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High-selective pyrolysis of naphtha in the fast-mixing reactor
Fuel Processing Technology 106 (2013) 48–54
Contents lists available at SciVerse ScienceDirect
Fuel Processing Technology journal homepage: www.elsevier.com/locate/fuproc
High-selective pyrolysis of naphtha in the fast-mixing reactor M.G. Ktalkherman a,⁎, I.G. Namyatov b, V.A. Emel'kim a, K.A. Lomanovich a a b
Khristianovich Institute of Theoretical and Applied Mechanics, Russian Academy of Science, Siberian Branch, Novosibirsk 630090, Russia Institute of Chemical Kinetics and Combustion, Russian Academy of Science, Siberian Branch, Novosibirsk 630090, Russia
a r t i c l e
i n f o
Article history: Received 28 April 2012 Received in revised form 15 June 2012 Accepted 15 June 2012 Available online 4 July 2012 Keywords: Naphtha Pyrolysis Olefins Fast-mixing reactor
a b s t r a c t The process of naphtha pyrolysis in the heat-carrier at ultra-short time of feedstock / heat carrier mixing is studied. The experiments are carried out within the temperature range at the reactor inlet of 1240–1400 K. Detailed data on the temperature and product composition distribution along the reactor axis are obtained. Experimental results are compared with the calculation. Significant enhance of ethylene yield in our experiments with relation to furnace pyrolysis method is found. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Thermal decomposition of hydrocarbons for the production of basic petrochemicals (ethylene, propylene) is carried out in steam-cracking process. Liquid (naphtha, gas oil) and gaseous (ethane, propane, butane) hydrocarbons are the feedstock for pyrolysis. The feedstock is heated in the convection section of the furnace; it mixes with the superheated steam and then enters a fired tubular reactor (radiant coil), where the endothermic decomposition of feedstock takes places within the controlled residence time, resulting in light olefins and by-products formation. Then the mixture enters the transfer-line exchanger from which the cooled mixture passes through compression, and gas separation units. The yield of the most valuable petrochemical – ethylene – is the determining factor in the total effectiveness of the process. Ethylene content in pyrolysis products and other mixture components is presented in [1–4], where the authors also consider the operation conditions of modern ethylene plants. Along with the conventional method of furnace pyrolysis, alternative methods were studied. The results of early investigations are presented in [5–9], recent results can be found in [10,11]. Among non-traditional methods of pyrolysis, the catalytic method and feedstock pyrolysis in the heat-carrier flow is the most interesting. Comparing to modern steam-cracking units, the catalytic pyrolysis process in large-scale plants [12,13] is realized at lower temperatures, with approximately the same ethylene yield, whereas the propylene yield is much higher.
⁎ Corresponding author. E-mail address: [email protected] (M.G. Ktalkherman). 0378-3820/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2012.06.017
The other way of process intensification, predominantly purposed for ethylene yield gain, is associated with the temperature increase in the reaction zone. It has been known for a long time that the ethylene concentration increases in the pyrolysis products as the temperature in the reaction zone rises [14], but existing technology is constrained by temperature limitations of coil tube material. This limitation can be overcome if the heat is supplied to feedstock not through the tube wall, but is directly transferred via mixing with the high-enthalpy carrier gas with the heat storage sufficient to realize the pyrolysis at high temperatures. All experiments in the heat-carrier flow show the ethylene yield higher than in the conventional method. Since the heat-carrier temperature is much higher than its optimum value in the reaction zone, the key problem in this case is the feedstock / heat carrier mixing time. Taking into account that the feedstock residence time in the reaction zone is much shorter than in the conventional method, the mixing time must be minimal in order to reduce the duration of feedstock residence in the high-temperature zone of the mixing region, because it results in uncontrolled reactions prior to the reactor inlet. In different works, this problem was solved by different methods. In VNIIOS experiments [6,7], the liquid feedstock was injected along the reactor axis into the heat-carrier flow moving with the speed of 300–400 m/s. Hence the processes of feedstock evaporation, mixing, and reaction were not spatially separated. In the ACR reactor [8,9] of the Laval nozzle shape, the liquid feedstock was sprayed from the convergent duct walls, upstream the nozzle throat. Droplets evaporation, mixing with the heat carrier, as well as partial decomposition of the feedstock occurs in the supersonic flow as the temperature decreased downstream. In a certain cross section of the divergent part of the channel, the flow quickly transits to the subsonic one in the shock-wave system, its temperature increases, and the pyrolysis process continues at the high temperature. In the experiments [11], the mixing and reaction zones
M.G. Ktalkherman et al. / Fuel Processing Technology 106 (2013) 48–54
are completely separated. Gaseous feedstock (ethane) and heat carrier (superheated steam) are accelerated separately up to supersonic speeds; they mix at low temperatures due to the interaction between parallel flows of different density. As the mixing process is over, the flow transits quickly into the subsonic mode in the shock-wave system in the same way as in [8,9], and the reactions start up at the prescribed initial temperature. Method of hydrocarbon pyrolysis in high-temperature heat carrier differing from the above mentioned ones is proposed in [15]. This method is based on the possibility to realize the ultra-short time of feedstock/heat carrier mixing. The mixer geometry satisfying these conditions was estimated experimentally in [16] on a gas-dynamic setup. Study of liquified petroleum gases pyrolysis in the lab-scale fast-mixing model reactor demonstrated the viability of this method [17]. The combination of the increased temperature and short mixing time raises significantly both the ethylene and total ethylene-propylene yields in comparison with the furnace pyrolysis. The objective of this study is to examine the process of liquid feedstock pyrolysis in the fast-mixing reactor. Two tasks were under consideration: • Investigation of naphta pyrolysis in the fast-mixing reactor into wide range of residence time at high feedstock heating rates and high temperatures in reacting zone, • Development of a numerical model for prediction of products yield and its testing by experiment.
2. Method of high-temperature pyrolysis in the fast-mixing reactor The method of high-temperature pyrolysis investigated is conceptually similar to those studied before. The difference lies in the manner of feedstock and heat carrier mixing. The flow sheet of the process is shown in Fig. 1. Main components of the unit are: the combustion chamber, mixing device, and reactor itself. The combustion products of stoichiometric fuel-oxygen mixture diluted by superheated steam are used as the heat carrier. The feedstock is injected into the heat-carrier stream by radial jets, mixes quickly and then enters the reactor channel. At the outlet, the products quenches quickly. The liquid feedstock is preliminary evaporated. Superheated steam may also be added to the feedstock. The realization of the proposed model strongly depends on the feedstock/heat carrier mixing rate. The set of experiments [16] was performed in order to develop effective mixing chambers; the characteristics of radial jets mixing with the transverse flow were studied in these experiments. The results (with inert flows) were then used to choose the mixer geometry in the tests with liquefied petroleum gases. This approach turned out to be successful. It permitted to study the pyrolysis at the high initial mixture temperature [17]. We used the similar method in the naphta pyrolysis device.
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3. Experiment 3.1. Model device Fig. 2 shows a setup in the configuration used in these tests. Opposite to previous experiments [17], the unit of feedstock evaporation is added. Main parts consist of the burner, combustion chamber, mixer, and reactor. The reactor consists of two sections of 40 and 80 mm in diameter, 1.15 m and 1.52 m in length, respectively. The combustion chamber is water-cooled; the reactor walls are covered with a mullite wool layer (10 mm). All device parts are made of stainless steel like Steel 321. The mixer geometry was chosen in accordance with the results of experiments [16] with inert flows. The quantity of injecting nozzles remained the same as in the tests of [17] (n=8), their diameter d was increased to 1mm in order to realize under these tests conditions the optimum value of the jet penetration, corresponding the jets collision in central area of mixer chamber. The mixer diameter D is 15 mm. In general, the mixer geometry approximated to one version analyzed in [17]. Prior to the start of the main experiments, the mixing quality was monitored at the distance of L/D=1 from the injection cross section. In these model tests nitrogen was used as injected gas and jet penetration parameter corresponded to the main experiments conditions. The area-averaged root-mean-square deviation of the injected gas concentration from the average value not exceeded 4 – 5%. The vaporizer unit is a cylindrical vessel, in which naphta was filled up prior to the test; it also includes an electrical nitrogen heater and evaporator wherein the naphta was sprayed in the hot nitrogen stream. Naphta was driven out from the vessel by nitrogen from a bottle. The heater presents a spiral stainless-steel tube. Its length is 4.5 m, outer diameter 8mm, wall thickness 1mm. The tube is all around covered with a thermal-insulation layer. The naphta-nitrogen hot mixture enters the receiver connected with the reactor mixer by eight tubes. In order to provide the uniform distribution of the mass flow rate between injecting nozzles, a sonic orifice was installed in each line. 3.2. Measurement technique and experimental conditions The hydrogen-air combustion products (with small hydrogen excess over stoichiometry) were used as the heat-carrier. Hydrogen excess was needed to prevent possible incomplete utilization of the air oxygen. During the experiment, the flow rates of naphta, nitrogen, air, hydrogen were measured as well as the pressure in the combustion chamber and mixer receiver, cooling water flow rate and it heating in the cooling contour, the temperature of the naphta-nitrogen mixture, and temperature distribution along the reactor axis. The temperature was detected by K-type thermocouples (junction diameter of 0.7mm), regarding the correction for emission. The gas samples were collected into 150 ml syringes. The sampler is a copper tube of 6 mm in diameter, with a stainless-steel capillary of 1 mm in diameter inside. The 90-degree
Fig. 1. Block flow diagram.
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Fig. 2. Experimental fast-mixing reactor.
angled capillary nose was entered into the tube mouth and flush-sealed on the wall. The capillary intake was oriented toward the flow. The sampler's tube passed through the reactor walls. Pyrolysis products quenching occurred directly in the capillary cooled by the water flowing in the tube. Samples from each station were collected with two syringes, and the mixture composition was analyzed with two gas chromatographs: Crystallux-4000 M (TCD detector) and Agilent 6850 (FID detector). Aside from hydrocarbons, the content of hydrogen, carbon oxide, and nitrogen in the mixture was defined. Total amount of the detected hydrocarbons reached 42. Fig. 3 shows the typical temperature history of experiment to illustrate the sequence of operation. First, nitrogen flow rate was adjusted; then the electric heater was switched on, and walls of the heater-evaporator-receiver system were heated. A dashed line in Fig. 3 shows the temperature in the receiver. At the moment t=6 min, the combustion chamber was initiated, the heating of the combustion chamber, mixer, and reactor started. Solid lines in Fig. 3 mean the
indications of the thermocouples located on the reactor axis. At t= 18min (an arrow), the naphta supply started. Three minutes later the sampling started, within the sampling process all operating parameters were almost unchanged. Then the supply of hydrogen and naphta was stopped, the current source was off, and for a certain period of time the device was cooled by air and nitrogen. The main parameters characterizing the operation conditions are presented in the Table 1. The following parameters characterize the feedstock used in the experiments: density is 714.1 kg/m 3 (15 °С), initial boiling point is 35 °С, and final boiling point is 167 °С. 110 components were detected in the naphta composition. Group composition is: normal hydrocarbons, 34.03%, aromatics, 4.83%, iso+cycles, 54.81%, fraction С1–С4, 6.13%, olefins, 0.2%.
4. Pyrolysis modeling The calculations were carried out on the one-dimensional model. The point of calculation origin was located at the distance of L/D=1 from the point of jets injection. The flow in this cross section was considered as uniform, the process of feedstock/heat carrier mixing was assumed to be so fast that the feedstock decomposition in the mixing
Table 1 Operation conditions. Parameter
Test No 1
2
3
Adiabatic temperature of combustion products (К) 2400 2400 2400 Temperature at mixer inlet (K) 1800 1750 1750 Temperature of naphta/nitrogen mixture (K) 505 500 560 a Flow temperature after mixing (K) 1240 1320 1400 Pressure in combustion chamber (МPа) 0.156 0.152 0.152 Pressure in reactor (МPа) 0.1 0.1 0.1 Heat carrier/naphta ratiob 6.3 7.3 10.8 Mixing time (ms) ≈0.05 ≈0.05 ≈0.05 c Residence time (ms) 36 34 33 a b
Fig. 3. Temperature temporal history in experiment.
c
Calculated on the assumption of inert mixing. Heat-carrier composition involves injected nitrogen. Calculated for the last sampling point.
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area can be neglected. Temperature distribution along the reactor axis was assigned experimentally. The temperature in the initial point was determined from the heat balance equation for the mixed flow. The package CHEMKIN-ll and a kinetic model including 764 reactions with 197 particles were used. The kinetic model was combined from three parts. The model of butane pyrolysis [18] approved before [17] was used as a basis for the kinetics. Additive reactions included saturated and unsaturated hydrocarbons with carbon atoms quantity up to 11 [19] and cyclic hydrocarbons — cyclohexane, methylcyclohexane [20], etc. The particles containing oxygen and respective reactions were excluded. In order to simplify the scheme, paraffins with the atoms number of 7 and above were joined in groups. For example, the group “nC7H16” contained all normal and isomers of non-cyclic hydrocarbons with the carbon atoms number 7. Non-aromatic cyclic hydrocarbons were divided into two groups — the “cyclohexane” and the “methylcyclohexane” group. The “cyclohexane group” contained the cyclic hydrocarbons without associated methyl, ethyle, and other admixtures, the “methylcyclohexane group” included all the other cycles. Respectively, in the initial conditions the mass concentrations of the particles in one group were summed. The naphtha composition used in simulation is presented in Table 2. 5. Experimental results and discussion Three experiments were carried out; the air and hydrogen flow rates were almost constant, and the temperature of the flow after mixing, calculated on the assumption of inert feedstock/heat carrier mixing (Тinert), varied due to the variation of naphta and nitrogen flow rates. In every case, the naphta temperature before the injection was higher than the final boiling temperature. Figs. 4–6 show the varying temperature and principal pyrolysis products along the reactor axis. Coordinate х=0 corresponds to the distance of 15 mm from the injection cross section (L/D=1), where, as was expected, the feedstock and heat carrier mixing should finish for the greater part, and the reaction should begin. Let us consider the results of the experiment 2 in detail. In this test, the volume concentration of naphta in the heat-carrier composition is 0.03. The remaining experimental parameters are presented in the Table 1. The upper horizontal axis shows the distance from reactor inlet corresponding to the residence time. The products mass fraction presented in Fig. 5 is approximately 95% of the all pyrolysis products, and total amount of the chromatographically detected substances exceeded 30. The data in Fig. 5 show the extremely high reaction rate at the initial stage of naphta thermal decomposition. Measurements make possible to analyze the process details. The first thermocouple was located at 15mm from the injection cross section (х=0), the first sampler was within 24mm from this cross section (х=9 mm). The temperature in the first point (1340 К) was close to the design temperature Тinert =
Fig. 4. Temperature and product yields vs. residence time and distance from reactor inlet at Tinert =1320 K.
1320 К, calculated on the assumption of the inert mixing. In the further points, however (х=30 and 48mm), the temperature varies insignificantly (1300 and 1335K, respectively) instead of the expected drop. It begins to decrease only upon this point (1304 К at х=78mm, t= 0.33ms). Such a temperature behavior can result from the incomplete mixing process within the distance L/D=1. The temperature on the axis here evidently differs from the average value, and on a certain distance L/D>1, the endothermic decomposition process passes in the
Table 2 Reduced naphtha composition. Particle
Mass fraction, %
Xylene Isopentane Decane Cyclohexane Methylcyclohexane Hexane Heptane Octane Nonane Pentane Isohexane 3-Methylpentane
3.2 4.16 2.12 2.83 24.4 8.76 16.2 15.9 8.25 5.95 5.11 3.12
Fig. 5. Temperature and product yields vs. residence time at Tinert =1240 K.
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The propylene yield decreases along the reactor (as was mentioned above, the maximum propylene yield is observed near the reactor inlet). The butadiene yield also decreases with the temperature increase. The low level of high-molecule compounds C5+ should also be noted. But this is typical only for the late stage of naphta decomposition. In the early stage, the content of fraction C5+ in the pyrolysis products is much higher. More than 20 components of fraction C5+ with the total concentration of 40, 10, and 5% were identified in the first sampling point at Тinert =1240, 1320, and 1400 K, respectively. As the residence time increases, fraction С5+ decreases significantly. 6. Calculation results
Fig. 6. Temperature and product yields vs. residence time at Tinert =1400 K.
conditions of slightly varying temperature due to the heat supply to the axis area from neighboring areas with the higher temperature. When analyzing the dependence T(t), it can be concluded that the mixing time tmix ≈0.05ms is some insufficient for the complete feedstock/ heat carrier mixing. Moreover, since the inert mixing condition in this experiment (Т0 =1750 К, Тinert =1320 К) does not realize completely, the reactions partially begin in the mixing region. In particular, this is vindicated by the fact that in the first sampling point the ethylene concentration reaches 35%, whereas the propylene concentration is maximum (15%). As the value Тinert reduced to 1240 К (Fig. 4), the effect of above factors weakens, and the ethylene concentration is 25% in the first sampling point, but the maximum values in both cases differ insignificantly. As was already mentioned, the characteristic feature of the concentration profiles in Figs. 4–6 is the fast gain in the concentrations at the initial stage of the process. Then, at t≥25 ms, the dependencies Ci (t) smoothen. Almost permanent level of the concentrations within great distances from the inlet is maintained by the relative low flow temperature (Тb1100 K). Figs. 4–6 does not reflect any influence of the secondary reactions which would result in the ethylene concentration reduction. Fig. 7 illustrates the temperature influence on the pyrolysis products composition. The gas analysis data are presented in the points of maximum ethylene concentration. In various tests, these points correspond to the residence time t=24–34ms. High ethylene yield has been found in all experiments. As the temperature grows, the maximum ethylene concentration in the pyrolysis products rises to 49.4% at Тinert =1400K.
Fig. 7. Effect of temperature on the product yields.
Simulation model testing is illustrated in Fig. 4–6, wherein the calculation results are compared with the measurements. Under the assumptions made, the comparison can be correct if the reactions in the mixing zone do not take place. In our experiments, this assumption was satisfied approximately. As is seen from Figs. 4–6, the calculation results describe quite well the behavior of the concentration profiles of the major components of the reacting mixture even near the reaction area beginning. For the other components, their total concentration is 10–15%, the calculation results are in worse agreement with the experiment (Fig. 4b). A number of factors influence the correctness of the used model: kinetic scheme and kinetic constants chosen, hydrocarbon composition admitted for the calculations. Taking the sufficient accuracy of the used model into account, we decided to determinate the influence of mentioned above temperature non-uniformity at the reactor inlet (depended by mixing incompletion and heat transport to reactor wall) on the pyrolysis process. For this we performed simulation of naphtha pyrolysis in adiabatic reactor at assumption of instantaneous feedstock/heat carrier mixing. The effects of these factors on pyrolysis process can be established comparing the results of that simulation with calculation using experimental profile T(x), influenced by the mixing process and heat boundary layer. As before, the starting temperature was equal to Tinert The parameters of experiment 2 were used as an example. Note that in the case of adiabatic reactor with instantaneous mixing, no experimental information is used, the simulation results describe directly the kinetic process with the chosen model. The comparison results are presented in Fig. 8. Here, the experimental and simulated temperature profiles are shown, as well as the simulated profiles of ethylene and propylene concentrations. Under the experimental conditions, the temperature in the reaction area, as was already mentioned, depends on both heat absorption due to feedstock decomposition and the feedstock/heat carrier mixing continuing behind the mixer, and heat transfer to the wall.
Fig. 8. Comparison of simulations for adiabatic (dashed lines) and experimental (solid lines) reactors. Symbols — experimental results.
M.G. Ktalkherman et al. / Fuel Processing Technology 106 (2013) 48–54
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The influence of endothermicity alone reflects the profile T(t) in the adiabatic reactor, more sharp than it is in the experiment. Approximately in 5 ms, the simulated temperature approaches to the experimental one, which further becomes lower than it is in the adiabatic reactor. Such a situation results from the heat transfer through the reactor wall. Experimental values of the ethylene concentration in the reaction area beginning exceed the corresponding values in the adiabatic reactor, since the flow temperature in the experiment is higher, and, outward from the inlet, the experiment is in good agreement with both numerical calculations. As follows from Fig. 8, the simulation at assumptions of instantaneous feedstock/heat carrier mixing (ideal case) is in general in good agreement with the calculation based on experimental profile T(t). 7. Influence of heat carrier composition on pyrolysis process As will readily be observed from Figs. 1 and 2, the heat carrier composition in our model experiments is not consistent with the block flow diagram. The combustion products of stoichiometric H2–air mixture (namely, N2+H2O) are used for simplicity of the performance of the experiments instead of CO2+H2O, according to Fig. 1. Since these mixes differ in heat capacities, temperature history in the reactor and concentration profiles must be different, too. Effects of the heat carrier composition on the pyrolysis process have been studied by numerical simulation. A reasonable good consistence between calculations and experiments with the model heat carrier, indicating on the sufficient accuracy of the kinetic model used, was taken into account. Parameters of test no. 2, used as a base for simulation of the naphtha pyrolysis in the heat carrier, are the combustion products of the CH4/ O2-stoichiometric mixture diluted by superheated steam. Steam temperature is 500K. Heat carrier temperature is 1800K. Nitrogen in injected stream is replaced by the steam. Jet temperature (naphtha+steam) is 500K. Pressure in reactor is 0.1MPa. Temperature at reactor inlet is 1340K. For these conditions, the ratio of heat carrier / naphtha is 7.1 (steam in jets was considered as the heat carrier, too). This value corresponds to the experimental conditions. The reactor walls were assumed to be heat- insulated. Naphtha/heat carrier mixing is considered to be instantaneous. The simulation results (temperature distribution and ethylene and propylene yields versus residence time) are shown in Fig. 9 along with similar ones for the adiabatic reactor with model heat carrier (Fig. 8). The data of test no. 2 are plotted, too. As expected, the temperature level in the reaction zone is higher in the case of actual heat carrier due to the higher heat capacity, but this distinction is comparatively inconsiderable in these conditions. In accordance to that, ethylene and propylene profiles are not far different from each other. Hence, the calculation predicts only slight effects of distinction in actual composition of heat carrier from model (experimental) one on the yields of more valuable pyrolysis products.
Fig. 9. Effects of heat carrier composition on product (a) and temperature (b) distributions. Solid lines, simulation with actual heat carrier (CO2+H2O), dashed lines, simulation with model heat carrier (N2+H2O), symbols, experiment.
The theoretical model used for the analysis of naphtha pyrolysis, agrees quite well with the experiment, taking the flow model simplicity into account. Predicted ethylene yield for the case of practicable heat carrier is close to the experimental value. Nomenclature x distance from the injection nozzles d injection nozzle diameter D mixer diameter L mixing length t time tmix mixing time T temperature Tinert inert mixing temperature
8. Conclusion References The detailed measurements of the composition of the naphta pyrolysis products in the model fast-mixing reactor demonstrated the possibility to realize the pyrolysis process in the heat-carrier with the temperature much higher than it is in the conventional method. The flow parameters at the reactor inlet are close to those in the case of instantaneous feedstock/heat carrier mixing. Under the conditions of the temperature drop downstream, the kinetic process provides the products composition featuring the high ethylene selectivity. In the tests performed with model heat carrier composition, the ethylene yield reached almost 50% per pass at the initial process temperature of 1320K. This value is much higher than it is in conventional naphtha pyrolysis method.
[1] T. Ren, M. Patel, K. Blok, Olefins from conventional and heavy feedstock: energy use in steam cracking and alternative processes, Energy 31 (2006) 425–451. [2] C.P. Bowen, Stone & Webster ethylene technology In: R.A. Meyers (Ed.), Handbook of petrochemicals production processes, McGraw-ill, 2005, pp. 6.21–6.49. [3] S. Borsos, S. Ronczy, KBR Score™ ethylene technology, In: R.A. Meyers (Ed.), Handbook of Petrochemicals Production Processes, McGraw-ill, 2005, pp. 6.51– 6.63. [4] H. Zimmermann, R. Walzle, Ethylene, http://media.wiley.com/product_data/excerpt/ 55/35273038/3527303855.pdf. [5] L.F. Albright, B.L. Crynes, W.H. Corcoran, Pyrolysis: Theory and Industrial Practice, Academic, New York, 1983. [6] T.N. Mukhina, N.L. Barabanov, S.B. Babash, V.A. Men'shikov, G.L. Avrekh, Pyrolysis of Hydrocarbon Feedstock, Khimiya, Moscow, 1987. (in Russian). [7] S.P. Chernykh, New processes of organic synthesis, Khimiya, Moscow, 1989. (in Russian).
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[8] G.R. Kamm, D. Milks, J.D. Kearns, H.I. Britt, C.R. Khavarian, US Patent 4 136 015, 1977. [9] R.L. Baldwin, G.R. Kamm, Make ethylene by ACR process, Hydrocarbon Processing 61 (11) (1982) 127–130. [10] Hertzberg, A. Mattick, D.A. Russel, US Patent 5 300 216, 1993. [11] A.T. Mattick, C. Knowlen, D.A. Russel, A. Hertzberg, Pyrolysis of hydrocarbons using a shock wave reactor, In: 21st International symposium of shock waves, Great Keppel Island, Australia, 1997, paper 3800. [12] VNIIOS, Catalytic pyrolysis of petroleum fractions to produce ethylene and propylene, http://www.vniios.ru/english/1.htm 2003. [13] M. Tallman, ACO™, The Advanced Catalytic Olefin processes, http:// technologyconference.kbr.com/2009/Dubai/conference. [14] H.S. Glick Hertzberg, Studies of kinetics in a single-pulse shock tube In: A. Ferri (Ed.), Fundamental data obtained from shock-ube experiments, Pergamon Press, 1961, pp. 180–202. [15] M.G. Ktalkherman, I.G. Namyatov, Pyrolysis of hydrocarbons in a heat-carrier flow with fast mixing of the components, Combustion, Explosion & Shock Waves 44 (2008) 529–534.
[16] M.G. Ktalkherman, V.A. Emel'kim, B.A. Pozdnyakov, Influence of the geometrical and gas-dynamic parameters of a mixer on the mixing with of radial jets colliding, Journal of Engineering Physics and Thermophysics 83 (2010) 539–548. [17] M.G. Ktalkherman, I.G. Namyatov, V.A. Emel'kim, High-temperature pyrolysis of liquefied petroleum gases in fast-mixing reactor, International Journal of Chemical Reactor Engineering 9 (A69) (2011) 1–23. [18] N.M. Marinov, W.J. Pitz, C.K. Westbrook, A.M. Vincitore, M.J. Castaldi, S.M. Senkan, Aromatic and Polycyclic Aromatic Hydrocarbon Formation in a Laminar Premixed n-Butane Flame, Combustion and Flame 114 (1998) 192–213. [19] C.K. Westbrook, W.J. Pitz, O. Herbinet, H.J. Curran, E.J. Silke, A Detailed Chemical Kinetic Reaction Mechanism for n-Alkane Hydrocarbons from n-Octane to n-Hexadecane, Combustion and Flame 156 (2009) 181–199. [20] W.J. Pitz, C.V. Naik, T.N. Mhaolduin, C.K. Westbrook, H.J. Curran, J.P. Orme, J.M. Simmie, Modeling and experimental investigation of methylcyclohexane ignition in a rapid compression machine, Proceedings of the Combustion Institute 31 (1) (2007) 267–275.
Contents lists available at SciVerse ScienceDirect
Fuel Processing Technology journal homepage: www.elsevier.com/locate/fuproc
High-selective pyrolysis of naphtha in the fast-mixing reactor M.G. Ktalkherman a,⁎, I.G. Namyatov b, V.A. Emel'kim a, K.A. Lomanovich a a b
Khristianovich Institute of Theoretical and Applied Mechanics, Russian Academy of Science, Siberian Branch, Novosibirsk 630090, Russia Institute of Chemical Kinetics and Combustion, Russian Academy of Science, Siberian Branch, Novosibirsk 630090, Russia
a r t i c l e
i n f o
Article history: Received 28 April 2012 Received in revised form 15 June 2012 Accepted 15 June 2012 Available online 4 July 2012 Keywords: Naphtha Pyrolysis Olefins Fast-mixing reactor
a b s t r a c t The process of naphtha pyrolysis in the heat-carrier at ultra-short time of feedstock / heat carrier mixing is studied. The experiments are carried out within the temperature range at the reactor inlet of 1240–1400 K. Detailed data on the temperature and product composition distribution along the reactor axis are obtained. Experimental results are compared with the calculation. Significant enhance of ethylene yield in our experiments with relation to furnace pyrolysis method is found. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Thermal decomposition of hydrocarbons for the production of basic petrochemicals (ethylene, propylene) is carried out in steam-cracking process. Liquid (naphtha, gas oil) and gaseous (ethane, propane, butane) hydrocarbons are the feedstock for pyrolysis. The feedstock is heated in the convection section of the furnace; it mixes with the superheated steam and then enters a fired tubular reactor (radiant coil), where the endothermic decomposition of feedstock takes places within the controlled residence time, resulting in light olefins and by-products formation. Then the mixture enters the transfer-line exchanger from which the cooled mixture passes through compression, and gas separation units. The yield of the most valuable petrochemical – ethylene – is the determining factor in the total effectiveness of the process. Ethylene content in pyrolysis products and other mixture components is presented in [1–4], where the authors also consider the operation conditions of modern ethylene plants. Along with the conventional method of furnace pyrolysis, alternative methods were studied. The results of early investigations are presented in [5–9], recent results can be found in [10,11]. Among non-traditional methods of pyrolysis, the catalytic method and feedstock pyrolysis in the heat-carrier flow is the most interesting. Comparing to modern steam-cracking units, the catalytic pyrolysis process in large-scale plants [12,13] is realized at lower temperatures, with approximately the same ethylene yield, whereas the propylene yield is much higher.
⁎ Corresponding author. E-mail address: [email protected] (M.G. Ktalkherman). 0378-3820/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2012.06.017
The other way of process intensification, predominantly purposed for ethylene yield gain, is associated with the temperature increase in the reaction zone. It has been known for a long time that the ethylene concentration increases in the pyrolysis products as the temperature in the reaction zone rises [14], but existing technology is constrained by temperature limitations of coil tube material. This limitation can be overcome if the heat is supplied to feedstock not through the tube wall, but is directly transferred via mixing with the high-enthalpy carrier gas with the heat storage sufficient to realize the pyrolysis at high temperatures. All experiments in the heat-carrier flow show the ethylene yield higher than in the conventional method. Since the heat-carrier temperature is much higher than its optimum value in the reaction zone, the key problem in this case is the feedstock / heat carrier mixing time. Taking into account that the feedstock residence time in the reaction zone is much shorter than in the conventional method, the mixing time must be minimal in order to reduce the duration of feedstock residence in the high-temperature zone of the mixing region, because it results in uncontrolled reactions prior to the reactor inlet. In different works, this problem was solved by different methods. In VNIIOS experiments [6,7], the liquid feedstock was injected along the reactor axis into the heat-carrier flow moving with the speed of 300–400 m/s. Hence the processes of feedstock evaporation, mixing, and reaction were not spatially separated. In the ACR reactor [8,9] of the Laval nozzle shape, the liquid feedstock was sprayed from the convergent duct walls, upstream the nozzle throat. Droplets evaporation, mixing with the heat carrier, as well as partial decomposition of the feedstock occurs in the supersonic flow as the temperature decreased downstream. In a certain cross section of the divergent part of the channel, the flow quickly transits to the subsonic one in the shock-wave system, its temperature increases, and the pyrolysis process continues at the high temperature. In the experiments [11], the mixing and reaction zones
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are completely separated. Gaseous feedstock (ethane) and heat carrier (superheated steam) are accelerated separately up to supersonic speeds; they mix at low temperatures due to the interaction between parallel flows of different density. As the mixing process is over, the flow transits quickly into the subsonic mode in the shock-wave system in the same way as in [8,9], and the reactions start up at the prescribed initial temperature. Method of hydrocarbon pyrolysis in high-temperature heat carrier differing from the above mentioned ones is proposed in [15]. This method is based on the possibility to realize the ultra-short time of feedstock/heat carrier mixing. The mixer geometry satisfying these conditions was estimated experimentally in [16] on a gas-dynamic setup. Study of liquified petroleum gases pyrolysis in the lab-scale fast-mixing model reactor demonstrated the viability of this method [17]. The combination of the increased temperature and short mixing time raises significantly both the ethylene and total ethylene-propylene yields in comparison with the furnace pyrolysis. The objective of this study is to examine the process of liquid feedstock pyrolysis in the fast-mixing reactor. Two tasks were under consideration: • Investigation of naphta pyrolysis in the fast-mixing reactor into wide range of residence time at high feedstock heating rates and high temperatures in reacting zone, • Development of a numerical model for prediction of products yield and its testing by experiment.
2. Method of high-temperature pyrolysis in the fast-mixing reactor The method of high-temperature pyrolysis investigated is conceptually similar to those studied before. The difference lies in the manner of feedstock and heat carrier mixing. The flow sheet of the process is shown in Fig. 1. Main components of the unit are: the combustion chamber, mixing device, and reactor itself. The combustion products of stoichiometric fuel-oxygen mixture diluted by superheated steam are used as the heat carrier. The feedstock is injected into the heat-carrier stream by radial jets, mixes quickly and then enters the reactor channel. At the outlet, the products quenches quickly. The liquid feedstock is preliminary evaporated. Superheated steam may also be added to the feedstock. The realization of the proposed model strongly depends on the feedstock/heat carrier mixing rate. The set of experiments [16] was performed in order to develop effective mixing chambers; the characteristics of radial jets mixing with the transverse flow were studied in these experiments. The results (with inert flows) were then used to choose the mixer geometry in the tests with liquefied petroleum gases. This approach turned out to be successful. It permitted to study the pyrolysis at the high initial mixture temperature [17]. We used the similar method in the naphta pyrolysis device.
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3. Experiment 3.1. Model device Fig. 2 shows a setup in the configuration used in these tests. Opposite to previous experiments [17], the unit of feedstock evaporation is added. Main parts consist of the burner, combustion chamber, mixer, and reactor. The reactor consists of two sections of 40 and 80 mm in diameter, 1.15 m and 1.52 m in length, respectively. The combustion chamber is water-cooled; the reactor walls are covered with a mullite wool layer (10 mm). All device parts are made of stainless steel like Steel 321. The mixer geometry was chosen in accordance with the results of experiments [16] with inert flows. The quantity of injecting nozzles remained the same as in the tests of [17] (n=8), their diameter d was increased to 1mm in order to realize under these tests conditions the optimum value of the jet penetration, corresponding the jets collision in central area of mixer chamber. The mixer diameter D is 15 mm. In general, the mixer geometry approximated to one version analyzed in [17]. Prior to the start of the main experiments, the mixing quality was monitored at the distance of L/D=1 from the injection cross section. In these model tests nitrogen was used as injected gas and jet penetration parameter corresponded to the main experiments conditions. The area-averaged root-mean-square deviation of the injected gas concentration from the average value not exceeded 4 – 5%. The vaporizer unit is a cylindrical vessel, in which naphta was filled up prior to the test; it also includes an electrical nitrogen heater and evaporator wherein the naphta was sprayed in the hot nitrogen stream. Naphta was driven out from the vessel by nitrogen from a bottle. The heater presents a spiral stainless-steel tube. Its length is 4.5 m, outer diameter 8mm, wall thickness 1mm. The tube is all around covered with a thermal-insulation layer. The naphta-nitrogen hot mixture enters the receiver connected with the reactor mixer by eight tubes. In order to provide the uniform distribution of the mass flow rate between injecting nozzles, a sonic orifice was installed in each line. 3.2. Measurement technique and experimental conditions The hydrogen-air combustion products (with small hydrogen excess over stoichiometry) were used as the heat-carrier. Hydrogen excess was needed to prevent possible incomplete utilization of the air oxygen. During the experiment, the flow rates of naphta, nitrogen, air, hydrogen were measured as well as the pressure in the combustion chamber and mixer receiver, cooling water flow rate and it heating in the cooling contour, the temperature of the naphta-nitrogen mixture, and temperature distribution along the reactor axis. The temperature was detected by K-type thermocouples (junction diameter of 0.7mm), regarding the correction for emission. The gas samples were collected into 150 ml syringes. The sampler is a copper tube of 6 mm in diameter, with a stainless-steel capillary of 1 mm in diameter inside. The 90-degree
Fig. 1. Block flow diagram.
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Fig. 2. Experimental fast-mixing reactor.
angled capillary nose was entered into the tube mouth and flush-sealed on the wall. The capillary intake was oriented toward the flow. The sampler's tube passed through the reactor walls. Pyrolysis products quenching occurred directly in the capillary cooled by the water flowing in the tube. Samples from each station were collected with two syringes, and the mixture composition was analyzed with two gas chromatographs: Crystallux-4000 M (TCD detector) and Agilent 6850 (FID detector). Aside from hydrocarbons, the content of hydrogen, carbon oxide, and nitrogen in the mixture was defined. Total amount of the detected hydrocarbons reached 42. Fig. 3 shows the typical temperature history of experiment to illustrate the sequence of operation. First, nitrogen flow rate was adjusted; then the electric heater was switched on, and walls of the heater-evaporator-receiver system were heated. A dashed line in Fig. 3 shows the temperature in the receiver. At the moment t=6 min, the combustion chamber was initiated, the heating of the combustion chamber, mixer, and reactor started. Solid lines in Fig. 3 mean the
indications of the thermocouples located on the reactor axis. At t= 18min (an arrow), the naphta supply started. Three minutes later the sampling started, within the sampling process all operating parameters were almost unchanged. Then the supply of hydrogen and naphta was stopped, the current source was off, and for a certain period of time the device was cooled by air and nitrogen. The main parameters characterizing the operation conditions are presented in the Table 1. The following parameters characterize the feedstock used in the experiments: density is 714.1 kg/m 3 (15 °С), initial boiling point is 35 °С, and final boiling point is 167 °С. 110 components were detected in the naphta composition. Group composition is: normal hydrocarbons, 34.03%, aromatics, 4.83%, iso+cycles, 54.81%, fraction С1–С4, 6.13%, olefins, 0.2%.
4. Pyrolysis modeling The calculations were carried out on the one-dimensional model. The point of calculation origin was located at the distance of L/D=1 from the point of jets injection. The flow in this cross section was considered as uniform, the process of feedstock/heat carrier mixing was assumed to be so fast that the feedstock decomposition in the mixing
Table 1 Operation conditions. Parameter
Test No 1
2
3
Adiabatic temperature of combustion products (К) 2400 2400 2400 Temperature at mixer inlet (K) 1800 1750 1750 Temperature of naphta/nitrogen mixture (K) 505 500 560 a Flow temperature after mixing (K) 1240 1320 1400 Pressure in combustion chamber (МPа) 0.156 0.152 0.152 Pressure in reactor (МPа) 0.1 0.1 0.1 Heat carrier/naphta ratiob 6.3 7.3 10.8 Mixing time (ms) ≈0.05 ≈0.05 ≈0.05 c Residence time (ms) 36 34 33 a b
Fig. 3. Temperature temporal history in experiment.
c
Calculated on the assumption of inert mixing. Heat-carrier composition involves injected nitrogen. Calculated for the last sampling point.
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area can be neglected. Temperature distribution along the reactor axis was assigned experimentally. The temperature in the initial point was determined from the heat balance equation for the mixed flow. The package CHEMKIN-ll and a kinetic model including 764 reactions with 197 particles were used. The kinetic model was combined from three parts. The model of butane pyrolysis [18] approved before [17] was used as a basis for the kinetics. Additive reactions included saturated and unsaturated hydrocarbons with carbon atoms quantity up to 11 [19] and cyclic hydrocarbons — cyclohexane, methylcyclohexane [20], etc. The particles containing oxygen and respective reactions were excluded. In order to simplify the scheme, paraffins with the atoms number of 7 and above were joined in groups. For example, the group “nC7H16” contained all normal and isomers of non-cyclic hydrocarbons with the carbon atoms number 7. Non-aromatic cyclic hydrocarbons were divided into two groups — the “cyclohexane” and the “methylcyclohexane” group. The “cyclohexane group” contained the cyclic hydrocarbons without associated methyl, ethyle, and other admixtures, the “methylcyclohexane group” included all the other cycles. Respectively, in the initial conditions the mass concentrations of the particles in one group were summed. The naphtha composition used in simulation is presented in Table 2. 5. Experimental results and discussion Three experiments were carried out; the air and hydrogen flow rates were almost constant, and the temperature of the flow after mixing, calculated on the assumption of inert feedstock/heat carrier mixing (Тinert), varied due to the variation of naphta and nitrogen flow rates. In every case, the naphta temperature before the injection was higher than the final boiling temperature. Figs. 4–6 show the varying temperature and principal pyrolysis products along the reactor axis. Coordinate х=0 corresponds to the distance of 15 mm from the injection cross section (L/D=1), where, as was expected, the feedstock and heat carrier mixing should finish for the greater part, and the reaction should begin. Let us consider the results of the experiment 2 in detail. In this test, the volume concentration of naphta in the heat-carrier composition is 0.03. The remaining experimental parameters are presented in the Table 1. The upper horizontal axis shows the distance from reactor inlet corresponding to the residence time. The products mass fraction presented in Fig. 5 is approximately 95% of the all pyrolysis products, and total amount of the chromatographically detected substances exceeded 30. The data in Fig. 5 show the extremely high reaction rate at the initial stage of naphta thermal decomposition. Measurements make possible to analyze the process details. The first thermocouple was located at 15mm from the injection cross section (х=0), the first sampler was within 24mm from this cross section (х=9 mm). The temperature in the first point (1340 К) was close to the design temperature Тinert =
Fig. 4. Temperature and product yields vs. residence time and distance from reactor inlet at Tinert =1320 K.
1320 К, calculated on the assumption of the inert mixing. In the further points, however (х=30 and 48mm), the temperature varies insignificantly (1300 and 1335K, respectively) instead of the expected drop. It begins to decrease only upon this point (1304 К at х=78mm, t= 0.33ms). Such a temperature behavior can result from the incomplete mixing process within the distance L/D=1. The temperature on the axis here evidently differs from the average value, and on a certain distance L/D>1, the endothermic decomposition process passes in the
Table 2 Reduced naphtha composition. Particle
Mass fraction, %
Xylene Isopentane Decane Cyclohexane Methylcyclohexane Hexane Heptane Octane Nonane Pentane Isohexane 3-Methylpentane
3.2 4.16 2.12 2.83 24.4 8.76 16.2 15.9 8.25 5.95 5.11 3.12
Fig. 5. Temperature and product yields vs. residence time at Tinert =1240 K.
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The propylene yield decreases along the reactor (as was mentioned above, the maximum propylene yield is observed near the reactor inlet). The butadiene yield also decreases with the temperature increase. The low level of high-molecule compounds C5+ should also be noted. But this is typical only for the late stage of naphta decomposition. In the early stage, the content of fraction C5+ in the pyrolysis products is much higher. More than 20 components of fraction C5+ with the total concentration of 40, 10, and 5% were identified in the first sampling point at Тinert =1240, 1320, and 1400 K, respectively. As the residence time increases, fraction С5+ decreases significantly. 6. Calculation results
Fig. 6. Temperature and product yields vs. residence time at Tinert =1400 K.
conditions of slightly varying temperature due to the heat supply to the axis area from neighboring areas with the higher temperature. When analyzing the dependence T(t), it can be concluded that the mixing time tmix ≈0.05ms is some insufficient for the complete feedstock/ heat carrier mixing. Moreover, since the inert mixing condition in this experiment (Т0 =1750 К, Тinert =1320 К) does not realize completely, the reactions partially begin in the mixing region. In particular, this is vindicated by the fact that in the first sampling point the ethylene concentration reaches 35%, whereas the propylene concentration is maximum (15%). As the value Тinert reduced to 1240 К (Fig. 4), the effect of above factors weakens, and the ethylene concentration is 25% in the first sampling point, but the maximum values in both cases differ insignificantly. As was already mentioned, the characteristic feature of the concentration profiles in Figs. 4–6 is the fast gain in the concentrations at the initial stage of the process. Then, at t≥25 ms, the dependencies Ci (t) smoothen. Almost permanent level of the concentrations within great distances from the inlet is maintained by the relative low flow temperature (Тb1100 K). Figs. 4–6 does not reflect any influence of the secondary reactions which would result in the ethylene concentration reduction. Fig. 7 illustrates the temperature influence on the pyrolysis products composition. The gas analysis data are presented in the points of maximum ethylene concentration. In various tests, these points correspond to the residence time t=24–34ms. High ethylene yield has been found in all experiments. As the temperature grows, the maximum ethylene concentration in the pyrolysis products rises to 49.4% at Тinert =1400K.
Fig. 7. Effect of temperature on the product yields.
Simulation model testing is illustrated in Fig. 4–6, wherein the calculation results are compared with the measurements. Under the assumptions made, the comparison can be correct if the reactions in the mixing zone do not take place. In our experiments, this assumption was satisfied approximately. As is seen from Figs. 4–6, the calculation results describe quite well the behavior of the concentration profiles of the major components of the reacting mixture even near the reaction area beginning. For the other components, their total concentration is 10–15%, the calculation results are in worse agreement with the experiment (Fig. 4b). A number of factors influence the correctness of the used model: kinetic scheme and kinetic constants chosen, hydrocarbon composition admitted for the calculations. Taking the sufficient accuracy of the used model into account, we decided to determinate the influence of mentioned above temperature non-uniformity at the reactor inlet (depended by mixing incompletion and heat transport to reactor wall) on the pyrolysis process. For this we performed simulation of naphtha pyrolysis in adiabatic reactor at assumption of instantaneous feedstock/heat carrier mixing. The effects of these factors on pyrolysis process can be established comparing the results of that simulation with calculation using experimental profile T(x), influenced by the mixing process and heat boundary layer. As before, the starting temperature was equal to Tinert The parameters of experiment 2 were used as an example. Note that in the case of adiabatic reactor with instantaneous mixing, no experimental information is used, the simulation results describe directly the kinetic process with the chosen model. The comparison results are presented in Fig. 8. Here, the experimental and simulated temperature profiles are shown, as well as the simulated profiles of ethylene and propylene concentrations. Under the experimental conditions, the temperature in the reaction area, as was already mentioned, depends on both heat absorption due to feedstock decomposition and the feedstock/heat carrier mixing continuing behind the mixer, and heat transfer to the wall.
Fig. 8. Comparison of simulations for adiabatic (dashed lines) and experimental (solid lines) reactors. Symbols — experimental results.
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The influence of endothermicity alone reflects the profile T(t) in the adiabatic reactor, more sharp than it is in the experiment. Approximately in 5 ms, the simulated temperature approaches to the experimental one, which further becomes lower than it is in the adiabatic reactor. Such a situation results from the heat transfer through the reactor wall. Experimental values of the ethylene concentration in the reaction area beginning exceed the corresponding values in the adiabatic reactor, since the flow temperature in the experiment is higher, and, outward from the inlet, the experiment is in good agreement with both numerical calculations. As follows from Fig. 8, the simulation at assumptions of instantaneous feedstock/heat carrier mixing (ideal case) is in general in good agreement with the calculation based on experimental profile T(t). 7. Influence of heat carrier composition on pyrolysis process As will readily be observed from Figs. 1 and 2, the heat carrier composition in our model experiments is not consistent with the block flow diagram. The combustion products of stoichiometric H2–air mixture (namely, N2+H2O) are used for simplicity of the performance of the experiments instead of CO2+H2O, according to Fig. 1. Since these mixes differ in heat capacities, temperature history in the reactor and concentration profiles must be different, too. Effects of the heat carrier composition on the pyrolysis process have been studied by numerical simulation. A reasonable good consistence between calculations and experiments with the model heat carrier, indicating on the sufficient accuracy of the kinetic model used, was taken into account. Parameters of test no. 2, used as a base for simulation of the naphtha pyrolysis in the heat carrier, are the combustion products of the CH4/ O2-stoichiometric mixture diluted by superheated steam. Steam temperature is 500K. Heat carrier temperature is 1800K. Nitrogen in injected stream is replaced by the steam. Jet temperature (naphtha+steam) is 500K. Pressure in reactor is 0.1MPa. Temperature at reactor inlet is 1340K. For these conditions, the ratio of heat carrier / naphtha is 7.1 (steam in jets was considered as the heat carrier, too). This value corresponds to the experimental conditions. The reactor walls were assumed to be heat- insulated. Naphtha/heat carrier mixing is considered to be instantaneous. The simulation results (temperature distribution and ethylene and propylene yields versus residence time) are shown in Fig. 9 along with similar ones for the adiabatic reactor with model heat carrier (Fig. 8). The data of test no. 2 are plotted, too. As expected, the temperature level in the reaction zone is higher in the case of actual heat carrier due to the higher heat capacity, but this distinction is comparatively inconsiderable in these conditions. In accordance to that, ethylene and propylene profiles are not far different from each other. Hence, the calculation predicts only slight effects of distinction in actual composition of heat carrier from model (experimental) one on the yields of more valuable pyrolysis products.
Fig. 9. Effects of heat carrier composition on product (a) and temperature (b) distributions. Solid lines, simulation with actual heat carrier (CO2+H2O), dashed lines, simulation with model heat carrier (N2+H2O), symbols, experiment.
The theoretical model used for the analysis of naphtha pyrolysis, agrees quite well with the experiment, taking the flow model simplicity into account. Predicted ethylene yield for the case of practicable heat carrier is close to the experimental value. Nomenclature x distance from the injection nozzles d injection nozzle diameter D mixer diameter L mixing length t time tmix mixing time T temperature Tinert inert mixing temperature
8. Conclusion References The detailed measurements of the composition of the naphta pyrolysis products in the model fast-mixing reactor demonstrated the possibility to realize the pyrolysis process in the heat-carrier with the temperature much higher than it is in the conventional method. The flow parameters at the reactor inlet are close to those in the case of instantaneous feedstock/heat carrier mixing. Under the conditions of the temperature drop downstream, the kinetic process provides the products composition featuring the high ethylene selectivity. In the tests performed with model heat carrier composition, the ethylene yield reached almost 50% per pass at the initial process temperature of 1320K. This value is much higher than it is in conventional naphtha pyrolysis method.
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