Methanol Synthesis

Supplementary information for the course: Catalysis, Theory and Applications J a n u a r y 2 0 0 2 / P a u l C. C. J . Ka Ka m e r a n d G a d i R o th th e n b e r g

METH ANOL ANOL SYNTHE SYNTHE SIS

comp iled led by E. K. K. Poel Poelss a n d D. S. Bran ds

1 0 . 1 I n t r o d u c t i on on Catalytic processes for the synthesis of methanol have existed since the twenties. At fir s t h ig h p r e s s u r e p r o c e s s e s w e r e u s e d , w h i ch c h w e r e in in t r o d u c e d b y BA BAS F i n 1 9 2 3 . Th Th e r a w m a t e r i a l fo fo r t h e s e p r o c e s s e s , c a l le le d s y n t h e s is g a s , w a s a m i x t u r e o f C O , H 2 a n d frequ en tly also CO 2 . Th Th e m e t h a n o l s y n t h e s i s r e a c t io io n s a r e h ig h ly ly e xo xo t h e r m i c : CO + 2H 2 CO2 + 3 H2

➔ ➔

CH3 OH C H 3 OH + H 2 O

∆H 2 9 8

= - 9 0. 0 . 8 kJ kJ / m o l 9 . 6 kJ kJ / m o l ∆H 2 9 8 = - 4 9.

The catalysts used were based on ZnO-Cr 2O3 compounds and the process was o p e ra r a t e d a t t e m p e r a t u r e s b e t we we e n 3 0 0 a n d 4 0 0 °C a n d a t p r e s s u r e s b e t w e e n 2 5 0 a n d 350 bar. This is the well-known high T h e z in in c - c h r o m it e c a t a ly s t h a d high press ure p r o c e s s . Th a very high resistance to catalyst poisoning, especially towards sulphur, which was q u i t e a b u n d a n t i n t h e e a r l y c o a l - b a s e d s y n t h e s i s g a s e s . L a t e r , I C I d e v e l o p e d t h e low  pressure p r o c e s s , w h i c h w a s m a d e p o s s i b l e b y t w o d e v e l o p m e n t s w h i c h t o o k p l a c e simultaneously. It had become possible to produce large amounts of pure synthesis gas, essentially free of poisons like sulphur and chlorine and a better catalyst was d i s c ov ove r e d : t h e c o m b i n a t i o n C u / Zn O w h ic h w a s c le le a r l y m u c h m o r e a c t iv ive . Th Th e p r o b le le m w i t h t h is C u - c a t a ly s t w a s it s s e n s i t iv ivit y u n d e r p r o c e s s c o n d i t io io n s . I t b e c a m e possible during the mid-sixties to produce a catalyst that was stable under proc ess conditions by using support materials like Al 2 O 3 a n d C r 2 O 3 . W i t h t h e l o w p r e s s u r e process the synthesis takes place at pressures of 50-100 bar and temp eratures of  2 2 0 - 2 8 0 °C . M o d e r n m e t h a n o l s y n t h e s i s p la la n t s u s e a Cu C u / Zn O / Al 2 O 3 catalyst. Reproducible preparation of this catalyst is very difficult, as the catalyst precursor must consist of  s e v e r a l p h a s e s , o f w h i ch ch t h e m i x ed ed m e t a l h y d r o xy xy ca ca r b o n a t e s r o s a s it e a n d h y d r ot ot a l c it it e are the most important for achieving the desired activity and stability. A high catalyst activity activity is relate relate d to a h igh igh copp er su rface rface a rea or sm all crysta llite lite size com com bined with with in t i m a t e c o n t a c t w i t h t h e z in in c p r o m o t e r . Th Th e d e c r e a s e in a c t i vi vit y is is c o n n e c t e d w it it h t h e loss of copper surface area by crystal growth, although the re lationship is not unambiguous. Th e s y n t h e s is g a s n e e d e d fo fo r t h e p r o c e s s c a n b e p r e p a r e d in in s e ve ve r a l wa wa y s . In In t h e p a s t steam reforming of naphtha was frequently used. Another possibility was the partial oxidation of vacuum residues. Nowadays most synthesis gas is produced by reacting

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methane with steam. Natural gas and the gas which is released in oil production are particularly cheap raw materials. The synthesis gas is obtained by steam reforming (over a Ni/  α- Al2 O 3 cata lyst at 1 100 K): CH4 + H2 O

➔

CO

+ 3 H2

Methanol is an important chemical with a world-wide production volume of some 14 million tons per annum. It is mainly used as a raw material for variou s chemical products, of which formaldehyde is the most important. Methanol is also frequently u s e d in t h e p r o d u c t io io n o f e n e r g y c a r r ie ie r s a n d a d d i t iv ive s f or or t r a n s p o r t a t i on on fu e l. l. O n e o f   the increasing uses for methanol is in the production of methyl tertiary butyl eth er (M TB TB E ) w h i ch ch is u s e d t o in in c r e a s e t h e o c t a n e n u m b e r o f le a d fr e e g a s o li li n e . A s u m m a r y of th e ap plications plications of met h an ol is giv given en in tab le 10.1: Applications ons of m ethan ol T a b le le 1 0 . 1 : Applicati _____________________________________ ___________________ _____________________________________ _________________________________ ______________ en ergetic (6%) n on -en ergetic (94 %) _____________________________________ ___________________ _____________________________________ _________________________________ ______________ - met h an ol ad ditives ditives in gasolin gasolin e (1-2%) - forma ldeh yde (52%) - en gin gin e fu el (1%) - ace tic acid (6%) - m et e t h yl yl t er e r t ia ia r y b u t yl yl e th t h e r (M (M TB E) E )(4 %) %) - di d im e t h yl yl t er e r ep e p h t h a la la t e( e (4 %) %) - raw ma terial for synt h etic fu el - met h yl h alides alides (4%) - methyl amines (4%) - methyl methacrylate (4%) - solvent (8%) - other s (8%) _____________________________________ ___________________ _____________________________________ _________________________________ ______________

10.2 Mechanism The kinetics of the synthesis of methanol has been studied for a large range of  catalysts, temperatures and pressures. An attempt has been made t o clarify the m e c h a n is m b y p o s t u la t i n g a s e r ie ie s o f r e a c t i on on s t e p s (a d s o r p t io io n , r e a c t i on on , d e s o r p t i on on ) from which a theoretical rate equation could be derived. Comparing the theoretical e q u a t io io n w it it h e x p e r i m e n t a l d a t a w o u l d t h e n g iv ive n a n e x p e r im im e n t a l r a t e e q u a t i on on . T h e mechanisms deduced from these kinetic studies can be subdivided into three groups (see also fig. 10.1): I Carbon Carbon dow n hydrogenation in which first a formyl complex [HCO] is formed w h i ch ch is b o n d e d t o a n M -s -s i t e b y t h e C - a t o m ; O x y g e n d ow ow n h ydrogen II ydrogen ation in wh ich format e [HCOO [HCOO - ] is initially formed which is t h e n b o n d e d t o a n M -s -s i t e b y O ; III ydrogen ation in wh ich ich ad sorbe d forma forma ldeh yde [HCHO] [HCHO] is formed formed . Side on h ydrogen The representation of the active site as "M" is a simplification as disagreement exists in lit e r a t u r e c o n c e r n i n g t h e n a t u r e a n d c h a r g e o f t h e c o p p e r (o (o r n o b l e m e t a l) s p e c i e s involved.

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Formyl and formate are found as intermediates in various investigations, whe reas formaldehyde is only rarely identified. It is not simple to prove which mechanism is correct. Mixed reaction pathways have been suggested as well (see below). It is even p o s s i b le le t h a t a g en en e r a lly va va l id id m e c h a n is m d o e s n o t e x is is t . I carbon down hydrogenation:

H

formyl

O

H H

C

O

H

O

H H

H

C

M

M

M

O

C

H

C

H

H

M + CH3OH

M

II oxygen down hydrogenation: hydrogenation:

formate H

H O-

CO

M

4H

C O - O M

C H3

H

O-

O-

+ H 2O

M

M

H-

H2

M III side on hydrogenation: hydrogenation:

O 2H C

O M

or

C

+ CH3OH

H C

O

H

2H

+ CH OH 3

M + CH3OH

M

M

formaldehyde

Fig. 10 .1:

S u m m a r y o f p ro ro po po s e d m e t h a n o l s y n t h e s is is m e c h a n is is m s .

An a t t e m p t h a s b e e n m a d e t o e lu lu c id id a t e t h e m e c h a n is m fo r Rh Rh / TiO 2 b y u s i n g a 1 3 1 6 1 2 1 8 mixture of  C O and C O. If methanol formation were to take place via d i s s o c ia ia t i ve ve a d s o r p t io io n t h e n t h e is is o t o p e d is is t r i b u t i o n i n t h e p r o d u c t m i x t u r e w o u l d b e : 1 3 CH 1 6 O H 3 1 3 CH 1 8 O H 3 1 2 CH 1 8 O H 3 1 2 CH 1 8 O H 3

25% 25% 25% 25%

If dissociation of CO does not take place and O-scrambling therefore also does not o c cu c u r , t h e d is t r i b u t i o n w o u l d b e : 1 3 CH 1 6 O H 3 1 2 CH 1 8 O H 3

50% 50%

As t h e l a t t e r is is t h e c a s e a d s o r p t i on on is n o n - d is is s o c ia ia t i ve ve , w h i ch c h is i n a g r e e m e n t w it it h t h e h ypothes es of fig. ig. 10 .1.

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Th e o x y g e n d ow in c o r r e c t b e c a u s e O - s c r a m b l in in g w o u l d t h e n a l s o oc oc c u r . ow n r o u t e i s a l s o in In t h e gr gr o u p : H C O

- O

M

the two O-atoms are equal and therefore it is unlikely that O-scrambling does not t a k e p la c e . Th is investigation investigation was condu cted with with Rh-cat alysts. It It is is n ot a priori c e r ta ta i n t h a t C u o r ZnO based catalysts would operate according to the same mechanism as noble metal catalysts do. Mechanism I is probably correct for the noble metal catalysts which form relatively strong metal-carbon bonds (Pt, Pd, Rh). In addition it is known from single crystal studies that CO adsorbs onto these metals with the carbon side towards t he metal. On the other hand, from fundamental studies the most likely adsorption mode of CH 3 O - a p p e a r s t o b e o x y g e n d o w n . F o r t h i s r e a s o n s o m e a u t h o r s s u g g e s t a t r a n s i t io io n fr fr o m m e c h a n is m I t o a n o x yg yg en en d o wn wn p a t h w a y a t a n i n t e r m e d ia ia t e s t a g e . F or o r Zn Zn O a n d Zn Zn O / C r 2 O 3 catalysts mechanism II is thought to be more likely. The large dipole moment and relatively irregular electron distribution of oxides make it more plausible for CO to adsorb via its O atom. The formate sp ecies has been identified during both methanol synthesis and decomposition over these cataly sts. O n e o f t h e i m p o r t a n t s y s t e m s a p p li lie d in i n in d u s t r y, y , C u / Zn O / Al 2 O 3 i s a l s o t h o u g h t t o belong to this group. Th e g r o u p a t IC IC I Ka t a lc o Re Re s e a r c h p r o p o s e s a m e c h a n i s m o ve ve r C u / Zn O / Al2 O 3 c a t a l ys ys t s w h e r e C O 2 is t h e p r e d o m i n a n t r e a g e n t i n t h e s y n t h e s i s ga ga s . C O in in t h e i r vi vi e w only serves to keep the copper surface in the correct, reduced zerovalent state, forming CO 2 i n t h e p r o c e s s . T h u s C O 2 a n d H 2 a d s o r b o n t o t h e C u t o f o r m a f o r m a t e species, which is progressively hydrogenated towards a methoxy-group and subsequently to methanol. This oxygen down type mechanism leaves little room for the promoting role of the ZnO, other than keeping the copper in a well-d ispersed state. The current hypothesis of the ICI group seems to be that Zn exists as a Zn -H hydride under reaction conditions, somehow supplying hydrogen for the reaction by spill-over. Other possible structures for the active site include small epitax ial Cu c lu lu s t e r s o n t o p o f t h e C u - Z n - O m i x ed e d o x id id e ; c op op p e r c a t io io n s o n a s i m il ila r m a t r ix ix ; a d u a l Cu-Zn site. It seems the debate will continue for some time. 1 0 . 3 C o m m e r c ia ia l p r o c e s s e s A scheme of the synthesis loop of the process is given in Fig. 10.2. Synthesis gas is mixed with recycled unreacted gas, pressurised in the compressor until the operational pressure is reached, then preheated in a heat exchanger until a

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temperature close to the reaction temperature is obtained and subsequently fed into t h e s y n t h e s i s r e a c t or or . The formation of methanol is determined thermodynamically. Depending on the p r o c e s s c o n d i t io io n s u s e d , t h e p r o d u c t g a s c o n t a i n s 4 - 8 v ol o l - % m e t h a n o l . A la r g e p a r t o f   the heat of condensation of methanol is transferred to the reactor feed in a feedeffluent heat-exchanger before the gas is cooled to room temperature in a water cooler. Approx. 95% of the methanol condenses here; separation of the gas phase takes place in a separator. After passing through the recycle compressor the gas is mixed with fresh synthesis gas and subsequently re-introduced into the reacto r. A fixed percentage of the gas leaving the separator is continuously purged. This is to p r e v e n t i n e r t g a s e s t h a t d o n o t c o n d e n s a t e ( m a i n l y N 2 a n d C H 4 ) from building up in t h e s y n t h e s is l oo oo p .

make-up syngas

220°C

vent recycle

separator CH3OH

Fig. 10 .2:

cold shot

compressor

heat exch.

condenser

to distillation

270°C reactor

Schem e of a methan ol sy nthes is loop. loop.

The crude methanol is purified by means of distillation. In the topping c ol o lu m n g a s e s and impurities with a low boiling point are removed; a second refining c o l u m n removes the heavier products and water. At present methanol is produced almost exclu exclu sively sively via th ree processes : th e ICI ICI,, th e Lu Lu rgi an d th e Mitsu Mitsu bish i process es. These processes differ mainly in their reactor designs and the way i n which the p r o d u c e d h e a t i s r e m o ve ve d , s e e F i g. g. 1 0 . 3 . The ICI design consists of a number of adiabatic catalytic beds, and cold gas is used t o c o ol ol t h e r e a c t a n t g a s e s b e t w e e n t h e b e d s . Th Th i s g iv ive s r is e t o t e m p e r a t u r e p r o fi file s in th e reactor as illus illus tra ted in Fig. Fig. 10.4, in in wh ich also the concen tra tion tion profil profile of th e r e a c t a n t s a n d t h e p r o d u c t s a r e i n d ic ic a t e d . Th Th e h ig h e s t t e m p e r a t u r e i s r e a c h e d i n t h e fir s t c a t a ly s t b e d . Th Th e Lu r g i a n d M it it s u b i s h i r e a c t o r s h a ve a m u c h f la la t t e r t e m p e r a t u r e profi profile, as th e react or is is a lmost isoth erm al owing owing to its its cooling cooling on on th e sh ell side by th e generation of valuable high-pressure steam. As a result of the flat temperature profile, catalyst deactivation will be lower. However, in both processes the cat alyst still d e a c t i va va t e s , w h i ch c h m e a n s t h a t t h e p r o d u c t i vi vit y d e c r e a s e s a n d t h e p la la n t c a p a c i t y is n o longer met. Fortunately, the catalyst productivity is a function of the process UNIVERSITEIT

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pressure, so increasing the process pressure can compensate for the loss in activity. On doing so the plant can produce its design value over longer periods (2-3 years) . No wa wa d a y s la la r g e m e t h a n o l p la la n t s e x c lu lu s i ve ve ly ly u s e c e n t r i fu fu g a l co co m p r e s s o r s t o b r i n g t h e synthesis gas to the desired operating pressure, which varies from 70 to 100 bar, depending on the activity of the catalyst, the lower pressure being used with fr esh c a t a l ys ys t a n d t h e h i gh gh e s t p r e s s u r e a t t h e e n d o f t h e c a t a l ys ys t l if ife t im im e . SYNGA

SYNGA

HIGH PRESSURE

COOLING WATER

ICI

LURGI MITSUBISHI

F i gu gu r e 1 0 . 3 :

Some poss ible ble d es igns of a m ethanol sy nthes is reactor eactor.

F i gu gu r e 1 0 . 4 :

Temperature, Temperature, m ethanol and carbon carbon m onoxide/ onoxide/ carbon carbon d ioxide oxide  profi  profilles in an ad iab atic atic reactor. reactor.

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