Sulfuric Acid Plants for Copper Converter Gas

However, this paper is primarily concerned with sulfuric acid pro duction from copper smelters where most of the sulfur dioxide is in a gas stream whi...
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4 Sulfuric Acid Plants for Copper Converter Gas J. B. RINCKHOFF Davy Powergas Inc., Lakeland, Fla. 33803

Conventional sulfuric acid plants have traditionally been used to recover sulfur dioxide from smelter gases, but these are inadequate to meet the proposed sulfur dioxide emission standards. Double absorption, which removes sulfur trioxide from the partially converted sulfur dioxide gas stream, reduces the sulfur dioxide emission to less than 500 ppm in the undiluted stack gas. Two double absorption plants using Lurgi technology have been operating with copper converter gas since early 1973. In spite of the wide and frequent varia­ tions in gas volume and sulfur dioxide concentration, these plants have consistently maintained sulfur dioxide emission levels well below 500 ppm. This paper presents data on the design and operating conditions for these plants.

Ύ^ΐιβ smelting of nonferrous metals, primarily copper, zinc, and lead, generally causes sulfur dioxide emissions varying from as much as 15% to less than 1% sulfur dioxide depending on the type of operation. Some sulfur dioxide is recovered as sulfuric acid. T h e nonferrous smelter industry—either in operation or under construction in 1974—has a total sulfuric acid production capacity of about 15,000 tons/day. About one third of this output comes from lead and zinc smelters which produce a reasonably steady gas stream containing 5-14% sulfur dioxide, depend­ ing on the type of roaster or sinter machine used. W i t h a steady gas flow and sulfur dioxide concentration, designing a sulfuric acid plant to use this off-gas presents few problems except for cleaning the gas in the purification section of the plant. A

However, this paper is primarily concerned with sulfuric acid pro­ duction from copper smelters where most of the sulfur dioxide is in a gas stream which varies widely and frequently, both in gas volume and in sulfur dioxide concentration. This gas stream presents a real challenge 48

4.

Sulfuric Acid

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Plants

49

to t h e s u l f u r i c a c i d p l a n t designer, e s p e c i a l l y i n v i e w of the

proposed

Environmental Protection Agency ( E P A ) regulation w h i c h w o u l d limit s u l f u r d i o x i d e emissions f r o m the a c i d p l a n t to a n average of 650 p p m o v e r a 6-hr p e r i o d . I n the c o n v e n t i o n a l process w h i c h has b e e n u s e d for m a n y years, the m a x i m u m c o n v e r s i o n of s u l f u r d i o x i d e to s u l f u r t r i o x i d e is 9 8 % . T h i s process w i l l r e d u c e the s u l f u r d i o x i d e i n a gas c o n t a i n i n g

8.0%

s u l f u r d i o x i d e to a b o u t 1800 p p m i n the stack gas l e a v i n g the absorber. W i t h a gas c o n t a i n i n g 4 %

sulfur dioxide, 9 8 %

conversion w i l l

reduce

the stack gas to 850 p p m . B o t h of these concentrations are greater t h a n the p r o p o s e d E P A l i m i t a t i o n of 650 p p m , a n d a different a p p r o a c h is required. T h i s has b e e n a c c o m p l i s h e d i n the d o u b l e - c a t a l y s i s process d e v e l oped by Bayer and Lurgi.

D a v y P o w e r g a s , w h o is a L u r g i licensee for

this process, b u i l t the o n l y t w o p l a n t s of this t y p e i n the U n i t e d States w h i c h use c o p p e r converter gas. T h e y h a v e k e p t s u l f u r d i o x i d e emissions w e l l b e l o w the g u a r a n t e e d 500 p p m l e v e l . I n the c o n v e n t i o n a l p l a n t s u l f u r d i o x i d e is c o n v e r t e d to s u l f u r t r i o x i d e i n a series of three or f o u r catalyst b e d s w i t h c o o l i n g b e t w e e n t h e beds to r e m o v e the heat of r e a c t i o n . T h e o v e r a l l c o n v e r s i o n is l i m i t e d b y t h e e q u i l i b r i u m for the r e l a t i v e p a r t i a l pressures of s u l f u r d i o x i d e , s u l f u r t r i o x i d e , a n d o x y g e n a n d the t e m p e r a t u r e of t h e converter exit gas. T h i s e q u i l i b r i u m is e q u i v a l e n t to a b o u t 9 8 . 5 %

conversion.

I n the d o u b l e - c a t a l y s i s p l a n t a major p o r t i o n of the s u l f u r t r i o x i d e is r e m o v e d f r o m the gas i n a n i n t e r m e d i a t e a b s o r p t i o n t o w e r after t h e second stage of c o n v e r s i o n .

T h e b a l a n c e of the gas, w h i c h is r e t u r n e d

to the c o n v e r t e r for the final t w o stages of c o n v e r s i o n , is a v e r y w e a k s u l f u r d i o x i d e gas w i t h a h i g h oxygen-to-sulfur d i o x i d e ratio. T h e e q u i l i b r i u m c o n d i t i o n s for this gas l e a v i n g the c o n v e r t e r are v e r y close to 1 0 0 % c o n v e r s i o n of the t o t a l s u l f u r d i o x i d e e n t e r i n g the converter.

In

steady state o p e r a t i o n , w h i c h is not p o s s i b l e w i t h c o p p e r converter gas, over 9 9 . 8 % c o n v e r s i o n of s u l f u r d i o x i d e to s u l f u r t r i o x i d e is e x p e c t e d i n double-catalysis plants. Capper Smelter

Operation

A b r i e f d e s c r i p t i o n of a t y p i c a l c o p p e r smelter o p e r a t i o n w i l l h e l p i n u n d e r s t a n d i n g the e x t r e m e l y v a r i a b l e n a t u r e of the s u l f u r d i o x i d e gas s t r e a m to b e processed.

T h e c o p p e r concentrates d e l i v e r e d to the smelter

are a m i x t u r e of c o p p e r a n d i r o n sulfides. I n c u r r e n t p r a c t i c e these are processed i n t w o or three steps to p r o d u c e 9 9 % b l i s t e r c o p p e r . I n some smelters the concentrates are first p a r t i a l l y roasted, r e m o v i n g of

the s u l f u r .

T h i s produces

containing 4 - 1 4 %

a r e l a t i v e l y strong, steady

gas

20-50% stream

s u l f u r d i o x i d e , d e p e n d i n g o n t h e t y p e of roaster u s e d .

50

S U L F U R

R E M O V A L

A N D

R E C O V E R Y

T h e p a r t i a l l y r o a s t e d m a t e r i a l , or g r e e n concentrates if n o r o a s t i n g step is i n c l u d e d , is t h e n c h a r g e d to t h e r e v e r b e r a t o r y f u r n a c e . H e r e the c h a r g e is m e l t e d , a n d a m a t t e c o n t a i n i n g 2 5 - 5 0 %

copper and 4 0 - 2 0 %

i r o n settles to the b o t t o m w h i l e a n i r o n slag floats to the t o p .

S o m e of

the s u l f u r i n the c h a r g e is b u r n e d b u t a d d i t i o n a l fossil f u e l firing is r e q u i r e d to p r o v i d e t h e necessary heat. an additional 1 0 - 3 0 %

I f the c h a r g e has b e e n roasted,

of the s u l f u r w i l l be b u r n e d .

W i t h green

con-

centrate f e e d a b o u t 2 0 - 4 0 % of the s u l f u r w i l l be b u r n e d . T h e off-gas f r o m the r e v e r b e r a t o r y f u r n a c e is a steady flow b u t it contains o n l y V i — 2 V i % sulfur dioxide. The verters.

final

step, a b a t c h o p e r a t i o n , takes p l a c e i n the c o p p e r

con-

M a t t e , w i t h d r a w n f r o m the r e v e r b e r a t o r y f u r n a c e , is c h a r g e d

to the c o n v e r t e r a l o n g w i t h a siliceous

fluxing

material.

A i r is b l o w n

t h r o u g h the m o l t e n charge, a n d the i r o n sulfide is selectively o x i d i z e d to f o r m i r o n o x i d e a n d s u l f u r d i o x i d e gas.

T h e iron oxide combines

with

the flux to f o r m a slag. W h e n s l a g g i n g is essentially c o m p l e t e the a i r b l o w i n g is s t o p p e d a n d the slag is s k i m m e d off.

A d d i t i o n a l matte and

flux is c h a r g e d to the converter, a n d the o p e r a t i o n is r e p e a t e d u n t i l a f u l l c o n v e r t e r charge is c o m p l e t e d .

T h i s m a y r e q u i r e f o u r to eight separate

charges, d e p e n d i n g o n the c o m p o s i t i o n of the matte.

The

individual

slag b l o w s m a y e a c h last as l o n g as 1. h r . N o a i r is b l o w n for a b o u t 15 m i n b e t w e e n charges w h i l e the slag is b e i n g s k i m m e d a n d a n a d d i t i o n a l c h a r g e is a d d e d . W h e n the c h a r g e is c o m p l e t e a n d essentially a l l of the i r o n has b e e n slagged, the final c o p p e r b l o w is started to b u r n t h e r e m a i n i n g c o p p e r sulfide. T h i s c o p p e r b l o w m a y last for s e v e r a l h o u r s before a l l the s u l f u r is r e m o v e d a n d a 9 9 % b l i s t e r c o p p e r is o b t a i n e d . T h e c o n verter is t h e n u n l o a d e d a n d m a d e r e a d y for the next charge.

Figure 1

shows t y p i c a l v a r i a t i o n s i n s u l f u r d i o x i d e concentrations i n the c o n v e r t e r gas d u r i n g slag a n d c o p p e r b l o w s

for

a plant w i t h

two

converters

operating. M o s t smelters h a v e at least three converters, a n d one or m o r e c o n verters s h o u l d b e o n the o p e r a t i n g c y c l e at a l l times to p r o v i d e a r e a s o n a b l y constant gas

flow.

T h e o p e r a t i n g t i m e for one c y c l e m a y b e f r o m

6 to 12 h r , d e p e n d i n g o n the o p e r a t i o n of the p a r t i c u l a r smelter a n d the t y p e of concentrates b e i n g treated. T h e gas flow d u r i n g the f u l l converter c y c l e is i n t e r m i t t e n t because it is necessary to shut off the air b l o w d u r i n g slag s k i m m i n g a n d c h a r g i n g . T h e s u l f u r d i o x i d e c o n c e n t r a t i o n i n t h e gas d i r e c t l y a b o v e the c o n verter c h a r g e is close to t h e o r e t i c a l — a b o u t 1 6 % d u r i n g slag b l o w s a n d 2 1 % d u r i n g copper blows.

T h e converter hoods, h o w e v e r , are not t i g h t

because the converter m u s t b e r o t a t e d for c h a r g i n g a n d s k i m m i n g . F r o m 100 to 3 0 0 % a i r w i l l leak i n to p r e v e n t a p p r e c i a b l e s u l f u r d i o x i d e leakage i n t o the w o r k i n g area a r o u n d the converters.

I n a t y p i c a l o p e r a t i o n the

4.

Sulfuric Acid

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Pfonts

ELAPSED

Figure 1.

51

TIME - HOURS

Variation of sulfur dioxide concentration converter gas

in a typical

copper

off-gas f r o m t h e c o n v e r t e r m a y average a b o u t 4 % s u l f u r d i o x i d e d u r i n g the slag b l o w s a n d 8 % d u r i n g t h e c o p p e r b l o w . 60-80%

T h i s gas w i l l c o n t a i n

of t h e t o t a l s u l f u r c o n t a i n e d i n t h e concentrates

c h a r g e d d i r e c t l y to t h e r e v e r b e r a t o r y f u r n a c e .

i f t h e y are

I f t h e concentrates are

p a r t i a l l y roasted before s m e l t i n g o n l y a b o u t 3 0 - 5 0 %

of the s u l f u r w i l l

b e i n t h e c o n v e r t e r off-gas. Plant Design

Considerations

F r o m t h e p o i n t of v i e w of the s u l f u r i c a c i d p l a n t designer i t w o u l d be advantageous i f the smelter h a d a roaster g e n e r a t i n g a h i g h s t r e n g t h s u l f u r d i o x i d e gas f o r t h e base l o a d . T h i s w o u l d r e d u c e t h e effect of t h e w i d e swings i n t h e s u l f u r d i o x i d e c o n c e n t r a t i o n of the c o n v e r t e r gas. U n f o r t u n a t e l y , most o f t h e e x i s t i n g roasters p r o d u c e

a very weak sulfur

d i o x i d e gas, a n d t h e s u l f u r i c a c i d p l a n t m u s t b e d e s i g n e d to use o n l y the c o n v e r t e r gas. F o r this e x a m p l e of a c i d p l a n t d e s i g n considerations, assume t h a t a smelter has three c o p p e r converters a n d t h a t the a c i d p l a n t m u s t b e a b l e to t a k e a l l of t h e c o n v e r t e r gas a n d m a i n t a i n a s u l f u r d i o x i d e l e v e l of less t h a n 500 p p m i n t h e a c i d p l a n t stack gas. T h e m a x i m u m gas flow f r o m

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REMOVAL

A N D RECOVERY

e a c h converter is 30,000 s t a n d a r d e u f t / m i n ( S C F M ) , a n d no m o r e t h a n t w o converters w i l l be o n the l i n e at a n y t i m e .

T h e m a x i m u m sulfur

d i o x i d e c o n c e n t r a t i o n w i t h the m a x i m u m gas flow w i l l b e 8 % . is to be a u t o t h e r m a l w h e n o p e r a t i n g w i t h as l o w as 4 %

The plant

sulfur dioxide.

T h i s means that no f u e l f i r i n g w i l l be r e q u i r e d to k e e p the p l a n t i n t h e r m a l b a l a n c e i f the gas has at least 4 % less t h a n 4 %

sulfur dioxide.

F o r gas c o n t a i n i n g

s u l f u r d i o x i d e , a f u e l - f i r e d i n d i r e c t heater w i l l b e r e q u i r e d .

T o m e e t these c o n d i t i o n s t h e a c i d p l a n t m u s t b e d e s i g n e d to h a n d l e 60,000 S C F M of gas. W h e n this gas contains t h e m a x i m u m of 8 % d i o x i d e the a c i d p r o d u c t i o n rate is e q u i v a l e n t to 960 t o n s / d a y .

sulfur

S i n c e this

c o n d i t i o n m a y persist for at least 1 h r , the catalyst l o a d i n g a n d the a c i d coolers m u s t b e d e s i g n e d for this a c i d p r o d u c t i o n rate. D e f i n i n g the m i n i m u m s u l f u r d i o x i d e c o n c e n t r a t i o n for the p l a n t establishes c e r t a i n other d e s i g n c r i t e r i a . If o n l y one converter is o n the fine w i t h a slag b l o w p r o d u c i n g 4 %

s u l f u r d i o x i d e gas, the e q u i v a l e n t

a c i d p r o d u c t i o n rate is o n l y 240 t o n s / d a y .

T h i s is s t i l l not the m i n i m u m

o p e r a t i n g rate since t h e r e w i l l b e p e r i o d s , p r e s u m a b l y short, w h e n t h e s u l f u r d i o x i d e c o n c e n t r a t i o n decreases to 1 % or e v e n zero.

S o m e of the

gas-to-gas heat exchangers m u s t be s i z e d to m a i n t a i n the a u t o t h e r m a l r e q u i r e m e n t w i t h 60,000 S C F M

of 4 %

sulfur dioxide.

The

gas-cooling

f a c i l i t i e s i n t h e p u r i f i c a t i o n section of the p l a n t m u s t b e able to r e d u c e t h e w a t e r content of t h e gas sufficiently to p e r m i t the p r o d u c t i o n of 9 3 % sulfuric acid w i t h 4 % gas to a b o u t 85 ° F .

s u l f u r d i o x i d e gas.

T h i s necessitates c o o l i n g the

F i g u r e 2 shows the r e q u i r e d gas t e m p e r a t u r e

v a r i o u s p l a n t elevations a n d s u l f u r d i o x i d e

for

concentrations.

A n o t h e r m a j o r c o n s i d e r a t i o n i n the d e s i g n of the p l a n t is the n a t u r e a n d q u a n t i t y of i m p u r i t i e s i n the gas stream d e l i v e r e d to the a c i d p l a n t . N o a c i d p l a n t c a n operate s a t i s f a c t o r i l y w i t h a n i n a d e q u a t e gas p u r i f i c a t i o n section. T h e gas w i l l n o r m a l l y b e d e l i v e r e d to the a c i d p l a n t at about 600° F after most of the dust has b e e n r e m o v e d i n electrostatic p r e c i p i tators. H o w e v e r , t h e gas m a y s t i l l c o n t a i n dust, as w e l l as s u l f u r t r i o x i d e , halogens, arsenic, a n d other m e t a l l i c vapors. Gas

Purification I n the p u r i f i c a t i o n section of the a c i d p l a n t the gas first enters a w e a k

a c i d s c r u b b e r w h e r e its t e m p e r a t u r e is r e d u c e d to a b o u t 130° F b y w a t e r e v a p o r a t i o n . It is t h e n c o o l e d to a b o u t 85° F to r e d u c e its w a t e r content to t h e r e q u i r e d l e v e l .

F i n a l l y , it is c l e a n e d i n electrostatic m i s t p r e -

cipitators w h e r e the last traces of dust a n d the a c i d m i s t f o r m e d t h e s u l f u r t r i o x i d e i n the gas are r e m o v e d .

from

T h e t y p e of e q u i p m e n t u s e d

i n t h e p u r i f i c a t i o n section w i l l v a r y s o m e w h a t w i t h i n d i v i d u a l p l a n t c o n d i t i o n s a n d operator

preferences.

4.

Sulfuric

RiNCKHOFF

Figure 2.

Acid

53

Fiants

Gas cooling required to produce 93%

or 98%

sulfuric

acid

G A S S C R U B B I N G A N D C O O L I N G . If the gas contains fluorine, t w o s c r u b b i n g towers i n series m a y b e r e q u i r e d to r e m o v e the

fluorine

completely.

T h i s step is necessary p r i m a r i l y to protect the catalyst i n the

contact

section of

second

the a c i d p l a n t .

Excess

scrubbing liquor from

the

s c r u b b e r is t r a n s f e r r e d to the first s c r u b b e r , a n d the w e a k a c i d p u r g e f r o m the system is t a k e n f r o m the first s c r u b b e r . Since

fluorine

w i l l attack the u s u a l a c i d - p r o o f refractory m a t e r i a l s ,

the first s c r u b b e r , w h i c h is subject to the h i g h e s t fluorine content, is l i n e d w i t h c a r b o n b r i c k . T h e s c r u b b e r m a y be either a v e n t u r i - t y p e or a n o p e n s p r a y - t y p e t o w e r i n w h i c h the gas is q u e n c h e d to its s a t u r a t i o n t e m p e r a ture. W e a k a c i d is r e c i r c u l a t e d over this t o w e r to r e m o v e as m u c h of the dust a n d fluorine as possible, as w e l l as to q u e n c h the gas. I n the second t o w e r t h e gas m u s t be c o o l e d to meet the r e q u i r e d w a t e r b a l a n c e for a c i d p r o d u c t i o n . T h i s is u s u a l l y a p a c k e d - or t r a y - t y p e t o w e r w i t h l i q u o r coolers i n the r e c i r c u l a t e d w e a k a c i d stream. T h e t e m p e r a t u r e to w h i c h the gas m u s t be c o o l e d is d e t e r m i n e d b y its s u l f u r d i o x i d e content, the p r o d u c t a c i d strength d e s i r e d , a n d the e l e v a t i o n of the p l a n t a b o v e sea l e v e l . A s s h o w n i n the chart, e a c h of these factors has a n i m p o r t a n t effect o n the r e q u i r e d t e m p e r a t u r e .

A n a l l o w a n c e has

54

SULFUR

REMOVAL

AND RECOVERY

b e e n m a d e i n the c a l c u l a t i o n to p e r m i t the a d d i t i o n of some w a t e r to the s t r o n g a c i d system to p r o v i d e m o r e If the

fluorine

flexible

c o n t r o l of a c i d strength.

or dust content of the gas is n o t excessive, a single

p a c k e d - o r t r a y - t y p e t o w e r w i t h c o o l i n g of the r e c i r c u l a t e d l i q u o r c a n be u s e d f o r b o t h gas s c r u b b i n g a n d c o o l i n g . once-through reasons.

D i r e c t c o o l i n g of the gas w i t h

c o o l i n g w a t e r is no longer c o n s i d e r e d p r a c t i c a l for

two

F i r s t , the w a t e r w i l l b e c o n t a m i n a t e d w i t h d u s t a n d the a c i d

f o r m e d f r o m t h e s u l f u r t r i o x i d e i n the gas, m a k i n g d i s p o s a l a p r o b l e m . S e c o n d , some of the s u l f u r d i o x i d e w i l l be a b s o r b e d i n the w a t e r , a n d the a m o u n t of s t r i p p i n g a i r r e q u i r e d to recover this a n d a v o i d a n u i s a n c e w i l l d i l u t e the m a i n gas stream excessively. M a t e r i a l s of c o n s t r u c t i o n are a n i m p o r t a n t c o n s i d e r a t i o n i n the gas s c r u b b i n g system because

fluorine

a n d c h l o r i n e m a y b e present.

s c r u b b i n g l i q u o r is u s u a l l y less t h a n 1 0 % and

The

sulfuric acid below

135 ° F ,

a stainless steel ( s u c h as 20 a l l o y ) w o u l d be suitable for

pumps,

v a l v e s , a n d l i q u o r coolers i f halogens are not present.

T h e alternatives

are glass- or p l a s t i c - l i n e d , g r a p h i t e or h i g h e r alloys. T h e towers are u s u a l l y a c a r b o n steel s h e l l w i t h a n i m p e r v i o u s m e m b r a n e a n d a n a c i d b r i c k l i n i n g , a l t h o u g h p l a s t i c c a n b e u s e d i n some areas. ELECTROSTATIC MIST

PRECIPITATORS.

The

gas

l e a v i n g the

scrubbers

is essentially free of halogens a n d dust b u t i t still contains a c i d m i s t . T h e a m o u n t of a c i d m i s t d e p e n d s p r i m a r i l y o n c o n d i t i o n s i n t h e smelter.

In

gas f r o m c o p p e r converters, the s u l f u r t r i o x i d e content m a y v a r y f r o m 2 to 1 0 % of the t o t a l s u l f u r oxides. T h e a m o u n t of s u l f u r t r i o x i d e f o r m e d d e p e n d s l a r g e l y o n the t e m p e r a t u r e a n d t i m e the gas contacts t h e i r o n o x i d e i n the dust a n d the scale o n the c a r b o n steel

flues.

T h e sulfur

t r i o x i d e c o m b i n e s w i t h the m o i s t u r e i n t h e gas to f o r m s u l f u r i c a c i d v a p o r . W h e n the gas is c o o l e d i n the scrubbers, most of this v a p o r condenses as a finely d i v i d e d a c i d m i s t , a l t h o u g h some of i t is a b s o r b e d scrubber liquor. be

i n the

S u l f u r i c a c i d m i s t , w h i c h is g e n e r a l l y c o n s i d e r e d

particles less t h a n 5 μ, is v e r y

difficult to

remove

from

a

to gas

stream, so o n l y a p o r t i o n of the m i s t w i l l b e r e m o v e d i n t h e s c r u b b e r . If the r e m a i n i n g m i s t w e r e a l l o w e d to enter the contact section of the a c i d p l a n t it w o u l d corrode the c a r b o n steel ducts a n d heat a n d the m a i n b l o w e r .

exchangers

It m u s t , therefore, be r e m o v e d as c o m p l e t e l y as

possible i n the p u r i f i c a t i o n section of the p l a n t .

T h i s is a c c o m p l i s h e d

i n the electrostatic m i s t p r e c i p i t a t o r s . T h e s e p r e c i p i t a t o r s are u s u a l l y m a d e of sheet l e a d . T h e y

resemble

a v e r t i c a l t u b u l a r exchanger w i t h h i g h voltage d i s c h a r g e electrodes

sus­

p e n d e d i n the center of e a c h of the 10-in. d i a m e t e r tubes.

gas,

flowing

The

u p w a r d t h r o u g h the tubes, is exposed to a c o r o n a d i s c h a r g e f r o m

the electrodes w h i c h d r i v e s t h e mist p a r t i c l e s to the g r o u n d e d t u b e w a l l s . T h e c o l l e c t e d a c i d runs d o w n the tubes a n d is c o l l e c t e d i n the l o w e r

4.

RiNCKHOFF

h e a d e r as 5 - 1 0 %

Sulfuric Acid

55

Plants

s u l f u r i c a c i d . T h e a c i d , w h i c h is s a t u r a t e d w i t h s u l f u r

d i o x i d e a n d w h i c h contains the last traces of d u s t f r o m the gas, is u s u a l l y r e t u r n e d to the s c r u b b e r c i r c u l a t i n g system. T h e gas l e a v i n g the scrubbers m a y c o n t a i n as m u c h as 100 m g / S C F of s u l f u r i c a c i d as a c i d m i s t , a n d t w o m i s t p r e c i p i t a t o r s are u s u a l l y i n stalled i n series to o b t a i n 9 9 % r e m o v a l efficiency.

T h i s efficiency c o u l d

b e o b t a i n e d i n a single l a r g e r u n i t g i v i n g the same t o t a l r e s i d e n c e t i m e b u t this w o u l d be less r e l i a b l e . T h e efficiency is a f u n c t i o n of

power

i n p u t to the d i s c h a r g e electrodes, a n d this is l i m i t e d b y the voltage at w h i c h a r c i n g occurs i n the tubes.

E n t r a i n e d a c i d d r o p l e t s i n the gas

stream w i l l aggravate the a r c i n g a n d r e q u i r e r e d u c e d i n p u t voltage w h i c h lowers the u n i t efficiency. W i t h t w o units i n series the v o l t a g e is r e d u c e d o n l y o n the first u n i t so that the o v e r a l l efficiency is affected o n l y s l i g h t l y . T h e i m p u r i t i e s r e m o v e d f r o m the gas i n the p u r i f i c a t i o n system m u s t be p u r g e d f r o m t h e system, a n d the p u r g e is n o r m a l l y t a k e n f r o m t h e s c r u b b i n g tower. M a k e u p w a t e r is u s u a l l y r e q u i r e d to p r o v i d e this p u r g e because the gas e n t e r i n g the s c r u b b e r n o r m a l l y contains less w a t e r t h a n the c o o l e d s a t u r a t e d gas l e a v i n g the m i s t p r e c i p i t a t o r s . T h e q u a n t i t y of m a k e u p w a t e r r e q u i r e d is the t o t a l of the a m o u n t a d d e d to t h e gas stream a n d the a m o u n t r e q u i r e d to m a i n t a i n the a c i d a n d / o r the dust c o n c e n t r a t i o n i n the s c r u b b e r l i q u o r b e l o w a selected l i m i t i n g figure. T h e p u r g e stream is saturated w i t h s u l f u r d i o x i d e a n d is d i s c h a r g e d f r o m the system

Figure 3.

Flow diagram of a double-catalysis

sulfuric acid plant

56

SULFUR

REMOVAL

A N D RECOVERY

t h r o u g h a s t r i p p e r w h e r e most of the s u l f u r d i o x i d e is r e c o v e r e d i n a s m a l l stream of s t r i p p i n g a i r w h i c h is a d d e d to t h e m a i n gas stream. T h e gas l e a v i n g the m i s t p r e c i p i t a t o r s s h o u l d c o n t a i n o n l y

sulfur

d i o x i d e , o x y g e n , n i t r o g e n , a n d w a t e r v a p o r , b u t there w i l l be some traces of i m p u r i t i e s w h i c h are not h a r m f u l to the contact section of the a c i d p l a n t . If the gas has not b e e n a d e q u a t e l y c l e a n e d , the contact section the p l a n t w i l l h a v e c o n t i n u i n g o p e r a t i n g p r o b l e m s .

Improper

of

operation

of the m i s t p r e c i p i t a t o r s w i l l p e r m i t a c i d m i s t to enter the c o n t a c t section.

T h i s w i l l result i n c o r r o s i o n a n d sulfate a c c u m u l a t i o n i n the gas

ducts, the b l o w e r , a n d the heat exchangers.

T h e c o r r o s i o n is p a r t i c u l a r l y

severe i n the heat exchangers b e c a u s e the tube w a l l t e m p e r a t u r e , w i t h h o t s u l f u r t r i o x i d e gas i n the tubes, exceeds 3 0 0 ° F . T h e sulfate a c c u m u l a t i o n on the d u c t w a l l s is n o r m a l l y not a serious corrosion p r o b l e m

but

it e v e n t u a l l y breaks loose a n d is c a r r i e d onto the catalyst beds i n the converter.

T h i s sulfate, a n d also a n y dust that m a y pass t h r o u g h

the

p u r i f i c a t i o n section, w i l l g r a d u a l l y b l i n d the catalyst beds. T h i s reduces the c o n v e r s i o n

efficiency

a n d increases

the pressure d r o p t h r o u g h

the

plant.

Contact

Section

I n t h e contact section of the p l a n t , the saturated gas is first d r i e d b y contact w i t h 9 3 % a c i d a n d t h e n the s u l f u r d i o x i d e i n the gas is o x i d i z e d to s u l f u r t r i o x i d e .

T h e s u l f u r t r i o x i d e is a b s o r b e d i n 9 8 %

w h e r e it c o m b i n e s w i t h the free w a t e r present sulfuric acid.

acid

to p r o d u c e a d d i t i o n a l

F i g u r e 3 is a t y p i c a l flow d i a g r a m of a

double-catalysis

s u l f u r i c a c i d p l a n t o p e r a t i n g w i t h s u l f u r d i o x i d e gas f r o m c o p p e r

con-

verters. T o o b t a i n essentially c o m p l e t e o x i d a t i o n of s u l f u r d i o x i d e to s u l f u r t r i o x i d e excess o x y g e n i n the gas a b o v e the s t o i c h i o m e t r i c r e q u i r e m e n t is necessary.

T h e v o l u m e of o x y g e n i n the gas s h o u l d at least be e q u i v a l e n t

to that of the s u l f u r d i o x i d e , a n d it is p r e f e r a b l e to h a v e a s l i g h t l y h i g h e r ratio. T h e r e are times d u r i n g the o p e r a t i n g c y c l e of t h e c o p p e r w h e n the oxygen-to-sulfur

converter

d i o x i d e r a t i o is l o w e r t h a n is desirable.

Dur-

i n g these p e r i o d s d i l u t i o n a i r m u s t be m i x e d w i t h t h e gas e n t e r i n g the d r y i n g t o w e r of the c o n t a c t section of the p l a n t to increase its o x y g e n content. T h e gas is d r i e d b y c o n t a c t w i t h r e c i r c u l a t e d 9 3 % passes u p t h r o u g h the p a c k i n g i n t h e d r y i n g tower.

a c i d as the gas

T h e a c i d absorbs

t h e m o i s t u r e f r o m the gas a n d is h e a t e d b y m o i s t u r e c o n d e n s a t i o n

and

b y the resultant a c i d d i l u t i o n . T h i s heat is r e m o v e d b y p u m p i n g the a c i d t h r o u g h coolers before i t is r e t u r n e d to the t o p of the tower.

4.

RiNCKHOFF

Sulfuric Acid

57

Phnts

S i n c e d r i e d gas is not corrosive, c a r b o n steel ducts are u s e d for t h e r e m a i n d e r of the p l a n t . T h e gas leaves the d r y i n g t o w e r at a b o u t 1 1 0 ° F , a n d after p a s s i n g t h r o u g h a n e n t r a i n m e n t separator i t goes to the m a i n blower.

T h e b l o w e r p r o v i d e s sufficient s u c t i o n to d r a w the gas t h r o u g h

the p u r i f i c a t i o n section a n d the d r y i n g t o w e r a n d sufficient pressure to d e l i v e r i t t h r o u g h the b a l a n c e of the p l a n t .

T h e t o t a l pressure

drop

t h r o u g h a c l e a n d o u b l e - c a t a l y s i s p l a n t , i n c l u d i n g the p u r i f i c a t i o n system, is u s u a l l y a b o u t 200 i n . W G (7.25 p s i ) , a n d a n a d d i t i o n a l 2 5 - 3 0 i n . W G is u s u a l l y a d d e d for the d e s i g n of the b l o w e r to a l l o w for pressure b u i l d u p i n the system. Conversion. T h e gas leaves the b l o w e r at a b o u t 175° F a n d is h e a t e d i n the s h e l l side of a series of gas-to-gas s h e l l a n d t u b e heat exchangers to the r e q u i r e d c o n v e r t e r i n l e t t e m p e r a t u r e of 8 2 0 ° F . I n the converter, f o u r catalyst beds are a r r a n g e d one a b o v e the other w i t h d i v i s i o n plates b e t w e e n the beds. T h e gas l e a v i n g e a c h catalyst b e d is c o o l e d i n the t u b e side of the s h e l l a n d t u b e heat exchangers to the d e s i r e d t e m p e r a t u r e before e n t e r i n g the next b e d .

T h e t e m p e r a t u r e rise

i n the catalyst beds w i l l v a r y c o n s i d e r a b l y w i t h the w i d e v a r i a t i o n i n t h e sulfur d i o x i d e content of the feed gas. T h e q u a n t i t y of catalyst i n s t a l l e d i n each b e d w i l l be d e t e r m i n e d b y the m a x i m u m s u l f u r d i o x i d e flow, w h i c h is u s u a l l y a n 8 % gas. T h e r e f o r e the catalyst l o a d i n g is greater t h a n r e q u i r e d for a w e a k e r s u l f u r d i o x i d e gas, a n d a h i g h e r c o n v e r s i o n w i l l b e o b t a i n e d o n the first b e d . T h i s is e n h a n c e d b y the l o w e r t e m p e r a t u r e rise o b t a i n e d w i t h a w e a k s u l f u r d i o x i d e gas for a g i v e n percentage c o n v e r s i o n of s u l f u r d i o x i d e to s u l f u r t r i o x i d e . F o r e x a m p l e , a p p r o x i m a t e l y 6 5 % of a n

8%

s u l f u r d i o x i d e gas w i l l be c o n v e r t e d to s u l f u r t r i o x i d e i n the first b e d w i t h a t e m p e r a t u r e rise of 2 8 5 ° F .

With a 4%

s u l f u r d i o x i d e gas a b o u t

8 5 % w i l l b e c o n v e r t e d w i t h a t e m p e r a t u r e rise of o n l y 190 ° F .

The dif-

ferences a n d t e m p e r a t u r e rises w i l l be less o n subsequent beds.

The

extent of c o n v e r s i o n approaches e q u i l i b r i u m o n each b e d , a n d the gas m u s t be c o o l e d after e a c h b e d so that f u r t h e r c o n v e r s i o n c a n b e a c h i e v e d i n the next b e d . Absorption. I n a t y p i c a l d o u b l e - c a t a l y s i s p l a n t , the gas l e a v i n g the second

stage of c o n v e r s i o n passes to the interstage a b s o r p t i o n

tower

w h e r e the s u l f u r t r i o x i d e is a b s o r b e d f r o m the gas. W i t h a n 8 %

sulfur

d i o x i d e gas m o r e t h a n 8 5 % of the s u l f u r d i o x i d e has b e e n c o n v e r t e d to s u l f u r t r i o x i d e at this p o i n t , a n d w i t h a 4 %

gas over 9 5 % has b e e n c o n -

v e r t e d . T h e gas l e a v i n g the converter m u s t be c o o l e d at least to 450° F b u t not b e l o w 300° F before e n t e r i n g the absorber.

T h e heat a v a i l a b l e

f r o m this c o o l i n g is u s e d to reheat the gas r e t u r n i n g f r o m t h e absorber to t h e t h i r d stage of c o n v e r s i o n . O n e of the m a j o r p r o b l e m s i n d e s i g n i n g a d o u b l e - c a t a l y s i s p l a n t is to m a k e i t a u t o t h e r m a l w h e n o p e r a t i n g w i t h a w e a k s u l f u r d i o x i d e gas.

The

58

SULFUR REMOVAL

A N D RECOVERY

o n l y heat a v a i l a b l e to the c o n v e r t e r - h e a t exchanger system is the exot h e r m i c heat of r e a c t i o n of o x i d i z i n g s u l f u r d i o x i d e to s u l f u r t r i o x i d e a n d the heat of c o m p r e s s i o n i n t h e m a i n gas b l o w e r .

Some of the heat c a n

b e r e c o v e r e d f r o m the gas g o i n g f r o m the converter to the absorbers b u t this gas stream, w i t h its s u l f u r t r i o x i d e content, s h o u l d not b e

cooled

b e l o w 300° F to a v o i d possible a c i d c o n d e n s a t i o n i n the heat exchanger. I n a c o n v e n t i o n a l t y p e p l a n t w i t h a single absorber it is p r a c t i c a l to d e s i g n the p l a n t to b e a u t o t h e r m a l w i t h a gas c o n t a i n i n g as little as 3 % s u l f u r d i o x i d e because heat is o n l y lost w h e n the gas goes to the absorber p l u s t h e n o r m a l heat lost to atmosphere.

I n the d o u b l e - c a t a l y s i s process,

h e a t is lost f r o m t h e gas s t r e a m g o i n g to e a c h of the t w o absorbers.

In

the n o r m a l d e s i g n of absorber, i f the t o t a l gas stream leaves the interstage a b s o r b e r at 1 7 0 - 1 8 0 ° F to be r e h e a t e d to converter t e m p e r a t u r e it w o u l d be i m p o s s i b l e for the p l a n t to be a u t o t h e r m a l w i t h 4 % s u l f u r d i o x i d e gas. A s o l u t i o n to this p r o b l e m , d e v e l o p e d a n d p a t e n t e d b y L u r g i , uses a v e n t u r i - t y p e interstage absorber. T h e a c i d a n d gas flows are c o - c u r r e n t d o w n t h r o u g h the v e r t i c a l v e n t u r i so that the gas leaves the

absorber

at essentially the same t e m p e r a t u r e as the a c i d . W i t h s u i t a b l e adjustment of the a c i d c i r c u l a t i n g rate the gas exit t e m p e r a t u r e c a n be m a i n t a i n e d at 250 ° F . 4%

T h i s a d d i t i o n a l 7 0 - 8 0 ° F insures a u t o t h e r m a l o p e r a t i o n w i t h

s u l f u r d i o x i d e gas i n a d o u b l e - c a t a l y s i s p l a n t . S o m e a d d i t i o n a l heat

a d v a n t a g e c a n be g a i n e d b y b y p a s s i n g a p o r t i o n of the gas d i r e c t f r o m the second to the t h i r d stage.

T h i s , h o w e v e r , increases the q u a n t i t y of

s u l f u r t r i o x i d e i n t h e gas l e a v i n g the converter, t h e r e b y r e d u c i n g the m a x i m u m c o n v e r s i o n that c a n b e a c h i e v e d . S u l f u r t r i o x i d e a b s o r p t i o n i n the interstage absorber is essentially c o m p l e t e , a n d v e r y little passes to t h e final stages f r o m this source.

T h e absorber c i r c u l a t i n g a c i d is m a i n -

t a i n e d at 9 8 % s u l f u r i c a c i d to o b t a i n g o o d a b s o r p t i o n . T h e heat r e s u l t i n g f r o m a b s o r b i n g s u l f u r t r i o x i d e a n d c o o l i n g the gas is r e m o v e d f r o m the a c i d i n coolers b e f o r e the a c i d is r e t u r n e d to the absorbers. T h e gas e n t e r i n g the t h i r d stage of the converter has a v e r y l o w s u l f u r d i o x i d e content w i t h a h i g h o x y g e n - t o - s u l f u r d i o x i d e r a t i o so t h a t a h i g h c o n v e r s i o n is p o s s i b l e i n the last t w o stages.

Adequate facilities

for c o o l i n g b e t w e e n these stages are i m p o r t a n t , so that the final converter exit t e m p e r a t u r e c a n b e l o w to p r o v i d e the best e q u i l i b r i u m c o n d i t i o n s for m a x i m u m overall conversion.

T h i s c o o l i n g also p r o v i d e s a

safety

factor against upsets i n o p e r a t i o n of the first t w o beds w h i c h w o u l d r e q u i r e m o r e t h a n n o r m a l c o n v e r s i o n i n the last t w o beds w i t h a h i g h e r t e m p e r a t u r e rise. T h e c o n v e r t e r exit t e m p e r a t u r e is a c o n t r o l l i n g factor i n the degree of c o n v e r s i o n that c a n b e a c h i e v e d , a n d it s h o u l d b e m a i n t a i n e d as l o w as possible, p r e f e r a b l y b e l o w 8 0 0 ° F . T h e gas l e a v i n g the final stage of t h e converter is c o o l e d i n heat exchangers b e f o r e g o i n g to the final absorber.

T h e heat r e c o v e r e d i n

4.

RiNCKHOFF

Sulfuric Acid

Phnts

59

these exchangers a n d i n those c o o l i n g the gas after the first a n d t h i r d stages of the c o n v e r t e r is u s e d to p r e h e a t the sulfur d i o x i d e gas l e a v i n g the b l o w e r to the r e q u i r e d c o n v e r t e r inlet t e m p e r a t u r e . I n the final absorber, w h i c h is s i m i l a r to the d r y i n g t o w e r , the s u l f u r t r i o x i d e is a b s o r b e d f r o m the gas.

T h e r e m a i n d e r of the gas, w i t h a

s u l f u r d i o x i d e content b e l o w 5 0 0 p p m , is v e n t e d to the atmosphere t h r o u g h a d e m i s t e r w h i c h removes a c i d m i s t . Automatic Controls. A c e r t a i n a m o u n t of a u t o m a t i c c o n t r o l of process o p e r a t i n g c o n d i t i o n s is necessary to c o p e w i t h f r e q u e n t large changes i n s u l f u r d i o x i d e content of the gas. A s u l f u r d i o x i d e a n a l y z e r , i n d i c a t i n g t h e s u l f u r d i o x i d e content of the gas at the m a i n b l o w e r , controls t h e a d m i s s i o n of d i l u t i o n a i r to the d r y i n g t o w e r w h e n the sulfur d i o x i d e content of the f e e d gas exceeds 8 % .

A s e c o n d a n a l y z e r records t h e s u l f u r

d i o x i d e content of the gas l e a v i n g t h e final absorber. T h e t e m p e r a t u r e of the gas e n t e r i n g e a c h of the first t w o catalyst beds is a u t o m a t i c a l l y c o n t r o l l e d b y a d j u s t i n g the heat exchanger bypasses. T h e a c i d strength a n d levels i n e a c h of the three p u m p tanks s e r v i n g t h e d r y i n g a n d a b s o r p t i o n towers are c o n t r o l l e d a u t o m a t i c a l l y b y r e g u l a t i n g t h e v a r i o u s cross transfers, a d d i n g w a t e r , a n d d e l i v e r i n g the p r o d u c t a c i d to storage. T h e s e controls t a k e care of the adjustments for w i d e changes i n a c i d p r o d u c t i o n , f r e e i n g the operator to m a k e the m i n o r changes w h i c h o p t i m i z e the o p e r a t i o n . R E C E I V E D April 4, 1 9 7 4