Title:
Continuous digester with strainer valve cycling
United States Patent 3881985


Abstract:
Methods for the continuous pulping of a compacted mass of woods chips in an upright elonated reaction vessel having a wood chip infeed for feeding wood chips and liquid substantially continuously into the lower inlet end of the vessel and discharging wood chips substantially continuously from the upper end of the vessel, the mass of wood chips being advanced upwardly through the vessel, for treatment in liquid submergence, by generating liquid propelling pulses by utilizing a recirculation system connected between an intermediate strainer and the wood chip infeed and having a valve and a cycling timer for operating it, such propelling pulses being effective to cause the rate of movement of the submergence liquid relative to the wood chips in the mass to vary cyclically to produce a cyclic advancing movement of the liquid effective to advance the wood chips throughout the length of the column from the input to the output end.



Inventors:
Simmons, Frederick William (Amherst, NH)
Vogel, Karl Edwin (Hollis, NH)
Application Number:
05/420669
Publication Date:
05/06/1975
Filing Date:
11/30/1973
Assignee:
IMPROVED MACHINERY, INC.
Primary Class:
Other Classes:
162/41, 162/243
International Classes:
D21C3/24; (IPC1-7): D21C3/24
Field of Search:
162/237,238,243,248,251,41,42,17,19
View Patent Images:



Primary Examiner:
Bashore, Leon S.
Assistant Examiner:
D'andrea Jr., Alfred
Parent Case Data:


This application is a continuation-in-part of our application Ser. No. 196,391 filed Nov. 8, 1971; which was a continuation of our application Ser. No. 37,436 filed May 14, 1970; which in turn was a continuation of our application Ser. No. 641,935 filed May 29, 1967, all of which are now abandoned.
Claims:
What is claimed is

1. In a process for the continuous pulping of a mass of wood chips in an upright elongated reaction vessel

Description:
Its invention relates to the continuous digesting of wood chips in an upright elongated digester vessel, and more particularly to novel methods for continuously advancing a compacted mass of wood chips throughout the length of such a vessel by generating liquid propelling pulses through the cycling of a valve connected between an intermediate strainer and the wood chip-liquid inlet to the vessel.

Upflow digesters of the type shown and described in U.S. Pat. No. 2,878,116 and Pulp and Paper, "New Continuous Digester" of September, 1959 have proved to be highly successful because of their thermal stability and their unique ability to provide chip drainage, immediately preceded, if desired, by high temperature countercurrent washing, as compared with downflow systems. A short stroke bottom piston is utilized in these digesters to provide the force both for creating and maintaining the lower end of the compacted mass of wood chips and advancing the entire mass upwardly throughout the length of the digester in spite of the presence of multiple strainers and countercurrent flow and drainage zones and the like.

More recently, as described in U.S. Pat. No. 3,700,548, issued Oct. 24, 1972, the compaction of wood chips on the lower end of the chip mass and the advancing of the entire mass upwardly through the digester has been accomplished by advancing liquid upwardly through the lower portion only of the chip mass at a liquid flow rate effective to create a force which will propel upwardly the entire chip mass extending throughout the length of the digester, preferably by providing liquid propelling pulses as produced by a liquid propelling pulse generating means effective to cause the rate of movement of the submergence liquid relatively to the wood chips in at least a portion of said mass to vary cyclically. These liquid propelling pulses provide an average net propelling pulse force acting in the submerged chip mass which is highly effective to advance the wood chips throughout the length of the wood chip column from the input to the output end thereof. The liquid flow also may serve to position the lower end of the chip mass so that the level of compacted chips deposited thereon may be kept uniform.

A unique advantage of upflow digesters is their capability of providing a drainage zone at the top, for draining spent liquor from the cooked chips prior to further treatment, in order to increase the efficiency of chemical recovery. However, the substantial weight of the drained chips in such a drainage zone opposes the advance of the chip mass lower in the digester.

Too, in multiple zone digesters, a number of strainers are provided around the perimeter of the digester at various locations therealong for withdrawing liquor in order to accomplish heating, treating or washing of the chip mass in the various zones. These also have the adverse effect of retarding the advance of the compacted chip mass, as chips tend to cling to the strainer surfaces because of the pressure of the liquor being withdrawn.

Furthermore, the countercurrent flow zones, used to advantage in such multiple zone digesters for carrying out a number of sequential process steps such as heating and washing, produce retarding forces on the advancing chip mass.

Because of these factors, the forces which must be created, in order to oppose the resistance within the vessel, must be quite high if the chip mass is to be advanced. For this reason, the short stroke bottom piston heretofore utilized in commercial upflow digesters has been relatively massive and hence a major factor in their cost and mechanical complication, while in the case of liquid propulsion in the absence of pulses, high flow rates and hence large pumps and strainers have been needed. In either of these systems, the application of the high forces applied at the lower end of the chip mass has tended to compact portions of the chip mass thereabove to an undue degree which caused undesirable variations in the pulping reaction. Although the propelling pulse system of U.S. Pat. No. 3,700,548 is not subject to these disadvantages, the specific pulse generating means thereof requires, for feeding chips into the digester, the use of a constant displacement positive sealing pump of the type shown and described, for example, in U.S. Pat. No. 2,908,226, which pump is of relatively high cost and is subject to wear problems not present in much less expensive types of pumps, such as centrifugal vane pumps, for example.

Accordingly, it is a major object of the present invention to provide novel methods for propelling the compacted mass of wood chips throughout the length of the digester vessel while maintaining a much more uniform compaction of the chips in the mass than has heretofore been possible.

In general, such is accomplished in a process for the continuous pulping of a mass of wood chips in an upright elongated reaction vessel having an entrance end and an exit end, wood chip-liquid infeed means including a pump adjacent the entrance end of the vessel for feeding wood chips and liquid into the entrance end of the vessel and onto the entrance end of the mass, wood chip discharge means adjacent the exit end of the vessel for discharging the wood chips, propelling liquid strainer means including a strainer valve, and liquid recirculating means connected between the strainer valve and the infeed means, by feeding substantially continuously the wood chips and a liquid into the reaction vessel through the infeed means to maintain a columnar mass of compacted wood chips therein, advancing the wood chip mass to move it throughout the length of the vessel, meanwhile treating the same in liquid submergence, and substantially continuously discharging the wood chips from the output end of the column at the discharge means, and, according to the invention, cyclically operating the strainer valve to generate liquid propelling pulses, wherein the valve cyclically opens to recirculate liquid from the strainer means to the infeed means, and alternately cyclically closes to cause liquid propelling pulses to be generated throughout the submerged mass of wood chips to cause the rate of movement of the submergence liquid relative to the wood chips in the mass to vary cyclically to produce a cyclic advancing movement of the liquid effective to advance the wood chips throughout the length of the column from the infeed to the discharge means.

For the purpose of fully explaining the invention, reference is now made to the following detailed description of a preferred embodiment thereof, together with the accompanying drawing showing a diagrammatic side elevation, partly in section, of apparatus useful in practicing the methods of the invention.

Referring to the drawing, the pressurized vessel 12 therein shown is generally cylindrical except for its lower tapered bottom end portion 13 and may be, for example, of the order of 10 feet or more in diameter and 100 feet in length. Preferably, vessel 12, although of generally uniform cross-sectional area above its tapered bottom end portion 13, has a slight taper to provide a gradually upwardly increasing cross-sectional area, a taper of about three-eighths inch of diameter per foot of length being suitable. A wood chip-liquid infeed is provided at the lower entrance end of vessel 12 in the form of a pipe 15 having a tangential inlet 16 to which is fed, by means of a motor 17 driving a pump 18, preferably of the recessed vane centrifugal type of U.S. Pat. No. 2,635,548, a wood chip-liquid mixture from a supply tank 20. At the upper end of the vessel 12 is provided an inlet line 14 for supplying steam under pressure from any suitable source. There is also provided a discharger for the pulped wood chips in the form of a scraper 22 which establishes a predetermined upper level of the wood chip mass by discharging drained pulped wood chips through discharge outlet 24 to a discharge mechanism (not shown) such as is described in U.S. Pat. No. 3,206,356.

Within vessel 12, generally along the vertical axis thereof, are provided a series of three concentric injection pipes 41, 43 and 53 having outlets 42, 44 and 54, respectively, at successively higher levels. Also within vessel 12 are provided strainers 46 and 48, with their valves 52 and 51, respectively, together with said outlets establishing a plurality of zones within the vessel by advancing liquid for either co-current or countercurrent flow within said zones as desired. For example, as shown in the drawing, there is provided a lowermost chip compaction zone with a superposed impregnation and heating zone followed by a heating and cooking zone with co-current and coutercurrent flow and a countercurrent washing zone with an uppermost drainage zone, the liquid flows being shown by the single arrows and the wood chip movement by the double arrows. A liquid level controller 50 is provided near the upper end of vessel 12 to establish the liquid level below scraper 22 and the upper level of the wood chip mass as by operating valve 52 of upper strainer to provide the chip drainage zone.

A chip mass control system is provided for controlling the position of the lower end of the chip mass normally spaced above the lower end of the vessel for compacting chips thereon continuously to establish and maintain said lower end at a desired level, in order that the jamming of wood chips within inlet pipe 15 may be avoided. The apparatus components of such system include a lower liquid strainer 30 spaced a substantial distance preferably of about 1/2 to 11/2 times the vessel cross-section diameter above the inlet pipe 16 and below scraper 22 defining a chip compaction zone therebelow and a connecting pipe 34 and 38 and valves 69 and 75, the functions of which are hereinafter described, for controlled recirculation of liquid through supply tank 20 and pump 18 to inlet 16.

The lower tapered vessel portion 13 of upwardly increasing cross-sectional area is utilized, in conjunction with the upward flow of propelling liquid, normally to maintain the lower end L of the columnar mass of wood chips within the tapered vessel portion, suitably positioned below the propelling liquid strainer 30 and above vessel inlet 16. This is made possible both because of the rapid pressure change with distance which occurs as a result of resistance to fluid flow within the compacted chip mass, as contrasted to the lack of such pressure change in the liquid filled column below the chip mass, and the velocity change with distance which occurs throughout the tapered vessel portion 13, such being greatest at liquid inlet 16 and progressively decreasing upwardly thereform as the vessel cross section increases. A pair of sensors 62, 64 may be provided near the lower end of vessel 12 to control the position of the lower end of the chip mass through controller 68 as by operating valve 69 to vary the flow of recirculating liquid and hence the rate of movement of the wood chip mass.

In accordance with the present invention, novel liquid pulse generating means are provided for causing the rate of movement of the submergence liquid relatively to the wood chips in the mass to vary cyclically for propelling the entire columnar chip mass through the vessel. Specifically, such means includes the wood chip-liquid infeed means includng supply tank 20, pump 18 and pipe 15 with inlet 16 adjacent the entrance end of vessel 12, propelling liquid strainer 30 positioned in a wall of vessel 12 spaced between inlet 16 and the discharge means, for removal of liquid from vessel 12, strainer valve 75 operated by cycle timer 77 and liquid recirculating means including pipes 34 and 38 connected between the strainer valve and tank 20 of the infeed means, so that removal of recirculation liquid from strainer 30 to supply tank 20 is cyclically interrupted. The variations in recirculating liquid are absorbed at supply tank 20, the level of which varies throughout the distance Δh, but such variations are small enough so that the flow of liquid and chips through pump 18 to vessel inlet 16 is not significantly affected. Nor does the change ΔH in the upper level in vessel 12 seriously affect the liquid flow below strainer 30, although upward flow of liquid occurs through the wood chip mass above strainer 30 during the portion of the cycle in which valve 75 is closed.

It is a particular feature of the invention that it makes possible the use, as wood chip-liquid infeed pump 18, of a centrifugal vortex pump, characterized by a relatively small pressure change over a wide range of flow rates, to the extent that the pump outlet pressure is nearly constant from zero flow up to the maximum flow rate of the pump. In addition, such a pump is uniquely suitable for pumping a mixture such as wood chips and water, and is much less costly than the so-called thick stock pumps heretofore used for the purpose. Nevertheless, such pumps, as well as conventional centrifugal pumps, are useful with the system of the invention.

It has been established, in the commercial operation of digesters of the type described in U.S. Pat. No. 2,878,116, that the wood chips are present therein in the form of a columnar mass of contacting and compacted but discrete wood chips through which liquid can flow more or less independently of the wood chips in the mass, but subject to a flow resistance due to the presence of the wood chips. Because the wood chips are present in the mass in discrete form but in contact with one another, the wood chips within the mass can be moved throughout the length of the vessel solely by pushing on the bottom end surface of the mass within the vessel regardless of the direction of liquid flow. Such operation in fact occurs in digesters of the type described in said U.S. Pat. Nos. 2,878,116 and 3,061,007, the latter describing a digester in which the top portion of the chip mass extends out of submergence into a drainage zone above the surface of the liquid in the vessel, so that the weight of such portion is completely unsupported by the liquid. With such digesters, however, the degree of wood chip compaction tends not to be uniform throughout the length of the chip column, so that the wood chip digestion is not as uniform as is desired. Furthermore, the foraminous reciprocating piston tends to be unduly massive.

As disclosed in U.S. Pat. No. 3,700,548, the entire compacted chip mass may also be advanced uniformly by subjecting it, preferably throughout its entire submerged length, to cyclical liquid advancing pulses created in the submergence liquid. By so doing, non-uniform chip compaction throughout the length of the chip column is eliminated. Furthermore, since the forces needed at the lower inlet end of the wood chip column for carrying out chip accretion and compaction thereon are relatively small, such accretion and compaction may be accomplished either by the mechanical piston of said Patents, but with much less massive apparatus, or with the propulsion liquid approach of U.S. Pat. No. 3,700,548, but with much lower flow rates and propulsion column lengths. In a downflow system, the action of gravity will serve to form a compacted chip mass.

Furthermore, by applying the principle of tapering the lower portion 13 of vessel 12, suitably high fluid flow velocities are readily created in the tapered liquid column at the inlet end of the vessel so that the free wood chips are lifted away from the lower end thereof, yet almost any desired low velocity may be created and maintained within the tapered columnar chip mass for propelling it. In addition, the use of a tapered lower vessel portion provides self-regulating control of the lower end of the chip mass to an adequately uniform predetermined level within tapered vessel portion 13. This apparently occurs because of the large pressure changes which occur within the lower end of the tapered columnar wood chip mass as a result of increasing liquid velocities caused by decreasing vessel cross section area. Thus, as chips accrete on the bottom of the chip mass, its level L will move downwardly. The added increment will create increasingly higher propelling forces due to the higher liquid velocities through it as the vessel cross section decreases. This has been found to result, in a fairly short time interval, of the entire chip mass moving upward, whereupon the process repeats itself. Self-regulating level control of the lower end L of the chip mass may be thus achieved.

Both the taper and the length of the propelling zone in relation to the diameter or other transverse dimension of the chip mass is important in maintaining flow uniformity and in preventing channelling, that is, the opening of large liquid flow channels through the chip mass as tends to occur between closely spaced fluid inlets and outlets. It is preferred that the propelling zone be of at least the same order or somewhat greater than that of the transverse dimension of the chip mass, but certainly not less than about one-half of said dimension, as with short propelling zones, the necessary liquid flow velocity tends to become unduly high. It is also preferred that the vessel taper to a cross-sectional area of at most about one half to one third that of its maximum cross-sectional area at the upper end of the propelling zone at propelling liquid strainer 30.

In general, the use of liquid advancing pulses must be effective to cause the rate of movement of the submergence liquid relatively to the wood chips in the mass to vary cyclically between an advancing movement of the liquid at a rate at least not less, and preferably greater, than that of the alternating intervening movement of the submergence liquid. In fact, as hereinafter more fully explained, it is desirable that such advancing liquid pulse flow take place in turbulent flow, with the intervening pulse in laminar flow. However, in considering the precise nature of the pulses, the effect of the vessel configuration on the resistance to movement of the compacted chip mass therein must be taken into consideration, and the most important of these considerations has to do with the amount and direction of taper of the vessel. Thus, neglecting the effects of strainers, continuous liquid flows, length of chip column out of submergence, and the like, an untapered vessel will have equal resistance to movement of the chip mass therein in either direction, whereas a tapered vessel will have a lower resistance to such movement in the direction of increasing cross-sectional area than in the opposite direction. In the case of a vessel in which the vessel cross-section increases in the direction of advance of the wood chip mass, it does not appear to be essential in theory that the advancing pulse produce a greater speed of liquid movement than the intervening pulse, although it is desirable that it be at least as great and preferably greater since the amount of vessel taper is preferably limited in order that a preferred amount of chip compaction occur, which we have found does not occur if the taper much exceed about one inch in vessel cross-section per foot of length. In an untapered vessel, it appears to be essential that the advancing pulse produce a much greater rate of liquid movement than that provided by the intervening pulse.

The use of an asymmetric pulse has the further advantage of exploiting the difference between laminar and turbulent liquid flow for producing the advancing force within the chip mass. This result is made possible because, as is known from hydraulic theory, the rate of fluid flow resistance increases much more rapidly with increased flow velocity within the turbulent flow region than it does within the laminar flow region. Thus, within the region of laminar flow, the resistance to flow is directly proportional to the flow velocity, whereas within the turbulent flow region it increases as the power n of the flow, where n is greater than unity. The resistance to flow created by the presence of the columnar mass of chips, in which the liquid pulse of the present invention acts, can be determined by measuring the difference Δ P between the fluid Pressure P1 at the bottom of the chip column and the fluid pressure P2 at the top of the chip column. With fluid flow increasing from zero, the resistance Δ P will rise proportionally throughout the laminar flow region, and then rather abruptly change in a short transition region to a much more rapid rise as the flow enters the turbulent region. In this way, the short of the turbulent flow region for a specific chip mass can readily be determined so that a fluid flow well within it, that is beyond the transition region, can readily be selected. Since, as stated in said U.S. Pat. No. 3,700,548, turbulent flow occurs within a compacted chip mass at far lower flow rates than had heretofore been assumed, liquid propulsion of the entire columnar chip mass can be achieved using pulsed liquid flow within the chip mass at flow rates not too high to be provided by conventional means, yet high enough to be within the turbulent flow region. These flows create suitably high flow resistance within the columnar chip mass to move it as desired so that the compacted wood chips can be advanced upwardly throughout the entire length of the columnar mass maintained within the vessel. The propelling force so created is sufficiently high to move the chip mass even if there be countercurrent liquid flow throughout portions of the wood chip mass and a wood chip drainage zone at the upper end of said mass. The ability to provide such a highly desirable multiple zone upflow system with its inherent economics in heating, chemical retention and chip drainage is a particularly important feature.

The wood chip advancing force is generally proportional to the difference between the advancing pulse liquid flow rate (raised to the power n) and the intervening liquid flow rate (raised to the power n). This difference will be pronounced if only the advancing pulse be in turbulent flow, where n is greater than about 1.5, usually within the range of about 1.5 to 2.5, as is preferred. Although the actual difference depends greatly on the porosity of the chip mass and so varies depending on the chip size, compressibility of the mass and the amount of sawdust or silvers included, the value needed for a particular digester can be determined from the characteristics and dimensions of the chip mass treatment zones maintained therein, the condition of such zones as to non-submergence or submergence, and the forces created because of factors such as the presence of strainers and fluid flow forces co-current or countercurrent.

To operate the digester of the drawing according to the present invention, it is simply necessary to provide fluid pulses by cyclically operating strainer valve 75 to generate liquid propelling pulses, wherein said valve cyclically opens to recirculate liquid from strainer 30 to the infeed means, and alternately cyclically closes to cause liquid propelling pulses to be generated in axial liquid upflow throughout the entire submerged portion of the mass of wood chips to cause the rate of movement of the submergence liquid relative to the wood chips in the mass to vary cyclically to produce a cyclic advancing movement of the liquid effective to advance the wood chips throughout the length of the column and movement of the liquid alternately to raise and lower the upper level of said submergence liquid through the limited distance Δ H. This will become apparent by consideration of the drawing, in which it can be seen that, with strainer valve 75 open, liquid flow occurs from high pressure in vessel 12 through strainer 30 into ambient atmospheric pressure at tank 20 and then back into vessel 12 after repressurization by pump 18, adding to the flow of the wood chip-liquid input in the lower portion of vessel 12 between inlet 16 and strainer 30 and there being again diverted for recirculation. When strainer valve 75 is closed, the recirculation flow acts as a pulse to increase the flow throughout the wood chip mass above strainer to advance it upwardly.

The cyclical operation is carried out at a frequency of preferably about 1 to 30 cycles per minute, but at least about 5 to 10 cycles per hour by valve timer 77, for example, a closed time of about 1 second and an open time of about 5 seconds, to produce a rapid height change Δ H in the level of submergence liquid in vessel 12 of about 1 to 6 inches when valve 75 is closed, followed by a slower restoring of its level when valve 75 is open.

The advancing movement of the liquid through the entire submerged portion of the chip mass is at a rate preferably in turbulent flow, while the intervening movement is in laminar flow, providing a wood chip advancing movement, when opposed by the resistance within said vessel to advancing movement of said mass, substantially greater than any wood chip retreating movement produced by the intervening liquid movement through said chip mass, when opposed by the resistance within said vessel to retreating movement of said mass. Such resistance in the case of the preferred tapered vessel of increasing cross-section in the direction of wood chip advance is so much greater than it is in the opposite direction of chip mass movement that the chip mass advances upwardly in stepwise manner without intervening retreating steps.

The position L of the lower end of the compacted chip mass may be predetermined by varying the recirculation through pipe 34 and supply tank 20. To accomplish this result, the recirculation is increased if the lower end of the chip mass moves downwardly and is decreased if the lower end of the chip mass moves upwardly, such being controlled by the tapered bottom portion 13 as well as by sensors 62 and 64 and valve 69. Alternately, the bottom level L of the chip mass may be controlled by varying the pulse flow rate, frequency, or relative duration.

Otherwise, the digester operates in a conventional manner, with uncooked chips entering through tangential inlet 16 and being discharged at outlet 24, the other liquid flows within the digester being at conventional values as may be needed for treatment of the wood chips within the vessel.

With pulse propulsion, the advancing pulse of liquid acts on the submerged portion of the columnar mass of compacted wood chips throughout its entire length. Thus, if vessel resistance tends to compact one section of the mass to a greater extent than another, the pressure drop through such section will increase. This will increase the propulsive force in that section which will in turn act to decrease the packing. Conversely, if a section of the mass becomes loosely packed, the propulsive force will decrease, since if the liquid flows freely through the loose chips, no propulsive force will be created. As a result, a loose section of the mass will not be advanced until the following chips in the mass advance to recompact such loose section. If, in actual operation of the digester of the drawing, any unusual resistance of movement of the chip mass is encountered, such as from a partial plug of chips clinging to a strainer, the amplitude or rate of the advancing pulse can be increased to provide additional liquid movement as needed to free the plug. The invention is also useful in downflow digesters, since the forces it creates are not limited merely by the force of gravity acting on the chips.

Various modifications of the invention within the spirit thereof and the scope of the appended claim will be apparent to those skilled in the art.