Seeking advice on carbon ammonium crystal formation in acidic gas pipelines
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The acidic gas line at the top of the methanol regeneration tower in the low-temperature methanol washing process often gets clogged; once blocked, the sulfur recovery process is shut down and the line is cleared. It’s very troublesome. Performance and reasons: 1. The ammonia concentration at the inlet of the ammonia washing tower was 15 PPM, while it was 5 PPM at the outlet. 2. The ammonia level in the lean methanol stream kept rising, reaching up to 140 PPM; the ammonia content in the fourth reflux tank reached 1.6%. After crystallization caused blockages, the pressure in that tower increased, while the pressure of the acidic gas dropped to 0.15 MPa. 3. A carbon ammonium crystal pipeline was installed, but when this pipeline was activated, large amounts of methanol were introduced into the sulfur recovery unit, making it difficult to control the air supply there; moreover, this led to coking and blockages in the catalysts. Solution: Redirect the acidic gas to the flare, shut down the deep cooler, and raise the temperature to 65–70°C to eliminate crystallization. However, sulfur recovery is considered an environmental protection facility, and shutdowns or startups require approval and registration with the environmental protection authorities. Question: 1. Is the ammonia washing tower not meeting the requirements, resulting in high ammonia levels at the outlet? 2. Is there any way to reduce or prevent blockages in acidic pipelines? 3. How to determine the air supply amount and avoid coking when acidic gases are not sent to the flare but directly fed into the sulfur recovery unit? 4. What should be done in case of high levels of methanol and ammonia?
It seems that water enters the boiler at the inlet, and at the outlet there’s no other way except to release steam...
1. The ammonia content at the outlet should be 1 ppm; 5 ppm is a bit high. Add some wash water to see if there is a fault with the tower； 2. Methanol with high ammonia content can be discharged intermittently or continuously from the bottom of the thermal regeneration recycle tank and the bottom of the methanol separation tank, provided that it is sold rather than recovered. 3. Theoretically, the temperature in the combustion furnace needs to exceed 1500°C in order to completely decompose ammonia; I suspect your system cannot achieve this. It is important to ensure that the concentration of ammonia in the acidic gases does not exceed 1%, otherwise it will cause problems when sulfur is recovered and ammonium sulfate is formed ； 4. By addressing the first 3 issues, the level of poor-quality methanol and ammonia will naturally decrease.
Check the operation status of the ammonia washing tower in the previous process.
The ammonia content is high in the 4th reflux tank; as a result, the methanol produced there contains 0.7% H2S, making it unsellable due to the high sales risks! We have replaced 400 tons of methanol, but the effect lasted only one month. It doesn’t work well. Hot washing has a significant impact on sulfur recovery, and it can easily cause coking in the sulfur recovery process; this is exactly what gives us the most trouble at the moment. The ammonia washing tower was inspected just half a year ago; the tray plates are in good condition, and the water flow rate has been increased to 150% of the design value
Could you explain in more detail, the washing part, that area? Where is the best location to install heat tracing?
The ammonia washing tower is also under analysis; the highest level at the outlet is 6 PPM, which doesn’t seem normal
Let me describe our operating conditions: we have confirmed corrosion of the internals in the ammonia scrubber, and currently the ammonia levels at the inlet and outlet are around 70 ppm. When the pressure difference from the tops of the four towers to the sulfur recovery unit reaches 30 Kpa, warming begins, and the liquid level in the cryocooler is gradually reduced. Currently, we have an exhaust gas monitoring system installed on the WSA chimney; whenever the temperature rises, nitrogen oxide levels increase. If all of the cryocoolers are turned off, nitrogen oxide levels exceed the allowed limits, and such a situation lasting 30 minutes is considered an environmental accident by the operations department. So now, rewarming is always done gradually. During the rewarming process, the temperature in the WSA furnace rises; this allows for a reduction in the amount of natural gas used for heating, or an increase in the volume of hot air supplied, in order to maintain the desired furnace temperature. The situation described by the original poster does not occur. At present, we do not monitor the ammonia content in the methanol-rich mixture, and the laboratory is also unable to determine the ammonia content in liquids. It’s a bit troublesome, but the device is very effective; it only needs to be warmed up once or twice a month.