Managing the Threat of Corrosion in Hydroprocessing Units - The Relevance of Adequate Design

Question 1:

Importance of maintaining specific range of temperature to Hot HP separator in hydrotreating units?

Relation of H2 gas and salts ie: Ammonium Bisulfide and Ammonium chloride  solubility with temperature of the gas/liquid mixture upstream of Hot HP separator?

My Response:

This is a key issue in the hydrotreating units, especially those units processing heavier feedstocks which tends to present higher contaminants content like sulphur and nitrogen that will produce the NH4HS (Ammonium Bisulphide) and NH4Cl (Ammonium Chloride) under specific conditions.

For hydrotreating units processing lighter feeds, like naphtha, the hydrotreated stream tends to present physical properties very different from the water and a single separation vessel can be applied. For heavier feeds like LCO (Light Cycle Oil), and Gasoils two separating vessels are applied once the hydrotreated feed present physical properties closer with the water leading to a harder separation process which demands higher interface area, in these cases are applied a High Pressure (HP) separation vessel (Usually a vertical vessel) where it's separated the recycle gas (H2, H2S, and NH3) from the liquid phase which concentrates water, hydrocarbons and dissolved H2S and NH3. The liquid phase is fed to a Low Pressure (LP) separation vessel (Usually a horizontal vessel), as aforementioned this is the common configuration for high severity hydrotreating units.

In both cases the range of operating temperature aims to avoid the deposition of ammonium salts which tends to produce precipitations and corrosion which can reduce the lifetime of the process unit and led to accidents, generally the salt precipitation and the corrosion process is controlled through water injection in the post reaction section under temperatures above the precipitation temperature of the salts. In the literature is available some precipitation charts for the Ammonium Bisulphide (NH4HS) and NH4Cl (Ammonium Chloride). The Ammonium Bisulphide can precipitate in temperatures close to 140 oC while the Ammonium Chloride precipitate close to 250 oC, this is especially concerning for processing units with high chloride concentration in the feedstocks once the water injection needs to be made at higher temperature leading the necessity of the use of noblest metallurgy in the processing unit aiming to minimize the corrosion process. It's fundamental a deep characterization of the feedstock of the hydrotreating unit in order to measure the expected concentration of Ammonium Bisulphide and Ammonium Chloride based on the concentration of Sulphur, Nitrogen and Chloride in the feed, with this information it's possible to design an adequate water injection system to prevent the salt deposition and corrosion process.

Question 2 :

According to the Inspection Guidelines for Corrosion Control in Hydroprocessing Reactor Effluent Air Cooler (REAC), we need to ensure that at least 25% of the wash water is liquid. My question is how do we calculate it practically?

My Response:

This a fundamental issue to ensure adequate management of hydroprocessing assets, according to the literature and the API RP 932-B between 20 to 25 % of the wash water injected to the process need to remain in the liquid phase to ensure a real capacity to remove the NH4HS (ammonium bisulfide) and NH4Cl (ammonium chloride).

The wash water flow rate is calculated based on the feed flow rate of the processing unit, the literature quotes a minimum flow rate of 5,0% of the feed stream, but this depends on the design of the hydroprocessing unit. Most severe hydrotreating units processing heavier and high contaminants content of sulfur, nitrogen, and chloride tend to demand higher flow rates of wash water. In this sense, hydroprocessing units processing cracked feeds and residue will demand more wash water.

It's important to consider that a good parameter to estimate if the wash water flow rate is adequate is the concentration of NH4HS in the sour water which can be measured in the separator vessel, again according to the specialized literature, the concentration should be around 6,0 to 8,0 % (maximum). Another way to verify the quantity of wash water injected is to measure the free water flow rate downstream of the injection point.

Further the discussed above it's important to consider a verification of another important topics related to the wash water injection system as described below:

- The presence of oxygen in the wash water can cause corrosion and the oxygen concentration in the wash water should be below than 50 ppbw;

- It's important to ensure symmetry in the piping arrangement of the air coolers in order to ensure adequate wash water distribution and non flowing sections which accelerate corrosion;

- The velocity in the tubes needs to controlled aiming to avoid the corrosion-erosion phenomena, according to the literature the velocity in tubes should be controlled in the range of 3,0 to 6,0 m/s;

- At last, taking into account the chloride concentration in the feed. Chloride can lead to corrosion due to HCl formation in aqueous phase and accelerate the NH4Cl corrosion and fouling.

Dr. Marcio Wagner da Silva is Process Engineer and Stockpiling Manager on Crude Oil Refining Industry based in São José dos Campos, Brazil. Bachelor’s in chemical engineering from University of Maringa (UEM), Brazil and PhD. in Chemical Engineering from University of Campinas (UNICAMP), Brazil. Has extensive experience in research, design and construction to oil and gas industry including developing and coordinating projects to operational improvements and debottlenecking to bottom barrel units, moreover Dr. Marcio Wagner have MBA in Project Management from Federal University of Rio de Janeiro (UFRJ), in Digital Transformation from PUC RS, and is certified in Business from Getulio Vargas Foundation (FGV).