Patent Publication Number: US-2013248127-A1

Title: Method and arrangement for measuring and controlling chip and/or liquid level

Description:
TECHNICAL FIELD 
     The invention relates to a method and an arrangement for measuring and controlling a chip and/or liquor level. The invention relates particularly, though not exclusively, to measuring of the surface level of chips in a steam-liquid phase digester and a hydraulic digester. 
     BACKGROUND ART 
     In a continuous pulp digester the chips are fed as a regular flow to the digester together with the liquor while a corresponding amount of pulp and liquor is blown out of the digester. There are two types of continuous digesters, the liquid-steam phase digester and the hydraulic digester. 
     In the liquid-steam phase digester the liquid surface is lower than the chip surface wherein a steam phase is formed in an upper portion of the digester. A steam supply is provided via a top of the digester. The chips can be fed from outside the digesting volume, in the level of the steam phase, to a chip screw which is feeding the chips upwards to the top of the digester. The chips are dropping outside the chip screw from the top of the digester to the digesting volume. 
     The hydraulic digester is filled with a digesting liquid, i.e., under hydraulic pressure. The chips are brought to the upper portion of the digester by means of filling liquor and the upper screw in a cylindrical screen presses the chips into the digesting liquid to the actual digesting volume. Then the chips proceed downwards in an impregnation zone and after that reach a cooking zone. In the cooking zone the temperature is increased to a cooking temperature. 
     The surface height of the chips is measured in the liquid-steam phase digester and the hydraulic digester in order that the cooking process can proceed undisturbed. The chip surface of the digester can be influenced by blow flow, bottom scraper rotation speed and chip screw rotation speed which is regulating the chip feed. The blow flow is affecting the mass flow which is exiting the digester and the rotation speed of the bottom scraper is affecting the consistency of the mass in the bottom of the digester. The described control quantities can be applied as different combinations. 
     The liquor surface can rise above the chip surface in the liquid-steam phase digester what is causing an abnormal situation. The situation may arise for example due to disruption in the feed of the chips what may be caused by a blockage of the input path of the chips. 
     In the regulation of the chip surface the surface height is in the prior art measured with mechanical or, in the liquid-steam phase digester, with radiometrical sensors. A precipitation which is accruing onto walls of the steam phase, and scum, are disturbing the radiometrical measurement. The mechanical surface height sensors are blades which are mounted inside the digester and measuring torque. The chips on the blade are causing to the blade a torque which is increasing when the load against the blade is increasing. There are many measuring variations in the blade measurements because the height of the chip pile is differing in different locations of the digester. Further the precipitation is accumulating on the blades which is increasing measuring uncertainty in the course of time. 
     A mechanical chip surface meter is used in the prior art as a prolongation of the upper screw of the hydraulic digester wherein torque is measured by members which are lead through the upper screw and sunken into the mass. Precipitation is accruing to the mechanical parts wherein the measuring uncertainty is increasing. 
     An object of the invention is to provide an alternative method and arrangement for measuring the surface height of the chips. An object of the invention is to eliminate or at least minimize drawbacks in the prior art. Still an object of the invention is to increase reliability of surface height measuring of the chips, improve surface height regulation of the chips and decrease service need of surface height measuring equipment. 
     SUMMARY 
     According to a first aspect of the invention there is provided a method for measuring and controlling a chip level and/or a liquor level height of a liquid-steam phase digester or a hydraulic digester which digester comprises a chip screw for feeding chips into a digesting volume, and the method comprising mounting electrode pairs in certain heights and vertically at distances from each other in the digesting volume of the digester, the electrode pairs being formed of electrodes, connecting the electrode pairs to electric circuits, measuring in each electric circuit a current or voltage value during operation of the digester, determining, based on the measurement, type of material layer (for example, steam, chips, combination of chips and liquor, or liquor) located in a certain height, based on the electrical conductivity of the materials in the digesting volume. 
     Preferably the method comprises arranging a potential difference (AC, DC) between the electrodes of the electrode pairs and measuring the current in the electric circuit. 
     Preferably the method comprises supplying current to the electric circuit of the electrode pairs and measuring the voltage in the electric circuit. 
     Preferably the method comprises using the determined height information of the chip level and/or the liquor level as control for the digesting process and repeating the measuring and control steps of the method. 
     Preferably the method comprises arranging a changing potential difference (AC, DC) between the electrodes of the electrode pairs which is preventing generation of precipitation on electrode surfaces. 
     According to a second aspect of the invention there is provided an arrangement for measuring and controlling a chip level and/or a liquor level height of a liquid-steam phase digester or a hydraulic digester which digester comprises a chip screw for feeding chips into a digesting volume, and the arrangement comprises electrode pairs which are formed of electrodes and mounted in certain heights and vertically at distances from each other in the digesting volume of the digester, and the electrode pairs are connected to electric circuits such that, during operation of the digester, a current or voltage value measured in each electric circuit is determining the type of material layer (for example, steam, chips, combination of chips and liquor, or liquor) located in a certain height, based on the electrical conductivity of the materials in the digesting volume. 
     When the electric circuits are in an electrified state, the locations of the vertical chip and liquor zones can be determined based on the electrical conductivities of the chips, the liquor and the combination of chips and liquor which are at least approximately known. 
     Preferably the arrangement comprises a circuit measuring means which is during operation of the digester arranged to measure current of the circuits when potential differences (AC, DC) are arranged between the electrode pairs. Preferably the arrangement comprises voltage supplying means for generating potential difference (AC, DC) in the circuits of the electrode pairs. Preferably the circuit measuring means comprises voltage supplying means for generating potential difference (AC, DC) in the circuits of the electrode pairs. 
     Preferably the arrangement comprises a circuit measuring means which is during operation of the digester arranged to measure voltage of the circuits when current is supplied in the circuits. Preferably the circuit measuring means comprises current measuring means for measuring current of the circuits. 
     Preferably the arrangement comprises an output for communicating the height information of the chip level and/or the liquor level from the circuit measuring means for the control of the digesting process. 
     Preferably the electrode pairs, more preferably measuring rods, are arranged under the chip screw. Preferably the electrode pairs, more preferably the measuring rods, are arranged under a chip basket. 
     Preferably the arrangement comprises one or more measuring rods to which is/are mounted electrodes for the electrode pairs. Preferably the measuring rod/rods is/are mounted under the chip screw. 
     Preferably the electrode pair is formed in the same measuring rod of electrodes which are mounted in two different heights. 
     Preferably the electrode pair is formed in two or more measuring rods of electrodes which are mounted in the same height or different heights. 
     Preferably the electrode pair is formed in the same measuring rod of electrodes which are mounted in the same height and are at a horizontal distance from each other, preferably at opposite sides of the rod. 
     Preferably the vertical distances of the electrode pairs form the neighboring electrodes are in the upper end of the measuring rod (in the gas phase) larger than in the bottom end of the measuring rod. 
     With the method and the arrangement the chip surface height measured in the liquid-steam phase digester or the hydraulic digester can be exploited in improving the fluency of the cooking process. The measured chip surface and/or liquor surface height can be used as a control quantity when controlling the blow flow; the bottom scraper rotation speed; the chip screw rotation speed and/or material flows of the digester such as supply of liquor to the digesting volume. 
     By the method and the arrangement the level height measurement of masses in the digesting volume of the continuous pulp digester can be improved, the reliability of the surface height measurement can be increased and the reliable lifetime of the measuring means in the digesting volume can be improved, the surface height regulation of the masses in the digesting volume can be improved and the service need of surface height measuring means can be reduced. By the method and the arrangement the problem of the so called floating chips can be recognized which can exist both in the liquid-steam phase digester and in the hydraulic digester. Then the chips don&#39;t sink under the liquor but are floating on the liquor surface. A region is forming under a thin chip bed where exist sole liquor what can also be detected in the surface height measurement. Generally the chips are then too dry and the chips can block quickly the chip feeding openings until the chip feeding screw and the process is stopping. This failure can be detected proactively by the surface height measurement and there may be time left for making some improvements in order to prevent the blockage. 
     Different embodiments of the present invention will be illustrated or have been illustrated only in connection with some aspects of the invention. A skilled person appreciates that any embodiment of an aspect of the invention may apply to the same aspect of the invention and other aspects alone or in combination with other embodiments as well. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described, by way of example, with reference to the accompanying drawings, in which: 
         FIG. 1  shows the method and the arrangement in connection with a liquid-steam phase digester; 
         FIG. 2  shows the method and the arrangement in connection with a hydraulic digester; 
         FIG. 3  shows a first alternative placement of the measuring electrodes; 
         FIG. 4  shows a second alternative placement of the measuring electrodes; 
         FIG. 5  shows the method and the arrangement in an abnormal situation; 
         FIG. 6  shows details of a first measuring rod; and 
         FIG. 7  shows details of a second measuring rod. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, like numbers denote like elements. It should be appreciated that the illustrated drawings are not entirely in scale, and that the drawings mainly serve the purpose of illustrating embodiments of the invention. 
       FIGS. 1 and 3  to  5  show a liquid-steam phase digester  100  which comprises a chip screw  102  in an upper portion  101  of the digester. Feed of the chips into the digester  100  is illustrated with dashed line arrows. The chips are fed from outside a digesting volume through an input channel  103 , in the height of a steam phase, to a chip screw  102  which is feeding the chips upwards to a top  104  of the digester. A motor  105  which is located in the top  104  of the digester is rotating the chip screw  102  which is located in a screen basket  106 . During lifting motion of the chip screw the liquid which is separated from the chips is lead through the screen basket  106  to a collecting channel  107  and further outside the digester (arrow  108  in  FIG. 1 ). The chips are dropping outside the chip screw  102  from the top  104  of the digester to the digesting volume. 
     The steam phase is extending around the chip screw  102  and to a steam volume  109  below the chip screw. An upper surface of a chip layer  111  is called chip surface  110 . The boundary surface between the chip layer  111  and a combined chip and liquor layer  113  is called liquor surface  112 . 
     The arrangement comprises electrode pairs which are formed of electrodes and mounted at certain heights and vertically at distances from each other in the digesting volume of the digester  100 , and the electrode pairs are connected to electric circuits  300  such that, during operation of the digester, a current or voltage value measured in the electric circuit is determining the type of material layer (for example, steam, chips, combination of chips and liquor, or liquor) located at certain height, based on electrical conductivity of the materials in the digesting volume. The electric circuits  300  are measured by a circuit measuring means  301  to which preferably the electric circuits of all electrode pairs are connected. 
     Additionally the arrangement comprises an output  302  for communicating the height information of the chip level  110  and/or the combined chip and liquor level  113  and/or the liquor level  115  from the circuit measuring means  301  to be exploited in the control of the digesting process such as when controlling the blow flow; regulating the bottom scraper rotation speed; regulating the chip screw rotation speed and/or regulating the material flows of the digester such as supply of the liquor into the digesting volume. 
     In the method and the arrangement a voltage (preferably a constant voltage) is regulated between the electrodes of the electrode pairs forming the measuring circuit which electrodes are mounted in the digesting volume and the current in the measuring circuit is measured. Alternatively a constant current can be supplied to the measuring circuit and the voltage can be measured. The voltage is preferably constant voltage DC but also pulsed DC or alternating voltage AC can come into question wherein accruing of precipitation on the electrodes can be decreased. In the method and the arrangement the chip surface and the liquor surface height can be measured by one or more measuring rods  2 ,  3 ,  4  based on electrochemical measurement and mounted in the digester  100 . 
     In  FIGS. 1 and 6  the measuring rod  2  comprises electrodes E of which two electrodes are connected in the measuring circuit and placed in the same measuring rod in different heights, for example, both electrodes E 4 -E 5  in the steam volume  109 ; and E 5 -E 6  at both sides of the chip surface  110 . Electric properties of a gas or a mass or a liquid existing in each height of the digesting volume can be determined with the voltage or current which is connected to the electrodes E. The surface height of the chips or liquor can be deduced from the measured properties such as electric conductivity in a normal operation situation and an abnormal situation wherein for example liquor can be accrued to a layer above the chips. 
     Electrodes E are preferably arranged in an upper part  2 ′ of the measuring rod  2  in a region of the steam phase of the digesting volume with larger distances than in a lower part  2 ″ of the measuring rod in an actual location region of the mass and the liquid in a height of the chip layer  111  and the combined chip and liquor layer  113  (liquid phase). The electrodes which are located more sparsely, in a height of an unlikely location height of the mass, are saving costs of the construction. The electrodes E placed in different heights in the measuring rod  2  are denoted with an indexing E(1+n), where n=1, which is continuing downwards from top. For the sake of clarity only a part of the electrodes are named in the figures. 
     The distance between the electrodes is preferably 30 to 40 cm in the upper end  2 ′ of the measuring rod located in the steam volume (particularly in the upper portion of the steam volume), and lower than that the distance is smaller. Naturally, the distance between the electrodes must not be constant in any region of the measuring rod when the location of the electrodes is known in the vertical direction of the digesting volume, in order to control the cooking process based on the measuring results of the surface heights. 
       FIG. 2  shows the method and the arrangement in connection with the hydraulic digester  200 . Feed of the chips into the digester  200  is illustrated with dashed line arrows. The chips are fed through an input channel  203  to a chip screw  202  which is located in an upper portion  201  of the digester  200 . The chip screw  202  is feeding the chips downwards from a top  204  of the digester. A motor  205  which is located in the top  204  of the digester is rotating the chip screw  202  which is located in a screen basket  206 . During rotating motion of the chip screw the liquid which is separated from the chips is lead through the screen basket  206  to a collecting channel  207  and further outside the digester (arrow  208  in  FIG. 2 ). The chips are dropping under the chip screw  202  to an upper portion  209  the digesting volume which is filled with liquor. An upper surface of a chip layer  111  is called chip surface  110 . In the hydraulic digester there can be found only the upper portion filled with liquor and a lower portion which is covered with the combined chip and liquor layer. The chip surface must not rise too high wherein the digester starts to become blocked. On the other side the chip surface must not drop too low wherein the capacity of the cooking process collapses and the quality of the fiber suffers from the excessive alkali dosage. 
     A space  220  through the rotation motor  205  and the chip screw  202  is arranged in the digester  200  for placing the measuring rod  2  in the digesting volume on the rotation axis  202 ′ of the chip screw. The space can exist for the known mechanical surface height measuring apparatus wherein the digester is easy to rebuild according to the arrangement of the invention. 
     In connection with  FIG. 2  it is referred to  FIG. 1  where a corresponding measuring rod  2 , electrodes E, electric circuits  300 , measuring means and output  302  are described. The digesting volume  209  in  FIG. 2  is deviating from the digesting volume in  FIG. 1  below the chip screw  202  in that the digesting volume  209  is filled with liquid. 
     The electrical conductivity in different layers of the digesting volume changes depending on the conductivity in the material which is located between the electrodes of the electrode pair. A closed electric circuit is substantially not generated in the steam volume  109  (if at least one electrode is in the steam volume), i.e., there flows no current. There flows current in the chip layer  111  depending on the amount of liquor in the chips when a potential difference is generated in the electric circuit. In an abnormal situation ( FIG. 5 ) the amount of current in the liquor layer  115  above the chip layer  111  is larger related to the combined chip and liquor layer  113  because the undiluted fresh liquor is conducting electricity better than the combined layer. 
     The potential difference to be connected between the electrode pairs can be about 2V wherein the current in the electric circuit can be more than 100 mA, maximally about 2 A. The current is preferably at a mA level when current is supplied to the electric circuit. 
     The examples of arranging one measuring rod and electrode pairs described in connection with  FIGS. 1 ,  3 ,  5 ,  6  and  7  can be applied also with the hydraulic digester  200  in  FIG. 2 , preferably when the measuring rod is placed on the rotation axis of the feeding screw. Examples of arranging electrodes to pairs are described in the following. 
     In  FIG. 1 , the electrodes E 1  . . . E 5  are located in the steam volume  109 , the electrodes E 6  . . . E 11  in the chip layer  111  below the steam volume and the electrodes E 12  . . . E 14  are located in the liquor layer  113  below the chip layer  111 . The electrodes are connected in electric circuits such that there are formed electrode pairs. 
     In  FIG. 2 , the electrodes E 1  . . . E 14  are arranged to electrode pairs corresponding to  FIG. 1 , for example, E 1 -E 2 , in certain heights vertically at distances from each other such that the electrodes of the electrode pair connected in one electric circuit  300  are placed in different heights in the same measuring rod. 
       FIG. 3  shows an example of placing electrodes in two or more measuring rods  2 ,  3 . Two electrodes A, B which are connected in a measuring circuit are placed in separate measuring rods  2 ,  3  in the same height, for example, the electrodes A 2 -B 2  in the steam volume  109 ; and A 9 -B 9  in the chip layer  111 . Naturally, electrodes located in two or more measuring rods and connected to the same measuring circuit can locate in different heights. 
     In  FIG. 3 , the electrodes placed in neighboring measuring rods  2  and  3  are denoted in the same height in the following way: in the first measuring rod  2  with an indexing A(1+n), where n=1 which is continuing from above downwards, and in the second measuring rod  3  with an indexing B(1+n), where n=1 which is continuing from above downwards. 
     In  FIG. 3 , the electrode pairs A 1 -B 1  . . . A 5 -B 5  are located in the steam volume  109 , the electrode pair A 6 -B 6  in the height of the chip surface  110 , the electrode pairs A 7 -B 7  . . . A 11 -B 11  in the chip layer  111  and the electrode pairs A 12 -B 12  . . . A 14 -B 14  in the chip-liquor layer  113  below the chip layer  111 . 
       FIGS. 4 and 7  show two electrodes which are connected in the same measuring circuit and placed in the same height in the same measuring rod, for example, both electrodes C 1 -D 1  in the steam volume  109 . 
     In  FIG. 4 , the neighboring electrodes placed in the same measuring rod  2  at opposite sides are denoted in the same height with an indexing C(1+n) and D(1+n) where n=1 which is continuing from above downwards. 
     In  FIG. 4 , the electrode pairs C 1 -D 1  . . . C 5 -D 5  are located in the steam volume  109 , the electrode pair C 6 -D 6  in the height of the chip surface  110 , the electrode pairs C 7 -D 7  . . . C 11 -D 11  in the chip layer  111  and the electrode pairs C 12 -D 12  . . . C 14 -D 14  in the chip-liquor layer  113  below the chip layer  111 . 
       FIG. 5  shows the method and the arrangement in an abnormal operation situation of the liquid-steam phase digester  100 . 
     The surface of the liquor in the digester is risen above the chip surface wherein the liquor layer is above the chip layer when the chip layer  111  is sunken abnormally deep inside the liquor. The situation may arise for example due to disruption in the feed of the chips what may be caused by a blockage of the input path of the chips. 
     In  FIG. 5 , the electrodes E 1  . . . E 4  are located in the steam volume  109 , the electrodes E 5  . . . E 6  in the liquor layer  114  above the chip-liquor layer  113 , the electrodes E 7  . . . E 14  are located below the chip surface  110 . 
     Examples of arrangements in which a constant voltage is regulated in electric circuits which are connected between an electrode pair, and the current in the electric circuits is measured is shown in Tables 1 to 4. The voltage can be connected to all measuring circuits at the same time. It is preferable to measure the current of the electrode pairs in the arrangement alternately in suitable time intervals (to poll alternately the electric circuits of the electrode pairs) for example in intervals of some seconds. In the measurement can be determined a) if the current flows or not, and if desired, additionally b) the amount of the current. 
     Table 1 shows the relations of the voltage and the current when the digester  100  is in the normal state which is shown in  FIG. 1 . 
     
       
         
           
               
               
             
               
                   
               
               
                 electrode pairs, constant voltage 
                 current 
               
               
                   
               
             
            
               
                 E1-E2 . . . E5-E6 
                 0 
               
               
                 E6-E7. . . E10-E11 
                 small, increases downwards 
               
               
                 E11-E12 
                 &gt;small 
               
               
                 E12-E13 . . . 
                 largest 
               
               
                   
               
            
           
         
       
     
     Table 2 shows the relations of the voltage and the current when the digester  100  is in the normal state which is shown in  FIG. 3 . 
     
       
         
           
               
               
               
             
               
                   
                   
               
               
                   
                 electrode pairs, constant voltage 
                 current 
               
               
                   
                   
               
             
            
               
                   
                 A1-B1 . . . A5-B5 
                   0 
               
               
                   
                 A6-B6 
                 ~0 
               
               
                   
                 A7-B7 . . . A11-B11 
                 small 
               
               
                   
                 A12-A12 . . . 
                 largest 
               
               
                   
                   
               
            
           
         
       
     
     Table 3 shows the relations of the voltage and the current when the digester  100  is in the normal state which is shown in  FIG. 4 . 
     
       
         
           
               
               
               
             
               
                   
                   
               
               
                   
                 electrode pairs, constant voltage 
                 current 
               
               
                   
                   
               
             
            
               
                   
                 C1-D1 . . . C5-D5 
                   0 
               
               
                   
                 C6-D6 
                 ~0 
               
               
                   
                 C7-D7 . . . C11-D11 
                 small 
               
               
                   
                 C12-D12 . . . 
                 largest 
               
               
                   
                   
               
            
           
         
       
     
     Table 4 shows the relations of the voltage and the current when the digester  100  is in the abnormal state which is shown in  FIG. 5 . 
     
       
         
           
               
               
               
             
               
                   
                   
               
               
                   
                 electrode pairs, constant voltage 
                 current 
               
               
                   
                   
               
             
            
               
                   
                 E1-E2 . . . E4-E5 
                 0 
               
               
                   
                 E5-E6 . . . E6-E7 
                 largest 
               
               
                   
                 E7-E8 . . . E10-E11 
                 &lt;previous 
               
               
                   
                 E11-E12 
                 &lt;previous 
               
               
                   
                 E12-E13 . . . 
                 &lt;previous 
               
               
                   
                   
               
            
           
         
       
     
       FIG. 6  shows a side view and a top view of a portion of the measuring rod  2  in more detail. The measuring rod  2  comprises a body  10 , an isolation layer  11  which is surrounding the body and an electrode E 1  outside the isolation layer. Preferably the body is metal, for example, stainless steel which is suitable in the circumstances of the digester. The material of the isolation layer  11  can be isolating material polytetrafluoroethylene (PTFE, Teflon) which is suitable in the circumstances of the digester without softening too much.  FIG. 6  shows a circular single electrode E 1  which is preferably of stainless steel and fixed to the body by a screw  12 . An electric wire is leading from the electrode to the electric circuit  300  inside the body  10 . 
       FIG. 7  shows a side view and a top view of a portion of the measuring rod  4  in more detail. The measuring rod  4  comprises the body  10  and the isolation layer  11  corresponding to  FIG. 6 . Additionally the measuring rod comprises in the same height two electrodes C 1  and D 1  which are connected to the electric circuit  300  by wires which are lead inside the body. The electrodes shown in  FIG. 7  are designed rounded in their shapes such that they resist flow marginally what is reducing the precipitation. If desired, the electrodes comprise above and below chamfers  13 . 
     The foregoing description provides non-limiting examples of some embodiments of the invention. It is clear to a person skilled in the art that the invention is not restricted to details presented, but that the invention can be implemented in other equivalent means. Some of the features of the above-disclosed embodiments may be used to advantage without the use of other features. 
     As such, the foregoing description shall be considered as merely illustrative of the principles of the invention, and not in limitation thereof. Hence, the scope of the invention is only restricted by the appended patent claims.