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

465

23Chemical Technologies

23.1Introduction

It is not possible to comprehensively cover chemical refining technologies in a single chapter oreven in a single book. One only has to look at some of the standard references on chemicals suchas ethene [1], propene [2], benzene [3], and toluene and xylenes [4] to appreciate the breadth ofthe field. The books by Asinger on alkane and alkene conversion technologies [5, 6] are likewisefounts of information on conversion processes relevant to typical Fischer–Tropsch compounds.The scope of the present discussion is very limited by comparison and focuses on n-1-alkeneproduction and autoxidation. The extraction and synthesis of n-1-alkenes from syncrude is oneof the few instances where considerable effort has been expended to develop Fischer–Tropschspecific technologies. Autoxidation is discussed because of its wide general applicability tosyncrude and its potential use in alkane activation at moderate processing conditions.

Industrially, the refining of Fischer–Tropsch syncrude to chemicals can be divided into twomain categories:

1) Chemical extraction from Fischer–Tropsch syncrude. These are processes that were devisedto extract and purify compounds already present in the syncrude. Some of these technologiesare standard separations that may also be encountered in crude oil refineries and petro-chemical complexes. For example, the purification of ethene and propene, the purificationof ethanol from an ethanol–water mixture, and vacuum fractionation of wax are industriallypractised in crude-oil-based facilities too. In all of these examples, the separations are nottrivial, and the product matrix is not necessarily unique to syncrude. The extraction technolo-gies that are discussed are those that are unique to the separation of chemicals from a typicalsyncrude matrix. These technologies are all aimed at n-1-alkene extraction (Sections 23.2.1and 23.2.2).

2) Chemical synthesis from Fischer–Tropsch syncrude. Syncrude, like conventional crude oil,can form the basis for a petrochemical complex. In many instances, the technologies that areemployed require purified feed to meet specific product or technology requirements. Afterpurification, syncrude-derived feed may have different trace level compounds compared tocrude oil, but it does not require a different type of petrochemical technology. Dependingon the trace impurities, it may render syncrude incompatible with some of the chemicaltechnologies, but normally some benefit can be derived from the sulfur- and nitrogen-free

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

466 23 Chemical Technologies

nature of syncrude. Short of reviewing chemical production in general, there are fewsyncrude-specific conversion technologies. The discussion on conversion technologies forchemical production is limited to the conversion of 1-heptene into 1-octene (Section 23.2.3),the production of alcohols from distillate-range n-1-alkenes (Section 23.2.4), and autoxidation(Section 23.3). Alkene to alcohol conversion by hydroformylation is widely practised, andin a Fischer–Tropsch context, it is differentiated by the feed purification that is required.Autoxidation has been applied with syncrude since the first industrial Fischer–Tropschfacilities were constructed. It has a much wider applicability than just for chemical produc-tion. Autoxidation is considered as a refining technology with a broad range of potentialapplications in Fischer–Tropsch syncrude refining. Some of the technologies that werediscussed in previous chapters also have use in the production of chemicals. To name a few,the oxygenate conversions (Chapter 17), wax hydroisomerization (Section 18.4), and aromaticalkylation (Chapter 19), all double as both fuel refining and petrochemical technologies. Areview of these technologies is not repeated here.

23.2Production of n-1-Alkenes (Linear α-Olefins)

The technologies for the production of n-1-alkenes (linear α-olefins) from Fischer–Tropsch syn-crude are niche applications. The global market size of all even-numbered n-1-alkenes, excluding1-butene, is in the order of 4 million tons per year, and the main uses are comonomers forpolyethylene production, hydroformylation to produce plasticizer- and detergent-range alcoholsand polyalphaolefin (PAO) synthetic lubricants (Table 23.1) [7].

Most of the industrial production of n-1-alkenes is by ethene oligomerization, and it hastwo important consequences for the n-1-alkene market. Firstly, the market developed aroundeven-numbered n-1-alkenes only and although odd-numbered n-1-alkenes may be useful, suchcompounds are not produced by ethene oligomerization. Secondly, most (not all) etheneoligomerization processes yield a Poisson-like distribution of n-1-alkenes, rather than a specificcarbon number. The product values of the different fractions are therefore interdependent.

Table 23.1 Industrial applications of n-1-alkenes.

Carbon number range Applications of n-1-alkenes

C4 –C8 Polymers and polyethylene comonomersC6 –C8 Carboxylic acids and thiolsC6 –C10 Plasticizer alcoholsC10 –C12 Polyalphaolefins and lubricant additivesC10 –C12 Amine oxides and aminesC10 –C16 Detergent alcohols and nonionic surfactantsC10 –C18 SurfactantsC10 –C30C Oil field chemicalsC20 –C30C Wax replacements

23.2 Production of n-1-Alkenes (Linear α-Olefins) 467

The extraction and purification of n-1-alkenes from Fischer–Tropsch syncrude are different,because both odd- and even-numbered n-1-alkenes can in principle be extracted. However,attempts to market the odd-numbered 1-pentene met with limited success, because it was asingle supplier chemical [8]. Some specifics relevant to the application of n-1-alkene recoveryfrom Fischer–Tropsch syncrude are as follows:

1) All n-1-alkene extraction technology has been developed for recovery from high-temperatureFischer–Tropsch (HTFT) syncrude only. These technologies can be adapted for use withlow-temperature Fischer–Tropsch (LTFT) syncrudes, but neither the carbon number distri-bution nor the alkene content favors n-1-alkene recovery.

2) Unless there is a dramatic increase in the global market for n-1-alkenes, it is unlikely thatfuture Fischer–Tropsch facilities will be able to exploit the niche opportunity that existedin the Sasol Synfuels facility (Section 9.5). This does not imply that one cannot exploit thebeneficial properties of the n-1-alkenes within the refinery, for example, for lubricating baseoil production. It is just that there is a high risk of oversupply when the n-1-alkenes areconsidered final products.

3) Some of the main advantages of n-1-alkene extraction from syncrude are that the productioncost is not linked to the ethene price and that single carbon number production is possible.The latter advantage is being eroded by the development of single carbon number n-1-alkenetechnologies from ethene [9].

4) The complexity of extraction increases with increasing carbon number. The complexity isalso dependent on the origin of the syncrude fraction, with HTFT condensate fractions beingcleaner than HTFT light oil fractions (Figure 5.6).

5) There are different impurities in the n-1-alkenes purified from Fischer–Tropsch syncrudethan those in the n-1-alkenes produced from ethene. This may affect some applications.In this respect, the production of n-1-alkenes from ethene has an advantage, because theethene is more easily purified than syncrude and the ethene oligomerization productsare consequently free of nonhydrocarbon contaminants. This situation is analogous to theadvantage that indirect liquefaction technology has over direct liquefaction technology – itis easier to purify synthesis gas (or ethene in this case) than it is to purify a solvent-refinedliquid, catalytic liquefaction liquid, or pyrolysis oil (or syncrude in this case).

6) In LTFT refineries, the production of n-1-alkenes by thermal cracking (Chapter 21) may bea contender for HTFT n-1-alkene extraction. Although thermal cracking of LTFT syncrudeproduces a range of products as is produced by ethene oligomerization, it has the same feedcost advantage as HTFT syncrude over ethene.

23.2.1Extraction of 1-Pentene and 1-Hexene

The same process is employed for 1-pentene and 1-hexene extraction from Fischer–Tropschsyncrude. The commercial extraction process has been described in literature (Figure 23.1) [10].The subsequent description will use 1-hexene extraction as an example.

The first step in the process is to prepare a C6-cut that is rich in 1-hexene (not shown inFigure 23.1). The design of this step depends on the origin of the feed in the Fischer–Tropschgas loop (Table 23.2) [11]. When the material is derived from the HTFT condensate, the C6-cutcan be prepared by topping and tailing the feed. The condensate material is not oxygenate free,


HTFTC6-cut

Etherification

MethanolMethanolrecovery

C6-narrow cut Extractive distillation

Recycle of lean NMP

CycloalkenesOxygenates

1-Hexene

Alkanes

Light products

Heavyproducts

Figure 23.1 Extraction of 1-hexene from Fischer–Tropsch syncrude.

Table 23.2 Composition of typical HTFT C6-cut ma-terial obtained from different stages of cooling in theFischer–Tropsch gas loop.

Compound Composition of C6-cut (mass%)

HTFT condensate HTFT light oil

C5 and lighter 1.2 0.04-Methyl-1-pentene (4M1P) 9.2 3.43-Methyl-1-pentene (3M1P) 10.9 3.4Methylpentanes 4.5 5.42,3-Dimethyl-butene 0.8 0.74-Methyl-cis-2-pentene 0.2 0.02-Methyl-1-pentene (2M1P) 4.2 5.31-Hexene 55.7 68.7n-Hexane 6.5 2.02-Ethyl-1-butene (2E1B) 0.5 0.72- and 3-Hexene isomers 2.0 0.8Methyl-cyclopentenes 0.6 0.62-Methyl-2-pentene (2M2P) 0.8 0.1C7 and heavier 2.2 0.1Oxygenates 0.7 8.8

but it contains much less oxygenates than the material in the HTFT light oil. When 1-hexene is

extracted from light oil, which contains percentage level oxygenates, it is necessary to first remove

the oxygenates in the C6-cut. The next step is an etherification process, which benefits from

an oxygenate-free feed. The oxygenates inhibit etherification and double bond isomerization,

thereby reducing the effectiveness of this unit [12]. Even though the HTFT condensate is not

subject to oxygenate removal in industrial practice, there is a benefit to be derived from doing so.


N OMelting point = −24 °CBoiling point = 202 °C

Density = 1027 kg m−3

Figure 23.2 Structure of N-Methyl-2-pyrrolidone (NMP).

On account of their polar nature, oxygenates have a strong affinity for the polar extractantN-methyl-2-pyrrolidone (NMP) (Figure 23.2) [13]. NMP also has a high density, which makes itideal for liquid–liquid extraction applications. NMP can therefore be considered as a solvent forextractive removal of oxygenates from HTFT naphtha, and it is indeed industrially applied inn-1-alkene purification [10, 14, 15]. The main drawback of NMP as solvent for Fischer–Tropschapplications is its basic nitrogen. In syncrude fractions that contain carboxylic acids, subsequentrecovery of the NMP is very difficult [14]. Although this is not a problem in 1-hexene purification,it is an issue in 1-octene purification (Section 23.2.2).

The C6-cut is already rich in 1-hexene, but for comonomer applications, the purity mustbe very high. The alkene isomers that are close boiling to the n-1-alkene present a separationchallenge, because these isomers cannot be separated by temperature (distillation) or polarity(extraction). For n-1-alkene extraction, the close boiling alkene isomers (Table 23.3) [16, 17]must be reactively converted to isomers that are less close boiling. This is accomplished byetherification with methanol (Section 17.3). During the etherification reaction, alkenes with aCDC on a tertiary carbon are selectively converted into the corresponding methyl ether or aredouble bond isomerized. The combined conversion to ethers and double bond isomers thatare not close boiling to the n-1-alkene is quite high. In the case of 1-hexene, the conversion of2-ethyl-1-butene can be >99% and the conversion of 2-methyl-1-pentene can be 92–96% [10, 11].The conversion is equilibrium limited.

The product from the etherification reactor is separated to produce a light product, a heavy prod-uct and a C6-narrow cut for further purification. This separation can be performed in two distilla-tion columns or in a single divided wall column. The distillation takes place at near-atmosphericconditions [10]. The methanol is recovered and recycled to the etherification reactor.

The C6-narrow cut contains around 94% 1-hexene and further purification is performedby NMP extraction, using an NMP:C6 mass ratio of 15 : 1 [10]. The purpose of the extractivedistillation is to remove the close boiling alkane isomers, and other compounds that are stillpresent in small concentrations. Three extractive distillation columns are employed in series.

Table 23.3 Hydrocarbon isomers present in HTFT syncrudethat are close boiling to 1-pentene and 1-hexene, respectively.

C5 hydrocarbons Boiling point (◦C) C6 hydrocarbons Boiling point (◦C)

3-Methyl-1-butene 20.0 2-Methylpentane 60.32-Methylbutane 27.8 2-Methyl-1-pentene 60.71-Pentenea 30.0 3-Methylpentane 60.32-Methyl-1-butene 31.2 1-Hexene 60.3n-Pentane 36.1 2-Ethyl-1-butene 64.7cis-2-Pentene 36.9 cis-3-Hexene 66.4

aUnderlined entries indicate the target compound for purification.


Most of the alkanes are recovered overhead from the first column. The purified 1-hexene, about98.5% pure, is recovered overhead from the second column. The third column serves as asolvent stripper, where the cycloalkenes and oxygenates that contaminate and build up in theNMP are removed from the NMP solvent. The NMP is then recycled for extraction.

23.2.2Extraction of 1-Octene

The complexity of syncrude increases with carbon number. This is partly due to the proliferationof isomers that is possible for each carbon number. A more important aspect from an extractionpoint of view is oxygenate partitioning (Section 5.3.3).

The carbon number difference between hydrocarbons and oxygenates in the same boilingrange is around 2–6 depending on the oxygenate class. The C7 –C10 heavy naphtha fraction ofthe HTFT light oil is just heavy enough so that there are C3 –C6 oxygenates partitioning into theoil phase that are coboiling with the heavy naphtha. The concentration of oxygenates is in thesame order of magnitude as that of the C5 –C6 light naphtha from the light oil (Table 23.2), butthe heavy naphtha also contains carboxylic acids. This presents a problem for NMP extraction,which requires the carboxylic acids to be removed first.

One approach to acid removal is to neutralize the carboxylic acids with an appropriatewater-soluble base and then remove the carboxylate salts in the aqueous phase. This methodologywas applied in the design of the first 1-octene extraction unit at Sasol Synfuels (Figure 23.3)[14]. The neutralization process employed a potassium carbonate (K2CO3) in water solution(Equation 23.1).

2 RCOOH C K2CO3(aq) C 2 RCOO�KC(aq) C H2O C CO2 (23.1)

The design intent was for neutralization to be limited to the potassium carboxylate and potassiumbicarbonate (KHCO3), which could be regenerated in a thermal regenerator [14]. The bicarbonateis still a base and as indicated by Equation 23.1, the reaction can proceed to completion, convertingthe carbonate into water and carbon dioxide.

It has been reported that the industrial thermal regenerator used for this process was prone tosludge buildup, which had to be disposed of [14]. This resulted in a loss of potassium from thesystem.

Another problem with the K2CO3 neutralization approach is that the potassium carboxylatesare surface active. The carboxylates in the carbon number range of the butanoates areamphophilic (i.e., they have hydrophilic and hydrophobic character). The phase behavior ofC2 –C6 potassium carboxylates in water–alcohol mixtures indicates that the carboxylates maybe found in both the aqueous and organic phases [18]. Separation by water extraction isnot necessarily quantitative and the potassium carboxylates in the organic phase may causedownstream problems, as was indeed industrially found (Section 9.5.4) [18, 19].

Once the carboxylic acids are removed from the light oil, a C8-cut can be prepared (Figure 23.3).Topping and tailing of the light oil takes place in a divided wall distillation column, instead oftwo columns, and produces a C8-cut. The C8-cut contains hydrocarbons and nonacid oxygenates.The oxygenate compounds are removed by extractive distillation using NMP as a heavy-boilingsolvent. The oxygenates can then be separated from the NMP and the NMP is recycled.


HTFTnaphtha

Superfractionation Extractive distillation

Recycle of NMP

Cycloalkenes

1-Octene

Methylheptene

n-Octane

C9 and heavier

C7 and lighter

Thermalregenerator

K2CO3 (aq)

C7–C10

Recycleof NMP

Extractivedistillation

Oxygenate free C8-cut

Oxygenates

Figure 23.3 Extraction of 1-octene from Fischer–Tropsch syncrude, original Train I design.

The oxygenate-free C8-cut is then processed in two superfractionation columns to remove closeboiling lighter and heavier compounds from the 1-octene (Table 23.4) [16, 17]. The C8-narrowcut thus produced is rich in 1-octene. The C8-narrow cut from superfractionation is furtherpurified by extractive distillation with NMP to remove the cycloalkenes that are still present inthe 1-octene. The cycloalkenes are undesirable when 1-octene is employed as comonomer [14].The final 1-octene purity is better than 97%.

A different approach has been suggested to remove the oxygenates from the Fischer–Tropschlight oil, without first removing the carboxylic acids. It was found that all of the oxygenates couldbe removed from the C8-cut by azeotropic distillation with ethanol and water [15]. By doing so,the two-step oxygenate removal can be consolidated in a single step, leading to a more efficientdesign. It also avoids potential problems with metal carboxylates.

The azeotropic distillation methodology was followed in the design of the second 1-octeneextraction unit at Sasol Synfuels (Figure 23.4) [14, 20]. The separation of the oxygenate-freeC8-cut by superfractionation and NMP extraction is analogous to that described before. The maindifference between this part of the design of the first (Figure 23.3) and second (Figure 23.4)1-octene extraction units, is heat integration and optimization. An overall energy saving of 30%was achieved. This helped to offset the additional processing cost associated with the productionof a higher quality 1-octene product, and the net overall energy consumption of the second1-octene extraction unit is 15% higher than that of the first [14].


Table 23.4 Some of the close boiling hydrocarbon isomers to1-octene that may affect 1-octene extraction.

C8 hydrocarbons Boiling point (◦C)

2-Methylheptane 117.62-Methyl-1-heptene 118.23-Methylheptane 119.02-Ethyl-1-hexene 120.01-Octenea 121.31,2,3-Trimethylcyclopenteneb 121.6trans-4-Octene 122.3cis-4-Octene 122.52-Methyl-2-heptene 122.6cis-3-Octene 122.9trans-3-Octene 123.3cis-2-Octene 125.0trans-2-Octene 125.6n-Octane 125.7

aUnderlined entry indicates the target compound for purification.bThere are various C3-alkylcyclopentanes close boiling to 1-octene.

HTFTnaphtha

Superfractionation Extractive distillation

Recycle of NMP

Cycloalkenes

1-Octene

Methylheptene

n-Octane

C9 and heavier

C7 and lighter

C7–C10 Azeotropicdistillation

Oxygenate free C8-cut

Oxygenates

C8-cut

Figure 23.4 Extraction of 1-octene from Fischer–Tropsch syncrude, Train II design.


23.2.3Production of 1-Octene from 1-Heptene

The process to produce 1-octene from Fischer–Tropsch-derived 1-heptene consists of five steps(Figure 23.5) [21].

The first step is to extract the 1-heptene from the Fischer–Tropsch syncrude and purify it toa sufficient degree so that it is compatible with the hydroformylation technology. The extractionprocess is similar to that described for 1-octene extraction (Section 23.2.2), but it is not necessaryto produce such a narrow 1-heptene cut, since purification of the final product is performed inthe last step of the process.

The hydroformylation reaction is conducted in the liquid phase with a homogeneous catalyst,usually a Co- or Rh-based organometallic catalyst (Section 16.3.6). Being a homogeneous catalyticprocess, catalyst recovery and recycling are important to the design and more so when anexpensive catalyst is employed. Hydroformylation of 1-heptene to octanal (Equation 23.2) is thefirst of three conversion steps, and it is followed by hydrogenation of the octanal to 1-octanol(Equation 23.3) and then the dehydration of the 1-octanol to 1-octene (Equation 23.4).

CH3(CH2)4–CHDCH2 C CO C H2 ! CH3(CH2)4–CH2–CH2–CHDO (23.2)

CH3(CH2)5–CH2–CHDO + H2 ! CH3(CH2)5–CH2–CH2–OH (23.3)

CH3(CH2)5–CH2–CH2–OH ! CH3(CH2)5–CHDCH2 C H2O (23.4)

Since the process involves a stepwise synthesis, the efficiency of the overall process is multiplica-tive. One advantage of the many reaction steps is that each conversion step enables the efficientremoval of some impurities [21]. For example, better separation of close boiling isomers to1-octene can be achieved before the final dehydration of 1-octanol to 1-octene. The final productpolishing step requires only the separation of some internal and branched alkenes producedduring the dehydration reaction.

This configuration places an additional selectivity burden on the dehydration reaction andspecifically on the avoidance of double bond isomerization. The dehydration reaction andits associated catalysis make it difficult to avoid double bond isomerization at high per passconversion (Section 17.2.2). Although alumina is the preferred dehydration catalyst [22], it iscapable of complex reaction chemistry [23].

Extraction of1-heptene

1-Octenepurification

Hydroformylation

Alcoholdehydration

HTFTnaphtha

Naphtha(to refinery)

CO + H2

SelectivehydrogenationH2

1-OcteneH2O

Figure 23.5 Conversion of 1-heptene from Fischer–Tropsch syncrude into 1-octene.


23.2.4Distillate-Range n-1-Alkene Extraction

The same basic principles that have been applied to the extraction of n-1-alkenes from heavynaphtha (Section 23.2.2) can also be applied to distillate-range materials. The Fischer–Tropschsyncrude matrix is more complex and obtaining a pure n-1-alkene is more difficult.

In some applications, a partial purification is all that is required. For example, the productionof detergent-range alcohols from Fischer–Tropsch distillate requires alkene purification but notn-1-alkene separation. Hydroformylation of the mixed alkene product, followed by aldehyde toalcohol hydrogenation (similar to what is shown in Figure 23.5), yields an alcohol product thatreflects the degree of branching of the Fischer–Tropsch syncrude. Specifically, the Rh-catalyzedhydroformylation of HTFT distillate-range alkenes, as industrially applied at Sasol Synfuels(Section 9.5), produces a product with little branching in the 2- and 3-position relative to thealcohol functionality [20].

23.3Autoxidation

Autoxidation refers to conversion processes involving air oxidation without a catalyst. It is differentfrom selective catalytic oxidation reactions that require the presence of a catalyst to proceed.Selective catalytic oxidation is performed in many petrochemical processes [24, 25]. Although themechanism of some catalytic oxidation processes involves free radical chemistry, autoxidationprocesses proceed exclusively by free radical chemistry and autoxidation are inherently lessselective than catalytic oxidation. The main advantages of autoxidation over catalytic oxidationare process simplicity, low sensitivity to contaminants and a lower risk of complete oxidation toCO2. Selective oxidation in general, which includes autoxidation, has the further advantage thatit employs air as O2 source, which is cheap and readily available and does not require very severeoperating conditions.

There are different autoxidation regimes (Section 23.3.1), which have very different industrialapplications. The regimes that are of interest in Fischer–Tropsch refining are autoxidationreactions taking place at moderate to low temperature, where some control over the productselectivity can be achieved. Autoxidation naturally takes place at ambient conditions too and isresponsible for product degradation during storage (Sections 14.3.7 and 15.3.8) [26, 27]. This typeof autoxidation is usually undesirable, although the chemistry is the same. This again highlightsthe importance of selectivity and control in autoxidation.

The challenge of selectivity in autoxidation of hydrocarbons has been outlined by Emanuel[28], who pointed out that selectivity control is possible through, among others, limited initiation,differences in gas-phase and liquid-phase kinetics, and exploitation of the sequential nature ofthe reaction network. Selectivity control and limitations to selectivity control are explored duringthe discussion on reaction chemistry (Section 23.3.2).

Fundamentally, there is synergy between autoxidation and Fischer–Tropsch refining. TheFischer–Tropsch syncrude already contains oxygenates. Introduction of additional oxygenatefunctionality into syncrude through autoxidation does not affect the refining complexity. Potentialrefining applications of autoxidation in general, have already been discussed (Section 16.3.7).

23.3 Autoxidation 475

Fischer–Tropsch syncrude-specific refining applications of autoxidation by air that can beconsidered are as follows:

1) Improving the quality of diesel fuel. Surface-active polar compounds provide boundary layerlubricity, and oxygen is the most effective of the heteroatoms in providing such lubricity(Section 15.3.4). Mild autoxidation can introduce alcohol and carbonyl functionalities to pro-vide lubricity. Although carboxylic acids are more effective [29], the production of carboxylicacids through autoxidation involves C–C scission and runs the risk of producing corrosiveshort-chain carboxylic acids. In addition to boundary layer lubricity, the hydroperoxidesformed during autoxidation also provide cetane number improvement [30]. The latter istypically not required for Fischer–Tropsch-derived diesel fuel.

2) Improving the density of diesel fuel. In countries where the diesel fuel specification includesa lower density limit for better engine emission control, it becomes difficult to produceon-specification diesel fuel from Fischer-Tropsch syncrude in high yield [31]. One of thestrategies for improving diesel fuel density is autoxidation, which not only provides lubricityand cetane number as mentioned before, but also results in increased density (Table 23.5)[32].

3) Lubricant base oil production. Thermal oligomerization of distillate-range Fischer–Tropschsyncrude fractions can produce good quality lubricant base oil (Section 19.3.6). This typeof conversion requires high temperature. Attempts to perform thermal oligomerization atlower temperatures were abandoned because of low productivity of the peroxide-initiator,which made the process uneconomical [33]. Autoxidation can introduce hydroperoxides intosyncrude at a lower cost than traditional initiators such as di-tert-butyl peroxide (DTBP).Combined autoxidation and thermal oligomerization is an alternative pathway for lubricantbase oil production from straight-run Fischer–Tropsch distillates that are rich in n-1-alkenes.The reaction proceeds at low temperature and a 10% yield of peroxide oil has been obtainedwithin 2 h during autoxidation of 1-hexadecene at 130 ŽC [34]. In fact, this type of oxidativeoligomerization is well known for the conversion of unsaturated bio-oils by air blowing intoprotective coatings, oilcloth, linoleum, and ink [35].

Table 23.5 Change in Fischer–Tropsch distillate propertiesafter autoxidation at 160 ŽC for 90 min with oxygen.

Fuel property Fischer–Tropsch-derived distillate

Unoxidized Autoxidized

Distillate yield from feed (mass%) – 95Density (kgÐm�3) 773.2 811.5Oxygen content (mass%) 0.2 5.4Lubricity, HFRR wear scar (µm) 570 365Distillation profile (ŽC)

T10 238 237T50 304 304T90 352 364


AutoxidationAlkanesSelective

hydrogenation

Alkane to alcoholAlkane to carbonyl

Carbonyl toalcohol

Dehydration

Alcohol toalkene

Alcohols

Alkenes

Figure 23.6 Conversion of alkanes by selective autoxidationand hydrogenation into alcohols or by subsequent dehydra-tion into alkenes.

4) Alkanes can be activated by autoxidation at low temperature. Once the alkanes have beenconverted into oxygenates, further conversion is easier. A conceptually simple process hasbeen proposed to employ selective autoxidation for the production of alcohols and alkenes(Figure 23.6) [36]. There are similarities with the 1-heptene to 1-octene conversion process(Figure 23.5), with autoxidation replacing n-1-alkene extraction and hydroformylation andthe product being an internal alcohol or alkene. The dehydration step can also be replacedwith another acid-catalyzed conversion, for example, aromatic alkylation. If the last stepis omitted, alcohols are obtained as final products. Depending on the chain length of thestarting material, the alcohols can be employed as solvents, plasticizers, or detergents. Notall solvent applications require 1-alcohols.

5) Carboxylic acid (fatty acid) production. Prolonged autoxidation of syncrude, or autoxidationunder more severe conditions, yields carboxylic acids as products. This type of conversionhas been extensively used in the past for the production of carboxylic acids and carboxylatesoaps from Fischer–Tropsch slack wax (Chapter 6) [5]. Changes in the soap market after theSecond World War made autoxidation of slack wax for soap manufacturing uneconomicaland it was discontinued [37]. However, the same process may also be applied to producelighter carboxylic acids. Ethanoic acid (acetic acid) and propanoic acid (propionic acid) arevaluable commodity chemicals and these light carboxylic acids are more easily recoveredfrom syncrude autoxidation products than by extraction from the Fischer–Tropsch aqueousproduct.

6) Wax oxidation to produce oxidized waxes. There are many applications of oxidized waxproducts, which include emulsifiers, polishes, ink carriers, and surface coating for variousapplications. This process is industrially practised with Fischer–Tropsch waxes [38, 39], andis discussed in more detail (Section 23.3.3).

7) Autoxidation for the conversion of refinery waste products. Heavy or solid-rich waste productscan be autoxidized to produce lighter oxygenates that can be recovered as products. Becauseautoxidation is noncatalytic, contaminants that would otherwise foul catalytic systems do notaffect autoxidation. Even when oxidation inhibitors are present, it is possible to break downorganic compounds by prolonged autoxidation. Autoxidation can also be employed duringwastewater treatment to reduce the chemical oxygen demand by preoxidizing the organicspecies. This has the added benefit of making the material more susceptible to conversionby microorganisms.


23.3.1Autoxidation Regimes

The three main variables that affect product selectivity within any autoxidation application aretemperature, oxygen availability, and duration of exposure. Different autoxidation regimes canbe identified for different applications. On the two extremes are the very-high-temperatureoxidation processes that are employed for total or partial combustion, as is found duringsynthesis gas production (Chapter 3) and the ambient-temperature autoxidation that affectsproduct degradation during storage. Of practical interest in refining are the autoxidation regimesin the temperature range 75–475 ŽC:

1) High temperature and low oxygen availability. High-temperature (>300 ŽC) thermal con-version does not require O2. These conditions are typical of thermal residue upgradingtechnologies such as visbreaking (Section 16.6.2) and coking (Section 16.6.4). Toward thelower end of this temperature range, radical recombination reaction, or retrograde conden-sation, dominates. In the absence of a hydrogen-donor solvent, chain degradation takes placewith the production of refractory aromatic products and coke production is promoted. Whena suitable hydrogen-donor solvent is available, these conditions can be employed for directliquefaction. For example, the direct thermal liquefaction of coal is usually conducted in thetemperature range 425–460 ŽC [40]. The rate of conversion is increased by the introductionof some O2 to produce oxygenates. The oxygenates have lower homolytic bond dissociationenergies (Tables 21.2 and 21.3) and are more readily thermally cracked. This autoxidationregime has little value in the Fischer–Tropsch refinery, although it may have use in thecoproduction of products from the raw material used for synthesis gas generation.

2) High temperature and high oxygen availability. At temperatures >300 ŽC and in the presenceof sufficient O2, a carbon-based material is rapidly oxidatively degraded. Under theseconditions, carboxylic acids and light hydrocarbons are readily produced by a combination ofoxidation and thermal cracking. In the absence of a suitable diluent, retrograde condensationresults in the formation of more refractory aromatic products, but with sufficient exposuretime these compounds may ultimately also be oxidatively degraded. This is the type ofautoxidation employed in gasification and reforming.

3) Medium temperature (180–300 ◦C). This autoxidation regime is used in processes suchas asphalt oxidation to decrease its softening point (harden the asphalt) [41]. The asphaltconversion is regulated by a combination of oxygen availability and exposure time. Theautoxidation strips out some lighter material and promotes retrograde condensation of themore reactive, oxidized asphalt to produce a more refractory product. In this way, muchof the oxygen that has been incorporated during autoxidation is again lost through chaindegradation and the production of lighter oxygenates, such as carboxylic acids. To employ thisautoxidation regime for oxygenate production, retrograde condensation must be suppressedor prevented when thermal cracking becomes significant or when the feed is alkene rich.Alkene-rich feed autoxidation in this temperature regime leads to a rapid increase in viscositydue to oxidative oligomerization [35].

4) Moderate temperature (150–180 ◦C). The moderate-temperature autoxidation regime isemployed for the production of oxidized waxes and other oxygen-functionalized hydrocarbons(Section 23.3.3). Selectivity can be regulated somewhat by oxygen availability and exposuretime, but the temperature is generally too high to eliminate the production of secondary


oxidation products. Some chain degradation takes place and carboxylic acids and esters areusually produced in abundance, but the temperature is too low for thermal cracking andretrograde reactions are not significant in alkane-based feed materials. Alkene-based feedmaterials undergo oxidative oligomerization.

5) Low temperature (<150 ◦C). Low-temperature autoxidation with low oxygen availabilityis selective toward primary oxidation products (alcohols, carbonyls, and hydroperoxides).Although oxygen is incorporated into the product, the conditions are such that little chaindegradation takes place. Operating in this regime can be very selective, but with continuedexposure secondary reactions eventually take place. By limiting the exposure time and theoverall extent of conversion, the probability of secondary reactions can be reduced. Whenoxygen availability is increased in combination with exposure time, secondary oxidationproducts (acids and esters) are produced along with some chain degradation. However, thetemperature is too low for thermal cracking, and with alkane-rich feed materials, retrogradereactions are not significant. For alkene-rich materials, the upper temperature threshold oflow-temperature autoxidation is 130 ŽC, not 150 ŽC, and below this temperature there is ameasurable induction period before oxidative oligomerization takes place [42].

23.3.2Reaction Chemistry

Autoxidation takes place by a free radical mechanism. As is the case with all free radical reactions,there are three distinct reaction steps: initiation, propagation, and termination. In the case ofautoxidation, initiation is through the action of molecular oxygen (O2) and it can take place evenat ambient conditions, albeit very slowly. Initiation takes place when molecular oxygen abstractsa hydrogen atom from the hydrocarbon (Equation 23.5).

RH C O2 ! Rž C žOOH (23.5)

This is a slow reaction even at moderate temperatures (150–180 ŽC). Two radicals are generated.The concentration of the hydroperoxy radical (žOOH) remains low and it rapidly abstracts ahydrogen to form hydrogen peroxide (Equation 23.6).

RH C žOOH ! žR C H2O2 (23.6)

The alkyl radical (Rž) that is formed by initiation (Equation 23.5) and the subsequent hydroperoxyradical reaction (Equation 23.6) determine the rate of O2 uptake. If this radical is stable, O2

uptake is slow, since further O2 uptake is autocatalytic (Equations 23.7–23.9).

Rž C O2 ! ROOž (23.7)

RH C ROOž ! Rž C ROOH (23.8)0RCHDCH2 C ROOž ! 0R(Cž)H–CH2–OOR (23.9)

The activation energy for the O2 uptake reaction (Equation 23.7) is very small and the O2 uptakereaction is generally fast [27]. The main products from initiation are, therefore, the alkylperoxyradical (ROOž) and the hydroperoxide. The concentration of the alkylperoxy radical is determinedby the extent of the initiation reaction (Equation 23.5). This is a slow reaction, and despite theimportance of the alkylperoxy radical, the alkylperoxy radical is not the species that governs


Con

cent

ratio

n

O2

cons

umpt

ion

Time

O2 conversion

Induction period

Organicperoxides

Oxygenateproducts

Figure 23.7 Progress of autoxidation over time, in-dicating the induction period and buildup of organicperoxides, whereafter the rates of O2 consumption andproduct formation rapidly increase.

autoxidation. Autoxidation is governed by the hydroperoxides (Equation 23.8) and alkylperoxides(Equation 23.9). During the induction period, as the autoxidation progresses, the concentrationof hydroperoxides and alkylperoxides gradually increases (Figure 23.7). During the inductionperiod, before the rate of autoxidation becomes significant, some oygenates (mainly ketones andalcohols) are formed as final products. The rate of O2 consumption then rapidly increases as theperoxides decompose and increase the concentration of free radicals in the reaction mixture.

The chemistry of the organic peroxides is extensive and it is described in the monumentalwork edited by Swern [43–45]. The organic peroxides all contain the labile peroxide bond (O–O).Decomposition of the peroxide bond, by either homolytic bond dissociation (Equation 23.10) orother processes, such as radical-induced dissociation (Equation 23.11), determines the rate andnature of the products formed by autoxidation.

ROOH ! ROž C žOH (23.10)

ROOH C ROOž ! ROH C ROž C O2 (23.11)

At temperatures <180 ŽC, the contribution of thermally induced dissociation is not as significantas induced dissociation, either homolytically or otherwise. To quote Haitt [46]: ‘‘. . .the evidenceappears to offer incontrovertible proof that the homolysis of RO2H can be assisted by olefins,aldehydes and ketones, carboxylic acids, alcohols, amines and amine salts, sulfur compounds,bromide ions, aralkanes, or another molecule of hydroperoxide; in other words, by practicallyanything.’’

There are two important observations relating to autoxidation reaction chemistry of initiationand autocatalytic O2 consumption. Firstly, the contribution of the induction period to the overallproduct yield is minimal. The conditions during this time may be harsher to speed up theautoxidation. The induction period can also be practically eliminated by the use of small amountsof metal ions [47, 48] or, as Hiatt put it, ‘‘practically anything’’ that assists in peroxide homolysis[46]. Secondly, the organic peroxides are the common intermediates of autoxidation products.The primary products from hydroperoxide autoxidation are alcohols and ketones (Figure 23.8).The reactions involve a mixture of propagation and termination steps and the selectivity isinfluenced by the reaction parameters. Low temperature favors alcohol formation over ketoneformation. This is due to the lower free radical concentration and the higher probability ofhydrogen abstraction by the alkoxy radical (ROž). In n-alkanes, the position of the oxygenate


− HOR C R′

OOH

H

R C R′

O

H

R C R′

OH

H

″R+

R C R′

O

+ ″ROOH

R C R′

O

+ ″ROH

″RH

″RO

″ROO

Figure 23.8 Alcohols and ketones as primary autoxidation products.

functionality is of equal probability along the chain, with the exception of terminal carbons, whichare not oxidized as primary products [49], (i.e., little or no 1-alcohols and aldehydes are formed).

In alcohols, the hydrogen on the carbon containing the alcohol functionality can be oxidativelyremoved to produce ketones. This is a secondary reaction, but the product is indistinguishablefrom ketones that are formed as primary autoxidation products. The ketones are also keyintermediates during the formation of acids and esters. Autoxidation results in the introductionof a peroxide functionality on the carbon adjacent to the carbonyl group.

Carboxylic acids and esters are secondary autoxidation products. The carboxylic acids and estersare usually produced in a close to 1 : 1 ratio. During the autoxidation of Fischer–Tropsch waxes,it was found that carboxylic acid formation was completely suppressed at low O2 availability andlow temperature, but esters were still produced in low concentration [39]. The overall reactionnetwork for hydrocarbon autoxidation is presented in Figure 23.9.

Further oxidation of the secondary products yields CO2. The CO2 is partly produced by thedecarboxylation of the carboxylic acids, but not exclusively so [50]. Oxidative decarboxylationof carboxylic acids proceeds only to a limited extent. It was also shown that esters could bedecarboxylated.

23.3.3Fischer–Tropsch Wax Oxidation

A number of studies described the autoxidation of Fischer–Tropsch waxes specifically [38, 39,51–53]. Industrially, Fischer–Tropsch wax oxidation is conducted by batch-mode autoxidation.The autoxidation takes place in a bubble column reactor with internal heating and coolingcoils for heat management. The process, strictly speaking, is a semibatch process, with the

Alcohols

Ketones

Carboxylic acids

Esters

PeroxidesHydrocarbonO2

Figure 23.9 Reaction network of hydrocarbon autoxidation.


LTFT wax

Step 1Wax

preheating

Step 2Autoxidation

(initiation)

Step 3Wax

cooling

Step 4Autoxidation(propagation)

Step 5Wax

cooling

Oxidized wax

N2 N2Air Air AirLiOH orCa(OH)2

Figure 23.10 Workflow of commercial batch-modeoxidized wax production at Sasol 1. (1) The loadedwax is preheated to the autoxidation temperature un-der N2. (2) The first phase of autoxidation is mainlyneeded to initiate autoxidation. (3) The temperature

is then adjusted to the autoxidation temperature forthe propagation period. (4) The second phase of au-toxidation during which most of the wax oxidationtakes place. (5) The oxidized wax is cooled downunder nitrogen for unloading.

Fischer–Tropsch wax being a stationary phase (batch), while air is continuously bubbled throughthe molten wax. The steps involved in batch-mode wax oxidation are shown in Figure 23.10 [36].

The wax conversion is performed in two steps. The induction of the autoxidation processis slow in the absence of a catalyst or initiator. For this reason, the first oxidation period isusually conducted at a higher temperature to reduce the duration of the induction period. Mostof the autoxidation takes place during the second oxidation period, which controls the oxidationselectivity and final product properties. Batch times may be preset, or can be determined basedon the analysis of the product. The duration of the second autoxidation period is typically adjustedin accordance with the wax conversion, as indicated by the acid number of the oxidized wax.Some wax grades are saponified after autoxidation to convert some of the acids into soaps (metalcarboxylates). This takes place at the end of the second period of autoxidation.

The determination of the acid number [54] is a comparatively easy laboratory method thatcan be performed in a short period of time. Most other oxidized wax analysis methods arequite cumbersome [55–57]. Assessing the extent of oxidation by the acid number is of coursenot a true reflection of the actual extent of oxidation (Section 23.3.2). Spectrometric methodsbased on infrared (IR) and near-infrared (NIR) analyses have been successfully employed for thecharacterization of oxidized Fischer–Tropsch waxes [39, 58, 59]. Such spectrometric methodsprovide a more accurate reflection of the overall extent of oxidation and are less time consumingthan wet chemical analyses.

The properties of the oxidized wax are mainly determined by the temperature programme,operating conditions, and wax feed. Some variation is introduced in product quality by residualoxidized wax from the previous batch that affects initiation time; corrosion products can catalyzeautoxidation [47, 48], and handling conditions on unloading and storage of the oxidized wax mayallow further autoxidation. The production procedure (Table 23.6) and properties (Table 23.7)of some of the commercial grades of oxidized Fischer–Tropsch waxes produced by batch-modeautoxidation are given [36].

In general, the nature of batch-mode autoxidation is such that oxygen availability is notcontrolled during the second autoxidation period. Carboxylic acids are consequently produced.Apart from the carboxylic acid functionality in the oxidized wax, oxidation also results in theformation of short-chain carboxylic acids and other oxygenates. The short-chain oxygenates arestripped from the oxidized wax by the air that is bubbled through the reactor and the off-gas is


Table 23.6 Production procedure for different commer-cial oxidized waxes according to the workflow shown inFigure 23.10.

Step Description Oxidized wax grades Saponified oxidized wax grades

A1 A6a A28.1 A2 A3 A14 A28.2

Fe-LTFT wax feed H2-wax H2-wax C105-wax H2-wax H2-wax H2-wax C105-waxSaponificationchemical

None None None LiOH LiOH Ca(OH)2 LiOH

1 Wax preheatingtime (min)

10–15 10–15 10–15 10–15 10–15 10–15 10–15

2 Autoxidation,phase 1 time(min)

80 80 80 80 80 80 80

Autoxidationtemperature (ŽC)

175 180 175 175 175 175 175

3 Wax cooling time(min)

40 0 40 40 40 40 40

4 Autoxidation,phase 2 time(min)

>80 >80 >80 >80 >80 >80 >80

Autoxidationtemperature (ŽC)

140 180 140 140 140 140 140

Acid number (mgKOH/g)

27–29 35–45 27–29 21–23 25–28 25–27 21–23

5 Unloadingtemperature (ŽC)

100–120 130 110–130 110–120 120–130 100–120 110–120

� Average batchtime (min)

400 – 450 380 470 – 390

aHighly oxidized wax, no nitrogen used during its production.

rich in oxygenates. These oxygenates and light alkanes resulting from oxidative cleavage of thewax must be recovered before the N2-rich air can be vented to atmosphere.

The composition of the off-gas depends on the severity of autoxidation (Table 23.8) [36]. Thisstream is corrosive on account of the short-chain carboxylic acids. In a Fischer–Tropsch refinery,the condensed material can in principle be coprocessed with the Fischer–Tropsch aqueousproduct.

Continuous-mode wax oxidation has been suggested as an alternative to batch-mode waxoxidation [39]. The continuous-mode process involves the cocurrent feed of wax and air in apipe-reactor (a long process line rather than a reactor vessel). With this type of design, there is thepossibility to adjust the temperature over the length of the reactor and there is also the possibilityof bleeding in air at different positions along the length of the reactor (analogous to interbedinjection in fixed-bed reactors). This type of autoxidation and reactor design can overcome someof the problems associated with batch-mode wax oxidation:


Table 23.7 Typical properties of different commercial oxi-dized waxes that are produced from Sasol Fe-LTFT-derivedH2- and C105-waxes.

Property LTFT wax feed Oxidized wax grades Saponified oxidized wax grades

H2 C105 A1 A6 A28.1 A2 A3 A14 A28.2

Acid number(mg KOH/g)

<0.1 – 27 37 28 11 10 7 12

Ester number(mg KOH/g)

– – 28 65 27 14 24 26 –

Penetration at25 ŽC, ASTMD1321 (mm)

<0.1 <0.1 0.6 2.5 – <0.4 0.25 0.2 –

Congealing point,ASTM D 938 (ŽC)

98 104 87 79 94 89 91 86 97

Melting point,ASTM D 127 (ŽC)

– – 96.5 – – 101 103.5 – –

Latent heat offusion (kJÐkg�1)

217 245 192 126 174 182 185 179 175

Specific heat capacity (kJÐkg�1ÐK�1)

At 25 ŽC 1.8 1.9 2.3 3.1 2.2 2.5 2.1 2.6 2.1At 130 ŽC 2.3 2.7 3.0 2.6 2.6 3.8 2.8 – 3.0At 170 ŽC 2.5 2.8 2.9 2.6 2.5 3.8 2.6 1.6 2.8

Averagemolecular mass(gÐmol�1)

785 1100 700 720 890 750 780 750 1090

Density (kgÐm�3)

At 25 ŽC 940 950 950 950 940 960 980 980 960At 100 ŽC – – 825 842 – 814 829 846 –At 110 ŽC – – 819 830 810 805 824 836 –At 120 ŽC – – 813 820 – 797 821 832 800

Viscosity (cP)

At 125 ŽC – – 12 25 19 20 32 44 36At 135 ŽC 6–10 18 10 20 16 14 22 38 28At 145 ŽC – – 8.5 17 14 11.5 15 32 21

Surface tensionat 130 ŽC (NÐm�1)

– – 0.0284 0.0291 0.0222 0.0288 0.0336 0.0294 0.0281

1) The initiation period and product properties in continuous-mode oxidation are not dependent

on variables such as the batch-to-batch variations in cleanliness of the reactor and product

handling.

2) Control over oxidation selectivity in larger scale batch reactors is limited by equipment

constraints such as heating and cooling rates between periods, operating range for airflow,

and bubble hydrodynamics. These are not constraints in continuous-mode oxidation.


Table 23.8 Typical composition of the off-gas frombatch-mode autoxidation of Fischer–Tropsch waxes. Normaloperation refers to the production of oxidized waxes such asA1 and severe operation refers to the production of oxidizedwaxes such as A6.

Description Normal operation Severe operation

Flow rate (kgÐh�1 per ton wax)Organics 9 19Water 27 52Remaining air 1124 1089

Organics composition (mass%)Alcohols 4 5Carbonyls 5 5Carboxylic acids 74 72Alkanes 12 13Other organics 5 5

3) In continuous-mode autoxidation, the oxygen availability is inherently limited by consump-tion and can be regulated by intermediate addition. This avoids overoxidation and providesselectivity control that is more difficult to achieve with batch-mode oxidation. To achieve thesame conversion-dependent control in batch-mode autoxidation, the air must be diluted withnitrogen and the ratio must be controlled over time.

4) Hydrocarbon chain degradation and carboxylic acid formation can be almost completelysuppressed during continuous-mode oxidation, which is an advantage for autoxidationapplications that rely on alcohol intermediates.

5) It is possible to produce oxidized wax grades with continuous-mode autoxidation that cannotbe produced by batch-mode autoxidation.

6) In batch-mode oxidation, where air is introduced at the bottom of the bubble column reactor,foaming presents an operating problem. The oxidized wax is surface active, because theoxygen-containing functionality is polar. This causes the already oxidized wax to concentrateat the gas–liquid interface and stabilize the air bubbles. This not only leads to stable foamformation but also keeps the oxidized wax in contact with air, which in turn can result inoveroxidation.

7) Continuous operation and steady state control are possible in continuous-mode autoxidation,whereas batch-mode control is time dependent.

23.3.4Syncrude Process Technology

The application of autoxidation with a paraffinic Fischer–Tropsch-derived feed is no differentfrom the general application of autoxidation to alkanes. The situation is different when applying

References 485

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1) The presence of low levels of metals in syncrude, specifically iron or cobalt, has a potentiallybeneficial effect on autoxidation, by increasing the rate of peroxide decomposition. Many ofthe other nonalkane species may fulfill a similar role. This may result in autoxidation with avery short induction period.

2) The oxygenates present in syncrude are susceptible to further autoxidation (Figure 23.9).3) The alkenes present in syncrude are not only susceptible to autoxidation but are likely to

result in oxidative oligomerization. This is advantageous only when oil production is theobjective.

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