Influence of Dimethyl Disulfide on Coke Formation during Steam

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Ind. Eng. Chem. Res. 2007, 46, 4134-4148

Influence of Dimethyl Disulfide on Coke Formation during Steam Cracking of Hydrocarbons Jidong Wang,† Marie-Franc¸ oise Reyniers,* and Guy B. Marin Laboratorium Voor Petrochemische Techniek, UniVersiteit Gent, Krijgslaan 281 S5, B-9000 Gent, Belgium

The influence of dimethyl disulfide (DMDS), which is widely used as an additive in ethylene plants, on coke formation during the steam cracking of hydrocarbons was investigated in a continuous-flow stirred-tank reactor (CSTR) setup with n-hexane as the feed and in a pilot-plant setup with ethane as the feed. Both of the reactors were made of Incoloy 800HT. Experiments were carried out at conditions relevant to industrial steam crackers. DMDS was applied by presulfidation, continuous addition, and presulfidation followed by continuous addition. Application of DMDS suppresses CO production. The influence of DMDS on coke formation was found to depend on the application method and the amount of DMDS used. SEM examination of the coke samples obtained from the steam cracking of n-hexane indicated that application of DMDS leads to a significant change in the coke morphology. EDX analysis indicated that application of DMDS causes a significant change in the metal content and distribution in both the alloy surface and the coke layers. The mechanism of the influence of DMDS on the coke formation is discussed. 1. Introduction Steam cracking of hydrocarbons is the most important process for the production of olefins and aromatics, the building blocks of the chemical industry. Steam cracking is carried out at high temperatures (873-1173 K) in tubular reactors constructed of heat-resistant Fe-Ni-Cr alloys. An inherent problem associated with steam cracking is coke formation. The amount of coke deposited on reactor walls depends on the type of feed, the operating conditions, and the composition of the alloy. Coke accumulation on the inner surface of the cracking coils and on the transfer-line heat exchanger (TLE) hampers the heat transfer from the furnace to the process gas and increases the pressure drop over the coils. As a result of coke formation, steam cracking furnaces have to be removed from service regularly for decoking, thus lowering the on-stream time and therefore the production capacity. The use of steam as a diluent gives rise to the formation of CO, which is a poison for the catalyst, usually Pd supported on alumina, used in downstream acetylene, methylacetylene, and propadiene hydrogenation. To control the production of CO, sulfur-containing compounds such as dimethyl disulfide (DMDS) are used. For gas feeds such as ethane, a large dose of DMDS is applied to presulfide the reactor surface prior to the introduction of the hydrocarbon feed into the reactor. Then, a small maintenance dose of DMDS is added continuously with the hydrocarbon feed during the cracking. For naphtha, a large dose of DMDS is applied only to presulfide the reactor surface. The sulfur contained in the naphtha is then relied upon to maintain a stable and low production of CO. The reduction of CO in the effluent by the application of sulfur-containing compounds is often interpreted as an indication of reduced coking. This implies that sulfur-containing compounds are able to inhibit coke formation in steam cracking. However, on the basis of experimental observations, Froment,1 * To whom correspondence should be addressed. E-mail: [email protected]. Fax: 0032/9/264 49 99. Tel.: 0032/9/264 56 77. † Current address: College of Chemical Engineering, Beijing University of Chemical Technology, 15 BeiSanhuan East Road, ChaoYang District, Beijing, 100029 China.

Reyniers and Froment,2 and Dhuyvetter et al.3 pointed out that the amount of CO produced during the steam cracking of hydrocarbons does not have any direct correlation with the amount of coke formed. Also, gas-phase reactions of hydroxyl radicals with unsaturated molecules can lead to the formation of CO.4 Therefore, the CO production in steam cracking is not a reliable measure for coke formation. In fact, the influence of sulfur-containing compounds on coke formation during the steam cracking of hydrocarbons is still far from clear, as evidenced by the contradictory experimental findings that have been reported.2-10 From the available literature, it can be stated that the influence of sulfur on coke deposition is complex and depends not only on the application method, i.e., presulfidation, continuous addition, combined presulfidation and continuous addition, but also on the nature and amount of sulfur compound used, as well as on the composition of the metal surface on which the coke is deposited. It has been reported that coke deposition on the surface of a reactor could be reduced by presulfidation.3,5,11-13 However, Trimm and Turner14 found that presulfidation could lead to promotional or inhibiting effects and that the effect of presulfidation on coke formation depends on the nature of the material on which the coke is deposited. In addition, the amount of sulfur adsorbed on metals or alloys was also found to have a pronounced effect on coke formation.14-16 Tong et al.17 reported that presulfidation suppresses coke formation only in the initial stage. With increasing time on stream, the rate of coke formation increased and exceeded the rate of a blank run. The high coking rate could be still observed in the subsequent blank run performed after burning off the coke. The reports on the influence of the continuous addition of S-containing compounds on coke formation are contradictory. A suppressing effect has been reported by Bajus et al.5-8 and Depeyre et al.,9 whereas a promotional effect was reported by Velenyi et al.9 and Reyniers and Froment.2 Trimm et al.14 found that the effect of continuous addition of H2S on coke formation in the steam cracking of propane depends on the material on which the coke is deposited. Reyniers and Froment2 found that the effect of continuous addition of CS2 on coke formation in the steam cracking of n-hexane was associated with the surface state of the alloy (Inconel 600) on which the coke was deposited.

10.1021/ie061096u CCC: $37.00 © 2007 American Chemical Society Published on Web 01/18/2007

Ind. Eng. Chem. Res., Vol. 46, No. 12, 2007 4135

With continuous addition of 50 ppm CS2, the promotional effect was more pronounced on the prereduced Inconel 600 surface than on the preoxidized Inconel 600 surface. From studies with various S-containing compounds as additives, Reyniers and Froment2 and Tan et al.18 concluded that the effect of continuous addition of S-containing compounds on coke formation is related not only to the amount of sulfur used but also to the molecular structure of the S-containing compounds. Presulfidation followed by continuous addition of S-containing compounds was also found to influence coke formation in steam cracking. Reed19 reported that presulfidation with dimethyl sulfide (DMS) followed by continuous addition of DMS increased coke deposition on the inner surface of an Incoloy 800H tubular reactor during the steam cracking of ethane. However, coke deposition in the presence of sulfur was more evenly distributed in the cracking coil, and consequently, the actual run length was longer than that without the application of DMS. On the basis of experiments performed in a pilotplant unit, Dhuyvetter et al.3 found that presulfidation followed by continuous addition of DMDS does not have a significant influence on coke deposition in the steam cracking of naphtha, although the production of CO could be significantly suppressed. Herrebout and Grootjans20 found that the effect of S-containing compounds on coke formation is closely related to their molecular structure. Coke deposition during the steam cracking of naphtha and propane on presulfided Incoloy 800H in the presence of S-containing compounds with an aromatic nature such as thiophene as additives was significantly lower than that with nonaromatic compounds such as DMDS. To suppress CO production during steam cracking, DMDS is widely used in industrial ethylene plants because of its stable nature and easy handling. With DMDS, CO production is effectively inhibited, but the effect of DMDS on coke formation is ambiguous. To understand the effect of DMDS on coke formation in steam cracking, a further investigation is still worthwhile and necessary. In this study, the influence of DMDS on coke formation and CO production during the steam cracking of n-hexane in a continuous-flow stirred-tank reactor (CSTR) setup was examined. Under the conditions used in the CSTR, mainly light gaseous components are formed, as is also the case in ethane cracking. Therefore, ethane cracking was investigated in a pilot-plant setup. Steam cracking of the hydrocarbons was carried out at conditions relevant to industrial ethylene furnaces. 2. Experimental Section 2.1. CSTR. The CSTR setup was described previously by Reyniers and Froment.2 The reactor is made of Incoloy 800HT and has a volume of 5.23 × 10-6 m3. Distilled water and n-hexane were evaporated, mixed, and then preheated to about 843 K before entering the reactor. The preheated feed entered the reactor at high velocity through 24 narrow channels with a diameter of 1 mm each, drilled at an angle of 15° with respect to the vertical, so that complete mixing could be achieved. Before the start of a cracking run, the mixture of n-hexane and steam was bypassed to a condenser, and nitrogen was flowed through the reactor until conditions were stabilized. Then, a sliding valve was set in such a position that the n-hexane/steam mixture was admitted to the reactor. To determine the rate of coke formation, the electrobalance technique was used. A hollow cylinder made of Incoloy 800HT with a surface area of 7.565 × 10-4 m2 was suspended on the arm of a Cahn 2000 electrobalance and positioned in the center of the reactor. This allowed the coke deposition on the cylinder to be measured continuously. With the cylinder mounted in the reactor, the

surface-to-volume ratio was 445.0 m-1. The rate of coke formation was obtained per unit surface area and per unit time. The initial coking rate was calculated from the amount of coke deposited between 15 and 30 min; the asymptotic coking rate was calculated from the amount of coke deposited during the last 120 min. After cracking, the effluent was cooled, but not condensed, in a heat exchanger in which quench oil (Ultratherm 330SCB) was circulated. The effluent was then conducted into a cyclone, in which the tar and coke particles were separated from the gas and in which nitrogen, the internal standard for the gas chromatographic analysis, was added. This gas stream was sent to on-line analysis. For the analysis of the cracking products in the CSTR setup, three gas chromatographs were used. The internal standards allowed the three GC analyses to be related to one another. Peak identification and integration were performed using XChrom software provided by Labsystems. The conversion and product yields were calculated from the absolute flow rate of the effluent components using eqs 1 and 2, respectively

X)

F0A - FA

Yi )

F0A Fi F0A

× 100

× 100

(1)

(2)

where F0A is the mass flow rate of the reactant, FA is the mass flow rate of the reactant in the effluent, X is the conversion of the reactant, Fi is the mass flow rate of product i, and Yi is the yield of product i. The concentration of CO in the effluent was continuously measured with an infrared analyzer (Teledyne IRA, model 711). For this purpose, the effluent was cooled with three water coolers and a methanol cooler at 263 K to remove the heavy components. Based on the volumetric flow rate of the effluent and the CO concentration, the yield of CO can be calculated. 2.2. Experimental Procedures and Conditions in the CSTR Setup. In industrial practice, the surface of the cracking coils is initially in an oxidized state, which is realized by decoking with a mixture of air and steam. To be in line with this practice, the microreactor and the cylinder were oxidized with air at standard preoxidation conditions, i.e., oxidation in air (13 NL h-1) at 1023 K for 14 h. Cracking of n-hexane (Chem-Lab NV, 99.0%) was carried out at 1053-1148 K using a flow rate of n-hexane of 40 g h-1 and steam dilution of 0.5 g of steam/g of n-hexane. The coke deposit was burned off at 1073 K first with a mixture of air (13 NL h-1) and nitrogen (13 NL h-1) for 15 min to remove most of the coke. Then, the nitrogen flow was stopped, and only air (13 NL h-1) was used for 15 min. Finally, the temperature was raised to 1173 K in 15 min and was maintained at this temperature for 30 min. The completion of decoking was followed by measuring the mass loss of the cylinder. Presulfidation with DMDS (Aldrich, 99.0%) was carried out in steam. The desired amount of DMDS was dissolved in water and fed to the evaporator with a syringe pump (ISCO 500D). The flow rate of water was 20 g h-1. After presulfidation, the temperature of the reactor was brought to the cracking temperature in a nitrogen stream in approximately 1 h. Then, a cracking run was started. For continuous addition, the desired amount of DMDS was directly dissolved in the n-hexane feed. The detailed conditions for the application of DMDS in the CSTR setup are given in Table 1.

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Table 1. Application Conditions of DMDS in the CSTR Setup application method presulfidation + continuous addition parameter

presulfidation

continuous addition

presulfidation

continuous addition

FH2O (g h-1) DMDS [ppm (wt)] FDMDS (g h-1) temperature (K) duration (h)

20 500-1000 (1.0-2.0) × 10-2 973-1123 0.5-2

2-300 (8.0 × 10-5)-(1.2 × 10-2) 1148 -

20 500 1.0 × 10-2 973 0.5

0.5-300 (2.0 × 10-5)-(1.2 × 10-2) 1148 -

All data reported in this study were reproduced in at least two repeat runs, and the values reported pertain to the average values over the repeat runs. Mass balances closed to within 3%. 2.3. Pilot-Plant Setup. The pilot-plant unit was described in detail in a previous article.3 The furnace, built of silica/alumina brick (Li23), is 4 m long, 0.7 m wide, and 2.6 m high. It is fired by means of 90 premixed gas burners, mounted with automatic fire checks and arranged on the side walls in such a way as to provide a uniform distribution of heat. The fuel supply system comprises a combustion controller for the regulation of the fuel-to-air ratio and the usual safety devices. The furnace is divided into seven separate cells that can be fired independently so that any type of temperature profile can be set easily. The cracking coil used in this study is made of Incoloy 800HT. It is 12.8 m long and has an internal diameter of 9 mm. The surfaceto-volume ratio of the pilot reactor is 444.4 m-1, which is practically the same as that of the CSTR. The dimensions were chosen to achieve turbulent flow conditions in the coil with reasonable feed flow rates. Twenty thermocouples and five manometers are mounted along the coil to measure the temperature and pressure of the reacting gas. After cracking, a precisely known amount of nitrogen was injected into the effluent, which helped to quench the effluent and served as an internal standard for the gas chromatographic analysis. The effluent was then quenched to 423 K in a concentric heat exchanger by means of cooling oil. A small fraction of the effluent was sampled for GC analysis. Peak identification and integration were performed using XChrom software provided by Labsystems. Calculations of the product yields were based on the mass flow rates of the effluent components. The conversion of ethane and the yields of the cracking products were calculated using eqs 1 and 2, respectively. After removal of the heavy components, the CO and CO2 concentrations in the effluent were also continuously measured by means of infrared analyzers (Fuji Electric Co. Ltd., ZRH 2GPP2-7AAYYRRAY). 2.4. Experimental Conditions in the Pilot-Plant Setup. In the radiant coil, cracking and coke deposition are considered to occur only in the cells where T > 873 K. For the temperature profiles used in this study, the rector surface area available for coke deposition amounted to 0.34 m2. Ethane with a purity of 99.5% (Air Liquide) was used as the hydrocarbon feed. The main impurities in ethane are ethylene (0.2%) and higher hydrocarbons (0.2%). To be in line with industrial practice, presulfidation with DMDS was performed in the presence of steam. The detailed presulfidation conditions are given in Table 2. To add DMDS precisely and uniformly, a 20 wt % DMDS solution in n-hexane was used during presulfidation. For continuous addition of 2 ppm DMDS, a 0.1 wt % DMDS solution in n-hexane was used. For continuous addition of 1001000 ppm DMDS, a 10% DMDS solution in n-hexane was used. The DMDS solution was introduced at the inlet of the reactor by means of an ISCO 500D syringe pump.

Table 2. Experimental Conditions in the Pilot-Plant Setup parameter

blank

PS-I

PS-II PS-III PS-IV

823 873 1023 1093 1113 1143 1.7 4000 1 750 8.82

823 873 1023 1093 1113 1143 1.7 4000 1 750 8.82

Presulfidation temp profile CIT cell 3 COT cell 3 COT cell 4 COT cell 5 COT cell 6 COT cell 7 COP (bar) Fsteam (g h-1) duration (h) DMDS in steam [ppm (wt)] amount of DMDS added (g of DMDS m-2 h-1)a

-

Cracking Fethane (g h-1) 3003 steam dilution (kg kg-1) 0.385 continuously added DMDS (ppm) 0 temp profile CIT cell 3 973 COT cell 3 1023 COT cell 4 1063 COT cell 5 1093 COT cell 6 1113 COT cell 7 1143 COP (bar) 1.7 duration (h) 6

823 873 1023 1093 1113 1143 1.7 4000 1 2000 23.53

823 873 1023 1093 1113 1143 1.7 4000 1 4000 47.06

3003 3003 3003 0.385 0.385 0.385 2 100 500

3003 0.385 1000

973 1023 1063 1093 1113 1143 1.7 6

973 1023 1063 1093 1113 1143 1.7 6

973 1023 1063 1093 1113 1143 1.7 6

973 1023 1063 1093 1113 1143 1.7 6

a Surface area used to calculate amount of DMDS added (g of DMDS m-2 h-1) was 0.34 m2.

After presulfidation, the cracking coil was heated to the cracking temperature profile as specified in Table 2 in the presence of a steam flow of 4 kg h-1. Then, the flow rate of steam was set to the desired value for cracking, and ethane was introduced. Upon the introduction of ethane, the temperature in the cracking coil decreased by about 20 K because of the endothermic nature of the cracking reactions. After about 20 min, the temperature in the cracking coil reached the set value. In this study, ethane was cracked at a coil outlet pressure (COP) of 0.17 MPa, a coil outlet temperature (COT) of 1142 K, and a steam dilution of 0.385 kg of steam/kg of ethane. These cracking conditions were maintained for 6 h. Decoking of the cracking coil was performed with a steam/ air mixture at the conditions specified in Table 3. At the start of the procedure, the cracking coil was heated to 1073 K under a nitrogen flow, and then steam was introduced. After 3 min, the nitrogen flow was stopped, and air was admitted. Once most of the coke had been removed, the temperature of the coil was increased to 1173 K. When practically all of the coke had been burned off, the steam flow was stopped, and further decoking occurred in air only. The standard decoking time was 100 min. During decoking, the CO and CO2 concentrations in the effluent were determined by means of infrared analyzers (Fuji Electric Co. Ltd., ZRH 2GPP2-7AAYY-RRAY). The volumetric flow rate of the effluent was measured using a metal tube flowmeter (Brooks, MT3809, 5512/CB 101000A). The concentrations of

Ind. Eng. Chem. Res., Vol. 46, No. 12, 2007 4137 Table 3. Decoking Conditions in the Pilot-Plant Unit

before start start CO2 < 1 mol % CO2 < 0.1 mol %

FH2O (kg h-1) 0 1.008 1.008 0

Fair (NL h-1) 0 662 662 662

F N2 (NL h-1) 662 0 0 0

CO and CO2 and the flow rate of the effluent were automatically recorded every 10 s. These data were used to determine the total amount of coke deposited on the reactor surface. 2.5. Scanning Electron Microscopy and Energy-Dispersive X-ray Analysis. The morphologies of the coke samples were studied using scanning electron microscopy (SEM) (JEOL JSM540). The surface composition of the alloy treated under various conditions and the metal content in the coke layer were determined using energy-dispersive X-ray (EDX) analysis (Noran Instruments Series II X-ray analyzer). EDX analysis was carried out at an accelerating voltage of 10 kV and an acquisition time of 15 min. The metal content obtained by EDX pertains to a small surface fraction only. Because the metal content and composition in the coke layer can vary from point to point, the values presented in this work are the mean values of 10 analyses at various locations over the surface. The absolute amount of metal in the coke layer was estimated by the external calibration method using Incoloy 800HT as a standard. The penetration depth of the electron beam was estimated using the empirical formula as proposed by Kanaya and Okayama21

h)

0.0276WaE1.67 Z0.89F

(3)

where h is the penetration depth (µm), Wa is the molar weight 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 3, the penetration depth differs depending on the element. Typical values for various elements at E ) 10 kV are as follows: C, 1.7 µm; Cr, 0.56 µm; Fe, 0.50 µm; Ni, 0.44 µm. Therefore, for the metals considered in this work, the analytical results typically refer to a penetration depth of approximately 0.5 µm. The thickness of the coke layer was determined using eq 4

L)

MC × 10-6 dS

(4)

where L is the thickness of the coke layer (µm), MC is the amount of coke deposited on the cylinders (g), d is the density of coke (1.78 × 106 g m-3), amd S is the surface area of the cylinder (7.565 × 10-4 m2). The density of the coke was taken from Bennett and Price.22 The value was obtained from the measurement of coke produced in an industrial naphtha cracker. 3. Results and Discussion 3.1. Influence of DMDS on the Steam Cracking of n-Hexane. In this study, the effects of three methods of applying DMDS, i.e., presulfidation, continuous addition, and presulfidation followed by continuous addition, on the CO production and coke formation during the steam cracking of n-hexane in a CSTR setup were evaluated. The detailed conditions for the application of DMDS are given in Table 1. Cracking of n-hexane was carried out at an n-hexane feed rate of 40 g h-1 and a steam dilution of 0.5 kg of steam/kg of n-hexane. The conversion of

Tout,cell3 (K) 1023 1023 1023 1023

Tout,cell4 (K) 1073 1073 1173 1173

Tout,cell5 (K) 1073 1073 1173 1173

Tout,cell6 (K) 1073 1073 1173 1173

Tout,cell7 (K) 1073 1073 1173 1173

n-hexane and main product yields in a blank run at 1148 K are reported in Table 4. The conversion of n-hexane amounted to 38%, and the yields of ethylene and propylene were 14 and 6 wt %, respectively. The mass of coke deposited on the preoxidized Incoloy 800HT cylinders as a function of time during the steam cracking of n-hexane in the CSTR is shown in Figure 1. At the beginning of the cracking runs, the rate of coke formation was high. With increasing time on stream, it decreased and reached a stable value, the asymptotic coking rate. Increasing the cracking temperature resulted in a higher amount of coke deposited on the cylinders and, consequently, a higher coking rate. Coke formation during steam cracking is a complex phenomenon. It is generally accepted that mechanisms of three types, i.e., heterogeneous catalytic, heterogeneous noncatalytic, and homogeneous noncatalytic, contribute to the coke deposition occurring during steam cracking.1,2 The high initial coking rate is believed to be associated with the catalytic wall effects. Figure 2 provides a schematic representation of the processes involved in the catalytic stage in coke formation during the thermal cracking of hydrocarbons. Catalytic coke formation on metals involves surface reactions, diffusion, and precipitation of carbon.23-25 A significant characteristic of catalytic coke formation is the appearance of filamentous coke as shown in Figure 3. The filamentous coke formed at 1148 K had a diameter of 3-5 µm and a length of 10-30 µm and was scattered on the surface that has been in contact with the gas stream. In addition to coke filaments, coke globules with a diameter of 1-2 µm were also formed (Figure 3B). Most of the surface in contact with the gas stream was covered by globular coke. The coke globules probably are undeveloped coke filaments that, because of encapsulation of the metal particles by coke, were unable to develop into coke filaments. For catalytic coke formation, the nature of the material on which the coke is deposited is an important factor. Coke deposition on different materials has been found to proceed at different rates.14,25 Moreover, the composition of the metal surface is also important. Significant differences in the rate of coke formation between prereduced and preoxidized Inconel 600 alloy surfaces have been reported by Reyniers and Froment.2 The surface composition of the Incoloy 800HT cylinders as determined by EDX is reported in Table 5. Fresh Incoloy 800HT mainly consists of Fe, Cr, and Ni and small amounts of Mn, Si, Ti, and Al. Preoxidation results in a significant enrichment of Cr, Mn, and Ti and a significant depletion of Fe and Ni on the surface. The change in the surface composition upon preoxidation indicates the formation of an oxide layer that contains mainly the oxides of Cr and Mn, which is consistent with previous reports.27,28 Formation of such an oxide layer is desired for the protection of the alloy. The composition of the oxide layer was estimated in this work using a thermodynamic calculation program, EKVICAL.29 In the calculations, the metal composition of the oxide layer as determined by EDX (Table 5, preoxidation) was used as the input condition. In addition to the oxides of the concerned metals, complex metal oxides, i.e., NiFe2O4, NiCr2O4, MnFe2O4, MnCr2O4, NiTiO3, and NiTiO3, were also selected as possible oxidation products. The thermo-

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Table 4. Results of Steam Cracking of n-Hexane in the CSTR Setup blank temperature (K) n-hexane (g h-1) steam flow (g h-1) dilution (kg kg-1) residence time (s) conversion (%) mass balance (%) H2 CO CH4 C2H6 C2H4 C2H2 C3H8 C3H6 C4H8 C4H6 1-C5H10 cy-C5H6 benzene toluene styrene naphthalene coking rate (mg m-2 s-1) initialc asymptoticd

1148 41.2 20.3 0.49 0.10 38 97.6 0.8 4.6 4.8 1.1 13.8 0.1 0.2 6.0 3.2 0.8 0.8 0.1 0.6 0.1 0.1 0.0

presulfidationa 1148 41.3 20.3 0.49 0.10 37 99.1 product yield (wt %) 0.9 3.6 5.0 1.2 14.1 0.1 0.2 6.0 3.2 0.8 0.8 0.1 0.6 0.1 0.1 0.0

0.29 0.27

0.23 0.21

continuous addition (100 ppm)

presulfidationa + CAb (100 ppm)

1148 40.9 20.5 0.50 0.10 37 98.4

1148 41.00 20.2 0.49 0.10 38. 98.9

0.5 0.4 5.1 1.2 15.6 0.2 0.3 6.2 3.2 0.9 0.8 0.2 0.9 0.1 0.2 0.0

0.5 0.3 5.6 1.2 16.5 0.2 0.3 6.5 2.6 1.1 0.7 0.3 1.1 0.2 0.2 0.0

3.00 3.06

5.70 5.79

a Presulfidation conditions (PS-I): T ) 973 K, H O ) 20 g h-1, DMDS in H O ) 500 ppm (wt), t ) 0.5 h. b CA ) continuous addition. c Calculated 2 2 from the amount of coke deposited between run times of 15 and 30 min. d Calculated from the amount of coke deposited during the last 120 min of the run.

Figure 1. Mass of coke deposited on the preoxidized Incoloy 800HT cylinders as a function of time during the steam cracking of n-hexane in a CSTR. Cracking conditions: T ) 1053-1148 K, n-hexane ) 40 g h-1, δ ) 0.5 kg of steam/kg of n-hexane.

Figure 2. Schematic representation of the process for catalytic coke formation during the steam cracking of hydrocarbons.

dynamic data were provided by the EKVICAL29 database, except for data for MnCr2O4(s), which were not available in the database. The thermodynamic data for MnCr2O4(s) were taken from the report of Jung.30 At 1023 K and a total pressure of 0.1 MPa, the calculated equilibrium molar composition of

the oxide layer is 48.3% MnCr2O4, 37.9% Cr2O3, 8.7% TiO2(rutile), 2.5% Fe2O3, 2.0% NiTiO3, 0.6% SiO2, and 0.1% Al2TiO5. The oxidation behavior of an Fe-16Cr alloy containing a small amount of Mn oxidized at 923-1123 K in air was investigated by Jian et al.31 They found that the oxide layer has a duplex microstructure, with MnCr2O4 on top of Cr2O3. Hansson and Somers32 also reported that oxidation of a commercial Fe-Cr alloy with 22 wt % Cr at 1173 K in Ar/9% H2/1% H2O (PO2 ) 9.8 × 10-20 MPa) and in air/1% H2O (PO2 ) 0.0208 MPa) formed an oxide layer consisting of MnCr2O4 on top of Cr2O3. Therefore, it seems reasonable to assume that, in this study, oxidation of the Incoloy 800HT alloy produced an oxide layer consisting mainly of MnCr2O4 and Cr2O3. With increasing time on stream, the metal surface is encapsulated by coke, and the catalytic activity of the metal decreases. The main route for coke formation is then taken over by the heterogeneous noncatalytic mechanism, which leads to a constant so-called asymptotic coking rate. In this mechanism, coke formation results from the reactions of coke precursors in the gas phase with active sites located in the coke matrix.33 Radicals, unsaturated molecules, and aromatics are important coke precursors.34,35 The homogeneous noncatalytic mechanism leads to coke formation in the gas phase, so-called gas-phase coking.36,37 Molecular and/or radical reactions in the gas phase result in the formation of high-molecular-weight polyaromatic compounds that can be liquid or even solid at the high temperatures prevailing in cracking coils. The soot particles can collide with the wall and subsequently be integrated into the coke layer. Gas-phase coking can be neglected at typical cracking conditions unless very heavy feedstocks or cracking temperatures above 1193 K are used.1 Of these three mechanisms, heterogeneous noncatalytic coke formation is the most important because it operates over essentially the entire cracking run. In steam cracking, two processes, i.e., the metal-catalyzed removal of carbonaceous intermediates and/or coke by steam


Figure 3. Micrograph of coke deposited on a preoxidized Incoloy 800HT surface during the steam cracking of n-hexane. Cracking conditions: T ) 1148 K, n-hexane ) 40 g h-1, δ ) 0.5 kg of steam/kg of n-hexane. Table 5. Surface Composition of the Incoloy 800HT Cylinders and the Metal Contents in the Coke Layers as Determined by EDXa composition (wt %) asymp coking rate (mg m-2 s-1)

h (µm)

metal content (wt %)

Cr

fresh alloy preoxidation presulfidation

-

-

Incoloy 800HT 20.7 71.5 70.6

blank (1098 K)b blank (1123 K)b blank (1148 K)b presulfidationc CAd 5 ppm DMDSe presulfidationc + CAd 5 ppm DMDSe

0.08 0.14 0.26 0.21 0.34 3.69

1.5 2.3 3.6 3.0 5.3 42.4

44.7 35.6 22.2 15.8 18.4 6.7

Fe

Ni

Mn

Si

Ti

Al

46.1 2.2 6.7

31.2 0.9 2.5

0.9 21.2 16.0

0.3 0.1 0.4

0.5 4.1 3.6

0.3 0.1 0.2

3.6 3.9 6.6 9.4 5.0 19.9

1.2 1.7 4.9 7.5 2.8 17.3

11.6 25.1 21.4 14.9 24.4 13.1

0.4 0.6 1.3 1.2 0.8 3.5

0.4 0.4 0.4 0.5 0.4 0.7

0.1 0.2 0.5 0.4 0.3 0.9

Coke 82.6 68.1 65.0 66.2 66.3 44.6

a Results reported as averages over 10 points on the surface. b Cracking conditions: n-hexane ) 40 g h-1, δ ) 0.5 kg of steam/kg of n-hexane. c Presulfidation conditions (PS-I): T ) 973 K, H2O ) 20 g h-1, DMDS in H2O ) 500 ppm (wt), t ) 0.5. d CA ) continuous addition. e Cracking conditions: T ) 1148 K, n-hexane ) 40 g h-1, δ ) 0.5 kg of steam/kg of n-hexane.

reforming and the slower noncatalytic gasification of coke by steam, are important for the production of CO.1 The high initial CO production and lower ethylene yield observed by Froment1 in the steam cracking of ethane was attributed to the steam reforming associated with the catalytic activity of the metal wall. As the metal surface becomes covered by coke, its catalytic activity diminishes. In industrial crackers, the temperature at the coke-gas interface exceeds 1173 K, and direct gasification of coke by steam can contribute to nonzero asymptotic CO production.1,38 Also, at the high temperatures prevailing in cracker coils, the coke is not impervious; therefore, the influence of the metal can still be felt during the asymptotic stage. 3.1.1. Influence of Presulfidation. After overnight oxidation in air at 1023 K, the oxidized Incoloy 800HT cylinder was presulfidized at the conditions specified in Table 1. Then, cracking of n-hexane was performed under conditions identical to those used for the blank runs at 1148 K. The conversion of n-hexane and the product yields obtained upon presulfidation at T ) 973 K, H2O ) 20 g h-1, DMDS in H2O ) 500 ppm (wt), and t ) 0.5 h (PS-I) are presented in Table 4. As compared to the results observed for a blank run, presulfidation at the above-specified conditions does not have a significant influence on the conversion of n-hexane and the product selectivities. This is also the case for presulfidation at the other conditions specified in Table 1. Representative curves of CO concentration in the effluent as a function of time on stream during cracking are shown in Figure 4. Upon presulfidation at conditions PS-I, the peak of the CO concentration that appears at the beginning of the blank run is largely suppressed. However, in the asymptotic stage, the CO

Figure 4. CO concentration in the effluent as a function of time on stream during the steam cracking of n-hexane in a CSTR. Presulfidation conditions (PS-I): T ) 973 K, H2O ) 20 g h-1, DMDS in H2O ) 500 ppm (wt), t ) 0.5 h. Cracking conditions: T ) 1148 K, n-hexane ) 40 g h-1, δ ) 0.5 kg of steam/kg of n-hexane.

concentration in the effluent decreases by only 20%. The amount of coke deposited on the cylinders and the rates of coke formation as a function of time on stream are shown in Figures 5 and 6, respectively. For the surface presulfidized at PS-I, the variation of the coking rate with time on stream is similar to that observed for the preoxidized surface, i.e., coke formation is initially high, and with increasing time on stream, it decreases to reach the asymptotic coking rate. Similar curves are also observed for the surfaces presulfidized at other presulfidation conditions.

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Figure 5. Amount of coke deposited on the cylinders as a function of time on stream during the steam cracking of n-hexane in a CSTR. Presulfidation conditions (PS-I): T ) 973 K, H2O ) 20 g h-1, DMDS in H2O ) 500 ppm (wt), t ) 0.5 h. Cracking conditions: T ) 1148 K, n-hexane ) 40 g h-1, δ ) 0.5 kg of steam/kg of n-hexane.

Figure 6. Rate of coke formation as a function of time on stream during the steam cracking of n-hexane in a CSTR. Presulfidation conditions (PSI): T ) 973 K, H2O ) 20 g h-1, DMDS in H2O ) 500 ppm (wt), t ) 0.5 h. Cracking conditions: T ) 1148 K, n-hexane ) 40 g h-1, δ ) 0.5 kg of steam/kg of n-hexane.

The rates of coke formation and the CO yields obtained under other presulfidation conditions are summarized in Table 6. Presulfidation under these conditions also shows a significant suppressing effect on the initial CO production, and in the asymptotic stage, the CO yield decreases by only 20% on average. This indicates that presulfidation of the oxidized Incoloy 800HT surface alone is not an effective method for suppressing CO formation during the entire run. The data with regard to coke formation indicate that, when the presulfidation was performed at 1023 K for 1 h, the coking rate increased with increasing concentration of DMDS in steam from 500 to 1000 ppm. When the presulfidation was performed with 500 ppm DMDS for 1 h, the coking rates at 1073 and 1123 K were higher than those at 973 and 1023 K. When the presulfidation was performed at 973 K with 500 ppm DMDS, the coking rate showed an increasing trend for presulfidation lasting longer than 1 h. The lowest coking rate was obtained under the following conditions: T ) 973-1023 K, DMDS in H2O ) 500 ppm (wt), duration ) 0.5-1 h. SEM photomicrographs of the coke deposited on the presulfided Incoloy 800HT surface are shown in Figure 7. As for the blank run, two types of coke, i.e., coke filaments and coke globules, are present. Their dimensions and populations do not show a significant change as compared to those of the blank.

The surface composition of the presulfided Incoloy 800HT cylinders was determined with EDX and is presented in Table 5. Presulfidation at conditions PS-I results in a significant depletion of Mn and a significant enrichment of Fe and Ni as compared to preoxidation only. The influence of sulfur on the composition of the oxide layer that is preformed upon the oxidation of alloys (Fe-25 wt % Cr-20 wt % Ni) was investigated by Baxter et al.39 They reported that exposure of the preoxidized alloy to a sulfur-containing gas at elevated temperature led to an increase in the concentration of Fe in the preformed oxide layer. In this study, the increase in the content of Fe in the oxide layer after presulfidation as determined using EDX is in line with the report of Baxter et al.39 The increased Fe and Ni contents in the oxide layer upon presulfidation of the oxidized Incoloy 800HT seem contradictory to the wellknown fact that Fe and Ni are the active metals catalyzing CO production and coke formation. During presulfidation, steam and DMDS and/or its decomposition products react with the metal oxides on the surface of the Incoloy 800HT cylinder. Sulfidation of the oxide layer formed from the oxidation of an Fe-45Cr alloy was investigated by Zhou et al.40 at 1173 K. They reported that formation of metal sulfides from the preoxidized Fe-45Cr alloy depended on the partial pressure of sulfur in the gas phase. When the partial pressure of sulfur was lower than 10-9 MPa, only chemisorbed sulfur on the surface could be detected by Auger electron spectroscopy. To evaluate the possibility of the formation of metal sulfides from the oxide layer, the EKVICALC program29 was used to calculate the equilibrium composition formed from the H2O/DMDS mixture used for presulfidation and the possible presulfidation products from the oxide layer under the conditions used in this study. The important gas-phase equilibria considered are

H2 + 1/2O2 ) H2O

(R1)

H2 + 1/2S2 ) H2S

(R2)

O2 + 1/2S2 ) SO2

(R3)

2SO2 + O2 ) 2SO3

(R4)

The calculation results indicate that, at a total pressure of 0.1 MPa, sulfur exists mainly as H2S below 773 K. Starting at 773 K, the main sulfur-containing component gradually shifts from H2S to SO2 until 973 K. Above 973 K, sulfur exists mainly as SO2. The partial pressure of O2 is (3.95 × 10-17)-(4.67 × 10-13) MPa; that of S2 is (1.46 × 10-12)-(3.55 × 10-9) MPa. For the sulfidation reaction of a metal oxide

z M1x M2y Oz + (x + y/2)S2 ) xM1S + yM2S + O2 (R5) 2 conversion to the metal sulfide depends on the partial pressures of O2 and S2. Based on the thermodynamic data,29,31 phase diagrams as a function of the partial pressures of O2 and of S2 can be constructed. From the phase diagrams, it is clear that, under the presulfidation conditions used in this study, MnCr2O4, Cr2O3, and Fe2O3 are stable. Further, calculations concerning the presulfidation products from metal oxides to metal sulfides indicate that no stable metal sulfides can be formed from the oxide layer. During presulfidation, the mass of the cylinder was monitored using the electrobalance. The minimum detectable change of mass was set at 0.05 mg. No noticeable change in the mass of the cylinder was observed. Taking the atomic radius

Ind. Eng. Chem. Res., Vol. 46, No. 12, 2007 4141 Table 6. Influence of Presulfidation on Coke Formation and CO Production during the Steam Cracking of n-Hexanea change (%) CRb,c

T (K) 1148 1023 1023 1023 973 1073 1123 973 973

conc (ppm) 500 750 1000 500 500 500 500 500

t (h) 1 1 1 1 1 1 0.5 2

CO (wt %) 4.56 3.6 3.2 3.7 4.1 3.6 3.9 3.6 4.2

CRb,d

initial (mg m-2 s-1)

asymptotic (mg m-2 s-1)

Blank 0.29 Presulfidation 0.23 0.27 0.31 0.23 0.27 0.26 0.23 0.26

asymptotic CRb

CO

initial CRb

0.27

-

-

-

0.21 0.24 0.25 0.21 0.24 0.23 0.21 0.24

-22 -30 -19 -11 -21 -14 -21 -7

-21 -7 7 -21 -7 -10 -21 -10

-22 -11 -9 -22 -11 -15 -21 -12

a Cracking conditions: T ) 1148 K, n-hexane ) 40 g h-1, δ ) 0.5 kg of steam/kg of n-hexane. b CR ) coking rate. c Calculated from the amount of coke deposited during the first 30 min of the run. d Calculated from the amount of coke deposited during the last 120 min of the run.

Figure 7. Micrograph of coke deposited on a presulfided Incoloy 800HT surface during the steam cracking of n-hexane in a CSTR. Presulfidation conditions: (A,B) T ) 973 K, H2O ) 20 g h-1, DMDS in H2O ) 500 ppm (wt), t ) 0.5 h; (C,D) T ) 973 K, H2O ) 20 g h-1, DMDS in H2O ) 500 ppm (wt), t ) 2 h. Cracking conditions: T ) 1148 K, n-hexane ) 40 g h-1, δ ) 0.5 kg of steam/kg of n-hexane.

of sulfur as 1.03 × 10-10 m,41 the mass of a monolayer of chemisorbed sulfur on the cylinder surface (7.565 × 10-4 m2) amounts to only 1.21 × 10-3 mg. This mass cannot be detected by the electrobalance even if multilayers of sulfur are adsorbed on the surface. On the basis of the experimental observations and the thermodynamic calculations, it can be stated that presulfidation of the preoxidized Incoloy 800HT surface under the conditions used in this study results in the presence of chemisorbed sulfur only. In steam cracking, chemisorption of hydrocarbon molecules on the reactor surface is the first step for coke formation. Chemisorbed sulfur on metal oxides can influence the adsorption and reaction of hydrocarbons on the metal oxides. It has been

found that the amount of sulfur adsorbed by the metal also exerts an influence on coke deposition. Depending on the amount of sulfur adsorbed, the catalytic activity of the metal toward coke formation can either increase or decrease.14-16 Trimm and Turner14 reported a strong dependence of both the initial and the asymptotic coking rates on the amount of predeposited sulfur on prereduced stainless steel (75.5% Fe, 17.5% Cr, 9.4% Ni, 0.7% Mn, 0.18% C). The deposition of coke was significantly reduced by small amounts of sulfur. Once the predeposited sulfur exceeded 10-12 g m-2, the rate of coke formation was higher than that on the unsulfided stainless steel. In this study, the amount of sulfur is only 1.6 mg m-2 if it is assumed that a

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monolayer of sulfur atoms is adsorbed on the cylinder surface. This amount of sulfur is in the range defined by Trimm and Turner. The metal content in the coke layer in contact with the gas phase was determined using EXD and is presented in Table 5. It can be seen from the data for the blank runs performed at different temperatures that the thickness of the coke layer increases with increasing cracking temperature whereas the metal content in the coke layer decreases linearly with increasing thickness of the coke layer. These results indicate that the metal content in the coke layer is inversely proportional to the thickness of the coke layer. The main metal constituents in the coke layer are Cr, Mn, Fe, and Ni. At 1148 K, the thickness of the coke layer observed for a presulfidation run was 3.0 µm. According to the correlation of the metal content with the thickness of the coke layer as obtained from the blank runs, it was expected that the metal content would be 28.2 wt %. However, the observed metal content for the presulfidation run was only 15.8 wt %. This can be taken as an indication that presulfidation suppresses the diffusion of metals into the coke layer. From the above discussion, it can be stated that presulfidation of the oxidized Incoloy 800HT produces chemisorbed sulfur on the metal oxide layer, which suppresses the coke formation in steam cracking by weakening/blocking the adsorption of hydrocarbon molecules and by reducing the diffusion of metals into the coke layer. 3.1.2. Influence of Continuous Addition of DMDS. A series of tests with continuous addition of increasing amounts of DMDS to the feed (2, 5, 10, 20, 50, 100, and 300 ppm relative to n-hexane) were carried out in the CSTR setup for the following cracking conditions: n-hexane ) 40 g h-1, δ ) 0.5 kg of steam/kg of n-hexane, T ) 1148 K. In Table 4, the conversion of n-hexane and the product yields with continuous addition of 100 ppm (wt) DMDS are reported. As compared to the blank run, continuous addition of 100 DMDS does not cause a significant change in the conversion of n-hexane but results in a decrease of the yields of H2 and CO and a slight increase in the ethylene yield. As pointed out by Froment,1 these changes in product selectivities are a reflection of the suppression of the steam reforming reactions that lead to the formation of H2 and CO from hydrocarbons. A typical curve of the CO concentration in the effluent as a function of time on stream with continuous addition of 100 ppm DMDS is shown in Figure 4. Continuous addition of DMDS suppressed both the initial and the asymptotic CO production. The asymptotic CO yield as a function of the amount of continuously added DMDS is shown in Figure 8. With increasing amount of continuously added DMDS, the CO yield first shows an exponential decrease from 2 to 50 ppm (wt) DMDS. Further increasing the amount of DMDS from 50 to 300 ppm (wt) does not lead to a further decrease of the CO yield. As compared to a blank run, continuous addition of as little as 2 ppm (wt) DMDS decreases the CO yield by about 50%. Continuous addition of 100-300 ppm (wt) DMDS decreases the CO yield by 90%. This indicates that continuous addition of DMDS is an effective method for suppressing the formation of CO. In addition to its influence on the production of CO, DMDS also exerts an influence on the coke formation occurring during the steam cracking of n-hexane. The amount of coke deposited on the preoxidized Incoloy 800HT cylinder and the coking rate as a function of time on stream with continuous addition of 100 ppm DMDS are shown in Figures 5 and 6, respectively.

Figure 8. Asymptotic CO yield and asymptotic coking rate as functions of the amount of continuously added DMDS during the steam cracking of n-hexane. Cracking conditions: T ) 1148 K, n-hexane ) 40 g h-1, δ ) 0.5 kg of steam/kg of n-hexane.

Unlike the blank run and the presulfidation runs, the coking rate with continuous addition of 100 ppm DMDS does not show a significant change with increasing time on stream. The asymptotic coking rate is 11 times higher than that of the blank. The asymptotic coking rate as a function of the amount of continuously added DMDS is shown in Figure 8. Continuous addition of 2 ppm (wt) DMDS does not affect the coke formation; the coking rate is essentially the same as that of the blank run. Above 5 ppm (wt) DMDS, the coking rate is significantly altered. With increasing amount of continuously added DMDS from 5 to 20 ppm, the asymptotic coking rate shows a sharp increase. From 20 to 300 ppm, a small, but significant, further increase in the coking rate is observed. The morphologies of the coke formed with various amounts of continuously added DMDS were examined by SEM. The typical morphologies of the coke formed with continuous addition of DMDS are presented in Figure 9. Continuous addition of DMDS results in significant changes in the morphology of the coke formed in the steam cracking of n-hexane. When the amount of continuously added DMDS is less than 5 ppm (wt), three types of coke can be observed. In addition to the globular coke and the needle-shaped filamentous coke as shown in Figure 3, braid-shaped filamentous coke (Figure 9A), which is scattered over the surface, was also formed. The braid-shaped coke was never observed in the absence of DMDS. From 10 to 300 ppm (wt) DMDS, the filamentous coke and the globular coke were no longer formed; the coke on the surface in contact with the gas stream changed to a porous coke, as shown in Figure 9B, that consisted of large amounts of small coke particles and voids. The metal contents in the coke layer formed with continuous addition of various amount of DMDS were determined with EDX and are shown in Figure 8. With increasing amount of DMDS from 2 to 20 ppm, the metal content in the coke layer decreased by 90%. Above 20 ppm, the metal content was 0.6-2 wt %. A number of studies concerning the influence of continuous addition of S-containing compounds on coke formation in the steam cracking of hydrocarbons have been reported. It should be mentioned, however, that a direct comparison of the results reported in the literature is complicated because of the use of different alloys, different feeds, different pyrolysis conditions, etc. Bajus et al.5-8,49 investigated the effects of elemental sulfur and various S-containing compounds including dibenzyl disulfide, dibenzyl sulfide, thiophene, 1-butanethiol, and carbon disulfide on coke deposition in the thermal cracking of a


Figure 9. SEM image of coke deposited on a preoxidized Incoloy 800HT surface during n-hexane cracking with the application of DMDS. Cracking conditions: T ) 1148 K, n-hexane ) 40 g h-1, δ ) 0.5 kg of steam/kg of n-hexane.

reformer raffinate at 1093 K in a stainless steel reactor (75.5% Fe, 17.5% Cr, 9.4% Ni, 0.7% Mn, 0.18% C). Coke deposition was reduced by continuous addition of those sulfur-containing compounds. The authors assumed that, in each case, the sulfurcontaining compounds decomposed to hydrogen sulfide, which reacted with the metal surface to form metal sulfides. The reduction of coke was attributed to the inhibiting effect of a metal sulfide layer. Depeyre et al.9 investigated the effect of (C2H5)2S on coke deposition during the steam cracking of n-nonane in a quartz reactor. Addition of 0.005-0.4 wt % of S as (C2H5)2S in the n-nonane feed suppressed both the initial and the asymptotic coking rates on the quartz surface at T ) 1073 K and H2O/C9H20 ) 0.4 kg of steam/kg of n-nonane. The suppressing effect of (C2H5)2S on coke deposition was assumed to originate from the reduction of the secondary cracking products such as acetylene, butadiene, and aromatics. Velenyi et al.10 reported that continuous addition of H2S increased the coke deposition on an oxidized Incoloy 600 surface. The amount of coke increased by a factor of 10 with continuous addition of 100 ppm (mol) H2S. The promotional effect of H2S on coke deposition on an Fe/Ni (1:4) alloy in steam cracking of ethane at 1038 to 1198 K was also observed by Tan and Baker.18 Reyniers and Froment2 investigated the influence of various sulfur-containing compounds (hydrogen sulfide, carbon disulfide, thiophene, benzothiophene, and DMDS) on coke formation in the steam cracking of n-hexane in a CSTR setup. All S-containing compounds showed a promotional effect on the coke deposition on both a prereduced and a preoxidized surface of Fe-Cr-Ni alloys at 1058 K. For instance, continuous addition of 73 ppm (wt) DMDS increased the coking rate by a factor of about 30 on prereduced Inconel 600. The high initial coking rate was assumed to be the result of grain-boundary embrittlement caused by the corrosive attack of sulfur, which led to a fast precipitation of coke. The higher asymptotic coking rate was proposed to be due to the interference of thiol radicals, produced from the decomposition of sulfur-containing com-

pounds, with the coke surface. Incorporation of the thiol radical into the coke enhances the reactivity and increases the number of the active sites on the coke matrix. The difference in the coking rate observed for different S-containing compounds was assumed to originate from the ability of the various compounds to produce H2S upon their thermal decomposition under the cracking conditions. When DMDS is used as an additive in the steam cracking of n-hexane, its thermal decomposition is inevitable. At 735-833 K, the main decomposition products are methanethiol, hydrogen sulfide, methane, hydrogen, ethylene, and ethane. At temperatures of 931-983 K, the main products are methane, methanethiol, and hydrogen sulfide.42 In addition, thioformaldehyde and carbon disulfide can also be formed at temperatures of 9001000 K.43 The influence of continuously added phosphoruscontaining compounds on the decomposition of n-hexane was reported in a previous paper by Wang et al.44 With continuous addition of 74 ppm P as dioctyl phenylphosphonate and 100 ppm P as hexamethyl phosphoric amide (HMPA) and tripiperidinophosphine oxide, the decomposition of n-hexane in steam cracking was accelerated. The accelerating effect of these phosphorus-containing compounds mainly originates from the initiation reaction, as the P-C and P-N bonds in the phosphoruscontaining compounds are weaker than the C-C bonds in n-hexane. In DMDS, the bond dissociation enthalpy (BDE) of the S-S bond is 309 kJ mol-1, and the BDE of the S-C bond is 238 kJ mol-1. In methanethiol, the BDE of the S-C bond is 313 kJ mol-1.45 All of these bonds are significantly weaker than the C-C bond (368 kJ mol-1) in hydrocarbons.46 Therefore, it is expected that DMDS would act as an initiator in the thermal cracking of n-hexane, causing an increase in the conversion of n-hexane. However, in this study, continuous addition of DMDS was found not to cause any significant change in the conversion of n-hexane, which is consistent with previous observations by Reyniers and Froment2 and Dhuyvetter et al.3 The reason for the unpromoted decomposition of n-hexane by DMDS is likely

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that, before entering the reactor, most of the DMDS has decomposed to H2S in the preheating section where the temperature is around 843 K. This assumption is supported by thermodynamic calculations of the equilibrium composition obtained from the DMDS/steam mixture. At 843 K, the most abundant sulfur-containing species is H2S. The BDE of the H-S bond in H2S is 376 kJ mol-1,45 which is slightly higher than that of the C-C bond in n-hexane. Therefore, H2S cannot act as an initiator for the decomposition of n-hexane in steam cracking. This assumption is consistent with the observation that DMDS does not show any significant influence on the conversion of n-hexane. As reported by Bajus,49 diethyl disulfide showed a similar behavior in heptane steam cracking. Not only did diethyl disulfide not accelerate the steam cracking of heptane, but on the contrary, it caused a retardation. With continuous addition of DMDS, steam, hydrocarbons, and sulfur-containing species are in contact with the preoxidized Incoloy 800HT surface at the beginning of a cracking run. Competitive chemisorption of these species on the surface occurs. However, chemisorption of sulfur-containing species is expected to be faster and stronger because of the strong metalsulfur bond.46 The presence of sulfur might results in grainboundary embrittlement of an alloy that contains Ni under the steam cracking conditions.47 As a consequence, the initial coking rate in the presence of sulfur is promoted.2 Because coke formed at a high rate generally contains more structural deficiencies, which, in fact, are the active sites in the coke layer, the rate of coke formation via the heterogeneous noncatalytic mechanism in the asymptotic stage is also higher. HS radicals produced from the decomposition of H2S, originating from DMDS cracking, can also exert an influence on coke formation during the asymptotic stage. HS radicals can be incorporated into the coke surface via recombination reactions with radical sites on the coke layer, thus modifying the nature of the radical sites on the coke surface. As a result, hydrogen abstraction from the coke surface by gas-phase active species such as H and methyl radicals is facilitated. In addition, more radical sites can also be generated as a result of the presence of sulfur in the coke matrix.2 The high asymptotic coking rate and the lower metal contents in the coke layer observed for the continuous addition of higher amounts of DMDS (Figure 8) imply that the influence of continuously added DMDS on coke formation originates mainly from the interference of the HS radical with the heterogeneous noncatalytic radical reactions responsible for the growth of the coke layer in the asymptotic stage. The porous coke shown in Figure 9, obtained with continuous addition of higher amounts of DMDS, also supports the idea that continuously added DMDS results in rapid growth of the coke layer via the heterogeneous noncatalytic mechanism. The observed high coking rate and low CO yield for continuous addition of DMDS in the steam cracking of n-hexane demonstrate that the CO yield is not directly correlated with the coke formation in steam cracking. 3.1.3. Influence of Presulfidation Followed by Continuous Addition of DMDS. To evaluate the influence of combined presulfidation and continuous addition of DMDS, a series of tests with presulfidation followed by continuous addition of various amounts of DMDS to the feed (0.5, 2, 5, 20, 50, 100, and 300 ppm (wt) relative n-hexane) were carried out in the CSTR setup. Presulfidation was performed at conditions of PSI, i.e., T ) 973 K, H2O ) 20 g h-1, DMDS in H2O ) 500 ppm (wt), duration ) 0.5 h. The conversion of n-hexane and the product yields with presulfidation followed by continuous addition of 100 ppm DMDS are reported in Table 4. As for the

Figure 10. CO yield and asymptotic coking rate as functions of the amount of continuously added DMDS in the combination of presulfidation followed by continuous addition during the steam cracking of n-hexane. Presulfidation conditions: T ) 973 K, H2O ) 20 g h-1, DMDS in H2O ) 500 ppm (wt), t ) 0.5 h. Cracking conditions: T ) 1148 K, n-hexane ) 40 g h-1, δ ) 0.5 kg of steam/kg of n-hexane.

continuous addition of DMDS, presulfidation followed by continuous addition of 100 ppm DMDS does not cause a significant change in the conversion of n-hexane but results in a decrease in the yields of H2 and CO and a slight increase in the yield of hydrocarbon products. The change in the product yields with presulfidation followed by continuous addition of DMDS can also be attributed to the reduced steam reforming reactions that lead to the formation of H2 and CO from hydrocarbons.1 A representative curve of the CO concentration in the effluent as a function of time on stream is shown in Figure 4. Presulfidation followed by continuous addition of DMDS results in a reduction of CO in both the initial and the asymptotic stages. The asymptotic CO yield as a function of the amount of continuously added DMDS is shown in Figure 10. With increasing amount of continuously added DMDS, the CO yield first shows an exponential decrease from 0.5 to 50 ppm DMDS. Further increasing the amount of continuously added DMDS from 50 to 300 ppm does not lead to a further decrease of CO. As compared to a blank run, presulfidation followed by continuous addition of as little as 0.5 ppm (wt) DMDS decreases the CO yield by about 40%; presulfidation followed by continuous addition of 50-300 ppm (wt) DMDS decreases the CO yield by 90%. This means that presulfidation followed by continuous addition of DMDS is a more effective method for the suppression of CO than presulfidation alone. Presulfidation followed by continuous addition of DMDS also influences the coke formation during the steam cracking of n-hexane. The amount of coke deposited on the presulfided Incoloy 800HT surface and the coking rate as a function of time on stream with continuous addition of 100 ppm DMDS are shown in Figures 5 and 6, respectively. As compared to that of the blank run, the coking rate of presulfidation followed by continuous addition of 100 ppm DMDS is significantly higher and does not show a significant change with time on stream. The asymptotic coking rate as a function of the amount of continuously added DMDS in the combination of presulfidation followed by continuous addition is presented in Figure 10. With increasing amount of continuously added DMDS from 0.5 to 20 ppm (wt), the asymptotic coking rate shows a sharp increase. However, further increasing the amount of continuously added DMDS from 20 to 300 ppm does not cause a significant variation of the asymptotic coking rate. Upon presulfidation followed by continuous addition of 20-300 ppm DMDS, on average, the coking rate is approximately 24 times the value observed for the blank run.


Figure 11. Effect of presulfidation on CO production and coke formation during the steam cracking of n-hexane in the presence of DMDS. Presulfidation conditions: T ) 973 K, H2O ) 20 g h-1, DMDS in H2O ) 500 ppm (wt), t ) 0.5 h. Cracking conditions: T ) 1148 K, n-hexane δ ) 40 g h-1, δ ) 0.5 kg of steam/kg of n-hexane.

The morphologies of the coke samples obtained with presulfidation followed by continuous addition of DMDS were investigated by SEM. Presulfidation followed by continuous addition of 0.5 ppm (wt) DMDS did not cause a significant change of the coke morphology as compared to that of the blank. Presulfidation followed by continuous addition of 2 ppm (wt) DMDS resulted in a coke morphology similar to that observed for continuous addition of 2-5 ppm (wt) DMDS alone. A representative image of the coke obtained with presulfidation followed by continuous addition of 5-300 ppm DMDS is shown in Figure 9C, which is similar to those observed for continuous addition of 10-300 ppm DMDS. By comparing the CO yield and coking rate of presulfidation followed by continuous addition of DMDS with those of continuous addition of DMDS alone, the effect of presulfidation in the presence of sulfur on CO production and coke formation can be deduced. Taking the CO yield and the asymptotic coking rate of continuous addition of DMDS as references, the effects of presulfidation on the CO production and on the coking rate are shown in Figure 11. It can be seen that the effects of presulfidation on CO production and coke formation depend on the amount of continuously added DMDS. Presulfidation results in a reduction of the CO production throughout the range of continuously added DMDS from 0.5 to 300 ppm (wt). The most pronounced reduction is observed for 5-20 ppm DMDS. When the amount of continuously added DMDS is less than 5 ppm or more than 50 ppm, the suppressing effect of presulfidation on CO is only marginal. Presulfidation gives rise to an increase in the coke deposition in the presence of 2-100 ppm (wt) DMDS. The most pronounced increase is observed for continuous addition of 5 ppm DMDS. Above 100 ppm DMDS, the effect of presulfidation on coke formation in the presence of DMDS is much less pronounced. The metal content and distribution in the coke layer with continuous addition of 5 ppm DMDS and presulfidation followed by continuous addition of 5 ppm DMDS were determined using EDX and are presented in Table 5. As compared to continuous addition of 5 ppm DMDS alone, presulfidation followed by continuous addition of 5 ppm DMDS results in a 7-fold increase in the rate of coke formation and an 8-fold increase in the thickness of the coke layer. However, the metal content in the coke layer decreases by only a factor of 3.6, implying that presulfidation followed by continuous addition

of 5 ppm DMDS promotes the diffusion of metals into the coke layer. The amounts of the individual metal elements in the coke layer are also significantly different. Presulfidation followed by continuous addition of 5 ppm DMDS causes reductions of Cr and Mn and increases of Fe, Ni, and Si in the coke layer. This could imply that presulfidation combined with continuous addition of DMDS promotes the diffusion of Fe, Ni, and Si from the alloy into the coke layer, leading to a higher coking rate. 3.2. Influence of DMDS on the Steam Cracking of Ethane in a Pilot Plant. The influence of DMDS on the steam cracking of ethane was investigated in a pilot-plant setup. DMDS was applied by presulfidation followed by continuous addition of DMDS. In the industrial ethylene furnace, a temperature profile exists along the cracking coils. To simulate this situation, during presulfidation with DMDS, a temperature profile was also established along the cracking coil of the pilot-plant setup. Presulfidation with DMDS was performed using steam as the carrier gas for 1 h with a coil outlet pressure of 0.17 MPa. The concentration of DMDS in the steam was varied from 750 to 4000 ppm. After presulfidation, the temperature of the coil was set to the cracking temperature profile in the presence of 4 kg h-1 steam, and a cracking run was started. During cracking, the amount of continuously added DMDS was varied from 2 to 500 ppm based on ethane. The results are summarized in Table 7. Under the cracking conditions specified in Table 2, the ethane conversion amounted to 70%. An ethylene yield of 52% and a CO yield of 1.67% were observed. The amount of coke deposited during the 6-h cracking run was 1.56 g. Presulfidation followed by continuous addition of DMDS did not result in a significant change in the conversion of ethane or the product selectivities. Presulfidation with 750 ppm DMDS followed by continuous addition of 2 ppm DMDS reduced the CO yield by 70%. Presulfidation with 750 ppm DMDS followed by continuous addition of 100 ppm DMDS reduced the CO yield by 95%. Further increasing the amount of DMDS in both presulfidation and continuous addition did not cause a significant further reduction of CO. The CO concentration in the effluent as a function of time on stream is shown in Figure 12. The CO spike appearing at the beginning of the blank run was significantly suppressed upon presulfidation with 750 ppm DMDS followed by continuous addition of 2 ppm DMDS. Further increasing the amount of

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Table 7. Results with the Application of DMDS as an Additive in the Steam Cracking of Ethane in the Pilot-Plant Unit run blank Ve/Fo (m3 s mol-1) residence time (s) conversion (%) mass balance (wt %) amount of coke (g/6 h) H2 CO CO2 CH4 C2H2 C2H4 C2H6 C3H8 C3H6 propadiene n-C4H10 t-2-C4H8 1-C4H8 i-C4H8 c-2-C4H8 methyl acetylene 1,3-C4H6 1-C5H10 1,3-cyclopentadiene benzene toluene styrene indene naphthalene

PS-I

PS-II

PS-III

PS-IV

0.084 0.084 0.081 0.284 0.282 0.287 70.0 69.7 68.2 97.9 99.0 1.6 1.9 30.6 product yield (wt %) 5.09 5.75 5.06 1.67 0.51 0.08 0.25 0.04 0.00 3.67 3.84 3.62 0.52 0.54 0.49 51.97 53.03 52.14 30.00 30.27 31.78 0.07 0.07 0.07 1.06 1.11 1.04 0.03 0.03 0.02 0.21 0.22 0.22 0.06 0.06 0.06 0.13 0.13 0.13 0.01 0.01 0.01 0.04 0.04 0.05 0.04 0.04 0.04 2.01 2.02 1.91 0.03 0.04 0.20 0.23 0.43 0.42 0.04 0.06 0.02 0.03 0.01 0.01 0.01 0.01 -

0.085 0.291 68.8 98.1 48.9

0.083 0.308 69.0 100.0 76.2

5.05 0.08 0.00 3.89 0.44 52.86 31.20 0.06 1.08 0.02 0.20 0.06 0.13 0.01 0.05 0.04 2.00 0.02 0.24 0.42 0.04 0.02 0.02 0.01

5.14 0.05 0.00 4.00 0.38 52.73 33.29 0.07 1.07 0.02 0.21 0.06 0.13 0.01 0.05 0.03 1.84 0.03 0.23 0.45 0.05 0.03 0.01 0.01

DMDS in presulfidation and continuous addition completely eliminated the CO spike. The concentration of CO in the effluent was low and stable. Application of DMDS during ethane cracking showed a significant effect on coke formation. Presulfidation with 750 ppm DMDS followed by continuous addition of 2 ppm DMDS increased the amount of coke deposited on the surface of the cracking coil during 6 h of cracking by approximately 20%. Presulfidation with 750 ppm DMDS followed by continuous addition of 100 ppm DMDS increased the coke by a factor of 18.6. Further increasing the amount of DMDS in both presulfidation and continuous addition of DMDS increased the amount of coke further. The amount of coke as a function of continuous added DMDS in the steam cracking of ethane in the pilot-plant setup is shown in Figure 13. With continuous addition of 100-1000 ppm DMDS, the amount of coke increased linearly, independent of the presulfidation conditions. This implies that the significant

Figure 13. Amount of coke as a function of the amount of continuously added DMDS during the steam cracking of ethane in the pilot-plant setup.

increase in coke formation with the application of DMDS mainly originates from the continuously added DMDS. The effects of DMDS on the coke formation and CO production in the steam cracking of ethane as observed in the pilot-plant setup are consistent with the observations from the CSTR setup in the steam cracking of n-hexane. Reed19 studied the influence of presulfidation followed by continuous addition of DMS on coke deposition during ethane cracking (COT ) 1173 K, δ ) 0.5 mol mol-1, τ ) 0.5 s) in a tubular reactor (Incoloy 800H, length ) 2.4 m, i.d. ) 4.57 mm). It was found that presulfidation followed by continuous addition of 25-100 ppm sulfur as DMS resulted in an increase in coke formation by a factor of 4-9. In this study, the promotional effects of presulfidation followed by continuous addition of DMDS on coke formation as observed in both the CSTR setup and the pilot-plant are in line with the observations of Reed.19 The influence of presulfidation followed by continuous addition of DMDS on coke formation during naphtha cracking was studied by Dhuyvetter et al.3 in a pilot-plant setup with a cracking coil made of Incoloy 800HT. A 0.5-h presulfidation step with 700 ppm DMDS followed by continuous addition of 588 ppm (wt) DMDS decreased the coke deposition by 15%. Application of higher amounts of DMDS in presulfidation (1390 ppm) and in continuous addition (1175 ppm) gave practically the same amount of coke as compared to a blank run. A comparison of the results obtained by Dhuyvetter et al.3 for naphtha cracking with those obtained for ethane cracking by Reed and by us in the present study seems to indicate that the influence of DMDS on coke formation in the steam cracking of naphtha is different from that of ethane cracking. In naphtha cracking, more and heavier aromatic hydrocarbons are present in the effluent. Differences in adsorption on the metal surface between heavier aromatic hydrocarbons and light gaseous olefins might contribute to the observed difference. 4. Conclusions

Figure 12. CO concentration in the effluent as a function of time on stream during the steam cracking of ethane in the pilot-plant setup.

The results obtained in this study indicate that the influence of DMDS on coke formation in the steam cracking of n-hexane depends on the application method and the amount of DMDS applied. Coke formation during steam cracking is not directly related to the production of CO. Application of DMDS has no influence on the conversion of n-hexane. Presulfidation with DMDS eliminates the CO peak appearing at the beginning of a cracking run. CO production in the asymptotic stage of cracking is not suppressed. However, continuous addition of DMDS or presulfidation followed by continuous addition of DMDS are both effective in suppressing


CO production. For asymptotic coke formation during n-hexane cracking, the following conclusions can be made: Presulfidation of the oxidized Incoloy 800HT surface with DMDS at T ) 9731123 K, DMDS in H2O ) 500-750 ppm (wt), and t ) 0.5-2 h results in a decrease in coke formation by 10-20%. On both preoxidized and presulfided Incoloy 800HT surfaces, continuous addition of trace amounts of DMDS [2 ppm (wt)] significantly promotes coke deposition. In the presence of DMDS, the effect of presulfidation on CO production and on asymptotic coke formation depends on the concentration of DMDS in the feed. In the presence of 2-300 ppm DMDS, presulfidation suppresses CO production, whereas coke formation is promoted with DMDS concentrations up to 100 ppm in n-hexane feed. When the concentration of DMDS in the n-hexane feed is higher than 100 ppm, the effect of presulfidation on coke formation is much less pronounced. Continuous addition and presulfidation followed by continuous addition of DMDS leads to a significant change in the coke morphology. In the absence of DMDS addition, needle-shaped filamentous coke and globular coke are mainly formed. If less than 5 ppm DMDS is added continuously, braid-shaped filamentous coke is also formed in addition to the needle of shaped filamentous coke and globular coke. When the amount of continuously added DMDS is higher than 5 ppm, only porous coke is formed. Presulfidation results in an increase in the contents of Fe and Ni in the oxide layer. The reduction in coke formation upon presulfidation can be attributed to the presence of adsorbed sulfur, which weakens/blocks the adsorption of hydrocarbons on the surface and suppresses the diffusion of metals into the coke layer. The high rate of coke deposition on both preoxidized and presulfidized Incoloy 800HT surfaces with continuous addition of higher amounts of DMDS likely originates from the interference of HS radicals, originating from DMDS decomposition, with the heterogeneous noncatalytic mechanism during the asymptotic stage of cracking via radical termination and hydrogen abstraction reactions. The suppressing effect of DMDS on CO production and the promotional effect of continuous addition of DMDS on coke formation as observed in the CSTR setup were confirmed by tests performed in a pilot-plant for the steam cracking of ethane. The results obtained in this study indicate that the influence of DMDS on coke formation during the steam cracking of ethane is different from that occurring during the steam cracking of naphtha. Acknowledgment J.W. gratefully acknowledges the financial support of BASF Antwerp N.V. List of Symbols d ) density of coke, g m-3 E ) energy of the electron beam, kV FA ) mass flow rate of the reactant in the effluent, g h-1 Fi ) mass flow rate of product i, g h-1 F0A ) mass flow rate of the reactant, g h-1 h ) penetration depth of electron beam, µm L ) thickness of the coke layer, µm MC ) amount of coke deposited on the cylinders, g S ) surface area of cylinder, m2 Wa ) molar weight of the element, g mol-1 X ) conversion of the reactant, %

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ReceiVed for reView August 18, 2006 ReVised manuscript receiVed November 23, 2006 Accepted November 29, 2006 IE061096U

Influence of Dimethyl Disulfide on Coke Formation during Steam

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