Review pubs.acs.org/EF
A Review of the Unconventional Methods Used for the Demetallization of Petroleum Fractions over the Past Decade Annie C. Jenifer, Princy Sharon, Arushi Prakash, and Priya C. Sande* Department of Chemical Engineering, Birla Institute of Technology & Science (BITS), Pilani, Rajasthan 333031, India ABSTRACT: Metals in petroleum fractions have a deleterious effect on the refining equipment and catalyst. They corrode the equipment, plug the catalyst pores, and create irregularities in the system. With reference to today’s petroleum industry, the fast depletion of light crude has created the need to process heavier crudes with much higher metal content. Hence, the need to remove traces of harmful metals and restrict their accumulation in feed stocks has become a pressing issue. In this context, we review developments over the past decade in regard to the methods of metal removal with a focus on unconventional methods, such as the use of molecularly imprinted polymers, supercritical fluids, and irradiation. Other methods of demetallization discussed include the usage of nanocatalysts, micro-organisms, and certain solvents. The removal of vanadium and nickel were reviewed most, because of the abundance of these elements in the crude.
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INTRODUCTION Crude oil contains traces of up to 1000 ppm of various metals such as vanadium (V), nickel (Ni), cobalt (Co), copper (Cu), and arsenic (As).1−3 Metals are generally concentrated in the higher boiling fractions of crude oil (above 540 °C or 1000 °F).4,5 Among the metals, Ni and V are the most abundant in natural petroleum, of which V constitutes ∼80% of metal concentration.4,6,7 Ni is generally found in the +2 valence state.8 Both Ni and V are found as porphyrinic structures and all vanadium present is reportedly porphyrinic.9,10 Reportedly, all of the vanadium detected by very high-resolution mass spectra techniques was associated with porphyrins.11,12 The relative amount of metals (in all forms) to other major impurities present in crude is shown in Table 1.
the highly polar and highly aromatic portions of crude, making separation even tougher.16 Upon degrading porphyrin compounds, metals are released into the aqueous phase and become water-soluble.17 Metal porphyrin trapped within asphaltene aggregates hinder their reactivity and extraction.18 A further complication is that the porphyrins are highly volatile, aggravating demetalization processes.19 Several porphyrin structures were studied in the context of demetallization, as shown in Table 2. These metal compounds were detected using atomic absorption spectroscopy (AAS), electron spin resonance (ESR), neutron activity analysis, and inductively coupled plasma (ICP) emission spectroscopy.20−23 The presence of metals is undesirable, mainly because they cause poisoning and deactivation of the catalysts used in the refining processes.2,4,9,17,26−29 Metal contaminants in FCC feeds are potentially more harmful than sulfur, which can be converted to gaseous forms and readily handled, whereas the nonvolatile metal contaminants tend to accumulate in the unit during cracking and eventually get deposited on the catalyst, together with the coke.33 Metals deposit on the catalyst and decrease catalyst surface area by plugging their pores, thereby rendering them inactive.34 Metals promote dehydrogenation reactions during the cracking sequence, which results in an increase in the production of coke and dry gas and decrease in gasoline production.28−30,34 The hydrotreatment process has a lower catalyst lifetime when heavy metals are present in the crude, because of the above-mentioned reasons. High amount of metals in the crude makes production costly by increasing the amount of hydrogen needed for the process and by increasing yield loss due to carbon rejection. Guard reactors, employed with appropriate catalysts, are used before the hydrocracking units to control the metal content in the feed for hydrocracking. Up to 90% of metal removal can be achieved using catalysts containing Ni, Co, Mo.35
Table 1. Typical Concentration of Some Impurities in Different Types of Petroleum Feedstocka Impurity Concentration extra light light heavy extra heavy a
S (wt %)
N (wt %)
Ni, V (ppm)
0.02−0.2 0.05−4 0.1−5 0.8−6.0
0−0.01 0.02−0.5 0.2−0.8 0.1−1.3
