Determining Dissolved Water in Liquefied Gases CHARLES W. PERRY U. S. Industrial Alcohol C o . , Baltimore, Md.
T
HE production and use of the many liquefied gases commercially available today frequently bring up the problem of amounts of dissolved water. by Instances of trouble from this impurity are the freezing Of pressure regulators in the domestic use of liquefied petroleum gases, the freezing of controls in solid carbon dioxide manufacture,and the attendant corrosion with liquefied hydrogen sulfide. I n order to eliminate these difficulties it is necessary to have a n accurate analytical procedure for studying the behavior of this impurity during production and use. In some a study of this sort may reveal of dehydrating, such as fractionation O f a n azeotrope O f water-liquid gas, to replace'niore expensive and less effective methods ( 2 ) .
The source of sample A is shown inverted in Figure 1, so that liquefied &asis led Up t o the point of vaporization, valve c, where the function of the steam jet, B , is to prevent dehydration due to the expansion cooling effect. This jet also has the function of vaporizing any liquid that goes beyond valve C, thus preventing pressure surges. By proper manipulation of needle valves C and E, the pressure O f the gas to be tested can be adjusted that T1o sudden surges occur. If connections and apparatus of ordinary size are used, a positive reading of 0.25 to 1 inch of mercury Kill suffice to overcome the pressure drop through the remainder of the testing train. The connections between the differentparts of the aPparatus are 0.125-inch copper or glass tubing, as far as possible, \yith a minimum of rubber for flexibility, if the gas being tested does not attack this substance.
The method described below has been developed for the analysis of dissolred water, is accurate to within *0.005 per cent water content, and may be used down to 0.005 per cent, although below 0.01 per cent the accuracy is not very great. Its manipulation is easily learned, and the apparatus is ordinarily available in any laboratory except for the glass freezing bulb and the necessary liquid air or similar refrigerating gas. Any gas with a boiling point a few degrees lower than the one being tested may be used, if the same pressure is used for both the tested and cooling gases. Analyses are made by freezing the water out of solution by subjecting the vapor to a low-temperature area of condensed gas kept in that state by the cooling medium. The necessary equipment is shown connected for use in Figure 1.
Test Procedure At the start of a test the entire train is hooked up to the source of sample and gas is allolTed to run through the apparatus for
several minutes. The freezing bulb must be dry and clean at the beginning. After flushing the bulb out for a few- minutes it is removed from the train, held in an upright' position (if the gas being tested is heavier than air), and stoppered at bot'li the exit and entrance tubes. The bulb is novi n-eighed carefully on an analytical balance, using a counterbalancing bulb for accuracy. Since the quantity of moisture to be extracted from a reasonable quantity of gas is small, the bulb must be handled very carefully at all times. During this period a short piece of glass t'ubing should be substituted for the freezing bulb and the gas allorved to course through the testing train a t normal pressure and atmospheric temperature. After the weighing is completed In / e t the test is ready to begin. Careful manipulation is necessary during tlie first part of a test. The gas is shut off temporarily and the meter reading is recorded. Then the gas is turned on slo~vlyand condensed in the freezing bulb as fast as it enters the system, thus keeping the meter hand from moving as far as possible. -4 slight forward movement of the counter is t o be desired rather than a backward one, since the latter indicates a too rapid condensation of gas and consequent danger of contaminat,ion by back suction of air or moisture from the exit. The condensation is brought about by raising the thermos container of refrigerating gas around the freez-
Equipment
A is the source of sample, and B is a small jet of steam impinging on the 0.125-inch needle expansion valve, C, &-heremost of the expansion to atmospheric pressure takes place. It is necessary to keep only 5 to 13 pounds pressure on the Bourdon gage, D, between needle valves C and E. Valve E is the final regulation of gas pressure into the testing apparatus, measured by the mercury C-tube, F . S e x t in line is the freezing bulb, G, where the moisture is collected, and which is protected from meter moisture by the phosphorus pentoxide U-tube, H. The Pyrex freezing bulb is shown in larger size in Figure 2, and consists simply of a three-turn glass spiral sealed into a chamber of similar material with an exit tube at the top. The bottom of the spiral fits into a small hemispherical depression and comes to within 0.125 inch of the depression bottom. The final piece of apparatus in the train is the usual type of wet-test meter, J . I is a widemouthed thermos container of the refrigerating gas.
:
FIGERE FREEZISG 2 . HCLB PYREX 513
ing bulb. nate raising I t map and belorrering found that of alterthe
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INDUSTRIAL AND ENGINEERIKG CHEMISTRY
VOL. 10, NO. 9
FIGURE3.
AZEOTROPISM O F
PROPANE-WATER
LTIXTURE
AT
80" F.
Ye O F SAMPLE VAPORIZED
thermos is necessary to prevent too rapid liquefaction. After the liquefied gas fills the Pyrex bulb approximately half full, or covers the spiral, the thermos is adjusted to such a height below the bulb that the liquid level remains constant. Gas will then be condensed as fast as it leaves the system, and the rate of flow should be adjusted t o approximately 0.02 to 0.05 cubic foot per minute. For dissolved water contents of from 0.01 to 0.1 per cent by weight, 1 or 2 cubic feet of gas \ d l ordinarily be sufficient for an accurate determination, although within limits the greater the sample tested the better will be the accuracy. When sufficient gas has passed through the cold area to secure a visible amount of frozen moisture in the spiral, the flow is stopped and the part of the train preceding the bulb is closed off by a pinchcock. The thermos is removed from the outside of the bulb and the condensed gas inside is allowed to boil off to dryness. Enough pressure will be built up spontaneously to run the meter and measure this additional quantity of gas-a necessary procedure, since this is part of the tested material. I t will be found that additional moisture in the form of frost is left in the bottom of the outer glass chamber when the last of the tested sample has boiled off. At this point the bulb is removed from the train, sealed with the same stoppers as before, and again weighed, after carefully drying the outside. Just before this second weighing the exit tube stopper should be opened for a moment to relieve any pressure above atmospheric brought about by warming the gas in the bulb to room temperat>ure. I t is necessary to perform both weighings with the gas in the bulb at approximately the same temperature and pressure.
inverted; then by testing successive fractions with the sample upright a history of the vaporization of a batch of the liquefied gas is obtained. I n the case of propane-water mixtures azeotropism is found to exist, as shown by Figure 3, the curve of which was obtained by the above method. Such information is valuable when dehydration on a large scale is necessary. Some liquefied gases form hydrates, as in the case of propane, which takes on 6 molecules of water to give the solid white compound C8Hs6H20. The effect of low temperature on this type of compound is not known, but the moisture ordinarily collected is probably free dissolved water, and other means would be necessary for determining molecular water. It may be desirable to vary the pressure of the gas in the testing train for certain gases. This gives the investigator choice of test temperatures for any given cooling medium. Constructing apparatus for this is usually not worth while, however.
The increase in weight is due to the moisture extracted from the volume of gas measured b y the meter, if there has been no interference from other factors, such as the condensation of heavy fractions dissolved in the gas. The type of bulb shown in Figure 2 is sufficiently efficient to freeze out substantially all the moisture in the gas. This has been checked by placing two bulbs in series during a test, where the first bulb duplicated results with a single bulb on the same sample, and the second increased in weight by a negligible amount compared to the first. To substantiate the statement that accuracy within 10.005 per cent is obtainable, Table I gives check tests on liquefied petroleum gases. In Figure 1 the sample is shown inverted so that the liquid phase is maintained up to the vaporizing point, valve C. This procedure is necessary if error due t o aseotropism is to be avoided. One of the chief values of this testing method is to study the azeotropism of such gas-water mixtures. An average analysis of the liquid is obtained by testing the sample
100% propane
TABLE I. CHECKTESTSON LIQUEFIED PETROLEVM GASES T e s t No.
Sample Propane-propylene mixture
1
2 3 4
1
2
Weight P e r C e n t of in Liquefied G a s 0.040 0.037 0.040 0.041 0.020 0.025
This method of water determination may be applied t o any gases with boiling points between that of the refrigerating gas and approximately 0 O E'. Above this temperature errors due to water loss become sufficiently great to prevent an accurate determination. For practical testing the method is recommended as being easily adaptable to routine work and of reasonable accuracy. I t has been found superior in accuracy to similar determinations by phosphorus pentoxide absorption, cobalt bromide color indication, and wet- and dry-bulb thermometry, as recommended by other investigators ( I , 2 ) .
Literature Cited (1) Bragg, L.B.,Gas Age-Record, 66, 153-6 (1930). (2) Haohmuth, K. H.,Western Gas, 8,55-6,62,64 (January, 1931). RECEIVED February 11, 1938.
