Patent Application
20110198049
In conventional facilities producing chemical pulp utilizing continuous digesters, the manner of handling the extracted black liquor from the digester is to pass the black liquor to a pre-evaporation system comprising two flash tanks and a series of exchangers and then to pass the resulting liquor to other equipment in an evaporation/recovery phase. See, e.g.,
According to the present invention, one embodiment of a continuous cooking and recovery system, process and method utilizes one or more streams of black liquor extracted from the cooking system (i.e., the cooking system, which is also known as the digester system or digester plant, includes the chip bin through the digester—and optionally including an impregnation vessel) in an evaporation/recovery system after any pre-heater (if applicable) and without any pre-evaporation system (i.e., pre-evaporation equipment being equipment such as flash tanks, reboilers and exchangers where steam is generated as the black liquor is cooled prior to sending the black liquor to the evaporation system in the recovery area).
According to another embodiment of the invention, black liquor from the digester is sent to a flash tank and then to the evaporation/recovery system without using any flashing steam condenser.
The embodiments of the invention result in the improvement of the steam economy of the fiberline system, improved recovery of the various ingredients used in the system, reduction of foul condensate and malodorous gases, and/or reduced equipment and system requirements.
To assist in an understanding of the technology in the field and area of the invention, attention is directed to the attached documents, which are incorporated herein by reference: U.S. Pat. Nos. 6,346,166, 6,176,971, 6,132,555, 4,789,428 and 4,897,157, patent document nos. WO96/32531 and WO96/12848, and pages 183-184 of the book entitled Present and Future Developments of Continuous Cooking and its FIG. 12.2. FIG. 12.2 (simplified as
The current state of the art utilizes a fiberline system with a chip bin 100 where steaming of wood chips or other cellulose material may or may not be occurring, liquid is added to form a slurry, followed by pressurization of the slurry (this section is also referred to as the feed system), fed to a treatment vessel or vessels (could be an impregnation vessel, a pre-hydrolysis process or other vessels), followed by a digester 5 (this section is also referred to as the cooking system). Currently, at least one black liquor stream (typically at a temperature of 110-150° C.) is withdrawn from the cooking system. The extracted black liquor stream or streams are used as a source of heat to “pre-heat” white liquor, other black liquor streams, and/or other liquid streams being sent to the feed and cooking systems. The extracted black liquor stream (or streams) is then sent to the pre-evaporation system, e.g., two or more flash tanks 10, 20 and/or reboilers where steam is produced from the hot black liquor as the liquor is cooled, typically to temperatures of about 95-110° C. At this point, the black liquor is sent to the evaporator system “EVAP” in the recovery area.
Once in the recovery area (
The WBL is introduced into the evaporator system at the effect which is most appropriate (e.g., effect IV in
One of the embodiments of the subject invention eliminates the pre-evaporator system. See
It has also been discovered that the steam produced in this inventive evaporator system is cleaner than that produced in a pre-evaporator system at the digester area. The cleaner steam is suitable for use as steam for the chip bin 100 and can be drawn off the evaporator system between effects so that the steam going to the chip bin 100 has a temperature suitable for this use and thereby eliminates a steaming vessel, thus allowing for more efficient use of the steam produced in the mill.
An advantage of this inventive process is the reduction of contaminates introduced into the chip bin in the steam, which, in turn, reduces contaminates in the gas vented out of the chip bin. The cleaner steam produced in the evaporation system and used in the chip bin 100 reduces adverse emissions from the mill.
This inventive process also allows for the elimination of the WBL storage tank external to the train of the evaporator system. The WBL storage tank can become part of the evaporator system train, not external to the training, thereby saving costs.
The exemplary evaporator system shown in
Optionally, a pressurized fiber filter 40 can be located in the line between the digester and the evaporator system to allow the removal of fiber from the black liquor stream to a level of about 40 ppm leaving the filter, The fiber material removed from the filter would be in the form of a slurry to be returned to the digester 5 or feed system.
In current systems that may arguably have a filter somewhat like this, the temperature of the stream entering the filter is too low (95-130° C.); thus, this stream material must be returned to the feed system to be reheated to cooking temperature prior to further treatment. However, in the inventive process, the higher temperature of the black liquor stream from the digester to the evaporator system means the filtered material is at a higher temperature. This higher temperature material is closer to the cooking temperature and therefore can be returned to the top of the digester, reducing the overall need for heat addition. Depending on the temperature of the material, a small portion of the stream from the digester could be sent through a cooler 50 and to a smaller tank 30 and to the evaporator system at the appropriate effect.
The inventive system has never been contemplated or used before because it has never been considered to “connect” the digester/cooking area and the recovery area because of the distance between the two areas in the mill and other factors. Another reason is the reliability of the separation of each area. It is undesirable to have one of these areas “upset” that would cause a shutdown in the other area. Particularly detrimental are upsets at the recovery area that would cause the digester to be shut down. In short, loosing one area could cause extreme problems in the other area. The inventive process overcomes these problems.
In another embodiment of the invention, a chemical pulping system, method and process utilizes a flashing tank. The prior art is first described, followed by the inventive embodiments.
In chemical pulping, cooking liquor at a high temperature is discharged from a continuous digester. Typically, the discharged amount is 8-12 m3/ADMT and the temperature on average is about 130-170° C. The cooking liquor discharged from the digester, i.e., black liquor, flows to an evaporation plant (containing pre-evaporation and evaporation systems), where, among other things, almost all the water present in the black liquor is removed in several evaporation effects connected in series prior to black liquor combustion. Various black liquor evaporation effects use fresh steam and flashing steam obtained from effects having a higher temperature and pressure. in the evaporation plant, the black liquor is led into a non-pressurized storage tank prior to the actual evaporation. Prior to the storage tank, the temperature of the black liquor is decreased below 100° C. for preventing boiling in the tank. Between the digester and the storage tank of the evaporation plant, a very large amount of heat—1-4 GJ/ADMT—is removed from the black liquor.
Typically, several flashing tanks are used for removing heat from the black liquor, wherein part of the hot black liquor converts into flashing steam. If it was possible to remove by flashing the total amount of heat mentioned above, the amount of water flashing, i.e., vaporizing, would be 0.5-1.7 m3/ADMT. In practice, the temperature of the black liquor in the flashing tank decreases at best to a level of about 110° C., whereby the amount of water being vaporized is between 0.3-1.3 m3/ADMT. Final cooling below 100° C. is typically performed indirectly in a heat exchanger, simultaneously providing hot water for pulp washing.
The amount of heat transferred into the flashing steam is typically utilized in chip steaming. In the chip steaming, the chips are heated and the air contained therein is removed by means of steam, whereby significant advantages are achieved for the digesting process. Typically, steam is consumed during chip steaming, depending on the chip properties and the steaming method, in the amount of about 0.5-2 GJ/ADMT. As the above calculations indicate, the situation often is such that flashing steam is available in excess of steaming purposes.
Typically, this excess flashing steam is led to a separate flashing steam condenser, where water vapor is condensed and simultaneously hot water is obtained. The flashing steam contains compounds referred to as non-condensable gases (methanol, turpentine, various sulfur compounds etc.), which are removed from the condenser in gaseous form and at high concentration. These gases are called concentrated malodorous gases. A chemical pulp mill also produces so-called weak malodorous gases, wherein the content of malodorous compounds is maintained under explosive concentration. Condensed flashing steam contains other compounds in addition to water, and it is called foul condensate. The non-condensable gases generated in the flashing steam condenser and the foul condensate are led into other plants of the mill for further treatment, because if released into the atmosphere they would cause a significant emission problem. As the concentrated malodorous gases and foul condensate are toxic and explosive, the treatment thereof is dangerous and requires particular care.
It often happens, e.g., due to low capacity of the flashing steam condenser, that the black liquor is not flashed to the maximum level of about 110° C., but the black liquor exits the flashing tank at a higher temperature. In that case, the flashing steam removal from the black liquor is below maximum and the need for evaporation in the evaporation plant increases. In view of the overall energy efficiency of the mill, it is advantageous to remove as much flashing steam from the black liquor as possible, provided that sensible objects of use are found, where it can be used to replace fresh steam with high efficiency.
Various techniques have been used for chip steaming. Conventionally, in addition to atmospheric steaming, a short-term pressurized steaming has been performed in a so-called steaming vessel or chip chute. Pressurized steaming is usually performed at a pressure of 1.0-1.5 bar. Pressurized steaming consumes a great amount of steam. Therefore, in connection with pressurized steaming, also the black liquor steaming is performed in two stages so that from the first stage the flashing steam is led into a steaming vessel and from the second stage into an atmospheric chip bin with a remarkably longer steaming retention time. In modern pulp mills, steaming in the chip bin is so efficient that pressurized steaming has been omitted and typically only one black liquor flashing phase is used. This way, the process is simpler and the investment costs are lower.
In modern fiberlines, the amount of hot water used in pulp washing has significantly decreased and often there is no reasonable use for the whole amount of hot water generated in the flashing steam condenser and the black liquor cooler, but the hot water has to be recooled and removed from the plant. Thus, part of the heat amount of the black liquor is lost. The objective of this embodiment of the invention is to utilize the excess flashing steam in a simple way, and simultaneously prevent the generation of concentrated malodorous gases and foul condensates in the digesting plant.
The solution according to this embodiment of the invention does not use a flashing steam condenser, but the excess flashing steam is led directly to a suitable evaporation stage of the evaporation plant, where it partly replaces fresh steam and thus improves the steam economy of the evaporation plant. Simultaneously, generation of foul condensate and concentrated malodorous gases in the area of the digester plant is prevented and the treatment thereof can to that extent be restricted to the evaporation plant, where they are inevitably treated anyway. A process devoid of a flashing steam condenser and treatment of concentrated malodorous gases and foul condensate is naturally smaller than a conventional process, including a favorable decrease in investment costs.
Although this embodiment is a novel and good solution and prevents generation of concentrated malodorous gases and foul condensate in the digester plant, it may have some drawbacks that can be addressed as follows. If flashing steam is available in greater amounts than needed for steaming, there is no way of utilizing this excess steam. Another disadvantage becomes obvious during shutdowns of the plant, whereby the flashing steam may not be led to steaming and no flashing of black liquor takes place. For this reason, the constructional pressure of the flashing tank should be significantly higher than normal in order to resist the high pressure resulting from the high temperature. Further, the cooling capacity of the black liquor cooler has to be much higher than normal, so that the black liquor may be cooled under 100° C. even in this situation. There is a further risk that in a situation where the flashing steam is not discharged from the flashing tank, concentrated malodorous gases are enriched in the flashing tank. When flashing steam is taken into use again, a large amount of concentrated malodorous gases may erupt. This forceful and sudden eruption of concentrated malodorous gases may cause dangerous situations in the steaming area or devices connected thereto. In this embodiment according to the invention, no enrichment would take place because flashing steam is continuously discharged to the evaporation plant.
The use of flashing steam in evaporation has also been practiced in connection with various pre-evaporation solutions. In these solutions, black liquor is flashed in several stages and flashing steam from a preceding stage is used for flashing of black liquor in the following stage. The flashing steam used in chip steaming is typically obtained from some pre-evaporator, and not directly from the flashing tank. In addition to the pre-evaporation plant, the mill also has a normal evaporation plant, where the dry solids content of the black liquor is increased to a level high enough for combustion. In practice, the pre-evaporation plant solutions have often been complicated processes and prone to black liquor foaming. For these reasons, pre-evaporation plants are undesireable.
When using the inventive systems, processes and methods of the invention, some may argue that the operation of the digester plant and the evaporation plant are more dependent on each other than normal. For example, if the amount of flashing steam led into the evaporation plant varies significantly, it may result in an irregular operation of the evaporation plant. This problem may be easily solved by stabilizing the steam flow going to the evaporation plant by means of an optional flow controller 16 and allowing the amount of steam for steaming to change in various fault situations of the digester plant. The amount of steam required for steaming may easily be compensated for in various fault situations by means of fresh steam without any practical negative effect on the overall steam economy.
Another challenging situation might occur if the evaporation plant is not in operation while the digester plant is operating. This situation is very rare and short-term because the black liquor storage tank between the digester plant and the evaporation plant fills quickly and would not permit the digester plant to operate without the operation of the evaporation plant. In this situation, the previously presented solution may be utilized, where flashing steam is used for chip steaming only. However, the sizing of the flashing tank and the black liquor cooler should be engineered for a situation where no flashing steam is removed from the black liquor. Another solution is to lead the excess flashing steam to a condenser in the evaporation plant. Possible condensers include, e.g., a surface condenser following the evaporation plant or a condenser for a stripper used in cleaning foul condensates from the evaporation plant. During short-term disturbances in the evaporation plant, these condensers can preferably receive steam. In case of a longer-term shutdown of the evaporation plant, there is no reason to operate the digester plant.
In principle, the flashing steam from the flashing tank 1 may led into any of the evaporation stages in the evaporation plant, where the pressure is low enough. Because the typical operational pressure in the flashing tank is about 0.2-0.5 bar overpressure, the evaporation stages operating at atmospheric or lower pressure are suitable. The most preferable solution, however, is to lead the steam into an evaporation unit having the highest possible pressure, because that provides the largest possible saving of fresh steam. In a typical modern 7-stage evaporation plant, it would be most advantageous to lead the flashing steam to the third evaporator stage. In that case, the flashing steam would be present and working in five evaporation stages and allow for saving fresh steam in a proportion of 5:7. In other words, one kilogram of flashing steam would decrease the consumption of fresh steam by 5/7 kilos. The use of flashing steam in the evaporation plant is most advantageous when the evaporation plant has at least six evaporation stages.
The principles of the invention may also be applied in situations where there is more than one flashing tank and also a pressurized steaming phase is used. When several flashing tanks are used, higher-pressure fractions may be separated from the total amount of flashing steam available, and the higher-pressure fractions may be used for various purposes. Nevertheless, the use of several flashing stages gives only a marginal benefit compared to the simple solution based on a single flashing stage according to the invention because, with the solution utilizing one flashing tank, all the flashing steam generated can be efficiently utilized in steaming and in the evaporation plant. “Efficient use” of flashing steam means that it replaces fresh steam usage with high efficiency. For this reason, the solution according to the invention is most advantageous when using one flashing tank.
While the invention has been described in connection with what are presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments, but on the contrary, is intended to cover various modifications and equivalent arrangements and variations included within the spirit and scope of the appended claims.
This application is based upon U.S. Provisional Patent Application No. 60/729,239, filed on Oct. 24, 2005, the entire disclosure of which is incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 60729239 | Oct 2005 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 11548911 | Oct 2006 | US |
| Child | 13096923 | US |
