Study On Supercritical Fischer-Tropsch Synthesis And Supercritical Ammonia Synthesis On Fused Iron Catalysts

The conventional FTS reaction has been performed in either gas phase or liquid phase reaction media. Gas phase reactions exhibit higher reaction rates and diffusivities when compared to liquid phase reactions, but are hampered by inadequate heat removal resulting in excessive methane formation. Liquid-phase reactions have superior heat removal capabilities when compared to gas phase reactions and are therefore able to maintain a constant reaction temperature without deactivation. However, liquid-phase FTS suffers from mass transfer limitations, requiring relatively large reactor volumes. The ideal FT synthesis medium would, therefore, be one with gas-like transport properties and liquid-like heat capacity and solubility characteristics. Such a desired combination of fluid properties is possible with supercritical reaction media.The performance of supercritical phase Fischer-Tropsch synthesis over fused iron catalysts was evaluated in the fixed bed reactor. Catalysts were characterized by means of XRD, TPH, EDS and so on to provide better insight into the relationship therein. Main results from this work were as follows.It has been observed that CO conversion, 1-olefin selectivity, and heavy hydrocarbon production can be enhanced in supercritical phase FTS process compared to gas phase FTS. After 48hrs' continuous running of the reaction in supercritical phase FTS, the catalyst does not show any sign of deactivation and deposition of inactive carbon on catalyst always much lower than that in gas phase FTS.Higher CO conversion in supercritical phase FTS results from: 1) The higher solubility and diffusivity of syngas in the supercritical phase FTS have an important role in the enhancement of concentration of syngas on catalyst active site; 2) The supercritical phase FTS operation results in the enhanced extraction of heavy hydrocarbons from catalyst pores and surfaces thereby enhancing pore diffusivity and creating more active sites; 3) The deposition of inactive carbon on fused iron catalyst in supercritical phase FTS process could be well controlled and this resulted in the creating more active site on catalyst.Compared to gas phase FTS, no significant different effect of temperature, pressure, syngas flow rate and syngas feed ratio on the supercritical phase FTS has been observed. Optimum reaction temperature and pressure are T/T_c=1.0-1.1 and P/P_c=1.0-1.5, respectively. Our results have also shown that paraffins are promising candidates for supercritical solvent because they do not poison the catalysts and are stable under the reaction conditions.Compared to the results without recycle of the solvent, CO conversion and hydrocarbon products distribution is similar to, but CH₄ selectivity is higher than those with recycle of the solvent.Compared to gas phase FTS, no significant different effect of the content of K and Al and the change in Fe²⁺/Fe³⁺ on the supercritical phase FTS has been observed. But higher CO conversion and better products selectivity is achieved in the supercritical phase FTS.we have done supercritical ammonia synthsis research systematically using 17 differenct supercritical media, and investigated the influence of reaction conditions. It has been found that supercritical media is decomposed under reaction conditions when using Fe_1-xO and Ru/AC catalysis. The decomposition products deactivate the catalysts. From experimental results, we conslude that the activation temperature of catalyst is the key factor in supercdtical ammonia synthsis.