Solids chemical conversions

The wide range of soHd lubricants can generally be classified as either inorganic compounds or organic polymers, both commonly used in a bonded coating on a matching substrate, plus chemical conversion coatings and metal films. Since solid-film lubricants often suffer from poor wear resistance and inabihty to self-heal any breaks in the film, search continues for improved compositions. 

H. Alter and J. J. Duim, Jr., Solid Waste Conversion to Energy Current European and U.S. Practices, Marcel Dekker, Inc., New York, 1980, Chapt. 5 H. Alter andj. A. CampbeU, in J. L. Jones and S. B. Radding, eds.. The Preparation and Properties ofDensified Refuse-Derived Fuel, Thermal Conversion of Solid Wastes andBiomass, American Chemical Society, Washington, D.C., 1980, pp. 127—142. 

A number of three-phase processes (processes in which contact is established between a gaseous phase, a liquid phase, and a solid-particle phase in order to promote chemical conversion and the transfer of momentum, heat, and mass) are becoming increasingly important in the process industries. 

The area of interest covered by this paper is limited to processes in which chemical conversion occurs, as in the processes noted above. Gas-liquid-particle processes in which a gaseous phase is created by the chemical reaction between a liquid and a solid (for example, the production of acetylene by the reaction between water and carbide) are excluded from the review. Also excluded are physical separation processes, such as flotation by gas-liquid-particle operation. Gas absorption in packed beds, another gas-liquid-particle operation, is not treated explicitly, although certain results for this operation must necessarily be referred to. 

O.l-l.O Fine Accumulation particles Coagulation from smaller particles, chemical conversion of gases to solids, some mineral material... 

Van Swaaij, W. P. M., and Zuiderweg, F. J., The Design of Gas-Solids Fluidized Beds - Prediction of Chemical Conversion, Proc. Int. Symp. on Fluidization and itsAppl., Ste Chimie Industrielle, Toulouse (1973)... 

Our library synthesis was carried out with a set of 27 tube-shaped solid phase synthesis support, called MicroTubes. These supports are prepared by radiation grafting of polystyrene ( — 350 pmol) onto polypropylene tubes, chemically functionalizing the polystyrene with aminomethyl groups to afford about 55 imol of amine per tube, inserting a reusable Rf ID tag into each tube, and heat-sealing the tube ends to prevent loss of the tag. The chemical conversion of all 36 aminomethyl tubes was carried out simultaneously using standard procedures with rink amide linker, each with —46 pmol of available amine per tube.1 2... 

MRI to characterize hydrodynamics within reactors is already established. The extent to which the potential of MR to study both hydrodynamics and chemical conversion is fully realized will depend on our ability to integrate the well-established MR spectroscopy techniques in liquid- and solid-state NMR into imaging pulse sequences, and still provide quantitative data in the magnetically heterogeneous environments typical of catalysts and reactors. 

The performance and scalability of the various techniques is most easily compared in a side-by-side format. With respect to experimental procedures, it is now recognized that many chemical conversions (e.g., formation of C-N or C-C bonds) that were reported to require solid supports with catalytic activity and microwave irradiation (and thus introduced environmental concerns) do not require such auxiliaries or irradiation. They occur exothermally at low temperatures with quantitative yields and without solvent-consuming workups even on a large scale. 

It is also quite reasonable to treat a reactive medium as a two-component material.140,141 The initial state of the reactive mixture is a low-viscosity liquid, which passes into a uniform solid material as a result of chemical reactions. The ratio of solid-to-liquid components is determined by the degree of chemical conversion. This ratio is an important property of a reactive mixture, and can range from 0 to 1. The fundamental characteristic of such a two-component material is its specific free energy. This thermodynamic function is assumed to be the sum of the free energies of both components calculated from the degree of conversion ... 

In a typical case, the barrier widths in heavy-particle tunneling reactions correspond to transfer distances that are much smaller than that for hydrogen transfer and are not usually realized at van der Waals interreactant spacings in solids. Therefore, chemical conversions associated with heavy-particle tunneling are rare, often occurring in exoergic reactions where d is much smaller than the geometric transfer distance. A few examples of these reactions are cited in Section 9.2. 

Chemical dynamics at low temperatures is connected with elementary reactions that surmount potential energy barriers separating reactants from products in the absence of thermal activation. The first experimental evidence of this type of reactions was obtained in the early 1970s in studies of solid-state conversion of free radicals. These investigations clearly demonstrated that there is a sufficiently sharp transition from Arrhenius-like exponential temperature dependence, characteristic of thermal activation, to much weaker power-like temperature dependence down to the low-temperature limit of the rate constant. 

A number of composition analyzers used for process monitoring and control require chemical conversion of one or more sample components preceding quantitative measurement. These reactions include formation of suspended solids for turbidimetric measurement, formation of colored materials for colorimetric detection, selective oxidation or reduction for electrochemical measurement, and formation of electrolytes for measurement by electrical conductance. Some nonvolatile materials may be separated and measured by gas chromatography after conversion to volatile derivatives. 

The more recent theories of chemical conversions [59-61] take into account the fact that the process of overcoming the activation barrier involves a cooperative change of more than one degree of freedom for the starting reagents subsystem. For the surface processes this is expected to lead to a need for considering the dynamics of the solid atom motion and, at least, the model should include information on Debye frequencies for its atoms (see, e.g., Ref. [62]). An additional inconvenience of the models for the elementary surface processes is associated with the fact that the frequencies of the surface atom oscillations differ from those inside the solid. Consideration of the multiphonon contributions to the probabilities that the elementary process can take place results in a significant modification of its rate constant up to the complete disappearance of the activation form of the temperature dependence [63,64]. 

In the context of integrated chemical processes however, reactive filtration should be specified as the combination of the solids handling unit operation filtration device and the unit operation chemical reactor . This comprises the separation of solids or aerosols from a fluid stream and the chemical conversion of undesired compounds carried by the stream in one instead of two unit operations. The chemical reaction can proceed either continuously during the separation or stepwise after a certain amount of solids or aerosols has been deposited. Two general cases for the application of a reactive filtration unit may be distinguished ... 

Flotation is a depletion process which obeys a time law of the 1st order and is described by the flotation rate constant, kF. The target quantity, solids discharge A , corresponds to the chemical conversion, X, in 1st order reactions ... 

This chapter generalizes the results of the studies of traveling waves of chemical conversion observed by the authors in a wide variety of solids at liquid-nitrogen and -helium temperatures. 

However, we would like to point here not to the differences between the equilibrium tunneling mechanism and the above examples of mechanisms of the nonequilibrium type in low-temperature chemical conversions, but, on the contrary, to a simplifying assumption which relates them but which has to be rejected in a number of cases—and that is the subject matter of this chapter. In the above models the solid matrix itself was considered, in essence, from a special point of view, namely, as an ideal system, devoid of defects, which is in mechanical equilibrium. In other words, the fact that the systems in question are significantly out of equilibrium with respect to their mechanoenergetic state was ignored. This property of the experimentally studied samples was the result of both their preparation conditions and the ionizing radiation. 

According to the hypothesis, chemical conversions in a solid occur on the surface of a new crack (or in the layer adjacent to it), that is, the reaction rate in such a reacting system is a certain function of the specific surface area S (active surface area per unit volume). As noted above, the positive feedback in this model manifests itself in the fact that the rate of formation of cracks (i.e., the active surface growth rate in the sample volume) is proportional to the reaction velocity. Therefore, the equation describing the formation of a new surface can be written in a form analogous to that of a branched-chain process ... 

From the theoretical analysis it follows that acting on the strength characteristics of the solid matrix of a frozen reactant mixture may be an effective means for testing the concepts developed. This was an impetus for a study of the effect of high pressures on the dynamic characteristics of the autowave regimes of chemical conversion,19 since it is known that uniform compression of solid materials results in significant strengthening. 

Therefore, neither the appreciable plastic deformation (both in the case of uniform compression and of local fracture) of the solid reaction systems studied nor their static state of high stress is a factor conditioning the critical phenomena and autowave processes observed during the chemical conversion in the systems. In other words, this series of experiments has provided another telling argument for the decisive role of brittle fracture in the mechanism of the phenomena considered. 

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