NAM automatic dosing system under different loads for stable urea production

Abstract

This invention discloses an automatic NAM dosing system for stable urea production under different loads, belonging to the field of urea treatment technology. This system adds a built-in dissolving tank, a centrifuge, a NAM / urine pump, a dissolving liquid metering and regulating valve, and a load calculation module to the existing process. The load calculation module calculates the dosing flow rate and the required amount of dissolving liquid based on the production load of the melt pump and the target NAM concentration. The dissolving liquid metering and regulating valve diverts urea solution from the first-stage evaporation as the dissolving liquid, which is then fed into the centrifuge in the built-in dissolving tank to mix, dissolve, and separate with the NAM solid. The NAM / urine pump pressurizes the separated dissolved additive and injects it after the melt pump outlet and before the static mixer, mixing it with the molten urea for granulation. This invention solves the problem of long dissolution time of the synergist in traditional processes, which causes decomposition and bubble formation; simultaneously, by using the evaporated urine as the additive solvent, it solves the problem of increased moisture content caused by the addition of synergist during production.

Description

Technical Field

[0001] This invention belongs to the field of urea treatment technology, specifically relating to an automatic NAM dosing system for stable urea production under different loads. Background Technology

[0002] Urea is an important nitrogen fertilizer and industrial chemical, traditionally synthesized by reacting ammonia with carbon dioxide under high temperature and pressure. Its production process mainly includes key steps such as raw material processing, high-pressure synthesis, product purification, and granulation and packaging. In urea industrial production, various additives are often added to improve product quality and functionality, such as substances that increase particle strength and slow-release synergists (e.g., NAM).

[0003] NAM (nitrification inhibitor / urease inhibitor complex) has been widely used as a urea slow-release synergist. Studies have shown that adding NAM to urea can effectively inhibit the activity of urease and nitrifying bacteria in the soil, slow down urea hydrolysis and the conversion of ammonium nitrogen to nitrate nitrogen, thereby prolonging the fertilizer effect period, improving nitrogen use efficiency, reducing nitrogen loss and greenhouse gas emissions, and significantly improving the agricultural performance of urea products. Especially in areas prone to nitrogen loss, such as high rainfall, high temperature, or sandy soils, adding NAM can significantly reduce nitrogen leaching and denitrification losses, improving the economic benefits of fertilizers. In traditional addition processes, the NAM solution is usually pumped from the storage tank into the second evaporator of the urea evaporation process. NAM mixes with urea in the evaporator and subsequent processes, and is evenly distributed in the urea granules, thereby achieving the purpose of prolonging the fertilizer effect and improving utilization efficiency.

[0004] However, the existing urea production system's additive process has the following technical defects: 1. NAM solution usually uses water as a solvent, which can easily lead to a high water content in the final urea product, affecting product quality; 2. In traditional processes, the prepared NAM solution is stored for 20-24 hours at a time. During this period, water and NAM will decompose under high temperature, resulting in the loss of NAM active ingredients and the generation of gas (forming bubbles). This not only affects the addition effect but may also cause corrosion to the equipment. 3. Traditional addition methods use a fixed addition ratio, which cannot adjust the NAM addition amount in real time according to the actual production load (molten urea flow rate) of the urea plant. This results in large fluctuations in the NAM content in the product, affecting the consistency of the sustained-release effect. Summary of the Invention

[0005] In view of this, the purpose of the present invention is to provide an automatic NAM dosing system for stable urea production under different loads, in order to solve the above-mentioned technical problems.

[0006] To achieve the above objectives, the present invention provides the following technical solution: An automatic NAM dosing system for stable urea production under different loads includes a heater, a primary evaporator / separator, a secondary evaporator / separator, a urea melting pump, a static mixer, and a urea granulation tower. It also includes a centrifuge with a built-in dissolving tank, a dissolving liquid metering and regulating valve, a NAM urea pump, and a load calculation module. The automatic NAM dosing system performs the following operations during operation: The dissolving solution metering control valve diverts a portion of the urea solution from the first stage of evaporation and heating separation completed in the upstream urea synthesis process as NAM dissolving solution according to the real-time demand of NAM dissolving solution, and uses the remaining urea solution to continue to complete the second stage of heating and evaporation separation to obtain molten urea; Molten urea is fed into a urea melting pump through a pipeline, where it is pressurized and transported towards the urea granulation tower. The real-time delivery flow rate of the urea melting pump is recorded as the production load, and the dosing flow rate of the NAM urea pump is constrained according to the production load and the target concentration of NAM. The real-time demand for NAM solvent is determined based on the addition flow rate, and then the real-time demand for NAM solution is calculated. An additional stirring and centrifugation process is added, in which solid NAM and dissolved NAM are introduced during the rotation of the stirring centrifuge, and the dissolved NAM is separated from the solid NAM immediately. The separated dissolved additive is then pumped to the direction of the urea granulation tower by a NAM urea pump pressurization. The injection point of the dissolved additive is set after the outlet of the urea melting pump and before the static mixer. The soluble additive and molten urea are rapidly mixed using a static mixer to obtain a mixture containing the additive, which is then directly fed into the urea granulation tower for granulation.

[0007] Furthermore, the NAM solution is a 95% aqueous urea solution.

[0008] Furthermore, the extraction amount of NAM solution is determined based on the amount of solid NAM additive used in subsequent synthesis processes, and the extraction is completed at a ratio of 2:1 between NAM solution and solid NAM additive.

[0009] Furthermore, the water content of molten urea should be less than 1%.

[0010] Furthermore, the dissolved additives are separated and transported to the urea granulation tower within 10-15 minutes.

[0011] Furthermore, the dissolution temperature of the NAM solution and the NAM solid is controlled at 90-120 degrees Celsius.

[0012] Furthermore, the volume of the built-in dissolving tank in the stirred centrifuge is related to the historical average production load. The volume of the dissolving tank corresponding to a higher historical average production load is greater than the volume of the dissolving tank corresponding to a lower historical average production load.

[0013] Furthermore, the load calculation module performs the following operations: The mass flow rate of the urea melting pump, pre-set for this production run, is recorded as the production load. And obtain the NAM content of the urea granules expected to be produced during this production, denoted as the target concentration of NAM. ; According to production load and target concentration Constraint on the mass flow rate of the NAM urine pump : The dosage flow rate is taken as the lower limit of the NAM solvent output required by the stirred centrifuge, and is denoted as the real-time demand for NAM solvent. ; Obtain the solubility ratio R of NAM solution to solid NAM additive, combined with the real-time demand for NAM solvent. The real-time demand for NAM solution was calculated. : In the formula, the ratio of NAM solution to solid NAM additive is 2:1, i.e., R=2.

[0014] Furthermore, the load calculation module also includes a feedback correction unit for receiving the actual NAM concentration of the mixture measured by an online concentration analyzer installed at the outlet of the static mixer. The concentration deviation was calculated. : If concentration deviation If the preset deviation range is not met, the mass flow rate of the NAM urine pump will be adjusted according to the PI control algorithm. Make corrections and recalculate the real-time demand for NAM solution based on the corrected mass flow rate.

[0015] Furthermore, the load calculation module also includes a feedforward control unit, which communicates with the load setting device of the urea melting pump to obtain the setting change signal of the production load in real time. When a change in the production load setting value is detected, the feedforward control unit immediately recalculates the mass flow rate of the NAM urea pump and the real-time demand of the NAM solution based on the new production load setting value, and synchronously outputs control commands to the NAM urea pump and the solution metering regulating valve.

[0016] The beneficial effects of this invention are as follows: 1. Utilizing rotary centrifugal separation technology, a system for instant addition of synergists was developed. This solves the problem of traditional processes where synergists are dissolved and used for 20-24 hours at a time, causing synergists to decompose and form bubbles, affecting the effect and corroding equipment. The volume of the dissolution system tank is reduced, and a stirring centrifuge is added to dissolve the additives during rotation and immediately separate the solution from the solid additives. The dissolved additives are separated and transported to the urea granulation system within 10-15 minutes, meaning that the production process is completed before the material decomposes. When cooled to below 70°C, the reaction stops, and the cavitation of the melt pump caused by bubbles generated during the additive dissolution process is avoided. 2. For the first time, urine after evaporation is used instead of water as a solvent for additives. The water-containing urine (5%) has good fluidity due to the presence of water, is not easy to solidify, and has a water content of no more than 6%, which solves the problem of increased water content caused by the addition of synergists in production; moreover, urea is between organic and inorganic substances, and has better miscibility with organic additives, making the additives easier to dissolve. 3. By setting the additive injection point after the urea melting pump, the natural temperature drop of the molten urea during transportation is utilized to ensure that the urea temperature at the injection point is lower than the outlet temperature of the second-stage evaporator. At the same time, the pressurization of the melting pump increases the boiling point of the liquid. The combined effect of these two factors significantly reduces the risk of NAM decomposition, ensures the accuracy and stability of the addition amount, and also shortens the NAM solution travel time. 4. By monitoring the urea melting pump's delivery flow rate (i.e., production load) in real time, the load calculation module dynamically calculates the NAM addition flow rate, enabling the NAM addition amount to change synchronously with urea production, ensuring that the NAM content in the product remains stable within the target range.

[0017] Other advantages, objectives, and features of the invention will be set forth in the following description and will be apparent to those skilled in the art in some respects, or may be learned by practice of the invention. The objects and other advantages of the invention may be realized and obtained by means of the structures particularly pointed out in the written description and the accompanying drawings.

[0018] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0019] The accompanying drawings are provided to further illustrate the invention and form part of the specification. They are used in conjunction with embodiments of the invention to explain the invention and do not constitute a limitation thereof. In the drawings: Figure 1 This is a schematic diagram of the dosing process of an automatic NAM dosing system for stable urea production under different loads, as described in an embodiment of the present invention. Detailed Implementation

[0020] The preferred embodiments of the present invention will be described below with reference to the accompanying drawings. It should be understood that the preferred embodiments described herein are for illustration and explanation only and are not intended to limit the present invention.

[0021] like Figure 1 As shown, this invention proposes an automatic NAM dosing system for stable urea production under different loads, including a heater, a primary evaporator / separator, a secondary evaporator / separator, a urea melting pump, a static mixer, and a urea granulation tower. It also includes a centrifuge with a built-in dissolving tank, a NAM urea pump, a dissolving liquid metering and regulating valve, and a load calculation module. The automatic NAM dosing system performs the following operations during operation: S1: Extraction of NAM solution The dissolving solution metering control valve diverts a portion of the urea solution from the first stage of evaporation and heating separation completed in the upstream urea synthesis process as NAM dissolving solution according to the real-time demand of NAM dissolving solution, and uses the remaining urea solution to continue to complete the second stage of heating and evaporation separation to obtain molten urea.

[0022] S2: Delivery of molten urea Molten urea is fed into a urea melting pump through a pipeline, where it is pressurized and transported towards the urea granulation tower.

[0023] S3: Real-time acquisition of production load and calculation of added flow rate The real-time flow rate of the urea melting pump is recorded as the production load, and the NAM dosing flow rate of the urea pump is constrained based on the production load and the target NAM concentration. It is worth noting that the target NAM concentration refers to the expected amount of NAM in the produced urea granules.

[0024] S4: Calculation of NAM solvent and solution requirements The real-time demand for NAM solvent is determined based on the addition flow rate, and then the real-time demand for NAM solution is calculated.

[0025] S5: Rapid dissolution and separation via stirring and centrifugation An additional stirring and centrifugation process is added, in which NAM solid and NAM solution are introduced during the rotation of the stirring centrifuge, and the NAM solution is separated from the NAM solid in real time. The separated dissolved additive is then pumped to the direction of the urea granulation tower by a NAM urine pump pressurization. The injection point of the dissolved additive is set after the urea melt pump outlet and before the static mixer.

[0026] S6: Static Mixing and Granulation The soluble additive and molten urea are rapidly mixed using a static mixer to obtain a mixture containing the additive, which is then directly fed into the urea granulation tower for granulation.

[0027] The NAM solution is a 95% aqueous urea solution. This concentration of urea solution has a low crystallization temperature (approximately 80-90℃) and can efficiently dissolve NAM solids while avoiding the introduction of excessive moisture.

[0028] The extraction amount of NAM solution is determined based on the amount of solid NAM additive used in subsequent synthesis processes, and extraction is performed at a mass ratio of NAM solution to solid NAM additive of 2:1. This ratio ensures sufficient dissolution of NAM without causing excessive waste of solution.

[0029] The moisture content of molten urea should be less than 1% to ensure the strength and storage stability of the urea particles after granulation.

[0030] The dissolved additives are separated and transported to the urea granulation tower within 10-15 minutes. This time window minimizes the decomposition loss of NAM at high temperatures.

[0031] The dissolution temperature of NAM solution and NAM solid is controlled between 90-120℃. This temperature range ensures rapid dissolution of NAM while avoiding excessive temperature that could lead to NAM decomposition.

[0032] The volume of the dissolving tank built into the centrifuge is related to the historical average production load. The volume of the dissolving tank corresponding to a higher historical average production load is greater than the volume of the dissolving tank corresponding to a lower historical average production load. Specifically, the effective volume V (liters) of the dissolving tank and the historical average hourly production load L (tons / hour) are related by the following formula: V = k × L, where k is an empirical coefficient with a value ranging from 0.5 to 1.5.

[0033] The load calculation module performs the following operations: The mass flow rate of the urea melting pump, pre-set for this production run, is recorded as the production load. And obtain the NAM content of the urea granules expected to be produced during this production, denoted as the target concentration of NAM. ; According to production load and target concentration Constraint on the mass flow rate of the NAM urine pump : The dosage flow rate is taken as the lower limit of the NAM solvent output required by the stirred centrifuge, and is denoted as the real-time demand for NAM solvent. ; Obtain the solubility ratio R of NAM solution to solid NAM additive, combined with the real-time demand for NAM solvent. The real-time demand for NAM solution was calculated. : In the formula, the ratio of NAM solution to solid NAM additive is 2:1, i.e., R=2.

[0034] It is worth noting that since NAM urine pumps typically use volumetric flow rate as the control parameter, the mass flow rate needs to be converted to volumetric flow rate. in The density (kg / L) of the dissolving additive at the operating temperature can be obtained by actual measurement with an online density meter or by referring to a table based on temperature and concentration.

[0035] The load calculation module also includes a feedback correction unit, which receives the actual NAM concentration of the mixture measured by an online concentration analyzer installed at the outlet of the static mixer. The concentration deviation was calculated. : If concentration deviation If the preset deviation range is not met, the mass flow rate of the NAM urine pump will be adjusted according to the PI control algorithm. Make corrections and recalculate the real-time demand for NAM solution based on the corrected mass flow rate.

[0036] The preferred correction method is: The proportionality coefficient The value ranges from 0.5 to 2.0, and the integral coefficient is... The value range is 0.05-0.2min -1 Then, based on the corrected mass flow rate, the real-time demand for NAM solution is recalculated, and the instructions for the solution metering valve and NAM urine pump are adjusted accordingly to form a closed-loop control, ensuring the stability of NAM content in the product.

[0037] The load calculation module also includes a feedforward control unit, which communicates with the load setting device of the urea melting pump to obtain the setting change signal of the production load in real time. When a change in the production load setting value is detected, the feedforward control unit immediately recalculates the mass flow rate of the NAM urea pump and the real-time demand of the NAM solution based on the new production load setting value, and synchronously outputs control commands to the NAM urea pump and the solution metering and regulating valve.

[0038] To further illustrate the present invention, three load conditions are provided below: Example 1 (Normal Operating Conditions): This embodiment uses a 300,000-ton-per-year urea unit (designed hourly output of 37.5 tons / hour) to produce stable urea with a target NAM content of 0.5%.

[0039] System Configuration: Effective volume of the built-in dissolving tank in the stirred centrifuge: Based on the historical average production load of 37.5t / h, take the coefficient k=1.0 and V=37.5 liters.

[0040] NAM solution: 95% urea solution taken from the outlet of the primary evaporator separator.

[0041] Dissolution temperature: 105℃.

[0042] The mass ratio of NAM solid to solution is 1:2.

[0043] Operating steps: 1. Extraction of dissolving solution: The load calculation module calculates the dosage mass flow rate based on the current production load (37.5 t / h) and the target NAM concentration (0.5%). Then calculate the required amount of solvent: Based on a NAM solid:solution ratio of 1:2, approximately 62.81 kg / h of NAM solid is required. The system extracts NAM solid from the outlet branch pipe of the first-stage evaporator separator. A 95% urea solution is fed into the dissolving tank of a centrifuge with a stirrer. It is worth noting that the volumetric flow rate should be calculated as needed.

[0044] 2. Rapid Dissolution and Separation: Solid NAM is added from the silo to the dissolution tank via a screw feeder at a rate of 62.81 kg / h. A centrifuge rotates at 300 rpm, with built-in impellers promoting solid-liquid contact. The temperature inside the dissolution tank is maintained at 105°C (maintained by heating). The NAM solid dissolves completely within 5-8 minutes. The centrifuge speed is then increased to 1200 rpm, using centrifugal force to separate the dissolved liquid (containing dissolved NAM) from the undissolved solids. The clear liquid flows out through the bottom outlet, while a very small amount of undissolved solids are retained.

[0045] 3. Production Load Monitoring and Dosing: A mass flow meter is installed at the outlet of the urea melting pump, and the real-time molten urea flow rate is measured to be 37.5 t / h. Based on this, the load calculation module calculates the required NAM dissolving additive flow rate: This translates to a flow rate of approximately 154.5 L / h for the dissolving additive solution (since the concentration of the dissolving solution is 95% and the total volume changes slightly after NAM dissolution, the actual density is approximately 1.2 kg / L). The NAM urine pump (metering pump) delivers the dissolving additive to the injection point at this flow rate.

[0046] 4. Injection and Mixing: The injection point is located 2 meters after the urea melt pump outlet and before the static mixer. The pressure of the dissolving additive is 0.6 MPa (0.45 MPa higher than the melt pump outlet pressure) to ensure smooth injection. The mixing time in the static mixer (SV type, 8 mixing units) is approximately 4 seconds, after which the mixture enters the granulation tower for spray granulation.

[0047] Product testing: Samples were taken and analyzed after granulation. The NAM content in the product was 0.49%-0.51%, and the moisture content was 0.3%, which meets the superior grade standard of GB / T2440-2017. After 72 hours of continuous operation, no signs of pipe blockage or equipment corrosion were found.

[0048] Example 2 (Low Load Condition): This embodiment simulates the device operating at 60% load (22.5 tons / hour), still producing stable urea with a target NAM content of 0.5%. The system configuration and general requirements have already been described under the above normal operating conditions and will not be repeated here.

[0049] Operation process: The load calculation module detected that the molten urea flow rate had dropped to 22.5 t / h and automatically calculated... .

[0050] Solution required: .

[0051] The system automatically adjusts the speed of the NAM solid feeder and the metering control valve of the dissolving liquid, and sets the flow rate of the NAM urine pump to 112.5 kg / h.

[0052] Since the volume of the dissolving tank is fixed at 37.5 liters, the residence time of the NAM solution in the dissolving tank under low load is slightly extended (about 10 minutes), but it is still far lower than the 20 hours of the traditional process. The NAM decomposition rate test shows that it is only 0.2% (the traditional process is 5%-8%).

[0053] Product testing: The NAM content of the product is 0.496%-0.504%, with a pass rate of 100%.

[0054] Example 3 (High load and low target NAM concentration): This embodiment simulates the production of stable urea with a NAM content of only 0.3% (suitable for long-acting fertilizers in sandy soils) under 110% load (41.25 tons / hour). The system configuration and general requirements have been described under the above normal operating conditions and will not be repeated here.

[0055] Operation process: Load calculation module: .

[0056] Solution required: .

[0057] System response: The amount of solution extracted decreases (due to the reduction in target concentration), and the NAM solid feed is reduced accordingly.

[0058] Due to the high flow rate of urea under high load, the mixing effect of the static mixer still meets the requirements, and the online concentration analyzer (near-infrared) shows that the NAM content fluctuates between 0.298% and 0.302%.

[0059] The granulation tower is operating normally and there is no blockage.

[0060] Example 4 (Feedback Correction) Based on Example 1, the online concentration analyzer at the outlet of the static mixer measured... ,deviation The PI controller calculates the correction factor. ,Pick Assuming the integral term is 0.00005 and the correction factor is approximately 1.00015, then... The rate was adjusted from 188.44 to 188.47 kg / h and recalculated. 5 minutes later It has recovered to 0.50%.

[0061] Comparative Example The traditional water-based solvent process involves preparing a 20% NAM solution with water, adding it to the inlet of the second-stage evaporator, and storing it in a tank for 24 hours. Comparative results: The product had a water content of 0.7% (exceeding the standard), a NAM decomposition rate of 7.2%, and a NAM content of only 0.23% (target 0.5%). Pitting corrosion occurred in the heating tubes of the second-stage evaporator. In contrast, the products from the embodiments of this invention have a water content ≤0.35%, a NAM decomposition rate <1%, and a NAM content deviation ≤±0.01%.

[0062] Example 5 (Feedforward + Feedback Control in Emergency Load Increase Scenario) In this embodiment, the simulation device was operating stably at a load of 37.5 t / h when it suddenly received an emergency task to increase the load to 41.25 t / h (+10%) within 1 minute.

[0063] Traditional feedback scheme: If only downstream concentration feedback is relied upon, the NAM concentration will drop from 0.50% to 0.45% within about 3 minutes after the load is increased, resulting in about 1.8 tons of non-compliant products (concentration below 0.48%). Subsequently, the PI controller slowly recovers, with a total transition product of about 2.5 tons.

[0064] The feedforward + feedback scheme of this invention: While the DCS system issues the command "load increased to 41.25 t / h", it simultaneously... Send to the feedforward control unit. The feedforward unit immediately calculates: Originally Originally The command was sent synchronously to the NAM urine pump along with the pump speed increase command (response time <0.5 seconds). Since the mechanical response times of the NAM urine pump and the molten urea pump are similar (approximately 2-5 seconds), their flow rates increased synchronously. The online concentration analyzer showed that during the entire load increase process, the maximum deviation in NAM concentration was only 0.48%-0.51%, and no non-conforming products were produced. The feedback correction unit made only extremely fine adjustments (<0.5%), and the system quickly stabilized.

[0065] To further verify the technical advantages of this invention, the present invention's solution (corresponding to the improved version of solution 6 below: 95% urine as solvent, added after the melt pump outlet, and equipped with a load adaptive control and rapid dissolution and separation system) is compared with six different production solutions in the prior art. The key parameters and results of each solution are as follows: Comparative Example 1 (Scheme 1: Steam condensate is used as a solvent and added before the two-stage evaporation heater) Solvent: Steam condensate (pure water); Point of application: Before the second-stage evaporator heater; Main impacts: Biuret: No increase; Moisture content: The product has a high moisture content due to the introduction of additional moisture. Production conditions: High steam consumption, requiring both heat preservation steam and dehydration steam; the solution passes through both a two-stage evaporation system and a molten urea pump, resulting in heavy equipment load; Conclusion: The product quality is guaranteed, but the solution path is long, energy consumption is high, and the introduction of moisture affects the product's storage stability.

[0066] Comparative Example 2 (Scheme 2: Steam condensate is used as a solvent and added before the molten urea pump) Solvent: Steam condensate; Point of addition: Before the molten urea pump; Main impacts: Biuret: Increased by 3.5‰ × 2 × 100% = 0.7% (seriously exceeding the standard); Moisture content: Significantly increased; product moisture content is high. Production conditions: Steam consumption is slightly high, only heat preservation steam is needed; the solution only passes through the molten urea pump; Conclusion: The product has high moisture content and is not feasible. The increase in biuret is too large, making it impossible to meet the standards for superior-grade urea.

[0067] Comparative Example 3 (Scheme 3: 75% urine as solvent, added before the two-stage evaporation heater) Solvent: 75% urea solution; Point of application: Before the second-stage evaporator heater; Main impacts: Biuret: No increase; Moisture content is reduced compared to pure water solvent, but some moisture is still introduced; Production conditions: The solution viscosity is slightly high, which increases the difficulty of preparation and transportation and makes it prone to clogging; the steam consumption is low, requiring both heat preservation steam and dehydration steam; the solution passes through both a two-stage evaporation system and a molten urea pump. Conclusion: The product quality is good, but the solution path is long and the high viscosity can easily lead to pipeline blockage risk.

[0068] Comparative Example 4 (Scheme 4: 75% urine as solvent, added before the molten urea pump) Solvent: 75% urea solution; Point of addition: Before the molten urea pump; Main impacts: Biuret: Increase of 3.5‰ × 2 × 25% = 0.175%; Moisture content: Still too high; Production conditions: Same as Scheme 3, but the solution has high viscosity and is prone to clogging; steam consumption is low, requiring only insulation steam; the solution only passes through the molten urea pump. Conclusion: The product has high moisture content, making the proposed solution unfeasible. The increase in biuret is significant, and the moisture content exceeds the standard.

[0069] Comparative Example 5 (Scheme 5: 95% urine as solvent, added before the two-stage evaporation heater) Solvent: 95% urea solution (close to the solvent used in this invention); Point of application: Before the second-stage evaporator heater; Main impacts: Biuret: No increase; Moisture: Because the solvent is highly concentrated urine, very little moisture is introduced; Production conditions: The solution viscosity is slightly high, which brings difficulties to the equipment and transportation, and is prone to clogging; the steam consumption is low, and both heat preservation steam and dehydration steam are required; the solution passes through both a two-stage evaporation system and a molten urea pump. Conclusion: The product quality is acceptable, but the solution path is long and the high viscosity leads to a serious risk of clogging, resulting in poor system stability.

[0070] Comparative Example 6 (Scheme 6: 95% urine as solvent, added before the molten urea pump) Solvent: 95% urea solution; Point of addition: Before the molten urea pump; Main impacts: Biuret: Increased by 3.5‰ × 2 × 5% = 0.035% (slight increase, still within the acceptable range); Moisture content: extremely low, meeting the requirements for superior grade; Production conditions: The solution viscosity is slightly high, but it only passes through the molten urea pump; steam consumption is low, only heat preservation steam is needed; Conclusion: The product quality is acceptable, but there is still a slight increase in biuret, and the load adaptation problem remains unresolved.

[0071] The comparison between the present invention and various comparative examples is summarized in the following table: The comparison shows that: Using pure water as a solvent (Comparative Examples 1 and 2) would lead to an increase in product moisture content or a serious exceedance of biuret, which is not feasible; Although using 75% urine as a solvent (Comparative Examples 3 and 4) reduces the introduction of water, it has high viscosity, is prone to clogging, and will still produce a lot of biuret when added before the pump. Using 95% urine as a solvent, but adding it before the second stage (Comparative Example 5), the risk of blockage is extremely high, and the solution path is long; Although the closest comparative example to this invention, 6 (95% urine + added before pump), has low moisture content and controllable biuret increment, NAM and urea still have a relatively long contact time in the pump and in the post-pump pipeline, which poses a certain risk of side reactions and has no load self-adaptation capability.

[0072] In summary, the automatic NAM dosing system for stable urea production under different loads provided by this invention is suitable for new urea plants or the technical upgrading of existing plants. Through innovative designs such as using urea solution as a solvent, rapid dissolution via stirring and centrifugation, and adaptive load control, this system significantly reduces NAM decomposition losses and product moisture content, improves dosing accuracy and production flexibility, and offers high economic and environmental benefits. It is particularly suitable for producing differentiated products such as stable urea and controlled-release fertilizers.

[0073] Finally, it should be noted that the above preferred embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail through the above preferred embodiments, those skilled in the art should understand that various changes can be made to it in form and detail without departing from the scope defined by the claims of the present invention.

Claims

1. A NAM automatic dosing system under different loads for stable urea production, comprising a heater, a first evaporation separator, a second evaporation separator, a urea melting pump, a static mixer and a urea prilling tower, characterized in that, It also includes a centrifuge with a built-in dissolving tank, a dissolving liquid metering and regulating valve, a NAM urine pump, and a load calculation module; the NAM automatic dosing system performs the following operations during operation: The dissolving solution metering control valve diverts a portion of the urea solution from the first stage of evaporation and heating separation completed in the upstream urea synthesis process as NAM dissolving solution according to the real-time demand of NAM dissolving solution, and uses the remaining urea solution to continue to complete the second stage of heating and evaporation separation to obtain molten urea; Molten urea is fed into a urea melting pump through a pipeline, where it is pressurized and transported towards the urea granulation tower. The real-time delivery flow rate of the urea melting pump is recorded as the production load, and the dosing flow rate of the NAM urea pump is constrained according to the production load and the target concentration of NAM. The real-time demand for NAM solvent is determined based on the addition flow rate, and then the real-time demand for NAM solution is calculated. An additional stirring and centrifugation process is added, in which solid NAM and dissolved NAM are introduced during the rotation of the stirring centrifuge, and the dissolved NAM is separated from the solid NAM immediately. The separated dissolved additive is then pumped to the direction of the urea granulation tower by a NAM urea pump pressurization. The injection point of the dissolved additive is set after the outlet of the urea melting pump and before the static mixer. The soluble additive and molten urea are rapidly mixed using a static mixer to obtain a mixture containing the additive, which is then directly fed into the urea granulation tower for granulation.

2. A NAM automatic dosing system under different loads for stable urea production according to claim 1, characterized in that, The NAM solution is a 95% aqueous urea solution, and the water content of the molten urea should be less than 1%.

3. A NAM automatic dosing system under different loads for stable urea production according to claim 1, characterized in that, The extraction amount of the NAM solution is determined based on the amount of solid NAM additive used in subsequent synthesis processes, and the extraction is completed at a ratio of 2:1 between the NAM solution and the solid NAM additive.

4. The automatic NAM dosing system for stable urea production under different loads according to claim 1, characterized in that, The dissolved additive is separated and transported to the urea granulation tower within 10-15 minutes.

5. The automatic NAM dosing system for stable urea production under different loads according to claim 1, characterized in that, The dissolution temperature of NAM solution and NAM solid is controlled at 90-120 degrees Celsius.

6. The automatic NAM dosing system for stable urea production under different loads according to claim 1, characterized in that, The volume of the built-in dissolving tank in the stirred centrifuge is related to the historical average production load. The volume of the dissolving tank corresponding to a higher historical average production load is greater than the volume of the dissolving tank corresponding to a lower historical average production load.

7. The automatic NAM dosing system for stable urea production under different loads according to claim 1, characterized in that, The load calculation module performs the following operations: The mass flow rate of the urea melting pump, pre-set for this production run, is recorded as the production load. And obtain the NAM content of the urea granules expected to be produced during this production, denoted as the target concentration of NAM. ; According to production load and target concentration Constraint on the mass flow rate of the NAM urine pump : The dosage flow rate is taken as the lower limit of the NAM solvent output required by the stirred centrifuge, and is denoted as the real-time demand for NAM solvent. ; Obtain the solubility ratio R of NAM solution to solid NAM additive, combined with the real-time demand for NAM solvent. The real-time demand for NAM solution was calculated. : In the formula, the ratio of NAM solution to solid NAM additive is 2:1, i.e., R=2.

8. The automatic NAM dosing system for stable urea production under different loads according to claim 7, characterized in that, The load calculation module also includes a feedback correction unit, used to receive the actual NAM concentration of the mixture measured by an online concentration analyzer installed at the outlet of the static mixer. The concentration deviation was calculated. : If concentration deviation If the preset deviation range is not met, the mass flow rate of the NAM urine pump will be adjusted according to the PI control algorithm. Make corrections and recalculate the real-time demand for NAM solution based on the corrected mass flow rate.

9. The automatic NAM dosing system for stable urea production under different loads according to claim 7, characterized in that, The load calculation module also includes a feedforward control unit, which communicates with the load setting device of the urea melting pump to obtain the setting change signal of the production load in real time. When a change in the production load setting value is detected, the feedforward control unit immediately recalculates the mass flow rate of the NAM urea pump and the real-time demand of the NAM solution based on the new production load setting value, and synchronously outputs control commands to the NAM urea pump and the solution metering regulating valve.

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