Autothermal methanol conversion

Figure 2.16 Methanol conversion as a function of temperature hydrogen selectivity vs. methanol conversion for autothermal methanol reforming [39] (by courtesy of ACS).

Chen et al. [36] performed a comparison of micro structured steel and aluminum plates with a conventional monolith by varying the GHSV. Full conversion could be maintained for autothermal methanol reforming in the micro structures up to a GHSV of 40 000h 1, whereas conversion dropped to 80% at 20 000h 1 at the monolith. Even at 186 000 h, still 95% conversion could be achieved in the stainless-steel micro reactor. No significant performance differences were observed between the steel and aluminum plates. 

Keeping in mind these insights, we turn to a second application of the model, the estimation of the lifetime of a catalyst bed. This is an important consideration in the design of any reactor and is particularly critical in transportation applications where maintenance intervals must be well known. As an initial approach to developing metrics for this analysis we define Ae time in service or service life, at constant wall temperature, as the duration when the overall conversion in the reactor (for full power output) is greater than 85%. This conversion was chosen because it is close to the autothermal point of operation where the burning of unreacted methanol will just balance the endothermic heat of reaction. A reference catalyst mass was determined by requiring a methanol conversion of 85% at the wall temperature of 240"C for fresh catalyst. 

At 290 °C reaction temperature and a feed composition of 9 vol.% methanol and 11% water, which corresponded to S/C 1.2,65% methanol conversion could be achieved at 99% hydrogen selectivity over CuZns samples treated by acid leaching for 20 min. Under autothermal conditions, more than 25% methanol conversion was achieved at S/C 1.2 and 0/C 0.3, while the oxygen was fully converted. Later, Homy et al. improved their catalyst by doping with chromium [482]. At S/C 1.0 and 0/C 0.25, axial temperature profiles were determined over the reactor to determine the hot spot formation. The hot spot did not exceed 3 K due to the high heat conductivity ofthe brass. A fixed catalyst bed showed a hot spot of about 20 K under comparable conditions. 

Figure 27. Methanol synthesis under autothermal operation (reverse flow reactor) A. B) Temperature and conversion pr< period C) Reaction path for reverse flow operation and for a two-stage adiabatic reactor with interstage cooling [46]...

Another example is monolithic-type reactors, which have found their main application in the field of combustion. A monolith bed allows better autothermic operations with a minimal pressure-drop. This concept was used to improve performances in commercial methanol into formaldehyde conversion by adding a... 

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 methanol reforming takes place between the membrane mbe and the second jacket. The hydrogen through the membrane is recovered as CO-free stream in the permeate side. Air is injected in the second jacket to produce heat by reaction with the unrecovered hydrogen and unconverted reactants. The produced heat is transferred by the jacket wall to the reforming zone. In this way, an autothermal reaction can be carried out with a complete conversion and integrated H2 purification. 

Lattner and Harold [56] performed autothermal reforming of methanol in a relatively big fixed-bed reactor carrying 380 g BASF alumina-supported copper/zinc oxide catalyst modified with zirconia. The 01C ratio was set to 0.22 while the SIC ratio varied from 0.8 to 1.5. The axial temperature profile of the reactor, which had a length of 50 cm, was rather flat, the hot spot temperature did not exceed 280° C which was achieved by the air distribution system through porous ceramic membrane tubes. More than 95% conversion was achieved. Very low carbon dioxide formation was observed for this reactor only 0.4 vol.% was found in the reformate. However, the WHSV calculated from the data of Lattner and Harold [56] reveals a low value of only 6 l/(h gcat) for the highest CHSV of 10 000 h reported. 

Compared with other hydrocarbon fuels such as ethanol, methane, and gasohne, SR of methanol exhibits a relatively high theoretical conversion efficiency, low conversion temperature, and low by-product formation [117]. Partial oxidation and autothermal reforming (ATR) of methanol are also possible, but play a minor role. 

Meyer et cd. described the development of a multi-fuel processor by International Fuel Cells, LLC [627]. Methanol and gasohne (quality California reformulated gasoline grade II) were the major fuel alternatives. The technology chosen consisted of feed desulfurisation, autothermal reforming and catalytic carbon monoxide removal by two water-gas shift stages and two preferential oxidation reactors. The system had a power equivalent of 50 kW. However, performance data were only provided with respect to the autothermal reformer Desulfurisation proved to increase the reformer conversion up to 98%. No residual heavy hydrocarbons then remained in the product. The hot spot of the autothermal reformer approached 1000 °C. 

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. 

The equilibrium of the methanol synthesis reaction severely limits the conversion in the conventional process. The equilibrium conversion is very sensitive to temperature. The high recycling rate is costly and requires oxygen instead of air in the autothermal reforming or partial oxidation step. The development of low-temperature and continuous methanol removal processes mentioned brieffy in Section 1.3.1, would be very attractive [6365]. Hi single-pass conversion could also be attained with a two-step process methanol carbonylation to methyl formate followed by methyl formate hydrogenolysis to 2 mol methanol [6669]. Research in these areas has yielded promising results. 

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