The present invention relates to a method and an arrangement for producing chemical pulp at a continuous digester plant of a chemical pulp mill.

Prior art utilizes a fiberline system with a chip bin, where wood chips or other cellulosic material is steamed and liquid is admixed therein for forming a slurry, after which the slurry is pressurized (this section is also referred to as a feed system), fed into a treat- ment vessel or treatment vessels (which may be an impregnation vessel, a pre- hydrolysis process or other vessels), whereafter follows a digester. At present, at least one black liquor stream is extracted from the cooking system (typically at a temperature of 120-180°C). The extracted black liquor stream or streams can be used as a heat source for preheating white liquor, other black liquor streams and/or other liquid streams being led to feed and cooking systems. The extracted black liquor stream (or streams) can then be led to a pre-evaporation system, i.e. to one or several flash tanks, wherein steam is generated from hot black liquor as it is cooled, typically to a temperature of approximately 105-125 °C. After optional additional cooling to 90-95 °C the black liquor is led to an evaporator system of the recovery zone. The thus generated flash steam can be used at another location in the pulping process. Flash steam can e.g. be used for direct preheating of chips prior to cooking.

The above described flashing process has the disadvantage that the generated steam contains volatile compounds, including sulfur compounds, which are not desirable in wood chip presteaming. Typically, wood chips are steamed at atmospheric or slightly higher pressure so that residual gases are not absorbed into the wood chips, but they are collected and treated. The treatment is typically combustion in the non-condensable gas (NCG) system of the mill. This collection and treatment system becomes especially significant when the used steam contains volatile compounds, including sulfur com- pounds, which have a disadvantageous impact for the environment, including detrimental odor. Therefore, it is advantageous to use such a heat source that minimizes or eliminates the introduction of volatile compounds into the steaming process. In addition, an explosion risk exists, if concentrated gases entrained in flash steam are introduced into low concentration gases. Therefore, it is advantageous to find for flash steam other objects of use than pretreatment, such a steaming, of cellulosic material being fed into digestion. Processes have been presented for producing clean steam for pretreating cellulosic material. US-patent 6,722, 130 discloses a system for producing clean steam from black liquor. First the pressure of the black liquor is decreased for producing a second black liquor having a higher concentration and black liquor vapor, which is condensed to condensate. This condensate is heated with the first black liquor and steam-flashed for producing clean steam to be used in a chip bin.

WO 2007073333 discloses a system and a method for producing steam at a digester plant of a chemical pulp mill. The pressure of hot pressurized black liquor obtained from a digester is decreased in a first stage for producing black liquor steam that is used for chip pretreatment in a second preheating stage. Clean steam for pretreating chips in the first preheating stage is generated by re-heating the black liquor, the pressure of which was decreased prior to the final subsequent pressure decrease, where the increased black liquor steam volume is led into a steam converter for producing clean steam.

Known solutions offer various systems for using flash steam and for producing cleaner steam for heating needs of a digester plant and for improving the energy economy of a chemical pulp mill.

Steam is also needed for heating the fibrous material to cooking temperature in a digester, typically in a vapor phase of the digester. In known systems, live steam obtained from the turbine plant of the mill is typically used for this purpose. The steam is condensed in the fibrous material, and the thus generated condensate cannot be recovered However, the condensate of live steam is clean, and thus it could be recovered for reuse. US- patent 6346166 presents that the volume of flash steam obtained by flash evaporation of black liquor is increased by operatively connecting a jet ejector to a steam outlet of the flash tank. Steam at a higher pressure is introduced to the jet ejector from another source, whereby a mixture of flash steam and another steam is formed, which can be used as a steam source for heating fibrous material in a vapor phase digester. In this solution, too, live steam is consumed in the digester for heating the fibrous material. US- patent publication 8512514 discloses a process, in which a black liquor stream is extracted from the digester, which stream is evaporated in an evaporation device using live steam as an indirect heating medium. The steam generated from black liquor in the evaporation is used for heating the fibrous material at the top of the digester, whereby live steam requirement is decreased. An object of the present invention is to avoid the above mentioned problems and to provide an improved and alternative method of heating fibrous material at the top of the digester by utilizing the heat of black liquor as efficiently as possible. One object is also to improve the overall energy economy of a chemical pulp mill. Another object is to utilize flash steam of the black liquor at a location that is advantageous in view of its composition.

The present invention provides a new method, in which the use of live steam for heating fibrous material in a digester can be decreased. The new method comprises the following steps: a) a slurry formed of comminuted cellulosic fibrous material and liquid is led into a digester, heated, and the material is treated with a treatment chemical at a temperature of over 130 °C for producing treated material and black liquor;

b) at least one black liquor stream is extracted from the digester and the black liquor is led into an indirect heat exchange relationship with steam in a heat exchanger for heating the black liquor;

c) the heated black liquor is led into a flashing vessel, wherein flash steam and

flashed black liquor are generated; and

d) flash steam from step c) is used for heating the fibrous material in the digester in step a).

According to an embodiment, the method comprises a step, wherein the flashed black liquor from step c) is led into subsequent heat recovery stages, e.g. into a second flashing vessel, whereby a second flash steam and a second flashed black liquor are gener- ated. Heat can be recovered from the flashed black liquor also or alternatively e.g. in a kettle reboiler or in a black liquor cooler. The first mentioned allows to produce as known per se steam from a clean liquid for pretreating fibrous material and the latter allows heating water fractions of the mill.

A central feature of the new method is that extraction liquor from the digester is led through an indirect heat exchanger, wherein its heat is increased with live steam, typically with intermediate pressure steam, the pressure of which is typically 6-17 bar (g). By means of intermediate pressure steam, depending on its pressure, the temperature of the black liquor is typically increased into a level of approximately 160-190 °C, e.g. to 175-185 °C. Most advantageously the invention is utilized in a vapor phase digester. At a higher temperature, more flash steam is obtained from the same amount of liquor, but then a larger heat exchange area is needed in the heat exchanger, which can increase the equipment costs. After this the heated black liquor is led into a flashing vessel, where it is allowed to flash into steam that is used at the top of the digester for heating the fi- brous material instead of live steam.

The flashed black liquor can be led from the first flashing vessel into a subsequent flashing vessel, typically with other extraction liquor or other extraction liquors extracted from the digester, and the black liquor is flashed therein again. The flash steam of this second flashing vessel can be used in a vapor reboiler for producing clean steam, which can be used for steaming the fibrous material, such as wood chips, prior to the digester.

Black liquor is extracted from the digester at one or several levels depending on the cooking process, e.g. between cooking and washing zones. Extraction liquor from the digester, which is heated in a heat exchanger, can be either upper extraction liquor, lower extraction liquor, or if e.g. there are several lower extraction screens, extraction liquor from the uppermost screen row. Black liquor can be led into the heat exchanger also from two or more screen levels. According to a preferred embodiment, black liquor at the highest temperature is heated for optimizing the consumption of heating medium, steam, in the heat exchanger. The temperature of the black liquor stream extracted from the digester is dependent on the cooking mode. If there are more than one black liquor stream, the black liquor that is extracted at the end of the cooking stage from the lower part of the digester typically has the highest temperature.

According to an embodiment, in step b) at least two black liquor streams are extracted from the digester, which streams are at different temperatures. The black liquor stream at a higher temperature is heated in the heat exchanger, and the second black liquor stream at a lower temperature is led to heat recovery, such as into a second heat exchanger or a second flashing vessel.

The present invention can be applied in connection with both a single vessel digester and a two-vessel digester. Advantageously the digester is a vapor phase digester, whereby the flash steam is led into a gas space at the top of the digester for heating the fibrous material. The flash steam can also be fed into the upper part of a hydraulic digester with a steam injector, as described in WO-publication 2016/038251. An essential feature of the invention is heating of the extraction liquor with subsequent flashing, whereby the digester does not necessary need steam from outside the balance area, but the steam originates from internal circulation. Further, in an indirect heat exchanger the condensate of live steam can be recovered and led back to the boiler plant as boiler water.

By indirectly heating the liquor being led from the digester into a flash tank with steam, typically with medium pressure steam, the amount of generated flash steam can be in- creased (compared to a situation where the black liquor is not heated) and thus produce economically an adequate amount of heating steam the digester needs and decrease the use of direct live steam in the digester for heating the fibrous material. Flashed liquor can also be recirculated back via the heat exchanger into a flashing vessel, whereby the amount of flash steam can be further increased.

Easily volatile sulfur compounds entrained in flash steam are returned from the black liquor into the upper part of the digester, which compounds may to some extent be accumulated in the digester. This is not necessarily a negative phenomenon, but sulfur compounds may have impacts that accelerate the cooking reaction and improve the yield.

Description of the drawing:

Figure 1 illustrates schematically an embodiment according to the invention, and Figure 2 illustrates schematically another embodiment according to the invention,

Detailed description of preferred embodiments

A slurry formed of comminuted cellulosic fibrous material, such as wood chips, and liquid is led into the top of the digester 1 via line 2. Prior to the digester the slurry can alternatively be fed for treatment into a treatment vessel or vessels (can be an impregnation vessel, pre-hydrolysis vessel or other vessels, not shown). The figure illustrates only those components that are important in view of the invention. In this case the digester is a vapor phase digester. Also steam is added to the top of the digester via line 3.

The digester 1 comprises at least one or more screen arrangements 4, 5 and 6, via which black liquor can be extracted. Black liquor having a temperature of typically 120- 180 °C and a dry solids content of 12-17 %, is extracted via a screen 4 and line 7 and led into a flashing vessel 9. Line 7 is provided with a heat exchanger 8, in which the black liquor is heated indirectly with steam, typically medium pressure steam from line 24. The black liquor has typically been pressurized in the digester to a pressure of 5 - 15 bar (abs) and its temperature corresponds to the temperature of cooking treatment (approximately 130 - 180 °C). According to a preferred embodiment, black liquor at the highest temperature is heated for optimizing the consumption of heating medium, steam, in the heat exchanger 8. Live steam is condensed in the heat exchanger 8, and the thus generated live steam condensate in line 25 is clean. The condensate can be led into a recovery boiler plant of the mill, where it can be used as feed water.

In the flashing vessel 9 the pressure of the black liquor is decreased for producing flash steam and for producing flashed black liquor, the dry solids content of which typically increases in the flashing. The construction of the flashing vessel 9 is substantially conventional and it has a feed line 10 for hot black liquor, an outlet 1 1 for flash-evaporated vapor, and an outlet 12 for cooled black liquor at a lower pressure. Flashing vessels are typically operated so that the pressure prevailing in them is lower than the pressure of black liquor led thereto, but higher than the pressure at the top of the digester, typically approximately 4-7 bar (abs). Flashing vessels are containers designed for this purpose, which facilitate the pressure decrease of hot, pressurized black liquor, whereby liquid, typically water, is vaporized from the black liquor very quickly, into steam, whereby also the concentration of spent cooking chemical and the products of a cooking reaction (dissolved solids) is increased. The steam produced in this quick evaporation is discharged via a steam discharge opening 1 1 into line 3 at a pressure prevailing in the vessel 9, e.g. 4-7 bar (abs) and at a saturation temperature corresponding to the prevailing pressure, e.g. approximately 140 - 180 °C. After flash evaporation the remaining liquid, which typically also is at a temperature of approximately 160 °C settles onto the bottom of the vessel 9, from where it is discharged from an outlet 12 via line 13.

The flash steam from flashing vessel 9 is led via line 3 to the top of the digester 1 to be used as heating steam. The temperature of the steam, e.g. 140-180 °C, 140-180 °C, is typically higher than the cooking temperature, so that the steam that is led via the steam inlet of the digester, which inlet is in contact with the flashing vessel, heats the fibrous material to the cooking temperature. The flashed black liquor is led via line 13 to another flashing vessel 15 of the mill, where the black liquor is further flashed. A portion of the black liquor can be circulated via line 14 and heat exchanger 8 to flashing, and thus it is possible to further increase the amount of steam being generated in the flashing vessel 9. Black liquor is supplied into the second flashing vessel 15 also from the digester from digester screens, in this case from washing screen 6 via line 16. The liquid remaining after the flashing, which typically is at a temperature of approximately 100 - 140 °C, settles on the bottom of vessel 15, from where it is discharged via line 17. The discharge line 17 can be provided with a black liquor cooler 18, in which heat can be recovered from the black liquor. Thereby water in line 26 is typically heated for producing hot water, which discharges via line 28. The flashed, cooled black liquor is led via line 27 to the evaporation plant of the mill, where the black liquor is concentrated to a high solids content prior to combustion in a recovery boiler.

The flash steam from the second flashing vessel 15 is led via line 19 into a reboiler 20, wherein the energy contained therein is recovered. The steam is set in an indirect heat exchange contact with "clean" liquid free of volatile compounds for heating the liquid to a temperature exceeding the boiling temperature for producing clean steam. The reboiler is preferably a falling film tube reboiler. Flash steam is condensed inside the tubes, while clean liquid boils on the outer surface of the tubes for producing clean steam. Clean steam in line 22 is typically used for pretreating the fibrous material to be fed into the digester, such as for steaming wood chips. The clean liquid is led via line 21 into the reboiler 20. It can typically comprise condensate from an evaporator, demineralized water, or adequately clean water fraction, such as e.g. hot water of the mill. The clean steam produced in the reboiler contains a substantially smaller amount of non-condensible gases than steam produced by direct flashing of black liquor. Clean steam in line 22 is preferably used for wood chips preheating by means of steam. Flash steam from the black liquor flashing vessel 15 contains volatile compounds, such a sulfur compounds. These compounds enter the foul condensate and into the stream of concentrated non-condensible gases, CNCG, generated in the reboiler. The foul condensate is sent from the reboiler via line 23 to the evaporation plant where it is treated in a manner known per se. Heat can be recovered from the flashed black liquor exiting from the first flashing vessel also in other ways than additional flashing.

The produced pulp is discharged from the bottom of the digester via line 29. Figure 2 illustrates an alternative solution for leading black liquor from the digester into the first flashing vessel 9. Unlike the solution of figure 1 , black liquor is led into the first flashing vessel also from a second screen. The digester screen 6 comprises two screen levels 6a and 6b. At the end of the cooking zone black liquor is extracted via the upper screen row 6a into line 30 and via line 31 further into the flashing vessel 9. The temperature of this black liquor stream can be already adequately high so that it does not need heating prior to flashing. However, preferably the black liquor stream is led into heat exchanger 8 via lines 30 and 32.

Through a lower row of screens 6b black liquor is extracted via line 33 to heat recovery, e.g. flashing.

Example

In a prior art solution, where live steam is used for heating fibrous material, intermediate pressure steam is supplied to the top of the digester e.g. approximately 320 kg/adt. From the end of the cook, one extraction liquor fraction is taken from an extraction screen, which is e.g. 5,5 m ³ /adt at a temperature of 146 °C. Heat is recovered from this black liquor e.g. by flashing and the flash steam is used for steaming the chips.

In the solution according to the invention, the extraction liquor fraction taken from the end of the cook is divided by means of two separate screens into two streams. The amount of one of the streams is e.g. 3,0 m ³ /adt at a temperature of 169 °C and the amount of the other stream is 2,5 m ³ /adt at a temperature of 120 °C. The cooler stream of these is taken to conventional heat recovery. The hotter stream is taken into a heat exchanger, where it is heated indirectly with intermediate pressure steam to a tempera- ture of 180 °C, whereby steam is consumed in the amount of approximately 50 kg/adt. After this the heated liquor is taken into a flash tank, where the pressure of the liquor decreases to a pressure of 4,5 bar(g) and the temperature to a level of 158 °C. Thereby flash steam is generated in the amount of approximately 1 10 kg/adt, which is taken to the top of the digester, where the pressure level is approximately 4 bar(g). Thus, in this example the solution according to the invention provides a saving of 110 kg/adt of direct intermediate pressure steam at the top of the digester by using 50 kg/adt of indirect intermediate pressure steam in the heat exchanger. Simultaneously, black liquor is evaporated by 100 kg/adt reducing the evaporation need of the evaporation plant and hot condensate of intermediate pressure steam is recovered 50 kg/adt.

The present invention provides at least the following advantages: the total consumption of steam in the digester plant is decreased;

live steam condensate is recovered when flash steam can be used in the digester instead of live steam;

the equipment that is needed is simple.