Organic Chlorides in Petroleum Products

Organic chlorides are those compounds where one or more chlorine atoms are directly connected with carbon atom.

Examples:
  • Methylene dichloride (CH2CL2)
  • Methyl chloride (CH3CL)
  • Chloroform, also called trichloromethane (CHCL3)
  • Carbon tetrachloride (CCL4)
  • 1, 1, 1 trichloroethane (CH3-CCL3) dry cleaning solvent
  • Ethylene dichloride (CH2CL-CH2CL)
  • Carbonyl chloride (COCL2)
Inorganic Chlorides:

Inorganic chlorides are those compounds where one or more chlorine atoms are directly connected with metals like sodium, calcium, aluminum, iron copper, magnesium, lithium and many more.

Examples:
  • Sodium chloride or common salt (NACL)
  • Lithium chloride (LICL)
  • Aluminum chloride (ALCL3)
  • Carbon tetrachloride (CCL4)
  • Calcium chloride (CACL2)

Total chlorides is the sum of inorganic and organic chlorides. (Total chlorides = inorganic chlorides + organic chlorides)

Significance of organic chlorides

Presence of organic chlorides in crude oils is extremely damaging during refining. The organic chlorides are not naturally occurring in crude oils but get introduced at producing sites, pipelines or tanks during cleaning process. During hydrotreating or reforming, organic chlorides are converted into hydrochloric acid and this acid accumulates in condensing regions of the refinery causing rapid corrosion. It is extremely important for the refineries to have a good knowledge of the organic chlorides in crude oils, particularly when transfer of custody is involved. This information is furnished by laboratories capable of testing organic chlorides. The results must be obtained by use of the proper method, right equipment and trained technician. Laboratory personnel involved with testing organic chlorides must have a thorough understanding of the test procedure they are asked to run organic chlorides. Organic chlorides in crude oil during distillation concentrate in heavy naphtha fractions and can cause extreme damage if not properly analyzed and reported. Correct reporting of organic chlorides before refining helps engineers to take proper corrective action.

As far as inorganic chlorides are concerned, a desalter unit in the refinery can easily remove salt and hence the inorganic chlorides.

In summary, organic chlorides even in a very small concentration (a few PPM) can cause considerable damage to the refinery equipment.

Lab testing of organic chlorides in crude

There are several approved ASTM & UOP methods for testing total, inorganic and organic chlorides. Selection of the best method for a given crude oil or its naphtha fraction depends on the mutual agreement between the lab and the customer requesting it.

Common industry test methods:
ASTM D 4929 - 4929A or D 4929B Test method of organic chloride in crude oil
ASTM D 4929A Potentiometric Titration
Naphtha fraction of crude oil washed repeatedly with caustic to remove H2S, is treated with sodium biphenyl which converts organic chlorides to inorganic chlorides. The excess sodium biphenyl is decomposed. The mixture acidified and aqueous phase separated and evaporated to a desired volume. The aqueous solution is then titrated potentiometrically.
ASTM D 4929B Combustion and Microcoulometric Titration

The washed naphtha fraction of crude oil is injected into a flowing stream of oxygen through a combustion tube maintained at 800°C. The chlorine is converted into chlorides and oxychlorides, which then flow into a titration cell where they react with silver ions. The silver ions thus consumed are coloumetrically replaced. The current required to replace the silver ions is a measure of the chlorine present in the sample. Refer to the method for calculations.

ASTM D 5808 Determining organic chloride in aromatic hydrocarbons and related chemicals by microcoulometry
ASTM 5808 Organic Chloride by Microcoulometry

This is used for cleaner products. The liquid sample is injected into a combustion tube maintained at 900°c having a flowing stream of 50% oxygen and 50% argon gas. Oxidative pyrolysis converts organic chlorides to hydrogen chloride that then flow into a titration cell where it reacts with silver ions. The silver ions thus consumed are coloumetrically replaced and the current required in this case is a measure of the organic chlorides. Refer to the method for calculations.

UOP 588 & UOP 779 (similar to D4929A & D4929B) Total, inorganic and organic chloride in hydrocarbons by potentiometric titration
ASTM D 7536

This is a nondestructive method based on x-ray fluorescence technique. This is essentially for total chlorine, requires minimum sample preparation and can be completed in 5-10 minutes for each analysis. The method can be extended to measure organic chlorides by careful washing of the naphtha fraction and separating the organic chlorides with similar techniques as discussed above and then running through the x-ray.