A method and system for controlling crystallization and evaporation of sodium chlorate

Abstract

The application belongs to the technical field of evaporation control, and specifically discloses a sodium chlorate crystallization evaporation control method and system. The method can adjust the feed temperature according to the change trend of the flashing intensity in advance by detecting the pressure change rate in the crystallizer, thereby reducing the change of the amount of generated mist caused by the fluctuation of the flashing intensity. On this basis, the gas-liquid separation space at the upper part of the crystallizer is divided into three layers. The mist concentration and the advancing state of the mist front in each layer are detected in real time. Differentiated adjustment is performed according to the layer position of the mist front: the circulating flow is adjusted in the near-liquid surface layer, the discharge rate is adjusted in the intermediate transition layer, and the liquid level is adjusted in the near-outlet layer. The method can be targeted to intervene according to the actual rising state of the mist, which helps to slow down the rising process of the mist and reduce the pollution of the mist to the demister and the condensing equipment.

Description

Technical Field

[0001] This invention belongs to the field of evaporation control technology, and specifically discloses a method and system for controlling the evaporation of sodium chlorate crystals. Background Technology

[0002] Sodium chlorate is an inorganic chemical product widely used in papermaking, water treatment, and metal smelting. Industrially, sodium chlorate is produced by electrolyzing a sodium chloride solution. After electrolysis, the electrolyte is evaporated, concentrated, and cooled to crystallize, yielding sodium chlorate crystals. Vacuum evaporation crystallization is currently the mainstream process for sodium chlorate production. This process lowers the boiling point of the solution under vacuum conditions, evaporating water to achieve a supersaturated state of sodium chlorate, thus precipitating crystals. It features low energy consumption and continuous production capability.

[0003] During the vacuum evaporation crystallization process of sodium chlorate, the solution in the evaporator crystallizer is in a vacuum boiling state. After the hot electrolyte enters the crystallizer, it undergoes violent flash evaporation, generating a large number of bubbles. After the bubbles rise to the liquid surface and break, they form droplets carrying the liquid. The droplets in the crystallizer first interfere with the measurement accuracy of the liquid level gauge, causing the liquid level control to be distorted. When the droplets continue to rise to the demister, the fine crystals they carry form scale on the surface of the demister, causing blockage. When the droplets enter the condensation equipment with the secondary steam, the salt crystallizes and forms scale on the surface of the heat exchange tubes, resulting in a decrease in heat exchange efficiency.

[0004] To address the aforementioned issues, patent application CN110975314A proposes a closed-loop process system and method for sodium chlorate crystallization evaporation water. This solution mainly addresses mist entrainment in two ways: first, by installing a shell-and-tube heat exchanger inside the atmospheric condenser to cool uncondensed steam and prevent non-condensable gas from entraining mist into the vacuum pump system; second, by simplifying the structure of the evaporation crystallizer, namely by eliminating the straight cylindrical section and conical hopper above the evaporation liquid surface and removing the demister, thereby reducing the adhesion area of ​​crystals and the probability of crystallization.

[0005] However, although the above solutions address the issue of mist entrainment during sodium chlorate crystallization and evaporation, their solutions mainly involve hardware structural improvements to reduce the generation of droplets. When the amount of droplets generated exceeds the design expectations due to fluctuations in operating conditions, the hardware structure cannot change the rising process of the droplets during operation through adjustment methods. The droplets that have already been generated will continue to rise and affect liquid level detection, the smoothness of the demister, and the heat exchange efficiency of the condensing equipment. Summary of the Invention

[0006] To solve the above-mentioned technical problems, or at least partially solve them, the present invention provides a method and system for controlling the evaporation of sodium chlorate crystallization. By dividing the gas-liquid separation space in the upper part of the crystallizer into three layers and detecting the mist concentration in each layer in real time, and combining the movement state of the droplet front and the gas phase flow rate at the steam outlet, the method implements phased differential adjustment to change the movement state of the droplets during the rising process.

[0007] The objective of this invention can be achieved through the following technical solutions: In a first aspect, this invention proposes a method for controlling the evaporation of sodium chlorate crystallization, comprising: real-time detection of the pressure change rate inside the crystallizer as the degree of flash evaporation;

[0008] When the flash evaporation degree is higher than the previous cycle for multiple consecutive sampling cycles and the cumulative change exceeds the limit, the feed temperature setting value is lowered.

[0009] The upper gas-liquid separation space of the crystallizer is divided vertically into a near-liquid surface layer, an intermediate transition layer, and a near-outlet layer.

[0010] The concentration of droplets is detected in each layer, and the current layer position of the droplet front is determined based on the time series of droplet concentration. The vertical propulsion velocity and propulsion acceleration of the droplet front are also calculated.

[0011] Differential adjustments are made based on the location of the droplet front in the layer: when it is near the liquid surface, the circulation flow rate is adjusted according to the droplet concentration; when it is in the intermediate transition layer, the discharge rate is adjusted according to the propulsion speed and propulsion acceleration; and when it is near the outlet layer, the liquid level is adjusted according to the interception efficiency determined by the ratio of the droplet concentration near the outlet layer to the droplet concentration at the steam outlet.

[0012] After differential adjustment, the gas phase flow rate at the steam outlet is detected. If the flow rate exceeds the upper limit, the vacuum pumping rate and the supply of heating medium are reduced simultaneously until the gas phase flow rate falls back below the upper limit.

[0013] Secondly, the present invention proposes a sodium chlorate crystallization evaporation control system, comprising the following modules: a detection module, which collects in real time the detection signals of pressure inside the crystallizer, mist concentration in each layer and gas phase flow velocity at the steam outlet, and calculates the pressure change rate based on the pressure acquisition signal;

[0014] The flash control module determines the degree of flashing based on the rate of pressure change. When the degree of flashing is higher than that of the previous cycle for multiple consecutive sampling cycles and the cumulative change exceeds the limit, the feed temperature setpoint is lowered.

[0015] The leading edge tracking module divides the gas-liquid separation space above the crystallizer into a near-liquid surface layer, an intermediate transition layer, and a near-outlet layer. Based on the time series of droplet concentration in each layer, it calculates the vertical propulsion velocity and propulsion acceleration of the droplet leading edge to determine the layer position of the droplet leading edge.

[0016] The differential adjustment module performs differential adjustment based on the layer position of the droplet front and the propulsion speed and acceleration: when it is in the near liquid surface layer, the circulation flow rate is adjusted according to the droplet concentration; when it is in the intermediate transition layer, the discharge rate is adjusted according to the propulsion speed and acceleration; and when it is in the near outlet layer, the liquid level is adjusted according to the interception efficiency determined by the ratio of the droplet concentration in the near outlet layer to the droplet concentration at the steam outlet.

[0017] When the gas flow rate exceeds the limit, the end-effector module synchronously reduces the vacuum pumping rate and the heating medium supply in fixed steps until the gas flow rate falls back below the upper limit.

[0018] Combining all the above technical solutions, the positive effects of this invention are as follows:

[0019] This invention, by real-time detection of the pressure change rate within the crystallizer and adjusting the feed temperature accordingly, can proactively adjust based on the changing trend of flash evaporation intensity, reducing fluctuations in droplet generation caused by flash evaporation intensity fluctuations. Furthermore, by dividing the upper gas-liquid separation space of the crystallizer into three layers and implementing differentiated adjustments based on the layer position of the droplet front: adjusting the circulation flow rate near the liquid surface layer, adjusting the discharge rate in the intermediate transition layer, and adjusting the liquid level near the outlet layer, targeted intervention can be made based on the actual rising state of the droplets, helping to slow down the droplet's ascent and reduce contamination of the demister and condensation equipment. Attached Figure Description

[0020] The present invention will be further described with reference to the accompanying drawings, but the embodiments in the drawings do not constitute any limitation on the present invention. For those skilled in the art, other drawings can be obtained based on the following drawings without creative effort.

[0021] Figure 1 This is a flowchart illustrating the method steps of Embodiment 1 of the present invention;

[0022] Figure 2 This is a flowchart illustrating the implementation of step S4 in this invention;

[0023] Figure 3 This is a schematic diagram illustrating the differential adjustment performed according to the layer position of the droplet front in this invention;

[0024] Figure 4 This is a module connection diagram of Embodiment 2 of the present invention;

[0025] Figure 5 This is a schematic diagram illustrating the linkage between the degree of flash evaporation and the feed temperature setpoint in this invention;

[0026] Figure 6 This is a schematic diagram illustrating the dynamic changes in the droplet leading edge height, propulsion speed, and propulsion acceleration in this invention. Detailed Implementation

[0027] 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.

[0028] Example 1

[0029] This invention proposes a method for controlling the evaporation of sodium chlorate crystals, see reference. Figure 1 The process includes the following steps: S1, Real-time detection of the rate of pressure change inside the crystallizer as the degree of flash evaporation.

[0030] In the vacuum evaporation crystallization process of sodium chlorate, the generation of droplets originates from the intense flash evaporation that occurs when the hot electrolyte enters the low-pressure crystallizer. Specifically, the boiling point of the solution decreases under vacuum conditions, and the vaporization nuclei in the superheated solution are rapidly activated, generating a large number of bubbles inside the solution and on the surface of the suspended crystals. These bubbles rise to the gas-liquid interface and rupture, forming droplets that carry the liquid with them. This invention reduces the amount and rate of bubble generation by adjusting the flash evaporation intensity.

[0031] Considering that the intensity of flash evaporation is mainly determined by the rate of pressure drop—the faster the pressure drop, the greater the superheat of the solution, and the more intense the flash evaporation—this invention uses the rate of pressure change as a parameter characterizing the degree of flash evaporation.

[0032] In a preferred embodiment of the present invention, the degree of flash evaporation is obtained as follows: during each sampling period, a pressure signal is collected by an absolute pressure transmitter installed in the gas phase space at the top of the crystallizer or on the steam outlet pipeline.

[0033] The pressure collected in the current sampling period is read from the pressure collected in the previous sampling period. The absolute value of the difference between the two is calculated as the ratio of the sampling time interval to obtain the pressure change rate, which is used as the flashing degree of the current sampling period.

[0034] Since the pressure in the crystallizer continuously decreases during the vacuuming process, the direct calculation of the difference is negative. Taking the absolute value represents the rate of pressure decrease, reflecting the speed at which the pressure decreases within the crystallizer. The larger the value, the greater the pressure drop per unit time, the higher the degree of solution overheating, and the stronger the flash evaporation drive.

[0035] S2. When the flashing degree is higher than the previous cycle for multiple consecutive sampling cycles and the cumulative change exceeds the limit, the feed temperature setting value is lowered.

[0036] After defining the flash degree, as the pressure inside the crystallizer continues to decrease, the flash degree gradually increases. When the flash degree continues to rise and the cumulative change exceeds the limit, it indicates that the bubble formation rate is accelerating. If not intervened in time, the amount of spray generated after the bubbles burst will continue to increase. Considering that an increase in feed temperature will increase the heat brought in by the liquid and increase the evaporation, thereby increasing the burden on the vacuum system and accelerating the pressure drop rate, this invention adopts the method of lowering the feed temperature setpoint. By reducing the superheat of the liquid when it enters the crystallizer, the pressure drop trend is slowed down. The purpose is to slow down the further increase in flash intensity, rather than directly eliminating the spray that has already formed.

[0037] The specific implementation is as follows: After each sampling period ends, the flashing degree of the current sampling period is compared with the flashing degree of the previous sampling period. If the current flashing degree is higher than the flashing degree of the previous sampling period, it indicates that the flashing degree is on an upward trend, so the continuous increase count is incremented by one. Conversely, if the current flashing degree is lower than the previous sampling period, it indicates that the upward trend is interrupted, so the continuous increase count is cleared to zero and the cumulative change is set to zero.

[0038] When the count increases continuously to one and the current flash level is higher than the previous sampling period, the flash level of the previous sampling period is recorded as the baseline value. The reason for this is that the first sampling period with continuous increase has an increment relative to its previous period. Using the previous period as the baseline can fully reflect the total change since the flash level started to rise, and avoid the cumulative change being underestimated due to improper selection of the baseline point.

[0039] The difference between the flashing degree in the current sampling period and the baseline value is used as the cumulative change, reflecting the total increase in flashing degree since it began to rise continuously.

[0040] When the count is continuously increased to the threshold and the cumulative change exceeds the change limit, the feed temperature setpoint is lowered by one temperature step. The threshold can be determined based on the system sampling period, preferably 3 to 5 sampling periods. The temperature step is preferably 0.5 to 2℃. The change limit is set as follows: During the normal production stage when the crystallizer is operating stably, historical flash evaporation data for multiple sampling periods are continuously collected. The difference between the maximum and minimum values ​​in the historical flash evaporation data under normal operating conditions is taken as the fluctuation range. For example, 20% to 30% of the fluctuation range is taken as the change limit.

[0041] The flash evaporation level is recalculated in the next sampling period, and the above judgment is repeated. If the condition is still met, the level is lowered until the continuous increase in the count does not reach the number of times threshold or the cumulative change does not exceed the change limit, at which point the lowering stops.

[0042] In the above operation, the continuous increase in the count reflects the persistence of the upward trend of the flash degree, and the cumulative change reflects the actual magnitude of the increase in the flash degree. Combining the two can avoid false triggering caused by a single fluctuation or a small cumulative change, and ensure that the flash degree is identified as abnormally increasing only when the trend is continuous and the magnitude is sufficient, thereby triggering the feed temperature adjustment.

[0043] It is important to understand that the feed temperature setpoint is lowered only to the minimum allowable feed temperature. When the feed temperature setpoint is lowered to the minimum allowable value and the flash evaporation still meets the adjustment conditions, the feed temperature will not be lowered further. Instead, droplets will continue to be controlled through droplet front detection and differential adjustment in subsequent steps.

[0044] like Figure 5 As shown, the correlation between the degree of flash evaporation and the feed temperature setpoint varies with the sampling time in a working condition sequence consisting of 10 consecutive sampling times (t1 to t10). The degree of flash evaporation at each sampling time is calculated according to the aforementioned method, and the corresponding feed temperature setpoint is recorded simultaneously.

[0045] Specifically, during the time interval from t1 to t5, the degree of flash evaporation shows a continuous upward trend. This change indicates that the rate of pressure change inside the crystallizer is accelerating, and the flash evaporation effect is gradually strengthening. In response to this trend, the present invention gradually lowers the feed temperature setpoint according to a predetermined step size. From time t6 onwards, as the adjustment of the feed temperature setpoint takes effect, the degree of flash evaporation begins to gradually decrease and eventually tends to stabilize.

[0046] S3. Divide the gas-liquid separation space at the top of the crystallizer into a near-liquid surface layer, an intermediate transition layer and a near-outlet layer in the vertical direction.

[0047] Adjusting the feed temperature in S2 can reduce the variation in droplet generation caused by flash intensity fluctuations. However, in actual production, due to uncontrollable factors such as feed fluctuations, and the feed temperature being limited by the lower limit of the process, it is difficult to completely avoid droplet generation by simply adjusting the feed temperature. A small amount of droplets will still inevitably form at the gas-liquid interface.

[0048] Therefore, the present invention first divides the gas-liquid separation space into layers.

[0049] As one feasible approach, the layer division process is as follows: obtain the real-time height of the liquid level inside the crystallizer and the installation height of the steam outlet, calculate the vertical distance between the liquid level and the steam outlet, and the vertical distance is the total height of the gas-liquid separation space;

[0050] Based on the bottom of the vertical distance, the gas-liquid separation space is divided into the near-liquid surface layer, the intermediate transition layer and the near-outlet layer in sequence along the vertical direction upward.

[0051] The specific height boundaries of each layer can be preset according to the structural parameters and process conditions of the crystallizer. In one example, the height of the near-outlet layer is determined based on the steam outlet pipe diameter, and the height of the near-outlet layer is 0.5 to 1.0 times the steam outlet pipe diameter to ensure that this layer can effectively cover the airflow influence area near the steam outlet; the height of the near-liquid surface layer is 0.3 to 0.6 times the crystallizer cylinder diameter, and this height range can effectively cover the initial jet area of ​​droplets generated by bubble bursting at the liquid surface; the height of the intermediate transition layer is the remaining height after subtracting the near-liquid surface layer and the near-outlet layer from the total height of the gas-liquid separation space.

[0052] In the above settings, the height of the near-outlet layer is set to be less than that of the near-liquid surface layer and the intermediate transition layer. The reason is that the near-outlet layer is adjacent to the steam outlet and is the last space area before the droplets enter the steam outlet. Setting the height of this layer to be smaller allows the droplets to be quickly identified once they enter this layer, so that the liquid level can be adjusted in time, increasing the chance of the droplets settling in the near-outlet layer.

[0053] It should be noted that the layer division remains effective during crystallizer operation. The layer division step is described after the flash evaporation control step in this invention for ease of description and does not constitute a limitation on the actual execution order of the two steps.

[0054] S4. Detect the concentration of droplets in each layer, determine the current layer position of the droplet front based on the time series of droplet concentration, and calculate the vertical propulsion velocity and propulsion acceleration of the droplet front.

[0055] After the gas-liquid separation space is divided into layers, the concentration of droplets in each layer can be detected in real time during the droplet ascent. The droplet concentration refers to the concentration of suspended droplets in the space, which is a quantitative representation of the state of droplet existence. This is used to track the advancing position of the droplet front and provide a basis for subsequent differentiated regulation.

[0056] See Figure 2 As shown, the following is an explanation in conjunction with a specific embodiment: First, multiple fog concentration detection points distributed horizontally are set up in each layer. As the droplets are affected by factors such as airflow distribution and internal structure of the equipment during their ascent, their horizontal distribution in the same layer is not uniform. By setting up multiple detection points distributed horizontally, the fog concentration changes at different horizontal positions in the layer can be fully captured, avoiding missed detections or misjudgments caused by single-point detection.

[0057] In practice, the detection point can employ an optical turbidity sensor or a conductivity-type droplet sensor, whose detection signal is converted into a concentration value by a transmitter. Considering that sensor scaling is a common problem in salt-containing crystallization environments, to ensure long-term stable detection, a sensor with a self-cleaning structure or a sensor employing a non-contact measurement principle can be used. Those skilled in the art can make a reasonable choice based on specific operating conditions. Furthermore, this application determines the droplet front position based on the rate of concentration change rather than an absolute measurement value. Drift caused by scaling is slow and gradual, while the droplet signal is sudden. The two are distinguishable on a time scale, and long-term reliability can be ensured through routine maintenance methods such as regular cleaning or replacement of the sensor window.

[0058] Secondly, concentration values ​​at each detection point are collected in each sampling cycle. For each layer, the maximum concentration of all detection points in the current sampling cycle is taken as the current representative concentration of the layer, and the maximum concentration of the previous sampling cycle is taken as the previous representative concentration. The reason for using the maximum value is that the droplet front refers to the farthest position reached by the droplets in the vertical direction. The droplet concentration at the farthest position reached by the droplet front is usually the local highest value in that layer. Therefore, taking the maximum value can capture the signal of the droplets arriving at the layer first, ensuring that the determination of the front position is not lagging.

[0059] Next, the difference between the current representative concentration and the previous representative concentration is divided by the sampling time interval to obtain the concentration change rate of the layer. The change rate reflects how fast the concentration of droplets in the layer increases per unit time. A positive value indicates that the concentration of droplets is rising, and a negative value indicates that it is falling. The larger the value, the faster the droplets enter the layer.

[0060] Finally, the concentration change rate of each layer is compared, and the layer with the largest concentration change rate is selected. The layer with the largest concentration change rate is the layer with the fastest increase in droplet concentration in the current sampling period, corresponding to the position where the droplets are invading. If the layer with the largest concentration change rate is higher than the layer where the droplet front is located in the previous sampling period and the concentration change rate is positive, it indicates that the droplet front has advanced to a higher layer. In this case, the lower boundary height of the layer with the largest concentration change rate is taken as the current height of the droplet front. The reason for using the lower boundary height is that the layer is a height range, and when the droplets enter the layer, they first reach its lower boundary. Therefore, the lower boundary height represents the lowest position that the droplet front has actually advanced to, and the physical meaning is the clearest.

[0061] Conversely, if the layer with the largest concentration change rate is not higher than the layer where the previous cycle's front edge was located, or if the concentration change rate is not positive, it indicates that the droplet front edge has not made substantial progress and its position remains unchanged. The position of the droplet front edge reflects the actual height to which the droplets rise in the gas-liquid separation space, and is the direct basis for judging whether the droplets have reached each layer and triggered corresponding adjustment measures.

[0062] Meanwhile, the height values ​​of the droplet front in the current sampling period and the previous N consecutive sampling periods are recorded, with N ranging from 5 to 10. The least squares method is used to perform linear fitting on the above N height values, and the slope of the fitted line is used as the propulsion speed in the current sampling period, reflecting the speed at which the droplets rise, and is used to determine whether the droplets are continuously moving upward.

[0063] The same method was used to linearly fit the propulsion speed values ​​for N consecutive sampling periods. The slope of the fitted line was taken as the propulsion acceleration of the current sampling period, which reflects the changing trend of the droplet rising speed. A positive acceleration indicates that the droplets are accelerating upward, which means that the propulsion momentum of the droplets is increasing; a negative or zero acceleration indicates that the rise of the droplets is slowing down or has stopped.

[0064] The above tests yielded three state parameters: the current layer position of the droplet front, its propulsion speed, and its propulsion acceleration. These parameters serve as the basis for subsequent differential adjustments.

[0065] like Figure 6 As shown, it reflects the dynamic changes in the height, velocity, and acceleration of the droplet front as a function of the sampling time, and marks the boundary between the near-liquid surface layer and the intermediate transition layer to characterize the evolution of the layer in which the droplet front is located.

[0066] Specifically, during the sampling interval from t1 to t3, the droplet front was located near the liquid surface; from t4 to t7, the droplet front entered the intermediate transition layer; and from t8 onwards, the droplet front further advanced to the near-exit layer. Throughout the upward advancement process, the advancing velocity showed a trend of first increasing and then decreasing, while the advancing acceleration gradually decreased from a positive value and eventually turned negative. This pattern indicates that the upward momentum of the droplet front was suppressed under the effect of differential regulation.

[0067] Therefore, it can be seen that by comprehensively monitoring three key parameters—droplet leading edge height, propulsion speed, and propulsion acceleration—this invention can dynamically identify the current evolution stage of droplets, thereby providing a reliable basis for subsequent implementation of phased adjustment and control strategies.

[0068] S5. Perform differentiated adjustments based on the layer position of the droplet front: when it is near the liquid surface layer, adjust the circulation flow rate according to the droplet concentration; when it is in the intermediate transition layer, adjust the discharge rate according to the propulsion speed and propulsion acceleration; when it is near the outlet layer, adjust the liquid level according to the interception efficiency determined by the ratio of the droplet concentration near the outlet layer to the droplet concentration at the steam outlet.

[0069] After obtaining the droplet frontal state parameters in S4, differentiated adjustment strategies can be implemented based on the specific location of the droplet front. Since droplets pass through the near-liquid surface layer, intermediate transition layer, and near-exit layer sequentially from generation to escape, the movement state, hazard level, and control objectives of droplets are different at each stage. Therefore, this invention implements targeted regulation according to the stages of droplet propagation.

[0070] See Figure 3 As shown, the first stage: located in the near-liquid surface layer.

[0071] When droplets are generated at the gas-liquid interface and just enter the near-liquid surface layer, their initial kinetic energy mainly comes from the jet impact when bubbles burst. If the droplet concentration continues to rise, it indicates that the amount of droplets generated by bubble bursts is constantly increasing. If not intervened in time, the droplets will continue to propel upwards. Therefore, the control measure adopted in this stage is to reduce the circulation flow rate of the circulating pump.

[0072] It should be noted that the effect of adjusting the circulation flow rate in this step is on the fluid shear state in the liquid surface region, not the overall evaporation or boiling intensity of the crystallizer. While the circulation flow rate primarily serves to ensure uniform heat exchange and crystal suspension, its magnitude also determines the flow state of the liquid surface within the crystallizer; a higher flow rate results in stronger surface shear disturbance. The initial kinetic energy of droplets after detaching from the liquid surface mainly comes from the jet impact during bubble bursting, and the intensity of surface shear disturbance directly affects the intensity of the jet impact. Therefore, by slightly reducing the circulation flow rate to reduce surface disturbance as droplets just enter the near-surface layer, the initial kinetic energy imparted to the droplets by bubble bursting can be reduced, thereby weakening their ability to continue upward movement at the starting point of their ascent.

[0073] The control method is as follows: compare the current height of the droplet front with the height of the upper boundary of the near-liquid layer. If the current height of the droplet front is lower than or equal to the height of the upper boundary of the near-liquid layer, it is determined that the droplet front is located within the near-liquid layer.

[0074] The average concentration of each detection point in the near-liquid surface layer during the current sampling period is calculated as the average concentration of the near-liquid surface layer. The average value is used here instead of the maximum value because the adjustment decision of the circulation flow rate should be based on the overall droplet accumulation level of the layer area, rather than local extreme values. The near-liquid surface layer is the area where droplets are just formed. The concentration difference of each detection point mainly reflects the spatial distribution of the bubble bursting position. Taking the average value can truly reflect the overall droplet generation intensity of the layer area and avoid incorrect adjustment due to local fluctuations at a certain point. The concentration increase is calculated by comparing it with the average concentration of the near-liquid surface layer in the previous sampling period.

[0075] When the concentration increase is greater than zero, it indicates that the overall droplet concentration near the liquid surface is still rising and the amount of droplets generated continues to increase. At this time, the circulation flow rate of the circulation pump needs to be reduced, with a single reduction of 2% to 5% of the current circulation flow rate, preferably 3%.

[0076] When the concentration increase is zero, it indicates that the droplet concentration near the liquid surface has stabilized and the amount of droplets produced has stopped increasing. At this point, there is no need to adjust the circulation flow rate; simply maintain the current flow rate.

[0077] Phase Two: Located in the intermediate transition layer

[0078] If the concentration increase of droplets in the near-liquid surface layer remains greater than zero after circulation flow adjustment, it indicates that the circulation flow adjustment has failed to effectively suppress the continuous rise of droplets, and the droplets will continue to rise into the intermediate transition layer. In the intermediate transition layer, droplets are under the combined influence of gravity settling and gas phase drag. If the droplet rising velocity continues to increase, it means that the gas phase drag is greater than the gravity settling capacity, and the droplets are accelerating upwards. If not intervened in time, the droplets will advance to the near-exit layer in a short period of time, increasing the risk of escape.

[0079] Therefore, the control measures adopted in this stage are to reduce the liquid level by increasing the discharge rate. This is because the liquid level is determined by both the feed rate and the discharge rate during the operation of the crystallizer. Increasing the discharge rate directly affects the liquid level and has a fast response speed. Compared with decreasing the feed rate, it causes less disturbance to the crystallization process. Specifically, when the feed rate remains constant, increasing the discharge rate makes the amount of liquid discharged from the crystallizer per unit time greater than the amount of liquid entering. The total amount of material in the crystallizer decreases, and the liquid level drops accordingly, thereby expanding the gas-liquid separation space and increasing the residence time of droplets in the intermediate transition layer.

[0080] The control method is as follows: compare the current height of the droplet front with the height of the upper and lower boundaries of the intermediate transition layer. If the current height of the droplet front is higher than or equal to the lower boundary of the intermediate transition layer and lower than the upper boundary of the intermediate transition layer, then the droplet front is determined to be within the range of the intermediate transition layer.

[0081] Obtain the propulsion velocity and acceleration of the droplet front;

[0082] When the propulsion speed and propulsion acceleration are both positive, it indicates that the droplets are not only rising, but the rising speed is also continuously accelerating. At this time, the feed rate is kept constant, and the discharge rate is increased to lower the liquid level, thereby prolonging the residence time of the droplets in the intermediate transition layer. The discharge rate can be increased by 3% to 8% of the current value each time, preferably by 5%. This adjustment range is within the normal operating range of crystallizer liquid level adjustment.

[0083] Conversely, if the propulsion speed or propulsion acceleration is not positive, it indicates that the droplets are not accelerating upwards or have stopped propulsing. No intervention is needed at this time, and the current discharge rate and feed rate should be maintained unchanged.

[0084] It should be noted that the accelerated rise of droplets is an occasional and temporary state. Once the propulsion speed or acceleration turns non-positive, the adjustment stops immediately. Therefore, this adjustment is not a long-term operating state, but a brief intervention. Within an adjustment range of 3% to 8%, the temporary drop in liquid level will not have a lasting impact on the overall material and thermal balance of the crystallizer, while the resulting benefit of increased gas-liquid separation space and droplet settling is immediate and direct.

[0085] Phase 3: Located near the exit layer

[0086] If the droplets maintain a positive propulsion velocity and acceleration after level adjustment in the intermediate transition layer, it indicates that the level adjustment in the intermediate transition layer has failed to effectively block the upward movement of the droplets. The droplets will continue to advance into the near-outlet layer, which is the spatial region before the droplets enter the steam outlet. Once the droplets penetrate the near-outlet layer, they will escape directly from the crystallizer with the secondary steam. Therefore, a level reduction operation is performed in this stage to further expand the gas-liquid separation space and increase the chance of droplet settling. This adjustment, together with the steam outlet gas phase flow rate control in S6, constitutes a control measure for the final stage of droplet ascent.

[0087] The control method is as follows: compare the current height of the droplet front with the height of the lower boundary of the near-exit layer. If the current height of the droplet front is higher than or equal to the height of the lower boundary of the near-exit layer, it is determined that the droplet front is located within the near-exit layer.

[0088] The ratio of the near-outlet droplet concentration to the steam outlet droplet concentration is used as the interception efficiency. The physical meaning of this ratio is as follows: Under normal interception conditions, the near-outlet droplet concentration should be higher than the concentration at the steam outlet, meaning that the vast majority of droplets are intercepted within the near-outlet layer, with only a very small amount penetrating to the steam outlet; at this point, the ratio is relatively large. When the interception efficiency decreases, the difference between the near-outlet layer concentration and the steam outlet concentration narrows, and the ratio decreases. Therefore, the ratio objectively reflects the near-outlet layer's ability to intercept droplets.

[0089] The interception efficiency is continuously monitored for multiple sampling cycles. If the interception efficiency decreases continuously and the decrease in the current sampling cycle is greater than the decrease in the previous sampling cycle, it is determined that the interception efficiency is continuously deteriorating. This means that the droplets are accelerating their penetration into the near-outlet layer. If no timely intervention is made, the droplets will quickly escape to the steam outlet. At this time, the liquid level reduction operation is initiated to expand the gas-liquid separation space. The liquid level reduction amount can be 50-100 mm each time, preferably 80 mm.

[0090] Once the interception efficiency stops decreasing and rebounds after two consecutive sampling cycles, it indicates that the interception function is recovering. At this point, gradually restore the liquid level by increasing it in increments of 20-50 mm each time, increasing it once per sampling cycle until it returns to the normal operating value.

[0091] S6. After differential adjustment, detect the gas phase flow rate at the steam outlet. If the flow rate exceeds the upper limit, simultaneously reduce the vacuum pumping rate and the heating medium supply until the gas phase flow rate falls back below the upper limit.

[0092] After differentiating the droplet ascent process, if the interception efficiency near the outlet layer continues to decline and fails to recover in time, the droplets will continue to penetrate the near-outlet layer and enter the steam outlet pipe, at which point the droplets have actually escaped. In this case, it is necessary to directly detect the droplet escape status at the steam outlet and take end-of-pipe remedial measures to reduce the gas phase carrying capacity, namely, simultaneously reducing the vacuum pumping rate and the supply of heating medium.

[0093] The specific control method is as follows: after the differential control is executed, the gas phase flow rate at the steam outlet is detected. The gas phase flow rate at the steam outlet can be directly measured by a flow meter installed on the steam outlet pipe. The detected gas phase flow rate is compared with the upper limit value of the flow rate.

[0094] When the gas phase velocity is greater than the upper limit of the velocity, it indicates that the droplets are being carried away by excessive steam and escape. At this time, the vacuum pumping rate is reduced by a fixed step, for example, 3% to 5% of the current vacuum pumping rate, to reduce the gas phase velocity at the steam outlet. At the same time, the heating medium supply is reduced by a fixed step, for example, 3% to 5% of the current heating medium supply, to match the reduction in evaporation intensity and avoid the pressure inside the crystallizer from increasing due to only reducing the pumping rate.

[0095] After each sampling period, the gas phase velocity is re-detected and compared with the upper limit of the velocity. If the gas phase velocity is still greater than the upper limit of the velocity, the above-mentioned downward adjustment operation is repeated until the gas phase velocity is less than or equal to the upper limit of the velocity and then the downward adjustment stops.

[0096] The above-mentioned upper limit of flow velocity is set as follows: Under the condition that the droplet front has not reached the near-outlet layer, the gas phase flow velocity at the steam outlet is recorded as the reference flow velocity; the reference flow velocity is multiplied by a safety factor greater than 1, and the product is used as the upper limit of flow velocity. The safety factor is introduced to reserve a margin for normal process fluctuations in gas phase flow velocity. The specific value is set by the operator according to the actual on-site operating conditions.

[0097] In a further preferred embodiment, after the reduction is stopped, since the decrease in vacuum pumping rate and heating medium supply is only a temporary measure to address droplet escape, maintaining this state for a long time will lead to insufficient evaporation intensity in the crystallizer, affecting normal crystallization efficiency and production capacity. Therefore, after the risk of droplet escape is eliminated, both need to be gradually restored to normal operating values. The restoration sequence is to first restore the heating medium supply, and then restore the vacuum pumping rate. This is because restoring the heating medium supply can promptly replenish the heat required for evaporation, avoiding a sudden drop in evaporation intensity. After the evaporation intensity is restored, the pumping rate is gradually increased to avoid a sudden increase in gas phase flow rate due to the pumping rate being restored first, which could trigger droplet escape again.

[0098] Both phases of recovery involve gradually increasing the value by a fixed step size, with one step size increased for each sampling period, until the value is restored to the normal operating value.

[0099] Example 2

[0100] See Figure 4 As shown, the present invention proposes a sodium chlorate crystallization evaporation control system, including the following modules: a detection module, which collects the detection signals of pressure inside the crystallizer, mist concentration in each layer and gas phase flow velocity at the steam outlet in real time, and calculates the pressure change rate based on the pressure acquisition signal;

[0101] The flash control module is connected to the detection module. It determines the degree of flashing based on the rate of pressure change. When the degree of flashing is higher than the previous cycle for multiple consecutive sampling cycles and the cumulative change exceeds the limit, the feed temperature setpoint is lowered.

[0102] The leading edge tracking module, connected to the flash evaporation control module, divides the gas-liquid separation space above the crystallizer into a near-liquid surface layer, an intermediate transition layer, and a near-outlet layer. Based on the time series of droplet concentration in each layer, it calculates the vertical propulsion velocity and propulsion acceleration of the droplet leading edge to determine the layer position of the droplet leading edge.

[0103] The differential adjustment module, connected to the leading edge tracking module, performs differential adjustment based on the layer position of the droplet leading edge and its propulsion speed and acceleration: when it is near the liquid surface layer, the circulation flow rate is adjusted according to the droplet concentration; when it is in the intermediate transition layer, the discharge rate is adjusted according to the propulsion speed and acceleration; and when it is near the outlet layer, the liquid level is adjusted according to the interception efficiency determined by the ratio of the droplet concentration near the outlet layer to the droplet concentration at the steam outlet.

[0104] The end-efficiency module is connected to the detection module and the differential adjustment module respectively. It is used to synchronously reduce the vacuum pumping rate and the heating medium supply in fixed steps when the gas phase flow rate exceeds the limit, until the gas phase flow rate drops back to below the upper limit value.

[0105] The above embodiments can be implemented, in whole or in part, by software, hardware, firmware, or any other combination thereof. When implemented using software, the above embodiments can be implemented, in whole or in part, in the form of a computer program product.

[0106] Those skilled in the art will recognize that the modules and algorithm steps of the various examples described in conjunction with the embodiments disclosed herein can be implemented in electronic hardware, or a combination of computer software and electronic hardware. Whether these functions are implemented in hardware or software depends on the specific application and design constraints of the technical solution. Those skilled in the art can use different methods to implement the described functions for each specific application, but such implementation should not be considered beyond the scope of this application.

[0107] In addition, the functional modules in the various embodiments of this application can be integrated into one processing module, or each module can exist physically separately, or two or more modules can be integrated into one module.

[0108] The above description is merely a specific embodiment of this application, but the scope of protection of this application is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

[0109] Finally, the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for controlling the evaporation of sodium chlorate crystals, characterized in that, Includes the following steps: The rate of pressure change inside the crystallizer is monitored in real time to determine the degree of flash evaporation. When the flash evaporation degree is higher than the previous cycle for multiple consecutive sampling cycles and the cumulative change exceeds the limit, the feed temperature setting value is lowered. The upper gas-liquid separation space of the crystallizer is divided vertically into a near-liquid surface layer, an intermediate transition layer, and a near-outlet layer. The concentration of droplets is detected in each layer, and the current layer position of the droplet front is determined based on the time series of droplet concentration. The vertical propulsion velocity and propulsion acceleration of the droplet front are also calculated. Differential adjustments are made based on the location of the droplet front in the layer: when it is near the liquid surface, the circulation flow rate is adjusted according to the droplet concentration; when it is in the intermediate transition layer, the discharge rate is adjusted according to the propulsion speed and propulsion acceleration; and when it is near the outlet layer, the liquid level is adjusted according to the interception efficiency determined by the ratio of the droplet concentration near the outlet layer to the droplet concentration at the steam outlet. After differential adjustment, the gas phase flow rate at the steam outlet is detected. If the flow rate exceeds the upper limit, the vacuum pumping rate and the supply of heating medium are reduced simultaneously until the gas phase flow rate falls back below the upper limit.

2. The method for controlling the evaporation of sodium chlorate crystallization as described in claim 1, characterized in that: The determination of the degree of flash evaporation includes the following: Pressure is collected during each sampling cycle by a pressure detection element located inside the crystallizer; Read the pressure collected in the current sampling period and the previous sampling period, calculate the ratio of the absolute value of the difference between the two to the sampling time interval, and use this ratio as the degree of flash evaporation.

3. The method for controlling the evaporation of sodium chlorate crystallization as described in claim 1, characterized in that: The process of lowering the feed temperature setpoint is as follows: After each sampling period, the flashing degree of the current sampling period is compared with the flashing degree of the previous sampling period: If the current flashing level is higher than the flashing level in the previous sampling period, the continuous increase count will be incremented by one; otherwise, the continuous increase count will be reset to zero and the cumulative change will be set to zero. When the count is continuously increased to one and the current flashing degree is higher than the previous sampling period, the flashing degree of the previous sampling period is recorded as the baseline value. The difference between the flashing degree of the current sampling period and the baseline value is used as the cumulative change. When the count is continuously increased to the threshold and the cumulative change exceeds the change limit, the feed temperature setpoint will be lowered by one temperature step. The flash evaporation level is recalculated in the next sampling period, and the above judgment is repeated. If the condition is still met, the level is lowered until the continuous increase in the count does not reach the number of times threshold or the cumulative change does not exceed the change limit, at which point the lowering stops.

4. The method for controlling the evaporation of sodium chlorate crystallization as described in claim 1, characterized in that: The specific method for dividing the upper gas-liquid separation space of the crystallizer into a near-liquid surface layer, an intermediate transition layer, and a near-outlet layer along the vertical direction is as follows: Obtain the real-time height of the liquid level inside the crystallizer and the installation height of the steam outlet, and calculate the vertical distance between the liquid level and the steam outlet; Based on the bottom of the vertical distance, the gas-liquid separation space is divided into the near-liquid surface layer, the intermediate transition layer and the near-outlet layer in sequence along the vertical direction upward.

5. The method for controlling the evaporation of sodium chlorate crystallization as described in claim 4, characterized in that: The process of determining the current layer position of the droplet front based on the time series of droplet concentration, and calculating the vertical propulsion velocity and propulsion acceleration of the droplet front, is carried out as follows: Multiple fog concentration detection points distributed horizontally are set up in each layer area; In each sampling period, the concentration value of each detection point is collected. For each layer region, the maximum concentration of all detection points in the current sampling period is taken as the current representative concentration, and the maximum concentration of the previous sampling period is taken as the previous representative concentration. Divide the difference between the current representative concentration and the previous representative concentration by the sampling time interval to obtain the concentration change rate of the layer region; Compare the concentration change rate of each layer, and take the layer with the largest concentration change rate. If it is higher than the layer where the droplet front is located in the previous sampling period and the concentration change rate is positive, then the lower boundary height of the layer with the largest concentration change rate is taken as the current height of the droplet front. If it is not higher than the layer where the droplet front is located in the previous sampling period or the concentration change rate is not positive, then the position of the droplet front remains unchanged. Record the height of the droplet front over multiple consecutive sampling periods, calculate the vertical distance the droplet front traverses per unit time as the propulsion velocity, and calculate the rate of change of the propulsion velocity over time as the propulsion acceleration.

6. The method for controlling the evaporation of sodium chlorate crystallization as described in claim 1, characterized in that: The specific method for adjusting the circulation flow rate based on the mist concentration when the liquid level is near the liquid surface is as follows: If the current height of the droplet front is lower than or equal to the height of the upper boundary of the near-liquid layer, then the droplet front is determined to be within the near-liquid layer. The average concentration of each detection point in the near-liquid layer during the current sampling period is calculated as the average concentration of the near-liquid layer. This is then compared with the average concentration of the near-liquid layer in the previous sampling period to calculate the increase in concentration. When the concentration increase is greater than zero, reduce the circulation flow rate of the circulation pump; when the concentration increase is zero, keep the circulation flow rate unchanged.

7. The method for controlling the evaporation of sodium chlorate crystallization as described in claim 1, characterized in that: The process of adjusting the discharge rate based on the propulsion speed and propulsion acceleration when the material is located in the intermediate transition layer is as follows: If the current height of the droplet front is higher than or equal to the lower boundary height of the intermediate transition layer and lower than the upper boundary height of the intermediate transition layer, then the droplet front is determined to be within the range of the intermediate transition layer. Obtain the propulsion velocity and acceleration of the droplet front; When the feed rate and the feed acceleration are both positive, the feed rate is kept constant while the discharge rate is increased; otherwise, the current discharge rate and feed rate are maintained.

8. The method for controlling the evaporation of sodium chlorate crystallization as described in claim 1, characterized in that: The specific method for adjusting the liquid level based on the ratio of the mist concentration near the outlet layer to the mist concentration at the steam outlet when the liquid level is near the outlet layer is as follows: If the current height of the droplet front is higher than or equal to the lower boundary height of the near-exit layer, then the droplet front is determined to be within the near-exit layer. The ratio of the mist concentration near the outlet layer to the mist concentration at the steam outlet is calculated as the interception efficiency. The interception efficiency is continuously monitored for multiple sampling cycles. If the interception efficiency decreases continuously and the decrease in the current sampling cycle is greater than the decrease in the previous sampling cycle, the liquid level is adjusted downward. After the interception efficiency stops decreasing and recovers for two consecutive sampling cycles, the liquid level is gradually restored to the normal operating value.

9. The method for controlling the evaporation of sodium chlorate crystallization as described in claim 1, characterized in that: The specific method for simultaneously reducing the vacuum pumping rate and the heating medium supply until the gas phase flow rate falls below the upper limit value is as follows: After differential adjustment, the gas phase flow rate at the steam outlet is detected and compared with the upper limit value of the flow rate. When the gas flow rate is greater than the upper limit of the flow rate, the current vacuum pumping rate is reduced by a fixed step, and the current heating medium supply is reduced by a fixed step. After each sampling period, the flow rate is re-detected and compared. If it is still greater than the upper limit of the flow rate, the flow rate is adjusted downward until it is less than or equal to the upper limit of the flow rate.

10. A sodium chlorate crystallization evaporation control system, characterized in that, Includes the following modules: The detection module collects real-time detection signals of pressure inside the crystallizer, mist concentration in each layer, and gas phase flow velocity at the steam outlet, and calculates the pressure change rate based on the pressure acquisition signal. The flash control module determines the degree of flashing based on the rate of pressure change. When the degree of flashing is higher than the previous cycle for multiple consecutive sampling cycles and the cumulative change exceeds the limit, the feed temperature setpoint is lowered. The leading edge tracking module divides the gas-liquid separation space above the crystallizer into a near-liquid surface layer, an intermediate transition layer, and a near-outlet layer. Based on the time series of droplet concentration in each layer, it calculates the vertical propulsion velocity and propulsion acceleration of the droplet leading edge to determine the layer position of the droplet leading edge. The differential adjustment module performs differential adjustment based on the layer position of the droplet front and the propulsion speed and acceleration: when it is in the near liquid surface layer, the circulation flow rate is adjusted according to the droplet concentration; when it is in the intermediate transition layer, the discharge rate is adjusted according to the propulsion speed and acceleration; and when it is in the near outlet layer, the liquid level is adjusted according to the interception efficiency determined by the ratio of the droplet concentration in the near outlet layer to the droplet concentration at the steam outlet. When the gas flow rate exceeds the limit, the end-effector module synchronously reduces the vacuum pumping rate and the heating medium supply in fixed steps until the gas flow rate falls back below the upper limit.

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