An Analysis of the Convergence Pressure Concept For Hydrocarbon And Hydrocarbon Non-Hydrocarbon Systems
Authors: Robinson D.B. 
Published: March 2010

This paper evaluates the convergence pressure concept for the correlation of K-factors in hydrocarbon systems. It is shown that, even in the case of binary and ternary systems containing non-hydrocarbon components, the concept breaks down and leads to unacceptable errors in the predicted K-factors.

INTRODUCTION

THE CONCEPT OF CONVERGENCE PRESSURE as a parameter for the correlation of K-factors has been accepted by worker's in the hydrocarbon processing industry for many years. Working charts of tables for these K-factors are available in many forms, but undoubtedly the most systematic and comprehensiv treatment for hydrocarbon systems is the one prepared by the Natural Gas Processors Suppliers Association and made available through their Engineering Data Book Although the concept originated more than two decades ago, and although its shortcomings have long been recognized, it is still widely accepted and widely used. Calculations based on K-factors obtained using the convergence pressure principle will continue to be made for many years to come.

Within the past five to ten years, several alternative methods for allowing for the effect of temperature, pressure and composition on K-factor calculations have been suggested. Significant among these are correlations based directly on an equation of state such as the RWR equation, on the regular solution theory incorporated into the Chao-Seader(3) correlation or on the recent work of Prausnitz and Chueh(9). It can be argued that these approaches are more sound theoretically, but in spite of this they all have their limitations ill one way or another. For this reason, many situations exist in which K-factors based on convergence pressure are still as reliable as any.

The purpose of this paper is to critically evaluate the concept by making some comparisons between K-factors for hydrocarbons in the presence of other hydrocarbons with the same hydrocarbons in the presence of hydrogen sulphide OR carbon dioxide, evaluated at the same convergence pressure, pressure and temperature. The analysis will show that the basic convergence pressure postulate breaks down even in the case of binary and ternary systems when non-hydrocarbons are present.

CONVERGENCE PRESSURE CONCEPT

The convergence pressure concept has been clearly explained by Katz and Kurata(7) for a binary system, The convergence pressure for a binary system is the critical pressure of the particular binary mixture that has the temperature in question as its critical temperature. Thus, for a given binary system, the convergence pressure depends only on temperature. Basically, the postulate means that the K-factor for component A in an A-B binary should be the same as for component A in an A-C binary wherever the systems have the same temperature, pressure and convergence pressure.

The basic convergence pressure concept has also been analyzed quantitatively by Holland(8) for ternary and more complex systems. It is shown that for a ternary system only one graph of log P vs log K, exists for each component at a given temperature and convergence pressure. Thus, for ternary systems the concept should be just as valid as it is for binary systems.