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A New Methodology to Measure the Solids Dispersion in High Pressure Slurry Bubble Column Reactor

Lu Han, Department of Chemical Engineering, Washington University in St. Louis, One Brookings Drive, Urbauer Hall 208, Campus Box 1198, St. Louis, MO 63130 and Muthanna H. Al-Dahhan, Department of Chemical Engineering, Washington University, One Brookings Drive, Saint Louis, MO 63130.

Slurry bubble column (SBC) reactors are cylindrical vessels in which gas is distributed through a sparger into a suspension of liquid with solid particles. Due to the simple structure and excellent heat and mass transfer characteristics, SBCs have been widely implemented in various industrial processes, especially with recent rising interests in gas conversion processes such as Fischer-Tropsch synthesis of liquid hydrocarbons and liquid phase synthesis of methanol (Parkinson, 1997; Dudukovic et al., 1999; Krishna et al., 2000). The mixing and distribution of the solids phase plays an important role in the reactor performance. Although the solids dispersion in SBCs has been the focus of a number of studies over decades, the studies are limited to steady state and the reported measurement method is mostly limited to sample-withdrawing or bed sedimentation. The majority of the studies measured the solids axial distribution by withdrawing samples from ports (Nakao et al., 2000; Zhang et al., 2002; etc.), while some others by settling the solids onto several shutter plates (Matsumo et al., 1989 and 1992). These methods are invasive and limited to steady state measurements. Besides, all of the previous solids dispersion studies were performed at ambient pressure. Hence, the pressure effect needs to be investigated by performing measurements at high pressure in SBCs.

This work presents a new methodology to determine the solids phase axial dispersion and distribution using the gamma-ray based computer automated radioactive particle tracking (CARPT) technique. CARPT measures the solids motion by tracking a single radioactive tracer particle made similar in size and density to the solids phase. Lagrangian trajectory data of the particle was obtained by the CARPT, which were further used to generate response curves of virtual batch solids tracer. The solids axial dispersion coefficient was determined by using the transient sedimentation-dispersion model (SDM) and the dynamic response data. As a comparison, the steady state method was also used to determine the solids dispersion. The solids concentration distribution data used in the steady state method were obtained statistically from the occurrence data of the radioactive tracer particle. The good fitting results of both methods show the solids distribution in slurry bubble column can be well described by the sedimentation-dispersion model at both ambient and high pressures. The quantitative Ds results from the transient method were found to be much less sensitive to ust values and more trustable than the results of the steady state method, which are directly affected by the ust estimation. The solids axial dispersion increases with the superficial gas velocity due to both the higher solids circulation velocity and the higher solids axial eddy diffusivity. The solids axial dispersion is enhanced at high pressure due to the dominant effect of higher solids circulation velocity, although the solids axial eddy diffusivity and turbulence are reduced.

Keywords: Slurry bubble column, Solids distribution, Axial dispersion, Radioactive particle tracking

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