Industrial View of the State-of-the-Art in Phase ... - ACS Publications

3 Industrial View of the State-of-the-Art in Phase Equilibria

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T. S. KROLIKOWSKI Union Carbide Corp., Chemicals and Plastics Div., S. Charleston, W.Va. 25303

Twenty-five years ago, in 1952, there was a series of articles in Chemical Engineering Progress (1) entitled: "Industrial Viewpoints on Separation Processes". In the section on phase equilibrium data, it was noted that "The complete representation of such data for mixtures containing more than three components becomes impractically complex". Simplified calculations for multicomponent systems were recommended, and i f the predicted values did not agree with experimental data, a system of minor correction factors should be devised. In the same year, one of the annual review articles in Industrial and Engineering Chemistry (2) mentioned that the BWR equation of state seemed to provide the most accurate method thus far developed for estimating K-factors for hydrocarbon systems. Use of the equation was deemed tedious, and a procedure for using the equation in a simplified form suitable for rapid equilibrium calculations was to be presented. Charts based on the procedure were available from the M. W. Kellogg Co., New York. Another review article (3) observed that automatic computers have entered the field of ditillation calculations. The author remarks: "The difficulty is that the machines cannot evaluate the errors in the assumptions set up by the operator, and therefore the value of the numbers produced by the machine gives a false impression of accuracy". That statement is as valid today as i t was twenty-five years ago. On the other hand, the industrial approach to phase equilibria has changed over the years. In this state-of-the-art report, I will describe our present practices and concerns. This presentation will be subdivided according to the sequence presented in Figure 1 - HOW, WHAT and WHY, WHERE. HOW are phase equilibria problems treated in an industrial situation? WHAT methods and correlations are used, and WHY are these techniques used? WHERE should future development work be directed? Of necessity, the conditions described here are based

62 In Phase Equilibria and Fluid Properties in the Chemical Industry; Storvick, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.

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Industrial View of Phase Equilibria


HOW? WHAT AND WHY? WHERE ? Figure 1.

Sequence of presentation

In Phase Equilibria and Fluid Properties in the Chemical Industry; Storvick, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.

63

64

PHASE

EQUILIBRIA

A N D FLUID PROPERTIES

IN C H E M I C A L

INDUSTRY

p r i n c i p a l l y on my own work environment. They may not be u n i v e r s a l l y t r u e , but they a r e c e r t a i n l y r e p r e s e n t a t i v e of the c u r r e n t s t a t e of a f f a i r s i n i n d u s t r y .


HOW Are Phase E q u i l i b r i a Problems Treated? The t e c h n i c a l p o p u l a t i o n i n i n d u s t r y can be d i v i d e d i n t o the computer people and the non-computer people. The non-computer people tend to use simple c o r r e l a t i o n s , g e n e r a l i z e d models and graphs, and estimates based on t h e i r experience or i n t u i t i o n . The computer people, who are i n the m a j o r i t y , have the c a p a b i l i t y of developing very complex models. S e v e r a l years ago a t Union Carbide, an e f f o r t was i n i t i a t e d to improve the e f f e c t i v e n e s s of the computer system used by these i n d i v i d u a l s f o r design purposes. I would l i k e to spend some time d e s c r i b i n g the system as i t now e x i s t s . Subprogram L i b r a r y . The b a s i s of the system i s the Engineering Subprogram L i b r a r y . The l i b r a r y c o n s i s t s of computer subroutines which have been w r i t t e n to perform engineering process and design c a l c u l a t i o n s and to s o l v e v a r i o u s types of mathematical problems. The subroutines a r e w r i t t e n i n accordance w i t h a standard format, and they use c o n s i s t e n t technology. They are f u l l y documented i n a manual so that they can be e a s i l y used by other programmers. There a r e three kinds of thermodynamic and p h y s i c a l property subroutines: monitor s u b r o u t i n e s , method s u b r o u t i n e s , and i n i t i a l i zation subroutines. Their i n t e r - r e l a t i o n s h i p i s i l l u s t r a t e d i n F i g u r e 2. There i s a monitor subroutine f o r every p r o p e r t y ; vapor molar volume, vapor f u g a c i t y c o e f f i c i e n t s , l i q u i d a c t i v i t y c o e f f i c i e n t s , e t c . Suppose a main program needs the value of property 1. I t w i l l c a l l the monitor subroutine f o r property 1 and supply a computation method code. The monitor subroutine checks the code and c a l l s the a p p r o p r i a t e method subroutine to perform the c a l c u l a t i o n s of property 1. I f the method subroutine r e q u i r e s the value of property 2, i t c a l l s the monitor subroutine f o r property 2 and so f o r t h . The monitor subroutines a l l o w us t o w r i t e very general main programs w i t h a v a r i e t y of options f o r c a l c u l a t i n g thermodynamic and p h y s i c a l p r o p e r t i e s . I t i s a l s o very easy to add a new method to the system; one simply i n c l u d e s a new c a l l statement i n the monitor s u b r o u t i n e . The main program does not have to be reprogrammed when adding a new method. The t h i r d type of subroutine i s the i n i t i a l i z a t i o n subroutine which c a l c u l a t e s the constant parameters a s s o c i a t e d w i t h a method s u b r o u t i n e ; f o r i n s t a n c e , the Redlich-Kwong equation of s t a t e c o n s t a n t s . The main program c a l l s the r e q u i r e d i n i t i a l i z a t i o n subroutines once f o r any given s e t of components. Code S t r u c t u r e . As mentioned e a r l i e r , a monitor subroutine checks a method code to determine the c a l c u l a t i o n a l method. A f l e x i b l e scheme has been developed f o r s p e c i f y i n g method codes. The codes r e q u i r e d f o r v a p o r - l i q u i d e q u i l i b r i u m c a l c u l a t i o n s a r e shown i n F i g u r e 3. Each column represents a s e t of f i v e codes t r a n s m i t t e d to a monitor subroutine f o r the property designated


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MAIN PROGRAM

>

< Downloaded by NORTH CAROLINA STATE UNIV on May 9, 2015 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0060.ch003

MONITOR SUBROUTINE PROPERTY 1

1 V INITIALIZATION SUBROUTINE PROPERTY 2,

METHOD SUBROUTINE PROPERTY 1

MONITOR SUBROUTINE PROPERTY 2

METHOD SUBROUTINE PROPERTY 2

Figure 2. Structure of the subroutine system

EQUILIBRIUM VAPOR PRES.

I.D. 1

ACT. FUG.

COEF.

* 7

11

19

15

ENTHALPY

COOES

LIQUID

K. E Q.

VAPOR FUG. COEF. 27

23

MIXING

CODES

LIQUID VAPOR 35

31

39

THERM0

— 1 1 1 1 11 1 1 1 1 1 1 1 11 1 1 1 1 11 1 11 1 11 1 11 1 1

11 t

h

s

u

b

i i i i i

2

1 1 1 11 1 1 1 1 11 1 1 1 1 11 1 11

1

1 1 11 1 1 1 1 11 1 1 1 1 11 1 1 1 1 11 1 11 1 1

THSUB4

111

1

11

11 1 1 1 1 11 1 1 1 1 11 1 11 1 11 1 11 1 1 1 1 1 1 1 11 1 1 1 1

11

1 11 1 11 1 11 1

LIQ

LIQ

ACT. COEF. 19

ACT. COEF. 19

— REGULAR SOLUTION — LIQUID VOLUME

BASIC CODE

1

1

SUBCODES

1 1

— WILSON EQUATION

MODEL

POLYNOMIAL

— UNMODIFIED — LIQUID VOLUME POLYNOMIAL

Figure 3.

Method code structure


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

66

MOLECULAR WEIGHT NORMAL

BOILING & FREEZING POINTS

CRITICAL CONSTANTS


LIQUID DENSITY

AT REFERENCE TEMPERATURE

HEATS OF FORMATION FOR VARIOUS STATES PITZER'S ACENTRIC FACTOR REFRACTIVE INDEX AT REFERENCE TEMPERATURE SOLUBILITY IN VARIOUS SOLVENTS AT REFERENCE TEMPERATURE SURFACE TENSION AT REFERENCE TEMPERATURE Figure 4.

Examples of single-valued properties in the date bank

PERFECT GAS HEAT CAPACITY Cp = A + BT + CT + DT + ET 2

VAPOR

3

4

PRESSURE

In P = A - B/(T + C) + D In T + ET

N

LIQUID VISCOSITY ln^t= A + B/T + C In T LIQUID THERMAL CONDUCTIVITY In k = A + BT + CT 2

Figure 5. Examples of correlations whose constant parameters are in data bank



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above i t . The f i r s t code i s the b a s i c method code; each method i s assigned a b a s i c method code. The other four codes are method subcodes; the d e f i n i t i o n of the subcodes depends on the b a s i c method code. L e t ' s look a t the l i q u i d a c t i v i t y c o e f f i c i e n t codes as an example. A b a s i c method code equal t o 2 designates the Regular S o l u t i o n model. Then, the f i r s t subcode i s the b a s i c method code f o r the l i q u i d volume c o r r e l a t i o n s r e q u i r e d i n t h i s model, and the next code i s the l i q u i d volume method subcode, i f one i s r e q u i r e d . On the other hand, suppose the b a s i c method code f o r l i q u i d a c t i v i t y c o e f f i c i e n t i s equal to 4, which denotes the W i l son Equation. Since s e v e r a l m o d i f i c a t i o n s of the Wilson Equation have been programmed i n our system, the f i r s t subcode i n d i c a t e s which v a r i a t i o n should be used. Now, the second subcode i s the b a s i c method code f o r the l i q u i d volume c a l c u l a t i o n s r e q u i r e d i n t h i s model, f o l l o w e d by any r e q u i s i t e volume method subcodes. The methods s p e c i f i e d by subcodes a r e r e s t r i c t e d to that p a r t i c u l a r a p p l i c a t i o n . Thus, one l i q u i d volume method can be used i n the a c t i v i t y c o e f f i c i e n t c a l c u l a t i o n s , another f o r the Poynting c o r r e c t i o n , and a t h i r d f o r pipe s i z i n g c a l c u l a t i o n s . I f a subcode i s m i s s i n g , the b a s i c method code f o r that property w i l l be s u b s t i t u t e d . I n the l i q u i d a c t i v i t y c o e f f i c i e n t examples, i f the subcode f o r the l i q u i d volume method i s m i s s i n g , the b a s i c method code s p e c i f i e d f o r l i q u i d volume c a l c u l a t i o n s w i l l be used. I f the b a s i c method codes are m i s s i n g , d e f a u l t values a r e assigned. The d e f a u l t method code f o r a property i s the s i m p l e s t model r e q u i r i n g the l e a s t input data, e.g., i d e a l - g a s model f o r vapor f u g a c i t y c o e f f i c i e n t s . "Data Bank. Another p a r t o f the computer system i s a Data Bank which serves as a r e p o s i t o r y f o r pure component data. The Data Bank contains s i n g l e - v a l u e d p r o p e r t i e s o f the type shown i n F i g u r e 4. I t a l s o contains the constant parameters and a p p l i c a b l e ranges f o r property c o r r e l a t i o n s o f the s o r t i l l u s t r a t e d i n F i g u r e 5. Only p r o p e r t i e s r e q u i r e d f o r engineering c a l c u l a t i o n s are s t o r e d i n the Data Bank. A l l o f the values i n the Data Bank are i n t e r n a l l y c o n s i s t e n t ; the vapor pressure c o r r e l a t i o n s do p r e d i c t the normal b o i l i n g p o i n t s and the c r i t i c a l p o i n t s , the enthalpy and entropy values are based on the same reference s t a t e , etc. Every v a l u e i n the Data Bank has a reference number assoc i a t e d w i t h i t . These r e l a t e to a l i s t of references s t o r e d i n a separate computer f i l e . The references a r e comprehensive - every Data Bank v a l u e can be reproduced from the i n f o r m a t i o n given i n the r e f e r e n c e . The references i n c l u d e a l l o f the data sources and the d a t a - r e d u c t i o n methods used i n o b t a i n i n g the Data* Bank values. Main Programs. There a r e a v a r i e t y o f programs which c a l l upon the Engineering Subprogram L i b r a r y and the Data Bank. S e v e r a l of these programs w i l l be b r i e f l y d e s c r i b e d here. VLEFIT i s a f i t t i n g program which uses v a p o r - l i q u i d e q u i l i b r i u m (VLE) data t o determine the best values f o r the a d j u s t a b l e parameters


68

P H A S E EQUILIBRIA

INPUT

A N D F L U I D PROPERTIES

I N C H E M I C A L INDUSTRY

DATA

OPTIONS:

P-T-X P-T-X-Y P-T-Y


P-T-X-AH

M I X

T-X-tf P - T - X - USER

VARIABLE

ALLOWABLE OBJECTIVE FUNCTIONS:

P Y X

Figure 6.

Vapor-liquid equilibrium datafittingprogram (VLEFIT)

ABSORPTION EXTRACTIVE AZEOTROPIC

DISTILLATION DISTILLATION

FRACTIONATION RECTIFICATION STRIPPING LIQUID - LIQUID

EXTRACTION

Figure 7. Scope of the MMSP (Multicomponent Multistage Separation Processes) programs



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i n VLE models. I t w i l l handle up t o a quaternary system. The input data options and the a l l o w a b l e o b j e c t i v e f u n c t i o n s are shown i n F i g u r e 6. Any combination of i n p u t data and o b j e c t i v e f u n c t i o n s can be used i n a given run, and weighting f a c t o r s can be attached t o both the input data and the o b j e c t i v e f u n c t i o n s . The MMSP programs were developed f o r the s i m u l a t i o n , design, and c o n t r o l of Multicomponent M u l t i s t a g e Separation Processes. These i n c l u d e the operations l i s t e d i n F i g u r e 7. These processes may be c a r r i e d out i n c o n v e n t i o n a l columns (one feed and two prod u c t s ) , i n complex columns ( m u l t i p l e feeds, m u l t i p l e products, and m u l t i p l e i n t e r s t a g e c o o l e r s or h e a t e r s ) , o r i n a s e r i e s of c o n v e n t i o n a l o r complex columns. A u s e f u l d i a g n o s t i c o p t i o n f o r checking VLE models i s a l s o a v a i l a b l e i n the MMSP programs. Most VLE models are based on b i n a r y parameters. The data used i n o b t a i n i n g the b i n a r y parameters may be very l i m i t e d and/or a t c o n d i t i o n s removed from those of i n t e r e s t . Therefore, i t i s wise to examine the p r e d i c t i o n s of the model f o r the b i n a r y p a i r s before attempting a multicomponent s i m u l a t i o n . This o p t i o n i n MMSP w i l l use the VLE model to produce y-x, T-x-y, P-x-y, a-x, and K-x diagrams f o r the b i n a r y systems. Two of these p l o t s f o r the acetone-water system are shown i n F i g u r e 8. Another program IPES (Integrated Process Engineering System) i s a process s i m u l a t o r . I t has the a b i l i t y of modeling processes of any s i z e , from a s i n g l e d i s t i l l a t i o n column to a complex p l a n t . IPES i s based on a b u i l d i n g b l o c k concept. The process i l l u s t r a t e d i n the flowsheet of F i g u r e 9-A would be modeled i n IPES u s i n g the scheme i n F i g u r e 9-B. Each b l o c k corresponds to a u n i t or o p e r a t i o n i n the a c t u a l process. The b l o c k s i n IPES i n c l u d e mixer-splitters, reactors, flach units, d i s t i l l a t i o n units, e x t r a c t o r s , heat exchangers, compressors, c o n t r o l u n i t s , and economic u n i t s f o r s i z i n g and c o s t i n g process equipment. The method codes d e s c r i b e d e a r l i e r can be s p e c i f i e d on an o v e r a l l b a s i s o r i n d i v i d u a l l y f o r each b l o c k . WHAT Models Are Used And WHY? An e f f i c i e n t computing system i s capable of producing meaningf r e s u l t s only i f a p p r o p r i a t e models are used. Therefore, a t t h i s p o i n t , I would l i k e t o d e s c r i b e the techniques we use f o r VLE, f o r l i q u i d - l i q u i d e q u i l i b r i u m (LLE), and f o r other s e p a r a t i o n processes This w i l l i n c l u d e examples of t y p i c a l systems, the problems they pose, and how we attempt t o cope w i t h them. Vapor-Liquid E q u i l i b r i u m Models. I w i l l begin by d e s c r i b i n g the models a v a i l a b l e i n our system f o r VLE c a l c u l a t i o n s - the f i e l d i n which we have had the most experience, and i n which the most e f f o r t has been expended. The c a l c u l a t i o n s are based on VLE r a t i o s - K v a l u e s . The K v a l u e of a component i n a mixture i s r e l a t e d t o i t s f u g a c i t y i n the l i q u i d phase and i t s f u g a c i t y i n the vapor phase. The equations f o r determining the f u g a c i t i e s are given i n F i g u r e 10. The vapor f u g a c i t y i s expressed i n the u s u a l f a s h i o n i n terms of a f u g a c i t y c o e f f i c i e n t , 0 . There are In Phase Equilibria and Fluid Properties in the Chemical Industry; Storvick, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.


A N D F L U I D PROPERTIES IN C H E M I C A L INDUSTRY


70

Figure 8.

Diagnostic options available in MMSP


co Downloaded by NORTH CAROLINA STATE UNIV on May 9, 2015 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0060.ch003


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A N D F L U I D PROPERTIES I N C H E M I C A L INDUSTRY


RECYCLE

CONDEN SATE

STEAM BOTTOMS

REC

FEED

HFD

EHXOI HEAT .USTM

EMSOI MIX

RXFD i

PROD ERXOI ROUT EFLOI CFD EDXOI * REAC FLSH COLM HVY

i

CSTM

ECR02I CONT

Figure 9.

LOTS

Process simulation program



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73

three expressions f o r the l i q u i d f u g a c i t y . The f i r s t i s the t r a d i t i o n a l approach most o f t e n found i n textbooks based on a s t a n dard s t a t e equal t o the t o t a l system p r e s s u r e ; t h i s i s the o p t i o n most f r e q u e n t l y e l e c t e d . I n the second approach, the standard s t a t e i s a f i x e d r e f e r e n c e p r e s s u r e . I n the t h i r d equation, the _ l i q u i d f u g a c i t y i s expressed i n terms o f a f u g a c i t y c o e f f i c i e n t , 0L. A l t e r n a t e l y , the K v a l u e can be c a l c u l a t e d using an e m p i r i c a l c o r r e l a t i o n depending only on pressure and temperature. The f u g a c i t y c o e f f i c i e n t s are determined from equations o f s t a t e . The equations a v a i l a b l e f o r the vapor phase f u g a c i t y coeff i c i e n t s are l i s t e d i n F i g u r e 11. The two B-W-R equations and the Soave equation are a l s o used f o r l i q u i d phase f u g a c i t y c o e f f i c i e n t s . The Soave equation and the Hayden-0 Connell v i r i a l equat i o n are very recent a d d i t i o n s to the system. Therefore, i f they are e l i m i n a t e d from c o n s i d e r a t i o n , the Prausnitz-Chueh m o d i f i c a t i o n of the Redlich-Kwong equation and the BWR equations have r e c e i v e d the most usage. The l i q u i d r e f e r e n c e f u g a c i t y and the l i q u i d a c t i v i t y c o e f f i c i e n t models are l i s t e d i n F i g u r e s 12 and 13. A t the present time, Henry's Law f o r s u p e r c r i t i c a l components can be used only w i t h the UNIQUAC equation; the unsymmetric convention i s not i n c l u d e d i n the other l i q u i d a c t i v i t y c o e f f i c i e n t models. Vapor pressure w i t h a Poynting c o r r e c t i o n i s u s u a l l y used f o r the l i q u i d r e f e r e n c e f u g a c i t y . The Wilson equation and the NTRL equations are the most commonly u t i l i z e d l i q u i d a c t i v i t y c o e f f i c i e n t models. UNIQUAC and UNIFAC have j u s t been added to the system, and i f they f u l f i l l our e x p e c t a t i o n s , they w i l l become the most commonly used models. Enthalpy models are a l s o necessary i n VLE design c a l c u l a t i o n s . The procedures f o l l o w e d f o r vapor and l i q u i d enthalpy c a l c u l a t i o n s are s p e c i f i e d i n F i g u r e 14. The equation of s t a t e approach l - ( i ) i s the most popular f o r vapor enthalpy. For l i q u i d enthalpy, the equation of s t a t e approach l - ( i ) , the Yen-Alexander c o r r e l a t i o n l - ( i i ) , and method 2 o f mole f r a c t i o n averaging the pure component e n t h a l p i e s are used w i t h about equal frequency. Method 3 f o r l i q u i d enthalpy i n c l u d e s a heat o f mixing term, A H ^ ; i t i s not used very o f t e n because we have d i s c o v e r e d t h a t heats of mixing p r e d i c t e d by the l i q u i d a c t i v i t y c o e f f i c i e n t models are u n r e l i a b l e unless such data were i n c l u d e d i n the f i t t i n g procedure. A l l o f the property monitor subroutines have a group o f codes which a l l o w i n d i v i d u a l s to supply t h e i r own u s e r - s u b r o u t i n they f i n d the a v a i l a b l e methods inadequate. The user-subroutines have f i x e d names and argument l i s t s which are d e s c r i b e d i n the documentation f o r the monitor s u b r o u t i n e s . Why do these l i s t s c o n t a i n so manv models - some mediocre models, i n f a c t . New models are always being added to the s y s tem, but o l d models are never d i s c a r d e d . Once a process has been s u c c e s s f u l l y designed u s i n g a c e r t a i n model, t h a t model must always be a v a i l a b l e f o r f u t u r e process c a l c u l a t i o n s , expansion o f 1

m

x


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P H A S E EQUILIBRIA A N D F L U I D PROPERTIES I N C H E M I C A L INDUSTRY

k, = y/XI =

I.

VAPOR:

(II/x,)/

(

Industrial View of the State-of-the-Art in Phase ... - ACS Publications

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