1. Six correlations were developed, each presented in the form of four nomographic charts, which provide a rapid method for the calculation of the pressure and temperature at the cricondenbar, the cricondentherm, and the critical point of any multicomponent paraffinic hydrocarbon mixture to within an average accuracy of 1.19%.
2. These three key points define the phase boundaries in the critical region, which are the most difficult to predict. The remainder of the border curve at lower pressures can be calculated from equilibrium constants ("K" values) derived from the modified solution laws using fugacities, Thus, these six correlations permit the rapid calculation of the complete phase diagram of any multicomponent paraffinic hydrocarbon mixture.
3. The basis of the critical pressure correlation, which is very similar to the other five correlations, is a rectilinear plot of Pc, the critical pressure of the binary mixtures of methane, versus Mave., the average molecular weight of these mixtures on a weight basis. On this diagram, lines of constant weight fraction of methane (WCH4) in the various systems were straight and intersected at a common point with the highly significant coordinates of (16, 673.1), the molecular weight and critical pressure of methane. Similar diagrams were obtained with every other member of the paraffin hydro¬carbon series for which data were available.
4. Equations were developed to represent each of these straight line diagrams for all six correlations. Their form can be illustrated by the equation for the critical pressure (psia) diagram of the binary mixtures of methane:
Pc = 673.1 + (137 wCH4 1.073 − 3,07)(Mave. − 16.04)
When wCH4 & $61; 0, this equation reduces to that for the line connecting the critical pressures of the pure paraffin hydrocarbons, in pounds per square inch absolute. This line also represents the pseudo-critical pressure, on a weight basis, of any mixture, P pew , which is defined by the equation,
Ppcw = ∑wiPci
where, wi = weight fraction of any component "i"
Pci = critical pressure of any component "i"
5. The method for the calculation of the critical pressure of multicomponent paraffinic hydrocarbon mixtures, also analogous to the other five correlations, consists of summing up the increases in the critical pressure of the multicomponent mixture, Pcd , above that predicted on an additive basis, caused by each component of the mixture except the heaviest. The sum of these increases is then added to the pseudo-critical pressure of the mixture to give the desired critical pressure of the multicomponent system. The equations for the increase (more precisely, the difference) in the critical pressure of a multicomponent mixture, caused by each light hydrocarbon present, are derived by subtracting the 'zero' line equations from the corresponding general equations for the binary mixtures of each light component. The form of these equations is typified by the equation for the increase in the critical pressure, PcdCH4, of any multicomponent mixture above Its value of Ppcw , caused by methane present, which is:
PcdCH4 = 137 wCH4 1.073 (Mave. − 16.04)
6. For simplicity and ease of calculation, a nomograph was designed for the latter equation along with three others for the increase in critical pressure caused by ethane, propane, and n-butane. These four nomographs are used for the prediction of the critical pressure of the vast majority of multicomponent paraffinic hydrocarbon mixtures. Five other sets of four nomographic charts were made in similar fashion for the rapid calculation of the oricondenbar and cricondentherm pressures, and the cricondenbar, cricondentherm, and critical temperatures. The relatively few multicomponent mixtures which can not be handled by these nomographs are those with the heavier liquid components, which are also devoid of methane, ethane, propane, and n-butane,
7. For these latter uncommon multicomponent mixtures, general equations were derived for the critical region values of all multicomponent mixtures of paraffinic hydrocarbons, which are liquid at ordinary room conditions of temperature and pressure, without experimental data. Along with the equations developed for the gaseous members of the series, these equations obviate the present need for painstaking, laborious experimental determination of phase relations. Furthermore, physical limitations often render experimental measurement of critical region data impossible, such as in the case of compounds which decompose at temperatures below the critical point. Also, mixtures of compounds heavier than n-decane have critical temperatures which are too high, while those of methane mixtures above 0.5 weight fraction of methane are too low, to be determined by the ordinary laboratory procedures.
8. The oomplete border curves of 11 multicomponent hydrocarbon mixtures of known composition were experiment¬ally determined. Each mixture was composed of from two to six components, chosen from the first six straight-chain members of the paraffin hydrocarbon series (methane through n-hexane).
9. The correlation of the Bureau of Mines, Benedict's critical pressure equation, and the Kellogg fugacity charts were extensively tested with the experimental data of the 11 multicomponent hydrocarbon mixtures of this investigation. None of these correlations satisfact¬orily represented the critical pressures, or any other of the critical region values, of all multicomponent paraffin!c hydrocarbon mixtures. The Benedict-Webb-Rubin eight constant equation of state, of which the Kellogg Fugacity charts are a graphical representation, are too complex and prohibitively time-consuming for use by process design engineers.
10. An exhaustive literature survey of the densities, compressibility factors, and the changes in the compress¬ibility factor due to pressure and temperature of the first six straight-chain paraffin hydrocarbons (methane through n-hexane) was made and presented.
11. A method was developed for the calculation of the changes in the compressibility factor of gases due to pressure and temperature, when no experimental values of the compressibility factor are available in the literature.