Conference Overview - ACS Symposium Series (ACS Publications)

1 Conference Overview STEPHEN A . NEWMAN

Downloaded by UNIV OF BRIGHTON ALDRICH LIB on May 26, 2014 | http://pubs.acs.org Publication Date: October 29, 1980 | doi: 10.1021/bk-1980-0133.ch001

Foster Wheeler Energy Corporation, Livingston, NJ 07039

The theme of the symposium has relevance to workers engaged in a wide variety of activities; it is anticipated that the con cepts described in the specific papers will be useful in areas superficially different from each other, yet in a fundamental manner, indeed very similar. The invited conference papers have been organized into reasonably related subject matter, yet run the gamut from the very practical to the very theoretical with the intent of providing the theorist an appreciation of the prac tical problems facing the technologist,and the technologist, an awareness of the theoretical tools he can use to solve his pro blems. Several applied areas of concern, all involved with water, served as focal points for the meeting. Two of these, pollution control and coal processing, have current social significance and are subjects that everyone can readily appreciate. Practi cal insights into the fields were given in the industrially ori ented review papers, and the role of thermodynamics in contri buting to the solution of the many specific problems encountered in carrying out an engineering design were brought out in other papers. As support to the applied thermodynamic techniques de scribed in the application-oriented sessions, one session was devoted to theory. It is this fundamental work that enables the thermodynamic practitioner, be he either academic or industrial, to develop in a rational matter his correlations which are ab sorbed into the design techniques applied by process engineers. A very fine example was provided by the extensive use of Profes sor Pitzer's electrolyte activity coefficient theory within se veral acid gas phase equilibrium models. Another important group o f papers provided i n s i g h t i n t o s e v e r a l very s u c c e s s f u l c o o p e r a t i v e i n d u s t r i a l programs, those sponsored by the Gas Research I n s t i t u t e , the American Petroleum I n s t i t u t e , the Gas Processors A s s o c i a t i o n and the American I n s t i t u t e o f Chemical Engineers. The d e s i g n a t i o n i n f r a t e c h n o l o g y has been a p p l i e d t o those aspects o f t e c h n i c a l i n v e s t i g a t i o n and knowledge t h a t permeate a given i n d u s t r y , yet are n o t s p e c i f i c 0-8412-0569-8/ 80/47-133-001 $05.00/0 © 1980 American Chemical Society

In Thermodynamics of Aqueous Systems with Industrial Applications; Newman, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.


2

THERMODYNAMICS OF AQUEOUS SYSTEMS W I T H INDUSTRIAL

APPLICATIONS

enough t o i n d i v i d u a l c o n s t i t u e n t s t o j u s t i f y e x t e n s i v e i n v e s t i g a t i o n on t h e i r own. F r e q u e n t l y , the problems are so complex and d i f f u s e t h a t i t would be e s s e n t i a l l y u s e l e s s f o r a s i n g l e company t o embark on an i n v e s t i g a t i o n t h a t would take many manyears o f investment w i t h very l i t t l e monetary r a t e - o f - r e t u r n . C e r t a i n segments o f the chemical engineering i n d u s t r y have found a s o l u t i o n t o such problems by c o n s o l i d a t i n g i n t e r e s t s i n c o o p e r a t i v e research o r g a n i z a t i o n s such as those sponsored by A P I , GPA, GRI and AIChE. The growth o f these i n d u s t r i a l c o o p e r a t i v e groups i s encouraging; besides conserving resources and e l i m i n a t i n g d u p l i c a t i o n o f e f f o r t , these groups provide a forum f o r exchange o f ideas and n o n - p r o p r i e t a r y technology. The involvement of academia i n these o r g a n i z a t i o n s , u s u a l l y as research cont r a c t o r s , a l l o w s u n i v e r s i t y researchers an i n t i m a t e i n s i g h t i n t o r e a l - w o r l d problems and h e l p s c l o s e the breach between academia and i n d u s t r y t h a t has become so p r e v a l e n t i n recent y e a r s . A l l o f these f a c t o r s s u b s t a n t i a l l y i n c r e a s e output per u n i t i n p u t and by i t s very d e f i n i t i o n promote an i n c r e a s e i n p r o d u c t i v i t y . One very important element missing from the scene u n t i l r e c e n t l y has been government. Government's involvement has o b v i o u s l y been i n c r e a s i n g and w i l l become a welcome a d d i t i o n by p r o v i d i n g a d d i t i o n a l cohesiveness as w e l l as f i n a n c i a l backing and t e c h n i c a l e x p e r t i s e to f l e d g l i n g groups. As an i n t r o d u c t i o n t o the t e c h n i c a l aspects o f the conference, the r e s u l t s o f some s t u d i e s conducted by the w r i t e r on two r e l e v a n t s u b j e c t s are presented below. The f i r s t commentary i s concerned w i t h the design o f sour-water s t r i p p e r s and the e f f e c t s o f thermodynamic data on these designs; the second commentary i s concerned w i t h the c a l c u l a t i o n o f e n t h a l p i e s o f steam-containing m i x t u r e s , e s s e n t i a l t o the design o f c o a l p r o c e s s i n g and r e l a t e d p l a n t s · Sour-water Thermodynamic Data For the past s e v e r a l years there has been c o n s i d e r a b l e i n t e r e s t i n the phase e q u i l i b r i a o f sour-water systems. T h i s i n t e r e s t l a r g e l y stems from the need t o design sour-water s t r i p pers so t h a t t h e i r l i q u i d e f f l u e n t s a t t a i n the very low l e v e l s mandated by c u r r e n t environmental r e g u l a t i o n s . The EPA l i q u i d emission l e v e l g o a l s , based on e c o l o g i c a l e f f e c t s , are ten PPM (mass) f o r ammonia and two PPM (mass) f o r hydrogen s u l f i d e . The g u i d e l i n e s f o r tower design e s t a b l i s h e d many years ago ( f o r example, Û.06 kgs s t r i p p i n g steam per l i t e r o f hot feed) are outmoded and are not exact enough i n an e r a when s o p h i s t i c a t e d data and c a l c u l a t i o n methods can be a p p l i e d t o the problem once cons i d e r e d so complicated t h a t o n l y e m p i r i c a l r u l e s were accepted p r a c t i c e , f o r l a c k o f anything b e t t e r . A number o f models have been developed t o d e s c r i b e the chemistry o c c u r r i n g i n sour-water s t r i p p i n g and a b s o r p t i o n . The many simultaneous chemical r e a c t i o n s whose s o l u t i o n r e s u l t s i n the establishment o f the phase e q u i l i b r i a o f the com-



1.

NEWMAN

Conference Overview

3

ponents are l i s t e d i n Figure 1. This system i s i n s e v e r a l r e spects c o n s i d e r a b l y more complicated than many hydrocarbon and chemical systems i n that i t i s necessary to contend w i t h both chemical and phase e q u i l i b r i a i n e s t a b l i s h i n g the design. Each o f the c i t e d r e a c t i o n s and Henry's law constants, besides being temperature dependent, are a l s o f u n c t i o n s o f the type and concent r a t i o n o f acid-gas components i n s o l u t i o n and, i n some cases, the i o n i c s t r e n g t h o f the s o l u t i o n . In a p r a c t i c a l sense, the most important reason f o r c o n s i d e r i n g these e l e c t r o l y t e r e a c t i o n s i s t h a t o n l y those p o r t i o n s of ammonia, hydrogen s u l f i d e , and carbon d i o x i d e t h a t remain i n molecular, rather than i o n i c form, can be s t r i p p e d from s o l u t i o n . The o b j e c t of the c a l c u l a t i o n s , t h e r e f o r e , i s the determination of the i o n i c and molecular d i s t r i b u t i o n of species as a f u n c t i o n o f temperature,pressure,and composition. The designer can employ a computer program embodying the c a l c u l a t i o n s to c a r r y out h i s design, or he can use c h a r t s such as those shown i n Figure 2 as a b a s i s f o r h i s s t r i p p i n g or abs o r p t i o n c a l c u l a t i o n s . The two c h a r t s shown can be used e i t h e r to c a l c u l a t e the ammonia and hydrogen s u l f i d e concentrations i n the l i q u i d - p h a s e from a knowledge o f t h e i r p a r t i a l pressures i n the gas phase; o r , a l t e r n a t e l y , the p a r t i a l pressures can be d i r e c t l y read from the c h a r t s given the s o l u t i o n c o n c e n t r a t i o n of the d i s s o l v e d gases. C a l c u l a t i n g the l i q u i d - p h a s e composition from t h a t o f the gas r e q u i r e s a t r i a l - a n d - e r r o r procedure. A l i q u i d - p h a s e molar r a t i o o f the a c i d gases i s assumed, and from the known p a r t i a l pressures, a l i q u i d - p h a s e composition i s read from the graphs and a c a l c u l a t e d l i q u i d - p h a s e molar r a t i o i s obtained. This c a l c u l a t i o n i s repeated two or three times u n t i l the c a l c u l a t e d l i q u i d - p h a s e molar r a t i o equals the assumed v a l u e , and at t h i s p o i n t the e q u i l i b r i u m l i q u i d - p h a s e composition has been determined. For most sour water s t r i p p e r design work, a computer i s used to perform the c a l c u l a t i o n s . S e v e r a l o f the proposed sourwater modules were incorporated i n t o a tower program and a s e r i e s of designs on a t y p i c a l sour-water s t r i p p e r have been undertaken. Comparisons o f data methods with each other such as those shown i n Figure 3 f o r the Wilson (1) and Mason-Kao (2) methods demonstrate that the Wilson method generates the smaller ammonia and hydrogen s u l f i d e p a r t i a l pressures. In p r a c t i c a l terms, Wilson's p r e d i c t i o n s imply a greater amount o f s t r i p p i n g steam to achieve a d e s i r e d l e v e l of a c i d gas removal when compared to the Mason-Kac p r e d i c t i o n s . U n f o r t u n a t e l y , such comparisons are only s e m i - q u a n t i t a t i v e from a designer's viewpoint and i t i s nenessary to employ the f u l l s t r e n g t h o f a r i g o r o u s tower comput e r s i m u l a t i o n to demonstrate the s i g n i f i c a n c e that the data d i f ferences imply i n terms of equipment s p e c i f i c a t i o n and u t i l i t y comsumption. Some i n s i g h t i s provided i n the comparisons dep i c t e d i n Figures 4 and 5.


4

THERMODYNAMICS O F AQUEOUS SYSTEMS W I T H INDUSTRIAL

APPLICATIONS

Qc

SOUR-WATER,—c FEED -

PPM NH , H S, C0 = ? 3

2

2

QR SUMMARY OF CHEMICAL EQUILIBRIA INVOLVED IN CALCULATING NH -H S-C0 -H 0 VLE 3

C0 + H 0 Downloaded by UNIV OF BRIGHTON ALDRICH LIB on May 26, 2014 | http://pubs.acs.org Publication Date: October 29, 1980 | doi: 10.1021/bk-1980-0133.ch001

2

2

K

l

2

2

2

HC07+H+

HCOT* COT + H+ κ NH + H+ NH+ -K NH3+HCO3 H NC0 +H 0 h HS HS +H+ 2

3

3

4

2

2

2

2

HS-

Ke

S +H+ S= =

H0 H+ + OHI OF ALL ELECTRONIC CHARGES = Ο K ?

2

PNH

3

=

HNH

3

NH

3

Ρco2 = Hco C0 2

; PH S = 2

HH S 2

HS 2

2

IPC Science and Technology

Figure 1.

Figure 2.

Sour-water absorber/stripper VLE (4)

Typical design data charts for NH -H S-H 0 3

2

2

VLE



1.

NEWMAN

Conference Overview

The PPM ammonia remaining i n the bottoms product stream o f the s t r i p p e r tower are shown i n Figure 4 as a f u n c t i o n o f the kilograms of steam i n j e c t e d i n t o the tower. The number o f theo r e t i c a l stages used i s shown as a cross-parameter. What i s observed i s t h a t i n a p r a c t i c a l sense, the Wilson and Mason-Kao methods y i e l d e s s e n t i a l l y the same ammonia p u r i t y i n the s t r i p p e d water product, whereas very s u b s t a n t i a l d i f f e r e n c e s are obtained when the c l a s s i c a l van Krevelen c o r r e l a t i o n i s a p p l i e d to design the wastewater s t r i p p e r . Using the amount o f hydrogen s u l f i d e remaining i n the tower bottoms as the c r i t e r i o n , F i g u r e 5 shows s i m i l a r design comparisons. Large d i f f e r e n c e s between the methods are observed; however, the Mason-Kao p r e d i c t i o n s and van Krevelen r e s u l t s are very s i m i l a r . The Wilson method generates a more c o n s e r v a t i v e design i n t h a t to achieve a designated l e v e l o f hydrogen s u l f i d e p u r i t y a designer would s p e c i f y a g r e a t e r number o f t r a y s , or a l t e r n a t e l y , provide f o r more s t r i p p i n g steam. E n t h a l p i e s o f Steam-containing M i x t u r e s For those engaged i n designing p l a n t s f o r p r o c e s s i n g syngas streams, c o n s i s t i n g o f mixtures o f hydrogen, carbon monoxide, carbon d i o x i d e , n i t r o g e n and methane, there i s a constant need to c a l c u l a t e the e n t h a l p i e s of mixtures o f these gases w i t h steam. These data are r e q u i r e d i n the design o f r e a c t o r s , heat exchangers and s e p a r a t i o n equipment. U n f o r t u n a t e l y , d e s p i t e the prevalence of such systems i n design work, u n t i l very r e c e n t l y , there have been no experimental c a l o r i m e t r i c data w i t h which to assess the v a l i d i t y o f proposed design c o r r e l a t i o n s . With the i n c r e a s e d i n t e r e s t i n s u b s t i t u t e gas processes i n which e n t h a l p i e s f o r mixtures c o n t a i n i n g as much as 50 percent water, at high p r e s s u r e s , are r e q u i r e d , e f f o r t s to o b t a i n experimental enthalpy data f o r these systems have been i n i t i a t e d , and s e v e r a l measurement programs are underway. Some data comparisons were made o f s e v e r a l p r e d i c t i v e methods w i t h steam mixture enthalpy data obtained by P r o f e s s o r Wormald ( 3 ) . To provide a b a s i s o f comparison, Figure 6 i l l u s t r a t e s how three methods c u r r e n t l y i n vogue i n the thermodynamics world perform i n p r e d i c t i n g the enthalpy departures from i d e a l i t y o f methane. The p r e d i c t i o n s o f the Lee-Kesler e q u a t i o n - o f - s t a t e seem t o best r e p l i c a t e the d a t a , w i t h a maximum e r r o r of 1.2 kJ/kg. A p o r t i o n o f the enthalpy-temperature diagram f o r steam i s presented i n F i g u r e 7. F r e q u e n t l y , i n performing heat balance c a l c u l a t i o n s f o r syngas p r o c e s s i n g , i t i s necessary to develop e n t h a l p i e s f o r steam i n mixtures a t c o n d i t i o n s t h a t do not conform t o pure component c o n d i t i o n s . Design engineers f r e q u e n t l y develop t h e i r steam e n t h a l p i e s from the pure component data by e i t h e r using s a t u r a t i o n values or by e x t r a p o l a t i n g i n t o the pure component dome region from the l i n e a r p o r t i o n o f the s u p e r c r i t i c a l i s o b a r s . Some advocates o f t h i s l a t t e r procedure


5

APPLICATIONS


THERMODYNAMICS OF AQUEOUS SYSTEMS W I T H INDUSTRIAL

IPC Science and Technology

Figure 4.

Predicted PPM NH Kao; (

3

in tower bottoms (( ) van Krevelen) (4)

) Wilson; (

) Mason-


1.

NEWMAN

Conference Overview

7 100.0,

100.0

χ

\


t S s η,

* 10.0

ο t

\ \ \ %

g

\

Έ

\

* 10.0

s

\

\

\ \Ν

\

ν

X

\ s

X \

\ Ν

\% 0.15

Figure 5.

0.20 0.25 K6S STEAM/L FEED

0.30

1.0

0.15

0.201 0.25 KGS STEAM/L FEED

Predicted PPM H S in tower bottoms (( Kao; ( ) van Krevelen) 2

5000

10,000

) Wilson; (

0.30

) Mason-

15,000

Figure 6. Comparisons of predicted and experimental enthalpy departures for pure methane at 598 Κ



8

THERMODYNAMICS O F AQUEOUS SYSTEMS WITH INDUSTRIAL

500

550

600

Figure 7.

Figure 8.

650

TEMPERATURE/K

700

APPLICATIONS

750

800

Enthalpy diagram for steam

Comparisons of predicted and experimental enthalpy departures for pure steam at 598 Κ



1.

NEWMAN

Conference Overview

use enthalpy values at the p a r t i a l pressure of steam, o t h e r s , at the t o t a l system pressure. In the enthalpy diagram, the temperature at which the o n l y p u b l i s h e d steam mixture c a l o r i m e t r i c data were obtained i s i n d i c a t e d . The pressure range o f the data spans from i d e a l gas to s a t u r a t i o n p r e s s u r e , about 12,000 kPa. Figure 8 d e p i c t s how the three popular e q u a t i o n - o f - s t a t e methods c i t e d p r e v i o u s l y perform on pure steam. From a theoret i c a l viewpoint, none of the methods has the foundation to handle mixtures o f polar/non-polar components. Although the agreement w i t h experimental data i s not very s a t i s f a c t o r y f o r any o f the methods, the Lee-Kesler e q u a t i o n - o f - s t a t e does best. I t was a l s o found t h a t by s l i g h t l y a d j u s t i n g the a c e n t r i c f a c t o r o f water, improvement i n the r e p r e s e n t a t i o n o f the enthalpy of steam can be obtained by t h i s method a t 598 K, the c o n d i t i o n s o f the experimental mixture d a t a , and at other temperatures as well. Figure 9 provides a comparison of the p r e d i c t i o n s of emp i r i c a l methods w i t h Wormald's data f o r a 50/50 mole percent mixture o f steam and methane. As can be seen, the f r e q u e n t l y used a r t i f i c e s of c a l c u l a t i n g mixture e n t h a l p i e s by blending the pure component e n t h a l p i e s a t e i t h e r t o t a l or p a r t i a l pressures are very i n a c c u r a t e . L i k e w i s e , the assumption of i d e a l gas ent h a l p y f o r the r e a l gas m i x t u r e , e q u i v a l e n t to a zero enthalpy departure on the diagram, i s an e q u a l l y poor method. In Figure 10 are shown comparisons of the equation of s t a t e methods w i t h the experimental data. The Lee-Kesler methods r e present the data the best. Again, i f the water a c e n t r i c f a c t o r determined to best represent the pure steam enthalpy data i s a p p l i e d to the mixtures, f u r t h e r improvement i s noted f o r the p r e d i c t i o n s by the Lee-Kesler method. Use o f i n t e r a c t i o n cons t a n t s w i t h i n the Lee-Kesler, or other models, would undoubtedly provide even b e t t e r r e p r e s e n t a t i o n o f the data. Although comparisons f o r the steam-methane system have been presented, s i m i l a r trends were noted f o r the other b i n a r y s y s tems p r e v i o u s l y p u b l i s h e d by Wormald, namely mixtures of steam w i t h n i t r o g e n , carbon d i o x i d e , n-hexane, and benzene. P r o f e s s o r Wormald, i n a paper presented a t t h i s conference, has provided a d d i t i o n a l steam mixture enthalpy data and some c o r r e l a t i o n work he has done on the data using an a s s o c i a t i o n model.


9

THERMODYNAMICS O F AQUEOUS SYSTEMS W I T H INDUSTRIAL

200-1

APPLICATIONS

PURE COMPONENT / DATA / (TOTAL PRESSURE) /


EXPERIMENTAL 100-

o >Î3 PURE COMPONENT DATA (PARTIAL PRESSURE) PRESSURE/kPa 5000

10,000

15,000

Figure 9. Comparisons of predicted and experimental enthalpy departures for an equimolar steam-methane mixture at 598 Κ (equation-of-state methods)


1.

NEWMAN

Conference

11

Overview

Literature Cited

1. Wilson, G.M., "A New Correlation of NH , CO and HS Vola tility Data from Aqueous Sour Water Systems." Final Report to API Committee on Refinery Environmental Control under EPA Grant No. R804364010, Thermochemical Institute, BYU, Provo, Utah, February 9, 1978. 3

2

2

2. Mason, D., "Vapor-Liquid Equilibria in the NH-CO-HS-HO System", Project 8979, "Preparation of a Coal Conversion Systems Technical Data Book," Quarterly Reports by Institute of Gas Technology for U.S.Department of Energy Contract ΕΧ76-C-01-2286, February and May, 1978.


3

2

2

2

3. Wormald, C.J., "Thermodynamic Properties of Some Mixtures Containing Steam." Paper presented at National Physical Laboratory Conference on Chemical Thermodynamic Data on Fluids and Fluid Mixtures, Teddington, Middlesex, U.K., September 11-12, 1978. Conference Proceedings published by IPC Science and Technology Press. 4. Newman, S.A., "Novel Applications of Phase Equilibria Methods to Process Design Problems." Paper presented at National Physical Laboratory Conference on Chemical Thermo dynamic Data on Fluids and Fluid Mixtures, Teddington, Middlesex, U.K., September 11-12, 1978. Conference Pro ceedings published by IPC Science and Technology Press. 5. Lee, B.-I. and Kesler, M.G., AIChE Journal, 1975, 12(5), 510. 6. Peng, D.-Y. and Robinson, D.B., Ind. Eng. Chem., Fundam, 1976, 15, 59 7. Soave, G., Chem. Eng. Science, 1972, 27, 1197. 8. van Krevelen, D.W., Hoftijzer, P.J. and Huntjens, F.J., Recueil des Travaux Chimiques des Pays-Bas, 1949, 68, 191. RECEIVED

January 17, 1980.


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