metal mole ratio was about 0.05)to approximately 515 nm when the ratio exceeded 3. Thus there is a change in the identity of the major fluorophore as the ligand-to-metal ion mole ratio is increased. Maximum fluorescence intensity was recorded when the ligand-to-metal ion mole ratio was ca. 0.4 which suggests that a lower thorium-8-quinolinol complex, ThQ or perhaps ThQz, is more intensely fluorescent than the 3 :1 and 4 :1 chelate species. Red shifts in the wavelength of maximum fluorescence with increasing ligand concentration as well as very high luminescence emission intensity from lower chelate species have been reported for 8-quinolinol chelates of some other metal ions (9, 16, 18) and for aluminum ion-flavanol (21) F. L. Urbach and A. Timnick, ANAL.CHEM., 40,1269 (1968).
complexes (21). The formal equilibrium constant for Reaction 1 was calculated from these [BQ and 8-quinolinol concentration] data. The K’ at pH 10 is approximately 10-6 and corresponds to a stepwise formation constant, K4,of 1Olo for tetrakis(8-quinolino1ato)-thorium(1V). RECEIVED for review August 19, 1969. Accepted January 19, 1970. The authors are grateful to the National Science Foundation for partial financial support (Grant GP-8585). Gordon M. White held a National Aeronautics and Space Administration Fellowship 1965-1968. Paper presented at the Fourth Middle Atlantic Regional Meeting, ACS, Washington, D.C., February 1969.
NOTES
Determination of n-Paraffins in Gas Oils by Molecular Sieve Adsorption N. Y. Chenl and S . J. Lucki Mobil Research and Development Corporation, Research Department, Paulsboro Laboratory, Paulsboro, N. J . 08066 THEFLOW PROPERTIES (viscosity, viscosity index, fluidity, pour point, etc.) of heavy petroleum fractions depend heavily on the content of n-paraffins. In our studies we have encountered the analytical problem of determining the amount and distribution of normal paraffins in gas oil range material. O’Connor and Norris, in 1960 (1) reported that normal paraffins can be determined in the gasoline to light gas oil range (c6 to C2,,) by adsorption on a column of molecular sieves. Also, the amount of straight chain hydrocarbons below ~ - C Zcan Z be determined gas chromatographically by the combined use of molecular sieve and silicone g u m rubber columns. However, these techniques were found not to be applicable to straight chain hydrocarbons boiling above 650°F. O’Connor et al., in 1962 (2) reported a batchwise adsorption method for the determination of normal paraffins in CZO to c 3 2 paraffin waxes by molecular sieve adsorption. However, we found that their method does not give accurate results in the presence of cyclic and polycyclic aromatics or when the size of the n-paraffin molecule exceeds n-C36. Two methods for the determination of n-paraffins in gas oils having end point above 900°F have been developed and are described here.
c
“i, OUTL ET
H,O
Nz INLET
CONDENSER
1
50 M L BULB
Figure 1. Adsorption apparatus EXPERIMENTAL
Equipment. The apparatus used for the adsorption of normal paraffins, Figure 1, consists of a 50-ml adsorption bulb, HzO condenser, and a Nz inlet tube. 1 Current address, Mobil Research and Development Corporation, Research Department, Central Research Division Laboratory, Princeton, N. J. 08540
(1) J. G . O’Connor and M. S . Norris, ANAL.CHEM., 32,701 (1960). (2) J. G . O’Connor, F. H. Burow, and M. S. Norris, ibid., 34, 82 (1962). 508
ANALYTICAL CHEMISTRY, VOL. 42, NO. 4, APRIL 1970
Materials. SOLVENTS.Iso-octane, research grade (99.99 mol %) was obtained from Phillips Petroleum Company. Benzene and acetone, Reagent Grade, were obtained from J. T. Baker Co. ADSORBENT.Molecular sieve type 5A, ‘18 X 1/16-inch pellets (containing 30 clay binder) were obtained from Linde Co. Samples of sieve used in this experimentation were dehydrated for a minimum of 6 hours at 500 “C and kept in a dry box until prior to use. HYDROCARBON CHARGES.Normal docosane and n-hexatriacontane were obtained from Humphrey Wilkinson Co. and contained 5.6 wt % and 4.8 of isoparaffins, respectively.
Gas oils used, including a Kuwait dewaxed raffinate from a 200-sec paraffinic type neutral, an Amal gas oil 650/850 O F fraction, and a wide cut Mid-Continent gas oil, were laboratory samples. Analysis. All analyses were performed on a Model 720 F&M Scientific Company dual column-temperature programmed chromatograph. The gas chromatographic column used was a 4-ft, 1I4-inchcopper column packed with 20 wt UCW-98 on Chromosorb W. Adsorption Methods. METHODA (Iso-OCTANEREFLUX). Approximately 2 grams of n-paraffin containing gas oil was charged to a 50-ml adsorption bulb and weighed to the nearest 0.1 mg. Iso-octane (research grade) (25 ml) was added to the flask to give a solution of gas oil in iso-octane. To this solution, freshly dehydrated Linde 5A molecular sieve pellets were introduced in the weight ratio of 10 grams of sieve to each gram of gas oil or about 40 grams of sieve per gram of estimated n-paraffins. The mixture was then refluxed for 17 hours to ensure complete sorption of n-paraffins present in the gas oil sample. METHODB (HIGHTEh%PERATURE-NO SOLVENT). In this method the gas oil was charged to the adsorption bulb and Type 5A molecular sieve was added directly. No solvents were used during the adsorption step. The gas oil and molecular sieve were then mixed by rotating the bulb until the walls of the bulb and the molecular sieve appeared dry. The adsorption apparatus was then purged with nitrogen gas entering in at the top of the condenser. The bulb was submerged in an oil bath which was raised to 200 OC and maintained at that temperature for 2 hr. A nitrogen purge of -10 cc/min was used during the entire adsorption step to provide a nonoxidative atmosphere for the gas oil. After the adsorption, the bulb was removed from the oil bath and cooled to room temperature. Recovery of Nonnormal Fraction. Complete recovery of the nonnormal fraction in the sample is essential in the present method. In adsorption method A, after reflux the bulb was cooled to room temperature and the solution filtered into a 250-ml Erlenmeyer flask. The molecular sieve containing the adsorbed normal paraffins was then extracted as described below. In adsorption method B, only the extraction steps are necessary. The molecular sieve was extracted successively with four portions of 25 ml each of the following three solvents, ciz., iso-octane, benzene, and acetone. The extract was filtered into a 250-ml Erlenmeyer flask (or combined wlth the filtrate in method A and stripped of all solvents at 61-70 "C with a stream of nitrogen (-200ml/min) blowing aboce the liquid to aid in rapid removal of solvents. The residue that remained was cooled to room temperature and weighed to the nearest 0.1 mg. Using this method of stripping, an essentially solvent-free residue was obtained as indicated by chromatographic analyses. The difference in weight between the gas oil charged and recovered represents the amount of normal paraffins in the gas oil.
z
DISCUSSION
Demonstration of Complete Absorption of n-Paraffins by SA Molecular Sieve. A study was made to show the length of time required for complete absorption of n-paraffins of different molecular weights. A blend of n-docosane (CZJ and n-hexatriacontane (c36) in a Kuwait dewaxed raffinate was used. The composition of the blend was as follows: n-Cnz (iso-paraffin free basis) mC36 (iso-paraffin free basis) Total paraffins in Kuwait dewaxed raffinate n-Paraffin free Kuwait dewaxed raffinate Total
14.1 Wt 11.6 Wt 1 . 7 Wt 72.6 Wt 100.0 Wt
z z z z z
Approximately 2 grams of the above blend and 25 ml of isooctane were refluxed wi.h 20 grams of 5A molecular sieve.
3 ADSORPTION T I M E , H O U R S
Figure 2. Effect of normal paraffin molecular weight on adsorption time Twenty-microliter samples of the solution were taken at intervals of 4, 7, 11, and 17 hours for chromatographic analysis. Data in Figure 2 show that a minimum of 17 hours was required to completely adsorb the n-Css while 11 hours were needed for complete adsorption of n-Cn. The rate of adsorption of n-paraffins from a heavy gas oil iso-octane mixture is apparently much slower than that reported by O'Connor et al. (2) who found that pure normal paraffins (CZIto C ~ Zand ) those present in petroleum waxes were completely adsorbed from a paraffin-iso-octane solution under reflux conditions in less than 3 hours. The time for adsorbing n-paraffins from a heavy gas oil mixture can be shortened considerably by raising the temperature to 200 "C as in our method B. Complete removal of n-Czz and mC36 paraffin was accomplished in 2 hours or less. Recovery of Nonnormal Fraction. To ensure complete removal of n-paraffin from the gas oil mixture, a ratio of 40 grams per gram of n-paraffin is employed, or the total nparaffin content in the zeolite should not exceed 2.5 wt The use of such high sorbentln paraffin ratio brings in the problem of surface absorption of the nonnormal fraction which must be removed from the sorbent during the recovery step. The following experiment demonstrated the effect of surface adsorption on the recovery of the nonnormal fraction in the gas oil. Kuwait dewaxed raffinate, 2.1420 grams, (containing 98.3 wt nonnormal hydrocarbons) was treated with 22 grams of Linde 5A pellets according to adsorption method B at 200 "C for 2 hours, and then extracted successively with 4 portions of 25 ml each of iso-octane, benzene, and acetone. The cumulative recovery of nonnormal hydrocarbons after each solvent extraction is :
z
After iso-octane extraction After benzene extraction After acetone extraction
89.0 95.4 100.0
The amount of nonnormal hydrocarbons not extracted by iso-octane corresponds to approximately 1 wl of the zeolite used. Thus, an error in n-paraffin content of no less than 40 could be expected if only iso-octane were used. This is particularly important when the sample contains significant amounts of aromatic and polynuclear aromatic hydrocarbons as in most gas oils. Comparison of Absorption Methods A and B. The nparaffin contents in the above calibration blend as determined
z
ANALYTICAL CHEMISTRY, VOL. 42, NO. 4, APRIL 1970
509
Table 111. Carbon Number Distribution of Gas Oils WCMC G.O. Amal G.O. 650/850 "F Carbon Cyclics & Normal Cyclics & Normal no. isoparaffins paraffins isoparaffins paraffins 10-12 3.7 1.7 0.0 0.0 14-18 26.7 1.7 0.0 0.3 18-20 13.5 4.5 1.7 0.5 20-22 11.0 8.8 1.7 2.5 22-24 7.2 10.1 1.4 4.6 24-26 1.1 6.5 12.3 4.5 26-28 6.5 0.2 13.7 3.5 28-30 4.9 1.1 11.4 3.7 3.8 30-32 0.6 7.4 2.9 2.3 32-34 0.7 3.3 1.6 1.1 34-36 1.1 0.6 0.7 36-38 0.6 0.4 0.4 0.4 3840 0.2 0.1 0.4 0.0 40-42 0.2 0.0 0.0 0.0 0.0 42-44 0.0 0.2 0.0 88.2 74.8 11.8 25.2 14
16
18
20
2 2 24 26 28 33 32 34 36 38 40
CARBON
NUMBER
Figure 3. Chromatograms of Amal Gas Oil Top: as received Bottom: rz-paraffin free
Table I. Experimental Results on Calibration Blend Method A Method B Actual iso-octane reflux 2 hr-200 "C 27.4 28.4, 28.0 27.9 Table 11. n-Paraffin Content of Typical Gas Oils Wt n-Paraffin Method A Method B Wide Cut Mid-Continent Gas Oil 11.8 Amal650/850 OF Gas Oil ... 24.9,'25.6
by both adsorption methods are shown in Table I. With twogram samples, both methods are accurate to about + l wt n-paraffin. Accuracy could improve with larger samples. Analyses of Some Typical Gas Oils. Results on a wide cut Mid-Continent gas oil and a 650/850 O F Amal gas oil are shown in Table 11. As expected, the Amal gas oil contains more than twice the n-paraffins in the Mid-Continent gas oil. Carbon Number Distribution. The carbon number distribution of both the normal paraffins and the cyclic isoparaffinic portion of the gas oil sample was determined in the following way. The gas oil sample was first analyzed by high temperature gas chromatography to give a general breakdown of carbon number distribution. After 5A adsorption and extraction, the recovered nonnormal fraction (containing the cyclic and iso-paraffinic fraction of the gas oils) was again analyzed in the same manner. Figure 3 shows a set of chromatograms of
+
510
ANALYTICAL CHEMISTRY, VOL. 42, NO. 4, APRIL 1970
650/850 O F Amal gas oil before and after n-paraffin removal. Superimposed on the bottom of the chromatogram is the carbon number scale determined from a known blend of n-paraffins. The fraction of hydrocarbon in each carbon number range could be allocated by its area under the curve using the disc integrator, and the total area normalized by the n-paraffin content as determined by the method described in this report. Table 111 shows the result on a wide cut Mid-Continent gas oil and a 6501850 " F Amal gas oil.
CONClLUSIONS Two equally effective adsorption methods have been developed for the quantitative determination of normal paraffins in gas oils. In one method, n-paraffin adsorption is effected by refluxing an iso-octane solution of the gas oil in the presence of 5A molecular sieve for 17 hours, while in the second method the gas oil is mixed with the molecular sieve and the mixture heated at 200 "C for 2 hours. Following these initial treatments, the molecular sieves are successively extracted with iso-octane, benzene, and acetone. Solvents are evaporated from the extracts leaving behind the nonnormal (cyclic and branched) fractions of the gas oils. The difference in weight between the original gas oil sample and the nonnormal fraction represents the amount of n-paraffins in the gas oil. High temperature gas chromatographic analyses of the original gas oil and the nonnormal fraction were used to determine the individual amounts of n-paraffins related to carbon number in the gas oils. These methods are particularly useful when samples contain significant amounts of aromatic and polynuclear aromatic hydrocarbons as do most gas oils. With 2-gram samples both methods are accurate to about *l wt % n-paraffin. Accuracy could be further improved with larger samples. RECEIVED for review November 10, 1969. Accepted January 19, 1970.
