US2993827A - Method and apparatus for controlling wood pulp digesters - Google Patents

Description

July 25, 1961 A. E. BENNETT 2,993,827

METHOD AND APPARATUS FOR CONTROLLING woon PULP DIGEsTERs Filed Feb. 11, 1954 HNDREW E, BEN/V577' LLL A-H-y.l

Patented July 25, 11.961

2,993,827 METHOD AND APPARATUS FOR CONTROLLING WOOD PULP DIGESTERS Andrew E. Bennett, Weymouth, Mass., assignor, by mesne assignments, to Worthington Corporation, New

York, N.Y., a corporation of Delaware Filed Feb. 11, 1954, Ser. No. 409,699 6 Claims. (Cl. 162-61) This invention relates to the control of pulp digesters, and concerns a method and apparatus for governing the heat input to the digester during the cooking process, the method and apparatus embodying this invention being particularly applicable to digesters of the direct steamheated type, with or without forced circulation, wherein the sulphate or soda process is utilized, although the invention is also adapted for use with the sulphite process for cooking wood pulp.

As is well-known to those skilled in the art, a digester is filled with wood chips of the character suited to the product and is then charged with cooking liquor until the major portion of the chips are covered. When the sulphate process is utilized, the liquor is in the form of water to which carbonates, sulphides and hydroxides have been added to provide a caustic solution -best adapted for the piupose. Heat is then applied to the mass by the introduction of steam at the bottom of the digester, it being common practice to raise the cook temperature as rapidly as possible without injuring the product to a selected maximum value in accordance with a predetermined vapor pressure-temperature curve at which value the cook temperature is preferably maintained until the pulp has attained the consistency desired.

It will be understood by those skilled in the art that the efficient operation of a pulp digester depends upon penetration of the cooking liquor into all of the chips and that this in turn depends upon proper circulation through the entire mass. And it has been found that during the heating up process of a sulphate digester quantities of air together with Volatile matter, such as turpentine or the like, which vaporize as the cooking process proceeds, seriously impedes circulation, and that these gases, although in gradually diminishing amounts, continue to accumulate for the balance of the cook. It therefore becomes necessary to relieve `the volatile gases during the cook cycle if the digester temperature is to conform to the vapor pressure-temperature curve. This may be accomplished manually or preferably by automatic means, a preferred form of automatic gas relief being disclosed in the United States Patent to I. M. Mc- Alear, No. 2,490,533, dated December 6, 1949.

While some form of gas relief is necessaryfor the satisfactory operation of a pulp digester, the other factor essential to eiiicient wood pulp cooking relates to the method and/ or apparatus for governing the heat input to the digester in order that the mass may be raised to the maximum cook temperature without upsetting the operation of the digester and injuring the product. And I have found that in direct steam-heated digester cooking the method and apparatus utilized for admitting steam to the digester has, so far as known to me, resulted under many cook conditions in injuring the product, increasing the rejects, and prolonging the cook schedule. As is Well-known to those skilled in the art, the cook conditions vary with the kind of wood used, its sap cntent, and the size, in respect to height and width, of the digester. It is common practice for the mill personnel to predetermine the kind of solution required for the wood chips being processed, the pressure-temperature curve to be employed, and the length of cook necessary to obtain the desired end product. This procedure is well understood and is customary throughout the industry. Having determined the pressure-temperature curve best suited to the cook, the operator attempts to govern the cook in accordance with that schedule either by manual control of the steam admission valve or preferably by automatic control of said valve, and when automatic control is utilized a time cycle controller provided with a cam cut in accordance with the predetermined cook curve may be employed.

Heretofore, so far as known to me, attempts have been made to conform to the predetermined cook schedule by using the pressure at the top or the upper part of the digester as a reference point, or by varying the rate of steam input to the digester in accordance with a rate of How curve which it is figured will provide a temperature rise to the mass to conform to that desired by the engineer in charge. ln automatic control when the top pressure method is followed, the time cycle controller responds to the top pressure and throttles the steam input valve as required to vary the top pressure as called for by the cam. When the rate of ow method is utilized, the reference is to changes in rate of flow only, the steam valve Ibeing governed to vary the rate of flow as prescribed by the cam without regard to the pressure conditions in the digester. While I have found that the rate of flow type of control is superior in some respects to the control which responds to changes in pressure in the upper part of the digester, both of these controls have failed to give the desired results for the reasons now to be set forth.

When the digester has been filled with wood chips of the kind required Ifor the product, hot cooking liquor is introduced at =a temperature which may be approximately degrees F., the surface of the liquor being raised to a predetermined level somewhat below the top of the digester. Since the chips are at atmospheric temperature, the liquor is cooled on contact with the chips and at this stage the liquor is well below the boiling point. Steam at a relatively high rate is then introduced to the bottom of the digester and since the mass is reasonably cool, the steam condenses rapidly. As the bottom of the mass becomes heated, circulation commences at the bottom of the digester and the steam gradually permeates further up into the mass, thereby resulting in further condensation. It is to be understood that closely packed wood chips tend to restrict the natural circulation of the liquid portion of the mass. Therefore during kthe pressure-temperature rise, if the rate of steam admission to the bottom of the digester exceeds the condensing rate, an excessive pressure is immediately created which may result in lifting the liquor and compressing the gases at the top of the digester. Thus a so-called false pressure is created at the top of the digester, which pressure does not correspond to the temperature of the mass at that point. Under these conditions the relief valve is either manually or automatically opened and when this occurs the liquor may rise to a level at which it enters the relief line, usually connected at the neck or dome of the digester, and when the liquor passes through the screen with which the relief line is provided in order to prevent the escape of wood chips, the chips may become lodged against the screen in su'icient quantity to plug it. It then becomes necessary to either automatically or manually blow steam back through the screen into the digester in order to clean the screen. Under these conditions, if the pressure in the upper part of the digester is the reference for governing the valve by which steam is admitted to the bottom of the digester, since this pressure is excessive, the valve is immediately closed. And this is true whether the valve which admits steam to the bottom of the Vdigester is manually operated or is governed by a time cycle pressure controller. Therefore the valve remains closed until the top pressure again returns to a value called for by the cook curve or by the cam of the time cycle controller. When the rate of ilow method is employed, if the rate of steam input exceeds that required by the cook conditions, excessive pressure is immediately created at the bottom of the digester, and if the pressure is suiiicient to raise the cook liquor in the manner noted above, the same false pressure conditions will be established.

It will be clear from the above that not only a portion of the cooking liquor has been lost, but since steam admission has been stopped, the circulation already established has been reduced. Under some conditions the upset referred to is repeated many times during the temperature rise period unless the rate of steam admission is sufliciently reduced to prevent it.

I have found from actual tests carried out with digesters in commercial operation that the diiculties referred to may be substantially eliminated by means of the method and apparatus embodying this invention. There is no substantial loss of cooking liquor, rejects have been re-` duced from 121/2% to 2% in'thirty-two permanganate stock, a high quality uniform product is obtained, and a very consider-able saving in cooking time results. Experiments were conducted with a digester which is l2 in diameter by 40 high. The digester was charged with chips and cooking liquor in the usual manner and two recorders were employed, one recording the digester top pressure and the other recording the digester bottom pressure. Before steam was admitted to the bottom of the digester, the bottom recorder indicated a pressure of 16 p.s.i., due to the weight of the mass, the pressure at the top being p.s.i. Without taking into consideration the effect of the static head pressure, in order to heat up the mass and at the same time to obtain substantially equal pressures at the top and bottom of the digester, the steam input valve was operated manually and steam, contrary to the usual custom, was admitted at a relatively slow rate until the top pressure reached 4 p.s.i. and the bottom pressure increased 4 p.s.i. above the static head pressure of 16 p.s.i., thereby providing a total bottom pressure of 20 p.s.i. The steam valve was then opened wide, as is the usual practice at the start of the cook, and the bottom pressure immediately rose from 20 p.s.i. to 40 p.s.i. Two minutes later the top pressure started to rise above 4 p.s.i. and stopped rising at 12 p.s.i., since the steam inlet valve was then manually closed. It will be noted that the pressure at the bottom of the digester immediately increased 20 p.s.i. and that after a lag of two minutes the top pressure started to rise and gradually increased 8 p.s.i. Thus a response to top pressure not only creates a lag in the control of the steam admission valve but it is not a true reference to the pressure condition which is essential to the proper operation of the cook, namely, to the pressure at the bottom of the digester where false pressure conditions originate.

After the experiments noted above, the digester was placed on automatic control. A time cycle pressure conk troller responsive to changes in bottom pressure was employed to govern the steam admission valve, the time cycle cam being cut to provide a pressure rise curve as best suited to the cook. Since the static head pressure was 16 p.s.i., the cam was cut to provide an initial pressure of approximately 16 p.s.i. The maximum pressure afforded by the cam was 118 p.s.i. which provided a top vapor pressure of p.s.i. since the initial static head pressure is normally increased by approximately 2 p.s.i. due to the weight of condensate added by steam condensation during the pressure rise period. Reference to bottom digester pressure was utilized, for if the steam input exceeds the condensing rate and tends to build up a pressure materially in excess of that called for by the cam, the steam `flow is reduced by throttling the steam admission Valve. And it will be understood that a material increase in pressure at the bottom of the digester is a pressure increase which may result in lifting the level of the cooking mass to the relief line.

Successive tests were carried out on the basis referred to above in the same digester under varying cook conditions. A large volume of steam was admitted at the start of the cook, as called for by the cam and governed by the pressure conditions at the bottom of the digester, until the bottom pressure reached 30 p.s.i. The rate of flow of steam was then gradually reduced by the cam until a maximum bottom pressure of approximately 118 p.s.i. was obtained, the pressure being maintained at this value until reduced at the end of the cook. The pressure rise was created over a period of one hour and the maximum pressure was held for a period of one and a half hours. The cooks which were governed in accordance with the method and apparatus embodying this invention produced a high grade of uniform pulp with no loss of cooking liquor, a low percentage of rejects, and materially reduced the cooking period heretofore normally required.

1t is an object of this invention to provide an improved method and apparatus for governing the cooking of wood pulp in digesters of the steamheated type, with or without forced circulation, whereby loss of cooking liquor and rejects are substantially eliminated, uniform high grade pulp is produced, and the time period required is materially reduced.

The invention will be more fully understood from the following description when taken in connection with the accompanying drawings in which:

B1G. 1 is a view, partly in diagram, of a pulp digester of the direct steamheated type showing a time cycle pressure controller for governing the steam input to the digester in accordance with my invention together with a preferred form of automatic gas relief mechanism;

FIG. 2 is a front elevation, enlarged, of the time cycle pressure controller shown in FlG. 1, and

FIG. 3 is a front elevation, enlarged, of the controller which governs the gas relief mechanism shown in FIG. 1.

Having reference to the drawings and particularly to FIG. 1, there is shown a digester 10 having the usual detachable cover 11 which is mounted on the digester neck 12 to provide for charging the digester with wood chips, and having a cone-shaped bottom 13 provided with a pipe 14 equipped with a hand Valve 15 through which heated cooking liquor may be introduced and the digester lled to a level as indicated by dotted line 16. The pipe 14 and lvalve 15 are also used to discharge the finished pulp at the end of the cook.

The digester 10 is provided with a steam admission line 17 which connects a source of steam supply, not shown, with a perforated ring type manifold 18, or equivalent steam distribution device, in the upper portion of the bottom 13 by means of a plurality of pipes 19. Steam admission to the digester through the pipe 17 is varied by a valve 20 as governed by a time cycle pressure controller 21 in a manner hereinafter to be described. The digester 10 is also provided with a relief line 22, having a connection 221, preferably connected with the neck 12 of the digester, and provided with the usual strainer or screen 2 3 to prevent the escape of wood chips'. The relief line is equipped with a relief valve 24 to permit the relief of gases from the digester and has connected therewith a blow-back line 25 communicating with a source of steam supply, not shown, and having a valve 26 for blowing steam into the upper part of the digester when required.

As herein illustrated, automatic gas relief means are provided, preferably the means shown and described in the aforesaid United States patent to l. M. McAlear. It has been found that when relatively large quantities of gas are intermittently relieved from the digester, this in itself creates a tendency for the mass to surge and carry over slugs of liquid, thereby causing some of the pulp to lodge against the screen, whereby it may become clogged. It is therefore preferable to provide means for gradually bleeding off the air and non-aqueous vapors as they are released, thereby reducing the surging tendency. Thus circulation is improved and the average digester temperature may be made more nearly to adhere to the vapor temperature of the cooking liquor.

Having reference to FIGS. l and 3 the gas off control includes the pressure-temperature relation controller 30 which may be of any usual construction, but is preferably similar to the controller described in detail in the patent to J. M. McAlear referred to above. It is provided with two helical coils 31 and 32 of which coil 31 is connected with the pressure in the dome of the digester as indicated at 33 by means of a pipe 34, and the coil 32 is responsive to chang in temperature at a point downstream of the strainer 23 by means of a temperature sensitive bulb -35 which is connected with the controller by capillary tubing 36 and filled with the usual expansible fluid. The bulb is preferably mounted in a side connection 37 with which an elbow 3S forming a part of the relief line 22 is provided. The coils 31 and 32 are adapted to jointly actuate a ilapper 40 in respect to a nozzle 41 by means of a ratio movement, generally indicated at 42. The nozzle 41 is mounted on a support member 43 carried by the extension 44 of a circular member `45 which is rotatably mounted on a controller back plate 46 by means of a fixed bearing ring 47. The circular member 45 has a gear segment 48 which meshes with the pinion 49 secured to a shaft 50 by which the support member 43 may be rotated to any selected position. The apper is pivoted at 51 to the extension 44 and is positioned in respect to the nozzle 41 by means of a follower arm 52 which cooperates with a cam 53.

The ratio movement 42 by which the ratio between the temperature, in terms of pressure, of uids as they leave the digester and the fluid pressure in the top of the digester may be established and maintained, is comprised essentially of four members which are so arranged in respect to their relative positions and movements as substantially to constitute a parallelogram under all conditions of setting and control. These members comprise an arm 55 rigidly secured to a pivot shaft 56 to which a pointer 57 is also rigidly secured; an arm 58 rigidly connected to a pointer 59 which is in turn pivotally mounted on the shaft 56; and a lever 66 and link 61, the lever being pivoted at one end to the free end of the arm 55 and at its free end being arranged to carry the cam 53, and the link 61 being pivotally connected with the lever and arm 53 as shown. The cam 53 may be adjustably secured to the lever 60 by means of a frictional mounting such as a screw 62 or the like. The coils 31 and '32 are arranged to rotate arms 58 and 55, respectively, arm 58 being operatively connected with the coil 31 by an arm 67 and link 63 and responsive to changes in pressure at the top of the digester through the pipe 84, and the arm 55 being operatively connected with the coil 32 by means of an arm 78 and link 79 and responsive to changes in relief line temperature through the capillary 36. A setting index pointer *64 is secured to the circular portion 45 of the assembly support and together with the pointers 57 and 59 cooperates with a scale 65. The selected ratio setting is obtained by turning the index pointer 64 to establish the initial relation between the nozzle 41 and the flapper 40. Rotation of the assembly support extension 44 counterclockwise to move the setting indicator y64 from 0 on the scale 65 increases the ratio, and as the index pointer is turned clockwise and approaches `0, the differential is decreased. A simple method of establishing the ratio setting for a given liquor solution is to set the indicator index pointer 64 at a position at which the liquor solution vapors just start to bleed through the relief valve.

The nozzle 41 is supplied from a source of operating fluid under regulated pressure, not shown, through a pipe 68 having a restriction 69 of less capacity than that of the nozzle, and the pipe 68 is in operative connection, at a point downstream of the restriction 69, with the relief valve 24 by means of a pipe 70' in which a three-way valve 71 is mounted. The relief valve 24 is actuated by a diaphragm 72 which partially defines a pressure chamber 73 to which the pipe 76 is connected. The diaphragm 72 is backed by a spring 74 and the parts are so arranged that the valve opens on an increase of pressure in the diaphragm chamber 73.

The valve 26 in the blow-back pipe 25 is of similar construction to the relief valve 24 and like that valve is adapted to open on an increase of pressure in its diaphragm chamber 75. The chamber 75 is in communication with a source of operating uid under regulated pressure, not shown, by means of a pipe 76 in which a three-way valve 77 is mounted.

The three-way valves 71 and 77 are solenoid operated and may be similar in detail to those shown and described in the said United States patent to J. M. McAlear. The valve 71 is arranged so that when its solenoid 80` is deenergized, the diaphragm chamber 73 of the relief valve 24 is connected with the nozzle pressure in the pressure-temperature relation controller 30, and when the solenoid is energized, the diaphragm chamber is cut oif from the nozzle pressure and connected with atmosphere. On the other hand, the valve 77 is arranged so that when its solenoid 81 is deenergized, the diaphragm chamber 75 of the blow-back valve 26 is connected with atmosphere, and when the solenoid is energized, the diaphragm chamber is connected with the source of uid operating pressure. Means for governing the operation of the solenoid valves is in the form of a pressure switch 82 which is governed by the nozzle pressure in the pipe '70 to which it is connected by means of a pipe 83. The switch 82 may include a bello-ws, not shown, which is opposed by a spring, also not shown, set to permit the bellows to expand when a predetermined pressure is exceeded and, by means of a member 84, closes switch members 35 and 86 which connect a suitable source of electric current with the solenoids '80 and 81 all as shown.

In operation, when the cook is started, steam is introduced into the bottom of the digester through the inlet valve 20 as governed by the time cycle pressure controller 21 in a manner hereinafter to be described. The pressure is raised in accordance with a predetermined schedule to a selected maximum, is maintained at that maximum over a given period, and is then reduced at the end of the cook. As the pressure is raised, air and other non-aqueous vapors collect at the top of the digester and enter the relief line 22. It may be assumed, for example, that the pressure at the top of the digester is l0 p.s.i. and that the temperature of the liquor vapor at 10 p.s.i. is 240 degrees F. As air and other non-aqueous vapors enter the relief line, the temperature at the bulb 35 will fall below 24() degrees F., the helical coil 32 will wind up, turning pointer 57 and arm 55 clockwise, thereby raising the cam 53 and apper 4i). The nozzle pressure will then increase and open the relief valve u2,4 to proportionally increase the flow of gases from the digester. As the air and volatile vapors escape and are replaced by vapors of the liquid solution, the temperature at the bulb 35 rises, coil 32 unwinds turning pointer 57 counterclockwise to lower the cam 53 and flapper 40 and reduce the opening of the relief valve until the ratio established by the ratio setting of the controller 30 is restored.

It wiil be understood that as the top pressure in the digester rises above 10 p.s.i., the coil 31 unwinds, turns the pointer 59 counterclockwise and with it the arm 5S, lever 60 and cam 53, thereby tending to raise the flapper 40 to increase the operating pressure and open the` relief valve. However, if the temperature at the bulb 35 increases at the same ratio, the temperature responsive coil 32 will also unwind and turn the pointer 57 counterclockwise, so that the arm 55 lowers the cam 53 and tends to reduce the nozzle pressure and close the relief valve 24. Thus, under these conditions, irrespective of the pressure in the top of the digester, the dapper-nozzle relation remains undisturbed and the opening of the relief valve is not varied.

The blow-back mechanism functions to clear the screen 23 should occasion for doing so arise. It may be assumed that the pressure-temperature relation controller has an output pressure operating range from 0 to 2() p.s.i., that the relief valve '24 is closed at a pressure of 3 p.s.i. and is fully open at a pressure of 14 p.s.i., and that the pressure switch 82 is set to close the switch contacts at 16 p.s.i. and to keep the contacts closed until the pressure again drops below that value. It may also be assumed that the blow-back valve 26 is closed at a pressure of 3 p.s.i., is fully open at a pressure of 14 p.s.i., and that the fluid pressure in the pipe 76 is supplied at 20 p.s.i. Should the strainer 23 become clogged suiciently to interfere with the flow of gases and render the relief valve ineffective to maintain the required pressure-temperature ratio, the temperature at bulb 35 will fall, thereby causing the controller 30 to increase the output pressure proportionally to the increase in the pressure-temperature ratio. When the pressure-temperature differential expands to a value at which an output pressure of 14 p.s.i. is reached, the relief valve 24 is opened wide. :If the temperature at the bulb continues to fall, when the operating pressure arrives at 16 p.s.i., the pressure switch S2 will close the circuit and will energize the solenoid 80 of the valve 71, thereby cutting olf the diaphragm chamber 73 of the relief valve 24 from the nozzle pressure of the controller and venting the diaphragm chamber to atmosphere. At the same time the three-way valve 77 will close the vent to atmosphere of the diaphragm chamber 75 of the blowback valve 26 and will admit fluid operating pressure to the said diaphragm chamber. Thus the Valve 26 will open wide to admit steam .to the strainer 23, and the relief valve 24 will close to prevent the escape of steam through the relief line. The blast of steam will not only tend to clean the strainer but at the same time will raise the temperature at the bulb 35, and when the temperature at the bulb arrives at a value within the selected pressure-temperature range at which the controller reduces the output pressure below 16 p.s.i., the switch will open the contacts 35 and 86, thereby closing the blow-back valve 26 and opening the relief valve 24 wide. If the strainer has been freed by the blast of steam, -the relief valve will function to maintain the set pressure-temperature relation. 1f however, the blast of steam fails to clear the strainer, the temperature at the bulb will again drop, and the cycle just described will be repeated until the strainer yis cleared and normal operation is resumed. While the steam blow-back mechanism is preferably provided in connection with the gas relief controller, I have found that the tendency of the cooking mass to surge and enter the relief line is substantially eliminated by the method and apparatus embodying this invention `for governing the admission of steam to the bottom of the digester.

In accordance with my invention, I preferably employ means for automatically governing the admission of steam to the digester in the form of the time cycle pressure controller 21 which is responsive to changes in pressure at Ithe bottom of the digester through a pipe and varies the bottom pressure of the digester in accordance with a predetermined cook schedule.

The controller may be of any well-known construction, a simple form of mechanism which serves the purpose of this description being herein illustrated. Referring to FIG. 2, the controller generally comprises a Bourdon coil 91, a proportioning device 92, which together with the coil positions a dapper 93 `in respect to a nozzle 94, and a cam 95 -by which the set point of the controller is varied in accordance with the predetermined cook schedule. The coil 91 is of the spiral type, as herein shown, and is secured at its outer end to a block 96 which is mounted on the back plate 97 of the controller. The inner end of the coil 91 is adapted to rotate a shaft 98 which is journalled at its inner end Iin a bottom member, not shown, secured to the back plate 97, and at its outer end in an outer member 99 also mounted on the back plate. The fixed end of the coil 91 is connected by means of the pipe 90, a seal chamber 150, and a pipe 151 to the cone bottom 13 of the digester so that the coil is subjected to the bottom pressure therein.

.A water purging system may be employed to prevent the pipe 151 from becoming clogged with wood fibre. For this purpose the seal chamber is connected with a pipe 152 having a suitable source of water supply under pressure, not shown. The pipe 152 may include a strainer 153, a restriction 154 downstream of the strainer, and a reducing valve 155 downstream of the restriction, the parts being illustrated diagrammatically. It may also include a check valve 156, also diagrammatically shown, to prevent charging liquor, when at a higherpressure than the water supply pressure, from being forced back through ythe supply line in the event of failure of the reducing valve 155. The pipe 151 includes a shut-oif valve 157 which together with the water purging system functions in a manner to be described.

Referring again to the controller 21, an arm 160 is secured to the shaft 9S so that it rotates therewith. The arm 160 is adapted to actuate an angle lever 161 about its fixed pivot 8 by means of a floating link 2. The angle lever 161 includes an indicator arm 101 which is adapted to cooperate with a suitable scale 102, and also includes an actuating arm 100 which positions the flapper 93 in respect to the nozzle 94 in a manner to be described. The floating link 2 consists of two members 3 and 5 which are adjustably connected as indicated at 4 to enable the length of the lever to be varied and therefore the controller to be zeroed in a manner to be described. And means for calibrating the movement of the angle lever 161 as required for any selected range of coil pressure change may consist of a series of openings 6 in the arm 160 and a series of openings 7 in the pointer 101, one end of the link 2 being pivoted at 9 in a selected opening in the arm 160, and the other end of the link being pivoted lat in a selected opening in the indicator 101. In operation, an increase in bottom pressure in the digester causes the actuating arm 100 and pointer 101 to rotate counterclockwise, and on a decrease in the lbottom pressure, the said members rotate in the opposite direction.

The proportioning device 92 may include a bellows 105 which is sealed at one end to a xed mounting member 106 and at the other end to a disk 107. The proportioning device also includes a helical coil spring 103 which is confined between a fixed mounting member 109 at one end and a disk 110 at the other end, the disks 107 and 110 being connected by means of a rigid post 111. The interior of the bellows 105 communicates through a pipe 112 with a uid operating pressure line 115 which connects with a source of regulated fluid pressure, not shown, by means of a pipe 116 having a restriction 117.

The line 115 also connects with the nozzle 94 which may be adjusted lengthwise of a mounting member 118 secured to the back plate 97, the line 115 having a flexible portion 119 to permit the nozzle to be moved to selected positions on the said member.

Fluid operating pressure in the line 115 is varied by means of the iiapper 93 which has a pivotal connection 120 to the post 111 of the proportioning device at one end, and at the other end has a slot and pin pivotal connection 121 with one end of a floating lever 122. It will be understood that the capacity of the restriction 117 in the pipe 116 is less than that of the nozzle 94 so that the operating pressure in the line 115 is determined by the position of the flapper 93 in respect tol the nozzle. The other end of the floating link 122 has a slot and pin pivotal connection 123 with the free end of the actuating arm 100, and midway its length has a pivotal mounting `124 on the upwardly extending arm 125 of a set point lever 126. One end of the set point lever 126 has a pivotal connection 127 with a bracket 128 secured to the back plate 97, and the other end of the lever provides an index pointer 129 which cooperates with a suitable control point setting scale 130. The lever 126 is rotatably positioned about its pivot 127 by means of the cam 9S which coacts with a roller 131 mounted on a depending portion 132 of the lever, the parts being held in engagement by means,

of an extension spring 133 which is connected with the lever at one end and at the other end is connected to a pin 134 iixed to the back plate. The cam 95 is mounted on a shaft 135 which is driven by any well-known type of clock mechanism, not shown. The cam may have a rotation period suited to the cooking conditions, a four hour rotation period for example, being common in sulphate digester cooking.

The steam admission valve 20 is actuated by a diaphragm 140 which partially defines an operating pressure chamber 145 having a connection with the operating pressure line 115 by means of a pipe 141. The diaphragm 140 is backed by a spring 142 which may be adjusted for any suitable opera-ting range, such as from 3 p.s.i. to l5 p.s.i. As herein shown the valve is of the reverse acting type, being closed at 3 p.s.i. and fully open at l5 p.s.i. It will be understood that the output pressure range of the controller may be from (l p.s.i. tor 20 p.s.i. in accordance with the usual practice, and it will also be understood that while the simplified construction herein illustrated does not include a pilot commonly used to speed up the valve action, any well-known type of pilot may be utilized for this purpose if desired.

In controlling a digester cook by the apparatus herein above described, the cam is cut to conform to the cook schedule laid out by the mill personnel. It is customary in accordance with the practice at some mills to provide at the start of the cook a pressure rise of substantially 30 p.s.i. as rapidly as is possible without disturbing the cook, since the more rapidly the steam can be introduced, the better the circulation. From that point on the rate of pressure rise depends on the type of wood, the circulation of the liquor, and the end product obtained. The pressure-temperature rise period may vary from half an hour to three hours or longer and the holding period from one half an hour to live hours depending on the mill practice and on the cooking conditions.

In carrying out the experiments referred to above, the controller 21 was located at the bottom of the digester and the cam 95 was cut to provide an initial pressure which substantially corresponds to the static head pressure of the mass. This pressure is indicated by the pointer 1 on the scale 102 or, in the event that a recording chart is used, by the recording pen. Thus since the static head pressure was 16 p.s.i., the cam was cut to provide a starting pressure substantially at that value. Due to the gradual accumulation of condensate during the pressure rise period, the cam may also be cut to allow for the additional static head of liquid, normally about 2 p.s.i.

1G Since the cook schedule called for a rise in digester' pr'essure over a periodof one hour from a starting pressure of 0 p.s.i. to a maximum holding pressure of 100 p.s.i. as measured at the top of the digester, at which pressure the cook is held for a period of one and a half hours, the cam was arranged for a starting pressure of substantially 16 p.s.i. and a maximum pressure of substantially 118 p.s.i. By this means the cook pressure required at the top of the digester and the temperature corresponding thereto is in accordance with the predetermined cook schedule.

When the cook was started, the relief mechanism was set to continuously bleed off the non-aqueous vapors to maintain the selected pressure-temperature relation at the top of the digester. The cam was set a-t its starting point and under the conditions just referred to, the roller 1311 positions the pivotal connection 124 with the lever 122 at a point at which a control setting value of substantially 16 p.s.i. is provided all as shown on the index scale 130. Since the Bourdon coil 91 is subjected to the static head pressure of 16 p.s.i. as indicated on the scale 102, the ilapper 93 is positioned in respect to the nozzle 94 to provide a minimum output pressure in the diaphragm chamber 145 of the valve 20. The valve therefore remains closed. However, as the cam rotates counterclockwise, the pivotal connection 124 is raised, thereby rotating the oating link 122 in a counterclockwise direction around its pivotal connection 123. This movement of the link 122 rotates the flapper 93 clockwise about its pivotal connection at the proportioning device, thus increaseing the nozzle pressure. The increase in nozzle pressure is transmitted to the proportioning bellows 105 which expands and returns the iiapper 93 to a throttling relation with the nozzle 94 at an increase in output pressure which increase is proportional to the increase in the pressure setting established by the cam 95. The diaphragm of the steam valve 20 responds to this increase in output pressure and the valve is opened in proportional relation thereto to admit steam to the digester. As the pressure increases in the bottom of the digester, the Bourdon coil 91 rotates the actuating arm 100 counterclockwise, thereby swinging the link 122 clockwise and tending to move the flapper away from the nozzle to reduce the increase in output pressure initiated by the cam. When the lefthand pivotal connection 121 of the apper moves down, the bellows 105 contracts, raises the righthand connection 120 of the apper, and modulates the pressure change established by the coil 91 on a basis which varies with the width of the proportioning band. It will be understood that the width of the proportioning band may be varied by positioning the nozzle 94 along the support member 118, and that when the nozzle is positioned to the left, as viewed in FIG. 2, the proportioning band is narrowed and vice versa. Thus when the bottom pressure increases to the value established by the cam, the coil 91 functions together with the proportioning device 92 to maintain the pressure substantially at that value. If the rate of seam input-exceeds the rate of condensation in the digester, and thereby tends to provide a pressure which is in excess of that established by the cam, the coil 91 turns the actuating arm 100 counterclockwise and reduces the nozzle pressure, whereby the steam input is decreased and the pressure returned substantially to the established setting. Thus a material excess in bottom pressure is prevented and the mass is not permitted to be lifted to a point where the liquor escapes through the relief line 22 and tends to clog the strainer 23y with chips or partly digested pulp.

It will be understood by those skilled in the art that when the controller 21 is located above the digester as herein dagrammatically illustrated, the purging system -described above may be employed to lill the pipe 90 with water. This is accomplished by setting the reducing valve 1155 at a pressure required to raise the water to the top of the pipe 90. At this time the valve 157 is closed. The connection between the pipe 90 and the coil 91 is broken and when the pipe is filled the end of the pipe is again secured to the coil so that a sealed joint is provided. Whe-n the digester has been charged and the liquor is at the level as indicated by the `dotted line i6 for example, and the rst cook is started, the valve `i157 is opened. This permits a small flow of water, as determined by the restriction 154, to pass into the bottom of the digester, thereby clearing the pipe 151. When the pressure in the bottom of the digester reaches the pressure at which the reducing valve 155 is set, the reducing valve closes and the purging water is automatically shut off. Since the pipe 52 is connected with the seal chamber 150 at a point above the connection of pipe 90, air which enters with the water through the pipe 152 is prevented from passing into the pipe 90 and is vented through the pipe 151 into the digester. The controller 21 may be calibrate to correct for the difference in liquid hea-d in the digester and in the pipe 90 by means of the lengthwise adjustment provided by the floating link 2. Thus the controller may be zeroe-d at the liquor level indicated by the dotted line ld. Furthermore the controller 2l can be calibrated to compensate for the rise in liquid head due to steam condensation accumulating during the pressure rise period by suitably positioning the ends of the floating link 2 on the arm 160 and on the indicator 101. On this basis the cam 95 can be cut to operate from an initial bottom pressure of zero to the maximum holding pressure desired.

It will be understood that While I have shown and described a time cycle controller responsive to changes in the pressure `at the bottom of the digester, a controller responsive to changes in temperatureV at that point in the digester may be equally well employed, since variations in temperature vary in proportional relation to variations in pressure. And it will be further understood that while I have herein shown and described automatic means for governing a pulp digester cook in accordance with my invention, when hand control is relied upon, the operator may vary the steam input as required to cause the pressure-temperature conditions at the bottom of the digester to conform to the cook schedule-by reference to a suitable recorder, indicator or the like responsive to said pressuretemperature conditions.

I regard the invention herein shown and described as an important advance in the art of digester cooking since, so far as known to me, there has never been any adequate method or means for governing the admission of steam to the digester, which means function together with a suitable gas relief mechanism to prevent thecooking mass from surging and upsetting the cook in the manner herein described.

Having thus described my invention, what I claim as new and desire lto secure by Letters Patent of the United States is:

l. The method of controlling the cooking of wood chips and cooking liquor disposed in a direct steam heated digester at a given static head comprising, admitting steam to the, lowermost portion of the digester over a selected period of time in accordance withk a predetermined bottom pressure temperature cook schedule, varying the admission of steam during the cooking schedule as the pressure temperature conditions vary at the lowermost portion of the digester, and relieving non-condensable gases formed in the digester from the uppermost portion ofthe digester during said selected period of time.

2. The method of controlling the cooking of a wood chip and cooking liquor mass at a given static head in a digester of the direct steam heated type comprising, admitting steam to the lowermost portion of the di gester over a selected period of time to raise the heat of the wood pulp and cooking liquor in accordance with a predetermined bottom pressure temperature cook schedule, varying the admission of steam duringthe cooking schedule in accordance. With the rise curve for the, pressure temperature conditions at the lowermost portion of the digester, controlling variationsY in the amount of admitted steam as a function of the differential between the bottom pressure and the top pressure plus the given static head of the mass, andrelieving non-condensable gases formed in the digester from the uppermost portion of the digester during said rise period.

3. The method of controlling the cooking of wood chips and cooking liquor disposed in a direct steam heated digester at a given static head comprising admitting steam to the lowermost portion of the digester in accordance with a predetermined bottom pressure temperature cook schedule over a selected period of time, varying the admission of steam during the cooking schedule in accordance with the rise in pressure temperature conditions at the lowermost portion of the digester, controlling variations in the admitted steam to prevent the amount of steam from materially exceeding the condensing rate for the cooking schedule, and relieving non-condensible gases formed in the `digester from the uppermost portion of the digester during said selected period of time.

4. The method controlling the cooking of wood chips and cooking liquor disposed in a direct steam heated digester at a given static head as set forth in claim 3 which includes controlling variations in the steam admitted during the rise in pressure temperature conditions at the lowermost portion of the digester so that the pressure of the digester at the lowermost portion does not exceed the pressure at the uppermost portion of the digester by an amount materially in excess of the given static head pressure in the digester.

5. The combination With a direct steam heated digester having wood chips and cooling liquor disposed therein at a given static head of, a steam admission line communicating With a source of steam and connected to the lowermost portion of the digester, a valve in the steam admission line to regulate the flow of steam to the digester, and a controller including, means for varying the setting of said controller in accordance with a predetermined cook schedule over a selected period of time, means connected between said cont-roller and said valve and to actuate the valve in proportional relation to changes in the controller effected by said rst mentioned means, and means connected to the lowermost portion of the digester Yresponsive to changes in pressure temperature conditions at the lowermost portion of the digester, said last mentioned means operatively associated with said first mentioned means to vary admission of fthe steam whereby the bottom pressure temperature conditions remain substantially in accordance with the setting eifected by the rst mentioned means, and relief means connected to the uppermost section of the digester to permit escape of noncondensible gases from the digester.

6. The combination with a direct steam heated digester having wood chips and cooling liquor disposed therein at a given static head of, a steam admission line communicating with a source of steam and connected to the lowermost portion of the digester, a valve in the steam admission line to regulate the ilow of steam to the digester, and a controller including, a cam for varying the pressure setting of said controller, said cam cut to provide a rise in pressure at the bottom of the digester over a selected period of time substantially parallel to the rise in pressure at the top of the digester plus a constant pressure diiierential equal to the given sta-tic head, a source of power for said controller, first mentioned means associated with said controller for varying the power from said source to said valwe in proportional relation to changes in pressure setting effected by said cam, second means associated with said first means responsive to changes in pressure at the bottomy of the digester to maintain the said bottom pressure in accordance with the setting effectedv by said cam, and relief means connected to the uppermost section 2,993,827 13 14 of the digester to permit escape of non-oondensable gases 2,402,705 Short .lune 25, 1946 from the digester. 2,490,533 MCAlear Dec. 6, 1949 OTHER REFERENCES Ehrisman: Automatic Digester Control for Alkaline Pulp Cooking, Tappi, Sec., pp. 195-19'8, 342-345, Nov.

References Cited in the file of this patent UNITED STATES PATENTS

Claims (1)

1. THE METHOD OF CONTROLLING THE COOKING OF WOOD CHIPS AND COOKING LIQUOR DISPOSED IN A DIRECT STEAM HEATED DIGESTER AT A GIVEN STATIC HEAD COMPRISING, ADMITTING STEAM TO THE LOWERMOST PORTION OF THE DIGESTER OVER A SELECTED PERIOD OF TIME IN ACCORDANCE WITH A PREDETERMINED BOTTOM PRESSURE TEMPERATURE COOK SCHEDULE, VARYING THE ADMISSION OF STEAM DURING THE COOKING SCHEDULE AS THE PRESSURE TEMPERATURE CONDITIONS VARY AT THE LOWERMOST PORTION OF THE DIGESTER, AND RELIEVING NON-CONDENSABLE GASES FORMED IN THE DIGESTER FROM THE UPPERMOST PORTION OF THE DIGESTER DURING SAID SELECTED PERIOD OF TIME.

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