| Countercurrent experiments were carried out according to the results of the preliminary calculations.
The full length of the column was used as stripping section. First experiments showed next to no separation concerning the hydrolysable chlorides. The color of the raw isocyanate was changed from brown to yellowish in the extract, whereas the raffinate became darker. Subsequent extractions showed different behavior. There was no remarkable difference between the extractions with pure carbon dioxide and those with 20% propane concerning the separation efficiency. However, due to the higher solubility, higher PFIC flow rates could be realized.
Table 10: Calculated numbers of equilibrium stages (nth) and solvent to feed ratios.
The results of the countercurrent extractions led to the assumption that there were three different classes of HC. The existence of low volatile HC was indicated by high HC values in very dark raffinate samples taken during the start up of extractions when the feed supply was started while the solvent was already circulated in the extraction plant. The relatively low separation efficiency at already lower HC concentrations in the third extraction indicated a chlorinated substance with similar behavior as the PFIC in the separation. Following this theory a third class of chlorinated substances must have existed providing higher separation factors than those determined during the phase equilibrium measurements. GC-MS analysis of extract and raffinate samples with high HC concentrations obtained from the countercurrent extractions support the assumption.
Extract fractions showed only signals of chlorinated compounds of low molecular weight, such as chloro-benzene, whereas in the raffinate fractions only chlorinated compounds of higher molecular weight could be detected.
Fig. 22: SIM chromatogram.
The various chlorinated isocyanates distribute differently in the countercurrent separation. An illustration is presented in Fig. 23.
Fig. 23: Chlorinated substances (HC) during countercurrent separation.
Fig. 24: TIN-product as extract
To conclude, the extraction of chlorinated compounds with supercritical carbon dioxide and supercritical carbon dioxide-propane mixtures is possible. The addition of propane was shown to be profitable mainly due to the increase of solubility, which severely affects the economic efficiency. With a simple modification of the separation section a constant propane concentration was obtained, thus avoiding the need for complex feedback control. However, the removal of chlorinated substances (HC) is not possible in a single operation unit, since their different molecular weights obviously influence their distribution in the coexisting phases. The assumption of three different classes of chlorinated substances according to their volatility was shown to be valid. Two of these substance classes, the high volatile and low volatile HCs could be removed successfully. The third class of chlorinated substances, which is of similar volatility to the PFIC, is a more challenging separation problem.
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