Gas methanol synthesis

TABLE 9.17. Other Applications of Steam Reforming Process. 

Process Lean town gas 0X0 alcohol synthesis gas Direct iron ore reducing gas 

Synthesis gas from natural gas feeds has, however, an excess of hydrogen but may be used directly if a high purge rate from the synthesis loop is acceptable. 

It is possible to adjust the ratio of hydrogen to caibon oxides by the addition of caibon dioxide to the synthesis gas, either just before the methanol synthesis loop, or directly into the feed for the steam reformer. It is also possible to use a secondary reformer using oxygen rather than air, after the primary reformer. The amount of oxygen used is adjusted to provide the stoichiometric ratio of hydrogen and carbon oxides and, at the same time, reduce the amount of inert methane in synthesis gas. (Table 9.9) 

Typical steam reformers are used to produce synthesis gas for 0X0 reactions. It is usual to operate with a steam/carbon ratio of less than two with a recycle of sufficient carbon dioxide to provide a carbon dioxide/methane ratio of about two so that an appropriate composition of the gas product is obtained. 

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As an example of the application of a fixed-bed tubular reactor, consider the production of methanol. Synthesis gas (a mixture of hydrogen, carbon monoxide, and carbon dioxide) is reacted over a copper-based cat dyst. The main reactions are... 

Mechanisms for both reactions are proposed, which are supported by the identification of several intermediate reactions. The role of various organic and inorganic reaction promoters is discussed. Amongst these, phosphine oxides are exceptionally efficient in that they induce high reaction rates combined with high selectivities. Reactions a) and b) potentially allow two-step, methanol/ /synthesis gas-based routes to ethyl acetate and proprio-nic acid, respectively. 

Kvaerner Process Methanol Synthesis gas Efficient, modern, reliable process operates at low pressure/ ... 

Rotary Ki1n Soda Ash Catalyst High Pressure No Oxygen Required 3 ODT/D Wood Chips MBG ( 50 BTU/scf) Methanol synthesis gas Small industrial plants, pulp/ paper mi 11s PDU designed startup mid 1980... 

Catalytic partied oxidation is a process which produces a "stoichiometric" methanol synthesis gas by reacting natural gas with "minimum" steam and oxygen in a catalytic vessel with or without recovered hydrogen. 

The various routes to methanol synthesis gas all produce hydrogen and carbon oxides in different ratios. 

Common names for mixtures of CO/H2 are derived from their origin such as water gas (CO + H20) from steam gasification of coal and crack gas (CO + 3H ) from the steam reforming of methane, or from their application such as methanol synthesis gas (CO + 2H2) for the manufacture of methanol and oxo gas (CO + H2) for hydroformylation reactions. Another synthesis gas is ammonia synthesis gas (N2 + 3H2) which does not contain CO as a higher constituent. 

The following types of processes may be taken into account for purifying and conditioning methanol synthesis gas derived from coal ... 

A gas produced by high-temperature gasification of a coal slurry [2.24], for instance, and containing some 10 % CO2, 45 % CO and 36 % H2 (in addition to sulfur components and a few inerts) could be shifted without any catalysts simply by quenching with water to obtain a syngas which would be appropriate for methanol production. To this end, 0.55 kg of water would have to be added to the raw gas containing some 0.4 kg of steam per m of gas at approximately 15(X) °C. The temperature of the mix would in this way be adjusted to approximately 950°C and about one fifth of the CO in the raw gas would be converted. The converted gas would then contain 32 % CO2, 19.5 % CO and 40 % H2 so that -after the CO2 has been washed out to approximately 3 % - the methanol synthesis gas will have a stoichiometric ratio of 2.07. 

CO conversion is preferred to the CO2 conversion by all copper catalysts applied in practice. Therefore one endeavours to produce methanol synthesis gas with the highest possible CO content and lowest possible CO2 content. 

In more recent reactor concepts, the hot synthesis gas at the outlet of the autothermal reformer is used for the heating of the catalyst tubes in the primary reformer. In general, this system is called convective reformer. Figure 5.6 shows an example of the so-called tandem reformer for the generation of methanol synthesis gas. 

Figure 13. Production of Methanol Synthesis Gas. Two step Reforming with Heat Exchange Reformer (Topsoe, 1979).

Following feed pretreatment, the next step in the preparation of methanol synthesis gas is the steam reforming of the natural gas to form a mix of hydrogen and carbon oxides. Two principal reactions take place in the steam reformer reforming [Eq. (29)] and water-gas shift [Eq. (20)]. The predominant reaction taking place is the steam reforming of methane [Eq. (21)]. 

The methanol synthesis gas exiting the reformer is much hotter than required for the methanol synthesis reaction. To increase the efficiency of the process this waste heat is recovered back into the process just as the heat in the reformer flue gas is recovered to increase furnace efficiency. The process gas leaves the reformer typically at about 860880°C and first enters a steam boiler for the recovery of excess process heat. The steam raised in the boiler (after it has been superheated) is then used to provide motive power for compressor turbines and/ or process steam to the reformer. 

Tjatjopoulos, G. J., Vasalos, I. A., 1998. Feasibility analysis of ternary feed mixtures of methane with oxygen, steam, and carbon dioxide for the production of methanol synthesis gas. Industrial Engineering Chemistry Research 37 (4), 1410—1421. 

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