New Method for Determining Free-Water Content in Fuel John Bitten I 7 T Research Institute, I O West 35 Street, Chicago, 111. 60616 A new, simple, and reliable method has been developed for determining free water in Ii uid hydrocarbon fuels. It is based on the adsorption o the free water by preconditioned cellulose pads. Adsorbed water in the pad is determined electronically using a calibrated vacuum tube ohmeter. The calibration was done by correlating instrument readings with pad moisture values determined in Karl Fischer equipment. An accuracy of &0.5 ppm free water can be obtained with a fuel sample of 500 to 3000 ml containing 0 to 20 ppm of free water.
9
sample containing free water is passed through these cellulose pads, the pads ‘adsorb only free water. The amount of free water can be determined by cutting up the pads and titrating the total water content in methanol with Karl Fischer equipment. Another method is to use a calibrated instrument which measures electronically the moisture content in the pad. This paper is concerned mainly with the development of the instrumental method. EXPERIMENTAL
PRESENT JET
obtain maximum fuel utilization by flying at extreme altitudes. Under such conditions, dispersed or free water in jet aircraft fuels may form ice crystals that block fuel lines, screens, or orifices in the fuel system. To minimize this hazard, a limit of 0 mg of dispersed water per liter of fuel has been established ( I ) by the military service. Current methods used for determination of this water have been unsatisfactory. The standard method used for military specifications (1) is the Karl Fischer analysis. However, various authors have indicated that this method is not sufficiently reliable (2-4). Indeed the ASTM manual states that the repeatability is 11 ppm ( 5 ) . Other methods that have been used are based on light transmission or scattering (6). Unfortunately, air bubbles, dirt, and water-droplet size are sources of serious interference. Recently, a free-water-in-fuel detector ( 4 , 7, 8) has been developed by the Naval Aeronautical Engine Laboratory. It is currently being used on many military bases, and it has been considered for use as a standard in military specifications. This method consists of allowing a fuel sample to flow through a cellulose pad or disk coated with sodium fluorescein. When the exposed pad is viewed under ultraviolet light, the areas of the pad that have contacted free water fluoresce. The amount of fluorescence is proportional to the amount of free water in the fuel and can be estimated by comparing the test samples to a set of printed standards that also fluoresce under ultraviolet light. This method appears to have promise, but no evaluation or standardization has appeared in the open literature. Recently a method has been developed at IIT Research Institute that is rapid, simple, and accurate. This method also uses a cellulose pad and it is the subject of this paper. AIRCRAFT
PRINCIPLE OF METHOD
If cellulose pads are suspended in fuel stored over water, the pads adsorb water until an equilibrium water content is reached. Based on the temperature and fuel properties, they attain a certain constant moisture content. If a fuel (1) U. S. Military Specification, MIL-F-8901A, June 11, 1963. (2) L. R. Beynon, Filtration Separation, 3, 418 (1966). (3) L. Gardner and G. Topal, Mufer.Res. Std., 1, 112 (1961). (4) R. K. Johnson and C . M. Monita, Tech. Repr. AFAPL-TR-6639, April 1966. (5) Am. SOC.Testing Mater., ASTM Std., ASTM-D-1744-64, 1964. (6) M. D. Foster, Filtration Separation, 4, 577 (1967). (7) T. Thyrum, U. S. Patent 3066221 (Nov. 27, 1962). (8) U. S. Military Specification, MIL-S-8182 (WP), July 9, 1965. 960
ANALYTICAL CHEMISTRY
Equipment. The main instrument used in this determination is the Hart Moisture Meter (Model Kl01-336, manufactured by Hart Moisture Meters, West Islip Boulevard, P.O. Box 277, West Islip, Long Island, N. Y . ) (9). This simple, portable instrument, which consists of a batteryoperated vacuum-tube ohmeter, determines the electrical resistance of the pad. The pad is clamped (at approximately 125 psi) in the instrument between two 3/8-in. diameter electrodes, and after the circuit is adjusted, the resistance is read from the meter. This is done by pressing the lettered button which most nearly zeros the ammeter dial. Hence a reading does not consist of the actual resistance value but of a letter and an ammeter number which is used t o zero the instrument. Before the instrument can be used for a particular application it must be calibrated for that system. The cellulose pads used had a 47-mm diameter and they were made from No. 726 filter paper (Filpaco Industries, Chicago, Ill.). During sampling, the pads were held in specially designed holders that could be easily and quickly disassembled ; however, any holder such as the commercial units designed to hold membranes, can be used. The fuel flow rate through the pads was never greater than 1 liter/min. Pad preparation consisted of placing a bunch of weighed pads into a gallon jar containing a fuel-water emulsion having a predetermined amount of free water. The jar was then hand-shaken until the fuel was clear. These pads can be stored in the fuel for months without any harmful effects. They are removed from the fuel just before being used for a water determination. In all tests the pads are evaluated immediately after they have been removed from the storage jar or the test holder. This is done to minimize any moisture loss from the pads t o a low humidity atmosphere. Evaluations of the pad method were made in an experimental test stand for evaluating single filter/separator elements. The test stand is a 100-gal. recycling fuel system designed to evaluate fuel filters and fuel monitors at flow rates up t o 50 gal/min. The water-injection system consists of a Milton Roy “mini-pump,” which feeds a metered amount (0 to 5 ml/min) of water into an eye of a peripheral turbine pump. This pump, which recirculates fuel from the main system, injects an extremely well-dispersed fuel-water emulsion back into the main fuel system. With this method, a known constant free-water concentration of 0 to 20 ppm by volume could be easily maintained in the fuel stream. Calibration. The Hart Moisture Meter was calibrated with constant weight pads which contained various amounts of water, Constant weight refers to pads which have the same weight when they have a zero moisture content. In the calibration work, test pads were obtained by flowing various (9) K. W. Hardacker, R. D. Rawcliffe, TAPPI, 35 (6), 168A (1952).
M O I S T U R E C O N T E N T of P A D mg ( K A R L FISCHER A N A L Y S I S )
Figure 1. Hart pad reading us. moisture content Fuel, JP-4; pad wt, 0.490
=k 0.002
amounts of fuel through them. Before cutting up the pads and titrating the water in the pieces placed in the Karl Fischer equipment, a Hart reading was obtained on all test pads. The Hart reading generally required no more than 2 to 3 seconds. A correlation for JP-4 fuel is shown in Figure 1. After obtaining correlations similar to Figure 1, a study was made t o determine the effects of sample volume and free water content upon accuracy of the analysis. For the Karl Fischer determinations, the equilibrium water content was determined by cutting up pads which had been stored in water-saturated fuel, and then titrating the pieces in the Karl Fischer equipment. There was essentially no variation in the moisture content of these pads. After a measured amount of test fuel was filtered through similar test pads, they were cut up and the total amount of adsorbed water titrated in the Karl Fischer equipment. The free-water content in the fuel was obtained by subtracting the equilibrium water content from the total water content. For the Hart determinations, Hart moisture readings on the pads were taken before fuel sampling and before the Karl Fischer titration. The free water content in the fuel was determined using the data in Figure 1. Results of the evaluations are listed in Tables I and 11. The final evaluation was a comparison of the known freewater content in JP-4 fuel in a test rig, with the free-water content determined by the pad method. The original weight of the pads was not constant and the original moisture content of the pads was either in equilibrium with the fuel or in excess of the equilibrium value. Results are given in Table 111. RESULTS
Results of the studies summarized in Figure 1 and Tables I and I1 show that there is essentially no difference between Karl Fischer pad and Hart pad results. Tables I and I1 show a few determinations where they differ by 1 ppm. However, with aircraft fuels, a free-water determination which furnishes a result within k l ppm of the actual free water present, is considered to be exceptionally accurate. The probability also exists that repeat analyses would eliminate these few differences between the Karl Fischer pad and Hart pad methods. In general, one may conclude that the difference between the two methods is less than 0.5 ppm. This pertains t o a free-water concentration of 0 to 20 ppm and a fuel-sample volume of 500 to 3000 ml.
gram; temp., 75" F; pad diam., 1.875 in,
Table I. Comparison of Determinations of Free-Water Content in Cellulose Pads by Karl Fischer Titration and by Hart Moisture Meter Readings (Temperature: 75' F) Free-water pick-up Free-water content, mg/liter Sample by IITRI pad," mg Karl Karl volume, Hart Fischer Fuel ml Fischer Hart JP-4
500 500 1000 1000 1000 1500 3000 750 1000 1000 1000 1000 1000 1500 2000 2000 2000
0.1 1.8 6.6 9.6 1.0 0 7.0 Aviation 13.9 gasoline 4.1 9.7 12.0 5.5 12.1 17.8 12.3 11.0 12.2 0. Original pad moisture was 48 to
0 1.8 6.8 9.8 0.9 0 7.2 15.0 5.0 8.8 12.5 5.0 11.3 17.5 12.5 11.3 12.5 50 mg.
0.2 3.6 6.6 9.6 1.0 0 2.3 18.6 4.1 9.7 12.0 5.5 12.1 11.8 6.1 5.5 6.1
0 3.6 6.8 9.8 0.9 0 2.4 20.0 5.0 8.8 12.5
5.0 11.3 11.7 6.2 5.6 6.2
Table 11. Effect of Sample Volume on Pickup of Constant Concentration of Free Water by Cellulose Pads (Temperature: 75 F) Free-water pick-up Free-water content, by IITRI Pad, mg mg/liter Sample volume, Karl Karl Fuel ml Fischer Hart Fischer Hart O
JP-4
500 1000 1000 1500 2000
3.6 7.4 7.0 9.4 12.6
3.7 6.3 6.3 10.0 12.5
Aviation gasoline
500 750 1000 2000
2.6 3.4 5.5 9.4
2.5 3.7 5.0 8.9
7.2 7.4 7.0 6.3 6.3
Av
6.8 5.2 4.6 5.5 4.7
Av
5.0
VOL. 40, NO. 6, M A Y 1968
7.4 6.3 6.3 6.7 6.3 -. 6.6 5.0 4.9 5.0 4.5 4.9
_ .
961
Table 111. Comparison of Material-Balance Values of Free Water Fed in a Test Rig With Values Determined by IITRI Pad Method (Temperature: 75' F; Fuel: JP-4) Free-water determination Free-water IITRI pad method content, Time of feed rate, mg/liter run, min mg/liter 30 12.0 12.3 9.0 30 9.5 5.5 5.1 44 3.6 3.5 36
weight pads since one uses a difference reading on the same pad. Also the condition of the unused pad is satisfactory if its moisture content is equal to or greater than the equilibrium saturation value of the water-saturated fuel. In this work, the pad constants for JP-4 and aviation gasoline were respectfully 1.8 and 2.1 mg of water per Hart unit. A study of the variation in the pad constant in different types of fuel was not made. However, most fuels probably fall within this range. The final evaluation of the Hart pad or IITRI pad method showed that it can be used to determine within 0.5 ppm the actual amount of free water present in a fuel system. This is evident in Table 111. CONCLUSION
In this presentation it is evident that there are two methods for the free-water determination. One is the Karl Fischer pad determination. Because it is a destructive test, it is necessary to use a group of constant weight pads which have been preconditioned to a known moisture condition. The other test is the Hart pad method which must first be calibrated with the Karl Fischer pad determination. It is a nondestructive method which is based on data shown in Figure 1, Hence one may evaluate the pad before and after sampling and obtain the amount of free water in the sample by multiplying the difference of the two readings by the slope of the line in Figure 1. In this method it is not necessary t o use constant
A simple, accurate and reliable method suitable for determining free water in fuel has been developed. The accuracy of the method is not affected by a lack of homogeneity or the amount of dissolved water in the fuel, and all the main reasons for the inaccuracies in the Karl Fischer method are eliminated. One disadvantage of the method is that it cannot be used for fuel systems containing salt water as free water. All the additives normally found in fuel, however, do not affect the method. RECEIVED for review January 22, 1968. Accepted February 21, 1968.
Determination of Trace Sulfur in Hydrocarbons by Pyrolysis and Hydrogenation L. L. Farley and R. A. Winkler Chevron Research Co., Richmond, Calif. 94802 A rapid accurate method for the determination of sulfur in the fractional parts per million range by means of pyrolysis and hydrogenation is described. A 2-ml sample is vaporized in a hydrogen stream, pyrolyzed in a hot zone, and passed over a platinum catalyst at 1 2 0 0 O C. The sulfur is converted to H,S, absorbed in a dilute zinc acetate solution, and reacted with N,N-dimethylphenylenediamine to form methylene blue. The spectrophotometric finish covers the range from 0.2 to 5.0 ppm with an accuracy of *lo% of the amount present. The catalyst has an exceptionally long life for the relatively large samples that are used. An analysis requires about 1 hour.
IN A SEARCH for a technique suitable for the determination of fractional part per million sulfur contents of petroleum liquids, no existing method was found which was entirely adequate. The procedures which approached the required sensitivity and precision were those suggested by Schluter ( I ) and Kat0 (2) and their coworkers. Schluter pyrolyzed a 0.2-OS-gram sample in a hydrogen stream and reduced the sulfur over a nickel catalyst at 1200" C. The colorimetric finish used gave a sensitivity of 5 ppm. Kat0 used the same (1) E. P. Schluter, Jr., E. P. Parry, and George Matsuyama, ANAL.CHEM.,32,413 (1960). (2) Motohiko Kato, Iwao Fujishima, and Tsugio Takeuchi, Bunseki Kuguku, 11,178 (1962) [C.A., 57, 155% (1962)l. 962
ANALYTICAL CHEMISTRY
procedure in a closed system to reduce atmospheric contamination and obtained a sensitivity of 1 ppm. Of the other approaches investigated, none were considered able to measure sulfur in the fractional ppm range. X-ray fluorescence (3) can be extended to the 1- to 5-ppm range, but matrix effects may cause variations. The Wickbold (4) oxyhydrogen burner operates well also in this range but becomes lengthy and loses sensitivity rapidly below 2 ppm. The Raney nickel method (5) is sensitive to 0.1 ppm, but olefins interfere. Microcoulometry has been used (6-8) to determine small amounts of H2S but was not used for this work in an effort to keep the instrumentation as simple as possible. The procedure discussed in this work extends the range to at least 0.2 ppm and is designed to operate as high as 5 ppm with an accuracy of + l o % of the sulfur present throughout this entire range. Appropriate changes in sample size can expand both ends of the sensitivity range. A platinum-on-quartz catalyst was developed and was shown to have a longer life than the previously used nickel catalyst. The injection system (3) T. C . Yao and F. W. Porsche, ANAL.CHEM., 31, 2010 (1959). (4) R. Wickbold, Angew. Chem., 64, 133 (1952). (5) Lawrence Granatelli, ANAL.CHEM.,31, 434 (1959). (6) V. T. Brand and D. A. Keyworth, Ibid.,37,1424 (1965). (7) R. L. Martin and J. A. Grant, Zbid.,37,644 (1965). (8) D. F. Adams, G. A. Jensen, J. P. Steadman, R. K. Koppe, and T. J. Robertson, Zbid.,38, 1094(1966).
