3 Ideal gases

The ability to calculate the performance of a gas producing system, including the reservoir and the piping system, requires knowledge of many gas properties at various temperatures & pressures. If the natural gas is in contact with liquids, such as condensate or water, the effect of the liquids on gas properties must be evaluated.

The understanding of the behavior of gases with respect to pressure and temperature changes is made clearer by first considering the behavior of gases at conditions near normal conditions of pressure and temperature, that is:

1 psia = 6894,757 Pa

SPE – society of petroleum engineers

Tabl. 1 - Transfer of-in degrees Kelvin

of (Kelvin) in (Kelvin) (Celsius) [°C]=[°K]-273.15 [°K]=[°C]+273.15 (Fahrenheit) ) [°F]=[°K]*9/5-459.67 [°K]=([°F]+459.67)*5/9 (Rankine) [°R]=[°K]*9/5 [°K]=[°R]*5/9 (Delisle) [°De]=(373.15-[°K])*3/2 [°K]=373.15-[°De]*2/3 (Newton) [°N]=([°K]-273.15)*33/100 [°K]=[°N]*100/33+273.15 (Reaumur) [°Re]=([°K]-273.15)*4/5 [°K]=[°Re]*5/4+273.15 (Romer) [°Ro]=([°K]-273.15)*21/40+7.5 [°K]=([°Ro]-7.5)*40/21+273.15

At these conditions the gas is said to behave ideally, and most of the early work with gases was conducted at conditions approaching these conditions. An ideal gas defined as one in which:

1. the volume occupied by the molecules is small compared to the total gas volume;

2. all molecular collisions are elastic;

3. there are no attractive or repulsive forces among the molecules.

Early Gas Laws

The basis for describing ideal gas behavior comes from the combination of some of the so-called gas laws proposed by early experiments.

Boyle`s Law. Boyle observed experimentally that the volume of an ideal gas is inversely proportional to the pressure for a given weight or mass of gas when temperature is constant. This may be express as:

Charles` Law. While working with gases at low pressure, Charles observed that the volume occupied by a fixed mass of gas is directly proportional to its absolute temperature, or

Avogadro`s Law. Avogadro`s Law states that under the same conditions of temperature and pressure, equal volumes of all gases contain the same number of molecules. This is equivalent to the statement that at a given temperature and pressure one molecular weight of any ideal gas occupies the same volume as one molecular weight of another ideal gas. It has been shown that there are 2.73x10²⁶ molecules/lb-mole (6.02x10²³ molecules/kg-mole) of ideal gas and that one molecular weight in pounds of any ideal gas (at 101.325kPa and 273,15 K) occupies a volume of 379.4 cu ft (lb-mole) (22.41 litres = 22.41 x 10^-3 cubic meters for kg-mole).

1 kg = 2.2046 lb

1 ft=0.3048 m

1 m³=1000 l

One mole of a material is the quantity of that material whose mass, in the system of units selected, is numerically equal to the molecular weight.

This is means that:

- One mole of any ideal gas, that is6.02x10²³ molecules/kg-mole of any gas, will occupy the same volume at a given pressure and temperature (a volume of 379.4 cu ft (22.41 litres).