Article pubs.acs.org/IECR
Recycling and Recovery Routes for Spent Hydroprocessing Catalyst Waste H. Al-Sheeha, Meena Marafi, Vira Raghavan, and Mohan S. Rana* Kuwait Institute for Scientific Research, P.O. Box 24885, Safat 13109 Ahmadi, Kuwait ABSTRACT: This study investigates the recovery of Mo, V, and Ni metals from the industrial spent hydroprocessing catalyst. These catalysts are not viable to regenerate mainly due to the metal deposition. The study was carried out on industrial spent residue hydroprocessing (ARDS) catalysts that contained high levels of metals. In the extraction process, metals were recovered through pyrometallurgical and hydrometallurgical routes. The possibility of recycling of total spent catalyst (TSC) was studied using various steps such as deoiling, drying, grinding, sieving, and decoking. In the subsequent steps, the digested spent catalysts were treated with acid−base reactions in order to separate the various components of the spent catalyst. Using various leaching reaction conditions such as acid−base concentration, reaction pH in aqueous as well as organic mediums was studied. The metals were leached out in the solution while the alumina support was recovered as bulk solid in the form of boehmite. The recovered alumina is further treated hydrothermally and recovered as boehmite. Samples were characterized by surface area, pore volume, and pore size distribution measurements. Hence, recovery of valuable metals from the spent catalysts is an attractive option for their recycling and utilization. Therefore, TSC recovery is not only important from an environmental point of view but also very vital from an economic viewpoint.
1. INTRODUCTION In recent years, due to the depletion of light crude, poor quality (high metals, high asphaltene, and low API gravity) crude oil is being explored and processed in petroleum refineries. It has been also estimated that about 80% of the petroleum streams pass though hydroprocessing in order to improve fuel quality. Thus, depending on the type of feedstock and process unit severity, the cycle length of a fixed bed hydroprocessing catalyst is typically between 6 months and 2.5 years. Moreover, in order to maintain the quality of the product, time-on-stream is compensated by a progressive increase in catalyst bed temperature. When the catalyst fails to produce the product of the required quality, the catalyst is deactivated and requires regeneration or replacement. Usually, catalyst deactivation is due to three main causes: carbon (coke) laydown, active phase sintering, and metal poisoning. In recent years, catalysts have played a key role in the refining of petroleum to produce clean fuels and many other valuable products.1,2 Therefore, hydroprocessing catalysts have become widely accepted and are even preferred by the industry due to a number of reasons. One reason is that they result in purification of various petroleum streams, particularly for the upgrading of heavy oils and residues.3 In the residue hydrotreating process, the catalysts which consist of Mo with promoter Co (Ni) on alumina support enhance the removal of undesirable impurities such as sulfur, nitrogen, and metals by hydrodesulfurization, hydrodenitrogenation, and hydrodemetallization reactions, respectively.4−6 Hence, catalysts used for this process are deactivated rapidly by coke and metal (V and Ni) deposits and have a short life (6−12 months). Spent hydroprocessing catalysts are the major solid wastes of refinery industries and contain various hazardous components, such as Al, V, Mo, Co, Ni, As, and Fe as well as nonmetallic elements such as elemental sulfur, carbon, and oils that are potentially hazardous to the environment.7 In the refinery, one © 2013 American Chemical Society
of the main causes of industrial pollution is the discharge or disposal of spent hysroprocessing catalysts, which contains valuable components of the catalyst. Spent hydroprocessing catalysts recovery have gained importance due to environmental regulations which registers spent catalyst as hazardous waste materials. For example, Kuwait refinery produces ca. 6000 ton/y spent catalyst from bottom of barrel (ARDS) processes, which contain valuable metals such as molybdenum, vanadium, nickel, cobalt, etc. Thus, besides the environmental issues with disposal, spent catalysts containing high metal concentrations can serve as secondary raw materials. A variety of processing approaches for recovering metals from the spent catalysts has been proposed.2 Techniques such as direct smelting, calcination and smelting, chlorination, and salt roasting are applied for metal recovery from spent hydroprocessing catalysts.8,9 Also, many reagents, such as NaOH, H2SO4, NH3, (NH4)2SO4, and oxalic acid with H2O2 and Fe(NO3)2, have been tested.10−12 Hence, the disposal of spent catalyst poses an unavoidable environmental issue,13 which essentially requires high capital investment, huge swaths of land, and massive efforts. Kuwait, at present, is producing around 2.6 million bbl/d (Mbpd) of conventional crude oil. It has a refining capacity of about 936 000 bbl/d in its three refineries, which is likely to increase to 2 Mbpd with the addition of the fourth refinery. Kuwait has a total atmospheric residue (AR) hydrotreating capacity of around 216 000 bpd.14 The Kuwait oil industry has already embarked on plans to expand the exploitation of Kuwait heavy oil reserves, including Lower Fars. Plans are on to construct a Received: Revised: Accepted: Published: 12794
June 17, 2013 August 12, 2013 August 14, 2013 August 14, 2013 dx.doi.org/10.1021/ie4019148 | Ind. Eng. Chem. Res. 2013, 52, 12794−12801
Industrial & Engineering Chemistry Research
Article
The recovery of metals and support from spent catalyst obtained from the Kuwait refinery hydroprocessing ARDS unit (reactors three and four) is described in Figure 1, by using three different options. The separation of metals was carried out in a typical experiment; about 5 g of the powdered decoked spent catalyst was mixed with the selected acid/base reagent and agitated in an ultrasonic bath for the required period of time and temperature and then filtered. Each process has various stages in order to recover metals (Mo, Ni, V) as well as alumina. The spent catalyst was deoiled, decoked, crushed, and ground to fine powder (size citric acid > H 2SO4 > HNO3 ≫ CH3COOH
Oxalic acid was highly effective for Mo and V extraction but showed very poor activity for Ni extraction, which followed the order: EDTA > citric acid = H2SO4 > HNO3 > CH3COOH > 12799
dx.doi.org/10.1021/ie4019148 | Ind. Eng. Chem. Res. 2013, 52, 12794−12801
Industrial & Engineering Chemistry Research
Article
Figure 7. Effect of complexing agent on the metal recovery (at 50 °C and 6 h and 10% concentration of acid).
The dechelation process takes place by using H2SO4 (pH = 1) to break the metal complex, and EDTA recovered as H4EDTA in the solid form and aqueous solution contains metals in leached liquor (Mo, V, Ni, and Al). The EDTA recovery is about 95%, which makes this process highly economically viable. The effect of different acids/complexing agents indicates that the organic acids are substantially more significant than the inorganic acids for extraction. Apart from the extraction capacity, organic acids showed advantages such as low concentration, high efficiency of metal extraction, ease of stripping, and low corrosive environment. Thus, organic acid based processes are more effective, recoverable, and reusable, which has great potential for full scale application.
oxalic acid. In case of aluminum extraction, both organic and inorganic acids showed poor activity. Using acid leaching, the recovery of Al was only 37%, 29%, and 25% with oxalic, sulfuric, and nitric acids, respectively. The effect of different organic acids can be understood on the basis of their Ka (acid dissociation constant) values, which is a good indicator of the degree of ionization. The used organic acid has monoprotic (acidic acid, Ka = 1.8 × 10−5), diprotic (oxalic acid, Ka = 5.9 × 10−2 and 6.4 × 10−5), triprotic (citric acid, Ka = 8.4 × 10−4, 1.8 × 10−5, 4.0 × 10−6), and tetraprotic acid (EDTA, Ka = 1.0 × 10−2, 2.2 × 10−3, 6.9 × 10−7, 5.5 × 10−11). The lower the Ka value, the greater is the acid strength and the smaller is the conjugated base produced by the acid. Therefore, Ka values are referred to as the ionic strength of the solution that is required for the interaction of a particular metal cation, e.g., [Mn+ + EDTA4− ↔ [M(EDTA)](n−4)+]. The presence of more than one metal ion leads to different formation constants of the leaching solvent (e.g., metal−EDTA complexes). In the case of EDTA, a pair of unshared electrons capable of complexing with a metal ion is contained on each of two nitrogens and four −COOH groups (six complexing groups). The amino nitrogens are more basic and are protonated (−NH3+) more strongly than carboxylate groups, which bind to the metal atoms by losing their protons. The results of the metal extraction studies with different acids showed that EDTA was very effective in the extraction of the three major metals, namely, Mo, V, and Ni present in the spent catalyst. Citric acid was moderately active in the extraction of all three metals with extraction percentages in the range of 88−93%. The inorganic acids were relatively less effective than the organic acids in the extraction of Mo and V, but they were found to be equally good for Ni extraction. Large amounts of organic acids (oxalic and citric) are required to recover metals, which make it less costeffective. However, EDTA can be recovered by H 2SO4 treatment; thus, this process can be efficiently used in the commercial applicability of the metal recovery of spent catalyst.
4. CONCLUSIONS The recovery of spent hydroprocessing catalyst not only is to reuse or recover the catalysts components but also lead to the environmentally safe disposal of the waste spent catalyst. Three main metal recovery options were studied by using pyrometallurgical and hydrometallurgical options in a combination in order to maximize the metal recovery. The recovery of boehmite or γ-alumina after recovering the metals (V, Mo, Ni) is an economical process that provides a method of total recycling of the spent catalyst waste material and a complete solution to the environmental problem of the hazardous spent catalyst waste management. The ammoniacal solution (basic) leaching showed poor separation of metal and produced impure alumina. The soda roasting method revealed that Na2CO3 is more effective than NaOH leaching, as molybdenum and vanadium are selectively extracted over nickel and aluminum. The residue can be further separated as nickel and bulk of solid alumina with high purity. The inorganic acids were relatively less effective than the organic acids for the extraction of Mo and V, but they were equally good for Ni extraction. Oxalic acid exhibited the highest leaching efficiency toward Mo and V 12800
dx.doi.org/10.1021/ie4019148 | Ind. Eng. Chem. Res. 2013, 52, 12794−12801
Industrial & Engineering Chemistry Research
Article
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metals but very poor recovery for Ni. EDTA is very effective for Mo, V, and Ni recoveries.
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AUTHOR INFORMATION
Corresponding Author
*Address: Kuwait Institute for Scientific Research (KISR), Petroleum Research Center, Ahmadi, P.O. Box 24885, Safat 13109, Ahmadi, Kuwait. Tel.: +965 2495-6890. Fax: +965 23980445. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors wish to thank the Kuwait Foundation for the Advancement of Science (KFAS) for their financial support (project: PF037C). We gratefully acknowledge the helpful suggestions of Dr. A. Stanislaus during the initial stage of this research.
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REFERENCES
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