(142bq) Hydrodynamics and Dry Pressure Drop in Structured Packing

Anup Sunkarwar, Vimal Kumar

Department of Chemical Engineering, Indian Institute of Technology Roorkee

Email: anup.14023@gmail.com, vksinfch@iitr.ernet.in

In recent years supercritical fluids have been used in the extraction of natural raw materials in food, pharmaceutical, beverages industries, etc. (Krukonis, 1994; Gamse, 2005). The liquid phase supercritical fluid extraction (SFE) has been carried out in packed columns with structured packing due to their relatively high surface area, free volume, high mass transfer efficiency and high column capacity. As compared to the unstructured packing, structured packing has high cost and low capacities at high flow rates. However, with an accurate design columns with structured packing can be design with low cost and high separation efficiency.

It is difficult to assess the efficiency and influence of shape and geometry in structured packing at severe conditions (at high temperature and pressure) during the process. Computational fluid dynamics (CFD) tools can be very helpful in predicting hydrodynamic behavior and pressure drop under severe conditions also reduces number of experiments to predict the efficiency of the process. An extensive literature is available where hydrodynamics and pressure drop is studied using computational fluid dynamic (Hodson et al., 1997; Von Scala et al., 1999; Petre et al., 2003a and 2003b). Fernandes et al. (2008) computationally estimated the dry pressure drop in the Sulzer EX structured gauze packing considering two approaches: the simpler model with two corrugated packing sheets with periodic boundary conditions and the complex model with thirteen packing sheets representing one-third of the actual packing element, tightly inserted into a cylindrical tube. They compared the pressure drop results with experimental data for supercritical carbon dioxide of Meyer (1998), Olano (1995) and Stockfleth and Brunner (1999). They further extended the dry pressure drop work to wet pressure drop calculations using computational fluid dynamic tool in a whole Sulzer EX packing element (Fernandes et al., 2009).

In the present work the hydrodynamics and dry pressure drop numerically simulated in structured packing (Flexipac 1Y, Sulz packing) using commercial computational fluid dynamic software, FLUENT 6.3, ANSYS. Also the hydrodynamics and pressure drop is studied in a newly proposed zigzag structured design considering supercritical fluid extraction using CO₂. The zigzag pattern resulted into centrifugal forces and hence better interaction between the fluid elements, which may increase the efficiency of the extraction process.

It is observed that the present simulated predictions using realizable k-ε model considering pressure gradient effects is in good agreement with the results reported in the literature. The prediction were analysed in terms of resistance factor (Ψ) versus Reynolds number for the comparison of different models and literature results. The realizable k-ε model considering pressure gradient effects model is used to study the pressure drop inside the newly developed geometry. The new geometry with the same computational domain of two sheet packing, but with the changed geometric parameter with the zigzag orientation is developed. The effect of the geometric packing parameter variation on the dry pressure drop friction term inside the newly developed structured packing is studied. It has been observed that the zigzag pattern structured packing resulted into small pressure drop as compared to the Sulz EX packing. Also the wall effects of the column with the zigzag patterned geometry are reduced, which is one of the reasons that the pressure drop is less as compared to the other structured packings. The flow is developed only in the inner channels for the sulz packing while it is developed in the entire computational domain of the zigzag packing leading to the less pressure drop as compared to the sulz packing. Fluid element interaction between different fluid elements is more in the zigzag packing as compared to sulz EX packing due to the zigzag orientation where the flow experiences maximum directional changes. Therefore the zigzag patterned packing have less pressure drop and better mixing performance. Further work is proposed for the development of entire column and to carry out the simulations for CO₂absorption process.

References

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