Patent Publication Number: US-2003221804-A1

Title: Lignins derived from black liquor

Description:
BACKGROUND OF THE INVENTION  
       [0001] Liquor containing sodium hydroxide is employed to digest wood chips to produce a pulp and, depolymerize, disperse and dissolve lignins from the wood to yield a black liquor containing sodium hydroxide and depolymerized lignins. A recovery boiler is used to recover the sodium compounds. Lignins contained in concentrated black liquor, upon combustion, furnishes energy contained in the lignins to provide steam.  
       [0002] A state of the art Kraft pulping digestion process employs white liquor containing sodium hydroxide and sodium sulfide and creates a black liquor. After concentration of the black liquor, by means of a multiple-effect evaporator, the concentrated black liquor is sent to a recovery boiler to produce steam and a molten smelt containing sodium carbonate. The smelt of melted sodium compounds flows to a vat where it dissolves into green liquor. Green liquor, removed from the vat, is slaked by calcium oxide to convert the sodium carbonate to produce sodium hydroxide. Calcium oxide, as a result, is converted to calcium carbonate which settles at the bottom of a white liquor classifier tank. The resulting calcium carbonate, after substantial separation of adhering white liquor, is transported to a lime kiln where the calcium carbonate is converted to calcium oxide. Thus the black liquor recovery cycle is completed. The pulping operation can be accomplished in batch form or continuous form, for example, a Kamyr continuous digester. Accordingly, it is believed, that only a well defined recovery method has been developed.  
       [0003] Therefore, an object of this invention is to obviate many of the limitations and disadvantages of the prior art to digest wood chips and form a black liquor containing sodium hydroxide.  
       [0004] This invention relates to a method of recycling a solution containing sodium hydroxide without using a lime kiln and a recovery boiler producing a smelt.  
       [0005] A significant object of this invention is to produce lignins significantly free of sodium compounds from a black liquor.  
       [0006] Another object of this invention is to utilize a multiple-effect evaporator, previously required to concentrate black liquor, to remove water from liquor containing sodium hydroxide.  
       [0007] With the above and other objects in view, this invention relates to the novel features and alternatives and combinations presently described in the brief description of the invention.  
       PHRASEOLOGY APPLIED IN THE INVENTION.  
       [0008] Wood chips in the presence of white liquor, containing sodium hydroxide at an elevated temperature, will digest wood chips to result in cellulose pulp and depolymerize, disperse and dissolve lignins to form black liquor. Lignins are not well structured polymers. When a lignin polymer is fractured the term “depolymerized lignins” is often utilized even though no definite “lignin monomer” exists. The term “depolymerized lignin” is used interchangeably to designate lignins. The term lignins is commonly applied to in a black liquor containing sodium hydroxide and depolymerized lignins. Sodium sulfide, anthraqinone and polysulfide polymers, provide catalysts to delignify wood chips.  
       [0009] Black liquor, when extracted by an organic solvent to extract depolymerized lignin from the black liquor will form an extractate of an organic solvent of depolymerized lignin and an insoluble raffinate. The term “extractate” establishes a function and the ending ate, such as filtrate. Lignins are prone to self condensation under acidic conditions. Lignins removed from the previously formed extractate, with bound sodium compounds, and mixed with an acidic solution will produce a sodium salt solution divided from the lignins to produce lignins free of sodium and a solution of sodium salts. The solution containing sodium salts is treated in a salt splitting procedure to remove salts and form a sodium hydroxide solution and an acidic solution. The sodium hydroxide solution, combined with the raffinate, containing sodium hydroxide, is evaporated to remove water to concentrate the solution. The concentrated solution, referred to as an “active liquor,” regularly contains about 40% sodium hydroxide to about 10% sodium hydroxide. A salt splitting operation often utilizes a bipolar membrane to remove sodium from the salt and form a sodium hydroxide solution. As a result sodium hydroxide is created from the salt. Concentrated liquor thus formed is skimmed of tall oil. Tall oil resembles a soap having chemically bound sodium. A multiple-effect evaporator, previously required to concentrate black liquor, is capable of being applied to remove water from the solution containing sodium hydroxide to produce “white liquor” for recycle.  
       [0010] For supplementary particulars on lignin, for example, refer to Pulp And Paper Technology, pages 33-36. For information on tall oil, for example, refer to Kirk-Othmer, Encyclopedia Of Chemical Technology, Fifth Edition, Volume 23, pages 615-622. Water splitting is described, in a bulletin provided by Aqualytics, in which a salt solution is converted to an acid and a hydroxide.  
       BRIEF DESCRIPTION OF THE INVENTION  
       [0011] The present invention, in its broadest aspect, is a method to yield lignins separated from a black liquor containing sodium hydroxide. This invention defines a method to produce a lignin, substantially free of sodium compounds, and a liquor made suitable for reuse to depolymerize, disperse and dissolve additional lignins. The method comprises: providing a black liquor, and an organic solvent. The method, depicted herein, originates from a black liquor containing lignins dispersed and dissolved in a liquor containing sodium hydroxide. Upon subjecting black liquor to extraction by an organic solvent, an extractate of lignins contained within the organic solvent is formed and a raffinate containing sodium hydroxide is also formed. Consequent to separating the extractate, with lignins of bound sodium compounds from the raffinate, and adding an acidic solution, such as one derived from carbon dioxide contained in a flue gas, to the extractate forms sodium salts and renders the lignins insoluble within the extractate. By means of separating the lignins insoluble within the organic solvent and separating the sodium salts contained within the organic solvent, an organic solvent for recycle is obtained. The organic solvent is often selected from the group of hydrocarbons, organic halogens and alcohols including an individual or a combination thereof. The neutralized extractate forms a phase of a solution containing sodium salts and an insoluble phase of lignins. Separation of the salt phase will provide lignins substantially free of sodium salts. The salt phase is subjected to a salt splitting technique to convert to a solution of sodium hydroxide and an acidic solution.  
       [0012] Key Features of this Invention are:  
       [0013] Separating lignins from sodium compounds contained within a black liquor. Extraction of a black liquor by an organic solvent creates an extractate containing depolymerized lignins.  
       [0014] Producing depolymerized lignins, substantially free of sodium, from a black liquor. Depolymerized lignins, substantially free of sodium, is suitable to fire boilers to avoid explosions due to the presence of sodium.  
       [0015] Recycle substantially all of the sodium compounds as sodium hydroxide and sodium sulfide.  
       [0016] Concentrated active liquor, containing sodium hydroxide, is subjected to recycle for employment to digest wood chips, depolymerize, disperse and dissolve lignin from wood chips to create black liquor.  
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0017] Features that are considered characteristic of this invention are set forth in the appended claims. This invention, however, both as to its origination and method of operations as well as additional advantages will best be understood from the following description when read in conjunction with the accompanying drawings in which:  
     [0018]FIG. 1 is a flow sheet denoting the invention as set forth in the appended claims.  
     [0019]FIG. 2 is a flow sheet denoting a method to separate tall oil from a concentrated liquor containing sodium hydroxide employing a multiple effect evaporator for concentration.  
     [0020]FIG. 3 is a flow sheet denoting a method for acidifying lignins to form condensed lignins.  
     [0021]FIG. 4 is a flow sheet denoting a method of settling the extractate to form lignins.  
     [0022]FIG. 5 is a flow sheet denoting a method of centrifugal division of the extractate to form lignins.  
     [0023]FIG. 6 is a flow sheet denoting a method for gasification of lignins 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
     [0024] The flow diagram of FIG. 1 illustrates the preferred embodiment of the present invention. In the diagram, rectangles represent stages or functions of the present invention and not necessarily separate components. Arrows indicate direction of flow of material in the method.  
     [0025] Referring to FIG. 1, black liquor  10 , containing depolymerized lignins and sodium hydroxide, is conveyed to counter flow extraction stage  12 , within the extraction is achieved by recycled organic solvent  26 A to produce a raffinate  16 , and an extractate  14 . Extractate  14 , is conveyed to a heat exchanger  20  for exchanging heat from extractate  14  to establish heated recycled organic solvent  26 A and cooled extractate  14 A to exchange heat to cooled organic solvent  26 . Cooled extractate  14 A, containing insoluble lignins is conveyed to separation of lignins stage  22  to form cooled recycled organic solvent  26  and lignins  24 . Recycled organic solvent  26 A readily dissolves depolymerized lignins from black liquor  10  and forms extractate  14 , containing depolymerized lignins, transferred within recycled organic solvent  26 A, and a raffinate  16 , previously employed as an aqueous sodium hydroxide liquor for depolymerization of lignins. Accordingly recycled organic solvent  26 A, is utilized to extract lignins from black liquor  10 .  
     [0026] Separation of lignins stage  22  can be a supplied centrifuge, settling tank or any equipment utilized to separate insoluble lignins and form recycled organic solvent. Organic solvent  26  is ordinarily selected from the group of hydrocarbons, organic cyclic compounds, organic halogens and alcohols including an individual or a combination thereof. Pulp washing, composed of weak black liquor, is customarily combined with black liquor  10 .  
     [0027] Referring to FIG. 2, a flow sheet is portrayed denoting a method to concentrate a raffinate  16 , conveyed to a multiple-effect evaporator stage  18 , to remove water  28 , and form concentrated raffinate  30 A, containing tall oil. Concentrated raffinate  30 A, containing tall oil, is conveyed to a skimming tall oil stage  32  to skim tall oil  34  and form concentrated raffinate  30 , substantially devoid of tall oil. Heretofore skimmed tall oil  34 , can be reacted with an acid to form crude tall oil. Concentrated raffinate  30 , regularly contains about 40% sodium hydroxide to about 10% sodium hydroxide. Concentrated raffinate  30  is employed to depolymerize, disperse and dissolve lignins contained within wood chips, to form additional black liquor  10 .  
     [0028] Referring to FIG. 3, lignins  24  is conveyed to lignins condensation stage  36  and combined with acid  38 , to transform lignins to self condensation lignins to form condensed lignins mixture  40  for transport to condensed lignins separation stage  42  to form condensed lignins  40 B and additionally separate sodium salt solution  40 A. Lignins condensation stage  36 , regularly releases gaseous hydrogen sulfide  38 A which is reacted with sodium hydroxide to form sodium sulfide.  
     [0029] Referring to FIG. 4, a flow sheet is portrayed denoting a method to transport cooled extractate  14 A, containing insoluble lignins, to a settling tank  22 A, to settle and provide lignins  24  and cooled organic solvent  26 . Cooled extractate  14 A is generally cooled from cooled water obtained from a cooling tower  
     [0030] Referring to FIG. 5, a flow sheet is portrayed denoting a method to convey cooled extractate  14 A, containing insoluble lignins, to a centrifugal separation stage  22 B, to separate and provide lignins  24  and cooled organic solvent  26 .  
     [0031] Referring to FIG. 6, a flow sheet is portrayed denoting a method to convert lignins  24  by steam  48  within gasification stage  46 , to produce gasification products  50  to function as a fuel.