15 Thermophysical Properties: Their Effect on Cryogenic
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Gas Processing D. G. ELLIOT, P. S. CHAPPELEAR, R. J . J. CHEN, and R. L . MCKEE McDermott Hudson Engineering, Houston, TX 77036
The changing economic climate in the natural gas processing industry has precipitated the design and construction of gas processing plants to recover 80 percent or more of the contained ethane. Typically, high ethane recoveries are attained by employing a cryogenic process utilizing a turboexpander. Historically, the availability of fundamental thermophysical property data required to design gas processing plants has lagged behind the design and construction of such facilities (1). The cryogenic process is no exception. Due largely to the efforts of the Gas Processors Association, pertinent experimental data has been taken and correlated by several methods. These thermophysical property correlations are generally available to the gas processing industry. It is the authors' intent to illustrate the relationship between process design and accurate prediction of K-values, enthalpy, entropy, and CO solubility. 2
Scope In 1970, White et al (2) demonstrated the importance of accurate K-value correlations in cryogenic plant design by comparing predicted product recoveries for a typical cryogenic plant using several correlations available at that time. The various K-value correlations gave predicted recoveries for ethane from 25.7% to 45.0% and for propane from 66.7% to 90.7%. Since that time much progress has occurred in correlations. Existing correlations have been refined and new correlations have been introduced. It is the authors intent to demonstrate the importance of accurate thermophysical properties on plant design and to compare the properties predicted by generally available correlations. This has been done by comparing predictions both from K-value correlations and enthalpy/entropy correlations for the major components in a typical cryogenic process. In addition, the predicted CO concentrations near the conditions of CO solids formation, a condition limiting ethane recovery in many plants, are presented. The correlations examined, listed in Table I, are of three 1
2
2
289
Storvick and Sandler; Phase Equilibria and Fluid Properties in the Chemical Industry ACS Symposium Series; American Chemical Society: Washington, DC, 1977.
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P H A S E EQUILIBRIA A N D F L U I D PROPERTIES IN C H E M I C A L
INDUSTRY
TABLE I CORRELATIONS Description
Reference
K-values from Hadden convergence pressure concept (1972 e d i t i o n ) w i t h CH4, C2H5, 3 3 from b i n a r y data a t low temperatures. K-values (convergence) computer c a l c u l a t e d from GPA C o e f f i c i e n t curve f i t . K-values (binary) hand calculated.
(4)
Symbol
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Empirical:
GPA CONV
C
H
HUDSON
McDermott Hudson p r o p r i e t a r y c o r r e l a t i o n f o r K-values of CH4, C 2 H 6 , and C 3 H 8 . Uses GPA CONV f o r h e a v i e r components .
MARK V
Wilson m o d i f i c a t i o n of the Redlich-Kwong equation of s t a t e ; computer program marketed by PVT, I n c .
Equation of State: van der Waals
BWR
(6)
LEE
Lee e t a l . c o r r e l a t i o n as programmed i n the GPA computer package K & H.
SOAVE
Soave m o d i f i c a t i o n of the (8) Redlich-Kwong equation of s t a t e , as programmed i n the GPA computer package K & H.
P-R
Peng-Robinson m o d i f i c a t i o n (9) of the Redlich-Kwong equat i o n of s t a t e . Values computer c a l c u l a t e d from c o r r e l a t i o n by P-R d i s t r i b u t e d by GPA.
SH BWR
Starling-Han m o d i f i c a t i o n of the Benedict-WebbRubin equation of s t a t e as programmed i n the GPA computer package K & H.
Storvick and Sandler; Phase Equilibria and Fluid Properties in the Chemical Industry ACS Symposium Series; American Chemical Society: Washington, DC, 1977.
(Z)
(10) (11)
15.
ELLIOT E T A L .
Cryogenic Gas Processing
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TABLE I (Continued)
Iipe
Symbol
Description
Reference
Conformal S o l u t i o n :
K VAL
1972 program f o r K-value by CHEMSHARE f o r h i g h methane content streams. Based on b i n a r y K data f o r t e n r e f e r e n c e systems.
PROP-75
Property-75 R i c e U n i v e r s i t y shape f a c t o r c o r r e s ponding s t a t e s program f o r p h y s i c a l properties using methane and pentane as r e f e r e n c e . 1975 r e v i s i o n of computer program d i s t r i b u t e d by GPA.
K DELTA
T. W. Leland corresponding (13) states c o r r e l a t i o n using data f o r b i n a r y systems as r e f e r e n c e . Computer program marketed by S i m u l a t i o n Science Inc. ( S S I ) .
Storvick and Sandler; Phase Equilibria and Fluid Properties in the Chemical Industry ACS Symposium Series; American Chemical Society: Washington, DC, 1977.
(12)
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292
P H A S E EQUILIBRIA
A N D F L U I D PROPERTIES
IN C H E M I C A L INDUSTRY
types: e m p i r i c a l , equation of s t a t e , and conformal s o l u t i o n . E m p i r i c a l c o r r e l a t i o n s are graphs of experimental data p l o t t e d a g a i n s t a c o r r e l a t i n g parameter. Equation of s t a t e c o r r e l a t i o n s a r e d i v i d e d i n t o two types: one i s s i m i l a r to the van der Waals equation w i t h a s m a l l number of parameters; the other i s based on the m u l t i p l e cons t a n t BWR equation. A l l equation of s t a t e c o r r e l a t i o n s i n v o l v e the computation of the parameters f o r the equation of s t a t e which w i l l represent the multicomponent mixture. These computations a r e based on parameters determined f o r the pure components, combining r u l e s , and p o s s i b l y b i n a r y i n t r a c t i o n parameters. The conformal s o l u t i o n or corresponding s t a t e s c o r r e l a t i o n computes pseudo-reduced c o n d i t i o n s f o r the mixture to conform to the r e f e r e n c e substance. The r e f e r e n c e substance may be a pure component or a m i x t u r e ; i t s p r o p e r t i e s can be given i n t a b u l a r or equation format. The computer programs used i n t h i s study a r e c u r r e n t v e r s i o n s . Other programming of the same equations may g i v e s l i g h t l y d i f f e r e n t r e s u l t s . The a b b r e v i a t i o n s l i s t e d i n Table I are used throughout the t e x t . Design B a s i s and Process D e s c r i p t i o n P r o p e r t i e s p r e d i c t e d by these c o r r e l a t i o n s determine not only the economic f e a s i b i l i t y of gas p r o c e s s i n g i n s t a l l a t i o n s ; they a l s o d i c t a t e the d e s i g n ( s i z e ) of i t s major components. These components are u s u a l l y the compressor, turboexpander, heat exchangers, e x t e r n a l r e f r i g e r a t i o n ( i f any), and demethanizer. A t y p i c a l arrangement f o r a cryogenic gas p l a n t of these components i s shown i n F i g u r e 1. The f o l l o w i n g process c o n d i t i o n s are s e l e c t e d : 1. 2. 3. 4. 5. 6. 7. 8.
I n l e t f l o w r a t e of 70 MMscfd I n l e t gas pressure of 250 p s i a I n l e t gas temperature of 120°F High pressure separator a t -80°F and 800 p s i a Expander o u t l e t pressure a t 200 p s i a Expander i s e n t r o p i c e f f i c i e n c y a t 80% (commonly c a l l e d adiabatic) Compressor i s e n t r o p i c e f f i c i e n c y at 75% (commonly c a l l e d adiabatic) I n l e t gas composition i n Table I I .
In t h i s t y p i c a l p l a n t , the i n l e t gas i s compressed from 250 p s i a to 815 p s i a . I t i s then cooled by an a i r c o o l e r to 120°F f o l l o w e d by exchanging heat w i t h the demethanizer r e b o i l e r and the p l a n t r e s i d u e gas. Any a d d i t i o n a l r e f r i g e r a t i o n r e q u i r e d to c o o l the i n l e t gas to -80 F b e f o r e e n t e r i n g the h i g h pressure separator i s f u r n i s h e d from an e x t e r n a l source. The l i q u i d separated i n the h i g h pressure separator i s f e d to the demethanizer where the methane i s s t r i p p e d out. Energy i s recovered from the h i g h pressure separator vapor by reducing the p r e s s u r e through a turboexpander. The expander o u t l e t i s much c o l d e r and p a r t i a l l y condensed. This energy i s converted to
Storvick and Sandler; Phase Equilibria and Fluid Properties in the Chemical Industry ACS Symposium Series; American Chemical Society: Washington, DC, 1977.
15.
293
Cryogenic Gas Processing
ELLIOT E T AL.
TABLE I I - COMPOSITIONS USED IN CALCULATIONS
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Component
I n l e t Gas Mol % Mol/Day
Residue Gas Mol % Mol/Day
L i q u i d Product Mol % Mol/Day 0
Nitrogen
0.40
738
0.44
738
Carbon D i o x i d e
0.52
959
0.30
498
2.91
461
Methane
90.16
166,308
98.52
166,124
1.16
184
Ethane
4.69
8,651
0.70
1,181
47.15
7,470
Propane
1.85
3,412
0.04
73
21.07
3,339
Isobutane
0.79
1,457
9.20
1,457
Normal Butane
0.51
941
5.94
941
Isopentane
0.27
498
3.14
498
Normal Pentane
0.18
332
2.10
332
Hexane P l u s *
0.63
1,162
7.33
1,162
100.00
15,844
100.00 *A11 c a l c u l a t i o n s
184,458 100.00
168,614
used n-Heptane f o r the Hexane P l u s f r a c t i o n .
RESIDUE GAS -167"F 200 PSIA
(-B0T
800 PSIAQ
HP. SEPARATOR
