Abstract:
A central screen element being located substantially parallel with the vertical axis of a pulp digester is provided. In the vertical direction, the screen element comprises at least one zone including one screen segment for removing a liquid stream from the digester and at least one outlet for introducing a liquid stream to the digester. The zones may be adjacent or separated and liquid streams may be independently removed and introduced, as required by the properties and the stage of the cooking process. A screen element may be used in both continuous and batch digesters.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention concerns a screen arrangement of a digester, and more precisely, a screen element being located substantially parallel with the vertical axis of the digester, in the middle of the digester, and process chemical being arranged to flow through said screen simultaneously either into the digester or out of the digester, depending on the determinations of the process and the stage of the process. The field of use comprises especially feeding of process chemicals to the digester and removal of those from the digester when producing pulp or paper stock from wood chips in a batch or continuously operating digester. 
       PRIOR ART 
       [0002]    In the technique of prior art, process chemicals are moved with respect to the material to be processed, for example when digesting pulp from wood chips so, that mounted to the inner surface of the process vessel there is a screen with apertures of the kind that chemicals can flow through the screen surface, but the material to be processed is not able to pass through the screen surface. The chemical flow can in the technique of prior art be directed either inwards to or outwards from the vessel, and the material to be processed, for example wood chip, can be either stable or movable to some direction, depending on the need. 
         [0003]    At present, the pulp produced chemically must as well have a good strength as a low kappa number, in other words, have a low lignin content after the digestion. For achieving the both of these properties, the digestion conditions must be carefully optimized. Among these digestion conditions are the correct alkali distribution, suitable temperature profile, adequate amount of liquid, and the amount of unwanted gradients being as small as possible, especially f.ex. in the radial direction of a continuous digester. This requires big circulation and expansion flows. 
         [0004]    Even a partial clogging of for example expansion and circulation screens causes channelling and disturbed flows in the digester. For this reason it is not always possible to maintain such digestion conditions, that the pulp to be processed would be strong, pure and homogenous. This causes increased consumption of raw material, energy and chemicals, which, in turn, increases production costs and environmental load. 
         [0005]    As a result of the clogging, the screens can also be broken, when the support constructions fail. On average, this causes need for renovation of screens. 
         [0006]    Screens nowadays available on the market for the continuous digestion usually comprise a plurality of vertical bar screens in an arrangement resembling for example a chessboard and attached to the inner surface of the digester in places determined by the process specification. The screen bars of the bar screens have an angular form, for example as a metal bar worked in the form of T, or having a circular form. These kinds of screen constructions are disclosed in patents WO 94/19533 and WO 01/31117. Their location on the inner surface of the digester is very traditional. 
         [0007]    Correspondingly, one problem of the screens used for the digestion in prior art is the fact that they clog easily, when the chip particles cling to the slots. When the process chemical flow sucks the chips, sticks and pre-digested pulp against the screens, this and the radial component of the pressure resulted from the chip column cause a resultant force pushing the particles to the slots of the screens said particles clogging superpositioned from bottom upwards causing blocking, which in turn disturbs the plug flow and causes channelling of the flow. When the packing degree of the digester is normal, the wood chips cling to each other inside the plug flow and also on the edges of the plug flow. Small sticks and chip particles follow the quicker radial liquid flow and cling to the sharp edges of the screen bars. New sticks and chip particles cling more easily to these clung particles resulting in clogging of a wider screen area. The clogging increases continuously as the screen area decreases. As a result of the strong clogging, the flows slow down also on the backside of the screen (between the screen and the digester jacket), resulting in that heavier and heavier particles are carried along to the backside of the screen and block the flow area between the screen and the jacket. Due to the location of the screens and the way of blocking, they are difficult to clean, and it is tried to be avoided as far a possible, for example by changing the construction types of the screens to the same place on the inner surface of the digester, on the casing of the pressure vessel. 
         [0008]    Solutions for preventing the clogging have been developed recently for example by changing the direction of the screen slots, as described in the patents WO 95/16817, U.S. Pat. No. 6,039,841, U.S. Pat. No. 6,344,112 and U.S. Pat. No. 6,165,323. Patents FI 54509 and FI 105 931 disclose methods for cleaning the screens. 
         [0009]    Problems related to the correct alkali distribution, suitable temperature profile, adequate liquid amount and optimal flowing conditions still exist, and no solution has been suggested by locating the screens for example into the middle of the digester. It is, however, clear that by optimizing the conditions mentioned above, also clogging of the screens can be decreased. 
         [0010]    One of the biggest problems is to provide an even distribution of the process chemical to the whole chip column without disturbing the flows. In a continuous digester, wood chips are fed from the upper part of the digester and the processed pulp is removed from the lower part of the digester. Cooking chemicals are fed via a longitudinal centre pipe mounted in the middle of the digester to different areas of the digester, determined according to the process technique. However, the direction of flow is traditionally from the centre of the digester towards the jacket of the digester, where the screens are located. Typically the additional screen capacity is mounted to the jacket of the digester. 
         [0011]    U.S. Pat. No. 3,475,271 discloses a digester especially meant for cuttings. In this invention, there is located a rotating centre screen in the middle of the digester, near the bottom thereof. The screen can have a cylindrical or conical form so that the diameter of the cone grows in the plug flow direction of the chip column. The washing liquor is fed to the digester via screens on the walls of the pressure vessel and it flows towards said central screen, through which it is removed to recycling. This solution aims at providing horizontal displacement wash on the bottom of the digester. Problems are caused, however, by the location of the central screen with respect to the feeding point of the washing liquor. The speed of the flow directed away from the wall of the digester towards the central screen is increased due to the reduced flowing area. Correspondingly, the radial force vector towards the screen located in the middle of the digester increases rapidly, even with low flow rates of liquor. As a result of that, the central screen disclosed in U.S. Pat. No. 3,475,271 can be easily clogged. Its weakness is the limited possibility of its construction with respect to the multipurpose operation. In addition to the suction only backflushing is possible. In that respect the solution is traditional. 
         [0012]    In the conventional batch digestion method, the digester is filled with chips and digestion chemical. After that, the content of the digester is heated by circulating the chemical with a pump and by heating it in the circulation pipe with a heat exchanger. In this digestion method, the digester is equipped with only one screen (circulation screen) mounted in the lower part of the digester, to the inner surface of the jacket. The location of the screen is unprofitable, as for having quickly homogenous digestion. Thus, among others, digestion time is wasted. 
         [0013]    In a more advanced batch digestion method, the thermal content of the previous digestion batches is recovered for the following digestions. This is achieved by displacing the hot liquor in the digester with cooler and purer liquor. The hot liquor displaced from the digester is partly used as a digestion chemical of the following batches and partly for heating the new digestion chemical in the heat exchangers. For these kinds of digesters another screen is typically needed in addition to the above-mentioned recycling screen in order to perform the required displacements. Typically these so called displacement screens have been mounted to the upper part of the digester on the inner surface of the jacket. The displacement batch digestion method is prominently more energy-efficient than the conventional batch digestion method and provides the possibility to produce pulp with better quality. As a result of this, it has become more popular and there is a need to change the existing conventional batch digestion methods to this kind. 
         [0014]    Firstly, because the screens are located only to the inner surface of the vessel, it is difficult to have the chemical evenly distributed to the chip column in the whole vessel. As a result, the quality of the product received with this method is easily uneven. Secondly, as the process chemical flow is effected extending to the whole area of the screen, the flow can change uncontrollably, as the single chip pieces of the chip column to be processed are moving during the digestion for example due to settling. Thirdly, these kinds of screen constructions attached to the walls of a process vessel and typically to those of a pressurized process vessel are expensive. In order to increase the capacity of the digester, however, the increasing of the screen area is unavoidable. And fourthly, if increasing of a screen becomes necessary afterwards for improving the process, the installing of a screen of prior art type into an existing process vessel is difficult, expensive and slow. 
         [0015]    The object of the present invention is to eliminate disadvantages stated in connection with the both digestion methods by using a central screen element wherein the flow of the process chemical can be directed to a predetermined point either inwards and/or outwards, and which can be installed into a process vessel easily also afterwards. 
       OBJECTS AND A BRIEF DESCRIPTION OF THE INVENTION 
       [0016]    The problems discovered in the technique of prior art can be solved in accordance with the present invention by providing a tubular screen element having one or a plurality of zones in the longitudinal direction. Each zone comprises a screen segment, through which process chemical can be sucked away from the digester, and at least one outlet, through which chemical is supplied to digester. The zone can be also referred to as screen zone. In the transversal direction per each zone, the central screen element must have at least two channels in order to allow the flow of the process chemical in different directions also simultaneously. The screen element is installed inside the process vessel, substantially parallel with the vertical axis, in the middle. 
         [0017]    This kind of a screen element installed in the middle of the digester increases the screen area of the digester, thus improving the screen capacity. As the screen element has a plurality of zones together with multiple channels, the feed and removal of process chemicals can be optimized so, that the digestion is generally better. The optimizing of the feed and removal means firstly, that the chemical can be fed to the right points in the longitudinal direction of the digester, and also to the middle of the cooking chip column. This is to ensure that there is enough of the process chemical and that it is evenly distributed throughout the whole chip column. On the other hand, the feed and removal flows have influence on the movements of the chip column and on the flows of the whole digester, and in the changing digestion conditions the best possible flowing conditions for each digestion stage are provided by adjusting the places of the feeds and removals. This, for its part, improves the quality and homogeneity of the pulp. 
         [0018]    The figures described in the following are only intended for describing the preferred embodiments of the present invention, without limiting the scope of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0019]      FIG. 1  shows one embodiment of the structure of the central screen element in accordance with the present invention, 
           [0020]      FIG. 2  shows one embodiment of the location of the screen zones of the central screen element, in the longitudinal direction of the screen element. 
           [0021]      FIG. 3  illustrates the operation of the central screen element in the campaign operation of a continuous digester. 
           [0022]      FIG. 4  shows a displacement batch digester comprising a central screen element in accordance with the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0023]    The central screen element in accordance with the invention can be implemented with many different constructions. The following descriptions of the apparatus are meant to clarify the construction and operation of the screen element. They are in no way limiting the scope of the invention, which only becomes apparent from the enclosed claims. 
         [0024]    The screen element has a tubular form having the height bigger than the diameter. The tubular form in this connection means “part of a machine” in which there is arranged space for the flows of liquid. The digesters ready in use and to be produced in the future are the so called tailor made digesters, whereby also this central screen element in accordance with the invention must be designed for each digester separately, so that it corresponds optimally to the dimensioning and operating conditions of the apparatus in question. In the dimensioning of the central screen element the size of the digester, already existing screen constructions, the capacity of the digester, pipe units and connections as well as other eventual special features of the operation of the digester must be taken into consideration. 
         [0025]    The central screen element is divided in the longitudinal direction into one or a plurality of screen zones. The number of the screen zones, their size and location with respect to the whole central screen element is dependent on the digester, to which the screen element will be installed, and on the digestion method to be used, as well as on the technical determinations of the process. The screen zones can locate in the central screen element sequentially in the vicinity of each other or at a distance from each other, on the area of digestion zone and/or other area meant to be inside each digestion vessel. Also the elements of the screen zone, screen segment and at least one outlet can be located in the zone sequentially in the vicinity of each other or at a distance from each other. The outlet can also be located in the area of the screen segment, behind the screen bars in the flow space. Due to the screen zones, the process chemical can be fed to the digester and removed from the digester at optimal points and thus it is possible to support the operation of the existing screens located on the jacket of the digester. The central screen element provides more screen area for the digester, thus increasing the screen capacity of the digester and, additionally, it can be used for controlling the flows inside the digester and thus for improving the quality of the produced pulp. 
         [0026]    In the transversal cutting direction the central screen element is divided into two or more channels. The meaning of the channels is to provide for the flowing of the liquor containing process chemical in and out to/from the central screen element simultaneously without causing disturbances in the operation. For making the flows possible in different directions simultaneously, the flows in different directions must have their own channels. Thus, there must be at least two channels per each zone of the central screen element. It is possible that the amount of the outlets in the zone is more than one, for example two. This is to provide a correct alkali distribution in changing digestion conditions. Also the traditional backflushing of the screen segments is possible. 
         [0027]    The same basic idea of the central screen element can be utilized in both digestion methods, in other words as well in the batch digestion as in the continuous digestion. In both of these methods, the wood chips to be digested are continuously moving from the upper part of the digester towards the lower part. In the batch digestion this is resulted from the settling of the pulp to be processed and the thickening thereof towards the lower part of the digester. So, considering a single wood chip to be digested, it is continuously moving downwards. The speed of the movement of the chip during the digestion is rather slow and the length of path it goes is short, but the motion is continuous. In a continuous digestion the motion is significantly quicker, because in this process, new wood chips are fed from the upper part of the digester into the digester, and the produced pulp is removed from the lower part as a continuous operation during the digestion. 
         [0028]    Also in the continuous digestion the pulp to be processed is thickening towards the lower part of the digester, when the wood chips are digested. 
         [0029]    Although the motions have different speeds and they are caused at least mostly by a different factor, it plays a very significant role in both of the digestion methods. As a result of the motion, big requirements are set to the operation of the screens and the flows of the digester. They should be able to operate optimally during the whole digestion, even in changing digestion conditions. The solutions of prior art are not able to adapt sufficiently according to the changing conditions. With the solution in accordance with the present invention, these demanding, changing conditions can be taken into consideration as well with respect to the operation of the screens as with respect of the flows. 
         [0030]      FIG. 1  shows as an example a vertical cross sectional drawing of one central screen element of the invention and a section X-X in the transverse direction of the same central screen element. In the case of this example, the screen element is divided in the longitudinal direction into two screen zones (A) and (B), and in the transverse direction into five channels (flows  1 ,  2 ,  3 ,  4  and  5 ). As it can be seen in the drawing, in this construction the flow to the digester can take place in three points, outlets ( 1 . 1 ), ( 2 . 1 ) and ( 3 . 1 ) with flows ( 1 ), ( 2 ) and ( 3 ), respectively. The flow out from the digester can take place in two separate screen zones (A) and (B) in the screen element through the screen segments ( 5 . 1 ) and ( 4 . 1 ), with process chemical flows ( 5 ) and ( 4 ), respectively. For improving the uniformity of the digestion, for compensating the motions of the chip column and for decreasing the clogging, the flows in and out can be conducted through different channels independently from each other as required by the changing digestion conditions. The flow can be conducted to one or more zones at a time. The flow to the lower zone or away from it is directed via a channel through the upper zone, in the axial direction of the central screen element. In other words, the chemical can be fed to the digester for example simultaneously at all three points, outlets ( 1 . 1 ), ( 2 . 1 ) and ( 3 . 1 ), and removed through the both screen segments ( 4 . 1 ) and ( 5 . 1 ). If required by the digestion conditions, however, for example only one of the screen segments, ( 4 . 1 ) or ( 5 . 1 ) can be in use. All different combinations are available due to separate channels. In the batch digestion for example as a result of settling of the pulp, the supply of the chemical through the outlet ( 3 . 1 ) is stopped when the upper surface of the chip column passes the outlet ( 3 . 1 ) of the central screen element. On the other hand, at the final stage of the digestion, the supply of the chemical through the outlet ( 1 . 1 ) is increased by diluting in order to decrease the thickening of the pulp, for example in connection with blowing. 
         [0031]    The screen element in accordance with the invention can be installed either into a new digester or as a retrofitting into an existing digester. The screen element is installed into the digester substantially parallel with the vertical axis, in the middle. The screen element is installed into the digester substantially parallel with the vertical axis on the central axis of the digester, in order to have symmetrical and equal long paths for the radial flows in the digesting chip column. 
         [0032]    The central screen element is installed inside the digester arranging the supporting so that the support disturbs the digestion process as less as possible. 
         [0033]    The supporting can be implemented at any point of the digester, and if necessary, the supporting can also be made to several different points. Preferably the central screen element is attached to the digester at the upper part of the digester and if necessary, the lateral supporting is made also to the lower part of the digester. It is especially important to take care of the supporting of the central screen element in a batch digestion vessel, where in connection with the filling of the chips and filling of the impregnation liquor, the chip column can heavily move upwards and. break the constructions. The connection of the central screen element to the liquid flows is preferably made at the upper part of the screen element. For this reason, when installing the screen element into an existing digester, the existing pipe connections must be taken into consideration when designing and sizing the apparatus. 
         [0034]    The central screen element can be made of any material suitable for that purpose, like austenitic and ferrite-austenitic steels. 
         [0035]    The construction of the screen segment of the screen zone can be a round bar screen (as described in Patent WO01/31117), which is flow-technically efficient and would significantly increase the hydraulic capacity of the screen. On the other hand, also any other possible constructions implementing the characteristic features in accordance with the invention are covered by the scope of the invention. The screen element can be implemented for example with a perforated plate or a slotted plate. The choice is often based on a plurality of criteria, among others on the price. 
         [0036]    The transversal cross section X-X of  FIG. 1  illustrates one embodiment in accordance with the invention, showing the channel construction of the central screen element. Also other channel constructions, for example pipe constructions for conducting the flows to different zones of the central screen element and away from those are possible and are covered by the scope of the invention. The word channel in this connection refers to a construction providing for the flow of the process chemical inside the tubular central screen element. Multiple channels refer to a channel construction providing for flows of the process chemical in different directions also simultaneously. When the screen segments are for example located far from each other, it is profitable to arrange the liquids to flow in the standard pipes in the central screen element, in the portions between the screen segments. 
         [0037]      FIG. 2  illustrates one embodiment in accordance with the invention of the central screen element in a continuously operating digester. The central screen element of  FIG. 2  comprises four screen zones, the zones A, B, C and D. The screen zones are located as well sequentially close to each other (A and B) as at a distance from each other (C and D). 
         [0038]      FIG. 3  illustrate the operation of a central screen element in accordance with the invention in a campaign operation of a continuously operating digester, where for example softwood chips and hardwood chips are processed in turn. 
         [0039]    Digestion of Softwood Chips (Upper Drawing): 
         [0040]    A traditional radial displacement flow ( 6 ) from the outlet ( 6 . 1 ) like traditionally from the central pipe, flows radial towards the screen ( 7 ) of the wash cycle mounted onto the jacket of the digester. The washing liquor displaces the contaminated cooking liquor away from the plug flow. 
         [0041]    In this example, because the production capacity has been increased over the nominal capacity, the screen capacity of the digester is too small and disturbs the operation of the digester. 
         [0042]    A retrofitted central screen and its lower screen zone (A) can be taken into use for increasing the screen capacity. A pressure difference over the screen of the screen zone, suitably adjusted, causes that a part of the flow ( 6 ) can be sucked away from the plug flow as a flow ( 5 ) through the screen segment ( 5 . 1 ). 
         [0043]    When the pressure differences over a traditional screen ( 7 ) located on the jacket and over a screen segment ( 5 . 1 ) are in right proportion to each other, the radial flow ( 6 ) distributes to the both screens and the plug flow is purified as a whole more efficiently. The location of the screen segment ( 5 . 1 ) slightly below the screens ( 7 ) of the jacket contributes to the distribution of the radial flows. 
         [0044]    The dilute connections ( 8 ) and ( 9 ) of the bottom act in a traditional way for controlling consistency of the blow. 
         [0045]    Digestion of Hardwood Chips (Lower Drawing): 
         [0046]    A traditional radial displacement flow ( 6 ) from the outlet ( 6 . 1 ) like traditionally from the central pipe, flows radial towards the screen ( 7 ) of the wash cycle mounted onto the jacket of the digester. The washing liquor displaces the contaminated cooking liquor away from the plug flow. 
         [0047]    When hardwood is digested in a continuously operating digester and the capacity is increased over the nominal production of the digester, problems are caused by the following matters, especially on the area of the wash cycle of the digester: a) strong packing in the bottom area of the digester and b) high density of the digested chips on the screen surfaces, extending about from 100 mm to 200 mm from the screen surface towards the central axis of the digester. The bigger the pressure difference over the screen is, the higher is the density and the worse the filterability. 
         [0048]    For achieving the objectives of production in the conditions defined by the second wood quality (in this case the hardwood), the traditional radial flow ( 6 ) from the outlet ( 6 . 1 ) is supported for example with two radial flows ( 2 ) from the outlet ( 2 . 1 ), and ( 3 ) from the outlet ( 3 . 1 ), and the upper screen zone (B) as well as the lower zone (A) of the central screen element will be taken into use for increasing the screen capacity simultaneously. 
         [0049]    A maximized screen capacity allows the use of a smaller pressure difference for displacing the same amounts of liquid away from the plug flow. 
         [0050]    The strongest of the radial flows is the traditional flow ( 6 ) from the outlet ( 6 . 1 ), and supporting that are the smaller radial flows ( 3 ) from the outlet ( 3 . 1 ) and flow ( 2 ) from the outlet ( 2 . 1 ). Mutual differences between the flows and the mutual pressure differences over the screen surface of different screens like the screen of the jacket ( 7 ) and the screen segments ( 4 . 1 ) and ( 5 . 1 ) are adjusted so that the plug flow is purified as efficiently as possible. 
         [0051]    In a problem situation for example the flow ( 4 ) to the screen segment ( 4 . 1 ) and flow ( 5 ) to the screen segment ( 5 . 1 ) can be closed and the radial flows ( 3 ) from the outlet ( 3 . 1 ) and ( 2 ) from the outlet ( 2 . 1 ) can be emphasized for solving the problems caused by the strong packing. 
         [0052]    It is also possible to use alternated the flow ( 4 ) to the screen segment ( 4 . 1 ) and flow ( 5 ) to the screen segment ( 5 . 1 ) and radial flows ( 3 ) from the outlet ( 3 . 1 ) and ( 2 ) from the outlet ( 2 . 1 ) optionally simultaneously or according to a programmed alternation. 
         [0053]    The radial flow ( 1 ) from the outlet ( 1 . 1 ) is suitable for adjusting the consistency of the bottom of the digester in addition to the traditional dilute connections ( 8 ) and ( 9 ). 
         [0054]    If for easing the packing of the bottom, the buoyancy force resulted from the counter current cooking would be wanted below the screens-of the wash cycle for supporting the chip column, in the bottom part of the digester, the radial flow ( 1 ) from the outlet ( 1 . 1 ) can be utilized also in that, without disturbing the other operations. Only the mutual pressure differences of the screens ( 7 ) and the screen segments ( 4 . 1 ) and ( 5 . 1 ) over the screen shall be adjusted according to the conditions. The radial flow ( 1 ) from the outlet ( 1 . 1 ) must be forced to flow upwards towards the lowering plug flow and towards the screen ( 7 ). 
         [0055]      FIG. 4  illustrates one embodiment of the invention, wherein the central screen element is used in batch digestion. Wood chips are fed to the digester from the upper part of the digester. By means of the packing steam of the chip fill said steam being discharged from the nozzles located into the upper part of the digester, the chips can be evenly distributed and packed into the digester. The process chemical is fed to the digester through the lower part ( 16 ) of the digester. The air brought along with the wood chips is removed from the digester as a result of the process chemical fill through the pipe connections ( 17 ) located in the upper part of the digester and/or the screen segments ( 4 . 1 ) and ( 5 . 1 ) of the central screen element ( 11 ). During the digestion, a part of the liquor is discharged from the digester and new chemical is supplied instead. Through the central screen element in accordance with the invention, liquor can be removed just at the wanted point of the digester through the screen segments ( 4 . 1 ) and/or ( 5 . 1 ). During the digestion, the process chemical is circulated so that a part of it is removed from the digester either through the screens ( 10 ) attached to the jacket of the digester or through the screen segments ( 4 . 1 ) and ( 5 . 1 ) of the central screen element ( 11 ) as flows ( 4 ) and ( 5 ) and the liquor is returned to the digester either to the lower part ( 12 ) of the digester or to the upper part ( 13 ) of the digester, and/or through the central screen element ( 11 ) to a wanted point of the digester through the outlets ( 1 . 1 ), ( 2 . 1 ) and/or ( 3 . 1 ), as flows ( 1 ), ( 2 ) and/or ( 3 ). Due to the central screen element in accordance with the invention, chemical can be removed simultaneously at several points of the digester. The returning of the process chemical can be made in the batch digester in accordance with the invention also at several points of the digester, thus providing even distribution of the process chemical to the whole pulp column, and simultaneously the settling of the pulp into the lower part of the digester during the digestion can be decreased. After the digestion is ready, in other words when the kappa number of the pulp meets the requirements, the hot liquor is replaced in the displacement batch digestion by cooler and cleaner liquor. In a batch digester in accordance with the present invention, having a central screen element installed therein, the displacing liquor can be fed also through it from the outlets ( 1 . 1 ), ( 2 . 1 ) and/or ( 3 . 1 ) as flows ( 1 ), ( 2 ) and/or ( 3 ). In this way the displacement can be more carefully controlled, for example for decreasing the overhead digestion of the upper end caused by a long displacement time. The displaced liquor is removed through the displacement screens ( 14 ) installed to the upper part of the digester, and/or through the screen segments ( 4 . 1 ) and/or ( 5 . 1 ) as flows ( 4 ) and/or ( 5 ) of the central screen element. For discharging the digested pulp settled on the bottom from the digester, the pulp column must be supplied with dilution liquor. In the solution in accordance with the invention, liquor can be fed either through the nozzles ( 15 ) located in the lower part of the digester or from the outlet ( 1 . 1 ) of the central screen element, as flow ( 1 ), or through both of them. 
         [0056]    In case the conditions of digestion with respect to the wood material and to the technical determinations of the process cause the chip column to settle strongly downwards along with the progressing digestion, the upper outlet ( 3 . 1 ) and the upper screen segment ( 4 . 1 ) of the central screen element ( 11 ) in accordance with the invention can be disengaged, because they would operate in a gas space. 
         [0057]    One embodiment in accordance with the invention is to change a traditional batch digester to a displacement batch digester by installing a central screen element into the digester. In that case, during the change of liquids, air or liquor is discharged through the screen and during the digestion cycle the liquor of the upper cycle returns to the digester through the same, as described above. 
         [0058]    The multi-function screen element in accordance with the present invention, due to its location in the middle of the digester and to its eventual multiple zones, improves the alkali distribution during the whole digestion process as well in a batch digester as in a continuously operating digester. The control of the flows enabled by the multiple channels in the different zones of the central screen element, independently from each other either into the digester or out of the digester improves the optimization of the flows in the digestion conditions changing continuously caused by the digesting wood chips.