Process for producing liquid ammonia by ammonia distillation

1# joy168 Ammonia Recovery Technology (III) – Production Principle: Both the vent gas from spherical tanks and the gas from synthesis processes contain a certain concentration of ammonia. First, ammonia in the vent gas is absorbed using water in a ammonia purification tower, resulting in ammonia water of a specific concentration. Then, in an ammonia stripping tower, steam is used to distill out the ammonia from this ammonia water, which is subsequently condensed into liquid ammonia and sent to ammonia storage tanks or other units that require ammonia. Ammonia is a substance that dissolves easily in water, and it is miscible with water. Ammonia has a high vapor pressure at room temperature; to absorb all the ammonia from the vent gas, it is necessary to do so under certain pressure. 1. Absorption of ammonia: The partial pressure of ammonia above an ammonia solution is commonly expressed by the following formula: lg PNH3 = –1750/T + 1.1 lgM + 7. Here, PNH3 represents the partial pressure of ammonia above the solution, in mmHg ； T — Temperature, K ； M — the concentration of ammonia in the solution, in mol/L. When the ammonia-removed gas is used as city gas, it is required that the ammonia content in the gas be ≤50 ppm. In practice, the pressure for ammonia removal during production is set at 1.3–1.5 MPa (gauge pressure), which corresponds to an ammonia partial pressure of less than 70–80 Pa, or 0.5–0.6 mmHg. Use desalinated water at room temperature for ammonia removal; take the temperature of the desalinated water as T = 30°C = 303 K ； With the ammonia partial pressure PNH3=0.5 mmHg, substituting this value into the equation gives M=0.041 mol/L=0.82 t t. That is, the ammonia concentration in the wash water at the top of the pure ammonia tower must not exceed 0.82 ppm; in the design, ammonia-free deionized water is added to the tower top to ensure that the ammonia content in the gas after purification is ≤50 ppm. The ammonia content in the gas released from the spherical tank is approximately 40%, with a partial pressure of around 4000 mmHg. Assuming the temperature of the ammonia water circulating at the bottom of the tower to be 70°C = 343 K, the theoretical concentration of ammonia in this water can reach 35 mol/L = 700 ppm. Since the lower part of the net ammonia tower uses an ammonia water circulation absorption method, the actual controlled ammonia water concentration is 130–140 t/t. The current net ammonia tower is of three-stage design; the upper stage is equipped with three layers of sieve plates, and ammonia-free deionized water is used to ensure that the ammonia content in the gas remains within acceptable levels ； Diluted ammonia water (concentration ≥25 tt) is added from the bottom of the ammonolysis tower in the middle section ； The lower section contains circulating ammonia water, in order to increase the concentration of concentrated ammonia water and reduce the flow rate of ammonia water sent to the ammonia hydrolysis tower. The water and dilute ammonia added in the upper and middle sections flow downward stage by stage within the pure ammonia tower, eventually merging into the circulating ammonia in the lower section. 2. Distillation of ammonia: In an ammonia-water system, the boiling point of the solution changes depending on its composition under a certain pressure. This system is an ideal solution; at 1.6 MPa, the saturated vapor pressures of various components at corresponding temperatures are shown in the table below: Temperature (°C): 70, 80, 90, 113, 150, 160, 175. Ammonia in the gas phase (%): 99, 98, 97, 92, 70, 60, 40. Ammonia in the liquid phase (%): 57, 49, 45, 34, 17, 14, 8. Relative volatility: 75, 51, 9.2, 7.8. To keep the ammonia content in the waste liquid very low, it is noted that the boiling point of water at 1.6 MPa is 202°C. Therefore, the temperature in the ammonia stripping tower must be close to the boiling point of water in order to ensure a low ammonia content. The bottom temperature of the tower is set at 200°C during design. As can be seen from the table above, 14% concentrated ammonia has a boiling point of 160℃ ； 8% concentrated ammonia water has a boiling point of 175°C. The ammonia content at the outlet of the circulating pump in the net ammonia tower can reach 130–140 tt, which is approximately 11–12%, corresponding to a boiling point of around 165°C. Increasing the ammonia concentration of the feed to the ammonolysis tower allows the temperature of the feed liquid to be reduced, which helps to cut back on steam consumption. In the design, the feed temperature is set at 140°C, which constitutes a subcooled liquid feed in order to prevent ammonia from vaporizing in the pipes. To achieve an ammonia concentration in the top product of over 99.5%, a certain amount of recycled ammonia must be added to the tower top. Refering to the liquid ammonia vapor pressure table, the saturated vapor pressure of liquid ammonia at 43°C is 17.2 kg/cm2 (absolute pressure), which corresponds to a gauge pressure of 1.6 MPa. When the pressure in the ammonia hydrolysis tower is set at 1.6 MPa, the temperature of the product at the top of the tower is 43°C. This ammonia hydrolysis tower features feed at the middle section; the ammonia concentration in the feed is 130–140 tt, and the temperature is above 140°C ； A tubular reboiler is installed at the lower part, and it is indirectly heated using 2.5 MPa medium-pressure steam ； No separate condenser is installed at the tower top; the gas exiting the tower top is sent to the horizontal cooler in the chiller system for condensation. The ammonia used for reflux is returned from a 100 m3 liquid ammonia storage tank and then pumped into the tower top using an ammonia pressure pump. The current process does not recover ammonia from the ammonia recovery unit as liquid ammonia; instead, the gas coming from the top of the tower (containing about 70% ammonia and the rest water vapor) is sent directly to the absorption unit for carbonization. There, it is converted into concentrated ammonia solution, which is then used to produce ammonium carbonate. The products at the top of the tower are gaseous ammonia and water vapor; therefore, no ammonia is recycled back to the top of the tower. If the production volume of ammonium carbonate is high enough to achieve a balance between ammonia and water, it is also possible to directly feed 130–140 tt of concentrated ammonia solution into the old carbonization unit, in which case the ammonia hydrolysis tower can be discontinued. (IV) Brief description of the process 3. Ammonolysis tower process: The concentrated ammonia solution with an ammonia content of 130–140 tt, supplied by the circulating pump of the pure ammonia tower, is preheated to above 140°C in two series-connected preheaters A/B (tag number E002). It then enters the stripping section of the ammonolysis tower (tag number R002), where it comes into countercurrent contact with the hot gas stream rising from the bottom of the tower, thereby enabling the removal of ammonia gas from the ammonia solution. The ammonia gas stream containing a certain amount of water vapor rises and comes into countercurrent contact with the recycled ammonia coming from the upper distillation section; once the desired ammonia concentration is achieved, it is drawn out from the top of the tower and sent to an ammonia condenser for condensation. Part of the condensed ammonia is exported as a finished product, while another part is returned to the system as reflux ammonia. A portion of the liquid in the bottom of the tower enters the reboiler (tag number E004), is heated with medium-pressure steam, and then returned to the ammonolysis tower ； A portion of the stream is sent to the shell side of preheaters B/A, where it exchanges heat in counterflow with the ammonia water flowing inside the tubes, thereby preheating the ammonia water ； At the same time, the reactor broth itself is cooled. The kettle liquid coming out of the preheater enters the water cooler (tag number E003) where it is further cooled to room temperature, after which it is sent to the dilute ammonia tank to be used cyclically as the absorption liquid in the pure ammonia tower. The designed operating pressure of the ammonolysis tower is 1.6 MPa; since no ammonia reflux is applied at present, the distillation section cannot function. Due to reasons such as the vapor at the top of the tower not being sent to the horizontal cooler, a pressure reduction operation is now being carried out, with the actual operating pressure at 0.4–0.5 MPa. In practice, in order to reduce steam consumption, the ammonia concentration in the reactor liquid is kept high, usually above 20 wt%,; therefore this reactor liquid cannot be discharged to avoid environmental pollution. 4. Steam system: The medium-pressure steam used in the hydrolysis tower comes from the medium-pressure steam main in the boiler room, with a pressure of 2.5 MPa and a temperature of 430°C℃ ； The waste steam is returned to the low-pressure steam main at a pressure of 1.0 MPa. 5. Desalinated water: The desalinated water used in the ammonia purification tower comes from the high-pressure desalination water network at the desalination station, with a pressure of 3.0 MPa and at normal temperature ； During operation, desalinated water is also supplied from this pipeline network for use in devices such as the net ammonia tower, dilute ammonia tank, and ammonolysis tower. 6. Cooling water circuit: This position is equipped with two water coolers: the water cooler for the circulating absorption liquid in the pure ammonia tower (tag number E001) and the water cooler for the residual liquid in the ammonolysis tower (tag number E003). All cooling water comes from either the old desulfurization cooling circulation system or the carbonization cooling circulation system; after cooling the corresponding fluids, it returns to either the old desulfurization or carbonization circulation systems. 8. Ammonia and ammonia water process: When the product recovered from the ammonolysis unit is liquid ammonia, the gaseous ammonia extracted from the top of the ammonolysis tower is first sent to the horizontal ammonia condenser in the refrigeration section; after being condensed into liquid ammonia, it enters a 100 m3 liquid ammonia storage tank ； The reflux ammonia used in the ammonolysis tower comes from a 100 m3 liquid ammonia storage tank. When the product recovered from ammonia hydrolysis is gaseous ammonia containing water vapor, it is sent directly to the carbonation absorption system for further processing into concentrated ammonia solution to produce ammonium carbonate. The ammonia water with a concentration of 130–140 tt produced by the net ammonia tower is sent not only to the ammonolysis tower but also to other units that require ammonia water, such as those for carbonization, desulfurization, and desalination. (5) Pressure test of ammonia recovery equipment 1. Preparatory work before the pressure test 1.1 Prepare a pressure test plan and obtain approval from the relevant departments. 1.2 Before the pressure test, the following documents must be reviewed: (1) Equipment factory certificate of conformity ； ⑵Certificate of Conformity for Equipment Accessories and Internal Parts ； ⑶Design parameters and on-site compensation records ； ⑷Material certificates for the assembly equipment, equipment assembly records, welding procedure records, non-destructive testing reports, etc. 1.3 Before conducting a pressure test on the equipment, it is necessary to carry out at least one thorough inspection of the equipment in question, based on the design and installation drawings. This inspection should verify that the equipment installation and pipe connections meet the design requirements, and that pipes, valves, and other accessories are all properly installed. 1.4 Thoroughly inspect each tower, heat exchanger, pipeline, and pump for mechanical impurities, and remove them immediately once detected. 1.5 Install the covers for manholes and access holes properly; for those components indicated on the drawings as not being able to withstand test pressures, they should be removed or isolated using blind flanges before conducting the pressure test. If individual pressure testing of certain equipment is required, blind flanges must also be installed at the relevant connection ports. Minimum thickness of the test insertable blind flange: 8 mm for DN125, and 6 mm for those below DN100. 1.6 Check whether the air source, water source, pressure testing pump, and other tools required for the pressure test are all ready. 1.7 During the pressure test, all fastening bolts in various parts must be properly installed. Two pressure gauges should be used during the test; they should be placed at the highest and lowest points of the equipment. The pressure gauges must be calibrated, with an accuracy level of not less than 1.5, and their maximum range should be 1.5 times the test pressure. 2. Pressure test media: compressed air and clean potable water. 3. During the equipment pressure testing process, the pressure-bearing components must not be modified; any defects identified during testing must be rectified after the pressure is released, and testing must be conducted again after these corrections are made. 4. Test pressure: If air is used as the medium for the final pressure test, the test pressure shall be 1.15 times the design pressure, and shall not be less than p + 0.1 MPa ； If water is used as the final pressure testing medium, the test pressure shall be 1.25 times the design pressure, and shall not be less than p + 0.1 MPa. In the formula, p represents the design pressure, in MPa ； σ] —— Allowable stress of the material at test pressure, MPa ； σ]t—— Allowable stress of the material at the design temperature, MPa ； The ratio of σ]/σ]t does not exceed 1.8 at most. 5. Test procedures and precautions: 5.1 The pressure test can be carried out in stages, with inspections and checks performed at (1) below the operating pressure, (2) at the operating pressure, (3) at the design pressure, and (4) at the test pressure. 5.2 When testing with air, inspections shall be carried out at pressures of 1.0 MPa, 1.6 MPa, and the design pressure; any obvious leakage points must be eliminated before continuing to increase the pressure ； The pressure is raised to the test pressure and held for 30 minutes; then, once the pressure is reduced to the design pressure, a thorough inspection is conducted. If there are no leaks or seepages, the pressure testing is considered successful. Inspection is generally carried out by applying soapy water to welds, seals, or areas of suspicion; if no bubbles form at the area where the soapy water is applied, it is considered leak-free. 5.3 When conducting a pressure test with water once, necessary inspections should be carried out at regular intervals as water is added, and any leaks detected must be repaired promptly ； After filling with water, conduct a thorough inspection; only if there are no leakage points at normal pressure can the pressure be increased to the test pressure. ⑴Hydraulic testing should generally be carried out before painting and insulation ； ⑵The water temperature for the hydrostatic test must not be lower than 5℃ ； ⑶During the hydrostatic test, the outer surface of the equipment must be dry. The test should be carried out at an ambient temperature of above 5°C; otherwise, anti-freezing measures must be taken ； ⑷During the hydrostatic test, air must be completely removed from the highest points of all equipment and systems, ensuring that they are filled with water. The pressure is then increased slowly to the specified level, and the pressure is held constant for 30 minutes. After that, the pressure is reduced to the design level, and this condition is maintained for at least another 30 minutes, during which time any leaks or seepages are checked for. ⑸After the hydrostatic test, the water should be drained promptly; it must not be discharged near the equipment foundation. 6. During the pressure test, if any abnormal noises are heard, if the pressure drops rapidly, or if there are any other abnormalities such as failures in the pressurization device, the test must be stopped immediately. The cause of these issues should be identified, and once they are resolved, the test can be resumed. 7. Test records: During the testing process, it is necessary to promptly record in the relevant test record books the personnel involved in the tests, the test times, the data from each test stage, as well as the processing procedures involved. (VI) System inspection, purging, and cleaning of pipelines A. System inspection of pipelines 1. After the pipelines are installed, tests for strength, tightness, etc. shall be carried out in accordance with the design specifications, in order to assess the quality of installation of the pipeline system and its connections. 2. Test medium: clean water. 3. Conditions to be met before testing: (1) The pipeline system shall have been constructed completed, and shall meet the design requirements as well as the relevant regulations for pipeline installation. ⑵The brackets and hangers have been installed, are properly configured, and secured firmly. ⑶The welding and heat treatment tasks have been completed and passed inspection; the welds and other areas that require inspection have not been painted or insulated. ⑷Before the test, all welded flanges and other joints were ensured to be easy to inspect. ⑸Remove all temporary clamps, plugs, blind flanges, etc. from the pipeline. ⑹The coordinates, elevations, slope, and pipe foundation layer of the buried pipes have been rechecked and found to be satisfactory; the temporary reinforcement measures used for testing have been inspected and confirmed to be safe and reliable. ⑺The pressure gauges used for testing have been calibrated, with an accuracy of not less than grade 1.5; the full-scale value of these gauges is 1.5–2 times the maximum test pressure, and there should be no fewer than 2 such pressure gauges. 4. Before the test, use compressed air to remove debris, rust, and slag from the pipes; if necessary, rinse with water at a flow rate of 1–1.5 m/s until the water discharged is clean. 5. Before the test, systems, equipment, instruments, and pipeline accessories that cannot participate in the test should be isolated; safety valves should be removed, and areas where blind plates are installed must have clear markings and records. 6. Before testing the pipeline system, isolation blind plates should be installed in conjunction with the pipelines that are in operation. When valves are used to isolate steam pipelines, the temperature difference between the two sides of the valve should not exceed 100°C. 7. If a leak is detected during the test, it must not be repaired under pressure; the test should be repeated after the defect is corrected. 8. After the test is successful, the test medium should be discharged properly. 9. Carefully fill out the \"Pipeline System Testing Record\": 10. Precautions: (1) The water used for hydrostatic testing should be clean water (fresh water or deionized water). When filling the system with water, open the exhaust valves at various heights to remove all air. Once the water tank is full, close the exhaust valves and the water inlet valve. The test pressure should be increased gradually; once it reaches a certain value, the process should be stopped to inspect the pipeline, and only if there are no issues should the pressure be increased further. Maintain at the test pressure for 20 minutes. ⑵In pipeline systems with a large pressure difference, the highest pressure shall apply, but the pressure at the lowest point must not exceed the capacity of the pipeline fittings and valves. ⑶On-site, the medium-pressure desalinated water can be introduced into the system to serve as a hydrotesting pipeline as well as a heating and make-up water pipeline. B. Purging and cleaning of the piping system 1. Purging and cleaning should be carried out after the piping system has passed the strength test. 2. The purging sequence is from the main pipe to the branch pipes. 3. Before cleaning, the instruments in the system should be protected, and components such as throttle valves and check valves should be removed and stored properly; they should be reinstalled after the cleaning is completed. 4. Equipment and pipelines that are not allowed to be purged should be isolated from the purging system. 5. During purging, dirt inside the pipes must not enter the equipment, and dirt expelled by the equipment should generally not enter the pipes either. 6. The pipeline purging should have sufficient flow rate; the purging pressure must not exceed the design pressure, and the flow velocity should be no lower than the operating flow velocity, generally not less than 20 m/s. 7. When cleaning, use a wooden hammer to tap the pipes; pay special attention to tapping the welds, dead corners, elbows, and the bottom of the pipes, but be careful not to damage them. 8. Before purging, the stability of the pipeline supports should be considered, and they should be reinforced if necessary. 9. Fill in the “Piping System Purging Record” and “System Sealing Record” carefully. 10. The flow rate for water flushing is generally 1.5 m/s; the water flushing should be carried out sequentially, and it is considered satisfactory when the color and transparency of the water at the outlet are the same as those at the inlet. 11. After flushing the pipeline, the water should be drained completely; compressed air can be used to dry it if necessary. 12. Air purging, used on ammonia gas pipes; inspection is carried out at the exhaust outlet using a cloth or a board coated with white paint. It is considered satisfactory if no rust, dust, moisture, or other contaminants are found on it within 5 minutes. 13. Steam purging is used in steam and its condensate pipelines. Before purging, the pipe should be warmed up slowly; after maintaining a constant temperature for one hour, purging is carried out. Then it is allowed to cool down naturally to ambient temperature, after which it is heated up again, the pipe is warmed up once more, and a second purge is performed while maintaining a constant temperature. This is repeated in this manner, usually no less than three times. 14. The exhaust pipe for steam purging should be inclined with its opening facing upward to ensure safe discharge; the exhaust pipe must be equipped with secure supports ； The diameter of the exhaust pipe should not be smaller than that of the pipe to be purged, and its length must be sufficient. 15. Steam purging of insulated pipes should generally be carried out before insulation; if necessary, local heat protection measures can be employed. 16. Inspection method: The inspection should be carried out on the painted plate installed on the exhaust pipe; the surface of this painted plate must be smooth and even, with a width of 5–8% of the exhaust pipe’s diameter, and a length equal to the inner diameter of the pipe ； Replace the painted plate twice in succession and conduct inspections; it is considered satisfactory if there are no visually visible impact marks on the plate, or if the number of impact points is no more than ten, with each point being no larger than 1 mm. (7) Startup, shutdown, and normal operation A. Initial startup (follow these steps for startup after major repairs): 1. Pre-startup inspections: ⑴ All equipment and pipelines must be inspected; operations can proceed only after cleaning, pressure testing, and replacement are completed successfully. ⑵Check all electrical instruments and adjust the valves to ensure they operate properly. ⑶Inspect the safety valve and apply a lead seal. ⑷Check whether various tools, protective equipment, fire-fighting equipment, and communication devices are all available. ⑸Check whether the motor rotation direction is correct; the pump unit has passed the trial run. ⑹Before starting the vehicle, close all valves connected to the outside world within the system, and drain any water accumulated in the existing pipelines and equipment. 2. Purging of the pure ammonia system: To prevent explosions resulting from the mixing of gas released from the synthesis unit and spherical tanks with the air inside the equipment, an inert gas is introduced into the system after pressure testing is successful, in order to remove the air from the equipment and pipelines. The gases released before and after the ammonia recovery tower are analyzed; if their compositions are identical and the oxygen content is less than 0.5%, the results are considered satisfactory if the composition remains unchanged after three analyses. If there is no source of inert gas available, the raw gas, that is, the vent gas, can be used cautiously for purging; however, the gas flow rate must be strictly controlled during this process. Open flames and smoking are strictly prohibited in the area, and valves must always be operated by hand ； If an F wrench must be used, it must be handled with care; it is strictly prohibited to let the F wrench collide with the valve, as this could cause sparks and explosions. The purging of the ammonia vaporization system is covered in the startup and normal operation of the ammonia vaporization system. 3. Commissioning, starting, and stopping of circulating pumps and multi-stage pumps 3.1 Checks before starting ⑴ Before starting, check that all bolts, pipelines, and connections are securely fastened. ⑵Check that all instruments and valves are functioning properly. ⑶Check whether the lubricating oil in the pump is in adequate quantity and whether the oil level gauge is in good condition. ⑷Check whether the motor is rotating in the correct direction. 3.2 Starting of the centrifugal pump ⑴ Rotate the pump shaft by more than 2 full turns before starting; the rotation should be smooth, with no signs of collision between the pump and the motor. ⑵Open the inlet valve of the pump and close the outlet valve. ⑶Open the pump’s exhaust valve to release the air inside the pump, and close it once the pump is filled with liquid. ⑷Start the motor; once the pump is operating properly and the pressure is stable, open the valve at the pump outlet as well as the valves on the output pipeline to an appropriate degree, depending on the flow rate required by the process. 3.3 Pump operation inspection: After the pump starts operating, a comprehensive inspection should be carried out at regular intervals to ensure its proper functioning. During the first 5 minutes of pumping after starting the pump, the operator must not leave the site; thereafter, inspections should be carried out every 15 minutes for the first 2 hours, and then every hour after 2 hours of normal operation. Pay special attention to checking the pump’s outlet pressure and flow rate, as well as the lubrication and temperature rise of the motor and the pump; also check for any abnormal noises coming from the pump and motor ； The pump should be stopped for maintenance when it fails. 3.4 Shutting down the centrifugal pump ⑴ Close the pump’s outlet valve to minimize flow, but do not close the pump’s inlet valve. ⑵Press the pump’s stop button to shut down the motor. ⑶Close the outlet valve of the pump. ⑷Close the pump’s suction valve after the pump has come to a complete stop. ⑸Pumps that are not expected to be in use for an extended period should have the liquid inside them drained, especially in winter, the dilute ammonia solution or clean water inside the pump should be removed. 3.5 Pump switching ⑴ Start the standby pump following the pump startup procedures; switch pumps only after its operation is normal and the pressure has risen to a high level. ⑵Slowly open the outlet valve of the standby pump while closing the outlet valve of the pump in use ； Until the flow rate of the standby pump reaches the requirement of the process, close the outlet valve of the pump in use. ⑶Stop the pump in use following the normal shutdown procedures. ⑷If the stopped pump malfunctions, notify the maintenance staff and other personnel immediately for repairs. B. Commissioning and normal operation of the ammonia purification system 1. Commissioning of the ammonia purification system 1.1 Normal commissioning 1.1.1 Inspection tasks before commissioning ⑴ Check for any defects in electrical appliances and equipment. ⑵Check that all instruments and pressure gauges are present and accurate. ⑶Check whether each valve operates smoothly. ⑷Check the liquid levels in the dilute ammonia tank and the pure ammonia tower (an appropriate amount of deionized water can be added to these tanks during initial startup). ⑸Check that all pumps are properly lubricated. 1.1.2 Commissioning of the net ammonia system ⑴ Close all vent valves and drain valves. ⑵When the hydrogen extraction system is not in operation and gas users are not using it, the relevant valves in the system must be closed and blind flanges installed. ⑶Start the multi-stage pump and the circulation pump to establish liquid-phase circulation. (At the beginning of operation, if deoxygenated water free of ammonia is used in the ammonia purification tower, the circulation pump can be operated alone for a short period of time to carry out the first stage of ammonia purification. ) ⑷ Open the inlet and outlet valves of the external cooler to supply water to it. ⑸Open the inlet valve for the vent gas of the ammonia stripping tower. ⑹When the pressure in the net ammonia tower reaches the operating pressure, the vent valve of the tower should be opened appropriately to keep the system pressure within the specified range. ⑺Analyze the gas composition after removing ammonia; once it meets the requirements, open the corresponding valves of the hydrogen extraction system or other user systems to send the gas backward or outward. ⑻After the ammonia concentration is reached at the specified level, the ammonia distillation system can be started or ammonia can be supplied to other ammonia users. 2. Operating parameters: ⑴ Operating pressure: 1.5–1.7 MPa; when the 100 m3 liquid ammonia tank is under reduced pressure for hydrogen extraction, the operating pressure of the pure ammonia tower should be kept at ≤1.3 MPa. ⑵Operating temperature: 30–40°C ⑶ Ammonia concentration after ammonia purification: 120–140 tt ⑷ Concentration of dilute ammonia water entering the tower: ≤25 tt ⑸ Ammonia content in the gas after ammonia purification: ≤50 mg/Nm3 ⑹ Liquid level in the ammonia purification tower: 1/2–2/3 ； Level of dilute ammonia solution in the tank: 1/2 – 2/3. ⑺ Temperature of ammonia after cooling: 30 – 40°C. ⑻ Outlet pressure of the multi-stage pump: <2.5 MPa; current ≤ 35.5 A. ⑼ Outlet pressure of the circulation pump: <0.4 MPa; current ≤ 11 A. 3. Key points for normal operation: ⑴ Regularly monitor the motor current, outlet pressure, and temperature rise of the bearings of both the multi-stage pump and the circulation pump, to ensure that each pump operates properly and delivers the required flow rate. ⑵Maintain stable liquid levels in the net ammonia tower and dilute ammonia tank to prevent pump cavitation. ⑶Maintain the stability of the pressure of each pump, the pressure of the feed gas, and the flow rate of the feed gas. ⑷Adjust the circulation water volume of the external heat exchanger at appropriate times to ensure effective cooling. ⑸Adjust the volume of the ammonia water circulation tank outside the tower at appropriate times to ensure optimal ammonia removal, so that the ammonia content in the gas after purification remains within the specified limits; if these limits are exceeded, the reasons must be identified promptly. ⑹Regularly monitor the pressure of the spherical tank to release air, and operating under overpressure is strictly prohibited. ⑺Maintain stable gas supply, and strengthen communication between teams when the load changes. ⑻When the 100m3 liquid ammonia tank needs to release gas under reduced pressure to this system, first reduce the operating pressure of the pure ammonia tower to below 1.30 MPa, then open the inlet valve for gas release from the 100m3 tank in the pure ammonia tower (remove the blind plate if it is in place first); immediately close the inlet valve once the pressure has been reduced, to prevent ammonia solution from the pure ammonia tower from flowing into the 100m3 liquid ammonia tank. ⑼Strictly control the amount of deionized water added at the top of the net ammonia tower to maintain system water balance, and prevent ammonia-containing water from being discharged. C. Commissioning and normal operation of the ammonolysis system 1. Pre-commissioning inspections ⑴ All equipment and pipelines have been inspected, tested, purged, and cleaned, and all have passed the checks. ⑵Inspect all electrical instruments and adjust the valves to ensure smooth operation. ⑶Inspect the safety valve and apply a lead seal. ⑷Check whether various tools, protective equipment, fire-fighting devices, and communication signals are all available ； ⑸Checked whether the rotation direction of the return pump motor was correct, and conducted a single-unit test run of the pump. ⑹Before starting the vehicle, close all valves that connect the system to the outside environment, and drain any water accumulated in the existing equipment and pipes. ⑺After contacting the dispatching team and relevant departments (boilers, chillers, carbonization, etc.), the startup phase begins. 2. Introduce cooling water: Open the inlet and outlet valves of the water cooler, and pay attention to venting at the high point. 3. Fluid filling: Add deionized water through the water addition valve to maintain the liquid level at the same level as the upper tube sheet of the reboiler. 4. Introducing steam (purging and heating): After warming the pipes, slightly open the inlet valve on the main pipe, open the drain valve on the separator’s bypass line, and slowly heat the reboiler in order to warm up and purge the ammonia decomposition tower. Open the vent valve of the ammonia decomposition tower, and control the heating rate at around 15–30°C/h. Once the temperature rises above 100°C and all the sampled gas has condensed, the purging process is considered complete; at that point, close the vent valve of the ammonia decomposition tower. Continue to increase the temperature; once it reaches above 140°C, proceed with the normal startup procedure. During the heating process, the heating rate must be strictly controlled to prevent damage to equipment, pipes, or fittings due to too rapid heating ； During the heating process, increase routine inspections and perform heat-tightening on some bolts. 5. Pressure increase in the ammonolysis tower: As the temperature in the ammonolysis tower rises, water inside the tower vaporizes, and the pressure gradually increases as well. Before the addition of ammonia, the relationship between pressure and temperature inside the tower can be roughly considered to be that of the saturated vapor pressure of pure water: Temperature (°C): 100, 110, 120, 130, 140, 150; Pressure (MPa, absolute): 0.100, 0.146, 0.202, 0.275, 0.368, 0.485. Temperature (°C): 160, 170, 180, 190, 200, 202; Pressure (MPa, absolute): 0.630, 0.808, 1.022, 1.280, 1.586, 1.653. As can be seen from the table, when the temperature in the ammonolysis tower rises to 140°C, the pressure inside the tower is 0.368 MPa (absolute). 6. Slightly open the feed valve to allow liquid to enter the ammonolysis tower. 7. Control the liquid level at the normal position using a level regulator to keep the system in a circulating state. 8. Adjust the pressure through the pressure control system of the ammonia reflux tower, gradually raising it to 1.6 MPa (absolute). 9. Start one reflux pump following the normal startup procedure; when starting the pump, make sure to coordinate with the synthesis team to keep the valve for the liquid ammonia in the 100 m3 storage tank in the fully open position. Note: The return pump is a piston-type positive displacement pump. Before starting the pump, it is necessary to open the pump’s inlet valve and the bypass valve first before powering on the motor. Once the pump is operating properly, open the outlet valve while reducing or fully closing the bypass valve; use the bypass valve to regulate the flow rate of the return fluid, and ensure that the pump’s outlet pressure and motor current remain within the specified range. 10. Allow a slight amount of ammonia to flow back in order to gradually reduce the temperature of the gas exiting the ammonolysis tower to around 43°C; be careful not to use an excessive amount of ammonia, as this will cause the pressure to rise rapidly. 11. When the temperature of the outlet gas phase is below 70°C, the valve for the horizontal cooler of the de-icing unit can be opened, while the valve for the ammonia removal tower should be closed; it is however necessary to ensure that the gas phase temperature drops rapidly to around 43°C. 12. Control the feed rate appropriately and increase the steam usage, but be careful that the pressure in the low-pressure steam pipeline system does not exceed 1.0 MPa. 13. To control the liquid level, the cooled liquid can be sent to the circulation tank (dilute ammonia tank), or it can be fed into the pure ammonia tower via the outlet of multi-stage pumps; when the temperature is too high and makes pure ammonia production difficult, the liquid can be discharged through backwashing. 14. Gradually bring all system parameters back to normal, with a bottom temperature of 200°C, a top temperature of 43°C, and an inlet material temperature above 140°C. 15. When the content of inert gases in gaseous ammonia is high, it is necessary to contact the chiller team to remove the inert gases from the coil cooler in order to prevent overpressure. 16. Under the condition where no ammonia is recycled to the ammonolysis tower and the product from this tower is sent directly to the absorption unit, the distillation section does not function. Due to reasons such as the vapor from the tower top not being sent to the horizontal cooler, a pressure reduction operation is now being carried out, with the actual operating pressure at 0.4–0.5 MPa ； To reduce steam consumption during operation, the ammonia concentration in the reactor liquid is high; therefore, the reactor liquid cannot be discharged directly into the drain. 2. Increase and decrease: Increase or decrease the amount of material fed into the tower, and correspondingly increase or decrease the amount of ammonia reflux and the steam supplied to the reboiler. Strictly prevent overpressure in the low-pressure steam network caused by adjustments to steam consumption. 3. Key points for normal operation: ⑴ Strictly control the pressure in the ammonolysis tower, as well as the temperatures at the bottom and top of the tower, at the feed point, and in the residual liquid. ⑵The amount of steam added and the amount of ammonia recycled must be adjusted to match the amount of material fed into the tower ； Excessive steam addition results in a low concentration of residual liquid, but it requires an increased amount of ammonia to be recycled, thereby invisibly increasing the load on the boiler and refrigeration system ； Excessive amount of recycled ammonia improves product purity, but it increases the load on the refrigeration condenser. ⑶Constantly monitor the motor current of the return pump, the outlet pressure, and the temperature rise of the motor bearings to ensure normal operation of the pump and proper flow rate of the fluid. ⑷Adjust the circulation water volume of the external heat exchanger at appropriate times to ensure effective cooling. 4. Normal driving: If the system is in heat retention and pressure maintenance mode, it will return to normal operation after a brief cycle. If the system has been depressurized or cooled down, follow the relevant provisions listed under 1.4 in the original startup procedure. D. Shutdown 1. Normal shutdown: 1.1. For the ammonia purification system: Open the vent valve of the tower, close the valves at the ball tank and at the inlet for release gas from the synthesis unit, as well as the main line valves of the system; follow the standard procedure for shutting down the pumps, and close the inlet valve of the dilute ammonia tank as well as the inlet and outlet valves of the cooler. 1.2 Ammonolysis system: Close the steam valve, feed valve, level control valve, pressure control valve, etc.; stop the reflux ammonia pump following the pump shutdown procedure, and close the reflux ammonia valve. When it is required that the system remain in a heat-retained state, an appropriate amount of steam is added to keep the bottom temperature around 160°C ； At the same time, control the pressure in the ammonolysis tower properly, and overpressure must be strictly avoided. 2. Emergency shutdown: (1) Shutdowns caused by power loss, steam interruption, or cooling water failure can be handled as normal shutdowns. ⑵Handle it as a normal shutdown due to reasons such as instrument failure; if necessary, use manual control to prevent overpressure. ⑶If the instrument air supply to the ammonolysis tower is interrupted, it can be stopped manually or by contacting the dispatch team. ⑷If the ammonolysis tower has to be shut down for special reasons, the reflux ammonia pump can be stopped immediately, the steam valve, feed valve, and level control valve should be closed, and the pressure control valve should be switched to the pure ammonia tower as appropriate. 3. Major repair shutdown or long-term shutdown: For shutdowns carried out to perform maintenance on the entire system, the normal shutdown procedures can be followed. The ammonia in the pure ammonia tower is sent to the carbonization unit or discharged into the dilute ammonia tank under system pressure, after which inert gas is used to purge the relevant equipment and pipelines. Depending on the maintenance tasks, the ammonia water treatment in the tank is arranged by the workshop or the Technical Production Department. For the ammoxidation tower system, the accumulated liquid within the system should be drained through backwashing and the temperature should be reduced gradually. Once the temperature of the tower is close to that of the deionized water, deionized water can be used to flush the system; after proper replacement, it can then be taken for maintenance. (VIII) Process Parameters (Revised in January 2006)
Serial No. | Parameter Name | Unit | Parameter Range | Category | Remarks
1 | Operating pressure of the pure ammonia tower (normal) | MPa | 1.5–1.7 | Plant-controlled |
2 | Pressure of the pure ammonia tower during depressurization of the 1000 m³ storage tank | MPa | ≤1.3 | Plant-controlled |
3 | Pressure in the ammonia evaporation tower | MPa | ≤0.50 | Workshop-level; no reflux ammonia used |
4 | Concentration of ammonia water in the liquid output from the pure ammonia tower | tt | ≥120 | Workshop-level |
5 | Concentration of ammonia water in the residual liquid from the ammonia evaporation tower | tt | ≤25 | Workshop-level |
6 | Purity of hydrogen gas produced | % | 80–93 | Plant-controlled; varies with production balance |
7 | Pressure in the hydrogen gas buffer tank | MPa | 0.4±0.1 | Workshop-level |
8 | Pressure in the gas buffer tank | MPa | 0.02–0.04 | Plant-controlled |
9 | Ammonia content in the gas | mg/Nm³ | ≤50 | Plant-controlled |
10 | Methane content in the gas | % | ≥20 | Workshop-level |

(IX) Abnormal Conditions, Accident Diagnosis, and Handling
| Cause of Abnormal Condition | Handling Method |
|---------------------------|------------------|
| Pump fails to generate pressure or draw liquid | 1. Gas remaining inside the pump not removed properly.<br>2. Liquid level in the ammonia water tank or tower is too low, causing the pump to draw air.<br>3. Pump is running in reverse. | 1. Remove all gas from the pump.<br>2. Raise the liquid level.<br>3. Check the rotation direction and inform the electrician to swap the motor connections. |
| Leakage at the pump gasket | 1. Gasket is too loose.<br>2. Quality of the sealing material is poor. | 1. Tighten the bolts securing the gasket.<br>2. Replace the gasket. |
| Excessive heat or smoking at the gasket | Gasket is too tight. | Loosen the gasket and the gasket securing bolts. |
| Noise inside the pump | Solid particles have entered the pump or the impeller is loose. | Stop the pump, disassemble it for inspection, and carry out repairs. |
| Excessive current in the pump motor | Gasket is too tight. | Loosen the gasket and the gasket securing bolts. |
| Ammonia content in the gas exceeds the limit | 1. The amount of gas released into the system suddenly increases, or the ammonia content in that gas is too high.<br>2. The amount of absorbent liquid or desalinated water used is too low.<br>3. Temperature of the absorbent liquid is too high.<br>4. There is uneven flow inside the pure ammonia tower.<br>5. Ammonia water enters the gas system.<br>6. Pressure in the pure ammonia tower is too low.<br>7. Overflow occurs in the pure ammonia tower. | 1. Instruct the previous process to stabilize the gas release rate and control ammonia content.<br>2. Increase the circulation volume and the amount of desalinated water used.<br>3. Lower the temperature of the absorbent liquid.<br>4. Repair the pure ammonia tower to eliminate uneven flow.<br>5. Control the liquid level carefully to prevent liquid carryover.<br>6. Slightly increase the pressure in the pure ammonia tower.<br>7. Reduce the amount of fluid processed. |
| Overpressure in the ammonolysis system | 1. The valve for releasing ammonia gas from the cooler is not fully open.<br>2. The gas-phase ammonia pipeline is blocked.<br>3. Excessive amount of reflux ammonia.<br>4. Instrumentation malfunction.<br>5. Temperature at the bottom of the tower is too high.<br>6. Overflow occurs in the ammonolysis tower.<br>7. Overpressure in the refrigeration unit. | 1. Open the valve for releasing ammonia gas.<br>2. Check the opening degree of pipelines and valves, as well as whether the valve seats are damaged.<br>3. Slightly reduce the amount of reflux ammonia.<br>4. Contact the instrumentation technician for assistance.<br>5. Slightly reduce the steam usage.<br>6. Reduce the flow rate and drain the tower contents.<br>7. Contact the refrigeration unit for repair. |
| High temperature at the top of the ammonolysis tower | 1. Insufficient amount of reflux ammonia.<br>2. Excessively high pressure.<br>3. Liquid carryover or overflow occurs. | 1. Increase the amount of reflux ammonia.<br>2. Adjust the pressure appropriately.<br>3. Reduce the amount of fluid processed. |
| Low temperature of the feed material entering the ammonolysis tower | 1. Insufficient steam usage.<br>2. Low circulation volume. | 1. Increase the steam usage.<br>2. Increase the circulation volume of nitrogen. |
| Overflow occurs in the ammonolysis tower | 1. Excessive amount of secondary steam.<br>2. Large amount of feed material.<br>3. Liquid level control is too strict.<br>4. Preheating temperature is too low.<br>5. Blockage in the system. | 1. Slightly reduce steam usage to lower the load.<br>2. Slightly reduce the amount of feed material.<br>3. Lower the liquid level.<br>4. Increase the preheating temperature.<br>5. Shut down the tower for maintenance. |
| Overflow occurs in the pure ammonia tower | 1. Excessive amount of gas released.<br>2. Excessive circulation volume.<br>3. Liquid level control is too strict.<br>4. The circulating ammonia water contains impurities and has high viscosity.<br>5. Blockage in the packing or trays. | 1. Reduce the amount of air blown into the tower.<br>2. Slightly reduce the circulation volume.<br>3. Control the liquid level inside the tower carefully.<br>4. Replace the liquid with fresh water.<br>5. Shut down the tower for maintenance. |

This process is actually quite simple; it is a typical distillation setup. In other words, steam is used to raise the temperature of the ammonia solution to its boiling point; a condenser is installed at the top of the tower to convert gaseous ammonia into liquid ammonia, and the purity of the gaseous ammonia exiting from the top of the tower is controlled by recycling a portion of the liquid ammonia. The equipment configuration is a typical distillation process: reboiler + distillation column + top condenser.

This post was last edited by Chemical Gas Purification on 2011-1-23 at 15:58. I agree with the view expressed in the third comment, but it should be noted that the operating pressure of the ammonia vaporization tower must be above 1.7 MPa; otherwise, liquid ammonia will not be able to condense. It must be ensured that ammonia remains in a liquid state at 40 degrees under this pressure, and not in a gaseous state.

If the final product in an actual factory is liquid ammonia, the aforementioned process equipment is essential. If the final product is urea, it is sufficient to send the ammonia solution from the net ammonia tower to a desorption-hydrolysis unit for recovery. There is also the option of producing carbon ammonia and sending it directly to the ammonia tank.

Isn’t it a bit of a waste of steam? I know that many factories prefer to secretly discharge the ammonia that they can’t sell into the sewers, rather than using ammonia vaporization equipment.

This post was last edited by Chemical Gas Purification on 2011-1-23 at 16:00. Approximately 200 KG of steam is required to evaporate one ton of dilute ammonia solution; first, the dilute ammonia solution is concentrated and then evaporated

This post was last edited by Chemical Gas Purification on 2011-1-23 at 16:01. Does what the original poster is referring to involve recovering ammonia from vent gases? It operates on the same principle as the vent gas system used in the ammonia synthesis process. Released gas (with a cooler at the bottom of the tower) – Absorption tower (absorption by desalinated water) – Heat exchanger (ammonia solution) – Distillation tower (reboiler) – Gaseous phase as pure ammonia – Water cooler – Liquid ammonia.

This post was last edited by Chemical Gas Purification on 2011-1-23 at 16:03. There is no waste; the water used, including steam, is recycled. Moreover, an appropriate amount of water also needs to be added during distillation. After heat exchange with steam, it enters the boiler feedwater system, and can be reused after chemical treatment. You don’t need to worry about waste, unless you are distilling ammonia alone.

How can one say that? It is essential to comply with environmental protection requirements in business operations; recovering ammonia can still generate considerable profits