Carbon transport limits

GP 9[ [R 16]The extent of internal transport limits was analysed for the wide fixed-bed reactor, using experimental data on carbon monoxide conversion and matter and process parameter data for the reactants [78]. The analysis was based on the Weisz modulus and the Anderson criterion for judging possible differences between observed and actual reaction rates. As a result, it was found that the small particles eliminate internal transport limitations. 

Increasing the GHSV from 2 105 to 1.2 106 h 1 led to a 10% drop in methane conversion and deteriorated hydrogen and carbon monoxide yields, which were at least partially attributed to mass transport limitations. To check for mass transport limitations, the channel dimensions were varied from 60 to 120 pm width at a constant depth of about 130 pm. No effect of the channel diameter on conversion and selectivity was found at 900 °C. 

Nafion content in the catalyst layer plays an important role in electrode performance. Incorporation of Nafion ionomer into carbon-supported catalyst particles to form the catalyst layer for the gas diffusion electrode can establish a three-dimensional reaction zone, which has been proven by cyclic voltammetric measurements. An optimal Nafion content in the catalyst layer of the electrode may minimize the performance loss that arises from ohmic resistance and mass transport limitations of the electrode [6],... 

Interphase inhibition [52] occurs where the inhibitive layer has a three-dimensional structure situated between the corroding metal and the electrolyte. The interphase layers generally consist of weakly soluble compounds such as oxides, hydroxides, carbonates, inhibitors, etc. and are considered to be porous. Non-porous three-dimensional layers are characteristic of passivated metals. The inhibitive efficiency depends on the properties of the three-dimensional layer, especially on porosity and stability. Interphase inhibition is generally encountered in neutral media, either in the presence or absence of oxygen. In aerated solutions, the inhibitor efficiency may be correlated with the reduction in the oxygen transport limited current at the metal surface. 

Catalyst particles with diameters of about 0.3—0.6 mm were found to be suitable to avoid significant transport limitations. Stabilization of the PR-F catalyst was performed by running the pre-reforming reaction at high steam to carbon molar ratios (>2) at 763 K and 40 bar over a period of 582 h. SEM-EDX data indicated that the surface was sulfur and carbon free after this treatment. 

Another way to change concentration of active material is to modify the catalyst loading on an inert support. For example, the number of supported transition metal particles on a microporous support like alumina or silica can easily be varied during catalyst preparation. As discussed in the previous chapter, selective chemisorption of small molecules like dihydrogen, dioxygen, or carbon monoxide can be used to measure the fraction of exposed metal atoms, or dispersion. If the turnover frequency is independent of metal loading on catalysts with identical metal dispersion, then the observed rate is free of artifacts from transport limitations. The metal particles on the support need to be the same size on the different catalysts to ensure that any observed differences in rate are attributable to transport phenomena instead of structure sensitivity of the reaction. 

FT synthesis kinetics are similar on Co and Ru catalysts and reflect similar CO activation and chain growth pathways on these two metals. These kinetic expressions are consistent with the stepwise hydrogenation of surface carbon formed in fast CO dissociation steps (26). Chemisorbed CO and CH t species are the most abundant reactive intermediates, as expected from the high binding energy of CO and carbonaceous deposits on Co and Ru surfaces (7, 76-80). As a result, reaction orders in CO remain negative at the usual inlet pressures but can become positive as CO reactants are depleted by transport limitations within pellets or by high levels of conversion within the catalyst bed. 

CO reactants and the H2O product of the synthesis step inhibit many of these secondary reactions. As a result, their rates are often higher near the reactor inlet, near the exit of high conversion reactors, and within transport-limited pellets. On the other hand, larger olefins that are selectively retained within transport-limited pellets preferentially react in secondary steps, whether these merely reverse chain termination or lead to products not usually formed in the FT synthesis. In later sections, we discuss the effects of olefin hydrogenation, oligomerization, and acid-type cracking on the carbon number distribution and on the functionality of Fischer-Tropsch synthesis products. We also show the dramatic effects of CO depletion and of low water concentrations on the rate and selectivity of secondary reactions during FT synthesis. 

Experimental number distributions (Fig. 16) and chain termination probabilities (/3 and jSn) (Fig. 17) on Co catalysts at low values of bed residence time (<2 s, <10% CO conversion) are accurately described by the model. We reported previously a similar agreement on Ru catalysts (4). The model quantitatively describes the observed curvature of carbon number distribution plots (Fig. 16) and also the constant values of j3/y and the decreasing values of )3o observed as hydrocarbon size increases (Fig. 17). Such effects arise from the higher intrapellet fugacity and the higher residence time of larger a-olefins within transport-limited pellets. 

Chain growth during the Fischer-Tropsch synthesis is controlled by surface polymerization kinetics that place severe restrictions on our ability to alter the resulting carbon number distribution. Intrinsic chain growth kinetics are not influenced strongly by the identity of the support or by the size of the metal crystallites in supported Co and Ru catalysts. Transport-limited reactant arival and product removal, however, depend on support and metal site density and affect the relative rates of primary and secondary reactions and the FT synthesis selectivity. 

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