ARTICLE pubs.acs.org/IECR
Accelerated Deactivation of Hydrotreating Catalysts by Coke Deposition Marcelo Edral Pacheco,† Vera Maria Martins Salim,‡ and Jose Carlos Pinto*,‡ † ‡
Plantas Piloto, CENPES, PETROBRAS, Rio de Janeiro, 21941-915, Brazil Programa de Engenharia Química/COPPE, Universidade Federal do Rio de Janeiro, Cidade Universitaria, CP 68502, Rio de Janeiro, 21941-972, Brazil ABSTRACT: Pilot plant experiments are designed to allow for evaluation of the impact caused by increasing the severity of key operation variables on deactivation of hydrotreating (HDT) catalysts, through accelerated runs performed in short periods of time. One of the main aspects of this work is the use of a kinetic approach in order to observe the impact of accelerated deactivation conditions upon catalyst performances during hydrodesulfurization (HDS), hydrodenitrogenation (HDN), and aromatic hydrogenation (HDA) reactions. This approach allows for evaluation of catalyst deactivation for each of the analyzed reactions. Results obtained with a reference catalyst indicate that the proposed methodology is able to deactivate the catalyst effectively, leading to proper representation of actual industrial deactivation at the end of a catalyst life cycle.
1. INTRODUCTION The study of catalyst deactivation is one of the major issues in the practice of industrial catalytic processes, as it constitutes a problem that impacts refinery profits all over the world. Catalyst deactivation is inevitable. However, some of its consequences may be slowed, avoided, or even reversed.1,2 Deactivation can be caused by a number of different mechanisms and can be classified into three main groups: (i) poisoning by strong adsorption on active sites; (ii) degradation, due to thermal (sintering), chemical (modification of active sites), or mechanical (attrition/crushing) effects; (iii) fouling (coking and/or metals deposition, which leads to physical coverage of the active surface).2,3 Fouling is the main type of deactivation for hydrotreating catalysts. The relative contribution of coke and metals to catalyst deactivation depends on several factors, such as the origin of the feed, type of catalyst, operating conditions, type of reactor, and position of catalyst in the fixed bed. The contribution of metals to deactivation becomes evident for heavy feeds; however, coke deposition is the most important deactivation mechanism for middle distillate hydtrotreating (HDT) catalysts (typically NiMo/γ-Al2O3 or CoMo/γ-Al2O3).47 Under commercial operating conditions, HDT catalyst activity is maintained by constantly raising the reactor temperature. Deactivation can then be monitored by the continuous increase of the reactor temperature as a function of time. Industrial HDT catalysts are deactivated very slowly at plant sites, losing 0.5 3 °C of catalyst activity per month.79 Start of run temperature is determined by the lowest value necessary for hydrotreated products to meet commercial specifications. End of run temperature is set either by the metallurgical limits of the reactor materials or by thermodynamic limits associated with aromatics hydrogenation reaction reversibility.10 Typically, for a middle distillate HDT unit, cycle lengths range from 1 to 6 years. Given the very long cycle lengths of usual commercial catalysts, it is almost impracticable to study the catalyst activity r 2011 American Chemical Society
decay at normal operation conditions. One alternative is the development of a pilot plant procedure for accelerated deactivation, in order to obtain relevant information about activity decay before the industrial usage of the catalyst. Accelerated deactivation consists of submitting the catalyst to high severity reaction conditions through short periods of time. Weissman and Edwards11 exposed commercial NiMo/Al2O3 and CoMo/Al2O3 catalysts to differing degrees of reaction length and severity using a variety of naphtha and gas oil feeds. After characterization of the spent catalysts they concluded that deactivation was caused primarily by suppression of active sites by carbon-containing deposits on the support. Structural changes in MoS2 structure did not seem to contribute to deactivation. They also concluded that the type of feed impacted the nature of carbon deposits and the activity loss. Tanaka et al.12 performed accelerated deactivation tests of a laboratory prepared CoMo/Al2O3. Results indicated that carbonaceous deposition was the major cause of the catalyst deactivation. However, the effect of carbonaceous deposition might have been overestimated because of the higher reaction temperatures during the accelerated aging. Results indicated that accelerated aging tests did not fully represent practical deactivation. Gamez et al.13,14 submitted samples of a CoMo/Al2O3 catalyst to aging in a pilot plant and in a commercial reactor. No significant variation of textural properties and very slight sintering of the active phase were observed, both showing no correlation with catalytic activity. They observed that, as time on stream increased, the structure of the carbonaceous deposits shifted toward increasing aromaticity. The fraction of active surface covered by aromatic species correlated well with the observed decline of catalyst activity. Received: November 22, 2010 Accepted: March 31, 2011 Revised: March 26, 2011 Published: March 31, 2011 5975
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Industrial & Engineering Chemistry Research Koh et al.15 investigated the accelerated deactivation of CoMo/Al2O3 in the hydrodesulfurization (HDS) of dibenzothiophene. Characterization of deactivated catalysts identified two types of carbonaceous species: the reactive coke and the refractory coke. They observed that the amounts of refractory coke present on the catalyst determined the overall activity. They suggested that the reaction periods of laboratory and industrial processes could be correlated with each other based on the amounts of refractory coke. Kallinikos et al.16 investigated the activity of an HDS catalyst performing experiments at industrial conditions in a small-scale laboratory hydrotreater. They concluded that miniscale reactors can be very effectively used to estimate catalyst deactivation with reduced cost and time. More recently, Guichard et al.17,18 investigated the coke deposits of various spent Ni(Co)MoP/Al2O3 catalysts in order to elucidate the modifications undergone on the catalysts in operating conditions. They concluded that there is an important difference between the deactivation of the NiMoP/Al2O3 catalysts and that of the CoMoP/Al2O3 catalysts. According to their observations coke deposit is the main cause of CoMo catalyst deactivation. However, for NiMo catalysts, deactivation is due to the promoter segregation. One can see that previously published material regarding accelerated deactivation of HDT catalysts is focused mainly on identification of deactivation mechanisms by characterization of coked catalyst obtained in both laboratory scale and industrial scale. This strategy has some disadvantages mainly due to difficulties associated with the correct choice of a representative sample of the industrial deactivated catalyst. Long-term deactivation in industrial reactors is very likely to involve multiple causes. Many of those causes are related to factors inherent to the industrial operations, such as process configuration (number of reactors, number of catalytic beds inside each reactor), changes at the composition of the feed, impact of the relative positioning of the catalyst inside the industrial reactor, and bad liquid distribution through the catalytic bed. For this reason, the main objective of this work is the development of a pilot plant procedure for acceleration of HDT catalyst deactivation, in order to obtain relevant information about activity decay before the industrial usage of the analyzed catalyst. One of the main aspects of this work is the use of a kinetic approach in order to observe the impact of accelerated deactivation conditions upon catalyst performances as an alternative to the traditional approach based on catalyst characterization. This approach allows for evaluation of catalyst deactivation for each of the analyzed reactions: HDS, hydrodenitrogenation (HDN), and aromatic hydrogenation (HDA).
2. EXPERIMENTAL SECTION Experimental data presented in this work were obtained in fixed bed pilot plants. Hydrocarbon feed was pumped by a high precision metering pump. Hydrogen flow was controlled by a mass flow meter. Feed and hydrogen were mixed and followed the upward direction through the tubular reactor. The reactor was equipped with four internal thermocouples (J-type) and four independent heating zones. The reactor outlet effluent went to the high pressure separator for gas/liquid separation. The gas phase was recovered continuously at the top of the separator under pressure control. The liquid product was recovered continuously at the bottom under level control and sent to a low
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pressure separator for N2 stripping in order to remove the residual H2S dissolved in the hydrotreated products. Tests were carried out isothermically, using 40 g (∼55 mL) of fresh reference NiMo/Al2O3 catalyst (19.0 wt % MoO3, 5.5 wt % NiO, 4.1 wt % P2O5). Catalyst was diluted in silicon carbide with a volumetric ratio of 1:1.Tests were carried out isothermically, using 40 g of fresh reference NiMo/Al2O3 catalyst (19.0 wt % MoO3, 5.5 wt % NiO, 4.1 wt % P2O5). Catalyst was diluted in silicon carbide with a volumetric ratio of 1:1. Process feed and hydrotreated products were characterized in terms of sulfur content (ASTM D-5354), nitrogen content (ASTM D-4629), and aromatics content (ASTM D-5186), in order to allow for independent analysis of the deactivation effects on HDS, HDN, and HDA reactions separately. The process feed consisted in a mixture of 60 wt % heavy diesel, 10 wt % fluid catalytic cracking light cycle oil, and 30 wt % delayed coke gas oil, sampled from an industrial HDT unit processing Marlim crude (d20/4 °C = 0.8871, S = 5466 mg/kg, N = 1874 mg/kg, total aromatics = 38.7%, initial boiling point = 144 °C, and final boiling point = 480 °C (ASTM D-2887)). Considering the distillation range from the feedstock, the predominant sulfur species are thiophenes, benzothiophenes, and dibenzothiophenes. Sterically hindered dibenzothiophene species are the most refractory sulfur components for HDS reactions.19 The predominant nitrogen species are carbazoles (nonbasic organic nitrogen) and acridines (basic organic nitrogen).6 The most common aromatic species present in the feedstock are the diaromatics (naphthalenes) and triaromatics (anthracene and phenanthrene).10 The characterization of the metal contents of the processed feed indicated values below the lower detection limit of the methodology (ASTM D-4927-05). These results, together with the short duration of the pilot plant runs, indicate that deactivation by metals deposition may be considered negligible. Each pilot plant run for the proposed accelerated deactivation procedure comprises the following steps: (i) activation and stabilization of the catalyst; (ii) evaluation of initial catalyst activity at conditions representative of the industrial units (T = 360 °C, P = 8.2 106 Pa, liquid hourly space velocity (LHSV) = 1.0 h1, H2/oil = 700 Nm3/m3); (iii) accelerated deactivation performed at higher severity operational conditions; (iv) evaluation of catalyst residual activity (at the same conditions presented in step ii). The catalysts were sulfided in the liquid phase, using a hydrotreated feedstock spiked with a sulfiding agent (CS2), in order to reach a sulfur content of 2% by weight. At the end of activation, the catalyst is very active. In order to avoid premature coke formation, catalyst stabilization must be performed before using real process feeds. This step consisted of processing a less reactive middle distillate feed (without cracked components) for 30 h, at the same operational conditions of step ii. Initial and residual catalyst activity evaluation steps are responsible for generating information that allows for quantification of activity loss. The accelerated deactivation step (iii) consists of two consecutive stages, named “A” and “B”. Stage A consists of submitting the catalyst to a higher reaction temperature, in order to raise coke formation.6,20 Stage B is performed at the same higher temperature and lower ratios between H2 and oil feed flow rates, H2/oil. The objective of stage B is to “age” the coke initially formed by increasing its aromaticity.2123 Experiments are designed to allow for evaluation of the effects caused by key operation variables on catalyst deactivation: 5976
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Table 1. Experimental Design of Pilot Plant Tests for Accelerated Deactivation of HDT Catalysts run
T (°C)
tA (h)
tB (h)
H2/oil (Nm3/m3)
LHSV (h1)
1
400
48
48
150
1.5
2 3
380 420
48 48
48 48
150 150
1.5 1.5
4
400
24
48
150
1.5
5
400
72
48
150
1.5
6
400
48
24
150
1.5
7
400
48
72
150
1.5
8
400
48
48
250
1.5
9
400
48
48
50
1.5
10 11
400 400
48 48
48 48
150 150
0.5 2.5
12
400
48
48
150
1.5
13
400
48
48
150
1.5
for n = 1: ki ¼ LHSV ln
Cci Cp i
for n 6¼ 1:
ð2Þ
! LHSV 1 1 ki ¼ ni 1 ni 1 ðCpi Þ ðCci Þni 1
temperature (T); relative hydrogen concentration (given by H2/ oil); duration of deactivation stages A and B (tA, tB); liquid hourly space velocity (LHSV). The values evaluated for each operational variable were selected in order to accelerate catalyst deactivation and simultaneously prevent other deactivation mechanisms (but coke deposition) to become relevant. The experimental design, shown in Table 1, includes three replicates (runs 1, 12, and 13) in order to allow for evaluation of experimental errors. Deactivation can be represented in terms of the normalized reaction temperature (Tnorm) for each experimental run and each of the studied reactions. Tnorm is defined as the necessary temperature for the deactivated catalyst to achieve the same conversion obtained at the initial activity test, as shown in eq 1. The use of Tnorm is well established as a monitoring and predicting tool of catalyst activity through a life cycle at industrial units.2426 Tnormi ð°CÞ ¼
that it is caused mainly by the decrease of the number of catalytic sites30).
1 R kiresidual 1 ln 273:15 þ Eati Tresidual kiinitial ð1Þ
where R is the universal gas constant (R = 8.314 J/(mol 3 K)), k is the apparent rate coefficient for the analyzed reaction, Eat is the apparent activation energy (J/mol), i represents the studied reactions (HDS, HDN, HDA), and Tresidual is the reactor temperature at the residual catalyst activity evaluation step. The following hypotheses are assumed for Tnorm determination: (i) PFR reactor; (ii) constant hydrogen partial pressure through the reactor; (iii) apparent kinetic rate coefficients given by power-law models represented in eqs 2 and 3; (iv) irreversible reactions with the apparent reaction orders of 1.6 for HDS and 1.0 for HDN and HDA (For the industrial feed considered in the paper, these were the reaction orders that allowed for the best fits of available sulfur, nitrogen, and aromatic contents of the hydrotreated products after the initial catalyst activity evaluation step. It is important to note that these values are in accordance with previous published information available in the literature.2729); (v) apparent reaction rates following the Arrhenius law; (vi) apparent energies of activation and reaction orders remaining the same after deactivation (suggesting
ð3Þ
where Cp and Cc are the product and feed concentrations of the components to be hydrotreated and n is the apparent reaction order for each individual studied reaction (HDS, HDN, and HDA). The determination of catalyst activity loss for each specific reaction comprises the following steps: (a) calculation of the apparent kinetic rate coefficients given by eq 2 or 3, based on feed and product characterization (b) calculation of the apparent energy of activation, based on feed and product characterizations obtained at the initial catalyst activity evaluation step (for this purpose tests were performed at three different temperatures: 340, 360, and 380 °C in runs 1, 12, and 13) (c) calculation of Tnorm values using eq 1 (d) calculation of the temperature raise (ΔTraise) necessary for the deactivated catalyst to achieve the same conversion obtained at the initial activity, given by eq 4 ΔTraise ð°CÞ ¼ Tnormi ð°CÞ Tinitiali ð°CÞ
ð4Þ
where Tinitial is the reactor temperature at initial catalyst activity evaluation step.
3. RESULTS AND DISCUSSION 3.1. Evaluating Whether the Apparent Energy of Activation Remains the Same after Deactivation. In order to
evaluate whether the apparent energy of activation remains the same after deactivation, the evaluations of the initial and residual catalyst activity steps were performed at three different temperatures (340, 360, 380 °C) at runs 1, 12, and 13. This allowed for determination of the apparent energy of activation after the accelerated deactivation step. Table 2 shows that initial and residual apparent energies of activation were statistically equivalent for all analyzed reactions. Consequently, the hypothesis that assumes that the apparent energy of activation remains the same after deactivation seems to be valid. 3.2. Accelerated Deactivation. Tnorm and catalyst activity loss (given by the ratio between apparent kinetic rate coefficients obtained at the residual (kf) and initial (ki) activity evaluation steps) are presented in Table 3. Results obtained at runs 1, 12, and 13 (replicates) in Table 3 show that the proposed experimental methodology leads to excellent reproducibility. Assuming the normal distribution of experimental fluctuations and the 95% confidence level,31 experimental errors for Tnorm were equal to 2 °C for all reactions. Considering the kf/ki ratio, the experimental errors were equal to 0.09 for HDS, 0.03 for HDN, and 0.04 for HDA. Tnorm values presented in Table 3 clearly indicate that the proposed methodology is able to deactivate middle 5977
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Table 2. Initial and Residual Apparent Energies of Activation and Associated Errors for HDS, HDN, and HDA Reactions apparent energy of activation (kJ/mol) initial
residual
residual
residual
(runs 1, 12, 13)
run 1
run 12
run 13
HDS
176 ( 32
188 ( 12
194 ( 13
HDN HDA
82 ( 2 80 ( 6
83 ( 11 71 ( 22
95 ( 17 72 ( 23
96 ( 20 90 ( 33
Table 3. Results of Accelerated Deactivation Pilot Plant Tests Tnorm (°C)
kf/ki
run
HDS
HDN
HDA
HDS
HDN
HDA
1
364
371
371
0.82
0.77
0.77
2
365
369
366
0.78
0.81
0.87
3
369
385
383
0.63
0.55
0.59
4
368
370
372
0.64
0.76
0.73
5 6
362 370
369 374
376 375
0.92 0.60
0.81 0.71
0.69 0.70
7
364
376
372
0.82
0.68
0.75
8
371
375
370
0.56
0.69
0.80
9
364
369
372
0.81
0.81
0.76
10
364
376
379
0.83
0.68
0.65
11
362
371
369
0.88
0.77
0.81
12
366
372
373
0.74
0.75
0.73
13
366
370
373
0.75
0.78
0.73
Figure 1. ΔTraise values obtained at each experimental run for HDS, HDN, and HDA reactions.
distillate HDT catalysts. ΔTraise values greater than 0 were obtained for every reaction at every run as shown in Figure 1. ΔTraise values were given by eq 4 considering a Tinitial value of 360 °C. For HDN and HDA reactions, most ΔTraise values were in the 816 °C range, representing an activity loss of 2030%. However, at run 3 (where the accelerated deactivation step was conducted at 420 °C), ΔTraise values of 25 °C could be obtained, representing an even higher activity loss of 4045%. These results indicate that the accelerated deactivation methodology is very effective, since high activity loss can be obtained through short time pilot plant deactivation tests in a very reproducible way. One can also see that ΔTraise values for HDS reaction were systematically lower than those for HDN and HDA reactions. This could be associated with a different deactivation behavior
Figure 2. WABT profile during cycle lengths of two HDT middle distillate industrial units processing reference feed with reference catalyst.
for this reaction or just with a natural consequence of the different apparent kinetic rate equation considered for the HDS reaction. Interpretation of deactivation for the HDS reaction was partially impacted by experimental limitations associated with N2 stripping of the hydrotreated products, in order to remove the residual H2S dissolved in the liquid product stream. This led to difficult characterization of the low sulfur levels observed at the hydrotreated products obtained at runs 48 and 10. Consequently, deactivation results for HDS reactions were not considered during discussions presented below. Comparison of the ratio between apparent kinetic rate coefficients obtained for the residual and initial activities (kf/ki) indicates that the activity loss caused by accelerated deactivation seems to be uniform for HDS (when considering only runs with no stripping limitations), HDN, and HDA reactions for the reference catalyst. Therfore, smaller ΔTraise values for HDS probably arose from the different reaction order (1.6) considered for this reaction, when compared to HDN and HDA (1.0). In order to verify the uniformity of catalyst decay for HDS, HDN, and HAD reactions, reaction temperature was progressively raised in all runs after evaluation of the residual catalyst activity in order to reach the same specific gravity of the product obtained in the beginning of the run. Then, samples of the hydrotreated products were taken and sulfur, nitrogen, and total aromatics contents were determined. In all cases, the obtained sulfur, nitrogen, and total aromatics contents were similar to the initial values, confirming the initial assumption, as reported elsewhere.32 (As a consequence, it was shown that the in-line monitoring of specific gravity can be used to provide real-time inferences about the nitrogen, sulfur, and aromatics contents of hydrotreated products.) 3.3. Comparing Results Obtained in the Pilot Plant with Industrial Catalyst Deactivation Data. One of the main challenges to establishing a relationship of similarity between pilot plant and industrial deactivation consists of selecting an industrial catalyst sample that actually represents the residual activity throughout the catalytic bed. The content and nature of contaminants, temperature profiles, and hydrogen partial pressure may be significantly different throughout the industrial reactor. These factors are directly associated with loss of catalytic activity, and consequently, deactivation is strongly influenced by the relative positioning of catalyst samples obtained from industrial reactors. Two reference catalyst samples obtained at the end of the industrial unit B campaign shown in Figure 2 were selected in 5978
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Table 4. Characterization of Industrial Deactivated Catalyst Samples IND1
IND2
total carbon (%)
13.3
12.0
aromatic carbon (%) aliphatic carbon (%)
24 76
51 49
MoO3 (%)
19.6
19.8
NiO (%)
5.4
5.3
P2O5 (%)
4.0
4.2
