(343e) Liquid Flow Distribution in Catalytic Distillation Columns: Use of High Energy X-Ray Tomography

Catalytic distillation is a type of process intensification that utilises close coupling of separation and chemical reaction systems. The combination of heterogeneously catalysed chemical reactions and distillation in one single apparatus has several advantages. Such a hybrid process offers the potential for large savings of capital, energy, materials and significant reduction of waste products that satisfies the increasing demand for sustainable technology. Various catalyst packing configurations based on a regular alternation of zones with weak permeability (catalytic zones) and of high permeability (liquid and vapour transport zones) have been reported in the literature. These sandwich structures allow postponing the flooding beyond usual operating flow rates.

However, designing catalytic distillation columns remains a challenge. The introduction of separation and reaction zones in one single apparatus leads to complex interactions between hydrodynamics, vapour-liquid equilibrium, vapour-liquid mass transfer, internal diffusion and chemical kinetics. Different models of the process as well as new structured catalytic packings have been developed. However, to take advantage of these potentialities requires a better understanding of complex multiphase flow phenomena occurring in catalytic distillation packings.

X-ray tomography has been shown to be an efficient non-intrusive tool to see inside and to adequately image the liquid and vapour flow distribution in packed columns. In this paper, we report on the use of a high energy and high resolution X-ray tomograph able to penetrate through stainless steel packing and dense catalytic zones and to achieve a spatial resolution of a few hundreds of microns.

Description of the X-ray tomography set-up

The generator is a Baltograph CSD450 constant potential generator (Balteau NDT, BE). It may be operated between 30 and 420 kV. The X-ray source is an oil cooled, bipolar TSD420/3 tube (Comet, CH and Balteau NDT, BE), producing a 40° aperture fan beam. Its minimum focus size is 0.8x0.8 mm following norm IEC336 - EN12543. Its intensity may be varied between 2 and 8 mA. The detector is a X-Scan 0.4f2-512-HE (Detection Technology, FI). It is constituted by a linear array of 1280 photodiodes each coupled with a CdWO4 scintillator. The detector is 512 mm long with a 0.4 pixel pitch, which corresponds to a spatial resolution at the centre of the object of approx. 360 µm.

Reconstruction of object cross sections are obtained by a classical linear back projection algorithm adapted to the fan beam geometry implemented in the Fourier domain. In order to eliminate the background noise affecting reconstructed images, various numerical treatments such as thresholding, opening, ... are applied to raw images. 3D reconstructions are performed by stacking 2D reconstructed sections, boxfiltering and computing the isosurfaces (i.e. surfaces of equal X-ray attenuation) of the resulting 3D matrix.

The test rig allows manipulation of rather large objects (max. height: 4m, max. diam: 0.45 m). During the tomographic measurements, the column rotates slowly around its vertical axis, while the X-ray source and the detector remain fixed. The entire scanning unit is located inside a room surrounded by 0.6 m thick concrete wall to prevent exposing operating personnel to X-ray radiation.

The colum used in this study has a hight of 4 m. and an inner diameter of 0.1 m. It is made of transparent PVC. The packing (1.8 m high - 0.1 m diameter) is constituted by the superposition of, from the bottom to the top, one Mellapak 752Y element, five Katapak-SP12 elements and two MellapakÔ 752Y elements. These packings are manufactured by Sulzer Chemtech, CH. They are made of stainless steel. Baskets in the KatapakÔ SP12 elements are filled with 1 mm polypropylene spheres. Both types of packings are 0.10 m diameter and 0.2 m high. Baskets and corrugated sheets. Multiple point source (approx. 4000 drip points/m2) and single point source (approx. 130 drip point/m2) distributors were used to feed the liquid at the top of the column. An air-water system was used. The superficial velocities of the liquid and gas phases ranged between 10 and 40 m3/m2.h and 0.5 and 2 m/s, respectively.

Experimental results

Tomographic measurements have been realised in packing cross sections situated at different heights between the top and the bottom of the packed column. To obtain the sole liquid contribution, the projection data obtained on the dry column are subtracted, before reconstruction, from those obtained at the same height on the irrigated column. The resulting projection data are then used to liquid distribution image. Image analysis techniques allow separating liquid holdup fractions inside and outside baskets of the Katapak SP12 elements. The evolution of these fractions versus gas and liquid superficial valocities as well as versus packing height is examined. Most of the liquid flows inside the catalyst baskets. The pressure drop for dry and irrigated packing has been measured and correlated to the liquid holdup evolution. Comparison with published results and correlation are given.

Conclusions

These X-ray imaging results reported in this paper indicate the technique has significant potential to provide insight into vapour-liquid hydraulics prevailing in complex catalytic distillation metallic packings. This information should greatly assist on-going efforts to develop fundamentally rigorous hydraulic models.