Fischer-Tropsch Refining (DE KLERK:FISCHER-TROPSCH O-BK) || Oligomerization

369

19Oligomerization

19.1Introduction

Alkene oligomerization was identified as one of the key Fischer–Tropsch refining technologies[1, 2]. This is understandable considering the large amount of alkenes in syncrude (Table 1.2)and especially the amount of alkenes in the lighter syncrude fractions. It is difficult to conceiveof a high-temperature Fischer–Tropsch (HTFT) refinery without an alkene oligomerization unitto convert the gaseous alkenes into liquid products. Although low-temperature Fischer–Tropsch(LTFT) synthesis produces less alkenes, there are still percentage levels of alkenes present inthe gaseous products from LTFT synthesis. Furthermore, the total amount of light alkenesthat are produced may increase with time on stream as the LTFT catalyst deactivates (Sections4.5.5–4.5.7). Depending on the Fischer–Tropsch catalyst, such deactivation may actually be tothe advantage of the refinery if it employs oligomerization or alkylation technology [3].

Oligomerization refers to one or more consecutive addition reactions between alkenes. Inthis book, alkene addition is collectively referred to as oligomerization, unless it is important toindicate a specific multiple of the addition reaction. It is a catch-all term that includes dimerization(addition reaction of two alkenes), trimerization, tetramerization, and higher multiples of additionreactions. Refinery processes employing oligomerization is often colloquially called polymerizationprocesses, which should not be confused with true polymerization to produce plastics such aspolyethylene and polypropylene. The term polymerization will not be employed here, despite itsubiquitous use in oligomerization and refining literature.

As refining technology, oligomerization was originally developed to convert the gaseousproducts from crude oil cracking operations into liquid products. Oligomerization units startedappearing in second-generation crude oil refineries (Section 2.4.2), and some refineries to thisday still have oligomerization units. The main product from these units is high-octane olefinicmotor-gasoline. In Fischer–Tropsch refineries, alkene availability is not constraining, but incrude oil refineries the limited availability of alkenes restricts the continued widespread use ofoligomerization technology. The capital and operating cost of aliphatic alkylation (Section 16.4.5)as alternative is more than double that of oligomerization, and, as a result, oligomerization is stilla refining technology selected by some crude oil refiners despite its higher alkene demand [4].

There are some general objectives when oligomerization technology is employed, but in mostcases the application (Section 16.4.4) is closely linked to the selection of the oligomerizationtechnology and catalyst. In this respect, oligomerization is different from most other refining

Fischer–Tropsch Refining, First Edition. Arno de Klerk. 2011 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2011 by Wiley-VCH Verlag GmbH & Co. KGaA.

370 19 Oligomerization

Table 19.1 Alkene oligomerization technologies relevant forfuel refining. Examples of some commercially available tech-nologies are given.

Catalyst Technology Supplier Main fuels application

Solid phosphoric acid CatPoly UOP Motor-gasoline, jet fuelInAlk UOP Motor-gasoline

Amorphous silica-alumina Polynaphthaa IFP/Axens Diesel fuelSelectopola IFP/Axens Motor-gasoline

Montmorillonite Octol-Ab Huls/UOP Motor-gasolineH-ZSM-5 zeolite MOGD ExxonMobil Diesel fuel

COD PetroSA Diesel fuelH-ZSM-22 or -57 zeolite Emogas ExxonMobil Motor-gasoline, jet fuelAcidic resin NExOCTANE Fortum Oy Motor-gasolineHomogeneous nickel Dimersol Gc IFP/Axens Motor-gasoline

aAlso available with a zeolite-based catalyst.bAlso available with an Ni-promoted catalyst for butene dimerization to a more linear product.cDimersol E for ethene oligomerization and Dimersol X for butene dimerization to a more linear product.

technologies. For most refinery conversion processes, the products are broadly speaking similar,irrespective of the technology or catalyst selection. For oligomerization, different applicationsrequire different catalyst and technology combinations (Table 19.1) [5].

Potential applications of different oligomerization technologies and catalysts with Fischer–Tropsch syncrude include the following:

1) The most important general application of oligomerization is to convert normally gaseousalkenes into liquids. Preferably, this conversion should target a specific product, but in thecoarsest sense it can be used to increase the liquid syncrude yield from a Fischer–Tropschfacility. It is consequently a useful technology for a syncrude upgrader, as well as for arefinery to improve its carbon efficiency.

2) Some oligomerization technologies can double as deoxygenation technologies. In suchapplications, straight-run oil fractions can be oligomerized and deoxygenated to produceolefinic products with little remaining oxygenates that can more easily be fractionatedand refined. By doing so, the boiling point broadening is significantly reduced. Onespecific implementation of this type is the conversion of the total syncrude product overan oligomerization and deoxygenation active acid catalyst directly after Fischer–Tropschsynthesis [6].

3) The naphtha to distillate ratio in the refinery can be manipulated through oligomerization.The selection of the oligomerization technology will favor one over the other, but someoligomerization catalysts also allow the manipulation of the naphtha to distillate ratiothrough the operating conditions of oligomerization.

4) The primary product of alkene oligomerization in most refineries is good quality olefinicmotor-gasoline, also called polymer gasoline (Table 2.5). For this purpose solid phosphoricacid (SPA) catalyzed oligomerization is preferred, although alternative catalysts can beselected.

19.1 Introduction 371

5) Selective isobutene dimerization followed by alkene hydrogenation is called indirect alky-lation, because it produces an alkylate-equivalent product, namely a high-octane paraffinicmotor-gasoline [7, 8]. Indirect alkylation units became increasingly prevalent after theinclusion of 2-methoxy-2-methylpropane (MTBE) in motor-gasoline was prohibited in partsof the United States [9]. This made some installed etherification capacity for MTBE produc-tion redundant. These units and their associated isobutene feed became prime candidatesfor conversion into resin-catalyzed isobutene dimerization processes. Selective isobutenedimerization can also be performed over SPA. Straight-run Fischer–Tropsch syncrude islean in isobutene, and isobutene dimerization should typically be considered in conjunctionwith another refining technology that produces isobutene, such as skeletal isomerization(Chapter 18) or catalytic cracking (Chapter 21).

6) Another form of indirect alkylation is selective n-butene dimerization over SPA followed byalkene hydrogenation [10]. This technology is well matched to Fischer–Tropsch syncrude,because it does not require isobutene. The oligomerization and hydrogenation steps canalso be combined in a single unit by employing a metal-promoted SPA catalyst, for example,Fe/NiO/SPA [11, 12].

7) Most oligomerization technologies produce a kerosene range product that is branched.This is a natural consequence of the oligomerization mechanism. When this product ishydrogenated, it becomes an iso-paraffinic kerosene (IPK), which is a good jet fuel blendingcomponent. The branching is required for the demanding cold-flow characteristics ofjet fuel (Section 14.3.3). The IPK derived from SPA-catalyzed Fischer–Tropsch alkeneoligomerization is the only synthetic product that has been qualified for inclusion insemisynthetic jet fuel, and it is also a component qualified for fully synthetic jet fuel(Section 14.2.1).

8) Oligomerization and aromatic alkylation (Chapter 20) can be combined in a single unitto produce a synthetic jet fuel meeting Jet A-1 specifications and the minimum aromaticrequirement for synthetic jet fuel [13]. This process makes use of an SPA catalyst.

9) Oligomerization at higher temperatures with H-ZSM-5 is able to generate aromatics asa side reaction during oligomerization. It is therefore in principle possible to operatesuch an oligomerization process in a way so that the process, even without an aromaticco-feed, can meet the minimum aromatic requirement for synthetic jet fuel. It was reportedthat the hydrogenated kerosene from H-ZSM-5 oligomerization met all standard Jet A-1specifications [14]. With mild metal promotion of the zeolite, this process may even befurther improved.

10) Refinery benzene reduction can be accomplished by co-feeding the benzene with alkenesto an appropriate acid-catalyzed refinery process, of which oligomerization has beenshown to be the most suitable [15]. This process has been successfully demonstrated onindustrial scale in a Fischer–Tropsch refinery using an SPA-catalyzed oligomerizationunit [16].

11) Distillate range material can be produced by oligomerization. Depending on the oligomer-ization technology, it is possible to convert gaseous and naphtha range alkenes into distillate[17]. The distillate properties depend on the technology. When ZSM-5-catalyzed oligomer-ization is employed, the branching in the distillate is limited and the hydrogenated distillatehas a high cetane number and good cold-flow properties [18–20].


12) Selective dimerization of 1-butene over a Ni-containing catalyst gives a product that has a lowdegree of branching [21]. The octene mixture thus produced is employed as feed material forhydroformylation and hydrogenation to produce isononanol, which is a plasticizer alcohol.The synergy between this application and a Fischer–Tropsch-based facility is obvious, with1-butene, CO, and H2 being readily available.

13) Various homogeneous catalyst systems are industrially employed for ethene oligomerizationto produce a range of n-1-alkenes for various chemical uses [22]. There are also specifichomogeneous catalysts for the selective trimerization of ethene to 1-hexene, which is acomonomer used in plastics [23], and selective tetramerization of ethene to 1-octene isbeing commercialized [24].

14) In the past, detergent range alkenes have been produced by tetramerization of propene overSPA [25]. The degree of branching of this product is very high, and more linear alkenes fromother routes are generally preferred. In a Fischer–Tropsch refinery, selective dimerizationof hexenes and heptenes may be considered for this purpose.

15) Synthetic polyalphaolefin (PAO) lubricants are produced by the oligomerization ofn-1-alkenes. Lubricant base oils have been prepared from Fischer–Tropsch-derived alkenes[26]. Lubrication oil production by oligomerization of Fischer–Tropsch 1-alkenes is still ofinterest [27].

19.2Reaction Chemistry

The reaction chemistry of alkene oligomerization cannot be described in generic terms withoutsome reference to the catalyst or process. There is not a single mechanism and, even whenalkene addition is catalyzed by the same general mechanism, different catalysts may still produceproducts with very different characteristics [28].

Broadly speaking, there are four different mechanisms of industrial relevance for alkeneoligomerization (Figure 19.1):

1) Classic Whitmore-type carbocation mechanism. Brønsted acid-catalyzed alkene oligomer-ization takes place through this mechanism. The first step in this mechanism involvesthe protonation of an alkene to yield a free carbocation intermediate. This carbocationintermediate is capable of all the side reactions associated with such chemistry. Dependingon the length of the carbon chain, double bond isomerization, skeletal isomerization, andcracking by β-scission may take place (Figure 18.2). These are all monomolecular reactionsand are always in competition with the addition reaction that is bimolecular. After the alkeneaddition reaction, the addition intermediate is still a carbocation, which is still capable offurther alkene addition as well as the aforementioned side reactions. The final product is analkene and it is also capable of further protonation and further reaction. Zeolites and acidicresin catalysts oligomerize alkenes by a carbocation mechanism.

2) Ester-based mechanism. Some acid catalysts form strong formal σ -bonds with the protonatedintermediate, and the mechanism involves a polarized acid ester, rather than the equivalentcarbocation. Transitions that would otherwise involve a primary carbocation intermediatebecome possible when the α-carbon of the alkene is bonded to the acid, since the α-carbonis no longer a primary carbon. The polarized intermediates are weaker electrophiles than

19.2 Reaction Chemistry 373

(c)− H + H

(b)

O

PO OH

OH

O

PO OH

OH

d+

d−

(a) +

+

+

(d)

M HL

L

+ML

L

ML

L

ML

L

ML

L

b -H

1,2-Insertion b Hydride elimination

− M HL

L

+ H+

− H+

+ H3PO4

− H3PO4

− H3PO4

+ H3PO4

− H+

+ H+

Figure 19.1 Oligomerization mechanisms illustratedby propene dimerization: (a) classic Whitmore-typecarbocation mechanism, (b) ester-based mechanism,(c) free radical mechanism, and (d) organometallic

insertion mechanism where L denotes an arbitraryligand. Stereochemistry, side reactions, and rear-rangements of the intermediates are not shown.

carbocations and reactions can be more selective, but typical acid-catalyzed side reactionscan still occur. Phosphoric acid–based catalysts oligomerize alkenes by an ester mechanism.

3) Organometallic insertion mechanism. Some metals are capable of forming coordinationcomplexes with alkenes. This chemistry forms the basis for Ziegler–Natta and metallocenepolymerization catalysis. Chain growth proceeds by 1,2-insertion, and β-hydride eliminationterminates it. The metal most often encountered in general alkene oligomerization is Ni.For ethene oligomerization specifically, a variety of metal complexes are used, which includemetals such as Ni, Al, Zr, Ti, and Cr. The ligands on the organometallic catalyst can bedesigned to catalyze oligomerization in a very specific way, making it useful for very selectivecatalysis. Homogeneous and heterogeneous nickel-based catalysts can oligomerize alkenesby the insertion mechanism to yield more linear products than is found during acid catalysis.

4) Free radical mechanism. The free radical oligomerization of alkenes is initiated by theformation of a free radical species that propagates by addition to another alkene. Thechain growth is terminated when the radical recombines with another radical, or abstractsa hydrogen atom to initiate a new radical chain. This chemistry is also encountered infree radical polymerization. The process is thermally initiated, and co-feeding compoundswith low bond dissociation energy, such as peroxides, can lower the initiation temperature.


The side reactions occurring during free radical oligomerization are usually fewer thanfor acid-catalyzed oligomerization. Double bond isomerization occurs by intramolecularhydrogen transfer (Figure 16.5). Skeletal isomerization and cracking is not prevalent, exceptat high temperatures (>400 ◦C) where C–C bond scission becomes significant. There is nocatalyst involved in free radical oligomerization.

Alkene oligomerization is highly exothermic, and heat management is crucial in/duringoligomerization. Each alkene addition reaction is accompanied by a heat release of85–105 kJ·mol−1 at standard conditions. To put this into perspective, during etheneoligomerization, every 1% conversion is equivalent to an adiabatic temperature increase of12–13 ◦C.

Oligomerization is favored by low temperature and high pressure, as one would expect fromLe Chatelier’s principle. It is not customary to think of oligomerization as an equilibrium-limitedprocess, but at high temperature, oligomerization is subject to equilibrium limitations. At lowtemperature (<200 ◦C), equilibrium favors oligomerization, and the reaction rate is usually nothigh enough to reach the equilibrium limitation. Over acid catalysts, there is a transition betweenkinetically limited oligomerization and thermodynamically controlled oligomerization in thetemperature region 200–250 ◦C. The point of transition is determined by the activity of thecatalyst; more active catalysts will reach the transition point at a lower temperature.

19.3Catalysis

There are many oligomerization technologies to choose from for fuels refining (Table 19.1) andeven more for chemicals refining. Over the years, there have been different waves of technologydevelopment, starting with thermal (noncatalytic) oligomerization, which was followed bycatalytic oligomerization employing phosphoric acid and other liquid acid catalysts. Later on,various amorphous and zeolitic silica–alumina catalysts were developed, as well as acidic resincatalysts. Each catalyst type enabled the production of new or different products and contributedto the smorgasbord of options, rather than displacing the older technologies or catalysts. In fact,for many Fischer–Tropsch-based applications, SPA is still the catalyst of choice.

Overviews of homogeneous and heterogeneous oligomerization catalysis can be found in booksand reviews spanning many decades [5, 29–37]. Only some of the catalyst types are discussedhere. For some commercial technologies, there is insufficient information available to performa proper evaluation of the catalyst type for syncrude conversion.

In order the help with technology selection for refining by oligomerization, some generalpointers are given below:

1) Carbon number distribution. Both catalyst and operating conditions affect the carbonnumber distribution and thereby the yield of the different product types. The most importantcontrolling factors are kinetic limitations at lower temperatures, thermodynamic limitationsat higher temperatures, and catalyst-related effects that promote desorption over adsorption(or radical transfer in the case of thermal oligomerization).

2) Alkene composition. Some catalyst types are sensitive to the composition of the alkenes inthe feed. Such feed sensitivity can be in the form of preferential conversion of particular

19.3 Catalysis 375

alkenes, or it can affect the nature of the oligomerization product. If the product is formed insuch a way that the carbon double bond is in a sterically hindered position, further reactionmay be difficult.

3) Concentration of inerts. The amount of inert material can have a dramatic effect onoligomerization selectivity. Inert material reduces the probability of multiple additionreactions before desorption. Increasing the inert concentration may therefore cause a shiftin product distribution to lighter products. Inert material also helps with heat management.More inert material reduces the adiabatic temperature rise, which may be beneficial foroligomerization selectivity.

4) Oxygenates. Not all oligomerization catalysts tolerate oxygenates. Oxygenates are reactivecompounds and are more strongly adsorbed by acid catalysts. The oxygenates may alsobe catalyst poisons for some catalyst types, and the water generated during deoxygenationreactions may affect catalyst performance.

5) Catalyst geometry. Catalysts with a pore-constrained geometry can affect the nature ofthe oligomerization products through transition-state selectivity or by imposing diffusionlimitations. This can be useful if the degree of branching in the product must be limited.

6) Acid strength. The operating temperature window, propensity for side reactions, and catalystdeactivation rate can all be affected by the acid strength of the catalyst. In some cases, strongacid sites may activate alkanes for aliphatic alkylation reactions, or catalyze hydrogen transferreactions.

7) Mechanism. The dominant reaction mechanism is dependent on the catalyst, but temperaturecan affect it. Selecting the most appropriate oligomerization mechanism for the productsrequired is an effective way to narrow the technology selection.

19.3.1Solid Phosphoric Acid

SPA catalysts are actually supported liquid-phase catalysts. The SPA catalyst is prepared byimpregnating kieselguhr (diatomaceous earth), which is a natural silica source, with phosphoricacid and then extruding and calcining the mixture. Alkene oligomerization takes place in theviscous phosphoric acid layer on the support.

Reaction takes place by an ester mechanism (Figure 19.1b) and the catalytic behavior is complex[38]. Catalysis is influenced by the amount of water or oxygenates in the feed, which affects the hy-dration (acid strength) of the catalyst. Catalysis is also affected by the chain length of the alkene andoperating conditions. Shorter alkenes form stable phosphoric acid esters, and the alkyl phosphoricacid esters of the shorter alkenes become the primary reaction intermediates. This affects the prod-uct selectivity. The relative stabilities of the different alkyl phosphoric acid esters also determineintramolecular rearrangement and the likelihood that it will participate in an addition reaction.

In most refineries that employ SPA-catalyzed oligomerization, the primary product ishigh-octane olefinic motor-gasoline. The conversion is insensitive to the feed composition andoperating conditions (Table 19.2) [39–43]. Similar observations were reported for the oligomer-ization of Fischer–Tropsch-derived alkenes (Table 9.3). Different C2 –C4 alkenes converted atdifferent conditions result in olefinic motor-gasolines within a narrow octane number range,usually with a RON of 95–97 and MON of 81–82. This reflects the insensitivity of octane number


Table 19.2 Olefinic motor-gasoline produced from dif-ferent alkene feedstocks by phosphoric acid–catalyzedoligomerization.

Description Phosphoric acid oligomerization with different alkenes

Ethene [39] Propene [40] n-C4 [41] Mixed C4 [42] C2 –C4 [43] C3 –C4 [43]

Operating conditionsTemperature (◦C) 296–324 204–232 177 205–230 204–232 204Pressure (MPa) 3.6 1.7 0.8 5.5 1.4 0.7

PerformanceAlkene conversion (mass%) 65–73 90–95 72 75–95 67–94 72–89Naphtha selectivity (mass%) 59–71 100a 77 100 100a 100a

Olefinic motor-gasolineDensity at 15.6 ◦C (kg·m–3) 711 732 708 732 734 722RON 96 94.6 – 97.1 – –MON 82 81.0 81 84.0 82 81Reid vapor pressure (kPa) 45 69 – 14 59 76T90 distillation (◦C) – 193 – 128 198 207T95 distillation (◦C) 183 – ∼175 – – –FBP distillation (◦C) 203 226 – 198 249 237

aComplete product employed as olefinic motor-gasoline, at a 175 ◦C cut point; the naphtha to distillate ratio is around83 : 17.

to the branching structure in branched alkenes. The alkene oligomers produced by differentfeeds and at different conditions are not isostructural.

In a Fischer–Tropsch refinery, there is a need to produce a good quality alkylate-equivalentblending component for motor-gasoline. The considerable impact of feed composition and oper-ating conditions on SPA oligomerization becomes apparent when the olefinic motor-gasoline ishydrogenated (Table 19.3) [44]. There are some important aspects of SPA-catalyzed oligomeriza-tion to be kept in mind when oligomerizing syncrude to produce a hydrogenated motor-gasoline:

1) Hydrogenated RON and MON are very sensitive to the structure of the oligomer.2) The best quality hydrogenated motor-gasoline is produced by dimerization of butenes with as

little lighter and heavier alkenes in the feed as possible [44]. For mixed alkene feed materials,the hydrogenated octane numbers are much poorer.

3) It is not necessary to have a high isobutene content in the C4 feed to obtain a goodquality motor-gasoline, because SPA catalyzes butene skeletal isomerization through theester mechanism at low temperature [45]. Nevertheless, increasing the isobutene contentwill improve the quality of the motor-gasoline beyond which can be obtained by n-butenesonly.

4) Low temperature (160 ◦C or less) combined with a high level of phosphoric acid hydrationfor weaker acidity produces the best quality of motor-gasoline from butene oligomerization[46]. The same conditions also increase the naphtha to distillate ratio, thereby maximizingthe yield of high-quality product.

19.3 Catalysis 377

Table 19.3 Hydrogenated motor-gasoline quality producedby alkene oligomerization over SPA at 3.8 MPa and with acut point of 185 ◦C between naphtha and distillate. Catalysthydration was in the range 104–108% H3PO4.

Description Phosphoric acid oligomerization with different alkenes

C3-only C3 –C4 C3 –C5 C4-rich

Feed compositionPropene 56 57 26 26 0 0 0n-Butenes 0 17 32 32 51 51 61Isobutene 0 2 3 3 5 5 6C5 alkenes 0 0 5 5 4 4 4Alkanes 44 24 34 34 40 40 29

Operating conditionsTemperature (◦C) 200 191 180 160 180 160 160Space velocity (h–1) 1.3 – 0.7 0.7 0.7 0.7 2

PerformanceAlkene conversion (mass%) 85 – 81 75 93 82 63Naphtha selectivity (mass%) – – 82 82 78 83 91

Hydrogenated motor-gasolineDensity at 20 ◦C (kg·m–3) – – 720 720 719 720 719RON 47 56 70 74 81 86 90MON – – 76 78 83 88 89

5) Although motor-gasoline quality and yield are improved by higher catalyst hydration, ahydration level lower than 103% H2PO4 may lead to increased phosphoric acid loss due toleaching [30].

6) The oligomerization temperature should be decreased as the isobutene content in thefeed increases. Very high octane number alkylate-type products can be obtained whenoperating at low temperature (<140 ◦C) with an isobutene-rich feed: for example, the UOPInAlk-process (‘‘indirect alkylation’’) [7].

SPA-catalyzed oligomerization can also produce a highly branched kerosene range productwith very low freezing point (<–47 ◦C), which makes it a good blending material for jet fuel.Although some jet fuels can be produced by butene oligomerization, mixed C3 –C4 and C3 feedmaterials are better for jet fuel production.

Oligomerization over SPA mechanistically limits the carbon number distribution. The solubilityof heavier alkenes in the phosphoric acid phase is limited, and longer chain alkenes do not formsufficiently stable phosphoric acid esters [38]. Both factors contribute to limit the distillate yieldfrom SPA oligomerization. Although naphtha range alkenes can be converted over SPA, it is apoor catalyst for naphtha range alkene conversion for the reasons mentioned. It has been foundthat the >175 ◦C distillate yield from the oligomerization of a mixed alkene syncrude feed withnaphtha recycling is a function of the propene conversion only (Equation 19.1) [47].

Distillate yield = 0.7 · [propene converted] (19.1)


Complete conversion of C3-only feed into distillate is possible with recycle operation [25]. The dis-tillate thus produced is essentially a kerosene. Distillates produced by SPA oligomerization havelow cetane numbers (typically 25–30) but excellent cold-flow properties. Such distillates makegood jet fuel but poor diesel fuel. SPA-catalyzed oligomerization should therefore not be con-sidered for diesel fuel production, although the production of diesel fuel from Fischer–Tropschsyncrude is practised industrially (Section 9.4.1).

Although phosphoric acid is used as catalyst for ethene hydration (Section 17.4), SPA is not wellsuited for oligomerization of oxygenate-rich syncrude. Oxygenates typically present in syncrudeaffect SPA oligomerization in a number of ways:

1) Oxygenates inhibit oligomerization [48]. The oxygenates are not only stronger nucleophilesthan the alkenes, but are also more soluble in the phosphoric acid phase. Oxygenateconversion often leads to dehydration. The water thus formed increases the hydration stateof the SPA catalyst and reduces its acid strength.

2) Oxygenates participate in various acid-catalyzed side reactions [49].3) Oxygenates undermine the structural integrity of the SPA catalyst [48]. This is not a problem

at low oxygenate content but, as the oxygenate content increases and water release increases,phosphoric acid leaching increases (minor effect), and the mechanical strength of the catalystdecreases. In contact with Fischer–Tropsch naphtha, this leads to collapse of the catalystbed.

The average catalyst lifetime obtained during industrial operation of SPA with Fischer–Tropschsyncrude is 620 kg product per kilogram catalyst [50] and, with proper operation, lifetimes on theorder of 600–900 kg product per kilogram catalyst can be achieved [51]. In comparison to otherheterogeneous catalysts used for oligomerization, the lifetime of SPA is low. This is cited as oneof the primary reasons for selecting other catalyst types. Because SPA is a cheap catalyst, it isnot regenerated, which raises concern over the environmental impact of SPA. Yet, the processis, environmentally speaking, surprisingly benign. The catalyst is produced from natural silicasource without the need of templating molecules, and the spent catalyst can be disposed of in abeneficial way.

By neutralizing the spent SPA catalyst with ammonia, ammonium phosphate fertilizer can beproduced for agricultural use [52]. This is practised on a commercial scale in South Africa withthe spent SPA catalyst from the Fischer–Tropsch syncrude refinery. Spent SPA therefore doesnot generate solid waste.

19.3.2H-ZSM-5 Zeolite

Oligomerization over H-ZSM-5 (MFI-type zeolite) is usually conducted around 200–320 ◦Cand 5 MPa. At lower temperatures, H-ZSM-5 catalyzes oligomerization with limited cracking,resulting in the formation of oligomers that are multiples of the monomer [53]. Under suchconditions, the catalyst is rapidly deactivated by the accumulation of heavy oligomers that restrictaccess to the catalyst. By operating at a sufficiently high temperature, oligomerization andcracking reactions equilibrate and are thermodynamically controlled. This has some importantconsequences for oligomerization:

19.3 Catalysis 379

1) The carbon number distribution becomes equilibrated and is not sensitive to that of thefeed. It has been shown that feed materials ranging from C2 to C10 alkenes all result inoligomerization products with similar carbon number distribution [17].

2) Extraction of chemicals from a feed to an oligomerization unit will not affect the yield ofthe different product fractions, because the carbon number distribution is equilibrated. Thisprinciple was demonstrated by a study to ascertain the impact of 1-hexene extraction fromthe HTFT feed to an H-ZSM-5 oligomerization unit [54].

3) Although the carbon number distribution is equilibrated, it is a limited ‘‘equilibrium,’’ whichdoes not extend to the alkene isomers. The feed insensitivity with respect to carbon numberdistribution does not imply insensitivity with respect to isomer distribution. The impact offeed composition, catalyst age, and operating conditions on motor-gasoline quality can besignificant [20].

4) The naphtha to distillate ratio of the product can be controlled by the operating conditions.There are bounds to the control though. At high pressure, the reaction may again becomekinetically constrained and the heavy oligomers predicted from thermodynamic control maynot be obtained [55]. At high temperature, hydrogen transfer reactions may take place,leading to the formation of aromatics and alkanes. These reactions become signification attemperatures above 300 ◦C. This can be advantageous for jet fuel production.

5) A wide range of feed materials can be employed for oligomerization, and oligomerizationover H-ZSM-5 is not restricted to gaseous alkenes. In fact, it was a specific design intentof the ‘‘Conversion of Olefins to Distillate’’ (COD) process to convert gaseous and naphtharange alkenes in Fischer–Tropsch syncrude [18].

Oligomerization takes place by classic Brønsted acid catalysis (Figure 19.1), but the zeolitepore structure of H-ZSM-5 imposes a geometric constraint on oligomerization. The H-ZSM-5catalyst has an MFI zeolite structure with 0.51 × 0.55 nm and 0.53 × 0.56 nm channels [56].The pore-constrained geometry reduces the degree of branching of the oligomers by imposingtransition-state selectivity on the mechanism [53]. Reactant and transition-state selectivity effectsof H-ZSM-5 are also discussed in Section 21.4.1.

The lower degree of branching has a direct impact on the product quality (Table 19.4) [14,18–20, 57]. The more linear structure of the distillate range material is advantageous for dieselfuel production. The lower degree of branching improves the cetane number, while the presenceof at least some branching improves the cold-flow properties of the diesel fuel. The olefinicmotor-gasoline properties is not as good as that obtained by SPA oligomerization (Table 19.2), butas is explained, the lower degree of branching is only partly to blame for the poorer octane number.

The octane number of the olefinic motor-gasoline seems extraordinarily low for a branchedolefinic product, even though the degree of branching is not very high. This is a consequence of thefeed selection. When a naphtha range syncrude is employed as feed material for oligomerization,the naphtha range alkanes are not converted and are retained in the naphtha fraction. Sincesyncrude contains mainly linear alkanes, the resulting olefinic naphtha range product has a loweroctane number on account of the alkanes.

Despite the low coking propensity of H-ZSM-5 catalysts, the catalyst has to be regeneratedevery three to six months by controlled coke burnoff. The catalyst lifetime extends over multiplecycles. With time on stream and as the catalyst ages over time, the strongest acidic sites aredeactivated first. This is beneficial, since it allows skeletal isomerization of the shorter chain


Table 19.4 Olefinic motor-gasoline and hydrogenateddiesel fuel properties obtained from the H-ZSM-5-catalyzedoligomerization of various alkene-rich feed materials.

Fuel properties MOGDprocess

COD process H-ZSM-5 pilot plant

FCC C3 –C4 HTFT C3 –C6 Propene HTFT C5 –C6 HTFT C7 –C9

Olefinic motor-gasolineDensity at 15.6 ◦C (kg·m–3) 730 – 738 – – –RON 92 81–85 85 80 76–80 66–67MON 79 74–75 75 – – –

Hydrogenated diesel fuelDensity at 15.6 ◦C (kg·m–3) 779 787a 801a – – –Cetane number 52–56 52–54 51 40 45–48 41–43Viscosity at 40 ◦C (cSt) 2.5 2.55 – – – –Cold filter plugging point (◦C) – <–35 – <–25 <–25 –Pour point (◦C) <–50 <–51 – – – –T90 distillation (◦C) 342 320 323 365 345–352b

FBP distillation (◦C) – 358 361 388 – –

aDensity at 20 ◦C.bT95 distillation.

alkenes to improve the octane number without cracking, and it reduces distillate cracking toimprove the distillate yield. Single-pass distillate yield is around 65%, which can be improved toaround 84% with naphtha recycle operation [14].

Oligomerization activity over H-ZSM-5 is reduced by water and oxygenates in the alkene feed[58]. Water and alcohols result in catalyst inhibition, and catalyst activity is restored when thewater and alcohols are removed from the feed. Carboxylic acids and ketones result in strongerinhibition as well as deactivation. This can be explained by the acid-catalyzed interconversionof carbonyls and carboxylic acids, combined with carbonyl aromatization (Figure 16.4). Whenalcohols are present in the mixture, the deactivating effect of carboxylic acids is suppressed [59].

In industrial practice, H-ZSM-5 oligomerization is applied with oxygenate-containing HTFTfeed despite some inhibition.

19.3.3Amorphous Silica–Alumina

Amorphous silica–alumina (ASA), like H-ZSM-5, is composed of SiO2 and Al2O3, but there area number of important differences that affect oligomerization. In order to differentiate these twocatalysts, let us look at the differences that are expected to affect oligomerization behavior:

1) ASA by definition does not have the crystalline nature of H-ZSM-5. Oligomerization is notsubject to the transition-state limitations imposed by a pore-constraining geometry. Theproducts should therefore be more branched than those obtained by H-ZSM-5.

19.3 Catalysis 381

2) The acid strength of ASA is lower than that of H-ZSM-5 with the same SiO2 to Al2O3

ratio. It has also been found that lower acid strength sites are in fact more effective foroligomerization than higher acid strength sites [60]. As a consequence, the activity of ASAmay not be lower, but the cracking propensity is likely to be lower and one would anticipatethat a heavier product can be produced at comparable operating conditions.

3) The ratio of hydrogen transfer to oligomerization activity of ASA is an order of magnitudehigher than that of H-ZSM-5 [61]. This implies that ASA is more prone to producealkanes, dienes, cycloalkanes, cycloalkenes, and aromatics. The density of the product fromASA-catalyzed oligomerization is likely to be higher than that from H-ZSM-5.

4) Oligomerization over ASA should take place by the standard Brønsted acid-catalyzed carboca-tion mechanism (Figure 19.1a). However, it has been reported that the ASA oligomerizationis cis-selective [62], and that the dimerization products from 1-butene have a higher blendingoctane number than those from 2-butenes [63]. There is consequently some macroscopicevidence that the oligomerization mechanism over ASA may involve some ester-like inter-mediates (Figure 19.1b).

The behavior predicted from catalysis fundamentals was indeed observed during the oligomer-ization of Fischer–Tropsch syncrude over ASA catalysts (Table 19.5) [64, 65]. The higher degreeof branching is reflected in the fuel properties, and the distillate in general was much heavier.The hydrogen transfer activity was quite high, and at 175 ◦C the alkane yield reached 5% andmonotonically increased with increasing temperature [64]. This reduced the distillate yield andmotor-gasoline quality by converting naphtha range alkenes into alkanes.

With Fischer–Tropsch feed materials that were low in oxygenates, catalyst cycle lengths of100–120 days could be obtained with a start-of-run temperature of 180 ◦C [64]. At similarstart-of-run conditions, feed materials containing 1–4% oxygenates resulted in cycle lengths of80–90 days [65]. Oligomerization activity could be completely restored at the end of each cycleby controlled oxidative regeneration. It was also recommended that the start-of-run temperatureof ASA catalysts should be reduced to 110 ◦C with Fischer–Tropsch-derived feeds. Under suchstart-up conditions, cycle lengths of eight months have been obtained with oxygenate-containingfeed [65]. ASA catalysts are tolerant to oxygenates, are able to perform partial deoxygenationof the feed, and are active for carbonyl and carboxylic acid interconversion at typical operatingconditions.

Oligomerization over ASA competes with H-ZSM-5 as oxygenate-tolerant distillate producingtechnology. The distillate yield over ASA is lower, and the ultimate yield that can be obtained byrecycle operation varies from 63 to 76% depending on the feed [64]. The main difference is indistillate density, a quality that is lacking in Fischer–Tropsch distillate for diesel fuel production[66]. Although ASA-derived distillate has a much higher density, the cetane number is also muchlower, detracting from its use for the production of synthetic diesel fuel.

19.3.4Acidic Resin

The use of acidic resin catalysts for oligomerization is mainly aimed at the productionof high octane number alkylate-equivalent motor-gasoline by selective isobutene dimeriza-tion. There are many different types of acidic resins that can be employed, and sulfonated


Table 19.5 Fuel properties of products obtained by theoligomerization of different Fischer–Tropsch-derived feed ma-terials over amorphous silica–alumina.

Description ASA oligomerization with different alkene feeds

C3 –C6 HTFT C7 –C10 SPA C5-105 ◦C HTFT C5-105 ◦C HTFTcondensate oligomers alumina treated water washed

Operating conditionsTemperature (◦C) 140–235 180–210 180–230 225–280Pressure (MPa) 6 3.5 6 6Space velocity (h–1) 0.5 0.5 0.5 0.5

Feed oxygenate content (%) <0.01 0.05 0.5–0.8 0.5–3.6Distillate yield, >177 ◦C (%) 65–67 52–60 52–57 52–55Olefinic motor-gasoline

Density at 15.6 ◦C (kg·m–3) 707 708–714 – –RON 92–94 78–82 76 74MON 71–72 – – –Reid vapor pressure (kPa) 72 – – –Alkene content (g Br/100 g) 82–114 44–62 66 44

Hydrogenated motor-gasolineRON 76–85 – – –

Diesel fuel, hydrogenatedDensity at 15.6 ◦C (kg·m–3) 810–816a 809–810 810 810Cetane number 28–29 29–30 37 37Viscosity at 40 ◦C (cSt) 2.8–3.4a 3.5–3.6 2.5 2.8Cold filter plugging point (◦C) –15a <–20 <–20 –Pour point (◦C)T90 distillation (◦C) 329–348 363–364 347 346FBP distillation (◦C) 434–451 462–463 452 448

aUnhydrogenated properties.

styrene–divinylbenzene-based resins are commonly used in industrial oligomerization processes.The process is moderated by the addition of polar compounds, typically 2-methyl-2-propanol(tert-butanol), to maximize dimerization selectivity and limit heavy oligomer formation [67, 68].

Dimerization is conducted in the liquid phase at <100 ◦C. The product from selective isobutenedimerization can be hydrogenated to give a high-octane alkylate-equivalent product that is richin trimethylpentanes. The motor-gasoline from this type of indirect alkylation has an RON of99–101 and MON of 96–99 [8].

Oligomerization of a mixture of n-alkenes and branched alkenes results in selective conversionof the branched alkenes, with little n-alkene conversion taking place. Acidic resin catalysts are quitecapable of isomerizing and oligomerizing n-alkenes, but, under typical selective dimerizationconditions, these reactions are effectively suppressed. Acidic resin–catalyzed oligomerizationof syncrude can therefore be performed, and the products are anticipated to be typical ofthose formed by a Brønsted acid–catalyzed carbocation mechanism (Figure 19.1a). There

19.3 Catalysis 383

is consequently no specific benefit to be derived from using acidic resin catalysts for theoligomerization of syncrude.

19.3.5Homogeneous Nickel

Homogeneous catalysts can be designed to enable very selective conversion. Catalyst additionrate can also be varied to compensate for feed rate or feed poisons, and such processes can dealwell with variable load operation. The main drawback of homogeneously catalyzed processes isthe separation of the catalyst from the product.

The industrial homogeneous-catalyzed oligomerization processes make use of a nickel-basedZiegler-type catalyst system, and alkene oligomerization takes place by the organometallicinsertion mechanism (Figure 19.1d) [21]. Typical process conditions are 40–50 ◦C, 1–3 MPa, and1–5 h residence time [35]. A number of process variants are commercially available to producenaphtha range products (Table 19.6) [35, 69, 70]:

1) Dimersol E is used for the oligomerization of ethene and the light off-gas (C2 –C3 alkenes)from fluid catalytic cracking or steam cracking units to produce olefinic motor-gasoline.

2) Dimersol G is used for the oligomerization of propene and C3 –C4 alkene mixtures to produceolefinic motor-gasoline.

3) Dimersol X is used for butene dimerization to produce octenes with a low degree of branchingfor plasticiser alcohol manufacturing. The olefinic motor-gasoline quality obtained by thisprocess is low. Reduced catalyst consumption and improved selectivity can be achieved byemploying the Difasol modification of this process, which conducts oligomerization in abiphasic medium with ionic liquids [71, 72].

The organometallic catalyst is sensitive to any impurities that will complex with the nickel;among others, it is sensitive to dienes, alkynes, water, and sulfur, which should not exceed 5–10µg g−1. Attention must be paid to feed pretreatment when using any of these technologies withFischer–Tropsch-derived feed. The Dimersol E process was successfully employed for variableload ethene conversion from HTFT synthesis (Section 9.5.3). Unless there are significant variable

Table 19.6 Olefinic motor-gasoline properties obtained byhomogeneous nickel-catalyzed oligomerization of C2 –C4

alkenes.

Fuel properties Dimersol E Dimersol G Dimersol Xa

Alkene feed range C2 C3 –C4 C4

Olefinic motor-gasolineDensity at 15.6 ◦C (kg·m–3) 720 692–700 730RON 93–94 96–96.5 79MON 79–80 80–82 71Reid vapor pressure (kPa) – 48 5

aCalculated from composition data.


load constraints, there is no advantage in using homogeneous nickel oligomerization for olefinicmotor-gasoline production over the use of SPA oligomerization.

There is a competitive advantage to employ the Dimersol X and Difasol technologies forchemical production from Fischer-Tropsch-derived butenes. Fischer–Tropsch butenes have alow isobutene content and high 1-butene content, which increases selectivity to linear dimers.

19.3.6Thermal Oligomerization

Thermal oligomerization is a noncatalytic process following a free radical mechanism(Figure 19.1c). The mechanism is not skeletally isomerizing, and skeletally isomerized productsare formed only at higher temperatures (>400 ◦C) due to the combined effects of thermaloligomerization and thermal cracking. On the basis of its mechanism, some attributes ofthermal oligomerization can be inferred:

1) Thermal oligomerization is less efficient than catalytic oligomerization. This is one of thereasons for the displacement of thermal by catalytic processes in general.

2) Oxygenates are beneficial for thermal oligomerization on account of their lower homolyticbond dissociation energies [73]. The oxygenates will act as radical initiators.

3) The oligomerization product will have a low degree of branching. The degree of branchingwill reflect the carbon number and degree of branching of the feed material. Lighter andmore branched alkenes will lead to a more branched product, whereas a more linear andheavier alkene feed will lead to a less branched product.

4) The olefinic motor-gasoline quality from thermal oligomerization will be lower than thatfrom acid-catalyzed processes. Under thermal oligomerization conditions that do notpromote substantial aromatization, olefinic motor-gasoline from thermal oligomerizationhas an MON of 76–78 [74, 75].

5) The distillate range product will benefit from the low degree of branching. Even witha comparatively light C5 –C6 Fischer–Tropsch feed, the distillate obtained by thermaloligomerization had a cetane number of ≥54 after hydrogenation [76].

6) Thermal oligomerization should be able to produce good quality lubricant base oils fromlonger chain alkenes. The mechanism suggests that oligomerization of n-1-alkenes shouldyield PAO-like lubricant base oils. Since the alkenes in Fischer–Tropsch syncrude are mainlyn-1-olefins, it should make a good feedstock for lubricant base oil production. This was indeedfound in practice, with purified n-1-alkenes and cracked Fischer–Tropsch wax producinggood quality lubricating oils (Table 19.7) [77]. Although C5 –C6 HTFT naphtha is a poor feedmaterial for lubricating oil production, it was nevertheless possible to produce an oil with aviscosity index of 100 [76]. Equal or better results were reported with HTFT material fromthe Hydrocol process (Chapter 7) [77].

Thermal oligomerization of Fischer–Tropsch gaseous and naphtha range alkenes givesproducts that have characteristics similar to that of H-ZSM-5 oligomerization (Table 19.4).However, thermal oligomerization is not as efficient as catalytic oligomerization and it requireshigher operating temperatures than catalytic oligomerization. Attempts to reduce the operatingtemperature by making use of radical initiators such at di-tertiary-butyl peroxide failed because

19.4 Syncrude Process Technology 385

Table 19.7 Thermal oligomerization at around 320 ◦C and10 h residence time to produce lubricant base oils fromdifferent alkene-based feed materials.

Description Thermal oligomerization with different alkene feeds

Ethene 1- 1- 1- 1- 1- CrackedHexene Octene Decene Dodecene Hexadecene wax

Product yield (mass%)Atmospheric distillate + gases – 74 25 2 5 15 –Vacuum distillate – 16 30 56 45 30 –Lubricating oil 40 10 45 42 50 55a 34

Lubricating oil propertiesb

Density at 15.6 ◦C (kg·m–3) 837.8 829.4 833.8 835.3 829.4 836.8 –Viscosity index 115 85 128 143 ∼154 146 137Viscosity at 37.8 ◦C (cSt) 58.0 16.6 29.8 31.9 27.7 39.0 –Viscosity at 98.9 ◦C (cSt) 8.1 3.4 5.4 6.1 5.8 7.4 4.8Pour point (◦C) <–34 <–54 <–57 –29 –4 +24 –34

aThermal oligomerization at 350 ◦C.bLubricating oils were unhydrogenated.

of low initiator productivity [78]. Thermal oligomerization is therefore not recommended for theproduction of transportation fuels.

However, thermal oligomerization has potential as technology for the production of lubricantbase oils from straight-run Fischer–Tropsch syncrude. There is no need to remove oxygenates,which simplifies the process considerably. The oxygenates are actually beneficial. In this respect,thermal oligomerization has an advantage over catalytic routes for lubricant base oil production.Catalyst systems like AlCl3 [26] and BF3 [79], which were previously used for lubricant baseoil production from Fischer–Tropsch n-1-alkenes, are sensitive to water and oxygenates. Usingsuch catalytic technologies to produce lubricant base oils from syncrude requires extensive feedpretreatment.

19.4Syncrude Process Technology

There are three important differences in syncrude composition relative to crude oil that arepertinent to oligomerization: alkene availability is comparatively high; the dominant alkenespecies for each carbon number is the n-1-alkene; and there is usually oxygenates or waterpresent with the alkenes. Some guidelines are given below:

1) Oligomerization technologies that rely on a branched alkene feed are at a disadvantagewhen used in a Fischer–Tropsch refinery. The disadvantage may be overcome by employingskeletal isomerization (Chapter 18) or catalytic cracking (Chapter 21) technologies thatproduce branched alkenes. However, if such conversion processes must be installed inorder to make the oligomerization technology work, one should reconsider the choice


of oligomerization technology. Often, there will be a different but overall more efficientrefining pathway.

2) Catalysts and technologies that are very sensitive to oxygenates or water are at a disadvantagewhen applied in a Fischer–Tropsch refinery. In some instances, the alkenes are availablefrom a processing step that removes oxygenates and water (e.g., ethene purification)and then oxygenate tolerance is not an issue. However, if extensive feed pretreatment isrequired to make the oligomerization technology work, one should reconsider the choice ofoligomerization technology. Conversely, some oligomerization catalysts are also effectivedeoxygenation catalysts.

3) Some oligomerization technologies benefit from n-1-alkenes in the feed. Depending onthe refining objective, it is worthwhile considering such oligomerization technologies first.These technologies are oligomerization of butenes over SPA to produce motor-gasoline,oligomerization of butenes over homogeneous nickel to produce chemicals, and thermaloligomerization of heavier naphtha and distillate range straight-run syncrude to producelubricant base oils.

4) The carbon number distribution of the feed to an oligomerization process can be critical tothe quality of the product. In such cases, care should be taken to properly fractionate thefeed.

5) Alkene oligomerization is very exothermic, and heat management is crucial. Some catalystshave a threshold temperature where mechanistic changes take place. In such cases, it isimportant to keep the temperature within the desired mechanistic regime.

6) When the oligomerization selectivity is very temperature sensitive, or when it is importantto maximize dimerization selectivity, better control can be achieved by employing a reactivequench with dilute alkene feed (Figure 19.2). In this way, the reactive short-chain alkenefeed is always diluted in an alkane matrix and the alkene concentration is lower. It alsoallows similar-sized catalyst beds to be employed, which holds an advantage for mechanicalintegrity of the catalyst. Different bed heights place a disparate amount of mechanical stresson the catalyst at the bottom of the largest catalyst bed.

Oligomerization

Alkene-richfresh feed Alkane-rich

recycle

LPG orlight alkanes

Olefinicproduct

Figure 19.2 Reactive quenching with a dilute alkene feedto the top of the oligomerization reactor as a strategy toimprove heat management and dimerization selectivity.

19.4 Syncrude Process Technology 387

7) In cases where cycle length is restricted by pressure drop, it is often found that the topof the first catalyst bed is fouled. It may also happen that the cycle length is determinedby deactivation of the first catalyst bed. In these instances, it is advantageous, from anoperational point of view, to place the first oligomerization bed in a separate vessel. Oneshould also consider having two such beds in parallel for swing-mode operation. Metalcarboxylates in the syncrude feed and reactive species, such as oxygenates, may justify sucha configuration.

8) Oligomerization by SPA is a good technology for producing high octane number olefinicmotor-gasoline, high octane number alkylate-equivalent paraffinic motor-gasoline, andhigh-quality jet fuel. All these products can be produced in high yield. SPA-catalyzedoligomerization can also be employed to coprocess benzene to reduce refinery benzenelevels. For these applications, SPA-based oligomerization is a proven and recommendedtechnology with Fischer–Tropsch syncrude.

9) In refinery designs where SPA-catalyzed oligomerization is employed for alkenes spanningmore than one carbon number, one should consider the use of two separate oligomerizationunits. Butenes should be oligomerized separately from other alkenes if hydrogenatedmotor-gasoline is a final product.

10) Oligomerization by H-ZSM-5 is a good technology for producing high-quality distillaterange material in high yield. It has the further advantage that it is capable of processingoxygenate-containing Fischer–Tropsch naphtha, as well as gaseous alkenes. It can thereforedouble as an oligomerization and deoxygenation process. For these applications, it is arecommended Fischer–Tropsch oligomerization technology and it has been industriallyproven with Fischer–Tropsch syncrude.

11) When an oligomerization technology is employed with a naphtha range syncrude feed, onemust carefully consider the effect of the naphtha range alkanes on the final naphtha rangeproduct. The alkanes are not converted. When the product is a blending component formotor-gasoline, it may be worthwhile to fractionate the naphtha range product in such away that the fraction containing the feed alkanes is separated from the rest of the product(Figure 19.3). This alkane-containing fraction may then be separately refined to upgrade its

Syncrude

Heavy key:C7-alkanes

Light key:C7-alkenes

Heavy key:C7-alkanes

Light key:C6-alkenes

Oligomerization

Recycle

Product(to productfractionation)

Alkane-rich(to refinery)

Syncrude(to refinery)

Figure 19.3 Oligomerization with naphtha range feed,illustrating the separation of feed alkanes in the productfor separate downstream processing and partial recycle toaid heat management. This type of configuration is benefi-cial if olefinic motor-gasoline is one of the products fromoligomerization.


octane number. Ideally, one would like to restrict the alkanes in the feed to C6 and lightermaterial, but this is not always possible. Part of the alkane-containing product fraction mayalso be used as recycle to improve heat management.

12) Product fractionation after oligomerization can be employed to improve the properties ofthe products. For example, if both olefinic and paraffinic motor-gasoline are produced,a cut point at 105–110 ◦C will maximize the octane number that can be obtained byhydrogenating the overhead fraction. This cut point allows the trimethylpentenes to beseparated from the dimethylhexenes and less branched products. In an analogous way, it ispossible to find optimum cut points for lighter and heavier carbon number cuts. There aretwo learning points from this example. Firstly, the cut point affects quality, and, secondly,it may be worthwhile investing in sharper product fractionation after oligomerization.

13) Acid-catalyzed oligomerization processes are capable of carbonyl and carboxylic acid in-terconversion. If the syncrude feed contains either compound class, the material ofconstruction of the oligomerization reactor and downstream equipment should be selectedto cope with corrosive short chain carboxylic acids.

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Fischer-Tropsch Refining (DE KLERK:FISCHER-TROPSCH O-BK) || Oligomerization