PRIOR APPLICATION 
     This application is a U.S. national phase application based on International Application No. PCT/SE2009/050288, filed 19 Mar. 2009 claiming priority from Swedish Patent Application No. 0800645-4, filed 20 Mar. 2008. 
     TECHNICAL FIELD 
     The present invention relates to a feed system for a continuous digester in which wood chips are cooked for the production of cellulose pulp. 
     BACKGROUND AND SUMMARY OF THE INVENTION 
     In older conventional feed systems for continuous digesters, high-pressure pocket feeders have been used as sluice feeders for pressurisation and transport of a chips slurry to the top of the digester. 
     The  Handbook of Pulp , (Herbert Sixta, 2006) discloses this type of feeding with high-pressure pocket feeders ( High Pressure Feeder ) on page 381. The big advantage with this type of feed is that the flow of ships does not need to pass through pumps, but is instead transferred hydraulically. At the same time it is possible to maintain a high pressure in the transfer circulation to and from the digester without losing pressure. The system has however demonstrated some disadvantages in that the high-pressure pocket feeder is subjected to wear and must be adjusted so that the leakage flow from the high-pressure circulation to the low-pressure circulation is minimized. Another disadvantage is that during transfer, the temperature must be kept low so that bangs related to steam implosions do not occur in the transfer. 
     As early as 1957, U.S. Pat. No. 2,803,540 disclosed a feed system for a continuous chip digester where the chips are pumped from an impregnation vessel to a digester in which the chips are cooked in a steam atmosphere. Here, a part of the cooking liquor is charged to the pump to obtain a pumpable consistency of 10%. However, this digester was designed for small scale production of 150-300 tons pulp per day (see col 7, r. 35). 
     Also, U.S. Pat. No. 2,876,098 from 1959 discloses a feed system for a continuous chip digester without a high-pressure pocket feeder. Here the chips are suspended in a mixer before they are pumped with a pump to the top of the digester. The pump arrangement is provided under the digester and here the pump shaft is also fitted with a turbine in which pressurised black liquor is depressurised to reduce the required pump energy. 
     U.S. Pat. No. 3,303,088 from 1967 also discloses a feed system for a continuous chip digester without a high-pressure pocket feeder, where the wood chips are first steamed in a steaming vessel, followed by suspension of the chips in a vessel, whereafter the chips suspension is pumped to the top of the digester. 
     U.S. Pat. No. 3,586,600 from 1971 discloses another feed system for a continuous digester mainly designed for finer wood material. Here, a high-pressure pocket feeder not used either, and the wood material is fed with a pump  26  via an upstream impregnation vessel to the top of the digester. 
     Similar pumping of finer wood material to the top of a continuous digester is also disclosed in EP157279. 
     Typical for these embodiments of digester systems from the late 50&#39;s to the beginning of the 70&#39;s is that these were designed for small digester houses with a limited capacity of about 100-300 tons pulp per day. 
     U.S. Pat. No. 5,744,004 shows a variation of feeding wood chips into a digester where the chips mixture is fed into the digester via a series of pumps. Here, so called DISCFLO™ pumps are used. A disadvantage with this system is that this type of pump typically has a very low pump efficiency. 
     The previously mentioned  Handbook of Pulp  also discloses on page 382 an alternative pump feed of chips mixtures called TurboFeed™. Here three pumps are used in series to feed the chips mixture to the digester. This type of feed has been patented in U.S. Pat. No. 5,753,075, U.S. Pat. No. 6,106,668, U.S. Pat. No. 6,325,890, U.S. Pat. No. 6,336,993 and U.S. Pat. No. 6,551,462; however in many cases, U.S. Pat. No. 3,303,088 for example, has not been taken into consideration. 
     U.S. Pat. No. 5,753,075 relates to pumping from a steaming vessel to a processing vessel. 
     U.S. Pat. No. 6,106,668 relates specifically to the addition of AQ/PS during pumping. 
     U.S. Pat. No. 6,325,890 relates to at least two pumps in series and the arrangement of these pumps at ground level. 
     U.S. Pat. No. 6,336,993 relates to a detail solution where chemicals are added to dissolve metals from the wood chips and then drawing off liquor after each pump to reduce the metal content of the pumped chips. 
     U.S. Pat. No. 6,551,462 essentially relates to the same system already disclosed in U.S. Pat. No. 3,303,088. 
     A big disadvantage with the systems with multiple pumps in series is limited accessibility. If one pump breaks down, the whole digester system stops. With 3 pumps in series and a normal accessibility for each pump of 0.95, the total systems accessibility is just 0.86 (0.95*0.95*0.95=0.86). 
     Today&#39;s modern continuous digesters with capacities over 4000 tons pulp per day use digesters that are 50-75 meters high, where a gauge pressure of 3-8 bar is established in the top of the digester in the case of a steam phase digester, or 5-20 bar in the case of a hydraulic digester. The continuous digester systems are designed to, during the main part of operation, typically well over 80-95% of operation, run at nominal production, which makes it necessary, in regard to operational costs, for the pumps to be optimized for nominal production. 
     A typical digester system with a capacity of about 3000 tons with a feed system with the so called “TurboFeed™” technology requires about 800 kW of pumping power. It is obvious that these systems must have pumps that run at an optimized efficiency close to their nominal capacity. Such a feed system requires 19,200 kWh (800*24) per 24 hours, and at a price of 50 Euro per MWh, the operational cost comes to 960 Euro per 24 hours or 336,000 Euro per year. 
     The systems must also be able to be operable within 50-110% of nominal production which places great demands on the feed system. 
     This means that a system supplier must offer pumps that are large enough to handle 4000 tons and that may also be operated within a 2000-4400 ton interval. Such a pump operated at 50% of its capacity is far from optimised, but it is necessary to at least temporarily be able to operate the pump at limited capacity in case of temporary capacity problems, for example further down the fibre line. 
     If this system supplier offers digester systems that can handle nominal capacities of 500-5000 tons, then pumps must be designed in a number of different pump sizes so that each individual installation can offer, from a power consumption and energy perspective, optimised transfer at nominal production. This makes the pumps very expensive, as normally a very limited series of pumps are manufactured in each size. To be able to meet demands of reasonably short delivery times, the system supplier must stock pumps in all pump sizes, which is very expensive. 
     The digester feed should also be able to guarantee optimal feeding to the top of the digester even if the flow in the transfer line is reduced to 50% of nominal flow. 
     This is difficult, because the flow rate in the transfer lines should be maintained above a critical level, as well-steamed chips have a tendency to sink against the direction of the transfer flow if the speed becomes too low. 
     A corrective measure that can be used at low rates, is to increase the dilution before pumping so that a lower chips concentration is established. This is however not energy efficient as it forces the feed systems to pump unnecessarily high volumes of fluid, which increases the pump energy consumption per produced unit of pulp. 
     Each pump has a construction point (Best Efficiency Point/“BEP”) at which the pump is intended to work. At this “BEP”, shock induced loss and frictional loss are, in the case of centrifugal pumps, at their lowest which in turn leads to that the pumps efficiency is highest at this point. 
     A first aim of the present invention is to provide an improved feed system for wood chips wherein optimal transfer can be achieved within a broader interval around the digesters design capacity. 
     Other aims of the present invention are;
         improved efficiency of the feed system;   improved accessibility;   lower operational costs per pumped unit of chips;   constant chip concentration during pumping regardless of production level;   a limited range of pump sizes that can cover a broad span of the digesters production capacity;   simplified maintenance;   lower installation costs compared to feed systems with high-pressure pocket feeders or multiple pumps in series;       

     The above mentioned aims may be achieved with a feed system according to the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a first system solution for feed systems for digesters with a top separator; 
         FIG. 2  shows a second system solution for feed systems for digesters without a top separator; 
         FIGS. 3-6  show different ways of attaching pumps to an outlet in a pre-treatment vessel; 
         FIG. 7  shows the feed system&#39;s connection to the top of a digester without a top separator; and 
         FIG. 8  shows a top view of  FIG. 7 ; 
         FIG. 9  shows a third system solution for feed systems for digesters without a top separator; 
         FIG. 10  shows a fourth system solution for feed systems for digesters with a top separator, and 
         FIG. 11  shows how the transfer lines from each pump in the systems in  FIGS. 9 and 10  may be combined to form one single transfer line. 
         FIG. 12  shows a second alternative of how the transfer lines from each pump may be combined to form one single transfer line, and 
         FIG. 13  shows a third alternative of how the transfer lines from each pump may be combined to form one single transfer line. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following detailed description, the phrase “feed system for a continuous digester” will be used. “Feed system” herein means a system that feeds wood chips from a low-pressure chips processing system, typically with a gauge pressure under 2 bar and normally atmospheric, to a digester where the chips are under high pressure, typically between 3-8 bar in the case of a steam phase digester or 5-20 bar in the case of a hydraulic digester. 
     The term “continuous digester” herein means either a steam phase digester or a hydraulic digester even though the preferred embodiments are exemplified with steam phase digesters. 
     A basic concept is that a feed system comprises at least 2 pumps in parallel, but preferably even 3, 4 or 5 pumps in parallel. It has been shown that a single pump can feed a chips suspension to a pressurised digester, and it is therefore possible to exclude conventional high-pressure pocket feeders or complicated feed systems with 2-4 pumps in series. 
     The pumps are arranged in a conventional way on the foundation at ground level to facilitate service. 
     With the above outlined solution it is possible to provide feed systems for digester production capacities from 750 to 6000 tons pulp per day, with only a few pump sizes. This is very important, as these pumps for feeding wood chips at relatively high concentration are very specific in regard to their applications, and pumps that are able to handle production capacities of 4000-6000 tons pulp per day are very large and only manufactured in very limited series of a few pumps per year. The cost for these pumps therefore becomes a crucial factor for a digester system. 
     The table below shows an example of how it is possible to cover a production interval of 750-6000 tons with only two pump sizes optimised for 750 and 1500 tons pulp, respectively, per day; 
     
       
         
               
             
               
               
               
             
           
               
                   
               
               
                 PUMP PROGRAM 
               
             
          
           
               
                 Nominal Production 
                 750 
                 1500 
               
               
                 Capacity (ton per day) 
                 pump 
                 pump 
               
               
                   
               
               
                  750 
                 1 unit 
                   
               
               
                 1500 
                     2 units 
               
               
                 2250 
                 1 unit 
                 1 unit     
               
               
                 (2250 alt) 
                 (3 units*) 
                 — 
               
               
                 3000 
                 — 
                 2 units 
               
               
                 (3000 alt) 
                 (4 units*) 
               
               
                 3750 
                 1 unit 
                 2 units 
               
               
                 4500 
                 — 
                 3 units 
               
               
                 (4500 alt) 
                 (2 units*) 
                 (2 units*) 
               
               
                 5250 
                 1 unit 
                 3 units 
               
               
                 6000 
                   
                 4 units 
               
               
                   
               
               
                 (X unit* = 1: st alternative) 
               
             
          
         
       
     
     This table clearly shows how it is possible, with the concept according to the present invention, to cover production capacities between 1500-6000 tons with only 2 optimised pump sizes while using a single pump installation in smaller digester systems with a capacity of 750 tons. Continuous digesters with a capacity of 750 tons are seldom used for new installations today, because batch digester systems are often more competitive for these capacities. A certain after market may exist for older digester systems with a low capacity where expensive feed systems with high-pressure pocket feeders are still used. 
     First Embodiment 
       FIG. 1  shows an embodiment of the feed system with at least 2 pumps in parallel. The chips are fed with a conveyor belt  1  to a chips buffer  2  arranged on top of an atmospheric treatment vessel  3 . In this vessel, a lowest liquid level, LIQ LEV , is established by adding an alkali impregnation liquid, preferably cooking liquor (black liquor) that has been drawn off in a strainer screen SC 2  in a subsequent digester  6 , and possibly adding white liquor and/or another alkali filtrate. 
     The chips are fed with normal control of the chip level CH LEV  which is established above the liquid level LIQ LEV . 
     The remaining alkali content in the black liquor is typically between 8-20 g/l. The amount of black liquor and other alkali liquids that are added to the treatment vessel  3  is regulated with a level transmitter  20  that controls at least one of the flow valves in lines  40 / 41 . With this alkali impregnation liquor the wood acidity in the chips may be neutralised and impregnated with sulphide rich (HS − ) fluid. Spent impregnation liquor, with a remaining alkali content of about 2-5 g/l, preferably 5-8 g/l, is drawn off from the treatment vessel  3  via the withdrawal strainer SC 3  and sent to recovery REC. If necessary, white liquor WL may also be added to the vessel  3 , for example as shown in the figure, to line  41 . The actual remaining alkali content depends on the type of wood used, hardwood or softwood, and which alkali profile that is to be established in the digester. 
     In the case where a raw wood material that is easy to impregnate and neutralise is used, for example raw wood material such as pin chips or wood chips with very thin dimensions and a quick impregnation time, vessel  3  may in extreme cases be a simple spout with a diameter essentially corresponding to the bucket formed outlet  10  in the bottom of the vessel. Required retention time in the vessel is determined by the time it takes for the wood to become so well impregnated that it sinks in a free cooking liquor. 
     After the chips have been processed in vessel  3  they are fed out from the bottom of the vessel where also a conventional bottom scraper  4  is arranged, driven by a motor M 1 . 
     According to the invention, the chips are fed to the digester via at least 2 pumps  12   a ,  12   b  in parallel, and these pumps are connected to a bucket formed outlet  10  in the bottom of the vessel. The bucket formed outlet  10  has an upper inlet, a cylindrical mantle surface, and a bottom. The pumps are connected to the cylindrical mantle surface. 
     To facilitate pumping of the chips mixture, the chips are suspended in a vessel  3  to create a chips suspension, in which vessel is arranged a fluid supply via lines  40 / 41 , controlled by a level transmitter  20  which establishes a liquid level LIQ LEV  in the vessel, and above the pump level by at least 10 meters, and preferably at least 15 meters and even more preferably at least 20 meters. Hereby a high static pressure is established in the inlet to pumps  12   a  and  12   b  so that one single pump can pressurise and transfer the chips suspension to the top of the digester without cavitation of the pump. The top of the digester is typically arranged at least 50 meters above the level of the pump, usually 60-75 meters above the level of the pump while a pressure of 5-10 bar is established in the top of the digester. 
     To further facilitate the feeding to the pumps, a stirrer  11  is arranged in the bucket formed outlet. The stirrer  11  is preferably arranged on the same shaft as the bottom scraper and driven by the motor M 1 . The stirrer has at least 2 scraping arms that sweep over the pump outlets arranged in the bucket formed outlet&#39;s mantle surface. Preferably a dilution is arranged in the bucket formed outlet, which may be accomplished by dilution outlets (not shown) connected to the upper edge of the mantle surface. 
       FIGS. 3-6  show how a number of pumps  12   a - 12   d  may be connected to the outlet&#39;s cylindrical mantle surface and how the stirrer  11  may be fitted with up to 4 scraping arms. The pumps may preferably be arranged symmetrically around the outlets cylindrical mantle surface with a distribution in the horizontal plane of 90° between each outlet if there are 4 pump connections (120° if there are 3 pump connections and 180° if there are 2 pump connections). This way it is possible to avoid an uneven distribution of the load on the bottom of the vessel and its foundation. In practice, shut-off valves (not shown) are also arranged between the outlet&#39;s  10  mantle surface and the pump inlet and a valve directly after the pump to make it possible to shut off the flow through one pump if this pump is to be replaced during continued operation of the remaining pumps. 
     In  FIG. 1  the chips are fed by pumps  12   a ,  12   b  via transfer lines  13   a ,  13   b  (only two shown in  FIG. 1 ) to the top of the digester  6 .  FIG. 1  shows a conventional top separator  51  arranged in the top of the digester. The transfer lines  13   a ,  13   b , preferably 2, both open into the bottom of the top separator, where, driven by motor M 3 , a feeding screw  52  drives the chips slurry up under a dewatering process against the top separators withdrawal strainer SC 1 . Drained chips will then be fed out from the upper outlet of the separator in a conventional way and fall down into the digester. In the case a hydraulic digester is used, the top separator is turned up-side down, and feeds the chips down into the digester. 
     The drained liquid from the top separator  51  is led through a line  40  back to the processing vessel  3 , and may preferably be added to the bottom of the processing vessel, to there facilitate feeding out under dilution. 
     Alternatively, line  40  may be connected to the position for the outlet of line  41  in the processing vessel  3  and line  41  may be connected to the position for the outlet of line  40  in the processing vessel  3 , according to the concept CrossCirc™. In a variation, the flow of line  40  and  41  may be mixed at the intersection of lines  40  and  41  in  FIG. 1 . 
     The digester  6  may be fitted with a number of digester circulations and the addition of white liquor to the top of the digester or to the digester&#39;s supply flows (not shown). The figure shows a withdrawal of cooking liquor via strainer SC 2 . The cooking liquor drawn off from strainer SC 2  is known as black liquor and may have a somewhat higher content of remaining alkali than black liquor that is normally sent directly to recovery and normally drawn off further down in the digester. The cooked chips P are then fed out from the bottom of the digester with the help of a conventional bottom scraper  7  and the cooking pressure. 
     Second Embodiment 
       FIG. 2  shows an alternative embodiment which does not include a top separator. Instead the transfer lines  13   a ,  13   b  (only two are shown in  FIG. 1 ) open directly into the top of the digester. Excess liquid is then drawn off with a digester strainer SC 1  arranged in the digester wall.  FIGS. 7 and 8  show this in more detail. The remaining parts of this embodiment correspond to the digester system shown in  FIG. 1 . 
       FIG. 8  shows how  4  transfer lines  13   a ,  13   b ,  13   c  and  13   d  may open directly into the top of the digester. These outlets may preferably be arranged symmetrically in the top of the digester with a distribution in the horizontal plane of 90° between each outlet if there are 4 outlets (120° if there are 3 outlets and 180° if there are 2 outlets). The outlets are suitably arranged at a distance of 60-80% of the digester radius.  FIG. 7  shows how the transfer lines  13   a ,  13   b  and  13   c  open directly down into the top of the digester and thereby distribute the chips over the cross section of the digester. In this case a steam phase digester is shown where steam ST and/or pressurised air P AIR  is added to the top of the digester, in which a chips level CH LEV  is established above the liquid level LIQ LEV  in the top of the digester. Excess liquid is drawn off with a strainer SC 2  and collected in a withdrawal space  51  before being led back via line  41 . An advantage with the second embodiment, but also with the first embodiment, is that each pump may closed independently while the remaining pumps may continue pumping at optimal efficiency and without requiring modification of the feed system itself. 
     Third Embodiment 
       FIG. 9  shows an alternative embodiment for the feed system to a continuous digester without a top separator where each pump  12   a ,  12   b  pumps the chips suspension through a first section  13   a ,  13   b  of a transfer line to the top of the digester, and the first sections of the transfer lines from at least 2 pumps are combined at a merging point  16  to form a combined second section  13   ab  of the transfer line before this second section is led towards the top of the digester. To maintain a constant flow rate, a supply line  15  is also connected to the merging point  16 . In this embodiment black liquor is taken from line  41  and may be pressurised with a pump  14 . However, because the black liquor has already reached a full digester pressure, the need to pressurise the liquor is limited. All other characterizing parts of the system correspond to the system shown in  FIG. 2 . 
     Fourth Embodiment 
       FIG. 10  shows an alternative embodiment for the feed system to a continuous digester with a top separator where each pump  12   a ,  12   b  pumps the chips suspension through a first section  13   a ,  13   b  of a transfer line to the top of the digester, and the first sections of the transfer lines from at least 2 pumps are combined at a merging point  16  to form a combined second section  13   ab  of the transfer line before this second section is led towards the top of the digester. To maintain a constant flow rate, a supply line  15  is also connected to the merging point  16 . In this embodiment black liquor is taken from line  40  and may be pressurised with a pump  14 . However, because the black liquor has already reached a full digester pressure, the need to pressurise the liquor is limited. All other characterizing parts of the system correspond to the system shown in  FIG. 1 . 
       FIG. 11  shows an example of how supply lines  15   a ,  15   b  that are used in both the third and the fourth embodiment may be connected to merging points  16 ′ in the case  4  pumps  12   a - 12   d  are used. An advantage with this supply arrangement is that it is possible to guarantee optimal speed in the combined flow in the second section  13   ac / 13   bd  and in the combined flow in the final third section  13   abcd  of the transfer line. 
     It is critical that the rate of the flow up to the digester is well over 1.5-2 m/s so that the chips in the flow do not sink down towards the feed flow and cause plugging of the transfer line. The flow in the transfer line should suitably be maintained between 4-7 m/s to make sure that the chips are transferred to the top of the digester. 
     If, for example, pump  12   a  would be shut down due to repair or a desired capacity reduction, the flow in addition line  15   a  may be increased so that the flow rate in the second section  13   ac  is maintained. 
     In these combined line systems for transferring chips suspensions it is advantageous that the lines after the merging points  16 ,  16 ′,  16 ″ have a flow cross section that is equal to or greater than the sum of the incoming lines, to avoid pressure loss in the transfer lines. Suitable equations for flow areas A may be:
 
 A   13bd ≧( A   13d   +A   13b ),  and
 
 A   13abcd ≧( A   13bd   +A   13ac ).
 
     In a transfer line where the first section has a diameter of for example 100 mm and an established flow rate of 5 m/s, a flow rate of 4.4 m/s is established if a second section that combines 2 lines with diameter 100 mm has a diameter of 150 mm. With a subsequent combination of 2 such lines with a diameter of 150 mm to a third section with a diameter of 250 mm, a flow rate of 3.18 m/s may be established. All these flow rates have a margin towards the critical lowest flow rate. 
     The supply lines  15   a ,  15   b  may also have connections directly after each pump outlet, so that the line between pump and merging point is kept flushed during the time that the pump is shut down or operated at a reduced capacity. The addition of extra fluid may also be combined with a further dilution of the chips suspension before the pumps, for example on the suction side of the pumps or in the bottom of vessel  3 . 
       FIG. 12  shows a cross-sectional view of a second embodiment of how lines  13   a - 13   d  from the pumps may be combined to form one single transfer line  13   abcd . Here, the supply line  15  for dilution liquid provides a vertical part of the transfer line towards the top of the digester, and each line  13   a ,  13   b ,  13   c ,  13   d  from each pump is connected successively, one by one, to this vertical part of the transfer line at different heights. At each supply position, the chip flow is added in a conical part of a diameter increase in the transfer line. As is indicated by the dashed alternatives  13   b   ALT / 13   d   ALT , the connections from the pumps may instead be shifted from side to side on the transfer line. 
       FIG. 13  shows a cross-sectional view of a third embodiment of how lines  13   a - 13   d  from the pumps may be combined to form one single transfer line  13   abcd . Here, the supply line  15  for dilution liquid provides a vertical part of the transfer line towards the top of the digester, and each line  13   a ,  13   b ,  13   c ,  13   d  from each pump is connected at the same height to this vertical part of the transfer line. Preferably the supply position for the chip flow is arranged in a conical part of a diameter increase in the transfer line and each connected line is oriented upwards and inclined at an angle in relation to the vertical orientation in the interval 20-70 degrees. The Figure shows only the connections  13   a ,  13   b ,  13   c , as connection  13   d  is in the part that is cut away in this view. 
     The invention is not limited to the above mentioned embodiments. More variations are possible within the scope of the following claims. In the embodiments shown in  FIGS. 2 and 9 , in some applications the strainer SC 1  and the return line  40  may for example be omitted, preferable for cooking of wood material with a higher bulk density, such as hardwood (HW), that for a corresponding production volume require less liquid during transfer. 
     In the case where a raw wood material that is easy to impregnate and neutralise is used, for example raw wood material such as pin chips or wood chips with very thin dimensions and a quick impregnation time, vessel  3  may in extreme cases be a simple spout with a diameter essentially corresponding to the bucket formed outlet  10  in the bottom of the vessel. 
     If the chips fed into the vessel  3  are already well steamed, the liquid level LIQ LEV  may be established above a chips level CH LEV . 
     In the embodiments shown, an alkali pre-treatment was used in vessel  3 , but it is also possible to use a process where this pre-treatment comprises acid pre-hydrolysis. 
     While the present invention has been described in accordance with preferred compositions and embodiments, it is to be understood that certain substitutions and alterations may be made thereto without departing from the spirit and scope of the following claims.
The feed system is for a continuous digester where at least two pumps are arranged in parallel at the bottom of a pre-treatment vessel and a stirrer is provided in direct connection to inlets to pumps. The system makes it possible to provide a feed system with an improved accessibility and operational reliability, and to operate the main part of the pumps at optimal efficiency even if the production capacity is reduced.