JP2017507024A - Evaporator - Google Patents

Abstract

The invention relates to an evaporator of a multi-effect evaporation plant for black liquor in a pulp mill, the evaporator including a housing in which a liquid film flowing heat transfer surface and an inlet for heated steam First and second evaporating sections each comprising: a supply for introducing black liquor to be evaporated into the first and second evaporating sections and onto the heat conducting surface of the heat conducting unit, respectively An inlet line; a discharge line connected to the housing for extracting the evaporated black liquor; a steam inlet of the heat exchange unit of the first evaporation section, connected to the steam outlet of the utility can in front of the multi-effect evaporation plant; And a steam outlet line for steam generated by evaporation, the steam line connected to a steam pressure increasing device to direct at least a portion of the steam therein. And the discharge line from the pressure increasing device is connected to the steam inlet of the heat exchange unit of the second section, the housing comprising a partition between the evaporation sections, whereby the partition connects the lower part of the housing to a common steam space Is divided into two liquid compartments, in which a heat exchange unit is located, and a common vapor space is connected to the vapor outlet line. [Selection] Figure 1

Description

  The present invention relates to an evaporator for concentrating the black liquor that evaporates in a black liquor evaporation plant of a chemical pulp factory, and a novel method using vapor compression evaporation for pre-evaporation of black liquor. Is what you do.

  Due to combustion in the recovery boiler, black liquor is most commonly concentrated in a multiple effect evaporation (MEE) plant. The numbering of the utility cans is based on the order defined by their vapor flow. Most commonly, the evaporation plant is operated such that the evaporator under the highest pressure accepts, for example, fresh steam as a heat source, and the fresh steam boiles the liquid on the liquid side of the evaporator, thereby allowing the evaporator to Steam is generated having a pressure lower than the pressure of the steam introduced on the steam side. The generated steam is used to heat the evaporator under the next lower pressure as the black liquor flows through the evaporation plant countercurrent to the heated steam or steam. Similarly, the same process takes place in third effect cans, fourth effect cans, etc., but these processes reduce the pressure and temperature of the steam from the last effect can with respect to the steam flow and are technically Until there is no value to continue in both terms and overall economics. The vapor at the lowest pressure is usually condensed in a condenser using water or air. Typically, the evaporation plant contains 5-7 utility cans. One of the conventional evaporation unit types is a so-called liquid film flow-down type evaporator, in which black liquid flows downward on the surface of the heating element in the form of a thin film for high heating efficiency. be introduced.

  Vapor compression evaporation is also used in the concentration of black liquor to obtain combustion black liquor, also called mechanical vapor recompression MVR, which is the pressure of vapor evaporated from solution, Therefore, the steam can be used as a heating medium for the same utility can because the saturation temperature is raised either high by steam by an ejector or mechanically by a compressor or fan. Compression is required to increase the saturation temperature of the vapor to create a sufficient temperature difference in the evaporation effect. The increase in saturation temperature must be greater than the boiling point increase. Vapor compression evaporation is mainly used when the boiling point rise is low because the power demand of the compressor or fan correlates with the pressure increase. The limiting factor of mechanical vapor recompression in the evaporation of black liquor is an increase in the boiling point of black liquor. As the dry solids content in the black liquor increases, the boiling point rises rapidly and the usable temperature difference decreases. In practice, the use of fan evaporators in the sulfate pulp industry is limited to pre-evaporators where black liquor is evaporated to a dry solids content of about 20-25%. The pre-evaporation is followed by the multi-effect evaporation described above, increasing the dry solids content of the black liquor to the preferred dry solids content, typically 75-85%, and even 90% for combustion in the recovery boiler. . Typically, a liquid film falling evaporator operates efficiently even with a small temperature difference and is suitable for use as a compression evaporator.

  The economic application of vapor compression evaporation relies on the fact that the cost of electrical energy is sufficiently low compared to the cost of available thermal energy. The advantage of this evaporation method is that no external steam source is required.

  As the cost of additional fuels and green power (this term generally refers to power generated outside of fossil fuels) continues to increase, various plant solutions to reduce factory steam consumption ) Is becoming more cost effective. As steam consumption decreases, mill balance is reduced in additional fuel, or more power is available in the condensing section of the turbine for power production. Pulp mills tend to sell more electricity to the outside due to increasing interest in economics. In addition to chemical pulp mills, other production plants, such as paper mills, are also aiming for more efficient and more economical energy consumption.

  If a fan is used, the energy consumption of a mechanical vapor compression pre-evaporation plant, also called a fan pre-evaporation plant, is, for example, compared to an evaporation plant with seven utility cans connected in series. When calculated in terms of money, it is often as low as 70% or less. Early fan pre-evaporation plants have been used primarily in sulfate pulp mills to obtain additional capacity of the evaporation plant. Only part of the total evaporation required is done in the fan pre-evaporation plant because the excessively high dry solids content of the black liquor to be evaporated increases the boiling point and causes the fan or compressor This is because a high pressure increase is required, and energy consumption increases.

  WO 2009/053518 discloses the arrangement of evaporators in a multi-effect evaporation plant used for the evaporation of black liquor. In this arrangement, black liquor evaporation using mechanical vapor recompression is incorporated into a single effect can in a multi-effect evaporation plant. The evaporator of the evaporation plant comprises a vapor compression evaporation section and an evaporation section, which are arranged in a common housing. When two different black liquors are evaporated, the space for the secondary vapor is divided by the intermediate wall, which equalizes the pressure between the two sections formed by the intermediate wall in the housing One or more openings for. Each of the secondary steam is discharged via a separate discharge conduit, one discharge conduit being connected to the next evaporative effect can in the direction of the steam flow, and a portion of the steam is directly removed as a heating medium. One discharge conduit for secondary steam is connected to a pressure increase device, such as a fan, to increase the steam pressure and return a portion of the increased steam to the vapor compression section of the evaporator plant . Incorporation of vapor compression evaporation into a single evaporative effect can in accordance with the WO publication reduces steam consumption compared to a normal multi-effect evaporative plant, but further improves steam efficiency Is needed.

  It is an object of the present invention to provide a method for integrating vapor compression evaporation into a multi-effect black liquor evaporation plant in a more advantageous manner from the standpoint of thermal efficiency and economy.

To achieve these objectives, the present invention relates to a multi-effect evaporation plant evaporator for black liquor in a pulp mill,
Including a housing, within the housing,
First and second evaporation sections each including a heat exchanger unit having a liquid film downflowing heat transfer surface and an inlet for heated steam;
A supply inlet line for introducing the black liquor to be evaporated into the first and second evaporation sections and onto the heat transfer surface of the heat transfer unit, respectively;
A discharge line, each connected to the housing to extract evaporated black liquor,
A steam inlet of a heat exchanger unit of the first evaporation section, connected to a steam outlet of a previous utility can in a multi-effect evaporation plant, and a steam outlet line for steam generated by evaporation;
This steam line is connected to a steam pressure increasing device to direct at least part of the steam there,
The discharge line from the pressure increasing device is connected to the steam inlet of the heat exchanger unit of the second section.

  In an evaporator according to the invention, the housing comprises a partition between two evaporation sections, whereby the partition divides the lower part of the housing into two liquid compartments with a common vapor space, in which the heat exchanger unit is located. Is done. A common steam space is connected to the steam outlet line. Typically, the black liquor supplied to the evaporating section each has a different dry solids content, such as dilute black liquor and black liquor from the MEE plant utility can.

  The present invention is configured such that the heat exchanger unit of the vapor compression evaporation and the heat exchanger unit of the multi-effect evaporation plant are arranged in the same housing (tank), that is, in a common evaporator. Thus, a single evaporator body has two heat exchanger units, one of which uses the steam from the steam pressure increaser as a heating medium, and the other in a multi-effect evaporation plant. Use the steam generated in the preceding utility can. As the black liquor is evaporated on the outer surface of the heat exchanger unit, the vapor generated by the evaporation enters the same space defined by the housing, typically in the housing and more typically in the upper part of it. It is discharged via one or more conduits located in the part. A portion of this steam is used in subsequent evaporation effect cans at lower pressures, and the pressure of the portion of steam is increased and returned to the unit of the vapor compression evaporation plant as heated steam. An essential component of the present invention is a common vapor space, in which only the black liquor compartments are separated from one another by partitions. This allows the vapor generated in one evaporation section to flow to the other evaporation section and the black liquor evaporated in the other evaporation section to be heated. For example, if the temperature of the dilute black liquor is low, the dilute black liquor from which the vapor from the evaporation section connected to the MEE plant is to be preheated can be preheated, and thus a separate preheater can be unnecessary.

  The preferred heat conducting surface of the heat exchanger unit is a liquid film downflow evaporator, the evaporation surface being formed of a number of lamellae or tubes. Preferably, the black liquor to be evaporated flows on the outer surface of the lamella or on the inner surface of the tube, while the heating steam is directed inside the lamella or outside the tube. For example, in a plate-type liquid film-flowing lamellar evaporator, almost all or all of the vertical part of the evaporation surface is in contact with the surrounding vapor space, so that the free space releases the vapor generated on those surfaces Therefore, it is left between the heat conduction elements. In an evaporator operating on the principle of free liquid film flow, steam flows freely in a space defined by a housing tank, and a heat conducting element unit is located inside the tank. The generated steam flows in a free space between the heat-conducting element unit and the housing vessel wall, essentially vertically upward, towards the upper part of the housing vessel. Usually, the steam is collected in a drop separator and allowed to flow out of the upper part of the tank. Each of the effect cans in an evaporation plant with multiple effect cans connected in series is one heat exchanger unit arranged inside the housing, or several parallel connected heat exchange surface units each arranged inside the housing Includes units.

  According to a preferred embodiment of the present invention, the second evaporation section functions as a black liquor pre-evaporator. The supply line of the second evaporation section is connected to the lean black liquor tank of the pulp mill. Thus, according to the present invention, the MVR pre-evaporator is integrated with one of the MEE evaporator effect cans (typically 2-7 effect cans), which are placed in the same evaporator body. The The effect can is divided into black liquor sections, one section for dilute black liquor pre-evaporation and the other for regular MEE black liquor. The evaporation surface is also divided into sections, one section for dilute black liquor pre-evaporation and the other for regular MEE black liquor. Vapor fraction evaporated in the pre-evaporation section is compressed in a steam compressor and is used as heating steam in the pre-evaporator section after the compressor. Vapor from the previous MEE utility can is used as heating steam in the MEE section, and the vapor content evaporated in this section continues to the next MEE utility can. In this case, the common steam space is further connected to a conduit for directing part of the steam in the direction of the steam flow to the next utility can of the multi-effect evaporation plant.

  Typically, the heat exchanger unit is located above the liquid compartment and the divider is substantially vertical and extends to the level of the lower end of the unit.

  Typically, the vapor pressure increasing device is a fan or a compressor.

  According to one embodiment, the steam inlet of the heat exchanger unit of the vapor compression evaporation section (second evaporation section) is connected to the steam outlet of one utility can of the multi-effect evaporation plant via a line with a bypass valve. This also increases the flexibility of the evaporation system. The bypass valve is closed when the MVR pre-evaporation section and the steam compressor are operating. However, if the steam compressor is not used for some reason, the bypass valve is opened and the MVR section is heated by steam from another utility can in the MEE plant. Thus, depending on the factory energy situation, the pre-evaporation is switched from MVR evaporation to steam operation. This two-section evaporator can operate even if the steam compressor is inoperable or there are other obstacles in the evaporation plant.

  Typically, the discharge of non-condensable gas generated in the heat exchanger unit of the vapor compression evaporation, ie the exhaust, is performed by the respective devices of the multi-effect evaporation plant.

  Typically, the discharge and treatment of the condensate generated in the heat exchanger unit of the vapor compression evaporation is carried out in dedicated equipment of a multi-effect evaporation plant.

The present invention will be described in more detail with reference to the accompanying drawings.
FIG. 1 schematically shows in detail an evaporator according to a preferred embodiment of the invention,
FIG. 2 schematically shows the connection state of the evaporators in the multi-effect evaporation plant according to the present invention,
FIG. 3 schematically shows another preferred evaporator according to the invention.

  FIG. 1 shows an evaporator with a housing 2, in which a heat exchanger unit 4 of a vapor compression evaporation section and a heat exchanger unit 3 of a multi-effect evaporation section are arranged, both operating on the principle of liquid film flow To do. In this embodiment, the heat conducting surface unit is formed of a number of lamellae, heated steam is supplied inside the lamella, and the black liquor to be evaporated flows over the external surface of the lamella, whereby the black liquor is transferred to the heat exchanger Heated by indirect contact with the steam inside the unit. A tube can also be used in place of the lamella, whereby the black liquor flows on its inner or outer surface. The lower part of the housing 2 includes a partition 6 and the bottom is divided into two black liquor compartments 7 and 8.

  The dilute black liquor (feed liquor) to be evaporated is supplied to the black liquor compartment 8 via the conduit 5, from which the black liquor is positioned on the heat transfer unit 4 via the conduit 11 by the pump 10. It is pumped to the dispensing device 19 that is present. In the dispensing device, the black liquor flows to the outer surface of the lamella through the openings or portions corresponding to the openings, where the black liquor is evaporated. The evaporated black liquor is collected in the black liquor compartment 8 and is discharged from there through the line 13 to the subsequent evaporation effect can.

  The heat transfer unit 3 of the multi-effect evaporation plant receives steam as a heating medium, typically via a conduit 12, from a previous evaporation effect can operating at a higher pressure. The heat-conducting surface unit 4 of the vapor compression evaporation section receives the vapor from the vapor pressure increasing device 9 via line 14 as a heating medium.

  Since the heat exchanger unit 4 of the vapor compression evaporation section and the heat exchanger unit 3 of the multi-effect evaporation plant are located in the same housing 2, each of the vapors evaporated from the black liquor on their outer surfaces Flows into a common space 15 in which the two heat exchanger units are located. A conduit 10 (correctly “14”) is connected thereto. The conduit 14 is provided with a vapor pressure increasing device 9, which is typically a compressor or a fan. In the steam pressure increasing device 9, the steam pressure is increased to a level at which the steam can return to the same utility can as the heating medium and returns to the steam compression heat transfer surface unit 4. The branch conduit 16 is also connected to the conduit 14, and a part of the steam discharged from the housing 2 is taken out as a heating medium to other evaporation effect cans via the branch conduit.

  Black liquor is introduced via line 17 from the other evaporation effect cans into the black liquor compartment 7 of the multi-effect evaporation section. From the black liquor compartment 7 the black liquor is further led to the heat exchanger unit 3 of the multi-effect evaporation plant. The black liquor is conveyed to the upper part of the unit by the pump 10 via the line 11 and flows downward on the outer surface thereof. The evaporated black liquor is guided to the next evaporation effect can through the line 21. The evaporated secondary steam flows to the common steam space in the housing 2 as described above. Condensate is discharged via lines 19 and 20 from the interior of the heat exchanger unit to the condensate treatment of the multi-effect evaporation plant. Non-condensable gas generated in the unit is also discharged via the common line 18 for further processing. An advantage of the present invention is that the condensate treatment and non-condensable gas treatment are performed in the same equipment, i.e. equipment in a multi-effect evaporation plant, and vapor compression evaporation does not require a separate separate equipment. .

  The advantage of the present invention is that, compared to known couplings, the possibility of using equipment such as transport lines for black liquor and steam, condensing systems and vacuum systems of multi-effect evaporation plants is more It is expensive.

  FIG. 2 shows a black liquor evaporation plant in which seven multi-effect cans are connected in series. In this case, the evaporation plant comprises continuous utility cans 1-7, which operate at pressures and temperatures that continuously decrease in the direction of steam flow. The last evaporative effect can 1A, 1B of black liquor includes steps IA and IB. The evaporator shown in FIG. 2 is a lamellar evaporator with a falling film, but other evaporators suitable for black liquor evaporation can be used in this case as well. FIG. 2 uses the same reference numerals as FIG. 1 where possible.

  For evaporation, factory fresh steam is fed through channels 30 to steps IA and IB of the evaporation effect can I, where steam warms the black liquor and condenses it at the same time. In the evaporation effect can I, the vapor is separated from the black liquor, and this vapor is conveyed to the effect can 2A through the channel 31 as a heating medium. In the evaporative effect can 2A, the vapor at a lower temperature in the evaporative effect can I is separated, and this vapor is further guided to the subsequent evaporative effect can 3A through the channel 32. Therefore, in the evaporative effect cans 3A, 4A, 5A and 6A, the secondary vapor separated from the black liquor is transported to the subsequent evaporative effect cans 4A, 5A, 6A and 7A to warm and evaporate the black liquor, respectively. Is done.

  The dilute black liquor (feed liquid) is introduced into the vapor compression evaporating section of the effect can 4A via the line 5 and flows from there to the effect can 7A via the line 13. The black liquor from the effect can 7A is introduced into the effect can 6A via the line 40 for evaporation, and further introduced into the effect can 5A via the line 41 for the formation of the intermediate black liquor in the line 42. . The intermediate black liquor is further led to the effect can 4A, from which the black liquor is the last evaporative effect can with two steps IA and IB connected successively to the liquid side via evaporative effect cans 3A and 2A It is conveyed to 1A and 1B. The black liquor is first evaporated in step IB, from which the black liquor is directed to step IA to evaporate the black liquor to a high dry solids content, ie about 75-90%. The concentrated black liquor is discharged for combustion via line 43.

  In the last utility can 7A, the generated steam is conveyed via line 33 to the vacuum system of the evaporation plant, where it is cooled by cooling water, for example in a surface condenser (vacuum condenser) (not shown). .

  In accordance with the present invention, the vapor compression evaporation section is incorporated into the first evaporation effect can with respect to the direction of black liquor flow in a multi-effect evaporation plant. The method of incorporation is similar to that shown in more detail in FIG. The effect can 4A comprises a vapor compression evaporation section heat exchanger unit 4 and a multi-effect evaporation section heat exchanger unit 3, which operate on the principle of liquid film flow and are located in a common housing. ing. The bottom portion of the housing includes a partition that divides the bottom into two black liquor compartments. The diluted black liquor (feed liquid) 5 is introduced into the bottom part of the evaporator housing, from which the black liquor is pumped to the outer surface of the heat exchanger unit 3 via the distributor, whereby the black liquor is moved over the surface. As it flows downward, the vapor is evaporated therefrom. The vapor compression section functions as a black liquor pre-evaporator.

  The heat exchanger unit 3 of the multi-effect evaporation section operating on the principle of liquid film flow is located in a common housing of the effect can 4A. Black liquor is introduced from the second evaporative effect can through line 42 into the heat exchanger unit 3 of the multi-effect evaporative plant located in the housing 2. The black liquor is conveyed by a pump to the upper part of the unit and flows downward on its outer surface.

  The secondary steam formed by the evaporation in the effect can 4A is discharged from the upper part of the evaporator through the conduit 14. A part of the steam is conveyed to the next evaporative effect can 5 </ b> A through the line 16 in the steam flow direction. Part of the steam is led again to the steam pressure increasing device 9 arranged in the conduit 14, where the pressure of the steam is increased, so that this steam serves as a heating medium for the heat-conducting surface unit 4 of the steam compression evaporation plant. Can return to '. Condensates 19 and 20 (FIG. 1) generated in the heat exchanger unit are processed in the condensate treatment system of the multi-effect evaporation plant.

  FIG. 3 shows an embodiment in which the steam inlet of the heat exchanger unit 4 of the vapor compression evaporation section (second evaporation section) is also connected via line 45 to the steam outlet of the utility can of the multi-effect evaporation plant. , Line 45 is provided with a bypass valve 46, which increases the adaptability of the evaporation system. When the MVR pre-evaporation section and the steam compressor 9 are in operation, the bypass valve 46 is closed and the valves 47 and 48 downstream and upstream of the compressor are open. When the steam compressor is stopped, the bypass valve is open. The second evaporation section is then heated by steam from one effect can of the multi-effect evaporation plant, which is conducted via lines 45 and 14 '.

By connecting a black liquor vapor compression pre-evaporator to a multi-effect evaporation plant according to the present invention, at least the following advantages are obtained.
The capacity of the pre-evaporator can be selected so that the coniferous black liquor sweetening is performed in the pre-evaporator.
• No external heat (such as fresh steam) is required for preheating black liquor.
• When the temperature of the dilute black liquor is low, the heat required for preheating is conducted from the normal evaporating section to the preevaporating section in the form of vapor.
Depending on the energy situation of the factory, the pre-evaporation can be switched from MVR evaporation to steam operation.
• Significant cost savings compared to stand-alone pre-evaporator:
* Only one evaporator body shared with the MEE body * Condensate collection tank shared with the MEE utility can * No separate supply or discharge pump is required.
* A separate vacuum system is not required and a system shared with MEE is used.
* Installed with MEE utility cans.

Although the invention has been shown and described in relation to what is presently considered to be the most practical and most preferred, it will be apparent to those skilled in the art that many modifications can be made within the scope of the invention, The scope of the invention should be accorded the broadest interpretation according to the appended claims to cover all corresponding systems and methods.

Claims (6)

An evaporator of a multi-effect evaporation plant for black liquor in a pulp mill,
Including a housing, within the housing,
First and second evaporation sections each including a heat exchanger unit having a liquid film downflowing heat transfer surface and an inlet for heated steam;
A supply inlet line for introducing the black liquor to be evaporated into the first and second evaporation sections and onto the heat transfer surface of the heat transfer unit, respectively;
A discharge line connected to the housing for extracting the evaporated black liquor,
A steam inlet of the heat exchange unit of the first evaporation section, connected to the steam outlet of one utility can of the multi-effect evaporation plant, and a steam outlet line for steam generated by evaporation;
This steam line is connected to a steam pressure increasing device to direct at least part of the steam there,
The discharge line from the pressure increasing device is connected to the steam inlet of the heat exchange unit of the second section,
The housing comprises a partition between the first and second evaporation sections, whereby the partition divides the lower part of the housing into two liquid compartments having a common vapor space, in which the heat exchanger unit is located and a common An evaporator in a multi-effect evaporation plant characterized in that the steam space is connected to a steam outlet line.   The evaporator according to claim 1, characterized in that the second evaporation section functions as a black liquor pre-evaporator and the supply line of the evaporation section is connected to a lean black liquor tank of a pulp mill.   According to claim 1 or 2, characterized in that the common steam space is further connected to a conduit for directing a part of the steam in the direction of steam flow to the next utility can of the multi-effect evaporation plant. Evaporator.   Evaporator according to claim 1, characterized in that the heat exchanger unit is located above the liquid compartment, the partition is substantially vertical and extends to the level of the lower end of the heat exchanger unit.   An evaporator according to any of the preceding claims, characterized in that the vapor pressure increasing device is a fan or a compressor. The evaporator according to claim 1, wherein the steam inlet of the heat exchanger unit of the second evaporation section is further connected to the steam outlet of one effect can of the multiple effect evaporation plant via a line with a bypass valve.

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