Solid-gas systems

Prior to determination of an isotherm, all physisorbed material has to be removed from the surface of the adsorbent. This is best achieved by exposure of the surface to high vacuum, the exact conditions required (temperature and residual pressure) being dependent on the particular gas-solid system. In routine determinations of surface area it is generally advisable not to remove any chemisorbed species which may be present thus, the hydroxylated oxides are usually outgassed at 1S0°C. Microporous adsorbents such as zeolites or active carbons however require higher temperatures (350-400 C, say) for complete removal of physisorbed material from their narrowest pores. An outgassing period of 6-10 hours (e.g. overnight) is usually sufficient to reduce the residual pressure to 10 Torr. 

A manual entitled Reporting Physisorption Data for Gas/Solid Systems with Special Reference to the Determination of Surface Area and Porosity has been prepared as a provisional publication by Commission 1.6 of the International Union of Pure and Applied Chemistry (lUPAC). The purpose of the manual is to draw attention to problems involved in reporting physisorption data and to provide guidance on the evaluation and interpretation of isotherm data. The general conclusions and recommendations are very similar to those contained in Chapter 6. 

Sing, K. S. W., Everett, D. H., Haul, R. A. W., Moscou, L., Pierotti, R. A., Rouqudrol, J. and Siemieniewska, T., Reporting physisorption data for gas solid systems with special reference to the determination of surface area and porosity. Pure Appl. Cliem., 1985, 57(4), 603 619. 

Most microscopic theories of adsorption and desorption are based on the lattice gas model. One assumes that the surface of a sohd can be divided into two-dimensional cells, labelled i, for which one introduces microscopic variables Hi = 1 or 0, depending on whether cell i is occupied by an adsorbed gas particle or not. (The connection with magnetic systems is made by a transformation to spin variables cr, = 2n, — 1.) In its simplest form a lattice gas model is restricted to the submonolayer regime and to gas-solid systems in which the surface structure and the adsorption sites do not change as a function of coverage. To introduce the dynamics of the system one writes down a model Hamiltonian which, for the simplest system of a one-component adsorbate with one adsorption site per unit cell, is... 

The wall-particle Nwp represents the heat transfer process between the wall and a contacting particle. For gas-solid systems, where thermal resistance of the gas gap dominates,... 

Most processes employing fluidization technology are concerned with gas/solid systems. We are so accustomed to the bubbling phenomenon of such systems that we often lose sight of other less common yet nonbubbling systems or fail to assess the consequences of the suppression of bubbles in gas/solid systems (Kwauk, 1992). 

Anon, Dynamics of Gas/Solids Systems, Pneumatic Handling ofBulkMat., Part A Fundamentals, Bulk Solids Handling Unit, Thames Polytechnic, London (1983)... 

Although microwave activation of catalytic reactions has been the subject of many studies (Sects. 10.3.1 and 10.3.2), the mechanism of these reactions is not yet fully understood. In heterogeneous catalytic liquid/solid and gas/solid systems many results have revealed significant differences between the rates of conventionally and microwave heated reactions. As a rule, at the same temperature microwave heated reactions were faster than conventional and their rate enhancement was over one or-... 

Our discussion of multiphase CFD models has thus far focused on describing the mass and momentum balances for each phase. In applications to chemical reactors, we will frequently need to include chemical species and enthalpy balances. As mentioned previously, the multifluid models do not resolve the interfaces between phases and models based on correlations will be needed to close the interphase mass- and heat-transfer terms. To keep the notation simple, we will consider only a two-phase gas-solid system with ag + as = 1. If we denote the mass fractions of Nsp chemical species in each phase by Yga and Ysa, respectively, we can write the species balance equations as... 

We treat each of these three cases in turn to obtain, as far as possible, analytical or approximate analytical rate expressions, taking both mass transfer and reaction into account. Each of these cases gives rise to important subcases, some of which are developed further, and some of which are left to problems at the end of the chapter. In treating the cases in the order above, we are proceeding from special, relatively simple, situations to more general ones, the reverse of the approach taken in Section 9.1 for gas-solid systems. 

If it is necessary to predict the minimum fluidization velocity the following correlation [Grace (1982)] may be used for gas-solid systems. 

In order to understand the properties of a fluidised system, it is necessary to study the flow patterns of both the solids and the fluid. The mode of formation and behaviour of fluid bubbles is of particular importance because these usually account for the flow of a high proportion of the fluid in a gas-solids system. 

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