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Petroleum Notes for the Classroom
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- Acetylene is the simplest hydrocarbon with a triple bond. In the days before oil gained widespread acceptance as the main feedstock of chemical industry, acetylene was the predominant building block of industrial organic chemistry. The calcium carbide process was the sole route for acetylene production until 1940, when thermal cracking processes using methane and other hydrocarbons were introduced. At first, these processes used an electric arc; then, in the 1950s, partial oxidation and regenerative processes were developed.
- Today, only three processes remain for the commercial production of acetylene: the calcium carbide route, in which the carbide is produced electrically, the arc process, and the partial oxidation of natural gas. Other once popular processes have become uneconomical as the price of naphtha has increased.
- Acetylene is made : Thermodynamic and Kinetic Aspects: The production of acetylene from hydrocarbons, e.g, 2CH4 ---> C2H2+ 3H2H(298k)= 376.4kJ/mol requires very high temperatures and very short reaction times. Figure 1 also shows that the free energy of acetylene decreases as temperature increases, whereas the free energies of the other hydrocarbons increase. Above about 12308C, acetylene is more stable than the other hydrocarbons. The temperature at which the acetylene line intersects an other line in Figure 7 is higher the shorter the chain length of the hydrocarbons. Acetylene production from methane requires higher reaction temperatures than production from heavier hydrocarbons.
- The selection of raw materials depends upon the availability of raw materials and their prices. In the U.S.A. , near Niagra falls, where hydroelectricity is cheap, carbide process is used for the manufacture of acetylene. Where natural gas is available, it is used for the manufacture of acetylene. In Germany, during the world war II and well after that coal gas was used for the manufacture of acetylene. Higher hydrocarbons are used as raw materials when acetylene and ethylene, both are required as products
- By the partial oxidation of methane or higher hydrocarbons: Methane is found in natural gas and coal gas. Hence, both can be used for the manufacture of acetylene.
General Flow Scheme For Partial Combustion Processes
- The partial combustion processes for light hydrocarbons, from methane to naphtha, all follow similar schemes. The feed and a certain amount of oxygen are preheated separately and introduced into a burner. There they pass through a mixing zone and a burner block into the reaction zone, where they are ignited. On leaving the reaction zone the product mixture is cooled rapidly, either by water or oil. Cooling by water is easier, and more common, but it is thermally less efficient than cooling by oil. Alternatively, the gases can be cooled with light hydrocarbon liquids, which leads to additional acetylene and ethylene formation between 1500 and 8008C. These processes are usually called two-step processes.
Partial Combustion Processes.
- In this group of processes, part of the feed is burnt to reach the reaction temperature and supply the heat of reaction. The necessary energy is produced where it is needed. Almost all carbon- containing raw materials can be used as feedstocks: methane, ethane, natural gas liquids (NGL), liquefied petroleum gas (LPG), naphtha, vacuum gas oil, residues, and even coal or coke. Natural gas is especially suitable because it is available in many parts of the world and because its only other uses are for heating and for the production of synthesis gas. Only under the conditions of acetylene synthesis can methane be transformed into another hydrocarbon in a single process step, and this is the essential reason for using the thermodynamically unfavorable acetylene synthesis.
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