Ammonium Polysulfide (APS)
Process gas from refinery coking and FCC plants often contain hydrogen cyanide. This acidic gas is absorbed in the amine and desorbed in the regenerator, where it causes corrosion in the overhead equipment. The ammonium polysulfide is added to convert the cyanide (CN-) into thiocyanate (SCN-). Unlike cyanide, thiocyanate is not volatile, non-corrosive and accumulates in the caustic amine as a component of the heat stable salts.
Among the main processes where HCN is found we have:
1. Crude oil processing: fluidized catalytic cracking (FCC), hydrocrackers, Flex coking, oil gasification, sludge incineration, gas plant effluents, sour water, sour gas, and sour gas removal systems.
2. Coke processing: Combustion (incomplete), delayed and fluidized coking units, gasification, gaseous effluents and water scrubbers.
3. Chemical production: Nitrile compounds, polymers, specialty chemicals, agricultural chemicals and liquid and gaseous effluents.
Cyanide Contamination Process.
To understand how Ammonium Polysulfide (APS) injection works in corrosion control, it is important to know the corrosive reaction mechanism that occurs due to the presence of cyanides.
HCN is a highly volatile acid. In the presence of ammonia and H2S at high pressure, HCN solubilizes in condensed water. In aqueous solution, the dissociation of this acid occurs at the pH of the solution (see Eq. 1) and is enhanced at higher amounts of ammonia present.
Free cyanide: HCN(gas) <---> HCN(ac) <---> CN- + H+ (Eq. 1)
In an amine plant, HCN is absorbed by alkanolamine solutions and released jointly with ammonia (NH3) absorbed in the regenerating tower, while in the top condenser of the tower, gaseous NH3 and HCN are reabsorbed in the reflux condensate water.
In this sense, the NH3 dissolved in the reflux water provides the necessary alkalinity to absorb and retain acid gases in solution, and in the absence of a water purge in the reflux condensate water these acids would be trapped in the top of the amine regeneration system, promoting corrosion and the formation of heat-stable salts.
It is necessary to know that in presence of the cyanide ion (CN-) the iron sulfide (FeS) passivation layer (FeS) is not formed, and even the layer that is already formed tends to be destroyed by it, so the carbon steel of remains susceptible to corrosion by atomic hydrogen, making the metal more brittle and susceptible to breakage.
Cyanide Corrosion Process Control in Amine Plants
To reduce corrosion in the top of the regeneration tower of amines plants, caused by both cyanides and ammonia, several methods are employed:
1. Purge of the reflux water to reduce the concentration of ammonium cyanide and bisulfide at the top of the regenerator.
2. Water flushing upstream of the amine treatment system to remove HCN and NH3 from the feed gas.
3. APS injection at the top of the regenerator system.
4. Stripping corrosive components from the regenerator reflux.
5. Use of corrosion resistant materials in the regenerator top system.
6. Combination of the above. Injection of APS in the wash water. The dosing of APS in the wash water, upstream of the absorber, reduces the corrosive potential of the inlet gas to the plant, avoiding not only corrosion in the plant, but also the formation of heat-stable salts, thus reducing the plant’s efficiency.
By washing, the NH3 present is dissolved, and the polysulfide reacts with the CN- to form thiocyanates (SCN-) which are soluble in water and non-corrosive. The chemical reaction representing the above is:
2HCN + (NH4)2Sx –> 2(NH4) SCN + HS- + H+ + Sx-3 x = 3, 4 or 5
The APS in the scrubbing system can reduce the CN- concentration to less than 10 ppmw, which is low enough to prevent sulfur stress cracking (SSC) and induced hydrogen cracking (HIC).
GP GasPack is the distributor of Tessenderlo Kerley, Inc. for its product Polysulfide of Ammonium (APS-Cyntrol® 2040).