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
- 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:
- gravity readings must be corrected to 15 DegC (60 DegF)
- petroleum liquid get less dense at higher temperature
- 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
- 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.
- 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.
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