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  1. 1. Advances and Prospects of Isobutane Alkylation on Solid CatalystsBlock 2, Forum 9 paperAdvances and Prospects of Isobutane Alkylation on SolidCatalystsJohannes A. Lercher, TU MuenchenRoberta Olindo, Technische Universität MünchenCarsten Sievers</author><affiliation>Abstract:Alkylation of isobutane with n-butene is one of the important routes to provide high octanecomponents that can substitute aromatic molecules in gasoline. Traditionally, this route iscatalyzed by sulfuric and hydrofluoric acid. Solid acid catalysts have been exploredfrequently, but have not been commercially implemented to date. This is related to thenecessity of frequent regenerations, complex handling of solid catalysts in a liquid-solidsystem and inherent differences in the reactivity towards n-butene and isobutane. The maindrawback in the use of solid catalysts for isobutane alkylation is their rapid deactivation, whichso far has prevented their industrial application.Over the last years, significant progress has been made in this field, leading to the successfuldevelopment of alkylation technology on a pilot stage. Insight into surface chemistry, theadvances in catalyst preparation and the integration with novel reactor technology led toremarkable progress. Catalysts, which surpass the lifetime of sulfuric acid by three orders ofmagnitude, show potential for economic feasibility based on frequent mild regenerations. Thisprogress has been made possible by enhancing the key quality of the solid catalysts, which isthe hydride transfer between the surface bound species and the isobutane.The lecture will review the potential and limitations of current chemical and engineeringconcepts and will give an outlook to new developments in isobutane alkylation.IntroductionThe alkylate produced from isobutane/butene alkylation is an excellent blending componentfor gasoline. In 2005 the worldwide alkylation capacity amounted to approximately 2 millionbpd and it is expected to grow further Figure 1. In fact, due to increasingly strict legislation,the concentration of alkenes and aromatics in the gasoline will be more and more limited inthe next years. Additionally, methyl-tertiary-butyl ether (MTBE), a high-octane-numberoxygenate, is likely to be prohibited as a gasoline compound.HFH2SO4total050010001500200025001930 1950 1970 1990 2010YearAlkylatecapacity/thousandbpbAlkylatecapacity/thousandbpdYearFigure 1. World alkylation capacity (Taken from [1]).Copyright © World Petroleum Congress – all rights reserved
  2. 2. Advances and Prospects of Isobutane Alkylation on Solid CatalystsBlock 2, Forum 9 paperCurrently, only liquid acid-catalyzed processes are operated on an industrial scale withapproximately equal market shares for processes using sulfuric and hydrofluoric acid. Both ofthese catalysts suffer from serious disadvantages.Anhydrous HF is a corrosive and highly toxic liquid with a boiling point close to roomtemperature. Therefore, refineries with HF alkylation plants are under pressure to installexpensive mitigation systems minimizing the dangers of HF leaks. Moreover, authorities inmany industrialized countries have ceased to license new HF alkylation plants.Sulfuric acid is also a corrosive liquid, but not volatile, making its handling easier. Its majordisadvantage is the high acid consumption in the alkylation process, which can be as muchas 70–100 kg of acid/ton of alkylate. The spent acid contains water and heavy hydrocarbonsand has to be regenerated, usually by burning. The cost of such a regenerated acid is about2–3 times the market price for freshly produced sulfuric acid [2].In the last 30 years considerable efforts have been made to replace the existing liquidcatalysts by solid materials, which are environmentally benign and easier to handle. Amongthem the most promising candidates seem to be zeolites. Pioneering work on this field wasdone by groups at Mobil Oil [3] and Sun Oil [4] using rare earth exchanged zeolites. Ingeneral, all large-pore zeolites are active alkylation catalysts, giving product distributionssimilar to those characteristic of the liquid acids, but, due to their rapid deactivation, anefficient regeneration procedure is necessary. So far this has been the obstacle tocommercialization. Other solid materials tested in alkylation include sulfated zirconia,supported Brųnsted and Lewis acids, heteropolyacids and organic resins.The aim of this paper is to present a general overview on research development for alkylationprocesses. A special emphasis will be given only on recent results as a detailed review hasbeen published recently [5]. Further reviews have been published on both liquid and solid acidcatalysts [6] and on solid acid catalysts alone [7].Mechanistic aspectsInitiationIndependently of the acid used, the mechanism of isobutane/butene alkylation is essentiallythe same [8]. The reaction is initiated by the addition of the acid to the double bond of anolefin (reaction 1). With sulfuric and hydrofluoric acids alkyl sulfates and fluorides are formed.In zeolites this reaction leads to the formation of surface alkoxides rather than free carbeniumions [9]. However, in literature these species are often referred to as carbenium ions.Branched alkenes do not form esters. It is believed that they easily protonate and polymerized[10].Direct activation of isobutane by protonation is only observed at temperatures significantlyhigher than those used in typical alkylation reactions [11]. When the olefin is activated it canabstract a hydride from isobutane to form a tert-butyl carbenium ion (reaction 2).(1)(2)Alkene addition and isomerizationOnce tertiary carbenium ions have been formed, they can undergo electrophilic addition tofurther alkene molecules forming a larger carbenium ion. When 2-butene is used as alkylationagent the reaction mainly yields trimethylpentane (TMP) isomers. The primary product of theaddition would be 2,2,3-TMP. However, large amounts of other TMP isomers are usuallyobserved indicating that isomerization of the primary products plays a significant role. The+ HX+X+ X-++ + +Hydride transferCopyright © World Petroleum Congress – all rights reserved
  3. 3. Advances and Prospects of Isobutane Alkylation on Solid CatalystsBlock 2, Forum 9 paperaddition of a carbenium ion to an olefin is exothermic and of all the reaction steps contributesmost to the overall heat of reaction.(3)Hydride transferIntermolecular hydride transfer (reaction 4), typically from isobutane, to an alkylcarbeniumion/alkoxi group, transforms the species into the corresponding alkane and forms a new tert-butyl cation to continue the chain sequence. Hydride transfer is the elementary step thatensures the perpetuation of the catalytic cycle. A sufficiently high hydride transfer activity isnecessary to prevent oligomerization.The hydride donor does not necessarily have to be isobutane. In sulfuric acid catalyzedalkylation hydrides from conjunct polymers are transferred to carbenium ions. Sulfuric acidcontaining 4-6 wt% of conjunct polymers produces a much higher quality alkylate than acidswithout acid-soluble oil [12]. Similar compounds are also formed in zeolite catalyzedalkylation. Several groups proposed that these compounds might be responsible for thedeactivation of zeolite catalysts [13,14]. Despite the importance of this elementary step, thereare few studies, in which the kinetics of hydride transfer was investigated directly [11,15,16].Platon and Thomson proposed a test reaction, in which the hydride transfer occurs between acyclohexylcarbenium ion and isobutane using the fact that cyclohexene is moderately stableagainst polymerization [17,18].Oligomerization and crackingThe overall product distribution is governed by the relative rates of alkene addition andhydride transfer. As depicted in Figure 2, the ratio between rate constant for the addition of anolefin k3 and the rate constant for the hydride transfer step k2 determines whether the catalystwill effectively catalyse the alkylation or deactivates quickly through multiplealkylation/oligomerization reactions [19].C4+OC8+ C12+ C16+O OC8 C12k1k2 k4k3 k5P P...C4+OC8+ C12+ C16+O OC8 C12k1k2 k4k3 k5P P...Figure 2: Pathway to oligomerization products (adapted from [19]).Not only alkene addition but also the reverse reaction, i.e., cracking, is observed underalkylation conditions. This explains the detection of products with carbon numbers, which arenot multiples of four. The fragments are formed following the β-scission rule (reaction 5). Ingeneral, oligomerization and cracking products exhibit octane numbers lower than the TMPs.Average RON values of 92-93 for C5-C7 and of 80-85 for C9-C16 have been reported [20].The reactions discussed so far can occur in various combinations and proportions allowingthe formation of a large variety of compounds.(4)+ + +++++Copyright © World Petroleum Congress – all rights reserved
  4. 4. Advances and Prospects of Isobutane Alkylation on Solid CatalystsBlock 2, Forum 9 paper(5)DeactivationOligomerization is the main cause for catalyst deactivation. Although a certain amount ofconjunct polymers is beneficial during sulfuric acid catalyzed alkylation the catalyst becomesinactive when their concentration exceeds a certain critical level. During liquid-phasealkylation compounds with single or conjugated double bonds and five- and six memberedrings are observed [14,21]. Usually, one would not expect aromatic molecules to form underthe conditions of alkylation reactions. However, a series of condensation, hydride abstractionand cyclization reactions might lead to the formation of alkyl-substituted rings (reactions 6 and7).(6)(7)Developments of new alkylation catalystsZeolitesAdsorption of hydrocarbonsOne of the major differences between acidic zeolites and liquid acids is their selective andstrong chemisorption of unsaturated compounds. Therefore, considerable concentrations ofalkenes are found in the zeolite pores, even if the alkene concentration in the bulk is kept at alow level. This favors oligomerization of the alkene and thus deactivation of the catalyst.Consequently, the control of the alkene concentration is the key for providing zeolite catalystswith reasonable lifetimes.An important feature for the adsorption of hydrocarbons in zeolites is the linear increase of theheat of adsorption with increasing chain length [22] (Figure 3a and b). The value of this increase depends on the size and shape of zeolite pores.As a result different apparent activation energies are observed for the transformation ofhydrocarbons with different chain lengths. However, the same intrinsic activation energieswere found [23]. As a consequence of the increase of the heat of adsorption with the chainlength, the rate of desorption decreases by four orders of magnitude with each butene unitadded (Figure 3c). The tremendous difference illustrates the difficulties encountered for removingoligomers from the zeolite pores.Figure 3b demonstrates that, in addition to the heat of adsorption from physisorption, arelatively small amount of heat is generated by the directed interaction of the Brųnsted acidsites with the sorbate (chemisorption).++ ++ H+++HT++ H++HT +Copyright © World Petroleum Congress – all rights reserved
  5. 5. Advances and Prospects of Isobutane Alkylation on Solid CatalystsBlock 2, Forum 9 paper2 4 6 8 10020406080100120Carbon numberHeatofadsorption[kJ/mol]H-MFI MFI H-FAU FAUa)b)100102104106108101010121014kC8/kC12kC8/kC16kC8/kC20relativedesorptionratesc)2 4 6 8 10020406080100120Carbon numberHeatofadsorption[kJ/mol]H-MFI MFI H-FAU FAUa)b)100102104106108101010121014kC8/kC12kC8/kC16kC8/kC20relativedesorptionratesc)100102104106108101010121014kC8/kC12kC8/kC16kC8/kC20relativedesorptionratesc)Figure 3: a) Hydrocarbon in interaction with zeolite framework. b) Heat of adsorption as a function ofcarbon number for zeolites MFI and FAU in the acidic and non-acidic form. c) Relative desorption ratesof a C12, C16 and C20 alkane compared to octane.AcidityUnlike for liquid acids, zeolite acid sites vary substantially in nature and strength. Differentreaction steps require a different acid strength. Only weak acid sites are needed for doublebond isomerization in alkene, whereas somewhat stronger sites are necessary foroligomerization. Alkylation needs moderately strong acid sites and very strong acid sites arerequired for cracking.It is beyond discussion that Brųnsted acid sites are the active sites for alkylation. However,different opinions exist about the ideal strength of these sites [24,25]. It is well known that thestrong Brųnsted acid sites deactivate first. Once this has happened, the selectivity shiftstowards oligomerization and, so, the deactivation process accelerates (Figure 4). This isexplained by the fact that the olefinic oligomers add to existing carbenium ions generating solarge hydrocarbon frameworks that desorption via hydride transfer is not possible.0204060801000 5 10 15 20Time on stream (h)Conversion(%)0204060801000 5 10 15 20Time on stream (h)Conversion(%)Figure 4: Typical time-on-stream behavior of a LaX zeolite during isobutane/butene alkylation.Taken from reference [26].Lewis acid sites in zeolites result from a partial destruction of the zeolite lattice. Duringcalcination aluminum is partially hydrolyzed from the zeolite lattice and forms extra-frameworkaluminum oxide species (EFAL). This is enhanced in presence of water. Some of the speciesformed in this way exhibit strong Lewis acidity. Another source of Lewis acid sites are metalions on ion-exchange positions. However, most of these metals exhibit Lewis-acidity weakerthan aluminum species. It has been shown in several studies that Lewis acid sites areresponsible for the formation of unsaturated compounds in zeolite pores [21,24]. Theincreased concentration of these compounds leads to premature catalyst deactivation [25].Copyright © World Petroleum Congress – all rights reserved
  6. 6. Advances and Prospects of Isobutane Alkylation on Solid CatalystsBlock 2, Forum 9 paperSilicon/aluminum ratioThe aluminum content of a zeolite influences its acidity. The decrease of the Si/Al ratio leadsto an increase of the concentration of Brųnsted acid sites, while the strength of the individualacid sites decreases. In contrast to this general trend, several authors have found that theratio between strong and weak acid sites increases with decreasing Si/Al ratio [27-29]. Thisleads to a beneficial effect on the alkylation performance, because low Si/Al zeolites favor thehydride transfer [30,31]. However, a low Si/Al ratio reduces the thermal stability of the zeoliteleading to materials, which are more prone to dealumination.Exchange with rare earth cations could be used to induce increased acidity and stability[32,33].Structure types of zeolitesFor zeolite catalyzed alkylation large pore zeolites are needed. In materials with small poresdiffusion limitations become a major obstacle and higher operating temperatures are needed.With large pore materials the product distribution resembles the typical alkylate from liquidcatalyzed processes. Yoo et al. compared various large pore zeolites and found that zeoliteBEA exhibited the best time-on-stream behavior in both lifetime and TMP selectivity [34].ZSM-12 also showed a long lifetime but obviously catalyzed oligomerization instead ofalkylation. USY, MOR and LTL were found to deactivate quickly. The authors concluded thatzeolites without periodic expansions do not allow extensive coke formation and hencedeactivate at a slower rate. In contrast to these results, other groups found significant cokedeposition in H-BEA zeolite [25]. However, it has to be mentioned that it is difficult to separatethe influences of the pore structure and acidity of the respective samples.Sulfated zirconiaBesides zeolites, a variety of solid acids has been tested in alkylation reaction. Among thosesulfated zirconia received a considerable interest due to its strong acidity and activity forisomerization of short linear alkanes at temperatures below 150°C. Corma et al. showed thatsulfated zirconia already catalyzed alkylation at 0°C while only dimerization was observedover BEA [35]. However, at 50°C a high selectivity to cracking products (65 wt%) wasobserved on sulfated zirconia while BEA was selective for TMP.Like zeolites, sulfated zirconia suffers from rapid deactivation [36]. Additionally, significantloss of sulfur can affect the catalytic stability. Nevertheless, Xiao et al. reported a lifetime ofsulfated zirconia catalysts of 70 h [37].Other solid catalystsAnother solid material tested for alkylation, is Nafion-H, a perfluorated sulfonic acid resin withan H0 value comparable to sulfuric acid. Unsupported Nafion-H suffers from its low surfacearea [38]. Botella et al. showed that Nafion-H can be supported on porous carriers or directlyincorporated into silica [39]. Using these methods a material with acidity comparable to zeoliteBEA was obtained, which produces oligomers at low temperatures and saturated products athigher. Organic resins can also be used to support HF [40], leading to a considerablereduction of the HF volatility.Chlorinated alumina was one of the first solids tested in alkylation [41]. The acidity of thesesamples can be moderated with Li+or Na+cations to prevent excessive cracking and improvetime-on-stream behavior. Other solid materials tested for isobutane alkylation includefluorinated alumina [42], silica supported 12-tungstophosphoric (H3PW12O40) acid [43] andsilica supported perflouralkanedisulphonic acids [44].Copyright © World Petroleum Congress – all rights reserved
  7. 7. Advances and Prospects of Isobutane Alkylation on Solid CatalystsBlock 2, Forum 9 paperIonic liquidsIn addition to the development of solid catalysts, ionic liquids have been tested for alkylation.Yoo et al. used 1-alkyl-3-methylimidazolium halides-aluminum chloride for isobutane/2-butenealkylation [45]. The best sample of this series showed a higher activity than sulfuric acid, butTMP selectivity was lower. The authors concluded that an ionic liquid with a higher Brųnstedacidity is needed to obtain a competitive catalyst.In a recent publication Olah et al. showed that HF acid can be immobilized as pyridiniumpoly(hydrogen fluoride) [40]. Compared to pure HF the volatility was reduced by 95%, whilethe activity remained high.Influence of process conditionThe choice of appropriate reaction conditions is crucial for optimized performance inalkylation. The most important parameters are: reaction temperature, feed paraffin/olefin ratio(P/O), olefin space velocity (OSV), olefin feed composition and reactor design. Changingthese parameters will induce similar effects independently of the chosen catalyst.Nevertheless, the sensitivity towards changes is different for the individual catalysts. Table 1summarizes the most important parameters employed in industrial operations for differentacids. The values given for zeolites refer to operation in a slurry reactor. It is observed thatzeolites can be operated at the same or higher severity (with respect to P/O and OSV) thanliquid acids.Table 1: Typical values of important process parameters. The numbers for the liquid acids are taken from references[46-48]. The values given for zeolites refer to operation in a slurry reactor.HF H2SO4 ZeolitesReaction temperature (°C) 16-40 4-16 50-100Feed paraffin/olefin ratio (mol/mol) 11-14 7-10 6-15Olefin space velocity (kg Olefin/kg Acid h) 0.1-0.6 0.03-0.2 0.2-1.0Exit acid strength (wt.-%) 83-92 89-93 -Acid per reaction volume (vol.-%) 25-80 40-60 20-30Catalyst productivity (kg Alkylate/kg Acid) 1000-2500 6-18 600The required reaction temperature is determined by the activation energy of the individualreaction steps on a given catalyst. The viscosity of sulfuric acid and the solubility ofhydrocarbons in HF are further constraints in liquid phase alkylation. Zeolites have to beoperated at a significantly higher temperature due to the higher stability of the surface esterscompared to the esters of sulphuric acid [25]. However, at too high temperatures significantamounts of unsaturated products will be formed, which lead to rapid catalyst deactivation.To obtain a high quality alkylate and prevent rapid catalyst deactivation, the ratio between thehydride transfer and oligomerization rates must be high. This can be achieved at sufficientlyhigh P/O ratio and a low OSV [28,47]. On the other hand, at high P/O ratios more isobutanehas to be recycled, which leads to increased separation costs. Therefore, a suitable balancehas to be found to optimize the overall economics of the process.It has been shown that the deposition of coke and, therefore, the catalyst deactivation can bereduced when the reaction is conducted under supercritical or near-critical conditions insteadthan in liquid phase [49,50]. This in turn could beneficially decrease the frequency, andtherefore the cost, of catalyst regeneration. Isobutane, the most abundant species present inthe alkylation reactor, has critical pressure and temperature of 36.5 bar and 135°C. Underthese conditions the occurrence of cracking reaction lowers the octane number and thereforethe alkylate quality. For this reason various groups have investigated the use of inert solventswhich, added to the reactant mixture, allows reaching supercritical conditions or near-criticalcondition at lower temperature and pressure [51]. Among the inert solvents, carbon dioxidereceived the most attention [50,52]. Ginosar et. al compared various solvents underCopyright © World Petroleum Congress – all rights reserved
  8. 8. Advances and Prospects of Isobutane Alkylation on Solid CatalystsBlock 2, Forum 9 papersupercritical condition and concluded that light hydrocarbons are superior to carbon dioxidefor isobutane/butene alkylation [53].Industrial processesLiquid acid-catalyzed processesCurrently all industrial alkylation processes use liquid acid catalysts. Liquid acid-catalyzedprocesses require intensive mixing of acid and hydrocarbon phases in order to obtain asufficiently large phase boundary. On the other hand an efficient phase separation after thereaction is required. Even after 50 years of industrial operation the mixing properties ofsulfuric acid and hydrocarbons remains a subject of research [54,55]. In addition, newmethods for adding the olefins still have potential to improve the existing processes.As alkylation is an exothermic reaction, a complex cooling system is necessary. The HF-catalyzed processes are operated at temperatures between 16 and 40°C: Therefore thereactor can be cooled with water. H2SO4-catalyzed processes operate at lower temperature(4-18°C) and therefore require more complex cooling system, which typically utilize theprocessed hydrocarbon steam itself.Sulfuric acid-catalyzed processesTwo licensors offer sulfuric acid alkylation units, i.e. Stratco and ExxonMobil. Stratco hasdeveloped the Effluent Refrigerated Alkylation Technology [47]. A scheme of the reactor isshown in Figure 5. It is a horizontal pressure vessel containing an inner circulation tube, aheat exchanger tube bundle to remove the heat of reaction, and a mixing impeller in one end.The hydrocarbon feed and recycle acid enter on the suction side of the impeller inside thecirculation tube. This design ensures the formation of a fine acid emulsion and preventssignificant temperature differences within the reactor. A portion of the emulsion is withdrawnfrom the reactor and flows to the acid settler. The acid, being the heavier of the two phases,settles to the lower portion of the vessel and is returned to the suction site of the impeller. Thehydrocarbon effluent from the top of the acid settler is expanded and partially evaporated. Thecold two-phase hydrocarbon is passed through the tube bundle in the Contactor reactor andremoves the heat of reaction.A - CONTACTOR REACTOR SHELLB - TUBE BUNDLE ASSEMBLYC - HYDRAULIC HEADD - MOTORE - IMPELLERF - CIRCULATION TUBECOOLANTINCOOLANTOUTEMULSIONTO SETTLER ACIDA HCF BDCEA - CONTACTOR REACTOR SHELLB - TUBE BUNDLE ASSEMBLYC - HYDRAULIC HEADD - MOTORE - IMPELLERF - CIRCULATION TUBECOOLANTINCOOLANTOUTEMULSIONTO SETTLER ACIDA HCF BDCEFigure 5: Stratco® ContactorTMreactor used in sulfuric acid-catalyzed alkylation [47].The second process using sulfuric acid is ExxonMobil’s auto-refrigerated process [46]. Thereactor consists of a large horizontal vessel divided into a series of reaction zones, eachequipped with a stirrer (Figure 6). The alkene feed is premixed with recycle isobutane and fedin parallel to all mixing zones. The acid and additional isobutane enter only the first zone andCopyright © World Petroleum Congress – all rights reserved
  9. 9. Advances and Prospects of Isobutane Alkylation on Solid CatalystsBlock 2, Forum 9 papercascade internally to the other zones. The heat of the reaction is removed by evaporatingisobutane plus added propane from the reaction zones. Thus, cooling coils are not necessary.Figure 6: ExxonMobil auto-refrigerated alkylation process. Taken from [46].Hydrofluoric acid-catalyzed processesConocoPhillips offers a process where the hydrocarbon mixture is introduced in a non-cooledriser-type reactor through nozzles at the bottom and along the length of the reactor[56](Figure 7). The acid is injected at the bottom. Perforated trays provide a high dispersion ofthe hydrocarbons in the acid phase. The reaction mixture enters the settler where the acid iswithdrawn at the bottom and cooled in a heat exchanger to remove the heat of reaction. Thehydrocarbons are separated in a fractionation section.WateroutReactorstandpipeReactor riserAdditionalhydrocarboninjection pointslocated invarious locationsHydrocarbonfeed bottominjection nozzlesWaterinWateroutReactorstandpipeReactor riserAdditionalhydrocarboninjection pointslocated invarious locationsHydrocarbonfeed bottominjection nozzlesWaterinFigure 7: ConocoPhilips HF alkylation reactor [56].Copyright © World Petroleum Congress – all rights reserved
  10. 10. Advances and Prospects of Isobutane Alkylation on Solid CatalystsBlock 2, Forum 9 paperThe UOP HF alkylation process uses a vertical reactor-heat exchanger (Figure 8). Theisobutane–alkene mixture enters the shell of the reactor through several nozzles, and HFenters at the bottom of the reactor. The reaction heat is removed by cooling water, whichflows through cooling coils inside the reactor.alkenefeedRecycle i-C4Regenerated HFto settlercoolingwaterHF fromsettleralkenefeedRecycle i-C4Regenerated HFto settlercoolingwaterHF fromsettlerFigure 8: UOP HF alkylation reactor [5].Over recent years, the industry has addressed the issue of the potential environmental impactof HF leaks and has developed effective mitigation strategies. Two general strategies havebeen employed, the first related to the installation of remotely operated isolation valves, watercurtain/cannon systems and rapid acid dump systems. The second strategy relates to the useof vapor reduction additive. The advantage of this mitigation system, with respect to thetraditional methods of risk mitigation, is that its efficiency does not rely on rapid detection andlocalization of HF leaks. Examples of this strategy are the OUP/Texaco Alkad process andthe Philipps/Mobil ReVap technology [1,57]. To face the risks related to HF-catalyzedalkylation plants, an even more drastic solution consists in converting existing HF alkylationunits to use H2SO4. An example of this technology is the Stratco AlkySafe. Recently, it hasalso be claimed the possibility to convert an HF alkylation plant to use solid alkylationcatalystsSolid acid-catalyzed alkylation.RegenerationThe overall scheme for solid acid-catalyzed alkylation processes is similar to that for liquidacid-catalyzed processes, the main difference being the presence of a regeneration unit.Periodic regeneration is necessary for all solid alkylation catalysts developed, when employedat commercially feasible conditions. To extend the lifetime of the solid catalysts, severalregeneration methods have been developed, such as combustion or hydrocracking, extractionwith liquid or supercritical fluid, and hydrogenative regeneration.The main inconvenience of the regeneration by combustion of organic deposits in air is thereorganization of the hydrocarbons left on the catalyst, changing from an aliphatic to anaromatic structure, during temperature increase [58]. For this reason to regenerate thecatalyst it is necessary to reach temperature as high as 600°C. This causes dealuminationand degradation of the zeolite and in turn requires catalysts with extremely high temperatureCopyright © World Petroleum Congress – all rights reserved
  11. 11. Advances and Prospects of Isobutane Alkylation on Solid CatalystsBlock 2, Forum 9 paperstability. Alternatively Querini suggested a first treatment with ozone (which reduces theamount of coke and converts it to a structure easier to be burnt, followed by a treatment eitherin H2 or in He to complete the operation [58]. However, the use of ozone in large quantities foroxidative regeneration in ana industrial process is hard to imagine.Hydrogenative treatment was first patented by Union Carbide as early as 1974 [59,60]. Herethe activity of a fixed bed zeolite containing a metal hydrogenation component (Pt) was fullyrecovered by periodic regeneration either with hydrogen dissolved in a liquid hydrocarbon(typically isobutane) or with gas-phase hydrogen. The efficiency of the method was proved for12 and 19 cycles alkylation/regeneration. For both treatments a temperature of 65°C wasreported in the application examples, although it was claimed that temperatures between 27and 300°C could be employed. The treatment pressure was the same used in the alkylationreaction (33 bar) for liquid-phase regeneration while increasing pressures (from 1 to 33 atm)were required for gas-phase regeneration.Since 1974 several patents have claimed the possibility to extend the useful lifetime of solidacid catalyst by periodic hydrogenative treatments which represents variations of the originalmethods from Union Carbide [61-66]. Up to date hydrogenative regeneration is scarcelydocumented in the open literature. Recently Weitkamp’s group reported that the alkylationactivity of a 0.4Pt/La-X or 0.4Pt/La-Y can be completely recovered in a cycled regenerationprocess where step of alkylation reaction are alternated with hydrogenative gas-phaseregeneration steps performed at 300°C with 15 bar H2 [67,68].Solid alkylation catalysts can also be regenerated by washing with hydrocarbons. Ginosar etal. [69-72] in a systematic manner the regeneration of a USY zeolite with light hydrocarbons(C3-C5). They found supercritical conditions to be more effective than liquid phase and near-critical conditions. Isobutane was reported to be the most effective reactivation fluid with theadditional advantage that it is an alkylation reactant. Supercritical reactivation could providefull activity recovery only if performed on catalysts moderately deactivated. The activityrecovery from catalyst completely was not higher than 80%. An alkylation process where thecatalyst is periodically regenerated by extraction with a mixture of C8-C15 saturatedhydrocarbons has been patented [73].Processes with solid catalystsThe UOP AlkyleneTMprocess is schematically shown in Figure 9. It utilizes a vertical riser, inwhich the feed, freshly regenerated catalyst and recycled isobutane are injected from thebottom [74]. At the top of the reactor the catalyst is separated from the products. It sinks downinto the reactivation zone where it is regenerated in a mixture of isobutane and hydrogen. Toprovide complete reactivation a small amount of catalyst is withdrawn and regenerated batch-or semi-batch-wise in a reactivation vessel in vapor phase. The catalyst, which is referred toas HAL-100TM, might be alumina-supported AlCl3 modified with alkali metal ions and ahydrogenation metal such as Ni, Pd or Pt [75].IsobutanerecycleReactivationwash zoneFeedtreatmentReactivationwash zoneAlkylateAlkylenereactorReactivationvesselLightendsFractionationsectionOlefinfeedLPGi-C4/H2i-C4/H2IsobutanerecycleReactivationwash zoneFeedtreatmentReactivationwash zoneAlkylateAlkylenereactorReactivationvesselLightendsFractionationsectionOlefinfeedLPGi-C4/H2i-C4/H2Figure 9: UOP AlkyleneTMsolid acid-catalyzed alkylation process [74].Copyright © World Petroleum Congress – all rights reserved
  12. 12. Advances and Prospects of Isobutane Alkylation on Solid CatalystsBlock 2, Forum 9 paperLURGI and Süd-Chemie AG have developed a solid acid-catalyzed alkylation process calledLurgi Eurofuel®. The reactor technology uses a new type of reactive distillation. Isobutane andsuspended catalyst enter at the top of the tower while the alkene with premixed isobutane isintroduced in stages (Figure 10). The evolved heat of reaction is most likely dissipated byevaporation of the reaction mixture. Thus, the temperature is controlled by the overallpressure and the composition of the liquid. The catalyst–reactant mixture is agitated by theboiling mixture of alkylate and isobutane. At the bottom of the column the catalyst isseparated and intermittently regenerated by exposition to hydrogen-rich operating conditions.The catalyst is faujasite derived, with a high concentration of sufficiently strong Brųnsted acidsites and a minimized concentration of Lewis acid sites. It also contains a hydrogenationfunction.C4= +iC4iC4SteamCoolingwaterCoolingwaterAlkylateSteamC4= +iC4iC4SteamCoolingwaterCoolingwaterAlkylateSteamFigure 10: Lurgi Eurofuel®solid acid-catalyzed alkylation process [76].Akzo Nobel and ABB Lummus recently stated a solid acid-catalyzed alkylation process,referred to as AlkyCleanTMin a pilot plant in Finland [77]. The demonstrative unit incorporatesthree reactors where one is used for alkylation alternating with another in mild regenerationand a third reactor undergoing high temperature regeneration. Hydrogen is used toregenerate the catalyst. The catalyst is reported to be a “true solid acid” without halogen ionaddition. In the patent describing the process [61], a Pt/USY zeolite with alumina binder isemployed.Fixed-bed alkylation (FBATM) process of Haldor Topsųe uses liquid triflic acid, which issupported on a porous material [78-80]. This allows the use of a simple fixed-bed reactor as ifthe catalyst was truly solid. The acid in the bed is concentrated in a distinct band, the“reactive band”. The liquid hydrocarbon phase passes through the reactor in plug flow. In thetop of the reactive band the olefin comes into contact with a supported acid molecule andforms an ester. The ester is less strongly adsorbed on the solid support than the acid andtends to move down with the flow direction into the acid-rich zone Figure 11. The acid in theacid-rich zone catalyses the reaction of the ester with isobutane to form alkylate and an acidmolecule. The acid readsorbs on the support and the alkylate molecule flows downward withthe hydrocarbon phase. The reaction mechanism results in a downward but slow movementof the reactive band, in the direction of the hydrocarbon flow. The spent acid can bewithdrawn from the reactor without interrupting the production. The acid is regenerated in aproprietary acid recovery unit, which produces some oil as a by-product.Copyright © World Petroleum Congress – all rights reserved
  13. 13. Advances and Prospects of Isobutane Alkylation on Solid CatalystsBlock 2, Forum 9 paperSolid supported material(particulate porous solid)Direction of movementof reactive bandAcid-rich zoneEster formation zoneFlow of Hydrocarbon phase(olefin feed and isoparaffins)SLPCatalystzoneSolid supported material(particulate porous solid)Direction of movementof reactive bandAcid-rich zoneEster formation zoneFlow of Hydrocarbon phase(olefin feed and isoparaffins)SLPCatalystzoneFigure 11: Reaction zone in Haldor Topsųe’s alkylation process [78].ConclusionsThe alkylation mechanism, the influence of the catalyst type and reaction conditions showthat, in essence, the chemistry is identical with all the examined liquid and solid acid catalysts.Differences in the importance of individual reaction steps originate from the variety of possiblestructures and distributions of acid sites of solid catalysts. Changing process parametersinduces similar effects with each of the catalysts. However, the sensitivity to a particularparameter depends strongly on the catalyst. All the acids deactivate by the formation ofunsaturated polymers, which are strongly bound to the acid.Liquid acid-catalyzed processes are mature technologies, which are not expected to undergodramatic changes in the near future. Solid acid-catalyzed alkylation now has been developedto a point where the technology can compete with the existing processes. Catalystregeneration by hydrogen treatment is the method of choice in all the process developments.Some of the process developments eliminate most, if not all, the drawbacks of the liquid acidprocesses. The verdict about whether solid acid-catalyzed processes will be applied in thenear future will be determined primarily by economic issues.Bibliographic references(1) Pryor, P. S., PTQ Winter 2004, 69.(2) Furimsky, E. Catal. Today 1996, 30, 223.(3) Garwood, W. E.; Venuto, P. B. J. Catal. 1968, 11, 175.(4) Kirsch, F. W.; Potts, J. D.; Barmby, D. S. J. Catal. 1972, 27, 142.(5) Feller, A.; Lercher, J. A. Adv. Catal. 2004, 48, 229.(6) Corma, A.; Martķnez, A. Catal. Rev. 1993, 35, 483.(7) Weitkamp, J.; Traa, Y. In Handbook of Heterogeneous Catalysis; Ertl, G., Knözinger, H.,Weitkamp, J., Eds.; VCH: Weinheim, 1997; Vol. 4; pp 2039.(8) Feller, A.; Zuazo, I.; Guzman, A.; Barth, J. O.; Lercher, J. A. J. Catal. 2003, 216, 313.(9) Kazansky, V. B. Catal. Today 1999, 51, 419.(10) Albright, L. F.; Spalding, M. A.; Nowinski, J. A.; Ybarra, R. M.; Eckert, R. E. Ind. Eng. Chem. Res.1988, 27, 381(11) Sanchez-Castillo, M. A.; Agarwal, N.; Miller, C.; Cortright, R. D.; Madon, R. J.; Dumesic, J. A. J.Catal. 2002, 205, 67.(12) Li, K. W.; Eckert, R. E.; Albright, L. F. Industrial & Engineering Chemistry Process Design andDevelopment 1970, 9, 441.(13) Feller, A.; Barth, J.-O.; Guzman, A.; Zuazo, I.; Lercher, J. A. J. Catal. 2003, 220, 192.(14) Pater, J.; Cardona, F.; Canaff, C.; Gnep, N. S.; Szabo, G.; Guisnet, M. Ind. Eng. Chem. Res.1999, 38, 3822.(15) Yaluris, G.; Rekoske, J. E.; Aparicio, L. M.; Madon, R. J.; Dumesic, J. A. J. Catal. 1995, 153, 54.(16) Beirnaert, H. C.; Alleman, J. R.; Marin, G. B. Ind. Eng. Chem. Res. 2001, 40, 1337.(17) Platon, A.; Thomson, W. J. Appl. Catal. A-Gen. 2005, 282, 93.(18) Platon, A.; Thomson, W. J. Catal. Lett. 2005, 101, 15.(19) Simpson, M. F.; Wei, J.; Sundaresan, S. Ind. Eng. Chem. Res. 1996, 35, 3861.(20) Albright, L. F. Chemtech 1998, 40.Copyright © World Petroleum Congress – all rights reserved
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  15. 15. Advances and Prospects of Isobutane Alkylation on Solid CatalystsBlock 2, Forum 9 paper(77) DAmico, V. J. v. B., E.H.; Nat, P.J.; Nousiainen, H.; Jakkula, J. NPRA Annual Meeting, 2002,San Antonio, Tx, USA.(78) Hommeltoft, S. I. Appl. Catal. A-Gen. 2001, 221, 421.(79) Hommeltoft, S. I. Ind. Eng. Chem. Res. 2003, 42, 5526.(80) Jonsdottir, J. H. S.-A. P., PTQ summer 1998.Copyright © World Petroleum Congress – all rights reserved