(606e) 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.

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.