Fractionation of Certain Aroma tic Hyd roca rbo ns with M oIe c uIa r Sieve Ads o rbe nts BEVERIDGE J. MAlR and MUTHU SHAMAIENGAR Chemical and Petroleum Research laboratory, Carnegie Instifute of Technology, Pittsburgh, Pa.
b A method has been developed for separating certain aromatic hydrocarbons according to the shape and size of the molecules. The mixture of aromatic hydrocarbons is introduced into a column containing the Molecular Sieve adsorbent, the portion which is not adsorbed is eluted with iso-octane, and the adsorbed portion is desorbed and brought out of the column with superheated steam or with ethyl alcohol a t 75" C. The method was tested with mixtures of pure aromatic hydrocarbons and with petroleum fractions comprised, respectively, of mononuclear, dinuclear, and trinuclear aromatics, in the Cl,to CQjrange.
B
and his associates (1-4) have shown that certain natural and synthetic zeolites may be used as Molecular Sieve adsorbents for the separation of hydrocarbons and other compounds. The separation depends on the configuration of the molecules. Molecules with critical dimensions significantly less than those of the pores of the adsorbent diffuse readily into the pores and are rapidly adsorbed; molecules with critical dimensions larger than those of the pores cannot enter and are not adsorbed; molecules with critical dimensions only slightly less than those of the pores are adsorbed very slon-1y. The critical dimension of the molecules which controls the process is the mavimum diameter normal to the longest axis. Barrer has shown that n-paraffins, a t least those boiling in the gasoline range, can be separated quantitatively from branched paraffins, cycloparaffins, and aroniatics, using zeolites with pore diameters near 5-9. units as adsorbents. The straight chain molecules enter the pore of the adsorbent, the others do not. Synthetic Molecular Sieve adsorbents with pore diameters near 4-, 5-, lo-, and 13-A. units are now available (Linde Co., Tonawanda, h'. Ye), Nelson. Grimes, and Heinrich (6) and Schn-artz and Brasseaux (i"), have described the use of the 5 4 . material for the analytical determination of the normal paraffin content of petroleum naphthas. All of the hydrocarbons occurring in natural petroleum. except the normal ARRER
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ANALYTICAL CHEMISTRY
paraffins, have molecular diameters in excess of &A. units. Some have niolecular diameters greater than 10- or 13-A. units, and these can be separated with the corresponding Molecular Sieve adsorbents from smaller molecules. This report describes the fractionation, with Molecular Sieve adsorbents, of niixtures of pure aromatic hydrocarbons and certain aromatic petroleum fractions in the C l ~to Cas range, according to the shape and size of the molecules. GENERAL DESCRIPTION
The mixture of aromatic hydrocarbons is introduced into an adsorption column and caused t o flow slowly into the Molecular Sieve adsorbent. This is followed with 2, 2, 4-trimethylpentane (iso-octane) which is passed slowly through the adsorbent and serves to bring out of the column the components which are not adsorbed. The portion of the aromatic material tvhich has entered the pores of the adsorbent is very strongly adsorbed, but can be desorbed and brought out of the column with steam. To do this, the adsorbent is wet with water, the temperature is raised t o between 175" and 220" C. and steam is passed through the colunm. Alternatively, the adsorbed material may be desorbed and brought out of the column with liquid ethyl alcohol (and presumably with other polar organic compounds having the proper size and shape of molecule) a t a temperature near 75" C. I n this manner, the original charge is separated into two portions. Aromatic hydrocarbons with molecules larger than the pores of the adsorbent are contained in the iso-octane eluent, those with molecules appreciably smaller than the pores of the adsorbent are contained in the portion recovered by steaming or by desorption with ethyl alcohol. Those aromatics with molecules only slightly smaller than the pores of the adsorbent are distributed between the two portions. APPARATUS AND PROCEDURE
The rate of adsorption is very slow when the critical diameter of the molecules approaches the diameter of the pores of the adsorbent. Because the results depend on the rate and manner
in which the experiments are conducted, it is desirable to standardize a procedure and to control carefully the movement of the material through the adsorbents. Somewhat better control was obtained with upward rather than dom-nward flow of the charge and eluent. Positions 1 and 2 (Figure 1) show the final form of the apparatus assembled for the adsorption and desorption parts of the experiment. Assuming the column to be packed and assembled as in position 1, the charge of aromatic material is placed in the funnel, K , and the stopcocks, G and J , opened to permit i t to flow to the flask, I . Stopcock G is then closed. Nitrogen pressure in line L is adjusted to the desired value and admitted to flask I . As a result the charge rises in the capillary, H , and passes into the packed column, D. After the charge has entered the adsorbent, a 10-ml. portion of eluent (isooctane) is introduced to wash out the funnel and flask, and pressure is reapplied to bring the washings into the column. The flask, I , is finally filled with isooctane and the liquid caused to move slowly up the column. Small adjustments of pressure are made to maintain the movement of the liquid front at a constant rate of 20 em. per hour. Under these conditions the hydrocarbon, dodecahydrochrysene, is completely adsorbed on Molecular Siere adsorbent 13X but only slightly adsorbed on 1OX. When the refractive index of the liquid issuing from the delivery tube, B, has decreased to the value for pure iso-octane, the adsorption part of the experiment is finished. The aromatic material is separated from the isooctane bv azeotropic distillation with e x e s ethyl alcohol in a small distilling column. After the iso-octane has passed completely into the distillate, the residue is diluted with 8 volumes of water, a small amount of sodium carbonate is added, and the mixture is refluxed gently t o break up the emulsion which is usuallv formed. The aromatic oil is recovered, washed once with water, and again recovered. Assuming that steam is to effect the desorption, the column is first wet with water. This avoids the high temperatures which result when steam is passed directly into the column. The column is prepared for the addition of water by replacing the delirery tube, B , with the funnel. -4;the flask, I , is removed and replaced with a receiver. Water, added through -1,desorbs and
POSITION 2 .
POSITION I.
Table 1.
Percentage of Hydrocarbon Adsorbed on Molecular Sieve Adsorbents 1OX and 13X Approximate '7% Adsorbed on Same Formula lox 13X n-Dec lbeiiaenc Cdn 100 100 100 CI~HIB 0 1,3,&'hethylbencene 1OQ 50 O-Decyl-I1 2,3,4-tetrahydronnphthnlene] CsbHn '100 2-Butyl-l-hcx y lindsn ClOHW 2-I3utyl-5-hesylindan CisHa 00 100 1,2,3,~,6,6,7,8,13,14,15,IODodecahyrIrochrymnc ClhHl4 5 100
A
0 E C
, . I
displaces tlie iso-octane. The temusr.11. The formvr consistcd of 14 X rnture rises momentarily ubove the 30-mcsh particles and the lnttrr of Ciling point of water nntl iso-octane, l/zrinch pellets crushed so that thc so t h a t n vapor zone of nbout 0.5 em. largw particles npprnrcrl to IN 14 mesh exists IRtween the adwincing w t e r in sizr. The pore volumrs, drtorniined front nnd the retreating renr end of tlic hy rquilibrntion with t h r snturntrd iso-oetunc. After water h t i ~ryqxetired vapor of Iwnzmr, wrrr found to be in the filtrate the rolumn is tiasembled 0.31 and 0.36 ml. per prnm for Tyws 8s s h o i v ~in i m i t i o n 2, Figure 1 S t e m 13X nnd IOX, rrpcctivrly. The pnckcd is passel tiirougli the column for 21) volunin hold 130 grnnix of thtx 13X or hours a t 11 ternixtrnturc of 175' C., and tiiiully for 2 Iioiirw tit 220' C, Initinllv, 145 grrrins of tlir 1OX citlnorlwbiit. Es11 xmnN m o u n t of iso-oc*t:tne is Iwou ht crpt in 11 few insbincw, whim) inauffinut of tlic ooliimti with a t e m . Tfris c h i t nitrtrrinl WIW avnihrt~lc,thr chnrgo is usually sh:irl)ly xcpnrcitetl from the ronsistcd of nn 8- or n-nil. portioti of the aromatic oil, so tlirit the latter nmlw srtniplr. only to I* ncp:ir:ital from water. E:tllyl Iilcollr~l tnny rrlso 1x2 URN1 t o affect tlie tlcwrptioii of tiic iiromntic EXPERIMENTS .WTH INDIVIDUAL materid. At 2.5' 6 . tlcsorptiori with HYDROCARBONS ethvl nlcoliol is usiurllg inro1nplcte m i teni~miturcsiicnr the imiIing point Sis liy(1roc:irI)otis w r v tcatrd: n(atmit 7.5' C.) itre iiccwsnry, Tlie Iwyllnwzrnr, I ,8,htrir~tliyllw~iizpnr, 6 desorption inny l ~ !ncwnil)lishctl in lccyl [ 1,2,3,-! tc~tr:iIiytIroliii;,Iltlinlenc~ rcvcn1l steps 11.9 fnllo\va: l
