Polyurethane bag film urea tower air cooling device and method

Abstract

This invention discloses a tower-type air-cooled cooling device and method for polyurethane-coated urea film, belonging to the field of controlled-release fertilizer processing technology. The tower body of this invention is sequentially arranged from top to bottom as a slow-cooling pre-cooling zone, a critical curing zone, and a strong-cooling dispersion zone. The device also includes three supply fans, one exhaust fan, four pulse valves, a material flow detection device, a temperature sensor group, and a controller. The four pulse valves at the bottom of the strong-cooling dispersion zone are diagonally grouped along the circumference of the tower body. The method of this invention sets different target cooling rates for the three zones based on the glass transition temperature and critical cooling rate of the polyurethane film. The controller alternately controls the diagonally grouped pulse valves according to the first and second on / off cycles, and adjusts the operating frequency of the supply fans in each zone using a combination of feedforward and feedback. This invention simultaneously suppresses film cracking and particle adhesion within the same device, significantly improving film integrity, anti-adhesion dispersion effect, and consistency of controlled-release performance compared to existing processes.

Description

Technical Field

[0001] This invention belongs to the field of controlled-release fertilizer processing technology, specifically relating to a polyurethane-coated film urea tower-type air-cooled cooling device and method. Background Technology

[0002] Polyurethane-coated urea is one of the mainstream controlled-release fertilizers. Its preparation process involves spraying isocyanate and polyol onto the outer surface of urea granules in a rotary drum reactor or sputtered bed reactor, where an addition polymerization reaction occurs, forming a polyurethane film on the urea granules. The polyurethane film-forming reaction is exothermic, and the reaction temperature is typically higher than the glass transition temperature of the polyurethane film used. After film formation, the coated urea granules are in a softened state, and the polyurethane film on the outer layer of the granules has a certain degree of stickiness, making it easy for granules to adhere and further agglomerate. If the material after film formation is not cooled promptly and the granules are not kept dispersed during the cooling process, the film layer of the coated urea will develop microcracks due to insufficient thermal stress relaxation, losing its controlled-release function, or it will become unusable due to the formation of agglomerates that cannot be properly screened, thus affecting the product's yield and production capacity.

[0003] Currently, the coating urea cooling devices used in the industry mainly include three types: drum coolers, conventional fluidized bed coolers, and single-stage tower coolers. Drum coolers rely on cooling air being circulated inside a slowly rotating drum for heat exchange, but suffer from uneven cooling, easy adhesion and scaling on the inner wall of the drum, and low heat exchange efficiency. Conventional fluidized bed coolers use a single air chamber for uniform ventilation, resulting in acceptable particle dispersion, but this type of cooler makes it difficult to control the cooling rate in stages. During the process of crossing the glass transition temperature, the viscoelasticity and mechanical properties of the polyurethane film undergo significant changes. If the cooling rate is too fast during this stage, the film will develop microcracks due to insufficient thermal stress relaxation and lose its slow-release effect; if the cooling rate is too slow, the film will remain in a softened state for a long time, accumulating and causing particle adhesion and agglomeration. Single-stage tower coolers have a relatively simple structure, but the entire process is a single cooling zone, making it difficult to simultaneously meet the conflicting process requirements of preventing film cracking and preventing particle adhesion.

[0004] Therefore, existing technologies for cooling polyurethane-coated urea have the following shortcomings: First, existing cooling devices struggle to precisely control the cooling rate in segments according to the thermal properties of the polyurethane film, particularly its glass transition temperature. Consequently, the contradictory failure modes of film cracking and particle adhesion are difficult to suppress simultaneously within the same device. Second, existing cooling devices employ relatively simple air inlet and distribution methods, making it difficult to provide directionally controllable shear momentum for already adhered particle clusters. Cooling air tends to form fixed flow channels within the particle bed, making it difficult to maintain consistent particle dispersion across different spatial locations. Third, existing cooling devices often employ open-loop control or a single feedback control method, resulting in poor adaptability to fluctuations in material inlet temperature, changes in material mass flow rate, and seasonal variations in ambient temperature. Material temperature overshoot is prone to occur during the start-up and disturbance occurrence phases, and the sustained-release performance of the finished product is difficult to maintain consistently between batches. Summary of the Invention

[0005] To address the problems existing in the background art, the present invention provides a tower-type air-cooled cooling device for polyurethane-coated urea, used for cooling polyurethane-coated urea, including a tower body, three air supply fans, one exhaust fan, four pulse valves, a material flow detection device, a temperature sensor group, and a controller.

[0006] The tower body is divided into three zones from top to bottom: Zone 1, Zone 2, and Zone 3. Zone 1 is a slow cooling and pre-cooling zone, Zone 2 is a critical solidification zone, and Zone 3 is a strong cooling and dispersion zone. An upper annular flow equalization baffle is provided between Zone 1 and Zone 2, and a lower annular flow equalization baffle is provided between Zone 2 and Zone 3.

[0007] The top of the tower body is equipped with a feed inlet, and the bottom of the tower body is equipped with a conical discharge section and a discharge outlet from top to bottom. The bottom end of the conical discharge section is equipped with a rotary discharge valve. The side wall of the first zone is equipped with an air outlet and a first air inlet, with the air outlet located above the upper annular flow equalization baffle. The side wall of the second zone is equipped with a second air inlet. The bottom of the third zone is equipped with four third air inlets at equal intervals along the circumference of the tower body.

[0008] The three air supply fans include a first air supply fan, a second air supply fan, and a third air supply fan. The first air supply fan is connected to the first air inlet via a pipeline, the second air supply fan is connected to the second air inlet via a pipeline, and the third air supply fan is connected to the four third air inlets via branch pipelines. The exhaust fan is connected to the air outlet via a pipeline. The exhaust fan is used to keep the inside of the tower under negative pressure relative to atmospheric pressure.

[0009] Four pulse valves are installed on the pipes of the four third air inlets. Two pulse valves that are diagonally opposite each other along the circumference of the tower body form the first group, and the other two pulse valves that are diagonally opposite each other along the circumference of the tower body form the second group.

[0010] The material flow detection device is located at the feed inlet and is used to detect the mass flow rate of polyurethane-coated urea. The temperature sensor group includes temperature sensors located at the feed inlet, zone 1, zone 2, zone 3, and discharge outlet respectively; the controller is electrically connected to 3 air supply fans, 4 pulse valves, rotary discharge valve, material flow detection device, and the temperature sensor group respectively.

[0011] In a preferred embodiment, both the upper annular flow equalization baffle and the lower annular flow equalization baffle are provided with material discharge holes at their centers. An airflow distribution plate is provided around the material discharge holes, and the airflow distribution plate has a plurality of airflow holes. The upper surface of the airflow distribution plate slopes downward from its outer edge toward the material discharge holes. The material discharge holes allow the polyurethane-coated urea to pass downward. The airflow holes allow cooling air to pass through, and the diameter of the airflow holes is smaller than the particle size of the polyurethane-coated urea.

[0012] The first air inlet and the second air inlet are offset from each other in the circumferential orientation of the tower body; the four pulse valves are electrically adjustable butterfly valves, which are driven by the pulse width modulation signal output by the controller and used to adjust the opening degree; the air outlet is provided with a wire mesh, the pore size of which is smaller than the particle size of the polyurethane-coated urea, and the wire mesh is used to prevent the polyurethane-coated urea from being discharged from the tower body with the airflow.

[0013] The present invention also provides a polyurethane-coated film urea tower-type air-cooling method, comprising the following steps:

[0014] S1. Determine the glass transition temperature and critical cooling rate of the polyurethane film contained in the polyurethane-coated urea, and input the glass transition temperature and the critical cooling rate into the controller;

[0015] S2. The controller sets the target cooling rates for Zone 1, Zone 2, and Zone 3 respectively based on the critical cooling rate;

[0016] S3, The controller determines the material mass flow rate. Based on the target cooling rates of the first zone, the second zone, and the third zone, the initial frequency values ​​of the three fan supply fans are determined respectively.

[0017] S4. The controller starts the three supply fans, the exhaust fans and the four pulse valves, so that the pulse valves of the first group and the pulse valves of the second group are alternately turned on and off in diagonal groups, forming a periodic turbulent airflow in the third zone;

[0018] S5. The polyurethane-coated urea is continuously fed into the tower body through the feed inlet, so that the polyurethane-coated urea flows sequentially through the first zone, the second zone, and the third zone.

[0019] S6. The controller collects the material temperatures at the feed inlet, in zone 1, in zone 2 and in zone 3 measured by the temperature sensor group according to the sampling period, adjusts the working frequency of the three blowers respectively in a combination of feedforward and feedback, and adjusts the pulse frequency of the four pulse valves according to the material temperature at the feed inlet, the material temperature in zone 3 and the glass transition temperature.

[0020] S7. The controller controls the rotary unloading valve to discharge the cooled polyurethane-coated urea through the discharge port.

[0021] Furthermore, step S1 includes the following sub-steps:

[0022] S11. Using a differential scanning calorimeter, thermal analysis is performed on the polyurethane film sample at a heating rate of 10℃ / min. The glass transition temperature of the polyurethane film is determined from the midpoint temperature of the glass-to-elastic transition step in the obtained heat flow-temperature curve, and this glass transition temperature is denoted as... , The unit is ℃;

[0023] S12. The critical cooling rate of the polyurethane film is denoted as... , The unit is ℃ / s. Determined by one of the following methods: Method 1, using the differential scanning calorimeter, thermal analysis is performed on the polyurethane film sample at heating rates of 0.5℃ / s, 1℃ / s, 2℃ / s, 3℃ / s, and 5℃ / s, respectively, where the polyurethane film is located... The highest heating rate among those corresponding to the relaxation peaks nearby without distortion is selected as the [value]. The value of; Method 2, based on the chemical composition and thickness of the polyurethane film, The values ​​are taken as empirical values ​​within the range of 1.5℃ / s to 3℃ / s;

[0024] S13, Set the glass transition temperature and the critical cooling rate Input the controller to obtain the cooling rate reference of the method.

[0025] Furthermore, the target cooling rates of the first zone, the second zone, and the third zone are respectively denoted as... , , , , , The units are all ℃ / s; S2 includes the following sub-steps:

[0026] S21, The controller sets the target cooling rate of the first zone. Set as the critical cooling rate The polyurethane-coated urea is pre-cooled in the first zone at a cooling rate lower than the critical cooling rate by 0.4 to 0.6 times.

[0027] S22, The controller sets the target cooling rate of the second zone. Set as the critical cooling rate The polyurethane-coated urea is cooled at a rate of 0.9 to 1.0 times the critical cooling rate within the second zone, allowing it to pass through the glass transition temperature at a rate not exceeding the critical cooling rate. The polyurethane film is then cured in a glassy state.

[0028] S23, The controller sets the target cooling rate of the third zone. Set as the critical cooling rate The polyurethane-coated urea is cooled to the target outlet temperature in the third zone by 1.5 to 2.0 times the concentration of the polyurethane-coated urea.

[0029] Furthermore, step S3 includes the following sub-steps:

[0030] S31, The controller calculates the downward linear velocity of the polyurethane-coated urea within the tower body using the following formula. :

[0031] ;

[0032] In the formula, The downward linear velocity of the polyurethane-coated urea within the tower body. The unit is m / s; The mass flow rate of the material; The bulk density of the polyurethane-coated urea is given. The unit is kg / m 3 ; The effective cross-sectional area of ​​the tower body is... The unit is m 2 ; The percentage of the polyurethane-coated urea in the tower body by volume. For dimensionless parameters, The value range is from 0.05 to 0.70. The specific value is obtained through the cold calibration of the device;

[0033] S32, the controller calculates the following formula: Cooling air volume of the zone :

[0034] ;

[0035] In the formula, For the district number, Choose 1, 2, or 3; For the first Cooling airflow in the area The unit is m 3 / s; The mass flow rate of the material; The specific heat capacity of the polyurethane-coated urea is given. The unit is J / (kg·K); For the first The target cooling rate of the zone; For the first Material bed height in the zone The unit is m; For the density of cooling air, The unit is kg / m 3 ; The specific heat capacity of cooling air. The unit is J / (kg·K); For the first The allowable temperature rise of the cooling air in the zone The unit is K; The downlink linear velocity obtained from S31;

[0036] S33. The controller, based on the airflow-frequency characteristic curves of the three fans, calculates the cooling airflow obtained in S32. Converted to the corresponding number Initial frequency of the district's ventilation fans , The unit is Hz. Take 1, 2, and 3; the initial frequency values ​​of the air supply fans corresponding to Zone 1, Zone 2, and Zone 3 are respectively denoted as... , , .

[0037] Furthermore, step S4 includes the following sub-steps:

[0038] S41, the controller operates according to the initial frequency value. , , Start the first air supply fan, the second air supply fan, and the third air supply fan respectively, so that the first air supply fan supplies cooling air to the first zone, the second air supply fan supplies cooling air to the second zone, and the third air supply fan divides the cooling air into four paths through the branch pipeline and sends them to the upstream side of the four pulse valves.

[0039] S42. The controller starts the exhaust fan, so that the inside of the tower is under negative pressure relative to atmospheric pressure, so that the cooling air inside the tower is discharged through the air outlet.

[0040] S43. The controller alternately controls the pulse valves of the first group and the pulse valves of the second group according to the first on-off cycle and the second on-off cycle. The first on-off cycle is when the pulse valves of the first group are open and the pulse valves of the second group are closed. The second on-off cycle is when the pulse valves of the first group are closed and the pulse valves of the second group are open, so that the airflow direction in the third zone is periodically deflected to obtain the periodic disturbed airflow.

[0041] Furthermore, step S5 includes the following sub-steps:

[0042] S51, the polyurethane-coated urea is expressed at the material mass flow rate. The feed continuously enters the first zone through the feed inlet;

[0043] S52. The polyurethane-coated urea undergoes convective heat exchange with cooling air in the first zone, according to the target cooling rate. Cool the target temperature to above the glass transition temperature. The temperature is adjusted to obtain the pre-cooled material;

[0044] S53. The pre-cooled material enters the second zone through the upper annular flow equalization baffle and is cooled at the target rate. To achieve the target cooling temperature across the glass transition temperature The polyurethane film is then cured in a glassy state to obtain the cured material.

[0045] S54. The cured material enters the third zone through the lower annular flow equalization baffle, and is disturbed by the periodic turbulent airflow, cooling at the target rate. The urea is cooled to the target outlet temperature to obtain the cooled polyurethane-coated urea.

[0046] Furthermore, the material temperature at the feed inlet collected by the temperature sensor group is recorded as follows: The material temperatures in zones 1, 2, and 3 collected by the temperature sensor group are respectively recorded as follows: , , , , , , The units are all in °C; S6 includes the following sub-steps:

[0047] S61, the controller calculates the following formula: The material set temperature of the zone :

[0048] ;

[0049] In the formula, For the district number, Choose 1, 2, or 3; For the first The material set temperature in the zone, The unit is ℃; For the first The material inlet temperature of the zone, The unit is ℃. The value of is equal to , The value of is equal to , The value of is equal to ; For the first The target cooling rate of the zone; For the first Material residence time in the area, The unit is s. according to Calculated;

[0050] S62, the controller operates according to the sampling period. The material temperatures in the first, second, and third zones are collected. , , The sampling period The value range is from 1s to 5s; and the value of the first is calculated according to the following formula. Material temperature deviation in the zone :

[0051] ;

[0052] In the formula, For the district number, Choose 1, 2, or 3; For the first The material temperature deviation in the current sampling period. The unit is ℃; The temperature sensor group collects the first data in the current sampling period. The material temperature in the zone; The first obtained for S61 The material set temperature in the zone;

[0053] S63. The controller calculates the first step according to the incremental proportional-integral-derivative control algorithm using the following formula: Frequency increment of the district ventilation fan in the current sampling period :

[0054] ;

[0055] In the formula, For the district number, Choose 1, 2, or 3; For the first The frequency increment of the district's ventilation fans in the current sampling period The unit is Hz; For the first The proportion coefficient of the area, The unit is Hz / ℃; For the first The integral coefficient of the region, The unit is Hz / (℃·s); For the first The differential coefficients of the region, The unit is Hz·s / ℃; The current sampling period obtained by S62 is the first... Material temperature deviation in the zone; For the first The material temperature deviation in the area during the previous sampling period; For the first The material temperature deviation in the area during the previous sampling period; The sampling period is defined as follows; when or When the corresponding sampling period does not exist, the value of the corresponding variable is 0;

[0056] S64, the controller updates the following formula: Operating frequency of district ventilation fans and the operating frequency Output to the corresponding number The district's ventilation fans:

[0057] ;

[0058] In the formula, For the district number, Choose 1, 2, or 3; For the first The operating frequency of the district's ventilation fans during the current sampling period. The unit is Hz; For the first The operating frequency of the district's ventilation fans in the previous sampling period. The value taken in the initial calculation is the [number]. Initial frequency value of the district ventilation fan ; The current sampling period obtained by S63 is the first... Frequency increment of district-level ventilation fans;

[0059] S65, the controller calculates the pulse frequency of the four pulse valves according to the following formula. :

[0060] ;

[0061] In the formula, The pulse frequency of the four pulse valves. The unit is Hz. The value range is limited to 0.25Hz to 2.0Hz. When the calculated value according to the above formula is lower than 0.25Hz... Taking 0.25Hz, when the calculated value according to the above formula is higher than 2.0Hz... Select 2.0Hz; Based on the pulse frequency, The unit is Hz. The value range is from 0.3Hz to 0.8Hz; The temperature response coefficient, For dimensionless parameters, The value range is from 0.6 to 1.2; The temperature of the material in the third zone is collected by the temperature sensor group in the current sampling period; It is the glass transition temperature; The temperature of the material at the feed inlet is collected by the temperature sensor group during the current sampling period;

[0062] S66, The controller calculates the duty cycle of the four pulse valves according to the following formula. :

[0063] ;

[0064] In the formula, The duty cycle of the four pulse valves is... It is a dimensionless parameter; The mass flow rate of the material detected by the material flow rate detection device in the current sampling period; The rated material mass flow rate of the device. The unit is kg / s. The value range is 0 to Correspondingly The value range is from 0.4 to 0.5;

[0065] S67, the controller operates according to the pulse frequency obtained in S65. The duty cycle obtained from S66 Pulse control signals are output to the four pulse valves, causing the four pulse valves to operate at the specified pulse frequency. and the duty cycle It operates alternately according to the first on / off cycle and the second on / off cycle.

[0066] Furthermore, step S7 includes the following sub-steps:

[0067] S71. The cooled polyurethane-coated urea is collected from the bottom of the third zone through the conical discharge section;

[0068] S72. The controller controls the rotary unloading valve to rotate according to a preset unloading frequency, so that the polyurethane-coated urea collected in the conical discharge section is continuously discharged through the discharge port.

[0069] The beneficial effects achieved by this invention are as follows:

[0070] This invention designs a tower-type air-cooled cooling device for polyurethane-coated urea. Inside the same tower, three functional zones are set from top to bottom along the height direction: a slow cooling pre-cooling zone, a critical curing zone, and a strong cooling dispersion zone. The target cooling rate of each zone is limited by different proportions according to the glass transition temperature and critical cooling rate of the polyurethane film. This allows the polyurethane-coated urea to smoothly decrease its inlet high temperature in the slow cooling pre-cooling zone at a cooling rate lower than the critical cooling rate. In the critical curing zone, the urea passes through the glass transition temperature and completes the glass curing of the polyurethane film at a cooling rate not exceeding the critical cooling rate. In the strong cooling dispersion zone, the urea is cooled to the target outlet temperature at a cooling rate higher than the critical cooling rate. Because the slow cooling pre-cooling zone applies a cooling rate below the critical cooling rate to the material, the thermal stress in the inner and outer layers of the film can be relaxed in time, and the initiation of microcracks in the initial stage of cooling is suppressed. Because the critical curing zone limits the cooling rate to near the critical cooling rate, neither too fast nor too slow, the residual stress in the polyurethane film when crossing the glass transition temperature is controlled, and the residence time of the film in the viscous temperature zone is also compressed. Thus, the two contradictory failure modes of film cracking and particle adhesion are suppressed simultaneously in the same device. Because the strong cooling dispersion zone applies a cooling rate above the critical cooling rate to the cured material, the temperature of the material at the discharge port is further reduced to a level suitable for subsequent screening and packaging, and the finished product is less likely to absorb moisture and clump in subsequent processes.

[0071] This invention arranges four third air inlets and four corresponding pulse valves at equal intervals along the circumference of the tower body at the bottom. The four pulse valves are divided into a first group and a second group based on their diagonal relative positions on the circumference of the tower body. A controller alternately controls the two groups of pulse valves according to a first on / off cycle and a second on / off cycle. The two pulse valves in the first group and the two pulse valves in the second group are distributed along two mutually perpendicular diagonal lines on the circumference of the tower body. The main airflow direction corresponding to the opening of the first group and the opening of the second group are respectively located on these two mutually perpendicular diagonal lines. The main airflow direction at the bottom of the tower thus periodically switches between these two perpendicular diagonal lines according to the pulse frequency. At any given moment, one group of pulse valves is open while the other is closed, ensuring a continuous supply of cooling air without interruption during the main airflow switching process. This prevents localized heat regeneration caused by cooling interruption in the particle bed within the strong cooling dispersion zone. Under the action of switching of the mainstream direction, the particle bed continuously bears the shear stress of directional change. The particle clusters formed due to the initial surface adhesion are gradually broken up under the cumulative effect of alternating directional shear. The dispersion effect of different spatial positions in the particle bed tends to be consistent. Compared with the fixed flow channel formed by the constant air distribution method, the stability of the particle anti-sticking and dispersion effect is improved.

[0072] In the method step, this invention directly calculates the required cooling air volume for each zone based on the material mass flow rate, target cooling rate, and tower geometry parameters, and then converts it into the initial frequency value of the air supply fan for each zone. During the startup phase, the controller drives the air supply fan of each zone to operate near the static operating point required for thermal balance using the initial frequency value. The controller also collects the material temperature of each zone according to the sampling period, calculates the frequency increment based on the initial frequency value using an incremental proportional-integral-derivative control algorithm, and updates the operating frequency of the air supply fan of each zone accordingly. Because the initial frequency calculation directly incorporates the real-time measurable disturbance variable of material mass flow rate and the pre-designable process variable of target cooling rate, the device approaches its steady-state operating point during startup, thus compressing material temperature overshoot during startup. The feedback correction loop only outputs the frequency increment rather than the absolute frequency value, and the inverter internally maintains the operating frequency itself. This suppresses the impact of the cumulative error of integral action on the control output. The system has the ability to eliminate steady-state deviations online for unmodeled disturbances such as material temperature jumps at the inlet, material mass flow rate fluctuations, and seasonal changes in ambient temperature. The pulse frequency is adaptively adjusted according to the margin of the material temperature relative to the glass transition temperature in the strong cooling dispersion zone, and the duty cycle is adaptively adjusted according to the material mass flow rate. A dynamic match is formed between the intensity of the disturbance airflow and the cooling load, thereby improving the device's adaptability and robustness to changes in operating conditions.

[0073] The aforementioned three-section partitioned tower structure, pulsed diagonal grouping alternating air distribution structure, segmented airflow feedforward calculation, and composite control method combining feedforward and feedback work together to simultaneously achieve polyurethane film integrity protection, particle anti-adhesion dispersion, reduced unit material cooling energy consumption, and consistent batch-to-batch nutrient release rate during continuous operation of the polyurethane film-coated urea tower-type air-cooled cooling device and method of this invention. The cooling rate is strictly constrained in the viscous temperature range and moderately increased after glass curing, and the unit material cooling energy consumption is correspondingly reduced without loss of film integrity. Because particle clusters are continuously dispersed in the strong cooling dispersion zone, the adhesion rate of the finished product remains at a low level throughout the entire steady-state operating range of the device. Film integrity is maintained under different implementation conditions, and the relative standard deviation of nutrient release rate in the sustained-release performance evaluation of different batches of finished products is at a low level, ensuring batch-to-batch consistency of the sustained-release performance of the finished product. This invention is adaptable to polyurethane films with different chemical compositions and film thicknesses, as well as operating conditions with different rated capacity and material mass flow rates. Attached Figure Description

[0074] Figure 1 Examples 1, 2, and 3 are different from Comparative Examples 5 and 6. A comparison of film integrity and adhesion rate under different values, where (a) shows the film integrity rate as a function of... The scatter plot of the change, (b) shows the adhesion rate as a function of... A scatter plot showing the changes.

[0075] Figure 2 Examples 1, 1, and 4 are in Comparison of material temperature response in three zones under a step disturbance of +10℃, where (a) is the material temperature in zone 1. Response curve, (b) shows the material temperature in zone 2. Response curve, (c) is the material temperature in zone 3. Response curve graph.

[0076] Figure 3 This is an error bar graph showing the change in particle clustering rate as a function of device operating time for Examples 1, 2, and 3.

[0077] Figure 4 The bar charts are comparison charts of the comprehensive performance indicators of Examples 1 to 4 and Comparative Examples 1 to 6, where (a) is a comparison chart of adhesion rate, (b) is a comparison chart of film integrity rate, (c) is a comparison chart of cooling energy consumption per ton of material, and (d) is a comparison chart of the relative standard deviation of nutrient release rate over 28 days.

[0078] Figure 5 This is a schematic diagram of the structure of the polyurethane-coated film urea tower-type air-cooled cooling device of the present invention. Detailed Implementation

[0079] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings. In addition, the forms of the various structures described in the following embodiments are merely illustrative. The present invention is not limited to the structures described in the following embodiments. All other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0080] Before describing this embodiment in detail, a brief explanation of the specific terms and concepts involved is provided. Polyurethane-coated urea refers to controlled-release fertilizer granules obtained by the addition polymerization reaction of isocyanate and polyol on the outer surface of urea particles, followed by curing to form a film, with the outer layer covered by a polyurethane film. The glass transition temperature is denoted herein as... It is the characteristic temperature at which amorphous polymer materials transition from a rubbery state to a glassy state. It is the critical temperature point at which the macroscopic mechanical properties and specific heat capacity of the thin film undergo a step change. The critical cooling rate is denoted as in this paper. It is a polyurethane film that traverses The highest tolerable cooling rate that does not produce microcracks or film adhesion within the specified range. Differential scanning calorimetry (DSC) is a standard thermal analysis instrument based on differential heat flow rate measurement to determine the thermal behavior of materials, such as melting, crystallization, and glass transition. Pulse width modulation (PWM) signal refers to a digital control signal that adjusts the average output of an actuator by periodically adjusting the duty cycle of a rectangular pulse. Incremental proportional-integral-derivative (PID) control algorithm, also known as incremental PID algorithm, differs from positional PID algorithm in that the output is the increment of the control quantity rather than the full quantity. Actuators with internal holding functions, such as frequency converters, have better compatibility with incremental output.

[0081] The polyurethane-coated urea tower-type air-cooled cooling device of the present invention is used to cool polyurethane-coated urea, and includes a tower body, three supply fans, one exhaust fan, four pulse valves, a material flow detection device, a temperature sensor group, and a controller. See also... Figure 5The tower body is a vertical cylindrical container, preferably constructed from rolled and welded carbon steel plates lined with stainless steel plates. The outer wall of the tower body should be covered with rock wool or rigid polyurethane foam insulation to reduce the impact of environmental thermal disturbances on the temperature field of the material inside the tower. The tower body is divided into three zones from top to bottom: Zone 1 is a slow cooling pre-cooling zone, Zone 2 is a critical solidification zone, and Zone 3 is a strong cooling dispersion zone. An upper annular flow equalization baffle is installed between Zone 1 and Zone 2, and a lower annular flow equalization baffle is installed between Zone 2 and Zone 3. The installation positions of the upper and lower annular flow equalization baffles inside the tower body determine the height ratio of each zone. The height of each zone should be determined based on the material's downward linear velocity and the designed residence time, ensuring that the axial dimension of Zone 2 is slightly larger than that of Zones 1 and 3, thus guaranteeing sufficient heat exchange time for the material near its glass transition temperature.

[0082] The top of the tower body has a feed inlet, and the bottom of the tower body has a conical discharge section and a discharge port from top to bottom. A rotary discharge valve is located at the bottom of the conical discharge section. The cone angle of the conical discharge section should be close to 45° to allow the cooled polyurethane-coated urea to smoothly collect along the conical surface to the discharge port under its own weight. The rotary discharge valve uses a star-shaped rotor as the material conveying component. Under the control of the controller, the star-shaped rotor rotates at a certain speed, which on the one hand isolates the interior of the tower body from the downstream section, and on the other hand allows the material to be discharged in batches and evenly, thereby maintaining a negative pressure environment inside the tower body.

[0083] The side wall of Zone 1 is equipped with an air outlet and a first air inlet. The air outlet is located above the upper annular flow equalization baffle, allowing the cooling air entering from the bottom to collect at the top of Zone 1 after passing through the material layer for convective heat exchange and then being discharged from the air outlet. The side wall of Zone 2 is equipped with a second air inlet. The bottom of Zone 3 has four third air inlets at equal intervals along the circumference of the tower body, with each pair of third air inlets spaced 90° apart.

[0084] Three air supply fans are designated as the first, second, and third air supply fans, all preferably centrifugal blowers equipped with frequency converters. The first air supply fan is connected to the first air inlet via piping and supplies cooling air to Zone 1. The second air supply fan is connected to the second air inlet via piping and supplies cooling air to Zone 2. The third air supply fan is connected to four third air inlets via branch piping. At the outlet of the third air supply fan, the branch piping divides the airflow into four paths, each leading to one of the four third air inlets at the bottom of Zone 3. An exhaust fan is connected to the outlet via piping. During operation, the exhaust fan maintains a negative pressure state inside the tower relative to atmospheric pressure. This prevents urea dust from leaking out of the tower through gaps into the workshop and creates a stable upward airflow, guiding the cooling air to flow counter-currently through the material layer along a path opposite to the downward flow of the material. The temperature gradient advantage of this counter-current flow is fully utilized along the path and directly applied to this device, thereby improving the heat exchange efficiency between the cooling air and the material.

[0085] Four pulse valves are installed on the pipes of the four third air inlets. Two pulse valves diagonally opposite each other along the circumference of the tower body form the first group, and the other two pulse valves diagonally opposite each other along the circumference of the tower body form the second group. Accordingly, the two pulse valves in the first group are 180° apart from each other on the circumference of the tower body, and the two pulse valves in the second group are also 180° apart from each other. The pulse valves in the first group and the second group are offset from each other by 90° on the circumference of the tower body.

[0086] The material flow detection device is located at the feed inlet and is used for online detection of the mass flow rate of polyurethane-coated urea. , The unit is kg / s. The material flow detection device should preferably be a speed sensor linked to the feed screw feeder, or an impact solid flow meter. The temperature sensor group includes temperature sensors located at the feed inlet, zone 1, zone 2, zone 3, and discharge outlet. Each temperature sensor should preferably be an armored PT100 or PT1000 platinum resistance thermometer. The armored housing effectively prevents dust abrasion and chemical corrosion, and the linearity of the platinum resistance thermometer meets the accuracy requirements of this device for temperature measurement. The controller is electrically connected to three blowers, an exhaust fan, four pulse valves, a rotary discharge valve, the material flow detection device, and the temperature sensor group. The controller receives the material mass flow signal from the material flow detection device, receives the temperature signals from each measuring point from the temperature sensor group, and outputs corresponding frequency control signals, switching signals, pulse width modulation signals, and speed control signals to the frequency converters of the three blowers, the exhaust fan, the four pulse valves, and the rotary discharge valve. The controller should be implemented using an industrial control platform such as a programmable logic controller, a distributed control system, or an industrial computer with real-time expansion capabilities. The internal operation of the controller involves the various calculation, acquisition, and output logics described in S1 to S7 below.

[0087] See appendix Figure 3 Both the upper and lower annular flow equalization baffles have material discharge holes at their centers. An airflow distribution plate with several airflow holes is located around the material discharge holes. The upper surface of the airflow distribution plate slopes downwards from its outer edge towards the material discharge holes. This slope guides the dispersed material from the upper section to the upper surface of the airflow distribution plate to the center, and then conveys it back to the lower section through the material discharge holes, thus maintaining the centralization of the material's downward path. The material discharge holes allow polyurethane-coated urea to pass downwards; the airflow holes allow cooling air to pass upwards. The diameter of the airflow holes is smaller than the particle size of the polyurethane-coated urea to prevent material from leaking into the lower section and forming a bypass. The opening ratio of the airflow holes on the airflow distribution plate should be matched with the required cooling airflow for each zone. The opening ratio of the airflow distribution plate above zone 1 should be smaller, and the opening ratio of the airflow distribution plate above zone 3 should be larger, thereby coordinating with the airflow distribution of the fans in each zone.

[0088] The first and second air inlets are offset from each other circumferentially within the tower body. The airflow directions of Zone 1 and Zone 2 do not coincide. After the cooling air enters the tower body through the first and second air inlets, the airflow direction rotates axially within the tower body, resulting in a spiral upward airflow. During the descent, the material is continuously scourd by the changing airflow, improving the lateral uniformity of material distribution. The four pulse valves are electrically adjustable butterfly valves, driven by pulse width modulation signals output from the controller and used to adjust their opening. The valve opening of the electrically adjustable butterfly valves can rapidly open and close near the average position determined by the duty cycle of the pulse width modulation signal, achieving a clear on / off switch while maintaining a certain flow area during the transition, thus avoiding momentary interruptions in the cooling airflow. The air outlet is equipped with a wire mesh. The mesh aperture is smaller than the particle size of the polyurethane-coated urea. The wire mesh is used to prevent the polyurethane-coated urea from being discharged from the tower with the airflow. At the same time, it also intercepts the fine polyurethane film fragments that may fall off during the film formation process, thereby maintaining the cleanliness of the air outlet duct and exhaust fan.

[0089] The polyurethane-coated urea tower-type air-cooling method of the present invention is implemented using the aforementioned polyurethane-coated urea tower-type air-cooling device. The steps of the method are described in detail below.

[0090] In step S1, the glass transition temperature and critical cooling rate of the polyurethane film contained in the polyurethane-coated urea are determined, and the glass transition temperature and critical cooling rate are input into the controller. Step S1 further includes sub-steps S11 to S13.

[0091] In S11, a differential scanning calorimeter was used to perform thermal analysis on the polyurethane film sample at a heating rate of 10℃ / min. The glass transition temperature of the polyurethane film was determined by the midpoint temperature of the glass-to-elastic transition step in the obtained heat flow-temperature curve. The glass transition temperature was denoted as... , The unit is °C. Polyurethane film during traversing... At this time, the molecular backbone segments transition from a frozen state to a freely rotating state, and the specific heat capacity then undergoes a step-like jump. This jump is represented as an approximately S-shaped step on the heat flow-temperature curve obtained from DSC testing; the temperature corresponding to the midpoint of the step is taken as... , can make The determination is not affected by the drift of the step start and end points caused by minor deviations in the test rate, ensuring the repeatability of the measurement results. The test was conducted at a heating rate of 10℃ / min, and the measured values ​​were... It has consistency and traceability at the regulatory level.

[0092] In S12, the critical cooling rate of the polyurethane film is denoted as... , The unit is ℃ / s. Determined using one of the following methods. In Method 1, a differential scanning calorimeter is used to perform thermal analysis on the polyurethane film sample at heating rates of 0.5℃ / s, 1℃ / s, 2℃ / s, 3℃ / s, and 5℃ / s, respectively, when the polyurethane film is located at... The highest heating rate among those corresponding to the relaxation peaks nearby without distortion is selected as the [value]. The value of . The morphology of the relaxation peak originates from the polyurethane molecular chain segments in The stress relaxation response in the vicinity shows distortion in the relaxation peak, indicating that the stress inside the film was not released in time through molecular rearrangement at the applied heating rate, and the film layer is already in a boundary state of microstructural instability. The highest heating rate at which the relaxation peak is not distorted is selected as the upper limit of the cooling rate that the film can withstand without structural damage; this upper limit is used as the critical cooling rate in engineering applications. In Method Two, based on the chemical composition and thickness of the polyurethane film, The values ​​are taken as empirical values ​​within the range of 1.5℃ / s to 3℃ / s; for samples with low crosslinking density and thin film layers, It is advisable to select the upper limit of this range; for samples with high crosslinking density and thick film, It is advisable to select the area near the lower limit of this interval.

[0093] In S13, the glass transition temperature is... and critical cooling rate Input controller, and This serves as the benchmark for the cooling rate in this method, and subsequent settings for the target cooling rate of each zone and calculations of the pulse frequency are all based on this benchmark.

[0094] In S2, the controller sets the target cooling rates for zones 1, 2, and 3 based on the critical cooling rate. The target cooling rates for zones 1, 2, and 3 are denoted as follows: , , , , , The units are all ℃ / s. S2 further includes sub-steps S21 to S23.

[0095] In S21, the controller sets the target cooling rate for zone 1. Set as critical cooling rate The temperature is 0.4 to 0.6 times higher than the critical cooling rate, allowing the polyurethane-coated urea to be pre-cooled in Zone 1 at a rate below the critical cooling rate. The material's temperature upon entering the tower from the inlet is typically higher than the critical cooling rate. In many cases, the polyurethane film is in a highly elastic state, and its outer surface still retains a certain degree of adhesion. If a cooling rate close to the critical cooling rate is applied in Zone 1, the temperature difference between the outer surface of the material and the counter-current cooling air will induce significant thermal stress. The inner and outer layers of the film will shrink asynchronously, and microcracks may initiate in the initial stage. (The last part, "0.4 to 0.6 times," appears to be an unrelated fragment and is omitted from the translation.) set up This allows Zone 1 to only perform the pre-cooling task of reducing the high inlet temperature, enabling the film temperature to drop steadily. The transition temperature zone above and near the surface provides stable starting conditions for subsequent critical curing.

[0096] In S22, the controller sets the target cooling rate for zone 2. Set as critical cooling rate The temperature is 0.9 to 1.0 times higher, allowing the polyurethane-coated urea to pass through the glass transition temperature in Zone 2 at a cooling rate not exceeding the critical cooling rate. The polyurethane film undergoes glassy curing. In region 2, the film transitions from a highly elastic state to a glassy state, with molecular chain segments changing from freely rotating to frozen, resulting in a step change in properties such as specific heat capacity, free volume, and refractive index. If the cooling rate in region 2 is lower than the lower limit of the specified region, the film... If the residence time in the vicinity is too long, the cumulative effect of surface adhesion will further develop the adhesion between particles; if the cooling rate of the second zone is higher than the upper limit of the zone, the stress relaxation of the film molecules will be insufficient, the residual stress in the cured film will increase, and the probability of microcracks will increase accordingly. Limited to The cooling rate is 0.9 to 1.0 times that of the previous generation, and is located near the upper boundary that will not induce film cracking. The temperature rise duration in the second zone is thus minimized, and the film curing quality and anti-adhesion requirements are coordinated.

[0097] In S23, the controller sets the target cooling rate for zone 3. Set as critical cooling rate The temperature is 1.5 to 2.0 times higher, allowing the polyurethane-coated urea to be cooled to the target outlet temperature in Zone 3. The material's temperature upon entering Zone 3 is already below [temperature missing]. The film layer has completed vitrification and curing, and its mechanical properties are stable, allowing it to withstand high cooling rates without damage. (Higher...) This increases the required cooling airflow in Zone 3, and the high-speed airflow at the bottom of the tower provides sufficient momentum for the subsequent pulsed diagonal alternating disturbances. The target outlet temperature should be 3°C to 5°C above the workshop ambient temperature to prevent moisture absorption and clumping during the screening and packaging process.

[0098] In S3, the controller determines the material mass flow rate. Based on the target cooling rates of Zone 1, Zone 2, and Zone 3, the initial frequency values ​​of the three fan supplies are determined respectively. S3 further includes sub-steps S31 to S33.

[0099] In S31, the controller calculates the downward linear velocity of the polyurethane-coated urea within the tower body using the following formula. :

[0100] ;

[0101] In the formula, The downward linear velocity of polyurethane-coated urea within the tower. The unit is m / s; For material mass flow rate, The unit is kg / s; The bulk density of polyurethane-coated urea. The unit is kg / m 3 ; The effective cross-sectional area of ​​the tower body, The unit is m 2 ; The volume percentage of polyurethane-coated urea within the tower. For dimensionless parameters, The value ranges from 0.05 to 0.70. The specific value is obtained through the cold calibration of the device.

[0102] The above formula is derived from the principle of conservation of mass. The mass of material flowing down through any horizontal section of the tower per unit time should be equal to the mass flow rate of the material supplied at the inlet. When the material inside the tower is expressed as a percentage by volume Occupy effective cross-section The bulk density is At that time, the mass of material passing through the horizontal cross-section per unit time is also equal to If we make them equal, then... Divide both sides by The above formula is thus obtained. Cold calibration of the device should be performed before feeding. The calibration method involves continuously feeding tracer material with a known mass flow rate, recording the ratio of the time difference between the tracer material's flow rate from the inlet to the outlet to the effective height of the tower, and then using this ratio to deduce the corresponding operating conditions of the device. The value is then fixed in the controller.

[0103] In S32, the controller calculates the first... Cooling air volume of the zone :

[0104] ;

[0105] In the formula, For the district number, Choose 1, 2, or 3; For the first Cooling airflow in the zone The unit is m 3 / s For material mass flow rate; The specific heat capacity of polyurethane-coated urea, The unit is J / (kg·K); For the first The target cooling rate of the zone; For the first Material bed height in the zone The unit is m; For the density of cooling air, The unit is kg / m 3 ; The specific heat capacity of cooling air, The unit is J / (kg·K); For the first Permissible temperature rise of cooling air in the zone The unit is K; The value range is from 10K to 20K; The value range is 15K to 25K; The value range is 20K to 30K; The downlink linear velocity obtained from S31.

[0106] The above formula is derived from the first... The gas-solid steady-state energy balance of the region is derived. Sensible heat released by materials in the zone per unit time Press Calculate, where For materials in the first Duration of stay within the area Sensible heat absorbed by cooling air per unit time Press Calculate; let ,have Divide both sides by , that is, The calculation formula. Follow The reason for the increase in temperature is that as the tower descends, the material temperature tends to approach the outlet temperature, and the driving temperature difference between the material and the incoming cold air relatively decreases. At this point, a larger allowable temperature rise is needed to maintain a reasonable air volume. However, in Zone 1, the material temperature is high, and the air can achieve a smaller temperature rise through short-distance convection heat exchange. The upper limit should not be too high, so as to avoid the air volume being designed too conservatively. The value should be between 1400 J / (kg·K) and 1500 J / (kg·K), which represents the typical specific heat capacity of polyurethane-coated urea in the operating temperature range; The value of should be taken as approximately 1005 J / (kg·K), which is the isobaric specific heat capacity of dry air at normal pressure; It is advisable to select the value based on the converted value of the cooling air inlet temperature according to the ideal gas law.

[0107] In S33, the controller, based on the airflow-frequency characteristic curves of the three fans, calculates the cooling airflow obtained in S32. Converted to the corresponding number Initial frequency of the district's ventilation fans , The unit is Hz. Take 1, 2, and 3; the initial frequency values ​​of the air supply fans corresponding to zones 1, 2, and 3 are respectively denoted as... , , The air volume-frequency characteristic curve is either the factory test curve attached to the fan manufacturer's nameplate or a field curve calibrated under actual pipeline conditions before the unit is put into operation. It reflects the monotonically increasing relationship between the fan's air volume and the inverter's output frequency under a given pipeline resistance. Compared to the traditional method of starting the unit at a fixed frequency and then gradually adjusting, using an initial frequency value... This ensures that the blower operates near the static operating point required for thermal equilibrium from the very beginning of startup, thus reducing the overshoot of the outlet material temperature during the startup phase.

[0108] In S4, the controller activates three supply fans, an exhaust fan, and four pulse valves, causing the pulse valves in the first and second groups to alternately open and close in diagonal groups, creating periodic turbulent airflow in the third zone. S4 further includes sub-steps S41 to S43.

[0109] In S41, the controller operates according to the initial frequency value. , , Start the first, second, and third air supply fans respectively, so that the first air supply fan supplies cooling air to zone 1, the second air supply fan supplies cooling air to zone 2, and the third air supply fan divides the cooling air into 4 paths through branch pipelines and sends them to the upstream side of the 4 pulse valves.

[0110] In S42, the controller starts the exhaust fan, creating a negative pressure inside the tower relative to atmospheric pressure, allowing the cooling air inside the tower to be discharged through the air outlet. The negative pressure value inside the tower relative to atmospheric pressure should be maintained between 5Pa and 15Pa. If the negative pressure value is too high, the cooling air will converge too quickly towards the air outlet inside the tower, affecting the uniformity of air distribution in the horizontal direction. The air intake distribution of the side air inlets in Zones 1 and 2 will tend to concentrate towards the air outlet side. If the negative pressure value is too low, it will not be enough to overcome the leakage of urea dust along the joints of the tower.

[0111] In S43, the controller alternately controls the pulse valves of the first and second groups according to the first and second on / off cycles. The first on / off cycle is when the pulse valves of the first group are open and the pulse valves of the second group are closed, and the second on / off cycle is when the pulse valves of the first group are closed and the pulse valves of the second group are open, causing the airflow direction in Zone 3 to be periodically deflected, resulting in periodically disturbed airflow. The two pulse valves in the first group are located at a 180° diagonal position on the circumference of the tower body. When the first group is open, the mainstream airflow line formed extends along the diagonal line connecting the two pulse valves in the tower body. The two pulse valves in the second group are deflected 90° relative to the first group. When the second group is open, the mainstream airflow line formed extends along the other diagonal line. The alternating opening of the first and second groups causes the mainstream direction of the cooling air at the bottom of the tower to switch periodically between two mutually perpendicular diagonal lines. Under the action of switching mainstream directions, the particle bed in Zone 3 undergoes continuous swaying, and the initially adhered particle clusters are broken up by repeated shear stress. Compared to the full pulse mode where all four channels are simultaneously switched on and off, the diagonal grouping alternating switching method ensures that one group of pulse valves remains open at any given moment, maintaining a continuous supply of cooling air and preventing localized heat resurgence caused by cooling interruption in the tower bottom section.

[0112] In step S5, polyurethane-coated urea is continuously fed into the tower body through the feed inlet, causing the polyurethane-coated urea to flow sequentially through zone 1, zone 2, and zone 3. Step S5 further includes sub-steps S51 to S54.

[0113] In S51, polyurethane-coated urea is expressed as a material mass flow rate. It continuously enters Zone 1 through the feed inlet.

[0114] In S52, polyurethane-coated urea undergoes convective heat exchange with cooling air in zone 1, according to the target cooling rate. Cool the target temperature above the glass transition temperature The temperature is adjusted to obtain the pre-cooled material. In Zone 1, the convective heat exchange is carried out between the material particles and the cooling air that enters from the first air inlet and flows upward. The relative motion between the material and the cooling air is counter-current, and the temperature gradient of the counter-current flow gradually decreases from bottom to top, making the heat exchange driving force more evenly utilized along the process.

[0115] In S53, the pre-cooled material enters the second zone through the upper annular flow equalization baffle, and cools according to the target cooling rate. Cooling the target temperature across the glass transition temperature The polyurethane film undergoes glass curing to obtain the cured material. After pre-cooling, the material enters Zone 2 through the central material discharge hole of the upper annular flow equalization baffle. The airflow holes of the airflow distribution plate on the outer edge of the upper annular flow equalization baffle allow the cooling air from Zone 2 to pass upwards into Zone 1 under the negative pressure of the exhaust fan. As the material passes through the material discharge hole, it is guided by the inclined surface of the airflow distribution plate, resulting in a centered and reorganized material distribution, thus maintaining the lateral uniformity of the material bed in Zone 2.

[0116] In S54, the cured material enters zone 3 through the lower annular flow equalization baffle, where it is disturbed by periodic airflow and cooled at the target rate. The target temperature is lowered to the target outlet temperature to obtain cooled polyurethane-coated urea. The cooling air in Zone 3 mainly comes from the grouped diagonal pulse airflow formed by the four bottom third air inlets after being regulated by pulse valves. After passing through the particle bed of the cured material, the cooling air merges with the cooling air entering from Zone 2 and rises to the outlet for discharge.

[0117] In S6, the controller collects the material temperatures at the inlet, zone 1, zone 2, and zone 3 measured by the temperature sensor group according to the sampling period. It adjusts the operating frequencies of the three blowers using a combination of feedforward and feedback methods, and adjusts the pulse frequencies of the four pulse valves based on the material temperature at the inlet, the material temperature in zone 3, and the glass transition temperature. The material temperature at the inlet collected by the temperature sensor group is recorded as... The material temperatures collected by the temperature sensor group in zones 1, 2, and 3 are recorded as follows: , , , , , , The units are all in °C. S6 further includes sub-steps S61 to S67.

[0118] In S61, the controller calculates the first... The material set temperature of the zone :

[0119] ;

[0120] In the formula, For the district number, Choose 1, 2, or 3; For the first The material set temperature in the zone, The unit is ℃; For the first The material inlet temperature of the zone, The unit is ℃; The value of is equal to ; The value of is equal to ; The value of is equal to ; The first set for S2 The target cooling rate of the zone; For the first Material residence time in the zone The unit is s. according to Calculated; For the first Material bed height in the zone; The downward linear velocity obtained from S31.

[0121] The above formula reflects the ideal situation where the material is cooled at the target rate. In the Duration of stay within the area The outlet temperature that should be reached afterward; due to the first The material outlet of the district is the first Material entrances for each area, and other areas Connecting the sections along the height of the tower, based on this , , This forms a temperature gradient that decreases progressively along the height of the tower, with the temperature drop in each segment strictly corresponding to the product of the target cooling rate and the residence time in that segment.

[0122] In S62, the controller operates according to the sampling period. The material temperatures in zones 1, 2, and 3 were collected. , , Sampling period The value range is from 1s to 5s; and the value of the first s is calculated according to the following formula. Material temperature deviation in the zone :

[0123] ;

[0124] In the formula, For the district number, Choose 1, 2, or 3; For the first The material temperature deviation in the current sampling period. The unit is ℃; For the temperature sensor group, the number of samples collected in the current sampling period The material temperature in the zone; The first obtained for S61 The set temperature of the material in the zone. Sampling cycle. It is advisable to select the appropriate time period based on a trade-off between the material's thermal response time constant and the control system's scanning cycle. If the value is too small, the control output will fluctuate frequently due to signal noise interference. An excessively large value will cause the control system to lag in its response to disturbances.

[0125] In S63, the controller calculates the first step according to the incremental proportional-integral-derivative control algorithm using the following formula. Frequency increment of the district ventilation fan in the current sampling period :

[0126] ;

[0127] In the formula, For the district number, Choose 1, 2, or 3; For the first The frequency increment of the district ventilation fan in the current sampling period The unit is Hz; For the first The proportion coefficient of the area The unit is Hz / ℃; For the first The integral coefficient of the region, The unit is Hz / (℃·s); For the first The differential coefficients of the region, The unit is Hz·s / ℃; The current sampling period obtained by S62 is the first... Material temperature deviation in the zone; For the first The material temperature deviation in the area during the previous sampling period; For the first The material temperature deviation in the area from the previous sampling period; The sampling period; when or When the corresponding sampling period does not exist, the value of the corresponding variable is 0.

[0128] The above formula is the incremental form of the standard PID control algorithm, derived from the positional PID algorithm through differential operations. The incremental PID output is a correction to the control quantity of the previous cycle. The frequency converter and other actuators internally maintain the frequency itself, so the accumulated error has relatively little impact on the control output, thus ensuring the robustness of the control process. Proportional term Rapid response to changes in deviation; integral term Eliminate steady-state deviation; differential term The second-order difference of the deviation is predicted to dampen the overshoot. , , It is advisable to set the temperature according to the step response characteristics of the controlled object during the initial stage of equipment commissioning, using engineering trial and error or relay oscillation method; for the temperature-sensitive object of this equipment, Values ​​should be taken in the range of 0.5 Hz / ℃ to 2 Hz / ℃. Values ​​should be taken in the range of 0.02 Hz / (℃·s) to 0.1 Hz / (℃·s). The value should be taken in the range of 0.1 Hz·s / ℃ to 0.5 Hz·s / ℃.

[0129] In S64, the controller updates the following formula: Operating frequency of district ventilation fans and operating frequency Output to the corresponding number District ventilation fans: ;

[0130] In the formula, For the district number, Choose 1, 2, or 3; For the first The operating frequency of the district ventilation fans during the current sampling period. The unit is Hz; For the first The operating frequency of the district ventilation fans in the previous sampling period. The value taken in the initial calculation is the [number]. Initial frequency value of the district ventilation fan ; The current sampling period obtained by S63 is the first... The frequency increment of the district's ventilation fans.

[0131] The composite control method combining feedforward and feedback described in S61 to S64, with the initial frequency value obtained from S3... As the main body of the feedforward output, the PID feedback correction from S62 to S64 serves as an online supplement to the feedforward. The feedforward section directly incorporates pre-obtainable disturbance variables such as material mass flow rate, target cooling rate, and tower geometry parameters into the calculation, allowing the system to skip the long transition process from zero to steady state in traditional steady-state PID during startup. The feedback section uses the difference between the measured temperature and the set temperature as input to perform online correction for unknown disturbances not reflected in the feedforward calculation, such as model errors, sensor drift, and seasonal changes in ambient temperature. As a result, steady-state deviations are eliminated, and the tracking accuracy of material temperature in each zone is maintained under conditions such as material flow fluctuations and inlet temperature jumps.

[0132] In S65, the controller calculates the pulse frequency of the four pulse valves using the following formula. :

[0133] ;

[0134] In the formula, The pulse frequency of the four pulse valves, The unit is Hz. The value range is limited to 0.25Hz to 2.0Hz; when the calculated value according to the above formula is lower than 0.25Hz... Take 0.25Hz; when the calculated value according to the above formula is higher than 2.0Hz. Select 2.0Hz; Based on the pulse frequency, The unit is Hz. The value range is from 0.3Hz to 0.8Hz; The temperature response coefficient, For dimensionless parameters, The value range is from 0.6 to 1.2; The temperature of the material in zone 3 is collected by the temperature sensor group in the current sampling period; The glass transition temperature obtained from S11; This refers to the material temperature at the feed inlet collected by the temperature sensor group during the current sampling period.

[0135] In the above formula Reflects the current relative temperature of materials in Zone 3 High output, Reflects the relative temperature of the feed material High output, business For the The normalized temperature margin for Zone 3 is approximately 1 when the material first enters Zone 3, and gradually approaches 0 as cooling progresses; according to the above formula, when the material in Zone 3 still has a relatively high temperature... A higher value results in a higher swaying frequency of the pulsed turbulent airflow, which has a stronger dispersing effect on initially agglomerated particles; when the material in zone 3 approaches the target outlet temperature... By using a lower value, the airflow switching frequency slows down, reducing mechanical wear on the particle bed caused by high-frequency disturbances, and maintaining the integrity of the film surface. As a normalization benchmark, the above formula can be adaptively adjusted for polyurethane films with different chemical compositions, improving cross-product versatility. The upper and lower limits prevent the valve life from being shortened due to excessively frequent or slow switching of the pulse valve under abnormal operating conditions, and avoid resonance between the pulse signal and the response characteristics of the blower.

[0136] In S66, the controller calculates the duty cycle of the four pulse valves using the following formula. :

[0137] ;

[0138] In the formula, The duty cycle of the four pulse valves. Dimensionless parameter; This refers to the mass flow rate of the material detected by the material flow detection device during the current sampling period. The rated material mass flow rate of the device, The unit is kg / s; The value range is 0 to Correspondingly The value range is from 0.4 to 0.5.

[0139] The above formula reflects a monotonic linear relationship. The closer the material mass flow rate is to the rated value, the higher the proportion of the opening time of the first and second group pulse valves to a single complete cycle, and the corresponding increase in the average supply of cooling air per unit time, maintaining a dynamic match between the cooling air and the material load. Because The upper limit is limited to 0.5. There is a transition gap between the first group and the second group, where both groups are closed. Within a single pulse cycle, the direction of the mainstream airflow at the bottom of the tower undergoes two clear switches rather than instantaneous reversals. The particle bed responds relatively smoothly to the mainstream switch, and the impact stress on the material caused by the instantaneous interruption of airflow is reduced.

[0140] In S67, the controller operates according to the pulse frequency obtained in S65. Duty cycle obtained from S66 Pulse control signals are output to four pulse valves, causing the four pulse valves to operate at a pulse frequency. and duty cycle It operates alternately according to the first on / off cycle and the second on / off cycle as described in S43. The two pulse valves in the first group receive... For frequency, with The pulse control signal is the duty cycle pulse, and it opens and closes synchronously according to the first on / off cycle; at the same time, the pulse control signals received by the two pulse valves in the second group are lagging behind the first group in phase by half a cycle, and the second group opens and closes synchronously according to the second on / off cycle; the opening intervals of the first and second groups do not overlap within one pulse cycle, and only one group of pulse valves is in the open state at the bottom of the tower at any given time, and the mainstream direction of the disturbed airflow in the third zone is according to... The frequency switches periodically between two mutually perpendicular diagonal lines.

[0141] In step S7, the controller controls the rotary discharge valve to discharge the cooled polyurethane-coated urea through the discharge port. Step S7 further includes sub-steps S71 and S72.

[0142] In S71, the cooled polyurethane-coated urea is collected from the bottom of Zone 3 via a conical discharge section. The inner wall of the conical discharge section should preferably be made of polytetrafluoroethylene coating with a low coefficient of friction or polished stainless steel plate, so that the material can slide smoothly down the conical surface by its own weight without lingering on the conical surface; the cone angle of the conical discharge section should preferably not be lower than the angle of repose between the material and the inner wall to prevent the material from forming bridging blockages in the discharge section.

[0143] In S72, the controller controls the rotary discharge valve to rotate according to a preset discharge frequency, so that the polyurethane-coated urea collected in the conical discharge section is continuously discharged through the discharge port. The discharge frequency should be proportional to the material mass flow rate. The controller can match the data collected by the material flow detection device. The value is divided by the single-turn volume of the rotary discharge valve, and the discharge frequency is dynamically calculated accordingly. The accumulation height of the material in the conical discharge section is thus maintained near a stable target value. The star rotor blades of the rotary discharge valve always keep the material sealed to the inner wall during rotation, and the negative pressure environment inside the tower is not destroyed by the airflow short circuit at the discharge port.

[0144] This method utilizes the characteristic thermophysical parameters of polyurethane films. and As the design benchmark for the entire process, the material undergoes three stages of cooling: pre-cooling, critical solidification, and strong cooling dispersion. Each stage applies a graded cooling rate to the material, preventing film cracking while reducing the residence time of the material in the viscous temperature zone. At the bottom of the tower, diagonally alternating pulsed turbulence provides periodic shear momentum for particle dispersion in the third zone, continuously breaking up solidified but potentially slightly agglomerated particle clusters. The feedforward stage calculates the initial value of the blower frequency using material flow rate, target cooling rate, and tower geometry parameters as known quantities, ensuring the unit approaches steady-state operation during startup. The feedback stage uses temperature deviation as input and corrects the frequency based on an incremental PID algorithm, maintaining temperature tracking accuracy in each zone under disturbances such as material flow fluctuations, inlet temperature jumps, and seasonal changes in ambient temperature. The pulse frequency and duty cycle are determined based on the relative temperatures of the material in the third zone. The margin and current material flow rate are adaptively adjusted to achieve a dynamic match between the turbulence intensity and the cooling load. With each step working in tandem, the polyurethane-coated urea achieves both film integrity protection and particle anti-agglomeration and dispersion during continuous tower cooling, thus improving the device's adaptability to operating disturbances.

[0145] Example 1: This example implements the method flow described in steps S1 to S7 of the present invention on the apparatus of the present invention. The apparatus is configured for an annual production capacity of 20,000 tons of polyurethane-coated urea, with an inner diameter of 1.4m in the tower and material bed heights in zones 1, 2, and 3. , , The dimensions are 1.0m, 1.6m, and 0.8m respectively; the three air supply fans are variable frequency centrifugal blowers, with rated powers of 5.5kW, 11kW, and 15kW for the first, second, and third air supply fans respectively; the exhaust fan has a rated power of 22kW; the four pulse valves are electrically adjustable butterfly valves; the temperature sensor group uses armored PT100 platinum resistance thermometers; the controller uses an industrial PLC. The rated material mass flow rate of the device... Take 0.7 kg / s.

[0146] In S1, differential scanning calorimetry (DSC) was used to perform thermal analysis on the polyurethane film sample at a heating rate of 10 °C / min, as per S11. The temperature was determined by the midpoint temperature of the glass-to-elastic transition step in the heat flow-temperature curve. The temperature was 40℃; thermal analysis was performed on the same batch of thin film samples according to method S12 with heating rates of 0.5℃ / s, 1℃ / s, 2℃ / s, 3℃ / s, and 5℃ / s respectively. The nearby relaxation peak remains intact at 2℃ / s and below, but becomes distorted at 3℃ / s and above. Therefore, [the following is taken]. It is 2.0℃ / s; according to S13, ℃ and ℃ / s input controller.

[0147] In S2, set the first zone according to S21. for That is, 1.0℃ / s; set the second zone according to S22. for That is, 1.9℃ / s; set the third zone according to S23. for That is, 3.5℃ / s.

[0148] In S3, the downward linear velocity inside the tower is calculated using Formula 1 according to S31. , where the bulk density Take 760kg / m 3 Effective cross-sectional area Determined by the inner diameter of the tower body and the proportion of material volume. The cold-state calibration value of the device is taken as 0.55; the cooling air volume of zone 1, zone 2 and zone 3 are calculated according to formula 2 in S32 respectively. , , The specific heat capacity of coated urea is... Take 1450 J / (kg·K) and the specific heat capacity of air at constant pressure. Take 1005 J / (kg·K), the density of the incoming air Based on an inlet air temperature of 25℃, the allowable temperature rise is... , , Take 15K, 20K, and 25K respectively; according to S33, based on the air volume-frequency characteristic curves of the three fans, , , Reverse lookup for initial frequency value , , , serving as the feedforward baseline for subsequent operations.

[0149] In S4, press S41 with the initial frequency value. , , Start the first, second, and third air supply fans respectively; start the exhaust fan by pressing S42 to bring the relative atmospheric pressure inside the tower to a negative pressure of 10Pa; press S43 to make the first group of two diagonal pulse valves and the second group of two diagonal pulse valves alternately open according to the first on-off cycle and the second on-off cycle, forming a periodic turbulent airflow in the third zone.

[0150] In S5, according to S51 to S54, the polyurethane-coated urea is used to... kg / s of urea continuously enters the tower body through the feed inlet and flows sequentially through Zone 1, Zone 2, and Zone 3 to obtain cooled polyurethane-coated urea.

[0151] Sampling period in S6 Take 2 seconds, and calculate the set temperature of the material in each zone according to Formula 3 using S61. ;Calculate the material temperature deviation in each zone using Formula 4 according to S62. ;Calculate the frequency increment using the incremental proportional-integral-derivative control algorithm described in Formula 5 according to S63. PID parameters in zone 2 , , The PID parameters for Zone 1 and Zone 3 are taken as close as possible using the engineering tuning method, with values ​​of 1.2 Hz / ℃, 0.05 Hz / (℃·s), and 0.3 Hz·s / ℃ respectively. The operating frequency of the blower is updated according to formula 6 in S64. Calculate the pulse frequency using Formula 7 according to S65. The fundamental pulse frequency Take 0.5Hz, temperature response coefficient Take 1.0; calculate the duty cycle according to Formula 8 in S66. Press S67 to output pulse control signals to the four pulse valves.

[0152] In S7, S71 causes the cooled material to collect in the conical discharge section; S72 causes the material to be continuously discharged by the rotary discharge valve at a preset discharge frequency. After continuous operation to a steady state, the outlet material temperature is maintained at around 30℃, the adhesion rate is 1.5%, the film integrity rate is 98.0%, the cooling energy consumption per ton of material is 9.0 kWh / t, and the relative standard deviation of the nutrient release rate after 28 days is 2.8%.

[0153] Example 2 differs from Example 1 in that the polyurethane film used has a higher crosslinking density and a thinner film layer, as measured according to S11. The temperature was 35℃, measured according to method S12. It is 1.8℃ / s; set according to S21 to S23 respectively. , , Material mass flow rate The flow rate was 0.5 kg / s; the remaining equipment configuration and procedures were the same as in Example 1. Under steady-state conditions, the adhesion rate was 1.8%, the membrane integrity rate was 97.5%, the energy consumption per ton was 8.5 kWh / t, and the relative standard deviation of the nutrient release rate over 28 days was 3.0%.

[0154] Example 3 differs from Example 1 in that the polyurethane film used has a lower crosslinking density and a thicker film layer, as measured according to S11. The temperature was 50℃, measured according to method S12. The speed is 2.5℃ / s; set according to S21 to S23 respectively. , , Material mass flow rate The concentration was 0.65 kg / s; the rest was the same as in Example 1. The adhesion rate under steady state was 1.2%, the membrane integrity rate was 98.5%, the energy consumption per ton was 10.0 kWh / t, and the relative standard deviation of the nutrient release rate over 28 days was 2.5%.

[0155] Example 4, the difference between this example and Example 1 is that: according to method two in S12, the sample is directly taken. The empirical value is 2.0℃ / s. The temperature was determined to be 42℃ according to S11. The coefficient values ​​are the same as in Example 1; material mass flow rate Taking 0.28 kg / s, it is equivalent to 40% is used to examine the duty cycle in S66. The adaptability of the device as the material flow rate decreases linearly; otherwise, it is the same as in Example 1. Under steady-state conditions, the adhesion rate is 1.6%, the membrane integrity rate is 97.8%, the energy consumption per ton is 9.5 kWh / t, and the relative standard deviation of the nutrient release rate after 28 days is 2.9%.

[0156] Comparative Example 1 uses a single-stage tower cooler. The tower body is not divided into sections, does not have annular flow equalization baffles, and does not have pulse valves. A single shared fan delivers air evenly from the bottom of the tower. There is no feedforward or feedback combined control; the fan frequency is set only based on manual experience. The remaining equipment scale and cooling object are the same as in Example 1. Under steady-state conditions, the adhesion rate is 12.0%, the film integrity rate is 85.0%, the energy consumption per ton is 14.0 kWh / t, and the relative standard deviation of the nutrient release rate after 28 days is 8.5%.

[0157] Comparative Example 2: The configuration of this comparative example device is exactly the same as that of Example 1, but the diagonal grouping and alternating on / off of the pulse valves described in S43 is cancelled. Instead, four pulse valves are always kept fully open, and the cooling air is constantly supplied to the third zone; the rest of the methods are the same as in Example 1. Under steady-state conditions, the adhesion rate is 4.0%, the film integrity rate is 96.0%, the energy consumption per ton is 10.5 kWh / t, and the relative standard deviation of the nutrient release rate after 28 days is 5.5%.

[0158] Comparative Example 3: The configuration of this comparative example device is exactly the same as that of Example 1, but the diagonal grouping alternation in S43 is changed to four-way synchronous on / off, that is, four pulse valves open and close simultaneously, with the pulse frequency and duty cycle the same as in Example 1; the rest is the same as in Example 1. Under steady-state conditions, the adhesion rate is 3.0%, the membrane integrity rate is 96.5%, the energy consumption per ton is 11.0 kWh / t, and the relative standard deviation of the nutrient release rate after 28 days is 4.8%.

[0159] Comparative Example 4: The configuration of this comparative example device is exactly the same as that of Example 1, but the feedforward link described in S3 is removed in the control strategy of S6, and only the incremental PID feedback described in S61 to S64 is retained; the fan uses 80% of its rated frequency as the initial operating frequency; the rest is the same as in Example 1. Under steady-state conditions, the adhesion rate is 2.2%, the membrane integrity rate is 97.0%, the energy consumption per ton is 10.0 kWh / t, and the relative standard deviation of the nutrient release rate after 28 days is 4.0%.

[0160] Comparative Example 5: The configuration of this comparative example device is exactly the same as that of Example 1, but the target cooling rate of the second zone in S22 is changed. Set as That is, 3.0℃ / s, which exceeds the limit specified in step S22 of this invention. to The upper limit of the range; the rest is the same as in Example 1. Under steady state, the adhesion rate is 1.0%, the membrane integrity rate is 80.0%, the energy consumption per ton is 9.5 kWh / t, and the relative standard deviation of the nutrient release rate after 28 days is 6.5%.

[0161] Comparative Example 6: The configuration of this comparative example device is exactly the same as that of Example 1, but the target cooling rate of the second zone in S22 is changed. Set as That is, 1.0℃ / s, which is lower than the value defined in step S22 of this invention. to The lower limit of the range; the rest is the same as in Example 1. Under steady state, the adhesion rate is 15.0%, the membrane integrity rate is 94.0%, the energy consumption per ton is 8.8 kWh / t, and the relative standard deviation of the nutrient release rate after 28 days is 7.0%.

[0162] Experiment Example 1: The experimental subjects in this experiment are Example 1, Example 2, Example 3, Comparative Example 5, and Comparative Example 6, a total of 5 schemes, and additional tests were conducted on schemes other than those mentioned above. Four operating points of 0.7, 0.8, 1.1, and 1.3 were selected to obtain... The complete trend from 0.5 to 1.5. After each operating point was run continuously for 30 minutes under steady-state conditions, 500g of material was randomly sampled at the discharge port.

[0163] Glass transition temperature With critical cooling rate The determination method is based on the glass transition temperature determination by differential scanning calorimetry (DSC) for plastics; the determination method for adhesion rate is based on the agglomeration rate determination method for controlled-release fertilizers, where the sample is passed through a 2mm sieve, and the adhesion rate is the ratio of the mass of agglomerated material on the sieve to the total mass of the sample; the film integrity rate is determined by optical microscopy counting method, where no less than 100 particles are randomly selected and observed under 100x magnification, and the percentage of particles without visible cracks or peeling is the film integrity rate.

[0164] Experimental results are as follows Figure 1 As shown. Figure 1 (a) represents different The comparison of membrane integrity is shown on the horizontal axis. The vertical axis represents the film integrity rate; Figure 1 (b) represents different The adhesion rate comparison is shown on the horizontal axis. The vertical axis represents the adhesion rate; Examples 1, 2, and 3 are illustrated with different symbols compared to Comparative Examples 5 and 6, and the steps defined in S22 of this invention are indicated by vertical dashed lines. Interval.

[0165] Depend on Figure 1 As shown in (a), the film integrity rate increases with... The changes exhibited a single-peak characteristic of first rising and then falling, with the maximum film integrity rate of 97.5% to 98.5% occurring in [the period described]. The three data points are within the range of 0.9 to 1.0 and correspond to Examples 1, 2, and 3; when As the pH value gradually increased from 0.5 to 1.0, the membrane integrity rate steadily improved from 94.0% to 98.5%. This improvement stemmed from the material's... The shortening of residence time in the nearby viscous temperature zone and the reduction of initial adhesion between particles; when As the value increased from 1.0 to 1.5, the film integrity rate plummeted from 98.5% to 80.0%. This decline was due to the cooling rate exceeding the critical value that the polyurethane film could withstand. Molecular chain segments crossing Insufficient stress relaxation during the process led to the accumulation of residual stress within the membrane, inducing numerous microcracks. Figure 1 As shown in (b), the adhesion rate increases with... Monotonically decreasing, comparative ratio 6 in The adhesion rate was as high as 15.0%, decreasing to 1.2% to 1.8% after entering the 0.9 to 1.0 range. This decrease reflects the effect of increased cooling rate. The shortened residence time nearby effectively inhibits the accumulation of adhesive on the particle surface.

[0166] It can be seen that, The range up to 1.0 is the only interval that simultaneously satisfies both an adhesion rate of less than 2% and a film integrity rate of more than 97.5%. Step S22 of this invention... The limitation is the optimal trade-off between simultaneously suppressing two contradictory failure modes: adhesion failure and film cracking failure. The three data points in Examples 1, 2, and 3 are respectively located at the median, lower limit, and upper limit of the above interval, and all four indicators are within the optimal plateau region; the glass transition temperature of this invention... The location of the viscous temperature zone and the critical cooling rate were determined. Material passage was calibrated The physical upper limit of the cooling rate, S2 step Based on this, the target cooling rates for the three zones are set using three sets of ratios: 0.4 to 0.6 times, 0.9 to 1.0 times, and 1.5 to 2.0 times. and The intrinsic physical relationship between the two thin film thermophysical parameters enables simultaneous suppression of two failure modes, adhesion and film cracking, within the same device.

[0167] Experimental Example 2 is used to verify the effectiveness of the initial feedforward frequency value described in step S3 and the feedforward-feedback composite control method described in step S6 compared to a single control strategy. The experimental subjects are three schemes: Example 1, Comparative Example 1, and Comparative Example 4. Each scheme operates under its own steady-state conditions. At time s, a step disturbance of +10°C is applied to the material at the feed inlet, that is... The temperature is instantly increased from 80℃ to 90℃, and the temperature sensor group continuously collects the material temperature in zones 1, 2, and 3 at a sampling period of 1 second. , , The data collection time was 180 seconds.

[0168] Experimental results are as follows Figure 2As shown. Figure 2 (a) is Material temperature in zone 1 under step disturbance The response curve Figure 2 (b) represents the material temperature in zone 2. The response curve Figure 2 (c) represents the material temperature in zone 3. The response curves; the three curves correspond to Example 1, Comparative Example 1, and Comparative Example 4 respectively in the same coordinate system, and the material set temperature of the corresponding area is marked by a horizontal dashed line in each figure. .

[0169] Depend on Figure 2 As can be seen from (a), (b), and (c), Comparative Example 1 uses a single-segment open-loop method. The tower body relies solely on its own thermal inertia to buffer the inlet temperature disturbance. The material temperatures in the three zones rise exponentially within 180 seconds after the disturbance without falling back, and the steady-state deviation eventually stabilizes in the range of 7°C to 9°C. The fundamental reason is that the thermal inertia of the tower body only slows down the time-domain propagation speed of the disturbance, but does not have the ability to eliminate the disturbance itself. Comparative Example 4 retains only the incremental PID feedback loop and cancels the feedforward channel. During the startup phase, the PID accumulates correction from zero. The material temperatures in all three zones show an overshoot of 3°C to 4°C and undergo a period of oscillation and decline, only returning to near the set value after 120 seconds. The overshoot originates from the fact that the feedback loop needs to wait for the material temperature to change before triggering correction to identify the disturbance. The feedback channel cannot respond to the measurable disturbance of the inlet temperature within the same sampling period. Example 1 uses a composite control method combining feedforward and feedback. The feedforward loop in Within the same sampling period when a step occurs, the required cooling air volume for each zone is recalculated according to Formula 2, and the air volume increment is mapped to the fan frequency increment and sent to the frequency converter. The feedback loop follows up to correct the residual between the feedforward estimate and the actual temperature. The maximum deviation of the material temperature in the three zones is compressed to about 2°C and returns to near the set value within 60 seconds.

[0170] Example 1, compared to Comparative Example 1, reduces the steady-state deviation from 7°C to 9°C to zero; compared to Comparative Example 4, it reduces the overshoot from 3°C to 4°C to approximately 2°C; and the settling time is shortened from over 120 seconds to less than 60 seconds. The feedforward-feedback composite control described in step S6 of this invention improves disturbance suppression capability compared to single control. The contribution of the feedforward element is manifested in the immediate compensation for measurable disturbances. Step S3 utilizes the material mass flow rate according to Formula 2. Target cooling rate Tower geometric parameters With air physical properties , , Directly calculate the required cooling air volume for each zone; among these parameters... The material flow rate is collected in real time by the material flow detection device, and the other parameters are calibrated before the device is put into operation. Therefore, the feedforward calculation can be completed and output within the same sampling period when the disturbance occurs. The contribution of the feedback loop is reflected in the online elimination of unmodeled disturbances and model residuals. The incremental PID algorithm in step S63 only outputs the frequency increment and not the absolute frequency value. The frequency converter internally maintains the frequency itself, and the cumulative error of the integral action will not cause an impact on the control output.

[0171] Experimental Example 3: This experiment was used to verify the beneficial effect of diagonal grouping alternating pulses relative to constant air distribution and four-way synchronous pulses on particle anti-sticking and dispersion described in step S43 of the present invention. The experimental subjects were three schemes: Example 1, Comparative Example 2, and Comparative Example 3. Each scheme was run continuously for 60 minutes. During the run, 500g samples were taken from the discharge port every 10 minutes. Each sampling point was sampled three times to examine batch-to-batch repeatability. The clustering rate index was determined according to the method for determining the agglomeration rate of controlled-release fertilizers. Samples were passed through a 2mm sieve, and the ratio of the mass of agglomerated material on the sieve to the total mass of the sample was used as the clustering rate. The clustering rate results for each sampling point were reported as the arithmetic mean and standard deviation of three parallel samplings.

[0172] Experimental results are as follows Figure 3 As shown. Figure 3 The horizontal axis represents the device's operating time. The vertical axis represents the clustering rate of each sampling point. The three curves with error bars correspond to the diagonal grouping alternating pulses of Example 1, the steady wind without pulses of Comparative Example 2, and the four-way synchronous pulses of Comparative Example 3, respectively. The error bars represent the standard deviation of the three parallel samplings.

[0173] Depend on Figure 3 It can be seen that the steady-state air distribution method in Comparative Example 2 has a high agglomeration rate in the early stage of operation. After 40 minutes, the agglomeration rate stabilizes at 4.0% to 5.0%, and the length of the error bar at each time point is the largest among the three methods. This indicates that the local deflection channel formed by the cooling air passing through the particle bed along a fixed path causes differences in the dispersion effect at different positions in the particle bed. Particles inside the deflection channel have difficulty acquiring shear momentum, while particles outside the deflection channel are better dispersed, resulting in an unstable overall dispersion effect. The agglomeration rate of the four-way synchronous pulse method in Comparative Example 3 stabilizes at around 3.0% after 30 minutes, which is an improvement over the steady-state air distribution. The pulse switching causes the particle bed to rise and fall as a whole, and the relative positions of the particles are rearranged. However, the airflow is completely interrupted instantaneously when the four channels are opened and closed synchronously. The mainstream direction of the cooling air at the bottom of the tower only switches between two states: with wind and without wind. The directionality of shear momentum is insufficient. In Example 1, the clustering rate of the diagonally grouped alternating pulse method decreased to 1.5% to 1.6% after 20 minutes and tended to stabilize. The error bar was significantly shorter than that of the two comparative examples. The mainstream direction of the cooling air at the bottom of the tower was determined by the pulse frequency. The particle bed is subjected to shear stress with changing direction in the horizontal plane by periodically switching between two perpendicular diagonal lines. The clusters formed initially due to surface viscosity are gradually broken up under the cumulative effect of alternating shear stress.

[0174] Example 1 reduced the steady-state clustering rate from 4.0% to 5.0% to 1.5% to 1.6% compared to Comparative Example 2, and Comparative Example 3 reduced the steady-state clustering rate from 3.0% to 1.5% to 1.6%. The difference between the two examples shows that diagonal grouping and alternating on / off cycles synergistically contribute to the particle dispersion effect. In the device of this invention, the four third air inlets are arranged at equal intervals along the circumference of the tower and diagonally grouped by pulse valves as a physical premise. The alternating control of the first and second on / off cycles in step S43 is a timing mechanism, forming a directionally controllable periodic shear airflow in the particle bed of the third zone. The directionality of the shear airflow is a necessary momentum source for effectively breaking up the agglomerated clumps. Comparative Example 2 lacks the timing mechanism, and Comparative Example 3 lacks the diagonal grouping effect in the physical premise. Neither of them can provide directional shear, and therefore their clustering rates are higher than those of Example 1.

[0175] Experiment Example 4: The experimental subjects in this experiment are 10 schemes, namely Examples 1 to 4 and Comparative Examples 1 to 6. Each scheme was run continuously for 90 minutes, and samples were taken in 3 batches. 500g of samples were taken from each batch, and four indicators were measured: adhesion rate, film integrity rate, cooling energy consumption per ton of material, and relative standard deviation of nutrient release rate over 28 days. The arithmetic mean of the results of each batch was taken as the final indicator of the scheme.

[0176] The methods for determining the adhesion rate and film integrity rate are the same as in Experiment 1. Cooling energy consumption per ton of material is the ratio of the cumulative power consumption of the supply and exhaust fans to the cumulative output mass during operation; power consumption is directly read from the power meter. The relative standard deviation of the 28-day nutrient release rate is determined by the 25℃ water bath immersion method. Ten samples (20g each) are randomly selected from the same batch of finished products, and each sample is independently immersed for 28 days before the nitrogen release rate is measured. The relative standard deviation of the nitrogen release rate of the ten samples is used as the dispersion index.

[0177] Experimental results are as follows Figure 4 As shown. Figure 4 (a) Figure 4 (b) Figure 4 (c) Figure 4 The middle (d) columns show the relative standard deviation of adhesion rate, film integrity rate, cooling energy consumption per ton, and nutrient release rate for 10 examples (Examples 1 to 4 and Comparative Examples 1 to 6), respectively. Examples are distinguished by warm-colored columns, while comparative examples are distinguished by cool-colored columns.

[0178] Depend on Figure 4As can be seen from (a), (b), (c), and (d), regarding the adhesion rate, the four warm-colored columns in Examples 1 to 4 are all controlled within 2% and are very close to each other, while the single-stage steady-state cooling in Comparative Example 1 reaches as high as 12.0%, and Comparative Example 6... The deviation from the lower limit of the range defined in step S22 of this invention reached 15.0%. Comparative Examples 2 and 3, due to the absence of diagonal alternating pulses or diagonal groupings, remained at 3.0% to 4.0%. Regarding film integrity, Examples 1 to 4 were concentrated between 97.5% and 98.5%, while Comparative Example 1, due to the lack of segmented cooling, only achieved 85.0%, and Comparative Example 5, due to... The deviation from the upper limit of the range defined in step S22 of this invention is only 80.0%. Regarding energy consumption per ton, Examples 1 to 4 range from 8.5 kWh / t to 10.0 kWh / t, while Comparative Example 1 reaches 14.0 kWh / t due to mismatch between airflow and heat load. Comparative Example 3 requires additional compensation from the blower at the same outlet temperature due to the instantaneous interruption of airflow during the synchronization pulse, reaching 11.0 kWh / t. Regarding the relative standard deviation of nutrient release rate over 28 days, Examples 1 to 4 range from 2.5% to 3.0%, while Comparative Example 1 reaches 8.5% and Comparative Example 6 reaches 7.0%. The dispersion of release rate is inversely correlated with the integrity rate of the membrane layer; the lower the integrity rate, the worse the batch consistency of the release rate.

[0179] The above description is merely 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 polyurethane-coated film urea tower-type air-cooled cooling device, characterized in that, The system includes the tower body, three air supply fans, one exhaust fan, four pulse valves, a material flow detection device, a temperature sensor group, and a controller. The tower body is divided into three zones from top to bottom: Zone 1, Zone 2, and Zone 3. Zone 1 is a slow cooling and pre-cooling zone, Zone 2 is a critical solidification zone, and Zone 3 is a strong cooling and dispersion zone. An upper annular flow equalization baffle is provided between Zone 1 and Zone 2, and a lower annular flow equalization baffle is provided between Zone 2 and Zone 3. The top of the tower body is equipped with a feed inlet, and the bottom of the tower body is equipped with a conical discharge section and a discharge outlet from top to bottom. The bottom end of the conical discharge section is equipped with a rotary discharge valve. The side wall of the first zone is equipped with an air outlet and a first air inlet, with the air outlet located above the upper annular flow equalization baffle. The side wall of the second zone is equipped with a second air inlet. The bottom of the third zone is equipped with four third air inlets at equal intervals along the circumference of the tower body. The three air supply fans include a first air supply fan, a second air supply fan, and a third air supply fan. The first air supply fan is connected to the first air inlet via a pipeline, the second air supply fan is connected to the second air inlet via a pipeline, and the third air supply fan is connected to the four third air inlets via branch pipelines. The exhaust fan is connected to the air outlet via a pipeline. The exhaust fan is used to keep the inside of the tower under negative pressure relative to atmospheric pressure. Four pulse valves are installed on the pipes of the four third air inlets. Two pulse valves that are diagonally opposite each other along the circumference of the tower body form the first group, and the other two pulse valves that are diagonally opposite each other along the circumference of the tower body form the second group. The material flow detection device is located at the feed inlet and is used to detect the mass flow rate of polyurethane-coated urea. The temperature sensor group includes temperature sensors located at the feed inlet, zone 1, zone 2, zone 3, and discharge outlet respectively; the controller is electrically connected to 3 air supply fans, 4 pulse valves, rotary discharge valve, material flow detection device, and the temperature sensor group respectively.

2. The polyurethane-coated film urea tower-type air-cooled cooling device according to claim 1, characterized in that: Both the upper annular flow equalization baffle and the lower annular flow equalization baffle are provided with material discharge holes at their centers. An airflow distribution plate is provided around the material discharge holes, and the airflow distribution plate has a plurality of airflow holes. The upper surface of the airflow distribution plate slopes downward from the outer edge of the airflow distribution plate toward the material discharge holes. The material discharge holes allow the polyurethane-coated urea to pass downward. The airflow holes allow cooling air to pass through, and the diameter of the airflow holes is smaller than the particle size of the polyurethane-coated urea. The first air inlet and the second air inlet are offset from each other in the circumferential orientation of the tower body; the four pulse valves are electrically adjustable butterfly valves, which are driven by the pulse width modulation signal output by the controller and used to adjust the opening degree; the air outlet is provided with a wire mesh, the pore size of which is smaller than the particle size of the polyurethane-coated urea, and the wire mesh is used to prevent the polyurethane-coated urea from being discharged from the tower body with the airflow.

3. A tower-type air-cooling method for polyurethane-coated urea, characterized in that: The method is implemented using the polyurethane-coated film urea tower-type air-cooled cooling device as described in claim 1 or 2, and includes the following steps: S1. Determine the glass transition temperature and critical cooling rate of the polyurethane film contained in the polyurethane-coated urea, and input the glass transition temperature and the critical cooling rate into the controller; S2. The controller sets the target cooling rates for Zone 1, Zone 2, and Zone 3 respectively based on the critical cooling rate; S3, The controller determines the material mass flow rate. Based on the target cooling rates of the first zone, the second zone, and the third zone, the initial frequency values ​​of the three fan supply fans are determined respectively. S4. The controller starts the three supply fans, the exhaust fans and the four pulse valves, so that the pulse valves of the first group and the pulse valves of the second group are alternately turned on and off in diagonal groups, forming a periodic turbulent airflow in the third zone; S5. The polyurethane-coated urea is continuously fed into the tower body through the feed inlet, so that the polyurethane-coated urea flows sequentially through the first zone, the second zone, and the third zone. S6. The controller collects the material temperatures at the feed inlet, in zone 1, in zone 2 and in zone 3 measured by the temperature sensor group according to the sampling period, adjusts the working frequency of the three blowers respectively in a combination of feedforward and feedback, and adjusts the pulse frequency of the four pulse valves according to the material temperature at the feed inlet, the material temperature in zone 3 and the glass transition temperature. S7. The controller controls the rotary unloading valve to discharge the cooled polyurethane-coated urea through the discharge port.

4. The polyurethane-coated film urea tower-type air-cooling method according to claim 3, characterized in that, S1 includes the following sub-steps: S11. Using a differential scanning calorimeter, thermal analysis is performed on the polyurethane film sample at a heating rate of 10℃ / min. The glass transition temperature of the polyurethane film is determined from the midpoint temperature of the glass-to-elastic transition step in the obtained heat flow-temperature curve, and this glass transition temperature is denoted as... , The unit is ℃; S12. The critical cooling rate of the polyurethane film is denoted as... , The unit is ℃ / s. Determined by one of the following methods: Method 1, using the differential scanning calorimeter, thermal analysis is performed on the polyurethane film sample at heating rates of 0.5℃ / s, 1℃ / s, 2℃ / s, 3℃ / s, and 5℃ / s, respectively, where the polyurethane film is located... The highest heating rate among those corresponding to the relaxation peaks nearby without distortion is selected as the [value]. The value of; Method 2, based on the chemical composition and thickness of the polyurethane film, The values ​​are taken as empirical values ​​within the range of 1.5℃ / s to 3℃ / s; S13, Set the glass transition temperature and the critical cooling rate Input the controller to obtain the cooling rate reference of the method.

5. The polyurethane-coated film urea tower-type air-cooling method according to claim 4, characterized in that, The target cooling rates of the first zone, the second zone, and the third zone are respectively denoted as... , , , , , The units are all ℃ / s; S2 includes the following sub-steps: S21, The controller sets the target cooling rate of the first zone. Set as the critical cooling rate The polyurethane-coated urea is pre-cooled in the first zone at a cooling rate lower than the critical cooling rate by 0.4 to 0.6 times. S22, The controller sets the target cooling rate of the second zone. Set as the critical cooling rate The polyurethane-coated urea is cooled at a rate of 0.9 to 1.0 times the critical cooling rate within the second zone, allowing it to pass through the glass transition temperature at a rate not exceeding the critical cooling rate. The polyurethane film is then cured in a glassy state. S23, The controller sets the target cooling rate of the third zone. Set as the critical cooling rate The polyurethane-coated urea is cooled to the target outlet temperature in the third zone by 1.5 to 2.0 times the concentration of the polyurethane-coated urea.

6. The polyurethane-coated film urea tower-type air-cooling method according to claim 5, characterized in that, S3 includes the following sub-steps: S31, The controller calculates the downward linear velocity of the polyurethane-coated urea within the tower body using the following formula. : ； In the formula, The downward linear velocity of the polyurethane-coated urea within the tower body. The unit is m / s; The mass flow rate of the material; The bulk density of the polyurethane-coated urea is given. The unit is kg / m 3 ; The effective cross-sectional area of ​​the tower body is... The unit is m 2 ; The percentage of the polyurethane-coated urea in the tower body by volume. For dimensionless parameters, The value range is from 0.05 to 0.

70. The specific value is obtained through the cold calibration of the device; S32, the controller calculates the following formula: Cooling air volume of the zone : ； In the formula, For the district number, Choose 1, 2, or 3; For the first Cooling airflow in the area The unit is m 3 / s; The mass flow rate of the material; The specific heat capacity of the polyurethane-coated urea is given. The unit is J / (kg·K); For the first The target cooling rate of the zone; For the first Material bed height in the zone The unit is m; For the density of cooling air, The unit is kg / m 3 ; The specific heat capacity of cooling air. The unit is J / (kg·K); For the first The allowable temperature rise of the cooling air in the zone The unit is K; The downlink linear velocity obtained from S31; S33. The controller, based on the airflow-frequency characteristic curves of the three fans, calculates the cooling airflow obtained in S32. Converted to the corresponding number Initial frequency of the district's ventilation fans , The unit is Hz. Take 1, 2, and 3; the initial frequency values ​​of the air supply fans corresponding to Zone 1, Zone 2, and Zone 3 are respectively denoted as... , , .

7. The polyurethane-coated film urea tower-type air-cooling method according to claim 6, characterized in that, S4 includes the following sub-steps: S41, the controller operates according to the initial frequency value. , , Start the first air supply fan, the second air supply fan, and the third air supply fan respectively, so that the first air supply fan supplies cooling air to the first zone, the second air supply fan supplies cooling air to the second zone, and the third air supply fan divides the cooling air into four paths through the branch pipeline and sends them to the upstream side of the four pulse valves. S42. The controller starts the exhaust fan, so that the inside of the tower is under negative pressure relative to atmospheric pressure, so that the cooling air inside the tower is discharged through the air outlet. S43. The controller alternately controls the pulse valves of the first group and the pulse valves of the second group according to the first on-off cycle and the second on-off cycle. The first on-off cycle is when the pulse valves of the first group are open and the pulse valves of the second group are closed. The second on-off cycle is when the pulse valves of the first group are closed and the pulse valves of the second group are open, so that the airflow direction in the third zone is periodically deflected to obtain the periodic disturbed airflow.

8. The polyurethane-coated film urea tower-type air-cooling method according to claim 7, characterized in that, S5 includes the following sub-steps: S51, the polyurethane-coated urea is expressed at the material mass flow rate. The feed continuously enters the first zone through the feed inlet; S52. The polyurethane-coated urea undergoes convective heat exchange with cooling air in the first zone, according to the target cooling rate. Cool the target temperature to above the glass transition temperature. The temperature is adjusted to obtain the pre-cooled material; S53. The pre-cooled material enters the second zone through the upper annular flow equalization baffle and is cooled at the target rate. To achieve the target cooling temperature across the glass transition temperature The polyurethane film is then cured in a glassy state to obtain the cured material. S54. The cured material enters the third zone through the lower annular flow equalization baffle, and is disturbed by the periodic turbulent airflow, cooling at the target rate. The urea is cooled to the target outlet temperature to obtain the cooled polyurethane-coated urea.

9. The polyurethane-coated film urea tower-type air-cooling method according to claim 8, characterized in that, The material temperature at the feed inlet collected by the temperature sensor group is recorded as follows: The material temperatures in zones 1, 2, and 3 collected by the temperature sensor group are respectively recorded as follows: , , , , , , The units are all in °C; S6 includes the following sub-steps: S61, the controller calculates the following formula: The material set temperature of the zone : ； In the formula, For the district number, Choose 1, 2, or 3; For the first The material set temperature in the zone, The unit is ℃; For the first The material inlet temperature of the zone, The unit is ℃. The value of is equal to , The value of is equal to , The value of is equal to ; For the first The target cooling rate of the zone; For the first Material residence time in the area, The unit is s. according to Calculated; S62, the controller operates according to the sampling period. The material temperatures in the first, second, and third zones are collected. , , The sampling period The value range is from 1s to 5s; And calculate the first one according to the following formula Material temperature deviation in the zone : ； In the formula, For the district number, Choose 1, 2, or 3; For the first The material temperature deviation in the current sampling period. The unit is ℃; The temperature sensor group collects the first data in the current sampling period. The material temperature in the zone; The first obtained for S61 The material set temperature in the zone; S63. The controller calculates the first step according to the incremental proportional-integral-derivative control algorithm using the following formula: Frequency increment of the district ventilation fan in the current sampling period : ； In the formula, For the district number, Choose 1, 2, or 3; For the first The frequency increment of the district's ventilation fans in the current sampling period The unit is Hz; For the first The proportion coefficient of the area, The unit is Hz / ℃; For the first The integral coefficient of the region, The unit is Hz / (℃·s); For the first The differential coefficients of the region, The unit is Hz·s / ℃; The current sampling period obtained by S62 is the first... Material temperature deviation in the zone; For the first The material temperature deviation in the area during the previous sampling period; For the first The material temperature deviation in the area during the previous sampling period; The sampling period is defined as follows; when or When the corresponding sampling period does not exist, the value of the corresponding variable is 0; S64, the controller updates the following formula: Operating frequency of district ventilation fans and the operating frequency Output to the corresponding number The district's ventilation fans: ； In the formula, For the district number, Choose 1, 2, or 3; For the first The operating frequency of the district's ventilation fans during the current sampling period. The unit is Hz; For the first The operating frequency of the district's ventilation fans in the previous sampling period. The value taken in the initial calculation is the [number]. Initial frequency value of the district ventilation fan ; The current sampling period obtained by S63 is the first... Frequency increment of district-level ventilation fans; S65, the controller calculates the pulse frequency of the four pulse valves according to the following formula. : ； In the formula, The pulse frequency of the four pulse valves. The unit is Hz. The value range is limited to 0.25Hz to 2.0Hz. When the calculated value according to the above formula is lower than 0.25Hz... Taking 0.25Hz, when the calculated value according to the above formula is higher than 2.0Hz... Select 2.0Hz; Based on the pulse frequency, The unit is Hz. The value range is from 0.3Hz to 0.8Hz; The temperature response coefficient, For dimensionless parameters, The value range is from 0.6 to 1.2; The temperature of the material in the third zone is collected by the temperature sensor group in the current sampling period; It is the glass transition temperature; The temperature of the material at the feed inlet is collected by the temperature sensor group during the current sampling period; S66, The controller calculates the duty cycle of the four pulse valves according to the following formula. : ； In the formula, The duty cycle of the four pulse valves is... It is a dimensionless parameter; The mass flow rate of the material detected by the material flow rate detection device in the current sampling period; The rated material mass flow rate of the device. The unit is kg / s. The value range is 0 to Correspondingly The value range is from 0.4 to 0.5; S67, the controller operates according to the pulse frequency obtained in S65. The duty cycle obtained from S66 Pulse control signals are output to the four pulse valves, causing the four pulse valves to operate at the specified pulse frequency. and the duty cycle It operates alternately according to the first on / off cycle and the second on / off cycle.

10. The polyurethane-coated film urea tower-type air-cooling method according to claim 9, characterized in that, S7 includes the following sub-steps: S71. The cooled polyurethane-coated urea is collected from the bottom of the third zone through the conical discharge section; S72. The controller controls the rotary unloading valve to rotate according to a preset unloading frequency, so that the polyurethane-coated urea collected in the conical discharge section is continuously discharged through the discharge port.

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