Determination of Trace Water in Butane by Gas Chromatography A. A. Carlstrom, C. F. Spencer, and J. F. Johnson, California Research Cotp., Richmond, Calif.
petroleum refining procIwateresses,content it is desirable to know the of butane a t the trace N CERTAIN
level. The electrolytic water analyzer (2, 3) has been used successfully on hydrocarbon gases. but generally is not applicable below 1 p.p.m. and is subject to some interference from other conipounds. Conventional gas chromatography is usually not sensitive enough to determine water in this range directly. Therefore, a trapping arrangement (Figure 1) vias used to concentrate the water until it could be conveniently measured (1). The trap consists of 1 foot of I/d-inch copper tubing packed with 30% polyethylene glycol, molecular weight about 200, on 20- t o 30-mesh insulating firebrick. It is maintained a t 10" C.
during trapping, as water will not emerge a t the exit a t this temperature for over 4 hours at a flow rate of 100 cc. per minute of butane gas. After the water is trapped from a fixed volume of butane, as measured by a wet-test meter located after the trap, the butane stream is bypassed. The trap is then heated to 90" C., and the sample valve is adjusted to allow water vapor to be removed from the trap by a helium stream into the gas chromatograph. The gas lines are arranged so that the helium flows through the trap in the opposite direction from the flow of butane during sampling. This minimizes broadening and distortion of the water peak in the chromatograph. While it TT ould be possible to go directly to a detector without a n analyzer column, this would introduce large base line drifts due t o temperature instability. The water is analyzed with a Perkin-
GA5
GAS INLET
GAS CLTLE-
SAMPLING
c
i G 4 S CHROMATOGRAPH
Figure 1.
Table 1. Retention Times for Compounds Often Found in Butane Streams Retention Time, Compound Jlin. Hydrogen aulfide 0 1 Ammonia 0 6 Water 11 5 Benzene 11 1-Hexene 02 Cyclohexane 0 3 Isoprene 0 n-Hexane 0 . 12 Methyl mercaptan 5 0 n-Butane 0 1
Elmer VaDor Fractometer Model 154B. using a 2-foot column with the same composition as the trap. \F7ith a flow rate of 100 cc. per minute and a column temperature of 80" C., the retention time for water is 11.5 minutes. The peak is symmetrical. A voltage regulator is used for stability of the recorder. and a preamplifier is used to increase the sensitivity. Table I lists retention times for several compounds often found in butane streams. The lack of interference of these compounds is one of the advantages of this method.
Table II. Calibration Results for Air Saturated with Water Saturated Area, A4ir,Cc. Temp., C. Water, y Sq. In. Ka 27.2 0.61 22.46 24.5 29.2 0.69 25.5 23.66 33.9 2.68 27.4 78.99 25.8 1.33 27.0 51.48 35 9 1.82 50.62 26.7 28.4 0.71 25.03 26.5 = area x attenuation micrograms water ' O
~
Trap assembly
1056
ANALYTICAL CHEMISTRY
Table 111. Reproducibility of Results on Two Samples of Butane Area X, s'ample Attenuation Size, Water , Liters sq. In. P.P.M.
Sample 10 1.OO
1.OO
0.50 1 .00 1 .OO
13 20 1 2 . 10 6.40 11.60 13 20
174 159 169 153 174
Sample 2b 1.00 1.00 2.00 3.00 1.68
1.19 1 26 2.41 3.60 1.74
15.8 16.7 14.8 16.0 13.8
Average 166 =I=26 p.p.m. a t 95% confidence level. *Average 15.4 f 2.8 p.p.m. at 95% confidence level.
A most important precaution is to make sure that all lines are conipletely dried before analysis. This is done by flushing all lines with nitrogen or helium which has been previously dried with Linde hIolecular Sieve 5A in a 2foot drying column a t room temperature. Heating the lines with infrared lamps and heating tapes hasten the drying, but this still takes from 12 to 24 hours. A continuous flow of sample is then essential and ensures that equilibrium conditions, as evidenced by repeatable measurements, have been established. The apparatus should be located as close to the sampling point as possible. Calibration results for air saturated nith water are shown in Table 11. The amounts of water in these calibrations correspond to 1 to 3 p.p.m. by weight of water in a 10-liter sample of butane. The results of several determinations on tn-o different samples are shown in Table 111. The average percentage deviations for the two samples are similar, Variation in the amount of butane trapped without change in the measured concentration establishes the quantitative trapping of the nater. This method has been used extensively for water levels in butane down to 0.2 p.p.m., the approximate limit of detectability with a 10-liter sample. Lower concentrations of wvnter probably can be measured by increasing the capacity of the trap to allow trapping larger samples. K a t e r content of other condensable gases can be measured rrith minor variations in the method. LITERATURE CITED
(1) Eggertsen, F. T., Nelsen, F. M., Aix.4~.CHEX 30, 1040 (1958). (2) Keidel, F. A,, Zbid., 31, 2043 (1959). (3) Taylor, E. S., RejTig. Eng. 64,Yo. 8, 41 (1956).
