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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?

Dec-2023

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Yang Shi, Sinopec Shanghai Gaoqiao Petrochemical Company, 332181739@qq.com

I have found a possible solution to the problem, but I am not sure it‘s the right one.

We can find the saturated vapour pressure of water at the temperature of the hydroprocessing reactor effluent air cooler. We can also get the mole flow rate of the hydroprocessing reactor effluent air cooler stream by DCS data and laboratory analysis data. Then [(the mole flow rate of the injected wash water) / (the mole flow rate of the hydroprocessing reactor effluent air cooler stream)] × (the pressure of the hydroprocessing reactor effluent air cooler) = (the saturated vapour pressure of water at the temperature of the hydroprocessing reactor effluent air cooler), by solving this equation we can calculate the amount of water that could turn into water vapour at the hydroprocessing reactor effluent air cooler. I think if we injected more water than this amount, the additional water will keep in the liquid phase.

Jan-2024
Marcio Wagner da Silva, Petrobras, marciows@petrobras.com.br

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 an 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.

Dec-2023