1. The manufacture of ethene:<br />Ethene, commonly known as ethylene, is an organic chemical that is largely manufactured in industries today. It provides as a basis, a raw material for the production of many synthetic substances such as organic chemicals and familiar household objects. Items such as milk cartons and plastic bins comprise of a long chain of numerous ethene molecules joined together and is known as the polymer polyethene. As for the manufacture of ethene, ethene can be extracted from crude oil and natural gas. The composition of natural gas and crude oil are summarized in the table below. <br />Table 14. Natural Gas and Crude Oil - composition<br />Natural GasCrude OilMain constituent is methane CH4Mainly comprised of alkanesSmall amount of ethane present (<10%)Consists of a large range of alkanes (CH4 – C70H142) with numerous isomersCan contain smaller amounts of other compounds and alkanes such as N2, CO2, H2S, propane, butane and pentane Small amounts of aromatic hydrocarbons (benzene and toluene) and organic compounds that contain sulfur, oxygen or nitrogen atoms are also present.<br />Hydrocarbons exist in both natural gas and crude oil, however ethene is found in neither. Therefore in petroleum refineries, the production of ethene results from the chemical change and the physical separation of the two raw materials. This is done through fractional distillation.<br />2286001448435The alkanes in natural gas/crude oil undergo thermal (or steam) cracking to produce unsaturated hydrocarbons and other products. The ‘feedstock’, which is either ethane, propane, naphtha, or gas-oil fractions, along with steam, is passed through a metal coiled tubing in a furnace at a temperature range of 750 – 900 oC. The gases (ethene and H2/CH4) are then liquefied in the cooler at a temperature of -100 oC and then separated by fractional distillation resulting in high purity ethene. This process is illustrated by the diagram below (figure 1).<br />Figure 14. Thermal cracker – production of unsaturated hydrocarbons.<br />Reactions for the cracking of ethane and propane are shown below.<br />C2H6 (g) C2H4 (g) + H2 (g) ∆H = + 138 KJ/mol<br />C3H8 (g) C2H4 (g) + CH4 (g) ∆H = + 81 KJ/mol<br />Using Le Chatelier’s principle it can be said that the formation of products (ethene) is favoured at high temperatures, since the reaction is endothermic, and is favoured at low pressure, as there are more molecules of gas on the product side than the reactant side. However, the rate of reaction decreases as the pressure decreases, therefore the reaction is carried out at a pressure just below atmospheric pressure and at a high temperature.<br />Ethene can also be obtained from the dehydration of ethanol, which is denoted by the equation below, although the opposite reaction is usually used more in industries, the hydration of ethene to produce ethanol. As the name suggests, water is removed from the ethanol molecule, OH and H atoms are removed and the two free bonds then join together to form ethene. The synthesis of ethene from ethanol occurs when the alcohol is heated at 170 degrees in the presence of a catalyst. Catalysts that are commonly used for this reaction include concentrated acids (e.g. sulfuric acid) and zeolite catalyst, although compounds like aluminium oxide can also be used. <br /> H2SO4<br />C2H5OH (l) ==> C2H4 (g) + H2O (g)<br />Cellulose, a raw material that is widely available, can contribute to the petrochemical industry and be used to produce plastics. Ethene can be manufactured by first converting the cellulose to glucose which is converted to ethanol which is then transformed into ethene. However this procedure is so far impractical as the energy cost from the many chemical conversion would be very high. <br />