Effects of Light Olefins Formation during Catalytic Pyrolysis of n-Heptane

China Petroleum Processing and Petrochemical TechnologyScientifc Research2011,Vol. 13, No. 4, pp 8-14December 30 2011Effects of Light Olefins Formation during CatalyticPyrolysis of n-HeptaneCheng Xiaojie; Xie Chaogang; Wei Xiaoli(SINOPEC Research Institute of Petroleum Processing, Bejing 100083)Abstract: The infuence of zeolite structure and process parameters (including reaction temperature and catalyst/oil ratio)on rules for formation of ethylene and propylene in the course of catalytic pyrolysis of n-heptane was studied in a smal-scale fixed fuid catalytic cracking unit. Test results have revealed that compared to the USY zeolite and Beta zeolite, thecatalytic pyrolysis of n-heptane in the presence of the ZRP zeolite catalyst can result in higher yield and selectivity of ethyl-ene and propylene, while a higher reaction temperature and a higher catalystoil ratio can promote the formation of ethyleneand propylene during catalytic pyrolysis of n-heptane. The ethylene formation reaction is more sensitive to the changes inreaction temperature, whereas the changes in catalyst/oil ratio are more infuential to the propylene formation reaction. Thispaper has made a preliminary exploration into the different reaction pathways for formation of ethylene and propylene onzeolites with different structures.Key words: n-heptane catalytic pyrolysis light olefins reaction pathway1 Introductionoutput of ethylene and propylene than that provided bythe steam cracking process, despite its effective contribu-Ethylene and propylene are important organic chemicaltion to the increase in the propylene/ethylene ratio, for itfeedstocks. In 2010 the global demand for ethylene andstill delivers a sizable amount of gasoline component. Inpropylene was 115 million metric tons and 70 millionorder to tackle the above mentioned problems after tak-metric tons, respectively, while the propylene /ethyleneing into account the current situation of China's crudedemand ratio increased to 0.61, with the propylene /ethyl-oil production and petroleum refining industry as wellene production ratio decreased to 0.34. It is projected thatas the ethylene production cost, it is recognized that thein the future years to come the global propylene demandsteam cracking and catalytic pyrolysis units will still usewill be growing at a rate exceeding 5% a year, which willnaphtha as the feedstock for many years to come, and thebe higher than the demand increase for ethylel-2. Adevelopment of naphtha catalytic pyrolysis technologypresent more than 95 percent of ethylene and over 60 per-would be an important direction for the growth of lightcent of propylene are produced worldwide by the steamolefins industry in a bid to increase the product yield andcracking technology', but the propylene /ethylene outputimprove the product distribution.ratio delivered by steam cracking units is much lowerThe straight-run (SR) naphtha fraction contains hydro-than the propylene /ethylene demand ratio. Since the gapcarbons that are divided into aliphatic hydrocarbons,between the current production of ethylene and propyl-naphthenic hydrocarbons and aromatic hydrocarbons,ene through steam cracking technology and the marketand clear understanding of the possible chemical reac-demand will constantly widen, the development of newions and reaction pathways of these hydrocarbons hastechnology for manufacture of light olefins is extremelyimportant and practical significance for design of newurgent. The heavy oil catalytic pyrolysis technology (suchcatalysts, selection and optimization of process. The nor-as DCC and CPP techniques) as another important meansmal alkanes account for over 30 m% of SR naphtha andfor producing propylene has been producing propylenethat accounts for around 30% of total propylene produc-Corrresponding Anthor' Prnfessonr Xie Chaogang, Telephone:tion'l. However, catalytic pyrolysis still has a lower total+86-10-823687中国煤化工c.com.YHCNMHG8●Cheng Xiajie, et al. Efects of Light Olefns Formation during Catalytic Pyrolysis of n-Heptaneare mainly distributed in the C。C, fraction. Compared2.3 Test methodto other hydrocarbons, normal alkanes are characterized The catalytic pyrolysis of n-heptane was carried out inby a single molecular structure that undergoes reaction a small-scale fixed fluidized catalytic cracking reactoraccording to a simple reaction pathway. By focusing on (ACE), with its catalyst jinventory equating to 9 grams. Athe rules of catalytic pyrolysis reactions of n-alkanes as a definite amount of catalyst was loaded in the reactor, ingroundbreaking point it is possible to help delve into the which the preheated feedstock was injected by means ofchemistry relating to the catalytic pyrolysis reactions of an injection pump to take part in the catalytic cracking re-the SR naphtha. This article intends to use normal heptane action. The compounds remaining inside the catalyst wereas a model compound in the study on the infuence of ze- then stripped out by nitrogen gas prior to coke burningolite structure and process parameters (including reactionunder air fow and regeneration of the catalyst. The reac-temperature and catalyst/oil ratio) on the rules on forma-tion products after passing through the condensation/tion of ethylene and propylene in the course of catalyticcooling system were separated into gascous productspyrolysis of n-heptane in order to provide a reference forand liquid products. The detailed composition of FCCthe development of technology for catalytic pyrolysis ofgas products was obtained through online chromato-naphtha.graphic analysis of gaseous products. The compositionof liquid products after collecting and weighing was ob-2 Experimentaltained through analysis by an off-line chromatograph.The fue gas obtained during burning of coke deposits2.1 Feedstockon the catalyst was routed to the co converter to beThe feedstock applied in the experiments was n-heptanefully transformed into CO, which was then measured(analytically pure reagent) with a purity exceeding 99%by an online infrared CO2 analyzer to calculate the cokemanufactured by the ACROS ORGANICS Reagent Com-yield.pany.3 Results and Discussion2.2 CatalystsThe catalysts contained 50% zeolite made in laboratory3.1 Infuence of zeolite structure on catalyticwith the active components in the catalyst comprisingpyrolysis of n-heptaneUSY zeolite, Beta zeolite, and ZRP zeolite, respectively.The influence of zeolite structure on the heptane con-The conditions for hydrothermal treatment of catalystsversion rate, the yield and selectivity of ethylene andand their main physico chemical properties are presentedpropylene during catalytic pyrolysis of n-heptane in thein Table I.presence of the USY zeolite, the Beta zeolite and the ZRPTable 1 Conditions for hydrothermal treatment ofzeolite, respectively, at a reaction temperature of 600 C,catalysts and their main physico -chemical propertiesa catalyst/oil mass ratio of 6, and a weight hourly spaceDatavelocity of8 h', wih the test results presented in Table 2.ltemZRPBetaUSY 二It can be seen from the data listed in Table 2 that the ZRPChemical composition, m%:catalyst achieved the highest ethylene and propyleneAl2O,52.145.756..yield equating to 2.00 % and 5.72 %, respectively, and the51.542.5Beta zeolite showed a medium ethylene and propylenePhysical property:yield that was 0.74% and 2.65%, respectively, whereasSpecific surface area, m'/g20617682the USY zeolite gave the lowest ethylene and propylenePore volume, mL/g0.1800.2530.235yield that was merely 0.69 % and 1.15 %, respectively.The sum of selectivity of ethylene and propylene duringHydrothermal treatment800心, 10h| 800C, 14h | 800 C, 17hconditionscatalytic pyrolys中国煤化工the fllwMicroreactor activity, %7Sing order: ZRP 2CNMH G, which was9●China Petroleum Processing and Petrochemical Technology2011,13(4):8-14Table 2 Yield and seletivity of ethylene and propylene obtained on catalysts with different structuresYield, %Seletivity, %Propylene/ethyleneCatalystConversion, %EthylenePropyleneyield ratioUSY12.610.69 .1.155.479.091.67Beta17.920.72.654.1314.85 .3.58ZRP27.602.005.727.220.722.86consistent with the conversion rate of n-heptane achievednonlinear zigzag channel measuring around 0.56*0.65during its catalytic pyrolysis reaction.nm with its dimension ranging between that of the USYThe difference in the yield and selectivity of ethylene andzeolite and the ZRP zeolite. Therefore, the selectivity ofpropylene existing during catalytic pyrolysis of n-heptaneethylene and propylene achieved by the Beta zeolite ison catalysts with different pore structures can be dis-also between that of the USY zeolite and the ZRP zeolite.cussed in the context of the following circumstances.Secondly, it is necessary to mention the function of acidFirstly, it is necessary to mention the shape selectiv-centers on the catalyst. The catalytic pyrolysis reaction ofity of zeolite pores which feature the catalytic reactionhydrocarbons takes place under the action of acid centers,that takes part inside‘catalyst crystal lttices', and theand it is generally recognized that the Bronsted acids onstructure and size of zeolite pores have a great impact on the catalyst are the major centers of pyrolysis activity.catalytic reactions. The USY zeolite'l is composed of aThe infrared spectroscopic analysis of acidity of differ-series of spherical cavities with a diameter of around 1.2ent zeolite catalysts is presented in Table 3. It can be seennm, and each spherical cavity connects through a pore offrom the data listed in Table 3 that the ZRP zeolite has the12-membered oxygen ring (with a free pore aperture ofhighest amount of acid centers, and the Beta zeolite as-0.74 nm) with other four similar cavities to form a space sumes the second place, while the USY zeolite pssessesskeleton of cubical networks. This open space skeletonthe least amount of acid centers. Therefore the conversionstructure with macroporous channels can make most n-of n-heptane on the USY zeolite was the lowest, whichheptane (with a critical molecular diameter of 0.4- -0.5was equal to about 12.61 % (as shown in Table 2). Fur-nm) and molecules of reaction products freely enter thesethermore, it can be seen from data listed in Table 3 thatcavities, resulting in decreased yield and selectivity of the ZRP zeolite has an amount of Lewis acidity which isethylene and propylene since the reaction products areclose to that of the Beta zeolite, but the amount of Br8n-not constrained by diffusion. The ZRP zeolite is a shape-sted acid sites of which is more than twice of those of theselective zeolite with a MFI structure classifies as theBeta zeolite. In particular, the strong Br8nsted acid cen-mesoporous zeoltel4l composed of 10-membered oxygenters of the ZRP zeolite are four times the strong Bronstedrings that have no cages in the framework except a space acid centers on the Beta zeolite, and such a large amountof around 0.9 nm at the intersection of two pores providedof Bronsted acid centers is the main cause responsible forwith a free pore diameter of0.5- 0.6 nm. Since the pore the high conversion of n-heptane during catalytic pyroly-diameter is relatively small, the difusion of larger reac- sis reation in the presence of the ZRP zeolite.tion products molecules is contolled by the zeolite pore,Table 3 Infrared spectroscopic analysis of zeolitewhich means that the small pore diameter hinders thecatalysts through pyridine adsorptionformation and accumulation of large molecular productsAcids anount at 200 C Acids amount at 350 CTotalType ofamount ofand only allows for the existence of small transitionalzeoliteLewisBr'nstedLewis| Bronstedacidsmolecular reactions leading to the formation of moleculeswith chain-like structure. Therefore, the ZRP zeolite can103.4115.119.5128.3achieve higher (ethylene+propylene) selectivity. The169.820.011.6189.8linear channels in Beta zeolites1 have a pore diameter中国煤化工45.04.3measuring around 0.57x0.75 nm and a pore diameter ofYHCNMH G10●.Cheng Xiaojie, et al. Effects ofLight Olefins Formation during Catalytic Pyrolysis of n-HeptaneIt can be seen from data listed in Table 2 that on three dif-a hydrocarbon molecule cannot be easily protonized be-ferent zeolite catalysts, the propylene/ethylene ratio in thecause of their higher bond energy, and the C- -H bonds onproducts of n-heptane pyrolysis is always greater than 1,other positions have close bond energy. Hence this articledenoting that the propylene yield is higher than the eth-intends to discuss the possibilities on the pathways forylene yield. This fact has revealed that under the currentcraking reactions of protonized C- -H bonds on the C2-reaction conditions in the presence of these three zeoliteC4 carbon atoms.catalysts n-heptane follows a similar pyrolysis reactionIt can be seen from Figure 1 that protons originated frompathway. According to the carbenium mechanism, whenthe Bronsted acid centers attack the C- H bonds on C2the hydrogen protons from the acid centers of the catalystcarbon atoms to form penta-coordinated carbonium ionsoattack the n-heptane molecules, there are four positions(Figure 1), which could have three possible pyrolysis re-on the C- -H bonds that are susceptible to proton attacksaction pathways'" as shown below.as shown in Figures 1- -3. The terminal C- -H bonds ofThe reaction pathway①can via cleavage ofC- H bondsDH2+cn田jc4.，C、②CH4. 思CHz③田CH2Figure 1 Reaction pathways for pyrolysis of protonized C- H bonds at C2 atom of normal heptaneform hydrogen and tri-coordinated carbonium ions withpenta-coordinated carbonium ions as shown in Figure 2,the same number of carbon atoms as the feedstock; thethe penta-coordinated carbonium ions formed therebyreaction pathway②can via cleavage ofC- C bonds andalso follow three pyrolysis reaction pathways. The reac-C- -H bonds form methane and tri-coordinated carboniumtion pathway①can via cleavage of C--H bonds formions of C6; and the reaction pathway❸can via cleavagehydrogen and tri-coordinated carbonium ions having theofC- C bonds and C- H bonds form ethylcarboniumsame number of carbons as that of the feedstock; the reac-ions and Cs alkanes. Thereby upon attacking C- -H bondstion pathway②can via cleavage ofC- C bonds and C-on C2 carbon atoms by protons there is great possibilityH bonds form ethane and tri- coordinated carbonium ionson formation of ethylcarbonium ions that would form eth-of Cs; and the reaction pathway③can via cleavage ofylene after desorption of protons.C- C bonds and C- -H bonds form propylcarbonium ionsUpon attacking C- H bonds of C3 carbon atoms by pro-and C4 alkanes. Therefore upon attacking C- H bonds ontons originated from the Bronsted acid centers to formC3 carbon atoms by protons there is a great possibility on8~c、2cn, + C\C、c中国煤化工Figure 2 Reaction pathways for pyrolysis of protonized C- H bonds.MYHCNMHG11●China Petroleum Processing and Petrochemical Technology2011,13(4):8-14formation of propylcarbonium ions that would form pro- pathway ①can via cleavage of C- H bonds form hydro-pylene after desorption of protons.gen and tri-coordinated carbonium ions having the sameUpon attacking C- H bonds of C4 carbon atom by pro-number of carbons as that of the feedstock; and the reac-tons originated from the Bronsted acid centers to formtion pathway②can via cleavage ofC- C bonds and C-penta-coordinated carbonium ions as shown in Figure 3,H bonds form propane and tri-coordinated carbonium ionsthe penta-coordinated carbonium ions formed therebyof C. Under this circumstance it is most probably to formmay follow two pyrolysis reaction pathways. The reactiona lot of propane and Cx olefins.①c田Cc/HH田Figure 3 Reaction pathways for pyrolysis of protonized c- -H bonds at C4 atom of normal heptaneIt can be learmned from analysis of reaction pathways forpyrolysis of n-heptane that upon attacking C- -H bonds3.2 Influence of reaction temperaturelocated at different positions of hydrocarbon moleculesby hydrogen protons originated from catalyst acid centersThe influence of reaction temperature on the yield andthere is difference in ethylene/propylene ratio of the py-selectivity of ethylene and propylene formed during cata-rolysis products. The fact that the propylene yield in thelytic pyrolysis of n-heptane at a catalyst/oil ratio of 6 andproducts of n-heptane pyrolysis obtained on three zeolitea weight hourly space velocity of 8 h"' was investigatedcatalysts always exceeds the ethylene yield has indicatedin the presence of the ZRP zeolite catalyst, with the testthat under the experimental conditions the protonationresults presented in Table 4.location mainly exists at the C- H bonds of C3 carbonWith respect to the hydrocarbon cracking reaction, a 10 C .Table 4 Infuence of reaction temperature on formation of ethylene and propylene during catalytic pyrolysis of n-heptaneReactionYield, %Seletivity,%Conversion, %Propylenc/ethylene yield ratiotemperature, CEthylenePropylene50017.300.492.222.8112.814.5655020.773.695.1217.793.4727.602.005.727.2520.722.86rise in the reaction temperature would increase the reac-that of the conversion rate. When the reaction tempera-tion rate by 10%- 20%. It can be seen from data litedture increased from 500 C to 600 C, the sum of ethylenein Table 4 that the conversion rate during n-heptaneand propylene yield also increased from 2.71% to 7.72%,pyrolysis reaction increased with an increasing reactionwhereas the sum of ethylene and propylene selectivitytemperature, and the higher the reaction temperature, thesurged from 15.62% to 27.97%. This fact has indicatedmore significant the increase in conversion rate would be.that an increase in reaction temperature could be condu-When the reaction temperature was in the range betweencive to the formation of ethylene and propylene through500- 550 C and between 550- -600 C, the conversionn-heptane pyrolysis. It is worth mentioning that with anrate had increased by 3.47% and 6.83%，respectively.increase in reaction temperature there is a difference inCorrespondingly, the change of ethylene and propylene scope of increase in the.vield of ethvlene and propyleneyields in reaction products revealed the same tendency asdespite an sir中国煤化Ihylene and pro-TYHCNMH G12●.Cheng Xiaojie, et al. Effects of Light Olefins Formation during Catalytic Pyrolysis of n-Heptanepylene yields, as evidenced by a gradually decreasing3.3 Infuence of catalyst/oil ratiow(propylene)/w(ethylene) ratio. For example, when theIn an attempt to avoid affecting the catalyst fuidizationreaction temperature increased from 500 C to 600 C,state inside the reactor, a kaolinite sample that had beenthe propylene/ ethylene yield ratio dropped from 4.56 tosubjected to deactivation treatment was used as the dilut-2.86, indicating that in the course of n-heptane pyrolysising agent. Upon maintaining the total weight of catalystreaction higher temperature was conducive to increasingand kaolinite sample in the reactor at 9 grams, the cata-ethylene and propylene yields, however, the ethylene for-lyst/oil ratio for the reaction was regulated through chang-mation reaction was more sensitive to changes in reactioning the catalyst dosage at a constant feedstock rate. Thetemperature. Furthermore, it can be learmed from analysisinfuence of catalystoil ratio on formation of ethylene andof the reaction pathway for pyrolysis of normal heptanepropylene was investigated during catalytic pyrolysis of(as shown in section 3.1) an increasing reaction tempera-n-heptane in the presence of the ZRP catalyst at a reactionture can change the route of normal heptane pyrolysistemperature of 600 C and a weight hourly space velocityreaction to enhance the possibility for attacking the C- -Hof8 h', with the test results presented in Table 5.bonds on C2 atoms by protons, resulting in more remark-It can be seen from data listed in Table 5 that conver-able scope of increase in ethylene yield.sion rate of n-heptane achieved during catalytic pyrolysisTable 5 Influence of catalyst/oil ratio on formation of ethylene and propylene during catalytic pyrolysis of n-heptaneYield, %Selectivity, %Catalystoil ratioConversion, %Propylene/ ethylene yicld ratioEthylenePropylene0.3314.500.811.345.619.221.641.3316.18 .0.912.2614.002.493.3322.191.383.966.2317.842.866.0027.602.005.727.2520.72reaction increased with an increasing catalystoil ratio4 Conclusionscoupled with an increase in ethylene and propylene yield.For example, when the catalyst/oil mass ratio increased (1) Catalytic pyrolysis of n-heptane in the presence offrom 0.33 to 6.00, the sum of ethylene and propyleneZRP zeolite with smaller pore size and more Bronstedyield increased from 2.15% to 7.72%, and the sum ofacid sites can give rise to a higher yield and selectivity ofethylene and propylene selectivity increased from 14.83%ethylene and propylene.to 27.97%. The increased catalyst/oil ratio could increase(2) Higher reaction temperature and higher catalystoilthe access of reactants to catalytic active centers, whichratios can promote catalytic pyrolysis of n-heptane toyas conducive to the formation of carbonium ions and form ethylene and propylene. The ethylene formationpropagation of carbonium ion reactions, leading to thereaction is more sensitive to changes in reaction tempera-formation of smaller olefin molecules. Furthermore, when ture, whereas the catalyst/oil ratio has a more significantthe catalyst/oil mass ratio increased to 6.00 from 0.33,impact on propylene formation reaction.the scope of increase in ethylene and propylene yield was (3) The n-heptane catalytic pyrolysis reaction pathway on147% and 327%, respectively, and the scope of increase different kinds of zeolites is similar. In the course of cata-in propylene yield was far more than that of ethylenelytic pyrolysis, the hydrogen protons intend to attack theyield, while the propylene/ ethylene yield ratio increased C- -H bonds on the C3 carbon atoms to initiate reactionsfrom 1.64 to 2.86, indicating that the propylene formationinvolving carbonium ions to form more propylene, andreaction was more sentive to the changes in the number of with an increasing qotion tomnortura the _nositions that中国煤化工active centers in the reaction system.are attacked by I_shift towardsTYHCNMHG13..China Petroleum Processing and Petrochemical Technology2011,13(4):8-14the terminal carbon atoms to enhance the probability for(in Chinese)attacking C2 carbon atoms, resulting in an gradual in- [4] Chen Junwu. Catalytic Cracking Process and Engineeringcrease in ethylene yield.[M]. Beijing: China Petrochemical Press, 2005: 184- 186 (inChinese)References[5] Pang Xinmei, Gao Xiaobui, Zhang Li, et al. Performance[1] Wang Jianming. Advances in technology and commercialof β zeolite in catalytic cracking reactions [小] Acta Petroleiapplication of catalytic pyrolysis for manufacture of low-Sinica (Petroleum Processing Section), 2009, 25(S1): 77-79carbon olefins[J].Advances in Chemical Industry, 2011,30(5): 911-917 (in Chinese)[6] Haag W 0, Dessau R M. Duality USY of Mechanism for[2] Zhou Yingfei, Qian Baizhang. Present status and forecast onAcid-Catalyzed Paraffin Cracking [C]/ Proceeding of the 8thpetrochemical products market in China [D. PetrochemicalCongress on Catalysis. Berin : Dechema, 1984: 305- 313Science and Technology Marke, 2010, 3(5); 1-6 (in Chinese) [7] Long Jun, Wei Xiaoli. Study on mechanism for formationo of[3] Wang Hongqiu, Zheng Yidan, Liang Chuan. Advances indry gas in the course of catalytic cracking []. Acta Petroleilight olefins production technology and analysis of its pros-Sinica (Petroleum Processing Section), 2007, 23(1): 1-7(inpects [小. Chinese and Overseas Energy, 2010, 15(8): 62-67Jilin Fuel Ethanol Company to Launch Cellulose Ethanol ProjectThe Jilin Fuel Ethanol Company is engaging in precon- companies are also looking for cooperative partners instruction preparatory work on the commercial project for China to jointly construct cellulose ethanol plants. SINO-manufacturing ethanol from cellulose, and the project PEC intends to cooperate with COFCO and Novozymesconstruction work is to be commenced soon. It is leamed in the production of cellulose ethanol in China, with thethat in China there are five fuel ethanol enterprises, project proposal already being submitted for review andamong which four enterprises manufacture ethanol from approval.grains and one enterprise produces ethanol from cassava The Jilin Fuel Ethanol Company after having comparedin Guangxi province. To fulfill the rising public opinion the process routes and technical characteristics of cel-on grain security the fuel ethanol industry should follow lulose ethanol production technology owned by the Ca-a non-grain synthesis route in the future. Major compa- nadian logen Company, the American DDCE Companynies at home and abroad on the basis of successful pilot-and the Finnish Chempolis Company has finally selectedscale technical tests have been devoting their attention on the technology for manufacture of cellulose ethanolthe construction of 10-kt/a-class commercial production from corn stalks licensed by Chempolis. This technologyunits. The Canadian logen Company and its American through isolation of cellulose and hemicelluloses fromcounterpart are planning to jointly invest $700 million to lignin in com stalks followed by deep processing of eachconstruct a 130 kt/a cellulose ethanol plant in 2012, and component separately can yield individual products, re-the DuPont DDCE Company plans to invest 850 millionspectively. The isolated cellulose after enzymolysis andRMB to construct a 75 kt/a commercial cellulose ethanolre-fermentation can yield ethanol, and the hemicellulosesunit in 2013. Furthermore, Shell, DuPont and other major after acidolys中国煤化Ifurfural.YHCNMH G