Review of Demetallisation

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from 1.1 to 2.8 wt.%. In another study on the isolation andcharacterization of nickel and vanadyl porphyrins in ArabianHeavy crude residue [535 C+], Ali et al. [17]reported 189 ppmof vanadium and 62 ppm of nickel in the residue.

Vanadium and nickel are thought to occur in petroleum in twoforms; porphyrinic and nonporphyrinic. Little is known about the

nature of nonporphyrins. However, the porphyrins have beenextensively studied not only because of their deleterious effects

but also for their considerable role as geochemical markers[17].Vanadium, nickel and iron are sometimes collectively known asheavy metals. The heavy metals (Me) in crude oil residue areagglomerated in asphaltenes in the form of porphyrin compounds(Fig. 1). The molecular weight of this type of compound varies

between 420 and 520 i.e., from C27N4C33N4[18].The metalloporphyrins were the first compounds claimed to be

of conclusive biological origins. Treibs discovered in the year1934 that a wide variety of petroleum and bitumen contain

porphyrins[19]. The subject of metal porphyrins in petroleum

was reviewed extensively and published[20

22].Crude oils containing large proportions of metals arefrequently treated for their removal, as these substances tendto accumulate in the residuum during distillation and affect its

properties adversely. Some of the organometallic compoundsmay also volatilize at refinery distillation temperatures in thehigher-boiling distillates. The presence of metal contaminants inFCC feeds presents another and potentially more serious

problem because although sulfur can be converted to gaseousforms which can be readily handled in an FCC unit, the non-volatile metal contaminants tend to accumulate in the unit andduring the cracking process they are deposited on the catalysttogether with the coke. Because both nickel and vanadium

exhibit dehydrogenation activity, their presence on the catalystparticles tends to promote dehydrogenation reactions during thecracking sequence and this result in increased amounts of cokeand light gases at the expense of gasoline production. Vanadiumand nickel seriously affect cracking, when they accumulate onthe particles of the catalyst over time, causing alteration in thecatalyst structure [23]. Also, small amounts of nickel andvanadium in the charge stocks poison clay and synthetic cata-lysts. Metals in fuel oils produce ash, when the oil is burned. Ashdeposits in engines result in abrasion of the moving parts of theengines, and the ash is injurious to the walls of the boilers andfurnaces. Residues are demetallized due to these and several

more disadvantages of the large concentration of the metals inthe heavy petroleum residue.

The metals in crude oil are usually present in twocombinations[4]:

i. Zinc, titanium, calcium, and magnesium are usually present

in combination with naphthenic acid as soaps.ii. Vanadium, copper, nickel, and part of the iron are present as

oil soluble porphyrin-type compounds.

Distillation concentrates the metallic compounds in theresidue. The majority of the metal compounds may be

precipitated along with the asphaltenes by hydrocarbonsolvents. Thus the removal of asphaltenes with n-alkanereduces the metal content of the oil by up to 95%.

Since the most common metal contaminants are nickel andvanadium, which are generally present in the form of porphyrinsor asphaltenes and are concentrated in residues, much work has

been devoted to the treatment of crude oil residues for theirremoval. Both physical and chemical methods are used. Thephysical method is essentially deasphalting. The chemicalmethod includes thermal processes such as visbreaking andcoking; and chemical treatment. In the deasphalting process, thelighter oils are physically separated from heavier asphaltenes bymixing the heavy oil/residue with a very low boiling solventsuch as propane, butane or isobutene. The thermal processes are

basically the reshuffling of the hydrogen distribution in theresidue to produce lighter products containing more hydrogenwhile the asphaltenes and metals are removed in the form ofcoke or visbreaking residue.

2. Physical methods

distillation, solvent extraction and

filtration

Distillation separates crude oils into fractions according toboiling point, so that each of the processing units following willhave the feedstock that meet their particular specifications. Themetallic constituents concentrate in the residues.

The short-path distillation of atmospheric residues fromCalifornia crude oil at 358 C removed 98% of metallopetropor-

phyrins. The vapor phase contained metal complexes 92% ofwhich were metallopetroporphyrins. The hydrotreatment of thisdistillate diluted with gas oil in a fixed bed of a low-activity

catalyst removed all the metallopetroporphyrins. The spectro-scopic analysis of metallopetroporphyrins remaining in thedistillate-gas oil after very mild treatment suggested that theywere degraded to chlorins which were intermediates in eitherthermal or catalytic residuum demetallization[24].

Yamada et al. [25] described a method of extraction by heatingthe oils to 177238 C, refluxing the oils with an organic solvent,and centrifuging the mixture. Thus, a residual oil containing159 ppm V was heated [3/min to 182216 C for 1 h], refluxedwith hexane, and then centrifuged to give a hexanesoluble portioncontaining 0 ppm V, vs. 105 ppm when the hexaneoil mixturewas not heated.

The lowering of kinematic viscosity and concentrations ofmetals in heavy oils by visbreaking was improved by adding

R''

R'''

R''

R

R

N

N

N

NMe

R'

Fig. 1. Example of porphyrin complex.

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aqueous ammonia solution to the oil. Thus, visbreaking Melonescrude at 232 C and 6895 kPa for 260 s with equal weight of watergave a product with viscosity 410 cP. The removal of Ni, V, and Sin this case was 12%, 13%, and 5%, respectively. When the watercontained also 0.7% ammonia (on crude wt.) the product viscosity

became 180 cP and the removal of Ni, V, and S was 26%, 20%,

and 14%, respectively. The use of ammoniacontaining waterinstead of water only decreased coke deposition in the visbreakerfrom 0.61% to 0.27%[26].

The extraction methods of deasphalting with liquid hydrocar-bon or gasses such as propane, butane or isobutene are veryeffective and used by refineries for removal of metals fromvacuum residues. In practice, a solvent deasphalting unit asso-ciated with an oil refinery mixes residual oil produced by arefinery with a light hydrocarbon solvent in which the asphaltenesand most of the metals remain insoluble and are removed from the

product stream. Thus, removal of the asphaltenes with n-pentanereduced the vanadium contents of the residual fuel oil by up to

95% with substantial reductions in the amount of iron and nickel[27].Observation of porphyrinmetal complexes in the propane

deasphalted raffinate of a mid-continent crude oil prompted anintensive investigation of their identities and characteristics.Chromatographic and solvent-extraction methods were combinedto isolate the complexes. The vanadium and nickelporphyrincomplexes were identified by spectrophotometric analysis of theextracts[28].

Ni- and V-containing metalloporphyrins in heavy petroleumfeedstocks were removed by solvent extraction with a solventchosen from g-butyrolactone, acetonitrile, phenol, furfural, 2-

pyrrolidone, dimethylformamide, pyridinewater mixtures,

ethylene carbonate, propylene carbonate, ethylene trithiocarbo-nate, and dimethylsulfoxide: the solvent was regenerated bycontacting it with a highly aromatic oil stream. Solventextraction was carried out at 2793 C; regeneration was carriedout at b52 C lower than the extraction step, suitably withvacuum gas oils, vacuum residues, and (preferably) catalyticcracking residues. A heavy Arab vacuum residue [b. 510677 C] was extracted 3 times with sequential portions of 1:2(wt. ratio) 1-butyrolactone residue, resulting in 97% removal ofvanadium. The choice of solvent depends on a solubility

parameter characterized by certain values of dispersion forces,dipole-coupled forces, and H-bonding forces. Preferred solvents

are ethylene carbonate, propylene carbonate, ethylene trithio-carbonate, and dimethylsulfoxide[29].

A petroleum atmospheric residue was deasphalteddemetal-lated by precipitation with individual n-C5-8alkanes, petroleumether, EtOAc, or BuOAc. EtOAc gave the highest metal (V+Ni)removal of 89.5 wt.% at a solventfeed ratio of 8:1. The metals,

particularly V, were concentrated mainly in the asphalteneprecipitate[30].

Treatment of fractions derived from Boscan asphaltenes withlithiumethylenediamine, with Raney nickel, and electrolysis inlithium chlorideethylene diamine effected reduction, desul-furation, decrease in vanadium and metalloporphyrin contents,and altered solubilities. Loss in vanadium correlated with loss inmetalloporphyrins[31].

In recent years, processes based on membrane technologythat reduce the viscosity, asphaltene content, and the sulfur andmetals content of both heavy oil and residues have beendescribed. Most of these methods use membranes for therecovery of solvent from an initial extraction, phase separation,or dilution step. Only a few reports are available on the direct

use of membranes for the removal of asphaltene, sulfur, andmetals from heavy oil.

Kutowy et al.[32]presented a method of filtration to removehigh molecular weight contaminants and inorganic substancesfrom aliphatic hydrocarbon liquids by passing through amembrane system having an outer layer, on the high-pressureside, of a microporous membrane of an aliphatic hydrocarbon-swellable, polysulfone compound. The viscosity of thehydrocarbon liquid was reduced by heating or by mixing witha solvent to b600 cP before being contacted with the membrane.The method was found to be especially suitable for removal of

N, S, Al, Cr, Cu, Ni, V, and asphaltenes from spent diesel,

lubricating oil, crude oil, heavy oils or bitumen.Arod et al.[33]of Energie Atomique, France, described theultrafiltration of a vacuum residue at high temperature [330 C]using a ceramic membrane with an average pore diameter of10 nm. The membrane was operated at a cross-flow velocity of5.6 m/s and a trans-membrane pressure of 500 kPa. Theasphaltene content of the heavy oil was reduced from 6.3 to4.1 wt.%. The vanadium content was also reduced (from 195 to90 ppm) and the permeate flux was reportedly at 667 L/m2 perday. However the effect of time on-stream and membranefouling was not examined, and it is not clear if the permeate fluxwas stable during the operation.

Osterhuber[34]of Exxon developed a method for upgrading

heavy oils by solvent dissolution and ultra-filtration at highpressure. The process is especially suitable for removing tracemetals (mainly Ni and V) and lowering the Conradson CarbonResidue (CCR) of the resulting oil. The process includes thesteps of diluting the heavy oil with a solvent (such as toluene)and subjecting the resulting mixture to an ultra-filtration stepusing selected membranes. Preferred membranes include thoseof modified cellulose and polyvinylidine fluoride, such as

Nuclepore type F. In one of the examples, the author tested theultrafiltration of Arabian Heavy vacuum residue mixed withtoluene at a weight ratio of 1:1. The vanadium and nickelcontents of the feed were reduced from 180 and 43 to 25.8 and

7.1 ppm in the permeate fraction.Duong et al. [35] of the University of British Columbia,

described an ultra-filtration study on Cold Lake heavy oilusing ceramic membranes with average pore diameter of lessthan 0.1 m. The ultra-filtration experiments were carried outunder relatively mild conditions [600 kPa and 110 C]. Theresults showed a significant reduction in density, viscosity, and

Ni and V contents. However, the permeate flux data showed astrong effect of fouling that significantly reduced the permeateflux but increased asphaltene retention. Experimental datashowing the effects of membrane pore diameter and cross flowvelocity were interpreted in terms of membrane fouling,initially by the porerestriction mechanism and later with gellayer formation.

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3. Chemical methods

The basic chemical concept of demetallization is toselectively remove the metal from the organic moiety withminimal conversion of the remaining petroleum. The deme-tallization of metalloporphyrins by acid is a reversible reaction

and can be represented by the following equation:

PMHXPHMX

Treatment of petroleum fractions with sulfuric acid (H2SO4)has been used commercially for many years. The objective ofthis treatment was to remove sulfur, nitrogen, metals andvarious hydrocarbon types to improve the quality of products. Itwas first employed for refining animal and vegetable oils, andsince the beginning of the petroleum industry, about onehundred and fifty years ago, sulfuric acid was successfully usedin various purification steps of petroleum products. A number of

patents for the use of sulfuric acid were issued at the verybeginning of the petroleum industry [3638]. An earlyreference to the use of sulfuric acid was made by Marcussonand Eickmann in 1908, who first precipitated the asphaltenesfrom asphaltic materials by treatment of the sample with low

boiling naphtha, followed by fractionation of the naphtha-soluble material with concentrated sulfuric acid[15].

In the treatment of petroleum residues with sulfuric acid,some of the acid is almost always reduced to sulfur dioxide.This sulfur dioxide may also react with some of the unsaturatedhydrocarbons, forming various addition products and thuscomplicating still further the nature of the basic reactions. The

basic reactions of sulfuric acid with olefins and substituted

aromatics have been discussed extensively and it is a generalknowledge that the reaction products may be of the type ofsulfones, polysulfones, aromatic sulfonic acids and/or hetero-

polymeric gums. The fact that sulfuric acid reacts with andpromotes reactions of hydrocarbons is a drawback to its use asan agent for removing metals, sulfur and nitrogen from oil. Thehydrocarbon reactions increase the quantity of acid required anddecrease the yield of fuel products.

Hydrofluoric acid has been found to be the most effectiveacid as a demetallization reagent. Metals can be removed (90%)with a high yield of the liquid fraction (8590 wt.%).Hydrochloric, sulfuric, sulfonic, polyphosphoric, hydrobromic,

and other acids also have some activity, particularly in thepresence of hydrocarbon solvents. Unfortunately, acids have anumber of disadvantages, including extensive side reactions and

product contamination[39].Demetallization of petroleum feedstocks was carried out by

mixing and demulsification of hydrocarbon oils with aqueousHCl or HNO3solutions containing a demulsifying agent at 2014 C, which transferred the metal to the aqueous phase.Demetallization was carried out at pH 27, which wascontrolled by addition of ammonia water, at a 350:100 vol.ratio of water to hydrocarbon oils, and a 0.35:1 molar ratio ofHCl or HNO3[40].

Kukes and Battiste [41] of Phillips Petroleum usedphosphorus acid (H3PO3) to remove Ni and V from heavy

oils. It is believed that H3PO3reacts with the metals containedin the hydrocarbons to form oil insoluble compounds that can beremoved either by filtration, centrifugation, or settling/decan-tation. The demetallization of Venezuelan Monagas oil (Vcontent 335 ppm and Ni content 98 ppm) was studied in a stirredautoclave using variable amounts of phosphorus compounds at

atmospheric pressure and various temperatures for a period ofone hour. After cooling the mixture in the autoclave, it was

passed through a glass filter and analyzed for Ni and V byatomic absorption spectrometry and plasma emission spectrom-etry. H3PO3 was more effective in removing Ni and V,

particularly V from heavy oil, than a known demetallizingagent, phosphoric acid (H3PO4). In the range of 24 wt.% acid,H3PO3was far superior to H3PO4in removing Ni and V.

Eidem[42]demonstrated a method for reducing the metalscontents of petroleum feed stock by H3PO4. 250 g sample ofvacuum residue [b.N538 C] was stirred and heated under anitrogen gas blanket to 150 C, 1 wt.% aq. H3PO4was added,

the mixture was blended under nitrogen, and the temperaturegradually raised to 260 C; the sample, after 44 min wasseparated into maltenes fraction and asphaltenes fraction. Thetotal contents of Fe, Ni, and V in the maltenes and asphaltenesfractions of H3PO4treated residue were b0.06, 14.5, and 9.70(vs. 64, 101, and 63.3, respectively in the original residue).

Heavy hydrocarbon feedstocks were demetallized by contactat 375450 C with active carbon that has previously beentreated with a highly acidic oxidizing fluid. Thus, a commercialactive carbon (C) was contacted at 2025 C with a solution of3.3 wt. parts conc. H2SO4and 1 wt. part conc. HNO3in a ratioof 4 cm3 solution/g active C. The temperature rose to 130 C,and after 20 min the C was washed free of acid and dried at

140150 C. The residual oils from both the atmosphericdistillation and the vacuum flashing of a Gulf Coast crude oilwere combined to give a feedstock containing 33 ppm Ni and99 ppm V. The feedstock (10 wt. parts) was combined with 1 wt.

part treated active C, and the mixture was heated 5 h at 400 C.Upon cooling the product yielded two phases separable bydecantation: a thick dense lower phase (20 vol.%) and a morevolatile, less-dense, less-viscous upper phase (80 vol.%). Theupper phase contained 5 ppm Ni and 45 ppm V. When the

procedure was repeated in the absence of the treated active C,the upper low-metal-containing phase was reduced in volume to30 vol.%[43].

The concentration of metal contaminants (e.g., Ni and V) inhigh-boiling petroleum distillates was reduced by treating anatmospheric residue in a contacting zone with SO2 or SO2

precursors (e.g., H2SO3) at 93232 C for 0.015 h. Thetreated residue was vacuum distilled and separated into adistillate [b.p.=271 C] having relatively low metals contentand a residue having a relatively high metal content. Thus, anatmospheric residue was treated with SO2 at 173 C for 2 h.After heat soaking, the product vacuum distillate [b. b312 C]had a V content of 1.36 wt. ppm, vs. 1.70 wt. ppm for thedistillate prepared without SO2treatment[44].

Michlmayr [45] of Chevron Research Co. studied theupgrading of metal-contaminated oils containing vanadiumand nickel. The oil contained 80 ppm of vanadium, 25 ppm of



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nickel, and 4 ppm of iron. The oil stream was subjected to anextraction with an aqueous solution of ferric or stannic saltssuch as ferric or stannic chloride soluble in acidified water. Theaqueous solution was maintained at a pH at least sufficientlyacidic to inhibit precipitation of the agent as hydroxide or basicsalt. The experiments demonstrated the effectiveness of ferric

slats as agents for removing vanadium and nickel. Ferric nitratein nitric acid was very effective in the demetallization; howeverit caused some increase in the viscosity of the oil, presumably

by promoting the oxidation and/or polymerization of theunsaturated components of the oil.

Garwood[46]presented a process for the removal of sulfurby treatment with H2SO4 and the removal of vanadium bytreatment with MgSO4. Residual oil from a storage tank in anelectric generating plant was contacted with 90 wt.% sulfuricacid at a ratio of 0.005 gallon acid/gallon of oil. The oil feed ratewas 0.5 gallon/min, and the rotational speed of the contact ro-tor was 7500 rev./min. Total sulfur was reduced from 2.6 to

0.02 wt.%. The desulfurized oil was then treated with 20 wt.%aqueous solution of magnesium sulfate to remove the sodiumand vanadium compounds. The ratio of solution used was aboutone volume of solution per 10 volumes of oil. Vanadium wasreduced from 395 to 24 ppm, and sodium was reduced from 21to 2 ppm.

Metal salts were removed from crude petroleum andpetroleum fractions by extraction into an aqueous solutionusing a demetallization agent and a demulsifying agent. Thefeedstock was emulsified at 30140 C for 0.5 s to 10 min inorder to transfer the metal to the aqueous phase. Thedemetallization agent is a C1-4-fatty acid-containing byproduct

prepared by air oxidation of paraffin waxes. Solvent extraction

was carried out at a 350:100 vol. ratio of the water phase(containing 1040 wt.% fatty acids) to the hydrocarbon phase,at a 0.510:1 molar ratio of demetallization agent to total Ca2+.Demulsifying agent was present at a ratio of 1100 ppm to thehydrocarbon phase[47].

Greaney and Polini[48]reported a process for demetalliza-tion of a petroleum stream by contacting a metal-containing

petroleum feed in the presence of an aqueous base selected fromGroup IA and IIA hydroxides and carbonates and NH4OH and/or (NH4)2CO3, an oxygen containing gas, and a phase-transferagent at 180 C for a time sufficient to produce a treated

petroleum feed having a decreased metal content. The method

enhances the value of petroleum feeds that traditionally havelimited use in refineries due to their metals, e.g., Ni and Vcontents.

Hurter [49] studied the removal of trace metals and ashcontaminants from vacuum residue or used lubricating oil. Thehydrocarbon stream was treated with an aqueous solution of ananion capable of reacting with the metal to form water-solublesalts such as aqueous solutions of ammonium chloride or nitricacid and NaCl. The reaction between the anion solution and thecontaminated oil was conducted at an elevated temperature,

preferably at or just below the boiling point of water. The ratioof solution to oil was about 1:9. The ash phase formed wasseparated by a centrifuge filter to remove particulate matter witha specific gravity such that it floated in the oil. A centrifuge

filter to remove particulate matter with a specific gravity suchthat it floated in the oil separated the ash phase formed. Fe, Ca,

Na, and Al showed maximum reduction for a mixture oflubricating oil, fuel oil, and lubricating oil sludge.

The use of boron trifluoride ether as a precipitant for as-phaltenes was studied experimentally[50]. When 2.5% to 4% of

this reagent was mixed with Kuwait crude oil, the reagentsettled with about 8% of the asphaltenes and aromatichydrocarbons. The tar layer was then distilled to recover

boron compound. The metal contents of the crude oil wasreduced by about half and the percentage of carbon residue byabout 30%.

Gould [51] described oxidative demetallization of petro-leum asphaltenes and residua. Cold Lake asphaltenes,Arabian Heavy asphaltenes and Cold Lake vacuum residuumwere treated with a variety of oxidizing agents. Of these,reagents such as air at 100 C and NaOH/air were found tohave no appreciable demetallization activity while oxidants

such as sodium hypochlorite and peroxyacetic acid exhibitedhigh demetallization activity coupled with the ability toremove or destroy petroporphyrins. The sodium hypochlorite,however, was found to suffer from the disadvantage ofcausing chlorine incorporation into the feed. This oxidativedemetallization appears to be rather unselective reaction with

both metals and porphyrin removal being proportional to theamount of oxidant used. Various peroxy acids were found to

be effective.The demetallization was done by adding 0.53 wt.% cumene

hydroperoxide at 80250 C to petroleum fractions b. N250 C,which resulted in increased amounts of asphaltenes, containing9396% of the initial amount of heavy metals and thus the

remainder of the petroleum fractions for cracking was purifiedfrom catalytic poisons. The increased amounts of asphalteneswere due to polymerization, polycondensation, and oxidationcaused by cumene hydroperoxide[52].

Ni and V were removed from heavy petroleum oil bypyrolysis at 182288 C in the presence of a hydrogenatingcompound containing N rings, and removal of a residue fromthe pyrolysis product. Thus, a 20:60 (g) mixture of a vacuum-distillation residue (288 ppm V) and 1,2,3,4-tetrahydroquino-line was autoclaved [5 ml/min to 216 C for 60 min], cooled,and filtered, 83% of the V was removed [53].

Ni and V were removed from hydrocarbon feedstocks, par-

ticularly heavy residues destined to be hydrocracked, byblending them with ZnCl2 and TiCl4 [2.04.5 lb/bbl (2.04.5 ppb) oil] and treating the resulting mixtures with hydrogenat approx. 288482 C. 70% of the Ni and V contaminants wereconverted to oil-insoluble forms with coke formation beingb3 wt.%. Thus, a heavy Iranian residue [b. 262558 C; 9 wt.%C residue; 41 ppm Ni and 126 ppm V] was hydrogenated at343 C, 10342500 kPa, 2 h residence time, and 7000 rpmstirring rate in the presence of ZnCl2 [4.2 ppb]. The resulting

products were an oil [b. = 260 C; yield 96.9 vol.%; containing11 ppm Ni and 20 ppm V] and a coke product (0.8 wt.%).Experiments showed that ZnCl2 and TiCl4 were superior toFeCl3 and AlCl3 as catalysts in demetallating the heavyresiduum[54].



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Strong chlorinating compounds such as Cl2, SOCl2, or inor-ganic salts like FeCl2, SnCl2, ZnCl2, TiCl4, RuCl3, CrCl3,COCl2, or their aqueous solutions have been employed fornickel and vanadium removal from heavy oils. Up to 70% of themetals are reported to be removed at temperatures ranging from40 to 300 C. The metals were converted to insoluble

constituents and removed by filtration. The use of thesereagents, however, results in chlorine and metal incorporationinto the production and, therefore, degrades rather than improvesthe quality of the oil (U.S. Patent nos: 4148717, 4043900,4039432, 3996130, 3483117).

Baird and Bearden [55]used sodamide to desulfurize anddemetallize heavy hydrocarbons at elevated temperatures in the

presence of H2. The salt produced after the reaction wasseparated from the desulfurized feedstock by filteration.Safaniya atmospheric resid was treated with 9.9% sodamideat 3447.5 kPa and 370 C, resulting in 68% desulfurization and77% demetallization. A detailed method for sodamide regen-

eration by electrolysis from the sodium sulfur salt was alsopresented. Additionally, recycling for reaction with additionalfeed was described. The methods include salt conversion tosodium polysulfide, nitrate, or chloride and electrolysis thereof.Another improved process for residual oil treatment withmolten sodium under H2pressure between 1013 and 2026 kPaand above 400 C was studied by Bearden [56]. Sodiumtreatment of Safaniya resid reduced sulfur to 0.2% and Ni+ V toless than 1 ppm compared to 3.91 wt.% and 97 ppm in the feed,respectively.

Myers et al.[57] reported an improved sodium desulfurizationand demetallization process that was developed by a consortiumof three companies comprising Imperial Oil Resources (Esso),

Exxon R&E Company, and AEA Technology. The technologywas used for the treatment of high-sulfur bitumen.

Greaney et al. [58] of Exxon demonstrated a method fordecreasing the metal contents of petroleum streams by forminga mixture of the petroleum fraction containing these metals andan essentially aqueous electrolysis medium and passing anelectric current through the mixture at a voltage, pH, and timesufficient to remove metals such as Fe, Ni, and V from thestream. The electrochemical cell used in this study was acommercially available coulometry cell consisting of a mercury

pool cathode, a platinum wire anode, a standard calomelreference electrode, and a glass stirring paddle. One of the

examples in the patent discusses the electro-demetallization ofArabian Light atmospheric residue having an API of 14 degrees.A sample weighing 1.7 g of this residue was mixed with 10 mltoluene and added to an aqueous solution of 40 wt.% tetra-butylammonium hydroxide (20 ml) placed in the electrochemicalcell. The solution was purged under nitrogen [1 atm(101.33 kPa)]. The applied potential was set at 2.5 V andthe solution was stirred. After 18 h, the stirring was stopped andthe aqueous/residue mixture was allowed to separate. Thetoluene was evaporated and the treated residue was analyzed forvanadium, nickel, and iron, which showed removal of 53%,50%, and 65%, respectively.

V and Ni were removed from heavy oils and otherhydrocarbon feeds by treatment with red phosphorus (P).

Thus, red P (0.55.0 wt.%) added to the feed in the presence ofair or hydrogen for 1 h reacted with Ni and V in the feed at204 C and 6894.8 kPa to form oil-insoluble compounds thatwere removed by physical separation methods. Ni and V were072% and 60100% removed respectively and the productcontained 1009000 ppm P[59].

Phosphorus compounds, both organic and inorganic, werefound to be effective for vanadium removal from heavy oils andresidues at temperature in excess of 370 C. The rate ofdemetallization with the phosphorus compounds was found todepend on the asphaltene. The solubility of the phosphoruscompounds and particularly the steric effects of the groupattached to the phosphorus are very important parameters. Theeffectiveness of the additives declined in the following order:

RO3PNAr2POHNAr3PNAr3PONNH42HPO4

where R = alkyl and Ar = phenyl groups.

The phosphorus compounds were found to be very activeand selective towards vanadium removal from heavy oils eitherat elevated temperatures [390420 C] or at lower temperatures[260370 C] if most of the asphaltenes were removed. Partialdissociation of the vanadium compounds from the asphalteneswas thought to be a limiting step in the chemical demetallizationreactions at elevated temperatures[60]. It was also interesting tonote that whereas the phosphorus compounds are effective atvanadium removal, the nickel compounds remained almostunaffected.

Organophosphate esters and light solvents were used toremove metals from heavy petroleum refining residues. Thus,99.0% V and 89.8% Ni removal was accomplished from a

Primrose crude residue (b.N343 C, containing 116 ppm Ni,375 ppm V, and 13.9% Ramsbottom C) by hot extraction[210 C, 6205.5 kPa] with n-pentane containing di-methyl

phosphite (0.057:1 agentoil ratio)[61].Demetallization tests were carried out with more than 200

chemicals (chelating, methylating, halogen-containing, inor-ganic oxides, redox, and P-containing) agents for heavy

petroleum crudes oils. P-containing compounds reacted withV components of asphaltenes forming insoluble products whichcan be removed. Thus, treating Monagas pipeline oil with 3%P4S3at 400 C removed 90% V and 18% Ni. The most activedemetallation agents such as inorganic oxides, unfortunately,

produced also a variety of side reactions (e.g., cracking,polymerization, element incorporation)[60].Heavy hydrocarbon oils were demetallized by contacting the

feed oils with an aqueous solution containing chelating agentsuch as EDTA,N-(hydroxyethyl)ethylenediamido triacetic acid,

N-[2-{bis(carboxymethyl)amino}-ethyl]-N-(2-hydroxyethyl)glycine, diethylenetriamine pentaacetic acid or its salts. Theaqueous solution was preferably adjusted to a pH of =2. thus avacuum distillation residual oil was treated with a 27% EDTAaqueous solution (pH 4.5), resulting in the removal of Ca 99%,Fe 35%, Ni 4% and V 3%[62].

A demetallization method for raw hydrocarbons (e.g., crudeoils) comprised (1) dosing the feed with aqueous solution of achelating agent containing phosphate salts, an emulsion breaker,



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and solid wetting agents, and (2) applying an electric field forliquidliquid separation to effectively remove Na, Ca, Mg, Fe,etc. metals. Preferred phosphate salt was (NH4)3PO4; emulsion

breakers were AP 221 and BP 2040[63].Vanadium (8092%) was removed from petroleum crude

(0.03 wt.% vanadium) by complexing with 0.0010.09 mol%

TiCl4, FeCl3, ZnCl2, CuCl2, CuBr2, CuSO4, or AlCl3 at 3090 C for 560 min. There was a correlation between the redox

potentials of the ligands and their reactivities [64].Petroleum residual oil was treated with molten Sn or Sn

alloys [450600 C]. The molten Sn or Sn alloys adsorb heavymetals in the residual oil. Thus, 650 g Sn was melted [500 C] ina reactor and into this was sprayed [at 1.6 ml oil/min, with106 ml min/Ar(g) for 30 min] a residual oil containing V 110,

Ni 33, and S 5 ppm through nozzles at the bottom of the moltenSn. The product was gas 8.7%, coke 14.8%, oily material76.5%. The oily material contained V 7 and Ni 3 ppm and the

by-product coke contained V 177 ppm. When Pb was used, the

product was gas 8.4%, oily material 78.4%, and coke 12.9%.The oily material contained V 27 and Ni 10 ppm and the cokecontained V 600 ppm[65].

Recently, Reynolds[66]suggested various methods for thepretreatment of heavy oils to remove metals before subjectingthe oils through the high costs processes of hydrogen addition.Surrogate metal solutions containing the metal bound as

petroporphyrin, or petroporphyrin fractions isolated fromheavy crude oils were treated with a variety of chemical agents,and then washed with aqueous solutions to remove the metal.The most effective reagents were found to be maleic acid indimethylformamide, montmorillonite in 1-metyl naphthalene(1-MN), CF3SO3H in 1-MN and FSO3H in 1-MN. A more

detailed examination and effectiveness of low concentrations ofsuperacids, fluorosulfonic acid, and trifluoromethane sulfonicacid, as demetallation agents for nickel and vanadium porphyrincompounds was conducted.

4. Catalytic hydroprocessing

Catalytic hydroprocessing is a hydrogenation process used toremove compounds containing nitrogen, sulfur, oxygen and/ormetals from liquid petroleum fractions. These compoundsadversely influence equipment and catalysts in the refinery. Areduction in the amount of metals in the oil is accomplished by

the process of hydrodemetallization (HDM), where themolecules that contain metals lose these atoms by reactions ofhydrogenation. The products of HDM reactions can accumulatein the catalyst pores, causing the formation of deposits whichend up obstructing those pores irreversibly, blocking access tothe catalyst sites and leading to a progressive loss of catalyticactivity [67]. Therefore, hydroprocessing units are installed

prior to units for processes such as catalytic reforming andcatalytic cracking so that the expensive catalyst is notcontaminated by untreated feedstock. One of the most practicaland effective methods of feedstock demetallization (especiallyfor vanadium and nickel) is the use of HDM catalysts. It has

been shown that the most active HDM catalysts are thoseprepared from synthetic aluminium oxide or natural aluminium

silicate enriched with the oxides of molybdenum, cobalt andnickel. The natural aluminosilicate activated with sulfuric acidwas found to be best at removing vanadium and nickel [68].Further, HDM catalysts with very small amounts of oxides ofactive metals belonging to sixth and seventh subgroups of the

periodic table of the elements were found to be several times

more effective than the usual hydrodesulfurization catalysts.The use of model compounds in reaction kinetic studies has

provided valuable insight into the fundamental processesoccurring in resid HDM. The reaction of Ni and V porphyrinsunder commercial HDM conditions involves a sequentialmechanism on two distinct types of catalytic sites. The

porphyrins are initially hydrogenated, forming precursorspecies which subsequently undergo ring cleavage reactions,depositing the metal on the catalyst surface:

MPMPH2 Y deposithydrocarbon

where P represents the starting porphyrin. Metal depositionoccurs from the dehydrogenated metalloporphyrin intermediate(M-PH2). The resulting metal deposition ultimately deactivatesthe catalyst through fouling and pore blockage.

Investigation of nonporphyrin metallic compounds has notbeen very extensive, and their behavior during HDM are notknown. It is expected that these compounds will behave roughlythe same as porphyrin compounds. There are indications that thenonporphyrin metallic compounds may cause more severe

problems than the porphyrins.Catalytic method reported by Wieckowska et al. [69]for the

demetallation of petroleum, petroleum products, and petroleumrefining residues comprised contacting the feed with a SiO2and/

orAl2O3, or silicagel-adsorbent (pore size 1001000) activatedwith a mineral acid (esp. H2SO4) followed by contacting the feedwith an Fe catalyst at 249397 C with simultaneous introductionof H2at a flow rate equal to that of the feed. Absorbent-catalystwas prepared by treating silica gel 100 g with H2SO4 30 g, dryingof the activated silica gel, and combining 1/3 of the activatedadsorbent with 2/3 volume parts of a catalyst containing Fe2O3.The activated adsorbent and catalyst were placed in a reactorfollowed by feeding heavy petroleum distillate 100 kg containingV, Ni, and Fe totally 80 ppm. Hydrogen 300 m3 was fed to thereactor and the temperature was kept at 397 C. The contact timewas 1.8103 s. The product contained V, Ni, and Fe, 8 ppm in

total amount.Bowes et al. [70] of Mobil oil described a method to

demetallize and desulfurize residual oils by adding to the oil anaromatic solvent and contacting the mixture in the presence ofhydrogen with an alumina having an average pore size greaterthan about 220 . Two samples of Arabian Heavy and Lightresids were mixed with ortho-xylene in ratios of 1:8 and 1:4.The samples were demetallized and desulfurized by pressuriz-ing them in an autoclave for one hour at 350 C and 6890.1 kPa.

Aldridge et al. [71] of Exxon achieved the removal ofmetallic contaminants from heavy hydrocarbons using vanadi-um oxide supported on activated carbon. A schematic of the

process and the effects of process variables were provided inthis patent. Several examples cited in the patent showed



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removal of vanadium and nickel from heavy hydrocarbons suchas a 2030% cut of Arabian Heavy vacuum resid at 5268 kPaand 290 C. The reaction was highly selective with minimaloccurrence of other reactions. Hydrogen consumption was only50 to 150 scf/bbl (1.4164.248 m3/bbl). The results showed thatthe activity of vanadium removal can be increased by increasing

the percentage of vanadium on the activated carbon support.Rankel[72]of Mobil oil used activated carbon catalysts to

treat heavy oils in the presence of hydrogen. The main objectivewas to reduce the content of nickel and vanadium in the feed-stock and achieve conversion of the carbon residue for pro-ducing a lighter oil. An atmospheric resid containing 4.2 wt.%sulfur, 104 ppm vanadium, and 32 ppm nickel was treated in atrickle bed micro unit reactor. The trickle bed was charged withdifferent types of commercially activated carbon catalysts andoperated at 400 C under hydrogen pressure. The hydrotreating

process typically achieved at least about a 23% reduction of theoriginal metal content (i.e., Ni and V content) together with

significant reductions in the sulfur content and Conradson Car-bon Residue.A process for the hydrotreating of heavy hydrocarbons in

supercritical fluids was demonstrated by Piskorz et al.[73]ofNatural Resources Canada. This single-step process usessupercritical fluids and activated carbon as a catalyst. Examplescited include the treatment of Athabasca bitumen (sulfur content5.44 wt.%, Ni+V 300 ppm) with n-dodecane as a solvent in ahydrogen atmosphere. Sulfur content was reduced to 1.16%with complete demetallization. Three runs were performed atdifferent pressures, all with a 1:1 by mass mixture of bitumenand n-dodecane as feed; the hydrogen feed ratio was 1220 m3

hydrogen (STP) per m3 of liquid feed. The same weight of

catalyst was used in all tests.Heavy oil is hydrodesulfurized and demetallized by

treatment in a first stage with hydrogen in the presence of acatalyst with fine pores, and in a second stage in the presence ofa catalyst with larger pores and a specified pore size dis-tribution. Thus, second stage treatment of heavy oil containing53.4 ppm Vover a catalyst containing CoO 4.5%, MoO316.0%,and Al2O379.5% with pore vol. 0.525 mL/g and average poreradius 84 removed approximate 60% V, compared withapproximate 40% for a catalyst with pore volume 0.47 mL/gand average pore radius 35 [74].

A CoMo catalyst for hydrodesulfurizationhydrodemetal-

lization of petroleum distillation residues is supported on Al2O3 which has been calcined at 8711093 C for 0.2510 h, to induce the formation of high-temp.- or-phase Al2O3.The catalyst has total pore vol. 0.40.65 cm3/g and has 10% ofits pore volume in pores with diameter 150300 . Examples ofthe preparation of the catalyst from and Al2O3are given in[75].

Mobil Oil Corp., USA developed a catalyst for demetallizingand desulfurizing hydrocarbons. The catalyst contained 110wt.% of an Fe-group metal (Co or Ni) and 525 wt.% of aGroup VIB metal as the oxides or sulfides on a calcined [373497 C] support containing 85% Al2O3(boehmite) and 0.57.0wt.% of a rare earth. At least 60% of the pore volume of thecatalyst was comprised of 80200 pores. The demetallized

hydrocarbons then underwent catalytic cracking, hydrocrack-ing, or coking. Thus, a catalyst contained 3.5 wt.% CoO, 10 wt.%MoO3, and 1.5, 3, or 6 wt.% rare earth. A Lago atmosphericresiduum containing 235 ppm V and 2.12 wt.% S was treatedover the catalyst after it had been calcined at varioustemperatures [281466 C ] V removal was 56.671.1 wt.%

and S removal 48.755.9 wt.%[76].Residfining, an Exxon catalytic desulfurization-demetalliza-

tion process for heavy petroleum fractions, was extended by aseries of catalyst improvements and engineering to includehydroconversion of distillation residues [b. N566 C][77].

The heavy petroleum oils, containing heavy metals (e.g., Ni)and S compounds, were catalytically hydrodesulfurized [300450 C, 50250 kg/cm2, 0.14.0 h/1 liquid space velocity,2001500:1 H-oil vol. ratio] and then demetallized in a mag-netic field. Thus, a vacuum distillation residue was hydrode-sulfurized [400 C, 150 kg/cm2, 0.5 h/1 liquid space velocity,1000:1 H-oil vol. ratio) in a fluidized bed of zeolite catalyst and

then demetallized in a magnetic field. About 80% of the Scompounds and 45% of the Ni were removed[78].Heavy and sour petroleum fractions were hydrodemetallized

in a bed of sulfurized Al2O3 catalyst. Thus, a catalyst wasprepared by passing (CH3)2S2 [250 C, 1 h/1 space velocity)through a bed of Al2O3granules (surface area 188 m

2/g, porevol. 0.77 cm3/g). The catalyst was then used to hydrodeme-tallize a refining residue (d. 1.025, Conradson carbon 18%, S5%, V 120 ppm, Ni 55 ppm). At 410 C the removal of S, V, and

Ni reached 15%, 85%, and 68%, respectively[79].Chen and Massoth[80] performed hydrodemetallization of

vanadium and nickel porphyrin model compounds over sulfidedcobaltmolybdenum/alumina catalyst in a batch stirred auto-

clave at several temperatures with changing hydrogen (H)pressure and initial porphyrin concentration. A hydrogenatedintermediate leading to deposited metal was found for bothreactants. The time course of the reaction followed pseudo firstorder in reactant concentrations atN350 C but followed lowerorder at lower temperatures. Runs at different initial concentra-tions showed that reaction was inhibited by adsorption ofreactant and products. HDM rates increased with temperatureand H pressure and were very low without catalyst or H present.An apparent activation energy of 24 kcal/mol for the overalldisappearance was obtained for both reactants. Kinetic analysisof HDM of Ni porphyrin (NiP) showed, in addition to a pathway

through the hydrogenated intermediate, an apparent directpathway to hydrocarbon products. The latter was interpreted interms of a direct conversion of adsorbed NiP to products in asingle adsorption step, without desorption of hydrogenatedintermediate. Evidence was obtained to show that the reactionorder in porphyrin was less than unity at a lower temperature,increasing to first order above about N350 C.

Crude oils that could not be easily or economicallytransported or processed using conventional facilities, withmicrocarbon residue content (ASTM D 4530)N0.1 wt.%, totalacid no. (TAN)N0.1, and with N10 wt.% material boiling in thevacuum gas oil range (ASTM D 5307), were hydrotreated andhydrodemetalated in the presence of a supported Group VIBmetal catalyst, esp. Mo and W on Al2O3, with median pore



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diameterN180 . Additional elements of the catalysts werederived from Groups VIB, Group VIII and Group VA elements.The crude oil product had a microcarbon residue content ofb90% of the initial microcarbon residue content, b10% of theinitial TAN, and 70130% of the initial vacuum gas oil content[81].

A cracking unit for cracking of petroleum refining residuesincluded a riser reactor and a stripper/separator with adjustableoutlets in flow communication with separate regeneration unitsfor regeneration of cracking catalysts and adsorbents. Theadsorbents, which were used for removal of Conradson carbon

precursors, high-molecular weight nitrogen compounds, andimpurity metals (esp. Ni and V) in the residues were injectedinto the riser reactor such that they came into contact with freshresidue feedstock prior to coming into contact with fresh and/orregenerated cracking catalyst. Suitable adsorbents were selectedfrom microspheres derived from calcined clay, calcined andcrushed coke, magnesium oxide, silica-alumina, and a residue

cracking additive for removal of Conradson carbon precursorsand metals[82].The kinetics of hydrodemetallization (HDM) of vanadyl

etioporphyrin (VO-EP) was studied in a batch autoclave at543 K and 5 MPa of total pressure, with white oil as solvent and

presulfiding CoMo/A1203 (TK 710) as catalyst. The mostwidely accepted kinetic model comprised of only dihydroge-nated intermediate (VOEPH2) did not fit the experimental data.A new model with two reversible hydrogenation steps and alumped irreversible hydrogenolysis step was proposed, andfollowed the concentration trace of reactants (VOEP and VOEPH2) very well for most reaction times [83].

A feasibility study of the decomposition and demetallation

of metalloporphyrins by ultrasonic irradiation is presented ina paper by Tu and Yen [84]. Two representative modelcompounds, NiTPP and VOTPP, were investigated in thisultrasonic process on the laboratory scale. The extent of thedecomposition was detected by UVvis. The metals weremeasured by ICP/MS. In the initial investigation, thedecomposition of metalloporphyrins, which were dissolvedin different solventwater mixtures, was performed under theultrasonication process. Among these solvents, the chlorinat-ed-type solvents (e.g., chloroform and dichloromethane)achieved a higher efficiency because they generated moreoxidizing species under sonication at 20 kHz frequency. Other

additives such as surfactant and hydrogen peroxide, whichaffect the decomposition efficiency, were also investigated.Under optimal condition, the decomposition efficiencyreached about 90% in 1 h for both model compounds. Anoxidative intermediate existed for both metalloporphyrinsunder ultrasonication. The decomposition reaction rates ofthese two compounds followed pseudo-first-order in reactantconcentration and were inhibited by initial feed concentration.The dependence of the rate constants on the different initialconcentrations could be determined by the Langmuir Hinshel-wood equation.

The influence of the hydrodynamic effects on the plug flowmodel deviation in the hydrodesulfuration (HDS) and hydro-demetallation (HDM) reactions of a petroleum residue was

evaluated by Callejas and Martinez[85]. The removal of sulfur,vanadium, and nickel from a heavy residual oil was examined,using a commercial catalyst. The possibility of incorporatingchemical and hydrodynamic complexity in the kinetics analysisof hydrotreating reactions in a pilot trickle bed reactor werediscussed. The study was done in a small pilot scale trickle bed

reactor in continuos operation at 375 C and 10 MPa of partialhydrogen pressure with a commercial NiMo/alumina catalyst.The nickel removal reaction, a firstorder liquid-limited reaction,was used to test the predictions of several models, whichincorporate the influence of the hydrodynamics on the catalystutilization. For this task, additional experiments in a stirred tankreactor at the same conditions were done in order to determinethe value of the effectiveness factor for denickelation reactions.Comparison of model predictions and experimental dataindicated that the use of a hydrodynamic parameter in themodels improved the data fit.

5. Metal passivators

Although most of the metallic constituents of crude oil areconcentrated in the residues, some of the organometallic com-

pounds are actually volatilized at refinery distillation tempera-tures and appear in the feed to the FCC cracking units. Metalcontaminants in the feedstock tend to deposit on the matrix ofFCC catalysts wherein they catalyze the combustion of carbonmonoxide (CO). Generally, only a portion of the total (2530%) deposited metal is active. Nickel in FCC feed and ironscale are predominant sources of such contaminants. Iron scalein the flue gas lines, cyclones, and dilute phase of theregenerator can be a cause of after burning problems. Other

metal contaminants such as lead, sodium, and vanadium willact as poisons to the active precious metal contained in COoxidation promoters. Significant increases in contaminantlevels will increase the severity and usage rates for CO

promoters. Because the compounds of these metals cannot, ingeneral, be removed from the cracking unit as volatilecompounds the usual approach has been to passivate them orrender them innocuous under the conditions that areencountered during the cracking process. One passivationmethod has been to incorporate additives into the crackingcatalyst or separate particles that combine with the metals andtherefore act as traps or sinks so that the active zeolite

component is protected. The metal contaminants are removedtogether with the catalyst withdrawn from the system during itsnormal operation and fresh metal trap is added together withmakeup catalyst so as to affect a continuous withdrawal of thedeleterious metal contaminants during operation. Dependingupon the level of the harmful metals in the feed to the unit, theamount of additive may be varied relative to the makeupcatalyst in order to achieve the desired degree of metals

passivation.Antimony and tin are used to passivate the activity of

vanadium and nickel on FCC catalyst. Their purpose is toreduce gas make caused by metals catalyzed dehydrogenation.Their effect on CO promotion catalysts is to reduce their activityas well. Thus, the increased use of passivators will increase the



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severity of the promotion application and the possibility ofafterburn problems[86].

The existing metal passivator are classified into twocategories in terms of function, i.e., the single-function metal

passivator which is nickel passivator or vanadium passivatorcomposed of single effective metal like Sb, Sn or Bi; and

difunction metal passivator composed of composite metals likeSbSn, SbBi, SbRe, etc which can simultaneously passivatenickel and vanadium. There are two ways for the addition ofmetal passivators: one is to add it to the reactor with the catalyticcracking feedstock and this method is usually applied for theliquid metal passivator. The other is to add it to the reactor withthe catalyst and this method is usually applied for solid metal

passivator such as vanadium trapping agent. Additivesproposed for this purpose include the alkaline earth metalsand rare earths such as lanthanum and cerium compounds asdescribed in U.S. Patents Nos. 4,465,779; 4,519,897;4,485,184; 4,549,958; 4,515,683; 4,469,588; 4,432,896; and

4,520,120. These materials which are typically in the oxideform at the temperatures encountered in the regeneratorpresumably exhibit a high reaction rate with vanadium toyield a stable, complex vanadate species which effectively bindsthe vanadium and prevents degradation of the active crackingcomponent in the catalyst.

6. Conclusions

The metal-containing compounds in petroleum are verydetrimental to petroleum processing. They cause rapid catalyst

poisoning and other problems in refining. The metals aretypically removed in petroleum refineries by a solvent

deasphalting process. This process also removes a large amountof convertible material along with the metal-containing species.

The literature search revealed that various research organiza-tions, oil companies, and individual researchers are exploringnon-conventional options for metal removal from residual oil.These options include chemical treatment with acids and alkali,selective oxidation/solvent extraction, the photocatalytic meth-od, electrochemical treatment, and novel thermal and catalyticmethods. In general, these efforts are at the laboratory to small

pilot plant stages, and no dates have been set for commercial-ization. The only processes which have been proven capable ofeffecting substantial removal of metals are hydrotreating and

gasification. Hydrotreating may not be economical or practicalfor heavy, high metal contents resids. If metals could be removedin a pretreatment step by one of the above reviewed methods,then heavy resids could be more economically upgraded bytraditional methods of catalytic cracking. Ability to removemetals prior to hydrotreatment increases the amount and thetypes of fuels that might be processed.

Acknowledgement

The authors would like to thank the Higher EducationCommission, Govt. of Pakistan for financial support. The facilitysupport provided by H. E. J. Research Institute of Chemistry,University of Karachi is gratefully acknowledged.

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Documents

Review of Demetallisation