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Processing oil and gas
Longford is the onshore receiving point for all of the crude oil and gas produced by the offshore platforms in Bass Strait.
Crude oil and gas consist primarily of various combinations of two common elements, hydrogen and carbon. Hydrogen and carbon atoms combine in many ways to form molecules called hydrocarbons which are packed with energy.
Hydrocarbons range from very light gas molecules like methane (C1) - which is commonly known as natural gas - to liquids like pentane (C5) and heavier hydrocarbons (C6+) which are used to make petrol, oils and heavier tar-like bitumen.
When the hydrocarbons arrive at Longford from Bass Strait they contain other compounds such as water, carbon dioxide, nitrogen and hydrogen sulphide (rotten egg gas). However, they have already been sorted into two basic streams - the heavier molecules arrive as unstabilised crude oil and the lighter molecules arrive as raw gas.
In their raw condition these hydrocarbons are not suitable for use or further transportation.
The Longford plants remove impurities and process the hydrocarbons into stabilised products. These consist of methane, which is sold directly to customers as natural gas, heavier gas liquids (ethane and liquified petroleum gas - LPG), and stabilised crude oil. The LPG and crude oil are sent for further processing and storage at Long Island Point on Westernport Bay.
Crude oil
When unstabilised crude oil arrives at Longford from Bass Strait it is under high pressure and contains considerable amounts of dissolved gas, similar to liquids in a soft-drink can. The Crude Oil Stabilisation Plant heats up the crude and releases the pressure thereby extracting the gas and leaving the flat (stabilised) liquid, again just like releasing the gas from a soft-drink can by opening it.
Stabilised crude is pumped via pipeline 190 kilometres to Long Island Point for further processing and storage prior to sale to refineries.
The vapours (gas) are transferred to the gas stream for processing in gas plants.
Natural gas
The first step in gas processing is to remove the water and hydrogen sulphide from the raw gas. This is done by passing the gas through high-tech "sieves" which absorb water and hydrogen sulphide from the gas. The process is then one of physical separation of gas liquids (ethane, propane and butane) and gas condensate (pentanes and heavier oil) from the gas stream. Longford uses two basic techniques to achieve this separation.
Gas Plant 1 is a lean oil absorption plant. This means that the separation of natural gas is achieved through absorption of the gas liquids and condensate into a special oil, which has an affinity for these components but not for methane. This is a cyclical process similar to the way clothes are dry cleaned. The lean oil acts in a way solvents do in dry cleaning. The solvent is passed through the garment (or in Longford's case, the gas) and it absorbs the dirt (valuable gas liquids and condensate). The solvent is then processed to remove the dirt (gas liquids and condensate) so that it can be used over again.
Gas Plants 2 and 3 use a cryogenic process. The separation of raw gas into methane, gas liquids and stabilised crude is achieved by subjecting the gas to very low temperatures. The gas is cooled to as low as -90°C. At this temperature everything but methane, and a small fraction of the ethane, become liquid and so can be separated.
Did you know...?
Natural gas has very little odour. The typical smell normally associated with gas is actually caused by the chemical "mercaptan". Mercaptan is added as a safety mechanism during gas processing. Its distinctive and unpleasant odour allows people to identify the presence of what is otherwise a colourless and odourless gas.
Which gas is which?
Gas liquids are a mix of ethane, propane and butane. Ethane is separated and used in the manufacture of detergents and plastics such as polythene and polystyrene, which in turn are used for plastic food wraps, plastic bags and polystyrene foam.
Propane is the liquified gas that is commonly supplied in bottles used for cooking, camping and barbecues. It is also used for industrial purposes such as metal cutting and welding and refrigeration. Butane is used as a propellant for pressure pack cans and is also mixed with propane to become the commercial Auto LPG (liquified petroleum gas) which is used in cars.
Methane (natural gas) is also known as "dry gas" or "sales gas" and is the most plentiful and commonly used gas. Methane is typically supplied via pipeline to industry for heating and power generation and to households for cooking, heating and hot water.
