The invention relates to a process for removing pure aromatic hydrocarbons from hydrocarbon mixtures containing these compounds by extraction in the presence of reactive substances, substantially aromatic-free raffinates being obtained at the same time.
The starting materials for the removal according to the invention, of pure aromatics are mixtures of aromatic and nonaromatic hydrocarbons from which the pure aromatics cannot be removed by simple thermal distillation as a consequence of similar boiling points. The mixtures may contain the aromatics, for example benzene, toluene and xylene, either together or individually. Mixtures of the type mentioned are produced, for example, in refineries and petrochemical plants.
It is known that nonaromatics can be separated from aromatics by extraction using selective solvents. Extraction is taken to mean both classical liquid-phase extraction and extractive distillation. In both methods, the aromatics are selectively extracted from the hydrocarbon by a solvent. In liquid-phase extraction, the removal of the low-aromatic raffinate from the high-aromatic solvent takes place by mechanical separation in an extractor; in extraction distillation, the separation takes place by distillation in a column. In both cases, isolation of the pure aromatic(s) takes place in a downstream column (stripper) by distillation of the aromatic(s) from the solvent. The low-aromatic solvent is fed back from the stripper bottom into the liquid-phase extractor or into the extractive-distillation column.
In both process variants for obtaining pure aromatics from aromatic-containing hydrocarbon mixtures, the specific cost of separation is determined by the following factors:
a) by the aromatic concentration in the starting mixture,
b) by the aromatic yield, and
c) by the quality demands on the pure aromatic(s) or on the raffinate.
For example, in the case where the aromatic concentration of the input stream is constant and the quality demands on the pure aromatic(s) with respect to nonaromatics and traces of solvent, and the quality demands on the raffinate with respect to traces of solvent and aromatic content likewise remain the same, the energy cost increases with the desired yield of aromatics. The ideal point from the process engineering point of view is then determined by the energy costs on the one hand and by the proceeds from sale or further processing of the aromatics and the raffinate on the other hand.
The present invention now makes it possible to reduce the specific cost of separation at the same aromatic yield or to increase the aromatic yield at the same specific cost of separation, it also being possible, if desirable, to produce operating states between the extreme values mentioned. Since legal restrictions with respect to the aromatic content in carburettor fuel are expected in the future, it is necessary to reduce the aromatic content in the low-aromatic raffinate as far as possible, which at the same time means preferably giving priority to increased aromatic yield at the same specific cost of separation.