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    methanol pair is best adapted for operating cycles with small
    evaporating temperature variations (up to 40 C), whereas the
    adsorption cycles with zeoliteewater pair, a larger range evapo-
    rating temperature (70 C, or more) is needed.2.1. DubinineAstakhov equation
    To describe adsorption in microporous materials with poly-
    modal distributions, Dubinin and Astakhov [14] have proposed an
    isotherm that is a log-linear form of DubinineRudishkevich equa-
    tion, which can be expressed as follows:Being the latent heat, the first termof Eq. (6), the other terms do
    correspond to the energy that most specifically concerns the
    adsorption binding forces. For temperature around 5 C, the value
    of L is about 1200 kJ/kg. According to Srivastava and Eames [16], the
    isosteric heat of adsorption for activated carbonemethanol ranges
    from 1800 to 2000 kJ/kg. So, the energy resulting from the binding
    forces in adsorption corresponds to something from 33% to 40% of
    the isosteric heat; the rest would come as a result of capillary
    condensation inside the micropores.2.3. Kinetics of adsorption
    In most cases, adsorption in microporous materials is mainly
    controlled by the diffusion that takes place inside porous structure,
    for on the surface of the grains, diffusion happens too fast. In the
    case of materials with bidisperse structure, such as activated
    carbon, two diffusion mechanisms may be found: a gaseous phase
    diffusion through transport pores (mesoporous and macroporous
    diffusion) and an adsorbed phase diffusion in micropores. The
    relative importance of these mechanisms on the global diffusion
    effect greatly depends on pressure. According to Dubinin and
    Erashko [17], for pressures lower than 10 hPa, mesoporous and
    macroporous diffusions are seen to be most predominant, whereas
    for pressures greater than 10 hPa, microporous diffusion tends to
    control the mass transfer process.
    Thomas and Glauckauf, cited by Sakoda and Suzuki [18],
    proposed an approach based on two important hypotheses: the
    temperature of the grain is uniform, and the concentration on the
    solid surface is equivalent to an equilibrium concentration. These
    hypotheses have established, for modeling the mass transfer
    resistance of the adsorption kinetics, the following linear equation:where Di is the diffusion coefficient, aeq is the equilibrium
    concentration (given by an isotherm) and rg is the average radius of
    the grain.
    In both mesopores and macropores, different mechanisms
    contribute simultaneously to the diffusion process, the relative
    influence of which does depend on the pores dimensions. Among
    them, fourmainmechanisms are identified: superficial diffusion Ds,
    molecular diffusion Dm, Knudsen’s diffusion DK, and Poiseuille’s
    diffusion DP. For an overall diffusion analysis, an effective diffusion
    coefficient De is normally considered, so that De ¼ f(Ds, Dm, DK, DP).
    Notwithstanding the importance of the kinetics theory for
    modeling adsorption processes, the practice demonstrates that the
    mass transport resistance can be neglected in systems under
    moderate thermal power. For methanol adsorption in activated
    carbon, Kariogas and Meunier [19] have shown that the mass
    resistance is negligible when the incident energy is lower than
    50 W/kg of adsorbent. In other words, for modeling adsorption
    processes related to low-grade energy sources, such as the case of
    solar energy, it will not be necessary to consider diffusion through
    the porous medium. Moreover, for many engineering applications,
    it may be taken into account the existence of an instantaneous
    equilibrium between the adsorbed and the gaseous phases. This
    can be represented by means of an isotherm, such as the Dubi-
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