2 Methanol: Manufacture and Uses
Downloaded by NORTH CAROLINA STATE UNIV on November 14, 2012 | http://pubs.acs.org Publication Date: June 15, 1981 | doi: 10.1021/bk-1981-0159.ch002
THOMAS F. KENNEDY and DEBORAH SHANKS Celanese Chemical Co., Inc., Dallas, TX 75247
This paper covers the current technology of methanol production, reviews how the energy crisis and the escalation of hydrocarbon feedstocks impact that technology, and describes conventional, new, and potential uses for methanol. Methanol is a chemical intermediate and solvent produced from several feedstocks and is consumed in a variety of end uses. Prior to the development of a synthetic route to methanol, commercial quantities were obtained from the destructive distillation of wood or other biomass. Now, with interest focused on conservation and the use of renewable resources, methods have been proposed to use biomass again as a methanol feedstock. While some of these proposals hold great appeal, they are still speculative and beyond the scope of this paper. (Methanol from wood is discussed in the following chapter.) Methanol P r o d u c t i o n Methanol i s produced by the c a t a l y t i c r e a c t i o n of carbon monoxide and hydrogen ( o r synthesis gas) i n a converter according to the r e a c t i o n : CH3OH CO + 2H Figure 1 d e p i c t s a t y p i c a l methanol synthesis scheme 0
[FEEDSTOCK
SYNGAS PRODUCTION 1 HEAT RECOVERY |Figure 1.
»| COMPRESSION
METHANOL SYNTHESIS
• PURIFICATION OF CRUDE METHANOL
Methanol synthesis summary
Synthesis Gas. There are three p r i n c i p a l routes to synthes i s gas: steam reforming, p a r t i a l o x i d a t i o n , and c o a l g a s i f d c a -
0097-6156/81/015 9-0019$05.00/ 0 © 1981 American Chemical Society In Monohydric Alcohols; Wickson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
Downloaded by NORTH CAROLINA STATE UNIV on November 14, 2012 | http://pubs.acs.org Publication Date: June 15, 1981 | doi: 10.1021/bk-1981-0159.ch002
20
MONOHYDRIC ALCOHOLS
t i o n . Steam reforming i s l i m i t e d t o l i g h t e r hydrocarbon feeds t o c k s , p r i m a r i l y methane and naphtha. P a r t i a l o x i d a t i o n can use not only those l i g h t feedstocks, but a l s o heavier feedstocks l i k e r e s i d u a l o i l . Coal g a s i f i c a t i o n , a p p l i c a b l e t o a l l types of c o a l , a n t h r a c i t i c to l i g n i t i c , i s an emerging technology w i t h s e v e r a l competing processes (JL). Thus a v a r i e t y of hydrocarbons, ranging from n a t u r a l gas to c o a l , are used i n methanol production. Regardless of the feedstock used to prepare the synthesis gas, i t i s necessary to remove s u l f u r so that the converter c a t a l y s t i s not poisoned. Before n a t u r a l gas or naphtha i s reformed, the feedstock i s d e s u l f u r i z e d . In the p a r t i a l o x i d a t i o n and c o a l g a s i f i c a t i o n processes, the feedstock i s f i r s t o x i d i z e d and the r e s u l t i n g synthesis gas i s d e s u l f u r i z e d before e n t e r i n g the converter. Methanol S y n t h e s i s . By whatever means produced, s y n t h e s i s gas i s then compressed f o r feed to the converter. There are two routes f o r methanol s y n t h e s i s , a high pressure process r e q u i r i n g compression to about 340 atmospheres, and a low pressure process r e q u i r i n g compression i n the range of 50 t o 100 atmospheres (2,2A). Because of the inherent economic advantages of the low pressure technology, high pressure u n i t s are i n the process of being phased out or converted to low pressure. I C I and L u r g i have the two p r i n c i p a l low pressure technologies p r a c t i c e d today f o r methanol s y n t h e s i s . They d i f f e r p r i m a r i l y i n the technique used to remove the heat of r e a c t i o n and thus c o n t r o l temperatures w i t h i n the converter. This d i f f e r e n c e r e s u l t s from converter design. Converter e x i t gas c o n t a i n i n g methanol i s cooled by heat exchange with c o o l i n g water. The condensed methanol and water mixture i s then separated. This crude methanol i s p u r i f i e d i n a two or three column d i s t i l l a t i o n . The f i r s t column separates l i g h t ends from methanol. The second column separates methanol from water and f u s e l o i l s . I f very low ethanol content i s r e q u i r e d , a t h i r d column can be used f o r f i n a l p u r i f i c a t i o n . Impact of Energy C r i s i s . The two p r i n c i p a l steps i n methan o l p r o d u c t i o n , s y n t h e s i s gas p r e p a r a t i o n and methanol s y n t h e s i s , have been g r e a t l y impacted by the c o n t i n u i n g energy c r i s i s . Synthesis gas production has been i n f l u e n c e d by the r e l a t i v e value of i t s hydrocarbon feedstocks, and methanol s y n t h e s i s by the improvements i n the energy balances of the newer low pressure technologies. Most s y n t h e s i s gas production f o r methanol i s based on steam reforming of n a t u r a l gas. U n t i l r e c e n t l y , many u n i t s outside the U.S. u t i l i z e d naphtha as a feedstock. Steam reforming of naphtha produces a s^mthesis gas c o n t a i n i n g hydrogen and carbon i n a r a t i o c l o s e t o t h e o r e t i c a l requirements f o r methanol s y n t h e s i s . As the p r i c e of crude o i l has skyrocketed, the p r i c e of naphtha has f o l l o w e d and made naphtha uneconomic versus n a t u r a l gas PS P inet^anol feed. In the U.S. n a t u r a l gas has always been the economic choice because of the
In Monohydric Alcohols; Wickson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
Downloaded by NORTH CAROLINA STATE UNIV on November 14, 2012 | http://pubs.acs.org Publication Date: June 15, 1981 | doi: 10.1021/bk-1981-0159.ch002
2.
KENNEDY
21
Methanol Manufacture and Uses
A N D SHANKS
r e l a t i v e abundance of reserves that e x i s t e d on the Gulf Coast. While the energy c r i s i s has caused a tremendous increase i n the p r i c e of n a t u r a l gas, the even more dramatic increase i n world o i l p r i c e s makes n a t u r a l gas the economic choice f o r feedstock over naphtha. However, steam reforming of n a t u r a l gas produces a hydrogen to carbon oxides r a t i o of about 3:1. To s a t i s f y the s t o i c h i o m e t r i c requirements f o r the methanol synthesis r e a c t i o n , an e x t e r n a l source of carbon, u s u a l l y carbon d i o x i d e , can be added to the feed, and/or excess hydrogen can be purged from unreacted synthesis gas and used as a f u e l f o r the reformer. P a r t i a l o x i d a t i o n has more r e c e n t l y a t t r a c t e d a t t e n t i o n because of i t s a b i l i t y to u t i l i z e the l e a s t valuable p o r t i o n of the crude o i l b a r r e l (3). P a r t i a l o x i d a t i o n of r e s i d u a l o i l generates synthesis gas w i t h a hydrogen to carbon oxides r a t i o of about 1:1. To adjust the synthesis gas to the required composition, a p o r t i o n of the gas stream i s sent to a s h i f t converter where CO and water are converted to hydrogen and C 0 according to the water gas s h i f t r e a c t i o n : CO + H 0 — • C 0 + H The carbon d i o x i d e i s removed before r e t u r n i n g the hydrogen t o the make up gas stream. The e s c a l a t i o n of o i l p r i c e s has caused even r e s i d u a l o i l p r i c e s to r i s e to a point where c o a l has a t t r a c t e d i n t e r e s t as a feedstock f o r synthesis gas i n methanol production. Several p l a n t s now e x i s t i n other parts of the world based on c o a l g a s i f i c a t i o n . Coal i s g a s i f i e d i n the presence of oxygen and steam at high temperatures. Like synthesis gas prepared from r e s i d u a l o i l , the hydrogen to carbon oxides r a t i o i s about 1:1 and must be adjusted to a higher r a t i o u t i l i z i n g the water gas s h i f t r e a c t i o n . Table I summarizes the feedstock, process, and hydrogen:carbon oxides r a t i o of the competing feedstocks. 2
2
2
2
TABLE I METHANOL FEEDSTOCKS
FEEDSTOCK
SYNGAS MANUFACTURE PROCESS
HYDROGEN:CARBON OXIDES RATIO
1.
NATURAL GAS
STEAM REFORMING
3:1
2.
NAPHTHA
STEAM REFORMING
2:1
3.
RESIDUAL FUEL OILS
PARTIAL OXIDATION
1:1
4.
COAL
GASIFICATION
1:1
The energy p i c t u r e f o r the f u t u r e remains cloudy, and the r e l a t i v e costs of competing feedstocks d i f f i c u l t to p r o j e c t w i t h
In Monohydric Alcohols; Wickson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
22
MONOHYDRIC
ALCOHOLS
any c e r t a i n t y . I n c r e a s i n g l y i t appears that coal w i l l be the long term economic choice f o r synthesis gas feedstock. Yet, i n those areas of the world where n a t u r a l gas i s i n excess supply, gas remains the more a t t r a c t i v e feedstock f o r s y n t h e s i s gas i n methanol production. The c a p i t a l costs are much lower. The hydrogen:carbon oxides r a t i o i s b e t t e r f o r methanol s y n t h e s i s , and the s u l f u r removal step i s minimized. Oxygen i s not r e q u i r e d , and the s o l i d s handling problem i s e l i m i n a t e d . For these reasons, i t i s l i k e l y that methanol p l a n t s w i l l be b u i l t o u t s i d e the U.S. i n the 1980 s i n areas of excess n a t u r a l gas s u p p l i e s , and methanol from these p l a n t s w i l l supply part of U.S. demand. As p r e v i o u s l y s t a t e d , the high pressure process to convert synthesis gas to methanol i s being phased out i n favor of the low pressure process because of the l a t t e r * s inherent economic advantages. These advantages i n c l u d e lower c a p i t a l c o s t s , lower energy requirements and lower maintenance c o s t s . The f i r s t commercial synthesis of methanol had been c a r r i e d out i n a reactor at a pressure of about 340 atmospheres. High pressure processes use a zinc/chromium c a t a l y s t that i s rugged and poison r e s i s t a n t . However, these c a t a l y s t s e x h i b i t poor a c t i v i t y , n e c e s s i t a t i n g high temperatures (325°-375° C) and pressures f o r commercial use ( 4 ) . Most e x i s t i n g u n i t s are low pressure processes or are being converted to low pressure technology. Reduction i n s y n t h e s i s pressure r e q u i r e s a r e d u c t i o n i n temperature. Lower temperatures n e c e s s i t a t e a more a c t i v e c a t a l y s t . The a c t i v i t y and s e l e c t i v i t y of copper based c a t a l y s t s f o r methanol s y n t h e s i s had been known p r i o r to commercial u t i l i z a t i o n ( 2 ) . The stumbling block, preventing e a r l i e r use of the c a t a l y s t , i s the s e n s i t i v i t y of the c a t a l y s t to s u l f u r . This has been solved by the development of methods to remove s u l f u r from n a t u r a l gas before i t i s reformed ( 4 ) . Since the f i r s t low pressure methanol plant was s t a r t e d up i n 1966 ( 2 ) , most new p l a n t s have been of that design, and the energy c r i s i s has r e s u l t e d i n the ongoing conversion of the remaining high pressure u n i t s to the low pressure process.
Downloaded by NORTH CAROLINA STATE UNIV on November 14, 2012 | http://pubs.acs.org Publication Date: June 15, 1981 | doi: 10.1021/bk-1981-0159.ch002
f
Current A p p l i c a t i o n s Methanol has long been an important item of commerce, r e s u l t i n g from the a v a i l a b i l i t y of low cost raw m a t e r i a l s coupled w i t h the development and refinement of an e f f i c i e n t s y n t h e t i c process. Despite having been d i r e c t l y and g r e a t l y impacted by the ongoing energy c r i s i s , methanol remains a r e l a t i v e l y inexpens i v e s o l v e n t and chemical intermediate w i t h a myriad of uses. While many of these uses are mature w i t h only minimal growth f o r e c a s t , newer end uses continue to be commercialized, spurred by economic f a c t o r s , namely a v a i l a b i l i t y and low c o s t . Today, over three b i l l i o n g a l l o n s of methanol are produced and consumed i n the world a n n u a l l y w i t h the U.S. accounting f o r n e a r l y one
In Monohydric Alcohols; Wickson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
Downloaded by NORTH CAROLINA STATE UNIV on November 14, 2012 | http://pubs.acs.org Publication Date: June 15, 1981 | doi: 10.1021/bk-1981-0159.ch002
2.
KENNEDY
A N D SHANKS
Methanol Manufacture and Uses
23
t h i r d of the t o t a l . Growth i s f o r e c a s t to approximate twice the r a t e of growth i n GNP (5). Conventional uses of methanol account f o r 90% o f present consumption and i n c l u d e formaldehyde, dimethyl t e r e p h t h a l a t e , methyl methacrylate, methyl h a l i d e s , methylamines and various solvent and other a p p l i c a t i o n s . Newer uses f o r methanol that have r e v i t a l i z e d i t s growth and outlook i n c l u d e a new technology f o r a c e t i c a c i d , s i n g l e c e l l p r o t e i n , methyl t e r t i a r y b u t y l ether-(MTBE), and water d e n i t r i f i c a t i o n . P o t e n t i a l uses f o r methanol include i t s use as a c a r r i e r f o r coal i n p i p e l i n e s , as a source of hydrogen or synthesis gas used i n d i r e c t reduction of i r o n ore, as a d i r e c t a d d i t i v e to or a feedstock f o r g a s o l i n e , peak power shaving and other f u e l r e l a t e d p o s s i b i l i t i e s . Table I I l i s t s the world methanol demand by end use i n 1979. The l a r g e s t and o l d e s t chemical intermediate use f o r methanol i s formaldehyde. Over h a l f of the methanol c u r r e n t l y consumed i n the world goes i n t o formaldehyde production. Formaldehyde i s produced by the c a t a l y t i c o x i d a t i o n or the o x i d a t i v e dehydrogenation of methanol* The major o u t l e t f o r formaldehyde i s amino and phenolic r e s i n s . These r e s i n s are i n turn used i n the manufacture of adhesives f o r wood products, molding compounds, binders f o r thermal i n s u l a t i o n and foundry r e s i n s . Formaldehyde i s a l s o consumed i n the production of a c e t a l r e s i n s , p e n t a e r y t h r i t o l , neopentyl g l y c o l , t r i m e t h y l o l p r o p a n e , methylenediphenyldiisocyanate (MDI), and textile treating resins. Dimethyl t e r e p h t h a l a t e (DMT) i s produced e i t h e r by the e s t e r i f i c a t i o n of t e r e p h t h a l i c a c i d or the e s t e r i f i c a t i o n of monoraethyl t e r e p h t h a l a t e produced by o x i d a t i o n of methyl p - t o l u a t e . DMT i s consumed i n the production of polyethylene t e r e p h t h a l a t e , the polymer used i n the manufacture of p o l y e s t e r f i b e r s , f i l m s and b o t t l e r e s i n s . Terephthalic a c i d (TPA) i s a l s o used i n the production of polyethylene t e r e p h t h a l a t e but does not consume methanol. Since TPA i s c o n t i n u i n g t o increase i t s share of the market, DMT i s expected t o e x h i b i t slower growth than the o v e r a l l market f o r polyethylene t e r e p h t h a l a t e . Methyl methacrylate, accounting f o r 4% of methanol consumption, i s produced by the cyanohydrin process u t i l i z i n g methanol. Methyl methacrylate i s used to produce a c r y l i c sheet, surface coating r e s i n , and molding and e x t r u s i o n powder. A l s o , there e x i s t minor miscellaneous uses such as m o d i f i c a t i o n of a c r y l i c f i b e r and p o l y e s t e r r e s i n . Methanol consumed i n the production of methyl h a l i d e s and methylamines accounts f o r 8% of consumption. Methyl c h l o r i d e i s made by the r e a c t i o n of h y d r o c h l o r i c a c i d and methanol. Methylene c h l o r i d e and chloroform are produced by c h l o r i n a t i n g methyl c h l o r i d e . Methylamines are produced by c a t a l y t i c a l l y r e a c t i n g ammonia w i t h methanol. Methyl c h l o r i d e i s used i n the production of s i l i c o n e s and tetramethyl l e a d . Methylene
In Monohydric Alcohols; Wickson, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
Downloaded by NORTH CAROLINA STATE UNIV on November 14, 2012 | http://pubs.acs.org Publication Date: June 15, 1981 | doi: 10.1021/bk-1981-0159.ch002
24
MONOHYDRIC
ALCOHOLS
TABLE I I
WORLD METHANOL DEMAND 1979
END USE
Formaldehyde
PERCENT OF TOTAL METHANOL CONSUMPTION
52
Dimethyl Terephthalate
4
Methyl Methacrylate
4
Methyl Halides and Methyl Amines
8
Acetic Acid
6
MTBE
4
Single C e l l Protein Solvents Miscellaneous
