Heterogeneous Catalysis - American Chemical Society

26 Methanol: Bright Past-Brilliant Future? Downloaded by UNIV OF CALIFORNIA SAN DIEGO on August 25, 2015 | http://pubs.acs.org Publication Date: June 3, 1983 | doi: 10.1021/bk-1983-0222.ch026

ALVIN B. STILES University of Delaware, Department of Chemical Engineering, Center for Catalytic Science and Technology, Newark, DE 19711

Methanol presently i s a large volume chemical used p r i m a r i l y for chemical purposes. The possibility is increasing that methanol itself, as a blend or after further processing, w i l l become a major l i q u i d fuel i n the future. This chapter surveys the history over the past 75 years of methanol synthesis from CO, H and CO2. Factors considered are first the c a t a l y s t developments but also describing the improvements i n gas p u r i f i c a t i o n , reactor design, c a t a l y s t life, operat i n g pressure and p r o d u c t i v i t y . 2

The very f i r s t methanol was probably produced by nature herself when l i g h t n i n g struck the f i r s t plant which contained l i g n i n and which emerged from the primordial sea. The rapid p y r o l y s i s of the l i g n i n content of t h i s p r i m i t i v e plant very l i k e l y generated methyl or methoxy radicals which combined with hydroxyl r a d i c a l s or protons to form methanol (Figure 1). In medieval times the alchemist also p y r o l i z e d wood i n his r e t o r t and i n so doing generated gases some of which were condensible to l i q u i d s and some of the l i q u i d was methanol or, i n those days, termed wood a l c o h o l . It may seem anachronistic but wood p y r o l y s i s was the method used i n World War I to make wood alcohol for methyl ester solvents. The esters were the solvents for the pyroxylin used to strengthen and tighten the f a b r i c s on the wings of those early planes. Germany, however, had two much more serious problems - one r e l a t i n g to t h e i r i s o l a t i o n from sources of petroleum and the other from n i t r a t e s . As a

0097-6156/83/0222-0349$07.25/0 © 1983 American Chemical Society

In Heterogeneous Catalysis; Davis, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on August 25, 2015 | http://pubs.acs.org Publication Date: June 3, 1983 | doi: 10.1021/bk-1983-0222.ch026

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HETEROGENEOUS CATALYSIS

Figure 1. P r i m i t i v e Scene o f C y n t h i a S. Boyd.

First

Me OH

Production


-

by


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consequence, t h e i r p o l i c y makers determined t h a t they must do two t h i n g s , one o f which was t o d e r i v e l i q u i d f u e l s such as methanol and r e l a t e d p r o d u c t s and the second t o d e r i v e ammonia. These must a l l be s y n t h e s i z e d from i n t e r n a l l y a v a i l a b l e raw m a t e r i a l s . The m o t i v a t i o n , o f c o u r s e , was t o f r e e Germany from the need t o b r i n g i n n i t r a t e f o r f e r t i l i z e r and e x p l o s i v e s from C h i l e and a l s o t o p r o v i d e l i q u i d fuels from internally available sources. It i s apparent t h a t t h e s e German developments were n e c e s s i t a t e d by the g e o g r a p h i c s i t u a t i o n o f Germany and by the f a c t t h a t economic and p o s s i b l y a g g r e s s i v e t e n d e n c i e s were d i c t a t i n g such a move. Ammonia s y n t h e s i s from e l e m e n t a l N and H was developed by Haber a p p r o x i m a t e l y i n 1906 and the f i r s t p a t e n t on the p r o c e s s appeared i n 1910. Following closely therea f t e r was the s y n t h e s i s o f methanol from CO and hydrogen which i n t u r n was d e r i v e d from c o a l a v a i l a b l e i n t e r n a l l y i n Germany. We w i l l subsequently t a l k about the f i r s t methanol u n i t s which were r e a l l y p u r i f i c a t i o n u n i t s f o r gas b e i n g used down-stream f o r ammonia s y n t h e s i s . 2

Early Historical Present

Factors

2

as

they

Relate

to

the

I t i s f r e q u e n t l y d e s i r a b l e t o l o o k a t the h i s t o r i cal background o f a p r o c e s s t o see why the p r e s e n t o p e r a t i n g c o n d i t i o n s have been s e l e c t e d , but i t i s a l s o important t o l o o k a t t h e s e f a c t o r s because t h e r e may be reasons f o r a g a i n a d o p t i n g o r a d a p t i n g t h e s e early conditions. The e a r l i e s t s y n t h e t i c methanol was s y n t h e s i z e d from carbon monoxide, carbon d i o x i d e , and hydrogen which were d e r i v e d from c o a l by the water-gas r e a c t i o n . From the e a r l i e s t days o f methanol and ammonia synthesis, the interrelationship between a d j a c e n t ammonia and methanol p l a n t s was evident. In some o f the v e r y e a r l i e s t methanol p l a n t s , the s y n t h e s i s gas was most c o n v e n i e n t l y and e c o n o m i c a l l y produced as a carbon monoxide, carbon d i o x i d e , hydrogen, and n i t r o g e n m i x t u r e . T h i s gas was produced i n gas s e t s i n which c o a l o r coke was f i r s t heated t o i n c a n d e s c e n c e i n a c o n v e r t e r ; then steam o r steam p l u s a i r was passed over the i n c a n d e s c e n t coke t o produce the aforementioned gases. This gas c o n t a i n e d ash as w e l l as s u l f u r compounds and o t h e r contaminants. The p u r i f i c a t i o n p r o c e d u r e cons i s t e d o f removal o f the ash and s u l f u r compounds by scrubbing. In t h i s c o m b i n a t i o n p r o c e s s shown i n F i g u r e 2, the



352

V O L A T I L E S - H . NHJJ, BTX NAPHTHALENE, T A R S 2

PRODUCT GASES COAL

N

1

COKE OVENS


2t

CO, C O . H 2

2

HOT GASES FROM AIR BLOW GAS GENERATING SETS

PART I O F GAS GENERATING CYCLE

CHECKERWORK (LARGE MASS OF FIREBRICK OR CERAMIC) ALSO HEATED TO INCANDESCENCE

AIR HEATING COKE TO INCANDESCENCE (ABOUT 2 min of 5 min CYCLE) EXHAUST COOL AIR BLOW GASES TO ATMOSPHERE

• HOT STEAM - AIR MIXTURE INTO INCANDESCENT COKE

N , CO, H 2

N . CO, C Q , H 2

2

PART 2 OF GAS GENERATING CYCLE STEAM AND AIR AT AMBIENT TEMPERATURE INTO HOT CHECKERWORK (ABOUT 3 min of 5 min CYCLE)

C H 3 O H , RESIDUAL CO, H , 2

2

N

2

2

FROM GAS SETS

co

2

CH30H

Θ-φ

REMOVAL SCRUBBER

SYNTHESIS CONVERTER

3

1

' C 0 RECOVERY 2

CONDENSER

I CH30H

CH3OH

CONDENSER

SEPARATOR

LIQUID METHANOL TO STORAGE

Figure 2 .

NH3

METHANATION UNIT FOR CO REMOVAL

SYNTHESIS UNIT

H . N , CH 2

2

NH SEPARATOR 3

PURGE GAS 4

LIQUID NH TO STORAGE 3

RECYCLE GAS

Flow sheet f o r a p l a n t s e q u e n t i a l l y making meth a n o l then ammonia from c a r b o n monoxide, nitrogen and hydrogen d e r i v e d from c o a l ( c a . 1920's).



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c l e a n e d gas c o n t a i n i n g c a r b o n monoxide, hydrogen, and n i t r o g e n was f i r s t compressed t o o p e r a t i n g c o n d i t i o n s which c o u l d have been as h i g h as c a . 1000 atm. and was t h e n passed over a methanol s y n t h e s i s c a t a l y s t t o remove the carbon monoxide and a p a r t o f hydrogen so t h a t the gas e f f l u e n t which then p a s s e d i n t o the ammonia c o n v e r t e r would have the p r o p e r 1:3 r a t i o o f n i t r o g e n : hydrogen. The methanol was condensed from the gas stream b e f o r e i t p a s s e d i n t o the ammonia conv e r t e r , and any r e s i d u a l c a r b o n monoxide was removed from the n i t r o g e n - h y d r o g e n stream by methanation (CO + 3H^ ^CH^+ HO). The pure ammonia s y n t h e s i s gas was p a s s e d over the ammonia s y n t h e s i s c a t a l y s t i n a downstream c o n v e r t e r . There were many problems w i t h t h i s type o f o p e r a t i o n , some r e l a t i n g t o the s e a s o n a l n a t u r e o f the s a l e s demand f o r b o t h methanol, used l a r g e l y as a n t i f r e e z e a t t h i s time, and ammonia, used as a f e r t i l i z e r . When methanol i s s y n t h e s i z e d from s y n t h e s i s gas c o n t a i n i n g n i t r o g e n ( n i t r o g e n i s almost always p r e s e n t ) , t h e r e i s always the s i m u l t a n e o u s s y n t h e s i s o f o r g a n i c amines. These are o b j e c t i o n a b l e because they g i v e an o b j e c t i o n a b l e b a s i c p r o p e r t y t o the methanol and cause an o f f e n s i v e odor. The h i g h temperatures and p r e s s u r e s used i n early plants dictated specialized equipment which c o u l d be f a b r i c a t e d o n l y by t h o s e who had e x p e r i e n c e i n , and f a c i l i t i e s f o r , m u n i t i o n s manufacture. T h i s r e s u l t e d i n a l l o y s and d e s i g n o f t e n showing t h e common p a r e n t a g e w i t h the m u n i t i o n s o f the p e r i o d between WW I and WW I I . S i n c e then, s p e c i f i c f a b r i c a t i o n t e c h n i q u e s have been d e v e l o p e d which have made i t p o s s i b l e t o f a b r i c a t e the l a r g e r s i z e c o n v e r t e r s and i n t e r c o n n e c t i n g p i p i n g and the more e a s i l y assembled and d i s a s s e m b l e d f a c i l i t i e s now r e q u i r e d . These earlier facilities were a l s o p l a g u e d by c o r r o s i o n due t o the s u l f u r compounds, t o carbon d i o x i d e (as c a r b o n i c a c i d s o l u t i o n ) , o r t o hydrogen e m b r i t t l e m e n t , o r t o i r o n c a r b o n y l f o r m a t i o n from the h i g h p r e s s u r e - l o w temperature c a r b o n monoxide cont a i n i n g gases. These problems were f o r the most p a r t s o l v e d , but t h e i r o n c a r b o n y l problem remains and seems t o d e f y an economic s o l u t i o n . The s y n t h e s i s gas p r e p a r a t i o n phase o f methanol s y n t h e s i s i s p r e s e n t l y the most i n need o f major i n n o v a t i o n s and developments. As n a t u r a l gas and liquid hydrocarbons become more costly and less abundant, the l e s s e a s i l y p r o c e s s e d s o u r c e s o f c a r b o n must be used. First t o be c o n s i d e r e d w i l l be the




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s o u r c e s which have been h i s t o r i c a l l y i m p o r t a n t , next the ones most w i d e l y u t i l i z e d p r e s e n t l y , and f i n a l l y s p e c u l a t i o n as t o the s o u r c e s and p r o c e s s e s o f the future. The e a r l i e r s o u r c e s i n c l u d e d f e r m e n t a t i o n b y - p r o d u c t gases (Commercial S o l v e n t s C o r p . ) , coke oven b y - p r o d u c t gas, and s t e e l f u r n a c e o f f gases. The coke oven b y - p r o d u c t gas i s d e f i c i e n t i n carbon mon o x i d e and carbon c o n t a i n i n g m o l e c u l e s whereas the o f f gas from b l a s t f u r n a c e s i s h i g h i n c a r b o n monoxide and d e f i c i e n t i n hydrogen. These l a t t e r s o u r c e s o f hydrogen and carbon monoxide have been c o n s i d e r e d and t o some e x t e n t used, but because o f t h e many d i f f i c u l t i e s i n v o l v e d i n t h e i r u t i l i z a t i o n , they b o t h a r e o f v e r y minor importance p r e s e n t l y . However, as the need intensifies f o r complete c o n s e r v a t i o n o f our energy r e s o u r c e s , they may i n t h e f u t u r e by u t i l i z e d rather than burned as i s almost u n i v e r s a l l y the present case. The p r e p a r a t i o n o f carbon monoxide and hydrogen from coke o r c o a l by the b l u e - g a s and blow-run gas t e c h n i q u e was f r e q u e n t l y used. T h i s p r o c e s s was used by Ε. I duPont Nemours a t the B e l l e , West V i r g i n i a p l a n t when the p l a n t was f i r s t p l a c e d i n o p e r a t i o n i n the l a t e 20 s and c o n t i n u e d as the s o u r c e o f s y n t h e s i s gases f o r both methanol and ammonia u n t i l the l a t e 40's. The p r o c e d u r e used and a diagram o f the f a c i l i t i e s are shown i n F i g u r e 3. In a d d i t i o n t o the gas s e t s which were used f o r the gas g e n e r a t i o n , a complete s e t o f coke ovens was a l s o r e q u i r e d f o r the c o n v e r s i o n o f the c o a l t o coke. At one time the c o n v e r s i o n o f c o a l d i r e c t l y t o s y n t h e s i s gases was the b a s i c p r o c e d u r e , but because o f d i f f i c u l t i e s w i t h compounds b e i n g v o l a t i l i z e d from the c o a l which l a t e r condensed i n the gas l i n e s t o p l u g them, the p r o c e dure was a l t e r e d t o u s i n g coke which a v o i d e d t h e s e problems. The duPont Company a t t h a t time a d v e r t i s e d t h a t the p r o d u c t s o f the p l a n t were from c o a l , a i r , and water, which i s something we f i n d many p e o p l e p r e s e n t l y s p e c u l a t i n g about w i t h o u t r e a l i z i n g t h a t t h i s had as a matter o f f a c t been an i n d u s t r i a l p r o c e s s i n the 20's. I t c o u l d h a r d l y be c o n s i d e r e d a v i a b l e p r o c e s s o f the p r e s e n t , but t h e r e are c e r t a i n f e a t u r e s o f the o p e r a t i o n and p r o c e s s which p r e s e n t interesting backgrounds which we certainly must a s s e s s i n the p r e s e n t l i g h t o f needs t o d e r i v e f u e l s and c h e m i c a l s from d o m e s t i c a l l y available fossil energy. Steam-hydrocarbon r e f o r m i n g has become the most f r e q u e n t l y used method f o r s y n t h e s i s gas p r e p a r a t i o n , and u s u a l l y n a t u r a l gas, t r e a t e d t o remove a l l com1



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EXHAUST GASES TO ATMOSPHERE VALVE OPEN ONLY DURING AIR BLOW

••VALVE OPEN ONLY DURING STEAM BLOW PORTION OF CYCLE CO. H MIXTURE 2

VALVES GAS COOLER AND SCRUBBER

COKE COKE ADDED PERIODICALLY FROM TRAP HOPPER

HIGH PRESSURE SCRUBBER

COMPRESSOR

u C 0 + SULFUR COMPOUNDS 2

AIR HEATING COKE TO INCANDESCENCE

STEAM PREHEATED BY CHECKERWORK HEATED BY OFF GAS DURING AIR BLOW

SCRUBBED CO. H

METHANOL SYNTHESIS CONVERTER

RESIDUAL CO2 +

ASH

2

SULFUR COMPOUNDS!

MIXTURE

TO CONDENSER C H 3 O H SEPARATOR

AND STORAGE

Figure 3. Flow sheet f o r a p l a n t p r o d u c i n g methanol from carbon monoxide and hydrogen m i x t u r e s d e r i v e d from c o a l by the b l u e gas p r o c e s s .



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ponents except methane and n i t r o g e n , i s the p r e f e r r e d hydrocarbon. The steam-hydrocarbon r e f o r m i n g p r o c e s s i s h i g h l y d e v e l o p e d and w i l l o p e r a t e f o r months o r even y e a r s w i t h o u t i n t e r r u p t i o n , e x c e p t f o r normal outages s c h e d u l e d f o r b o i l e r i n s p e c t i o n , r o u t i n e maintenance, and o t h e r a t t e n t i o n which i s p l a c e d on a d e f i n a b l e schedule. The heat b a l a n c e and u t i l i z a t i o n are w e l l e n g i n e e r e d o r d i n a r i l y so t h a t t h e r e i s l i t t l e waste, and what heat i s unused on the f u r n a c e s i d e o f the r e f o r m e r i s s u b s e q u e n t l y r e c o v e r e d f o r use t o genera t e steam. The b a s i c e q u a t i o n i s : CH

+ HO

= 3H

+ CO

0

but f o r methanol s y n t h e s i s the f o l l o w i n g r e l a t i o n s h i p between the hydrogen and carbon monoxide i s n e c e s sary : 2H

2

+ CO

= CH 0H 3

T h e r e f o r e , the f o l l o w i n g s i m u l t a n e o u s o p e r a t i o n i s performed i n the r e f o r m e r by a d d i n g carbon d i o x i d e i n t o the f e e d gas: C0

o

+ CH.

= 2C0

+ 2H

0

+

H_0

The l a t t e r r e a c t i o n , o f c o u r s e , produces excess c a r bon monoxide r e l a t i v e t o t h a t needed f o r methanol, and as a consequence, i n p r a c t i c e , the f e e d t o a r e f o r m e r comprises n a t u r a l gas, steam, and carbon dioxide. S y n t h e s i s Gas

Purification

The steam-carbon d i o x i d e - h y d r o c a r b o n c o n v e r s i o n i s conducted over a c a t a l y s t such as n i c k e l ( o x i d e ) on alumina. T h i s type o f c a t a l y s t can be purchased i n q u i t e s i m i l a r c o m p o s i t i o n from a number o f c a t a l y s t vendors. In the case i n which the f e e d s t o c k i s p r o c e s s e d over a c a t a l y s t as i n steam-hydrocarbon r e f o r m i n g , i t i s e s s e n t i a l t h a t the gas be p u r i f i e d , a t l e a s t t o some e x t e n t , p r i o r t o i t s passage o v e r the r e f o r m i n g c a t a l y s t , p a r t i c u l a r l y i f the c a t a l y s t i s o f the t y p i c a l c o m p o s i t i o n o f s u p p o r t e d and promoted n i c k e l (oxide). In steam hydrocarbon r e f o r m i n g , the methane ( n a t u r a l gas) i s u s u a l l y d e t o x i f i e d u s i n g an adsorbent such as carbon on which i s impregnated s u i t a b l e c h e m i c a l a d s o r b e n t s such as e l e m e n t a l i r o n o r copper. There are a t l e a s t two o f t h e s e m e t a l l i z e d carbon d e s u l f u r i z e r s i n p a r a l l e l w i t h one on



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the l i n e w h i l e the o t h e r i s b e i n g r e g e n e r a t e d . Reg e n e r a t i o n i s e f f e c t e d by p a s s i n g steam o r steam p l u s a s m a l l amount o f oxygen ( p a r t s p e r m i l l i o n range) o v e r the a d s o r b e n t i n such a way t h a t the s u l f u r i s removed as s u l f u r d i o x i d e o r hydrogen s u l f i d e . Heavy hydrocarbons s i m u l t a n e o u s l y adsorbed by the c a r b o n are removed as such t o be f l a r e d , r e c o v e r e d , f o r s a l e , o r combusted t o d e r i v e u s e f u l h e a t . I f there are s o l i d s such as sodium c h l o r i d e and i t s a s s o c i a t e d m i n e r a l s e n t r a i n e d w i t h the n a t u r a l gas, o b v i o u s l y t h e s e i m p u r i t i e s must be removed; water s c r u b b i n g can be used. Steam used i n the o p e r a t i o n must be f r e e o f s o l i d s s i n c e the entrainment o f s o l i d s i n the m i s t from b o i l e r s i s not an i n f r e q u e n t cause o f p h y s i c a l l y p o i s o n i n g the r e f o r m i n g c a t a l y s t a t l e a s t i n the upper (upstream) p a r t o f the r e f o r m e r t u b e s . Hydrocarbons themselves can be troublesome. F i r s t , the steam-to-carbon r a t i o must be kept above an e x p e r i e n c e range, and second, unplanned f o r h i g h e r hydrocarbons must be a v o i d e d because h i g h e r h y d r o c a r bons are more s u s c e p t i b l e t o d e h y d r o g e n a t i o n ( c r a c k ing, c a u s i n g carbon d e p o s i t i o n ) than are the lower ones such as methane, ethane, and even propane. I f , f o r some reason, the n a t u r a l gas has a surge in h i g h e r hydrocarbon c o n t e n t , the e f f e c t can be d i s a s t r o u s i n t h a t the n i c k e l c a t a l y s t w i l l q u i c k l y become c o a t e d w i t h a s o o t y carbon, which quickly d e a c t i v a t e s i t . A second r e s u l t i s t h a t the c a r b o n may be d e p o s i t e d i n t e r s t i t i a l l y i n the c a t a l y s t body c a u s i n g i t t o d i s i n t e g r a t e t o powder, which o f c o u r s e s t o p s f l o w through the tube so t h a t when naphtha i s used i n s t e a d o f n a t u r a l gas, the problems a r e s i m i l a r but more s e v e r e , and p r o p e r m o d i f i c a t i o n s a r e made i n the c a t a l y s t t o make i t r e s i s t a n t t o s e v e r e p o i s o n i n g and carbon d e p o s i t i o n c o n d i t i o n s . These c a t a l y s t s usually c o n t a i n lower amounts o f n i c k e l and may contain such promoters as uranium, potassium, or other a l k a l i or a l k a l i n e earth oxides. Inasmuch as alkali metal o x i d e s m i g r a t e from the c a t a l y s t a t h i g h e r temperatures and p r e s s u r e s , i t i s n e c e s s a r y t o o p e r a t e the naphtha r e f o r m e r s a t m i l d e r c o n d i t i o n s t h a n when n a t u r a l gas i s the f e e d . In such c a s e s , i t may be d e s i r a b l e t o o p e r a t e two r e f o r m e r s , one f o r the c o n v e r s i o n o f naphtha t o l i g h t e r h y d r o c a r b o n s , f o l l o w i n g t h i s w i t h a c o n v e r t e r d e s i g n e d and o p e r a t e d more l i k e the n a t u r a l gas r e f o r m e r p r e v i o u s l y described . Throughout the p a s t decade o r two t h e r e have been s e v e r a l s h i f t s t o and from steam-hydrocarbon


358


r e f o r m i n g v s . h y d r o c a r b o n p a r t i a l combustion as the 'Ore economical p r o c e d u r e f o r s y n t h e s i s gas genera tion. The e q u a t i o n f o r the p a r t i a l o x i d a t i o n r e a c t i o n i s as f o l l o w s :

r

HO + C H + 0 •H +CO+CO +H 0 (Exothermal) ζ n. ^n.! ζ ζ ζ ζ ζ The r a t i o o f i n g r e d i e n t s i n the f e e d , t h a t i s , water, hydrocarbon, and oxygen, i s a d j u s t e d so t h a t the d e s i r e d r a t i o o f carbon monoxide p l u s carbon d i o x i d e and hydrogen are o b t a i n e d i n the e f f l u e n t gas. The temperature and p r e s s u r e , o f c o u r s e , i n f l u e n c e t h i s r a t i o so t h a t a l l o f t h e s e f a c t o r s must be c o n s i d e r e d when the f e e d gas r a t i o s are e s t a b l i s h e d . A number o f p o t e n t i a l s o u r c e s of s y n t h e s i s gases have been advanced i n r e c e n t y e a r s , many due t o the 1972 energy c r i s i s i n the U.S. These s o u r c e s r e q u i r e much e f f o r t and development b e f o r e they become a p p l i cable. However, the p r o d u c t i o n o f s y n t h e s i s gas i s a c r u c i a l c o n s i d e r a t i o n because o f the l a r g e impact i t has on the c o s t o f the methanol p r o d u c t .


- -

C a t a l y s t Developments In commercial o p e r a t i o n c a t a l y s t l i f e , as w e l l as a c t i v i t y , must be c o n s i d e r e d . In the earlier i n s t a l l a t i o n s o f methanol s y n t h e s i s p l a n t s , t h e r e was l i t t l e gas p u r i f i c a t i o n o f the s c a l e and e f f e c t i v e ness t h a t i s p r e s e n t l y used, and as a r e s u l t the c a t a l y s t s or o p e r a t i n g c o n d i t i o n s t h a t were used were n e c e s s a r i l y more rugged and s e v e r e than i s p r e s e n t l y the c a s e . In o t h e r words, the f a c t t h a t r e c e n t meth a n o l i n s t a l l a t i o n s o p e r a t e under m i l d e r temperature and p r e s s u r e c o n d i t i o n s i s a t t r i b u t a b l e not o n l y t o improved c a t a l y s t s but a l s o t o the f a c t t h a t the s y n t h e s i s gases p r e s e n t l y employed have been d e t o x i f i e d t o the e x t e n t t h a t c a t a l y s t s t h a t have l o n g been known are now p r a c t i c a b l e . M i t t a s c h and S c h n e i d e r . The f i r s t p a t e n t f o r the synthesis o f methanol was granted i n Germany to M i t t a s c h and S c h n e i d e r i n 1913 (\) . The c a t a l y s t s which they d e s c r i b e d were o x i d e s o f cerium, chromium, manganese, molybdenum, t i t a n i u m , and z i n c which had been " a c t i v a t e d " by i n c o r p o r a t i n g a l k a l i e s such as sodium and p o t a s s i u m c a r b o n a t e s . The p r o d u c t s were methanol, h i g h e r a l c o h o l s and s a t u r a t e d and u n s a t u r ated hydrocarbons. P r e s s u r e s and temperatures were 100-200 atmospheres and 300° t o 400°C. These were


26.

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the temperatures and p r e s s u r e s used s i v e l y i n t h i s and o t h e r e a r l y work.


359

Methanol: Bright Past—Brilliant Future almost

exclu-

F i s c h e r and T r o p s c h . The next p a t e n t was i s s u e d t o F i s c h e r and Tropsch and was g r a n t e d i n 1922 ( 2 ) , 9 y e a r s a f t e r the M i t t a s c h - S c h n e i d e r p a t e n t . The c a t a l y s t s which were c l a i m e d by F i s c h e r and Tropsch were e l e m e n t a l n i c k e l , s i l v e r , copper and i r o n and most s p e c i f i c a l l y i r o n p l u s cesium and rubidiumhydroxides. The p r o d u c t s of the F i s c h e r - T r o p s c h c a t a l y s t s a t t h a t time were methanol , h i g h e r a l c o h o l s , o t h e r oxygenated p r o d u c t s but no hydrocarbons e i t h e r saturated or unsaturated. The name t h a t was a s s i g n e d t o t h i s p r o d u c t was " s y n t h o l . " F i s c h e r and T r o p s c h used a s l i g h t l y h i g h e r temperature (420°C) and p r e s s u r e (134 atm.) than M i t t a s c h and S c h n e i d e r employed. M i t t a s c h , P i e r and W i n k l e r . The next p a t e n t t o i s s u e was a l s o i n Germany and was g r a n t e d t o Mittasch, P i e r and W i n k l e r i n 1923 (3) . The c a t a l y s t s t h a t were taught f o r use a t 380-420°C and 200 atm. p r e s sure were as f o l l o w s : C a t a l y s t A:

copper (Cr

+ 3

)

o x i d e , z i n c o x i d e , chromium o x i d e and

optionally

manganese (Cr

+ 6

oxide.

C a t a l y s t B:

z i n c o x i d e , chromium o x i d e

C a t a l y s t C:

z i n c o x i d e , chromium manganese o x i d e .

C a t a l y s t D:

z i n c o x i d e and chromium t r i o x i d e as t r i v a l e n t chromium.

oxide

).

+2 (Cr ) and

I t w i l l be q u i c k l y r e c o g n i z e d t h a t t h e s e a r e t h e c a t a l y s t s which v e r y c l o s e l y approach t h e modern methanol s y n t h e s i s c a t a l y s t s . The one c o n t a i n i n g manganese was s i m i l a r t o t h a t used c o m m e r c i a l l y 30-40 years ago f o r the s y n t h e s i s o f h i g h e r alcohols. S t r a n g e l y t h e p r o d u c t d e s c r i b e d by t h e i n v e n t o r s i s i d e n t i f i e d as pure methanol. We w i l l c o n s i d e r subs e q u e n t l y t h e f a c t t h a t the manganese does have a s t r o n g tendency f o r the p r o d u c t i o n o f h i g h e r a l c o h o l s which would c a s t some doubt on t h e i d e n t i t y o f t h e p r o d u c t s o f a l l o f t h e s e c a t a l y s t s as b e i n g pure methanol.



360


M i t t a s c h , P i e r and Winkler. The next catalyst p a t e n t was a l s o g r a n t e d t o M i t t a s c h , P i e r and W i n k l e r and was i s s u e d i n 1925 (4) . Some o f the c a t a l y s t s which are taught are r a t h e r e x o t i c i n l i g h t o f p r e s e n t knowledge and c o n s i s t o f A) Cr2 O 3 p l u s ;;inc o x i d e ; B) z i n c o x i d e p l u s uranium o x i d e ; C) z i n c o x i d e p l u s vanadium p e n t o x i d e ; D) z i n c o x i d e p l u s t u n g s t e n o x i d e ; E) cerium o x i d e p l u s manganese o x i d e and l a s t l y , F) copper o x i d e and z i n c o x i d e . The p r o d u c t from r e a c t i o n s a t 380-420°C and 200 atm. p r e s sure was said t o be methanol p l u s v a r i o u s o t h e r oxygenated p r o d u c t s . Lewis and F r o h l o c h . Another modern catalyst c o m p o s i t i o n was d e s c r i b e d by Lewis and F r o h l i c h i n 1928 ( 5 ) . T h i s c a t a l y s t had the c o m p o s i t i o n o f z i n c o x i d e , copper o x i d e and aluminum o x i d e . The p r o d u c t t h a t was o b t a i n e d when s y n t h e s i s gas was passed o v e r t h i s c a t a l y s t a t 275-350 C and 100 atm. p r e s s u r e was methanol. Lazier. L a z i e r (6) i n v e n t e d and a s s i g n e d t o the DuPont Company the next c a t a l y s t t o be d e s c r i b e d . T h i s was c o v e r e d by a p a t e n t i s s u e d i n 1931 and the c a t a l y s t , i t s e l f , was z i n c o x i d e , manganese o x i d e , and chromium o x i d e . Depending on the manganese r a t i o and o p e r a t i n g c o n d i t i o n s (350-475°C and 267-900 atm. p r e s s u r e ) t h i s c a t a l y s t c o u l d produce 20 t o 50% h i g h e r a l c o h o l s i n a d d i t i o n t o methanol. Those f a m i l i a r w i t h modern methanol and methanolhigher alcohol synthesis processes w i l l quickly reco g n i z e t h a t t h e s e e a r l y i n v e s t i g a t o r s have p r e t t y w e l l d i s c l o s e d the c h e m i c a l c o m p o s i t i o n o f p r e s e n t l y used c a t a l y s t s f o r t h e s e o p e r a t i o n s . The improvements t h a t have been made s i n c e the e a r l y i n v e s t i g a t o r s i d e n t i f i e d the c o m p o s i t i o n s have l a r g e l y been i n the way the components are put t o g e t h e r and the p h y s i c a l c h a r a c t e r i s t i c s o f the c a t a l y s t s . The e a r l y i n v e s t i g a t o r s q u i t e o f t e n simply took o x i d e s and then heated them t o v e r y h i g h temperatures i n the presence o f an a l k a l i . M e l t i n g or s i n t e r i n g was considered desirable. In f a c t , i n the DuPont Company we had w i t h us i n the c a t a l y s t development a r e a a p e r s o n who had been w i t h the N i t r o g e n F i x a t i o n Labor a t o r y i n the e a r l y 20's. He had h i s g r o u n d i n g i n f u s e d , promoted i r o n o x i d e ammonia s y n t h e s i s c a t a lyst. He was the type who f e l t t h a t a l l c a t a l y s t s , whether they were f o r methanol, h i g h e r a l c o h o l s o r ammonia, a l l s h o u l d be f u s e d . The r e s u l t was t h a t " c a t a l y s t s " were produced but a l l were q u i t e poor p h y s i c a l l y and c a t a l y t i c a l l y .



26.

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361


D e s p i t e the g r e a t s t r i d e s i n c a t a l y s t d e v e l o p ment, my personal f e e l i n g and recent observations have s t r e n g t h e n e d t h i s f e e l i n g t h a t t h e r ^ i s s t i l l a g r e a t d e a l t h a t c o u l d be done t o f u r t h e r improve c a t a l y s t s f o r methanol and h i g h e r a l c o h o l s servicer-, S i n c e we have d i s c u s s e d c a t a l y s t s , the ne>r: t h i n g t h a t we p l a n t o d i s c u s s i s the c a t a l y t i c r e a c t o r s and i t would be out o f c h a r a c t e r f o r me not, a t t h i s time, t o emphasize the f a c t t h a t when c a t a l y s t s are b e i n g developed the c a t a l y t i c s c i e n t i s t s and the p r o j e c t e n g i n e e r d e s i g n i n g the c o n v e r t e r must work very c l o s e l y together. I t has f r e q u e n t l y been a costly situation when they do not work t o g e t h e r until the p l a n t d e s i g n i s e s s e n t i a l l y frozen, at which time i t may be found t h a t some o f the c h a r a c t e r i s t i c s o f the c a t a l y s t , such as h i g h e x o t h e r m i s i t y , were not a d e q u a t e l y c o n s i d e r e d and the r e a c t o r i s not d e s i g n e d t o d i s s i p a t e t h i s e x t r a heat l o a d . An o f t e n n e g l e c t e d c o n s i d e r a t i o n i s t h a t the c a t a lytic scientist and reactor designer must work c l o s e l y d u r i n g developmental stages, e s t a b l i s h i n g abrasion resistance, density, particle size, gas d i s t r i b u t i o n , p o r o s i t y and heat d i s s i p a t i o n . Methanol U n i t Designs and

Types

The f i r s t commercial methanol u n i t s u s u a l l y were d e s i g n e d i n such a way t h a t they c o u l d accommodate the gases which c o n t a i n e d l a r g e amounts o f n i t r o g e n and r e l a t i v e l y small amounts o f CO or 2methanol p r o c e s s was really a purification process f o r the ammonia s y n t h e s i s gas and the gas was first p r o c e s s e d through the Claude type methanol u n i t which i s shown i n F i g . 4. You w i l l see that there are f i r s t two c o n v e r t e r s i n p a r a l l e l , t h e n a second s e r i e s o f c o n v e r t e r s i n p a r a l l e l , then a condenser system f o r the removal o f the a l c o h o l and a combining o f the gas streams which then pass through the two a d d i t i o n a l c o n v e r t e r s making a t o t a l o f 6. F i n a l l y the e f f l u e n t p a s s e s t o a second condenser system which removes the methanol. The a l c o h o l - f r e e gases pass t o a methanator which p u r i f i e s the ammonia s y n t h e s i s gas by methanation o f the carbon monoxide and carbon d i o x ide. A f t e r removal o f the water vapor the mixed gas (N :3H2) i s passed through an ammonia s y n t h e s i s s y s tem of e s s e n t i a l l y the same d e s i g n as the p r e v i o u s l y d e s c r i b e d Claude u n i t . The ammonia i s condensed and the u n r e a c t e d gases are burned o r are r e c y c l e d t o the s y n t h e s i s gas p r o c e s s . C 0

2


T

n

e


2

2

5 % CO < I %C 0 Z0% N 75% H

2

/ \

Liquid Methanol Draw- off

Liquid Methanol Separator

3

Figure 4 . Claude CH 0H

Methanol Reactors

Methanol Condensor

NH

Methanol Reactors

3

Draw-off

CH3OH

Liquid

2

Methanation to remove CO to < 5 p p m

Combined P r o c e s s ,

*

Liquid Methanol Separator

Methanol Condenser

2

24% N 76%H 2 % CH*


except

for refrigerated condensers

to C H 3 O H

3

• N H reactor system similiar

H >

§

M

W

5«

H W

w

w

26.

STILES


363


Casale R e c i r c u l a t i o n Unit

The second t y p e s y n t h e s i s u n i t i s the modern recycling unit and i s known as the C a s a l e . The C a s a l e r e c y c l e u n i t i s shown i n F i g . 5 and c o n s i s t s o f a l o o p i n which t h e r e i s a gas m i x t u r e c o m p r i s i n g a p p r o x i m a t e l y 10% CO, 3-5% carbon d i o x i d e and t'ne remainder hydrogen. The gas p a s s e s i n t o the r e a c t o r (which w i l l l a t e r be d e s c r i b e d ) then emerges from the r e a c t o r i n t o a heat r e c o v e r y u n i t and t h e r e a f t e r a condenser; the c o o l e d gases e n t e r a methanol s e p a r a tor from which the methanol i s withdrawn a t the bottom and the u n r e a c t e d gas d i s c h a r g e s a t the t o p . The u n r e a c t e d gases pass t o the r e c y c l e pump s u c t i o n but on the way a t e e w i t h a v a l v e p e r m i t s the w i t h drawal o f a c e r t a i n amount o f u n r e a c t e d gases t o p r e v e n t the b u i l d - u p o f e x c e s s i v e q u a n t i t i e s o f i n e r t gases. The r e c y c l e pump d i s c h a r g e s gas a t a s u f f i c i e n t l y h i g h p r e s s u r e t h a t i t w i l l a g a i n e n t e r the reactor. However, b e f o r e e n t e r i n g the r e a c t o r , makeup gas ( t o compensate f o r a l c o h o l formed and gas purged) i s added as the s t o i c h i o m e t r i c c o m p o s i t i o n , 33% CO, 67% hydrogen. A problem r e l a t i n g t o t h i s system i s the f a c t t h a t l a r g e q u a n t i t i e s o f gas must be r e c i r c u l a t e d a t h i g h e x p e n d i t u r e s o f energy. Consequently, i t is e s s e n t i a l t o have as much r e a c t i o n as p o s s i b l e o c c u r per pass. A second, and r e l a t e d problem, i s the o b v i o u s need t o minimize p r e s s u r e drop through the converter. T h i s need i s t r a n s l a t e d i n t o o p t i m i z i n g c a t a l y s t bed depth and diameter and p a r t i c l e s i z e and shape of the catalyst while o b t a i n i n g maximum approach t o e q u i l i b r i u m , F i g . 6. Reactor

Types

Claude. The r e a c t o r s used w i t h the Claude type u n i t are b a s i c a l l y a c y l i n d r i c a l column o f c a t a l y s t whereas r e a c t o r t y p e s used i n the C a s a l e u n i t are much more complex. Multitubular, The o l d e r type i s the m u l t i t u b u l a r r e a c t o r shown i n F i g u r e 7. Although t h i s i s mentioned as the o l d - t y p e because i t was used decades ago, i t has r e c e n t l y come back i n t o f a v o r i n the Lurgi units. I n s t e a d o f the i n t e r n a l heat exchanger s e r v i n g as a gas p r e h e a t e r , the r e a c t o r tubes are surrounded w i t h a l i q u i d which removes the heat and can be c y c l e d through a b o i l e r f o r heat r e c o v e r y . There are many problems w i t h t h i s r e a c t o r i n t h a t the



364


PURGE GAS TO BURN UNREACTED GAS H + SOME CO + CO2 + CH4+ N

CONDENSER

2

2

METHANOL CONVERTOR

CIRCULATING COMPRESSOR

SEPARATOR

CRUDE CH3OH Ho + ~ 1 0 % CO + INERTS

N , C H , ETC. 2

4

M A K E - U P GAS ~ 2 H : CO OR C 0 : H EQUIVALENT AT SYNTHESIS PRESSURE 2

Figure 5 . C a s a l e MeOH S y n t h e s i s

2

2

Loop.




to GTS

366



T C

4

Γ"

Crude Product 8 Unreacted Gas

5 \ J

Π3

Ο Ο Ο

ο

•Catalyst

ο ο ο ο ο ο ο ο ο ο ο ο ο ο ο ο

Inside Surface of Pressure Shell

Feed Gas Cold Shot Gas Bypassing Internal Heat Exchanger Figure 7 .

Typical

Tubular Synthesis Reactor.



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tubes tend t o become warped and heat exchange between tubes becomes i n f e r i o r . The major problem i s the s e a l i n g o f the tubes a t the tube s h e e t s . Improperly s e a l e d tubes may r e s u l t i n gas p a s s i n g through t h e s e poor s e a l s from the c a t a l y s t s i d e o f the tube i n t o the b o i l e r w i t h a p o s s i b l e over p r e s s u r i n g o f the boilers. The second r e a c t o r t o be d e s c r i b e d i s the t r a y t y p e , F i g u r e 8, which a l l o w s the c a t a l y s t t o be put on i n d i v i d u a l t r a y s a t c e r t a i n depths throughout the reactor. In the u s u a l d e s i g n o f t h i s r e a c t o r c o l d s y n t h e s i s gases are added between t r a y s t o d r i v e the r e a c t i n g gases t o a lower temperature f a v o r i n g a better equilibrium. The most r e c e n t c a t a l y s t c o n v e r t e r (the Wentworth p r o c e s s , F i g u r e 9 ) , i s y e t t o be i n s t a l l e d i n an o p e r a t i n g p l a n t but i t i s a n t i c i p a t e d t h a t w i t h i n the next few y e a r s a t l e a s t two p l a n t s employing t h i s system w i l l be i n o p e r a t i o n . This i s also a tray type system but i t i s d e s i g n e d i n such a way t h a t c a t a l y s t s most e f f i c i e n t f o r o p e r a t i o n a t g i v e n tempe r a t u r e s , are charged on each t r a y w i t h the i n i t i a l catalyst b e i n g low temperature, the next c a t a l y s t b e i n g medium temperature, the t h i r d c a t a l y s t b e i n g h i g h temperature. T h e r e a f t e r , the gas i s removed from the c o n v e r t e r and heat i s r e c o v e r e d and the gas i s r e t u r n e d t o the c o n v e r t e r a t a temperature i d e n t i cal t o t h a t p e r t a i n i n g when the gas went i n t o the f i r s t tray of c a t a l y s t s . The gas p r o c e e d s from the f i r s t t r a y t o the second t r a y which has the temperat u r e and c a t a l y s t c h a r a c t e r i s t i c s o f the second t r a y i n the f i r s t series. T h e r e a f t e r , the gas i s a g a i n c o o l e d and heat r e c o v e r e d and the gas then passes back i n t o the c o n v e r t e r f o r a low temperature pass o v e r the c a t a l y s t o f the type i n t r a y s 1 and 4. This f a v o r s low temperature e q u i l i b r i u m and h i g h c o n v e r s i o n and thus minimizes r e c y c l i n g and i s r e p o r t e d by the advocates as r e d u c i n g energy consumption by app r o x i m a t e l y 30%. C a t a l y s t L i f e Then and

Now

F i g u r e 10 shows c a t a l y s t l i f e o f the e a r l y opera t i o n s and the p r e s e n t o p e r a t i o n s which are g e n e r a l l y set f o r a p p r o x i m a t e l y two y e a r s b e f o r e r e c h a r g e . The remarkable e f f e c t o f c a t a l y s t and p r o c e s s d e v e l o p ments can r e a d i l y be seen.



368


Figure 8. Tray Type S y n t h e s i s C o n v e r t e r .


26.

369


STILES


1 T°C

C, CATALYST

T°C + 6 0 °

C

2

CATALYST

T°C+I20°

C

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e

T C

e

T C+60°

C, CATALYST

— ι — C

2

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Γ T°C

HEAT RECOVERY

C, CATALYST

HEAT EXCHANGE

HEAT EXCHANGE

0±b τί.

Figure 9.

P r e f e r r e d Example o f New

R e a c t o r System.



370 HETEROGENEOUS CATALYSIS


26.

STILES


Gas

371


P u r i t y Then and Now

One o f t h e i n i t i a l problems w i t h a l c o h o l s y n t h e sis i s t h e f a c t t h a t gases were r e l a t i v e l y impure. T h i s r e q u i r e d the use o f h i g h temperatures and r e l a tively crude c a t a l y s t s . These f u n c t i o n a t 450 whereas over t h e y e a r s t h e t r e n d has been toward lower temperature and more a c t i v e c a t a l y s t s which a r e a l s o more temperature s e n s i t i v e . P r e s e n t temperature t r e n d s a r e toward a p p r o x i m a t e l y 250 C, p e r m i s s i b l e when the s y n t h e s i s gas p u r i t y i s e x c e l l e n t as i n t h e case p r e s e n t l y i n d i c a t e d by F i g u r e 11 p l o t t i n g t h e sulfur level. O p e r a t i n g P r e s s u r e Then and Now I n i t i a l l y , the r e a c t o r s i n t h e days o f t h e e a r l y methanol and ammonia s y n t h e s e s o p e r a t e d a t a p p r o x i mately 1000 atmospheres o r 15,000 p s i . T h i s , over the y e a r s , has been reduced t o 12,000, then t o 5,000 and then t o 750 b u t p r e s e n t l y t h e t r e n d i s back up toward 2,500 p s i ( F i g u r e 1 2 ) . I t i s l i k e l y that, when s i n g l e l i n e 5,000 o r 10,000 t o n p e r day p l a n t s are c o n s t r u c t e d , the optimum p r e s s u r e w i l l be i n the range o f 2,500 t o 4,000 p s i . The economics o f p r e s sure and s i z e a r e a l l c l o s e l y r e l a t e d t o many f a c t o r s such as c a p i t a l investment, r e a c t o r s i z e , s y n t h e s i s gas g e n e r a t i n g p r e s s u r e , i n t e r c o n n e c t i n g p i p i n g s i z e , method o f g e n e r a t i n g s y n t h e s i s gas, and problems o f maintenance. Methanol

P r i c e s Then, Now and i n t h e F u t u r e

F i g u r e 13 shows the p r i c e o f methanol over t h e l a s t 55 y e a r s . The p r e s e n t p r i c e quoted i s about 72 c e n t s but spot market p r i c e can be as low as f i f t y eight to sixty cents. P r o d u c t Uses and F o r e c a s t s f o r t h e F u t u r e The uses o f methanol in o r d e r o f volume a r e as f o l l o w s :

relative

descending

Formaldehyde Dimethyl t e r e p h t h a l a t e Solvents Methyl h a l i d e s Methyl amines Methyl m e t h a c r y l a t e I n h i b i t o r f o r formaldehyde Acetic acid MTBE, F u e l s - i n c r e a s i n g r a p i d l y


372



,

0

n

Gas Purity in ppm S

Figure 11. Gas P u r i t y i n ppm

S.

Figure 12. O p e r a t i n g P r e s s u r e Then and

Now.


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374

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CATALYSIS

Capacity

The annual p r o d u c t i o n c a p a c i t y f o r U.S., Canada and New Zealand i s t o become a p p r o x i m a t e l y 2,500 m i l l i o n g a l l o n s i n 1983. T h i s i s an i n c r e a s e o f about 140% i n s i x y e a r s . This capacity i s approxi mately 6.5 days e q u i v a l e n t o f g a s o l i n e consumption i n the U.S. a n n u a l l y . I t i s e v i d e n t t h a t i f one o f o u r l i q u i d f u e l s i s t o be methanol i n any major p r o p o r t i o n , o r d e r s o f magnitude i n c r e a s e s i n c a p a c i t y w i l l be c a l l e d f o r . C u r r e n t r e s e a r c h and road t e s t s o f a u t o m o b i l e s f u e l e d w i t h 100% methanol a r e v e r y en c o u r a g i n g both e c o n o m i c a l l y and o p e r a t i o n a l l y . This i s a s e p a r a t e and l e n g t h y s u b j e c t . Literature 1. 2. 3. 4. 5. 6. 7.

Cited

A. Mittasch and C. Schneider, German Patent 293, 787, 1913 (U.S. Patent 1,128,804; Feb. 16, 1915). H. Fischer and H. Tropsch, German Patent 411,216; 1922. A. Mittasch, M. Pier and K. Winkler, German Patent 441, 433; 1923. A. Mittasch, M. Pier and Κ. Winkler, European Patent 299, 714; March 26, 1925. W. K. Lewis and P. K. F r o h l i c h , Ind. and Eng. Chem. 1928, 20, 287. W. L a z i e r , U . S . Patent 1,829,046; Oct. 27, 1932. A. B. S t i l e s , A. I . Ch. Ε . J., 1977, 23, 362.

RECEIVED January 25, 1983


Heterogeneous Catalysis - American Chemical Society

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