Enhancement factor reaction

The algebraic form of the expression (9.24) for the enhancement factor is specific to the particular reaction rate expression we have considered, and corresponding results can easily be obtained for other reactions in binary mixtures, for example the irreversible cracking A—2B. ... 

In any event the value of iri the presence of a chemical reac tion normally is larger than the value found when only physical absorption occurs, 7c . This has led to the presentation of data on the effects of chemical reaction in terms of the reaction factor or enhancement factor defined as... 

As discussed later, the reaction-enhancement factor ( ) will be large for all extremely fast pseudo-first-order reac tions and will be large tor extremely fast second-order irreversible reaction systems in which there is a sufficiently large excess of liquid-phase reagent. When the rate of an extremely fast second-order irreversible reaction system A -t-VB produc ts is limited by the availabihty of the liquid-phase reagent B, then the reac tion-enhancement factor may be estimated by the formula ( ) = 1 -t- B /VCj. In systems for which this formula is applicable, it can be shown that the interface concentration yj will be equal to zero whenever the ratio k yV/k B is less than or equal to unity. 

FIG. 23-28 Enhancement factor E and Hatta niimher of first- and second-order gas/liqiiid reactions, numerical solutions hy several hands. 

The turnover number of methylmalonyl-CoA epimerase is 100 sec and thus the enzyme enhances the reaction rate by a factor of 10. ... 

FXa complexes bind to and neutralize tissue factor/ FVIIa complexes, the key starting point of the extrinsic clotting cascade (see earlier) (Fig. 7). Heparin is able to enhance this reaction by direct binding to the complex and by releasing TFPI from the unaltered vessel wall, which then can access the TF-exposing surface. 

Figure 4.23. Comparison of predicted and measured enhancement factor A values for some of the early studies of catalytic reactions found to exhibit the NEMCA effect.1,19 Reprinted with permission from Elsevier Science.1...

This causes a 380% increase in rn2co and a 413% increase in rCo- The corresponding enhancement factors are AH2co=-17.5, ACo=-3. There is also a 190% increase in r< H4 with an enhancement factor ACh4= 0.3, but this rate increase has been shown56 to be Faradaic and due to the electrocatalytic reaction ... 

The reaction was investigated under atmospheric pressure and at temperatures 500°C to 600°C, where the only product was CO, as Pd, contrary to Rh, does not adsorb C02 dissociatively.59 This difference in reaction pathway is also reflected in the NEMCA behaviour of the system, since in the present case CO formation is enhanced (by up to 600%) not only with decreasing catalyst potential and work function, but also enhanced, although to a minor extent, via catalyst potential increase (Fig. 8.56). Enhancement factor A values up to 150 were measured. The reaction exhibits typical inverted volcano behaviour, which is characteristic of the weak adsorption of the reactants at the elevated temperature of this investigation, and thus of promotional rule G4. 

This same sensitivity can, however, be misleading. Even minor radical routes to a particular reaction product could give rise to intense polarized n.m.r. signals which could obscure the normal monotonic increase of the signal due to product formed by a non-radical route. This problem can be overcome in some cases by estimation of the spectral enhancement factor. Again, it is not possible to justify a firm, threshold value, but as a useful rule of thumb when enhancements fall below about 100 then the possibility of an important alternative non-radical route to the same product should be carefully investigated. 

Any fast reaction can enhance mass transfer. Consider a very fast, second-order reaction between the gas-phase component A and a liquid component B. The concentration of B will quickly fall to zero in the vicinity of the freshly exposed surface and a reaction plane, within which b = Q, will gradually move away from the surface. If components A and B have similar liquid-phase diflusivities, the enhancement factor is... 

Silica compounds are generally processed in conventional internal mixers, preferably with intermeshing rotors. These mixers are designed and optimized for carbon black-fiUed compounds in which mixing is based only on physical processes. When a silica-silane reinforcing system is used, additionally a chemical reaction, the sUanization, occurs. One of the main influencing factors of the silanization reaction is the concentration of ethanol in the compound as well as in the mixer [25,26]. As the silanization finally reaches an equilibrium, low concentrations of ethanol in the compound are expected to enhance the reaction rate. 

The enhancement factor of CO2 defined as ratio of the flux of CO2 with chemical reaction to that without chemical reaction is shown as follows ... 

It has been observed that under reaction conditions mass transfer is often significantly faster than would be expected based on the film model. This is modelled by introducing an enhancement factor, E. In case the concentration in the bulk liquid, ca, is zero, the rate of mass transfer of A now becomes ... 

The interpretation is straightforward. At reaction conditions the concentration in the film is lowered by reaction, and, as a consequence, the driving force for mass transfer increases. In a homogeneous system this results in high values of Ha. In a slurry reactor this enhancement can occur if the catalyst particles are so small that they accumulate in the film layer. Table 5.4-4 summarizes expressions for the reaction rate or enhancement factor for various regimes. 

The parameter p (= 7(5 ) in gas-liquid sy.stems plays the same role as V/Aex in catalytic reactions. This parameter amounts to 10-40 for a gas and liquid in film contact, and increases to lO -lO" for gas bubbles dispersed in a liquid. If the Hatta number (see section 5.4.3) is low (below I) this indicates a slow reaction, and high values of p (e.g. bubble columns) should be chosen. For instantaneous reactions Ha > 100, enhancement factor E = 10-50) a low p should be selected with a high degree of gas-phase turbulence. The sulphonation of aromatics with gaseous SO3 is an instantaneous reaction and is controlled by gas-phase mass transfer. In commercial thin-film sulphonators, the liquid reactant flows down as a thin film (low p) in contact with a highly turbulent gas stream (high ka). A thin-film reactor was chosen instead of a liquid droplet system due to the desire to remove heat generated in the liquid phase as a result of the exothermic reaction. Similar considerations are valid for liquid-liquid systems. Sometimes, practical considerations prevail over the decisions dictated from a transport-reaction analysis. Corrosive liquids should always be in the dispersed phase to reduce contact with the reactor walls. Hazardous liquids are usually dispensed to reduce their hold-up, i.e. their inventory inside the reactor. 

Riehl et al. also characterized the CL system lucigenin-hydrogen peroxide-A-methylacridone in the presence of different cationic surfactants such as HTAC, S-ClV-dodecyl-A lV-dimethylammonio) propane-1-sulfonate, and DODAB [41], Enhancement factors (ratio between CL intensity in the presence of organized medium and CL intensity in the absence of organized medium) of CL intensity were found of 3.4, 2.5, and 1.6, respectively. The alterations in CL intensity are explained in terms of the effect of the different surfactants on the rate of the reaction and on excitation efficiency. 

Finally, Yamada and Suzuki made a comparative study of the use of DDAB, HTAB, STAC, and CEDAB to improve the sensitivity and selectivity of the determination of ultratraces of Cu(II) by means of the CL reaction of 1,10-phenanthroline with hydrogen peroxide and sodium hydroxide, used as detection in a flow injection system [46]. Of the four cited surfactants it was found that CEDAB causes the greatest enhancement of the chemiluminescent signal (Fig. 12) (an enhancement factor of 140 with respect to the absence of surfactant). 

The same authors studied the CL of 4,4,-[oxalylbis(trifluoromethylsulfo-nyl)imino]to[4-methylmorphilinium trifluoromethane sulfonate] (METQ) with hydrogen peroxide and a fluorophor in the presence of a, p, y, and heptakis 2,6-di-O-methyl P-cyclodextrin [66], The fluorophors studied were rhodamine B (RH B), 8-aniline-l-naphthalene sulfonic acid (ANS), potassium 2-p-toluidinylnaph-thalene-6-sulfonate (TNS), and fluorescein. It was found that TNS, ANS, and fluorescein show CL intensity enhancement in all cyclodextrins, while the CL of rhodamine B is enhanced in a- and y-cyclodextrin and reduced in P-cyclodextrin medium. The enhancement factors were found in the range of 1.4 for rhodamine B in a-cyclodextrin and 300 for TNS in heptakis 2,6-di-O-methyl P-cyclodextrin. The authors conclude that this enhancement could be attributed to increases in reaction rate, excitation efficiency, and fluorescence efficiency of the emitting species. Inclusion of a reaction intermediate and fluorophore in the cyclodextrin cavity is proposed as one possible mechanism for the observed enhancement. 

Enhancement factor E. For reaction occurring only in the liquid film, whether instantaneous or fast, the rate law may be put in an alternative form by means of a factor that measures the enhancement of the rate relative to the rate of physical absorption of A in the liquid without reaction. Reaction occurring only in the liquid film is characterized by cA - 0 somewhere in the liquid film, and the enhancement factor E is defined by... 

Figure 9.8 Enhancement factor, E (Ha, ,), for fast gas-liquid reaction (in liquid film) reaction A(g) + bB(9 - products (B nonvolatile)...

Note that the enhancement factor E is relevant only for reaction occurring in the liquid film. For an instantaneous reaction, the expressions may or may not involve E, except that for liquid-film control, it is convenient, and for gas-film control, its use is not practicable (see problem 9-12(a)). The Hatta number Ha, on the other hand, is not relevant for the extremes of slow reaction (occurring in bulk liquid only) and instantaneous reaction. The two quantities are both involved in rate expressions for fast reactions (occurring in the liquid film only). 

The signal enhancement due to this approach can, in principle, be as high as 105-fold - that is, equal to the reciprocal Boltzmann factor however, the experimentally achievable enhancement factors typically range between 10 and 103. Thanks to this increase in sensitivity, the PHIP phenomenon, therefore, provides for a powerful tool to investigate the fate of the dihydrogen, the catalysts, and of the substrates during hydrogenation reactions. 

Situation 4 very fast reaction, Ha> 3 and Eboundary layer. The hydrogen concentration in the bulk of the liquid falls to zero. Thus, all the catalyst in the bulk is useless. For instantaneous reactions, Ha 3, E=EX and the reaction takes place in a narrow plane located somewhere in the boundary layer the larger Ea0 the closer to the interface the reaction plane. If the limiting enhancement factor E is very high, it is said that the reaction takes place at the gas-liquid interface. Such a case is referred to as surface reaction . 

The numerical solution of these equations is shown on the plot which is due to van Krevelen Hoftijzer (Trans Instn Chem Engrs 32 S360, 1954). The plot is of the enhancement factor E against the Hatta number (3 which is defined in P8.02.01. The parameters along the curves are of a ratio, a = CbLDb/CaLD0. The uppermost curve is for a first order reaction. 

A solute A in a gas phase reacts with B in the liquid phase by a second order reaction. Some results of a numerical solution by Perry Pigford (Ind Eng Chem 45 1247, 1953) for the enhancement factor are fitted by the equation... 

Enhanced surfactant flooding, 23 532 Enhancement factor, gas absorption with reaction, 1 47-48 Enhancement programs, aquatic organisms, 3 183, 198 Enhancement reagents, 12 102 Enhancer, 10 688 Enichem oxo-alcohols, 17 725 Enkaid, molecular formula and structure, 5 92t... 

The equations (23) to (27) can only be solved numerically. However such a numerical solution gives less insight in the factors governing the transport and conversion processes. Therefore we consider another approach. In this approach, the transport and conversion of component A are calculated under the assumption that no reaction of ozone with component B takes place. The enhancement factor for mass transfer of ozone, EA, can now be given by the equation ... 

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