volume 10.1021bk-1980-0133 issue 1980 [doi 10.1021%2fbk-1980-0133.ch020] newman, stephen a.; barner,...

20 Two- and Three-Phase Equilibrium Calculations for Coal Gasification and Related Processes

D.-Y. PENG and D. B. ROBINSON

Department of Chemical Engineering, University of Alberta, Edmonton, Alberta, Canada T6G 2G6

The gasificiation of coal, shale-oil, or other lower grade hydrocarbon base stocks inevitably leads to the production of process streams which contain a very wide range of paraffinie, naphthenic, aromatic and olefinic hydrocarbons in the presence of associated non-hydrocarbons such as hydrogen, nitrogen, carbon dioxide, hydrogen sulfide and ammonia. These streams are often contacted with water at process conditions which normally lead to either gas - water - rich liquid equilibrium or gas -water - rich liquid - hydrocarbon rich liquid equilibrium. The processing conditions and stream compositions which may lead to the formation of these different phases and the distribution of the components between phases are of great importance to the design engineer. For this reason the establishment of reliable procedures for predicting the behavior of these mixtures in the one-, two-, and three-phase regions is a matter of considerable importance.

In an earlier paper (1), the authors presented an efficient procedure for predicting the phase behavior of systems exhibiting a water - rich liquid phase, a hydrocarbon - rich liquid phase, and a vapor phase . The Peng-Robinson e q u a t i o n o f s t a t e (2) was used t o r e p r e s e n t the b e h a v i o r o f a l l t h r e e phases . I t has the f o l l o w i n g f o r m :

v v-b " v(v+bj + b(v-b) where a(T) = a α

= KL . a (T) ( 1 )

a c = 0 . 4 5 7 2 4 - j ±

= 1 + K ( 1 - T R ^ 2 ) ( 2 )

0-8412-0569-8/80/47-133-393$05.50/0 © 1980 American Chemical Society

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

394 THERMODYNAMICS OF AQUEOUS SYSTEMS WITH INDUSTRIAL APPLICATIONS

κ = 0.37464 + 1.54226ω - 0.26992ω2 (3)

R T c b = 0.07780 ψ-±

c For m i x t u r e s

b = I x. b. ( 5 )

A l t h o u g h the c a l c u l a t e d phase c o m p o s i t i o n s f o r the h y d r o carbon - r i c h l i q u i d phase and the vapor phase showed e x c e l l e n t agreement w i t h the e x p e r i m e n t a l d a t a , the c a l c u l a t e d hydrocarbon c o n t e n t s o f the aqueous l i q u i d phase was c o n s i s t e n t l y s e v e r a l o r d e r s o f magnitude lower than the r e p o r t e d e x p e r i m e n t a l v a l u e s . I t was s p e c u l a t e d t h a t a d d i t i o n a l t empera tu re - dependent i n t e r a c t i o n parameters would be r e q u i r e d to b r i n g the p r e d i c t e d va lues and the e x p e r i m e n t a l r e s u l t s i n t o q u a n t i t a t i v e agreement ; n e v e r t h e l e s s , no a t t empt was made a t t h a t t ime t o t r y to a c c o m p l i s h t h i s .

In t h i s s t u d y , i t has been p o s s i b l e t o d e v i s e a p rocedure wh ich can be used t o gene ra te r e l i a b l e phase c o m p o s i t i o n s f o r both the hydrocarbon - r i c h phase and the aqueous phase ove r a wide range o f t empera tu re and p r e s s u r e . Moreove r , the c a l c u l a t i o n p rocedure has been s u c c e s s f u l l y a p p l i e d t o non-hydrocarbon -wate r systems w i t h q u a n t i t a t i v e r e s u l t s .

C a l c u l a t i o n P r o c e d u r e . Wi th the e x c e p t i o n o f two s i g n i f i c a n t m o d i f i c a t i o n s , the c a l c u l a t i o n p rocedure used i n t h i s s tudy was b a s i c a l l y the same as t h a t used p r e v i o u s l y .

The f i r s t m o d i f i c a t i o n concerns the use o f Eqn. ( 2 ) f o r w a t e r . When d e v e l o p i n g the o r i g i n a l c o r r e l a t i o n f o r oh and κ as e x p r e s s e d by Eqn . ( 2 ) and ( 3 ) , wa te r was no t i n c l u d e d as one o f the components , and c o n s e q u e n t l y the p r e d i c t e d vapor p r e s s u r e s f o r wa te r were no t as good as e x p e c t e d . Thus i n o r d e r to c o r r e l a t e the vapor p r e s s u r e o f wa te r more a c c u r a t e l y o ve r the e n t i r e t empera tu re range^ Eqn. ( 2 ) was m o d i f i e d f o r t h i s compound a t t empera tures where T R ^ < 0 . 8 5 as f o l l o w s :

ah = 1 . 0 0 8 5 6 7 7 + 0 . 8 2 1 5 4 ( I - T J * ) ( 6 ) ν K

A t t empera tu res where T R2 >_ 0 . 8 5 , Eqn. ( 2 ) s t i l l a p p l i e s .

The second m o d i f i c a t i o n concerns the c o r r e l a t i o n o f the c o m p o s i t i o n o f the aqueous l i q u i d phase . In o r d e r to a c c o m p l i s h t h i s , a tempera ture - dependent i n t e r a c t i o n parameter was used f o r the aqueous l i q u i d phase and the p r e v i o u s tempera ture -independent parameter was used f o r the non-aqueous l i q u i d phase and the vapor phase . Thus f o r the aqueous - l i q u i d phase Eqn . ( 4 ) becomes

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20. PENG AND ROBINSON Coal Gasification and Related Processes 395

where τ-jj i s a tempera tu re - dependent i n t e r a c t i o n pa ramete r . The i n t r o d u c t i o n o f t h i s parameter f o r each aqueous b i n a r y

p a i r means t h a t the i n t e r a c t i o n between the water m o l e c u l e and the gas m o l e c u l e i n the aqueous l i q u i d phase i s much d i f f e r e n t from t h a t i n the nonaqueous phases . For a l l the aqueous b i n a r i e s wh ich have been examined i n t h i s s t u d y , the tempera tu re -dependent i n t e r a c t i o n parameters take on n e g a t i v e v a l u e s a t ambient tempera ture and m o n o t o n i c a l l y i n c r e a s e as tempera tu re i n c r e a s e s . T h i s i n d i c a t e s t h a t the a t t r a c t i o n energy between the water m o l e c u l a r and the o t h e r m o l e c u l e s dec reases as the tempera ture i n c r e a s e s .

Non-Hydrocarbon - Water B i n a r i e s

Of the many p o s s i b l e non-hydrocarbon - wate r b i n a r y systems which a re r e l a t e d to s u b s t i t u t e gas p r o c e s s e s , the d a t a on o n l y the wa te r b i n a r i e s c o n t a i n i n g H 2 S , C 0 2 , N 2 , and NH 3 were used i n t h i s s t u d y . The t r ea tmen t o f hyd rogen , a quantum g a s , i s d i f f e r e n t from t h a t o f the o t h e r g a s e s . A sepa ra t e paper w i l l dea l w i t h the c o r r e l a t i o n o f the da ta on hydrogen m i x t u r e s .

Hydrogen S u l f i d e - Water Sys tem. The da ta o f S e l l e c k e t a l . (J3) were used to e v a l u a t e the i n t e r a c t i o n parameters f o r the hydrogen s u l f i d e - wate r s y s t e m . The d a t a i n c l u d e the c o m p o s i t i o n o f both phases a t tempera tures f rom 100°F to 340°F and p r e s s u r e s from 100 t o 5000 p s i a i n the c o e x i s t i n g vapor and aqueous l i q u i d -hydrogen s u l f i d e - r i c h l i q u i d - vapor e q u i l i b r i u m l o c u s .

A s i n g l e , c o n s t a n t i n t e r a c t i o n parameter has been de te rmined f o r the hydrogen s u l f i d e - r i c h phases . T h i s d e t e r m i n a t i o n was based on the three-phase p r e s s u r e - tempera tu re l o c u s . Wh i l e i n v e s t i g a t i n g the three-phase r e g i o n , i t was noted t h a t the t h r e e -phase l o c u s and the c o m p o s i t i o n o f the hydrogen s u l f i d e - r i c h phase were r a t h e r i n s e n s i t i v e to the tempera tu re - dependent aqueous phase i n t e r a c t i o n pa ramete r . F u r t h e r m o r e , the c o m p o s i t i o n o f the aqueous phase was r e l a t i v e l y independent o f the c o n s t a n t i n t e r a c t i o n pa ramete r . For these r e a s o n s , the s o l u b i l i t y o f hydrogen s u l f i d e i n the aqueous l i q u i d was c o r r e l a t e d a t the same t ime as the parameter was be i ng de te rmined f o r the hydrogen s u l f i d e - r i c h phases .

The c a l c u l a t e d and e x p e r i m e n t a l gaseous and l i q u i d phase c o m p o s i t i o n s a re shown i n F i g u r e s 1 and 2 r e s p e c t i v e l y .

Carbon D i o x i d e - Water Sys tem. The da ta o f Wiebe and Gaddy (1? §) w e r e u s e d e x c l u s i v e l y i n t h i s s t u d y to de te rm ine the i n t e r a c t i o n parameters f o r the carbon d i o x i d e - wa te r b i n a r y s ys t em. These d a t a cove r the tempera tu re and p r e s s u r e range from 12°C to 100°C and from 25 atm to 700 atm r e s p e c t i v e l y . As w i t h the H2S - H2O s y s t e m , a c o n s t a n t i n t e r a c t i o n parameter has been o b t a i n e d f o r the gaseous phase and the carbon d i o x i d e - r i c h

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4000

0 I 1 L 0.80 0.85 0.90 0.95 1.00

MOLE FRACTION HYDROGEN SULFIDE

Figure 1. Experimental and predicted vapor phase compositions for the hydrogen sulfide-water system (( ) P-R prediction; data from Ref. 3: (Φ) 340°F; (O)

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PENG AND ROBINSON Coal Gasification and Related Processes 397

MOLE FRACTION HYDROGEN SULFIDE

Figure 2. Experimental and predicted aqueous liquid phase compositions for the hydrogen sulfide-water system (( ) P-R prediction; data from Ref. 3: (Φ)

160°F; (A) 220°F; (U> 280 T; Cf) 340°F)

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l i q u i d phase . A t each t e m p e r a t u r e , the s o l u b i l i t y o f carbon d i o x i d e i n wa te r can be c o r r e l a t e d a c c u r a t e l y th rough the whole p r e s s u r e range u s i n g one i n t e r a c t i o n parameter f o r the aqueous phase . The e q u i l i b r i u m aqueous l i q u i d and vapor c o m p o s i t i o n s f o r t h i s b i n a r y a t two tempera tu res a r e shown i n F i g . 3.

M a l i n i n ( 7 ) , Todhe ide and Franck (8) and Takenouchi and Kennedy (9) r e p o r t e d e q u i l i b r i u m d a t a f o r t h i s system a t t empera tu res up to 350°C and p r e s s u r e s t o 3500 b a r s . However, the vapor phase d a t a o f these au tho r s do no t a lways agree w i t h each o t h e r . The aqueous phase d a t a have been used to ex tend the tempera tu re - dependent i n t e r a c t i o n parameter to 300°C.

N i t r o g e n - Water Sys tem. The i n t e r a c t i o n parameters f o r the n i t r o g e n - wa te r sys tem have been e v a l u a t e d u s i n g the da t a o f Wiebe and Gaddy ( 1 0 ) , P a r a t e l l a and Sagramora (21), R igby and P r a u s n i t z (12)and 0 ' S u l l i v a n and Smith ( 1 3 ) . As w i t h the two p r e v i o u s s y s t e m s , o n l y one c o n s t a n t i n t e r a c t i o n parameter was n e c e s s a r y to c o r r e l a t e the vapor phase c o m p o s i t i o n w h i l e the i n t e r a c t i o n parameter f o r the aqueous l i q u i d phase i n c r e a s e d m o n o t o n i c a l l y w i t h t e m p e r a t u r e . A compar i son o f the c a l c u l a t e d and e x p e r i m e n t a l vapor phase and aqueous l i q u i d phase c o m p o s i t i o n s i s g i v e n i n Tab le I.

Ammonia - Water Sys tem. I n t e r a c t i o n parameter f o r the ammonia - wa te r system was o b t a i n e d u s i n g the da ta o f C l i f f o r d and Hunter (14) and o f M a c r i s s e t a l . ( 1 5 ) . A s i n g l e - v a l u e d parameter was capab l e o f r e p r e s e n t i n g the c o m p o s i t i o n o f the l i q u i d phase r e a s o n a b l y w e l l a t a l l t e m p e r a t u r e s , however , the c a l c u l a t e d amount o f wa te r i n the vapor phase i n the ve r y h i gh ammonia c o n c e n t r a t i o n r e g i o n was somewhat l ower than the da t a o f C l i f f o r d and Hunter and M a c r i s s e t a l . Edwards e t a l . (16) have a p p l i e d a new thermodynamic c o n s i s t e n c y t e s t t o the d a t a o f M a c r i s s e t a l and have conc luded t h a t the da t a appear t o be i n c o n s i s t e n t and t h a t the r e p o r t e d wate r c o n t e n t o f the vapor phase i s too h i g h .

The e x p e r i m e n t a l d a t a and the c a l c u l a t e d r e s u l t s a r e g i v e n i n F i g . 4 . Hydrocarbon - Water B i n a r i e s

The i n t e r a c t i o n parameters f o r b i n a r y systems c o n t a i n i n g wa te r w i t h methane, e t h a n e , p ropane , η-butane, n-pentane, n-hexane, η-octane, and benzene have been de te rm ined u s i n g da t a f rom the l i t e r a t u r e . The phase b e h a v i o r o f the p a r a f f i n - wa te r systems can be r e p r e s e n t e d ve r y w e l l u s i n g the m o d i f i e d p r o c e d u r e . However, the a r o m a t i c - wa t e r sys tem can no t be c o r r e l a t e d s a t i s f a c t o r i l y . P o s s i b l y a d i f f e r e t n type o f m i x i n g r u l e w i l l be r e q u i r e d f o r the a r o m a t i c - wa te r s y s t e m s , a l t h o u g h t h i s has not as y e t been e x p l o r e d .

Methane - Water Sys tem. I n t e r a c t i o n parameters were gene ra ted f o r the vapor phase and the aqueous l i q u i d phase f o r the methane -

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PENG AND ROBINSON Coal Gasification and Related Processes 399

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TABLE I. Expe r imen ta l and C a l c u l a t e d Aqueous L i q u i d and Vapor Phase Compos i t i ons f o r the N i t r o g e n - Water Sys tem.

* 3 * 3 P r e s s u r e , a tm. x N χ 10 y Χ 10 E x p t . C a l c . E x p t . C a l c .

Τ = 25°C

22 .20 1.529 1.502 25 0 .280 0 .278 30 .50 1.149 1.123 38 .19 0.941 0 .919 50 0 .542 0.537

100 1.015 1.004 200 1.812 1.795 300 2.455 2.458 500 3.558 3.555 800 4 .909 4 .869

1000 5.720 5.604

Τ = 50°C

20.81 6 .260 6 .190 25 0.219 0 .220 36 .93 3 .680 3.640 50 0.436 0 .428 59 .04 2.420 2 .410 75 .99 1.956 1.952

100 0 .812 0 .810 200 1.470 1.470 300 2.034 2.032 500 2.982 2 .968 800 4.181 4 .084

1000 4 .900 4.701

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Tab le I - c o n t i n u e d . * 3 * 3

P r e s s u r e , a tm. χ N χ 10 y Χ 10

E x p t . C a l c . E x p t . C a l c .

Τ = 75°C

25 0 .204 0 .203

41 .66 10 .09 10.12

50 0.397 0 .398

6 0 . 3 5 7.21 7.25

88 .55 5.23 5.23

100 0 .760 0 .760

200 1.390 1.395

300 1.936 1.942

500 2.872 2.859

800 4 .052 3.948

1000 4 .747 4.544

Τ = 100°C

25 0 .214 0.206

50 0 .415 0 .410

56 .42 19 .94 19 .94

78 .44 15 .03 14.89

100 0 .792 0 .792

100.o9 12.19 12 .09

200 1.462 1.470

300 2.042 2 .060

500 3.044 3.052

800 4 .294 4 .223

1000 5.003 4 .857

* Mole F r a c t i o n

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0 0.2 0.4 0.6 0.8 10

MOLE FRACTION AMMONIA

Figure 4. Experimental and predicted vapor and liquid phase compositions for the ammonia-water system (( ) P-R prediction; data from Ref. 15: liquid— (A) 300°F; (f) 200°F; (*) 100°F; vapor—(A) 300°F; (V) 200°F; (O) 100°F)

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wate r b i n a r y system u s i n g e x p e r i m e n t a l da t a r e p o r t e d by S u l t a n o v e t a l . (17_, 1 8 ) , O lds e t a l . ( 1 9 ) , and C u l b e r s o n and McKet ta ( 2 0 ) .

The vapor-phase mole f r a c t i o n s o f wa te r o f O lds e t a l . (19) can be r e p r e s e n t e d ve r y w e l l u s i n g the Peng-Robinson e q u a t i o n o f s t a t e i n c o n j u n c t i o n w i t h a c o n s t a n t i n t e r a c t i o n parameter o ve r the tempera tu re range from 100°F to 460°F. The same i n t e r a c t i o n parameter can be used to rep roduce the da ta o f S u l t a n o v e t a l . (18) up to 300°C w i t h good r e s u l t s . However, a t h i g h e r tempera tures the c a l c u l a t e d wa te r c o n t e n t i n the vapor phase d e v i a t e d somewhat f rom t h e i r d a t a .

The tempera tu re - dependent i n t e r a c t i o n parameters were de te rmined f rom 77°F to 680°F u s i n g the da t a o f C u l b e r s o n and McKet ta (20) and o f S u l t a n o v e t a l . (1_8). T h i s parameter i n c r e a s e s w i t h tempera ture and appears t o converge to the v a l u e o f the c o n s t a n t parameter used f o r the vapor phase as the c r i t i c a l t empera tu re o f wa te r i s app roached .

The e x p e r i m e n t a l and c a l c u l a t e d r e s u l t s f o r t h i s b i n a r y system a t 250°C a re p resen ted i n F i g u r e 5.

Ethane - Water Sys tem. The da ta used f o r the d e t e r m i n a t i o n o f the i n t e r a c t i o n parameters f o r the ethane - wa te r b i n a r y a r e those o f C u l b e r s o n and McKet ta ( 2 1 ) , C u l b e r s o n e t a l . (22) and Reamer e t a l . (230 ·

A c o n s t a n t i n t e r a c t i o n parameter was capab l e o f r e p r e s e n t i n g the mole f r a c t i o n o f wate r i n the vapor phase w i t h i n e x p e r i m e n t a l u n c e r t a i n t y ove r the tempera ture range f rom 100°F to 460°F. As w i t h the methane - wa te r s y s t e m , the tempera tu re - dependent i n t e r a c t i o n parameter i s a l s o a m o n o t o n i c a l l y i n c r e a s i n g f u n c t i o n o f t e m p e r a t u r e . However, a t each s p e c i f i e d t e m p e r a t u r e , the i n t e r a c t i o n parameter f o r t h i s system i s n u m e r i c a l l y g r e a t e r than t h a t f o r the methane - wa te r s y s t em. A l t h o u g h i t i s p o s s i b l e f o r t h i s b i n a r y to form a three-phase e q u i l i b r i u m l o c u s , no e x p e r i m e n t a l da t a on t h i s e f f e c t have been r e p o r t e d .

F i g u r e 6 i l l u s t r a t e s the c a l c u l a t e d and e x p e r i m e n t a l e q u i l i b r i u m phase c o m p o s i t i o n s a t 220°F f o r t h i s b i n a r y s y s t em.

Propane - Water Sys tem. The i n t e r a c t i o n parameters f o r the propane - wa te r system were o b t a i n e d ove r a t empera tu re range f rom 42°F t o 310°F u s i n g e x c l u s i v e l y the da t a o f Kobayashi and Ka tz ( 2^ ) . T h i s i s because among the a v a i l a b l e l i t e r a t u r e on the phase b e h a v i o r o f t h i s b i n a r y s y s t e m , t h e i r d a t a appear to g i v e the most e x t e n s i v e i n f o r m a t i o n . A c o n s t a n t i n t e r a c t i o n parameter was o b t a i n e d f o r the p r o p a n e - r i c h phases a t a l l t e m p e r a t u r e s . The magni tude o f the tempera tu re - dependent i n t e r a c t i o n parameter f o r t h i s b i n a r y was l e s s than t h a t f o r the ethane - wate r b i n a r y a t the same t e m p e r a t u r e . Aza rnoosh and McKet ta (25) a l s o p r e sen t ed e x p e r i m e n t a l da t a f o r the s o l u b i l i t y o f propane i n wa te r ove r about the same tempera tu re range as t h a t o f Kobayashi and Ka tz bu t a t p r e s s u r e s up to 500 p s i a o n l y . However, a d i f f e r e n t s e t o f tempera ture - dependent parameters

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404 THERMODYNAMICS OF AQUEOUS SYSTEMS W ITH INDUSTRIAL APPLICATIONS

Figure 5. Experimental and predicted vapor and liquid phase compositions for methane-water system at 250°C (( ) P-R prediction; (A) (17); (A) (IS))

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MOLE FRACTION

Figure 6. Experimental and predicted vapor and liquid phase compositions for the ethane-water system at 220°F (( ; P-R prediction; (A) (21); (O) (23)) D

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406 THERMODYNAMICS OF AQUEOUS SYSTEMS W ITH INDUSTRIAL APPLICATIONS

would be r e q u i r e d to a c c u r a t e l y c o r r e l a t e t h e i r r e s u l t s . The wate r c o n t e n t o f the propane - r i c h phases i n the

aqueous l i q u i d - propane l i q u i d - vapor r e g i o n a re i l l u s t r a t e d i n F i g u r e 7.

η-Butane - Water Sys tem. Reamer e t a l . (26) have de te rmined the c o n c e n t r a t i o n o f wa te r i n the n-butane - wa te r system i n the vapor and the η-butane l i q u i d phases i n the th ree-phase r e g i o n . Reamer e t a l . (27) have p u b l i s h e d e x p e r i m e n t a l da ta f o r the s o l u b i l i t y o f η-butane i n wa te r and o f wa te r i n η-butane i n the two-phase r e g i o n c o v e r i n g a tempera ture range f rom 100°F to 460°F and a t p r e s s u r e s up to 10 ,000 p s i a . LeB re ton and McKet ta (28) have p r e sen t ed the r e s u l t s o f an e x p e r i m e n t a l s t udy on the s o l u b i l i t y o f η-butane i n wa te r a t f o u r t empera tu res bu t a t p r e s s u r e s up t o o n l y 1000 p s i a . Wh i l e the r e p o r t e d th ree-phase p r e s s u r e s f rom these two sources agree v e r y w e l l , the da t a on the s o l u b i l i t y o f η-butane i n wa te r show marked d i f f e r e n c e s . The s o l u b i l i t y v a l u e s p r e sen t ed by LeB re ton and McKet ta a re c o n s i s t e n t l y lower than those r e p o r t e d by Reamer e t a l . In v iew o f the f a c t t h a t the da t a o f Reamer e t a l . cove red a much b roade r range o f both tempera tu re and p r e s s u r e , t h e i r da t a were used f o r d e t e r m i n i n g the i n t e r a c t i o n parameters f o r t h i s s ys tem.

As w i t h the f i r s t t h r e e p a r a f f i n - wa te r s y s t e m s , o n l y a c o n s t a n t parameter was r e q u i r e d to c o r r e l a t e the hydroca rbon -r i c h phases a l t h o u g h a t empera tu re - dependent parameter was n e c e s s a r y to f i t the aqueous - l i q u i d phase d a t a .

The e q u i l i b r i u m c o m p o s i t i o n o f the n-butane - wa te r b i n a r y i n the th ree-phase r e g i o n a r e i l l u s t r a t e d i n F i g u r e 8.

n-Pentane - Water Sys tem. S c h e f f e r (29) has p r e s e n t e d the th ree-phase l o c u s f o r a m i x t u r e o f i-pentane and n-pentane o ve r a t empera tu re range f rom 150°C to 187.1°C. However, no c o m p o s i t i o n a l measurements were r e p o r t e d . Namiot and B e i d e r (30) r e p o r t e d the s o l u b i l i t y o f n-pentane i n wa te r a t t h r e e t empera tu res a l o n g the th ree-phase l o c u s . I n t e r a c t i o n parameters f o r the n-pentane - wa te r system were de te rm ined u s i n g these d a t a .

n-Hexane - Water Sys tem. The n-hexane - wa te r system i s the l i g h t e s t p a r a f f i n - wa te r b i n a r y where the vapor p r e s s u r e l o c u s o f the hydroca rbon i n t e r s e c t s t h a t f o r pure w a t e r . The e x p e r i menta l phase b e h a v i o r da ta a v a i l a b l e i n the l i t e r a t u r e f o r t h i s sys tem cove r a wide range o f t empera tu re and p r e s s u r e . U n f o r t u n a t e l y these d a t a do not c o r r o b o r a t e each o t h e r and n o t i c e a b l e d i s c r e p a n c i e s e x i s t . The da t a o f S c h e f f e r ( 3 1 ) , Reber t and Hayworth ( 3 2 ) , and S u l t a n o v and S k r i p k a ( 3 3 j were employed i n d e t e r m i n i n g the i n t e r a c t i o n parameter f o r the hydrocarbon - r i c h phases . A un ique va l ue f o r t h i s i n t e r a c t i o n parameter c o u l d not be o b t a i n e d because o f the d i s c r e p a n c i e s among the d a t a . However, a t e n t a t i v e v a l u e , based on the e x t r a p o l a t i o n o f the v a l ue s f o r o t h e r p a r a f f i n - wa te r i n t e r a c t i o n p a r a m e t e r s , has been a s s i g n e d to the c o n s t a n t i n t e r a c t i o n pa ramete r . The i n t e r a c t i o n parameters f o r the aqueous l i q u i d phase were de te rmined u s i n g the d a t a o f

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20. P E N G AND ROBINSON Coal Gasification and Related Processes 407

0 2 4 6 8 10 12 14

MOLE FRACTION WATER χ 103

Figure 7. Experimental and predicted water content of propane liquid and vapor phases for the propane-water system along the three-phase locus (( ) P-R

prediction; data from Ref. 24: (O) vapor; (Φ) liquid)

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50 100 150 200 250 300 350 TEMPERATURE °F

Figure 8. Experimental and predicted water content of n-butane and vapor phases for the n-butane-water system along the three-phase locus (( ) P-R prediction;

(·) (2D)

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Kudchadker and McKet ta ( 3 4 ) . T h e i r s o l u b i l i t y da ta i n the vapor - l i q u i d r e g i o n are b e l i e v e d to be i n e r r o r p r o b a b l y due to t h e i r i n c o r r e c t p rocedure o f smooth ing the raw d a t a . However, t h e i r da t a i n the l i q u i d - l i q u i d r e g i o n seem to be a c c e p t a b l e . The da t a o f Rebe r t and Hayworth (32) were used t o ex tend the tempera tu re - dependent i n t e r a c t i o n parameters t o tempera tu res above the c r i t i c a l p o i n t o f n-hexane.

O the r Hydrocarbon - Water Sys tems. I n t e r a c t i o n parameters were gene ra ted f o r the benzene - wa te r s y s t em. The da t a used were those o f S c h e f f e r (3]_), Reber t and Kay (35)> and C o n n o l l y ( 3 6 ) . As w i t h the a l k a n e - wate r s y s t e m s , the i n t e r a c t i o n parameters f o r the aqueous l i q u i d phase were found to be tempera ture - dependent . However, the c o m p o s i t i o n s f o r the benzene - r i c h phases c o u l d no t be a c c u r a t e l y r e p r e s e n t e d u s i n g any s i n g l e v a l u e f o r the c o n s t a n t i n t e r a c t i o n pa ramete r . The c a l c u l a t e d wa te r mole f r a c t i o n s i n the hydrocarbon - r i c h phases were a lways g r e a t e r than the e x p e r i m e n t a l v a l ues as r e p o r t e d by Reber t and Kay ( 3 5 ) . The f i n a l v a l ue f o r the c o n s t a n t i n t e r a c t i o n parameter was chosen t o f i t the t h r e e phase l o c u s o f t h i s s y s t em . N e v e r t h e l e s s , the c a l c u l a t e d th ree-phase c r i t i c a l p o i n t was about 9°C lower than the e x p e r i m e n t a l v a l u e .

I n t e r a c t i o n parameter was a l s o gene ra ted f o r the hydroca rbon -r i c h phases o f the n-octane - wa te r s y s t em. The d a t a o f K a l a f a t i and P i i r (37) were u s e d . There were no da ta a v a i l a b l e f o r the wate r - r i c h l i q u i d phase f o r t h i s b i n a r y .

Expe r imen ta l s o l u b i l i t y da t a a re a v a i l a b l e f o r some h i g h e r a l k ane - wate r systems ( s e e , f o r example , S k r i p k a e t a l . , ( 3 8 ) ) . However, these da ta e i t h e r cove r o n l y a ve ry l i m i t e d tempera tu re range o r c o n t a i n r e s u l t s f o r one phase o n l y . No a t t empt has been made to de te rmine the i n t e r a c t i o n parameters f o r wa te r - h y d r o carbon systems where the hydroca rbon i s l a r g e r than n-oc tane .

The tempera tu re - dependent i n t e r a c t i o n parameters de te rm ined f o r s e v e r a l a l k ane - wate r systems a r e p l o t t e d i n F i g u r e 9 . The v a l u e s f o r the hydrogen s u l f i d e - carbon d i o x i d e - , and n i t r o g e n -wate r b i n a r i e s a r e g i v e n i n F i g u r e 10 . I t can be seen t h a t a s y s t e m a t i c t r e n d e x i s t s f o r these pa r ame te r s . The i n t e r a c t i o n parameter i n c r e a s e s w i t h t he s i z e o f the m o l e c u l e and f u r t h e r m o r e i t appears t o converge r a p i d l y as the carbon number i n c r e a s e s . A t a g i v e n tempera tu re and p r e s s u r e , the s o l u b i l i t y o f a l k a n e s i n wa te r g e n e r a l l y dec reases as the m o l e c u l a r we igh t o f the h y d r o carbon i n c r e a s e s . The amount o f n-octane and h e a v i e r hydrocarbons d i s s o l v e d i n wa te r streams r e s u l t i n g f rom s y n t h e t i c gas p r o c e s s e s i s b e l i e v e d t o be i n s i g n i f i c a n t . The c a l c u l a t i o n o f the s o l u b i l i t y o f these compounds i n wa te r under these c o n d i t i o n s by u s i n g e x t r a p o l a t e d v a l ues f rom i n t e r a c t i o n parameters o f l i g h t e r p a r a f f i n - wate r b i n a r i e s p r o b a b l y w i l l no t cause l a r g e e r r o r s .

Three-Phase L o c i . F i g u r e 11 shows the th ree-phase l o c i f o r the a l k a n e - wa te r s y s t ems . No e x p e r i m e n t a l th ree-phase da ta were a v a i l a b l e i n the l i t e r a t u r e f o r the ethane - wa te r b i n a r y .

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-0.4 \-

0 I 1 ι ι I 0 0.2 0.4 0.6 0.8

T R ( P C H C / P C H 2 O )

Figure 9. Temperature-dependent interaction parameters for selected paraffin-water binary systems

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Reduced Temperature

Figure 10. Temperature-dependent interaction parameters for nitrogen, carbon dioxide, and hydrogen sulfide with water

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0 100 200 300 400 500 600 TEMPERATURE, °F

Figure 11. Experimental and predicted three-phase loci for selected paraffin-water binary systems (( ) P-R prediction; (V) (24); (·) (21); ( 0 ) (39); (O) (29);

(k) (3l);(A)PV;W(3V)

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N e v e r t h e l e s s , a c a l c u l a t e d l o c u s i s i n c l u d e d f o r comple teness and t o i n d i c a t e the p o s s i b l e r e g i o n o f th ree-phase e q u i l i b r i u m .

As was ment ioned e a r l i e r , the th ree-phase da t a r e p o r t e d by S c h e f f e r (29) f o r pentane - wa te r were f o r a " b i n a r y " composed o f wa te r ancf a m i x t u r e o f i-pentane and n-pentane. As shown i n the f i g u r e , these da t a a re bounded by the c a l c u l a t e d l o c i o f the i-pentane - wa te r and n-pentane - wa te r s y s t ems .

C o n c l u s i o n . The m i x i n g r u l e f o r use w i t h the Peng-Robinson e q u a t i o n o f s t a t e has been m o d i f i e d to i n c l u d e tempera tu re -dependent i n t e r a c t i o n pa rame te r s . Both the c o n s t a n t and the tempera tu re - dependent i n t e r a c t i o n parameters c o v e r i n g a wide range o f tempera tures have been de te rmined f o r hydrocarbon -wate r systems i n c l u d i n g methane - w a t e r , e thane - w a t e r , propane -w a t e r , n-butane - w a t e r , and n-hexane - wa te r and non-hydrocarbon -wate r systems i n c l u d i n g hydrogen s u l f i d e - w a t e r , carbon d i o x i d e -w a t e r , n i t r o g e n - w a t e r , and ammonia - w a t e r . The i n c l u s i o n o f these tempera tu re - dependent parameters has g r e a t l y improved the a c c u r a c y o f p r e d i c t i o n s o f th ree-phase and two-phase e q u i l i b r ium f o r systems i n v o l v i n g w a t e r .

Acknowledgement. The f i n a n c i a l s u p p o r t r e c e i v e d f rom the N a t i o n a l S c i e n c e and E n g i n e e r i n g Research C o u n c i l o f Canada i s s i n c e r e l y a p p r e c i a t e d .

Abstract

Two-constant equation of state phase behavior calculations for aqueous mixtures often require the use of temperature dependent binary interaction parameters. The methods used for evaluating these parameters for some of the typical aqueous binary pairs found in coal gasification and related process streams are described. Experimental and predicted phase compositions based on these methods are illustrated for aqueous pairs containing CO2, H2S, NH3, and other gases.

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2. Peng, D.-Y., Robinson, D.B., Ind. Eng. Chem. Fundam., 1976, 15, 59.

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10. Wiebe, R., Gaddy, V.L., Heins, C., J. Am. Chem. Soc., 1933, 55, 947.

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Promyshlennost, 1971, 16 (4), 6. 18. Sultanov, R.G., Skripka, V.G., Namiot, A. Yu., Gazovaya

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volume 10.1021bk-1980-0133 issue 1980 [doi 10.1021%2fbk-1980-0133.ch020] newman, stephen a.; barner,...

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