High TAN Crudes
High TAN Crudes
(OP)
As with most refiners today we are processing heavier crude slates, with much higher TAN. Our vacuum tower bottoms pumps have experienced significant corrosion - believed to be due to naphthenic acid attack. These pumps are to be replaced and it is requested to upgrade the metallurgy. The pumps are already constructed of 317L SS - which is generally considered to be resistant to nap acid. Our next option is to upgrade to Alloy 20. I was just wondering if any of you out there are experiencing similar issues, and what metallury are you using in this service. Thanks in advance.





RE: High TAN Crudes
Steve Jones
Materials & Corrosion Engineer
http://www.pdo.co.om/pdoweb/
RE: High TAN Crudes
RE: High TAN Crudes
1) Are you sure it is naphthenic acid corrosion (for the vac bottoms) ? I believe I saw some data that suggest the TAN for vac bot is lower than the gas oils, and the sulfur provide some protection.
2) Maybe it is temperature related. What is your vac bot temp ? If above 350 C, you should perhaps quench it.
RE: High TAN Crudes
http://antwrp.gsfc.nasa.gov/apod/astropix.html
RE: High TAN Crudes
RE: High TAN Crudes
Naphtenic acid is the generic name to define all organic acid contained in the Crude. The majority of these acid are known by the chemical formula R (CH2) nCOOH where R, is a ciclopentano ring and n, is typically bigger that 12. However, a multiplicity of other organic acid composites are also present in crude oil and until to date nor all had been analysed. Typical Naphtenic Crudes come from: California, Venezuela, North Sea, west Africa, India, China, Russia.
TEMPERATURE
Naphtenic corrosion occurs at temperatures between 200ºC and 400ºC.
CONDITIONS FOR the OCCURRENCE OF (NAC)
In general, the conditions that promote the occurrence of Naphtenic corrosion are: TAN, temperature, metallurgy of the equipment and velocity of the flow and presence of H2S.
TAN (Total Acid Number)
As general rule when the TAN of the feed is less than 0.5mg KOH/g Naphtenic corrosion will not be expected. If the rate of sulphur in crude is too much low or either less than 0,2% in weight, there is a strong probability of Naphtenic corrosion occurrence. In this last case, number TAN of crudes to process, between the feed and the cuts, will have to be placed between 0.3 and 1.0.
The methods of Mitigation of Naphtenic corrosion are; make blending of the crude mix in order to lower nº TAN of the feed, injection of phosphorous inhibitors and make "upgrade" of known resistant alloys “immune” to naphtenic corrosion such austenitic 317L.
Some are of the opinion that continuous operations Technologies of sulphur-based corrosion inhibitor, in monitoring naphthenic acid corrosion have the same value as phosphate ester programs and are not so prone for catalyst poisoning by iron compounds.
Luis Marques
RE: High TAN Crudes