Chemical reactions mass difference

The spatio-temporal variations of the concentration field in turbulent mixing processes are associated wdth very different conditions for chemical reactions in different parts of a reactor. This scenario usually has a detrimental effect on the selectivity of reactions when the reaction time-scale is small compared with the mixing time-scale. Under the same conditions (slow mixing), the process times are increased considerably. Due to mass transfer inhibitions, the true kinetics of a reaction does not show up instead, the mixing determines the time-scale of a process. This effect is known as mixing masking of reactions [126]. 

From the previous discussion, equilibrium relations required for process circuit analysis are evidently in ortant. To achieve equilibrium requires equipment infinite in size, which is a physical and economical in jossibility. We must be satisfied wifii an economical approach to equilibriimi conditions. In some cases, because of rapid mass transfer or chemical reaction, the difference between actual and equilibrium conditions is insignificant. 

In many processes, diffusion rates are altered by homogeneous chemical reactions. A homogeneous reaction is one that takes place in solution. In that case, the RA term in equation (1-64), representing the volumetric rate of formation of component A by chemical reaction, is different from zero. The following examples illustrate some of the effects of homogeneous chemical reactions on mass transfer rates. 

Figure 3-13. Coefficients of selectivity (x) characterizing isotopic effect in non-equilibrium plasma-chemical reactions of different molecules as a function of relative molecular mass difference AA//A/ of the molecular isotopes. Numbers on curves represent activation energies of specific plasma-chemical reactions (in electron volts) circles represent positions of the specific plasma-chemical reactions of vibrationally excited molecules.

An example of wave propagation in an RD column is shown in Figs. 10.21 and 10.22. Constant pattern waves can be observed in case of ideal phase equilibrium, kinetically controlled mass transfer and a single reversible reaction close to chemical equilibrium (compare Figs. 10.21). In contrast, at low Da in the kinetically controlled regime of the chemical reaction a different, more complex type of dynamic behavior can be observed (compare Fig. 10.22). The behavior in the kinetic regime is not sufficiently understood today and needs further research. 

Dunham and Edie [98] established a mathematical model of the stabilization process for 12-60k PAN fiber and checked theory with experiments using 3k and 12k 1.22 d tex Courtauld SAF PAN fiber (6% MA, 1% ITA) by embedding a thermocouple in the fiber bundle. The governing equations for the model are based on the rates of chemical reactions, mass balances on reacting species, radial mass transfer and radial heat transfer within the bundle. They showed that the fiber bundle can be as much as 15°C above the stabilization oven temperature and the model predicted the measured temperatures quite well, except for, as would be expected, run-away reaction conditions. Samples stabilized below 230°C did not exhibit a skin core effect, but above 245°C, exhibited distinct skin core differences which were observed by reflected light microscopy. Hence diffusion appears to limit the stabilization rate above 245°C but not below 230°C. Bundles larger than 12k tended to bum when stabilized much above 230°C. The model would not hold for temperatures above 245°C. 

Figure 34 Conversion vs. dimensionless rate constant from simple one-dimensional, two-region, axially uniform model for first-order chemical reaction with different overall voidages of 0.90 and 0.96 and different values of the dimensionless core-annulus interregion mass transfer coefficient.

Reactive scattering or a chemical reaction is characterized by a rearrangement of the component particles within the collision system, thereby resulting in a change of the physical and chemical identity of the original collision reactants A + B into different collision products C + D. Total mass is conserved. The reaction is exothemiic when rel(CD) > (AB) and is endothermic when rel(CD) < (AB). A threshold energy is required for the endothemiic reaction. 

Measurements usually consist of a unit and a number expressing the quantity of that unit. Unfortunately, many different units may be used to express the same physical measurement. For example, the mass of a sample weighing 1.5 g also may be expressed as 0.0033 lb or 0.053 oz. For consistency, and to avoid confusion, scientists use a common set of fundamental units, several of which are listed in Table 2.1. These units are called SI units after the Systeme International d Unites. Other measurements are defined using these fundamental SI units. For example, we measure the quantity of heat produced during a chemical reaction in joules, (J), where... 

The dimensions of permeabiUty become clear after rearranging equation 1 to solve for P. The permeabiUty must have dimensions of quantity of permeant (either mass or molar) times thickness ia the numerator with area times a time iaterval times pressure ia the denomiaator. Table 1 contains conversion factors for several common unit sets with the permeant quantity ia molar units. The unit nmol/(m-s-GPa) is used hereia for the permeabiUty of small molecules because this unit is SI, which is preferred ia current technical encyclopedias, and it is only a factor of 2, different from the commercial permeabihty unit, (cc(STP)-mil)/(100 in. datm). The molar character is useful for oxygen permeation, which could ultimately involve a chemical reaction, or carbon dioxide permeation, which is often related to the pressure in a beverage botde. 

Over 25 years ago the coking factor of the radiant coil was empirically correlated to operating conditions (48). It has been assumed that the mass transfer of coke precursors from the bulk of the gas to the walls was controlling the rate of deposition (39). Kinetic models (24,49,50) were developed based on the chemical reaction at the wall as a controlling step. Bench-scale data (51—53) appear to indicate that a chemical reaction controls. However, flow regimes of bench-scale reactors are so different from the commercial furnaces that scale-up of bench-scale results caimot be confidently appHed to commercial furnaces. For example. Figure 3 shows the coke deposited on a controlled cylindrical specimen in a continuous stirred tank reactor (CSTR) and the rate of coke deposition. The deposition rate decreases with time and attains a pseudo steady value. Though this is achieved in a matter of rninutes in bench-scale reactors, it takes a few days in a commercial furnace. 

It is important to understand that when chemical reactions are involved, this definition of Cl is based ou the driving force defined as the difference between the couceutratiou of un reacted solute gas at the interface and in the bulk of the liquid. A coefficient based ou the total of both uureacted and reached gas could have values. smaller than the physical-absorption mass-transfer coefficient /c . 

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. 

A special type of substituent effect which has proved veiy valuable in the study of reaction mechanisms is the replacement of an atom by one of its isotopes. Isotopic substitution most often involves replacing protium by deuterium (or tritium) but is applicable to nuclei other than hydrogen. The quantitative differences are largest, however, for hydrogen, because its isotopes have the largest relative mass differences. Isotopic substitution usually has no effect on the qualitative chemical reactivity of the substrate, but often has an easily measured effect on the rate at which reaction occurs. Let us consider how this modification of the rate arises. Initially, the discussion will concern primary kinetic isotope effects, those in which a bond to the isotopically substituted atom is broken in the rate-determining step. We will use C—H bonds as the specific topic of discussion, but the same concepts apply for other elements. 

An important mixing operation involves bringing different molecular species together to obtain a chemical reaction. The components may be miscible liquids, immiscible liquids, solid particles and a liquid, a gas and a liquid, a gas and solid particles, or two gases. In some cases, temperature differences exist between an equipment surface and the bulk fluid, or between the suspended particles and the continuous phase fluid. The same mechanisms that enhance mass transfer by reducing the film thickness are used to promote heat transfer by increasing the temperature gradient in the film. These mechanisms are bulk flow, eddy diffusion, and molecular diffusion. The performance of equipment in which heat transfer occurs is expressed in terms of forced convective heat transfer coefficients. 

The chemical engineer is concerned with the industrial application of processes. This involves the chemical and microbiological conversion of material with the transport of mass, heat and momentum. These processes are scale-dependent (i.e., they may behave differently in small and large-scale systems) and include heterogeneous chemical reactions and most unit operations. Tlie heterogeneous chemical reactions (liquid-liquid, liquid-gas, liquid-solid, gas-solid, solid-solid) generate or consume a considerable amount of heat. However, the course of... 

The rate of a chemical reaction can be described in any of several different ways. The most commonly used definition involves the time rate of change in tlie amount of one of the components participating in tlie reaction tliis rate is usually based on some arbitrary factor related to tlie reacting system size or geometry, such as volume, mass, or interfacial area. Tlie definition shown in Eq. (4.6.7), wliich applies to homogeneous reactions, is a convenient one from an engineering point of view. 

The energy liberated in nuclear reactions is of such magnitude that mass differences are relatively easy to detect. The same mass-energy considerations pertain to chemical reactions. However, the energies involved are millions of times smaller, and mass differences are virtually impossible to detect. Discussions of energies involved in chemical reactions do not include mass energy. Nevertheless, there is eveiy reason to believe that mass energy is involved. 

These operations are characterized by different reaction engineering properties. The transport of momentum, heat, and mass take place by different rates in the different operations, and the yield and selectivity obtained for a given chemical reaction will depend upon the type of operation employed. The operations also differ with respect to more loosely defined characteristics, such as ease of operation, and it can be noted in particular that some operations have been studied with considerably more thoroughness than others, and may consequently be designed with greater accuracy and reliability. 

A slightly different approach was taken by Gill (G15), who considered the case of a bubble moving through a stationary liquid with mass transfer accompanied by simultaneous first-order chemical reaction. His assumptions were as follows ... 

In many instances, two or more miscible liquids must be mixed to give a product of a desired specification, such as, for example, in the blending of petroleum products of different viscosities. This is the simplest type of mixing as it involves neither heat nor mass transfer, nor indeed a chemical reaction. Even such simple operations can however pose problems when the two liquids have vastly different viscosities. Another example is the use of mechanical agitation to enhance the rates of heat and mass transfer between the wall of a vessel, or a coil, and the liquid. Additional complications arise in the case of highly viscous Newtonian and non-Newtonian liquids. 

In processing, it is frequently necessary to separate a mixture into its components and, in a physical process, differences in a particular property are exploited as the basis for the separation process. Thus, fractional distillation depends on differences in volatility. gas absorption on differences in solubility of the gases in a selective absorbent and, similarly, liquid-liquid extraction is based on on the selectivity of an immiscible liquid solvent for one of the constituents. The rate at which the process takes place is dependent both on the driving force (concentration difference) and on the mass transfer resistance. In most of these applications, mass transfer takes place across a phase boundary where the concentrations on either side of the interface are related by the phase equilibrium relationship. Where a chemical reaction takes place during the course of the mass transfer process, the overall transfer rate depends on both the chemical kinetics of the reaction and on the mass transfer resistance, and it is important to understand the relative significance of these two factors in any practical application. 

Isotopic molecules will have force fields which are identical to a high degree of accuracy. The vibrational amplitudes, on the other hand, will be mass-dependent, which means that the steric requirements of isotopic molecules will be slightly different. For this reason it is to be expected quite generally that isotopic molecules will respond differently to the change in steric conditions imposed by a chemical reaction, and hence that their reaction rates will differ somewhat. 

It is possible to take advantage of the differing characteristics of the periphery and the interior to promote chemical reactions. For example, a dendrimer having a non-polar aliphatic periphery with highly polar inner branches can be used to catalyse unimolecular elimination reactions in tertiary alkyl halides in a non-polar aliphatic solvent. This works because the alkyl halide has some polarity, so become relatively concentrated within the polar branches of the dendrimer. This polar medium favours the formation of polar transition states and intermediates, and allows some free alkene to be formed. This, being nonpolar, is expelled from the polar region, and moves out of the dendrimer and into the non-polar solvent. This is a highly efficient process, and the elimination reaction can be driven to completion with only 0.01 % by mass of a dendrimer in the reaction mixture in the presence of an auxiliary base such as potassium carbonate. 

In addition to the magnetic differences between the deuteron and proton, however, their mass difference may also cause observable effects. A well known example is found in the theory of chemical reactions, where the so called kinetic isotope effects (KIE s) are an important source of information about reaction mechanisms. Also in the field of ESR, such effects may arise, although these have been much less studied than the KIE s. 

A mass spectrometer provides an example of a molecular beam, in this case a beam of molecular ions. Molecular beams are used in many studies of fundamental chemical interactions. In a high vacuum, a molecular beam allows chemists to study the reactions that take place through specifically designed types of collisions. For example, a crossed-beam experiment involves the intersection of two molecular beams of two different substances. The types of substances, molecular speeds, and orientations of the beams can be changed systematically to give detailed information about how chemical reactions occur at the molecular level. Chemists also have learned how to create molecular beams in which the molecules have very little energy of motion. These isolated, low-energy molecules are ideal for studies of fundamental molecular properties. 

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