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Ind. Eng. Chem. Res. 2008, 47, 6870–6877
Alkylate Technology Selection for Fischer-Tropsch Syncrude Refining Arno de Klerk*,† and Philip L. de Vaal‡ Fischer-Tropsch Refinery Catalysis, Sasol Technology Research and DeVelopment, P.O. Box 1, Sasolburg 1947, South Africa, and Department of Chemical Engineering, UniVersity of Pretoria, Pretoria 0001, South Africa
Technology selection to produce alkylate from straight run Fischer-Tropsch syncrude has been investigated. Alkylate is a high octane paraffinic motor-gasoline component and can be produced by direct alkylation (olefin addition to isobutane) or indirect alkylation (isobutene dimerization followed by hydrogenation). Neither isobutane nor isobutene is abundant in the light fraction of Fischer-Tropsch syncrude, which is rich in linear alpha-olefins. Direct alkylation (HF and H2SO4) and indirect alkylation (acidic resin and solid phosphoric acid) based flowschemes were evaluated in terms of alkylate yield, octane number, compatibility to Fischer-Tropsch derived feed, and environmental friendliness. It was found that the refining focus determined the selection. Indirect alkylation with solid phosphoric acid was found to be the best in terms of Fischer-Tropsch feed compatibility, environmental friendliness, and least refining complexity. The highest alkylate yield could be obtained by a combination of partial olefin hydrogenation, hydroisomerization, and direct alkylation. Butene skeletal isomerization in combination with indirect alkylation yielded an alkylate with the highest octane number. Introduction Aliphatic alkylation is a process whereby a branched paraffin is alkylated with an olefin to produce a highly branched paraffinic product. Most current industrial processes are based on sulfuric acid (H2SO4) or hydrofluoric acid (HF) catalyzed alkylation of isobutane with C3-C5 olefins.1–4 Alkylate originally referred only to the product from aliphatic alkylation, but its meaning in a refining context has since broadened to include any high-octane paraffinic motor-gasoline blending component that is rich in 2,2,4-trimethylpentane. Processes based on isobutene, as opposed to isobutane, are called indirect alkylation processes and consist of selective olefin dimerization followed by olefin hydrogenation.5–8 Alkylate is a valuable blending component, since it improves the octane number of the paraffinic base stock motor-gasoline. This is important, since the high-octane compound classes that can be used to increase the octane number of motor-gasoline are generally limited by legislation governing motor-gasoline specifications. For example, Euro-VI motor-gasoline is limited to 35% aromatics, 18% olefins, and 10-15% oxygenates depending on the nature of the oxygenate. It is therefore not surprising that most modern crude oil refineries include an alkylate production unit.9 The selection of an appropriate alkylate production technology for application in a Fischer-Tropsch refinery is not obvious, since neither isobutane nor isobutene is present in significant quantities in Fischer-Tropsch derived syncrude. The light fraction of Fischer-Tropsch syncrude is rich in linear hydrocarbons, especially linear R-olefins (Table 1).10 This is not to say that isobutane and isobutene are abundant in crude oil, but most crude oil refineries include thermal and/or catalytic cracking units to upgrade residue fractions. These units produce the light gases that contain the molecules necessary for both direct and indirect alkylation. Although cracking units can be added to a Fischer-Tropsch refinery, there are more environ* Corresponding author. Tel: +27 16 960-2549. Fax: +27 11 5223517. E-mail:
[email protected]. † Sasol Technology Research and Development. ‡ University of Pretoria.
mentally friendly refining pathways available to the refiner of Fischer-Tropsch syncrude.11,12 Alkylate technology selection for the refining of FischerTropsch syncrude has been investigated. Rather than focusing on a purely economic comparison, which can quickly become outdated, the technology options have been evaluated in terms of efficiency. Aspects that are considered include alkylate yield and quality, fundamental compatibility of each technology to Fischer-Tropsch syncrude, and environmental friendliness. Refining Technologies Most commercially available refining technologies have been developed for crude oil or crude oil refinery derived feed materials. Fischer-Tropsch syncrude differs from crude oil on a molecular level1,2 and the evaluation of refining technologies for use with Fischer-Tropsch syncrude requires a fundamental understanding of the conversion mechanism. Differences in isomer distribution have a significant effect on the alkylate quality being produced in each type of technology and will consequently affect the evaluation. As background, a short description of the main direct and indirect alkylation technologies will be given. In addition to Table 1. Composition of C3-160 °C Fischer-Tropsch Syncrude on Carbon Atom Basis Relative to the Total Syncrude Product Derived from Cobalt- and Iron-Based Low Temperature Fischer-Tropsch (LTFT) Synthesis, as Well as Iron-Based High Temperature Fischer-Tropsch (HTFT) Synthesisa selectivity (%) propene propane butenes 1-butene 2-butenes isobutene butanes n-butane isobutane C5-160 °C a
Co-LTFT, slurry 220 °C
Fe-LTFT, slurry 240 °C
Fe-HTFT, fluidized 340 °C
2 1 2 1.8 0.2 0.9 2-butenes32 but with less of a difference in reaction rate between isobutene and the n-butenes than with acidic resin catalysts. Codimerization between isobutene and n-butenes is consequently unavoidable if the feed contains n-butenes. In Fischer-Tropsch refining context where the isobutene content of the C4-cut is low, this is an advantage, because the process requires less n-butenes to be skeletally isomerized. Very high quality alkylate can be produced by indirect alkylation with SPA, with the hydrogenated product having a RON of 101 and MON of 96.6 The process has flexibility with respect to the n-butene content of the feed that can be varied from 30-80%.6 The codimerization of isobutene and n-butenes (not propene) does not significantly affect the alkylate quality, as long as the process is properly controlled by keeping the reaction temperature low, typically below 140 °C. At higher operating temperatures the isobutene derived alkylate quality quickly deteriorates due to trimerization and cracking.32 In general the reaction temperature should be decreased with increasing isobutene content in the feed (Figure 3). Propene in the feed can also deteriorate the alkylate quality. Propene forms a stronger ester bond with the phosphoric acid than the butenes, and it will become the dominant carbocation source.33 When the propene reacts with n-butenes, less branched (lower octane number) species are produced. The ester-based mechanism that is operative during solid phosphoric acid catalyzed dimerization of butenes has another peculiar feature. There is a low temperature skeletal isomerization pathway to convert 1-butene to isobutene.17 Isobutene
Ind. Eng. Chem. Res., Vol. 47, No. 18, 2008 6873 Table 4. Yield Structure of the Commercial Lyondell Olefin Isomerization Process Using Their Improved Second-Generation Catalyst at 40% and 44% Conversion yield (mass %)
Figure 3. Hydrogenated motor-gasoline quality from butene dimerization over solid phosphoric acid is influenced by three parameters, namely, isobutene content of the feed, operating temperature in relation to the isobutene content, and the propylene content of the feed. Motor-gasoline quality is expressed in terms of the research octane number (9) and motoroctane number (0).6,32,35
Figure 4. Block flow diagrams of the direct alkylation (HF and H2SO4 catalyzed alkylation) configurations evaluated.
Figure 5. Block flow diagrams of the indirect alkylation (acidic resin and solid phosphoric acid dimerization) configurations evaluated.
is not desorbed from the catalyst, but the product from 1-butene dimerization over SPA is rich in trimethylpentenes and the hydrogenated naphtha has a RON of 86-88 and MON of 86-88.32 It implies that indirect alkylation of butenes over SPA
product
40% conversion
44% conversion
light gases propene isobutene C5 and heavier
