Oligomerization of Fischer-Tropsch Tail Gas over H-ZSM-5

Fischer-Tropsch synthesis produces a product that invariably contains C-4 and lighter material. The C-2-C-4 fraction has a high olefin content. The light hydrocarbons in the Fischer Tropsch tail gas can be separated from the unconverted synthesis gas, but the added cost and complexity of doing so by pressure distillation is often considered unjustified. In this study the possibility of reactive recovery of the C-2-C-4 olefins from the Fischer Tropsch tail gas by oligomerization over H-ZSM-5 was investigated. Two specific issues were investigated for this unconventional application of this industrially practiced technology, namely, to determine the impact of low olefin partial pressure on productivity and to determine whether acid catalyzed CO reactions, such as the Koch reaction, were taking place. Catalyst and reactor system performance was evaluated using model propylene oligomerization. A catalyst productivity of 0.42 g.(g(cat))(-1).h(-1) was achieved with a 39% C3H6 feed at 190 degrees C, 3 MPa, and weight hourly space velocity (WHSV) of 0.9 h(-1). These results were comparable to previous reports in the literature. The same catalyst was employed to evaluate conversion of a model Fischer Tropsch tail gas mixture containing 17.4% H-2, 7.5% CO, 68.0% CH4, 0.65% C2H4, and 6.45% C3H6. The operating range 205-278 degrees C, 3 MPa, and WHSV 1-1.5 h(-1) was investigated. At 237-278 degrees C and WHSV 1.5 h(-1), catalyst productivity remained around 0.32-0.36 g.(gcat)(-1).h(-1). Despite an olefin partial pressure of only 0.22 MPa, better than 85% olefin conversion was achieved. The liquid product was olefinic and contained 70-80% naphtha and 20-30% distillate. No evidence was found that CO reacted with the olefins in the feed, either by the Koch reaction to produce carboxylic acids, or to form ketones. Deposits formed on the catalyst and deactivation was observed over a period of 240 h. The nature of the deposits varied from top to bottom in the packed bed. Analysis of the spent catalyst indicated that liquid filled pores preceded hydrogen transfer and aromatization to form coke-like deposits over time.