Laboratory Investigation of Adipic Acid Degradation in Flue Gas

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Downloaded by UNIV OF NEW SOUTH WALES on February 2, 2016 | http://pubs.acs.org Publication Date: July 1, 1982 | doi: 10.1021/bk-1982-0188.ch011

Laboratory Investigation of Adipic Acid Degradation in Flue Gas Desulfurization Scrubbers J. C. TERRY, J. B. JARVIS, D. L. UTLEY, and Ε. E. ELLSWORTH Radian Corporation, Austin,TX78766 The addition of adipic acid to FGD scrubber liquor results in improved limestone utilization and enhanced SO sorption kinetics. During scrubber op eration, however, adipic acid is lost from the system in the liquid and solid phase purge streams and by chemical degradation. In order to assess the effects of scrubber operating parameters on the loss of adipic acid, a series of laboratory tests have been per formed. These include both bench-scale closed loop scrubber tests and batch mode jar tests. The param eters investigated includepH,adipic acid concentra tion, trace metal catalysts, degree of sulfite oxida tion, sulfite concentration, SO gas loading, and scrubber slurry temperature. Results indicate that the amount of adipic acid coprecipitated with the scrubber solids decreases significantly with increas ing sulfate in the solids. Chemical degradation increased dramatically with forced sulfite oxidation. Chemical degradation was also found to increase with increasing adipic acid concentration and temperature. Reduced chemical degradation was observed with Mn in the liquor. pH also affected chemical degradation but only when Mn was present. 2

2

+2

+2

The addition of adipic acid to limestone-based FGD wet scrub bers results in improved limestone utilization and enhanced SO2 sorption kinetics. The use of adipic acid was first proposed by Rochelle (1) and has been tested by the EPA in pilot systems at the Industrial Environmental Research Laboratory, Research Triangle Park, North Carolina and at the TVA Shawnee Test Facility at Paducah, Kentucky. Adipic acid in the concentration range of 1,000-2,000 mg/1 has been found effective as a scrubber additive. During scrubber operation, however, adipic acid is lost from the system in the liquid and solid phase purge streams and by chemical degradation (2,3). 0097-6156/82/0188-0221$6.50/0 © 1982 American Chemical Society In Flue Gas Desulfurization; Hudson, John L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

FLUE

222

GAS

DESULFURIZATION

Tests conducted at the Shawnee Test F a c i l i t y i n d i c a t e d that a d i p i c a c i d added to t h e i r limestone FGD scrubber d i d not degrade at pH's below 5. Since these unexpected but f a v o r a b l e r e s u l t s were important to the f u t u r e a p p l i c a t i o n of a d i p i c a c i d as an FGD a d d i t i v e , independent v e r i f i c a t i o n was d e s i r e d . Radian was cont r a c t e d by the EPA to c a r r y out a systematic study of the e f f e c t s of scrubber operating c o n d i t i o n s on a d i p i c a c i d degradation. The s p e c i f i c o b j e c t i v e s of the study were t o : • setup a bench-scale S 0 scrubber capable of closed loop operation; • v e r i f y the quenching of a d i p i c a c i d degradation at low pH observed at Shawnee; and • conduct parametric s t u d i e s to more f u l l y c h a r a c t e r i z e the e f f e c t s of key operating v a r i a b l e s on a d i p i c a c i d degradation. The t o p i c s presented i n t h i s paper i n c l u d e a d e s c r i p t i o n of the bench-scale system, the experimental approach, and the r e s u l t s of degradation t e s t i n g . A l s o included are the r e s u l t s of batch p r e c i p i t a t i o n experiments designed to study c o p r e c i p i t a t i o n of a d i p i c a c i d i n scrubber waste s o l i d s .


2

Experimental Approach The design of the bench-scale system was i n f l u e n c e d by two important c o n s i d e r a t i o n s : • The d e s i r e to simulate the operation of f u l l - s c a l e FGD systems which would allow r e s u l t s from the bench-scale system to be used to a n t i c i p a t e a d i p i c a c i d degradation i n l a r g e r systems, and • The need f o r accurate mass balance determinations s i n c e c e r t a i n sets of operating c o n d i t i o n s were a n t i c i p a t e d to produce low degradation r a t e s . The experimental equipment design which r e s u l t e d from the above c o n s i d e r a t i o n s i s i l l u s t r a t e d s c h e m a t i c a l l y i n Figure 1. In general terms, the operation of the system c o n s i s t e d of c o n t a c t i n g s y n t h e t i c or b o i l e r f l u e gas i n a packed scrubber with r e c i r c u l a t ing s l u r r y from the hold tank. The pH of the hold tank s l u r r y was maintained at a constant l e v e l by a d d i t i o n of reagent grade C a C 0 or limestone. At the s t a r t of each run, v a r i o u s m a t e r i a l s were added to the hold tank. These m a t e r i a l s included deionized water, a d i p i c a c i d , calcium s u l f i t e seed c r y s t a l s , NaCl, MnS0i+*H20, F e ( 8 0 4 ) 3 , f l y ash, e t c . depending on the purpose of the t e s t . The r e s u l t i n g t h i n s l u r r y was c i r c u l a t e d through the scrubber where i t was contacted with f l u e gas. This procedure was continued f o r s e v e r a l hours, allowing time f o r the mass of s o l i d s i n the hold tank to _ increase and the c o n c e n t r a t i o n of i o n i c species ( p a r t i c u l a r l y S O 3 ) to reach a constant l e v e l . At t h i s p o i n t , a q u a n t i t a t i v e amount of s l u r r y was withdrawn from the hold tank. 3

2

In Flue Gas Desulfurization; Hudson, John L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.


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Figure 1.

>0 , N (AIR)

/ \ / \

Λ

V

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2

REACTOR (HOLD TANK)

α ?

VARIBLE SPEED MOTOR

ABSORBER (PACKED

iff

-VENT

DOWNSTREAM IMPINGERS

BLEED STREAM OR MAKE-UP WATER ADDITION

Diagram of closed loop SQ scrubber.

WATER BATH

0-120V AC

LIMESTONE ADIPIC ACID

100-4.82 l/min HHX3—? S O , N

ROTOMETER

1.5-49.3 l/min

ROTOMETER

ROTOMETER .025-875 l/min

.4-16.9 l/min

ROTOMETER

MANIFOLD PRESSURE GAUGE

2

2

H 0 SATURATOR COLUMN

DYNA SCIENCES S 0 ANALYZER RANGES 0-1000 ppm 0-3000 ppm 0-10,000 ppm

t



224

FLUE

GAS

DESULFURIZATION

A n a l y s i s f o r a d i p i c a c i d i n both the s o l i d and l i q u i d phase, plus data f o r the hold tank volume, s l u r r y density, and weight percent s o l i d s , provided s u f f i c i e n t i n f o r m a t i o n to c a l c u l a t e the inventory of a d i p i c a c i d remaining i n the hold tank. At the end of the run, a second sample was withdrawn from the hold tank and the a d i p i c a c i d inventory was again c a l c u l a t e d . The absolute amount of a d i p i c a c i d which had degraded during the run was represented by the d i f f e r e n c e between the i n i t i a l and f i n a l i n v e n t o r i e s . Several f e a t u r e s were incorporated i n t o the design of the bench-scale u n i t which f a c i l i t a t e d the study of the e f f e c t s of ope r a t i n g c o n d i t i o n s on a d i p i c a c i d degradation. Some of the more important f e a t u r e s are described below: • Synthetic f l u e gas, rather than combustion gases, was used during the laboratory phase of t e s t i n g . This allowed an independent determination of the e f f e c t on degradation of components of f l u e gas such as f l y ash, percent 0 , and c e r t a i n s u l f u r species. • Calcium carbonate, rather than limestone was used as the source of a l k a l i n i t y f o r b a s e l i n e t e s t i n g . Again, t h i s permitted independent study of limestone components (notably trace metals) on a d i p i c a c i d degradation. In a d d i t i o n to the measurements required f o r determining the a d i p i c a c i d degradation r a t e , a v a r i e t y of other v a r i a b l e s were monitored : • pH (continuously), • I n l e t and o u t l e t S0 concentration, • Percent o x i d a t i o n i n the s o l i d s , • Temperature (hold tank and gas i n l e t ) , • Limestone consumption r a t e , • Limestone u t i l i z a t i o n , • L i q u i d to gas r a t i o (L/G), • S u l f i t e concentration, • C h l o r i d e c o n c e n t r a t i o n (used as a t r a c e r ) , and • Trace metals c o n c e n t r a t i o n ( i f added to system). With t h i s i n f o r m a t i o n , the a d i p i c a c i d degradation r a t e and other aspects of o p e r a t i o n with a d i p i c a c i d can be r e l a t e d to s p e c i f i c process v a r i a b l e s . And, r e s u l t s obtained with the benchs c a l e system can then be used to a n t i c i p a t e a d i p i c a c i d degradat i o n rates i n f u l l - s c a l e systems. 2

2

Bench-Scale Scrubber Test Results The r e s u l t s of bench-scale t e s t s were obtained during three phases of experimentation. The f i r s t phase consisted of l a b o r a tory t e s t i n g at Radian. The second phase consisted of t e s t i n g at the Shawnee t e s t f a c i l i t y i n Paducah, KY, and at the C i t y U t i l i t i e s of S p r i n g f i e l d Southwest Power S t a t i o n i n S p r i n g f i e l d , MO. The t h i r d phase consisted of a d d i t i o n a l laboratory t e s t i n g at Radian. A t o t a l of 43 bench-scale runs were performed. The r e s u l t s


11.

TERRY

ET AL.

225

Adipic Acid Degradation

of these runs, i n c l u d i n g the observed degradation r a t e s and a sum mary of run c o n d i t i o n s , a r e l i s t e d i n Table I . Twenty-one of these runs were b a s e l i n e t e s t s i n which the v a r i a b l e s under con s i d e r a t i o n were l i m i t e d t o the SO2 a b s o r p t i o n / o x i d a t i o n r a t e , the a d i p i c a c i d c o n c e n t r a t i o n , and pH. CaC03 was used as the a l k a l i n e species f o r a l l b a s e l i n e t e s t s . The ranges f o r the v a r i a b l e s i n v e s t i g a t e d i n the b a s e l i n e t e s t s were:


Variable S0

2

Range

absorption r a t e

2.07 - 7.86 g S0 /hr 2

Percent s o l i d s o x i d a t i o n (a measure of the t o t a l absorbed S 0 that i s o x i d i z e d to s u l f a t e )

9.7 - 100%

Adipic acid concentration

876 - 10,080 ppm

2

pH

4.6 - 5.5

The r e s u l t i n g a d i p i c a c i d degradation r a t e s ranged from 15.4 to 600 mg/hr. A computer program was used to perform a s t a t i s t i c a l a n a l y s i s on the b a s e l i n e t e s t data. This a n a l y s i s showed that the a d i p i c a c i d degradation r a t e was a f u n c t i o n of the o v e r a l l S0 o x i d a t i o n r a t e and the a d i p i c a c i d concentration. The degradation r a t e was found not to depend on pH, a t l e a s t over the range tested. The s u l f i t e i o n c o n c e n t r a t i o n was a l s o included i n t h i s analysis. However, the s u l f i t e c o n c e n t r a t i o n was g e n e r a l l y a f u n c t i o n of pH and was a l s o found not to i n f l u e n c e the a d i p i c a c i d degradation r a t e . A weighted l e a s t squares a n a l y s i s was used to c o r r e l a t e the s i g n i f i c a n t v a r i a b l e s . The r e s u l t i n g c o r r e l a t i o n , which represents the best f i t of the experimental data, i s given below: 2

a d i p i c a c i d degradation r a t e , mg/g S0 removed = 2

η nn

M H

H W

FLUE


234

GAS

DESULFURIZATION

i s p l o t t e d against the r e c i p r o c a l of the absolute temperature. The use of i s appropriate here only because the percent oxidat i o n and the average a d i p i c a c i d c o n c e n t r a t i o n f o r the three runs are c l o s e . From the slope of the l i n e shown i n F i g u r e 4, the Arrhenius a c t i v a t i o n energy was determined to be 10.6 Kcals/mole. This a c t i v a t i o n energy i s t y p i c a l f o r an organic r e a c t i o n i n which the r a t e i s c o n t r o l l e d by a chemical r e a c t i o n rather than a p h y s i c a l process such as d i f f u s i o n . In the a n a l y s i s of the b a s e l i n e t e s t r e s u l t s , the s u l f i t e i o n c o n c e n t r a t i o n was not found to have an e f f e c t on the a d i p i c a c i d degradation r a t e . However, i n these t e s t s , the e q u i l i b r i u m s u l f i t e i o n c o n c e n t r a t i o n was a strong f u n c t i o n of pH. To o b t a i n an independent determination of the e f f e c t of the s u l f i t e i o n c o n c e n t r a t i o n , the s u l f i t e c o n c e n t r a t i o n was increased v i a the a d d i t i o n of sodium ions (as N a 2 S 0 i t ) . In t h i s way, the e f f e c t of d i f f e r e n t s u l f i t e i o n concentrations could be evaluated at the same pH. A comparison of the degradation rates from these t e s t s with the values p r e d i c t e d from the b a s e l i n e c o r r e l a t i o n i n d i c a t e s that the s u l f i t e i o n c o n c e n t r a t i o n has l i t t l e , i f any, impact on the a d i p i c a c i d degradation r a t e mechanism. Coprecipitation

of A d i p i c A c i d

i n Scrubber

Solids

A major f a c t o r i n determining the degradation r a t e of a d i p i c a c i d i s d i f f e r e n t i a t i n g between p h y s i c a l l o s s of a d i p i c a c i d and a c t u a l chemical degradation. There are at l e a s t two mechanisms f o r p h y s i c a l l o s s of a d i p i c a c i d i n c l u d i n g l i q u o r l o s s with the wet f i l t e r cake and c o p r e c i p i t a t i o n with calcium s u l f i t e hemihydrate and gypsum s o l i d s . The bench-scale data, as w e l l as e a r l i e r work (2, 3), i n d i c a t e d that a d i p i c a c i d c o p r e c i p i t a t i o n with scrubber s o l i d s was a p o t e n t i a l l y important f a c t o r i n mass balance c a l c u l a t i o n s . F u r t h e r , the bench-scale data, shown i n F i g u r e 5, i n d i c a t e d that the amount of c o p r e c i p i t a t e d a d i p i c a c i d was a strong f u n c t i o n of the l e v e l of s u l f i t e o x i d a t i o n . In order to f u r t h e r c h a r a c t e r i z e t h i s mechanism f o r a d i p i c a c i d l o s s , two s e r i e s of batch p r e c i p i t a t i o n experiments were performed. The t e s t s were designed to study: • The mechanism f o r a d i p i c a c i d c o p r e c i p i t a t i o n (surface a d s o r p t i o n versus o c c l u s i o n or s o l i d s o l u t i o n formation), • The e f f e c t of l i q u i d phase a d i p i c acid c o n c e n t r a t i o n on a d i p i c a c i d c o n c e n t r a t i o n i n the scrubber s o l i d s , and • The e f f e c t of s u l f i t e to s u l f a t e r a t i o ( o x i d a t i o n f r a c t i o n on a d i p i c a c i d c o p r e c i p i t a t i o n . In the f i r s t s e r i e s of t e s t s 1.0 M N a S 0 and 1.0 M NaaSOt* were separately dripped i n t o s t i r r e d s o l u t i o n s of 0.5 M CaCl2 at 50 C. In t e s t s with Na2SÛ3, a n i t r o g e n blanket was maintained over the s o l u t i o n s and b o i l e d deionized water was used to prepare reagents. The pH was maintained at 4.6 and 5.5 by dropwise a d d i t i o n of h y d r o c h l o r i c a c i d . To t e s t the surface a b s o r p t i o n mechanism, about 3,000 ppm a d i p i c a c i d was added to the s l u r r i e s a f t e r 2

3



Figure 5. Relationship of adipic acid content of bench-scale solids to percent oxidation.


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Laboratory Investigation of Adipic Acid Degradation in Flue Gas

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