Will salt form in the stabilizer downstream of the low-pressure fractionator in the hydrogenation unit?
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At present, after the newly built hydrogenation units began operation, changes in weather cause the temperature to drop to 36°C with air cooling and to 21°C with water cooling. This results in a large difference between the pressure at the top of the tower and the reflux pressure, necessitating a reduction in tower pressure for operation. As the temperature continues to drop, this pressure difference grows even larger, forcing the shutdown of feed supply and the use of steam to heat the air cooling and water cooling systems. What is the exact reason for this? Are there any similar situations?
Based on the description, it is indeed a typical ammonium salt crystallization issue. It is recommended to pay close attention to the following points: 1. The chlorine content in the raw materials should be recorded hourly; when it exceeds 3 ppm, the water injection volume needs to be increased in advance ； 2. It is recommended to equip air coolers with online flushing interfaces; after installing rotating nozzles on the surface of the finned tubes in our systems, the salt deposition problem was significantly reduced ； 3. When using steam purging for temporary treatment, care must be taken to control the temperature gradient, as too rapid an increase in temperature may cause the lining to peel off. Is the location of your water injection point before the heat exchanger or after the mixer? This has a significant impact on the anti-crystallization effect.
Based on the description, it is indeed a pressure difference issue caused by ammonium salt crystals; our facility encountered a similar situation last year as well. Here are two additional points of experience for reference: 1) It is recommended to check the quality of the water used for injection; we once experienced increased crystallization due to high levels of chloride ions in the injection water, and the situation improved significantly after switching to deionized water ； 2) For temporary handling, it is possible to try installing temporary heating along the pipeline at the top of the tower, which is safer and more controllable than using steam purging for the entire system. Is the pH value at your water injection point monitored? This parameter is also crucial for preventing crystallization. The specific procedures still need to be adjusted according to the actual conditions of your equipment.
It’s not the stabilizer tower where salt accumulation occurs; rather, it’s the air coolers. The salts carried into the stabilizer tower from lower-pressure areas are transported to the air coolers through evaporation. The low temperature in the air coolers is just right for salt to crystallize, and the salts then crystallize within the air cooler tubes. If maintenance is approaching, it is recommended to have a desalination water flushing line added through the design firm. It’s also necessary to check whether all the inlet and outlet valves of the air coolers are present; if this issue arises again in the future, it’s possible to reduce the processing volume and clean the air coolers in batches, which will yield better results. If there is still time before maintenance, rubber hoses can be used for flushing, with the water being sent back to the return tank for disposal. It’s best to avoid using steam directly to heat the air coolers and water coolers, as this can lead to safety problems.