Abstract:
A method for heating a chip slurry in a pulping system including an impregnation vessel and a chemical digesting vessel, the method including: impregnating chips in the impregnation vessel with a liquor; transporting the chips from the impregnation vessel to an upper elevation of the digester vessel; extracting liquor from a lower elevation of the digester vessel, wherein the extracted liquor has a temperature substantially higher than a temperature of the chips being transported to the upper elevation of the digester vessel, and adding extracted liquor from the lower elevation of the digester vessel to the chips being transported from the impregnation vessel to the digester vessel.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to the field of chemical treatment of comminuted cellulosic fibrous material (generally referred to herein as “chips”). In particular, the invention relates to heating chips in a pulping system and processing black liquor in the system, which includes an impregnation vessel and a chemical digesting vessel. 
     To produce pulp in a chemical pulping process, the lignin bonds between cellulose fibers are dissolved by “cooking” wood chips (or other cellulosic fibrous material) in a digester vessel containing chips and cooking liquor. White liquor is generally referred to as the virgin liquids and cooking chemicals before being introduced to the chip slurry. Black liquor, typically produced during the digesting process, is the combination of the lignin residue with water and the chemicals used for digesting the chips to form pulp. Black liquor is extracted from the pulping system, such as from the digester vessel. Black liquor can be introduced to the cooking process and removed during the cooking process. 
       FIG. 1  shows a conventional chemical digesting system  100 , that processes wood chips. A continuous feed of wood chips is transported from a chip feed system (not shown) via line  101  to an impregnation vessel  102 . Chips are pretreated with cooking liquor in the impregnation vessel  102 . From the impregnation vessel  102 , chips and liquor flow in a slurry through line  106  to the top inlet  103 , e.g., top separator, of a chemical digester vessel  104 . The top separator  103  in the upper internal region of digester vessel  104  feeds the chips to the inner chamber of the digester vessel. 
     As the chips and liquor flow through the top separator  103 , a portion of the black liquor is extracted from the digester vessel via liquor circulation line  108 . The liquor extracted from the top separator flows through an in-line drain  110 , which directs the extracted liquor to a heat recovery unit  114  via line  112  or to a liquor circulation line  116 . 
     A conventional liquor circulation line  116  transports a portion, e.g. 90%, of the liquor passing through the in-line drain  110  to the chip discharge at the bottom of the chip impregnation vessel  102  or to the chip slurry line  106 . A liquor pump  118  may be used to move the liquor through the line  116  and to the chip slurry flowing from the impregnation vessel and to the digester vessel. The extracted liquor in line  116  is added to the pretreated chips discharged from the impregnation vessel and being transported via line  106  to the digester vessel. 
     The extracted liquor is introduced to the chip slurry in line  106  at or near the bottom of the impregnation vessel to increase the liquor content of the slurry for transport through the line. The liquor from line  116  is introduced to the chip slurry to reduce the concentration of chips in the slurry and facilitate chip transport through the chip slurry line  106 . The slurry flows from the impregnation vessel through line  106  to the top of the digester vessel  104 . 
     Conventionally, a small portion, e.g., 10%, of the extracted liquor flows via line  112  to the heat recovery system  114 , e.g., the heat exchanger. The temperature of the extracted liquor flowing through the in-line drainer  110  and heat exchanger is, for example, about 110° C. (Celsius). Heat energy from the extracted liquor may be used in a heat exchanger to generate, for example, low pressure steam, e.g., at 1 bar, for use in the puling mill. 
     The liquor concentration L/W (ratio of liquor to chips by weight) in the impregnation vessel may be 2.5. In the chip slurry line  106 , the liquor concentration (L/W) is increased by the addition of liquor (line  116 ) to the chips in the bottom of the impregnation vessel or an initial region of the chip slurry line  106 . The addition of liquor and aid increase I the L/W in transporting the chips through the chip slurry line  106 . When the slurry enters the digester vessel  104 , the ratio of liquor to chips (L/W) is reduced by, for example, extracting liquor at the top separator. 
     Heat is often added to the chips in the digester vessel. Generally, a digester vessel operates at a higher temperature than does an impregnation vessel. For example, an impregnation vessel may operate at a temperature of 110° C. and the digester vessel may operate at temperature of 140° C. The chip slurry enters the top separator at a lower temperature then the temperature in the digester vessel. Heat, such as medium pressure steam via line  134 , is added to the vessel to increase the temperature of the chips in the digester vessel. Medium pressure steam  146  is typically at a pressure of 10 to 12 bar and a temperature of 180° C. to 190° C. The temperature of the digester vessel tends to be lowest at the top inlet of the vessel and progressively increases in a downward direction through the vessel. 
     The digester vessel  104  generally includes multiple elevations of screens, including screens at upper elevations of the digester vessel (where upper refers to screens  120 ,  121  that are above the lower elevations of screens  122 ). Screen(s)  122  (generally referred to as wash screens) at lower elevations of the digester vessel may be near the bottom of the digester vessel, such as in the lower third (⅓) to one quarter (¼) of the digester vessel. Liquor is extracted through the upper elevation screen  120  and fed via liquor line  124  to the heat recovery system  114 . Liquor to be recirculated through the digester is removed by upper elevation screen  121  and fed to a liquor recirculation line  126 . 
     Other sources of liquor for circulation in the digester may include wash liquor (W.L.)  140  and a cold blow liquor system  142 . The was liquor  140 , cold blow liquor  142  and liquor extracted from screen  121  are combined in line  126  and pumped  127  to a heater  44 . The liquor for recirculation is heated in the heater  144  that uses a steam source  146  for heat energy. Typically, the circulation liquor is reintroduced via line  126  to the vessel at a different, e.g. higher, elevation from which the liquor was extracted. 
     The liquor extracted from lower elevation(s) of screens  122  may be, via line  128 , circulated to the digester at a higher elevation or discharged to the heat recovery system. The liquor from the lower elevation(s) of screens  122  tends to be hotter, e.g., 140° C., than liquor extracted from the top separator  103  (via line  108 ) and liquor extracted from screens  120  at upper elevations (via lines  124 ,  126 ). The hot liquor (line  128 ) from the lower region of the digester is recirculated to the top of the digester, via line  132  and pump  130 , without adding heat to the liquor. Below the lower elevation screens and near the pulp discharge, cold blow, e.g., cooled wash liquor, is added to reduce the temperature of the pulp being discharged from the digester through line  145 . The cold blow is provided from a source  142  of cooling liquor and passes through a cold blow cooler  143  and lines to direct the cold blow to the bottom of the digester. 
     Generally a large portion, e.g., 70%, of the liquor extracted via line  128  from the lower elevation screens  122  flows to the heat recovery system  114 . For example, liquor extracted to line  128  from the lower elevation of screens  122  may be divided such that a first portion, e.g., 70% to 80%, of liquor flows via line  128  to the heat recovery system  114 ; a second portion, e.g., 10% to 18%, of the liquor is pumped  130  to line  126  for recirculation to the top of the digester vessel, and a third portion, e.g., 8% to 12%, of the liquor is pumped  130  via line  132  to recirculation line  126 . 
     The excess black liquor, e.g. waste liquor, extracted from screens in the digester vessel and not reintroduced to the vessel or to the chip slurry line  106 , flows through the heat recovery system  114 . A portion of all streams of excess liquor, e.g., from in-line drainer and upper and lower extraction screens, tend to flow through the heat recovery system where heat energy from the excess liquor is converted to steam, typically low pressure steam, for other uses in the mill. Low pressure steam is typically at 1 bar or less at a temperature of about 100° C., such as in a range of 90° C. to 110° C. 
     To increase the temperature of the chips in the digester vessel, medium pressure steam  146  is often added to the digester vessel via steam line  136  and heater  144 . The medium pressure steam comes in at a temperature of 180° C. to 190° C. and increases the temperature of the chips in the digester vessel to promote the chemical reactions for digestion, e.g., breaking the lignin that bonds together the cellulosic fibers in the chip. Medium pressure steam requires energy to generate. Further, evaporators are generally needed to remove condensate resulting from the steam injected into the digester vessel. 
     There is a long felt need to reduce the energy requirements for a pulping system, including systems having an impregnation vessel and digester vessel. In particular, there is a long felt need for techniques to add heat to the chips in the digester vessel, which improve the energy efficiency of the pulping system and reduce the need for steam from external sources such as steam source  146 . 
     BRIEF DESCRIPTION OF THE INVENTION 
     Hot black liquor extracted at the lowest or lower screens in a digester is added to the chip slurry being transported from an impregnation vessel to a digester vessel. The hot liquor adds heat to the chip slurry before the slurry enters the top separator. The chips enter the digester vessel at a higher temperature that would occur if hot black liquor were not added. The heated chip slurry in the top separator reduces heat energy to be added to the digester vessel. The energy requirement of the digester vessel is reduced because hot black liquor is added to the chip slurry before the chips enter the digester. 
     A method has been developed for heating a chip slurry in a pulping system including an impregnation vessel and a chemical digesting vessel, the method comprising: impregnating chips in the impregnation vessel with a liquor; transporting the chips from the impregnation vessel to an upper elevation of the digester vessel; extracting liquor from a lower elevation of the digester vessel, wherein the extracted liquor has a temperature substantially higher than a temperature of the chips in the impregnation vessel, and adding extracted liquor from the lower elevation of the digester vessel to the chips to be or being transported from the impregnation vessel to the digester vessel. 
     A method has been developed for heating a chip slurry in a pulping system including an impregnation vessel and a chemical digesting vessel, the method comprising: impregnating chips in the impregnation vessel with a liquor; transporting the chips from the impregnation vessel to a top separator of the digester vessel; extracting liquor from a lower elevation screen of the digester vessel, wherein the extracted liquor has a temperature at least 20 degrees Celsius higher than a temperature of the chips in the impregnation vessel, and adding the extracted liquor from the lower elevation screen of the digester vessel to the chips to be or being transported from the impregnation vessel to the digester vessel. 
     An apparatus has been developed for heating a chip slurry in a pulping system including an impregnation vessel and a chemical digesting vessel, the apparatus comprising: a chip transport conduit extending from the impregnation vessel to the digesting vessel, wherein chips in the chip transport conduit flow from the impregnation vessel to the digesting vessel; a lower elevation screen on the digesting vessel, wherein hot liquor is extracted from the lower elevation screen; and a first liquor conduit extending from the lower elevation screen to the chip transport conduit, wherein liquor extracted from the lower elevation screen is added to the chip transport conduit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a conventional chemical chip digesting system including an impregnation vessel and a chemical digesting vessel. 
         FIG. 2  is a schematic diagram of a chemical chip digesting system including an impregnation vessel and a chemical digesting vessel, wherein hot black liquor extracted from a lower region of the digester vessel is added to the chip slurry flowing from the impregnation vessel to the digester vessel. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A system and method has been developed to recover heat from the excess black liquor extracted from the lower sets of screens or lowest set of digester screens (generally referred to as the wash screens) to heat the black liquor in the chip slurry circulation line between the top of the digester and the bottom of the impregnation vessel. 
       FIG. 2  is a schematic diagram of a chemical chip digesting pulping system  10  including an impregnation vessel  12  and a chemical digesting vessel  14 . The impregnation vessel receives a supply feed  23  of chips at an upper inlet  15 . The impregnation vessel  12  is upstream of the digester vessel  14 , in the direction of chip flow through the pulping system. The impregnation vessel pre-treats a chip with cooking liquor in the vessel. The impregnation vessel is a pretreatment vessel for the chemical digester vessel. 
     Chips and liquor (the chip slurry) discharged from a bottom outlet  16  of the impregnation vessel flow through a chip circulation line  18  to a top separator  20  in the upper region of the digester vessel. As the chip slurry flows through the top separator, a portion of the liquor is extracted through screens in the top separator and flows out the digester through a liquor circulation line  21 . 
     The liquor concentration (L/W) in the impregnation vessel  12  may be lower than the liquor concentration in the digester vessel. The liquor concentration in the transport stream (line  18 ) increases the L/W concentration levels in the impregnation vessel and digester vessel to transport the chips from the impregnation vessel through line  18  to the digester vessel  14 . In the digester, liquor from the incoming chip feed stream (line  18 ) is extracted from the top separator  20  to a liquor circulation line  21  and flows to an in-line drain  22 . The extraction of liquor from the top separator reduces the L/W ratio for the chips in the digester to a L/W level below that in line  18 . A portion, e.g., about half of the flow, of the liquor passing from line  21  through the in-line drain  21  flows through a liquor circulation line  24  and is returned to the chip stream flowing from the impregnation vessel to the digester. The returned liquor in liquor circulation line  24  is added to the bottom  16  of the impregnation vessel or to an initial portion of the chip circulation line  18 . A pump  17  may force the liquor through line  24  and into the chip slurry in line  18 . The returned liquor is added to increase the portion of liquor, e.g., increase the L/W ratio, in the chip slurry flowing through line  18  between the impregnation vessel and digester vessel. 
     A second portion of the liquor flow through the in-line drain  22  flows through liquor line  26  to a heat recovery system  28 , e.g., heat exchanger. The volume of the second portion of the excess liquor extracted from the top separator may be about one-half (½) of the volume of excess liquor flowing through the in-line drain  22 . 
     A relatively large portion of black liquor passing through the in-line drain  22  (such as one-half of the liquor passing through the drain) is directed to the heat exchanger  28 , as compared to portion of liquor from the in-line drain that flows to the heat exchanger of the conventional system shown in  FIG. 1 . A relatively small portion of the black liquor flowing through the in-line drain (such as one-half of the liquor passing through the drain) flows through line  24 , where it mixes with hot black liquor (from line  42 ) and is added to the chip slurry line  18 . The portion passing through the in-line drain that is directed to chip transport line  18  may vary depending on the amount of liquor needed in the chip transport line  18 , and sent to heat recovery. 
     The chips and cooking liquor discharged to the digester vessel from the top separator  20  flow down through the digester vessel  14 . Screens  30 ,  31 ,  32  at various elevations of the digester vessel extract black liquor from the chip slurry in the vessel. The screens are conventional and well-known in the art of chemical pulp digesting. The extraction and circulation of liquor from the upper screens  30  and  31  is convention and, as an example, is similar to the extraction and circulation shown in  FIG. 1  of the screens  120 ,  121 . The black liquor extracted from extraction screens  30  at upper elevations of the digester vessel extract black cooking liquor that may flow to the heat recovery system  28  via liquor line  34 . Liquor from the recirculation screens  31  at upper elevations may also be circulated to an upper region  38  of the vessel, e.g., the top separator via liquor circulation line  36 . In addition, white liquor (W.L.)  52  may be added to the liquor circulation line(s)  50  to ensure sufficient liquor in the top of the digester vessel. 
     The lower screen(s)  32 , e.g., wash screens, in the digester vessel  14  provide a substantial flow of extracted hot black liquor. The lower screens may be in the lower third (⅓) or quarter (¼) of the height of the digester vessel. For example, the rate of black liquor extracted from the lower screen(s) for the chip slurry line  21  may be three (3) to four (4) times the rate of the black liquor flowing from the lower screen to the top separator to liquor line  21 . A portion of the black liquor from the lower screen, e.g., 25%, may flow to upper regions of the digester vessel through lines  48  and  50 . 
     The hot black liquor extracted from the lower screen(s)  32  is extracted via liquor wash line  40 . A portion, e.g., 70% to 80%, of the extracted hot liquor flows from line  40  to line  42  that conveys the hot liquor to the chip slurry flowing from the impregnation vessel  12  to the digester vessel  14  via chip circulation line  18 . A pump  41  may boost the pressure of the hot black liquor to the pressure of lines  24  or line  18 . The hot black liquor from the lower screens  32  may be mixed with the cooler liquor from top separator in line  24 . The mixed streams of black liquor are added to the chip transport line (conduit)  24  near the impregnation vessel and/or start of the chip transport. 
     Of the extracted black liquor from the wash screens  32  (line  40 ), about 30% to 80%, more preferably 50% to 70%, and most preferably 70% to 75%, of the black liquor is circulated to the circulation line between the top of the digester and the bottom of the impregnation vessel downstream of the in-line drainer. 
     The temperature of the black liquor extracted from the lower screen(s)  32  is generally hotter than the temperature of the black liquor extracted from the top separator to lines  21  and  24 . For example, the temperature of the black liquor extracted from the lower screen(s)  32  is typically about 140° C. and the temperature of the black liquor extracted from the top separator tends to be about 110° C. 
     Below the lower elevation screens and near the pulp discharge, cold blow, e.g., cooled wash liquor, is added to reduce the temperature of the pulp being discharged from the digester through line  145 . The cold blow is provided from a source  142  of cooling liquor and passes through a cold blow cooler  143  and lines to direct the cold blow to the bottom of the digester. 
     The hot black liquor may be added to liquor line  24  to allow the hot black liquor to mix with the liquor from the top separator before the mixed liquor streams are added to the chip transport conduit  18 . The temperature of the mixed liquor in line  24  is substantially hotter, e.g., at least 20° C. hotter, than the chip slurry in the impregnation vessel  12 . 
     The hot liquor from line  24  and pump  17  heats the chips in line  18 . The increased temperature of the chips in line  18  reduces the energy required to heat the chips in the digester vessel. The hot black liquor from the lower screen(s)  32  is used to raise the temperature of the chips to a cooking temperature or towards the cooking temperature. Due to the reduced need for heat energy, the quantity may be reduced of medium pressure steam  44  introduced via steam line  46  to the top of the digester vessel. Medium pressure steam is typically steam at 10 to 12 bar and a temperature of 180° C. to 190° C. 
     The hot black liquor from conduit  42 , with optional pump  41 , is added under pressure to the liquor in conduit  24  such that there is substantially no flashing, e.g., vaporization. The mixture of black liquor in conduit  24  is pumped  17  under pressure to the chip transport conduit  18  and, preferably, without flashing the liquor. 
     The liquor added to the chip transport conduit  18  increases the liquor to wood ratio (L.W.) in the conduit  18 . The L.W. ratio in the impregnation vessel  12  and digester  14  is typically lower than the L.W. ratio in conduit  18  because of the liquor added to the conduit and extracted in the top separator. 
     By using the liquor, e.g., wash liquor, line  40  to heat the chip slurry in line  24 , the amount of medium pressure steam added to the top of the digester may be substantially reduced, such as a reduction of about 40%. Reducing the amount of medium pressure steam needed to heat the chips in the top of the digester vessel results in a corresponding reduction in the energy requirements of the pulping process. Further, reducing the amount of medium pressure steam injected into the digester vessel also reduces the water condensate to the digester and thereby reduces the need for evaporator operations. 
     While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.