Abstract:
A system and method for digesting cellulosic material to extract fermentable sugars, lignin, and pulp is disclosed. One embodiment comprises a continuous digester comprising a cellulosic material feed section including a pre-steam and impregnation zone, a sugar extraction zone, a lignin extraction zone and a cooking zone, the continuous digester to impregnate the cellulosic material with a mild acid solution and continuously digest the cellulosic material to extract fermentable sugars, lignin, and pulp. Another embodiment comprises a method for receiving cellulosic material in a continuous digester, removing air from the cellulosic material, impregnating the cellulosic material with a mild acid, hydrolyzing hemicellulose in the cellulosic material to fermentable sugars, extracting the fermentable sugars from the cellulosic material, cooking the cellulosic material to extract lignin from the cellulosic material, washing the cellulosic material in a hot alcohol wash, a hot water wash, and a cold water wash, and discharging pulp.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the priority of U.S. Provisional patent application Ser. No. 61/180,067, filed May. 20, 2009. 
     
    
     BACKGROUND 
       [0002]    The disclosed embodiments relate generally to the art of cellulosic material digesters, and more particularly to a continuous tube reactor to extract fermentable sugars, lignin and pulp from cellulosic material. 
         [0003]    In a typical continuous pulp digester, the wood chips and the white liquor are fed into the upper end of a vertically aligned digester, with the interior of the digester defining a cylindrical digesting chamber maintained at a relatively high pressure (e.g. 200 PSI) and high temperature (e.g. approximately 380.degree. F.). The mixture of chips and white liquor moves slowly and downwardly through the digester so that the total dwell time within the digester is generally between about two to four hours. During the period that the wood chips are in the digester, the white liquor reacts with the material in the wood chips to break down certain organic compounds in the wood chips so as to “delignify” the pulp. 
         [0004]    At several locations along the length of the digester, portions of the liquid are extracted, either to be re-circulated back into the digester, sent to an evaporator, or possibly to be processed elsewhere in the system. To retain the wood chips that are being processed in the digester, the liquid is extracted through sets of screens which are generally placed in sets at vertical locations circumferentially around the digester. 
       SUMMARY 
       [0005]    Accordingly, a system and method for continuous tube reactor is disclosed. 
         [0006]    One embodiment comprises a continuous digester comprising a cellulosic material feed section including a pre-steam and impregnation zone, a sugar extraction zone, a lignin extraction zone and a cooking zone, the continuous digester to impregnate the cellulosic material with a mild acid solution and continuously digest the cellulosic material to extract fermentable sugars, lignin, and pulp. Another embodiment comprises a method for receiving cellulosic material in a continuous digester, removing air from the cellulosic material, impregnating the cellulosic material with a mild acid, hydrolyzing hemicellulose in the cellulosic material to fermentable sugars, extracting the fermentable sugars from the cellulosic material, cooking the cellulosic material to extract lignin from the cellulosic material, washing the cellulosic material in a hot alcohol wash, a hot water wash, and a cold water wash, and discharging pulp. 
         [0007]    This Summary is provided to introduce concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. Furthermore, the claimed subject matter is not limited to implementations that solve any or all disadvantages noted in any part of this disclosure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  shows an isometric view of one form of a digester. 
           [0009]      FIG. 2  shows a schematic view of a continuous tube reactor related to a lignin and ethanol plant. 
           [0010]      FIG. 3  shows a schematic view of a continuous tube reactor related to a bleached pulp mill. 
           [0011]      FIG. 4  shows a conventional Kraft process. 
           [0012]      FIG. 5  shows a sectional view of a tube reactor showing the inlet and extraction perforations that cooperate with the extraction rings. 
           [0013]      FIG. 6  shows a plurality of extraction rings. 
           [0014]      FIG. 7  shows fluid flow across a reactor tube at various porosity levels. 
           [0015]      FIG. 8  shows the energy efficiency of continuous cooking versus batch cooking. 
           [0016]      FIG. 9  shows a schematics view of a tube reactor. 
           [0017]      FIG. 10  shows a side cross-sectional view of a positive displacement pumping system. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    Disclosed embodiments illustrate aspects of a continuous tube reactor. In some embodiments, a continuous tube reactor  100  converts higher grade cellulosic feedstock (material) in a relatively continuous process to fermentable sugars for ethanol production, to relatively chemical free lignin for products conventionally made from fossil base material, and to pulp for paper making and other products. In some embodiments, a continuous tube reactor converts lower grade cellulosic material into lignin and fermentable sugars. 
         [0019]    In some embodiments, a continuous tube reactor may use a multi-stage chemical “cracking” process to split cellulosic feedstock into three commercial end-products, fermentable sugars for ethanol production, lignin for plastics and other conventionally fossil-fuel-base products, and pulp for paper making. Additionally, this cracking process may be accomplished with a single pressure vessel and may use existing auxiliary equipment. In this way, a continuous tube reactor may provide transformational harvesting and use of sustainable renewable cellulosic materials, transformational production of alternative fuels, transformational production of pure lignin and products thereof, and transformational production of pulp, as will be described in more detail in the following detailed description with reference to the attached figures. 
         [0020]      FIG. 1  shows an isometric view of one form of a digester. In some embodiments, a continuous tube reactor may comprise a continuous wood chip or other cellulosic material (feedstock) feed section, a pre-steaming and impregnation section where air is removed and the feedstock impregnated with mild acid solution, and the actual digester sections. In some embodiments all section of the continuous tube reactor may have a tubular shape. 
         [0021]    With reference to the example embodiment illustrated in  FIG. 1 , an  8 ″ diameter pilot-scale continuous tube reactor  100  may be about 110 ft long and have 47 inlet/extraction rings. In contrast, conventional pulp digesters have from 3 to 6 inlet/extraction rings. The additional inlet/extraction rings allow a very accurate process control. The embodiment in  FIG. 1  shows a 24 ft long section consisting of second and third cooking zones and an end wash zone in front at left, with a 21 ft long section of a mild acid hydrolysis and first cooking zones behind it. Further behind these sections there are three twenty feet long tube sections that form 60 ft long chip feed, pre-steaming and impregnation sections. The cellulosic feedstock is processed in this embodiment pilot-scale tube reactor in a continuous manner. 
         [0022]    In the present embodiment, all process auxiliary tanks, blow tank for pulp discharge, sugars and lignin concentrators, lignin recovery centrifuge, air compressor, and vacuum evaporation equipment and condensers for ethanol recycling are mounted on a 32 ft long goose-neck trailer. This allows a pilot-scale continuous tube scale reactor to be transported and re-assembled within a relatively short time, such as a week or two from a pulp mill, saw mill or ethanol plant to another to allow multiple end-users to experiment with their own feedstock, laboratories, and in case of ethanol facilities, with their own fermentation and distillation processes. 
         [0023]    For high-grade woody cellulosic feedstock some embodiments of a continuous tube reactor may have six process zones for unbleached pulp, including a zone to hydrolyze hemicelluloses to fermentable sugars and extract them, three cooking zones to dissolve and extract lignin in three stages, a three-stage wash zone consisting of hot alcohol wash, hot water wash, and cold water wash, and a pulp discharge zone. In some embodiments, for bleached pulp production one or more bleaching zones may added between cooking zones and an end wash zone. 
         [0024]    In one example embodiment, cellulosic material may be converted to fermentable sugars, lignin and pulp in one single pass through a horizontal tubular pressure vessel using a solution comprising 60% ethanol and 40% water for a transportation and process liquid. For example, an embodiment process liquid may be a mixture of around 60% ethanol and 40% water at 350°-400° F. and a process pressure of 300 psig. In this example, each of the process zones may have several process liquid inlet/extraction rings to provide heat and process liquid input and dissolved organic matter extraction as well as process consistency and porosity control within the entire length of the reactor, as will be explained more fully in the following detailed description. 
         [0025]      FIG. 2  shows a schematic view of an embodiment continuous tube reactor  200  related to a lignin and ethanol plant. For low-grade woody  204  and any grassy  202  cellulosic feedstock for production of lignin  234  and ethanol  248 , as shown in  FIG. 2 , the continuous tube reactor  200  may have nine process zones. For example, continuous tube reactor  200  may have two zones to hydrolyze hemicelluloses to fermentable sugars and extract them as shown to the left of block  206  and in the middle of block  206 , and two zones to hydrolyze cellulose to fermentable sugars and extract them as shown in the sections of block  206  immediately to the right of the hemicellulose hydrolization sections. Additionally, continuous tube reactor  200  may include four cooking zones to dissolve and extract lignin in four stages as shown in block  206 , and a residual fibrous matter discharge zone as shown on the far right of block  206 . 
         [0026]    In the present embodiment continuous tube reactor  200 , the continuous tube reactor will process almost all of the feedstock into lignin  234  and fermentable sugars without any pulp production except that a small percentage of the feedstock is left at the end of the reactor in a fibrous stage to facilitate a cross-flow extraction process of dissolved sugars and lignin. In this way, part of the residual fibrous matter can be recycled back to the reactor feed end, if desired, or all of it can be burnt to produce steam  212  and power  220  for the plant. Some of the residual fibrous matter has to be always burnt to get rid of the inorganic salts and ashes in the feedstock. 
         [0027]    As can be seen in  FIG. 2 , the process ethanol may be recycled through evaporation and condensation in a conventional manner. Additionally, ethanol may be burned in a bio-fuel boiler  210  with the residual blow-out matter will be made up by a corresponding amount from the fresh ethanol produced in a conventional manner by fermentation of the sugars and distillation of the freshly produced ethanol. The dissolved pure chemical free lignin  234  may then be recovered through evaporation, flash drying  230  and bag house  232  operation. 
         [0028]    Organic vapors from evaporation and blow tank will be burned in a conventional bio-fuel boiler  210  as well as the organic extractives from the impregnation liquid. In this way, small amounts of fresh make-up water may be used since much of the moisture in the feedstock can be recycled through evaporation, as shown in  FIG. 2 . Additionally, the acidity of the hydrolyzed sugar stream may be buffered with lime  224  before fermentation  222 . 
         [0029]      FIG. 3  shows a schematic view of a continuous tube reactor  300  related to a bleached pulp mill  FIG. 4  shows a conventional Kraft process  400 . We now demonstrate a reduction in energy-related emissions including greenhouse gases by comparing the conventional modern Kraft process  400  to the continuous tube reactor bleached pulp mill diagram in  FIG. 3 . 
         [0030]    The continuous tube reactor  300  eliminates the recovery boiler  438 , causticising  448  and lime kiln  452 , which are generally responsible for ¾ of the emissions and greenhouse gases of a Kraft process  400  mill. In this way, by combining the cooking, washing and bleaching processes into one continuous process inside multiple, relatively small diameter continuous tube reactors  300 , all the process liquids may be circulated counter-currently upstream until being extracted to an evaporation plant, and the emissions and greenhouse gases may be dropped to less than 10% of a Kraft process  400  mill&#39;s values. 
         [0031]    Additionally, a continuous tube reactor  300  uses very clean ethanol for cooking liquor instead of sodium and sulfur-containing Kraft process  400  chemicals, further allowing for reduced evaporation plant emissions. Also, a Kraft process  400  mill operates by burning lignin and other dissolved organic matter to recover costly cooking chemicals from ashes through causticising and lime kiln processes. 
         [0032]      FIG. 5  shows a sectional view of a tube reactor showing the inlet and extraction perforations  502  that cooperate with the extraction rings  506 .  FIGS. 5 and 6  relate to reducing the porosity of feedstock within a tube reactor that may be used in either a continuous or a batch process and are utilized in continuous tube reactor  900  as illustrated and described with reference to  FIG. 9 . 
         [0033]      FIG. 6  shows a partial sectional view of plurality  600  of extraction rings  662 ,  664 , etc. In  FIG. 6 , the fluid flow including feedstock is indicated by arrow  660  is to be passed through the tube reactor (not shown). In general, a tube reactor may be provided with openings therearound to communicate with the various passageways and the plurality  600  of fluid transport rings. 
         [0034]    With reference to  FIG. 6  in detail, in the right-hand portion of  FIG. 6  is a fluid passage ring  662  which is a packing ring. As described above, when it is desired to lower the porosity of the feedstock at any point along the entire tube reactor assembly, a positive displacement pump (as shown in  FIG. 10 ) or cylinder will release a known volume of fluid through a packing ring  662  while the main drive system of the entire unit is adding additional fluid to the entrance portion of a digester or continuous tube reactor. In other words, in a preferred form, the various fluid biasing members are positive displacement pumps or cylinders, and if one unit of water enters the digester at an entrance portion, a downstream packing ring can simultaneously extract one unit process liquid (volumetric unit), thereby controlling the compression of the feedstock at that particular location adjacent to that particular packing ring. 
         [0035]    As further shown in  FIG. 6 , an array of countercurrent displacement rings  664  may be utilized to extract dissolved organic matter from the feedstock at various stages throughout the tube reactor process. Such a detailed description of one form of a countercurrent flow is described in U.S. Pat. No. 5,680,995, which is incorporated by reference and is a patent invented by the same inventor of this application. One preferred method of passing fluid through countercurrent displacement rings  664  will be described below with reference to  FIG. 10 . 
         [0036]      FIG. 7  shows fluid flow across a reactor tube at various porosity levels. By way of illustration, if porosity (a measure of the void spaces in a material as a fraction, between 0-1, or as percentage between 0-100%, typically 0.01 for solid granite to more than 0.5 for peat and clay) is too high, a situation occurs as shown in the left-hand portion  710  and  740  where a displacement washing may flow in a non-uniform manner across a pipe cross section. For example, with reference to  710 , a displacement washing may flow from a typically 90 degree open section at 6 o&#39;clock to the typically 180 degree open section at 12 o&#39;clock, such that there is not a sufficient amount of resistance to the flow and whereby the flow is not complete and not traveling through the 4.30 to 3 and 7.30 to 9 o&#39;clock lateral regions. Essentially, the fluid takes a tunnel flow approach and does not provide adequate coverage of the feedstock due to too high of porosity of the feedstock. 
         [0037]    Now looking at  720  and  750  in  FIG. 7 , it can be seen that there is a lower porosity where the flow from 4.30 and 7.30 o&#39;clock to 3 and 9 o&#39;clock is in somewhat more of a steady stratified manner where the across-tube flow of the inlet displacing liquid is reaching the 3 and 9 o&#39;clock locations in a more evenly distributed fashion to extract the displaced liquid at the top. And now looking at  730  and  760 , it can be seen that the even lower porosity has a much more even cross-flow strata velocities to perform a more complete cross-sectional displacement of the fluid to be extracted. 
         [0038]    Therefore, having a lower porosity is advantageous for displacing the liquid contained within the digester with new fresh liquid from one peripheral location along the tube reactor.  FIG. 8  shows the energy efficiency of continuous cooking versus batch cooking. 
         [0039]      FIG. 9  shows a schematic view of a batch process embodiment of a continuous tube reactor  900 . In general, continuous tube reactor  900  includes a pre-impregnation section  910 , then a reactor section  950  and an end a washing section  980 . The pre-impregnation section  910  in one form can be filled up with feedstock, thereafter filled with water, where the leading plug  930  remains in place by fill water in the downstream sections  950  and  980 . A typical feedstock may be any cellulosic material, from woody materials to grasses, and other possible feed materials. After the feedstock is inserted, an end cap containing a trailing plug  932  may be fastened to the end of the tube  910 . Thereafter, water may be placed in the section  910  to fill the void spaces between feedstock particles. The trailing piston  932  has water pressure or otherwise a biasing force placed thereon to bias it. 
         [0040]    In one form, a biasing system include pumping water behind trailing plug  932 , whereby the pressure over the surface area creates a force which pressurizes the water between the leading and trailing plugs. In this way, leading piston  930  may remain intact as long as fill water in the reactor section  950  and washing section  980  is not relieved through a relief valve. 
         [0041]    In the embodiment illustrated in  FIG. 9 , various openings are provided along the tube  900 , thereby allowing liquid to escape in a controlled manner through positive displacement pumps or cylinders and leaving the feedstock therein. In this way, the ratio of water to the raw feedstock may be reduced, in turn lowering the feedstock porosity. 
         [0042]    After the feedstock porosity is adjusted to a defined level, the feedstock/water mixture may be heated to a temperature and then pre-impregnated with a water and chemical solution. After a time the leading piston and all the feedstock material between the leading and trailing plugs may then be allowed to travel forward by setting the relief valve at the end of the washing section  980  to relieve at a desired pressure. The speed of the travel of feedstock may be controlled by the water input volume per unit of time behind the trailing plug  932   
         [0043]    There are four process zones in the reactor section  950  as shown in example embodiment  900 , with each process zone ending with a packing/extraction section. In general, the packing/extraction section has a series of rings including an initial packing ring and several subsequent countercurrent displacement rings described further herein. At the beginning of the first process zone, named mild acid hydrolysis zone  956 , the porosity of the feedstock is first lowered to a level by extracting some liquid out with a so-called packing ring. The volume of extraction may be controlled by measuring it with a positive displacement pump or cylinder. 
         [0044]    After the packing ring, the initial impregnation fluid may be extracted with a mild acid hydrolysis liquid, such as an acid hydrolysis liquid containing 60% alcohol, 1% sulfuric acid, and 39% water, by way of example. The extraction may be accomplished by a measured amount of mild acid hydrolysis liquid being pumped into the last of the several displacement rings with a positive displacement pump or cylinder. In some embodiments, several of these pumps or cylinders may be “ganged” to operate together so that the liquid that is displaced within the first extraction ring will flow into the inlet side of the next pump or cylinder to be delivered to the second displacement ring and so on. In this manner the fresh ingoing liquid will counter-currently “wash” out the liquid to be extracted; however, the porosity remains relatively constant throughout the entire extraction section since all input/output liquid volumes are same by virtue of all displacement pumps or cylinders being of same volume. 
         [0045]    This newly input mild acid hydrolysis liquid converts the feedstock hemicelluloses to sugars, which are then extracted in a similar manner at the end of the process zone after a packing ring has first reduced the porosity again by a desired value corresponding to the amount that the feedstock has shrunk during its travel through the processing zone. Again a packing ring may be followed by four or more displacement rings which in a countercurrent flow pattern extract the processed sugars to a sugar concentrator tank to be further processed in a conventional manner through fermentation to ethanol. 
         [0046]    Some embodiments may utilize three subsequent process zones with their own packing and extraction sections following the above sugar extraction section. In these embodiments, these process zones may be a first lignin extraction zone, a second lignin extraction zone, and a third lignin extraction zone, and numbered  974 ,  942 , and  978  respectively. In a preferred embodiment, the process liquid in these zones is 60% ethanol and 40% water. 
         [0047]    After de-lignification, there may be an additional packing ring followed by a four-ring hot alcohol countercurrent displacement wash, another four-ring counter-current hot water wash, and finally a four-ring counter-current cold-water wash. Thereafter, a discharge section  988  may discharge the feedstock to a blow tank utilizing the pressure within the system to bias the material outward. 
         [0048]    With the foregoing description in place, there will now be a more detailed discussion of lowering the porosity. In general, throughout the system, the porosity values may change given the various states of the feedstock. As the feedstock is processed through the various zones its particle size and shape is reduced and the particles soften due to the organic matter being dissolved from the feedstock. To avoid porosity increase the feedstock may be packed at the end of each process zone to maintain proper and uniform displacement liquid cross-flow. 
         [0049]    In general, with each positive displacement piston within an embodiment, there may be a pressure drive system upstream of the system that provides sufficient pressure for passing the feedstock and the intermediate pistons through the tube reactor assembly. In order to control the porosity in the manner described above, the feed drive may provide a certain displacement of a known quantity of fluid. 
         [0050]    In an example embodiment, the feed drive is a positive displacement device which positively displaces a prescribed amount of fluid into the system. Along the way, various packing rings in the system controls the porosity through the entire tube reactor. Basically, in order to pack the feedstock and lower the porosity, the intermediate packing rings will be extracting liquid at intermediate stages. In other words, at the very end portion of the system there is a pressure-relief-valve-based extraction whereby if (for example) one unit of water is pumped in by the feed pump or cylinder behind the trailing plug and one unit of water is removed at a packing ring, then the particles of the feedstock before that particular packing ring will move closer to one another (wherein the trailing piston will advance forward), and effectively the porosity of the feedstock in that section is lowered. 
         [0051]      FIG. 10  shows a side cross-sectional view of a positive displacement pumping system. As shown in  FIG. 10  there is a positive displacement pumping assembly  1000 . In general, the assembly is provided with a central shaft  1002  that is operatively connected to a plurality of drive pistons  1004 A- 1004 E (where—the drive pistons in sum will commonly be referred to as the drive pistons  1004 ). The central shaft is further connected to a drive piston  1006  which is powered by hydraulic fluid passing through the valve assembly  1008 . In general, the drive piston  1006  applies a force to the central shaft  1002 , which in turn biases the plurality of drive pistons  1004 , which in turn bias fluid contained within the first and second chambers  1010  and  1012 . 
         [0052]    In this way,  FIG. 10  shows the chambers  1010  and  1012  on either side of the drive piston  1004 A, wherein each of these chambers are configured to intake fluid through an intake line  1014  through the check valves  1016  and  1018 . Further, the fluid within the chambers  1010  and  1012  is configured to be discharged through the output line  1020 , which receives fluid through either the check valves  1022  or  1024  depending upon which direction the drive piston  1004  is traveling in. 
         [0053]    Therefore, referring now back to  FIG. 6 , it can generally be appreciated that the pump assembly  1000  can be connected to the various cross-flow rings for distributing fluid therethrough. For example, the first cross-flow ring in the array will pass fluid from the pump assembly that has already been processed through the forward process rings in a countercurrent flow-like manner as described in detail in U.S. Pat. No. 5,680,995. 
         [0054]    However, the pump assembly provides operation of multiple drive pistons in a positive displacement manner that inject and withdraw a prescribed amount of fluid to maintain the fluid content within the system. Fluid can thus be extracted at prescribed intervals based on either gauges or known properties of the feedstock to adjust the porosity as described above. In general, a pump assembly having, for example, a single piston can be utilized for extracting fluid from a packing ring  662  as shown in  FIG. 6 . As shown in  FIG. 10 , the pump assembly  1000  has the additional feature of extracting injecting fluid simultaneously by having the substantial simultaneous movement of the drive pistons  1004 , which are all commonly connected to the central shaft  1002 . 
         [0055]    It will further be understood that the configurations and/or approaches described herein are exemplary in nature, and that these specific embodiments or examples are not to be considered in a limiting sense, because numerous variations are possible. The specific routines or methods described herein may represent one or more of any number of processing strategies. As such, various acts illustrated may be performed in the sequence illustrated, in other sequences, in parallel, or in some cases omitted. Likewise, the order of any of the above-described processes is not necessarily required to achieve the features and/or results of the embodiments described herein, but is provided for ease of illustration and description. 
         [0056]    The subject matter of the present disclosure includes all novel and nonobvious combinations and subcombinations of the various processes, systems and configurations, and other features, functions, acts, and/or properties disclosed herein, as well as any and all equivalents thereof.