Packed columns, multiphase flow

Sampling of materials consisting of more than one phase at the sampling point is particularly difficult. Common examples are the sampling of liquid or vapour from a plate absorber or distillation column or from a packed column, or the sampling of a multiphase mixture flowing in a pipe or duct. In such cases it is not always known whether the liquid or vapour has been sampled or some combination of both. 

Porous media play an important role in a wide number of disciplines. Geologists are interested in the flow of water through aquifers, and the flow of oil in reservoirs. Chemical engineers are interested in the effectiveness of porous catalyst particles and in multiphase flow in packed columns. Biologists are interested in the flow of blood through organ tissues. Porous media are also important in the fields of soil science (transport of water and nutrients to roots), environmental science (soil remediation and wastewater treatment),... 

Computational fluid dynamics (CFD) is rapidly becoming a standard tool for the analysis of chemically reacting flows. For single-phase reactors, such as stirred tanks and empty tubes, it is already well-established. For multiphase reactors such as fixed beds, bubble columns, trickle beds and fluidized beds, its use is relatively new, and methods are still under development. The aim of this chapter is to present the application of CFD to the simulation of three-dimensional interstitial flow in packed tubes, with and without catalytic reaction. Although the use of... 

The mass balances (Eqs. (10.3) and (10.4)) assume plug-flow behavior for both the vapor and the liquid phase. However, real flow behavior is much more complex and constitutes a fundamental issue in multiphase reactor design. It has a strong influence on the column performance, for example via backmixing of both phases, which is responsible for significant effects on the reaction rates and product selectivity. Possible development of stagnant zones results in secondary undesired reactions. To ensure an optimum model development for catalytic distillation processes, we performed experimental studies on the nonideal flow behavior in the catalytic packing MULTIPAK [77]. 

Regular flow patterns are provided by the segmented flow in a single capillary or in multi-channel microreactors. Miniaturized packed-bed microreactors follow the paths of classical engineering by enabling tridde-bed or packed bubble column operation. M ost of the microstructured multiphase reactors are at the research stage. Due to the small reaction volumes th will find their appHcation mainly in small-scale production in the fine chemical and pharmaceutical industries. 

The above discussion and examples illustrate that reactor scale models have not advanced much during the past decades and this hinders our ability to reduce risk in implementation of new more efficient catalytic technologies. To make progress, it is necessary to develop improved and more accurate descriptions of flow and mixing in typical multiphase reactors. Multiphase reactors frequendy encountered in practice (Fig. 1.10) such as risers, bubble columns, fluidized beds, packed beds, and stirred tanks are opaque so that not all flow visualization tools (Table 1.6) are suitable. 

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