Determination of Total Nitrogen in Petroleum Distillates by Catalytic Hydrogenation CHARLES WANKAT AND J. G. GATSIS Uniaersal Oil Products Co., Riverside, Ill.
Currently available procedures for determination of total nitrogen in petroleum distillates were found unsatisfactory when the nitrogen content was below 0.01% (100 p.p.m.). A high pressure catalytic hydrogenation process for determining total nitrogen in petroleum distillates in the range of 1 to 100 p.p.m. consists essentially of quantitatively reducing organic nitrogen to ammonia by high pressure hydrogenation in an autoclave with a nickel catalyst and absorbing the resulting ammonia on acidic alumina which is in admixture with the catalyst proper. The absorbed ammonia is determined by a conventional Kjeldahl distillation of the catalyst mixture. Interference due to sulfur is eliminated by proper choice of sample size. Data are presented on synthetic samples containing the types of nitrogen compounds found in petroleum as well as results on a variety of light petroleum distillates, both virgin and cracked.
APPARATUS
High-pressure autoclave, 3600-ml. capacity. An Ipatieff-type rotating autoclave was used in this Iahoratory. Glass bomb liner with cap. Kjeldahl flask, 800-ml. Kjeldahl distillation apparatus. REAGENTS
COP nickel-kieselguhr hydrogenation catalyst. This material is available in small quantities from the Universal Oil Products Co., Des Plaines, 111. Pure alumina cylindrical pills. The blank on 100 grams of pills must show less than 2 p.p.m. of nitrogen. If the available commercial product is not suitable, the ammonia content may be decreased by heating a t 1200" F. for about 16 hours. Heated Harshaw alumina pills were found satisfactory. tert-Butyl chloride. Sodium hydroxide, 50% aqueous solution. Boric acid solution, saturated. Standard sulfuric acid, 0.1 S. Methyl purple indicator, aqueous solution. approximately 0.1 %. PROCEDURE
R
ICCEXT publications on the Kjeldahl (4, 6 ) and the ter Rleulen (3) procedures for the determination of total nitrogen in petroleum distillates have demonstrated the utility of these methods for nitrogen in the range upwards of 0.01% (100 p.p.m,). Howevei, attempts in this laboratory to use the Kjeldah1 method on naphthas containing less than 0.017 0 nitrogen failed, largely because the amount of sample which can be digested is small, with the consequence that the ammonia blank for the reagents usually exceeded that recovered from the sample. The ter Meulen method q-as not tried in this laboratory, but the recent work of Holov-chak, Wear, and Baldeschwieler ( 3 ) showed that the lower limit of detection was 100 p.p.m.. with a possibility that refinement of technique would enable detection of 10 p.p.m. An entirelv different approach then seemed advisable. The procedure sought was one which would enable detection of nitrogen in the range of 1 to 100 p.p.m. present in petroleum distillate fractions. The low nitrogen concentration dictated that the reagent blank be negligible, and that a large sample, up to 1 liter, be used to give a convenient and accurate analysis. These requirements seemed to be fulfilled by a catalytic hydrogenation technique in which the organic nitrogen is quantitatively converted to ammonia The first efllort5 along these lines were with a high pressure bench scale hydrogenation apparatus in which the naphtha was pumped, along with hydrogen, over a suitable catalyst bed. These preliminai y results showed considerable promise for the use of UOP nickel-kieselguhr catalyst for the quantitative reduction of organic nitrogen to ammonia, but the technique had to tie abandoned for several practical reasons-it was difficult to recovei all the ammonia and the practical aspect of repeated heating and cooling of the reactor vewel, changing of catalyst, etc., made it very difficult to maintain a leak-free system. Although it v a s ahandoned, this v-oik led to the idea that ammonia might be iecovered by using an acidic solid in admixture with the hydrogenation catalyst. The batch-type reaction run in an autoclave proved to be a successful technique for low nitrogen concentrations in petroleum distillates.
The sample size for catalytic hydrogenation is governed by the concentration of the total nitrogen and t,otal sulfur in the sample being analyzed. Tables I and I1 can be used as a guide in choice of sample size; sulfur is usually the limiting factor and therefore Table I is generally used. If the nitrogen content is high as determined by a preliminary analysis, Table I1 is used. For distillates which have both high sulfur and high nitrogen contents, judicious use of both tables is necessary. The sample sizes listed are conservative estimates in order to ensure quantitative determination :inti are not intended as inflexible limitations.
Table I.
Sample Size for Distillates Containing Sulfur
Table 11.
Total Sulfur, %
Sample, RII.
0.01-0.1 0 1-0 5 0.5-2 0 >2 0
1000
500 250 200
Sample Size for Sulfur-Free Distillates (0.01% sulfur or IP-s)
Total Sitrogen, P.P.11.
Satiiple. JII.
0-20 20-50 > 50
1p30 000 250
Introduce into a bomb liner 100 grams of alumina pills, 50 grams of UOP nickel-kieselguhr pills, the appropriate volume of sample, and 1 ml. of tert-butyl chloride. Place the charged bomb liner in an Ipatieff-type rotating autoclave and charge with hydrogen to a pressure of 750 pounds per square inch gage. Heat the bomb to 450" C. Continue heating a t this temperature for 4 hours. After the run is completed, cool the bomb to room temperature and depresmrize t o atmospheric pressure. Remove the bomb liner and separate the solids from the liquid product by filtration. Place the solids in an 800-ml. Kjeldahl flask. Rinse the bomb liner with a t least two 50-nd. portions of distilled water and transfer quantitativelr to the Kjeldahl flask. Add 300 ml. of
1631
ANALYTICAL CHEMISTRY
1632 distilled water to the Kjeldahl flask to bring the total volume to 400 ml. Cool the contents of the flask in an ice bath. Place a 500-ml. Erlenmeyer flask containing 25 ml. of boric acid solution and 5 drops of methyl purple indicator under the condenser, so that the tip is well beneath the surface of the liquid. Slowly pour 80 ml. of cooled sodium hydroxide solution down the inclined neck of the Kjeldahl flask. Connect the flask to the distillation condenser. Heat the distillation flask so that rapid boiling is established and maintained. When the volume of liquid in the receiving flask reaches approximately 250 ml. lower the receiver to expose the condenser tip and rinse it ob with distilled water. After about 1 minute of additional distillation, turn off the heat and allow the condenser tube to drain. The distillate may contain a hydrocarbon layer formed by hydrocarbon adsorbed on the catalyst. To facilitate the titration, carefully decant the bulk of the hydrocarbon layer, avoiding loss of aqueous solution. Titrate the distillate with standard sulfuric acid until the green color fades into gray and just beyond to the point that the gray assumes the first faint tinge of purplish pink. Make a blank determination of the catalyst-alumina mixture following the same distillation procedure and using the same amounts of reagents as in the analysis. The ammonia should be less than 2 p.p.m. based on the total catalyst-alumina mixture. An ammonia content of alumina pills greater than 2 p.p.m. can be decreased by heating the pills overnight in a muffle furnace a t 1200" F.
For very accurate work the amount of nitrogen in the form of basic nitrogen in the hydrogenated liquid product must be accounted for. The procedure used was titration with perchloric acid in anhydrous medium (8, 6). Ordinarily the amount of nitrogen found in the liquid hydrocarbon after hydrogenation is less than 1% of the total and this operation is usually unnecessary. PRECAUTIONS
Safety limits for pressure of the autoclave must not be exceeded during the determination, Generally, pressures attained in the course of the hydrogenation do not exceed 200 atmospheres a t 450' C., although in one analysis in this laboratory on a sample of cyclohexane the pressure went higher and the temperature had to be lowered. Hydrocarbon is adsorbed on the catalyst and is distilled during the distillation procedure. Care must be taken to keep the distillation receiver away from flame. CALCULATIONS
where A
B N V G
( A - B ) ( N ) (14,000) Nitrogen, p.p.m. = ( V )(GI = volume of standard acid required to titrate distillate milliliters = volume of standard acid required to titrate blank, milliliters = normality of standard acid = volume of sample, milliliters = specific gravity of sample RESULTS AND DISCUSSION
The basis of the method of analysis is the quantitative reduction of the organic nitrogen to ammonia by high pressure hydrogenation with a nickel catalyst. The resulting ammonia is absorbed on alumina pills which are in admixture with the catalyst proper, In order to obtain effective absorption an acidic alumina was needed, which was achieved by inclusion of a small amount of ten-butyl chloride with the hydrocarbon sample prior t o the hydrogenation reaction. The organic halide dehydrohalogenates during the heating period to split out hydrogen chloride, which then reacts with the alumina. The resulting material is effective in quantitative absorption of the ammonia on the catalyst-alumina mixture. Following the hydrogenation step, the mixture is separated from the hydrocarbon sample and the ammonia is determined by a conventional Kjeldahl distillation.
As a test of the ability of the acidic alumina to hold the inmonia quantitatively, analyses were made of both the effluent reactor gases and the recovered hydrogenated liquid hydrocarbon; these were in addition to the determination of ammonia held on the catalyst mixture. rlmmonia was determined in the gases by the method of Donn and Levin (1) and basic nitrogen was determined in the liquids hy titration with perchloric acid in glacial acetic acid (2, 6). These analyses were made on a large number of samples; in all cases no ammonia was found i n the gases. Basic nitrogen in the hydrogenated liquid hydrocarbon amounted to 1% or less of the total nitrogen in most caqes, although a few runs on high-sulfur stocks showed 2 to 5%. The analytical method can therefore safely disregard ammonia in the reactor gas, and basic nitrogen in the recovered hydrogenated liquid hydrocarbon can be easily titrated, if maximum accuracy is desired. All the results reported here include this nitrogen as well as that recovered from the acidic solid. In the preparation of synthetic samples three organic nitrogen compounds were used as representative of the kind of materials that might be expected to occur naturally in petroleum distillates: pyridine, 2-methylpyrrole, and .Y-ethylcarbazole (7, 8). It was anticipated that Y-ethylcarbazole would be a severe test of the proposed method in view of the fused ring structure, and accordingly this material was used in most of the work on synthetic samples. Subsequent results on partly sulfur-deactivated catalyst showed the refractory nature of the carbazole. Results on synthetic naphtha samples of low sulfur contcnt are presented in Table 111, where a series ranging from 4.5 to 543 p.p.m. of nitrogen was used. The data show that hydrogenation of these nitrogen compounds to form ammonia and its adsorption on the catalyst-alumina mixture can be considered as quantitative for this analytical method. Comparison of values for nitrogen found versus nitrogen added shows that the procedure is accurate, considering the low concentrations of nitrogen.
Table 111. Analysis of Synthetic Hydrocarbon Samples of Low Sulfur Content
Nitrogen Compound Pyridine 2-Nethylpyrrole S-Ethvlcarbazole
(1s-octane solvent) N -4dded, Volume for .4nalysie, M I . P.P.M. 1000 250 1000 1000 700 1000 1000 1000 1000 1000
343 39.3 22.4 154 154 98.2 48.6 19.2 15.4 4.5
N
Found, P.P.M. 529 37.0 18.9 133 14 1 97.3 47.9 20.0 18.1 5.2
Error, P.P.11. S
-
14 -2.3 -3.5 -21 13 -0.9 -0.7 +o. 8 +2.7 +0.7
-
Because sulfur occurs naturally and is known to poison nickel hydrogenation catalyst, the effect of organic sulfur compounds 17-as investigated as summarized in Table IV. Thiophene and tert-hexyl mercaptan were used in order roughly to simulate naturally occurring sulfur. Sulfur is shown to have a definite effect in decreasing ammonia formation, but this can be negated by appropriate choice of sample size, since the mass of sulfur in proportion to nickel can be varied and the sulfur poisoning eliminated. The synthetic samplesused here were purposely made up as high-sulfur samples, 0.5 to 1.9% sulfur, to give a severe test. In use of the method on petroleum distillates the sulfur content will very seldom be as high as this. The tables giving choice of sample size in accordance with sulfur and nitrogen contents are included to ensure freedom from the sulfur poisoning effect. The procedure should work in the presence of olefins where depletion of hydrogen by saturation of the double bond might cause difficulty. The latter item was felt to be insignificant in view of the high hydrogen pressure, 760 pounds per square inch gage,
V O L U M E 25, NO. 11, N O V E M B E R 1 9 5 3 ~
~~
1633
_. .
with t h e partially sulfurpoisoned catalyst the latter was reduced, while the N-ethylcarCharge Stock Composition Hydrocarbon S bazole gave poor ammonia reK (Added as N-Ethylcarbazole), Total S o , RSH S O , Solution Found, Error, covery. Comparison of lines Solvent P.P.M. Wt. % Wt. % Used, hll. P.P.M. P.P.hI. S 1 and 2 shows the sulfur Iso-octane 19.5 h'il Nil 1000 17.0 -2 5 21.6 3.89 0.28 900 6.9 -14 7 poisoning effect,, while lines 4 22 pyrrole) 4 (as 2-methyl0.93 0.35 1000 18.9 -3.5 and 5 demonstrate that by Iso-octanr diisobutylene, 1 : 1 99.1 1.86 0.69 1000 6.8 -92.3 proper choice of sample size 99 1 1.86 0.69 250 99.8 +Q 7 the effect of sulfur poisoning 29.2 0 52 0.15 1000 25.4 -3 8 Iso-octane-diisobutylene, 7 : 1 26.2 0.48 0.18 1000 22.2 -4 0 is eliminated. a Sulfur added as thiophene and tert-hexyl mercaptan. The method was applied to a high-sulfur cracked naphtha, a Santa Maria coker distillate containing 2.35% sulfur, in further st'udy of sulfur interference. The analyses as given in but was nevertheless investigated. The results are summarized in Table IV, where it is shown that the method works nicely even Table V show that the excellent precision is obtained when t,he in the synthetic naphtha cont,aining a high concentration of olesize of sample taken for analysis is regulated to allow for the fin. sulfur poisoning effect. 4 synthetic sample having properties similar to the coker distillate was prepared to check the accuracy The data of Table J V (lines 2 and 3) show that dV-et,hylcarof the analysis. The results show that accuracy as n-ell as prehazole is refractory in comparison to 2-methylpyrrole-that is, cision is excellent. .4large number of petroleum distillates have been analyzed, in-~ cluding virgin gasolines and naphthas, cracked gasolines and naphthas, and S o . 2 burner oils. Some typical results are given Table V. Analysis of Santa Maria Coker Distillate in Table VI. The virgin gasolines showed a considerable range S Found, Remarks Strniple Volume, 1\11, P.P.hl. of nitrogen contents, from less than 1 to 15.0 p.p.m. The ratio S.31. coker distillate 1000 500 111) 120, Sample too large of basic nitrogen to total nitrogen varied considerably in the 250 130 'I cracked mat.erials where basic nitrogen almost equaled total nitro250 128 Correct amount gen in some cases, whereas in others it was only one half of the 150 130 13 1 (added Of Simulated S.M.coker 230 total nitrogen. In virgin naphthas basic nitrogen was as little diatillate 139) as one fifth of the total nitrogens. (The basic nitrogen referred Properties of Samples to is naturally occurring and is not a result of the hydrogenation.) S.X. coker distillate Variations in the mechanics of running this analysis are pos52.3 API gr. a t 60' F. Boiling range 1.58' t o 398' F E P sible-for example, smaller hydrogenation bombs can be used 0.006 Hydrogen sulfide, wt. c/L Mercaptan sulfur, wt. % 0.082 along with corresponding reduction in sample size, catalyst, Total sulfur, wt. % 2.35 Bromine KO. 67 alumina, and normality of the t'itrating acid. The limit of deBasic nitrogen, p.p.m, 62 tection may be lowered below a few parts per million nitrogen as Siiniilated S.M. coker distillate Solvent, 1: 1 mixture of i3o-octane studied. The problem would be t'o obtain reagents containing and diisobutylene far less ammonia; it would not involve measurement of small Sulfur d as thiophene, % amounts of ammonia produced, as there are available procedures 0.067 1 94 S as tert-hexyl mercaptan, % Nitrogen, p.p.m. for t'his purpose. 40 As N-ethylcarbazole Table 11.. Effect of Organic Sulfur and Olefins
1
60 39 139
Pyridine 2-inet hylpyrrole Total
ACKh-OWLEDGMENT
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Table VI.
-4nalysis of Petroleum Distillates
Type of Stock Virgin gasolines and naphthas Source Calif. naph 288-360O F Calif. naph:: 168;230° E': Penn. S.R. gasoline Wyo. med. naph. 31ich. S.R. gasoline Middle East naph. (Kuwait) Penn. S.R. naph. Penn. S.R. gaso. Venezuela hvy. naph. Middle East hvy. naph. Mich. S.R. naph. Penn. S.R. gaso. Wyo. med. naph. Cracked gasolines and naphthas Mid-Cont. med. naph. Visbreaker naph. Wyo. R. Tex. Calif. cracked naph. Santa hIaria coker distillate Calif. therm. cracked naph. No. 2 fueloil components Calif. therm. side cut Wyo. cat side cut Mid. Cont. therm. side cut Miscellaneous Tech. meth lcyclopentane Tech. cyclogexane C.P. benzene Crude coal tar benzene Tech. n-heptane
Total S , P . P. hI. 7 3 5 9 15.0 4 6 5.0 3.1 3.5 2.0
