Multistage flash evaporation and vacuum low-temperature negative-pressure black liquor concentration system based on industrial waste heat

Abstract

The utility model relates to a multistage flash evaporation and vacuum low-temperature negative-pressure black liquor concentration system based on industrial waste heat, which belongs to the technical field of flash evaporation and comprises at least one heat exchange unit and at least two flash evaporation units sequentially connected in series. The concentration medium outlet of the flash evaporation unit located at the last effect is communicated with the concentration medium inlet of the heat exchange unit equivalent to the flash evaporation unit of the last effect located at the last effect, and when the number of the heat exchange units is at least two, the heat exchange units are sequentially connected in series. The flash evaporation unit located at the first effect communicates with the heat exchange unit equivalent to the flash evaporation unit located at the first effect through a heating unit. The concentration medium generated by the last-effect flash evaporation unit flows back to the previous-effect heat exchange unit to serve as a heat source, energy recycling is achieved, the requirement for external heat input is reduced, and therefore the energy efficiency of the whole system is improved; the plurality of heat exchange units which are sequentially connected in series can more effectively utilize temperature gradient to transfer heat, so that the heat can be more uniformly transferred to each flash evaporation unit, and the heat transfer effect is enhanced.

Description

Technical Field

[0001] This invention belongs to the field of flash evaporation technology, and in particular relates to a multi-stage flash evaporation and vacuum low-temperature negative pressure black liquor concentration system based on industrial waste heat. Background Technology

[0002] Black liquor is the waste liquid produced during the alkaline pulping process. It is commonly called black liquor because of its dark brown color. Currently, paper mills mainly concentrate black liquor through pulping until its concentration reaches 75%, then send it to an alkali furnace for combustion and recover the caustic soda for use in the following process.

[0003] Against the backdrop of global industrialization and economic development, the market demand for paper products continues to grow, driving the continuous expansion of the pulping industry. In the pulping process, black liquor treatment has become a crucial step, directly impacting resource recycling, equipment energy consumption, and enterprise costs.

[0004] Currently, pulp mills mainly use multi-effect evaporation (MVR) or a combination of mechanical vapor recompression (MVR) and MVR for black liquor concentration. Both processes rely on fresh steam as a heat source; however, the sheer volume of black liquor produced during pulping leads to persistently high fresh steam consumption. Actual surveys show that the cost of treating black liquor using MVR is approximately 32-40 yuan per ton of steam, while the cost of the combined MVR and MVR process is 15-20 yuan per ton of steam. Such high processing costs not only increase the operational burden on pulp mills but also severely impact their economic efficiency and energy utilization, becoming a factor restricting their development.

[0005] From a technological perspective, both black liquor treatment processes are highly complex and large-scale. This presents numerous challenges for equipment installation, maintenance, shutdown, and routine upkeep. Furthermore, the complexity of the processes increases the difficulty and cost of maintenance; if the equipment is shut down for repairs, various problems may arise in the subsequent pulping and cooking stages, affecting production continuity and stability.

[0006] Therefore, pulp mills urgently need to find more energy-efficient and effective black liquor treatment solutions in order to reduce energy consumption and costs through process optimization. This has become an important research direction in the current pulp industry. Utility Model Content

[0007] To solve the above-mentioned technical problems, the technical solution adopted by this utility model is: a multi-stage flash evaporation and vacuum low-temperature negative pressure black liquor concentration system based on industrial waste heat, comprising: at least one heat exchange unit and at least two flash evaporation units connected in series. The concentration medium outlet of the flash evaporation unit located in the last effect is connected to the concentration medium inlet of the equivalent heat exchange unit of the flash evaporation unit in the previous effect of the flash evaporation unit 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. The medium outlet of the heat exchange unit located in the first effect is connected to the medium inlet of the vacuum disc low-temperature dryer.

[0008] Furthermore, the vacuum exhaust pipe of the vacuum disc low-temperature dryer is connected to the second condenser through a pipeline, and the second condenser is connected to the medium waste condensate tank through a pipeline. A vacuum pump is connected to the medium waste condensate tank.

[0009] Furthermore, the heat exchange unit is connected to the first condenser via a pipeline, and the condenser is connected to the heavy waste condensate tank via a pipeline. A vacuum pump is connected to the heavy waste condensate tank.

[0010] Furthermore, it also includes a liquid storage tank, which is provided with a concentration medium inlet. The liquid storage tank is connected to the medium inlet of the flash evaporation unit located in the first effect or the medium inlet of the flash evaporation unit located in the last effect via a pipeline.

[0011] Furthermore, the liquid storage tank is connected to the medium inlet of the heating unit via a pipeline.

[0012] Furthermore, it also includes a pressure stabilizing unit for maintaining the pressure within the flash unit at the end of its lifespan.

[0013] Furthermore, the voltage stabilizing unit is a condenser.

[0014] Furthermore, the heating unit is a heat exchanger or a heat pump.

[0015] Furthermore, the heat source of the vacuum disc low-temperature dryer is waste heat.

[0016] Furthermore, the steam generated by the flash evaporation unit enters the shell side / tube side of the heat exchange unit, and the concentrated medium of the flash evaporation unit enters the tube side / shell side of the heat exchange unit.

[0017] The beneficial effects of this utility model are:

[0018] This application achieves energy reuse by returning the concentrated medium generated by the final-effect flash evaporation unit to the heat exchange unit of the previous effect as a heat source, reducing the need for external heat input and thus improving the energy efficiency of the entire system; multiple heat exchange units connected in series can more effectively utilize the temperature gradient for heat transfer, so that heat can be transferred more evenly to each flash evaporation unit, enhancing the heat transfer effect.

[0019] This application utilizes waste heat from the plant area as the heat source for concentration throughout the entire process, without using any fresh steam, which can greatly reduce the operating costs and expenses of pulp mill black liquor concentration.

[0020] This application uses a vacuum disc dryer to concentrate black liquor. It can perform thermal concentration according to the target product to produce black liquor powder or high-concentration black liquor product, thereby improving the economic recovery value of the production process.

[0021] This application reduces the processing steps of downstream processes by classifying and collecting condensate from low-concentration black liquor process sections and high-concentration black liquor process sections. Attached Figure Description

[0022] Figure 1 This is a system diagram of this utility model. Detailed Implementation

[0023] 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.

[0024] 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.

[0025] The present invention will be further described below with reference to the embodiments and accompanying drawings: A multi-stage flash evaporation and vacuum low-temperature negative pressure black liquor concentration system based on industrial waste heat includes: at least one heat exchange unit and at least two flash evaporation units connected in series. The concentration medium outlet of the flash evaporation unit located in the last effect is connected to the concentration medium inlet of the equivalent heat exchange unit of the flash evaporation unit in the previous effect of the flash evaporation unit 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. The medium outlet of the heat exchange unit located in the first effect is connected to the medium inlet of the vacuum disc low-temperature dryer.

[0026] The term "first-effect flash unit" refers to the flash unit where the concentrated medium first enters from the heat exchange unit. The term "last-effect flash unit" refers to the flash unit where the concentrated medium first enters from the flash unit. "Equivalent" means that there is a steam medium transfer relationship between the flash unit and the heat exchange unit, i.e., they are equivalent. The first-effect flash unit is connected to the equivalent heat exchange unit of the first-effect flash unit via a heating unit, i.e., the first-effect heat exchange unit. "Heat exchange units connected in series" 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 a pulp mill alkali furnace, green liquor, etc. Using a vacuum unit and a pressure stabilizing unit helps maintain a suitable operating environment and promotes the production of high-quality products. The vacuum disc low-temperature dryer includes: a dryer body, on which a medium inlet pipe, a vacuum exhaust pipe, and a medium outlet pipe are provided. The heat source can be the same as that of the heating unit, thus making full use of the waste heat resources in the field. Currently, most fields have excess waste heat, and a good heat outlet is beneficial for energy utilization. The medium discharge pipe is used to discharge the dried medium. The vacuum disc low-temperature dryer is a device specifically designed for drying materials under low-temperature conditions. It combines the advantages of vacuum technology and disc drying technology. By using vacuum technology, the boiling point of water is lowered, allowing black liquor to evaporate water rapidly at a lower temperature, achieving a highly efficient concentration effect. Papermaking black liquor may contain valuable chemical substances or components that need to be retained. The low-temperature operating environment can prevent these components from being damaged or lost due to high temperatures, ensuring the quality of the concentrated product. Compared with high-temperature drying, low-temperature operation can reduce scaling inside the equipment and reduce the risk of corrosion to equipment materials, extending the service life of the equipment. Using vacuum to lower the boiling point of water for evaporation consumes less energy than traditional high-temperature evaporation methods, which helps to save energy and reduce emissions.

[0027] The vacuum exhaust pipe of the vacuum disc low-temperature dryer is connected to the second condenser via a pipeline. The second condenser is connected to the medium waste condensate tank via a pipeline. A vacuum pump is connected to the medium waste condensate tank. The second condenser is a plate condenser or a shell-and-tube condenser. The second condenser is used to condense the exhaust steam generated by the vacuum disc low-temperature dryer. The vacuum pump creates a negative pressure environment in the system. In addition, the vacuum pump group can also extract the odor generated by black liquor concentration. The medium waste condensate tank is discharged externally through the condensate drain pump II. The waste heat source inlet and outlet of the vacuum disc low-temperature dryer can be connected to the waste heat source inlet and waste heat source return water of the heating unit.

[0028] The heat exchange unit is connected to the first condenser via a pipeline, and the condenser is connected to the heavy waste condensate tank via a pipeline. A vacuum pump is connected to the heavy waste condensate tank to condense the incompletely condensed flash vapor into condensate and then discharge it into the heavy waste condensate tank for storage. Once the heavy waste condensate tank reaches a certain liquid level, it is discharged through the condensate drain pump I.

[0029] It also includes a liquid storage tank, which is equipped with a concentration medium inlet. The liquid storage tank is connected to the medium inlet of the flash evaporation unit located in the first effect or the medium inlet of the flash evaporation unit located in the last effect through a pipeline. It can also be connected to the medium inlet of the flash evaporation unit located between the first effect and the last effect. The specific connection method can be determined according to the site conditions.

[0030] The liquid storage tank is connected to the medium inlet of the heating unit through a pipeline. When the temperature of the liquid medium is too low, it can be preheated by the heating unit to reach the temperature set by the flash evaporation unit, thus making it applicable to a wide range of scenarios.

[0031] 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 treatment unit or an absorption treatment unit. The condensing treatment unit, i.e., the condenser, can specifically be a plate heat exchanger, a flue gas heat exchanger, a flash heat exchanger, etc. The liquid medium after flashing inside the flash unit located in the last effect is transported to the concentrated medium outlet of the flash unit located in the last effect and the equivalent heat exchange unit of the flash unit in the previous effect located in the last effect. The medium in the heat exchange unit is heated by its equivalent flash exhaust steam. However, if the flash exhaust steam in the flash unit located in the last effect cannot be absorbed in time, it will cause a pressure imbalance in the system, which then needs to be adjusted and absorbed by the pressure stabilizing unit. The condensing treatment unit can be a condenser. The specific method of the condenser will not be elaborated further without application. The absorption treatment unit can use lithium bromide solvent, i.e., a substance with water-absorbing properties. It can also be said that the method is divided into physical method or chemical method.

[0032] The heating unit is a heat exchanger or a heat pump. The heat source of the vacuum disc low-temperature dryer is waste heat, or it can be an indirect contact heat exchanger, in which the waste heat water and the circulating dilute black liquor exchange heat in a non-contact indirect manner.

[0033] 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.

[0034] 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.

[0035] The above connection methods can be combined arbitrarily and selected according to the physical and chemical properties of the liquid medium. That is, they are arranged sequentially from top to bottom. The flash evaporation unit in the first effect and the heat exchange unit equivalent to the flash evaporation unit in the first effect are located at the upper and lower ends respectively. They can be connected by pipes of appropriate diameter or through holes in the shell. The movement between different cavities is achieved by the gravity of the medium itself, thereby reducing the investment of 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.

[0036] The steam generated by the flash evaporation unit enters the shell side / tube side of the heat exchange unit and is connected. The concentrated medium of the flash evaporation unit enters the tube side / shell side of the heat exchange unit. The flow of the concentrated medium between different units is controlled by gravity or pump drive, which not only ensures the stable operation of the system, but also allows for flexible adjustment of operating conditions according to actual needs.

[0037] Firstly, steam enters the shell side of the heat exchange unit, while the concentrated medium enters the tube side.

[0038] 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.

[0039] Secondly, steam enters the tube side of the heat exchange unit, while the concentrated medium enters the shell side.

[0040] 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.

[0041] 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.

[0042] Work process:

[0043] The initial concentrated material enters the storage tank through the medium inlet for storage, and 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 I-effect heat exchange unit and exchanges heat with the waste heat medium unit for another round of circulation and concentration until the set concentration standard is reached. 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 within its heat exchange tubes. The resulting condensate is collected in a collection tank and discharged by a condensate pump.

[0044] Waste heat medium enters the heating unit through the input end, performs indirect heat exchange and replenishment of the circulating medium, and then exits the heating unit through the medium return end.

[0045] After the multi-stage flash evaporation concentration system concentrates the liquor to a certain concentration standard, it is discharged through the black liquor discharge pump and connected to the medium feed pipe of the vacuum low-temperature disc dryer. After drying in the vacuum low-temperature disc dryer until the concentration or drying standard is reached, it is discharged from the vacuum low-temperature disc dryer through the medium discharge pipe, thus completing the drying process.

[0046] 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 multi-stage flash evaporation and vacuum low-temperature negative pressure black liquor concentration system based on industrial waste heat, comprising: The system comprises 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. 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. The system is characterized in that the medium outlet of the heat exchange unit located in the first effect is connected to the medium inlet of the vacuum disc low-temperature dryer.

2. The black liquor concentration system based on industrial waste heat and vacuum low-temperature negative pressure according to claim 1, characterized in that, The vacuum exhaust pipe of the vacuum disc low-temperature dryer is connected to the second condenser through a pipeline, and the second condenser is connected to the medium waste condensate tank through a pipeline. A vacuum pump is connected to the medium waste condensate tank.

3. The black liquor concentration system based on industrial waste heat and vacuum low-temperature negative pressure according to claim 1, characterized in that, The heat exchange unit is connected to the first condenser via a pipeline, and the first condenser is connected to the heavy waste condensate tank via a pipeline. A vacuum pump is connected to the heavy waste condensate tank.

4. The black liquor concentration system based on industrial waste heat and vacuum low-temperature negative pressure according to claim 1, characterized in that, It also includes a liquid storage tank, which is provided with a concentration medium inlet. The liquid storage tank is connected to the medium inlet of the flash evaporation unit located in the first effect or the medium inlet of the flash evaporation unit located in the last effect via a pipeline.

5. The black liquor concentration system based on industrial waste heat and vacuum low-temperature negative pressure according to claim 4, characterized in that, The liquid storage tank is connected to the medium inlet of the heating unit via a pipeline.

6. The black liquor concentration system based on industrial waste heat and vacuum low-temperature negative pressure according to claim 1, characterized in that, It also includes a pressure stabilizing unit for maintaining the pressure within the flash unit at the end of its lifespan.

7. The black liquor concentration system based on industrial waste heat and vacuum low-temperature negative pressure according to claim 6, characterized in that, The voltage stabilizing unit is a condenser.

8. The black liquor concentration system based on industrial waste heat and vacuum low-temperature negative pressure according to claim 1, characterized in that, The heating unit is a heat exchanger or a heat pump.

9. The black liquor concentration system based on industrial waste heat and vacuum low-temperature negative pressure according to claim 1 or 2, characterized in that, The heat source for the vacuum disc low-temperature dryer is waste heat.

10. The black liquor concentration system based on industrial waste heat and vacuum low-temperature negative pressure according to claim 1, characterized in that, 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.

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