Shift reactors

In a typical PAFC system, methane passes through a reformer with steam from the coolant loop of the water-cooled fuel cell. Heat for the reforming reaction is generated by combusting the depleted fuel. The reformed natural gas contains typically 60 percent H9, 20 percent CO, and 20 percent H9O. Because the platinum catalyst in the PAFC can tolerate only about 0.5 percent CO, this fuel mixture is passed through a water gas shift reactor before being fed to the fuel cell. 

The reaction produces additional hydrogen for ammonia synthesis. The shift reactor effluent is cooled and tlie condensed water is separated. The gas is purified by removing carbon dioxide from the synthesis gas by absorption with hot carbonate, Selexol, or methyl ethyl amine (MEA). After purification, the remaining traces of carbon monoxide and carbon dioxide are removed in the methanation reactions. 

Fig. 8.25. Cycle E2. Open IGCC plant with shift reactor and CO2 removal (after Chiesa and Consonni 13 ).

A process worker is monitoring the rise in temperature in reactor A. An exothermic reaction occurs producing an alarm requiring the opening of a valve on a circuit which provides cooling water to the reactor. Instead of opening the correct valve, he operates another valve for reactor B, which is the reactor which he monitors on most shifts. Reactor A is destroyed by a runaway reaction. 

The scheme of commercial methane synthesis includes a multistage reaction system and recycle of product gas. Adiabatic reactors connected with waste heat boilers are used to remove the heat in the form of high pressure steam. In designing the pilot plants, major emphasis was placed on the design of the catalytic reactor system. Thermodynamic parameters (composition of feed gas, temperature, temperature rise, pressure, etc.) as well as hydrodynamic parameters (bed depth, linear velocity, catalyst pellet size, etc.) are identical to those in a commercial methana-tion plant. This permits direct upscaling of test results to commercial size reactors because radial gradients are not present in an adiabatic shift reactor. 

The process begins with a gasification process that converts coal into carbon monoxide and hydrogen. Part of this gas is sent to a water-gas shift reactor to increase its hydrogen content. The purified syngas is then cryogenically separated into a carbon monoxide feed for the acetic anhydride plant and a hydrogen-rich stream for the synthesis of methanol. 

TonkovichA.L.ZilkaJ.L.PaMont, M.J. VangY. VegengP., Micro-channelchemicalreactorforfuelprocessing applications -1. Water gas shift reactor, Ghem.Eng.Sd. 54(1999)2947-2951. 

Various technologies have been investigated to reduce the concentration of CO in fuel gas exiting the shift reactor to 10 ppm or less. Among the candidates are membrane separation, methanation, and... 

Fischer-Tropsch synthesis requires a stochiometric H2 CO ratio of 2.1 1. If coal or biomass are used as feedstock, the raw syngas contains much less hydrogen than needed. Hence, CO is reacted with water to form C02 and hydrogen in the shift reactor. As the C02 cannot be used in the Fischer-Tropsch synthesis, part of the carbon for fuel production is lost in this process. If external hydrogen is added to increase the H2 CO ratio, the carbon of the coal or biomass is more effectively used and the hydrocarbon product yield is improved. 

In a first step, biomass is converted to synthesis gas by gasification. If Fischer-Tropsch (FT) synthesis is applied, a H2 CO ratio of about 2.1 1 is required for a maximum yield of liquid hydrocarbons. To adjust the H2 CO ratio, CO shift reactors are used to convert a part of the CO to H2 and C02 according to the following reaction ... 

To reach a better CO conversion, it is possible to add a low-temperature shift reactor, which increases the CO2 capture rate (see also Fig. 10.3). If both clean CO2 for storage and clean hydrogen for fuel cell applications are required, a combination of a C02-capture plant (e.g., absorption with Rectisol) and a PSA plant is necessary. If only pure hydrogen is required, a PSA unit would be sufficient (and is standard practice), but the C02 stream would be contaminated by impurities, such as H2, N2 or CO, which have to be removed for geological storage. 

Due to the operating requirements of PEM stack technology, shift reactors and a carbon monoxide removal step are required to produce reformate of sufficient quality. Similarly, the stack operating temperature and its humidity requirements require a water management system as well as radiators for heat rejection. Some developers are developing pressurized systems to the benefit from higher reactant partial pressures on both anode and cathode. Fuel processing for PEM APU systems is identical to that needed in residential power or propulsion applications. 

POX reactor exit temperatures vary widely. Noncatalytic processes for gasoline reforming require temperatures in excess of 1,000°C. These temperatures require the use of special materials and significant preheating and integration of process streams. The use of a catalyst can substantially reduce the operating temperature allowing the use of more common materials such as steel. Lower temperature conversion leads to less carbon monoxide (an important consideration for low temperature fuel cells), so that the shift reactor can be smaller. Lower temperature conversion will also increase system efficiency. 

When used in a PEFC system, the reformate must pass through a preferential CO catalytic oxidizer, even after being shifted in a shift reactor. Typically, the PEFC can tolerate a CO level of only 50 ppm. Work is being performed to increase the CO tolerance level in PEFC. At least two competing reactions can occur in the preferential catalytic oxidizer ... 

A fuel processor for PEFC application contains sulfur removal, an ATR-enhanced UOB reformer, advanced shift reactors, a steam generation system, a product gas cooler, a PROX system, a gas compressor, an air compressor, an anode-off gas oxidizer, and a control system. Goal efficiency (LHV H2 consumed by fuel cell/LHV fuel consumed by fuel processor) is 69 to 72%. H2 concentration is presently >50% (dry). 

A particular version of the PEFC is the direct methanol fuel cell (DMFC). As the name implies, an aqueous solution of methanol is used as fuel instead of the hydrogen-rich gas, eliminating the need for reformers and shift reactors. The major challenge for the DMFC is the crossover of methanol from the anode compartment into the cathode compartment through the membrane that poisons the electrodes by CO. Consequently, the cell potentials and hence the system efficiencies are still low. Nevertheless, the DMFC offers the prospect of replacing batteries in consumer electronics and has attracted the interest of this industry. 

The TC biodiesel fuel processor instead consists of a two-reactor system, in which one reactor, the cracker, is used for production, while the other one is being regenerated by gasification of the deposited sohd C with steam yielding H, COj, CO and CH (Ledjefif-Hey et al., 2000). The product gas streams of the TC and the gasification unit are combined, cooled down to the shift inlet temperature and then fed to the shift reactor and the CO-purification step. First, the cracking of biodiesel takes place for the production of H ... 

Adjustment of the C0 H2 ratio is effected by the shift reaction (iv) which proceeds over a chromium-promoted iron catalyst at 700-800°F (370-425°C) or over a reduced copper/zinc catalyst at 375" 50°F (190-230 C) and the fraction of crude gas sent through the shift reactor is calculated from the initial gas composition and specific downstream requirements. The latter are i1 lustrated by... 

Fuel supply is usually from liquid hydrogen or pressurized gaseous hydrogen. For other fuels, a fuel processor is needed, which includes a reformer, water gas shift reactors and purification reactors, in order to decrease the amount of CO to an acceptable level (below a few tens of ppm), which would otherwise poison the platinum-based catalysts. This equipment is still heavy and bulky and limits the dynamic response of the fuel cell stack, particularly for the electric vehicle in some urban driving cycles. 

Advanced water-gas shift reactors using sulphur-tolerant catalysts to produce more hydrogen from synthesis gas at lower cost. 

A high temperature water-gas shift reactor 400°C) typically uses an iron oxide/chromia catalyst, while a low temperature shift reactor ( 200°C) uses a copper-based catalyst. Both low and high temperature shift reactors have superficial contact times (bas on the feed gases at STP) greater than 1 second (72). 

A similar study reports the results of adding 100 ppm thiophene to As in the Palm et al. study,the catalyst is not described rather, it is identified only as a commercial naphtha reforming catalyst, presumably Pt-based. In their reactor, the reformate from the ATR step passes through separate high and low temperature shift reactors before being analyzed. Thus, it was not possible to determine the effect of sulfur on the reforming step alone, nor was any post-reaction characterization of the catalyst reported, for example to determine coke or sulfur content. Figure 16 shows the observed deactivation, as measured by a decrease in H2 and CO concentrations. 

The high activity of Rh compared to conventional Ni-based catalysts may also lead to a lower operating temperature of the reformer, eliminating high-and low-temperature shift reactors and minimizing the O/C. At 550°C (O/C = 1, S/C = 3.0, and GHSV = 179,290 h ), Newson et al. obtained a H2 yield of... 

It is of interest to assess the process potential of methanol production by a direct partial oxidation of methane. This way the steam reformer and the shift reactor can be saved, and the catalytic methanol reactor can be replaced by a noncatalytic partial oxidation reactor. It is estimated that direct partial oxidation is competitive if a conversion of methane of at least 5.5% can be obtained with a methanol selectivity of at least 80%. 

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