The role of sodium polysulfide in the injection water for hydrocracking

To form a protective film to prevent corrosion caused by high-pressure air cooling

1. Form a protective film on the air-cooled tube bundle. 2. Convert corrosive CN ions into non-corrosive SCN ions

Form hydrocyanic acid on the air-cooled surface to prevent corrosion

Sodium polysulfide and ammonium polysulfide are injected into the reaction product stream upstream of the reaction product cooler to resist cyanide corrosion. The reaction of polysulfides with cyanides produces thiocyanates. Additionally, polysulfides can inhibit sulfur corrosion of steel through ammonium polysulfide, and this is achieved by altering the adhesion and composition of the corrosion products of iron sulfide. Therefore, the injection of polysulfides can reduce corrosion. Typically, polysulfides are introduced into the system by injecting them into the reaction water. Polysulfides cannot stop pitting inside blocked pipes, but they reduce scale accumulation; when the system remains clean, the injection of polysulfides can minimize blockages and corrosion. It should be noted that polysulfides must be added to the injection water, and the pH value of the water must be ≥7.5. A pH value below 7.5 causes polysulfides to decompose; oxygen also leads to the decomposition of polysulfides, resulting in other products of corrosion reactions such as elemental sulfur, and it can even cause pipe blockages. Therefore, it is necessary to strengthen the monitoring of the quality of water used in the reaction process. When polysulfides are used, since they interfere with the analysis of oxygen, the water sampling point is located upstream of the polysulfide injection point. The polysulfide solution should be kept away from air before injection; the polysulfide tank should be covered with an inert gas or a layer of oil on the surface of the polysulfide to prevent it from coming into contact with air.

The retardant forms a film on the inner surface of the tube bundle, serving a protective role.

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1. Form a protective film on the air-cooled tube bundle. 2. Convert corrosive CN ions into non-corrosive SCN ions

There are two functions of adding sodium polysulfide to high-pressure injection water: 1. To prevent corrosion caused by cyanides； 2. Sodium polysulfide forms a hard layer of iron sulfide on the inner surface of the air cooler tubes, which is sufficient to prevent further corrosion.