还原条件对高温费托合成熔铁催化剂性能的影响

Effects of reduction conditions on fused iron catalyst for high temperature Fischer-Tropsch synthesis

为了探索熔铁催化剂还原过程中物相变化及对高温费托反应性能的影响,采用SEM(扫描电子显微镜)、EDS(X射线能谱仪)、BET和原位XRD(X射线衍射)对催化剂进行了表征。并在类似工业条件(340℃、2.4MPa、H_(2)/CO比例为3.8)下,于固定床反应器上评价了费托反应性能。分析了不同升温过程、氢气分压、还原空速和还原时间对高温费托熔铁催化剂物相结构、晶粒尺寸、还原度等的影响。结果表明:起始升温速率影响显著,>2.4℃/min时容易使催化剂局部温度过高进而还原速率过快,导致α-Fe晶粒快速长大并团聚。氢气分压影响还原度有转折点,>70%时还原度变化趋势相同;对α-Fe晶粒尺寸的影响表现在低氢分压时更稳定,40h内40%H_(2)/Ar条件仅增加1nm。高还原空速(10000h^(-1))使得还原催化剂表面孔道分布、孔径尺寸更均匀,还原效率及极限还原度更高;而低还原空速(1000~2000h^(-1))则使得还原催化剂表/体相结构稳定,晶粒尺寸更稳定,费托合成反应活性高。还原时间在其他条件固定下有最佳值,420℃恒温还原到极限还原度后,α-Fe晶粒尺寸随还原时间延长继续缓慢增长。通过研究获得了适宜的还原条件为:还原升温速率0.4℃/min(200~350℃)~0.8℃/min(<200℃),空速5000h^(-1),还原时间<30h。

In order to investigate the phase change of fused iron catalyst during the reduction process and its effect on the performance in high temperature Fischer-Tropsch reaction,the catalyst was characterized by SEM,EDS,BET and in-situ XRD.The performance of Fischer-Tropsch reaction was evaluated in a fixed-bed reactor under 340℃,2.4MPa,and H_(2)/CO=3.8,similar to industrial conditions.The effects of different heating processes,hydrogen partial pressure,reduction space velocity and reduction time on the phase structure,crystal size and reduction degree of the fused iron catalysts were analyzed.The results showed that the initial heating rate was quite significant.When the initial heating rate was over 2.4℃/min,reduction rate was too fast,and theα-Fe grains grown and agglomerated rapidly.The influence of hydrogen partial pressure on the reduction degree was not monotonical,and the change trend of reduction degree is the same when it is>70%.α-Fe was more stable at low hydrogen partial pressure,which,under 40%H_(2)/Ar,increased only by 1nm within 40h.The high reduction space velocity of 10000h^(-1) made the catalyst surface pore distribution and pore size more uniform,and the reduction efficiency and limit reduction degree higher.The low reduction space velocity 1000—2000h^(-1) gave the reduction catalyst stable surface/bulk structure,more stable crystal size and higher reaction activity.After reduced at 420℃to the limiting reduction degree,theα-Fe continued to grow slowly with the increase of reduction time.The suitable reduction conditions were obtained as:reduction heating rate in between 0.4℃/min(200—350℃)—0.8℃/min(<200℃),space speed of 5000h^(-1),and reduction time less than 30h.