How to clean fouling in a stripping tower

It is best to carry out a chemical cleaning, using professional cleaning agents for soaking and cleaning. Mechanical cleaning can affect the distributor seal.

The scaling is mainly caused by oil contaminants and methylammonium salts in liquid ammonia; using acid or organic acids for cleaning should resolve the issue. Additionally, if a major overhaul is not necessary, online flushing with CO2 gas can also be effective, but the liquid level control valve of the stripping tower must be able to close completely; otherwise, this method cannot be used.

The last edit to this post was made by lxq700918 on 2009-7-23 at 10:25. The main component responsible for scaling is likely oily substances, which form carbon deposits on the surface of the stripping tower. Both chemical and mechanical cleaning methods can meet the production requirements; however, it is important to carry out pre-coating passivation after cleaning to prevent equipment corrosion

1. It’s best to ignore it if it doesn’t affect operation. 2. Chemical cleaning is possible, but it is necessary to assess the impact of the chemical cleaning agents on the material of the equipment. After cleaning, check its sealing performance carefully to avoid bigger problems as a result of minor issues.

This post was last edited by long198600 on 2009-7-23 at 12:11, in response to the moderator’s request to provide a detailed explanation of \"the reasons for the decrease in the stripping efficiency of the CO2 stripper and the methods to address it.\" After more than a year of operation of the first urea production unit, it was found that the stripping efficiency of the stripper had declined from over 78% at the start of operation to below 75% by August 2006; the ammonia content in the stripped liquid exceeded 11%, and the low-pressure level rose to 0.32 MPa. As a result, the production volume had to be reduced to 500 t/day. To this end, through careful analysis and bold experimentation, we have developed methods to address the decline in the stripping efficiency of stripping towers under conditions of long pauses, short pauses, and no pauses, achieving good results. 1 Process of the high-pressure system in a CO2 stripping urea plant: Liquid ammonia is pressurized by a high-pressure ammonia pump and then sent to a high-pressure ammonia heater, from where it passes through a high-pressure ejector to the high-pressure ammonium methane condenser ； CO2 gas mixed with inert air is compressed by a CO2 compressor, desulfurized, and dehydrogenated before entering the lower part of the stripping tower. It comes into countercurrent contact with the synthetic liquid flowing down from the top liquid distributor, which leads to a decrease in the ammonia partial pressure and thus causes the decomposition of methylammonium. The heat required for this decomposition is provided by medium-pressure steam at 2.45 MPa outside the tubes. The stripping liquid is sent to the low-pressure circuit via a level control valve. The stripping gas, along with the liquid ammonia and methylammonium coming from the high-pressure ejector, enters the top of the high-pressure methylammonium condenser where it mixes together; from there, it is distributed into the condensation tubes, allowing ammonia and CO2 to react to form methylammonium. The heat released during this reaction generates low-pressure steam at 0.4 MPa outside the tubes. The amount of methylammonium that condenses is regulated by controlling the pressure of this low-pressure steam. The mixture of methylammonium liquid, ammonia, and carbon dioxide exiting from the bottom of the high-pressure methylammonium condenser enters the bottom of the synthesis tower, where methylammonium is dehydrated to form urea. The heat required for this process is provided by the heat of condensation of ammonia and carbon dioxide, thereby maintaining thermal balance in the synthesis tower. The synthetic liquid enters the stripping tower through a downcomer, while the inert gas along with unreacted ammonia and carbon dioxide go into the high-pressure scrubber. The inert gas, which contains a small amount of ammonia and carbon dioxide, exits the high-pressure scrubber and is then sent to a low-pressure absorption tower at a pressure of 0.4 MPa for further absorption before being released. 2 Analysis of the reasons for the decline in the stripping efficiency of the CO2 stripping tower. We found that as the operating time of the stripping tower increased, its stripping efficiency gradually declined. By April 2006, this efficiency was around 75%; the low-pressure level was significantly too high, forcing the production volume to be limited to around 550 t/d. In particular, after a long shutdown in August 2006 and subsequent restart, the stripping efficiency dropped below 74%, the ammonia content in the stripping liquid reached 12%, the temperature of the liquid exiting the stripping tower was 175°C, and the nickel content in the final product also increased, to the point where production could no longer be sustained. After ruling out other factors, we believe that there are mainly two reasons for the decline in stripping efficiency. 2.1 The small holes in the φ2.3mm distributor of some stripping tubes become clogged due to fine particles resulting from the wear of the packing in the ammonia pump and ammonium methanate pump. These particles move back and forth with the plunger of the reciprocating pump, passing through the guide sleeve of the plunger and the gap between the plunger and the cylinder, eventually reaching the liquid ammonia and ammonium methanate solutions. There they get trapped in the small holes of the stripper distributor, thereby blocking those holes and causing uneven distribution of urea and ammonium methanate solutions through the stripper distributor; as a result, the flow rate of liquid in each stripping tube varies. When the pores of the distributor are blocked, the liquid flow rate in the stripping column is low; as a result, no liquid film may form or only a thin liquid film exists in the upper longitudinal sections of the stripping column. This results in little resistance to the gas, allowing a larger amount of gas to pass through. If the liquid outlet temperature of the stripping tower is high while the steam pressure in the heating chamber of the stripping tower remains unchanged, it can also cause corrosion of the stripping tubes. In a stripping column with a high liquid flow rate, the liquid film thickens, the gas channels decrease, resistance increases, and as a result the amount of gas that can pass through is reduced. The gas-liquid ratio decreases, the decomposition rate of methylammonium falls, the temperature of the liquid exiting the stripping column rises, and the stripping efficiency declines. Therefore, the gas flow and liquid flow within the stripping tube influence each other; for a single stripping tube, there is a mutual influence between the liquid flow and gas flow inside it. 2.2 Contamination of the stripper tower’s distributor or stripping tubes by oil: When the plant operates for extended periods, sometimes for months without shutdown, oil, which has a lower density than urea or ammonium solutions, accumulates in the upper layers of the urea tower. The longer the operation period, the thicker the oil layer becomes. One scenario is that during operation, if the liquid level in the synthesis tower is lowered too much, oil contaminants can enter the stripping tower, blocking the small holes in the distributor or entering the stripping tubes and reducing their heat transfer efficiency, which in turn leads to a decrease in the stripping efficiency. Another scenario occurs when, prior to long-term shutdown discharge, the oil layer at the top of the synthesis tower accumulates in the downcomer as the liquid level in LRA-201 drops. During the discharge process, if the pressure in the stripping tower decreases or there is a slight leak in HV-201, oil can enter the stripping tower, thereby contaminating its distributor or stripping tubes and reducing the efficiency of stripping. As a result, the stripping efficiency is poor every time the plant is restarted after a long shutdown. This situation does not occur during short stops, as steam at a certain pressure is supplied to the sides of the stripping tower and the high-pressure methane cooler, preventing ammonia and CO2 from condensing in these units. As a result, the urine present in the synthesis tower and the downcomer does not flow into the stripping tower. Alternatively, during short stops, high-pressure flushing water must be added to the downcomer of the synthesis tower at regular intervals, to force the urea, ammonium methoxide solution, and oil contaminants back into the synthesis tower. 3 Treatment Methods 3.1 Treatment Method during Prolonged Downtime Before increasing the CO2 pressure after the temperature-raising passivation is completed, high-pressure flushing water is used to add water to the stripping tower so that the LRC-203 reaches its full liquid level. Start the CO2 compressor to maintain the pressure in PRC-204 at 3.0 MPa; simultaneously, continuously supply flushing water into the stripping tower through the CO2 pipeline. Quickly open the main valve connecting the CO2 pipeline to the stripping tower, so that large amounts of CO2 gas carrying flushing water can enter the stripping tower rapidly, thereby backwashing the small holes in the tower’s distributor. Repeat this process 3–4 times for significant results. 3.1.1 Precautions for long-term shutdown backwashing: First, ensure that the pressure of PRC-204 is not too high, to prevent the lifting plate of the vapor distributor in the stripping tower from being blown over. Secondly, the temperature of CO2 entering the stripping tower is kept at around 120°C, so as to prevent a drop in the temperature of the various equipment in the high-pressure system. Third, open fully the 3″ manual vent valve and the HV-201 valve on the gas-phase pipeline of the synthesis tower, so that CO2 gas can be quickly discharged from the system through the gas-phase vent valve of the synthesis tower, thereby preventing overheating at the top of the synthesis tower. 3.2 Handling methods during short pauses: According to the operating procedures, when starting up after a short pause, CO2 gas is first introduced; at this point, the HV-201 valve has not yet been opened, and there is no liquid level in the distributor. Some of the CO2 gas passes directly through the distribution holes in the stripper, and then exits the stripper through the equilibrium holes on the fixed plate of the distributor’s vent pipe. This situation is essentially equivalent to backwashing. Therefore, the stripping efficiency is very good every time the vehicle is started after a short stop. However, since only 70% of the capacity is utilized during driving for feeding, and the majority of the CO2 gas passes through the flow-limiting orifice plates in the distributor’s lift pipes, with only a small portion passing through the small holes in the distributor, the backwashing effect is poor. Therefore, it is necessary to increase the amount of CO2 gas; by raising the gas flow rate to 150% of the load, a large volume of CO2 gas can pass through the small holes in the distributor, repeated 2–3 times. 3.2.1 Precautions for short-term shutdown backflushing: First, keep the pressure of PRC-204 above the system pressure by 2.0 MPa or more, and add high-pressure flushing water to the CO2 pipeline leading to the stripping tower; this will yield better results. Secondly, the temperature of the CO2 gas must be raised above 100°C, as the temperature of the high-pressure flushing water at this point is not high enough; this prevents cold shock from occurring and causing damage to the equipment. 3.3 Handling methods when not stopping operation: As the urea production cycle lengthens, the stripping efficiency gradually decreases, leading to unstable production and increased consumption. If operations are stopped specifically to perform back-flushing or to remove and clean the stripper distributor due to the reduced stripping efficiency, certain economic losses will occur. Based on our successful experience with backflushing during long and short stops, we have proposed a method for backflushing the stripper distributor without shutting down the plant. First, start the backup CO2 unit; both units are then brought online simultaneously, and the load is increased to around 180%. After that, the HV-201 valve is quickly closed, allowing a large amount of CO2 gas to pass through the small holes in the stripper distributor for 1–2 minutes. Finally, the backup unit is shut down, and the gas flow is maintained at the original load level. Comparison table of the stripping efficiency and liquid outlet temperature of the non-stop back-purge stripper before and after back-purging. Before back-purging / After back-purging: Item, Date – August 20, August 21, August 22, August 24, August 25, August 26. Stripping efficiency: 73.5%, 74.2%, 74.7%, 78.6%, 78.2%, 77.9%. Liquid outlet temperature: 175.4°C, 174.7°C, 175.1°C, 168.2°C, 167.5°C, 168.7°C. 3.3.1 Precautions for non-stop back-purging: It is necessary to maintain a certain liquid level in the stripper to prevent, after HV-201 is closed, the stripper from having no liquid level, which could allow high-pressure CO2 gas to enter the low-pressure system and cause overpressure in that system. Secondly, due to the closure of valve HV-201, no synthesis fluid reaches the stripping tower; therefore, it is necessary to open the vent valve HV-901 to keep the steam pressure on the shell side of the stripping tower at PIC-904 below 1.2 MPa, thereby preventing the stripping tubes from overheating. Thirdly, during the backwashing process, the pressure in the high-pressure system increases only slightly; therefore, it is sufficient to increase the opening degree of HV-202 by just a bit compared to its original setting. After backwashing, the pressure actually decreases. 4 Summary We performed backblowing on the small holes in the stripper distributor under conditions of long pauses, short pauses, and no pause, thereby improving the stripping efficiency. We used each method several times, and the results were all significant. In particular, by performing back-purging of the stripping tower without shutting it down, the stripping efficiency increased from 74% to 78%, while the temperature of the liquid exiting the stripping tower dropped from 175°C to 168°C. This achieved unexpected results: it enabled the production facility to operate stably without shutdowns, and it also helped to reduce equipment corrosion, lower consumption, and avoid unnecessary downtime losses.

Can physical cleaning be done using a high-pressure water gun? Are there experienced cleaning companies for chemical cleaning? Please, fellow sailors, help out as much as you can

Physical cleaning methods can be used, but water quality and pressure must be strictly controlled; it is best to hire an experienced construction team to minimize damage to the equipment；

The consequences of not cleaning are that energy consumption increases, which is not worth it；

This post was last edited by lxq700918 on 2009-7-24 20:48. The gray scale is a chelate of metals such as titanium, nickel, and chromium; it is relatively hard and brittle. The chemical cleaning is not effective ；

It is likely carbon deposition and scaling caused by the high temperatures in the stripping tower; a chemical cleaning method would be appropriate.