I
I. HAROLD SILBERBERG,' KENNETH A. KOBE, and JOHN J. McKETTA University of Texas, Austin, Tex.
Determine Compressibility Factors
by the Burnett Method IN
THE Burnett method for precise P-V-T measurements only pressure and temperature are measured. Neither volume nor mass measurements are necessary. Compressibility factors are determined directly from the experimental pressures by either graphical or analytical means. It is not necessary that composition of the experimental gas be known accurately; in fact, it is not even necessary to know the identity of experimental gas. Experimental equipment, methods of operation, and analysis of data have been discussed. In graphical evaluation of the data, pTNp is plotted against pressure, p,, and the zero-pressure ordinate p0/q is obtained by extrapolation. The compressibility factor, z, at pressure p, may be calculated from the following equation.
PJ'
(Pa/zekr (1) Shape of the p,Nr isotherm for each run is similar to the shape of the compressibility factor isotherm. If the latter
parison of these data with those of previous investigators shows the high accuracy which may be obtained with this method.
I I THE COMPLETE ARTICLES on equipment and methods of determining compressibility factors are available in October 1959 issue of the Journal of Chemical Engineering Data, beginning on page 31 4
Nomenclature
p
= absolute pressure
N z r
= = =
apparatus constant compressibility factor, pV/RT number of expansions in Burnett equipment 0 = initial state of a run RECEIVED for review July 30, 1959 ACCEPTED August 4, 1959
I
=
approaches linearity at low pressures, which is generally the case, extrapolation can be made with great accuracy. As any error in p,/zO is introduced directly into each zr, accurate low pressure measurements are essential unless the isotherm exhibits linearity to relatively high pressures. Because N is raised to the power r in Equation 1, the effect of errors in N is considerably magnified. As a first approximation, the error in p,Nr from an error of N is r times that error. Accurate knowledge of the equipment constant N is the key to accurate compressibility determinations with this method. The analytical treatment of data (7) is the most complete treatment of methods for obtaining compressibility factors and virial coefficients by the Burnett method.
Figure 1. A graphicalmethod is used to obtain po/zo
PRESSURE,
e,
ATM.
Compressibility of lsopentane
The P-V- T relationships of isopentane have been studied with the Burnett method at high pressures ( 2 ) . Com1
Present address, Magnolia Petroleum
Co., Field Research Laboratories, Dallas, Tex.
Figure 2. Experimental compressibility factor isotherms of isopentane. At lower pressures, these isotherms are consistent with respect to temperature within 0.03% VOL. 51, NO. 10
OCTOBER 1959
1325
