FI123103B - A method and system for using black liquor expansion steam - Google Patents

Description

BACKGROUND OF THE INVENTION AND SUMMARY OF THE INVENTION BACKGROUND OF THE INVENTION

In conventional chemical pulp mills using steams, the black liquor removed from the digester 5 tends to be fed to a pre-evaporator system consisting of two tanks and a series of changers, and subsequently channeled to the remaining evaporator / recovery stage devices. See. for example, Figure 1.

In accordance with the present invention, one embodiment, process and method of the leaching and recovery system utilizes one or more black liquor streams removed from the cooking system 10 (i.e., the cooking system, also known as a cooker system or cooker, comprising an area from a chip silo to a cooker after any pre-heater (if applicable) and without a pre-evaporation system (i.e. pre-evaporation systems such as expansion tanks, boilers and exchangers that generate steam as the black liquor cools before being directed to the evaporation system in the recovery area).

The characteristic features of the method according to the invention are set forth in claim 1. The features of the system according to the invention are in turn reflected in claim 8. Various embodiments of the invention will be apparent from the dependent claims.

According to another embodiment of the invention, the black liquor from the digester is fed to the expansion vessel and then to the evaporator / recovery system without using an expansion-steam condenser.

cv, Invention of the Invention ™ ° d ° t Improve fiber line steam economy and recovery of 5 different materials used in the system, reduce dirt condensation and odor gases and / or reduce

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- 25 BRIEF DESCRIPTION OF THE DRAWINGS

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Figure 1 is a schematic representation of a conventional fiber line system.

Fig. 2 is a schematic view of the pre-evaporator and evaporator tin of a conventional fiber line system.

Fig. 3 is a schematic view of an evaporator portion of a fiber line system 30 according to an embodiment of the present invention.

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Fig. 4 is a schematic view of an evaporator portion of a fiber line system according to an embodiment of the present invention.

Figure 5 is a schematic view of a portion of a conventional fiber line system.

Figure 6 is a schematic view corresponding to Figure 5 of an embodiment of the present invention.

Fig. 7 is a schematic view similar to Fig. 5 of another embodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS AND FIGURES OF THE INVENTION

For a better understanding of the technology within the scope of the present invention, reference is made to the following publications: US Patents 6,346,166, 6,176,971, 6,132,555, 4,789,428 and 4,897,157, WO96 / 32531 and WO96 / 12848, and pages 183-184 of Develop and Continuous Cooking and its Figure 12.2. Figure 12.2 (in this simplified form as Figure 1) is an example of the current state of the art for pre-evaporator and evaporator systems and includes the following technical terms: Fig. 1 is a dashed box around the expansion tanks 10 and 20 and a replacement set near the top of the figure with the text "PREFACE." • Preheaters - see fig. 1, with the text "preheat", in the form of a "kitchen preheater", designed to use part of the removed black liquor heat to "preheat" white liquor, other black liquor streams, or other liquid streams to the digester or feed system.

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^ · Evaporator system - see fig. 1 is a dashed portion marked "DARK". This area has a series of switches, which are referred to as phases. Each of the steps 25 produces steam from the incoming black liquor. These steps are labeled I to N, of which I operates at the highest temperature and N at the lowest temperature.

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The prior art employs a fiber line system with wood chips 100 where wood chips or other cellulosic material can optionally be steamed to which material is added.

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30 liquids to form a slurry followed by pressurizing the slurry (this portion is also referred to as a feed system), is fed to a treatment vessel (s) (which may be an impregnation vessel, a pre-hydrolysis process or other) followed by a machine 5 (also referred to as a cooking system) . Currently, at least one black liquor stream (typically at a temperature of 110-150 * 0) is removed from the cooking system. The removed mushy liquor stream or streams are used as a heat source for the "preheating" of white liquor, other black liquor streams to the feed and cooking systems and / or other liquid streams. The removed black liquor stream (or streams) is then fed to a pre-evaporation system, i.e. two or more expansion vessels 10, 20 and / or boilers where hot black liquor generates steam when cooled, typically to a temperature of about 95-110 * O. At this point, the black liquor is introduced into the recovery zone evaporation system 10 ("SHARES").

In the capture area (Figure 2), the "cool" black liquor entering therein is stored in a "thin black liquor" (WBL) container 30 before being fed to an evaporator system. The evaporator system comprises a series of steps (numbered I-VI in Figure 2), each operating at different temperatures and / or conditions to allow steam to be produced during lean black liquor cooling and concentration. The black liquor solids concentration leaving Stage I, the highest temperature stage, is about 75% and is fed to a recovery boiler (not shown) for incineration, whereas the last stage VI liquid stream is the lowest-temperature condensate (65 ° C in Figure 2) used throughout factory.

The lean black liquor is introduced into the evaporator system at the most convenient stage (e.g., 20 in Figure 2, step IV), i.e., the stage where the lean black liquor temperature allows expansion to produce steam, typically in the third or fourth stage. After this entry point, the black liquor stream is split and flows in both directions (some of the black liquor goes to the hotter stages allowing evaporation of the liquid and concentrating the cm solids, the other part of the black liquor goes to the lower temperature stage and is heated to a higher temperature for centrifugation). The solids content of the black liquor from the first step is set (e.g., 75% solids as in Figure 2).

| One embodiment of the present invention eliminates the pre-evaporator system. See. Figures 3 and 4. In the process of the present invention, a lean is introduced into the recovery zone

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§ 30 black liquor has a higher temperature, so lean black liquor can be fed to a stage of the evaporator system which allows for higher steam production.

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It has also been found that the steam produced by the evaporator system of the present invention is purer than that produced by the pre-evaporator in the digester zone. The cleaner steam 4 is suitable for use as the steam in the chips silo 100 and can be removed from the evaporator system between stages so that the temperature of the steam entering the chips silo 100 is appropriate, eliminating the need for steam produced in the factory.

An advantage of the process of the present invention is the reduction of impurities in the steam delivered to the chips silo, which in turn reduces the impurities in the chips silo exhaust gas. The cleaner steam produced in the evaporator system and used in the chips silo 100 reduces harmful factory exhaust gases.

This process of the invention also allows for the removal of the 10 lean black liquor storage tank outside the evaporator system. The lean black liquor storage tank can become part of the evaporator system assembly and not remain outside, saving costs.

The exemplary evaporator systems shown in Figures 3 and 4 have other improvements over conventional systems of Figures 1 and 2. The temperature of the black liquor stream from the digester 5 to the evaporator system 15 is higher. This flow can be directed directly to the appropriate stage of the evaporator system to enhance the plant's steam economy.

Alternatively, a pressurized fiber filter 40 may be disposed between the digester and the evaporator system to allow removal of fibers from the black liquor stream such that the level of fiber after the filter is about 40 ppm. The fibrous material removed from the filter would be a suspension which would be returned to the digester 5 or feed system.

In current systems, which apparently may have a filter of this type, the flow temperature of the filter to the filter is too low (95-130) and therefore the flow medium cm must be returned to the feed system for reheating before further treatment.

In the process of the invention, in turn, due to the higher temperature of the black liquor stream g from the digester to the evaporator system, the temperature of the filtered material is also higher. The temperature of this higher temperature material is closer to the cooking temperature and can therefore be returned to the top of the digester, thereby reducing the overall need for heat increase. Depending on the temperature of the material, a small portion of the flow from the digester to the g could be directed through the condenser 50 to a smaller container 30 and a suitable stage of the haihogg 30 system. This system has never been carefully considered or used. 'combines' the cooking zone / cooking zone and the recovery zone due to the distance between these 5 zones and other factors. Another reason is the reliability of the separation of these areas. It is not desirable to "disturb" one of these zones to cause the other zone to shut down. Particularly harmful are the stoppages in the recovery zone which would force the machine to shut down. In short, the loss of one zone-5 zone would cause extremely difficult problems in the other zone. The process of the present invention solves these problems.

In another embodiment of the invention, the chemical pulp mill system, method and process utilize an expansion vessel. In the following, prior art will be described, followed by embodiments of the invention.

10 In chemical pulping, hot soup is removed from the spout. The amount removed is typically 8-12 m3 / ADMT and the average temperature is about 130-170 ° C. The black liquor from the digester flows into the evaporator (which includes the pre-evaporator and evaporator systems). almost all the water contained in the black liquor is removed in a series of evaporator stages connected in series before the black liquor is burned. The various evaporation stages of the black liquor use fresh steam and expanded steam obtained from the steps at higher temperature and pressure. In the evaporation plant, the black liquor is led to a non-pressurized soil storage tank before the actual evaporation. Before the storage tank, the temperature of the black liquor is lowered to below 100 ° C to prevent boiling in the tank. Very much heat is removed from the black liquor between the digester and the evaporator storage tank at -1 -4 GJ / ADMT.

Typically, several expansion tanks are used to remove heat from the black liquor, where part of the hot black liquor is converted into expansion steam. If it were possible to remove all the above mentioned amount of heat by swelling, the amount of water to be swelled, i.e. evaporated, would be 0.5 - 1.7 m3 / ADMT. In practice, the temperature of the black liquor in the expansion tank drops at its best to about 110 ° C, whereby the amount of water evaporated is between 0.3 and 1.3 ^ 25 m3 / ADMT. Final cooling to a temperature below 100 ° C is typically performed in an unfavorable σ> 9 cross in the heat exchanger, thereby providing hot water for pulp washing at the same time.

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The amount of heat transferred to the expansion steam is typically used for the steaming of the chips. logging

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In steaming the chips, the chips are heated and the air contained therein is removed by steam, thereby providing significant advantages for the cooking process. Steam consumption of chips at σ> § 30 is usually about 0.5 - 2 GJ / ADMT, depending on the characteristics of the chips and the method of steaming. As these calculations suggest, it is often the case that expansion steam is more available than is required for steaming.

6 This excess expansion steam is generally conducted to a separate expansion steam condenser, where water vapor condenses, thereby providing hot water. The expansion vapor contains mixtures known as non-condensable gases (methanol, turpentine, various sulfur compounds, etc.) which are removed from the condenser in the form of gas and in high concentrations. These gases are called concentrated odor gases. The chemical pulp mill also produces so-called dilute odor gases whose odor concentration remains below the explosive concentration. Condensed expansion steam contains not only water but also other ingredients and is called dirty condensate. The non-condensed gases and dirty condensate generated in the expansion steam condenser are led to further treatment at the other plants of the mill, since, when discharged to atmosphere 10, they would cause a significant emission problem. Because of the toxic and explosive nature of strong odor gases and dirty condensates, their handling is dangerous and requires special care.

Due to the low capacity of the expansion steam condenser, for example, it is often the case that the black liquor does not expand to a maximum temperature of about 110 ° C, but exits the expansion tank at a temperature higher than -15 ° C. This removes the expansion steam from the black liquor below the maximum level and the need for evaporation at the evaporation plant increases. For the overall efficiency of the plant's electrical economy, it is advantageous to remove as much expanded steam as possible from the black liquor, provided that it has reasonable uses where it can be effectively used to replace fresh steam.

20 Several techniques have been used to steam the chips. Traditionally, in addition to atmospheric pressure steaming, short-term pressurized steaming has been used in so-called steam vessels or chips. The pressure for pressurized steaming is usually 1.0 to 1.5 bar. Pressurized steam consumes a great deal of steam. Therefore, in the case of pressurized steaming, the expansion of the black liquor is also carried out in two stages, so that from the first stage the expansion steam is introduced into the steaming vessel and from the second stage to the atmospheric pressure chips silo, where the steaming retention time is considerably longer. In modern pulp mills, the steaming in the chips silo is so effective that pressurized steam has been dispensed with and typically only one black liquor expansion step is used. Bridge

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this way, the process is simpler and investment costs lower.

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§ 30 Figure 5 shows a typical system for utilizing the heat of the black liquor leaving the digester at a modern pulp mill. The black liquor from the digester passes through the expansion tank

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night 1 through tube 4. The black liquor can be removed from the digester into the tube 4 from more than one zone. As the pressure drops, the expansion vapor separates from the black liquor, which is conveyed via conduit 5 to the atmospheric pressure vaporization step and, where appropriate, conduit 2 through conduit 2 to more than one condenser for condensation of the expansion vapor. The condensing steam condenser receives cold water (about 0-30 ° C) or warm water (about 40-60 ° C) through the conduit 11. As the steam condenses, the temperature of this water rises and the water is discharged as hot water (about 65-90 ° C) through pipe 12 to the factory hot water system. The condensed vapor exits as dirt condensate via conduit 10 typically into the digester condensate system and from there to the evaporation condensate reservoir. The non-condensable gases, i.e. the strong odorous gases, are discharged through the pipe 9 to the system for collecting and treating the concentrated gases. In the expansion vessel, the swollen and cooled 10 black liquor exits via pipe 7 to the black liquor cooler 3. There, the black liquor temperature continues to drop and from there, the black liquor is discharged through the conduit 8 to the black liquor storage tank. The water used to cool the black liquor enters the condenser via conduit 13 and exits to the hot water system via conduit 14.

With modern fiber lines, the amount of water used for pulping is significantly reduced, and often not all of the hot water generated in the expansion steam condenser and black liquor cooler is used sensibly, but the hot water must be re-cooled and removed from the plant. This will lose some of the heat content of the black liquor. The purpose of this embodiment of the invention is to utilize excess expansion steam in a simple manner while preventing the production of concentrated odor gases and dirty condensate in the cooking plant.

The solution according to this embodiment of the invention does not use an expansion steam condenser, but the excess expansion steam is led directly to a suitable evaporation stage of the evaporation plant where it partially replaces the fresh vapor and thus improves the evaporation rate of the evaporation plant. At the same time, the formation of dirty condensate and strong odorous gases by boiling with sludge is prevented and their treatment can be restricted to the evaporation plant, where they have to be treated anyway. Naturally, the process that lacks the expansion steam condenser and the treatment of concentrated non-odorous gases and dirt condensate is smaller than the conventional process, which also results in lower investment costs.

| Fig. 6 shows a solution according to this embodiment of the invention for expanding black liquor oo and utilizing expanded steam. The black liquor from the digester enters the expansion tank 1 through pipe 4. The black liquor can be removed from the digester (or diggers)

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g to tube 4 from more than one zone. As the pressure in the expansion tank 1 drops,

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from the black liquor, the expansion vapor is discharged through a conduit 5 to an atmospheric vaporization step and through a conduit 15 to a condenser suitable for any purpose 8 or another advantageous object. The expansion steam condenser is not used. As a result, the production of dirty condensate and strong odor gases in the cooking plant can be prevented. In the expansion tank, the expanded and cooled black liquor exits via pipe 7 to the black-lye condenser 3. In the condenser 3, the temperature of the black liquor is further reduced and the black 5 is removed to the evaporator via pipe 8 (e.g., black liquor storage). The water used to cool the black liquor enters the condenser via conduit 13 and exits to the hot water system via conduit 14.

Fig. 7 shows another embodiment in which the expansion steam is supplied only to the chipping steam and without the expansion steam condenser. In this embodiment, the black liquor from the digester 10 enters the expansion vessel 1 through a conduit 4. Black liquor from the digester (s) can be removed from more than one zone. When the pressure in the expansion tank 1 drops, the expansion steam is separated from the black liquor, and the expansion steam is passed through the pipe 5 only to the atmospheric pressure evaporation stage. Since the expansion steam condenser is not used, the generation of dirty condensate and concentrated odor gases in the cooking plant can be prevented. In the expansion tank, the swollen and cooled black liquor is discharged through pipe 7 to the black liquor cooler 3. There, the black liquor temperature continues to drop and the black liquor is removed to the evaporator via pipe 8 (e.g., the black liquor storage tank). The water used to cool the black liquor enters the condenser via conduit 13 and exits the hot water system via conduit 14.

20 Although this embodiment is a novel and good solution and prevents the formation of concentrated odor gases and dirty condensate in the cooking plant, it may have some disadvantages, which are discussed below. If the expansion steam is available in larger amounts than is necessary for the steaming, there is no way to utilize this excess steam. Another disadvantage occurs during cm plant shutdowns, in which the expansion steam cannot be led to steaming and no expansion of the black CM 25 liquor occurs. Therefore, the structural pressure of the expansion vessel should be substantially higher than normal in order to withstand the high pressure generated by the high temperature. In addition, the cooling capacity of the black liquor cooler must be much higher than normal so that the black liquor can be cooled down to a temperature below 100 ° C also here.

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Annex. There is also a risk that, in the situation where the expansion vapor is not removed from the expansion vessel, the strong odor gases are enriched in the expansion vessel. When the expansion steam is reintroduced, a large amount of concentrated odorous gases may be released. This strong and sudden discharge of concentrated odorous gases can cause hazards in the steaming zone or associated equipment. In this embodiment of the invention, enrichment would not occur because the expansion steam is continuously removed to the evaporation plant.

The use of expansion steam for evaporation has also been implemented in connection with various pre-evaporation solutions. In these solutions, the black liquor is expanded in several steps and the expansion steam of the preceding 5 steps is used to evaporate the black liquor in the next step. The expansion steam used to steam the chips is usually obtained from a pre-evaporator and not directly from the expansion tank. In addition to the pre-evaporation plant, the plant also has a standard evaporation plant where the dry matter content of the black liquor increases to a level sufficient for combustion. In practice, pre-evaporation solutions have often been complex processes prone to black-lye lye dripping. For these reasons, pre-evaporators are not desired.

When using the systems, processes and methods of the invention, one may argue that the operation of the digester and the evaporator are more interdependent than usual. For example, if the amount of expansion steam delivered to the evaporator varies significantly, this may result in malfunction of the evaporator. This problem can be easily solved by venting the vapor flow to the evaporator with the optional flow regulator 16 and allowing the amount of steam used for the steaming to vary in different fault situations in the cooker. The amount of steam required for steaming can easily be offset by virtually no steam, without any negative impact on the overall steam economy.

Another challenging situation can arise if the evaporator is not in operation when the boiler is 20 in operation. This situation is very rare and short-lived because the black liquor storage tank between the cooker and the evaporator fills up quickly and prevents the cooker from functioning if the evaporator fails. In this situation, the solution presented earlier can be utilized in which the expansion steam is used only for the steaming of the chips. However, the dimensioning of the expansion tank and mushroom cooler should be designed with the situation where the expansion steam is not removed from the ^ 25 black liquor. Another solution is to introduce excess expansion σ> 9 steam into a condenser in the evaporator. Possible such condensers are e.g.

^ Surface condenser after evaporation or cleaning of dirty condensates from evaporation | condenser for a stripper. During short periods of malfunction of the evaporator, these oo condensers can advantageously receive steam. There is no reason to keep the boiler running during the longer period of steaming at § 30.

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In principle, the expansion steam from the expansion vessel 1 can be conducted to any stage of the evaporation plant where the pressure is sufficiently low. Since the typical operating pressure of the expansion vessel is about 0.2-0.5 bar overpressure, the steps of the evaporator operating at atmospheric pressure or below are suitable. However, the most advantageous solution is to direct the steam to the evaporator unit with the highest possible pressure, since this achieves maximum savings in the amount of fresh steam. In a typical modern 7-stage evaporator, it would be most desirable to conduct the expansion steam into the third evaporator-5. Then, expansion steam would be available in five evaporation steps, and fresh steam would be saved at a ratio of 5: 7. In other words, one pound of expansion steam would reduce the consumption of fresh steam by 5/7 pounds. The use of expansion steam at the evaporation plant is most advantageous when the evaporation plant comprises at least six evaporation steps.

The principles of the invention can also be applied in situations where there is more than one pressure vessel and in addition a pressurized steaming step is used. When multiple expansion vessels are used, higher pressurized fractions can be separated from the total amount of available expansion steam and used for various purposes. However, the use of multiple expansion tanks provides only marginal benefit over the simple solution based on the single expansion step of the invention, since using a single expansion tank 15, all the resulting expansion steam can be effectively utilized in the steaming and evaporation plant. The '' efficient use '' of expanded steam means that it is very effective in replacing the use of fresh steam. For this reason, the solution according to the invention is most advantageous when using a single expansion vessel.

While the invention has been described in the context of what is currently considered to be the most practical and preferred embodiments thereof, it should be understood that the invention is not intended to be limited to the disclosed embodiments, but is intended to embrace various modifications and corresponding arrangements and modifications.

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Claims (11)

A process for using the expansion steam of black liquor 5 from a chemical pulp digester system comprising the steps of: (a) producing black liquor in a digester system, the temperature is 10 to about 130Ό or higher, and the black liquor is passed to a suitable evaporator step as determined by the temperature of the black liquor, (c) expanding the black liquor in the evaporator system to produce steam 15 for use in the cooking system. The method of claim 1, wherein the temperature of the black liquor fed to the evaporator system is about 130Ό to 170 <. The method of claim 1, wherein the temperature of the black liquor fed to the evaporator system is about 130Ό to 150Ό. The method of any one of claims 1 to 3, wherein the temperature of the black liquor fed into any stage of the evaporator system is about 130Ό to 150Ό. O 25 C \ J cf) The method of any one of claims 1 to 4, wherein the temperature of the black liquor fed into any stage of the evaporator system is about 130Ό or higher. CVJ X IX The method of any one of claims 1 to 5, further comprising using a cS 30 pressurized fiber filter between the digester system and the evaporator system. O) o CO o The method of claim 6, wherein the filter enables the fiber to be removed from the black liquor stream so that after the filter the fiber level is about 40 ppm. 8. Chemical pulp system comprising: (a) a digestion system capable of producing black liquor, (b) an evaporator system comprising a multi-stage evaporator having at least six 5 steps, wherein this chemical pulp system does not have a pre-evaporator system, and wherein the black liquor system can be expanded to produce steam, and at least part of the steam may be used to steam the chips in the chips silo and / or lead to indirect heat exchangers in the digester system to preheat the white liquor and / or filtrates for use in the digester system. The system of claim 8, further comprising a pressurized fiber filter between the digester system and the evaporator system. The system of claim 9, wherein the fiber filter permits removal of fibers from the black liquor stream so that the level of fiber after the filter is about 40 ppm. The system of claim 8, comprising two or more digesters. 20 CM δ CM σ> cp CM X DC CL CO CM O) o CO o o CM