Phase Equilibria in Hydrocarbon E. R. KENNEDY, B. H. SAGE, AND W. N. LACEY California Institute of Technology, Pasadena, Calif.
XIV. Joule-Thomson Coefficients of n-Butane and n-Pentanel
T
HE Joule-Thomson coefficient is , may defined as (bT/~ P ) xwhich
be considered to be the change in temperature resulting from a very small change in pressure occurring a t constant heat content. Some of its uses in thermodynamic c a l c u l a t i o n s have been indicated in a previous paper of this series ( 3 ) . Direct experimental information relating to the Joule-Thomson coefficients of hydrocarbon gases has been seldom reported in the literature. Pattee and Brown (1) give values for n-pentane and painter's naphtha for temperatures above 200" F. obtained by evaluation of changes of heat content (enthalpy) resulting from throttling the gases, using pressure drops ranging from 35 to 991 pounds per square inch. The methods and apparatus employed in the present investigation have been previously reported ( 3 ) . The determination consisted e s s e n t i a l l y in circulating a sample of gas continuously through a porous thimble, a constant small pressure drop being automatically maintained through the thimble and the corresponding change i n t e m p e r a t u r e being measured. The only modification of the apparatus found necessary was variation of the permeability of the porous thimble to give suitable pressure drops a t optimum gas velocities. No variation in the value of the coefficient was observed until the rate of flow had been decreased to such an extent that heat conduction through the thimble wall began to have an appreciable effect upon the temperature of the slowly moving gas on the downstream side.
Materials The sample of n-butane used in this work was obtained from the Phillips Petroleum Company, and the following special analysis was submitted with it: 99.7 per cent %-butane, 0.3 per cent isobutane. Results obtained with this sample showed no appreciable difference from corresponding values using
e FIGURE1. (Tow) EXPERIMENTAL RESULTSFOR n. ., BUTANE GAS FIGURE2. (Tov Center) EFFECT OF TEMPERATURE O N CoEFFICIENTS FOR n-BUTANE
FIGURE 3. FIQURE
4.
(Lower Center)
EXPERIMEXTAL RESULTS
FOR n-PENTASE GAS Bottom) EFFECTOF TEMPERATURE' OX bOEFFICIESTR FOR ?I-PENTANE
Systems Joule-Thomson coefficients were experimentally determined for n-butane gas at a series of temperatures from 70" to 220" F. and at pressures from atmospheric to those approaching the saturation pressures. Similar measurements for n-pentane were made, the lowest temperature used being 130" F. The results are compared with those for propane at equal reduced temperatures and pressures. From the experimental data were calculated values of specific heat at constant pressure and of isothermal change of heat content with change of pressure.
a less pure sample, indicating that small amounts of impurities of similar character do not cause serious impairment of accuracy. This less pure and conmaterial was used in earlier studies (4,6) sisted of 99.21 per cent n-butane, 0.18 per cent isobutane, and 0.61 per cent isopentane. The n-pentane was obtained from the same company. Its analysis showed it to consist of 99.3 per cent of n-pentane and 0.7 per cent of isopentane. The same material was used in earlier work (6).
Experimental Results Measurements were made upon n-butane gas a t six temperatures from 70" to 220" F. and a t pressures as close to saturation as feasible, the highest being approximately 200 pounds per square inch absolute a t 220° F. The experimental results were o b t a i n e d by two investigators independently, upon several different samples a t each temperature. Readings were taken subsequent to increase and to decrease of total pressure. The pressure drop across the porous thimble was in all cases approximately 1 pound per square inch. These results are shown by the points in Figure 1. The precision of the various measurements was the -. same as in the similar study of Propane ( 3 ) * B1gure 2 shows values taken from the curves of Figure 1 replotted on a p vs. temperature diagram. The saturation curves of these two figures were constructed from vapor pressure data, as yet unpublished, by extrapolation of the isothermal curves of Figure 1 to the corresponding vapor pressures and then connecting these points by a smooth curve. Values read from large-scale curves corresponding to Figure 1 are presented in Table I. It is believed that the accuracy of these values is approximately 1 per cent. The measurements on n-pentane were made in a manner entirely analogous to those on n-butane. In this case, however, the liquid pentane was boiled for a short time under reduced pressure to eliminate dissolved air before trans-
FIGURE5. ( T o p ) SPECIFIC HEaT .4T CONSTANT PRESSURE FOR '%BUTANE GAS PRESFIGURE6. crop Center) SPECIFICHEAT A T CONSTANT SURE FOR '+PENTANE GAS FIGURE 7. (Lower Center) ISOTHERMAL CHANGE IN HEATCONTENT OF n-BUTANE GASWITH CHANGE OF PRESSURE, IN B. T. u. PERPOUND/POUND PERSQUARE INCH ~
~
~
~ ~ E ~ & A $~ PER POUND/POUND PER
~
~
~~
)
~
SQUARE INCH
0
ferring the gas into the apparatus. The low vapor pressure of n-pentane a t 70" and 100" F. precluded satisfactory measurements a t those temperatures because of the small mass velocities obtainable in the circulation system. The maximum pressure studied at 220" F. was approximately 70 pounds per square inch. The results are shown in Figures and 4,and taken from the smooth curves are listed in Table 11. The values are believed to be accurate to
1 Previous articles in this series appeared during 1934 and 1935, and i n January, February, April, and May, 1936.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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TABLEI. JOULE-THOMSON COEFFICIENTS Abs. Pressure Lb./sq. in.
Satd. ga8 Atm. 20 40 60 80 100 125 150 175 200 225
70' F. c
OF ?+BUTANE
100" F. 130' F. 160' F. 190' F. ' F . per pound per square i n c h
GAS
220' F. 7
(0.3750) (0.3336) (0.2984) (0.2712) (0.2501) (0,2295) 0,2343 0.1996 0,1430 0,1173 0.2900 0.1708 0.2486 0.1472 0.1200 0.2086 0.8164 0.1772 0.3012 0.2410 0.1634 0.1310 0.1988 0.1778 0,1412 0.2706 0.2184 0.2954 0,1910 0.1512 0.2362 0,2032 0.1608 0.2526 0.2176 0.1720 0.2330 0,1830 0,2476 0.1947 0.2069 0,2188
I per cent. The dashed-line curves in Figures 3 and 4 represent estimated values resulting from short extrapolations. Vapor pressure values used in locating the line for saturated gas were taken from a previous publication (6). In Figures 1 to 8, inclusive, the results for each of the materials were plotted with the same scale and coordinate range (except the pressure scales of Figures 1 and 3) in order that the differences could be visualized readily.
TABLE 11. JOULE-THOMSON COEFFICIENTS OF %-PENTANE GAS Abs. Pressure
Lb./sq. in.
70 80 90
130' F. , -
160' F.
190' F. F . per lb. per sq. in.
220' F.
0.2432 0.2528 (0.2620)
Values of the specific heat a t constant pressure for each substance in the gas state were calculated from the experimental data presented above. These calculations involved the solution of the following equation from atmospheric pressure to each pressure concerned, as described previously for propane ( 3 ) :
I n the case of n-butane, the values of CP a t atmospheric pressure were obtained from a previous investigation (4). Those for n-pentane were not available and so were esti-
VOL. 28, NO. 6
mated from the corresponding values for propane and nbutane (4). The variation from one member of the series to the next is small enough that an uncertainty of only about 1 per cent is involved in this estimation. Furthermore, such uncertainty would cause only a small vertical displacement of the specific heat curves without affecting their slopes. The values obtained from such calculations are shown in Figures 5 and 6. The curves for saturated liquid were drawn for comparison, being constructed from information taken from earlier papers of this series (2, 3 ) . A further use may be made of the Joule-Thomson coefficients in calculating isothermal changes of heat content with change of pressure by a method which is independent of P-V-T measurements and which therefore offers a valuable check upon values obtained through use of those measurements. Since dH
(sp)
= -PcP
the values of p and CP previously discussed can be combined to obtain the desired result, Such calculations were the basis for the curves shown in Figures 7 and 8. Values of (bH/~ P ) T are useful because, by suitable integration, isothermal changes in heat content can be calculated for various changes in pressure. The Joule-Thomson coefficients of the three gases propane, n-butane, and n-pentane furnish a sufficient basis for a comparison. Such a cornparison can be made advantageously by reference to Figure 9. The values of p are there plotted against reduced pressure (ratio of actual pressure to critical pressure) for a series of reduced temperatures. The solid lines show the curves for n-butane, and the marked points indicate values obtained from the n-pentane curves presented above. The agreement for these two hydrocarbons is good. However, values for propane (Sj,as illustrated by the dashed-line curves for reduced temperature of 0.80 and 0.85, show marked deviation from the other curves. This is not surprising, for it is to be expected that the first few members of homologous series of compounds differ from the rest when compared on a reduced basis. The curves for propane and n-pentane a t the lower pressures lie upon opposite sides of those for n-butane, as would be expected. Since the curves for n-butane and n-pentane so nearly coincide, it follows that the values of p for these two substances must be nearly equal a t their critical temperatures.
Acknowledgment The authors wish to acknowledge indebtedness to the American Petroleum Institute for financial support of the investigation, which was carried on under its Research Project 37. J. A. Davies assisted materially by making a number of the experimental measurements. D. C. Webster contributed to the preparation of the figures.
Literature Cited Pattee, E. C., and Brown, G. G . ,IND. EKG.CHEM.,
26, 511 (1934). Sage, B. H., Backus, H. S., and Vermeulen, T., I b i d . , 28, 489 (1936). Sage, B. H., Kennedy, E. R., and Lacey, W. N., I b i d . , 28, 601 (1936). Sage, B. H., and Lacey, W. N., I b i d . , 27, 1484 (1935).
FIGURE 9. COMPARISON OF COEFFICIENTS FOR THREEGASES AT THE SAME REDUCED PRESSURES AND TEMPERATURES
(5) I b i d . , 28, 106 (1936). (6) Sage, B. H., Schaafsma, J. G., and Lacey, W. X.. Ibid., 27, 48 (1935).
RECEIVED March 16, 1936,
