Lecture 4 – Crude Oil Properties

Some of the most important physical properties of crude oil include:

 

oAPI

oAPI = (141.5/SG 15oF) – 131.5   The purpose of this equation was to extend the range of the specific gravity scale.  Crude oil SG changes, although small, may be important.

Crude Oils oAPI = 10 – 50

 

Higher oAPI, more paraffinic crude, higher yields of gasoline.

Lower oAPI, more aromatic crude, lower yields of gasoline.

 

Viscosity

Resistance to flow, usually measured @ 100oF in centistokes (kinimatic viscosity)

 

Pour Point

Measured by ASTM D-97 – temperature at which oil ceases to flow.  Diesel may contain waxes, smaller than candle wax, which could solidify in cold weather.

 

Flash Point

Temperature above which the oil will spontaneously combust.  Fractions in vacuum tower are the least combustible.  They are the heaviest.

 

Vapor Pressure

Measured by ASTM D-323.  Also know as Reid vapor pressure (RVP).  True vapor pressure is usually 5-9% > RVP

 

Carbon Residue

The solid residue (%wt) remaining after heating to coking temperatures (700-800oC)

ASTM D-524 Ramsbottom Carbon

ASTM D-189 Conradson Carbon

CCR incr. then Asphaltene incr.

 

Salt Content

Measured by ASTM – 3230 (lb NaCl/1000 bbl)

Desalting is necessary because NaCl content > 10 lbs/1000 bbl leads to corrosion

 

Metals

Measured by EPA Method 3040  These include Ni, V, Ag, Hg, Na, and Ca. Metals can cause catalyst deactivation and corrosion.

 

Sediment and Water

Measured by ASTM D – 96  These inorganic particles can lead to operational problems.

 

Acidity

            Measured by ASTM – 664

 

Sulfur

Measured by ASTM D – 129, 1552, 2622

Sour crudes > 0.5 wt% and sweet crudes < 0.5 wt%.  Today it is difficult to find crudes below 1% sulfur.

 

TBP Distillation Data

            Butanes and lighter                 55-175 oF

            Light Gasoline                         175-300 oF

            Light naphtha                          300-400 oF

            Heavy naptha                         400-500 oF

            Kerosene                                 500-650 oF

            Atmosphere Gas Oil               650-800 oF

            Light Vacuum Gas Oil            800-1000 oF

            Hvy. Vacuum Gas Oil            1000 oF

            Vacuum Residue                     > 1000 oF

 

Specifications and Environmental Regulations for Gasoline and Diesel

Engine knock is caused by the spontaneous ignition of fuel (higher octane gas reduces engine knock).  Gasoline engines use spark ignition.  Octane number is determined by (Research Octane + Motor Octane)/2.  Research octane is measured at low rpm, characteristic of city driving.  Motor octane is measured at high rpm, characteristic of highway driving.  The octane #s are reported by comparison of the gasoline’s performance to that of a binary mixture of:

            n-heptane (0)

            2,2,4 – trimethylpentane (100)

 

Octane Rating             Fuel Type       Compression Ratio

         87                             regular                   9

         89                             plus                                  

         93                             super                     12

 

Octane # of straight run crude oil is ~ 40

EPA regulations limiting benzene to 1%, aromatics to 10%, and sulfur to 30 ppm

 

Diesel fuel is rated by a cetane number.  The desirable range for the cetane number is between 40-50

 

EPA regulations limiting sulfur content to 50 ppm in diesel.