7 replies to this topic

[Arsal]

Is there any range defined for TAN of crude oil.at what TAN we said Crude is acidic and need to be special treatment?

IS Any range of Heavy Naphtha TAN at which we said naphtha is acidic and create corrosion problem at unit?

I have checked Heavy naphtha TAN i.e 0.16 mg KOH/g of oil. and we are facing severe corrosion problem at hydrotreator?but i did not know the specified range?

[P.K.Rao]

There is no specific range to call a crude acidic or non acidic. They are only low acidic or high acidic or non acidic (which contains no acidity at all).

The acidity of the crude oil can be due to inorganic acidity and organic acidity. Inorganic acidity is caused by inorganic acids like hydrochloric acid which is not naturally found in crude oils. It is formed during processing due to the hydrolysis of magnesium chloride which can be present as  a salt. Acidity of naphtha is caused by this. This creates corrosion in the column top and condenser. To prevent this, corrosion inhibitor is dosed  at the top. Some light organic acids  like formic acid and acetic acid may also cause this corrosion.

The acidity in heavy fractions of kerosene range and above is due to naphthenic acids which are corrosive at  high temperatures, typically at flash zones.

Hydrotreatment removes organic acids also.  So the corrosion you are experiencing may not be due to organic acids.  Check the naphtha TAN after washing with plain distilled water. If the acidity  persists, it could be organic acidity otherwise it is due to inorganic acidity.

You can also check various anions present in naphtha/overhead condensate by Ion Chromatography. You may be having one in your lab.

Hope this will help you. 

[Arsal]

Pk rao thank you for such useful info.
We are actually facing severe corrosion problems including several time air cooler leakages stripper reflux drum low thickness and overhead stripper piping as well at below 250 f temp.I am talking about the heavy naphta hydrotreater unit licences by UOP.tail water of product separator and stripper reflux drum having low PH some time below 3and normally 3.while total chloride are always above 1000 ppm and total iron are also above 1000 ppm.we increases wash water rate from 3 vol% to 5 vol% but no change.organic chloride in the feed is less than 1 ppm and we also minimize PERC injection at platformer .now we r totally confused what's going there?amine is not preferred to neutralize because of nitrogen is poison to platformer catalyst.

Edited by rao arsalan, 03 June 2015 - 12:19 PM.

[P.K.Rao]

Chloride is high. A 1000 ppm chloride (if present as HCl) does give a pH of 3 or less.

Since pH is logarithmic, dilution (3 % to 5% wash water) will alter the pH by only around 0.2. If you desire to increase the pH by 1, the wash water requirement would be 10 times. To increase the pH by 2, the wash water requirement would be 100 times. The wash water should have a pH of 6.8 to 7.2 and no chloride. Please check the wash water also for chloride and pH.

You have to find the source for chloride (which is usually present as HCl). May be coming  from the gas by leaching. Check the gas entering into the system for chloride.

[Arsal]

Recycle gas has a chloride in the range if 0.3-1 mole ppm daily checked by drager tube.as catalyst vendor suggest to inject ammonia to neutralize acid.but NH3 will form NH4Cl because we have high chlorides system.actually we Inject percholoroethylene at plat former which is obviously leach out in the recycle gas because system has 40 mol ppm moisture as well.we already decrease chloride injection from 2ppm to .3 ppm.kindly suggest if any solution or step to minimize chloride and iron ??wash water PH is: 6.5 and chloride is 4 ppm(less than 10)

Edited by rao arsalan, 04 June 2015 - 09:57 AM.

[P.K.Rao]

I suggest, please contact your process licensor who knows every millimeter of your plant.because it needs a further close study for the ingress of chloride and low pH.

[Arsal]

Thanks;

[gegio1960]

After sharing p.k.rao considerations I'd like to add my 2 cents of (design) experience (I met high TAN crudes from North Sea and West Africa about 20 years ago):

- corrosion by high TAN is very different from corrosion by inorganic acids: the first is very quick and higly destructive;

- corrosion by high TAN is generally localized in Hevy Atmospheric Gasoil (HGO), Atmospheric Residue (AR), Vacuum Gasoils (VGOs) zones;

- an acidic crude has TAN > 1;

- special construction materials and/or special chemicals are needed to avoid serious and immediate problems in case of acidic crude processing.

good luck!