WHAT IS LIQUEFIED PETROLEUM GAS

WHAT IS LIQUEFIED PETROLEUM GAS

Chemically, Liquefied petroleum gas or liquid petroleum gas (LPG or LP gas) is a mixture of two flammable but nontoxic gases called propane and butane. Both of these are hydrocarbons (their molecules are made from different combinations of hydrogen and carbon atoms. propane molecules (C₃H₈) have eight hydrogen atoms attached to three carbon atoms, while butane molecules (C₄H₁₀) have ten hydrogen atoms bonded to four carbon atoms. LPG sometimes contains a variation of butane called isobutane, which has the same component atoms (four carbons and ten hydrogens) connected together in a slightly different way. In the northern hemisphere winter, the mixes contain more propane, while in summer, they contain more butane. In the United States, primarily two grades of LPG are sold: commercial propane and HD-5. These specifications are published by the Gas Processors Association (GPA) and the American Society of Testing and Materials (ASTM). Propane/butane blends are also listed in these specifications.

Exactly which of the gases is present in LPG depends on where it comes from, how it is supplied, and what it is being used for. LPG typically contains a mixture of butane and propane gases, and tiny quantities of other gases are also naturally present. Since LPG is normally odorless, small amounts of a pungent gas such as ethanethiol (also known as ethyl mercaptan) are added to help people smell potentially dangerous gas leaks, which might otherwise go undetected. Pure butane tends to be used more for small, portable LPG supplies in such things as boats and gas-powered barbecue stoves. Since butane doesn't burn well at low temperatures, portable canisters often contain a blend of 20 percent propane and 80 percent butane; propane has a much lower boiling point so it's less affected by freezing temperatures and generally better for year-round outdoor use in cold climates. Larger household tanks are more likely to contain a majority of propane (typically 90 percent propane in North America).The internationally recognized European Standard is EN 589. In the United States, tetrahydrothiophene (thiophane) or amyl mercaptan are also approved odorants, although neither is currently being utilized.

As its boiling point is below room temperature, LPG will evaporate quickly at normal temperatures and pressures and is usually supplied in pressurised steel vessels They are typically filled to 80–85% of their capacity to allow for thermal expansion of the contained liquid. The ratio between the volumes of the vaporized gas and the liquefied gas varies depending on composition, pressure, and temperature, but is typically around 250:1. The pressure at which LPG becomes liquid, called its vapour pressure, likewise varies depending on composition and temperature; for example, it is approximately 220 kilopascals (32 psi) for pure butane at 20 °C (68 °F), and approximately 2,200 kilopascals (320 psi) for pure propane at 55 °C (131 °F). LPG is heavier than air, unlike natural gas, and thus will flow along floors and tend to settle in low spots, such as basements. There are two main dangers from this. The first is a possible explosion if the mixture of LPG and air is within the explosive limits and there is an ignition source. The second is suffocation due to LPG displacing air, causing a decrease in oxygen concentration.

COMPARISION BETWEEN LPG AND CNG

LPG is composed primarily of propane and butane, while natural gas is composed of the lighter methane and ethane. LPG, vaporised and at atmospheric pressure, has a higher calorific value (94 MJ/m³ equivalent to 26.1kWh/m³) than natural gas (methane) (38 MJ/m³equivalent to 10.6 kWh/m³), which means that LPG cannot simply be substituted for natural gas. In order to allow the use of the same burner controls and to provide for similar combustion characteristics, LPG can be mixed with air to produce a synthetic natural gas (SNG) that can be easily substituted. LPG/air mixing ratios average 60/40, though this is widely variable based on the gases making up the LPG. The method for determining the mixing ratios is by calculating the Wobbe index of the mix. Gases having the same Wobbe index are held to be interchangeable.

LPG-based SNG is used in emergency backup systems for many public, industrial and military installations, and many utilities use LPG peak shaving plants in times of high demand to make up shortages in natural gas supplied to their distributions systems. LPG-SNG installations are also used during initial gas system introductions, when the distribution infrastructure is in place before gas supplies can be connected. Developing markets in India and China (among others) use LPG-SNG systems to build up customer bases prior to expanding existing natural gas systems.

LPG-based SNG or natural gas with localized storage and piping distribution network to the house holds for catering to each cluster of 5000 domestic consumers can be planned under initial phase of city gas network system. This would eliminate the last mile LPG cylinders road transport which is a cause of traffic and safety hurdles in Indian cities. These localized natural gas networks are successfully operating in Japan with feasibility to get connected to wider networks in both villages and cities.

ENVIRONMENTAL EFFECTS

Commercially available LPG is currently derived from mainly from fossil fuels. Burning LPG releases carbon dioxide, a greenhouse gas. The reaction also produces some carbon monoxide. LPG does, however, release less CO2 per unit of energy than does coal or oil. It emits 81% of the CO2 per kWh produced by oil, 70% of that of coal, and less than 50% of that emitted by coal-generated electricity distributed via the grid. Being a mix of propane and butane, LPG emits less carbon per joule than butane but more carbon per joule than propane. LPG burns more cleanly than higher molecular weight hydrocarbons because it releases less particulates

FIRE/EXPLOSION AND RISK MANAGEMENT

LPG must be stored in pressure vessels. These containers are either cylindrical and horizontal or spherical. Typically, these vessels are designed and manufactured according to some code. In the United States, this code is governed by the American Society of Mechanical engineers (ASME). LPG containers have pressure relief valves, such that when subjected to exterior heating sources, they will vent LPGs to the atmosphere or a flare stack

If a tank is subjected to a fire of sufficient duration and intensity, it can undergo a boiling liquid expanding vapor explosion .This is typically a concern for large refineries and petrochemical plants that maintain very large containers. In general, tanks are designed that the product will vent faster than pressure can build to dangerous levels.

One remedy, that is utilized in industrial settings, is to equip such containers with a measure to provide a fire-resistance rating. Large, spherical LPG containers may have up to a 15 cm steel wall thickness. They are equipped with an approved pressure relief valve. A large fire in the vicinity of the vessel will increase its temperature and pressure, following the basic gas laws. The relief valve on the top is designed to vent off excess pressure in order to prevent the rupture of the container itself. Given a fire of sufficient duration and intensity, the pressure being generated by the boiling and expanding gas can exceed the ability of the valve to vent the excess. If that occurs, an overexposed container may rupture violently, launching pieces at high velocity, while the released products can ignite as well, potentially causing catastrophic damage to anything nearby, including other containers.

People can be exposed to LPG in the workplace by breathing it in, skin contact, and eye contact. The Occupational Safety and Health Administration (OSHA) has set the legal limit (Permissible exposure limit) for LPG exposure in the workplace as 1000 ppm (1800 mg/m³) over an 8-hour workday. The National Institute for Occupational Safety and Health (NIOSH) has set a recommended exposure limit (REL) of 1000 ppm (1800 mg/m³) over an 8-hour workday. At levels of 2000 ppm, 10% of the lower explosive limit, LPG isimmediately dangerous to life and health.

REFERENCES

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