Kinetic Study of the Hydrodesulfurization of a Heavy Gasoil in thePresence of Free Fatty Acids Using a CoMo/γ-Al2O3 CatalystAndreas Vonortas and Nikos Papayannakos*

School of Chemical Engineering, National Technical University of Athens, Heroon Polytechniou 9, 157 80 Zografos, Athens, Greece

*S Supporting Information

ABSTRACT: The effect of an acidic vegetable oil (AVO) on the hydrodesulfurization (HDS) reaction of an atmospheric heavygasoil over a commercial CoMo/γ-Al2O3 catalyst was studied. The coprocessing of the two liquid feeds was carried out in atrickle-bed bench-scale reactor at the typical HDS reaction conditions used in most refineries; namely, the reaction temperatureranged from 310 to 350 °C, while the total pressure was kept constant at P = 33 bar. The AVO content in the liquid feed variedfrom 0 to 20 wt %, and it was found that its presence inhibited the HDS reaction in all different conditions tested, which reflectsan obvious decrease in the HDS reaction rate constant. Catalytic selectivity concerning the hydrodeoxygenation reaction wasdetermined via analysis of the gaseous products. Decarboxylation cannot be distinguished from a decarbonylation mechanismbecause of the simultaneously occurring water−gas shift reaction. It was obvious that the overall selectivity for decarboxylationand decarbonylation reaction paths decreased with increasing AVO content in the feed and increased with the reactiontemperature. During the experimental tests, the catalyst activity was measured and deactivation was found to be negligible.

1. INTRODUCTION

The dwindling fossil fuel reserves and the current legislationconcerning the use of renewable resources as fuels, especially inthe transportation sector, lead to increased interest in theresearch and development of new technologies for theproduction of liquid fuels that are not dependent on crudeoil. The European Union has set the objective of 20%substitution of conventional fuels with renewable ones by theyear 2020.1 Moreover, several environmental concerns haveemerged from the intensive use of petroleum and itsderivatives, making the use of alternative sources imperative.Vegetable oils and fats are an abundant source for producingbiofuels resembling petroleum-derived diesel.Vegetable oils have been investigated in the last decades with

the purpose of producing fuels fully compatible withpetroleum-derived diesel. Biodiesel was the first biofuelintroduced in the market that could be used in blends withdiesel or neat in diesel engines because of its similar propertiesto conventional petrodiesel and lower SOx and CO emissionsfrom its combustion.2 It is produced by transesterification oftriglycerides or esterification of carboxylic acids with a low-molecular-weight alcohol. Its application is accompanied bycertain drawbacks mainly related to its high oxygen content3

and its conventional production process, which requiresvegetable oils and fats with low acidity because of the basichomogeneous catalyst used, methanol, and a continuouslyconsumed base as the catalyst.4

An alternative route for the production of biocomponentsresembling diesel is the hydrotreatment (HDT) of vegetableoils and their derivatives. This is a promising process that canpartially substitute diesel fuel. Although the HDT of vegetableoils is a more complex process than their transesterification, thenecessary infrastructure and hydrogen already exist in oilrefineries, making this new technology attractive. Theresearcher’s interest has been focused on shedding light on

all different aspects concerning this new technology. Researchwork has already been carried out with both neat vegetable oilsand blends with petrodiesel to determine optimal operatingconditions,5 suitable catalysts,6 reaction mechanisms,7 catalystdeactivation,8 and final fuel properties.9,10 The cetane numberof fatty acid methyl ester is slightly higher than 50,11 while thatof the product from hydroprocessing of vegetable oils reachesor even exceeds 80.12 It was found that vegetable oils can beconverted into useful hydrocarbons at milder reactionconditions (temperature, pressure, etc.) than gasoil’s hydro-desulfurization (HDS). Corma et al.5 found that, in mixtures ofsunflower oil and heavy vacuum oil, the former was totallyconverted, while the latter reached only 41% HDS conversionat 350 °C, indicating that even lower temperatures are adequatefor full hydrodeoxygenation (HDO) of triglycerides. In acomparison of various supported catalysts,6 with noble metalsas the active phase or typical of HDS processes, the latterproved to be more active in the HDO reaction. The use ofsulfided NiMo and CoMo catalysts in the HDT of triglycerideshas been studied in a number of publications,13−15 and theirgreat potential of being used as HDO catalysts has beenindicated. This process is currently applied industrially by avariety of companies worldwide like Neste Oil, UOP/Eni,Emerald Biofuels, Syntroleum, etc., with more companiesshowing interest in investing in this new technology.Cohydroprocessing of gasoil and vegetable oils is a viable

process for the production of biofuel and petroleum dieselmixtures, and it is easily adopted by current gasoil HDT units.For a biofuel content of less than 10%, the produced fuel hasacceptable cold-flow properties, while the cetane number and

Received: February 14, 2014Revised: May 7, 2014Accepted: May 9, 2014Published: May 9, 2014

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cetane index are considerably improved. Currently, there isgreat interest in using cheap biooils, not appropriate for food, asraw materials, and this imposes the development of effectivemethods for converting such waste or nonedible vegetable oilsinto useful hydrocarbons. Therefore, it is imperative to clarifythe impact of the oxygenated molecules and the free fatty acids(FFAs) on the performance of the HDS reactor, including thesulfur concentration in the final product, hydrogen con-sumption, and catalyst deactivation. Although the possibilityof using FFAs in the feed mixture has been referred to in theliterature,16,17 there is a lack of quantitative effects of theirpresence on the reactor performance. Determination of theimpact of the presence of FFAs on the HDS kinetic parameterswill allow the modeling and simulation of hydrotreaters,enabling existing units to be used and new units to be designed.The scope of this work is to study the cohydroprocessing of

mixtures composed of an atmospheric heavy gasoil and anacidic vegetable oil (AVO) using a bimetallic CoMo/γ-Al2O3catalyst under typical HDS conditions. The focal points are thestudy of the effect of the presence of oxygenated and acidicmolecules on the HDS kinetics and estimation of the influenceof various parameters, like temperature, vegetable oil/gasoilratio in the feed, and H2S presence, on the selectivity of theHDO reaction. The latter was investigated by analysis of thegaseous products at several reaction conditions. For compar-ison purposes, a refined palm oil was also tested in blends withthe same atmospheric heavy gasoil. For estimation of the H2Simpact on the selectivity of the HDO reactions, a set ofexperiments was conducted using blends of a hydroprocessedand desulfurized diesel and AVO.

2. EXPERIMENTAL SECTION2.1. Experimental Apparatus and Conditions. Experi-

ments were carried out in an integral, isothermal, bench-scale,tubular reactor. A detailed description of the experimentalapparatus was presented in a previous paper.18 A total of 42experiments were performed for determination of the effect ofthe presence of carboxylic acids and triglycerides on the HDSkinetic constants and 19 experiments for investigation of theHDO reaction pathways. In total, 65 experiments wereperformed for 610 h. The experiments were performed atthree different reaction temperatures (310, 330, and 350 °C)and a constant total pressure of 33 bar, which correspond totypical mild HDS conditions. The gas flow at the reactor inletwas kept constant at approximately 20 NL/h. The liquid flowstested were 28, 40, and 56 g/h.The catalyst’s sulfidation was performed in situ, using a

mixture of dimethyl disulfide (DMDS) and light gasoil with a 2wt % sulfur concentration. The procedure followed included aninitial step of catalyst pretreatment at 100 °C and 35 bar for 2h. After the first step, the mixture of DMDS and light gasoil waspumped to the catalytic bed at a rate of 40 g/h, while thetemperature was increased from 100 to 300 °C with 20 °C/h.Consequently, the temperature was raised from 300 to 380 °Cwith a rate of 10 °C/h and kept constant at 380 °C for 2 hmore. After that, the experimental program started.2.2. Feedstocks and Materials Used. The gaseous feed

was hydrogen of industrial purity (99.95%) provided by AirLiquide, while the liquid feed was neat gasoil, blends of gasoilwith AVO (2.5%, 5%, and 10%), blends of gasoil with a refinedpalm oil, and finally blends of diesel with AVO. A total of 40 gof commercial CoMo/γ-Al2O3 was used (cylindrical particleswith an average length of 3.5 mm and a diameter of 1.3 mm),

diluted with 60 g of fine SiC inert particles (46 grit, supplied byAlpha Aesar) to avoid liquid maldistribution over the solidcatalyst and wall effects. Gasoil and ultradesulfurized diesel usedin this work were provided by Hellenic Petroleum Inc.(Greece), accompanied by their physical properties anddistillation curves. Analysis of the fatty acid composition forboth vegetable oils was provided by Minerva Co., which alsodonated the vegetable oils. The AVO and refined palm oilcomposition is presented in Table S1 in the SupportingInformation (SI), while in Table S2 in the SI, the basic physicalproperties of both heavy gasoils used are summarized. Thedistillation curve of the heavy gasoil used in this work ispresented elsewhere.19

2.3. Analysis of the Products. The sulfur content of thefinal product was determined using an Antek 9000 series sulfuranalyzer. Ester bond conversion was determined fromsaponification number analysis (ASTM D 94-07) and wasproven to be practically total (above 99.5%) for all of theexperimental conditions applied in this study. The final acidityof the samples derived from the HDT of feeds containing AVOwas measured according to the method described by Pasias etal.20 A suitable amount of the liquid sample was weighed andadded directly to the solvent mixture, ethanol and diethyl ether,along with the titration indicator. This mixture was thensubsequently titrated with 0.17 M KOH until the solutionchanged color. The key parameter for accurate determination ofthe acidity is the amount of the sample added initially. In ourwork, it was found that this amount was 2 g. It was found thatfull conversion of the acid molecules had been achieved at allexperimental conditions. Gas-phase analysis was performedusing a Hewlett-Packard HP 6890 series gas chromatographwith a 5973 series mass selective detector. The analysisindicated the presence of CO and CO2 due to decarbonylation,decarboxylation, and water−gas shift (WGS) reactions, C3H8from the HDO of the triglyceride molecules, and CH4 from themethanation reaction. The partial pressure of H2S in the gasphase, when present, was calculated by its mass balance and theconversion of organic sulfur bearing molecules in gasoil.Catalyst deactivation during the experimental tests was checkedby repetition of a standard experiment that was performed atregular intervals and estimated by measuring the product sulfurconcentration. Four experiments were conducted, with thereaction temperature kept constant at T = 330 °C and thepressure at P = 33 bar, to check the repeatability of theexperimental procedure and that of the analysis. It was provedthat the analysis results did not differ by more than ±3% that issatisfactory for this process.

3. RESULTS AND DISCUSSION3.1. Catalytic Selectivity. It is well-known that HDO

reactions follow three distinct reaction pathways, i.e., decarbon-ylation, decarboxylation, and hydrogenation.7 The catalyticselectivity for these three pathways is crucial for the processeconomics because it is strongly related with hydrogenconsumption. In the case of triglycerides, the hydrogenationpath consumes 4 mol of hydrogen per ester bond, so it is themost hydrogen-consuming pathway; decarbonylation consumes2 mol per ester bond, whereas decarboxylation consumes 1 molper ester bond. In the case of carboxylic acids, thehydrogenation path consumes 3 mol of hydrogen per carboxylicbond, and this is again the most hydrogen-consuming pathway.Decarbonylation consumes 1 mol per bond, whereasdecarboxylation consumes none. It should be noted that

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hydrogenation of multiple bonds is a faster reaction that occursat much higher rates in our experimental conditions, and forthis reason, the effect of this reaction on hydrogenconsumption is the same, irrespective of the HDO reactionpath followed. Given this fact, it is obvious that decarboxylationis the most desirable reaction pathway, while hydrogenation isthe least. However, it should be noted that the hydrogenationpath produces hydrocarbons with a carbon atom more than theother two paths, leading to higher biofuel yield. Thisconsideration concerns the liquid-phase reactions. However,reactions that can increase hydrogen consumption can alsooccur among the molecules in the gaseous phase. Forestimation of the catalytic selectivity of the HDO reaction,gas-phase analysis was performed.Analysis of the gaseous phase was presented in detail in

previous papers,18,19 and only a brief description is includedherein. Gas samples were collected from the gas outlet of theseparator vessel located exactly after the reactor’s outlet. Acubic equation of state was employed to calculate the amountof gaseous species dissolved in the liquid phase, and then byfitting of the experimental values, the mole fractions at thecatalyst bed exit were calculated. By conversion of these valuesto molar flows, estimation of the catalytic selectivity concerningthe HDO reaction was achieved. Two reactions were assumedto occur in the gaseous phase: WGS and the methanationreaction of CO. The WGS reaction was proven to be atequilibrium by calculating the equilibrium constant for thethree temperatures using the van’t Hoff equation andcomparing them with those resulting from the experimentallymeasured gas-phase composition of the exit gas. No cracking ofother hydrocarbon molecules was taken into consideration forthe production of methane. This is due to the mildexperimental conditions. For support of this assumption, twoexperiments were conducted at a maximum experimentaltemperature of 350 °C, one with neat heavy gasoil and theother with neat diesel, while samples of the gaseous outletstream were collected and analyzed. This analysis indicated noproduction of methane or lighter hydrocarbons via crackingreactions at our experimental conditions.In Figure 1, the trend of the catalytic selectivity for the HDO

reaction, with the reaction temperature kept constant (330 °C)and the vegetable oil content varied in the initial liquid feed, isdepicted. On the Y axis, the percentage of the total ester bondsfollowing the two paths that yield alkane molecules with onecarbon atom less than the parental fatty acid, namely,

decarbonylation plus decarboxylation (DCN/DCX), is repre-sented. It is obvious that, with an increase in the vegetable oilcontent, the DCN/DCX selectivity decreases for the threefeeds tested in this work. By a comparison of the deoxygenationselectivity for the two vegetable oils in the gasoil matrix, it isobserved that only minor differences exist, with the AVOhaving almost 10% more ester bonds following the hydro-genation pathway in all contents examined and the refined palmoil mostly following the DCN/DCX pathways. The twovegetable oils have almost similar fatty acid distribution,19 withC16 and C18 alkyls dominating in the initial triglycerides orFFAs of the refined palm oil and AVO, respectively. Thus, itappears that, irrespective of the types of vegetable oil molecules,esters, or acids, the selectivity trends remain unchanged.On the other hand, when the points showing the effect of the

different matrices, desulfurized diesel or gasoil, are focused on,it is seen that the presence of H2S in the gaseous phase and,consequently, on the catalyst surface strongly affects the HDOselectivity. When H2S is absent, a strong shift of the HDOselectivity (almost 30%) toward the hydrogenation pathway isobserved, indicating an enhancement effect by this molecule onthe two other paths. This tendency was also observed by Senolet al.21 when model compounds of aliphatic esters or carboxylicacids were used to study the influence of a sulfiding agent, H2Sor CS2, on the HDO of these model compounds. In their work,H2S shifted the HDO selectivity of both types of moleculesfrom the hydrogenation path to the other two paths, yieldingmore C6 hydrocarbons as main products. The in situ generatedH2S in our case ranges from 9350 ppm, which corresponds tofull conversion of the sulfur contained in the liquid phase, to7450 ppm, which corresponds to the highest final sulfurconcentration measured in our work, as can be seen in Table S3in the SI. As for the two other paths, it is not possible to saywhether H2S inhibits more the decarbonylation path than thedecarboxylation path or vice versa because of the simultaneousoccurrence of the WGS reaction, as mentioned before.In Figure 2, the effect of the reaction temperature on the

catalyst selectivity for the HDO reaction is presented forconstant vegetable oil initial content in the liquid feed (10 wt%). The continuous increase of the number of ester bondspreferring the two paths DCN/DCX with increasing temper-ature is evident, although the slope is less steep when gasoil wasused as the matrix for the feed mixtures.

Figure 1. HDO reaction selectivity with varying vegetable oil contentin the feed mixture (where D, G, A, and P denote diesel, gasoil, AVO,and refined palm oil, respectively).

Figure 2. HDO reaction selectivity with varying temperature andconstant vegetable oil content (where D, G, A, and P denote diesel,gasoil, AVO, and refined palm oil, respectively).

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Again, the difference of the effect of the vegetable oil ormolecule type is not very significant, but the absence of H2Sstrongly affects the HDO selectivity, promoting the hydro-genation pathway as the temperature decreases.CH4 was also found to be present in the gaseous stream. It is

an important molecule produced during the HDO reactionbecause its production increases hydrogen consumption andthus affects process economics. CH4 is produced via themethanation reaction of carbon oxides with hydrogen at theprevailing experimental conditions. Although both CO andCO2 can produce CH4, taking into account both methanationreactions in our calculations is not possible because of the WGSreaction that appears to be at equilibrium at our experimentalconditions18 and because of which the two methanationreactions are not independent. Therefore, only the methanationreaction of CO is assumed to occur, as was clearly suggested byDonnis et al.7 For estimation of the effect of H2S on theformation of CH4 during the HDT, the CH4 flow rate at thereactor outlet is presented in Figures 3 and 4 for variation of the

reaction temperature and vegetable oil content in the feed. InFigure 3, CH4 production is presented at constant temperature(T = 330 °C) by varying vegetable oil content in the liquid feedfrom 5 to 20 wt %. CH4 production increased with increasingvegetable oil content in the liquid feed. This is attributed to the

fact that higher vegetable oil content results in higher COproduction because of the existence of more oxygen-bearingmolecules in the feed mixture. When gasoil was fed to thereactor with either of the two vegetable oils, similar results wereobtained for CH4 production.When diesel/AVO blends were fed to the reactor, CH4

production was slightly reduced, indicating a weak enhance-ment of CH4 production by the presence of H2S. The sametrend was observed when the reaction temperature wasincreased from 310 to 350 °C and the vegetable oil contentwas kept constant in the feed (10 wt %), as shown in Figure 4.In Figures 5 and 6, the percentage of CO converted to CH4

is given. CO conversion is continuously decreasing with

increasing AVO content, indicating that CH4 production ratesare not proportional to CO produced from the HDO or WGSreactions. This might be due to the presence and varyingconcentration of the other two oxygenate molecules, H2O andCO2, and/or to a decrease of the vegetable oil moleculesfollowing the decarbonylation/decarboxylation mechanism withincreasing AVO content (Figure 1), which results in less COavailable for the methanation reaction.H2S inhibits the methanation reaction by almost 10%, while

the effect of the different vegetable oils is rather negligible. With

Figure 3. CH4 production with varying vegetable oil content (whereD, G, A, and P denote diesel, gasoil, AVO, and refined palm oil,respectively).

Figure 4. CH4 production with varying temperature and constantvegetable oil content (where D, G, A, and P denote diesel, gasoil,AVO, and refined palm oil, respectively).

Figure 5. CO conversion to CH4 (%) with varying vegetable oilcontent (where D, G, A, and P denote diesel, gasoil, AVO, and refinedpalm oil, respectively).

Figure 6. CO conversion to CH4 (%) with varying temperature andconstant vegetable oil content (where D, G, A, and P denote diesel,gasoil, AVO, and refined palm oil, respectively).

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increasing temperature, CH4 production is enhanced, indicatingtemperature as the key parameter for CO methanation.C3H8 was also present in the gaseous stream but only in the

case of gasoil/palm oil blends, verifying triglyceride moleculecleavage.3.2. Kinetic Analysis. In the cohydroprocessing of

vegetable oil and gasoil mixtures, the effect of the presence ofvegetable oil on the desulfurization rates is a crucial parameterbecause it affects the hydrotreating reactor performance. Thesulfur compounds in the gasoil mixtures with vegetable oilsderive only from the gasoil fraction and not from the sulfur-freerenewable feeds. Gasoil contains a large number of sulfur-bearing compounds, and although first-order kinetics is valid forthe desulfurization reaction of each sulfur-containing com-pound,22 nth-order kinetics is usually applied to describe thedesulfurization rates of their mixtures. Thus, for estimation ofthe effect of the presence of the vegetable oil in the feed on theHDS of the sulfur-bearing molecules in the mixtures, thedesulfurization reaction rate constants were determined in eachcase for constant reaction order.The equation used to describe the HDS of the sulfur-bearing

molecules is of the power-law form

=− ε⎜ ⎟⎛

⎝⎞⎠R k

ERT

P Cexp nHDS 0 H S2

where k0 is the frequency factor in kgoil·ppmS1−n/kgcat·barH2

, Eα

is the activation energy in J/mol, PH2is the hydrogen partial

pressure in bar, CS is the sulfur concentration in ppmw, and n isthe reaction order.For calculation of k0 and Eα, the mass balance describing

sulfur elimination was solved using the Runge−Kutta method.The mass balance used is

= −C

mRM

dd

S

cat

HDS

oil

where mcat refers to the catalytic mass used in grams and Moilrepresents the liquid flow fed to the reactor in g/h.The Simplex method for nonlinear programming was used

for the fitting of the experimental data. The reaction order wasassumed to be a test variable, while frequency factor k0 andactivation energy Eα were optimized for each experimentalseries. As an objective function, the absolute value of therelative error of the final sulfur concentration was used:

=−

×C C

Cerror % 100S,exp S,est

S,exp

where CS,exp represents the experimental values of the sulfurconcentration measured in the final products and CS,estrepresents the estimated values from reactor simulation. Themodel was proven to be in good agreement with theexperimental data.Figure 7 depicts the parity plot for gasoil/AVO HDS

experiments. The majority of the experimental points residewithin the ±5% error range. The minimum overall relative errorwas taken into account for defining the optimum HDS reactionorder n, frequency factor k0, and activation energy Eα, the lattertwo for each AVO content and the former for all experimentalpoints. The value for n was found to be 1.4, which is typical forHDS processes of gasoil feeds.Activity tests throughout the experimental work indicated

that no deactivation occurred during the experimental tests run

with AVO or refined palm oil (Figure 8). In a previous work ofour group,23 the study of the performance of the same

commercial CoMo catalyst, at the same experimentalconditions but with a feed consisting of a different gasoil anda refined palm oil, showed the typical deactivation trendoccurring during the HDS process. Figure 8 compares the twodeactivation curves for the two experimental tests. The absenceof catalyst deactivation in the present work indicates that thepresence of AVO or palm oil in the feed does not have anegative effect on catalyst activation while the deactivationobserved in the previous study can be attributed to the differentgasoil matrices used. Because the basic characteristics of the twogasoils are not significantly different, as shown in Table S2 inthe SI, other structural characteristics like the aromaticity,which promotes coke formation and catalyst deactivation,should be the reason for the observed deactivation in theprevious work.Variation of the frequency factor and activation energy with

the AVO content in the feed mixture is given in Figure 9. It isobvious that both parameters decrease with increasing AVOcontent. An almost linear decrease of the activation energy withthe AVO content in the feed mixture is observed, with a 17%decline when AVO increases from 0 to 10 wt %. The decreasein the frequency factor is very steep, about 98%, resulting in adecrease in the global reaction rate constant. In a previousstudy,19 NiMo/Al2O3 was tested for the same feed systems.Although in that case the decrease of the HDS activation

Figure 7. Parity plot of the estimated sulfur concentrations versus thecorresponding experimental data for the gasoil/AVO mixtures withAVO content of 0−10 wt %.

Figure 8. Comparison of the two experimental activity curves for thetwo different gasoils.

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energy was almost linear and a 10% decline can be calculatedfor the same increase of the AVO content in the feed, thedecrease in the frequency factor is also linear and equal to 65%for the same range of the AVO content in the feed, namely, 0−10 wt %. Although in that case the effect of the presence ofAVO in the feed on HDS global reaction rate constant, kHDS, isnegligible, in the case of the CoMo/Al2O3 catalytic system, theeffect is important.In Figure 10, the effect of the presence of AVO in the feed on

the calculated global reaction rate constant for the HDS

reaction, kHDS, is presented. It is obvious that a continuousdecrease in this parameter is observed with increasing AVO inthe feed. This decrease is smooth for the lowest temperaturetested (310 °C) and becomes steeper with increasingtemperature. For low AVO content, lower than 5 wt %, thedecrease appears steeper than that for higher AVO concen-trations in the feed. An analogous observation has been referredto in a previous publication,18 where a refined vegetable oil wasused to prepare the feed mixtures using the same CoMo/Al2O3catalyst. Although the general tendency of the dependence ofthe global reaction rate constant on the vegetable oil content isthe same, a distinct difference exists. In the case of using therefined vegetable oil, a steep, 50%, decrease of kHDS wasestimated when the vegetable oil content increased to 5 wt %.Then, the value of kHDS was leveled off up to 10 wt % vegetableoil. In the case of using AVO in the feed mixtures, the first

decrease is calculated as about 45% and then a further smalldecrease of about 20% is calculated from 5 to 10 wt % AVO.This different dependence of kHDS on the vegetable oil contentcan be attributed to hindrances of the sulfur-bearing moleculesby the competitively adsorbed triglycerides and FFAs or thespecies produced from cleavage (HDO) of the oxygenatemolecules, namely, CO, CO2, and H2O, on the same catalystactive sites. This phenomenon, as has already been shown fromother investigators,24,25 is similar to the strong adsorption oforganic nitrogen-bearing molecules present in gasoil on thecatalytic sites responsible for the HDS reaction. For clarificationof the inhibition mechanism and the effect of each molecule onHDS, further research is needed.Tests with the same feeds but with a NiMo/Al2O3 catalyst

19

showed that the presence of both vegetable oils in the feed hadno effect on the HDS reaction rates. The different perform-ances of the two catalytic systems can be explained on the basisof the existing knowledge of the HDS mechanisms on thecorresponding active sites. The NiMo catalyst’s hydrogenationactivity is higher, and HDS occurs via an indirect mechanisminvolving as a first-step hydrogenation of the sulfur-bearingmolecules and then direct desulfurization. CoMo catalystscontain active sites for direct and indirect HDS.26,27 DirectHDS occurs via the adsorption of −S− in the sulfur-bearingmolecules on the pertinent active sites, and sulfur removalfollows by breaking the S−C bonds. It appears that, althoughthe active sites promoting indirect HDS are not affected by theoxygenate compounds, the active sites promoting direct HDSthrough −S− adsorption are greatly affected by the oxygenatesdue to the competitive adsorption of −O− contained inoxygenates. This competitive adsorption is attributed to thesimilarity of the sulfur and oxygen atoms, which indicates asimilar affinity of the oxygen- and sulfur-bearing molecules withthe same active sites of direct HDS.

4. CONCLUSIONS

In this study, a commercial CoMo/γ-Al2O3 catalyst was usedfor the study of HDT of neat gasoil and its mixtures with AVO(2.5, 5, 10, and 20 wt %) in a micropilot HDT reactor at typicalHDS conditions. The catalytic selectivity for the HDOreactions was estimated by gas-phase analysis and datatreatment and showed that DCN/DCX pathways are enhancedby decreasing the vegetable oil content or increasing thereaction temperature. Gas-phase analysis indicated the presenceof CH4, which is attributed to methanation of CO. Theconversion of the latter to CH4 increased with increasingvegetable oil content in the feed mixture and decreased with thetemperature. H2S was found to have a considerable impact onthe selectivity of the HDO reactions. Its presence promotes theDCN/DCX pathways, while its absence promotes the thirdreaction path, the hydrogenation path. This observation isimportant because it shows clearly that coprocessing, whereH2S is present, is more attractive from an economic point ofview than the hydroprocessing of neat vegetable oil feedstocks.The effect of the vegetable oil and FFA molecules on the HDSreaction was studied by using a simple power-law kineticequation. Oxygenated molecules proved to have an adverseimpact on the hydrotreater performance at all experimentalconditions. The catalyst activity appeared to be constant for thewhole experimentation time, and oxygenate molecules do notpromote catalyst deactivation.

Figure 9. Dependence of the frequency factor (kgoil·ppmS1−n/kgcat·

barH2) and activation energy (J/mol) on the AVO content.

Figure 10. AVO content effect on the global HDS reaction rateconstant.

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■ ASSOCIATED CONTENT

*S Supporting InformationFatty acid composition of both vegetable oils used in this work(Table S1), physical properties of both atmospheric heavygasoils (Table S2), experimental conditions and final sulfurcontent for each experiment used for determination of thekinetic constants presented in this work (Table S3). Thismaterial is available free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATION

Corresponding Author*Tel.: +30 210 772 3239. Fax: +30 210 772 3155. E-mail:npap@central.ntua.gr.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

This research has been cofinanced by the European Union(European Social Fund, ESF) and Greek national fundsthrough the Operational Program “Education and LifelongLearning” of the National Strategic Reference FrameworkResearch Funding Program: Thales. Investing in knowledgesociety through the European Social Fund. Dr. A. Zerva andProf. C. Philippopoulos are acknowledged for their contribu-tion to analysis of the exit gases.

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Industrial & Engineering Chemistry Research Article

dx.doi.org/10.1021/ie5006492 | Ind. Eng. Chem. Res. 2014, 53, 9646−96529652