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Recent Trends in the Cleaning of DieselFuels via Desulfurization ProcessesS. A. Hanafi

a& M. S. Mohamed

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Egyptian Petroleum Research Institute, Nasr City, Cairo, EgyptPublished online: 15 Dec 2010.

To cite this article:S. A. Hanafi & M. S. Mohamed (2011) Recent Trends in the Cleaning of DieselFuels via Desulfurization Processes, Energy Sources, Part A: Recovery, Utilization, and Environmental

Effects, 33:6, 495-511, DOI: 10.1080/15567030903058360

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Energy Sources, Part A, 33:495511, 2011

Copyright Taylor & Francis Group, LLC

ISSN: 1556-7036 print/1556-7230 online

DOI: 10.1080/15567030903058360

Recent Trends in the Cleaning of Diesel Fuelsvia Desulfurization Processes

S. A. HANAFI1

and M. S. MOHAMED1

1Egyptian Petroleum Research Institute, Nasr City, Cairo, Egypt

Abstract This article is a selective review of new approaches and emerging tech-nologies for ultra-clean (ultra-low sulfur) diesel fuels. The issues of diesel deep

desulfurization are becoming more serious because the crude oils are getting higher

in sulfur content, while the regulated sulfur limits are becoming lower and lower.Deep reduction of diesel sulfur (from 500 to

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496 S. A. Hanafi and M. S. Mohamed

of all the crude oils refined in the five regions of the U.S. (known as the five PetroleumAdministration for Defense Districts [PADDs]) increased from 0.89 wt% in 1981 to 1.42wt% in 2001, while the corresponding PI gravity decreased from 33.74 in 1981 to 30.49PI in 2001. The problem for diesel desulfurization is also somewhat more serious inthe U.S. because a higher proportion of light cycle oil (LCO) from the fluid catalyticcracking (FCC) is used in the diesel pool in the U.S., which has higher contents of morerefractory sulfur compounds (see below). H2 demand increase is another challenge tothe refinery operations. Hydrogen deficits are processing restraints and will impact futurehydrotreating capabilities and decisions (Choi et al., 2004; Schlogl et al., 2010).

The heightened interests in ultra-clean fuels, are also due to the need for using newemission control technology of IC engines (especially those for diesel fuels), and forusing on-board or on-site reforming of hydrocarbon fuels for new fuel cell vehicles (Maet al., 2005).

This article is a selective overview on new design approaches and associated catal-ysis and chemistry as well as processes for deep desulfurization of hydrocarbon fuels,

particularly diesel fuels.

2. Design Approaches to Ultra-deep Desulfurization of Diesel

Approaches to ultra-deep desulphurization include: (1) improving catalytic activity bynew catalyst formulation for hydrodesulfurization (HDS) of 4,6-DMDBT; (2) tailoringreaction and process conditions; (3) designing new reactor configurations; and (4) devel-oping new processes. Design approaches for ultra-deep HDS focus on how to remove4,6-DMDBT more effectively. One or more approaches may be employed by a refineryto meet the challenges of producing ultra-clean fuels at an affordable cost. Some selectedapproaches are elaborated below.

2.1. Improving Catalytic Activity by a New Catalyst Formulation

The catalyst development has been one of the focuses of industrial research and devel-opment for deep HDS. For example, new and improved catalyst have been developedand marketed by Akzo Nobel, Criterion, Haldor-Topsoe, United catalyst/Sud-Chemie,and Exxon Mobil. Akzo Nobel currently markets four CoMo desulphurization catalysts:KF 752, KF 756, and KF 757, which have been available for several years, and KF848, which was announced in 2000. KF 752 can be considered to be typical of anAkzo Nobel catalyst of the 19921993 timeframe, while KF 756 and KF 757 catalysts

represent improvements. Akzo Nobel estimates that under typical conditions (e.g., 500ppmw sulfur), KF 756 is 25% more active than KF 752, while KF 757 is 50% moreactive than KF 752 and 30% more active than KF 756 (Gerritsen and Stoop, 2000).However, under more severe conditions (e.g.,

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Recent Trends via Desulfurization Processes 497

Criterion Catalyst Company announced two new lines of catalyst. One is calledcentury, and the other is called centinel (Salvator et al., 2003). These two lines of catalystsare reported to be 4570 and 80% more active, respectively, at desulfurizing petroleumfuel than conventional catalysts used in the mid-1990s. These improvements have comeabout through better dispersion of the active metal on the catalyst substrate.

2.2. Tailoring Reaction and Processing Conditions

Tailoring process conditions aims at achieving deeper HDS with a given catalyst in anexisting reactor without changing the processing scheme, with no or minimum capitalinvestment. The parameters include those that can be tuned without any new capitalinvestment (space velocity, temperature, pressure), and those that may involve somerelatively minor change in the processing scheme or some capital investment (expansionin catalyst volume or density, H2S scrubber from recycled gas, improved vapor-liquiddistributor) (Tai-sheng, 2004). First, space velocity can be decreased through increasing

the catalyst bed volume or reducing the flow rate of liquid feedstock to increase thereactant-catalyst contact time. More refractory sulfur compounds would require lowerspace velocity for achieving deeper HDS. Second, temperature can be increased, whichincreases the rate of HDS. Higher temperature facilitates more of the high activation-energy reaction. Third, H2 pressure can be increased. Fourth, improvements can bemade in vapor-liquid contact to achieve uniform reactant distribution, which effectivelyincreases the use of surface area of the catalyst. Finally, the concentration of hydrogensulfide in the recycle stream can be removed by amine scrubbing. Since H2S is aninhibitor to HDS, its build-up in high-pressure reactions through continuous recyclingcan become significant.

2.3. Designing New Reactor Configurations

Industrial reactor configuration for deep HDS of gas oils in terms of reaction orderand effect of H2S has been discussed (Chou and Rost, 2003). The reactor design andconfiguration involve one-stage and two-stage desulphurization as shown in Figure 1.Hydrogen sulfide strongly suppresses the activity of the catalyst for converting therefractory sulfur compounds that should occur in the major downstream part of a co-current trickle-bed reactor during deep HDS. The normally applied Co-current trickle-bed single reactor is, therefore, not the optimal technology for deep HDS. However, asecond reactor can be used, particularly to meet lower sulfur levels. Adding a second

reactor to increase the degree of desulfurization is an option, and both desulphurizationand hydrogenation in the second reactor can be improved by removing H2S and NH3from the exit gas of the first reactor before entering the second reactor. This last technicalchange is to install a complete second stage to the existing one-stage hydrotreater. Thissecond stage would consist of a second reactor and a high pressure hydrogen sulfidescrubber between the first and second reactors (Terrence, 2003). The compressor wouldalso be upgraded to allow a higher pressure to be used in the new second reactor (Tai-Sheng, 2004). Assuming use of the most active catalysts available in both reactors, thatconvert from one-stage to a two-stage hydrotreater could produce 5 ppmw sulfur relativeto a current level of 500 ppmw today.

A new type of reactor design is to have two or three catalyst beds that are normallyplaced in separate reactors, within a single reactor shell and have both co-current andcounter-current flows. This new design was developed by ABB Lummus and Criterion

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Figure 1. Single-stage process flow diagram.

called Syn Technology. With this technology, in a single reactor design, the initial portionof the reactor will follow a co-current design, while the last portion of the reactor willbe counter-current. Traditional reactors are co-current in nature. The hydrogen is mixedtogether with the distillate at the entrance to the reactor and flow through the reactortogether. Because the reaction is exothermic, heat must be removed periodically. Thisis sometimes done through the introduction of fresh hydrogen and distillate at one ortwo points further down the reactor. The advantage of co-current design is practical; iteases the control of gas-liquid mixing and contact with the catalyst. The disadvantage isthat the concentration of H2 is the highest at the front of the reactor and lowest at theoutlet. The opposite is true for the concentration of H2S. This increases the difficulty

of achieving extremely low-sulfur levels due to the low H2 concentration and high H2Sconcentration at the end of the reactor. A new solution to this problem is to designa counter-current reactor, where the fresh H2 is introduced at one end of the reactorand the liquid distillate at the other end (Figure 2). Here, the hydrogen concentration ishighest (and the H2S concentration is lowest) where the reactor is trying to desulfurizethe most difficult compounds. The difficulty of counter-current design in the case ofdistillate hydrotreating is vapor-liquid contact and the prevention of hot spots within thereactor.

In a two-reactor design, the first reactor will be co-current, while the second reactorwill be counter current. ABB Lummus estimates that the counter current design canreduce the catalyst volume needed to achieve 97% HDS by 16% relative to a co-currentdesign. The impact of the counter-current design is even more significant when aromaticscontrol (or cetane improvement) is desired in addition to sulfur control.

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Figure 2. Reactor design for deep HDS.

2.4. Developing New Processes

Among the new process concepts, design approaches for ultra-deep desulphurization arefocused on the following: (1) adsorption and sulfur atom extraction remove sulfur byusing reduced metals to react with sulfur to form metal sulfides at elevated temperaturesunder H2 atmosphere without hydrogenation of aromatics; (2) selective adsorption ofsulfur compounds remove sulfur by selective interaction with sulfur compounds in thepresence of aromatic hydrocarbons under ambient or mild conditions without hydrogen;(3) oxidation and extraction oxidize sulfur compounds by liquid-phase oxidation reactionswith or without ultrasonic radiations, followed by separation of the oxidized sulfur com-pounds; and (4) biodesulfurization attacks sulfur atoms by using bacteria via microbialdesulphurization.

2.4.1. Reactive Adsorption for Sulfur Capture and S-Zorb Process. Phillips Petroleumconducted an internal study of its refineries and concluded the use of hydrotreatingtechnologies to reach ultra-low sulfur levels to be a cost-prohibitive option. A prospectivediesel desulphurization process, S-Zorb diesel, was announced by Phillips Petroleum,which is an extension of their S-Zorb process for gasoline (at 377502C, 7.021.1kg/cm2). S-Zorb for diesel contacts highway diesel fuel (typically) with about 350ppmw sulfur, with a solid sorbent in a fluid bed reactor at relatively low pressuresand temperature in the presence of hydrogen (Figure 3). The sulfur atom of the sulfur-containing compounds adsorbs onto the sorbent and reacts with the sorbent. PhillipsPetroleum uses a proprietary sorbent that attracts sulfur-containing molecules and removesthe sulfur atom from the molecule. The sulfur atom is retained on the sorbent while thehydrocarbon portion of the molecules is released back into the process stream. Hydrogen

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Figure 3. S Zorb unit.

sulfide is not released into the product stream and, therefore, prevents recombinationreactions of hydrogen sulfide and olefins to make mercaptans, which would otherwiseincrease the effluent sulfur concentration. Based on the principle, it appears that thesorbent is based on reduced metal that reacts with sulfur to become metal sulfide.The spent sorbent is continuously withdrawn from the reactor and transferred to theregenerator section. In a separate regeneration vessel, the sulfur is burned off of thesorbent and SO2 is sent to the sulfur plant. The cleaned sorbent is further reduced byhydrogen and the regenerated sorbent is then recycled back to the reactor for removingmore sulfur. The rate of sorbent circulation is controlled to help maintain the desiredsulfur concentration in the product. Because the sorbent is continuously regenerated,Phillips estimates that the unit will be able to operate 45 years between shutdowns.Because untreated distillate can contain several percent of sulfur, Phillips believes that

the S-Zorb process for diesel could get overwhelmed by the amount of sulfur that isadsorbing onto the sorbent. Thus, the S-Zorb process may not be able to treat untreateddistillate streams but would likely be used to treat distillate containing 500 ppmw ofsulfur or less.

2.4.2. Selective Adsorption for Deep Desulfurization at Ambient Temperature (SARS).

An alternate process for deep desulfurization of distillate fuels (diesel, gasoline, and jetfuels) is based on selective adsorption for removal of sulfur compounds at ambient con-ditions without using H2 (Kim et al., 2006). Figure 4 illustrates the known coordinationgeometries of thiophene in organometallic complexes, which indicate likely adsorptionconfigurations of thiophenic compounds on the surface of adsorbents. Both thiopheniccompounds and non-sulfur aromatic compounds can interact with metal by -electrons.However, in Figure 4, only two types of interaction of thiophene with metal involve

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Figure 4. Know coordination geometries of thiophene in organometallic complexes, indicating

likely adsorption configurations of thiophenic compounds on the surface of adsorbents.

sulfur atom in thiophene, the s bonding interaction between the sulfur atom and onemetal atom, and the S3 bonding interaction between the sulfur atom and two metalatoms (Song, 2003).

2.4.3. New Integrated Process Based on Adsorption. By using conventional HDS pro-cesses, the refiners need to process 100% of the fuel for dealing with sulfur compoundsthat account for less than 0.3 wt% of the feed with a 500-ppmw sulfur level. Figure 5shows the flow diagram of the proposed concept of the new integrated desulfurization

Figure 5. The proposed adsorption process for deep desulfurization.

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process, which consists of selective adsorption for removal of sulfur compounds (SARS)followed by HDS of concentrated sulfur compounds. The subsequent HDS of sulfur com-pounds removed by selective adsorption is much easier than conventional HDS of dieselstreams for two reasons. First, it is more concentrated, and thus, reactor utilization is moreefficient. Second, the rate of HDS reaction is faster because of the removal of aromaticsthat inhibit the HDS by competitive adsorption in the hydrogenation sites. Third, andmost importantly for practical application, the reactor volume can be made substantiallysmaller because the amount of fuel to be processed is smaller by 95% or more.

2.4.4. Adsorption of Polar Nitrogen Compounds before Diesel HDS. Adsorption ofpolar substances can be used as a pretreatment to remove nitrogen before HDS of fuels.Feedstocks for diesel fuel, such as atmospheric gas oil and LCO from FCC, contain somenitrogen compounds. SK in Koria has developed a new process, SK HDS pretreatmentprocess, which enables the refiners to produce economically ultra-low sulfur-diesel ofbelow 10 ppmw. It is known that nitrogen compounds can inhibit HDS on a catalyst

surface due to competitive adsorption. Adsorption removal of nitrogen compounds isused as a pretreatment before conventional HDS processing. Using a solid adsorbent, thisprocess is based on the adsorptive removal of nitrogen containing compounds (NCCs)from the feedstock prior to conventional HDS units. As is well known, NCC inhibitsthe activity of HDS catalysts. In the SK HDS pretreatment process, the feedstock to theHDS unit is pretreated, and 90% or more of NCC in the feedstock is removed, resultingin higher desulfurization in a conventional HDS unit. It was found that the degree ofimprovement in HDS is proportional to the degree of NCC removal. SK HDS pretreatmentprocess claims to be a cost-effective method for refineries to choose for 10 ppmw ultralow sulfur diesel (ULSD) production. The SK HDS pretreatment technology has a simpleprocess configuration (Figure 6). The process consists of parallel vertical vessels withadsorbent beds, an adsorption solvent circulation system, two stripping columns andassociated pumps, and an overhead system. The diesel feedstock is passed over one ofthe adsorbent vessels followed by stripping to remove a small amount of desorption

Figure 6. SK HDS pretreatment process.

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solvent. Nitrogen bearing NCCs are adsorbed in the adsorber. In the desorption cycle,the adsorbed NCCs are removed from the off-line adsorbent bed and stripped in thesecond stripper column. The desorption solvent is recycled in the process. Pretreateddiesel blend is further processed in the downstream HDS unit. The rejected NCC stream,about 4 vol% for SKs LCO treatment case, can be blended either into the refinery fueloil pool or with marine diesel (Ryu et al., 2004).

2.4.5. Oxidation and Extraction for Desulfurization.With respect to the commercialoxidation process, Petrostar announced a desulfurization technology, which removessulfur from diesel fuels using chemical oxidation. Desulfurization of diesel fuel is ac-complished by first forming a water emulsion with diesel fuel. In the emulsion, the sulfuratom is oxidized to a sulfone using catalyzed peroxyacetic acid. With an oxygen atomattached to the sulfur atom, the sulfur-containing hydrocarbon molecules become polarand hydrophilic and then move into the aqueous phase. Like biodesulfurization, some ofthe sulfones can be converted to a surfactant that could be sold to the soap industry at an

economically desirable price; the earnings made from the sales of the surfactant couldoffset some of the costs of oxidative desulfurization (Liotta and Han, 2003).

2.4.6. Pre-FCC Feed Hydrotreating for Sulfur Reduction in Light Cycle Oil. The dieselfuel is produced form several blending stocks, of which light cycle oil (LCO) from FCCis a major blending stock that contributes to the sulfur in the diesel pool. Sulfur couldbe removed from distillate material early or late in the refining process. Early in theprocess, the most practical place to remove sulfur is prior to the FCC unit. The FCCunit primarily produces gasoline, but it also produces a significant quantity of LCO.LCO is high in aromatics and sulfur, and contains a relatively high fraction of thesterically hindered dibenzothiophene (DBT)-type compounds. Many refineries alreadyhave an FCC feed hydrotreating unit; the LCO from these refineries could contain amuch lower concentration of sterically hindered DBT compounds than refineries nothydrotreating their FCC feed (Rashid, 2007). Adding an FCC feed hydrotreater is muchmore costly than distillate hydrotreating. Just on the basis of sulfur removal, FCC feedhydrotreating is more costly than distillate hydrotreating, even considering the need toreduce gasoline sulfur concentrations, as well. This is partly due to the fact that FCCfeed hydrotreating by itself is generally not capable of reducing the level of diesel fuelsulfur to those being considered in this rule. However, FCC feed hydrotreating providesother environmental and economic benefits.

FCC feed hydrotreating decreases the sulfur content of gasoline significantly, as well

as reduces sulfur oxide emissions from the FCC unit. Economically, it increases the yieldof relatively high value gasoline and LPG from the FCC unit and reduces the formationof coke on the FCC catalyst. For individual refiners, these additional benefits may offsetenough of the cost of FCC hydrotreating. Figure 7 depicts a simplified representation ofthe combined FCC-feed hydrotreating and FCCU.

2.4.7. Undercutting LCO. It is conceivable that the sulfur-rich fraction, if they havea narrow boiling range, could be separated out in distillation operation. The primarystumbling block preventing the simple desulfurization of distillate to sulfur levels meetingthe 15 ppmw cap is the presence of sterically hindered compounds, particularly thosewith two methyl or ethyl groups blocking the sulfur atom. These compounds are found inthe greatest concentration in LCO, which itself is highly aromatic. These compounds canbe desulfurized readily if saturated. However, due to the much higher hydrogen cost ofdoing so, it is better economically if this can be avoided. Because these compounds are

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Figure 7. Simplified flow diagram of FCC hydrotreater and FCCU.

inherently large in molecular weight due to their chemical structure, they distill near thehigh end of the diesel range of distillation temperatures. Thus, it is possible to segregatethese compounds from the rest of the cracked stocks via distillation and avoid the needto desulfurize them. Once separated, this LCO material could be mixed into the refinerystreams currently being used to produce off-highway diesel fuel and heating oil. Whilethis heavy LCO material could be shifted to other markets, this does not necessarily haveto be the case. Under certain conditions, this material can be cycled to the FCC unit. Forthis to be feasible, the refiner must hydrotreat the FCC feed at a pressure sufficient todesulfurize the sterically hindered sulfur-containing compounds, and the feed hydrotreatermust have sufficient excess capability to handle the additional material. This materialcould also be sent to an existing hydrocaracker, if sufficient capacity existed, and wasconverted into gasoline blend-stock. Or, it could be hydrotreated separately under moresevere conditions to remove the sulfur. This would entail higher hydrogen consumptionper barrel of treated material because of some aromatic saturation. However, the amountof material being processed would be small (Gerristen et al., 2000).

2.4.8. Biodesulfurization. At the beginning of the 1990s, the research in biodesulfur-

ization intensified due to the implementation of more stringent regulations on the sulfurcontent in fuels. Biodesulfurization became relevant only since 1992 when several aerobicbacteria were isolatedmost of them belonging to the genus Rhodococcusand wereable to selectively extract the sulfur atom from the DBT molecule without degrading itscarbon skeleton. Therefore, the energetic value of the fuels is not affected, since DBTis not degraded but only transformed into 2-hydroxybiphenyl (2HBP). This product isrecycled to the organic phase constituted by the fuel itself, while the sulfur is eliminatedin the form of inorganic sulfate in the aqueous phase containing the biocatalyst (Figure 8).The first isolated and patented Rhodococcus strain, R. erythropolis IGTS8, is the basisof the biodesulfurization process proposed by ENCHIRA Biotechnology Corporation(ENBC), formerly Energy Biosystems Corporation (EBC) (Rasmus et al., 2010). Themetabolic pathway for desulfurization has been elucidated and the enzymes and genesimplicated have been isolated (Rafael et al., 2002).

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Figure 8. Metabolic pathway of DBT desulphurization by Rhodococcus erythropolis IGTS8.

Three main factors have limited the implementation of the microbial desulfurization,including the technology proposed by Energy Biosystems Co.: (1) the large amounts ofwater necessary for the microbial metabolism; (2) the long residence time in the processthat makes the reactor volumes unthinkable; and (3) the limited microbial metabolizationon the large variety of chemical structures found in the organosulfur compounds existingin the petroleum. None of these problems have been solved yet.

This limitation might be addressed by using enzymes instead of whole microorgan-isms. Enzymes require less water than microorganisms to be active and stable in organicsolvents and, theoretically, only a film of water covering their surface should be sufficientfor catalysis to occur. The fuel itself could be the organic solvent, minimizing the additionof water.

An enzymatic process for fuel desulfurization has been described (Lang and Lin,2000). This method is based on the biocatalytic oxidation of organosulfides and thio-

phenes contained in the fuel with hemoproteins to form sulfoxides and suflones, followedby a distillation step in which these oxidized compounds are removed from the fuel.This enzymatic process could be applied after a conventional hydrodesulfurization

using a reactor with the immobilized enzyme. The challenge is still to have an enzymaticpreparation able to perform the sulfur oxidation directly in the petroleum fraction withoutthe addition of water, and stable under the conditions found in the refinery.

3. Future Trends

3.1. Fischer-Tropsch Gas-To-Liquids (GTL)

Another source of low aromatic, high hydrogen content diesel is gas-to-liquidsconversion,an area receiving much interest recently. In this case, diesel is derived from natural gas

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Figure 9. Exxons AGC-21 process.

rather than petroleum in catalytic processing equipment most likely located at a gasreserve rather than at a refinery.An example of Fischer-Tropsch (F-T) diesel is that produced by the advanced gas

conversion process, AGC-21, developed by Exxon for converting natural gas to liquidiso-paraffins virtually free of typical emission-causing impurities. The AGC-21 process,as shown in Figure 9, contains multiple new technologies within three main processblocks: synthesis gas generation, hydrocarbon synthesis, and hydroisomerization. Thisprocess, which has been described previously, first brings about the conversion of naturalgas or methane to hydrogen and carbon monoxide. This is accomplished by contactingmethane with steam and limited oxygen in a catalyzed fluid bed reactor where steamreforming and catalytic partial oxidation occur in a single vessel.

The hydrogencarbon monoxide syngas is then converted almost exclusively to linearparaffins in a novel slurry reactor using a high productive, cobalt-based hydrocarbonsynthesis catalyst. These hydrocarbons are produced at high levels of syngas conversion.The full-range, primarily normal paraffin, product contains significant 650F C waxymaterial that is a solid at room temperature and melts above 250F and which is unsuitablefor pipeline or transporting in conventional crude carriers. The final step, accomplishedwith proprietary catalysts in a fixed or trickle bed reactor, mildly isomerizes any normalparaffins to slightly branched molecules, which makes excellent feeds for refineries andchemical plants. This step, along with hydrocarbon synthesis, can be altered to control theselectivity and yield of liquids from the process (Edit et al., 1994; Bigger and Tomlinson,

2004; Ansell et al., 1994).

3.2. Biofuel

Biodiesel is a renewable and biodegradable fuel refined from vegetable oil (or animal fat).It is rapidly gaining momentum in the U.S. as an alternative fuel source for diesel engines.Compared to petroleum diesel, biodiesel is environmentally friendly and is governmentmandated. It reduces carbon monoxide, carbon dioxide, sulfur dioxide, hydrocarbons,and other particulate matter emissions that cause respiratory damage. Biodiesel alsoeliminates the cloud of dense, black smoke normally associated with diesel vehicles. Italso has better lubricity than diesel fuel because of its higher viscosity.

There are two processing routes for converting vegetable oil into diesel. The conven-tional processing root for diesel production is via transesterification, where a vegetable

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Figure 10. Simplified transesterfication of vegetable oil into biodiesel (alkyl ester).

oil or animal fat is reacted under heat with an alcohol, in the presence of a catalyst.The chemical reaction products are alkyl esters, commonly referred to as a biodiesel andglycerol (Figure 10). However, it must be emphasized that, even though the transesterifi-cation process is relatively straightforward, homemade biodiesel is not going to generate

the highest-quality product, it will most probably contain impurities, such as residualalcohol, moisture, and unreacted vegetable oil.The hydroprocessing route uses hydrogen to remove oxygen from the triglyceride

molecules (Stockle, 2007; Noweck, 2007). This is the route used in the new processto produce green diesel. While hydroprocessing was clearly the chemistry preferred forrefinery applications, how to implement it in a process design is not obvious. Two optionscan be considered:

Co-processing in an existing distillate hydroprocessing unit (Figure 11). Building a stand-alone unit as shown in Figure 12.

In the process shown in Figure 12, vegetable oil is combined with hydrogen, brought toreaction temperature, and then sent to the reactor where the vegetable oil is converted tothe green diesel product. This product is separated from the recycled gas in the separator,and the liquid product is sent to a fractionation section.

Figure 11. Alternative vegetable oil hydroprocessing routes to transportation fuels.

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Figure 12. The new green diesel process converts vegetable oil into fuels.

Biodiesel and bioethanol based on seeds and fruits of plants alone are not capableof meeting the growing energy demand. For this purpose, second generation biofuels areneeded. These are made from feedstock based on the whole plant and biomass. Expertsbelieve that biomass could satisfy one-third of the world energy demand.

By 2050, only about 50% of the energy will still originate from petroleum andnatural gas, and the balance will proceed from the coal and, increasingly, from biomass.

4. Desulfurization Unit for Fuel Cell Technologies

Fuel cell technology is a new highly efficient alternative to fossil fuels for producingelectrical power, while having the additional benefit of not polluting the environment.Since fuel cells rely on chemistry and not combustion, emissions from this system typeare much smaller than emissions from the cleanest combustion processes. Fuel cellswork when electrochemical energy converts hydrogen (H2) and oxygen into water, thus

producing electricity. The proton exchange membrane (PEM) fuel cell has the broadestapplication potential with fuel cell technologies and its subject to extensive research.However, H2 is difficult to store and distribute due to diffusion problems and explosionrisks. Consequently, it is necessary to use a specialized high pressure storage tank.

This problem is addressed by utilizing a reformer that turns hydrocarbon or alcoholfuels into H2. The reformer extracts H2 from H2-rich fuels like methanol, natural gas,petroleum, gasoline, and even biogas or biodiesel, and directly feeds the H2 gas to thefuel cell.

The core of the PEM fuel cell is the platinum ruthenium anode-catalyst (Figure 13).However, this is very sensitive to numerous impurities like sulfur and can be easilydamaged. Fuel cell longevity is mostly dependent on feedstock impurities. Thus, if H2isto be produced from H2-rich fuels using a reformer, the sulfur in the hydrocarbon fuelsmust be removed from extending the fuel cells life.

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Figure 13. Proton exchange membrane (PEM) fuel cell.

Recent developments have demonstrated that it is possible to make feedstock for fuelcells practically sulfur-free by using a desulfurization unit. During this process stage, atotal sulfur analyzer may be used to record sulfur concentrations in the desulfurizationunits outlet stream over time, producing breakthrough curves (Figure 14). As a resultof these measurements, the quality (residual sulfur) and capacity (life cycle time) of thedesulfurization unit are evaluated (Mathiak et al., 2006).

5. Conclusion

Although the petroleum refining industry is very mature, ongoing changes in legislativerequirements and engine design will continue to challenge the industry to make changes in

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Figure 14. Typical breakthrough curve.

fuel composition. Significant advances in catalysis science and technology will be neededto meet the challenge. This provides room to obtain competitive advantage through thedevelopment of superior technology options and, as a result, there is intense industryactivity to meet the clean fuels challenge of providing cleaner fuels to the consumerfrom more difficult feedstocks at the lowest cost.

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Choi, K.-H., Korai, Y., and Mochida, I. 2004. Preparation and characterization of nano-sized

como/Al2O3 catalyst for hydrodesulfurization.Applied Catalysis A 260:229236.

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