CFD Simulations Of The Local-Flow And Mass-Transfer In The Structured Packing

Better understanding of the multi-scale flow behavior and mass-transferphenomena that happened in structured packing columns is of great importance forpredicting their mass-transfer efficiency more accurately. In this thesis, the liquidfilm flow and mass-transfer mechanism at small scale, gas and liquid distributionpattern at large scale, and a multi-scale associated computational mass-transfermethod have been studied preliminary.The local rivulet film flow is the most popular behavior for distillation process instructured packing column. Using multi-scale coupled computational mass-transfermethod to predicting the column performance needs a better understanding of thiskind of flow. Therefore, a two-dimensional two-phase flow CFD model using thevolume of fluid (VOF) method is presented. Two important momentum source termsworking on the film flow, surface tension and interface stress shear, are considered.Based on the simulated results, the influences of the plate structures, liquid propertiesand gas and liquid velocities on the film flow are discussed. A mass-transfer CFDmodel for gas stripping is also presented based on the two-phase flow CFD model.The simulation results indicated that making the interfacial waves more violent is aneffective means to enhance the separation efficiency. Three-dimensional two-phaseflow CFD models for the single rivulet film flow and structured packing rivulet filmflow are also proposed based on the two-dimensional CFD flow model. For validatingthe corresponding theories, three-dimensional LDA technique is used to measure therivulet film flow. The experimental results of the free surface profile shows that themodel proposed here can predict the liquid flow on structured packing surface moreaccurately comparing with other literature models.A new macro liquid distribution model is established, where the wall effect,liquid mixing and allocation process at the crunodes and the liquid transversedispersion are taken into account. Some parameters of the model were regressedusing the experimental data obtained in a two-sheet testing rig for structured packingof Mellapak 250X/Y and 350X/Y. The experimental data include liquid flowdistribution and liquid holdup results for the four types of packings mentioned above.The local rivulet widths used in the model are calculated by the single rivulet flowCFD model, which improves the model precision greatly comparing with the modelsusing empiricism formulae. The local liquid flow characteristics can be obtainedconveniently by this liquid distribution model. A pseudo-single-phase gas flow CFDmodel is established for predicting the macro gas distribution in structured packing.A hybrid arithmetic which combined the gas CFD model, the macro liquiddistribution model and some classical mass-transfer theories has been proposed forpredicting the mass-transfer efficiency of structured packing. The hybrid arithmetic bylocal small-scale mass-transfer process estimating macro-scale mass-transferefficiency is a kind of multi-scale computational mass-transfer method. The simulatedHETP agreed with the experimental HETP very well under the conditions of normaland vacuum pressure. This hybrid arithmetic needn't introduce any experimental data,gas-liquid properties and packing structural parameters are enough. The hybridarithmetic of mass-transfer process provides a new approach for the design of theequipment and optimization of the process of large-scale structured packing columns.