Available Energy Analysis of a Sulfuric Acid Plant - American

6 Available Energy Analysis of a Sulfuric Acid Plant K. R A V I N D R A N A T H and S. T H I Y A G A R A J A N

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Larsen & Toubro Limited, Bombay, India

Available energy concept is applied to analyse a sulphuric acid plant. First law and second law analyses are compared. Second law analysis pin points available energy consumptions and losses. Possible improvements by reducing availability consumptions and losses are presented. S u l p h u r i c a c i d p l a n t i s w e l l known t o p r o d u c e a l o n g w i t h s u l p h u r i c a c i d a n e q u i v a l e n t amount o f steam. T h e p o t e n t i a l e n e r g y a v a i l a b l e w i t h t h e b a s i c raw m a t e r i a l sulphur c a l l s f o r a high degree e v a l u a t i o n o f energy recovery i n s u l p h u r i c acid plants. The e f f i c i e n c y o f e n e r g y c o n v e r s i o n a n d u t i l i s a t i o n i n t h i s p r o c e s s c a n n o t b e e v a l u a t e d b a s e d o n f i r s t law of thermodynamics a l o n e a n d t r u e energy d i s s i p a t i o n s can be brought out by u s i n g a v a i l a b l e energy a n a l y s i s . S e c o n d law a n a l y s i s i s a p p l i e d t o a 1 0 0 t o n n e s p e r day d o u b l e - c o n t a c t d o u b l e - a b s o r p t i o n (DC-DA) s u l p h u r i c acid plant i n order t o b r i n g out true energy conversion e f f i c i e n c i e s a n d c o n s u m p t i o n s b a s e d o n work a v a i l a b i l i t y o f v a r i o u s s t r e a m s . S e c o n d law e f f i c i e n c i e s a r e c o m p a r e d w i t h t h o s e o f f i r s t law t o p i n p o i n t true l o s s e s and i n e v i t a b l e consumptions i n energy conversion processes. B a s e d o n s e c o n d law a n a l y s i s a l t e r n a t i v e s a r e worked o u t f o r i m p r o v i n g o v e r a l l energy c o n v e r s i o n e f f i c i e n c y by recovering thermal energy i n a c i d coolers f o r power g e n e r a t i o n a n d f o r p r e h e a t i n g b o i l e r f e e d water · S u l p h u r i c Acid System A b l o c k d i a g r a m o f a t y p i c a l DC-DA s u l p h u r i c

acid

0097-6156/83/0235-0119S06.00/0 © 1983 American Chemical Society In Efficiency and Costing; Gaggioli, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

SECOND LAW ANALYSIS OF PROCESSES

120

p l a n t c o n s i s t i n g o f s e v e n s e c t i o n s i s shown i n F i g u r e 1* F o r c a r r y i n g o u t t o t a l e n e r g y a n a l y s i s o f t h i s p l a n t t h e s e seven s e c t i o n s a r e grouped i n t o f i v e major blocks* 1.


2.

3*

4*

5*

Sulphur preparation and combustion: M e l t i n g of s o l i d sulphur to l i q u i d , conversion of molten s u l p h u r t o SO2 g a s b y u s i n g d r y a i r a n d p a r t i a l recovery o f the combustion heat i n a waste heat boiler • C o n v e r s i o n I : F i r s t p a s s c o n v e r s i o n o f SO2 t o SO3 i n t h r e e b e d s o f V2O5 c a t a l y s t a n d i n t e r m e d i a t e heat removal i n waste heat b o i l e r , economiser and heat exchangers* Orying & A b s o r p t i o n I : Moisture removal from p r o c e s s a i r b y d r y i n g w i t h s u l p h u r i c a c i d a n d SO3 removal i n i n t e r m e d i a t e a b s o r p t i o n tower a f t e r f i r s t pass conversion* C o n v e r s i o n I I : S e c o n d p a s s c o n v e r s i o n o f SO2 t o SO3 a f t e r r e h e a t i n g t h e g a s e s f r o m i n t e r m e d i a t e absorber and subsequent heat removal i n a heat exchanger• A b s o r p t i o n I I : SO3 r e m o v a l i n f i n a l a b s o r p t i o n t o w e r b e f o r e v e n t i n g o u t i n e r t gas t h r o u g h s t a c k and a c i d c o o l i n g i n various cascade c o o l e r s *

F i r s t Law A n a l y s i s Input and o u t p u t e n t h a l p i e s o f v a r i o u s streams a c r o s s e a c h s e c t i o n f o r a 1 0 0 TPD DC-DA s u l p h u r i c a c i d p l a n t w i t h 10% s u l p h u r d i o x i d e f e e d t o c o n v e r t e r , 99.8% c o n v e r s i o n e f f i c i e n c y a n d 99.9% a b s o r p t i o n e f f i c i e n c y a r e shown i n T a b l e I * If the e f f i c i e n c y i s c a l c u l a t e d based on thermal energy e n t e r i n g and l e a v i n g each s y s t e m , i t works o u t t o b e 94 t o 98% f o r a l l s e c t i o n s , a c c o u n t i n g f o r 2-6/S h e a t l o s s e s * On t h e o t h e r h a n d , i f o n l y n e t u s e f u l energy from each system i s considered i t works out t o b e 8 9 t o 96% f o r s u l p h u r p r e p a r a t i o n , c o m b u s t i o n a n d c o n v e r s i o n s e c t i o n s ( 1 , 2 a n d 4 ) a n d 5*9 t o 0.1% f o r d r y i n g and a b s o r p t i o n s e c t i o n s (3 and 5 ) . T h i s d e n o t e s that the energy e f f i c i e n c i e s a r e a t a l a r m i n g l y low l e v e l s f o r s e c t i o n s 3 & 5* O v e r a l l e f f i c i e n c y b a s e d o n net u s e f u l o u t p u t i s o n l y 3 8 . 7 % . Out o f t o t a l t h e r m a l l o s s e s o f 6 1 . 3 % , l o s s e s i n warm w a t e r a r e a s h i g h a s 52% b a s e d o n f i r s t l a w a n a l y s i s * S e c o n d Law A n a l y s i s The d e g r a d a t i o n

i n t h e q u a l i t y o f e n e r g y a s i t moves

In Efficiency and Costing; Gaggioli, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.


COO LI NO WATER PROCESS

MOLTEN SULPHUR

SULPHUR COMBUSTION % WASTE HEAT RECOVERY

3

INTERMEDIATE ABSORPTION

SO2 CAS

I PASS CONVERSION It WASTE HEAT RECOVERY

(S0 +S > > •z

1

X

H

0

>

X

z

>

5 a

73

122


I . E n t h a l p y B a l a n c e o f A 1 0 0 TPD S u l p h u r i c Acid Plant E n t h a l p y v a l u e s i n K 3 / h r (' 0 0 0 ) D a t u m Temp.:25°C Enthalpy Output stream Enthalpy Input stream


Table

1 • Sulphur preparation & Combustion: 201 .0 DM w a t e r Steam e x p o r t Dry a i r 268.5 Hot S02 g a s B o i l e r I Uater 3728.8 Deaerated uater R e a c t i o n h e a t 12 6 2 4 . 0 Heat l o s s e s Total 1 6822.3 Net u s e f u l Efficiency % 2 • Conversion I 5234.0 SO3 g a s Hot SO2 g a s Dilution a i r 26.6 Boiler I uater Deaerated u a t e r 1742.0 Steam e x p o r t C o l d SO2 g a s 454.0 Hot SO2 g a s R e a c t i o n heat 4055.0 Heat l o s s e s Total 11 511 . 6 Net u s e f u l Efficiency % 3. D r y i n g & A b s o r p t i o n I SO3 g a s 2205.0 C o l d SO2 g a s Uet a i r 11 6.5 Dry a i r Cooling uater 51 8 2 . 5 Sulphuric acid R e a c t i o n heat 6 6 3 8 .0 Warm u a t e r Total 14142.0 Net u s e f u l Efficiency % 4. C o n v e r s i o n I I SG2 g a s SO3 g a s 1 632.6 R e a c t i o n heat 118.0 Heat l o s s e s Total 1750.6 Net u s e f u l Efficiency % 5. A b s o r p t i o n I I SO3 g a s Sulphuric acid 1688.0 Cooling uater 751 .6 Stack gas R e a c t i o n heat 155.0 LJarm u a t e r Total 2594.6 Net u s e f u l Efficiency % Net u s e f u l o u t p u t 11505.0 (38.7%) Uarm u a t e r l o s s 15447.4 (52.0^) Stack gas l o s s 4 5 1 . 3 ( 1 .SjS) Heat l o s s e s 2 3 2 1 .3 ( 7 . 8 $

7957.0 5234.0 1742.0 1889.3 1 6822.3 14933.0 88.8 2205.0 3728.8 3456.0 1 632.6 489.2 11511.6 11022.4 95.8 454.0 292.0 89.3 13306.7 14142.0 835.3 5.9 1 688.0 62.6 1750.6 1688.0 96.4 2.6 451 .3 2140.7 2594.6 2.6 0.1



6.

RAVINDRANATH AND

THIYAGARAJAN

Sulfuric Acid Plant

123

through v a r i o u s p r o c e s s u n i t s i s a s s e s s e d by c a l c u l a t ing t o t a l a v a i l a b l e energy o f input and output streams of each s y s t e m . T o t a l a v a i l a b i l i t y c o n s t i t u t e s b a s i c a l l y chemical, thermal, pressure a v a i l a b i l i t i e s and e l e c t r i c e n e r g y . The b a s i s f o r d e t a i l e d a v a i l a b l e energy c a l c u l a t i o n s o f a sulphuric acid plant i s given i n T a b l e 1 1 . ( 1 .2) . The a v a i l a b l e e n e r g y f l o w t h r o u g h f i v e m a j o r s e c t ions o f s u l p h u r i c a c i d p l a n t i s g i v e n i nF i g u r e 2. The major i n p u t s t o t h i s system a r e s u l p h u r and pouer, u i t h d e m i n e r a l i s e d (DM; u a t e r , u e t a i r , p r o c e s s u a t e r and c o o l i n g u a t e r from e n v i r o n m e n t . The u s e f u l o u t p u t s from the system a r e s u l p h u r i c a c i d and steam. Losses to environment i n c l u d e heat l o s s e s from various equipments,bloudoun u a t e r , steam from d e a e r a t o r v e n t , uarm u a t e r a n d s t a c k g a s . Process information o f various streams (streams 1 to 16) and t h e i r c h e m i c a l , thermal a n d p r e s s u r e a v a i l a b i l i t i e s a r e g i v e n i n Table I I I . Pouer i n p u t s t o t h e s y s t e m ( s t r e a m s 17 t o 2 0 ) a n d a v a i l a b i l i t y l o s s e s ( s t r e a m s 21 t o 2 9 ) a r e g i v e n i n T a b l e I V . T h e s t r e a m s marked a s i n p u t s from environment i n F i g u r e 2 a r e c o n s i d e r e d t o have z e r o a v a i l a b i l i t y * For e a c h i n p u t a n d o u t p u t s t r e a m o f a l l s e c t i o n s , a v a i l a b i l i t y i s c a l c u l a t e d . The d i f f e r e n c e b e t u e e n t h e output a v a i l a b i l i t y o f the t o t a l product ( i n c l u d i n g the l o s s e s ) a n d t h e i n p u t a v a i l a b i l i t y i s c o n s i d e r e d as a v a i l a b i l i t y consumed i n t h e p r o c e s s i n o r d e r t o e f f e c t the c o n v e r s i o n p r o c e s s . The r a t i o o f a v a i l a b i l i ty o f useful product t o t o t a l input a v a i l a b i l i t y i s c o n s i d e r e d a s e f f e c t i v e n e s s o f the system. F o reach o f the f i v e s e c t i o n s c o n s i d e r e d i n a v a i l a b l e energy f l o u diagram, the a v a i l a b i l i t y o f input and output streams, l o s s e s , consumption and e f f e c t i v e n e s s are c a l c u l a t e d . The f i v e m a j o r s e c t i o n s a r e b r o k e n d o u n f u r t h e r i n t o a number o f c o m p o n e n t s t o p i n p o i n t t h e a r e a s o f s i g n i f i cant a v a i l a b i l i t y consumptions. These r e s u l t s are g i v e n i n T a b l e V. R e s u l t s And D i s c u s s i o n From t h e a v a i l a b i l i t y a n a l y s i s , o v e r a l l e f f e c t i v e n e s s o f a s u l p h u r i c a c i d p l a n t u o r k s out t o be 4 9 % compared t o o v e r a l l e f f i c i e n c y o f 3 9 % based on f i r s t l a u . A l s o e f f e c t i v e n e s s i ns e c t i o n s 2 and 4 ( f i r s t and second pass conversion) i s a s h i g h as 86-92% thus l e a v i n g l e s s p r o s p e c t s f o r improvements i nthese s e c t i o n s . In section 1 (sulphur preparation and combustion), t h e e f f e c t i v e n e s s i s 75% and the a v a i l a b i l i t y consumption


124


Table 11.Basis f o r A v a i l a b i l i t y

Calculations

1 • Chemical a v a i l a b i l i t y ( a ) r

Atmospheric E q u i l i b r i u m {Stable condensed phases c o n s t i t u e n t s mole f r a c t i o n ! a t standard c o n d i t i o n s of 298"K & 1 a t m . 0.7567 H 0 0.2035 CaSO, 2 H 0 H 0 0.0303 CaC0„ CCL 0.0003 2


2

2

fora

Formulas

Q

a_ ( s o l i d ) S so (g)

i n kcals/q.mole • 191 .09 1 2 2 . 0 4 + R To I n Xo u„ 1 0 5 . 6 0 5 + RTo I n Xi»U3 -

a

Q n

2

2

a

a

S0

( 3

9

)

n

H S0 (l) 2

74.40

4

'H20

(Q)

2 . 0 7 1 7 + RT I n X

(9)

0.1 6518 + RT_ I n X..

(o)

0 . 9 4 3 2 8 + R To I n X u

o

2. Thermal a

-

T

u n

Kg n

0

a v a i l a b i l i t y (a-j. k c a l / q . m o l e ) - B T ) ( T - T ) + ( e / 2 - ^ P ) ( T * - T * ) + c / 3 ( T3 - T3v ^)

(A

r

o

Q

- AT I n f -+ Q

'o

0-T - V)-D( - I ) 1

r

T^ A B C x10 SO, 7.70 0.0053 - 0 . 8 3 SO SO3 1 3 . 7 0 0.00642 0 02 8.27 0.00258 0 N 6.5 0.001 0 Pressure a v a i l a b i l i t y ( a )

1

T

2

3

2

2

' O 6

0 x 1 0 0 -0.312 -0.1877 0

p

a

p

= R T I n [^p^j Q

kcals/g.mole


J

T

6.

RAVINDRANATH AND THIYAGARAJAN

C o

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CO

3

CD

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ID

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ON

CO

• O N r* Lf) r-

co CO ra CO

G

co


CD U

4->

3 j ON co O O CM «- in t—

• 1

1 CM o-

•H T3 C

o o (0 Q) L> O (4

CL

CO wD GO

m

to

CM ON

CO VO ON • • CM 1 VO ON

r-

Available Energy Analysis of a Sulfuric Acid Plant - American

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