FI13409Y1 - Arrangement for producing chemical pulp at a digester plant of a chemical pulp mill - Google Patents

Abstract

Arrangement for producing chemical pulp in a continuous digester, which arrangement comprises - a conduit (2) for conveying a slurry of finely divided cellulosic fibrous material and liquid to a digester (1) for treating the finely divided cellulosic fibrous material with a treatment chemical at a temperature exceeding 130 °C in order to produce treated material and black liquor, characterized in that the arrangement comprises - at least a first line (7) extending from the boiler (1) to lead at least one flow of expansion black liquor to indirect heat exchange conditions with steam in a heat exchanger ( 8) for the purpose of heating the black liquor, - a line to lead the expansion black liquor heated in the heat exchanger (8) from the heat exchanger (8) to a first expansion vessel (9) to form expansion steam and expanded black liquor, and - a line (3) drawn from the first expansion vessel (9) to the boiler (1) to lead expansion steam to the boiler (1) for the purpose of heating the cellulosic fibrous material led to the boiler (1).

Description

ARRANGEMENT FOR PRODUCING PULP IN A PULP MILL COOK-

MOLLA

The present invention relates to an arrangement for producing chemical pulp in a continuous cooking plant at a pulp mill.

In known technology, a fiber line system with chip silos is used, where wood chips or other cellulosic material is steamed and liquid is added to it to form a suspension, after which the suspension is — pressurized (this part is also referred to as a feed system), fed into a treatment vessel or vessels (which can be an impregnation vessel, a pre-hydrolysis process or other dishes), followed by the kettle. Nowadays, at least one black liquor flow is removed from the cooking system (typically at a temperature of 120-180°C). The removed black liquor flow or — flows can — be used as a heat source for preheating the white liquor, other black liquor flows and/or other liquid flows that are led to the feed and cooking systems. The removed black liquor flow (or flows) can then be led to a pre-evaporation system, i.e. one or more flash tanks, where steam is formed from the hot black liquor as it cools, typically to a temperature of about 105-125 °C. After possible additional cooling to 90-95 *C, the black liquor is led to the evaporator system of the recovery zone. The expansion steam thus produced can be used elsewhere in the pulp production process. Expansion steam, for example, can be used to directly preheat the wood chips before cooking.

S 25

S The expansion process described above has the disadvantage that the produced steam contains volatile compounds, including sulfur compounds, which are not desirable

E tavia in the pre-evaporation of wood chips. Typically, wood chips are steamed il-

N at atmospheric pressure or slightly higher pressure, so that

S 30 —residual gases are not absorbed into the wood chips, but are collected and treated

S county. Treatment is typically incineration in the factory's non-condensable gas (NCG) system. This collection and treatment system becomes particularly significant when the used steam contains volatile compounds, including sulfur compounds, which have an adverse environmental effect, including a harmful smell. It is therefore advantageous to use a steam source that minimizes or eliminates the introduction of volatile compounds into the steaming process. In addition, there is a risk of explosion if concentrated gases enter the dilute gases along with the expansion steam. Thus, it is advantageous to find uses for the expansion steam other than the pretreatment of the cellulose-based material fed into the soup, such as steaming.

Processes have been presented for producing clean steam for the pretreatment of cellulosic material. US patent US 6,722,130 shows a system for producing clean steam from black liquor. First, the pressure of the black liquor is reduced in order to create another black liquor with a higher concentration and black liquor steam, which is condensed to condensate. This condensate is heated with the first black liquor and the expansion is evaporated in a chip silo — to produce usable clean steam.

WO 2007073333 presents a system and method for producing steam in a boiler of a chemical pulp mill. The pressure of the hot pressurized black liquor from the boiler is reduced in the first stage to produce black liquor steam, which is used to pre-treat the wood chips in the second pre-heating stage. Clean steam for the pretreatment of wood chips in the first preheating stage is generated by reheating the black liquor, the pressure of which was reduced before the final next pressure reduction, where the increased amount of black liquor steam is led to the steam converter

S 25 — for blades to produce clean steam.

S

& Well-known solutions offer different systems for the use of expansion steam and

I to produce cleaner steam for the heating needs of the boiler room and improve the energy economy of the pulp mill.

N 30 o

N Steam is also needed to heat the fibrous material to cooking temperature in the cooking

S where, typically in the vapor phase of the boiler. In known systems, fresh steam from the factory's turbine plant is typically used for this purpose. The steam condenses into fibrous material, and the condensate formed in this case cannot be recovered. However, the fresh steam condensate is clean and thus could be recovered for reuse. US patent 6346166 shows that the amount of expansion steam obtained by expansion evaporation of black liquor is increased by functionally connecting the jet ejector to the steam outlet of the expansion tank. The jet ejector is supplied with higher-pressure steam from another source, resulting in a mixture of expansion steam and other steam, which can be used as a steam source for heating the fibrous material in the vapor phase digester. In this solution as well, fresh steam is used in the boiler to heat the fiber material. US patent publication 8512514 describes a process in which a black liquor flow is removed from the boiler, which is evaporated in an evaporator using fresh steam as an indirect heating agent.

During evaporation, the steam generated from the black liquor is used to heat the fibrous material at the top of the boiler, thus reducing the need for fresh steam.

One purpose of the present invention is to avoid the aforementioned problems and to provide an improved and alternative arrangement for heating the fibrous material in the upper part of the boiler by utilizing the heat of the black liquor as efficiently as possible. One purpose is also to improve the overall energy economy of the pulp mill. One other purpose is to utilize the expansion steam of the black liquor in an advantageous destination in terms of its composition.

The present invention offers a new arrangement in which the use of fresh steam for heating the fibrous material in the cooker can be reduced. The new organization & 25 — includes: a

Forming of the chopped cellulose-containing fiber material and liquid of the S line

In order to lead this sludge to a digester for treating the shredded cellulose-containing fibrous material with a treatment chemical at a temperature above 130 °C

N to produce treated material and black liquor,

O

5 30 — -line's chopped cellulose-containing fibrous material and the form of the liquid

In order to lead the sludge to the digester for treating the shredded cellulose-containing fibrous material with a treatment chemical to produce the material treated at a temperature of more than 130 *C and black liquor, -at least one of the first line led away from the digester to lead at least one expansion black liquor flow to the heat exchanger in an indirect heat exchange relationship with steam to heat the expansion black liquor, -the line for leading the expansion black liquor heated in the heat exchanger from the heat exchanger to the first expansion vessel to form expansion steam and expanded black liquor, and -from the first expansion vessel to the boiler to lead the expansion steam of the line led to the boiler to heat the fiber material led to the boiler.

In one embodiment of the arrangement according to the invention, a second line is led from the boiler to the first line led away from the boiler before the heat exchanger to remove the second expansion black liquor flow from the boiler and connect it to the expansion black liquor flow of the first line. The heat from the expanded black liquor can also be recovered or alternatively, for example, in a kettle reboiler or a black liquor cooler. In the first mentioned, it is possible to produce steam in itself — from a pure liquid for the pretreatment of fiber material, and in the latter, water fractions of the factory can be heated. @ The central feature of the new arrangement is that the boiler expansion black liquor

N is passed through an indirect heat exchanger, where its temperature is raised

S with fresh steam, typically with intermediate pressure steam, the pressure of which is typically 6-17

N 25 — bar (g). With intermediate pressure steam, depending on its pressure, the temperature of the black liquor

E is typically raised to a level of around 160-190 *C, for example 175-185 *C. Edul-

At least the invention is applied in a vapor phase boiler. At higher temperatures

In boiler I, more expansion steam is obtained from the same amount of lye, but

S then a larger heat transfer surface is needed in the heat exchanger, which > 30 can increase equipment costs. After this, the heated black liquor is led to the expansion vessel, where it is allowed to expand into steam, which is used at the top of the boiler to heat the fibrous material instead of direct steam.

The expanded black liquor can be led from the first expansion vessel to the second expansion vessel, typically with other expansion liquor or other expansion liquors removed from the boiler, and the black liquor is expanded there again. The expansion steam from this second expansion vessel can be used in a boiler (Vapor Reboiler) to generate clean steam, which can be used to steam fibrous material such as wood chips before the boiler.

Black liquor is removed from the cooker at one or more levels depending on the cooking process, for example between the cooking and washing zones. Boiler expansion liquor, which is heated in the heat exchanger, can be either top expansion liquor, bottom expansion liquor, or if, for example, there are several bottom expansion sieves, then expansion liquor of the top row of sieves. Black liquor can also be led to the heat exchanger from two or more sieve levels. According to the advantageous embodiment, the black liquor at the highest temperature is heated in order to optimize the consumption of the heating medium, steam, in the heat exchanger. The temperature of the black liquor flow removed from the boiler depends on the boiler model. If there is more than one black liquor flow, then typically — the highest temperature is with the black liquor, which is removed from the bottom of the boiler at the end of the cooking phase.

The present invention can be applied both in connection with a single-pot cooker and a two-pot cooker. Advantageously, the cooker is a vapor phase cooker, in which case the

N steam is led into the gas space at the top of the boiler through the fiber mater-

S 25 — aala to warm up. The expansion steam can also be fed to the hydraulic boiler

N to the upper liquid phase with a vapor injector as described in WO

E 2016/038251. The essential feature of the invention is the heating of the expansion liquor

N nen and then expanding it, so it is not necessary in the cooker

I steam from outside the balance area, but it originates from the internal circulation. Li-

S 30 — in the indirect heat exchanger, the condensate of the fresh steam is recovered > and can be taken back to the boiler plant as boiler water.

By indirectly heating the lye going from the boiler to the expansion tank with steam, typically with intermediate pressure steam, the amount of expansion steam generated can be increased (compared to a situation where the black liquor is not heated) and thus cheaply produce enough heating steam for the needs of the boiler and reduce the use of direct fresh steam in the boiler to heat the fibrous material. The expanded lye can also be recycled back through the heat exchanger to the expansion vessel, in which case the amount of expansion steam can be increased further.

Along with the expansion steam, easily volatile sulfur compounds from black liquor return to the top of the boiler, which can accumulate in the boiler to some extent. This is not necessarily a negative phenomenon, but sulfur compounds can have effects that speed up the cooking reaction and improve yield. —Description of the drawing:

Figure 1 schematically shows an embodiment of the invention, and

Figure 2 schematically shows a second embodiment of the invention.

Detailed description of affordable implementations

The sludge formed by the shredded cellulose-containing fiber material, such as wood chips, and the liquid is led to the top of the digester 1 along line 2. Before the boiler, the suspension can optionally be fed for processing until processing- e]

S 25 to or containers (can be impregnation container, pre-hydrolysis process or others

N vessels, not shown). Only those components that are important for the invention are shown in the drawing. In this case, the cooker is a vapor phase cooker.

I Steam is also added to the top of the kettle along line 3. a

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S 30 Boiler 1 includes at least one or more sieve arrangements 4, 5 and 6, which

Black liquor can be removed through N. Black liquor with a typical temperature of

At a temperature of 120-180 *C and a dry matter content of 12-17%, it is removed through sieve 4 and line 7 and led to the expansion tank 9. Line 7 is equipped with a heat exchanger 8, where the black liquor is heated indirectly with steam, typically intermediate pressure steam from line 24. The black liquor is typically pressurized in the cooker to a pressure of 5 - 15 bar (abs), and its temperature corresponds to the temperature of the cooking process (about 130 - 180 *C). According to the advantageous embodiment — the black liquor at the highest temperature is heated in order to optimize the consumption of the heating medium, steam, in the heat exchanger 8. The fresh steam condenses in the heat exchanger 8, and the resulting fresh steam condensate in line 25 is clean. The condensate can be led to the factory's boiler plant, where it can be used as boiler feed water.

In the expansion tank 9, the pressure of the black liquor is reduced to produce expansion steam and to produce expanded black liquor, the dry matter content of which typically increases during expansion. The expansion tank 9 is essentially conventional in structure, and has an inlet 10 for hot black liquor, an outlet opening 11 for the steam evaporated by expansion and an outlet 12 for the cooled, lower-pressure black liquor. Expansion tanks are typically used in such a way that the pressure in them is lower than that of the black liquor supplied to them pressure, but higher than the pressure at the top of the boiler, typically around 4 -7 bar (abs). Expansion tanks are designed for this purpose — tanks that enable the pressure of the hot, pressurized black liquor to be reduced, whereby a liquid, typically water, evaporates from the black liquor very quickly into steam, which also increases the concentration of the cooking chemical used and the products of the cooking reaction (dissolved solids). The steam produced in this rapid evaporation is removed from the steam outlet 11 lin-

S 25 jaan3 in the tank 9 at the prevailing pressure, for example 4 - 7 bar (abs) and val-

N at the saturation temperature corresponding to the pressure, for example 140 -180 & °C. The liquid remaining after expansion-evaporation, which is typically

I also e.g. at a temperature of about 160 *C, settles on the bottom of the container 9, from where it *is removed from the outlet 12 along the line 13.

N 30 o

N The expansion steam from the expansion vessel 9 is directed via line 3 to the boiler 1

S puff for use as heating steam. The temperature of the steam, e.g. 140-180 °C, is typically higher than the cooking temperature, so the steam, which is led through the steam supply opening of the cooker, which is connected to the expansion vessel, heats the fibrous material to the cooking temperature. The expanded black liquor is sent along line 13 to the factory's second expansion vessel 15, where the black liquor is further expanded. Part of the black liquor can be recycled through line 14 and — through the heat exchanger 8 to the expansion, and thus further increase the amount of steam generated in the expansion vessel 9.

Into the second expansion vessel 15, black liquor is also brought from the kettle from the kettle strainers, in this case from the washing strainer 6 along line 16. After the expansion-evaporation, the remaining liquid, which is also typically at a temperature of about 100 - 140 *C, settles at the bottom of the container 15, from which it is removed along the line 17. The discharge line 17 can have a black liquor cooler 18, where heat can be recovered from the black liquor. In this case, the water in line 26 is typically heated to produce hot water, which exits through line 28. The expanded, cooled black liquor is sent along line 27 to the factory's evaporation plant, where the black liquor is concentrated to a high dry matter content before it is burned in the recovery boiler.

The expansion steam from the second expansion vessel 15 is led along the line 19 to the boiler 20, where its energy is recovered. The steam is brought into an indirect heat exchange relationship with a "pure" liquid 21 free of volatile compounds to heat it to a temperature higher than its boiling point to produce pure steam. The boiler is preferably a falling membrane tube boiler. The expansion steam condenses on the inside of the pipes, while

S 25 pure liquid boils on the outside of the tubes to produce pure steam.

N Clean steam in line 22 is typically used for the pretreatment of fiber & fiber material fed into the cooker, such as steaming wood chips. = * Clean liquid is led through line 21 to boiler 20. It can typically: 30 be condensate from the evaporator, demineralized water, or sufficiently pure water

N fractions, such as hot water in a factory. Produced in a kettle

S clean steam contains essentially less non-condensable gases than steam produced by direct black liquor expansion. The clean steam of line 22 is preferably used for preheating wood chips with steam. The expansion steam coming from the mustalipä expansion vessel 15 contains volatile compounds, such as sulfur compounds. These compounds enter the dirty condensate and the flow of concentrated noncondensable gases (CNCG) formed in the boiler. The dirty condensate is sent from the boiler via line 23 to the evaporator, where it is processed in a manner known per se. Heat can also be recovered from the expanded black liquor, which leaves the first expansion vessel, in a way other than additional expansion.

The mass produced is removed from the bottom of the digester via line 29.

Figure 2 shows an alternative solution to lead the black liquor from the boiler to the first expansion vessel 9. Unlike the solution in Figure 1, the black liquor is led to the first expansion vessel from the second sieve as well. The cooking sieve 6 comprises two sieve levels 6a and 6b. At the end of the cooking zone, the black liquor is removed through the upper sieve row 6a to line 30, and through line 31 to the expansion vessel 9. The temperature of this black liquor flow may already be high enough, so it does not need to be heated before expansion. Advantageously, however, the black liquor flow is led to the heat exchanger 8 via lines 30 and 32.

Through the lower sieve line 6b, black liquor is removed from the washing zone via line 33 for heat recovery, for example for expansion. & 25

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AN Example: <Q

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I In a solution according to known technology, where fresh steam is used * to heat the fiber material, intermediate pressure steam is driven to the top of the boiler: 30 — for example, about 320 kg/adt. From the end of the soup, take one through a strainer

N expansion black liquor fraction, which is, for example, 5.5 m3/adt at a temperature of 146 °C.

S Heat is recovered from this black liquor, for example by expansion, and the expansion steam is used to roast wood chips.

In the solution according to the invention, the expansion black liquor fraction taken from the end of the soup is divided into two streams using two separate sieves. The amount of the second flow is, for example, 3.0 m3/adt at a temperature of 169 °C and the amount of the second — flow is 2.5 m3/adt at a temperature of 120 °C. The colder flow is sent to traditional heat recovery. The hotter flow is taken to the heat exchanger, where it is heated by indirect intermediate pressure steam to a temperature of 180 *C, whereby steam is consumed at about 50 kg/adt. After this, the heated lye is taken to the expansion tank, where the pressure of the lye drops to 4.5 bar(g) and the temperature to 158 *C. In this case, expansion steam of about 110 kg/adt is generated, which is taken to the top of the boiler, which has a pressure level of about 4 bar(g). Thus, in this example, the solution according to the invention saves 110 kg/adt direct intermediate pressure steam at the top of the boiler by using 50 kg/adt indirect intermediate pressure steam in the heat exchanger. At the same time, 110 kg/adt of black liquor evaporates, reducing the need for evaporation in the evaporator, and 50 kg/adt of hot intermediate pressure steam condensate is recovered.

The present invention achieves the following advantages: - — the total steam consumption of the kitchen is reduced; - — fresh steam condensate is recovered when expansion steam can be used in the boiler instead of fresh steam; - — advantageous sulfur compounds in terms of the cooking process can be recycled back into the cooking; and e]

S 25 - — the required equipment is simple.

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Claims (6)

PROTECTION REQUIREMENTS 1. Arrangement for producing chemical pulp in a continuous digester, which arrangement comprises - a line (2) for conveying a slurry of finely divided cellulosic fibrous material and liquid to a digester (1) for treating the finely divided cellulosic fibrous material with a treatment chemical at a temperature exceeding 130 °C in order to produce treated material and black liquor, characterized in that the arrangement comprises - at least a first line (7) extended from the boiler (1) to lead at least = a flow of expansion black liquor = to — indirect heat exchange relationship with meadow in a heat exchanger (8) for the purpose of heating the black liquor, - a line to lead the expansion black liquor heated in the heat exchanger (8) from the heat exchanger (8) to a first expansion vessel (9) to form the expansion bed and expanded black liquor, and - a line (3) extended from the first expansion vessel (9) to the digester (1) to lead the expansion bed to the digester (1) for the purpose of heating the cellulosic fibrous material led to the digester (1). 2. Arrangement according to protection requirement 1, where a second line (30, 32) is drawn to the first line (7) extended from the boiler (1) before the heat exchanger (8) to draw off a second flow of expansion black liquor from & the boiler (1) and combine it with the flow of expansion black liquor in the first N. & 25 the lead (7). ? No N 3. Arrangement according to protection requirement 1 or 2, where a line (24) is drawn I i to the heat exchanger (8) to lead free leg, which is intermediate pressure steam, to the N heat exchanger (8) in order to constitute its heat source. S 0) 30 N O N 5 4. Arrangement according to any of the preceding protection claims, where a line (13) is extended from the first expansion vessel (9) to a second expansion vessel (15) to form the second expansion bed and second expanded black liquor. 5. Arrangement according to any of the preceding protection requirements, where a line (14) is extended from the first expansion vessel (9) and provided with a pump, with which expanded black liquor withdrawn from the first expansion vessel (9) is circulated through the line (14) back to the heat exchanger (8) and from there further back to the first expansion vessel (9). 6. Arrangement according to any of the preceding protection requirements, where the arrangement comprises at least a first line (7) extended from the boiler and a second line (30, 31, 33) extended from the boiler, and where the black liquor in that line (7) of these two, where the temperature of the black liquor is higher, is led to treatment in the heat exchanger (8) and the black liquor in the line (30, 31, 33) where the temperature of the black liquor is lower, is led to heat recovery. e] N O N O <Q NN N I = N N O < 0) N O N 5