API 572: Crude Distillation Unit Chloride Attack a short, condensed passage

API 572 is the recommended Practices for Pressure Vessels Inspection. As for any API recommended practice, this document is written in a very detailed manner. Many valuable information exists throughout the document. A first diagonal reading with an experienced eye can easily reveal important insights. From API 510 examination point of view questions based on API 572 are the most difficult. This is basically because of the nature of this document and the large amount of information hidden between lines.

API 572 for example gives many recommendations on how to conduct inspection and where to focus efforts along with some hints on damage mechanisms' susceptible locations.

One effective way to study this recommended practice is to collect for example these hints and try to expand further into the matter using other available sources.

In this article I will illustrate an example of such approach. By the end and when you reread the same hint you will be surprised that in your head the related 5 lines pictures a whole big story.

Our journey starts with 3 sentences in paragraph 9.4.3 of API 572.

Always and as usual try to make figures and pictures to summarize concepts. These figures are essential for the revision purposes and assimilating information through visual memorization.

This passage refers to a specific damage mechanism “Chloride attack” and specifies two locations within “Crude Distillation Unit” where this damage is susceptible.

Now go to API 571 and look at the end of the book for the PFD of the Crude Distillation Unit”. Locate than, on the PFD, the locations stated in the passage. Try to see what damage mechanisms related to chloride attack are highlighted there.

Two damage mechanisms are listed in these locations: Ammonium Chloride Corrosion and HCl corrosion.

As usual, make a figure for revision purposes. You can easily say that this issue of chloride attack is a concern for distillation tower overhead system.

The term Chloride Attack is a generic term. Using the above PFD extract and a cross check of API 571 chapters we can deduce the following figure:

You can notice that there are many chemical entities playing essential roles in the chloride attack.

Basically, to better understand these kinds of chlorides attack we should understand first of all where the Chloride comes from and how Ammonium Chloride and Hydrogen chloride form.

The HCl is evolved from the hydrolysis of MgCl2 and CaCl2 to form HCl during heating in the crude oil preheat train and associated heaters. NaCl does not hydrolyze to a great extent under normal CDU furnace conditions. Therefore, NaCl generally is not considered a significant contributor to corrosion's issues of the crude distillation tower overhead.

We can deduce automatically that if we can remove these inorganic chlorides from the crude feed, the formation of HCl will be inhibited and thus the risk of Chloride Attack will be reduced. For that reason, there is an equipment installed upstream of the unit furnace ensuring such function which is the desalter.

Now we know how HCl is formed. But what are the differences between HCl corrosion and ammonium chloride corrosion. Basically, and with advanced search (for example based on guidance of Nace Paper 34109) we can deduce that HCl corrosion requires presence of water (at and below dew point) and ammonium chloride corrosion do not require the presence of aqueous phase.

HCl, formed by hydrolysis in the furnace, often combines with NH3 to form solid NH4Cl salt in CDU distillation tower overhead systems.

NH4Cl salts usually form above the aqueous dew point, and they remain dry and non-corrosive as long as the process temperature is sufficiently above the dew point. A typical suggestion for this temperature difference is minimum 20 to 25 Kelvin. The reason for this limit is that NH4Cl is very hygroscopic and can absorb moisture, even if water has still not condensed. If NH4Cl salt deposits become wet, they can be very corrosive to many kinds of materials provoking localized under-deposit corrosion.

The below figure summarizes this fact:

In the upcoming lines and figures I will try to study parts of API 571 dealing with chlorides Attack which are chapters 3.6 and 3.37. We will see only parts dealing with crude distillation units.

Corrosion by Ammonium Chlorides depends essentially on the presence of HCl and NH3, so one effective way to decrease the probability of such damage to occur is by well controlling the concentration of these species.

That is why a well-designed and performing desalter is the first barrier to combat Ammonium Chlorides corrosion by eliminating inorganic chlorides salts from the crude feed.

Another solution is to inject NaOH to neutralize the hydrochloric acid (HCl) that could had been formed in the heater by hydrolyses of inorganic chlorides salts.

Neutralizing amines can also react with hydrogen chloride to form amine hydrochlorides that can act in a similar fashion as ammonium chloride. The right selection and dosing of such neutralizer is primordial for the integrity of the CDU overhead system.

If ammonium chloride pre-exists and salts deposit above the water dew point then a water wash injection may be required to dissolve them. This is because ammonium chloride is very soluble in water.

Another additional corrosion mitigation solution for overhead system is the use of filmers. The filmer causes a thin layer of process hydrocarbon to adhere to the metal surfaces that it contacts. It is the layer of process hydrocarbon that is actually the protective film, not the filmer itself; the filmer is there to ‘stick’ the hydrocarbon to the metal.

By preventing any corrosive species contacting the metal surfaces, the corrosion cell is disrupted, and the corrosion rate will decrease.?

After reviewing chapter 3.6 of API 571 dealing with Ammonium Chloride and Amine Hydrochloride Corrosion we should move to chapter 3.37 which deals with Hydrochloric Acid Corrosion.

I will try to summarize the chapter into some illustrations as follow:

At the end I should say that this article was the result of days of research and hours of readings. I was trying to demonstrate that what is stated in API 572 is a very condensed information. Understanding what is meant by a simple sentence such the one stated in this article requires a deep searching and dedication. Conducting such exercise will surely enhance your technical background and extent your standards’ library. Many of these information will be not a concern during API examination. Nevertheless, and from practical point of view, it is very useful whenever a real case is faced.

This article was based on illustration rother then a writing. Illustrations are based on what is written in API 571 and some other references and open-source documents.

After this exercise we had highlighted the following points:

• Types of Chloride Attacks in Crude Oil Units
• Locations of Chloride Attack in CDU Units
• Inorganic Chlorides Salts
• Hydrolysis of Inorganic Chlorides Salts in the CDU furnace
• The important role of a desalter in CDU Units
• Difference between Hydrochloric acid corrosion and Ammonium Chloride corrosion
• Corrosion control and mitigation measures for each type of chloride attack
• Inspection techniques
• Caustic Injection design considerations
• Nickel based alloy.
• Effect of oxidizing agent on Nickel based alloy corrosion resistance.
• The concept of ISO Corrosion Curves
• Injection Points distribution patterns
• Corrosion under deposits
• Fouling problems
• Corrosion Inhibitors Filmers principle...

Each point can be detailed in a separate article. I encourage any interested people to dig further into the matter. Finally, I can ensure you that in my mind the link between Chloride attack and Overhead systems in Crude Distillation unit is well formed and will never be forgotten.