Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – Gas – vapor or mist contact
Reexamination Certificate
1998-11-17
2001-01-16
Chin, Peter (Department: 1731)
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
Gas, vapor or mist contact
C162S068000, C162S017000, C162S019000, C162S041000, C162S052000, C162S237000, C162S246000, C162S249000
Reexamination Certificate
active
06174411
ABSTRACT:
BACKGROUND AND SUMMARY OF THE INVENTION
In the art of continuous digesting of comminuted cellulosic fibrous material to produce cellulose pulp, from which paper products are made, there are essentially two types of digesters: the hydraulic digester and the dual-phase or vapor-phase digester. A hydraulic digester is a pressure-resistant vessel which is completely filled with comminuted cellulosic fibrous material and liquid; any introduction or removal of liquid from the vessel affects the typically super-atmospheric pressure within the vessel. A vapor-phase digester is not completely filled with liquid but includes a section at the top containing super-atmospheric steam. Since this gas zone is compressible compared to the liquid zone below it, the pressure within a vapor-phase digester is typically determined by the pressure of the gas present at the top of the digester. Prior art vapor-phase digesters are illustrated in U.S. Pat. Nos. 3,380,883; 3,429,773; 3,532,594; 3,578,554; and 3,802,956.
The reaction of pulping chemicals with comminuted cellulosic fibrous material to produce a chemical pulp requires temperatures ranging between 140-180° C. Since at atmospheric conditions 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 5 bars gauge (i.e., at least approximately 70 psi gauge).
One principal distinction between the method of operation of these two types of digesters is the way the contents of the digesters are heated to the desired 140-180° C. In the hydraulic digester, the slurry of comminuted cellulosic fibrous material, typically wood chips, and cooking liquor is typically heated by means of heated liquid circulations, i. e. one or more recirculation loops. Liquid is typically removed from the digester, for example, by using an annular screen assembly and pump, heated with steam by means of an indirect heat exchanger, and re-introduced to the material in the vessel using a centrally located pipe. In the vapor-phase digester, the chips are typically heated by exposing the chips to steam. This steam heating is typically performed as the chips are introduced to the steam-filled zone at the top of the digester.
In addition to the method of heating, the operation of the hydraulic and vapor-phase digester also differs in the methods used to monitor and control the level of chips and liquid in the vessel. Since the hydraulic digester is completely filled with liquid, only the level of chips need be monitored. The level of chips in a hydraulic digester is typically monitored using mechanical paddles, the deflection of which is detected by electronic strain gauges or similar devices. Typically two or more, preferably three or more, of these electro-mechanical devices are located on the inner surface of the hydraulic digester. The presence or absence of chips at the level of the paddle is determined by the degree of deflection or agitation of each paddle by the chips. The agitation of each paddle is detected by the strain gages and an approximate level of chips in the hydraulic digester, expressed as a percent, is determined via a mathematical algorithm. The operator can vary the chip level by varying the input of chips or output of pulp from the hydraulic digester.
In a vapor-phase digester, two levels must be monitored and controlled: the level of chips, similar to the hydraulic digester, and the level of the liquid. However, unlike the hydraulic digester, in a vapor-phase digester the chips are not submerged in liquid at the top of the digester. By the nature of the vapor-phase digester, which requires the direct exposure of chips to steam for heating, the chip level in a vapor-phase digester is above the level of the liquid. This unsubmerged (exposed) chip level is typically detected by a gamma-radiation emitter/detector device mounted on the side of the digester vessel. The liquid level in a vapor phase digester is detected by conventional liquid pressure detecting devices, for example a “dp cell”.
Furthermore, chips are introduced to the two types of digesters using different mechanical devices. Wood chips, or other comminuted cellulosic fibrous material, are typically fed to the inlet of a continuous digester using a separate feed system. The feed system typically includes equipment for de-aerating, heating, pressurizing, and introducing cooking liquor to the chips before transferring a slurry of chips and liquor to the digester. In the case of the hydraulic digester, this slurry of chips and liquor is introduced in a downward-directed screw-type conveyor, known in the art as a “top separator”. In the vapor-phase digester, since the slurry is introduced to a gas space, the slurry of chips and liquor is transferred upward in a screw-type conveyor in which the chips and liquor overflow the top of the conveyor and fall freely in the steam-filled atmosphere. This upward flow and overflow of chips and liquid is ideally suited to the vapor-phase digester because it prevents the escape of gas as the slurry is introduced to the digester while providing a weir-type reservoir for removing excess liquid. This device is known in the art as an “inverted top separator”. Both devices remove excess liquid from the slurry so that it can be returned to the feed system (e.g. conventional high pressure feeder) as a source of slurrying liquid. The functions of these devices are similar, but they have distinct applications to their respective type of digester.
Conventionally, the construction and operation of hydraulic and vaporphase digesters are also distinctly different. No one of ordinary skill in the art would consider operating one type of digester in the fashion of the other, at least without significant modification to the respective digester. For example, the vapor-phase digester does not typically have the same number of annular screens or liquor circulations required for heating in a hydraulic digester. Also, the hydraulic digester typically does not have a device for detecting the level of chips above the liquid level that a vapor-phase digester requires. Furthermore, the two types of top separators are different in construction and mode of operation.
There are several disadvantages to the vapor-phase digester in comparison to the hydraulic digester. For example, exposing wood chips to direct steam can be harmful to the chip fibers. The typically sudden increase in temperature of a chip due to exposure to steam can cause non-uniform treatment of the chip. For example, if the chip is not uniformly impregnated with cooking chemical, the increased temperature can cause non-uniform reaction of cooking chemical with the cellulose and non-cellulose components of the chip. This can be manifest in reduced pulp quality, for example, causing reduced paper strength, or in non-uniform delignification. The more uniform heating and treatment provided by the liquid-filled hydraulic digester is less prone to cause non-uniform treatment of the chip while submerged in a liquor.
The vapor-phase digester is also sensitive to variations in the relative chip and liquid levels. Since the principle means of heating the chips to cooking temperature in a vapor-phase digester is retention time in the steam atmosphere, any loss in this retention time means a loss in heating. Therefore, in a vapor phase digester, the chip level must always be maintained sufficiently above the liquid level to ensure proper heating. A loss of retention time in the steam atmosphere results in less heating of the chips which is manifest as increased uncooked chip particles, or “rejects” in the resulting pulp. For this reason, the operator of the vapor-phase digester must continuously monitor and regulate the level of liquid relative to the level of chips. This problem does not exist in a liquid-filled hydraulic digester which heats using liquid circulations.
Also, the chip pile above the liquid level in a vapor phase digester promotes a non-uniform pressure distribution and hence non-
Henricson Kaj O.
Laakso Richard
Stromberg C. Bertil
Andritz-Ahlstrom Inc.
Chin Peter
Fortuna Jos'e A.
Nixon & Vanderhye P.C.
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