Fischer–Tropsch synthesis: Effect of ammonia in syngas on the Fischer–Tropsch synthesis performance of a precipitated iron catalyst

Abstract

The effect of ammonia in syngas on the Fischer-Tropsch Synthesis (FTS) reaction over 100 Fe/5.1 Si/2.0 Cu/3.0 K catalyst was studied at 220-270 o C and 1.3 MPa using a 1-L slurry phase reactor. The ammonia added in syngas originated from adding ammonia gas, ammonium hydroxide solution or ammonium nitrate (AN) solution. A wide range of ammonia concentrations (i.e., 0.1-400 ppm) was examined for several hundred hours. The Fe catalysts withdrawn at different times (i.e., after activation by carburization in CO, before and after cofeeding contaminants, and at the end of run) were characterized by ICP-OES, XRD, Mössbauer spectroscopy, and synchrotron methods (e.g., XANES, EXAFS) in order to explore possible changes in the chemical structure and phases of the Fe catalyst with time; in this way, the deactivation mechanism of the Fe catalyst by poisoning could be assessed. Adding up to 200 ppmw (wt. NH 3 /av. Wt. feed) ammonia in syngas did not significantly deactivate the Fe catalyst or alter selectivities toward CH 4 , C 5+ , CO 2 , C 4-olefin and 1-C 4 olefin, but increasing the ammonia level (in the AN form) to 400 ppm rapidly deactivated the Fe catalyst and simultaneously changed the product selectivities. The results of ICP-OES, XRD and Mössbauer spectroscopy did not display any evidence for the retention of a nitrogen-containing compound

Effect of H2S in syngas on the Fischer–Tropsch synthesis performance of a precipitated iron catalyst

“…The results of Mössbauer spectroscopy reflect three important aspects about the behavior of sulfur: 1) χ-Fe 5 C 2 phases were unstable at low temperature conditions, and a fraction of them converted to έ-Fe 2.2 C; 2) the formation of bulk Fe-S compounds as a deactivation mechanism in the current context was ruled out, consistent with the results of XRD; 3) co-feeding sulfur for several hundreds of hours did not alter the distribution of the iron phase, indicating that sulfur adsorbed on iron sites in a uniform manner. έ-Fe 2.2 C and χ-Fe 5 C 2 exhibited better stability at low and high temperatures, respectively, which is consistent with our earlier study [9] and that of a study in the open literature [31]. The lower amounts of iron magnetite found in the used sample at 260 o C relative to the one at 270 o C could be ascribed to a lower extent of oxidation of χ-Fe 5 C 2 by water (3Fe 5 C 2 + 32H 2 O = 5Fe 3 O 4 + 6CO 2 + 32H 2 ).…”

“…To understand the deactivation mechanism by sulfur in this study, XRD results of the used catalysts that were exposed to 1.0 ppm H 2 S at 270 o C and 260 o C, as well as clean conditions (i.e., no H 2 S) are provided in Figure 8. Fe 3 O 4 and Fe 5 C 2 phases were detected at 2θ of 30 o , 35.5 o and 40 o , 42.6 o , 45.0 o and 50.5 o respectively, which were also observed on the Fe catalysts exposed to ammonia [9]. Iron-sulfur compounds with sufficient long range order were not detected by XRD, and the distributions of iron carbides and magnetite did not change significantly after sulfur poisoning.…”

“…From an economic perspective, gas purification is the most costly process in the BTL technology, covering 65-70% of total cost [7]; identifying a potential poison's limit in syngas can significantly reduce gas purification costs by avoiding excessive gas-purification. In our most recent efforts, the effects of alkali halide and alkali bicarbonate compounds [8], ammonia [9], HCN [10], and hydrohalic acids (HX) i.e., HF,HCl,and HBr [11], on the catalytic behavior of an Fe catalyst (100Fe/5.1Si/2Cu/3K) have been extensively studied under commercially relevant FTS conditions. It was found that the tolerance levels of the catalyst to each of these poisons varied significantly.…”

“…Ammonia, alkali halide and alkali bicarbonate compounds were found to be the weakest poisons to the Fe catalyst. When the level of ammonia in the syngas feed attained as high as 200 ppm [9] or when the concentrations of alkali halide and alkali bicarbonate compounds were less than 40 ppm [8], no significant changes in Fe catalyst performance were observed during 400 to 1200 h of continuous testing; instead, the HX limits were determined to be low, i.e. 1-2 ppm for HCl and HBr and 10 ppm for HF [11], indicating that the catalyst was much more susceptible to hydrohalic acids relative to ammonia and alkali compounds.…”

Fischer‒Tropsch Conversion of Biomass‐Derived Synthesis Gas to Liquid Fuels

“…In the same study they found no effect of ammonia on the activity of iron catalysts. This is supported by Ma et al 80 , who did not find any negative effect of ammonia on the catalytic activity or selectivity for iron catalysts.…”