What Happens When Gases are Compressed?

Some common terms you may be familiar with include liquified petroleum gas (LPG) and compressed natural gas (CNG). These are the most common examples of compressed gas used in our daily lives. Gas compression changes the state of the gas, i.e., temperature, pressure, volume and gas compression are done to increase the pressure. There are three styles of gas compression methods in industrial applications. These are piston compressors, rotary screw compressors, and rotary vane compressors. 

Why are Gases Compressible?

Gas particles have a large intermolecular space among solids, liquids, and gases. They do not have any specific arrangement among them. Gases have free movement, and they occupy the vessel’s shape they are kept in. Due to large intermolecular space, they are more compressible than solids and liquids.

Gases are less dense than solids and liquids, and their pressure is independent of their properties and nature, due to which gases are more compressible than others. Another reason gases are compressible is that the intermolecular forces among the gas particles are negligible compared to solids or liquids, and this makes them easily compressible.

Factors Involved with Gas Compression

Factors involved with gas compression depend upon the type of gas compression process and the changes in the gas’s physical state. 

Adiabatic Compression

Adiabatic compression is when there is no heat transfer between the system and the surroundings.  While compressing the gas adiabatically, the gas temperature increases, increasing the gas pressure. 

Adiabatic compression is defined as:

PVγ = constant

Here, γ is the ratio of specific heat (Cp/Cv).

Cp is the specific heat at constant pressure (P) conditions, and Cv is the specific heat at constant volume (V) conditions.

There is no heat transfer (dQ = 0), but some amount of heat is lost in experimental conditions. A few practical examples of adiabatic compression are gas turbines and internal combustion engines. 

Adiabatic compression works on the principle of the first law of thermodynamics:

dQ = dU + dW

As dQ = 0 

dU = -dW = -PdV

This states that an increase in internal energy leads to a decrease in the volume of the gas. Internal energy increases as the temperature increases. 

Isothermal Compression

Isothermal compression is when the temperature (T) of the system remains constant. Thermal equilibrium is maintained to carry on this process. An isothermal process is expressed as:

PV = constant

=> P1V1 = P2V2

Polytropic Compression

A polytropic compression can be expressed as: 

PVn = constant

Here n is the polytropic index. Its value can be from 0 to ∞ depending upon the process. 

For n=0, P = constant, it shows an isobaric process.

For n= 1, PV= constant, it shows an isothermal process.

For n -> ∞, shows an isochronic process where volume is constant.

Gas Compression Methods

Gas compression is conducted using different kinds of compressors. The type of compressor used depends upon the application of the system. Some compressors are:

• reciprocating compressors
• rotary compressors (rotary screw compressors and rotary vane compressors) 
• centrifugal compressors
• balance piston-centrifugal compressors 

Kinetic Theory of Gases

The kinetic theory of gases involves the behaviour of gases. Maxwell and Boltzmann postulated the kinetic theory of gases. It explains the pressure, volume, temperature, and movement of gases. The kinetic theory of gases has the following assumptions: 

• All gases consist of molecules that move randomly in all directions
• The size of a molecule is much smaller than the average separation between the molecules
• The molecules exert zero force on each other or on the walls of the container, except during collision
• All collisions between two molecules are perfectly elastic
• The molecules obey Newton’s Laws of Motion
• When gas is left for a sufficient amount of time, it comes to a steady state

Conclusion 

The properties of gas molecules are highly beneficial. It is the kinetic theory of gases that postulated these properties, and one of the most valuable properties of gases is their ability to compress. Gases can be compressed due to large intermolecular forces, less force of attraction, and low density. There are different types of processes of gas compression according to the state of the gas.