VULCAN Processes for Alky Feed Pre-treatment

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Application Description
The Impact Of Poor Quality Olefin Feedstocks
The Importance Of Alky Unit Pre-treatment
Typical Process Conditions
VULCAN VIG Catalyst Morphology
Selective Hydrogenation of Acetylenes and Alkenes
Alkylation Reaction Chemistry
ALKYLATION CHEMISTRY AND PROCESS VARIABLES
What Are VULCAN Processes
FIXED BED PROCESSES
Advantages
Dis-Advantages
VULCAN UltraPurification Guards
VULCAN UltraPurification Impurities
VULCAN Sulfur Guards
VULCAN Guards - Prediction Of Sulfur In Feed
Basic HDS Reactions
Mechanisms for DBT desulfurization
Relative Reactivities of Three benzothiophene molecules

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VULCAN Processes for Alky Feed Pre-treatment

  1. 1. Gerard B. Hawkins Managing Director
  2. 2.  The aim of the alkylation unit is to produce high RON alkylates  Alkylates are important in the refinery gasoline pool. They are richer in branched chain high octane blending components than either reformate or isomerate  Alkylation refers to the reaction between low MR olefins and isoparaffins  eg. n-butene + isobutane dimethyl hexanes  Alky units are therefore located downstream of FCC units  The quality of the alkylate produced depends on the olefin feedstock: Isobutylene > butenes > propene > pentanes
  3. 3.  Most refiners use mono-olefin butene from the FCC as their feedstock into the alky unit  Di-olefins in the alky feed present problems to the refiner and require pre-treatment  Di-olefin spec to the alky unit is 100 ppmW  Excessive di-olefin leads to greater acid consumption Increased acid consumption is a serious problem to refiners, resulting in greater costs to the refiner and increasing governmental/environmental pressure
  4. 4.  Alky unit pre-treatment technology is catalyst based  FCC feed [containing di-olefin] over VULCAN VIG Series Pd/Al2O3  Most desirable product is but-2-ene  Historically ratio of but-2-ene to but-1-ene has been 3 to 4  Catalyst selectivity improvements mean today ratios are at 10 to 12  Directionally, ratios must improve for the sake of the gasoline pool  The role of the catalyst should be: a]To selectively hydrogenate feed to the alky b]To isomerize feed to the alky
  5. 5.  Temperature 40oC  Pressure 160 psi  Feed Flow-rate 32 m3/hr  Feed density 0.55 Kg/L  Volume 7 m3  LHSV 4.6
  6. 6. Acetylenes may be reduced to olefins or alkanes under mild reaction conditions (20 to 100ºC, 1 to 10 atmospheres H2 pressure) in the presence of other reducible functional groups. Note: Palladium and platinum are the preferred catalytic metals for these reactions. Palladium is the most selective metal for conversion of acetylenes to olefins. Selective Hydrogenation of Acetylenes and Alkenes
  7. 7. Olefins are easily reduced to alkanes. Platinum group metals exhibit the following general order of activity: Pd > Rh > Pt >> Ru. Strained olefins are reduced more easily than unstrained olefins; exocyclic double bonds are reduced more easily than endocyclic double bonds. In molecules containing more than one double bond, the least hindered bond generally will be reduced preferentially. Selective Hydrogenation of Acetylenes and Alkenes
  8. 8. A complication in the hydrogenation of alkenes can be double bond migration and cis-trans isomerization. Selective Hydrogenation of Acetylenes and Alkenes The tendency of the platinum group metals to promote these reactions during hydrogenation is as follows: Pd > Rh > Ru > Pt.
  9. 9. Double bond migration and cis-trans isomerization tend to be faster than olefin hydrogenation with palladium catalysts. Isomerization is impeded by conditions that minimize catalyst acidity and increase hydrogen availability at the catalyst surface (i.e. increased pressure and agitation, lower catalyst and/or metal loadings, and catalysts of low intrinsic activity). Platinum is useful when double bond migration is to be avoided. Selective Hydrogenation of Acetylenes and Alkenes
  10. 10. Palladium: 5% Pd/C 5% Pd/CaCO3 Platinum: 5% Pt/C Rhodium: 5% Rh/C Ruthenium: 5% Ru/C Recommended VULCAN VIG Series PGM Catalysts: Selective Hydrogenation of Acetylenes and Alkenes
  11. 11. The alkylation of isobutane with light olefins is an acid- catalyzed reaction that follows well-known Carbo-cation chemistry. A general representation of the reaction mechanism is shown in Figure 1.
  12. 12. The initial C4 carbo-cation is formed by the Protonation of a light olefin on the solid catalyst surface. The second step, often Referred to as isobutane activation, is the reaction (alkylation) of an olefin with this C4 carbo-cation on the catalyst surface and the subsequent hydride abstraction from isobutane by the resulting C8 carbo-cation.
  13. 13. In liquid acid systems, and particularly those using sulfuric acid, key Reactions are promoted by the Intimate mixing of acid with hydrocarbon and by adequate isobutane-to Olefin (I/O) ratios.
  14. 14. The key process variables in alkylation are the reaction temperature, the I/O ratio, reactant contact time with the catalyst, and the catalyst-to-olefin (C/O) ratio. The effect that each of these variables has on process performance in the Alkylene process is similar to the effect they have in conventional liquid acid technologies: ·
  15. 15. Reaction temperature. Alkylate product quality, particularly octane, improves as the reaction temperature is reduced. Refrigeration is typically required to optimize alkylation reaction conditions. The optimum temperature is chosen by balancing the improvement in product quality against the increased utility consumption and capital investment for lower temperature operation.
  16. 16. · Isobutane-to-olefin (I/O) ratio. Alkylate product quality improves with higher I/O ratios because olefin polymerization reactions are minimized as a result of the higher isobutane concentration present in the second reaction step. The result is that the alkylate product is enriched in the higher-octane trimethylpentane isomers rather than the lower- octane olefin oligomers. Generally, the I/O ratio determines the capital and operating costs for the process. Lower I/O ratios reduce these costs and improve the process economics. The optimum I/O ratio is chosen by balancing the improvement in product quality against the higher capital and operating costs resulting from increasing the I/O ratio. ·
  17. 17. Catalyst-to-olefin (C/O) ratio. The ratio of active catalytic sites to olefins is a key parameter related to catalyst activity and stability. A higher C/O ratio results in less deactivation per pass through the reactor. In liquid acid systems, such as the UOP HF Alkylation process, this ratio is expressed as the acid-to-hydrocarbon ratio. Liquid acid alkylation units are usually operated at a constant acid circulation rate. In solid acid alkylation units, the additional catalyst required to maintain a high C/O ratio must be balanced against improved catalyst stability.
  18. 18. Reactant contact time. The reactant contact time with the catalyst has a significant impact on product quality because undesirable secondary reactions, such as isomerization and cracking, occur at longer contact times. These secondary reactions act to degrade alkylate product quality. Determining the correct contact time that will maximize alkylate product quality, while still achieving high olefin conversion, is critical in optimizing the process design. ·
  19. 19.  Processes to remove impurities to extremely low concentrations  Use of fixed bed technology, employing catalysts, catalytic absorbents and regenerable adsorbents
  20. 20.  Low capital cost  Easily retrofitted  No environmental impact  Dispose of spent material by recycling  Minimal operator attention  No feed losses  Impurities removed to ppb levels  Flexible and robust design
  21. 21.  Only remove reactive species  May have a high operating cost.
  22. 22. •Liquid or Gas duty •High Capacity •Sharp Absorption Profile •Effective in Dry Streams
  23. 23. •Sulfur compounds •Halogen compounds •Organometallics •Mercury •Nitrogen compounds •Unsaturated hydrocarbons •Oxygenates
  24. 24. CAPACITY DEPENDENT ON THE NATURE OF THE SULFUR SPECIES: H2S FULL REMOVAL RSH FULL REMOVAL RSSR PARTIAL REMOVAL THIOPHENE NO REMOVAL THIOPHENE DOES NOT "POISON" GUARD
  25. 25. MO + H2S → MS + H2O MO + COS → MS + CO2 2 MS + Hg → M2S + HgS
  26. 26. Cu /Zn PRODUCT: SINCE RSH / H2S IS THE PREDOMINANT SULFUR SPECIES FOUND IN NAPHTHA, THE MIXED METAL OXIDE (Cu / Zn) OFFERS THE MOST COST EFFECTIVE GUARD. A Ni BASED MATERIAL MAY BE REQUIRED IN SELECTIVE CASES TO POLISH LESS REACTIVE SULFUR SPECIES, i.e., THIOPHENE
  27. 27. VULCAN GUARDS PREDICTION OF SULFUR IN FEED
  28. 28.  Mercaptans: RSH + H2 → RH + H2S  Sulfides: R2S + 2H2 → 2RH + H2S  Disulfides: (RS2) + 3H2 → 2RH + 2H2S  Thiophenes: C4H4S + 4H2 → C4H10 + H2S  Aromatics: ArS + 2H2 → Aromatic + H2S (excludes ring saturation)
  29. 29. The chemical reaction mechanism essentially involves the direct desulfurization (DSD) path and to a lesser extent the more hydrogen consuming hydrogenation route (HYD)..

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