(62a) Multiphase Distribution and Holdup Investigations in Trickle Bed Reactors and Upflow Moving Bed Reactors Using Gamma-Ray Computed Tomography (CT) Technique

Multiphase flow reactors are gas-liquid-solid contacting equipment widely used in many fields such as petrochemical, petroleum, fine chemicals and biochemical industries. Trickle bed reactors, in which gas and liquid phase reactants flow in downward direction over a bed of solid catalyst particles, have quite versatile applications such as oxidation reactions, petroleum processing, hydrogenation reactions, esterification, F-T synthesis etc. Moving bed reactors are typically vertical reactors with gas and liquid flow upward through catalyst bed in hydrodesulfurization, hydrodenitrogenation, hydrodemetalization etc.

Phase distributions and holdups in multiphase flow reactors can affect many other design parameters such as wetting efficiency, heat and mass transfer coefficients etc. The phase holdup in the bed also controls the phase residence time and conversion of the reactants. It is therefore essential to understand how gas/liquid distribute at different axial levels and how gas/liquid holdup varies along such as (1) column diameter, particle size and shape, (2) operating conditions like gas/liquid flowrates, (3) physical and chemical properties of fluids.

Proper and highly accurate investigations of phase distributions and holdups are imperative for the design and diagnoses of multiphase flow reactors. Gamma-ray computed tomography is a non-invasive technique that provides the cross-sectional images at different axial levels by rotating the gamma source and its detectors around the object. It can visualize and quantify the phase distributions and holdup profiles for multiphase flow reactors which cannot be measured by other techniques. The Cs-137 source and its detectors are arranged and designed to provide a 40° gamma-ray fan beam with 5 mm height in the horizontal plane. This fan beam can cover columns up to 24 in. (0.6 m) in diameter for scanning. The detectors are shielded with lead collimators which has 5×2 mm fine apertures to obtain narrow gamma-ray beams and minimize the scattered gamma-ray in order to achieve better spatial resolution. The data acquisition system, which converts the detection of γ photons to the number of photon counts in computer, consists of NaI crystal detectors, preamplifiers, pulse processors and stepper motors. All the parameters such as the sample time, sample frequency etc. can be specified in the input text file which commands the motor controller to move the detector plates and source plate to a precise distance. While scanning, all the photon counts will be recorded in the output text file for future data analysis. For one CT scanning, Cs-137 source has 197 source positions (197 views). At each source position, the detector array moves counter clockwise for 20 times in 0.13° angular steps which are total 21 positions (21 projection measurements). Computer will collect the photon counts for each projection. After finishing the scan, the data of 62055 projections (197 views × 15 detectors × 21 projection measurement per detector) of gamma-ray that pass through the column will be written to the specified output text file. The main goals of CT data analysis are obtaining the phase distribution images (cross-sectional image for each level) and phase holdup profiles. The original data collected from CT scan are processed by alternating minimization (AM) algorithm to depict the attenuation values of the cross-sectional images.

Different multiphase flow reactors such as trickle bed reactor, upflow moving bed reactor have been deeply researched under many selected conditions using the Gamma-ray CT technique at Multiphase Flow Reactor Engineering and Application Laboratory (MFReal) in Missouri University of Science and Technology. The phase distributions at different axial levels can be easily obtained with high resolution images. Azimuthally averaged phase holdup profiles and local linear holdup profiles for different directions are the main achievements of Gamma-ray CT technique. Some other parameters such as wetting efficiency and internal liquid holdup inside porous catalyst are also able to be obtained combining with some special image processing methodologies.