Liquid salt extraction of aromatics from process feed streams

#This invention relates to a process for the extraction of aromatics from mixed aliphatic/aromatic hydrocarbon feeds such as isomerization process feed streams, cat naphthas, lube stocks, and the like. More particularly, this invention is concerned with the use of liquid salts, in particular, quaternary ammonium, phosphonium and arsonium salts of halides, acids, or more complex anions, as extracting agents for aromatics from paraffin feedstreams. Such salts show the required solubilities in various solvents and low volatility to allow for a distinct selectivity advantage over more conventional solvents, particularly sulfolane, which is commercially used. Such salts also show the exceptionally low volatilities typical of salts, thus minimizing losses by vaporization and facilitating recovery of extractate.

#In certain instances, aromatics in hydrocarbon feedstreams can act as catalyst poisons such as in isomerization feedstreams. Then, the aromatics must be removed prior to the paraffin isomerization reaction. In this instance, sulfolane cannot be used since it is partially miscible with the paraffinic hydrocarbon and would also tend to poison the isomerization catalyst. By contrast, liquid salts which have a high affinity for aromatic hydrocarbons and are insoluble in the paraffinic hydrocarbon/carrier, are excellent extractants since they are readily separated from the paraffinic hydrocarbon feedstream and pose no catalyst poisoning hazard.

#Specific objects of this invention are to provide a process utilizing liquid salts, having a high extractive selectivity for aromatic compounds and high solvation capacity, are readily synthesized from low cost, readily available materials, and can easily be stripped or separated from the extraction phase, and which have high thermal and chemical stability.

#In order to test the extractive efficiency of the process of the present invention, four liquid phosphonium salts, among others, were synthesized and compared to sulfolane in extraction experiments. The salts employed were tri-n-butyl-2-ethylhexylphosphonium chloride, tri-n-butyl-n-octylphosphonium bromide, tri-n-butylphenylphosphonium methanesulfonate with 4.5 volume percent methanesulfuric acid, 0.3 volume percent H.sub.2 O and tri-n-butyl-3-methylbutylphosphonium benzenesulfonate with 4.1 volume percent benzenesulfonic acid, 0.8 volume percent H.sub.2 O.

#The apparatus for the extractions was developed with two objectives in mind: keeping the time to reach equilibrium at a minimum, and facilitating sampling of both phases in the extraction vessel. The apparatus consisted of a 200 milliliter flask fitted with reflux condenser and a temperature-controlled oil bath, magnetic stirrer, an inlet for an inert atmosphere. The flask was charged with equal volumes (50 cc) of solvent and a feed solution made up as 10 volume percent benzene and heptane. The separate phases were stirred vigorously for a specified time, allowed to separate and the supernatant raffinate decanted. Comparative benzene concentrations were determined for feed and raffinate by infrared spectroscopy. The ratio of benzene to heptane in solvent was determined by gas chromatography retention. From these data, a measure of extractive capability was determined. Table summarizes the results of the above-mentioned experiments. As can be seen, the selectivity values for tri-n-butyl-2-ethylhexylphosphonium chloride, tri-n-butyl-n-octylphosphonium bromide were all significantly higher than those for sulfolane. The capacities of the two sulfonate salt mixtures were also higher than those for sulfolane.

#For some purposes, the costs of the solvent material, in particular, the tetraalkylphosphonium salts are particularly preferred as solvents for extraction of aromatics, whereas for other purposes the aryltrialkylphosphonium salts are particularly preferred as solvents for aromatics extraction. The same is likewise true for the analogous quaternary ammonium salts which are less preferred than the phosphonium salts due to their stabilities, etc.