Irreversability/reversibility

Deactivation of zeolite catalysts occurs due to coke formation and to poisoning by heavy metals. In general, there are two types of catalyst deactivation that occur in a FCC system, reversible and irreversible. Reversible deactivation occurs due to coke deposition. This is reversed by burning coke in the regenerator. Irreversible deactivation results as a combination of four separate but interrelated mechanisms zeolite dealu-mination, zeolite decomposition, matrix surface collapse, and contamination by metals such as vanadium and sodium. 

Although substrates may enhance or inhibit their own conversion, as noted in Section 10.4.1, other species may also affect enzyme activity. Inhibitors are compounds that decrease observable enzyme activity, and activators increase activity. The combination of an inhibitor or activator with an enzyme may be irreversible, reversible, or partially... 

Thrombin inhibition Irreversible Reversible Irreversible Reversible... 

The properties of nanocomposite systems, whose microstructures aim at reproducing real systems, have been examined in various numerical modelling studies [127, 128], In general, the essential features of the hysteresis cycles may be satisfactory reproduced. In particular, soft layer reversal is quantitatively accounted for, which is expected for reversible phenomena. By contrast, the calculated high-field irreversible reversal of the hard phase magnetization is not reproduced in general. Such discrepancy illustrates the already mentioned difficulty to describe irreversible processes. 

Potential at half-height — (in voltammetry) This is a diagnostic criterion in -> linear scan voltammetry. The potential at half-height Ep/2 is the potential at which the current is equal to one-half of the peak current fp Ep/2 = h (/=/p/2)- I he first of two potentials at half-height, the one that precedes the peak potential (Ep) is considered only. If a simple electrode reaction is reversible (- reversibility) and controlled by the planar, semi-infinite - diffusion, the absolute value of the difference between Ep/2 and Ep is equal to 56.6/n mV and independent of the - scan rate. If the -> electrode reaction of dissolved reactant is totally irreversible (-> reversibility), the difference Ep/2 - Ep is equal to 47.7/an mV for the cathodic process and -47.7/(l - a)n mV for the anodic process. 

The scan rate is an important parameter for potential sweep methods such as CV or LSV The current is proportional to the square root of the scan rate in all electrochemical systems—irreversible, reversible, and quasi-reversible systems. Figure 4.4 shows the LSV for EMI—TFSl using a glassy carbon (GC) [49]. Note in this figure... 

A. controlled by the end product, catalyzed by a specific enz)une. irreversible, reversible. 

Hybrids may be reversible or irreversible. Reversible hybrids act like sponges, soaking up the hydrogen. They are usually solids. These alloys or intermetallic compounds release hydrogen at specific pressures and temperatures. They may be replenished by adding pure hydrogen. 

Pollutant release is determined by the type of binding, which can be reversible or irreversible. Reversible binding is in equilibrium with the dissolved species and is reduced with decreasing solution concentration. It is also affected by other changes in solution composition. Irreversibly bound pollutants are not in equilibrium with dissolved species. Normally, a clear distinction between irreversibly and reversibly bound forms is not possible, and in most cases the binding strength lies between these two extremes (Brady and Boms 1997). Characterization becomes even more complicated if the integrity of the matrix depends... 

Inhibitors can be grouped broadly in two categories, reversible and irreversible. Reversible inhibitors are weakly bonded to the surface and therefore can be removed relatively easily (by dialysis or simple dilution). Irreversible inhibitors are essentially those that cannot be easily removed. Several categories of reversible enzymes are possible the most important are competitive, noncompetitive, and substrate. Given in Table 20.5 are brief descriptions of these types of inhibition along with their effects on the constants of the MM model, as reflected in the original MM equation (see Levenspiel, 1993, for derivations). 

A reversible inhibitor (in contrast to one that is irreversible) reversibly binds to an enzyme. An eqnilibrinm is established therefore, the inhibitor can be removed from the enzyme by shifting the eqnilibrinm ... 

The reaction time depends on the kinetics and therefore on the reaction type. If the reaction is simple, complex, irreversible, reversible, or multiple, one may integrate the equation using analytical or numerical methods. 

Therefore, to calculate the reactor volume or inlet and outlet flows in the reactor we should know the rate constants, which are usually irreversible, reversible, multiple, complex, enzymatic, or polymeric. 

The equation above is valid for any gas or liquid system. The reaction rate will depend on the reaction kinetic model. The reaction may be irreversible, reversible, simple, multiple, elementary or not, enzymatic or polymeric. If the reaction occurs with varying the number of moles or variable volume, one should take into account the factor b defined previously. 

But if there is no volume variation sa = 0), we obtain the same expression for space time (Equation 14.38), meaning that the mean residence time is equal to the space time. When there is volume variation ba 0), the mean residence time and space are different. The rate expression depends on the reaction order and the factor Sa for any kind of reaction (irreversible, reversible multiple, etc.). In short ... 

The annihilation rate constant in PS II is much the same as in PS I, as found by analogous measurements but with an irreversible reaction scheme. This correlates with the similar structure and pigment composition of the two antenna systems of higher plants and shows that this functional parameter is rather independent of the mechanism (irreversible/ reversible) and the rate of trapping. If in analogy to the theory of exciton dynamics for... 

Enzyme-catalyzed reactions can be strongly inhibited by the presence of other compounds. This effect sets the basis for many enzyme assays enabling, in the presence of a substrate, the determination of the concentration of the inhibitor (analyte). There are two mechanisms of inhibition reversible and irreversible. Reversible inhibition can be further distinguished as competitive, noncompetitive, uncompetitive, and mixed type of inhibition. A brief scheme of the most important mechanisms is depicted in Figure 8. 

Under physiological conditions the pathway of purine biosynthesis de novo is believed to be irreversible. Reversibility of some reactions can, however, be demonstrated under some conditions of incubation in vitro. If Ehrlich ascites tumor cells are incubated with formate- C in the absence of glucose, for example, the 2-position of the purine ring may contain 8 to 10 times as much C as does the 8-position (SO), although these two positions should be equally labeled if net synthesis only had occurred. Apparently inosinate can be reversibly converted to phosphoribosyl amino-imidazole carboxamide, which in reforming inosinate incorporates radioactive formate. 

An inhibitor is a compound that decreases the rate of an enzyme-catalyzed reaction. Moreover, this inhibition can be reversible or irreversible. Reversible enzyme inhibition can be competitive, uncompetitive, or linear mixed type, each affecting Ks and Vmax in a specific fashion. In this chapter, each type of reversible inhibition is discussed in turn. This is followed by two examples of strategies used to determine the nature of the inhibition as well as to obtain estimates of the enzyme-inhibitor dissociation constant (Ki). 

The results of the previous section are now applied and different kinetic systems examined. Stoichiometric equations are written for irreversible, reversible, simultaneous, and consecutive reactions. Each is followed by the describing rate equation. The next section contains a detailed analysis of the different methods of solution for several of these kinetic systems. The units and notations of the various reaction velocity constants employed are carefully reviewed at the conclusion of this section. 

Chromophores can be classified in two categories reversible and irreversible. Reversible chromophores, often named molecular switches, undergo... 

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