(121b) Tomography Gamma-Scan Applications for Packed Towers

Packed columns are widely used in distillation, absorption and stripping processes.  It is well known that liquid maldistribution can severely reduce column efficiency and operability.  Maldistribution creates localized disparity between the desired liquid/vapor ratios; reduces the effective packing efficiency for mass and heat transfer, translating to a high HETP and causes under-irrigated or dry areas that can result in localized coking or sintered chemical deposits within the packed bed.

For packed towers a grid-scan, 3 or 4 equal-distant scans would be done to qualitatively measure the liquid distribution.  While this gamma scan procedure is fine for screening to detect macro problems of severe liquid maldistribution there are more subtle patterns of maldistribution better diagnosed by a more comprehensive method.

 A tomography scan provides cross-sectional images of three-dimensional (3D) objects. The most prevalent use of tomography is still in medical applications, for instance the Computerized Tomography (CT) device that is used for non-destructive examination of the human body. Gamma-ray tomography takes advantage of the penetrating properties of high-energy gamma radiation to generate a high-resolution cross sectional density profile through a packed bed.

This paper will describe gamma-ray computer-aided tomography (CAT-Scan) for measuring the density profile across a packed bed showing its liquid distribution.  Applications of CAT-Scans for diagnosing, monitoring and solving maldistribution and coking problems in refinery Vacuum and FCC Main Fractionating towers will be included in the presentation.