Abstract:
A method for chemically digesting cellulosic fibrous material including: adding a liquor to cellulosic fibrous material to form a slurry in a chip feed system; transporting the slurry from the chip feed system to a batch digester vessel; determining when a chip level rises above an extraction screen in the batch digester vessel; after the chip level rises above the extraction screen, extracting liquor from the batch digester vessel while the transport continues of the slurry into the batch digester vessel; adding cooking liquor to slurry in the batch digester vessel; ceasing the transport of the slurry into the batch digester vessel and thereafter converting the cellulosic material in the vessel to a pulp, and discharging the pulp from the batch digester vessel before restarting the transporting of the slurry into the digester vessel.

Description:
CROSS RELATED APPLICATION 
       [0001]    This application claims the benefit of application Ser. No. 61/362,037 filed Jul. 7, 2010, which is incorporated in its entirety by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    This invention relates to a method and system for feeding comminuted cellulosic fibrous material (referred to as “chips”) to a treatment vessel, such as a batch digester. In particular, the invention relates to creating a slurry, providing heat for the slurry and cooking liquor addition to the batch digester to which the chips being fed. 
         [0003]    The terms “wood”, “chips” and “wood chips” refers generally to comminuted cellulosic fibrous material, such as chipped hardwoods and softwoods, and other lignocellulosic material. 
         [0004]    A batch digesting system may include a chip feed system and a batch digester vessel. The chip feed system intermittently delivers chips, with liquor and optionally steam heat, to the batch digester vessel. The conventional process cycle of a batch digester vessel includes the sequential steps of: (i) filling the vessel with chips from the chip feed system, (ii) adding steam and liquor to the batch digester vessel (the liquor may be added simultaneously with the filling of the chips); (iii) capping the filled vessel and operating the vessel at conditions, e.g., above-atmospheric temperature and pressures, to digest, e.g., “cook”, the chips in the vessel and thereby convert the chips to pulp, and (iv) discharging, e.g., emptying, the pulp and liquor from the vessel. The “cycle time” is the period required for steps (i), (ii), (iii) and (iv) of the process. The cycle is repeated to successively process “batches” of chips in the digester vessel. Between capping the digester vessel and discharging the pulp from the vessel, time is needed to extract liquor through screens of the vessel and add steam heat to the chips in the vessel. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0005]    A system and method has been conceived to reduce the cycle time of batch digester vessel. The cycle time is reduced by extracting liquor from the vessel while chips continue to flow into the vessel and prior to the capping of the vessel. The vessel is filled such that a chip level is established to be above the top of an extraction screen in the vessel. Liquor is extracted through the extraction screen while the chips slurry continues to fill the batch digester vessel. Optionally, cooking liquor and steam may be added to the digester vessel while liquor is extracted from the vessel and while the chip slurry continues to fill the vessel. Cooking liquor and steam may also be added to the vessel after the desired volume of chips have been added to the vessel and the slurry stops filling the vessel. The described filling method may result in a shorter cycle time than the cycle time of conventional batch digester operations. 
         [0006]    Reducing the cycle time allows for an increase in the volume of chips that may be processed by a batch digester vessel during a given period. Similarly, reducing the period during which chips are retained in a digester vessel during each cycle may allow for reduced cycle times and smaller digester vessels to achieve a desired flow rate of chips through a batch digesting system. 
         [0007]    Liquor is added to the chips in the chip feed system to transport the chips (as a chip slurry) through the conduits, e.g., pipes, of the chip feed system. The proportion of liquor to chips in the chip slurry needed to transport the chips is substantially greater than the proportion of liquor to chips needed during cooking. The ratio of liquor to chips tends to be lower when cooking the chips in the vessel than is the ratio used to transport chips to the vessel. To reduce the liquor to chip ratio liquor is removed from the batch digester vessel. It is conventional to start the removal of liquor from a batch digester vessel after the vessel is capped. What is not conventional is to start the removal of liquor before the vessel is capped and while chips are being fed into the vessel. 
         [0008]    The cycle time for a batch digester vessel may be reduced by removing, e.g., extracting, liquor as the vessel is filled with chips and before the vessel is capped to start of the digesting operation. The extraction of liquor during filling may begin after a minimum level of chips is established in the digester vessel. The minimum chip level may be at an elevation in the vessel at or above the top of the extraction screen. 
         [0009]    Extracting liquid from the batch digester after establishment of minimum chip level and while chips continue to flow into the vessel allows for the liquid level to be drawn down in the vessel. As the liquid level is drawn down, space is formed in the batch digester vessel to add cooking liquor after the chips have been transported to the vessel. 
         [0010]    The chemical composition of liquor used to transport chips, e.g., water, may be different than the chemical composition of liquor used to cook the chips in the vessel. If water or other non-cooking liquor is used to transport the chips to the vessel, cooking liquor is added to the vessel. In an embodiment of the present invention, cooking liquor may be added to the vessel before the vessel is capped. 
         [0011]    Cooking liquor may be added to the chips in the chip feed system and used to transport the chips to the vessel. If cooking liquor is used as the transport liquor, the extraction of liquid from the batch digester vessel during the filling of the vessel may be regulated to ensure that sufficient cooking liquor remains in the vessel. Further, steam may be added to the chips in the chip feed system to start the heating the chips prior to the capping of the batch digester vessel. This early addition of steam may also shorten the cycle time. 
         [0012]    A method has been conceived for chemically digesting cellulosic fibrous material, the method comprises: forming a slurry of cellulosic fibrous material; optionally adding heat energy (via steam for example) to the cellulosic fibrous material or the slurry; transporting the heated slurry through a high pressure transfer device and into a batch digester vessel; filling the batch digester vessel with the slurry to establish a minimum chips level; extracting transport liquor from the batch digester; optionally replacing the transport liquor with cooking liquor and when appropriate stopping extraction of the transport liquor from the batch digester vessel to provide sufficient cooking liquor in the batch digester vessel; ceasing the filling of the digester vessel and thereafter converting the cellulosic material in the vessel to a pulp, and discharging the pulp from the batch digester vessel before restarting the transporting of the slurry into the digester vessel. 
         [0013]    A method for chemically digesting cellulosic fibrous material has been conceived comprising: forming a slurry of cellulosic fibrous material; transporting the slurry through a high pressure transfer device and into a batch digester vessel and forming a chip level in the vessel; determining when the chip level exists above an extraction screen in the batch digester vessel; after establishing the chip level, extracting liquor from the batch digester vessel while the feeding continues of the slurry to the batch digester vessel; adding cooking liquor to the batch digester vessel; ceasing the transport of the slurry into the batch digester vessel and thereafter converting the cellulosic material in the vessel to a pulp, and discharging the pulp from the batch digester vessel before restarting the transporting of the slurry into the digester vessel. 
         [0014]    The steps of transporting the slurry of chips and liquor, filling the vessel with the slurry, converting the chips to pulp in the vessel and the discharging the pulp from the vessel are sequentially repeated. The high pressure transfer device may be one or more chip pumps which pressurizes the slurry in series or parallel as needed to meet the design of the system (meet static head requirements, flexibility for production turndown or increase, the need to feed multiple vessels, or other design specific requirements). The step of adding heat may include adding steam heat to the cellulosic material or slurry in a chip feed system coupled to the digester vessel and including the high pressure transfer device. The steam heat may be recovered heat from the extracted cooking liquor. The ratio of liquor to cellulosic material (L/W by weight) may be in a range of five to eight as the slurry flows through the high pressure transfer device, and in a range of three to five during the conversion of the cellulosic material to the pulp in the vessel. 
         [0015]    A method has been conceived for chemically digesting cellulosic fibrous material comprising: adding a liquor to cellulosic fibrous material to form a slurry in a chip feed system; transporting the slurry from the chip feed system to a batch digester vessel; determining when a chip level rises above an extraction screen in the batch digester vessel; after the chip level rises above the extraction screen, extracting liquor from the batch digester vessel while the transport continues of the slurry into the batch digester vessel; ceasing the transport of the slurry into the batch digester vessel and thereafter converting the cellulosic material in the vessel to a pulp, and discharging the pulp from the batch digester vessel before restarting the transporting of the slurry into the digester vessel. 
         [0016]    The method may include adding heat energy to the cellulosic fibrous material or the slurry before the slurry is transported into the batch digester vessel. A cooking liquor may be added to the slurry before the slurry enters the batch digester vessel. 
         [0017]    Liquor may be extracted from the batch digester vessel during the transport of the slurry into the vessel. The extracted liquor may be used to heat the liquor added to the chip feed system. Steam heat may be added to the cellulosic material or slurry in the chip feed system. The steam heat is added at a lower end of a chip tube coupled to an inlet of a high pressure transfer device. 
         [0018]    A liquor to cellulosic material ratio on a weight basis may be in a range of five to eight as the slurry flows through a high pressure transfer device of the chip feed system, and in a range of three to five during the conversion of the cellulosic material to the pulp in the batch digester vessel. 
         [0019]    A method has been conceived for batch digesting method for pulping cellulosic fibrous material, the method comprising: adding a liquor to cellulosic fibrous material to form a slurry in a chip feed system; transporting the slurry from the chip feed system to an upper inlet to a batch digester vessel; as the slurry is being transported into the batch digester vessel and after a liquid or chip level in the vessel rises above an extraction screen in the vessel, extracting liquor from the batch digester vessel; ceasing the transport of the slurry into the batch digester vessel; while the transport of the slurry is ceased, converting the cellulosic material in the vessel to a pulp, and discharging the pulp from the batch digester vessel. The steps of transporting, ceasing and converting are performed sequentially and separately, and may be performed repeatedly. 
         [0020]    Heat energy may be added to the cellulosic fibrous material or slurry before the slurry is transported to the upper inlet of the batch digester vessel. In addition, a cooking liquor may be added to the material in an impregnation vessel of the chip feed system and before the slurry enters the batch digester vessel. Further, heat may be transferred from liquor extracted from the batch digester vessel to the liquor added in the chip feed system. In addition, cooking liquor may be added to the cellulosic fibrous material in the chip feed system and the batch digester vessel. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]      FIG. 1  is a schematic diagram of a chip feed and steaming system coupled to a batch digester vessel for chemically processing cellulosic fibrous material, wherein the figures show a cycle of operation of the batch digester vessel. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0022]      FIG. 1  is a schematic diagram of a chip feed system  10  and a batch digester vessel  12  for chemically processing cellulosic fibrous material. The batch digester vessel  12  operates in a repeating sequence of filling, cooking and discharge. 
         [0023]    During the filling step, chips from the chip feed system flow into a top inlet  46  of the vessel. Cooking liquor  14  or transport liquor  60  flows with the chips into the vessel. Cooking liquor may also be added separately to the vessel during the filling step. In addition, steam  20  may be added via conduit  40  to the vessel during the filling step. The flow of chips into the vessel ceases at the end of the filling step. 
         [0024]    The cooking step follows the filling step. After the top inlet is capped, e.g., sealed, the cooking step involves impregnating the chips in the vessel with the cooking liquor, maintaining the chips under conditions, e.g., 5 to 10 kilogram/centimeter squared (kg/cm 2 ) and at elevated temperatures of 140 to 200 degrees Celsius (° C.), favorable to cooking of cellulosic fibrous material. During the cooking step, the cooking chemicals, e.g., alkaline solutions of sodium sulfate (kraft process) or sodium hydroxide, delignify the chips and allow for separation of the fibers in the chips to produce pulp. After the cooking step, the pulp  54  is discharged from the vessel during the discharge step. 
         [0025]    Chips  22  may be supplied to the chip feed system. For example, the wood yards of conventional pulp mills store wood chips  22  in open chip piles or in chip storage silos. Chips  22  are conveyed by conventional means, e.g., a conveyor or front-end loader (not shown), to a chip bin  24  of the chip feed system. 
         [0026]    The chip bin  24  may be a DIAMONDBACK® chip bin marketed by the Andritz Group and as disclosed in U.S. Pat. No. 5,000,083, or other conventional chip storage vessel. The chip bin may have near an outlet a one-dimensional convergence and side relief and a rotary or vibratory discharge mechanism. The chip bin  24  may have an upper inlet connected to an airlock  26  which monitors and controls the flow of chips into the bin. The conveyor for the chips  22  feed the chips to the inlet of the airlock. If the chip bin is pressurized, a vent  28  at the top of chip bin regulates the pressure in the bin, such as in a range of 5 to 10 kg/cm 2 . 
         [0027]    The chip bin  24  may be operated at atmospheric or super-atmospheric pressure, for example at 0.1 to 5 bar. If the chip bin is operated at super-atmospheric pressure, a pressure isolation device  30 , e.g., a rotary valve low pressure feeder, may be located at the chip inlet (possibly instead of the air lock) of the chip bin to prevent the release of pressure from the bin. The low pressure feeder may be a star-type isolation device or a screw-type feeder having a sealing capacity. The pressure isolation device isolates the pressurized chip bin from the unpressurized chip supply  22  that is at atmospheric pressure. 
         [0028]    The chip bin  24  discharges to a metering device for example a Chip Meter sold by Andritz Group or other screw-type metering device. The metering device  32  may discharge chips to a pressure isolation device  36  and thereafter to an optional chip impregnation vessel  34 . 
         [0029]    White cooking liquor  14  may be added to one or more of the chips in the chip bin  24 , pressure isolation device  36 , impregnation vessel  34  and batch digester vessel via central liquor pipe  64 . The cooking liquor may be, by way of examples, kraft white, black or green liquor. White cooking liquor  14  may be a strongly alkaline solution including sodium hydroxide and sodium sulfide used for Kraft pulping. As an alternative to or in addition to adding cooking liquor to the chip bin, cooking liquor may be added to a chip impregnation vessel that temporarily holds the chips at atmospheric pressure or at an elevated pressure. The addition of white liquor to the chip feed system via conduits  52  starts the process of impregnation of the chips with the cooking liquor before the chips enter the batch digester vessel  12 . 
         [0030]    To feed the chips to the vessel, liquor, such as transport liquor  60 , e.g., water, and white cooking liquor  14 , is injected in the chip feed system to establish a slurry. Transport liquor  60  may be water, for example. The ratio of liquor to chips, e.g., cellulosic material, on a weight basis may be in a range of five to eight for transporting the chips to the vessel. The ratio of the liquor to chips may be in a range of three to five during the cooking process in which the cellulosic material is converted to the pulp. In view of the higher ratio of liquor to chips for transport as compared to cooking, a substantial amount of liquor is removed from the chips after they have entered the vessel. 
         [0031]    A portion of the liquor added in the chip feed system may be liquor extracted from the batch digester vessel, such as through screens  16  or a top separator  18  in the vessel. Conduit  62  transports extracted liquor from the vessel  12  to the chip feed system, such as to the discharge of the chip bin  24 . 
         [0032]    Extraction of liquor from the batch digester vessel occurs during the chip filling step and starts after the chip level  61  in the vessel  10  has risen above the top of the extraction screens  16 . Liquor is extracted while chips continue to be fed into the vessel. The extraction of the liquor as the batch digester vessel is filled with a desired mass of chip allows the liquid level in the vessel to be drawn down prior to the addition of cooking liquor. Optionally, while liquor is extracted from the screens  16  into conduit  62 , white cooking liquor  14  may added to the vessel. Further, liquor may be extracted and additional cooking liquor added during the cooking step. 
         [0033]    Steam  20  may be injected via conduit  40  to the chips in the feed system  10  to provide heat energy to the chips and increase the temperature of the chips to or near a cooking temperature, e.g., between 140° C. to 180° C., before the chips enter the batch digester vessel. By adding liquor and optionally steam to the chips in the chip feed system, the process time may be reduced in the batch digester vessel as compared to the conventional approach of adding steam after the chips enter the batch digester vessel and extracting liquor only after the vessel has been capped for the cooking step. Steam  20  may also be added directly to the batch digester vessel via conduit  41   
         [0034]    The period during which the chips are impregnated in the impregnation vessel  34  depend on the level of chip impregnation desired to be reached before the chips are fed to the batch digester vessel. The period of chip impregnation may be very short, e.g., 0.1 second (as the chips enter the digester vessel) to long, such as a two (2) hour retention period in the impregnation vessel  34 . If the period for chip impregnation is shorter than the period in the cycle for cooking in the digester vessel, the cooking liquor may be added to the impregnation vessel after the chips fed to the impregnation vessel and at a time selected to achieve the desired impregnation period before the chips are fed to the digester vessel. The impregnation vessel may be large if needed to retain the chips in the vessel for a period longer than the period in the cycle for cooking in the digester vessel. The addition of cooking liquor in the chip feed system may also be accomplished without an impregnation vessel such as by adding the liquor to the chip bin  24  via conduit  40 , to the chips in prior to the pumps  44  coupled to conduit  42  or to another component of the chip feed system such as the pressure isolation device  36 . 
         [0035]    Steam, such as fresh steam  20  or flashed steam  38  produced from the evaporation of waste liquor, may be added to chip bin  24 , to the chip impregnation vessel  34  or to other sections of the chip feed system  10 . Steam conduits  40  direct the steam from the steam supply  20  of fresh steam or flashed steam  38  from a flash tank  39  to one or more components, e.g., chip bin  24  and impregnation vessel  34 , of the chip feed system  10 . 
         [0036]    The steam  20  added in the chip feed system increases the temperature of the chips before entering the digester vessel and thereby reduces the period needed to heat the chips in the batch digester vessel. The steam also facilitates removal of air from the chips. By way of example, the steam may be added to the chip feed system moments before the chips enter the digester vessel or as much as 2 hours before the chips enter the digester vessel. 
         [0037]    A slurry of chips and liquor is fed through conduits  42  from the chip bin  24 , the chip impregnation vessel  34  (if present), the pressure isolation device  30  or  36  to a high pressure transfer device  44 , such as one or more chip pumps, such as disclosed in U.S. Pat. No. 5,753,075, the entirety of which is incorporated by reference. These pumps, as described in U.S. Pat. No. 5,753,075 may be arranged in series or parallel as needed to meet specific design criteria, such as the necessary head for the pumps, volume of material to pump, and flexibility in operations of the one or multiple batch digesters. Another example of a high pressure transfer device is disclosed in U.S. Pat. No. 5,236,285, the entirety of which is incorporated by reference. The pressure isolation device may include a chip tube that receives at an upper inlet the chips and a lower outlet that feeds the chips to a pump. The chip slurry is pressurized in the high pressure transfer device  44  to a pressure substantially equal to the pressure in the digester vessel  12 . 
         [0038]    From the high pressure transfer device  44 , the chips and liquor slurry flows to a top inlet  46  of the digester vessel. The chips and liquor flow into the digester vessel through the top inlet and fill the digester vessel. The chips and liquor may be added simultaneously to the digester in a chip slurry. 
         [0039]    The quantity of liquor added the chip feed system  10  to transport the chips from the chip bin  24  to the digester vessel  12  may be in a range of 5 to 8 times the quantity of chips by weight (that is a liquor to wood ratio of 5:1 to 8:1, L/W of 5:1 to 8:1 or L/W of 5 to 8 by weight). This range of L/W of 5 to 8 of liquor to chips is particularly suitable where the high pressure transport device is one or more chip pumps. 
         [0040]    The quantity of liquor needed for treating, e.g., cooking, the chips in the batch digester vessel is typically 3 to 5 times liquor to chips (a L/W ratio of 3 to 5). The quantity of liquor needed for chip treatment in the digester vessel may be less than the quantity of liquor needed to transport the chips through the chip feed system, particularly if the chip pumps form the high pressure transfer device. 
         [0041]    Liquor can be extracted from the digester vessel through screens  16  that may extend partially or fully around the circumference of the vessel and be at a mid-elevation of the vessel. The positioning and arrangement of the screens is a matter of design choice for the designer of the digester vessel. Alternative types of screening devices for extracting liquor from the digester vessel include a top separator  18  that may be included in the vessel at the top inlet  46 . These screening devices extract liquor from the chip slurry in the digester vessel while blocking chip fibers from leaving the vessel. Liquor may be extracted from the screens  16  separately from the top separator  18 . Liquor may be extracted from screen  16  after the liquor level in the vessel  10  rises above the screen  16 , even though the top separator is not immersed in liquor. 
         [0042]    Some of the liquor may be extracted via screen as the chip slurry fills the digester vessel and before the start of the formal batch cooking process. As the chip slurry flows through the top inlet  46  and accumulates in the digester vessel, the slurry will fill the vessel and form a liquor liquid level and a chip pile in the vessel. When the chip pile level  61  exceeds the elevation of the screens  16 , a portion of the liquor may be extracted through the screens. 
         [0043]    The extracted liquor may be used to heat, via a heat exchanger  56 , the white liquor  14  that supplies liquor to conduits  52  connected to one or more of the chip bin  24 , impregnation vessel  34  and other components in the chip feed system  10 . If a top separator  18  is present, a portion of the liquor may be extracted as the chip slurry flows through the inlet  46  and into the vessel. The amount of liquor removed as the chip slurry fills the vessel may be such that the amount of liquor remaining in the vessel is sufficient for cooking the chips, e.g., in a range of L/W of 3 to 5 liquor to chips weight ratio. 
         [0044]    The extraction of liquor is started before the chip slurry has fully filled the batch digester vessel  12 . The extracted liquor may be replaced by cooking liquor. In particular, the transport liquor may be extracted and drawn down to allow replacement by cooking liquor once the extraction has stopped. By extracting liquor before the formal cooking process is started in the batch digester vessel, the time taken in reducing the liquor amount to the that needed for cooking does not delay (or at least minimizes any delay) the start of the formally cooking process. Additional time is saved in the batch digesting cycle by filling the digester vessel with chips and liquor simultaneously, rather than by the conventional sequential processes of separately adding chips and liquor to the batch digester. Cycle time may also be saved by preheating the chips in the chip feed system rather than waiting to heat the chips after they fill the vessel. 
         [0045]    After the batch digester vessel  12  has received an appropriate or specified volume or amount of chips, the vessel is capped such that the flow of chips and liquor into the vessel ceases. After the vessel has been capped, the vessel digests, e.g., cooks, the chips using the cooking liquor, heat and pressure to dissolve or remove lignin from the chips and convert the chips to pulp. After completion of the digesting process, the pulp  54  is discharged from the vessel for further processing. 
         [0046]    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.