System involving chemical reactions

Most interesting chemical reaction systems involve multiple reactions, which we wiU consider in the next chapter. In this section we examine several reaction systems that involve nearly a single reaction and are both industrially important and involve some interesting reaction engineering issues. 

There are many systems that can fluctuate randomly in space and time and cannot be described by deterministic equations. For example. Brownian motion of small particles occurs randomly because of random collisions with molecules of the medium in which the particles are suspended. It is useful to model such systems with what are known as stochastic differential equations. Stochastic differential equations feature noise terms representing the behavior of random elements in the system. Other examples of stochastic behavior arise in chemical reaction systems involving a small number of molecules, such as in a living cell or in the formation of particles in emulsion drops, and so on. A useful reference on stochastic methods is Gardiner (2003). 

A single-channel manifold also can be used for systems in which a chemical reaction generates the species responsible for the analytical signal. In this case the carrier stream both transports the sample to the detector and reacts with the sample. Because the sample must mix with the carrier stream, flow rates are lower than when no chemical reaction is involved. One example is the determination of chloride in water, which is based on the following sequence of reactions. ... 

Water generally is used for gases fairly soluble in water, oils for light hydrocarbons, and special chemical solvents for acid gases such as CO9, SO9, and H9S. Sometimes a reversible chemical reaction will result in a veiy high solubility and a minimum solvent rate. Data on actual systems are desirable when chemical reactions are involved, and those available are referenced later under Absorption with Chemical Reaction. ... 

In 1966, in a paper that now is considered a classic, Danckwerts and Gillham [Tmns. Inst. Chem. Eng., 44, T42 (1966)] showed that data taken in a small stirred-ceU laboratoiy apparatus could be used in the design of a packed-tower absorber when chemical reactions are involved. They showed that if the packed-tower mass-transfer coefficient in the absence of reaction (/cf) can be reproduced in the laboratory unit, then the rate of absorption in the l oratoiy apparatus will respond to chemical reactions in the same way as in the packed column even though the means of agitating the hquid in the two systems might be quite different. 

Step 4 Define the System Boundaries. This depends on the nature of the unit process and individual unit operations. For example, some processes involve only mass flowthrough. An example is filtration. This unit operation involves only the physical separation of materials (e.g., particulates from air). Hence, we view the filtration equipment as a simple box on the process flow sheet, with one flow input (contaminated air) and two flow outputs (clean air and captured dust). This is an example of a system where no chemical reaction is involved. In contrast, if a chemical reaction is involved, then we must take into consideration the kinetics of the reaction, the stoichiometry of the reaction, and the by-products produced. An example is the combustion of coal in a boiler. On a process flow sheet, coal, water, and energy are the inputs to the box (the furnace), and the outputs are steam, ash, NOj, SOj, and CO2. 

The backward reaction tends to increase the resistance to mass transfer. If the backward reaction rate is very small compared with the forward reaction rate, the transfer rate is at its highest value. Then, as the backward reaction rate is increased, the transfer rate begins to decline. When the backward reaction rate approaches infinity, the chemical reaction exerts no influence on the mass transfer and the system behaves as if no chemical reaction is involved. 

If the reactant solid is porous, the reactant fluid would diffuse into it while reacting with it on its path diffusion and chemical reaction would occur in parallel over a diffuse zone. The analysis of such a reaction system is normally more complex as compared to reaction systems involving nonporous solids. Here also it is important to assess the relative importance of chemical reaction kinetics and of mass and heat transport. 

In discussions of systems in which only a single chemical reaction is involved, one may use the words yield and conversion as complementary terms. However, in dealing with multiple reactions, conversion refers to the proportion of a reagent that reacts, while yield refers to the... 

The photochemistry of the polluted atmosphere is exceedingly complex. Even if one considers only a single hydrocarbon pollutant, with typical concentrations of nitrogen oxides, carbon monoxide, water vapor, and other trace components of air, several hundred chemical reactions are involved in a realistic assessment of the chemical evolution of such a system. The actual urban atmosphere contains not just one but hundreds of different hydrocarbons, each with its own reactivity and oxidation products. 

Before ozone - and PAN were identified as specific phytotoxic components of the photochemical complex, researchers used a number of artificial chemical reaction systems to simulate the ambient photochemical-oxidant situation. These efforts involved a number of irradiated and nonirradiated reaction systems unsaturated hydrocarbon-ozone mixtures, unsaturated hydrocarbon-NOx mixtures, and dilute auto exhaust). Most research before 1960 involved one or more of these reaction systems. This research has been well reviewed " - 451.459.488.505 extenslvely covered here. Although the... 

Thus we see from these examples that almost aU the important chemical reactions in the petroleum and chemical industries require the skilled processing of feedstocks to produce specific products. These industries are based on the successful handling of simultaneous reaction systems involving an almost infinite set of chemical reactions. 

Parallel reactions play an important role in chemical reaction systems that involve selectivity. An example is the selective noncatalytic reduction of NO (SNCR), which is a widespread secondary measure for NO control. In this process NO is reduced to N2 by injection of a reducing agent such as NH3 into the flue gas in a narrow temperature range around 1000°C. The process is characterized by a selectivity in the reaction pathways as shown by the parallel (global) steps... 

One must be aware of the fact that when the set pressure of the protection device is reached, the pressure increase does not stop immediately, but continues to increase to the maximum pressure before decreasing. These two pressure levels, set pressure and maximum pressure, have to be defined during the design procedure. The design can be for two different scenarios, the physical scenario where no chemical reaction is involved and the chemical scenario where a chemical reaction determines the behavior of the system. 

It can be concluded from the simple analysis above that impinging streams can only be used for gas-liquid reaction or chemical absorption systems involving fast reaction(s) in liquid for success. 

A chemical reaction always involves bond-breaking/making processes or valence electron rearrangements, which can be characterized by the variation of VB structures. According to the resonance theory [1, 50], the evolution of a system in the elementary reaction process can be interpreted through the resonance among the correlated VB structures corresponding to reactant, product and some intermediate states. Because only symmetry-adapted VB structures can effectively resonate, all VB structures involved in the description of a reaction will thus retain the symmetry shared by both reactant and product states in the elementary process. Therefore, we postulate that the VB structures of the reactant and the product states for concerted reactions should preserve symmetry-adaptation, called the VB structure symmetry-adaptation (VBSSA) rule. 

In certain chemical and biological as well in some unit operations, foaming occurs to such an extent that the process is severely impaired or even comes to a complete standstill. For example, chemical reaction systems tend to foam if a gas is formed in nascent state, because such minute gas bubbles do not coalesce to form larger ones and therefore remain in the system. Expulsion of residual monomers after emulsion polymerization often involves serious foaming problems because, in this case, very fine gas bubbles are formed in a material system which contains emulsifiers, i.e. foam producing surfactants. 

Numerous physical transformations can be considered as a system changes from one energy state to another. Chemical reactions also involve reactants and products that have different energies. As a result, it is important to understand the relationship between equilibrium and energy. 

Generalized prediction methods for hL and HL do not apply when chemical reaction occurs in the liquid phase, and therefore one must use actual operating data for the particular system in question. A discussion of the various factors to consider in designing gas absorbers and strippers when chemical reactions are involved is presented by Astarita, Savage, and Bisio, Gas Treating with Chemical Solvents, Wiley (1983) and by Kohl and Nielsen, Gas Purification, 5th ed., Gulf (1997). 

It can be inferred from the above descriptions that chemical reactions may involve processes characteristic of one or all of. these categories in such fashion as to become almost impossible of simple description or classification. Because of this near infinity of possible behaviors of reacting systems, we shall restrict our discussion in the present chapter to the most general methods for the mathematical description of such systems. At the present stage, this is all that can be done to provide a basis for their study. As the experimenter will easily discover, kinetic systems when investigated in detail display an anarchistic tendency to become unique laws unto themselves. 

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