Abstract:
Method is disclosed for treating wood chips including a heater for preheating the wood chips in the presence of steam, a beater to mechanically process the wood chips into wood fibers, a cyclone for separating the wood fibers from steam and volatile organic substances produced in the beater, a blower line for transporting the wood fibers, steam and volatile organic substances from the beater to the inlet of the cyclone, a drying conduit connected to the lower outlet of the cyclone for carrying wood fibers therefrom, a sluice valve associated with the lower outlet of the cyclone for controlling the removal of the wood fibers from the cyclone, and a processing conduit connected to the upper outlet of the cyclone for the steam and the volatile organic substances whereby the volatile organic substances separated in the cyclone can be separated and heat associated with the steam separated in the cyclone can be recovered.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method designed to produce and treat wood fibers. 
     BACKGROUND OF THE INVENTION 
     One problem which is generally encountered in connection with plants for producing and treating wood fibers is the emission to the surrounding atmosphere of volatile organic substances, i.e. Volatile Organic Compounds (VOC), as well as formaldehyde from the raw wood, and from the size which is used in the process. One object of the present invention is to solve this problem. Another object of the present invention is to recover thermal energy in these processes. 
     SUMMARY OF THE INVENTION 
     These and other objects have now been accomplished by the invention of apparatus for the treatment of wood chips comprising a preheater for preheating the wood chips in the presence of steam thereby producing volatile organic substances from the wood chips therein, a beater for mechanically processing the wood chips whereby wood fibers are released from the wood chips and the steam and the volatile organic substances are contained therein, a cyclone for separating the wood fibers from the steam and the volatile organic substances, the cyclone including an inlet, an upper outlet and a lower outlet, a blower line for transporting the wood fibers, the steam and the volatile organic substances from the beater to the inlet of the cyclone, a drying conduit connected to the lower outlet of the cyclone for carrying the wood fibers therefrom, a sluice valve associated with the lower outlet of the cyclone for controlling the removal of the wood fibers from the cyclone, and a processing conduit connected to the upper outlet of the cyclone for the steam and the volatile organic substances whereby the volatile organic substances separated in the cyclone can be separated and heat associated with the steam separated in the cyclone can be recovered. In a preferred embodiment, the apparatus includes a scrubber for separating the volatile organic substances from the steam separated in the cyclone, and a heating coil for recovering heat from the steam separated in the scrubber. 
     In accordance with a preferred embodiment of the apparatus of the present invention, the apparatus includes a steam ejector associated with the lower outlet of the cyclone for transporting the wood fibers in the dryer conduit, and a mixing chamber attached to the steam ejector for mixing flue gas and drying air and supplying the mixture of flue gas and drying air to the steam ejector. 
     In accordance with one embodiment of the apparatus of the present invention, the apparatus includes an injector associated with the lower outlet of the cyclone, and an air compressor for supplying compressed air to the injector for transporting the wood fiber, the steam and the volatile organic substances in the drying conduit. 
     In accordance with another embodiment of the apparatus of the present invention, the preheater includes an upper portion including a wood chip entrance and an outlet, a lower portion, and a steam inlet for supplying steam to the lower portion of the preheater whereby the steam can countercurrently contact the wood chips entering the wood chip entrance in the preheater and the volatile organic substances and steam can exit from the outlet from the preheater, and a scrubber connected to the outlet from the preheater whereby the volatile organic substances can be separated from the steam and additional heat can be recovered from the steam. 
     In accordance with another embodiment of the apparatus of the present invention, the apparatus includes a heat exchanger connected to the outlet from the cyclone and a condensate tank connected to the heat exchanger whereby the volatile organic substances can be separated in the gaseous state for incineration and the heat can be recovered from the steam. Preferably, the heat exchanger comprises a first heat exchanger, and the apparatus includes a second heat exchanger and a third heat exchanger connected to the condensate tank for providing heat for drying the wood fibers and for cooling the condensate in the condensate tank. Preferably, the apparatus includes at least one mixing chamber whereby the drying conduit supplies drying air to at least one of the second and third heat exchangers. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be more fully appreciated with reference to the following detailed description, which, in turn, refers to the attached drawings in which: 
     FIG. 1 a  is schematic representation of the process performed in a portion of a plant designed in accordance with the present invention; 
     FIG. 1 b  is a schematic representation of another portion of the process performed in a plant designed in accordance with the present invention; 
     FIG. 2 is a top, elevational, partial view of a portion of the plant shown in FIG. 1 a  in an enlarged scale and in an alternative embodiment; 
     FIG. 3 is a top, elevational, partial view of the same part of the plant shown in FIG. 2 with yet another embodiment shown therein; and 
     FIG. 4 is a schematic representation of the process performed in an alternative embodiment of a plant designed in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION 
     The plant shown in FIGS. 1 a  and  1   b  comprises a number of parts, including a fiber production part A, a steam separator part B, two fiber drying stages, C 1  and C 2 , and an energy subplant D. The plant will be described with regard to its design/construction while simultaneously explaining its operation in order to avoid needless repetition. 
     In fiber-production part A, the wood chips are preheated in an alkalinizing bin  1 , preferably using steam at atmospheric pressure. From bin  1 , the chips are fed by a plug-type screw conveyor  2  which compresses and dewaters the chips as they are conveyed to chip preheater  3 . Chip preheater  3  heats the chips with heat obtained from condensing steam that is supplied through steam inlet  50  located in the lower part of the preheater, the pressure, temperature and time having been present and adapted to the raw wood chips. Preheater  3  has a top outlet  56  where released organic emissions are degassed from the wood along with air during heating, thereby improving heat transfer between the steam and the chips. The greater part of the emissions, i.e. volatile organic substances (VOC and formaldehyde), is released in the preheater and separated at a high concentration in the top of the preheater and conveyed together with steam and air through line  57  to scrubber  11  where solid particles are separated along with certain condensable organic substances and volatile waste gases, and where heat is recovered from the steam. Since the steam is supplied through inlet  50  at a low level the chips, which enter from the top of the preheater, can be washed in counterflowing steam during condensation. 
     The electrical energy that is added in order to free fibers from the chips in beater  4  is converted, for the most part, to steam in connection with the mechanical processing of is the preheated chips to produce free fibers or fiber bundles. During processing, a certain amount of organic emissions are released from the wood, and they are conveyed forward in blower line  5  in the gaseous state together with the steam. In blower line  5  the fibers, fiber bundles and steam are conveyed at high speed to inlet  52  of cyclone separator  6 . If size is to be used, it is added in blower line  5  at  51  thereby sizing the fiber. Emissions of volatile organic substances are also released from the size, and together with the fiber and the steam, they are conveyed to cyclone separator  6  for separation from the fiber. Bottom outlet  53  on cyclone separator  6  is connected, by means of sluice valve  7 , to conveyance line  55  in which the fiber is sent to a fiber drier. However, drying of the fiber can also take place in line  55  due to the fact that the transport medium is drying air (see especially FIGS.  2  and  4 ). Consequently, line  55  will hereinafter be called conveyance/drying line  55 . Upper connection  54  on the cyclone separator is connected to scrubber  10  that separates the fibers and organic substances from the steam obtained from the cyclone separator. 
     Cyclone separator  6  can be included in several alternative basic embodiments. In alternative embodiment  1 , which is shown in FIG. 1 a , steam ejector  8  is connected downstream from sluice valve  7 . Steam ejector  8 , which is supplied with steam at  67 , handles further conveyance of the fiber together with preheated drying air and flue gases that are sucked from mixing chamber  9 . This mixing chamber  9  receives flue gases through line  66 , which runs from energy subplant D in the plant and also receives drying air through valve  58 . In this alternative embodiment, the fiber is already being dried while it is being conveyed to drier stage C 1 . 
     In an alternative embodiment  2 , which is shown in FIG. 2, conveyance/drying line  55  is connected directly to sluice valve  7  so that hot air is supplied directly to the drier line at  59 . In an alternative embodiment  3 , which is shown in FIG. 3, the fiber is transported to the drier stage by means of compressor  31  which is supplied with conveyance air at  60  and feed injector  32 . 
     In drier stage C 1 , the drying air is heated as indicated by arrows  61  in air/hot-water coil  18  and in air/steam coils,  19  and  20 , and also in mixing chamber  21  used for flue gas obtained from energy subplant D. The suspension of steam, air and volatile organic substances (VOC and formaldehyde) that arrives at scrubber  11  from preheater  3  is washed free of solid particles in scrubber  11  using condensate pumped from condensate tank  26  by means of pump  28 . Parts of released emissions from the wood are condensed and leave scrubber  11  together with the scrubber water. Steam leaving scrubber  11  is used during condensation to heat the drying air in air/steam coil  20 . Condensate from coil  20  leaves separator  14  where volatile organic emissions proceed by means of regulator valve  15 , suction fan  22  (FIG. 1 b ) and duct  30  to incinerator  62  in energy subplant D. 
     Steam obtained from cyclone separator  6  that contains organic emissions released in connection with fiber production and sizing is washed free of solid particles in scrubber  10  using condensate from condensate tank  26 . Parts of the aforesaid emissions are condensed and leave scrubber  10  together with the scrubber water. The washed steam from scrubber  10  is sent to heating coil  19  where it is used to heat drying air  61 . In heating coil  19  the steam is condensed, and the condensate is sent to separator  16  from which volatile non-condensable emissions are sent to incinerator  62  for incineration by means of regulator valve  17 , suction fan  22  and duct  30 . 
     The condensate in condensate tank  26  is transported by pump  27  to heat the drying air in heating coil  18 . The condensate leaves coil  18  at a temperature of about 40° C. by means of separator  12  where the remaining emissions of volatile organic gases are sent to incinerator  62  by means of valve  13 , suction fan  22  and duct  30 . 
     Condensate from separator  12  is sent to tank  23 , which contains a decanter insert. Condensate consisting of emission remnants (terpenes) released from the wood substance is decanted and transported by pump  24  and pipe  29  to incinerator  22  where it is used to moisten solid fuel  63  and grindings  64 , which are also transported here. 
     The level in condensate tank  23  is regulated by means of pump  25 . Water that proceeds by means of pump  25  is a) used if so desired to heat drying air, as indicated by arrows  65 , in drier stage C 2  by means of heating coil  40 , or b) used in the rest of the process wherever needed, or c) sent out directly for purification. Drying air  61  and drying air  65  in drier stages C 1  and C 2  are heated to the final temperature together, if so desired, with flue gas from the energy subplant by means of mixing chambers  21  and  41 , or using some other heating medium. 
     FIG. 4 shows an alternative embodiment of the plant, which is somewhat simplified relative to the previously described alternative embodiments. Here, fiber-production part A is the same as in the embodiments previously described. In steam separator part B, on the other hand, the emissions released in preheater  3  are sent together with steam and air through line  70  to heat exchanger  73  and condensate tank  76  in order to separate condensable organic substances and volatile waste gases and recover heat from the steam. Bottom outlet  53  on cyclone separator  6  is connected, by means of sluice valve  7 , to injector  32  which is located in conveyance/drying line  55  that runs to the first drier stage C 1 . Upper connection  54  on cyclone separator  6  is also connected to heat exchanger  73  in order to heat the drying air. 
     In drier stage C 1 , the drying air is heated, as indicated by arrows  87 , in the following manner: a) heat exchanger  75 , b) mixing chamber  74  together with mixed-in drying air leaving cyclone separator  83  in drier stage C 2 , c) heat exchanger  73  together with steam from cyclone separator  6  and preheater  3  and d) mixing chamber  72  using the necessary supplementary drying energy introduced at  85 . In drier stage C 2 , the drying air is heated in heat exchanger  78  and also by means of heating coil  84 , alternatively using a heating medium supplied at  86 . 
     An alternative to the aforesaid drying air heating arrangement can be provided by eliminating mixing chamber  74 , where mixed-in drying air from drier stage C 2  is used. Another alternative can be provided by eliminating heat exchangers  75  and  78  which are intended for cooling condensate sent from condensate tank  76  to condensate tank  79  and replacing them with some other form of cooling. 
     The suspension of steam, air and volatile organic substances (VOC and formaldehyde) which arrives at heat exchanger  73  from preheater  3  and cyclone separator  6  is condensed and sent out as condensate to tank  76 , where volatile organic emulsions in the gaseous state are sent to the energy subplant for incineration through  93 . Parts of the emulsions mentioned above are condensed and transported together with the condensate by pump  77  to heat exchanger  75  used for drier stage C 1  and also to heat exchanger  78  used for drier stage C 2  and they transfer parts of their heat content to the drying air. Transport to cyclone separator  83  is provided by fan  82 . 
     Condensate from heat exchanger  78  is sent at a temperature of about 40° C. to condensate tank  79 , from which remaining emulsions of volatile organic gases are sent by means of  93  to the energy subplant for incineration. The level in condensate tank  79  is regulated by means of pump  80 . If so desired, water from pump  80  is used, by means of  89 , in the rest of the process wherever needed or is sent out directly for purification. 
     Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims.