Methanol production

Scheme of the industrial methanol synthesis from synthesis gas

The production of methanol is a large-scale industrial process that is carried out in several variants for the production of methanol from synthesis gas . The chemist Matthias Pier succeeded 1923, the industrial production of methanol from synthesis gas at high pressure to zinc oxide - chromium oxide - catalysts . Until then, methanol was only obtained by dry distillation of wood . In addition to ammonia , another basic product of industrial chemistry was thus accessible through high pressure processes. The fluidized bed process developed by Winkler for the gasification of fine-grained lignite provided the required synthesis gas on a large scale.

history

Methyl phenyl ether group in coniferyl alcohol

Methanol was first obtained by dry distillation of wood . As recently as 1930, 50% of the methanol in the USA was obtained by this process. To do this, wood is heated to around 500 ° C in iron containers. The methyl-phenyl-ether groups of the coniferyl and sinapyl alcohol units present in the lignin split with the absorption of water in methanol and the phenolic residue. Charcoal is obtained as a solid residue , the gaseous and liquid products are drawn off and partially condensed. In addition to methanol, the resulting aqueous distillate mainly contains acetone , acetic acid and methyl acetate . The separation of these components and the final drying required several neutralization, distillation and drying steps. The yield of methanol in the dry distillation was about 10% of the mass of the starting product.

From 1913 onwards, BASF chemists experimented with the hydrogenation of carbon monoxide at pressures of around 100 bar and temperatures of 350 ° C. As products, they received a mixture of pure hydrocarbons and oxidation products such as alcohols , aldehydes and ketones . Mittasch, Pier and Winkler resumed this work in 1923 and developed a process from it that delivered pure methanol at pressures of 200 to 300 bar and temperatures of 350 to 400 ° C.

Raw material

Heinrich Koppers

The production of synthesis gas for the production of methanol takes place today mainly through steam reforming or the partial oxidation of natural gas and the gasification of coal. In North America and Europe, natural gas is mostly used as a raw material; in China and South Africa, synthesis gas production is based on coal or lignite. Depending on the carbon monoxide to hydrogen ratio, the products are called water gas (CO + H ₂ ), synthesis gas (CO + 2 H ₂ ) or cracked gas (CO + 3 H ₂ ).

Steam reforming of natural gas

The process of allothermal catalytic conversion of natural gas , which consists predominantly of methane , with steam to carbon monoxide and hydrogen is called steam reforming. The reaction (1) of methane and water to form carbon monoxide and hydrogen is endothermic , which means that energy must be supplied so that the process in the direction of carbon monoxide formation takes place. Before the reaction, all catalyst poisons such as hydrogen sulfide must be removed, for example using the Rectisol process . The methane gas is preheated to around 420 to 550 ° C and passed through tubes impregnated with nickel.

${\displaystyle (1)\ \mathrm {CH} _{4}+\mathrm {H} _{2}\mathrm {O} \ \rightleftharpoons \ \mathrm {CO} +3\;\mathrm {H} _{2};\ \Delta H(300\;\mathrm {K} )=206\;\mathrm {kJ} /\mathrm {mol} ,}$

Since four molecules are formed from two molecules according to equation (1), a low pressure favors this reaction according to the Le Chatelier principle , a pressure increase favors the reverse reaction . According to equation (2), the water-gas shift reaction , the carbon monoxide reacts further with water vapor, so that in addition to carbon monoxide, carbon dioxide is also formed.

${\displaystyle (2)\ \mathrm {CO} +\mathrm {H} _{2}\mathrm {O} \ \rightleftharpoons \ \mathrm {CO} _{2}+\mathrm {H} _{2};\ \Delta H(300\;\mathrm {K} )=-41{,}2\;\mathrm {kJ} /\mathrm {mol} .}$

High temperatures favor the shift of the so-called Boudouard equilibrium to the left, which is between carbon monoxide on the left, carbon dioxide and carbon on the right:

${\displaystyle (3)\ 2\;\mathrm {CO} \ \rightleftharpoons \ \mathrm {CO} _{2}+\mathrm {C} ;\ \Delta H(300\;\mathrm {K} )=-172{,}6\;\mathrm {kJ} /\mathrm {mol} .}$

Partial oxidation of natural gas

In the case of partial oxidation, a substoichiometric mixture of natural gas and oxygen is partially oxidized in a reformer according to equation (4), with synthesis gas being produced.

${\displaystyle (4)\ \mathrm {2\ CH_{4}+O_{2}\longrightarrow 2\ CO+4\ H_{2};\ \Delta H(300\;K)=-35,7kJ/mol} }$

The process can be carried out purely thermally at temperatures between 1200 and 1800 ° C. and pressures from 30 to 80 bar or catalytically using platinum or rhodium on alumina catalysts. The temperature range of the catalytic process is between 800 and 900 ° C at normal pressure. A disadvantage of the process is the need for pure oxygen, which requires prior air separation and thus has a decisive influence on energy costs. In addition, in contrast to steam reforming, the hydrogen obtained is of purely fossil origin, the inexpensive hydrogen source water is not used.

Autothermal reforming of natural gas

By combining steam reforming and partial oxidation, the efficiency of the overall process can be optimized. By precisely metering oxygen and water, the yield of synthesis gas can be maximized while reducing external energy costs. The process is carried out on an industrial scale, for example as the Lurgi-Combined Reforming Process, with which the world's largest methanol plant of the Atlas Methanol Company in Trinidad is operated in 2004 with a plant capacity of 5000 tons per day.

Coal gasification

Principle of the Winkler generator

The coal gasification is carried out as an autothermal process with water vapor and air. The energy for the endothermic water gas reaction is supplied by burning part of the coal. Lime or basic alkali salts are added to the coal to reduce the sulfur content in the gas .

${\displaystyle (5)\ \mathrm {C\ +\ H_{2}O\ \rightleftharpoons \ CO\ +\ H_{2}} ;\ \Delta H(300\;\mathrm {K} )=+119\ \mathrm {kJ/mol} \,}$

The desired carbon monoxide to hydrogen ratios are set by means of the water-gas shift reaction according to equation (2). The carbon dioxide produced is separated off using, for example, ethanolamine .

Depending on the type of coal to be processed, various coal gasification techniques have become established. One of the oldest processes is fluidized bed gasification in the Winkler generator developed by Fritz Winkler . The process is suitable for lignite and is carried out in the temperature range from 850 to 1100 ° C.

In the Lurgi pressurized gas process , coal or lignite is gasified in counterflow at increased pressure . The temperatures must be below 1000 ° C in order to avoid liquefaction of the ash . This process produces valuable by-products in large amounts of hydrocarbons such as naphtha, oils, tar and phenols.

In the Koppers-Totzek process , which is suitable for all types of coal, the coal is gasified in cocurrent at high temperatures of 1500 to 1600 ° C. The coal must be ground very finely for this. Due to the high temperatures, no methane or higher hydrocarbons are produced.

Procedure

The production of methanol can be divided into the steps of synthesis gas production, raw methanol production and processing of the raw methanol. The synthesis gas can be obtained from a number of different fossil and renewable raw materials such as coal, lignite , heavy petroleum fractions , garbage , peat , wood , biogas or sewage sludge . Biomass can be gasified using the biomass-to-liquid (BTL) process. According to current knowledge, the gasification of giant Chinese reeds has a positive net energy balance per hectare of cultivation area, which is around 4.4 times as high as the production of biodiesel from rapeseed oil . In the case of processes that produce an excess of hydrogen, the additional use of carbon dioxide as a carbon source is possible. The steam reforming and the partial oxidation of natural gas , according to current estimates, the largest economically usable hydrocarbon source is next to the coal the main supplier of syngas.

In addition to the investment costs , the decisive factors for the selection of the process are the efficiency , the energy requirement and the carbon dioxide emissions both for the provision of the raw material and for the actual process. The efficiency for the production of methanol is between approx. 40% for entrained flow gasification of coal and 70% for processes based on natural gas.

The processes for producing methanol from synthesis gas are classified according to the reaction pressure. A distinction is made between three pressure areas. The high pressure process, which is no longer used today, works at pressures of 250 to 350 bar and temperatures of 360 to 380 ° C. The medium pressure process uses a pressure of 100 to 250 bar at a temperature range of 220 to 300 ° C and the low pressure process is carried out at a pressure of 50 to 100 bar and temperatures between 200 and 300 ° C. Each process works with special catalysts and carbon monoxide to hydrogen ratios.

The original high-pressure process was based on a zinc oxide-chromium oxide mixed catalyst. The catalyst components were present in a molar ratio of approximately 2.3 to 1. The catalyst was not particularly active and required high temperatures and pressures, but was relatively insensitive to catalyst poisons such as sulfur compounds. At the time of the development of the high-pressure process in the early twenties, these could not yet be separated on an industrial scale. The temperature in the catalyst bed therefore had to be controlled within narrow limits, since above a temperature of around 380 ° C the catalyst components form a spinel phase and lose their activity as a result. The catalyst was used in fixed beds supported by internals. Cold gas could be fed in between the beds to control the temperature. The conversion was about 10 to 15% per pass. For further processing, the methanol was separated off and some of the gas was returned to the process after the inert components had been separated off.

Today, methanol is produced on an industrial scale from synthesis gas using the low or medium pressure process. The resulting raw methanol is partially contaminated with by-products. If the raw methanol is used for combustion in the energy sector, the purity of the raw methanol is sufficient. For further processing in the chemical industry, the methanol has to be worked up by distillation. Low-boiling components such as dimethyl ether are separated off in a low-boiling column. The higher-boiling fractions are separated off as bottoms in a further distillation stage in a high-boiling column, with methanol being drawn off at the top.

Copper-zinc oxide-aluminum oxide catalysts are mainly used for the production of methanol from synthesis gas, with newer generations of the catalyst being doped with cesium. The catalysts have a relatively long service life, which is usually several years. Decreasing activity, which is usually caused by catalyst poisons such as sulfur, halogens or iron pentacarbonyl , can be compensated for for a while by increasing the reaction pressure. Elevated temperatures also lead to irreversible damage to the catalyst due to sintering of the active copper species.

catalysis

The following equations can be formulated for the formation of methanol from carbon monoxide, carbon dioxide and hydrogen:

${\displaystyle (6)\ \mathrm {CO} +2\;\mathrm {H} _{2}\rightleftharpoons \ \mathrm {CH} _{3}\mathrm {OH} ;\ \Delta H(300\;\mathrm {K} )=-90{,}8\;\mathrm {kJ} /\mathrm {mol} ,}$

${\displaystyle (7)\ \mathrm {CO} _{2}+3\;\mathrm {H} _{2}\rightleftharpoons \ \mathrm {CH} _{3}\mathrm {OH} +\mathrm {H} _{2}\mathrm {O} ;\ \Delta H(300\;\mathrm {K} )=-49{,}6\;\mathrm {kJ} /\mathrm {mol} .}$

Both of these reactions are exothermic. According to the principle of Le Chatelier, low temperatures and a pressure increase lead to a shift of the equilibrium to the right.

Copper-zinc oxide-aluminum oxide catalysts are used for the synthesis of methanol. These are made by co-precipitation from copper, zinc and aluminum hydroxycarbonates. Further steps are washing, aging, drying, calcination and activation by reduction in a hydrogen / nitrogen stream, whereby the copper oxide produced during the calcination is reduced to the metal. The zinc oxide is in the wurtzite structure. As separate individual components, the compounds are not catalytically active.

According to kinetic data, methanol synthesis on copper-zinc oxide-aluminum oxide catalysts proceeds according to a Langmuir-Hinshelwood mechanism . Both carbon monoxide and hydrogen adsorb on the catalyst surface and react quickly to form a surface-bound formyl species. The rate-determining step was the reaction of the methoxy species formed by further hydrogenation with adsorbed hydrogen to form methanol. The formation of a surface-bound formyl species was detected by infrared spectroscopy. Some percentage of carbon dioxide in the feed gas is beneficial for the reaction and at a concentration of about 1% carbon dioxide the activity is highest. The exact function of the carbon dioxide has not been conclusively clarified.

Kinetic investigations led to the formulation of a rate equation of the type:

r = kp _CO ^{0.2 - 0.6} p _{H 2} ^0.7 Φ _{CO 2}

the optimal partial pressure ratio of CO ₂  : CO being 0.01 to 0.03.

Conduct of proceedings

Low pressure process

After processes were known to minimize the sulfur content in the synthesis gas, the companies ICI and Lurgi advanced the development of low-pressure processes based on copper-zinc oxide-aluminum oxide catalysts. The contact is established by impregnation with the metal salts and then reduced. The copper is present in contact as metallic copper . The copper recrystallizes at higher temperatures and thus becomes catalytically inactive. Precise compliance with the reaction temperatures is therefore of great importance for the service life of the catalyst. Dimethyl ether , methyl formate and ethanol are obtained as by-products of the reaction in the ppm range and are distilled off. The steam generated by the reactor cooling during the process is used.

The reactor types used in the low-pressure process must allow effective heat dissipation in order to maintain the catalyst activity. Lurgi used a fixed-bed tube bundle reactor in which the catalyst bed is located in tubes surrounded by boiling water. The tube diameter was designed so that the temperature in the catalyst bed can be kept constant at about 5 to 10 ° C. The resulting steam is used to drive the compressors or to distill the raw methanol.

Medium pressure process

The medium-pressure processes use both copper-zinc oxide-alumina catalysts, such as the low-pressure process, and catalysts based on oxidic copper-chromium-zinc or chromium-zinc catalysts. The disadvantage of the higher pressures is offset by higher space-time yields . A corresponding process was developed, for example, by the Vulcan company, which works at a pressure of 150 to 250 bar and a temperature range of 220 to 300 ° C. with a Cu-Zn-Al catalyst, or by the Haldor-Topsoe company.

Letterpress process

In a methanol plant, between six and eight million kilojoules of heat must be dissipated per cubic meter of catalyst. The high demands on the temperature control in the catalyst bed have led to a number of process variants. The heat dissipation is realized through various reaction engineering processes. Direct feeding in of cold quench gas, intermediate cooling of the products after partial conversion or direct cooling of the catalyst bed by means of cooling coils built into the bed are possible.

Furthermore, attempts were made to combine the production of methanol with processes for generating energy. One of these processes is the liquid phase methanol process (LPMeOH process). It is based on a catalyst oil slurry through which synthesis gas from coal gasification is passed. At around 235 ° C, methanol forms in the reactor, which leaves the reactor in vapor form together with the unconverted synthesis gas. The presence of the catalyst in oil-suspended form means that the temperature of the catalyst slurry can be controlled within narrow limits. The process was investigated on the initiative of the United States Department of Energy with the aim of converting the synthesis gas produced in power plant processes into methanol outside of peak demand times. The methanol can be stored and later used again to generate energy. So far, the process has only been implemented in a test facility.

Products

→ Main article : methanol

Methanol is a widely used solvent, energy carrier and raw material for the chemical industry. It is a colorless, neutral, polar liquid that can be mixed with water and many organic solvents in any ratio. Inorganic salts also dissolve in methanol. Methanol is flammable and poisonous.

variants

Processes are intensively investigated in which methane is oxidized directly to methanol without the detour via the synthesis gas. Such a process would eliminate the costly steam reforming step. Since the methyl group in methanol is easier to oxidize than in methane, selective oxidation is difficult at high conversions. Mixed metal oxide catalysts, such as those based on molybdenum and bismuth, were investigated. To avoid further oxidation, pure oxygen is used in a high methane excess. The experiments so far indicate that the direct oxidation of methane to methanol takes place according to a Mars-van-Krevelen mechanism via lattice oxygen. However, the selectivities and conversions achieved are still too low for an economical implementation of the process.

Another possible way is the direct esterification of the resulting methanol with strong acids. The esters are less sensitive to further oxidation. The methanol is released in a further step by saponification . Experiments with platinum, cobalt, palladium, copper and mercury salts as catalysts in sulfuric or trifluoroacetic acid were successful, but with low conversions. Working up the esters has proven to be uneconomical; no process has yet been developed up to commercial use.

Methanol can be produced from methane with the aid of methane monooxygenase using enzyme catalysis . Methanotrophic bacteria , such as Methylococcus capsulatus , use this reaction as an energy source when methane is the only food source. A large-scale implementation of this reaction path is currently not being investigated.

The chemist and Nobel laureate George Olah developed a process according to which methanol is produced from carbon dioxide and water by supplying an electric current as the reverse of the reaction occurring in fuel cells; this makes it possible to “recycle” CO ₂ . The current problem is to achieve the necessary high current densities for large-scale production.

The further development of the known synthesis gas and methanol processes seems to be promising. Japanese scientists have developed a catalyst with a two to three times higher conversion than conventional catalysts in the conversion of carbon dioxide and hydrogen into methanol. The process requires cheap hydrogen, e.g. B. by electrolysis of water with ideally climate-neutral electricity from renewable energies, arguably nuclear power, or in the long term possibly fusion energy generated electricity. Biotechnical or photocatalytic processes for hydrogen production, as well as the sulfuric acid-iodine cycle in connection with process heat (e.g. from high-temperature nuclear reactors such as the dual fluid reactor ) are conceivable as future prospects.

literature

• F. Asinger : Methanol, chemical and energy raw material . Akademie-Verlag, Berlin, 1987, ISBN 3-05-500341-1 .
• E. Supp: How to Produce Methanol from Coal . Springer-Verlag, 1990, ISBN 0-387-51923-8 .
• JG Speight: Synthetic Fuels Handbook: Properties, Process, and Performance . Mcgraw-Hill Professional Publishing, 2008, ISBN 0-07-149023-X , ISBN 978-0-07-149023-8 .
• Martin Bertau, Heribert Offermanns , Ludolf Plass, Friedrich Schmidt, Hans-Jürgen Wernicke: Methanol: The Basic Chemical and Energy Feedstock of the Future: Asinger's Vision Today , 750 pages, Verlag Springer; 2014, ISBN 978-3-642-39709-7

Web links

Wiktionary: Methanol  - explanations of meanings, word origins, synonyms, translations

Commons : Methanol  - collection of pictures, videos and audio files

• Methanol Institute, Arlington (English)

Individual evidence

1. ↑ Peter Klason, Gust. V. Heidenstam, Evert Norlin: Investigations on charring. In: Journal for Applied Chemistry. 23, 1910, p. 1252, doi: 10.1002 / ange.19100232702 .
2. ^ Walter Fuchs: Die Chemie des Lignins , Springer Verlag, ISBN 978-3-642-89726-9 , pp. 151-162.
3. ^ ^{A b} Friedrich Asinger: Chemistry and technology of paraffin hydrocarbons . Akademie Verlag, 1956, pp. 71-75.
4. ^ Rectisol underwear. (No longer available online.) In: linde-le.com. Formerly in the original ; Retrieved January 8, 2010 .  ( Page no longer available , search in web archives )  
5. ^ Reforming . In: lurgi.com . Archived from the original on October 18, 2006. Retrieved January 8, 2010.
6. ↑ The ecological balance of biofuels: Energy versus biotopes (PDF; 103 kB) Ministry for the Environment, Climate and Energy, Baden-Württemberg . Archived from the original on January 10, 2014. Retrieved January 9, 2010.
7. ↑ L. Guczi (Ed.): New Trends in CO Activation . Elsevier, Amsterdam 1991, ISBN 978-0-08-088715-9 , pp. 268 ( limited preview in Google Book search).
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10. ↑ JC Lavalleya, J. Sausseya and T. Raïsa: Infrared study of the interaction between CO and H2 on ZnO: Mechanism and sites of formation of formyl species , in: Journal of Molecular Catalysis , 1982 , 17  (2-3), p 289-298 ( doi: 10.1016 / 0304-5102 (82) 85040-2 ).
11. ↑ methanol . In: linde-process-engineering.com . Archived from the original on October 15, 2006. Retrieved January 8, 2010.
12. ↑ Michael Felleisen: Process control technology for the process industry . Oldenbourg Industrieverlag, 2001, ISBN 3-486-27012-5 , p. 112 ( limited preview in Google Book search).
13. ↑ Haldor Topsoe - Processes . www.topsoe.com. Retrieved January 8, 2010.
14. ↑ Evalyn Mae C. Alayon, Maarten Nachtegaal, Marco Ranocchiari, Jeroen A. van Bokhoven: Catalytic conversion of methane to methanol using Cu Zeolites . In: Chimia . tape 66 , no. 9 , 2012, p. 668-674 , doi : 10.2533 / chimia.2012.668 , PMID 23211724 .
15. ↑ W. Choi, J. Park, M. Kim. H. Park and H. Hahm: Catalytic Partial Oxidation of Methane to Methanol , in: Journal of Industrial and Engineering Chemistry , 2008 , 7  (4), pp. 187-192.
16. ^ Martin Bertau, Heribert Offermanns , Ludolf Plass, Friedrich Schmidt, Hans-Jürgen Wernicke: Methanol: The Basic Chemical and Energy Feedstock of the Future: Asinger's Vision Today , 750 pages, Verlag Springer; 2014, ISBN 978-3-642-39709-7 , p. 52.
17. ↑ Kevin Bullis: Methanol instead of hydrogen. In: Technology Review. July 2006.
18. ↑ Patent DE69808983 : Methanol synthesis and reforming catalyst consisting of copper, zinc and aluminum. Published August 7, 2003 , Inventors: Hideo Fukui, Masayuki Kobayashi, Tadashi Yamaguchi, Hironori Arakawa, Kiyomi Okabe, Kazuhiro Sayama, Hitoshi Kusama. ‌