In the last article, I said heat changes the material’s phase.

Heat solid and get liquid. Heat liquid and get vapour (gas).

(I will try to remove the confusion between these two words – vapour and gas later in this article).

In other words, cool the gas, and you’ll get liquid. Cool the liquid you’ll get solid. An exception is dry ice, the solid form of CO2 carbon dioxide gas. When it melts, it directly changes to gas—no liquid state in between.

So heat is one parameter which changes the phase of the material.

Is there any other parameter which can do that?

Yes, it is the pressure which can do that. If you compress a gas, the gas will turn into a liquid.

This process is called liquefaction by compression.

And if you remove the applied pressure, the liquid will become gas again.

  • At what pressure will this happen?
  • Will this pressure remain constant under all atmospheric temperature conditions?
  • Will this pressure be the same for all gases?
  • Can all gases be compressed to liquid?

These questions pop up at this time along with several new terms, like permanent gas, liquefiable gas, critical pressure, and critical temperature.

What is critical temperature?

The pressure at which a gas becomes liquid depends on the temperature of the gas at that moment, which may or may not be equal to an atmospheric temperature higher the temperature, the higher the pressure required. At some temperatures, and beyond that at any higher temperature, it becomes impossible to liquefy the gas by compression. This temperature is called the Critical temperature. If gas is compressed to liquid at this temperature, the pressure at which the gas has liquefied is known as Critical pressure.  

Let us see how SMPV rules define these terms.

Petroleum and Explosives Safety Organisation (PESO) is the department of the Government of India. They have published rules for the storage and transportation of pressurised liquids and gases under the title “Static and Mobile Pressure Vessels (Unfired) Rules 1981”. All aspects of LPG are controlled by these rules.

“Critical temperature” means the temperature above which gas cannot be liquefied by the application of pressure alone, but the wording “by application of pressure alone “must be adequately understood.

Just to recap, gas can be liquefied in two ways, by cooling it and by compressing it. The third way is the combination of these two. If the gas temperature exceeds its critical temperature, you must cool it below its critical temperature. This is cooling and compressing. While defining critical temperature, SMPV assumes that liquefaction is done by compression process only without cooling.

Several new terms come in at this stage, like permanent gas, liquefiable gas, low-pressure liquefiable gas, and high-pressure liquefiable gas. 

Every gas can be liquefied by compressing it or by, cooling it, or combining the two.

But all gases are not called liquefiable gas. Gases like oxygen, nitrogen, argon, and helium are some of the gases that fall into the category of permanent gases. Methane and natural gas are also Permanent gases. What is the difference between these gases and gases like CO2, Propane, Butane, and ammonia, called liquefiable gases?

Liquefaction of gases by compression alone is a more convenient process. However, when a gas is compressed, it becomes hot. It is essential to cool it if it has not turned into liquid and then compress it further. Otherwise, you will need much higher pressure, equal to critical pressure, for liquefying the gas. Cooling it using water as a cooling medium, using some type of heat exchanger, is very easy if the temperatures are above 0°C. This is impossible when the gas temperature needs to be below zero degrees centigrade to its critical temperature. Any gas or liquid will tend to attain atmospheric temperature when stored. It will cool if its temperature is higher than the atmospheric temperature. And will heat if its temperature is lower than the atmospheric temperature. This is a natural process because of thermodynamic laws.

Now recollect the definition of critical temperature. Gases mentioned in the first group have critical temperatures much below zero degrees centigrade. For example, O2 minus 119°C, N2 minus 147°C, natural gas minus 162°C.

These gases cannot be liquefied unless cooled to their critical temperatures. Unfortunately, it is not an easy process. Special coolants and complex equipment are required to cool PGs to liquefaction, and they also need special storage arrangements.

That is the reason they are called permanent gases. SMPV rules define permanent gases as: “permanent gas” means a gas whose critical temperature is lower than minus 10°C (−10°C).

So gases having critical temperatures above minus 10°C are called liquefiable gases.

SMPV rules define: “liquefiable gas” means any gas that may be liquefied by pressure above at minus 10 °C (−10°C), but will be completely vaporised when in equilibrium with normal atmospheric pressure (760 mm Hg) at 30°C. 

The second clause of this definition stipulates that if pressure is reduced to 0 psig,(1 atmosphere absolute), the liquid should turn into gas (This will be called vapour).

Take CO2, for example. Its critical temperature is 31°C, very close to 30°C, and its critical pressure is 78 ata. It means that if it is in liquid condition, it is at 78 ata pressure. If the gas is at 30 °C, it can be slightly less pressure. Now if pressure is reduced to zero, all the liquid must convert back to gas.

What happens in the case of permanent gases? At atmospheric pressure, they are already in a gaseous state. And even after applying any pressure, they will remain in a gaseous state. But in the case of liquefiable gases, as soon as pressure is applied, the liquefaction process will start. A part of the gas will turn into liquid. As you increase the pressure, more and more gas will turn into liquid. Depending on the temperature, gas will completely turn into liquid at some pressure. If the temperature is less than critical, gas will turn into liquid completely at pressure less than critical pressure. This is called the saturation pressure at that temperature. Every gas will have a chart showing saturation pressure at different temperatures. At any temperature, the phase will change at saturation pressure. This is also termed Vapour pressure at that temp.

Let me introduce one more term: “Low-pressure liquefiable gas.”

As defined in IS 3107, “Low-pressure liquefiable gas: A liquefiable gas having a critical temperature above 70°C. By this definition, CO2 is a high-pressure liquefiable gas. On the other hand, propane and Butane are low-pressure liquefiable gases.

I will write more about saturation pressure and temperature in the following article. These terms are more important for understanding the behaviour of LPG under varying atmospheric conditions.

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