Economic methanol synthesis

The various processes involving carbon monoxide steam reforming, water-gas shift, methanation, and methanol synthesis illustrate the operation of thermodynamic constraints and some of their attendant economic consequences. 

In this paper the various routes to methanol synthesis are summarized and compared with a new catalytic partial oxidation route. Also compared are quench, steam raising and tube cooled converters, along with comparative energy and economic stannaries of the various routes. 

New technologies for methanol synthesis are presently being developed. These new systems, which have been tested in bench or demonstration units, use either low temperature liquid-gas phase or solid-gas phase methanol reaction techniques. However, the suitability for cormvercialization of these new technologies is uncertain due to either questionable economical advantages or insufficient development therefore, this paper will address the conventional methanol synthesis loop. 

Kuczynski and Westerterp have proposed an elegant means of removing the reaction products from a system [10, 11]. In the area of methanol synthesis, where the conventional process is limited in conversion per pass to about 30%, they proposed to adsorb the product onto an amorphous silica-alumina powder that trickles through the solid catalyst packed bed. At the bottom of the bed, this soKd absorbent is collected and depressurized to yield the methanol product. Even though the process looked economically viable, it has, to the best of our knowledge, not seen any commercial application. 

In industrial operation it is necessary, for economic reasons, to recover as much as possible the heat produced by exothermic reactions. One obvious way of doing this, mentioned earlier in Section 11.3, is to preheat the feed by means of the reacting fluid and/or the effluent. When the heat of reaction is sufficient to raise the temperature of the feed to such a value that the desired conversion is realized in the reactor without further addition of heat, the operation is called auto-thermic. Some of the most important industrial reactions like ammonia and methanol synthesis, SO2 oxidation, and phthalic anhydride synthesis, the water gas shift reaction can be carried out in an autothermic way. Coupling the reactor with a heat exchanger for the feed and the reacting fluid or the effluent leads to some special features that require detailed discussion. 

The effect of coexisting SC solvent on the methanol synthesis was shown in Fig. 5. The total carbon conversion siginificantly increased from 45 to 60% when the amount of 2-propanol increased from 0 to 23 mol% whereas the selectivity of methanol slightly decreased. This indicated that the coexisting 2-propanol in SC n-hexane enhaneed the reation activity of methanol synthesis. As the small amount of 2-propanol was used therefore, the S5mthesis was considered as an economic process. A small amount of cosolvent added to SCF possibly had a significant impact on the economics of SC separation processes [22]. 

The high vapour pressure of the glycolethers has the effect that only very small solvent quantities leave the system together with the clean gas and with the sulfur and CO2 offgases, whose recovery in a secondary scrubber does not seem economically justifiable at first glance. However, even traces of this solvent have to be eliminated from the clean gas before it can be used for methanol synthesis so that an after-treatment is necessary at least at this point. 

In the above section, the importance of carbon monoxide and carbon dioxide conversion and the technically attainable approach to the equilibrium has been described. However, these two parameters alone do not decide upon the optima-tion for the production of methanol from a specific synthesis gas. The methanol yield from the synthesis gas is of quite decisive importance for economically producing methanol on a commercial scale. Its this yield on which depend the quantity of synthesis gas which must be produced horn coal, cleaned, conditioned and compressed and the quantity of CO2, CO and H2 which must be removed from the methanol synthesis as purge gas and thus is lost to methanol production by the direct route. 

Whereas formerly zinc oxide chrome oxide catalysts were in general applied for methanol synthesis under high pressures - about 300 to 400 bar - which featured high temperature resistance and relative insensitivity to catalyst poisons, above all sulfur in many forms, today catalysts on a copper basis are used exclusively. Sometimes they are termed Blasiak catalysts in specialised literature and a number of works [3.14] exist on the first test results with this catalyst type. The copper-based catalysts permit methanol to be synthesized in an economic manner at pressures between 50 and 100 bar and temperatures around 230 to 270°C. The fact that it was first applied commercially about 1970 was mainly due to people not being in a position in the forties and fifties to obtain the necessary... 

The data produced were instrumental in establishing a proper phenomenological model for the bubble column. This model was able to predict both liquid and gas tracer curves obtained in an AFDU pilot-plant column in LaPorte, Texas, for three different reaction systems methanol synthesis, dimethyl ether synthesis, and Fischer Tropsch synthesis (Chen et ah, 1998 Devanathan et ah, 1990 Gupta et al., 2001a, 2001b). Unfortunately, the desire to use improved science in scale-up gas-to-Hquid fuels processes to large diameter bubble columns disappeared when Hquefaction of natural gas became more economically attractive. 

Methanol is an important multipurpose intermediate traditionally used for production of various chemicals [57], It is currently produced from syngas, which is industrially generated via catalytic steam or autothermal reforming of methane [13-15]. Figure 23.7 schematically illustrates commercial and alternative routes for methanol formation from methane. Despite the fact that syngas production and methanol synthesis are highly optimized processes, strong economic and environmental interests exist in direct oxidative conversion of methane to methanol. 

Reaction (5) is only slightly exothermic with a heat of reaction comparable to that of the shift reaction (reaction (2)). With a catalyst selective for the selective oxidation, it would be possible to produce synthesis gas suitable for e.g. methanol synthesis at low temperature without the large heat production and heat recovery required in using present reforming technology. With such a catalyst, it might be possible to establish more economic flow schemes. 

The attractive characteristics of DMC hydrogenation to methanol, namely solvent free, waste-free, atom-economical, mild pressure and temperature, high turnover numbers (TON), and high selectivity, represent an ultimate green process and make this catalytic transformation an attractive technology for methanol synthesis [80-82]. Some representative examples catalysed by complex 6 are presented in Fig. 11 (vide supra). 

The methanol earbonylation process can be integrated quite naturally into a large-scale methanol plant [9], The reactant CO can be obtained readily by separation from the synthesis gas used in methanol synthesis. With the large proven reserve of natural gas in the world, this process is in a very secure position compared with the hydrocarbon-based processes. It is also easily adaptable to the use of coal as a source of raw material, since there are proven economic routes to produce synthesis gas from coal... 

---