Phase Equilibria in Hydrocarbon Systems

B. H. SAGE, H. D. EVANS,. AND W. N. LACEY. California Institute of Technology, Pasadena, Calif. THE latent heat of vaporization is of importance in...
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Phase Equilibria in Hvdrocarbon Systems J

J

Latent Heat of Vaporization of Propane and n-Pentane'

The latent heat of vaporization was measured experimentally for propane at temperatures from 100" to 170' F. and for n-pentane from 80' to 200' F. The apparatus employed for these measurements is described in detail. The results are presented in both tabular and graphical form. A comparison is made between the results obtained and the data of other investigators.

B. H. SAGE, H. D. EVANS, AND W. N. LACEY California Institute of Technology, Pasadena, Calif.

HE latent heat of vaporization is of importance in establishing the thermodynamic behavior of pure substances in the two-phase region. Although this quantity may be calculated from volumetric and vapor pressure data by means of the Clapeyron equation, it can be measured direct in certain regions more accurately. This measurement also affords a desirable check upon the consistency of thermodynamic data related to the saturated liquid and gas. For this reason an experimental investigation was made of the latent heat of vaporization of propane and n-pentane a t temperatures from 80" to as high as 200" F. This work is part of a general study of the heat of mixing of the lighter paraffin hydrocarbons. Dana and eo-workers ( 2 ) measured the latent heat of vaporizat,ion of propane from -40" to 68" F. with an estimated uncertainty of approximately one per cent. These data were obtained directly by measuring the weight of propane evolved from a calorimeter as a result of the addition of a known quantity of electrical energy. These investigators also determined the specific volume of the saturated liquid and saturated gas, and measured the vapor pressure of propane throughout the above-mentioned temperature interval. They compared values of the latent heat of vaporization calculated from these volumetric and pressure data with the directly measured values. Pressure-volume-temperature measurements from the authors' laboratory (13) also permitted the calculation of the latent heat of vaporization of propane by the use of the Clapeyron equation. These data indicated values approximately 6 B. t. u. per pound lower than values obtained by Dana and eo-workers ( 2 ) by extrapolation of their measurements a t lower temperatures. This large discrepancy caused doubt as to the proper value of the latent heats of vaporization for propane a t temperatures below 160" F. The heat of vaporization of commercial n-pentane a t temperatures between -4" and 86" F. was measured by Griffiths and Awbery (3) who expressed their results as a linear function of the temperature. They also reported data upon the specific volumes of saturated gaseous n-pentane a t temperatures from 32" to 88" F. The specific volumes of the saturated liquid and the saturated gas of n-pentane as well as the vapor. pressure were measured by Young (15) a t tempera-

tures from 70" to the critical temperature (387' F.). The measured values of Griffiths and Awbery (3) are in only fair agreement with values calculated from the pressure-volumetemperature measurements reported by Young. The vapor pressure and the volumetric behavior of liquid n-pentane were studied in the authors' laboratory a t temperatures between 70" and 220" F. (1.2). These data were in good agreement with the measurements made by Young.

T

Materials and Method The propane and n-pentane used in this investigation were obtained from the Phillips Petroleum Company. The special analyses furnished showed that the propane did not contain more than 0.03 mole per cent of impurities. The sample of n-pentane contained 99.3 mole per cent n-pentane and 0.7 mole per cent isopentane. The propane was used without further purification, but the n-pentane was subjected to a fractionation in a column packed with glass rings in order to remove dissolved air and some of the isopentane. The purity of these samples was substantiated by the degree of constancy of their vapor pressure throughout the course of isothermal condensation. The presence of the isopentane in the sample of n-pentane was not of serious consequence since its latent heat of vaporization is not greatly different from that of n-pentane. It is believed, therefore, that the presence of the impurities indicated above did not cause any appreciable uncertainty in the reported values of the latent heat of vaporization. The method employed is similar to that used by Osborne and eo-workers (5, 6, 7) in their investigation of the latent heat of vaporization of water and ammonia. Since Osborne ( 5 ) has already made an excellent thermodynamic analysis of the behavior of such a device, it is unnecessary to derive the expressions that were employed in reducing the experimental data to the desired thermodynamic quantities. I n principle, the method consisted of adding a known amount of electrical energy to a calorimeter containing liquid and gas phases of the substance in question and determining the weight of material that was withdrawn as saturated gas in order to maintain the temperature and pressure within the calorimeter a t constant values. Under ideal conditions in which there is no superheat of the gas or liquid during the isothermal evaporation process, the latent heat of vaporiza-

1 This i5 the twenty-fourth paper in this series. Previous articles appeared during 1934 to 1938, inclusive, and in Maroh, 1939.

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FIGURE 1.

DL4GRAM OF

APPARATUS

tion is simply related to the electrical energy added by means of the following expression:

However, additional correction terms are required when the actual process digresses from the above-mentioned ideal vaporization process. I n the present instance the maximum total of all of the corrections amounted t o only 0.6 per cent of the latent heat of vaporization. These corrections were applied i n a manner similar to that proposed by Osborne (6). Information concerning the specific heat of the saturated liquid and saturated gas as well as data upon the specific volume and isothermal enthalpy-pressure coefficient ( b H / bP)= of both of the phases is required in the evaluation of these corrections. I n addition, the heat capacity of the empty calorimeter must be known in order to determine the net energy added to the system when digressions from a n isothermal vaporization process are encountered.

Apparatus The apparatus developed for making the measurements reported in this paper is depicted in Figure 1 : The calorimeter bomb, A , was mounted within the vacuum jacket, B. The space between the calorimeter and the jacket was maintained at a pressure below 10-7inch of mercury by means of the oil diffusion pump, C. This pressure was measured by means of the McLeod gage, E. It was found that the quantity of condensable vapor within the vacuum space was sufficiently small so that the pressure measurements by means of the McLeod gage gave a satisfactory indication of the resistance to heat transfer by conduction through the space from the bomb to the jacket. The temperature of the jacket was maintained at the desired value by means of the small oil bath, F , of low heat capacity, which was agitated by means of stirrer G. A four-]unction thermocouple was mounted with one set of junctions on the exterior surface of calorimeter bomb A and the other on the interior surface of vacuum jacket B. This double-wire copper-constantan (No. 40 B. and S. gage) thermocouple was employed to measure the temperature difference between the bomb and jacket during the course of a given set of measurements. A similar two-junction thermocouple was mounted on the surface of the bomb and the interior of the jacket. This latter thermocouple was connected to a high-sensitivity low-resistance galvanometer. The light beam from this galvanometer actuated a photoelectric relay which in turn controlled the electrical energy added to the jacket bath. This arrangement was used when the heat capacity of the

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calorimeter was being evaluated and permitted the automatic maintenance of a temperature difference of nearly zero between the interior surface of the jacket and the exterior of the calorimeter bomb. In general, temperature differences of not more than 0.02' F. were encountered in the course of a particular set of measurements of this type. For the actual measurements of latent heat of vaporization, the temperature of the jacket oil bath was controlled at a constant value by means of a four-junction thermocouple operating between an ice bath and the thermostat. This four-junction thermocouple in conjunction with a potentiometer actuated the high-sensitivity galvanometer which controlled the same photoelectric relay mentioned in connection with the differential control apparatus. This control device maintained the temperature of the vacuum jacket within 0.01" F. of the desired value. Oil bath F was placed within an air thermostat which was maintained a t a temperature approximately 2" F. below that of the calorimeter. Calorimeter bomb A was connected to the motor-operated valve, H , by means of a small stainless-steel tube ap roximately 0.02 inch in inner diameter. This tube was in good tierma1 contact with the jacket, avoiding any thermal gradient in this tube adjacent to the calorimeter bomb. All of the tube outside of oil thermostat bath F was enclosed in an isobaric steam jacket to avoid any condensation in this part of the a paratus. The work of Osborne and co-workers em hasized (67 the importance of avoiding such condensation. &e weighing bomb, J , was immersed in the condenser thermostat bath, K, which was maintained at a predetermined temperature, lower than that of the calorimeter bomb. This temperature was so chosen that a pressure differential of approximately 4 pounds per square inch was maintained across the motor-operated valve, H , which was remotely controlled from the location of the temperature measuring equipment. The calorimeter could be evacuated through valve L. The ressure existing within the calorimeter was measured by means o r a pressure balance, N , which was connected to it through the oil-mercury interface in the U-tube, M. The exterior of tube M , as well as that of the tubing connecting it with the calorimeter bomb, was steam-heated to avoid condensation. The details of the construction of the calorimeter bomb are presented in Figure 2: The bomb was machined from solid bar stock which, after heat treatment, exhibited an elastic limit in excess of 200,000 pounds per square inch. The hemispherical ends were threaded to the cylindrical section and were soldered to it with block tin. Adequate agitation of the contents of the calorimeter was desirabIe in order to avoid undue su erheating of the liquid and inequalities in the temperature witEin the calorimeter. This was accomplished by placing a small centrifugal agitator, Y, within the calorimeter bomb, similar in some respects to that employed by Osborne and Van Dusen (7). This agitator circulated the liquid upward around shield W past heater P. The ump was mounted upon jewel bearings and was driven externallyg y means of a small (0.01-inch) iano wire which was enclosed in the stainless-steel tube, &. Jutside the vacuum jacket this piano wire passed through the packing gland, R, of Fi re 1 and was driven by means of the electric motor, S. A vage, T,was provided above the packing gland to permit the addition and withdrawal of samples of liquid from the calorimeter. The ap aratus was so designed that less than 0.03 cubic inch (0.4 cc.) of free space existed in stainless steel tube & and packing gland R. This quantity was small in comparison with the volume of the calorimeter which was ap roximately 18.3 cubic inches (300 cc.). Heater P ofpFigure2 was constructed of approximately 3 feet of No. 36 constantan wire enclosed within a small (0.03-inch inner diameter) stainless-steel tube. It was so constructed that all of the constantan wire was within the calorimeter bomb and only relatively large (No. 28 B. and S. gage) copper leads were brought out of the calorimeter. The voltage impressed upon the heater was measured by means of otential leads connected to the heater at the exterior surface of t f e calorimeter bomb. Since this electromotive force was approximately 9 volts, a volt box with a resistance of 104 ohms was employed in conjunction with a recently calibrated Leeds & Northrup type K-2 potentiometer for its measurement. Intercomparisons of the various unsaturated cadmium standard cells employed in the laboratory indicated that this voltage was measured with an uncertainty of not more than one part in ten thousand. The current flowin through the heater was established by measuring the potentia? across a standard resistance connected in series with heater P. It is believed that the current flowing was measured with an uncertainty of not more than two parts in ten thousand. Each of these quantities (i. e., the current and the volta e) was corrected for the flow of current through the volt box and for the resistance of the heater leads from the potential connections to the heater. The

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time during which the electrical energy was added to the calorimeter was determined by means of an electrical timing device o erated by the power supply to the laboratory. It is believed t f a t in the period involved in a given measurement (more than 1000 seconds) no uncertainties greater than 0.05 per cent were involved in this quantity. Storage batteries of adequate capacity were employed as the current source of the calorimeter heater. Changes in the rate of energy addition of more than 0.1 per cent were usually not encountered in the course of a given set of measurements. These batteries were maintained in closed circuit for a period of a t least one hour before their use with the calorimeter in order to avoid initial changes in voltage upon closing the circuit. It is believed that the energy added to the heater within the calorimeter was known with an uncertainty of not more than 0.1 per cent. The temperature of the liquid within the calorimeter was estabhhed by means of the single-junction copper-constantan thermocouple, V , of Figure 2. This thermocouple was of threewire construction and was calibrated accurately after its installation within the apparatus against mercury-in-glass thermometers which had been standardized recently at the U. S. Bureau of Standards. The electromotive force measurements for all of the precision thermocouples used in connection with the calorimeter were made by means of a White double potentiometer with which temperature differences as small as 0.005" F. could be detected. The standard cell for this instrument was enclosed in a brass thermostat block which showed a maximum variation of a p proximately 0.1 O F. throughout any particular month. Three standard cells were placed in this thermostat block and were so arranged tis to permit their direct intercomparison. Such a comparison indicated a probable uncertainty in standard cell voltage of a proximately one part in ten thousand during the entire course o f t h e present investigation. The use of a thermostat container for the standard cells was found to be justified because of the rather large daily fluctuations in the temperature of the laboratory which were found to result in nonequilibrium changes in standard cell voltage of as much as four parts in ten thousand. It is believed that the thermocouples employed permitted the establishment of the temperature of the contents of the apparatus with an uncertainty of not more than 0.1' F. relative to the International Platinum Scale. Changes in temperature in the course of any one measurement of less than 0.001 ' F. were easily measurable. The temperature of the gas leaving the calorimeter was established by means of a single-junction, three-wire, copper-constantan thermocouple attached to the outlet tube of the calorimeter approximately one inch from the calorimeter bomb. The cold junctions of this thermocouple and of the main thermocouple V were immersed in an agitated ice bath. In order to avoid temperature gradients along the agitator drive tube &, a small manually controlled heater, U (Figure l), was installed. This heater was so controlled that a differential two-junction thermocouple operating between the lower end of this agitator tube and vacuum jacket B indicated no measurable temperature difference between these two points. All of the other metallic connections to the calorimeter bomb passed through oil bath F and therefore were at substantially the same tem erature as the calorimeter bomb. TEe material evaporated from the calorimeter was condensed in weighing bombs which had capacities of approximately 2 cubic inches. These bombs were weighed in air with a nearly identical tare, and the change in weight was determined by means of calibrated brass weights. Additions and withdrawals of the contents of the calorimeter by such means indicated that the quantity of material withdrawn could be determined with an uncertainty of not more than 6 X 10-6 pound (3 mg.). Therefore, it is believed that the weight of material evaporated was established with an accuracy of 0.1 per cent for all of the measurements reported in this investigation.

Procedure The apparatus was first evacuated through valve L of Figure 1. A known weight of the hydrocarbon in question was then introduced through valve T. A sufficient quantity of material was added so that the liquid level would be approximately 0.75 inch above the top of the circulation shield. The bomb and jacket were then brought to the desired temperature, and the various adjustments made to avoid a p preciable thermal leakage from the apparatus. The weighing bomb was attached and immersed in the condenser thermostat bath which was adjusted a t the proper temperature relative to the calorimeter. Measurements as a function of time were made of the temperature difference between the jacket

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and bomb, and of the temperatures of the outlet tube and the interior of the calorimeter bomb. The vacuum jacket was maintained a t a constant temperature by the photoelectric control previously described. The agitator was then started and electrical energy added to the calorimeter heater. The motor-operated valve was opened sufficiently to maintain a nearly constant temperature of the contents of the calorimeter. During this evaporative period readings were taken of the temperature difference between the calorimeter bomb and jacket, the temperature of the outgoing gas, and the temperature of the contents of the calorimeter. Each of these quantities was measured at approximately 30second intervals. During this period measurements of the voltage impressed upon the calorimeter heater and the current flowing through it were also taken a t intervals of approximately one minute. The electrical energy was added for a period of approximately 20 minutes, which corresponded to the removal of from 0.022 to 0.044 pound (10 to 20 grams) of material from the calorimeter. The heater current was then shut off, and readings of the temperature difference and the temperatures were continued until static equilibrium had been reestablished. The motor-operated valve was closed gradually during this period in such a fashion as to maintain-a nearly constant bomb temperature. The pressure existing within

FIGURE 2 . CALORIMETER BOMB

the bomb was also measured a t periodic intervals. HOWever, it was found that the temperature of the evaporating liquid corresponded within small limits (0.05' F.) with the value predicted from the vapor pressure measurements. This indicated that the evaporation from the liquid was taking place substantially a t equilibrium. However, approximately 0.2" F. difference in temperature was encountered between the exit gas and the evaporating liquid. The experimental results yielded information concerning the electrical energy furnished to the heater and the weight of material removed from the calorimeter. The temperature of the contents of the calorimeter bomb and of the exit gas, as well as the temperature difference between the calorimeter jacket and the exterior of the bomb, were also available as

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functions of time. From these data the latent heat of vaporization could be calculated. The isobaric heat capacity and the isothermal enthalpy-pressure coefficient of gaseous propane required in the evaluation of the small correction terms were taken from published data (IO,14). Similar data for n-pentane were also taken from the literature (4, 14). Information concerning the isobaric heat capacity and the isothermal enthalpy-pressure coefficient of liquid propane HEATSOF VAPORIZATION TABLE I. LATENT -Latent

Temp. F. 103.4 103.7 109.5 109.8 110.0 110.1 114.4 118.1

HeatDeviaExptl. Smoothed tion Temp. 7 - B . t. u./Zb.% O F . A. Provane 1 3 2 . 9 4 133.98 -0.78 ii8.4 130.7 133.16 133.84 - 0 . 5 1 130.9 130.34 130.92 - 0 . 4 4 144.4 132.20 130.76 1.10 144.5 132.20 130.66 1.18 129.58 130.61 - 0 . 7 9 162.6 128.60 128.36 0.11 163.2 128.07 126.37 167.0 1.35 €3.. -

82.3 103.7 104.3 111.5 114.4 120.5 122.4 127.8

156.12 152.58 152.69 151.65 150.51 149.58 148.69 148.73

155.90 152.58 152.49 151.24 150.67 149.55 149.18 148.11

-Latent

HeatDeviation

Exptl. Smoothed -B. t . u./Zb.-

%

128.22 126.21 119.84 119.23 119.26 119.10 110.21 110.80 110.67 110.74 99.56 98.97 99.39 98.17 94.48 95.45

1.59 0.51 0.13 -0.53 0.06 0.60 1.24 -0.81

n-Pentane .. - . . ..__.

0.14 0.0 0.13 0.27 -0.10 0.02 -0.33 0.42

131.0 135.1 149.0 154.3 158.2 180.4 180.5 200.0

147.76 146.35 142.92 142.63 141.21 135.65 136.61 130.38

147.44 146.62 143.50 142.27 141.33 135.79 135.72 130.41

0.22 -0.18 0.40 0.25 -0.09 -0.10 0.66 -0.02

and n-pentane were obtained from earlier investigations (IO,11, IS, l e ) . It is believed that these data are of sufficient accuracy so that they introduce no uncertainties in the resulting values of the latent heat of vaporization greater than 0.03 per cent. Information concerning the specific volume of the saturated liquid and gas is also required in the solution of Equation 1. The specific volume of saturated liquid propane was taken from unpublished direct measurements from the authors' laboratory which are in satisfactory agreement with earlier data (IS). The specific volume of gaseous propane was based upon some recent direct measurements for superheated propane gas made in the same laboratory, which are in agreement with values reported by Beattie and coworkers (1). These data for the superheated gas were used as a basis for determining the specific volume of the saturated gas from the Joule-Thomson coefficients already available (IO). The specific volume of saturated n-pentane liquid was taken from an earlier experimental investigation (12). The specific volume of the saturated gas was based upon the data of Rose-Innes and Young (8) relating to the specific volume of the superheated gas, in conjunction with available JouleThomson measurements (4). It is believed that these volumetric data were accurate enough so that no uncertainties greater than 0.2 per cent were introduced by them in the evaluation of the latent heat from the experimental data.

Results The latent heat of vaporization of propane was determined a t sixteen temperatures between 100" and 170" F. The experimental results are recorded in Table IA, together with smoothed values that were obtained by residual interpolation of the experimental data. The deviation of each of the experimental points from these interpolated values has also been included in Table I. The average deviation of the recorded experimental values for propane was 0.77 per cent. All of the experimental results are included, with the exception of a single measurement during which a small leak developed in the weighing bomb. The experimental results were obtained in part with a calorimeter bomb in which there

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was no internal agitator, but it was impossible to distinguish any systematic variation between the measurements obtained with this equipment and those made with the apparatus described in this paper. The volumetric term indicated in Equation 1 amounts to approximately 20 per cent of the latent heat of vaporization of propane at a temperature of 160" F. This large factor introduces a possible added uncertainty in the data recorded in Table I a t the higher temperatures. However, it is believed that no uncertainties larger than 0.2 per cent have been introduced by this term in the calculation of the latent heat of vaporization from the experimental results even at temperatures as high as 167" F. I n general, the direct measurement of the latent heat of vaporization becomes increasingly difficult as the critical state is approached, and it is therefore not surprising that discrepancies somewhat larger than would be indicated by , the precision of measurement were encountered in the experimental results. Table I1 records interpolated values of the latent heat of vaporization at even temperatures throughout the range covered by the experimental investigation. For comparison, the smoothed results published by Dana and eo-workers (2) are included. The results tabulated are somewhat above the temperature range covered by their investigation, and some added uncertainty probably exists due to this extrapolation. Figure 3 presents the experimental results obtained in this laboratory together with the measurements of Dana et al. I n the temperature region covered by the experiments of the latter investigators, the curve was drawn through their smoothed points. There is no appreciable inconsistency between the two sets of experimental measurements. TABLE11. COMPARISON OF LATENTHEATOF VAPORIZATION RESULTS FOR PROPANE Exptl., Dana. Equation 2, Exptl., Equation 2, et a1.a Authors Temp. Authors Author6 OF. E . t. u./lb.a F. -B. t. U./lb.60 153.0 152.6 130 119.7 119.4 70 , .. 149.5 148.7 140 113.6 112.8 80 146.0 144.6 150 107.2 105.6 90 142.5 140.2 160 100.4 100 135:6 138.5 135.6 170 93.3 110 130.7 134.0 130.6 180 85.8 120 125.3 129.0 125.3 0 The highest temperature at which experimental measurements were made was 67' F. Temp.

Exptl., Authors

--... ...

Published data (IO, 11, 1.4) relating to the isobaric heat capacity and the isothermal enthalpy-pressure coefficient of saturated liquid and gaseous propane permit the calculation of the change in the latent heat of vaporization with temperature by means of the following equation: L=LA+

Table I1 includes values of the latent heat of vaporization based upon Equation 2 and the experimentally determined value of the latent heat a t 100" F., which was taken as the reference value, LA. The agreement between the experimental results and those calculated from Equation 2 is satisfactory when it is considered that they are based primarily upon different types of experimental measurements. The latent heat of vaporization of n-pentane was determined experimentally a t fourteen temperatures between 80" and 200" F. The individual experimental results are recorded in Table IB, together with smoothed values obtained by residual interpolation. The deviations of the experimental values from the smoothed data are also included. These measurements are somewhat more consistent than were

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the values for propane, the average deviation of the experimental values from the smoothed data being only 0.22 per cent. This improved accuracy i s to be expected since n-pentane was farther below its critical temperature under these conditions than was propane, and therefore the heat of vaporization was larger and the correction terms were much smaller. Smoothed values of the latent heat of vaporization of n-pentane at even temperatures are recorded in Table 111. For comparison, values of the latent heat of vaporization calculated by Young (16) from his volumetric data are included. The latter results were interpolated by residual methods to the even temperatures recorded in Table 111. The agreement between the experimental results and the values reported by Young is considered satisfactory, although the discrepancies a t some temperatures are larger than the estimated experimental uncertainty in the present measurements.

TEMPERATURE

RESULTS FOR ?I,-PENTANE

90

100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250

Exptl. Author's

,ti:,

154.7 153.2 151.5 149.7 147.7 145.6 143.3 140.9 138.4 135.9 133.2 130.4

... ...

Exptl., Clapeyron Griffiths Equation, and Awbery Young B. t . u . / l b . 158.3 ... 156.0 153,6 li5:8 151.2 154.0 152.2 148.8 ... 150.2 ... 148.2 146.2 144.0 141.7 ... 139.4 ... 137.0 ... 134.5 132.0 ... ... 129.3 126.6 124.0 120.9 118.0 115.0

... .... ..

'5

HEATOF VAPORIZATION FOR PROPANE Ii'IGURE 3. LATENT

TABLE 111. COMPARIBON OF LATENTHEATOF VAPORIZATION Temp. F. 60 70 80

767

Equation 2, Authors

161.3 159.3 157.3 155.3 153.2 151.1 148.9 146.6 144.3 141.8 139.3 138.7 134.1 131.3 128.4 125.4 122.3 119.1 115.8 112.4

possibly to entrainment of small particles of unvaporized liquid in the gas stream from the calorimeter. It is believed, however, that the values for the latent heat sf vaporization of propane recorded in Table I1 do not involve any absolute uncertainty greater than 1.5 per cent. The values for npentane reported in Table I11 are somewhat more accurate and probably do not involve a n uncertainty greater than 1 per cent. The errors of measurement of the primary quantities and of evaluation of the correction terms taken together were approximately 0.4 per cent in each case.

Acknowledgment This work was done as a part of a general program of research relating to the thermodynamic behavior of hydrocarbon mixtures, which is being carried out by Research Project 37 of the American Petroleum Institute. The financial assistance and cooperation of the institute are appreciated. Lee T. Carmichael was of great assistance in connection with the construction and operation of the calorimeter.

Nomenclature Griffiths and Awbery determined experimentally the latent heat of vaporization of a commercial pentane, consisting primarily of n-pentane, a t temperatures from -4" to 86" F. A part of the results of their experimental work is included in Table 111. I n general, they obtained values of the latent heat of vaporization which were approximately 6 B. t. u. per pound less than the values based either upon Young's volumetric data or upon the experimental measurements reported from this investigation. Within the accuracy of the Griffiths and Awbery measurements, they found a linear variation in the latent heat of vaporization with temperature. The isobaric heat capacity and the isothermal enthalpypressure coefficient for saturated liquid and gaseous n-pentane (4,8, 1 2 , I d ) were used in conjunction with Equation 2 to evaluate the change in the latent heat of vaporization with temperature. The experimental result a t 100" F. was taken as a reference point in calculating the values based upon Equation 2 which are recorded in Table 111. The agreement of these values with those measured directly is considered satisfactory. It is difficult to estimate with accuracy the absolute uncertainty involved in the experimental results reported in this paper. The precision of the directly measured quantities was distinctly better than the consistency of the final experimental results. This difference is probably due to traces of condensation in the packing of the motor-operated valve and

L Vd

= heat of va orization, B. t. u./lb. = specific voyume of saturated gas. cu. ft./lb.

specific volume of saturated liquid, cu. ft./lb.

v b

=

&.

= electrical energy added to calorimeter, B. t. u.

Q

=

= = P = Pi' = H = CP, = CP, = Am

T

energy lost from calorimeter due t o temperature differences between it and the surroundings, B. t. u. change in weight of contents of calorimeter, lb. absolute temperature (thermodynamic scale), " F. abs. pressure, lb./sq. in. abs. vapor pressur e, lb./sq. in. abs. enthalpy, B. t. u./lb. isobaric heat capacity of saturated gas, B. t. u. /lb./O F. isobaric heat capacity of saturated liquid, B. t. u. / lb./" F.

Literature Cited (1) Beattie, Kay, and Kaminsky, J. Am. Chem. Soc., 59, 1589 (1937). (2) Dana, Jenkins, Burdick, and Timm, Refrig. Eng., 12,387 (1926). (3) Griffiths and Awbery, Proc. P h y s . SOC.(London), 44,121 (1932). (4) Kennedy, Sage, and Lacey, IND. ENG.CHEM.,28,718(1936). (5) Osborne, Bur. Standards J. Research, 4, 609 (1930). (6) Osborne, Stimson, and Fiock, Ibid., 5, 411 (1930). (7) Osborne and Van Dusen, U. S. Bur. Standards, Sci. Paper, 315 (1919). (8) Rose-Innes and Young, Phil. Mag., [ 5 ]47, 353 (1899). (9) Sage, Backus, and Vermeulen, IND. ENQ.CHEM.,28,489 (1936). (10) Sage, Kennedy, and Lacey, Ibid., 28,601 (1936). (11) Sage and Lacey, Ibid., 27, 1484 (1935). (12) Sage, Lacey, and Schaafsma, Ibid., 27, 48 (1935). (13) Sage, Schaafsma, and Lacey, Ibid., 26, 1218 (1934). (14) Sage, Webster, and Lacey, Ibid., 29, 1309 (1937). (15) Young, Sci. Proc. Royal Dublin SOC.,12,374 (1910).