Classifications

• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
  • D21C11/00—Regeneration of pulp liquors or effluent waste waters
  • D21C11/0085—Introduction of auxiliary substances into the regenerating system in order to improve the performance of certain steps of the latter, the presence of these substances being confined to the regeneration cycle
  • D21C11/0092—Substances modifying the evaporation, combustion, or thermal decomposition processes of black liquor
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
  • D21C3/00—Pulping cellulose-containing materials
  • D21C3/02—Pulping cellulose-containing materials with inorganic bases or alkaline reacting compounds, e.g. sulfate processes
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
  • D21C11/00—Regeneration of pulp liquors or effluent waste waters
  • D21C11/10—Concentrating spent liquor by evaporation
  • D21C11/106—Prevention of incrustations on heating surfaces during the concentration, e.g. by elimination of the scale-forming substances contained in the liquors
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
  • D21C3/00—Pulping cellulose-containing materials
  • D21C3/22—Other features of pulping processes
  • D21C3/226—Use of compounds avoiding scale formation
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
  • Y02P40/00—Technologies relating to the processing of minerals
  • Y02P40/40—Production or processing of lime, e.g. limestone regeneration of lime in pulp and sugar mills

Definitions

• the present invention relates to a method for producing pulp f om 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 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 ttoee-dimensional phenolic polymer network that holds the cellulose fibres together and imparts rigidity.
• Lignin comprises about 28wt% of softwood and about 20wt% hardwood. It is selectively removed during chemical pulping and subsequent bleacl mg without significantly degrading the cellulose fibres. Extractives account for about 3wt % of softwoods and about 5wt % of hardwoods. They include plant hormones, resin and fatty acids.
• 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.
• 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 rrr ture 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".
• the treatment chemicals sodium sulphide and sodium hydroxide
• the treatment chemicals are then regenerated f om the black liquor by a process whose main piece of equipment is a so-called To linson furnace.
• Black liquor at about 65% dry solids content is sprayed into the furnace.
• 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 burns under reducing conditions at a temperature of 750°-1050°C. and the recovered chemicals, mainly Na 2 CO 3 andNa 2 S, 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.
• the maximum recommended bed temperature is 760°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 2 CO 3 and Na 2 SO 4 which have to be subjected to recaustication to regenerate white liquor.
• US-A-3523864 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°C (1300-1400°F) and contains solid reaction products in which Na 2 SO 4 becomes reduced to Na 2 S.
• the intermediate bed is at 648-704°C (1200- 1300°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 2 CO 3 and Na 2 SO 4 which become deposited on the surface of the CaO pellets together with combustion gases and organics.
• the upper bed receives recycled CaCO 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.
• US-A-4011129 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.
• graminaceous crop residues are the materials left over after annual agricultural crops have been harvested for their primary or intended purpose. They include cereal straws, such as wheat, rice, barley and oats; seed grass straws such as flax and rye; the crushed stalks of sugar cane known as bagasse; sorghum and com stalks; and other agricultural residues e.g. cotton linters, the short fibres adhering to cotton seed after cotton ginning
• cereal straws such as wheat, rice, barley and oats
• seed grass straws such as flax and rye
• the crushed stalks of sugar cane known as bagasse
• sorghum and com stalks sorghum and com stalks
• other agricultural residues e.g. cotton linters, the short fibres adhering to cotton seed after cotton ginning
• pulp from straw and bagasse is being used in high proportions for paper-making - up to 90% for high quality printing and writing paper. For example, in China, over 85% of
• Straw can be pulped by chemical processes and by a combination mechanical and chemical process (chemi-mechanical pulping).
• chemi-mechanical pulping For the cooking of non-wood raw material sodium hydroxide alone is recommended as the active chemical because most non-wood fibre does not contain sticky resins and the sodium sulphide catalyst is unnecessary.
• the major part of chemical pulp production from this class of raw materials is performed with a process called the soda process in which raw material is heated together with a highly alkaline cooking liquor contenting sodium hydroxide to a temperature of 140-170°C under pressure. Under these conditions, the main portion of lignin dissolves.
• Sodium hydroxide can be recovered from the resulting black liquor and organic substances in the black liquor can be used as fuel for energy generation.
• black liquor from the soda process can be burnt even under highly oxidising conditions.
• Chemical recovery therefore involves evaporating the black liquor to a suitable content of dry matter and burning the evaporated liquor by means of excess oxygen.
• the inorganic combustion residue consisting mainly of sodium carbonate, is dissolved in water and re-causticised with burnt lime to regenerate sodium hydroxide, wliich is recycled.
• slaked lime Ca(OH) 2 has been used in achnixture with NaOH as active chemicals in white liquor because it also serves as a digesting agent and is of lower cost.
• a process for recycling NaOH/Ca(OH) 2 black liquors has not been described and such black liquors have in the past merely been discharged untreated.
• silica in straw and other non-wood cellulosic agricultural products presents difficulties for chemical recovery.
• Wheat straw contains 4-10 w% silica as small crystals embedded in the straw.
• Rice straw has an even higher silica content, 9-14 wt%.
• Other cereals such as barley, oat and rye straw have 1 - 6 wt% silica.
• Wood on the other hand has a silica content of less than 1 wt%.
• soda process as applied to straw pulping most of the silica in the straw reacts with the sodium hydroxide to form water-soluble sodium silicate, which remains in the black liquor in addition to lignin and other organic compounds.
• Black liquor of high silica content gives rise to scaling (coating equipment with a glass like substance) especially in an evaporative process.
• a modified wood-based recovery system may be used if the silica content of the cereal straw is less than 5 - 6 wt%, but at higher capital and operating costs.
• products of higher silica content, especially for rice straw there has been up to now no process that is technically and commercially viable.
• Our WO 03/014467 discloses a method for treating raw elongate material suitable for use in a paper making plant comprising: extracting contrary material from the raw material; crushing the raw material from which contrary material has been removed to remove nodes; splitting the crushed raw material lengthways; supplying the split raw material to a co-rotating screw conveyor divided into a plurality of zones and processmg said material in said conveyor to produce pulp and a black liquor effluent; supplying treatment material to at least one zone; controlling the temperature and/or pressure of at least one zone; and spraying concentrated black liquor into a processing vessel in the form of a fluidised bed reactor for treatment of said black liquor, said processing vessel being part of treatment material and energy recovery means.
• the alkali supplied to the co- rotating screw conveyor to bring about pulping may include sodium hydroxide and additionally calcium hydroxide, which has the effect of precipitating silica onto the cellulosic fibres and preventing silica from entering the black liquor as calcium silicate.
• WO 03/014467 further describes a process for treating black liquor in which black liquor effluent arising from the pulping process is collected in a digestion liquor storage tank and concentrated to 30-70% solids using a standard evaporator designed for concentration purposes. If the black liquor effluent has a solids concentration of 30% or above it may be treated directly in the processing vessel el ⁇ rinating the evaporation step.
• the concentrated black liquor is moved to a reactor vessel at a temperature in excess of 90°C using a pipe or an enclosed twin- screw transport system.
• the enclosed transport system is used to minimise the loss of organic components through vaporisation.
• a temperature in excess of 90°C is required to decrease the viscosity of the black Uquor so that it will transport without resistance.
• the black liquor is treated in a toroidal fluidised bed reactor vessel.
• a limit of 650°C is specified for the upper temperature of the fluidised bed, in practice a maximum temperature of 610°C has only ever been used. This is because at temperatures above 600°C, volatilisation of the inorganic alkali metal species present in the black liquor (e.g. Na and K) has been demonstrated to occur in other processes. When these species are in the vapour phase, additional process equipment is required to recover them at increased overall cost.
• a temperature of above 650°C can be used in the above mentioned black liquor recovery process. Recent experiments have shown that the loss of inorganic species from the black liquor when heated to between 650 and 700°C or even 725°C in the fluidised bed, was minimal, i.e. the losses were not economically significant, and hence no additional equipment was required to ensure their recovery.
• a method for treating black liquor which comprises heating the black liquor at a temperature above 650°C in a reactor containing an alkaline earth metal oxide, e.g. calcium oxide.
• the black liquor may react with the alkaline earth metal oxide to form a mixture of sodium hydroxide and sodium carbonate and alkaline earth metal carbonate and a volatile gas and liquid component which contains a combustible component and can be used as a fuel as in conventional treatment processes, e.g. a boiler.
• the invention provides a method of treating graminaceous materials which reduces or overcomes the problems associated with a high silica content in the resulting black Uquor.
• the invention further provides a method of converting graminaceous raw material to pulp for paper or board, said method comprising: digesting said raw material with a white liquor based on sodium hydroxide and further comprising calcium hydroxide in an amount effective to substantially convert silica of said raw material to calcium silicate; recovering pulp and black liquor substantially free of soluble silicate; heating the black liquor in a fluidized bed reactor containing calcium oxide for catalysing conversion of organic content of said black liquor to gas and for providing recovered solids including sodium values of said white liquor and calcium oxide; and regenerating said white liquor using said recovered soUds.
• the invention relates to the use of calcium hydroxide as an additive in the soda process digestion of a graminaceous starting material to form pulp for inhibiting scaling during black liquor concentration and recovery when processing black liquor at least partly from pulp washing.
• FIG. 1 is an overall block diagram of a process for making pulp from wheat straw according to the invention
• FIG. 2 is a schematic view of a roller arrangement for use in a raw material pre-treatment process forming part of the pulp manufacturing process of Fig. 1
• FIG. 3 is a schematic view of the construction of a self-cleaning pin roller that may be used in the roller arrangement of Fig. 2
• FIG. 4 is a schematic view of a possible embodiment of a co-rotating twin screw conveyor that may be used for converting straw to pulp in the process of Fig. l
• FIG. 5 is a block diagram of a preferred black liquor effluent treatment apparatus that may be used in the process of Fig. 1.
• a method is therefore preferably employed which crushes the nodes, opens out the straw stem lengthways in a gentle fashion and feeds the raw material into a digester in a positive, metered and continuous process.
• the straw to be treated passes from a delivery conveyor 9 to a straw pre-treatment station 10 where the stems are crushed between rollers, contrary material is extracted, and the stems are spUt longitudinally.
• the conditioned straw is then supplied to a digesting or pulping station 12 where it is subjected to mechanical work in the presence of aqueous alkali (white liquor) whilst subject to elevated temperatures and pressures.
• the resulting black liquor is then passed to an effluent treatment station 14 where it is heat treated to provide solids that can form a so-called green liquor That liquor in turn is contacted with lime derived from a CaCO 3 feed and regenerated to white liquor for recycling to the pulping station 12.
• Off-gas recovered from heat-treating the black Uquor can be used for generating steam and for process heat.
• a solids bleed of CaCO 3 sludge is removed to avoid excessive build-up of trace metals in the white liquor.
• straw is the raw material from which pulp is to be made, it may take the form of chopped straw, straw that has been subjected to a longimdinal splitting or shredd ng operation, or straw that has been both longituclinally split and or shredded and chopped.
• the straw is passed along a conveyor belt 101 where dust, heavy items such as stones and other contraries such as plastic siring are removed.
• the straw is then passed into a feed hopper 103 which feeds the straw into to an arrangement of knurled rollers 105 and 107 which crush the nodes in the straw stem and rollers with pins which open the straw stem out lengthways in a gentle fashion.
• first and second counter-rotating knurled crushing roUers 105 and 107 to crush the straw nodes.
• the crushed material then passes through two counter-rotating intermediate rollers 109 and 111 that prevent any contrary materials from damaging the rollers below.
• the straw then passes through two more rollers 113 and 115, this time rotating in the same direction. These latter rollers are provided with pins that open and shred the straw lengthways and act in cooperation with a feed shoe.
• this system leaves the straw as shortened and opened out shredded material without nodes. This will facilitate quicker chemical and steam penetration and so faster and more uniform pulping, whilst treating the fibres gently so preserving their length. This results in the production of an improved quality of pulp including a very significant reduction in visible "shiners" in the paper sheet, due to dispersion of parenchyma cells, improved drainage, a higher tensile and tear strength, a higher pulp yield and a reduced demand for pulping chemicals.
• the treated straw then drops from the pinned roUers 113 and 115 into feed hopper 117 leading to either a conveyor or blower system (not shown) that feeds the treated straw into a live bottom bin for buffer storage of the prepared material prior to pulping.
• a conveyor or blower system not shown
• the above discussed pinned and knurled or fluted roller opening and feeding system is specifically designed for straw but, with minor modifications, could
• the pinned roUers can also be constructed to be self-cleaning when used with longer fibered cellulosic raw materials such as hemp and flax. This is to prevent the material wrapping around the rollers and fouling the apparatus.
• a schematic of the functioning part of a pin roUer is shown in FIG. 3.
• the pin roller 120 has an outer surface having a large number of radially extending pins 122. This is used with a matching perforated or woven belt 124 on which the material 126 being treated is carried.
• the pins 122 pick up the material 126 and as the belt 124 leaves the pins, it takes off the material keeping the pin roller 120 free from tangled fibres.
• the preferred feed for the digestion stage of the present process is straw or other grarninaceous plant stem material that has been longitudinaUy split and/or shredded.
• the white Uquor used for digestion makes easy contact with the graminaceous plant stem material and with any remaining node material, dissolving silica therein and digesting tignin and other alkali-sensitive materials.
• the raw material is heated together with an alkaline cooking liquor containing sodium hydroxide to a temperature in the range from 140 to 170°C or in some instances up to 180°C under pressure.
• the cooking liquor should have a high alkaline concentration.
• the main portion of Ugnin will be dissolved from the raw material; however, also the main portion of any silicon in the raw material wiU react with the sodium hydroxide, forming water-soluble sodium silicate.
• the black liquor produced in cooking will contain silicate ions in addition to lignin and other organic compounds.
• the preferred white liquor used is therefore of the soda type (i.e. no sodium sulphide) and additionally contains calcium hydroxide in an amount effective to precipitate the sUica.
• calcium hydroxide is present in the white liquor with which the graminaceous feed is treated, or calcium hydroxide is added shortly after the sodium hydroxide and in either case the amount of calcium hydroxide should be sufficient to precipitate siUca onto the straw fibres as calcium silicate to reduce the soluble silicate content of the resulting black Uquor.
• An effective amount of calcium hydroxide should be present to convert substantially all the silica or a desired portion thereof into insoluble silicate, much of which precipitates on the straw fibres, before extraction of black Uquor from partiaUy or completely digested straw is initiated.
• the straw or other non-wood cellulosic material may be digested using a continuous digester, e.g. a single tube or multi-tube screw-fed digester, for example a Pandia digester available from Lenzingtechnik GmbH & Co KG.
• a continuous digester e.g. a single tube or multi-tube screw-fed digester, for example a Pandia digester available from Lenzingtechnik GmbH & Co KG.
• the use of fast cooking of the plant fibre in a horizontal tube continuous digester with screw feeder is reviewed in Atchison, J.E., Rapid Cooking Horizontal Tube Continuous Digester with Screw Feeder - Now the World Standard for Pulping Non-Wood Plant Fibers, 1990 Pulping Conference Proceedings. It is explained that this technology was developed initiaUy for the pulping of bagasse, but is also applicable to other forms of non-wood plant material including wheat straw and rice straw.
• the white liquor may be of the soda or Kraft type, and for straw or bagasse pulping typically 12-14 wt%.NaOH or 6-7wt% NaOH and 6-7 wt Na 2 S are used based on the weight of the dry straw, with pulping temperatures of 170 - 180°C, pulping pressures of 7 - 9 bar and cooking times stated to be 10-15 minutes in order to produce a chemical pulp, and 3-5 minutes for a semi-chemical pulp. As explained above these figures are in the inventors' experience not obtainable in practice.
• a preferred pulping process for bagasse or straw uses a horizontal tubular digester in which straw transport is by a conveyor based on a co- rotating and intermeshing double screw system.
• a digester can be made physically small for its processing capacity and consequently carries a lower capital cost than competing technologies.
• a significant advantage is that pulping can be conducted with low amounts of water, perrmtting black Uquor to be produced at higher concentration and reducing or removing the need for subsequent black liquor evaporation.
• Shredded and/or chopped straw is continuously fed from storage into the barrel of the digester where white Uquor and steam are injected through ports in the barrel or instead of steam injection electrical heating external to the barrel is provided.
• twin screw arrangement transport along the barrel is primarily by the termeshing flights of the two screws, whereas in a single screw conveyor transport is because the material is trapped between the advancing screw flights and the static barrel wall.
• the single screw arrangement is therefore less efficient because of friction and it can give rise to slippage (pressure in the barrel causes the straw to slip between the screw and the barrel wall) and surging.
• a temperature in the downstream cooking zones of the digester of not less than 165°C was found preferable in order to achieve a well- disintegrated pulp. It was concluded that it is possible to achieve Kappa numbers in the low forties from a relatively short 40mm diameter twin screw, in less than one minute and with caustic additions below 10%, which represents a quality consistent with a semi-chemical pulp. In a full-scale machine it is envisaged that the cooking temperature would be 170°C which is usual for making semi-chemical pulp using a continuous screw digester.
• twin-screw extruder it may be possible to achieve fully digested chemical pulp suitable for printing or writing papers without an additional cooking stage with a retention time of e.g. about 1 minute and a cooking pressure of 7 bar.
• the twin-screw extruder may be used to produce serm-chemical pulp in a first stage, which pulp may be converted into chemical pulp in a second cooking stage e.g. with 30 minutes additional cooking at a pressure of 7 bar before pulp de- watering.
• An embodiment of a twin-screw co-rotating digester 131 appears in Fig 4.
• G ⁇ arninaceous raw material (straw, flax, hemp, bagasse), wood chips or any other cellulosic raw material from buffer storage can be pulped.
• the raw material is drawn into the digester 131 in which the screw profiles are specially designed with two outer sections 133 and 134 having flights going in a first direction while in a middle section 135 the flight direction is reversed.
• the flights of the conveyor screws are manufactured from hardened steel with a deep cut flight to improve the size of the region where raw material is positively conveyed as explained above and are specially designed to minimize fibre damage.
• This particular design results in a reduced energy demand, which means that a smaller drive shaft and gearbox can be used, which also reduces capital cost.
• the design of the screw profile and the reduced drive shaft size also allows throughput of raw material to be increased by an anticipated 400% over conventional co -rotating twin screws.
• one embodiment of the conveyor has a first zone 137 to which the raw material is fed through a feed hopper 139.
• the flights of the conveyor screws in zone 1 are designed to be as open as possible in order to accept the material into the unit.
• cooking liquor which is preferably a soda process white liquor not containing significant sulphide but containing Ca(OH) 2 can be added through an inlet 143 and steam can be introduced into the second zone of the digester barrel through inlet 145.
• reaction to form insoluble CaSiO 3 is highly favoured over the competitive reaction to form soluble Na 2 SiO 3 , so that the silica is retained on the fibres as calcium silicate or enters the black Uquor as insoluble calcium silicate.
• soluble silica content there is no significant difference as regards soluble silica content between the black liquor from grarninaceous materials treated with NaOH/Ca(OH) 2 and the black liquor from wood pulping, which is amendable to treatment by known methods.
• the precipitated calcium silicate once formed, is not prone to re-dissolve under the conditions encountered in subsequent pulp processing operations including bleaching, de- watering and paper or board manufacture none of which employ low pH values (pH ⁇ 4), and it simply provides an innocuous part of the ash content of the pulp, being chemically similar to Wollastonite which can be used as a filler in paper-making and also being similar to china clay which is a complex silicate.
• the only difference, so far as subsequent processing is concerned is that the pulp from graminaceous materials may have an ash content of e.g. 3-4% whereas the ash content of wood pulp is usuaUy about 1%.
• the effluent treatment station 14 described below which gasifies the black liquor using a fluidized bed of CaO naturally produces a white liquor containing Ca(OH) 2 as well as regenerated NaOH, this black liquor both digesting the feed and precipitating calcium siUcate on the cellulosic fibres so that it does not enter the black Uquor in harmful quantities.
• the partially digested raw material passes to a third zone 147 where the conveyor screws have a reversed flight region 135 which acts as a braking zone for the advancing raw material which forms a plug of material being processed, with a high pressure zone thus being generated upstream of the plug.
• the barrel wall has a perforated region 149 through which some of the cooking Uquor becomes squeezed out.
• the action of the white liquor on the raw material is to rapidly solubilize all or much of the readily accessible lignin content of the raw material which dissolves with soluble hemicellulose and other dissolvable organic solids.
• a portion of the white liquor which may typicaUy correspond to about half of the white liquor initially added exits the region 149 as a black liquor stream of high solids content, typically about 30 wt% soUds.
• Removal of the easy lignin, soluble hemiceUulose and other soluble organic materials in this black Uquor stream means that these materials are no longer present to impede alkali attack on lignin remaining in the partly digested raw material in subsequent zones, assists the later stages of digestion, and also provides a solids-rich black liquor stream that contributes to recovery of a final combined black liquor stream of relatively high solids content.
• a remainder of the barrel and conveyor screws define a fourth zone 151 and a fifth zone 132 leading to an outlet 153 for pulp. In Zone 4 temperature and pressure are increased as indicated so that digestion continues through residual white liquor on the pulp which also serves to lubricate the partly digested starting material as it progresses along the digester barrel.
• Zone 5 temperature and pressure are reduced in preparation for the material leaving the twin-screw digester.
• the material travels through the tivin-screw unit in between 2-3 minutes.
• the screw speed may be around 200 rpm.
• the digester is suitably of modular construction which facilitates making changes to both screw and barrel configurations. This should be a cost-effective way to make use of one standard twin-screw unit to process many different types of cellulosic raw materials and/or to produce different grades of pulp simply by changing the screw and barrel configurations. Machine speeds of between 50-500 rpm may be used.
• a speed of 50-250 rpm has been used in practice. The speed needs to be adjusted for the raw material used and the pulp quality required.
• the twin- screw digester can be built in such a way that chemicals and liquids can be injected and liquids or steam can be vented or removed in each zone, which is a standard feature of twin-screw extruders. It has further been realized that a sophisticated gearbox and drive of the type conventionally used in twin-screw extruders is not necessary to suitably pulp fibres. A simple gearbox and drive can be used, reducing the capital cost and energy consumption. It is anticipated that the pulping system wtil consume less than half the energy of a conventional twin screw used for this purpose.
• the feed zone into the conveyor screws is enlarged compared to the other zones to aUow the raw material to be fed freely into said zone to increase the throughput of the conveyer.
• the area within the co-rotating twin screw conveyor may be continually decreased which has the effect of continually increasing the pressure within the zones.
• the co- rotating mtermesr ⁇ ng-twin screw extruder is set up with five zones as described below.
• the pulp exiting from the twin screw may have approximately 50% moisture content and would be expected to have a Kappa Number of 30-40. This is an unbleached chemical pulp ready for bleaching using standard methods and suitable for printing and writing papers. A semi-chemical pulp with a higher kappa number suitable for use in fluted packaging can also be produced if required. The result is a function of the rpm and the flight design or time spent in the twin-screw extruder together with the pressure, temperature and amount of pulping chemicals used. It is expected that kappa-numbers as low as 20 may be achievable using a single screw.
• Atchison discloses the use of a two horizontal tube digesters disposed one above another with the discharge outlet of the upper digester feeding partly digested material into the inlet of the second digester. In practice, in the prior art, it has often been necessary to use three such digesters ananged in series.
• twin-screw digester produces significant breakdown of the graminaceous or other starting material, and it is expected that only a single further twin-or single-screw digester stage will be needed to achieve a chemical pulp ready for bleaching so that installation size and capital cost may be reduced.
• a Kappa Number of 14-20 may be achieved after this further digestion.
• Pulp from the twin-screw digester or from a subsequent further digestion stage is then washed for further recovery of black Uquor before further treatment e.g. bleaching in the case of a chemical pulp. Washing is normally a multi-stage operation in which e.g. the pulp is successively contacted with wash liquid and passed through a plurality of dewatering stages arranged in series e.g.
• calcium silicate is relatively water-insoluble and high melting, WoUastonite melting at 1540°C, well above the bed temperatures for fluidized bed gasification of black liquor and well above the melting temperature of sodium silicate. Under the process conditions contemplated herein, it is expected that any calcium silicate in the black liquor will remain as discrete particles, and wiU not either volatilise or promote agglornentation of the fluidized bed used in the recovery process.
• Each dewatering stage may take place in a screw-type press in which an elongated rotating screw fits within a forarmnous sleeve that in turn is contained within a housing forming means for collecting wash liquor that has passed through the sleeve, pulp being advanced longitudinally of the screen by the rotating screw and being subjected to a squeezing action e.g. with the cross-sectional area of the channel representeded by the screw thread or the spacing between adjacent screw threads dhnfnishing from the inlet end of the screen towards the outlet, so that the pulp collected becomes compressed and liquid is progressively squeezed from the pulp.
• the present invention provides a treatment process to recover organic and inorganic chemicals and energy from black liquor effluent arising from the pulping of cellulosic raw materials to make paper. It is specifically intended to be used with the above described pulping process but could be used alone to treat black liquor from other pulping processes. It is designed to be economically viable at small throughputs.
• the black Uquor may be treated to volatilise the organic component thereof in a fluidized bed reactor under oxidizing conditions in the presence of a stochiometric amount of oxygen or oxygen-conta ⁇ ring gas such as air.
• the black liquor is gasified (partiaUy oxidixed) to a synthesis gas having as components inter alia CO 2 , CO, H 2 O, and H 2 usually together with methane and, C 2+ components under pyrolysing or partial oxidizing conditions in the presence of a sub-stochiometric amount of oxygen or free-oxygen- conta ing gas.
• Such gas may be a mixture of steam and combustion gas from a natural gas boiler, or a rnixture of steam and combustion gas from a boiler supplied with cleaned recycled synthesis gas supplemented with natural gas as required.
• the gases act as fluidising medium for the bed, and the bed material consists of or comprises CaO which catalyses the gasification process and also the gasification of any char which may form as by-product within the bed.
• the amount of oxygen in the gas mixture supplied to the bed should be sufficient to support partial oxidation and maintain bed temperature, but insufficient to convert the sodium and/or calcium hydroxide content of the black liquor entirely to carbonate, it being believed possible to conduct the reaction so that at least some of the NaOH remains as such in the bed
• the oxygen content of the fluidizing gas may be ⁇ 5% oxygen and usuaUy about 1.5-2% oxygen, giving an oxygen content in the off-gas above the bed of ⁇ 1%, typically about 0.8%, all by volume.
• Thermal degradation of the organic matter in black Uquor begins above 200°C producing water vapor, CO 2 , CO, H 2 , light hydrocarbons, tar and in the case of Kraft and other sulfur containing liquors, tight sulfurous compounds (e.g. mercaptans).
• 600°C devolatilisation is essentiaUy complete with the char residue containing fixed carbon, some hydrogen and most of the inorganic matter.
• Char composition can vary widely and depends upon both processing conditions (e.g. temperature) and fuel characteristics. Straw black liquor has a lower calorific value than wood black liquor, which should be taken into account when designing the fluidized bed reactor.
• TGA temperature ramp ihe ⁇ nogravimetric analysis
• a spouted fluidized bed differs from a bubbling fluidized bed in that it is designed with a central jet that forces material from the base of the bed along the central axis before it is allowed to settle back down along the walls of the vessel.
• the bed is typically conically shaped rather than cylindrical to assist with the bulk circulation movement. It may exhibit greater bed stability and fewer tendencies to agglomerate than a bubbling fluidized bed.
• the reactor is preferably a toroidal fluidized bed reactor; such reactors are described in US Patents 4479920 and 4559719 (the disclosures of which are incorporated herein by reference) and are available from Torftech Limited of Reading, UK (www.torftech.com).
• Such a reactor is intended to overcome the problems of conventional fluidised bed reactors as regards control of temperature and rate of heat transfer within the bed resulting from the random nature of lateral movement of the bed particles in a bed that remains essentially static and is fluidised by a vertical flow of gas/air mixture. Both a bubbling fluidized bed and a spouting fluidized bed suffer from these problems. In the experience, of the inventors, agglomeration occurs when there are hot spots in the bed. A toroidal fluidised bed minirnises this risk and the active CaO bed will also help.
• the solution proposed by Torftech and which is followed according to a preferred aspect of the invention, as applied to the treatment of black liquor, comprises providing a reactor having a processing region provided with a mass of particulate material consisting of or containing calcium oxide; supplying heated fluidizing gas to the processing region so as to generate a swirling flow of fluid within said processing region, the fluid of said swirling flow of fluid causing the particulate material to assume a compact band and circulate about an axis of said processing region in a turbulent manner, the fluidizing gas mcluding oxygen for at least partially combusting organic material in the black liquor; feeding the black liquor into said compact band of particulate material and treating the black liquor in said bed so as to gasify organic materials in said black Uquor; recovering organic material from said black liquor as off-gas from said bed; and recovering inorganic material from said black liquor as soUds from said bed.
• each particle travels to and fro inside the processing region along the full periphery of the compact band so that uniform processing conditions may be obtained.
• the motions of the particles within the particulate mass are deterrnined by the combined effects of the fluid flow, gravity, and the centrifugal forces created by the swirling of the fluid, and the result is a thorough and continuous mixing of these particles and matter to be processed, on the supply of such matter into the band of particles. Consequently, a very efficient processing operation may be achieved using only a shallow band of particles.
• the process gas stream impacts on and minimises the insulating microscopic gas layer around each particle. As a result, the heat and mass transfer rate is greater than in other types of reactor, which should permit faster and more effective processing.
• Tests have been carried out using a semi-industrial scale reactor of the above toroidal fluidized bed type. Where the black liquor to be treated is converted into solids form, the soUds were fed through a two-stage screw feeder (one dosing screw, one feeding screw). Where the black liquor to be treated was in the form of a liquid, it was fed using a centifugal pump either through the top of the reactor or through a two-phase nozzle (supplied with Ar orN 2 ) directly above and perpendicular to the bed surface. The off-gas and some solid material exited through the top of the reactor and passed through a length of duct into a cyclone and venturi scrubber before being passed to an afterburner chamber operating at 850°C.
• the reactor chamber was of a high temperature ceramic (not strictly required for this work) and was of 400 mm at the base rising to 500 mm at the top and was of height 850mm.
• the distributor through which fluidizing gas entered the reactor chamber from beneath the fluidized bed comprised a number of parallel plates aligned at an angle, between which plates the fluidizing gases passed.
• the free surface area of the distributor was 30%.
• the effect of the distributor was to impart a severe swirling motion witirin the bed, which caused high local gas velocities and high turbulence without blowing particles out of the top of the reactor.
• the nominal fluidizing velocity used for all experiments was 10 m/s, which adequately supported a calcium oxide bed of mean particle size 1.5 mm.
• This solution is then treated as described above.
• the processing temperature used in these experiments ranged between 550 and 725°C. In general an increase in temperature increased the rate of conversion of the black liquor to reaction products.
• the majority of the inorganic content of the black Uquor could be recovered in solid form at higher temperatures than previously believed possible. Typical recoveries were in excess of 90% where experimental data was measured within 95% confidence Umits.
• CaO is important for the gasification reaction. It also prevents solid carbon from forming (i.e. char and tars) and forms complexes with sulfur (specifically CaS), although this may been seen as very much a beneficial side reaction. It is preferably added in the preferred amount CaO:DS (dry solids) of 0.2:1 to 0.4:1, mostpreferably about 0.35:1. At aratio of 1.2:1 it lowers the amount of hydrocarbons formed considerably by tying up carbon as CaCO 3 Thus, in a prefened embodiment, black liquor of soUds content 10-40% e.g.
• 15-30% may be supplied directly to a toroidal fluidized bed reactor containing either calcium oxide alone or calcium oxide and an inert material, and suppUed with steam and combustion gas from a burner supplied with recycled synthesis gas from the reactor supplemented with natural gas as required.
• An evaporation plant and a calciner to recover CaO from CaCO 3 are desirable, and a small quantity (about 10%) of the CaCO 3 stream will need to go to waste to prevent build-up of heavy elements which are naturaUy drawn up form the soil by the plants. This material could however be sent for local re-processing and used to make bricks or in cement works.
• Black Uquor effluent arising from the pulping process is coUected in a digestion liquor storage tank 301 and concentrated to 30-70% solids using a standard evaporator 302 designed for concentration purposes. If the black liquor effluent comes from the co-rotating twin-screw conveyor at a solids concentration of 30% or above it may be treated directly in the processing vessel eliminating the evaporation step.
• the concentrated black liquor is moved to a reactor vessel 304 at a temperature in excess of 90°C. using an enclosed twin-screw transport system 303.
• the enclosed transport system is used to minimize the loss of organic components through vaporization.
• a temperature in excess of 90°C. is required to decrease the viscosity of the black liquor so that it becomes easy to transport.
• the black Uquor is treated in the reactor vessel 304 in either of two methods.
• the black Uquor is introduced into a toroidal fluidized bed reactor 304 by spraying the concentrated liquor into the chamber of the reactor in which a bed of fluidized material is supported.
• the material may be an earth oxide such as lime at a ratio of 0.3:1 of lime to black liquor dry solids.
• the mean particle size of the earth oxide may be between 1 and 4 mm.
• the reactor may operate under stoichio etric or sub-stoicbiometric conditions.
• black Uquor effluent is pre-mixed in the twin screw conveyor 303 with an earth oxide such as time (CaO) in the ratio e.g.
• the reactor may operate under stoichiometric or sub-stoicbiometric conditions.
• the ratio of earth oxide, e.g. lime to black liquor dry solids may be in the range 0.2 to 1.3:1 lime to black Uquor dry soUds.
• the earth oxide may be suppUed by a standard calciner 308.
• the chamber of the toroidal fluidized bed reactor 304 is maintained within the temperature range 300 to 750°C and preferably 650-750°. where the necessary chemical reaction takes place in the space of seconds.
• a portion of the soUds within the toroidal fluidized bed reactor 304 may be recycled via the screw feeder 303 back to the reactor 304.
• the black liquor is converted by a chemical reaction to: (1) Sodium hydroxide and sodium carbonate and Ume within the fluidized bed reactor 304.
• the bed will overflow through a central discharge point and the overflowing material is then dissolved in a dissolving tank 305 to recover sodium hydroxide as green liquor in the traditional manner known as re-causticisation.
• the green Uquor is then filtered using a known filter 306 to form a calcium carbonate sludge and white liquor (containing sodium hydroxide and calcium hydroxide) for reuse in the pulping process.
• a known filter 306 to form a calcium carbonate sludge and white liquor (containing sodium hydroxide and calcium hydroxide) for reuse in the pulping process.
• re-causticisation can take place in the reactor.
• the gas is coUected to power a botier 309 that will produce energy and steam for use in the pulp miU process line.
• the gas containing combustible components may be recycled to the fluidized bed reactor to provide heat for the chemical recovery reaction.
• the calcium carbonate sludge may be dried to remove some water and sent to a second calciner reactor 308, which may be a toroidal fluidized bed reactor.
• This reactor may operate at a temperature of around 1100°C where calcium carbonate CaCO 3 is converted back to calcium oxide CaO for re-use in the black liquor effluent chemical recovery process.
• Approximately 10% of the fluidized bed material generated may need to be removed from the process continuously in order to prevent the build up of heavy metals and other materials in the process.
• black liquor effluent below 30% solids can also be processed using this method (and has been tested). However energy consumption is greater and so this is not preferred.
• the black liquor treated in the fluidized bed could be a Kraft liquor or a mixture of the soda/Ca(OH)2 black liquor and Kraft liquor or black liquor from the soda/anthraquinone process.

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• 2003
  • 2003-11-03 GB GBGB0325578.3A patent/GB0325578D0/en not_active Ceased
• 2004
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