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Europaisches Patentamt

European Patent Office

Office europeen des brevets

@ Publication number: 0 0 1 8 0 2 8 A 1

EUROPEAN PATENT APPLICATION

@ Application number: 80200296.4

@ Date of filing: 29.03.80

© mt.ci.*:C01C 1 / 1 2 . C 0 1 B 3 1 / 2 0 , B01 D 1 9 / 0 0

® Priority: 16.04.79 US 30374 @ Applicant: STAMICARBON B.V., Postbus 10, NL-6160 MC Geleen(NL)

@ Date of publication of application: 29.10.80 ® Inventor: Goorden, Josephus Johannes Petrus Maria, Bulletin 80/22 Haagstraat 29, NL-6133 VR Slttard (NL)

@ Representative : Hatzmann, Marinus Jan et al, ® Designated Contracting States: AT BE CH DE FR GB IT OCTROOIBUREAU DSM P.O.Box 9, NL-6160 MA Geleen

NLSE (NL)

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@ Process for the separation of ammonia and carbon dioxide from mixtures containing ammonia, carbon dioxide and water.

A process for the separation of substantially pure NH3 and substantially pure CO2 from a mixture containing NH3, CO2 and water is disclosed.

In this process NH3 is separated by rectification in an NHa-separation zone (3), the remaining liquid phase is fed to a CO2-separation zone (18) where gaseous CO2 is separated by rectification, and the resulting liquid phase is fed from the bottom of this zone to a desorption zone (10), where virtually all NH3 and CO2 still contained in this liquid phase are separated as a gaseous mixture.

To improve the efficiency of the separation and to decrease the sensitivity of the process to fluctuations in the feed to the NH3-separation zone (3), the process is characterized in that so much gaseous CO2 or a gaseous mixture containing CO2 is fed to the bottom of the NH3-se- paration zone (3) that the composition of the liquid phase released in the bottom of the NHa-separation zone (3) is not on the side of the boundary line rich in NH3.

Preferably also an amount of inert gas is introduced together with the CO2.

P r o c e s s for the s e p a r a t i o n of s u b s t a n t i a l l y pure NH3

and s u b s t a n t i a l l y pure C02 from a m i x t u r e c o n t a i n i n g NH3,

C02 and w a t e r , in which NH3 is s e p a r a t e d by r e c t i f i c a t i o n

in an N H 3 - s e p a r a t i n g zone , the r e m a i n i n g l i q u i d phase i s

fed to a C 0 2 - s e p a r a t i n g zone where g a s e o u s C02 is s e p a r a t e d

by r e c t i f i c a t i o n , and the r e s u l t i n g l i q u i d phase is fed f r o m

the b o t t o m of t h i s zone to a d e s o r p t i o n zone , where v i r t u a l l y

a l l NH3 and C02 s t i l l c o n t a i n e d in t h i s l i q u i d phase a r e

s e p a r a t e d as a g a s e o u s m i x t u r e .

In some c h e m i c a l p r o c e s s e s m i x t u r e s c o n t a i n i n g ammonia and c a r b o n d i o x i d e , and s o m e t i m e s a l s o c o n t a i n -

ing w a t e r , . a r e o b t a i n e d as b y - p r o d u c t s . For i n s t a n c e ,

in the s y n t h e s i s of m e l a m i n e from u r e a , a gas m i x t u r e , is o b t a i n e d w h i c h , in a d d i t i o n to m e l a m i n e , a l s o c o n - t a i n s ammonia and c a r b o n d i o x i d e in amoun t s of a t l e a s t

1.7 t ons per ton of m e l a m i n e . Such m i x t u r e s a re a l s o

o b t a i n e d in the p r e p a r a t i o n of u r ea from ammonia a n d

c a r b o n d i o x i d e , r e s u l t i n g from the d e c o m p o s i t i o n o f

b y - p r o d u c t ammonium c a r b a m a t e , and i t s s e p a r a t i o n f r o m

the u r e a p r o d u c t . In o r d e r to e f f e c t i v e l y u t i l i z e t h i s

ammonia and c a r b o n d i o x i d e a f t e r s e p a r a t i n g i t from t h e

me lamine or u r e a , fo r e x a m p l e as r e c y c l e to a u r e a

s y n t h e s i s p r o c e s s , i t i s in most c a s e s n e c e s s a r y t o

r a i s e the g a s e s to a h i g h e r p r e s s u r e . C o m p r e s s i o n o f

s u c h m i x t u r e r e q u i r e s s p e c i a l m e a s u r e s to p r e v e n t t h e

c o n d e n s a t i o n of ammonia and c a r b o n d i o x i d e and t h e

d e p o s i t i o n of s o l i d ammonium c a r b a m a t e t h e r e b y f o r m e d .

For t h i s r e a s o n , such gas m i x t u r e s a re u s u a l l y

a b s o r b e d in w a t e r or in an a q u e o u s s o l u t i o n , which r e -

s u l t s in the f o r m a t i o n of an ammonium c a r b a m a t e s o l u t i o n

which can be pumped to the u r e a s y n t h e s i s r e a c t o r ,

s o m e t i m e s b e i n g c o n c e n t r a t e d by d e s o r p t i o n and r e p e a t e d

a b s o r p t i o n at a h i g h e r p r e s s u r e . A d i s a d v a n t a g e o f t h i s p r o c e d u r e i s t h a t the w a t e r , r e c y c l e d to t he u r e a r e a c t o r t o g e t h e r w i t h t he ammonia and c a r b o n d i o x i d e , has an u n f a v o r a b l e e f f e c t on the u rea s y n t h e s i s r e a c t i o n .

I t has been p r o p o s e d to s e p a r a t e l y remove t h e ammonia and c a r b o n d i o x i d e from the b y - p r o d u c t m i x t u r e s , and to s e p a r a t e l y r e c y c l e them in o r d e r to a v o i d t h e

f o r m a t i o n and d e p o s i t i o n of ammonium c a r b a m a t e . H o w e v e r , t he b i n a r y s y s t e m of ammonia and c a r b o n d i o x i d e f o r m s

a maximum b o i l i n g a z e o t r o p e a t a m o l a r a m m o n i a - t o - c a r b o n d i o x i d e r a t i o of a b o u t 2 : 1 , and t h e r e f o r e c a n n o t b e

s e p a r a t e d by s i m p l e d i s t i l l a t i o n . Th i s phenomenon a l s o

o c c u r s in t he t e r n a r y s y s t e m of ammonia, c a r b o n d i o x i d e and w a t e r , and the t e rm a z e o t r o p e as used h e r e i n s h o u l d be u n d e r s t o o d to i n c l u d e t h i s phenomenon in t he t e r n a r y s y s t e m as w e l l . Also as used h e r e i n , w i t h r e s p e c t t o

such b i n a r y or t e r n a r y m i x t u r e , the t e rm " r i c h " w i t h

respec t to ammonia shall be unders tood to mean that when h e a t

is applied to a mixture 'rich' in ammonia, subs t an t i a l l y p u r e gaseous ammonia e s c a p e s , until the remaining mixture has a

composi t ion on the boundary line (to be defined h e r e i n a f t e r ) .

On the other hand the term ' lean' with respec t to ammonia m e a n s that the mixture is not 'rich' in a m m o n i a .

Converse ly the term 'rich' with respec t to carbon d i o x i d e

means that when heat is applied to a mixture 'rich' in c a r b o n

dioxide, subs t an t i a l l y pure carbon dioxide e s c a p e s . The t e rm ' lean' with respec t to carbon dioxide means, that the mix ture

is not 'rich' in carbon d i o x i d e .

Figure 1 shows the ammonia -ca rbon d i o x i d e - w a t e r

ternary system at constant p ressure in a t r i a n g u l a r

diagram. The system is divided into two areas by l i n e

I I I which is t e r m e d h e r e i n the " b o u n d a r y l i n e " , a n d

which r e p r e s e n t s the a z e o t r o p i c c o m p o s i t i o n at c o n - s t a n t p r e s s u r e a t v a r y i n g w a t e r c o n c e n t r a t i o n s . T h i s

b o u n d a r y l i n e c a n n o t be t r a n s g r e s s e d by means of n o r m a l

d i s t i l l i n g or r e c t i f y i n g t e c h n i q u e s . Thus when a l i q u i d m i x t u r e r i c h w i t h r e s p e c t t o

ammonia, t h a t i s , f a l l i n g w i t h i n t h e a r e a I on F i g u r e 1 , i s r e c t i f i e d , g a s e o u s ammonia e s c a p e s u n t i l the l i q u i d

c o m p o s i t i o n r e a c h e s the b o u n d a r y l i n e . When a m i x t u r e r i c h w i t h r e s p e c t to c a r b o n d i o x i d e , f a l l i n g w i t h i n

a r e a I I on F i g u r e 1 is r e c t i f i e d , g a s e o u s c a r b o n d i o x i d e e s c a p e s u n t i l the l i q u i d c o m p o s i t i o n r e a c h e s

the b o u n d a r y l i n e . Once the l i q u i d m i x t u r e c o m p o s i t i o n is at t he b o u n d a r y l i n e , f u r t h e r r e c t i f i c a t i o n o r d i s t i l l a t i o n at c o n s t a n t p r e s s u r e r e s u l t s in a g a s e o u s m i x t u r e of a l l c o m p o n e n t s , bu t t he c o m p o s i t i o n of t h e

r e m a i n i n g l i q u i d m i x t u r e does no t l e a v e the b o u n d a r y l i n e . See a l s o , P . J . C . K a a s e n b r o o d , C h e m i c a l R e a c t i o n

E n g i n e e r i n g , P r o c e e d i n g s of t he F o u r t h E u r o p e a n

Symposium, S e p t . 9 - 1 1 , 1968, p u b l i s h e d by P e r g a m o n P r e s s ( 1 9 7 1 ) , p a g e s 3 1 7 - 3 2 8 .

V a r i o u s m e t h o d s have been p r o p o s e d to ge t a r o u n d

t h i s a z e o t r o p e , a l l of which e n t a i l t he s e p a r a t i o n o f

the a m m o n i a - c a r b o n d i o x i d e m i x t u r e s i n t o t h e i r c o n s t i t u -

e n t s . S e p a r a t e r e c o v e r y of ammonia is most i m p o r t a n t , i t b e i n g the most v a l u a b l e c o m p o n e n t .

Some of t h e s e p r o c e s s e s a re b a s e d o n s e l e c t i v e

a b s o r p t i o n of e i t h e r t he ammonia or the c a r b o n d i o x i d e

in a l i q u i d . The N e t h e r l a n d s P a t e n t A p p l i c a t i o n No .

1 4 3 , 0 6 3 , fo r e x a m p l e , d e s c r i b e s a p r o c e s s in w h i c h

ammonia is a b s o r b e d i n a n a q u e o u s s o l u t i o n of a n ammonium s a l t of a s t r o n g a c i d , such as ammonium

n i t r a t e , a t an e l e v a t e d p r e s s u r e . S e l e c t i v e a b s o r p t i o n of c a r b o n d i o x i d e by w a s h i n g a gas m i x t u r e w i t h a n

a q u e o u s a l k a n o l a m i n e s o l u t i o n , such as m o n o e t h a n o l a m i n e

i s d i s c l o s e d in German P a t e n t S p e c i f i c a t i o n No. 6 6 9 , 3 1 4 .

However , a l l of t h e s e p r o c e s s e s have the d r a w b a c k t h a t

t he a b s o r b e d c o m p o n e n t must t h e r e a f t e r be r emoved f r o m the a b s o r b e n t and p u r i f i e d .

I t has f u r t h e r been p r o p o s e d to s e p a r a t e a m m o n i a and c a r b o n d i o x i d e from m i x t u r e s of ammonia, c a r b o n d i o x i d e and w a t e r by d i s t i l l i n g o f f most of the a m m o n i a

in a f i r s t s t e p f o l l o w e d by d i s t i l l i n g o f f t he c a r b o n d i o x i d e in a s e c o n d s t e p c a r r i e d ou t a t a h i g h e r s y s t e m

p r e s s u r e . The t e r m " s y s t e m p r e s s u r e " as used h e r e i n

means t he sum of t he p a r t i a l p r e s s u r e s of ammonia , c a r - bon d i o x i d e and w a t e r . P r o c e s s e s of t h i s k i n d a r e d e s c r i b e d in U.S. P a t e n t N o s . 3 , 1 1 2 , 1 7 7 and 4 , 0 6 0 , 5 9 1 and in B r i t i s h P a t e n t Nos. 9 1 6 , 9 4 5 and 1 , 1 2 9 , 9 3 9 .

U.S. P a t e n t No. 3 , 1 1 2 , 1 7 7 d e s c r i b e s a p r o c e s s i n

wh ich in a f i r s t s t e p c a r r i e d ou t a t a s y s t e m p r e s s u r e of b e t w e e n 1 and 5 a t m o s p h e r e s a b s o l u t e , c a r b o n d i o x i d e

gas is s e p a r a t e d from a m i x t u r e of ammonia, c a r b o n

d i o x i d e and w a t e r , wh ich m i x t u r e is l e a n w i t h r e s p e c t to ammonia. The r e m a i n i n g l i q u i d i s t h e n s t r i p p e d w i t h , f o r i n s t a n c e , m e t h a n e a t an o v e r a l l p r e s s u r e of 1

a t m o s p h e r e a b s o l u t e . Th i s r e s u l t s in a l o w e r i n g of t h e

s y s t e m p r e s s u r e and in the e s c a p e of ammonia and some c a r b o n d i o x i d e , so t h a t a m i x t u r e of m e t h a n e , a m m o n i a

and c a r b o n d i o x i d e w i t h an o v e r a l l p r e s s u r e of 1

a t m o s p h e r e a b s o l u t e i s o b t a i n e d . In o r d e r to r e m o v e

t r a c e s of c a r b o n d i o x i d e c o n t a i n e d in the gas m i x t u r e ,

p a r t of the m i x t u r e is c o n d e n s e d , c a u s i n g the c a r b o n

d i o x i d e to be a b s o r b e d by t he l i q u i d a m m o n i a .

U.S. P a t e n t No. 4 , 0 6 0 , 5 9 1 d i s c l o s e s a p r o c e s s f o r

r e c o v e r i n g ammonia from a q u e o u s m i x t u r e s a l s o c o n t a i n i n g

C02 and H20 w h e r e i n the m i x t u r e is f i r s t d e a c i d i f i e d

by s t r i p p i n g ou t C02 a t an e l e v a t e d p r e s s u r e . T h e

r e m a i n i n g l i q u i d i s t h e n s t r i p p e d of a l l NH3, C02 a n d

H20, and the r e s u l t i n g gas m i x t u r e is s c r u b b e d o r

s t r i p p e d a t a r e d u c e d p r e s s u r e , r e l a t i v e to the d e a c i d i - f i c a t i o n s t e p , to y i e l d a gas s t r e a m of pu re a m m o n i a .

A s i m i l a r p r o c e s s is d e s c r i b e d in B r i t i s h P a t e n t

9 1 6 , 9 4 5 w h e r e i n an a m m o n i a c a l l i q u o r is d e a c i d i f i e d a t

an e l e v a t e d p r e s s u r e in a column w h e r e i n the a s c e n d i n g

gas i s washed of ammonia by c o l d w a t e r , t h e r e b y y i e l d i n g g a s e o u s c a r b o n d i o x i d e . T h e . r e m a i n i n g l i q u i d is t h e r e a f t e r e x p a n d e d i n t o a s t r i p p e r w h e r e i n i t i s

f r e e d of a l l a m m o n i a and c a r b o n d i o x i d e . The r e s u l t i n g

g a s e o u s m i x t u r e is washed or s c r u b b e d w i t h a l l or a p o r t i o n of the a m m o n i a c a l l i q u o r f e e d , b e f o r e t h e

l a t t e r is d e a c i d i f i e d , y i e l d i n g a s u b s t a n t i a l l y p u r e ammonia s t r e a m .

In B r i t i s h P a t e n t 1 , 1 2 9 , 9 3 9 , a gas m i x t u r e c o n - s i s t i n g of ammonia and c a r b o n d i o x i d e , r i c h w i t h r e s p e c t to ammonia, i s a b s o r b e d in w a t e r or an a q u e o u s s o l u t i o n .

Ammonia is d i s t i l l e d from the r e s u l t i n g a q u e o u s s o l u t i o n

a t a t m o s p h e r i c p r e s s u r e . The r e m a i n d e r of the s o l u t i o n is t h e n s u b j e c t e d to f r a c t i o n a l d i s t i l l a t i o n at a

p r e s s u r e of b e t w e e n 5 and 20 a t m o s p h e r e s a b s o l u t e w i t h

h e a t i n g in o r d e r to remove the c a r b o n d i o x i d e .

These l a t t e r p r o c e s s e s a re b a s e d on the p r i n c i p l e t h a t c h a n g i n g the p r e s s u r e of a s y s t e m of a m m o n i a , c a r b o n d i o x i d e and w a t e r makes i t p o s s i b l e to s e p a r a t e ou t ammonia a t the l ower p r e s s u r e and c a r b o n d i o x i d e

a t the h i g h e r p r e s s u r e . In t h e s e " p r e s s u r e d i f f e r e n t i a l "

p r o c e s s e s the s y s t e m p r e s s u r e , i n the c a r b o n d i o x i d e

s e p a r a t i o n zone s h o u l d be a t l e a s t t w i c e t h a t in t h e

ammonia s e p a r a t i o n zone . P r e f e r a b l y the r a t i o b e t w e e n

t he s y s t e m p r e s s u r e s in the ammonia s e p a r a t i o n and t h e

the c a r b o n d i o x i d e s e p a r a t i o n - z o n e s s h o u l d be b e t w e e n

a b o u t 1:5 and 1 : 2 0 , i f the s e p a r a t i o n is to p r o c e e d

s m o o t h l y .

However , the p r e s s u r e d i f f e r e n t i a l p r o c e s s e s h a v e

the d r a w b a c k t h a t i f the ammonia and c a r b o n d i o x i d e

m i x t u r e is a v a i l a b l e a t a p r e s s u r e of more t h a n 1

a t m o s p h e r e , i t f i r s t has to be e x p a n d e d to 1 a t m o s p h e r e . Gaseous ammonia is t hen r e l e a s e d h a v i n g a maximum

p r e s s u r e of 1 a t m o s p h e r e , or even l ower in the e v e n t a l a r g e amount of a n o t h e r gas is p r e s e n t . If t h i s ammonia is to be s u b j e c t e d to f u r t h e r p r o c e s s i n g , s u c h

as in a u r e a s y n t h e s i s p r o c e s s , i t has to be r a i s e d

to a h i g h e r p r e s s u r e . The c o m p r e s s i o n e n e r g y r e q u i r e d fo r t h i s i s q u i t e s u b s t a n t i a l . F u r t h e r m o r e , the c a r b o n d i o x i d e c o n c e n t r a t i o n in t he ammonia has to be k e p t e x t r e m e l y low in o r d e r to a v o i d the f o r m a t i o n a n d

d e p o s i t i o n of s o l i d ammonium c a r b a m a t e in the c o m p r e s s o r and h i g h p r e s s u r e l i n e s .

An a l t e r n a t e p r o c e s s no t r e q u i r i n g t h i s p r e s s u r e d i f f e r e n t i a l is d i s c l o s e d in the p u b l i s h e d D u t c h

p a t e n t a p p l i c a t i o n 7612163. The p r o c e s s t h e r e i n d e s c r i b e d

p e r m i t s the r e c o v e r y of ammonia and c a r b o n d i o x i d e

s e p a r a t e l y from such m i x t u r e s , w i t h o u t the need f o r

such a p r e s s u r e d i f f e r e n t i a l , i f the ammonia and c a r b o n

d i o x i d e c o n t a i n i n g f e e d s u p p l i e d to a c a r b o n d i o x i d e

s e p a r a t i o n zone is d i l u t e d by the a d d i t i o n of w a t e r i n

an amount of b e t w e e n 0.2 and 6 t i m e s , by w e i g h t , t h e

t o t a l w e i g h t of such ammonia and c a r b o n d i o x i d e c o n t a i n -

ing f e e d . For s i m p l i c i t y t h i s l a t t e r p r o c e s s w i l l b e

r e f e r r e d to h e r e i n as a " d i l u t i o n p r o c e s s . "

A c c o r d i n g to one e m b o d i m e n t of the d i l u t i o n p r o -

c e s s , ammonia s u b s t a n t i a l l y f r e e of c a r b o n d i o x i d e a n d

w a t e r is f i r s t s e p a r a t e d from a m i x t u r e of a m m o n i a , c a r b o n d i o x i d e and p o s s i b l y w a t e r , r i c h , w i t h r e s p e c t to ammonia, in an ammonia s e p a r a t i o n zone . From t h e

r e s i d u a l l i q u i d p h a s e l e a v i n g the b o t t o m of t h i s

ammonia s e p a r a t i o n zone , c a r b o n d i o x i d e is s e p a r a t e d in the c a r b o n d i o x i d e s e p a r a t i o n zone , w h e r e i n t h e

r e s i d u a l l i q u i d p h a s e from the ammonia s e p a r a t i o n z o n e fed to the c a r b o n d i o x i d e s e p a r a t i o n zone is d i l u t e d

w i t h b e t w e e n 0.2 and 6 t i m e s i t s w e i g h t of w a t e r .

A c c o r d i n g to a n o t h e r e m b o d i m e n t of the d i l u t i o n

p r o c e s s , c a r b o n d i o x i d e s u b s t a n t i a l l y f r e e of a m m o n i a

and w a t e r is f i r s t s e p a r a t e d in a c a r b o n d i o x i d e s e p a r a - t i o n zone from a m i x t u r e of ammonia , c a r b o n d i o x i d e

and p o s s i b l y w a t e r , which m i x t u r e i s l e a n w i t h r e s p e c t to ammonia. From t he b o t t o m of t h i s c a r b o n d i o x i d e

s e p a r a t i o n zone , t he r e s i d u a l l i q u i d p h a s e is fed to a d e s o r p t i o n zone w h e r e i n v i r t u a l l y a l l ammonia and c a r b o n

d i o x i d e a re d e s o r b e d and the r e s u l t i n g gas p h a s e i s

i n t r o d u c e d i n t o t he ammonia s e p a r a t i o n zone . Ammonia ,

s u b s t a n t i a l l y f r e e of c a r b o n d i o x i d e and w a t e r , i s

r e c o v e r e d from t h i s r e s u l t i n g gas p h a s e in the a m m o n i a

s e p a r a t i o n zone , and the r e s u l t i n g l i q u i d p h a s e i s

s u p p l i e d to t he c a r b o n d i o x i d e s e p a r a t i o n z o n e .

D i l u t i n g w a t e r i s added to t h i s c a r b o n d i o x i d e s e p a r a - t i o n zone in an amount of b e t w e e n 0.2 and 6 t i m e s , b y

w e i g h t , the c o m b i n e d t o t a l w e i g h t of t he i n i t i a l m i x t u r e

to be s e p a r a t e d , p l u s t he r e s i d u a l l i q u i d p h a s e from t h e

ammonia s e p a r a t i o n zone fed i n t o the c a r b o n d i o x i d e

s e p a r a t i o n z o n e . A c c o r d i n g to a n o t h e r e m b o d i m e n t of the d i l u t i o n

p r o c e s s , where the ammonia and c a r b o n d i o x i d e c o n t a i n i n g m i x t u r e to be s e p a r a t e d a l s o c o n t a i n s a s u b s t a n t i a l

q u a n t i t y of w a t e r , i t may be a d v a n t a g e o u s to f eed t h i s

m i x t u r e i n i t i a l l y to t he d e s o r p t i o n zone w h e r e i n t h e

ammonia and c a r b o n d i o x i d e a re d e s o r b e d , and, t o g e t h e r w i t h some w a t e r v a p o r , a re i n t r o d u c e d i n t o the a m m o n i a

s e p a r a t i o n zone . Ammonia, s u b s t a n t i a l l y f r e e of c a r b o n

d i o x i d e and w a t e r v a p o r , is o b t a i n e d from the top o f

the ammonia s e p a r a t i o n zone , and the r e s i d u a l l i q u i d

p h a s e , c o n t a i n i n g ammonia, c a r b o n d i o x i d e and w a t e r , is i n t r o d u c e d i n t o the c a r b o n d i o x i d e s e p a r a t i o n z o n e .

D i l u t i n g w a t e r is a l s o i n t r o d u c e d i n t o the c a r b o n d i o x i d e s e p a r a t i o n zone in an amount of b e t w e e n a b o u t 0.2 to 6 t i m e s , by w e i g h t , the t o t a l q u a n t i t y of t h e r e s i d u a l l i q u i d phase from the ammonia s e p a r a t i o n z o n e fed i n t o the c a r b o n d i o x i d e s e p a r a t i o n z o n e . C a r b o n d i o x i d e , s u b s t a n t i a l l y f r e e of ammonia and w a t e r , i s o b t a i n e d from the top of t he c a r b o n d i o x i d e s e p a r a t i o n zone, and the r e s i d u a l l i q u i d p h a s e from t h e b o t t o m o f the c a r b o n d i o x i d e s e p a r a t i o n zone is f e d t o t h e d e s o r p t i o n z o n e .

In b o t h the p r e s s u r e d i f f e r e n t i a l and d i l u t i o n

p r o c e s s e s , t he c o m p o s i t i o n o f t h e r e s i d u a l l i q u i d

phase from t he b o t t o m of the ammonia s e p a r a t i o n z o n e s h o u l d r e a c h or c l o s e l y a p p r o x i m a t e the b o u n d a r y l i n e ,

or a z e o t r o p i c c o m p o s i t i o n u n d e r the p r e v a i l i n g

c o n d i t i o n s , in o r d e r to m a x i m i z e the e f f i c i e n c y of t h e

s e p a r a t i o n . Th is means t h a t the amount of a m m o n i a

s t i l l p r e s e n t in t he l i q u i d phase is c o n s i d e r a b l e , which is u n d e s i r a b l e w i t h r e s p e c t to e n e r g y c o n s u m p t i o n . r h i s ammonia n e g a t i v e l y i n f l u e n c e s t he e f f e c t i v e n e s s o f

the s e p a r a t i o n in t he c a r b o n d i o x i d e s e p a r a t i o n zone t o

which i t i s f e d , which in t u r n r e q u i r e s a s u b s t a n t i a l

amount of e x t r a e n e r g y in the d e s o r p t i o n z o n e .

I t i s t h e r e f o r e an o b j e c t of t he p r e s e n t i n v e n t i o n

to p r o v i d e an i m p r o v e d p r o c e s s fo r t he s e p a r a t i o n o f

s u b s t a n t i a l l y p u r e ammonia and s u b s t a n t i a l l y pure c a r b o n

d i o x i d e f rom m i x t u r e s t h e r e o f w i t h w a t e r , which p r o c e s s p e r m i t s s i g n i f i c a n t s a v i n g s in e n e r g y .

F u r t h e r m o r e , the f l u c t u a t i o n s in the f eed to the ammonia

s e p a r a t i o n zone have a s t r o n g i n f l u e n c e on t h e e n e r g y e f f i c i e n c y of the s e p a r a t i o n . C o n s e q u e n t l y the s e p a r a t i o n r e q u i r e s a v e r y s e n s i t i v e m e a s u r i n g and c o n t r o l s y s t e m .

I t is t h e r e f o r e a n o t h e r o b j e c t of the p r e s e n t i n v e n t i o n t o

p r o v i d e an improved p r o c e s s of the above d e s c r i b e d k ind w h i c h is v i r t u a l l y i n s e n s i t i v e to f l u c t u a t i o n s in the c o m p o s i t i o n o f

the feed to the ammonia s e p a r a t i o n z o n e .

The p r o c e s s a c c o r d i n g to the p r e s e n t i n v e n t i o n is c h a r a c t e r i z e d

in t h a t so much g a s e o u s CO2 or a g a s e o u s m i x t u r e c o n t a i n i n g C02 is f e d to the b o t t o m o f the N H 3 - s e p a r a t i o n zone t h a t the c o m p o s i t i o n of t h e l i q u i d p h a s e r e l e a s e d in the b o t t o m o f the N H 3 - s e p a r a t i o n zone i s

n o t o n t h e s i d e of the b o u n d a r y l i n e r i c h in NH3. R e f e r r i n g t o

F i g u r e 1, s u f f i c i e n t c a r b o n d i o x i d e s h o u l d be i n t r o d u c e d t o

move the c o m p o s i t i o n of the r e s i d u a l l i q u i d p h a s e removed f r o m

the ammonia s e p a r a t i o n zone from a r e a I to a c o m p o s i t i o n l y i n g

on the b o u n d a r y l i n e or in a r e a I I .

The amount o f c a r b o n d i o x i d e r e q u i r e d to a c h i e v e

this ob jec t ive ranges genera l ly between about 0.01 and 0 . 2 ,

p r e f e r a b l y b e t w e e n a b o u t 0 .02 and 0.1 t i m e s by w e i g h t the amount of r e s i d u a l l i q u i d p h a s e r emoved f rom t h e ammonia s e p a r a t i o n zone , which l i q u i d p h a s e is t h e r e - a f t e r fed i n t o t he c a r b o n d i o x i d e s e p a r a t i o n zone . T h e t o t a l a m o u n t * o f such c a r b o n d i o x i d e c o n t a i n i n g g a s e o u s p h a s e i n t r o d u c e d i n t o the b o t t o m of t he a m m o n i a s e p a r a t i o n zone d e p e n d s , among o t h e r f a c t o r s , upon t h e d e s i r e d e f f e c t . However the amount of c a r b o n d i o x i d e

t h a t can be i n t r o d u c e d has an u p p e r l i m i t d e f i n e d b y the p o i n t a t wh ich s o l i d ammonium c a r b a m a t e or o t h e r s o l i d compounds b e g i n to d e p o s i t in t he c o l u m n s a n d

l i n e s , wh ich w i l l o c c u r i f too much c a r b o n d i o x i d e i s u s e d .

When c a r r i e d ou t in c o n j u n c t i o n w i t h t he me thod o f t h i s i n v e n t i o n , t h e e n e r g y c o n s u m p t i o n of t h e k n o w n

s e p a r a t i o n p r o c e s s e s becomes c l e a r l y l o w e r , and t h e e f f e c t i v e n e s s and e f f i c i e n c y of the s e p a r a t i o n b e c o m e s

v i r t u a l l y i n s e n s i t i v e to f l u c t u a t i o n s in t he c o m p o s i t i o n of t he f e e d to t he ammonia s e p a r a t i o n z o n e . F u r t h e r m o r e , t h i s i m p r o v e d p r o c e s s can r e s u l t in a d e c r e a s e in t h e

q u a n t i t y of the l i q u i d f l ows wh ich have to be r e c y c l e d t h r o u g h o u t the v a r i o u s zones of t he s e p a r a t i o n .

P r e f e r a b l y t h i s c a r b o n d i o x i d e c o n t a i n i n g gas p h a s e is o b t a i n e d by u s i n g a p a r t of the c a r b o n d i o x i d e o f f -

gas from the c a r b o n d i o x i d e s e p a r a t i o n z o n e . H o w e v e r ,

use can be made of a gas m i x t u r e c o n t a i n i n g c a r b o n

d i o x i d e and ammonia. In such c a se i t i s n e c e s s a r y t h a t

the c a r b o n d i o x i d e to ammonia r a t i o in such a gas m i x -

t u r e be g r e a t e r t h a n the c a r b o n d i o x i d e to ammonia r a t i o

of the gas in e q u i l i b r i u m w i t h the r e s i d u a l l i q u i d p h a s e o b t a i n e d from the b o t t o m of the ammonia s e p a r a t i o n z o n e .

O t h e r w i s e , i t w i l l not be p o s s i b l e to t r a n s g r e s s t h e

b o u n d a r y l i n e .

In a p r e f e r r e d embod imen t of the i n v e n t i o n , t h e

c a r b o n d i o x i d e c o n t a i n i n g g a s e o u s p h a s e c o n t a i n s , i n a d d i t i o n to the c a r b o n d i o x i d e , an amount of an i n e r t

g a s , p r e f e r a b l y a i r or n i t r o g e n . The w e i g h t r a t i o o f c a r b o n d i o x i d e to i n e r t gas is p r e f e r a b l y in the r a n g e of b e t w e e n 30 :70 and 7 0 : 3 0 .

I n t r o d u c i n g an i n e r t gas in an amount g r e a t e r t h a n

n o r m a l l y i n t r o d u c e d w i t h g a s e s used to p a s s i v a t e the s t a i n l e s s s t e e l m a t e r i a l s (such as a i r or oxygen) o f f e r s the a a v a n t a g e t h a t the p r e s e n c e of such i n e r t c'as t e n d s to r e d u c e t h e

amount of ammonia r e m a i n i n g in the r e s i d u a l l i q u i d p h a s e removed from the ammonia s e p a r a t i o n zone. As n o t e d

a b o v e , t h i s r e s u l t s in an a d v a n t a g e o u s r e d u c t i o n i n

e n e r g y c o n s u m p t i o n , e s p e c i a l l y in t he d e s o r p t i o n s t e p . A f u r t h e r a d v a n t a g e of t he p r e s e n c e of a d d i t i o n a l

amount s of i n e r t gas is t h a t the b o t t o m t e m p e r a t u r e o f

the ammonia column is l o w e r e d , t h u s p e r m i t t i n g a m o r e economic use of the h e a t c o n t e n t of the v a r i o u s l i q u i d and gas s t r e a m s o b t a i n e d d u r i n g the s e p a r a t i o n s .

A t h i r d a d v a n t a g e i s t h a t the gas to l i q u i d r a t i o

is more f a v o r a b l e , w i t h the c o n s e q u e n c e t h a t t h e

s e p a r a t i o n e f f i c i e n c y is i m p r o v e d . With such a n

i m p r o v e d s e p a r a t i o n e f f i c e n c y , e i t h e r the t o t a l a m o u n t of m i x t u r e t h a t is s e p a r a t e d can b e i n c r e a s e d , or t h e

columns used fo r the s e p a r a t i o n can be r e d u c e d in s i z e .

M o r e o v e r , the d e p e n d e n c e of the e n e r g y c o n s u m p t i o n r e q u i r e d fo r the s e p a r a t i o n s on the e x a c t c o m p o s i t i o n of t he l i q u i d p h a s e removed from the b o t t o m of t h e

ammonia separat ion zone is greatly r e d u c e d .

Preferably the p res su res in the various separa t ion z o n e s should be between about 50 and 5000 kPa. A pressure of a b o u t

1800 kPa or more, depending upon the temperature of the a v a i l a b l e

cooling water, as referred to her inaf ter , in the ammonia s e p a r a t i o n zone o f f e r s the a d v a n t a g e t h a t t he s e p a r a t e d a m m o n i a

can be l i q u i f i e d r e l a t i v e l y e a s i l y and i n e x p e n s i v e l y

by c o o l i n g w i t h c o o l i n g w a t e r . In the d i l u t i o n t y p e

p r o c e s s , the s y s t e m p r e s s u r e in the c a r b o n d i o x i d e

s e p a r a t i o n zone can be the same as in the ammonia s e p a r a t i o n

zone, but can be h i g h e r as w e l l . P r e f e r a b l y , h o w e v e r , t h e

sys t em p r e s s u r e in the c a r b o n d i o x i d e s e p a r a t i o n zone and t h e

d e s o r p t i o n zone w i l l not be g r e a t e r than t w i c e the s y s t e m

p r e s s u r e in the ammonia s e p a r a t i o n z o n e .

The t e m p e r a t u r e s m a i n t a i n e d in the v a r i o u s s e p a r a - t i o n zones depend on the p r e s s u r e s , the c o m p o s i t i o n o f

the f e ed and the p u r i t y r e q u i r e d of the p r o d u c t s to b e

s e p a r a t e d . In the d i l u t i o n p r o c e s s , t h e s e t e m p e r a t u r e s , where r e c t i f y i n g co lumns a re used in the s e p a r a t i o n

z o n e s , g e n e r a l l y r a n g e w i t h i n the l i m i t s s t a t e d in t h e

f o l l o w i n g t a b l e :

The t e m p e r a t u r e s in the d e s o r p t i o n zone a re a l s o d e t e r -

mined by the p r e s s u r e s , the c o m p o s i t i o n of the f eed t o

t he zone and the p u r i t y r e q u i r e m e n t s (amount of ammonia

and c a r b o n d i o x i d e ) for the d e s o r p t i o n w a t e r r e m o v e d from the b o t t o m of t h i s zone . G e n e r a l l y t h e s e t e m p e r a - t u r e s a re c h o s e n to be h i g h e r t h a n the b o i l i n g p o i n t o f

the l i q u i d phase to be d e s o r b e d at the p r e s s u r e s u s e d .

When u s i n g the p r o c e s s of t h i s i n v e n t i o n in c o n j u n c - t i o n w i th a p r e s s u r e d i f f e r e n t i a l t y p e o f p r o c e s s , t h e

s y s t e m p r e s s u r e in the c a r b o n d i o x i d e s e p a r a t i o n z o n e s h o u l d be at l e a s t t w i c e the s y s t e m p r e s s u r e in t h e

ammonia s e p a r a t i o n zone . However , fo r the s e p a r a t i o n

to p r o c e e d s m o o t h l y i t is p r e f e r a b l e t h a t the r a t i o o f

the s y s t e m p r e s s u r e in the c a r b o n d i o x i d e s e p a r a t i o n

zone to the s y s t e m p r e s s u r e in the ammonia s e p a r a t i o n

zone be w i t h i n the r a n g e of b e t w e e n a b o u t 1:5 and 1 : 2 0 .

The t e m p e r a t u r e s m a i n t a i n e d in the v a r i o u s s e p a r a t i o n

zones a g a i n depend upon the p r e s s u r e c h o s e n , t h e

c o m p o s i t i o n of the f e ed and the p u r i t y r e q u i r e d of t h e

p r o d u c t s to b e - s e p a r a t e d . I t may a l s o be of some a d v a n t a g e , when u s i n g . a

p r e s s u r e d i f f e r e n t i a l t y p e of p r o c e s s , to f eed some

d i l u t i n g w a t e r i n t o the c a r b o n d i o x i d e s e p a r a t i o n

zone to a s s i s t in t he s e p a r a t i o n . In such case t h e

amount of d i l u t i n g w a t e r added d e p e n d s upon many f a c t o r s

i n c l u d i n g the p r e s s u r e d i f f e r e n t i a l used b e t w e e n t h e c a r b o n d i o x i d e and ammonia s e p a r a t i o n z o n e s , and t h e c o m p o s i t i o n of the v a r i o u s f e e d s .

With the t e rm ' s u b s t a n t i a l l y p u r e ' is meant t h a t t h e

ma jo r p a r t of the gas or l i q u i d s e p a r a t e d in the s p e c i f i c z o n e

c o n s i s t s of o n e c o m p o n e n t , i . e . NH3 or C02 . P r e f e r a b l y i n t h e s e p a r a t e d NH3, the amount of CO2 i s n o t

g r e a t e r than 500 p a r t s per m i l l i o n (by w e i g h t ) and t h e a m o u n t

of w a t e r is no t g r e a t e r than 7500 p a r t s per m i l l i o n (by w e i g h t ) ,

more in p a r t i c u l a r not g r e a t e r t han 100 and 2000 p a r t p e r m i l l i o n (by w e i g h t ) r e s p e c t i v e l y .

In the C02 the amount of NH3 is p r e f e r a b l y not g r e a t e r

than 500, more in p a r t i c u l a r 100 p a r t s per m i l l i o n (by w e i g h t ) .

The amount of w a t e r is r a t h e r u n i m p o r t a n t bu t from e c o n o m i c

p o i n t of view s h o u l d no t e x c e e d 20 ,000 more in p a r t i c u l a r 2 ,000 p a r t s per m i l l i o n (by w e i g h t ) .

The choice to which separa t ion step the mixture to be s e p a r a t e d i s ' in i t i a l ly fed depends i .a . upon the composi t ion of this m ix tu re . Thus, if it is rich with respect to ammonia, (in area I on Figure 1), then it is most advan tageous ly fed to the ammonia separat ion s t e p . On the other hand, if it is lean with respect to ammonia, (on boundary line III or within area II on Figure 1), then it is m o s t advan tageous ly fed init ially to the carbon dioxide separat ion s t e p . Finally, if the mixture to be separated is rich with respect to ammonia and also contains a subs tan t ia l quanti ty of water, it may be m o s t

advan tageous to feed it init ially to the desorpt ion s t e p .

The ammonia and c a r b o n d i o x i d e zones may c o n s i s t

of any t ype of a p p a r a t u s s u i t a b l e fo r s e p a r a t i n g t h e

g a s e o u s a n d . l i q u i d c o m p o n e n t s in a c c o r d a n c e w i t h t h e

i n v e n t i o n i n c l u d i n g , bu t no t l i m i t e d t o , r e c t i f i c a t i o n ,

d i s t i l l a t i o n and w a s h i n g c o l u m n s . However , r e c t i f i c a -

t i o n c o l u m n s , or co lumns h a v i n g r e c t i f i c a t i o n z o n e s , a re most s u i t a b l e f o r use in t h e . i m p r o v e d p r o c e s s o f t h i s i n v e n t i o n .

Some v a r i a t i o n s may be made in the f lows b e t w e e n

the v a r i o u s s e p a r a t i o n zones of t he s e p a r a t i o n p r o c e s s .to which the p r e s e n t p r o c e s s i s a p p l i e d , and s t i l l b e

w i t h i n the s cope and i n t e n t of t h i s i n v e n t i o n .

F i g u r e 1 shows a d i a g r a m o f t h e NH3/CO2/H2O s y s t e m

a t c o n s t a n t p r e s s u r e , which was d e s c r i b e d i n d e t a i l

a b o v e .

F i g u r e 2 shows a d i a g r a m for the p r o c e s s a c c o r d i n g

to t he i n v e n t i o n u s e d in c o n j u n c t i o n w i t h a d i l u t i o n

p r o c e s s in which the NH3 and the CO2 s e p a r a t i o n z o n e s

are o p e r a t e d a t a l m o s t the same p r e s s u r e .

F i g u r e 3 shows a d i a g r a m of a s i m i l a r p r o c e s s i n which t he s e p a r a t i o n s a re e f f e c t e d a t d i f f e r e n t

p r e s s u r e s .

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

One p r e f e r r e d e m b o d i m e n t of t h i s i n v e n t i o n i s i l l u s t r a t e d in F i g u r e 2 w h e r e i n t he c a r b o n d i o x i d e a d d i t i o n i s a p p l i e d to a d i l u t i o n p r o c e s s in w h i c h the ammonia and c a r b o n d i o x i d e s e p a r a t i o n zones a r e o p e r a t e d a t s u b s t a n t i a l l y t he same p r e s s u r e . In t h i s

F i g u r e 2, t he ammonia s e p a r a t i o n zone is c o m p r i s e d o f

N H 3 - r e c t i f y i n g column 3, the c a r b o n d i o x i d e s e p a r a t i o n

zone of C O 2 - r e c t i f y i n g column 18 and the d e s o r p t i o n - zone of d e s o r b e r 1 0 .

A m i x t u r e of NH3, CO2 and H2O i s fed to an NH3- r e c t i f y i n g column 3 t h r o u g h c o n d u i t 1 and pump 2 .

Through c o n d u i t 31, a C 0 2 c o n t a i n i n g gas i s i n t r o d u c e d i n t o the b o t t o m o f t h e NH3 r e c t i f y i n g column 3. NH3 is d i s c h a r g e d from t he top of column 3 t h r o u g h c o n d u i t

4. T h i s NH3 can be c o n d e n s e d by ( deep - ) c o o l i n g i n

c o n d e n s e r 5. A n o n - c o n d e n s e d g a s e o u s m i x t u r e of NH3 and i n e r t gas e s c a p e s from the c o n d e n s e r . Th i s i n e r t

gas comes from the a i r i n t r o d u c e d i n t o t he i n s t a l l a t i o n

to p a s s i v a t e t he s t r u c t u r a l m a t e r i a l s used in v e s s e l s

and c o n d u i t s i n o r d e r to r e d u c e c o r r o s i o n to an a c c e p t - ab le l e v e l . N a t u r a l l y , oxygen or an o x y g e n - r e l e a s i n g s u b s t a n c e may be used f o r t h i s p u r p o s e i n s t e a d of a i r .

In one e m b o d i m e n t of t he i n v e n t i o n the amount of a i r

i n t r o d u c e d is f a r in e x c e s s of t he amount n e c e s s a r y t o

keep the m a t e r i a l s in a p a s s i v a t e d s t a t e . In t h i s

embodiment the a i r h e l p s to i m p r o v e the e f f i c i e n c y o f

the s e p a r a t i o n . P a r t of the a i r is fed to t h e

N H 3 - r e c t i f y i n g column 3 t h r o u g h c o m p r e s s o r 6 a n d

c o n d u i t s 7 and 8 and p a r t to d e s o r b e r 10 t h r o u g h c o n d u i t

9. The g a s e o u s m i x t u r e from c o n d e n s e r 5 is f r e e d o f

NH3 in s c r u b b e r 11 by w a s h i n g i t w i t h w a t e r s u p p l i e d

t h r o u g h c o n d u i t 32, w h i l e an amount of a b s o r p t i o n h e a t

i s r emoved by c o o l i n g p a r t of the r e s u l t i n g a q u e o u s NH3 s o l u t i o n t h a t i s d i s c h a r g e d t h r o u g h pump 12 in a r e c y c l e c o o l e r 13 and r e t u r n i n g i t to s c r u b b e r 11 t h r o u g h c o n d u i t

14. The s o l u t i o n t h e n formed is r e t u r n e d to N H 3 - r e c t i - f y i n g co lumn 3 t h r o u g h c o n d u i t 1 5 .

The i n e r t gas i s d i s c h a r g e d t h r o u g h c o n d u i t 16 a n d

fed to the b o t t o m of C O 2 - r e c t i f y i n g column 1 8 t h r o u g h . c o n d u i t 17. I f so d e s i r e d , i t may a l s o be v e n t e d c o m -

p l e t e l y or p a r t l y t h r o u g h c o n d u i t 19. P a r t of t h e

NH3 l i q u i f i e d in c o n d e n s e r 5 f l o w s back t h r o u g h c o n d u i t

20 to the N H 3 - r e c t i f y i n g column to be used as r e f l u x .

A s o l u t i o n of NH3 and C 0 2 i n w a t e r is d i s c h a r g e d f r o m

the b o t t o m of t he N H 3 - r e c t i f y i n g column 3 t h r o u g h c o n - d u i t 2 1 .

Th i s s o l u t i o n i s p a s s e d i n t o the C O 2 - r e c t i f y i n g co lumn 18, wh ich is o p e r a t e d a t v i r t u a l l y the s a m e

p r e s s u r e as t he N H 3 - r e c t i f y i n g column 3. An amount o f

d i l u t i n g w a t e r i s i n t r o d u c e d i n t o column 18 by way o f

l i n e 2 5 . The b o t t o m p r o d u c t of d e s o r b e r 10 is r e m o v e d

by w a y o f pump 22 and c o n d u i t 23. In o r d e r to o b t a i n

a b e t t e r h e a t d i s t r i b u t i o n , t h i s d e s o r p t i o n w a t e r i s

f i r s t a l l o w e d to g i v e o f f p a r t of i t s h e a t i n t h e

b o t t o m of t he C 0 2 - r e c t i f y i n g column 18. The r e m a i n d e r

of t he h e a t r e q u i r e d fo r the r e c t i f i c a t i o n is s u p p l i e d

h e r e by means of h e a t i n g c o i l s 24 and, fo r i n s t a n c e ,

s t e a m . The l i q u i d . f l o w from c o n d u i t 23 is d i s c h a r g e d , ialthough part of it can be used as diluting w a t e r .

An a d d i t i o n a l amount of w a s h i n g w a t e r is fed to c o l u m n

18 t h r o u g h c o n d u i t 26 t o . r e m o v e the NH3 as c o m p l e t e l y

as p o s s i b l e from the C02. A gas c o n s i s t i n g of C02 a n d i n e r t ga s , i f any, e s s e n t i a l l y f r e e of NH3, e s c a p e s from the top of column 18 t h r o u g h c o n d u i t 27. T h e b o t t o m p r o d u c t , or r e s i d u a l l i q u i d p h a s e , of column 1 8 , which is a d i l u t e d s o l u t i o n of NH3 and C02 in w a t e r ,

is passed through conduit 28 to desorber 10. Virtually all NH3 a n d

CO2 are removed in desorber 10 by hea t ing , for ins tance w i t h

steam in heat ing coils 29. The resul t ing desorpt ion water i s

vir tual ly free of NH3 and CO2 and is d i scharged through conduit 23.

The gaseous mixture of NH3. CO2 and H2O formed in d e s o r b e r

10 flows to NH3-rec t i fy ing column 3 through conduit 30. In the a b o v e

embodiment of the process according to the invent ion, use was m a d e

of a mixture of NH3, CO2 and H2O, having a composi t ion lying in

the area rich in NH3. If the composi t ion of this starting mixture w a s lean with respect to NH3, then the feed stream would have b e e n

intial ly introduced into CO2 rect i fying column 18.

Another preferred embodiment of the invention is i l l u s t r a t e d

in Figure 3 wherein the CO2 r ec t i f i ca t ion is effected at a p r e s s u r e higher than the NH3 r e c t i f i c a t i o n .

This figure is vir tual ly the same as Figure 2, and t h e

reference figures have the same meaning. The d i f ferences l i e

in the fact that a compressor A and a pump B, r e s p e c t i v e l y ,

are ins ta l led in conduits 17 and 21 to raise the p ressure of t h e

flows of gas and l iquid, r e s p e c t i v e l y . Furthermore, conduit 30

comprises a reducing valve C through which part of the g a s e o u s mixture from desorber 10 is reduced in p re s su re . Here t h e

desorpt ion column 10 is operated at vir tual ly the same s y s t e m

pressure as the C02 r ec t i f i ca t ion . Conduit 18 moreover c o m p r i s e s

a reducing valve D which is used to lower the pressure of part o f

the air fed i n .

A third embodiment of the invention ut i l iz ing the p r e s s u r e di f ferent ia l process can also be effected in the appara tus i l l u s t r a t e d

in Figure 3. In such case the pressure at which the C O 2 - s e p a r a t i o n is effected is at least 2 t imes, but preferably 5 times as high a s the pressure at which the N H 3 - s e p a r a t i o n is e f fec ted . In this c a s e

no diluant need to be added to the C 0 2 - s e p a r a t i o n zone as is t h e

case in the dilution type p r o c e s s . The invention will be further descr ibed by means of t h e

following examples , which are for i l lus t ra t ive purposes on ly , and the scope of the invention is not res t r ic ted t h e r e t o .

Example 1

S u b s t a n t i a l l y pu re NH3 and s u b s t a n t i a l l y p u r e

C 0 2 w e r e s e p a r a t e d from a m i x t u r e of NH3, CO2 and H2O in an i n s t a l l a t i o n h a v i n g a c o n f i g u r a t i o n as shown i n

F i g u r e 3 .

The r e f e r e n c e f i g u r e s r e l a t e to F i g u r e 3 a s d e s c r i b e d above in r e l a t i o n to F i g u r e 2 and p e r c e n t a g e s a re p e r c e n t a g e s by w e i g h t . The p r e s s u r e s m e n t i o n e d r e l a t e to t he NH3/C02/H20 s y s t e m p r e s s u r e . The a c t u a l

p r e s s u r e may be s l i g h t l y h i g h e r owing to the p r e s e n c e of i n e r t g a s .

At a p r e s s u r e of 1800 kPa 56489 k g / h of a s o l u t i o n of NH3 and C02 in w a t e r , the c o m p o s i t i o n of which i s 32.8% NH3, 18.3% CO2 and 48.9% w a t e r , a re added to t h e

N H 3 - r e c t i f y i n g column 3. 635 k g / h of a i r a re a d d e d

by way of c o m p r e s s o r 6, 248 k g / h b e i n g i n t r o d u c e d i n t o the N H 3 - r e c t i f y i n g co lumn 3 and 387 k g / h i n t o d e s o r b e r

10. By way of l i n e 31, 2000 k g / h of C02 a re i n t r o d u c e d in the N H 3 - r e c t i f y i n g column 3. 27 ,026 k g / h of a

g a s e o u s m i x t u r e . c o n s i s t i n g of 58.2% NH3, 14.1% C02 27 .7%

H20 and 1.5% i n e r t gas from d e s o r b e r 10 a re e x p a n d e d

t h r o u g h v a l v e C and a l s o added to the N H 3 - r e c t i f y i n g column 3. 3 8 , 1 8 0 kg /h of g a s e o u s m i x t u r e c o n s i s t i n g o f

98.0% NH3, 0.3% H20 and 1.7% i n e r t gas a re d i s c h a r g e d from the top of t h i s column 3. P a r t of t h i s g a s e o u s m i x t u r e i s l i q u i f i e d by c o o l i n g in c o n d e n s o r 5, f r o m

which 17 ,824 k g / h of l i q u i d NH3 are r e t u r n e d to c o l u m n

3 as a r e f l u x , and 18 ,528 k g / h of l i q u i d NH3 a re d i s -

c h a r g e d . 2 ,464 k g / h of g a s e o u s m i x t u r e c o n s i s t i n g o f

74.2% NH3 and 25.8% of i n e r t gas e s c a p e from c o n d e n s o r

5. Th is m i x t u r e is washed in s c r u b b e r 11 w i t h 2200 k g / h

of w a t e r . Heat is r emoved f rom s c r u b b e r 11 v i a r e c y c l e c o n d e n s o r 13. Per h o u r , 4029 kg of a s o l u t i o n c o n -

s i s t i n g of 45.4% NH3 and 54.6% H20 a re r e t u r n e d to t h e

N H 3 - r e c t i f y i n g co lumn . 635 kg /h of i n e r t gas a re p a s s e d

t h r o u g h c o n d u i t s 16 and 17 and c o m p r e s s o r A to t h e

C O 2 - r e c t i f y i n g column 1 8 .

69 ,180 k g / h of a l i q u i d h a v i n g a c o m p o s i t i o n o f

22.7% NH3, 23.4% CO2 and 53.9% H2O a re p a s s e d f rom t h e

b o t t o m of t he N H 3 - r e c t i f y i n g column 3 t h r o u g h c o n d u i t 21 and pump B to the C 0 2 - r e c t i f y i n g column 18. C o l u m n

18 i s f e d , t h r o u g h c o n d u i t 25, and t h r o u g h c o n d u i t 26 w i t h 34 ,079 k g / h of w a t e r .

A g a s e o u s m i x t u r e of 1 2 , 3 3 7 k g / h e s c a p e s from t h e

top of C 0 2 - r e c t i f y i n g column 18 and c o n s i s t s , i n t e r

a l i a , of 93.2% C02 and c o n t a i n s l e s s t h a n 100 ppm NH3. 90 ,913 k g / h of a s o l u t i o n 78.5% H20, 17.3% NH3 and 4 .2%

C O 2 a r e p a s s e d from t he b o t t o m of t he column 18 t o d e s o r b e r 10. The c o m p o s i t i o n of t h i s l i q u i d is on t h e s i d e of t he b o u n d a r y l i n e t h a t is r i c h in CO2.

In t he d e s o r b e r t he s o l u t i o n is v i r t u a l l y f r e e d o f

NH3 and CO2 by means of s t e a m . A t o t a l of 63 ,892 k g / h of l i q u i d a re d i c h a r g e d from the d e s o r b e r , which l i q u i d

may be. used, for ins tance , for.the absorpt ion of NH3 and CO2, o r ,

partly, as diluting water in the C O 2 - s e p a r a t i o n z o n e .

Example 2

S u b s t a n t i a l l y pu re NH3 and s u b s t a n t i a l l y pu re CO2 were s e p a r a t e d f r o m - a m i x t u r e of NH3, C02 and H20 in a n i n s t a l l a t i o n h a v i n g the c o n f i g u r a t i o n of F i g u r e 3 ,

w h e r e i n C02 as w e l l as a i r were i n t r o d u c e d in t h e

N H 3 - r e c t i f y i n g co lumn. The r e f e r e n c e f i g u r e s r e l a t e

to F i g u r e 3 and p e r c e n t a g e s a re p e r c e n t a g e s by w e i g h t .

At a p r e s s u r e of 1800 kPa, 5 1 , 9 7 2 k g / h of a s o l u t i o n of NH3 and C02 in w a t e r , h a v i n g a c o m p o s i t i o n

of 33.4% NH3, 18.2% C02 and 48.4% w a t e r , a r e added t o

the N H 3 - r e c t i f y i n g co lumn 3. 2 ,387 k g / h of a i r a r e

i n t r o d u c e d i n t o the p r o c e s s by way of c o m p r e s s o r 6 ,

2000 kg /h b e i n g fed to the N H 3 - r e c t i f y i n g column 3 a n d

387 kg /h to d e s o r b e r 10. By way of l i n e 31, 2000 k g / h

of C02 are i n t r o d u c e d in the N H 3 - r e c t i f y i n g column 3 .

22 ,928 k g / h of a g a s e o u s m i x t u r e c o n s i s t i n g o f

57.7% NH3, 16.0% C02, 26.3% H20 and 1.5% i n e r t g a s from d e s o r b e r 10 a re e x p a n d e d t h r o u g h v a l v e C a n d

a l s o i n t r o d u c e d i n t o N H 3 - r e c t i f y i n g column 3. 3 2 , 4 1 5

k g / h of a g a s e o u s m i x t u r e c o n s i s t i n g of 98.0% NH3, 0.3% H20 and 1.7% i n e r t gas a r e d i s c h a r g e d f r o m t h e

top of t h i s co lumn. P a r t of t h i s g a s e o u s m i x t u r e i s

l i q u i f i e d by c o o l i n g in c o n d e n s e r 5, from which 1 5 , 0 5 7

k g / h of l i q u i d ammonia a re r e t u r n e d to column 3 as a r e f l u x and 1 7 , 3 5 8 k g / h of l i q u i d NH3 a r e d i s c h a r g e d . 9 ,252 kg /h of t he r e s i d u a l n o n - c o n d e n s e d g a s e o u s m i x -

t u r e , c o n s i s t i n g o f 74.2% NH3 and 25.8% i n e r t g a s ,

e s c a p e from c o n d e n s e r 5. This r e s i d u a l m i x t u r e i s

washed in s c r u b b e r 11 v i a r e c y c l e c o n d e n s e r 13. P e r

h o u r , 3 ,829 kg of a s o l u t i o n c o n s i s t i n g of 82 .0%

NH3 and 18,0% of H20 a re r e t u r n e d to the N H 3 - r e c t i f y i n g co lumn . 635 k g / h of i n e r t gas a re p a s s e d t h r o u g h c o n d u i t s 1 7 a n d c o m p r e s s o r A to the C 0 2 - r e c t i f y i n g column 1 8 .

62 ,042 k g / h of a r e s i d u a l l i q u i d p h a s e h a v i n g a

c o m p o s i t i o n of 22.3% NH3, 24.6% C02 and 53.1% H20 a re p a s s e d from the b o t t o m of the N H 3 - r e c t i f y i n g column 3 t h r o u g h c o n d u i t 21 and pump B to C 0 - - r e c t i f y i n g column 1 8 .

Column 18 is a l s o f e d , t h r o u g h c o n d u i t 25, a n d

t h r o u g h c o n d u i t 26, w i t h 20 ,486 k g / h of w a t e r . A

g a s e o u s m i x t u r e of 1 1 , 4 8 2 k g / h e s c a p e s from the t o p of C 0 2 - r e c t i f y i n g column 18, and c o n s i s t s , i n t e r a l i a , of 93.2% C02 and c o n t a i n s l e s s t h a n 100 ppm NH3. 79 ,817 kg /h of a s o l u t i o n of 77.9% H20, 17.3% NH3 a n d 4.8% C02 a re p a s s e d from the b o t t o m of column 18 t o d e s o r b e r 10. The c o m p o s i t i o n of t h i s l i q u i d is on t h e s i d e of the b o u n d a r y l i n e t h a t is r i c h in CO2.

A t o t a l of 55 ,889 k g / h of l i q u i d a re d i s c h a r g e d from the d e s o r b e r , which l i q u i d may be u s e d , f o r

ins tance , for the absorpt ion of NH3 and CO2, or, partly, a s

diluting water in the C O 2 - s e p a r a t i o n zone . I n this d e s o r b e r

the solution is vir tual ly freed of NH3 and CO2 by means of s t e a m .

Example 3

S u b s t a n t i a l l y pu re NH3 and CO2 were s e p a r a t e d from m i x t u r e s t h e r e o f u s i n g a p r o c e s s as d e s c r i b e d i n

F i g u r e 3, bu t w h e r e i n the t r a n s g r e s s i o n of the b o u n d a r y l i n e was no t e f f e c t e d by the a d d i t i o n of w a t e r to t h e

C02 co lumn, bu t r a t h e r by a p r e s s u r e d i f f e r e n t i a l b e -

tween the ammonia s e p a r a t i o n zone and the c a r b o n d i o x i d e

s e p a r a t i o n zone . Thus , t he NH3 column was o p e r a t e d a t

a p r e s s u r e of b e t w e e n 100 and 500 kPa and the C02 c o l u m n

was o p e r a t e d a t a p r e s s u r e b e t w e e n 1500 and 3000 k P a .

R e s u l t s c o m p a r a b l e to t h o s e of the above e x a m p l e s

were o b t a i n e d in t h i s w a y .

Examples 4, 5 and C o m p a r a t i v e Example 6

Examples 4 and 5 were c a r r i e d out s u b s t a n t i a l l y t h e

same as Examples 1 and 2, and the amount of e n e r g y

n e c e s s a r y was d e t e r m i n e d . Th i s amount of e n e r g y w a s

compared w i t h a p r o c e s s (Example 6) w h e r e i n no CO2, and on ly an amount of a i r n e c e s s a r y to keep t h e

m a t e r i a l s in a p a s s i v a t e d s t a t e , were added to the C02

co lumn. T a k i n g the e n e r g y c o n s u m p t i o n of Example 6 a s 100%, the r e s u l t s were as f o l l o w s :

1 . P r o c e s s fo r the s e p a r a t i o n of s u b s t a n t i a l l y pu re NH3 a n d

s u b s t a n t i a l l y pure C02 from a m i x t u r e c o n t a i n i n g NH3, CO2 a n d

w a t e r , in which NH3 is s e p a r a t e d by r e c t i f i c a t i o n in an NH3- s e p a r a t i n g zone, the r e m a i n i n g l i q u i d phase is fed to a C 0 2 - s e p a r a t i n g zone where g a s e o u s C02 is s e p a r a t e d by r e c t i f i c a t i o n , and the r e s u l t i n g l i q u i d phase is fed from the b o t t o m of t h i s

zone to a d e s o r p t i o n zone , where v i r t u a l l y a l l NH3 and C02 s t i l l c o n t a i n e d in t h i s l i q u i d phase are s e p a r a t e d as a

g a s e o u s m i x t u r e , c h a r a c t e r i z e d in t h a t so much g a s e o u s C02 o r

a ga seous m i x t u r e c o n t a i n i n g CO2 i s fed to the b o t t o m of t h e

N H 3 - s e p a r a t i n g zone t h a t the c o m p o s i t i o n of the l i q u i d p h a s e r e l e a s e d in the b o t t o m of the N H 3 - s e p a r a t i n g zone is not o n the s i d e of the b o u n d a r y l i n e r i c h in NH3.

2. P r o c e s s a c c o r d i n g to c l a i m 1, c h a r a c t e r i z e d in t h a t p a r t o f

the g a s e o u s CO2 from the C O 2 - s e p a r a t i o n z o n e i s i n t r o d u c e d

i n t o the b o t t o m of the N H 3 - s e p a r a t i o n z o n e . 3. P r o c e s s a c c o r d i n g to c l a i m 1, c h a r a c t e r i z e d in t h a t the a m o u n t

(by w e i g h t ) of g a s e o u s C02 or g a s e o u s m i x t u r e c o n t a i n i n g C02 r a n g e s b e t w e e n 0.2 and 0.01 t i m e s the amount of l i q u i d p h a s e r e l e a s e d from the b o t t o m of t he N H 3 - s e p a r a t i o n z o n e .

4. P r o c e s s a c c o r d i n g to c l a i m 3, c h a r a c t e r i z e d i n t h a t s a i d a m o u n t

of gas r a n g e s b e t w e e n 0.1 and 0 . 0 2 t i m e s s a i d l i q u i d p h a s e . 5. P r o c e s s a c c o r d i n g to c l a i m 1, c h a r a c t e r i z e d i n t h a t an a d d i t i o n a l

amount of i n e r t gas is i n t r o d u c e d i n t o the b o t t o m of the CO2- s e p a r a t i o n z o n e .

6. P r o c e s s a c c o r d i n g to c l a i m 5, c h a r a c t e r i z e d in t h a t the r e l a t i v e

amounts (by w e i g h t ) of C02 and i n e r t gas r ange b e t w e e n 3 0 - 7 0

and 7 0 - 3 0 . 7. P r o c e s s a c c o r d i n g to c l a i m 1, s u b s t a n t i a l l y as d e s c r i b e d

h e r e i n b e f o r e .