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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

#### 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.

#notes