INDUSTRIAL AND ENGINEERING CHEMISTRY
May 15, 1934
CHECKING VALIDITY OF HYDROGEN-ELECTRODE POTEXTIALS It is common practice, when determining hydrogen-ion activity, to use two or more electrodes together. It is considered that the agreement of several electrodes is a satisfactory indication that they have reached the desired potential. With identical preparation of the catalysts we may expect them to have similar properties and that the effect of polarizing impurities on all electrodes may be the same-that the potentials will show close agreement even though they be far from the equilibrium value. The only certainty that we are reading the true equilibrium potential is by the agreement of electrodes of widely different catalytic activities. The prepa-
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ration of electrodes of low activity is best accomplished by allowing bright electrodes to age in hydrogen a few days. They should not, however, be allowed to become too inactive, as an activity less than 0.001 makes the catalyst impractical for the purpose. For most ordinary work such refinement is not necessary. LITERATURE CITED (1) Beans and Hammett, J. Am. Chem. SOC.,47, 1215 (1925). (2) Hammett and Lorch, Ibid., 55, 70 (1933). RECBIVED December 30,1033. In part based upon a diasertstion submitted by Arthur E. Loroh to the Faculty of Pure Science of Columbia University in partial fulfilment of the requirements for the degree of doctor of philosophy, May 16, 1932.
A Method for Measuring the Dew Point of Natural Gases A. MICHELSAND G. W. NEDERBRAGT, Laboratorium Bataafsche Petroleum My., Amsterdam, Holland
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GREAT quantity of natural gas escapes from the wells in oil fields. The greater part of this gas is methane, and the remainder is mostly a mixture of saturated hydrocarbons up to hexane, of which the higher-boiling constituents are valuable for addition to the low-boiling fractions of the natural oil. The separation of the higher homologs in the gas is usually carried out by compressing to about 20 atmospheres at most, and cooling to about - 10' C. The object of the work described here was a study of the C
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change in slope, until it becomes zero at the critical point of the mixture. It follows that the dew pressure can be determined by measuring the isotherm.
TEE APPARATUS The use of an ordinary Cailletet piezometer, in which the gas is compressed into a narrow glass capillary tube, would obviously be unsatisfactory, because, as soon as liquid is formed, small drops might stick to the wall of the capillary tube and be enclosed by the mercury when this rises. A diagrammatic sketch of the apparatus used by the suthors is given in Figure 1. Two high-pressure gas cylinders, A and B (volume about 600 cc.), are connected with the steel capillary C, which reaches the bottom of the gas cylinders. Both cylinders, when in use, are partly filled with mercury. The gas under investigation fills the upper half of cylinder A. The top of B can be filled with nitrogen from cylinder E. Cylinder A is placed in a thermostat, the temperature of which can be regulated to within O.OIo while
H
FIGURE1. DIAGRAM OF APPARATUS
dew point of a gas mixture with a view to establishing the most efficient conditions for the extraction of the higher homologs. THEMETHOD When an ideal gas (Boyle gas) is compressed at constant temperature, the product, pv, is independent of the pressure. I n the case of a real gas, however, the value of pv changes with pressure, so that if pv is plotted against p , a curve is obtained which runs smoothly up to the pressure a t which condensation of the gas starts. At this point there is a break and the curve becomes a straight line perpendicular to the p axis, as it is impossible to increase the pressure a t constant temperature until all the gas has been liquefied. With a mixture of gases there is also a break in the curve at the condensation point, but the value of p does not remain constant during condensation and therefore the pv-p curve does not become perpendicular to the p axis. The difference in the slope of the two parts of the curve on each side of the break depends on the composition of the gas and also on the temperature. The higher the temperature the smaller the 1
Thirty-fourth publication of the v. d. Waals fund, Amsterdam, Holland.
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I FIGURE 2. DIAGRAM OF THERMOSTAT
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ANALYTICAL EDITION,
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cylinder B stands on one scale pan of a b a l a n c e . The steel capillaries D and C (1.2 mm. outer, 0.8 mm. inner diameter) are flexible enoughto give a balance sensitivity of 4 scale divisions per gram. To compress t h e gas under investigation the tap leading from cylinder E is opened, and the pressure of nitrogen in cylinder B increases, forcing t h e mercury through ca illary C i n t o gomb A. The decrease FIGURE 3. ISOTHERM AT 0" c. i n v o l u m e of the gas in A is equal to the volume of the mercury displaced from B, which can be calculated from the loss in weight of B. For accurate measurements of the pressure a ressure balance was used. Tube F leads to a leveling gage, Gfwhich is coupled to the press, H , and from here to the pressure balance, K . When equilibrium is reached, the pressure of the gas in A can e calculated from the load on the pressure balance, corrections being applied for the hydrostatic pressures of oil between K and the oil level in G, of nitrogen between G and the mercury level in B, and of the mercury between the levels in A and B .
TEMPERATURE CONTROL The bomb which contains the gas to be examined is placed in a well-insulated thermostat. Above room temperature heating control as described by one of the authors ( I ) is used. Between room temperature and 0" the thermostat is cooled with the help of a cooling coil and the temperature is regulated in the same way. As it is essential for good reguIation of temperature that the amount of heat removed from the thermostat by the cooling coil shall not fluctuate too rapidly, an arrangement was made whereby cooling liquid of constant temperature was introduced a t constant rate into the cooling coil (Figure 2). Kerosene was used as the cooling liquid, being precooled in another copper coil, D , placed in a second double-walled vessel, B. The kerosene was circulated by a piston pump, C, running a t constant speed.
In practice it is preferable not to place coil D directly in ice, but to cool it with water of nearly 0" C. The arrangement used is shown in Figure 2. Spiral D was surrounded by the tinned copper gauze, E, and the space between E and the wall of the vessel filled with chopped ice. The vessel was then further filled with water, until level F was reached. An Archimedes screw pump, G, sucked the water past the windings of the coil and poured it out again a t H into tray I , the bottom of which is riddled with small holes, thus giving a reasonable distribution of the water over the top of the ice. MEASUREMENTS AND RESULTS Measurements were carried out on natural gas, obtained from the Dutch East Indies. The gas had been passed through a commercial condensation apparatus and was stored in a steel bomb of 50-liter capacity. The measuring apparatus was to be filled directly from this bomb and as some of the higher homologs might have condensed in the bomb,
FIGURE 5.
ISOTHERM AT
21'
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it was heated up to 40" C. just before filling. The analysis of the gas, as carried out with a Podbielniak rectifying apparatus, was as follows: MOLE PERCENT 0.6 0.3 1.2 74.9
12.0 6.7 1.9
1.3 0.8 0.1 0.2
Measurements were carried out a t temperatures of 0", lo", and 21" C. The results are shown in Figures 3, 4, and 5 in which the values of pv are plotted against p . The values of pv were obtained by multiplying the pressure in atmospheres by the volume of the gas in an arbitrary unit. By this method the normal volume of the gas-i. e., the volume a t 1 atmosphere and 0"-was not measured; therefore it was not possible to express the figures in standard units. From the figures it can be seen that a t 0' the gas starts to condense a t a pressure of 15.5 atmospheres and a t 10" C. a t 25.5 atmospheres, whereas there is no obvious break in the 21 " C. curve. Therefore the critical temperature of the mixture is probably below 21" C. ACKNOWLEDGMENT The authors wish to thank the Bataafsche Petroleum Company, and especially the Research Department for their collaboration which enabled this work to be undertaken. LITERATU~E CITED 15
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FIGURE4. ISOTHERM AT 10' C.
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(1) Michels, A., and Michels, C.,Phil. Trans. Roy. SOC.(London), 231, 419 (1933). RECEIVED January 18, 1934.
