Naphthenic Acid content in products

[Freddy MartinezParticipant]

Dear all,

literature is showing that Naphthenic Acid corrosion phenomena are mainly experienced in CDU and VDU units, especially for HGO, LVGO, HVGO and VR cuts.

I wasn’t able to find any article or literature of such type of corrosion in DCU’s.

What I understand is that this may be an issue (at high TAN values) for the DCU feed side up to the heater (i.e. for the VR side only).

Are Naphthenic Acids destroyed in the heater/coke drums? And if so, we should reasonably expect no issues for the fractionator side and the downstream products (Naphtha, LCGO, HCGO).

Can anyone be of help on this subject?

I am dealing with a VR with a TAN value of max 1.7 mg KOH/g.

Thank you,
Andrea

[hectorParticipant]

Naphthenic Acid
 
Susceptible materials: Carbon steels Environmental conditions: Temperatures in the range 220C to 400C Expected failure mode: Leakage  
Description
 
Naphthenic acid is the collective name for all organic acids present in crude oil. A large number of acids can exist spanning a wide boiling range. However, the majority is within the heavy gas oil range of boiling point (375 – 425C) and exist in sour rather than sweet crude oils. Naphthenic acid content is often defined by the amount of KOH required (in mg) to neutralise 1 gram of the acidic oil, otherwise known as the Total Acid Number (TAN) (refer to ASTM D664 or D974). For instance, corrosion is noted with crudes and sidecuts with TAN’s in the range 0.5 to 6 mg KOH/g.
 
Naphthenic acid corrosion occurs at high temperatures above 220C by the direct reaction of the acid with steel to produce iron naphthenates, which are oil soluble and non-protective, so that attack is made significantly worse under high liquid velocities and turbulence. Naphthenic acids are most corrosive around 280C, although the acids decompose above 400C, where corrosion becomes negligible. For a given temperature, corrosion rate is proportional to TAN, while, for a given TAN, corrosion rate triples for each 40C increase in temperature.
 
Carbon, low alloy, and stainless steels (with up to 12%Cr), all exhibit uniform attack (up to 20mm/yr) often in the form of sharp sided grooves/ripples. In contrast, austenitic stainless steels, e.g. 304, tend to pit in the presence of naphthenic acid. Molybdenum-additions (> 2.3%Mo) to stainless steels are recommended for corrosive naphthenic crudes. Material selection should consider attack by other mechanisms, e.g. hydrogen sulphide, and the products of cracked naphthenic acid, e.g. formic, acetic and carbonic acids. Susceptible process units and locations include:
 
Crude and vacuum distillation units;
Sometimes in thermal and catalytic cracking units;
In reformer units: top part of reactor and upstream of reactor, during hydrodesulphurization of gas oil and heavier feedstock;
Furnace coils, transfer lines, vacuum columns (overhead condensers), side stream coolers, pumps (especially where velocities exceed );
Under high velocity (> 30 m/s), turbulence, or impingement conditions;
Elbows, weld reinforcements, pump impellers, thermowells, steam injection nozzles; and
Towers where freshly formed acid runs down metal surfaces
 
Methods of avoidance
 
Blend oils to reduce TAN to <1.0;
Replace carbon steel with 5Cr 0.5Mo steel for temperatures above 230C;
Type 316/317 for most conditions (i.e. >2.3% Mo), ideally stabilised with Ti, e.g. 316Ti;
Weld with Type 309Mo, the molybdenum is required for corrosion protection
Aluminised steels, Alloys 600 and 800 are also resistant, but with limited experience
 
Further reading
 
Piehl RL: “Naphthenic acid corrosion in crude distillation units”, Materials Performance, January 1988, p37-43.
Slavcheva E, Shone B, Turnbull A: “Review of naphthenic acid corrosion in oil refining”, British Corrosion Journal, Vol.34, No.2, 1999, p125-131.

[Anonymous]

There is a company in Calgary, Alberta, Canada (Endurance Technologies Inc.) that alonizes (alluminizes) tubes and parts.

[Anonymous]

There is a company in Calgary, Alberta, Canada (Endurance Technologies Inc.) that alonizes (alluminizes) tubes and parts.

[Anonymous]

Andrea,
 
That is what is typically seen in the industry.  After the heater does not tend to be a problem with Nap acids.  This is also mentioned by other companies.  See the NACE Meeting minutes for other peoples comments in the Information exchange of the STG 34 group.  Hope that helps.

[Author]

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