Advanced Techniques in Naphtha Isomerization Process

ISOMERIZATION OF LIGHT NAPHTHA Guide: Dr. PREETI AGHALAYAM Co-Guide: Dr. S SIVASANKER NITIN JAIN CA10M004

Contents of aromatics and olefins in a gasoline should be reduced for environmental protection. The loss of octane number caused by reduction of aromatics and olefins should be compensated by addition of some compounds that have high octane numbers. The other high-octane gasoline blend stocks will be surely required under these regulations, which will put too great a burden on refinery. Therefore, the production of paraffin based high-octane gasoline blend stock such as isomerate from isomerization of light naphtha might be a key technology for gasoline supply to cope with future gasoline regulations They burn easily and cleanly, and they are good blending components for clean motor gasoline.

Pentane C5 compounds equilibrium Butane or C4 compounds equilibrium Hexane or C6 compounds equilibrium The equilibrium compositions of C4, C5, and C6 paraffins as a function of temperature. If the goal is isomerization of paraffins, then the least desirable components - normal butane, pentane and hexane - are favored at increasing temperature. Fuel Processing Technology, 35 (1993) 183-200

Component (R + M)/2 n-pentane 61.8 i-pentane 93 N-hexane 24 2,3DMB 103.3 operating at as low a temperature as is catalytically practicable to produce the high-octane isomers is desirable.

Component Composition, LV-% Composition, LV-% feed Product i-C5 10.3 26.9 n-C5 24.8 8.4 i-C6 23.2 47.8 n-C6 25.6 5.7 Cyclics 5.6 11.2 Benzene 10.5 0 RON clear 68.8 82.8

FriedelCrafts catalysts :-The catalysts used for industrial isomerization processes are such as AlCl3 with additives such as SbCl3 and HCl. The catalysts are strongly acidic and very active even at 300 390 K. The low-temperature activity is very favorable since the equilibrium shifts to branched isomers at lower temperature. The Friedel-Crafts catalysts represented a first-generation system. Although they permitted operation at low temperature, and thus a more favorable isomerization equilibrium, they lost favor because these systems were uneconomical and difficult to operate. High catalyst consumption and a relatively short life resulted in high maintenance costs and a low on-stream efficiency. Corrosion rates were excessive, plugging of catalyst beds and equipment was common, Main reaction :-Isomerization of alkanes by acids proceeds through elementary steps nRH + H+ nR+ + H2 (1) nR+ isoR+ (2) isoR+ + nRH nR+ + isoRH (3) Side reactions Alkenes are formed by deprotonation of carbenium ions. CnH2n+1+ CnH2n+ H+ (4) Alkenes react easily with carbenium ions to form those with longer carbon chains C4 + C8 + C12+ (5) When the number of carbon atoms is 7 or more, the carbenium ions undergoes scission to form alkenes and c arbenium ions, whose carbon numbers are different from the starting alkane C8+ C=3+ C5+ (6) C8+ C=5+ C3+ (7)

Bifunctional mechanism:- Solid acids loaded with transition metal (Pt) are used in the industrial isomerization processes. The catalysts are often called bifunctional. One function is a function as an acid catalyst, the other being a function as a hydrogenation dehydrogenation catalyst. it is supposed that alkane is dehydrogenated on metallic sites to the corresponding alkene, which is isomerized by acid sites into a branched alkene. The branched alkene is then hydrogenated into the branched alkane again on the metallic sites nC5H12 nC5H10 + H2 (on Pt) (8) the equilibrium conversion of paraffin in Reaction 8 is low at paraffin isomerization conditions, sufficient olefin is present to be converted to a carbonium ion by the strong acid site (Reaction 9): nC5H10 + H+ nC5H11+ (on solid acid) (9) Reaction 8 undergoes a skeletal isomerization, probably through a cycloalkyl intermediate as shown in Reaction 10: nC5H11+ isoC5H11+ (on solid acid) (10) The isoparaffinic carbonium ion is then converted to an olefin through loss of a proton to the catalyst site (Reaction 11): isoC5H11+ isoC5H10 + H+ (on solid acid ) (11) In the last step, the isoolefin intermediate is hydrogenated rapidly back to the analogous isoparaffin isoC5H10 + H2 isoC5H12 (on Pt) (12) The rate-determining step of the isomerization is the rearrangement of carbenium ions, reaction (10).

Impact of desulfurization on zeolite catalysts The catalyst must be able to catalyze a number of different reactions critical to the isomerization process including: 1. Dehydrogenation of the light paraffin 2. Skeletal isomerization of the paraffin cation 3. Ring opening of cyclic paraffins (C5-C6 isomerization) However, they also have some disadvantages, such as their high cost due to the utilization of noble metals, large investment and operating cost because of the required presence of hydrogen, high reaction temperature for Pt-supported zeolite catalysts, and environmental concerns for Pt-supported chlorinated-alumina catalysts.

Fig.Isomerization of pentane over mordenite and Pt/mordenite. Numbers in parentheses show the selectivity for branched isomers. 523 K, 30 kg cm 2, H2/n-C5 = 2.5. Y. Ono / Catalysis Today 81 (2003) 3 16

Bifunctional catalysts :- chloride alumina catalysts, that of course operate at low temperature but highly unselective towards iso-paraffins as well high catalysts/reactants ratio, corrosions in reactor or pipes and several environmental issues Mordenite Catalysts works at high temperature above 2500C give rise to still low selectivity and less thermodynamically favoured iso-paraffins products as well as high operating cost, cracking products deactivation .

Sulfated Zirconia:-Sulfation of metal oxides introduces quite strong Brnsted acidity and, in general, enhances the catalytic activity in acid catalyzed reactions. Par-Isom process Catalyst comparison of calculated RON T. Kimura / Catalysis Today 81 (2003) 57 63

Temperature dependency between conventional catalysts and the Pt/SO4 2/ZrO2 catalyst in n-pentane isomerization T. Kimura / Catalysis Today 81 (2003) 57 63

Ionic liquid:-Because the properties of ionic liquids (miscibility with water and other solvents, dissolving ability for metal salts, polarity, viscosity, density, etc.) can be tuned by an appropriate choice of the anion and the cation, ionic liquids can be considered as designer solvents [27]. Ionic liquids are good solvents for many organic and inorganic compounds. Higher rates and better selectivity in selected ionic liquids than in classical solvents have been observed in the case of Friedel-Crafts reactions. Disadvantage:- 1-moisture sensivity 2-Reaction time are very large 3-Separation of products after reaction 4-High consumption of catalysts 5-Recycle of catalysts are not possible because of low activity The strategy of combination of polyoxometalate (POM) anions with appropriate cations has been regarded as a feasible one to afford promising ionic liquids, for they might possess the advantages of both IL and POMs in the same material