Automated Kjeldahl Determination of Nitrogen in Petroleum Robert N. Heistand
Sun Oil Company, Marcus Hook, Pa. 19061 The lengthy, involved procedure used to determine trace nitrogen in petroleum has been fully automated. Sulfuric acid extraction, continuous Kjeldahl digestion, and indophenol blue color development are employed. The method can handle most petroleum distillates having end points less than 700 O F and nitrogen levels from 0 to 200 ppm. Straight-run gasoline, kerosenes, naphthas, naphthalenes, gas oils, light crude distillates, specialty oils, and lube stocks have been analyzed and compared against accepted results. At the 25 ppm N level, repeatability (hl n) is 1.4 ppm N and accuracy (mean error) IS 0.1 ppm N. Total time required to complete the analysis of a single sample i s 30 min. About 40 samples can be analyzed in 1 day.
NATURALLY OCCURRING ORGANIC NITROGEN COMPOUNDS, frequently present in petroleum distillates, can seriously reduce the catalytic activity of the commonly used reforming catalysts and produce an inhibiting effect on catalytic cracking. These nitrogen compounds are also one of the chief causes of product instability. Since only traces of these compounds can cause catalyst or product degradation, a precise, accurate, and rapid method for determining nitrogen in trace amounts has been the objective of many investigators. One of the main problems in the determination of trace nitrogen in petroleum distillates is in obtaining a sufficient amount of nitrogen to overcome errors caused by deviations in reagent blanks. Although considerable effort has been spent by many investigators in purifying reagents in order to obtain lower and more reproducible blanks, this effort did not entirely solve the problem. Large samples were needed. Direct analysis of large samples was done by King and Faulconer ( I ) using the ter Meulen method and by Noble (2) using the Kjeldahl method. More recently, the lengthy, direct treatment of large samples has been avoided by employing a preliminary concentration step. Many substances have been suggested for removing and concentrating nitrogenous material from petroleum samples. Some of these that have appeared in the literature are: silica gel (3) sulfuric acid (4), FeC13-ZnCle (5), alumina (6), silica coated with sulfuric acid (7), pumice coated with sulfuric acid (8), and FeClrZnClz with sulfuric acid coated pumice (9). Although these procedures involving preliminary concentrations are somewhat slow (3 to 5 samples completed in a day), they do offer improvements over direct treatment methods. In order to speed up this important analysis, laboratory automation and instrumentation are needed. A study of this (1) R. W. King and W. B. M. Faulconer, ANAL.CHEM.,28, 255 (1956). (2) E. D.Noble, ibid., 27, 1413 (1955). (3) G.R.Bond and C. G. Harriz, ibid., 29,177 (1957). (4) 0.I. Milner, R. J. Zahner, L. S. Hepner, and W. H. Cowell, ibid., 30, 1528 (1958). (5) G. K. Hartung and D. M. Jewell, Anal. Chim. Acta, 27,219 (1962). (6) L.L.Farley, J. F. Guffy, andR. A. Winkler, ANAL.CHEM.,36, 1061 (1964). (7) I. J. Oita, ibid., 38,804 (1966). (8) P.Gouverneur, Anal. Chim. Acta, 26,212(1962). (9) A. J. Smith, F. F. Cooper, Jr., J. 0. Rice, and W. C. Shaner, Jr., ibid., 40,341 (1968).
approach resulted in a method suitable for the determination of trace nitrogen in light petroleum distillates. The result, an automated extraction Kjeldahl method, is the automation of the sulfuric acid extraction-concentration technique of Milner et al. (4), combined with the automated Kjeldahl determination procedure of Catanzaro, Goldgraben, and Gasko (10). EXPERIMENTAL
Apparatus. The Technicon AutoAnalyzer is used. The modules needed include sampler 11, pumps I and 11, continuous digestor, colorimeter with a 50-mm flow cell, range expander, and recorder. An extraction coil is fashioned from Technicon glassware-a 28 turn, 2.4-mm i.d. mixing coil, an “h” connector, a pulse chamber, and tubing (see Figure 1). This extractor-separator is used, as pictured, in an upright position. Concurrent extraction occurs with both streams, the petroleum sample and the sulfuric acid, flowing downward. The allglass design of the extractor-separator is necessary to prevent leaks. Reagents. The extraction acid was prepared by adding 2 liters of sulfuric acid (96%) to 1 liter of water. All water used in this method is N-free (deionized-distilled). The digest acid was prepared by dissolving 12 grams of selenous acid (H2Se03) in 200 ml of water, adding 80 ml of perchloric acid (70%), and diluting to 4 liters with sulfuric acid (96%). The sodium hydroxide solution contained 400 grams of NaOH/liter. Sodium phenate was prepared by dissolving 250 grams of phenol in water, carefully adding 400 grams of 50% (w/w) sodium hydroxide, and diluting to l liter with water. “Clorox” is a suitable sodium hypochlorite solution. Solvents for sample dilution and standard preparation must have less than 1 ppm N. Transformer oil, Sun Transformer D, or equivalent, and isooctane, Phillips ASTM grade, or equivalent, are suitable. Standards. For the reference standard dilute 500 grams of transformer oil to 1 liter with isooctane. For the Calibration standards, weigh 431.8 mg of 8-hydroxyquinoline, 235.5 mg of 2,3-dimethylquinoxalne, and 349.9 mg of indole into 500 ml of the reference standard to make a stock solution (1 ml = 0.25 mg of N). Take suitable aliquots of the stock solution and dilute with the reference standard to prepare a series of calibration standards having 0, 1, 2.5, 5, 10, 17.5, 25, 37.5, and 50 mg of N/liter or 0, 2, 5, 10, 20, 35, 50, and 100ppm N. Procedure. The flow diagram of the automated method is shown in Figure 2. Petroleum samples are dissolved in isooctane such that 10 ml of the final volume contains 5 grams of sample. They are fed intb the extractor-separator between samples of the reference standard at the rate of 10 petroleum samples an hour. Air is pumped in the intervals between the samples. In the extraction stage, nitrogenous material is concentrated in the extract acid. After settling out in the last few coils of the extractor, a portion of the extract acid is drawn off the bottom of the separator. The balance of the acid, the air, and the spent oil flow through the side arm to waste. The extract and digest acids are mixed and fed into the digestion helix. After the Kjeldahl digestion, the mixture is cooled, diluted with water, aliquotted, and neutralized with excess sodium hydroxide. (10)E. W. Catanzaro, G. R. Goldgraben, and R. M. Gasko, Aufomat.Anal. Chem. Technicon Symp., p 241 (1966). ANALYTICAL CHEMISTRY, VOL. 42, NO. 8, JULY 1970
903
MIfMin. SAMPLE SAMPLE EXT.ACID AIR
-
0.80
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400.C
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AIR
0.42
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FIC
-
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,
300. C
-
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630 Mu
Figure 1. Extractor-separator Figure 2. Flow diagram
Then, the reagents, sodium phenate and sodium hypochlorite, are added and mixed, The indophenol blue color is measured at 630 nm in a 50-mm flow cell. The resulting signal is expanded and recorded on the stripchart. Total time between sample pickup and recorder readout is ahout 28 min. A calibration curve is prepared daily because of changes in reagents and, more important, changes in the pumping characteristics of the pump tubes. Useful life of these tubes, particularly the “acid flex” tubing, varies widely. Calihration standards, like the petroleum samples, are placed between reference standards on the sample tray. Ahout once every hour during a sample run, the base line and calibration curve are checked by using a reference and a calibration standard in place of samples. No calculations are needed for samples having less than 100 ppm N since the calibration curve is prepared by plotting ppm N us. % T. The method may be extended to samples having greater than 100 ppm N by using less than 5 grams/l0 ml and multiplying the result obtained from the calibration curve by the appropriate factor. RESULTS AND DISCUSSION
Samples. All samples are dissolved in isooctane such that 5 grams of sample are contained in 10 ml of the final volume. This eliminates the need to measure the density of the samples in order to calculate the ppm N on a weight basis. Dilution with isooctane also decreases the volatility of the light petroleum distillates, decreases the density of the oils for better separation, and lowers the amount of unsaturation to prevent excessive extraction of carbonaceous material into the sulfuric acid. A minimum sample of 5 grams is needed for a single analysis. Samples of unknown origin or composition, particularly the higher-boiling, darker, more viscous distillates, should he tested before accepting automated results. Should the samples dissolve in sulfuric acid or fail to separate easily, excessive charring will take place in the digestion helix. This can lead to low results due to incomplete nitrogen conversion or high results due to extraneous colored matter from the charred material. A clear, colorless digestate is needed at all times. 904
ANALYTICAL CHEMISTRY, VOL. 42, NO. 8, JULY 1970
In any event, the initial results from unknown material should be checked by an acceptable method. For the most part, all petroleum samples distillingbelow 700 “Fand having less than 200 ppm N were analyzed with no trouble and the data ohtained were comparable to acceptahle data. In this complex and lengthy system, tailing is a problem. Using a slow sampling rate, small holdup volumes in the separators, and the shortest lengths possible of nonsegmented sample streams helped eliminate the errors caused by the tailing of the previous sample. The insertion of an oil containing 0 ppm N as a wash between all samples, calibration standards, and base-line checks practically eliminated the effects of tailing. Since tailing still occurs to the extent that any sample contains approximately 1-3 of the preceding sample, adjacent samples Wering fivefold in their nitrogen values are not reported, hut reanalyzed in a different order. This keeps errors due to tailing less than 5 % relative. A longer wash time would probably eliminate tailing errors completely but would require custom design and fabrication of the sampler module. Extraction. Several variables were studied to obtain optimum separation of the petroleum and acid phases after extraction. Many types of extractor-separators were tried before choosing the type shown in Figure 1. The efficiency of the extraction was determined by analyzing the extracted oil using the modified Gouvemeur method described hy Smith et ai. (9). Variables studied were total flow rates, oil:acid ratios, acid strength, temperature, nitrogen concentration, and nitrogentypes. Total flow rate through the extractor-separator should be less than 8 ml/min. Although a large oil:acid ratio would offer a convenient means of concentrating the nitrogen in the acid phase, this ratio must he kept below 10. At ratios above 10, the efficiencyfalls off rapidly-90 efficient at 20 oil :acid ratio and 70% at 40. The strength of the sulfuric acid, between 60 and 96 wt %, has little effect on the extraction efficiency. No significant drop in efficiency for either basic or
Table I. Repetitive Analyses of a Known Standard p = 24.3 ppm N Modified Gouverneur Automated (MG) (AEK) Analyzed values, ppm N 23, 25, 24 24, 23, 24
150 '
27, 25, 23 23, 23, 22 23
"/
Table 11. Nitrogen Content of Various Petroleum Samples Nitrogen content, pprn N Modified Auto Material Gouverneur (MG) Ext Kjel (AEK) Straight run gasoline
