Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
pubs.acs.org/IECR
Role of Presulfidation and H2S Cofeeding on Carbon Formation on SS304 Alloy during the Ethane−Steam Cracking Process at 700 °C Anand Singh,† Scott Paulson,† Hany Farag,‡ Viola Birss,† and Venkataraman Thangadurai*,† †
Department of Chemistry, University of Calgary, 2500 University Drive Northwest, Calgary, Alberta T2N 1N4, Canada Centre for Applied Research, NOVA Chemicals Corporation, 2928 16 Street Northwest, Calgary, Alberta T2E 7K7, Canada
‡
ABSTRACT: The influence of presulfidation and H2S cofeeding on the carbon formation on SS304 alloy in the ethane−steam cracker was investigated in a laboratory-scale quartz reactor setup. SS304H coupons and SS304L powder samples were exposed to ethane−steam and dry ethane in varying H2S content (0−50 ppm), and the SS304 samples were characterized by scanning electron microscopy. This study shows that H2S cofeeding decreases catalytic carbon formation; while it increases the pyrolytic carbon formation during ethane−steam cracking. Preoxidation, presulfidation, and addition of steam to ethane feed also reduces the amount of catalytic carbon formed on the SS304H surface in short-term experiments (4 h). Presulfidation and addition of H2S to ethane feed significantly influences the shape and size of the carbon formed on the surfaces of investigated metal alloys. Presulfidation and H2S cofeeding reduced spalling of the SS304H coupon surface during coking/decoking and thermal cycling.
1. INTRODUCTION Ethylene is produced by cracking ethane in the presence of steam at high temperatures, typically at ∼800−900 °C, and the cracking of ethane occurs primarily by gas-phase homogeneous radical chain reactions, although heterogeneous wall effects exist.1 The ethane−steam gas mixture is first preheated to 590− 650 °C by passing through high-temperature alloy (Fe−Ni− Cr) tubes in the convection section of the ethane cracker. The preheated gases then flow through tube coils in the radiant section of the ethane cracker where the endothermic cracking takes place. The radiant coils are constructed from hightemperature-resistant Fe−Ni−Cr alloys and suspended in a large gas-fired furnace. The ethane-cracking reaction is favored by low pressure as the ethane-cracking reaction produces an increase in the number of moles.1 Low reactant partial pressures favor primary reactions producing ethylene over unwanted secondary reactions. Steam is used in the industry to act as a diluent to decrease reactant partial pressure and enhance the conversion of ethane. Steam helps to suppress coke formation and transfer heat to the reactant gases. Steam is also easy to remove from the product gases by cooling. To reduce unwanted secondary reactions, the residence time in the radiant coils is kept at ∼0.1−0.5 s.1,2 The gases exiting the radiant coils are rapidly quenched in a transfer line exchanger (350−600 °C) to prevent further conversion of valuable products like ethylene and butadiene and to extract heat from the effluent gases.2,3 The effluent gases are then fractionated and purified in the downstream processes to separate desired products.4 Along with the desired ethylene, undesired CO and coke are also formed during ethane−steam cracking. The CO increases the carbon activity5 and is a poison for the downstream © XXXX American Chemical Society
catalysts, usually Pd supported on alumina that is used in hydrogenation of acetylene, methyl-acetylene, and propadiene.6 The coke deposits on the inner walls of the reactor tubes reducing their inner diameter and increases the pressure drop across the reactor tubes, resulting in undesirable bimolecular reactions, including combination and polymerization reactions producing higher hydrocarbons and coke.1,7 The pressure buildup in the reactor reduces the yield of ethylene. The coke deposits reduce the heat transfer to the gas phase due to the low thermal conductivity of coke, and lower heat flux causes lower conversion of the ethane.8 The heat load of the furnaces is increased to ensure adequate heat transfer to maintain ethane conversion levels, until the tube temperature reaches the tolerance limit of the alloy. When the pressure or temperature reaches a critical value, the ethane-cracking operations must be shut down and the tubes cleaned by passing steam and air through the coils. Presently, the decoking has to be done typically every 30−90 days, and each ethane cracker is in the decoking mode for about 5−15% of the time. Hence, understanding and reducing the coke formation in the furnace tubes is very important to ethylene manufacturers. The carbon formation in ethylene furnaces can be classified as catalytic and pyrolytic. The catalytic formation of carbon fibers has been observed on metal surfaces under carburizing conditions, i.e., at high carbon activity. For Fe-based alloys, Fe3C and Ni can act as catalysts for carbon fiber formation.9 The hydrocarbon adsorbs on the surface of the alloys and Received: October 7, 2017 Revised: December 15, 2017 Accepted: January 2, 2018
A
DOI: 10.1021/acs.iecr.7b04136 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
sulfur compounds on coke formations is still not clear. There are contradictory reports in the literature on the effect of sulfur compounds on the formation of coke.6 A suppressing effect has been reported by Bajus et al.18,19 and Depeyre et al.,20 whereas a promotional effect was reported by Velenyi et al.21 and Reyniers and Froment.4 Hence, further understanding of the effect of sulfur compounds on the formation of coke is critical in ethane−steam-cracking processes. In this paper, the effect of presulfidation and continuous addition of H2S to ethane feed on catalytic and pyrolytic carbon formation on SS304H coupons at conditions prevailing in the convection section of an ethane-cracking furnace have been studied.
undergoes subsequent dissociation and dehydrogenation leaving behind adsorbed carbon. The adsorbed carbon dissolves and diffuses through the metal and finds a favorite facet to precipitate out.9 The continuous precipitation of carbon from the Fe3C or Ni particles results in the formation of carbon fibers. Pyrolytic coke is the product of radical-type reactions and may be caused by reactions of coke precursors in the gas phase with active sites on coke matrix.6,10,11 The actives sites on the coke matrix are created by interaction of the coke with gasphase radicals like hydrogen, methyl, ethyl, and allyl radicals. Gas-phase radicals, unsaturated hydrocarbons, and aromatics are important coke precursors, which contribute to the growth of the coke layer by elementary reactions with the coke surface.6,10,11 Pyrolytic coke may also form by condensation of high boiling polymeric aromatic hydrocarbons (PAHs) formed by radical and molecular polymerization reactions.12,13 PAHs build in size until their vapor pressure becomes high enough that, even at pyrolysis temperatures, they condense, forming liquid droplets.12 Operating conditions, including temperature, pressure, residence time, steam content, and feedstock composition, play a significant role in determining the amount and type of carbon formed on the metal surface. Generally, the amount of coke deposited and the coking rate increases with increasing cracking temperature.6 In the convection (590−650 °C) and radiant (800−900 °C) sections, the coke formation may be due to both metal catalysis and interaction of coke precursors in the gas phase with active sites on coke. At the start of an ethanecracking run and on a fresh steel surface the coke formed is predominantly catalytic.6 The fibers formed by the catalytic process can trap the pyrolytic carbon formed in the gas phase and also interact with the coke precursors in the gas phase to form more pyrolytic carbon through gas-phase−solid reactions.14 As the metal surfaces get covered with coke, the catalytic activity decreases and the coke formed is predominantly a pyrolytic mechanism. Both catalytic and pyrolytic coke formation are significant in ethane crackers, and strategies should be adopted to control both types of carbon formation. The coke formation rate increases with increasing reactor pressure and ethane concentration.12 As the residence time is increased, the coke formation rates go through a maximum and then decrease.15 Such a maximum may be caused by the formation of maximum concentrations of coke precursors at intermediate conversions of the hydrocarbon feedstock. At higher residence times, further conversion of the hydrocarbon feedstock occurs and the concentrations of the precursors begin to fall.15 The feedstock composition plays a significant role in determining the coke formation rates. The presence of different hydrocarbon species in the gas influences pyrolytic carbon formation.16 Sulfur-containing species, such as sulfides (hydrogen sulfide (H2S), dimethyl sulfide (DMS), dimethyl disulfide (DMDS)), mercaptans, and polysulfides, have been conventionally used in industrial practice to reduce coke formation in pyrolysis furnaces. At conditions existing in the ethane crackers, the DMDS and DMS convert mostly into H2S.6 A significant amount of the surface of the metals is covered by adsorbed sulfur, even at very low parts per million levels of H2S.17 The effect of sulfur on coke deposition is influenced by many factors including continuous addition, presulfidation, composition of alloys, amount, and nature of the sulfur compounds.6 However, in the reaction tubes of an ethane-cracking furnace, the role of
2. EXPERIMENTAL SECTION Stainless steel 304H coupons (20 mm × 10 mm × 1.7 mm or 10 mm × 4 mm × 1.7 mm) and SS304L powder (ALFA AESAR, 100 mesh) were used in this work. SS304H and SS304L have similar alloy composition, except for a slightly lower carbon content in case of SS304L. Compared to the SS304H coupons, the SS304L powder has a larger surface area for reaction, which helps to better understand the surface changes caused by preoxidation, presulfidation, and cofeeding H2S with ethane; and the effect of these surface changes on carbon formation. The as-received SS304H coupons were rinsed with acetone followed by ultrasonication in methanol− water solution for further experiments. SS304L powder was used as received. The flow rates of ethane (CP grade, 99%, Gas innovations/PRAXAIR), ethane + 50 ppm of H2S (PRAXAIR), and He + 26 ppm of H2S (PRAXAIR) gases were controlled by mass flow controllers (Alicat Scientific). The desired steam content in the reactant gas was achieved by passing the gas through a water bubbler system maintained at the desired temperature. The H2S-containing gases bypassed the humidifier in all experiments. The stainless steel gas lines were heated from the humidifier to the furnace to prevent steam condensation. The reactant gases were heated to 700 °C inside a 19 mm i.d. quartz tube reactor heated by a horizontal tube furnace. A K-type thermocouple positioned in the middle of the furnace and exterior to the quartz reactor tube was used to control the furnace temperature. The temperature profile inside the quartz reactor tube was measured by a movable thermocouple, and the furnace temperature was adjusted to reach the desired temperature. During the ethane exposure experiments the movable thermocouple was removed from the setup. After each experiment the quartz tube reactor was cleaned by heating to 900 °C under a flow of air (25 mL/min) to ensure that any deposited carbon was completely oxidized and removed. The SS304H coupons or SS304L powder was placed inside the quartz tube reactor and exposed to the reactant gases at different operating conditions. Fresh samples were used in all of the experiments. The coke formation tests were conducted on as-received, preoxidized, and presulfided samples separately. The surface preoxidation was performed by exposing the samples to a mixture of air (30 wt %) and steam (70 wt %) at 700 °C for 24 h. Presulfidation was done by exposing the preoxidized samples to He (24 wt %) and steam (76 wt %) in the presence of 10 ppm of H2S for 24 h at 700 °C. The chemical changes occurring on the SS304L powder after preoxidation and presulfidation were analyzed by powder X-ray diffraction (Bruker D8 Advance powder X-ray Diffractometer with Cu Kα radiation (40 kV and 40 mA)) and X-ray photoelectron spectroscopy (PHI VERSAPROBE 5000-XPS). B
DOI: 10.1021/acs.iecr.7b04136 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
than unity, and hence, the thermochemical stability boundary for the formation of oxides, sulfides, and sulfates will be shifted to higher values of pO2 and pS2.27 Also, in alloys, mixed phases containing FeCr2O4 and MnCr2O4 may become stable under the above conditions. To understand the changes occurring on alloy surfaces at different operating conditions, SS304L powder was exposed to different operating conditions and analyzed by powder XRD, XPS, and TGA. The mass of SS304L powder increased by 30% on preoxidation and is due to the formation of various metal oxides. PXRD of preoxidized powder shows diffraction peaks due to Fe2O3, Cr2O3, (Fe0.6Cr0.4)2O3, NiO, NiCr2O4, NiFe2O4, Ni1.25Fe1.85O4, FeCr2O4, Cr1.3Fe0.7O3, Fe3O4, and NiCrFeO4 (Figure 1). As-received SS304L shows mainly diffraction peaks
The chemical changes occurring on as-received, preoxidized, and presulfided SS304L powders under reducing conditions were analyzed in a thermogravimetric analyzer (TGA) (METTLER TOLEDO). A gas mixture of Ar (50 mL/min) + humidified (0.3% H2O) He/10%H2 (5 mL/min) was passed over 0.1 mg of the sample, while the temperature was ramped from 25 to 700 °C at 10 °C/min, soaked at 700 °C for 20 h, and cooled to 25 °C at 10 °C/min. The coke formation experiments were done either with dry ethane or with ethane−steam (weight ratio 70/30) in varying H2S content (0−50 ppm). The total gas flow rate during preoxidation, presulfidation, and coke formation experiments was 100 mL/min. The coke formation experiments were of either short (2 and 4 h) or long (24 and 100 h) duration to study the predominant formation of catalytic or pyrolytic carbon, respectively. In the short-duration experiments, the samples were exposed to dry ethane for a period of 2 h or to ethane−steam for a period of 4 h. In the long-duration experiments, the samples were exposed to dry ethane for a period of 24 h or to ethane−steam for a period of 100 h. In the short-term experiments carbon formed only on the alloy surface and not on the quartz tube, showing that catalytic carbon had formed. In the long-term experiments carbon could be observed on the quartz reactor tube surfaces showing that pyrolytic carbon had also formed in addition to catalytic carbon. The amount of carbon formed on the SS304L powder samples was analyzed by measuring the mass change directly using an analytical balance. The amount of carbon deposited on some of the SS304H coupons was analyzed using temperatureprogrammed oxidation in a thermogravimetric analyzer (SETARAM TAG 16 TGA/DSC dual chamber balance). The microstructure of the carbon formed on both the powder and the coupon samples was analyzed by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) using a ZEISS Sigma VP field emission scanning electron microscope equipped with an Oxford INCA X-Act EDXS unit.
Figure 1. XRD pattern of as-received SS304L powder showing a (●) face-centered-cubic iron (Fm3̅m) structure and preoxidized and presulfided SS304L powders showing peaks corresponding to (■) Fe 2 O 3 , Cr 2 O 3 , and (Fe 0.6 Cr 0.4 ) 2 O 3 , ( ⧫ ) NiCr 2 O 4 , NiFe 2 O 4 , Ni1.25Fe1.85O4, FeCr2O4, Cr1.3Fe0.7O3, Fe3O4, and NiCrFeO4, and (▲) NiO.
3. RESULTS AND DISCUSSION 3.1. Influence of Preoxidation and Presulfidation on SS304L Powder. In the convection and radiant sections of the ethane cracker, the oxygen partial pressure (pO2) and sulfur partial pressure (pS2) varies as a function of gas temperature, operation mode such as preoxidation, presulfidation, or ethane−steam cracking, as well as along the length of the tube. The feed enters the convection section at 400 °C and leaves at ∼650 °C, and the tube metal temperature is around 700 °C. During preoxidation, the pO2 is about 10−2 atm in the convection section. Cr2O3, Fe2O3, and NiO are thermodynamically stable at 700 °C.22 During presulfidation, the pO2 and pS2 varies along the length of the convection section. At the entrance of the convection section, where the gas temperature is around 400 °C, the pO2 and pS2 are on the order of 10−12 and 10−10 atm, respectively. At the exit, where the gas temperature is around 650 °C, the pO2 and pS2 are on the order of 10−8 and 10−7 atm, respectively. Under these pO2 and pS2 at 400−650 °C, Fe2(SO4)3, Cr2(SO4)3, and NiSO4 are thermodynamically stable.23−25 The equilibrium pO2 and pS2 values may be established by dissociation of H2O and H2S, and computed as described in the lierature.26 These stable phases were determined based on the thermochemical stability diagrams calculated for pure metals.23−25 However, in the case of alloys, the activity of the component elements is less
due to elemental Fe (Figure 1). Under the present preoxidation conditions, it has been suggested that a duplex oxide structure consisting of an outer layer of MnCr2O4 and sublayer of Cr2O3 might have formed.28 MnCr2O4 was not identified in the XRD, either because it is absent or is only a few nanometers thick. Presulfidation of the preoxidized powders caused an additional mass increase of 2%, and no additional compounds were detected by XRD (Figure 1). However, XPS analysis confirms formation of metal sulfates after presulfidation. Figure 2a and 2b shows the S 2p and O 1s XPS spectra, respectively, of presulfided and preoxidized SS304L powder. The presulfided powder shows a weak sulfur (S 2p) signal relative to the background signal measured for the preoxidized SS304L powder. XPS analysis of a SS304H coupon exposed to ethane−steam and 1 ppm of H2S also showed similar results, with a S 2p peak at a binding energy of 169.29 eV.29 The binding energy of this peak is consistent with S in a sulfate group. The O 1s XPS spectra of the presulfided powder show a dominant shoulder at 532 eV, compared to the preoxidized powder. The binding energy of this peak is also consistent with oxygen in a sulfate group.30 Previous studies on presulfidation have proposed that sulfates and sulfides cannot form during the presulfidation process, and sulfur could be in the form of a C
DOI: 10.1021/acs.iecr.7b04136 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
Figure 2. (a) S 2p and (b) O 1s XPS spectra of presulfided (green) and preoxidized (red) SS304L powder.
chemisorbed layer on the alloy surface.6 However, the pO2 and pS2 used in that study6 were on the order from 10−16 to 10−12 and 10−11 to 10−8 atm, respectively, which is significantly lower than that used in the present study. It should be important to note that Fe2(SO4)3, Cr2(SO4)3, and NiSO4 are thermodynamically stable under the employed pO2 and pS2 regime in this work.23−25 Thus, this work shows that during presulfidation, sulfur in addition to being chemisorbed on the alloy surface may also be present in the alloy as sulfates. The pO2 and pS2 of the gas phase varies along the length of the convection section during ethane−steam cracking. When 10 ppm of H2S is added to ethane−steam, the pO2 and pS2 at the entrance of the convection section (∼400 °C) are on the order of 10−12 and 10−10 atm, respectively, whereas in the middle of the convection section (∼500 °C) the pO2 and pS2 increase to 10−10 and 10−9 atm, respectively. At temperatures above 600 °C, ethane cracking starts to occur and H2 is produced and results in a reduction in the pO2 and pS2 as conversion increases. At the exit of the convection section the conversion is around 1−2%, and the corresponding pO2 and pS2 at the exit of the convection section was calculated to be on the of order 10−18 and 10−11 atm, respectively, for the gas compositions used in this study. Nishiyama et al. calculated the pO2 in an actual ethane cracker to be
