(412a) a-Olefins Synthesis from Fischer-Tropsch Reaction in a Trickle Bed Reactor

Abstract: Recently, there is a world-wide increasing interest in the Fischer-Tropsch (FTS) as a method to produce synthetic liquid fuels and chemicals from coal, natural gas or biomass in view of dwindling petroleum reserves. The FTS product spectrum consists of a complex multicomponent mixture of linear and branched hydrocarbons and oxygenated products. The main products are linear paraffins and α-olefins. Linear α-olefins are very important chemical intermediate for a number of industrial and consumer products, including polymers, synthesis fluids, surfactants, additives and specialty chemicals. For example, C5?C8 α-olefins are used as the comonomers in ethylene polymerization and the longer chain α-olefins provide premium synthetic lubricants. Therefore, significant economic benefit can be achieved by increasing the α-olefin selectivity of the raw FTS products. In general, modern FT synthesis is operated in the gas phase-fixed bed system, in the gas phase-fluidized bed system and in the liquid phase-slurry system. In the gas phase-fixed bed system, catalyst bed and catalyst pores are plugged by a waxy product to suppress the catalytic performances. One can avoid this problem, if the fixed bed is washed with solvent continuously or intermittently during the operation. The continuous washing system is a kind of ??trickle bed reactor'' system. Therefore, the washing material is a kind of solvent of the reactor. It is expected that the solvent will affect the FTS reaction. One of the present authors has found that the supercritical solvent affects the FTS reaction to a marked extent, especially for the selectivity of olefins, α-olefin, CO2, as well as the minimization of waxy product in the catalyst bed. The minimization effect can be attributed to an extraction effect of supercritical hexane (or pentane). The extraordinarily high selectivity of olefin in the super critical fluid (SCF) phase reaction has been explained by the well-balanced extraction of α-olefin from the catalyst surface and its transfer inside of the catalyst pores. In recent studies of FTS on Ru- and Co-based catalysts, Fan and Fujimoto has introduced a supercritical phase into the conventional gas?solid phase FT process. Results show that the α-olefin content decreases with an increase in the carbon number, but this phenomenon is more moderate than in the gas phase- or liquid phase-FTS. We deduce that the reaction media will remarkably affect the α-olefin selectivity due to the different capabilities of extraction and transportation. In this study, our major aim is to obtain a deeper insight into the effect of solvent on the α-olefin selectivity and to assist the product distribution towards the desired α-olefin. For this purpose, we have undertaken a thorough investigation of the effects of solvents over 20 wt.% Co/SiO2 catalyst under the typical reaction conditions of 513 K, 4.5 MPa and H2/ CO = 2. Not only supercritical n-hexane, but also near critical fluids and other kinds of fluids such as n-octane, n-decane, n-dodecane, n-hexadecane, iso-octane and decahydronaphthalene are studied. The effect of carbon chain length, molecular structure or content of solvent on the CO conversion, product distribution and especially on the α-olefin content in the FTS products, are discussed in details.The results show that the selected solvents do not obviously affect the catalytic activity or chain growth factor, but do exhibit remarkable influence on the α-olefin content. In the case of n-hexane solvent, the α-olefin content decreased markedly with carbon number. The n-decane solvent gives the highest α-olefin selectivity which is independent of carbon number on the C6+ products at the level of about 40%. This result is due to quicker desorption, diffusion and more suppressed re-adsorption for primary α-olefin. With the solvent of the same carbon number, an n-paraffin gives a higher α-olefin selectivity than a branched paraffin or a cyclic paraffin.