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RE: Ammoniun Polysulfide Injection

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In some our refinery we get the same problem and we worked with the vendor of the polysulfide, and they gave to us some guideline, that we followed and the problem was eliminated.
There is some guideline for polysulfide use

For use of polysulfide, for either reinforcing the stability of the sulphide film, or reacting with cyanide, are discussed below.
Polysulfide is always added as an aqueous solution; therefore the water wash guideline apply for injection of polysulfide into a vapour stream. The most important principles are:
1. Maintenance of < 2%w ammonium hydrosulfide (NH4SH) in the aqueous phase in carbon steel equipment
2. Annular flow in heat exchanger tubes
3. > 50% of polysulfide aqueous solution remaining as liquid after equilibrium flash at injection point

In addition, there are criteria relating to temperature and pH

1. The temperature at the injection point should be a maximum of 230ºF after equilibrium
2. The pH of the polysulfide solution, after injection, should not be allowed to fall below about pH 8.0. This is a concern especially for injection into hot vapour stream, about 200ºF

Below about 38ºF, ammonium polysulfide, tends to salt out of the concentrated solution (storage tank)

The temperature at and downstream of the polysulfide injection point should be not exceed 230ºF. If the polysulfide solution is flashed above about 230ºF, polysulfide will decompose to H2S and elemental sulphur. For instance, when an ambient temperature polysulfide solution is suddenly heated to 230ºF, the pH will decrease about 1 pH unit(depend on receiving stream vapour composition, and the resulting pH may be low enough that elemental sulphur will drop out. Also, if a large amount of water is evaporated in the injection zone, it is very likely that the pH of the remaining aqueous solution will drop below the stable range for polysulfide and elemental sulphur will be formed

Below pH of about 8, polysulfide starts to decompose to form elemental sulphur and the corrosion protective sulphide film on carbon steel surface becomes unstable, also, at high pH in presence of carbon dioxide can be converted to carbonate

The minimum polysulfide dosage to achieve corrosion control is primarily temperature dependent: 12 ppm of active polysulfide sulphur at 150ºF versus 250 ppm at 230ºF aprox. The total active sulphur in the combined feed to any sour water should not be greater than 25 ppm. Decomposition of polysulfide in neutral pH sour water stripper bottoms can be plugging of reboiler tubes by elemental sulfur

Could describe to me as it is the system of injeccion of polisulfuro where they inject puro(tambor of fraccionadora head, interstage of the compressor, etc)

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