Recycling technology for co-production of urea from melamine byproduct methylamine liquid and tail gas

Abstract

The invention relates to the technical field of co-production of melamine and urea, in particular to a recycling technology for co-production of urea from melamine byproduct methylamine liquid and tail gas. Ammonia and carbon dioxide in the melamine reaction process are condensed to form methylamine liquid, the methylamine liquid is conveyed to a high-pressure system of a urea device through a high-pressure plunger pump to serve as a raw material for synthesizing urea, meanwhile, a small amount of non-condensable gas is conveyed to a low-pressure system of the urea device, and resource utilization of tail gas is achieved. According to the method, the net consumption value of urea in the melamine production process can be remarkably reduced, emission of carbon dioxide and ammonia is reduced, meanwhile, the stability of a urea device is guaranteed, and remarkable environmental protection benefits and economic benefits are achieved.

Description

Technical Field

[0001] The present invention relates to a tail gas recovery and utilization technology in the field of chemical production, and particularly to a recovery and utilization technology for co-producing urea from by-product methylamine solution and tail gas in melamine production. Background Art

[0002] In the European high-pressure process for melamine production, a high-pressure tail gas (about 1621 kg / h, pressure of 10.45 MPa g) is generated during the co-production process of methylamine solution and urea. This tail gas mainly consists of ammonia (49.48%), carbon dioxide (41.72%), (1.39%), and a small amount of air (7.4%). Since the tail gas contains carbon dioxide, if it is directly incorporated into the melamine production device, it will cause fluctuations in the system pH value, thereby affecting the quality of melamine products. This tail gas is usually directly discharged, which not only causes environmental pollution but also wastes the carbon dioxide resources therein. Currently, the main methods for treating the tail gas from high-pressure melamine plants include co-producing urea and co-producing ammonium bicarbonate and soda ash, etc., and the processes are relatively mature. The by-product methylamine solution (41635 kg / h, pressure of 10.45 MPa g) produced during melamine production contains ammonia (42.43% wt), carbon dioxide (43.47% wt), and water (14.06% wt). Since the main raw material for melamine production is urea, co-producing with the urea plant can directly introduce ungranulated molten urine to produce melamine, and the tail gas generated by the melamine plant can be returned to the urea plant to produce urea. This can not only reduce production costs and consumption but also solve the problem of environmental pollution. However, in practical applications, when melamine and urea are co-produced, due to the water content in the tail gas, it is easy to cause an imbalance in the water-carbon ratio in the urea production system, thereby triggering the problem of instability in the urea plant system. The present invention addresses the above problems and proposes an innovative tail gas and methylamine solution recovery and utilization system and method. Summary of the Invention

[0003] The purpose of the present invention is to address the above deficiencies and aims to provide an innovative tail gas and methylamine solution recovery and utilization system and method, which can make the high-pressure methylamine solution from the ammonia recovery and water treatment section have a higher temperature and a lower water content, avoid the imbalance of the water-carbon ratio in the urea production system after co-production, can transport the high-pressure tail gas flashed from the methylamine receiver to the low-pressure system of the urea plant, realize the resource utilization of the tail gas, ensure the stability of the urea plant system, and at the same time avoid environmental pollution and carbon emissions.

[0004] To achieve the above purpose, the present invention adopts the following technical solutions:

[0005] A recovery and utilization technology for co-producing urea from by-product methylamine solution and tail gas in melamine production, characterized by including the following steps:

[0006] Step A: The tail gas generated during the synthesis of melamine (containing only NH3 and CO2) is mixed with the tail gas from the decomposer and the carbamate solution with a relatively high water content transported by the high-pressure carbamate pump, cooled in a cooler, and then forms a carbamate solution and a small amount of uncondensed tail gas in the receiving storage tank. At the same time of cooling, 0.35 MPa g steam is by-produced for heat recovery.

[0007] Step B: The carbamate solution in the liquid phase is pressurized to 16.5 MPa(g) by a plunger booster pump and sent directly to the pool condenser of the urea plant as a raw material for synthesizing urea.

[0008] Step C: Part of the non-condensable gas cooled by the cooler is transported to the low-pressure system (pre-evaporator) of the urea plant after pressure regulation.

[0009] Step D: How to transport the tail gas to the urea plant:

[0010] 1. Tail gas transportation system

[0011] - Use pipes made of urea-grade stainless steel with high corrosion resistance to transport the high-pressure tail gas to the low-pressure system (pre-evaporator) of the urea plant.

[0012] - The pipes are designed with a full jacket structure and insulated with 0.8 MPa g saturated steam to prevent the tail gas from crystallizing due to temperature reduction during transportation.

[0013] - Set a flushing and draining point for high-pressure flushing water (12 MPa g) every 30 meters for cleaning pipe blockages or shutdown replacement.

[0014] 2. Pressure reduction and two-phase flow control

[0015] - Set the tail gas pressure reducing valve near the equipment of the urea low-pressure system (pre-evaporator) to avoid premature pressure reduction of the tail gas resulting in condensation into a liquid phase and forming a two-phase flow, ensuring the safety of the transportation process.

[0016] 3. Pressure monitoring and interlock protection

[0017] - Set two pressure measurement points behind the pressure reducing valve to monitor the tail gas transportation status in real time.

[0018] - When high-pressure values are detected simultaneously by the two pressure measurement points, trigger an alarm on the monitoring screen and interlock the pressure reducing valve to close slightly to prevent overpressure of the urea low-pressure system (pre-evaporator) from causing equipment damage.

[0019] 4. Anti-backflow and anti-blockage protection

[0020] - Install a check valve and a cut-off valve at the connection of the tail gas to the urea plant to prevent the carbamate solution in the urea low-pressure system (pre-evaporator) from flowing back and crystallizing to cause pipe blockage during the shutdown of melamine.

[0021] - The pressure ratings of the check valve and the shut-off valve are the same as those of the tail gas pipeline to ensure system compatibility.

[0022] 5. Pipeline layout optimization

[0023] - The pipeline layout has a slope of 4‰, sloping from the melamine unit to the urea unit, ensuring that the residual liquid can flow by gravity to the low-pressure system (pre-evaporator) of the urea unit during flushing and replacement.

[0024] Furthermore, in step A, while the tail gas is cooled, low-pressure steam at 0.35 MPa is by-produced, and this steam is used to heat other process media in the heating system, reducing the overall consumption during the melamine production process.

[0025] The beneficial effects of the present invention are:

[0026] As can be seen from the above technical solutions, this application includes step A: the melamine tail gas is cooled and recovered into ammonium carbamate solution and non-condensable tail gas; step B: the ammonium carbamate solution is pressurized by a high-pressure plunger pump and sent to the high-pressure area of the urea unit for co-production; step C: the non-condensable tail gas passes through pressure regulation and is transported through a pipeline to the low-pressure system of the urea unit to recover ammonia and carbon dioxide; step D: the tail gas is transported to the urea unit.

[0027] Beneficial effects: Resource utilization: By transporting the high-pressure tail gas to the low-pressure system (pre-evaporator) of the urea unit, recovering the carbon dioxide therein for urea production, realizing resource recycling and reducing carbon emissions.

[0028] Environmental benefits: Avoiding environmental pollution caused by direct discharge of tail gas, meeting the environmental protection requirements of energy conservation and emission reduction.

[0029] System safety: By optimizing the position of the pressure reducing valve, setting pressure monitoring and interlock protection, ensuring the safety of the tail gas transportation process.

[0030] Anti-blocking design: Through measures such as jacket insulation, high-pressure flushing water points and check valves, effectively preventing pipeline blockage and ensuring the long-term stable operation of the system.

[0031] The present invention can cool the waste gas in the melamine production process to form ammonium carbamate solution and collect it in the receiving storage tank, send the liquid phase to the high-pressure system of the urea unit to recover the effective components; send the gas phase of the receiving storage tank to the low-pressure system of the urea unit, making the water content of the treated tail gas lower, avoiding the imbalance of the water-carbon ratio in the urea production system after co-production, and ensuring the stability of the urea unit system. Brief description of the drawings

[0032] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following will briefly introduce the drawings required for the description of the specific embodiments or the prior art. In all the drawings, similar elements or parts are generally identified by similar reference numerals. In the drawings, the elements or parts are not necessarily drawn to scale.

[0033] Figure 1 It is a schematic structural diagram of the present invention. Specific embodiments

[0034] The following will further elaborate in detail on the technical solution of a recovery and utilization technology for co-producing urea from by-product methylammonium solution and tail gas in melamine in conjunction with the embodiments.

[0035] Example 1:

[0036] As Figure 1 shown, it includes the following steps:

[0037] Step A: The tail gas (containing only NH3 and CO2) generated by melamine in the reaction systems 1-1 / 1-2 is mixed with the tail gas from the decomposer 2 and the relatively high-moisture-content methylammonium solution transported by the high-pressure methylammonium pump 3, cooled in the cooler 4, and then forms methylammonium solution and a small amount of uncondensed tail gas in the receiving storage tank 5. At the same time of cooling, 0.35 MPa g steam is by-produced for heat recovery.

[0038] Step B: The formed liquid-phase methylammonium solution is pressurized to 16.5 MPa(g) by the plunger booster pump 7 and sent directly to the pool condenser 8 of the urea plant as a raw material for synthesizing urea;

[0039] Step C: Part of the non-condensable gas cooled by the cooler 4 is transported to the low-pressure system (pre-evaporator) 10 of the urea plant through the pressure regulator 9.

[0040] Step D: How to transport the tail gas to the urea plant:

[0041] 1. The high-pressure tail gas (10.45 MPa g) generated during the production of melamine by the European technology high-pressure method is transported to the low-pressure system (pre-evaporator) 10 of the urea plant through high-corrosion-resistant urea-grade stainless steel pipes.

[0042] 2. The pipeline adopts a full-jacket design and is filled with 0.8 MPa g saturated steam for heat preservation to prevent the tail gas from crystallizing during transportation.

[0043] 3. A high-pressure flushing water point (12 MPa g) is set every 30 meters for cleaning pipeline blockages or shutdown replacement.

[0044] 4. The tail gas pressure reducing valve 9 is set at a position close to the equipment of the urea low-pressure system (pre-evaporator) 10 to avoid condensation and two-phase flow problems.

[0045] 5. Set two pressure measuring points after the pressure reducing valve 9 to monitor the tail gas pressure in real time. When the pressure is abnormal, trigger an alarm and interlock the pressure reducing valve to close slightly.

[0046] 6. Set a check valve and a cut-off valve at the connection point of the tail gas to the low-pressure system 10 of the urea plant to prevent the crystallization of methylamine solution by backflow.

[0047] 7. The pipeline layout is designed with a slope of 4‰ to ensure that the residual liquid flows by gravity to the low-pressure system (pre-evaporator) of the urea plant during flushing and replacement.

Claims

1. A recovery and utilization technology for co-producing urea from by-product methylammonium solution and tail gas of melamine, characterized in that: It includes the following steps: Step A: Tail gas cooling and formation of carbamate solution Cool the tail gas (mainly containing NH3 and CO2) generated by the melamine reaction system by mixing it with the tail gas from the decomposer and the carbamate solution with a relatively high water content transported by the high-pressure carbamate pump, to form a carbamate solution and a small amount of uncondensed tail gas; By-product 0.35MPa g low-pressure steam is produced during the cooling process; Step B: Carbamate solution pressurization and transportation Pressurize the formed liquid-phase carbamate solution to 16.5MPa(g) with a plunger booster pump and transport it to the pool condenser of the urea plant; Step C: Non-condensable gas treatment and transportation Transport part of the non-condensable gas (high-pressure tail gas) cooled by the cooler to the low-pressure system (pre-evaporator) of the urea plant through pressure regulation; Step D: Design of the tail gas transportation system (1). Use pipes made of urea-grade stainless steel with high corrosion resistance. The pipes are designed with a full-jacket structure and are insulated with 0.8MPa g saturated steam; (2). Set high-pressure flushing water points (12MPa g) every 30 meters; (3). Set the tail gas pressure reducing valve near the equipment of the urea low-pressure system (pre-evaporator); (4). Set two pressure measuring points behind the pressure reducing valve to monitor the tail gas pressure in real time; (5). Set a check valve and a cut-off valve at the connection of the tail gas to the urea plant; (6). The pipeline layout has a slope of 4‰.

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