Doctor test for Diesel fuel

Doctor test is a standard Test Method for Qualitative Analysis for Active Sulfur Species in Fuels and Solvents. this test method is intended primarily for the detection of Mercaptans in motor fuel, kerosene, and similar petroleum products. This method may also provide information on hydrogen sulfide and elemental sulfur that may be present in these sample types.

Doctor Test result

Doctor test indicates the presence of H2S and Mercaptans by showing a Negative (-ve) result if the contents of H2S and Mercaptans are negligible and don't cause harmful effect on equipment. If the result is positive (+ve), then the test shows that the contents of H2S and Mercaptans will cause damage to equipment.

Factors Affecting Test result

1.      Stripping Steam flow rate to the stripping tower

2.      Stripping tower pressure.

3.      Stripper bottom/top temperature.

The proper action to adjust the fuel to give "-ve" value is to increase the bottom temperature as much as possible in the stripping tower.

The most common standards methods for laboratory test are :

1- IP 30: Detection of mercaptans, hydrogen sulfide, elemental sulfur and peroxides - Doctor test method.

2- ASTM D4952 - Standard Test Method for Qualitative Analysis for Active Sulfur Species in Fuels and Solvents (Doctor Test)

3- ASTM D3227 - standard Test Method for (Thiol Mercaptan) Sulfur in Gasoline, Kerosene, Aviation Turbine, and Distillate Fuels (Potentiometric Method) - we use this method if the sample containing from 0.0003 % to 0.01 % by mass of mercaptan sulfur. Organic sulfur compounds such as sulfides, disulfides, and thiophene, do not interfere. Elemental sulfur in amounts less than 0.0005 % by mass does not interfere. Hydrogen sulfide will interfere if not removed.

4- UOP41 - 07 Doctor Test for Petroleum Distillates