Available Energy Analysis of a Sulfuric Acid Plant - ACS Symposium

Nov 11, 1983 - Larsen & Toubro Limited, Bombay, India. Efficiency and Costing. Chapter 6, pp 119–133. DOI: 10.1021/bk-1983-0235.ch006. ACS Symposium...
0 downloads 0 Views 1MB Size
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

Downloaded by TUFTS UNIV on November 27, 2015 | http://pubs.acs.org Publication Date: November 11, 1983 | doi: 10.1021/bk-1983-0235.ch006

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.

Downloaded by TUFTS UNIV on November 27, 2015 | http://pubs.acs.org Publication Date: November 11, 1983 | doi: 10.1021/bk-1983-0235.ch006

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.

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

SECOND LAW ANALYSIS OF PROCESSES

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

Downloaded by TUFTS UNIV on November 27, 2015 | http://pubs.acs.org Publication Date: November 11, 1983 | doi: 10.1021/bk-1983-0235.ch006

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

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

Downloaded by TUFTS UNIV on November 27, 2015 | http://pubs.acs.org Publication Date: November 11, 1983 | doi: 10.1021/bk-1983-0235.ch006

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

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

124

SECOND LAW ANALYSIS OF PROCESSES

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

Downloaded by TUFTS UNIV on November 27, 2015 | http://pubs.acs.org Publication Date: November 11, 1983 | doi: 10.1021/bk-1983-0235.ch006

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

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

J

T

6.

RAVINDRANATH AND THIYAGARAJAN

C o

•H D

CO

3

CD

XT

ID

NJ

ON

CO

• O N r* Lf) r-

co CO ra CO

G

co

Downloaded by TUFTS UNIV on November 27, 2015 | http://pubs.acs.org Publication Date: November 11, 1983 | doi: 10.1021/bk-1983-0235.ch006

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-