Back

Question

Submit an Answer

I wanted to know the possible problems and consequences of processing high acidity crude oil (HIGH TAN) and what is the solution? Isn't it better to process these types of crude oil in Delayed Coker or Visbreaker units?

Feb-2025

Answers

Abbas Ezzat, Pharos university, abbassezzat@yahoo.com

It is imperative that nanphthenic acid attack on crude oil distillation tower represents a serious problem in refinery operations with high acid value crudes. the following factors influence the severity of such attack on the tower components: 1. Feed specs of Crude oil and its TAN concentration, which should be below 0.5 KOH/ g 2. Temperature profile which enhances corrosion impacts on the tower components 3.Tower 0perating parameters , such as flowrates. pressure profile, water content, ...... 4. Proper selection of inhibitors 5. Material selection optimization.

Mar-2025
Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com

In simplistic terms naphtenic acid corrosion happens above 400F if the TAN is above a minimum threshold. Rule of thumb is for low S crude keep crude TAN <0.5 and for high S crude you can go up to 1 TAN. The reality is that the rules of thumb aren't very good. NAC is a function of TAN, S temperature, shear stress, and metallurgy. People generally don't run crude to a Coker or visbreaker for economic reasons. Even for Canadian bitumen only about half of it is vacuum residue, so running a high TAN crude to a Coker effectively reduces refinery heavy crude capacity. Most of the acids will be cracked in the Coker furnace, but often they are just cracked into lighter acids that are still corrosive. The front end of the Coker up into the furnace can still see NAC. Every thing said about Cokers also applies to visbreakers as well.

Mar-2025
Marcio Wagner da Silva, Petrobras, marciows@petrobras.com.br

The main concern to processing high acidity crude oil is related to the occurrence of naphthenic acid corrosion which represents a significant threat to process safety and asset management policies. Naphthenic corrosion occurs in processing units that process bottom barrel streams that tend to present high concentrations of naphthenic acids and operate under high temperatures. Due to these characteristics, the naphthenic corrosion phenomenon is observed in crude oil distillation units and residue upgrading units like delayed coking. A very relevant characteristic of oils for refining hardware is naphthenic acidity. Naphthenic acidity is determined based on the amount of KOH required to neutralize 1.0 gram of crude oil, normally a mixture of crude oils is sought in the refinery load so that it does not exceed 0.5 mg KOH/g, above this reference, the bottom sections of the distillation units can undergo a severe corrosive process, leading to shorter periods of operational campaign and higher operating costs in addition to problems associated with integrity and safety. Naphthenic acidity is directly linked to the concentration of oxygenated compounds in the crude oil that tend to be concentrated in the heavier fractions, giving instability and odor to the intermediate currents. The sulfur content in crude oil is another key factor in the naphthenic corrosion phenomena. In crude oils with a sulfur content higher than 2,0 %, a protective layer of iron sulfide (FeS) is formed in the metal surface that is insoluble, avoiding the attack by naphthenic acids. In this sense, refiners processing very low sulfur crude oils with high total acid number (TAN) can face severe issues with naphthenic corrosion in their refining hardware. In crude oil distillation units, the bottom section of the atmospheric column and the vacuum distillation column are the most common regions where naphthenic corrosion is observed while in delayed coking units the phenomenon is observed in the bottom section of the main fractionator’s column. The carbon steel, series 300 and 400 stainless steel, and nickel alloys tend to suffer naphthenic corrosion. Among the actions to control the naphthenic corrosion in the refining hardware is the blending of crude oils aiming to keep the TAN and sulfur content inside the adequate limits. Other alternatives are the injection of neutralizers or corrosion inhibitors in the processing streams and the selection of materials with higher resistance to naphthenic acid attack. Refiners processing crudes with high acidity and low sulfur normally apply chromium and molybdenum alloys in the bottom sections of crude oil distillation columns and transfer pipes as well as in residue upgrading units. In extreme conditions, it’s possible to consider applying stainless steel 317 L that presents high resistance to naphthenic corrosion, this decision needs to consider the higher capital investment due to the high cost of this material. As inspection and monitoring strategies, ultrasound and X-ray assay are normally applied to identify localized corrosion and thickness loss as well as sacrifice metals coupons to determine the corrosion rates. Naphthenic acid corrosion is not expected to be a serious issue for the stockpiling assets; however, the knowledge of this corrosion mechanism is important for addressing possible issues related to the pipelines dedicated to fed or withdrawing hydrocarbons for crude oil distillation and delayed coking units. Considering this information, the best way to deal with high acidity crudes is to blend the available crude oils to reach an acceptable TAN (lower than 0.5 mg KOH/g) or to adequate the metallurgy of the processing units aiming to raise the resistance to the naphthenic acid corrosion.

Feb-2025