US3201489A - Recovery of aromatic hydrocarbons - Google Patents

Description

United States. Patent 0. l

3,201,489 RECOVERY OF AROMATIC HY DROCARBONS Donald Fred Knaack, Wilmington, Del., assignor to E. I. du Pont de Nemours and Company, Wilmington, lDeL, a corporation of Delaware 1 N Drawing. Filed Oct. 27, 1960, Ser. No. 65,298 2 Claims. (Cl. 260-674) The present invention relates to the recovery of aromatic hydrocarbons from hydrocarbon mixtures containing such and more particularly to the separation of aromatic hydrocarbons from mixtures thereof with saturated hydrocarbons and the separation of specific aromatic hydrocarbons from mixtures of aromatic hydrocarbons.

, It had heretofore been known that olefinic hydrocarbons could be separated from hydrocarbon mixtures containing such by passing the hydrocarbon stream through a silver fluoborate solution. The fluoborate reacts with the olefin to form a soluble complex. Saturated hydrocarbons are not affected by the silver fluoborate and pass through the solution unchanged. The olefin silver fluoborate complex formed is unstable and is readily decomposed at elevated temperatures and/ or reduced pressures, releasing the olefin. The complex can also be decomposed by extreme dilution through addition of water. The reac tion of the silver fluoborate with theolefin is extremely rapid and proceeds to an extremely high degree of completion, so that the separation of the olefin is very effective. The separated olefin, furthermore, has a purity of greater than 99%. However, it was heretofore believed that the separation was only etfective with olefins, i.e., compounds containing an ethylenic type of double bond, since the operability of the process was based on the rapid reaction of the olefinic double bond with the silver fluoborate. I

It is an object of the present invention to separate aromatic hydrocarbons from hydrocarbon mixtures containing such. 'It is'another object to separate mixtures of aromatic hydrocarbons. A further object is to separate fluid mixtures of aromatic hydrocarbons and olefinic hydrocarbons. Still another object is to provide a process for the separationof aromatic hydrocarbons from mixtures containing such using selective complex formation. Other objects will become apparent hereinafter.

In accordance with the present invention, fluid aromatic hydrocarbons are separated from fluid mixtures of aromatic hydrocarbons differing in absorptivity, fluid mixtures of aromatic hydrocarbons and saturated hydrocarbons and fluid mixtures of aromatic and olefinic hydrocarbons by contacting the hydrocarbon mixture with an aqueous solution of a silver salt selected from the class consisting of silver fluoborate, silver fluosilicate and mixtures thereof, and thereafter recovering the aromatic hydrocarbon absorbed by the solution.

In a preferredembodiment of thepresent invention, the silver salt solution is modified by the addition of a stable metal salt wherein the cation has a charge to ionic radius ratio of greater than one and wherein the anion is selected from thegroup consisting of fluoborate, fluosilicate and mixtures thereof. The ratio of valence or charge to ionic radius is readily calculated from published data. Thus,

both can be found'in Therald Moellers textbook, In-

organic Chemistry, published by John Wiley and Sons, Inc., 1952, on pages 140 to 142. In particular, the metals 3,2hlA89 Patented Aug. 17, 1965 in Group H of the Periodic Table of Elements found in said textbook having atomic numbers from 4 to 56 inclusive, and copper, lead and lithium, are highly suitable. The molar ratio of silver to the secondary metal is not critiIcal butis preferably maintained between 1:1 and 10:

The concentration of the silver salt is not critical but generally concentrated solutions of 4 to 12 molar are preferred, since the separation is more complete at these concentrations. For certain separations, however, it may be preferred to employ lower concentrations of the silver salt. The preferred anion of the silver salt or the secondary metal salt is fluoborate because of its higher activity in the system of the present invention. The anion of the silver and the secondary metal salt need not be the same and it is quite feasible to carry out the process with mixtures of anions in the aqueous solution, i.e., mixtures such as fluoborate and fiuosilicate.

The process of the present invention is preferably employed .in the separation of fluid aromatic hydrocarbons from fluid hydrocarbon mixtures containing aromatic hydrocarbons admixed with saturated hydrocarbons, i.e., saturated aliphatic and/or cycloaliphatic hydrocarbons. Olefinic hydrocarbons, i.e., hydrocarbons which contain non-aromatic unsaturation, also form complexes with the silver salt solutions. The absorptivity of such olefins differs from aromatic hydrocarbons. Depending on the concentration of the silver salt solution it is possible to either preferentially absorb the olefin or preferentially absorb the aromatic hydrocarbon, e.g., in a mixture of cyclohexene and benzene cyclohexene is preferentially absorbed at low concentrations (l- -4 molar silver fluo borate), Whereas at high concentrations (6-12 molar silver fluoborate) benzene is preferentially absorbed. The ef fect of concentration will shift somewhat with the addition of the secondary metal salt. Thus by proper design it is also possible to separate olefinic hydrocarbons from aromatic hydrocarbons. The term fluid is used through out to denote a liquid or gaseous state of the material at the temperature employed in the absorption step. The term aromatic hydrocarbon as employed in the description of this invention denotes hydrocarbon compounds containing no other unsaturation. Hydrocarbons containing ethylenic unsaturation are defined as olefinic hydrocarbons. The compounds principally separated by the process of the present invention are therefore benzene, toluene, xylenes, and fluid homologs and isomers of these homologs. The process of the present invention is particularly-applicable to the separation of these aromatic hydrocarbons from mixtures with saturated aliphatic and/ or cycloaliphatic hydrocarbons which are difficult to separate by distillation becauseof close boiling points or formation of azeotropes with the aromatic hydrocarbons, e.g., aliphatic and cycloaliphatic hydrocarbons having from 6 to 12 carbon atoms. The process is furthermore of utility where the aromatic hydrocarbon occurs only in small concentration in a hydrocarbon stream and it is ability to be absorbed by the salt solution. As will be apparent hereinafter, aromatic hydrocarbons differ in their ability to be absorbed depending on the concentration of the absorbing solution. Thus, at low concentrations, xylenes are not absorbed whereas benzene and toluene are. Hence, this difference in absorption provides a method for separating aromatic hydrocarbons from each other. Where the absorptivity is the same, it will be apparent that such aromatic hydrocarbons cannot be separated by the process of the invention claimed.

More than one hydrocarbon can be separated when simultaneously admixed with saturated hydrocarbons. Thus, both benzene and toluene can be separated from hydrocarbon mixtures containing same in a single pass through the absorber.

The process of the present invention is characterized by extreme simplicity of operation in that it is only necessary to bring the hydrocarbon mixture in contact with the aqueous solution containing the silver salt for a suificient length of time to allow the formation of the complex. Where the hydrocarbon mixture is gaseous at absorption temperatures it is passed through an absorption tower containing the aqueous solution which may be continuously regenerated or used batchwise. If the hydrocarbon mixture is liquid at the absorption temperature, the separation can readily be carried out in an agitated vessel in which the hydrocarbon mixture and the aqueous salt solution are admixed for a sufiicient time to allow transfer of the aromatic hydrocarbon into the aqueous phase. The absorption of the aromatic hydrocarbon is normally carried out at room temperature; however, it is possible to employ temperatures in the range of to 50 C. Desorption is accomplished by heating the aqueous solution, reducing the pressure over the aqueous solution, or by a combination of both. Suitable desorption temperatures at atmospheric pressure are 80 C. or higher. At reduced pressures, the use of lower temperatures will result in substantially complete recovery of the absorbed aromati hydrocarbon. 1

Certain impurities, such as carbon dioxide, carbon monoxide, oxygen, hydrogen, nitrogen or noble gases, have only a very small elfect on the operability of the described process. Where such impurities become major components of the mixture to be separated, it may be desirable to remove these impurities in a prior step. It has been found that the concentration of acetylene should be maintained at a low level in order to prevent interference of silver acetylide with the separation process practiced. Concentrations of acetylene should remain on the average below 1%. Where the acetylene in the gas to be separated is continuously at levels above 1%, it is preferred to hydrogenate the acetylene to saturated compounds which do not interfere with the separation prior to contacting the hydrocarbon stream with the salt solution.

The following tables show the absorption of various aromatic hydrocarbons by the silver salt solutions employed in the process of the present invention. The absorption was determined by agitating a mixture of the salt solution and the aromatic hydrocarbon at 24:1" C. until the amount of the aromatic hydrocarbon absorbed reached a constant value.

Table I.Abs0rpti0n of benzene absorbed by the silver solution.

Table II.Absorpti0n of toluene Moles of Toluene/ Mole of Silver Absorption Solution AgBF4 AgBF4-L8 1W (B154) A ar.

Table III Moles oi Xylene/ Liter oi Solution Moles of Xylene/ Mole of Silver Aromatic Absorption Solution Hydrocarbon M AgBFq M AgBF4-1.8 M Mg(BF4)2.

AgBF

o-Xylene a NlO .4 M AgBFq No measurable absorption of either ortho-, meta-, or para-xylene occurred when 2.0 molar silver fluoborate solutions, 2.0 molar silver fiuoborate-1.8 molar magnesium fluoborate solutions, or 4.0 molar silver fluoborate solutions were employed. Hence, the difference in absorptivity of benzene and xylenes at 2 to 4 molar concentration of silver fluoborate provides a method for the separation of benzene from mixtures of benzene and xylenes.

The separation of benzene from a mixture comprising 2% benzene, 49% cyclohexane and 49% n-hexane (by volume) is shown in Table IV. The hydrocarbon mixture, 20 ml., was agitated with 30 ml. of the silver solutions shown in Table IV for 5-15 minutes at 24 C. In each of the separations, no cyclohexane or n-hexane was Ultraviolet spectrophotometry was employed to determine the amount of aromatic hydrocarbon absorbed.

Table IV Absorption solution: Percent of benzene absorbed 5.9 M AgBF, 19 5.9 M AgBF -1.8 M Mg(BF 50 11.8 M AgBF 78 Greater than 90% of the benzene absorbed could be recovered from the absorption solution by heating the solution to 99 C. and distilling off the benzene. It will be apparent that with more efiicient equipment, such as countercurrent absorption towers, much higher percentages of the benzene will be extractable.

The foregoing separation was repeated using the following absorption solutions:

Table V Absorption solution: Percent of benzene absorbed N 4 5.8 N Ag SiF -L8 M Mg(BF 18 11.6 N Ag SiF 13 The separation of benzene and cyclohexene from cyclohexane and the preferred absorption of one unsaturated compound as compared to the other unsaturated compound depending on the concentration of the absorbing solution is shown in Table VI. A hydrocarbon mixture containing 10% cyclohexene, 10% benzene, and of cyclohexane, 20 ml., was agitated with 10 ml. of the silver salt solutions shown in Table V1 for 5 to 15 minutes at 24 C. In each of the separations no cyclohexane was absorbed.

' Table VI Absorption Solution Grams Grarns/ Amount of Material Absorbed Moles/ ml. lite liter X X x N Cyclohexone 0. 12 12. 0 0. 10 MAgBF {cenlzelilie 0. 843 4. 3 0. 055 yo 0 exene 0. 97 9. 7 0.12 10 M AgBF enzene o. 24 24. 0 0.31 steers: ti? as as? Cyclohexene O. 34 34. 0 0. 41 10 M AgBm {Benlzekllie 0. 86.0 1. 10 yo 0 exene 0. 48. 0 0.59 10 M AgBF Benzene 2 47 247. 0 3.17 e The amount of benzene p1 esent was determined by ultraviolet spectrophotometry The results show the preferred absorption of cyclo- I claim:

hexene using concentrated solutions of silver fluoborate. Table VII illustrates the absorption of benzene using taining, by volume percent, 2% benzene, 2% mesitylene (1,3,5-trimethylbenzene), 48% n-hexane, 48% cyclohexane. To ml. of the absorption solution shown in the table was added 20 ml. of the mixture described above. The mixture was shaken for 15 minutes and the Ultraviolet spectrophotometry was used to determine the N0 mesitylene,

Substantially all of the absorbed benzene could be recovered by heating.

aromatic hydrocarbon is desorbed, and the stripped silver solution is recycled to the absorption tower.

The process set forth in claim 1 wherein the salt solution comprises silver fluoborat-e modified with mag- References Cited by the Examiner UNITED STATES PATENTS 1/49 Friedman et al 260 677 2,913,505 11/59 Van Raay et al. 260-677 2,953,589 260674 X 260-677 OTHER REFERENCES Toluene Soluble Copper and Silver Fluoborates, by J. C. Warf, J. American Chemical Society, vol. 74, p. 3702-3703 relied on, 1952.

Complex Fluorides, The Properties of Silver Salts and ALPHONSO D. SULLIVAN, Primary Examiner.

Claims (1)

1. A PROCESS FOR SEPARATING FLUID AROMATIC HYDROCARBONS FROM FLUID MIXTURES OF AROMATIC HYDROCARBONS WITH SATURATED HYDROCARBONS, WHICH COMPRISES CONTACTING SAID MIXTURE WITH AN AQUEOUS SOLUTION OF A SILVER SALT WHEREIN THE ANION IS SELECTED FROM THE CLASS CONSISTING OF FLUOBORATES, FLUOSILICATES AND COMBINATIONS THEREOF, AND WHEREIN THE SILVER SALT SOLUTION IS MODIFIED BY THE ADDITION OF A SECOND METAL SALT WHEREIN THE CATION IS A METAL OF GROUP II OF THE PERIODIC TABLE OF ELEMENTS AND HAS AN ATOMIC NUMBER FROM 4 TO 56 INCLUSIVE, AND THE ANION IS SELECTED FROM THE GROUP CONSISTING OF FLUOBORATES, FLUOSILICATES AND COMBINATIONS THEREOF, AND THEREAFTER RECOVERING THE AROMATIC HYDROCARBON FROM SAID SOLUTION.