| The extraction of hydrolysable chlorinated compounds from an isocyanate formulation using supercritical carbon dioxide and mixtures of carbon dioxide and propane as solvents was investigated applying the usual methods. Information about the distribution of the components between the coexisting phases and about the mutual solubilities was obtained from phase equilibria. An approximate solution for the number of theoretical stages was determined for the assumed quasi-binary separation problem to reduce chlorinated compounds. Based on the results it can be said that the assumption (based on the usual analytical method) that one chlorinated substance is the main source of hydrolysable chloride was not true. An extensive development of a new analytical procedure lead to an estimation of the distribution of the unknown hydrolysable chlorinated compounds. Based on this knowledge, a new two-step purification of the isocyanates was proposed.
Polyfunctional isocyanates (PFIC) are of widespread use in polymer chemistry especially as key intermediates in formation of polyurethanes by polyaddition with polyfunctional alcohols.
Due to the toxic and explosive compounds used in the phosgene-based formation the production of isocyanates is practiced by only a limited number of companies. In the phosgene-based production of isocyanates typical by-products and intermediates are chlorinated organic compounds.
The possible formation of HCl by hydrolysation of those compounds forces their removal from isocyanates prior to application in formation of polyurethanes, especially when this is done at the ultimate consumer. The aim of this investigation was to remove the chlorinated compounds by supercritical fluid fractionation. Supercritical fluid fractionation overcomes the typical drawbacks of a thin film evaporation such as high temperatures, when dealing with compounds of high molecular weight.
The raw polyfunctional isocyanate used in this study had a low vapor pressure due to its relatively high molecular weight and was contaminated with hydrolysable chlorides to a content. Since the isocyanate fraction was pretreated in different purification operations, it was assumed that the residual contamination was due to a single class of chlorinated substances.
If the purification of the isocyanate is possible by fractionation with supercritical carbon dioxide or with mixtures of carbon dioxide and propane can be examined. Propane can be used in order to enhance the solubility and hence the operating efficiency.
Considering the different solubility of propane and carbon dioxide in the liquid phase it was necessary to assure a constant concentration of propane in the supplied solvent. Since the separation of extract is forced by decreasing the pressure and hence reducing the solvents density, the phase equilibrium of the propane – carbon dioxide system can be used to achieve the desired propane concentration in the solvent. Phase equilibrium data of the binary system carbon dioxide – propane is well documented. According to phase equilibrium, the propane concentration in the gas phase of a two phase mixture can be controlled over a wide range by changing the temperature of the system. In order to use this effect the separation section of the extraction plant was modified. Leaving the separator the solvent passed a cooler and entered the separating column. The column was now used as a saturator circulating the propane-rich liquid phase countercurrently to the carbon dioxide-rich gas phase. The temperature of the saturator was controlled using a profiled heat exchanging hose and a thermostat. The liquid phase was pumped by a high pressure gear pump and the liquid level was manually controlled observing the phase boundary line by means of two sapphire windows and supplying or releasing propane and propane-rich liquid phase respectively. Propane was supplied using a cooled air driven pump.
Fig. 21: Separation section incorporating a flash separator and saturation of carbon dioxide
with propane.
The composition of gas phase was found to be dependent only on gas phase solubility in the investigated range of temperatures and pressures. It was tried to enhance gas phase solubility by adding propane as a co-solvent without reducing selectivity. There is an increase of gas phase solubility of about 1% at propane concentrations of 20% and 30% with a simultaneous increase in selectivity.
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