CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]    This application is a continuation-in-part of parent U.S. patent application Ser. No. 10/283,369 filed on Oct. 30, 2002, and is based on, and claims domestic priority benefits under 35 USC §119(e) from, U.S. Provisional Application Serial No. 60/334,963 filed on Dec. 4, 2001, the entire content of each prior filed application being expressly incorporated hereinto by reference. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention is related generally to chemical wood pulping processes and systems. In particular, the present invention relates to processes and systems for handling of knots in such chemical wood pulping processes.  
         BACKGROUND OF THE INVENTION  
         [0003]    Screw-type chip meters are typically used to determine the chip feed rate to a digester employed in chemical wood pulping processes. The chip meters therefore are employed to feed uncooked wood chips in a controlled manner to the downstream pre-cooking and cooking operations, for example, chip steaming, impregnation vessels and/or digester vessels. A chip chute typically is employed to convey the uncooked wood chips to such downstream operations.  
           [0004]    Conventionally, knots present in the wood chips are typically removed by screening following at least some cooking. Thus, in many pulp mills, a drainer is provided so as to remove the knots from the pulp, such as the system suggested in U.S. Pat. No. 3,886,035 1 . Such knots may then be refined or re-cooked so as to minimize waste and to recover any fiber content that may be present therein. (See, U.S. Pat. No. 4,002,528.) Conventional knot recovery techniques typically involve returning the knots separated from the pulp to the chip bin which requires expensive recycling equipment. Alternatively, knot drainers may be located at the top of the chip bin, but such an arrangement requires excess energy be used to pump all the filtrate carrying the knots recovered after cooking back to the chip bin for reprocessing.  
           [0005]    Another knot-recycling technique has recently been proposed in U.S. Pat. No. 5,672,245. In this regard, the &#39;245 patent proposes to separate knots by means of a screen downstream of the pulp digesters and then redirect such separated knots, following their treatment in a knot bin with black liquor, by pumping the knot suspension to the chip chute upstream of the high pressure feeder.  
         SUMMARY OF THE INVENTION  
         [0006]    According to the present invention, novel processes and systems are provided for the return of knots removed by conventional means, such as screening in the knotter or screen room, directly (without pumping) to an essentially atmospheric pressure feed system prior to being cooked in the digester operations associated with a chemical wood pulping process. More specifically a flow of undercooked wood chips is metered into a chip vessel or chute without pumping upstream of the digester operations. The processes and systems of the present invention will involve the return of the knots to the chip feed system by feeding the knots into the chip handling system operating at essentially atmospheric pressure, instead of the conventional technique of returning the knots to a higher elevation chip bin. Knots from the knotter or screen room are most preferably returned to the chip feed system through a knot drainer or separator associated operatively with the chip screw or chip chute.  
           [0007]    The present invention is therefore especially well suited for use with current Lo-Level® Feed Systems commercially available from Andritz Inc. of Glens Falls, N.Y. (See also, U.S. Pat. Nos. 5,476,572; 5,700,355; 5,968,314; 5,766,418; 6,368,453, and 6,436,233.)  
           [0008]    The use of the present invention should result in lower energy costs by allowing the knot drainer or separator to be placed at a lower elevation (e.g., attached physically to a chip screw or chip chute instead of the higher elevation top of the chip bin). Additional energy savings are incurred by eliminating the need for the pressurization of the knot stream to the pressure of the conventional feed system.  
           [0009]    These and other aspects and advantages will become more apparent after careful consideration is given to the following detailed description of the preferred exemplary embodiments thereof. 
       
    
    
     BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS  
       [0010]    Reference will hereinafter be made to the accompanying drawing FIGURE which is depicts one particularly preferred system for treating knots in accordance with the present invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0011]    As shown in the accompanying drawing FIGURE, one particularly preferred system in accordance with the present invention includes a feed system  10  for introducing, steaming, slurrying and pressurizing comminuted cellulosic fibrous material, for example, hardwood or softwood chips, and feeding the slurry to a continuous digester system (not shown). These systems are disclosed in U.S. Pat. Nos. 5,476,572; 5,622,598; 5,635,025; 5,700,355; 5,766,418; and 5,968,314 and are marketed under the trademark LO-LEVEL® by Andritz Inc. of Glens Falls, N.Y.  
         [0012]    Though comminuted cellulosic fibrous material may take many forms, including sawdust; grasses, such as straw or kenaf; agricultural waste, such as bagasse; recycled paper; or sawdust, for the sake of simplicity, the term “chips” will be used when referring to comminuted cellulosic fibrous material; but any and all of the listed materials, and others not listed, may be processed by the present invention. Also, though a continuous digester may be referenced below and in the accompanying FIGURE, it is understood that the present invention as also applicable to feeding several continuous digesters or one or more discontinuous or batch digesters.  
         [0013]    As shown in the FIGURE, chips  13  are introduced to the system, for example, via a conveyor (not shown) from a chip storage facility, for example, a woodyard, via an isolation and metering device  14 . For example, the FIGURE illustrates a screw-type isolation device  14  as described in U.S. Pat. No. 5,766,418. The device  14 , driven by an electric motor (not shown), introduces the chips to chip retention and steaming vessel  16 . Though various types of vessels are known in the art, vessel  16  is preferably a DIAMONDBACK® Steaming vessel as marketed by Andritz Inc. and described in U.S. Pat. Nos. 5,500,083; 5,617,975; 5,628,873; and 4,958,741, or a CHISELBACK™ vessel marketed by Andritz Inc. as described in U.S. Pat. No. 6,199,299. This vessel typically includes a gamma-radiation level-detection system, a regulated vent for discharging gases which accumulate in the vessel and one or more steam introduction conduits  16 ′. The pressure in the vessel  16  may be slightly below atmospheric pressure or slightly above atmospheric pressure, that is, the pressure in vessel  16  may vary from about −1 to 2 bar gage (that is, about 0 to 3 bar absolute).  
         [0014]    During treatment with steam in vessel  16 , the air that is typically present in the chips is displaced by steam and the heating of the chips is initiated. The removal of air from the cavities within the chips permits the more efficient diffusion of cooking chemical into the chip and minimizes the buoyant forces on the chip during subsequent processing.  
         [0015]    The steamed material is discharged from the bottom of the vessel  16  to a metering device  17 , for example, a star-type metering device or Chip Meter as sold by Andritz Inc., though any type of metering device may be used. The metering device  17  is typically driven by an electric motor (not shown) and the speed of rotation of the metering device is typically controlled by operator input to define a set rate of introducing chips to the system. The chips discharged by the metering device  17  are introduced to a vessel in the form of a vertical conduit or pipe  18 , for example, a Chip Tube sold by Andritz Inc. Cooking chemical and other liquids are typically first introduced to the chips in conduit  18  by means of one or more conduits  19  such that a level of liquid is established in conduit  18  and a slurry of chips and liquid is present in the bottom of conduit  18 . This level of liquid is typically monitored and controlled by a level detection device, for example, a gamma-radiation level detection device or a “d-p” cell. The metering device  17  typically does not act as a pressure isolation device, though it may, and the pressure in conduit  18  typically varies from 0 to 2 bar gage (or 1 to 3 bar absolute).  
         [0016]    Conduit  18  discharges the slurry of chips and liquid by means of a radiused section  20  to the inlet of slurry pump  21 . Though any slurry pump can be used, pump  21  is preferably a Hidrostal® screw centrifugal pump sold by Wemco Pump of Salt Lake City, Utah or a pump provided by Lawrence Pumps Inc. of Lawrence, Mass. Slurry pump  21 , driven by electric motor  21 ′, pressurizes and transfers the slurry in conduit  18  via conduit  22  to the low pressure inlet  23  of a high pressure transfer device  24 . This high pressure transfer device is preferably a High-pressure Feeder as sold by Andritz Inc. High-pressure feeder  24  includes a pocketed rotor mounted in a housing typically having a low-pressure inlet  23 , a low-pressure outlet  25 , a high-pressure inlet  26  and a high-pressure outlet  27 . The low-pressure outlet  25  typically includes a screen plate (not shown) which minimizes the passage of chips out of low-pressure outlet  25  while allowing the liquid in the slurry to pass out outlet  25  to conduit  28 , though as disclosed in U.S. Pat. No. 6,199,299, the screen in the low-pressure outlet of feeder  24  may be omitted. The chips which are retained in the feeder by the screen are slurried with high-pressure liquid provided by pump  29 , preferably a Top Circulation Pump (TCP) provided by Andritz Inc., to inlet  26  via conduit  30 . The slurry is discharged out of high-pressure outlet  27  into conduit  31  and to the digester  32  of digester system  12  at a pressure of between about 5 and 15 bar gage, typically between about 7 to 12 bar gage.  
         [0017]    The digester (not shown) may be a single or multiple-vessel digester and may be a hydraulic or steam-phase digester. The digester may also consist or comprise one or more batch digesters. The cellulose material with added cooking chemical is treated under temperature and pressure in the digester and essentially fully-treated chemical cellulose pulp is discharged into a conduit at the bottom of the digester. Though many types of processes may be performed in the digester, preferred processes includes those described in U.S. Pat. Nos. 5,489,363; 5,536,366; 5,547,012; 5,575,890; 5,620,562; 5,662,775; 5,824,188; 5,849,150; 5,849,151; 6,086,712; 6,132,556; 6,159,337; 6,280,568; and 6,346,167 and marketed by Andritz Inc. under the trademark LO-SOLIDS®. The process performed in the digester may also be one of the processes disclosed in U.S. Pat. Nos. 5,635,026 or 5,779,856 and marketed under the name EAPC™ cooking by Andritz Inc.  
         [0018]    As shown in the FIGURE, excess liquor in the slurry in conduit  31  at the top of the digester is separated from the slurry by a liquor separator and returned to the feed system  10  by means of conduit  34 . The liquid in conduit  34  is pressurized by pump  29 , driven by electric motor  29 ′, and provides the pressurized slurrying liquid introduced to the high-pressure inlet  26  of feeder  24  via conduit  30 . Feeder  24  is typically driven by an electric motor (not shown), the speed of which is monitored and controlled.  
         [0019]    As shown in the FIGURE, the liquid discharged from the low-pressure outlet  25  of high-pressure feeding device  24  passes via conduit  28  to a cyclone-type separator  35  which isolates undesirable material and debris, such as sand, stones, etc., from the liquid in conduit  28 . Separator  35  is preferably a Sand Separator as sold by Andritz Inc. Liquid having little or no undesirable material or debris is discharged from separator  35  and is passed through a liquor separating device  37  via conduit  36 . At least some liquid is removed from the liquid separator  35 , which is preferably an Inline Drainer as sold by Andritz Inc., via conduit  38  and sent to vessel  39 . Vessel  39  is preferably a Level Tank as sold by Andritz Inc. Liquid is discharged from vessel  39  to conduit  40  and pump  41  and is supplied to the digester as liquor make-up as needed via conduit  42 . Pump  41  is preferably a Make-Up Liquor Pump (MLP) as sold by Andritz Inc. The sand separator  35 , level tank  36 , and in-line drainer  37  can be omitted without interfering with the ultimate function of the feed system  10 .  
         [0020]    The liquid discharged from separator  37  into conduit  43  may be supplemented with cooking chemical, for example, kraft white, green, orange (that is, liquid containing polysulfide additives) or black liquor, prior to being introduced to tank  45 . Tank  45  is preferably a Liquor Surge Tank as sold by Andritz Inc. and described in U.S. Pat. No. 5,622,598. The cooking chemical may be heated or, preferably, cooled as needed by a heat exchanger (not shown). Some of the liquid in conduit  43  may bypass tank  45  and be introduced via conduit  19  to conduit  18  as described above. Tank  45  communicates with conduit  18  and the inlet of pump  21  via conduits  47  and  20 . As disclosed in U.S. Pat. No. 6,368,453, tank  45  may comprise or consist of an integral vessel concentric with conduit  18 .  
         [0021]    Important to the present invention, a knot separator or drainer  50  is provided so as to supply knots via conduit  52  to the conduit  18  downstream of the metering device  17  without the use of a pump or other process equipment. Most preferably the knot separator  50  is a Model KW Secondary Knofter from Andritz Inc. The knot separator  50  separates knots from the inlet supply of slurried knots and liquor pumped from a primary knotter or a knot screener introduced via inlet conduit  54 . Within the knot separator  50  (operating at essentially atmospheric pressure), a vertically oriented screw-type conveyor is driven by motor  55  and lifts separated knots with sufficient liquid to allow the knots to flow into the discharge chute  56  connected to the conduit  52 . The separated liquor is transferred via chute  58  to chamber  60  and thereafter discharged via conduit  62 . Gas may be vented from the chamber  60  via conduit  64 .  
         [0022]    The present invention therefore involves the return of the knots directly, without pumping or other processing, to the chip feed system  10  by feeding the knots into the conduit  18  at under essentially atmospheric conditions.  
         [0023]    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. Thus, for example, instead of using a high pressure feeder, it is possible to use a transfer device consisting of pumps, such as a transfer device of the kind disclosed in U.S. Pat. No. 5,753,075 and U.S. Pat. No. 6,325,890.
Processes and systems are provided for cooking wood chips containing knots in a digester to produce a chemical pulp. Specifically, a low pressure slurry of comminuted cellulosic fibrous material is formed in a vessel operating at essentially atmospheric pressure by mixing therein a liquid and the comminuted cellulosic fibrous material. The low pressure slurry is thereafter supplied to a low pressure inlet of a high pressure transfer device while liquid is removed from the slurry through a low pressure outlet thereof. A high pressure liquid is supplied to the transfer device (e.g., via a slurry pump) so as to form a high pressure slurry of the comminuted cellulosic material which is then discharged from a high pressure outlet of the transfer device. Knots are removed via a knot separator from a slurry of undercooked knot-containing cellulosic material at essentially atmospheric pressure. The removed knots are thereafter transferred to the vessel so that the knots become part of the low pressure slurry supplied to the low pressure inlet of the transfer device.