Fuel 90 (2011) 3571–3576

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Fuel

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Non-catalytic hydrodesulfurization and hydrodemetallization of residua

Sergio Ramírez, Jorge Ancheyta ⇑, Guillermo Centeno, Gustavo MarroquínInstituto Mexicano del Petróleo, Eje Central Lázaro Cárdenas Norte 152, Col. San Bartolo Atepehuacan, Mexico City 07730, Mexico

a r t i c l e i n f o

Article history:Received 24 September 2010Received in revised form 26 January 2011Accepted 20 February 2011Available online 3 March 2011

Keywords:Non-catalyticResiduaHydrothermal desulfurization

0016-2361/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.fuel.2011.02.026

⇑ Corresponding author.E-mail address: jancheyt@imp.mx (J. Ancheyta).

a b s t r a c t

Non-catalytic hydrodesulfurization (NHDS) and hydrodemetallization (NHDM) i.e. hydrothermal desul-furization and demetallization of heavy crude and atmospheric residue was studied in two differentbench-scale units equipped with fixed-bed reactors in series operated in adiabatic and isothermal modes.The reactors were loaded with inert material (silicon carbide). Different feedstocks were used for thermalhydrodesulfurization tests: 13�API heavy crude oil, 21�API crude oil, atmospheric residue from the 13�APIheavy crude oil, and atmospheric residue from the 21�API crude oil. The effects of pressure, residencetime, temperature and type of feed on NHDS and axial reactor temperature profiles were examined.The effect of reaction variables is explained in terms of quenching of the reaction by hydrogen additionand changes in reaction selectivity. The results indicate that selectivity toward the reaction of NHDS ascompared with NHDM do not show important changes when the temperature was increased. Tempera-ture was found to be the main variable that affected the extent of thermal desulfurization and demetal-lization reactions. The different reactor temperature profiles were explained with the type of reactionoccurring in the different sections of the reacting system.

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1. Introduction Various researchers have studied thermal reactions during

Thermal hydrodesulfurization and hydrocracking are unavoid-able phenomena that occur simultaneously during catalytic hydro-processing of heavy oils. Depending upon the reaction conditions,type of feed and experimental set-up, the extent of thermal reac-tions can be such that the behavior of catalysts is masked or notinterpreted properly. The composition and properties of the feedplay an important role in NHDS since paraffinic material tends tobe easier to crack thermally or catalytically, while aromatic feedsare more difficult to convert because the heaviest part tend toagglomerate, thus forming coke [1]. Heavy oils and residua containhydrocarbons that are characterized by large amounts of heteroat-oms and asphaltenes. The nature and chemical structure of thesecomplex components are also other factors that strongly affectthe extent of thermal reactions. For instance, asphaltenes withsmall content of aromatic rings and high number of alkyl sidechains are more feasible to convert [2].

The apparent activation energy of the thermal reaction is higherthan that of the catalytic reaction and it can be the rate limiting step[3]. During thermal hydrotreating, heavy molecules crack into smal-ler ones and therefore favoring the catalytic action for sulfur andother heteroatoms removal [4]. Thermal reactions occur in the spacebetween catalysts particles, in the liquid phase, and also in the gasphase and they are non-sensitive to hindrance restrictions [5].

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hydroprocessing of heavy oil fractions. Marafi et al. [6] conducteda series of experiments at different space velocity and temperature.They concluded that thermal reactions must be taken into accountfor an accurate hydroprocessing data analysis. Juraidan et al. [7]showed that non-catalytic hydrodesulfurization may change from0.35 up to 21% in the range of 330–410 �C of reactor temperature.This means that this thermal reaction is indeed important and itseffects must be taken into consideration when studying the cata-lytic hydrotreating of heavy oils. Available literature is limited toevaluating only the effect of reactor temperature on heteroatomsremoval, and not much emphasis has been put on the effects offeedstock properties, pressure and residence time.

The objective of the present work is to study the thermalhydrodesulfuration of various heavy oils at different reactionconditions. The experiments were carried out in two differentbench-scale units equipped with fixed-bed reactors in seriesoperated adiabatically and isothermally. The reactors were loadedwith a catalytically inert material.

2. Experimental

2.1. Crude oils and residua

Two heavy crude oils with 21 and 13�API and their respectiveatmospheric residua were used for non-catalytic hydrodesulfuriza-tion experiments. The atmospheric residua were obtained by frac-tionation of crude oils in a single stage distillation unit based on

Table 1Properties of the feedstock.

21�APIcrude oil

13�APIcrude oil

AR-21�API AR-13�API

Specific gravity, 60/60�F 0.9260 0.9812 1.0206 1.0505API Gravity 21.31 12.71 7.14 3.20Sulfur, wt.% 3.57 5.22 4.6 6.21Ramsbottom carbon, wt.% 10.87 16.07 17.66 21.48nC7 insolubles, wt.% 11.32 18.78 17.74 25.10Ni, ppm 53.4 84.0 86.6 117.4V, ppm 298.1 423.0 489.0 578.2TBP distillation, �CIBP/5% 23/84 51/136 340/383 374/41010/15% 128/170 199/252 410/435 446/47520/25% 206/242 299/340 454/475 504/52930/35% 280/317 378/421 497/524 551/-40/45% 352/386 468/51750/55% 428/46960% 512Recovery at 538 �C, vol.% 62.9 46.99 36.0 27.3K factor 11.30 11.32 11.59 11.61

3572 S. Ramírez et al. / Fuel 90 (2011) 3571–3576

ASTM D-1160 method. A distillation column with one theoreticalplate was loaded with crude oil in a glass flask. The distillation col-umn was operated in manual mode and the separation of the prod-uct was made without rectification equipment to simulate a flashdistillation system. The first stage of product separation startedat atmospheric pressure. The temperature of the liquid in the bot-tom distillation flask reached approximately 275 �C. The tempera-ture was increased slowly to avoid the thermal cracking of thesample. In a second stage, the column was operated at 100 mmHg of pressure, and finally the sample was distilled at 2 mm Hgof vacuum pressure. The recovered volume percentages of atmo-spheric residue were 70.5 and 56.1 for the 13 and 21�API crude oils,respectively. The main properties of the crude oils and their corre-sponding atmospheric residua are shown in Table 1.

It is observed that in general the heaviest feeds possess thehighest amount of impurities (sulfur, Ramsbottom Carbon andmetals) and the lowest recovery of distillate products at 538 �C.The values of the characterization factor (K) are similar for thetwo crude oils (�11.3) and also for both (residua �11.6), whichis due to the dependence that K value has on boiling temperaturerange and specific gravity, (K = MeABP1/3/sg, where MeABP = MeanAverage Boiling Point, and sg = Specific gravity). The higher valuesof K indicate that the residua tend to be more paraffinic than thecrude oils. Thus, it is anticipated that residua will exhibit higherextent of non-catalytic hydrodesulfurization.

2.2. Experimental set-up

The experimental tests were carried out in two high pressurebench-scale units equipped with fixed-bed reactors as shown inFig. 1. The unit with one reactor was used to determine the reac-tion kinetics and to study the effect of temperature and liquid-hourly space velocity (LHSV). The reactor was loaded with200 mL of silicon carbide and operated in isothermal mode. Vari-ous electric resistances were used to keep the temperature at thedesired value. The gas is analyzed by means of an on-line gas chro-matograph. The liquid product is recovered in an accumulator forfurther analysis.

The effects of temperature, pressure and feed on NHDS werestudied at adiabatic conditions in a two fixed-bed unit. Hydrogenwas fed to the reactors by means of a high pressure compressorand quantified by mass flow meters. High pressure discharge pumpis used to introduce the feed to the reactors. The feed flow is mea-sured and controlled by weight on an electronic scale. The reaction

section consists of two stainless steel reactors (500 mL) that oper-ated as fixed-bed either in adiabatic mode or in isothermal mode.For these experiments the reactors were operated as adiabatic anddown-flow. 500 mL of inert material (silicon carbide) was loadedinto the reactors. Silicon carbide has a density of 3.2 g/cm3 and itis highly chemically inert. It possesses high thermal conductivity,strength and low thermal expansion coefficient (4.0 � 10�6/K)and does not experience phase transitions that would cause dis-continuities during thermal expansion [8].

The adiabatic operation of the reactors was achieved by turningoff the electric resistances of the reactor wall and controlling onlythe temperature at the entrance of the reactor bed. The oven of thereactors was equipped with a second insulation to avoid heatlosses to the ambient.

The unit has three separators; one operated at high temperatureand pressure, another at high pressure, and the third one at lowpressure in which light fraction and hydrotreated product were ob-tained. Water was added to the high pressure separator for neu-tralization of the hydrogen sulfide and ammonia formed duringthe reaction. High purity hydrogen (99.9 mol%) was used for allhydrotreating experiments.

2.3. Reaction condition and analytic techniques

The experiments in the one-reactor unit were conducted at100 kg/cm2 and 890 m3/m3 H2-to-oil ratio (5000 ft3/bbl), andvarying the temperature and LHSV in the ranges of 380–420 �Cand 0.2–0.6 h�1, respectively. The feed used for this study wasAR-13�API.

The following conditions were used to study the effect of thetype of feedstock in the two-reactor unit: 70 and 100 kg/cm2 pres-sure, 890 m3/m3 (5000 ft3/bbl) hydrogen-to-oil ratio and 0.5 h�1

LHSV. The reaction temperature was varied in the range of 380–420 �C. To avoid plugging, it was necessary to heat up the feed ves-sel, separators and pipe lines at 140 �C.

Gas product was analyzed on-line by means of an Agilent 6890model gas chromatograph fitted with TCD.

Specific gravity 60/60�F of crude oils and residua were mea-sured by using a pycnometer according to ASTM D-70 method. En-ergy Dispersive X-ray Fluorescence Spectrometry technique wasused for sulfur content measurements (ASTM D-4294) with a mod-el SLFA-2100 HORIBA spectrometer. Coke-forming tendency of thefeedstocks was measured as Ramsbottom Carbon (ASTM D-524)which determines the amount of carbon residue left after evapora-tion and pyrolysis of oil. The amount of asphaltenes was measuredas insolubles in n-heptane according to ASTM D-3279 method.Atomic absorption was used for determining Ni and V contentswith a model AA Series Solar spectrometer. Distillation curve ofstabilized feeds (TBP Distillation) was determined by means ofASTM D-2892 method.

3. Results and discussion

3.1. One-reactor unit

These tests were conducted to analyze the sulfur and metals(Ni, V) contents in the product at different space velocity and tem-perature and were used to estimate kinetic parameters of the non-catalytic hydrodesulfurization and hydrodemetallization reactions.The isothermal reactor was assumed to operate as ideal plug-flow,so that the corresponding design equation was employed.

The NHDS and NHDM reactions were modeled with the power-law approach. By solving the two equations (reactor model andkinetic model) the following expressions were found to calculatethe sulfur and metals conversions:

Fig. 1. Simplified diagrams of the bench-scale units.

S. Ramírez et al. / Fuel 90 (2011) 3571–3576 3573

NHDScal ¼ 1� kðn� 1ÞLHSV

Sn�1f þ 1

� � 1n�1

ð1Þ

NHDMcal ¼ 1� kðn� 1ÞLHSV

Mn�1f þ 1

� � 1n�1

ð2Þ

where NHDScal and NHDMcal are the calculated sulfur and metalsconversion respectively, k the rate constant, n the reaction order,LHSV the space velocity, Sf and Mf are the sulfur and metals con-tents in the feed, respectively. k and n may be different for eachreaction.

The best set of kinetic parameters was determined by using anoptimization algorithm based on the sum of square errors betweenexperimental and calculated conversions for sulfur and metals. Acomparison between the experimental results and those calculatedwith Eqs. (1) and (2) is presented in Figs 2 and 3, respectively. Good

agreement between the two values is observed. The reaction orderand activation energy for the NHDS reaction were found to be 1.9and 48.2 kcal/mol, respectively. For NHDM reaction, they were 1.4and 42 kcal/mol, respectively.

Marafi et al. [6] also conducted an NHDS evaluation with Boscancrude having higher sulfur and metals content (S = 5.54 wt.%,Ni + V = 1353 ppm) and similar reaction conditions than thoseused in our kinetic study. For the NHDS reaction they reportedreaction order of 1.74 and activation energy of 25.42 kcal/mol.The activation energy reported by these authors is about a half ofthat determined by us, while for the case of NHDM, they report val-ues of between 1.38 and 1.79 for reaction order and 43.83 and50.665 for the activation energy. The observed differences areclearly due to the type of feed used in both experiments.

Figs 2 and 3 also show the effect of both LHSV and temperatureon NHDS and NHDM. As expected, it is observed that at low tem-perature and high LHSV the non-catalytic conversion of sulfur is

0

5

10

15

20

25

30

370 380 390 400 410 420 430

NH

DS,

%

Temperature, °C

Fig. 2. Non-catalytic sulfur conversion of AR-13�API feed at different temperatureand LHSV. (d) 0.6 h�1, (j) 0.5 h�1, (s) 0.4 h�1, (h) 0.3 h�1, (N) 0.2 h�1, (—)predicted.

0

1

2

3

4

5

6

7

370 380 390 400 410 420 430

NH

DM

, %

Temperature, °C

Fig. 3. Non-catalytic metals conversion of AR-13�API feed at different temperatureand LHSV. (j) 0.5 h�1, (s) 0.4 h�1, (h) 0.3 h�1, (—) predicted.

Fig. 4. Total thermal hydrodesulfurization (NHDS2) of heavy crude oil andatmospheric residua as a function of temperature and pressure: (�) 13�API crudeoil, (N) AR-13�API, (�) AR-21�API, (j) 21�API crude oil.

3574 S. Ramírez et al. / Fuel 90 (2011) 3571–3576

minimal (<2%). However at the most severe conditions(LHSV = 0.2 h�1 and T = 420 �C) as high as 25% of NHDS was ob-tained, while NHDM was kept below 6%.

3.2. Two-reactor unit

To evaluate the extent of non-catalytic hydrodesulfurizationreaction in the two-reactor unit, the following expression wasused:

NHDSi ¼Sf � Si

p

Sf

!� 100 ð3Þ

where Sf and Sip are the sulfur contents (in wt.%) in the feed and in

the product respectively, and i (i = 1, 2) represents the number ofreactor. NHDS1 is the removal of sulfur in reactor 1, while NHDS2

is the accumulated removal of sulfur in the two reactors. This equa-tion does not take into account the possible change in liquid massflow rate which is expected to be minimal.

Fig. 4 shows the results of non-catalytic hydrodesulfurizationobtained at the exit of reactor 2 for the four feeds at operatingpressures of 100 and 70 kg/cm2. It is observed that, in general,the higher the temperature the higher the sulfur removal. The or-der of NHDS with respect to the feeds is the following

13�API crude oil � 21�API crude oil � AR � 21�API

< AR � 13�API

The NHDS of the heavier feedstock is higher than the NHDS ofthe other feedstocks. The type of sulfur compounds present in

the residua are the same as those of the crude oil from which theywere obtained. Also, asphaltenes and metals are the same in bothcrude oil and residua, after separating the light fraction, the totalamount of these compounds is concentrated in the residua. Thismeans that the only difference between the two types of feeds(crude oil and residue) is the light distillate separated from thecrude oil by distillation. The presence of light fractions and theircomposition is then affecting sulfur removal. Thermal cracking ismore feasible for heavy paraffinic hydrocarbons compared withlighter ones. During cracking of heavy molecules the sulfur at-tached to them is released, that is why residua exhibited higherNHDS than crude oils [9,10].

At low temperature (<390 �C) the level of NHDS is relativelysmall, however when temperature is higher than 390 �C NHDS rap-idly increases. This behavior is due to the high values of activationenergy of thermal desulfurization reactions [3]. These results agreewith those reported by Juraidan et al. [7] for non-catalytic hydro-thermal of Boscan crude.

The path for thermal reactions is through the mechanism of freeradicals [3], therefore, it is expected that at higher hydrogen partialpressure in the reactor, the saturation reaction should be favored,and in this way hydrogen quenches the free radicals, subsequentlyimpeding higher conversions [11]. However, within the range ofreaction conditions studied in this work the effect of pressure onNHDS was insignificant, which indicates that the prevailing effectis that given by the temperature.

The effect of residence time can be examined by sampling andanalyzing the product obtained at the exit of reactor 1. The resultsof NHDS1 are shown in Fig. 5. In general, the same behavior as thatfound at the exit of reactor 2 was observed. The lighter feedstocksshowed similar NHDS meanwhile the NHDS of the heavier feed-stock is considerably higher. The difference is that NHDS1 < NHDS2,which is due to the shorter residence time in reactor 1. It should bementioned that between the two reactors a hydrogen quenchingstream is added to avoid excessive reactor delta-T and also tomaintain the same hydrogen-to-oil ratio in both reactors. Thismeans that the NHDS profile of reactor 2 is not a continuation ofthat of reactor 1. Both reactors have similar inlet temperatureshence, the NHDS profiles are similar.

Fig. 5. Reactor 1 thermal hydrodesulfurization (NHDS1) of heavy crude oil andatmospheric residua as a function of temperature and pressure: (�) 13�API crudeoil, (N) AR-13�API, (�) AR-21�API, (j) 21�API crude oil.

S. Ramírez et al. / Fuel 90 (2011) 3571–3576 3575

The temperature dominates the NHDS of the four feedstockshowever, the nature and composition of the feeds also affectNHDS. The heaviest feed (AR-13�API) has higher sulfur conver-sion, which is due to its paraffinic nature (highest K value,

0

20

40

60

80

100

120

140375 390 405 420 435

Rea

ctor

leng

th, c

m

380 C, 100 Kg/cm2 0

20

40

60

80

100

120

140375 390

Temp

420 °C,

Temperature,°C

°

Fig. 6. Axial temperature profile along the two reactors as a function of feedstock, tempecrude oil.

Table 1). Kin et al. [4] conducted hydrodesulfurization tests ofbitumen-derived heavy oils with Na impregnated on aluminaand explained that sulfur removal is the result of thermal reac-tions involving aliphatic sulfides present in heavy oils. Sulfurconversion was reported to be from 3 up to 15% in the range of370–410 �C. Sanford [12] suggested that the first step of conver-sion mainly involves the breaking of labile carbon-to-carbonbonds to produce distillate. Additional conversion results fromthe formation of aromatic biradical intermediate, which is formedfrom hydroaromatic structures. Hydrogen transfer to the aro-matic-carbon radical center stops condensation reaction leadingto coke formation. The hydrogen radical produced as part of thehydrogen-transfer plays an important role in the process reactingwith a condensed aromatic center, finally producing significantamounts of gas and distillate from large aromatic molecules.The reaction mechanism proposed by Yang et al. [10] and Dong[11] considers that the conversion of paraffinic chains is the firststep for hydrothermal reactions. Therefore more paraffinic feedsare more labile to react under NHDS conditions (high tempera-ture and hydrogen pressure). These large and branched moleculesare easier to hydrocrack [10] compared with residua. Crude oilscontain higher amounts of molecules of low molecular weightwhich are more stable and consequently NHDS is smaller.

Compared with catalytic HDS in which higher sulfur removal isexpected when the pressure is increased, for NHDS (thermal, non-catalytic) the sulfur removal is governed by free radical mechanismand the presence of hydrogen may quench the free radicals in-volved during sulfur removal. The heavier and more reactive feeds(those that generate higher number of free radicals) are more sen-sitive to the increase of reaction pressure as that causes saturationof free radicals [12].

405 420 435

erature,°C

100 Kg/cm2

0

20

40

60

80

100

120

140375 390 405 420 435

420 °C, 70 Kg/cm2

Temperature,°C

rature and pressure: (�) 13�API crude oil, (N) AR-13�API, (�) AR-21�API, (j) 21�API

1.5

2.0

2.5

3.0

3.5

370 380 390 400 410 420 430

NH

DS/

NH

DM

Temperature, °C

Fig. 7. NHDS/NHDM ratio: (j) 0.5 h�1, (s) 0.4 h�1, (h) 0.3 h�1.

3576 S. Ramírez et al. / Fuel 90 (2011) 3571–3576

Fig. 6 shows the axial temperature profiles along the two reac-tors as function of feedstock at different reaction conditions. Theprofiles were plotted for three different conditions: low tempera-ture–high pressure, high temperature–high pressure and high tem-perature–low pressure. The length of the bed in each reactor is50 cm and both reactors operate in down-flow mode (the arrowsindicate the flow direction from top to bottom of the reactor).For all the feeds, the hydrocarbon enters the first reactor about3 �C lower than the temperature set point, and then it reachesthe desired value, after which it remains constant through thebed. Near the exit of the reactor, there is a temperature changeof 5–8 �C. At the exit of the reactor the temperature decreasesprobably due to heat losses at that point. The product obtainedfrom reactor 1 is mixed with hydrogen and the mixture entersreactor 2 through an isolated pipeline.

At the entrance to reactor 2 the hydrogen-hydrocarbon mixtureis heated to reach again the set point temperature, which is thesame as that at the entrance of reactor 1. Temperature profileswithin reactor 2 show a decrease in temperature values. It seemsthat endothermic reactions are taking place from the entrance upto 60–70% of the length of reactor 2, and in the remaining sectionthere is a release of heat causing an increase of the set point reactortemperature of about 15–25 �C.

This changing behavior from endothermic to exothermic reac-tions is explained as follows. Two types of reactions commonlytake place in thermal processes; (1) scission reactions that areendothermic and produce small molecules from larger molecules,and (2) condensation that are exothermic and converts smallmolecules to larger molecules. Therefore, in the present experi-mental set-up, within the first reactor, the reaction is more scis-sion-dominated and once smaller molecules are formed theyundergo condensation reaction thus increasing the temperatureof reactor 2.

The results obtained at high temperatures showed an importantchange in the selectivity of the thermal reactions of each feed. Theconversion of those reactive species susceptible to be hydrocracked(hydrodesulfurized) causes the dehydrogenation reaction to bepromoted resulting in the adsorption of heat. When the reactor isoperated at low temperature, the conversion of the reactive speciesis lower and thermal NHDS is the prevailing reaction. Operatingthe reactor at different temperatures involves changes in the equi-librium constant of the hydrogenation-dehydrogenation reactionsof aromatic and naphthenic species. This phenomenon has beendemonstrated by numerous researchers. For instance, Demireland Wiser [13] have reported values of equilibrium constants forhydrogenation reaction of a number of aromatic compounds. Thevalues of equilibrium constant change from 107 to 10�4 in therange of 325–425 �C for phenanthrene hydrogenation reaction.This phenomenon involves a change in the thermal behavior ofthe reacting system, in which the reactions go from exothermicto endothermic when the temperature is increased.

The detailed study and modeling not only of thermal but also ofcatalytic hydrocracking reaction is very complex due mainly to thecomplexity of the feed composition and the difficulty of its charac-terization, and also the complexity of the great variety of reactionsoccurring during thermal catalytic hydrocracking [14].

The ratio of sulfur removal/metals removal (NHDS/NHDM)against the temperature is depicted in Fig. 7. The results showedan increase of this ratio from 2 to 3.5 in the 380–420 �C tempera-ture range. High LHSV, i.e. short residence time, causes the NHDSreaction extent to be higher than that of NHDM, which implies thatNHDM reaction requires more residence time to proceed. It is well-known that metals (Ni and V) are mainly concentrated in heavymolecules (asphaltenes), which are unsaturated in nature, whilesulfur is either non-asphaltenic or forming bridges within theasphaltene molecules. It can be then expected that sulfur is easier

to remove than metals. In addition, the concentration of sulfur inthe feed is by far much higher than that of the metals(5.869 wt.% sulfur, 0.06924 wt.% metals). As for the effect of tem-perature, Fig. 7 indicates that straight lines at each space velocityare obtained when it is increased in the range of 380–420 �C, whichmeans that the non-catalytic removals of sulfur and metals are notselective. Both reactions increase their rate when the temperatureis increased as any other chemical reaction, but they are not ori-ented to the removal of sulfur in spite of this element is much moreconcentrated in the feed than metals, and can be located in moreaccessible location in the asphaltene molecule.

4. Conclusion

Thermal hydrodesulfurization is an important reaction duringhydrotreating of heavy oils. The extent of thermal hydrodesulfuri-zation and hydrodemetallization reactions is related to variousreaction conditions: feedstock quality, temperature, residence timeand pressure. Temperature is the main variable that commands theextent of thermal reactions. High pressure inhibits thermal desul-furization due to quenching of free radicals by working in a rich-hydrogen atmosphere. The degree of thermal hydrodesulfurizationcan be as high as 25 wt.% at reactor temperature of 420 �C andLHSV of 0.2 h�1, while thermal hydrodemetallization is lower than6%. At longer residence time the reaction becomes more exother-mic due to the condensation reactions that convert the small mol-ecules formed in the first part of the reacting system into largermolecules.

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