Question

under what conditions gas can be liquefied? In which from LPG is filled gas cylinder

Answer 1

LPG always exists in a Liquid-Vapour equilibrium state in a container (barring pipe lines). That means LPG is never filled in a cylinder to 100% of its containing capacity. Usually about 85% of the vessel is filled. The remaining space is occupied by the vapour phase of the LPG. when there is more evaporation in the cylinders, more vapour gets generated, and since the space for vapour is limited, pressure inside the cylinder will increase. Due to the increase in pressure, the vapour will get condensed into liquid. When vapour becomes liquid the pressure in the cylinder drops, which causes the liquid to vaporise. Thus, there will be a continuous liquid-vapour transformation in the cylinder, which is called a state of ‘dynamic equilibrium’. The LPG is never at rest you know!

Now the valve is fitted on the top of the cylinder, which, when opened, only allows the vapour to come out of the valve and is used by us. This escaping of vapour causes the pressure drop in the cylinder and more liquid will become vapour. Thus we actually vaporise the contents of the cylinder and use only the gaseous form of LPG for our heating purposes. Hence it is commonly called gas.

There are applications which requires more heat and hence more LPG to be drawn from cylinders than produced by natural evaporation. In such applications, cylinders fitted with LOT (Liquid Off-Take) Valves which take liquid from the cylinder directly and the vaporisation is done outside of the container using ‘vaporisers’ .

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Answer 2

Gases can be liquefied by increasing pressure and decreasing temperature on it .
LPG is filled in cyclinder in liquid form.