Emission reduction device for ammonia in granulation tail gas of urea production device

Abstract

The utility model discloses an emission reduction device for ammonia in granulation tail gas of a urea production device, which comprises a combined tower body, a deamination section arranged at the lower part in the combined tower body, an ammonia recovery section arranged at the upper part in the combined tower body, and a partition plate and a gas rising cap arranged between the deamination section and the ammonia recovery section, the gas phase between the deamination section and the ammonia recovery section is communicated, and the liquid phase is not communicated. The process device is short in process, compact in equipment arrangement, small in size, light in weight, capable of being flexibly installed in a granulation tower or a granulation plant, low in investment cost and operation cost and long in equipment service life.

Description

Technical Field

[0001] The utility model belongs to the field of chemical equipment, and particularly relates to a process device for reducing ammonia emission in urea granulation tail gas. Background Art

[0002] Agricultural production cannot do without fertilizers, especially nitrogen fertilizers. It can be said that nitrogen fertilizers are the "staple food" of agriculture, and most of the nitrogen fertilizers are urea. The global annual production of urea exceeds 200 million tons. Urea emits a large amount of tail gas during the production process, and the pollutants in the tail gas are urea dust and free ammonia. The environmental protection emission standards of various countries have different requirements for the emission concentration indicators of dust and ammonia. Developed countries have very low requirements, generally at 30mg / Nm 3 Below, some countries and regions even require 10mg / Nm 3 the following.

[0003] Urea dust and free ammonia pollutants are mainly generated during the granulation process in the urea production process. There are two main ways to granulate urea. One is natural granulation through a granulation tower. More than 90% of China's total urea production capacity uses the natural granulation process of a granulation tower. The other is mechanical granulation, including circulating fluidized bed granulation and drum granulation. The application in China is relatively small, and the proportion in other parts of the world is higher than that in China. Because the natural granulation process of the granulation tower has low investment and operating costs, more than half of the urea production in the world uses this process.

[0004] The natural granulation process of the granulation tower is that the molten urea is sprayed into fine droplets through the nozzle in the granulation tower. The molten urea droplets are countercurrently contacted with the air coming up from the bottom of the granulation tower to complete the crystallization and cooling process. When the molten urea droplets are ejected from the small holes of the granulator and in the process of crystallization, fine urea dust will inevitably be generated. The large dust will fall to the bottom of the tower, and the small dust will be discharged from the granulation tower with the hot air and enter the atmosphere to produce urea dust pollution. The concentration is generally 80-300 mg / m 3 Free ammonia is dissolved in molten urea during the urea concentration process. In the pipeline from the concentration equipment to the granulator, the molten urea will condense into a by-product biuret and generate free ammonia. The longer the residence time, the more biuret and free ammonia will be generated.

[0005] Usually, the free ammonia content in the molten urea after concentration in the molten urea concentration device is 400-600ppm (wt). The length of the pipeline from the concentration device to the granulator is generally 70-130 meters. Free ammonia is released in the process of urea polymerization to generate by-product biuret in the pipeline. Generally, the free ammonia content before the granulator is 800-1100ppm (wt). During the heat release and cooling process of the molten urea droplet crystallization, except for the free ammonia remaining in the solid urea product, which is generally less than 100ppm (wt), the remaining free ammonia will be released in the molten urea droplet crystallization process, enter the cooling air, and finally discharged into the atmosphere from the top of the granulation tower, with a general concentration of 70-150mg / Nm 3 In addition, the free ammonia in the molten urea is released in the form of gas during the crystallization of the molten urea droplets, and tiny droplets are brought out during the release process to form urea dust. The more free ammonia content, the more dust is generated during the release process. If the urea dust and free ammonia in the tail gas from the granulation tower are pollutants and enter the atmosphere without treatment, they will inevitably pollute the environment.

[0006] Whether it is fluidized bed mechanical granulation or rotary drum mechanical granulation, the granulation process uses small-sized urea as "seed crystals". By spraying molten urea on the surface of the "seed crystals", cooling it with air sent by the blower, spraying molten urea while cooling, the "seed crystals" gradually grow into larger particles of urea. The amount of dust generated by the mechanical granulation process is very large, accounting for about 5% of the output, with a large proportion. This part of dust must be dust-removed during the granulation process design. Since the amount of dust generated by mechanical granulation is very large, it is economically reasonable to use wet washing to wash the urea dust into urea solution for recycling.

[0007] Urea is a substance that easily absorbs water and has a relatively high solubility in water. The tail gas containing urea dust can reach 30mg / Nm through wet scrubbing process (i.e. water washing). 3 By reducing the concentration of the detergent, it can also reach 10mg / Nm 3 Below, this requires a large amount of clean water to be added, and the recovery of very dilute urine also requires additional energy consumption. Another process for treating urea dust is dry dust removal, that is, using filter bags or filter screens to filter urea dust. The denser the filter bag or filter screen, the better the filtering effect. The urea dust in the exhaust gas can be treated to 5mg / Nm 3 The following shows that the dust removal effect is very good, and no water needs to be added during the dust removal process. The exhaust gas discharged is unsaturated gas. Even in winter, it is not visually visible that there is exhaust gas discharged from the granulation device (while the exhaust gas discharged by wet scrubbing is saturated, and white exhaust gas can be clearly seen, especially in winter). However, dry dust removal is also unable to deal with free ammonia in the exhaust gas.

[0008] Regardless of the granulation process, whether it is a simple wet dust removal process using water washing or a dry dust removal process, the purpose of treating urea dust can be achieved, but the purpose of removing ammonia cannot be achieved. To achieve the purpose of removing ammonia by the water washing method, it is necessary to add acid washing on the basis of water washing, and use sulfuric acid, hydrochloric acid, nitric acid, etc. to react with ammonia to generate ammonium salts to achieve the purpose of treating ammonia in the tail gas. Regardless of the granulation process, the amount of tail gas from granulation is very huge. For every ton of urea product produced, 8000 to 12000 Nm 3 For a urea plant with an annual capacity of 800,000 tons, the total tail gas volume is between 640,000 and 960,000 Nm 3 / h. Adding acid washing to treat these tail gases will inevitably increase the resistance drop of the washing equipment, that is, increase the wind pressure of the tail gas fan and increase power consumption. In addition, the washing equipment is huge, and sulfuric acid or hydrochloric acid washing has high requirements on the material of the equipment, and the equipment investment also increases significantly.

[0009] For the granulation process of granulation tower, whether it is wet washing or dry dust removal, adding acid washing is extremely difficult, especially for old equipment that did not take exhaust gas treatment into consideration when designing the granulation tower. Adding water washing and dry dust removal during the transformation process has already used up the safety margin when designing the granulation tower. It is almost impossible to add acid washing facilities.

[0010] Therefore, the removal of ammonia from the granulation tail gas of urea production equipment is an environmental problem that is difficult to achieve and has high costs (investment and operation) worldwide. Utility Model Content

[0011] The purpose of this application is to address the defects of the prior art and to provide an efficient ammonia removal process device with a short process, compact layout, small footprint and low operating cost. Ammonia removal is carried out before granulation, which fundamentally solves the global industry problem of high ammonia emission concentration in granulation tail gas, and can achieve ultra-low concentration emissions of ammonia in urea granulation tail gas, or even zero emissions.

[0012] In order to achieve the above-mentioned purpose, the present application adopts the following technical scheme: a device for reducing the emission of ammonia in the granulation tail gas of a urea production device, characterized in that it comprises a combined tower body, a deammoniation section at the lower part and an ammonia recovery section at the upper part arranged in the combined tower body, and a partition plate and a gas lifting cap arranged between the deammoniation section and the ammonia recovery section, wherein the gas phase is connected with the ammonia recovery section, but the liquid phase is not connected;

[0013] A deamination layer is provided in the deamination section, and the deamination layer is a packing structure or a tower tray, or a combination of a tower tray and a packing structure; a molten urea inlet is provided above the deamination layer, and a hot gas inlet is provided below the deamination layer;

[0014] A demisting layer and a recovery layer are arranged from top to bottom in the ammonia recovery section, and a washing liquid inlet and an outlet are respectively arranged above and below the recovery layer.

[0015] Furthermore, the packing structure adopts random packing or structured packing; the tower tray adopts sieve plate tray or floating valve tray.

[0016] Furthermore, a gas heater is arranged outside the hot gas inlet.

[0017] Furthermore, a molten urea outlet is provided at the bottom of the deammoniation section, and the molten urea outlet is connected to a granulator.

[0018] Furthermore, the recovery layer is a packing structure or a tower tray, or a combination of a tower tray and a packing structure, and a washing liquid inlet and outlet are respectively arranged above and below the recovery layer.

[0019] Furthermore, a washing liquid circulation pipeline is arranged outside the recovery layer, between the washing liquid inlet and outlet, and an acid mixer and a circulating washing pump are arranged on the washing liquid circulation pipeline.

[0020] The present application discloses a device for reducing ammonia in the granulation tail gas of a urea production device. The deammoniation section and the ammonia recovery section are arranged in a combined tower body. The combined tower body is provided with an upper section and a lower section, a total of two sections. The upper and lower sections are separated by a partition and a gas lifting cap to achieve gas phase communication but no liquid phase communication.

[0021] The lower section in the combined tower body is a molten urea free ammonia removal section, which is provided with a molten urea inlet, a molten urea outlet and a hot gas inlet.

[0022] A gas heater is arranged on the hot gas inlet pipe at the lower part of the combined tower body, and steam or electricity is used to heat the gas to a temperature higher than the crystallization temperature of the molten urea. The gas used can be air or other inert gas, preferably air, and can be compressed air or ambient air pressurized by a blower. The gas is heated to 130-150°C, preferably 135-145°C.

[0023] The acid used for free ammonia recovery can be sulfuric acid, nitric acid, hydrochloric acid, phosphoric acid, etc., which can be selected based on the availability of raw materials of the user.

[0024] By using the ammonia emission reduction device for granulation tail gas of a urea production device of the present application to remove ammonia from the granulation tail gas, most of the free ammonia in the molten urea is separated before the molten urea enters the granulator, and the free ammonia in the molten urea entering the granulation device is greatly reduced, and the free ammonia in the molten urea before the granulator can be reduced from 800 to 1100 ppm (wt) to less than 100 ppm (wt). Even if all the free ammonia in the molten urea in the granulator enters the granulation tail gas, the ammonia content in the granulation tail gas does not exceed 12 mg / Nm3 If the free ammonia in the urea granules is deducted, the free ammonia content in the granulation tail gas should be less than 10 mg / Nm 3 , meeting the most stringent exhaust emission standards at home and abroad. The process device of this patent application has a short process, compact equipment layout, small size, light weight, can be flexibly installed in the granulation tower or granulation tower plant, low investment cost and operation cost, and long equipment service life. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 The present invention is a schematic diagram of the structure of a device for reducing ammonia in granulation tail gas of a urea production device according to an embodiment. DETAILED DESCRIPTION

[0026] In order to enable those skilled in the art to better understand the present application, the technical solutions in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are only embodiments of a part of the present application, not all embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in the field without creative work should fall within the scope of protection of the present application.

[0027] It should be noted that the terms "including" and "having" and any variations thereof in the specification and claims of the present application and the above-mentioned drawings are intended to cover non-exclusive inclusions. For example, a process, method, system, product or apparatus comprising a series of steps or units is not necessarily limited to those steps or units clearly listed, but may include other steps or units that are not clearly listed or inherent to these processes, methods, products or apparatuses.

[0028] See also Figure 1 The deamination section 2 and the ammonia recovery section 3 are arranged in a combined tower body 1. A partition 7 and a gas lift cap 8 are arranged in the combined tower body to separate the lower deamination section and the upper ammonia recovery section, with the gas phase communicating and the liquid phase not communicating.

[0029] A deamination layer is provided in the deamination section. The deamination layer is a packing or a tray, or a combination of a tray and a packing. A molten urea inlet 10 is provided above the deamination layer, and a hot gas inlet 17 is provided below the deamination layer. Molten urea enters the deamination section from the molten urea inlet, and performs mass and heat transfer with the hot gas entering from the hot gas inlet in the deamination layer, thereby extracting free ammonia in the molten urea.

[0030] The bottom of the deamination section is provided with a molten urea outlet 11. The molten urea outlet is connected to a granulator 18, and the molten urea after gas stripping is discharged from the molten urea outlet and granulated by the granulator.

[0031] A gas heater 5 is arranged outside the hot gas inlet, and the gas heater uses steam or electricity to heat the gas to 130-150° C.; the gas is air or inert gas.

[0032] A demisting layer 4 and a recovery layer 19 are arranged from top to bottom in the ammonia recovery section. The demisting layer is used to eliminate the entrainment of mist and foam in the exhaust gas, and the demisting layer adopts a wire mesh demister. A washing water inlet 13 and a tail gas outlet 12 are arranged above the demisting layer, and the washing water entering the washing water inlet is used to clean the demisting layer. The washing water used can be desalted water or the purified process condensate of the urea device. The acidic solution reacts with the ammonia in the gas in the recovery layer to generate ammonium salt, and the ammonia in the gas is recovered. The washing liquid inlet 16 and outlet 15 are respectively arranged above and below the recovery layer.

[0033] A washing liquid circulation pipeline is arranged outside the recovery layer, between the washing liquid inlet and outlet, and an acid mixer 9 and a circulating washing pump 6 are arranged on the washing liquid circulation pipeline. After the acid solution washes the gas in the recovery layer, it enters the washing liquid circulation pipeline through the washing liquid outlet, and the recovered liquid enters the acid mixer through the circulating washing pump, and after mixing with the acid solution added to the acid mixer, it enters the recovery layer through the washing liquid inlet for recycling.

[0034] The molten urea sent from the molten urea feed pipeline enters the upper part of the lower ammonia removal section 2 of the combined tower body 1 through the molten urea feed port 10. The gas for ammonia removal is treated by the hot gas device 5, and the temperature of the gas is heated to above 130°C. It enters the lower part of the ammonia removal section 2 through the hot gas inlet 17. The hot gas and the molten urea transfer mass and heat in the ammonia removal section to extract the free ammonia in the molten urea. The free ammonia in the molten urea before the granulator can be reduced from 800 to 1100 ppm (wt) to less than 100 ppm (wt). Even if all the free ammonia in the molten urea in the granulator enters the granulation tail gas, the ammonia content in the granulation tail gas does not exceed 12 mg / Nm 3 If the free ammonia in the urea granules is deducted, the free ammonia content in the granulation tail gas should be less than 10 mg / Nm 3 .

[0035] The ammonia-containing gas leaving the ammonia removal section 2 enters the lower part of the ammonia recovery section 3 through the gas lift cap 8 on the partition 7. The ammonia in the gas is washed by washing water and sulfuric acid in the ammonia recovery section 3, and the ammonium sulfate solution byproduct is generated and discharged from the washing liquid outlet. The washing water is first used for demisting the demisting layer 4 at the top of the ammonia recovery section to avoid acid mist in the discharged tail gas. Sulfuric acid is added to the acid mixer 9 and mixed with the circulating washing liquid. The circulating washing liquid in the tower enters the inlet of the circulating washing pump 6 from the circulating washing liquid outlet 15, and the circulating washing of the ammonia recovery section is realized by the circulating washing pump 6. The circulating washing liquid at the outlet of the acid mixer 9 enters the upper part of the ammonia recovery section through the circulating washing liquid inlet 16.

[0036] The washing water used for ammonia recovery is added through the washing water inlet 13, and the amount of washing water added is determined according to the amount of ammonia recovered. By controlling the pH value of the circulating washing liquid, the ammonia concentration in the tail gas leaving the ammonia recovery section is not higher than 10mg / Nm 3 It can be discharged directly into the atmosphere, or sent into the granulation tail gas for mixing and then discharged into the atmosphere together.

[0037] The lower ammonia removal section 2 and the upper recovery layer 19 of the combined tower body 1 can use fillers such as random packing, structured packing, etc.; they can also use trays, such as sieve plate trays, float valve trays, etc.; or a combination of fillers and trays. The demisting section 4 uses a wire mesh demister or other high-efficiency demisters.

[0038] The above is a further detailed description of the present application, which shall not be regarded as a limitation on the specific implementation of the present application. For ordinary technicians in the technical field to which the present application belongs, simple deduction or replacement without departing from the concept of the present application shall fall within the protection scope of the present application.

Claims

1. A device for reducing ammonia emissions from granulation tail gas in a urea production device, characterized in that: The utility model comprises a combined tower body, a deamination section at the lower part and an ammonia recovery section at the upper part arranged in the combined tower body, and a partition plate and a gas lifting cap arranged between the deamination section and the ammonia recovery section, wherein the deamination section and the ammonia recovery section are connected in gas phase but not in liquid phase; A deamination layer is provided in the deamination section, and the deamination layer is a packing structure or a tower tray, or a combination of a tower tray and a packing structure; a molten urea inlet is provided above the deamination layer, and a hot gas inlet is provided below the deamination layer; A demisting layer and a recovery layer are arranged from top to bottom in the ammonia recovery section, and a washing liquid inlet and an outlet are respectively arranged above and below the recovery layer.

2. The device for reducing ammonia in granulation tail gas of a urea production device according to claim 1, characterized in that: The packing structure adopts random packing or structured packing; the tower tray adopts sieve plate tower tray or floating valve tower tray.

3. The device for reducing ammonia in granulation tail gas of a urea production device according to claim 1, characterized in that: A gas heater is arranged outside the hot gas inlet.

4. The device for reducing ammonia in granulation tail gas of a urea production device according to claim 1, characterized in that: A molten urea outlet is provided at the bottom of the deammoniation section, and the molten urea outlet is connected to a granulator.

5. The device for reducing ammonia emission in granulation tail gas of a urea production device according to claim 1, characterized in that: The recovery layer is a packing structure or a tower tray, or a combination of a tower tray and a packing structure. A washing liquid inlet and an outlet are respectively arranged above and below the recovery layer.

6. The device for reducing ammonia emission in granulation tail gas of a urea production device according to claim 5, characterized in that: A washing liquid circulation pipeline is arranged outside the recovery layer, between the washing liquid inlet and outlet, and an acid mixer and a circulating washing pump are arranged on the washing liquid circulation pipeline.

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