to the chips in the pressurized steaming process, typically in a Steaming Vessel.
Gases are also introduced to the Steaming Vessel from the outlet of the vessel which typically discharges to a vertical conduit or chute leading to a transfer device. For example, the outlet of the Steaming Vessel may discharge chips to a conduit leading to a star-type feeding device, for example, a High Pressure Feeder (HPF) sold by Ahlstrom Machinery, or to a slurry-type pump, for example, a LO-LEVEL® pump also sold by Ahlstrom Machinery. The conduits leading to these devices typically contain liquids containing sulfur compounds which also contribute TRS gases to the Steaming Vessel. Thus, the vessel below the LPF typically contains pressurized gases containing TRS compounds.
As a result, the outlet of the LPF typically is exposed to pressurized gases containing TRS compounds. These gases, if left unchecked, can be carried by the rotation of the LPF to the inlet of the LPF and released to the metering device and chip bin above. In addition, as discussed above, some LPF devices also include an exhaust port for discharging any accumulated gases from the LPF housing. Again, these TRS gases can typically be re-introduced upstream, for example, in the chip bin, and collected in the chip bin gas relief conduit. In conventional systems, this gas relief is directed to the Non-Condensable Gas (or NCG) collection system for destruction or re-use.
However, some pulp mills, typically older pulp mills, either do not have an NCG collection system or have an NCG collection system of limited capacity. Therefore, in such mills, it is undesirable to vent the TRS-laden gas streams in and around the LPF to the chip bin or to NCG treatment. In such systems, it is more desirable to re-introduce the TRS-laden streams to the feed system in a manner and form that does not allow the gases to escape to the atmosphere or be introduced to the NCG system. The present invention addresses this problem by removing the TRS-laden gases from the feed system and reintroducing these gases at a location downstream from where they were removed so that little or no TRS-laden gases are released to the atmosphere or must be treated or destroyed.
The broadest embodiment of this invention comprises or consists of a method and apparatus for minimizing the release of malodorou
Grace Todd S.
Lebel David J.
Andritz-Ahlstrom Inc.
Nguyen Dean T.
Nixon & Vanderhye P.C.
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