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.
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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
RiNCKHOFF
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
52
SULFUR
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
