Physical Properties of Petroleum Products CHAPTER Physical Properties of Petroleum Products BY DR. GHULAM ABBAS

Physical Properties of Petroleum Products The physical properties are density, surface tension, vapor pressure, flash point, viscosity, pour point and heat of combustion, etc.. Why should we know these properties? An oil producer/refiner not only know the properties as a complete data but also must know the environmental effects/hazards and for health safety measures as well. For example, it should be known that how oil behaves in environment when spilled in the soil.

Physical Properties of Petroleum Products Oil producers and refiners typically do not know to what extent their oils will evaporate and how quickly; to what extent dispersibility will occur naturally, whether emulsions will form; if the oil is likely to sink or submerge; the viscosity of the oil, at ambient temperatures, as it evaporates; the health hazard to on-site personnel from volatile organic compounds; and the toxicity to marine or aquatic organisms.

Oil Density Density is defined as the mass per unit volume of a substance. It is most often reported for oils in units of g/mL or g/cm3, and less often in units of kg/m3. Density is temperature-dependent. Oil will float on water if the density of the oil is less than that of the water.  This will be true of all fresh crude oils, and most fuel oils, for both salt and fresh water. Bitumens and certain residual fuel oils may have densities greater than 1.0 g/mL and their buoyancy behaviour will vary depending on the salinity and temperature of the water. 4

Oil Density The density of spilled oil will also increase with time, as the more volatile (and less dense) components are lost. After considerable evaporation, the density of some crude oils may increase enough for the oils to sink below the water surface.  Two density-related properties of oils are often used: specific gravity and American Petroleum Institute (API) gravity. Specific gravity (or relative density) is the ratio, at a specified temperature, of the oil density to the density of pure water. API is the major United States trade association for the oil and natural gas industry and sets STANDARDS for petroleum products. One of the most important standard set by API is the measurement of petroleum density.

Specific gravity and API density Specific gravity is the ratio of the density of one substance to the density of a reference substance, usually water. API gravity is the standard specific gravity used by oil industry, which compares the density of oil to that of water through a formula: API gravity = (141.5/specific gravity) – 131.5 It has no units. It is inversely related to density and specific gravity, which means the denser an oil is, the lower its API gravity will be. API value of water is equal to 10, which means any oil with an API above 10 will float on water while any oil with an API below 10 will sink. 6

Specific gravity and API density API gravity is very important in industry, because its value is used to classify oils as light, medium, heavy and extra heavy. The API values for each weight of oil are as follows: light____ API > 31.1 Medium ____ API between 22.3 and 31.1 Heavy ___ API < 22.3 Extra heavy ___ API < 10.0 The price of a crude oil is usually based on its API gravity, with high gravity oils have much higher prices.

Reid Vapor Pressure (RVP) Vapor pressure is an important physical property of volatile liquids. It is the pressure that a vapor exerts on its surroundings. Its units are kilopascals. For volatile petroleum products, vapor pressure is used as an indirect measure of evaporation rate/volatility. Vapour pressure can be measured by a variety of methods including Reid, dynamic, static and vapor pressure balance. The standard test method for crude oils is the Reid vapor pressure. 8

Reid Vapor Pressure (RVP) To obtain the RVP of petroleum products and crude oils, a chilled sample is placed in a special vapor chamber that is heated to 37.8 oC. The reading then records the observed constant pressure in the chamber. The higher a fuel's RVP, the more quickly it evaporates. The more quickly a fuel evaporates, the more it contributes to the ozone layer destruction.

Evaporation Evaporation of spilled oil is a major process which contributes to the spoil the weather. Pure compounds evaporate at constant rates, while oils, which are composed of thousands of compounds, do not. Rapid initial loss of the more volatile fractions is followed by progressively slower loss of less volatile components. Using a simple pan evaporation technique, the rate of evaporation rate can be determined for various oils.   10

Viscosity and surface tension Viscosity is a measure of a fluid's resistance to flow; the lower the viscosity of a fluid, the more easily it flows. Like density, viscosity is affected by temperature. As temperature decreases, viscosity increases. The SI unit of dynamic viscosity is the millipascal-second (mPa.s) or centipoise (cP).  Viscosity is a very important property of oils because it affects the rate at which spilled oil will spread, the degree to which it will penetrate shoreline substrates. 11

Viscosity and surface tension Interfacial tension is the force of attraction between the molecules at the interface of two fluids. At the air/liquid interface, this force is often referred to as surface tension. The SI units for interfacial tension are millinewtons per metre (mN/m). The surface tension of an oil, together with its viscosity, affects the rate at which an oil spill spreads.

Spreading Coefficient = SWA - SOA – SWO Air/oil and oil/water interfacial tensions can be used to calculate a spreading coefficient which gives an indication of the tendency for the oil to spread. It is defined as:  Spreading Coefficient = SWA - SOA – SWO where SWA is water/air interfacial tension, SOA is oil/air interfacial tension, and SWO is water/oil interfacial tension.  Spreading to a thin slick is likely if the spreading coefficient of an oil is greater than zero, and the higher the spreading coefficient, the faster the spreading will occur .  13

The flash point of a fuel is the temperature to which the fuel must be heated to produce a vapour above the liquid fuel that is ignitable when exposed to an open flame under specified test conditions. In North America, flash point is used as an index of fire hazard. Flash point is an extremely important factor in relation to the safety of spill cleanup operations. Gasolines and other light fuels can be ignited under most ambient conditions and therefore pose a serious hazard when spilled. Many freshly spilled crude oils also have low flash points until the lighter components have evaporated or dispersed. 

Ignition temperature and POUR point Ignition temperature: sometimes called "autoignition temperature", this is the minimum temperature at which the material will ignite without a spark or flame being present. The pour point of an oil is the lowest temperature at which the oil will just flow, under standard test conditions. The pour point of the oils is therefore an indication, and not an exact measure, of the temperature at which flow ceases.  15

HEAT OF COMBUSION The heat of combustion (ΔHc0) is the energy released as heat when a substance undergoes complete combustion with oxygen. The chemical reaction for combustion is typically that of a hydrocarbon fuel reacting with oxygen derived from atmospheric air to form gaseous carbon dioxide, water vapor and heat. For example: methane + oxygen carbon dioxide + water vapor The heat of combustion may be quantified with these units: ●  energy per mole of fuel, such as kilojoule per mol (kJ/mol) or British thermal unit per pound-mol (Btu/lb-mol) ●  per mass of fuel, such as megajoule per kilogram (MJ/kg) or British thermal unit per pound (Btu/lb).

HEAT OF COMBUSION Expressions for the heat of combustion of fuels The heat of combustion of a fuel is commonly referred to as the heating value or the caloric value and briefly defined as the amount of heat released when a unit amount of the fuel is completely combusted. The heating value is a unique characteristic of each specific fuel.

HEAT OF COMBUSION

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