1. CHAPTER – 1 PART-2 PROCESSES IN THE OIL REFINERY Islamic University in Madinah Department of Chemistry PREPARED BY DR. KHALID AHMAD SHADID CHEMISTRY DEPARTMENT – ISLAMIC UNIVERSITY IN MADINAH 1

2. CRACKING Crude oil is separated into its different components by a process called fractional distillation. The products of fractional distillation are often converted into other, even more useful hydrocarbons by a process called cracking. Although all of the fractions produced from crude oil have their uses, some of the fractions are produced in greater quantities than needed, whilst others are not produced in sufficient quantities. Petrol and naphtha approximate supply% are much less than approximate demand% 2

3. This can be corrected by breaking up some larger hydrocarbons in fuel oil into the smaller ones found in gas oil, or by breaking up some hydrocarbons in kerosene into the smaller ones found in petrol, naphtha or the liquefied petroleum gases. In other words the larger fractions (for which supply exceeds demand) can be broken up into smaller fractions (for which demand exceeds supply). The process by which this is carried out is called cracking. CRACKING 3

4. cracking is important for two reasons: i) It converts low-demand fractions into higher demand fractions ii) It makes useful hydrocarbons not naturally found in crude oil There are two types of cracking: thermal cracking and catalytic cracking. Both involve the breaking of C-C bonds to form smaller molecules. C-C bonds are weaker than C-H bonds and so break more easily when heated. CRACKING 4

5. In thermal cracking, the bonds are broken using a high temperature (400 – 900 o C) and a high pressure (70 atmospheres). The high temperatures mean that the molecule breaks near the end of the chain, giving a high percentage of small alkenes such as ethene. Most thermal cracking reactions involve the formation of one of more small alkane molecules and one alkene molecule. Naphtha (C 7 – C 14 ) is usually used as the starting material. THERMAL CRACKING 5

6. THE FREE-RADICAL CHAIN REACTION Initiation: Generates a radical intermediate. Propagation: The intermediate reacts with a stable molecule to produce another reactive intermediate (and a product molecule). Termination: Side reactions that destroy the reactive intermediate. 6

7. RADICAL MECHANISM REACTIONS OF ETHANE Radical mechanism pyrolysis of hydrocarbons can be subdivided into three phases: Initiation: Propagation Steps: Homolytic Fission 7

8. Termination RADICAL MECHANISM REACTIONS OF ETHANE 8

9. RADICAL MECHANISM REACTIONS OF PROPANE Initiation: Propagation Steps: 9

10. Beta fission C-C in propyl radical to produce ethylene with methyl free radical While C-H fission in isopropyl radical takeover to produce propene with hydrogen radical RADICAL MECHANISM REACTIONS OF PROPANE 10

11. RADICAL MECHANISM REACTIONS OF PROPANE Termination 11

12. RADICAL MECHANISM REACTIONS OF NAPHTHA Initiation: Homolytic fission in heating Propagation Steps: Beta fissions to form ethylene molecules 12

13. RADICAL MECHANISM REACTIONS OF NAPHTHA Fragmentation process in propagation steps: Fragmentation process in propagation steps: H removal from main alkane by methyl free radical which formed from previous steps to form nonyl free radical and methane H removal from main alkane by methyl free radical which formed from previous steps to form nonyl free radical and methane H removal from main alkane by ethyl free radical which formed from previous steps to form nonyl free radical and ethane with ethylene H removal from main alkane by ethyl free radical which formed from previous steps to form nonyl free radical and ethane with ethylene H radical removes hydrogen atom from main molecule H radical removes hydrogen atom from main molecule 13

14. Beta  -fissions to form ethylene molecules RADICAL MECHANISM REACTIONS OF NAPHTHA 14

15. Butene but not ethylene are formed RADICAL MECHANISM REACTIONS OF BRANCHED ALKANES 15

16. RADICAL MECHANISM REACTIONS OF CYCLIC ALKANES Butadiene formation In aromatic compounds: Thermal cracking only occurs with the side chain of hydrocarbon, not with aromatic ring 16

17. CATALYTIC CRACKING Catalytic cracking, including fluidic catalytic cracking, are processes in which a chemical catalyst is used to increase the yield of the thermal cracking process. Acidic catalyst like Aluminum silicate or zeolite catalyst added to the feed stock stream allows the breakdown of long chain molecules into lighter, short chain hydrocarbon molecules at temperature 460-520 o C, and a slight pressure (slightly greater than 1 atmosphere). The aim of catalytic cracking is: 1. to increase gasoline by converting Heavy fuel and fuel oil into gasoline. 2. to produce better quality of gasoline and octane number of gasoline 3. To produce some important chemicals like: propene, butene, and hydrocarbon gases. 17

18. Feed stock of Heavy fuel and fuel oil contain high complicated mixture of chemicals. Hence its difficult to study the path of chemical reactions. In describing main reactions which is lead to decrease in molecular weight for feed stock. Mechanism of catalytic cracking: 1. Primary reactions 2. Secondary reactions MECHANISM REACTIONS OF CATALYTIC CRACKING 18

19. 1. Primary reactions: formation of carbonium ion (intermediate). 2. Secondary reactions: (Rearrangement, dehydrogenation), producing aromatic compounds from hydrocarbon chains … etc. Primary reactions: 1. Primary reactions 1. Primary reactions: from alkane chain like Hexadecane (C 16 H 34 ) carbonium ion can form from: A) an alkene which formed from thermal cracking, then protonation of alkene by catalyst to form carbonium ion MECHANISM REACTIONS OF CATALYTIC CRACKING 19

20. B) from dehydration of Hydride ion H¯ of alkane through catalyst C) the carbonium ion subtract Hydride anions of methylene groups in main alkane. Hence more stable secondary carbonium ions formed. D) Fission of carbonium ion in adjacent bond, this called  Beta fission: Fission of Hexadecane carbonium ion to from Heptene with primary carbonium ion MECHANISM REACTIONS OF CATALYTIC CRACKING 20

21. Fission catalytic for branched alkane chains and cyclic alkanes, also catalytic fission if aromatic hydrocarbon substituted with two or more alkyl groups Mechanism of Fission include protonation of aromatic ring to produce sigma complex sigma (  ), which undergoes to further fission to give carbonium ion and benzene. Carbonium ion will lose proton to form an alkene This type of production has high octane number (good quality of fuel). The main production is Ethylene, Benzene and branched alkenes. MECHANISM REACTIONS OF CATALYTIC CRACKING 21

22. 2. Secondary reactions 2. Secondary reactions The primary reaction in catalytic cracking undergoes for Structural Conversions: 1. shifting of double bond in alkenes into middle of chain. Then a branched alkene will be more stable with high octane number. (Protonation of double bond followed by elimination of a proton from adjacent carbon of carbonium ion) 2. further breaking on alkenes that formed in primary reactions through protonation of alkenes to form carbonium ions, then formation of a small molecules of alkenes and alkanes. MECHANISM REACTIONS OF CATALYTIC CRACKING 22

23. 3. conversion of cycloalkanes (cyclohexanes) into alkenes and aromatic hydrocarbons by a series reactions of deprotonation. MECHANISM REACTIONS OF CATALYTIC CRACKING 23

24. MECHANISM REACTIONS OF CATALYTIC CRACKING 4. Formation of an aromatic hydrocarbons from alkenes cyclic chains Main production from secondary reactions of catalytic cracking is Benzene, Toluene, Xylenes, and small alkenes like ethylene. 24

25. MECHANISM REACTIONS OF CATALYTIC ALKYLATION Catalytic Alkylation is the reaction of isobutane with alkenes to form higher branched alkanes (“alkylate”). The aim is the production of gasoline components with a high octane number from low molecular weight alkenes (propene, butenes, and pentenes) and isobutane. The advantages of catalytic alkylation are that gas-phase molecules are eliminated and a valuable liquid product is formed, that is, gasoline with high octane number (87 up to 98). 25

26. Normally acid catalyzed using H 2 SO 4 or anhydrous HF. MECHANISM REACTIONS OF CATALYTIC ALKYLATION 38% 16% 4% 25% 26

27. Initiation Step: Protonation of propene by a catalyst following by subtracting hydride ion H¯ of Isobutane by carbonium ion of propene. The alkylation of Tertiary Carbonium ion of butyl react with a propene to form secondary carbonium ion of dimethyl pentyl MECHANISM REACTIONS OF CATALYTIC ALKYLATION 27

28. The secondary carbonium ion of dimethyl pentyl undergoes to series of rearrangements reactions through shifting of 2,1 Hydride ion or methyl group and the formation of carbonium ions, while more stable carboinum ion will dominate MECHANISM REACTIONS OF CATALYTIC ALKYLATION Propagation Steps: 28

29. MECHANISM REACTIONS OF CATALYTIC ALKYLATION Subtracting of Hydride ion from isobutene by secondary methyl pentyl carbonium ion to form a branched alkane with a tertiary butyl carbonium ion. Propagation Steps: Tertiary butyl carbonium ion react with propene and then a series of reactions will take over. 29

30. MECHANISM REACTIONS OF CATALYTIC ALKYLATION Side reactions of catalytic Alkylation operation: 1.A proton shifting from a tertiary butyl carbonium ion into propene molecule to form isobutene molecule 2.Reaction of the formed isobutene above with tertiary butyl carbonium ion to give tertiary carbonium ion of 8 carbon atoms. 30

31. 3.Tertiary carbonium ion of 8 carbon atoms subtracting hydride ion, from isobutane to give a branched alkane of 8 carbon atoms, and hence another tertiary carbonium ion will form. 4.The 2 nd and 3 rd steps will regenerate. Final product of catalytic alkylation is a branched side chain of alkanes. MECHANISM REACTIONS OF CATALYTIC ALKYLATION 31

32. MECHANISM REACTIONS OF CATALYTIC REFORMING Catalytic reforming is a key process in the production of gasoline components with a high octane number. It also plays an important role in the production of aromatics for the chemical industry. Furthermore, catalytic reforming is a major source of hydrogen. Feedstocks are straight-run naphtha and other feeds in the gasoline boiling range (about C6–C11). During catalytic reforming the change in molecular weight of the feed is relatively small, as the process mainly involves internal rearrangement of hydrocarbons. Reactions of Catalytic Reforming Reactions of Catalytic Reforming 1) Dehydrogenation of cyclic hexanes to form Aromatic compounds 32

33. Reactions of Catalytic Reforming Reactions of Catalytic Reforming 2) Reaction of Dehydrogenation with formation of cyclopentanes which substituted to form aromatic compounds. (Dehydroisomerisation). 3) Rearrangement of alkanes (Isomerisation) MECHANISM REACTIONS OF CATALYTIC REFORMING 33

34. Reactions of Catalytic Reforming Reactions of Catalytic Reforming 4) Dehydrocyclization of alkane to form aromatic compounds. 4) Hydrocracking Reaction Hydrocarbon with less than 6 carbon atoms can not converted to aromatic compounds. MECHANISM REACTIONS OF CATALYTIC REFORMING 34

35. MECHANISM REACTIONS OF CATALYTIC REFORMING 1.Dehydrogenation of cyclic hexanes to form Aromatic compounds: Done in seconds on surface catalyst which is difficult to study the mechanism 2. Dehydrogenation with forming: Mechanisms: 1.Dehydrogenation on surface of catalyst on site of hydration 2.Formation of substituted cyclopentene 3.Protonation and carbonium ion formation. 1.Rearrangement followed by cyclisation on the surface of cidic catalyst. 2.Formation carbonium ion of cyclohexyl 3.Proton Subtractin and Conversion into cyclohexene 1.Dehydrogenation on surface of catalyst on site of hydration 35

36. MECHANISM REACTIONS OF CATALYTIC REFORMING 3. Rearrangement of alkanes (Isomerisation)/ Rearrangement on two sites of catalyst Mechanisms: 1.Dehydrogenation of alkane on site of hydration 2.Protonation of alkene formed on acidic site of catalyst. 3. Formation of carbonium ion. 1.Rearrangement of carbonium ion, followed by subtracting of proton. 2.Formation of isomer of above alkene. 1.Hydration of alkene to form an alkane isomer with above alkane. 36

37. MECHANISM REACTIONS OF CATALYTIC REFORMING 4. Dehydrocyclization (Aromatization) of alkane to form aromatic compounds. Mechanisms: Metal clusters of Pt incorporated into the channels of zeolite L have been found to catalyze the dehydrocyclization of heptane with a high activity and selectivity. Reaction temperature=750 K, H 2 /n-alkane molar ratio=6 and atmospheric pressure 1.Bond formation due to Dehydrogenation of alkane with a surface od catalyst. While all other Carbon in a gases form. 2.Cyclisation of alkene by a combination of one of carbon bond with a carbon in a gases form (dehydration). 3.A continuing dehydration of alkene to form an aromatic hydrocarbon. 37

38. The term steam reforming is used to describe the reaction of hydrocarbons with steam in the presence of a catalyst. This process used to synthesis a synthetic gas to prepare Ammonia and methanol, and also used as a source of hydrogen gas, which is used in hydrogenation processes. The process used to rearrange alkanes containing C1 to C4, particularly methane. The process is also used, for the preparation of synthesis gas from naphtha. the process involves, hydrocarbon gas and water vapor to pass, on the catalyst surface. Basic interaction of the process of steam reforming of methane molecule, for example: STEAM REFORMING 38

39. Formation of carbon STEAM REFORMING Carbon formation in steam reformers must be prevented for two main reasons. Firstly, coke deposition on the active sites of the catalyst leads to deactivation. Secondly, carbon deposits grow so large that they can cause total blockage of the reformer tubes, resulting in the development of “hot spots”. Hence, the reforming conditions must be chosen such that carbon formation is strictly limited. 39

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