Reactor achievable, chemical

The focus of the examples given in this chapter is clearly on micro reactors for chemical processing in contrast to p-TAS or Lab-Chip systems for bioanalytical applications. In the latter microfluidic systems, the fluidic requirements are somehow different from those in micro reactors. Typically, throughput plays only a minor role in p-TAS systems, in contrast to micro reactors, where often the goal is to achieve a maximum molar flux per unit volume of a specific product. Moreover, flow control plays a much greater role in p-TAS systems than in micro reactors. In... 

In a more recent study using dedicated multimode microwave reactors for chemical synthesis, which enable temperature and power control, it was demonstrated that microwave irradiation could be effectively employed to couple aromatic carboxylic acids to polystyrene Wang resin [25], if the symmetrical anhydride procedure was used, and not the three-component O-acylisourea activation method [19]. Almost quantitative loading was achieved in l-methyl-2-pyrrolidone (NMP) at 200 °C within 10 min under... 

The structure of this review is as follows. Section II focuses on the basic principles of MRI techniques, and then more advanced techniques which have been used to study catalytic reactors will be introduced in Section III. To illustrate the use of these techniques examples will be given from the field of catalysis, although not necessarily at the scale of the reactor and, in some cases, data for model systems will be shown. Section IV describes methods used to achieve chemical mapping. Section V focuses exclusively on previous research which has used MRI techniques to spatially resolve chemical composition in fixed-bed reactors. 

Initially, reaction times were varied from 2 minutes to 24 hours at 573 K and 673 K. These experiments revealed that 30 minutes reaction time was sufficient for the mixtures to achieve chemical reaction equilibrium. Therefore, further experiments were all performed with 30 minutes reaction time. Figures 1 and 2 provide a summary of smoothed results as discussed later. Approximate densities (g/cm3) were 0.97, 0.93,0.89, 0.83, 0.75, 0.64, 0.36, and 0.15 for temperatures (K), 373,423, 473, 523, 573, 623, 673,723, respectively. All metals were found to be soluble in the aqueous 1.6 M HN03 solution at room temperature. Considering eqn. (1), the metal must react to a form that is insoluble in the liquid phase for the recovery to increase. Moreover, according to the experimental procedure, the reacted metal must be insoluble in the liquid phase at room temperature as the batch reactor is cooled before analysis. 

Various parameters are considered when selecting a reactor for chemical reactions, such as the number of phases involved, the differences in the physical properties of the participating phases, the post-reaction separation, the inherent reaction nature (stoichiometry of reactants, the intrinsic reaction rate, isothermal/adiabatic conditions, etc.), the residence time required, and the mass and heat transfer characteristics of the reactor. For a given reaction system, the first four aspects are usually controlled only to a limited extent if at all, while the remaining serve as design variables to optimize reactor performance. High rates of heat and mass transfer improve effective rates and selecti-vities and the elimination of transport resistances. This enables the reaction to achieve its chemicaT potential in the optimal temperature and concentration window. Transport processes can be ameliorated by increasing interfacial surface areas and short diffusion paths. These are easily attained in micro-stmctured reactors. 

Concern about the potential diversion of separated reactor-grade plutonium has led to a reduction ia U.S. governmental support of development of both plutonium recycle and the Hquid metal reactor. This latter ultimately depends on chemical reprocessing to achieve its long-range purpose of generating more nuclear fuel than it bums ia generating electricity. 

The three chemical reactions in the toluene—benzoic acid process are oxidation of toluene to form benzoic acid, oxidation of benzoic acid to form phenyl benzoate, and hydrolysis of phenyl benzoate to form phenol. A typical process consists of two continuous steps (13,14). In the first step, the oxidation of toluene to benzoic acid is achieved with air and cobalt salt catalyst at a temperature between 121 and 177°C. The reactor is operated at 206 kPa gauge (2.1 kg/cm g uge) and the catalyst concentration is between 0.1 and 0.3%. The reactor effluent is distilled and the purified benzoic acid is collected. The overall yield of this process is beheved to be about 68 mol % of toluene. 

A large number of radiometric techniques have been developed for Pu analysis on tracer, biochemical, and environmental samples (119,120). In general the a-particles of most Pu isotopes are detected by gas-proportional, surface-barrier, or scintillation detectors. When the level of Pu is lower than 10 g/g sample, radiometric techniques must be enhanced by preliminary extraction of the Pu to concentrate the Pu and separate it from other radioisotopes (121,122). Alternatively, fission—fragment track detection can detect Pu at a level of 10 g/g sample or better (123). Chemical concentration of Pu from urine, neutron irradiation in a research reactor, followed by fission track detection, can achieve a sensitivity for Pu of better than 1 mBq/L (4 X 10 g/g sample) (124). 

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. 

Chemical reactions obey the rules of chemical kinetics (see Chapter 2) and chemical thermodynamics, if they occur slowly and do not exhibit a significant heat of reaction in the homogeneous system (microkinetics). Thermodynamics, as reviewed in Chapter 3, has an essential role in the scale-up of reactors. It shows the form that rate equations must take in the limiting case where a reaction has attained equilibrium. Consistency is required thermodynamically before a rate equation achieves success over tlie entire range of conversion. Generally, chemical reactions do not depend on the theory of similarity rules. However, most industrial reactions occur under heterogeneous systems (e.g., liquid/solid, gas/solid, liquid/gas, and liquid/liquid), thereby generating enormous heat of reaction. Therefore, mass and heat transfer processes (macrokinetics) that are scale-dependent often accompany the chemical reaction. The path of such chemical reactions will be... 

Adiabatic Reaction Temperature (T ). The concept of adiabatic or theoretical reaction temperature (T j) plays an important role in the design of chemical reactors, gas furnaces, and other process equipment to handle highly exothermic reactions such as combustion. T is defined as the final temperature attained by the reaction mixture at the completion of a chemical reaction carried out under adiabatic conditions in a closed system at constant pressure. Theoretically, this is the maximum temperature achieved by the products when stoichiometric quantities of reactants are completely converted into products in an adiabatic reactor. In general, T is a function of the initial temperature (T) of the reactants and their relative amounts as well as the presence of any nonreactive (inert) materials. T is also dependent on the extent of completion of the reaction. In actual experiments, it is very unlikely that the theoretical maximum values of T can be realized, but the calculated results do provide an idealized basis for comparison of the thermal effects resulting from exothermic reactions. Lower feed temperatures (T), presence of inerts and excess reactants, and incomplete conversion tend to reduce the value of T. The term theoretical or adiabatic flame temperature (T,, ) is preferred over T in dealing exclusively with the combustion of fuels. 

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