(595e) Nitrogen Co-Feeding in Fischer Tropsch Synthesis: The Effect to the Reaction Rate and Product Selectivity at Higher Reaction Rates

Our previous study, “Effects of Nitrogen Co-Feeding in Fischer Tropsch Synthesis At Low CO Conversion”, showed that diluting syngas with N₂ has a positive effect on the activity and selectivity of products at low CO conversion. In the current study, we pursue similar kind of experimental work for Fischer Tropsch Synthesis (FTS), with an aim of investigating the effect of nitrogen co-feeding in FTS at high CO conversion.

We started the reaction as per previous experiment at 60 (NTP)ml/min, 20 bar and the temperature of 200 ^oC (for 48 hours) for syngas FTS. Then, we increased the reaction temperature to 220 ^oC and started the investigation of the effect of N₂ co-feeding in FTS at this temperature by repeatedly switching between two kinds of feed gases into the reactor: (1) with only syngas at 20 bar, 60 (NTP)ml/min, and (2) with N₂ co-feeding syngas at 25 bar, 75 (NTP)ml/min. In order to keep the “reaction kinetics” the same in all runs, we kept the molar flow rate of syngas as well the partial pressure of the CO and H₂ constant while varying the molar flow rate of N₂ and the reactor pressure. The reactor was run for around 2800 hours at 220 ^oC while alternating the experimental conditions from syngas to syngas diluted with N₂. The experimental results show that the increase in temperature from 200 ^oC to 220 ^oC resulted in the increase of CO conversion from 16% to 46%. However, the catalyst deactivation was observed with time going from a CO conversion of 46% down to a CO conversion of 12%. Thereafter, the CO conversion remained fairly constant approximately 12% for a period of 1800 hours.

Our data demonstrate that N₂ co-feeding in FT reaction decreases the selectivity to the undesired lights hydrocarbons, especially for CH₄. It was also noticed that the rates of light paraffin (C₂ – C₅) formation as a function of time on stream (TOS) decreased significantly at high CO conversions and remained fairly constant at CO conversion average 12% for both conditions. However, the rates of formation of light olefins as a function of TOS appear to be quite constant for the high CO conversions and a significant increase in the rates of formation of light olefins (C₂ – C₅) as a function of TOS is observed for low CO conversion. Further experimental results, such as product selectivity and paraffin to olefin ratios are calculated and compared.