In the former case the rate of evaporation will approach evaporation conditions in a closed space. In a closed chamber evaporation will take place more or less rapidly at first. After some time, however, the space above the liquid will become partly filled with stray molecules that have escaped through the surface film. These, after escape, move about indiscriminately in the chamber and are reflected from its walls and from each other. Some return to the liquid, and once they fall on its surface they may be attracted into the interior. It thus happens that a certain stage is ultimately attained at which as many molecules return to the liquid per second as leave it and an equilibrium is established; at this stage evaporation may be said to have ceased. There is no further loss to the liquid or gain to the vapor outside it; there is, however, a continual exchange going on, new molecules are being continually projected from the surface and others are falling into the liquid in equal numbers. The chamber is then said to be filled with saturated vapor, or the vapor is said to be saturated; in any stage before this final stage has been reached the vapor is said to be nonsaturated. A saturated vapor is thus one that is in equilibrium with its own liquid. If there is any means of escape, however, as from the bunghole of the drum, some of the molecules, in wandering about inside the drum, will make their exit through the bunghole and into the atmosphere, never returning to the liquid. There will be thus a continual flow of molecules from the surface of the liquid to the atmosphere, and evaporation will continue in this manner at a steady rate as long as the temperature is maintained constant. If the area of the hole be expanded to the size of the surface of the liquid, much faster evaporation will take place, because there will be a much wider avenue of escape opened for the wandering molecules. When natural-gas condensates or any liquids are exposed to the air, as in transferring the condensates from accumulator tanks to storage tanks or to drums, evaporation takes place much more rapidly than when the condensate is lying quietly in the container. This increased rate results from the much greater surface of the liquid that is exposed to the air. The authors of this publication found that from 2,040 gallons of condensate transferred from a storage tank to a loading station at a railroad siding by means of a pipe line 1,500 feet long, there was lost by evaporation 747 gallons, or 36 per cent. This loss occurred at the plant at which the tests shown in Tables 10 and 11 were conducted. The material was allowed to run down the pipe line by gravity to the loading stations. This experiment shows that evaporation rates may be much faster than in the tests conducted by the authors in glass cylinders. Evaporation losses may be also hastened when the atmospheric temperature is high. The atmospheric temperature, when the authors conducted the tests, ranged from 40° to 70° F. (4° to 21° C.), but most of the time the temperature ranged between 50° and 60° F. (10° to 16° C.). During the summer time temperatures are much higher and evaporation losses will be correspondingly greater. In colder weather they would be smaller. CURVES SHOWING EVAPORATION LOSSES. The authors have plotted certain curves to show the evaporation losses resulting when natural-gas condensates and mixtures of con LOSS IN VOLUME, PER CENT. 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 PERIOD EXPOSED, HOURS. FIGURE 11.-Evaporation losses of natural-gas condensates. 19 20 21 22 23 24 densates and naphtha are exposed to the air. The curve results shown in figure 11 are plotted from data from Table 10. It will be observed that the rate of evaporation of condensates of high specific gravity is rapid for the first eight hours. The rate of evaporation of a condensate with a specific gravity of 79° B. is fairly uniform. A condensate with a specific gravity of 98° B. underwent a slower rate of evaporation than four condensates of lower specific gravity. This difference was probably due in part to the fact that during the tests of condensates of high specific gravity the temperature of the atmosphere was about 10° F. (6° C.) lower. In figure 12 are shown the evaporation losses that resulted when two natural-gas condensates, with gravities of 98° and 83.5° B., were mixed with naphtha in different proportions. The curves were prepared from Tables 10 and 11. Figure 13 shows the evaporation losses resulting when a condensate with a specific gravity of 93° B. was exposed to the air and when it was mixed with naphtha. The curves were prepared from Table 9. Figure 14 shows the evaporation losses when a condensate, with a specific gravity of 95° B., was exposed to the air and when it was FIGURE 12.-Evaporation losses when two natural-gas condensates were mixed with naphtha in different proportions. mixed with naphthas of different specific gravities and exposed. Smaller losses resulted when a naphtha with a specific gravity of 60° B. was used than when a naphtha with a specific gravity of 44° B. was used. VAPOR-PRESSURE TESTS. Figures 15, 16, 17, and 18 show the results of tests made by the authors to determine the pressures exerted by the vapors of condensates and of blends. In making the tests a small steel container of about 2liter capacity was fitted with a pressure gage. A hole was bored in the bottom of the container for the introduction of the liquid. This hole could be securely closed with a small threaded steel plug. To determine the pressure exerted at different temperatures, the con tainer was placed in a water bath heated by a gas burner. Pressure, temperature, and specific-gravity observations were made. In figure 15, curve 1 shows the vapor pressures of a freshly drawn condensate having a specific gravity of 93° B. The vapor pressures ranged from 19 pounds per square inch at 55° F. (13° C.) to 48 pounds per square inch at 100° F. (38° C.). After the test represented by curve 1 had been completed, the steel plug was removed from the container and the liquid allowed to volatilize until it had lost 10 per cent of its original volume. Then the plug was again inserted and the vapor pressures were again noted for differ 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 FIGURE 13.-Evaporation losses from a condensate with a specific gravity of 93° B., and from the same condensat mixed with kerosene and with naphtha. Sudden bend in lower curve at 7 hours due to accidental violent agitatio of mixture, causing more rapid loss. ent temperatures, as shown by curve 2. Curves 3, 4, and 5 show the vapor pressures at different temperatures after the original condensate had lost 20, 30, and 40 per cent of its original volume. It will be observed that the greatest drop in vapor pressure occurred after the first 10 per cent loss. This is due to the fact that the liquefied gases, propane and butane, were escaping in largest quantities during the first part of the exposure. There was not much difference in the vapor pressures after 30 and 40 per cent losses, as shown by curves 4 and 5. Figure 16 shows the vapor pressures of a condensate obtained at the same plant as that represented in figure 15, but at a different time. It will be observed that the condensate represented by curve 1, figure 16, and that represented by curve 1, figure 15, were of the same specific gravity, but produced appreciably different vapor pressures at the same temperatures. In other words, condensates of same specific gravity may produce quite different vapor pressures. A striking instance of such a variation can be noticed by comparing curve 3, figure 16, representing the vapor pressure of a condensate with a specific gravity of 78° B., with curve 5, figure 15, representing the vapor pressure of a condensate with a specific gravity of 88° B. The 50 per cent condensate, 50 per cent 44°B. kerosene. Resultant specific gravity, 67° B. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 PERIOD EXPOSED, HOURS. FIGURE 14.-Evaporation losses from a condensate with a specific gravity of 95° B., and from the same condensate mixed with kerosene and with naphtha. condensate with a specific gravity of 88° B. had a lower vapor pressure than the condensate with a specific gravity of 78° B. However, the condensate represented by curve 5, figure 15, had weathered long enough to lose 40 per cent of its original volume, and the condensate represented by curve 3, figure 16, was freshly drawn. From the former, liquefied gases and liquids of low boiling point had been largely removed, and in the latter there may have been enough of the liquefied gases to exert a pronounced vapor pressure, but not enough to greatly affect the specific gravity. After exposure to the atmosphere for an hour or even less, the condensate with a specific gravity |