Chemical reactions instantaneous

Perturbation or relaxation techniques are applied to chemical reaction systems with a well-defined equilibrium. An instantaneous change of one or several state fiinctions causes the system to relax into its new equilibrium [29]. In gas-phase kmetics, the perturbations typically exploit the temperature (r-jump) and pressure (P-jump) dependence of chemical equilibria [6]. The relaxation kinetics are monitored by spectroscopic methods. 

The well-known phenomenon of smell-fatigue is explained by the theory that actual chemical reaction takes place between the odoriferous body and some reacting material in the nose thus it can easily be conceived that some sort of addition reaction takes place and that directly the osmoceptor in the nose becomes saturated no further reaction is possible and no further odour can be appreciated until fresh osmoceptor has been formed. Ruzicka has suggested that two such osmoceptors are involved since substances inspired in a concentrated state have odours different to those perceived in a dilute condition. He suggests that one osmoceptor reacts more readily than the other and in consequence is the more readily saturated or consumed, this osmoceptor is responsible for the sensation produced when dilute odours are inspired. If the odour be concentrated, the first osmoceptor is saturated almost instantaneously and then the sensation produced is the result of the reaction between the odoriferous substance and the second osmoceptor. 

In batch operations, mixing takes place until a desired composition or concentration of chemical products or solids/crystals is achieved. For continuous operation, the feed, intermediate, and exit streams will not necessarily be of the same composition, but the objective is for the end/exit stream to be of constant composition as a result of the blending, mixing, chemical reaction, solids suspension, gas dispension, or other operations of the process. Perfect mixing is rarely totally achieved, but represents the instantaneous conversion of the feed to the final bulk and exit composition (see Figure 5-26). 

Characteristic length [Eq. (121)] L Impeller diameter also characteristic distance from the interface where the concentration remains constant at cL Li Impeller blade length N Impeller rotational speed also number of bubbles [Eq, (246)]. N Ratio of absorption rate in presence of chemical reaction to rate of physical absorption when tank contains no dissolved gas Na Instantaneous mass-transfer rate per unit bubble-surface area Na Local rate of mass-transfer per unit bubble-surface area Na..Average mass-transfer rate per unit bubble-surface area Nb Number of bubbles in the vessel at any instant at constant operating conditions N Number of bubbles per unit volume of dispersion [Eq. (24)] Nb Defined in Eq. (134)... 

Consider the case when the equilibrium concentration of substance Red, and hence its limiting CD due to diffusion from the bulk solution, is low. In this case the reactant species Red can be supplied to the reaction zone only as a result of the chemical step. When the electrochemical step is sufficiently fast and activation polarization is low, the overall behavior of the reaction will be determined precisely by the special features of the chemical step concentration polarization will be observed for the reaction at the electrode, not because of slow diffusion of the substance but because of a slow chemical step. We shall assume that the concentrations of substance A and of the reaction components are high enough so that they will remain practically unchanged when the chemical reaction proceeds. We shall assume, moreover, that reaction (13.37) follows first-order kinetics with respect to Red and A. We shall write Cg for the equilibrium (bulk) concentration of substance Red, and we shall write Cg and c for the surface concentration and the instantaneous concentration (to simplify the equations, we shall not use the subscript red ). 

Two gaseous streams are available for use in carrying out a chemical reaction. The first contains pure A and is produced at a rate of 400 ft3/min. The second contains 50% B (remainder is an inert material) and has a flow rate of 200 ft3/min. These streams are mixed instantaneously and fed to a flow reactor. Both streams are at the same temperature (86 °C) and pressure (1 atm) and these quantities remain unchanged during the instantaneous mixing process. The gases behave ideally. A and B react to form an addition product... 

Nevertheless, chemical methods have not been used for determining ionization equilibrium constants. The analytical reaction would have to be almost instantaneous and the formation of the ions relatively slow. Also the analytical reagent must not react directly with the unionized molecule. In contrast to their disuse in studies of ionic equilibrium, fast chemical reactions of the ion have been used extensively in measuring the rate of ionization, especially in circumstances where unavoidable irreversible reactions make it impossible to study the equilibrium. The only requirement for the use of chemical methods in ionization kinetics is that the overall rate be independent of the concentration of the added reagent, i.e., that simple ionization be the slow and rate-determining step. 

Finally, to conclude our discussion on coupling with chemistry, we should note that in principle fairly complex reaction schemes can be used to define the reaction source terms. However, as in single-phase flows, adding many fast chemical reactions can lead to slow convergence in CFD simulations, and the user is advised to attempt to eliminate instantaneous reaction steps whenever possible. The question of determining the rate constants (and their dependence on temperature) is also an important consideration. Ideally, this should be done under laboratory conditions for which the mass/heat-transfer rates are all faster than those likely to occur in the production-scale reactor. Note that it is not necessary to completely eliminate mass/heat-transfer limitations to determine usable rate parameters. Indeed, as long as the rate parameters found in the lab are reliable under well-mixed (vs. perfect-mixed) conditions, the actual mass/ heat-transfer rates in the reactor will be lower, leading to accurate predictions of chemical species under mass/heat-transfer-limited conditions. 

Both parts (a) and (b) of Example 6-1 illustrate that rates of molecular collisions are extremely large. If collision were the only factor involved in chemical reaction, the rates of all reactions would be virtually instantaneous (the rate of N2-02 collisions in air calculated in Example 6-l(a) corresponds to 4.5 X107 mol L-1 s-1 ). Evidently, the energy and orientation factors indicated in equation 6.4-2 are important, and we now turn attention to them. 

Fast reaction. If chemical reaction is sufficiently fast, even though not instantaneous, it is possible for it to occur entirely within the liquid-film, but not at a point or plane. This case is considered as a special case of reaction in both bulk liquid and liquid film in Section 9.2.3.4. In this situation, NA = ( rA). 

Internal Detonations or Explosions An internal detonation or explosion may occur due to several scenarios. Air leakage into the system may cause a combustible mixture to form, undesired chemical reactions may occur, and extremely rapid vapor expansion may occur. These almost instantaneous events have to be carefully protected against as many overpressure devices do not react quickly enough to prevent the vessel from rupturing. 

Most of the theoretical works concerning dynamical aspects of chemical reactions are treated within the adiabatic approximation, which is based on the assumption that the solvent instantaneously adjusts itself to any change in the solute charge distribution. However, in certain conditions, such as sudden perturbations or long solvent relaxation times, the total polarization of the solvent is no longer equilibrated with the actual solute charge distribution and cannot be properly described by the adiabatic approximation. In such a case, the reacting system is better described by nonequilibrium dynamics. 

Baldyga, J. (1989). Turbulent mixer model with application to homogeneous, instantaneous chemical reactions. Chemical Engineering Science 44, 1175-1182. 

In the case of supersonic combustion ramjet devices, instantaneous ignition must occur because the flow time in the constant area duct that comprises the ramjet chamber is short. As noted in Chapter 1, supersonic combustion simply refers to the flow condition and not to any difference in the chemical reaction mechanism from that in subsonic ramjet devices. What is unusual in supersonic combustion because of the typical flow condition is that the normal ignition time is usually longer than the reaction time. To assure rapid ignition in this case, many have proposed the injection of silane (SiH4), which is... 

When an automobile collision activates an air bag, sodium azide, NaN3(g), decomposes to form sodium, Na(s), and nitrogen gas, N2(g). (The gas inflates the bag.) This chemical reaction occurs almost instantaneously. It inflates the air bag quickly enough to cushion a driver s impact in a collision. 

Why do some reactions occur slowly while others seem to take place instantaneously How do chemists measure, compare, and express the rates at which chemical reactions occur Can chemists predict and control the rate of a chemical reaction These questions will be answered in Chapter 6. 

Distinguish between the instantaneous rate and the initial rate of a chemical reaction. Under what circumstances would these two rates be the same ... 

Mass transfer with instantaneous chemical reaction... 

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