INTERFACIAL APPLICATIONS IN ENVIRONMENTAL ENGINEERING - CHAPTER 5 pdf

Materials In evaluating the catalysts for gasoline catalytic cracking desulfurization, the feed is FCC gasoline distillate at higher than 100°C, provided by Shengli cal Factory, whose su

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Studies on Catalysts/Additives

for Gasoline Desulfurization via

Catalytic Cracking

C Y LI, H H SHAN, Q M YUAN, C H YANG, J S ZHENG,

Shandong Province, People’s Republic of China

I INTRODUCTION

For a reaction catalyzed by a solid catalyst, at least one reactant must adsorb andthe reaction happens on the active sites on the surface by either a Langmuir–Hinshelwood or Rideal–Eley mechanism Obviously, the rupture of the bonds

of the reactants and the formation of the bonds of the products bear close ship to the surface properties of the catalyst If there are no interactions betweenthe reactants and the catalyst surface, then the catalytic reaction will not occur.The development of new catalysts used to be for improving production effi-ciency, reducing production cost, or producing new products With civilizationand the advancement of humankind, however, the aims for developing catalystshave changed gradually, and more and more catalysts have been used to eliminateharmful materials The treatment of polluted water and waste gases needs cata-lysts; the automotive emissions converter is a typical example In refineries, pro-ducing low-sulfur, low-olefin, and high-octane-number environmentally benigngasoline also requires catalysts

relation-Sulfur in gasoline is not only a direct contributor to SOxemissions; it is also apoison affecting the low-temperature activity of automobile catalytic converters.Therefore, it inﬂuences volatile organic compounds, NOx, and total toxic emis-sions [1] Consequently, developed countries limit the content of sulfur in gaso-line stringently In the United States, sulfur content will be lower than 30µg/g

in 2005 In China it will be reduced to 300µg/g from the present 800 µg/g.About 90% of sulfur in gasoline originates from FCC gasoline, so reducingthe sulfur content of FCC gasoline is the main target of sulfur removal Several

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et al [1–3], can be added into the reaction-regeneration system of FCC ently, based on the real situation, to improve the cracking of sulfur compounds

expedi-in the gasolexpedi-ine range The maximum of sulfur reduction is about 40%, compared

to the sulfur content of gasoline produced without adding the additive, if theadditive is combined with the specially developed FCC catalyst [4]

This is a cheap sulfur-removal technique, and we have done some work on

it In this chapter, we not only introduce the results from our studies on sulfurremoval additives, but also give the results on the mechanism of sulﬁde crackingand the catalysts of gasoline cracking desulfurization

II EXPERIMENTAL

A Materials

In evaluating the catalysts for gasoline catalytic cracking desulfurization, the feed

is FCC gasoline distillate at higher than 100°C, provided by Shengli cal Factory, whose sulfur content is 1650µg/g, measured via the burning lightmethod The feed used to evaluate the sulfur removal additives of FCC gasoline

Petrochemi-is VGO (vacuum gas oil), supplied by Shenhua Reﬁnery The properties of VGOare listed inTable 1

All the chemicals used to prepare the catalysts or additives are analyticallypure

B FCC Catalyst, Catalyst/Additive Preparation and

Characterization

The regenerated FCC catalyst used in the experiments, also provided by ShengliPetrochemical Factory, is Vector60SL, whose BET surface area and microactivityare 112 m2/g and 70, respectively

Both the catalysts for gasoline catalytic cracking desulfurization and the sulfurremoval additives of FCC gasoline were prepared via coprecipitation combinedwith impregnation First, we used coprecipitation to make the colloid of mixedmetal hydroxides; then the USY powder, bought from Zhouchun Catalyst Fac-tory, was added in with continuous stirring After being aged for 12 h, the colloidwas dried at 100°C for more than 20 h and then calcined at 700°C for 6 h.TM

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TABLE 1 Properties of Shenghua VGO

X-ray diffraction and BET surface area of the catalyst/additive were measured

by D/MAX-III X-ray diffractometer and ASAP2010, respectively

C Apparatus

with PONA7531 column and FID detector) Between the sampling inlet and thecolumn is a minireactor with a 2-mm inner diameter The pulsed liquid sample

is gasiﬁed at the sampling inlet and carried by gas to the catalyst bed to react

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FIG 1 Schematic of on-line pulse-reaction chromatography.

with products that go directly into the column after distributary and that are lyzed with FID To ensure that the sample was pulsed to gasify quickly andcompletely in the experiments of thiophene cracking, the temperature at the sam-pling inlet was controlled at 250°C The ﬂow rate of the carry gas (highly pure

ana-N2), the amount of the USY zeolite used, and the quantity of thiophene pulsedwere 30 mL/min, 14 mg, and 1µL, respectively

Furthermore, thiophene/n-heptane (sulfur content 0.33% and gasoline

distil-late at over 100°C were used as the raw materials to react in a ﬁxed-bed reactorwith 25 g of catalyst to validate the results obtained from on-line pulse-reaction

chromatography Ten grams of thiophene/n-heptane or the gasoline distillate was

pumped into the reactor within 1 min Sulﬁdes in the liquid product collected inthe condenser were analyzed via Varian3800 chromatography combined withCB80 column and a PFPD detector The sulfur content of the liquid product wasalso measured via the burning-light method

The apparatus for MS transient response has been described elsewhere [5].Thirty milligrams of USY was placed in the middle of the quartz reactor Toquicken response time, the other space of the reactor was ﬁlled with 0.3- to 0.45-

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mm quartz sand The efﬂuents were detected with a quadrupole mass ter (AMTEK QuadLink 1000) with a minimum dwell time of 3 milliseconds.

The octane numbers of the gasoline distillate before and after reaction wereanalyzed by HP5890 The sulfur content deposited on the catalysts was measured

1 min The efﬂuent from the reactor was collected with three condensers in seriesimmersed in ice/water bath The uncondensed gas was collected via the draining-water method Distilling the liquid product, we obtained the gasoline The gasand the gasoline were all analyzed with the HP5890 to obtain the hydrocarboncomposition and the octane number The sulfur content of the gasoline is alsomeasured via the burning-light method The carbon content of the catalyst wasdetermined by chromatography

III RESULTS AND DISCUSSION

A Sulfur Distribution and Sulﬁdes in FCC Gasoline

The FCC gasoline was cut to narrow distillates; the sulfur content measured viathe burning-light method is listed inTable 2.The sulfur content of the distillate

at 80–100°C is 507.9 µg/g, about twice that at 60–80°C, while the sulfur content

at 100–120°C is almost twice that at 80–100°C The sulfur content of the late at over 140°C is more than 1600 µg/g Obviously, sulfur content increaseswith the boiling point of distillate and concentrates in the high-boiling-point dis-tillates

distil-Because the distillate of IBP-60°C is too ‘‘light’’ and that of 160-EP is too

‘‘heavy,’’ their sulfur content is difﬁcult to determine accurately by this method

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TABLE 2 Sulfur Distribution in FCC Gasoline

Distillation range,°C IBPl–60 60–80 80–100 100–120 120–140 140–160 160–EP

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TABLE 3 Type and Distribution of Sulﬁdes in the Gasoline Before and AfterDesulfurization via Catalytic Cracking

Before After Before AfterUSY/Al2O3/ZnO desulfur desulfur desulfur desulfur Sulfur removal(1/2/2, wt.) (%) (%) (µg/g) (µg/g) (%)Total 100 100 1650 288 82.5Thiophene 0.69 8.02 11.5 23.1 ⫺102Mercaptans 0.32 0.08 5.35 0.243 95.52- and 3-methylthiophene 14.4 43.6 238 126 47.5Thioethers and disulﬁdes 11.1 3.72 184 10.7 94.2

Sulﬁdes in the distillate at over 100°C were analyzed by chromatography with

a PFPD detector; the results are shown in Table 3 In the distillate, the sulfurexisting as mercaptans, thioethers, and disulﬁdes accounts for less than 12% ofthe total sulfur, and the rest exists as different alkylthiophenes The greatestamount is C2-substituted thiophene (including different 2-methylthiophenes andethylthiophenes), while the amount of thiophene is small Therefore, to lower thesulfur content of the distillate via catalytic cracking, we must study how to makethiophene and alkylthiophenes crack effectively

B Cracking of Thiophene and Sulﬁdes

in FCC Gasoline

Fourteen milligrams of USY was placed in the reactor of on-line pulse reactionchromatography; the height of the catalyst bed was about 4 mm When the tem-perature of the reactor was increased to 490°C in 30 mL/min N2gas ﬂow andTM

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FIG 2 Chromatograph of thiophene reacting over the USY zeolite at 490°C.

the chromatography was stable, a pure thiophene pulse was generated The carbon products were propane, propylene, isobutane, 1-butene, and 2-butene(Fig 2)

hydro-Thirty milligrams of the USY zeolite was used in the experiments on the MStransient response The ﬂow rate of the Ar carry gas was also 30 mL/min At

490°C, 2 µL of thiophene was pulsed; the results are shown in Figure 3 The

FIG 3 Transient responses of thiophene pulsed over the USY zeolite catalyst at 490°C

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characteristic peak of H2S (m/e⫽ 34) was detected and appeared almost neously with that of thiophene This proves that thiophene can crack over theUSY zeolite to produce H2S Furthermore, benzothiophene (m/e⫽ 134) was alsodetected, and it appeared a little later than thiophene and H2S.

simulta-The formations of butane, butene, and H2S indicate that the ring of thiophenecan open and that S can be removed during the cracking reaction Furthermore,hydrogen transfer must happen simultaneously; otherwise, only high-unsaturatedhydrocarbon can be produced If a thiophene cracks to a butene and a H2S, itmust obtain six hydrogen atoms Under the experimental conditions, no H2partic-ipated in the reaction So the hydrogen can be obtained only via hydrogen transferamong thiophene molecules or thiophene and hydrocarbon fragments In addition,after several thiophene pulses, signiﬁcant coke deposited on the USY zeolite,which illustrates that dehydrogenation of hydrocarbons or hydrocarbon fragments

or sulﬁdes must take place during the reaction

In the reaction, that propane, propylene, and benzothiophene can be formedshows that the reactions of thiophene over the USY zeolite are very complexand that maybe other sulﬁdes can also be formed So we performed the followingexperiment

Thiophene/n-heptane (sulfur content: 0.33%) were used as the raw material

to react in a ﬁxed-bed reactor with 25 g USY zeolite at 490°C After reaction,the sulfur content of the liquid product was reduced to 0.13%, and 61% sulfurhad been removed Obviously, the cracking desulfurization of thiophene is thedominant reaction Sulﬁde analysis by chromatography with a PFPD detectorshows that in the liquid product there are thiophene, 2-melthylthiophene, 3-meth-ylthiophene, benzothiophene, and a little dimethylthiophene and trimethylthio-phene, where unconverted thiophene, benzothiophene, 2-methylthiophene, and3-methylthiophene account for 67%, 20%, 5%, and 3%, respectively (Fig 4)

That indicates that, except for cracking, thiophene can form other sulﬁdes, andbenzothiophene and 2-methylthiophene are easy to be produced

The other conditions were the same as for Figure 2, and thiophene pulses weregenerated in the on-line pulse-reaction chromatography apparatus at differenttemperatures The conversion of thiophene at different temperatures is depicted

monotonically, but has a maximum of about 400°C Luo et al [6] also reportedthat there is a maximum conversion of thiophene at 400°C when thiophene/etha-nol crack over HZSM-5 This means that hydrogen transfer may play a veryimportant role in thiophene cracking [7] Hydrogen transfer is an exothermicreaction, and high temperature restrains the reaction Cracking, however, is anendothermic reaction, and high temperature promotes the reaction That 400°C

is the optimal temperature for thiophene cracking indicates that hydrogen transfer

is an important elementary step of thiophene cracking Otherwise, the conversion

of thiophene should increase with temperature

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FIG 4 Products of thiophene reacting over the USY zeolite catalyst at 490°C analyzedwith a PFPD detector.

FIG 5 Relationship between thiophene conversion and temperature

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2 Cracking of Sulﬁdes in FCC Gasoline over a Catalyst

of Gasoline Tracking Desulfurization

USY zeolite has good cracking activity for sulﬁdes but bad selectivity (we discussthis in detail in Sec III.C), so we chose a USY/ZnO/Al2O3catalyst for gasolinecracking desulfurization to carry out the experiments that investigate the cracking

of various sulﬁdes in the FCC gasoline distillate at over 100°C

In the gasoline distillate, more than 88% of the sulfur exists in thiophenespecies with different alkyl substitutions(Table 3).After the gasoline distillatereacted over the catalyst at the same conditions (400°C and catalyst/oil ⫽ 2.5),the sulfur content was reduced to 288µg/g, and 82.5% of the sulfur was removed

In Table 3, the percentages of sulfur removed as mercaptans, thioethers, fides, C2-substituted thiophene, C3-substituted thiophene, and C4-substituted thio-phene are all larger than this value This indicates that the sulfur existing inthese sulfides is easier to remove via cracking However, the sulfur existing in2-methylthiophene and 3-methylthiophene is relatively more difficult to removeand the percent sulfur removed is only 47% In the table we can also see thatthe amount of thiophene, although small, has increased more than 100% In ouropinion, this does not mean thiophene cannot desulfurize via cracking, but it maymean that alkylthiophenes can form thiophene via dealkylization

disul-We also investigated the effect of temperature on sulfur removal In Figure

6, we can see that sulfur content has a minimum value between 390°C and 420°C

It seems that high temperature is not favorable for sulﬁde cracking Obviously,the result is consistent with that of pure thiophene cracking

FIG 6 Relationships between the yield and sulfur content of gasoline and temperatureover the catalyst for gasoline desulfurization via catalytic cracking (catalyst/oil⫽ 2.5)

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Then it decomposes to produce H2S via pyrogenation or catalysis The viewpoint

of Wang et al [8] is completely different They thought the thiophene ﬁrst obtainshydrogen to form tetra-hydrogen-thiophene, and then tetra-hydrogen-thiophenedecomposes to produce H2S In our experiments, however, we did not ﬁnd tetra-hydrogen-thiophene or even molecules larger than benzothiophene Saintigny et

al [9] studied the mechanism of thiophene cracking over acid catalyst in theoryand suggested that one of the CES bonds breaks ﬁrst on acid sites to formsurface HCDCECHCCHESH, whose CES bond then breaks toHCDCECDCH and H2S In the mechanism, hydrogen transfer does not hap-pen If so, cracking will be the only reaction and high temperature will favorthiophene cracking however, our experimental results show that about 400°C

is the optimum temperature for thiophene(Fig 5)desulfurization via cracking.Therefore, the cracking desulfurization of thiophene must be limited by otherreactions Based on our experimental results, we suggested that thiophene cracks

by the mechanisms described inFigures 7–9

Thiophene ﬁrst obtains proton on the B acid sites of USY to form carboniumion, and then the CES bond at the β-position breaks, for its bond energy is 268kJ/mol, the weakest among those of CEH, CEC, and CCC Thus, the ring

of thiophene opens to form mercaptan species with two double bonds (Fig 7).After carbonium ion isomerization and hydrogen transfer, the remaining CESbond of the mercaptan at theβ-position breaks, and H2S and dibutene are pro-duced Through hydrogen transfer, dibutene can convert to butene and even bu-tane

Besides butane and butene, propylene is also produced during the cracking ofthiophene In our opinion, the formation of propylene has a close relation withthe formation of methylthiophene In Figure 7, if the mercaptan with two doublebonds from the ring opening of thiophene polymerizes with thiophene at theα-position, then species A is produced After carbonium ion isomerization andhydrogen transfer(Fig 8), the CES bond at the β-position of A breaks, and

H2S and 2-butenylthiophene are produced 2-butenylthiophene cracks at theposition to 2 methylthiophene and propylene after hydrogen transfer

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