The Schwank Group

-Fischer-Tropsch Selectivity Control-

James Bucher and Johannes Schwank

FT Selectivity to Heavy HCs

Selectivity of hydrcarbons in the C11-C15 range
in a dual bed reactor system as a function
of varied H2:CO ratio in the second reactor

Synthetic transportation fuels potentially have significant advantages over fuels derived from petroleum feedstocks. Such advantages include the abilities to custom design the fuel according to the desired combustion characteristics, to produce sulfur free fuels for use in reformer applications, and to generate fuels from abundant carbonaceous materials like biomass.

The Fischer-Tropsch reaction provides a means for synthesizing transportation fuels from carbon monoxide and hydrogen, a mixture commonly known as synthesis gas. The reaction as performed industrially yields a wide range of hydrocarbon products, many of which are undesired when the yield of fuels is to be maximized.

Through a unique reaction engineering approach, it may be possible to greatly improve the selectivity of this reaction in the gasoline and diesel range (C8 through C15) in order to make the process more efficient and economical on a smaller scale. This approach would include downstream co-feeding of different chemical species as well as different catalyst and temperature regimes.

The current study examined the effect of downstream synthesis gas feeding on normal Fischer-Tropsch product distributions. Two different synthesis gas composition regimes were explored: a hydrogen deficient regime, in which the H2/CO ratio of the synthesis gas feed was 1.0, and a hydrogen rich regime, in which the H2/CO ratio of the synthesis gas feed was 2.0.

Within each composition regime, the product distributions of downstream feeding and top feeding cases were compared. In the downstream feeding case, synthesis gas was fed into the entrances of a first series reactor. The effluent from this reactor was then fed along with fresh synthesis gas into a second series reactor. In the top feeding case, synthesis gas was simply fed into the first reactor and flowed through both series reactors.

The results of the aforementioned experiments showed that the selectivity and yield of hydrocarbons in the C8-C15 range increased when the downstream synthesis gas feeding strategy was employed. Further work is necessary to confirm the results of these initial experiments and reconcile the data to a theoretical model that was developed to describe the behavior of this system.

FT Reactor Feed Scenarios
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