Accumulation of liquid hydrocarbons during Fischer-Tropsch fixed-bed synthesis and process optimization based on periodical pore filling and pore draining

在费托合成（FTS）中，合成气（CO和H2）在基于Fe或CO的多相催化剂上转化为更高的碳氢化合物。对于所谓的低温费托合成（ft） （T < 250°C），到目前为止，通常认为在反应的初始阶段，催化剂的孔隙在几天内完全充满了液体反应产物（蜡）。这导致了一个不必要的但不可避免的孔隙有效系数的降低，并返回到较低的活性和更高的甲烷选择性，特别是，如果在技术固定床反应器的情况下使用毫米颗粒（压降）。在一个将于2020年秋季完成的dfg项目的过程中，我们可以证明，催化剂（钴）的孔隙并不总是完全填充，填充时间甚至可能持续几个月，尤其是如果链生长概率很高（> 0.9）。填充过程取决于H2/ co比、压力和温度。所有的实验都是用单催化剂颗粒在磁悬浮天平上进行的。在申请的新研究项目（申请继续资助）中，这项研究应继续进行，并转移到固定床反应器中的FTS。此外，还应研究一种基于FTS填充孔和氢解排孔的新型瞬态工艺。基于实验和理论研究（建模），应该回答以下两个主要问题：1)固定床反应器中CO转化率和气相组成的局部（轴向）差异是否会影响FTS初始阶段局部孔隙填充程度，以及这如何影响催化剂的有效活性和选择性（这里以钴为基础）？2)做一个固定的暂态操作。床反应器，即从孔隙填充（FTS）到孔隙排水（在不含CO的原料气中进行氢解）的周期性切换，导致更高的平均反应速率和更高的液态烃形成速率。固定床反应器的颗粒部分/完全充满不同程度的CO转化率（高达40%）的较高碳氢化合物。在一定时间后停止FTS，对不同轴向位置的颗粒进行采样和分析（即填充程度，孔隙中液相组成）。氢解实验将获得快速排孔过程的最佳反应条件。孔隙填充率也应直接通过在固定床反应器（强制流动）的磁悬浮天平中进行原位测量来研究。最后，在FT固定床反应器中高碳氢化合物的积累以及交替的瞬态过程（孔隙的填充/排水）应该根据所提出的瞬态过程的优化进行建模。

In Fischer-Tropsch synthesis (FTS), syngas (CO and H2) is converted to higher hydrocarbons over heterogeneous catalysts based on Fe or Co. In case of the so-called low-temperature FTS (T < 250 °C), it was until now generally assumed that the pores of the catalyst are completely filled with liquid reaction products (wax) within a couple of days in the initial phase of the reaction. This leads to an unwanted but unavoidable decrease of the pore effectiveness factor and in return to a lower activity and a higher selectivity to methane, above all, if mm particle are used as in case of technical fixed-bed reactors (pressure drop).In the course of a DFG-project, which will be completed in autumn 2020, we could show that the pores of the catalyst (cobalt) are not always completely filled and the filling time may even last several months, above all, if the chain growth probability is high (> 0.9). The filling process depends on the H2/CO-ratio, pressure and temperature. All experiments were conducted with single catalyst particles in a magnetic suspension balance.Within the applied new research project (application for continuation of funding), this research should be continued and transfered to FTS in a fixed bed reactor. In addition, a promising new transient process based on pore filling by FTS and pore draining by hydrogenolysis should be investigated. Based on experimental and theoretical studies (modelling), the following two main questions should be answered: 1) Do local (axial) differences in CO conversion and gas phase composition, respectively, in a fixed bed reactor have an influence on the local pore filling degree during the initial phase of FTS, and how does this affect the effective activity and selectivity of the catalyst (here based on cobalt)? 2) Does a transient operation of a fixed-.bed reactor, i.e. a periodical switch from pore filling (FTS) to pore draining (hydrogenoloysis in case of a feedgas without CO) lead to a higher mean reaction rate and a higher rate of formation of liquid hydrocarbons?The following examinations are intended: The particles of a fixed-bed reactor are partly/completely filled with higher hydrocarbons at different degrees of CO conversion (up to 40%). The FTS is stopped after a certain time, and particles at different axial positions are sampled and analyzed (i.e. filling degree, composition of liquid phase in the pores). Experiments on the hydrogenolysis will gain the best reaction conditions with regard to a rapid pore draining process. The rate of pore filling should also be investigated directly by in situ measurements in a magnetic suspension balance with a fixed-bed reactor (forced flow). Finally, the accumulation of higher hydrocarbons in the FT fixed-bed reactor as well as the alternating transient process (filling/draining of pores) should be modelled with regard to an optimization of the proposed transient process.