Around half goes to domestic and commercial fuel — cooking, water heating and space heating, delivered in cylinders and bulk. Petrochemical use is the next largest, covering propane dehydrogenation to propylene, steam cracking of propane and butane to ethylene, and butane oxidation to maleic anhydride. Beyond those, LPG serves as autogas for vehicles, as hydrocarbon aerosol propellant, and as natural refrigerant in the form of R-290 propane and R-600a iso-butane.

It depends on temperature, and the answer is unforgiving. Propane boils at −42 °C, so it develops adequate vapour pressure and vaporises reliably from a cylinder even in hard frost. Butane boils at around −0.5 °C, so below about 5 °C it stops vaporising at a useful rate and the appliance starves regardless of how much liquid remains in the cylinder. Butane carries more energy per litre and generally costs less, so warm climates and indoor applications favour it. Most markets supply a blend adjusted seasonally, and specifying the wrong split for the climate presents to the end customer as a supply failure rather than a specification error.

Propane and butane are odourless, so a leak would give no warning until the atmosphere reached the flammable range. Ethyl mercaptan is added at the terminal at a level chosen so that the smell becomes obvious at roughly one fifth of the lower flammable limit, giving a substantial safety margin. Odorant fade is the phenomenon where mercaptan is adsorbed onto rust, mill scale or new steel pipework and its concentration in the gas falls — most often in new or long-idle installations. The consequence is that a leak may be present without being smelt, which is why odour is never accepted as the sole confirmation that a space is gas-free and why gas detection is used for confined space work.

A boiling liquid expanding vapour explosion happens when a vessel containing liquefied gas is heated externally, typically by a fire. The liquid boils and pressure rises; the relief valve lifts but cannot keep pace, and meanwhile the vessel wall above the liquid level has no liquid to cool it and weakens. When the wall fails, the remaining superheated liquid flashes to vapour almost instantly and ignites, producing a fireball and hurling vessel fragments a considerable distance. It is the reason bulk LPG installations are designed around relief capacity, fire protection or insulation, fixed water spray and generous separation distances, and it is why fire crews attending an LPG incident prioritise cooling the vessel.

Both, and the split has shifted significantly. Historically LPG was mainly a fuel, and petrochemical use was opportunistic — cracking propane when it was cheaper than naphtha. The growth of on-purpose propane dehydrogenation changed that: PDH plants consume propane continuously to make propylene independently of naphtha cracker economics, and that has created structural petrochemical demand. Butane has its own dedicated outlet in maleic anhydride. The practical consequence for a buyer is that fuel-grade and feedstock-grade LPG have different specifications — olefin content, sulphur and above all odorant status — and they are not interchangeable in either direction.

Every cargo includes a Certificate of Analysis covering composition and C3/C4 split, vapour pressure, ethane and lighter, C5 and heavier, olefins, total sulphur, hydrogen sulphide, copper strip corrosion, residue on evaporation, free water and odorant status; a Safety Data Sheet; UN 1075 Class 2.1 dangerous goods documentation; an origin certificate; and destination-market documentation including any autogas or food-grade propellant certification required.