12 Sulfur Dioxide Vapor Pressure and pH of Sodium Citrate Buffer

12 Sulfur Dioxide Vapor Pressure and pH of Sodium Citrate Buffer Solutions with Dissolved Sulfur Dioxide G A R Y T. R O C H E L L E Downloaded by UNIV OF ROCHESTER on May 28, 2014 | http://pubs.acs.org Publication Date: October 29, 1980 | doi: 10.1021/bk-1980-0133.ch012

Department of Chemical Engineering, University of Texas at Austin, Austin, T X 78712

Aqueous scrubbing followed by steam stripping is a potentially attractive method of desulfurizing stack gas with the production of concentratedSO (1,2). SO is absorbed from stack gas containing 500 to 5000 ppmSO by an aqueous solution at 30° to 60°C. The solution is regenerated by stripping with steam at 80° to 150°C. Liquid water is easily condensed from the stripper overhead vapor, leaving concentrated SO . This process has not received commercial acceptance because it generally re quires an excessive amount of steam for stripping. Sodium citrate was recognized as a potential aqueous absor bent for absorption/stripping as early as 1934 (3, 4). It has recently reappeared in work by the U. S. Bureau of Mines (5), in process development sponsored by Peabody, Inc., and in a process offered by Flakt, Inc. (6). This paper reports on work which is part of a development program on absorption/stripping sponsored by the Electric Power Research Institute. 2

2

2

2

B u f f e r s i n the pH range o f 3.5 t o 5.5 provide f o r r e v e r s i b l e S O 2 a b s o r p t i o n as b i s u l f i t e ( H S O 3 ) by the acid/base r e a c t i o n : S0 (g) + H 0 ^ 2

2

+

H + HS0~

In a p e r f e c t l y - b u f f e r e d s o l u t i o n the S O 2 vapor pressure w i l l be d i r e c t l y p r o p o r t i o n a l t o the t o t a l c o n c e n t r a t i o n o f S O 2 and b i s u l f i t e , g i v i n g a l i n e a r e q u i l i b r i u m r e l a t i o n s h i p . I n simple a l k a l i s u l f i t e s o l u t i o n without added b u f f e r , the e q u i l i b r i u m r e l a t i o n s h i p i s h i g h l y n o n l i n e a r , because IT*" accumulates as S O 2 i s absorbed. Under these c o n d i t i o n s i s i t not p o s s i b l e t o c a r r y out r e v e r s i b l e S O 2 a b s o r p t i o n / s t r i p p i n g i n a simple system, r e s u l t i n g i n greater steam requirements than expected w i t h a l i n e a r e q u i l i b r i u m r e l a t i o n s h i p . Weak a c i d b u f f e r s such as sodium c i t r a t e have been proposed to " s t r a i g h t e n " the e q u i l i b r i u m r e l a t i o n ship and thereby reduce u l t i m a t e steam requirements ÇL, _2, 7) · C i t r a t e b u f f e r i s a t t r a c t i v e because i t i s e f f e c t i v e over a wide range, from pH 2.5 to pH 5.5 i n concentrated s o l u t i o n s . Johnstone, e t a l , (2) found that the r a t i o o f S 0 vapor 2

0-8412-0569-8/80/47-133-269$05.75/0 © 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.

270

THERMODYNAMICS OF AQUEOUS SYSTEMS WITH INDUSTRIAL APPLICATIONS

pressure to H 2 O vapor pressure ( P S O 2 / P H 0 O ) over weak a c i d b u f f e r s o l u t i o n s was g e n e r a l l y independent o f temperature. The r e a c t i o n of H S O 3 w i t h IT to g i v e gaseous S O 2 has an enthalpy change of about 10.8 kcal/g-mole ( 8 ) . The e x t r a c t i o n of H from a weak a c i d u s u a l l y r e q u i r e s n e g l i g i b l e enthalpy change. Therefore the net enthalpy change f o r d e s o r p t i o n of gaseous S O 2 from a weak a c i d i s about 10.8 kcal/g-mole, almost equal to the enthalpy of water vaporization: +

HS0~ + H

+

+ S0 (g) + H 0(1) 2

Downloaded by UNIV OF ROCHESTER on May 28, 2014 | http://pubs.acs.org Publication Date: October 29, 1980 | doi: 10.1021/bk-1980-0133.ch012

HA + H

+

2

+ Α"

ΔΗ = 10.8 ΔΗ =

HS0~ + HA -> S0 (g) + H 0(1) + A~ 2

2

kcal/g-mole

0

ΔΗ = 10.8

kcal/g-mole

I t i s expected that sodium c i t r a t e s o l u t i o n w i l l behave as a t y p i c a l weak a c i d b u f f e r . Both s o l u t i o n pH and Ps02/ H2^ l^ be independent of temperature. Under these c o n d i t i o n s , the steam requirements w i l l g e n e r a l l y be independent of the s t r i p p e r oper a t i n g pressure and temperature (J., _2) . This work was c a r r i e d out to c o n f i r m minimal temperature de pendence of Ps02^ H20 over sodium c i t r a t e s o l u t i o n s and to d e t e r mine the dependence of PgQ /Pjj o °n s o l u t i o n composition. Mea surements of pH as a f u n c t i o n of temperature and s o l u t i o n compo s i t i o n have been performed i n order to separate the e f f e c t s of the s p e c i f i c b u f f e r on i S02/ H20' Design c a l c u l a t i o n s are p r e sented to estimate the steam requirements on t y p i c a l a p p l i c a t i o n s . p

s n o u

P

2

2

>

p

Experimental Methods S o l u t i o n P r e p a r a t i o n . S o l u t i o n s were prepared from reagent grade c i t r i c a c i d monohydrate, sodium c i t r a t e d i h y d r a t e , N a H S 0 3 , Na2S04, NaCl, and s t a n d a r d i z e d NaOH s o l u t i o n . Hydroquinone (0.1 wt %) was added to i n h i b i t o x i d a t i o n of s o l u t i o n s w i t h NaHS03. The NaHSÛ3 was analyzed by i o d i n e t i t r a t i o n and was t y p i c a l l y 97-98% of the expected S O 2 content. S e v e r a l of the s o l u t i o n s used 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 experiments were analyzed f o r t o t a l S O 2 and found to c o n t a i n 5 to 10% l e s s than the nominal concent r a t i o n . Nominal c o n c e n t r a t i o n s were used i n p r e s e n t i n g and a n a l y z i n g the d a t a , unless noted otherwise. Therefore, c o r r e l a t e d v a l u e s of P S O 2 be 5 to 10% low f o r a given s o l u t i o n composition. m a v

pH Measurements. A Ag/AgCl combination e l e c t r o d e was used f o r a l l measurements at 25°C. A thalamid combination e l e c t r o d e was used at 55° and 95°C. I t was c o n d i t i o n e d i n pH 4.00 b u f f e r at 55° or 95°C f o r at l e a s t 24 hours before use. The e l e c t r o d e s were s t a n d a r d i z e d at the measurement temperature by p h t h a l a t e b u f f e r at pH 4.00 and phosphate b u f f e r at pH 7.00. Response of the e l e c t r o d e s to c i t r a t e b u f f e r s r e q u i r e d 15 t o 30 minutes f o r


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271

Sulfur Dioxide Vapor Pressure


a s t a b l e r e a d i n g . I n l a t e r experiments secondary c i t r a t e b u f f e r s were used t o s t a n d a r d i z e the e l e c t r o d e s and thereby reduce e q u i l i brium time. Dynamic S a t u r a t i o n . Dynamic s a t u r a t i o n was the primary method used 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 (VLE) determinations. T y p i c a l apparatus i s shown i n f i g u r e 1. N 2 gas was sparged through a s o l u t i o n o f known, r e l a t i v e l y constant composition, then analyzed f o r S O 2 . T y p i c a l l y 1 t o 5 l i t e r s o f N 2 o r N 2 / S O 2 was sparged through a coarse, f r i t t e d - g l a s s d i s p e r s i o n tube sub merged 7 t o 15 cm i n 125 t o 300 ml o f s o l u t i o n i n a s i n g l e - s t a g e g l a s s o r s t a i n l e s s s t e e l s a t u r a t o r . Gas l e a v i n g the s a t u r a t o r was assumed to be i n e q u i l i b r i u m w i t h the s o l u t i o n . Comparable r e s u l t s were obtained i n experiments u s i n g three s a t u r a t o r s i n s e r i e s and i n experiments u s i n g S O 2 a b s o r p t i o n r a t h e r than des o r p t i o n . The s a t u r a t o r was maintained ± 1°C. Most data a t 25° to 55°C were taken a t atmospheric pressure. Data a t 55° to 150°C were taken at 4.4 t o 12 atm. S O 2 content of the gas was determined by sparging i n t o a 125 ml gas washing b o t t l e c o n t a i n i n g a known amount of I 2 i n acetate b u f f e r a t pH 4-5.5 w i t h a s t a r c h i n d i c a t o r . N 2 f l o w was measured by a wet t e s t meter. Water content of the gas l e a v i n g the s a t u r a t o r was estimated using a m o d i f i e d Raoult's law. Constants f o r vapor pressure lowering were obtained from Weast ( 8 ) . Constants f o r NaH2Citrate, Na2HCitrate, N a 3 C i t r a t e , and NaHS03 were assumed to be equal t o those f o r NaH2P04, N a 2 H P 0 4 , Na3P04, and NaCl, r e s p e c t i v e l y . The a c t i v i t y o f water was assumed t o be independent of temperature and i s given by: ^ (1 - a ) 7 6 0 = 30.0 [ N a C i t ] + 23.5 [Na HCit] + 21.0 [NaH Cit] un

0

0

0

+25.2 ([NaHS0 ] + [NaCl]) 3

+ 25.0 ( [ N a S 0 ] + [ 2

4

N a

S 2

2°3

] )

The lowest water a c t i v i t y encountered i n these experiments was about 0.88 (2 M C i t r a t e ) . The vapor pressure of water over the s o l u t i o n i s g i v e n by: P

H 0

=

Ρ

*Η 0 Η 0

2

st

2

n

2

e

where Pj^O * vapor pressure of pure water a t the s a t u r a t o r temperature. Assuming that the vapor l e a v i n g the s a t u r a t o r i s an i d e a l gas the S O 2 vapor pressure i s g i v e n by:


272


ri SO ρ

= S 0

a n d

(ρ

± n

2 n

S0

+

a r e

t h e

_ ρ

T

\

2

m

e s

H

o f

s

)

2° o

a n d

N

a n d

P

i s

where n g o N2 °l 2 2 T the totalpressure. Under most c o n d i t i o n s Pr^O l e s s than 1 0 % of P-p. Therefore, P S O 2 r e l a t i v e l y i n s e n s i t i v e to e r r o r s i n Ρ Η 0 · 2

w

a

s

i s

2

SO? E l e c t r o d e . A gas-sensing S O 2 e l e c t r o d e marketed by I o n i c s , Inc. was used to provide a d d i t i o n a l VLE data a t 25°C as a f u n c t i o n of composition. Aqueous S 0 e q u i l i b r a t e s across a p o l y meric membrane w i t h a f i l l i n g s o l u t i o n c o n t a i n i n g about 0 . 1 M NaHSO^. I o n i c species do not d i f f u s e across the membrane. A small combination g l a s s e l e c t r o d e measures the pH of the f i l l i n g s o l u t i o n . The S O 2 a c t i v i t y (PsOo) p r o p o r t i o n a l to the a c t i v i t y of Η"*" ( 1 0 ~ P ^ ) , because the b i s u l f i t e a c t i v i t y i s constant:


2

i s

HS0 " + "Ws0 (g) + H 0 3

H

P

2

S0

=

K

a

2

2

a

H+ HS0~ a s

o w

a s

The S O 2 e l e c t r o d e has a l i n e a r response to P g 0 2 ^ ^ ^ atm. Water a l s o d i f f u s e s across the polymer membrane to a l i m i t e d extent. Therefore the e l e c t r o d e response i s unstable and u n r e l i able i f there i s a s i g n i f i c a n t d i f f e r e n c e between the osmotic pressure of the f i l l i n g s o l u t i o n and the unknown s o l u t i o n . To p a r t i a l l y a l l e v i a t e t h i s problem, data were taken w i t h f i l l i n g s o l u t i o n s c o n t a i n i n g 0 , 1 . 0 , and 2 . 0 M a d d i t i o n a l K C l . The response of the S O 2 e l e c t r o d e i s a c t u a l l y read as v o l tage, (mV). Two constants are needed t o convert t h i s t o Ρ : so l o g

p

so

= 2

C

+

2

d

We have used the t h e o r e t i c a l value of d at 25°C which i s 59.16 mV. The constant c must be determined by measurement o f a known s o l u t i o n each day the e l e c t r o d e i s used. A l t e r n a t i v e l y , the e l e c t r o d e can be used t o provide r e l a t i v e P^Q f o r a s e r i e s o f measurements. 2 pH Behavior A s e m i e m p i r i c a l c o r r e l a t i o n of pH measurements w i t h 156 s o l u t i o n s gave the f o l l o w i n g r e l a t i o n s h i p :


12.

273


ROCHELLE

a

+

l

a

2

f+

a 3

[

A n i

°n] °*

5

(1)

T

2.77 ± 0.05 3.60 ± 0.05 -0.53 The

± 0.03

t o t a l c o n c e n t r a t i o n of anions i s given by: [ A n i o n ] = [ C i t r a t e ] + [ N a S 0 ] + [NaCl] + [NaHSO^


T

2

4

+

N e g l e c t i n g H and assuming that S 0 i s present as b i s u l f i t e , the f r a c t i o n n e u t r a l i z a t i o n i s d e f i n e d as: 2

+

[Na ] f

=

=

- [S0 ] - [Cl~] - 2 [ S 0 ] 2

4

3[Citrate]

This c o r r e l a t i o n i n c l u d e s s o l u t i o n s over the f o l l o w i n g range of conditions : f = 0.20 - 0.833 [ C i t r a t e ] = 0.05 - 2.0 M [ N a S 0 ] = 0 - 1.5 M 2

4

[NaHS0 ] = 0 - 1.0 M 3

[NaCl] = 0 - 1.9 M The

standard d e v i a t i o n of pH p r e d i c t i o n i s 0.12 pH u n i t s . In these s o l u t i o n s pH i s more s t r o n g l y c o r r e l a t e d w i t h t o t a l anion c o n c e n t r a t i o n than w i t h i o n i c s t r e n g t h . Thus 1 M Na2S04 and 1 M NaCl have about the same e f f e c t on the pH of a s o l u t i o n at a given f r a c t i o n n e u t r a l i z a t i o n . F i g u r e 2 shows pH a t 50% n e u t r a l i z a t i o n as a f u n c t i o n of anion c o n c e n t r a t i o n i n the s o l u t i o n s which are p r i m a r i l y c i t r a t e , N a 2 S 0 4 , or NaCl, as w e l l as i n mixed s o l u t i o n s . The e f f e c t of s o l u t i o n a n i o n i c c o n c e n t r a t i o n i s probably r e l a t e d to e f f e c t s on a c t i v i t y c o e f f i c i e n t s and i o n p a i r forma t i o n o f more h i g h l y charged b u f f e r species. In more concentrated s o l u t i o n s , the a c t i v i t y of the h i g h l y charged species i s reduced by both i o n i c s t r e n g t h and i o n p a i r formation. The e f f e c t on l e s s charged, a c i d i c species i s l e s s . Therefore, as s o l u t i o n s become more concentrated, the a c t i v i t y of b a s i c species i s r e duced r e l a t i v e to that of a c i d i c species, and at a given f r a c t i o n


274


Regulator

Thermometer ^Pressure Gauge

Μ Λΐ Needle ΛΛ

Λ

\

Valve

Shutoff Valve


N

Saturator

2

Graduated Cylinder

I

I I Ο ο ο _0

Absorber

Gas Trap

Carrier Gas

Figure 1.

Figure 2.

A pparatus for dynamic saturation

Dependence of pH on total anion concentration ((%) infinite dilution; (O) NaCl; ( Q ) Na S0 ; (A) sodium citrate; (V) mixed solution) 2

4


12.

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275


n e u t r a l i z a t i o n the pH decreases. I n concentrated s o l u t i o n s p o l y v a l e n t anions such as S O 4 " and c i t r a t e " probably form i o n p a i r s such as NaS04~ and N a 2 c i t r a t e . Therefore the e f f e c t i v e i o n i c s t r e n g t h and N a c o n c e n t r a t i o n a v a i l a b l e f o r i o n p a i r formation v a r i e s w i t h t o t a l anion c o n c e n t r a t i o n r a t h e r than i o n i c s t r e n g t h . The e f f e c t of f r a c t i o n n e u t r a l i z a t i o n on pH i s i l l u s t r a t e d i n f i g u r e 3 f o r a s o l u t i o n of constant a n i o n i c c o n c e n t r a t i o n . This corresponds to t i t r a t i n g c i t r i c a c i d w i t h NaOH. The t i t r a t i o n curve i s very n e a r l y l i n e a r from pH 2.2 to about pH 5.5 w i t h a slope of 3.60. The e f f e c t s of the three f u n c t i o n a l b u f f e r groups of c i t r i c a c i d are smeared so t h a t no S-shape or i n f l e c t i o n p o i n t s are apparent. As shown i n f i g u r e 4, t i t r a t i o n w i t h HCI or a b s o r p t i o n of S O 2 as b i s u l f i t e r e s u l t s i n a d i f f e r e n t dependence of pH on f r a c t i o n n e u t r a l i z a t i o n because the t o t a l anion c o n c e n t r a t i o n i s increased. The slope of pH versus f i s t y p i c a l l y g r e a t e r than 4.0 and i s modeled by equation ( 1 ) . As shown i n Table I , the ΔΗ values of the b u f f e r r e a c t i o n s corresponding roughly to K i , K 2 , and K 3 (16.7, 50.0, and 83.3% n e u t r a l i z a t i o n , r e s p e c t i v e l y ) a l l have absolute values l e s s than 2.0 kcal/g-mole. The r e a c t i o n s corresponding to K 2 and K 3 , which are most r e l e v a n t f o r S O 2 a b s o r p t i o n / s t r i p p i n g , had ΔΗ values of -0.7 and +0.1 kcal/g-mole, r e s p e c t i v e l y . The c a r e f u l data of Bates and P i n c h i n g ( 9 ) i n d i l u t e c i t r a t e s o l u t i o n s g i v e ΔΗ values between 25° and 50°C of -0.01, -0.002, and +0.03 k c a l / g mole, r e s p e c t i v e l y f o r K i , K 2 , and K 3» Because o v e r a l l tem perature e f f e c t s i n S O 2 a b s o r p t i o n / s t r i p p i n g are on the order o f 10 kcal/g-mole, we can n e g l e c t the enthalpy change of the b u f f e r reaction. +


+

a

a

a

a

a

S0

?

a

a

a

Vapor Pressure

S O 2 vapor pressure was determined i n e i g h t s e r i e s of e x p e r i ments (Tables I I and I I I ) w i t h a t o t a l o f about 80 s o l u t i o n s over the f o l l o w i n g range of c o n d i t i o n s : f = 0.40 - 0.80 [ C i t r a t e ] = 0.2 - 2.0 M [NaHS0 ] = 0.025 - 1.0 M 3

[Na S0 ] = 0 - 1 M 2

4

[ N a S 0 ] = 0 - 0.6 M 2

2

3

Τ = 25 - 168°C A s e m i e m p i r i c a l c o r r e l a t i o n of the data g i v e s :




276

Figure 4.

Dependence of pH on fraction neutralization—titration with HCI orS0 (0) 2


(A)

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277


+

Table I : Heat of Reaction of H w i t h Concentrated Sodium C i t r a t e B u f f e r s Solution composition ^—)

a Fraction neutralization


0.167

0.500

0.30 NaCl 1.50 0.60 C i t r a t e 3.00

+ -

Mean

- 2.0 ± 1.7

0.30 NaCl 1.50 0.50 Na2S0 0.15 C i t r a t e 0.50 0.75 1.00 2.00 4

0.833

, . Q ΔΗ (kcal/g-mole)

C

- 0.9 - 0.4 +0.2 - 0.7 - 1.5 - 0.1 - 1.0 - 0.9 C

C

C

Mean

- 0.7 ± 0.5

0.30 NaCl 1.50 0.50 Na2S0 0.0667 C i t r a t e 0.333 0.50 1.00 2.00

+ + + + -

Mean

+ 0.1 ± 0.6

4

+

0.7 0.3 0.4 0.7 0.1 0.2 0.4 0.9C C

C

=

[Na ] - [ C l ~ ] - 2 [ S 0 ]

a

4

fraction neutralization = b

ς

1.6 0.2 2.8 3.8

3[Citratel

C h l o r i d e and s u l f a t e s o l u t i o n s c o n t a i n 0.00667 M C i t r a t e . A l l s o l u t i o n s c o n t a i n NaOH as i n d i c a t e d by f r a c t i o n n e u t r a l i z a tion

ΔΗ c a l c u l a t e d from pH values a t 25° and 95°C

^ΔΗ c a l c u l a t e d from pH values at 25° and 55°C unless noted


278


Table I I : SO2 Vapor Pressure Obtained by Dynamic S a t u r a t i o n a t 25° t o 158°C, S e r i e s 1 S o l u t i o n Composition (M)

so

Citrate


0.5

r

0.533

0.25

0.583

1

0.80

0.700

a

0.533

0.20

mean

13.7 + 1.0

24 55 75 95 115 135

55 75 95 115 135

25 168 mean

1.0

0.164

b

0.612

J

14.8 12.1 14.7 14.2 14.0 12.4 14.3 13.2

mean 0.5

χ 10

24 51 92 117 148 mean

Calc Ρ

2

the h u m i d i f i e d i n l e t s t a c k gas (1). This steam requirement i s i n dependent of the s t r i p p e r temperature, but assumes that the s t r i p per feed i s preheated to i t s b o i l i n g p o i n t , that there are an i n f i n i t e number of stages i n the absorber and s t r i p p e r , and that the e q u i l i b r i u m curve i s l i n e a r . 2

2

2

2

2

p

2

o f

2


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285


The second c h a r a c t e r i s t i c of e q u i l i b r i u m data a f f e c t i n g steam requirements i s the n o n l i n e a r i t y of PsOo/ HoO versus [SO2]. That n o n l i n e a r i t y i s q u a n t i f i e d p r i m a r i l y by the dependence of K on f. F i g u r e 6 shows p r e d i c t e d and measured values of P s 0 ^ HoO [δ0 ] f o r 1 M c i t r a t e s o l u t i o n s w i t h 1.5,2.0,and 2.5 M Na . Performance of a simple a b s o r p t i o n / s t r i p p i n g system can be determined by use of a McCabe-Thiele diagram. E q u i l i b r i u m f o r both s t r i p p e r and absorber can be represented as a s i n g l e l i n e when p l o t t i n g Ps02/ H?0 versus [SO2]. M a t e r i a l balance gives o p e r a t i n g l i n e s f o r the absorber and s t r i p p e r . The slope of the absorber o p e r a t i n g l i n e i s the r a t i o of l i t e r s of c i r c u l a t i n g s o l u t i o n to moles of H2O i n the s a t u r a t e d s t a c k gas. The slope of the s t r i p p e r o p e r a t i n g l i n e i s the r a t i o of l i t e r s of c i r c u l a t i n g s o l u t i o n to moles of steam. The steam requirement i n moles per mole of SO2 absorbed i s equal to the i n v e r s e of the r a t i o , Ps02/ H20> P °f stripper. F i g u r e 7 i l l u s t r a t e s how minimum steam requirements can be estimated f o r 1 M c i t r a t e w i t h 2.0 molar N a i n a simple absorp t i o n / s t r i p p i n g system to remove 90% of the S 0 from stack gas at 55°C c o n t a i n i n g 3000 ppm SO2. The o p e r a t i n g l i n e s f o r the absorber and s t r i p p e r are s t r a i g h t , assuming that the H 0 vapor r a t e i s constant throughout the absorber and throughout the s t r i p p e r . With an i n f i n i t e number of stages the absorber i s pinched at the top and bottom. Using l i v e steam, the s t r i p p e r pinches i n the middle because of the n o n l i n e a r i t y of the e q u i l i b rium curve. The gas l e a v i n g the s t r i p p e r would have Pso /* H20 equal to 0.0173, g i v i n g a minimum steam requirement of 57.8 moles H20/mole SO2 (16.3 kg/kg). I f the e q u i l i b r i u m were l i n e a r the minimum steam requirement would be 50 moles H20/mole SO2. Thus, n o n l i n e a r i t y i n c r e a s e s the steam requirement by a f a c t o r of 1.16. F i g u r e 8 i l l u s t r a t e s the performance of a system w i t h three e q u i l i b r i u m stages i n the absorber and s i x i n the s t r i p p e r . The a c t u a l steam requirement i s 147 moles/mole SO2 (41.3 kg/kg). The use of a f i n i t e number of stages i n c r e a s e s the steam requirement a f a c t o r of 2.5 from the case of i n f i n i t e stages w i t h a non linear equilibrium. Table IV g i v e s minimum steam requirement ( i n f i n i t e stages) at s e v e r a l d i f f e r e n t s o l u t i o n c a p a c i t i e s . The f a c t o r a t t r i b u able to e q u i l i b r i u m n o n l i n e a r i t y i n c r e a s e s as more SO2 i s absorbed, because the b u f f e r c a p a c i t y i s consumed to a g r e a t e r extent. Any c a p a c i t y f o r S 0 a b s o r p t i o n can be achieved by v a r y i n g Na c o n c e n t r a t i o n (pH) i n the s o l u t i o n . At low pH ([Na] = 1.5 M) the s o l u t i o n c a p a c i t y f o r SO2 a b s o r p t i o n i s s m a l l , but the n o n l i n e a r i t y f a c t o r i s a l s o s m a l l (1.05). S o l u t i o n c a p a c i t y can be i n c r e a s e d by o p e r a t i n g at higher pH ([Na] = 2.5 Μ ) , but n o n l i n e a r i t y i s more severe (1.32). As shown by case 3 i n Table IV, the minimum steam r e q u i r e ment i n an optimized system i s not s e n s i t i v e to the magnitude o f PS0 over the s o l u t i o n , but only to i t s dependence on temperature p

c

2

p

v

e

r

s

u

s

2

+


p

p

a

t t

n

e t o

t

n

e

+

2

2

>

2

2

9




286

\S02J Figure 6.

(g-moles/liter)

Dependence of P on solution composition—1.0M citrate, 25°C ((O) SO electrode; f Q ) dynamic saturation) 802

2



Figure 7. Minimum steam requirement, simple absorption/stripping with live steam, 3000 ppm SO in at 55°C, 90% removal, l.OM citrate, 2.0M Na g


THERMODYNAMICS OF AQUEOUS SYSTEMS WITH INDUSTRIAL

APPLICATIONS


288


12.

ROCHELLE

289


and S O 2 s o l u t i o n c o n c e n t r a t i o n . I f the constant a i n the equation f o r K i s 0.30 r a t h e r than 0.46, the minimum steam re quirement i n c r e a s e s by only 3%. 4

c

Minimum steam requirement, i n l e t Table IV / P ο = 0.02, 90% S 0 removal, Ρ i n f i n i t e stages, 1.0 M C i t r a t e


s o

H

2

[Na] (M)

Steam Requirement Moles H 0/Mole S 0

1.5 2.0 2.0 2.5

52.4 57.8 59.5 65.8

2

a

Value of a

4

2

taken to be 0.30

Nonlinearity Factor

Capacity Moles S 0 / l i t e r

1.05 1.16 1.19 1.32

0.103 0.173 0.262 0.368

r a t h e r than

2

0.46.

Conclusions 1.

Temperature dependence of pH and P s o / ^ H 0 over sodium citrate buffer solutions i s insignificant. Composition dependence of pH and P s o / H 0 rep resented as a f u n c t i o n of f r a c t i o n n e u t r a l i z a t i o n and t o t a l anion c o n c e n t r a t i o n . A c t u a l steam requirement w i t h t y p i c a l s t a c k gas should be about 41 kg/kg S 0 . Optimized steam requirement i s r e l a t i v e l y i n s e n s i t i v e to s o l u t i o n pH. S o l u t i o n c a p a c i t y f o r S O 2 a b s o r p t i o n can reasonably vary from 0.1 to 0.4 g-moles S 0 / l i t e r . The S 0 gas sensing e l e c t r o d e i s an e f f e c t i v e t o o l 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 a t room temperature. 2

2

p

2

c

a

n D

e

2

2

4.

2

5.

2

Nomenclature a l, 3> a

a

a , a , a 6 2

4

a

Activity Constants i n c o r r e l a t i o n of pH and S 0 Constants w i t h a n i o n i c concent j t r a t i o n i n corr e l a t i o n of pH and Pgo^ MO-5 I n t e r c e p t f o r c a l i b r a t i o n of S 0 e l e c t r o d e . T o t a l c o n c e n t r a t i o n of c i t r i c a c i d and i t s anions, M Slope f o r c a l i b r a t i o n of S 0 e l e c t r o d e , mV Fraction neutralization E q u i l i b r i u m constant f o r S 0 a b s o r p t i o n as b i s u l f i t e , atm M" E q u i l i b r i u m constants f o r d i s s o c i a t i o n of c i t r i c acid, M Dependent v a r i a b l e i n c o r r e l a t i o n of S 0 vapor pressure, M~^ p

5

2

2

[Citrate] d f Κ

2

2

2

K

K

K

a l > a2> a 3

2


290

THERMODYNAMICS

M η Ρ

OF AQUEOUS SYSTEMS W I T H INDUSTRIAL APPLICATIONS

Molarity, gmol/liter Number o f g-moles Pressure o r p a r t i a l pressure, atm Vapor o r p a r t i a l pressure o f SO2, atm Vapor pressure of pure water, atm Vapor o r p a r t i a l pressure o f H2O, atm Total concentration of species (mostly bisulfite), M Concentration, M

Pfi 0 PHoO 2

[s6 ] 2

[]

Abstract

SO vapor pressure(PSO )wasmeasuredby dynamic saturation and by a gas-sensing SO electrode over solutions containing 0.5 to 2.0 M sodium citrate at pH 3.5 to 5 with up to 1 M NaHSO, Na SO , and NaCl, PSO was measured at 25° to 168°C; pH at 25° to 95°C. Both pH and the vapor pressure ratioPSO/PHOwere independent of temperature. The composition and temperature de pendence of the data are correlated by the semiempirical expres sions:


2

2

2

3

2

4

2

2

2

where f is the fraction neutralization of the citrate buffer. The steam requirement for simple absorption/stripping with 90% removal of SO from stack gas containing 3000 ppm SO at 55°C was estimated to be about 40 kg/kg SO . 2

2

2

Acknowledgements This work was p r i m a r i l y supported by c o n t r a c t s TPS 77-747 and RP 1402-2 w i t h the E l e c t r i c Power Research I n s t i t u t e . C u r t i s Cavanaugh, Richard U l r i c h , P u i L i n , and Michael Ragsdale have c o n t r i b u t e d experimental data as research a s s i s t a n t s . Legal Notice This work was prepared by the U n i v e r s i t y o f Texas a t A u s t i n as an account o f work sponsored by the E l e c t r i c Power Research I n s t i t u t e , Inc. ("EPRI"). N e i t h e r EPRI, members o f the EPRI, nor the U n i v e r s i t y of Texas a t A u s t i n , nor any person a c t i n g on behalf o f e i t h e r : a. Makes any warranty o r r e p r e s e n t a t i o n , expressed o r im p l i e d , w i t h respect to the accuracy, completeness, o r usefulness of the i n f o r m a t i o n contained i n t h i s r e p o r t , o r that the use o f


12.

ROCHELLE


291

any i n f o r m a t i o n , apparatus, method, or process d i s c l o s e d i n t h i s report may not i n f r i n g e p r i v a t e l y owned r i g h t s ; o r , b. Assumes any l i a b i l i t i e s w i t h respect to the use o f , o r for damages r e s u l t i n g from the use o f , any i n f o r m a t i o n , apparatus, method o r process d i s c l o s e d i n t h i s r e p o r t .


Literature Cited

1. Rochelle, G. T., "Proceedings: Symposium on Flue Gas Desul furization - Hollywood, FL", EPA-600/7-78-058b, p. 902 (1977). 2. Johnstone, H. F., H. F. Read, and H. C. Blankmeyer, Ind. Eng. Chem., 30, 101 (1938). 3. Boswell, M. C., U. S. Patent 1,972,074, Sept. 4, 1934. 4. Applebey, M. P., J. Soc. Chem. Ind. Trans., 56, 139 (1937). 5. Nissen, W. I., D. A. Elkins, and W. A. McKinney, "Proceedings: Symposium on Flue Gas Desulfurization - New Orleans", EPA600/2-76-136b, p. 843 (1976). 6. Farrington, J. F. and S. Bengtsson, "The Flakt-Boliden Pro cess forSO Recovery", presented at AIME annual Meeting, February 19, 1979. 7. Johnstone, H. F., Ind. Eng. Chem., 39, 3896 (1935). 8. Weast, R. C., "Handbook of Chemistry and Physics", 54th ed., CRC Press, pp. D-70, E-l (1973). 9. Bates, R. G. and G. D. Pinching, J . Am. Chem. Soc.,71, 1274 (1949). 2

RECEIVED

January 31, 1980.


12 Sulfur Dioxide Vapor Pressure and pH of Sodium Citrate Buffer

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