Torch Analyzer for Precise Determination of Hydrogen in Liquid Hydrocarbons David Liederman and John R. Glass Research Department, Paulsboro Laboratory, Mobil Research & Development Corp., Paulsboro, N . J . 08066
MODERNREFINERY OPERATING procedures require the precise knowledge of the chemically-combined hydrogen content of various process streams and products. ASTM Method D1018 ( I ) for determining hydrogen by burning the hydrocarbon and measuring the water formed gives good results for non-aromatic materials. However, the wick-type lamp used in this method is not satisfactory for aromatic stocks such as reformates. Beta-ray instruments and the mass spectrograph (2) are precise and are applicable to both aromatics and non-aromatics. However, because they are complex and require extensive calibration, they are not generally suitable for refinery laboratories. The traditional micro carbon-hydrogen method (3) i s not precise enough for our purpose5 and is very lengthy. This paper describes a relatively simple and inexpensive torch analyzer (4) which can be used to determine the hydrogen content of both aromatic and non-aromatic liquid hydrocarbons precisely and accurately. The elapsed time for one analysis is less than two hours. On a routine basis, 12 to 15 samples can be analyzed in 8 hours. EXPERIMENTAL
Apparatus. A diagram of the apparatus is shown in Figure 1. It consists principally of the torch, combustion chamber, sample pipet, an absorber, and means for regulating streams of oxygen. Oxygen flow is controlled through pressure regulators, float flowmeters, and capillary restrictors. A constant flow of 700 ml per minute of oxygen is supplied (1) “ASTM Standards on Petroleum Products and Lubricants,”
Method D1018, American Society for Testing and Materials, Philadelphia, Pa. (2) R. B. Jacobs, L. G. Lewis, and J. F. Piehl, ANAL.CHEM.,28, 324 (1956). (3) J. B. Niederl and V. Niederl, “Organic Quantitative MicroAnalysis,” John Wiley and Sons, New York, N. Y., 1942. (4) J. R. Glass and D. Liederman, U. S. Patent 3,207,585 (1965).
to the chamber, and a variable flow of 5 to 80 ml per minute is supplied to the torch. A detailed view of the torch-chamber assembly is shown in Figure 2; the pipet, shown in Figure 3, is held by a fuse spring clip. The sample is forced from the calibrated sample pipet through the sample capillary and into the torch by controlled pressure. Connections are made by a friction connection to Tygon tubing. A spark generator, like that used for vacuum leak testing, serves to ignite the mixture of sample and oxygen at the torch orifice. Once started, the flame is self-propagating. The heater is used to preheat the chamber in order to avoid water condensation and to aid in the combustion of high-boiling hydrocarbons. The water of combustion is collected in a 100-ml Turner absorption bulb filled with magnesium perchlorate. To assure that combustion is at equilibrium during the determination, the combustion is first stabilized; then a three-way stopcock with a Teflon (Du Pont) plug is used to switch the combustion products into the absorber at the start of sample measurement, and to the atmosphere when the required volume of sample is burned. The seal between the chamber and the stopcock is off-set to the top to prevent any carbon which may have been formed on start-up from being carried into the absorber. The apparatus can be operated for several days before cleaning is required. The torch itself presented several problems. The bore volume at the T-joint (Figure 2) must be kept small to prevent the flame from flashing back and burning in this space. Since it is normal for some flashing back to occur on starting and stopping the combustion, the quartz tubing is constricted on the oxygen and sample inlets to prevent flashing back to the Tygon joining tubes. These constrictions, and one at the flame tip, help to increase flame stability by smoothing sample flow into the torch. Procedure. Turn the chamber stopcock to exhaust. Adjust the oxygen pressure into the dryer to about 12 psig. At least 15 minutes before a sample is to be run, turn on the heater and the oxygen flow through the torch and chamber. Adjust the flow through the chamber to about 50 mlimin, and through the torch to an additional 30 ml/min.
PRESSURE REGULATORS
PURGERS
0.5mm BORE
1 DRYER
FLOW M ETE
- I 5 m m O F 0.008’’ R K GENERATOR
HEATER
OXYGEN L I N E PRESSURE REG U L A T 0 R
NOTE: RESTRICTORS OF 0 . 0 0 4 ” C A P I L L A R Y EXCEPT WHERE NOTED
Figure 1. Torch analyzer diagram
ANALYTICAL CHEMISTRY, VOL. 43, NO. 7, J U N E 1971
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SAMPLE P I P E T
Table I. Results for Standard Pure Compounds
TYGON TUBING
ZH
SAMPLE C A P I L L A R Y 15 mrn OF 0.004" I D TEFLON STOPCOCK c
--
fl/ I /
QUARTZ TUBE
m n n r
3 0 m m x 140rnm
ELECTRODES- 13cm OF 0.050" NICHROME WIRE EACH
SAMPLE
,
.
/
3XYGEN INLETS
ii
- SILICONE RUBBER
QUARTZ CAPILLARY I
CONSTRICTIONS
STOPPER
mm
TEFLON D I S K (NOTCHED AROUND PERIPHERY)
Figure 2. Torch-chamber assembly
i
2.2- 2.9 mm I.D. 6 mm 0.9.
ca 1.5 m l CALIBRATED TO 0,001 ml
ORIFICE- 0.5 TO 1.0
mm
I.D.
Figure 3. Sample pipet SAMPLE INLET
Benzene 7.72 7.72 7.69 7.74 7.73 7.72
Cyclohexane 14.37 14.37 14.36 14.37 14.38 14.35 Av 7.72 14.37 Theory 7.74 14.37 Accuracy Std dev = 0.032 Precision Std dev = 0.024
Heptane 16.11 16.18 16.13 16.18 16.13 16.09 16.14 16.10
Decalin 13.07 13.12 13.11 13.10 13.10 13.12
Visually ascertain that no water is condensing in the chamber. When the sample reaches the initial index mark on the sample pipet, quickly turn the chamber stopcock so that the gases pass through the absorber. If necessary, cover the tubing between the stopcock and absorber with a piece of asbestos cloth or paper to keep it hot so that all water will pass into the absorber. Note the room temperature from the thermometer suspended near the sample pipet. (Where the room temperature fluctuates widely and rapidly, it is desirable to thermostat the pipet.) When the sample reaches the lower index mark on the sample pipet, quickly turn the chamber stopcock so that the gases again exhaust to atmosphere. Immediately transfer the absorber to a purger which supplies dry oxygen at about 25 ml/min. Disconnect the sample pressure tube at the top of the pipet, and remove the pipet from the Tygon tube connecting it to the sample capillary (the flame may flash back harmlessly several times at this point). After the absorber has been purged for 1 hour, remove it and close the ports. Wipe it with a lint-free cloth and let it come to the temperature in the balance room. Vent it, then reweigh to the nearest 0.001 gram us. the tare. Calculate the hydrogen content of the sample.
A
SILICONE RUBBER SEPTA
I
RESULTS OXYGEN INLET
\
1
1
REGULATING WIRE
OXYGEN R E G U L A T l NG WIRE
Figure 4.
Improved torch
Attach the absorber to the purger and purge with oxygen (about 25 ml/min) for 15 minutes. Close the absorber ports and wipe the absorber with a lint-free cloth. Allow it to come to temperature in the balance room, vent it, and weigh to the nearest 0.001 gram against a similar absorber used as a tare. Open the absorber ports and butt the absorber inlet (by means of a 3/4-inchlength of silicone rubber tubing) up against the chamber outlet. Adjust the chamber oxygen flow to 700 ml/min. Fill the sample pipet with sample at room temperature, start the igniter, insert the pipet into the Tygon tube attached to the sample capillary, clamp it (avoid the inclusion of air bubbles), and attach the sample pressure tube. Adjust the sample and torch pressures to give a predominantly blue or bright white flame. If the flame flashes back into the torch tube, the ratio of torch to sample pressure is too high; if the flame is yellow, the ratio is too low. When the torch is burning properly, turn off the igniter. 980
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To determine precision and accuracy, the hydrogen contents of benzene, cyclohexane, heptane, and decalin standards, and also of typical samples, were determined. The benzene had been purified by recrystallization; the others by treatment with silica gel. The results for these compounds (Table I) yielded a standard deviation for accuracy of 0.032 hydrogen. Table I1 shows a comparison of results obtained by the torch analyzer and two beta-ray instruments on typical reformer charge stocks and reformates, and also on three highly aliphatic jet fuels having low vapor pressures. The latter were analyzed to demonstrate the versatility of the burner. The overall standard deviation for precision from the data in Tables I and I1 was 0.023% hydrogen. When the standard deviations of results from the two beta-ray instruments and the torch analyzer were compared, they were found to be approximately equal in precision. DISCUSSION OF POSSIBLE ERRORS
A number of operating variables which must be controlled to obtain accurate results are listed below. No significant amount of hydrogen gas or organic hydrogen in the combustion stream was found in oxygen produced from liquid air. Water in the oxygen is reduced to a noninterfering level by drying the gas stream with magnesium perchlorate. The sample volume is measured between two calibration marks on the pipet, and is therefore the same for all samples. Control of this volume is very important and depends upon
Table 11. Results for Typical Samples Hydrogen, 'Z Sample Number 1
2 3 4
Beta-ray Description Jet fuel Jet fuel Jet fuel Reformate Reformate Reformate Reformate Reformate Reformate Reformer charge Reformer charge
Cenco
Torch analyzere 14.99, 15.04 14.99, 15.06 15.07, 15.13 11.69, 11.68 10.16, 10.18 11.93, 11.92 10.83, 10.84 9.67, 9.66 11.67, 11.62 14.55, 14.63 14.64, 14.62 0.025
Mobil 14.9CP 14.97b 14.986 11.73, 11.69 10.31, 10.31 12.00, 12.04 10.91, 10.92 9.81, 9.87 11.74, 11.75 14.54, 14.59 14.51, 14.54 0.025
... ... ...
11.68, 11.66 5 10.14, 10.19 6 11.94, 11.93 7 10.88, 10.88 8 9.66, 9.69 9 11.67, 11.67 10 14.54, 14.63 14.58, 14.61 11 Std devc 0.028 Overall precision for torch analyzer (Tables I and 11) std dev = 0.023 a One of each pair of duplicates was run on a different day from the other, except for the jet fuels which were run on the same day. * Average of 3 results. c Calculated on the reformer charge and reformate only. several factors. Because of the slow drainage rate (approximately 0.1-0.3 ml per minute), we found negligible drainage error. Therefore, we could use the exact pipet volume which was determined by calibration with mercury. The temperature of the sample in the pipet must be known so that the weight of the sample can be calculated from its density. The stopcock leading to the combustion chamber must be turned at the exact moment that the liquid meniscus passes each of the calibration marks. The variations that will give an error of 0.01 % hydrogen are: drainage, 0.0015 ml; reading each mark, 0.0015 ml; temperature reading, 0.7 "C (1.3 OF); density, 0.0007 g/ml (0.2 degree API); and synchronization of turning the stopcock and the sample reaching the mark, 0.36 second. Incomplete combustion during the sample run can obviously cause serious errors on the low side. One of the advantages of the torch analyzer i s that the burning of the hydrocarbons is stabilized before the combustion products from the measured sample volume are carried through the absorber. Only during start-up or shut-down is the combustion likely to be incomplete, and at these times the resulting water is not being measured. Although decalin (Table I) can be determined quantitatively, more viscous or less volatile stocks are difficult to burn. An error of 0.01% hydrogen can occur for any of the following variations during the run; oxygen flow rate, 6 x ; sample flow rate, 6%; pressure drop through the absorber, 0.9 psi; and temperature variation in combustion chamber, 18 "C. There are several errors possible in measuring the water resulting from the combustion of the sample. Errors in collection can be caused either by leaks of combustion gases from the apparatus or condensation of water in the combustion chamber or connecting lines. A heater under the combustion chamber eliminates the latter problem. Absorption of the water from the combustion is quantitative with a single uncooled absorber containing magnesium perchlorate. One charge of the desiccant can be used to collect a total of about 7 grams of water from about 5 average samples. Errors in weighing can also occur. Although the absorber need not be cooled during the collection period, it must be cooled to room temperature before weighing, to avoid thermal errors. The error due to convection currents around a hot absorber can alone be equivalent to 0.274
hydrogen. The weight of the absorber varies also with the kind of gas in it; therefore, it is purged with dry oxygen before each weighing. An error of 0.01 % hydrogen can be caused by each of the following: water in tubes or chamber, 0.001 ml; leakage of combustion products before the adsorber, 4 ml; incomplete absorption, 1 mg; difference between room temperature and absorber temperature when removed from purging line and sealed, 4 "C ; and air instead of oxygen in absorber, 8 ml. IMPROVED TORCH DESIGN
After the work described in this report was completed, we had difficulty in obtaining suitable quartz torches from commercial sources. The improved torch design (Figure 4) uses interchangeable metal parts where the dimensions are critical, so that the need for highly skilled quartz working is eliminated. The oxygen flow is regulated by a 0.0113 X 2.2-in. nichrome wire in a 2.0-in. length of 0.016-in. i.d. hypodermic needle tubing. The wire is bent at a right angle 0.1 in. from the entrance end to hold it in place. The flow is controlled by the annular space between the wire and the inner wall of the tube. Similarly, the sample flow is regulated by a 0.0142 X 3.5-in. nichrome wire in a 3.3-in. length of 0.016-in. hypodermic needle tubing. The tubing has a 0.25-in. 0.d. metal spool silver-soldered on the end to fit the Tygon tubing. The hypodermic tubes are simply thrust through silicone rubber septa (Part No. 6-138 Barber Colman Co., 66 Hudson Street, Hoboken, N. J. 07030). The oxygen enters the quartz nozzle near the inside face of the septum. The sample enters about 20 mm further down the nozzle. The performance of this torch is equal to that of the best all-quartz ones. INTERFERENCES
To determine whether sulfur interfered, a blend of cyclohexane and thiophene was analyzed. The blend contained 2 . 4 2 x sulfur (far more than was expected in any sample) and 13.76% hydrogen. Analysis gave 13.82% hydrogen. This indicated no unacceptable interference from sulfur. Other elements are not likely to be present in sufficient quantity to interfere.
RECEIVED for review July 1, 1970. Accepted February 15, 1971. ANALYTICAL CHEMISTRY, VOL. 43,
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