10.5 Ethylene and propylene 10.5.1 Ethylene to form a s bond connecting the two carbons. The two unhybridized 2p orbitals – one from each carbon – Properties overlap to give a p molecular orbital. Therefore, the double bond in ethylene is composed of a s ϭ Ethylene (H2C CH2) is the largest volume component and a p component. building block for many petrochemicals and end Based on the orbital theory of molecules, 2p products such as plastics, resins, fibres, etc. The orbitals overlap to give p and p* orbitals; in ethylene, IUPAC (International Union of Pure and Applied however, only the p orbital is occupied at normal Chemistry) name is ethene. conditions. Electrons in the p bond are held less tightly and more easily polarized than electrons in a s Physical properties bond. The carbon-carbon double bond (sϩp) energy Ethylene is a colourless, flammable gas with a is 611 kJ/mol, which is less than twice the CϪC bond slight odour. Table 1 summarizes its physical, (s) dissociation energy of 736 kJ/mol found in ethane. thermodynamic and transport properties; additional The CϪH bond dissociation energy is 451 kJ/mol values are available in many references (Harrison and and the approximate acidity as measured by the Ϫ45 Douslin, 1971; Starling, 1973; Bonscher et al., 1974; dissociation constant Ka is 10 . Ethylene reacts with Vargaftik, 1975; Douslin and Harrison, 1976; TRC, electrophilic reagents like strong acids (Hϩ), halogens, Thermodynamics Research Center, 1986; Jacobsen, and oxidizing agents, but not with nucleophilic 1988). reagents such as Grignard reagents and bases. For the fundamental mechanisms of these reactions, consult Chemical properties the following references (Sykes, 1975; Carey, 1987). Ethylene is a very reactive intermediate and, Some important reactions are discussed below. Other therefore, is involved in many chemical reactions. The reactions not included in the following overview are chemistry of ethylene is based mainly around its primarily of academic interest and comprehensive double bond, which reacts readily to form saturated discussions are provided in various references (Miller, hydrocarbons, their derivatives and polymers. It is a 1969; Kniel et al., 1980). planar molecule with a carbon-carbon bond distance of 1.34 Å, which is shorter than the CϪC bond Polymerization (s bond) length of 1.53 Å found in ethane, a saturated Polymerization is one of the main reactions of molecule. ethylene, and polyethylene ranks as its major ϭ Ϫ᭤ ᎏ ᎏ polymer: nCH2 CH2 ( CH2CH2 )n. Very H H high-purity ethylene (Ͼ99.9%) is polymerized under CC specific conditions of temperature and pressure in H H the presence of an initiator or catalyst. This is an In ethylene, the carbon is in its sp2-hybridized exothermic reaction, and both homogeneous (radical state. Each carbon uses two of its sp2-hybridized or cationic) and heterogeneous (solid catalyst) orbitals to form s bonds with two hydrogen atoms. The initiators are used (Miller, 1969; Reichert and remaining sp2 orbitals – one on each carbon – overlap Geiseler, 1983; Ulrich, 1988). The products range VOLUME II / REFINING AND PETROCHEMICALS 551 BULK PRODUCTS AND PRODUCTION LINES IN THE PETROCHEMICAL INDUSTRY Table 1. Physical properties of ethylene Property Value Molecular weight, u 28.0536 Triple point Temperature, °C –169.164 Pressure, kPa 0.12252 Latent heat of fusion, kJ/mol 3.353 Normal freezing point Temperature, °C –169.15 Latent heat of fusion, kJ/mol 3.353 Normal boiling point Temperature, °C –103.71 Latent heat of vaporization, kJ/mol 13.548 Density of liquid mol/l 20.27 Ϫ104 d4 0.566 Specific heat of liquid, J/mol·K 67.4 Viscosity of the liquid, mPa·s (=cP) 0.161 Surface tension of the liquid, mN/m (=dyn/cm) 16.4 Specific heat of ideal gas at 25°C, J/mol·K 42.84 Critical point Temperature, °C 9.194 Pressure, kPa 5,040.8 Density, mol/l 7.635 Compressibility factor 0.2812 Gross heat of combustion at 25°C, MJ/mol 1.411 Limits of flammability at atmospheric pressure and 25°C Lower limit in air, mol% 2.7 Upper limit in air, mol% 36.0 Auto ignition temperature in air at atmospheric pressure, °C 490 Pitzer’s acentric factor 0.278 Dipole moment, D 0.0 Standard enthalpy of formation at 25°C, kJ/mol 52.3 Standard Gibbs energy of formation at 25°C for ideal gas at atmospheric pressure, kJ/mol 68.26 Solubility in water at 0°C and 101 kPa, ml/ml H2O 0.226 Speed of sound at 0°C and 409.681 kPa, m/s 224.979 Standard entropy of formation, J/mol·K 219.28 Standard heat capacity, J/mol·K 42.86 from a few hundred to a few million atomic mass to 350°C. These produce Low-Density unit in molecular weight. PolyEthylene (LDPE), a highly branched polymer Four types of basic reaction systems are of with densities from 0.91 to 0.94 g/cm3. commercial importance in the production • Low-pressure (0.1-20 MPa) polymerization at of polyethylene: temperatures of 50 to 300°C using heterogeneous • High-pressure (60-350 MPa) free radical catalysts such as molybdenum oxide or chromium polymerization using oxygen, peroxide or other oxide supported on inorganic carriers. These are strong oxidizers as initiators at temperatures of up used to produce High-Density PolyEthylene 552 ENCYCLOPAEDIA OF HYDROCARBONS ETHYLENE AND PROPYLENE (HDPE), which is more linear in nature, with Addition densities of 0.94 to 0.97 g/cm3. Many addition reactions with ethylene are • Low-pressure polymerization via ionic catalysts, important in the chemical industry. using Ziegler catalysts (aluminum alkyls and Halogenation-hydrohalogenation is used to produce titanium halides). various halides of ethylene, such as ethylene • Low-pressure polymerization with Ziegler catalysts dichloride, which is further cracked to produce supported on inorganic carriers. vinyl chloride, the monomer required for the A notable development in ethylene polymerization production of polyvinyl chloride (PVC): is the simplified low-pressure LDPE process. The CH ϭCH ϩCl Ϫ᭤ ClCH CH Cl pressure range is 0.7-2.1 MPa with temperatures less 2 2 2 2 2 than 100°C. The reaction takes place in the gas phase Vinyl chloride is obtained by the instead of the liquid phase as in the conventional dehydrochlorination of 1,2-dichloroethane in the gas LDPE technology. These new technologies require phase (500-600°C and 2.5-3.5 MPa): ultra-high-purity ethylene and many can use ClCH CH ClϪ᭤ CH ϭCHClϩHCl metallocene catalysts (Bennett, 1999). The physical 2 2 2 properties of the polymers can be modified by Oxychlorination of ethylene is carried out in a copolymerizing ethylene with other chemicals like fixed or fluidized bed at 220°C, with a suitable solid higher olefins, maleic anhydride, etc. Generally, chloride catalyst: linearity provides strength, and branching provides 2CH ϭCH ϩO ϩ4HClϪ᭤2ClCH CH Clϩ2H O toughness to the polymer. 2 2 2 2 2 2 Trichloroethylene and tetrachloroethylene are Oxidation important organic solvents that are produced by the Oxidizing ethylene produces ethylene oxide: further chlorination of 1,2-dichloroethylene in the gas CH ϭCH ϩ0.5O Ϫ᭤ CH CH phase, with the simultaneous dehydrochlorination in 2 2 2 2 2 the presence of a suitable chloride catalyst. O Oligomerization is used to produce a-olefins and The reaction is carried out over a supported linear primary alcohols. Hydration of ethylene metallic silver catalyst at 250-300°C and 1-2 MPa. produces ethanol. To produce ethylene glycol, ethylene oxide is Ethylbenzene, the precursor of styrene, is produced further reacted with ethylene in the presence of excess from benzene and ethylene. The ethylation of benzene water and an acidic catalyst at low temperatures is carried out in several different ways. In the older (50-70°C), followed by hydrolysis at relatively high technologies, the reaction is conducted in the liquid temperatures (140-230°C) and moderate pressures phase in the presence of a Friedel-Crafts catalyst (2-4 MPa). At low water concentration, polyethylene (AlCl3, BF3, FeCl3). The new processes all use zeolite glycol is obtained. catalysts. ABB Lummus Global and UOP (Universal Acetaldehyde can be obtained by the Wacker Oil Products) commercialized a process for liquid process in which a homogeneous CuCl2/PdCl2 system phase alkylation based on a zeolite catalyst (Horigome is used for the oxidation: et al., 1991). Badger and Mobil offer a similar process and also have a vapour phase alkylation process using CH ϭCH ϩ0.5O Ϫ᭤ CH CHO 2 2 2 3 zeolite catalysts (Lewis and Dwyer, 1977). A process The reaction is carried out in a bubble column at based on a catalytic distillation reactor also has been 120-130°C and 0.3 MPa. Palladium chloride is commercialized using zeolites (Ercan et al., 1998). reduced to palladium during the reaction and then is Almost all ethylbenzene produced is used for the reoxidized by cupric chloride. Oxygen converts the manufacture of styrene, which is obtained by reduced cuprous chloride to cupric chloride. dehydrogenation in the presence of a suitable catalyst Vinyl acetate is obtained by the vapour phase at 550-640°C and relatively low pressures oxidation of ethylene with acetic acid, which is (Lummus Crest, 1988). obtained by oxidation of acetaldehyde: Ethanol is manufactured from ethylene by direct catalytic hydration over a H3PO4/SiO2 catalyst at ϭ ϩ ϩ Ϫ᭤ CH2 CH2 CH3COOH 0.5O2 process conditions of 300°C and 7.0 MPa (diethyl Ϫ᭤ ϭ ϩ CH2 CHOCOCH3 H2O ether is formed as a by-product): CH ϭCH ϩH OϪ᭤ C H OH This process employs a palladium on carbon, 2 2 2 2 5 alumina or silica-alumina catalyst at 175-200°C and Ethylene can also be reacted to form propylene via 0.4 to 1.0 MPa.