CN1058704C - Method for prepn. of urea by carbon dioxide stripping method - Google Patents

Abstract

The invention is a method for preparing urea from coke oven gas by stripping ammonia from coke oven gas, and creates a new way for the metallurgical industry to recover ammonia from coke oven gas. Using recycled anhydrous ammonia as raw material is an economical and reasonable process route to produce urea, suitable for small-scale urea production. The invention solves the outlet problem of liquid ammonia in a coking plant, prevents environmental pollution, and produces fertilizer to support agricultural production. The present invention has the characteristics of simple process, less investment, less floor space, and convenient operation and management. Compared with the _CO2 stripping method of the Netherlands Stamicarbon Company, the number of equipment can be reduced by about 15%, and the investment can be reduced by about 10%. .

Description

Carbon dioxide stripping urea preparation method

The invention belongs to a method for preparing urea by stripping urea from liquid ammonia recovered from coke oven gas.

In the prior art, there are many urea production methods at home and abroad, mainly including aqueous solution circulation method, high-pressure _CO2 stripping method and ammonia stripping method. The aqueous solution circulation method is at the technical level in the 1960s. All domestic medium-sized nitrogen fertilizer plants adopt this method. The process of this method is relatively complicated, with many production processes, many equipment, and high investment. Newly built urea plants abroad are no longer used. adopted this method. The high-pressure CO ₂ stripping method is one of the advanced processes today. In the urea production process of Stamicarbon Company in the Netherlands, the stripping pressure of the CO ₂ stripping process is 14MPa. Stripping and condensing at a higher pressure, although it is beneficial to energy Recycling reduces steam consumption, but the stripping efficiency is poor. After stripping, the contents of _NH3 and _CO2 in the urine are 8% and 10% respectively, so a low-pressure decomposition system must be set up to recover the remaining _NH3 and _CO2 . The whole process has a lot of equipment, high investment and high operation requirements. Some special stainless steels are also used, which cannot be produced in China at present and need to be imported. The requirements for equipment installation, commissioning and operation management are also relatively high. This process is not suitable for small urea plants, and it is not very economical when the scale is below 150t/d.

The ammonia stripping method was developed by the Italian company SNAM, and it is widely used in large-scale factories in the world, with a daily output of 1,000-1,500 tons of urea. In recent years, our country will introduce several sets. This method has low energy consumption and does not require a high-rise frame, so the cost of urea products is also low. However, this method has a long process, a lot of high-pressure equipment, special titanium materials are required, and the control requirements are relatively complicated, so it is not suitable for use in small urea plants.

The purpose of the present invention is to create a new way to recover ammonia from coke oven gas, and to seek an economical and reasonable process route to produce urea with the recovered anhydrous ammonia as raw material, which is suitable for small-scale urea production plants.

Another object of the present invention belongs to the treatment of three wastes. In order to solve the outlet problem of liquid ammonia in coking plants, it not only prevents environmental pollution but also produces fertilizers to support agricultural production.

To produce urea from recovered ammonia in a coking plant belongs to the comprehensive utilization of by-products, which requires a short flow process, less equipment, low investment, and convenient operation and control. The _CO2 stripping urea preparation method of the present invention can meet these requirements.

The following will give a detailed introduction to the preparation method, process flow and advantages of the present invention in combination with the detailed description of the technical solution of the present invention and the description of the accompanying drawings.

The accompanying drawing of this specification is a process flow diagram of the urea preparation method by carbon dioxide stripping.

As shown in the figure, the raw material liquid ammonia 31 passes through the high-pressure liquid ammonia pump 15 and the liquid ammonia heater 13, and the ammonium carbamate (abbreviated as methylammonium) liquid 34 passes through the high-pressure methylammonium pump 16 and the methylammonium liquid heater 14, after pressurization and heating Enter the bottom of urea synthesis tower 1. The reaction is carried out under the conditions of a pressure of 20-22MPa and a temperature of 185-190°C. The production intensity in the tower is 8.4～10.5t/m ³ ·d. In order to prevent the back mixing of the reaction materials, several sieve plates with holes are arranged in the urea synthesis tower 1, the apertures of which are Φ7-9mm, and the opening ratio is 2-3%. There are 2 to 6 sieve plates. The lining material of the urea synthesis tower is improved 316L.

In the urea synthesis tower 1, NH ₃ /CO ₂ (molecular ratio)=4.5-5, this value should not be too high, otherwise a large amount of ammonia will be circulated. H ₂ O/CO ₂ (molecular ratio) in the tower = 0.9-1.2, the smaller the water-to-carbon ratio, the better, which can increase the conversion rate of urea, and about 62.5% of CO ₂ is converted into urea. The urea melt 35 after the synthesis reaction is drawn from the upper part of the tower, and is decompressed to 3.0-4.0 MPa through the automatic decompression regulating valve 24, and enters the top of the CO ₂ stripping tower 2 with a liquid distributor. The urea melt first passes through the sieve plate, and then enters the zigzag distributor that is not easy to be blocked, evenly enters the tubes, and flows downward along the inner wall in the form of a film.

The compressed CO ₂ gas 32 is sent to the bottom of the CO ₂ stripper 2, and is in countercurrent contact with the urea melt 35 flowing down the inner wall of the tube. The stripping column is heated with steam 27 and the condensate 28 is discharged. The excess ammonia and methylammonium in the solution are evaporated or decomposed as the liquid drops under the condition of heating, and the temperature of the solution in the tube is 155-160°C.

A sieve plate is provided in the _CO2 stripping tower, the aperture of which is Φ7-9mm, and the opening ratio is 5-25%. There are two sieve plates on the upper part of the tower to make gas-liquid contact to reduce the water content in the outlet gas. A sieve plate is installed at the lower part of the tower for adiabatic stripping to further improve stripping efficiency. The CO ₂ stripper 2 is a two-stage tube or single-stage tube falling film tower, the peripheral flow rate in the tower is 500-750kg/m·h, and the heat transfer coefficient K is 1400-1600KJ/m ² ·h·℃ . The tubes and shell materials of the CO ₂ stripper are all improved 316L.

Mixed gases 36 such as NH ₃ , CO ₂ , water vapor, and inert gases from the top of the stripping tower are mixed with the methyl ammonium liquid 33 at the bottom outlet of the absorption tower 4 when passing through the methyl ammonium condenser 17 to form a concentrated methyl ammonium liquid 34, which is passed through the methyl ammonium condenser 17. The high-pressure ammonium methyl ammonium pump 16 and the methyl ammonium liquid heater 14 return to the urea synthesis tower 1, and the unabsorbed gas 53 passes through the absorption tower 4 from bottom to top. Condensate and absorb in the internal cooling or external cooling tubes of the lower section of the absorption tower, so that most of the CO ₂ and NH ₃ react to form methyl ammonium solution, and a small amount of CO ₂ is sprayed into the space section in the middle of the tower and the upper packing layer and the top of the tower NH ₃ water countercurrent contact, in order to achieve the purpose of fine cleaning. The pressure of the absorption tower 4 is 3.0-4.0MPa, the temperature at the bottom of the tower is 110-120°C, the temperature at the top of the tower is 70-80°C, the superficial velocity in the tower is 0.02-0.03m/s, and the heat transfer coefficient K is 1800-2400KJ /m ² ·h·℃, NH ₃ /CO ₂ (molecular ratio) = 2.4-2.6, H ₂ O/CO ₂ (molecular ratio) = 0.9-1.2 in the methyl ammonium liquid in the tower.

The urea solution 41 drawn from the bottom of the _CO2 stripper 2 is decompressed through the automatic regulating valve 25, and the material enters the flash heater 6, the flash separator 7, and the flash condenser 8 in sequence.

The pressure of the flash heater 6 is 0.06-0.1 MPa, the heating temperature is 100-110°C, and the heat transfer coefficient K of the flash heater is 1600-2000KJ/m ² ·h·°C. When the urea solution 41 passes through the flash heater 6, it is heated by the steam 27 and then the condensate 28 is discharged, so that the residual NH ₃ and CO ₂ in the urine are further driven out, and the urea content in the urine at the flash outlet increases to 65-75% % (weight), the flash evaporation methylammonium decomposition rate is 97～99%, and the flash evaporation total ammonia steaming rate is 99～99.5%.

The flashed gas-liquid mixed liquid 42 from the top of the flash heater 6 passes through the flash separator 7, and the flashed gas 44 enters the flash condenser 8, and the NH in the condensed flash condensate ₄₅ is 16-18 % (weight), CO ₂ is 14 to 16% (weight). The flash condensate is put into the flash condensate tank 19, and then pumped into the middle of the absorption tower 4 with the flash condensate pump 18. The non-condensed gas 46 after flash evaporation and condensation is evacuated by the vacuum device 12 and then evacuated.

The ammonia-containing gas 37 exported from the top of the absorption tower 4 first enters the ammonia condenser 23 for condensation, and the condensed liquid ammonia 31 passes through the liquid ammonia intermediate tank 3 and flows back into the top of the absorption tower 4 as an absorbent. The uncondensed ammonia-containing gas 38 enters the ammonia cleaner 5 for cleaning again, so that most of the gas ammonia is washed and recovered by soft water 29, and the heat released by absorption is removed by the cooling water 30. The recovery pressure is 3.0-4.0MPa, and the temperature is 30-40°C. Below the ammonia cleaner 5 is a horizontal heat exchanger, and above it is a packed tower, and its heat transfer coefficient K is 800-1200KJ/m ² ·h·℃. After cleaning, NH in the bottom outlet ammonia ₃₉ is 10-15% (weight), and this part of ammonia is refluxed to the top of the absorption tower 4 as an absorbent. After cleaning, the CO ₂ in the gas 40 at the top outlet of the tower is <100ppm, and the NH ₃ is 2-4g/Nm ³ .

The flashed urine 43 coming out from the bottom of the flash separator 7 enters the evaporation device 11 by virtue of the potential difference, and the evaporated gas 49 exiting the evaporation device 11 is evacuated by the vacuum device 11 and then evacuated. The concentration of the urine 50 coming out of the evaporation device 11 is 99.5-99.8%, and it is sent to the granulation device 12 to be made into a granular urea product 51 .

The evaporative condensate 47 that comes out from the evaporator 11 first arrives at the evaporative condensate storage tank 22, and then divides into two paths into the ammonia distillation tower 9 after being boosted by the evaporative condensate pump 21, wherein one path is used as tower top feed, and the tower top feeds The raw material is 35-45°C, and the other way is to heat part of the evaporated condensate to 90-100°C through the heat exchanger 20 as the feed in the tower. The ammonia distillation pressure tower bottom of ammonia distillation tower 9 is 0.12～0.14MPa, and the tower top is 0.09～0.1MPa, and the ammonia distillation temperature tower bottom is 105～110 ℃, and the tower top is 80～90 ℃. 27 heat. After the ammonia is distilled, the waste liquid 48 at the bottom of the tower is released from the bottom of the ammonia distiller and discharged into the sewer through the heat exchanger 20 . The gas 52 at the top outlet of the ammonia distillation tower 9 contains NH ₃ , CO ₂ and H ₂ O, which are condensed in the flash condenser 8 .

In the past, the ammonia in coke oven gas was mainly used to produce ammonia water. Because of the high cost and low sales price, every ton of ammonia water would suffer a loss. In recent years, as the demand for ammonia water has been decreasing year by year, it is seriously unsalable in the off-season, and some factories have to discharge the ammonia water to the outside, causing serious environmental pollution.

The invention can open up a new way for the metallurgical industry to effectively utilize by-products. It is suitable for comprehensive utilization of recovered ammonia in coking plants of various iron and steel companies. Ammonia in coke oven gas is used to produce liquid ammonia. The production cost of this liquid ammonia is lower than that of ordinary small nitrogen fertilizer plants. This liquid ammonia is then used as a raw material for urea production. CO ₂ , another main raw material for urea production, is The flue gas containing 10-20% CO ₂ in the large amount of waste gas produced by using coal gas for heating and combustion can make use of the CO ₂ in the waste gas. This is an economical and reasonable process route, suitable for small-scale urea production .

The invention makes comprehensive utilization of ammonia and flue gas in coke oven gas, not only can produce urea, supports agricultural production, but also prevents pollution of three wastes and improves environmental conditions.

The invention has the characteristics of simple process, less investment, less occupied area, convenient operation and management, and can reduce the investment by about 10% compared with the _CO2 stripping method of the Netherlands Stamicarbon company. Due to the improvement of the stripping efficiency of the present invention, the two-stage decomposition system in the traditional process is omitted, so the equipment is reduced. Since the operating pressure is greatly reduced, the requirements for materials are not high, and the number of equipment is reduced by about 15% compared with the CO ₂ stripping method of the Netherlands Stamicarbon Company. The equipment of the present invention can be centrally arranged in two buildings, and the height of the main frame is only 23 meters, while the main frame of the _CO2 stripping method of the Dutch Stamicarbon company needs a height of 70 meters. In the urea production method of the present invention, the number of operating posts is reduced due to the simplification of process equipment, which is convenient for operation and management.

Since the stripping heating temperature required for _CO2 stripping in the present invention is lower than 160°C, this temperature is just the temperature that general urea stainless steel can withstand, so it is not necessary to use the special stainless steel required for high-pressure _CO2 stripping.

The present invention can also be extended to small synthetic ammonia plants, using NH ₃ and CO ₂ from synthetic ammonia plants as raw materials, and the method of the present invention can produce granular urea products.

The following examples illustrate the present invention, but do not mean to limit the present invention.

Example 1:

Urea synthesis tower Φ800×11500mm, CO ₂ stripping tower Φ500×11700mm, absorption tower Φ700×9600mm, flash heater Φ273×2700mm, ammonia cleaner Φ325×2000mm, ammonia distillation tower Φ273×11200, raw material liquid ammonia 1032.5kg/ h, CO ₂ gas volume is 696.48Nm ³ /h. Liquid ammonia and circulating ammonia are heated to 180°C and enter the urea synthesis tower, and methyl ammonium liquid is heated to 180°C and enter the urea synthesis tower. The NH ₃ in the inlet material of the synthesis tower is 1596.56kg/h, and the methylammonium liquid is 4942.32kg/h. NH ₃ /CO ₂ (molecular ratio) = 4.5, H ₂ O/CO ₂ (molecular ratio) = 1.1. The pressure in the tower is P=20MPa, and the temperature t=183°C. The _CO2 conversion rate is 62.5%.

The molten product at the outlet of the urea synthesis tower is decompressed and enters the CO ₂ stripping tower, where it is stripped with P=3.5MPa, 99% CO ₂ gas with a gas volume of 696.48m ³ /h (containing 0.7% O ₂ ). The pressure of the stripper is P=32MPa, the temperature t=155°C, and it is heated with steam. The total ammonia output rate in the tower is 95.8%, and the methylammonium decomposition rate is 85.5%.

The urea solution at the bottom outlet of the stripper contains 56.77% urea, 3.52% NH ₃ and 3.63% CO ₂ . After decompression, it enters the flash heater. The flash heater is heated by steam. Under the conditions of pressure P=0.066MPa and temperature t=105°C, the total ammonia evaporation rate is 99.37%, the methylamine decomposition rate is 97.08%, and the outlet urine concentration after flash evaporation is 70%.

The mixed gases such as NH ₃ and CO ₂ from the top of the stripping tower enter the absorption tower, the pressure of the absorption tower is P=3.1MPa, the middle section of the tower is filled with flash condensate, and the top of the tower is filled with liquid ammonia and dilute ammonia water from the ammonia cleaner. The temperature at the top of the tower is 78°C, the temperature of methylammonium at the bottom of the tower is t=110°C, and the heat generated by absorption is removed by the cooling water. The methylammonium liquid in the tower contains 40.46% of NH ₃ , 42.07% of CO ₂ , NH ₃ /CO ₂ (molecular ratio)=2.5, H ₂ O/CO ₂ (molecular ratio)=1.1.

The methylammonium liquid coming out of the bottom of the absorption tower is pressurized to 20MPa with a high-pressure methylammonium pump, 17.5Nm ³ /h of air is added to it, and then heated to 180°C to enter the urea synthesis tower.

The ammonia-containing gas at the top outlet of the absorption tower contains a large amount of NH ₃ and CO ₂ <100ppm. The gaseous ammonia is first condensed by the ammonia condenser, and the liquid ammonia flows back into the absorption tower. The uncondensed gas enters the ammonia cleaner, washes it with soft water, and operates under the pressure P=3.3MPa and temperature 35°C to obtain 13.4% dilute ammonia water to flow back into the absorption tower, and the tail gas venting meets environmental protection requirements.

70% of the urine from the bottom of the flash separator enters the evaporation equipment, and the urine concentration reaches 99.5% after two stages of evaporation. It is cooled and solidified in the granulation equipment to produce 1750kg/h of granular urea.

The total ammonia utilization rate of this embodiment is 96.05%.

Example 2:

The same equipment and process as in Example 1, the raw material liquid ammonia is 1228.50kg/h, and the CO ₂ gas volume is 835.28Nm ³ /h. Liquid ammonia and circulating ammonia are heated to 183°C and enter the urea synthesis tower, and methyl ammonium liquid is heated to 183°C and enter the urea synthesis tower. The NH ₃ 1915.87kg/h and methylammonium solution 5648.36kg/h in the inlet material of the synthesis tower. NH ₃ /CO ₂ (molecular ratio)=4.2, H ₂ O/CO ₂ (molecular ratio)=0.9, pressure P=20.5MPa in the tower, temperature t=188°C, CO ₂ conversion rate 64.7%.

The molten product at the outlet of the urea synthesis tower is decompressed and then enters the CO ₂ stripping tower, where it is stripped with P=4.0MPa, 99.5% CO ₂ gas with a gas volume of 835.28 Nm ³ /h (containing 0.5% O ₂ ). The stripping tower pressure P=3.6MPa, temperature t=158°C, heated by steam, the total ammonia steaming rate in the tower is 95.5%, and the methylammonium decomposition rate is 86.2%.

The urea liquid at the bottom outlet of the stripper contains 59.13% urea, 3.71% NH ₃ and 3.26% CO ₂ . The depressurized urine enters the flash heater. The flash heater is heated by steam. Under the conditions of pressure P=0.065MPa and temperature t=108°C, the total ammonia evaporation rate is 99.61%, and the methylammonium decomposition rate is 98.58%. The urea concentration in the outlet urine after flash evaporation is 73%.

Mixed gases such as NH ₃ and CO ₂ from the top of the stripping tower enter the absorption tower. The pressure of the absorption tower is P=3.4MPa. The flash condensate is added to the tower, and liquid ammonia and dilute ammonia water from the ammonia cleaner are added to the top of the tower. The temperature at the top of the tower is 73°C, the temperature of the methyl ammonium liquid at the bottom of the tower is t=120°C, and the heat generated by absorption is removed by the cooling water. The methyl ammonium liquid in the tower contains 39.37% of NH ₃ , 44.31% of CO ₂ , and 16.32% of H ₂ O, NH ₃ /CO ₂ (molecular ratio)=2.3, H ₂ O/CO ₂ (molecular ratio)=0.9.

Part of the methylammonium liquid coming out of the bottom of the absorption tower is pressurized to 20MPa with a high-pressure methylammonium pump, 21Nm ³ /h of air is added to it, and then heated to 185°C to enter the urea synthesis tower.

The ammonia-containing gas at the top outlet of the absorption tower contains a large amount of NH ₃ and CO ₂ <100ppm. The gaseous ammonia is first condensed through the ammonia condenser, and the liquid ammonia flows back into the absorption tower. The non-condensed gas enters the ammonia cleaner, washes it with soft water, and washes it with the evaporated condensate at a pressure of P=3.2MPa and a temperature of 35°C to produce 15.70% dilute ammonia water, which flows back into the absorption tower, and the tail gas is vented to meet environmental protection requirements .

73% of the urine from the bottom of the flash separator enters the evaporation equipment, and the concentration of the urine reaches 99.5% after two stages of evaporation, and 2100kg/h of large-grained urea products of Φ2-4mm are produced by drum granulation.

The total ammonia utilization rate of this embodiment is 97.26%.

Claims (2)

1, a kind of carbon dioxide stripping process urea production method comprises synthetic, stripping, absorption, flash distillation, evaporation, granulation, it is characterized in that:

(1) raw materials ammonia [31] and ammonium carbamate liquid [34], ammonium carbamate is called for short the first ammonium, enters urea synthesizer [1], reacts NH in urea synthesizer [1] under pressure 20～22MPa, 185～190 ℃ of conditions of temperature ₃/ CO ₂(molecular ratio)=4.5～5, H ₂O/CO ₂(molecular ratio)=0.9～1.2, the production intensity of tower is 8.4～10.5t/m ³D,

(2) urea melt after the building-up reactions [35] enters CO ₂The top of stripping tower [2], CO ₂Gas [32] is sent into the bottom of stripping tower [2], makes superfluous ammonia in the solution and first ammonium heating stripping when 155～160 ℃ of pressure 3.0～4.0MPa, temperature, and the peripheral flow rate in the tower is 500～750kg/mh,

(3) from CO ₂Come out in stripping tower [2] top contains NH ₃, CO ₂, water vapor mixed gas [36] when carbamate condenser [17] and the first ammonium liquid [33] that come out in [4] bottom from the absorption tower mix and generate dense first ammonium liquid [34], do not absorb gas [53] from passing through absorption tower [4] down, pressure in the tower is 3.0～4.0MPa, column bottom temperature is that 110～120 ℃, tower top temperature are 70～80 ℃, superficial velocity in the tower is 0.02～0.03m/s, the NH of dense first ammonium liquid ₃/ CO ₂(molecular ratio)=2.4～2.6, H ₂O/CO ₂(molecular ratio)=0.9～1.2,

(4) from CO ₂The urea liquid [41] that comes out in stripping tower [2] bottom enters flash heater [6] after reducing pressure by automatic decompression valve [25], and its pressure is 0.06～0.1MPa, and temperature is 100～110 ℃,

(5) liquid-vapor mixture [42] that comes out from flash heater [6] top NH through flash separator [7], the gas flash distillation phlegma [45] behind flash condenser [8] ₃Be 16～18% (weight), CO ₂Be 14～16% (weight),

(6) gas [37] of [4] top exit is first through the condensed liquefied ammonia of ammonia condenser [23] [31] feed liquor ammonia medial launder [3] from the absorption tower, gas reclaims remaining ammonia through ammonia cleaning tower [5] again, described pressure recovery is that 3.0～4.0MPa, temperature are 30～40 ℃, CO in the exit gas [40] after cleaning recovery ₂＜100ppm, NH ₃Be 2～4g/Nm ³, NH in the outlet ammoniacal liquor [39] ₃Be that 10～15% (weight) and liquefied ammonia [31] reflux respectively and enters top, absorption tower [4] as absorption agent,

(7) urine concentration of urea liquid [43] behind evaporation equipment [11] that come out of flash separator [7] bottom is 99.5～99.8% (weight), delivers to granulating equipment [12] and make granular urea product [51].

2, according to the described carbon dioxide stripping process urea production of claim 1 method, it is characterized in that: evaporation condensate [47] Fen Erlu that described evaporation equipment [11] comes out advances ammonia still [9], be that 0.12～0.14MPa, cat head are 0.09～0.1MPa at the bottom of the ammonia still process pressure column, be that 105～110 ℃, cat head are 80～90 ℃ at the bottom of the ammonia still process temperature tower, charging is 90～100 ℃ in 35～45 ℃ of cat head chargings, the tower.

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