JIN

^ecknical rlote

PB 151363

No. 4

THE VAPOR PRESSURES OF

SOME HYDROCARBONS IN THE

LIQUID AND SOLID STATE

AT LOW TEMPERATURES

by W. T Ziegler

s&

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

NATIONAL BUREAU OF STANDARDS

technical ^Lote

May 1959

THE VAPOR PRESSURES OF SOME HYDROCARBONS

IN THE LIQUID AND SOLID STATE AT LOW TEMPERATURES

by

W. T. Ziegler

NBS Technical Notes are designed to supplement the Bureau's regular publications program. They provide a means for making available scientific data that are of transient or limited interest. Technical Notes may be

listed or referred to in the open literature. They are for sale by the Office of Technical Services, U. S. Department of Commerce, Washington 25, D. C. Contents were formerly

NBS Report 6038, issued Feb. 13, 1959

DISTRIBUTED BY

UNITED STATES DEPARTMENT OF COMMERCE

OFFICE OF TECHNICAL SERVICES

WASHINGTON 25, D. C.

Price $ .75

-1-

THE VAPOR PRESSURES OF SOME HYDROCARBONS

IN THE LIQUID AND SOLID STATE AT LOW TEMPERATURES

by

W. T. Ziegler*

School of Chemical Engineering

Georgia Institute of Technology

SUMMARY

This paper presents a brief review of the literature pertaining

to the vapor pressure of eleven hydrocarbons at low temperatures.

These hydrocarbons are methane, ethane, propane, n-butane, isobutane (2-methyl propane), n-pentane, ethylene, propylene, 1-butene,

acetylene and cyclopropane. All of these except cyclopropane are

encountered in the purification of hydrogen from refinery sources.

Vapor pressure data are given for most of these hydrocarbons over

the pressure range 0. 001 to 1500 mm Hg. These data have been used

to estimate the triple-point pressures of the hydrocarbons where

these pressures were not found in the literature. Some data for the

vapor pressures of solid methane, ethane, n-butane, ethylene,

acetylene and cyclopropane are included.

It is recommended that vapor-pressure equations of Rossini

et al. ^ for ethane, propane, n-butane, isobutane, n-pentane, ethylene,

propylene and 1-butene be used for the range 1500 to 10 mm Hg, and

that the data of Tickner and Lossing 4 be used for all hydrocarbons

except methane for the range 10 to 0. 001 mm Hg. For liquid and

solid methane the equations of Armstrong, Brickwedde and Scott *®

appear to be the most accurate. However, they differ only very

slightly from the equations developed by Rossini et al. ^ For solid

ethylene below about 0. 03 mm Hg the vapor- pressure equation of

Lister and McDonald ? appears to be the most accurate.

* Consultant, NBS Cryogenic Engineering Laboratory, Summer 1958.
- 2 - INTRODUCTION

A knowledge of the vapor pressure of hydrocarbons is often

needed in the analysis of low- temperature processes such as the purification of hydrogen and the design of apparatus such as low-temperature

distillation or scrubbing equipment, cryogenic baths, etc. It is the

purpose of the present paper to collect in one place for ready reference

the vapor pressures of a number of hydrocarbons often encountered in

such studies. The eleven hydrocarbons included in this survey are

methane, ethane, propane, n-butane, isobutane (2-methyl propane),

n-pentane, ethylene, propylene, 1-butene, acetylene and cyclopropane.

In general the range covered is from 1500 mm down to approximately

0. 001 mm Hg. For the most part the data apply to the liquid range.

Some data for the vapor pressure of solid methane, ethane, n-butane,

cyclopropane, ethylene and acetylene are included. The data have been

used to estimate the triple-point pressures where this quantity was not

found in the literature.

The present survey is not intended to be a critical analysis of all

of the available data. However, it is believed that most of the available data have been examined. Principal reliance has been placed on

the extensive compilations of the National Bureau of Standards and the

data of Project 44 of the American Petroleum Institute ' for the

pressure range 10 to 1500 mm Hg, and the measurements of Tickner

and Los sing * for the range 10 to 0. 001 mm Hg. A number of other

papers dealing with the vapor pressure of these hydrocarbons have

^ D *7 been examined. Several of these papers ' ' emphasize the importance of taking the Knudsen thermo-molecular pressure effect into

account at pressures below about 0. 01 mm Hg if precise experimental

values of vapor pressure are desired. These papers will be discussed

in their appropriate place.

VAPOR PRESSURE OF HYDROCARBONS FROM 10 TO 1500 mm Hg

2 Rossini et al. have represented the vapor-pressure data for

many hydrocarbons over this pressure range by means of the Antoine

equation:

log

iop A - 77c (1)

where A, B, and C are constants for a given hydrocarbon, P is the

vapor pressure in mm Hg, and t is the temperature in °C.
- 3 -

If the temperature, T, is expressed in °K, equation (1) maybe written in the form

B

logs1rt P = A - r

10 T + C

1 (2)

The Antoine constants, as determined by Rossini, et al. , for nine of the hydrocarbons covered in this paper are given in

Table I. No vapor-pressure data are given by Rossini for acetylene

and cyclopropane.

TABLE I

Temp, at

which vapor

Hydro- pressure is

C

1 carbon Formula

CH4

State

Solid*

10 mmHg,°C

-195.51

A

7.69540

B C

Methane 532,20 275.00 1.84

Liquid 6.61184 389.93 266.00 -7.16

Ethane C2H6 Liquid -142.88 6.80266 656.40 256.00 -17.16

Propane C 3H8 Liquid -108.51 6.82973 813.20 248.00 -25.16

n- Butane C4H10 Liquid -77.76 6.83029 945. 90 240.00 -33.16

Isobutane C4H10 Liquid -86.42 6.74808 882.80 240.00 -33.16

n-Pentane C5H12 Liquid -50.1 6.85221 1064.63 232.00 -41.16

Ethylene C2H4 Liquid -153.22 6.74756 585.00 255.00 -18.16

Propylene C 3H6 Liquid -112.11 6.81960 785.00 247.00 -26.16

1-Butene C4*% Liquid -81.5 6.84290 926. 10 240.00 -33.16

* At the triple point -182.48°C, the pressure is 87.7 mm Hg.

Rossini et al. have used these equations to compute and tabulate

the temperatures corresponding to 30 pressures over the range 10 to

1500 mm Hg. The results of these calculations are shown in Table II.
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- 5 -

VAPOR PRESSURE OF HYDROCARBONS FROM 0. 001 TO 10 mm Hg

O A Tickner and Lossing ' have measured the vapor pressure of

eleven hydrocarbons and CO? as a function of temperature down to

a pressure of 0. 001 mm Hg using a mass spectrometer. These

investigators have presented their smoothed data as temperatures

for pressures of 0.001, 0.002, 0.005, 0.01, 0.02, 0.05, 0.1, 0.2,

0. 5, 1. 0, 2.0, 5.0 and 10. mm Hg. Their results for the hydrocarbons are presented in Table III.
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- 7 -

The vapor-pressure data reported by Tickner and Lossing

agree quite well with the tabulations of Rossini et al. in the region

of overlap (10 to 80 mm Hg, as judged from their plotted data).

In their experiments Tickner and Lossing used a 1. 6 cm

diameter tube connecting the low-temperature equilibrium chamber

with the mass spectrometer. They recognized the existence of the

Knudsen thermo-molecular pressure effect for pressure below about

0. 01 mm Hg but did not take it into account in their experiments.

Liang, using a technique which he had previously employed with

solid Kr and Xe, remeasured the vapor pressures of methane,

ethylene and acetylene at 77. 5° and 90. 0°K taking into account thermal

transpiration. From these measurements he concluded that small

corrections should be applied to the data of Tickner and Lossing

for these gases at the lowest temperatures but that for the. other hydrocarbons the corrections were probably smaller than the uncertainty

in the reported temperatures (+0. 3°C at or above -196°C).

7 The more recent measurements by Lister and McDonald of the

vapor pressure of solid ethylene, in which allowance was made for

thermal transpiration, agree to within 2% with the measurements of

Tickner and Lossing down to about 85°K (about 0. 005 mm). Below

85°K the values of Tickner and Lossing for ethylene are progressively

higher than those reported by Lister and McDonald, being about 10%

higher at 80°K.

o Delaplace has also reported measurements of the vapor

pressures of ethane, propane, isobutane, n-butane, ethylene,

propylene, isobutylene, 1-butene and acetylene down to about -196 C.

Unfortunately, no units are given in this paper for the tabulated pressures. If one assumes with Lister and McDonald that the units used

by Delaplace are baryes, then one finds approximate agreement with

the data of Tickner and Lossing for ethane, propylene and acetylene

and large discrepancies for the other hydrocarbons.

Almost no other data exist with which the data of Tickner and

Lossing may be compared. Such limited data as do exist will be discussed in the final section of this paper where additional comments

are made for the individual hydrocarbons.

The author is of the opinion that the data of Tickner and Lossing

can be used with considerable confidence down to about 0. 01 mm Hg.

Below this pressure level the pressures reported by them may be somewhat high because of neglect of thermal transpiration.
- 8 -

ESTIMATION OF TRIPLE -POINT PRESSURES

The triple-point pressures of only four of the hydrocarbons

discussed in this paper appear to have been measured. ^» ' The data

of Tickner and Lossing given in Table III have been used to estimate

the triple -point pressure either by interpolation or extrapolation of

the data using an empirical equation of the form

log

1() P = a -| (3)

where the pressure is expressed in mm Hg and the temperature is in

°K. The constants a and b in the appropriate equation were obtained

using data points taken from Table III immediately preceding the

triple point.

The triple-point temperatures and pressures are summarized

in Table IV. The calculations are discussed in more detail under

the individual hydrocarbons in the next section. It is recognized

that the estimated triple -point pressures of propane, propylene and

1-butene may be considerably in error.
- 9

-

TABLE IV

Triple-Point Temperatures and Pressures

Triple Point Triple Point

Temperature Pressure

Hydrocarbon °C °K mm Hg Referenc:e

Methane -182.48 90.68

90.66

87.7

87.50 + 0. 10

2

10

Ethane -183.27 89.89 6 x 10" 3

9

6.94 x 10" 3 Calc. Eq. (3)

Propane -187.69 85.47 4.07 x 10" 6 Calc. Eq. (8)

n- Butane -138.35 134.81 4.05 x 10" 3 Calc. Eq. (9)

Isobutane -159.60 113.56 1.52 x 10" 4 Calc. Eq. (ID

n-Pentane -129.72 143.44 4.66 x 10~ 4 Calc. Eq. (12)

Ethylene -169.15 104.01 0.9 9

0.854 Calc. Eq. (1)

0.729 Calc. Eq. (3)

Propylene -185.25 87.91 1.14x 10" 5 Calc. Eq. (14)

1-Butene -185.35 87.81 1.0 x 10~ 8 Calc. Eq. (15)

Acetylene -81.5 191.66 900 9

Cyclopropane -127.53 145.63 0.454 Calc. Eq. (16)
- 10 - COMMENTS ON VAPOR PRESSURE RELATIONS

FOR INDIVIDUAL HYDROCARBONS*

It is recommended that the vapor-pressure equations of Rossini

et al. ^ for ethane, propane, n-butane, isobutane, n-pentane, ethylene,

propylene and 1-butene be used for the range 10 mm to 1500 mm Hg,

and that the data of Tickner and Los sing be used for all hydrocarbons

except methane for the range 10 to 0.001 mm Hg. For liquid and

solid methane the equations of Armstrong et al. 1° appear to be the

most accurate. However, they differ only slightly from the equations

developed by Rossini et al. 2 For solid ethylene below about 0. 03 mm

Hg the vapor-pressure equation of Lister and McDonald ' appears to

be the most accurate. Sondak and Thodos

^

have represented the

vapor pressure of many saturated aliphatic hydrocarbons from "the

triple to the critical point" by a four-constant equation. They appear

not to have considered the work of Tickner and Lossing. * The

triple-point pressures of ethane and propane estimated from these

equations are approximately 20% greater than the values obtained

from the data of Tickner and Lossing.

Methane

Armstrong, Brickwedde and Scott have presented a review of

the vapor-pressure data available for both liquid and solid methane.

This paper should be consulted for a bibliography and critical discussion.

This paper includes extensive experimental data for the deuteromethanes as well as for methane, including careful measurements of

the triple-point temperatures and pressures.

10 Liquid. Armstrong et al. have represented the vapor pressure

of liquid methane above the triple point by two equations

log

10

P = 6.61184 -m^

b

(4)

and

5Q5 ^46

log10 P = 10. 68631 - y — - 0. 0348066 T °10 T

+ 1. 3338 x 10' 4 T

2

- 1.7869 x 10" 7 T

3

(5)

* All temperatures referred to in this section are in K and

pressures in mm Hg.
- 11 -

They recommend that equation (4), which is identical with that proposed by Rossini et al. ^ (see Table I), be used from the triple point

to 760 mm and that equation (5) be used for the remainder of the

liquid region to the critical point. Armstrong et al. have tabulated

the vapor pressure at one-degree intervals over the liquid range.

Solid. Armstrong et al. have adjusted the constants of the

empirical equation of Rossini et al. 2 (see Table I) very slightly to

give a fit with their choice of the triple-point temperature and pressure (90. 66 + 0. Q1°K, 87. 50 + 0. 10 mm Hg). The revised equation

is:

log6 10 P = 7.69540 --155i|SL (6) 1° T +1.842

This equation is based on experimental data of Brickwedde and

Scott down to 54°K. The equation is in excellent agreement with

the data of Tickner and Lossing 4 down to 0. 01 mm Hg. However 9 at 0.001 this equation gives a pressure about 6% higher than that

found by Tickner and Los sing.

Armstrong et al. have tabulated the vapor pressure of solid

methane at one-degree intervals down to 51°K.

Ethane

Solid. The triple-point pressure, calculated from equation (3),

and temperature given in Table IV, together with the experimental

data at 0. 001 mm Hg in Table III, have been used to obtain the following equation for the vapor pressure of solid ethane below the triplepoint temperature:

log10

P = 8.5812 ,965^40 (?)

This equation has been used to estimate the vapor pressures given

in Table V.
- 12 -

TABLE V

mperature Vapor Pressure, mm Hg

°K Obs. Calc.

Ref.4 Eq.(7)

89.89 a 6.94x 10" 3

88.76 5 x 10" 3 5.9 x 10" 3

85.46 2 x 10" 3 1.93x 10 -3

83.36 1 x 10" 3 1.00 x 10" 3

80 3.25 x 10" 4

77.4 1.28 x 10~4

70 6.16 x 10' 6

3

Triple point

Propane

Liquid. The vapor-pressure data of Tickner and Lossing

(Table III) for liquid propane at 0. 1 and 0. 001 mm Hg have been used

to obtain the empirical equation

log

1Q

P = 8.l6l73- 117

t

6 - (8)

For T = 115. 76°K this equation gives P = 0. 01006 mm Hg which maybe compared with the measured pressure of 0. 01 mm Hg. This

equation has been used to estimate the triple-point pressure given

in Table IV.
- 13 -

n- Butane

Liquid. The vapor-pressure data of Tickner and Los sing

(Table III) at 0. 5 and 0. 005 mm Hg have been used to obtain the empirical equation

log

10

P = 8. 47062 - l464

/

52

(9)

This equation has been used to obtain the triple-point pressure,

4. 05 x 10" 3 mm Hg, given in Table IV.

Solid. The triple-point pressure and temperature given in

Table IV, together with the experimental data at 0. 001 mm Hg in

Table III, have been used to obtain the empirical equation

log

10

P = 8.4460 - 146

T

1<2

(10)

Equation (10) has been used to estimate the vapor pressure of solid

n-butane at lower temperatures as shown in Table VI.

TABLE VI

Temperature Vapor Pressure, mm Hg

°K Obs. Calc.

Ref. 4 Eq. (10)

134. 81 a 4.05 x 10~ 3

127.66 1 x 10" 3

1 x 10" 3

120 1.86 x 10~ 4

110 1.45 x 10" 5

100 6.8 x 10" 7

Triple point
Isobutane

Liquid. The data of Tickner and Los sing given in Table III at

0. 01 and 0. 001 mm Hg have been used to obtain the empirical

equation

log,«P = 8.48776 - 1397, 6

& (11)

10 T

This equation has been used to estimate the triple-point

are, 1. 52 x I0~ 4 mm Hg given in Table IV.

Solid. No data for the vapor pressure of solid isobutane were

found.

n-Pentane

Liquid. The data of Tickner and Los sing given in Table III at

0. 02 and 0. 001 mm Hg have been used to obtain the empirical

equation

log

1() P = 8. 84223 - IZ^lii (12)

This equation has been used to estimate the triple-point pressure

given in Table IV.

Solid. No data for the vapor pressure of solid n-pentane were

found.

Ethylene

Liquid. It is felt that the empirical relation of Rossini et al.

given in Table I adequately represents the vapor -pressure data down

to the triple point. A short extrapolation of this equation down to the

triple point of 104. 01°K gave a triple-point pressure of 0.854 mm

Hg (see Table IV).

The data of Tickner and Los sing given in Table III for 10 and

1 mm Hg were used to evaluate the constants of equation (3). Extrapolation to the triple-point temperature gave a triple-point pressure of

0.729 mm Hg.
- 15 -

Solid. Lister and McDonald have measured the vapor pressure

of solid ethylene over the temperature range 77. 52° to 90. 09°K corresponding to a pressure range of 0. 432 x 10" -* to 27.8 x 10" ^ mm Hg.

They represented their data by the equation*

logl „ P x 10

3 = 1. 5477 - 1038. 1 (- - 0. 0110) °10 T

+ 16537 (I - 0.0110) 2

J. (13)

These investigators made a careful study of the thermal transpiration effects and corrected their measurements accordingly.

Comparison of their work with the measurements of Tickner and

Los sing''* showed that the two sets of measurements agree to within

2% above 85°K (P > 6 x 10" 3 mm Hg). Below this temperature the

disagreement becomes progressively larger reaching about 10% at

80°K, the values of Tickner and Los sing being higher, presumably

because they did not correct their measurements for thermal transpiration.

The measurements of Liang at 77. 5°K and 90. 0°K differ

significantly from those of Lister and McDonald and Tickner and

Lossing as may be seen from Table VII taken from the paper of

Lister and McDonald. ' TABLE VII

Temperature

°K

Vapor Pressure

mm Hg x 10-*

Lister and

McDonald

Tickner and

Lossing

Liang

77.52 0.431 0.53

90.09 27.87 26.9

90.0 30.0

The equation given by Lister and McDonald appears to contain

two typographical errors.
- 16 -

Propylene

Liquid. The data of Tickner and Los sing given in Table III for

0. 1 and 0. 001 mm Hg were used to obtain the empirical relation

log

10

P = 8. 13809 - 115

^

12

(14)

This equation has been used to estimate the triple-point pressure of

1. 14 x 10~5 mm Hg given in Table IV.

Solid. No data for the vapor pressure of solid propylene were

found.

1-Butene

Liquid. The data of Tickner and Los sing given in Table III for

0. 1 and 0. 001 mm Hg were used to obtain the empirical equation

log10 P = 8. 49889 - 1447 - 25

(15)

This equation has been used to estimate the triple-point pressure of

1. x 10-8 mm Hg given in Table IV.

Solid. No data for the vapor pressure of solid 1-butene were

found.

Acetylene

Solid. Tickner and Lossing compared their measurements

with those of Bourbo ** who has measured the vapor pressure of

solid acetylene over the range 89.65° to 169. 5°K. Unfortunately,

the papers of Bourbo were not available. However, Tickner and

Lossing stated that their vapor-pressure values were in good agreement with the values obtained by Bourbo, but were slightly lower

for a given temperature.
- 17 -

Cyclopropane

Liquid. No search was made for the vapor pressure of liquid

cyclopropane. The data of Tickner and Los sing given in Table HI

for 10. and 0. 5 mm Hg was used to obtain the empirical equation

log

1() P = 8. 15832 - 123

T

8 ' l (16)

This equation has been used to estimate the triple-point pressure

of 0. 454 mm Hg given in Table IV.

Solid. The data of Tickner and Los sing given in Table IV may

be used to obtain the vapor pressure of the solid down to approximately 115. 5°K where the vapor pressure is 0. 001 mm Hg. No other

vapor-pressure data for solid cyclopropane were found.
- 18 - REFERENCES

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148

(1950).

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ii n"l._. )..'-,)

I
NBS