How to prevent polysulfuric acid corrosion during the maintenance of hydrogenation units?

Methods to prevent corrosion cracking of austenitic stainless steels by polyoxysulfuric acid include: preventing the entry of oxygen and moisture during shutdown; using aluminum-infused steels as well as corrosion-resistant materials such as 347, 327, and B315; reducing residual stresses during welding; and carrying out alkali cleaning.

In hydrogenation units, hydrogen sulfide corrosion leads to the formation of ferrous sulfide (FeS). During unit maintenance (or shutdown), this ferrous sulfide reacts with moisture present in the water and oxygen in the air to produce sulfuric acid (H2SO4) and polythionic acids (H2SxO6, where x=3~6). These substances then react with ferrous sulfide to generate hydrogen sulfide again. That is: 3 FeS + 5O2 → Fe2O3·FeO + 3SO2; SO2 + H2O → H2SO3; H2SO3 + 1/2O2 → H2SO4; FeS + H2SO3 → mH2SxO6 + nFe2+; FeS + H2SO4 → FeSO4 + H2S; H2SxO6 + FeS → FeSxO6 + H2S. To prevent the formation of polythiosulfates: 1) Prevent air from entering the equipment; during shutdown for maintenance, some of the equipment can be filled with nitrogen, and blind flanges can be used for isolation. 2) It is advisable to consider using a passivator to comprehensively sulfide ferrous sulfide. The main hazard of polydithionic acid is to cause stress corrosion cracking in austenitic stainless steels, usually intergranular cracks.

I am the technical supervisor for hydrocracking units. Regarding corrosion cracking caused by polythionic acids, it mainly affects austenitic stainless steels. To prevent issues during startup and shutdown, the high-pressure reaction section should take the following measures: 1. Prevent air from entering the equipment within the reaction system; during shutdown for maintenance, the reaction system should be isolated under a slight positive nitrogen pressure. 2. The permanent light in the reaction heating furnace should remain on as much as possible to keep the temperature of the furnace tubes above 150 degrees. If the furnace is under maintenance, wash the walls of the sulfurization tube promptly with the prepared alkaline solution.

During operation of the hydrogenation unit, the high-temperature (H2 + H2S) medium reacts with steel to form FeS. When the equipment is shut down for maintenance and opened, the FeS on its inner surface comes into contact with air (O2) and water, resulting in the formation of polyoxysulfates. The reaction equations are as follows: 3 FeS + 5O2 → Fe2O3·FeO + 3SO2; SO2 + H2O → H2SO3; H2SO3 + 1/2O2 → H2SO4; FeS + H2SO3 → mH2SxO6 + nFe2+; FeS + H2SO4 → FeSO4 + H2S; H2SxO6 + FeS → FeSxO6 + H2S. Polyoxysulfates can cause stress corrosion cracking in austenitic stainless steel. Even at ambient temperature, cracking occurs very rapidly. Although stabilized austenitic stainless steels (TP321, TP347, etc.) have some resistance to polyoxosulfate stress corrosion, preventive measures are still necessary. The best approach is to keep the surface of austenitic steel (including weld overlays, cladding layers, linings, etc.) dry and free from contact with air; however, this is usually impossible. Therefore, the equipment should be cleaned with an alkaline solution before being turned on to neutralize acidic substances such as polyoxysulfuric acid. The key steps for stopping the alkaline cleaning process are as follows: ① When the system pressure is reduced, inert gas containing 5000 uL/L of ammonia should be used for purging before the temperature of the metal surface drops to the dew point. ②During shutdown periods, when the equipment is opened for cleaning and inspection, the stainless steel surface is rinsed with a solution of 1.5% to 2% sodium carbonate (Na2CO3) or sodium hydroxide (NaOH). ③In the areas cleaned with sodium hydroxide solution, it is essential to remove all traces of sodium hydroxide using softened water or condensed water. ④Adding 0.5% sodium nitrate to sodium carbonate or sodium hydroxide solutions can reduce the likelihood of chloride stress corrosion cracking in stainless steel; however, it is necessary to avoid adding an excessive amount of sodium nitrate, as this can pose a risk of stress corrosion cracking in carbon steel.

If the gas content in the pipeline network is high, the furnace tubes of the hydrogen heating furnace must also be alkali-washed! For high-pressure systems, alkali washing is sufficient; if no maintenance is required, nitrogen can be used to maintain pressure.

Excuse me, friend on the 4th floor: by \"keeping the heating furnace’s pilot light lit at all times\" do you mean to maintain the temperature of the furnace tubes above 150 degrees? If the furnace is under maintenance, wash the walls of the sulfurization tube promptly with the prepared alkaline solution. ”Why keep lighting the eternal lamp? What is the purpose of maintaining the furnace tube at 150 degrees? I’m not quite sure; please advise! Thank you

It is necessary to perform neutralizing cleaning on austenitic stainless steel that may be exposed to air

Neutralizing cleaning of high-pressure reaction circuits (other than the reactor) is now provided by specialized companies. However, the device should be designed to include the necessary flange connections left over from neutralization cleaning. The neutralized cleaning solution is circulated in the high-pressure loop of the equipment via temporary storage tanks and a temporary neutralization pump; this effectively prevents stress corrosion of austenitic stainless steel caused by polysulfuric acid

What’s said here is correct: http://bbs.hcbbs.com/viewthread.php?tid=70376&highlight=%C1%AC%B6%E0%C1%F2%CB%E1

For equipment operating in a high-temperature and high-pressure hydrogen environment, a certain amount of hydrogen is absorbed into the vessel walls during operation. During the shutdown process, if the cooling rate is too fast, the hydrogen that has been absorbed does not have enough time to diffuse out, resulting in supersaturated hydrogen remaining within the vessel walls. This can lead to subcritical crack propagation at temperatures below 150 degrees, posing a threat to the safe operation of the equipment.