Ind. Eng. Chem. Res. 1995,34,773-785
773
Influence of Metal Surface and Sulfur Addition on Coke Deposition in the Thermal Cracking of Hydrocarbons Marie-Franqoise S. G. Reyniers and Gilbert F. Froment" Laboratorium uoor Petrochemische Techniek, Uniuersiteit Gent, KrGgslaan 281, B9000 Gent, Belgium
Coke formation in the thermal cracking of hydrocarbons was studied in a pilot plant unit and in a microreactor with complete mixing of the gas phase, containing a hollow cylinder suspended a t the arm of an electrobalance. The morphology of the coke was studied by SEM, while EDX was used to determine the concentration of metals in the coke layer. The influence of the metal surface composition, of its pretreatment, and of the addition of various sulfur compounds on the coking rate and CO production was investigated for conditions typical for those in the cracking coil. The CO yield is not a measure of the coking rate. Sulfur compounds are very efficient in reducing the CO yield but promote coke formation.
Introduction
cracking is proportional to the amount of coke deposited in the process. It has been shown, however, that the In the thermal cracking of hydrocarbons, the most CO yield is not always a measure for coke production important source of olefins, coke formation by undesired (Froment (1991)). A comparison of the run length with side reactions hampers the heat transfer from the that of a blank run was not given. In British Patent furnace to the process gas and increases the pressure 695336, granted t o Esso Research and Engineering, it drop over the coil, which is detrimental for the ethylene is claimed that the coking in the cracking coil can be yield. As a result, regular decoking operations are controlled by the addition of sulfur in appropriate required, thus lowering the onstream time, and therequantities. Yet, in Dutch Patent 134715 of Aug 1972, fore, the production capacity. Further, reforming reacalso granted to Esso Research and Engineering, it is tions with steam, which is used as a diluent, catalyzed claimed that on-line decoking of ethane furnaces is by the coil material (Ni, Fe, Cr, Ti) leads to the possible by feeding pure ethane and steam, without any formation of carbon monoxide. This acts as a temporary sulfur. poison for the catalyst, usually Pd supported on aluA number of bench-scale experiments with sulfurmina, used in downstream acetylene, methylacetylene, containing additives have been reported. The observaand propadiene hydrogenation. tions are contradictory. Elemental sulfur (Bajus and Thermal crackers are constructed of heat-resistant Vesely (1980))and hydrogen sulfide (Kolts (1986))have Fe-Ni-Cr alloys. An inherent problem associated with been reported to reduce the deposition of carbon. these alloys is their tendency to promote the deposition Recently Velenyi (1991) reported an increase of the of carbonaceous materials. Since the trend in industrial coking rate on Inconel 600 in the pyrolysis of ethane operations is toward high seventy operation, i.e., higher with 25 and 100 ppmv hydrogen sulfide in the feed. temperatures and shorter residence times, the currently Depeyre et al. (1985) mentioned a reduction in the coke used materials all have a high nickel content. Knowldeposition in the thermal cracking of n-nonane in a edge of the coking behavior of alloys with various nickel quartz reactor by addition of either diethyl sulfide or contents can provide valuable information for the selecdimethyl sulfoxide. Crynes and Albright (1969),Dunkletion of construction materials. man and Albright (19761, and Ghaly and Crynes (1976) Naphtha or heavier feed fractions contain sulfur. In described the inhibiting effect of sulfur on the coke gas cracking a small amount of sulfur is added to the deposition in the cracking of propane, ethane, and feed. This reduces the CO yield in the outlet stream of propylene, respectively. Albright and Marek (1988) the furnace, which is often interpreted as an indication attributed the reduction of coking by the addition of of reduced coking. It is also reported that in the absence sulfur to the formation of a passivating layer of metal of sulfur the pressure drop rapidly increases with time. sulfides. Bajus and Baxa (1985) studied the effect of A major drawback of sulfur-containing additives is their various S-containing additives on coke formation during detrimental effect on the coil material. An understandpyrolysis of a reformer raffinate at 820 "C and 100 kPa ing of the role of sulfur in the coke formation and the (residence time = 0.25 s) in a stainless steel reactor (Fe CO production would contribute to the development of 72.5, Cr 17.5, Ni 9.4, Mn 0.7, C 0.18 wt %). In the noncorrosive additives which would prolong the run presence of sulfur compounds (dibenzyl disulfide, dibenlength of the furnace and eventually suppress the need zyl sulfide, thiophene, l-butanethiol, carbon disulfide) for off-line decoking. the coking rate was lower. The authors assumed that Precise information concerning the influence of sulfur in each case the sulfur compound was decomposed into additives on the coke formation in industrial units is hydrogen sulfide. The inhibition of coke deposition was scarce. Santiago et al. (1983) reported on tests perthen ascribed t o an inhibiting effect exerted by a layer formed in an industrial ethane cracker. Hydrogen of metal sulfides formed by interaction of hydrogen sulfide (10 ppm) was added in a first run, and 7 ppm S sulfide with the metallic wall. The stability of the metal as ethylmercaptan was added in a second run (T= 835 sulfides was not questioned, however. Trimm and "C, 6 = 0.4, ethane mass flow = 7400 kg/h). The drastic Turner (1981) emphasized the importance of the nature reduction of CO in the outlet stream of the furnace, from of the metals on which carbon deposition occurs. They 10 000 to 250 ppm upon addition of hydrogen sulfide, studied carbon formation during pyrolysis of propanel and even to 150 ppm with ethylmercaptan, was interhydrogen sulfide mixtures (1%v/v) at 805 "Con foils of preted as evidence of reduced coking. This would imply copper, nickel, iron, and stainless steel (Fe 70-74, Cr that the amount of carbon monoxide produced in the 17-19, Ni 9-11 wt %). In the presence of hydrogen 0888-58851951263~-0773$09.00/0 0 1995 American Chemical Society
774 Ind. Eng. Chem. Res., Vol. 34, No. 3, 1995 Table 1. Composition of Inconel 600 and Incoloy 800H Inconel 600 Incolov 800H 31 Ni 72.5 21 Cr 16 46 Fe 9.5 0.05 0.07 C Mn 0.8 0.6 0.6 Si 0.35 0.015 P 0.015 0.015 0.015 S 0.1 cu 0.1 0.35 0.3 Ti Al 0.3 0.25
sulfide the rate of carbon deposition was reduced on stainless steel and iron, slightly increased on copper, and strongly increased on nickel. They explained their results on the basis of the chemical nature of the surface on which carbon deposition is occurring; carbon on a surface may either dissolve in or encapsulate the material or may be gasified by reaction with hydrogen or steam. Any change of the chemical nature of the initial surface, for example, by sulfide formation, will have a repercussion on the relative importance of dissolution, encapsulation, and gasification. The inhibitor effect was attributed to the formation of a passivating layer of stable metal sulfides. The high rate of coke formation on nickel was believed to originate from the formation and decomposition of nickel sulfide, as a result of local variations in the ambient conditions. This would lead to a disruption of the surface and hence t o severe coking.
Experimental Procedure Electrobalance-Microreactor Unit. This is a specifically designed reactor with complete mixing, i.e.,
Figure 1. Flow sheet of the equipment.
operating under point conditions of partial pressure and temperature and is described in detail in a previous paper (Sundaram and Froment (1979)). To measure the rate of coking the electrobalance technique used in studies on catalytic coking (De Pauw and Froment (1975), Dumez and Froment (1976)) was adopted. The reactor is too heavy to suspend at the arm of the electrobalance, however. Therefore, a small hollow cylinder is inserted into the reactor and suspended at the arm of the Cahn 2000 electrobalance, thus allowing the coke deposition to be measured continuously. The reactor volume is 5.23 cm3, the surface of the cylinder is 7.565 em2, and the total surface-to-volume ratio is 4.644 cm-l. The reactor is made from Incoloy 800H and the cylinders used in the present study were made from Incoloy 800H and Inconel 600 (see Table 1). A schematic representation of the equipment is given in Figure 1. With n-hexane (99%),e.g., the flow rate is typically 40 g/h. It is set by pressurizing a glass vessel with nitrogen and forcing the liquid through a thermostated capillary. The flow rate is measured by means of a calibrated buret connected in parallel with the vessel. Water is fed at a rate of 20 g/h by means of a Pharmacia P-500 pump. The hydrocarbon and water are preheated, mixed, and bypassed to a condenser, while nitrogen flows through the reactor, until the conditions are stabilized. Then the sliding valve is set in such a position that the hydrocarbon-steam mixture is admitted to the reactor. The vapor enters the reactor at high velocity through 24 narrow channels drilled at an angle of 15" with respect t o the vertical (Figure 2). The small, hollow cylinder is suspended from one arm of the electrobalance and is positioned in the center of the reactor. The rate of coking is obtained as weight of coke per surface area of the cylinder and per unit time.
Ind. Eng. Chem. Res., Vol. 34, No. 3, 1995 776 1. Reactor 2. Preheat Section
3. Reactor Block 4. Insulation 5. Thermocouple 6. Suspension Wire 7. Cylinder
coolant -0
coolant 9 _
section and an internal diameter of 0.01 m ( S N = 400 m-l). The pilot unit can also be equipped with a shorter coil, made from Incoloy 800H, with a length of 5.78 m and an internal diameter of 11.5 mm (5“= 353 m-l). The exit gases are cooled in a TLE-type heat exchanger, and a small fraction is withdrawn for online GC analysis. Nitrogen is injected in the effluent and serves as an internal standard in the analysis. Coking runs were carried out with ethane, propane, and naphtha in the presence of steam and with or without addition of sulfur compounds. The runs lasted for 6 h. The coke was determined through controlled combustion as CO and C02 by means of Teledyne IR analyzers. The instantaneous CO and COz contents were integrated over the decoking period to yield the amount of coke deposited in the coil.
Experimental Results and Discussion
coolanl -d
effluenl
Figure 2. Details of the reactor.
The reactor effluent leaves the reactor through the tube containing the wire and further through a side exit. A flow of carbon dioxide from the balance chamber down the tube and through the side exit prevents the effluent flowing into the balance chamber. The effluent is cooled, but not condensed, in a heat exchanger by means of Ultratherm 330SCB and flows through a cyclone, in which tar is separated from the gas and in which nitrogen, the internal standard for the gas chromatografic analysis, is added. To obtain the gas-phase composition that led to the observed coke formation use is made of three gas chromatographs for online analysis. The internal standard permits relating the three GC analyses to one another. The reactor effluent is also analyzed for CO, after condensation of the hydrocarbons by means of a cooling unit at -20 “C, with a Teledyne IRA, Model 7 11,infrared analyzer. The cylinders in Inconel 600 and Incoloy 800H were turned on a lathe. According to specifications (norm NEN 3638) the surface roughness R , is then given by 1 pm < R, < 3.2 pm. The pretreatment of the reactor and the cylinder is strictly standardized and consists of either a reduction by hydrogen or an oxidation with air at 700 “C. The flow of hydrogen or air is 27 L/h (stp) and the duration of the pretreatment was 16 h. Before their use, the cylinders are electrolytically reduced for 15 min at 8 V. Tubular Pilot Unit. The pilot plant unit has been described in detail in a previous paper (Van Damme and Froment (1982)). Briefly, the furnace is divided into seven separate cells, fired independently by gas burners. This allows the setting of any type of temperature profile. The first two cells are used to generate steam and to preheat the hydrocarbon feed. Both gaseous and liquid hydrocarbons can be fed, The coil is made from Incoloy 800H and has a length of 21.75 m in the reaction
Influence of Coil Material. Carbon deposition on prereduced and on preoxidized cylinders of Inconel 600 and of Incoloy 800H in the steam cracking of n-hexane was investigated. Representative curves of coke vs time and coking rate vs time, determined by the electrobalance technique are shown in Figures 3 and 4. Initially the coking rate is high, but it gradually decreases with time to reach a constant value: the “asymptotic coking rate”. In one experiment the run length was extended to 200 h. The value of the asymptotic coking rate did not change over this time period. The high initial coking rate is associated with catalytic wall effects. It is generally accepted that catalytic carbon formation on metals involves surface reactions, diffusion, and precipitation of carbon (Baker (1973), Lobo and Trimm (19731, Figuerido (19891, Kock et al. (1985)). Initially a hydrocarbon molecule is chemisorbed on a metal crystallite on the surface. The CH, CH2, CH3, ..., groups present at the surface loose hydrogen atoms which recombine and desorb into the gas phase. Carbon atoms thus formed at the surface dissolve in and diffuse through the metal particle. The carbon accumulation in the particle causes a pressure buildup at the dislocations and the grain boundaries, which may exceed the tensile strength of the metal. The metal particle is then lifted from the surface and carbon crystallizes at the rear end of the particle. A growing carbon stem thus develops which carries the crystallite at its top. The precipitation of carbon can give rise to structural deficiencies in the carbon lattice, thereby creating reactive carbon centers along the filament skin. Hydrocarbon radicals and molecules from the gas phase are incorporated at these reactive sites, whereby lateral growth of the filaments occurs. In this way a porous layer of interwoven filaments is formed. Carbon migrating over the metal surface precipitates on the metal surface surrounding the carbon stem. Surface carbon may thus be formed, encapsulating the metal and preventing further growth. Dissociative chemisorption of water molecules on the metal particles produces highly reactive oxygen atoms at the surface. These react with hydrocarbons or carbon atoms at the surface to form carbon monoxide, which desorbs into the gas phase. This reaction prevents fast encapsulation of the metal particle at the top of the filament. Whether the “catalyst” is deactivated or not is a question of the relative rates of the processes involved, namely, surface carbon growth, gasification, dissolution, and diffusion. The properties of the metal are very important in this mechanism. First chemisorbed carbon atoms must be formed. The fact that copper is almost inert toward
776 Ind. Eng. Chem. Res., Vol. 34, No. 3, 1995
-
E
.-m
6
P
4
~=21-27%
Inconel600 ; PO
2 0 0
100
200
300
400
500
Time (min) Inconel600 Prereduction 789°C I \
Inconel600 lncoloy 800H lncoloy 800H Preoxidation 790°C Prereduction 786°C Preoxidation 785°C .. . ...,. , . . . - - - 4
Figure 3. Weight of coke deposited in the cracking of n-hexane (6 = 0.5 kg of steamkg of hexane, z = 21-27%) in the microreactor. .._
3-,
X
r
E c
= 21-27 %
2.5
2
u)
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m
Y
Q,
c
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L
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Inconel600 : PO
11
,
!
3
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.
61
L1
0
0 0.5 0
-1 0
I
I
I
I
100
200
300
400
500
Time (min) Inconel600 Prereduction 789°C
Inconel600 Preoxidation 790°C
_c_
4,
lncoloy 800H Prereduction 786°C
lncoloy 800H Preoxidation 785°C ---+
Figure 4. Rate of coke deposition in the cracking of n-hexane (6 = 0.5 kg of steamkg of hexane, z = 21-27%) in the microreactor.
carbon deposition can partly be explained by chemisorption properties different from these of nickel and iron. Molecular acetylene adsorption is weaker on copper than on iron and nickel (Geurts (1981)). Whereas the latter two metals can split acetylene into adsorbed fragments, copper cannot (acetylene desorbs at higher temperatures and does not dissociate). Acetylene is bound to iron and nickel by the Dewar-Chatt-Duncanson n-to-metal donation and metal-to-n* back-donation mechanism; the amount of back-donation is larger. Both effects weaken the C-C bond. On copper, the donation effect is dominated by a repulsive interaction between orbitals of nearly the same energy, i.e., the occupied acetylene valence orbitals (mainly nu and 3 ug) and the occupied Cu levels (3dI0,4s'). Dissolution and diffusion are also crucial steps. Filaments are not formed on metals that show a low solubility for carbon, only encapsulating carbon is formed. Platinum is an example of such a metal. The physical state of the
surface, for instance, roughness and grain size, has an important influence on this mechanism. Encapsulation of the metal particles reduces the rate of dehydrogenation of the chemisorbed hydrocarbon atoms. At this stage the catalytic activity of the metal particle diminishes and both carbon formation and CO production slow down. Another mechanism of carbon formation now gains in importance. The heterogeneous noncatalytic mechanism is the most important source of coke in a thermal cracker, since it operates practically over the complete run length. The coke in contact with the gas phase consists of well-defined layers. This points to growth from the coke-gas interface (Ranzi 1985)). Coke formation then results from the reaction of precursors in the gas phase with active centers on the surface. Important coke precursors are radicals, unsaturated molecules (ethylene, acetylene, butadiene, ...), and aromatics. Kopinke et al. (1993) have determined, by means of coking
Ind. Eng. Chem. Res., Vol. 34, No. 3, 1995 777
Scheme 1
Scheme 2
&+-qgI
experiments with 14C-labeledhydrocarbons, a scale for the coking tendency. Acetylene, anthracenes, cyclic naphthenes, and aromatics exhibit a high tendency for coke formation. Ethylene, though not the most reactive precursor significantly contributes because it is present in high concentration. Kopinke et al. (1988) observed no differences in relative rate constants for coke formation from a particular hydrocarbon on surfaces of different materials. This supports the idea that the active centers are radical in nature and located in the coke matrix. These radical sites originate from the abstraction of hydrogen from the partially dehydrogenated surface of the carbon layer by means of small reactive radicals (hydrogen, methyl, ...). This implies that the number of radical sites on the coke surface is also determined by the gas-phase composition. The coke macroradical grows by addition of unsaturated molecules and also by reaction with radicals from the gas phase. M e r dehydrogenation, graphitic layers with sp2 hybridization for C are formed (Reyniers and Froment (1994)) (Scheme 1). The hardness of the coke layer, observed in industrial practice, can be explained by cross-linking with sp3 or sp2 carbon atoms from different layers (Reyniers and Froment (1994)) (Scheme 2). The homogeneous noncatalytic mechanism or gasphase coking, which results from a sequence of molecu-
1
-1 0
&+w-
& 000
& 00
lar and/or radical reactions in the gas phase, leads to high molecular weight polynuclear aromatic compounds which can be liquid or solid even a t the high temperatures prevailing in the cracker coil. These soot particles may colloide with the wall and integrate in the coke layer (Wang et al. (1981)). Since gas-phase coking probably contributes very little to coke formation in a thermal cracker, especially a t temperatures below 900 "C and with lighter feedstocks, its contribution to the "asymptotic coking" under the currently used circumstances can be neglected. The use of steam as a diluent gives rise t o the formation of carbon monoxide. CO is not observed when nitrogen is used as a diluent. Representative curves of carbon monoxide yield vs time are shown in Figure 5. The CO yields vs time curves behave very much like the coking rate vs time curves. Two processes are
I
I
I
I
100
200
300
400
500
Time (min) lncoloy 800H lncoloy 800H Inconel600 Inconel600 Prereduction 789°C Preoxidation 790°C Prereduction 786°C Preoxidation 785°C I\
......, + .....
Figure 5. Amount of carbon monoxide produced in the cracking of n-hexane (6 microreactor.
---.+
= 0.5 kg of steamkg of hexane, x = 21-27%) in the
778 Ind. Eng. Chem. Res., Vol. 34, No. 3, 1995
correlation between the asymptotic coking rate and the bulk composition of the alloy was not apparent. The total metal content and the relative metal composition of the coke layer in contact with the gas phase were determined by EDX analysis. The absolute metal content, as well as the relative concentrations, varies throughout the surface. The mean values from 9 analyses at various locations at the surface are given in Table 2. The main metal constituents in the coke layer are chromium, iron, nickel, and titanium. These metals have been found before in coke layers (Baker et al. (1977)). Surprisingly, there seems t o be no correlation between the initial activity toward coke deposition and the metals content in the coke layer. The variation in the absolute metal concentrations as a function of the radial position on the top of filaments formed on a prereduced Inconel 600 surface is presented in Figure 6. For both Inconel 600 and Incoloy 800H the absolute metal concentration was highest at the edges and lowest in the center of the filament. For Incoloy 800H the absolute metal concentration along the top of the filament varied from 25 to 11wt %. The decrease was most strongly pronounced for chromium. A relative enrichment in iron towards the center of the filament was observed. In the case of Inconel 600 the absolute metal concentration varied from 4 to 10 w t %. Here too, the decrease was strongly pronounced for chromium. A relative enrichment in the nickel content was observed. Clearly, carbon deposition results from complex interactions between the components of the alloy surface and gas-phase hydrocarbons. The morphology of the coke layer deposited on the cylinder of the microreactor in the pyrolysis of n-hexane, with a run length of 6 h, was studied by SEM. Figure 7a,b shows SEM photomicrographs of the coke deposited on a prereduced Inconel 600 surface. The coke consisted mainly of coalesced globules (Figure 7a). In some cases needlelike filaments grew between the globular coke. In addition, several nicely shaped filaments (Figure 7b) with a diameter in the range 2.5-3 pm were formed. Three kinds of filaments were observed: filaments with a spiral shape, needlelike filaments and filaments with a layered structure. The metal particles on top of the
Table 2. Absolute Metal Content and Distribution (wt 70)of Metals in the Coke Layer in Contact with Gas Phase as Determined by EDX alloy Inconel 600 Incoloy 800H
metal pretreatment content reduction oxidation reduction
19.60 27.48 56.57
Cr
composition Fe Ni
Ti
64.55 4.35 29.70 1.40 79.70 1.69 12.19 6.42 84.06 9.17 1.35 5.18
important in describing this: the metal-catalyzed removal of carbonaceous intermediates and coke by steam reforming processes (Wagner (19921, Xu and Froment (1989)) and the slower noncatalytic gasification of coke by steam. Froment (1991)explained the high initial CO yield and the lower initial ethylene yield, observed during ethane cracking, by steam reforming associated with the catalytic activity of the wall. As the metal particles become covered with coke, their catalytic activity diminishes. In industrial crackers the temperature at the coke-gas interface exceeds 900 “C and direct gasification of coke by steam can contribute to the nonzero asymptotic CO production (Froment (1991), Pramanik and Kunzru (1985)). At the high temperatures prevailing in the cracker coil the coke is not impervious, so that the influence of the metals can still be felt during the asymptotic stage. The CO-yield is not a measure of the coke yield. This would only be the case if CO would result exclusivelyfrom the coke gasification. It is obviously mainly generated from catalytic steam reforming of coke precursors and of the feed. A comparison of the results obtained on Inconel 600 with those measured on Incoloy 800H points to a “remanent” catalytic effect in the coke formation and the CO production. Preoxidation of the surface increases the selectivity toward CO formation, the effect being most strongly pronounced for Inconel 600. The activity for coke deposition increases in the series Inconel 600 prereduced < Incoloy 800H preoxidized < Incoloy 800H prereduced < Inconel 600 preoxidized. The selectivity toward coke formation on Incoloy 800H is only slightly influenced by the pretreatment. On the other hand, preoxidation of Inconel 600 strongly increases the selectivity toward coke deposition. A direct
Inconel 600 Prereduction
Fe 11
I
-1.25
-0.75
Ti
A
I
-0.25
I
I
0.25
0.75
1.25
Radial Position (micrometer) Figure 6. Metal concentration vs the radial position on the top of a filament formed in n-hexane cracking (6 = 0.5 kg of s t e a d k g of hexane, x = 21-27%) on prereduced Inconel 600 (2’ = 785 “C).
Ind. Eng. Chem. Res., Vol. 34,No. 3, 1995 779
F i g u r e 7. SEM photomicrographs of coke deposited in n-hexane cracking (d = 0.5 kg of steamkg of hexane). (a, top left) Globular coke and spiral shaped filament formed on prereduced Inconel 600 (T= 785 "C). Magnification 3500 (reproduced at 68% of original size). (b, top right) Filament with a layered structure formed on prereduced Inconel 600 (T= 785 "C). Magnification 10 000 (reproduced a t 68% of original size). (c, bottom left) Coke agglomerate formed on prereduced Incoloy 800H (T= 776 "C).Magnification 3500 (reproduced a t 68% of original size). (d, bottom right) Filament formed on prereduced Incoloy 800H (T= 776 "C).Magnification 7500 (reproduced a t 68% of original size).
filaments were completely covered with coke. On a prereduced Incoloy 800H surface the appearance of the coke was somewhat different. The coke globules were smaller and interwoven agglomerates of braided filamentous coke were formed (Figure 7c). Analogous morphologies were reported by Marek and Albright (1982). There are fewer tall filaments, and their appearance points toward a nonuniform distribution of growth centers along the skin of the filament (Figure 7d). These photomicrographs reveal that there are more growth nuclei present on prereduced Incoloy 800H than on prereduced Inconel 600. Since the actual coke surface is much larger in this case the number of radical centers responsible for the growth of the coke in the asymptotic range is higher, leading to the observed higher asymptotic coking rate (see Figure 4). On a preoxidized Inconel 600 surface this effect seems to be even more pronounced. In this case no tall filaments were observed. Influence of Carbon Disulfide on Coke Deposition and CO Production. In commercial steam cracking of gaseous hydrocarbons, sulfur components are added to the feed. It is reported that the CO yield is thereby reduced, and this is often interpreted as evidence of reduced coking. To investigate the effect of sulfur ,on the coke formation and the CO production, cracking experiments with 50 ppm carbon disulfide (26.77 pg of S in the gas phase/m2 of metal surface) in a n-hexanelsteam feed were camed out in the elec-
trobalance-microreactor. Representative curves of coking rate and CO yield vs time obtained on prereduced and preoxidized Inconel 600 are given in Figures 8 and 9. It can be seen from Figure 9 that addition of 50 ppm carbon disulfide results in an important reduction of the CO yield, both in the initial and the asymptotic range, the effect being most pronounced for preoxidation. The same trends were observed on prereduced and preoxidized Incoloy 800H. Sulfur clearly acts upon the catalytic gasification of the steam-reforming process, responsible for the CO production. Sulfur components from the gas phase are more readily chemisorbed on the metal particles than water and hydrocarbon molecules. Coverage of metal sites by sulfur then leads to a lower concentration of active oxygen atoms at the surface and hence to a lower catalytic CO production (Wagner (1992), Rostrup-Nielsen (1984)). The detrimental effect of sulfur on the coke deposition is clearly demonstrated in Figure 8. Both the initial and the asymptotic coking rate exceed that of the corresponding blank, the effect being most pronounced on prereduced Inconel 600. On Incoloy 800H too, both initial and asymptotic coking rate exceeded that of the blank run; the effect was most pronounced on preoxidized Incoloy 800H. Sulfur compounds can alter the rate of radical reactions through their interference with hydrogen transfer and with termination reactions. No pronounced effect
780 Ind. Eng. Chem. Res., Vol. 34, No. 3, 1995 20 -1
I
Inconel600
PR ; CS2 ..PO ; cs2 8
.o.......?.. ....!.'
n-..... .o.....
...... CI"".U
0
I
100
200
PO
...... u...... ......n ...... ...... ~
D
I
PR
u"."'o
I
I
300
400
500
Time (min) Prereduction Preoxidation 789°C
n
790°C
......u .....
Prereduction
Preoxidation
50 ppm CS2,786"C 50 ppm CS2,789"c ...... .....
d
Figure 8. Rate of coke deposition in the cracking of n-hexane with and without carbon disulfide on Inconel 600 (6 = 0.5 kg of steamkg of hexane, z = 21-27%). 6 ?
5 h
-8
Inconel 600
-%,,,,
.
tl.,,
c+.g..
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0 0
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I
I
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I
100
200
300
400
500
Time (min)
-
Prereduction Preoxidation Prereduction Preoxidation 790°C 50 ppm CS2,786"C 50 ppm CS2,789"C 789°C i t
A
Figure 9. Amount of carbon monoxide produced in the cracking of n-hexane with and wthout carbon disulfide on Inconel 600 (6 = 0 5 kg of steamkg of hexane, x = 21-278)
on the rate of cracking or on the associated selectivities was observed in this work. Yet, the influence on coking reactions is quite pronounced: on prereduced Inconel 600 and on preoxidized Incoloy 800H the asymptotic coking rate increased by a factor 20 and 23,respectively; on preoxidized Inconel 600 and on prereduced Incoloy 800H the asymptotic coking rate amounted t o 8 and 10 times the values observed for the corresponding blank runs. Again, it is clear that the CO yield is not a reliable indicator for the coke formation (Froment (1991)). The results obtained in the microreactor were confirmed by pilot plant experiments in an Incoloy 800H coil (Table 3). In ethane cracking, for a coil outlet temperature of 805 "C, a residence time in the cracking coil of 0.8 s, a steam dilution of 0.3 kg/kg, and an ethane
conversion of 6l%, the CO yield in the absence of carbon disulfide was 6 wt %, while the C yield amounted to 0.018 g of coke1100 g of ethane fed. With 50 ppm carbon disulfide in the feed, the CO yield was reduced t o 0.3 wt %, while the C yield increased to 0.113 wt %. Figure 10 shows a SEM photomicrograph of the coke deposited after 6 h in the microreactor on a prereduced Inconel 600 cylinder in the cracking of n-hexane in the presence of steam and with 50 ppm carbon disulfide in the feed. A comparison with Figure 7 shows a pronounced change in morphology, due to the addition of carbon disulfide. Braided filamentous coke was predominantly formed. Influence of the Concentration and Nature of the Sulfur Compound. To evaluate the influence of the concentration and the nature of the sulfur compound
Ind. Eng. Chem. Res., Vol. 34,No. 3, 1995 781 Table 3. Results of Coking Experiments Performed in the Pilot Unit 5.78 5.78 5.78 5.78 coil length(m) 21 21 10 10 11.5 11.5 11.5 11.5 coil diameter(") 0.209 surface (sqm) 0.678 0.678 0.209 0.209 0.209 353 353 400 400 353 353 S N (m-l) feed C2Hs CzHs C3Hs C3Hs naphtha naDhtha 42 200 53 +io@ sulfur (ppm) DMDS DMDS sulfur compound cs2 4800 flow rate HC (g/h) 3000 3000 5900 5900 4800 2400 flow rate H20 (g/h) 1000 1000 1800 1800 2400 0.3 0.5 0.5 dilution (kgkg) 0.3 0.3 0.3 907 805 805 905 905 907 COT ("C) 100 res time (ms) 800 800 100 100 100 85 61 85 61 0.422b 0.422b conversion (%) 3.22 0.08 0.15 0.37 CO initial (wt %) 0.11 0.09 CO average (wt %) 6 0.3 2.63 0.06 42 1.84 22.7 19.4 C amount (gr/6 h) 0.067 0.146 0.018 0.113 0.005 0.06 C yield (wt %) 22.97 4.24 4.24 28.23 28.23 22.97 kg of HC/h m2 0.36 0.13 0.48 0.042 g of s/h 2.30 1.72 0.61 0.062 g of S/h m2 47.85 16.91 63.80 13.78 .DCP: - of S/m2 a Total S content = 253 ppm. Ratio propylene/ethylene.
Figure 10. SEM photomicrograph of coke deposited in n-hexane cracking with 50 ppm carbon disulfide on prereduced Inconel 600 (d = 0.5 kg of steamkg of hexane, T = 785 "C,microreactor). Magnification 3500 (reproduced at 70% of original size).
on the coke deposition and the CO production, cracking experiments with continuous addition of various concentrations of carbon disulfide, thiophene, benzothiophene, dimethyl disulfide, and hydrogen sulfide were performed in the microreactor with prereduced Inconel 600 cylinders. The performance of each additive was investigated a t temperatures ranging from 770 to 800 "C. In the entire temperature range the same trends were observed. Addition of increasing quantities (1, 5 , 20, 50, 150, 200 ppm) of carbon disulfide to the feed increased both the initial and the asymptotic coking rate. Addition of as little as 1ppm carbon disulfide already caused a 40% reduction of CO in the effluent, while the coking rate increased by a factor of 2. The amount of CO was reduced further by increasing concentrations of carbon disulfide. From 150 ppm carbon disulfide onward the CO yield amounted to only 7% of the value observed in a blank run. No further pronounced decrease was observed, indicating that from 80 pg of sulfur in the gas phase/m2 metal surface all the metal centers responsible for the-catalyticCO production are passivated by sulfur. Addition of increasing quantities of thiophene (100, 500,2500 ppm), benzothiophene (250,650 ppm), and of dimethyl disulfide (25, 75, 200 ppm) caused increasing initial and asymptotic coking rates, while a reduction in the CO yield was observed. Continuous addition of even as little as 10 ppm hydrogen sulfide resulted in excessive coke deposition, disrupting the measurement in the electrobalance-microreactor unit.
Although Trimm and Turner (1981) observed a decrease in the rate of coke deposition on presulfided foils of stainless steel with a nickel content of 9-11 w t %, on presulfided stainless steel (Fe 53.5, Cr 24.5, Ni 20.5, Mn 1.8, Si 0.55 wt %) with a higher nickel content the situation was found to be more complex (Trimm et al. (1981)). A strong dependency of both initial and asymptotic rates of coke deposition on the amount of predeposited sulfur was observed. The deposition of coke was markedly reduced by small amounts of sulfur. Once the predeposited sulfur exceeded (10-12) x lo6 pg/m2 the rate of deposition was higher than on an unsulfided foil. The actual rate of coke deposition exhibited a maximum a t a sulfur level of about 28 x lo6 pg/m2. The present experiments with continuous addition of S indicate that on prereduced Inconel 600 (72.5% Ni) the detrimental effect of sulfur is already felt upon addition of as little as 0.84 ppm sulfur to the feed (i.e., 0.54 pg of S in the gas phase/m2 metal surface). A comparison of the influence of the various sulfur compounds on the rate of coke deposition and on the CO production is given in Figures 11 and 12. The asymptotic coking rate on prereduced Inconel 600 as a function of the amount of sulfur in the feed is given in Figure 13. The same trends were observed at temperatures ranging from 770 to 800. "C. For a given concentration of sulfur in the feed the asymptotic coking rate decreases in the order dimethyl disulfide > carbon disulfide > benzothiophene > thiophene. The detrimental effect of DMDS on the coke deposition was confirmed by pilot plant experiments in an Incoloy 800H coil (Table 3). In propane cracking, the addition of 200 ppm S as DMDS had a pronounced effect on both CO and C yield. For a coil outlet temperature of 905 "C, a steam dilution of 0.3 kgkg, a residence time of 0.1 s and a propane conversion of 85%, the CO yield decreased from 2.63 to 0.06 w t %, while the C yield increased from 0.005 to 0.06 wt %. The same trend was also observed in naphtha cracking. The naphtha already contained 53 ppm S. For a coil outlet temperature of 907 "C, a steam dilution of 0.5 kgkg, the CO-yield amounted to 0.11 wt % and the C yield to 0.067 w t %. After addition of 200 ppm S as DMDS the CO yield was reduced to 0.09 wt %, while the C yield increased to 0.146 wt %. A comparison of the cracking conditions for the microreactor, the pilot unit and an industrial unit is given in Table 4. The S N ratio of the microreactorelectrobalance unit is of the same order of magnitude as that of the pilot unit. For a given sulfur concentration in the feed, the amount of sulfur fed per surface unit differs considerably for the various units. For ethane cracking conditions typically used in industrial practice, addition of 200 ppm S as DMDS corresponds to 968 pg S/m2, while for the pilot unit this value amounts to 47.85 pg S/m2 in propane cracking. The detrimental effect of sulfur on the asymptotic coking rate was also observed for higher sulfur concentrations in the feed. In the microreactor-electrobalance unit, for hexane cracking with addition of 5000 ppm carbon disulfide (2677 pg of S/m2), at 785 "C, the asymptotic coking rate on prereduced Inconel 600 amounted to 6 times the value observed in a blank run, while the CO yield decreased from 4 to 0.27 wt %. The thermal stability of the sulfur compounds follows the same order as their influence on the coking rate. Excessive coking was observed upon addition of hydrogen sulfide. These facts indicate that the detrimental effect of the sulfur compounds may be related to the
782 Ind. Eng. Chem. Res., Vol. 34,No. 3, 1995
40
-
Inconel 600 Prereduction
Dimethyldisulfide
Carbondisulfide Benzothiophene
0
200
100
400
300
500
Time (min) Blank 789
100 ppm Thiophene 787°C 38 ppm S
d
I
250 ppm Benzothiophene 789"C, 59.5 ppm S _4_
75 ppm Dimethyldisulfide 789"C, 51 ppm S
50 ppm Carbondisulfide
-.-
786"C, 42 ppm S A
Figure 11. Influence of carbon disulfide, dimethyl disulfide, thiophene and benzothiophene on the rate of coke deposition in the cracking of n-hexane (6 = 0.5 kg of steamkg of hexane, x = 21-27%) on prereduced Inconel 600.
3
, Inconel 600 Prereduction
.-e
l
{A,,,.
..-..A--.......
0 -
0
Thiophene
....................................... .......-...... .......................... t ......-t Benzothiophene
-
_
_
_
-8
Carbondisulfide Dimethyldisulfide
I
1
I
I
100
200
300
400
500
Time (min) Blank 789
100 ppm Thiophene 787"C, 38 ppm S ....... ......
-
250 ppm Benzothiophene 789"C, 59.5 ppm S
50 ppm Carbondisulfide
75 ppm Dimethyldisulfide
786% 42 ppm S
789"C,51 ppm S
......a......
-c-
Figure 12. Influence of carbon disulfide, dimethyl disulfide, thiophene, and benzothiophene on the production of carbon monoxide in the cracking of n-hexane (6 = 0.5 kg of steamkg of hexane, x = 21-27%) on prereduced Inconel 600.
thermal decomposition of the sulfur compound, leading to hydrogen sulfide. Wynberg and Bantjens (1959) observed a conversion of 40% for the thermal decomposition of thiophene in a Vycor glass tube at 800-850 "C and a residence time of 25 s. The conversion of thiophene to hydrogen sulfide amounted to 4% only. Under the conditions used in the present study with a residence time of 136 ms, the production of hydrogen sulfide from thermal decomposition of thiophene can be expected to be minimal. More likely, hydrogen sulfide then results from the addition of hydrogen radicals, resulting from the hydrocarbons, to thiophene (Horie et al. (1975)). Hanh et al. (1979) mentioned that benzothiophene remains unchanged at
675 "C and stated that benzothiophene is more susceptible for attack by hydrogen radicals than thiophene. The observed higher coking rate with respect to that observed by addition of thiophene can then be caused by an increased production of hydrogen sulfide. Carbon disulfide reacts readily with water above 150 "C and carbonyl sulfide is an intermediate in this reaction (Bacon and Boe (1945)). Since in these experiments steam was used as a diluent, carbon disulfide is partly converted t o hydrogen sulfide even before entering the cracking coil. Moreover, at temperatures above 150200 "C and in the presence of nickel sulfide, carbon disulfide is reduced by hydrogen t o give methanethiol, dimethyl sulfide, methane, and hydrogen sulfide (Phil-
Ind. Eng. Chem. Res., Vol. 34, No. 3, 1995 783
Inconel 600 Prereductlon -
.A
Dimethyldisulflde
r=7wc
A
.,"
..........................................
...___... 0 ._.-
.__ .._ _ _ _..................4
Benzolhiophene
* Thiophene
0
50
100
150
200
Sulfur concentration in the feed (wt ppm) Figure 13. Asymptotic coking rate in n-hexane cracking (6 = 0.5 kg of s t e a d k g of hexane, x = 21-27%) on prereduced Inconel 600 as a function of the sulfur concentration in the feed. Table 4. Comparison of Cracking Conditions feed f S (ppm) S compound flow rate HC (kg/hr) dilution (kgkg) COT ("C) 0 (ms) CO yield (wt %) C yield (wt %)
SN (m-1) kg of HC/h m2 g of S/h m2 pg of S/m2 a
micro
pilot
pilot
industrial
hexane 136 DMDS 0.04 0.5 785 136 0.27 0.06 445 17.39 2.33 88.14
naphthaa 200 DMDS 4.8 0.5 907 100 0.09 0.146 353 22.97 2.30 63.80
propane 200 DMDS 5.9 0.3 905 100 0.06 0.06 353 28.23 1.72 47.85
ethane 200 DMDS 3660 0.33 857 587 31.34 89.98 5.94 968.34
Total S content = 253 ppm.
ips Petroleum Co (1975)). During thermal decomposition of dimethyl disulfide at 400-500 "C and a residence time of 1.7 s, strong bands due to hydrogen sulfide and carbon disulfide were present in the PE spectrum of the pyrolysis products (Kroto and Suffolk (1972)). Bock et al. (1982) reported that thermal decomposition of dimethyl disulfide starts at 575-625 "C and yields hydrogen sulfide, methanethiol, thioformaldehyde, carbon disulfide, and methane. Hence, with dimethyl disulfide too, the formation of hydrogen sulfide is to be expected, even before entering the cracking coil, since the temperature in the preheat section reaches 600 "C. Although carbon deposition results from a series of complex interactions, a tentative explanation of the observed influence of sulfur addition is proposed in the following. The initial coking rate is associated with the catalytic mechanism consisting of (1)the formation of surface carbon atoms by chemisorption of hydrocarbon molecules from the gas phase on metal particles, (2) diffusion of carbon through the metal particle and buildup of carbon until the tensile strength of the metal is exceeded, and (3) detachment of the metal particle from the surface, with concurrent precipitation of carbon. Metal sites are covered by sulfur (cf. reduced CO production). At low surface coverage a monolayer of sulfur is formed. The surface metal-sulfur bonds are substantially more stable than the bulk metal-sulfide bonds (Bartholomew and Agrawal(1982)). After satu-
ration, deeper layers of metal are progressively attacked, with formation of bulk sulfides (Ng and Martin (1978)). Due to the induction of a positive charge (XN~ = 1.91, xs = 2.58), the metal sites are less active for back-donation and thus for scission of the C-C bond. Dissolutioddiffusion of carbon through the metal are crucial steps in the catalytic carbon formation. The influence of S on these steps is difficult t o evaluate, but the solubility and diffusion of C through the alloy surface will probably be affected by the presence of S. Bramley (1935) reported a marked influence on the diffusion of carbon through iron (1.8 x cm2 s-l a t 1000 "C) in the presence of sulfur (0.75 x cm2 s-l at 1000 "C). Investigations of sulfur uptake (Bramley (1935))by iron and steel in an hydrogen sulfide containing atmosphere at 1000-1100 "C showed that the diffision of sulfur follows Ficks law and that the diffusion rate is about 3% that of carbon a t the same temperature. With increasing carbon content the rate of diffusion of S decreases, while with increasing S content the rate of C diffusion decreases. Yet, the initial coking rate increases in the presence of sulfur. It is well-known that hydrogen sulfide attack on Ni-Cr-Fe alloys leads to grain boundary embrittlement. As a consequence, the tensile strength of the metal decreases and the detachment of the metal particles from the surface is facilitated. On prereduced Inconel 600 (72.5% Ni) the coverage effect of sulfur seems t o be completely overruled by corrosive attack, which is known to be virulent under reducing conditions, as encountered in steam cracking. For given operating conditions the corrosive attack increases with the nickel content of the alloy, which corresponds to a decreasing level of chromium, and this is regarded as being a significant factor (Fleetwood and Whittle (1970)). This results in a fast precipitation of carbon, which in turn gives rise t o an increased number of structural deficiencies, i.e., active centers, in the carbon lattice, leading to a large coke surface with a large number of potential radical centers for growth of the coke layer during the asymptotic stage. Moreover, sulfur compounds can alter the rate of the reactions responsible for the growth of the coke layer during the asymptotic range. Radical sites in the coke matrix, originating from hydrogen abstraction by small
784 Ind. Eng. Chem. Res., Vol. 34, No. 3, 1995 Table 7
Table 5 log A E ksoo~c (m3/kmols) (kJ/mol) (m3/kmols) ref
+
9.98
30.13
3.24 x lo8 c
CsH5-CH2CHi a Ivie and Forney (1988). Zhang et al. (1989). Chen and Froment (1991).
--
C6H5' + HS' C&SH C6HsSH H' C6H5S' + Hz C6H53 + CHFCHZ CsH5- SCHzCHz' a
log A E ksoo'c (m3/kmols ) (kJ/mol) (m3/mols) ref
10.1 9.6
13.3 0.2
2.93 x 3.89 x
lo9 lo9
a b
Westley (1982). Perrin et al. (1988).
Scheme 3
Table 6
- -- -
H2SIH'-HHS'+Hz H2S CH3' HS' + CHI CsH6 + HS' C6Hj* HzS C6Hj' + CHz=CHz C~HS-CHZCHZ.
log A E ksoo'c (m3/kmols ) (kJ/mol) (m3/kmols) ref 9.89 7.15 3.5 x 109 u 8.6 16.6 62.2 x lo6 b 9.98
30.13
3.24 x
lo8
c
a Westley (1982). Shum and Benson (1985). Chen and Froment (1991).
reactive radicals from the gas phase (H' and CH3*),react with unsaturated compounds. With benzene taken as a reference element for the coke matrix and ethylene as the coke precursor this can be written as in Table 5. Sulfur compounds interfere with this mechanism in two ways. Kinetic data indicate that hydrogen and methyl radicals can also react with hydrogen sulfide, originating from the decomposition of the sulfur compound. The SH radicals thus formed can in turn interact with the coke layer and create reactive centers in the coke matrix. No kinetic data are found for this reaction. However, hydrogen abstraction by SH radicals is expected t o have a higher activation energy (bond energy S-H = 90 kcaYmol compared to Ar-H = 104 kcdmol) than the abstraction by hydrogen (H-H = 104 kcaYmo1) and methyl radicals (H-CH3 = 101 kcaYmo1). Consequently, if the observed increase in the asymptotic coking rate results from hydrogen abstraction by SH radicals, its concentration should be at least of the same order of magnitude as that of the hydrogen and methyl radicals, since its reactivity is lower. In industrial thermal crackers, the mole fraction of hydrogen radicals amounts to 2.5 x loT7,while for methyl radicals a mole fraction of 33 x 10-6 is found. With this hydrogen radical concentration and based on thermodynamic data (Perrin et al. (1988)), the equilibrium mole fraction of SH radicals amounts t o 2.6 x for a n-hexane cracking experiment at 785 "C with 10 ppm hydrogen sulfide in the feed. Since the SH radical concentration is very low, compared to the concentration of hydrogen and methyl radicals, the contribution of this pathway t o the increase in the coking rate is probably small (Table 6). A more likely explanation of the increase in the asymptotic coking rate is the participation of SH radicals in termination reactions with the coke macroradical. Termination reactions are processes with a high kinetic efficiency, with a recombination rate close to the collision frequency. Radical positions in the coke matrix are thus modified into a thiol containing coke from which H abstraction by hydrogen and methyl radicals proceeds readily. This leads to a sulfur centered radical which reacts rapidly with unsaturated compounds from the gas phase (Table 7). Moreover, cleavage of one C-S bond then leads to the formation of two additional radical centers in the coke matrix, thus enhancing its reactivity toward gas-phase components (Scheme 3). Figure 12 reveals that the CO production decreases in the order thiophene > benzothiophene > carbon
disulfide > dimethyl disulfide. The presence of unshared electron pairs in sulfur compounds can lead to very strong chemisorption on the metal surface. Under reducing conditions, as encountered in steam cracking, the adsorption is typically dissociative, leaving a reduced sulfur atom strongly bonded to the surface (Batholomew and Agrawal (1982)). In the case of thiophene the sulfur atom is part of the aromatic ring and dissociative chemisorption requires breaking of the aromatic structure. Since in benzothiophene one aromatic ring can be kept intact, chemisorption is somewhat easier. In dimethyl disulfide the a effect enhances the reactivity of the sulfur as compared with carbon disulfide. As a consequence, for the same concentration of sulfur in the feed, the amount of sulfur actually adsorbed on the surface is highest for dimethyldisulfide. As mentioned earlier, the influence of sulfur on the CO production consists in the coverage of the metal sites by competitive chemisorption (Rostrup-Nielsen (1984)). The observed order is in agreement with this mechanism. The observation of the detrimental effect of sulfur compounds on the coke formation may seem to be in contradiction with current industrial practice. What is well established in industrial practice is the reduction of the CO yield by sulfur compounds. Data on coke formation are scarce or nonexistent, however, and often biased by the belief that the CO yield is proportional t o the coking rate. It has been reported that upon interruption of sulfur addition the pressure drop rapidly increases. What is really measured is the pressure drop over the coil and the TLE. It may well be that this is caused by increased coke formation in the TLE only. The temperature in the larger part of the TLE is considerably lower than in the cracking coil, and the catalytic phase is much more important there than in the coil itself. When the sulfur addition is discontinued, there is no passivation of the Ni and the Fe any more and the rapid catalytic coking is favored. Industrial plants are generally insufficiently instrumented to permit a clear distinction between the causes of the increased pressure drop. Direct observations are possible only after some maintenance or catastrophic shutdown. It should be stressed that the observations reported here relate to coil coking conditions only. Conclusions From the present study it can be concluded that the coke formation on Incoloy 800H and Inconel 600 in the cracking of n-hexane with steam at 770-790 "C depends on the composition of the surface, which is strongly influenced by the pretreatment. A correlation between
Ind. Eng. Chem. Res., Vol. 34, No. 3, 1995 785 the asymptotic coking rate and the bulk composition of the alloy is not apparent from the present work, however. Preoxidation increases the selectivity toward CO formation. The effect is most strongly pronounced for Inconel 600. For prereduced Inconel 600 the CO yield is lowered by injection of sulfur-containing molecules. This is explained by the well-known passivating effect of sulfur on metals such as Ni and Fe. On the other hand, S compounds increase the coke formation. This detrimental effect of the sulfur compounds can be related to their thermal decomposition, leading to hydrogen sulfide. The influence of sulfur on the catalytic coking mechanism is related to grain boundary embrittlement, caused by corrosive attack of hydrogen sulfide on Ni-Cr-Fe alloys. SH radicals, originating from interaction between gas-phase radicals and hydrogen sulfide, influence the radical reactions of the asymptotic coking mechanism through their interference in proton abstractions and in termination reactions, creating additional radical centers in the coke matrix, thus enhancing its reactivity toward gas-phase components. From the present study it can be concluded that the CO yield is not a reliable indicator for the coke production. The observations reported here, obtained in a microreactor and in a pilot reactor have important practical implications. The practice of adding dimethyl disulfide to gaseous hydrocarbon feeds may be questioned. Research on additives which reduce both the CO and the coke yields should be stimulated.
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@
Abstract published i n Advance ACS Abstracts, February
1, 1995.
