Reactions, coal with fluid phases

Type of Reaction and Application. An increased emphasis on gas-solid reactions has been evident for about a decade. Three of the papers in this symposium treat gas-solid reactions, two (13,18) dealing with coal combustion and the other (11) with catalyst regeneration. Of the four papers which consider solid-catalysed gas-phase reactions, one (15) deals with a specific application (production of maleic anhydride), and one (12) treats an unspecified consecutive reaction of the type A B C the other two (14,16) are concerned with unspecified first order irreversible reactions. The final paper (17) considers a relatively recent application, fluidized bed aerosol filtration. Principles of fluid bed reactor modeling are directly applicable to such a case Aerosol particles disappear by adsorption on the collector (fluidized) particles much as a gaseous component disappears by reaction in the case of a solid-catalysed reaction. 

In Aspen Plus, solid components are identified as different types. Pure materials with measurable properties such as molecular weight, vapor pressure, and critical temperature and pressure are known as conventional solids and are present in the MIXED substream with other pure components. They can participate in any of the phase or reaction equilibria specified in any unit operation. If the solid phase participates only in reaction equilibrium but not in phase equilibrium (for example, when the solubility in the fluid phase is known to be very low), then it is called a conventional inert solid and is listed in a substream CISOLID. If a solid is not involved in either phase or reaction equilibrium, then it is a nonconventional solid and is assigned to substream NC. Nonconventional solids are defined by attributes rather than molecular properties and can be used for coal, cells, catalysts, bacteria, wood pulp, and other multicomponent solid materials. 

Diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy has been proven to be an excellent means of characterizing coals and related materials. This report is devoted to the evaluation of the technique as a method for situ monitoring of the chemical structural changes wrought in reactions of coal with fluid phases. This technique does not require a supporting medium (matrix) which can contain chemical artifacts which inherently serve as a barrier for access to the solid coal. The rapid response of the Fourier transform infrared technique is further beneficial for kinetic studies related to combustion, liquefaction, gasification, pyrolyses, etc. Experimental equipment and techniques are described for studies over wide ranges of pressure (10 5 Pa to ca 1.5 x 10 kPa) and temperature (298 K to 800 K). 

Gas-solid reactions between a fluid and a solid are important in a number of applications such as coal gasification, metallic ore processing, and catalyst regeneration. They are related in many aspects to the gas-solid catalytic reactions we have treated in developing the concepts of catalytic effectiveness, but differ in the very important aspect that the solid itself (in the form of a porous matrix) is one of the reactants. Since the solid phase itself is involved in reaction, often conditions of diffusion/ reaction change with time of reaction and the overall process is an unsteady-state one. As with effectiveness factors, many variants on a theme can be envisioned, i.e., is the reaction fast or slow, does the particle porosity (hence D ff) change with reaction, are boundary layer transport effects of importance, etc. We will present in some detail the developments of Wen concerning these questions [C.Y. Wen, Ind. Eng. Chem., 60, 34 (1968) H. Ishida and C.Y. Wen, Amer. Inst. Chem. Eng. J., 14, 311 (1968)]. 

Bubble columns, in which the liquid is the continuous phase, are used for slow reactions. Drawbacks with respect to packed columns are the higher pressure drop and the important degree of axial and radial mixing of both the gas and the liquid, which may be detrimental for the selectivity in complex reactions. On the other hand they may be used when the fluids carry solid impurities that would plug packed columns. In fact, many bubble column processes involve a finely divided solid catalyst that is kept in suspension, like the Rheinpreussen Fischer-Tropsch synthesis, described by Kolbel [1], or the former I. G. Farben coal hydrogen process, or vegetable oil hardening processes. Several oxidations are carried out in bubble columns the production of acetaldehyde from ethylene, of acetic acid from C4 fractions, of vinylchloride from ethylene by oxychlorina-tion, and of cyclohexanone from cyclohexanol. 

Mass transfer with chemical reaction in multiphase systems" covers, indeed, a large area. Table 1 shows a general classification of the systems encountered. From the possible two-phase systems, solid-solid reactions, liquid-solid (reactive or catalytic) and gas-solid (reactive or catalytic) reactions are not discussed here. The first one was reviewed by Tamhankar and Doraiswamy (2) and gas-solid (reactive) systems, such as, coal gasification, calcination of limestone, reduction of ores, etc. have been treated in some detail in recent reviews (3-5). The industrially important fluid-solid catalytic processes were the topic of a previous Advanced Study Institute (6) and have been also discussed authoritatively elsewhere (5,7). Concerning solid (reactive)-liquid two-phase systems, only some interesting examples are presented in Table 2 (1). 

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