Design and Operation of a 1.7 MPa, 100,000 t/a Ammonia Synthesis Carbon Propylene Decarburization Unit

Thread Content

Since 1998, our institute has designed six sets of propylene carbonate decarboxylation units at normal temperature, with an absorption pressure of 1.7 MPa and an annual production capacity of 100,000 tons of synthetic ammonia; four of these units have been put into operation successively. Based on the performance over the past 2 years, the key technical and economic indicators have seen further improvements compared to those of the older units. A brief summary of the design and operation is provided below. 1 Design Overview 1.1 Main Design Parameters Absorption pressure: 1.7 MPa ； Vaporization pressure: 0.4 MPa ； Normal operating pressure: 0.03 MPa ； Stripping pressure: 0.004 MPa ； Temperature of the transformed gas entering the absorption tower: 35℃ ； Temperature of lean liquid entering the absorption tower: 38°C (summer) ； Transformed gas flow rate: 59,722 m3/h (standard conditions) ； Volume fraction of CO2 in the transformed gas: 27% ； Volume fraction of CO2 in the purified gas: ≤0.2% ； Volume fraction of CO2 in the mixed gas (normal and true mixture): ≥98% ； Mass concentration of H2S in the transformed gas: ≤10mg/m3 (standard conditions) ； Mass fraction of H2O in the solvent (in the system): ≤1% ； Volume fraction of CO2 in the lean liquid: ≤0.05% ； Ammonia solvent consumption: ≤0.5kg. 1.2 Process Flow The process flow is shown in Figure 1. http://www.nmtech.com.cn/jishuwang/upload/0604271630179110.jpg 1.3 Main technical measures: (1) Increase the height of the packing in the absorption tower and use high-efficiency packing to raise the saturation level of the solvent at the bottom of the tower as well as the equilibrium level at the top, thereby reducing the amount of solvent that needs to be recycled and improving the purity of the gas. (2) Adopt an air-pumping stripping process, reduce the stripping pressure, and improve the regeneration degree of the lean liquid (reduce the CO2 content in the lean liquid). (3) A four-stage desorption (flash, atmospheric desorption, vacuum desorption, air stripping) regeneration process is adopted. Compared to tertiary desorption (flash, vacuum desorption, stripping or flash, vacuum desorption, atmospheric pressure desorption), it achieves a higher degree of lean liquid regeneration and is energy-efficient. (4) Install purification gas washing and flash vapor washing. The mixed gas and stripping gas are subjected to two-stage washing (rough washing and fine washing) to reduce solvent entrainment losses. (5) Set up a full-flow filter. It is required that the H2S content in the syngas fed into the decarburization system be as low as possible, in order to keep the solution clean and prevent sulfur blockages. (6) Install a high-performance lean liquid pump-turbine unit to recover the energy from the rich liquid and reduce power consumption. 1.4 Main Equipment (1) Absorption tower: 1 unit, Ø3,800×48,660, carbon steel ； (2) Regeneration tower: 1 unit of carbon steel, dimensions Π5,000×38,400; upper section for normal dissolution, middle section for complete dissolution, lower section for air lift ； -. (3) Flash washing tower: 1 unit of Π1,000/Π1,600/Π3,800 (HT26525); the upper section is the flash vapor washing section with a diameter of Π1,000 made of stainless steel, the middle section is the purified gas washing section with a diameter of Π1,600 also made of stainless steel, and the lower section is the flashing section with a diameter of Π3,800 made of carbon steel ； (4) 1 stainless steel wash tower with dimensions of Φ2,000×23,735; the upper section is used for CO2 washing, while the lower section is used for stripping gas washing ； (5) Three liquid-poor liquid pump-turbine units: 250DK240 type pumps, WT1100—98 type turbines, SSS—30# clutches, and motors with a power of 1,050 kW at 6,000 V ； (6) Diluent pump: 2 units of 3P20 stainless steel, motor 11 kW 380V ； (7) Diluent pump: 2 units of IH50—32—160 stainless steel, with motors of 3kW at 380V ； (8) True solution fans: 2 units of L53LD, motors of type Y250M—4, 55 kW, 380 V ； (9) Stripping blower: 9–26, No.6.3–7, 2 units; motor Y250M–2, 55 kW, 380 V ； (10) Weak liquid water cooler: F=1,100 m2, carbon steel, 1 unit. 2 Operation Status 2.1 Operational Conditions The operation records of the two representative units (I, II) are shown in Tables 1 and 2. 2.2 Operation Analysis (1) When the volume of the transformed gas passed through the unit was 95% of the designed value, the volume fraction of CO2 in the purified gas remained at 0.1% throughout the day during summer. (2) When the volume flow rate of the transformed gas passing through Unit II is 120% of the designed value, the volume fraction of CO2 in the purified gas can still be maintained at 0.2%. (3) The WT1100 turbine installed in Unit I is currently the largest of its kind in China. Under normal operating conditions, the current drawn by the lean liquid pump decreases by 30A when the turbine is started; based on this, it can be calculated that the energy recovery unit saves 350 kwh of electricity per hour. (4) The actual power consumption index for ammonia production per ton by the plant operators is 108 kwh. (5) The ammonia loss due to gas-phase entrainment of the solvent is below 0.5 kg. 3 Conclusion (1) The propylene carbonate decarboxylation technique at room temperature can achieve a high level of gas purification even at low absorption pressures. (2) As the power consumption of the device decreases and solvent loss is reduced, traditional propylene carbonate decarboxylation at room temperature can also exhibit the advantages of low energy consumption and economic operation.