Patent GB-806421-A (GB)
Title: Heat economy in aromatic hydrocarbon solvent extraction processes
Inventor: STEPHENS JOHN HARLAN
IPC: C10G21/28
Filed: 1956-12-10
Abstract: <PICT:0806421/III/1> In a process for the hot liquid extraction of aromatic hydrocarbons from a mixture thereof with non-aromatic hydrocarbons using a selective solvent comprising a primary solvent which is less volatile than the aromatic hydrocarbons, and a secondary solvent which is more volatile than the primary solvent, the secondary solvent and aromatics are stripped from the fat solvent in a stripping zone and the secondary solvent is recovered from the vapours, passed in heat-exchange with at least one of the hot fat solvent and hot lean solvent streams, and is then introduced in an at least partially vaporized state and in such amount into the lower porton of the stripping zone that simultaneously aromatic hydrocarbons are substantially completely stripped from the lean solvent and the secondary solvent concentration in the lean solvent stream is restored to practically the original concentration in the extraction zone.  A 65-154.4 DEG  C. catalytically reformed straight run petroleum fraction containing approximately 7 per cent benzene, 25.6 per cent toluene, 30.6 per cent xylene and the balance non-aromatics is forced into an extraction column 7 at 260 cu. metres/day and 4.1 atmospheres by pump 3 via furnace 5 where it is heated to a temperature of 138 DEG  C. (i.e. above the boiling point of the secondary solvent at the pressure in the stripping zone).  The column 7 is supplied at an intermediate point therein with an aqueous diethylene glycol solution (8 per cent water) as solvent at 3,940 cu. metres/day and the above temperature and pressure.  The fat solvent is transferred to the top of stripper 23 by pump 16 via coil 19 in heat exchanger 18, the heat reduction being insufficient to prevent flash vaporization in the stripper.  The pressure differential between column 7 and stripper 23 is such that all the non-aromatic hydrocarbons together with some water (secondary solvent) flash vaporize and are removed by overhead line 24 and condenser 25 to receiver 28 from which the hydrocarbon layer is pumped by pump 31 to the bottom of column 7 via lines 29 and 32. The aromatic hydrocarbons and the remainder of the water are removed from stripper 23 as a side-cut and pass via line 33 and condenser 34 to receiver 37 whence the purified aromatics are removed as product via line 39.  The water collected in receivers 28 and 37 passes via lines 41 and 44 respectively to pump 46 which transfers it via lines 47 and 51, wherein any losses are compensated from line 53, heat exchanger 19, and line 55 to stripper 23 at the bottom thereof so that water comes into contact with reboiler coil 57 and the resultant high temperature steam released effects substantially complete removal of benzene from the stripper bottoms.  Coil 57 is maintained at substantially the same temperature as that at which column 7 is operated.  The lean solvent leaving stripper 23 by line 58 is pumped by pump 59 to the top of column 7 through line 8.  The raffinate is removed from column 7 by line 10 and valve 11 and the water separated therefrom is returned via line 12 and valve 13.  This material is reintroduced to column 7 through lines 9 and 10 to compensate any water deficiencies in the selective solvent, if necessary being augmented by water pumped into line 9 from line 47 by pump 48.  A yield of 138 cu. metres of aromatics containing less than 0.3 per cent non-aromatics both in the above operation and in a modification wherein heat exchange takes place between the lean solvent stream and the hot lean solvent stripper bottoms. Examples of charge stocks include distillate fractions of cracked naphthas, specific boiling range fractions of straight run petroleum distillates and reformed or hydroformed gasoline fractions.  The selective solvent may be a solution of a primary and a secondary solvent or it may contain more than one primary solvent.  Primary solvents mentioned are oxygen-containing organic compounds such as glycols and their ethers, esters, and etheresters; phenol, resorcinol, pyrocatechol, the cresols and thymol.  Secondary solvents include water, methanol, ethanol and acetone.  The secondary solvent should boil substantially below the primary solvent and preferably within 50 DEG  C. (either way) of the boiling point of the aromatic component to be extracted.