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Studies on recycling and utilization of spent catalysts: Preparation of active hydrodemetallization catalyst compositions from spent residue hydroprocessing catalysts
Applied Catalysis B: Environmental 71 (2007) 199–206 www.elsevier.com/locate/apcatb
Studies on recycling and utilization of spent catalysts: Preparation of active hydrodemetallization catalyst compositions from spent residue hydroprocessing catalysts Meena Marafi, Antony Stanislaus * Petroleum Refining Department, Petroleum Research and Studies Center, Kuwait Institute for Scientific Research, P.O. Box 24885, Safat, Kuwait Received 12 April 2006; received in revised form 31 July 2006; accepted 11 September 2006 Available online 10 October 2006
Abstract Spent catalysts form a major source of solid wastes in the petroleum refining industries. Due to environmental concerns, increasing emphasis has been placed on the development of recycling processes for the waste catalyst materials as much as possible. In the present study the potential reuse of spent catalysts in the preparation of active new catalysts for residual oil hydrotreating was examined. A series of catalysts were prepared by mixing and extruding spent residue hydroprocessing catalysts that contained C, V, Mo, Ni and Al2O3 with boehmite in different proportions. All prepared catalysts were characterized by chemical analysis and by surface area, pore volume, pore size and crushing strength measurements. The hydrodesulfurization (HDS) and hydrodemetallization (HDM) activities of the catalysts were evaluated by testing in a high pressure fixed-bed microreactor unit using Kuwait atmospheric residue as feed. A commercial HDM catalyst was also tested under similar operating conditions and their HDS and HDM activities were compared with that of the prepared catalysts. The results revealed that catalyst prepared with addition of up to 40 wt% spent catalyst to boehmite had fairly high surface area and pore volume together with large pores. The catalyst prepared by mixing and extruding about 40 wt% spent catalyst with boehmite was relatively more active for promoting HDM and HDS reactions than a reference commercial HDM catalyst. The formation of some kind of new active sites from the metals (V, Mo and Ni) present in the spent catalyst is suggested to be responsible for the high HDM activity of the prepared catalyst. # 2006 Elsevier B.V. All rights reserved. Keywords: Spent catalysts; HDM catalyst preparation; Waste catalyst management; Residue hydrotreating
1. Introduction Large quantities of catalysts are used in the petroleum refining industry for the purification and upgrading of various petroleum streams and residues. The catalysts deactivate with time and when the activity of the catalyst declines below the acceptable level, it is usually regenerated and reused. But, regeneration is not always possible [1,2] and after a few cycles of regeneration and reuse, the catalyst activity may decrease to very low levels and further regeneration may not be economically feasible. The spent catalysts are discarded as solid wastes [3,4]. The quantity of spent catalysts discharged from different processing units depends largely on the amount of fresh catalysts used, their life and on the amount of the
* Corresponding author. Tel.: +965 3980499; fax: +965 3980445. E-mail address: [email protected] (A. Stanislaus). 0926-3373/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2006.09.005
deposits formed on them during use in the reactors. In most refineries, a major portion of the spent catalyst waste comes from the residue hydrotreating and hydroprocessing units. This is because the catalysts used in these processes deactivate rapidly by coke and metal (V and Ni) deposits, and have a short life [5–7]. Furthermore, technology for regeneration and reactivation of the catalysts deactivated by metal fouling is not available to the refiners. The volume of spent hydroprocessing catalysts discarded as solid wastes has increased significantly in recent years due to a steady increase in the processing of heavier feedstocks containing higher sulfur, nitrogen and metal (V and Ni) contents together with a rapid growth in diesel hydrotreating capacity to meet the increasing demand for low sulfur fuels. At the same time, environmental laws concerning spent catalyst disposal have become increasingly more severe in recent years. Spent hydroprocessing catalysts have been classified as hazardous wastes by the Environmental Protection Agency
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in the USA [8,9]. The most important hazardous characteristic of spent hydroprocessing catalysts is their toxic nature. Chemicals such as V, Ni, Mo and Co present in the catalyst can be leached by water after disposal and pollute the environment. The hazardous nature of the spent catalysts is attracting the attention of environmental authorities in many countries and the refiners are experiencing pressures from environmental authorities for safe handling of spent catalysts. Several alternative methods such as disposal in landfills, reclamation of metals, regeneration/rejuvenation and reuse, and utilization as raw materials to produce other useful products are available to the refiners to deal with the spent catalyst problem [3,4,10– 13]. The choice between these options depends on technicalfeasibility and economic considerations. In recent years, increasing emphasis has been placed on the development of processes for recycling the waste catalyst materials as much as possible. Utilization of spent catalysts as raw materials in the production of other valuable products is an attractive option for recycling spent catalysts from environmental and economical points of view. Spent fluid catalytic cracking (FCC) catalysts have been successfully used in cement production [13,14]. Recently, a process for making highly stabilized non-leachable ceramic materials from spent catalysts has been reported by Sun et al. [15]. A few studies on the preparation of active catalysts from spent catalysts for various applications have been reported in the literature. Lee et al. [16] reported that active reforming catalysts can be prepared using the V, Ni and Mo containing extract obtained by leaching spent catalysts with citric acid. Furimsky [17] found that spent Co– Mo/Al2O3 and Ni–Mo/Al2O3 catalysts, after regeneration, can catalyze the decomposition of H2S. In a recent patented process, Choi et al. [18] used spent hydroprocessing catalysts to prepare active catalysts for reduction of nitrogen oxides. The use of spent catalysts in preparation of active hydrotreating catalysts, have been reported in a few earlier studies [19,20]. However, the spent catalysts used in these earlier works were from petroleum distillates hydrotreating units and contained Mo, Co and Al2O3 without V and Ni. Spent catalyst handling and utilization has been the subject of some investigation in our laboratory. In most of the previous studies, rejuvenation of spent residue hydroprocessing catalysts for reuse was addressed [21–27]. In the present work, utilization of spent residue hydroprocessing catalysts containing high levels of Vand coke together with Mo, Ni and Al2O3 in the preparation of active hydrodemetallization (HDM) catalysts has been considered. HDM catalysts are used in the front end reactors of petroleum residue hydrotreating processes to remove the metals such as V and Ni that are present in the residual oil. The catalyst usually possesses high activity for promoting HDM reactions together with some activity for sulfur removal by hydrodesulfurization (HDS) reaction. The catalysts were prepared by mixing and extruding the spent catalyst powder with boehmite in different percentages, and the effect of mix ratio between spent catalyst and boehmite on the key catalyst properties such as surface area, pore volume, pore size distribution and the metals (V, Mo and Ni) content of the
prepared catalyst samples was examined. The HDM and HDS activities of the catalysts were tested in a high pressure fixedbed microreactor unit using Kuwait atmospheric residue as feed and compared with that of a commercial HDM catalyst. 1.1. Experimental 1.1.1. Catalyst preparation from spent catalyst–boehmite mix The sequence of operational steps used for the preparation of catalyst extrudates from spent catalyst–boehmite mix in the present work are shown in Fig. 1. Boehmite used in the catalyst preparation experiments were obtained from Sasol, Germany. Spent catalyst was obtained from Kuwait National Petroleum Company (KNPC). The spent catalyst was first washed with naphtha and then extracted with toluene in a soxhlet apparatus to remove the residual oil. The oil free-spent catalyst was ground to fine powder in a grinding machine (Christison Particle Technologies Ltd., Model KM 100) and then sieved using a sieve shaker (Endecotts, model OCT Digital 4587-01) with appropriate sieves obtain particle sizes in the range 25– 90 mm similar to that of the boehmite powder. The chemical and physical properties of the oil-free spent catalyst powder were analyzed by various techniques. The characteristic of the spent catalyst are shown in Table 1. A laboratory kneading and extrusion machine (Type: LUK 2.5 AS) manufactured by Werner and Pfleiderer Gmbh & Co., Germany, was used for the preparation of catalyst extrudates from spent catalyst–boehmite mixtures. It contained a mixing chamber, two blades for mixing, and a drive unit with two threephase motors and gears, and a discharge screw. Three hundred grams of spent catalyst–boehmite mixture in the desired ratio was taken in the mixing chamber for each
Fig. 1. Operational steps in the preparation of catalyst extrudates from spent catalyst.
M. Marafi, A. Stanislaus / Applied Catalysis B: Environmental 71 (2007) 199–206 Table 1 Spent catalyst characteristics
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All catalysts were tested under the following operating conditions:
Catalyst property
Values
Mo (wt%) V (wt%) Ni (wt%) C (wt%) Al2O3 (wt%) Surface area (m2/g) Pore volume (ml/g) Side crushing strength (lb/mm)
4.5 9.7 4.0 15.3 43.5 18 0.10 2.9
experiment. Then 185 ml of dilute nitric acid (e.g., 2%), as a peptizing reagent, was added in drops at a constant rate to the boehmite powder, and the two were mixed and kneaded. The blades in the mixing chamber were counter rotating and turned at different speeds. They were designed and arranged for intensive mixing and kneading of the boehmite with the nitric acid to form a good, extrudable paste. At the end of the mixing and kneading time (20–30 min), the product was extruded by means of the discharge screw through a die containing several holes (1.5 mm in diameter) to form catalyst extrudates. The extrudates were dried at 110 8C in an oven for 24 h. The dried extrudates were calcined at 370 8C for 2 h, 450 8C for 3 h and 500 8C for 2 h. After calcinations the prepared extrudates were cooled in desiccators and characterized. 1.1.2. Catalyst characterization The concentrations of V, Mo, and Ni in the spent catalyst and in the prepared catalyst samples were determined by inductively coupled plasma atomic emission spectroscopy (Varian Liberty II ICP-AES). A scanning electron microprobe X-ray analyzer (model: EPMA JXA 8600MX) from Joel was used for measuring the distribution profiles of the metals across the catalyst pellets. Surface areas of the catalysts were determined by BET method using an autosorb adsorption unit manufactured by Quantachrome Corporation, USA. A mercury porosimeter (Quantachrome Poremaster-60) was used for pore volume and pore size distribution determination in catalyst samples. The method involves intrusion of mercury into the pores at high pressures of up to 60,000 psi. A side crushing strength measuring apparatus designed and manufactured by AKZO Nobel (Model No. 120794-108) was used to determine the side crushing strength of the catalyst pellets. 1.1.3. Catalysts activity testing Hydrotreating activities of the prepared catalysts were tested in a high pressure fixed bed microreactor unit using Kuwait atmospheric residue as feed. The feedstock contained 4.3 wt% sulfur, 0.27 wt% nitrogen, 69 ppm vanadium, 21 ppm nickel, 3.6 wt% asphaltenes and 12.4 wt% CCR. Thirty millilitres of the catalyst diluted with an equal amount of carborundum was used for each run. The catalysts were presulfided using 1% CS2 in straight run gas oil by a standard procedure [28], before introducing the feed. After presulfiding, the test conditions were adjusted to desired operating temperature, pressure, hydrogen to oil ratio and liquid hourly space velocity (LHSV).
pressure ¼ 120 bar;
LHSV ¼ 1 h1 ;
H2 =oil ratio ¼ 1000;
temperature ¼ 370 and 390 C For each run, product samples were collected every 24 h for analysis of S and metals (Ni, V) content. Sulfur content was determined using an Oxford Model 3000 XRF instrument. The concentrations of V and Ni in the oil were determined without ashing using a Varian Liberty Series II, ICP spectrophotometer. 2. Result and discussion 2.1. Characteristics of the catalysts prepared by mixing spent catalyst with boehmite The catalyst extrudates prepared with the addition of spent catalyst to boehmite in different proportions were subjected to chemical analysis to determine the concentration of Mo, Ni and V in them. The results are plotted in Fig. 2. It is seen that the concentration of the metals in the prepared catalysts increases linearly with increasing percentage of spent catalyst in the mix. A larger increase is noticed for V than the other metals. A catalyst prepared from a mix containing 20% spent catalyst and 80% boehmite contains 1.84% V, 0.91% Mo and 0.77% Ni. Increasing the amount of spent catalyst in the mix from 20 to 40% increases the V content to 3.8% whereas, the Mo and Ni concentrations increase to 1.85% and 1.47%, respectively. These results are consistent with the higher concentration of V than Mo and Ni in spent catalyst (Table 1). The distribution profiles of vanadium, and nickel within the pellets of spent catalyst and in the extrudates prepared by mixing different percentages of spent catalyst with boehmite are shown in Fig. 3a and b. It is seen that in the spent catalyst vanadium concentration is more near the outer edges than at the center of the pellet. Nickel distribution in the spent catalyst is more uniform than that of vanadium. In the prepared catalyst extrudates high edge concentration of vanadium is not seen, and
Fig. 2. Effect of increasing spent catalyst percentages mixed with boehmite on the concentration of metals in the prepared catalysts.
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Fig. 3. Vanadium and nickel distribution profiles within the pellets of spent catalysts and in the catalyst prepared by mixing different percentages of spent catalyst with boehmite: (a) vanadium and (b) nickel.
both V and Ni are more evenly distributed throughout the pellet cross-section with some spots having higher concentration than the others. The processes such as peptization, mixing, kneading and extrusion with an acid that are used to prepare the extrudates appear to influence the distribution of the metals in the alumina support. In Fig. 4, it is seen that the alumina extrudates prepared from boehmite alone without spent catalyst addition has a high surface area around 165 m2/g, and addition of small amounts (up to 15 wt%) of the spent catalyst to boehmite has no appreciable effect on the surface area, but higher amounts (>15%) reduce the surface area of the prepared catalysts. For example, the surface area of the catalyst gradually decreases from 165 to 120 m2/g when the amount of spent catalyst is increased progressively from 15 to 40 wt%. A further increase
in the amount of spent catalyst to 60 wt% leads to drastic reduction in the surface area to 50 m2/g. The pore volume data presented in Fig. 5 indicate a gradual decrease in pore volume with increasing spent catalyst content in the catalyst. Interestingly, a remarkable change in the pore size distribution of the catalysts is noticed with increasing spent catalyst content (Table 2). ˚ diameter region The volume of pores in the 100–200 A ˚ diameter decreases progressively while that in the 200–300 A shows a steady increase. Thus, for example, the pore volume ˚ pores for the contributions from the 100–200 and 200–300 A catalyst prepared with 10% spent catalyst are 0.41 and 0.09 ml/ g, respectively. Increasing the amount of spent catalyst from ˚ pores from 10% to 40% decreases the volume of 100–200 A ˚ pores 0.41 to 0.10 ml/g and increases the volume of 200–300 A
Fig. 4. Effect of mixing different percentages of spent catalyst with boehmite on the surface area of prepared catalysts.
Fig. 5. Effect of mixing different percentages of spent catalyst with boehmite on the pore volume of prepared catalysts.
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Table 2 Pore volume distribution in the catalyst extrudates prepared by using different percentages of spent catalyst Spent catalyst (wt%)
0 15 25 40 60
Pore size ˚ <50 A
˚ 50–100 A
˚ 100–200 A
˚ 200–300 A
˚ 300–500 A
˚ 500–1000 A
˚ >1000 A
Total pore volume (cm3/g)
cm3/g
%
cm3/g
%
cm3/g
%
cm3/g
%
cm3/g
%
cm3/g
%
cm3/g
%
0.001 0.007 0.011 0.005 0.000
0.13 1.29 2.11 1.04 0.00
0.006 0.032 0.034 0.038 0.030
0.88 5.59 6.53 7.52 8.26
0.519 0.418 0.262 0.100 0.105
77.36 73.04 49.84 20.07 28.82
0.087 0.071 0.168 0.280 0.189
13.00 12.35 31.92 56.17 51.91
0.026 0.019 0.020 0.043 0.022
3.91 3.23 3.88 8.62 6.10
0.013 0.007 0.008 0.007 0.003
1.89 1.19 1.50 1.36 0.83
0.019 0.019 0.022 0.026 0.016
0.86 1.15 0.98 1.78 0.58
0.671 0.573 0.525 0.500 0.363
from 0.09 to 0.28 ml/g. In agreement with this, the average pore diameter shows a gradual increase with increasing spent catalyst addition (Fig. 6). The spent catalyst used in the study contained 15.3% carbon and 9.7% vanadium. The surface area and pore volume of the spent catalyst were 18 and 0.1 ml/g, respectively (Table 1). The low surface area and pore volume values indicate that the catalyst is deactivated by pore blockage and fouling of the active catalytic sites by coke and metal deposits. Although, the coke deposits are removed during the calcinations of the extrudates in air, the metal deposits will remain in the pores. Therefore, a gradual decrease in the surface area and pore volume with the increasing amount of spent catalyst addition is not unexpected. It is well known that porosity in catalyst pellets and extrudates originates from the space between the particles. In large pore aluminas and catalysts, the primary particles are larger and larger spaces (pores) are formed between the particles when they are shaped into pellets or extrudates. On the other hand, in small pore materials, the primary particles are smaller and are more closely packed. The sizes of the primary particles and the nature of packing thus determine the pore size of catalyst materials. In spent catalysts, the coke and metal deposits can fill the pores and reduce the pore volume and pore-size. When a spent catalyst containing coke and metal deposits is mixed and extruded with alumina containing both narrow and wide pores, it is likely that a portion of the small particles of spent catalyst with its deposits fill the narrow pores.
The resulting extrudate material will have mainly large pores originally present in the alumina material. However, the pore size distribution data (Table 2) shows a large increase in ˚ diameter pores when the amount of spent the 200–300 A catalyst mixed with boehmite is increased above 20 wt%. These results clearly indicate that pores in this diameter region are created when the spent catalyst is extruded with boehmite and resulting extrudates are dried and calcined. When the spent catalyst is mixed with boehmite and extruded, the resulting catalyst extrudates will contain coke, metals and Al2O3 originally present in the spent catalyst. It is likely that the sizes of the g-Al2O3 crystallites in the spent catalyst are large and they create larger pores when extruded with boehmite. The removal of the carbonaceous matter from the extrudates during the calcination process can also create wide pores in the catalyst. Preparation of large pore alumina supports by mixing and extruding carbon black with boehmite followed by removal of the carbon black by combustion at temperatures around 500 8C from the extrudates has been reported in literature [29]. The side crushing strength of the prepared catalyst extrudates is also influenced by the spent catalyst percentage in the catalyst. In Fig. 7, it is seen that the change in side crushing strength is not appreciable up to 20% spent catalyst addition, but a further increase of spent catalyst amount in the mix to 40% leads to a remarkable increase in the side crushing strength of the prepared catalyst extrudates. The processes such as peptization, kneading, extrusion, drying and calcination that are used for the preparation of catalyst extrudates from
Fig. 6. Effect of mixing different percentages of spent catalyst with boehmite on the pore diameter of prepared catalysts.
Fig. 7. Effect of mixing different percentages of spent catalyst with boehmite on side crushing strength of prepared catalysts.
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boehmite and spent catalyst mix can have effects on the physical properties of the extrudate material, and in consequence can influence the mechanical properties. The total pore volume of the material is gradually reduced during extrusion with spent catalyst. It is generally known that the crushing strength of an extrudate material is inversely proportional to the porosity [30], and the present results are consistent with this. 2.2. Hydrotreating activities of the prepared catalysts The prepared catalysts were tested in a microreactor to evaluate their activity for promoting HDS and HDM reactions in residual oil hydrotreating process. Kuwait atmospheric residue was used as feed for the tests. All tests were conducted at two temperatures (i.e. 370 and 390 8C). The other test conditions and operating procedure are described in detail in the experimental section. Activity tests were also conducted on a reference commercial HDM catalyst and spent catalyst alone under similar operating conditions. The HDS activity of the prepared catalysts with different percentages of spent catalyst mixed with boehmite are presented in Fig. 8. The results demonstrate that the activity of the prepared catalysts for promoting the HDS reactions increases remarkably when amount of spent catalyst in them is increased up to 40 wt%. But a further increase in spent catalyst amount does not lead to any appreciable increase in the activity. Similar results are also noticed in Figs. 9 and 10 for metal (V and Ni) removal by HDM reactions. This is probably due to a large reduction in the surface area and porosity of the catalyst when the amount of spent catalyst is increased above 40%. Interestingly, the catalysts show significantly higher activity for metals removal (HDVand HDNi) than for sulfur removal (HDS). The HDS and HDM activities of the catalyst prepared by mixing 40% spent catalyst with boehmite are compared with that of spent catalyst alone and with that of a commercial reference HDM catalyst in Fig. 11. It is seen that the catalyst prepared from the spent catalyst–boehmite mix is more active than the reference catalyst for promoting both HDS and HDM reactions in residual oil hydrotreating. The spent catalyst alone
Fig. 8. HDS activity vs. spent catalyst percentage in boehmite.
Fig. 9. Vanadium removal activity (HDV) vs. spent catalyst percentage in boehmite.
has a very poor activity for promoting these reactions, probably due to its low surface area and porosity. The results of the present studies show that highly active catalysts for residual oil hydrotreating could be prepared from metals (V, Mo and Ni) containing spent catalysts by mixing them with boehmite. The scientific basis for the high hydrotreating activities of these catalysts is explained below.
Fig. 10. Nickel removal activity (HDNi) vs. spent catalyst percentage in boehmite.
Fig. 11. Comparison of the HDS, HDV and HDNi activities of spent, prepared and reference catalysts.
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Catalysts used for hydrotreating of petroleum distillates and residues normally consist of Mo supported on an alumina carrier with promoters such as Ni or Co [30–33]. Mo alone can promote hydrotreating reactions such as HDS and HDM but its activity is enhanced by the presence of Ni or Co. The synergy between Mo and Ni in promoting hydrotreating reactions has been explained on the basis of the formation of an active phase (Ni–Mo–S), which contains both metals [34,35]. In the catalysts prepared by mixing spent catalysts with boehmite, vanadium is present together with Mo and Ni. The metals (V, Mo and Ni) present in the spent catalysts are distributed and dispersed on the alumina support when they are mixed with boehmite, peptized with nitric acid kneaded and extruded. Vanadium either alone or in combination with Ni or Mo has been reported to exhibit some activity for promoting hydrotreating reactions [36–38]. Several studies have shown that NiV/Al2O3 and VMo/Al2O3 have fairly high catalytic activity for promoting HDM and HDS reactions [39,40]. The activities of these catalysts are; however, lower than NiMo/ Al2O3 catalysts. The catalysts prepared with the addition of up to 40 wt% spent catalyst to boehmite have large pores, and reasonably high surface area (>120 m2/g) and pore volume (>0.5 ml/g). Furthermore, some new active catalyst sites involving different combinations of the three metals (Mo, Ni and V) may be present in them [41]. The presence of such new kind of active sites together with the reasonably high surface area and porosity could be responsible for the high hydrotreating activity of the catalysts prepared by mixing spent catalyst with boehmite. 3. Conclusions In this work, a series of catalysts were prepared by mixing and extruding spent residue hydroprocessing catalysts with boehmite in different proportions. The prepared catalysts were characterized by chemical analysis and by surface area, pore volume, pore size and crushing strength measurements. The HDM and HDS activities of the catalysts were evaluated by hydrotreating tests in a microreactor using Kuwait atmospheric residue as feed. The important conclusions of the studies are as follows. Catalysts prepared from spent catalyst–boehmite blends contained vanadium, molybdenum and nickel. The concentration of the three metals increased linearly with increasing amount of spent catalyst. The surface area and pore volume of the catalysts decreased gradually with increasing amount of spent catalyst up to 40 wt% and then declined drastically, while the crushing strength showed the opposite trend. Catalysts prepared with the addition of up to 40 wt% spent catalyst to boehmite contained large pores together with fairly high surface area (>120 m2/g) and pore volume (>0.5 ml/g). A catalyst prepared by mixing 40 wt% spent catalyst with boehmite was relatively more active for promoting HDM and
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HDS reactions than a reference commercial HDM catalyst in residual oil hydrotreating. The method developed in this study for the preparation of active HDM catalyst from spent catalyst can be used for recycling spent residue hydroprocessing catalyst containing high levels of V and thereby to reduce the environmental problem of spent catalyst waste. Acknowledgements The authors thank the management of the Kuwait Foundation for the Advancement of Sciences (KFAS) for their financial support of the project. The assistance of Ms. Hanadi Al-Sheeha, Ms. Sara Al-Omani and Mr. Inian in the catalyst preparation and characterization experiments is gratefully acknowledged. References [1] [2] [3] [4] [5] [6] [7] [8] [9]
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Studies on recycling and utilization of spent catalysts: Preparation of active hydrodemetallization catalyst compositions from spent residue hydroprocessing catalysts Meena Marafi, Antony Stanislaus * Petroleum Refining Department, Petroleum Research and Studies Center, Kuwait Institute for Scientific Research, P.O. Box 24885, Safat, Kuwait Received 12 April 2006; received in revised form 31 July 2006; accepted 11 September 2006 Available online 10 October 2006
Abstract Spent catalysts form a major source of solid wastes in the petroleum refining industries. Due to environmental concerns, increasing emphasis has been placed on the development of recycling processes for the waste catalyst materials as much as possible. In the present study the potential reuse of spent catalysts in the preparation of active new catalysts for residual oil hydrotreating was examined. A series of catalysts were prepared by mixing and extruding spent residue hydroprocessing catalysts that contained C, V, Mo, Ni and Al2O3 with boehmite in different proportions. All prepared catalysts were characterized by chemical analysis and by surface area, pore volume, pore size and crushing strength measurements. The hydrodesulfurization (HDS) and hydrodemetallization (HDM) activities of the catalysts were evaluated by testing in a high pressure fixed-bed microreactor unit using Kuwait atmospheric residue as feed. A commercial HDM catalyst was also tested under similar operating conditions and their HDS and HDM activities were compared with that of the prepared catalysts. The results revealed that catalyst prepared with addition of up to 40 wt% spent catalyst to boehmite had fairly high surface area and pore volume together with large pores. The catalyst prepared by mixing and extruding about 40 wt% spent catalyst with boehmite was relatively more active for promoting HDM and HDS reactions than a reference commercial HDM catalyst. The formation of some kind of new active sites from the metals (V, Mo and Ni) present in the spent catalyst is suggested to be responsible for the high HDM activity of the prepared catalyst. # 2006 Elsevier B.V. All rights reserved. Keywords: Spent catalysts; HDM catalyst preparation; Waste catalyst management; Residue hydrotreating
1. Introduction Large quantities of catalysts are used in the petroleum refining industry for the purification and upgrading of various petroleum streams and residues. The catalysts deactivate with time and when the activity of the catalyst declines below the acceptable level, it is usually regenerated and reused. But, regeneration is not always possible [1,2] and after a few cycles of regeneration and reuse, the catalyst activity may decrease to very low levels and further regeneration may not be economically feasible. The spent catalysts are discarded as solid wastes [3,4]. The quantity of spent catalysts discharged from different processing units depends largely on the amount of fresh catalysts used, their life and on the amount of the
* Corresponding author. Tel.: +965 3980499; fax: +965 3980445. E-mail address: [email protected] (A. Stanislaus). 0926-3373/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2006.09.005
deposits formed on them during use in the reactors. In most refineries, a major portion of the spent catalyst waste comes from the residue hydrotreating and hydroprocessing units. This is because the catalysts used in these processes deactivate rapidly by coke and metal (V and Ni) deposits, and have a short life [5–7]. Furthermore, technology for regeneration and reactivation of the catalysts deactivated by metal fouling is not available to the refiners. The volume of spent hydroprocessing catalysts discarded as solid wastes has increased significantly in recent years due to a steady increase in the processing of heavier feedstocks containing higher sulfur, nitrogen and metal (V and Ni) contents together with a rapid growth in diesel hydrotreating capacity to meet the increasing demand for low sulfur fuels. At the same time, environmental laws concerning spent catalyst disposal have become increasingly more severe in recent years. Spent hydroprocessing catalysts have been classified as hazardous wastes by the Environmental Protection Agency
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in the USA [8,9]. The most important hazardous characteristic of spent hydroprocessing catalysts is their toxic nature. Chemicals such as V, Ni, Mo and Co present in the catalyst can be leached by water after disposal and pollute the environment. The hazardous nature of the spent catalysts is attracting the attention of environmental authorities in many countries and the refiners are experiencing pressures from environmental authorities for safe handling of spent catalysts. Several alternative methods such as disposal in landfills, reclamation of metals, regeneration/rejuvenation and reuse, and utilization as raw materials to produce other useful products are available to the refiners to deal with the spent catalyst problem [3,4,10– 13]. The choice between these options depends on technicalfeasibility and economic considerations. In recent years, increasing emphasis has been placed on the development of processes for recycling the waste catalyst materials as much as possible. Utilization of spent catalysts as raw materials in the production of other valuable products is an attractive option for recycling spent catalysts from environmental and economical points of view. Spent fluid catalytic cracking (FCC) catalysts have been successfully used in cement production [13,14]. Recently, a process for making highly stabilized non-leachable ceramic materials from spent catalysts has been reported by Sun et al. [15]. A few studies on the preparation of active catalysts from spent catalysts for various applications have been reported in the literature. Lee et al. [16] reported that active reforming catalysts can be prepared using the V, Ni and Mo containing extract obtained by leaching spent catalysts with citric acid. Furimsky [17] found that spent Co– Mo/Al2O3 and Ni–Mo/Al2O3 catalysts, after regeneration, can catalyze the decomposition of H2S. In a recent patented process, Choi et al. [18] used spent hydroprocessing catalysts to prepare active catalysts for reduction of nitrogen oxides. The use of spent catalysts in preparation of active hydrotreating catalysts, have been reported in a few earlier studies [19,20]. However, the spent catalysts used in these earlier works were from petroleum distillates hydrotreating units and contained Mo, Co and Al2O3 without V and Ni. Spent catalyst handling and utilization has been the subject of some investigation in our laboratory. In most of the previous studies, rejuvenation of spent residue hydroprocessing catalysts for reuse was addressed [21–27]. In the present work, utilization of spent residue hydroprocessing catalysts containing high levels of Vand coke together with Mo, Ni and Al2O3 in the preparation of active hydrodemetallization (HDM) catalysts has been considered. HDM catalysts are used in the front end reactors of petroleum residue hydrotreating processes to remove the metals such as V and Ni that are present in the residual oil. The catalyst usually possesses high activity for promoting HDM reactions together with some activity for sulfur removal by hydrodesulfurization (HDS) reaction. The catalysts were prepared by mixing and extruding the spent catalyst powder with boehmite in different percentages, and the effect of mix ratio between spent catalyst and boehmite on the key catalyst properties such as surface area, pore volume, pore size distribution and the metals (V, Mo and Ni) content of the
prepared catalyst samples was examined. The HDM and HDS activities of the catalysts were tested in a high pressure fixedbed microreactor unit using Kuwait atmospheric residue as feed and compared with that of a commercial HDM catalyst. 1.1. Experimental 1.1.1. Catalyst preparation from spent catalyst–boehmite mix The sequence of operational steps used for the preparation of catalyst extrudates from spent catalyst–boehmite mix in the present work are shown in Fig. 1. Boehmite used in the catalyst preparation experiments were obtained from Sasol, Germany. Spent catalyst was obtained from Kuwait National Petroleum Company (KNPC). The spent catalyst was first washed with naphtha and then extracted with toluene in a soxhlet apparatus to remove the residual oil. The oil free-spent catalyst was ground to fine powder in a grinding machine (Christison Particle Technologies Ltd., Model KM 100) and then sieved using a sieve shaker (Endecotts, model OCT Digital 4587-01) with appropriate sieves obtain particle sizes in the range 25– 90 mm similar to that of the boehmite powder. The chemical and physical properties of the oil-free spent catalyst powder were analyzed by various techniques. The characteristic of the spent catalyst are shown in Table 1. A laboratory kneading and extrusion machine (Type: LUK 2.5 AS) manufactured by Werner and Pfleiderer Gmbh & Co., Germany, was used for the preparation of catalyst extrudates from spent catalyst–boehmite mixtures. It contained a mixing chamber, two blades for mixing, and a drive unit with two threephase motors and gears, and a discharge screw. Three hundred grams of spent catalyst–boehmite mixture in the desired ratio was taken in the mixing chamber for each
Fig. 1. Operational steps in the preparation of catalyst extrudates from spent catalyst.
M. Marafi, A. Stanislaus / Applied Catalysis B: Environmental 71 (2007) 199–206 Table 1 Spent catalyst characteristics
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All catalysts were tested under the following operating conditions:
Catalyst property
Values
Mo (wt%) V (wt%) Ni (wt%) C (wt%) Al2O3 (wt%) Surface area (m2/g) Pore volume (ml/g) Side crushing strength (lb/mm)
4.5 9.7 4.0 15.3 43.5 18 0.10 2.9
experiment. Then 185 ml of dilute nitric acid (e.g., 2%), as a peptizing reagent, was added in drops at a constant rate to the boehmite powder, and the two were mixed and kneaded. The blades in the mixing chamber were counter rotating and turned at different speeds. They were designed and arranged for intensive mixing and kneading of the boehmite with the nitric acid to form a good, extrudable paste. At the end of the mixing and kneading time (20–30 min), the product was extruded by means of the discharge screw through a die containing several holes (1.5 mm in diameter) to form catalyst extrudates. The extrudates were dried at 110 8C in an oven for 24 h. The dried extrudates were calcined at 370 8C for 2 h, 450 8C for 3 h and 500 8C for 2 h. After calcinations the prepared extrudates were cooled in desiccators and characterized. 1.1.2. Catalyst characterization The concentrations of V, Mo, and Ni in the spent catalyst and in the prepared catalyst samples were determined by inductively coupled plasma atomic emission spectroscopy (Varian Liberty II ICP-AES). A scanning electron microprobe X-ray analyzer (model: EPMA JXA 8600MX) from Joel was used for measuring the distribution profiles of the metals across the catalyst pellets. Surface areas of the catalysts were determined by BET method using an autosorb adsorption unit manufactured by Quantachrome Corporation, USA. A mercury porosimeter (Quantachrome Poremaster-60) was used for pore volume and pore size distribution determination in catalyst samples. The method involves intrusion of mercury into the pores at high pressures of up to 60,000 psi. A side crushing strength measuring apparatus designed and manufactured by AKZO Nobel (Model No. 120794-108) was used to determine the side crushing strength of the catalyst pellets. 1.1.3. Catalysts activity testing Hydrotreating activities of the prepared catalysts were tested in a high pressure fixed bed microreactor unit using Kuwait atmospheric residue as feed. The feedstock contained 4.3 wt% sulfur, 0.27 wt% nitrogen, 69 ppm vanadium, 21 ppm nickel, 3.6 wt% asphaltenes and 12.4 wt% CCR. Thirty millilitres of the catalyst diluted with an equal amount of carborundum was used for each run. The catalysts were presulfided using 1% CS2 in straight run gas oil by a standard procedure [28], before introducing the feed. After presulfiding, the test conditions were adjusted to desired operating temperature, pressure, hydrogen to oil ratio and liquid hourly space velocity (LHSV).
pressure ¼ 120 bar;
LHSV ¼ 1 h1 ;
H2 =oil ratio ¼ 1000;
temperature ¼ 370 and 390 C For each run, product samples were collected every 24 h for analysis of S and metals (Ni, V) content. Sulfur content was determined using an Oxford Model 3000 XRF instrument. The concentrations of V and Ni in the oil were determined without ashing using a Varian Liberty Series II, ICP spectrophotometer. 2. Result and discussion 2.1. Characteristics of the catalysts prepared by mixing spent catalyst with boehmite The catalyst extrudates prepared with the addition of spent catalyst to boehmite in different proportions were subjected to chemical analysis to determine the concentration of Mo, Ni and V in them. The results are plotted in Fig. 2. It is seen that the concentration of the metals in the prepared catalysts increases linearly with increasing percentage of spent catalyst in the mix. A larger increase is noticed for V than the other metals. A catalyst prepared from a mix containing 20% spent catalyst and 80% boehmite contains 1.84% V, 0.91% Mo and 0.77% Ni. Increasing the amount of spent catalyst in the mix from 20 to 40% increases the V content to 3.8% whereas, the Mo and Ni concentrations increase to 1.85% and 1.47%, respectively. These results are consistent with the higher concentration of V than Mo and Ni in spent catalyst (Table 1). The distribution profiles of vanadium, and nickel within the pellets of spent catalyst and in the extrudates prepared by mixing different percentages of spent catalyst with boehmite are shown in Fig. 3a and b. It is seen that in the spent catalyst vanadium concentration is more near the outer edges than at the center of the pellet. Nickel distribution in the spent catalyst is more uniform than that of vanadium. In the prepared catalyst extrudates high edge concentration of vanadium is not seen, and
Fig. 2. Effect of increasing spent catalyst percentages mixed with boehmite on the concentration of metals in the prepared catalysts.
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Fig. 3. Vanadium and nickel distribution profiles within the pellets of spent catalysts and in the catalyst prepared by mixing different percentages of spent catalyst with boehmite: (a) vanadium and (b) nickel.
both V and Ni are more evenly distributed throughout the pellet cross-section with some spots having higher concentration than the others. The processes such as peptization, mixing, kneading and extrusion with an acid that are used to prepare the extrudates appear to influence the distribution of the metals in the alumina support. In Fig. 4, it is seen that the alumina extrudates prepared from boehmite alone without spent catalyst addition has a high surface area around 165 m2/g, and addition of small amounts (up to 15 wt%) of the spent catalyst to boehmite has no appreciable effect on the surface area, but higher amounts (>15%) reduce the surface area of the prepared catalysts. For example, the surface area of the catalyst gradually decreases from 165 to 120 m2/g when the amount of spent catalyst is increased progressively from 15 to 40 wt%. A further increase
in the amount of spent catalyst to 60 wt% leads to drastic reduction in the surface area to 50 m2/g. The pore volume data presented in Fig. 5 indicate a gradual decrease in pore volume with increasing spent catalyst content in the catalyst. Interestingly, a remarkable change in the pore size distribution of the catalysts is noticed with increasing spent catalyst content (Table 2). ˚ diameter region The volume of pores in the 100–200 A ˚ diameter decreases progressively while that in the 200–300 A shows a steady increase. Thus, for example, the pore volume ˚ pores for the contributions from the 100–200 and 200–300 A catalyst prepared with 10% spent catalyst are 0.41 and 0.09 ml/ g, respectively. Increasing the amount of spent catalyst from ˚ pores from 10% to 40% decreases the volume of 100–200 A ˚ pores 0.41 to 0.10 ml/g and increases the volume of 200–300 A
Fig. 4. Effect of mixing different percentages of spent catalyst with boehmite on the surface area of prepared catalysts.
Fig. 5. Effect of mixing different percentages of spent catalyst with boehmite on the pore volume of prepared catalysts.
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Table 2 Pore volume distribution in the catalyst extrudates prepared by using different percentages of spent catalyst Spent catalyst (wt%)
0 15 25 40 60
Pore size ˚ <50 A
˚ 50–100 A
˚ 100–200 A
˚ 200–300 A
˚ 300–500 A
˚ 500–1000 A
˚ >1000 A
Total pore volume (cm3/g)
cm3/g
%
cm3/g
%
cm3/g
%
cm3/g
%
cm3/g
%
cm3/g
%
cm3/g
%
0.001 0.007 0.011 0.005 0.000
0.13 1.29 2.11 1.04 0.00
0.006 0.032 0.034 0.038 0.030
0.88 5.59 6.53 7.52 8.26
0.519 0.418 0.262 0.100 0.105
77.36 73.04 49.84 20.07 28.82
0.087 0.071 0.168 0.280 0.189
13.00 12.35 31.92 56.17 51.91
0.026 0.019 0.020 0.043 0.022
3.91 3.23 3.88 8.62 6.10
0.013 0.007 0.008 0.007 0.003
1.89 1.19 1.50 1.36 0.83
0.019 0.019 0.022 0.026 0.016
0.86 1.15 0.98 1.78 0.58
0.671 0.573 0.525 0.500 0.363
from 0.09 to 0.28 ml/g. In agreement with this, the average pore diameter shows a gradual increase with increasing spent catalyst addition (Fig. 6). The spent catalyst used in the study contained 15.3% carbon and 9.7% vanadium. The surface area and pore volume of the spent catalyst were 18 and 0.1 ml/g, respectively (Table 1). The low surface area and pore volume values indicate that the catalyst is deactivated by pore blockage and fouling of the active catalytic sites by coke and metal deposits. Although, the coke deposits are removed during the calcinations of the extrudates in air, the metal deposits will remain in the pores. Therefore, a gradual decrease in the surface area and pore volume with the increasing amount of spent catalyst addition is not unexpected. It is well known that porosity in catalyst pellets and extrudates originates from the space between the particles. In large pore aluminas and catalysts, the primary particles are larger and larger spaces (pores) are formed between the particles when they are shaped into pellets or extrudates. On the other hand, in small pore materials, the primary particles are smaller and are more closely packed. The sizes of the primary particles and the nature of packing thus determine the pore size of catalyst materials. In spent catalysts, the coke and metal deposits can fill the pores and reduce the pore volume and pore-size. When a spent catalyst containing coke and metal deposits is mixed and extruded with alumina containing both narrow and wide pores, it is likely that a portion of the small particles of spent catalyst with its deposits fill the narrow pores.
The resulting extrudate material will have mainly large pores originally present in the alumina material. However, the pore size distribution data (Table 2) shows a large increase in ˚ diameter pores when the amount of spent the 200–300 A catalyst mixed with boehmite is increased above 20 wt%. These results clearly indicate that pores in this diameter region are created when the spent catalyst is extruded with boehmite and resulting extrudates are dried and calcined. When the spent catalyst is mixed with boehmite and extruded, the resulting catalyst extrudates will contain coke, metals and Al2O3 originally present in the spent catalyst. It is likely that the sizes of the g-Al2O3 crystallites in the spent catalyst are large and they create larger pores when extruded with boehmite. The removal of the carbonaceous matter from the extrudates during the calcination process can also create wide pores in the catalyst. Preparation of large pore alumina supports by mixing and extruding carbon black with boehmite followed by removal of the carbon black by combustion at temperatures around 500 8C from the extrudates has been reported in literature [29]. The side crushing strength of the prepared catalyst extrudates is also influenced by the spent catalyst percentage in the catalyst. In Fig. 7, it is seen that the change in side crushing strength is not appreciable up to 20% spent catalyst addition, but a further increase of spent catalyst amount in the mix to 40% leads to a remarkable increase in the side crushing strength of the prepared catalyst extrudates. The processes such as peptization, kneading, extrusion, drying and calcination that are used for the preparation of catalyst extrudates from
Fig. 6. Effect of mixing different percentages of spent catalyst with boehmite on the pore diameter of prepared catalysts.
Fig. 7. Effect of mixing different percentages of spent catalyst with boehmite on side crushing strength of prepared catalysts.
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boehmite and spent catalyst mix can have effects on the physical properties of the extrudate material, and in consequence can influence the mechanical properties. The total pore volume of the material is gradually reduced during extrusion with spent catalyst. It is generally known that the crushing strength of an extrudate material is inversely proportional to the porosity [30], and the present results are consistent with this. 2.2. Hydrotreating activities of the prepared catalysts The prepared catalysts were tested in a microreactor to evaluate their activity for promoting HDS and HDM reactions in residual oil hydrotreating process. Kuwait atmospheric residue was used as feed for the tests. All tests were conducted at two temperatures (i.e. 370 and 390 8C). The other test conditions and operating procedure are described in detail in the experimental section. Activity tests were also conducted on a reference commercial HDM catalyst and spent catalyst alone under similar operating conditions. The HDS activity of the prepared catalysts with different percentages of spent catalyst mixed with boehmite are presented in Fig. 8. The results demonstrate that the activity of the prepared catalysts for promoting the HDS reactions increases remarkably when amount of spent catalyst in them is increased up to 40 wt%. But a further increase in spent catalyst amount does not lead to any appreciable increase in the activity. Similar results are also noticed in Figs. 9 and 10 for metal (V and Ni) removal by HDM reactions. This is probably due to a large reduction in the surface area and porosity of the catalyst when the amount of spent catalyst is increased above 40%. Interestingly, the catalysts show significantly higher activity for metals removal (HDVand HDNi) than for sulfur removal (HDS). The HDS and HDM activities of the catalyst prepared by mixing 40% spent catalyst with boehmite are compared with that of spent catalyst alone and with that of a commercial reference HDM catalyst in Fig. 11. It is seen that the catalyst prepared from the spent catalyst–boehmite mix is more active than the reference catalyst for promoting both HDS and HDM reactions in residual oil hydrotreating. The spent catalyst alone
Fig. 8. HDS activity vs. spent catalyst percentage in boehmite.
Fig. 9. Vanadium removal activity (HDV) vs. spent catalyst percentage in boehmite.
has a very poor activity for promoting these reactions, probably due to its low surface area and porosity. The results of the present studies show that highly active catalysts for residual oil hydrotreating could be prepared from metals (V, Mo and Ni) containing spent catalysts by mixing them with boehmite. The scientific basis for the high hydrotreating activities of these catalysts is explained below.
Fig. 10. Nickel removal activity (HDNi) vs. spent catalyst percentage in boehmite.
Fig. 11. Comparison of the HDS, HDV and HDNi activities of spent, prepared and reference catalysts.
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Catalysts used for hydrotreating of petroleum distillates and residues normally consist of Mo supported on an alumina carrier with promoters such as Ni or Co [30–33]. Mo alone can promote hydrotreating reactions such as HDS and HDM but its activity is enhanced by the presence of Ni or Co. The synergy between Mo and Ni in promoting hydrotreating reactions has been explained on the basis of the formation of an active phase (Ni–Mo–S), which contains both metals [34,35]. In the catalysts prepared by mixing spent catalysts with boehmite, vanadium is present together with Mo and Ni. The metals (V, Mo and Ni) present in the spent catalysts are distributed and dispersed on the alumina support when they are mixed with boehmite, peptized with nitric acid kneaded and extruded. Vanadium either alone or in combination with Ni or Mo has been reported to exhibit some activity for promoting hydrotreating reactions [36–38]. Several studies have shown that NiV/Al2O3 and VMo/Al2O3 have fairly high catalytic activity for promoting HDM and HDS reactions [39,40]. The activities of these catalysts are; however, lower than NiMo/ Al2O3 catalysts. The catalysts prepared with the addition of up to 40 wt% spent catalyst to boehmite have large pores, and reasonably high surface area (>120 m2/g) and pore volume (>0.5 ml/g). Furthermore, some new active catalyst sites involving different combinations of the three metals (Mo, Ni and V) may be present in them [41]. The presence of such new kind of active sites together with the reasonably high surface area and porosity could be responsible for the high hydrotreating activity of the catalysts prepared by mixing spent catalyst with boehmite. 3. Conclusions In this work, a series of catalysts were prepared by mixing and extruding spent residue hydroprocessing catalysts with boehmite in different proportions. The prepared catalysts were characterized by chemical analysis and by surface area, pore volume, pore size and crushing strength measurements. The HDM and HDS activities of the catalysts were evaluated by hydrotreating tests in a microreactor using Kuwait atmospheric residue as feed. The important conclusions of the studies are as follows. Catalysts prepared from spent catalyst–boehmite blends contained vanadium, molybdenum and nickel. The concentration of the three metals increased linearly with increasing amount of spent catalyst. The surface area and pore volume of the catalysts decreased gradually with increasing amount of spent catalyst up to 40 wt% and then declined drastically, while the crushing strength showed the opposite trend. Catalysts prepared with the addition of up to 40 wt% spent catalyst to boehmite contained large pores together with fairly high surface area (>120 m2/g) and pore volume (>0.5 ml/g). A catalyst prepared by mixing 40 wt% spent catalyst with boehmite was relatively more active for promoting HDM and
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HDS reactions than a reference commercial HDM catalyst in residual oil hydrotreating. The method developed in this study for the preparation of active HDM catalyst from spent catalyst can be used for recycling spent residue hydroprocessing catalyst containing high levels of V and thereby to reduce the environmental problem of spent catalyst waste. Acknowledgements The authors thank the management of the Kuwait Foundation for the Advancement of Sciences (KFAS) for their financial support of the project. The assistance of Ms. Hanadi Al-Sheeha, Ms. Sara Al-Omani and Mr. Inian in the catalyst preparation and characterization experiments is gratefully acknowledged. References [1] [2] [3] [4] [5] [6] [7] [8] [9]
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