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Title:
METHOD AND ARRANGEMENT FOR STEAMING AND IMPREGNATING WOOD CHIPS IN A DOWN FLOW VESSEL
Document Type and Number:
WIPO Patent Application WO/2011/053203
Kind Code:
A1
Abstract:
The invention relates to a method and arrangement for steaming and impregnating wood chips in a single down flow vessel. A first upper volume of chips is established above a liquid level (LIQ_LEV) in the vessel, wherein steaming takes place. Said upper volume having a height (H1) of at least 2-5 meter. A second lower volume of chips is established below the liquid level, wherein an impregnation takes place. Said lower volume having a height (H1) of at least 2-5 meter.In order to reduce the torque demand on the bottom scraper are plug holding forces applied against the descending plug flow at a distance (H3) below the liquid level less than 20% of the height (H1) of said lower volume, such that a minimum free and unobstructed plug flow in the first volume is established which unobstructed plug flow is established at a vertical distance in said vessel exceeding 2-5 meter above the liquid level.

Inventors:
SAETHERAASEN JONAS (SE)
SAMUELSSON ANDERS (SE)
WILGOTSSON FREDRIK (SE)
NYSTROEM PER (SE)
Application Number:
PCT/SE2009/051228
Publication Date:
May 05, 2011
Filing Date:
October 28, 2009
Export Citation:
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Assignee:
METSO PAPER SWEDEN AB (SE)
SAETHERAASEN JONAS (SE)
SAMUELSSON ANDERS (SE)
WILGOTSSON FREDRIK (SE)
NYSTROEM PER (SE)
International Classes:
D21C1/00; D21C1/02; D21C3/24
Domestic Patent References:
WO2006006934A12006-01-19
Foreign References:
US5635025A1997-06-03
US5454490A1995-10-03
US6280569B12001-08-28
Attorney, Agent or Firm:
FURHEM, Hans (Box 1033, Karlstad, SE)
Download PDF:
Claims:
Claims

A method for steaming and impregnating wood chips, during the manufacture of chemical pulp in a continuous process, using a down flow vessel (30) where chips are descending down the vessel in a plug flow and where treated chips are continuously fed out at the bottom of the vessel, said vessel having an upper volume of chips above a liquid level (LIQ_LEV) established in the vessel, said upper volume having a height (HO) of at least 2-5 meter, or an equivalent retention time of the chips in the upper volume exceeding 30 minutes, and said vessel having a lower volume of chips below the liquid level, said lower volume having a height (H1 ) of at least 2-5 meter, or an equivalent retention time of the chips in the lower volume exceeding 30 minutes and where steam and impregnation liquid is supplied to the vessel characterized in that plug holding forces are applied against the descending plug flow at a distance (H3) below the liquid level less than 20% of the height (H1 ) of said lower volume, such that a minimum free and unobstructed plug flow in the first volume is established which unobstructed plug flow is established at a vertical distance in said vessel exceeding 2-5 meter above the liquid level, said distance depending on the type of wood being steamed, i.e. specific weight thereof, and the steaming effect in the chip pile above the liquid level.

A method according to claim 1 characterized in that below the position for applying the plug holding forces against the descending plug flow is an unobstructed plug flow established at a vertical distance in said vessel exceeding 2-5 meter below the position for applying the plug holding force against the descending plug flow.

An arrangement for steaming and impregnating wood chips, during the manufacture of chemical pulp in a continuous process, using a down flow vessel (30) where chips are descending down the vessel in a plug flow and where treated chips are continuously fed out at the bottom (10) of the vessel, said vessel having an upper volume of chips above a liquid level established in the vessel, said upper volume having a height (HO) of at least 2-5 meter or an equivalent retention time of the chips in the upper volume exceeding 30 minutes, and said vessel having a lower volume of chips below the liquid level, said lower volume having a height (H1 ) of at least 2-5 meter, or an equivalent retention time of the chips in the lower volume exceeding 30 minutes, and where steam and impregnation liquid is supplied to the vessel characterized in that baffles (25a,25b) protruding towards the center of the vessel are arranged on the interior wall of the vessel at a distance (H3) below the liquid level established, said baffles protruding towards the center of the vessel at a distance (D) being in the range 5- 20% of the interior diameter (D0) of the vessel at the height positions of the baffles (25a-25c). 4. The arrangement according to claim 3 characterized in that the upwardly directed face of the baffles (25a,25b) are sloping downwardly at an angle (a) from the wall of the vessel, and where said angle is in the range 5-30 degrees and preferably about 10 degrees. 5. The arrangement according to claim 4 characterized in that the baffles (25a, 25b) are integrated with means for supplying steam or impregnation liquid (BL2/ST) into the void volume formed below each baffle.

6. The arrangement according to claim 5 characterized in that the baffles (25a) have a continuous ring formed shape around the interior circumference of the vessel.

7. The arrangement according to claim 5 characterized in that the baffles (25b) have a discontinuous shape around the interior circumference of the vessel, with a multitude of baffles being arranged evenly distributed over the interior circumference of the vessel, and wherein each baffle only cover less than 30 degrees of the interior circumference and with a free space between neighboring baffles. 8. The arrangement according to any of preceding claims 2-7 characterized in that at least a second row of baffles (25c) are arranged at a distance (H4) below the first row of baffles (25a).

Description:
Method and arrangement for steaming and impregnating wood chips in a down flow vessel

Technical Field

The present invention concerns a method and an arrangement for steaming and impregnating wood chips, during the manufacture of chemical pulp in a continuous process, using a down flow vessel where chips are descending down the vessel in a plug flow and where treated chips are continuously fed out at the bottom of the vessel, said vessel having an upper volume of chips above a liquid level established in the vessel, said upper volume having a height of at least 2-5 meter, and said vessel having a lower volume of chips below the liquid level, and where steam and impregnation liquid is supplied to the vessel, according to the preamble of claim 1 and claim 2. State of the art

In old conventional Prior Art for producing chemical cellulose pulp with continuous digesters, it was common to use a pre-treatment arrangement with a chip bin, steaming vessel and an impregnating chip chute, before the cooking process is established in the digester. Steaming has been carried out in one or several steps in the chip bin and/or steaming vessel, prior to the subsequent formation of a slurry of the chips in an impregnation fluid or a transport fluid in process equipment separate from the steaming equipment. The extensive steaming was considered to be necessary in order to expel air and water bound in the chips, such that the impregnation fluid can fully penetrate the chips, and enables a stable plug flow through the digester.

Attempts have been made to integrate the chip bin with the impregnation vessel in order to obtain in this manner a simpler system. Already in US 2.803.540, a system was revealed in which the chips from a chip bin were fed to a vessel in which a combined steaming and impregnation was achieved. In this vessel, the chips were steamed at the upper part of the vessel and impregnation fluid at the same temperature was added at various levels in the vessel using distributing annular manifolds/headers located outside of the vessel wall, and having nozzles penetrating the wall. These principles were also applied in a process known as "Mumin cooking", which is described in "Continuous Pulping Processes", Technical Association of the Pulp and Paper Industry, 1970, Sven Rydholm, page 144.

A similar system with a common low pressure steaming and impregnation vessel is shown in US 3.532.594, which also was put into operation for example at the Skoghall mill in Sweden. Here was heated impregnation liquid added via a central pipe to the chip volume, but also showing an additional central pipe for steam supply. This system was later abandoned due to various reasons such as run ability problems, capacity problems in subsequent feeding system and far too high reject and shive content in the blown pulp.

A system is shown in US 5.635.025 in which the chips are fed without prior steaming to a vessel in the form of a combined chip bin, impregnation vessel and chip chute. Steaming of the chips takes place here, the chips lying above the fluid level, and a simple addition of impregnation fluid takes place trough the vessel wall below the liquid level. A further such system is revealed in US 6.280.567, in which the chips are fed without prior steaming to an atmospheric impregnation vessel in which the chips are heated by the addition of warm black liquor that maintains a temperature around 130-140 C. The added impregnation liquid is added via nozzles in a manner similar to what is shown in US 2.803. 540, i.e. using supply nozzles penetrating the wall of the vessel.

An alternative system is revealed by SE 523.850 in which pressurized black liquor is added to the upper part of the steaming vessel, whereby the black liquor after being subjected to a pressure reduction releases steam for the steaming process. The addition of the hot black liquor is made by a horizontal supply pipe penetrating the chip bed, and having a multitude of holes over the length of the pipe.

The technique with a common treatment vessel for steaming and impregnation at substantially atmospheric conditions is marketed by Metso Paper under the name of IMPBIN™. Several improvements of the concept have been patented in; • SE 518.738 (=US7381302), with impregnation liquids added at successively increasing temperature at positions in IMPBIN with higher static head;

• SE 528.448 (=EP1818445), with liquor circulations of IMPBIN separated from those in digester;

· SE 530.725 (=EP2065513), with cooling showers in top of IMPBIN for knocking down blow trough of malodorous gases.

One important feature in the IMPBIN™ systems is that a certain height of chips should be established above the liquid level in order to impose a downward thrust on the steamed chips as they sink down into the liquid level established. This downward thrust is necessary to establish a positive sinking motion of the chips as there may still remain a buoyancy force from individual chips. However, in some installations the total static height from the chip pile towards the bottom scraper is high which results in excessive torque demand on the bottom scraper motor. There are thus contradicting problems in establishment of the necessary downward thrust on the chip pile towards the liquid surface, and on the other hand excessive total load on the bottom scraper.

Solutions for reducing the pressure in the bottom of dry storage silos is known from DE1 .907.147, US 3.710.959, US 4.077.526 and US 5.454.490.

The solution published in 1970 by DE 1 .907.147 show the principle of a ring-shaped insert in order to reduce compression forces against the outlet.

Said US 3.710.959 showing inserts functioning like frictions brakes in form of smaller segments arranged on the interior wall above the bottom. Said friction brakes inducing an increased friction against the dry matter content stored in the silo, thus reducing the pressure in the bottom of the silo.

Said US 4.077.526 showing multiple types of friction brakes on the silo wall, including ring shaped inserts.

Said US 5.454.490 also showing conical inserts in form of cone frustums arranged around the entire circumference of the interior wall above the bottom. All these solutions are applicable for dry matter material stored in the silo.

In US 6.280.569 is shown compression rings for column relief in a continuous digester. The alleged purpose here is to reduce the friction forces against the wall, which is actually the opposite function as obtained from the friction brakes shown in any of DE1 .907.147, US 3.710.959, US 4.077.526 and US 5.454.490. The argued function in reduced friction is somewhat odd, as experience tell that the friction instead increase over any such compression ring. The compression rings shown in US 6.280.569 are also in their physical form identical to well known sacrificial anodes that are mounted on the digester wall in order to reduce corrosive wear.

The prior art solutions for dry material are based upon the fact that the "friction brakes" or baffles are to be arranged pressing towards the dry matter pile built up in the silo, and arranged close to the bottom of the silo.

The prior art solutions for wet material, i.e. liquid slurry of chips, are based upon the fact that these compression rings should decrease the friction between the chips and the wall of the digester, obtaining a more uniform flow of material in the vessel. None of these solutions are applicable for treatment vessels of the IMPBIN type, where a downward thrust from the chip pile above the liquid level is needed, and where the impregnation stage established below the liquid level should have a substantial retention time for a thorough impregnation. Yet another solution for impregnation vessels is shown in US 2009/00220244, where a convergence section is installed in the lower part of the impregnation vessel for the specific purpose of shielding an addition of hot liquid to the later phases of impregnation, avoiding establishment of heat currents from flowing upwards. The shield as outlined in this application is located closer to the bottom and has a substantial reduction in cross-sectional area of the flow path and is rather exposed for chip clogging or bridging in the outlet section, especially as the chips will be softened during the impregnation and more easily may block the plug flow.

The object and purpose of the invention

The principle object of the invention is to obtain an improved method and arrangement for steaming and impregnating wood chips, during the manufacture of chemical pulp in a continuous process, using a down flow vessel where chips are descending down the vessel in a plug flow and where treated chips are continuously fed out at the bottom of the vessel, said vessel having an upper volume of chips above a liquid level established in the vessel, said upper volume having a height of at least 3-5 meter, and said vessel having a lower volume of chips below the liquid level, and where steam and impregnation liquid is supplied to the vessel. A specific objective is to enable the greatest possible downward thrust from the chip pile established above the liquid level, allowing this thrust force to push the steamed chips downwardly into the liquid volume, which liquid volume impose a wet-steaming effect on the chips displacing any residual air bound in the chips, but first when this wet-steaming effect has been obtained at a certain depth into the impregnation liquid is a plug holding force applied against the descending plug flow now being immersed in the impregnation liquid.

Another specific objective is to establish a non disturbed dense packed chip pile above the liquid level, which chip pile is to be steamed by the hot impregnation liquids being added into the chip pile. When using the technique with addition of hot pressurized black liquor into the lower parts of the chip pile a compacted chip pile is beneficial for establishment of a even and thorough steaming effect over the entire cross section of the chip pile. The dense packed chip pile also enables establishment of a temperature gradient upwards in the chip pile, such that a so called "cold top" control of the chip pile is possible. "Cold top" control avoids continuous blow-trough of hot steam in evacuated gases and associated losses in energy in the chip steaming process.

The invention can advantageously be used when cooking hard wood and softwood wood chips, bagasse and other annual plants.

Short description of the invention

The characteristics of the invention are defined by the independent claims, and optional embodiments are defined in dependent claims in order of dependency of preceding claims. The invention is also disclosed in a number of preferred embodiments, but any specific feature of these embodiments could as such be included in the invention optionally, if not specifically defined as a necessary feature for the argued effect. Description of drawings

Figure 1 shows an impregnation vessel according to state of the art;

Figure 2 shows a detail of a first embodiment of the invention;

Figure 3 shows a detail of a second embodiment of the invention;

Figure 4 shows a detail of a third embodiment of the invention;

Figure 5 shows a detail of a fourth embodiment of the invention;

Figure 6 shows a detail of a fifth embodiment of the invention

Detailed Description of Preferred Embodiments

During following description will the term "treatment liquor" be used, and by this is meant one kind of treatment liquor that is intended to be added to the entire chip flow in the treatment vessel. This treatment liquor could comprise only of spent cooking liquor, i.e. black liquor, from a subsequent digester, but could also be a mixture of two or more of following liquids;

· black liquor,

• fresh cooking chemicals such as white liquor (and additives e.g. antraquinon),

• dilution liquids from subsequent wash stage (i.e. wash filtrate from such wash stages),

• steam (added directly in order to heat the treatment liquor).

The term "treatment liquid" will also be used, and by this is meant the treatment liquid established in or withdrawn from the vessel, which besides partially used treatment liquor also contains chip moisture or any other organic or inorganic content dissolved from the chips.

The term "treatment vessel" will also be used, and by this is meant any kind of treatment vessel used for treating chips in either form of delignification or impregnation state, i.e. the vessel could be an atmospheric combined steaming and treatment vessel like IMPBIN.

The term "comminuted cellulose material" will be used, which preferably could be in form of wood chips, but also more fragmented wood material such as sawdust or pin chips, all obtained from either hardwood or softwood or annual plants.

A prior art arrangement for the impregnation of chips during the manufacture of chemical pulp is shown in Figure 1 . In all essential parts this arrangement corresponds to the IMPBIN concept sold by Metso, including the method of wet- steaming chips in a substantially atmospheric pressure (±0,5 bar, i.e. not a pressure vessel). The arrangement comprises an essentially cylindrical impregnation vessel 30 arranged vertically into which unsteamed chips Ch are continuously fed into the top of the impregnation vessel via feed means, in the form of a small chip buffer 1 without steaming and a chute feed (preferably a sluice feeder) 2. The chips that are fed into the impregnation vessel are thus unheated chips that normally have the same temperature as the ambient temperature.

The pressure in the vessel can be adjusted as necessary through a control valve 31 arranged in a valve line 4 at the top of the impregnation vessel, possibly also in combination with control of the steam ST via input lines 5.

When atmospheric pressure is to be established, this valve line may open out directly to the atmosphere. It is preferable that a pressure is established at the level of atmospheric pressure, or a slight deficit pressure by the outlet 4 of magnitude -0.5 bar (-50 kPa), or a slight excess pressure of magnitude up to 0. 5 bar (50 kPa).

Input of a ventilating flow, SW_AI R (sweep air), may be applied at the top as necessary, which ensures the removal of any gases present or brought into the vessel by the in feed of comminuted cellulosic material.

The impregnated chips are continuously fed out via output means, here in the form of an outlet 1 0, with a bottom scraper 20 driven by a motor 22 via shaft 21 .

The level of the chips, CH_LEV, above the level of the liquid, LIQ_LEV, should preferably be at least 2 meters and preferably at least 5 meters when impregnating wood chips, or an equivalent retention time of the chips in the upper volume exceeding 1 0 minutes. The chip level is monitored by any suitable chip level meter means, and the output from such level meter controls the inlet feed rate of chips, here visualized as a valve V C H in the feed line for chips.

In the case of pulping raw material of low density, for example softwood, a corresponding increase in the height HO towards the upper part of the range is preferably established. In the case of pulping raw material of high density, for example hardwood and especially eucalyptus, a corresponding decrease in the height towards the lower part of the range is preferably established. The actual mini mum height established in this range 2-5 meters is also depending upon the temperature of the chips being fed into the vessel, as colder chips during winter operation may need more retention time in the steaming phase in the chip pile. This minimum height is important in order to provide an optimal passage of the chips in an even plug flow through the vessel, and establishment of a positive thrust force, indicated by F-marked arrow, on the chips that reach the liquid level and still may have some buoyancy effect left.

When impregnating primarily easily cooked types of wood during the warm season, such as eucalyptus and other annual plants, steaming with fresh steam can be essentially avoided if hot black liquor, typically spent cooking liquor from digester at cooking temperature in the range 130-150 degrees C, is added as the impregnation liquid, which black liquor during pressure release emits all the steam necessary. Fresh steam is thus not necessarily added to the chips that lie on top of the fluid level established by the impregnation fluid during normal steady-state operation. The invention can also be applied even if softwood with lower density is used as raw material, giving a markedly reduced need for steaming, that is, a reduced addition of fresh steam.

When treating primarily wood raw material that is difficult to cook, especially softwood having less density, and in operational cases with extremely low temperature of the chips, (such as during the cold season), the chips that lie above the fluid level established by the impregnation fluid can be heated by the addition of external steam ST such that a temperature of the chips approach 60-100° C in the chip pile before the chips reach the fluid level that has been established by the treatment liquor.

The treatment liquor added via supply pipe or pipes, here a common central pipe 7a, can also be established as a mixture from totally separate sources, that is, not from one common flow of black liquor. For example, the added liquor may also contain a wash filtrate.

The liquid level is monitored by any suitable liquid level meter means, and the output from such level meter controls the inlet valve V L i in the feed line for the impregnation liquors added. The chips immersed in the lower volume below the liquid level should have a sufficient retention time for a thorough impregnation, preferably such that said lower volume has a height H1 of at least 2-5 meter, or an equivalent retention time of the chips in the lower volume exceeding 1 0 minutes The treatment liquor added can also be a mixture of black liquor and an additive amount of fresh cooking chemicals, i.e. white liquor, with the object of establishing alkali profiles that are necessary for the process. A rapid initial consumption of alkali normally takes place, simply in order to neutralize the wood acidity, while it is desired to keep the final residual alkali after the impregnation stage at a certain level. The needed amount of additional alkali is mostly dependent on the level of wood acidity being released during steaming in the chip volume above the liquid level, and thus dependent on type of wood being treated (softwood or hardwood).

The vessel may be equipped without or with extraction screens 6 to allow liquid extraction (to recovery REC) early in the process.

In figure 2 is a detail view of a first embodiment of the invention. As is disclosed here is a ring shaped conical baffle 25a located a distance H3 below the liquid level, LIQ_LEV. In this and following embodiments of the invention the baffles 25a protrudes towards the center of the vessel at a distance D1 being in the range 5-20% and preferably 10% of the interior diameter of the vessel at the positions of the baffles. In a vessel with an interior diameter of 4 meter at the height positions of the baffles this distance D1 could thus preferably lie in the range 20 to 80 centimeter and preferably correspond to 40 centimeter.

In this first embodiment there is a withdrawal screen 6 at or close to the liquid level LIQ_LEV. Spent treatment liquid is withdrawn from this screen 6, typically to recovery REC, via conventional chambers arranged outside of the screen 6. A step-out SO is in a conventional manner established below the screen section, and the baffle 25a I located at a distance D2 below the step-out SO.

In this and following embodiments of the invention the upwardly directed face of the baffles 25a are sloping downwardly from the wall of the vessel, and preferably with an inclination angle a in the range 5-30 degrees, and preferably about 10 degrees. In the arrangement shown in figure 2 the baffle 25a have a continuous ring formed shape around the interior circumference of the vessel, and could preferably be built by a single deflection plate, leaving a void beneath the deflection plate.

The baffle 25a will thus introduce a plug holding force applied against the descending plug flow at a location at the distance H3 below the liquid level in said vessel such that a minimum free and unobstructed plug flow in the first volume above the liquid level is established which unobstructed plug flow is established at a distance exceeding 2-5 meter above the liquid level, said distance depending on the type of wood being steamed, i.e. specific weight thereof, and the steaming effect in the chip pile above the liquid level. In figure 3 is a second alternative embodiment shown but without a withdrawal screen. As there are no screen section, the baffle is arranged closer to the liquid level LIQ_LEV, such that the distance H3 equals the distance D2. The distance D2 is given by the expansion effect on the chip pile after a step-out. The chip pile expansion is indicated by the hashed line PE on the right hand side in figure 3.

In figure 4 is a third alternative embodiment shown but without a withdrawal screen or step-out. As there is no screen section or no step-out, the baffle is arranged even closer to the liquid level LIQ_LEV, such that the distance H3 equals the distance D2. According to the invention should a high downward thrust force, the F-marked arrow, be established down towards the liquid surface, and as close as possible beneath the liquid level should the baffle 25a in embodiments shown in figures 2-4 be located. This distance H3 is typically exceeding 0,2 meter, but less than 1 meter, if located in a vessel without a step out SO at the liquid level according to the embodiment shown in figure 4. If there is a step out SO in the vessel, as shown in figure 2 or 3, then the chip plug normally expands to the full vessel diameter, following the hashed line PE on the right hand side in figure 3, some 80-100 centimeter below the step out, indicated by the distance D2. The effect of this expansion could be observed on the interior wall below step-outs in installed digesters during shut-down, as the polishing effect from the chip pile on the interior wall is fully developed first at a distance some 80-100 centimeter below the step out.

Thus, if the vessel 30 lack any screen section or step out in the above lying liquid volume, this distance may typically lie in the range 0,2 to 1 meter, i.e. the indicated distance D2 in figure 4, and 1 -2 meter if a step out SO is located above in the liquid volume i.e. the indicated distance D2 plus the height of the screen section 6 in total corresponding to H3 in figures 2-3.

In figure 5 is a fourth alternative embodiment shown but where the baffles 25 are made from a plurality of individual baffle segments 25b located around the inner circumference of the vessel wall. In this embodiment the baffle segments 25b are all located at the same horizontal plane. All other details are corresponding to the embodiment shown in figure 2. In figure 6 is a fifth alternative embodiment shown but where the ring shaped baffle 25a according figure 2 is complemented by a second lower ring shaped baffle 25c at a distance H4 below the first upper shaped baffle 25a. Again, the distance D3 may typically lie in the range 0,2 to 1 meter below the step out generated by the first upper baffle 25a, such that the chip plug could expand to the full vessel diameter below the step out established by the first upper baffle 25a. In this embodiment are also a multitude of baffle segments 25d located around the inner circumference of the vessel wall above the liquid level. This additional baffle segments 25d used for supplying steam or hot treatment liquor at the periphery of the vessel, besides offering an additional load reduction on the bottom scraper.