catalyst Deactivation in hydro processing

7/28/2019 catalyst Deactivation in hydro processing

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Effect of Catalyst Deactivation and Reaction Time on

Hydrocracking Heavy Hydrocarbon Liquids

Marcos Millan,* Cristina Adell, Cecilia Hinojosa, Alan A. Herod, Denis Dugwell, andRafael Kandiyoti

Department of Chemical Engineering, Imperial College London, London SW7 2AZ, UK

ReceiVed September 15, 2006. ReVised Manuscript ReceiVed January 25, 2007

The activity and fouling behavior of a commercial NiMo/Al2O3 catalyst has been studied during thehydrocracking of a coal extract at short reaction times. The catalyst was precoated with a carbonaceous depositbeforehand in order to study the interaction between a coated catalyst and fresh feed. The conversion of theheavier fraction (boiling point above 450 C) of the feed steadily increased with time. However, the progressionin the amount of carbonaceous deposits on the coated catalyst was not uniform. A large initial deposition wasobserved when the catalyst and fresh feed were placed in contact. This initial deposition was reversible, andwithin the first 10 min of reaction, much of the deposit redissolved into the liquid phase with increasingreaction times. This bulk exchange of material between the deposits and the solution appears to be a mechanismwhich would help explain the sustained level of catalytic activity despite the large carbonaceous deposition.A harder more permanent deposit layer built up more gradually as reaction time increased.

1. Introduction

Catalysts employing molybdenum promoted by either cobaltor nickel, supported on alumina, are commonly employed forhydroprocessing in the oil industry. These catalysts suffer certaindegrees of deactivation during the first hours on stream andsettle down to operate at lower levels of activity. Theiravailability and widespread use make them suitable referencecatalysts in the study of more novel catalytic hydrocrackingmethods. Catalyst deactivation has a direct impact on theeconomic viability of heavy hydrocarbon upgrading processes,such as coal liquefaction and heavy oil hydroprocessing. As

the necessity for heavy hydrocarbon processing grows, thenecessity for understanding the processes involved in catalystdeactivation becomes more compelling.

Fouling, through the deposition of heavy hydrocarbons onactive surfaces and pore plugging, is thought to be responsiblefor the quick loss of catalytic activity affecting both thehydrogenation and cracking functionalities of NiMo basedcatalysts.1 The rate and extent of the deactivation processesappear to be strongly dependent on the characteristics of thesample being processed; deactivation is usually slower duringthe hydroprocessing of light feeds.

A commercial NiMo/Al2O3 commonly used in the oil industryas a hydrotreating catalyst has been extensively tested in thislaboratory, during hydrocracking studies of several heavy

petroleum-derived liquids and coal liquefaction extracts.2-5

Zhang et al.2,6 investigated the extent of hydrocracking of a coalliquefaction extract as a function of reaction time.

During initial experiments, high conversions were observedfor fractions of the feed with a boiling range above 450 C (the>450 C fraction), during the first 30 min of hydrocracking.This stage was followed by a continuous slowdown in thereaction rate up to 120 min.2,6 No significant changes inconversion were detected at reaction times longer than 120 min.In principle, this decrease in conversion rate could have beendue to catalyst deactivation. However, an alternative explanationwould be that in the batch system used in those studies, themost readily hydrocrackable fractions of the sample reactedwithin the first 120 min, leaving only the most intractablematerials. This would lead to loss of sample reactivity of the

batch, which would be undistinguishable from catalyst deactiva-tion. Both factors were evaluated in subsequent work by Begonet al.,7,8 and the loss of sample reactivity was identified as thepredominant cause of the observed slowdown in reaction rates.Although some degree of catalyst deactivation could be directlyobserved, the catalyst was still considerably active after 6 h ofreaction.

The changes in the extent of hydrocracking during the earlystages of the reaction (times shorter than 30 min) have alsobeen addressed by Begon et al.7 A somewhat unexpected patternof hydrocracking conversions was observed within the first 10min of the reaction. A drop in the >450 C boiling fraction

* Corresponding author. E-mail address: [email protected].(1) Furimsky, E.; Massoth, F. E. Deactivation of hydroprocessing

catalysts. Catal. Today 1999, 52, 381-495.(2) Zhang, S.-f.; Xu, B.; Herod, A. A.; Kandiyoti, R. Hydrocracking

reactivities of primary coal extracts prepared in a flowing-solvent reactor.Energy Fuels 1996, 10, 733-742.

(3) Zhang, S.-f.; Herod, A. A.; Kandiyoti, R. Effectiveness of dispersedcatalysts in hydrocracking a coal liquefaction extract: a screening study.Fuel 1997, 76, 39-49.

(4) Bodman, S. D.; Mc Whinnie, W. R.; Begon, V.; Suelves, I.; Lazaro,

M. J.; Morgan, T. J.; Herod, A. A.; Kandiyoti, R. Metal-ion pillared claysas hydrocracking catalysts (I): Catalyst preparation and assessment ofperformance at short contact times. Fuel 2002, 81, 449-459.

(5) Bodman, S. D.; Mc Whinnie, W. R.; Begon, V.; Millan, M.; Suelves,I.; Lazaro, M. J.; Herod, A. A.; Kandiyoti, R. Metal-ion pillared clays ashydrocracking catalysts (II): effect of contact time on products from coalextracts and petroleum distillation residues. Fuel 2003, 82, 2309-2321.

(6) Zhang, S.-f.; Xu, B.; Moore, S. A.; Herod, A. A.; Kandiyoti, R.Comparison of hydrocracking reactivities of coal extracts from a flowing-solvent reactor, a mini-bomb and a pilot plant. Fuel 1996, 75, 597-605.

(7) Begon, V.; Megaritis, A.; Lazaro, M. J.; Herod, A. A.; Dugwell, D.R.; Kandiyoti, R. Changes in sample reactivity and catalyst deactivationduring early stages of the hydrocracking of a coal extract. Fuel 1998, 77,1261-1272.

(8) Begon, V. Ph.D. thesis, Imperial College, University of London, 1998.

1370 Energy & Fuels 2007, 21, 1370-1378

10.1021/ef060466o CCC: $37.00 2007 American Chemical SocietyPublished on Web 03/17/2007


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content took place during the heatup of the sample to 440 C.Conversion increased in the next 5 min to show a maximum,which was followed by a diminution leading to a minimum inconversion at 10 min. After this period, the conversion wasobserved to increase monotonically as described in earlierreports.2,6

The minimum in conversion observed at 10 min was thoughtto be associated with the repolymerization of reactive moleculesand radicals released through thermal cracking during heatup

and the first 5 min of reaction time. It would appear that, duringthis early stage, the activation (by sulfidation) of the fresh NiMo/Al2O3 catalyst was still in progress and the catalyst not yet fullyactive.7

Deposition on the catalyst was found to be strong when freshcatalyst was placed in contact with fresh feed.7 The carbon-aceous layer coating the catalyst after a heatup only run (0min of holding at the peak temperature) represented almost 20%of the total filter cake weight. The amount of deposits steadilydecreased with reaction time, and after the 120 min run, only11% of the filter cake consisted of carbonaceous material. Thissuggested a process of reaction and redissolution of the depositsin the liquid phase.

When an already coated catalyst was reused in contact with

fresh feed in successive 120 min experiments, some newdeposition took place, leading to an increase in the total amountof carbonaceous deposit on the catalyst. However, the amountof deposit added to the carbonaceous layer became progressivelysmaller; thermogravimetric analysis (TGA) combustion tests onthe solid deposits showed that the carbonaceous layer tendedto become less reactive. The process was slow and in apparentagreement with observations suggesting the relative insensitivityof the amount of deposits to time on stream in continuousreactors.9-11

The early stages of the hydrocracking process are addressedagain in the present work with two key modifications toexperimental design. First, the Point of Ayr coal extract (alreadycontaining recycle solvent) is hydrocracked in the absence of

tetralin as solvent. This allowed the examination of productsfree of tetralin and products of the reactions of tetralin. Reactionconditions were therefore closer to conditions in the larger (pilot)scale process, where no extra hydrogen donor compound wasadded to the coal extract. Second, already coated NiMo/Al2O3catalyst samples have been used to study the processes takingplace in the first few minutes of contact between a coatedcatalyst and the fresh feed. In previous experiments, rapid earlydeposition on fresh catalyst surfaces tended to skew productcompositions. Precoating catalyst samples thus serves to betterrepresent (in a batch reactor) the process where fresh feed iscontinuously added to the pilot reactor and comes in contactwith a catalyst that has already seen a level of fouling anddeactivation.

2. Experimental

2.1. Hydrocracking Experiments. The microbomb reactor andthe experimental procedure have been described elsewhere.2,6 Thisreactor is operated in batch mode. A 1 g portion of hydrocrackerfeed from the liquid solvent extraction process development pilot

plant at Point of Ayr12 and 250 mg of a previously coated NiMo/Al2O3 catalyst (see below) were charged into a 5 mL reactor. Nosolvent was added to the reaction mixture. Reaction time, definedas the holding time at 440 C, was varied from 0 (heatupimmediately followed by cooldown) to 120 min. Independentexperiments were run for each reaction time. This was done in orderto avoid taking out sample during the experiment, which couldaffect the results due to the relatively small amount of sample used.

2.2. Precoating of Catalyst with Carbon Deposit. The NiMo/Al2O3 catalyst was coated with carbonaceous materials beforehand.

The starting material was fresh PBC-90D. This catalyst consists ofan active phase of MoO3 (8 wt %) and NiO (4 wt %) dispersed ona -Al2O3 support. It is preimpregnated with a proprietary organiccompound containing sulfur, whose composition is kept confidentialby the catalyst supplier. This compound releases H2S during heatupenabling catalyst activation. The particle size of this commercialcatalyst was less than 250 m.

A 1.5 g portion of coated NiMo/Al2O3 was prepared in two steps.First, 900 mg of the catalyst and 1.5 g of a coal tar pitch werecharged into the microbomb reactor and kept during 2 h at 350 Cand 170 bar hydrogen pressure. That process was repeated toduplicate the amount of coated catalyst available. The solidsrecovered from the two runs were washed with N-methyl-2-pyrrolidinone (NMP) to remove soluble materials and mixed. In asecond step, 1 g of the coal tar pitch was mixed with 1.5 g of the

catalyst mixture recovered from the previous duplicated runs. Thereaction took place at 370 C and 170 bar hydrogen pressure. NMPwas again used to wash the reaction mixture. The process wasrepeated to generate larger amounts of coated catalyst. Therecovered catalyst was heavily coated; its carbonaceous content wasbetween 30 and 35% of the total solids.

2.3. Thermogravimetric Analysis. A Perkin-Elmer TGA7thermogravimetric analyzer was used to determine both the >450C fraction of the liquid products, (f>450C)liq, and the feed, (f>450C)feed,and the carbonaceous deposition on the catalyst, fcarb. These valuesare applied in the calculation of conversion as shown below.

2.3.1. ThermograVimetric Analysis: >450 C Fraction. Conver-sions of the >450 C fraction were obtained from TGA data by amethod described in detail elsewhere.2 Briefly, it consists of heatingthe sample under a flow of helium and measuring the fraction of

the sample evaporated as a function of temperature on the TGApan. A calibration developed by Zhang et al.13 is applied to relatethe evaporation temperature on the TGA pan with the normalboiling point. According to this calibration, a TGA temperature of247 C corresponds to a boiling point of 450 C.

However, the use of no solvent in these hydroracking experimentsallowed modifications to the temperature program to be introduced.The original temperature program included a 2 h step in which thesample was kept at 50 C in order to evaporate tetralin. This stephas been eliminated to adapt the method to runs without tetralin.Instead, the sample is heated directly from 30 to 247 C, shorteningthe analysis time.

In addition, as no tetralin is introduced in the system, TGA dataobtained in the region of boiling points below 450 C is free fromsolvent-derived materials. This enabled the generation of boiling

point distributions in this region. They are obtained as the derivativeof the TGA weight vs temperature curve.

2.3.2. ThermograVimetric Analysis: Carbonaceous Depositionon the Catalyst. The carbonaceous deposits are burnt in a TGAunder a flow of air. Their fraction in the recovered catalyst iscalculated by difference with the remaining weight on the TGApan. The sample is heated up to 600 C and kept at that temperaturefor 30 min. Details on the procedure have been presentedelsewhere.14(9) Richardson, S. M.; Nagaishi, H.; Gray, M. R. Initial Coke Deposition

on a NiMo/-Al2O3 Bitumen Hydroprocessing Catalyst. Ind. Eng. Chem.Res. 1996, 35, 3940-3950.

(10) Benito, A. M.; Martinez, M. T. Catalytic hydrocracking of anasphaltenic coal residue. Energy Fuels 1996, 10, 1235-1240.

(11) Matsushita, K.; Hauser, A.; Marafi, A.; Koide, R.; Stanislaus, A.Initial coke deposition on hydrotreating catalysts. Part 1. Changes in cokeproperties as a function of time on stream. Fuel 2004, 83, 1031-1038.

(12) Kimber, G. M. Energy for the future - coal liquefaction for theEuropean EnVironment; Report No. Coal R078, Department of Trade andIndustry: United Kingdom, 1997.

(13) Zhang, S.-f. Ph.D. thesis, Imperial College, University of London,1995.

Hydrocracking HeaVy Hydrocarbon Liquids Energy & Fuels, Vol. 21, No. 3, 2007 1371


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As an already coated catalyst is reused in this work, the amountof material addedto the carbonaceous layer needs to be calculated.From TGA measurements, the carbonaceous contents as a fraction

of the total solids before ( fcarb and after ( fcarbF ) the run are known.

The fraction of carbonaceous layer due to new deposition on thecatalyst fcarb can be calculated as follows:

This value offcarb is added to the >450 C fraction in the liquid

products in runs where the catalyst has been reused.2.4. Definition of Apparent and Real Conversions. The

apparent conversion is obtained by taking into account only the>450 C fraction in the liquid products (CHCl3/CH3OH soluble):

where mfeed and mliq are the weight of the feed and the liquidrecovered after hydrocracking, respectively. The terms (f>450C)feedand (f>450C)liq are the fractions of material with boiling points above450 C in the feed and liquid products, respectively.

As high apparent conversions could be achieved by removal of>450 C material as deposits on the catalyst just as well as byhydrocracking, the evaluation of the catalyst effectiveness cannot

only be based on this value. The solids deposited on the catalystare considered as part of the >450 C fraction of the products tocalculate the real conversion. It is defined as follows:

where msol is the weight of solids recovered and fcarb is thecarbonaceous fraction of the solids deposited during the run.

2.5. Size Exclusion Chromatography (SEC). The SEC systemhas been described elsewhere.15 Briefly, a polystyrene/polydivi-nylbenzene SEC column from Polymer Laboratories Ltd (Mixed-Acolumn) was used in this work. The porosity range of this columnis not available due to manufacturer confidentiality. The linear rangeof log MM vs retention time is up to 15 million for this column,

according to a calibration based on polystyrene standards. Thecalibration of SEC columns has been discussed in detail elsewhere.16

A 0.5 mL/min flow of NMP is pumped through the system,which remains at room temperature. The signal is detected byultraviolet absorption at five wavelengths 280, 300, 350, 370, and450 nm. The latter is recorded by a variable wavelength Perkin-Elmer LC250 UV detector set at 450 nm whereas the rest aremeasured by a diode array detector. An evaporative light scatteringdetector is also connected in series with the UV ones. Data fromall six channels are collected, displayed, and saved in a PC.

Software developed in this laboratory is applied to normalizeintensities in order to allow comparison among runs where differentamounts of material were injected. In this work, area normalizationwas applied to chromatograms recorded by the 350 nm UV-Adetector.

2.6. UV Fluorescence Spectroscopy. A Perkin-Elmer LS50luminescence spectrometer has been used in the static-cell mode.The UV-F spectra have been recorded in three modes: synchronous,emission, and excitation. The emission spectrum is obtained by

exciting the sample at a constant wavelength and recording theemission in the full range of wavelengths. The excitation spectrumis recorded at a fixed wavelength while the excitation frequency ischanged over the range. In the synchronous spectrum, bothexcitation and emission are simultaneously varied but the wave-length difference is kept constant at 20 nm. In this mode, thefluorescence of sample solutions in NMP has been measuredbetween 250 and 800 nm. The samples were scanned at 240 nm/min.

2.7. Specific Surface Area Measurements. A Micromeritics

2000 ASAP surface area analyzer was used for specific surfacearea measurements. Approximately 200 mg of sample wereemployed in each analysis. The standard method based on theadsorption of nitrogen on the surface was applied.17 The sampleswere dried overnight at 150 C in a vacuum oven prior to theexperiment to remove moisture and adsorbed gases from the catalystpores. The model developed by Brunauer, Emmett, and Teller (BETmodel) was applied to calculate the surface area. Pore sizedistributions were obtained by the BJH method (developed byBarret, Joyner, and Halenda) based on the desorption isotherm.

Specific surface area measurements were applied to the freshNiMo/Al2O3, the coated NiMo/Al2O3 used as catalyst in these runs,and the catalysts recovered from the 0, 10, and 30 min runs.

3. Results

3.1. Boiling Point Distributions. As already mentioned,experiments in the absence of a donor-solvent (e.g., tetralin) inthe feed to the reactor allow solvent-free data to be collected inthe full range of boiling point distributions.

These distributions have been obtained from the derivativesof the TGA weight vs temperature curves used to determinethe boiling point distribution of the feed and the products. Thedata exhibited a marked dependence on the weight loaded onthe TGA pan. Larger amounts of sample on the TGA pandelayed the evaporation of the sample, which translated into ashift in the distributions toward higher boiling points. It hasbeen observed that a difference of 2 mg in weight loaded couldproduce shifts in the boiling point distributions of around 25C. Although in the present experimental array the amount ofsample loaded in the TGA pan cannot be controlled withsufficient precision, the method can still be useful in indicatinggeneral trends between samples. This is exemplified in Figure1 where boiling point distributions of the feed and the 0 (heatuponly) and 10 min hydrocracked products are presented. The feedshowed a larger proportion of material in the relatively highboiling point end of the distribution (peak with a maximumaround 350 C) than the 0 and 10 min hydrocracked products.The boiling point distribution has shifted toward lower valuesfrom feed to products, which show a larger proportion of theirmaterial under the peak at around 250 C.

3.2. Conversions. Table 1 shows the apparent and realconversions of the >450 C boiling fraction as a function of

reaction time. As described above, the difference between thesetwo conversions resides in the fact that the apparent conver-sion does not consider the carbonaceous deposits as part of the>450 C boiling fraction of the products.

By contrast, deposits are taken into account as part of the>450 C boiling materials in the calculation of the realconversion.

Hydrocracking of this fraction exhibited some progress duringthe reactor heatup to 440 C. Here, 13% of the >450 C boilingfraction was converted into lower boiling point materials within

(14) Begon, V.; Warrington, S. B.; Megaritis, A.; Charsley, E. L.;Kandiyoti, R. Composition of the carbonaceous deposits and catalystdeactivation in the early stages of the hydrocracking of a coal extract. Fuel1999, 78, 681-688.

(15) Lazaro, M. J.; Islas, C. A.; Herod, A. A.; Kandiyoti, R. Calibrationof Size Exclusion Chromatography in 1-Methyl-2-Pyrrolidinone for Coal-Derived Materials Using Standards and Mass Spectrometry. Energy Fuels1999, 13, 1212-1222.

(16) Karaca, F.; Islas, C. A.; Millan, M.; Behrouzi, M.; Morgan, T. J.;Herod, A. A.; Kandiyoti, R. The calibration of size exclusion chromatog-raphy columns: Molecular mass distributions of heavy hydrocarbon liquids.

Energy Fuels 2004, 18, 778-788.

(17) McDonnell, M. E.; Walsh, E. K. In A guide to materials charac-terization and chemical analysis; Sibila, J. P., Ed.; VCH Publishers: NewYork, 1988; pp 257-261.

fcarb )fcarbF-fcarb(1 -fcarb

F )/(1 -fcarb)

Apparent Conversion )mfeed(f>450C)feed - mliq(f>450C)liq

mfeed(f>450C)feed

Real Conversion )mfeed(f>450C)feed - mliq(f>450C)liq - msolfcarb

mfeed(f>450C)feed

1372 Energy & Fuels, Vol. 21, No. 3, 2007 Millan et al.


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that period. The real conversion steadily increased with timeup to 120 min, the longest time tested. The slowdown in thereaction rate observed after 60 min was probably due to loss ofsample reactivity as observed in previous work.2,6,7 By contrast,the apparent conversion showed a marked reduction between 0and 10 min to continuously increase thereafter.

The amount of carbonaceous material deposited on top ofthe precoated deposits on the catalyst during the hydrocrackingis also shown in Table 1. Despite the precoating, significantnew deposition of carbonaceous material took place on thecatalyst during the 0 min run. The amount of deposits sharplydecreased within the first 10 min of reaction (at temperature),clearly indicating redissolution of the carbonaceous layer,

thereafter presenting slower variations between 10 and 60 min.The negative additional carbonaceous deposition shown in

Table 1 for the 30 min run could be due to either experimentalerror or a small net dissolution of precoating material on thecatalyst taking place during that experiment. Although there wasan increase in the amount of carbonaceous deposit between 60and 120 min, the weight change was significantly slower thanduring the initial deposition.

The trend followed by the apparent conversion was the resultof two different processes simultaneously occurring in thereactor. First, longer reaction times allowed hydrocracking toproceed to greater extents, producing reductions in the >450C boiling fraction. On the other hand, carbonaceous materialdeposited on the catalyst during heatup partially redissolved in

the reaction mixture, producing a diminution in the total amountof carbonaceous deposit (Table 1). The deposits on the catalysttend to preferentially form from heavy fractions,1,18 andtherefore, their dissolution was likely to generate an increasein the amount of>450 C material in solution. Although someof it may have reacted on the catalyst and redissolved as lightermaterials, this cannot be told by the data. On the other hand,the increase in the >450 C fraction accompanying thedissolution of deposits is evident.

Within the first 10 min, the dissolution rate of the depositswas higher than the hydrocracking rate and consequently theapparent conversion dropped. After 10 min, both apparent andreal conversion increased. Although differences between con-versions at 0 and 5 min were within experimental error ((3 in

real conversion data), they were consistent with the processdescribed above and showed a drop in apparent conversion whilethe real conversion rose. Real conversions are not affected bythe redissolution of>450 C material since the deposits werealready taken as part of the >450 C fraction. These parallelevents, hydrocracking and redissolution of the deposits, explainthe difference in behavior followed by apparent and realconversions.

3.3. Size Exclusion Chromatography of the Products. SECchromatograms of the Point of Ayr hydrocracker feed andits hydrocracking products are shown in Figures 2 and 3. Similartrends have been recorded at all wavelengths. Only results fromdetection at 350 nm are shown. These data refer to liquidproducts and therefore exclude material deposited on the

catalyst. All chromatograms exhibited two peaks: one showingmaterial resolved by the column porosity and the other corre-sponding to material excluded from it. This is in line with thepattern generally shown by coal- and petroleum-derived heavyhydrocarbon liquids.15,19-24

(18) Suelves, I.; Lazaro, M. J.; Begon, V.; Morgan, T. J.; Herod, A. A.;Kandiyoti, R. Fractionation of coal extracts prior to hydrocracking: anattempt to link sample structure to conversion levels and catalyst fouling.

Energy Fuels 2001, 15, 1153-1165.(19) Begon, V.; Suelves, I.; Herod, A. A.; Dugwell, D. R.; Kandiyoti,

R. Structural effects of sample ageing in hydrocracked coal liquefactionextracts. Fuel 2000, 79, 1423-1429.

(20) Suelves, I.; Islas, C. A.; Herod, A. A.; Kandiyoti, R. Comparisonof fractionation methods for the structural characterization of petroleumasphaltenes. Energy Fuels 2001, 15, 429-437.

Figure 1. Boiling point distribution of the Point of Ayr hydrocracking feed and the 0 and 10 min hydrocracked products obtained by differentialthermogravimetry (DTG). The distributions have been area normalized.

Table 1. Apparent and Real Conversions of the 450

C Fractionand Deposition on the Catalyst as a Function of Reaction Timea

time(min)

apparentconversionb (%)

new carbonaceousdeposits (% of filter cake)

real conversionc

(%)

0 26 18 135 25 14 14

10 16 0 1630 26 6 2260 29 -2 30

120 45 12 37

a Experiments were carried out at 440 C and 190 bar hydrogen pressure.A previously coated catalyst was employed in these experiments. b Percentchange of original >450 C boiling material in the liquid phase. c Percentchange of original >450 C boiling material in the liquid phase minus freshcarbon caught up on the catalyst.



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All products showed a smaller excluded (large mass) peakthan that of the feed. The size of this peak decreased withreaction time. A shift of the high mass end of the retained peaktoward longer elution times was also observed with a progres-

sion in time in the series feed, 0 and 10 (these two were almostundistinguishable among themselves), 30, and 60 min. Thesechanges in the SEC chromatograms showed the expected overallreduction in molecular size with reaction time.

An exception to this general trend was found in the 5 minrun. The SEC chromatogram of the 5 min products showed aconsiderable shift of the retained peak toward longer elutiontimes than that of the 0 min run. The elution profile of this runis similar to that observed at 60 min. Retrogressive behaviorwas observed between 5 and 10 min, with a shift of the retainedpeak toward shorter retention times and an increase in theproportion of excluded material.

Two distinct processes are thought to have led to these results.The first is the natural progression in the breakup of largermolecules in the reactive environment within the reactor.Meanwhile, between 0 and 5 min, a large proportion of theheavier material originally contained in the feedstock was found

(21) Suelves, I.; Islas, C. A.; Millan, M.; Galmes, C.; Carter, J. F.; Herod,A. A.; Kandiyoti, R. Chromatographic separations enabling the structuralcharacterisation of heavy petroleum residues. Fuel 2003, 82, 1-14.

(22) Herod, A. A.; Lazaro, M. J.; Suelves, I.; Dubau, C.; Richaud, R.;Shearman, J.; Card, J.; Jones, A. R.; Domin, M.; Kandiyoti, R. Sizeexclusion chromatography of soots and coal-derived materials with 1-meth-yl-2-pyrrolidinone as eluent: observations on high molecular mass material.

Energy Fuels 2000, 14, 1009-1020.(23) Herod, A. A.; Shearman, J.; Lazaro, M. J.; Johnson, B. R.; Bartle,

K. D.; Kandiyoti, R. Effect of LiBr addition to 1-methyl-2-pyrrolidinonein the size-exclusion chromatography of coal-derived materials. EnergyFuels 1998, 12, 174-182.

(24) Zhang, S.-f.; Xu, B.; Herod, A. A.; Kimber, G. M.; Dugwell, D.R.; Kandiyoti, R. Effect of coal rank on hydrocracking reactivities of primarycoal extracts prepared in a flowing-solvent reactor. Fuel 1996, 75, 1557-1567.

Figure 2. Mixed-A column SEC chromatograms of the Point of Ayr hydrocracking feed and the hydrocracking products for 0, 5, and 10 minreaction time. All chromatograms have been area normalized.

Figure 3. Mixed-A column SEC chromatograms of the Point of Ayr hydrocracking feed and the hydrocracking products for 5, 30, and 60 minreaction time. All chromatograms have been area normalized.



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deposited on catalyst particles (Table 1). Between 5 and 10 min,much of the freshly deposited material is observed to dissolveback in the increasingly lighter liquid mixture. As mentioned

above in the discussion on conversions, deposits on the catalystsurfaces tend to be preferentially from the heavier materials andtheir redissolution would be likely to shift the SEC chromato-gram toward larger molecular sizes.

During these developments, measured real conversions of>450 C boiling material would not be immediately affectedby the redissolution of >450 C boiling material since thedeposits were already taken as part of the >450 C boilingfraction. These parallel events involving the hydrocracking ofthe overall sample mass and the redissolution of the depositsgo some way toward explaining differences in behavior shownby SEC chromatograms and the two conversions. In addition,it must be noted that not all material in the >450 C boilingfraction is expected to appear as large in SEC, since that

fraction can also contain relatively small molecules such as

benzopyrene. Therefore, a total correlation between SEC and

conversion data is not expected.

3.4. UV-Fluorescence Spectrometry. The UV-fluorescence(UV-F) spectra of the Point of Ayr hydrocracker feed and its

hydrocracked products are shown in Figures 4 and 5. The Point

of Ayr hydrocracker feed showed signal at longer wavelengths

than the hydrocracked products, indicating that higher concen-

trations of larger polynuclear aromatic rings are present in the

feed. The spectra of the 0, 10, 30, and 60 min runs were almost

undistinguishable. The spectrum of the 5 min run product was

clearly shifted toward shorter wavelengths. This suggests the

presence of (on average) smaller aromatic rings in solution than

those observed in the rest of the samples. Between 5 and 10

min, the spectrum shifted toward longer wavelengths as a

consequence of the dissolution of material which was deposited

Figure 4. Synchronous UV-fluorescence spectra of the Point of Ayr hydrocracking feed and the hydrocracking products for 0, 5, and 10 minreaction time. All spectra have been peak normalized.

Figure 5. Synchronous UV-fluorescence spectra of the Point of Ayr hydrocracking feed and the hydrocracking products for 5, 30, and 60 minreaction time. All spectra have been peak normalized.



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on the catalyst. This behavior is consistent with that shown bythe SEC chromatograms of the liquid products, as discussedabove.

3.5. Specific Surface Area Measurements. Specific surfacearea measurements were carried out on fresh NiMo/Al2O3catalyst, the coated NiMo/Al2O3 catalyst used as feed in theseexperiments, and the catalysts recovered from the 0, 10, and30 min runs (Table 2). Table 2 presents a summary of the results.A significant decrease in specific surface area takes place duringthe catalyst coating process. A slight increase in specific surfacearea was observed after the 0 min reaction time run (i.e., heatuponly). The variations in specific surface area observed thereafterwere found to be small. So whatever catalyst activity was

observed could not be attributed to increased specific surfaceareas, per se.

The changes in pore volume and average pore diameterfollowed the changes in carbonaceous content of the catalyst.Both parameters showed a marked decrease due to the coatingprocess and the 0 min reaction (i.e., during heatup), in line withthe large deposition taking place during the initial stages.However, this initial heavy deposition was clearly reversible.Increases in pore volume and pore diameter were seen for the10 min run, in agreement with the reduction in deposits observedby TGA.

4. Discussion

The extent of hydrocracking showed a continuous progres-sion, as expressed by the trend of the real conversion, between0 and 120 min. This marks a difference with previous work, 7

which showed a drop in conversion during the first 10 min ofreaction. Data published in that work did not consider the

carbonaceous deposits as part of the >450 C boiling fractionof the products in the calculation of conversion. This previousmethod of calculation did not therefore allow distinguishingbetween (i) reduction in the >450 C boiling fraction due tohydrocracking and (ii) removal of these materials from the liquidproducts as deposits on the catalyst. The need for differentiatingbetween these two processes has led to the definition of realconversion, as shown in the present work. In industrialprocesses, the amount of material deposited on the catalyst is

expected to be negligible in comparison with the amount offeed treated, and therefore, both conversions would be verysimilar. However, in experiments conducted with small amountsof sample, this difference is relevant and the fraction of thefeed forming deposits must be taken into account in order toevaluate a catalyst.

The real conversions, including the carbonaceous depositsas part of the >450 C boiling fraction, have been recalculatedhere using data from ref 8. Both conversions are plotted inFigure 6 as a function of reaction time. Apparent conversionrefers to that published in ref 7, whereas real conversion is therecalculated one, following the definitions presented in theExperimental Section (above).

There are two basic differences between the two works: (i)Tetralin was used as a donor-solvent. (ii) The catalyst was notcoated with carbonaceous material. In Figure 6, the drop inconversion at short reaction times was previously explained interms of a period needed to achieve the activation of the catalystby sulfidation.7 The fragments produced by thermal crackingof the sample at short reaction times were not hydrogenateddue to the absence of an active catalyst and recombined to formlarger molecules. As shown in Figure 6, the trends followed bythe conversion calculated in both ways are identical, and to thatextent, the conclusions of that work remain valid. The use inthis work of an already coated catalyst, which has been sulfidedduring the coating process, eliminated this activation period,and therefore, the real conversion was observed to increase

monotonically.Extensive deposition on fresh catalyst in the early stages ofthe hydrocracking process has been widely reported in theliterature.7,10,11,25,26 The present work shows that a relativelylarge deposition also takes place (in the presence of fresh feed)

Table 2. BET Specific Surface Area, Total Pore Volume, and

Average Pore Diameter of Fresh and Coated NiMo/Al2O3 and theCatalysts Recovered from Hydrocracking Runs

sampleBET specific surface

area (m2/g)tot pore

vol (cm3/g)avg pore

diam (nm)

fresh NiMo/Al2O3 78 0.28 14.7coated NiMo/Al2O3 45 0.12 11.00 min run 51 0.09 7.310 min run 52 0.13 10.230 min run 51 0.12 9.4

Figure 6. Apparent and real conversions as functions of reaction time obtained by Begon et al. 7



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even when a previously coated catalyst with a high carbonaceouscontent (30%) is used. This suggests that a fraction of the feed

has a strong tendency to adsorb on the solid surface providedby the catalyst.These fresh deposits seem to quickly redissolve in the liquid

mixture once the full reaction temperature is reached. Theredissolution process is relatively slow during the first 5 min,but it speeds up between 5 and 10 min. As a result, a weightequivalent to all the material initially deposited on the catalystwas transferred into the solution between 5 and 10 min. Clearly,the carbonaceous deposits are dynamic and material is ex-changed between them and the solution. The dynamic natureof the deposits probably contributes to explaining the sustainedactivity of the catalyst despite its extensive carbonaceouscoating. This bulk exchange of material between the depositsand the solution could be additional to other mechanisms

suggested in earlier work.27

It had been proposed there that thecarbonaceous deposits might act as a permeable layer, allowingthe exchange of hydrogen between the liquid phase and themolecules adsorbed on the catalyst surface.

The curves of apparent and real conversion as a function oftime are shown in Figure 7. The increase in real conversionslowed down in the second hour of hydrocracking due to thecompletion of the faster reactions with the subsequent loss ofreactivity of the feedstock.

Despite the use of a precoated catalyst, similar amounts ofmaterial deposited on the catalyst as those reported by Begonet al.7 on the fresh catalyst were observed. The absence oftetralin in the reactor made the feed more viscous, which mayhave had an effect on this observation.

5. Summary and Conclusions

The deactivation of a NiMo/Al2O3 catalyst in the early stagesof hydrocracking of a heavy hydrocarbon liquid has beenstudied. The use of an already coated catalyst and absence of adonor-solvent (e.g.) tetralin as solvent were two key elementsin the experiments that have been described. Instead, the

experiment relied on the donor ability of the recycle solventwhich had been a feature of the Point of Ayr pilot plant. 12 The

absence of an externally added donor-solvent (tetralin) in thereaction mixture enabled product characterization to be carriedout without interference from tetralin derivatives. Meaningfulboiling point distributions can now also be obtained by TGAin the region of boiling points below 450 C provided the weightloaded on the TGA pan can be precisely controlled.

The use of the precoated catalyst in a batch reactor went someway toward simulating a continuous process where fresh feedis continuously added onto already carbon-coated catalyst.Employing a batch reactor enabled the observation of theredissolution of the initial carbonaceous deposits into the bulkliquid. This redissolution process is not observable in continuousreactors due to the steady supply of heavy materials in the feed.

Although a precoated catalyst was used, the initial sample

deposition on solid surfaces was large. It appears that a fractionof the feed has a strong tendency to form deposits. However,the carbonaceous layer coating the catalyst was observed to bedynamic and exchanged material with the solution. The amountof deposits rapidly decreased due to redissolution of much ofthe coating material. This bulk exchange of material betweenthe deposits and the solution is a mechanism which contributesto explaining the sustained level of catalytic activity despitethe large carbonaceous deposition. However, this mechanismis not exclusive of others proposed in earlier works, such asthe permeability of the deposits to hydrogen, which wouldenable the hydrogenation of molecules on the catalyst surface.

The initial deposition caused some degree of pore blockagewhich led to a decrease in both the total pore volume and the

average pore diameter of the catalyst. The subsequent dissolutionof a fraction of the carbonaceous layer tended to partially reversethe pore blockage and allowed total pore volume and averagepore diameter to increase. On the other hand, the specific surfacearea did not present significant variation with hydrocrackingtime.

These changes in the amount of deposits affected the apparentconversions and the SEC and UV-F profiles obtained withinthe first 10 min. SEC and UV-F showed the presence of largermolecules in solution at 10 min in comparison with the 5 min

(25) Ancheyta, J.; Betancourt, G.; Centeno, G.; Marroquin, G.; Alonso,F.; Garciafigueroa, E. Catalyst Deactivation during Hydroprocessing ofMaya Heavy Crude Oil. 1. Evaluation at Constant Operating Conditions.

Energy Fuels 2002, 16, 1438-1443.(26) Ancheyta, J.; Betancourt, G.; Centeno, G.; Marroquin, G. Catalyst

Deactivation during Hydroprocessing of Maya Heavy Crude Oil. II. Effectof Temperature during Time-on-Stream. Energy Fuels 2003, 17, 462-467.

(27) Thomson, S. J.; Webb, G. Catalytic hydrogenation of olefins onmetals: a new interpretation. J. Chem. Soc., Chem. Commun. 1976, 13,526-527.

Figure 7. Apparent and real conversions as a function of reaction time obtained in the present work.



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products as a result of the redissolution of carbonaceous deposits.In addition, the apparent conversion, which is calculated withouttaking the deposits into account as carbonaceous material,dropped in the first 10 min. However, adding the depositedmaterial to the >450 C boiling material (in the conversioncalculation), conversions were found to steadily increase overthe 120 min period considered. This marks a difference withthe trend followed by conversion as a function of reaction time

in earlier experiments. This difference has been related to theuse of a precoated catalyst in the present study and, therefore,to the absence of the catalyst activation period by sulfidationobserved previously.

Acknowledgment. The support from EPSRC under Grant GR/R27471/01 is gratefully acknowledged.

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