isolation n-butane


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isolation n-butane

  2. 2. ‫َ‬ ‫ُ ْ َﺎﻧ َ ِﻠﻢ ﻟﻨ إ َ ﻋﻠﻤﺘﻨ إﻧ‬‫ﻗَﺎُﻟﻮا ﺳﺒﺤ َﻚ ﻻ ﻋ َ ََـﺎ ِﻻﱠ ﻣﺎ ﱠ ََـﺎ ِﱠﻚ‬ ‫ـ‬ ‫أَﻧﺖ اﻟﻌِﻴﻢ اﻟﺤﻜﻴﻢ‬ ‫ِ‬ ‫ْ َ َﻠ َ‬ ‫ﺻﺪق اﷲ اﻟﻌﻈﻴﻢ‬ ‫ﺳﻮﺭﺓ ﺍﻟﺒﻘﺮﺓ ﺍﻻﻳﺔ )23(‬
  3. 3. CERTIFICATEWe certify that we have read this thesis entitled "Improvement of Catalysts forHydroisomerization of Iraqi Light Naphtha" by Muayad MohammedHasan and as on Examining Committee examined the student in its contents andthat in our opinion it meets the standard of a thesis for the degree of Master ofScience in Chemical Engineering. Signature: Signature:Asst. Prof. Dr. Khalid A. Sukkar Asst. Prof. Dr. Shahrazad R. Raouf (Supervisor) (Chairman) Date: / / 2010 Date: / / 2010 Signature: Signature:Asst. Prof. Dr. Wadood T. Mohammed Asst. Prof. Dr. Saba A. Ghani (Member) (Member) Date: / / 2010 Date: / / 2010 Approved for the University of Technology – Baghdad Signature: Prof. Dr. Mumtaz A. Zablouk Head of Chemical Engineering Department Date: / / 2010
  4. 4. SUPERVISOR CERTIFICATIONI certify that this thesis entitled:- "Improvement of Catalysts forHydroisomerization of Iraqi Light Naphtha" Presented by MuayadMohammed Hasan, was prepared under my supervision in a partial fulfillmentof the requirements for the degree of Master of Science in Chemical Engineeringat the Chemical Engineering Department, University of Technology. Signature: Name: Asst. Prof. Dr. Khalid Ajmi Sukkar (Supervisor) Date: / / 2010In view of the available recommendations I forward this thesis for debate by theExamination Committee. Signature: Name: Asst. Prof. Dr. Khalid Ajmi Sukkar Deputy Head of Department of Chemical Engineering Date: / / 2010
  5. 5. CERTIFICATIONThis is to certify that I have read the thesis titled "Improvement ofHydroisomerization Process to Produce High Octane Gasoline usingModified Catalysts" and corrected any grammatical mistake I found.The thesis is therefore qualified for debate.Signature:Name:Date: / / 2010
  6. 6. Acknowledgment AcknowledgmentFirst of all praise be to god Who give me patience, strength and the mostimportant thing: faith to continue...I wish to present my sincere appreciation with deep respect to mysupervisor Dr. Khalid Ajmee Sukkar for his helpful efforts and adviceduring my work.My great gratitude is due to the Head and the staff of ChemicalEngineering Department of the University of Technology for their helpand assistance in providing facilities throughout this work.My respectful regards to Mr. Bushier Yosuf Sharhan for his kindness andhelpful efforts to making the characterization of my work.Finally my grateful thanks are due to my wife for her encouragement andsupport. I
  7. 7. Summary SummaryIn the presented work hydroisomerization of Iraqi light naphtha (produced in Al-Dura Refinery) has been investigated to produce isomers. Three types of catalystswere prepared Pt/HY, Pt/BaY, and Pt/Al 2 O 3 with 0.5wt% by impregnation withhexachloroplatinic acid.The catalytic unit was constructed from stainless steel and designed to carry outthe hydroisomerization process. The fixed bed reactor dimensions were O.D 3cm,I.D 2cm, and 21cm high. All experiments were made at atmospheric pressure andreaction temperature of 230, 250, 270, 290, and 310°C, WHSV 1.5, 3, and 4.5h-1,under constant H 2 /HC mole ratio of 4.The results show that the conversion of the main light naphtha components (n-pentane, n-hexane, 2-methylpentane, and 3-methylpentane) increases with increasein reaction temperature and decreases with increase in weight hour space velocity.Also, it was noted that the selectivity to isomers increase with Pt/HY, Pt/BaYcatalysts at low temperature and decrease at high temperature, while with Pt/Al 2 O 3 R R R Rcatalyst the aromatics products increase with increase in reaction temperature.Pt/HY catalyst gives higher selective isomerization than Pt/BaY catalyst which is(95%) and (89%) respectively at 270°C, and (1.5 hr-1). While, Pt/Al 2 O 3 catalyst P P R R R Rgives 64.7% as total conversion where 18% as aromatic products. The totalconversion for Pt/HY and Pt/BaY were about 50%. The following sequence forisomerization selectivity was concluded as: Pt/HY > Pt/BaY > Pt/Al 2 O 3R R R R II
  8. 8. SummaryA kinetic model was derived based on the present work results. Then, the kineticparameters such as K 1 , K 2 , K o , and activation energy (E) are calculated dependingon the present experimental work results.The results of model show that the values of apparent activation energyvary within a range of 22 and 23 kJ/mol for n-pentane, 20 to 24 kJ/mol forn-hexane, and 15 to 17 kJ/mol for 3mp isomerization reactions. On theother hand, the model pointed the reactivity order behaves as follows. 3-methylpentane > n-hexane > n-pentaneDerive an equations which are calculating the reaction rate constants (k 1 and k 2 )parameters as follows: k1= [(1+ Є) Ln – Єx] C iso = C A° [1- exp (- k 1 t) - [exp(-k 1 t) – exp(-k 2 t)] R R R R III
  9. 9. Contents CONTENTSSubject PagesAcknowledgments ISummary IIContents IVNomenclature VIII CHAPTER ONE : INTRODUCTION1. 1 Introduction 11. 2 Aims of the Work 3 CHAPTER TWO: LITERATURE SURVEY2. 1 Scope 42. 2 Gasoline Fuel and its Specifications 52. 3 Hydroisomerization Process 9 2. 3. 1 Catalysts for Hydroisomerization Process 14 16 Alumina 17 Zeolite2.4 Previous Work 20 IV
  10. 10. Contents2.5 Catalysts Preparation 27 2.5.1 Impregnation 28 2.5.2 Calcination 30 2.5.3 Reduction 302.6 Catalysts Characterization 31 2.6.1 X-ray Diffraction (XRD) 32 2.6.2 Surface Area 32 2.6.3 Scanning Electron Microscopy (SEM) 33 CHAPTER THREE: EXPERIMENTAL WORK3.1 Materials 343.2 Preparation of Modified Zeolites by Ion Exchange 37 3.2.1 Preparation of Barium- Zeolite 37 3.2.2 Preparation of HY- Zeolite 373.3 Catalysts Preparation 38 3.3.1 Preparation of Pt/ BaY and Pt/HY 38 3.3.2 Preparation of Pt/ AL 2 O 3 R R R 383.4 Experimental Unit 393.5 Procedure 423.6 Catalysts Characterization 45 V
  11. 11. Contents 3.6.1 X-Ray Diffraction Analysis 45 3.6.2 Surface Area 45 3.6.3 Scanning Electron Microscopy (SEM) 45 3.6.4 Energy Dispersive X-Ray (EDAX) Analysis 45 CHAPTER FOUR: KINETIC ANALYSIS4.1 Introduction 464.2 Model Development 484.3 Reactor Model 51 CHAPTER FIVE: RESULTS AND DISCUSSION5.1 Characterization of Catalysts 56 5.1.1 X-ray Diffraction 56 5.1.2 Scanning Electron Microscopy (SEM) Analysis 57 5.1.3 Energy Dispersive X-ray (EDAX) Analysis 58 5.1.4 Surface Area 605.2 Effect of Operating Conditions 61 5.2.1 Effect of Temperature 61 Effect of Temperature on Conversion of 61 light naphtha Effect of Temperature on Total Conversion 66 of light naphtha and Selectivity VI
  12. 12. Contents 5.2.2 Effects of WHSV 75 5.2.3 Effect of Time 785.3 Results of Kinetic Study 82 CHAPTER SIX: CONCLUSIONS AND RECOMMENDATIONS6. 1 Conclusions 906. 2 Recommendations 91 REFERENCES 92APPENDIX A (Volume Percent of Components) 106APPENDIX B (Concentration of Components) 118APPENDIX C (Conversion of Light Naphtha) 121APPENDIX D (Reaction Rate Constants) 123APPENDIX E (Percentage Selectivity and Conversion) 125APPENDIX F (Sample of Calculation) 126 VII
  13. 13. Nomenclature NomenclatureSymbols Definition Units Concentration of Normal CA R gm-mol/lit Paraffins at any Time Initial Concentration of CAo R RP gm-mol/lit Normal Paraffins C isoR Concentration of iso-Paraffins gm-mol/lit CN R Concentration of Olefin gm-mol/lit A integration constant (-) -r A R rate of reaction mole/gcat. hr T Time hr T Temperature K To P Initial Temperature KWHSV Weight Hour Space Velocity hr-1 P ko R Pre-Exponential Factor (-) k1 R R Rate Constant for Paraffins hr-1 P k2 R Rate Constant for Olefins hr-1 P E Activation Energy kJ/mole Molar Flow Rate of FA R mole/hr Component A Initial Molar Flow Rate of FAo R RP Component A mole/hr VIII
  14. 14. NomenclatureR Gas Constant atm-lit/gm-mol-KVA R Volume of Reactor cm3 PXR Conversion (-)Zt Length of Reactor cm Integration Step for the Reactor∆z Length (-) IX
  15. 15. Nomenclature Abbreviations RON Research Octane Number MON Motor Octane Number RVP Reid Vapor Pressure ASTM American Society for Testing Materials MTBE Methyl Tertiary-Butyl Ether UOP Universal Oil Product CompanyBUTAMER Butane Isomerization Unit MOR Mordenite i-C 5 R iso-Pentane C5 R n-Pentane C6 R n-Hexane 2MP 2-Methylpentane 3MP 3-Methylpentane2,2DMB 2,2-Dimethylbutane2,3DMB 2,3-Dimethylbutane 2,2DMP 2,2-Dimethylpentane 2,4DMP 2,4-Dimethylpentane X
  16. 16. Chapter One Introduction Chapter One Introduction1.1 IntroductionThe interest in improving the efficiency of the automotive motorsencourages the formulation of new catalysts and the development ofprocesses for gasoline.Due to the environmental restrictions a reduction in allowable of leadcompounds levels and toxic compounds such as aromatics, in particularbenzene, olefin, sulfur-containing components in automobile gasolinewere imposed, as a result it forced refineries to implement new octaneenhancement projects.Considering that branched-chain alkanes posses the greatest octanenumbers, the normal alkanes hydroisomerization is one of the mosteffective project decisions in a direction favoring the least initialinvestment approach as opposed to the best overall payout. The use ofgasoline containing higher content of these compounds is one alternativeto obtain clean fuel with high antiknock characteristics.In order to increase the gasoline octane number, major petroleumrefineries used different units such as catalytic reforming, cracking,alkylation, oligomerization, polymerization and isomerization(hydroisomerization) [Benadda et al., 2003, Nattaporn and James, 2007].It is important to mention here that the petroleum industry is looking foreconomical solutions to meet new regulatory specifications for producingenvironmentally clean fuels. Most of the implemented legislations require 1
  17. 17. Chapter One Introductiona reduction and a limitation on the concentration of benzene in thegasoline pool. This has increased the demand for high performance C 5and C 6 naphtha isomerization technology because of its ability to reducethe benzene concentration in the gasoline pool while maintaining orincreasing the pool octane.Light paraffin isomerization has been used historically to offset octaneloss from lead-phase out and to provide a cost-effective solution tomanage benzene in motor fuels. In the current refining environment,isomerate octane can be used to offset octane loss from MTBE phase-out[Anderson et al., 2004]. Therefore, the hydroisomerization of lightnaphtha (C 5 -C 6 fractions) is an industrially important process and is usedin the production of high octane gasoline blend stocks. The processinvolves the transformation (with minimal cracking) of the low octanenormal (and less branched) paraffin components into the high octaneisomers with greater branching of the carbon chain [Ravishankar andSivasanker, 1996, Andreas, 2003, Rachid et al., 2006, María et al., 2008].In Iraq there is no clear strategies to reduce the demand for leadedgasoline and aromatics (Benzene). Therefore, the hydroisomerizationunits are regarded a good solution and a good start point strategy indirection of clean fuels.The metal– acid bifunctional catalysts, such as alumina or zeolitesupported Pt catalysts, are used in hydroisomerization of light paraffins(n-pentane and n-hexane). It shows high efficiency in the isomerization ofalkanes. The isomerization of pentane and hexane is successfully carriedout using noble metals such as Pt- or Pd- supported on Al 2 O 3 , mordenite,beta zeolite, and silicon catalyst. However, difficulties are encounteredwith hydrocarbons larger than heptane because the cracking reaction 2
  18. 18. Chapter One Introductionbecomes more significant over these isomerization catalysts as the chainlength increases. So, some modification and pretreatment processes arerequired to increase the catalyst activity, selectivity and life time [Takeshiet al., 2003, Ping et al., 2009].The literature mentions many studies which were focused to investigatethe hydroisomerization of n-paraffins [Liu et al., 1996, Chica and Corma,1999, Yunqi et al., 2004, Salwa et al., 2007]. Few investigations haveused light naphtha as a feedstock for the process. On the other hand, manyauthors made a kinetic study on the hydroisomerization unit for n-hexaneand n-heptane [Runstraat et al., 1997, Annemieke et al., 1997, Franciscus,2002, Toshio, 2004, Matthew, 2008]. But only few studies dealling withthe hydroisomerization of light naphtha were published [Holló et al.,2002, Carsten, 2006].1.2 Aims of the Work The main aims of the present work are: 1- Preparation of modified zeolites (BaY and HY) by ion exchange method. 2- Preparation of Pt/ BaY and Pt/HY by impregnation method.3- Study the hydroisomerization of Iraqi light naphtha over bifunctional zeolite catalysts and test of the prepared catalysts activity and selectivity under different operating conditions of temperature, and WHSV.4- To make a mathematical model to describe the reaction kinetics of the hydroisomerization process.5- To estimate kinetics parameters under different operating conditions depending on the results of present experimental work. 3
  19. 19. Chapter Two Literature Survey Chapter Two Literature Survey2.1 Scope UThe hydroisomerization of light paraffins is an important industrial process toobtain branched alkanes which are used as octane boosters in gasoline. Thus,isoparaffins are considered an alternative to the use of oxygenate and aromaticcompounds, whose maximum contents are subjected to strict regulations inorder to protect the environment [Holló et al., 2002, Satoshi, 2003, Rafael et al.,2005].Hydroisomerization reactions are generally carried out over bifunctionalcatalysts, often containing platinum. The metal component aids in increasingthe rate of isomerization, besides lowering catalyst deactivation.The interest in the isomerization process is heightened with the phase out oftetraethyl lead in 1970s, following the phase out of leaded gasoline due to theintroduction of clean air act amendments of 1990 in the USA and similarlegislation in other countries. Aromatics and olefin react with NO X emission to R Rform ozone, thus contributing to smog formation [Maloncy et al., 2005].Therefore, in many plants refineries have to minimize benzene yield. In Europe,the aromatics content is limited since 2005 to content 35 vol% instead of 42vol% and benzene to approximately zero level [Liu et al., 1996, Goodarz et al.,2008].There are various approaches in petroleum refineries to obtain high octanenumber components, which include processes of cracking, reforming and 4
  20. 20. Chapter Two Literature Surveyisomerization. Catalytic cracking is the process for converting heavy oils intomore valuable gasoline and lighter products. The cracking process producescarbon (coke) which remains on the catalyst particle and rapidly lowers itsactivity. On the other hand, the catalytic naphtha reforming is the chemicalprocess which converts low octane compound in heavy naphtha to high-octanegasoline components, without changing carbon numbers in the molecule. Thisis achieved mainly by conversion of straight chain naphtha to iso-paraffins andaromatics over a solid catalyst. The isomerisation (hydroisomerization) is thechemical process which converts low octane compound in light naphtha to highoctane number components via rearrangement of the molecular structure of ahydrocarbon without gain or loss of any of its components. [Ulla, 2003,Northrop et al., 2007 ].The most widely applied alkane isomerization catalysts are chlorinated aluminasupported platinum and zeolite supported Pt or Pd. Also there are many ofdifferent catalysts in which the selectivity isomerization increases and thecracking decreases [Rachid et al., 2006].A comprehensive literature review is shown in this chapter to include: gasolinespecification, hydroisomerization process catalysts and characterization. 2.2 Gasoline Fuel and Its Specefications U UGasoline is one of petroleum fuels that consists of 5 carbons to 11 carbons inthe hydrocarbon compounds. Actually, gasoline contains up to 500hydrocarbons, either saturated or unsaturated hydrocarbons and othercompounds. Saturated hydrocarbon known as paraffin or alkane forms themajor component of low octane number gasoline. Unsaturated hydrocarbonincludes olefins or alkenes, isoparaffins or alkyl alkane, arenes or aromatics. 5
  21. 21. Chapter Two Literature SurveyOther compounds consist of alcohols and ethers [Lovasic et al., 1990, Carey,1992].Although there are several important properties of gasoline, the three that havethe greatest effects on engine performance are the Reid vapor pressure, boilingrange, and antiknock characteristics.The Reid vapor pressure (RVP) and boiling range of gasoline govern ease ofstarting, engine warm-up, rate of acceleration, loss by crankcase dilution,mileage economy, and tendency toward vapor lock. Engine warm-up time isaffected by the percent distilled at 158°F (70°C) and the 90% ASTM distillationtemperature. Warm-up is expressed in terms of the distance covered to developfull power without excessive use of the choke. Crankcase dilution is controlledby the 90% ASTM distillation temperature and is also a function of outsidetemperature [Takao, 2003].The octane number of the gasoline depends on the number of branch carbonatoms and the length of carbon atom chain. Octane number is a ratio of n-heptane to iso-octane part by volume and commercially is between 60:40 and40:60. n-heptane has octane number of zero while iso-octane has octane numberof 100. Higher octane rating is obtained by decreasing normal alkanes whileincreasing iso-alkanes and cyclic hydrocarbons. Although unsaturatedhydrocarbons have desirable octane rating, for example acetylene, benzene andtoluene, they are toxic and their content in the gasoline should be reduced.The octane number represents the ability of gasoline to resist knocking duringcombustion of the air-gasoline mixture in the engine cylinder. Gasoline musthave a number of the other properties in order to function properly and to avoiddamage to the environment [Antos et al., 1995, Tore et al., 2007]. 6
  22. 22. Chapter Two Literature SurveyOctane ratings in gasoline are conventionally boosted by addition of aromaticand oxygenated compounds. However, as a result of increasingly stringentenvironmental legislation, the content of these compounds in gasoline is beingrestricted and thus industry has been forced to investigate alternative processesto reach the required octane levels [Rafael et al., 2008].There are several types of octane numbers for spark ignition engines with thetwo determined by laboratory tests considered most common: those determinedby the ‘‘motor method’’ (MON) and those determined by the ‘‘researchmethod’’ (RON). Both methods use the same basic type of test engine butoperate under different conditions. The RON (ASTM D-908) represents theperformance during city driving when acceleration is relatively frequent, andthe MON (ASTM D-357) is a guide to engine performance on the highway orunder heavy load conditions.The difference between the research and motor octane is an indicator of thesensitivity of the performance of the fuel to the two types of driving conditionsand is known as the ‘‘sensitivity’’ of the fuel. On the other hand, the meanaverage of RON and MON is named rating. [Chica et al., 2001, Goodarz et al.,2008]. An overview of octane numbers of different hydrocarbons, given inTable (2.1).In the oil industry C 5 and C 6 paraffins are typically used in hydroisomerization R R R Runits to obtain high octane number components. Paraffins larger thanC 6 , such R Ras heptane are usually present in catalytic reforming feed streams and convertedinto aromatic compounds [Maloncy et al., 2005] . 7
  23. 23. Chapter Two Literature SurveyTable (2.1): Octane number for different hydrocarbons [Goodarz et al.,2008]. Compound MON RON n-butane 89.6 93.8 Iso-butane 97.5 98.6 n-pentane 62.6 61.7 Iso-pentane 90.3 92.3 n-hexane 26 24.8 2-methyl pentane 73.5 73.4 3-methyl pentane 74.3 74.5 2,3-dimethyl butane 94.3 94.6 n-heptane 0 0 2-methyl hexane 46.4 42.4 3-methyl hexane 55.8 52 3-ethyl pentane 69.3 65 2,2-dimethyl pentane 95.6 92.8 2,4-dimethyl pentane 83.8 83.1 3,3-dimethyl pentane 86.6 80.8 Iso-octane 100 100 8
  24. 24. Chapter Two Literature Survey2.3 Hydroisomerization Process U UOne of the important targets in the petroleum industry is the production ofbranched alkanes by skeletal isomerisation of n-alkanes using solid acidcatalysts. Environmental concerns are now promoting clean gasoline with highresearch octane number (RON) and low content of aromatics such as benzene.Isomerization of light straight run naphtha has the potential to satisfy theserequirements.The isomerisation process is catalytic reactions that involve rearrangement ofthe molecular structure of a hydrocarbon without gain or loss of any of itscomponents. This process uses light naphtha (C 5 -C 6 fractions) in the production R R R Rof high octane gasoline blend stocks. The process involves the transformation(with minimal cracking) of low octane normal (and less branched) paraffincomponents into high octane isomers with greater branching of the carbonchain. These types of processes are usually accomplished by bifunctionalcatalysts that have both metallic and acidic function [Ravishankar andSivasanker, 1996, Maha, 2007].The refineries of petroleum in the world include hydroisomerization unit.Figure (2.1) shows the position of hydroisomerization unit in a petroleumrefinery. It is important to mention here that many petroleum companiesdesigned hydroisomerization processes to produce high octane gasoline. 9
  25. 25. Chapter Two Literature Survey Fig. (2.1) Location of hydroizomerization process in a modern petroleum refinery [Ivanov et al.,2002]. 10
  26. 26. Chapter Two Literature SurveyFigure (2.2) shows a representative flow scheme hydroisomerization unit for thethe Penex™ process which provides highly isomerized light naphtha products. Figure (2.2) Penex Process Flow Scheme [Gary, 2001].Figure (2.3) shows the other flow scheme hydroisomerization unit for the thePenex DIH process. On the other hand, the Butamer™ process that is shown inFigure (2.4) provides highly isomerized butane products. 11
  27. 27. Chapter Two Literature Survey Figure (2.3) Penex DIH Process [Mikhail et al., 2001]. Figure (2.4) The Butamer™ process [Mikhail et al., 2001]. 12
  28. 28. Chapter Two Literature SurveyThe dual-functional catalysts used in these processes are platinum on chlorided-alumina support. These types of catalysts offer the highest activity to takeadvantage of higher thermodynamic equilibrium iso- to normal ratiosachievable at lower temperatures. In order to improve the performance of theseprocesses.If the normal pentane in the reactor product is separated and recycled, theproduct RON can be increased by about 3 numbers (83 to 86 RON) . If bothnormal pentane and normal hexane are recycled the product clear RON can beimproved to about 87 to 90. Separation of the normals from the isomers can beaccomplished by fractionation or by vapor phase adsorption of the normals on amolecular sieve bed. The adsorption process is well developed in several largeunits.On the other hand, it is important to mention here that the isomerization processis called hydroisomerization because its reaction requires H 2 gas to prevent R Rdeactivation of catalysts. In hydroisomerization process, some hydrocrackingoccurs during the reactions resulting in a loss of gasoline and the production oflight gas. The amount of gas formed varies with the catalyst type and age and issometimes a significant economic factor. The light gas produced is typically inthe range of 1.0 to 4.0 wt% of the hydrocarbon feed to the reactor. The maincomposition of these gases is methane, ethane and propane [Gary, 2001, Shi etal., 2008].Two types of hydroisomerization processes of alkanes were developed, havingdifferent objectives and technologies [Satoshi, 2003]:1. The isomerization of lower n-alkanes (C 5 -C 7 ) for the production of high- R R R R octane components and of n-C 4 to i-C 4 as feed for the production of R R R R alkylate. 13
  29. 29. Chapter Two Literature Survey2. The isomerization of the n-alkanes contained in paraffinic oils in order to produce a significant decrease in the freezing temperature and thus eliminate the need for dewaxing.2.3.1 Catalysts of Hydroisomerization ProcessThe first hydro- isomerization unit was introduced in 1953 by UOP, followed in1965 by the first BP unit, while in 1970 the first Shell Co. hydro-isomerization(HYSOMER) unit was started up. All these processes take place in the gasphase on a fixed bed catalyst containing platinum on a solid carrier. In the late1950s and early 1960s, chlorinated platinum loaded alumina was used as acatalyst. The major advantage of this catalyst was its low temperature activity(T< 200°C) due to its high acidity. However the catalysts were sensitivetowards water and oxygenates and in addition had corrosive properties.Furthermore, chlorine addition during the reaction is necessary to guaranteecatalyst stability [Gary, 2001, Maciej et al., 2002, Yunqi et al., 2004].In the Hysomer process zeolite based catalysts were used which had the majoradvantage of resistance to feed impurities. Industrially applied zeolites usedtoday are Pt-containing, modified synthetic (large-port) mordernite e.g. HS10 ofUOP, or HYSOPAR from Süd- Chemie. As higher hydrogen to hydrocarbonratios are needed recycle compressors and separators are required for thistechnology [Jens, 1982, Corma et al., 1995, Christian, 2005].The isomerization of hydrocarbons < C 6 is currently carried out very R Rsuccessfully using bifunctional supported platinum catalysts. However,difficulties are encountered with hydrocarbons larger than hexane since thecracking reactions become more significant over platinum catalysts as the chainlength increases [Cuong et al., 1995]. Catalysts used in state of the art 14
  30. 30. Chapter Two Literature Surveyisomerization-cracking reactors are bifunctional. They have a metal functionproviding de-hydrogenation and hydrogen activation properties that are usuallysupplied by group VIII noble metals like Pt, Pd, Ni or Co. The acid function isthe support itself and some examples include acid zeolites, chlorided aluminaand amorphous silica alumina. Noble metals have a positive effect on theactivity and stability of the catalyst. However they have a low resistance topoisoning by sulfur and nitrogen compounds present in the processed cuts[Busto et al., 2008].In order to prepare a suitable catalyst for hydroconversion of alkanes, goodbalance between the metal and acid functions must be obtained. Rapidmolecular transfer between the metal and acid sites is necessary for selectiveconversion of alkanes into desirable products [Vagif et al., 2003].Two of the attractive features of zeolite are that the catalysts are tolerant ofcontaminants and that they are regenerable. The chlorinated alumina catalystsare very sensitive to contaminants such as water, carbon oxides, oxygenates,and sulfur. Thus, feeds and hydrogen must be hydrotreated and dried to removewater and sulfur. Furthermore, the chlorinated alumina catalysts require theaddition of organic chloride to the feed in order to maintain their activities. Thiscauses contamination in the waste gas of hydrogen chloride, a scrubber isneeded to remove such contamination [Satoshi, 2003].The UOP BenSat process uses a commercially proven noble metal catalyst,which has been used for many years for the production of petrochemical-gradecyclohexane. The catalyst is selective and has no measurable side reactions.Because no cracking occurs, no appreciable coke forms on the catalyst toreduce activity. Sulfur contamination in the feed reduces catalyst activity, butthe effect is not permanent. Catalyst activity recovers when the sulfur isremoved from the system [Meyers, 2004]. 15
  31. 31. Chapter Two Literature Survey2.3.1.1 AluminaAlumina or aluminum oxide (Al 2 O 3 ) is a chemical compound with melting R R R Rpoint of about 2000°C and sp. gr. of about 4.0. It is insoluble in water andorganic liquids and very slightly soluble in strong acids and alkalies. Aluminaoccurs in two crystalline forms. Alpha alumina is composed of colorlesshexagonal crystals with the properties given above; gamma alumina iscomposed of minute colorless cubic crystals with sp. gr. of about 3.6 that aretransformed to the alpha form at high temperatures. Figure (2.5) shows theshape of Al 2 O 3 [Ulla, 2003]. R R R RThe most common form of crystalline alumina, α-aluminium oxide, is known ascorundum. If a trace of the element is present it appears red, it is known asruby, but all other colorations fall under the designation sapphire. The primitivecell contains two formula units of aluminium oxide. The oxygen ions nearlyform a hexagonal close-packed structure with aluminium ions filling two-thirdsof the octahedral interstices. Identifiers Aluminium oxide Figure (2.5) The shape of aluminium oxide 16
  32. 32. Chapter Two Literature SurveyTypical alumina characteristics include: Good strength and stiffness Good hardness and wear resistance Good corrosion resistance Good thermal stability Excellent dielectric properties (from DC to GHz frequencies) Low dielectric constant Low loss tangent2.3.1.2 ZeoliteZeolites are microporous crystalline solids with well-defined structures.Generally they contain silicon, aluminium and oxygen in their framework andcations, water and/or other molecules wthin their pores. Zeolites occur naturallyas minerals or synthetic, Figure (2.6) shows the shape of different types ofzeolites [Matthew, 2008].Because of their unique porous properties, zeolites are used in a variety ofapplications with a global market of several milliion tonnes per annum. In thewestern world, major uses are in petrochemical cracking, ion-exchange (watersoftening and purification), and in the separation and removal of gases andsolvents. Other applications are in agriculture, animal husbandry andconstruction. They are often also referred to as molecular sieves [Danny, 2002].Zeolites have the ability to act as catalysts for chemical reactions which takeplace within the internal cavities. An important class of reactions is thatcatalysed by hydrogen-exchanged zeolites, whose framework-bound protonsgive rise to very high acidity. This is exploited in many organic reactions,including crude oil cracking, isomerisation and fuel synthesis [Jirong, 1990]. 17
  33. 33. Chapter Two Literature Survey Figure (2.6) Structures and dimensions of different types of zeolite [Tirena, 2005].Underpinning all these types of reaction is the unique microporous nature ofzeolites, where the shape and size of a particular pore system exert a stericinfluence on the reaction, controlling the access of reactants and products. Thuszeolites are often said to act as shape-selective catalysts. Increasingly, attentionhas focused on fine-tuning the properties of zeolite catalysts in order to carryout very specific syntheses of high-value chemicals e.g. pharmaceuticals andcosmetics [Eisuke et al., 2005].The following properties make zeolites attractive as catalysts, sorbents,and ion-exchangers [Jirong, 1990, Liu et al., 1996, Danny, 2002]. 18
  34. 34. Chapter Two Literature Survey(1) well-defined crystalline structure.(2) high internal surface areas (>600 m2/g). P P(3) uniform pores with one or more discrete sizes.(4) good thermal stability.(5) highly acidic sites when ion is exchanged with protons.(6) ability to sorb and concentrate hydrocarbons.The tetrahedral arrangements of [SiO 4 ] -4 and [AlO 4 ] -5 coordination polyhedra R R P P R R P Pcreate numerous lattices where the oxygen atoms are shared with another unitcell. The net negative charge is then balanced by cations (e.g. K+ or P PNH 4 +). Small recurring units can be defined for zeolites named, ‘secondary R RP Pbuilding units [Tirena, 2005].The primary building blocks of all zeolites are silicon Si+4 and P Paluminum Al+3 cations that are surrounded by four oxygen anions O-2. P P P PThis occurs in a way that periodic three dimensional frameworkstructures are formed, with net neutral SiO 2 and negatively charged R RAlO 2 . R RThe negative framework charge is compensated by cation (often Na + ) R Ror by proton (H+) that forms bond with negatively charged oxygen P Panion of zeolite.The secondary building blocks differ between different types ofzeolites. In the top line of Figure (2.6) the structure of a faujasite typezeolite is shown. The secondary building block of this zeolite is asodalite cage, which consists of 24 tetrahedra in the geometrical form ofa cubo-octahedron. The sodalite cages are linked to each other via ahexagonal prism. 19
  35. 35. Chapter Two Literature Survey2.4 Previous Work UNumerous researchers which have dealt with hydroisomerization usingdifferent types of catalysts as follows:Diaz et al., [1983] studied the isomerization and hydrogenolysis of hexanes onan alumina-supported Pt-Ru catalyst. On ruthenium/ alumina catalysts, noisomer products were detected in C6 R R hydrocarbon reactions.Methylcyclopentane hydrogenolysis was selective as confirmed by the high 3-methylpentane/n-hexane ratios. Isomerization reactions on Pt(9.6 at.%)-Ru (0.4at.%)/Al 2 O 3 were studied between 220 and 300°C. Skeletal rearrangements R R R Rproceeded from 220°C where Pt is inactive for this type of reactions, Very lowapparent activation energies in isomerization reactions of Cs-labeledhydrocarbons were found for selective and nonselective cyclic mechanisms: 2-methylpentane 3- methylpentane and 2-methylpentane n-hexane, respectively. The results were explained using a bimolecular kineticmodel which can take into account the phenomenon as an increase either inhydrocarbon coverage or in hydrocarbon adsorption strength on the catalystsurface.Raouf, [1994] investigated hydroconversion (isomerization, cracking andcyclization of n-heptane) using three types of a crystalline zeolites as supports.It was noted platinum supported zeolite catalyst vary in their activity andselectivity towards n-heptane hydroconversion. Support types were found tobehave differently when impregnated with hexachloroplatinic acid. ApplyingH 2 PtCl 6 on acidic decationized and cationic zeolite type Y produce most active R R R Rcatalyst toward isomerization at lower temperature and for hydrocrackingathigher temperature. On the other hand, applying H 2 PtCl 6 on zeolite type X R R R Rproduce an active catalyst. The isomerizing activity is, however, lower than Ytype with moderate hydroisomerization and hydrocracking selectivity. While 20
  36. 36. Chapter Two Literature Surveyfor A type produces an active catalyst with low isomerizayion activity and ahigher cracking ability. catalytic activity of all types of Pt-zeolite catalystsstrongly depends on the Si/Al ratio. The order of the catalytic activity for thecatalysts is type Y > type X > Y type A.Ravishankar and Sivasanker [1996] studied the hydroisomerization of n-hexanewas carried out at atmospheric pressure in the temperature range 473-573 Kover Pt-MCM-22. The influence of Pt content, the SiO 2 /A1 2 O 3 ratio of R R R R R Rthezeolite and the reaction parameters on the isomerization efficiency of thecatalyst was investigated. The optimum Pt content for the reaction was found tobe around 0.5 wt.%. At a constant Pt content of 0.5 wt.%, increasing the A1content of the zeolite increased the catalytic activities andisomerization/cracking ratios. The studies suggest that the reaction proceeds bya bifunctional mechanism. Preliminary activity comparisons between Pt-H-MCM-22, Pt-H-β and Pt-Hmordenite are reported.Chica and Corma, [1999] tested The hydroisomerization of n-heptane todibranched and tribranched products for producing high octane gasoline hasbeen studied using unidirectional 12 Membered Ring (MR) zeolites withdifferent pore diameters, and zeolites with other pore topologies including onewith connected 12×10MRpores and two tridirectional 12 MR zeolites. Besidesthe pore topology, the crystallite size of the zeolite was seen to be of paramountimportance for improving activity and selectivity. In a second part of the work,a Light Straight Run naphtha including n-pentane and n-hexane and anotherfeed containing n-pentane, n-hexane, and n-heptane have been successfullyisomerized using a nanocrystalline Beta (BEA) zeolite. This can be a favorablealternative to the commercial zeolite catalyst based on mordenite (MOR),especially when n-heptane is present in the feed. They found, that with 21
  37. 37. Chapter Two Literature Surveyincreasing of reaction temperature within the range 240-380ºC, the conversion P Pof n-parafins increased. Also, the results clearly show that regardless of thezeolite used the reactivity follows the order n-heptane> n-hexane> n-pentane.Mordenite cracks n-heptane products very quickly, giving low selectivities tobranched products. While a larger unidirectional pore zeolite (SSZ-24) givesbetter results than H-mordenite, the 12 MR tridirectional zeolites are the bestcatalysts for the branching isomerization of n-heptane, owing to the fasterdiffusion rates of reactants and products through the micropores. The zeolitecrystal size has been found to be of paramount importance, because the catalyticactivity and selectivity of a nanocrystalline Beta zeolite was better than that ofBeta zeolites with larger crystallites.Shuguang et al., [2000] investigated the hydroisomerization of normalhexadecane using three Pt/WO 3 /ZrO 2 catalysts prepared by different methods. R R R RThey found that preparation of the catalyst by impregnation with H 2 PtCl 6 .6H 2 O R R R R R Rsolution and another calcinations at 500°C results in a highly active andselective platinum-promoted tungstate-modified zirconia catalyst(Pt/WO 3 /ZrO 2 ) for the hydroisomerization of n-hexadecane. The optimum R R R Rrange of tungsten loading to achieve high isomerization selectivity at high n-hexadecane conversion is between 6.5 and 8 wt%.Falco et al. [2000] studied the effect of platinum concentration on tungstenoxide-promoted zirconia over the catalytic activity for n-hexane isomerizationwas studied. Catalysts were prepared by impregnation of tungsten oxidepromoted zirconia reaching up to 1.50% platinum, followed by calcination at500℃. The n-hexane reaction was studied at 200℃, 5.9 bar, WHSV 4 and H 2 : R Rn-hexane (molar) ratio 7. It was found that catalytic activity and stabilityincrease for platinum concentrations above 0.05% because of higher hydrogen 22
  38. 38. Chapter Two Literature Surveyavailability at the surface, measured as a function of the methylcyclopentane/C 6 R Risomers ratio. Further increments in platinum concentration do not produceimportant modifications in catalytic activity or hydrogen availability.Srikant and Panagiotis, [2003] used Pt/H-ZSM-12 as a catalyst for thehydroisomerization of C 5 –C 7 n-alkanes and simultaneous saturation of benzene. R R R RThe performance of a Pt/H-ZSM-12 catalyst was compared with a Pt/H-betaand a Pt/H-mordenite catalysts having a similar Si/Al ratio. It was concludedthat both the paraffin conversion and benzene conversion activity of all thethree catalysts remain stable even in the presence of sulfur. However, the resultsshowed that the conversion levels over the Pt/H-ZSM-12 and Pt/H-Mor catalystare lower compared to the levels obtained in the absence of sulfur at the sametemperature.Abbass [2004], studied the transformation of n-hexane over0.5wt%Pt/HY-Zeolite at 250-325˚C and WHSV=1.6hr-1. The pressure P Pand hydrogen to feed mole ratio were kept constant at 1 bar and 2,respectively. He use three type of promoter to study the activity ofisomerization catalyst Sn, Ni and Ti .The comparison between preparedcatalysts shows that the total isomer yield during the process with Sn-Pt/HY-Zeolite catalyst was higher than the others and the total isomeryield reach 63.95% vol. He found that adding a 0.5 wt% of W and Zr toSn-Pt/HY-zeolite catalyst obtains co-metal promoters catalysts, and thetotal isomer yield reached to 81.14% vol. and 79.07% vol. respectively.The results show that the co-metal promoters enhanced the yield of theproduct more than that obtained by other types of promotersWong et al., [2005] Skeletal isomerization of npentane over Pt/HZSM5 andPt/WP/HZSM5 has been studied. Platinum (Pt) and Tungstophosphoric acid 23
  39. 39. Chapter Two Literature Survey(WP) have been immobilized on protonated ZSM5 by impregnation methodfollowed by calcinations at 823K. The state of WP on the zeolite surface wascharacterized by XRD, FTIR, pyridine adsorption FTIR, TG/DTA and BETsurface area techniques. Catalytic testing in npentane isomerization wasperformed in a continuous flow microreactor at 523K under hydrogen flow.Prior to the reaction, catalyst was treated by heating at 573K under oxygen (30min), nitrogen (10 min) and hydrogen (180 min) flow. Both of Pt/HZSM5 andPt/WP/HZSM5 shows high conversion of npentane and stable catalysts towardsthe deactivation compare to those of HZSM5. Although, Pt/HZSM5 andPt/WP/HZSM5 exhibit high catalytic activity, Pt/WP/HZSM5 catalyzed theisomerization of npentane more selectively compare to those of Pt/HZSM5dueto the presence of a strong acid.Jafar et al., [2006] investigated C 5 -C 6 isomerization in light straight run R R R Rgasoline over platinum/mordenite zeolite. They studied effects of hydrogenpartial pressure on catalyst activity and n-paraffins conversions at T=260°C andP=7-7.3 bar. They concluded that the activity increases with relatively sharpslope for n-pentane, n-hexane and n-heptane which show the positive effect ofhydrogen on decreasing deactivation. The behavior of the curves in thementioned pressure range shows that the activity is constant while increasingPH 2 . At T=270°C it seems as if the deactivation phenomenon takes place in R Rthe pressure less than PH 2 . Also, at this temperature and while PH 2 >8.5, the R R R Ractivity decreases evidently. By increasing the temperature, the slop of theinitial activity curve decreases but activity reduction is more evident in higherpressures.Rachid et al. [2006] investigated the present work is an evaluation of 1 wt.%Pd/sulfated zirconium pillared montmorillonite catalyst in thehydroisomerization reaction of two mfractions of light naphtha composed 24
  40. 40. Chapter Two Literature Surveymainly of C 5 and C 6 paraffins (feeds 1 and 2). Catalyst activity test was carried R R R Rout in a fixed-bed flow reactor at reaction temperature of 300 8C, underatmospheric hydrogen pressure and weight hourly space velocity of 0.825 h-1. P PThe reaction products showed high isomer and cyclane selectivity.Monobranched and multibranched isomers were formed as well as C5 and C6cyclane products. After the catalytic reaction, the total amount of benzene andcyclohexane decreased by 30% for the ‘‘feed 1’’ and by 40% for the ‘‘feed 2’’leading to methylcyclopentane formation in the products. A long-termperformance test catalyst for the two light naphtha fractions was also performedand we observed an improving of the research octane number (RON) by 15–17for, respectively, feeds 1 and 2.Rachid et al., [2006] the present work is an evaluation of 1 wt.% Pd/sulfatedzirconium pillared montmorillonite catalyst in the hydroisomerization reactionof two fractions of light naphtha composed mainly of C 5 and C 6 paraffins R R R R(feeds 1 and 2). Catalyst activity test was carried out in a fixed-bed flow reactorat reaction temperature of 300 8C, under atmospheric hydrogen pressure andweight hourly space velocity of 0.825 h-1. The reaction products showed high P Pisomer and cyclane selectivity. Monobranched and multibranched isomers wereformed as well as C 5 and C 6 cyclane products. After the catalytic reaction, the R R R Rtotal amount of benzene and cyclohexane decreased by 30% for the ‘‘feed 1’’and by 40% for the ‘‘feed 2’’ leading to methylcyclopentane formation in theproducts. A long-term performance test catalyst for the two light naphthafractions was also performed and we observed an improving of the researchoctane number (RON) by 15–17 for, respectively, feeds 1 and 2.Hadi [2007], studied the transformation of n-hexane over 0.3wt%Pt/HY-zeolite, 0.5wt% Pt/HY-zeolite, 1wt% Pt/HY-zeolite and 25
  41. 41. Chapter Two Literature Survey0.3wt%Pt/Zr/W/HY-zeolite catalysts at 240-270˚C and LHSV=1-3hr-1. P PThe pressure and hydrogen to feed mole ratio were kept atmosphericand 1-4, respectively. She concluded that the n-hexane conversionincreases with increasing temperature, decreasing LHSV and increasingPt content. Also isomerization rate is independent of the Pt loading thislead to the conclusion that dehydrogenation step is not rate limiting.The effect of the P H2 and P nC6 orders on the overall reaction rate was R R R Ralso studied by the author. She conclude that the value of hydrogenorder varies between -0.388 to -0.342, while the values of n-hexaneorder were 0.262 to 0.219. The values of E act, R isom R were also obtainedand found to be equal to 119.7 kJ/mole.Hadi also study the n-Hexane conversion enhancement by adding TCEand by co-impregnation with Zr and W using 0.3wt%Pt/HY-zeolitecatalyst, and found that by adding 435ppm of TCE a 49.5mol.%conversion was achieved at LHSV 1 h-1, temperature 270°C and H 2 /nC 6 P P R R R Rmole ratio= 4, while the conversion was 32.4mol.% on0.3wt%Pt/Zr/W/HY-zeolite at the same condition.María et al. [2008] studied Three different distillatednaphthas streamsprovidedby REPSOLYPF, being formed by n-paraffins, iso-paraffins, aromatics andnaphthenes, were isomerized using an agglomerated catalyst based on betazeolite.Methane and ethane were not observed as final products revealing thathydrogenolysis did not contribute to the cracking reaction. The highest overallparaffin conversion value was obtained when feed A was introduced to theprocess, due to its high molar composition of linear paraffins. It was observedthe presence of aromatic compounds (benzene and toluene) in the three feeds. Atotal hydrogenation of benzene was achieved, keeping the rest of the aromatic 26
  42. 42. Chapter Two Literature Surveycompounds under the limit imposed by legislation. Different naphtheniccompounds were obtained as a result of the hydrogenation of aromatic ones.Goodarz et al. [2008] investigated two types of beta zeolites, different amountsof platinum (0.2%, 0.5% and 1.2%) were loaded on the protonated form ofzeolite by incipient wet impregnation method applying hexachloroplatinic acidin 0.2N Cl- progressive ion solutions. Catalytic hydroisomerization reactions P Pwere carried out at atmospheric pressure in a fixed bed reactor with verticalplacing and downward flow at three different temperatures, various WHSV(weight hourly space velocity) and n-H 2 /n-HC (molar hydrogen/hydrocarbon) R Rratio. Increase in Si/Al ratio in zeolites structures from 11.7 to 24.5 promotedselectivity and yield. It was found that optimum platinum content depends onthe Si/Al ratio (zeolite acidity) in catalysts. Monobranched to dibranchedisomers ratio were correlated with a linear function of n-heptane conversion.Such a correlation was found to be valid for various Si/Al ratios, metal content,processing temperature and pressure, WHSV and hydrogen to hydrocarbonratio. Increase in WHSV, decreased n-heptane conversion, but enhancedisomers selectivity. On the other hand, increasing the ratio of hydrogen tohydrocarbon in the feed decreased conversion, while promoted isomersselectivity.2.5 Catalysts Preperation UA typical catalyst comprises one or more catalytically active componentssupported on a catalyst support. Typically, the catalytically active componentsare metals and/or metal-containing compounds. The support materials aregenerally high surface area materials with specific pore volumes and 27
  43. 43. Chapter Two Literature Surveydistribution [Lovasic et al., 1990, Raouf 1994, Novaro et al., 2000, Ramze,2008].Various methods for depositing catalytically active components on catalystsupports are known, the catalyst support may be impregnated with an aqueoussolution of the catalytically active components. The impregnated support maythen be dried and calcined. The catalytically active component may also bedeposited onto the catalyst support by precipitation, a catalyst support is firstimpregnated in an aqueous solution of a noble metal. The metal is thenprecipitated on to the support by contacting the impregnated support with anaqueous solution of an alkali metal salt [Iker, 2004].Many factors influence catalysts preparation, such as solution concentration,contact time, washing, temperature and method of reduction. Figure (2.7)illustrates the general procedure for catalysts preparation [Shuguang et al.,2000, Sergio et al., 2005].2.5.1 ImpregnationThe manner in which a metal is introduced to a support will influence itsdispersion as well as the nature of the metal-support interaction. Supportedcatalysts with low concentration of metal are generally prepared byimpregnation (or in some cases by ion exchanging). The choice of precursor saltis made both for its solubility in water, and preferred solvent, and for its abilityto disperse throughout the support. Impregnation of pore supported catalyst isachieved by filling the pores of support with solution of active species of metalsalt from which solvent is evaporated. The concentration of the metal contentcan be increased by successive impregnation with intermediate precipitationand thermal activation to isolubilize the supported species [Jensen et al., 1997,Shuguang et al., 2000]. 28
  44. 44. Chapter Two Literature Survey Figure (2.7): Typical arrangement of the catalysts preparation [Anderson, 1975]Impregnation with interaction occurs when the solute to be depositedestablishes a bond with the surface of the support at the time of wetting. Suchinteraction results in a near-atomic dispersion of the active species precursor.The interaction can be an ion exchange, an adsorption, or a chemical reactionsince ion exchanges occur much more frequently than the others [Lepage,1987]. 29
  45. 45. Chapter Two Literature Survey2.5.2 CalcinationCalcination means any thermal treatment carried out with the purpose ofdecomposing precursor compounds (usually with the evolution of gaseousproduct) and / or allowing solid-state reactions to occur among different catalystcomponents and / or making the catalyst sinter. The calcination temperature isusually not lower than that of operation at the industrial plant [Thomas, 2004].The type of calcination is assumed to be calcination in air, typically at atemperature higher than the anticipated temperatures of the catalytic reactionand catalyst regeneration.The objectives of calcination are to obtain:1- A well determined structure for the active agents or supports.2- The parallel adjustment of the texture with respect to the surface and pore volume.3- A good mechanical resistance if it does not already existAmong the various types of chemical or physico-chemical transformations thatoccur during calcination, the following are the most important:A- The creation of a generally macroporous texture through decomposition and volatilization of substances previously added to the solid at the moment of its shaping.B- Modifications of texture through sintering: small crystals or particles will turn into bigger ones.C- Modifications of structure through sintering.2.5.3 ReductionReduction process is the final step in activation of supported metal catalyst,which consists of the transformation of the metal precursor compound or itsoxide into the metallic state (metal atoms, small metal clusters). 30
  46. 46. Chapter Two Literature SurveyReduction involves reaction where the initiation process proceeds at distinctsites (potential centers) on the surface of solid, followed by propagation of thereaction zone from such a center through the solid, until complete conversion isachieved upon contact of a metal oxide with hydrogen, oxygen ions are created.The reaction process of oxides and halides can be represented by the followingequations [Vanden and Rijnten, 1979, Anderson et al., 1984]MO (s) + H 2 R R R M (s) + H 2 O (g) R R R R R R2MX (s) +H 2(g) R R R 2M (s) +2HX (g) R R R RThere are many factors affecting the reduction step, calcination of the depositedprecursor might cause several transformation and solid state reaction. Watervapor inhibits reduction by blocking nucleus forming sites.2.6 Catalysts Characterization U UCharacterization of the catalyst is a predominate step in every catalyst study andat every stage of the catalyst development. Critical parameters are measured notonly to check the effectiveness of each operation but also to providespecification for future products. Characterization might be studied orcontrolled in terms of support properties, metal dispersion and location andsurface morphology [Tirena, 2005].In general, the quality of any catalyst is determined by a number of factors,such as activity, selectivity for certain product, and stability. These parametersare themselves functions of pretreatment conditions of the catalyst preparationand reaction conditions. The interpretation of catalytic performance through themechanism of catalytic action depends on the study of the intrinsic chemicaland physical characteristics of the solid and a recognition of correlations 31
  47. 47. Chapter Two Literature Surveybetween some of these characteristics and catalytic performance [Sergio etal., 2005]. Table (2.4) offer presents the general physcochemical properties ofcatalysts and methods of measuring them.2.6.1 X-ray Diffraction (XRD)X-ray diffraction is a technique to identify the crystallinity of catalysts. Thistechnique is based on the knowledge that each compound in catalyst has adifferent diffraction pattern. The crystallinity can be determined by comparingthe intensity of a number of particular peaks to the intensity of the same peaksobtained by standard samples [Marı´a et al., 1997, Benitez et al., 2006].The diffraction pattern is plotted based on the intensity of the diffracted beams.These beams represent a map of reciprocal lattice parameter, known as Millerindex (hkl) as a function of 2θ, which satisfies Bragg equation:nλ = 2d sin θ -------------------------(2.1)where n is an integer number, λ is the wavelength of the beam d is interplanarspacing and θ is a diffraction angle. Equation (2.1) is obtained from Braggdiffraction as shown in Figure (2.8).2.6.2 Surface AreaIn practice, the surface area is calculated from the Brunauer-Emmett-Teller(BET) equation based on the physical adsorption of an inert gas at constanttemperature, usually nitrogen at the temperature of liquid nitrogen. Theprinciple of measurement consists in determining the point when a mono-molecular layer of gas covers the surface of the catalyst [Antonio et al., 2006]. 32
  48. 48. Chapter Two Literature Survey Figure (2.8) Bragg diffraction [Tirena, 2005].2.6.3 Scanning Electron Microscopy (SEM)Scanning electron microscopy is an extremely powerful technique for obtaininginformation on the morphology and structural characteristics of catalysts. Thereare some advantages in this technique, which are great depth of focus, thepossibility of direct observation of external form of real objects, and the abilityto switch over a wide range of magnification, so as to zoom down to fine detailon some part identified in position on the whole object [Shuguang et al., 2000]. 33
  49. 49. Chapter Three Experimental Work Chapter Three Experimental Work3.1 Materials UIn the present work, different materials and compounds are used asfollows:• Iraqi-Light-NaphthaIraqi light-naphtha is used as a feedstock in the present investigation. It wassupplied by Al-Dura Refinery (Baghdad). Table (3.1) shows the specificationsof Iraqi-light naphtha.• HydrogenHydrogen gas was obtained from Al-Mansour Factory/Baghdad witha high purity of (99.9%).• ZeoliteNaY-zeolite was supplied from Zeolyst International UWE Ohlrogge (VF)as an extrudate (2mm×4mm). The chemical analysis of this zeolite was doneby the General Establishment of Geological Survey and Mining, and theresults are shown in Table (3.2).• AluminaAlumina support (γ-Al 2 O 3 ) with spherical shape and average size of 3mm R R R Rwas supplied by FLUKA AG company.• Hexachloroplatinic AcidHexachloroplatinic acid (H 2 PtCl 6 .6H 2 O) was supplied by REIDEL- DE R R R R R RHAEN AG SEELZE -HANNOVER chemicals Ltd.This hexachloroplatinicacid contains 40 wt% of Pt and has a molecular weight of 517.92 g/mol. On 34
  50. 50. Chapter Three Experimental Workthe other hand, other chemicals used such as Barium Chloride (BaCl 2 ), R RAmmonium Chloride (NH 4 Cl) and Hydrochloric acid (HCl) were supplied R Rfrom FLUKA AG Company.In the present work the Iraqi light naphtha are used as a feedstock inhydroisomerization process to produce high octane gasoline. Table (3.3)shows the chemical composition of light naphtha. It is important to mentionhere that the main products of hydroisomerization process are i-pentane, 2,2-DMB, and 2,3-DMB. Table (3. 1) The propetries of Iraqi light naphtha. Property Data at 15.6℃ 0.702 API 80.5 Distillation I.B.P. 37℃ 5 Vol.% distillated 42℃ 10 Vol.% distillated 48℃ 20 Vol.% distillated 52℃ 30 Vol.% distillated 56℃ 40 Vol.% distillated 60℃ 50 Vol.% distillated 65℃ 60 Vol.% distillated 68℃ 70 Vol.% distillated 76℃ 80 Vol.% distillated 82℃ 90 Vol.% distillated 86℃ 95 Vol.% distillated 92℃ E.B.P. 124℃ Total distillate 96 Vol.% Total recovery residue 0.7 Vol.% Loss 3.3 Vol.% Octane Number 68.2 Sulfur Content < 3ppm (Desulfurized) Kinematic Viscosity at 25℃ 5.4 10-7 m2/s P P P P 35
  51. 51. Chapter Three Experimental Work Table (3.2): Chemical composition of zeolite Compound SiO 2 R AL 2 O 3 R R R Na 2 O R R CaO Fe 2 O 3 R R R MgO TiO 2 R L.O.I Percentage 45.85 20.50 12.00 0.140 0.060 0.120 0.010 19.14 Table (3.3) The composition of Iraqi light naphtha. Composition Vol.% n-Butane 0.20 iso-Pentane 3.80 n-Pentane 15.27 2,2DMB 7.20 2,3DMB 7.98 2MP 12.47 3MP 10.50 n-Hexane 12.74 2,2DMP 3.37 Cyclohexane 2.87 2,4DMP 5.65 Methylcyclopentane 3.34 Benzene 3.88 n-Heptane 1.85 Toluene 2.47 C7+ R RP 3.14 36
  52. 52. Chapter Three Experimental Work3.2 Preparation of Modified Zeolites by Ion Exchange: U3.2.1 Preparation of Barium- Zeolite:BaY form was prepared by ion exchanging of the parent zeolite NaY with(3N) barium chloride solution. Thus, 36.642 gm of barium chloride in 100ml distilled water was contacted with 20 gm of zeolite with stirring for 1 hrat 50℃. The batch of zeolite was left in the solution for 72 hr at 25 . Theexchanged barium zeolite was then filtered off, washed with deionized waterto be free of chloride ions and dried at 110℃ over night. The dried samplestemperature was increased to 550 ℃ at a rate of 10°C/min. The chemicalwere then calcined at 550℃ for 5 hr in the presence of O 2 . Then the R Ranalysis showed that a 82% of Na was exchanged by Ba in zeolite Y. It wasdone by the General Establishment of Geological Survey and Mining.3.2.2 Preparation of HY- Zeolite:HY form was prepared by ion exchanging of the parent NaY zeolite with(3N) ammonium chloride solution. Thus, 16.047 gm of ammonium chloridein 100 ml distilled water was contacted with 20 gm of zeolite with stirringfor 1 hr at 50℃. The batch of zeolite was left in the solution for 72 hr at25℃. The exchanged ammonium zeolite were then filtered off, washed withdeionized water to be free of chloride ions and dried at 110℃ over night.Then the temperature was increased to 500 ℃ at a rate of 10°C/min. TheThe dried samples was then calcined at 500℃ for 7 hr in presence of O 2 . R Rchemical analysis showed that a 87% of Na was exchanged by ammoniumchloride to form HY. It was done by the General Establishment ofGeological Survey and Mining 37
  53. 53. Chapter Three Experimental Work3.3 Catalysts Preparation U3.3.1 Preparation of Pt/ BaY and Pt/HYThe barium and hydrogen exchanged zeolites were loaded with 0.5 wt % Ptby impregnation with aqueous solution of hexachloroplatinc acid(H 2 PtCl 6 .6H 2 O). The platinum content of the catalyst was calculated from R R R R R Rthe weight of the support and the amount of the metal in impregnationsolution.Thus, 0.25 gm of hexachloroplatinc acid (40 wt % Pt) was dissolved in 25 mlof distilled water. Then the solution was added for 20 gm of the zeolitesample as drop wise with mixing for 2 hr at 25℃. The mixture was left atroom temperature for 24 hr, it was stirred intermediately during this time.The mixture was then slowly evaporated to dryness over a period of 8 hr byadditional 24 hr. Then the dried catalyst was calcined at 400 ℃ for 3 hr andheating on a heat mantle. The resulting catalyst was dried in air at 110℃ forreduced with hydrogen at 350℃ for 2 hr [Satoshi, 2003, Goodarz, 2008,Dhanapalan et al., 2008].The prepared catalysts at this time is called Pt/BaY and Pt/HY.3.3.2 Preparation of Pt/ AL 2 O 3 R R RThe γ-Al 2 O 3 (spherical shape with an average size of 3mm) was loaded with R R R R0.5 wt % Pt by impregnation with aqueous solution of hexachloroplatinc acid(40% Pt). Thus, 0.25 gm of hexachloroplatinc acid (40 wt % Pt) wasdissolved in 25 ml of distilled water. Then, the solution was added 20 gm ofγ-Al 2 O 3 sample as drop wise with mixing for 4 hr at 25℃. The mixture was R R R Rleft at room temperature for 24 hr, The mixture was stirred intermediatelyduring this time. The resulting catalyst was dried in air at 110℃ foradditional 24 hr. Then, the dried catalyst was calcined at 400℃ for 3 hr andreduced with hydrogen at 350℃ for 3 hr. 38
  54. 54. Chapter Three Experimental Work3.4 Experimental Unit U UThe experiments were carried out in a continuous catalytic unit. Figure (3.1)shows the general view of pilot plant for light naphtha hydroisomerizationprocess, and Figure (3.2) shows a schematic diagram of the apparatus. Thereaction was carried out in catalytic fixed bed tubular reactor, which is madeof stainless steel. The reactor dimensions were 2cm internal diameter, 3cmexternal diameter and 21cm height (reactor volume 66 cm3). The reactor was P Pcharged for each experiment with 20g of catalyst located in the middle zone,while, the upper and lower zones were filled with glass beads.The reactor was heated and controlled automatically using an electricalfurnace type Heraeushan (Germany) with maximum temperature of 1000 °C, P Pit was possible to measure the temperature of the catalyst bed usingcalibrated thermocouple sensor type K (iron-constantan) inserted into themiddle of the catalyst bed in order to measure and the control reactiontemperature.The reactor was fitted with accurate means for control of pressure, gas andliquid flow rate. The liquid (light naphtha) was pumped with a dosing pumptype Prominent (Beta/4- Germany). The liquid hydrocarbons were stored in aQVF storage tank with capacity of 2000cm3. The liquid flow was passed P Pthrough calibrated burette of 52cm3. P P 39
  55. 55. Chapter Three40 Experimental Work Figure (3.1): General view of pilot plant for light naphtha hydroisomerization process.
  56. 56. Chapter Three41 Experimental Work Figure (3.2): Schematic diagram of the experimental apparatus.
  57. 57. Chapter Three Experimental Work3.5 Procedure UTwenty grams of fresh catalyst was charged into the middle zone of thereactor. Iraqi light naphtha was fed to the dosing pump from a glass burettesupplied from a feed tank. Feed was pumped at atmospheric pressure. Thehydrogen gas flow to the unit was controlled by a calibrated gas hydrogenflowmeter. Downstream pressure was controlled with a back pressure valve.The hydrogen gas before it passed to the reactor passed through molecularsieve (5A) type to remove any impurities or moisture. The hydrogen gas wasmixed with hydrocarbon before the reactor inlet. The mixture was preheatedbefore entering the reactor to 150°C, and then passed through the catalyst P Pbed.The performance of catalysts was tested under different operatingtemperatures of (230, 250, 270, 290, and 310°C). The hydrogen to P P P Phydrocarbon molar ratio was kept constant at 4. The weight hourly spacevelocities (WHSV) was (1.5, 3, and 4.5hr-1). All types of catalysts were P Pactivated in the catalytic reactor before each run for 2 hr in a flow ofhydrogen. A pre-test period of one hour was used before each run to adjustthe feed rates and temperature to the desired values.The reaction products was cooled by cooling system and collected in theseparator in order to separate the non-condensed gases from the top of theseparator and the condensed liquid hydrocarbons from bottom of theseparator. Then, the products samples were analyzed using GasChromatograph type Shumids 2014 GC using flame ionization detector(FID). The column dimensions are 0.22mm internal diameter and length 25mand film thickness 0.2μm. The analyses were carried out under theconditions shown in Table (3.3), and the retentions time for the 42
  58. 58. Chapter Three Experimental Workhydrocarbons are shown in Table (3.4). It is important to mention here that,the calibration of gas chromatography was carried by injection the sameamount of a standard into the Gas Chromatography. Table (3.3): Gas chromatograph analysis conditions Temperature program for the column Initial temperature 50 °C Final temperature 120 °C Hold time 5 min Rate of temperature 5 °C/min Total time 20 min Other variables Pressure at the inlet column 1atm Pressure of hydrogen 55 KPa Injection temperature 180 °C Pressure of carrier gas N 2 R 5 atm Linear velocity 31.3 cm/min Split ratio 100 43
  59. 59. Chapter Three Experimental Work Table (3.4): Retention times of hydrocarbons in the catalytic isomerization of light naphtha reaction. Components Retention times (Sec) iso-pentane 1.676 n-pentane 1.724 2,2- dimethyl butane 1.924 2,3- dimethyl butane 1.927 2-methylpentane 1.954 3-methelpentane 1.994 n-hexane 2.037 2,2-dimethelpentane 2.580 Cyclohexane 2.699 2,4-dimethelpentane 2.815 Methylcyclpentane 2.983 Benzene 3.096 n-heptane 3.212 Toluene 4.884 44
  60. 60. Chapter Three Experimental Work3.6 Catalysts Characterization U3.6.1 X-Ray Diffraction Analysis.X-Ray diffraction analysis was done in the Lab of University of Manchesterin United Kendom. Analysis was carried out using Phillips X" Pert Pro PW3719 X-ray diffractometer with Cu Kα 1 and Cu Kα 2 radiation source R R R R(λ=1.54056 Å and 1.54439 Å) respectively. Slits width 1/8 and 1/4 havebeen applied. Tension=40 kV, Current=30 mA. The range of angles scannedwas (0 to 80) on 2θ.3.6.2 Surface Area and Pore VolumeThe catalysts surface areas and pore volume were determined using (BET)method, the apparatus used was Micromeritics ASAP 2400 located inPetroleum Research Center / Ministry of Oil (Baghdad).The surface area and pore volume of the catalysts was determined bymeasuring the volume of nitrogen gas adsorbed at the liquid nitrogentemperature (- 196 °C). The volume of gas adsorbed was measured from thepressure decrease that results from the adsorption of a dose of known volumeof gas.3.6.3 Scanning Electron Microscopy (SEM)Scanning electron microscopy (SEM) measurements were carried out using aPhillips SEM equipped with a XL30 Field Emission Gun, available at theLab of University of Manchester in UK.3.6.4 Energy Dispersive X-Ray (EDAX) AnalysisThe modified zeolite catalyst was subjected to the EDAX analyzer that wasdone in the Lab of University of Manchester in United Kendom andconnected with the SEM to measure the composition of the zeolite. 45
  61. 61. Chapter Four Kinetic Analysis Chapter Four Kinetic Analysis4.1 IntroductionThe main aim of the present study is to analyze the kinetics of hydroisomerizationprocess by assessing the effect of reaction time and reaction temperature on theperformance of the catalysts. The process feed involves light naphtha whichcontains many reactions. Therefore, the hydroisomerization reaction has threestages as follows: [Sergio et al., 2003, Antonio et al., 2006, Pitz et al., 2007,Marios et al., 2009]:1- Adsorption of n- paraffin molecule on dehydrogenation- hydrogenation site followed by dehydrogenation to n- olefins.2- Desorption of n- olefin from the dehydrogenation sites and diffusion to a skeletal rearranged site, which converts n- olefin into iso- olefin.3- Hydrogenation of iso- olefin into iso- paraffin molecule.In general, the hydroisomerization of n- paraffin can occur through the bifunctionalscheme shown below:n-Paraffin n-Olefin i- Olefin i- Parffin 46
  62. 62. Chapter Four Kinetic AnalysisThe hydroisomerization process of light naphtha is regarded as one of thecomplex chemical reactions network, where such types of reactions take on ametal and acid sites of catalysts [Antonio et al., 2006, Eric et al., 2007 ].Therefore, the mathematical modeling of the hydroisomerization process is avery important tool in petroleum refining industries. It translates experimentaldata into parameters used as the basis of commercial reactor process optimaization.In the hydroisomerization of alkanes it is supposed that the alkane isdehydrogenated to an alkene on the metal site. The alkene is then protonated on theacid site to a carbenium ion, which is subsequently isomerized to a branchedcarbenium ion. The branched carbenium ion gives the proton back to the acid site,the resulting branched alkene is hydrogenated on the metallic site. The branchedalkane is formed, and can be desorbed from the catalyst surface. The reactionmechanism scheme is shown in Figure (4.1) [Franciscus, 2002, Maha, 2007,Matthew, 2008].Figure (4.1) The general reactions mechanism for isomerization of n-alkane [Franciscus, 2002]. 47
  63. 63. Chapter Four Kinetic Analysis4.2 Model DevelopmentIn developing the model of the catalytic hydroisomerization kinetic the followingassumptions are taken into account:1. The system is isothermal and in steady state operation with first order reactions.2. The reaction is carried out in the gas phase with constant physical properties and without pressure drop.3. The temperature and concentration gradients in the radial direction can be neglected.The objective of kinetic study is to construct from the experimental results of theprocess, a mathematical formulation that can be used to predict the kineticparameters of the hydroisomerization process. Therefore, the main aim of thepresent work is to estimate the reaction parameters (reaction rate constant,activation energy and pre-exponential factor) depending on the experimental workresults under real isomerization conditions.In present work, it is suggested kinetic model for the reactions ofhydroisomerizayion for light naphtha (n-paraffin) can be considered by thefollowing scheme depending on the present model assumptions which can beformulated to the following equations: Figure (4.1) The suggested reactions of light naphtha isomerization of the present work. 48
  64. 64. Chapter Four Kinetic AnalysisLet C A denotes the mole fraction of n-paraffin present at any time t, R RC N the mole fraction of n-olefin, C iso the mole fraction of i-paraffin. R R R RThen, the mole balance can be formulated mathematically as follows: --------------------------------(4.2) R = k1CA R R R -------------------------------(4.3) = k 1 C A -k 2 C N R R R R R R RBy integration of equation (4.2) CA = CA° R R R RP P at t= 0 we getC A = C A ° exp (- k 1 t) R R R RP P R R -------------------------------------(4.4)Substituting the equation (4.4) in equation (4.3) yield: = k 1 C A ° exp (- k 1 t) - k 2 C N R R R RP P R R R R R -------------------------------------- R(4.5) RRearrangement of equation (4.5) gives: 49