ARTICLE pubs.acs.org/EF
Experimental Investigation of Initial Coke Formation over Stainless Steel, Chromium, and Iron in Thermal Cracking of Ethane with Hydrogen Sulfide as an Additive Seyed Mahdi Jazayeri and Ramin Karimzadeh* Chemical Engineering Faculty, Tarbiat Modares University, Post Office Box 14155-4838, Tehran, Iran ABSTRACT: The effect of injection of hydrogen sulfide, an available inexpensive byproduct in Iran’s petrochemical industry, on the rate of coke formation over chromium, iron, and stainless-steel (SS) coupons in thermal cracking of ethane was investigated. In a laboratory reactor, coke formation on the metal coupons was measured in the temperature range of 10981148 K after a fixed time of 900 s. Nitrogen was used as an inert diluent to exclude the concomitant coke oxidation in the commonplace steam-cracking process. The role of hydrogen sulfide was examined through an additive feeding policy either as a pure presulfidation of the coupons with a 200 ppmw concentration for 20 min or via continuous introduction of 25 and 50 ppmw amounts of hydrogen sulfide into the gas feed stream. The presulfidation reduced the rates of coke formation up to 20, 45, and 30% over Cr, Fe, and SS samples, respectively. However, the continuous injection of hydrogen sulfide showed a complex behavior depending upon the temperature, so that the rate of coke formation increased by 20120% at higher temperatures but decreased by 2530% at lower temperatures. The complex and contradictory behaviors of the H2S effect on the coke formation for the presulfidation and the continuous sulfidation scenarios were well-described through a simplified mechanism. An empirical model was developed to predict the rate of coke formation over the selected metal samples at given operating conditions and known H2S and ethylene concentrations. The model predictions were in good agreement with the experimental results.
1. INTRODUCTION Thermal cracking of hydrocarbons is the dominant process for the production of light olefins, such as ethylene and propylene, which are the base feedstocks for many chemical industries. In an industrial thermal cracking unit, the hydrocarbon feedstock and the process steam enter the convection zone of a furnace, where they are preheated. The mixture of hydrocarbon and diluent then passes through a reactor coil placed in the radiation zone of the furnace, and the temperature of the mixture is increased so that the endothermic cracking reactions take place. At the furnace exit, the effluent is quenched rapidly through a transfer-line heat exchanger (TLE) to avoid subsequent reactions. The effluent is then transferred to fractionation columns where the main products are separated from other components. The coke layer deposited on the inner surface of the very hot cracking coils reduces the heat transfer across the reactor wall and increases the pressure drop through the coil, which altogether bring about a reduction in the energy efficiency of the plant together with a diminution of the day-to-day and the overall (yearly) production because of the reduced product yields and the inevitable shutdowns of the furnace for decoking. To reduce coke formation during thermal cracking, sulfurbased additives, such as dimethyl disulfide (DMDS), are being widely exploited in thermal crackers via several policies, such as presulfidation, continuous addition, or a combination of both.1,2 Several research efforts have been devoted to analyzing the mechanism of coke formation in thermal cracking processes. In some of them, the kinetics of the coking has been studied,35 while in the others, the influencing parameters, such as operation time, reactor alloy, and coil geometry, were subjected to study.6,7 It has been shown already that the FeNiCr alloys used in r 2011 American Chemical Society
industrial coils of steam cracking have significant catalytic effects on coke deposition over the coil surface.2,8 The amount of coke deposited on the surface depends upon the type of feedstock as well as the operating conditions.9 Nevertheless, the scattered data reported in the literature do not exactly support one another, and the sources of the observed effects are not fundamentally clear. For instance, the continuous addition of C2H5S and (C2H5)2SO during the cracking of nnonane within a quartz tubular reactor and the introduction of dibenzyl sulfide and dibenzyl disulfide in cracking of heptane in a stainless-steel (SS) tubular reactor all caused a reduction in the rate of coke formation up to 70%,10 while a wide range of sulfurbased additives, such as DMDS, CS2, thiophene, and benzothiophene, caused an increase in the coking rate when continuously inserted into n-hexane stream during the cracking process in a mixed reactor of Incoloy 800H and Inconel 600.2 Whereas the coking rate increased by more than 85% for the continuous addition of DMDS during naphtha cracking in an Incoloy 800HT tubular reactor, a presulfidation of the inner surface of the reactor caused the rate of coke formation to reduce up to 35%.8 Similarly, the rate of coke formation in thermal cracking of naphtha with an Incoloy 800HT mixed-flow reactor decreased by more than 20% using a combination of S- and Si-type additives.11 A proper coke inhibitor should have a few capabilities, such as imposing larger reductions in coke formation and being easily removable from the product stream. Moreover, the additive must reduce the coke formation without significantly affecting the Received: April 5, 2011 Revised: July 20, 2011 Published: July 20, 2011 4235
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Figure 1. Experimental setup for studying of the influence of coke inhibitor additives in thermal cracking of hydrocarbons.
Table 1. Experimental Conditions of Ethane Pyrolysis in Presulfidation and Continuous Addition of Hydrogen Sulfide presulfidation
continuous addition 2.4 1023.1 102
3
Qethane (m /s) 3
Qnitrogen (m /s)
6 10
1.2 1022.4 102
diluent ratio (kg/kg) Qadditive + nirogen (m3/s)
3 103
0.50.9 04.2 103
3
Figure 2. Mixed-flow reactor used in this study.
conversion or selectivity for the main desirable products, although they might have some disadvantages, such as showing undesired effects, including corrosion, at the downstream units, particularly, in the separation section. Considering all of the positive and negative aspects of their use in the thermal cracking process, sulfur-based additives, such as DMDS, dimethyl sulfide, and thiophenes, have found their firm position as coke inhibitors in olefin plants. It is believed that the influence of sulfur-type additives on the coking rate depends upon their thermal stability under thermal cracking operation conditions.2 Most of the sulfur-based additives are decomposed even before entering the pyrolysis reactor, producing mainly hydrogen sulfide. Thermal decomposition of DMDS starts at 848898 K and results in hydrogen sulfide, methane thiol, thioformaldehyde, carbon disulfide, and methane.12 In the presence of nickel sulfide at temperatures of 423473 K, carbon disulfide is decomposed by hydrogen, thus producing methane thiol, dimethyl sulfide, methane, and hydrogen sulfide.13 Benzothiophene remains stable up to 948 K.14 Moreover, thiophenes are decomposed at ca. 10731123 K to hydrogen sulfide by only 4% conversion.14 In cracking reactions, however, hydrogen radicals emerged from hydrocarbons can attack thiophenes. This leads to ring opening and removing the sulfur atom from thiophenic compounds. As a consequence, a greater amount of hydrogen sulfide is produced.14 In a few previous works, hydrogen sulfide has been used as an additive in thermal cracking of hydrocarbons. The effect of hydrogen sulfide
hydrogen sulfide (ppmw)
200
050
temperature (K)
1073
10981148
run time (s)
1200
900
Table 2. Factors and Their Levels in the Three-Level Full Factorial Design
levels
levels
0
+1
1
x1 1098 1123 1148 1098
H2S concentration (ppmw) x2 DR (kg/kg)
continuous addition
1
factors temperature (K)
presulfidation
x3 0.5
0
25
50
0.7
0.9
0.5
0
+1
1123
1148
0.7
0.9
on the product distribution in thermal cracking of ethane was investigated by Scacchi et al.,15 who observed that the rate of coke formation and the yields of primary products, such as methane and ethylene, were reduced by adding hydrogen sulfide. Also, Rebick16 reported that the relative yields of methane, ethane, and ethylene decreased and the overall cracking rate increased with hydrogen sulfide in the pyrolysis of n-hexadecane within a quartz flow reactor. More recently, the growth of catalytic coke on FeNi particles in cracking of ethane in the presence of steam at temperatures of 10381198 K was found to be accelerated with increasing the concentration of hydrogen sulfide in the cracked gas.17 Similarly, the use of hydrogen sulfide in pyrolysis of propane had a significant effect on the rate of carbon formation.18 4236
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Table 3. Design Matrix for the Presulfidation Experiments rate of coke formation (105, kg/m2/s)
factor levels run
temperature (K)
DR (kg/kg)
Cr
Fe
SS
ethylene yield (wt %)
1
1098
0.5
4.72
11.14
8.36
47.71
2
1098
0.7
4.72
8.86
6.72
42.04
3
1098
0.9
3.56
8.28
5.94
40.01
4
1123
0.5
6.56
13.89
8.69
44.34
5
1123
0.7
6.31
12.03
7.17
37.63
6
1123
0.9
5.92
11.19
7.68
42.49
7
1148
0.5
8.69
15.53
10.81
44.72
8 9
1148 1148
0.7 0.9
8.25 7.56
15.39 14.28
10.58 9.94
42.79 42.91
Table 4. Design Matrix for the Continuous Addition Experiments rate of coke formation (105, kg/m2/s)
factor levels run
temperature (K)
DR (kg/kg)
H2S (ppmw)
Cr
Fe
SS
ethylene yield (wt %)
1 2
1098 1098
0.5 0.7
0 0
13.44 17.89
23.75 27.81
11.31 27.89
43.74 34.62
3
1098
0.9
0
28.33
27.08
52.89
39.35
4
1098
0.5
25
42.36
38.94
26.83
42.39
5
1098
0.7
25
36.14
30.56
26.50
45.24
6
1098
0.9
25
50.11
41.67
58.36
40.62
7
1098
0.5
50
17.33
18.11
19.56
40.9
8
1098
0.7
50
18.08
21.75
25.25
43.69
9 10
1098 1123
0.9 0.5
50 0
11.94 20.25
5.75 20.58
27.89 22.64
45.77 41.77
11
1123
0.7
0
50.19
33.44
58.39
43.29
12
1123
0.9
0
52.86
50.11
58.39
43.94
13
1123
0.5
25
5.89
8.08
10.28
40.61
14
1123
0.7
25
20.00
19.14
27.67
42.58
15
1123
0.9
25
23.67
26.50
29.25
46.57
16
1123
0.5
50
13.08
13.78
16.69
45.12
17 18
1123 1123
0.7 0.9
50 50
15.97 10.81
14.94 11.25
17.36 11.97
46.23 49.47
19
1148
0.5
0
3.94
14.72
4.81
40.08
20
1148
0.7
0
9.00
6.94
23.78
45.35
21
1148
0.9
0
5.64
8.50
7.42
47.53
22
1148
0.5
25
5.75
13.11
8.64
41.3
23
1148
0.7
25
8.50
13.25
7.47
44.72
24
1148
0.9
25
4.50
5.61
7.78
45.96
25 26
1148 1148
0.5 0.7
50 50
10.64 13.14
9.83 10.78
11.22 13.81
48.68 45.47
27
1148
0.9
50
32.4
56.7
48.3
44.86
No changes were detected in the products of pyrolysis reactions, but carbon formation was either inhibited or accelerated. As summarized by Tan and Baker,18 the addition of hydrogen sulfide with the feed reduces the initial rate of coke formation on SS and iron, increases the rates on copper, and has a very large accelerating effect on nickel, with the effects being attributable to the stability of the metal sulfide species. In summary, the previous studies indicate that coke formation is affected by the presence of additives via several parameters, such as the feed type, reactor throughput, addition technique,
and reactor alloy. The effects of operating conditions, such as the feed flow rate and additive concentration, have been welldescribed at different scales in the previous reports.1,6,8 As a conclusion, one can state that some research fell into the range where the sulfur additive alleviates the coke formation and the other research fell into an area where the additive intensifies the coke phenomenon. This implies that the concentration of the sulfurcontaining additive in the feed stream has to be carefully determined. The purpose of this work is to investigate the effect of injection of hydrogen sulfide as the surface pretreatment agent or the 4237
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Figure 4. Main effect plots of the temperature and DR for the rate of coke formation in presulfidation of (a) Cr, (b) Fe, and (c) SS.
Figure 3. Comparison of the rates of coke formation in presulfidation and pure cracking with DRs of (a) 0.5, (b) 0.7, and (c) 0.9.
additive on the rate of coke formation during ethane cracking over three samples of chromium, iron, and SS. Three process variables were taken into account: the reaction temperature, the concentration of the additive, and the nitrogen/ethane ratio. A kinetic model is also proposed in the same line on the basis of the results obtained from the experimental program implemented.
2. EXPERIMENTAL SECTION The experiments were performed in a laboratory unit consisting of three main sections: the feeding, the reactor, and the analysis sections, as outlined in Figure 1.
Ethane with 99.99% purity was used as the hydrocarbon feed in thermal cracking experiments, with nitrogen with 99.9% purity used as a diluent. A safe concentration of hydrogen sulfide in nitrogen (0.05 mol % hydrogen sulfide) was employed as an additive. First, the feedstock and the additive were mixed and run to the preheater part of the reactor. After preheating to 873 K, the reactions took place in a continuous stirred reactor, which was heated and encompassed by the walls of an electrical furnace. The reactor was made of quartz and had a volume of 20 106 m3. The designed dimensions enabled a well-mixed flow (Re > 1200) in the reaction area with reasonable feed flow rates (Figure 2.). To study the rate of coke formation, the amount of coke deposited on small coupons of iron, chromium, and SS with ordinary material specifications was measured by weighing the samples before and after the experiment. The metal samples were prepared with dimensions of 15 103 5 103 0.5 103 (m scales) and placed inside the reactor. The surface roughness of the material has a strong influence on the rate of coke formation. Then, as a pretreatment, the coupons 4238
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Figure 6. Contour plots of the rate of coke formation (kg m2 s1) in the presulfidation approach as a function of the temperature and DR for (a) Cr, (b) Fe, and (c) SS. Figure 5. Interaction effects between the temperature and DR in the presulfidation method for (a) Cr, (b) Fe, and (c) SS. were polished in a similar way before each experiment to provide the same roughness in all cases. The rate of coke formation was determined as the weight of the deposited carbon per unit sample surface area per time. The coking rate was calculated by measuring the extent of coke deposited after the first 900 s time span of each experiment. The reactor effluent was cooled immediately using the cooling water in a double-pipe heat exchanger. The heavier (liquid) products were condensed and removed from the cracked gas via passing the products through two condensers in series. Eventually, part of the cracked gas was sent to an Agilent 7890 refinery-gas analyzer (RGA) instrument for analysis, and the remaining gas was run to the flare and burned. A three-level full factorial design was used in both of the feeding policies to investigate the quadratic and higher order interactions of the
parameters. For the presulfidation experiments, two variables were chosen as the main factors, and for the continuous addition, three variables were chosen as the main factors. Thereby, the effects of the reaction temperature, nitrogen/ethane ratio, and concentration of hydrogen sulfide (in ppmw) as the additive on the rate of coke formation were investigated. More details of the experimental design and the analysis of the experimental results will be given in the Results and Discussion. The alteration range of the values in the experimental design implemented is shown in Table 1 for the two feeding policies of a pure pretreatment approach and a complete continuous addition method. This type of analysis enables ideally the means for (1) distinguishing the effect of any combination between the two pure additive feeding polices on the process performance and (2) determining the optimum concentration of the additive in the continuous addition policy to obtain a minimum coke formation in a specific range of operating variables. 4239
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Figure 7. Contour plots of the reduction percentage in the rate of coke formation in presulfidation as a function of the temperature and DR for (a) Cr, (b) Fe, and (c) SS.
3. RESULTS AND DISCUSSION Performing a full factorial experimental design enables detection of the influences of each process variable as well as their interactions on the rate of coke formation and the yields of the main products. Then, a three-level full factorial design has been applied to both of the feeding policies. In total, three variables have been used as the experimental design factors in each category: temperature, diluent ratio (DR), and additive concentration. The upper, lower, and medium (central) levels of these variables are given in Table 2. Moreover, the design matrix and the experimental observations for the rate of coke formation are shown in Tables 3 and 4 for the presulfidation and the continuous injection policies, respectively. 3.1. Effect of Presulfidation. To evaluate the influence of presulfidation on coke deposition, a series of tests were undertaken after presulfidation of the samples with 200 ppmw of
Figure 8. Main effect plots of the temperature, DR, and H2S concentration on the rate of coke formation in the continuous addition approach on (a) Cr, (b) Fe, and (c) SS.
hydrogen sulfide. The operating conditions of these experiments are shown in Table 1. It becomes evident by comparing the rates of coke deposition in Figure 3 that the observed rates of coke formation for all of the samples reduced when the surface was presulfided by means of hydrogen sulfide. The presulfidation of the surface decreased the amount of coke deposited within the reactor by 30, 55, and 40% for Cr, Fe, and SS, respectively. The main individual effects of the temperature and DR for the rates of coke formation in the presulfidation approach are summarized in Figure 4. The effects are more customarily shown in Figure 5 with confounded interactions. As evident, the rate of 4240
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Figure 9. Interaction effects between the H2S concentration and DR in the continuous addition approach at 1148 K for (a) Cr, (b) Fe, and (c) SS.
coke formation increases when the reaction temperature is increased and decreases when the DR is increased. The contour plots shown in panels a and b of Figure 6 reveal no significant interaction between the two parameters of the temperature and DR for Cr and Fe, as evidenced by almost parallel curves at constant rates. Nearly no sharp changes were observed in the rate of coke formation by changing the DR value. In other words, the effect of dilution is minor as compared to that of the temperature. On Figure 6c, however, one can note a weak interaction from the distortion of the response curves at low-to-moderate temperatures in the case of SS coking rates. This difference could be simply related to the fact that SS is an alloy with conjugated roles of its constituents (typically 2% Mo, 17% Cr, 12% Ni, and 65% Fe). It is, thus, anticipated that the behavior of SS is more complicated than that of pure metals, such as Cr and Fe. According to Figure 6, the minimum rate of coke formation is found at the lowest temperature and the highest DR for all
ARTICLE
Figure 10. Contour plots of the reduction percentage of the rate of coke formation in the continuous addition method as a function of the temperature and DR at 25 ppmw concentration of hydrogen sulfide over (a) Cr, (b) Fe, and (c) SS.
presulfided samples. Moreover, it appears that the rate of coke formation is highly temperature-dependent for Cr, while a weaker dependence upon temperature was obvious in the case of Fe and SS when compared to DR. The contour plots of the reduction percentage of the coking rate against two operating variables of temperature and DR for Cr, Fe, and SS are shown in Figure 7. These curves facilitate a direct comparison of the dependence of the reduction percentages upon the key process variables. As shown in this figure, the greatest reduction in the rate of coke formation over Cr is achieved by the simultaneous choice of high temperatures and low DR values (see Figure 7a). For presulfidation of Fe and SS, however, the opposite is true; i.e., low temperatures accompanied by large DRs are favored. Also worth mentioning is the curved behavior of the contour plots in both directions, indicating the presence of two-way interactions or squared effects of the two factors. 4241
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Figure 12. Schematic representation of the simplified mechanism for the effect of hydrogen sulfide on the coke formation during ethane cracking with (a) no sulfur compound added, (b) presulfidation of the surface, and (c) continuous injection of hydrogen sulfide.
Figure 11. Contour plots of the reduction percent in the rate of coke formation in the continuous addition method as a function of the temperature and DR at 50 ppmw concentration of hydrogen sulfide over (a) Cr, (b) Fe, and (c) SS.
When the contour plots of Figures 6a and 7a for Cr are compared, one can say that increasing the temperature brings about increased abatements in the coking rate, especially, for moderate-to-low temperatures, indicating that the presulfidation of Cr decreases the sensitivity of the coking to the process temperature. The observations from the other samples did not show such a distinct behavior. This discrepancy might be related to metal carbide formation and decomposition. It has been suggested19 that for iron and iron-based alloys, the metal carbide is decomposed into metal particles and carbon, with the mechanism involving unstable carbide as an intermediate phase. For Cr, however, metal dusting will start after chromium is tied up to oxides. The lowest reductions in coke formation were found in the case of Fe as the catalyst.
The alleviation of coke formation in presulfided samples can probably be associated with the adsorption of sulfur species on the metal surface, which inhibits the adsorption of the hydrocarbon species on the surface and suppresses the diffusion of metals up to the coke layer. The catalytic performance of the host metal center is affected by sulfur adsorption through two poisoning effects: (1) the blockage of the catalytic sites (geometric effect) and (2) the electronic field of adjacent sulfur species (electronic effect). The former plays an important role over the entire range of sulfur coverage and predominates in high-coverage regions,20 while the electronic effect is believed to be operative only at low sulfur coverages.21 The possibility for reconstruction of the metal surface is another compensating aspect encountered in sulfur addition reactions.17 The phenomenon of surface reconstruction or faceting is important in catalysis, especially, with regard to structure-sensitive reactions. 3.2. Effect of Continuous Injection of Additive. To evaluate the influence of the concentration of hydrogen sulfide in the hydrocarbon stream on the rate of coke formation, a series of 27 tests were planned and implemented. Table 1 presents the ranges of the operating conditions. The main effects on the rate of coke formation are shown in Figure 8. The interaction plots at different ratios of dilution are displayed in Figure 9 for 4242
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Table 5. Kinetic Parameters for the Rate of Coke Formation with No Treatment of the Surface or Additive Injected with the Hydrocarbon (Pure Thermal Cracking) Cr
Fe
SS
n
0.271
0.252
0.348
k0 (kg m2 s1 Pan)
0.838
0.963
0.264
Ec/R (K1)
7679.03
8838.54
10995.78
R2
0.87
0.85
0.83
Table 6. Kinetic Parameters for the Rate of Coke Formation with Presulfidation Cr
Fe
SS
n
0.625
0.625
0.625
k0 (kg m2 s1 Pan)
0.924
0.43
0.113
Ec/R (K1)
13294.36
11446.14
11620.07
R2
0.95
0.94
0.86
1148 K. It is clear from the main effects illustrated in Figure 8 that increasing the process temperature and the dose of hydrogen sulfide or decreasing the DR should increase the rate of coke formation. However, the intersecting interaction plots in Figure 9 indicate the presence of strong interaction effects between the selected operating variables. This means that the effect of a continual injection of hydrogen sulfide depends upon the level of the other variables. For example, the plots obtained in the case of Fe catalysis revealed that the priority of the constant-temperature curves at DR values of 0.5 and 0.7 is reversed when the concentration of hydrogen sulfide is about 30 ppmw. The HS radicals generated from the adsorption of hydrogen sulfide on the metal surface are the key players that increase the rate of coke formation. In fact, the bonds between the metal layers are weakened by the strong metalsulfur interaction, and this alters the arrangement of the atoms in the surface. Besides, the HS radicals can enter the coke layer reacting with the radical sites on the coke surface, whereby the nature of the radical sites on the coke surface will be changed. As a consequence, hydrogen abstraction from the coke layer is increased. This increase in hydrogen abstraction creates more abundant active sites on the surface, which are available to gas-phase radicals. The addition reactions are hence intensified on the surface, leading to higher rates of propagation of long-chain intermediates of coke formation. This increased activity can explain why the rate of coke formation is increased by the production of HS radicals. In addition, more radical sites are also generated as a result of the presence of sulfur in the coke matrix. The higher rates of coke formation with higher amounts of hydrogen sulfide observed in the continuous addition policy suggest that the influence of the continual injection of hydrogen sulfide on the coke formation originates mainly from the interference of the gas-phase HS radicals with noncatalytic radical reactions responsible for the growth of the coke filament. Figures 10 and 11 show the contour plots of the percentage of the change in the coking rate in terms of the temperature and DR for Cr, Fe, and SS in the approach of the continuous addition of hydrogen sulfide at the concentrations of 25 and 50 ppmw, respectively. As indicated in Figure 10, the rate of coke formation is increased in the presence of hydrogen sulfide at
Figure 13. Comparison between the kinetic model predictions and the experimental data for DR = 0.7.
most of the operating conditions. A slight reduction in the rate of coke formation is achievable at low temperatures, however. Furthermore, the DR has almost no significant effect in this concentration of H2S in comparison to the temperature on the coking rate. When the contour plots in Figures 10 and 11 are compared, it can be understood that a lower dosage of hydrogen sulfide as the additive is generally preferred from the viewpoint of coke inhibition. Indeed, when a higher degree of sulfidation is employed, a more profound increase in the rate of coke formation is encountered (see Figure 11). A marked feature in the coking behavior depicted in Figure 11 as opposed to that in Figure 10 is a nearly symmetric contour plot around the central amount of temperature. This means that we can chose either low or high temperatures along with a high concentration of H2S to reduce the amount of coke deposition. A large extent of dilution is also helpful in providing a safer margin for temperature. However, the reduction is still overshadowed by the negative role of hydrogen sulfide when compared to the presulfidation strategy. 3.3. Kinetic Model. The kinetic mechanism of coke formation in this research is formulated as an empirical model. This type of model is usually based on the experimental observations, 4243
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Table 7. Temperature Dependency Parameters of the Rate Coefficients Cr DR
Fe
SS
0.5
0.7
0.9
0.5
0.7
0.9
0.5
0.7
0.9
R1
0.08
0.073
0.066
0.025
0.037
0.045
0.046
0.026
0.034
R2
7.19 105
6.54 105
5.86 105
2.25 105
3.28 105
3.98 105
4.11 105
2.3 105
2.94 105 40.82
R3
102.53
66.11
22.18
67.52
52.71
50.25
63.55
56.41
R4
0.179
0.114
0.03642
0.119
0.092
0.087
0.111
0.099
0.071
R5
7.79 105
4.93 105
1.48 105
5.22 105
4.01 105
3.79 105
4.81 105
4.36 105
3.11 105
R2
0.99
0.99
0.95
0.98
0.99
0.98
0.99
0.99
0.98
Table 8. DR Dependency Parameters of the Rate Coefficients Cr
R1
R2
R3
R4
sum of degree of mean
SS source
squares freedom square
β1 γ1
0.0988 0.0369
1.471 103 0.0486
0.0577 0.0321
R2
0.99
0.99
0.4
β2
8.88 105
1.425 106
5.164 105
γ2
3.35 105
4.325 105
2.925 105
R2
0.99
0.98
0.41
β3
204.22
87.036
93.361
γ3
200.87
43.156
56.808
R2 β4
0.99 0.3593
0.85 0.1544
0.96 0.1626
T
618.1
2
309.0
0.099
DR 102.8 T DR 18.3
2 4
51.4 4.6
total
8
γ4 2
R5
Fe
Table 9. ANOVA for the Presulfidation Tests
0.356
0.079
DR T DR R2
0.86
0.95
1.58 104
3.58 105
7.07 105
γ5
1.58 10
5
R2
0.99
R2
5
4.25 10
285.6
2
142.8
20.4
2
10.2
2.6590 308.679
4
206.988
0.000
92.53
0.6647
0.000
6.61
0.002
0.86
0.000
83.61
0.000 0.002
13.91 2.48
8
0.98
739.200
103.356
0.99
0.93
SS T
where the overall rate of reaction is expressed in terms of one or two reaction products (as coke precursors) and the coefficients of the model are determined by using the statistical analysis of the experimental data.3,2224 A reaction mechanism is postulated for this empirical model as depicted in Figure 12. Here, the following general reaction is surmised to govern the rates of coke deposition on the surface of the samples (Figure 12a): C2 H4 f coke
contribution (%)
Fe
0.99
0.86
T
total
β5
6.84 10
R
Cr
R
4
percent F value
ð1Þ
The pure rate of coke formation on the basis of the above reaction may be written as Ec n ð2Þ rc, no H2 S ¼ k0 exp p RT C2 H4 For reasonable representation of the experimental data, the sensitivity of the model to partial pressure of ethylene was first examined and, next, the adjusted parameters were estimated from our experimental results. The kinetic data for the rate of coke formation in the absence of any sulfur-based additive are shown in Table 5. Table 6 shows similarly the kinetic parameters of coke formation for the presulfidation policy. As can be seen from this table, the exponent of the partial pressure, n, was calculated as 0.625 surprisingly for all of the samples investigated. If hydrogen sulfide is to be used for presulfidation of a metal surface before entering the hydrocarbon feedstock, the metal
250.9
2
125.5
0
80.39
46.2
2
23.1
0.001
14.80
T DR 15.0
4
3.8
0.047
4.81
total
8
DR
R2
312.160
20.067
0.98
surface may react with hydrogen sulfide, thus producing metal sulfide species. In such a manner, during the process of thermal cracking, not only does the catalytic effect of the metal effectively decrease, but there is also almost no HS radical to accelerate coke formation (Figure 12b). The rate of coke formation in the presulfidation approach is also calculated by means of eq 2, for which the adjusted parameters are shown in Table 6. The effect of a continuous introduction of hydrogen sulfide into the hydrocarbon stream on the rate of coke formation is rather complex, so that no proper mechanism was found to build and support a detailed kinetic model. Then, it was decided to present an empirical but well-established model equation for the prediction of the coking rate in the presence of hydrogen sulfide as the additive. As demonstrated by Kolts,25 at high temperatures applied in thermal cracking conditions, hydrogen sulfide is decomposed to HS radicals in the absence or presence of a metal as the catalyst but the rate of heterogeneous decomposition of hydrogen sulfide is larger than the corresponding homogeneous reactions in the gas phase. With the results of the growing population of active sulfur species in the gas phase, more hydrocarbon radicals will be generated. The coking rate is then expected to rise at 4244
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Table 10. ANOVA for the Continuous Addition Tests percent source
sum of squares
degree of freedom
mean square
R
contribution (%)
Cr T
23166.7
2
11583.3 0.0097
31.85
C
6049.9
2
3025.0 0.017
8.32
DR
3522.1
2
1761.0 0.05
4.84
29109.2
4
7277.3 0.046
40.02
4237.3
4
1059.3 0.05
5.83
C DR 4143.1 T C DR 2514.1
4 8
1035.8 0.001 314.3 0.002
5.70 3.46
TC T DR
total R2
72742.359
The rate coefficients k1 and k2 vary with the temperature and DR. The temperature dependencies in eq 4 were found to be of the following forms:
26
0.88
14414.6
2
7207.3 0.0001
31.58
C
6789.6
2
3394.8 0.007
14.88
708.9
2
354.4 0.049
1.55
TC T DR
12433.6 4011.5
4 4
3108.4 0.041 1002.9 0.05
27.24 8.79
C DR
1796.7
4
449.2 0.032
3.94
8
685.8 0.028
12.02
13462.4 0.038
30.28
T C DR 5486.2 total R2
45640.967 0.85
SS 26924.8
2
C
8849.6
2
4424.8 0.0015
9.95
DR TC
13977.3 15681.6
2 4
6988.7 0.048 3920.4 0.045
15.72 17.64
T DR
10168.6
4
2542.2 0.0195
11.44
C DR
8390.0
4
2097.5 0.0239
9.44
T C DR 4914.2
8
614.3 0.018
5.53
total R2
88906.177
26
0.91
higher levels of hydrogen sulfide after adsorption of the hydrocarbon radicals. The coke deposition affected by this mechanism is also observed experimentally to be rising with the amount of hydrogen sulfide. On the other hand, the partial real-time sulfidation of the surface in the continuous injection strategy can alleviate the catalytic effect of the metal surface and, hence, can result in a slight reduction of the coking rate. Therefore, the rate of coke formation either rises or decreases by the trade-off between these effects (Figure 12c). To cover these increasingdecreasing changes, the rate of coke formation in the continuous addition of hydrogen sulfide may be defined as rc, with H2 S ¼ 1 þ fincreasing ðppm H2 S, T, DRÞ rc, no H2 S fdecreasing ðppm H2 S, T, DRÞ
ð5Þ
k2 ¼ R3 þ R4 T þ R5 T 2
ð6Þ
Ri ¼ βi þ γi DR
26
T
k1 ¼ R1 þ R2 T
with the calculated parameters illustrated in Table 7. The coefficients in eqs 5 and 6 were observed to have linear dependencies upon DR as formulated below
Fe T DR
and linear functions for high and low sensitivities, respectively. Then, in accordance with the trends of the coking rate at several temperatures, DRs, and additive concentrations shown in Figure 13, the experimental data for the kinetics of the coke formation were correlated in the following mathematical expression: rc, with H2 S ¼ 1 þ k1 CH2 S 2 k2 CH2 S ð4Þ rc, no H2 S
ð3Þ
The dependency for each of the two functions upon the operating variables in eq 3 was empirically derived. We understood from our experimental observations that the increasing part of this equation has a stronger sensitivity to the hydrogen sulfide concentration than the decreasing part. The opposite is true for the temperature. Moreover, we decided to use quadratic
ð7Þ
The final results of the parameter estimation for the kinetic model have been summarized in Table 8. Figure 13 also demonstrates the results of the kinetic model against the experimental points. As can be seen from this figure, the model predictions are in excellent agreement with the experimental observations. The experimental data reported in Tables 3 and 4 were subjected to the analysis of variance (ANOVA) to evaluate the significance of the effects of the process variables on the rate of coke formation. Please see refs 26 and 27 for more details of the ANOVA technique. The results of ANOVA for these two feeding policies have been reported in Tables 9 and 10. According to the statistical analysis of the experimental data, a cubic polynomial equation was developed to represent the rate of coke formation for the response surface as a function of the temperature, concentration of the hydrogen sulfide additive, and DR. These analyses have been implemented on the basis of the coded variables xi defined by the linear transformation of the actual levels Xi with the arithmetic of the transformation given in eq 8. xi ¼
Xi Xi0 Xiþ1 Xi0
ð8Þ
The superscripts 0 and +1 in this equation stand for the medium and upper levels of the ith factor. The results in Table 9 indicate that the importance of the effect of the temperature decreased in the order Cr > Fe > SS, while a reverse behavior was found in the case of DR for the presulfidation. At the same time, the contributions of their first-order interactions were not significant for all of the three samples. For the continuous injection of the additive, however, the significant interaction of the additive concentration and temperature became highlighted, in the ANOVA table, to follow the decreasing trend of Cr > Fe > SS. This trend is connected, possibly, to the same role of the additive as occurred in the surface presulfidation. However, the rest of two- and three-way interactions of the parameters were, at most, of minor significance (see Table 10). Also worth noting is that the greatest effects of the concentration and DR were observed in the results 4245
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Energy & Fuels of Fe and SS, respectively. However, their contributions are less than half those of the temperature.
4. CONCLUSION The contributions of the temperature, DR, and H2S concentration to the coking rate in thermal cracking of ethane were identified by means of a set of three-level full factorial design of experiments applied to three metal samples of Cr, Fe, and SS. Also, the influence of the surface treatment policy was investigated using two different methods: a presulfidation of the surface and a continuous injection of the additive. The results obtained in this research showed that, by presulfiding each metal coupon with 200 ppmw hydrogen sulfide for 20 min prior to feeding the diluted ethane to the reactor, the rate of coke formation was significantly reduced for all examined metals in usual ranges of temperature and DR. This observation was related to the formation of the metal sulfide on the surface and, thereby, the reduction of the catalytic effect of the metal surface in coking. According to these results, the presulfidation of Cr was more effective when higher levels of temperature and lower ratios of dilution were used. The reverse argument was true for Fe and SS, however. It was found that the greatest reduction in the rate of coke formation over Cr was achieved by the simultaneous choice of high temperatures and low DRs. However, for presulfidation of Fe and SS, the opposite was favored. For the continuous introduction of hydrogen sulfide, at least two contradictory roles were relevant to the rate of coke formation: a promoting effect of hydrogen sulfide via facilitating the adsorption of hydrocarbon radicals on the metal surface and a reduced catalytic effect of the metal surface via sulfidation. A competition between these two parts may justify the dual effect of hydrogen sulfide on the coking rate at different operational conditions. A qualitative inspection of the experimental data resulted in a kinetic model for the coke formation in ethane cracking with hydrogen sulfide as the additive. The parameters of this model were estimated from the experimental data. The proposed empirical model was able to capture all of the trends over the entire operating range investigated. ’ AUTHOR INFORMATION Corresponding Author
*Telephone: +98-21-82883315. Fax: +98-21-88006544. E-mail:
[email protected].
’ NOMENCLATURE Symbols
C = concentration of hydrogen sulfide (ppmw) n = reaction order E = activation energy (J/mol) k0 = pre-exponential factor of the reaction rate coefficient (kg m2 s1 Pan) p = partial pressure (Pa) Q = volumetric flow rate (m3/s) R = ideal gas constant (8.314 J K1 mol1) r = rate of coke formation (kg m2 s1) T = reaction temperature (K) x = actual level of factors X = coded level of factors
ARTICLE
Greek Letters
R = diluent-ratio-dependent coefficients of the k equation β = constant in R correlation γ = coefficient of the DR in R correlation Superscripts
+1 = upper level 0 = medium level 1 = lower level Subscripts
0 = initial c = coke i = counter of factors or empirical coefficients Abbreviations
ANOVA = analysis of variance DR = diluent ratio RGA = refinery-gas analyzer SS = stainless steel
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