WO2014184443A1

WO2014184443A1 - A system and method to regulate consistency in a blow or pulp tank

Info

Publication number: WO2014184443A1
Application number: PCT/FI2014/050368
Authority: WO
Inventors: Kari Peltonen; Janne Vehmaa
Original assignee: Andritz Oy
Priority date: 2013-05-15
Filing date: 2014-05-15
Publication date: 2014-11-20

Abstract

The present invention relates to a system and method for blow tanks and other tanks that temporarily hold pulp. The invention particularly relates to regulating the consistency of the material in such tanks. Pulp discharged from the tank is recycled into the tank (12) through at least one feed conduit (44) arranged at one or more elevations around the tank. The feed conduit (44) includes a feed conduit inlet connected to an outlet of the circulation conduit (42) and a feed conduit outlet is configured to direct extracted pulp back into the tank (12).

Description

A SYSTEM AND METHOD TO REGULATE CONSISTENCY IN A BLOW OR PULP

TANK

BACKGROUND OF THE INVENTION

[0001 ] The present invention relates to blow tanks and other tanks that temporarily hold pulp and other similar materials. The invention particularly relates to regulating the consistency of the material in such tanks.

[0002] Blow tanks, storage towers and pulp tanks temporarily store pulp and other similar materials as part of processes to produce, for example, pulp. The tanks may be relatively large vessels that receive pressurized pulp from digester reactor vessels. Blow tanks may have capacities of 75 to 200 tons of a pulp-liquor slurry. The blow tanks allow the pressure on the pulp to be released as steam and discharge the pulp at atmospheric pressure. Blow tanks for pulp are commonly used in batch pulp cooking processes. Storage towers and pulp tanks are also typically large vessels that provide intermediate storage of slurries of materials in an industrial process, such as pulping.

[0003] In a typical conventional operating cycle for a batch cooking process, a digester reactor vessel is filled with wood chips and steam is added to pressurize and heat the chips. Warm black liquor is fed to the digester. The black liquor impregnates the chips. After impregnation, black liquor is removed from the digester reactor vessel and returned to an accumulator vessel. Thereafter, the digester reactor vessel is filled with hot black and white liquors to be used to cook the chips. The chips in the digester are cooked for the predetermined time and temperature to form pulp. After cooking, filtrate liquid is pumped into the digester reactor vessel to displace the hot liquor which is removed from the vessel. The pulp and filtrate form a slurry which is discharged from the digester reactor vessel and pumped under pressure to the blow tank.

[0004] These steps of the batch cooking operating cycle are performed sequentially. The pulp slurry mixture flows from the digester reactor vessel to the blow tank only during the discharge step of the process. During the other steps, pulp does not flow to the blow tank.

[0005] Due to the operating cycle for batch cooking, the flow of a pulp slurry to the blow tanks and other tanks is not a consistent stream and is rather an intermittent flow. The consistency of the pulp discharged e.g. from the digester may vary and it may be low. These features may cause flow channels in blow tanks and other tanks. A crack forms in the pulp which allows local pulp velocity differences. Friction between stagnant and flowing pulp generates a pressure gradient over the boundary causing the stagnant pulp to dewater into the boundary layer. The consistency of the stagnant pulp increases, and the flowing suspension undergoes dilution, which aggravates channeling. Accordingly, typically the consistency of the material, e.g., pulp slurry, in blow tanks, storage towers, and other pulp tanks tends to vary widely during an operating cycle, such as that of the batch cooking process. Wide variations in consistency are especially common in tanks located immediately downstream of a batch digester reactor vessel. The variations in pulp consistency may be greatest between the start and end of the step in which the pulp slurry is discharged from the digester reactor vessel and blows into the blow tank.

[0006] FIGURE 1 shows a conventional system in which pulp 10 from a batch digester reactor vessel flows into one or more inlets into a blow tank 12. As the pressure is released from the pulp, gas 14 is discharged from the tank. The pulp and liquor settle in the tank, and are discharged from a lower discharge outlet 16. A pump 18 moves the pulp and liquor slurry from the lower discharge outlet to other processes in a mill or other industrial pulping plant. Dilution liquid 20 may be pumped 22 into the bottom 24 of the blow tank and the conduits 26 for the discharged pulp. A flow indicator controller (FIC) 28 monitors the flow of pulp through the conduit 26 and adjusts a valve 30 to regulate the pulp flow from the blow tank. Similarly, a quantity indicator controller (QIC) 32 monitors the consistency of the pulp in the conduit 26 and adjusts valves 34 to regulate the amount of dilution liquid being added to the pulp flow.

[0007] Agitators 36, e.g., stirrers, are located in the bottom of the pulp tank and mix the dilution liquid into the pulp. The addition of the dilution liquid and mixing provides some means to control the consistency of the pulp as it is discharged from the blow tank. However, channeling of liquors in the blow tank can overwhelm the ability of the agitators to regulate the consistency of the discharged pulp and lead to large variations in the consistency of the pulp slurry discharged by the blow tank. The blow tank typically comprises a cylindrical section, an inverted frusto-conical section and a cylindrical dilution/mixing section. The narrowing flow path just downstream of the upper cylindrical section may cause arching of pulp and thus worsen channeling. [0008] Large variations in the consistency of pulp in a tank typically lead to difficulties in the processing of the pulp from the blow tank. A basic reason is that a low consistency slurry, e.g., five to nine percent (5-9 %) of pulp, is prone to forming liquor channels in a tank. These channels are unwanted as they disrupt the uniform treatment of pulp in the tank so that it is not possible to stabilize the pulp consistency and the pulp outflow from the tank. Further, channeling in a blow tank can cause the pulp discharged from the tank to have an excessively low consistency and adversely affect downstream processes such as pulp washing and pulp screening. Channeling in the blow tank may be due to low consistency pulp in the tank or low consistency of pulp flowing from the batch digester reactor vessel to the blow tank. There is a long felt need to develop processes to reduce variations in consistency in slurried materials held in blow tanks, storage towers and other pulp tanks.

BRIEF SUMMARY OF THE INVENTION

[0009] The present invention relates to a system to process pulp in a blow tank or a storage tank as recited in claims 1 and 9 and a method as recited in claim 16. Preferred optional features are recited in the dependent claims.

[0010] Novel methods and systems have been invented to treat pulp flowing in a blow tank and discharged from the blow tank. A portion of the pulp is recycled back into the tank. The pulp is recycled at least during periods in which there is a low consistency, e.g. below a threshold consistency level, of material in the discharge of the blow tank. Adding pulp back into the blow tank equalizes the consistency in the tank. Typically the pulp is fed to a blow tank from a batch digester.

[001 1 ] A circulation system of conduits, valves and pumps may be arranged to direct a portion of the pulp downstream of the blow tank back into the tank. The circulation system may be arranged to stabilize the discharge consistency of pulp from the blow tank, other pulp storage tank or another pulp tank or tower. The circulation system may also be used to avoid channeling of pulp down through the tank.

[0012] The circulation system may include an annular array of feed conduits, such as nozzles symmetrically arranged around a blow tank at at least one elevation. Preferably the nozzle are substantially equally spaced about the tank. The annular array may be positioned at or just above an inverted frusto-conical section of the blow tank. The annular array may be in the lower half or quarter (50% to 25%) of the height of the blow tank or of a cylindrical upper section of the tank. Further, nozzles to direct recycled pulp may be arranged in the inverted frusto-conical section of the tank. The pump for the circulation system may be the same pump 18 used to pump the pulp discharged from the blow tank to other processes or a separate pump dedicated to the circulation system.

[0013] Adding the pulp at low velocities into the tank and at various positions around the circumference of the tank prevents channeling of liquor down through the tank.

Preferably, the velocity of pulp flowing into the tank may be in a range of two tenths to one and a half meters per second (0.2 to 1 .5 m/s).

[0014] The circulation system may assist in achieving a uniform distribution of pulp, e.g., uniform consistency, across the entire frusto-conical section of the tank and/or upper cylindrical section. Pulp is preferably distributed equally to the cone area of the tank so that pulp can be removed from the whole tank area or tank volume.

[0015] The circulation system may achieve several benefits including, more uniform consistency of the pulp discharged from a blow tank, reduce variations in the

consistency of the discharged pulp, enhance the ability of the agitator 36 to achieve complete mixing and uniform consistency of the discharged pulp, reduction or elimination of channeling of suspensions in the tank, stabilizing the flow of pulp and the consistency of the flow downstream of a blow tank.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] FIGURE 1 is a side view of a conventional process using a blow tank.

[0017] FIGURE 2 is a side view of a novel process using a blow tank having a pulp recirculation system. Exemplary positions in the tank for pulp recirculation conduits are illustrated.

[0018] FIGURE 3 is a top down view of a novel process using a blow tank having a pulp recirculation system.

[0019] FIGURES 4 to 7 are side views of a novel process using a blow tank having various types of pulp recirculation systems. DETAILED DESCRIPTION OF THE INVENTION

[0020] FIGURE 2 shows a blow tank 12 to which pulp 10 is fed, for example, from a batch digester (not shown). The blow tank has a first embodiment of a pulp recirculation system 40 having conduits 42 directing pulp extracted from the pulp flowing through conduit 26 to arrays of feed conduits, such as nozzles 44 at various elevations on the blow tank. The feed conduit, such as a nozzle, is connected to an outlet of the circulation conduit 42, or the feed conduit, such as a nozzle, consists of the outlet of the conduit 42 for introducing recycled pulp through the tank wall. Each circulation conduit 42 includes a valve 46 that may be adjusted, for example, by a hand controlled indicator (HI) to regulate the pulp flow into the tank 12. The elevations for each of the nozzle arrays 44 may be, for example, (i) near the transition 50 between the inverted frusto-conical section 52 and the cylindrical section of the tank 56, (ii) at the lower quarter of the height (H) of the cylindrical section 56 and (iii) at the middle of the height of the cylindrical section 56.

[0021 ] The discharge of the tank is typically in a cylindrical section 58 below the frusto- conical section. The agitators 36 and discharge outlet 16 are conventionally in the lower cylindrical section 58 of the blow tank.

[0022] The upper cylindrical section 56 of a blow tank may function to release the pressure of the incoming pulp. The upper cylindrical section may have an upper gas phase region and a lower pulp-slurry region. The nozzle arrays 44 for the recycled pulp may be at and below the upper surface of the pulp-slurry region. The nozzles arrays feed recycled pulp along the inner wall of the blow tank, which is where stagnant higher- consistency pulp portions are known to occur. By introducing pulp at the inner wall of the tank, the feed pulp tends to push the higher-consistency pulp towards low-consistency areas. Thus the fed pulp tends to reduce the likelihood of channeling and stop channels when they occur.

[0023] The diameter of the upper cylindrical section 56 is typically greater than the diameter of the lower cylindrical section 58. The lower cylindrical section and possibly frusto-conical section 52 may function as a dilution zone in which dilution liquid is mixed with and dilutes the pulp in the blow tank. [0024] The discharging pump 18 may be dimensioned to have higher flow than the flow used for a conventional blow tank that lacked a recirculation system 40. The extra capacity of the pump 18 is used to provide sufficient pressure to cause a portion of the pulp to flow through the conduits 42 of the circulation system and reenter the blow tank.

[0025] The recirculated pulp returned into the upper cylindrical portion 56 of the tank may account from ten to 100 percent (10 to 100 %) of the pulp exiting the blow tank. The percentage of recirculated pulp will be low while the volume of the dilution liquid 20 into the blow tank is high, but preferably some pulp is recirculated to the tank all the time. The percentage of recirculated pulp will be high while the volume of the dilution liquid is low. A decrease in the volume of the dilution liquid needed in the tank is an indication of channeling, when a more dilute pulp flow is readily generated in the central portion of the blow tank, and is discharged from the tank.

[0026] The recirculated pulp flows slowly, e.g., at a low velocity, through the nozzle arrays 44 and into the blow tank. The velocity of pulp through the nozzles of the arrays 44 may be in a range of 0.2 to 1.5 m/s. The low velocity flow ensures that the pulp introduced through the nozzles does not form, e.g., drill, channels into the pulp already in the tank. The low velocity flow results in the recycled pulp providing a gentle push towards the center of the tank for pulp already in the blow tank. Pushing pulp that is not being recirculated toward the center of the tank allows for more uniform flow of all pulp in the tank. By having several nozzles in each array 44 and multiple elevations of arrays of nozzles, the effect of gently pushing and avoiding formation of channels by the recirculated pulp can be best achieved.

[0027] Figure 2 shows also an oval marking (a dash line 38) indicating an area of the blow tank where nozzles for recirculated pulp may be most suitably positioned. The oval marking identifies the cylindrical upper portion 56 of the tank near the transition 50 to the inverted frusto-conical section of the tank. For example, the arrays of nozzles may be positioned at one or more elevations within zero to twenty-five percent (25%) of the height of the cylindrical portion 56.

[0028] The flow and consistency of the pulp in the blow tank can be controlled by the recirculation system 40. If the consistency is out of the range, especially too low, then more pulp is returned back into the tank. It is preferable to use several recirculation connections into the tank having channeling problems caused by too large consistency variations. For example, low consistency peak causes less problems if it is removed with extended circulation flow.

[0029] FIGURE 3 is a top down view of the blow tank 12 and circulation system shown in Figure 2. The top down view shows the array of nozzles 44 arranged symmetrically around the circumference of the tank. Conduit lines 42 feed pulp to the nozzles. Each nozzle may have a respective valve 46 and controller 48. Alternatively, each array of nozzles (wherein an array is at a particular elevation) may be regulated by a valve. The number of nozzles 44 in each array and at each elevation depends on the desired flow of recirculated pulp to the tank.

[0030] The valves 46 may be adjusted to achieve a desired flow of recirculated pulp to the tank. For example, all or most valves 46 may be opened so that pulp flows through all or most nozzles 44 when there is a desire for more recirculated pulp such as when there is a low consistency peak in the pulp flowing to the blow tank from the digester reactor vessel. Alternatively, the valves may be closed or mostly closed to stop or reduce the flow of recirculated pulp to the blow tank when pulp flows evenly directly to the blow tank from the digester reactor vessel. The valves can also be opened periodically.

[0031 ] FIGURES 4 and 5 show side views of blow tank processes having a pulp recirculation system 60 that extracts pulp directly from the tank, such as from the inverted frusto-conical section 52 of the tank. This may be appropriate if the blow tank or pulp tower is high and operates at a relatively low pressure. The pulp may be extracted from the tank into a vertical inlet pipe (stand pipe) 66 and to a separate circulation pump 64, such as a medium consistency pump. The inlet pipe may be sized to ensure sufficient hydraulic head for the pump. The separate circulation pump may also extract pulp directly from the bottom cylindrical 58 portion of the blow tank and avoid the need for a vertical inlet pipe.

[0032] The circulation pump may move pulp into conduits 62 associated with arrays of nozzles 67 at various elevations on the blow tank, similar to the arrays of nozzles 44 shown in Figure 3. Valves 70, valve controllers 68 and flow indicators 78 may be used to regulate the circulation of pulp back into the blow tank. The circulation takes pulp from lower elevations in the blow tank and introduces the pulp back to the tank at higher elevations. As shown in Figure 4, the circulation of pulp may be solely to feed pulp directly back to the tank through the nozzles 67 at the sidewall of the tank.

[0033] As shown in Figure 5, a portion of the circulated pulp may also flow through conduit 72 directly to the conduit for pulp 10 extending between the digester reactor vessel and the blow tank. A valve 76 and valve controller 74 may be used to regulate the flow of circulated pulp into the conduit from the digester reactor vessel. For example, the controller 74 may open the valve 76 only when no pulp is discharged from the digester reactor vessel.

[0034] As shown in Figures 4 and 5, the pulp circulation conduit may be coupled to just the tank, to the tank and feed conduit from the digester reactor vessel or to just the feed conduit. For example, a medium consistency pump may be used if the circulation includes conduits 62 for the nozzles attached to the sidewall of the tank and to the conduit 72 that injects pulp into the feed conduit from the digester reactor vessel.

Adding pulp to the feed pipe from the digester reactor vessel will increase the average pulp consistency entering the blow tank and pulp flow through the nozzles will diminish or prevent channeling in the interior of the tank.

[0035] If the average pulp consistency from the blow tank to a subsequent process is relatively high, the medium consistency pump as the separate pump 64 may be more suitable than relying solely on the discharge pump 18 due to the performance of medium consistency pumps and to avoid diluting the pumping effect of the discharge pump. If the average pulp consistency is relatively low in the blow tank, the discharge pump 18 may be sufficient and is less expensive than adding a separate pump.

[0036] FIGURES 6 and 7 are side views of a blow tank process illustrating controls of the circulation of pulp to the tank to minimize variations in pulp consistency and to reduce channeling within the blow tank. The instrumentation and control system may include various flow indicators 80 and 82, flow indicator controllers 28, quantity indicator controllers 32 and associated valves and valve controllers. The controllers may be separate logic devices that may automatically adjust the valves or provide indicators for manual valve adjustments. The controllers may report to a control system 84 having a computer and a non-transitory storage medium with instructions to cause the control system to automatically make valve adjustments or adjust pump speed in response to the steps of the batch digesting process, flows measured by the flow controllers and pulp quantity, e.g., consistency, as measured by the quantity indicator 32.

[0037] As shown in Figure 6, a flow difference is calculated based on the difference between the total pulp discharge flow as measured by flow indicator 80 and the pulp flow downstream of the circulation system as measured by flow indicator 28. The flow difference is the pulp flow to the circulation system 40. The flow difference is compared to a desired circulation flow. To move the flow difference towards the desired circulation flow adjustments may be made by valve controllers 48 to the valves 46.

[0038] As shown in Figure 7, the pulp flow to each of the nozzle arrays may be measure directly by flow controllers 82. The valves 46 associated with each nozzle array may be adjusted by valve controllers 48 to achieve desired circulation pulp flows at each elevation of the nozzles.

[0039] The desired circulation flows may correspond to the steps of the batch digester cooking process and the pulp flowing through the main feed line directly from the batch digester reactor vessel. For example, when the volume of the dilution liquid is low the desired circulation pulp flow may be high. When the volume of the dilution liquid into the blow tank is high the desired flow of circulated pulp may be small or almost zero.

[0040] The new method and system may be applied also in a batch digester which typically has a conical or hemispherical bottom. Pulp is discharged from the digester by means of a pump. It is possible that the pulp does not flow steadily to the pump. In other words, the pulp has formed an open cavity around and above a discharge (pumping) opening. This phenomenon has been called arching of the pulp. Recirculation of pulp can be used in a vessel having a geometry that converges, for example, conically converges, to even out consistency differences and thus to minimize arching and channeling that may be created in the column of material being conveyed.

[0041 ] The novel method and system bring about several advantages. The consistency of the flow discharged from the blow tank or other tank to a following process stage is more stable. The recirculated and injected pulp flow makes mixing more effective in the whole tank area in the tank. Channeling in the tank is eliminated or decreased.

Consistency variations in the pulp flow before the tank can be stabilized. In a batch cooking process variations in the kappa number of pulp can be decreased when different pulp fractions coming from the digester are mixed more efficiently in the blow tank. [0042] While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.

Claims

Claims:

1 . A system to process pulp in a blow tank or storage tank, the system comprising:

a tank (12) including a tank inlet at an upper region of the tank (12) and a discharge outlet (16) at a lower region of the tank, wherein the inlet is configured to be coupled to a source of pulp (10);

a discharge conduit (26) coupled to the discharge outlet (16); a circulation conduit (42) having an inlet coupled to the discharge conduit (26) or the discharge outlet (16) and configured to circulate a portion of discharged pulp to the tank (12), and

a feed conduit (44) connected to the tank (12), including a feed conduit inlet connected to an outlet of the circulation conduit (42 and a feed conduit outlet configured to direct pulp from the discharge outlet (16) into the tank (12) at an elevation between the tank inlet and the discharge outlet.

2. The system of claim 1 , wherein the feed conduit (44) includes an array of feed conduits arranged symmetrically around a wall of the tank (12) at one or more elevations, arranged equidistantly around a wall of the tank (12), or a combination thereof.

3. The system of any of claims 1 to 2, wherein the nozzle (44) includes feed conduits arranged about a lower quarter (25%) of the height of the tank and below the middle (50%) of the height.

4. The system of any of claims 1 to 3, wherein the tank (12) is a tank including an upper cylindrical section (56), an inverted frusto-conical section (52), and a lower cylindrical section (58), wherein the feed conduit (44) is connected to the inverted frusto- conical section or connected to a transition zone (50) between the inverted frusto-conical section (52) and the upper cylindrical section (56) and/or to the upper cylindrical section.

5. The system of any of claims 1 to 4 further comprising a pump (18) coupled to the circulation conduits (42) and configured to pump the discharged pulp back into the tank.

6. The system of any of claims 1 to 5 further comprising a valve (46) and a controller (48) on each of the at least one circulation conduit (42) that regulates pulp flow into the tank (12).

7. The system of any of claims 1 to 6 further comprising a control system (84), flow indicators (80, 82), flow indicator controllers (28), quantity indicator controllers (32), and associated valves (46, 34) and valve controllers (48) on the circulation conduits (42) to adjust pump speeds and valve positions to control pulp flow in the circulation conduits (42).

8. The system of any of claims 1 to 7 further comprising a conduit (42) that directs a portion of the discharged pulp to re-enter the tank (12) through the inlet of the tank (12).

9. A system to process pulp in a blow tank or storage tank, the system comprising:

a tank (12) including a tank inlet at an upper region of the tank (12), and a discharge outlet (16) at a lower region of the tank, wherein the inlet is configured to be coupled to a source of pulp (10);

a vertical inlet pipe (66) including an inlet coupled to the tank (12) and an outlet coupled to a pump (64) to extract pulp from the tank (12);

a circulation conduit (62, 72) having an inlet coupled to the pump (64) and configured to circulate a portion of discharged pulp to the tank (12), and

a feed conduit (67) connected to the tank (12), including a feed conduit inlet connected to an outlet of the circulation conduit (62, 72) and a feed conduit outlet configured to direct extracted pulp back into the tank (12) at an elevation between the tank inlet and the inlet of the vertical inlet pipe.

10. The system of claim 9, wherein the feed conduit (67) includes an array of feed conduits arranged symmetrically around a wall of the tank (12) at one or more elevations, arranged equidistantly around a wall of the tank (12), or a combination thereof.

1 1. The system of claims 9 or 10, wherein the feed conduit (67) includes feed conduits arranged in a lower quarter (25%) of the height of the tank and below the middle (50%) of the height.

12. The system of any of claims 9 to 1 1 , wherein the tank (12) is a tank including an upper cylindrical section (56), an inverted frusto-conical section (52), and a lower cylindrical section (58), wherein the feed conduit (67) is connected to the inverted frusto- conical section or connected to a transition zone (50) between the inverted frusto-conical section (52) and the upper cylindrical section (56) and/or to the upper cylindrical section.

13. The system of any of claims 9 to 12 further comprising a valve (70) and a controller (68) on each of the at least one circulation conduit (62, 72) that regulates pulp flow into the tank (12).

14. The system of any of claims 9 to 13 further comprising a control system , flow indicators (78), flow indicator controllers (28), quantity indicator controllers (32), and associated valves (34, 70) and valve controllers (68) on the circulation conduits (62, 72) to adjust pump speeds and valve positions to control pulp flow in the circulation conduits (62, 72).

15. The system of any of claims 9 to 14 further comprising a conduit (72) that directs a portion of the discharged pulp to re-enter the tank (12) through the inlet of the tank (12).

16. A method to control a pulp consistency in a blow tank or storage tank, the method comprising:

feeding pulp (10) to an inlet of a tank (12);

discharging pulp through an outlet of the tank (12);

directing pulp back in the tank (12) through at least one circulation conduit (42, 62, 72); and

pumping discharged pulp into the tank (12) through at least one feed conduit (44, 67) arranged at one or more elevations around the tank(12) , the feed conduit (44, 67) including a feed conduit inlet connected to an outlet of the circulation conduit (42, 62, 72) and a feed conduit outlet configured to direct extracted pulp back into the tank (12).

17. The method of claim 16, wherein a flow difference in conduits (42, 62, 72, 26) coupled to the tank (12) is calculated; and

valve controllers (48, 68) and valves (46, 70, 34) are used to move the flow difference towards a predisposed desired circulation flow.

18. The method of claim 16 or 17, wherein the flow difference is calculated based on a difference between a total pulp discharge flow and a pulp flow downstream of the circulation conduits (42, 62, 72) as measured by flow indicators (80, 28).

19. The method of any of claims 16 to 18, wherein the calculation and comparison is performed by a control system (84) using value input from flow indicators (80, 82), flow indicator controllers (28), and quantity indicator controllers (32) on the circulation conduits (42, 62, 72), and the control system (84) uses the valve controllers (48, 68) to automatically adjust the valves or to provide indicators for manual valve adjustments.

20. The method of any of claims 16 to 19, wherein pulp flow velocity is adjusted for an array of feed conduits (44, 67) at different elevations arranged around the tank (12), adjusted at each elevation of the feed conduits (44, 67), or adjusted for each individual feed conduit (44, 67).

21. The method of any of claims 16 to 20, wherein 10% to 100% of the pulp discharged from the tank (12) is pumped back into the tank (12) at a low velocity between about two tenths to one and a half meters per second (0.2 to 1.5 m/s).

22. The method of any of claims 16 to 21 , wherein the pulp is fed to the blow tank from a batch digester.