1、Tech n cal Pu bl cat ion TP-22 Convergence Pressure and Vapor-Liquid Equilibrium Ratios Published by the Gas Processors Association June, 1999 6526 East 60th Street * Tulsa, Oklahoma 74145 Phone: 918/493-3872 - Fax: GPA Disclaimer GPA publications necessarily address problems of a general nature and
2、 may be used by anyon desiring to do so. Every effort has been made by GPA to assure accuracy and reliability of the informatioi contained in its publications. With respect to particular circumstances, local, state, and federal laws ani regulations should be reviewed. It is not the intent of GPA to
3、assume the duties of employers manufacturers, or suppliers to warn and properly train employees, or others exposed, concerning healtl and safety risks or precautions. GPA makes no representation, warranty, or guarantee in connection with this publication and hereb expressly disclaims any liability o
4、r responsibility for loss or damage resulting from its use or for thl violation of any federal, state, or municipal regulation with which this publication may conflict, or for an infringement of letters of patent regarding apparatus, equipment, or method so covered. “Copyright O 1999 by Gas Processo
5、rs Association. All rights reserved. No part of this report may be reproduced without the written consent of the Gas Processors Association.“ Convergence Pressure and Vapor-Liquid Equilibrium Ratios GPA Technical Publication TP-22 Prepared by Gas Processors Suppliers Association Editorial Review Boa
6、rd 6526 E. 60th Street Tulsa, OK 74145 June, 1999 Introduction This Technical Publication, TP-22, serves to archive the GPSA Convergence Pressure Vapor Liquid1 Equilibrium Ratios (K-data) along with other K-data charts. The convergence pressure approach to handling1 composition dependency of K-value
7、s was omitted when the eleventh edition of the GPSA Engineering Data, Book was published in 1998. The Editorial Review Board explained this change in the data book with the following statement: Previous editions of this data book presented extensive sets of K-data based on the GPSA Convergence Press
8、ure Pk method. A components K-data is a strong function of temperature and pressure and a weaker function of composition. The convergence pressure method recognized composition effects in predicting K- data. The convergence pressure technique can be used in hand-calculations, and it is still availab
9、le as computer correlations for K-data prediction. There is now general availability of computers. This availability coupled with the more refined K-value1 correlations in modern process simulators, has rendered the previous GPA convergence pressure charts outdated. The convergence pressure method i
10、s presented in dual metric (SI) and English (FPS) units. This TP contains the full explanatory text on convergence pressure from section 25 of the tenth Edition of the GPSA Engineering Data Book (the blue books) supplemented with the SI information from the corresponding material from section 18 in
11、the 1980 SI Engineering Data Book (the green book). All of the standard charts are from the 1980 SI Engineering Data Book (the green book). In addition, other charts from the 1957 edition relating to acid gas components in ethane and propane have been included. For quick trouble-shooting and evaluat
12、ion work a set of K-charts can be very helpful. They present an illustration of the effect of temperature and pressure on K-values for light hydrocarbons. A composite set has been retained in the eleventh edition of the data book for both the SI and FPS units. The charts represent decades of careful
13、 comprehensive research directed by GPA and GPSA. Accordingly, the Associations decided that they are to be retained as a resource available to the industry and academia. Also included in this TP are K-data for High-Boiling Hydrocarbons (Poettman charts), some hydrogen sulfide binanes and hydrogen b
14、inaries, The Equilibrium or K-value curves were first published in chart form by the NGAA and NGSMA (predecessors to the present day GPA and GPSA) in the early 1950s. Much of the basic data was compiled in tabular form under the direction of Dr. G. G. Brown, Dean of Engineering,University of Michiga
15、n. The data were taken from numerous published articles and unpublished experimental data. From these tabular data the first charts to be used by the industry were prepared by the Fluor Corporation Engineering Department under the guidance of Blaine Kuist, C. W. Leggett and C. K. Walker. Most of the
16、 painstaking draftsmanship was done by Orville McAdams, also of Fluor. Stuart Hadden developed the concept of convergence pressure, thereby making the charts useful for predicting K-values in a mixture containing any number of components. The charts were first published by the NGAA in 1955. In antic
17、ipation of needing additional information to expand the data and improve the accuracy. NGAA appointed the first Equilibrium Committee chaired by Elliott Organik, United Gas Corporation. Members of the committee were C. L. DePreister, California Research Corporation; Karl Hackmuth, Phillips Petroleum
18、 Company; James Kilmer, Stanolind Oil and Gas Company; H. H. Rackford, Humble Oil and Refining Company; Bryon Woertz, The Pure Oil Company; and Charles Webber, Sun Oil Company. In 1957 the charts were completely redrawn to provide data at lower temperatures and to increase the internai consistency o
19、f the K-values. Karl Hackmuth guided the chart revision. By then Byron Woertz had assumed chairmanship of the Equilibrium Committee. E.A. Gromatzky, The Texas Company; R. H. Jacoby, Pan American Petroleum Company; Herbert L. Stone, Humble Oil and Refining Company; and C. J. Walters. California Resea
20、rch Corporation had become members of the committee. This committee continued to act as a clearing house for new equilibria data and correlation methods. In 1958 these charts were published as Tables of Coefficients for use with early computers. Robert Norman and Brymier Williams presented a paper a
21、t the 57th Annual NGAA convention in Dallas April, 1958 that was the basis for the initial work that developed these machine computations. A set of equations was developed to represent each page in the Data Book. The NGAA prepared a reproduction of the data from these charts in two forms - point car
22、ds and coefficient cards. The point cards were IBM cards containing approximately 70,000 data points (pressure, temperature, composition and convergence pressure) punches on 17,000 cards. The coefficient cards were IBM cards that contained the coefficients for the equations used to empirically corre
23、late the point data. This required only 432 cards because each card contained seven coefficients. This drastically reduced the number of cards which in turn reduced the computer storage space required. Through the years the convergence curves have been modified and improved. They are still published
24、 in the GPSA Engineering Data Book and represent decades of careful and comprehensive research directed by GPA and GPSA. STDnGPA TP-ZZ-ENGL 1997 382qb97 00201178 1173 = TP-22 EQUILIBRIUM RATIO (K) DATA The equilibrium ratio (Ki) of a component i in a multi- component mixture of liquid and vapor phas
25、es is defined as the ratio of the mole fraction of that component in the vapor phase to that in the liquid phase. K,=- Yi xi For an ideal system (ideal gas and ideal solution), this equilibrium ratio is reduced to the ratio of the vapor pressure of component i to the total pressure of the system. pi
26、* Ki =- P This TP presents an outline procedure to calculate the liquid and vapor compositions of a two-phase mixture in equilibrium using the concept of a pseudobinary system and the convergence pressure equilibrium charts. Discussion of CO;! separation. alternate methods to obtain K values, and eq
27、uations of state follow. Finally, three sets of equilibrium rat:, charts are presented: K-values for specific binary systems K-values of various components based on different convergence pressures of hydrocarbon systems K-values of pseudocomponents based on normal boiling point and characterization
28、factor BINARY SYSTEMS DATA CHARTS The 1972 edition of the GPSA Engineering Data Book included for the first time several charts representing binary combinations of N2, Cl. C, C3, and n-C4. These data resulted from GPA-sponsored research at Rice University. Binary data with heavier components and aci
29、d gases have been added as revisions as they have become available. These charts are useful for low temperature (below - 100“F, -73“C) calculations. Below the critical temperature of methane (-1 16F -82“C), the convergence pressure of a system no longer has a real significance, since the K-values fo
30、r heavy components do not reach a value of 1 .O. Note that the critical data from these binary mixtures were NOT used to adjust the critical loci of Figure 5. Such an adjustment would necessitate complete cross-plotting and redetermination of all of the convergence pressure charts. Use of Binary Cha
31、rts - At temperatures below -100F -73T, a light hydrocarbon system is most easily approximated as a pseudobinary. Determine the average molecular weight on a methane-free basis; then interpolate the K-values between the two binaries whose heavy component lies on either side of this pseudocomponent.
32、This calculation becomes poor for systems with N2 content greater than 3 to 5 mole percent. The ternary methane-ethane-propane system is well defined.5 At temperatures below -75F -60C, (except for the highest pressures 700-800 psia 4800-5500 kPa(abs) at -75F -60“Cl and -100F -73“C. if f=XE(Mp) /xECM
33、i = ratio of ethane mole fraction in the liquid phase of the ternary system to the ethane mole fraction in the liquid phase of the methane- ethane system, then the ternary K-values can be calculated from: log KM = log KM(P) KM(P)IKM(E)1 Eq 3 log KP = log KP(M) log KP(M)/KgME)l Eq 5 Note that xE(MP)
34、I xE(M), and KAo, denotes K-value of A in the A-B binary system. These are found from pages 102 and 103. The infinite dilution KE(MP) and G, are given on page 104. These generalizations can be made about the ternary K- Addition of propane increases the methane K-value and decreases the ethane K-valu
35、e. Addition of ethane decreases the methane K-value and increases the propane K-value. The propane K-value is highest when at infinite dilution in the ternary (.e., a methane-ethane binary). The ethane K-value is highest when no propane is present (.e., a methane-ethane binary). values: These genera
36、lizations can be extended to other ternary (or Figure 1 Nomentclature K = qulibrium ratio, y/x y = mole fraction in the vapor phase L = ratio of moles of liquid to moles of total mixture N = mole fraction in the total mixture or system = pseudobinary Superscripts O = acentric factor = log Pv, - 1 .o
37、0 P = absolute pressure, psia kPa(abs) where. Pvr = reduced vapor pressure at Tr = 0.7 o0 = at infinite dilution Subscripts Pk = convergence pressure. psia kPa(abls) I = component P* = vapor pressure, psia kPa(abs) c = critical R = universal gas constant, (psia CU ft)Ab mole “R) (kPa m3 ) /(kmole K)
38、 M = methane T = temperature, OR or OF K or OC E = ethane V = ratio of moles of vapor to moles of total mixture P = propane x = mole fraction in the liquid phase _- 1 STD-GPA TP-ZZ-ENGL 1799 3824b99 0020479 30“ higher) systems: the heaviest component (propane in this case) is analogous to a “lean oi
39、l“ in an absorber system. The methane- ethane is acting in this case as a “natural gas“ with the heaviest component (ethane) being preferentially absorbed into the “lean oil“ (propane), with the resulting effects on the K-values. The liquid mole fraction of the middle (volatility) component is a use
40、ful parameter. The user should be aware that the composition effect on K-values is very pronounced in the regions covered by the binary and ternary data systems. Accurate values can be obtained only by cross-plots of K-value versus composition. Limiting conditions should be considered in the analysi
41、s and use of K-data. For example, the limits on a binary system are defined by the vapor pressure of the pure components, as shown by the Locus KG and Locus KS of page 102. The limits of a ternary system are the binary systems. The GPA/GPSA sponsors investigations iL systems of interest to gas proce
42、ssors. Detailed results are given in the annual proceedings and in various research reports and technical publications, which are listed in Section 1 of the Engineering Data Book. Example 1 - Binary System Calculation To illustrate the use of binary systerm K-value charts, assume a mixture of 60 mol
43、es of methane and 40 moles of ethane at -125OF -87“C and 50 psia 345 kPa(abs). From the chart on page 102, the K-values for methane and ethane are 10 and 0.35 respectively. This method is valid for either Ibmols or kmols. Solution Steps From the definition of K-value, Eq 1 : Y G2 = z= 0.35 x c2 Rewr
44、iting - - 0.35 yci - xc1 Solving xcl = 0.0674 ycl = 0.674 Hence xc2 = 0.9326 yo = 0.326 To find the amount of vapor in the mixture, let v denote moles of vapor. Summing the moles of methane in each phase gives: 0.674 v + 0.0674 (100 - v) = 60 v = 87.8 The mixture consists of 87.8 moles of vapor and
45、13.2 moles of liquid. K-VALUES CORRELATION BY CONVERGENCE PRESSURE The convergence pressure chart representation of K-values. used since 1957 by the GPA, provides a useful and rapid graphical approach for engineers. Within limits the values are sufficiently accurate to satisfy many calculations requ
46、ired by the practicing engineer. Moreover. these charts are widely used in industry and are generally preferred over most nomographs. The charts included in this TP are a mixture of past and present, as shown in Figure 2. The Pk charts for methane, ethane, and propane are based on extensive cross pl
47、ots of available experimental data. Properly used, they should represent both absorber oil and condensate systems adequately. Charts for components heavier than propane are selected from previous editions, based on general experience as to whether the 1957 or 1966 edition of the Engineering Data Boo
48、k gave the best results. The present convergence pressure charts do not have as many low temperature isotherms as in past editions. This is because no experimental data existed in these regions when the charts were prepared. Calculations of systems at very low temperatures and at high convergence pr
49、essures do not have much significance in hydrocarbon systems, and therefore should not be shown. As an example, refer to Figure 5. Assuming methane as the light componet and methylcyclohexane as the heavy, the critical locus line indicates that the lowest possible temperature for a 3000 Pk is -50F -46OC. This is about the practical limit in real systems; therefore, isotherms significantly below this would be imaginary. This deletion of low-temperature isotherms is intended to warn the user not to apply these charts beyond the ran
