Black liquid concentration system based on multi-stage flash evaporation and multi-effect evaporation coupling

Abstract

The utility model relates to a black liquid concentration system based on multi-stage flash evaporation and multi-effect evaporation coupling, which belongs to the technical field of flash evaporation and comprises a waste heat circulation self-heating type multi-stage flash evaporation system, a multi-effect evaporation system, a multi-stage flash evaporation system and a multi-stage flash evaporation system. The multi-effect evaporation system is used for conveying a concentration medium to the multi-effect evaporation system, industrial waste heat is adopted for black liquor concentration, waste heat in a paper pulp mill can be effectively utilized, recycling of energy is achieved, energy consumption of the paper pulp mill is reduced, the concentration of dilute black liquor can be increased from 1.5% to 30% by adopting a multi-stage flash evaporation concentration method for low-concentration black liquor, and the concentration of the dilute black liquor can be increased from 1.5% to 30% by adopting a multi-stage flash evaporation concentration method. Fresh steam is not used in the concentration process, the operation cost of black liquor concentration is reduced, the treatment cost can be reduced to 9 yuan / steamed ton to 13 yuan / steamed ton, due to the adoption of low-temperature concentration, substances such as cellulose, hemicelluloses and resin associates in the black liquor cannot be scaled and separated out, the cleaning and maintenance frequency of equipment is reduced, and the service life of the equipment is greatly prolonged.

Description

Technical Field

[0001] This invention belongs to the field of flash evaporation technology, and in particular relates to a black liquor concentration system based on the coupling of multi-stage flash evaporation and multi-effect evaporation. Background Technology

[0002] Black liquor is the waste liquid obtained after alkaline pulping in pulp mills. It is commonly called black liquor because of its dark brown color. Direct discharge of black liquor without treatment not only causes serious environmental damage but also wastes resources and severely restricts the development of the pulping industry. Therefore, black liquor is usually concentrated to a certain high concentration before being sent to an alkali furnace for incineration. This method effectively treats the black liquor and recovers the chemical alkali within it.

[0003] Currently, black liquor concentration systems in pulp mills are mainly divided into "multi-effect evaporation concentration systems" and "MVR+multi-effect evaporation systems." Both concentration methods have certain drawbacks. Multi-effect evaporation concentration systems consume a certain amount of fresh steam as the driving heat source for concentration. Field surveys of several pulp mills show that the current cost per ton of steam for pulp mills using multi-effect evaporation is 32-40 yuan, which is extremely expensive and significantly increases the processing costs for pulp mills. MVR+multi-effect evaporation systems improve steam utilization by using a compressor to compress secondary steam, thus improving evaporation efficiency to some extent. This method has been widely adopted in the chemimechanical pulp treatment of pulp mills. Field surveys show that the processing cost per ton of steam for MVR+multi-effect evaporation is 15-25 yuan, which is significantly lower than that of multi-effect evaporation. However, both methods consume a certain amount of steam to concentrate black liquor, resulting in high energy costs. Therefore, there is an urgent need for a black liquor concentration method that can reduce steam consumption, lower costs, and utilize waste heat from the plant. Utility Model Content

[0004] To solve the above-mentioned technical problems, the technical solution adopted by this utility model is: a black liquor concentration system based on multi-stage flash evaporation and multi-effect evaporation coupling, including: a waste heat circulation self-heating multi-stage flash evaporation system, wherein the concentration medium outlet of the waste heat circulation self-heating multi-stage flash evaporation system is connected to the multi-effect evaporation system for supplying the concentration medium to the multi-effect evaporation system.

[0005] Furthermore, the waste heat recycling self-heating multi-stage flash evaporation system includes: at least one heat exchange unit and at least two flash evaporation units connected in series. The concentrated medium outlet of the flash evaporation unit located in the last effect is connected to the concentrated medium inlet of the equivalent heat exchange unit of the flash evaporation unit in the previous effect located in the last effect. When there are at least two heat exchange units, the heat exchange units are connected in series. The flash evaporation unit located in the first effect is connected to the equivalent heat exchange unit of the flash evaporation unit located in the first effect through a heating unit.

[0006] Furthermore, the heat exchange method of the multi-effect evaporation system is countercurrent heat exchange, cocurrent heat exchange, or mixed heat exchange.

[0007] Furthermore, the multi-effect evaporation system includes a low-concentration section, a medium-concentration section, and a high-concentration section. The concentrated medium outlet of the heat exchange unit equivalent to the flash evaporation unit in the first effect supplies concentrated medium to the low-concentration section, the medium-concentration section, or the high-concentration section.

[0008] Furthermore, the multi-effect evaporation system also includes a semi-concentrated liquid black tank, wherein the concentrated medium outlet of the heat exchange unit in the first effect supplies concentrated medium to the semi-concentrated liquid black tank.

[0009] Furthermore, a discharge pump is provided between the waste heat recycling self-heating multi-stage flash evaporation system and the multi-effect evaporation system.

[0010] Furthermore, a drain pipe is provided on the high concentration section, and a drain pump is provided on the drain pipe.

[0011] Furthermore, the drain pipe is connected to the concentrated black liquid tank.

[0012] Furthermore, it also includes a condensate tank assembly, which is connected to the low-concentration section, the medium-concentration section, the high-concentration section, and the heat exchange unit.

[0013] Furthermore, the condensate tank assembly includes: a clean condensate tank unit, a lightly soiled condensate tank unit, a medium-soiled condensate tank unit, and a heavily soiled condensate tank unit, which are respectively used to collect condensate generated by the high-concentration section, the medium-concentration section, the low-concentration section, and the heat exchange unit.

[0014] Beneficial effects:

[0015] 1. Using industrial waste heat for black liquor concentration can effectively utilize the waste heat in pulp mills, realize the reuse of energy, and reduce the energy consumption of pulp mills.

[0016] 2. The multi-stage flash evaporation concentration method can increase the concentration of low-concentration black liquor from 1.5% to 30%, and fresh steam is not used in the concentration process, reducing the operating cost of black liquor concentration and reducing the treatment cost to 9 yuan / ton of steam to 13 yuan / ton of steam.

[0017] 3. Due to the use of low-temperature concentration, substances such as cellulose, hemicellulose, and resin compounds in the black liquor will not precipitate out as scale, reducing the frequency of equipment cleaning and maintenance and greatly extending the service life of the equipment. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the structure of this utility model. Detailed Implementation

[0019] In the description of this utility model, it should be understood that the terms "center," "longitudinal," "lateral," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," and "outer," etc., indicating orientation or positional relationships based on the orientation or positional relationships shown in the accompanying drawings, are only for the convenience of describing this utility model and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this utility model. Furthermore, the terms "first," "second," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, features defined with "first," "second," etc., may explicitly or implicitly include one or more of that feature. In the description of this utility model, unless otherwise stated, "a plurality of" means two or more.

[0020] In the description of this utility model, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "joining" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this utility model based on the specific circumstances.

[0021] The present invention will be further described below with reference to the embodiments and accompanying drawings:

[0022] Example: A black liquor concentration system based on multi-stage flash evaporation and multi-effect evaporation coupling includes: a waste heat recycling self-heating multi-stage flash evaporation system, wherein the concentration medium outlet of the waste heat recycling self-heating multi-stage flash evaporation system is connected to the multi-effect evaporation system for supplying the concentration medium to the multi-effect evaporation system.

[0023] The waste heat recycling self-heating multi-stage flash evaporation system includes: at least one heat exchange unit and at least two flash evaporation units connected in series. The concentrated medium outlet of the flash evaporation unit in the last effect is connected to the concentrated medium inlet of the equivalent heat exchange unit of the flash evaporation unit in the previous effect. When there are at least two heat exchange units, the heat exchange units are connected in series. The flash evaporation unit in the first effect is connected to the equivalent heat exchange unit of the flash evaporation unit in the first effect through a heating unit. The equivalent heat exchange unit of the flash evaporation unit in the first effect is provided with a medium outlet after concentration. The medium outlet is connected to the heating unit through pipelines. The medium inlet is connected to the multi-effect evaporation system. The "first-effect flash unit" refers to the flash unit into which the concentrated medium first enters. The "last-effect flash unit" refers to the flash unit into which the concentrated medium is finally discharged. "Equivalent" means that there is a steam medium transmission relationship between the flash unit and the heat exchange unit, that is, the two are equivalent. The heat exchange units are connected in series, which means that the shell side or tube side of multiple flash units is connected end to end. The heating unit can be a heat exchanger, a compression heat pump or an absorption heat pump. Its heat source can be flue gas from the alkali furnace of a pulp mill, green liquor, etc.

[0024] It also includes a black liquor storage tank, which is equipped with a concentration medium inlet. The black liquor storage tank is connected to the medium inlet of the flash evaporation unit located in the first-effect unit via pipeline. The black liquor storage tank is used to store the black liquor to be concentrated. Its material can be stainless steel, especially 304L or 316L grade stainless steel. This type of material can resist corrosive substances in black liquor, especially chloride ion corrosion resistance. Stainless steel also has good mechanical strength and processing performance, making it suitable for manufacturing large storage tanks. Alternatively, carbon steel with an anti-corrosion coating can be used: for cost-sensitive applications, carbon steel structures with internal epoxy resin or other types of anti-corrosion coatings can be used. This method can improve the container's ability to resist black liquor corrosion to a certain extent, but the coating condition needs to be checked regularly to ensure its effectiveness. Another option is to use fiberglass reinforced plastic (FRP): made of synthetic resin and glass fiber, it has the characteristics of being lightweight, high-strength, and chemically resistant, making it a good choice for treating black liquor under certain specific conditions (such as when the temperature is not particularly high). However, FRP may not be the best option for extreme conditions (such as very high temperatures); duplex stainless steel can be used instead. This material combines the advantages of both austenitic and ferritic structures, possessing not only good corrosion resistance but also maintaining high mechanical strength at high temperatures, making it suitable for more demanding working environments. Titanium and its alloys can also be used: although more expensive, their use may be necessary for extremely harsh working conditions, such as highly corrosive and / or high temperatures. Titanium has excellent corrosion resistance, making it particularly suitable for applications involving long-term contact with chloride-containing media. Its specific shape is not limited, depending on site or environmental conditions. The pipeline is equipped with valves for control switches, the specific form of which is not limited and can be determined according to the actual situation. A pump for driving the flow of the medium is also installed on the pipeline, the specific form of which is not limited and can be determined according to the actual situation.

[0025] It also includes a pressure stabilizing unit for maintaining the pressure within the flash unit located in the last effect. The pressure stabilizing unit is a condensing or absorption processing unit. Specifically, the condensing unit can be a plate heat exchanger, a flue gas heat exchanger, a flash heat exchanger, etc. The liquid medium after flashing inside the flash unit in the last effect is transported to the concentrated medium outlet of the flash unit in the last effect, which is equivalent to a heat exchange unit in the flash unit of the previous effect. The medium in the heat exchange unit is heated by its equivalent flash exhaust steam. However, the flash unit in the last effect... If the flash vapor cannot be absorbed in time, it will cause pressure imbalance in the system, which will then need to be adjusted and absorbed by the pressure stabilizing unit. The condensation treatment unit can be a condenser. The specific method of condenser will not be elaborated here. The absorption treatment unit can use lithium bromide solvent, which is a substance with water absorption properties. It can also be divided into physical method or chemical method. It also includes a vacuum unit for vacuum treatment of the flash unit and heat exchange unit. The vacuum unit specifically uses a vacuum pump. The specific model, quantity or combination of vacuum pumps can be selected according to the actual situation.

[0026] The concentrated medium between the multiple flash evaporation units can flow by gravity, and the concentrated medium between the heat exchange unit and the flash evaporation unit can flow by gravity; the concentrated medium between the multiple flash evaporation units can flow by a pump, and the concentrated medium between the heat exchange unit and the flash evaporation unit can flow by a pump.

[0027] The above connection methods can be combined arbitrarily and selected according to the initial concentration of black liquor; that is, they are set up sequentially from top to bottom, with the flash evaporation unit in the first effect and the equivalent heat exchange unit in the flash evaporation unit in the first effect located at the upper and lower ends respectively. They can be connected by pipes of appropriate diameter or through holes opened between the shells. The movement between different chambers is achieved by the gravity of the medium itself, thereby reducing the investment in pumps and saving the cost of the entire system. In large equipment or high flow systems, the cost of pumps is relatively high, so gravity method is preferred.

[0028] The steam generated by the flash evaporation unit enters the shell side / tube side of the heat exchange unit and is connected, while the concentrated medium of the flash evaporation unit enters the tube side / shell side of the heat exchange unit.

[0029] Firstly, steam enters the shell side of the heat exchange unit, while the concentrated medium enters the tube side.

[0030] Effects: In this configuration, steam flows in the shell side, transferring heat to the concentrated medium inside the tubes. This method is suitable for applications requiring high heat transfer efficiency because the steam has direct contact with a large heat exchange area. Furthermore, the shell-side design is easier to clean and maintain if the steam contains impurities or is prone to fouling. For concentrated media, the tube-side design facilitates control of flow rate and pressure drop, which helps improve heat transfer efficiency.

[0031] Secondly, steam enters the tube side of the heat exchange unit, while the concentrated medium enters the shell side.

[0032] Effect: In this configuration, steam flows inside the tubes, while the concentrated medium fills the shell-side space. This arrangement is suitable for handling high-viscosity or easily crystallizing concentrated media because the shell side provides a larger flow area, helping to reduce pressure loss and prevent blockage. Simultaneously, the steam flow within the tubes helps maintain a high heat transfer coefficient, especially when the steam velocity is sufficiently high.

[0033] The choice of configuration depends on the specific application requirements, including but not limited to: media properties (such as viscosity, corrosiveness, etc.), temperature and pressure conditions, economic and maintainability considerations, and the overall system design. In actual operation, safety factors and long-term operating costs must also be considered.

[0034] The heat exchange method of the multi-effect evaporation system is counter-current heat exchange, co-current heat exchange, or mixed heat exchange. Counter-current heat exchange: In this configuration, the medium to be concentrated and the heating medium (such as primary steam or secondary steam generated in the previous stage) flow in opposite directions. That is, if the material enters the last effect from the first effect, the heating steam flows from the last effect to the first effect. This method is beneficial for maintaining a high heat transfer temperature difference and is particularly suitable for processing materials whose viscosity varies greatly with temperature. Because the viscosity of a material may increase with increasing concentration, counter-current flow allows the material at a higher temperature to contact the heating surface at a lower temperature, helping to reduce the problem of decreased heat transfer efficiency due to excessive viscosity. Co-current heat exchange: In contrast to counter-current flow, in co-current heat exchange, the material and the heating medium flow in the same direction. This means that both start from the same end of the system and move towards the other end. This arrangement is suitable for situations where it is desirable to maintain a gradual increase in material temperature throughout the process, such as processing certain temperature-sensitive materials. However, for materials whose viscosity increases significantly with temperature, co-current flow may lead to poor end-stage performance because the material there is already very viscous, while the temperature of the heating steam is relatively low. Hybrid heat exchange: This is a design scheme that combines the characteristics of co-current and counter-current flow. In a multi-effect evaporation system, co-current flow is used in some stages, while counter-current flow is used in others. The purpose of this is to utilize the advantages of both modes; for example, co-current flow can be used in the early stages to ensure a stable temperature rise of the material, while counter-current flow can be switched in the later stages to overcome heat transfer difficulties caused by material concentration. Hybrid heat exchange provides more flexible operating conditions, but it also increases the complexity of the system design.

[0035] The multi-effect evaporation system includes a low-concentration section, a medium-concentration section, and a high-concentration section. The concentrated medium outlet of the heat exchange unit equivalent to the flash evaporation unit in the first effect supplies concentrated medium to the low-concentration section, the medium-concentration section, or the high-concentration section. The black liquor concentration in the low-concentration section is 10% to 30%, the concentration in the medium-concentration section is 30% to 45%, and the concentration in the high-concentration section is above 45%. Each of the low-concentration section, the medium-concentration section, and the high-concentration section contains multiple sets of flash evaporation units and heat exchange units.

[0036] The multi-effect evaporation system also includes a semi-concentrated liquid black tank, which is located at the outlet of the heat exchange unit of the first effect to supply concentrated medium to the semi-concentrated liquid black tank. The semi-concentrated liquid black tank can be set at the medium inlet end of each effect in the low-concentration section, medium-concentration section or high-concentration section. The number of tanks set depends on the actual situation. The function of the semi-concentrated liquid black tank is to buffer and neutralize the concentration. According to the current system setting, the liquid medium in the semi-concentrated liquid black tank has a solubility threshold range. When the threshold is met, the medium is directly supplied. If the threshold is not met, the concentration needs to be adjusted. When the concentration is high, liquid is taken from the front end of the semi-concentrated liquid black tank to neutralize the concentration. When the concentration is low, liquid is taken from the rear end of the semi-concentrated liquid black tank, i.e., the high-concentration section, to neutralize the concentration.

[0037] A discharge pump is provided between the waste heat circulation self-heating multi-stage flash evaporation system and the multi-effect evaporation system. The discharge pump is used to transport the medium concentrated to a certain extent by the waste heat circulation self-heating multi-stage flash evaporation system to the discharge pump provided between the multi-effect evaporation systems.

[0038] The high-concentration section is equipped with a drain pipe and a drain pump. The discharged medium can be sent to the crystallization stage for ultra-concentration evaporation or directly to the alkali furnace for combustion. The drain pipe is connected to the concentrated black liquid tank for storage. The concentrated medium can first be sent to the concentrated black liquid tank and then to the crystallization stage for ultra-concentration evaporation or directly to the alkali furnace for combustion.

[0039] It also includes a condensate tank assembly, which is connected to the low-concentration section, medium-concentration section, high-concentration section and heat exchange unit, and is used to collect the condensate generated by the flash steam condensation of the heat exchange unit in the low-concentration section, medium-concentration section and high-concentration section. The condensate tank assembly includes: a clean condensate tank unit, a lightly polluted condensate tank unit, a mediumly polluted condensate tank unit and a heavily polluted condensate tank unit, which are used to collect the condensate generated by the high-concentration section, medium-concentration section, low-concentration section and heat exchange unit, respectively.

[0040] Work process:

[0041] In the waste heat recycling self-heating multi-stage flash evaporation system, the initial concentrated material enters the storage tank through the medium inlet for storage. It is then pressurized by a feed pump and fed into the medium circulation system to replenish the solution, participating in the flash concentration process within the circulation system. The replenished concentrated medium sequentially passes through the I-effect flash evaporation unit, the II-effect flash evaporation unit, the IV-effect flash evaporation unit, and the III-effect heat exchange unit. Finally, it enters the heating unit and exchanges heat with the waste heat medium unit for another round of circulation and concentration until the set concentration standard is reached before being connected to the multi-effect evaporation process section. The IV-effect heat exchange unit acts as a pressure stabilizing unit, exchanging heat with the external environment to maintain a temperature gradient concentration and ensure stable system operation. Additionally, the flash exhaust steam generated by each flash evaporation unit enters the corresponding equivalent heat exchange unit to exchange heat with the concentrated liquid in its heat exchange tubes. The resulting condensate is collected in a collection tank and discharged by a condensate pump.

[0042] Furthermore, the waste heat medium enters the heating unit through the input end, performs indirect heat exchange and supplementation on the circulating medium, and then exits the heating unit through the medium return end.

[0043] Furthermore, the multi-effect evaporation system is divided into low-concentration, medium-concentration, and high-concentration sections based on the inlet and outlet concentrations of different effects.

[0044] Furthermore, depending on the concentration of the flash evaporation system, different evaporation stages can be selectively connected.

[0045] Furthermore, if connected to the low-concentration section, the black liquor, after being concentrated by the low-efficiency evaporator, passes through the medium-concentration section and the high-concentration section in sequence before being discharged into the concentrated black liquor tank, and then discharged to the alkali furnace for combustion via the discharge pump.

[0046] Furthermore, if connected to the medium-concentration section, the black liquor, after being concentrated by the medium-concentration section, is then concentrated by the high-efficiency section and discharged into the concentrated black liquor tank, and then discharged to the alkali furnace for combustion by the discharge pump.

[0047] Furthermore, if connected to a high-concentration section, the black liquor, after being concentrated in a high-efficiency section, is discharged into a concentrated black liquor tank and then pumped to an alkali furnace for combustion.

[0048] Furthermore, the vacuum pump is connected to the heavy sludge condensate tank unit, the medium sludge condensate tank unit, and the light sludge condensate tank unit, thereby creating a negative pressure environment for the entire system through pipelines and expelling the odor generated by the system.

[0049] The embodiments of this utility model have been described in detail above, but the content described is only a preferred embodiment of this utility model and should not be considered as limiting the scope of implementation of this utility model. All equivalent changes and improvements made in accordance with the claims of this utility model should still fall within the patent coverage of this utility model.

Claims

1. A black liquor concentration system based on multi-stage flash evaporation and multi-effect evaporation coupling, characterized in that, include: The waste heat recycling self-heating multi-stage flash evaporation system has its concentrated medium outlet connected to a multi-effect evaporation system for supplying concentrated medium to the multi-effect evaporation system.

2. The black liquor concentration system based on multi-stage flash evaporation and multi-effect evaporation coupling according to claim 1, characterized in that, The waste heat recycling self-heating multi-stage flash evaporation system includes: at least one heat exchange unit and at least two flash evaporation units connected in series. The concentrated medium outlet of the flash evaporation unit located in the last effect is connected to the concentrated medium inlet of the equivalent heat exchange unit of the flash evaporation unit in the previous effect located in the last effect. When there are at least two heat exchange units, the heat exchange units are connected in series. The flash evaporation unit located in the first effect is connected to the equivalent heat exchange unit of the flash evaporation unit located in the first effect through a heating unit.

3. The black liquor concentration system based on multi-stage flash evaporation and multi-effect evaporation coupling according to claim 2, characterized in that, The heat exchange method of the multi-effect evaporation system is countercurrent heat exchange, cocurrent heat exchange, or mixed heat exchange.

4. The black liquor concentration system based on multi-stage flash evaporation and multi-effect evaporation coupling according to claim 1, 2 or 3, characterized in that, The multi-effect evaporation system includes a low-concentration section, a medium-concentration section, and a high-concentration section. The concentrated medium outlet of the heat exchange unit, which is equivalent to the flash evaporation unit in the first effect, delivers concentrated medium to the low-concentration section, the medium-concentration section, or the high-concentration section.

5. The black liquor concentration system based on multi-stage flash evaporation and multi-effect evaporation coupling according to claim 4, characterized in that, The multi-effect evaporation system also includes a semi-concentrated liquid black tank, where the concentrated medium outlet of the heat exchange unit in the first effect supplies concentrated medium to the semi-concentrated liquid black tank.

6. The black liquor concentration system based on multi-stage flash evaporation and multi-effect evaporation coupling according to claim 1, characterized in that, A discharge pump is installed between the waste heat recycling self-heating multi-stage flash evaporation system and the multi-effect evaporation system.

7. The black liquor concentration system based on multi-stage flash evaporation and multi-effect evaporation coupling according to claim 4, characterized in that, The high-concentration section is equipped with a drain pipe, and the drain pipe is equipped with a drain pump.

8. The black liquor concentration system based on multi-stage flash evaporation and multi-effect evaporation coupling according to claim 7, characterized in that, The drain pipe is connected to the concentrated black liquid tank.

9. The black liquor concentration system based on multi-stage flash evaporation and multi-effect evaporation coupling according to claim 4, characterized in that, It also includes a condensate tank assembly, which is connected to the low-concentration section, the medium-concentration section, the high-concentration section, and the heat exchange unit.

10. The black liquor concentration system based on multi-stage flash evaporation and multi-effect evaporation coupling according to claim 9, characterized in that, The condensate tank assembly includes: a clean condensate tank unit, a lightly soiled condensate tank unit, a medium-soiled condensate tank unit, and a heavily soiled condensate tank unit, which are used to collect condensate generated by the high-concentration section, the medium-concentration section, the low-concentration section, and the heat exchange unit, respectively.

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