Co-current versus Countercurrent

Due to an always high water contents at the reactor inlet, the eventual internal water loop (as described in Fig. 8.3) should not be a problem. However, if co-current and countercurrent operation is compared (Fig. 8.28), a clear difference between these two operation modes is found only for the water concentrations, whereas the gain in space-time yield is limited to around 10 %. This can be attributed to the high recirculation rate of the liquid, whereas in both experiments the gas phase was vented after one pass. Therefore, the gas-liquid operation was of the cross-flow-type rather than pure co-current or countercurrent To identify the effect of pure co-current or countercurrent operation, the gas phase should also be recycled. 

Due to side reactions, in most of the experiments a higher octanol conversion than acid conversion is found. To investigate this phenomenon, an instantaneous selectivity is defined as the ratio of the rate of ester formation over the rate of 1-octanol-disappearance in one pass through the column  

The coincidence of the data points for several experiments can be explained by the similar concentrations obtained by the way the data are plotted. However, this implies that the water contents are also similar. As shown in Fig. 8.30, and as expected based on the similar stripping performance of most internals (see Fig. 8.24), the water contents are also comparable. 

At low inlet conversions, the single-pass conversions are so high that not all of the water produced in one pass can be stripped. Therefore, at the initial stages of 

In order to understand the maxima and the minimum of the instantaneous selectivity Smstr it is useful to plot the predictions of the kinetic model as represented by the two rate Equations (1) and (2). In Fig. 8.31, the S m( expected from the kinetics is plotted depending on the fraction of water stripped from the liquid. All these curves start at around 75 %, increase to near-unity, and then show a sharp drop at high inlet conversion levels. Depending on the fraction of water stripped, the slope of the initial increase and onset of the final drop of Smst differ. 

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Several investigations of these petrochemical processes address the question of co-current versus countercurrent operation [1, 9, 10]. Whereas reactive distillation is a countercurrent operation by definition, in HDS, HC and HDN countercurrent operation is favorable because the gas-liquid equilibrium (GLE) of H2S and ammonia lies to high extent on the gas side. However, this cannot be generalized. As shown in Fig. 8.3, an internal loop can occur if the gas flow, which stripped the... 

First, the effects of gas and liquid flows, co-current versus countercurrent operation, pressure and temperature were checked. As expected, based on the influence of these parameters on the vapor pressure or the vapor-liquid equilibrium (VLE), the fraction of water stripped by the nitrogen increases with increasing gas flows, decreasing liquid flows, lower pressures and higher temperatures. Countercurrent operation is more efficient than co-current operation, because the liquid phase at the inlet was already enriched with the compound which was to be separated. 

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