Patent Publication Number: US-6336993-B1

Title: Metal removal from comminuted fibrous material during feeding

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of application Ser. No. 09/063,429 filed Apr. 21, 1998, now U.S. Pat. No. 6,106,668 which in turn is a continuation-in-part of Ser. No. 08/738,239 filed Oct. 25, 1996, now U.S. Pat. No. 5,753,075. 
    
    
     BACKGROUND AND SUMMARY OF THE INVENTION 
     This invention relates to a method and system for feeding comminuted cellulosic fibrous material to a treatment vessel, such as a continuous digester. The invention simplifies and dramatically reduces the number of components needed when compared to the existing art. 
     U.S. Pat. Nos. 5,476,572, 5,622,598 and 5,635,025 and 5,766,418 introduced the first real breakthroughs in the art of feeding comminuted cellulosic fibrous material to a treatment vessel in over forty years. These patents and the application disclose several embodiments, collectively marketed under the trademark Lo-Level® feed system by Ahistrom Machinery Inc. of Glens Falls, N.Y., for feeding a digester using a slurry pump, among other components. As described in these patents and application, using such a pump to feed a slurry to a high-pressure transfer device dramatically reduces the complexity and physical size of the system needed, and increases the ease of operability and maintainability. The prior art systems employing a high-pressure transfer device, for example a High-Pressure Feeder as sold by Ahlstrom Machinery Inc., but without such a pump, are essentially unchanged from the systems sold and built since the 1940s and 1950s. 
     The present invention relates to an even more dramatic improvement to the methods and systems disclosed in the above-mentioned patent and applications. The present invention actually eliminates the need for transfer devices, such as a High-Pressure Feeder, by using high-pressure pumping devices to transfer a slurry of comminuted cellulosic fibrous material directly to a digester. 
     The reaction of pulping chemicals with comminuted cellulosic fibrous material to produce a chemical pulp requires temperatures ranging between 140-180° C. Since the aqueous chemicals used to treat the material would boil at such temperatures, commercial chemical pulping is typically performed in a pressure-resistant vessel under pressures of at least about 10 bars gauge (approximately 150 psi gauge). In order to maintain this pressure, especially when performing a continuous pulping process, special accommodations must be made to ensure that the pressure is not lost when introducing material to the pressure vessel. In the prior art this was accommodated by what is known in the art as a “High-Pressure Feeder”. This feeder is a specially-designed device containing a pocketed rotor which acts as a means for transferring a slurry of material from a low pressure to a high pressure while also acting as a valve for preventing loss of pressure. This complicated and expensive device has long been recognized as an essential component for introducing slurries of comminuted cellulosic material to pressurized vessels, typically at elevated temperatures, especially to continuous digesters. 
     According to the invention a system which replaces the High-Pressure Feeder—which has been recognized for over forty years as being essential to continuous digesting—is provided, greatly simplifying construction of a pulp mill. 
     According to one aspect, a system for producing chemical cellulose pulp from comminuted fibrous cellulose material, such as wood chips, comprises the following components: A steaming vessel in which comminuted fibrous cellulose material is steamed to remove the air therefrom. A superatmospheric pressure vertical treatment vessel having an inlet for a slurry of comminuted cellulose fibrous material at a top portion thereof and an outlet at a bottom portion thereof. And, pressurizing transfer means for pressurizing a slurry of material from the steaming vessel and transferring it to the treatment vessel inlet, the pressurizing transfer means consisting of one or more high pressure slurry pumps located below the top portion of the treatment vessel. 
     The one or more pumps preferably comprises first and second high pressure slurry pumps connected in series and each having a pressure rating, an inlet and an outlet, the first pump inlet operatively connected to the steaming vessel, the first pump outlet operatively connected to the second pump inlet, and the second pump having a higher pressure rating than the first pump. The slurry pumps may be helical screw centrifugal pumps, double-piston solids pumps, or other similar conventional pumping devices that are capable of pressurizing a slurry having a relatively high percentage of solids to (in one or more stages) a pressure of at least about 5 bar gauge. The pressurizing and transferring may also be effected by an one or more eductors, of conventional construction, driven by a pressurized fluid supply, such as supplied by conventional centrifugal pump. 
     One typical unit of measure that indicates the relative amount of solids in a slurry containing solids and liquid is the “liquid-to-solids ratio”. In this application, this ratio is the ratio of the volume of liquid being transferred to the volume of cellulose, or wood, material being transferred. Typical conventional centrifugal liquid pumps are limited to pumping liquid having a solids content of at most 3%. This 3% solids content corresponds to a liquid-to-solids ratio of about 33. In the slurry pumps of this invention, the liquid-to-solids ratio of the slurry being pumped is typically between 2 and 10, preferably between 3 and 7, and most preferably between 3 and 6. In other words, the slurry pumps of this invention transfer slurries having a much greater solids content than can be handled by a conventional pump. 
     A liquid return line may be provided from the top portion of the treatment vessel, containing liquid separated from the slurry at the top of the treatment vessel (preferably a continuous digester). The return line may be operatively connected to an inlet or outlet of one of the slurry pumps, either directly or indirectly. Preferably the liquid return line is connected to a pressure reduction means for reducing the pressure of liquid in the return line before the liquid passes to the inlet or outlet of the slurry pump. The pressure reduction means may take a variety of forms, such as a flash tank and/or a pressure control valve in the return line, or other conventional structures for effectively reducing the pressure of liquid in a line while not adversely affecting the liquid. Where a flash tank is utilized the liquid outlet from the flash tank is connected to the inlet to the first slurry pump, and the steam produced by the flash tank may be used in the steaming vessel. 
     Alternatively, the pressure reduction may be effected, or even avoided, by using an eductor which uses the pressurized return line liquor as its source of pressurized fluid. An eductor may be used in place of or in conjunction with one or more of the slurry pumps, or other devices, to transfer slurry to the digester. 
     A conventional chute, as well as other optional components, is preferably connected between the steaming vessel and the at least one slurry pump, the steaming vessel being located above the chute and the chute above the at least one slurry pump. The at least one slurry pump is typically located a distance at least 30 feet (about 10 meters) below the top of the digester, and typically more than about 50 feet (about 15 meters) below. 
     When the high pressure transfer device is eliminated it is desirable to utilize other mechanisms to retain one of the functions of the high pressure transfer device, namely providing pressure relief prevention should an aberrant condition occur, the high pressure transfer device typically preventing backflow of liquid from the digester into the feed system. Pressure relief preventing means according to the present invention are preferably distinct from the at least one slurry pump, although under some circumstances the inlets to or outlets from the slurry pumps may be constructed in a manner so as to provide pressure relief prevention. The pressure relief preventing means may comprise an automatic isolation valve in each of the slurry conduits transferring slurry from the pumps to the top of the treatment vessel and the return line from the treatment vessel, a conventional controller being provided connected to the isolation valves and operating the isolation valves in response to the pressure sensed by a pressure sensor associated with the slurry conduit feeding slurry to the top of the treatment vessel. The pressure relief preventing means may also comprise a check valve in the slurry conduit, and/or a variety of other valves, tanks, sensors, controllers, or like fluidic, mechanical, or electrical components which can perform the pressure relief preventing function. 
     The system may also comprise means for augmenting the flow of liquid to the inlet to the second slurry pump, or to any pump or transfer device, such as a liquid line having liquid at a pressure below the pressure at the second slurry pump inlet, a conduit between the liquid line and the inlet, and a liquid pump in the conduit. The liquid line may be the return line from the treatment vessel, and the conduit may be connected directly to the return line. The liquid return line may be connected to a flash tank as described above, and the conduit may be connected to the flash tank liquid outlet. 
     According to another aspect, a method of feeding comminuted cellulosic fibrous material to the top of a treatment vessel is provided. The method comprises the steps of: (a) Steaming the material to remove air therefrom and to heat the material. (b) Slurrying the material with a cooking liquor to produce a slurry of liquid and material. And, (c) pressurizing the slurry to a pressure of at least about 5 bar gauge at a location below the top of the treatment vessel (e.g. at least thirty feet below, preferably at least fifty feet below), and transferring pressurized material to the top of the treatment vessel, the pressurizing step consisting of acting on the slurry with one or more high pressure slurry pumps. 
     The method may comprise the further steps of: (d) returning liquid separated from the slurry at the top of the treatment vessel to the at least one pump; and (e) sensing the pressure of the slurry while being transferred to the top of the treatment vessel, and shutting off the flow of slurry to the top of the treatment vessel and the return of liquid from the top of the vessel if the sensed pressure drops below a predetermined value. There also may be the step (f) of flashing the liquid while returning in the practice of step (d) to produce steam, and using the steam in the practice of step (a). 
     In an additional embodiment, the concept of transferring a slurry of chips is extended back to the point where chips are introduced to the mill, that is, the Woodyard. Conventional pulp mills receive their supply of cellulose material, typically hardwood and softwood but other forms of cellulose material as described above may be handled, in various forms. These include as sawdust, as chip, as logs, as long de-limbed trees (that is, “long wood”), or even as complete trees (that is, “whole trees”). Depending upon the source of cellulose of the “wood supply”, the wood is typically reduced to chip form so that it can be handled and treated in a pulping process. For example, devices known as “chippers” reduce the long-wood or logs to chips that are typically stored in open chip piles or chip silos. This receipt, handling, and storage of the chips is performed in an area of the pulp mill referred to as the “woodyard”. From the Woodyard the chips are typically transferred to the pulp mill proper to initiate the pulping process. 
     In conventional Woodyards, the chips are stored in silos from which the chips are discharged, typically by means of a rotating or vibrating silo discharge device, to a conveyor. This conveyor is typically a belt-type conveyor which receives the chips and transfers them to the pulping treatment vessels. Since the Woodyard is typically at a distance from the pulping vessels, this conveyor is typically long. Such conveyors may have a length of up to one-half mile. In addition, treatment systems that do not employ the Lo-Level™ feeding system, as marketed by Ahlstrom Machinery and described in U.S. Pat. Nos. 5,476,572, 5,622,598, 5,635,025 and 5,766,416, require that the conveyor be elevated, typically to a height of at least 100 feet, in order to feed the chips to the inlet of the first pulping vessel. These conveyers, and the structures that support them, are very expensive and contribute a significant cost to the cost of a digester feed system. 
     In another embodiment, the concept of transferring a slurry of chips is extended back to the Woodyard. A preferred embodiment of this invention consists of a method of transferring comminuted cellulosic fibrous material to a pulping process, consisting of the following steps: (a) Introducing untreated chips to a first vessel. (b) Introducing slurrying liquid to the first vessel to create a slurry of material and liquid. (c) Discharging the slurry from the vessel to the inlet of at least one pressurizing and transferring device. (d) Pressurizing the slurry in the pressurizing and slurrying device and transferring the slurry to a treatment vessel. 
     The first vessel is typically a chip storage silo or bin. This bin preferably has a discharge having one-dimensional convergence without agitation or vibration, such as a DIAMONDBACK® bin as described in U.S. Pat. No. 5,000,083, though agitation or vibration may be used. This bin may also have two or more outlets which feed two or more transfer devices. This vessel may also be operated at superatmospheric pressure, for example at 0.1 to 5 bar. If the vessel is operated at superatmospheric pressure some form of pressure isolation device must be located at the inlet of the vessel to prevent the release of pressure. This device may be a star-type isolation device, such as a Low-pressure Feeder or Air-lock Feeder as sold by Ahlstrom Machinery, or a screw-type feeder having a sealing capacity as described in U.S. Pat. No. 5,766,416. 
     The slurrying liquid may be any source of liquid available in the pulp mill, including fresh water, steam condensate, kraft white, black, or green liquor or sulfite liquor or any other pulping-related liquid. This liquid may be a heated fluid, for example, hot water or steam, having a temperature of between 50 and 100° C. If the vessel is a pressurized vessel, liquid temperatures of over 100° C. may be used. Though not essential, this liquid may contain at least some active pulping chemical, for example, sodium hydroxide (NaOH), sodium sulfide (Na2S), polysulfide, anthraquinone or their equivalents or derivatives or surfactants, enzymes or chelates, or combinations thereof. 
     The pressurizing and transferring device of steps (c) and (d) is preferably a slurry pump, or pumps, but many other pressurizing and transferring devices may be used such as the piston-type solids pump or a high-pressure eductor. Preferably, more than one pressurizing and slurrying pump is used to transfer the slurry. These may be two or more slurry pumps, or any combination of slurry pump, piston-type pump, or eductor. This transfer system may also include one or more storage or surge tanks as well as transfer devices. Preferably, the one or more transfer devices include at least one device having de-gassing capability so that undesirable air or other gases may be removed from the slurry. Also, during transfer, the chips may be exposed to some form of treatment, for example, de-aeration or impregnation with a liquid, preferably a liquid containing pulping chemicals, such as those described above. The slurry may also be exposed to at least one pressure change or fluctuation during transfer, for example, such that the pressure of the slurry is varied from a first pressure to a second, higher pressure, and then optionally to a third pressure which is lower than the second pressure. As described in U.S. Pat. Nos. 4,057,461 and 4,743,338 varying the pressure of a slurry of chips and liquor improves the impregnation of the chips by the liquor. This pressure pulsation may be achieved by varying the outlet pressure of a set of transfer devices in series, or by controlled depressurization of the slurry between pumping. 
     In another embodiment, the material need not encounter liquid in the vessel, but may have liquid first introduced to it by means of an eductor located in or below the outlet of the vessel. This liquid is preferably pressurized so that the material and liquid form a pressurized slurry of material and liquid. 
     The treatment vessel of step (d) may typically be a steaming vessel as described above, preferably a DIAMONDBACK® steaming vessel. The vessel may also be a storage or surge tank in which the material may be stored prior to treatment. Since the transfer process may require excess liquor that is not needed during treatment or storage, some form of de-watering device may be located between the transfer device and the treatment vessel. One preferred dewatering device is a Top Separator, as sold by Ahlstrom Machinery. This Top Separator may be a standard type or an “inverted” Top Separator. This device may be an external stand-alone-type unit or one that is mounted directly onto the treatment vessel. An In-line Drainer, also sold by Ahlstrom Machinery, may also be used for the dewatering device. Preferably, the liquid removed from the slurry by means of the de-watering device is returned to the first vessel or to the transfer devices to act as the slurring liquid. This liquid may also be used where ever needed in the pulp mill. This liquid may be heated or cooled as desired. For example, this liquid may be heated by passing it in indirect heat exchange relationship with any heated liquid stream, for example, a waste liquid stream having a temperatures greater than 50° C. This liquid will also typically be pressurized using one or more conventional centrifugal liquid pumps. 
     In one preferred embodiment the treatment vessel of step (d) is a steaming vessel which feeds one or more transfer devices as described above. Though this system is preferably used in conjunction with a feed system not having a conventional High-pressure Feeder, this system may also be used with a feed system having a High-pressure Feeder. 
     The method and apparatus for feeding chips from a distant location, for example, a Woodyard, to a pulping process is not limited to chemical pulping processes, but may be used in any pulping process in which comminuted cellulosic fibrous material is conveyed from one location to another. The pulping processes that this invention is applicable to include all chemical pulping processes, all mechanical pulping processes, and all chemi-mechanical pulping or thermal-mechanical pulping processes, for either batch or continuous treatment. 
     According to another aspect there is provided a method of feeding wood chips to the top of a treatment vessel comprising the steps of: (a) Steaming the wood chips to remove air therefrom and to heat the material. (b) Slurrying the wood chips with a cooking liquor to produce a slurry of liquid and material. (c) Pressurizing the slurry to a pressure of at least about 5 bar gauge at a location at least thirty feet below the top of the treatment vessel and transferring pressurized wood chips to the top of the treatment vessel, the pressurizing step consisting essentially of acting on the slurry with one or more high pressure slurry pumps. And, (d) during the practice of the transferring step (c), treating the wood chips with polysulfide, anthraquinone or their equivalents or derivatives, surfactants, enzymes, chelants, or combinations thereof. 
     Where the treatment vessel is upstream of a continuous or batch digester, step (c) is typically practiced downstream of the treatment vessel. There may also be the further step (e), before the continuous or batch digester and substantially immediately after steps (a) and (b), of pressurizing the slurry at a location at least 30 feet below the top of the digester, and transferring pressurized wood chips to the top of the digester, the pressurizing step consisting of acting on the slurry with one or more high pressure slurry pumps. There may also be the step of returning liquid removed from the digester to the treatment vessel, and adjusting the temperature of the liquid while returning it to the treatment vessel. The step of removing liquid from the treatment vessel typically takes place at the top of the treatment vessel. 
     The method may also comprise the further step of returning liquid from downstream of the treatment vessel to the treatment vessel, and adjusting the temperature of the liquid, and the step of adjusting the temperature of the liquid may take place by passing the liquid through an indirect heat exchanger. The method may also comprise the further step of returning liquid separated from the slurry at the top of the digester to the one or more slurry pumps, pressurizing the slurry to transfer it to the digester, and adjusting the temperature of the removed liquid during recirculation. 
     The system and method herein not only reduce the size and cost of the system for transferring comminuted cellulosic fibrous material, but if the comminuted cellulosic fibrous material is treated during transfer, the number and size of the formal treatment vessels may be reduced. For example, this system may eliminate the need for conventional pretreatment or impregnation vessels prior to the digester. This system also has the potential for improving the over-all energy economy of the pulp mill. This and other aspects of the invention will become manifest upon review of the detailed description and figure below. 
     According to another aspect a method of treating comminuted cellulosic fibrous material using at least first and second series connected pumps, and at least first and second in series stations each with a solids/liquid separator is provided. The method comprises the steps of: (a) Pumping a slurry of comminuted cellulosic fibrous material using the series connected pumps. (b) Separating some liquid from the slurry at each station to substantially isolate liquor circulations and streams, and to recirculate removed liquid from at least one of the stations to upstream of one of the pumps. And (c) adding chemicals to the slurry upstream of each of the pumps, the chemicals including at least some chemical selected from the group consisting essentially of sodium hydroxide, sodium sulfate; polysulfide, anthraquinone, or their equivalents or derivatives; surfactants, enzymes, or chelants; or combinations thereof; so that pre-treatment of the material occurs during transfer of the material from each pump to each station. 
     There may be the further step of degassing the slurry at at least one of the stations. At least first, second and third series connected pumps and stations may be provided; and there may also be the further steps of: (d) Circulating liquid removed from the third station to a location upstream of the second pump, and (e) circulating liquid removed form the second station to a location upstream of the first pump (step (d) may be practiced downstream of the first station). There may also be the further step of passing the removed liquid, during the practice of at least one of steps (d) and (e), through a heat exchanger to change the temperature thereof. For example, the temperature of the removed liquid may be increased or decreased by from about 1 to about 10° C., depending upon the volume of the liquid and the amount of heating or cooling available. 
     Step (c) may be practiced by adding a different chemical, or combination of chemicals, upstream of each pump, so that significantly different treatments of the material of the slurry take place during transfer of the slurry from each pump to its associated station. Step (a) may be practiced to pressurize the slurry to a pressure of at least 5 bar. Also, there may be the further step of removing liquid from at least one of the stations through an eductor (also known as an ejector) instead of a flash tank and/or control valve. 
     According to another aspect of this invention, one treatment that can be used during the transfer of comminuted cellulosic fibrous material is the removal of metal ions. It is recognized in the art that the presence of certain metallic compounds or ions, for example, those containing iron, calcium, manganese, and others, can interfere with pulping and bleaching reactions or can precipitate as undesirable “scale” on the treatment equipment. It is also known the metal content of the cellulose material can be reduced by exposing the material to acidic liquids which can dissolve metal compounds or ions or to acidic to slightly alkaline conditions in the presence of a chelating agent (also known as a sequestering agent) which combine with certain metals and make them more easily isolated and removed, for example, by washing. According to the present invention, these deleterious metal-containing compounds and ions are removed from the cellulose material prior to the cooking process and bleaching process so that these metals do not interfere with these processes nor form scale on the equipment used to effect these processes. 
     According to this aspect of the invention, there is provided a method of treating a slurry of comminuted cellulosic fibrous material using at least first and second series connected pumps, and at least first and second in-series stations, each with a solids/liquid separator, in which the metal content of the material is reduced. The method comprises: (a) Pumping a slurry of comminuted cellulosic fibrous material using the series connected pumps. (b) Separating some liquid from the slurry at each station to substantially isolate liquor circulations and streams, and to recirculate removed liquid from at least one of the stations to upstream of one of the pumps. And (c) adding chemicals which dissolve or sequester metal containing compounds to the slurry at or upstream of at least one of the pumps, the chemicals including at least one chemical selected from the group consisting essentially of acids, chelating agents, and combinations thereof, so that at least some of the deleterious metals (e.g. at least about 10%, preferably about 20%-80%) present in the material prior to treatment are removed from the material. 
     There may further be (d) removing at least some of the liquid from the slurry during (a) or (b) to purge at least some (e.g. at least about 10%, preferably about 20%-80%) of the metal containing compounds from the liquor circulations. This liquid may be removed in a liquor separating device, for example, a conventional Top Separator or In-line drainer, or the liquid may simply be removed via a branch conduit in the circulation line. Also (d) may also be practiced at substantially the same time as and using substantially the same equipment in which (b) is practiced. There may also further be (e) introducing liquid to the circulation to substantially replace the liquid removed in (d). The liquid introducing procedure (e) may be practiced substantially immediately downstream of where (d) is practiced or elsewhere in the system. Also (e) may be practiced substantially in conjunction with (c) so that replacement liquid is introduced substantially with the treatment chemical. 
     This invention is preferably practiced before a further procedure (f) of treating the material with an alkaline liquid and (g) digesting the material in an alkaline digestion process; preferably (a)-(e) are practiced substantially immediately prior to (f) and (g). The alkaline liquid may comprise, for example, kraft white, green, or black liquor (which may contain yield or strength enhancing additives as described above). Thus, in a preferred embodiment of the invention, the chemical used to effect (c) is introduced at or upstream of the first pump and the chemical used to effect (f) is introduced at or upstream of the second pump. 
     According to another aspect a method of treating comminuted cellulosic fibrous material is provided comprising the steps of: (a) Pumping a slurry of comminuted cellulosic fibrous material using the at least first and second series connected pumps. (b) Separating some liquid from the slurry at each station to substantially isolate liquor circulations and streams, and to recirculate removed liquid from at least one of the stations to upstream of one of the pumps. (c) Adding treatment chemical to the slurry upstream of at least one of the pumps so that pretreatment of the material occurs during transfer of the material from that pump to its associated station. And (d) circulating liquid removed form the second station to a location upstream of the first pump. Where at least first, second and third pumps and stations are provided, there is the further step (e) of circulating liquid removed from the third station to a location upstream of the second pump. The details of the steps, or additional steps, may be as set forth above. 
     According to one aspect of the invention there is provided a system for producing chemical cellulose pulp from comminuted fibrous cellulose material, comprising: A steaming vessel in which comminuted fibrous cellulose material is steamed to remove the air therefrom. A superatmospheric pressure vertical treatment vessel having an inlet for a slurry of comminuted cellulose fibrous material at a top portion thereof and an outlet at a bottom portion thereof. Pressurizing transfer means for pressurizing a slurry of material from the steaming vessel and transferring it to the treatment vessel inlet, the pressurizing transfer means consisting of one or more high pressure slurry pumps, each having an inlet and outlet, located below the top portion of the treatment vessel. And means for circulating liquid from the outlet of at least one the high pressure slurry pump to the inlet thereof. 
     The recirculation means may be conduits and associated connections to other components, although any conventional structures which allow or provide this recirculation may be utilized including valves (in or apart from the conduits), tanks, ejectors, pumps, ducts, heat exchangers, or the like. 
     The system preferably further comprises a liquid return line from the top portion of the treatment vessel, the return line operatively connected to an inlet or outlet of one of the slurry pumps. 
     The system may also comprise a heat exchanger located in the return line, which preferably is a liquid-to-liquid indirect heat exchanger. While the heat exchanger may be used for cooling or heating liquid in a return line preferably it is connected to a source of cool liquid and cools the liquid in the return line, so that it is below the point where it will flash in the system. 
     The system may further comprise a slurrying vessel having an inlet operatively connected to the steaming vessel and an outlet operatively connected to the inlet of the one or more slurry pumps; the system may still further comprise a liquid return line from the top portion of the treatment vessel, the return line operatively connected to the slurry vessel, and the heat exchanger in the return line. 
     Preferably the at least one pump comprises at least two pumps, and each of the pumps has a recirculation means as described above. The recirculation means may comprise a first valve in a recirculation conduit, and a second valve between the pump outlet and the treatment vessel, and preferably each of the pumps has a recirculation means as described above associated therewith. 
     The treatment vessel may be a first treatment vessel, and the system may further comprise a second treatment vessel. The main conduit is connected to the outlet of the pump (or the last in a series of pumps), and a flow splitter is provided having an inlet and at least two outlets. The main conduit is connected to the flow splitter inlet, and one of the flow splitter outlets is connected to the first treatment vessel, and another outlet to the second treatment vessel. The first treatment vessel may also include two or more inlets and the at least two or more outlets of the flow splitter may be connected to the two or more inlets of the first vessel. The flow splitter may comprise a chamber having a substantially triangular shaped static baffle plate arrangement with a triangle apex substantially aligned with the inlet. 
     According to another aspect of the invention there is provided a method of feeding cellulosic material to the top of a treatment vessel comprising the steps of: (a) Steaming the material to remove air therefrom and to heat the material. (b) Slurrying the material with a cooking liquor to produce a slurry of liquid and material. And (c) pressurizing the slurry at a location at least thirty feet below the top of the treatment vessel and transferring pressurized material to the top of the treatment vessel, the pressurizing step consisting of acting on the slurry with two or more high pressure slurry pumps. 
     The method may also comprise (d) establishing a recirculation loop between each pump outlet and inlet during startup. For example, there may be a first valve in the recirculation loop and a second valve between each pump outlet and the treatment vessel, in which case (d) is practiced to open the first valve and at least partially (e.g. completely) close the second valve during startup. Then the method may further comprise (e) after startup closing the first valve and opening the second valve. The method may also further comprise returning the liquid from the treatment vessel to one of the pump inlets (preferably a first in-series pump) and partially cooling the cooling liquid (e.g. with an indirect liquid-to-liquid heat exchanger) so that the returning liquid has a temperature below the point it will flash during handling. 
     The method may be practiced further utilizing at least a second treatment vessel or a first treatment vessel having two or more inlets, and may further comprise statically splitting the flow of slurry from the outlet of the last of the pumps to direct part of the flow to each treatment vessel or the inlets of the first treatment vessel. 
     According to another aspect of the present invention there is provided a method of feeding comminuted cellulosic fibrous material to the top of a treatment vessel, comprising: (a) Steaming the material to remove air therefrom and to heat the material. (b) Slurrying the material with a cooking liquor to produce a slurry of liquid and material; (c) Pressurizing the slurry at a location at least thirty feet below the top of the treatment vessel and transferring pressurized material to the top of the treatment vessel, said pressurizing step consisting of acting on the slurry with one or more high pressure slurry pumps. And (d) establishing a recirculation loop between the pump outlet and inlet during startup. A first valve may be provided in the recirculation loop and a second valve between the pump outlet and the treatment vessel; and (d) may be practiced to open the first valve and at least partially close the second valve during startup; and the method may further comprise (e) after startup closing the first valve and opening the second valve. Cooling and returning liquid, and flow splitting, may also be practiced, as described above. 
     According to another aspect of the present invention there is provided a static flow splitter comprising: A static chamber. An inlet and at least two outlets connected to the chamber. And a substantially triangular shaped static baffle plate arrangement may be located within the chamber and have a triangle apex substantially aligned with the inlet. 
     It is the primary object of the present invention to provide a simple and effective system and method for feeding cellulose slurry to a treatment vessel, and also while achieving enhanced operability and maintainability. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a typical prior art system for feeding a slurry of comminuted cellulosic fibrous material to a continuous digester; 
     FIG. 2 illustrates another prior at system for feeding a slurry of comminuted cellulosic fibrous material to a continuous digester; 
     FIG. 3 illustrates one typical embodiment of a system for feeding a slurry of comminuted cellulosic fibrous material to a continuous digester according to this invention; 
     FIGS. 4 and 5 illustrate two other embodiments of systems according to the invention; 
     FIG. 6 is a schematic representation of another system that may be used for practicing a method according to the invention; 
     FIG. 7 is a schematic illustration of another typical system for feeding a slurry of comminuted cellulosic fibrous material to a digester, according to the invention; 
     FIG. 8 is a side view, with a portion of the near wall of the flow chamber cut away so as to illustrate the interior thereof, of an exemplary flow splitter according to the present invention; and 
     FIGS. 9 and 10 are top and end views of the flow splitter of FIG.  8 . 
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     Though the systems shown and described in FIGS. 1-3 are continuous digester systems, it is understood that the method and system of the present invention can also be used to feed one or more batch digesters, or an impregnation vessel connected to a continuous digester. The continuous digesters shown and which may be used with this invention are preferably KAMYR® continuous digesters, and may be used for kraft (i.e., sulfate) pulping, sulfite pulping, soda pulping or equivalent processes. Specific cooking methods and equipment that may be utilized include the MCC®, EMCC®, and Lo-Solids® processes and digesters marketed by Ahlstrom Machinery Inc. Strength or yield retaining additives such as anthraquinone, polysulfide, or their equivalents or derivatives may also be used in the cooking methods utilizing the present invention. 
     FIG. 1 illustrates one typical prior art system  10  for feeding a slurry of comminuted cellulosic fibrous material, for example, softwood chips, to the top of a continuous digester  11 . Digester  11  typically includes one liquor removal screen  12  at the inlet of the digester  13  for removing excess liquor form the slurry and returning it to feed system  10 . Digester  11  also includes at least one liquor removal screen  14  for removing spent cooking liquor during or after the pulping process. Digester  11  also typically includes one or more additional liquor removal screens (not shown) which may be associated with cooking liquor circulation, such as an MCC®, EMCC® digester cooking circulation, or a Lo-Solids® digester circulation having a liquor removal conduit and a dilution liquor addition conduit. Cooking liquor, for example, kraft white, black, or green liquor, may be added to these circulations. Digester  11  also includes an outlet  15  for discharging the chemical pulp produced which may be passed on to further treatment such as washing or bleaching. 
     In the prior art feed system  10  shown in FIG. 1, comminuted cellulosic fibrous material  20  is introduced to chip bin  21 . Typically, the material  20  is softwood or hardwood chips but any form of comminuted cellulosic fibrous material, such as sawdust, grasses, straw, bagasse, kenaf, or other forms of agricultural waste or a combination thereof, may be used. Though the term “chips” is used in the following discussion to refer to the comminuted cellulosic fibrous material, it is to be understood that the term is not limited to wood chips but refers to any form of the comminuted cellulosic fibrous materials listed above, or the like. 
     The chip bin  21  may be a conventional bin with vibratory discharge or a DIAMONDBACK® steaming vessel, as described in U.S. Pat. No. 5,500,083 and sold by Ahlstrom Machinery Inc., having no vibratory discharge but having an outlet exhibiting one-dimensional convergence and side relief. The bin  21  may include an airlock device at its inlet and a means for monitoring and controlling the level of chips in the bin and a vent with an appropriate mechanism for controlling the pressure within the bin. Steam, either fresh or steam produced from the evaporation of waste liquor (i.e., flashed steam), is typically added to bin  21  via one or more conduits  22 . 
     The bin  21  typically discharges to a metering device,  23 , for example a Chip Meter sold by Ahlstrom Machinery, but other forms of devices may be used, such as a screw-type metering device. The metering device  23  discharges to a pressure isolation device  24 , such as a Low-Pressure Feeder sold by Ahlstrom Machinery. The pressure isolation device  24  isolates the pressurized horizontal treatment vessel  25  from the essentially atmospheric pressure that exists above device  24 . 
     Vessel  25  is used to treat the material with pressurized steam, for example steam at approximately 10-20 psig. The vessel  25  may include a screw-type conveyor such as a Steaming Vessel sold by Ahlstrom Machinery. Clean or flashed steam is added to the vessel  25  via one or more conduits  28 . 
     After treatment in vessel  25 , the material is transferred to a high-pressure transfer device  27 , such as a High-Pressure Feeder sold by Ahlstrom Machinery. Typically, the steamed material is transferred to the feeder  27  by means of a conduit or chute  26 , such as a Chip Chute sold by Ahlstrom Machinery. Heated cooking liquor, for example, a combination of spent kraft black liquor and white liquor, is typically added to chute  26  via conduit  29  so that a slurry of material and liquor is produced in chute  26 . 
     If the prior art system of FIG. 1 does employ a DIAMONDBACK® steaming vessel as disclosed in U.S. Pat. No. 5,000,083, which produces improved steaming under atmospheric conditions, the pressurized treatment vessel  25  and the pressure isolation device  24  may be omitted. 
     The conventional High-Pressure Feeder  27  contains a low pressure inlet connected to chute  26 , a low pressure outlet connected to conduit  30 , a high-pressure inlet connected to conduit  33 , a high-pressure outlet connected to conduit  34 , and a pocketed rotor driven by a variable-speed electric motor and speed reducer (not shown). The low pressure inlet accepts the heated slurry of chips from chute  26  into a pocket of the rotor. A screen in the outlet, at  30 , of the feeder  27  retains the chips in the rotor but allows the liquor in the slurry to pass through the rotor to be removed via conduit  30  and pump  31 . As the rotor turns the chips that are retained within the rotor are exposed to high pressure liquid from pump  32  via conduit  33 . This high-pressure liquor slurries the chips out of the feeder and passes them to the top of digester  11  via conduit  34 . Upon reaching the inlet of digester  11  some of the excess liquor used to slurry the chips in conduit  34  is removed from the slurry via screen  12 . The excess liquor removed via screen  12  is returned to the inlet of pump  32  via conduit  35 . The liquor in conduit  35 , to which fresh cooking liquor may be added, is pressurized in pump  32  and passed in conduit  33  for use in slurrying the chips out of feeder  27 . The chips that are retained by the screen  12  pass downwardly in the digester  11  for further treatment. 
     The liquor removed from feeder  27  via conduit  30  and pump  31  is recirculated to the chute  26  above the feeder  27  via conduit  36 , sand separator  37 , conduit  38 , in-line drainer  39  and conduit  29 . Sand separator  37  is a cyclone-type separator for removing sand and debris from the liquor. In-line drainer  39  is a static screening device which removes excess liquor from conduit  38  and passes it through conduit  39 ′ and stores it in level tank  40 . Liquor stored in tank  40  is returned to the top of the digester via conduit  41 , pump  42  (i.e., the Make-up Liquor Pump), and conduit  43 . Fresh cooking liquor may also be added to conduits  41  or  43 . 
     FIG. 2 illustrates another prior art system  110  for feeding chips to a digester. This system uses processes and equipment described in U.S. Pat. Nos. 5,476,572, 5,622,598 and 5,635,025. This equipment and the processes they are used to effect are collectively marketed under the trademark Lo-Level™ by Ahlstrom Machinery. The components in FIG. 2 which are identical to those that appear in FIG. 1 are identified by the same reference numbers. Those components which are similar or which perform similar functions to those that appear in FIG. 1 have their reference numbers that appear in FIG. 1 prefaced by the numeral “1”. 
     Similar to the system of FIG. 1, chips  20  are introduced to steaming vessel  121  where they are exposed to steam introduced via conduit  22 . The vessel  121  discharges to metering device  123 , and then to conduit  126 , which is preferably a Chip Tube as sold by Ahlstrom Machinery. Cooking liquor is typically introduced to tube  126  via conduit  55 , similar to conduit  29  of FIG.  1 . Since the vessel  121  is preferably a DIAMONDBACK® steaming vessel as described in U.S. Pat. No. 5,000,083, no pressure isolation device,  24  in FIG. 1, or pressurized steaming vessel  25  in FIG. 1, are needed in this prior art system. As disclosed in U.S. Pat. No. 5,476,572 instead of discharging the slurry of chips and liquor directly to feeder  27 , a high-pressure slurry pump  51  fed by conduit  50  is used to transport the chips to the feeder  27  via conduit  52 . The pump  51  is preferably a Hidrostal pump as supplied by Wemco, or similar pump supplied by the Lawrence company. The chips that are passed via pump  51  are transported to digester  11  by feeder  27  in a manner similar to what was shown and described with respect to FIG.  1 . 
     In addition to using the pump  51  to pass the slurry to the feeder  27 , the system of FIG. 2 does not require the pump  31  of FIG.  1 . Pump  51  supplies the motive force for passing liquor through the feeder  27 , through conduit  30 , sand separator  37 , in-line drainer  39 , and conduit  129  to liquor level tank  53 . 
     The function of level tank  53  is disclosed in pending application Ser. No. 08/428,302, filed on Apr. 25, 1995. The tank  53  ensures a sufficient supply of liquor to the inlet of the pump  51 , via conduit  54 . This tank may also supply liquor to tube  126  via conduit  55 . This liquor tank  53  also allows the operator to vary the liquor level in the feed system such that, if desired, the liquor level may be elevated to the metering device  123  or even to the bin  121 . This option is also described in pending application Ser. No. 08/354,005, filed on Dec. 5, 1994. 
     FIG. 3 illustrates one preferred embodiment of a feed system  210  that simplifies even further the prior art feeding systems shown in FIGS. 1 and 2. In the preferred embodiment shown in FIG. 3, the high-pressure transfer device, component  27  of FIGS. 1 and 2, has been eliminated. Instead of transferring chips to the feeder  27  by means of gravity in chute  26  of FIG. 1 or via pump  51  in FIG. 2, at least one, preferably two, high-pressure slurry pumps  251 ,  251 ′ are used to transport the slurry to the inlet of the digester  11 . The components in FIG. 3 which are essentially identical to those that appear in FIGS. 1 and 2 are identified by the same reference numbers. Those components which are similar or which perform similar functions to those that appear in FIGS. 1 and 2 have their reference numbers that appear in FIGS. 1 and 2 prefaced by the numeral “ 2 ”. 
     Similar to the procedure in FIGS. 1 and 2, according to the embodiment of FIG. 3, chips  20  are introduced to steaming vessel  221 . The chips are preferably introduced by means of a sealed horizontal conveyor as disclosed in pending application Ser. No. 08/713,431, filed on Sep. 13, 1996. Also, the steaming vessel  221  is preferably a DIAMONDBACK® steaming vessel as described in U.S. Pat. No. 5,000,083 to which steam is added via one or more conduits  22 . The steaming vessel  221  typically includes conventional level monitoring and controls as well as a pressure-relief device (not shown). Vessel  221  discharges steamed chips to metering device  223 , which, as described above, may be a pocketed rotor-type device such as a Chip Meter or a screw-type device. 
     In one embodiment the metering device  223  discharges directly to conduit or chute  226 . However, in an optional embodiment, a pressure isolating device, such as a pocketed rotor-type isolation device, shown in dotted line at  224 , for example a conventional Low-pressure Feeder, may be located between metering device  223  and chute  226 . Though without the pressure-isolation device  224  the pressure in chute  226  is essentially atmospheric, with a pressure isolation device  224  the pressure in chute  226  may range from 1 to 50 psig, but is preferably between 5 to 25 psig, and most preferably between about 10 to 20 psig. Cooking liquor, as described above, is added to chute  226  (see line  226 ′ in FIG. 3) so that a slurry of chips and liquor is produced in chute  226  having a detectable level (not shown). The slurry in chute  226  is discharged via radiused outlet  250  to the inlet of pump  251 . The introduction of slurry to the inlet of pump  251  is typically augmented by liquor flow from liquor tank  253  via conduit  254  as described in pending application Ser. No. 08/428,302. 
     Pump  251  is preferably a centrifugal high-pressure, helical screw, slurry pump, such as a “Hidrostal” pump supplied by Wemco of Salt Lake City, Utah. The pump  251  may alternatively be a slurry pump supplied by the Lawrence Company of Lawrence, Mass. The pressure at the inlet to pump  251  may vary from atmospheric to 50 psig depending upon whether a pressure isolation device  224  is used. 
     In the preferred embodiment illustrated in FIG. 3, the outlet of pump  251  discharges to the inlet of pump  251 ′. Pump  251 ′ is preferably the same type of pump as pump  251  but with the same or a higher pressure rating. If two pumps are used, the pressure produced in the outlet of pump  251 ′ typically ranges from 150 to 400 psig (i.e., 345-920 feet of water, gauge), but is preferably between about 200 and 300 psig (i.e., 460-690 feet). If necessary, the liquor in the slurry in conduit  252  may be augmented by liquor from tank  253  via conduit  56  and liquid pump  57 . 
     Though the embodiment illustrated in FIG. 3 includes two pumps, only one pump, or even three or more pumps, in series or parallel, may alternatively be used. In these cases, the discharge pressure from the one pump, or from the last pump, is preferably the same as the discharge pressure from pump  251 ′ above. 
     The pressurized, typically heated, slurry is discharged from pump  251 ′ to conduit  234 . Conduit  234  passes the slurry to the inlet of continuous digester  11 . Excess liquor in the slurry is removed via screen  12  as is conventional. The excess liquor is returned to the feed system  210  via conduit  235 , preferably to liquor tank  253  for use in slurrying in conduit  250  via conduit  254 . The liquor in conduit  235  may be passed through a sand separator  237  if desired. This sand separator  237  may be designed for pressurized or unpressurized operation depending upon the mode of operation desired. 
     Unlike the prior art systems employing a High-Pressure Feeder ( 27  in FIGS. 1 and 2) which uses the pressure of the liquor returned via conduit  35  as an integral part of the method of slurrying from the High-Pressure Feeder to the digester  11 , it is not essential for the operation of the present invention that the pressurized recirculation  235  be returned to the inlet of the pumps  251 ,  251 ′. The energy available in the pressure of the flow in line  235  may be used wherever necessary in the pulp mill. However, in a preferred embodiment, the present invention does utilize the pressure available in conduit  235  to minimize the energy requirements of pumps  251  and  251 ′ as much as possible. 
     How the pressure in return line  235 , typically about 150 to 400 psig is used depends upon the mode of operation of the feed system  210 . If vessel  226  is operated in an unpressurized—essentially atmospheric—mode, the pressurized liquor returned in conduit  235  must be returned to essentially atmospheric pressure before being introduced to conduit  250 . One means of doing this is to use a pressure control valve  58  and a pressure indicator  59  in conduit  235 . The opening in valve  58  is controlled such that a predetermined reduced pressure exists in line  235  downstream of valve  58 . In addition, the liquor tank  253  may be designed so that it acts as a “flash tank” so that the hot pressurized liquor in conduit  235  is rapidly evaporated to produce a source of steam in vessel  253 . This steam can be used, among other places, in vessel  221  via conduit  60 . However, instead, in a preferred embodiment, the pressurized liquor in conduit  235  is used to augment the flow out of pump  251 ′, for example via conduit  61  and pump  62 . The pressure in conduit  235  may also be used to augment the flow between pumps  251  and  251 ′ in conduit  252  via conduit  63 , with or without pump  64  (a check valve may in some cases be used in place of or in addition to each of pumps  62 ,  64 ). By re-using some of the pressure available in line  235 , some of the energy requirements of pumps  251  and  251 ′ may be reduced. 
     Also, the heat of the liquor in line  235  can also be passed in heat-exchange-relationship with one or more other liquids in the pulp mill that need to be heated. 
     The pressurizing and transferring of pumps  251  and  251 ′ may instead by effected by a conventional eductor, for example, an eductor manufactured by Fox Valve Development Corporation. Or pumps  251 ,  251 ′ may be used in conjunction with an eductor for increasing the pressure in the inlet or outlet of the pumps. An eductor may also be used as a means of introducing liquid to the chips. For example, an eductor may be located in the outlet of or beneath vessel  226  and liquid first introduced to the chips by means of this eductor. The eductor may comprise a venturi-type orifice in one or more conduits  250 ,  252 , and  234  into which a pressurized stream of liquid is introduced. This pressurized liquid may be obtained from any available source but is preferably obtained from conduit  235 , upstream of valve  58 . An exemplary eductor is shown schematically at  70  in FIG.  3 . 
     The pumps  251  and  251 ′ need not be centrifugal pumps but may be any other form of slurry transfer device that can directly act on to pressurize and transfer a slurry of chips and liquor from the outlet of vessel  226  to the inlet of digester  11 . For instance, a solids pump as typically used in the mining industry may be used; for example, a double-piston solids pump such as the KOS solids pump sold by Putzmeister, or any other similar conventional pumping device may be used. 
     One function of the prior High-Pressure Feeder  27  of FIGS. 1 and 2 is to act as a shut-off valve to prevent possible escape of the pressure in the equipment and transfer conduits, for example, conduits  34  and  35  of FIG. 1, should any of the feed components malfunction or fail. In the feed system  210  according to the present invention, alternative means are provided to prevent such release of pressure due to malfunction or failure. For example, FIG. 3 illustrates a one-way (check) valve  65  in conduit  234  to prevent pressurized flow from returning to pump  251  or  251 ′. In addition, conventional automatic (e.g. solenoid operated) isolation valves  66  and  67  are located in conduits  234  and  235 , respectively, to isolate the pressurized conduits  234 ,  235  from the rest of the feed system  210 . In one preferred mode of operation, a conventional pressure switch  68  is located downstream of pump  251 ′ in conduit  234 . The switch  68  is used to monitor the pressure in line  234  so that should the pressure deviate from a predetermined value, the conventional controller  69  will automatically isolate digester  11  from feed system  210  by automatically closing valves  66  and  67 . These valves may also be automatically closed when a flow direction sensor detects a reversal of flow in conduit  234 . 
     While the pressure release preventing means  65 - 69  described above is preferred, other arrangements of valves, sensors, indicators, alarms, or the like may comprise the pressure release preventing means as long as such arrangements adequately perform the function of preventing significant depressurization of the digester  11 . 
     While the system  210  is preferably used with a continuous digester  11 , it also may be used with other vertical superatmospheric (typically a pressure of at least about 10 bar gauge) treatment vessels having a top inlet, such as an impregnation vessel or a batch digester. 
     FIG. 4 illustrates a further embodiment in which the concept of transferring chips is extended from the feed system of a digester to the Woodyard of a pulp mill. FIG. 4 illustrates a system  510  for feeding comminuted cellulosic fibrous material to a pulping process. It consists of a subsystem  410  for introducing chips from the Woodyard to system  510  and a subsystem  310  for treating and feeding chips to digester  11 . Subsystem  310  is essentially identical to the system  210  shown in FIG.  3 . 
     Again, the components in FIG. 4 which are identical to those that appear in FIGS. 1-3 are identified by the same reference numbers. Those components which are similar or which perform similar functions to those that appear in FIG. 1-3 have their reference numbers that appear in FIG. 1 prefaced by the numeral “ 3 ”. 
     The Woodyards of conventional pulp mills receive their wood supply in various forms as described above. Typically, the wood, or other comminuted cellulosic fibrous material, is converted to chip like form and stored either in open chip piles or in chip storage silos. In FIG. 4 the chip supply is shown as chip pile  80 . In a preferred embodiment of this invention the chips from pile  80  or some other storage vessel are conveyed by conventional means, e.g., a conveyor or front-end loader (not shown), and introduced  20  to vessel  81 . This vessel may be a DIAMONDBACK® vessel or any other conventional storage vessel. Vessel  81  may be operated at superatmospheric pressure, for example at 0.1 to 5 bar. If the vessel is operated at superatmospheric pressure, some form of pressure isolation device (not shown) may be located at the inlet of the vessel to prevent the release of pressure. This device may be a star-type isolation device, such as a Low-pressure Feeder or Air-lock Feeder as sold by Ahlstrom Machinery, or a screw-type feeder having a sealing capacity as described in co-pending application Ser. No. 08/713,431. 
     Liquid, for example fresh water, steam, liquids containing cooking chemicals is introduced to vessel  81  via one or more conduits  82  to produce a slurry of liquid and chips and to provide a detectable liquid level in vessel  81 . Means for monitoring and controlling the level of the liquid, and the level of the chips, in vessel  81  may be provided. This liquid may be a heated liquid, for example, hot water or steam, having a temperature of between 50 and 100° C. If the vessel is a pressurized vessel, liquid temperatures of over 100° C. may be used. Preferably, though not essentially, this liquid may contain at least some active pulping chemical, for example, sodium hydroxide (NaOH), sodium sulfide (Na2S), polysulfide, anthraquinone or their equivalents or derivatives or surfactants, enzymes or chelants, or combinations thereof. 
     From vessel  81 , the slurry is discharged to the inlet of slurry pump  85  via conduit  84 . The discharge from vessel  81  may be aided by a discharge device  83  (probably not necessary if a DIAMONDBACK® discharge is used). The flow of slurry in conduit  84  may also be aided by the addition of liquid via conduit  82 ′. The conduit  82 ′ may be the only mechanism for introducing liquid, so that a liquid level is present in conduit  84  or not in vessel  81 . Pump  85  may be any type of slurry pump discussed above, for example, a Wemco or Lawrence pump or their equivalents, any other type of solids or slurry transfer device. Though only one pump  85  is shown, more than one pump or similar devices may be used to transfer the slurry via conduit  86  to vessel  321 . The slurry transfer via conduit  86  may include one or more storage or surge tanks (not shown). Preferably, the one or more pumps  85  include at least one device having de-gassing capability so that undesirable air or other gases may be removed from the slurry. 
     The slurry discharged from pump  85  is transferred via conduit  86  to subsystem  810 . Subsystem  810  may be located adjacent subsystem  710 , that is, within about 30 feet of subsystem  710 , or may be spaced an appreciable distance from subsystem  710 , for example one-half mile or more away, depending upon the layout of the pulp mill. Hence, conduit  86  is broken to indicate an undetermined distance between subsystem  710  and subsystem  810 . 
     The pressure in conduit  86  is dependent upon the number of pumps and other transfer devices used and the height and distance that the slurry must be transferred. The pressure in conduit  86  may vary from about 5 psig to over 500 psig. 
     Also, during transfer, the chips may be exposed to some form of treatment, for example, de-aeration or impregnation with a liquid, preferably a liquid containing pulping chemicals, such as those described above. The slurry may also be exposed to at least one pressure fluctuation during transfer, such that the pressure of the slurry is varied from a first pressure to a second, higher pressure, and then to a third pressure which is lower than the second pressure. As described in U.S. Pat. Nos. 4,057,461 and 4,743,338 varying the pressure of a slurry of chips and liquor improves the impregnation of the chips with the liquor. This pressure pulsation may be achieved via varying the outlet pressure of a set of transfer devices in series, or by controlled depressurization of the slurry between pumping. 
     The slurry in conduit  86  is introduced to the inlet of vessel  321 . Though the vessel shown is a treatment, i.e., steaming, vessel, it may also be a storage vessel, an impregnation vessel, or even a digester. Since the transfer in conduit  86  typically requires that at least some excess liquid, that is not needed during treatment or storage, some form of de-watering device  87  may be located between the transfer device and the treatment vessel. One preferred dewatering device is a Top Separator, as sold by Ahlstrom Machinery. This Top Separator may be a standard type or an “inverted” Top Separator. This device may be an external stand-alone-type unit or one that is mounted directly onto the treatment vessel, as shown. Preferably, the liquid removed from the slurry by means of de-watering device  87  is returned to vessel  82  or to the inlet of the pump, or pumps,  85  via conduit  88  to aid in slurrying the chips. This liquid removed via device  87  may also be used where ever needed in the pulp mill. This liquid in conduit  88  may be heated or cooled as desired in a heat exchanger  90  and may be pressurized using one or more conventional centrifugal liquid pumps,  89 . The liquid in conduit  88  may be introduced to vessel  81  via conduit  82  and to conduit  84  via conduit  82 ′. 
     The treatment vessel  321  shown is a steaming vessel similar to vessel  221  shown in FIG. 3, for example a DIAMONDBACK® steaming vessel. The feed system  310  is otherwise similar to the system  210  shown in FIG.  3 . For example, chip feeding system  410 , feeds digester feed system  310 , which feeds digester  11 . Note that system  310  of FIG. 4 is simply one subsystem in the over-all system which feeds chips from the chip pile  80  to the digester  11 . This system may include one or more subsystems  310  for feeding to digester  11 . 
     FIG. 5 illustrates a further embodiment  610  that is an extension of the system  510  shown in FIG.  4 . The system  610  is a combination of three subsystems  710 ,  810  and  910 . Subsystem  710  is similar to the system  410  of FIG.  4 . Items in FIG. 5 that are essentially identical to those found in FIGS. 1 through 4 are identified by the same numbers. 
     Wood chips  20 , or some other comminuted cellulosic fibrous material, from chip pile  80  are introduced with or without pressure isolation to vessel  81 . The chips in vessel  81  may be treated with a gas, such as steam or hydrogen sulfide, or a liquid, such as water or a liquid containing cooking chemical, introduced by way of one or more conduits  82 . Vessel  81  may be any type of vessel, but is preferably a DIAMONDBACK® bin, as described above. The treated chips are discharged from vessel  81  into conduit  84 . Though any type of discharging mechanism can be used, the discharge of chips from vessel  81  is preferably performed without the aid of mechanical agitation or vibration, as is characteristic of DIAMONDBACK® chips bins. Conduit  84  may be any type of pipe or chute but is preferably a curved Chip Tube as described above. 
     Conduit  84  introduces the chips to the inlet of slurry pump  85 , which may be of the type supplied by Wemco or Lawrence, as described above. Typically, slurrying liquid is preferably first introduced to the chips in conduit  84 , for example, using the conduit  82 ′, to produce a level of liquid in vessel  81  or conduit  84 . The liquid introduced via conduit  82 ′, may be water or a liquid containing treatment chemicals such as kraft liquors, with or without strength or yield enhancing additives. Make-up liquor, for example, liquor containing these chemicals, is typically added via conduit  782 . 
     The slurry in conduit  86  is introduced to subsystem  810  via liquor separating device  887 , which is similar in operation to device  87  shown in FIG.  4 . The liquid removed via separator  887  can be returned to subsystem  710  via conduit  88  or can be used elsewhere in the pulp mill via conduit  888 . If returned to subsystem  710  via conduit  88  the liquor may be augmented with additional liquid or chemical via conduit  788 , heated via indirect heat exchanger  90  via conduit  790  and pressurized by pump  89  prior to being re-introduced to vessel  81  via conduit  82  or to conduit  84  via conduit  82 ′. Subsystem  710  may also include a liquor storage tank similar to tank  353  shown in FIG.  4 . Thus by the use of heater  90  and chemical addition  782  or  788 , the slurry of material transferred from subsystem  710  to subsystem  810  via conduit  86  may be heated to any desirable temperature while being treated with chemicals. For example, if the slurry in conduit  86  is heated to about 90° C. or above in the presence of alkali or sulfide, some pretreatment of the will occur during the retention time in conduit  86  prior to introduction of the slurry into subsystem  810 . Of course, lower temperatures and other chemicals may also be used in conduit  86 . 
     The chips retained by separator  887  are passed to vessel  821 . Vessel  821  may be a vessel similar to vessel  81 , but is preferably a tall cylindrical vessel, for example, 20 to 50 feet tall, in which a liquid level  823  is maintained. A gas space  824  may be maintained above level  823 . Vessel  821  may be maintained at atmospheric pressure or at super-atmospheric pressure, for example, at 0.2 to 10 bar gauge pressure (e.g. about 5 bar), depending on the treatment performed in vessel  821 . The temperature in vessel  821  may vary from 50 to 300° C., but is typically between about 50 and 150° C. Liquid may be introduced to vessel  821  via one or more conduits  822  or  860 . This liquid may contain cooking chemicals or additives as discussed above. These cooking chemicals or additives may be the same as those introduced in subsystem  710  or they may be different. For example, kraft cooking liquor containing a high concentration of sulfide ion or sulfidity may be introduced to subsystem  710  and kraft cooking chemical containing a lower concentration of sulfide ion or sulfidity may be introduce to the chips in subsystem  810 . In another example, a polysulfide-type additive may be introduced to the chips in subsystem  710  and an anthraquinone-type additive may be introduced in subsystem  810 . 
     The pressure within the vessel  821  may be monitored and controlled via pressure indicator and controller  825 . Excess pressure may be released via conduit  826 , for example, to a conventional noncondensable gas (NCG) treatment system or to vessel  81  for pretreatment. In addition, the pressure controller  825  can be used to regulate the pressure in vessel  821  to vary the pressure to effect pressure pulsation impregnation as described in U.S. Pat. Nos. 4,057,461 and 4,743,338. 
     The slurry is discharged from vessel  821  to conduit  850 . This discharge may be effected without agitation or vibration as in a DIAMONDBACK® chip bin, or it may be effected by agitation or vibration as is conventional. Conduit  850  introduces the slurry to the inlet of pump  851 , which may be similar to pump  85 , but typically will have a higher pressure rating. Additional liquid may be introduced to conduit  850  via conduit  854  to aid in introducing the slurry to the pump  851 . The slurry discharged from pump  851  is passed to subsystem  910  via conduit  886 . 
     The slurry in conduit  886  is introduced to subsystem  910  using the liquor separating device  987 . The separator  987  is similar to devices  887  and  87  (of FIG.  4 ). The liquor removed from device  987  may be returned by conduit  911  to subsystem  810  or may be used elsewhere in the pulp mill via conduit  988 . If returned to subsystem  810  via conduit  911 , the liquor may be augmented with additional liquid or chemical via conduit  912 , heated via indirect heat exchanger  890  via conduit  891  and pressurized by pump  889  prior to being re-introduced to vessel  821  via conduit  822  or  860  to conduit  850  via conduit  854 . The liquor in conduit  911  may also be introduced to subsystem  710 , for example, via a common connection with conduit  88  or  82 . Subsystem  810  may also include a liquor storage tank similar to tank  353  shown in FIG.  4 . Thus by using heater  890  and chemical addition  912 , the slurry of material transferred from subsystem  810  to subsystem  910  via conduit  886  may be heated to any desirable temperature while being treated with chemicals. For example, if the slurry in conduit  886  is heated to about 90° C. or above in the presence of alkali or sulfide, some pretreatment of the material will occur during the retention time in conduit  886  prior to introduction of the slurry into subsystem  910 . Of course, lower temperatures and other chemicals may also be used in conduit  886 . 
     The chips retained by separator  987  are passed to vessel  921 , which may be a vessel similar to vessels  81 , or a tall vessel similar to vessel  821 , or a vessel similar to vessel  321  of FIG.  4 . Vessel  921  may be maintained at atmospheric pressure, or at super-atmospheric pressure [for example, at 0.2 to 10 bar gauge, preferably 0.5 to 5 bar gauge pressure] depending on the treatment performed in vessel  921 . The temperature in vessel  921  may vary from 50 to 300° C., but is typically between about 50 and 150° C., preferably between about 80 and 120° C. Liquid may be introduced to vessel  921  via one or more conduits  922  or  960 . The introduced liquid may contain cooking chemicals or additives as discussed above. These cooking chemicals or additives may be the same as those introduced in subsystem  710  or  810  or they may be different. For example, kraft cooking liquor containing a high concentration of sulfide ion or sulfidity may be introduced to subsystem  810  and kraft cooking chemical containing a lower concentration of sulfide ion or sulfidity may be introduced to the chips in subsystem  910 . In another example, a polysulfide-type additive may be introduced to the chips in subsystem  710  and an anthraquinone-type additive may be introduced in subsystem  810 , and kraft white liquor may be introduced to the chips in subsystem  910 . Each or these liquors can be isolated from each other by the liquor separators  887  and  987 . 
     The slurry is discharged from vessel  921  to conduit  950 . This discharge may be effected without agitation or vibration using a discharge as in a DIAMONDBACK® chips bin, or it may be aided by agitation or vibration as is conventional. Conduit  950  introduces the slurry to the inlet of pump  951 , which may be similar to pumps  85  and  851 , but typically will have a higher pressure rating. Additional liquid may be introduced to conduit  950  via conduit  960  to aid in introducing the slurry to the pump  951 . The slurry discharged from pump  951  is passed to further treatment via conduit  986 , for example, to a digester (that is, a continuous or batch digester), or to further treatment in a subsystem similar to subsystems  810  or  910 , or subsystem  310  of FIG.  4 . However, the treatment effected in subsystems  710 ,  810  and  910  may be sufficient to produce an essentially fully-cooked pulp slurry in conduit  950  such that no further “pulping” need be performed. The pulp in conduit  950  may be passed directly to washing and/or bleaching. 
     As in subsystems  310 ,  810 , and  910 , excess liquor may be returned to subsystem  910  via conduit  913 . The liquor may be augmented with additional liquid or chemical via conduit  914 , heated via indirect heat exchanger  990  via conduit  991  and pressurized by pump  989  prior to being re-introduced to vessel  921  via conduit  922  or to conduit  950  via conduit  960 . The liquor in conduit  913  may also be introduced to subsystem  710  or  810 , for example, via a common connection with conduit  88  or  82  (not shown) or a common connection with conduits  911  or  822 , or similar conduits. Subsystem  910  may also include a liquor storage tank similar to tank  353  shown in FIG.  4 . 
     Thus, using heater  990  and chemical addition  914 , the slurry of material transferred from subsystem  910  to the subsequent subsystem or digester via conduit  986  may be heated to any desirable temperature while being treated with chemicals. For example, if the slurry in conduit  986  is heated to about 90° C. or above in the presence of alkali or sulfide, some pretreatment of the chips will occur during the retention time in conduit  986  prior to introduction of the slurry into the subsequent treatment device, for example to digester  11  of FIGS. 1 and 2. Of course, lower or higher temperatures and other chemicals may also be used in conduit  986 . 
     Also, though indirect heat exchangers  90 ,  890 , and  990  may each be supplied by their own separate source of heat, for example, separate sources of steam or hot water or hot effluent that would normally be discharged, heat exchangers  90 ,  890  and  990  may also be supplied with a common source of heat  915 . The source of heat  915  may be, for example, hot effluent or steam (low, medium or high pressure steam), and may be introduced to heat exchanger  990  and the residual heat transferred to heat exchanger  890  via conduit  992 . The residual heat from heat exchanger  890  may be passed to heat exchanger  90  via conduit  892 . Any residual heat remaining in conduit  92  may be used as needed in systems  710 ,  810  or  910  or elsewhere in the mill, or it may be discarded. For example, the liquid in conduit  92 , and any residual heat it may contain, may be introduced to vessel  81  or  821  via conduits  82  or  822  to recover and re-use as much of the available energy as possible. 
     Using a system  610  as shown in FIG. 5, a counter-current flow of treatment liquids can be established between each subsystem. For example, the liquid from upstream treatment can be returned to subsystem  910  via conduit  913 ; the liquid from subsystem  910  can be returned to subsystem  810  via conduit  911 ; and the liquid from subsystem  810  can be returned to subsystem  710  via conduit  88 . In addition some or all of these liquors can be removed and used elsewhere via conduits  888  and  988 . 
     The chemical addition at  788 ,  912 , and  914  is preferably sodium hydroxide, sodium sulfide; polysulfide, anthraquinone or their equivalents or derivatives; surfactants, enzymes, or chelants; or combinations thereof. For example, different treatment chemicals could be added at each of  788 ,  912 , and  914 , so that different treatments take place in each of the sections  710 ,  810 , and  910 . For example, polysulfide may be added at  788 , anthraquinone at  912 , and chelants and enzymes at  914 . The conduits at  788 ,  912 ,  914  need not be provided where illustrated in FIG. 5, but may be provided at any convenient location which facilitates impregnation, or other pretreatment, simultaneously with transport. For example, lines  788 ,  912 ,  914  may be added to the lines  790 ,  891 ,  991  before the heater exchangers  90 ,  890 ,  990 , respectively. 
     In one preferred embodiment, the slurry is treated in the system of FIG. 5 to remove undesirable metal-containing compounds or metal ions from the cellulose material. For example, in this embodiment the chemical added to the slurry is an acid and/or chelating agent. The acid is preferably sulfuric acid, sulfur dioxide, acetic acid, formic acid, oxalic acid, peroxy acids, Caro&#39;s acid, or their equivalents, or combinations thereof. Acidic bleach plant filtrates can also be used as the source of acid. The pH of the liquid during acid treatment typically varies from a pH of about 1 to a pH of about 7, but is preferably between a pH of about 2 and about 4. The temperature of the acid treatment may vary from about 0 to about 150° C., but is preferably between about 60 and about 90° C. The duration of the acid treatment may be 10 minutes to 6 hours, but is preferably about 30 to 120 minutes. The acid treatment may be followed by the addition of magnesium salts, for example, magnesium sulfate, to replenish the magnesium content of the material which under certain conditions has been found to be beneficial. 
     The chelating agent, that is, a solution containing polydendate ligand molecules, is preferably EDTA, DTPA, DTMPA, or their equivalents, or combinations thereof. The chelate charge is typically at most about 2 kg per ton of pulp but may range from about 0.5 to 5 kg per ton of pulp. During chelate treatment, the pH of the treatment liquid typically varies from a pH of about 2 to about 10, but is preferably between a pH of about 4 and about 8. The temperature of the chelate treatment may vary from 0 to 150° C., but is preferably between about 60 and 110° C. The duration of the chelate treatment may be 10 minutes to 6 hours, but is preferably between about 30 to about 90 minutes. 
     The chelation stages (Q) and the acid stages (A) are not mutually exclusive; both types of treatments may be used, for example, in succession (in either order) and repeatedly, during the transfer of the slurry of cellulosic material. Either treatment may also be practiced repeatedly. The successive treatments may or may not include a purge or washing stage between successive treatments. For example, some of the treatment sequences that may be practiced according to this invention include, but are not limited to, the following sequences: AA, QQ, AQA, QAQ, AAQ, QQA, AQQ, QAA, AAA, QQQ, AAQQ, QQM. Repetition or extension of these treatment sequences, as would be readily understood by those in the art, is also within the scope of this invention. Again, these sequences may or may not include a washing or purge between successive treatments. 
     In the embodiment shown in FIG. 5, the acid or chelating agent can be introduced via conduit  782 ,  788 ,  912 , and/or  914 , but the acid or chelate is preferably introduced to subsystem  710  via conduit  782  or to subsystem  810  via conduit  912 . If the acid orchelant is added to subsystem  810 , the metal removal treatment can be followed immediately by alkaline treatment in subsystem  910  prior to alkaline digestion in, for example, a digester (not shown) fed by conduit  986 , with or without the use of a conventional high-pressure feeder. 
     For example, after treatment or transport in subsystem  710 , acid or chelant can be introduced to subsystem  810  via conduit  912 ,  854 ,  860 , or  882 . The acidified/chelated slurry is pressurized by pump  851  and passed to liquor separator  987  via conduit  886 . The treatment liquor can be removed via separator  987  and returned upstream of the inlet of pump  851  or, preferably, removed from the system via conduit  988 . The metal-laden stream removed via conduit  988  can be passed to other treatment in the pulp mill or to disposal or to any suitable form of conventional metal recovery process. The liquid removed via conduit  988  may be removed simply through a branch conduit from conduit  911  or via a liquor separator, such as an In-line Drainer (not shown). The liquid in conduit  988  may also be removed directly from separator  987 . The volume of liquid removed via conduit  988  can be replaced, or “made up”, by liquid introduced via conduits  912 ,  854 ,  860  and/or  822 , for example, water, washer filtrate, black liquor, or bleach plant effluent, among other available liquids. Make-up acid or chelate may also be introduced, with or without make-up liquid, via one or more of the conduits  912 ,  854 ,  860 , and/or  822 . 
     In addition, according to this invention, the acid or chelant can also be introduced via conduit  782  or conduit  788  so that the metal removal treatment is practiced in the subsystem  710  and a second treatment is practiced in subsystem  810  prior to alkaline treatment in subsystem  910 . The second treatment in subsystem  810  may be a second acid or a second chelate treatment, or, if the treatment in subsystem  710  is an acid treatment, the treatment in subsystem  810  may be a chelate treatment, or vice versa. 
     Furthermore, since the pH of the acid or chelate treatment will typically be distinctly different from the pH of the alkaline treatment (for example, the alkaline treatment is typically practiced at a pH greater than 8, often greater than 10), in order to avoid excessive consumption of acid, chelate, and/or alkali, in one embodiment of the invention, the acid or chelate treatment in a first stage is followed by a wash or neutralization treatment in a following second stage, prior to the subsequent treatment, for example, prior to the introduction of alkaline liquids in a third stage. In the system shown in FIG. 5, the acid or chelate treatment can be practiced in subsystem  710 , a somewhat neutral wash or soaking of the material can be practiced in subsystem  810  and an alkaline treatment can be practiced in subsystem  910 . 
     For example, acid or chelant can be introduced via conduit  782  and the acidified/chelated slurry is pressurized by pump  85  and passed to liquor separator  887  via conduit  86 . The treatment liquor can be removed via separator  887  and returned to the inlet of pump  85  or, preferably, removed from the system via conduit  888 . The metal-laden stream removed via conduit  888  can be passed to other treatment in the pulp mill or to disposal or to a suitable conventional metal recovery process. The liquid removed via conduit  888  may be removed simply through a branch conduit from conduit  88  or via liquor separator, such as a conventional In-line Drainer (not shown). The liquid in conduit  888  may also be removed directly from separator  887 . The volume of liquid removed via conduit  888  can be replaced, or “made up”, by liquid introduced via conduits  788  and/or  782 , for example, water, washer filtrate, black liquor, or bleach plant effluent, among other available liquids. Make-up acid or chelate may also be introduced, with or without make-up liquid, via conduits  788  or  788  or both. 
     After acid or chelate treatment in subsystem  710 , subsystem  810  can be used to wash or neutralize the slurry prior to introducing the slurry to alkaline treatment in subsystem  910 . For example, essentially neutral to alkaline, preferably metal-free, liquid can be introduced to the slurry via conduit  912  or conduits  854 ,  860 , or  822 , to wash or increase the pH of the slurry during passage through vessel  821  and through conduits  850  and  886  prior to introducing the slurry to separator  987 . The neutralized or pH-adjusted liquid is removed from the slurry via separator  987  and the liquid can be returned to upstream of pump  851  via conduit  911  or removed via conduit  988 . Again, the liquid removed via conduit  988  may be removed via a simple branch conduit, via a liquor separator (e.g., a conventional In-line Drainer) or directly from separator  987 . 
     After metal removal in subsystem  710  and washing or neutralization in subsystem  810 , the cellulose material can be treated with alkaline cooking chemical, for example, kraft white, green, or black liquor (with or without additives as discussed above) in subsystem  910  prior to digestion with minimal excess use of chemical due to consumption of acids and/or chelants by alkali. 
     FIG. 6 schematically illustrates other desirable apparatus for practicing a desirable method according to the invention. Utilizing the system of FIG. 6 a slurry of comminuted cellulosic fibrous material (typically at a consistency of about 5-20%) is transported within a pulp mill at any locations within a fiber line, such as from the wood yard to a digester, with intermittent booster pumps in series. Each pump is associated with a station (treatment vessel) and a solids/ liquid separator is associated with each station (typically a conventional solid/liquid separator at the top of the station), to isolate liquor streams or circulations. Impregnation, or other pretreatment, is performed simultaneously during transit of the material, in the circulation lines (that is from one pump to its associated station), and the lines can be made very long (e.g. more than 100 yards, up to about a half a mile) to facilitate that pretreatment and impregnation. Preferably heat exchangers are utilized on the return lines, and degassing may be provided at one, more than one, or all of the transfer stations. Also, an eductor (ejector) can be used in place a flash tank and/or control valves through which liquor is removed and pressure reduced. Further, pressurized pulsation action may be associated with the configuration of pumps and stations, the pumps pressurizing the slurry to at least 5 bar (typically at least about 10 bar). Also, a wide variety of treatment chemicals may be utilized preferably added upstream of the pumps, including sodium hydroxide, sodium sulfide; polysulfide, anthraquinone or their equivalents or derivatives; surfactants, enzymes, or chelants; or combinations thereof. 
     The chip slurry  1000  is formed in any conventional manner (including by heat steam slurrying), and first, second and third booster pumps  1001 ,  1002 , and  1003  are connected in series. The pumps  1001 - 1003  are associated with stations (vessels)  1004 ,  1005 ,  1006 , respectively. Preferably each of the stations  1004 - 1006  has a liquid/solid separator associated therewith. In the embodiment illustrated in FIG. 6 separators  1007 ,  1008 ,  1009  are shown mounted at the top of each of the stations (treatment vessels)  1004 - 1006 , although the separator could be at another location, including the bottom. 
     Preferably chemical is added to the slurry at a number of different locations in the system, such as upstream at each of the pumps  1001 - 1003 . This is schematically illustrated by chemical addition at points  1010 ,  1011 , and  1012  in FIG.  6 . The same, or different, chemicals can be added at each of  1010 - 1012 . Preferably at least some of the chemical includes sodium hydroxide, sodium sulfide; polysulfide, anthraquinone or their equivalents or derivatives; surfactants, enzymes, or chelants; or combinations thereof. In the embodiment actually illustrated in FIG. 6, the chemical addition  1012  includes AQ laden white liquor (e.g. vessel  1006  is a continuous digester). 
     Instead of establishing circulation lines such as illustrated in FIG. 5, circulation is provided in the FIG. 6 embodiment, in the preferred form, so as to cause pseudo counter-current flow of the comminuted cellulosic fibrous material and liquid. While FIG. 6 illustrates three stations, any number of stations may be provided. In the embodiment in FIG. 6, the liquid removed from the separator  1007  in line  1013 , is used elsewhere in the mill, or treated for reuse. The liquid removed from separator  1008  passes in line  1014  to a point upstream of the pump  1001  (e.g. it is diverted by the valve  1015  either to the slurrying station  1000 , or to the infeed to the pump  1001 ) while liquid separated by the third separator  1009  is circulated in line  1016  to upstream of the pump  1002 , e.g. diverted by the valve  1017  to the first station  1004 , and/or to just upstream of the pump  1002 . Fresh liquor, from source  1012 , is added to the bottom of the vessel  1005 , or the intake of the pump  1003 . 
     In the return lines  1014 ,  1016 , conventional indirect heat exchangers  1018 ,  1019  may be provided which change the temperature of the liquid therein by at least 5° C. In the embodiment illustrated, the liquor is heated, but in some circumstances the liquid could be cooled instead of heated. A indirect heat exchanger  1020  may be also be associated with the chemical addition  1012 . 
     Liquor can be passed from the third station  1006  (which may be a digester—e.g. black liquor) through a conventional eductor (ejector)  1022 , rather than a flash tank and/or control valves. Each of the pumps  1001 - 1003  preferably pressurizes the slurry to a pressure of at least 5 bar (typically at least about 10 bar). 
     Degassing may also be associated with one, more than one, or all of the stations  1004 . This is schematically illustrated by the gas removal lines  1023 - 1025  in FIG.  6 . Degassing may be accomplished using any conventional degassing equipment, associated with the separator  1007 - 1009 , the inlet line, or the like. 
     FIG. 7 schematically illustrates a continuous digester feed system similar to the system illustrated in FIG.  3 . Some of the significant differences between the system of FIG. 7, and the method practiced thereby, and the system of FIG. 3, and the method practiced thereby, are the provision of a cooling heat exchanger and a return line from the digester to one or more pumps, a return conduit for introducing liquor directly into the chip tube (by bypassing the surge tank), and a recirculation conduit from the outlet of one or each slurry pump (including the first pump) ultimately to the inlet thereof (e.g. connected between the surge tank and the chip tube for the first pump) to establish a recirculation flow that is particularly desirable during the startup operation. 
     It is to be understood that though a continuous digester is illustrated in FIG. 7, the present invention is also applicable to a batch digester system. The system shown in FIG. 7 includes a feed system  1110  feeding a digester  1111 . The feed system  1110  includes an air-lock chip feed screw  1112 , for accepting wood chips  20 , and chip bin  1121 . Feed screw  1112  is preferably the device disclosed in U.S. Pat. No. 5,766,418 and bin  1121  is marketed under the name Diamondback® Steaming Vessel or Bin as discussed above. Other types of conventional steaming vessels, for example, horizontal screw conveyors or VibraBin vessels having a vibrating discharge, may also be used in place of a Diamondback Bin. 
     Similar to the system shown in FIG. 3, the system shown in FIG. 7 includes a metering device  1123 , such as a Chip Meter, a vertical conduit  1126 , such as a Chip Tube, and a liquor storage vessel  1153 , such as Liquor Surge Tank. Also, as shown in FIG. 3, the system of FIG. 7 includes a first pump, or pumping device,  1151  and a second pump, or pumping device,  1151 ′, which again, may be any type of pump or pumping device for pressurizing and transferring a slurry of comminuted cellulosic fibrous material and liquid. One preferred pumping device is a Hidrostal screw-feed-type pump provided by Wemco Pump of Salt Lake City, Utah, [http://www.wemcopump.com/Products/hidrostal/ details.html] or a pump provided by Lawrence Pumps Inc. of Lawrence, Mass. [http://www.lawrencepumps.com/]. Similar to the system shown in FIG. 3, the inlet of pump  1151  is in operative communication or is connected directly to the outlet of vertical conduit  1126  and the outlet of pump  1151  is in operative communication with or is connected to the inlet of pump  1151 ′. The outlet of pump  1151 ′ is operative communication with the inlet of digester  1111  via conduit  1134 . Excess liquor is returned from the digester  1111  to the feed system  1110  from the inlet of the digester, or from any other available source of liquid associated with the digester, via conduit  1135 . 
     Though not shown in FIG. 7, it would be recognized by those familiar with the art, that the present invention may also be practiced by having the one or more pumps  1151  feed two or more pumps  1151 ′ for feeding one or more digesters  1111 . This mode of operation may be particularly suitable for feeding a plurality of batch digesters, but may also be applicable to feeding two or more continuous digesters. One device that can be used to split the flow from one conduit to two or more conduits is shown in FIG.  8 . It is also recognized that the present invention may also incorporate the features of the inventions disclosed in U.S. Pat. No. 5,795,438, the disclosure of which is incorporated in its entirety by reference herein. 
     Liquor in conduit  1135  is returned to various locations in the feed system  1110 . The liquor in conduit  1135  is preferably returned to Chip Tube  1126  via conduit  1182  or to tank  1153  via conduit  1183  or to vessel  1121  via conduit  1184 . Since the liquor in conduit  1135  will typically have a temperature greater than 100° C. and the Chip Tube  1126  and vessel  1153  may operate at approximately atmospheric pressure, that is, −1 to 1 bar gage (that is, 0 to 2 bar absolute), to avoid undesirable rapid evaporation (that is, “flashing”), some form of cooling device  1136  is provided. This cooling device is preferably an indirect liquor-to-liquor cooling heat exchanger, and cools the liquid being returned to below the temperature at which it will flash. The cooling medium provided in conduit  1137  is typically any available cool liquid stream in the pulp mill. One preferred cooling medium is fresh water which is introduced via conduit  1137  to heat exchanger  1136  at one temperature and removed via conduit  1138  at a higher temperature. Cooking liquids, for example, kraft white, green, or black liquor (for example, via conduit  1150 ) may also be used as the cooling medium in heat exchanger  1136 . A bypass conduit  1135 ′ may also be used to divert liquor around heat exchanger  1136  when the heat exchanger is not needed or when it is being serviced. 
     The level of liquid in tank  1153  is typically controlled by a level control mechanism, for example, a level control mechanism using a d-p cell level indicator or a gamma radiation level indicator (not shown). The level in tank  1153  is typically controlled by varying the flow of liquid out of branch conduit  1181  which feeds pump  1160 , that is, the Make-up Liquor pump. Pump  1160  pressurizes and introduces this excess liquor to the top of the digester  1111  via conduit  1161 . 
     Liquor in conduit  1135  may also be introduced, with or with heating or cooling, upstream of pump  1151  via conduit  1163 . Conduit  1163  may have a valve F. The benefit of introducing pressurized liquid from conduit  1135  upstream of pump  1151  is discussed above in the description of FIG.  3 . The present invention also preferably includes a conduit  1156  between the outlet of pump  1151  and conduit  1154  which may have a valve E, so that liquor may flow from line  1156  to line  1154 . 
     Liquor may also be introduced to conduits  1134  and  1135  via conduits  1144  and  1145  during normal operation or during shutdown or startup of the system. For example, weak black liquor or “cold blow” liquor from pump  1140  may be introduced to conduits  1134  and  1135  to flush the lines during shutdown or to introduce additional liquor to the lines as needed, for example, for liquor-to-wood ratio control or black liquor pretreatment, during normal operation. Cooking liquor, for example, kraft white liquor, green liquor, black liquor, orange liquor, or liquor containing strength or yield enhancing additives, such as anthraquinone, polysulfide, chelants, surfactants, sulfur, or their derivatives and equivalents, may be added to feed system  1110  via conduit  1150  and pump  1152 . The liquor in conduit  1150  is preferably added to Chip Tube  1126  as shown, but can also be added to conduits  1134  or  1135 . 
     The system shown in FIG. 7 also includes several valves, either automatically controlled or manual, which isolate the flow of liquids and their pressures from each other. Valve A isolates the outlet of pump  1151  from the inlet of pump  1151 ′. Valve B isolates the outlet of pump  1151 ′ from the digester  1111 . Valve C in conduit  1134  isolates the feed conduit to the digester  1111  from the digester and valve D in conduit  1135  isolates the return conduit  1135  from the digester  1111 . These valves are especially important during upset conditions to isolate the hot pressurized liquids associated with the digester  1111  from the lower pressure feed system  1110  and from the surrounding personnel and adjacent machinery. 
     The valves A-F, along with selected other valves, can also be used to isolate liquor circulations to aid in start-up and shutdown procedures. For example, when valve A is closed and valve E in conduit  1156  is open, pump  1151  can be started and a closed circulation about pump  1151  can be established via conduit  1156 . Similarly, when valve A is closed and valves C, D, and F are opened and pump  1151 ′ is started, a circulation about pump  1151 ′ can be provided via conduit  1134 , the top of digester  111 , conduit  1135 , and conduit  1163 . (It is also possible to isolate the circulation about pump  1151  from the digester  1111  by inserting a conduit  1170 , with an appropriate valve G, in conduit  1170  between conduits  1134  and  1135 .) 
     The conduits  1156 ,  1154  (and preferably the isolating Valves A and E), and associated connections to other components, comprise means for circulating liquid from the pump  1151  outlet back to its inlet. While conduits are shown as such means it is to be understood that any conventional structures which provide this recirculation may be utilized, including tanks, ejectors, pumps, valves, ducts, heat exchangers, or the like. 
     Isolation of these circulations is especially advantageous during start-up and shutdown conditions when these isolations can be separately maintained. For example, during start-up, before the introduction of wood chips, the two pumps  1151 ,  1151 ′ can be operated to establish one circulation about pump  1151  via conduit  1156  and a second circulation about pump  1151 ′ passing through the digester top and conduits  1134  and  1135 . By so doing, the proper operation of each pump  1151 ,  1151 ′ can be verified and also the pressures and temperatures of each circulation can be isolated. For example, the temperature and pressure of the liquid in the circulation in conduits  1134  and  1135  can be raised to digester operating conduits, for example, 7-15 bar gage at 100-160° C., while the temperature of the circulation associated with pump  1151  and conduit  1156  can be maintained at lower conditions, for example, 1-3 bar gage at 60-120° C., Then when the conditions in each circulation agree, for example, the liquor in conduits  1134  and  1156  are both at about 10 bar gage and 120° C., valve A can be gradually opened while valve E is gradually closed and chips can be introduced to feed system  1110 . A similar situation can occur during shutdown or when the digester  1111  and/or feed system  1110  need to be isolated for servicing. 
     Feed system  1110  may also include a centrifugal separator for removing sand and debris, for example, a Sand Separator; a liquor/chips separator, for example, an In-line drainer; or a liquor storage vessel, for example, a Level Tank, if needed, as found in conventional systems. One or all of these devices may also be omitted from the embodiment shown in FIG.  7 . 
     Feed system  1110  may also include an integral Chip Tube and Surge Tank, as well as other simplifications to a feed system, as disclosed in co-pending application Ser. No. 09/520,761 filed on Mar. 7, 2000, the disclosure of which is incorporated by reference in its entirely herein. 
     FIGS. 8-10 illustrate another embodiment of the present invention for dividing the flow of slurry in a pipe line. FIG. 8 illustrates an elevation view, FIG. 9 a top view, and FIG. 10 a right-hand elevation view. The device  1200  shown in FIGS. 8-10, which is referred to as a static “flow divider” or “flow splitter”, can, for example, be inserted in conduit  34  of FIGS. 1 and 2, conduit  252  or  234  of FIG. 3, conduit  86  and  886  of FIGS. 4 and 5, or corresponding conduits in FIG. 6, or conduit  1134  in FIG.  7 . 
     The static flow splitter  1200  includes an inlet  1201  for a flow of a slurry of comminuted cellulosic fibrous material and liquid and two or more outlets  1202 ,  1203 . The inlet and outlets are preferably circular in cross section, but may be non-circular depending upon the needs of the installation, including elliptical, rectangular, square, or even triangular. The device  1200  includes a chamber  1204  for receiving the slurry from the inlet  1201  and discharging the slurry to the two or more outlets  1203 ,  1204 . The chamber  1204  can have any appropriate cross sectional shape, including round, elliptical, rectangular, square, or triangular, but the shape of the chamber preferably limits the areas in which material in the slurry can stagnate, for example, sharp corners are avoided. As shown in FIG. 8, one preferred shape of chamber  1204  is substantially triangular in which the outlets  1202 ,  1203  have centerlines that diverge from the centerline of the inlet  1201  by between about 30 and 60°, for example, by about 45°. 
     The chamber  1204  may also include one or more internal baffle plates  1210 ,  1211  (shown in phantom) in FIG. 8 to aid in directing the flow of slurry to the two or more outlets  1202 ,  1203 . These baffle plates  1210 ,  1211  may define a triangle with the wall  1212 , positioned opposite the inlet  1201  of device  1200 . The ends of the plates  1210 ,  1211  may be welded or otherwise attached to the walls  1213 ,  1214  of the chamber  1204 . In the embodiment illustrated in FIG. 8 the apex  1215  of the substantially triangular baffle plate arrangement  1210 ,  1211  is substantially aligned with the inlet  1201 . The flow splitter  1200  is static, i.e. has no moving parts (although the position of the baffle plate arrangement  1210 ,  1211  may be made adjustable). 
     The dimensions of device  1200  will vary depending upon the given or desired dimensions and production rate of the system in which it is used. The dimension, for example diameter, of the inlet  1201 , and the outlets  1202 ,  1203 , may range from 2 inches to 10 feet. For example, the inside diameter of the inlet and outlets is about 10 inches. The dimensions of the chamber  1204  will be essentially dictated by the dimensions of the inlet and outlet, an may also vary from about 2 inches to about 10 feet, for example, the chamber  1204  shown in FIGS. 8-10 has a width of about 13 inches. 
     Device  1200  is typically made of any appropriate material that can withstand the hot (for example, 400° F. or hotter), pressurized (for example, 300 psig or greater), corrosive (either acidic or alkaline) slurries that are typically handled in a pulp and paper mill, including metals and high-performance plastics. However, the device is preferably made of metal, in particular steel, and is preferably made from weldable stainless steel, for example 304L (having an ASTM designation ASTM-A240-304L), or its equivalents, or better. 
     In use, the inlet  1201  is connected to the conduit  34 ,  252 ,  234 ,  86 ,  856 ,  1134 , and one outlet  1202  is connected to the same conduit while the other outlet  1203  is connected to a conduit leading to the same or another digester (batch or continuous). Where only two outlets  1202 ,  1203  are provided preferably about one-half the inlet flow goes to each, although the plates  1202 ,  1203  may be dimensioned or positioned, so that a higher volume flow goes through one outlet  1202 ,  1203  than the other. 
     In the broadest aspect of this invention, a system and method are provided for the multistage transport and treatment of comminuted cellulosic fibrous material with the economical recovery and re-use of energy, including thermal energy. 
     The invention also specifically includes all narrower ranges within a broad range. For example, at a temperature of between 60-90° C. means 61-89° C., 68-74° C., 65-82° C., and all other narrow ranges within that broad range. 
     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.