The conversion of clean synthesis gas into a liquid fuel involves passing the syngas over a catalyst that promotes the desired synthesis reactions and then refining the raw product to obtain the final desired liquid fuel. Two basic designs for commercial synthesis reactors have been developed: gas‐phase (or fixed‐bed) and liquid‐phase (or slurry‐bed). Fixed‐bed reactors have a long commercial history, but liquid phase reactors have been gaining popularity in commercial applications because of attractive performance attributes and lower cost. Liquid phase reactors are now commercially offered for Fischer–Tropsch liquids (FTL), methanol, and DME synthesis. Liquid phase reactors for mixed‐alcohol synthesis are still under development.
Fixed‐bed and liquid‐phase reactor designs differ primarily in their handling of reactor temperature control. Synthesis reactions are exothermic, such that the reactor temperature increases as the reactions proceed if no heat is removed. Higher temperatures promote faster reactions, but maximum (equilibrium) conversion is favored by lower temperatures. Also, catalysts are deactivated when overheated. Thus, the temperature rise in a synthesis reactor must be controlled. In commercial practice, a reactor‐operating temperature of 250–280 °C for methanol, DME, or FTL synthesis balances kinetic, equilibrium, and catalyst activity considerations. For mixed‐alcohols synthesis, which is not yet a commercially established technology, higher reaction temperatures (300–400 °C) have been indicated with catalysts identified to date (Aden et al. 2005; Air Products and Chemicals, Inc. 2001).
A gas‐phase reactor incorporates the flow of syngas over a fixed‐bed of catalyst pellets. With this design it is difficult to maintain isothermal conditions by direct heat exchange (due to low gas‐phase heat transfer coefficients). To limit temperature rise, the synthesis reactions are typically staged, with cooling between reactor stages. Also, by limiting the initial concentration of CO entering the reactor (to 10–15 vol%) the extent of the exothermic reactions can be controlled. Control of the CO fraction is achieved in practice by maintaining a sufficiently high recycle of unconverted H2‐rich syngas back to the reactor.
In a liquid‐phase reactor, syngas is bubbled through an inert mineral oil containing powdered catalyst in suspension (Figure 9.22). Much higher heat release rates (i.e. extents of reaction) can be accommodated without excessive temperature rise as compared to a gas‐phase reactor because of more effective reactor cooling by boiler tubes immersed in the fluid. The vigorous mixing, intimate gas‐catalyst contact, and uniform temperature distribution enable a high conversion of feed gas to liquids in a relatively small reactor volume. Conversion by liquid‐phase FT synthesis is especially high. A single‐pass fractional conversion of CO of about 80% can be achieved (Bechtel Group, Inc. 1990), compared to less than 40% for conversion with traditional fixed‐bed FT reactors. For the FT reactor conditions it is assumed in simulations, the single‐pass CO conversion is about 65%.
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