Regression equations and coefficients for various versions of the GPA convergence-pressure charts are available from the GPA. The widespread availabihty and utihzation of digital computers for distillation calculations have given impetus to the development of analytical expressions for i regression equation and accompanying regression coefficients that represent the DePriester charts of Fig. It is analogous to the critical point for a pure component in the sense that the two. An alternative measure of composition is the convergence pressure of the system, which is defined as that pressure at which the Kvalues for aU the components in an isothermal mixture converge to unity. The Kellogg and DePriester charts and their subsequent extensions and generahzations use the molar average boiling points of the liquid and vapor phases to represent the composition effect. SI versions of these charts have been developed by Dadyburjor. For K as a function of T and P only, the DePriester charts provide good starting values for the iteration. One cannot calculate K values until phase compositions are known, and those cannot be known until the K values are available to calculate them. Ī trial-and- error procedure is required with any K-value correlation that takes into account the effect of composition. The Kellogg charts, and hence the DePriester charts, are based primarily on the Benedict-Webb-Rubin equation of state, which can represent both the liquid and the vapor phases and can predict K values quite accurately when the equation constants are available for the components in question. These charts are a simplification of the Kellogg charts and include additional experimental data. The easiest to use are the DePriester charts, which cover 12 hydrocarbons (methane, ethylene, ethane, propylene, propane, isobutane, isobutylene, /i-butane, isopentane, /1-pentane, /i-hexane, and /i-heptane). For example, several major graphical i light-hydrocarbon systems. However, for mixtures of compounds of similar molecular structure and size, the K value depends mainly on temperature and pressure. 4, the i complex function of temperature, pressure, and equilibrium vapor- and hquid-phase compositions. Campbell and Company, Norman, Oklahoma, USA, 2001.As discussed in Sec. M., “Gas conditioning and processing, Volume 2: Equipment Modules,” John M. Campbell and Company, Norman, Oklahoma, USA, 2001. “Gas conditioning and processing, Volume 1: Fundamentals,” John M. Campbell and Company, Norman, Oklahoma, USA, 1994. Lilly, “Gas conditioning and processing, Volume 3: Advanced Techniques and Applications,” John M. To learn more on applications of K-values and their impact on facilities calculation, design and surveillance, refer to JMC books and enroll in our G4 (Gas Conditioning and Processing) and G5 (Gas Conditioning and Processing – Special) courses. Many less than desirable systems results from failure to recognize this.
This is important, for in many systems a series of VLE calculations is made the output from one is the input to another. The compositional analysis on which the calculation was based will often be in error more than this. There are several methods available for this purpose (See pages 113-116 of Vol 1 of JMC Books).Īn experienced practitioner usually can predict the quantity of a specified liquid within 6% (for a specified analysis and conditions).
It is most important that the K-values be internally consistent. It is doubtful if one ever will encounter the analyses, flow rates and exact other conditions used as the design basis. This range, rather than one set of “magic” numbers, is then used to size equipment. Generally, crude oil and NGL phase behavior is handled by different models.Īll K-values are sensitive to composition, particularly the very volatile components like nitrogen, methane, and ethane.įor design purposes, several models may be used to determine a range of results. An experienced practitioner may have two or three different models or program available. There is no single K-Value correlation that is superior for all mixtures encountered. The accuracy of the results of calculations involving K-Values depends on the reliability of sampling, of the analysis of that sample, and the K-Value correlation used. Therefore, the following guidelines extracted from page 128 of Vol 1 of JMC book are suggested.
Again, the calculated liquid fractions by the Raoult’s law and Wilson correlations are close to each other but they deviate considerably from the GPA charts and the SRK EoS results.ĭue to the observation made in the previous section and other studies, care must be taken in selecting K-values correlations.
Similarly, for the same mixture shown in Table 1, a series of flash calculations for two isotherms were performed and the calculated liquid fractions (L/F) using different methods are compared in Table 5.