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 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 (sp) energy Ethylene is a colourless, flammable gas with a is 611 kJ/mol, which is less than twice the CC bond slight odour. Table 1 summarizes its physical, (s) dissociation energy of 736 kJ/mol found in ethane. thermodynamic and transport properties; additional The CH bond dissociation energy is 451 kJ/mol values are available in many references (Harrison and and the approximate acidity as measured by the 45 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 (H), 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 CC 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 polymer: nCH2 CH2 ( CH2CH2 )n. Very H H high-purity ethylene (99.9%) 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 20.27 104 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 CH Cl 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 Cl CH CHClHCl 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 CH O 4HCl2ClCH CH Cl2H 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 CH 0.5O 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 CH 0.5O 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 CH2 CH2 CH3COOH 0.5O2 process conditions of 300°C and 7.0 MPa (diethyl CH2 CHOCOCH3 H2O ether is formed as a by-product): CH CH H O 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. the metathesis of butene and ethylene (see below). The

VOLUME II / REFINING AND PETROCHEMICALS 553 BULK PRODUCTS AND PRODUCTION LINES IN THE PETROCHEMICAL INDUSTRY

butenes can be taken from steam cracker effluent or a involving radical and molecular species requires special refinery Fluid Catalytic Cracking (FCC) unit. Ethylene integration algorithms (Gear, 1971). To overcome these dimerization to butene can also be utilized, giving a numerical difficulties, various approximate methods direct conversion route of ethylene to propylene. This have been introduced in the past, such as pseudo route is expected to become more prevalent as the steady-state approximation for radicals (Semenov, 1959; propylene demand grows and ethylene production Boudart, 1968), and the errors associated with such from ethane pyrolysis becomes more common. techniques have been discussed by Sundaram and Froment (1978a). With modern computing power, such Biological properties approximate methods are no longer required. Ethylene is slightly more potent as an anesthetic Thermal cracking is a complex reaction and than nitrous oxide, but the smell of ethylene causes involves many thousands of chain reactions even for choking; therefore, it is no longer used as an anesthetic simple ethane cracking (Sundaram and Froment, agent. Diffusion through the alveolar membrane is 1978b; Dente and Ranzi, 1983); however, the reactions sufficiently rapid for equilibrium to be established can be classified into several known groups (Table 2). between the alveolar and the pulmonary capillary During initiation, two radicals are produced for blood with a single exposure. Ethylene is held in both each paraffin molecule. For example: cells and plasma, in simple physical solution. The C H 2CH lipoid stroma of the red blood cells absorbs ethylene, 2 6 3 but it does not combine with hemoglobin. It is Only a small fraction of reactant is involved in this eliminated from the body unchanged – primarily by step. When naphthenes are involved, diradicals are the lungs – and most elimination is complete within produced. For aromatics with side chains, ·H radicals three minutes of administration. are produced. Typically, this is the rate-controlling step Ethylene has been used in the controlled ripening under normal commercial operating conditions. of various fruits and vegetables since the 1930s. It In propagation, many types of reactions are causes the bleaching of green tissue, gives rise to foliar involved, including hydrogen abstraction, methyl abscission, suppresses certain types of dormancy, and abstraction, radical addition, radical decomposition promotes cellular swelling. For further information on and radical isomerization. this subject, consult references (Miller, 1969). In hydrogen abstraction, a hydrogen radical reacts with a molecule (primarily a paraffin) and produces a Manufacture via thermal cracking hydrogen molecule and a radical. In methyl abstraction, a methyl radical reacts to produce a Thermal cracking of hydrocarbons is the major radical and methane. Similar reactions with other route for the industrial production of ethylene. The radicals (ethyl and propyl) can also occur. chemistry and engineering of thermal cracking has In radical addition, some radicals like H, CH3, been reviewed by Kniel et al. (1980), Froment (1981), etc., are added to olefins (or diolefins) to form heavier Albright et al. (1983), Raseev (2003), and also in an radicals. Radical decomposition is one of the most earlier review by Sundaram et al. (1994). In thermal important types of reactions. In this case, a larger cracking, valuable by-products including propylene, radical decomposes to an olefin and a smaller radical. butadiene, and benzene are produced. Less valuable Radicals usually decompose at the beta-position of the methane and fuel oil are also produced in significant radical centre where the CC bond is the weakest. In quantities. An important parameter in the design of the case of naphthenes and aromatics, this may not be commercial thermal cracking furnaces is the the case and a CH bond may be the weakest. selectivity to produce the desired products. Finally, radical isomerization frequently occurs for large radicals and explains to a large extent the Mechanism, kinetics, conversion observed product distribution (i.e. many isomers). The thermal cracking of hydrocarbons proceeds via Radical termination is the reverse of initiation. a free-radical mechanism as proposed by Rice (1931). In addition to radical reactions, molecular and Since that discovery, many reaction schemes have been surface reactions also occur. proposed for various hydrocarbon feeds (Allara and According to Laidler (1965), radicals are classified Edelson, 1975; Sundaram and Froment, 1978b; Allara as b and m radicals. b radicals (e.g. H, CH3) undergo and Shaw, 1980; Dente and Ranzi, 1983; Willems and only addition reactions and do not decompose. m radicals Froment, 1988a, 1988b; Depeyre et al., 1989). Since (e.g. C4H9) undergo mainly decomposition. Some radicals are neutral species with a short life, their radicals, such as C2H5, can act as both b and m radicals. concentrations under reaction conditions are extremely The kinetics of thermal cracking is not simple and small. Therefore, the integration of continuity equations involves a series of elementary reactions. The order of

554 ENCYCLOPAEDIA OF HYDROCARBONS ETHYLENE AND PROPYLENE

Table 2. Examples of reactions occurring in thermal cracking

Frequency factor* Activation energy (thousands of t) (kJ/mol) A) Initiation 16 C2H6 2 CH3 4.0 10 366.1 B) Propagation H-abstraction 11 C2H6 H C2H5 H2 1.0·10 40.6 Methyl abstraction C2H6 CH3 C2H5 CH4 Radical addition 10 C2H4 H C2H5 1.0·10 6.3 Radical decomposition 13 C2H5 C2H4 H 3.2·10 167.4 Radical isomerization 14 1- C4H9 2- C4H9 5.2·10 171.5 C) Termination 8 2 C2H5 n-C4H10 4.0·10 0 D) Molecular reactions 7 C2H4 C4H6 cyC6H10 3.0·10 125.5 E) Surface reactions 9 C2H3 T C2H2 H T 2.0·10 131.8 T= third body collision molecule * For first order reactions the unit is s1 and for second order reactions l·mol1s1. These are typical values

11111212111111111component inletcomponent outlet the elementary reactions almost follows molecularity. X component inlet Energetically active small radicals, like H and C2H3, may involve a third body collision. Based on the main where inlet and outlet quantities are measured in initiation, propagation, and termination reactions, one weight units. can deduce the overall order of the reaction for the When a mixture is cracked, one or more decomposition of simple molecules like ethane components in the feed may also be formed as (Laidler, 1965). products. For example, in the co-cracking of ethane Most paraffin decomposition follows a first order and propane, ethane is formed as a product of rate-of-reaction, while olefin decomposition follows a propane cracking and propane is formed as a higher order rate-of-reaction. With the advent of product of ethane cracking. Therefore, the outlet modern computers, kinetic models for thermal term in the above equation contains the contribution cracking of hydrocarbons involving a few hundred to of formation from other feed components and thus a few thousand reactions and their mass and energy does not represent a true conversion. For simple balance equations can be solved in a few minutes. The mixtures, the product formation can be accounted following references provide a detailed review on this for and approximate true conversions can be topic (Allara and Edelson, 1975; Sundaram and calculated (Sundaram and Fernandez-Baujin, 1988). Froment, 1978b; Dente et al., 1979; Allara and Shaw, For liquid feeds like naphtha, it is impractical, if not 1980; Dente and Ranzi, 1983; Willems and Froment, impossible, to calculate the true conversion. Based 1988a, 1988b; Depeyre et al., 1989; Froment, 1992). on measured feed components, one can calculate a – The kinetic parameters of typical reactions occurring weighted average conversion (X ) as proposed by van in thermal cracking are given in Table 2. Camp et al. (1985): The terms conversion or severity are used to – X W X measure the extent of cracking. Conversion (X) can be i i th easily measured for a single component feed: where Xi is the conversion for the i feed component

VOLUME II / REFINING AND PETROCHEMICALS 555 BULK PRODUCTS AND PRODUCTION LINES IN THE PETROCHEMICAL INDUSTRY

and Wi is the weighting factor (weight or mol fraction resonance spectroscopy has been used to characterize usually). heavy feedstocks. Designers have employed other practical methods such as a key component conversion (e.g. n-pentane), Commercial furnaces kinetic severity factor (Zdonik and Green, 1970), or Thermal cracking of hydrocarbons is molecular collision parameter (Lohr and Schwab, accomplished in tubular reactors commonly known 1979) to represent severity. Alternatively, molecular as cracking furnaces, crackers, cracking heaters, weight of the complete product distribution has been etc. Several engineering contractors, including used to define conversion for liquid feeds: ABB Lummus Global, KBR, Linde, Stone and Webster (a Shaw Group Company), and Technip 23223Mf 1 offer cracking furnace technology. Two cracking M X1111e furnaces may share a common stack, and the height of the heater may vary from 30 to 50 m. 23223Mf 1 Before the 1960s, cracking tubes were arranged in 24.5 horizontal rows in a radiant chamber, leading to In the equation, Mf and Me are the molecular low ethylene capacity ( 20,000 mta, metric tonnes weight of the feed and of the dry (steam-free) effluent per annum). Modern designs use tubes arranged in respectively. Instead of molecular weight, hydrogen vertical rows, providing superior mechanical

content in the C5 product is also used: performance and higher capacity. Today, the capacity of a single cell furnace is well over 1111Y1 X 150,000 mta. Fig. 1 provides a sketch of a typical CY1 furnace. In the equation, Y=(H6)/(HF6), where HF is feed hydrogen content in wt%, H is hydrogen content Reaction

in the C5 product in wt%, and C is a constant for any The reaction proceeds in the pyrolysis coils of given feed. the radiant section of the furnace. Since coke is Instead of conversion, some producers prefer to use also formed during pyrolysis, steam is added as other identifications of severity, including coil outlet diluent to the feed. The steam minimizes the side temperature, propylene-to-methane ratio, propylene- reaction forming coke and also improves the to-ethylene ratio, or cracking severity index (Ross and selectivity to olefins by lowering the hydrocarbon Shu, 1979). Of course, all these definitions are partial pressure. The temperature of the somewhat dependent on feed properties and most are hydrocarbon and steam mixture entering the also dependent on the operating conditions. radiant chamber (known as the crossover When simple liquids like naphtha are cracked, temperature) is 500 to 700°C. Lower temperatures it may be possible to determine the feed are used for heavy feeds like Atmospheric Gas Oil components by Gas Chromatography combined (AGO) and Vacuum Gas Oils (VGOs) to minimize with Mass Spectrometry (GC-MS; van Camp et coking in the convection section, and higher al., 1985); however, when gas oil is cracked, temperatures are used for light gases like ethane complete analysis of the feed may not be possible. and propane, which are more refractory. Some Therefore, some simple definitions are used to incipient cracking can start as low as 400°C. characterize the feed. When available, paraffins, However, for light gases incipient conversion is olefins, naphthenes, and aromatics (PONA) quite low. Depending upon the residence time in content serves as a key property. When PONA is the radiant coil and the required feed severity, the not available, the Bureau of Mines Correlation coil outlet temperature is typically maintained Index (BMCI) is used: between 775 and 950°C. The combination of low residence time and low 111148,640 BMCI 473.7g456.8 hydrocarbon partial pressure produces high MABP selectivity to olefins at a constant feed conversion. where MABP is the Molal Average Boiling Point In the 1960s, the residence time was 0.5 to 0.8 s; (expressed in K) and g is the specific gravity of the by the 1990s, the residence time was typically 0.1 feed. to 0.2 s. Typical pyrolysis heater radiant coil Other properties like specific gravity, ASTM characteristics are given in Table 3. The typical distillation, viscosity, refractive index, Conradson temperature, pressure, conversion, and residence carbon, and bromine number are also used to time profiles across the reactor for naphtha characterize the feed. Even nuclear magnetic cracking are illustrated in Fig. 2.

556 ENCYCLOPAEDIA OF HYDROCARBONS ETHYLENE AND PROPYLENE

13 1- stack 1 2- induced draft fan 2 14 3- upper preheat coil 4- BFW preheat 5- SHP steam superheat coil 6- desuperheater BFW injection 3 7- lower mixed preheat coil 8- radiant coil 12 16 9- floor burner BFW 4 hydrocarbon feed 10- wall burner dilution 11- crossover manifold 5 steam 12- primary TLE 13- SHP saturated steam line SHP 14- steam drum steam 15 15- transfer line valve 6 16- secondary TLE 7 11 17- decoke valve 17 18- decoke bypass valve convection 19- decoke pot section 10 BFW: Boiler Feed Water SHP: Super High Pressure 18

gaseous radiant 19 fuel section 8 9

Fig. 1. Typical heater configuration.

Cracking reactions are endothermic (1,800 to exchanger is used to generate additional steam and 2,800 kJ/kg of ethylene produced) with heat sometimes to preheat the feed and dilution steam supplied by firing fuel gas and/or fuel oil in mixture. The outlet gas temperature from the TLE sidewall or floor burners. Sidewall burners usually varies from 350 to 650°C, depending upon the give uniform heat distribution, but the capacity of feedstock and the design. If the reaction mixture is each burner is limited (0.1-1 MW) and hence 40 to not cooled quickly, olefin selectivity is reduced due 200 burners are required in a single furnace. With to the many side reactions taking place in this modern floor burners, also called hearth burners, uniform heat flux distribution can be obtained for coils as high as 13.1 m, which are used extensively Table 3. Pyrolysis heater radiant coil characteristics in newer designs. The capacity of these burners is (single heater range) considerably large (1-4 MW) and thus only a few burners are required. The selection of burners Number of coils 2-176 depends on the type of fuel (gas and/or liquid), the Coil length, m 9-80 source of combustion air (ambient, preheated or

gas turbine exhaust), and the required NOx levels. Inside coil diameter, mm 30-200 The reaction mixture exiting the furnace is Process gas outlet temperature, °C 750-950 quickly cooled in quench coolers called Transfer Line Exchangers (TLEs). In earlier designs, direct Clean coil metal temperature, °C 900-1,080 quenching (spraying water or oil) was used for Maximum metal temperature, °C 1,040-1,150 most liquid feeds. Today, almost all designs employ indirect quenching, which generates valuable Average heat absorption, kW/m2 high-pressure steam. Direct quenching is used external area 50-120 only, in some designs, for very heavy feeds. Bulk residence time, s 0.1-0.6 Single-stage or two-stage cooling is used to achieve the desired degree of cooling in the TLE. Coil outlet pressure, kPa 150-275 In the first stage, the process gas is cooled in a Clean coil pressure drop, kPa 10-200 double pipe exchanger or in a shell and tube exchanger. In the second stage, a shell and tube Ethylene capacity, mta 20,000-250,000

VOLUME II / REFINING AND PETROCHEMICALS 557 BULK PRODUCTS AND PRODUCTION LINES IN THE PETROCHEMICAL INDUSTRY C) ° Fig. 2. Conversion, pressure, 0.20 100 1,100 250 temperature and residence max wall T time along the reactor 80 1,000 230 length for naphtha 0.15 cracking. 60 pressure 900 210 gas T 0.10 40 800 190 pressure (kPa) conversion (%) conversion temperature ( 0.05 residence time (s) 20 residence time 700 170 conversion 0 0 600 150 0 20406080100 coil length (%)

zone. After the TLE, further cooling is achieved by upon the plant capacity, gas turbines (15 to 70 directly quenching the furnace effluent with MW) are used to generate electric power. These quench oil. The cooled furnace effluent then turbines usually burn fuel gas with over 200% proceeds to the recovery section for further excess air. Therefore, the exhaust gas is not only separation. hot, but also rich in oxygen. Instead of directly generating steam from the exhaust gas by using Thermal efficiency waste heat boilers, the gas is fed to the cracking Since only a percentage between 35 and 50% of heaters as a source of combustion oxygen. fired duty is absorbed in the radiant section, the Typically, the exhaust gas temperature is 400 to flue gas leaving the radiant chamber contains 590°C with an oxygen content of 14 mol% or considerable energy, which can be extracted more. Typical energy savings of 10 to 30% are efficiently in the convection section of the furnace. reported (Albano et al., 1991; Cooke and Parizot, In the convection section, the feed is preheated 1991). Using hot combustion air requires special along with dilution steam to the desired crossover ducting and hence the investment cost of the heater temperature. Residual heat is recovered by is slightly higher. generating steam. The overall thermal efficiency of To reduce fuel consumption, air preheat has modern furnaces exceeds 93%, and a value of 95% been used in some plants. Flue gas leaving the is not uncommon. Modern heaters generate furnace stack passes through an air preheater and super-high pressure steam (11 MPa) compared to the preheated air is supplied to the burners. By old generation heaters, which produced 4-6 MPa of using mostly hearth burners, the ductwork and the steam. Since the steam produced in the heaters is investment cost can be minimized with air preheat used to drive turbines in the recovery section, and gas turbine exhaust. It is also possible to super-high pressure steam is preferred due to operate with 100% wall fired furnaces, which has higher efficiency. been proven in commercial operation (Albano The convection section is a series of cross flow et al., 1991). Economizers also have been used to exchangers with flue gas on one side and process increase the thermal efficiency. fluids on the tube side. Since mainly gas-to-gas heat transfer is involved, fin tubes are employed where Environment practical to improve the heat transfer rate. The Stringent environmental laws require that the

metallurgy of the tubes varies from carbon steel to emission of nitrogen oxides (NOx) and sulphur high temperature alloy depending on the service. oxides from furnaces be drastically reduced. In

When high overall efficiency is desired, condensation many parts of the world, regulations require NOx of acidic flue gases must be taken into account in the levels of 70 vol ppm or lower on a wet basis. selection of materials. Fouling of heat transfer surfaces Conventional burners usually produce 100 to 150

both inside and outside is unavoidable. Outside fouling vol ppm of NOx. Many burner vendors now supply is cleaned by steam lancing and inside fouling is low NOx and ultra-low NOx burners ( 40 ppm). usually cleaned by burning with air (and steam) or by Since NOx production depends upon the flame mechanical methods. temperature and quantity of excess air, meeting the In new plant designs, cogeneration of electricity required limits may not be possible through burner and steam is economically attractive. Depending design alone. Many new designs incorporate

558 ENCYCLOPAEDIA OF HYDROCARBONS ETHYLENE AND PROPYLENE

DeNOx units that employ catalytic methods to Kinetic models used for designs reduce the NOx level. Platinum-containing Due to many free radical and molecular monolithic catalysts are used (Boer et al., 1990). reactions, simplified kinetics was used in the past. Each catalyst performs optimally for a specific This is no longer necessary with modern temperature range and most of them work properly computing power. Laidler (1965) has generalized at around 400°C. Both the American Institute of the reaction order for overall feed decomposition Chemical Engineers (AIChE) and the European based on simple reactions for alkanes. Many Ethylene Producers Association hold regular researchers have correlated the overall meetings and the proceedings contain the current decomposition as an nth order reaction with most status of ethylene production and government paraffin molecules following the first order and regulations. A recent handbook published by John most olefin molecules following a higher order. Zink Company (Baukal Jr., 2001) provides the Fig. 3 shows the first-order rate constant for fundamentals of burner design and controlling paraffins as a function of carbon number. In emissions in a fired heater. general, isoparaffin rate constants are lower than normal paraffin rate constants. Note that the rate Product distribution constants are somewhat dependent upon In addition to ethylene, many by-products are conversion due to inhibition effects. That is, the also formed. Typical product distributions for rate constant often decreases with increasing various feeds from a typical short-residence-time conversion and the reaction order is not affected furnace are shown in Table 4. The product significantly. This has been explained by distribution is strongly influenced by the residence considering the formation of allyl radicals time, the hydrocarbon partial pressure, the (Buekens and Froment, 1968). To predict the steam/oil ratio, and the coil outlet pressure. product distribution, yields are often correlated as Generally, the higher the hydrogen content of feed, a function of conversion or other severity the higher the ethylene yield. Normal paraffins parameters (Fernandez-Baujin and Solomon, produce more ethylene and propylene than 1975). Detailed kinetic models have also been used isoparaffins. Aromatics produce very little ethylene (Ranzi et al., 1994; Tomlin et al., 1995). and propylene. Hydrocracked Vacuum Gas Oil Instead of radical reactions, models based on (HVGO) behaves like naphtha in terms of olefin molecular reactions have been proposed for the production and behaves like vacuum gas oil in cracking of simple alkanes and liquid feeds like terms of fouling characteristics. This feed is the naphtha and gas oil (Hirato and Yosida, 1973; unconverted oil from a hydrocracker, rich in Sundaram and Froment, 1977a, 1977b; Kumar and hydrogen but containing polynuclear aromatics. Kunzru, 1985; Zou et al., 1993). However, the Natural Gas Liquids (NGLs), also known as field validity of these models is limited and cannot be condensates, are cracked in many plants and extrapolated outside the range with confidence. behave almost like a mixture of naphtha and gas oil. The product distribution is similar to that of 30 full range naphtha feed except for coking. Since Z the end point of NGL is not well defined, the D

fouling (or coking) in the convection section and in C ° the TLE is of concern. Table 4 (in the eighth and ninth columns) shows 20 ) at 800 the effect on product distribution of varying 1 steam/oil ratio for a typical naphtha feed. Although this table shows the severity as maximum, it is theoretically possible to further increase the 10 severity and thus increase the ethylene yield.

Increasing the severity above these practical values rate constant (s produces significantly more fuel oil and methane with a severe reduction in propylene yield. The run 0 length of the heater is also significantly reduced. 246810 Beyond a certain severity level, the ethylene yield carbon number drops (after attaining a maximum); operating near Fig. 3. Overall first order reaction rate constants for paraffin or beyond this point results in extremely severe cracking. Z, Zdonik; D, Davis coking. (adapted from Froment, 1981).

VOLUME II / REFINING AND PETROCHEMICALS 559 BULK PRODUCTS AND PRODUCTION LINES IN THE PETROCHEMICAL INDUSTRY

Table 4. Product distribution obtained in a short residence time coil at 172 kPa

Light Full Full Full Light Hydrocracked FEED C H C H n-C H i-C H naphtha range range range atmospheric vacuum 2 6 3 8 4 10 4 10 naphtha naphtha naphtha gas oil gas oil

Specific gravity 0.662 0.726 0.726 0.726 0.8191 0.852 ASTM, °C IBP (Initial Boiling Point) 35.1 37.8 37.8 37.8 185.0 360.0 10 vol% 43.5 76.7 76.7 76.7 215.0 382.2 30 vol% 47.3 105.0 105.0 105.0 241.7 417.2 50 vol% 53.2 133.0 133.0 133.0 266.1 443.9 70 vol% 65.8 157.0 157.0 157.0 290.0 472.2 90 vol% 99.2 180.0 180.0 180.0 316.0 508.9 EBP (End Boiling Point) 148.9 199.0 199.0 199.0 335.0 536.1 BMCI 3.5 12.0 12.0 12.0 23.3 15.6 Paraffins, wt% 100.00 100.00 100.00 100.00 89.60 73.80 73.80 73.80 –– Naphthenes, wt% 7.70 18.00 18.00 18.00 –– Aromatics, wt% 2.70 8.20 8.20 8.20 –– Iso/normal ratio 0.80 1.00 1.00 1.00 –– Molecular weight, u 30.0 44.0 58.0 58.0 81.0 108.0 108.0 108.0 205.0 425.0

Feed H2 20.10 18.29 17.34 17.34 16.00 15.25 15.25 15.25 13.93 14.20 Steam/HC,wt/wt 0.30 0.30 0.40 0.40 0.50 0.50 0.50 0.75 0.75 0.75 Conversion 65% 95% 96% 95% Max Max Max Max Max Max ethylene ethylene propylene ethylene ethylene ethylene Yields, wt%

H2 3.93 1.56 1.17 1.31 1.00 0.91 0.75 0.91 0.63 0.65

CH4 3.82 25.30 21.70 23.80 18.00 15.70 12.60 15.30 11.20 12.60

C2H2 0.43 0.64 0.78 0.90 0.95 0.78 0.43 0.95 0.47 0.33

C2H4 53.00 39.04 39.20 15.50 34.30 30.80 25.50 32.20 26.50 29.00

C2H6 35.00 3.94 3.02 0.55 3.80 3.30 4.30 2.80 3.40 3.70

C3H4 0.06 0.53 1.15 3.55 1.02 1.00 0.56 1.15 0.80 0.95

C3H6 0.89 11.34 15.34 19.30 14.10 14.00 17.00 14.40 13.40 13.10

C3H8 0.17 5.00 0.16 0.33 0.35 0.28 0.45 0.22 0.25 0.24

C4H6 1.19 4.50 4.08 2.70 4.45 4.70 4.50 4.90 5.00 5.00

C4H8 0.18 0.80 1.69 16.15 3.70 3.80 6.50 3.81 3.70 3.40

C4H10 0.22 0.09 4.00 5.00 0.20 0.20 0.80 0.20 0.10 0.07

C5 0.27 1.61 1.38 1.40 2.10 2.93 4.95 3.10 2.75 1.90

C6-C8 non aromatics 0.39 0.31 1.45 0.35 0.80 1.80 6.40 2.20 1.20 1.40 Benzene 0.37 2.74 2.48 4.03 6.40 6.70 4.00 5.95 6.90 7.30 Toluene 0.08 0.67 0.52 1.63 2.30 4.00 3.80 3.90 3.20 3.65 Xylene + ethyl benzene 0.00 0.09 0.20 0.41 0.21 1.30 2.20 1.24 1.30 1.10 Styrene 0.00 0.51 0.23 0.42 0.75 0.82 0.65 0.75 0.79 0.65

C9-205°C 0.00 0.93 0.87 0.86 1.40 1.82 2.16 1.72 2.96 2.90 Fuel oil 0.00 0.40 0.58 1.81 4.17 5.16 2.45 4.30 15.45 12.06

Total 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00

560 ENCYCLOPAEDIA OF HYDROCARBONS ETHYLENE AND PROPYLENE

With the introduction of Gear’s algorithm for that the deposition rate decreases with time and integration of stiff differential equations, the attains a pseudo-steady state value (Sundaram et complete set of continuity equations describing the al., 1981). Though this is achieved in a matter of evolution of radical and molecular species can be minutes in bench scale reactors, it takes a few days solved even with a personal computer. There are in a commercial furnace. many articles dealing with kinetic models based on Initial coke deposition is affected by surface free radical reactions (Rice 1931, 1933; conditions and is commonly known as catalytic Trotman-Dickenson, 1965; Benson, 1968; Allara coke. The steady rate of coking following the and Edelson, 1975; Sundaram and Froment, 1978b; initial catalytic coking is known as thermal coking. Dente and Ranzi, 1983; Dean, 1985; Weast, 1987; However, the influence of surface conditions on Hillewaert et al., 1988; Ranzi et al., 1994; Pant and thermal coking is not yet clear. Kunzru, 1996). Some of them even used a Many inhibitors and additives tend to reduce pseudo-steady state approximation for radical the coking rate in bench scale units, but no concentrations that is not required. In fact, some significant influence has been found in radicals will never reach steady state (Sundaram commercial units. In order to take advantage of and Froment, 1978b). any small benefits, many producers use some sort of inhibitors – mostly sulphur compounds – to Run length suppress coking. Dimethyl disulphide (DMDS) or

Coke is produced as a side product, which other refinery gas containing H2S in the range of deposits on the radiant coil walls. This limits the typically 50 to 300 ppm is used. These heat transfer to the coils and increases the pressure compounds are used to crack gaseous feedstocks drop across the coil. The coke deposition not only and sulphur-deficient liquid feedstocks. Usually, limits the heat transfer, but also reduces the olefin naphtha and gas oil contain more than 100 ppm selectivity. Periodically, the heater must be sulphur, so sulphur is rarely added for these feeds. decoked. Typical run lengths are 15 to 100 days Sulphur addition, at least in ethane cracking, between decokings. Prediction of run length of a reduces CO formation and therefore reduces the commercial furnace is still an art, and various load on methanators in the recovery section. mechanisms are postulated in literature Sulphur is often injected continuously with (Proceedings [...], 1991-2004). Often heater hydrocarbon feed, although some producers prefer maintenance and operation have a more significant to pretreat the reactor with sulphur (usually influence on the run length than any other single higher than 300 ppm) for a few hours before variable in the unit. Coke also deposits in the TLE. injecting hydrocarbon feed, and do not use The mechanisms for coking in radiant coils and sulphur during operation. The beneficial effect of TLEs appear to be different for different feeds. sulphur pretreatment is still subjective. Though From the beginning of 1960s, Lichtenstein steam acts as a diluent, it also suppresses coke (1964) empirically correlated the coking factor of formation (Lee et al., 2004). the radiant coil to operating conditions. In many instances, the tube metal temperature Fernandez-Baujin and Solomon (1975) assumed controls the run length, although it is not the mass transfer of coke precursors from the bulk uncommon that the pressure drop across the coil of the gas to the walls was controlling the rate of – which is equally important for gas cracking – deposition. Goossens et al. (1980), Dente and limits the run length. Two-pass and single-pass Ranzi (1983), Plehiers et al. (1990), Kopinke et al. coils are very sensitive to fireside control (excess (1993a, 1993b), and Wauters and Marin (2002) air) and run length can be limited by the pressure developed kinetic models based on the chemical drop across the coil (or critical flow Venturi reaction at the wall as a controlling step. Bench pressure ratio used for distributing the flow), scale data of Sundaram et al. (1981) and others instead of the tube metal temperature limit. (Newsome and Leftin, 1980; Trimm and Turner, TLE coking is different from radiant coil 1981; Lee et al., 2004) appear to indicate that a cracking. Fernandez-Baujin and Solomon (1975) chemical reaction controls. However, flow regimes claim that condensation of coke precursors of bench scale reactors are so different from the contained in the fuel oil accounts for TLE fouling. commercial furnaces that the scale-up of bench Mass transfer of precursors to the film is assumed scale results cannot be applied confidently to to be the controlling factor. In contrast, Chen and commercial furnaces. For example, the coke Vogel (1973) and Dente et al. (1983, 1990) assume deposited on a controlled cylindrical specimen in a that the chemical reaction is the controlling Continuous Stirred Tank Reactor (CSTR) shows mechanism and have proposed a polymerization

VOLUME II / REFINING AND PETROCHEMICALS 561 BULK PRODUCTS AND PRODUCTION LINES IN THE PETROCHEMICAL INDUSTRY

mechanism similar to the Ziegler-Natta The inside of the convection tubes rarely foul, mechanism. Both models work well within the data but occasionally the unsaturated molecules in the range. Based on commercial experience for liquid feeds will polymerize and stick to the walls, hydrocracked vacuum gas-oil feed (Sundaram and thus reducing the heat transfer. This soft coke is Fernandez-Baujin, 1990), it appears that more than normally removed by mechanical means. In some polymerization, condensation occurs in a TLE. By cases, the coke can also be burned off with air and way of example, the typical TLE outlet gas steam. Normally, the outside surface of the temperatures as a function of time on-stream are convection section fouls due to dust and particles shown in Fig. 4 for various feeds. in the flue gas. Periodically (6 to 36 months), the TLE fouling for gaseous feedstocks is different outside surface is cleaned by steam or air lancing. from that for liquid feedstocks. With gas cracking With liquid fuel firing, the surface may require (especially with ethane), coke deposits on the TLE more frequent cleaning. inlet tubesheet, but does not significantly build up inside the tubes. With time, enough coke builds up Coke suppression technologies that the tubes are partially or fully blocked, Several companies are engaged in finding an reducing the heat transfer surface and giving rise to additive or modifying the radiant coil surface to higher outlet temperatures and high-pressure drop. suppress coke deposition (Tong et al., 1994; This can be minimized with large diameter tubes Brown et al., 1997; Redmond and Bergeron, – an advantage with double pipe exchangers, linear 1999; Magnan et al., 2002). In place of sulphur, exchangers, and quick quench exchangers. or sometimes in addition to sulphur, coke Generally, only a small temperature rise is suppressing additives are added to the observed from start- to end-of-run. hydrocarbon/dilution steam mixture before During the radiant coil decoking, the TLE is entering the coil. It is claimed that these also partially decoked (at least for ethane cracking, additives reduce the CO formation and prolong where most of the coke is deposited on the inlet the run length by a factor of two to four. Some tubesheet). For complete decoking, the furnace is additives have been used successfully in usually cooled down and the TLEs are opened and commercial gas cracking furnaces. The hydrojetted with high-pressure water. In some effectiveness of these additives in liquid cracking cases, the coke in the TLEs can also be burned off has yet to be proven. The cost of these chemicals and hence no mechanical cleaning is required needs to be evaluated considering the benefits of (BASF, 1980; Sliwka, 1981). The radiant coil is longer run lengths. Instead of adding a chemical, always cleaned by burning off the coke with steam some vendors have modified the radiant coil and air. Usually, all coils in a given heater are surface by a suitable coating. This reduces the decoked simultaneously and the effluent during catalytic coking, which is predominant at decoking is sent to a decoking pot or to a firebox start-of-run, and also reduces the adherence of for burning. gas phase coke, a dominant factor after a few C) °

Fig. 4. Typical TLE 650 (Transfer Line Exchanger) outlet temperatures as a function 600 of time on-stream for HVGO, high severity various feedstocks. 550

500

450 full range naphtha, high severity

naphtha, moderate severity 400 n-butane TLE outlet temperature ( ethane 350 steam

300 0501020 30 40 60 time on-stream (days)

562 ENCYCLOPAEDIA OF HYDROCARBONS ETHYLENE AND PROPYLENE

days of operation. Bench scale experiments show the feed notably affects the brazed aluminum as high as a ten-fold reduction in coking rate; exchangers in the recovery section, and arsenic and however, commercial tests show only a two-fold mercury affect the hydrogenation reactors increase in run length for shorter residence time (McPhaul and Reid, 1995). Sodium often reduces gas cracking heaters. Currently, the coating is the radiant coil run length and increases CO and

expensive and the life of the coating is not yet CO2 formation. Chlorides cause corrosion of the fully established. Often these coatings or surface tubes. Some heavy molecules, which are present in treatments can enhance the tube life. This topic the feed in ppm range, can cause severe coking in has been discussed at many ethylene producers’ the convection section and in the TLE. Some conferences (Proceedings [...], 1991-2004; contaminants (oxygenates) affect only the Health [...], 2001-2004). achievable olefin purity (ethylene or propylene). Nitrogen oxides in cracked gas effluents or feeds Metallurgy and mechanical engineering issues can produce explosive mixtures in the chilling train Thermal cracking is a matured technology (Halle, 1994). and hence recent improvements often come in the way of improved coil metallurgy. This subject Decoking cannot be addressed completely in this review. The heater requires periodic decoking to However, this is a significant discussion item in remove the coke laydown in the radiant coils, TLE, most conferences (Proceedings [...], 1991-2004; and/or convection section. This is normally Health [...], 2001-2004). The producers expect a accomplished in 12 to 48 hours by controlled long tube life. The radiant coils are usually made burning of the coke with a steam/air mixture in of high strength materials withstanding different proportions from start to end. The initial temperatures up to 1,150°C for several years of concentration of oxygen is kept low to control coke operation. Often the failure mode is carburization burn and avoid temperature overshoot, as steam/air of the radiant coils. In some cases, creep and decoking is an exothermic reaction. In other cases, bulging also contribute. Modern furnaces use the only steam is used for decoke, which gasifies the so-called micro alloys with nickel (30 wt%), coke. This reaction is endothermic and slow chromium (25 wt%) and the balance iron and a compared to steam/air decoke. Some proprietary few additives (Si, C, W, Mn). These are procedures use an air only step for decoking the proprietary alloys. Both wrought alloys and cast TLE; when decoking cannot burn off the coke alloys with smooth surfaces have been used in layer, it must be mechanically cleaned. commercial operation. For a detailed discussion In order to avoid contact between decoking air on related items, one should consult the and hydrocarbons, all designers and producers proceedings published annually in the adopt rigorous safety standards. These include Proceedings [...], 1991-2004 and Health [...], using interlocks to prevent unintended operations. 2001-2004 cited earlier. To improve the heat For example, when the heater is on decoke mode, transfer characteristics, fin tubes (Albano et al., the interlock prevents the feed hydrocarbon valves 1988; Barker and Jones, 2000) and elliptical from opening to eliminate the contact of tubes (Heynderickx and Froment, 1996) have hydrocarbons with air. been used and some are in the commercial operation for more than 15 years. Recovery and purification For gaseous and light naphtha feeds, the Feed impurities pyrolysis gas leaves the TLE at 300 to 400°C and While industrial furnaces can accept a wide for heavy liquid feeds (i.e. gas oil), at 550 to range of feedstocks, the feedstocks are rarely free 650°C. The lowest temperature is at the beginning of contaminants. Some contaminants present in of an operating cycle, when the exchangers are trace quantities (ppb to ppm range) can damage the clean. In order to minimize any further cracking furnace, resulting in plant shutdown. The for liquid feeds, the temperature must be reduced contaminants impact the pyrolysis heater or the quickly. This is achieved by direct quench using recovery section, or both. Most of the impurities quench oil. For naphtha-based plants, quenching is reduce run length; some to an extent that the run typically performed before reaching the oil quench length is reduced from days to hours. Again, these tower. In gas-oil plants, quenching is done are discussed in the various conference immediately after the TLE, resulting in two-phase proceedings cited earlier (Proceedings [...], flow in the transferline. For gaseous feeds, 1991-2004; Health [...], 2001-2004). Mercury in pyrolysis gas from the TLE is cooled by direct

VOLUME II / REFINING AND PETROCHEMICALS 563 BULK PRODUCTS AND PRODUCTION LINES IN THE PETROCHEMICAL INDUSTRY

quench in the water column before being routed to deposit in the system and also cause higher the compressor. Pyrolysis gas from liquid feeds is viscosity. When light feeds are cracked, the fuel cooled first by direct quench in an oil quench oil content is low and therefore high column followed by a water quench column. concentrations of unstable material are present. For all feeds, the effluent is separated into When gas oil is cracked, though the fuel oil desired products by compression in conjunction content is high, it is mainly unconverted feed. with condensation and fractionation at lower Hence the concentration of unstable material is temperatures. low and a higher bottom temperature can be Fig. 5 represents a typical flow diagram of an tolerated. ethylene plant for a naphtha feedstock. The Due to the small temperature difference quenched effluent enters the gasoline between the hot quench oil and the steam fractionator, where the pyrolysis gas and heavy generators, and the large amount of heat to be fuel oil cuts are separated. The function of the removed from the pyrolysis gas, the quench oil fractionator is to separate the pyrolysis gasoline flow has to be large and is typically in the range and pyrolysis fuel oil components and to cool the of 15 to 25 times the flow of feedstock to the cracked gas. The circulating quench oil is cooled heaters. Due to the presence of coke particles in to 185°C by generating dilution or low-pressure the quench oil, specially designed pumps are steam. The bottom temperature of the used. The dilution steam generator is one of the fractionator must be carefully controlled, since largest multiple-shell exchangers in the plant. In the quench oil is unstable at high temperatures. addition, spare pumps and exchangers are High residence time in combination with high needed due to severe fouling (Picciotti, 1977a, temperature results in polymerization that can 1977b, 1977c).

steam wash tower wash pyrolysis heater pyrolysis caustic and water water process water stripper process water quench tower gasoline fractionator fuel oil stripper condensate stripper

charge gas charge gas compressor compressor 1st - 3rd stages 4th and 5th stages

fresh feed fuel oil

ethylene propylene fractionator fractionator

dryer demethanizer debutanizer depropanizer converter deethanizer converter acetylene and propadiene methyl acetylene methyl

C4 product propylene fuel gas hydrogen methane ethane ethylene C3 LPG gasoline

Fig. 5. Schematic flow diagram of an ethylene plant using naphtha feedstock.

564 ENCYCLOPAEDIA OF HYDROCARBONS ETHYLENE AND PROPYLENE

The quench tower in essence operates as a dryers. Molecular sieve dryers completely partial condenser for the gasoline fractionator, remove water from the pyrolysis gas. Typically, condensing practically all of the steam and the there are two dryers with one in normal heavy pyrolysis gasoline components. Separation operation, while the other is being regenerated. of the water phase from the gasoline phase occurs The dryers are designed for 24 to 48 hours in a quench water drum. Hot quench water is used between successive regenerations. High-pressure as a process heat source for the recovery section. methane, heated with steam at 225°C, is the A portion of the gasoline phase is refluxed for the preferred regeneration medium. The pyrolysis gasoline fractionator, while the remainder is sent to gas is partially condensed at essentially constant the gasoline stripper for stabilization. This gasoline pressure over the stages of the cascade has a Research Octane Number (RON) of 95 to 99, refrigeration system to about 165°C, where and is usually blended with other gasoline only the hydrogen remains in the vapour state. products. The stage condensates (only one is shown The pyrolysis gas leaving the quench tower is in Fig. 5) are fed to the appropriate trays of the compressed to 3.5 MPa in a four- or five-stage demethanizer. Hydrogen (95 mol%) is withdrawn centrifugal compressor. The number of stages is from the lowest temperature stage separator. The determined by the maximum temperature demethanizer is designed for complete separation permissible for the fouling tendency of the of methane from ethylene and heavier pyrolysis gas. The compressor typically consists of components, and operates at nearly 0.7 MPa for three compressor casings driven by a single ABB Lummus Global’s low-pressure extraction/condensing turbine. For large plants, two demethanizer scheme. Condensing propylene turbines can be used. Water and hydrocarbons are refrigerant supplies heat to the reboilers and separated from the pyrolysis gas between stages, vaporizing refrigerant condenses the reflux. and recycled. The demethanizer overhead consists of methane

Acid gases (CO2 and H2S) are removed after (95 mol%) with some minor impurities the third or fourth compression stage. This is an of hydrogen, carbon monoxide, and traces optimum location since the gas volume has been of ethylene. Brazed aluminum plate-fin exchangers reduced significantly and the acid gases have not are used for the multi-pass cryogenic heat transfer contaminated any final products. When the services and are installed in a cylindrical or sulphur content of the feed is low, as in some rectangular carbon steel container, commonly naphtha feeds, scrubbing with a dilute caustic known as a cold box. This unit is filled with soda solution (typically 4 to 12% free caustic) is perlite or rockwool for insulation. cost-effective. Relatively weak solutions are The demethanizer bottoms, consisting of preferred to avoid the precipitation of sodium ethylene and heavier components, are sent to the salts and to minimize the formation of sodium deethanizer – a conventional tray-type fractionator complexes and ‘yellow oil’. The pyrolysis gas operating at a pressure of 2.4 to 2.8 MPa. An

leaving the scrubber contains less than 1 ppm overhead stream containing C2 hydrocarbons and a acid gases and is further treated by a water wash bottoms product of C3 and heavier hydrocarbons to remove any caustic carryover. A detailed are produced. Since acetylene is not usually analysis is given by Raab (1976). Plants designed recovered, the deethanizer overhead is heated to to process high sulphur content feeds (i.e. higher 20-100°C and hydrogen is added. The mixture is than 500 ppm) may contain a regenerative acid passed over a fixed bed of palladium catalyst for gas removal system upstream of the caustic acetylene hydrogenation. Due to the exothermicity scrubber. These systems employ of acetylene hydrogenation, multiple beds with monoethanolamine, diethanolamine, or Alkazid intermediate cooling are preferred. The acetylene as solvents with a standard absorber-desorber hydrogenation reactor effluent contains less than design. Depending upon the plant location, acid 1 ppm of acetylene, but does contain traces of gases are either sent to a fired heater or treated in methane and hydrogen. This is known as back-end a Claus unit for conversion of hydrogen sulphide acetylene hydrogenation and is preferred to to elemental sulphur. Coke suppression additives, front-end hydrogenation – particularly in the case

discussed earlier, somewhat decrease the CO2 of older catalysts with higher carbon-monoxide formation and can be economically attractive. (CO) sensitivity – due to higher selectivity and After acid gas removal, the pyrolysis gas from precise control as hydrogen is varied with the last stage of compression is cooled by acetylene concentration; temperature is varied propylene refrigerant and sent to the charge gas depending on catalyst activity.

VOLUME II / REFINING AND PETROCHEMICALS 565 BULK PRODUCTS AND PRODUCTION LINES IN THE PETROCHEMICAL INDUSTRY

Front-end acetylene hydrogenation is also The condensate stripper bottoms and the utilized in the ethylene flow scheme. This approach deethanizer bottoms are processed in the

requires a catalyst less sensitive to CO, and the depropanizer for a sharp separation of C3 reactor is located upstream of the demethanizer. hydrocarbons and C4 and heavier hydrocarbons. For this design, typically a deethanizer or The reboiler may be fouled by rubber-like depropanizer tower is located upstream of the polymers and require periodic mechanical demethanizer to remove heavy fractions before cleaning. To minimize this problem, two types of acetylene hydrogenation. The hydrogenation designs are used. In the first design, the bottoms catalyst is usually more sensitive to significant temperature is set low, which results in an variation in CO concentration, which is operating pressure requiring propylene experienced when cracking heaters are brought refrigerant instead of cooling water for the on-line after decoke operation. condensation of the overhead product. In the two During acetylene hydrogenation, there is a net tower system design, cooling water is used as the gain of ethylene since, under normal conditions, condensing medium for the high pressure tower more acetylene is hydrogenated to ethylene than and propylene refrigerant for the low pressure ethylene is hydrogenated to ethane. The catalyst is tower. This concept results in better overall deactivated over time due to coke and green oil energy efficiency, but involves higher capital (a polymer formed due to side reactions) investment. production and is therefore regenerated periodically The depropanizer bottoms are further

(six to twelve months). The kinetics of this reaction processed in the debutanizer for separation of C4 has been discussed by Lam (1988) and Cider and product from light pyrolysis gasoline. The Schoon (1991). For some catalysts, a trace amount debutanizer operates at a moderate pressure of of CO is used to control the selectivity of 0.4 to 0.5 MPa and is a conventional fractionator hydrogenation; however, the new generation of with steam-heated reboilers and water-cooled catalysts does not require CO addition. condensers. Acetylene can be recovered by using an The overhead of the depropanizer is sent to the absorption process with multiple towers. In the propylene fractionator. The methyl acetylene (MA) first tower, acetylene is absorbed in acetone, and propadiene (PD) are usually hydrogenated dimethyl formamide or methyl pyrrolidone (Lorber before entering the tower. A MAPD converter is et al., 1971; Stork et al., 1974). In the second similar to an acetylene converter, but operates at a tower, absorbed ethylene and ethane are rejected. lower temperature and in the liquid phase. Due to In the third tower, acetylene is desorbed. Since recent advances in catalysis, the hydrogenation is acetylene decomposition can result at certain performed at low temperatures (50-90°C) in trickle conditions of temperature, pressure and bed reactors (Stanley and Venner, 1991). Methyl composition, the design of this unit is critical for acetylene and propadiene are only rarely recovered. safety reasons. Catalytic distillation technology has been used to After acetylene hydrogenation/removal, the saturate MAPD in recent designs (Gildert et al., dried gas enters an ethylene-ethane separator (i.e. 1995). ethylene fractionator). This column contains 80 to Due to the low relative volatility, fractionation 150 trays, and a reflux ratio of 2.5 to 4.0 is typical of propylene and propane is even more difficult depending upon feed composition. A pasteurizing than the fractionation of ethylene and ethane. The section is usually provided at the top of the propylene fractionator operates at a pressure of fractionator for removal of residual hydrogen, 1.8 to 2.0 MPa, with nearly 160 trays required for a carbon monoxide, and methane to achieve very high purity propylene product. Often a two-tower high purity ethylene (99.95%). Propylene design is employed when polymer grade (high refrigerant is typically used as condensing and purity 99.9%) is required. A pasteurization reboiling medium. Condensing refrigerant vapour section may also be used when high purity is supplies heat to the reboiler, while the refrigerant required. The bottoms product contains mainly boiling under low-pressure generates the cooling propane, which can be recycled to the cracking required in the overhead condenser. An open heat heaters or used as fuel. pump can be used with the integration of the A typical specific energy consumption curve ethylene-ethane fractionator into the ethylene for various feedstocks processed at high severity is refrigeration system. The ethane withdrawn from shown in Fig. 6. The energy consumption for a the tower bottom is recycled to the heaters and naphtha-based plant has been reduced from 8,100

cracked to extinction. kcal/kg C2H4 in the 1960s to nearly 5,000 kcal/kg

566 ENCYCLOPAEDIA OF HYDROCARBONS ETHYLENE AND PROPYLENE

in 2003. This reduction was made possible by Feed saturation. When gas feeds like ethane improvements in cracking coil technology and and propane are cracked, dilution steam can be recovery section design. added via direct humidification in towers known as The energy consumption values do not include feed saturators. This design reduces the load on the the Olefins Conversion Technology (OCT) unit. dilution steam system and/or the medium pressure When OCT is integrated with an ethylene plant, steam level. the specific energy drops by up to 10% with Predemethanization. The conventional design similar investment cost. employs a single step, multiple-feed tower. Utilization of a second tower upstream of the Recent improvements existing primary tower reduces the load on the Some recent improvements not only reduce the primary tower, which reduces the propylene energy consumption, but also increase the capacity refrigeration power requirement as well as the of an existing plant. These approaches are propylene chilling loads. This approach typically discussed below. is not economical with low-pressure Large capacity cracking heaters. With large demethanizers. capacity ethylene plants, it is possible to consider Demethanizer overhead expander. high capacity heaters and still keep economical Incorporation of an expander into the conventional spare heater capacity. All technology suppliers high-pressure demethanizer system will eliminate are currently proposing large capacity heaters, in bottlenecks in the refrigeration system, the the range of 150,000-250,000 mta of ethylene demethanizer condenser, and the charge gas production. New computer tools, utilizing compressor. It reduces the operating cost by Computational Fluid Dynamics (CFD) lowering the refrigeration power requirement. techniques, have allowed designers and Multiple-feed deethanization and ethylene developers to better understand the aerodynamics fractionation. This approach debottlenecks the of the firebox side of large capacity heaters deethanizer, ethylene fractionator, and the (Platvoet et al., 2003). refrigeration systems, thereby reducing power Quench oil viscosity control. Increasing the consumption. bottoms temperature of the gasoline fractionator Tower internals and equipment modification. increases the bottoms liquid’s viscosity; Tower capacity expansion can be achieved through therefore, flux oil is often added to reduce the the use of random or structured packing or through viscosity. Effluents from ethane cracking can be the use of higher capacity trays, such as the UOP used as a viscosity control stripping medium, multiple downcomer tray. Packing, which reduces allowing a temperature of 195-230°C in the pressure drop and increases capacity, has been used gasoline fractionator bottom (depending on type in the gasoline fractionator, water quench tower, of feed mix). This reduces the quench oil caustic and amine towers, demethanizer, the upper pumping power, eliminates the flux oil addition zone of the deethanizer, debutanizer and (Stanley and Venner, 1991), and improves energy condensate strippers. Improved and redesigned efficiency. rotors of modern compressors save considerable power. The conventional tube bundles in the ethylene fractionator and the propylene 6,500 refrigeration condensers can be replaced with ) 4 extended surface tube bundles. H 6,000 2 Dephlegmators. These apparatuses accomplish 5,500 feed gas separation (i.e. fractionation) in a cold 5,000 box, in combination with heat transfer. Cryogenic 4,500 dephlegmators are brazed aluminum (plate-fin or 4,000 core) heat exchangers that are specially designed to operate as mass transfer devices. Depending upon 3,500 the feed composition, 5 to 15 theoretical stages can 3,000 be obtained with one dephlegmator. Application of specific (kcal/kg C energy 2,500 dephlegmators to ethylene plants is discussed by ethane butane gas oil propane naphtha HVGO Bowen (1991) and Nachenberg (1991). Dephlegmators are relatively more capital intensive Fig. 6. Specific energy consumption for a plant as compared to conventional plate-fin built in 2003. exchangers.

VOLUME II / REFINING AND PETROCHEMICALS 567 BULK PRODUCTS AND PRODUCTION LINES IN THE PETROCHEMICAL INDUSTRY

Pinch technology. This technology has been loop, constant composition, mixed refrigeration applied to optimize various designs (Greene et al., system and provides refrigeration typically at four 1994). Ethylene production by thermal cracking is temperature levels ranging from 40°C to an energy intensive process. Hence various optimal 140°C. This concept has been commercially arrangements of towers and refrigeration levels proven and has been extended to a tertiary have been proposed (Manley, 1996). refrigeration system that leads to even further Membrane technology. This technology has capital cost savings. A tertiary refrigeration system been used to partially separate hydrogen from other combines all refrigeration systems (methane, products to improve plant efficiency. Though the ethylene and propylene) into one system, providing economics of this approach is questionable, it may refrigeration from 40 to 140°C. This system be attractive for ethane cracking where hydrogen reduces capital cost significantly, improves concentration is the highest in the effluent mixture. operation reliability, and lowers maintenance costs. Catalytic distillation. Combining the Advanced computer control systems and hydrogenation and fractionation steps involved in training simulators. An ethylene plant contains the purification of olefins is a cost-effective more than 300 equipment items. With the advent innovation. In an ethylene plant, catalytic of modern computers, the plant operation can be distillation can be applied to MAPD simulated on a real-time basis and the results

hydrogenation, selective hydrogenation of C4 and displayed on monitors. Sophisticated C5 acetylenes and dienes, and to total mathematical models and control panels are used hydrogenation of C3/C4/C5 olefins and dienes to artificially simulate emergencies and train the (Stanley and Weidert, 2002). It can also be used operators to respond correctly when such

to selectively hydrogenate the C2 through C5 situations arise. In a similar way, computers are acetylenes and diolefins in a front-end used in a modern plant to control the entire depentanizer tower. This processing step removes operation. For the heaters, a model-based control 35 to 40% of the hydrogen contained in the system is gaining importance (Advanced Process heater effluent by chemical reaction rather than Control Handbook, 1991). Instead of simply by cryogenic separation, significantly reducing controlling the Coil Outlet Temperature (COT), compressor power and energy consumption. severity is actually controlled. The measurement

Catalytic distillation combines hydrogenation and of severity (either C3H6/C2H4 or C3H6/CH4 ratio), separation in one reactor, resulting in capital however, requires on-line effluent analysis using savings. A catalyst bed replaces a portion of the chromatographs, which have significant lag time. trays in the distillation tower. Since the reflux To overcome this lag time, sophisticated kinetic required for the distillation passes through the models are used to predict the severity for a catalyst bed, oligomers formed in the bed are given COT and compare it against the actual continuously washed, which increases the measurement as it becomes available. Although catalyst life and the selectivity to olefins. The COT is used as the set point, severity is the hydrocarbon and the hydrogen enter the reaction control variable (Stancato et al., 1991). This also zone as a two-phase mixture. Since the reaction provides a means to optimally adjust severity as takes place in the liquid phase, the potential for fouling in the radiant coil and in the TLE occurs. runaway associated with the highly exothermic hydrogenation reaction is also reduced. The Safety and environmental factors proper selection of the catalyst is crucial since a Care must be exercised in the design and highly active catalyst will also saturate the operation of equipment in an ethylene plant. While valuable propylene. ethylene is a colourless gas with a mild odour that Multi-component refrigeration. The is not irritating to the eyes or respiratory system, it low-pressure demethanizer uses a methane is a hydrocarbon and, therefore, a flammable gas. refrigeration system to provide the lowest level All vessels must be designed for handling the cooling to the cracked gas and the reflux to the liquids and gases during operation at the demethanizer. This system results in investment temperatures and pressures that will exist, and and operating cost advantages, compared to a safety and depressuring valves must be provided to high-pressure demethanizer system utilizing lowest relieve excessive pressure. The releasing of level ethylene refrigeration. The latest hydrocarbons into the air in large amounts must be developments further simplify this system by avoided due to health and fire hazards. To protect combining the ethylene and methane refrigeration the plant and personnel in case of fire, a complete into a binary refrigeration system. This is a closed fire fighting system is provided. Tanks are grouped

568 ENCYCLOPAEDIA OF HYDROCARBONS ETHYLENE AND PROPYLENE

to minimize fire and provided with foam makers Advanced cracking reactor and deluge systems. An Advanced Cracking Reactor (ACR) was Reviews at various stages of a project assure developed jointly by Union Carbide with Kureha safety, which is given constant attention in the Chemical Industry and Chiyoda Chemical plant design. Checklist and Hazard and Operability Construction (Wett, 1972). The key to this process (HAZOP) reviews are standard practice industry- is high temperature, short residence time, and low wide. Failure Modes and Effects Analysis (FMEA), hydrocarbon partial pressure, which improve the What-If, and Qualitative Fault Tree Analysis selectivity to ethylene significantly. Superheated methods have also been used when required. steam is used as the heat carrier to provide the heat

Stringent environmental laws exist in almost of reaction. The burning of fuel (H2 and CH4) with every country in the world. An ethylene plant will pure oxygen generates temperatures of 2,000°C produce liquid, gaseous and solid wastes, which are and the cracking reaction is carried out at 950 to disposed of in an environmentally safe manner, as 1,050°C (Hosoi and Keister, 1975; Kearns et al., dictated by local regulations. 1979; Baldwin and Kamm, 1982) with a residence Liquid wastes generated within the complex time of less than 10 ms. Since the residence time in consist of wastewater streams of relatively low the reactor is so low, a specially designed Ozaki organic content and process wastes of high organic quench cooler for rapid quenching is required. content. Wastewater from various units and A prototype was in operation for over 18 months operations are segregated according to the during the 1980s. Unfortunately, all very high wastewater characteristics, such as type of temperature processes produce high amounts of contaminants, concentration, and special treatment acetylene (2 wt%). Acetylene hydrogenation will or pretreatment requirements. A segregated sewer be a significant cost factor if there is no market system allows for the most efficient treatment of for acetylene. the wastewaters. Atmospheric emissions from the facility are Adiabatic cracking reactor either controlled or fugitive in nature. Controlled This principle is based on the injection of emissions, resulting from process venting, waste hydrocarbon feedstock into the flue gas at elevated incineration, decoking operations, and heater temperatures. Due to the high initial temperature firing, are released from stacks. Fugitive emissions (1,200°C), the feed is instantaneously vaporized occur from product loading and storage and and a very high rate of decomposition is obtained.

equipment leaks. Most modern plants use low NOx The temperature of the flue gas is controlled by burners and/or SCR (Selective Catalytic varying the oxygen/fuel ratio at the combustion Reduction) technology and send the decoking chamber and by the injection of steam in the effluents to the firebox. In general, all continuous combustion chamber. Due to the endothermic process vents are flared or combusted in the nature of the cracking process, the temperature furnaces. If required, the process vents are drops rapidly after the injection of the feed. A scrubbed prior to flaring to minimize acid gas substantial increase (over 10 wt%) in olefin yield emissions. Flow monitors are installed in major can be expected, but quenching the reaction to branches in the flare collection header to monitor desired conditions is still a problem. The process venting. A smokeless flare has a normal economics of the process are still not profitable. destruction efficiency of over 98%. This route to ethylene production has been During normal plant operation, certain solid analyzed by Dente et al. (1981, 1985) using wastes are generated. These wastes are treated in a mathematical models. Instead of hydrocarbon, solid waste disposal area to reduce their volume hydrogen has also been used as fuel, which and or toxicity prior to final disposal in a secured generates in situ dilution steam. landfill. Combustible wastes are incinerated in a slagging rotary kiln to reduce volume and toxicity. Fluidized bed cracking Lurgi developed the sand cracker (Schmalfeld, Other routes to ethylene manufacture 1963) using sand as the heat carrier while BASF used coke particles as the fluidizing medium In addition to conventional thermal cracking in (Steinhofer et al., 1963). Ube (Matsunami et al., tubular furnaces, other thermal methods and 1970) used inorganic oxide as the heat carrier, and catalytic methods to produce ethylene have been the Kunugi and Kunii process (Kunugi, 1980) used developed. None of these are commercialized as of a fluidized bed with coke as the heat carrier. present. Thermal regenerative cracking, jointly developed

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by Gulf Chemical (currently Chevron Phillips) and Cracking temperatures in catalytic pyrolysis are Stone & Webster (Ellis et al., 1981) uses solid heat somewhat lower than those observed with thermal carriers in the fluid bed. Other thermal processes pyrolysis. Catalytic pyrolysis is still dominated by are discussed by Hu (1982). free-radical reactions. As stated by Lemonidou and Vasalos (1989), most of these catalysts affect the Catalytic pyrolysis initiation of pyrolysis reactions and increase the In recent years, there have been many articles overall reaction rate of feed decomposition. The on catalytic pyrolysis. This should not be confused applicability of this process to ethane cracking is with fluid catalytic cracking, which is used in oil questionable, since the equilibrium of ethane to refining to produce gasoline. Catalytic pyrolysis is ethylene and hydrogen is not altered by a catalyst; aimed at producing primarily ethylene. There are therefore, the selectivity to olefins at lower catalyst many patents and research articles on this topic temperatures may be inferior to that of covering the last twenty years (Kikuchi et al., conventional thermal cracking. The suitability of 1985; Kolts and Delzer, 1986; Lemonidou et al., this process for heavy feeds, like condensates and 1987; Lemonidou et al., 1989; Lemonidou and gas oils, has yet to be demonstrated. Often these Vasalos, 1989; Chernykh, 1991; Basu and Kunzru, catalytic processes do not improve the selectivity to 1992; DeHertog et al., 1998; Picciotti, 2000; Jeong olefins significantly, but only reduce the energy et al., 2001). consumption in the hot section. Even after many Almost all catalysts produce higher amounts of years of research, there is limited commercial

CO and CO2 than normally obtained with interest for these technologies in the petrochemical conventional pyrolysis. This indicates that the industry. water-gas shift reaction is very active with these catalysts and this usually leads to some Membrane reactor deterioration of the olefin yield. Significant Another area of current activity uses amounts of coke have been found in these catalysts membranes in ethane dehydrogenation to shift the and thus there is a further reduction in olefin yield ethane to ethylene equilibrium. Technology has with on-stream time. Most of these catalysts are improved to produce ceramic and other inorganic based on low surface area alumina catalysts membranes (Champagnie et al., 1990) that can be (Lemonidou et al., 1989). A notable exception is used at high temperatures (600°C and above). In the catalyst developed in the former USSR addition, they can be coated with suitable catalysts (Chernykh, 1991). This catalyst primarily contains without blocking the pores of the membrane. vanadium as the active material on pumice and is Therefore, catalyst coated membranes can be used claimed to produce low levels of carbon oxides. for reaction and separation. LG Petrochemicals, Korea, recently developed a As an example, Champagnie et al. (1990) have similar catalytic process (Jeong et al., 2001). discussed the use of ceramic membranes for ethane DeHertog et al. (1998) and MITI (a Japanese dehydrogenation. The construction of a research consortium; Picciotti, 2000) have also commercial reactor, however, is difficult and a announced relatively low temperature catalysts sweep gas is required to shift the product primarily for naphtha cracking. composition away from equilibrium values. In China, fluid catalytic cracking (also known The achievable conversion also depends on the as deep catalytic cracking) has been used in permeability of the membrane. semi-commercial units to crack gas oils and heavy Another way to use membranes is for feedstocks to produce olefins (Chapin and Letzsch, separation only and not for reaction. In this 1996). Mainly zeolites and zeolite type catalysts method, a conventional, multiple, fixed-bed have been used. Cracking temperatures are still catalytic reactor is used for the dehydrogenation. high and the product distribution is almost After each bed, the hydrogen is partially separated identical to that obtained in a pyrolysis unit. using membranes to shift the equilibrium. Since However, these catalysts produce higher propylene separation is independent of reaction, reaction yields and slightly lower ethylene yields than those temperature can be optimized for superior achieved in thermal cracking, yielding a higher performance. propylene/ethylene ratio than possible with thermal Both concepts have been proven in bench scale cracking alone. Reportedly, this process has been units, but are yet to be demonstrated in commercial applied in one commercial unit outside China. The reactors. To improve the energy efficiency, an economics or the suitability to other light endothermic reaction (pyrolysis) on one side of the feedstocks is not reported. membrane and an exothermic reaction

570 ENCYCLOPAEDIA OF HYDROCARBONS ETHYLENE AND PROPYLENE

(combustion) on the other side has been proposed which are deterrents with conventional thermal (Choudhary et al., 2000). There are no commercial cracking furnaces. reactors yet. Oxidative coupling of methane Dehydrogenation of ethane The most stable paraffin is methane, which The dehydrogenation of paraffins is has a difficult C–H bond to break. During the equilibrium limited and hence requires high 1980s, new catalysts were developed to activate temperatures. Using this approach and methane, producing methyl radicals. These conventional separation methods, ABB Lummus methyl radicals combine to give ethane, which Global and UOP offer commercialized processes further undergoes pyrolysis reactions. This for the dehydrogenation of propane to propylene process and its economics have been discussed in (Vora et al., 1986). This technology has been detail (Preuss and Baerns, 1987; Baerns, 1991). applied commercially and has drawn significant According to these sources, this process is not interest in the Middle East with low-cost propane economical when conventional feedstocks feed. A similar concept is possible for ethane (naphtha, LPG, etc.) are inexpensive. The process dehydrogenation, but an economically attractive could be economical when methane is available commercial reactor has not been built. in abundance at extremely low cost, such as in Saudi Arabia and other geographic locations. Oxydehydrogenation of ethane Since this process does not depend upon crude Due to the limitations of ethane oil for raw feed, research has continued in many dehydrogenation equilibrium, research has focused countries, and it is possible that it may soon be on ways to remove one of the products – namely commercialized. Instead of direct coupling, hydrogen – by chemical methods. Hydrogen is oxidation of methane to methanol and then oxidized to water and there is no equilibrium to olefins has also been proposed limitation: (Nexant, 2002), but the process is not economically attractive. C2H6 O2 C2H4 H2O However, the same oxygen also oxidizes ethane Methanol to ethylene

and ethylene to CO2 and other oxygenated Methanol to ethylene economics tracks the products. Therefore, the selectivity to olefins is a economics of methane to ethylene. Methanol to serious consideration. Recent literature citations gasoline has been fully developed and, during this claim the development of low temperature, highly development, specific catalysts to produce ethylene selective oxydehydrogenation catalysts (Conway were discovered. Inui e Takegami (1982) and Inui and Lunsford, 1991; Laegreid, 1991). While this et al. (1991) have discussed the economics of this process has not yet been commercialized, it seems process and recently claimed to have a catalyst promising. (Ni/SAPO 34) with almost 95% selectivity to Recently, Schmidt et al. (2000) developed a ethylene. According to the authors, methanol is novel concept for ethane oxydehydrogenation. converted to dimethyl ether, which decomposes to Burning hydrogen with oxygen at catalytic sites ethylene and water. The method of preparation of (platinum gauze) results in very high temperature. the catalyst, rather than the active ingredient of the The reaction is very selective to hydrogen, which catalyst, has made the significant improvement in produces water. Water (steam) acts as a diluent yield. By optimizing the catalyst and process and also a heating medium. The exothermicity is conditions, the authors claim to maximize yields of used to heat the feed (ethane), which is ethylene, propylene, or both. This is still in the adiabatically cracked with the in situ steam bench/pilot scale stage and has yet to be applied on produced. This is a thermal process. The feed is a commercial scale. converted to products in less than 10 ms and hence the selectivity to ethylene is high. Since Dehydration of ethanol ethane cracking produces more hydrogen than The economics of this process depends upon required for oxidation to maintain the energy the availability and price of ethanol. High volume balance, there is no need to import hydrogen. For production of ethylene from ethanol, which is other feeds, however, it is less likely to be derived from fermentable raw materials, cannot economically attractive. The interesting feature of normally compete with ethylene produced in large, this process is that it significantly reduces the hydrocarbon-based olefin units. This process, production of greenhouse gases and pollutants, however, offers several advantages to a country

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with abundant fermentation materials, but limited on the Gulf Coast and other locations. This is hydrocarbon resources (Winfield, 1960). discussed in more detail in Section 10.5.2. Activated alumina and phosphoric acid on a suitable support became the choices for an Ethylene as a by-product industrial process. Zinc oxide with alumina has In refinery FCC units, a small amount of also been claimed to be a good catalyst. The actual ethylene is produced, but rarely recovered. Many mechanism of dehydration is not known, but a new FCC catalysts (generally ZSM 5) with plausible mechanism is given below: additives have improved the olefin production. As a result, some producers have integrated their 230°C 300-400°C ethylene ether ethanol ethylene refinery operation with an ethylene plant. Though propylene is produced in significant amounts and In industrial production, the ethylene yield is 94 is mostly recovered, ethylene, ethane and butenes to 99% of the theoretical value depending on the are formed in sufficient quantities to be processing scheme. Traces of aldehyde, acids, economically attractive for integration with an higher hydrocarbons and carbon oxides as well as ethylene plant. water must be removed. Fixed-bed processes developed at the beginning of the last century have Shipment and storage been commercialized in some countries, and small-scale industries are still in operation The supply and demand patterns for ethylene in Brazil and India. New fluid-bed processes have and its derivatives have changed significantly in been developed to reduce the plant investment the last 15 years. New ethylene projects are being and operating costs (Winter, 1976; Taso and Reilly, built in regions with cheaper feedstock with the 1978). primary objective of meeting demand in high growth regions. The trade of ethylene and its Ethylene from coal derivatives grew from nearly 4.5 million mta of net There are several possible routes to ethylene equivalent ethylene in 1990 to nearly 9 million mta from coal based on both conventional and in 2003. Of this, nearly one million mta is traded as emerging technologies. Synthesis gas derived from ethylene. The major exporting countries are in the coal gasification can be converted to hydrocarbons Middle East and the major importing countries are by the Fischer-Tropsch (FT) process. The in Western Europe and Asia/Pacific. The ethylene conventional FT process produces high molecular is mostly transported in refrigerated tanks with weight saturated and olefinic hydrocarbons and capacities ranging from 2,000-10,000 metric oxygenated compounds. Ethylene can be recovered tonnes. These tankers are of the semi-refrigerated directly or after pyrolyzing the ethane and naphtha type and transport liquid ethylene at atmospheric produced. In general, this is not an economical pressure and 104°C. The tankers include process; however, there is a plant based on this reliquefaction plants on board, since it is too route in South Africa (Dry, 1981). Some of the FT expensive to vent ethylene. catalysts developed during the last decade While the quantity of ethylene transported by maximize ethylene and propylene yields. However, international tankers is large, it accounts for only in Mobil’s MTG (Methanol To Gasoline) or MTO 1% of production. The majority of ethylene (Methanol To Olefins) processes, zeolite catalysts produced in the United States and Western are used and high olefin yields are reported (Inui, Europe is moved by integrated pipeline 1990). systems. In the US, the Gulf Coast produces and Propylene disproportionation consumes the majority of the US ethylene A commercial plant utilizing the production. The plants are located along the disproportionation of propylene to ethylene and southeast coast of Texas extending into Louisiana butene was built in 1966 by Gulf Oil of Canada, (CMAI, 1989). The plants are served by a system utilizing technology developed by Phillips: of pipelines connecting the producing and

consuming plants. The ethylene is transported as a 2C H C H C H 3 6 2 4 4 8 gas at very high pressures (5-5.7 MPa; Burdick and Since the above reaction is reversible, it can be Leffer, 1983). Since the critical temperature for used to produce either propylene or ethylene and ethylene is 9.2°C, the ethylene remains as a gas at butenes depending on relative prices. Commercial the elevated pressures. The pipelines are buried plants based on the reverse reaction have been built approximately 3 to 5 meters below the ground, so

572 ENCYCLOPAEDIA OF HYDROCARBONS ETHYLENE AND PROPYLENE

the surrounding ground temperature rarely less expensive, thinner-walled tanks. The ethylene approaches the critical temperature. is kept below its critical temperature and therefore Western Europe also operates an integrated lower pressure can be used. To keep the ethylene pipeline system. The system links the large below its boiling point (103.7°C), some ethylene producers and consumers in Germany, The is vaporized and then passed to a refrigeration Netherlands, France and Belgium. The pipeline plant for recovery (Burdick and Leffer, 1983). system is being expanded both in terms of ethylene Cryogenic storage tanks employ an outer and an and the addition of propylene. Expansion of the inner tank with the interspaces filled with an pipeline is being instituted to phase-out the use of insulating material (Perry and Chilton, 1973). rail tankers to transport ethylene. Producers feel For storage of very large quantities, that pipeline transport offers superior specification underground caverns or salt domes are used. The guarantees, which rail or barge tankers cannot offer cavern is created by dissolving the salt and since the operation is not directly under the user’s pumping out the brine (Perry and Chilton, 1973). control (Olefins [...], 1990). As the ethylene is pumped into the cavern, it A certain amount of storage is provided inside displaces the brine, which creates a pool that exerts operating plants to smooth out operational upsets. pressure on the ethylene. The ethylene is removed The capacity stored can range from a few hours to by returning brine from the pool. Since the caverns a few days of production, depending on the owner’s are located a few hundred meters below ground operating philosophy. Small amounts of ethylene level, temperatures are fairly constant and always are stored as a liquid in pressure tanks and held at above the critical temperature of ethylene (Burdick the required temperature by refrigeration available and Leffer, 1983). from the operating unit. This method of storage is only appropriate for a few hours of production Uses since the tanks must be expensive, heavy, thick walled vessels. Cryogenic storage is used for larger Almost all ethylene produced is consumed as amounts of ethylene, which allows for the use of feedstock for manufacturing other petrochemicals.

Table 5. Ethylene derivatives and world consumption*

1991 1996 2000** Polyethylene*** LDPE/LLDPE 19,250 24,516 29,024 HDPE 12,618 17,604 23,582

Ethylene dichloride 8,766 10,560 12,698

Ethylbenzene/styrene 4,213 5,163 6,349

Ethylene oxide 6,952 8,424 10,884

Ethanol 729 685

EPDM rubber**** 347 475

Oligomers 1,406 1,959

Vinyl acetate 925 1,113

Acetaldehyde 997 1,005

Others 1,872 1,143 8,163

Total 58,075 72,647 90,700

* Worldwide consumption as ethylene equivalent in 103 t/yr (SRI Consulting, 1997) ** Estimate from information supplied by CMAI (Chemical Market Association) *** LDPE, Low-Density PolyEthylene; LLDPE, Linear Low-Density PolyEthylene; HDPE, High-Density PolyEthylene **** EPDM, Ethylene Propylene Diene Monomer

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Specification and analysis Table 6. Specification for polymer Polyethylene is the predominant derivative of grade ethylene ethylene and requires very high purity (99.9%) monomer. A typical polymer grade specification of Ethylene, mol% 99.95 min ethylene is given in Table 6. Almost all ethylene Methane + ethane, mol% Balance plants use gas chromatography or gas chromatography combined with mass spectrometry Other impurities, mol ppm for analysis. For specific impurities like sulphur, Hydrogen 5.0 max water and other hydrocarbons and elements, ASTM standard tests are recommended (D2504-67, Acetylene 1.0 max D2505-67 2785-70, E203-75). The new generation Oxygen 1.0 max of polyethylene catalysts requires greater than 99.95% purity. Carbon monoxide 1.0 max Carbon dioxide 1.0 max Economic aspects Propylene 10.0 max In 2003, world capacity for the production of

C4+s 10.0 max ethylene was approximately 110.8 million metric tonnes. The US production capacity accounted for Water 2.0 max almost 25% of the world capacity, or approximately Total sulphur 2.0 max 27.7 million metric tons, followed by Western Europe with almost 22% or 24 million metric tonnes Methanol 5.0 max (Nakamura, 2004). Total chlorine 2.0 max While the US and Western Europe account for half of the world’s production capacity, the most rapid Dimethyl formamide (DMF) 1.0 max growth in capacity has been occurring in the Other compounds 5.0 max developing areas of the world. Table 7 shows the increase in capacity from 1996 through 2003 for the major regions of the world. As can be seen from this Only a very small amount has been used in the table, capacity growth in the developing countries will agricultural industry for ripening fruits. Table 5 lists be almost twice that of North America and Western major ethylene derivatives and the world consumption. Europe. Although some ethylene is shipped across the Ethylene is the first chemical in total market value oceans in large quantities, the preference is to ship among petrochemicals. Based on 2003 production first generation end-products, such as polyethylene, capacity of 110.8 million metric tonnes, the total ethylbenzene, etc. potential production value is approximately 83 billion

Table 7. Ethylene capacity and growth rate*

Growth rate, Region 1996 2003 %/year 1996-2003 North America 28.7 34.41 2.8 Western Europe 20.13 24.07 2.8 Asia + Oceania 19.80 29.35 6.9 Middle East and Africa 5.94 11.01 12.2 South and Central America 3.29 4.36 4.6

Eastern Europe 6.20 7.58 3.2 Total 84.06 110.78 4.5

* Capacity in 106 t/yr. Projected average growth rate is 3.2%/yr for the next five years. World-operating rate is 90% in 2003.

574 ENCYCLOPAEDIA OF HYDROCARBONS ETHYLENE AND PROPYLENE

Table 8. US ethylene produced from various feedstocks (Nakamura, 2004; Olefins [...], 1990; SRI Consulting, 1997)

Year Ethane/propane/ Naphtha (%) Gas oil (%) butane (%)

1987 74 15 11 1988 70 25 5 1989 69 26 5 1990 68 27 5 1991 71 24 5 1992 71 24 5 1996 76 20 4 2000 70 25 5 2003 70 27 3

dollars based on an ethylene price of 750 dollars per in the Middle East will be based on ethane cracking metric tonne. and, therefore, the feedstock mix will change in the The economics for the production of ethylene coming years. This change will also impact depends to a large extent on the prices for feedstocks production of coproducts like propylene, butadiene, and coproducts. In the US, the feedstocks of choice and benzene. As can be seen from Table 8, have been the lighter feeds like ethane, propane, and approximately 70% of US ethylene production is butanes, as opposed to Western Europe and the Far from ethane, propane, and butane. This percentage East, which have favoured naphtha (DeWitt and has remained fairly constant throughout the 1980s Company, 1989-1990). Table 8 shows the percentage and 1990s, and it is assumed that it will remain at of ethylene produced from the various feedstocks in this level. During the same time frame, the use of the US since 1987. naphtha as a feedstock has nearly doubled, The hydrocarbon feedstock used for the supplanting the use of gas oil as a feedstock. production of ethylene is shown in Fig. 7. Naphtha As an example, Table 9 provides the cost of cracking represents about 45% of production production of ethylene for ethane feedstock based on capacity, while nearly 35% of capacity is produced 2003 prices for the feedstocks and coproducts. This from ethane cracking. Most of the capacity addition cost of production is based on US Gulf Coast energy costs around the hot section. Investment cost and labour costs are not included. The plant capacity is ethane naphtha 800,000 mta of ethylene, which is an average 2003 35% 45% size for a new plant. Energy costs are often expressed as the specific energy consumption, which is defined as the fuel equivalent value of all utility imports and exports around the ethylene unit. The specific energy consumption includes the fuel fired in the furnaces, the electric power required, and the export of steam. Unconverted ethane and propane (formed as product) are recycled to extinction. Acetylene and MAPD are hydrogenated to ethylene/ethane and propane propylene/propane respectively. Similar tables 9% C4 can be constructed for other feedstocks. 3% other gas oil 3% 5% A detailed utility balance for the recovery section can be constructed on a similar manner. Typically, Fig. 7. Distribution of the different feed for ethylene more than 80% of the cost of production is production types in the world. associated with feed cost. Hence the gross margin

VOLUME II / REFINING AND PETROCHEMICALS 575 BULK PRODUCTS AND PRODUCTION LINES IN THE PETROCHEMICAL INDUSTRY

Table 9. Economic analysis around the ‘hot section’. 800,000 mta ethylene capacity based on ethane cracking

Unit cost Rate Revenue Cost (dollars/t) (mta) (dollars) (dollars) Feed

HC feed 169 985,416 166,535,304 Reacted steam 2,526 Total 987,942 Products

Hydrogen 578 87,181 50,390,618 Methane off gas 155 40,282 6,243,710 Ethylene (polymer grade) 500 800,000 400,000,000 Propylene 430 12,477 5,365,110

C4s 205 25,738 5,276,290

C5-gasoline 124 20,613 2,556,012 Fuel oil 155 529 81,995 Acid gases 1,122 Total 987,942 Utilities*

Super high-pressure steam 9 1,700,000 15,300,000 Fuel 110 36,000 3,960,000 Dilution steam 6 380,500 2,283,000 Boiler feed water 2 1,780,000 3,560,000 Electricity (dollars/MW)** 3 5,120 15,360 Miscellaneous (dollars/h) 10 87,600 Total revenue/cost 485,213,735 176,441,264 Net income before tax (dollars) 308,772,471

* Decoking is included. Calculations are done on 8,000 h/yr of operation to account for unscheduled plant shutdown and maintenance ** Rate is in MW

analysis shown above is sufficient for illustrative site-specific feedstock availability determines the purpose. type of plant. Ethane feed gives the lowest cost of production Another significant factor affecting capital and the highest return on investment. As the feeds investment is plant capacity. Prior to 1970, ethylene become successively heavier, the cost of production plant capacity was rarely greater than 300,000 mta. increases marginally, but the return on investment Since the late 1990s, 1,000,000 mta plants have decreases due to the increasingly higher capital become common. The incentive to build these larger investment required. Table 10 shows the effect on plants is the economic advantages of scale, resulting relative capital investment for various feedstocks as from the reduction in capital requirements per tonne well as for a range of capacities. However, of ethylene.

576 ENCYCLOPAEDIA OF HYDROCARBONS ETHYLENE AND PROPYLENE

Table 10. Relative ethylene plant investment and cost of production as a function of feedstock and plant capacity

Plant capacity (mta) Ethane Propane Butane Naphtha Gas oil Relative plant investment

300,000 1.17 1.35 1.40 1.70 1.93 500,000 1.00 1.15 1.20 1.45 1.55 600,000 0.96 1.10 1.15 1.39 1.58 800,000 0.84 0.97 1.01 1.22 1.39 1,000,000 0.76 0.89 0.92 1.12 1.30 Relative cost of production

300,000 1.05 1.15 1.16 1.15 1.48 500,000 1.00 1.09 1.11 1.08 1.40 600,000 0.99 1.08 1.09 1.06 1.38 800,000 0.96 1.06 1.07 1.04 1.35 1,000,000 0.93 1.03 1.04 1.01 1.33

By 2000, it was possible to build a single Propylene is a very reactive intermediate and, train (no duplication of compressors or other therefore, is involved in many chemical reactions. equipment, except for cracking heaters) ethylene The chemistry of propylene is based mainly around plant with a capacity of 1,300,000 mta. The its double bond that reacts readily to form saturated limitation above 1,300,000 mta became the hydrocarbons, their derivatives or polymers. Its suction volume to the charge gas compressor structural formula is: and/or refrigeration machine. There are ethylene plants currently under development with ethylene H H capacity close to 2,000,000 mta based on ethane C H cracking and 1,500,000 mta based on liquid H C C feedstock. Other factors limiting the size of ethylene plants are: larger equipment items H H require more costly field fabrication The carbon-carbon bond distance for the – as opposed to shop fabrication – limiting double bond is 1.341 Å. potential savings in investment; and increased risk due to large initial capital outlay Manufacture and changing market conditions. Most propylene is produced as a by-product of ethylene plants based on the thermal cracking of 10.5.2 Propylene hydrocarbons and from refinery Fluid Catalytic Cracking (FCC) units based on vacuum gas Properties oil/residue feeds. Other processes for on-purpose propylene production include: metathesis Propylene (CH3CH CH2) is a colourless, technology, propane dehydrogenation, natural gas practically odourless, flammable gas and is the or methanol to olefins, and olefins interconversion building block for many petrochemicals and end processes using fluidized or fixed bed reactors. products, mostly driven by growth in demand for engineered plastics. The IUPAC name is propene. Metathesis technology Table 11 summarizes its physical, thermodynamic, Metathesis technology is becoming one of the and transport properties. major routes for on-purpose production of

VOLUME II / REFINING AND PETROCHEMICALS 577 BULK PRODUCTS AND PRODUCTION LINES IN THE PETROCHEMICAL INDUSTRY

Table 11. Physical properties of propylene

Property Value

Molecular weight 42.078 Triple point Temperature, °C 185.3 Pressure, kPa 9.5·107 Latent heat of fusion, kJ/mol 3.004 Normal freezing point Temperature, °C 185.3 Latent heat of fusion, kJ/mol 3.004 Normal boiling point Temperature, °C 47.7 Latent heat of vaporization, kJ/mol 18.42 Density of liquid g/ml 0.5139 47 d 4 0.609 Specific heat of liquid, J/mol·K 92.1 Viscosity of the liquid, mPa·s (=cP) 0.187 Surface tension of the liquid, mN/m (=dyn/cm) 16.51 Specific heat of ideal gas at 25°C, J/mol·K 71.21 Critical point Temperature, °C 91.85 Pressure, kPa 4620.4 Volume, cm3/mol 181 Compressibility factor 0.278 Gross heat of combustion of gas at 25°C, MJ/mol 2.058 Limits of flammability at atmospheric pressure and 25°C Lower limit in air, mol% 2.0 Upper limit in air, mol% 11.1 Auto ignition temperature in air at atmospheric pressure, °C 460 Pitzer’s acentric factor 0.148 Dipole moment, D 0.366 Standard enthalpy of formation at 25°C, kJ/mol Gas 20.0 Liquid 1.7 Standard Gibbs energy of formation at 25°C For ideal gas at atmospheric pressure, kJ/mol 62.72

Solubility in water at 0°C and 101 kPa, ml/ml H2O 0.446 Ionization energy, eV 9.730.02 Heat of ionization, kJ/mol 959 Standard entropy of formation, J/mol·K 266.61 Standard heat capacity, J/mol·K 64.31

propylene. The word metathesis (from the Greek molecules. The chemistry of this process is based met©qesij) is derived from the words meta- (after, on the straight swapping of groups between two between) and t…qhmi (to place), and in chemistry acyclic olefins in the presence of transition-metal refers to the interchange of atoms between two compounds, which is also called cross-metathesis.

578 ENCYCLOPAEDIA OF HYDROCARBONS ETHYLENE AND PROPYLENE

This chemistry, based on heterogeneous catalysts, form a new olefin and a new metal carbene that was first used in industrial production in a plant propagates reaction (Fig. 8). Metal carbenes play a (Shawinigan Chemicals) started up in 1966 based key role in the exchange of groups around the on the Phillips Triolefin Process (now owned and carbon-carbon double bonds. licensed by ABB Lummus Global as Olefins Katz and McGinnis (1975) analyzed the Conversion Technology or OCT). This plant kinetics of olefin metathesis. This paper explained produced ethylene and 2-butene from the that if groups around the double bond are metathesis of propylene. As this is a reversible sufficiently different, one metal carbene product reaction, if ethylene and 2-butene are used as will be favoured over the other. In this case, only reactants (feeds), then propylene is produced. The one product will be formed as predicted by the metathesis of olefins was first observed in the conventional mechanism. However, if groups are 1950s by many researchers working with not sufficiently different, three products can form: heterogeneous catalysts. Herbert S. Eleuterio of two cross-products and one predicted by the DuPont first observed in 1956 that a mixture of conventional mechanism. propylene, ethylene and 1-butene was produced Heckelsberg et al. (1969) and Banks (1979) when propylene was passed over a molybdenum on searched a number of heterogeneous catalysts for aluminum catalyst. Peters and Evering (1960) of metathesis reactions and found that molybdenum Standard Oil also recorded formation of ethylene and tungsten oxides supported on high-surface-area and butenes when propylene was passed over silica were active for olefin metathesis at molybdenum oxide on aluminum treated with temperatures several hundred degrees higher than triisobutyl aluminum. In the late 1950s, researchers optimum for the alumina based catalysts. While at Phillips observed the formation of propylene and most transition metal catalysts were active for 2-pentene when they passed n-butenes over metathesis, carbon-monoxide-treated magnesium molybdenum hexacarbonyl supported on oxide catalyzed the propylene reaction without a high-surface-area g-alumina (Banks and Bailey, promoter (Katz and McGinnis, 1975). These new 1964; Banks, 1986). Based on various observations, catalysts broadened the reaction temperature Banks and Bailey (1964) discovered a reaction involving olefinic bonds being catalytically cleaved and recombined to form new olefinic products. cross metathesis They also found that supported molybdenum and tungsten oxides were more active than X M corresponding carbonyls. R R 1 C C 1 Similar unexpected reaction products were R1 R1 observed due to the cleavage and reformation of the double bonds of olefins. Calderon et al. (1967, 1968) at Goodyear Tire & Rubber named the reaction Olefin Metathesis. Mol et al. (1968) mechanism of the University of Amsterdam, also observed X X X similar reactions in the presence of heterogeneous M M M catalyst.

Mechanism, kinetics and conversion M M Researchers proposed various reaction mechanisms to explain the olefin metathesis (Mol et al., 1968; Pettit, 1971; Grubbs and Brunck, 1972; Biefeld et. al., 1973). In 1971, Yves Chauvin and Jean-Louis Herisson of Institut Français du overall reaction Pétrole suggested that olefin metathesis is initiated R1 R2 X R1 R1 R2 R2 by metal carbene (Chauvin and Herisson, 1971; M Chauvin, 1973). This work led to the 2005 Chemistry Nobel Prize for Yves Chauvin along R3 R4 R3 R4 R3 R4 with the work of Robert H. Grubbs and Richard R. Schrock. In this mechanism, they proposed that the metal carbene reacts with an olefin to form a Fig. 8. Proposed mechanism of the olefin metathesis metallacyclobutane intermediate, which breaks to reaction.

VOLUME II / REFINING AND PETROCHEMICALS 579 BULK PRODUCTS AND PRODUCTION LINES IN THE PETROCHEMICAL INDUSTRY

range, favouring the desired metathesis reaction unconverted recycle ethylene. This mixed feed is over competing side reactions. This resulted in heated prior to entering the metathesis reactor higher equilibrium conversions (reducing reactor where isomerization and equilibrium reactions take size) and the catalysts were more resistant to place. The catalyst promotes the reaction of poisons. ethylene and 2-butene to form propylene and Experimental studies showed that the primary simultaneously isomerizes 1-butene to 2-butene. products of propylene metathesis were ethylene The per-pass conversion of 2-butene is greater than and cis-/trans-2-butene (Heckelsberg et al., 60%, with overall selectivity to propylene 1968). It was also observed that 1-butene exceeding 90%. The reactions occurring are favoured production of ethylene and hexene, essentially isothermal. The OCT reactor effluent while 2-butene favoured propylene and pentenes. contains mainly propylene and unreacted feed. It is The conversion of mixed n-butenes was higher cooled and chilled prior to entering the product than either 1-butene or 2-butene metathesis by recovery section. itself, indicating significant reaction between Reaction. OCT converts 2-butene and ethylene these butenes. The earlier, four-centred reaction to propylene via metathesis. Two main equilibrium mechanism scheme helped in understanding the reactions take place: metathesis and isomerization. double-bond isomerization activity. In some Propylene is formed by the metathesis of ethylene applications, eliminating isomerization activity and 2-butene, and 1-butene is isomerized to prevented secondary metathesis reactions, which 2-butene, as 2-butene is consumed in the resulted in high selectivity for the desired metathesis reaction. In addition to the main products. In other applications, the mechanistic reaction, side reactions between olefins also occur scheme indicated the need for higher (Fig. 9). Based on the reaction stoichiometry, isomerization activity. 3 tonnes of propylene are produced from 2 tonnes The term conversion is used to measure the of butene and 1 tonne of ethylene. extent of conversion of the reactant. Conversion Run length. Coke is produced as a side product, (X) can easily be measured for a single component: which deposits on the catalyst and limits the 11111212111111111component inletcomponente outlet activity and, therefore, the conversion. Some feed X component inlet impurities can also deposit on the catalyst, limiting activity. The catalyst requires periodic regeneration Inlet and outlet quantities are measured in weight units. The yield data for metathesis process can be main reactions measured by three criteria: • Ethylene selectivity (mol%): C H 2-C H 2C H metathesis 11 11111222 propylene 2 4 4 8 3 6 2 ethylene reacted ethylene 2-butene propylene

• Butene selectivity (mol%): 1-C4H8 2-C4H8 isomerization 1-butene 2-butene 11 1111122 propylene 2 butene reacted • Propylene selectivity (wt%): typical side reactions 11111111111propylene propyleneC and heavier 5 C3H6 1-C4H8 C2H4 2-C5H10 propylene 1-butene ethylene 2-pentene Commercial reactor In ABB Lummus Global’s OCT, the metathesis 2-C4H8 1-C4H8 C3H6 2-C5H10 of olefins is accomplished in a fixed bed, adiabatic 2-butene 1-butene propylene 2-pentene reactor. The reaction is slightly exothermic. The catalyst is a mixture of magnesium and tungsten 1-C4H8 C2H4 3-C6H12 oxides. The vapour phase reaction occurs in an 1-butene ethylene 3-hexene ethylene-rich environment and proceeds to

equilibrium conditions. Fresh C4s plus unconverted Fig. 9. Scheme of the reactions involved in the synthesis C4 recycle are mixed with ethylene feed plus of propylene by metathesis route.

580 ENCYCLOPAEDIA OF HYDROCARBONS ETHYLENE AND PROPYLENE

to restore activity. For regeneration, the coke is Recovery and purification burned off in a controlled nitrogen-air atmosphere. Fig. 10 is a schematic flow diagram of the OCT Feed impurities. The metathesis catalyst is process. The OCT reactor effluent contains a sensitive to poisons in the feed such as arsine, mixture of propylene, unconverted ethylene and

mercury, oxygenates, mercaptans, nitriles, etc. butenes, and some C5 plus components from Therefore, the feed must be treated prior to by-product reactions. After cooling, the reactor entering the reactor. effluent is sent to the recovery section. This section The metathesis process can be designed to consists primarily of two towers. The first tower

accept butene feed from various sources. Ethylene separates unreacted ethylene from C3, and heavies feed can be polymer grade ethylene or a dilute and it is recycled to the metathesis reactor. The ethylene stream. Any saturated hydrocarbons, such second tower processes bottoms from the ethylene

as ethane and methane, do not react. Possible C4 recovery tower to produce a polymer grade streams sources include: a) FCC mixed C4s; b) propylene overhead product and a C4 recycle steam cracking mixed C4s; c) butadiene extraction stream. Purge streams containing non-reactive light C4 raffinate; d) MTBE (or other isobutene removal material and C4s and heavier are removed and sent process) C4 raffinate; e) butene from ethylene to OSBL (OutSide Battery Limit). dimerization process. Any acetylenes or dienes, if present in the feed, Safety and environmental factors promote coking reactions. If butadiene is present, it The safety and environmental considerations can be selectively hydrogenated to butenes to are similar to the ones discussed for ethylene plants provide additional butenes feed for the metathesis in par. 10.5.1. reactor. The Selective Hydrogenation Unit (SHU) offers a highly selective catalyst for the Other routes to propylene production hydrogenation of butadiene to butenes with Other processes are available for the production minimal saturation losses. of propylene. Some have been applied If isobutene is present in large concentration, commercially, while others are available for it promotes coking reactions and has low per pass commercialization. conversion. Depending on the quantity of

isobutane and isobutene in the C4 feed, the unit Catalytic cracking processes design may include a deisobutenizer to extend FCC processes are the second largest source of the reactor run length between regenerations and propylene as a by-product, supplying nearly 30% reduce the required OCT unit throughput, of total worldwide demand. FCC units are resulting in an overall lower capital cost plant. designed primarily to produce motor gasoline and The deisobutenizer is a catalytic distillation distillates, and yield of these products typically tower that isomerizes 1-butene to 2-butene varies from 60-80 wt% on feed. Propylene is (i.e. CDIsom technology) to maximize recovery produced as a by-product and the amount of of OCT feed. propylene (and light olefins) can be varied by

Fig. 10. Schematic flow ethylene feed recycle ethylene lights purge diagram of an olefins conversion unit (ABB propylene Lummus Global OCT product Process). guard bed ethylene column ethylene propylene column propylene metathesis reactor

C feed C recycle 4 4 C4

VOLUME II / REFINING AND PETROCHEMICALS 581 BULK PRODUCTS AND PRODUCTION LINES IN THE PETROCHEMICAL INDUSTRY

using specialized catalyst formulations and olefinic by-products. These technologies utilize adjusting operating severity. The reaction zeolite type catalysts and use either a reactor temperature can be as high as 700°C, depending on system similar to FCC processes or multiple fixed operating severity. The propylene yields can vary beds operating at elevated temperatures. The from 4-22 wt% of feed. FCC processes are offered technologies included in this category are by ABB Lummus Global, ExxonMobil, Kellogg Atofina/UOP’s Olefin Cracking process, Brown & Root, Shaw Stone & Webster, Shell ExxonMobil’s Olefins Interconversion, Kellogg International, UOP, etc. These processes are highly Brown & Root’s Superflex, Lurgi’s Propylur, and capital intensive and are more suited to a refinery others. These processes have higher capital costs environment as they are built primarily to meet than metathesis, and their economics are motor gasoline demand. Most of the potential dependent on feed pricing. opportunities from existing units are through additional recovery and yield enhancement. Propylene as a by-product A small amount of propylene is produced as a Propane dehydrogenation by-product in various processes: thermal cracking Propane dehydrogenation processes have been processes in refineries (visbreaking, delayed used in some plants that have access to inexpensive coking off-gas, etc.), coal gasification, propane feed. The dehydrogenation reaction is Fischer-Tropsch process, etc. The concentration carried out in the gas phase at sub- of propylene in these processes is relatively small atmospheric/atmospheric pressure and 550-650°C and is normally not economical to recover. in the presence of catalyst (Chromia-alumina, Pt-on-alumina or Pt-on-Zn-aluminate). The major Shipment and storage source of catalyst deactivation is rapid coke formation, which requires frequent regeneration. Currently, nearly 3,000,000 mta of propylene The conversion per pass is limited, resulting in and propylene derivatives are traded worldwide. significant recycle flow. The overall selectivity The major export is from North America to Asia, a towards propylene is typically 85% or higher. region short on propylene. Propylene is normally ABB Lummus Global, UOP, and Phillips shipped as a high-pressure gas or refrigerated Petroleum offer these technologies. These plants liquid in cargoes between 5,000 to 10,000 tonnes. are capital intensive and economics are dependent The shipping rates from North America to Asia in on propane feed pricing. 2003 are on average nearly 150 dollars per tonne. Therefore, import of propylene monomer is not a Natural gas or methanol to olefins long-term economic option for the derivative These processes were developed to convert plants. Most propylene trade is in propylene stranded gas – a cheap methane source – to olefins. derivatives, mainly polypropylene or acrylonitrile. They will compete with Gas To Liquid (GTL) Asia will remain the major importer in the projects based on project economics. Natural gas is foreseeable future, with the majority of the export first converted to methanol, then methanol is originating from North America and, eventually, converted to olefins. These processes can produce the Middle East. varying ratios of ethylene and propylene. In North America and Western Europe, UOP/Hydro and ExxonMobil have developed pipelines distribute most of the propylene, processes to convert methanol to olefins (ethylene although some is shipped by tankers, barges and and propylene), while Lurgi’s process converts tank trucks. Most of the propylene in North methanol to propylene and gasoline. UOP/Hydro’s America is stored in salt domes and an estimated process uses silicoaluminophosphate (SAPO) 3.5 million tonnes of underground storage space is catalysts. A significant issue for this route is the reserved for propylene. Aboveground storage large amount of water (nearly 56% in methanol) includes high-pressure spheres, bullets, and that needs to be handled in the processing steps refrigerated low-pressure storage tanks. and transported. These projects are highly capital and energy intensive. Uses

Olefins interconversion processes Nearly two-thirds of propylene produced This covers a broad range of developing worldwide goes into the manufacture of

technologies that convert olefinic C4s (and polypropylene resin. Other major propylene heavier) in naphtha streams to propylene and other derivatives include acrylonitrile, oxoalcohols,

582 ENCYCLOPAEDIA OF HYDROCARBONS ETHYLENE AND PROPYLENE

propylene oxide, and cumene. Some important Oxo chemicals chemical uses are discussed below. The addition of synthesis gas to the double bond of propylene produces butyraldehydes or Polypropylene butanols. These reactions are carried out at Polypropylene is the largest end user of propylene elevated temperatures and pressures in the and is the fastest growing of all propylene derivatives presence of catalysts (cobalt or rhodium). due to its versatility. It is used in injection molding n-Butanol is used as a solvent for lacquers and applications (automobile parts, household appliances, coatings and is an intermediate for several bottle caps, containers, etc.), blow molding chemicals. n-Butyraldehyde is converted to applications (bottles, containers, etc.), fibres (carpets, 2-ethylhexanol, which is commonly used as a sportswear, etc.), and films/sheets (packaging, plasticizer alcohol. Isobutyraldehyde is converted laminates, liners, etc.). Polypropylene has a very low to isobutanol, which is a solvent for surface specific gravity (0.90-0.91 g/cm3) with relatively high coatings. stiffness, tensile strength, heat deflection temperatures, and good clarity and stress crack resistance. These Cumene properties, together with lower cost, make Cumene is produced by the alkylation of polypropylene a favourable choice compared to metals benzene with propylene. This reaction takes place or other polymers where its properties suffice for the at elevated temperature and pressure in the intended application. presence of a catalyst (zeolite or phosphoric acid). The polypropylene resins are produced either The earlier Hock process is a vapour phase by gas or slurry-phase polymerization using mostly alkylation reaction. The new processes are mostly Ziegler-Natta-type catalysts, although liquid-phase (or two-phase alkylation) processes. metallocene-catalyzed polypropylene production is All cumene is consumed in phenol production, gaining ground because the resins have better which is used to manufacture phenolic resins, physical and mechanical properties. Most of the caprolactam, and bisphenol. production processes require high purity propylene (99.5%). Table 12. Typical specification for polymer Propylene oxide grade propylene Propylene oxide is used to make propylene glycols and urethane polyether polyols. It can be Propylene, mol% 99.95 min produced by either hydroperoxidation or Propane + ethane, mol% Balance chlorohydrin processes. In the hydroperoxidation process, a hydrocarbon is first converted with Other impurities, mol ppm oxygen or air to a hydroperoxide, which then reacts Hydrogen 10 max with propylene to produce propylene oxide and a significant amount of coproducts. In the Acetylene 1 max chlorohydrin process, propylene first reacts with Methyl acetylene (MA) 1 max hypochlorous acid to produce propylene clorohydrin, which is then dehydrochlorinated Propadiene (PD) 1 max (using caustic soda or slaked lime) to propylene Oxygen 2 max oxide. Carbon monoxide 0.1 max Acrylonitrile Carbon dioxide 2 max Acrylonitrile is a commodity chemical used in various applications including polyacrylonitrile Butenes 10 max fibres, plastics (acrylonitrile-butadiene-styrene C4+s (saturates) 50 max resins), adiponitrile, etc. It is produced by the catalytic ammoxidation of propylene, a vapour Water 2 max phase reaction at low pressure (50-200 kPa) and Total sulphur 2 max moderate temperatures (400-500°C) in the Methanol 5 max presence of catalysts (Bi2O3/MoO3 with iron and additives). Chemical grade propylene is typically Butadiene 1 max used as feed. One kilogram of acrylonitrile Ammonia 5 max requires 1.10 to 1.20 kilograms of propylene.

VOLUME II / REFINING AND PETROCHEMICALS 583 BULK PRODUCTS AND PRODUCTION LINES IN THE PETROCHEMICAL INDUSTRY

Specification and analysis – or approximately 15 million metric tonnes – in Polypropylene, the predominant propylene comparison to the fastest growing Asia/Pacific region, derivative, requires very high purity propylene which produces more than 30%, or approximately 18 (99.5%). Table 12 shows a typical polymer grade million metric tonnes. More than half of US propylene propylene specification. Almost all propylene supply is recovered from refinery sources. plants use gas chromatography or gas While the US and Western Europe currently chromatography combined with mass spectrometry account for over 50% of the world’s production for analysis. For specific impurities like sulphur, capacity, the most rapid growth in capacity has been water, and other hydrocarbons and elements, occurring in the developing areas of the world. Table ASTM standard tests are recommended 13 summarizes world consumption of various (D2504-67, D2505-67, D-2712, D-3246, D5623). propylene derivatives. Table 14 shows the capacity increase from 1996 through 2003 for the major regions Economic aspects of the world. It is expected that capacity growth in the developing countries will continue to be more than In 2003, world propylene production capacity was twice that of North America and Western Europe. approximately 57 million metric tonnes, with nearly Propylene is a chemical with very high total market two-thirds coming from steam crackers, 30% from value among the various petrochemicals. Based on refinery FCC units, and the balance from on-purpose 2003 production capacity, the total potential production propylene production technologies (propane value is approximately 29 billion dollars based on a dehydrogenation and metathesis). US production propylene price of 500 dollars per metric tonne. capacity accounts for almost 25% of world capacity The economics for the on-purpose production of propylene depends to a large extent on the prices for feedstock and propylene. The metathesis process can Table 13. Propylene derivatives and world be integrated with a steam cracker or with a refinery consumption (million mta) FCC unit.

2003 Polypropylene 35.5 Bibliography Acrylonitrile 5.1 Amigues P. et al. (1990) Propylene from ethylene and butane- 2, «Hydrocarbon Processing», 83, 79. Propylene oxide 5.0 Banks R.L. (1979) Olefins from olefins, «CHEMTECH», 9, Cumene 5.9 494-500. Bradshaw C.P.C. et al. (1967) Olefin dismutation. Reactions Oxo chemicals 7.1 of olefins on cobalt oxide molybdenum oxide-alumina, «Journal of Catalysis», 7, 269-276. Others 6.7 Casey C.P., Burkhardt T.J. (1974) Reactions of Total 65.3 (diphenylcarbene)pentacarbonyltungsten(o) with alkenes. Role of metacarbene complexes in cyclopropanation and

Table 14. Propylene capacity (106 t/yr) and growth rate

Growth rate, Region 1996 2003 %/yr 1996-2003 North America 15.2 17 1.6 Western Europe 13.1 14.1 1 Asia /Pacific 11.5 18.1 6.6 Middle East and Africa 1.6 2.5 6.5

South and Central America 2.3 2.75 2.5 Eastern Europe 3.2 3.4 1 Total 46.9 57.85 3

584 ENCYCLOPAEDIA OF HYDROCARBONS ETHYLENE AND PROPYLENE

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