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 CO₂ and 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 CO₂ reaction 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.