DAP production system and method

Abstract

The invention discloses a DAP production system and method.The DAP production system comprises a DAP pelletizer, an MAP pelletizer, a classifying screen and a crusher which are sequentially arranged, the DAP pelletizer and the MAP pelletizer are internally connected with a first tubular reactor and a second tubular reactor respectively, phosphoric acid with the low concentration reacts with ammonia in the first tubular reactor, and phosphoric acid with the low concentration reacts with ammonia in the second tubular reactor; then spraying the ammonium phosphate slurry on a material in a DAP granulator for granulation, burying an ammonia distributor in the material, ammonifying and neutralizing ammonium phosphate in the material, and feeding the neutralized and granulated granular DAP material into an MAP granulator; the method comprises the following steps of: reacting phosphoric acid with high concentration with ammonia in a second tubular reactor to generate ammonium phosphate slurry, spraying the ammonium phosphate slurry into an MAP granulator, forming MAP fine particles from the slurry in airflow in the MAP granulator, drying and granulating the material by using reaction heat, screening the dried material, and taking the qualified material as a target product.

Description

Technical Field

[0001] The present invention relates to a DAP production system and method, belonging to the technical field of diammonium phosphate production. Background Art

[0002] Diammonium phosphate (DAP), due to its high nutrient content and excellent physical properties, is particularly suitable for phosphorus-deficient soils and can be applied directly as a fertilizer. Annual domestic DAP production exceeds 13 million tons. DAP's nitrogen (N) content is higher than that of MAP (monoammonium phosphate, chemical name, MAP), almost double that of MAP, making it more suitable for crop production. However, DAP is less stable than MAP and is more susceptible to decomposition at high temperatures.

[0003] There are two common methods for producing DAP in the prior art: (1) Tank-type pre-neutralization-drum ammonia granulation process: When producing DAP, the molar ratio of ammonia to phosphoric acid in the pre-neutralization tank is controlled to be 1.3~1.4. The generated pre-neutralized slurry is pumped into the granulator and sprayed on the material. The ammonia distributor installed under the material layer of the granulator ammonia-neutralized the pre-neutralized slurry to a molar ratio of ammonia to phosphoric acid of 1.8. When using tank-type pre-neutralization, in order to maintain sufficient fluidity of the pre-neutralized slurry, the slurry has to contain more water. Obviously, this requires more energy consumption for increasing the return ratio and drying the product. Although a large amount of heat is released in the granulator, it is still not enough to completely dry the product. It is also necessary to dry it in another dryer. The energy consumption of drying the product accounts for about 40% of the total energy consumption. In order to ensure that the granules after granulation have a certain strength and do not deform before entering the dryer, it is usually necessary to maintain a return ratio of up to 5~6 times. Therefore, relatively large equipment is required, including large equipment such as granulation, drying, crushing and transmission systems, which will increase investment and operating costs.

[0004] (2) Single tubular reactor-rotary drum ammonia granulation process: When producing DAP, the molar ratio of ammonia to phosphoric acid in the tubular reactor is controlled to be 1.3~1.4. The generated pre-neutralized slurry can be directly sprayed onto the granulator material with the help of the pressure of the tubular reactor itself. The ammonia distributor installed under the granulator material layer ammoniates and neutralizes the pre-neutralized slurry to a molar ratio of ammonia to phosphoric acid of 1.8. The heat generated by phosphoric acid and ammonia in the tubular reactor and the heat released by continued ammonia in the granulator all enter the granulator. Part of this heat enters the water vapor and is discharged by the powerful exhaust fan, but this is not enough to reduce the material temperature to below 90℃. A cooling return device must be added to the granulation system. In order to reduce the granulator temperature to below 90℃, a large amount of cold return material is required, and the return ratio usually needs to be maintained at more than 4 times. Therefore, this process system consumes a lot of energy.

[0005] Therefore, it is necessary to improve the existing DAP production system, and it is urgent to provide a DAP production system and method with a short process, small footprint, low investment, low energy consumption and low return ratio. Summary of the Invention

[0006] The object of the present invention is to provide a DAP production system and method with a short process, small footprint, low investment, low energy consumption and small return ratio.

[0007] The DAP production system of the present invention adopts the following technical solution: a DAP production system, comprising a DAP granulator, a MAP granulator, a grading screen and a crusher arranged in sequence, wherein the DAP granulator and the MAP granulator are respectively connected to a first tubular reactor and a second tubular reactor, the first tubular reactor and the second tubular reactor both including an ammonia inlet, a phosphoric acid inlet and a slurry outlet, the slurry outlet of the first tubular reactor is located in the DAP granulator, an ammonia distributor is provided below the slurry outlet of the first tubular reactor in the DAP granulator, and the slurry outlet of the second tubular reactor is provided. The inlet is located in the MAP granulator, the top inlet of the MAP granulator is connected to the DAP material outlet at the bottom of the DAP granulator, the top outlet of the MAP granulator is connected to a cyclone separator, the grading screen includes a top inlet, a coarse particle outlet, a qualified particle outlet and a fine particle outlet arranged from top to bottom, the top inlet of the grading screen is connected to the MAP material outlet at the bottom of the MAP granulator, the coarse particle outlet of the grading screen is connected to the top inlet of the crusher, the bottom outlet of the cyclone separator, the fine particle outlet of the grading screen and the bottom outlet of the crusher are all connected to the top inlet of the DAP granulator through a conveying mechanism.

[0008] The grading screen is positioned higher than the MAP granulator, and a dry material elevator is provided between the top inlet of the grading screen and the MAP material outlet at the bottom of the MAP granulator.

[0009] The conveying mechanism includes a return conveyor and a return elevator connected to the return conveyor. The bottom outlet of the cyclone separator, the fine particle outlet of the grading screen, and the bottom outlet of the crusher are all connected to the inlet of the return conveyor. The outlet of the return conveyor is connected to the bottom outlet of the return elevator, and the top outlet of the return elevator is connected to the top inlet of the DAP granulator.

[0010] The DAP granulator and the MAP granulator are both drum granulators, and the DAP material outlet at the bottom of the DAP granulator is higher than the top inlet of the MAP granulator.

[0011] A spray head is provided at the slurry outlet of the first tubular reactor.

[0012] The production method of DAP of the present invention adopts the following technical scheme: A production method of DAP, which is carried out by using the above-mentioned DAP production system, comprises the following steps: (1) raw material feeding and DAP preparation section: ammonia and phosphoric acid from the boundary zone are respectively introduced into the ammonia inlet and phosphoric acid inlet of the first tubular reactor, the mass concentration of P2O5 in the phosphoric acid entering the first tubular reactor is greater than 40%, ammonia and phosphoric acid react to generate ammonium phosphate slurry, the ammonium phosphate slurry after the reaction is completed is sprayed from the slurry outlet of the first tubular reactor onto the material layer in the DAP granulator, ammonia gas enters the DAP granulator through the ammonia distributor, and ammonia enters the material The MAP in the material layer is further ammonified and neutralized inside the layer. The drum of the DAP granulator rolls to agglomerate the material into granules. After granulation, the granular DAP material enters the MAP granulator through the top inlet of the MAP granulator; (2) MAP preparation and DAP drying section: ammonia and phosphoric acid from the boundary area are respectively introduced into the ammonia inlet and phosphoric acid inlet of the second tubular reactor. The mass concentration of P2O5 in the phosphoric acid entering the second tubular reactor is greater than 46%, and greater than the mass concentration of P2O5 in the phosphoric acid entering the first tubular reactor. After the reaction of ammonia and phosphoric acid, ammonium phosphate slurry is generated and superheated steam is generated. The phosphorus after the reaction is completed is The ammonium slurry enters the MAP granulator from the slurry outlet of the second tubular reactor and is sprayed to the discharge end of the MAP granulator. The slurry forms MAP fine crystals with a particle size of 80μm-2mm in the air flow; the superheated steam sprayed from the second tubular reactor is used to dry the material in the MAP granulator; the dried MAP fine particles are transported into the cyclone separator from the top outlet of the MAP granulator with the hot air, and the MAP fine particles are completely separated in the cyclone separator. The separated MAP fine particles are discharged from the bottom outlet of the cyclone separator and returned to the DAP granulator as return material; the other dried MAP fine particles are mixed with Granular DAP is discharged from the MAP material outlet and enters the grading screen; (3) DAP discharging and MAP returning section: MAP fine particles and granular DAP are classified in the grading screen, and DAP coarse particles with a particle size greater than 4 mm are discharged from the coarse particle outlet and enter the crusher for crushing. The crushed dry material is discharged from the bottom outlet of the crusher as return material and returned to the DAP granulator as return material; MAP fine particles with a particle size less than 2 mm are discharged from the fine particle outlet and returned to the DAP granulator as return material; DAP qualified particles with a particle size between 2-4 mm are discharged from the qualified particle outlet and the grading screen as the target product.

[0013] Part of the MAP fine particles and granular DAP discharged from the MAP material outlet are lifted to the top inlet of the grading screen by the material elevator.

[0014] The MAP fine particles separated by the cyclone separator, the dry materials after the DAP coarse particles are crushed by the crusher, and the MAP fine particles discharged from the fine particle outlet of the grading screen are transported to the bottom inlet of the return material elevator through the return material elevator, and then lifted to the top inlet of the DAP granulator by the return material elevator.

[0015] The molar ratio of ammonia to phosphoric acid in the ammonium phosphate slurry generated in the first tubular reactor is NH3:H3PO4=13~15:10. Ammonia enters the material layer from the outlet of the ammonia distributor to further ammoniation and neutralize the MAP in the material layer. The molar ratio of ammonia to phosphoric acid in the material is NH3:H3PO4≤196:100. The granular DAP material discharged from the DAP granulator has a water content of less than 3% by mass.

[0016] The molar ratio of ammonia to phosphoric acid in the ammonium phosphate slurry generated in the second tubular reactor is NH3:H3PO4 less than 12:10; the temperature of the superheated steam ejected from the second tubular reactor is less than 150°C, and the water content of the granular DAP material discharged from the MAP granulator is less than 2% by mass; the ratio of return material to target product is 2~4:1, the water content of the target product is 1%~2% by mass, and the molar ratio of ammonia to phosphoric acid is NH3:H3PO4=18~19:10.

[0017] The beneficial effects of the present invention are as follows: the present invention first utilizes phosphoric acid with a relatively low concentration to react with ammonia in a first tubular reactor, and then sprays the ammonium phosphate slurry on the material in the DAP granulator for granulation; an ammonia distributor is buried in the material, and the ammonium phosphate in the material is ammoniated and neutralized, and the granulated DAP material after neutralization and granulation enters the MAP granulator; utilizes phosphoric acid with a relatively high concentration to react with ammonia in a second tubular reactor to generate ammonium phosphate slurry and generate superheated steam, and then sprays the ammonium phosphate slurry into the MAP granulator, and in the MAP granulator, the slurry forms MAP fine particles in the air flow, and the reaction heat is utilized to dry the material, and the granulated and dried material is screened, and the qualified material is used as the target product, namely, the DAP deboundary zone. The first tubular reactor of the present invention can use phosphoric acid containing a P2O5 mass concentration of 40% or more. The tail gas scrubbing liquid generated during production can be mixed with concentrated phosphoric acid and then used in the tubular reactor, and the scrubbing liquid does not need to be discharged. When the second tubular reactor uses phosphoric acid containing a P2O5 mass concentration of 46% or more, the high-concentration phosphoric acid reacts with ammonia to generate superheated steam. The superheated steam ejected from the second tubular reactor can dry the material in the MAP granulator without the need for external hot air heating.

[0018] This method utilizes phosphoric acid at varying concentrations and ammonia to react in a DAP granulator and a MAP granulator. The DAP produced in the DAP granulator is then fed into the MAP granulator, where excess liquid and heat are transferred. Excess heat from the DAP granulator is effectively utilized in the MAP granulator. The MAP fine particles, with a particle size of ≥80 μm, enter the cyclone separator with the hot air and are completely recovered by the cyclone for ammonia neutralization and granulation in the DAP granulator. This method can reduce the return ratio by three times, eliminates the need for product drying, and, at the same equipment scale, achieves a 50% increase in production capacity compared to a single tubular reactor-rotary drum ammonia granulation process.

[0019] The MAP in the MAP granulator has a large dispersion and high reactivity, and can be easily ammoniation-neutralized in the DAP granulator to the NH3:H3PO4 molar ratio required by DAP, and granulated in the DAP granulator. The temperature of the DAP granulator is controlled below 90°C, and the NH3:H3PO4 molar ratio in the product is 187:100, which is higher than the 18:10 of the tank pre-neutralization-rotary drum ammoniation granulation process and the single tube reactor-rotary drum ammoniation granulation process.

[0020] The present invention solves the problem that, in the prior art trough pre-neutralization-rotary drum amination granulation process for producing DAP, the slurry must contain a relatively high amount of water in order to maintain sufficient fluidity. This results in increased energy consumption for increasing the return ratio (5-6 times) and drying the product (energy consumption for drying the product accounts for approximately 40% of the total energy consumption), requiring larger equipment for the granulation, drying, crushing and transmission systems, and thus increasing investment and operating costs. The present invention also solves the problem that, in the single tubular reactor-rotary drum amination granulation process for producing DAP, the heat generated by phosphoric acid and ammonia in the tubular reactor and the heat released by continued amination in the granulator both enter the granulator. In order to reduce the temperature of the granulator to below 90°C, a relatively large amount of cold return material (a return ratio of more than 4 times) is required, necessitating the addition of a cooling return device to the granulation system.

[0021] The invention utilizes a double-tube reaction to solve the problems of high energy consumption, high investment and operating costs in the production of DAP by tank-type pre-neutralization and excessive heat load of the granulator in the production of DAP by a single-tube reactor. The invention rationally utilizes the reaction heat, has a low material return ratio, saves energy and reduces consumption, and protects the environment. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 This is an overall system diagram of a DAP production system according to an embodiment of the present invention; Figure 2 This is the performance assessment result of a 70 t / h DAP device in an application example of the present invention.

[0023] Among them, 1-first tubular reactor, 1.1-sprinkler, 2-DAP granulator, 2.1-ammonia distributor, 3-second tubular reactor, 4-MAP granulator, 5-dry material elevator, 6-grading screen, 6.1-coarse particle outlet, 6.2-qualified particle outlet, 6.3-fine particle outlet, 7-finished product conveyor, 8-crusher, 9-return material conveyor, 10-return material elevator, 11-cyclone separator. DETAILED DESCRIPTION

[0024] The technical solutions in the embodiments of this patent will be clearly and completely described below in conjunction with the drawings in the embodiments of this patent. Obviously, the described embodiments are part of the embodiments of this patent, not all of the embodiments. The components of the embodiments of this patent generally described and shown in the drawings herein can be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of this patent provided in the drawings is not intended to limit the scope of the patent claimed for protection, but merely represents selected embodiments of this patent. Based on the embodiments in this patent, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of this patent.

[0025] like Figure 1As shown, a DAP production system according to one embodiment of the present invention includes a DAP granulator 2, a MAP granulator 4, a grading screen 6, and a crusher 8, which are arranged in sequence. A first tubular reactor 1 and a second tubular reactor 3 are connected to the DAP granulator 2 and the MAP granulator 4, respectively. The first tubular reactor 1 and the second tubular reactor 3 each include an ammonia inlet, a phosphoric acid inlet, and a slurry outlet. A spray head is provided at the slurry outlet of the first tubular reactor 1. The slurry outlet of the first tubular reactor 1 is located within the DAP granulator 2. An ammonia distributor 2.1 is provided within the DAP granulator 2 below the slurry outlet of the first tubular reactor 1. The slurry outlet of the second tubular reactor 3 is located within the MAP granulator 4. The top inlet of the MAP granulator 4 is connected to the DAP material outlet at the bottom of the DAP granulator 2. The top outlet of the MAP granulator 4 is connected to a cyclone separator 11. The grading screen 6 includes, from top to bottom, a top inlet, a coarse particle outlet 6.1, a qualified particle outlet 6.2, and a fine particle outlet 6.3. The coarse particle outlet 6.1 and the qualified particle outlet 6.2 are located on the sides of the grading screen 6, while the fine particle outlet 6.3 is located at the bottom of the grading screen 6. The top inlet of the grading screen 6 is connected to the MAP material outlet at the bottom of the MAP granulator 4. The coarse particle outlet 6.1 of the grading screen 6 is connected to the top inlet of the crusher 8. The bottom outlets of the cyclone separator 11, the fine particle outlet of the grading screen 6, and the bottom outlet of the crusher 8 are all connected to the top inlet of the DAP granulator 2 via a conveying mechanism. Both the DAP granulator 2 and the MAP granulator 4 are rotary drum granulators. The DAP material outlet at the bottom of the DAP granulator 2 is higher than the top inlet of the MAP granulator 4. The first tubular reactor 1 and the ammonia distributor 2.1 are both located at the feed end of the DAP granulator. The first tubular reactor 1 is located above the material layer in the DAP granulator and the opening of the slurry outlet of the first tubular reactor 1 faces the material layer. The ammonia distributor 2.1 is located below the material layer in the DAP granulator. The second tubular reactor 3 is installed at the feed end of the MAP granulator. The second tubular reactor 3 is located above the material layer in the MAP granulator, and the slurry outlet opening of the second tubular reactor 3 faces the discharge end of the MAP granulator 4.

[0026] The grading screen 6 is a double-layer screen. The coarse particle outlet 6.1 on the side screens out DAP coarse particles, the qualified particle outlet 6.2 on the side screens out DAP qualified particles, and the fine particle outlet 6.3 at the bottom screens out MAP fine particles. The grading screen 6 is positioned higher than the MAP granulator 4. A dry material elevator 5 is provided between the top inlet of the grading screen 6 and the MAP material outlet at the bottom of the MAP granulator 4. The conveying mechanism includes a return conveyor 9 and a return elevator 10 connected to the return conveyor 9. The bottom outlet of the cyclone separator 11, the fine particle outlet 6.3 of the grading screen 6, and the bottom outlet of the crusher 8 are all connected to the inlet of the return conveyor 9. The outlet of the return conveyor 9 is connected to the bottom outlet of the return elevator 10, and the top outlet of the return elevator 10 is connected to the top inlet of the DAP granulator 2.

[0027] The method for producing DAP using the above-mentioned DAP production system comprises the following steps: (1) Raw material feeding and DAP preparation section: Ammonia and phosphoric acid from the boundary area are respectively introduced into the ammonia inlet and phosphoric acid inlet of the first tubular reactor. The mass concentration of P2O5 in the phosphoric acid entering the first tubular reactor is greater than 40% (usually 40%~55%). Ammonia and phosphoric acid react to form ammonium phosphate slurry. The molar ratio of ammonia to phosphoric acid in the ammonium phosphate slurry generated in the first tubular reactor is NH3:H3PO4=13~15:10. The ammonium phosphate slurry after the reaction is sprayed from the slurry outlet of the first tubular reactor onto the material layer in the DAP granulator. Ammonia gas enters the DAP granulator from the ammonia distributor, and ammonia enters the material layer to further ammoniation and neutralize the MAP in the material layer. The molar ratio of ammonia to phosphoric acid in the material is NH3:H3PO4≤196:100, usually NH3:H3PO4=185~196:100. The drum of the DAP granulator rolls to agglomerate the material into granules. After granulation is completed, the granular DAP material enters the MAP granulator through the top inlet of the MAP granulator; the water content of the granular DAP material is less than 3% by mass, usually 2%~3%.

[0028] (2) MAP preparation and DAP drying process: Ammonia and phosphoric acid from the boundary zone are respectively introduced into the ammonia inlet and phosphoric acid inlet of the second tubular reactor. The mass concentration of P2O5 in the phosphoric acid entering the second tubular reactor is greater than 46% (usually 46%~55%), and greater than the mass concentration of P2O5 in the phosphoric acid entering the first tubular reactor. Ammonia and phosphoric acid react to form ammonium phosphate slurry. The molar ratio of ammonia to phosphoric acid in the ammonium phosphate slurry generated in the second tubular reactor is NH3:H3PO4≤12:10, usually NH3:H3PO4=10~12:10; the ammonium phosphate slurry after the reaction is completed enters the MAP granulator from the slurry outlet of the second tubular reactor and is sprayed to the discharge end of the MAP granulator. The slurry forms particles with a diameter of 80μm in the air flow. The superheated steam ejected from the second tubular reactor is used to dry the material in the MAP granulator. A portion of the dried MAP fines (less than 40% by mass of the MAP fines, typically 20%-40%) are carried along with hot air (100°C) through the top outlet of the MAP granulator into a cyclone separator, where the MAP fines are completely separated. The separated MAP fines are discharged from the bottom outlet of the cyclone separator and returned to the DAP granulator as return material. Another portion of the dried MAP fines (more than 40% by mass of the MAP fines, typically 60%-80%) are discharged from the MAP material outlet together with the granular DAP and passed to a grading screen. The granular DAP material discharged from the MAP granulator contains less than 2% by mass, typically 1%-2%.

[0029] (3) DAP discharge and MAP return section: Part of the MAP fine particles and granular DAP are discharged from the MAP material outlet and lifted to the top inlet of the grading screen through the material elevator. The MAP fine particles and granular DAP are classified in the grading screen. The DAP coarse particles with a particle size greater than 4 mm are discharged from the coarse particle outlet and enter the crusher for crushing. The crushed dry material is discharged from the bottom outlet of the crusher as return material and returned to the DAP granulator as return material; MAP fine particles with a particle size less than 2 mm are discharged from the fine particle outlet and returned to the DAP granulator as return material; DAP qualified particles with a particle size between 2-4 mm are discharged from the qualified particle outlet of the grading screen as the target product, and the target product enters the de-bounding area of the finished product conveyor 7.

[0030] The ratio of return material to target product is 2~4:1, the mass percentage of water in the target product is 1%~2%, and the molar ratio of ammonia to phosphoric acid is NH3:H3PO4=18~19:10.

[0031] The MAP fine particles separated by the cyclone separator, the dry materials after the DAP coarse particles are crushed by the crusher, and the MAP fine particles discharged from the fine particle outlet of the grading screen are transported to the bottom inlet of the return material elevator through the return material elevator, and then lifted to the top inlet of the DAP granulator by the return material elevator.

[0032] The present invention first uses a relatively low concentration of phosphoric acid to react with ammonia in a first tubular reactor. This ammonium phosphate slurry is then sprayed onto the material in a DAP granulator for granulation. An ammonia distributor is embedded in the material to neutralize the ammonium phosphate in the material through ammoniation. The neutralized and granulated DAP granules then enter the MAP granulator. A higher concentration of phosphoric acid reacts with ammonia in a second tubular reactor to form an ammonium phosphate slurry, generating superheated steam. This ammonium phosphate slurry is then sprayed into the MAP granulator. In the MAP granulator, the slurry forms MAP fine particles in the airflow. The reaction heat is used to dry the material. The granulated and dried material is then screened, and qualified material is used as the target product—DAP debounding zone. The present invention utilizes a dual-tube reaction system and method to address the high energy consumption, investment, and operating costs of trough-type pre-neutralization DAP production, and the excessive heat load on the granulator in single-tube reactor DAP production. This system utilizes reaction heat efficiently, reduces material return ratios, and protects the environment, thereby saving energy and reducing consumption.

[0033] In a specific application example, the phosphoric acid entering the first tubular reactor in step (1) contains a P2O5 mass concentration of 40%, the molar ratio of ammonia to phosphoric acid in the ammonium phosphate slurry generated in the first tubular reactor is NH3:H3PO4=14:10, and the ammonia gas enters the ammoniation material of the DAP granulator through the ammonia distributor, and the molar ratio of ammonia to phosphoric acid is NH3:H3PO4 =187:100; the granular DAP material produced in the DAP granulator has a water content of 2.5% by mass; the phosphoric acid entering the second tubular reactor in step (2) has a P2O5 mass concentration of 54%, the molar ratio of ammonia to phosphoric acid in the ammonium phosphate slurry generated in the second tubular reactor is NH3:H3PO4=11:10, the MAP fine particles entering the cyclone separator account for 30% of the MAP fine particles by mass, the MAP fine particles discharged from the MAP material outlet account for 70% of the MAP fine particles by mass, and the granular DAP material discharged from the MAP material outlet has a water content of 1.3% by mass; in step (3), the ratio of return material to target product is 3:1, the target product has a water content of 1.3% by mass, and the molar ratio of ammonia to phosphoric acid is NH3:H3PO4=187:100. The performance evaluation results of the 70 t / h DAP device were carried out using this application example as shown below. Figure 2 As shown in the table.

Claims

1. A DAP production system, characterized by: It includes a DAP granulator, a MAP granulator, a grading screen and a crusher which are arranged in sequence. The DAP granulator and the MAP granulator are respectively connected to a first tubular reactor and a second tubular reactor. The first tubular reactor and the second tubular reactor both include an ammonia inlet, a phosphoric acid inlet and a slurry outlet. The slurry outlet of the first tubular reactor is located in the DAP granulator. An ammonia distributor is provided below the slurry outlet of the first tubular reactor in the DAP granulator. The slurry outlet of the second tubular reactor is located in the MAP granulator. The top inlet of the MAP granulator is connected to the DAP material outlet at the bottom of the DAP granulator. The top outlet of the MAP granulator is connected to a cyclone separator. The grading screen includes a top inlet, a coarse particle outlet, a qualified particle outlet and a fine particle outlet arranged from top to bottom. The top inlet of the grading screen is connected to the MAP material outlet at the bottom of the MAP granulator. The coarse particle outlet of the grading screen is connected to the top inlet of the crusher. The bottom outlet of the cyclone separator, the fine particle outlet of the grading screen and the bottom outlet of the crusher are all connected to the top inlet of the DAP granulator through a conveying mechanism.

2. The DAP production system according to claim 1, characterized in that: The grading screen is positioned higher than the MAP granulator, and a dry material elevator is provided between the top inlet of the grading screen and the MAP material outlet at the bottom of the MAP granulator.

3. The DAP production system according to claim 1, characterized in that: The conveying mechanism includes a return conveyor and a return elevator connected to the return conveyor. The bottom outlet of the cyclone separator, the fine particle outlet of the grading screen, and the bottom outlet of the crusher are all connected to the inlet of the return conveyor. The outlet of the return conveyor is connected to the bottom outlet of the return elevator, and the top outlet of the return elevator is connected to the top inlet of the DAP granulator.

4. The DAP production system according to claim 1, wherein: The DAP granulator and the MAP granulator are both drum granulators, and the DAP material outlet at the bottom of the DAP granulator is higher than the top inlet of the MAP granulator.

5. The DAP production system according to claim 1, wherein: A spray head is provided at the slurry outlet of the first tubular reactor.

6. A method for producing DAP, which is carried out using the DAP production system according to any one of claims 1 to 5, characterized in that: It includes the following steps: (1) Raw material feeding and DAP preparation section: Ammonia and phosphoric acid from the boundary zone are respectively introduced into the ammonia inlet and phosphoric acid inlet of the first tubular reactor. The mass concentration of P2O5 in the phosphoric acid entering the first tubular reactor is greater than 40%. Ammonia and phosphoric acid react to form ammonium phosphate slurry. The ammonium phosphate slurry after the reaction is completed is sprayed from the slurry outlet of the first tubular reactor onto the material layer in the DAP granulator. Ammonia gas enters the DAP granulator through the ammonia distributor. Ammonia enters the material layer to further ammoniation and neutralize the MAP in the material layer. The drum of the DAP granulator rolls to agglomerate the material into granules. After granulation, the granular DAP material enters the MAP granulator through the top inlet of the MAP granulator; (2) MAP preparation and DAP drying section: Ammonia and phosphoric acid from the boundary zone are respectively introduced into the ammonia inlet and phosphoric acid inlet of the second tubular reactor. The mass concentration of P2O5 in the phosphoric acid entering the second tubular reactor is greater than 46%, and greater than the mass concentration of P2O5 in the phosphoric acid entering the first tubular reactor. Ammonia and phosphoric acid react to form ammonium phosphate slurry and generate superheated steam. The ammonium phosphate slurry after the reaction is completed enters the MAP granulator from the slurry outlet of the second tubular reactor and is sprayed to the discharge end of the MAP granulator. The slurry forms MAP fine crystals with a particle size of 80μm-2mm in the air flow; The superheated steam ejected from the second tubular reactor is used to dry the material in the MAP granulator; the dried MAP fine particles are transported along with the hot air from the top outlet of the MAP granulator into the cyclone separator, where they are completely separated. The separated MAP fine particles are discharged from the bottom outlet of the cyclone separator and returned to the DAP granulator as return material; the remaining dried MAP fine particles are discharged from the MAP material outlet together with the granular DAP and enter the grading screen; (3) DAP discharging and MAP returning section: MAP fine particles and granular DAP are classified in the grading screen, and DAP coarse particles with a particle size greater than 4 mm are discharged from the coarse particle outlet and enter the crusher for crushing. The crushed dry material is discharged from the bottom outlet of the crusher as return material and returned to the DAP granulator as return material; MAP fine particles with a particle size less than 2 mm are discharged from the fine particle outlet and returned to the DAP granulator as return material; DAP qualified particles with a particle size between 2 and 4 mm are discharged from the qualified particle outlet and the grading screen as the target product.

7. The DAP production system according to claim 6, characterized in that: Part of the MAP fine particles and granular DAP discharged from the MAP material outlet are lifted to the top inlet of the grading screen by the material elevator.

8. The DAP production system according to claim 6, characterized in that: The MAP fine particles separated by the cyclone separator, the dry materials after the DAP coarse particles are crushed by the crusher, and the MAP fine particles discharged from the fine particle outlet of the grading screen are transported to the bottom inlet of the return material elevator through the return material elevator, and then lifted to the top inlet of the DAP granulator by the return material elevator.

9. The DAP production system according to claim 6, characterized in that: The molar ratio of ammonia to phosphoric acid in the ammonium phosphate slurry generated in the first tubular reactor is NH3:H3PO4=13~15:

10. Ammonia enters the material layer from the outlet of the ammonia distributor to further ammoniation and neutralize the MAP in the material layer. The molar ratio of ammonia to phosphoric acid in the material is NH3:H3PO4≤196:

100. The granular DAP material discharged from the DAP granulator has a water content of less than 3% by mass.

10. The DAP production system according to claim 6, characterized in that: The molar ratio of ammonia to phosphoric acid in the ammonium phosphate slurry generated in the second tubular reactor is NH3:H3PO4 less than 12:10; the temperature of the superheated steam ejected from the second tubular reactor is less than 150°C, and the water content of the granular DAP material discharged from the MAP granulator is less than 2% by mass; the ratio of return material to target product is 2~4:1, the water content of the target product is 1%~2% by mass, and the molar ratio of ammonia to phosphoric acid is NH3:H3PO4=18~19:10.

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