(42e) A Study of Low Temperature Fischer-Tropsch Synthesis by Switching Between CO2/H2/N2 and CO/H2/N2 Syngases Over a Cobalt Based Catalyst

Abstract:

The Fischer-Tropsch synthesis (FTS) provides a
well-established commercial technology
for conversion of syngas
to clean
transportation fuels and chemicals. The raw synthesis gas or syngas derived from coal, natural gas
or biomass is a mixture of H₂, CO,
CO₂ and CH₄. Because the syngas composition is dependent on many factors
such as gasifier type, operating conditions, gasifying agents, the composition of CO₂ in
the raw syngas varies from around 1% to 30%.  Although the need for CO₂ separation
before using the syngas in FTS is mentioned in the patent literature, for some
cases, recent process development studies discuss a potential cost advantage if
CO₂ is not removed before the synthesis step
^[1]. In addition, fixation of CO₂
into hydrocarbons through FTS, in an attempt to reduce CO₂ emission,
has become of greater interest in recent years. It is therefore interesting to
investigate the effect of CO₂ on a cobalt catalyst under
low-temperature FTS conditions.

In the present work, 10 wt % of Co/TiO₂
catalyst used in this study was prepared by impregnation of TiO₂
with a cobalt nitrate solution. The
experiment, called the cobalt-based catalyst stability testing during CO and CO₂
hydrogenation, was conducted by repeatedly switching between the two feed
gases, CO₂ feed (CO:H₂:N₂=30%:60%:10%)
and CO feed (CO₂/H₂/N₂
= 22.5%:67.5%:10%), into a micro plug flow reactor at 180-220^oC, 20bar and 30ml(NTP)/(min^.gcat).

For instance the CO or CO₂ reaction rate as a
function of time on stream for low temperature FTS is shown in Figure 1. The data shows that both of the CO and CO₂ are readily
hydrogenated over cobalt based catalyst and with an increase in temperature
both the CO and CO₂ reaction rates are increased. At the lower
temperature of 180°æ, the catalyst
reactivity for CO₂ reaction is close to that of CO. However, when we
increase the reaction temperature from 200 to 220^oC, a lower reactivity of CO₂ to CO is
obtained. It
is however quite interesting that with the reaction temperature at 180^oC: (1) when the CO₂
feed mixture is first introduced into the fixed bed reactor, the CO₂ reaction rate achieves its highest rate at that temperature; (2) after this, when the
feed gas is switched from CO₂ feed to CO feed and then switched back
to CO₂ feed, the CO₂ reaction
rate is two times lower than the
first time; 
(3) with
subsequent repeat switching between the two feed gases, both of the CO₂
and CO reaction rates remain constant and do not change for CO₂and CO reaction respectively, which indicates that the catalyst is not de-activated.

It
should be noted that as the feeds are changed back and forth between H₂/CO₂
and H₂/CO there
is no apparent catalyst deactivation. In this paper we further discuss the
catalyst activity, product selectivity, product distribution and paraffin to
olefin ratio (P/O) during switching between the two feed gases at
different reaction temperatures. The results provide some hints on how to
design FTS processes and FT catalysts to improve the product selectivity.

Figure 1: CO or
CO₂reaction rate as a function of time on-stream for a
Co/TiO₂ catalyst at 180-220^oC, 20bar and 30ml (NTP)/(min^.gcat).

References: [1] T. Riedel, G. Schaub Topics in
Catalysis, 2003, 26: 145-156.