HydrocarbonXater Solubilities at Elevated Temperatures


Solubilities of water in three petroleum fractions have been determined up to 280' C. and 940 pounds per square inch absolute at the total pressure of...
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HydrocarbonXater Solubilities at Elevated Temperatures and Pressures JOHN GRISWOLD AND J. E. ICASCH The University of Texas, Austin, Texas

cloud point temperature mas observed using a flue type heater similar to the one described by Eaton and Porter ( 2 ) . The results are plotted in Figure 1, with low-temperature data of Clifford ( I ) , Groschuff ( 3 ) and Hill (4) included. Above 200" C. trouble was experienced by bursting of the sealed glass tubes. The highest cloud temperature attained was 281" C. The pressure a t this temperature was approximately 945 pounds per square inch absolute, with 43 mole per cent water dissolved in the oil. At 300" C. the extrapolated solubility is 60 mole per cent water. Significant observations from Figure 1 are: (a) There is no regular trend of solubility with molecular weight for the six oils (whose molecular weights ranged from 95 to 425), and ( b ) all data on petroleum fractions lie viithin about 10" C. of the curve. Solubilities of

Solubilities of water i n three petroleum fractions have been determined up to 280' C. and 940 pounds per square inch absolute at the total pressure of the systems. The results (expressed on a mole basis) indicate that the solubility of water in straight-run petroleum fractions is substantially independent of molecular weight of the oil. This observation permits a correlation o f water dissolved in petroleum fractions in terms of steam pressure and temperature. At any given temperature and pressure, the solubility of water in oil is much greater than that of oil in water (both expressed on the mole basis). In both phases solubility of aromatic hydrocarbons with water is several times greater than the solubility o f these petroleum fractions with water.

IRECT or open steam is used in many petroleum refining operations such as stripping and fractional distillation. Gasoline-steam vapors are usually condensed together, and separation of the condensates is fairly rapid. Heavy oils withdrawn from a steam distillation or stripping operation are always wet, and the water separates much more slowly. Air blowing of heavy oils is sometimes resorted to in order to dry or "brighten" them. A review of the literature disclosed many mater solubility determinations a t temperatures to 100" C. and pressures (of water vapor) to atmospheric. One author determined solubilities of hydrocarbons in water to 300" C. at the total pressure of the system ( 5 ) ,but no data were found for solubilities of water in hydrocarbons at elevated temperatures and pressures, This article presents experimental data enabling a fairly accurate prediction of the amount of mater remaining in oil as a result of refinery operations involving direct contact with steam.

D

S O L U B I L I T Y O F WATER IN P E T R O L E U M F R A C T I O N S ( A T TOTAL PRESSURE

Solubility of Water in Oil Three petroleum products were studied-a cleaner's naphtha, a kerosene, and an S. A. E. 20 lubricating oil. While the base crudes were unknown and the refining histories of the products not available, they were in all probability typical straight-run materials from paraffin or mixed-base crudes. For the solubility determinations, weighed amounts of water and oil sample mere sealed into glass tubes, and the

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OF T H E S Y S T E M )

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TEMP.

DEGRYES 100

CENTIGRADE 140 200

FIGURE 1 804

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INDUSTRIAL A N D ENGINEERING CHEMISTRY

aromatic stocks from certain crudes, cracking, or other conversion operations are probably higher than those of the fractions from which the curve was drawn. The data on benzene form a separate curve as shown. Total pressure-temperature curves were determined for the three oil-water systems using a steel bomb immersed in a high temperature bath and connected to a pressure gage. Figure 2 was developed as follows: Vapor pressures of water were read from a steam table (6). The molal average boiling points of the oils a t atmospheric pressure were computed (8), and the pressures a t other temperatures were read from the Brown-Coats hydrocarbon vapor pressure chart. I n general, observed total pressures were greater than the vapor pressures of water alone and less than the sum of the vapor pressures of water and the oil. Using total pressure data for the naphtha-water system, Henry's law constants for water in oil were calculated from the relation: Kw = (P/x)~

805

HENRY'S LAW CONSTANT vs!) TEMPERATURE FOR WATER IN

(1)

where p = pressure of water vapor over two-phase mixture z = mole fraction of water soluble in oil Under saturated conditions the partial pressures of water and of oil must be the same for both phases. The units of K , and of p , are pounds per square inch absolute. Between 130" and 300" C., K , increased less than twofold, the pressure increased more than fortyfold, and the solubility increased twentyfold. These are general circumstances for which Henry's law is valid, and if this is the case, K w applies for conditions in which the steam is a t pressures below saturation or superheated. Accordingly, Figure 2 with Equation 1 may be used to predict the solubility of water in petroleum fractions for any pressure of steam a t temperatures up t o 300" C. (572 " F.). Extrapolation t o still higher temperatures should give results of the right order of magnitude.

Solubility of Oil in Water Water from refinery separators always contains small amounts of oil. This oil may be partly due to entrainment or incomplete separation, but there is also a definite solubility of oil in water. The data of Jaeger (6) were studied. Estimates of the average molecular weight of his oil samples were made, using the correlation of Watson, and the data were calculated to mole percentages. The results are plotted as Figure 3. It is evident that: ( a ) Solubility of petroleum fractions in water is not substantially independent of molecular weight (as for the oil phase) ; (b) the solubility of aromatic hydrocarbons in water is greater than that of petroleum fractions of the same molecular weight; and (c) the solubility of hydrocarbons in water (on the mole basis) is much lower than that of water in hydrocarbons. Since Jaeger's data on petroleum fractions show a consistent trend with molecular weight, Figure 3 with Henry's law may be used to obtain an approximation of the solubility of oil in water over a range of temperatures. Experimental Inspection tests on the oil samples are given in Table I. Molecular weights of the kerosene and lubricating oil were determined by a conventional ebulliometric method (7) and checked by Watson's correlation (8). Since the naphtha is somewhat volatile under conditions of the determination and there is no necessity of high accuracy for the molecular weight, the value for the naphtha was taken from the correlation. CLOUDPOINT DETERMINATIONS. The samples were tested for dryness by heating and were sealed in Pyrex glass tubes about 60 mm. long and 8 mm. 0. d. Tube wall thicknesses

120

I40

160

200 220 240 260 280 300

180

FIGURE 2

TABLE I. INSPECTION TESTS Naphtha Gravity, Viscosit:

Kerosene

Critioal pressure, lb./sq. in. aha. (8) Universal Oil Products characterization factor

328

285

12.2

Naphtha Initial b. p. 10% over

50 % 60 %

340 342 346

Kerosene 397 417 425 430 436 442 446

95

11.8

A. S. T. M. Distillations,

4%

Lube Oil

A . P. T

.a

7 0 7 over 80

1 $3point Recovered % Residue,

4

12.4

F. Naphtha 350 354 364 396 97.0 0.7

Kerosene 451 458 468 492 97.5 1.5

were 1 mm. for the low-temperature and 2 mm. for the hightemperature determinations. The tubes were first sealed a t one end, charged with water and then oil, mounted in an asbestos sheet with the lower end immersed in an ice bath, and sealed off at the top. With careful technique in the sealing operation, no losses of water or of oil could be detected. The following constants were determined for each tube: weight of empty tube, weight of tube plus water, weight of sealed tube, and total outside volume of sealed tube. These values, with the coefficients of expansion of the oil (Q),water, and Pyrex, permitted calculation of the amount of water remaining as vapor in the space above the oil a t the cloud point temperature. This was deducted from the total water to obtain the quantity dissolved in the oil. Since the data as obtained with different volumes of vapor space were all consistent, the vapor-space correction appears to have been

INDUSTRIAL AND ENGINEERING CHEMISTRY

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properly made. Eaton demonstrated that the presence of air in a sealed tube had a negligible effect on the critical temperature of petroleum oils, and there were no indications of air affecting the miscibility temperature in these experiments. Since the tops of the tubes were heated t o bright redness, most of the air was automatically driven out by the sealing operation. The type of heater used has been noted ( 2 ) . I n making a miscibility temperature (cloud point) determination, the sealed tube was heated and rotated, end for end. The water first dispersed in the oil, then disappeared. Slight temperature changes caused a cloud to appear and disappear in the oil a t a maximum temperature change of 2" C. The average of these values is reported in the tables. The thermometers were graduated in single degrees centigrade and were calibrated against a standard thermometer having a National Bureau of Standards certificate. Emergent stem corrections were made for all determinations. Since the thermometer bulbs and sealed tubes were of similar size, shape, and material, and the furnace jacket was well insulated, radiation errors should be negligible. VAPORPRESSURE DETERMINATIONG. The apparatus for vapor pressure determinations consisted of a steel bomb of about 200 cc. capacity, immersed in a high-temperature bath and connected to a 1500-pound Bourdon gage by a loop of small steel tubing. The gage had a 7-inch-diameter scale and was graduated in 10-pound intervals. Pressure readings were estimated to 1 pound. The gage was calibrated against

SOLUBILITY OF HYDROCARBONS AND PETROLEUM FRACTIONS IN TOTAL PRESSURE OF THE (JAEGER, 1923 1

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G. water/

Vol. 34, No. 7

TABLE 11. SUMMARIZED DATAOK NAPHTHA Solubility-

napht%a

Mole yo water

Cloud point, O C.

0.6404 1.2051 1.6570 2.3649

4.97 8.96 11.91 16.18

159 186 203 222

100

,

Total Pressure

C.

Lb./sq. in,

c.

abs.

154.5 177.5 191.0 194.5 218.5

83 138 193 215 338

243.0 274.0 292.5 296.0 296.5

Lb./sq. in. abs.

547 934 1254 1331 1342

TABLE 111. SUMMARIZED DATAON KEROSENE -SolubilityG. water/ kerosene

100 g.

Mole yo water

Cleud point C.'

0,1306 0.2313 0.5456 0.6508 0.8308 0,9029 1.3309 1.4377 1.8281 2.4502 3.1148 4.4436

1.24 2.18 4.98 5.89 7.39 7.98 9.00 12.14 14.94 19.06 23.04 34.97

112 135 169 177 185 191 203 207 216 22s 251 264

C. 122.9 142.3 162.0 176.5 186.2 195.9 206.1 217.0 223.2

Total Pressure Lb./sq. in. abs. c. 36 230.8 50 240.3 76 250.9 105 256.9 144 267.3 190 276, t3 241 285.3 315 295.0 356

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Lb./sq. in. abs. 407 491 610 666 796 936 1070 1256

TABLE IV. SUMMARIZED DATAON S. A. E. 20 LUBRICATING OIL 7

G . water{ 100 g. 011 0.1072 0.1365 0.2359 0.4369 0.8022 0.8070 0.8699 0.9286 1.6829 1.8115 2.0819 2.3011 2.4863 2.5174 2.8142 3,1869

Solubility Mole Yo water 2.52 3.19 5.3s 9.53 16.21 16.29 17.34 18.29 28.86 30.40 33.42 35.68 37.48 37.77 40.42 43.44

Cloud point, C. 124 137 151 189 226 208 215 215 250 259 267 272 269 273 274 281

-Total O

c.

126.0 144.6 161.4 182.4 202.8 207.7 217.1 226.5 237.7 245.6 254.1 266.9 274.3 285.2 295.1

Pressur-

Lb./sq. in. abs.

34 46 63 115 218 255 314 378 464 528 616 753 847 988 1154

a dead weight tester before and after the work. The bomb was mounted on a spring and agitated continually. For the kerosene and lubricating oil, a heavy petroleum oil was used as the bath liquid, and the temperatures were read with a thermometer. At the higher levels the data indicated that observed temperatures were '1 or 2" C. low even after thermometer stem corrections were applied. This error was reduced to less than 1" C. on the naphtha determinations by substitution of a low-fusing salt bath liquid (HTS) and taking the temperatures with a calibrated thermocouple. The procedure used for making the vapor pressure determinations was to charge the bomb with about 50 cc. of oil and 75 cc. of water, heat it to boiling (to sweep out the air), then connect i t to the gage. The bath temperature was slowly raised and held approximately constant for 15 to 30 minutes before each pressure reading was taken. With the amounts of oil and water charged, a two-phase mixture was present a t all times. The cloud point and vapor pressure data are summarized in Tables 11,111,and IV.

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Literature Cited

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DEG. CENTIGRADE

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150

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FIQURE 3

Clifford, J. IND.ESQ.CHEM.,13, 628, 631 (1921). Eaton and Porter, Zbid., 24, 819 (1932). Grosohuff, 2.Elektrochem., 17,348 (1911). Hill, J . Am. Chem. SOC.,45, 1143 (1923). Jaeger, Brenstof-Chem., 4, 259 (1923). Keenan and Keyes, "Thermodynamic Properties of Steam", New York, John Wiley & Sons, 1936. (7) Washburn and Read, J . Am. Chern. SOC.,41, 721 (1919). (8) Watson and Nelson, IND. ENQ.CEEM.,25, 880 (1933). (9) Watson et a!., Oil Gas J.,35, 85 (Nov. 12, 1936). (1) (2) (3) (4) (5) (6)

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