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REFINING PROCESSING THERMAL METHOD HISTORICAL DEVELOPMENT  1861: Cracking distillation (thermal decomposition with simultaneous removal of distillate) was noticed by chance by a stillman. It was recognized as a means of producing the valuable lighter product (kerosene) from heavier nonvolatile materials avec une technique very simple (1870 to 1900)  As the need for gasoline arose in the early 1900s, the necessity of prolonging the cracking process became apparent and pressure cracking evolved. The first successful method of converting heavier oils into gasoline was developed by Burton in 1912 which operated at about 400oC and 75 to 95psi. During the years 1914 to 1922, a number of successful continuous cracking processes were developed by which gas oil was continuously pumped through a unit that heated the gas oil to the required temperature, held it for a time under pressure, and then discharged the cracked material into distillation equipment where it was separated into gases, gasoline, gas oil, and tar.  As refining technology evolved throughout the 20th century, thermal processes, such as vacuum flashing, visbreaking, and coking, came into wide usage by refiners with the goal to achieve lighter products from the residuum or heavy distillate from a distillation unit or upgrade the heavies oils. Thermal Method 2 THEORETICAL BACKGROUND   Thermal processes are chemical transformations of pure hydrocarbons or petroleum fractions under the influence of high temperatures. Most of the transformations are cracking by a radicalic mechanism. The thermal processes comprise the following types of industrial processes:  PYROLYSIS (STEAM CRACKING): used for production of ethene and propene for the chemical industry. The pyrolysis of liquid feed stocks, leads also to butadiene, isoprene, and C6-C8 aromatics. Characteristic for the pyrolysis process are temperatures of about 900–950oC and low pressures (< 5 bar). At the present, pyrolysis is the most important thermal process.  VISBREAKING: Used for producing fuel oils from heavy residues. The process is characterized by relatively mild temperatures (around 500oC) and pressures of 15–20 bar. Recently, processes at much lower pressures, sometimes atmospheric, were also developed  COKING: Used for producing petroleum coke from heavy residues. There are two types of coking processes: the delayed coking realized at about 490oC, and a pressure of 5–15 bar in coke drums, and fluid coking realized at about 570oC and 2–3 bar, in a fluidized bed. Of some importance is the production of needle coke, which is used for the production of electrodes especially for electrometallurgy processes (e.g. aluminum) Thermal Method 3 THERMODYNAMICS  Thermodynamic calculations show that the thermal decomposition of alkanes of higher molecular weight may take place with high conversions even at relatively low temperatures. Thus, n-decane may convert to over 90% to form pentene and pentane at 350oC and 1 atmospheric pressure.  The great number of parallel–successive reactions that may take place results in the final product distribution being controlled by the relative rates of the reactions that take place and not by the thermodynamic equilibrium.  The situation is different for the lower alkanes. Thus, in order to achieve a conversion of 90% in the decomposition of butane to ethene and ethane at a pressure of 2 bar, a temperature of near 500oC is required. In these conditions the dehydrogenation reaction reaches a conversion at equilibrium of only about 15% Thermal Method 4 The thermodynamic equilibrium for reactions REACTION MECHANISMS     The initial chemical phenomenon, which take place at high temperatures, is the breaking of the hydrocarbon molecule in two free radicals The energy of the C–C bonds, for alkanes in general, is much lower than the energy of the C–H bonds, the reaction of breaking a C–C bond will be prevalent over breaking a C–H bond. Therefore, in the pyrolysis not only for ethane but also of other alkanes, a C–C bond will be preferentially broken. The cracking of the C–H bonds may be neglected, since their rate is approximately two orders of magnitude lower than for C–C bonds The energy of the C–C bonds of tertiary and especially quaternary carbon atoms is lower than the others; thus they will be preferentially cracked. The double and triple bonds have bonding energies much higher that the single C–C bond. The energy of C–C bonds in the  position to a double bond is higher than that of bonds in the  position, and significantly lower than the energy of the C–C bonds in alkanes In all cases, the initial step in the thermal decomposition of hydrocarbons is the breaking of a C–C bond. The cracking of various C–C bonds takes place with comparable rates, with the exception of those in the position to double bonds, to triple bonds, or to aromatic rings, the rate of which can be neglected Thermal Method 6 REACTION MECHANISMS Types of transformations undergone by the formed radicals:  isomerization:  decomposition:  the reverse of which is the addition to the double bond:  substitution: Thermal Method 7 KINETICS OF THERMAL PROCESSES  The kinetic equations can be deduced on the basis of the reaction mechanism. For the thermal decomposition of a hydrocarbon, the chain mechanism may be represented by the generalized scheme Thermal Method 8 The kinetics of coke formation The formation of coke during thermal processes evidences two different aspects: 1. The formation of coke deposits inside the pyrolysis tubes, where the feed and the reaction products are all in vapor phase 2. The formation of coke as a consequence of the reactions taking place in the liquid phase, even if a vapor phase is also present - as in the processes of visbreaking, delayed coking, and in all classical processes of thermal cracking  Thermal Method 9 Coke formation in pyrolysis processes  Coke forms gradually on the inner wall of the furnace tubes during pyrolysis. The rate of coke deposition increases as the molecular weight of the feed and its aromatic character increase. However, coke formation is exhibited even by very light raw materials such as ethane.  In the first stage, coke filaments are formed due to reactions on the metal surface (iron and nickel). This catalytic effect is stronger at the beginning of the cycle when the tubes are clean. Once the coke filaments have appeared, coke formation is amplified in subsequent stages, by two mechanisms: a) very small droplets of heavy liquid molecules resulting from reactions of dehydrogenation and condensation accumulate in the filaments and b) the trapping of methyl, ethyl, phenyl radicals and acetylene by free radicals existing on the coke surface.  The mechanism of coke formation during the pyrolysis of ethane and propane is represented by the following sequence: ethylene, propylene  cyclic olefins  benzene, alkyl-benzenes  condensed aromatics  coke Thermal Method 10 Coke formation during delayed coking and visbraking  In these processes, the initial stages of the thermal decomposition reactions have a specific character since they take place in liquid phase: 1 liter of gas at 500oC and 1atm contains approx. 1022 molecules while 1 liter of liquid contains approx. 1024 molecules  the concentrations in the liquid phase are equivalent to the concentrations in the gas phase at pressures of about 100 bar. Due to this effect, there will be an increase in the liquid phase of bimolecular reactions such as polymerization, condensation and the bimolecular interactions of thermal decomposition, as compared to the gas phase.  As a result, the cracking reactions in the liquid phase convert the feed to products of a higher average molecular mass such as asphaltenes, which remain mostly in the liquid phase and undergo increasingly advanced condensation and dehydrogenation, eventually becoming coke. Thermal Method 11 Coke formation during delayed coking and visbraking  The following chain mechanism was suggested for the polycondensation reaction, which transforms asphaltenes into coke precursors such as carboids where A is a molecule of asphaltene and M represents the light molecules that go into the gas phase. Reaction (a) is the chain initiation; reactions (b),. . .,(d) are the chain propagation and reaction (n) is the chain termination. is an inactive radical. Thermal Method 12 INFLUENCE OF OPERATING CONDITIONS     temperature, pressure, feedstock properties, steam introduced into the reactor Thermal Method 13 Influence of temperature      The influence of temperature must be examined from the point of view of both the modification of the thermodynamic equilibriums and of the relative rates of the decomposition and other reactions that take place within the process The increase in temperature has a favorable effect on the conversion to alkenes. Therefore, considerable efforts are made to increase the coil outlet temperatures (COT), in pyrolysis furnaces. Higher COT favor also the formation of acetylene from ethene, of butene from butane and of butadiene from butene. However, it is not possible to reach a maximum production of all these three hydrocarbons in the same time. The reason is in the difference among the rates of the reactions that produce them and in the fact that the desired products suffer further decomposition. Higher operating temperatures also favor the equilibrium for the formation of aromatic hydrocarbons from alkylcyclohexanes With increasing temperature the average molecular mass of the formed products decreases In the cracking processes of crude oil fractions, the reactions of the intermediary products resulting from the decomposition, have higher activation energies than the reactions of the feed. Higher operating temperatures will reduce the ratio of the rate constants and, also reduce the maximum concentration of the intermediary product. The time for obtaining the maximum will also be shorter Thermal Method 14 Rate constants for the conversion of asphaltenes to coke: 1-asphaltenes precipitation; 2-coke formation Thermal Method 15 Influence of pressure    The reduction of pressure favors the increase in conversion of alkane (ethane to ethene, propane to propene, 1-butane to butadiene) and in the dehydrogenations of the 6-carbon atoms ring cyclo-alkanes to aromatic hydrocarbons The polymerization of the alkenes, produced during the processes of cracking under high pressure, is favored by the increase in pressure The effect of pressure upon the kinetics of the radical chain decomposition may be analyzed also by examining its influence on the mechanisms of the following reactions:     cleavage of free radicals: The decomposition of the radicals is a succession of cracking steps that occur in  position, at time intervals corresponding to the medium life duration of the radicals. Since the reaction kinetics is of first order, the time during which the transformation took place to some extent does not depend on the concentration substitution reactions, i.e. the reactions of radicals with feed molecules: The substitution relations are of second order. Therefore, the time to reach a certain degree of transformation is inversely proportional to the concentration The increase of pressure, causes an increase of the concentration. The result is that the frequency of the substitution reactions is higher than that of the decomposition reactions. This means that the substitution reactions may take place before all the cleavages in  position took place. For this reason the average molecular mass resulting from the decomposition is higher at higher pressure. Thermal Method 16 Influence of pressure     The pressure also influences the precipitation of asphaltenes, which is a determining stage in the formation of coke at the temperatures of thermal cracking. By increasing the pressure, the liquid phase will become enriched in lighter hydrocarbons, in which asphaltenes are less soluble. This facilitates their precipitation and increases the rate of coke formation. This effect is the usual cause of coking in thermal cracking. However, if the system pressure exceeds the critical pressure of the hydrocarbons mixture in the vapor phase, an opposite effect may come into play. In supercritical conditions, the solubility of the heavy hydrocarbons is increased and their precipitation as well as coking will be retarded. As an end effect, the formation of coke becomes disfavored by the increase of pressure. With respect to coke formation, the processes of cracking at high pressure requires a thorough analysis in order to take into account the nature of the hydrocarbons present in the system and the pressure in the reactor. In the presence of other favorable process parameters such as high temperatures, a reduced pressure in the process may lead to a strong increase of the aromatization reactions especially by the dehydrogenation of cycloalkanes Thermal Method 17 Influence of pressure and temperature  The mentioned effects of the temperature and pressure on the thermal processes, resulted in the selection of two extreme regimes, determined by the objective of the process:    very high temperatures (limited only by the possibilities of technical implementation) and partial pressures as close as possible to the atmospheric, for pyrolysis processes. Here, the main objective is the production of ethene and propene. moderate temperatures (as low as possible, but sufficient for achieving reaction rates conducive to reasonable reactor sizes) and high pressures (limited by economic criteria), when the production of liquid fractions is targeted. The consequences of these two extreme regimes on the yields and the chemical character of the products, may be illustrated by the following data:   In the pyrolysis of liquid fractions, the gases represent approximately 73% for the pyrolysis of gasoline and about 50% for the pyrolysis of gas oil, whereas for the cracking at higher pressures, the maximal yield is about 12%. In the gases from pyrolysis (the fraction C1 –C4), ethene represents 30–35% by weight, propene 14–18%, butadiene about 5%, and unsaturated hydrocarbons represent around 75%. In the gases produced in the processes operating at high pressures, the alkenes generally represent about 25%. Thermal Method 18 Influence of Feed composion   The molecular mass and the chemical composition of the feedstock determine the conversion as well as the distribution of the products obtained from thermal processes. This connection between the characteristics of the feedstock and the result of the process is expressed quantitatively by the kinetic equations and the rate constants. Because the thermal cracking and the pyrolysis processes are strongly influenced by temperature, the overall conversion may be modified without difficulties by a small change in the temperature at the reactor exit. It must be taken into account that, in visbreaking and delayed coking, which are performed at temperatures around 5008C, the reaction rate doubles for a temperature increase of 12–158C. In pyrolysis, the doubling of the rate requires an increase of about 258C. A more important change of the temperature may be limited by the highest value tolerated by the tubes of the furnace. This is, among others, the reason for the special care paid to the selection of the value of the kinetic constants used in the design of the pyrolysis furnaces. This is of lesser importance for processes that use lower temperatures (thermal cracking, visbreaking, delayed coking, etc.). Thermal Method 19 Influence of Feed composion The variation of the conversion to GO for two feeds having different chemical character: 1-paraffinic, low in resins and asphaltenes; 2-naphthenic—aromatic, with more resins and asphaltenes Thermal Method 20