Inhibition Ammonia

The standard SCR LH model predicts the inhibition by NH3 as mentioned earlier in relation to Eq. (11.8). The standard SCR Redox model can also predict NH3 inhibition through the addition of another step that accounts for NH3 adsorption on Fe sites. This follows from recent work from the Milano group in which ammonia adsorption can result in hysteresis associated with NH3 feed transients, among other features. 

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Vanadia-titania ( 5 and other supported vanadia catalysts (9) can also be applied for the production of aromatic nitriles by ammoxidation of toluene and of the three xylene isomers allumina-supported V-Sb-based oxides seem to be the best catalysts (10). Detailed kinetic studies of toluene ammoxidation have been reported recently using different vanadia-titania catalysts ( 77,72). Ammonia inhibits toluene conversion, while benzonitrile yields (up to 80 % near 610 K) are mainly limited by... 

A minor transient feature was also manifested when ammonia was admitted to the reactor (t — 0 s) the NO outlet concentration immediately decreased, went through a weak minimum near 150 s and finally slightly increased, reaching steady state in correspondence of the end of the ammonia feed phase (tx 2,800 s). Again, the nitrogen evolution was symmetrical to that of NO. The same ammonia inhibition effect invoked to explain the enhancement in the deNOx conversion at ammonia shutdown can explain this transient behavior, too. In fact both features suggest the existence of an optimal ammonia surface concentration, which is lower than the coverage established at steady state. 

Accordingly, we conclude that the dual-site MR approach is compatible with the ammonia inhibition effects observed during unsteady SCR experiments, as well as with the oxygen dependence of the SCR kinetics at low temperatures, and can be successfully applied to simulate the complex dynamic behavior of... 

There is some evidence that cation and radical are not generated independently. If cation and radical are formed separately, inhibition of one active species will not influence the reactivity of other. The experimental results suggest the interrelation of cation and radical. For example, the addition of ammonia inhibits the cationic process, whereas the yield of radical polymerization even increases. Also, the content of... 

The nature of propagating species is definitely cationic since the addition of ammonia inhibits polymerization. The addition of water decreases the molecular weight of the polymer whereas DPPH does not... 

Glucose activation constant in specific growth rate expression (mM) Glutamine activation constant in specific growth rate expression (mM) Lactate inhibition constant in specific growth rate expression (mM) Ammonia inhibition constant in specific growth rate expression (mM) Lactate concentration (mM)... 

Ammonia inhibits the formation of partial (amm)oxidation products by competitve adsorption. It can also modify the degree of reduction of the surface and thereby shifting the selectivity of the reaction. To obtain a maximal selectivity to the partial (amm)oxidation products a molar ammonia/propene ratio of less than one should be applied. 

Lauterbiick, B., Ortner, M., Haider, R., Euchs, W. (2012). Counteracting ammonia inhibition in anaerobic digestion by removal with a hollow fiber membrane contactor. Water Research, 46, 4861—4869. 

The transient kinetic model of the standard SCR reaction over a commercial V-based catalyst for vehicles reported in Reference (101) is the only treatment available so far accounting both for the redox nature of the SCR catalytic mechanism and for the ammonia inhibition effect. It relies on a dual-site redox scheme, whereby ammonia is first adsorbed onto acidic sites, but reacts with NO on different redox sites associated with the vanadium component. The redox sites can, however, be blocked by excess ammonia. Adopting a Mars-Van Krevelen formal approach, the following modified redox (MR) rate expression was derived (27) ... 

Mod d Redox Kinetics (MR)— In order to take into account the redox nature of the Standard SCR reaction and the observed transient features indicating an ammonia inhibition effect, a new kinetic model was derived. 

When proceeding to the right, step (10.24) results in NH3 blocking sites Si, which are thus subtracted from the redox cycle. Accordingly, step (10.24) can in principle account for the observed ammonia inhibition. 

The parameter estimates provided by the fit based on MR kinetics were also used to simulate the experiments of Fig. 10.6 pulsed ammonia steps (1,000 ppm) were fed at 200 °C while feeding 1,000 ppm of NO, 2 % O2, 1 % H2O in He with a GHSV of 92,000 h . Two mns were performed with different pulse frequency. Notably, these were particularly demanding conditions with respect to the ammonia inhibition effect. The simulation results are also displayed in Fig. 10.6 (solid fines) it clearly appears that, on a purely predictive basis, the MR rate expression was able to follow the fast transients of these experiments. 

The MR rate expression Eq. (10.34) differs from the Eley-Rideal rate Eq. (10.18) only in its denominator, which accounts both for the adverse kinetic effect of NH3 and for the favorable O2 dependence at low ammonia coverage (as, e.g., at high temperature), the denominator tends to unity and Eq. (10.34) formally reduces to Eq. (10.18). Indeed, this is consistent with the experimental indications discussed above the ammonia inhibiting effect is particularly evident at low temperatures, but tends to disappear in the runs performed at temperatures of 250 °C and above, where ammonia coverage becomes lower. Likewise, the oxygen dependence is reportedly most manifest at low temperatures. 

Nielsen HB, Agelidaki I (2008) Strategies for optimizing recovery of the biogas process following ammonia inhibition. Bioresour Technol 99 7995-8001... 

Wiegant WM, Zeeman G (1986) The mechanism of ammonia inhibition in the thermophilic digestion of livestock wastes. Agric Wastes 16 243-253... 

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