Low energy consumption liquid caustic soda concentration evaporation process

Abstract

This invention relates to the field of chemical technology, and in particular to a low-energy-consumption liquid alkali concentration and evaporation process. The process includes the following steps: Step S1, raw material preheating and primary wastewater preheating: 32wt% raw material liquid alkali at 68-70℃ from the ion-exchange membrane electrolysis process is heat-exchanged with 42wt% intermediate alkali solution at 88-90℃ via plate heat exchanger A. This invention maximizes the recovery of low-grade heat energy generated by the process itself through a multi-stage heat exchange network design between "raw material alkali - intermediate alkali - finished alkali" and "condensate - wastewater - secondary steam", significantly reducing the dependence on high-grade generated steam. Secondly, the preheating and pre-concentration process of low-concentration acetylene production wastewater is organically integrated with the main alkali evaporation process, achieving effective recovery of alkali in the wastewater while saving energy, improving the environmental friendliness of the process. Moreover, the three-stage evaporation main framework is stable, and by adjusting the operating parameters of the equipment and the wastewater mixing ratio, it can adapt to different scales, raw materials, and product requirements.

Description

Technical Field

[0001] This invention relates to the field of chemical technology, specifically to a low-energy-consumption liquid alkali concentration and evaporation process. Background Technology

[0002] Caustic soda (NaOH) is an important basic chemical raw material. Caustic soda produced by ion-exchange membrane electrolysis is usually liquid caustic soda with a concentration of about 32%. In order to facilitate transportation, storage or use in the production of solid caustic soda (such as caustic soda flakes), it is often necessary to concentrate it to a high concentration (such as 50% or higher). Traditional liquid caustic soda concentration often adopts multi-effect evaporation or falling film evaporation processes, which have high energy consumption.

[0003] In the production of acetylene and other alkynes, a large amount of washing wastewater is generated. This wastewater contains a small amount of NaOH, and although its alkali concentration is extremely low (usually below 5%), direct discharge not only wastes resources but also increases the burden on environmental protection. Currently, the liquid alkali concentration system and the alkyne wastewater treatment system operate independently, and effective synergistic integration in terms of multi-stage heat utilization and material recycling has not yet been formed between the two. Summary of the Invention

[0004] The purpose of this invention is to provide a low-energy-consumption liquid alkali concentration and evaporation process to solve the problems mentioned in the background art.

[0005] To achieve the above objectives, the present invention provides the following technical solution: a low-energy-consumption liquid alkali concentration and evaporation process, the process comprising the following steps: Step S1, Raw material preheating and primary wastewater preheating: The 32wt% raw material alkali solution at 68-70℃ from the ion membrane electrolysis process is preheated to 80-85℃ by exchanging heat with the 42wt% intermediate alkali solution at 88-90℃ through plate heat exchanger A. At the same time, the ambient temperature wastewater from the production of 3-5% acetylene is preheated in the primary wastewater preheater using low temperature condensate at 50-60℃. Step S2, Primary Falling Film Evaporation and Secondary Preheating of Wastewater: After preheating in Step S1, the alkaline solution enters the primary falling film evaporator and is heated by 0.25-0.35MPa live steam. The rotating scraper forms a film and evaporates, generating secondary steam at 100-105℃. The output concentration is 36-38% and the temperature is 95-100℃. This output alkaline solution is used in plate heat exchanger B to perform secondary preheating of the wastewater after preheating in Step S1, raising the wastewater temperature to 70-75℃. Step S3, Secondary Falling Film Evaporation and Wastewater Pre-concentration: The 36-38% alkaline solution from step S2 enters the secondary falling film evaporator and is heated by the 90-95℃ secondary steam from step S4, concentrating it to 42-45% and 90-95℃ before returning it to S1. At the same time, the 70-75℃ wastewater is introduced into the -0.08MPa vacuum flash pre-concentrator, where sensible heat flash evaporation is used to increase the concentration to 8-10% and reduce the temperature to 50℃. Step S4, Three-stage falling film evaporation and mixing of wastewater and alkaline solution: 42-45% of the alkaline solution from step S3 enters the three-stage falling film evaporator and is heated to 50% by steam at a temperature of 85-90℃. The generated secondary steam at 85-90℃ is returned to step S3. After filtering and removing impurities from the 8-10% wastewater pre-concentrated in step S3, it is mixed online with 50% of the finished alkaline solution at a ratio of 5-15% of the total NaOH. The temperature after mixing is 80-85℃. Step S5, Finished Product Cooling and Energy Recovery: The mixed alkali solution is cooled to 70-75°C by heat exchange with the raw material alkali solution in plate heat exchanger A, and then cooled to the storage and transportation temperature before being sent to the finished product storage tank. The steam condensate from each stage is collected in stages. The high-temperature condensate is returned to the boiler room, and the low-temperature condensate is used for preheating the wastewater in step S1.

[0006] Preferably, in step S1, the raw material alkali liquid and the intermediate alkali liquid in plate heat exchanger A undergo countercurrent heat exchange, the raw material alkali liquid is heated by 20-22°C, and the intermediate alkali liquid is cooled accordingly to meet the preheating requirements, and the intermediate alkali liquid comes from step S3.

[0007] Preferably, in step S1, the low-temperature condensate used by the primary wastewater preheater comes from step S5, and the wastewater temperature is raised to 40-45℃ after preliminary preheating.

[0008] Preferably, in step S2, the primary falling film evaporator is a high-efficiency mechanically stirred thin film evaporator, and the rotating scraper makes the alkaline solution form a uniform liquid film.

[0009] Preferably, in step S2, the heat transfer area of ​​the primary falling film evaporator is 98-100 m². 2 It is made of 316L stainless steel.

[0010] Preferably, in step S3, the secondary falling film evaporator adopts a falling film structure with a heat transfer area of ​​88-90 m². 2 .

[0011] Preferably, in step S3, the vacuum flash pre-concentrator operates at an absolute pressure of 0.02 MPa, and the steam generated from the flash evaporation of wastewater has a low dust content, which can be used as a low-grade heat source for other process units or for condensation recovery.

[0012] Preferably, in step S4, the heating steam pressure of the three-stage falling film evaporator is 0.3-0.4 MPa, and the concentration at the concentration endpoint is precisely controlled at 50 wt%.

[0013] Preferably, in step S4, the heat transfer area of ​​the three-stage falling film evaporator is 78-80 m². 2 .

[0014] Preferably, in step S5, the storage and transportation temperature is <45℃.

[0015] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention maximizes the recovery of low-grade heat energy generated by the process itself through a multi-stage heat exchange network design between "raw material alkali - intermediate alkali - finished alkali" and "condensate - wastewater - secondary steam", significantly reducing the dependence on high-grade generated steam. Secondly, it organically integrates the preheating and pre-concentration process of low-concentration acetylene production wastewater with the main alkali evaporation process, achieving effective recovery of alkali in wastewater while saving energy, thus improving the environmental friendliness of the process. Moreover, the three-stage evaporation framework is stable, and by adjusting the operating parameters of the equipment and the wastewater mixing ratio, it can adapt to different scales, raw materials, and product requirements. Attached Figure Description

[0016] Figure 1 This is a schematic diagram of the process flow in this invention. Detailed Implementation

[0017] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0018] A low-energy-consumption liquid alkali concentration and evaporation process includes the following steps: Step S1, Raw Material Preheating and Primary Wastewater Preheating: The raw material liquid alkali from the ion-exchange membrane electrolysis process at a temperature of 68-70℃ is transported by a feed pump. It exchanges heat with intermediate alkali solution (from step S3) at 88-90℃ through a plate heat exchanger (code A). The raw material liquid alkali is preheated to 80-85℃. At the same time, the collected acetylene production wastewater (concentration of about 3-5%, temperature at room temperature) is sent to another heat exchange process through a wastewater pump. It is preheated in the primary wastewater preheater using low-temperature condensate (temperature of about 50-60℃, from step S5). The concentration of the raw material liquid alkali in the above process is 32wt%, the concentration of the intermediate alkali solution is about 42%, and the intermediate alkali solution comes from step S3, while the low-temperature condensate comes from step S5. Step S2, Primary Falling Film Evaporation and Secondary Preheating of Wastewater: After preheating in Step S1, the raw material alkali solution at 80-85℃ enters the primary falling film evaporator. The primary falling film evaporator adopts a high-efficiency mechanically stirred thin-film evaporator with a heat transfer area of ​​98-100m². 2The material is 316L stainless steel. The heating medium is live steam with a pressure of 0.25-0.35MPa (temperature 138-143℃). In the first-stage falling film evaporator, the alkaline solution forms a uniform liquid film under the action of the rotating scraper and evaporates rapidly to generate secondary steam. At this time, the temperature of the secondary steam is about 100-105℃, the concentration of the effluent alkaline solution increases to about 36-38%, and the temperature is about 95-100℃. The heat of this effluent alkaline solution is used to preheat the acetylene production wastewater after the preheating in step S1 in a second stage through another plate heat exchanger (code B). The temperature of the wastewater rises to 70-75℃. Step S3, Secondary Falling Film Evaporation and Wastewater Pre-concentration: The 36-38% alkaline solution from Step S2 enters the secondary falling film evaporator, which is also a falling film type with a heat transfer area of ​​88-90 m². 2 The heat source is secondary steam (temperature 90-95℃, from step S4). The alkaline solution is further concentrated to a concentration of about 42-45% and a temperature of about 90-95℃ in the secondary falling film evaporator. The heat of this alkaline solution has been used to preheat the raw material alkali solution in step S1. At the same time, the acetylene production wastewater, which has been preheated to 70-75℃ in step S2, is introduced into the vacuum flash pre-concentrator. The operating pressure is -0.08MPa (absolute pressure about 0.02MPa). Utilizing its own sensible heat and low-pressure environment, some water is flashed out, increasing the alkali concentration of the wastewater to 8-10% and reducing the temperature to about 50℃. The flashed steam can be used as a low-grade heat source for other purposes or for condensation. Step S4: Mixing the three-stage falling film evaporator with the alkaline wastewater: Allow 42-45% alkaline solution from step S3 to enter the three-stage falling film evaporator, which has a heat transfer area of ​​78-80 m². 2 The heating steam uses slightly higher pressure live steam (0.3-0.4MPa) or other reliable heat sources in the process. In the three-stage falling film evaporator, the alkaline solution is concentrated to the target concentration of 50% at a temperature of about 85-90℃. The secondary steam (temperature of about 85-90℃) generated by the three-stage falling film evaporator is introduced to the two-stage falling film evaporator in step S3 as a heat source. At the same time, the pre-concentrated acetylene production wastewater obtained in step S3 (containing 8-10% NaOH) is filtered to remove solid impurities and then precisely metered at a certain proportion (for example, 5-15% of the total feed amount based on the total amount of NaOH) and mixed with 50% of the finished alkaline solution from the outlet of the three-stage falling film evaporator in an online mixer. The mixing process is fast and uniform through turbulence. The temperature of the alkaline solution after mixing is 80-85℃. Step S5, Finished Product Cooling and Energy Recovery: The mixed 50% liquid alkali enters the finished product cooling system and exchanges heat with the raw liquid alkali at a temperature of 68-70℃ through the plate heat exchanger (code A) from step S1, reducing its own temperature to approximately 70-75℃, thus completing energy recovery. Then, as needed, it can be further cooled to the storage and transportation temperature (e.g., <45℃) through a cooling water heat exchanger and sent to the finished product storage tank. The steam condensate generated by each stage of the evaporator in the process (especially the live steam condensate, which is pure, pH neutral, and has low conductivity) is collected separately. Most of the high-temperature condensate is returned to the boiler room, and some of the low-temperature condensate is used for preliminary preheating of the acetylene production wastewater in the primary wastewater preheater of step S1, achieving multi-stage heat utilization. The entire evaporation system maintains the required vacuum level through a jet condenser or surface condenser located at the steam outlet of the final evaporator. For example, the operating pressure of the first-stage evaporator can be controlled between -0.06 and -0.09 MPa. Through vacuum system optimization, the boiling point of each effect evaporation is reduced, the heat transfer temperature difference is increased, and the waste heat of each low-temperature secondary steam and condensate is integrated as much as possible for the preheating of raw materials and wastewater. Live steam is only injected as the highest-grade heat source at key points in the first-stage falling film evaporator and the third-stage falling film evaporator, thereby maximizing the reduction of total fresh steam consumption.

[0019] The following are embodiments of the present invention: The original system's core equipment is retained as a backup, and a new low-energy liquid alkali concentration and evaporation system as described in this invention is constructed in parallel to achieve flexible switching and energy efficiency comparison between the old and new systems. The construction of the new system is strictly carried out according to the steps of this invention. S1: 32% raw material alkali solution from the ion membrane electrolysis process at 70°C is transported by a feed pump and preheated to 85°C by exchanging heat with 90°C, 42% intermediate alkali solution from step S3 through plate heat exchanger A. At the same time, room temperature acetylene production wastewater (concentration 5%) is pumped into the primary wastewater preheater and preheated by 60°C low-temperature condensate from step S5. S2: The preheated 85℃ alkaline solution enters the first-stage falling film evaporator (heat transfer area 100m²). 2 The material is 316L. It is heated by live steam at 0.35MPa (temperature 143℃). The alkaline solution evaporates under the action of rotating scraper, generating secondary steam at 105℃. The concentration of the discharged alkaline solution is increased to 38% and the temperature is 100℃. This hot alkaline solution passes through plate heat exchanger B, which preheats the wastewater after S1 to 75℃ in a secondary stage. S3: 38% alkaline solution enters the secondary falling film evaporator (heat transfer area 90m²). 2 Using the 95°C secondary steam from step S4 as a heat source, the mixture is concentrated to 45% at a temperature of 95°C. The heat has been recovered in S1. At the same time, 75°C wastewater is introduced into a vacuum flash pre-concentrator (operating absolute pressure 0.02MPa). After flash evaporation, the alkalinity concentration of the wastewater increases to 10%, and the temperature drops to 50°C. S4: 45% alkaline solution enters the three-stage falling film evaporator (heat transfer area 80m²). 2 The solution is heated with 0.4MPa live steam and concentrated to the target concentration of 50% at a temperature of 90℃. The 90℃ secondary steam generated is introduced to S3 as a heat source. The 10% pre-concentrated wastewater obtained from S3 is filtered and then mixed online with 50% finished alkali solution at a ratio of 10% of the total alkali content. The temperature after mixing is 83℃. S5: After mixing, the alkaline solution exchanges heat with the raw alkaline solution through plate heat exchanger A, and is cooled to 75°C. Then it is cooled to below 45°C by cooling water and sent to the finished product tank. The condensate from each stage is collected. The high-temperature condensate is returned to the boiler room, and some of the low-temperature condensate is used for preheating the wastewater in S1.

[0020] Example 2: S1: Directly apply the technology of this invention to construct a new system, which transports the raw material liquid alkali from the ion membrane electrolysis process at a temperature of 68°C and a concentration of 32% to plate heat exchanger A, where it exchanges heat with intermediate alkali liquid from the subsequent process and is then heated to 80°C; at the same time, collect acetylene production wastewater with a concentration of about 3% from the entire plant, introduce it into the first-stage preheater, and use low-temperature condensate from the subsequent process at a temperature of about 50°C for initial heating; S2: The alkaline solution, preheated to 80°C, enters the first-stage falling film evaporator (equipped with a heat transfer area of ​​98m²). 2 The alkaline solution is heated by live steam at 0.25 MPa (saturation temperature 138℃). After evaporation, the concentration of the alkaline solution is increased to 36%, the temperature reaches 95℃, and secondary steam at 100℃ is generated. The high-temperature alkaline solution at 95℃ is used to exchange heat with the wastewater in heat exchanger B, thereby raising the temperature of the wastewater to 70℃ in the second stage. S3: A 36% alkaline solution enters the secondary falling film evaporator (equipped with a heat transfer area of ​​88m²). 2 Using 90°C secondary steam from the three-stage evaporator as a heat source, the concentration is further increased to 42% while the temperature is maintained at 90°C. This intermediate alkaline solution is returned to S1 for raw material preheating. At the same time, 70°C wastewater is introduced into a vacuum flash tank (absolute pressure controlled at 0.02MPa) and self-evaporated using sensible heat to increase the wastewater concentration to 8% and reduce the temperature to 50°C. S4: 42% alkaline solution enters the three-stage falling film evaporator (equipped with a heat transfer area of ​​78m²). 2 The process involves heating with 0.3MPa live steam to concentrate the solution to the target concentration of 50% at a temperature of approximately 85℃. The resulting 85℃ secondary steam is then transported to S3 as a heat source. The 8% pre-concentrated wastewater obtained from S3 is filtered to remove solids and then precisely added at a ratio of 5% of the total NaOH in the system. This is then mixed with 50% of the finished alkaline solution in a pipeline mixer in a turbulent flow. The temperature after mixing is 80℃. S5: The mixed alkaline solution first exchanges heat with the low-temperature raw material alkali solution in heat exchanger A to cool down to 70°C, and then is deeply cooled to the storage and transportation temperature by cooling water heat exchanger before being put into storage. The vacuum system is specially optimized and the operating pressure of the first-stage evaporator is set to -0.09MPa to maximize the recovery of low-temperature heat energy.

[0021] Example 3: The system is built based on the three-stage evaporation process of this invention, with an interface reserved after the three-stage evaporator for easy future addition of evaporators to produce higher concentration alkali solutions. Currently, it is operated to produce 50% alkali. S1: Preheat the 32% raw material liquid alkali at 70℃ to 83℃, and preheat the 4% acetylene production wastewater at room temperature through a primary preheater. S2: An alkaline solution at 83°C enters the first-stage falling film evaporator (heat transfer area 99m²). 2 The process uses 0.30MPa live steam for heating, resulting in an output alkali concentration of 37% and a temperature of 98℃. The secondary steam temperature is 103℃, and this heat is used to heat the wastewater to 73℃ in a heat exchanger. S3: 37% alkaline solution enters the secondary falling film evaporator (heat transfer area 89m²). 2 Using secondary steam at 92℃ as a heat source, the alkali concentration is increased to 43% at 92℃. Simultaneously, wastewater at 73℃ is sent to a vacuum flash pre-concentrator (absolute pressure 0.02MPa) to flash evaporate using its own heat energy, increasing the alkali concentration to 9% and reducing the temperature to 50℃. S4: 43% alkaline solution enters the three-stage falling film evaporator (heat transfer area 79m²). 2 The process involves heating with 0.35MPa live steam to concentrate the solution to 50% at a temperature of 88℃. The generated 88℃ secondary steam is then recycled to S3. After solid-liquid separation, 9% of the pre-concentrated wastewater is mixed online with 50% of the finished alkali solution at a ratio of 15% of the total alkali content. Instantaneous homogenization is achieved using a high turbulent state, and the temperature after mixing is 85℃. S5: The mixed alkali solution is cooled down by the cooling system and then stored. The system design focuses on strengthening the pressure combination and heat balance between each effect, controlling the operating pressure of the first-stage evaporator at -0.06MPa, and ensuring that various low-temperature waste heats are fully integrated and utilized.

[0022] Table 1. Parameter Comparison Table for Various Embodiments; The above three embodiments strongly verify that the "low-energy liquid alkali concentration and evaporation process" constructed in this invention has strong versatility and outstanding energy-saving effect. With its systematic thermal energy integration strategy, this process can achieve stable operation with low steam consumption in various new or existing caustic soda evaporation units. The core advantages of this solution are specifically reflected in the following three aspects: Firstly, it achieves deep cascade utilization of energy. By constructing a precise and complex multi-stage heat exchange network between raw alkali → intermediate alkali → finished alkali and condensate → wastewater → secondary steam, it maximizes the recovery of low-grade heat energy generated by the process itself, thereby significantly reducing the dependence on high-grade generated steam. Secondly, it achieves the collaborative and resource-based treatment of waste. This technology innovatively combines the preheating and pre-concentration of alkyne production wastewater with the main process of caustic soda evaporation. While reducing energy consumption, it efficiently recovers the alkali in the wastewater, perfectly practicing the concept of circular economy and contributing to energy conservation and environmental protection. Third, it has flexible process design flexibility. Although it maintains stability with three-stage evaporation as the main framework, by adjusting the operating parameters (such as pressure and temperature) and integrated units (such as wastewater mixing ratio), the process can adapt to diverse scenarios with different production scales, raw material conditions and product requirements, showing excellent operational adaptability.

[0023] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0024] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A low-energy-consumption liquid alkali concentration and evaporation process, characterized in that, Its process includes the following steps: Step S1, Raw material preheating and primary wastewater preheating: The 32wt% raw material alkali solution at 68-70℃ from the ion membrane electrolysis process is preheated to 80-85℃ by exchanging heat with the 42wt% intermediate alkali solution at 88-90℃ through plate heat exchanger A. At the same time, the ambient temperature wastewater from the production of 3-5% acetylene is preheated in the primary wastewater preheater using low temperature condensate at 50-60℃. Step S2, Primary Falling Film Evaporation and Secondary Preheating of Wastewater: After preheating in Step S1, the alkaline solution enters the primary falling film evaporator and is heated by 0.25-0.35MPa live steam. The rotating scraper forms a film and evaporates, generating secondary steam at 100-105℃. The output concentration is 36-38% and the temperature is 95-100℃. This output alkaline solution is used in plate heat exchanger B to perform secondary preheating of the wastewater after preheating in Step S1, raising the wastewater temperature to 70-75℃. Step S3, Secondary Falling Film Evaporation and Wastewater Pre-concentration: The 36-38% alkaline solution from step S2 enters the secondary falling film evaporator and is heated by the 90-95℃ secondary steam from step S4, concentrating it to 42-45% and 90-95℃ before returning it to S1. At the same time, the 70-75℃ wastewater is introduced into the -0.08MPa vacuum flash pre-concentrator, where sensible heat flash evaporation is used to increase the concentration to 8-10% and reduce the temperature to 50℃. Step S4, Three-stage falling film evaporation and mixing of wastewater and alkaline solution: 42-45% of the alkaline solution from step S3 enters the three-stage falling film evaporator and is heated to 50% by steam at a temperature of 85-90℃. The generated secondary steam at 85-90℃ is returned to step S3. After filtering and removing impurities from the 8-10% wastewater pre-concentrated in step S3, it is mixed online with 50% of the finished alkaline solution at a ratio of 5-15% of the total NaOH. The temperature after mixing is 80-85℃. Step S5, Finished Product Cooling and Energy Recovery: The mixed alkali solution is cooled to 70-75°C by heat exchange with the raw material alkali solution in plate heat exchanger A, and then cooled to the storage and transportation temperature before being sent to the finished product storage tank. The steam condensate from each stage is collected in stages. The high-temperature condensate is returned to the boiler room, and the low-temperature condensate is used for preheating the wastewater in step S1.

2. The low-energy liquid alkali concentration and evaporation process according to claim 1, characterized in that: In step S1, the raw material alkali liquid and the intermediate alkali liquid in plate heat exchanger A undergo countercurrent heat exchange. The raw material alkali liquid is heated by 20-22°C, and the intermediate alkali liquid is cooled accordingly to meet the preheating requirements. The intermediate alkali liquid comes from step S3.

3. The low-energy liquid alkali concentration and evaporation process according to claim 1, characterized in that: In step S1, the low-temperature condensate used by the primary wastewater preheater comes from step S5, and the temperature of the wastewater is raised to 40-45℃ after preliminary preheating.

4. The low-energy liquid alkali concentration and evaporation process according to claim 1, characterized in that: In step S2, the first-stage falling film evaporator is a high-efficiency mechanically stirred thin-film evaporator, and the rotating scraper makes the alkaline solution form a uniform liquid film.

5. The low-energy liquid alkali concentration and evaporation process according to claim 1, characterized in that: In step S2, the heat transfer area of ​​the first-stage falling film evaporator is 98-100 m². 2 It is made of 316L stainless steel.

6. The low-energy liquid alkali concentration and evaporation process according to claim 1, characterized in that: In step S3, the secondary falling film evaporator adopts a falling film structure with a heat transfer area of ​​88-90 m². 2 .

7. The low-energy liquid alkali concentration and evaporation process according to claim 1, characterized in that: In step S3, the vacuum flash pre-concentrator operates at an absolute pressure of 0.02 MPa. The steam generated from the flash evaporation of wastewater has a low dust content and can be used as a low-grade heat source for other process units or for condensation recovery.

8. The low-energy liquid alkali concentration and evaporation process according to claim 1, characterized in that: In step S4, the heating steam pressure of the three-stage falling film evaporator is 0.3-0.4 MPa, and the concentration at the final concentration point is precisely controlled at 50 wt%.

9. The low-energy liquid alkali concentration and evaporation process according to claim 1, characterized in that: In step S4, the heat transfer area of ​​the three-stage falling film evaporator is 78-80 m². 2 .

10. The low-energy liquid alkali concentration and evaporation process according to claim 1, characterized in that: In step S5, the storage and transportation temperature is <45℃.

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