The rising demand for transportation fuels, and the depletion of fossil fuel resources has attracted the production of transportation fuels from renewable energy sources such as biomass. The biomass is first converted to syngas, which is further converted to liquid fuels via Fischer-Tropsch (FT) Synthesis. Iron and cobalt based catalysts are commonly used for Fisher-Tropsch Synthesis. Different technologies were developed for Fischer-Tropsch Synthesis, namely fixed bed, fluidized bed, and slurry reactors. Of the technologies mentioned, the slurry reactors are preferred because of high heat and mass transfer. Generally, the FT reactors are operated at a pressure > 20 bar and temperature > 220oC.
In a typical slurry FT reactor, the feed consists of 16 to 20 wt% catalyst. A fresh catalyst particle size ranges from 20 to 400 microns (James, N. et al., 2005; Sergio, M.,Harold, W., 2014; Basha, O., 2015). In a slurry FT reactor, the syngas will be bubbled through the catalyst slurry. The high rates of heat and mass transfer are due high degree of mixing. Due to high degree of mixing, the catalyst particles will undergo attrition and the exit catalyst particles size were reported in range of 5 to 120 micron depending on the operating conditions of the reactor (James, N. et al., 2005; Oluwaseyi, O., et al., 2006). The exit stream of a slurry reactor consists of liquid fuel and the catalyst. The catalyst particles need to be remove before they are charged into internal combustion engines, they create problems otherwise.
Different methodologies were proposed for the separation of FT catalyst from catalyst-wax slurry. The proposed methods include, batch settling, lamella settling, filtration (using cake filtration and filter press), magnetic field assisted settling (for iron catalyst).
The proposed work addresses a dynamic separation of catalyst particles from a slurry in a continuous split flow process, where the catalyst slurry fed is split in to upward and downward flows. The ratio of upward and downward velocity is a critical parameter. This dynamic split flow. For a proof of concept, the split flow settler is modelled using fundamental mass and momentum conservation equations considering the axial convection and dispersion under steady state conditions. The individual particle velocities were calculated using Richardson-Zaki model (Richardson, J.F., and Zaki, W.N., 1954). The solution of the modelled equations was obtained by using first order difference for the convection term and second order difference for the dispersion. The equations were coded in MATLAB®.
The developed model was validated against a static settler and compared results with the existing experimental results on particle segregation (Asif and Petersen, 1994). Different parameters for the split flow settler was studied namely, the slurry feed rate, particle size variation (5 to 93 microns) in the slurry, catalyst wt% in the slurry, average particle size in the feed, and the ratio of velocities between up and downward flow. It was found that, the ratio of velocities between upward and downward flow is an important parameter. It was found that all the most of the fines were removed from the fuel. The catalyst wt% in the upward flow can be brought down to less than 0.15%.
Dr. Udaya Bhaskar Reddy Ragula, “Dynamic Split-Flow Separation of Micron-sized Slurry Fischer-Tropsch Catalyst”, in AIChE Annual Meeting, Pittsburgh, USA, 2018.