SE2150125A1 - Indirect heating of digester - Google Patents

Abstract

The present invention relates to a method of producing chemical pulp in a continuous digester plant, which method comprises the following steps: a) leading a slurry formed of comminuted cellulosic fibrous material and liquid into a digester and treating the material with a treatment chemical at a temperature of over 130 °C for producing treated material;b) supplying a partial stream of incoming liquor to the digester (1), wash liquor from brown stock washing, leading the wash liquor stream into an indirect heat exchange relationship with steam in a heat exchanger (8) for heating the wash liquor;c) leading the heated wash liquor into a flashing vessel (9), wherein flash steam and flashed wash liquor are generated, andd) using flash steam from step c) for heating the fibrous material in the digester in step a).

Description

INDIRECT HEATING OF DIGESTER TECHNICAL FIELD The present invention relates to a method and an arrangement for producing chemical pulp at a continuous digester plant of a chemical pulp mill.

BACKGROUND Prior art utilizes a fiberline system With a chip bin, Where Wood chips or other cellulosicmaterial is steamed and liquid is admixed therein for forrning a slurry, after Which theslurry is pressurized (this section is also referred to as a feed system), fed into atreatment vessel or treatment vessels (Which may be an impregnation vessel, aprehydrolysis 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 l20-l80°C). The extracted black liquor stream or streamscan be used as a heat source for preheating White liquor, other black liquor streamsand/or other liquid streams being led to feed and cooking systems. The extracted blackliquor stream (or streams) can then be led to a pre-evaporation system, i.e. to one orseveral 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 additionalcooling 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 steamcontains volatile compounds, including sulfur compounds, Which are not desirable inWood chip presteaming. Typically, Wood chips are steamed at atmospheric or slightlyhigher pressure so that residual gases are not absorbed into the Wood chips, but they arecollected and treated. The treatment is typically combustion in the non-condensable gas(NCG) system of the mill. This collection and treatment system become especiallysignificant 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. Inaddition, an explosion risk exists, if concentrated gases entrained in flash steam areintroduced into low concentration gases. Therefore, it is advantageous to find for flashsteam other objects of use than pretreatment, such a steaming, of cellulosic materialbeing fed into digestion. Processes have been presented for producing clean steam forpretreating cellulosic material. US-patent 6,722, 130 discloses a system for producingclean steam from black liquor. First the pressure of the black liquor is decreased forproducing 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.

Steam is also needed for heating the f1brous material to cooking temperature in adigester, typically in a vapor phase of the digester. In known systems, live steamobtained from the turbine plant of the mill is typically used for this purpose. The steamis condensed in the fibrous material, and the thus generated condensate cannot berecovered. However, the condensate of live steam is clean, and thus it could be recovered for reuse.

WO 2017/212111 discloses a method of producing chemical pulp at a continuous plant,black liquor from the digester is heated and flashed and flash steam is used to heat material in the digester.

SUMMARY OF THE INVENTION An object of the present invention is to avoid the above mentioned problems and toprovide an improved and altemative method of heating f1brous material at the top of thedigester by utilizing parts of the wash liquor from the brown stock washing asefficiently as possible and whereby decrease the requirement of live steam. One object is also to improve the overall energy economy of a chemical pulp mill.

The present invention provides a new method, in which the use of live steam for heatingfibrous material in a digester can be decreased. The new method comprises the following steps: a) leading a slurry formed of comminuted cellulosic fibrous material and liquid into adigester and treating the material with a treatment chemical at a temperature of over 130°C for producing treated material; b) adding a partial stream of incoming liquor to the digester from brown stock washing(wash liquor), leading the wash liquor stream into an indirect heat exchange relationshipwith steam in a heat exchanger for heating the wash liquor; c) leading the heated wash liquor into a flashing vessel, wherein flash steam and flashedwash liquor are generated, and d) using flash steam from step c) for heating the f1brous material in the digester in step a).

A central feature of the new method is that a partial stream of wash liquor from thebrown stock washing is led through an indirect heat exchanger, wherein its heat isincreased with live steam, typically with interrnediate pressure steam, the pressure ofwhich is typically 6-17 bar (g). By means of interrnediate pressure steam, depending onits pressure, the temperature of the wash liquor is typically increased into a level ofapproximately 160-190 °C, e.g. to 175-185 °C. Most advantageously the invention isutilized in a vapor phase digester. At a higher temperature, more flash steam is obtainedfrom the same amount of liquor, but then a larger heat exchange area is needed in theheat exchanger, the equipment costs increases. After this the heated wash liquor is ledinto a flashing vessel, where it is allowed to flash into steam that is used at the top of thedigester for heating the f1brous material instead of live steam.An advantage according to the invention is to use wash liquor instead of for exampleblack liquor. Wash liquor is cleaner and contains fewer dry solids. When flashing theliquor, the dry solids is increased and in the circulation between the heat exchanger,flashing vessel, incoming and extracting liquor the dry solids needs to be extracted.Since wash liquor contains fewer dry solids a minor extraction is needed and a minorextraction, the less energy loss. This improves the overall energy economy of a chemical pulp mill.

The present invention can be applied in connection with both a single vessel digesterand 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 thef1brous material. The flash steam can also be fed into the upper part of a hydraulicdigester with a steam injector, as described in WO-publication 2016/038251. Anessential feature of the invention is heating of the wash liquor with subsequent flashing,whereby the digester does not necessary need steam from outside the balance area, butthe steam originates from intemal circulation. Further, in an indirect heat exchanger thecondensate of live steam can be recovered and led back to the boiler plant as boiler water.

By indirectly heating the wash liquor being led from the brown stock washing into aflash tank with steam, typically with medium pressure steam, the amount of generatedflash steam can be increased, compared to a situation where the wash liquor is notheated, and thus produce economically an adequate amount of heating steam thedigester needs and decrease the use of direct live steam in the digester for heating thef1brous material. Flashed wash liquor can also be recirculated back via the heatexchanger into the flashing vessel, whereby the amount of flash steam can be further increased.

Additional benefits and advantages of the invention will become readily apparent to the person skilled in the art in view of the detailed description below.

DRAWINGS The invention will now be described in more detail with reference to the appendeddrawing, wherein Fig. l discloses a schematic view of an arrangement according to the invention.DETAILED DESCRIPTION A slurry formed of comminuted cellulosic fibrous material, such as wood chips, and liquid is led into the top of a digester l via line 2. Prior to the digester l the slurry can altematively be fed for treatment into a treatment Vessel or Vessels (can be animpregnation Vessel, pre-hydrolysis Vessel or other Vessels, not shown). Figure 1 onlyillustrates those components that are important in View of the inVention. In this case thedigester 1 is a Vapor phase digester according to prior art. Steam is also added to the topof the digester 1 Via line 3 .According to the inVention a partial stream 70 of incoming liquor to the digester 1 issupplied from brown stock washing and is led to a circulation comprising a pump 4, aheat exchanger 8 and a flashing Vessel 9. The partial stream of wash liquor 70 is led tothis circulation and is pumped to the heat exchanger 8 through line 7. The wash liquor isled into the heat exchanger 8 in which the wash liquor is heated indirectly with steam,typically medium pressure steam from line 24.

The wash liquor has typically a pressure of 8-12 bar and its temperature corresponds to the temperature of washing treatment (approximately 80-90°C) According to a preferred embodiment, wash liquor at the highest temperature is heatedfor optimizing the consumption of heating medium, steam, in the heat exchanger 8. Livesteam is condensed in the heat exchanger 8, and the thus generated live steamcondensate 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.

After the heat exchanger 8 the pressure of the wash liquor is decreased for producingflash steam in the flashing Vessel 9. The construction of the flashing Vessel 9 issubstantially conVential and it has a feed line 10 for hot wash liquor, an outlet 11 forflash-evaporated Vapor and an outlet 12 for cooled wash liquor at a lower pressure.

Flashing Vessels are typically operated so that the pressure preVailing in them is lowerthan the pressure of the wash liquor led thereto, but higher than the pressure at the topof the digester 1, typically 4-7 bar. Flashing Vessels are containers designed for thispurpose, which facilitate the pressure decrease of hot, pressurized wash liquor, wherebyliquid, typically water, is Vaporized from the wash liquor Very quickly, into steam,whereby also the concentration of spent cooking chemicals 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 11 into the line 3 at a pressureprevailing in the flashing vessel 9 e.g. 4-7 bar and at a saturation temperaturecorresponding to the prevailing pressure, e.g. approximately 140 - 180 °C. After flashevaporation the remaining liquid, Which typically also is at a temperature ofapproximately 160 °C settles onto the bottom of the vessel 9, from Where it isdischarged from an outlet 12 and is led back to the pump 4. The supply of Wash liquorthrough line 70 and a discharge, through a line 71, is continuous to keep the levels ofpollutants at a stable level.The flash steam from flashing vessel 9 is led via line 3 to the top of the digester 1 to beused as heating steam. The temperature of the steam, e.g. 140-180 °C, 140-180 °C, istypically higher than the cooking temperature, so that the steam that is led via the steaminlet of the digester 1, Which inlet is in contact With the flashing vessel 9, heats thefibrous material to the cooking temperature. The flashed Wash liquor is circulated vialine 14 and heat exchanger 8 to flashing, and thus it is possible to further increase theamount of steam being generated in the flashing vessel 9.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.

Claims (6)

Claims

1. l. A method of producing chemical pulp in a continuous digester plant, Which method comprises the following steps: a) leading a slurry formed of comminuted cellulosic f1brous material and liquid into adigester and treating the material With a treatment chemical at a temperature of oVer 130°C for producing treated material; b) supplying a partial stream of incoming liquor to the digester (l), Wash liquor frombroWn stock Washing, leading the Wash liquor stream into an indirect heat exchangerelationship With steam in a heat exchanger (8) for heating the Wash liquor; c) leading the heated Wash liquor into a flashing Vessel (9), Wherein flash steam andflashed Wash liquor are generated, and d) using flash steam from step c) for heating the f1brous material in the digester in step a).

2. The method according to claim l, characterized in that in step b) live steam is used that is interrnediate pressure steam.

3. The method according to claim l or 2, characterized in that flashed Wash liquor from the flashing Vessel is circulated Via the heat exchanger back to flashing.

4. The method according to claim 3, characterized in that a partial stream of the flashed Wash liquor from the flashing Vessel is extracted from the circulation.

5. The method according to claim l, characterized in that the supply of incoming Wash liquor in step b) is continuous.

6. The method according to claim 4, characterized in that the extraction of flashed Wash liquor is continuous.