US3795577A - Continuous digester with pressure relief outlet valve cycling - Google Patents

Abstract

Apparatus for the continuous pulping of a compacted mass of wood chips in an upright elongated reaction vessel having an inlet for feeding wood chips substantially continuously into the lower end of the vessel and an outlet for discharging them 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 top steam outlet having a valve and a cycling timer for operating it, such propelling pulses causing 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 at a rate at least not less than that of the alternating internating movement of the liquid, such advancing movement of the liquid advancing the wood chips throughout the length of the column from the input to the output end thereof.

Description

March 5, 1974 CONTINUOUS DIGESTER WITH PRESSURE RELIEF OUTLET VALVE'CYCLING Original Filed May 29, 1967 STEAM fi% 1-.

77 & I a F DRAINAGE ZONE AH I i 24 P2 \T; "Hf

I COUNTER CURRE 50 Q WASHIAII/G ZONE I IHHIHH v I 7 n 53 1, H COOKING AND -TE HEATING 2L ZONE {MN 4 AP v IMPREKlNAT/ON i 4! 0 AND 5 i} HEATING 30 1 I 34 ZOIIVE l i i J wk :2 CHIP COMPACTION United States Patent US. Cl. 162-237 7 Claims ABSTRACT OF THE DISCLOSURE Apparatus for the continuous pulping of a compacted mass of wood chips in an upright elongated reaction vessel having an inlet for feeding wood chips substantially continuously into the lower end of the vessel and an outlet for discharging them 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 top steam outlet having a valve and a cycling timer for operating it, such propelling pulses causing 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 at a rate at least not less than that of the alternating intervening movement of the liquid, such advancing movement of the liquid advancing the wood chips throughout the length of the column from the input to the output end thereof.

This is a continuation of application Ser. No. 132,589, filed Apr. 8, 1971 (now abandoned), which was a continuation of application Ser. No. 37,435, filed May 14, 1970 (now abandoned), which in turn was a continuation of application Ser. No. 641,793, filed May 29, 1967 (now abandoned).

This invention relates to the continuous digesting of wood chips in an upright elongated digester vessel, and more particularly to novel apparatus and 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 pressure relief outlet valve.

Upfiow digesters of the type shown and described in US. 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 copending applications Ser. Nos. 508,531, now abandoned, and 548,225, now abandoned, of common assignor herewith, 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 a ICC 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 cylically. These liquid propelling pulses provide an average net propelling pulse froce 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, in either upflow or downflow vessels. 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 efliciency 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 cause undesirable variations in the pulping recation. Although the propelling pulse system of application Ser. No. 548,225 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 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. N

Accordingly, it is a major object of the present invention to provide novel methods and apparatus 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 by feeding the wood chips into the reaction vessel adjacent its entrance end to create and maintain a columnar mass of compacted wood chips therein; advancing them throughout the length of the mass while treating them in liquid submergence; and then discharging the treated wood chips from the other end of the column, the wood chip advance being accomplished by liquid propelling pulses. Such pulses are provided by propelling pulse generating means, including pulse timing means and vessel pressure relief valve means, preferably a steam outlet valve connected at the top of the vessel, responsive to the timing means effective to cause the rate of movement of the submergence liquid relatively to the wood chips to vary cyclically, providing a Wood chip propelling force eifective to advance the mass of wood chips continuously to the discharge means. The systenrpreferably also includes a propelling liquid recirculating strainer in the vessel spaced a substantial disstance from its infeed and a substantial distance from its discharge means for removal of liquid from the vessel. This establishes a liquid propelling zone within the vessel preferably in the lower portion of the wood chip mass below the propelling liquid strainer through which propelling liquid means, preferably of the centrifugal vane pump type, is effective to advance liquid through the propelling zone.

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 diagramatic side elevation, partly in section, of apparatus according to 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 ten feet or greater in diameter and one hundred 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 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 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 Pat. No. 3,206,356.

Within vessel 12, generally along the central 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 stainers 46 and 48, with their valves 51 and 52, 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 cocurrent and countercurrent 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 48 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 one-half to one and one-half times the vessel cross-section diameter above 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 cmss 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 therefrom 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, liquid pulse generating means are provided for causing the rate of movement of the submergenceliquid relatively to the wood chips in the mass to vary cyclically for propelling the entire columnar chip mass through the vessel. Specifically, such means consists of a top steam outlet line 73 having a steam pressure relief valve 75 operated by a cycle timer 77, so that reduction of steam pressure at the top of vessel 12 is cyclically effected. Any variations in recirculating liquid when this occurs are absorbed at supply tank 20, the level of which varies throughout the distance Ah, but such variations are small enough so that the flow of liquid and chips through pump 18 to vessel inlet 16 is not significantly alfected thereby. Nor does the change AH in the upper level in vessel 12 seriously affect the liquid flow below strainer 30, although up- Ward fiow of liquid occurs above said strainerduring the portion of the cycle in which valve 75 is closed.

It is a particular feature of the system of the invention that it makes possible the use as wood chip-liquid infeed pump 18 of a centrifugal vortex pump, characterized be 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 woo chips and water, and is much less costly than the socalled thick stock pumps heretofore used for the purpose. Conventional centrifugal pumps, are also useful with the cycling top pressure system of the invention.

It has been established in the commercial operation of digesters of the type described in 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 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 abovethe 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 application Ser. No. 548,225, the entire compacted chip mass may be advanced uniformly by subjecting it, preferably throughout its entire submerged length, to cyclical liquid advancing pulses created in the sub'mergence 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 application Ser. No. 508,531, 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 was as a result of increasing liquid velocities caused by decreasing vessel cross section area. Thus, as chips acrete on the bottom of the chip mass, its levels 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 sectiondecreases. This has been found to result, in a fairly short time interval, of the entire chip mass moving up- Ward, 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, that 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 crosssection 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 that 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 on which the liquid pulse of the present invention acts can be determined by measuring the difference (AP) between the fluid pressure (P at the bottom of the chip column and the fluid pressure (P at the top of the chip column. With fluid increasing from zero, the resistance (AP) 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 beginning 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 application Ser. No. 508,531, 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 of the present invention.

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 more 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 and the amount of sawdust or slivers 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 opening and shutting valve 75 at a frequency of preferably about 1 to 30 cycles per minute but at least about 5 to cycles per hour as by valve timer 77, for example, an open time of about one second and a closed time of about five seconds to product a rapid height change (AH) in the level of submergence liquid in vessel 12 of about one to six inches followed by a slower restoring of its level. These pulses act preferably in axial liquid upflow throughout the entire submerged portion of the chip mass to cause the rate of movement of the submergence liquid relatively to the wood chips in said mass to vary cyclically between an advancing movement of the liquid and an intervening movement of the liquid alternately to raise and lower the upper level of said submergence liquid through the limited distance (AH).

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 or the pulse frequency.

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 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 downfiow 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 claims will be apparent to those skilled in the art.

What is claimed is:

1. Apparatus for the continuous pulping of wood chips in a reaction vessel by feeding substantially continuously said wood chips and a liquid into said reaction vessel adjacent to the entrance end thereof to maintain a columnar mass of compacted wood chips therein; advancing the wood chips throughout the length of said mass meanwhile treating the same in liquid submergence, and substantially continuously discharging said wood chips from the exit end of said column, said apparatus comcomprising an upright elongated reaction vesselhaving an entrance end and an exit end,

wood chip liquid infeed means adjacent the entrance end of said vessel for feeding wood chips and liquid into the entrance end of said vessel and onto the end of said mass,

wood chip discharge means adjacent the exit end of said vessel, and

liquid propelling pulse generating means including pulse timing means and vessel pressure relief valve means connected to the top of said vessel and connected and responsive to said timing means to cause the rate of movement of the submergence liquid relatively to said wood chips to vary cyclically, providing a wood chip propelling force for advancing the wood chips continuously to said discharge means.

2. Apparatus as claimed in claim 1 wherein said wood chip discharge means is adjacent the upper end of said vessel and maintains a predetermined level of the top of said mass below the top of said vessel by discharging wood chips from the upper end of said column, and wherein said pressure relief valve means is connected to the top of said vessel above said predetermined level.

3. Apparatus as claimed in claim 2, wherein said wood chip-liquid infeed means includes a centrifugal pump having a small pressure change over a wide range of flow rates.

4. Apparatus for the continuous pulping of wood chips in a reaction vessel by feeding substantially continuously said Wood chips and a liquid into said reaction vessel adjacent to the lower entrance end thereof to maintain a columnar mass of compacted wood chips therein; advancing the wood chips upwardly throughout the length of said mass meanwhile treating the same in liquid submergence, and substantially continuously discharging said wood chips from said upper end of said column, said apparatus comprising an upright elongated reaction vessel having a lower entrance end and an upper exit end,

Wood chip liquid infeed means adjacent the lower entrance end of said vessel for feeding wood chips and liquid into the lower end of said vessel and onto the lower end of said mass,

wood chip discharge means adjacent the upper end of said vessel for maintaining a predetermined level of the top of said mass below the top of said vessel by discharging Wood chips from the upper end of said column,

said vessel having a tapered vessel portion of upwardly progressively increasing cross sectional area adjacent the lower endof said vessel with the lower end of said mass normally maintained within said tapered vessel portion by the upwardly progressively decreasing velocity of propelling liquid created within said tapered vessel portion, and

liquid propelling pulse generating means including pulse timing means and top pressure relief valve means connected to the top of said vessel above said predetermined level and connected and responsive to said timing means to cause the rate of movement of the submergence liquid relatively to said wood chips to vary cyclically, providing a wood chip propelling force for advancing the wood chips continuously upwardly to said discharge means.

5. In apparatus as claimed in claim 4, wherein said tapered vessel portion tapers to a minimum crosssectional area of about half of the maximum crosssectional area of said vessel within said propelling zone.

6. Apparatus for the continuous pulping of wood chips in a reaction vessel by feeding substantially continuously said wood chips and a liquid into said reaction vessel adjacent to the lower entrance end thereof to maintain a columnar mass of compacted wood chips therein; advancing the wood chips upwardly throughout the length of said mass meanwhile treating the same in liquid submergence, and substantially continuously discharging said wood chips from said upper end of said column, said apparatus comprising an upright elongated reaction vessel having a lower entrance end and an uper exit end,

wood chip-liquid infeed means including a centrifugal pump adjacent the lower entrance end of said vessel for feeding wood chips and liquid into the lower end of said vessel and onto the lower end of said mass, said pump having a substantially constant outlet pressure at varying flow rates,

wood chip discharge means adjacent the upper end of said vessel maintaining a predetermined level of the top of said mass below the top of said vessel by discharging wood chips from the upper end of said column,

propelling liquid strainer means in said vessel spaced a substantial distance of at least about one-half the cross section dimension of said vessel above said infeed means and a substantial distance below said discharge means for removal of liquid from said vessel establishing a liquid propelling zone within said vessel throughout the entire cross-sectional area thereof in the lower portion of said wood chip mass below said propelling liquid strainer means, and

liquid propelling pulse generating means including pulse timing means and top pressure relief valve means connected to the top of said vessel above said predetermined level and connected and responsive to said timing means to cause the rate of movement of the submergence liquid relatively to said wood chips to vary cyclically, providing a wood chip propelling force advancing the wood chips continuously upwardly to said discharge means.

7. Apparatus as claimed in claim 6 wherein said vessel has a lower tapered portion of upwardly progressively increasing cross-sectional area tapering at its lower end to a cross-sectional area of at most about half of its maximum cross-sectional area within said propelling zone, the lower end of said mass normally being maintained within said tapered vessel portion by the upwardly progressively decreasing velocity of propelling liquid created within said tapered vessel portion.

References Cited UNITED STATES PATENTS 2,635,548 4/1953 Brawley 103l03 A 2,845,347 7/1958 Byrne 162248 X 3,193,444 7/1965 Benjamin l62-237 X 3,700,548 10/1972 Rich 16217 ROBERT L. LINDSAY, JR., Primary Examiner A. DANDREA, 1a., Assistant Examiner US. Cl. X.R.

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