Oligomerization of Fischer−Tropsch Olefins:  Effect of Feed and Operating Conditions on Hydrogenated Motor-Gasoline Quality

Abstract

Oligomerization of Fischer-Tropsch-derived olefins over a solid phosphoric acid catalyst was studied to improve the quality of the hydrogenated motor-gasoline. It was shown that, with a narrow-cut Fischer-Tropsch butene mixture, rich in 1-butene and low in isobutene, a hydrogenated motor-gasoline with an RON and an MON of 86-88 could be produced at 80% olefin conversion. The influence of propene and pentenes was investigated, and a reactivity sequence for C 3 -C 5 mixtures could be established: isobutene > methylbutenes > n-butenes ) propene > n-pentenes. It was noted that 1-butene was closer in behavior to isobutene than to 2-butene and had to be treated as such to yield the highest-quality product. It was advantageous to oligomerize 1-butene-rich feed at 160°C or less and to exclude propene and n-pentenes from the reaction mixture. These effects could be explained.

An improved solid phosphoric acid catalyst for alkene oligomerization in a Fischer–Tropsch refinery

“…Since the FT process produces predominantly linear hydrocarbons, the high-quality diesel fuel produced can be used as a blending stock to upgrade lower quality diesels [1]. To increase the overall gasoline yield, the FT C 3 -C 5 olefins can be oligomerized over a solid phosphoric acid (SPA) catalyst to produce branched products which have higher octane numbers [2]. The need for an increased overall diesel yield can be addressed by C 3 -C 5 olefin oligomerization operated in the ''tetramer mode'' using SPA as catalyst [3], and the fraction thus produced can then be blended with the high-quality diesel.…”

Fischer–Tropsch Syncrude

“…28 The composition of some light fractions obtained according to Figure 4.3 are shown in Table 4.2. 26,29,30 Hydrogen can be recovered by pressure swing absorption (PSA), with the rest of the methane-rich gas being used for reforming. The C 2 -rich gas (not listed in Table 4.2) is a mixture of ethene and ethane, from which polymer-grade ethene is recovered by distillation.…”

Catalysis in the Upgrading of Fischer–Tropsch Syncrude

“…SPA has been found to be especially well suited for the upgrading of n-butenes to good quality hydrogenated motor gasoline, with straight run FTS-derived butenes giving a hydrogenated motor gasoline with a RON of 86 and a MON of 88. 43,44 Conversely, propene-derived motor gasoline has a very poor hydrogenated octane number, with a RON of less than 50. It is clear that C 3 and C 4 alkenes must be converted separately over SPA if part of the product is to be hydrogenated.…”

“…Test work at 160 1C and a total pressure of 3.8 MPa showed a drop in 1-pentene conversion at constant butene conversion with increasing 1-pentene concentration. 43 This suggested that the butenes, rather than the 1-pentene, were more readily protonated and became the primary carbocation source. A reasonable conversion of 1-hexene could be demonstrated at 200 1C and a total pressure of 6 MPa over SPA.…”

“…This unexpected conversion is responsible for the high octane numbers of hydrogenated motor gasoline from FischerTropsch butene OLI over SPA. 43 The ability of SPA to produce branched products from linear feed is clearly not Brønsted-like behaviour. SPA does not behave like a pure Brønsted acid catalyst, because the OLI mechanism involves the formation of phosphoric acid esters.…”