Method for concentrating potassium alkali through membrane distillation

Abstract

The invention provides a method for concentrating potassium alkali through membrane distillation, and aims to solve the technical problems of high energy consumption and heat efficiency bottleneck in the existing multi-effect evaporation technology. The method for concentrating potassium alkali by membrane distillation comprises the following steps: S10, pretreatment: filtering an initial potassium alkali solution and regulating pH and temperature; s20, membrane distillation and pulse flow operation are conducted, and efficient evaporation separation of water is achieved through low-grade heat energy; s30, seed crystal induction and product extraction are conducted, preliminary crystallization of a supersaturated solution is generated in the membrane distillation system, microcrystal nucleuses are generated, the crystal nucleuses are introduced into the step S20, and crystal mush is produced; and S40, fractional crystallization and solid-liquid separation: further concentrating and separating crystal mush from the membrane distillation system to obtain potassium alkali solid. And S50, periodic cleaning is conducted, when the flux of a certain stage in the step S20 is attenuated to 80% of the initial flux, cleaning is conducted, and acid washing, alkali washing and flushing are conducted in sequence. According to the method, low-grade heat energy is adopted during membrane distillation, and pulse flow operation is utilized, so that energy consumption is reduced, and heat efficiency is improved.

Description

Technical Field

[0001] This invention relates to the field of potassium alkali concentration technology, and more specifically to a method for concentrating potassium alkali by membrane distillation. Background Technology

[0002] Potassium hydroxide (potassium hydroxide, potassium carbonate, etc.) is an important basic chemical widely used in chemical, electronics, pharmaceutical, and agricultural industries. During its production or use, low-concentration potassium hydroxide wastewater or liquid is often generated. Direct discharge would lead to resource waste and environmental pollution; therefore, efficient concentration to recover the potassium hydroxide components has significant economic and environmental value.

[0003] Currently, industrial methods for concentrating potassium carbonate solutions mainly rely on thermal separation technologies, the most common of which is multi-effect evaporation. This technology improves thermal efficiency to some extent by utilizing the latent heat of steam multiple times. However, multi-effect evaporation technology has several inherent drawbacks:

[0004] Extremely high energy consumption: Its operation depends on a continuous supply of fresh steam or high-temperature heat source. Even with multi-efficiency design, its overall energy consumption is still huge and its operating costs are high.

[0005] Thermal efficiency bottleneck: Due to thermodynamic characteristics such as boiling point elevation, the marginal benefit of increasing the number of efficiency numbers for energy saving diminishes, and the equipment structure is complex. Summary of the Invention

[0006] To address the technical problems of high energy consumption and thermal efficiency bottlenecks in existing multi-effect evaporation technologies, this invention provides a method for concentrating potassium alkali by membrane distillation. This method utilizes low-grade heat energy during membrane distillation and employs pulsed flow operation to reduce energy consumption and improve thermal efficiency.

[0007] The technical solution of this invention is:

[0008] A method for concentrating potassium alkali by membrane distillation includes the following steps:

[0009] S10. Pretreatment: filtration of the initial potassium alkali solution and adjustment of pH and temperature.

[0010] S20, membrane distillation and pulsed flow operation, utilize low-grade heat energy to achieve efficient evaporation and separation of water;

[0011] S30, Seed induction and product extraction, to induce preliminary crystallization of a supersaturated solution in the membrane distillation system, generating microcrystal nuclei, and introducing the crystal nuclei into step S20 to produce crystal slurry;

[0012] S40. Fractional crystallization and solid-liquid separation further concentrate and separate the slurry from the membrane distillation system to obtain potassium alkali solid.

[0013] Optionally, the preprocessing step S10 includes:

[0014] S11. Fine filtration: The initial potassium alkali solution is filtered through a multi-stage filtration system to reduce the particulate matter content to less than 1 ppm.

[0015] S12. Dynamic pH and temperature control: pH and temperature are monitored by installing a pH meter and temperature sensor, and the pH meter and temperature are precisely adjusted by driving a micro metering pump and heat exchanger through a PID controller.

[0016] Optionally, the fine filtering step S11 includes:

[0017] The initial potassium alkali solution is passed sequentially through a basket filter, a bag filter, and a cartridge filter.

[0018] In step S12, dynamic pH and temperature control, the pH meter and temperature sensor are connected to a control system. This control system has a built-in database that stores the optimal pretreatment pH and temperature setpoints for potassium alkali solutions of different concentrations. Based on the initial concentration, the system automatically queries the database to set the pH and preheating temperature setpoints.

[0019] Optionally, step S20, membrane distillation and pulsed flow operation, specifically includes:

[0020] A hollow fiber membrane distillation system was used to perform triple-effect treatment on the pretreated initial potassium alkali solution.

[0021] During the process, the hot-side feed pump of the hollow fiber membrane distillation system is controlled by frequency conversion to generate periodic pulse flow.

[0022] Optionally, in step S20, the triple-effect treatment includes:

[0023] In the first effect, the hot side is the initial potassium alkali solution, and the cold side is distilled water;

[0024] The second effect is that the distilled water on the cold side of the first effect is the heat source for the hot side;

[0025] The third effect uses distilled water on the cold side of the second effect as the heat source for its hot side, and its cold side is connected to an external condenser.

[0026] Optionally, solar energy and / or a heat pump provide heat for the first effect;

[0027] The distilled water and concentrate at the third-effect cold side outlet exchange heat with the initial potassium alkali solution in the pretreatment step S10 through a heat exchanger.

[0028] Optionally, the S30 seed induction and product extraction includes:

[0029] S31. Pass the potassium alkali solution after partial membrane distillation and pulsed flow operation through an ultrasonic oscillator to obtain a crystal nucleus solution;

[0030] S32. Reintroduce the crystal nucleus solution into step S20 to produce crystal slurry in step S20;

[0031] S33. Periodically transport a portion of the crystal slurry to the graded crystallizer.

[0032] Optionally, the S40 fractional crystallization and solid-liquid separation includes:

[0033] S41. The crystal slurry is fed into the vacuum crystallizer;

[0034] S42. The crystals output from the vacuum crystallizer are sent into a centrifuge, and wet crystals are obtained after solid-liquid separation.

[0035] S43. Allow the wet crystals to enter the dryer and dry them with preheated air to obtain potassium alkali solid.

[0036] Optionally, in step S41, the crystal growth rate and particle size are controlled by controlling the cooling rate, stirring rate, and vacuum level.

[0037] Optionally, it also includes:

[0038] S50. Periodic cleaning: When the flux in a certain stage of step S20 decreases to 80% of the initial flux, cleaning is performed, including acid washing, alkali washing and rinsing in sequence.

[0039] Compared with the prior art, the beneficial effects of the present invention are:

[0040] The pretreatment in step S10 protects the membrane modules of downstream membrane distillation from membrane fouling and chemical degradation caused by suspended solids, impurities, or drastic pH fluctuations.

[0041] Through membrane distillation and pulsed flow operation in step S20, water is efficiently evaporated and separated using low-grade thermal energy.

[0042] Through seed induction and product extraction in step S30, preliminary crystallization of a supersaturated solution is achieved in the membrane distillation system, generating microcrystal nuclei to provide seed crystals for subsequent crystallization, thus significantly improving crystallization efficiency.

[0043] Through step S40, fractional crystallization and solid-liquid separation are performed to further concentrate and separate the crystal slurry, thereby obtaining a high-purity potassium alkali solid product.

[0044] This technical solution offers advantages in high efficiency and energy saving through multi-dimensional improvements to the energy system, process coupling, and operating mode. Attached Figure Description

[0045] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0046] Figure 1 This is a schematic diagram of the process flow of the present invention. Detailed Implementation

[0047] In the following description, only certain exemplary embodiments are briefly described. As those skilled in the art will recognize, the described embodiments can be modified in various ways without departing from the spirit or scope of the invention. Therefore, the drawings and description are considered to be exemplary in nature and not restrictive.

[0048] The following disclosure provides many different embodiments or examples for implementing various structures of the invention. To simplify the disclosure, specific examples of components and arrangements are described below. These are merely examples and are not intended to limit the invention. Furthermore, reference numerals and / or letters may be repeated in different examples; such repetition is for simplification and clarity and does not in itself indicate a relationship between the various embodiments and / or arrangements discussed. In addition, examples of various specific processes and materials are provided in this invention, but those skilled in the art will recognize the application of other processes and / or the use of other materials.

[0049] The embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

[0050] Example:

[0051] See Figure 1 This embodiment discloses a method for concentrating potassium alkali by membrane distillation, comprising the following steps:

[0052] S10. Pretreatment: filtration of the initial potassium alkali solution and adjustment of pH and temperature.

[0053] Step S10 specifically includes:

[0054] S11. Fine filtration: The initial potassium alkali solution undergoes a multi-stage filtration system to ensure that the particulate matter content is below 1 ppm.

[0055] In the specific implementation process, the initial potassium alkali solution is first passed through a 100μm-level basket filter to remove large particulate impurities. Then it passes through a 25μm-level bag filter, and finally through a 5μm-level cartridge filter, ultimately ensuring that the particulate matter content in the potassium alkali solution entering step S20 is less than 1ppm.

[0056] S12. Dynamic pH and temperature control: pH and temperature are monitored by installing a pH meter and temperature sensor, and the pH meter and temperature are precisely adjusted by driving a micro metering pump and heat exchanger through a PID controller.

[0057] Specifically, the pH meter and temperature sensor are connected to a control system that has a built-in database storing the optimal pretreatment pH and temperature setpoints for potassium alkali solutions of different concentrations. The solution concentration is typically calculated based on the conductivity of the tested solution. The system automatically queries the database based on the initial concentration to set the pH and preheating temperature setpoints.

[0058] S20, membrane distillation and pulsed flow operation, utilize low-grade heat energy to achieve efficient evaporation and separation of water.

[0059] First, a hollow fiber membrane distillation system is used for membrane distillation, and the cold side of each effect and the hot side of the next effect of the membrane distillation system are directly connected through pipelines and a low-pressure pump.

[0060] In terms of heat source selection, solar energy is preferred. When solar energy is insufficient, a high-temperature heat pump is used to supplement the heat, and it is required to ensure that the feed temperature on the hot side of the first effect is between 75-80℃.

[0061] During the process, the hot-side feed pump of the hollow fiber membrane distillation system is frequency-controlled to generate periodic pulsed flows (e.g., high-speed flow for 30 seconds, low-speed flow for 10 seconds). The pulsed flows generate strong shear forces and turbulence on the membrane surface, effectively disrupting the boundary layer, reducing concentration polarization and soluble salt deposition, thereby maintaining a high permeation flux.

[0062] The specific three-effect treatment includes:

[0063] In the first effect, the hot side is the pretreated initial potassium alkali solution, and the cold side is the distilled water formed by the condensation of steam generated in the second effect. Heat is transferred from the hot side to the cold side, and water vapor passes through the membrane pores, so the solution on the hot side of the first effect is initially concentrated.

[0064] In the second effect, the distillate from the cold side of the first effect still has a relatively high temperature, which serves as a heat source for the hot side of the second effect, further concentrating the solution in the second effect. This process is repeated, allowing for efficient reuse of heat.

[0065] In the third effect, the distillate from the cold side of the second effect serves as the heat source for the hot side. The cold side of the third effect is connected to an external condenser, typically supplied with water from a cooling tower, ultimately producing high-quality distillate.

[0066] Additionally, install a conductivity meter at the outlet of each effect to test the concentration of the concentrated liquid.

[0067] In step S20, the pulsed flow operation mode can effectively solve the membrane fouling problem.

[0068] S30, Seed induction and product extraction, causes preliminary crystallization of a supersaturated solution in the membrane distillation system, generating microcrystal nuclei, which are then introduced into step S20 to produce crystal slurry.

[0069] Specifically, it includes:

[0070] S31. When the concentration of the concentrated solution at the outlet of the third effect reaches near saturation (approximately 90% saturation), the conductivity meter or density sensor sends a signal, and then a portion (approximately 5%) of the concentrated solution that has been concentrated to near saturation is sent into the ultrasonic oscillator. Ultrasonic waves promote the formation of nuclei, resulting in a large number of tiny, uniform crystal nuclei in the solution.

[0071] S32. The slurry containing crystal nuclei is returned to the circulation line of the third effect to produce crystal slurry. The presence of crystal nuclei provides crystallization sites, allowing the subsequent concentration process to orderly precipitate tiny crystals on the membrane surface / system, rather than forming a dense fouling layer on the membrane surface.

[0072] S33. A certain amount of crystal slurry (solid-liquid mixture) is periodically extracted from the bottom of the third effect and transported to a staged crystallizer for crystallization. By periodically transferring the crystal slurry, the main site of crystal growth is shifted from the sophisticated membrane distillation system to the more robust and easier-to-handle solid crystallizer, thereby protecting the core and expensive membrane distillation system.

[0073] In addition, it also takes into account the goal of continuous production. Traditional membrane distillation systems must be shut down or drained and cleaned after reaching saturation, and cannot operate continuously.

[0074] S40. Fractional crystallization and solid-liquid separation further concentrate and separate the slurry from the membrane distillation system to obtain potassium alkali solid.

[0075] Specifically, it includes:

[0076] S41. The crystal slurry is fed into a vacuum crystallizer. In a vacuum environment, the boiling point of the solution decreases, and water evaporates further, which helps the crystals to continue growing.

[0077] In addition, the growth rate and particle size distribution of crystals can be controlled by precisely controlling the cooling rate, stirring speed and vacuum level, so as to produce large and uniform crystals that meet the requirements.

[0078] S42. The crystals output from the vacuum crystallizer are sent into a centrifuge, and wet crystals are obtained after solid-liquid separation.

[0079] The separated mother liquor is returned to the front end of the pretreatment step S10 and re-enters the membrane distillation system for concentration to achieve complete potassium recovery and zero liquid discharge.

[0080] S43. Allow the wet crystals to enter the dryer and dry them with preheated air to obtain potassium alkali solid.

[0081] S50. Process control and periodic cleaning: When the flux in a certain stage of step S20 decreases to 80% of the initial flux, cleaning is performed, including acid washing, alkali washing and rinsing in sequence.

[0082] Specifically, process control includes:

[0083] The control system described above incorporates a mechanistic model that includes mass conservation, energy conservation, mass transfer rate equations, and membrane fouling trend prediction.

[0084] The system collects data such as temperature, pressure, flow rate, and concentration for each effect in real time and inputs them into the mechanism model. It calculates the system state for a future period every 5 minutes and predicts the optimal set values ​​for temperature, pulse flow frequency, and circulation flow rate for each effect to maximize the "water production ratio".

[0085] Control execution: The controller sends the optimized setpoints to the underlying controller, which then executes them precisely, thus achieving optimal adaptive control based on the physical model.

[0086] Periodic cleaning includes:

[0087] First, use a dilute citric acid solution to circulate and clean, removing inorganic salt scale.

[0088] Then, a dilute sodium hydroxide solution is used for cyclic cleaning to remove organic contaminants.

[0089] Finally, rinse thoroughly with pretreated softened water until neutral.

[0090] Predictive cleaning triggered by relative flux decay rate is more scientific than timed cleaning and more classic and reliable than nanobubble cleaning.

[0091] In this embodiment, a heat energy supply and recovery system is also involved, which aims to provide stable, green, and low-cost heat energy and to deeply recover energy.

[0092] Specifically, solar energy is used as the primary heat source, employing a trough-type solar concentrator array to collect solar energy and heat the heat transfer oil to above 200°C. The high-temperature heat transfer oil then flows through a phase change material storage tank (such as a molten salt tank) to store the thermal energy.

[0093] The thermal storage tank provides a stable heat source for the first-effect heat exchanger, heating the feed liquid to the target temperature. Phase change thermal storage ensures at least 8 hours of continuous heating during nighttime or cloudy days.

[0094] The distillate and concentrate at the third-effect cold side outlet still have residual heat. The distillate and concentrate are then exchanged with the feed liquid in the pretreatment stage of step S10 through plate heat exchangers. After cooling, they enter the water storage tank or are output as by-products.

[0095] Finally, an auxiliary heat source is designed: an auxiliary heat pump. A high-temperature heat pump is used to extract low-grade heat from the cooled distillate water, which is then used to heat the first-effect feed liquid or for crystal drying, thereby maximizing energy utilization.

[0096] The heat supply and recovery system is controlled by an independent controller that logically links the heat transfer oil pump, valves and heat pump based on the temperature of the heat storage tank, solar irradiance and steam demand of the main process system.

[0097] Through the design of a combined solar energy, phase change thermal storage, and heat pump system, green, stable, and cascaded energy utilization has been achieved.

[0098] The embodiments described above are merely illustrative of specific implementations of the present invention, and while the descriptions are detailed, they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention.

Claims

1. A method for concentrating potassium alkali by membrane distillation, characterized in that, Includes the following steps: S10. Pretreatment: filtration of the initial potassium alkali solution and adjustment of pH and temperature. S20, membrane distillation and pulsed flow operation, utilize low-grade heat energy to achieve efficient evaporation and separation of water; S30, Seed induction and product extraction, to induce preliminary crystallization of a supersaturated solution in the membrane distillation system, generating microcrystal nuclei, and introducing the crystal nuclei into step S20 to produce crystal slurry; S40. Fractional crystallization and solid-liquid separation further concentrate and separate the slurry from the membrane distillation system to obtain potassium alkali solid.

2. The method according to claim 1, characterized in that, The preprocessing step S10 includes: S11. Fine filtration: The initial potassium alkali solution is filtered through a multi-stage filtration system to reduce the particulate matter content to less than 1 ppm. S12. Dynamic pH and temperature control: pH and temperature are monitored by installing a pH meter and temperature sensor, and the pH meter and temperature are precisely adjusted by driving a micro metering pump and heat exchanger through a PID controller.

3. The method according to claim 2, characterized in that: The fine filtering step S11 includes: The initial potassium alkali solution is passed sequentially through a basket filter, a bag filter, and a cartridge filter. In step S12, dynamic pH and temperature control, the pH meter and temperature sensor are connected to a control system. This control system has a built-in database that stores the optimal pretreatment pH and temperature setpoints for potassium alkali solutions of different concentrations. Based on the initial concentration, the system automatically queries the database to set the pH and preheating temperature setpoints.

4. The method according to claim 1, characterized in that, The specific steps of membrane distillation and pulsed flow operation in step S20 are as follows: A hollow fiber membrane distillation system was used to perform triple-effect treatment on the pretreated initial potassium alkali solution. During the process, the hot-side feed pump of the hollow fiber membrane distillation system is controlled by frequency conversion to generate periodic pulse flow.

5. The method according to claim 4, characterized in that, In step S20, the triple-effect treatment includes: In the first effect, the hot side is the initial potassium alkali solution, and the cold side is distilled water; The second effect is that the distilled water on the cold side of the first effect is the heat source for the hot side; The third effect uses distilled water on the cold side of the second effect as the heat source for its hot side, and its cold side is connected to an external condenser.

6. The method according to claim 5, characterized in that: Solar energy and / or heat pumps provide heat for the first effect; The distilled water and concentrate at the third-effect cold side outlet exchange heat with the initial potassium alkali solution in the pretreatment step S10 through a heat exchanger.

7. The method according to claim 1, characterized in that, The S30 seed induction and product extraction include: S31. Pass the potassium alkali solution after partial membrane distillation and pulsed flow operation through an ultrasonic oscillator to obtain a crystal nucleus solution; S32. Reintroduce the crystal nucleus solution into step S20 to produce crystal slurry in step S20; S33. Periodically transport a portion of the crystal slurry to the graded crystallizer.

8. The method for concentrating potassium alkali by membrane distillation according to claim 1, characterized in that, The S40 fractional crystallization and solid-liquid separation includes: S41. The crystal slurry is fed into the vacuum crystallizer; S42. The crystals output from the vacuum crystallizer are sent into a centrifuge, and wet crystals are obtained after solid-liquid separation. S43. Allow the wet crystals to enter the dryer and dry them with preheated air to obtain potassium alkali solid.

9. The method according to claim 8, characterized in that, In step S41, the crystal growth rate and particle size are controlled by controlling the cooling rate, stirring rate, and vacuum level.

10. The method according to claim 1, characterized in that, Also includes: S50. Periodic cleaning: When the flux in a certain stage of step S20 decreases to 80% of the initial flux, cleaning is performed, including acid washing, alkali washing and rinsing in sequence.

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