Corrosion by Naphthenic Acids in oil and gas.
Corrosion Has Many Sources and Naphthenic acids are just one source of the corrosive properties of crude or crude fractions.
Other naturally found crude components that can contribute to corrosion in the refining process are mineral acids, phenols, hydrogen sulfide, mercaptans, and carbon dioxide.
So exactly what are Naphthenic Acids?
• General term for all organic acids found naturally in crude oils.
• General structure is believed to be
1. What Makes Them Special?
• Naphthenic acid corrosion is very serious as Iron Naphthenate is soluble in oil.
• These effects can be seen even in highly alloyed metallurgies which are normally resistant to corrosive attack from sulfidic species.
2. How is Naphthenic Acid Concentration related to TAN (Total Acid Number)?
• There is no direct correlation. TAN is a measurement of all acidic species present in the crude.
• Many types of Crude with high TAN numbers can have a low naphthenic acid content, and vice-versa.
3. How is Naphthenic Acid Concentration Related to Corrosive Potential
• The corrosive potential from naphthenic acids is a function not only of concentration, but also of temperature, flow regime, velocity and metallurgy.
• Boiling points of the naphthenic acids.
And this means…..?
• Naphthenic acid concentration may be a serious source of corrosion for one process and have relatively benign effects for another.
• The density and viscosity of the liquid and the vapor in the pipe, the degree of vaporization in the pipe, and the pipe diameter are all factors affecting the corrosive activity of naphthenic acids.
CDU Furnace Tubes, Transfer Lines and Side Stream Piping
• Higher temperature increases likelihood. (kerosine/diesel, long resid)
• Two phase flow favors organic corrosion.
• Turbulence increases potential for organic corrosion.
Vacuum Column System
• Nap. acids are vaporized and condensed. This increases the TAN of the condensed material.
• Relatively low velocity. This means that corrosion is almost all in the liquid phase at the point of condensation.
• LVGO and HVGO are boiling ranges where nap acids are more prevalent and active.
And this Means?
• There is not a one-size-fits-all rule of thumb for how naphthenic acids will affect a given metallurgy in a given process.
• Depending on process conditions and metallurgy, small amounts of naphthenic acid can accelerate corrosion in specific areas of the refinery.
• Naphthenic acids are responsible for a specific type of corrosion in specific areas of the refining process.
• Naphthenic acid concentration is not necessarily a function of TAN magnitude.
Naphthenic Acid Analysis
- TAN- ASTM D664. Correlations not that great.
- UOP 565 and 587. Sulfur compounds are removed prior to potentiometric or color-indicator titration. Not very sensitive, and is not very specific to naphthenic acids.
- Mobil Method- LC extraction and IR quantification
- Fast Atom Bombardment –Mass Spec: This is very specific to naphthenic acids, and provides a very good profile of the MW distribution of the species that are present. (Energy and Fuels, 1991, Vol 5, pp371-375) although it’s also not a very easy technique and the amount of instrumentation is significant.
- Negative-Ion Micro-electrospray High-Field Fourier Transform Ion Cyclotron Resonance Mass Spectroscopy: - Very solid technique, but not widely available (Energy and Fuels 2001, Vol. 15, pp.1505-1511).
- Electrospray Ionization High-Field Asymmetric Waveform Ion Mobility Spectrometry-Mass Spectrometry:- Very little sample prep/No loss of volatile components such as formic or acetic acid (http://www.osern.rr.ualberta.ca/Downloads/conradsymp03/fedorak.pdf).
- SPE cartridge Extraction/Esterification/Mass Spec: - Provides a very solid analysis of C10+ naphthenic acids. Lighter acids can be lost in the esterification step (Analytical Chem., 2001, Vol. 73, pp 703-707).
So we can keep in mind that :
TAN is not an indication of corrosion potential of naphthenic acids in any given crude.
While more High Acid Crudes are being seen in the market place, Testing of Naphthenic Acid content in high TAN crude is essential to understanding corrosion potential of that crude.
Reference: - HIGH TAN CRUDE CONFERENCE held in Singapore on May 10, 2005