Reformate stream

Hydrogen-rich reformer stream Hydrogen-rich reformer stream 90-120. 002... 

PAFC. The weaknesses of this technology include the need to limit as much as possible the presence of sulfur in the fuel feed and in the reformate stream, in order to avoid catalyst poisoning and frequent corrosion problems at the primary cell stack coolant circuit, and the need to increase electrical generation efficiencies of cell stack assembly lifetime to at least 40,000 hours (5 years) furthermore, an increase in power densities is an area where technical development is necessary (see Lane et al., 1995). 

Table 85 Activity changes for Pt/ceria expressed as percentage of CO remaining (dry basis) before and after exposure of a reformate stream (3%CO, 15%COz, 48%H2, 39%N2 (dry basis) with 26%H20 and a WHSV of 20 000 h 1) to a shutdown condition at 60 °C435... 

Although the catalyst did not recover in the reformate stream, the authors did indicate that the carbonate species could be removed by air treatment at temperatures > 400 °C. Ruettinger and coworkers452 reported on the mechanism of aging of Pt/ceria-zirconia mixed oxide catalysts. In that case, the authors followed the activity of a 2% Pt/Ce02-Zr02 catalyst at 228 °C in a feed containing 5.92% CO, 7.4% C02, 31.82% H2, 28.86% N2, and 26% H20, and observed the deactivation data tabulated in Table 86. 

Table 87 Activity changes expressed in terms of the CO consumption rate (pmoles per g catalyst per second) during exposure to a reformate stream (10%CO, 22%HzO, 6%COz, 43%H2, 19%N2) with 15-30 mg catalyst at 240 °C437...

The additional issue for PEM is the minimization of steam needed for the fuel processor system. Since an APU is a mobile and/or remote unit, the need for external sources of water should be minimized. The reformate stream is further diluted by additional steam, if that water is not removed prior to the fuel cell stack. 

There are high temperature and low temperature methods to remove sulfur from a fuel reformate stream. Low temperature cleanup, such as hydrodesulfurizing (limited to fuels with boiling end points below 205°C), is less difficult and lower in cost so should be used where possible, certainly with low temperature cells. Sulfur species in the fuel are converted to H2S, if necessary, then the H2S is trapped on zinc oxide. As previously mentioned, a minimum bed volume of the zinc oxide reactor is achieved at temperatures of 350 to 400°C. Simple... 

There is an alternate process for recovering toluene from the reformate stream called azeotropic distillation. It also can be used to split toluene from the other hydrocarbons that have boiling points near toluene. Azeotropic distillation is like solvent extraction with an extra twist. The process can be more efficient than extraction when the toluene concentration is high. 

Refinery cat reformers produce a reformate stream with aromatics. That stream, with or-without the benzene-laden scream from the olefins plant, can be split apart in the various processing schemes in the BTX recovery facility. 

As mentioned earlier, reformate from a fuel processor often needs addition processing to reduce the carbon monoxide levels. Researchers at the Stevens Institute of Technology are developing a microscale preferential oxidation (FrOx) reactor to decrease the carbon monoxide level in the reformate stream to below 100 ppm. As part of their research, they used advanced computational fluid dynamic modeling. 

Recent fuel processor performance is summarized in Table 4. The fuel processors were operated at atmospheric pressure, and the water and methanol feed mixture was about 60 wt % methanol. The typical composition of the reformate stream was 72— 74% hydrogen, 24—26% carbon dioxide, and 0.5—1.5% carbon monoxide on a dry gas basis. The carbon monoxide levels were significantly below equilibrium (5.4% at 350 °C), but they still require additional cleanup for use in fuel cells. The fuel processor efficiency was calculated using eq 5 and was reported to be greater than 80%. It is interesting to note that increasing the power 5-fold, from 20 to 100 W, only resulted in a 50% increase in volume and a 33% increase in mass. 

The xylenes are very high-lonnage industrial chemicals and are raw materials or intermediate materials for numerous synthetic fibers, resins, and plastics. See also Xylene Polymers. A large amount of p-xylene goes into polyester fiber production, while substantial quantities of d-xylene are consumed by the manufacture of phthalic anhydride. The prime source of xylenes are petroleum refinery reformate streams in conjunction with benzene and toluene extraction. The xylenes occur mixed in these streams. 

A 200 mW methanol fuel processor was presented by Hu et al. [32], A 9% efficiency was determined for the device running at 1 vol.% carbon monoxide in the reformate stream. 

Figure 4 shows the theoretically predicted gas composition of the reformate stream for the UMR process with calcium oxide. The addition of calcium oxide substantially moderates the temperature drop during the reforming step and hence reduces the changes in reformate gas composition. 

Figure 4. The gas composition and temperature of die reformate stream from the UMR process with calcium...

Another limitation is dictated by equilibrium constraints for the reforming Reactions (1) and (2), i.e., they do not reach completion. As a result, the reformate stream contains unconverted methane, carbon monoxide and carbon dioxide. Typically, at lower temperatures (600°C) the amount of carbon monoxide in the reformate gas is reduced to 2%, however the amount of methane increases to 28%. At high temperatures (900°C), the amount of methane in the reformate is reduced to 2%, however the amount of carbon monoxide increases to 20%. In UMR reaction equilibrium is shifted favorably by the carbon dioxide sorbent. 

The parameters used to assess the UMR performance are methane conversion, and hydrogen-to-carbon-monoxide molar ratio in the reformate stream. 

The figures discussed above indicate that the optimum temperature for the reforming step in the UMR process is 700-850°C, the optimum pressure is less than 7 bar and the optimum calcium-to-carbon molar ratio during the reforming step is around one. Under these optimum conditions the methane conversion varies from 82 to 100% and the hydrogen-to-carbon-monoxide molar ratio in the reformate stream varies from 6 to 100. 

The pilot scale unit was also used to study the UMR process using natural gas as the feed-stock. The experimentally measured gas composition of the reformate stream during one cycle of the UMR process is shown in Figure 9. The gas composition data calculated using the model showed similar trends (see Figure 4). 

Bench scale experiments using diesel fuel have shown that for typical fuel sulfur concentrations there is no detectable sulfur in the reformate to at least the sub-ppm level. In the bench scale experiments, when sulfur was added to diesel fuel at a concentration of 2000 ppm by weight only 5 ppm hydrogen sulfide was detected in the reformate. In pilot scale experiments using diesel fuel, the sulfur concentration in the reformate stream was around 15 ppm. If the sulfur was not captured by the pilot-scale reactor, the sulfur concentration in the reformate stream would have been 37 ppm. 

Figure 7. Effect of temperature, pressure and calcium-to-carbon molar ratio on H2-to-CO molar ratio in reformate stream. The calcium-to-carbon molar ratio is for the entire reforming step (steam-to-carbon molar ratio of 3).

Toluene is converted into benzene by a catalytic hydrodealkylation (HDA) process at elevated temperature and pressure. The importance of this process is influenced by the relative value and demand for benzene, as benzene from this source is normally more costly than that isolated directly from refinery reformate streams. Benzene (along with xylenes) can also be obtained by the catalytic TDP. It has became favorable in recent years. Toluene consumption for toluene disproportionation versus HDA has changed from about 1/5 in 1990 to 2/1 in 2000. The volume of toluene that finds use as a solvent is expected to show a continued decline because of regulations controlling the emission of VOCs. 

Xylenes are obtained mainly (80%) from petroleum reformate streams in the form of mixed xylenes. A typical composition of this stream is about 18 percent /7-xylene, 40 percent w-xylene, 22 percent o-xylene, and 20 percent ethylbenzene. The major chemical uses of xylene, however, require the... 

One important feature of FCVs that remains crucial for their development is the fact that PEM fuel cells run on either pure hydrogen or a dilute hydrogen gas reformate stream (though direct-methanol fuel cells, still in an early stage of development, operate on methanol). This hydrogen can either be stored on board the vehicle in one of several ways, or generated from another fuel with an on-board reformer. 

The overall targeted reaction in the preferential removal of CO in the reformate stream is the oxidation of CO via a carefully metered amount of air ... 

The reforming process (as applied to a hydrocarbon or alcohol) yields a product stream that consists predominantly of hydrogen, carbon monoxide, carbon dioxide, water, unconverted feedstock, and trace by-products. This product stream mixture, called reformate, is unsuitable for direct use in low-temperature PEMFC and AFC, and some trace by-products (notably organosulfur compounds) will poison both high-temperature fuel cells and low-temperature fuel cells. A membrane for separating and purifying hydrogen from reformate must also be chemically compatible with the compounds in the reformate stream. 

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