High Gradient Magnetic Separation Of Catalyst/wax Mixture From Fischer-Tropsch Synthesis: Modeling And Experimental Study

Separating the iron-based catalyst/wax mixture from Fischer-Tropsch synthesis(FTS) products is one of the most important challenges in the development of FTSslurry bubble column reactor. High gradient magnetic separation (HGMS), with theadvantages of low cost, negligible pressure drop, high selectivity, excellent separationefficiency and ability to recycle the magnetic particles, is a promising technique forthe catalyst/wax separation.The mechanism of the high gradient magnetic separation (HGMS) process wasinvestigated in this dissertation. A three-dimensional mathematical model wasestablished to describe the magnetic field and magnetic force distribution withmulti-wires for HGMS of catalyst particles from FTS wax. The calculation resultsindicated that the magnetic force, as the dominant influence factor of the separationprocess, is determined by the distribution of the magnetic field. The magnetic force isproportional to the magnetic field strength and the magnetic field gradient. However,the distribution of magnetic field in the separator was significantly associated with thematrix arrangement patterns and the background uniform external magnetic fieldstrength.Based on the hydrodynamic software Ansys Fluent13.0, this dissertation studiedthe influence factors in the separation involving the wire interval, the matrix axialdistance, the arrangement angle as well as the back ground uniform magnetic fieldstrength in the column-wire-based unit by means of finite element analysis. It wasfound that both the excessive wire interval and the improper matrix arrangementpattern can lead to the formation of percolation channel in the separator. Thepercolation channel is a negative factor causing lower separative efficiency andinstability of the separation operations. Moreover, the magnetic force acting on aparticle is also proportional to the external magnetic field strength, which is a veryimportant influence factor when the magnetization of the matrix was not saturated.The simulation results shows that there are percolation channel with the arrangementangle of90°, and the optimal condition is the matrix with the wire interval of2mm,the axial distance of2mm, the arrangement angle of45°and the external magneticfield strength of398089A/m. A high gradient magnetic separation process was designed for the catalyst/waxmixture separation coming from a typical industrial process of F-T synthesis. Thehigh gradient magnetic separator adopted vertical uniform magnetic field generatedby electromagnetic, and the matrices with three kinds of wire intervals were used asinner parts. In order to validate the simulation results, experimental investigationswere also carried out to disclose the effects of four main factors involving the wireinterval, the axial distance between matrices, the arrangement angle between matricesand the external magnetic field strength on the separation efficiency in HGMS.Through the experimental optimization (the wire interval of2mm; the axialdistance of2mm; the arrangement angle of45°and the external magnetic fieldstrength of398089A/m), the magnetic iron content of the catalyst/wax mixture canbe reduced to less than30ppm, with the separation efficiency higher than99.95%which was in well accordance with the prediction of the established multi-wiremodels.