Hydrodynamic analogy approach

Up to now, a rigorous, purely theoretical analysis of separation processes, e.g. distillation and absorption, is unfeasible, due to the complexity of the process fluid dynamics in real equipment. Therefore, the design and optimisation tasks are usually based on the application of equilibrium and rate-based models. These models use a very simplified fluid-dynamic description and their realisation requires several experimental parameters to be estimated.

Therefore, we developed a basically new approach towards modelling of non-reactive and reactive separation processes in structured arrangements (e.g. structured packings). It is based on the so-called hydrodynamic analogies (HA) between complex flow patterns in real columns and simplified fluid-dynamic elements. These elements are described in a realistic way and combined to a simplified but physically relevant flow pattern of the whole apparatus. In this way, the method permits the application of rigorous partial differential transport equations for the modelling of equipment units with structured internals [1-3].

The developed HA model's have been successfully applied for the simulation of different distillation and absorption/desorption systems. They are able to determine HETP-values for a given system and to directly account for the influence of packing geometry [1].

Another process studied with the HA method is reactive stripping. With a film-flow monolith reactor is applied for a heterogeneously catalysed esterification. In this process, the by-product water shows an inhibiting and equilibrium-limiting effect. Therefore, it has to be removed by stripping from the liquid phase. The film flow is largely influenced here by the porous catalyst layer [3,4].

A further study is dedicated to the reactive distillation in catalytic packings. These packings are set of different basic elements: gause wire bags filled with catalyst allow the heterogeneous reaction of the liquid phase, whereas the corrugated sheet channels stay for the separation. Within this process, new flow patterns appear that interact and require new analogies to be developed [5].

The hydrodynamic analogies are also used for the description of non-reactive and reactive adsorption processes [6].
 
The development of hydrodynamic analogies is always based on a thorough observation and evaluation of the flow pattern which depends on physical properties and gas and liquid loads in the separation columns. These observations can be done visually, when working with glass apparatuses, or using tomographic equipment involving the phenomena inside the internals and catalytic bags [7].