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Study On Tuning Product Distribution Of Fischer-Tropsch Synthesis To Light Olefins

Posted on:2016-08-14Degree:DoctorType:Dissertation
Country:ChinaCandidate:Y LiuFull Text:PDF
GTID:1221330491961835Subject:Chemical Engineering and Technology
Abstract/Summary:PDF Full Text Request
Light olefins (C2-C4) are building blocks of the chemical industry and typically produced by steam cracking naphtha. The direct conversion of syngas to light olefins via Fischer-Tropsch synthesis (FTS) is a promising route to meet the increasing demand for chemical feed-stocks. Fischer-Tropsch synthesis has long been known as one of the key technologies for producing ultra-clean fuels from non-crude-oil feedstocks. In general, FTS products are almost always normal aliphatic hydrocarbons, and product distribution is wide and unselective. Therefore, it is a great challenge for FTS to selectively produce the specific products, especially to effectively form light olefins.Herein, our studies mainly focused on the development of iron-based catalyst with high light olefins selectivity, and the effect of the products mass transfer rate on the light olefins selectivity was also discussed. Furthermore, the effects of preparation method, textural properties, active metal size, promoters and surface carbide species of catalysts were investigated by many characterization technologies. The contents of the work consists of:(1) The effects of ethylene glycol modified co-precipitation were applied to develop a catalyst with higher activity and selectivity of light olefins in Fischer-Tropsch synthesis (FTS). The catalysts prepared by an ethylene glycol (EG) modified co-precipitation realized smaller and homogeneously distributed catalyst particles as 15-25 nm, which was two times smaller than that of the catalyst prepared from conventional co-precipitation. The Fe/Mn-EG catalyst has higher activity and enhanced selectivity to light olefins, as well as the doubled olefin to paraffin ratio (C2=-C4=/C20-C40), comparing to un-pretreated catalyst. Furthermore, the addition of magnesium promoter to Fe/Mn-EG catalyst inhibits the chain growth ability, and enhances the formation of light olefins (C2=-C4=), realizing the high selectivity of light olefins as 50.1%. The properties of catalyst structure, active phase, reduction and carburization of obtained catalysts were characterized by N2 physisorption, XRD, SEM, XPS, TPR and DRIFTS measurements.(2) The sole effect of pore size or iron particle size were investigated on formation of light olefins in Fischer-Tropsch synthesis (FTS). The various iron supported catalysts, with very similar iron particle size of ~8 nm and different pore size, were prepared by surface modification of silica support via ethylene glycol. Meanwhile, for comparison, the catalysts prepared from conventional impregnation method formed different iron particle size with different pore size, such as 8.1,13.7 and 17.5 nm, respectively. The iron active phase of different catalysts with similar iron particle size exhibited similar properties, contributing to solely comparing the effect of pore size or iron particle size on formation light olefins. The results indicate that smaller iron or iron carbide particles are advantageous in forming light olefins and the ratio of olefins to paraffins (O/P) in C2-C4 range is more sensitive to the pore size of the catalysts. The obtained catalysts were characterized by N2 physisorption, mercury intrusion porosimetry, XRD, TEM, XPS, TPR and Mossbauer spectroscopy.(3) We report on the conversion of synthesis gas to light olefins (C2-C4) with selectivity up to 60%, using catalyst that constitutes manganese nanoparticles homogeneously dispersed on Fe3O4 microspheres. The electronic state of surface carbonaceous species was affected by the Mn promoter, leads to the formation of special iron carbide (θ-Fe3C). By combining X-ray Absorption Fine Structure Spectroscopy (XAFS) and Mossbauer spectroscopy, it was found that the reactivity for light olefins formation was highly dependent on the content of θ-Fe3C. In addition, the special Fe3C-MnOx catalytic interfaces may also contribute to the formation of light olefins. These results demonstrate that the iron carbide phase and the location of promoters play a crucial role in the design of active and selective FTS catalysts.(4) The Rotating Packed Bed (RPB) reactor was used for the FTS reaction to control the product distribution, and the product distribution was significantly adjusted by the high gravity environment since the intensified mass transfer of reactants and products improved the formation of selected specific hydrocarbons, especially the light hydrocarbons. By combining precipitated iron-based catalysts and RPB reactor, it was found that the reactivity for light olefins formation was highly improved. The light olefins selectivity was up to 60% for the precipitated Fe/Mn-EG catalysts in the vertical type RPB reactor. In addition, this work proposes a new approach in the alternative route for the sustainable production of light olefins from synthesis gas.
Keywords/Search Tags:Fischer-Tropsch synthesis, light olefins, iron-based catalysts, manganese, iron carbide, pore size, particle size
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