Steam/carbon ratio

Because of a great practical importance of SMR as a major industrial process for manufacturing H2, the development of efficient steam reforming catalysts is a very active area of research. Nickel and noble metals are known to be catalytically active metals in the SMR process. The relative catalytic activity of metals in the SMR reaction (at 550°C, 0.1 MPa and steam/carbon ratio of 4) is as follows [12] ... 

Figure 6-12 CH4 Conversion as a Function of Fuel Utilization in a DIR Fuel Cell (MCFC at 650"C and 1 atm, steam/carbon ratio = 2.0, >99% methane conversion achieved... 

Carbon formation is also different for diesel and gasoline. The long chain hydrocarbons present in diesel or kerosene fuel are more difficult to reform than the shorter chain hydrocarbons present in gasoline, while aromatics in gasoline hinder the overall reaction rate. An example is found in the results of Ming et who showed that SR of n-Ci required a higher steam/ carbon ratio to avoid coke formation than i-Cg. The cetane number of the feed had little effect on carbon formation. Carbon formation can often be attributed to fuel pyrolysis that takes place when the diesel fuel is vaporized. This is considerably decreased when the steam content in feed increases. 

The catalyst for this reaction is normally nickel on a refractory or aluminate support. The steam reforming reaction is highly endothermic (-AH°298 = -206 kJ/mol) and high temperature, low pressure and high steam-carbon ratios (3-4 is commonly used) favour conversion [ ] ... 

Figure 2. Thermodynamic coke deposition vs. steam-carbon ratio.

When steam reforming of saturated hydrocarbons was carried out on nickel catalysts at a high steam - carbon ratio (eg >1.5 mol/atom) [1,8], no carbon deposition was observed after several hours of reaction. The same results (on N1/AI2O3) were observed during CO methanation at H2 CO ratios of 1 to 3 [4]... 

Computational results are plotted in Figures 2, 3 and 4. The effectiveness factors are very small at elevated temperatures in line with the observations of Van Hook (16). Note that this simulation has been kept as simple as possible for illustrative purposes. At steam-carbon ratios below about 1.4,carbon forming reactions should be considered. The water-gas shift reaction might also be a factor, but the experimental evidence suggests that both CO and CO2 are primary reaction products (16) in agreement with the assumed kinetics model. 

I.I.I. Thermodynamics, Operation, Pressure, Steam/ Carbon Ratio... 

The various support materials have different effects on potential carbon formation. This seems to go in parallel with the Lewis/Bronsted acidity. The main commercially used catalyst supports can be ranked as follows in decreasing order of carbon forming tendency (and thus in decreasing order of the important minimum practical steam/ carbon ratio) a-alumina > magnesium aluminate (spinel) > calcium aluminate > alkalized calcium aluminate [419],... 

The energy consumption figures discussed so far represent a thermodynamic analysis based on the first law of thermodynamics. The combination of the first and second laws of thermodynamics leads to the concept of ideal work, also called exergy. This concept can also be used to evaluate the efficiency of ammonia plants. Excellent studies using this approach are presented in [1061], [1062], Table 39 [1061] compares the two methods. The analysis in Table 39 was based on pure methane, cooling water at 30 °C (both with required pressure at battery limits), steam/carbon ratio 2.5, synthesis at 140 bar in an indirectly cooled radial converter. 

Topsoe offers two process versions. The first operates at steam/carbon ratio of 3.3 and with rather high residual methane content from the secondary reformer.. Shift... 

ICI AMV Process. The ICI AMV process [1034], [1083], [1111] - [1122], also operates with reduced primary reforming (steam/carbon ratio 2.8) and a surplus of process air in the secondary reformer, which has a methane leakage of around 1 %. The nitrogen surplus is allowed to enter the synthesis loop, which operates at the very low pressure of 90 bar with an unusually large catalyst volume, the catalyst being a cobalt-enhanced version of the classical iron catalyst. The prototype was commissioned 1985 at Nitrogen Products (formerly CIL) in Canada, followed by additional plants in China. A flow sheet is shown in Figure 110. 

Steam/carbon ratio Operating pressure, N/m (psia)... 

Pyrolysis temperature Gasification temperature Steam/carbon ratio gasifier Steam inlet t p ture gasifier Combustion temperature ... 

Carbon deposition is one of the luost serious problems of the steam reforming catalyst process (ref 1). The deposition of carbon on naphtha steam reforming catalysts depends ori the chemical composition of the hydrocarbon oil, the steam/carbon ratio in the feedstock, as well as the pi ocesa temperature and pressure, it is also affected by tlie presence of sulfur poisons Our past research of SNG catalysts ejiamined the nature of the carbon deposits as a function of the sulfur level on the catalyst (refs, 2 4). A small amount of sulfur was found to promote the formation of carbon that is non-reactive with steam and hydrogen under steam reforming reaction conditions. The continuous accumulation of this less reactive carbon [continuous carbon deposition (CCD)l on the catalyst surface leads to coke fouling Studies of the occurrence of CCD in our laboratory tests allow ua to predict, that coke fouling is likely to occur on the same catalyst used in real Indusl.rlal applications. 

Figure 5. Comparison of H2"TPR spectra of sulfur-poisoned Ni/Al203 catalysts used with feedstock steam/carbon ratios of 3 0 and 4.0 ...

Reforming or gasification produces syngas whose H2/CO ratio depends on the feedstock and process conditions such as feed steam/carbon ratio and reaction temperature and pressure. Water-gas shift reaction can further increase the H2/CO ratio of syngas produced from coal to the desired range for conversion to liquid fuels. This reaction is also an important step for hydrogen production in commercial hydrogen plants, ammonia plants, and methanol plants that use natural gas or coal as feedstock. 

Figure 2.4. Equilibrium methane conversions at different temperatures, steam/carbon ratios, and pressures obtained by thermodynamic calculations.

Rh-Ce catalyst exhibited a high ethanol conversion over 95% and high selectivity to H2 and CO in a very short residence time of <10ms under autothermal condition. Increasing steam/carbon ratio increased the H2 yield due to the participation of WGS reaction. The POE and WGS reactions were also performed in a two-stage reactor. 

Steam/carbon ratio is set by equilibrium considerations and carbon formation suppression. 

Depending on the product spectrum required, operating conditions are adjusted - the two important variables being the reactor temperature and the steam carbon ratio. Some indication of the different conditions required to produce gas for different applications is given in Figure 2. 

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