Môn Xúc tác CN lọc dầu, 2017-2018 N.T.Linh, Dept R&P, HUMG Bài tự học Chương 3- Chất xúc tác cho trình Reforming Yêu cầu: Đọc viết thu hoạch nội dung Bài thu hoạch viết độc lập, chép không chấp nhận Sinh viên nộp vào 7h ngày 11/12/2017 CATALYTIC REFORMING Background Catalytic reforming converts low-octane naphtha streams (20–50 RONC) to high-octane (90– 108RONC) gasoline blending stock using a dual-function catalyst with both metallic sites of Pt (or Pt with other metals as promoters) and acid sites supplied by the alumina support The traditional feed is heavy naphtha (primarily paraffins and naphthenes, 200–400°F, 93–204°C BP) from atmospheric distillation of crude oil Naphtha from delayed coking and hydrocracking and other streams high in naphthenes are also used The 150–200°F (66–93°C) stream from virgin naphtha, especially from paraffinic crude, will produce useful product if operation is at 200 psig or lower Hydrocracked naphtha in this same boiling range should not be used Also, naphtha from cracked stocks above 380°F (193°C) BP should be avoided, because it produces excessive coking The remarkable increase in octane number that reforming produces is due mainly to the conversion of essentially all the naphthenes and a sizeable portion of the paraffins to aromatics, and straightchain paraffins to branched isomers Aromatics can alternatively be recovered for petrochemical products, and operating conditions (low pressure and high temperature) and cut points of feedstocks can be tailored to maximize desired aromatics such as benzene, toluene, and xylenes With the 1990 Amendment of the Clean Air Act of 1970, reformulated gasoline must not contain significant amounts of benzene This goal can be accomplished in reforming by eliminating C6 hydrocarbons from feed to reformers operating for gasoline blend stock production Chemistry Refinery naphtha streams are composed of a large number of different molecular types in the range of C6 –C10 or C7–C10 A typical reformer product contains 300 different compounds Fortunately, this complexity can be comprehended because each compound type behaves similarly, and illustration of overall reactions N.T.Linh, Dept R&P, HUMG Page using compounds of a specific carbon number will be applicable to compounds with other carbon numbers Dehydrogenation of cyclohexanes to aromatics (quite endothermic, very fast): Dehydroisomerization of alkylcyclopentanes to aromatics (quite endothermic, rapid): Isomerization of paraffins (mildly exothermic, rapid): Dehydrocyclization of paraffins to aromatics (endothermic, slow): Hydrocracking of hydrocarbons (quite exothermic, slowest): Catalyst Types and Suppliers N.T.Linh, Dept R&P, HUMG Page The first catalytic reforming process, which used a molybdenum oxide-on-alumina catalyst, was jointly developed in 1939 by Standard Oil of New Jersey (Exxon), Standard of Indiana (Amoco), and M W Kellogg Company In 1949, UOP introduced the first process (Platforming ) using platinum-on-alumina as a dual-function catalyst having both acidic sites and metallic sites It soon became the catalyst of choice Over the years, a series of proprietary processes and catalysts have been developed, largely by refiners As primary patents have expired, a number of merchant catalysts are now available, often based on new promoters and other unique innovations Platinum remains the major active component of all modern naphtha reforming catalysts But, beginning in the 1970s, catalysts were introduced having one or more additional metallic components including rhenium, iridium, and tin These bimetallic and multimetallic catalysts exhibit greatly improved stability (cycle length) and selectivity Platinum-rhenium catalysts are the most widely used, because they are more coke tolerant and thus provide longer cycle times (times between regeneration) In addition, it has also been possible to reduce operating pressure and take advantage of yield enhancing equilibrium conditions and energy savings while maintaining attractive cycle times Typically, a Pt-Re catalyst will have two to four times the cycle time of a platinum-only catalyst Nature of the Catalyst Surface Perhaps no other catalyst has been studied so thoroughly Its astounding properties and complexity challenged many outstanding researchers to seek its mysteries Kinetic studies on pure components were interesting but led to little concrete knowledge Probing the nature of the catalyst surface characteristics proved most valuable and aided in tailoring catalyst recipes for improved catalysts and better operating procedures Extensive studies have permitted some useful generalizations: • Crystallite dispersion is an important prerequisite for successful catalyst use Platinum crystallites on fresh catalyst are typically less than 35 Å Practically speaking, the optimum preparative recipe and the redispersion procedures after regeneration may be unique for each catalyst type, and manufacturers or licensor recommendations for use should be carefully followed Many of the desired reactions are favored on small crystallites, while undesirable reactions such as hydrogenolysis, poisoning, and coke formation, are favored by larger crystallites where high coordination-number Pt atoms and larger ensembles of Pt atoms provide the geometry for multiple metal-carbon associations These complexes stabilize coke precursors on metal sites as well as precursors (e.g., cyclopentadiene) that migrate to the acid sites on the support and polymerize to polynuclear aromatics of low hydrogen content (0.05–0.1 H/C), i.e., coke • The acid function of the alumina support appears to be optimum at 0.7–0.9 wt% chlorine At these loadings, the rate of coke formation on the alumina surface is minimized A major coke precursor, methylcylopentane, is destroyed by hydrocracking at these higher acidity levels • The idealized edge of a crystallite for the several commercial catalysts may be represented as shown in Figure 18.4, with the fresh Pt/Re catalyst already sulfided which is then standard N.T.Linh, Dept R&P, HUMG Page procedure for this catalyst The Pt catalyst is shown partially coked and partially sulfided The Pt/Sn catalyst is shown fresh In all cases, the number of large ensembles (strings) of Pt atoms is reduced either by inert Sn atoms Re atoms made inert by sulfiding, or Pt atoms made inert by partial coking This situation favors the important dehydrogenation reactions, which are more rapid in small Pt ensembles, and discourages hydrogenolysis (which increases light ends) and coking reactions, both of which require larger assemblies of platinum • The iridium of the Pt-Ir catalyst is not inert, for it is an active dehydrocyclization catalyst (which is good) and an active hydrogenolysis catalyst (which is not good) But hydrogenolysis can be controlled to some degree by continuous addition of a sulfur compound in the feed (0–10 ppm) Although this action reduces the Pt ensemble size, desirable single-site hydrodecyclization is retained on both unsulfided Pt and iridium Deactivation Naphtha reforming catalyst is deactivated by coke formation, poisoning, loss of chloride, fines deposition, sintering of the metallic catalyst components, sintering of the support, and heavy metals deposition The effects of all but the last two can be remedied Since heavy metals and other poisons are restricted by feed pretreatment, coke is the main cause of short-term deactivation and determines the frequency of regeneration Long-term effects, such as support sintering, upsets in feed purity or temperature excursions, can reduce the ultimate catalyst life 5.1 Coke Formation Coke forms rapidly at the beginning of a run but, after about wt% is deposited, the coke laydown increases slowly at a steady rate As the reactor temperature is increased to compensate for loss in activity, a temperature is finally reached above which coke production accelerates, and the catalyst must be regenerated This marked increase in coking toward the end of the run can produce plugging in a fixed-bed reactor and ultimately unmanageable pressure drop The early formation of coke is due to coking on the faces of clusters, which does not affect edges or corners where the major desired reactions on Pt occur Subsequent coke formation occurs primarily on the alumina support which, because of its high surface area, can tolerate slow but constant increases in coke laydown over a longer time period Although metal activity does decline rapidly with coke laydown, sufficient activity remains for catalyzing all important metal site reactions Effect of Catalyst Type on Coke Formation Both coke formation and hydrogenolysis on Pt apparently occur on ensembles of contiguous metal atoms In the case of monometallic Pt catalysts, coke covers most of the Pt surface, leaving only islands of Pt for catalytic action These islands can contain sufficiently large ensembles of Pt to catalyze both hydrogenolysis and coking reactions Pt-Re, when sulfided, produces an inert ReS that separates the Pt into mostly very small ensembles that not catalyze hydrogenolysis or coking Thus, presulfided Pt-Re/Al2O3 catalyst produces less coke on the aggregate of metal sites N.T.Linh, Dept R&P, HUMG Page In fact, with increased Re loadings (skewed, more Re than Pt), this effect is accentuated, and even less coke-on-Pt is produced Since smaller Pt ensembles tend to weakly adsorb olefinic and aromatic coke precursors, these entities migrate to the acid sites of the support and form coke Thus, it is possible to produce more coke on a bimetallic catalyst such as Pt-Re-S/Al2O3 without excessively deactivating the metal portion The Pt-Re-S catalyst, however, apparently provides additional steric hindrances, perhaps partly because of its very broad distribution on the surface, and thereby makes it the most coke tolerant It is this high tolerance to coke deposits of Pt-Re-S that has made it the preferred catalyst for most reformers This characteristic permits operation at lower pressures and favors thereby higher aromatic yields as well as energy savings Alternatively, lower H2-to-oil may be used, or increased feed rate, or reduced catalyst The coke make on Pt-Ir catalyst is much less than Pt, Pt-Re, and Pt-Sn Iridium has a higher activity for hydrogenolysis, which serves to destroy coke precursors, but this quality causes higher production of light hydrocarbons and reduces yield However, a combined reforming operation has been proposed with Pt-Re in the initial beds followed by Pt-Ir The higher activity of Pt-Re for aromatization, which reaction occurs first, can then be used initially followed by Pt-Ir with its higher activity for dehydrocyclization in the later stages Effects of Operating Conditions and Feed Composition on Coke Formation The various catalysts differ in coke forming behavior, but all share similar qualitative effects of the major operating variablesloading Catalyst Poisons The major catalyst poisons that can occur in reformer feed streams are organic components containing sulfur, nitrogen, and metals (Pb, As, P) Organic nitrogen compounds are bases and will deactivate the important acidic function of the catalyst that is necessary for isomerization and cyclization reactions Sulfur compounds in sufficient quantity will deactivate the platinum sites, as will the metallic compounds Hence, all streams for reformer feed containing these poisons are first passed through hydrotreating units where the compounds are converted, respectively, to NH3, H2S and metallic deposits on the hydrotreating catalyst The gaseous components are removed from the product in a stripper and the product sent to the reformer Strict limits must be placed on residual poisons in the feed sent to the reformer Although sulfur compounds in the feed can seriously reduce Pt activity, controlled poisoning provides improved selectivity and activity In particular, hydrogenolysis reactions are inhibited by the poisoning of the most active sites Controlled addition of sulfur is practiced at start-up and strict limits on feed are imposed However, each catalyst type is affected differently, and special procedures for each must be followed for best results Chemisorption of sulfur occurs more easily on the face atoms (high-coordination number) of metal crystallites This adsorbed sulfur is deemed irreversible because it is strongly held and not easily removed by purging By contrast, the low-coordination-number edge and corner sites adsorb sulfur less strongly, and this sulfur is called reversible and is controlled by the following equilibrium N.T.Linh, Dept R&P, HUMG Page High H2 partial pressures cause the reaction to shift to the left, which provides a cleaning action that serves to maintain the activity of the edge and corner sites If feed sulfur exceeds the recommended limit, poor performance will result, including a decline in C5+ and H2 and aromatics yields Ultimately, H2S will appear in the recycle gas This poisoning can be reversed by continuing operation at a lower temperature provided low-sulfur feed has been reintroduced and allowed to cause desorption of the H2S Normal operation can be realized at the desired temperature when H2S no longer appears in significant amounts in the recycle gas If the temperature is raised prior to this condition, excess coke will form, and run time is thereby reduced Poisoning by nitrogen compounds is also reversible by procedures just described for sulfur poisoning If it goes undetected, the ammonia produced by the reforming conditions reacts with chloride on the catalyst to produce NH4Cl that deposits in downstream exchangers The loss of chloride further depletes active acid sites Lead and arsenic are irreversible poisons that form strong covalent bonds with the Pt sites and effectively deactivate them Other metals that also act as poisons, although not as severe as lead and arsenic along with approximate levels on the catalyst above which performance problems will ensue 5.2 Deactivation Due to Sintering During regeneration for coke removal, the oxidation reactions that occur produce high temperature and water vapor Both the metal and the alumina support sinter under these conditions, with the metal sintering accelerated by the high temperature and the oxidizing atmosphere The alumina sinters because of the high temperature and the presence of water vapor Fortunately, agglomerated alatinum can be redispersed and, although sintered alumina is irreversible, catastrophic sintering of alumina normally does not occur in one cycle Chloride and Water Control The chlorine content of the alumina, for which there is an optimum value (e.g., 0.9–1.2 wt%) for each catalyst type, sets the proper acidity of the alumina that supplies the acid-catalyst portion of the catalyst If chloride content becomes too large, excess hydrocracking occurs, and more light ends are produced but less hydrogen Water vapor is present in small amounts (1–2 ppm) If a surge in upstream units produces excess water (50 ppm), that water will strip a large portion of the chloride from the catalyst and deprive it of its important isomerization and cyclization reactions Feed water content is normally quite low (1–2 ppm) Chlorine is very slowly lost from the alumina and is replaced by a steady addition of an organic chloride or HCL (0.5–1.0 ppm of Cl in feed) Water is also added so as to produce 4–5 ppm in the feed for the purpose of distributing the chloride evenly over the catalyst bed.14 After regeneration, the rejuvenation step (see following section) also serves to rechloride the catalyst and, again, water addition accomplishes proper distribution Regeneration and Catalyst Rejuvenation As coke becomes excessive and sulfur poisoning (if it occurs) begins to deactivate the catalyst, regeneration must be initiated to burn off the coke Air is introduced along with added nitrogen to moderate the burning rate so as to prevent excessive temperatures (>550°C, 1022°F) and resultant N.T.Linh, Dept R&P, HUMG Page loss of support surface area and crushing strength.6 The burning process requires careful control to avoid excessively low temperatures that would produce incomplete regeneration After regeneration, the chloride content of the catalyst has declined due to reversal of the chlorination reactions on alumina, and the metallic crystallites have undergone considerable agglomeration A rejuvenation procedure is implemented following regeneration for the purpose of redispersing the metallic components and restoring chlorine to the alumina to maintain optimum acidity Rejuvenation procedures vary with the catalyst type, but some generalities based on the open literature are possible Metal redispersion is accomplished in an oxidizing atmosphere with chlorine as the primary reactant Metallic chlorides (e.g., PtCl2, IrCl3, and ReCl4) are formed from Pt, and perhaps some PtO and from IrO2 and Re2O7, all of which could be present after regeneration Evidence suggests that the chloride form for Pt might be PtIVOxCly, an oxychloride.8 In any event, the chloridation reactions are exothermic and reversible Based on postulates concerning chemical vapor transport,16 smaller crystallites, which have smaller mass per unit surface area, will reach higher temperatures than the larger crystallites when chloride formation occurs on the surface Once these temperature differentials are established, the forward reaction (Pt + Cl2 ⇔ PtCl2) will be favored on the lower-temperature larger crystallites and the reverse reaction on the smaller crystallites Thus, dispersion of larger crystallites occurs Chlorine can be added by various agents As shown in Figure 18.6, HCl is a common product when organic chlorides are employed at postregeneration conditions Since HCl and organic chlorides react with O2, free chlorine is made available The reaction of both HCl with O2 and HCl with alumina produces water, and control of water vapor pressure could have some effect on controlling the extent of chlorination and dispersion After regeneration and rejuvenation, the catalyst is reduced in hydrogen and then carefully presulfided according to manufacturer’s directions before placing it back in service N.T.Linh, Dept R&P, HUMG Page ... aromatics (endothermic, slow): Hydrocracking of hydrocarbons (quite exothermic, slowest): Catalyst Types and Suppliers N.T.Linh, Dept R&P, HUMG Page The first catalytic reforming process, which used a... on the aggregate of metal sites N.T.Linh, Dept R&P, HUMG Page In fact, with increased Re loadings (skewed, more Re than Pt ), this effect is accentuated, and even less coke-on-Pt is produced Since... energy savings Alternatively, lower H2-to-oil may be used, or increased feed rate, or reduced catalyst The coke make on Pt-Ir catalyst is much less than Pt, Pt-Re, and Pt-Sn Iridium has a higher