Effect of the Operating Pressure on Residual Oil Hydroprocessing

and Studies Centre, Kuwait Institute for Scientific Research, Post Office Box 24885, Safat 13109, Kuwait. Energy Fuels , 2012, 26 (12), pp 7257–...
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Effect of the Operating Pressure on Residual Oil Hydroprocessing Adel Al-Mutairi* and Abdulazim Marafi Petroleum Refining Department, Petroleum Research and Studies Centre, Kuwait Institute for Scientific Research, Post Office Box 24885, Safat 13109, Kuwait ABSTRACT: The atmospheric residue desulfurization (ARDS) process is one of the main processing units in Kuwaiti refineries. In the ARDS process, a graded catalyst system consists of different types of catalysts. The performance of the catalyst system is mainly influenced by feedstock quality and reaction conditions, such as temperature and pressure. In this study, the effect of operating hydrogen pressure on the life cycle of the catalyst is investigated using two feedstocks, Kuwait heavy crude atmospheric residue (KHC-AR) and Eocene-AR, using an analogous process condition that is generally used in refinery. The pilot-plant hydrotreatment results demonstrated an effect of hydrogen pressure on the catalyst performance for the hydrodesulfurization (HDS) activity and sustaining a target sulfur level of 0.6 wt % in the product. The required sulfur level in the product was adjusted by raising the temperature gradually to compensate for the catalyst deactivation. The activity test for two different feeds were studied up to 4000 h time-on-stream (TOS) at 120 and 150 bar hydrogen pressure. The results showed that the performance of the catalyst system under an operating pressure of 150 bar was better than that at 120 bar. The reaction rate is accelerated by high pressure because it reduces the effect of the coke deposition on the catalyst. [i.e., fluidized catalytic cracking (FCC)]. As an additional benefit, mild hydrocracking also occurs in the ARDS process with the conversion of residues to distillates, such as naphtha, kerosene, and diesel. Having heavy crude oil or their residue as a feed, the main concern for hydroprocessing is catalyst deactivation.10−13 The deactivation mainly depend upon the quality of feed, as well as operating conditions, particularly temperature and pressure. Deactivation by metal deposition has been reported for different feedstocks.10,13,14 The mechanism of deactivation by metal deposition is relatively well-understood, while coke formation is still not complete. Coke is the carbonaceous deposit that reduces the activity of the catalysts. Coke lay down on residual oil hydrotreating catalysts is more extensive during the very early stages of operation.12,15−17 The initial coking is reported to cause a large loss in surface area and a fairly rapid deactivation. In fixed-bed industrial hydrotreating reactor units, the reactor temperature is normally increased with the time-on-stream (TOS) to compensate for the loss of catalyst activity. Bartholdy and Cooper18 observed that more coke builds up and deactivation by coke occurs each time that the reactor temperature is increased to compensate for the loss of activity during the process. Coke deposits, together with the metal sulfide deposits, contribute to diffusional resistance to reacting molecules in the heavy oils by constricting the catalyst pore diameter and, thereby, reducing the effective catalyst life. It has been shown that coke deposition can account for more than 50% of the deactivation in resid upgrading. Asphaltenic oils have been found to have a higher propensity for coke formation.3 Residues with low asphaltene and carbon residue contents have been found to give longer cycle length or catalyst

1. INTRODUCTION Worldwide, the heavy consumption of conventional crude oil and the depletion of its reserves have increased the demand of using less desirable heavy feedstock and residues. Refiners have seriously focused on the processes for the upgrading of heavy crude oils and residues as the price of light crude oils (low sulfur) has increased significantly. Technology options available for the upgrading of heavy oils and residues can be broadly classified into two groups, namely, carbon rejection and hydrogen addition processes.1−3 Among the various processes used for residual oil upgrading by hydrogen addition, the atmospheric residue desulfurization (ARDS) process is widely used in many refineries worldwide.3−6 It is one of the major upgrading processes operating at Kuwait National Petroleum Company (KNPC). Since 1968, three ARDS units have been operating in existing KNPC refineries with a total capacity of around 200 000 barrels/day for more than 15 years. Each of these units consists of two trains of reactors, and each train consists of a series of four fixed-bed catalytic reactors operating in series.7 In Kuwait, the ARDS product (fuel oil) is mainly used in power generation. There is an increasing need for lowsulfur fuel oil (LSFO) to meet the increasing demand by the power generation sector, whereby heavy crude oils will be used as a part of its feedstock, such as Ratawi/Burgan, Eocene, and Lower Fars (LF). KNPC will enhance their ARDS capacity to about 650 000 barrels/day with the new refinery. The ARDS process will be used in the refinery for hydrotreatment of the atmospheric residues (AR) of Kuwait heavy crude (KHC) oil. In our previous study, an effect of feedstock on the catalyst life cycle was studied using AR of Kuwait export crude (KEC) and KHC oils.8,9 The ARDS process involves hydrotreatment of AR in a series of fixed-bed reactors containing catalysts. The unit works primarily as a desulfurization unit but also reduces the metals, asphaltenes, and nitrogen in the products, thereby ensuring a proper quality of purified feed for downstream conversion units © 2012 American Chemical Society

Received: July 11, 2012 Revised: October 1, 2012 Published: November 19, 2012 7257

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life because coke deposition on the catalyst is relatively lower with such feedstocks. Remarkable improvements in catalyst life by feed deasphalting have been reported in some studies.14,19,20 Among the operating conditions, feed quality and increasing hydrogen pressure can have a strong influence on reducing catalyst deactivation by coke deposition.21−24 The current work addressed preliminary studies to investigate the processability of AR from typical Kuwait heavy crude oils, namely, KHC (Ratawi/Burgan) and Eocene crude oils, for the production of LSFO under high pressure. The impact of feed quality as well as hydrogen pressure on the activity of the ARDS catalyst system is investigated using pilotplant testing units. Attention was made on the stability of the catalyst at two different pressures and their effect on catalyst cycle length and product quality.

Table 2. Reactor Loading for the Catalyst System Used in the Pilot-Plant Unit catalyst reactor number first top layer first bottom layer second top layer second middle layer second bottom layer total

2. EXPERIMENTAL SECTION

code CATBA CATBB CATBC CATBD CATBE

volume (mL)

weight (g)

weight percent (vol %)

HDM

56

26

1417

HDM/ HDS HDS

72

36

1923

97

64

3330

48

32

1715

48

33

1715

321

191

type

HDS/ HDN HDS/ HDN

100

reaction efficiency. To overcome this deficiency, the catalyst bed is diluted with inert particles of carborandum (1:1 ratio) to fill up the gaps between catalyst particles, to increase the oil residence time in the catalyst bed, and to increase the diffusion rate. The catalyst and their beds are shown in Figure 1.

2.1. Feedstock. Two different resid feedstocks, namely, KHC-AR and Eocene-AR, were used. They were fractionated from the original crude oils provided by Kuwait Oil Company (KOC). The feedstocks properties were compared in Table 1. It is seen that KHC-AR is

Table 1. Physicochemical Properties of AR from KHC and Eocene parameter TBP cut range (°C) yield on crude (wt %) yield on crude (%) density at 15 °C (g/mL) API gravity (deg) sulfur (wt %) nitrogen (wt %) kinematic viscosity at 100 °C (cSt) CCR (wt %) total metals (ppm) V (ppm) Ni (ppm) asphaltenes (wt %) pour point (°C)

KHC-AR

Eocene-AR

+360 56.44 62.26 1.0116 8.3 5.35 0.38 450.0 18.5 122 82.0 40.0 13.2 40.0

+360 64.7 68.9 1.0031 7.40 5.0 0.31 218.0 15.9 105 78.0 27.0 6.94 +18

heavier, with the content of metals (Ni + V), Conradson carbon residue (CCR), and asphaltenes. The KHC-AR contains more heteroatoms, such as sulfur, nitrogen, and oxygen. 2.2. Catalyst System. A typical commercial residue processing catalyst system was used in the study. It consisted of a combination of five catalysts for promoting hydrodemetallization (HDM), hydrodesulfurization (HDS), and hydrodenitrogenation (HDN) reactions. Reactor loading profiles for different catalysts in the pilot plant are shown in Table 2. A guard macroporous catalyst used for HDM is typically monometallic, while HDS and HDN mesoporous catalysts are bimetallic (CoMo/NiMo) in composition. 2.3. Hydroprocessing Reactor Unit Setup. The experimental work of the study was conducted in two different units. Both units have two reactors in series with similar catalyst loadings, as indicated in Table 2. The loading profile is exactly the same as the previous study, which was carried out at a lower pressure.8,9 The reactor was loaded using grading of the catalyst to prevent channeling and pressure drop during the processing to achieve the required performance of the ARDS process. The oil distribution and feed diffusion rate are the most challenging factors in pilot-plant reactors. Inadequate distribution of oil can cause channeling, which directly affects the reaction rate. To reduce the channeling effect, a layer of alumina ball (inert) is placed before the catalyst bed to enhance the oil distribution. The feed diffusion rate in the pilot plant is much lower than the commercial unit, which negatively impacts the

Figure 1. Loading of reactors 1 and 2. 2.4. Feed and Product Characterization. During the course of the test, product samples were collected from the outlets of the reactor and its analysis was carried out as per standard procedures available at our petroleum evaluation facility (PEF). The sulfur contents of feed and product liquid samples were determined using an Oxford 100 sulfur analyzer. An automatic Cosmo analyzer was used to determine asphaltenes. Metals (V and Ni) were analyzed using an X-ray fluorescence (XRF) analyzer. CCR was determined by a micro carbon residue instrument. 2.5. Catalyst Presulfiding. After loading and mounting the reactors, the unit was tested for leakage. The first leak test was conducted by nitrogen at 160 bar operating pressure, while the second leak test was conducted with hydrogen at a similar condition, to determine the major and minor leaks, respectively. As practiced in this laboratory, the catalyst presulfiding process was conducted by wetting the catalyst bed at 150 °C using gas oil that is spiked with carbon disulfide (CS2). The presulfiding procedure and conditions are described in Figure 2. 2.6. Test Procedure and Operating Conditions. Atmospheric residue feeds (KHC-AR and Eocene-AR) were introduced in situ to the sulfide catalyst, and the hydrotreating conditions were adjusted as 7258

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Figure 3. Comparison of the temperature profile in processing KHCAR at 120 and 150 bar for the total system with a target sulfur level of 0.6 wt %.

Figure 2. Presulfiding procedure. indicated in Table 3. At the start of the run, the bed temperatures of all reactors were adjusted at 370 °C. The temperatures were increased

WAT. For both of the pressures, the HDS conversion reached a steady state at about 1000 h TOS. However, a progressive deactivation indicated that deactivation continuously took place because of the carbon and metal deposition on the catalytic sites.24−27 In the case of Eocene-AR hydroprocessing, the deactivation and stability of the steady state were different from those of KHC-AR, indicating very stable deactivation after 1000 h TOS, as shown in Figure 4. The effect of different pressures (120 and

Table 3. Operating Conditions for ARDS Feedstock (KHCAR and Eocene-AR) process parameters process unit temperature (°C) pressure (bar) LHSV (h−1) H2/oil (mL/mL) H2 flow (nL/h) feed flow (mL/h)

08B/09B 350−410 150 0.28 680 62.5 92

gradually and periodically to maintain the target sulfur level at 0.6 wt %. Product samples were collected from both reactors at regular intervals during the run. The samples were analyzed daily for sulfur, metals, CCR, and asphaltenes. The rate constants for HDS, HDM, and asphaltene conversion (HDA) reactions were determined using the following standard nth order kinetic equation with n = 2 for HDS, n = 1.5 for HDM, and n = 2 for HDA:

k=

⎞ ⎛ LHSV ⎜ 1 1 − n − 1 ⎟⎟ ⎜ n−1 n − 1 ⎝ Sp Sf ⎠

where Sf is the sulfur weight percent in the feed and Sp is the sulfur weight percent in the product. The detailed kinetics data for different catalyst types, operating conditions, and the feedstocks have been reported by Marafi and co-workers.8,9

Figure 4. Comparison of the temperature profile in processing Eocene-AR at 120 and 150 bar for the total system with a target sulfur level of 0.6 wt %.

3. RESULTS AND DISCUSSION 3.1. Effect of the Hydrogen Pressure on the Variation of Weighted Average Temperature (WAT). The effect of the hydrogen pressure (120 and 150 bar) on the performance of the hydrotreating catalyst using KHC-AR is shown in Figure 3. The weighted average temperature of the overall system for achieving a target sulfur level of 0.6% in the hydrotreated product oil was plotted against TOS. The results showed that, with the TOS of the progress, the catalyst activity decreases because of the deactivation of the catalyst or the decreasing number of catalytic sites, increasing the temperature required to compensate for desulfurization to achieve the target sulfur level of 0.6 wt % in the product. In the same figure (Figure 3), an effect of the hydrogen pressure (120 and 150 bar) was also observed, where the lower hydrogen pressure required a higher

150 bar) is also indicative of deactivation, where a significant difference shows a more stable behavior at a higher pressure. After a closer look, the steady-state KHC-AR feed-treated catalyst has a lower steady state mainly because of the higher metal and asphaltene contents that uphold continuous deactivation, while the Eocene-AR feed-treated catalyst reached a steady state relatively faster and remained more stable with TOS mainly because of less metal, asphaltene, and CCR deposition during the hydroprocessing. For both feeds as a function of the H2 pressure, the catalyst life is extended at a high pressure (150 bar) compared to a lower pressure (120 bar). On the other hand, temperature changes have a higher rate for KHC-AR than Eocene-AR, and the reason is probably related to the high asphatene and metal contents in KHC-AR. Figure 5 indicated that the temperature 7259

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acidic properties of the catalyst. Therefore, a higher metal content feedstock shows faster and progressive deactivation with time. 3.2. Effect of the Hydrogen Pressure on Heteroatom Removal from Residues. The effect of the pressure on the removal of a heteroatom, such as S, N, and metals (Ni + V), is governed by the hydrogenation function of the catalyst, which is expected to involve a hydrogenation mechanism of the molecules, particularly porphyrine type of metal chelates. At a higher hydrogen pressure, the hydrogen function becomes more effective and hydrogenates rings, which promote the removal of heteroatoms. The extent of a high hydrogen pressure (stronger hydrogenation function) on asphaltene removal is also related to the hydrogenation reaction that prevents carbon deposition on the catalyst and enhances the life of the catalyst. Figure 6 show the influence of the hydrogen pressure on the HDM reaction rate constant. It is clearly noticed that processing KHC-AR or Eocene-AR at 150 bar increased the catalyst activity for metal removal because of the excess of hydrogen. On the other hand, similar increases in the HDS rate constant were noticed at 150 bar compared to 120 bar, as shown in Figures 7 and 8. Therefore, it was shown from these

Figure 5. Effect of the pressure on the WAT with variation of TOS (24−2000 h).

increase is selectively high in the case of lower pressure. The relationship also revealed that heavier feedstock required a higher temperature to obtain the required product specification (i.e., 0.6 wt %). When two different feeds are compared, KHCAR showed more unstable behavior with TOS (Figures 3 and 4). The difference is obvious where Eocene-AR hardly required temperature at 404 °C, while KHC-AR feed required about 408 °C to achieve 0.6 wt % S in the product. It is expected that, because of the higher CCR and asphaltene in KHC-AR, a higher temperature is required to hydrocrack larger molecules into smaller molecules. KHC-AR has a higher amount of metals, which may deactivate the catalyst at a faster rate than Eocene-AR feed. The faster deactivation behavior became more understandable in KHC-AR, where a higher temperature is required along with a higher pressure (Figure 5). Similar results are reported by Ancheyta et al.,17 confirming the three obvious segments of deactivation with TOS. They have reported that initial deactivation is due to the presence of acidic sites (carbon deposition), the middle of run deactivation (metal deposition) is sluggish, and the end of run deactivation (diffusion limitation) is fast. Therefore, the catalyst particular fraction is governed by different catalyst properties, and its ability for HDS, HDM, and HDA is also dictated by a wellbalanced combination of active-phase dispersion, textural, and

Figure 7. HDS system rate constant for KHC-AR at 120 and 150 bar.

Figure 6. Steady-state rate constant (middle of run) of (a) HDM reactions and (b) asphaltene (HDAasph) and Conradson (HDCCR) conversion reactions for Eocene-AR and KHC-AR at hydrogen pressures of 120 and 150 bar at the MOR (after 2 months). 7260

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The hydrogen function also can be deduce from the hydrotreated product volume, where more distillate yield is obtained at a higher pressure. The distillate yield depends upon the addition of hydrogen as well as cracked products of residue. Thus, product distribution with the higher hydrogen partial pressure (150 bar) generates a larger fraction of middle distillate. The distillate yield is another target in the residueupgrading process (ARDS system). The percent of conversion of 360 °C+ residues to distillate products during catalytic hydrotreating of KHC-AR at 120−150 bar hydrogen pressure showed an 8−12% increase in the conversion of residues to distillates. This is also in agreement with other results obtained from the pressure increase that will enhance the activity of the catalyst for hydrocracking of large molecules (asphaltene and CCR) into light distillates. 3.3. Effect of the Hydrogen Pressure on the Metals on the Catalyst (MOC). The metals (V and Ni) present in residual oil feed were removed by the hydrometallization reaction during the catalytic hydrotreating process. The metals were removed from the feed and deposited on the catalyst surface as metal sulfides. Figure 9a shows the amount of metals deposited on the catalyst during hydrotreating of KHC-AR at different pressures (120 and 150 bar). Similar results were also observed for Eocene-AR hydrotreating at pressures of 120 and 150 bar (Figure 9b). Because the rate of the HDM reaction was enhanced at a high hydrogen pressure, more metals would be removed from the feed at high-pressure operation than at lowpressure operation. On the other hand, higher accumulation of metal sulfide on the catalyst will lead to catalyst deactivation by pore plugging. However, in the present study, catalyst deactivation was found to be more rapid at low-pressure operation (120 bar) than at high-pressure operation, even though more metals were deposited on the catalyst at 150 bar. Hence, coke deposition on the catalyst appeared to play a more dominant role than metal deposition on catalyst deactivation. Apparently, a portion of metals (Ni and V) may deposit on the bare alumina support that may generate some active vanadium and nickel sulfide sites, and the nature of these catalytic sites could be explained as autocatalysis.31−34 Because coke deposition was suppressed at high hydrogen pressures, slower deactivation and longer catalyst life were noticed at highpressure operation. It is expected that the carbon deposition is more at a lower pressure and higher temperature. However, it is suggested that detailed characterization of the spent catalyst at different pressures may be important to reveal the exact deactivation nature of the catalyst and deposited species.

Figure 8. HDS system rate constant for Eocene-AR at 120 and 150 bar.

results that an increasing hydrogen pressure was found to have significant effects in residual oil, which increases the HDM and HDS reaction rate as well as enhances catalyst life. The enhanced rate of HDS and HDM can be explained on the basis of increased hydrogen availability in the reactor at higher hydrogen pressures. Hydrogen plays an important role in promoting various reactions, such as HDS, HDN, HDM, aromatic hydrogenation, asphaltene conversion, and free radical stabilization, during catalytic hydrotreating of residual oil. However, it is expected that the catalytic reaction was processed through the heterolytic dissociative mechanism.28−30 Hydrogenation of the aromatic rings or asphaltene is an important step in the HDS, HDM, HDN, and HDAsph reactions. The HDM reaction mechanism involved hydrogenation of aromatic rings in the initial steps and subsequent breaking of the metal− nitrogen bond.28 At a higher hydrogen pressure, more hydrogen is soluble in the feed and available on the catalyst surface for the reaction in the presence of catalysts. This would kinetically flavor the hydrogenation step and, consequently, increase the rates of all hydrotreating reactions. The hydrogenation reaction is also favored thermodynamically at high hydrogen pressures. Furthermore, at a higher hydrogen pressure, the coke forming free radicals and other coke precursors will be hydrogenated to a large extent. As a result, less coke will be formed on the catalyst during the reaction. Therefore, catalyst deactivation by coke formation will be reduced, and catalyst life will be prolonged at high hydrogen pressures.

Figure 9. Total MOC at 120 and 150 bar using (a) KHC-AR and (b) Eocene-AR feedstock. 7261

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The overall pressure effect provided evidence that the hydrogen pressure has a significant influence on the conversion, product quality, and catalyst deactivation. The high-pressure operating conditions specified higher metal deposition but lower coke formation tendencies, which is indicative of higher deactivation.

4. CONCLUSION In the present work, the effect of the hydrogen pressure on the residue hydroprocessing with different feedstock was studied. Residue conversion results indicated that the catalyst activity significantly depended upon the composition of the feedstock and the operating conditions that are used to achieve less than 0.6 wt % sulfur in the product. Using two different feedstock, it was concluded that the required sulfur can be achieved at a higher temperature with a lower pressure but with the expense of catalyst deactivation. Significant differences in deactivation were observed as a function of H2 pressure for both AR feedstocks. An increasing hydrogen pressure improved HDM and HDS reaction rates as well as enhanced catalyst life. Hence, an increase in the hydrogen partial pressure enhanced the quality of hydrotreated product significantly and obtained the desire product at lower temperature. The presence of a guard reactor and graded bed loading showed a high metal retention capacity on the fresh catalyst and achieved a satisfactory length of the catalyst life cycle. However, for the commercial applicability of the study, additional work is suggested to assess the impact of the usage of higher hydrogen pressure on the economics of the ARDS process because of high hydrogen consumption.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: 965-2495-6972. Fax: 965-2398-0445. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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