| Unit operations such as distillation, absorption, and stripping rely on the contacting of vapor and liquid phases to achieve a desired chemical separation. Existing models used to predict the performance of packed columns are primarily empirical. Such models were developed using measurements of column bulk properties, such as overall liquid holdup, pressure drop, and inlet and outlet stream compositions. Variations in column behavior on a local scale are not captured by existing models, and constitute an area in which model improvement has a potentially large impact. Development of improved models will require non-traditional data for validation. X-ray computed tomography (CT) has been evaluated as one possible technique for measuring local flow behavior in packed columns.; X-ray CT was used in this work to obtain images of an operating air-water contactor containing Mellapak 250Y stainless-steel structured packing. The image quality of two different X-ray scanner geometries, each with distinct temporal and spatial resolutions, was investigated. The void fraction and liquid holdup was calculated from CT images for a variety of liquid and vapor flow rates below the flood point. In general, average liquid holdup calculated from CT images was within 10 to 15% of holdup measured using traditional techniques. The axial variation in liquid holdup, particularly near packing element joints, was explored. Holdup was shown to vary throughout the column, and was greatest near the joint of adjacent packing elements. The region 0.25 to 0.4 inches above the interface exhibited holdup two to five times higher than the holdup measured in the packing element bulk.; Three-dimensional images of the packed bed were acquired for static and dynamic conditions. The packing surface area, interfacial area between vapor and liquid, and the effective fractional area of a packed bed were calculated using three-dimensional reconstruction of two-dimensional CT images. Packing surface area was in good agreement with vendor-supplied values. Interfacial area measurements were compared with experimental data for vapor-liquid reactive area. Finally, both time variation of liquid flow and measurement noise during the course of data acquisition were quantified by analyzing the variation of X-ray transmission data. |
