(388b) Simulation and Experimental Studies in Dual Source Gamma Ray Computer Tomography for Imaging Three Phase Systems

Multiphase systems are a reality in the chemical and biochemical industry and are used in abundance. These systems usually comprise of solids, liquids and gases. As hydrodynamic interaction of these phases' affects the performance of these systems it is important to understand it better to improve the efficiency of such systems. Tomography has been successfully employed to visualize the hydrodynamics of multiphase systems such as chemical and biochemical reactors in the past. Most of the tomography methods (gamma ray, x-ray and electrical capacitance and resistance) are successful in imaging dynamic systems with two phases (gas-liquid and liquid-solid) or have been extended to GLS (gas-liquid-solid) systems where the solid is stationary, covering different types of reactors and process equipments. Toye et. al. (2001) has successfully applied X-ray tomography to determine the liquid distribution and hold up in structured packing in a 0.6-m diameter. Roy. S et. al. (2004 and 2005) has successfully applied gamma ray tomography in monoliths of 2 inch diameter and other structured packing of 12 inch diameter to determine liquid and gas phase distribution.

Whoever, a significant number of chemical and biochemical reactors consists of dynamic three phases. A lot of research effort is directed towards the development of tomography techniques to image such dynamic systems with partial successes is specific systems with limited operating conditions.

Warsito et al (2001) have shown that it is possible to image three phase systems with the use of electrical capacitance tomography with a multi-criterion optimization technique using an analog neural network involving the use of a linear back projection algorithm. Warsito and Fan (2003) have applied this to a three phase slurry bubble column system. The hold up tomograms have been reconstructed assuming that uniform solids hold up in the gas-liquid part system of the column is the same as the in the three phase region where the gas bubbles are present. This assumption has a limited validity. Only at very high solids hold ups (around 40%) and hence this technique can't be applied in systems where the hold up is in the range of 10 -20%. George et. al. (2001) have applied electrical impedance tomography and gamma ray densitometry tomography in conjunction to determine hold up of dynamic gas, solid and liquid is a slurry bubble column system. However the gamma ray densitometry tomography reconstructs the holdup profiles assuming azimuthally symmetry of the phase distribution. Again, this assumption severely limits the scope of application of such a technique.

This works presents details of studies done to develop algorithms for imaging three phase chemical systems based on the Alternating Minimization technique developed by Benac and O'Sullivan (2002). Tomography data have been generated via computer programs for on theoretical three phase (gas liquid and solid) phantom. This data has been processed via the algorithm developed for image-reconstruction. The reconstructed images describe the phase distribution of the three phases. This study has been carried out for a combination of 137Cs - 60Co, and 75Se - 137Cs gamma ray emitting isotopes, with the goal to characterize the error and limitations of the reconstruction for a given combination of gamma ray sources. The results and key findings of this study will be presented and discussed in detail.

References:

1. Goerge D.L., Shollenberger K.A and Toczynski J.R., O'Hern T.J., and Ceccio S.L. (2001) Three phase material distribution measurements in a vertical flow using gamma ray- densitometry tomography and electrical- impedance tomography. International Journal of Multiphase flow. 27. pg. 1903-1930. 2. Roy S., Kemoun A., Al-Dahhan M.H., and Dudukovic' M.P., Skourlis T. B., Dautzenberg F. M., Countercurrent flow distribution in structured packing via computed tomography. Chemical Engineering and Processing, 44, 59-69, 2004 3. Roy S., Al-Dahhan M.H, Flow distribution characteristics of a gas?liquid monolith reactor Catalysis Today July 2005, accepted for publication. 4. Toye, D.; Crine, M.; L'Homme, G.; Olujic, Z.; Marchot, P. (2001), Measurement of the liquid distribution and hold-up in structured packing by X-ray tomography. Recents Progres en Genie des Procedes , 15(82, Information, Modelisation, Simulation et Optimisation), pg. 433-440. CODEN: RPGPEX ISSN: 1166-7478 5. Warsito, W.; Fan, L.-S.ECT (2003), imaging of three-phase fluidized bed based on three-phase capacitance model. Chemical Engineering Science 58(3-6), pg. 823-832. 6. Warsito, W.; Fan, L-S (2001), Neural network based multi-criterion optimization image reconstruction technique for imaging two- and three-phase flow systems using electrical capacitance tomography. Measurement Science and Technology 12(12), pg. 2198-2210.