Additional info – Q6

NATURAL GAS

Commodity

• Natural gas is a byproduct of crude oil production. It provides 23 percent (according to Energy Information Administration, 1999) of all energy consumed in the world. An economical, environmentally friendly and efficient energy source, natural gas is the cleanest-burning conventional fuel, producing lower levels of greenhouse gas emissions than do heavier hydrocarbon fuels, such as coal and oil. Natural gas fuels electric power generators, heats buildings and is used as a raw material in many consumer products, such as those made of traditional plastics.

Description/Technical Characteristics

• Natural gas is colourless, odourless, tasteless, shapeless and lighter than air. It is gaseous at any temperature over -161º C. When it is at its natural state, it is not possible to see or smell natural gas. For safety reasons, a chemical odorant that smells a little like rotten eggs, Mercaptan, is added to natural gas so that it can be smelled if there is a gas leak.

• Natural gas is a mixture of light hydrocarbons including methane, ethane, propane, butanes and pentanes. Other compounds found in natural gas include CO2, helium, hydrogen sulphide and nitrogen. The composition of natural gas is never constant, however, the primary component of natural gas is methane (typically, at least 90%), which has a simple hydrocarbon structure composed of one carbon atom and four hydrogen atoms (CH4).

• Natural gas is found throughout the world in reservoirs deep beneath the surface of the earth and floor of the oceans. It forms as pockets of gas over crude oil deposits or is trapped in porous rock formations. Natural gas can be found in oil deposits, as associated natural gas, although non-associated natural gas is often found without the presence of oil.

• When natural gas is cooled to a temperature of approximately -260°F at atmospheric pressure, it condenses to a liquid called liquefied natural gas (LNG). One volume of this liquid takes up about 1/600th the volume of natural gas. LNG weighs less than one-half that of water, actually about 45% as much. LNG is odourless, colourless, non-corrosive, and non-toxic. When vaporized it burns only in concentrations of 5% to 15% when mixed with air. Neither LNG, nor its vapour, can explode in an unconfined environment. Since LNG takes less volume and weight, natural gas is liquefied for ease of storing and transporting.

Uses

• Natural gas is used in residential users, commercial users, industry, power generation, natural gas vehicles and fuel cells.

Following graph shows natural gas use history and prospects in USA:

Source: Energy Information Administration

METHANE

• Methane is a chemical compound with chemical formula CH4. Methane is a colorless, odorless gas with a wide distribution in nature.

• At room temperature, methane is a gas less dense than air. • It melts at –183°C and boils at –164°C. • It is not very soluble in water. • Methane is combustible, and mixtures of about 5-15 percent

in air are explosive. • Methane is not toxic when inhaled, but it can produce

suffocation by reducing the concentration of oxygen inhaled. • A trace amount of smelly organic sulfur compounds (tertiary-

butyl mercaptan, (CH3)3CSH and dimethyl sulfide, CH3–S–CH3) is added to give commercial natural gas a detectable odor. This is done to make gas leaks readily detectible. An undetected gas leak could result in an explosion or asphyxiation.

• Methane is synthesized commercially by the distillation of bituminous coal and by heating a mixture of carbon and hydrogen.

• It can be produced in the laboratory by heating sodium acetate with sodium hydroxide and by the reaction of aluminum carbide (Al4C3) with water.

• In the chemical industry, methane is a raw material for the manufacture of methanol (CH3OH), formaldehyde (CH2O), nitromethane (CH3NO2), chloroform (CH3Cl), carbon tetrachloride (CCl4), and some freons (compounds containing carbon and fluorine, and perhaps chlorine and hydrogen).

• The reactions of methane with chlorine and fluorine are triggered by light. When exposed to bright visible light, mixtures of methane with chlorine or fluorine react explosively.

Uses of methane

• The principal use of methane is as a fuel. The combustion of methane is highly exothermic. CH4(g) + 2 O2(g) CO2(g) + 2 H2O(l) H = –891 kJ

• heat homes and commercial buildings• generation of electric power

ETHANE

• Ethane is a chemical compound with chemical formula C2H6. It is the only two-carbon alkane, that is, an aliphatic hydrocarbon. At standard temperature and pressure, ethane is a colorless, odorless gas. Ethane is isolated on an industrial scale from natural gas, and as a byproduct of petroleum refining. Its chief use is as petrochemical feedstock for ethylene production.

• In the laboratory, ethane may be conveniently prepared by Kolbe electrolysis.

• In this technique, an aqueous solution of an acetate salt is electrolysed.

• At the anode, acetate is oxidized to produce carbon dioxide and methyl radicals, and the highly reactive methyl radicals combine to produce ethane:

• CH3COO− → CH3• + CO2 + e−

• CH3• + •CH3 → C2H6

• Another method, the oxidation of acetic anhydride by peroxides, is conceptually similar.

• The complete combustion of ethane releases 1561 kJ/mol, or 51.9 kJ/g, of heat, and produces carbon dioxide and water according to the chemical equation

• 2 C2H6 + 7 O2 → 4 CO2 + 6 H2O + 3122 kJ/mol

• After methane, ethane is the second-largest component of natural gas.

• Ethane is most efficiently separated from methane by liquefying it at cryogenic temperatures.

• Various refrigeration strategies exist: the most economical process presently in wide use employs turboexpansion, and can recover over 90% of the ethane in natural gas.

• In this process, chilled gas expands through a turbine; as it expands, its temperature drops to about -100 °C.

• At this low temperature, gaseous methane can be separated from the liquefied ethane and heavier hydrocarbons by distillation.

• Further distillation then separates ethane from the propane and heavier hydrocarbons.

Uses of ethane• The chief use of ethane is in the chemical industry (usually uses

a catalyst to boost up the reaction), in the production of ethylene by steam cracking.

• Presently, INEOS operates a 1000 t/a ethane-to-vinyl chloride pilot plant at Wilhemshaven in Germany.

• Similarly, the Saudi Arabian firm SABIC has announced construction of a 30,000 t/a plant to produce acetic acid by ethane oxidation at Yanbu. This economic viability of this process may rely on the low cost of ethane near Saudi oil fields, and it may not be competitive with methanol carbonylation elsewhere in the world.

• Ethane can be used as a refrigerant in cryogenic refrigeration systems

NITROGEN

• Nitrogen is the largest single constituent of the Earth's atmosphere (78.082% by volume of dry air, 75.3% by weight in dry air).

• It is estimated to be the 7th most abundant chemical element by mass in the universe.

• Nitrogen occurs naturally in a number of minerals, such as saltpetre (potassium nitrate), Chile saltpetre (sodium nitrate) and sal ammoniac (ammonium chloride).

• At atmospheric pressure molecular nitrogen condenses (liquefies) at 77 K (−195.8 °C) and freezes at 63 K (−210.0 °C) into the beta hexagonal close-packed crystal allotropic form. Below 35.4 K (−237.6 °C) nitrogen assumes the alpha cubic crystal allotropic form. Liquid nitrogen, a fluid resembling water in appearance, but with 80.8% of the density (the density of liquid nitrogen at its boiling point is 0.808 g/mL), is a common cryogen.

Uses• To preserve the freshness of packaged or bulk foods (by delaying

rancidity and other forms of oxidative damage)• In ordinary incandescent light bulbs as an inexpensive alternative

to argon.[13]

• On top of liquid explosives as a safety measure• The production of electronic parts such as transistors, diodes,

and integrated circuits• Dried and pressurized, as a dielectric gas for high voltage

equipment• The manufacturing of stainless steel[14]

• Use in military aircraft fuel systems to reduce fire hazard, (see inerting system)

Effect

• Nitrogen dissolves in the bloodstream and body fats. • Rapid decompression (particularly in the case of

divers ascending too quickly, or astronauts decompressing too quickly from cabin pressure to spacesuit pressure) can lead to a potentially fatal condition called decompression sickness (formerly known as caisson sickness or more commonly, the "bends"), when nitrogen bubbles form in the bloodstream, nerves, joints, and other sensitive or vital areas

• Direct skin contact with liquid nitrogen will eventually cause severe frostbite (cryogenic burns).

• This may happen almost instantly on contact, depending on the form of liquid nitrogen. Bulk liquid nitrogen causes less rapid freezing than a spray of nitrogen mist (such as is used to freeze certain skin growths in the practice of dermatology

Heat of Combustion

• The heat of combustion (ΔHc0) is the energy released as

heat when a substance undergoes complete combustion with oxygen.

• Combustion is always exothermic, cH is negative. The chemical reaction for the combustion is typically that of a hydrocarbon fuel reacting with oxygen derived from atmospheric air to form carbon dioxide, water and heat.

• It may be quantified with these units: energy/mole of fuel (such as kJ/mol or Btu/lb-mol) energy/mass of fuel (such as MJ/kg or Btu/lb) energy/volume of fuel (such as MJ/m3 or Btu/ft3)

• The heat of combustion is traditionally measured with a bomb calorimeter. It may also be calculated as the difference between the heat of formation (ΔfH0) of the products and reactants.

Heat of Combustion of Some Common Substances

SubstanceHeat of Combustion (kJ/mol)

Combustion Reaction Hreaction (kJ/mol)

methane 890 CH4(g) + 2O2(g) ---> CO2(g) + 2H2O(l) H = -890

ethane 1560 C2H6(g) + 7/2O2(g) ---> 2CO2(g) + 3H2O(l) H = -1560

propane 2220 C3H8(g) + 5O2(g) ---> 3CO2(g) + 4H2O(l) H = -2220

butane 2874 C4H10(g) + 13/2O2(g) ---> 4CO2(g) + 5H2O(l) H = -2874

octane 5460 C8H18(g) + 25/2O2(g) ---> 8CO2(g) + 9H2O(l) H = -5460

methanol 726 CH3OH(l) + 3/2O2(g) ---> CO2(g) + 2H2O(l) H = -726

ethanol 1368 C2H5OH(l) + 3O2(g) ---> 2CO2(g) + 3H2O(l) H = -1368

propan-1-ol 2021 C3H7OH(l) + 9/2O2(g) ---> 3CO2(g) + 4H2O(l) H = -2021

butan-1-ol 2671 C4H9OH(l) + 6O2(g) ---> 4CO2(g) + 5H2O(l) H = -2671

Perfect Gas or Ideal Gas

• Experimental information about gases at low pressures i.e. Charles's law, Boyle's law and Avogadro's principle may be combined to one equation:

• P V=n R T • known as perfect gas equation. Where,

P= absolute pressure, T= absolute temperature, V= volume of the gas, n= number of moles, R is a constant, known as gas constant. (R=8314.51 J/(kmol.K))

• Any gas that obeys the above mentioned equation under all conditions is called a perfect gas (or ideal gas). A real gas (or an actual gas), behaves like a perfect gas only at low pressures. Some properties of actual gases such as specific heat at constant pressure and specific enthalpy are dependent on temperature but the variation due to pressure is negligible. There are empirical relations that calculate gas properties.