Acetylene or ethene is the smallest member of the hydrocarbon alkyne family, which is made of two hydrogen atoms and two carbon atoms that have a triple bond. Due to the presence of this triple bond, acetylene is known as an unsaturated chemical substance.
Acetylene is of particular importance in industry. It is needed as a raw material and precursor in the synthesis and preparation of various chemicals. Therefore, many industrial methods have been invented for the mass production of this substance. This material can be stored and transported.
History and discovery
Edmund Davy, who introduced it as “the new carburet of hydrogen”, discovered acetylene in 1836. This was an accidental discovery while trying to isolate potassium metal. By heating potassium carbonate with carbon at very high temperatures, he produced a residue known today as potassium carbide (K2C2), which reacted with water to release a new gas. It was rediscovered in 1860 by French chemist Marcellin Berthelot, who coined the name acetylene. Berthelot's empirical formula for acetylene (C4H2), as well as the alternative name "quadricarbure d'hydrogène", was incorrect because many chemists at the time used the wrong atomic mass for carbon (6 instead of 12).
Berthelot was able to prepare this gas by passing the vapors of organic compounds (methanol, ethanol, etc.) through the red-hot pipe and collecting the effluent. He also found that acetylene was mixed by sparking electricity through cyanogen and hydrogen gases. Bertello later obtained acetylene directly by passing hydrogen between the poles of a carbon arc. Preparation
Since the 1950s, acetylene has been made primarily by the partial combustion of methane. It is a recycled by-product in the production of ethylene by cracking hydrocarbons. In 1983, about 400,000 tons were produced with this method. Its presence in ethylene is usually undesirable due to its explosive properties and its ability to poison Ziegler-Natta catalysts. It is selectively hydrogenated to ethylene, usually using Pd-Ag catalysts of the 90,000 tpa acetylene plant, commissioned by BASF in the 1950s until the 2020s, when oil replaced coal as the main source of reduced carbon. As a result, acetylene (and the aromatic fraction from coal tar) was the main source of organic chemicals in the chemical industry. It is prepared by the hydrolysis of calcium carbide, a reaction discovered by Friedrich Weiler in 1862.
At atmospheric pressure, acetylene cannot exist as a liquid and has no melting point. The triple point in the phase diagram corresponds to the melting point (-80.8 °C) at the minimum pressure that liquid acetylene can exist at (1.27 atm). At temperatures below the triple point, solid acetylene can be converted directly to vapor (gas) by sublimation. The sublimation point at atmospheric pressure is -840°C.
At room temperature, the solubility of acetylene in acetone is 27.9 g/kg. For the same amount of dimethylformamide (DMF), the solubility is 51 g. At 20.26 bar, the solubility increases to 689.0 and 628.0 g for acetone and DMF, respectively. These solvents are used in pressurized gas cylinders.
Approximately 20% of acetylene is supplied by the industrial gas industry for welding and cutting oxyacetylene gas due to the high temperature of the flame. Combustion of acetylene with oxygen produces a flame over 3600 K (3330 °C; 6020 °F) and releases 11.8 kJ/g. Oxyacetylene is the hottest common fuel gas. Acetylene is the third hottest natural chemical flame after dicyanoacetylene at 5260 K (4990 °C; 9010 °F) and cyanogen at 4798 K (4525 °C; 8177 °F).
Oxyacetylene welding was a common welding process in the past decades. The development and advantages of electric arc welding processes have made oxy-fuel welding almost obsolete for many applications. The consumption of acetylene for welding has been significantly reduced. Oxyacetylene welding equipment, on the other hand, is quite versatile - not only because the torch is preferred for some types of iron or steel welding (such as certain artistic applications), but also because it can easily be used for soldering, welding It is suitable. Used for heating metal (for annealing or tempering, bending or forming), loosening corroded details, and other applications.
Bell Canada cable repair technicians still use portable acetylene fuel torch kits as a soldering tool for sealing lead sleeve joints in manholes and in some aerial locations.
Carbide lamp portable lighting
Combustion of acetylene produces a strong, bright light, and the ubiquity of carbide lamps led to the commercialization of acetylene in the early 20th century. Common applications included coastal lighthouses, streetlights, and automotive and mining headlights. In most of these applications, direct combustion is a fire hazard, and so acetylene has been replaced, first by incandescent lamps and years later by low power/high lumen LEDs. However, acetylene lamps still have limited use in remote or inaccessible areas and in countries with weak or unreliable central electrical grids.
Plastic and acrylic acid derivatives
Acetylene can be semi-hydrogenated to ethylene and provide a raw material for various polyethylene plastics. Another major use of acetylene, especially in China, is its conversion to acrylic acid derivatives. These derivatives form products such as acrylic fibers, glasses, paints, resins and polymers. Except in China, the use of acetylene as a chemical feedstock declined by 70% from 1965 to 2007 due to cost and environmental considerations.
In 1881, Russian chemist Mikhail Kucherov described the hydration of acetylene to acetaldehyde using catalysts such as mercury (II) bromide. Before the advent of the Walker process, this reaction was carried out on an industrial scale.
Acetylene polymerization with Ziegler-Natta catalysts produces polyethylene layers. Polystyrene, a chain of CH centers with alternating single and double bonds, was one of the first organic semiconductors discovered. Its reaction with iodine produces a material with high electrical conductivity. Although such materials are not useful, these discoveries led to the development of organic semiconductors, which were awarded the 2000 Nobel Prize in Chemistry to Alan J. Heger, Alan J. McDiarmid and Hideki Shirakawa were recognized.
Safety and health
Gaseous acetylene is not toxic, but when produced from calcium carbide, it can contain toxic impurities such as traces of phosphine and arsine, which give it a distinctive garlic-like odor. It is also highly flammable like most light hydrocarbons, hence its use in welding. Its unique hazard relates to its inherent instability, especially when under pressure: under certain conditions, acetylene can react in an exothermic addition-type reaction, yielding a number of products, usually benzene and/or vinyl acetylene, possibly in addition to carbon and hydrogen. To form consequently, acetylene, if initiated by extreme heat or a shock wave, can explode explosively if the absolute pressure of the gas exceeds about 200 kPa (29 psi). Most pressure regulators and gauges on equipment report gauge pressure, so the safe limit for acetylene is 101 kPagage, or 15 psig. Therefore, it is supplied and stored as a solution in acetone or dimethylformamide (DMF), contained in a gas cylinder with a porous filler (Agamassan), which makes it safe to transport and use due to proper handling. Acetylene cylinders should be used in an upright position to prevent acetone from escaping during use.
Acetylene production methods:
Two basic conversion processes are used to make acetylene. One is a chemical reaction process that takes place at normal temperature. The other is a thermal cracking process that occurs at very high temperatures.
Chemical reaction process
Acetylene may be formed by a chemical reaction between calcium carbide and water. This reaction produces a significant amount of heat, which must be removed to prevent the explosion of acetylene gas. There are different types of this process in which either calcium carbide is added to water or water is added to calcium carbide. Both of these changes are called wet processes because the excess amount of water is used to absorb the heat of reaction. The third type, called the dry process, uses only a limited amount of water, which then evaporates by absorbing heat.
1. Most high-capacity acetylene generators use a rotating screw conveyor to feed calcium carbide granules into the reaction chamber, which is partially filled with water. The granules are about 0.08 x 0.25 inches (2 mm x 6 mm) in size, which provides an adequate amount of exposed surface area to allow a complete reaction. The feeding rate is determined based on the desired gas flow rate and is controlled by a pressure switch in the chamber. If too much gas is produced at one time, the pressure switch opens and reduces the feed rate.
2. To ensure a complete reaction, the solution of calcium carbide granules and water is continuously stirred by a set of rotating paddles inside the reaction chamber. This also prevents any granules from floating to the surface, which could overheat and ignite the acetylene.
3. Acetylene gas bubbles to the surface and pulls out under low pressure. When it leaves the reaction chamber, the gas is cooled by water spray. This water spray also adds water to the reaction chamber to continue the reaction with the addition of new calcium carbide. After the gas cools, it passes through an arrester that prevents accidental ignition from equipment downstream of the chamber.
4. When calcium carbide reacts with water, it forms calcium carbonate slurry that sinks to the bottom of the chamber. Periodically, the reaction must be stopped to remove the formed slurry. Slurry is drained from the chamber and pumped to a pond, where calcium carbonate settles and water is removed. The concentrated calcium carbonate is then dried and sold for use as an industrial wastewater treatment agent, acid neutralizer, or soil softener for road construction.
Thermal cracking process
Acetylene may also be produced by heating various hydrocarbons to the point where their atomic bonds break or crack, in a process known as thermal cracking. After the hydrocarbon atoms are separated, they can be recombined to create different materials than the original starting materials. This process is widely used to convert oil or natural gas into various chemicals.
This process has many variations depending on the raw materials used and the method of increasing the temperature. Some cracking processes use an electric arc to heat the feedstock, while others use a combustion chamber that burns off some of the hydrocarbons to create a flame. Some acetylene is produced as a co-product of the steam cracking process used to make ethylene.
1. Natural gas, which is mostly methane, is heated to about 1200 degrees Fahrenheit (650 degrees Celsius). Preheating the gas makes it self-combust after reaching the burner and requires less oxygen for combustion.
2. The heated gas passes through a narrow tube called a venturi, where oxygen is injected and mixed with the hot gas.
3. The mixture of hot gas and oxygen passes through a diffuser that reduces its speed to the desired speed. If the speed is too high, the gas inlet will extinguish the flame in the burner. If the speed is too low, the flame can blow back and ignite the gas before it reaches the burner.
4. The gas mixture flows to the burner block, which contains more than 100 narrow channels. As the gas flows into each channel, it self-ignites, producing a flame that raises the gas temperature to about 2,730 degrees Fahrenheit (1,500 degrees Celsius). A small amount of oxygen is added to the burner to stabilize the combustion.
5. The burning gas flows into the reaction space just beyond the burner, where the high temperature causes about one-third of the methane to convert to acetylene, while most of the rest burns off. The entire combustion process takes only a few milliseconds.
6. Flaming gas is quickly extinguished with water sprays at the point where it is most converted to acetylene. The cooled gas contains large amounts of carbon monoxide and hydrogen, with smaller amounts of carbon soot, plus carbon dioxide, acetylene, methane, and other gases.
7. The gas passes through a water scrubber, which removes a large amount of carbon soot. Then the gas passes through the second scrubber, where it is sprayed with a solvent called N-methylpyrrolidinone, which absorbs acetylene, but does not absorb other gases.
8. The solvent is pumped to a separation tower where the acetylene is boiled off from the solvent and drawn off as a gas at the top of the tower, while the solvent is drawn off at the bottom.
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