PVT and Vapor Pressure Measurements on Ethane

New measurements of the vapor pressures and PVT properties of ethane are reported. PVT determinations have been made from near the triple point to 320 K at pressures to 33 MPa. The density range investigated extends to more than three times the critical density. The new measurements of the vapor pressures of ethane extend from 160 K to near the critical point.


Introduction
Liqu efied fuel gases, suc h as LNG, are expected to play an increasi n g role in sati sfying future energy requirements. Accurate th ermophysi cal prope rties data for these liquefi ed gas mixtures are necessary for the design of liqu efaction plants, transport equipment, shipping and rec eiving terminals, and for c us tody transfer. Th e near infinite variations in mixture compositions e nco untered with these fuel gases rule out completely experime ntal or s tri ctly co mputational approaches for de termining these properties. Calculation methods, based on accurate, wide range pure compon e nt data and selected mixtures data are bein g developed in a number of laboratories, and appear to offer the only reliable and economical approach for the gen eration of the necessary thermophysical properties.
This paper reports ne w measurements of vapor pressures and PVT properties of pure ethane. The measurements have been made as part of a comprehensive program to provide the required experimental data and to develop suitable calculation techniques for mixture prope rties determinations. PVT measurements have been made from near the triple point (90.348 K) [IF to 320 K at press ures up to 33 MPa. The density range extends to more than three time s the critical density. The new m eas ure ments of the vapor pressures extend from 160 K to near th e criti cal temperature (305 K).

Experimental Detail
To meas ure single-phase de nsities, th e gas expansion technique was us ed . A series of pressure-temperature observations are made on a nearly constant den sity sa mple of fluid confin ed in a cell of accurately calibrated volum e . When either the maximum press ure or maximum te mperature is reached, the fluid is ex· panded, to low press ure, into large calibrated volumes maintain ed at an accurately known te mperature above room te mperature. The d ensity can then be determin ed from th e cell volume and the compressibility factor (PV/RT) of the e thane at th e co nditions of the expansion volumes.
The e thane used was commercially available re-  [2][3][4][5] and have been describ ed in d etail [6][7][8]. Slight modification to existing apparatus was necessary because of the higher c riti cal temperature of ethan e. Those external parts of the system which contain ed fluid during a measureme nt were heated to well above the critical temperature (typi cally 330 K) in order to r educe the relative de n sity of the fluid residing in these parts, permitting a more accurate adjusted de nsity to be computed.    Although comparison with data from other sources is, in general, impossible without multiple interpolations , the agreement has been deduced by examining the density deviations of the various data sets [9,10] from densities calculated from an equation of state for ethane due to Goodwin [ll]. The agreement is found to be, in general, within the combined experimental error. Maximum difference occur in the critical region where the equation of state representation is expected to be less satisfactory and where the experimental densities are subject to increasing uncertainty. Estimated uncertainty in the experimental densities in this work is typically ± 0.1 percent at the lowest temperatures, increasing to ± 0.2 percent at higher temperatures and lower densities, becoming as muc~l as ± 1.0 percent in the critical region. New vapor pressure measurements also have been made at 5 K intervals from 160 to 300 K and are given in table 2. At each temperature, the pressure was measured at least twice with some ethane being removed from the cell between measurements. Identical pressure observations indicated that the two-phase ~ondition existed in the cell.
A vapor pressure equation of the form was fit to all available data for ethane [12]. Here, X = (1 -TdT) / (1-TdTc), and P and T are the pressure and temperature and t and c refer to the triple and critical points. Coefficients giving the best fit were found to be the following:

. Summary
We have made new wide-range measurements of th e vapor pressures and PVT properties of ethane. Th ese are th e only data currently available which cover the e ntire te mperature range from the triple point to 320 K. In addition, these data are the only accurate PVT data available for the compressed liquid below about 190 K. The data are being used along with other available data to r efin e the calc ulation of thermodyn a mi c functions for e thane and as input to, and as a c hec k upon, ne w calc ulation me thods for predi cting lique fi ed natural (fu el) gas properties bein g s tudied in thi s and other laboratories .