Ind. Eng. Chem. Res. 2009, 48, 10343–10358
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Coke Formation in the Transfer Line Exchanger during Steam Cracking of Hydrocarbons Kevin M. Van Geem, Inge Dhuyvetter, Serge Prokopiev, Marie-Franc¸oise Reyniers,* Dominique Viennet, and Guy B. Marin Laboratorium Voor Chemische Technologie, Ghent UniVersityt, Krijgslaan 281 (S5), B-9000 Ghent and Total Petrochemicals France, Usine de CarlingsSt-AVold, BP 90290, 57508 St AVold
Coke formation under transfer line exchanger conditions, that is, at temperatures from 623 to 873 K and atmospheric pressure, is studied in an electrobalance setup. The coking rate is initially very high (catalytic coking) and drops after a few hours to a constant value. At the studied conditions the observed coking behavior on 15Mo3 alloy pigs can be explained via a catalytic mechanism only, and contributions of the free-radical mechanism and the condensation mechanism are insignificant. Experiments with ethane and naphtha steam cracking effluents and with well-defined reaction mixtures show that the coking rate is independent of the partial pressure of ethene (0-2.7 × 104 Pa), ortho-xylene (0-1.0 × 104 Pa), heavy aromatic hydrocarbons (0-3.0 × 102 Pa), and also 1,3-butadiene (0-2.7 × 104 Pa). The rate of coke deposition depends only on the temperature and the ratio of the partial pressures of water to dihydrogen. The activation energy for initial coke formation was estimated to be ∼90 kJ/mol, a value close to the experimentally determined diffusion energy of carbon in iron. 1. Introduction Steam cracking of hydrocarbons is the main process for the production of a wide range of base chemicals such as ethene, propene, and benzene. The endothermic cracking process is carried out in the presence of steam in radiant coils constructed from high-temperature resistant alloys and suspended in large gas-fired furnaces. Feedstocks range from ethane to complex mixtures such as naphthas, gas oils, and even vacuum gas oils (VGO). First the hydrocarbon feedstock is preheated to 873-923 K in the convection section of the furnace. Then the process gas enters the reactor coil, where the temperature rises to 1050-1120 K at the reactor exit. To avoid losses of olefins by secondary reactions, the process gas leaving the reactor coil is then rapidly cooled in a transfer line exchanger (TLE).1 Startof-run TLE outlet temperatures typically range from 600 to 700 K depending on the type of feed. For naphtha cracking, the TLE outlet temperature typically increases 20-50 K during the first 2 days, and the increase then levels off to a few degrees per day. The TLE consists of a multitubular waste heat boiler that recuperates heat of the process gas by producing high pressure steam. Hence, cooling is a major factor contributing to the economics of an ethene plant because it prevents further reactions of valuable products such as ethene and butadiene and recovers heat from the effluent gases. TLEs are generally constructed in low-alloyed steels such as 15Mo3 (98.70 wt % Fe, 0.17 wt % C, 0.20 wt % Si, 0.60 wt % Mn, 0.03 wt % P, 0.30 wt % Mo).2 An inherent problem associated with the construction materials used in ethene plants is their tendency to promote the formation of a carbonaceous deposit, so-called coke, on the wall of the reactor coil and the TLE. The accumulation of coke reduces the tube cross sections increasing heat transfer resistances and pressure drops. Depending on the coke deposition rate, the furnace must be periodically shut down for decoking. Cleaning operations are carried out either mechanically or by * To whom correspondence should be addressed. Tel.: +32 9 264 45 17. Fax: +32 9 264 49 99. E-mail:
[email protected].
passing steam and/or air through the coils and TLEs to burn off the coke. The loss of the furnaces’ availability due to decoking, the decrease of the olefin selectivity, and the energy losses associated with the accumulation of coke on the reactor wall have important negative consequences on the economics of the cracking process.3 Moreover, carburization can lead to deterioration and/or damage of tube materials. Hence, coke formation and equipment fouling are a major challenge in steamcracking furnaces. Therefore ethene producers have been and continue to be interested in technologies to reduce coke formation in order to achieve longer furnace run lengths, for example, the use of specific additives4-6 and special alloys.7-9 Many efforts have been made by academics and industrial researchers to understand the coking problem.10-17 On the basis of various laboratory and industrial studies, several coking mechanisms have been proposed. In general, three coke formation mechanisms have been delineated: the catalytic mechanism, the free-radical mechanism, and the droplets condensation/tar deposition mechanism. The catalytic mechanism, producing filamentous coke with nickel and iron acting as a catalyst, is mainly important when clean metal surfaces are exposed to the gas phase at high temperatures. Although the details of the mechanism of filament formation are still heavily debated in literature,18-25 it is generally accepted that it involves a series of surface reactions, leading to the decomposition of the hydrocarbon and resulting in the formation of surface carbon atoms, followed by a sequence of solution, diffusion, and precipitation of carbon. The observation that the activation energy for carbon deposition in the form of filaments on several types of transition-metal catalysts corresponds reasonably well with the activation energy for carbon diffusion through the bulk of the metal has led to the conclusion that carbon diffusion through the solid catalyst particles is the rate-limiting step in filament formation. However, recently it has been suggested that liquid nanoparticles could initiate the growth of carbon nanotubes.20-22 The free-radical mechanism involves reactions between the solid coke and entities in the vapor phase, including free radicals, olefins, aromatics, etc.26,27 As radical coke formation decreases
10.1021/ie900124z CCC: $40.75 2009 American Chemical Society Published on Web 10/28/2009
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Figure 1. Simplified overview of the electrobalance setup for studying coke formation in the TLE.
rapidly with decreasing temperature, it can be expected that this mechanism mainly contributes to coke formation in the radiant coil and at the entrance of the TLE where the temperature is still high.28 The droplet condensation mechanism applies when heavy polynuclear aromatics, which are either present in the feed or formed as a result of secondary chemical condensation reactions, such as, for instance, Diels-Alder reactions, condense either directly on the wall or in the bulk gas phase, and then subsequently collect on the wall.29-33 This mechanism is mainly important when cracking heavier feedstocks, such as gas oils, vacuum residue, and bitumen, and where gases are cooled.34-41 At the inlet of the TLE, where both cracking and vapor cooling occurs, it is possible that all three mechanisms are involved; however, one of them may have a dominant role under particular conditions.16 There is a large body of literature on the mechanisms of coke formation/deposition under conditions relevant for the cracking coil.2-25,32 However, the reported studies on coke formation/deposition under TLE operating conditions are limited.2,29-41 Several operating variables, for example, steam dilution, TLE outlet temperature, feedstock composition, and coil outlet temperature, have an important influence on the amount and type of coke formed in the TLE. In general, the amount of high boiling components, as well as the aromaticity of the feed, is said to be of primary importance in determining the coking rates in radiant coils and TLE’s.15 Work by Kopinke and co-workers12,15,35 supports the dominant role of aromatic hydrocarbons, also in TLE coking. However, a detailed study of the effect of a wide range of operating conditions and of the composition of the cracked effluents on coke formation in the TLE has not been carried out yet. In this work the influence of these variables is investigated by performing experiments in a lab-scale reactor. Issues such as the role of the TLE temperature and the role of aromatic and unsaturated compounds on coke formation in the TLE for a given wall material and a given pretreatment are investigated. 2. Experimental Section 2.1. Electrobalance Setup. The experiments are carried out in an electrobalance setup. The unit is especially designed for studying coke formation under TLE conditions. It consists of
three parts: a feeding section, a reactor section, and an analysis section. A simplified flow diagram of the setup is shown in Figure 1. The reactor section consists of two reactors: a prereactor and an electrobalance reactor. In the prereactor, cracking is performed and the prereactor thus represents the reactor coil in the cracking furnace, while in the electrobalance reactor, coke formation under TLE conditions occurs. 2.1.1. Feed Section. In the feed section liquids, that is, water and hydrocarbons, are evaporated and mixed with the compressed gases, such as, helium, nitrogen, hydrogen, air, ethene. The flow rates are controlled by Brooks mass flow controllers (MFC). First the hydrocarbon feedstock and water are evaporated and preheated to 473 K in two separate evaporators. To limit flow oscillations, the evaporators are filled with glass pearls. After evaporation, water and hydrocarbons are led to a mixer heated at 473 K. Finally, the mixture is sent through a 3-way valve allowing stabilization of the flow rates and the temperature profile prior to start-up. Prior to start-up, the mixture of water and hydrocarbons is sent to the vent while nitrogen passes through to the prereactor. Nitrogen and hydrogen are fed between the mixer and the prereactor. The former is fed to control the partial pressures of the hydrocarbons, while the latter allows studying the effect of the hydrogen partial pressure on the coking rate and to determine the influence of the degree of reduction of the metal surface. It is also possible to feed air to the reactor section to burn off coke after each experiment. The effluent leaving the feed section can thus consist of a mixture containing hydrogen, nitrogen, water, and hydrocarbons. 2.1.2. Reactor Section. As stated previously, the reactor section consists of two different reactors: a prereactor, representing the cracking coil, and an electrobalance reactor, representing the TLE. The tubular prereactor has an internal diameter of 4.0 × 10-3 m and a total length of 1.375 m and can be used either for preheating or for cracking. Eight thermocouples measure the temperature profile in the prereactor. The prereactor is suspended in a cylindrical furnace and is heated by four infrared heating elements. The design allows setting any desired temperature profile. Prior to entering the electro-balance reactor, the effluent of the prereactor has already cooled down as the temperature of the transfer line between the prereactor and the electrobalance reactor is kept at 573 K. The electrobalance
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Figure 2. Detailed view of the electrobalance reactor: TEB, temperature in electrobalance reactor; Tw, temperature at the reactor wall; Tout, temperature at the reactor outlet.
reactor has a cylindrical shape, with the reaction gas flowing upward, and is made of Incoloy 800HT (Ni, 30-35; Cr, 19-23; and Fe, >39.5 wt %) and has an internal diameter of 2.4 × 10-2 m (see Figure 2). A hollow metal cylinder is suspended at the arm of a Cahn D-200 digital recording electro-balance allowing a continuous measurement of the mass of coke depositing on the metal surface of the cylinder. The cylinders are mechanically turned on a lathe from 15Mo3 or Incoloy 800HT alloy pigs. Their height is 15.0 mm, and outside and inside diameters are 15.0 and 14.6 mm, respectively; the mass of the fresh cylinders varies between 1.0 and 1.4 g, and their geometrical surface area amounts to 6.97 × 10-4 m2. The cylinder, as well as the suspending wire made of Kanthal (diameter 0.25 mm), is perfectly centered along the reactor axis to avoid oscillation. Moreover, before each experiment the wire is demagnetized and the cylinder is degreased and cleaned electrolytically to minimize the oscillations caused by the magnetic fields induced by the electric heating. The electrobalance is coupled to a computer to read out and store the mass data every second. The feed enters the electrobalance reactor through a perforated plate containing eight holes to realize a regular hydrodynamic flow pattern inside the electrobalance reactor. The effluent leaves the reactor via a perforated plate containing eight holes. To prevent the effluent of the electrobalance reactor entering the volume of the digital recording balance, a small helium flow (2.9 × 10-7 kg s-1) is sent through the tube containing the suspending wire. The helium flow leaves the electrobalance reactor together with the cracking effluent. The reactor is placed in an electrically heated furnace, allows to reach temperatures up to 1170 K. The maximum pressure in the reactor is limited to 1.5 × 105 Pa. For precise measurements of the pressure inside the reactor and of the pressure difference between the reactor and the electrobalance, accurate pressure transmitters are used (Huba-652). Five automatic data acquisition modules (ADAM) are used to send pressures and temperatures to the computer. Control of the setup and data-acquisition is performed using LABVIEW software to allow monitoring the data with 4 s time intervals
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and recording them with time intervals of 15 min. The electrobalance reactor is operated differentially, that is, the effect of conversion can be neglected. During each test, the differential conditions are verified by means of online GC-analysis of samples taken at the inlet and at the outlet of the electrobalance reactor. 2.1.3. Analysis Section. To monitor the chemical composition of the reaction mixture, two gas chromatographs (GC) are used: a ChromPack CP-9001 and a Carle 500. The C4- sample is analyzed on the Carle 500 automated refinery gas analyzer. The carrier gas is helium. Separation of nitrogen, carbon monoxide, carbon dioxide, and hydrocarbons up to C2 occurs on a Molsieve 5 Å (3 m × 3.0 mm) and Porapack N column (3 m × 3.0 mm) and a thermal conductivity detector (TCD) is used. Separation of C3 and C4 components occurs on a HPPLOT Al2O3-S25 column (50 m × 0.53 mm) and a flame ionization detector (FID) is used. The ChromPack CP-9001 chromatograph, equipped with a PONA column (50 m × 0.25 mm, 0.5 µm film), is used for quantitative determination of the C5+ hydrocarbons. A FID is used to measure the concentration of the hydrocarbons in the reaction mixture. Hydrogen and air are used to maintain the flame while He is used as carrier gas. Peak identification and integration is performed by a commercial integration package (XChrom of Labsystems). Calculations are based on the absolute flow rates of the effluent components. This is made possible by the injection of a precisely known nitrogen flow. From the peak areas of the Carle TCD, the experimentally determined calibration factors and the known flow rate of nitrogen, the flow rates of hydrogen, methane, COx, and C2 hydrocarbons are calculated. Using the methane flow rate thus calculated, the flow rates of the other components can be calculated. With these data a product distribution in terms of mass fractions can be determined. As the feed flow rate is known, yields and a material balance can also be calculated. This allows, among others, the determination of the pyrolysis gasoline fraction, so-called pygas, and the pyrolysis fuel oil fraction. The pygas fraction consists of C5+ to C10- hydrocarbons and can be used for production of aromatics (e.g., benzene) and gasoline. The pyrolysis fuel oil fraction consists of C10+ components, with naphthalene (10-25 wt %) as the most important component. 2.2. Operating Conditions. 2.2.1. Feedstock. Experiments with two different types of feedstock are carried out. In a first stage, the coke deposition under TLE conditions is measured for effluents obtained from cracking ethane or naphtha in the prereactor. The main characteristics of the naphtha feedstock are: Mm ) 111 g/mol, 20 wt % paraffins, 28 wt % iso-paraffins; 4 wt % olefins, 37 wt % naphthenes, and 15 wt % aromatics, specific density Fspec ) 0.7071. In a second stage, the coke deposition under TLE conditions is measured for well-defined mixtures. In this case, the prereactor is only used for preheating, not for cracking. The well-defined mixtures contain nitrogen, hydrogen, water, ethene, and ortho-xylene. Water is used as a diluent, while ethene is used as model component for unsaturated hydrocarbons and ortho-xylene as model component for aromatic hydrocarbons. To study the influence of heavy aromatic compounds on coke deposition under TLE conditions, acenaphthylene [C12H8], anthracene [C14-A], and phenantrene [C14-P]) are added. The influence of cycloaddition reactions on coke formation under TLE conditions is evaluated by adding a C4 raffinate fraction, containing mostly butadiene, to the welldefined mixtures. The C4-mixture used in this study consisted of 1.4 wt % isobutane, 8.1 wt % n-butane, 3.2 wt % trans-2-
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Table 1. Overview of the Range of the Experimental Conditions Used in the Prereactor and the Electrobalance Reactor
COTPR (K) COPPR (105 Pa) TEB (K) pEB (105 Pa) hydrocarbon flow rate (g s-1) dilution (kgH2O kgHC-1)
ethane
naphtha
well-defined mixtures
1098-1123 1.2 673-823 1.0 9.4 × 10-3
1068-1098 1.2 623-873 1.0 1.0 × 10-2
723 1.2 580-886 1.0 2.8 × 10-4 -1.8 × 10-2
0.1-1.5
0.3-0.5
0-1
butene, 13.4 wt % 1-butene, 26.7 wt % isobutene, 1.0 wt % cis-2-butene, 46.2 wt % 1,3-butadiene. 2.2.2. Pretreatment and Thermal Stabilization. To obtain reproducible coke data a standardized pretreatment procedure for the metal cylinder and the electrobalance reactor is used. As the condition of the metal surface in contact with the reaction mixture has an important influence on the coking rate,42 a fresh cylinder is used for each coking experiment and a number of standardized steps are carried out prior to each experiment to ensure that the state of the metal surface of the reactor and the metal cylinder are identical in each experiment. During all steps, a helium flow rate of 2.9 × 10-4 g/s is used as a blanketing gas to protect the electronics of the electrobalance. The in situ preoxidation of the metal cylinder is preceded by a thermal stabilization during which nitrogen (3.47 × 10-3 g/s) is sent through the electrobalance reactor. As the metal cylinders are made from a 15Mo3 alloy and this material corrodes easily, prior to their use the cylinders are protected from access of air and moisture by storage in a desiccator. The standardized pretreatment of the cylinders consists of (i) ultrasonic scouring during 5 min in diethyl ether, followed by 20 min in acetone; (ii) electrolytic reduction at -8 V in 0.3% (wt) aqueous solution of sulphuric acid; (iii) rinsing with distilled water, ethanol, and diethyl ether; (iv) drying and storage in a desiccator containing drierite; (v) oxidation in air (4.0 10-5 g s-1) at 643 K. Alternatively, a reducing pretreatment of the metal cylinder with hydrogen (3.0 × 10-6 g s-1) during at least 10 h can be used to study the effect of the surface state on the coking rate. However in most of the experiments an oxidizing pretreatment is used, as preoxidation is more representative for industrial practice.2 After pretreatment, a flow of nitrogen (3.47 × 10-3 g s-1) is sent to the reactor while the flows of the hydrocarbons and water are sent to the vent. 2.2.3. Experimental Conditions. After the pretreatment and stabilization the 3-way valve is switched and the reactor mixture is led to the prereactor. The experimental conditions in the electrobalance reactor are varied over a wide range for different prereactor effluents. An overview of the range of the experimental conditions used in the prereactor and the electrobalance reactor for the different feedstocks is given in Table 1. During a coking experiment several analyses of the effluent leaving the prereactor and the effluent leaving the electrobalance reactor are taken to confirm the differential conditions in the electrobalance reactor. After an experiment the cylinder is removed and the reactors are decoked. The temperature is increased to 1173 K in the prereactor and to 1023 K in the electrobalance reactor. The reactors are thermally stabilized for 1 h under a nitrogen atmosphere before burning off the coke. When the electrobalance reactor reaches 1023 K a mixture of nitrogen and air is sent through the reactor for 15 min. Next, the nitrogen flow is stopped and only a flow of 0.4 × 10-4 g s-1 of air is used to completely burn off the coke. Finally, the electrobalance reactor is cleaned with dichloromethane.
The absence of transport limitations have been verified using dimensional analysis and Fluent calculations. The results are given in Supporting Information and show that the flow regime is laminar and no transport limitations exist. 2.3. Scanning Electron Microscope and Energy Dispersive X-ray Analysis. The morphologies of the coke samples are studied using scanning electron microscopy (SEM). The surface composition of the alloy treated at various conditions and the metal content in the coke layer are determined using energy dispersive X-ray (EDX) analysis. EDX-analysis is carried out with an accelerating voltage of 10 kV and an acquisition time of 15 min.4,5 The penetration depth of the electron beam is estimated using the empirical formula as proposed by Kanaya-Okayama.43 h)
0.0276WaE1.67 Z0.89F
(5)
where h is the penetration depth (µm); Wa is the molar mass of the element (g mol-1); Z is the atomic number; F is the density of the material (g cm-3); and E is the energy of the electron beam (kV). According to eq 5, the penetration depth differs for each element. The typical values for various elements at E ) 10 kV are C, h ) 1.7 µm; Cr, h ) 0.56 µm; Fe, h ) 0.50 µm; Ni, h ) 0.44 µm, resulting in a penetration depth of approximately 0.5 µm for the metals concerned in this work. The thickness of the coke layer is determined using ∆)
∆mC dS
(6)
where ∆ is the thickness of the coke layer (10-6 m); MC is the amount of coke deposited on the cylinders (g); d is the density of coke (1.78 × 106 g m-3); and S is the surface area of the cylinder (7.565 × 10-4 m2). The density of the coke is taken from Bennett and Price.44 3. Results and Discussion 3.1. Coke Deposition in the TLE during Steam Cracking of Ethane and Naphtha. Steam cracking of ethane and naphtha was carried out in the prereactor. The prereactor effluent was then led to the electrobalance reactor operating under TLE conditions and the amount of coke deposited on a preoxidized 15Mo3 cylinder suspended in the electrobalance reactor was measured. The cracking conditions used in the prereactor and the composition of the prereactor effluent are specified in Table 2. The conversion for the ethane experiments ranges from 37% to 62% and the prereactor effluent consists almost entirely of C4- products, that is, the sum of the yields of C4- products for ethane cracking is always above 99.5 wt %. The mass balance for the ethane experiments is close within 0.5 wt %. During ethane cracking almost no pyrolysis gasoline, so-called pygas, (
