Trace Water Determination by Infrared Spectrometry J. W. FORBES Shell Development Co., Emeryville, Calif.
b The measurement of water contents of liquids in the few parts-per-million range by infrared spectrometry has been investigated. A laboratory method has been devised which involves conversion of water to acetylene with calcium carbide and a subsequent transfer, with a dry carrier gas, of the acetylene into an infrared cell where the acetylene is redissolved and measured in CCld solution. Also, an experimental model of an analyzer for determining continuously the trace water content of a plant stream was constructed. The results demonstrate the practicality of infrared absorption for determination of water in nearly anhydrous liquids.
T
of water-sensitive compounds aa catalysta for stere+ specific polymerizations has created a need for detesmining water at concentrations Mound 1 p.p.m. in various solvents, feeds, and reaction mixtures. Attempts to improve the sensitivity of existing chemical methods for water determination have not resulted in a satisfactory solution of the problem. The most highly refined Fischer techniques attain detection limits of around 1 p.p.m. but have other deficiencies, particularly thoae arising from contamination, which lead to ambiguous results. Solvents which are reduced in water content to the 1 p.p.m. range possess desiccative activity. This is also true of containera or handling equipment the dry solvent may contact. Depending upon such factors as volume-tosurface ratios, materials of construction, etc., the handling apparatus may receive from or give water to the material b e i i analyzed. Therefore, a practical solution can be achieved only by t a b all of these factors into consideration. Ideally the system should be of a flow type so that ateady state conditions exist during measurement or the handling equipment should have a very large volume to surface ratio (this is generally impractical). Commercially available electrolytic, continuous water analyzers have the desired sensitivity but are not applicable to unsaturated hydrocarbons in the liquid phase. The aim of this work is to evaluate the practicability of infrared absorption measure ments for the determination of water in hydrocarbons a t a concentration of 1 p.p.m. or less. HE USE
The following three approaches to the problem were conceived and studied to establish their relative merit: Direct measurement of the hydroxylstretching fundamental absorption of water in the hydrocarbon; Conversion of the water to acetylene by calcium carbide and direct measurement on the hydrocarbon of the =CH stretching fundamental absorption; Conversion of the water to acetylene, separation of the acetylene from the hydrocarbon, and measurement of the =CH absorption after redissolving in carbon tetrachloride or measurement of the stripped acetylene in the vapor P b . The near infrared region was also considered, but was discarded for the following reasons. The strongest water band in this region is the combination band at -1.94 microns, and its absorptivity is only -1.0 liter per molecm., whereas the absorptivity of the antisymmetric stretching band at -2.69 microns was determined to be 37 liters per mole-cm. in carbon tetrachloride. Therefore, in spite of the higher background a t 2.69 microns (on the average approximately three times greater), there is an over-all advantage of more than 10 in the ratio of peak height to background in the fundamental region. Direct Measurement of Absorption by Water in Hydrocarbons. The sensitivity of the direct infrared method depends primarily upon the hydrocarbon absorption a t the analytical wavelength since this absorption determines the maximum usable pathlength consistent with the proper operation of the spectrophotometer. To estimate the sensitivity of the direct method, the analytical wavelength of the free OH stretching vibration of the water molecule and the background absorption a t this wavelength of the completely anhydrous solvent were determined. As a case in point, an infrared spectrum waa obtained on liquid 1,bbutadiene which had been dried (over 4-A. sieve) and distilled directly into the infrared cell. This spectrum was compared with the spectrum of the butadiene containing water (no sieve treatment). The free water band was observed a t 2.69 microns A high resolution scan on a Beckman I R 7 more precisely determined this band to be at 3694 cm.-' F'rom these spectra, the sensitivity of
the infrared method for determining water in butadiene was calculated. For these calculations, the background absorption of dry solvent was measured, the peak intensity of the free water band waa determined to be 37 litem per mole-cm., the infrared instrument was assumed to operate satisfactorily if the reference path transmission is 30%T, and an absorbance change of 0.002was capable of being detected. [The molar absorptivity of dilute water (free) in carbon tetrachloride was determined using a tritium tracer technique. Anhydro- carbon tetrachloride was partially saturated with water of known tritium content, and ita infrared spectrum was obtained in the hydroxyl stretching region. The tritiated water in the carbon tetrachloride, contained in the infrared cell, was then determined by liquid scintillation counting. No excm water (water not in solution) was present during any of the measurementa as the partially saturated solutions were always removed from the excess tritium watercarbon tetrachloride mixture. The infrared cell waa flushed well with the tritiated CCL-H*O solution before it was sealed off. Nine independent experiments were made covering the concentration range of 50 p. p. m. down. The absorptivity obtained for the free hydroxyl stretching vibration at -2.7 microns was 37 litem per mole-cm., and was constant over the range studied. This value is used for all calibration data in this paper.] From the above considerations, the approximate lower limits of HaO concentration measurable in the following materials were estimated: l,&butadiene, -1-2 p.p.m. (background = 0.62 absorbance unit per cm.); benzene, -15-30 p.p.m. (background = 7.0 absorbance units per cm.) ; 1-butene, -2-3 p.p.m. (background = 1.3 absorbance units per cm.); 10% benzen*9wo 1-butene, 2-3 p. p. m. (background = 1.3 absorbance units per cm.). From this work it was obvious that the direct measurement of water at the level of 1 p.p.m. in the hydrocarbons would not be workable. The second alternative was then investigated. Direct Measurement on the Hydrocarbon of r C H Stretching Fundamental after Conversion of Water to Acetylene by Calcium Carbide. When water is contacted with calcium car-
>
VOL 34, NO. 9, AUGUST 1962
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bide, acetylene is produced according to the reactions 2H20 CaC2
+ CaCe = C2H, + Ca(OH),
+ Ca(OH)2 = C2H2+ 2CaO
The second reaction is slow and becomes significant only when partially hydrolyzed CaCpis allowed to stand for long periods. Therefore, if a hydrocarbon containing water is exposed to calcium carbide, the water will be converted quantitatively to acetylene. The usefulness of this technique was evaluated using benzene as an example. Because benzene has an absorption band a t approximately the same wavelength as that of dilute water' in benzene, direct water determination in benzene below about 15 p.p.m. is prohibited. However, after the wet benzene is allowed to contact CaC2, it is possible to observe absorption a t 3.05 microns owing to the acetylene C-H stretching mode. In this region, benzene has a background of only -1 absorbance unit per em. The peak absorptivity of the 3.05-micron acetylene hand was determined to be 89 liters per mole-em. This reduces the limit of the H,O determination (indirectly) to -1.0 p.p.m. in benzene. For 1,a-butadiene and 1butene, the background a t 3.05 microns is not significantly different than that a t -2.75 microns so this technique gives a large increase in sensitivity only in the case of benzene. However, the conversion to acetylene has an advantage with all solvents in that trace water contamination which occurs after the conversion will not affect the results because the =CH stretching vibration is far removed from the OH strctching fundamental. Conversion of Water to Acetylene and Measurement of Acetylene after Removal from Solvent. For greater sensitivity and general applicability of the method, the elimination of background absorption by the hydrocarbon is desirable. This was achieved by extraction of the acetylene from the hydrocarbon and subsequent measurement of the acetylene in a transparent solvent (CCL) or in the vapor phase. Redissolving the ace& yleue in CCh was chosen as a laboratory method, whereas vapor phase measurements were selected to demonstrate the practicability of the method for continuous stream monitoring. The laboratory method involves conversion of the water to acetylene with calcium carbide and a subsequent transfer, with a dry carrier gas (argon, dried calcium hydride), of the acetylene into a long-path infrared cell which has been filled with carbon tetrachloride. The apparatus is hermetically sealed and operates with a pressure build-up during the acetylene transfer. The sealed system eliminates all possibility of acetylene loss during a determination. 1126
ANALYTICAL CHEMISTRY
Figure 1 .
Complete apparatus for determinatioi
designed IO-em. infrared cell, a transfer line (can include a cold trap if needed) and a single unit which serves as the sample tube-carbide reactor-acetylene extraction unit. The assembled apparatus is shown in Figure 1. Sample-Reactor-Extractor Tube. The tube consists of a 3/4-inch stainless steel nipple 1.5 inches long yith a sint&ed stainless steel disk fitted into the lower end. The purpose of the sintered element is to disperse the dry argon into fine bubbles during t,he acetylene extraction step. The sintered element is located I/S-inch from the bottom of the tube to improve the gas dispersion. On the bottom of the nipple is a machined cap, to which a stainless steel needle-valve is attached. ';his is the valve through which the dry argon carrier gas is introduced. At the top end o f ,the nipple, is, another cap. This cap has a side hole~which,accommodates a safety pressure-relief device and a top hole which accommodates a needl-valve through which the tube is loaded with sample and through which the carrier gas passes out of the tube into the infrared cell during the acetylene extraction. Swage-Lok connectors are used a t the ends of the needlevalves to facilitate sample collection and insertion of the tube in the ertraction train. Transfer Line and Cold Trap. Tlris consists of a transfer tube with a coiled section for cold trapping if reanired. For most liauid hvdrocarGens, trapping is not hece&y since the small amounts of the hydrocarbons
carried over during the extraction do not interfere seriously with the absorption measurement a t 3.05 microns. For Cl's or lower boiling hydrocarbons, however, it is desirable to use a cold trap. Preliminary experiments on 1butene indicate that a satisfactory coolant is dry ice-acetone. Probably a better technique for Cl's would be to cool the reactor .tube during the extraction step and proceed in the same manner as with normaUy liquid hydrocarbons. Pressure Tight Infrared Cell. Figure 1 shows the pressure-tight 10om. path infrared cell. The interior construction is such that i t contains a 500-ml. gas dome and a 50-ml. liquid reservoir. The gas dome permits a large excess of gas flow over the minimum required to strip the sample of acetylene during the extraction. The liquid reservoir which lies below the gas dome is designed for the minimum volume which can receive the full beam of e, Beckman IR-4 spectrophotomekrLe., its wedge shape matches the convergence of the light beam. Fifty milliliters of liquid are slightly more than enough to fill the liquid reservoir. All crevices in which liquid might be held up have been eliminated, and the cell interior has been designed so that all liquid will drain into the liquid reservoir when the cell position is that which i t assumes in the spectrophotometer. The windows are '/,-inch fused quartz plates which are transparent in the 3-micron region. Preparation of Sample-Reactor-Extractor Tubes. To prepare the sample-reactor-eytractor tubes for sub-
-
sequent analysis, the following procedure is satisfactory: The top cap (the one at the tube end without a sintered stainless steel element) is removed, and approximately 40 ml. of 3/8-inch lump calcium carbide granules are introduced. The reactor is then assembled and checked for leaks using a helium leak detector. The sample-reactor-extractor tube is filled with about 75-ml. of carbon tetrachloride, which has been dried over 4-A. molecular sieves. The tuhe is then shaken for 4 hours, emptied, and purged with argon. This procedure is repeated to remove from the carbide all trapped acetylene which is present in various amounts depending upon the prior history of the calcium carbide. After the above procedure, the tubes will give a zero blank if an analysis is performed on an anhydrous sample. After the tubes have been prepared as outlined above, they are ready for repeated use with no further treatment. How often the carbide need be replaced is not known. We have worked daily with tubes over periods of several months with no noticeable deterioration in their efficiency. For the samples of lower water content for which this analysis is tailored, the carbide should last for long periods of time. A periodic check on the efficacy of the tubes should be made with a reference sample of known water content. Procedure for Analysis. An empty sample-reactor-extractor tube (SRE Tube) is purged with argon and dried over calcium hydride, a t 0.5 cu. feet per hour for 15 minutes. The tube is then evacuated to approximately 1 mm. of mercury and weighed to the nearest one-tenth of a gram. The weighed tube is fastened loosely to the sample source through Swage-Lok connectors, and the tip is flushed with the hydrocarbon to be analyzed. The connection is tightened, and the needle valve on the SRE Tube is opened, allowing around 50 to 75 ml. of sample to flow into the partially evacuated tube. The valve is closed and the tube disconnected from the sample supply. The filled tube is weighed again and placed in a shaker for 4 hours. After removal from the shaker, the tube is inserted between the dry argon source and the long-path infrared cell which has been filled with exactly 50-ml. of carbon tetrachloride. All of the needle valves are opened and the SRE Tube is swept at 0.5 cu. feet per hour with dry argon, until the infrared cell indicates 80 p.e.i. pressure on the gage. All the valves are closed and the infrared cell is disconnected, shaken by hand, and placed in the spectrophotometer. The acetylene absorption band a t approsimately 3.05 microns is then recorded. The method is useful in the range of 0.5 to several hundred p.p.m. H20. A standard deviation of 0.5 p.p.m. has
Table 1.
Blank, Grams/Liter 0.00377 0,00377 0,00377 0.00377 0.00377 0.00377
Parts Per Million 2.37 2.37 2.37 2.37 2.37 2.37
0.00863 0,00863 0.00863 0.00863 0.00863
5.41 5.41 5.41 5.41 5,41
0.0120 0.0120 0.0120 0.0120 0.0120
7.53 7.53 7.53 7.53 7.53
0.0157 0.0157 0.0157 0.0157
9.85 9.85 9.85 9.85
Water
0,0250 0.0250 0.0250 0,0250
15.7 15.7 15.7 15.7
0.0268 0.0268 0.0268 0.0268
16.8 16.8 16.8 16.8
Calibration Data
50-M1. sample size Absorbance, 3.05 4Hour Microns Correction 0.0238 -0.0060 0.0253 -0.0060 0.0364 -0.0060 0.0355 -0.0060 0.0287 -0.0060 0.0254 -0.0060 Mean 0.0485 -0.0060 0.0600 -0.0060 -0.0060 0.0660 -0.0060 0.0590 0.0680 -0.0060 Mean 0.0840 -0.0060 0.0824 -0.0060 0.0875 -0.0060 0.0810 -0.0060 0 0773 -0.0060 Mean 0.0977 -0.0060 -0.0060 0.1190 -0.0060 0.1007 0.1110 -0.0060 Mean 0.1670 -0.0060 0.1620 -0.0060 0.1680 -0.0060 0.1630 -0.0060 Mean 0.1770 -0.0060 0.1690 -0.0060 0.1780 -0,0060 0.1850 -0.0060
been found for a 50-ml. sample, and could probably be decreased proportionately by using larger samples. The time for an analysis is approximately 15 minutes of operator time plus a Phour reaction period. Calibration Data. A large stainless steel cylinder which was fitted with needle valves and Swage-Lok connectors a t the top and bottom was used as a reservoir to prepare and store standard water solutions. About eight gallons of reasonably dry carbon tetrachloride were added to the cylinder. The water content was determined directly by infrared absorption and was 2.37 p.p.m. Data on the acetylene yield were then obtained using the SRE Tubes in the manner outlined under Procedure for Analysis. The water content of the standard solution was increased in increments by adding more carbon tetrachloride of a higher water content to the original solution. At each step, the water content was again determined by infrared absorption. The resulting concentrations were 2.37, 5.41, 7.53, 9.85, 15.70, and 10.80 p.p.m. The data obtained a t the various water contents are tabulated in Table I. The constant &hour correction term that is subtracted from the acetylene band absorbance values was established experimentally. It represents the acetylene produced during a 4-hour
Corrected Absorbance 0.0178 0.0193 0.0304 0.0295 0.0227 0.0194 0.0232 0.0430 0.0540 0.0600 0.0530 0.0620 0.0544 0.0780 0.0764 0.0815 0.0750 0,0713 0.0764 0.0917 0.1130 0.0947 0.1050 0.1011 0.1610 0.1560 0.1620 0.1570 0,1590
Deviation
from Mean
0.1710
0.1630 0.1720 0.1790
0.0054 0.0039 0,0072 0.0063 0.0005
0.00?8 0.0114
0.0004
0,0056 0.0014 0.0076 0.0016 O.oo00 0.0051 0.0014 0.0051 0.0094 0.0119 0.0064 0.0039 0.0020 0.0030 0.0030 0.0020
0.0002
0.0082 0.0008 0.0078
shaking period when an anhydrous sample is analyzed and probably results from the slow reaction of the calcium hydroxide on the surface of the calcium carbide to produce acetylene. The standard deviation obtained a t each concentration does not reflect a concentration dependence in this range. The standard deviation pooled over all the concentration levels is 0.0052 absorbance unit or 0.0081 gram per liter, or 0.51 p.p.m. A plot of the mean absorbance values (3.05 microns) against the concentration of water in the samples analyzed is a straight line which passes through the origin. This indicates that the absorbance is linear with the water concentration between 0 and 16.8 p.p.m. by weight of water. For convenience, the absorbance value obtained a t the ace& ylene wavelength (3.05 microns) is corrected to a 50-ml. sample and the concentration of water is then read directly from a graph. For all the measurements, the Beckman IR-4 spectrophotometer was operated a t the following settings: scanning speed, 0.04; scale, 2 inches per micron; period, 2; slit schedule, 1.75 a t 14 microns; gain, 3.5% of full; double beam, NaCl prism. The resolution under these conditions was such that the half width of the acetylene band in CCl, solution at 3.05 microns was 33 cm.-'
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VOL 34, NO. 9, AUGUST 1962
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loo
Figure 2.
Apparatus diagram
Continuous Analysis Technique for Process Streams. Measurement and control of the water content of various hydrocarbons in the few parts-permillion range have become extremely important in certain plant operations, and a continuous analyzer for plant streams is highly desirable. The water to acetylene conversion method appeared adaptable to continuous monitoring, and therefore laboratory experiments were carried out to test the technique. The results obtained demonstrate the practicability of the method and indicate that adequate performance for plant use could be obtained from a more sophisticated design. In this continuous method, the water in a hydrocarbon stream is converted to acetylene by reaction with calcium carbide, and the acetylene is stripped simultaneously from the liquid by inert gas. The inert gas phase is monitored continuously for acetylene content by measuring its infrared absorption at an acetylene band. In these experiments, the transmittance a t 13.7 microns was recorded. A flow diagram of the apparatus is shown in Figure 2. The apparatus consisted of a 15 X */, inch steel reactor tube fdled with 8/8-inch lump of calcium carbide. The bottom of the reactor tube contained a sintered stainless steel disk for good dispersion of the stripping gas. At the bottom of the tube three lines entered; two of these were liquid sample supply linea and the third supplied the stripping gas (argon dried over calcium hydride in this case). All three lines were fitted with flow meters and had a common pressure head of 50 p.s.i. supplied from an argon cylinder. The common pressure head was used to eliminate back-flow in any of the lines. The samples tested were carbon tetrachloride containing 1.6 and 5.2 p.p.m. of HzO. Approximately eight gallons of each were contained in stainless steel cylinders. Carbon tetrachloride was used instead of a hydrocarbon because ita water content can be accurately determined and, therefore, known standards could be prepared. The liquid was allowed to flow continuously, a t approxi1128
ANALYTICAL CHEMISTRY
Siaried
,
Started
Equil.
~
t 0 0
I IO
I
I
I
1
I
I
I
I
J
20
30
40
50
60
70
80
90
100
Time, Minutes
Figure 3. Graphic illustration of experiment for continuous determination of water in carbon-tetrachloride
mately 1 gallon per hour, through the reactor tube and out the side near the top of the tube into a waste vessel. At the same time, an argon stream of approximately 0.2 cu. foot per hour w a ~ passed through the reactor tube and out the top of the tube into a Beckman multiple reflection 60-cm. micro gas cell (KBr windows) mounted in an IR-4 spectrophotometer. A cold trap (isopropyl alcohol solid COJ was mounted between the reactor tube and the gas cell to remove all carbon tetrachloride vapors. The spectrophotometer was set to record continuously the transmittance at the 13.7-micron acetylene band. A typical experiment is illustrated graphically in Figure 3. Starting with the sample stream containirig 5.2 p.p.m. HzO, equilibrium conditions were reached in about 10 mtnutes, with a signal of 20% transmission. ?his signal was constant for approximately 30 minutes during which time the gas and liquid flows were held constant. The analyaer was then switched to the 1.6p.p.m. HzO sample, and the liquid and gas flow rates were held constant at the same values as before. In 3 minutes the recorder began to respond to the change in sample composition and reached equilibrium in 15 minutes a t approximately 55% transmission. These conditions were held for 15 minutes, a t which time the analyzer was again switched to the 5.2-p.p.m. &O sample. The recorder again began to respond to the change after approximately 3 minutea, and equilibrium was reached in approximately 15 minutes. In this experiment the signal obtained was 0.124 absorbance unit per p.p.m. for the 5.2-p.p.m. sample and 0.129 absorbance units per p.p.m. for the 1.6p.p.m. sample. This indicates a linear response (within about 4%) with the experimental equipment where flow rates could not be controlled precisely. In B more sophisticated plant design, the
+
response times and signals obtained could be improved by operating closer to optimum values of the gas-liquid flow rates. Certainly faster flow rates would improve the response time, and probably greater carbide surface area (powdered CaCJ per unit volume of liquid would increase the signal. However, these variables were not explored since it was the purpose of this work only to investigate whether or not this approach to the development of a continuous plant analyzer is feasible. CONCLUSION
The results of these experiments demonstrate that the infrared method is adaptable to the continuous determination of the water content of plant streams. According to our experience, trace amounts of alcohols do not interfere. Isopropyl alcohol a t 50 p.p.m. and methanol in trace amounts apparently do not react a t a measurable rate. Sensitivity with this method in a plant instrument would certainly be as good as that obtained here. With proper instrumentation, and suitable scale expansion, 0.3 p.p.m. of &O could produce a full-scale deflection. If the other instrument variables could be held such that the noise was no more than 10% of the signal, then one could determine water to a t least j ~ 0 . 0 2 p.p:m. The sensitivity obtainable is flemble and in practice can be tailored to the plant stream of interest. ACKNOWLEDGMENT
The author expresses appreciation to D. E. LeGoullon for his valuable laboratory assistance during this work, V. P. Guinn for the liquid scintillation counting of the tritiated carbon tetrachloride solutions, and D. 0. Schissler for making numerous suggestions which helped clarify and improve the treatment in this manuscript. RECEIVED for review March 30, 1962. Aocepted June 4, 1962.
