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Physical Properties of Hydrocarbon (Hydrocarbon Exploration & Development Notes #3)

Hydrocarbon Exploration & Development Notes #2

Whenever a problem occurs in any part of the process, it most likely to be related to the physical laws of fluid. Therefore, let’s examine the physical properties of hydrocarbons:

  • density and liquid relative density
  • API gravity
  • gas relative density
  • miscibility
  • solubility
  • viscosity
  • vapor pressure
  • boiling point
  • reid vapor pressure (TVP and RVP)
  • heat and temperature
  • freeze point
  • flash point
  • combustion and flammability

1. Density and liquid relative density (RD)

  • density = mass / volume
  • relative density = specific gravity,  is a relative measure of density.
  • pressure has little effect upon the density of liquids
  • temperature is the only condition that needs to be considered when measuring relative density or specific gravity.
  • the relative density of a liquid is the ratio of its density at 60 DegF or 15 DegC to that of water at the same temperature.
  • Unit less
  • relative density of water is 1.0
  • RD = Density of liquid / density of water (measured at 60 DegF or 15 DegC)
  • petroleum products are generally lighter than water and therefore, RD<1.

2. API Gravity

  • unique oilfield used for crude oil and liquid product that are stored in atmospheric tanks.
  • not used for measuring product that are stored in pressure
  • API gravity is also known as API

API Gravity = ( 141.5 / RD ) – 131.5

RD = 141.5 / (API +131.5)

Mass = Volume X Density of water X RD of Liquid

3 things to remember about petroleum relative density are:

  1. gravity  readings must be corrected to 15 DegC (60 DegF)
  2. petroleum liquid get less dense at higher temperature
  3. a heavy liquid such as crude oil has high relative density and a low API gravity

3. Gas Relative Density

  • Gas RD is measured relative to air, which is arbitrarily given a relative density (gas) of 1.00
  • the density of a gas is directly proportional to its absolute pressure.
  • in a closed vessel, the pressure of the gas in it can be controlled. hence, gas RD may be measured using this fact
  • sample apparatus used = gas gravity balance

Absolute Pressure for air / Absolute Pressure for gas

  • RD = Mol weight of gas / Mol weight of air

4. Solubility

  • concentration = (weight of solute / weight of solvent) X 100
  • concentration = (weight of solute / volume of solvent) X 100
  • main factor affecting solubility is temperature
  • the solubility of some solids increase as temperature increases, while the solubility of others will decrease with an increase in temperature
  • the solubility of gases increases as temperature decreases.
  • the solubility of gas increases when the pressure of the gas above the liquid increases.

5. Miscibility

  • liquids that mix in each other completely and appear to have one phase are miscible (mix without separating)
  • all hydrocarbon are miscible
  • water mix with benzene to a small extent but it’s not miscible
  • the miscibility and solubility of a substance is very relevant to the oil industry when it comes to separating oil from water and water from oil.

6. Cloud Point and Pour Point

  • some crude oil and lubricating oils contain paraffin wax, which may cause the oil to solidify it its gets cold.
  • as oil containing wax is cooled, wax crystals will form when the temperature reaches the cloud point.
  • further cooling will cause additional wax formation and eventually, the oil will solidify.
  • pour point : temperature at which the oil turns to a solid.
  • a small concentration of wax can make a big difference in the temperature at which the oil will solidify.
  • Oil with 7% wax may solidify at normal room temperature, but, the same oil with no wax will remain a liquid.
  • there may be as much as 90 DegF difference in temperature between cloud point and the pour point.
  • the wax that forms at cloud point is soft, mushy material. It will flow if the velocity is high enough. It will settle out in a separator or other equipment when the velocity is very low
  • wax problem, heaters are often used to maintain higher temperature than room temperature (above cloud point), no wax will form
  • chemical depressant such as wax crystal modifiers are injected into the stream to lower or depress the temperature at which wax is forms.

7. Flash Point and Fire Point

  • 2 terms related to flammable materials that are important are flash point and fire point.
  • flash point is the lowest temperature of a fluid at which sufficient vapor is given off to form a combustible mixture.
  • flash point – product quality specification test and determines where products are stored and in what type of storage tank.
  • fire point: the lowest temperature at which enough vapor is given off to maintain combustion once it is ignited.




Categories: Oil & Gas Notes

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1 reply


  1. Physical Properties of Hydrocarbon Part 2 (Hydrocarbon Exploration & Development Notes #4) – aarif billah

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