Emission reduction device for ammonia in granulation exhaust gas from urea production device

Abstract

An emission reduction device for ammonia in granulation exhaust gas from a urea production device. The emission reduction device comprises a combined tower body (1), a deamination section (2) arranged at a lower portion inside the combined tower body (1), and an ammonia recovery section (3) arranged at an upper portion inside the combined tower body; and further comprises a partition plate (7) and a gas riser (8) that are arranged between the deamination section (2) and the ammonia recovery section (3), wherein the deamination section (2) and the ammonia recovery section (3) are in gas phase communication but not in liquid phase communication. The emission reduction device can be flexibly mounted in a granulation tower or a granulation plant, has low investment costs and operation costs, and has a prolonged service life.

Description

A device for reducing ammonia in urea production plant prilling tail gas TECHNICAL FIELD

[0001] The utility model belongs to the field of chemical equipment, especially relates to a kind of ammonia reduction process device for urea prilling tail gas. BACKGROUND

[0002] Agricultural production cannot do without chemical fertilizer, especially nitrogen fertilizer, which can be said to be the "main food" of agriculture, and most of the nitrogen fertilizer is urea, with global urea production exceeding 200 million tons per year. Urea emits a large amount of tail gas in the production process, and the pollutants in the tail gas are urea dust and free ammonia. Different countries have different emission standards for dust and ammonia concentration, and developed countries require very low dust and ammonia concentration, generally less than 30 mg / Nm 3 Some countries and regions even require less than 10 mg / Nm 3 .

[0003] Urea dust and free ammonia pollutants generated during urea production are mainly produced during granulation. There are two main ways of urea granulation, one is natural granulation through a prilling tower, more than 90% of China's total urea production capacity uses this process; the other is mechanical granulation, including circulating fluidized bed granulation and rotary drum granulation, which is less used in China but more used in other parts of the world. Because of the low investment and operating cost of the prilling tower natural granulation process, more than half of the world's urea production uses this process.

[0004] The prilling tower natural granulation process is to spray molten urea into fine droplets through a nozzle in the prilling tower, and the molten urea droplets contact the air coming from the bottom of the tower in a countercurrent manner to complete the crystallization and cooling process. In the process of spraying molten urea droplets from the small holes of the prilling machine and in the process of crystallization, fine urea dust is inevitably produced. Large particle size dust falls to the bottom of the tower, and small particle size dust is discharged from the prilling tower with hot air into the atmosphere, causing urea dust pollution, with a concentration of generally 80-300 mg / m 3 Free ammonia is dissolved in molten urea during urea concentration. From the concentration equipment to the prilling machine, the molten urea will condense into by-product biuret and generate free ammonia. The longer the residence time, the more biuret and free ammonia will be generated.

[0005] Generally, the free ammonia content in the concentrated molten urea from the melt urea concentration plant is in the range of 400-600 ppm (wt), and the length of the pipe between the concentration plant and the prilling machine is generally in the range of 70-130 meters. During the urea polymerization to form by-product biuret in the pipe, free ammonia is released, and the free ammonia content before the prilling machine is generally in the range of 800-1100 ppm (wt). During the crystallization exothermic and cooling of the molten urea droplets, the free ammonia left in the solid urea product is generally less than 100 ppm (wt), and the rest of the free ammonia is released into the cooling air during the crystallization of the molten urea droplets, and finally discharged into the atmosphere from the top of the prilling tower, generally at a concentration of 70-150 mg / Nm 3 In addition, the free ammonia in the molten urea is released in gaseous form during the crystallization of the molten urea droplets, and small droplets are carried out during the release process, forming urea dust, and the more the free ammonia content, the more dust produced during the release process. The urea dust and free ammonia in the tail gas from the prilling tower are pollutants that will inevitably pollute the environment if not treated before being discharged into the atmosphere.

[0006] Whether it is fluidized bed mechanical prilling or rotary drum mechanical prilling, the prilling process is to use small size urea as "seed", and the molten urea is sprayed on the surface of the "seed", and the air sent by the air blower is used for cooling, and the "seed" gradually increases to form larger particles of urea. The amount of dust generated by mechanical prilling is very large, about 5% of the output, and the specific gravity is large, so this part of the dust must be designed for dust removal during the design of the prilling process. Due to the large amount of dust generated by mechanical prilling, it is economically reasonable to use wet scrubbing to wash the urea dust into urea solution for recovery.

[0007] Urea is easy to absorb water and has a relatively large solubility in water. The tail gas containing urea dust can be treated by wet scrubbing process (i.e. water washing) to reach 30 mg / Nm 3 , and by reducing the concentration of the scrubbing liquid, it can also reach 10 mg / Nm 3 . Therefore, a large amount of clean water needs to be supplemented, and additional energy consumption is required to recover the very dilute urea solution. Another process for treating urea dust is dry dust removal, which uses filter bags or filter screens to filter down the urea dust. The more dense the filter bag or filter screen, the better the filtering effect, and the urea dust in the tail gas can be treated to 5 mg / Nm 3 . The dust removal effect is very good, and no water is needed during the dust removal process. The exhaust gas discharged is unsaturated gas, and even in winter, the prilling device cannot be seen from the visual point of view (while the wet scrubbing process discharges saturated exhaust gas, which can be clearly seen, especially in winter). However, dry dust removal cannot treat the free ammonia in the tail gas.

[0008] No matter which granulation process, whether it is simply using water washing wet dedusting process or using dry dedusting process, can achieve the purpose of treating urea dust, but can not achieve the purpose of removing ammonia. In order to achieve the purpose of removing ammonia by water washing method, it is necessary to increase pickling on the basis of water washing, using sulfuric acid, hydrochloric acid, nitric acid and ammonia to generate ammonium salt to achieve the purpose of treating ammonia in tail gas. No matter which granulation process, the granulation tail gas volume is very huge, and 8000-12000 Nm 3 / h of tail gas will be generated for each ton of urea product. For a urea plant with an annual capacity of 800,000 tons, the total tail gas volume is 640,000-960,000 Nm 3 / h. Treating these tail gases by pickling will inevitably increase the resistance drop of the washing equipment, that is, increase the air pressure of the tail gas fan and increase the power consumption. In addition, the washing equipment is huge, and the pickling of sulfuric acid or hydrochloric acid requires higher equipment material, and the equipment investment is also higher.

[0009] For the granulation process of the granulation tower, whether it is wet washing or dry dedusting, increasing pickling is difficult, especially for old devices that do not consider tail gas treatment when designing the granulation tower. In the process of transformation, increasing water washing and dry dedusting has already used up the safety margin when designing the granulation tower, and it is almost impossible to increase pickling facilities.

[0010] Therefore, the ammonia removal of the granulation tail gas of the urea production device is a difficult environmental protection problem to realize in the world, with high cost (investment and operation).

[0011] Application content

[0012] The purpose of the present application is to overcome the defects of the prior art, and to provide a high-efficiency ammonia removal process device with short flow, compact layout, small land occupation and low operation cost. The device removes ammonia before granulation, fundamentally solves the world-wide industry problem of high ammonia emission concentration in the granulation tail gas of urea, and can realize ultra-low concentration emission of ammonia in the urea granulation tail gas, even zero emission.

[0013] In order to achieve the above-mentioned purpose, the following technical scheme is adopted: an ammonia emission reduction device for urea production device granulation tail gas, characterized by: comprising a combined tower body, a deamination section and an ammonia recovery section arranged in the lower part and the upper part of the combined tower body, and a partition plate and a gas lifting cap arranged between the deamination section and the ammonia recovery section, the gas phase is communicated between the deamination section and the ammonia recovery section, and the liquid phase is not communicated;

[0014] The deamination section is provided with a deamination layer, and the deamination layer is a packing structure or a tray, or a combination of a tray and a packing structure; the upper part of the deamination layer is provided with a molten urea inlet, and the lower part is provided with a hot gas inlet;

[0015] The demisting layer and the recovery layer are arranged from top to bottom in the ammonia recovery section, and a washing liquid inlet and an outlet are arranged above and below the recovery layer respectively.

[0016] Further, the packing structure adopts random packing or structured packing; and the tray adopts sieve tray or float valve tray.

[0017] Further, a gas heater is arranged outside the hot gas inlet.

[0018] Further, a molten urea outlet is arranged at the bottom of the deamination section, and the molten urea outlet is connected to a granulator.

[0019] Further, the recovery layer is a packing structure or a tray, or a combination of a tray and a packing structure, and a washing liquid inlet and an outlet are arranged above and below the recovery layer respectively.

[0020] Further, a washing liquid circulation pipeline is arranged outside the recovery layer and between the washing liquid inlet and the outlet, and an acid mixer and a circulating washing pump are arranged on the washing liquid circulation pipeline.

[0021] The device for reducing ammonia in the granulation tail gas of a urea production device provided in the application, the deamination 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, and the upper section and the lower section are separated by a partition plate and a gas lifting cap, so that the gas phase is communicated and the liquid phase is not communicated.

[0022] The lower section of the combined tower body is a molten urea free ammonia removal section, and is provided with a molten urea inlet, a molten urea outlet and a hot gas inlet.

[0023] A gas heater is arranged on the hot gas inlet pipeline of the lower section 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, and air is preferred. Compressed air or air pressurized by a blower can be used, and the gas is heated to 130-150 DEG C, preferably 135-145 DEG C.

[0024] The acid used for free ammonia recovery can be sulfuric acid, nitric acid, hydrochloric acid, phosphoric acid and the like, and the specific selection is based on the raw material utilization condition of the user.

[0025] The device for reducing ammonia in the granulation tail gas of a urea production device provided in the application is used for removing ammonia in the granulation tail gas, and most of the free ammonia in the molten urea is separated before entering the granulator, so that the free ammonia in the molten urea entering the granulation equipment is greatly reduced, and the free ammonia in the molten urea before the granulator is reduced from 800-1100 ppm (wt) to below 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 granulation product is deducted, the free ammonia content in the granulation tail gas is less than 10 mg / Nm 3 , which meets the most stringent tail gas emission standard requirements at home and abroad. The process device of the patent has a short process, compact equipment arrangement, small volume, light weight, can be flexibly installed in the granulation tower or the granulation tower plant, low investment cost and operation cost, and long service life of the equipment. BRIEF DESCRIPTION OF DRAWINGS

[0026] Figure 1 is a structural schematic diagram of an ammonia emission reduction device for a urea production device granulation tail gas according to an embodiment. DETAILED DESCRIPTION

[0027] In order for those skilled in the art to better understand the present application, the technical solutions in the embodiments of the present application will be described clearly and completely below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, not all. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor should be within the scope of protection of the present application.

[0028] It should be noted that the terms "include" and "have" and any variations thereof in the specification and claims of the present application and the above-mentioned drawings are intended to cover non-exclusive inclusion, for example, a process, method, system, product or device including a series of steps or units does not have to be limited to only those steps or units clearly listed, but can include other steps or units not clearly listed or inherent to these processes, methods, products or devices.

[0029] Referring to 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 for dividing the lower deamination section and the upper ammonia recovery section, the gas phase is communicated, and the liquid phase is not communicated.

[0030] A deamination layer is arranged 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 arranged above the deamination layer, and a hot gas inlet 17 is arranged below the deamination layer. The molten urea enters the deamination section from the molten urea inlet, and the hot gas entering the hot gas inlet performs mass transfer and heat transfer in the deamination layer to extract the free ammonia gas in the molten urea.

[0031] A molten urea outlet 11 is arranged at the bottom of the deamination section. The molten urea outlet is connected to a granulator 18, and the stripped molten urea is discharged from the molten urea outlet and granulated by the granulator.

[0032] The hot gas inlet is provided with a gas heater 5, which uses steam or electricity to heat the gas to 130-150℃; the gas is air or inert gas.

[0033] A demisting layer 4 and a recovery layer 19 are provided in the ammonia recovery section from top to bottom. The demisting layer is used to eliminate entrainment of mist in the exhaust gas, and a wire mesh demister is used. A scrubbing water inlet 13 and an exhaust gas outlet 12 are provided above the demisting layer, and the scrubbing water entering the scrubbing water inlet is used to clean the demisting layer. The scrubbing water used can be desalted water, or purified process condensate from the urea plant. The acid solution reacts with the ammonia in the gas in the recovery layer to form ammonium salt, recovering the ammonia in the gas, and a scrubbing liquid inlet 16 and outlet 15 are provided above and below the recovery layer, respectively.

[0034] A scrubbing liquid circulation pipeline is provided outside the recovery layer, between the scrubbing liquid inlet and outlet, and an acid mixer 9 and a circulating scrubbing pump 6 are provided on the scrubbing liquid circulation pipeline. After the acid solution scrubs the gas in the recovery layer, it enters the scrubbing liquid circulation pipeline through the scrubbing liquid outlet, and the recovery liquid enters the acid mixer through the circulating scrubbing pump, mixes with the acid solution added to the acid mixer, and then enters the recovery layer through the scrubbing liquid inlet for recycling.

[0035] The molten urea from the molten urea supply pipeline enters the upper part of the lower ammonia removal section 2 of the combined tower body 1 through the molten urea inlet 10, the gas for ammonia removal is treated by the hot gas device 5 to heat the temperature of the gas to above 130℃, and then enters the lower part of the ammonia removal section 2 through the hot gas inlet 17, the hot gas and the molten urea perform mass transfer and heat transfer in the ammonia removal section to remove the free ammonia gas in the molten urea. The free ammonia in the molten urea before the prilling machine can be reduced from 800-1100ppm(wt) to below 100ppm(wt), even if all the free ammonia in these molten urea in the prilling machine enters the prilling tail gas, the ammonia content in the prilling tail gas does not exceed 12mg / Nm 3 , if the free ammonia in the urea particle product is deducted, the free ammonia content in the prilling tail gas is less than 10mg / Nm 3 .

[0036] The ammonia-containing gas leaving the ammonia removal section 2 enters the lower part of the ammonia recovery section 3 through the riser cap 8 on the partition 7, the ammonia in the gas is washed by the scrubbing water and sulfuric acid in the ammonia recovery section 3 to generate ammonium sulfate solution byproduct which is discharged from the tower through the scrubbing liquid outlet, the scrubbing water is first used for demisting in the demisting layer 4 at the top of the ammonia recovery section to avoid entrainment of acid mist in the exhaust gas. The sulfuric acid is added to the acid mixer 9 and mixed with the circulating scrubbing liquid, the circulating scrubbing liquid in the tower enters the circulating scrubbing pump 6 inlet from the circulating scrubbing liquid outlet 15, and the circulating scrubbing pump 6 realizes the circulating washing of the ammonia recovery section, and the circulating scrubbing liquid outlet of the acid mixer 9 enters the ammonia recovery section upper part through the circulating scrubbing liquid inlet 16.

[0037] The scrubbing water used for ammonia recovery is added through the scrubbing water adding port 13, and the amount of the added scrubbing water is determined according to the amount of the recovered ammonia. By controlling the pH value of the circulating scrubbing liquid, the ammonia concentration in the tail gas leaving the ammonia recovery section is not higher than 10 mg / Nm 3 , which can be directly discharged into the atmosphere or mixed with the granulation tail gas and then discharged into the atmosphere together.

[0038] The lower ammonia removal section 2 and the upper recovery section 19 of the combined tower body 1 can use fillers such as random packings, structured packings, etc.; can also use tower trays such as sieve tray, float valve tray, etc.; or use a combination of fillers and tower trays.

[0039] The above is a further detailed description of the present application, which cannot be considered as a limitation to the specific implementation of the present application. For ordinary skilled persons in the technical field to which the present application belongs, simple deductions or replacements without departing from the concept of the present application are within the protection scope of the present application.

Claims

1. An apparatus for abatement of ammonia in a prilling offgas of a urea production plant, characterized by: The combined tower body, the lower deamination section and the upper ammonia recovery section arranged in the combined tower body, and the partition plate and the riser cap arranged between the deamination section and the ammonia recovery section, the gas phase is communicated between the deamination section and the ammonia recovery section, and the liquid phase is not communicated; The deamination section is provided with a deamination layer, which is a packing structure or a tray, or a combination of a tray and a packing structure; the upper part of the deamination layer is provided with a molten urea inlet, and the lower part is provided with a hot gas inlet; The ammonia recovery section is provided with a demisting layer and a recovery layer from top to bottom, and is provided with a washing liquid inlet and an outlet above and below the recovery layer respectively.

2. The device for abatement of ammonia in the granulation off-gas of a urea production plant according to claim 1, characterized by the fact that: The packing structure uses a random packing or a structured packing; the tray uses a sieve tray or a float valve tray.

3. The device for abatement of ammonia in the granulation off-gas of a urea production plant according to claim 1, characterized by the fact that: The hot gas inlet is provided with a gas heater outside.

4. The device for abatement of ammonia in the granulation off-gas of a urea production plant according to claim 1, characterized by the fact that: The bottom of the deamination section is provided with a molten urea outlet, which is connected to a granulator.

5. The device for abatement of ammonia in the granulation off-gas of a urea production plant according to claim 1, characterized by the fact that: The recovery layer is a packing structure or a tray, or a combination of a tray and a packing structure, and is provided with a washing liquid inlet and an outlet above and below the recovery layer respectively.

6. The device for abatement of ammonia in the granulation off-gas of a urea production plant according to claim 5, characterized by the fact that: The outside of the recovery layer is provided with a washing liquid circulation pipeline arranged between the washing liquid inlet and the outlet, and is provided with an acid mixer and a circulating washing pump.

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