4040
Ind. Eng. Chem. Res. 2001, 40, 4040-4049
Simulation of the Effects of CCl4 on the Ethylene Dichloride Pyrolysis Process Byung-Seok Choi,† Joo Seok Oh,‡ Sang-Wook Lee,‡ Hwayong Kim,§ and Jongheop Yi*,† Environmental Materials and Process Laboratory, School of Chemical Engineering, Seoul National University, 151-742 Seoul, Korea, Research and Engineering Center, Hanwha Chemical Corporation, 305-345 Taejon, Korea, and Thermophysical Properties Laboratory, School of Chemical Engineering, Seoul National University, 151-742 Seoul, Korea
Modeling of the ethylene dichloride (EDC) pyrolysis reaction was performed with respect to 108 reversible elementary reactions with 47 molecular and radical species. Kinetic schemes and reaction pathways have been developed based on thermochemical kinetic theories, such as thermal decomposition of molecules and radical-chain reactions, especially the abstraction of H by a Cl radical. In particular, mass, energy, and momentum conservation equations in a gasphase plug reactor were solved simultaneously with the established reaction mechanisms using a numerical scheme for stiff ordinary differential equations. Because of the characteristics of Cl-catalyzed reaction mechanisms, Cl suppliers play key roles in promoting the reaction conversion. This study reports on the calculation and analysis of the effects of CCl4, as a promotor, on the process conversion to vinyl chloride monomer, concentration of the coking precursor, changes in the heat required, and the pressure drop. The study addresses quantitatively and qualitatively the reaction mechanisms for the pyrolysis of EDC and the predicted results from the calculation. Simulation results are in good agreement with commercial plant data and, as a result, they should be useful for modifying and optimizing the EDC pyrolysis process. Introduction The commercial significance of the vinyl chloride monomer (VCM) can be highlighted by the production of poly(vinyl chloride) (PVC), the world’s second most abundant plastic. Approximately 96% of the VCM production is used for the production of PVC. VCM was first produced commercially in the early 1900s via the reaction of HCl with acetylene.1 Ethylene became plentiful, and abundant supplies of low-cost liquefied petroleum gas became available in the early 1950s. Ethylene dichloride (EDC or 1,2-dichloroethane), produced from the direct chlorination and oxychlorination of ethylene, which can be thermally decomposed to VCM and HCl, then became on industrially important process. Large-scale commercial processes for EDC pyrolysis have been developed in the ensuing years.1 Such processes are operated at feed purities of 96-99 wt % EDC, a temperature range of 480-530 °C, and pressures of 6-35 atm. EDC conversion levels are normally maintained in the range of 50-65 wt % in order to optimize the economics among on-stream time, the cost of utilities, and conversions.2 Great care should be taken to ensure that EDC used for cracking to VCM is of high purity, because cracking is exceedingly susceptible to inhibition and fouling by trace amounts of impurities or additives. Some of these components, which can act either as promotors or as inhibitors, also have a synergic effect on undesirable coke formation.2 * To whom correspondence should be addressed. E-mail:
[email protected]. † Environmental Materials and Process Laboratory, School of Chemical Engineering, Seoul National University. ‡ Hanwha Chemical Corp. § Thermophysical Properties Laboratory, School of Chemical Engineering, Seoul National University.
Thus, it is necessary to add the proper amount of a promotor and to estimate the effects of the promotor on the performances of the process based on the reaction kinetics expressions and mechanisms. Studies on calculating the products with rigorous reaction mechanisms and individual reaction rate constants for thermal decomposition and chlorination of chlorinated hydrocarbons were reported from the early 1950s. In addition to the amounts of primary products, Kurtz calculated the amounts of byproducts resulting from interactions among the free-radical reaction intermediates and compared these values with the observed amounts, obtained from the experimental chlorination of methyl chloride.3 Techniques for the rapid and relatively quantitative estimation of thermochemical data and reaction rate parameters for a gas-phase reaction were summarized by Benson.4 Weissman and Benson published detailed results obtained from laboratory experiments and detailed modeling of the chlorine-catalyzed polymerization of methane.5 Ranzi et al. showed that the results of these previous studies could be applied to EDC pyrolysis reactions and proposed essential elements of a program for the simulation of a EDC pyrolysis operation.2,6 Although a series of previous studies have investigated the pyrolysis of chlorinated hydrocarbons and related kinetic modeling, the study is more comprehensive in nature. First, coupled integration of a continuity set with an energy and momentum equation for EDC pyrolysis reactions is performed, with respect to a typical coil-type plug reactor based on all elementary and reversible radical reaction mechanisms. The methods result in highly accurate predictions both for percent-order and ppm-order products quantitatively. Second, these results provide guidelines for the EDC pyrolysis unit considering the additive’s level up due to
10.1021/ie000836a CCC: $20.00 © 2001 American Chemical Society Published on Web 08/17/2001
Ind. Eng. Chem. Res., Vol. 40, No. 19, 2001 4041
Figure 1. Schematics of the EDC pyrolysis process. The dimensions of the firebox are 21.0 m long, 8.8 m high, and 3.4 m wide.
the lack of topical and quantitative study related with investigation on the additive CCl4 (carbon tetrachloride) of EDC pyrolysis process. Because CCl4 is known to be an efficient source of Cl radicals, it can be used to promote the pyrolysis reaction. Ranzi et al. reported that the addition of 1000 ppm of CCl4 led to a twofold increase in radical formation at 500 °C.6 The Cl radical, however, also acts as a promotor for undesirable coke formation. The detrimental consequences of the coking inside the reactor include an increase in the heat-transfer resistance from the furnace side and a pressure drop along the reactor and decreases in the process efficiency and VCM selectivity.7-9 Thus, the effects of CCl4 addition should be investigated in the view of the process. To analyze the effects of the addition of CCl4 as a promotor on the performances of the process, the establishment of molecular/radical reaction networks for EDC pyrolysis and the calculation of specific governing equations for the typical tubular reactor are performed. In this study, reaction mechanisms and networks based on catalysis by a Cl radical, thermal decomposition, and recombination are established together for the purpose. Rigorous sets of ordinary differential equations for a gas-phase plug reactor are solved simultaneously. The methodologies and results have the capacity for wide extrapolations and considerable flexibility in terms of components, mixtures, and operating conditions, and they are in good agreement with data obtained from an industrial operating unit. System Description The EDC pyrolysis process has four conventionally distinguishable sections: a radiation section, a convection section, a shock section, and a stack.10 A schematic diagram for this study is shown in Figure 1. As shown in the figure, the radiation section, the so-called firebox or furnace side of this study, contains two tubular reactors. The dimensions of the firebox are approximately 21 m long, 8.8 m high, and 3.4 m wide. The inside of firebox is heated by 88 burners distributed in four rows, which are placed on the largest two opposing
walls of the firebox. The heat required for the endothermic pyrolysis reactions is supplied via combustion of fuel from these burners. The burners are roughly in the form of the rectangular boxes through which pure methane or mixtures of methane and city gas are injected, while air is fed through a rectangular slit around the burners. The reason the firebox or furnace is more frequently referred to as the radiation section is that the temperature is so high that the main heattransfer mechanism is radiation.10 After the radiation section, the combustion gas flows through a shock area before passing into the convection section, where the heat is recovered by preheating the feed EDC stream. The combustion gas is emitted to the atmosphere through the stack after the convection section or heat recovery utilities that can have various forms, depending on the process designs.11 This study focuses on the modeling of the reactor including the pyrolysis reaction, which occurs in the tubular reactor which is located in the firebox. The overall length of the tubular reactor is approximately 320 m, and the diameter is 0.2 m. The reactor is a coiltype with 15 straight parts and 16 bends. The length of each straight part of the reactor is 19 m, and the length of each bend is 0.76 m. The data on the dimensions and scheme of the unit described above are obtained from a commercial operating plant. The investigated plant for this study has a capacity of 150 000 tons/year of VCM production with a liquefied natural ags consumption of $3.6 million/year. The EDC conversion level is maintained at approximately 58% under steady-state operating conditions with the recirculation of 390 molar ppm of CCl4. Although a small enhancement in the process conversion can result in considerable profit by the increase of a promotor, such effects need to be verified by simulation before any modification because detrimental results from the additive can often be significant. The objectives of this study are to calculate the effects of CCl4 concentration on conversion to VCM, concentrations of main coking precursors, change of heat required to get a temperature profile, and pressure drop. The EDC fed to the reactor is 96% pure and the level between 0 and 1200 ppm of CCl4 is added into the feed EDC stream to promote pyrolysis without changing the concentrations of other components, which are recycled from the process downstream and include trichloroethane, trichloroethene, and methyl chloride. Reaction Mechanisms and Kinetics At a temperature in the range of 425-550 °C, EDC undergoes thermal cracking to yield vinyl chloride and hydrogen chloride:
CH2ClCH2Cl f C2H3Cl + HCl
(1)
However, the molecular reaction or overall reaction described above is not representative of the actual chemical reaction steps. An analysis using the overall mechanism cannot consider real intermediate molecular and radical species. The pyrolysis reaction from EDC to VCM consists of complex Cl-catalyzed radical and molecular reactions.3 Because the pathways of the reactions are largely unknown, procedures for screening and simplifying assumptions are needed. The mechanisms of the reaction have been extensively investigated and shown to involve a sequence of free-radical intermediates.12 The reaction mechanisms of EDC pyrolysis
4042
Ind. Eng. Chem. Res., Vol. 40, No. 19, 2001
are composed of 108 elementary reactions and 47 species in this study. On the basis of chemical reaction equilibrium, all kinetic parameters for the reverse reactions are considered together in the modeling. Radical reactions consist of three steps: initiation, propagation, and termination. These reactions can be additionally categorized into eight main classes. The main classes are chain initiation reactions, H abstraction reactions, Cl abstraction reactions, radical addition reactions, radical decomposition reactions, purely radical reactions, purely molecular reactions, and chain termination reactions. The master sets of kinetic expressions are tabulated in Table 1. The initiation step for the EDC pyrolysis reaction involves the generation of Cl radicals from molecular species. EDC is the main source of supply for Cl radicals, but other impurities, such as CCl4, also generate Cl radicals.6
CH2ClCH2Cl f CH2ClCH2 + Cl
(2)
CCl4 f CCl3 + Cl
(3)
The abstraction of H by Cl radicals represents the main propagation step and the key point for analyzing the EDC pyrolysis reaction as well.3,4,6
CH2ClCH2Cl + Cl f CH2ClCHCl + HCl
(4)
When a Cl radical reacts with a chlorinated hydrocarbon, an H is abstracted from the chlorinated hydrocarbon, producing a chlorinated hydrocarbon radical. The reaction rate depends on the concentration of Cl radical and reaction temperature.3,4,6 Along the path of the radical decomposition reaction, the CH2ClCHCl radical decomposes to VCM and Cl radical again.
CH2ClCHCl f C2H3Cl + Cl
(5)
The EDC pyrolysis reactions consist not only of radical reactions but also of molecular thermal decomposition reactions. Some purely molecular reactions can play a significant role in this process. For example, the molecular dehydrochlorination of EDC has been extensively investigated and represents a relevant path for VCM under the usual operating conditions.6
CH2ClCH2Cl f C2H3Cl + HCl
(6)
Most side reaction products follow a reaction pathway similar to that of EDC pyrolysis. In particular, acetylene, which is regarded as a main precursor of coke generation, is involved in the same mechanisms of radical and molecular reactions. Acetylene is mainly obtained via dehydrochlorination of VCM. The first step in the formation of acetylene is H abstraction from VCM by Cl or a CH2Cl radical generated from other reactions.4,13
C2H3Cl + Cl f CHClCH + HCl
(7)
C2H3Cl + CH2Cl f CHClCH + CH3Cl
(8)
The decomposition of CHClCH radical to acetylene and Cl radical plays the key role in the production of acetylene.
CHClCH f C2H2 + Cl
(9)
Because the decomposition of CHClCH radical is an endothermic reaction, the rate of acetylene production increases with an increase of the temperature for EDC pyrolysis. Dehydrogenation of C2H4 is also known to be one of the acetylene production paths.14
C2H4 + CH2Cl f C2H3 + CH3Cl
(10)
C2H4 + Cl f C2H3 + HCl
(11)
C2H4 + CH2ClCHCl f CH2ClCH2Cl + C2H3
(12)
C2H3 radicals produced from H abstraction from C2H4 are continuously reacted with Cl or CH2Cl radicals to produce acetylene.
C2H3 + Cl f C2H2 + HCl
(13)
C2H3 + CH2Cl f C2H2 + CH3Cl
(14)
Some of molecular reactions can play a considerable role to form acetylene and HCl from VCM.4,5,13
C2H3Cl f C2H2 + HCl
(15)
Thus, a simplified kinetic scheme of pyrolysis and chlorination of EDC can be given as Figure 2. The major path for the formation of heavy species goes through the interaction of VCM or acetylene with the other components to form butadiene (C4H6), chloroprene (C4H5Cl), vinylacetylene (C4H4), and heavy chlorinated components.15 To formulate the above reaction mechanisms, NASAChemkin database format is used for the calculation of thermodynamic functions.16,17 Seven coefficients are used for the high- and low-temperature ranges. The heat capacity, enthalpy, and entropy for the kth species can be written in the following forms as a function of temperature:
Cpk/R ) a1k + a2kT + a3kT2 + a4kT3 + a5kT4
(16)
a3k 2 a4k 3 a5k 4 a6k a2k Hk ) a1k + T+ T + T + T + RT 2 3 4 5 T
(17)
Sk a3k 2 a4k 3 a5k 4 ) a1k ln T + a2kT + T + T + T + a7k R 2 3 4 (18) In addition, mixture-averaged thermodynamic properties are required for the analysis. The mixtureaveraged heat capacity, enthalpy, and entropy in molar units are calculated and used for the simulation. Rate expressions for the chemical reactions consider elementary reversible (or irreversible) reactions involving K chemical species, which can be represented in the general form as K
K
∑ ν′kiXk T k)1 ∑ ν′′kiXk
(i ) 1, ..., I)
(19)
k)1
The ith reaction stoichiometric coefficients νki are integer numbers, and Xk is the chemical symbol for the kth species. The prime indicates forward stoichiometric coefficients, and the double prime indicates reverse stoichiometric coefficients. Normally, an elementary
Ind. Eng. Chem. Res., Vol. 40, No. 19, 2001 4043 Table 1. Main Elementary Reactions for EDC Pyrolysis (Forward Reactions Only)a no.
reaction
A
b
E
1 2 3 4 5 6 7 8 9 10 11
Class 1. Chain Initiation Reactions CH2ClCH2CldCH2ClCH2 + Cl 1.01 × 1028 1.71 × 1038 C2H3CldC2H3 + Cl CH2Cl2dCH2Cl + Cl 1.02 × 1016 0.60 × 1016 CHCl3dCHCl2 + Cl CCl4dCCl3 + Cl 1.00 × 1016 CH3CldCH3 + Cl 1.26 × 1037 1.00 × 1016 C4H6Cl2 + MdC4H6Cl + Cl + M C4H5ClS + MdC4H5S + Cl + M 1.00 × 1016 1.00 × 1016 C4H5ClU + MdC4H5U + Cl + M C2H5CldC2H5 + Cl 1.00 × 1016 CH2ClCHCl2dCH2ClCHCl + Cl 1.00 × 1013
-4.6 -7.1 0.0 0.0 0.0 -6.9 0.0 0.0 0.0 0.0 0.0
86509.0 96370.0 76800.0 71000.0 70000.0 90540.0 66600.0 85900.0 94900.0 86200.0 77000.0
12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49
Class 2. H Abstraction Reactions CH2ClCH2Cl + CldCH2ClCHCl + HCl 1.00 × 1013 CH2ClCH2Cl + CH2CldCH2ClCHCl + CH3Cl 1.16 × 1011 1.00 × 105 CH4 + CH2ClCHCldCH2ClCH2Cl + CH3 C2H3Cl + CH2CldCH2CCl + CH3Cl 1.79 × 101 1.79 × 101 C2H3Cl + CH2CldCHClCH + CH3Cl C2H3Cl + CldCH2CCl + HCl 1.20 × 1014 C2H3Cl + CldCHClCH + HCl 1.20 × 1014 1.00 × 106 CH2ClCH2 + CH3CldC2H5Cl + CH2Cl C2H5Cl + CldCH2ClCH2 + HCl 2.50 × 107 CH3Cl + CldCH2Cl + HCl 9.30 × 106 1.26 × 1011 CH3Cl + CH3dCH2Cl + CH4 CH4 + CldCH3 + HCl 2.08 × 108 1.00 × 106 CH3Cl + CHCl2CHCldCH2ClCHCl2 + CH2Cl C2H4 + CH2CldC2H3 + CH3Cl 2.00 × 1012 C2H3Cl + CH3dCH2CCl + CH4 1.08 × 10 1.08 × 10 C2H3Cl + CH3dCHClCH + CH4 C2H3Cl + CHCl2dCH2Cl2 + CHClCH 1.00 × 106 1.00 × 106 C2H3Cl + CHCl2dCH2Cl2 + CH2CCl C4H4Cl + HCldC4H5ClS + Cl 1.00 × 106 C4H5Cl2U + HCldC4H6Cl2 + Cl 1.00 × 106 1.00 × 105 C4H5U + CH3CldC4H6 + CH2Cl C4H5U + HCldC4H6 + Cl 1.00 × 106 C4H5S + HCldC4H6 + Cl 1.00 × 106 1.00 × 1014 C2H4 + CldC2H3 + HCl C2H5Cl + CH3dCH2ClCH2 + CH4 4.40 × 102 4.40 × 102 C2H5Cl + CH3dCH3CHCl + CH4 C2H3Cl + CHCl2CHCldCHClCH + CH2ClCHCl2 1.00 × 106 C2H3Cl + CHCl2CHCldCH2CCl + CH2ClCHCl2 1.00 × 106 1.00 × 106 C2H3Cl + CHClCCldCHClCH + CHClCHCl C2H3Cl + CHClCCldCH2CCl + CHClCHCl 1.00 × 106 1.00 × 106 C2H3Cl + CH2ClCH2dCHClCH + C2H5Cl C2H3Cl + CH2ClCH2dCH2CCl + C2H5Cl 1.00 × 106 C2H3Cl + CH3CHCldCHClCH + C2H5Cl 1.00 × 106 1.00 × 106 C2H3Cl + CH3CHCldCH2CCl + C2H5Cl CH2ClCHCl + C2H4dCH2ClCH2Cl + C2H3 1.00 × 106 C2H6 + CH2ClCHCldCH2ClCH2Cl + C2H5 1.00 × 106 1.00 × 105 CH2ClCHCl + C2H3CldCH2ClCH2Cl + CHClCH CH2ClCHCl + C2H3CldCH2ClCH2Cl + CH2CCl 1.00 × 106
0.0 0.0 2.0 3.6 3.6 0.0 0.0 2.0 2.0 2.4 0.0 1.8 2.0 0.0 3.9 3.9 2.0 2.0 2.0 2.0 2.0 2.0 2.0 0.0 3.2 3.2 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0
3100.0 9000.0 25933.0 9620.0 14480.0 13300.0 13300.0 11408.0 680.0 3300.0 11600.0 2650.0 14158.0 12000.0 10490.0 12490.0 20158.0 17608.0 18283.0 29833.0 21733.0 23233.0 35158.0 7000.0 10340.0 9340.0 22408.0 19858.0 13708.0 11358.0 19558.0 17008.0 24616.0 20608.0 19258.0 13408.0 22408.0 19858.0
50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70
Class 3. Cl Abstraction Reactions CH2ClCH2Cl + CldCH2ClCH2 + Cl2 1.00 × 107 CH2ClCH2Cl + CH2CldCH2ClCH2 + CH2Cl2 1.00 × 106 4.00 × 105 CH2ClCH2Cl + CH3dCH2ClCH2 + CH3Cl CCl4 + CH3dCCl3 + CH3Cl 1.26 × 1012 CCl4 + CldCCl3 + Cl2 1.00 × 1014 1.00 × 106 C2H3Cl + CCl3dC2H3 + CCl4 C4H6Cl2 + CldC4H6Cl + Cl2 1.00 × 107 1.00 × 107 C4H5ClS + CldC4H5S + Cl2 C4H5ClU + CldC4H5U + Cl2 1.00 × 107 C4H5U + HCldC4H5ClU + H 1.00 × 106 1.00 × 106 C4H5S + HCldC4H5ClS + H CH3 + C2H3CldCH3Cl + C2H3 3.00 × 1011 C2H3Cl + CHClCCldC2H3 + C2HCl3 1.00 × 106 1.00 × 106 C2H3Cl + CH2CCldC2H3 + CCl2CH2 C2H3Cl + CH2ClCH2dC2H3 + CH2ClCH2Cl 1.00 × 106 1.00 × 106 C2H3Cl + CH3CHCldC2H3 + CH3CHCl2 Cl2 + C2H3dC2H3Cl + Cl 5.24 × 1012 CH2ClCHCl + CH3CldCH2ClCHCl2 + CH3 1.00 × 1011 1.00 × 106 CH3CHCl2 + CH2ClCHCldCH3CHCl + CH2ClCHCl2 CH2ClCHCl + C2H3CldCH2ClCHCl2 + C2H3 1.00 × 1012 1.00 × 1012 CH2ClCHCl + Cl2dCH2ClCHCl2 + Cl
2.0 2.0 2.0 0.0 0.0 2.0 2.0 2.0 2.0 2.0 2.0 0.0 2.0 2.0 2.0 2.0 0.0 0.0 2.0 0.0 0.0
28108.0 11283.0 16908.0 9900.0 20000.0 27808.0 17908.0 27900.0 36900.0 28858.0 41533.0 17983.0 14233.0 13783.0 17758.0 21508.0 -480.0 17950.0 16783.0 18550.0 6175.0
4044
Ind. Eng. Chem. Res., Vol. 40, No. 19, 2001
Table 1. (Continued) no.
reaction
A
b
E
71 72 73 74 75
Class 4. Radical Addition Reactions C2H2 + C2H3dC4H5U 7.94 × 108 C2H2 + C2H3dC4H5S 7.94 × 108 C2H3Cl + C2H3dC4H6Cl 2.28 × 1027 C2H3Cl + C2H3dC4H6 + Cl 2.13 × 108 C2H3Cl + CH2CCldC4H5ClS + Cl 7.94 × 1013
0.0 0.0 -4.6 1.7 0.0
6900.0 6900.0 11778.0 9157.0 12844.0
76 77 78 79 80
CH2ClCHCldC2H3Cl + Cl CHClCCldC2HCl + Cl CHClCHdC2H2 + Cl C4H5Cl2UdC4H5ClU + Cl C4H6CldC4H6 + Cl
Class 5. Radical Decomposition Reactions 1.58 × 1013 3.82 × 1028 1.50 × 1013 3.00 × 1013 3.00 × 1013
0.0 -5.0 0.0 0.0 0.0
20600.0 37441.0 23000.0 37000.0 41600.0
81 82
Class 6. Pure Radical Reactions CH2CCl + C2H3dC4H5S + Cl 3.46 × 1011 C2H3 + CH2ClCHCldC4H6Cl + Cl 1.41 × 1014
0.0 0.0
10443.0 10416.0
83 84 85 86 87
CH2ClCH2CldC2H3Cl + HCl C2H3CldC2H2 + HCl C4H6Cl2dHCl + C4H5ClS C4H6Cl2dHCl + C4H5ClU C2H5CldC2H4 + HCl
Class 7. Pure Molecular Reactions 1.43 × 1012 2.75 × 1017 3.98 × 1010 3.98 × 1010 3.20 × 1013
-0.7 -1.3 0.0 0.0 0.0
58920.0 69312.0 49000.0 51000.0 57600.0
Class 8. Chain Termination Reactions CH2Cl + CH2CldCH2ClCH2Cl 3.00 × 1038 CH2ClCHCl + CldCHClCHCl + HCl 1.00 × 108 CHClCCl + CldC2Cl2 + HCl 1.00 × 108 CH2Cl + CH2CldC2H3Cl + HCl 1.10 × 1024 CH2ClCH2 + CldC2H3Cl + HCl 1.10 × 1030 C2H3 + CldC2H2 + HCl 4.70 × 1025 C2H3 + CH2CldC2H2 + CH3Cl 1.00 × 1013 C2H3 + CH2ClCHCldC4H5ClS + HCl 1.98 × 1013 C2H3 + CH2ClCHCldC4H5ClU + HCl 1.98 × 1013 CH2CCl + C2H3dC4H5ClS 1.29 × 1012 C4H5Cl2S + CldC4H4Cl2 + HCl 1.00 × 107 C4H5Cl2S + CldC4H5ClS + Cl2 1.00 × 107 C4H5Cl2U + CldC4H4Cl2 + HCl 1.00 × 107 C4H5Cl2U + CldC4H5ClU + Cl2 1.00 × 107 C4H5Cl2S + CH3dC4H5ClS + CH3Cl 1.00 × 105 C4H6Cl + CldC4H5ClS + HCl 1.00 × 107 C4H5U + CldC4H4 + HCl 1.00 × 105 C2H5 + CldC2H4 + HCl 2.36 × 1023 C2H3 + CH2ClCHCldC4H6Cl2 1.21 × 1017 CH3CHCl + CH2ClCHCldC4H6Cl2 + HCl 3.69 × 1013 CH2ClCH2 + CH2ClCHCldCH2ClCH2Cl + C2H3Cl 1.00 × 105
-8.0 2.0 2.0 -3.2 -4.7 -3.2 0.0 0.0 0.0 0.4 2.0 2.0 2.0 2.0 2.0 2.0 2.0 -2.6 -1.2 0.0 2.0
9431.0 0.0 3080.0 8200.0 17464.0 11790.0 0.0 7127.0 7127.0 1565.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 9735.0 3103.0 10689.0 0.0
88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108
a k ) ATb exp(-E/RT) [mol/cm3‚s]. A: frequency factors, [1/s] for unimolecular reactions and [cm3/mol‚s] for bimolecular reactions. b: exponent of temperature. E: activation energies [cal/mol]. The reaction coefficients for backward reactions are calculated as in this text.
I
w˘ k )
νkiri ∑ i)1
(k ) 1, ..., K)
(20)
where
νki ) ν′′ki - ν′ki
(21)
The rate ri for the ith reaction is given by the difference of the forward and reverse rates as K
ri ) kfi Figure 2. Simplified reaction scheme for the pyrolysis of EDC.
reaction involves only three or four species; hence, the νki matrix is quite sparse for a large set of reactions. The production rate w˘ k of the kth species can be written as a summation of the rate of the variables for all reactions involving the kth species:
∏ k)1
K
[Xk]ν′ki - kri
[Xk]ν′′ ∏ k)1
ki
(22)
where [Xk] is the molar concentration of the kth species and kfi and kri are the forward and reverse rate constants of the ith reaction, respectively. The forward rate constants for the ith reactions are assumed to follow the Arrhenius temperature dependence relationship. On the basis of thermodynamics, the reverse rate constants are related to the forward rate constants through the equilibrium constants. The equilibrium
Ind. Eng. Chem. Res., Vol. 40, No. 19, 2001 4045
constants are obtained with the relationship of entropy and enthalpy changes of the reactions. Reactor Simulation The simulation of a pyrolysis reactor requires the integration of a set of continuity equations for the process gas species, along with the energy and momentum equations. These three equations, which were originally proposed by Plehiers, can be written as
dFk πdt2 ) w˘ k dz 4
[
(23) 2
πdt dT 1 Q(z) πdt + ) Σri(-∆Hi) dz ΣFkCpk 4 Fu
[
]
dpt ζ 2f du + Fu2 )dz dz dt πrb
]
(24) (25)
where Fk represents the molar flow rate of the kth species and dt is the inner diameter of reactor.18 Q(z) and ∆Hi are the heat flux from the furnace side and the heat of the ith reaction, respectively. f is the friction factor which describes the pressure drop in the straight tube, and the factor ζ is used for the supplementary pressure drop in the bends of radius rb as given by Nekrasov.19 A one-dimensional plug-flow reactor model is assumed under the consideration of high Reynolds number and low viscosity of the reaction side stream. A temperature profile for the reaction side is obtained from the commercial plant data, and the heat flux profile along the reactor axial position is calculated from the temperature profile and heat of reaction. Numerical Methods Stiffness occurs in the set of governing equations, because the value ratios of dependent variables are sufficiently large and the change of pressure is very stiff at the bending zone of the reactor. A numerical scheme for such a stiff problem is required to solve this set of equations. Of the higher-order methods for the stiff problem, the Kaps and Rentrop methods, which are referred to as the Rosenbrock algorithms, are implemented in this study.20 These methods have the advantage of being relatively simple to understand and easy to implement, and for moderate accuracies ( < 10-410-5 in the error criterion), they are competitive with the more complicated algorithms.20 For more stringent parameters, the Rosenbrock methods remain more reliable but are less efficient than competitors, such as the semiimplicit extrapolation method. Critical to the success of a stiff integration scheme is an automatic step-size adjustment algorithm. Kaps and Rentrop discovered an embedded or Runge-Kutta-Fehlberg method. Two estimates are computed: the real one and a lower-order estimate with different coefficients. The difference between the real and the lower-order methods leads to an estimate of the local truncation error, which can then be used for step-size control. The simulation program is coded with Fortran 90 language. The calculation time for each case is about 3.5 min of running time on a Pentium-III 500 MHz personal computer. Results and Discussion Operation parameters such as conversion, heat flux, the concentration of coking precursor, and the pressure
Figure 3. Temperature profile inside the tubular reactor. Table 2. Kinetic Parameters for the Initiation Reactions of Cl Radicalsa reaction
A
b
Ea (kcal/mol)
CH2ClCH2Cl f CH2ClCH2 + Cl CCl4 f CCl3 + Cl
1.00 × 1028 1.00 × 1016
-4.6 0.0
86.509 70.000
a
k ) ATb exp(-Ea/RT).
drop are investigated to evaluate the effects of CCl4 addition on the EDC pyrolysis process in this work. The concentration of CCl4 is changed from 0 to 1200 molar ppm; thus, the four simulated values are 0, 400, 800, and 1200 ppm in this study. Concentration levels of this promotor in an actual plant can be varied according to the plant configuration and operating conditions. The plant examined in this study maintains a level of about 390 molar ppm of CCl4 and a 58% conversion to control the economics and shutdown period due to decoking. Figure 3 illustrates the temperature profile of the reaction stream inside the tubular reactor, which is assumed to be maintained by controlling the fuel feeding rate at the furnace side. Open circles represent data for an actual process, and the solid line represents the numerical regression of a sixth-order polynomial as a function of the axial position of the reactor. Temperature increases very rapidly up to 70 m, and the overall profile appears convex in shape. The EDC pyrolysis reaction is known to start at about 700 K, and thus increasing gradient decreases from the position at 70 m where 700 K is reached, because heat is dominantly used for the endothermic reaction and not for the elevation of temperature from that position. The temperature reaches approximately 760 K at the outlet of reactor. The abstraction of H by a Cl radical is a major pathway in the EDC pyrolysis reaction although the reactions consist not only of radical reactions but also of thermal decomposition reactions. The initiation step in generating a Cl radical is a key step in the process. Thus, the reason for the addition of a small amount of CCl4 increases the conversion of the process, compared with the case without CCl4 addition, is that the rate and the equilibrium constant for a reaction donating a Cl radical from CCl4 are much greater than those for the reaction for donating a Cl radical from EDC. The activation energy for the reaction, CCl4 f CCl3 + Cl, is 70 kcal/mol, and that for CH2ClCH2Cl f CH2ClCH2 + Cl is 86.5 kcal/mol, as shown in Table 2. In addition, because the exponent of the temperature in the modified Arrhenius-type rate equation for the reaction donating
4046
Ind. Eng. Chem. Res., Vol. 40, No. 19, 2001
Figure 4. EDC conversion at the reactor outlet as a function of the concentration of added CCl4. Comparison between the conversions of the conditions of controlled temperature by heat flux change and controlled heat flux by temperature change.
Cl radical from EDC is negative, the reaction for donating a Cl radical from CCl4 becomes more significant with increasing temperature. The differences of reaction rate coefficients for donating a Cl radical from CCl4 and EDC increase as the temperature increases. The conversions as a function of the concentration of CCl4 addition are plotted in Figure 4. The figure compares the cases for conversions at conditions of controlled temperature profile by heat flux change versus a controlled heat flux profile as a result of temperature change. The square symbols represent results which were obtained by increasing the amount of added CCl4 to the EDC feed stream without changing the profile of the process temperature. On the other hand, the diamond symbols denote conversions achieved as the result of increasing the amount of added CCl4 to the EDC feed stream without changing the heat flux profile along the reactor. Figure 4 also shows that the simulated result is in good agreement with plant data for a 58% conversion with 390 ppm of CCl4 addition. The calculated value indicates the conversion to be 58.8% at this point. As shown in the figure, the increase in added CCl4 does not have a significant effect on the conversion without an increase in heat flux. This is because increasing CCl4 results in a temperature decrease. In the case of increasing CCl4 with the heat flux change in order to maintain the prescribed profile of process temperature, the data show that the final conversions of EDC increase from 52.4 to 65.4% with increasing CCl4 concentration from 0 to 1200 molar ppm. An increase of 13% in EDC conversion corresponds to an increase of 1900 ton/year of VCM production in the investigated plant being investigated here. The conversions as a function of supplied heat are shown in Figure 5. The square symbols represent results obtained by increasing the addition of CCl4 into the EDC feed stream without changing the profile of the process temperature. On the other hand, the diamond symbols in Figure 5 are obtained under almost the same conditions as those found for the diamond symbols in Figure 4, except that the concentration of CCl4 is constant at 400 ppm. Thus, the analysis with and without change of CCl4 addition under the condition of heat flux change can be performed, and the results are plotted in Figure 5. As shown in the figure, the conversion increases with
Figure 5. EDC conversion at the reactor outlet with and without change of CCl4 under conditions of heat flux change. Comparison between the conversions of the conditions of controlled temperature by heat flux change and the same conditions, except for a constant CCl4 concentration of 400 molar ppm.
Figure 6. Effects of CCl4 concentration added to the feed on EDC conversion as a function of the axial reactor position.
the addition of CCl4 at the same heat flux as that given from the furnace side. To quantify the effects of CCl4 addition on the operation characteristics such as conversion, heat flux, coking precursor’s concentration, and pressure drop, the concentration of CCl4 was altered from 0 to 1200 molar ppm and the stream temperature was maintained by changing the heat flux, as shown by the square symbols in Figures 4 and 5. Thus, all results from Figures 6-11 are obtained under these conditions. The conversion profiles along the axial position of the reactor are represented in Figure 6. These results were also obtained without changing the profile of the process temperature or by increasing CCl4 addition. It appears clear that conversion is predominantly affected by the temperature profile of the process. The process has a positive correlation with temperature, because the EDC pyrolysis reaction is endothermic. The cracking of EDC begins from the position at 70 m in the reactor because, at that position, the temperature reaches about 700 K and the initiation of the reaction begins at that temperature. The conversion increases at a constant rate from this position up to the end of the reactor. The conversion of EDC also increases with CCl4 concentra-
Ind. Eng. Chem. Res., Vol. 40, No. 19, 2001 4047
Figure 7. Effects of CCl4 concentration added to the feed on the heat flux profile as a function of the axial reactor position.
Figure 10. Effects of CCl4 concentration added to the feed on the concentration of acetylene as a function of the axial reactor position.
Figure 11. Effects of CCl4 concentration added to the feed on the pressure drop as a function of the axial reactor position. Figure 8. Total heat supplied from the furnace side as a function of the CCl4 concentration.
Figure 9. Concentration of acetylene at the reactor outlet as a function of added amounts of CCl4.
tion. A 65.4% yield of the final conversion product is achieved at 1200 ppm of CCl4 concentration, whereas a 52.4% yield is achieved when no CCl4 is added. Heat supplied from the furnace side should be increased with the CCl4 concentration as a function of the axial reactor position, to maintain the given temperature profile. Profiles of heat flux supplied from the
furnace side to the reactor are shown in Figure 7. Although the temperature profile is not changed, the heat flux profile should increase with the CCl4 concentration because the conversion of the endothermic EDC pyrolysis reaction increases with the addition of CCl4. Supplied heat fluxes up to 70 m of the reactor position are used to elevate the reaction stream temperature. Thus, the profiles of four cases are nearly the same up to this position. The heat flux profiles, as shown in the figure, decrease from the inlet of the reactor to the position at 70 m and then oppositely increase to 240 m because EDC is continuously converted to VCM and HCl within the range of the reactor position. Heat flux profiles reach a maximum point at a position of 240 m and decrease again from that position to the exit of the reactor, because a small decrease in the gradient of the temperature profile from the position of reactor to the exit reduces the heat flux required. The maximal heat flux difference between the condition of 1200 and 0 ppm of CCl4 reaches to about 1.02 kcal/m2‚s at the peak point of 240 m. The total heat supplied by the combustion of fuel fed from burners, which are located at the furnace side wall, should be increased because the required heat flux shown in Figure 7 increases with the CCl4 concentration in the reactor between 70 and 240 m positions. Figure 8 gives the values of total heat required from the furnace side as a function of the CCl4 concentration under conditions of controlled temperature profile. The results are calculated by a numerical integration of the profiles
4048
Ind. Eng. Chem. Res., Vol. 40, No. 19, 2001
in Figure 7 and multiplied by the outer surface area of the cylindrical reactor. As the concentration of added CCl4 increases, the amount of total heat also increases. Even though the supplied heat rises by 147 kcal/s, compared with the conditions of 1200 to 0 ppm of CCl4, a change in the process gas temperature does not exist because the increased heat is used only for conversion enhancement and not for temperature elevation. An improvement in the conversion of EDC by 13% is achieved because of the increase in heat by 147 kcal/s. One fact that should be considered is the heat flux profile shown in Figure 7. This may not be obtained, although the total heat supplied to the burner at the furnace side is the same as the results shown in Figure 8. Not only the total amount of heat but also the distributions of the fuel feeding rate through the 88 burners on the furnace walls determine the profile of the heat flux. Detailed studies of computational fluid dynamics and a pilot plant for the furnace side have been ongoing during the past decades. Schemes for operation of burners to achieve the above-mentioned heat flux profile are not discussed in this paper because this particular study focuses on the effects of promotors on the pyrolysis reaction. The increase in conversion by the promotor inevitably produces more byproducts and coking precursors. Coking detrimentally affects the performance of the pyrolysis process. The major consequences of the deposition of coke inside the reactor are an increase in the heattransfer resistance from the furnace side, a pressure drop along the reactor, and an accompanying decrease in the process efficiency and VCM selectivity. In particular, the shutdown period for decoking becomes shorter, with a penalty for the increased conversion. This is detrimental to the economy of the plant. For these reasons the amount of added CCl4 is limited. To determine the effects of CCl4 addition on the concentration of a coking precursor, acetylene was analyzed and reported in this paper. Because of the chemistry of this process, the prevailing mechanism of coking involves radical addition on the saturation of the polymeric coke film. Unfortunately, Because of the very short lifetime of the radical, its concentration cannot be analyzed in the plant. Acetylene, methyl chloride, and chloroprene are also known to be major molecular compounds associated with coke deposition. Among these compounds, acetylene was found to be a dominant precursor in the examined plant data. About 346 molar ppm of acetylene is being produced in the plant under normal conditions with 390 molar ppm of CCl4 addition and a 58% conversion of EDC. Figure 9 shows the calculated and plant concentration of acetylene as a function of the concentration of added CCl4. These results were also obtained under the assumption of a fixed process temperature profile. The final concentration of the produced acetylene is about 431 molar ppm when 1200 molar ppm of CCl4 is added. In comparison, the acetylene concentration is just 299 molar ppm when no CCl4 is added. Thus, the simulated result is in good agreement with the plant data represented as hollow circles in Figure 9. Axial profiles of the acetylene concentration in units of molar ppm are shown in Figure 10. The overall shapes of the profiles are similar to those of the EDC conversion profiles described in Figure 6 except that the formation starting position shifts from 70 to 90 m and the concentrations increase rapidly from the position of
240 m of the reactor position. It is considered that acetylene is produced after the cracking of some amounts of EDC above a temperature of 700 K. Acetylene is included in 6 reactions of the 108 elementary reactions established in this modeling work. The VCM molecule and C2H3Cl, C2H2Cl, C2H3, CH2Cl, and Cl radicals are directly related to the acetylene concentration. As the concentration of byproducts increases and the temperature rises, the concentration of acetylene increases very fast from the 240 m of the reactor. Because the acetylene production has a positive correlation with temperature, higher temperature favors the production of coking precursors as well as the EDC main conversion. For a simulation, the reactor inlet pressure is maintained constant at 11.3 atm. The pressure decreases in a stepwise manner because of 16 straight parts and 15 bends in the reactor. Figure 11 shows that pressure profiles as a function of the axial reactor length decrease and the total pressure drops over the reactor are approximately 0.49 atm. In addition, the calculated value agrees exactly with the pressure drop in the actual plant. Differences in the results among the simulated cases with the change of the CCl4 concentration from 0 to 1200 ppm are not apparent although overall pressure drops increase as the added CCl4 is increased. This is because only the gas-phase products are analyzed without consideration of a reduction in the crosssectional area of the reactor by coking with operation time. The pressure drop may increase dramatically when the inner diameter of the reactor is reduced by a deposited layer of coke on the inner reactor wall, and the velocity of the process stream becomes faster to cover the loss in conversion by reduction of the reactor cross-sectional area. Conclusions Feed for an EDC cracking process for the production of VCM needs to be high purity because these reactions are susceptible to both inhibition and fouling by the amounts of impurities. Some of these components act either as a promotor or as an inhibitor. Because the EDC pyrolysis is mainly catalyzed by the Cl radical and the initiation step of the radical is a key step in the process, the addition of a small amount of CCl4 resulted in an increase of the EDC conversion in the process. To rigorously estimate the effects of the addition of CCl4 as a promotor on the performances of the EDC pyrolysis process, the molecular/radical reaction networks were established and the calculation of governing equations for the typical commercial tubular reactor is performed in this study. A total of 108 reversible elementary reaction mechanisms with 47 speciess26 molecular and 21 radical compoundssis developed, based on the catalysis by Cl radical, thermal decompositions, and recombinations. Modeling results were validated against existing plant data. The results accurately predict the byproducts such as acetylene as well as the major products. EDC conversion, the total heat required from the furnace side, the concentration of coking precursors, and the pressure drop were estimated to increase with an increase of CCl4 addition. It is remarkable that a 13% increase in conversion could be achieved when the concentration of CCl4 promotor increased from 0 to 1200 ppm for the investigated plant. Because the pyrolysis is a highly endothermic reaction, the increase of the total heat input should be cooperated with promotor addition in order to achieve conversion enhancement in
Ind. Eng. Chem. Res., Vol. 40, No. 19, 2001 4049
the plant. This suggests that an optimization of the amount of CCl4 is required with an increase of the coking precursor, the total amount of heat required, and the conversion. Further studies of coke deposition on the reactor inner wall should be established to support the rigorous modeling of the pyrolysis process. Acknowledgment Financial support from the BK-21 of the Korea Ministry of Education and the Clean Technology Project by Ministry of Commerce, Industry and Energy is gratefully acknowledged. Nomenclature a1k-a7k ) NASA-Chemkin thermodynamic function coefficients Cpk ) heat capacity of the kth species [cal/mol‚K] dt ) inner diameter of the reactor [m] f ) friction factor Fk ) molar flow rate of the kth species [mol/s] Hk ) enthalpy of the kth species [cal/mol] ∆Hi ) heat of the ith reaction [cal/mol] kfi ) forward reaction rate constant [consistent unit] Kpi ) equilibrium constant [consistent unit] kri ) reverse reaction rate constant [consistent unit] pt ) pressure [atm] Q(z) ) heat flux from the furnace side to the reactor [cal/ m2‚s] R ) gas constant rb ) radius of the reactor bend [m] ri ) ith reaction rate [mol/s] Sk ) entropy of the kth species [cal/mol‚K] T ) temperature [K] u ) velocity [m/s] vki′ ) forward stoichiometric coefficient vki′′ ) reverse stoichiometric coefficient w˘ k ) production rate of the kth species [mol/s] Xk ) chemical symbol for the kth species z ) axial coordinate [m] F ) density [kg/m3] ζ ) supplementary friction factor for the reactor bend
Literature Cited (1) McPherson, R. W.; Starks, C. M.; Frayar, G. J. Vinyl Chloride Monomer...What You Should Know. Hydrocarbon Process. 1979, March, 75. (2) Ranzi, E.; Grottoli, M. G.; Bussani, G.; Che, S. C.; Zahng G. A New Simulation Program Predicts EDC Furnace Performances. Chim. Ind. (Milan) 1993, 75, 261. (3) Kurtz, B. E. Homogeneous Kinetics of Methyl Chloride Chlorination. Ind. Eng. Chem. Process Des. Dev. 1972, 11, 332. (4) Benson, S. W. Thermochemical Kinetics; John Wiley & Sons: New York, 1976.
(5) Weissman, M.; Benson, S. W. Pyrolysis of Methyl Chloride, A Pathway in the Chlorine-Catalyzed Polymerization of Methane. Int. J. Chem. Kinet. 1984, 16, 307. (6) Ranzi, E.; Dente, M.; Rovaglio, M. Pyrolysis and Chlorination of Small Hydrocarbons. Chem. Eng. Commun. 1992, 117, 17. (7) Borsa, A. G.; Herring, A. M.; McKinnon, J. T.; McCormick, R. L.; Yamamoto, S.; Teraoka, Y.; Natori, Y. Characterization of Coke Formed in Vinyl Chloride Manufacture. Ind. Eng. Chem. Res. 1999, 38, 4259. (8) Kopinke, F. D.; Zimmerman, G.; Reyniers, G. C.; Froment, G. F. Relative Rate of Coke Formation from Hydrocarbons in Steam Cracking of Naphtha. 2. Paraffins, Naphthenes, Mono-, Di-, and Cycloolefins, and Acetylenes. Ind. Eng. Chem. Res. 1993, 32, 56. (9) Reyniers, G. C.; Froment, G. F.; Kopinke, F. D.; Zimmerman, G. Coke Formation in the Thermal Cracking of Hydrocarbons. 4. Modeling of Coke Formation in Naphtha Cracking. Ind. Eng. Chem. Res. 1994, 33, 2584. (10) Detemmerman, T.; Froment, F. Three-Dimensional Coupled Simulation of Furnaces and Reactor Tubes for the Thermal Cracking of Hydrocarbons. Rev. Inst. Fr. Pet. 1998, 53, 181. (11) Brown, D. J.; Cremer, M. A.; Smith, P. J.; Waibel, R. T. Fireside Modeling in Cracking Furnace. 9th Ethylene Producers’ Conf. 1997, 1587. (12) Karra, S. B.; Senkan, S. M. A Detailed Chemical Kinetic Mechanism for the Oxidative Pyrolysis of CH3Cl. Ind. Eng. Chem. Res. 1988, 27, 1163. (13) Weissman, M.; Benson, S. W. Mechanism of Soot Formation in Methane Systems. Prog. Energy Combust. Sci. 1989, 15, 273. (14) Ranzi, E.; Dente, M.; Faravelli, T.; Mullick, S.; Bussani, G. Mechanistic Modeling of Chlorinated Reacting Systems. Chim. Ind. (Milan) 1990, 72, 905. (15) Borsa, A. G. Industrial Plant/Laboratory Investigation and Computer Modeling of 1,2-Dichloroethane Pyrolysis. Ph.D. Thesis, Colorado School of Mines, Golden, CO, 1999. (16) Kee, R. J.; Rupley, F. M.; Meeks, E.; Miller, J. A. ChemkinIII: A Fortran Chemical Kinetics Package for the Analysis of GasPhase Chemical and Plasma Kinetics; Sandia National Laboratories: Livermore, CA, 1996. (17) Coltrin, M. E. Surface Chemkin III: A Fortran Package for Analyzing Heterogeneous Chemical Kinetics at a Solid SurfaceGas-Phase Interfaces; Sandia National Laboratories: Livermore, CA, 1996. (18) Plehiers, P. M.; Reyniers, G. C.; Froment, G. F. Simulation of the Run Length of an Ethane Cracking Furnace. Ind. Eng. Chem. Res. 1990, 29, 636. (19) Froment, G. F.; Bischoff, K. B. Chemical Reactor Analysis and Design; John Wiley and Sons: New York, 1979. (20) Press, W. H.; Teukolsky, S. A.; Vetterling, W. T.; Flannery, B. P. Numerical Recipies in Fortran; Cambridge University Press: New York, 1992.
Received for review September 25, 2000 Revised manuscript received June 18, 2001 Accepted June 21, 2001 IE000836A
