LPG processing

LPG production and applications

LPG is comprised of a mixture of propane and butane. These are relatively light hydrocarbons, which are produced either from 'wet' natural gas sources that contain a mixture of butane, propane, methane and ethane molecules, or during the refining of crude oil where it is among the lightest fractions to be recovered.

To achieve liquefaction of LPG, it must either be cooled to approximately -42°C at atmospheric pressure or stored as liquid at a pressure exceeding its vapour pressure. This would be approximately 1000 kPa (10 bar) at around 30°C. LPG is therefore easier to liquefy and store than LNG, and this has led to its wide adoption as a highly transportable fuel.

Typical applications for LPG gas cylinders are as forklift truck fuel, as BBQ gas, for patio heaters or for cooking and heating in regional areas, where there is no piped gas network and the electricity distribution grid might struggle to cope with the high energy consumption of heating applications. LPG can also be used as an automotive fuel where the distribution and storage supply chain is slightly different.

LPG storage and distribution

When stored in large quantities, LPG is typically contained in pressurised 'bullets'. An example of this distribution and supply chain is where LPG is delivered in road tankers to storage tanks at roadside service stations where it can be decanted from these vessels into smaller tanks in cars and trucks.

LPG is commonly filled into low-pressure welded cylinders, which differ from the high-pressure seamless cylinders used for storage of air gases such as oxygen, nitrogen or argon. The welded cylinders are wider, have thinner walls than their high-pressure cousins and there is a clearly visible welded seam around the middle of the cylinder. During production of welded cylinders, the weld must undergo stringent quality control to ensure that there is no porosity in the weld that would result in leakage of LPG and cause a safety hazard. Leak detection gas mixtures containing helium or hydrogen can be used for this purpose. This shape and construction is chosen for LPG because it is significantly cheaper to produce than a high-pressure steel cylinder.



LPG fractionation involves the following principles:

1. Removal of the lightest hydrocarbon - ethane - to be used on the site as a fuel in combustion processes which generate heat and steam that are required on the gas processing facility. Ethane removal allows the production of LPG as a liquid fuel which enables convenient transportation.
2. Separation of the LPG fraction - propane and butane - from the heavier hydrocarbons: naptha and heavy crude oil.
3. Purification of the LPG using molecular sieves. Sulphur is removed so that the fuel will burn cleanly without the production of SO₂. Water is removed to avoid any reaction with residual sulphur chemicals which may cause acidic corrosion of the pipework, LPG tankers or storage tanks.
4. Fractionation of the heavier hydrocarbons into naptha and heavy crude oil. The naptha fraction can be used for the production of liquid transportation fuels such as petrol, diesel and kerosene at other petrochemical processing facilities.

The Santos Port Bonython LPG fractionation processing facility encompasses the following unit operations to achieve the above:

• Pipeline terminal for incoming stream flow rate & composition analysis. Knowledge of the incoming raw material is essential for production planning and process control. For transfers between operators it will also be the critical measurement for billing according to the product composition and associated financial value. Gas chromatographs fitted with TCD or FID detectors are the most common analytical equipment to use for this composition analysis. High purity gases such as Helium 5.0 grade, Hydrogen 5.0 grade and Zero grade Air will be used as carrier and detector gases for this instrumentation. Accurate calibration gas mixtures which are blended at a target composition to reflect the typical incoming feed stream will also be required.

• Pre-fractionation heaters. Combustion of ethane produces the energy required to heat up the incoming feed material and allow the distillation processes to function. Burner combustion control calibration gas mixtures will be used to ensure that the air / fuel mix in the burner is correct to achieve good stoichiometry. This maximises energy efficiency and minimises emissions. Emissions monitoring from the furnace gases using CEMS will also take place to ensure environmental compliance.

• Ethane distillation column for generation of plant fuel, eg for the pre-fractionation heaters above.

• De-propaniser, de-butaniser and naptha distillation columns. Process control in the distillation columns will rely on a combination of rapid response direct read temperature and pressure measurement. Additional chemical analysis using gas chromatography may be employed for longer term process optimisation, quality control and trouble shooting.

• Propane and butane molecular sieve purifiers to remove sulphur and water. The molecular sieves used in this process become loaded with water and sulphur chemicals and require periodic regeneration where they are back-flushed with hot dry gas to clean the molecular sieve. The purified gases emitted from the molecular sieve purifiers will be continuously analysed for moisture and sulphur content to ensure that the process is under control and that there is no breakthrough from the adsorbers. As soon as a small increase in the analysed moisture of sulphur concentration is detected the system can be reversed to regenerate the molecular sieves. For rapid response to changes in the process conditions, online process control analysers such as a pulsed ultraviolet fluorescence (PUVF) instrument for total sulphur analysis will be used. Calibration gas mixtures containing ppm or low % levels of SO₂ are required.

• Propane & butane chillers.

• LPG pressurised bullet storage tanks. Around these storage tanks, gas detection for explosive gas leaks will take place.

• Crude oil and naptha bulk storage tanks. Around these heavier hydrocarbon storage tanks, it will be common to use BTEX gas detection equipment to check for leaks for personal safety.

• Propane and butane bulk storage tanks