Effect of Operating Parameters on Composition of Dry Gas Obtained During Gasoline Cracking Process

China Petroleum Processing and Petrochemical Technology2010. Vol. 12. No. 3, Ppp 1-5Effect of Operating Parameters on Composition of Dry GasObtained During Gasoline Cracking ProcessSha Youxin; Long Jun; Xie Chaogang; Li Zheng; Wei Xiaoli( Research Institute of Petroleum Processing, SINOPEC, Beijing 100083)Abstract: The influence of operating parameters on ethylene content in dry gas obtained during catalytic cracking ofgasoline was investigated in a pilot fixed fluidized bed reactor in the presence of the MMC-2 catalyst. The resuts haveshown that the majority of dry gas was formed during the catalytic cracking reaction of gasoline, with a small proportionof dry gas being formed through the thermnal cracking reaction of gasoline. The ethyene content in dry gas formed duringthe catalytic cracking reaction was higher than that in dry gas formed during the thermal cracking reaction. The ethyleneformed during catalytic cracking of gasoline with a lower olefin content, which meant that the higher the amount ofcarbonium ions was produced during the reaction, the higher the ethylene content in the dry gas would be. An increasingreaction temperature could increase the percentage of dry gas formed during thermal cracking reaction in total dry gasproducts, leading to decreased ethylene content in the dry gas An increasing catalystoil ratio could be conducive to thecatalytic cracking reactions taking place inside the zeolite Y, leading to a decreased ethylene content in the dry gas.Adecreasing space velocity could be conducive to the catalytic cracking reactions taking place inside the shape sectivezeolite, leading to increased ethylene content in the dry gas.Key words: gasoline; operating parameter; catalytic cracking; duo gas; ethylene1 Introductionand a low space velocity to increase the dry gas yield toThe dry gas produced during the fluid catalytic crackingachieve the target for enhancing the ethylene yield. Actu-(FCC) process is a low add-value product and used to beally the dry gas contains in its composition hydrogen, .burned as fuel gas in the refinery, With the increased de-methane, ethane and ethylene, and the increase in dry gasmand for ethylene and the increased supply of heavy crudesyield will be inevitably accompanied with an increase in theworldwide, the ethylene product contained in dry gas at theyield of hydrogen, methane, and ethane, which have a lowrefinery has been atracting the attention of refiners. Theadd-value coupled with high hydrogen content. It is well-known that the catalytic cracking process is a process asso-technology for manufacture of ethylene through catalyticciated with redistribution of the carbon and bydrogencracking of heavy oil has become a new source of ethylene.elements. Since the formation of dry gas with low add-valueThe CPP processl developed by the Research Institute ofand high hydrogen content is not beneficial for efficientPetroleum Processing (RIPP) and the HCC process21 devel-utilization of the hydrogen element in the raw gas, it is anoped by the Luoyang Petrochemical Engineering Companyurgent task to think about how to suppress the hydrogen,(LPEC) all intend to obtain ethylene from the FCC dry gas.methane and ethane yields while increasing the ethyleneFurthermore, the reutilization of ethylene contained in theyield. Therefore the most efficient way is to increase thedry gas produced by the traditional FCC process has beenethylene content in the FCC dry gas.garnering special attention of researchers!. Thereby theethylene content of dry gas has become an important pa-Gasoline is an important intermediate product formed dur-rameter determining its value, because the higher the ethyl- ing catalytic cracking of heavy oil, and the secondary cata-ene content in the dry gas, the higher the value of dry gas lytic cracking of gasoline has a major impact on the ethylenewould be.content in dry gas. This paper intends to investigate theThe technology for production of ethylene by catalytic py-influe中国煤化工ilyic cakingofrolysis of heavy oil generally adopts the operating condi- CorresHC N M H Gelephone: +86-10-tions favoring a high temperature, a high catalyst/oil ratio 82368367; E-mail: shayx.shky@ sinopec comSha Youxin, et al. Effect of perating Parameters on Composition of Dry Gas Obtained DuringGasoline Cracking Processgasoline on the ethylene content in dry gas in an attempt topreheater, in which the feed oil was mixed with the atomizingdetermine to what extent the individual operating parametersteam and routed into the reactor flld with the catalyst tocan affect the ethylene content of dry gas.undergo catalytic cracking reaction. After termination ofcatalytic cracking reaction the catalyst was subjected to2 Experimentalsteam stipping, followed by regeneration via coke burm-off2.1 Feedstock and catalystin the presence of oxygen introduced into the reactor.The feedstock used in the experiment included the straight-2.3 Products analysisrun gasoline and the FCC gasoline with their PONA compo-The liquid products obtained during catalytic cracking reac-sition presented in Table 1 The catalyst used in the FCCtion were analyzed by a 6890 type gas chromatograph (madeprocess was a fresh MMC-2 catalyst, which was manufac-by the Agilent Company) to measure the mass fractions oftured by the Catalyst Factory of the Qilu Petrochemicalgasoline and diesel distillates, and the gasoline was ana-Company. This catalyst containing both the Y zeolite and alyzed by a 7890A type gas chromatograph (made by theshape-selective zeolite was subjected to hydrothermal treat-Agilent Company) to determine its PONA composition. Thement by 100% steam at 800 C for 14h prior to experiments.gas product was analyzed by a 7890A type gas chromato-The property of the catalyst after hydrothermal treatment isgraph to determine the composition and yield. The gas ob-shown in Table 2.tained after coke bum-off from the coked catalyst was ana-Table 1 Hydrocarbon group analysis of gasoline feedstock lyzed by an online flue gas analyzer to determine the CO2content of flue gas for converting into the coke deposit.Hydrocarbon group analysis, %ItemsSR gasolineFCC gasoline3 Results and DiscussionNormal paraffins22.09_5.02|Iso-paraffins23.7325.453.1 Infuence of thermal cracking reactionOlefins0.2140.28During the FCC process the catalytic cracking reaction takesNaphthenes43.987.40place simultaneously with the thermal cracking reaction. InAromatics9.8421.86order to determine the contributions of thermal cracking re-action to the yield of dry gas products, a blank test usingTable 2 Property of equilibrium catalyst MMC-2silica gel as the inert catalyst was carried out. Since the silicaDatagel with a particle size ranging from 60 mesh to 100 mesh didAl2O3 content, %53.6not have catalytic activity after high-temperature calcination,Bulk density, g/cm0.83the reaction taking place during the blank test could be re-Pore volume, cm/g0.183 .garded only as a thermal cracking reaction.Specific surface area, m2/g143Granularity, v%The straight-run gasoline and the FCC gasoline that repre-0--40μm12sented feedstocks with different olefin contents were sub-0-149 u.m92jected to FCC tests and blank tests, respectively, with thedry gas obtained thereof shown in Table 3. The dry gas2.2 Test methodyield obtained during catalytic cracking of two gasoline feed-The test equipment for conducting catalytic crack-Table 3 Yield of dry gas formed during catalytic cracking test anding experiments was a pilot fixed fluidized bed (FFB)blank test of gasoline%unit, designed and manufactured by the SINOPECFeedstockFCC gaoline| SR gasolineResearch Institute of Petroleum Processing. Prior toCatalytic cracking test2.84.55the catalytic cracking test a definite amount of cata-Blank test0.390.10lyst was loaded into the reactor, which was then Ratio of dry ga中国煤化工0.0preheated to the specified reaction temperature. The Note: Ratio= adry gaMYHC N M H Gailytic cacking est.feed oil was pumped by a gear pump into theReaction temperature: 550亡，space velocity: 7.5 h*'.2China Petroleum Processing and Petrochemical Technologystocks was evidently higher than that obtained during blank conversion of most hydrocarbons follow the carbenium iontests of these relevant feedstocks. The dry gas obtainedreaction mechanism, which means that the carbenium ionduring blank tests was mainly emanated from thermal crack- reaction is the main reason leading to apparent changes ining reaction of gasoline, whereas the dry gas obtained dur-the yield and composition of dry gas. According to the theory .ing catalytic cracking reaction of gasoline was emanated of Haag and Desaul671, the protonation tniggered by thefrom both of the catalytic cracking and the thermal cracking alkane during the catalytic reaction can form the penta-co-reactions of gasoline. The ratio between the dry gas ob-ordinated carbenium ions, which can be further cracked totained from the blank test and the dry gas obtained from the form the ti-coordinated carbenium ions and dry gas. Somecatalytic cracking test was very low, which could verify thatpapers have made a detailed elaboration on the pathway forthe share of dry gas formed during thermnal cracking of gaso- formation of the dry gas from the penta-coordinated car-line in the total yield of dry gas products was very low, and bonium ionsl-9]. This theory can explain how the dry gasthe dry gas was predominantlty emanated from the catalyticformed during the catalytic cracking experiments is emanatedcracking reaction of gasoline. Furthermore, gasoline with a from the catalytic cracking reaction, but cannot explain thehigh H/C ratio would have a low coke yield and would notreason leading to changes in the ethylene content of drylead to an obvious change in the proportion of catalyticgas.cracking reaction versus thermal cracking reaction causedIt can be realized that after comparing two cases the pres-by deactivation of the catalyst by coke deposits.ence of catalyst can lead to the formation of tri-coordinatedThe mass fraction of ethylene in dry gas obtained from cata- carbenium ions during the catalytic cracking reaction, andlytic cracking tests and blank tests of two gasoline feed-olefins contained in the FCC gasoline can be easily proto-stocks is presented in Table 4. The ethylene content in dry nated to give birth to more carbenium ions. Hence the moregas formed during the catalytic cracking of gasoline was carbenium ions are formed during the catalytic crackinghigher than that obtained during the thermal cracking ofprocess, the higher the ethylene content in the dry gas prod-gasoline. Some researchers assumed that the Lewis acid sites uct will be. It is possible to draw forth a conclusion thatof catalyst taking part in catalytic cracking reactions couldbesides the formation of dry gas in the a-fission of penta-promote the formation of free radicals41 from alkanes, andcoordinated carbenium ions there is another reaction path-the new free radicals formed thereby would undergo reac- way for the formation of ethylene originated from carbeniumtions according to the free radical reaction mechanism with-ions.out being influenced by the Lewis acid sites. Therefore, theLewis acid sites of the catalyst could only realize the pur-John and coworkers pointed out upon studying the cata-pose for increasing the conversion rate through fosteringlytic cracking reaction of raw diesel ditilate that ethylenethe formation of free radicals of alkanes to boost the dry gasappeared when the conversion rate was greater than 20%，yield, which actually would not affet the reaction equilibo-and would increase continuously with an increasing con-rium and the composition of dry gas'51. This concept is obvi-version rate. Therefore, ethylene was the secondary prod-ously not in agreement with the test resuts. The existenceuct after stabilization of the catalytic cracking process. Dur-of catalyst not only can increase the dry gas yield, but alsoing the catalytic cracking process ethylene was formed uponcleavage at the β-position of the primary carbonium ions,can obviously increase the ethylene content in dry gas.such as the primary n-butyl carbenium ions:Table 4 Mass fraction of ethylene in the dry gas obtainedduring catalytic cracking and thermal cracking of twoCH-CHr-CHxCH2"→CH-CH2*+CH= CH2 -196kJ/molgasoline feedstocks%FeedstockFCC gasolineSR gasolineSimilarly, the catalytic cracking reaction of pentene cartboniumCatalytice cracking test343ions can give bith to a propylene molecule and an ethyleneBlank test47molecule as shown in Figure 1. However, the probability forNote: Reaction temperature: 550 C, space velocity: 7.5 h"'.ocurren中国煤化工nall because theCompared to the thermnal cracking reaction, the presence of~ energy a|YHCNMHG$quitehigh,butcatalyst during the catalytic cracking reaction can make the this pathway does really exist. Since the sum of butene andSha Youxin, et al. Effect of Operating Parameters on Composition of Dry Gas Obtained During GasolineCracking ProcessCH- CH- CHr CH2*Cracking a B-ositinCH2=CH+CH,- CH'CHethylene content in total amount of dry gas was reduced,H,。because the ethylene content in dry gas formed during theCH,CH- CH- CHCracking at B-posion→CH- CH-CH+CH- CH* thermal cracking reaction was lower than that in the dry gasPathway Iformed during the catalytic cracking reaction.Figure 1 Pathways forformation of ethylene through pentenecrackingThe effect of the catalysUoil ratio and the weight hourlypentene yields is quite high in the FCC process aimed atspace velocity on the mass fraction of ethylene in dry gas isboosing he yield of low carton oeins, the amount of eth- presented in Figures 3 and 4. The mass fraction of etyleneylene formned during cracking of butene and pentene alsoin dry gas decreased with an increasing catalyst/oil ratio.increases, which can have an apparent impact on the com-With a decreasing space velocity the residence time of gaso-position of dry gas, as evidenced by the increased ethyleneline fraction in the catalyst bed was extended, resulting in ancontent in the dry gas, which is emanated from the enhancedincreasing mass fraction of ethylene in dry gas. An increas-reactions of carbenium ions during the catalytic crackinging catalystoil ratio made individual hydrocarbons to beprocess.easily in contact with more catalyst. A decreasing spacevelocity could extend the diffusion time of hydrocarbons in3.2 fctoftemperaturec,alystoil ratioandspace velocitythe catalyst bed, so that these hydrocarbons could be inon ethylene content in dry gascontact with more catalyst. An increasing catalystoil ratioThe change in operating parameters not only can changeand a decreasing space velocity all could promote catalyticthe dry gas yield, but also can change the dry gascracking reaction of bydrocarbons through enhancing thecomposition. The effect of reaction temperature on the masscontact of hydrocarbons with more catalyst. However, thesefraction of ethylene in dry gas is presented in Figure 2. Withtwo factors had different effects on the ethylene content inan increasing reaction temperature, the mass fraction of eth-dry gas, which might be caused by the unique compositionylene in dry gas was apparently decreased. The higher theof the MMC-2 catalyst.reaction temperature was, the larger the magnitude of reduc-tion in the mass fraction of ethylene would be. Inconsequence, despite the increase in the rates of catalytic78-cracking reaction and thermal cracking reaction acceleratedconcurently by an increased reaction temperature, the mag-台77-nitude for the increase in thermal cracking reaction rate thatwas not dependent on the catalyst was much larger and the6thermal cracking reaction accounted for a greater shareamong the whole reaction system, resulting in an increased13Tsshare of dry gas originated from thermal cracking reaction inCatalystoil ratiothe total dry gas yield. As it has been proved previously, theFigure 3 Effect of catalystoil ratio on ethylene mass fractionin dry gas82厂8078568-540580620Reaction temperature, c中国煤化工古十Figure 2 Effect of reaction temperature on ethylene mass1HCNMHGfraction in dry gasFigure 4 Effect of WHSV on mass fraction of C;Hs in dry gasChina Petroleum Processing and Petrochemical TechnologyBeing quite different from the conventional catalyst, the(2) The ethylene content in the dry gas formed during theMMC-2 catalyst contains two active ingredients, namely gasoline catalytic cracking reaction was higher than thatthe Y zeolite and the shape selective zeolite. The pore diam- obtained during gasoline thermal cracking reaction. Theeter of the Y zeolite is around 0.76 nm and that of the shape- higher the content of carbonium ions formed during the gaso-selective zeolite - about 0.54x0.56 nm, so that the oil vapor line catalytic cracking reaction, the higher the ethylene con-can more easily enter the channels of Y zeolite than those of tent in the dry gas would be.the shape-selective zeolite through diffusion. Upon increas-(3) During the gasoline catalytic cracking process an in-ing the catalystoill ratio, the oil vapor would easily enter theY zeolite through diffusion, although the number of activecreasing reaction temperature would decrease the ethylenesites in the shape-selective zeolite and the Y zeolite wascontent in the dry gas. An increasing catalystoil ratio wouldincreased at the same ratio. The newly formed ethylene couldbe conducive to the catalytic cracking reaction in the Yundergo the hydrogen transfer reaction to yield ethane inzeolite, resulting in decreasing ethylene content in the drythe pores of the Y zeolite, leading to a reduction of ethylenegas. A decreasing space velocity would be conducive to thecatalytic cracking reaction in the shape seletive zeolite, lead-content in the dry gas.ing to increasing ethylene content in the dry gas.When the space velocity dropped, the number of activeReferencessites in the shape-selective zeolite and the Y zeolite did not[1] Xie Chaogang, Wang Xieqing, Guo Zhixiong, et al. Develop-change, and gasoline would more easily be subjected toment and commercial tests of CPP technology for manufacture ofcatalytic cracking reaction in the shape-selective zeolite thanolefins[J]. Petroleum Processing and Petrochemicals, 2001,32(12):in the Y zeolite. 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Microporous andin dry gas.Mesoporous Materials, 2000, 35-36: 11-20[8] Long Jun, Wei Xiaoli. Study on mechanism of dry gas forma-4 Conclusionstion during catalytic cracking reaction[J]. Acta Petrolei Sinica(1) During the gasoline FCC process the dry gas was mainly(Petroleum Processing Section),2007.23(1):1 (in Chinese)emanated from the calyic cracking reaction, and the dry [9] Wei Xiali, Long Jun, Zhang Jiushun. Infuece of oerainggas formned during the thermal cracking reaction only ac- paramet中国煤化工compostion of drycounted for a small proportion in the tolal amountofdrygas gasJ].HCNMHGicals, 2007 38(4):34products.(in Chinese,5