255
V O L U M E 28, NO. 2, F E B R U A R Y 1 9 5 6 Table I.
Effect of Amine on Determination
A M P Added, ?/RIl.
NXIP Bdded, y/hIl.
N M P Found, y/R.II.
5018.8 5018.8 1003.0 1003.0
50 100 50 100
5 14 35
Replicate results of the determination of small amounts of 2-nitro-2-methyl-1-propanol in 2-amino-2-rnethyl-l-propano1, using the reflux modification, are given in Table 11. LITERATURE CITED
77
Table 11. Determination of 2-Nitro-2-methyl-1-propanol in 2-Amino-2-methyl-1-propanol Mixtures AMP Added, G .
NAIP Added, llg.
N M P Found, hlg.
0,5045 0.5045 0,5045 0,5046
5.0 1.0 0.5 10 0
4,960, 5.005 1.005,1.000 0.500.0.495 9 , 9 6 0 , Q .980
Beyer, G . F., J . Assoc. Ofic. Agr. Chemists 34, 745 (1951). B O O S , R. E., ANAL. CHEY.20, 964 (1948). Boyd, M. J., and Logan, M. A , J . Bid. Chem. 146, 279 (1942). Bricker, C. E., and Johnson, H. R., IND.ENG.CHEY.,ANAL.E D . 17, 400 (1945). Bricker, C. E., and Roberts, K. H., ANAL. CHEIM.21, 1331 (1949).
Bricker, C. E., and Vail, W. .1.,Ibid., 22, 720 (1950). Eegriwe, E., 2 . anal. Chem. 110, 22 (1937). Grebber, K., and Karabinos, J . V., J . Research A'atZ. B u r . Standards 49, 163 (1952).
mixture to such an extent that all of the formaldehyde was not released. The formaldehyde may be conveniently separated from the interfering aminohydroxy compounds by refluxing in alkaline bisulfite solution, The formaldehyde released from the nitrohydroxy compounds complexes with bisulfite and the complex reacts quantitatively with the chromotropic acid.
Hass, H. B., and Riley, E. F., Chem. Reus. 32, 373 (1943). Jones, L. R., and Riddick, J. A , , ANAL.CHEM.24, 1533 (1952). Klienert, T . , and Srepel, E., Mikrochemie w r . Mikrochim. Acta 33, 328 (1948).
.\lacFayden, D. A., J . Biol. Chem. 158, 107 (1945). Speck, J . C., A N ~ LCHEJI. . 20, 647 (1948). Turba, F., Haul, R., and Uhlen, G.. A n g e w . Chem. 61, 73 (1949). RECEIVED for review September 29,
1955.
Sccepted November 14, 1955.
Determination of Total Nitrogen in Reformer Charge Stock R.
W. KING
and
W. B.
M. FAULCONER
Research and Development Department, Sun
Oil Co.,
A method is described for the determination of traces of combined nitrogen in reformer feed stocks that is particularly suitable for the concentration range from 1 to 100 p.p.m. It is a modification of a procedure originally described by ter Meulen, and involves conversion of nitrogen compounds to ammonia by catalytic hydrogenation and colorimetric determination of ammonia with Nessler's reagent. Application is limited to petroleum stocks having end points lower thanP.50' F. The sensitiFity is about 1 p.p.m. of nitrogen. Repeatability and accuracj are of the same order of magnitude as the sensitivity in the range below 20 p.p.m. The equipment is relatively inexpensi\e and simple to operate. An anal?sis may he completed in a little less than 2 hours.
D
U R I S G the past decade the petroleum industry has hecome increasingly av-are of the serious problems caused by nitrogen-containing compounds. The presence of even trace quantities of these materials may affect adversely the processing, storage, and quality of petroleum products. The presence of nitrogen compounds reduces the activity of cracking catalysts. It has been demonstrated that in catalytic charge stocks they seriously decrease the conversion to gasoline ( 7 , 6). Other catalysts, such as those used in reforming, polymerization, and isomerization, are susceptible to poisoning by nitrogen compounds (2). The increased commercial use of platinum catalysts for reforming straight-run naphthas has made necessary the determination of total nitrogen in such materials. The nitrogen content is generally in the range below 20 or 30 p.p.m. and therefore lies below the reliable detectability limits of the conventional Kjeldah1 and Dumas methods ( 4 ) . The low nitrogen content of these stocks makes a negligible reagent blank essential. This requirement may be most conveniently met by a catalytic hydrogenation technique in which organic nitrogen is quantitatively converted to ammonia. A method embodying this principle has
Norwood, Pa.
recently been described by Wankat and Gatsis (IO). il sample up to 1 liter in volume is hydrogenated a t high pressure using a nickel catalyst. The resulting ammonia is adsorbed on alumina pellets, the alumina neutralized, and the ammonia distilled into boric acid and titrated. They reported excellent results on a number of naphthas using this technique. However, the method is time-consuming and requires the use of a large capacity autoclave a t pressures up to 200 atm. with the attendant dangers of operation. I n 1924 ter Meulen (6) described a semimicroprocedure for the determination of nitrogen by destructive hydrogenation. The method is based on the fact that when 10 to 200 mg. of an organic compound containing nitrogen are pyrolyzed in a stream of hydiogen and the products are passed over a heated nickel catalyst, the nitrogen is quantitatively converted to ammonia, n hich may be absorbed and determined by customary procedures. The ter Meulen method has been generally accepted in Germany, but has received little attention in this country because of the short life of the catalyst. Recently Holowhak, Wear, and Baldeschnieler ( 3 )reported on the application of this method to petroleum frxtions. These authors n-ere able to develop an improved catalyst which v a s resistant to poisoning by sulfur or halogens. They s h o m d that the lower limit of detection \vas about 100 p.p.m., but suggested that by increasing the sensitivity of determining ammonia by spectrophotometry it might be possible to analyze accurately samples containing as little as 10 p.p.m. of nitrogen. I t seemed desirable to attempt to extend the ter LIeulen method to enable detection and estimation of concentrations as low as 1 p.p.m., as it is a more rapid and convenient method than a high pressure hydrogenation technique. The idea of developing more sensitive means of detecting the ammonia produced was abandoned in favor of one which would allow larger quantities of sample to be analyzed than in the conventional procedure. I n addition to increasing the sensitivity by producing larger quantities of ammonia, such an approach would eliminate the rather tedious handling of semimicro quantities of volatile materials If one attempts to pyrolyze samples much larger than about 300
ANALYTICAL CHEMISTRY
256 mg. in the equipment described by Holomhak, Wear, and U:tldeschwieler (S), the catalyst rapidly becomes inactive, owing to ewessive accuniulation of carbon. An apparatus design which ~vouldprevent or minimize such carbon deposition, and a t the same time allow the pyrolysis of low boiling samples M ith a minimum of operator attention, seemed necessary. Equipment was ultimately developed employing a vertical reaction tube with separate pyrolysis and catalyst zones which permitted up to 3 grams of sample to be analyzed. The apparatus and procedure are well suited for routine work, and are readily adaptable to multiple determinations. 4PPARATU S
Gas Flow System. The general arrangement of the apparatus is shown in Figure 1. Hydrogen is supplied by a conventional pressure cylinder and reducing valve ( A , Figure 1). Gas flow is controlled by a needle valve, B, and microrotameter, C. The mercury ga e, D , permits an estimation of the back pressure existing &fin the system. The hydrogen stream enters the reaction tube, G, through the side arm in the tube cap. This side arm is fitted with a bypass line, E, to equalize pressure and allow the sample to be admitted a t a uniform rate from the sample pipet, F . During an analysis 1 to 3 grams of sample are allowed to drip from the pipet into the pyrolysis zone of the tube. This section contains a roll of nickel gauze, J , and is maintained a t 900" to 950" C. by the small furnace, K . As the droplets come in contact with the hot gauze pyrolysis takes place. Carbonaceous material is deposited on the gauze and the gaseous roducts are swept by hydrogen over the nickel-magnesium catafyst, M . The catalyst is maintained a t 350" to 360" C. by the furnace, L. The products of the catalytic reaction are swept into the absorber, P , where ammonia is scrubbed out and retained. The ammonia is then determined colorimetrically with Kessler's reagent (1, 9). Pyrolysis Furnace. The pyrolysis furnace consists of a 3.25inch length of 1-inch inside diameter Alundum tubing held in place within a cylindrical brass shell by end plates of 0.25-inch Transite. The Alundum tube is wound with a helical coil of 15 feet of No. 24 B and S gage Xichrome V, and the winding is completely covered with a layer of RA 1162 Alundum cement The annular space between the tube and the brass housing is filled with vermiculite as insulation. Such a unit nil1 attain a temperature of 950" C. in 10 minutes. The temperature of the pyrolysis furnace is measured with a Chromel-Alumel thermocouple and controlled by means of a variable autotransformer. Catalyst Furnace. A large furnace capable of maintaining a temperature of 350" to 360" C. is necessary for heating the catalyst zone. .4 Hevi-Duty furnace, Type FD 303, has proved suitable. The temperature is measured with a Chroniel-Alumel
thermocouple and controlled by means of a variable autotransformer. Quartz Reaction Tube and Cap. The quartz reaction tube is shown in detail in Figure 2. I t is 550 mni. long and consists of an 80-mm. pyrolysis section, a 270-mm. catalyst zone, and an exit section 5 mm. in diameter and 80 mm. long. The yrolysis and catalyst sections are 20 nim. in inside diameter. TEe upper end of the reaction tube is fitted with a quartz standard-taper 24/40 joint, and the exit end is provided with a 12/5 quartz spherical joint for connection t o the absorber system. Quartz joints are obtainable from the Cleveland Quartz Works, General Electric Co. A borosilicate glass standard-taper 21/40 joint fitted with a side arm and standard-taper 10/30 joint placed concentric with the reaction tube serves as a cap. The reaction tube is supported in the vertical furnace by means of a conventional laboratory clamp which grips the tube just below the top ground joint. Weighing Pipet. -1neighing pipet fitted with standard-tape] 10/30 joints to fit the cap and bypass is used to introduce the sample. Details of its construction are shown in Figure 2. Absorber. The absorber is constructed of borosilicate glass ~ c shown in Figure 3. RE4GENTS
Hydrogen. Magnesium oxide. LIerck reagent grade, low in sulfate, 19 suitable. Kickel nitrate hexahydrate, Merck reagent grade. Hydrochloric acid, approximately 0.021;, prepared with ani__ - monia-free water. Nessler's Reagent (1, 9). Dissolve 50 grams of potassium iodide in a minimum volume of cold distilled water Iamroximately 35 ml.), Slowly add a saturated solution of mere;& chloride until the first slight precipitate of red mercuric iodide persists. Add 400 ml. of a clarified 9'V solution of sodium hydroxide. Dilute to 1 liter with ammonia-free water and allow to clarify. The clear supernatant liquid is decanted and used. Standard ammonium chloride solution, 10 mg. of nitrogen per liter. Standard Yitrogen Blend. Prepare a blend of Eastman Kodak Co. white label quinoline and Knock Test Grade iso-octane t o contain 20 to 30 p.p.m. of nitrogen. This blend is used to check catalyst activity periodically. Ammonia-Free Kater. This may be conveniently prepared by redistilling laboratory-distilled water from a solution containing 1 cir 2 ml. of concentrated sulfuric acid. PROCEDURE
Preparation of Calibration Curve for Colorimetric Determination of Ammonia. Pipet 1-, 2-, 4-, 6-, 8-, and 10-ml. aliquots of
S 10/30 MICRO STOPCOCK
,8 MM 0 D GFiADUmED TUBING
I -P
a
TO
rl
8 MM
Figure 1. Schematic diagram of apparatus for determining trace quantities of nitrogen
.I215 SPhERlCAL JOINT
REACTION TUBE
Figure 2. cat a l p t
P. Absorber
REACTION TUBE CAP
Details of reaction tube, cap, and weighing pipet .411 dimensions in millimeters
:
257
V O L U M E 2 8 , N O , 2, F E B R U A R Y 1 9 5 6
2 MM BORE
SSCPCCCK
Sweep out the air with a strcam of hydrogen and reduce the catalyst by raising the temperature of the large furnace to 340" to 380" C. while maintaining a hydrogen flow of 30 to 40 ml. per minute. When reduction is complete (disappearance of condensation in the exit end of the tube) adjust the temperature of the furnace to 350" to 360" C. and maintain it at this temperature during the course of the determinations. The furnace may be turned o f f over week ends, but a slow rate of flow of hydrogen should be maintained continuously during nonoperating periods. hlake a blank determination by performing all operations as described in the following paragraph, but momentarily disconnecting the bypass instead of inserting the sample pipet. If the blank value is greater than 1-y of nitrogen, repeat the determination. With a new reaction tube filling the first blank is generally high, but will drop to a negligible value after an additional determination. It is necessary to run a. blank whenever fresh catalyst, is added to the reaction tube or fresh batches of reagents are used. Fill the sample pipet and weigh. With the catalyst furnace a t operating temperature add 20 ml. of ammonia-free water to the absorber. Add 1 ml. of 0.02*V hydrochloric acid to the absorber contents. Remove the bypass from the reaction tube cap, place the pipet in the cap, and place the bypass in the joint on the top of the pipet. Adjust the flow of hydrogen to a rate of 60 to 70 ml. per minute and raise the temperature of the pyrolysis furnace to 900" to 950' C. Introduce from 1 t o 3 grams of sample a t a rate of 1 drop every 12 to 18 seconds. After addition of the sample, raise the temperature of the yrolysis furnace to 980" to 990" C. for 10 minutes. Turn off anfallow to cool while purging the system for an additional 25 minutes with hydrogen.
r5MM0D
L
Figure 3. Details of absorber, all dimensions in millimeters
the standard ammonium chloride solution into 50-nil. voluuietrica flasks. Dilute almost to the necks with ammonia-free water, and pipet into each flask 1 ml. of Xessler's reagent. Swirl the flask during the addition of the reagent to prevent precipitation. Fill the flasks to the mark with ammonia-free water and shake until the solutions are thoroughly mixed. Allow to stand for 5 niinUtes, and measure the absorbance on a spectrophotometer a t 400 nip. Compensate for the blank color of the reagent by using a solut,ion of 1 ml. of reagent and 49 ml. of ammonia-free water in the reference cell. Prepare a calibration curve by plotting the concentrations of ammonia nitrogen in micrograms per 50 ml. against the rorresponding absorbances. A new calibration should be prepared whenever a new quantity of Nessler's reagent is made up. The successful use of Nessler's reagent depends upon cleanlincss and attention to detail. Glassware should be cleaned with chromic acid and rinsed with ammonia-free water just prior to use. Absorption cells may be dried after cleaning by rinsing with ethyl alcohol and blowing with a stream of filtered air. As the complex formed is colloidal in nature, it is necessary to standardize the procedure used for production of the color. Nessler"s reagent is best added in a squirt, as from an automatic pipet with a bulb; solut'ions should be a t a uniform and reasonably constant temperature, and the time interval allowed for color development should be adhered to rigidly.
The operations described above are not hazardous, provided all air is swept out of the system and reasonable care is taken to prevent entry of air during addition of the sample. If the operations pertaining to introduction of the weighing pipet are performed quickly, and some sample is always retained in the pipet, there need be no concern. As a precaution the operator should wear safety goggles during addition of the sample. Without discontinuing the flow of hydrogen, drain the absorber contents into a 50-ml. volumetric flask. Rinse the absorber with several small portions of ammonia-free r a t e r , adding the rinsings to the content's of the flask. Remove the pipet and weigh. Add 1 ml. of Nessler's reagent to the contents of the flask, dilute to the mark with ammonia-free water, mix well, and read the color developed on a spectrophotometer a t 400 mp, 5 minutes after addition of the reagent. Determine the number of micrograms of ammonia nitrogen present by comparison to the calibration curve. Calculate the nitrogen content as follows: iiitrogen, p.p.m. =
ammonia nitrogen, micrograms sample weight, grams
As the method depends upon catalysis, the activity of the catalyst must be checked when freshly prepared a.nd periodically thereafter using the synthetic blend of known nitrogen content. Sitrogen recovery should be a t least 95%.
Catalyst Preparation. The nickel-magnesium catalyst has been described by Holowchak, Wear, and Baldeschwieler ( 3 ) .
RESULTS AND DISCUSSION
Prepare a slurry of 125 grams of magnesium oxide in 1.25 liters of distilled water a t 50" C. Dense crystalline forms of magnesia are not suitable, The material used should be a finely divided, fluffy solid. Dissolve 400 grams of nickel nitrate hexahydrate in 4 liters of distilled water a t 50" C. Add the nickel nitrate solution slowly to the magnesia slurry with constant st,irring. Adequate mixing a t this point is essential to the preparation of a successful catalyst and a mechanical stirrer is desirable. Allow the precipitate to settle and decant the mother liquor. Transfer the precipitate to a Biichrier funnel, filter, and wash with distilled water until the washings show no more than a trace of nitrate ion. Score the precipitate in 0.25-inch squares, dry in air, and then a t 100" C. in vacuum. After vacuum drying, a suitable catalyst is generally very pale green in color. Screen the dried precipitate through 4- and &mesh screens, breaking into smaller pieces, if necessary. The catalyst retained on the 8-mesh sieve is used for filling t,he reaction tube. Follow instructions for preparing and reducing the cat>alystclosely, in order to obtain a reagent of high activity. Operation of Apparatus. Fill the cat,alyst zone of the reaction tube and assemble the apparatus. The sample pipet may be omitted and the bypass line connected directly to the reaction tube cap. Ignite the roll of nickel gauze and allow to cool. Remove the cap and drop the gauze into place in the pyrolysis zone.
Although the nickel gauze causes t,he major part of the carboil to be deposited in the pyrolysis zone, it was observed that after about a neek's operation, excessive back pressure developed in the apparatus. Upon inspection of the reaction tube it was found that approximately the top 2 inches of the catalyst xere covered with soft carbon. This port,ion was removed and replaced with fresh catalyst, and the apparatus was reassembled. This operation is now performed routinely n-henever back pressure becomes greater than 1.5 to 2 inches of mrrcury. The tendency for carbon to deposit on the catalyst can be alleviated by periodically removing the nickel gauze and burning off the carbon. In the preparation of synthetic samples, organic compounds which were representative of the type of material that might be expected to occur in petroleum m r e used. In addition, the applicability of the method for naturally occurring nit,rogen compounds was examined by analyzing blends of a shale naphtha whose nitrogen content had been well established by the Kjeldahl method ( 5 ) . -4s conventional procedures can be applied above 100 p.p.m., the test blends were prepared so as to cover the region
ANALYTICAL CHEMISTRY
258 below this amount. Emphasis was placed on the region 0 to 20 p.p.m., as it was anticipated that the nitrogen content of most virgin naphthas would fall within this range. Results on these synthetic naphtha samples are shown in Table I. The data show that the technique developed is entirely satisfactory for the determination of nitrogen in virgin naphthas even a t concentrations as low as 1 or 2 p.p.m. The sensitivity is about 1 p.p.m. The accuracy is of the same order of magnitude in the low range. Although the catalyst preparation used has been reported to be resistant to poisoning, a few experiments to determine the effect of the presence of organic sulfur compounds r e r e performed. The results are summarized in Table 11. Thiophene and benzyl mercaptan mere used to simulate naturally occurring sulfur. The synthetic samples were purposely made up to contain large amounts of sulfur to give a severe test. In the majority of petroleum distillates, the sulfur content would rarely be RO high. The data show that the method gives acceptable results when considerable quantities of sulfur are present. A few virgin naphthas of widely differing crude source have been analyzed. Some typical results are shown in Table 111. These naphthas show considerable variation in nitrogen content, from less than 1 to as high as 7 p.p.m. Such data may be helpful in crude evaluation studies. As expected, the nitrogen contents of naphthas derived from asphaltic crudes are considerably higher than those from other sources. 4 California naphtha of relatively high nitrogen content was fractionated in a small laboratory distillation column packed with glass helices. The fractions were then analyzed to check the distribution of nitrogen with boiling range. Approximately 85% of the nitrogen was found in the two fractions boiling above 373' F. These fractions constituted roughly 30% of the original naphtha. The fractions comprising the initial 70% of the sample could be combined to give a naphtha containing about 1.5 p.p.m. of nitrogen.
Table I. Analysis of Synthetic Blends Sample Iso-octane quinoline
+ Iso-octane + quinoline Iso-octane + quinoline Iso-octane + quinoline Iso-octane + quinoline
Boiling Range, F.
... ...
...
... ...
Xitrogen, P.P.M. Added Found 44.6 40.3 45.3 42.5 29 5 29.5 31.6 19.9 21.3 22.0 10.0 10.4 9.4 10.2 5.4 p.9 5.3
Straight-run naphtha
200-403
,
+ quinoline + indole + quinoline
200-405 260-405
82 ' 0
+ shale naphthaa
250-405
22 0
230-403
10 0
250-405
0 0
+ shale naphtha + shale naphtha + shale naphtha + shale naphtha
5.3
