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
  • D21C11/06—Treatment of pulp gases; Recovery of the heat content of the gases; Treatment of gases arising from various sources in pulp and paper mills; Regeneration of gaseous SO2, e.g. arising from liquors containing sulfur compounds
• • 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
  • D21C1/00—Pretreatment of the finely-divided materials before digesting
  • D21C1/02—Pretreatment of the finely-divided materials before digesting with water or steam
• • 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
  • D21C11/06—Treatment of pulp gases; Recovery of the heat content of the gases; Treatment of gases arising from various sources in pulp and paper mills; Regeneration of gaseous SO2, e.g. arising from liquors containing sulfur compounds
  • D21C11/08—Deodorisation ; Elimination of malodorous compounds, e.g. sulfur compounds such as hydrogen sulfide or mercaptans, from gas streams
• • 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
• • 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
  • D21C11/0042—Fractionating or concentration of spent liquors by special methods

Definitions

• the present invention concerns a system and a method for the generation of steam in a digester plant during the production of chemical cellulose pulp in accordance with the introductions to claims 1 and 4.
• the chips are to be first pre-treated with steam, not only for the expulsion of air but also of the preheating of the chips before the cook.
• the expulsion of air is important to ensure that the chips are fully impregnated with cooking fluid.
• Steam-treatment of chips takes place conventionally in several steps, in which untreated chips are first admitted to a first pre-treatment vessel, typically a chip bin, and there steam pre-treated at a lower first pressure, typically atmospheric pressure up to an excess pressure of approximately 1 bar, after which the pre-heated chips are sluiced in to a second pre-treatment vessel in which the chips are steam pre-treated at a considerably higher pressure, typically an excess pressure of 1-3 bar.
• Black liquor steam is used in both of these steam pre-treatment steps in older systems, but this involves an increased risk of the release of malodorous chemicals, since this black liquor steam contains gases known as TRS gases (totally reduced sulphur gases) from the cook.
• TRS gases totally reduced sulphur gases
• a system is revealed in US 6.722.130 for the generation of this pure steam from black liquor in which the pressure of the black liquor is first reduced in order to produce black liquor at atmospheric pressure and black liquor vapour, where this black liquor steam is condensed in subsequent steps and form the pure steam from this condensate.
• a system was revealed long ago in US 2.029.360 in which a steam converter is used in order to heat a pure process fluid for the generation of pure steam in a steam converter in the form of a heat exchanger.
• a variant was also revealed here in which the quantity of expelled clean steam in the heated clean process fluid can be increased by injecting steam into this heated process fluid. The system has no effect, however, on the dry solids content of the black liquor whose pressure has been reduced.
• the present invention aims to offer an improved system and method for the generation of pure steam from black liquor, where sufficient quantities of both black liquor steam and pure steam can be obtained, which quantities of steam are required for a steam pre-treatment process in several steps where pure steam is initially used, followed by pre-treatment with black liquor steam. Direct generation of black liquor steam from the black liquor is the most efficient form of steam generation.
• a second aim is to obtain an increased dry solids content of the black liquor when its pressure has been reduced, which dry solids content allows a reduced load in the recovery process.
• the evaporation capacity at older plants is often fully used and an increased production of cellulose pulp, which gives increased volume of black liquor that is sent to the evaporation process, cannot be considered unless the black liquor previous to the evaporation acquires a dry solids content that is greater than the previous content.
• the system according to the invention has the advantage that no new quantities of water are introduced in the process than those that are already present in the chips after the steam pre-treatment.
• a further aim is that the quantity of pure fresh steam that is removed from the steam network of the mill and which is added to the chips, and is in this way contaminated, is reduced.
• the pure steam that is required for the pre- treatment of the chips is obtained instead through heating of process water and in close association with the digester. It is for this reason unnecessary to remove steam from the steam production in the soda boiler (the steam network of the mill).
• the fresh steam that is taken from the steam network of the mill for heating the black liquor is not mixed with the chips, with the resulting contamination, and instead a pure condensate is obtained that can be returned to the soda boiler for generation of steam in the soda boiler and return to the steam network of the mill.
• the requirements for purity for the condensate that is used during steam generation are very high with the aim of avoiding clogging and precipitation in the system.
• Figure 1 shows schematically a system according to the invention.
• FIG. 1 shows a system for the generation of steam in a digester plant for the production of chemical cellulose pulp, according to the invention.
• the system comprises a digester vessel 4 in which chips are cooked in a pressurised hot cooking fluid.
• At least one withdrawal means is present in the digester vessel, in this case shown as a withdrawal strainer 10, for the withdrawal of hot, pressurised and spent cooking fluid, known as "black liquor" BL, from the digester vessel.
• the black liquor is led to a first pressure reduction means, here in the form of two pressure reduction tanks 11a, 11b coupled in series, where the pressure of the black liquor is reduced to a first lower pressure during the expulsion of a first quantity of black liquor steam BL-ST.
• the untreated chips CH are steam pre-treated in a first steam pre-treatment vessel 1 , typically a chip bin, in which the chips are pre-heated with a first steam at a first pre-treatment pressure, typically at a pressure that is equal to atmospheric pressure up to a pressure approximately 1 bar above atmospheric pressure.
• the pre-heated chips CHi are then fed by suitable sluice means 9 to a second steam pre- treatment vessel 2 in which pre-heated chips are pre-heated, before being cooked in the digester vessel, with a second steam at a second steam pre- treatment pressure, which second pre-treatment pressure is higher than the first pre-treatment pressure, typically between 1 - 3 bar.
• Formation of a slurry from the chips with cooking or impregnation fluid Liq follows in a conventional manner placing the chips mixture CS under pressure by a suitable feed arrangement 3 and feeding it to the top of the digester.
• the drawing shows a single-vessel digester plant, but the invention can be used on all forms of configurations of continuous digestion plants, for example, two-vessel digestion plants.
• the invention can, of course, also be used in a batch digestion plant, in which steam pre-treatment of the chips takes place in two steps, in which the first step uses pure steam for the steam pre-treatment. Only those components that are important for the invention are shown in the drawing, and other types of digester circulations can, of course, be present in the digester system.
• the feed arrangement 3 can be either of conventional type with sluice-feed with a high-pressure tap, with or without a top separator at the top of the digester.
• the feed arrangement may be also one with simple pumping technology, in which the mixture of chips CS flows through the pressurising pumps.
• the system comprises also a heating arrangement 13 for the black liquor after the pressure on the black liquor has been reduced in the first pressure-release arrangement 11a, 11b in which heating arrangement 13 the black liquor BL 2 the pressure of which has been reduced is heated to form reheated black liquor BL 2 H-
• This heating arrangement is preferably constituted by a heat exchanger in which the black liquor is heated indirectly by steam St, and from which heating arrangement is obtained a pure steam condensate Cond.
• An indirect heat exchanger is to be preferred since in this case no dilution of the black liquor by steam condensate is experienced, something that would otherwise entail an increased load on the evaporation system of the recovery plant.
• the black liquor BL 2 at this position has an increased dry solids content, relative to the dry solids content of the black liquor BL that was withdrawn from the digester, and thus it has an increased tendency for clogging any heat exchangers or reboilers that are used.
• the reheated black liquor BL 2H is subsequently led to a second pressure-reduction arrangement 12 in which the pressure of the reheated black liquor BL 2H is reduced while a second quantity of black liquor steam is expelled.
• This second quantity of black liquor steam is subsequently led to a steam converter 14 arranged to receive the second quantity of black liquor steam, which heats pure process fluid P. LJq in the steam converter while pure steam is formed.
• LJq can be recycled, as is shown in the drawing, over the heat-transfer surfaces of the steam converter through a pump 15 that withdraws heated process fluid from the bottom of the steam converter and leads this heated fluid to return in to the top of the steam converter, where the fluid is finely dispersed over the heat-transfer surfaces of the steam converter in a conventional manner.
• New pure process fluid P. LJq is added to the circulatory circuit through a level-controlled valve 16.
• the pure steam P-ST that is driven from the steam converter 14 is subsequently led to the first steam pre-treatment vessel 1 , while the first quantity of black liquor steam is led to the second steam pre-treatment vessel 2, each along the relevant steam lines.
• the hot pressurised black liquor BL typically maintains a temperature in the interval 130-160 0 C, corresponding to the cooking temperature in the digester, where the pressure on this black liquor is reduced in such a manner that a first quantity of black liquor steam BL-ST is generated while the black liquor BL 2 the pressure of which has been reduced obtains a temperature that is at least 15 0 C lower than the cooking temperature.
• the black liquor the pressure of which has been released is subsequently heated by at least 5 °C in order to form a re-heated black liquor BL 2 H. It is preferable that the black liquor the pressure of which has been reduced is heated by at least 10-15 0 C in order to form the re-heated black liquor BL 2H .
• the pressure of the re-heated black liquor BL 2H is again reduced in order to generate a second quantity of black liquor steam and black liquor BL 3 the pressure of which has been reduced, which black liquor BL 3 from which the pressure has been reduced maintains a second lower temperature, which second lower temperature is at least 5 0 C lower than the first lower temperature, preferably at least 10-15 0 C lower than the first lower temperature.
• This black liquor BL 3 is then led to the recovery plant.
• the reduction in pressure is regulated such that the second quantity of black liquor steam that is led to the steam converter maintains a temperature of at least 105-115 0 C.
• the pure steam that is produced in the steam converter is formed preferably from pure process water, preferably condensate from the pulp mill, which process water already maintains a relatively high temperature of 70-85 0 C.
• pure mains water can be used. It is not required, however, that this water satisfy the same high requirements for purity as the water that is used in the steam network of the mill.
• the condensate from the heat exchangers 13, 13b is, however, pure steam condensate and it can be returned to the steam network without any form of purification process.
• the process water can also, as is shown in Figure 1 , first be heated in a heat exchanger 20 with contaminated condensate, i.e. the contaminated condensate that is formed from the second quantity of black liquor steam in the steam converter 14.
• the pressure of the black liquor is reduced in pressure release tanks (of type 11a, 11b) to give a temperature of 123 0 C, i.e. a reduction in temperature of 32 0 C.
• the quantity of black liquor steam that was then obtained is more than sufficient for the steam pre-treatment in a second step.
• the temperature was raised from approximately 123 0 C to 137 0 C, i.e. a rise in temperature of approximately 14°C.
• This rise in temperature increased the quantity of pure steam that can be obtained from the steam converter in the system under consideration by more than 100%, from 3 kg/sec to just over 7 kg/sec.
• the second quantity of black liquor steam could be maintained at 110 0 C, and with a flow of just over 7.3 kg/sec, where the steam converter had a heat transfer surface of approximately 1 ,600 m 2 , a k-value of 2,000 W/(m 2 * 0 C) and a deltaT of 5.0 0 C.
• the condensate that had been added was heated by heater 20 to a temperature of just over 95 0 C and a quantity of steam corresponding in quantity to that expelled from the condensate (just over 7 kg/sec).
• This quantity of pure steam is that which was required in order to pre-treat in a satisfactory manner the chips in the first step to a temperature at the chips of approximately 100 0 C.
• the re-heating may take place through the addition of a quantity of heated cooking liquor to the black liquor, the pressure of which was reduced in the first step and which maintains approximately 123 0 C.
• Figure 1 shows such a variant in which spent cooking liquor, which typically maintains a temperature of approximately 110 0 C, is withdrawn in a lower withdrawal strainer 10b. This spent cooking liquor is subsequently led to a heat exchanger 13b where this spent cooking liquor is heated with fresh steam to a temperature in the interval 145-165 0 C.
• This heated spent cooking liquor is then mixed with the black liquor BL 2H whose pressure has been reduced in the first operation and that maintains a temperature of approximately 123 0 C, in such a manner and with such a quantity that the mixture reaches a temperature of approximately 137 0 C.
• the invention can be modified in a number of ways within the scope of the attached patent claims.
• the first reduction of pressure, which generates the black liquor steam may take place in a single step, or in more steps than two.
• the heating of the steam BL 2 from which the pressure has been removed can take place also through heat exchange with other hot process fluids than steam, or in combination with a subsequent heating using steam.
• the system according to Figure 1 has the advantage that no new quantities of water are introduced into the process than those that are already present with the chips after the steam pre-treatment, and then only those quantities of water that have been added in the first steam pre-treatment operation.
• the second steam pre-treatment operation is carried out fully and completely with regenerated steam, without the addition of any new quantities of fluid.

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Cited By (4)

Citations (3)

• 2005
  • 2005-12-21 SE SE0502851A patent/SE0502851L/sv not_active IP Right Cessation
• 2006
  • 2006-12-15 WO PCT/SE2006/050583 patent/WO2007073333A1/en active Application Filing

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