Fischer-Tropsch Synthesis and Product Upgrading Over Dual Bed Catalytic Reactor

Abstract:

            Converting biomass derived synthesis gas (syngas) to liquid hydrocarbons using Fischer-Tropsch (F-T) process is an emerging area of research. Traditionally, iron- and cobalt-based catalysts have been used as F-T catalysts. The advantages of Fe-based catalysts are that they are inexpensive, durable and  can be used over a wide range of H₂:CO ratios; Fe catalysts are  currently being used by Sasol to convert coal derived synthesis gas to a wide range of products. However, one of the problems with the use of Fe catalyst is that it forms hydrocarbon products rich with olefins.  The resulting hydrocarbon product is not stable due to a variety of oxidation and polymerization reactions.  This would require a post-treatment of the product to convert it to saturated hydrocarbons via a catalytic hydrogenation process.

The objective of this work is to develop a catalytic system for the single-step conversion of syngas to saturated hydrocarbons (paraffins). To accomplish this goal we studied syngas conversion over a dual catalyst bed comprised of a Fe catalyst coupled with a  hydrogenation catalyst. This allows the in-situ conversion of olefinic compounds produced by the Fe catalyst to paraffinic products which are more suitable for use as transportation fuel.  A number of Ni- and Co-containing hydrogenation catalysts were tested in combination with the Fe catalyst.

            Operational parameters including temperature, pressure, space velocity and syngas composition were varied and their effect on syngas conversion, selectivities towards light hydrocarbon gases, and liquid hydrocarbons were determined. The effect of addition of CO₂ to the H₂-CO mixture was also studied.  Among tested hydrogenation catalysts alumina-supported Ni-Mo catalyst demonstrated the highest activity and selectivity.

 Table 1. depicts the liquid product composition generated with the Fe catalyst alone and in a dual bed Fe + Ni-Mo/alumina arrangement.

Table 1. Composition of syn-crude generated over Fe and Fe + Ni-Mo/alumina catalysts at syngas composition: [H₂]/[CO]= 2, reactor temperature: 333^oC, pressure: 200 psig, and space velocity of about 4100  h^-1.

Product fraction, %	Catalyst
	Fe	Fe+Ni-Mo/Alumina
Gasoline range (C5-C10)	55.8	55.4
Kerosene/jet fuel range(C11-C12)	15.5	17.1
Diesel range (C13-C16)	15.3	16.8
Lube oil and wax range (C17-C26)	13.5	10.7
Olefin to paraffin molar ratio (for C8)	5.7	0.0

Results obtained, to date, indicate that:

1.      A highly paraffinic liquid product is obtained when a Ni-Mo/alumina hydrogenation catalyst is used in a combination and downstream of an iron catalyst.

2.      Presence of Ni-Mo catalyst did not markedly alter the distribution of liquid fractions in the product, but practically eliminated olefins from the raw product.

3.      Presence of CO₂ in the feed gas adversely affected syngas conversion and space time yield of liquid products.

Upon further development, this approach can provide a more facile and direct route to production of high quality liquid hydrocarbons from syngas. In this paper, a detailed description of these and other significant findings will be presented and discussed.