A catalytic treating process carried out in the presence of hydrogen to improve the properties of low viscosity-index naphthenic and medium viscosity-index naphthenic oils. It is also applied to paraffin waxes and microcrystalline waxes for the removal of undesirable components. This process consumes hydrogen and is used in lieu of acid treating. 

Diesel index is a measure of ignition quality of fuel. Diesel engine works on the principle of compression ignition. During compression adiabatically the air temperature reaches around 600 c, when the fuel is finely atomised form is fed in, it instantaneously explodes. Self ignition temperature is low for paraffins while it is high for aromatics. Thus a fuel rich in aromatics burns later causing ignition delay and it gives rise to what is known as diesel knock. For this reason all diesel fuels are processed to have a diesel index in the range of 45 to 55.

Diesel Index = (Aniline Point in oF X oAPI)/ 100

Significance: High aniline point indicates that the fuel is highly paraffinic and hence a high Diesel Index and a very good ignition quality. In case of aromatics the aniline point is low and the ignition quality is poor. 

Aniline point is defined as the minimum temperature at which equal volumes of anhydrous aniline and oil mix together. Aniline being an aromatic compound freely mixes with aromatic so a low aniline point indicates low diesel index (because of high percentage of aromatics).

Significance: High aniline point indicates that the fuel is highly paraffinic and hence has a high Diesel index and very good ignition quality. In case of aromatics the aniline point is low and the ignition quality is poor.

This test is useful for calculating Diesel Index.

This test is prescribed by Central Excise for distinguishing kerosene from Diesels for the purpose of charging duties. The test is similar to that of Smoke Point except in preconditioning the sample and wick and also in the final reading. The flame height is read at the end of the 15th minute after the lamp is lighted.

Smoke is an indication of clean burning quality of kerosene. Illumination depends upon the flame dimension although it is not related to flame height. Many paraffins may be gifted with better flame height but illumination may be poor. Smoke point is defined as the maximum height of flame in millimeters at which the given oil will burn without giving smoke. Different flame heights are obtained due to the presence of different components such as paraffins, naphthenes and aromatics. Aromatic contributes smoke, hence removal of aromatics increases the smoke point. Naphthenes with side chain one inevitably retained to give good illumination. In India, marketable kerosene should possess a smoke point of 18mm.

Fire point is the lowest temperature, corrected to one atmosphere pressure (101.3 kPa), at which the application of a test flame to the oil sample surface causes the vapour of the oil to ignite and burn for at least five seconds. For ordinary commercial lubricating oils, the fire point usually runs about 30oC above the flash point .The test is carried out in open cup rather then in a close one. ASTM D 92 method offers the advantage of open flash point and fire point determination. Low fire point petroleum products are potential fire hazards.

The flash point of a fuel is the temperature to which the fuel must be heated to produce a vapour/air mixture above the liquid fuel that is ignitable when exposed to an open flame under specified test conditions. Flash point is important primarily from a fuel-handling standpoint. Too low a flash point will cause fuel to be a fire hazard, subject to flashing, and possible continued ignition and explosion. In addition, a low-flash point may indicate contamination by more volatile and explosive fuels, such as gasoline. A very important reason to maintain the flash point as high as possible is due to the electrostatic hazards in pumping distillate fuels.

Flash point (BIS) requirements for some petroleum products are:

S.K.O.- 35 C
U.L.S.H.S.D.- 35 C
H.S.D.- 35 C
F.O.- 66 C
A.T.F.- 39 C

This test serves as measure of corrosivity of fuel with copper, brass, or bronze parts of a fuel system.

The copper strip is polished smoothly and immersed in the sample and put into a bomb. This is placed into a water bath which is maintained at the specified temperature for the specified time. The strip is removed from the sample, washed with iso-octane or normal heptane, and examined for evidence of etching or discoloration. The colour of the strip is compared with ASTM copper strip standards colour code.

Significance: Copper corrosion limits provides assurance that difficulties will not be experienced in deterioration of the copper and copper alloy fittings and connections that are commonly used in many types of utilization, storage and transportation equipments.

This test is required mainly for LPG, Naphtha, SKO, HSD, ATF and MS.

The main constituents of petroleum can be grouped into four categories:

Paraffin: A series of saturated straight chain or branched hydrocarbons, the lowest members of which are methane, ethane and propane.

Olefins: Double-bonded hydrocarbons that are not normally present in crude oil but are formed during refinery processing and vehicle combustion of fuel. Olefins help improve the octane rating, but their use may lead to gum formation or deposits in engine intake systems.

Aromatic: Unsaturated cyclic hydrocarbons are known as aromatics. Aromatics occur naturally in crude oil and can also be produced in some refining processes. Aromatics common in petrol include benzene, toluene and xylene. Controlling the level of aromatics directly limits evaporative losses and exhaust emissions.

Naphthenes: Naphthenes are a class of compounds that are saturated hydrocarbons typified by cyclic hydrocarbon molecular structure. The general formula for cyclic hydrocarbons is CnH2n.

Crude oils are complex mixtures containing many different hydrocarbon compounds that vary in appearance and composition from one oil field to another. Crude oils range in consistency from water to tar-like solids, and in color from clear to black. An "average" crude oil contains about 84% carbon, 14% hydrogen, 1%-3% sulfur, and less than 1% each of nitrogen, oxygen, metals, and salts. Crude oils are generally classified as paraffinic, naphthenic, or aromatic, based on the predominant proportion of similar hydrocarbon molecules. Mixed-base crudes have varying amounts of each type of hydrocarbon. Refinery crude base stocks usually consist of mixtures of two or more different crude oils.

Relatively simple crude oil assays are used to classify crude oils as paraffinic(West Texas crude, ,Brent), naphthenic (Louisiana crude, Nigerian Light), aromatic (Maya Crude), or mixed(Assam crude). One assay method (United States Bureau of Mines) is based on distillation, and another method (UOP "K" factor) is based on gravity and boiling points. More comprehensive crude assays determine the value of the crude (i.e., its yield and quality of useful products) and processing parameters. Crude oils are usually grouped according to yield structure.

Crude oils are also defined in terms of API (American Petroleum Institute) gravity. The higher the API gravity, the lighter the crude. For example, light crude oils have high API gravities and low specific gravities. Crude oils with low carbon, high hydrogen, and high API gravity are usually rich in paraffin's and tend to yield greater proportions of gasoline and light petroleum products; those with high carbon, low hydrogen, and low API gravities are usually rich in aromatics. The price of a crude oil is usually based on its API gravity, with high gravity oils commanding higher prices.

Crude oils that contain appreciable quantities of hydrogen sulfide or other reactive sulfur compounds are called "sour." Those with less sulfur are called "sweet." Some exceptions to this rule are West Texas crudes, which are always considered "sour" regardless of their H2S content, and Arabian high-sulfur crudes, which are not considered "sour" because their sulfur compounds are not highly reactive.