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bioweb.sungrant.org » Technical » Bioproducts » Bioproducts from Syngas » Methanol Derivatives

Products from Syngas—Methanol Fuels and Derivatives	Printer Friendly

In its simplest form, syngas (also called producer gas, town gas, blue water gas, and synthesis gas among others) is composed of carbon monoxide (CO) and hydrogen (H2), which provide the building blocks to produce a number of organic compounds. During the 1800’s, coal gasification was used for lighting and heating. The production of fuels and chemicals from syngas began in the early 20th century. Today, methanol and ammonia are produced from syngas, and in addition to hydrogen, constitute its major uses.    In principle, syngas can be produced from any hydrocarbon feedstock, including natural gas, naphtha, residual oil, petroleum coke, coal, and biomass. Under today’s conditions, the least expensive feedstock is natural gas. Using numerous synthesis pathways, a large number of organic compounds can be produced from syngas (figure 1).          Methanol (CH3OH) synthesis is a well-developed commercial catalytic process (equations 1-3) with high activity and very high selectivity (>99%) (Rostrup-Nielsen, 2000). Today, world methanol production is almost exclusively (90%) produced from syngas derived from the steam reforming of natural gas (Davenport, 2002). However, a variety of other feedstocks could be used, including biomass resources.   (equation 1)  CO + 2H2 à CH3OH               ΔHr = -90.64 kJ/mol   (equation 2)  CO2 + 3H2 à CH3OH + H2O     ΔHr = -49.67 kJ/mol   (equation 3)  CO + H2O à CO2 + H2           ΔHr = -41.47 kJ/mol     Most commercial methanol production from syngas uses copper based (Cu) catalyst formulations (predominately Cu/ZnO/Al2O3). The crude methanol (which contains up to 18% water, other alcohols, and other organic compounds) is purified using distillation. Several companies produce catalysts for methanol from syngas and a number of companies produce methanol.   Methanol (CH3OH) can be used as a transportation fuel, either pure (M100) or as an 85% blend (M85), or transformed to diesel, ethanol, or gasoline. But currently, its major use is as a key building block (intermediate) chemical that is used to produce other industrial organic chemicals such as formaldehyde (35% of global methanol production), methyl tert-butyl ether (MTBE) (25% of production), acetic acid (9% of production), and the remainder for other chemicals such as dimethyl ether, methyl amines, methyl halides (Davenport, 2002).

Formaldehyde
Formaldehyde is combined with phenol, urea, or melamine to make resins used in the manufacture of various construction board products, and its demand is driven by the construction industry. Formaldehyde is produced by dehydrogenating (equation 4) and partially oxidating (equation 5) methanol (Chen, 1995; Ruf, 1999).   (equation 4)   CH3OH à H2CO + H2     ΔHr = +84 kJ/mol   (equation 5)   CH3OH + ½ O2 à H2CO + H2O      ΔHr = -159 kJ/mol   Commercial production of formaldehyde uses one of three industrial processes (Reuss, 2003). The BASF process involves partial oxidation and dehydrogenation with air in the presence of a silver crystals, steam, and excess methanol at a temperature of 680-720°C. The conversion rate of methanol to formaldehyde is 97-98%. A variation of the BASF process uses crystalline silver or silver gauze at a temperature of 600-650 °C. The product is distilled and the unreacted methanol recycled. The conversion rate for this process is 77-87%. The third process (the Formox process) involves oxidation with a lean methanol-air mixture in the presence of a modified iron/molybdenum/ vanadium oxide catalyst [Fe2(MoO4)3] at a temperature of 250-400°C. The methanol conversion for this process is 98-99%. This reaction is more exothermic than the silver (Ag) process and heat removal is required (Satterfield, 1991). Excessive reactor temperatures cause volatilization of molybdenum oxide, which reduces the selectivity of the process. A high oxygen partial pressure is required to maintain catalyst activity. Catalyst activity is also lost in the presence of excess methanol. The estimated costs of production for each process is summarized in table 1 (Reuss, 2003). The BASF process is the most economical due to a liquid recirculation system that produces excess steam while simultaneous saving cooling water. For comparison, the price of formaldehyde given in Chemical Marketing Reporter for January 28th 2003 is $463/tonne ($0.21/lb).

Methyl tertiary-butyl ether (MTBE)
MTBE is used primarily (95%) as an oxygenate in gasoline, with North America consuming about 65% of global production. U.S. consumption of MTBE increased substantially following the Clean Air Act Amendments of 1990 but recent environmental concerns are leading states to phase out use of the additive. MTBE is also used in the petrochemical industry to produce isobutene, methacrolein, methycrylic acid, isoprene, and industrial solvents (Peters, 2003).   MTBE is produced by reacting isobutene with methanol in the presence of an acidic catalyst (equation 6)     (equation 6)  iso-C4H8 + CH3OH à (CH3)3COCH3       ΔHr = -37 kJ/mol       The reaction occurs in a liquid phase at temperatures of 30-100°C and pressure at 7-14 atm (100-200 psig) (Schädlich, 2003). Different catalysts can be used including solid acids, zeolites (H-ZSM-5), and macroporous sulfonic acid ion-exchange resins (e.g., Amberlyst-15) (Collignon, 1999). A molar excess of methanol is used to increase isobutene conversion and inhibit its dimerization and oligamerization. Conversion yields of up to 90% can be achieved at optimum reaction conditions.   Most commercial reactors are similar and consist of a reaction and a refining section. Commercial processes have been developed by Snamprogetti and Hüls (now Oxeno), Arco, IFP, CDTECH (ABB Lummus Crest and Chemical Research Licensing), DEA (formerly Deutsche Texaco), Shell (Netherlands), Phillips Petroleum, and Sumitomo (Peters, 2003). MTBE is produced by refinery/petrochemical plants using by-product isobutylene, by merchant plants which isomerize n- and iso-butane and dehydrogenate isobutane to isobutylene, and by tertiary butyl alcohol (TBA) plants which react by-product TBA from propylene oxide production with methanol (Lidderdale, 2001). The first is the cheapest way to produce MTBE at a cost of $6,000 - $10,000/BPD of capacity. The second is the most expensive at a cost of $20,000 - $28,000/BPD of capacity. Currently, there are over 140 MTBE plants with a total installed capacity of about 20 million tonnes/yr of which only 2 plants in the U.S. use the third manner to produce MTBE.   From 1980 to 2001, the average U.S. Gulf Coast price for MTBE fluctuated between $0.63/gallon (1998) and $1.27/gallon (1981) (Davenport, 2002).    Acetic Acid. The third most abundant chemical synthesized from methanol is acetic acid (equation 7).   (equation 7)   CH3OH + CO à CH3COOH                 Vinyl acetate (used to make latex emulsion resins for paints, adhesives, paper coatings, and textile finishing agents), acetic anhydride (used to make cellulose acetate fibers and cellulosic plastics), and terephthalic acid are all manufactured from acetic acid. Nearly one-half of the global production of acetic acid comes from methanol carbonylation (Wagner, 2002) and that share is expected to increase in the future (Cheung, 2003). In 1999, the world demand for acetic acid was 2.8 million tonnes (6.17 × 109 lb) (Wagner, 2002). The price of acetic acid was $992/tonne ($0.45/lb) (Chemical Marketing Reporter, January 28th 2003).   Two commercial processes are used to produce acetic acid from methanol. The BASF process uses a Co/iodine catalyst and process conditions of 250°C and 500-700 atm (3,000-10,000 psig), and 90% selectivity of acetic acid production from methanol. The Monsanto process uses a Rh/iodine catalyst [RhI2(CO)2] and process conditions of 180°C and 30-40 atm with over 99% selectivity. It is a liquid phase process initiated by the reaction of methanol with HI to yield methyl iodide. Insertion of the methyl iodide is the rate limiting step. Acetic acid is formed by the hydrolysis of the eliminated acetyl iodide (CH3COI) that also regenerates HI (King, 1985). The Monsanto process has displaced the BASF process and since 1973, all new installed capacity has used it (SRI, 1994). The catalytic system is very corrosive and requires expensive steel materials for construction. Complete recovery of the expensive Rh catalyst (~10-3 M) and recycling of the HI (~0.1 M) is paramount to maintain favorable process economics. The high cost of Rh has lead to the search for other, lower cost, metals that could be used as acetic acid catalysts.

Synthetic fuels
Methanol can be used to produce synthetic gasoline (the MTG process), olefins (the MTO process), and synthetic gasoline and diesel fuels (the MODG process).   The Methanol to Gasoline (MTG) process was developed by Mobil Oil Corporation and involves the conversion of methanol to hydrocarbons over zeolite catalysts (Keil, 1999). First, crude methanol (17% water) is super-heated to 300°C and partially dehydrated over an alumina catalyst at 27 atm to yield an equilibrium mixture of methanol, dimethyl ether, and water (75% of the methanol is converted). This effluent is then mixed with heated recycled syngas and introduced into a reactor containing ZSM-5 zeolite catalyst at 350-366°C and 19-23 atm to produce hydrocarbons (44%) and water (56%) (equations 8-10) (Hancock,1985; Wender,1996).   (equation 8)    2CH3OH à CH3OCH3 + H2O                                         (equation 9)    CH3OCH3 à C2-C5 olefins                                           (equation 10)  C2-C5 olefins à paraffins, cycloparaffins, aromatics          The overall MTG process usually contains multiple gasoline conversion reactors in parallel because the zeolites have to be regenerated frequently to burn off the coke formed during the reaction. The reactors are then cycled so that individual reactors can be regenerated without stopping the process, usually every 2-6 weeks (Kam, 1984). The selectivity to gasoline range hydrocarbons is greater than 85% with the remainder of the product being primarily liquid petroleum gas (LPG) (Wender, 1996). Nearly 40% of the gasoline produced from the MTG process is aromatic hydrocarbons consisting of 4% benzene, 26% toluene, 2% ethylbenzene, 43% xylenes, 14% trimethyl substituted benzenes, and 12% other aromatics (Wender, 1996). The shape selectivity of the zeolite catalyst results in a relatively high (3-5%) durene (1,2,4,5-tetramethylbenzene) concentration requiring distillation to reduce the level to less than 2% (MacDougall, 1991). This results in a high quality gasoline with a high octane number, but it also contains significant quantities of aromatics which are limited under the 1990 Clean Air Act Amendments (Owen, 1995).   The first commercial MTG plant began operation in 1985 in New Zealand (Mobil’s Motunui plant) and produced both methanol and high octane gasoline (14,500 BPD) from natural gas. In 1997, gasoline manufacturing was abandoned (presumably for economic reasons) and the plant now produces only methanol (2.43 million tonnes per year of chemical grade methanol for export when combined with the production of a nearby methanol plant at Waitara). http://www.techhistory.co.nz/pages/Petrochemical%20Decisions.htm. Additionally, a fluid bed MTG plant was jointly designed and operated near Cologne, Germany by Mobil Research and Development Corp., Union Rheinische Braunkohlen Kraftstoff AG and Uhde Gmb (Keil, 1999) and a demonstration plant (15.9 m3/day) operated from 1982 to 1985. Although, no commercial plants have been built, the fluid bed technology is ready for commercialization.   The estimated capital cost of the New Zealand MTG plant (14,450 BPD) was $767 million dollars (mid-1980) with a total project cost of $1,475 million dollars (1985) (Bem, 1985; Maiden, 1983). Production cost (including a return on investment) was estimated to be $0.96/gallon. The New Zealand government planned to sell the gasoline for $1.31/gallon (Bem, 1985). Initial studies performed prior to the design and construction of the plant estimated a gasoline price of $0.74/gallon (Bem, 1985). Table 2 summarizes other economic studies of the MTG process using coal as the gasification feedstock.        The MTG product has nearly the same emissions as gasoline from oil (Vermillion, 2001).   The methanol to olefins (MTO) and methanol to gasoline and diesel (MOGD) processes were also developed by Mobil to convert methanol to hydrocarbons based on zeolite catalysts. Since light olefins are intermediates in the MTG process, it is possible to optimize the methanol to olefins (MTO) synthesis. Higher reaction temperatures (~500°C), lower pressures, and lower catalyst (acidity) activity favors light olefin production (Keil, 1999). The rate of olefin production can be modified so that 80% of the product consists of C2 to C5 olefins rich in propylene (32%) and butenes (20%) with an aromatic rich C5+ gasoline fraction (36%) (MacDougall, 1991; Wender, 1996). The process can also be modified to produce high yields of ethylene and propylene (>60%).    In the methanol to gasoline and diesel (MOGD) process, olefins produced in the MTO process undergo further reactions (oligimerization, disproportionation and aromatization) to produce a gasoline product consisting of 3 wt% paraffins, 94 wt% olefins, 1 wt% napthenes, and 2 wt% aromatics (Tabak, 1990). The selectivity of gasoline and distillate from olefins is greater than 95% (Keil, 1999). A large-scale test run was performed at a Mobil refinery in 1981.   Neither the MTO nor the MOGD process is currently in commercial practice (Wender, 1996). However, UOP and HYDRO of Norway, licenses their own methanol to olefins process where the primary products are ethylene and propylene (Keil, 1999), http://www.uop.com/petrochemicals/processes_products/mto_intro.htm.  The process uses a fluidized bed reactor at 400 – 450°C and achieves an 80% carbon selectivity to olefins at nearly complete methanol conversion (Apanel, 2002). The operating parameters can be adjusted so that either more ethylene is produced (48 wt% ethylene, 31 wt% propylene, 9 wt% butenes and 1.5 wt% other olefins) or more propylene (45 wt% propylene, 34 wt% ethylene, 12 wt% butenes and 0.75 wt% other olefins) is produced.   The costs of producing methanol, olefins via MTO, and gasoline via MTG are estimated to be $0.45/gal ($0.68/lb), $0.16/lb, and $0.59/gal ($0.99/lb) respectively (Gradassi, 1998) for a natural gas price of $0.47/GJ, but the report provides few economic details.   The Topsoe Integrated Gasoline Synthesis (TIGAS) process (developed by Haldor Topsoe) sought to reduce the capital and energy costs of producing gasoline by integrating methanol synthesis with the methanol to gasoline (MTG) step into a single loop without isolating methanol as an intermediate (Keil, 1999; Topp-Jorgensen, 1987; Topp-Jorgensen, 1988). The process was designed for use in remote areas for the recovery of low cost natural gas. The Mobil MTG process uses different pressures for syngas production (15-20 atm; 221-294 psi), methanol synthesis (50-100 atm; 735-1470 psi), and the fixed bed MTG step (15-25 atm; 221-368 psi) (Wender, 1996). The TIGAS process involves modified catalysts and conditions such that the system pressure is the same eliminating the need for separate compression steps. This is accomplished by first producing a mixture of methanol and dimethyl ether (DME) so that only one recycle loop from gasoline synthesis back to MeOH/DME synthesis is used. A 1 MTPD demonstration plant was built in Houston, Texas in 1984 and operated for 3 years (Topp-Jorgensen, 1988). The gasoline yield for the TIGAS process, defined as the amount of gasoline produced divided by the amount of natural gas feed and fuel, was shown to be 56.5 wt% (Topp-Jorgensen, 1988). The TIGAS process, however, yields a lower quality gasoline with a lower octane number compared to MTG (MacDougall, 1991).   Dimethyl ether (DME). Commercial production of DME originated as a byproduct of high-pressure methanol production. It is used as the starting material to produce dimethyl sulfate which is used as an aerosol propellant. DME could also potentially be used as a diesel fuel or as a cooking fuel, a refrigerant, or a chemical feedstock (Gunda, 1995; Peng, 1999b; Shikada, 1999).    DME is produced by first synthesizing methanol from syngas (equation 11), and then dehydrating it (equation 12) using an acid catalyst (e.g., γ-alumina) at methanol synthesis conditions (Hansen, 1991; Peng,  1999a).   (equation 1)  CO + 2H2 à CH3OH                  ΔH = -21.6kcal/mole  (equation 2)   2CH3OH à CH3OCH3 + H2O    ΔH = -5.6 kcal/mole H2O + CO à H2 + CO2                                   ΔH = -9.8 kcal/mole  (water/gas shift) 3H2 + 3CO à CH3OCH3 +CO2                        ΔH = -58.6 kcal/mole (net reaction)   One product in each reaction is consumed by another reaction. Because of the synergy between the reactions, syngas conversion to DME gives higher conversions than syngas conversion to methanol (table 3) (Shikada, 1999).        The optimum H2:CO ratio for DME synthesis is lower than that for methanol synthesis and ideally should be around one (Peng, 1999a,b; Shikada, 1999).  Recent improvements to the DME synthesis process involve the development of bifunctional catalysts to produce DME in a single gas phase step (i.e., one reactor) (Ge, 1998; Peng, 1999b) and the use of a slurry reactor for liquid phase dimethyl ether (LPDME) synthesis (Brown, 1991; Sardesai, 1998).   At a natural gas costs of $0.6/MMBTU, DME can be produced for $0.67/gallon (Chaumette, 1999). In comparison, their estimate for methanol production at the same natural gas price is $0.35/gallon.

Methanol fuels (M100 and M85)
Neat methanol (M100) can be used as a transportation fuel in place of petroleum gasoline. It has a high heat of vaporization and relatively low heating value (about half of gasoline) that leads to a lower flame temperature compared to gasoline. This results in lower CO, NOx, and hydrocarbons emissions, but higher formaldehyde emissions. Methanol is miscible in water and in low concentration methanol mixtures, phase separation can occur. Methanol is more corrosive than gasoline, but less toxic and is not carcinogenic. Methanol was used as a gasoline blending agent prior to a mid-1980s EPA prohibition against it’s use in unleaded gasoline without the use of a cosolvent alcohol (Davenport, 2002) due to its poor solubility in gasoline and phase separation problems.   Currently, neat methanol is used as a fuel only in high-performance racing engines and airplanes that have been fully modified and adapted (i.e., replacement of plastic components in the fuel system, modified carburetor or fuel injection system, and preheating the fuel mixture) to operate on methanol (Fiedler, 2003).   M100 is being targeted for use in direct methanol fuel cells (DMFC) which allow the use of methanol as fuel without requiring a fuel processor to extract hydrogen from the methanol. Significant progress has been made on finding a better electrolyte material to prevent methanol crossover through the membrane. Operating temperatures of direct methanol fuel cells has increased to nearly 100°C. Improved anode catalysts have facilitated methanol oxidation and eliminated the need for an on-board hydrogen reformer. Several automakers have developed methanol powered fuel cell vehicles and currently, DaimlerChrysler and Honda both have prototype direct methanol fuel cell vehicles.  In 2002, one of DaimlerChrysler’s vehicles (NECAR 5) completed a 3,000 miles cross-country trip across the U.S.   The most common use of methanol as a fuel is M85 (a mixture of 85 vol% methanol and 15 vol% gasoline). The addition of gasoline provides color to the flame in case of fire (methanol flames are colorless) and reduces winter cold start problems and summer vapor lock [http://www.altfuels.org/m85.html]. Although automobiles have been manufactured that operate on M85 and some fleets continue to use M85, this fuel has failed to develop on a large-scale. Contributing factors include adverse consumer perceptions coupled with a lack of sufficient infrastructure to handle M85 even though only small changes are required at the gasoline fueling station (e.g., linings and seals in tanks, pumps, and dispensers) (Davenport, 2002).   Methanol is stable under normal storage conditions. It is not subject to hazardous polymerization reactions, but can react violently with strong oxidizing agents. The greatest hazard involved in handling methanol is the danger of fire or explosion (English, 1995).   When used in spark engines, higher methanol/gasoline blend ratios result in lower exhaust emissions of CO, HC, and NOx (Letcher, 1983). The addition of tertiary butyl alcohol (TBA) and methanol may reduce CO emissions by up to 40% and hydrocarbon emissions by up to 20% compared to conventional gasoline (Schädlich, 2003)

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