DOI: 10.1002/cctc.201402028

A Glimpse into the Molecular Journey inside an UltralowSulfur Diesel ReactorTushar V. Choudhary,* Kening Gong, Phillipp Ellison, and Ayyappan Subbiah[a]

Introduction

In view of the increasing global demand for ultralow sulfurdiesel (ULSD), extensive efforts are underway to improve theefficiency of sulfur removal from diesel feeds with high sulfurand nitrogen content.[1–6] Owing to the extremely largenumber of molecules present in real diesel feeds, most pub-lished studies use surrogate model feeds (blend of a few purecompounds) to develop superior catalysts and improve mecha-nistic understanding. Unfortunately, the practical efficiency ofthese studies is significantly hampered because at present nodefinitive information has been reported that would enableappropriate design of realistic model diesel feeds. Therefore, itwould be extremely useful if one could look inside an operat-ing ULSD reactor and quantitatively determine how the rele-vant sulfur, nitrogen, and aromatic compounds progress fromthe inlet to the outlet of the reactor, which would thus enablethe design of a realistic model diesel feed. Unfortunately,owing to the considerable complexity of the ULSD process,in situ generation of such information is not expected in theforeseeable future. Herein, we have combined carefully de-signed reactor experiments with ex situ molecular characteriza-tion methods to get a glimpse inside an operating ULSD reac-tor and gain insights into the design of realistic model dieselfeeds. This study is expected to significantly increase theimpact of future efforts in this important area by enabling thesuperior design of model diesel feeds for improved catalysisand mechanistic understanding.

Relevant information about the molecular journey of thesulfur, nitrogen, and aromatic compounds inside a ULSD reac-tor has not been reported previously because of the significantchallenges in generating appropriate data. Depending on theapproach used, obtaining this information could be practicallyinfeasible and/or fraught with errors. To overcome the chal-

lenges, sulfur, nitrogen, and hydrocarbon species were trackedalong the ULSD reactor by using the following methodology:1) an appropriate reaction temperature was identified toobtain ULSD (�10 ppm product sulfur) for a typical difficult-to-process diesel feed under realistic process conditions, 2) thefeed rate was varied at a constant catalyst loading while keep-ing all other process conditions constant to simulate productdiesel samples at 10, 17, 25, and 50 % points in the reactor aswell as at the outlet (100 %) of the reactor, and 3) the molecu-lar characterization of sulfur, nitrogen, and aromatic species inall the product samples was performed by using advanced an-alytical methods for diesel-range feeds.

Results and Discussion

Data were collected on the progression (based on distancepassed through reactor) of the relevant sulfur, nitrogen, andhydrocarbon species from the reactor inlet (feed: 0 % progres-sion) to the reactor outlet (ULSD product: 100 % progression),with intermediate points at 10, 17, 25, and 50 %. The total con-versions of sulfur, nitrogen, and aromatic compounds from theinlet to the outlet of the ULSD reactor are provided in Table 1.

Most of sulfur compounds are removed (�90 % conversion)in the top 10 % zone of the reactor (Table 1). Although the con-version of sulfur compounds such as sulfides, thiophenes, and

Rapid advances in increasing the efficiency of the ultralowsulfur diesel (ULSD) process are hindered owing to the lack ofrealistic understanding that could be obtained by lookinginside an operating ULSD reactor. Herein, quantitative informa-tion about the molecular journey of the sulfur, nitrogen, and

aromatic compounds from the inlet to the outlet of a ULSD re-actor has been provided. The improved understanding result-ing from this work is expected to significantly assist catalystdevelopment and mechanistic pathway determination effortsin this important area of research.

Table 1. Sulfur, aromatic compound, and nitrogen conversion inside theULSD reactor.

Progression[a] [%] Conversion [%]Sulfur Aromatic compounds Nitrogen

10 89.6 13.8 45.017 94.9 19.4 59.325 96.6 23.1 70.750 98.9 28.1 89.4

100[b] 99.9 45.1 99.1

[a] Based on distance passed through reactor. [b] Product sulfur con-tent = 11 ppmw.

[a] Dr. T. V. Choudhary, Dr. K. Gong, Dr. P. Ellison, Dr. A. SubbiahPhillips 66 CompanyPhillips 66 Research CenterBartlesville, OK 74004 (USA)Fax: (+ 1) 918-661-8761E-mail : tushar.v.choudhary@p66.com

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benzothiophenes is more than90 %, the conversion of dibenzo-thiophenes is significantly lower.The reactivity of sulfur com-pounds in the top 10 % zone ofthe reactor decreases in the fol-lowing order: sulfides> thio-phenes>benzothiophenes>phenenthrothiophene>dibenzo-thiophenes>4,6-dimethyldiben-zothiophene.

The order of reactivity ob-served for different sulfur com-pounds in the top 10 % zone ofthe ULSD reactor is in agreementwith the results of previous hy-drodesulfurization studies andcan be explained on the basis ofelectron density on the sulfuratom and steric hindrance ef-fects related to the structure of the molecule.[7, 8] For example,4,6-dialkyldibenzothiophenes have extremely low reactivity be-cause of steric hindrance (hinders access to active catalystsites) of the two alkyl groups in the highly hindered C4 and C6positions.[9–11] The drastically different rates of sulfur removalhave a strong effect on the distribution of sulfur species as thereaction progresses along the reactor. The change in the distri-bution of different sulfur molecules (as % of total sulfur in theliquid) with the conversion of the diesel feedstock to a ULSDproduct is shown in Figure 1. Some of the key observationsare as follows:

1) 10 % zone: Dibenzothiophenes, which representa minor component of sulfur compounds at theinlet of the reactor, become the dominant sulfurspecies in the liquid within this zone. Sulfides arealmost completely eliminated and the removal ofmost of the total sulfur compounds results in theproduction of hydrogen sulfide gas, which ishence the dominant sulfur species (of the fluidphases) in the ULSD reactor.

2) 17 and 25 % zones: Owing to the preferentialelimination of the easy-to-remove sulfur com-pounds, the concentration of dibenzothiophenesin the liquid product increases to more than 80 %whereas the concentration of benzothiophenesand thiophenes decreases drastically in the top10 % of the reactor.

3) 50 % zone: Sulfides, thiophenes, and phenanthro-thiophenes are completely removed halfwaythrough the ULSD reactor, and only dibenzothio-phenes along with few benzothiophenes remain in theliquid. Notably, in the first half of the ULSD reactor thediesel sulfur content decreases from 9200 to 112 ppmw(parts per million by weight) and in the second half the re-maining 112 ppmw sulfur content decreases to ULSD levels(11 ppmw).

4) 100 % zone (reactor outlet): The ULSD product (11 ppmproduct sulfur) exclusively consists of dibenzothiophenes.

In view of the importance of dibenzothiophenes (C0 diben-zothiophene: no side chain attached to dibenzothiophene, C1dibenzothiophene: one carbon side chain, etc.), detailed infor-mation on the progression of these compounds along theULSD reactor is provided in Figure 2. The reactivity of dibenzo-thiophenes decreases in the following order: C0>C1>C2>C3>C4 + .

Owing to lower steric hindrance, the C0 and C1 dibenzothio-phenes are much more reactive and completely disappear asthe reaction progresses to 17 and 50 % points in the ULSD re-actor, respectively. The C2 dibenzothiophenes are more reac-tive than the C3 dibenzothiophenes and the C3 dibenzothio-phenes are more reactive than the C4 + dibenzothiophenes

Figure 1. Distribution of sulfur species along the ULSD reactor.

Figure 2. Dibenzothiophene content along the ULSD reactor.

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because of the increasing statistical probability for the occupa-tion of the highly hindered C4 and C6 positions by side chains(alkyl groups).

Although most of the sulfur species are eliminated in thetop 10 % of the ULSD reactor, the corresponding total aromaticcompound conversion is only approximately 14 % (Table 1).The concentration of two- and three-ring aromatic compoundsdecreases, whereas the concentration of one-ring aromaticcompounds increases (Figure 3). This finding is attributed to

the fact that aromatic hydrogenation reactions are sequential(three-ring!two-ring!one-ring) and the reactivity of the firstaromatic ring that is hydrogenated typically increases with thetotal number of fused aromatic rings.[12, 13] Therefore, two- andthree-ring aromatic compounds continue to decrease from theinlet to the outlet of the reactor whereas one-ring aromaticcompounds pass through a maximum. Multi-ring(two- and three-ring) aromatic compounds, owing totheir stronger ability to adsorb on the catalyst sur-face, are significantly more inhibiting than one-ringaromatic compounds.[14, 15] The concentration of theseinhibiting compounds decreases from 26 % at theinlet to less than 5 % halfway through the reactor. Asexpected, the paraffin content remains relatively un-affected in the ULSD reactor and the cyclane contentincreases considerably.

The content of different nitrogen compounds (or-ganic nitrogen and ammonia) as the reaction pro-gresses from the inlet (feed) to the outlet (ULSDproduct) is presented in Figure 4. As ammonia isa product of nitrogen conversion, there is no ammo-nia at the inlet of the reactor. However, it becomesthe dominant nitrogen compound within the top10 % of the reactor. The reactivity of organic nitrogencompounds decreases in the following order: in-

doles>pyridine-type compounds>aniline-type compounds>carbazoles.

Compared with the approximately 14 % conversion observedfor aromatic species in the top 10 % zone, 45 % conversion isobserved for nitrogen species (Table 1). The indole, pyridine-type, and aniline-type fractions disappear quite rapidly in thetop 10 % of the reactor; however, the conversion of carbazolesis relatively slower. Halfway through the reactor, carbazoles aretherefore the main organic nitrogen species. However, overall

at this stage, in terms of nitrogen compounds, the re-actor is dominated by ammonia (�90 % conversionof nitrogen compounds).

Owing to the complexity of real diesel feeds, mostof the published studies continue to use surrogatemodel compounds to develop superior catalysts forincreasing the efficiency of the ULSD productionfrom difficult-to-process diesel feedstocks.[16] Themodel compound-based studies are typically target-ed toward simulating the bottom part of the ULSDreactor, in which the hardest-to-remove sulfur speciesare present predominantly.[17, 18] Given that a consider-able amount of ammonia is present in the bottomhalf of the reactor (Figure 3), it would seem thatmost of the model compound-based studies wouldinclude ammonia to accomplish a realistic simulationof the bottom part of the ULSD reactor. However, formore than 95 % of model compound-based studiesconducted previously, ammonia was not consideredas one of the components in the surrogate modelcompound blend for ULSD studies. One of the rea-sons for this is lack of cogent data that would dem-onstrate that ammonia could actually play an impor-

tant role under industrially relevant conditions. Owing to thehigh content of ammonia expected in the bottom half of theULSD reactor under industrially relevant conditions (Figure 3),it is important to understand the ammonia effect from theviewpoint of choosing the appropriate model compounds forthe diesel surrogate blend. Herein, studies were therefore me-

Figure 3. Distribution of hydrocarbon species along the ULSD reactor.

Figure 4. Nitrogen content along the ULSD reactor.

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ticulously conducted to obtain realistic information about theinhibition effect of ammonia in the bottom half of the ULSDreactor. As a first step, the feed for this study was collected bystopping the reaction halfway through the ULSD reactor. Sub-sequently, controlled reactor studies were conducted to simu-late the bottom half of the ULSD reactor with the collected“real” diesel feed in the presence and absence of ammonia. Aproduct sulfur of 5 ppm was obtained in the absence of am-monia, whereas a product sulfur of 11 ppm was obtained if anappropriate amount of ammonia (based on nitrogen content,see Figure 4) was introduced into the reactor. The difference inproduct sulfur for the two cases provides quantitative informa-tion on the inhibition effect of ammonia. This increase in prod-uct sulfur owing to the inhibition of ammonia halfway throughthe reactor corresponds to an increase in the total ULSD reac-tor size by 20 %; that is, the ammonia inhibition effect isstrong. Excellent reproducibility was obtained herein, whichconfirmed the strong ammonia inhibition effect.

Conclusions

Herein, we demonstrate that ammonia has a strong inhibitioneffect and should be included in model compound-based stud-ies to quantitatively distinguish between catalysts and/or gainrelevant mechanistic understanding if surrogate model dieselfeeds are used. Depending on the nature of the catalyst com-ponents (e.g. , promoters and supports), some catalysts are ex-pected to be more strongly inhibited by ammonia. Similarly, inthe absence of ammonia, obtaining relevant mechanistic infor-mation could be hampered because the inhibition effect ofammonia could suppress a particular pathway more than theother. The inclusion of ammonia as one of the blend compo-nents should therefore increase the effectiveness of the modelcompound-based studies aimed at improving mechanistic un-derstanding and developing superior catalysts in this criticalarea of research. On the basis of the above discussion andquantitative information about the compounds present at dif-ferent points in the reactor, it should now be possible todesign an effective model diesel feed (essentially comprisingof six components: ammonia, hydrogen sulfide, 4,6-dimethyldi-benzothiophene, ethylcarbazole, toluene, and naphthalene) forfuture studies aimed at increasing the efficiency of sulfur re-moval from difficult-to-process feeds. From an overall realisticmodel feed design viewpoint, it clearly makes practical senseto remain as close to the real system as possible and avoid ad-ditional unknowns (e.g. , before this study, the effect of ammo-nia was an unknown under realistic conditions). The data ob-tained herein can also be used to design appropriate modelfeedstocks for oxidative desulfurization studies. Model com-pounds are routinely used as surrogates to advance catalysisareas that involve complex feedstocks. This study reemphasiz-es that a significant understanding of the real catalysis systemis essential to design a model compound blend that will ap-propriately represent the complex feedstock.

Experimental Section

Sulfur analysis method

In the first step, the nonaromatic sulfide molecules were selectivelyoxidized to sulfoxides and separated through adsorption over silicagel. The nonaromatic sulfides were removed to avoid interferencein the quantitative analysis of other sulfur species. The sample(1 g) was reacted with tetrabutylammonium periodate (0.15 g) intoluene/methanol (5:1, 12 cm3) at 358 K for 1 h. After washing thesample with water (2 � 5 mL), it was separated into two fractionson a silica gel column by elution with toluene (nonsulfide fraction)followed by toluene/methanol (4:1) to obtain a sulfide fraction (assulfoxides). The second fraction containing sulfoxides was quantita-tively analyzed for total sulfur. The first fraction was analyzed byusing comprehensive two-dimensional GC with a dimethylpolysilox-ane (nonpolar) column coupled to a 50 % diphenyl 50 % dimethyl-polysiloxane (polar) column. The nonpolar column was separatedaccording to boiling point, and a modulator reintroduced thesample onto the second column, which separated according to po-larity; a sulfur chemiluminescence detector and a flame ionizationdetector were used for sulfur detection and hydrocarbon detec-tion, respectively. Amounts were calculated by using an externalstandard, with the response of 2,5-dimethylthiophene and 2-ethyl-thiophene (average) used for all thiophenes, benzothiophene forall benzothiophenes, and dibenzothiophene for all dibenzothio-phenes. The amounts of sulfur species determined from this analy-sis were combined with the amount of nonaromatic sulfide andnormalized to the total sulfur of the sample. The repeatability ofthis method was found to have a mean deviation of �5 %. Theaforementioned method was applicable only for diesel boilingrange fractions.

Nitrogen analysis method

Nitrogen species were separated from hydrocarbon species intodifferent fractions through solid-phase extraction with silica gel(5 g silica gel in 20 cm3 tubes), and the analysis was performedwith a GC column and a nitrogen chemiluminescence detector.The sample (10 cm3) was loaded on silica gel and treated with hep-tane (20 cm3), and the collected sample was discarded. The whole-fraction nitrogen sample was obtained by eluting with toluene/methanol (2:1, 20 cm3) and after concentrating (to 0.5 cm3) bygentle heating. A new sample (10 cm3) was loaded on silica geland treated with heptane (20 cm3), and the collected sample wasdiscarded. Nitrogen fractions 1 and 2 were each obtained by usingheptane/toluene (1:1, 10 cm3) and concentrating. Fractions 3 and 4were each obtained by using toluene (10 cm3) and concentrating.Fractions 5 and 6 were each obtained by using toluene/methanol(2:1, 10 cm3) and concentrating. The analysis was performed witha 5 % phenyl methyl silicone (DB-5) GC column treated for usewith nitrogen compounds and connected to a nitrogen chemilumi-nescence detector. Multiple compounds were used as calibrationstandards (e.g. , aniline, toluidine, indole, acetonitrile, pyridine, andcarbazole). The detector response for calibration compounds wasfound to be less than 10 % (�5 % mean deviation) over the courseof the analysis. The sum of the areas of fractions (typically between90 and 110 %) from the analyzers was normalized to the total areaof the whole fraction. The nonpolar nitrogen indoles and carb-azoles were in fractions 1–3, whereas the polar nitrogen, aniline,and pyridine types were in fractions 4–6. The pyridine-type fractionincludes quinolines, isoquinolines, and phenanthridines. The re-peatability of this method was found to have a mean deviation of

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�6 %. The aforementioned method was applicable only for dieselboiling range fractions.

Hydrocarbon analysis

Hydrocarbon species (aromatic compounds, paraffins, and cy-clanes) were analyzed by using nitric oxide ionization spectrometryevaluation at Triton Laboratory, and details about the analysis havebeen reported elsewhere.[19]

Hydrotreating experiments

The hydrotreating experiments were conducted in pilot units atthe Phillips 66 Research Center. As these pilot units are routinelyused to conduct optimization research for more than 15 commer-cial ULSD units, extensive efforts have been made over the pastdecade to ensure that the pilot reactors have no issues related towall effects, catalyst wetting, axial dispersion, and so on and canaccurately simulate commercial performance. The reactor wasa carbon steel reactor of 19 mm inner diameter with a maximumallowable working pressure of 23.5 MPa and a maximum tempera-ture of 811 K. The bottom layer (20.3 cm deep) consisted of R268inert (12.7 cm deep, 3 mm particle diameter) beneath Alundum(7.6 cm deep, 0.5 mm particle diameter), which served as a supportto the main catalyst. The main section of the bed contained a ho-mogeneous mixture of the catalyst (16 cm3, 2 mm particle diame-ter) and Alundum (59 cm3, 0.5 mm particle diameter). The heightof the main section was 30.5 cm and regular calibrations were per-formed to ensure uniform temperature in the main catalyst zone.The feed used herein was obtained from one of the Phillips 66 re-fineries and the properties are listed in Table 2. This particular feed

was selected because it was representative of the typical difficult-to-treat diesel feedstocks (contains significant amounts of lightcycle oil and coker diesel). Although the molecular profile ina ULSD reactor was expected to depend on the nature of dieselfeedstock and process conditions, the present study involvinga typical diesel feed and realistic operating conditions provideda wealth of previously unavailable information of generically ap-plicability to advancing ULSD research efforts.

A commercial CoMo catalyst was used for hydrotreating experi-ments. Presulfiding was accomplished with a straight run distillatefeed spiked with dimethyl disulfide (total sulfur content of both

feed and spiking agent was 2 wt %). Presulfiding was performed ata pressure of 3.45 MPa in three steps. The temperature was in-creased gradually from 283 to 519 K and held for 4 h. Then, thetemperature was held at 561 K for 10 h. In step 3, the presulfidingtemperature was held at 611 K for 6 h. The experimental methodfor obtaining information about the molecular profile of sulfur, ni-trogen, and aromatic compounds along the ULSD reactor was asfollows: The pressure was set to 6.9 MPa, the liquid hourly spacevelocity was maintained at 1 h�1 (feed rate: 16 cm3 h�1) and H2/oilratio at 506 N m3 m�3, and the temperature was varied until theULSD product (11 ppm product sulfur) was obtained. The tempera-ture corresponding to the ULSD product was identified as 636 K.Then, feed rates that corresponded to different zones of the reac-tor (10, 17, 25, 50 %) were identified. All other conditions werekept constant (e.g. , pressure, temperature) and only the feed ratewas varied to simulate products from different zones into the reac-tor. The feed rates were estimated by using Equation (1):

Target feed rate ¼ ULSD feed rateprogression ½%� in ULSD reactor

� 100 ð1Þ

The design of this study was based on daily predictions of com-mercial sulfur removal performance by using a single set of kineticparameters (single calibration), despite varying feed and processconditions.[20]

The products at each stage were analyzed for sulfur, nitrogen, andhydrocarbons. For the effectiveness of ammonia inhibition studies,the required quantity of the product was collected at a feed ratethat provided the product halfway through the reactor. This prod-uct was then used as a feed for subsequent experiments. Experi-ments were then conducted at a feed rate twice the feed rate ofthe ULSD reactor to simulate the bottom part of the ULSD reactorin the presence and absence of ammonia. Ammonia (in the formof tert-butylamine) and hydrogen sulfide (in the form of dimethyldisulfide) were added to the feed equivalent to the amountformed halfway through the reactor. Excellent reproducibility wasobtained for the studies, and catalyst deactivation was not a signifi-cant factor for this study on the basis of the relatively small timeand benign process conditions (high hydrogen partial pressureand relatively low temperature).

Keywords: ammonia · desulfurization · heterogeneouscatalysis · reaction mechanisms · sulfur

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Table 2. Feed properties.

Density[a] 0.8871 g cm�3

Contentsulfur 9200 ppmwnitrogen 658 ppmwparaffins 26 %cyclanes 28 %aromatic compounds 46 %

SIMDIS[b] D2887 temperature10 %[c] 473 K50 %[c] 566 K90 %[c] 634 K

[a] At 288.7 K. [b] Simulated distillation method. [c] Distillation tempera-ture based on weight (i.e. , the temperature at which x % of the weight ofthe sample has a lower boiling point).

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alysis by Sulfides, Denmark, 2010.

Received: February 10, 2014

Published online on && &&, 0000

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FULL PAPERS

T. V. Choudhary,* K. Gong, P. Ellison,A. Subbiah

&& –&&

A Glimpse into the Molecular Journeyinside an Ultralow Sulfur DieselReactor

Keeping on the ultralow: Quantitativeinformation about the journey of sulfurmolecules inside an operating ultralowsulfur diesel reactor is provided. This in-formation coupled with other new in-sights obtained under realistic industrialconditions is expected to significantlyincrease the effectiveness of futurestudies aimed at increasing the efficien-cy of the ultralow sulfur diesel process.ppmw = parts per million by weight

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