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Methods for producing pulp and treating black liquorRelated Patent Categories: Paper Making And Fiber Liberation, Processes Of Chemical Liberation, Recovery Or Purification Of Natural Cellulose Or Fibrous Material, With Regeneration, Reclamation, Reuse, Recycling Or Destruction Of Digestion Fluid, Digestion Fluid Reused On Different Fibrous MaterialsMethods for producing pulp and treating black liquor description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20060201641, Methods for producing pulp and treating black liquor. Brief Patent Description - Full Patent Description - Patent Application Claims
[0001] This application is a continuation-in-part of U.S. patent application Ser. No. 10/773,870, filed Feb. 6, 2004, which claims priority to PCT International Application No. PCT/GB2002/003641, filed Aug. 7, 2002, published as WO 2003/014467 on Feb. 20, 2003, which claims priority to GB 0119237.6, filed Aug. 7, 2001, to each of which priority is claimed, and each of which is incorporated by reference in its entirety herein. This application is also a continuation of PCT International Application No. PCT/GB2004/050023, filed Nov. 3, 2004, published as WO 2005/045126, which claims priority to UK Patent Application No. 03 25578.3, filed Nov. 3, 2003, to each of which priority is claimed, and each of which is incorporated by reference in its entirety herein. FIELD OF THE INVENTION [0002] The present invention relates to a method for producing pulp from graminaceous fibres and also to a method of treatment of black liquor that may be a by-product of said pulp production method or may have arisen otherwise e.g. Kraft black liquor or a mixture of soda black liquor with Kraft black liquor. BACKGROUND TO THE INVENTION The Kraft process [0003] The principal component of wood is long straight translucent cellulose fibres based on chains of glucose molecules that make up about 42 wt % of softwoods and 45 wt % of hardwoods. Hemicelluloses form a further component of wood and are short, branched chains of glucose and other sugar molecules that are relatively soluble in water and are removed during the pulping process. The cellulose fibres are held together by lignin which is a three-dimensional phenolic polymer network that holds the cellulose fibres together and imparts rigidity. Lignin comprises about 28 wt % of softwood and about 20 wt % hardwood. It is selectively removed during chemical pulping and subsequent bleaching without significantly degrading the cellulose fibres. Extractives account for about 3 wt % of softwoods and about 5 wt % of hardwoods. They include plant hormones, resin and fatty acids. [0004] The Kraft or sulphate process is preferred for the chemical pulping of wood because it can deal effectively with the resin component of many woods. It uses sodium hydroxide as the main cooking chemical and sodium sulphide as catalyst, and it gives stronger final pulp than the soda process, which employs sodium hydroxide alone. Anthraquinone is often used as an auxiliary catalyst in both the Kraft and soda processes. In the Kraft process, chips are cooked in a digester under heat and pressure with "white liquor" (in this case aqueous sodium hydroxide and sodium sulphide) to dissolve the lignin selectively. After 2 to 4 hours, the cooked mixture of pulp, spent pulping chemicals and wood waste is discharged from the digester. The resulting pulp is separated form a mixture of pulping chemicals and waste referred to as "black liquor". [0005] The treatment chemicals (sodium sulphide and sodium hydroxide) are then regenerated from the black liquor by a process whose main piece of equipment is a so-called Tomlinson furnace. Black liquor at about 65% dry solids content is sprayed into the furnace. During their descent, the black liquor droplets lose the remaining water by evaporation and the solids undergo pyrolysis to form a char bed at the bottom of the furnace. The char bed bums under reducing conditions at a temperature of 750.degree.-1050.degree. C. and the recovered chemicals, mainly Na.sub.2CO.sub.3 and Na.sub.2S, are drained from the furnace as a smelt which is dissolved in water to produce so-called green liquor, the precursor of the white liquor. The gases generated during pyrolysis and burning of the char are fully combusted at a higher location in the furnace. Flue gases must be thoroughly scrubbed to remove mercaptans that form under the process conditions. The furnace is provided with a suitable heat exchanger to recover heat from the hot combustion gases for steam and electricity generation. [0006] Although useful recovery of chemicals and energy can be achieved in commercial operation, the use of a Tomlinson furnace presents a number of problems. For example, inadvertent contact between water or dilute black liquor and the inorganic smelt may result in an explosion. Also, high char bed temperatures lead to increasing emission of sodium salts and excessive fouling of the steam pipes in the upper part of the furnace. Furthermore, the technology currently used to treat black liquor effluent is, depending on local economic conditions, only viable on a scale of not less than 60,000 tonnes of pulp production per annum, which may be compared with the typical scale of a modern wood pulp mill which is over 360,000 tonnes of pulp production per annum. Treatment of straw and other gramminaceous materials is, of course, on a much smaller scale inter alia because long-distance transport of bulky agricultural residues such as straw is uneconomic. Fluidised Bed Recovery in the Kraft Process [0007] To solve these problems, and also to reduce capital investment and increase the energy efficiency of the recovery operation, a number of Kraft recovery process have been described in which the smelt-water explosion hazard is eliminated and the emission of sodium salts reduced by maintaining the inorganic chemicals in solid rather than molten form. [0008] This principle was disclosed in U.S. Pat. No. 3,309,262 (Copeland et al) which discloses a process for treating black liquor in a reaction vessel containing a fluidized bed of solid particles consisting substantially entirely of residual inorganic materials derived from the black liquor. The process comprises: [0009] (a) concentrating the black liquor by evaporation to a solids content of 20-45 wt %, said liquor having a combustible content sufficient to support autogenous combustion; [0010] (b) spraying the concentrated black liquor into free space above the bed so that substantial evaporation is achieved within said free space, and the remaining further concentrated atomised black liquor flows into the fluidised bed; [0011] (c) maintaining fluidity of the bed by introducing at a speed of 30-150 cm/sec (1-5 ft/sec) oxygen-containing fluidizing gas in an amount sufficient to effect complete elimination of organic material as off-gas by substantially total autogenous combustion within the fluidised bed; [0012] (d) maintaining the bed at a non-smelting temperature below the eutectic temperature of the residual chemical mixture within the bed but in a temperature range of about 540-982.degree. C. to form gaseous combustion products above the bed and agglomerates within the bed from the residual inorganic materials of the black liquor that are of sufficient weight to prevent their entrainment in the fluidizing gas; [0013] (e) discharging the agglomerates from the fluidized bed; and [0014] (f) discharging the off-gas from above the bed. [0015] For sodium-based waste liquor the maximum recommended bed temperature is 760.degree. C. (although the inventors are aware that this value was exceeded in practical operation). Introduction of the black liquor as a mixture of coarse and fine droplets is recommended in order to combine rapid evaporation of water, an efficient scrubbing action that reduces dust loading, and promotion and control of agglomeration of the bed particles. Oxidising conditions within the reactor are maintained to prevent the formation of hydrogen sulphide gas, and conversion of organic material into combustible gas is not disclosed. The end products are Na.sub.2CO.sub.3 and Na.sub.2SO.sub.4 which have to be subjected to recaustication to regenerate white liquor. Although it has been reported in the patent literature that there have been attempts to commercialise the Copeland process, the inventors are aware that it is prone to severe bed agglomeration, especially when treating black liquor of relatively low calorific value from the cooking of straw, and that the process has since fallen into disuse for lack of technical and commercial viability. The experience of the inventors is that simple fluidized beds of the kind disclosed by Copeland are subject to unacceptable agglomeration, which makes operation impractical for anything beyond a short start-up period. [0016] U.S. Pat. No. 3,523,864 (Osterman) discloses a recovery process for Kraft black liquor based on a reaction vessel having lower, intermediate and upper fluidized beds disposed one above another and each formed by pellets of CaO. The lower bed operates at 704-760.degree. C. (1300-1400.degree. F.) and contains solid reaction products in which Na.sub.2SO.sub.4 becomes reduced to Na.sub.2S. The intermediate bed is at 648-704.degree. C. (1200-1300.degree. F.) and is fed with black liquor and preheated air in an amount of about 30% of that required for complete combustion to produce Na.sub.2CO.sub.3 and Na.sub.2SO.sub.4 which become deposited on the surface of the CaO pellets together with combustion gases and organics. The upper bed receives recycled CaCO.sub.3 which becomes calcined to regenerate CaO and provide the material for the fluidised beds which descends progressively from upper to lower beds. Overhead combustion gases are partly recycled as fluidising gas and after cyclone treatment are partly fed to a steam generator. Again all three beds are of the simple bubbling type, and the intermediate bed is subject to unacceptable agglomeration for the reasons already given. [0017] There are two further reasons for the absence of commercial utilization of these low-temperature fluidized bed processes: firstly the relatively high temperature required for fast and complete conversion of Na.sub.2SO.sub.4 to Na.sub.2S and secondly the ease of formation of H.sub.2S when Na.sub.2S is contacted with combustion gases below the melting point of the inorganic salts. So, while high temperatures favour the reduction, the above alternative processes require a relatively low temperature just below the melting point of the inorganic salt mixture. The consequence is that in fluid bed processes operating in the reducing mode, most of the Na.sub.2S formed is rapidly converted to H.sub.2S (and some COS) according to the overall reaction Na.sub.2S+CO.sub.2+H.sub.2O.fwdarw.. Na.sub.2CO.sub.3+H.sub.2S resulting in a low recovery of solid Na.sub.2S. [0018] For the sake of completeness, there should be mentioned U.S. Pat. No. 4,011,129 (Tomlinson) which discloses a method for increasing the chemical recovery capacity of a Kraft recovery furnace by injecting solid pellets of sodium sulphate and sodium carbonate directly onto the char bed in the reducing zone of the furnace while maintaining the temperature and reducing atmosphere in that zone, thereby forming a smelt containing sodium sulphide and sodium carbonate from the injected pellets. These pellets may be produced from a further quantity of black liquor in an auxiliary incinerator such as a fluid bed combustion unit, which permits recovery capacity to be increased without needing the construction of a further recovery furnace. Production of Non-Wood Cellulosic Pulp [0019] The use of agricultural residues from graminaceous annual crops could provide a solution to many problems of concern to the pulp and paper industry including fibre supply, farmers' concerns over the cost and availability of disposal alternatives, and consumer concerns over limited forest resources. Continue reading about Methods for producing pulp and treating black liquor... Full patent description for Methods for producing pulp and treating black liquor Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Methods for producing pulp and treating black liquor patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. Sign up (takes 30 seconds). 2. Fill in the keywords to be monitored. 3. Each week you receive an email with patent applications related to your keywords. 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