Patent Publication Number: US-2004040684-A1

Title: Arrangement for feeding pulp to a bleaching tower

Description:
[0001] The present invention relates to an arrangement in connection with bleaching pulp having a consistency of 7-20%, preferably 8-16% at a pulp mill, the arrangement comprising: a bleaching tower having at least one inlet opening and at least one outlet opening for pulp, a transfer line comprising a feed line operatively connected to the inlet of the tower and a discharge line operatively connected to the outlet of the tower, and at least one pump arranged in the feed line.  
       [0002] Bleaching of pulp produced from wood fibers is performed in bleaching towers, where the pulp is bleached with chemicals, such as oxygen, peroxide, hydrogen peroxide, peracids and chlorine dioxide. The bleaching can be effected at any consistency, but it is usually done in the consistency range of 8-20%. The present invention relates to the bleaching of wood fibers in vertical pressurized or unpressurized bleaching towers. The invention relates in particular to the bleaching of pulp produced by chemical pulping methods such as kraft or soda cooking of wood chips.  
       [0003] The conventional system for charging and discharging bleaching towers is shown in FIG. 1, which will be explained in detail in order to clarify the conventional technology. Pulp to be bleached is usually supplied from a filter or press  1  at a consistency of 8-16%. From the filter or press the pulp falls into a pipe type chest called standpipe  2  that has a diameter of 0.5 to 1.0 meter. At the bottom of the standpipe there is a so-called MC pump  3  designed for pumping pulp at medium consistency, i.e. at a consistency range of 8-16%. The pump is equipped with a fluidizing and gas separation means  4 . The MC-pump is rather expensive due to its complicated construction.  
       [0004] From the pump the pulp flows to a fluidizing chemical mixer  6 , where bleaching chemicals  5  are mixed with the pulp. From the mixer the pulp is supplied to a distributor  7 , which divides the pulp flow into three or four separate flows that are directed to a bleaching tower  8 . The mixer  6  is of fluidizing type, such as a mixer described in U.S. Pat. No. 5,813,758. The distributor is also of fluidizing type and a typical distributor is described in U.S. Pat. No. 4,964,950. The fluidization requires a relatively great amount of energy and the only function thereof is to create turbulence.  
       [0005] In the bleaching tower  8 , the bleaching chemicals react with the pulp and the pulp becomes brighter. After the bleaching, the pulp is discharged from the tower by a fluidizing discharger  9 . A typical discharge apparatus is described in EP patent 594740.  
       [0006] In FIG. 1 the pulp is fed from the degassing pump  3  to the mixer  6 , where chemicals and/or steam  5  may be added to the pulp. Chemicals, such as chlorine dioxide and steam are to be added after the pump, as the gas removal system of the pump would otherwise remove chemicals and gases from the pulp. Thus it is necessary to have a separate mixer, although the pump as such would be a good mixer. Another problem is that the chemicals and steam have to be added against the high pressure generated by the pump.  
       [0007] In the system of FIG. 1 there is only one pump feeding the pulp through the system consisting of the mixer  6 , the distributor  7 , the tower  8  and the discharger  9 . Each element causes pressure losses, although there are rotating devices consuming electricity when the pulp is being fluidized. In the system of FIG. 1 the pump often has to raise the pressure to more than 10 bar, even more than 12 bar.  
       [0008] An object of the invention is to solve the above-mentioned problems and to provide a more economical and less energy-consuming system for filling and discharging bleaching towers.  
       [0009] The new system is based on the use of a turbine or propeller type device in a feed line and/or discharge line of a bleaching tower. A characteristic feature of the invention is that the arrangement further comprises at least two propeller or turbine type devices for increasing the pressure of the pulp flowing in the transfer line in order to decrease the head of the pump. This arrangement decreases the head requirement of the at least one pump.  
       [0010] The expression “propeller or turbine type apparatus or device” is in this connection used to refer to an arrangement of wings or blades mounted on a shaft in a flow channel for pulp. The propeller or turbine type device is arranged inside the flow channel in such a way that the wings or blades extend and rotate substantially close to the inner surface of the flow channel in order to create a substantial increase in the pressure of the flow through the device. A single wing or blade alone does not extend to the entire circumference (360 degrees) around the shaft. Preferably, there are more than one wing or blade. Only the positioning of several blades or wings on the shaft with open areas between them creates a situation where the construction is open enough to prevent plugging etc. A real saving in energy can only be reached when the head requirement of the pump or pumps in a flow channel is reduced according to the invention.  
       [0011] According to a preferred embodiment of the invention at least two, preferably three, turbine or propeller type devices operating at a pulp consistency of 7-20%, preferably 8-16%, are used in feeding the pulp flow through the bleaching tower.  
       [0012] According to another preferred embodiment of the invention the turbine or propeller type devices together raise the pressure by at least one bar, preferably by more than two bar, and most preferably by more than three bar.  
       [0013] According to another preferred embodiment of the invention at least one propeller or turbine type device is arranged in a chamber located in the feed line, said chamber having at least one inlet opening and a plurality of outlet openings for the pulp flowing through the feed line, each of said outlet openings being connected to one of the inlet openings of the tower by means of a pipe.  
       [0014] According to another preferred embodiment of the invention the feed line comprises a standpipe having an outlet and a first pump located downstream of the standpipe, and a first propeller or turbine type device is arranged at the outlet of the standpipe and upstream of the pump and at least one second turbine or propeller type device is arranged downstream of the pump.  
       [0015] According to another preferred embodiment of the invention at least one propeller or turbine type device is arranged in a chamber having at least one inlet opening connected to the outlet opening of the bleaching tower and an outlet opening connected to the discharge line of the bleaching tower.  
       [0016] According to another preferred embodiment of the invention at least one propeller or turbine type device is arranged both at the inlet and at the outlet of the bleaching tower.  
       [0017] According to yet another preferred embodiment of the invention the first propeller or turbine type devices in the pulp flow direction is equipped with a gas removal system, the second device is not equipped with gas removal and the third device is equipped with a gas removal system.  
       [0018] According to the invention, preferably the feed channel of the bleaching tower is provided with a centrifugal pump, which preferably is devoid of a gas removal system. The pump is placed after the first turbine or propeller type device and raises the pressure of the pulp by at least five bar. This pump is necessary in installations having large towers and operating at high pressures. There are, however, bleaching applications, where the head requirement of the system is not high, such as DO towers with short reaction times or smaller prereactors. In such cases where the head requirement is low, the centrifugal pump may be omitted. In these cases a separate mixer is usually needed, but a distributor dividing the pulp flow to separate flows before the bleaching tower can be used as a mixer. If the retention time of a D stage or other bleaching stage is very short, the stage can be carried out in a flow channel after the preceding process stage. In that case, two turbine or propeller type devices may be arranged in series. Chlorine dioxide or other bleaching chemical is mixed with the pulp in the latter device and the pulp flows to the next stage, such as a washer.  
       [0019] According to a preferred embodiment of the invention the arrangement comprises a propeller or turbine type device located in a flow channel for increasing the pressure of the pulp flowing in said flow channel, and the device is connected to a washer or press, the flow channel between the device and the washer or press serving as a bleaching reactor, and means for introducing chemicals to the pulp downstream of the device for mixing chemicals into the pulp. 
     
    
    
     [0020] The present invention will be described in more detail, by way of example, with reference to the accompanying figures, of which  
     [0021]FIG. 1 illustrates a prior art system for feeding pulp to a bleaching tower and discharging the tower;  
     [0022]FIG. 2 illustrates a preferred embodiment for filling and discharging a bleaching tower according to the invention;  
     [0023]FIG. 3 illustrates a connection between a standpipe and a feed channel of a bleaching tower according to the invention;  
     [0024]FIG. 4 illustrates an outlet and a discharge channel of a bleaching tower according to the invention;  
     [0025]FIG. 5 illustrates a preferred arrangement of the outlets of the devices in FIG. 4 and FIG. 8;  
     [0026]FIG. 6 illustrates a preferred embodiment of the arrangement at the outlet of the bleaching tower of FIG. 2;  
     [0027]FIG. 7 illustrates a preferred system for distributing the pulp into the bleaching tower in FIG. 2;  
     [0028]FIG. 8 illustrates a device used in connection with the invention; and  
     [0029]FIG. 9 illustrates a device used in connection with the invention. 
    
    
     [0030] In FIG. 2, pulp to be bleached is usually supplied from a filter or press  10  at a consistency of 7-20%, preferably 8-16%. From the filter or press, the pulp falls into a standpipe and therefrom flows to a bleaching tower  16 . A feed channel or line  22  of the bleaching tower is provided with a pump  12 , which preferably is a conventional centrifugal pump with an open impeller. The pump is devoid of a gas removal system. From the pump the pulp is supplied to a distributor, which divides the pulp flow into at least two separate flows going to the bleaching tower  16 . Th distributor is also equipped with a propeller or turbine type device  14 .  
     [0031] According to the invention the feed channel  22  is provided with a propeller or turbine type device  11 . In this new system gas  17  is removed from the pulp in the first turbine or propeller type device  11 . As the pump  12  is not gas-removing, it can be used to raise pressure and mix chemicals and/or steam  18 . Typical chemicals to be added are chlorine dioxide, peroxide, peracids and sodium hydroxide. This way the need for one expensive mixer is eliminated. A mixer  13  may, however, be used after the pump  12  for mixing large quantities of chemicals or gases  19 , such as oxygen or ozone, which could cause problems in the pump  12 . Thus one characteristic feature of the system of the present invention is that chemicals and/or steam  18  are added after the first turbine or propeller type device  11  but before the centrifugal type pump  12 . Preferably, the first turbine or propeller type device  11  raises the pressure by less than two bar in order to keep the feeding pressure for the chemicals and/or steam low.  
     [0032] In the bleaching tower  16  the bleaching chemicals react with the pulp and the pulp becomes brighter. After the bleaching the pulp is discharged from the tower through a discharge channel or line  21  provided with a propeller or turbine type device  15  in the same way as the feed channel is provided with the device  11 . Also here the gases  20  are preferably removed.  
     [0033] Prior art has described systems with two centrifugal pumps in series in the feed channel of a bleaching tower. The idea is to have a low pressure after the first pump and add steam between the two pumps. These systems are, however, expensive solutions as a centrifugal pump is much more expensive and energy-consuming than the relatively simple and low-cost turbine or propeller type devices.  
     [0034] When the mixer  13  is needed in the system, it is preferably a propeller or turbine type device like apparatuses  11 ,  14  and  15 . The arrangement for filling and discharging a bleaching tower may thus comprise a fourth turbine or propeller type device  13  placed between the first and the second devices. The invention may utilize even more than four turbine or propeller type devices. Such a situation may occur for example when a bleaching stage is effected in two or more successive towers.  
     [0035] In the system of FIG. 1 there is only one pump feeding the pulp through the system consisting of the mixer  6 , the distributor  7 , the tower  8  and the discharger  9 . Each element causes pressure losses, although there are rotating devices consuming electricity when the pulp is being fluidized. In the system of FIG. 1 the pump often has to raise the pressure to more than 10 bar, even to more than 12 bar.  
     [0036] In the new system of FIG. 2 each of the devices  11 ,  14  and  15  raises the pressure. Thus the pressure increase needed in the pump  12  is reduced so that the pressure increase is usually below 10 bar, even below 8 bar. The saving in energy in a normal size pulp mill, i.e. 1000-2000 ton/day, may thus be 100-200 kW per bleaching tower system. As there usually are 4-6 towers per pulp bleaching line at the mill, these savings in energy may be quite considerable.  
     [0037]FIG. 3 shows the arrangement for feeding the pulp from the washer or press  10  of FIG. 2 further in the system at a consistency of 7-20%, preferably 8-16%. There is a standpipe  31  having a diameter of 1-3 meter receiving the pulp from the washer or press. At the outlet of the standpipe there is a turbine or propeller type device  32  having a diameter of between 0.2 and 0.7 meters, preferably 0.3 to 0.4 meters. During rotation, the diameter of the device is such that it substantially covers the cross-sectional area of the space in which the device is located. The rotational speed is 1500 to 3000 rpm, preferably 1700-2500 rpm. The turbine or propeller type device  32  connects the standpipe to a flow channel  34  directing the pulp to a centrifugal pump  33 . The flow channel typically has a diameter of less than 50% of the diameter of the standpipe. The device  32  is provided with a system for removing air or gas  35  separated by the centrifugal forces in the propeller or turbine. There are also means  37 ,  38 ,  39  for feeding chemicals and/ or steam to the flow channel  34  between the turbine or propeller type device  32  and the centrifugal pump  33 .  
     [0038] The arrangement of FIG. 3 has many advantages over conventional systems. The system is less costly and uses less energy than conventional systems. Further, it allows the use of standpipes of larger diameter than in the conventional systems, thus facilitating the operation and control of the pulp mill.  
     [0039]FIG. 4 illustrates the device  14  of FIG. 2 for distributing the pulp into the partial flows directed to the bleaching tower  16  of FIG. 2. The system comprises the following elements.  
     [0040] a feed channel  41  leading the pulp to a turbine or propeller type device  42  having a rotating shaft  46 . The propeller or turbine  42  has a diameter of 0.2 to 0.5 meters, usually about 0.3 meters. The rotational speed is 1500 to 3000 rpm, preferably 1700-2500 rpm;  
     [0041] the turbine or propeller  42  is located in a chamber  40  of the flow channel  41 , and at the outlet or pressure side of the turbine or propeller the chamber has at least two outlets  48 ,  49 , preferably four outlets, so that these outlets are influenced by the turbulence or agitation caused by the turbine or propeller  42 . Through these outlets the pulp flows to the bleaching tower ( 16  in FIG. 2);  
     [0042] the outlets  48 ,  49  are preferably provided with regulating or closing valves  47 ,  45  that preferably are of ball valve type;  
     [0043] stationary elements  43  are preferably installed at the backside  44  of the propeller or turbine  42 . The stationary element has at least one of the following two functions: to cause turbulence that improves mixing and prevents gas from gathering around the rotating shaft  46  and/or to direct the pulp flow towards the outlets  47 ,  48 .  
     [0044] The device described in FIG. 4 has many advantages over conventional solutions. The pulp can be distributed into more than two partial flow channels leading to the bleaching tower in such a way that there is no actual pressure loss. The rotational movement is used to raise pressure while distributing the pulp between the outlets. The device will not separate gas and thus it is applicable also at the inlet of bleaching towers using gaseous chemicals like oxygen.  
     [0045]FIG. 7 shows another preferred system for distributing the pulp into the bleaching tower  16  of FIG. 2. It is a characteristic feature of th system that it comprises a turbine or propeller type rotor  84  followed by at least two  88 ,  89 , but preferably four, outlets leading the pulp via separate flow channels to the bleaching tower. The system comprises the following elements:  
     [0046] a feed channel  81  leading the pulp at a consistency of 7-20%, preferably 8-16%, to the turbine or propeller type device  84 . The feed channel has a vertical part  82  forming a chamber;  
     [0047] a shaft  83  rotating the turbine or propeller  84 , said shaft  83  extending at least partly in the flow channel  81 ,  82 ;  
     [0048] at least two, but preferably four, outlets  88 ,  89  located in a space  85  downstream of the device  84 . The outlets are arranged in the space  85  close to the turbine or propeller so that the outlets  88 , 89  of the chamber  82  are influenced by the agitation or turbulence created by the rotating turbine or propeller  84 . The outlets are preferably provided with regulating or closing valves  87 ,  86 .  
     [0049] The system of FIG. 7 has several advantages. As the shaft and the outlets are on different sides of the rotor it is easy to arrange the outlets symmetrically and create a situation where the flow is evenly distributed between the outlets. The space  85  behind the rotor can be formed optimally with respect to the outlets. It is advantageous to make the space  85  as small as possible. The space preferably has the geometry of a three- or four-sided truncated pyramid. The valves are fitted to the sides of the pyramid.  
     [0050] Another advantage is that the vertical part of the flow channel  82  below the device can be made as low as possible in order to reduce the height of the construction located below the bleaching tower. This will reduce the reduction in head as the pulp is transferred upwards into the tower.  
     [0051]FIG. 5 shows a typical symmetric arrangement of the outlets of the apparatuses of FIG. 4 and FIG. 7. There are four outlets  51 ,  52 ,  53 ,  54  arranged symmetrically around the upper part of the flow channel ( 41  in FIG. 4 and  82  in FIG. 7). In the middle there are a rotating shaft  55  ( 46  in FIG. 4 and  83  in FIG. 7) and stationary elements  56  creating turbulence or directing the flow. The elements  56  can be arranged at a few locations only as shown in FIG. 5, or they can cover the total circular area around the axis. Stationary elements for creating agitation or directing the pulp flow can be used in the systems according to FIGS. 4 and 7.  
     [0052]FIG. 6 shows a preferred embodiment of the device  15  at the outlet of the bleaching tower  16  of FIG. 2. The system comprises the following elements:  
     [0053] a discharge channel or line  67  connected to the bleaching tower  68 , in which channel or line the pulp flows at a consistency of 7-20%, preferably 8-16%. The discharge channel comprises a round pipe  64  attached to the roof  63  of the bleaching tower. The flow channel typically has a diameter of between 0.8 and 0.2 meters, preferably 0.2 to 0.5 meters.  
     [0054] the chamber of the discharge channel is provided with a turbine or propeller type device  61 . The turbine or propeller  61  is preferably equipped with a system  62  for removing gas that is separated from the pulp suspension due to centrifugal forces at the back of the blades of the propeller or turbine or around the shaft. The same type of gas removal system can be used in devices  11  and  15  of FIG. 2.  
     [0055] a pulp outlet  66  from the chamber located close to the backside (pressure side) of the turbine or propeller. The outlet is located so that it is exposed to the turbulence or agitation caused by the turbine or propeller.  
     [0056] at least one regulating valve  69  is preferably provided in the outlet  66  and close to the chamber  64 .  
     [0057] positioned prior to the turbine or propeller  61  and rotating therewith there can be a mixing bar  65  extending further in the bleaching vessel  68  than the actual turbine or propeller  61 . The function of the bar is to facilitate the flowing of the pulp into the channel  67 .  
     [0058] The apparatus of FIG. 6 has many advantages. For example, energy is not lost for unnecessary fluidization and mixing. While being pressurized, the pulp suspension is directed towards the outlet in a smooth way so as to keep the outlet open. The pulp suspension is directed towards the discharge channel by the rotating propeller or turbine, possibly supported by the rotating bar. The gas removal makes the bleaching tower more stable to operate. The gas to be removed is either formed during the bleaching reactions or it is part of the chemicals added to the pulp prior to the bleaching tower.  
     [0059]FIGS. 8 and 9 show devices equipped with a turbine or propeller capable of raising the pressure by more than 0.5 bar, preferably by more that 1 bar. These devices can be used in flow channels of the bleaching tower arrangement shown in FIG. 2, but also in many other applications where a good mixing result is needed. The devices can be placed in series arbitrarily after each other to form a series of two or more devices. The placing in series is easy as each device raises the pressure, and thus the pressure loss through many mixers will not be problematic.  
     [0060] The apparatuses of FIGS. 8 and 9 are characterized in that the pulp flows at a consistency of 1 to 20%, preferably 7-20%, most preferably 8-16%, in a flow channel  91 ,  101 , 102  to a turbine or propeller type device  93 ,  104  and chemicals, preferably bleaching chemicals, are added to the flow channel  91 ,  101 ,  102  or to the turbine or propeller type device  93 ,  104 . Stationary elements  94 ,  98 ,  105 ,  106  are preferably located after the turbine or propeller  93 ,  104  to create agitation or turbulence for increasing mixing efficiency. It is advantageous that the central part  98 ,  106  of the stationary element is fixed and closed and prevents the flow from passing through so that the element forces the flow to bypass it. Gas is thus prevented from gathering into the center of the flow channel due to centrifugal forces. After having passed the propeller or turbine the pulp flows in flow channels  96  and  97 .  
     [0061] According to a preferred embodiment of the invention the devices of FIGS. 8 and 9 are devoid of systems for gas separation from the turbine or propeller. The rotational speed of the turbine or propeller is 1000 to 3000 rpm, typically 1500 to 2500 rpm.  
     [0062] Typical bleaching chemicals to be mixed with the pulp in the mixers of FIGS. 8 and 9 are chlorine dioxide, peroxide, peracids, sodium hydroxide, sulfuric acid, oxygen and ozone.  
     [0063] The device of FIG. 8 is further characterized in that the shaft  95  rotating the turbine or propeller  93  extends at least partly in the flow channel  96 ,  97  downstream of the turbine or propeller  93 . A valve  92  is preferably located in the flow channel  91  close to the turbine or propeller  93 . As can be seen in FIG. 8, this has two additional advantages. The valve being influenced by the rotor will have a low loss of head. The shaft can be arranged so that the flow channels  91  and  97  are arranged at an angle with respect to each other. This eliminates an expensive pipe elbow.  
     [0064] The device of FIG. 9 is further characterized in that the shaft  103  rotating the turbine or propeller  104  is placed at least partly in the flow channel  101 ,  102  leading the pulp to the turbine or propeller  104 . A valve  107  is preferably located in the flow channel  108  close to the turbine or propeller  104 . As can be seen in FIG. 9 this has two further advantages. The valve being influenced by the rotor will have a low loss of head. The shaft can be arranged so that the flow channels  101  and  108  are at an angle with respect to each other. This eliminates not only an expensive pipe elbow but also the flow friction caused by the elbow. A suitable angle between the flow channels  101 ,  91  and  108 ,  97  is more than 30 degrees, preferably 80 to 100 degrees.  
     [0065] It is understood that the invention is not limited to the particular construction and arrangement of parts herein illustrated and described, but encompasses all various embodiments and modifications thereof within the scope and the spirit of the appended claims.