US2845347A

US2845347A - Apparatus and method for digesting

Info

Publication number: US2845347A
Application number: US355662A
Authority: US
Inventors: Hugh J Byrne
Original assignee: Individual
Current assignee: Individual
Priority date: 1953-05-18
Filing date: 1953-05-18
Publication date: 1958-07-29
Anticipated expiration: 1975-07-29

Description

July 29, 1958 H. J. BYRNE 3 3 r APPARATUS AND METHOD FOR DIGESTING Filed Ma 18, 1955 3' Sheets-Sheet INVENTOR. HUGH J. BYRNE J ch &

ATTORNEY July 29, 1958 H. J. BYRNE APPARATUS AND METHOD FOR DIGESTING Filed May 18. 1953 3 Sheets-Sheet 2 INVENTOR.

HUGH J. BYRNE A'TTORNEY July 29, 1958 H. J. BYRNE 3 APPARATUS AND METHOD FOR DIGESTING 3 Sheets-Sheet :5

Filed May 18, 1955 INVENTORL HUGH J. BYRNE 3 1 HE Mu( W ATTORNEY United States Patent 2,845,347 APPARATUS METHOD FOR DIGESTING Hugh J. Byrne, Seattle, Wasl.

Application May 18, 1953, Serial No. '355,662

20 Claims. (Cl. 92-7) My present invention relates to apparatus and method for digesting materials, particularly fibrous materials such as wood chips used in the manufacture of pulp and paper products. The invention is of utility in the formation of any fibrous material by the digestion of a raw material such as woo d chips, straw, stalks of fibrous plants, or the like. The present invention comprises, in essence, an improvement upon standard apparatus and conventional methods of digesting materials, such as dsclosed in the patents to Christensen, 1,942,685, and Richmond, 2,163,437, for example, being concerned particularly with the problem of preventing the screens within the digester from becoming clogged.

The present invention achieves its highest utility in the wood pulp industries, and may be utilized in the digesting of wood pulp by any Chemical method, such as the sulfite process (arrmionia base, magnesium base, calcium base, etc.), and the sulfate or Kraft process, soda process, semi-chemical process, etc. In such processes a large pressure vessel is charged with a quantity of wood chips or other fibrous raw materials, and subjected to the digesting action of a heated liquor, acidic or basic, as the case may be, under pressure, until the fibrous material is broken down into individual fibers or small bundles of fibers as desired, and Chemical Components which may remain in part adhered to the fibers or bound therein, and in part are dissolved in or Suspended in the cooking liquor. The fiber is then subjected to further treatment by known processes and formed into paper, pulp, or other fibrous articles.

Widely varyng circulating systems are associated with the digesters in an eflort to achieve uniformity of digestion. It is quite common to circulate the cooking liquor from the digester through a heat exchanger and back to the digester, as dsclosed in the Christensen patent. In order to prevent the escape of fibrous material from the digester when the cooking liquor is withdrawn from the digester by means of a pump, the suction inlet to the pump is protected by a screen such as dsclosed in the Christensen and Richmond patents mentioned above. Since the cooking liquor is Corrosive the digester shells are sometimes lined with a protective lining such as ceramic, or stainless steel. The reheated liquor may be returned to the digester at different points in an efort to prevent overheating and charring, or undercooking of 'the fibrous material at various levels in the digester. Instead of using a heat exchanger in the liquor circulating system heat may be added by introducing 'steam to the digester at different points.

When external shell and tube heaters are not used, a common arrangement is to provide the suction inlet screen for the liquor at a higher level in the digester and to introduce steam at the bottom of the digester, and to return the liquor at either the top or the bottom or both, in an attempt to achieve substantial uniformi ty of cooking action. Usua lly, in such cases, live steam is admitted at the bottom of the digester and the cooking ice liquor is returned at the top of the digester. This ara rangement is subject to several disadvantages, -as follows:

First, there is a tendency to overcook the lower layers of fibrous material and to undercook the upper layers;

second, the steam is `absorbed by the liquor while passing.

upwardly through the fibrous material, causing imperfect digestion due to the liquor becoming more nearly neutral in the lower portion of the digester; and third, the practice often results in the 'Creation of water hammer which may tear loose the protectve lining, or cause other mechanical failure. An object of the present invention is to provide means for recirculating the cooking liquor, with or without external heating thereof, while controlling the heat and shortening the cooking time in a manner to prevent unequal digestion, overheating or undercooking in different areas, and water hammer.

A further object of the present invention is to lessen the cost of installation and upkeep of the suction screen for the liquor circulating system in a digester of the' character described. In some digesters a whole or par-- tial ring of stainless steel screens has been Situated around the interior of the digester shell at an intermediate level'.

through which cooking liquor is withdrawn from the di-- gester, the screens usually being perforated by a plu-- rality of small holes in the order of one-eighth to threeeighths of an inch in diameter. The perforations in the: screen must be small enough that they do not permit' the passage of more than a limited amount of separated fibers, but must be large enough that they do not clog: easily. The resulting compromise is such that a plu-- rality of fibers and small pieces of the raw material are: attracted to the screen and build up bridges over the screen openings, or otherwise plug them, the formation progressing until a mass of interlaced, fibrous material plastered onto the front of the screen prevents removal of cooking liquor through it. As a result circulation is dirninished and later stops and cooking must continue without circulation, the resulting pulp being of poorer quality than desired.. The diminishing flow of cooking liquors through the circulating system results in prolongation of the cooking time and increases the likelihood of uneven cooking throughout the digester. A further object of the present invention is to provide means for agitating the accumulated mass of fibers in front of the inner side of the digester screen whereby clogging of the screen is prevented so that a constant flow of cooking liquor through the screen into the circulating system is achieved, thus improving uniformity of digestion throughout the digester `and diminishing the average length of cooking time.

A further object of the present invention is to provide means whereby a screen of less overallsurface area, or one having a fewer number of perforations, or a plurality of spaced, small screens, may be substituted for the present annular ring of closely perforated screens completely surrounding the inten'or of the digester shell, thus reducing the cost of installation and maintenance.

A further object of' the present invention is to provide means for introducing a fluid in proximity to the inner surface of the circulating system screen to achieve the dual purpose of maintaining adequate circulation and controlling the character of the cooking liquor. The fluid introduoed may bea portion of the cooking liquor, an nert gas such 'as nitrogen, air, steam, or an active control gas or other fluid such as carbon dioxide -for acidic cooking liquors or ammonia for caustic cooking liquors. Thus the present invention permits a large choice of introduced fluid for the purpose of fluffing oragitating the mass of fiber adjacent to the screen, which otherwise could block its apertures, and controlling the strength or character of the cooking liquor, while introducing the fluid at the most advantageous point to achieve uniformity of digestion and eliminate water hammer.

A further object of present invention is to provide a method of achieving uniformity of digestion and shortening the time of cooking in a digester by introducing a fluid under pressure at a plurality of points around the interior of the digester at an intermediate level.

A further object of the present invention is to provide a method for controlling the character and quality, such as the pH value of digesting liquor, by introducing controlled amounts of an active control fluid during the process of digeston.

Further objects of the present invention are to reduce the temperature rise period in a digester and thereby reduce the' overall time cycle; to prolong the time that adequate circulation can be maintained, and thus reduce the overall time cycle and produce more uniformly cooked pulp; to reduce the chemical demand by achieving uniformity of cooking over the entire cooking period; otherwise to improve the result and dirninish the cost of digesting pulp; and to reduce costs by eliminating the eX- pense of replacing condensate with boiler feed make-up water in systems where recirculated liquor or gases are used for screen scavenging.

The foregoing and other objects and advantages of the present invention may be more thoroughly understood by reference to the accompanying drawings, taken in connection with the following specification.

Fig. l is a schematic representation of a digester and circulating system including a heat exchanger, with a part of the digester shell broken away to illustrate a form of the present invention.

Fig. 2 is an enlarged, vertical section taken radially of the digester substantially along line 2-2 of Fig. 1.

Fig. 3 is a partial vertical section along a chordal plane of the digester showing a modification of the apparatus of the present invention.

Fig. 4 is a horizontal, schematic section through a digester having a further modification of the apparatus of the present invention.

Fig. 5 is an enlarged detail view taken from the plane 5-5 of Fig. 4.

Fig. 6 is a horizontal section taken substantially along line 6- 6 of Fig. 5.

Fig. 7 is a schematic representation of a modified form of digester and circulating system embodying the present invention.

Fig. 8 is a schematic representation of a further modified form of digester and circulating system embodying the present invention.

Fig. 9 is a partial vertical section substantially corresponding to Fig. 2 illustrating a further modification of a portion of the apparatus.

In Fig. l I have illustrated a digester 10 which comprises a cylindrical shell having a rounded dome 11 and a conical bottom 12, representative of a conventional digester. In accordance with conventional practice the shell of the digester is fabricated of structural 'steel so as to withstand pressures, and the interier thereof is lined with a corrosion-resistant lining of suitable nature. The digester is provided with a filling neck 13 at the top which may be closed by a suitable lid 14. It will be understood that the interior of the digester is filled with appropriate raw material and the desi'ed cooking liquor.

A conventional circulating system is associated with the digester shell, including a suction screen ring 15 horizontally disposed within the shell in the form of an inwardly protruding, annular ring at an intermediate level within the shell. Cooking liquor is withdrawn through the screen and a suction pipe 16 having a valve 17 therein by means of a pump 18. The pump discharges through a discharge line 19 having a check valve 20 and a valve 21 therein. The discharged liquor passes through tubes in a shell and tube heat exchanger 22 having steam 4 admitted thereto through an entr'ance tube 23 having a valve 24 therein, the condensate being withdrawn through a discharge tube 25 having a valve 26 therein. The heated liquor passes from the heat exchanger into either or both of two return lines, one line 27 leading to the top of the dome 11 through a valve 28, and the other line 29 leading to the bottom of the cone 12 through a valve 30 and check valve 31. Admisson of liquor at the bottom of the cone is achieved through a T-fitting 32 forming a portion of a blow line 33 having a valve 34 therein. At the top of the dome there is provided a vent line 35 having a pressure relief valve 36 therein for the purpose of withdrawing waste gases. The digester and circulating system, as described, is conventional and representative of such systems.

lt is common practice to form the screen ring 15 (Fig. l) of a plurality of screen segments 40 made of stainless steel, each screen segment comprsing a stainless steel hacking plate 41 (Fig. 2) which may be held against the lining 42 of the digester shell by means including clamps 43 which are held in position by nuts 44 engaging studs 45 mounted on the digester shell, or other suitable means. Each screen segment preferably comprises a plate 46 perforated by suitable perforations 47, the plate being held in outwardly bowed condition by internal ribs 48, which are welded to the hacking plate 41. Individual segments engage each other at their vertical edges so as to form a continuous screen around the interier of the digester. Individual segments or the entire screen may be replaced when excessively worn or damaged. The screens illustrated are representative of practical installations, but scavenging apparatus of this invention may be applied to screens of any shape or size.

In accordance with the present invention I am enabled to reduce the outward bulge of the screen, keep the upper and lower legs thereof at acute inclinations to the wall to prevent the material from hanging up on the screen during blowoff, reduce the number of perforations, and reduce the number of screen segments. As illustrated in Fig. 1, the screen ring may be continuous, comprising a plurality of abutted screen segments, but the fabrication thereof may be simplified and the cost reduced by reducing the total screen area.

In accordance with the form of the invention illustrated in Fig. 1, I achieve the objects of the invention by providing a scavenging ring 50 adjacent to the surface of the screen ring which surface is convex axially of such ring and concave circumferentially of that ring, the scavenging ring being perforated at suitable intervals whereby a fluid under pressure may be introduced in a direction tangential to the screen surface and at a plurality of points, preferably, but not necessarily, in alignment with vertical rows of perforations to prevent clogging of such screen ring perforations. A form of such scavenging ring is illustrated comprising a continuous ring 50 of one and one-quartcr inch tubing having upwardly directed three-sixtcenths inch diameter holes 51 on two-inch centers in alignment with vertical rows of perforations 47. In a typical example the minimum inner diameter of the screen ring is forty-four and one-quarter inches, measured radially of diametrically opposed screen segments, and the mean diameter of the scavenging ring is forty-five inches, so that the jet openings 51 therein, located substantially along vertical diameters of the scavenging ring. slightly underlie the innermost portion of the screen ring bulge. The scavenging pipe is, in this. form of the invention, located below the lowermost row of perforations in the screen segments and its external surface is spaced from the adjacent surface of the screen ring by a distance of approximately three-quarters of an inch. Preferably the scavenging ring is made of two semicircular pipe sections 52 joined together by a pair of unions 53, in order to facilitate assembly. The scavenging ring is supported by a plurality of supporting arms 54, which may be Conveniently attached to certain one's of the screen mounting clamps 43. The scavenging ring is connected to .a riser pipe 55 which is connected by a pipe union 56 to the upwardly turned arm of an inlet pipe 57 having a valve 58 therein.

A desired fluid, either steam, inert gas, digester liquor,`

or a control gas or other fluid which will combine with the cooking liquor, may be introduced continuously or intermittently under pressure through the scavenging ring. The jets or fluid under pressure sweep upwardly across the screen and fluffor dislodge the mat of fibers or chips formed in front of the perforations 47, thus constantly, or as often as desired, clearng the perforations of the screen so that the circulation may be maintained at a high rate. Regardless of what fluid is introduced at the intermediate level in the digester, greater uniformity is achieved due to the maintenance of a more uniform temperature throughout the digester, as distinguished from prior practices where, for example, steam was admitted at the bottom of the digester. This eliminates condensation weakening the liquor at the bottom of the digester, greater circulation throughout the digester is achieved, and results in greater uniformity and less cooking time being required.

In the form of the invention illustrated in Fig. 3, a digester having a screen ring, asipreviously described, is provided with a scavenging ring 60 located above the screen ring in substantially the same manner that the scavenging ring 50 is located below the screen ring in .Fig. 1, in this instance the jet openings being directed downwardly across the face of the digester screen.

In Figs. 4, 5 and 6 I have illustrated a modification of screen and scavenging inlet Construction, of less expense since a plurality of widely spaced screens may be used. In this modification a digester shell 61 having a liner 62 is provided with a plurality of spaced screens, each comprising a mounting plate 63 and a perforated, frustropyramidal screen 64 in which a plurality of `perforations 65 are, provided. Each of the screens is suitably maintained circumferentially around the interior of the shell by suitable clamps and the interior thereof communicates 'with a branch 66 of a circulating manifold 67 surrounding the exterior of the shell, the manifold being connected to the circulating pump inlet tube 68. Each mounting platel63 has welded thereto or otherwise suitably aflixed thereto, a scavenging fluid inlet tube 69 connected to a vertical jet head 70havng a plurality of jet outlets 71, preferably, but not necessarily, in horizontal alignment, respectively, With rows of the screen perforations 65. The tubes 69 are connected to an inlet manifold 72 with which communicates the fluid inlet line 73. r

As so far described the modifications favor the use of steam as a scavenging and heating medium or a gas for scavenging or chemical control purposes.

A modification of the system illustrated in Fig. 7 favors the use of an inert gas or an active control gas to keep the screens clean and to control the character of the cooking liquor. In this modification a digester 80 is provided with a screen ring 81 adjacent to which is a scavenging ring 82, it being appreciated that the screen ring and scavenging ring are representative of the various previously described modifications. A gas vent line 83 is provided and liquor is circulated through suction line 84, pump 85 and discharge line 86 having branches leading to the top and bottom of the digester, suitable valves being schematically illustrated. The shell and tube heat interchanger is omitted. The blow line 87 is provided with the usual valve 90 and a branch inlet line 88 for the introduction of heat controlling steam, if desired,

i having a valve 89. The scavenging ring is connected to a fluid admission line 91 in which there is a valve 92. This system favors the use of an inert gas such as nitrogen, or an active control gas such as carbon dioxide for acid cooks, or ammonia for caustic cooks, in order to control the acidty or caustcity (pH value) of the cooking liquor.

6 Steam may be used if desired in 'such instances, admitting all steam for scavenging and heating purposes through scavenging ring 82 and dispensing with the conventional steam inlet line 88' of conventional systems. The control of cooking temperature may be achieved by preheatng the gas ntroduced through the scavenging ring An alternative arrangement is disclosed in Fg. 8 which favors the use of the circulated liquor to scavenge the screen as it does not alter cooking liquor strength. In this figure a digester 93 is shown, having a screen ring 94, scavenging ring 95 and the usual vent line 96 and blow line 97. To avoid clogging, the scavenging ring jets should be larger than the screen ring apertures, the former being 4 diameter andthe latter diameter for example; The liquor is withdrawn through the screen ring and the sucton line 98 incorporating valve 99 by pump 100 from which it is 'discharged through check valve 101, valve 102, shell and tube heater 103, and one or any combination of a plurality of

return lines

104, and 106 as desired. The return line 104 leads to the top of the digester through a valve 107. The return line 105 leads to the scavenging ring 95 through a valve 108. The return line 106 leads to the bottom of the cone through a valve 109 and check valve 110. The vent line may be closedby the conventional valve 111 and the blow line may be closed by the conventional valve 112. The shell of the heater may be provided with steam through an admission line 113 having a valve 114, the condensate being withdrawn through an exit line 115 and condensate trap 116. For increased pressure of the scavenging liquor, line 105 may be connected directly to the outlet of pump 100 or may include a booster pump.

In Fig. 9 I have illustrated another modification of the screen and scavenging ring arrangement. In this installation the scavenging ring pipe is placed in the space between the hacking plate 131 and the screen ring plate 132. The scavenging pipe has short branches 133` which terminate at the adjacent surface of the screen plate 132,

-each surrounding one plate aperture 134. scavenging fluid projected through such apertures by the branches 133 blasts the chips and pulp toward the center of the digester to clear apertures above and below apertures 134 to enable liquor to flow out through apertures 135. The scavenging ring thus located is protected from damage by pressure surges in the digester.

I have illustrated a number of different types of circulating systems to demonstrate the versatility of the present invention in that various types of fluid may be used in connection with a number of diflerent types of arrangement of screen and scavenging ring fluid inlet means. In some systems it may be desirable to circulate a portion of the liquor through the scavenging ring rather than introducing steam, achieving all heating by use of an external heat exchanger from which condensate may be salvaged, this being of extreme value in plants where boiler feed makeup Water is expensive. A further point in favor of 'circulating a portion of the liquor through the scavenging ring rather than introducing steam is that ordinarily a digester is charged with sufiicient liquor at sufficient strength to complete the whole cooking operation, with the result that the inital cooking of the external surfaces of the Wood chips or other material may be too drastic, which disadvantage may be eliminated by introducing a milder liquor at first and then introducing a control gas or control lquid to keep up the strength of the liquor or to increase its strength at a subsequent period during the cooking operation.

Further advantages of introducing a control gas or liquor are that the liquor strength is controlled and that dilution of the liquor as the cook progresses is prevented, as would be the case with the use of steam, and better quality of high grade pulp is achieved. On the other hand, an inert gas, heated or unheated, has the same advantages as externally heating the cooking liquor in a heat excha'nger and maybe desirable in some instances of one type of fluid over another, especially for the production of different types of pulp.

In all modicatons it will be apparent that the introduced fluid will force circulation of the adjacent mass of chips and fiber, which in themselves additionally tend to prevent the formation of a mat and prevent fouling the openings at the surface of the screen ring.

I claim:

1. A digester comprising a pressure vessel in which fibrous material and a cooking liquor may be placed for creating a fibrous pulp, a circulating system communicating with said pressure vessel and adapted to withdraw cooking liquor from the pressure vessel and return it thereto, screen 'means comprisng a plurality of screen segments arranged circumferentially about the interior of said pressure vessel at an intermediate level therein and providing a barrier at the inlet of said circulating system to prevent the escape of more than a limited amount of fine, fibrous material from the pressure vessel, each of said screen segments having a plurality of rows of perforations therein, and scavenging means comprising a fluid conduit entering said pressure vessel and having a plurality of jet orifices closely adjacent said screen segments, each of said jet orifices being directed toward 'certan perforations of one of said screen segments whereby a fluid may be forced through said jet orifices to dislodge accumulations of fibrous material from the surfaces of the screen segments.

2. In a digester including a vessel in which fibrous material may be cooked in liquor to produce pulp, and liquor circulating means communicating with a liquor discharge aperture below the surface of the liquor in the vessel during such cooking and a liquor feed aperture for such vessel spaced from the discharge aperture, for withdrawing cooking liquor from the body of cooking liquor in the vessel and returning it to such body, the combination of screen means located within the vessel and below the surface of the liquor therein during cooking and separating such liquor discharge aperture from the portion of the vessel containing the fibrous material and cooking liquor mixture to strain the liquor withdrawn from the cooking liquor body into the circulating means, and a scavenging fluid conduit including, within such vessel, an internal conduit portion below the surface of the liquor in the vessel during cooking, adjacent to said screen means and having a jet orifice directed generally toward said screen means and operable to project a jet for agitating the mass of fibre adjacent to said screen means and preventing clogging of such means by fibrous material.

3. In a digester including a vessel, the walls of which are curved, in which fibrous material may be cooked in liquor to produce pulp, and liquor circulating means communicating With a liquor discharge aperture below the surface of the liquor in the vessel during such cooking and a liquor feed aperture for such vessel spaced from the discharge aperture, for withdrawing cooking liquor from the body of cooking liquor in the vessel and returning it to such body, the combination of screen means located within the vessel on a curved wall thereof below the surface of the liquor therein during cooking and separating such liquor discharge aperture from the portion of the vessel containing the fibrous material and cooking liquor mixture to strain the liquor withdrawn from the cooking liquor "body into the circulating means,

and a fluid conduit including, within such vessel, an internal conduit portion below the surface of the liquor in the vessel during cooking, adjacent to said screen means and having a jet orifice directed close alongsde and generally parallel to said screen means.

4. In a digester including a vessel in which fibrous material `may be cooked in liquor to produce pulp, and liquor circulating means communicating with a liquor discharge aperture below the surface of the liquor in the vessel during such cooking and a liquor feed aperture for such vessel spaced from the discharge aperture, for

withdrawing cooking liquor from the body of cooking liquor in the vessel and returning it to such body, the combination of screen means located within the vessel and below the surface of the liquor therein during cooking and separating such liquor discharge aperture from the portion of the vessel containing the fibrous material and cooking liquor mixture and having a multiplicity of perforations arranged in rows to strain the liquor withdrawn from the cooking liquor body into the circulating means, and a scavenging fluid conduit including, within such vessel, an internal conduit portion below the surface of the liquor in the vessel during cooking, adjacent to said screen means and having a plurality of narrow jet orifices directed along and generally parallel to the rows of screen means perforations, respectively, and adjacent thereto and operable to project jets for preventing clogging of said screen means by fibrous material.

5. In a digester, the combination defined in claim 4, wherein the screen means comprises a screen plate having a bulging side nearer the centerline of the vessel, the internal conduit portion being located closely adjacent to the lower edge of said screen plate and the jet orifices being directed upwardly along the respective rows of perforations of said bulging screen plate side.

6. In a digester, the combination defined in claim 4, wherein the screen means compn'ses a screen plate having a bulging side nearer the centerline of the vessel, the internal conduit portion being located closely adjacent to the upper edge of said screen plate, and the jet orifices being directed downwardly along the respective rows of perforations of said bulging screen plate side.

7. In a digester including a vessel, the walls of which are curved, in which fibrous material may be cooked in liquor to produce pulp, and liquor circulating means communicating with a liquor discharge aperture below the surface of the liquor in the vessel during such cooking and a liquor feed aperture for such vessel spaced from the discharge aperture, for withdrawing cooking liquor from the body of cooking liquor in the vessel and returning it to such body, the combination of a plurality of screen plates on one of the curved walls of the vessel below the surface of the liquor therein during cooking, spaced horizontally along such wall and separating such liquor discharge aperture from the portion of the vessel containing the fibrous material and cooking liquor nu ture, and having a multiplicity of perforations to strain the liquor withdrawn from the cooking liquor body into the circulating means, and a scavenging fluid conduit ncluding, within such vessel, a conduit portion disposed alongside one of said screen plates and having a plurality of jet orifices therein directed alongside said one screen plate and operable to project fluid jets therefrom for preventing clogging of said screen plateiby fibrous material.

8. In a digester including a vessel, the walls of which are curved, in which fibrous material may be cooked n liquor to produce pulp, and liquor circulatingmeans communicating with a liquor discharge aperture below the surface of the liquor in the vessel durng such cooking and a liquor feed aperture for such vessel spaced from the discharge aperture for withdrawing cooking liquor from the body of cooking liquor in` the vessel and returning t to such body, the combination of a pluralty of screen plates projecting inward from a curved sde wall of the vessel below the surface of the liquor therein during cooking, spaced horizontally along such wall and separating such liquor discharge aperture from the portion of the vessel containing the fibrous material and cooking liquor mixture, and having a multiplicity of perforations arranged in horizontal rows to strain the liquor withdrawn from the cooking liquor body into the circulating means, and a scavenging fluid conduit including, within such vessel, a conduit portion disposed alongside one of said screen plates and having a plurality of narrow jets orifices therein directed along and generally parallel to the rows of screen plate perforations, respectively, and adjacent thereto and operable to project fluid jets therefrom for preventing clogging of said screen plate by fibrous material.

9. In a digester including a vessel, the walls of which are curved, with its axis extending substantially vertically in which fibrous material may be cooked in liquor to produce pulp, and liquor circulating means disposed externally of such vessel and communicatng with a liquor discharge aperture in a curved side wall of the vessel below the surface of the liquor in the vessel during such cooking and a liquor feed aperture in such vessel spaced from the discharge aperture for withdrawing cooking liquor from the body of cooking liquor in the vessel and returning it to such body, the combination of screen means of arcuate shape having its center substantially coinciding with the axis of the vessel, and a scavenging fluid conduit entering the vessel and including, within such vessel, an internal conduit portion of arcuate shape corresponding to the arcuate shape of said screen means, having a plurality of jet orifices spaced lengthwise of it and directed alongside and generally parallel to said screen means and operable to project jets for preventing clogging of said screen means by fibrous material.

10. In a digester including a vessel, the walls of which are curved, with its axis extending substantially vertically in which fibrous material may be cooked in liquor to produce pulp, and liquor circulating means disposed externally of such vessel and communicating with a liquor discharge aperture in one of the curved walls of the vessel below the surface of the liquor in the vessel during such cooking and a liquor feed aperture in such vessel spaced from the discharge aperture, for withdrawing cooking liquor from the body of cooking liquor in the vessel and returning it to such body, the combination of screen means of annular shape having its center substantially coinciding with the axis of the vessel and the side of said screen means nearer the center of the vessel is convex in radial planes of the vessel, and a scavenging fluid conduit entering the vessel and including, within such vessel, an internal conduit portion of annular shape corresponding to the annular shape of said screen means, having a plurality of jet orifices spaced circumferentially of it and directed alongside and generally parallel to said screen means and operable to project jets for preventing clogging of said screen means by fibrous material.

11. In a digester including a vessel in which fibrous material may be cooked in liquor to produce pulp, and liquor circulating means communicatingwith a liquor discharge aperture below the surface of the liquor in the vessel during such cooking and a liquor feed aperture for such vessel spaced from the discharge aperture, for withdrawing cooking liquor from the body of cooking liquor in the vessel and returning it to such body, the combination of screen means spaced inwardly from a wall of such vessel below the surface of the liquor therein during cooking and separating such liquor discharge aperture from the portion of the vessel containing the fibrous material and cooking liquor mixture, and having a multiplicity of perforations to strain the liquor Withdrawn from the cooking liquor body into the circulating means, and a scavenging fluid conduit including, within such vessel, an internal conduit portion located between said screen means and the adjacent portion of the vessel wall and having a jet orifice disposed substantially in contact with the adjacent side of said screen means for directing a jet of fluid from said orifice through a perforation of said screen means.

12. In the production of pulp, the method of cooking fibrous materials in a body of cooking liquor in a vessel which comprises straining cooking liquor through a screen beneath the surface of the body of cooking liquor in the vessel and withdrawing such strained liquor from such cooking liquor body, and, at a location beneath the surface of the body of cooking liquor in the vessel and adjacent to such screen, directing a fluid jet to agitate the fibrous material adjacent to the screen and prevent clogging of such screen by fibrous material to enable continued flow of liquor through the screen.

13. The method defined in claim 12, in which the liquor withdrawn from the body of cooking liquor in the vessel is returned to such body of cooking liquor, and the fluid of the jet is supplemental to such liquor returned to the body of cooking liquor.

14. The method defined in claim 12, in which the fluid of the jet is heated prior to being supplied to the vessel.

'15. The method defined in claim 12, in which the fluid of the jet is cooking liquor withdrawn through the screen and being returned to the body of cooking liquor in the vessel.

16. The method defined in claim 12, in which the fluid of the jet is steam.

17. The method defined in claim 12, in which the fluid of the jet is an inert gas.

18. 'I'he method defined in claim 12, in which the fluid of the jet has a pI-l value farther from pI-I 7 but at the same side of pH 7 as the pH value of the body of liquor in the vessel, and the supplemental fluid acts to alter the pH value of such body of liquor to a value farther from pH 7.

19. In the production of pulp, the method of cooking fibrous materials in a body of cooking liquor in a vessel which comprises straining cooking liquor through a screen located beneath the surface of the body of cooking liquor in the vessel and, adjacent to such screen, projecting a jet of hot gas into the mass of fibrous material and thereby eflecting heating of the liquor and agitation of such fibrous material adjacent to the screen.

20. In the production of pulp, the method of cooking fibrous materials in a body of cooking liquor in a vessel which comprises straining cooking liquor through a screen located beneath the surface of the body of cooking liquor in the vessel and, adjacent to such screen, projecting a jet of steam into the mass of fibrous material and thereby efiecting heating of the liquor and agitation of such fibrous material adjacent to the screen.

References Cited in the file of this patent UNITED STATES PATENTS 1,942,685 Christensen Jan. 9, 1934 2,072,776 Svensson Mar. 2, 1937 2,163,437 Richmond June 20, 1939 2,179,796 Morterud Nov. 14, 1939 2,608,91O McCrystle Sept. 2, 1952

Claims

20. IN THE PRODUCTION OF PULP, THE METHOD OF COOKING FIBROUS MATERIALS IN A BODY OF COOKING LIQUOR IN A VESSEL WHICH COMPRISES STRAINING COOKING LIQUOR THROUGH A SCREEN LOCATED BENEATH THE SURFACE OF THE BODY OF COOKING LIQUOR IN THE VESSEL AND, ADJACENT TO SUCH SCREEN, PROJECTING A JET OF STEAM INTO THE MASS OF FIBROUS MATERIAL AND THEREBY EFFECTING HEATING OF THE LIQUOR AND AGITATION OF SUCH FIBROUS MATERIAL ADJACENT TO THE SCREEN.