Ammonia/Ethanol Mixture for Adsorption Refrigeration

Adsorption refrigeration has become an attractive technology due to the capability to exploit low-grade thermal energy for cooling power generation and the use of environmentally friendly refrigerants. Traditionally, these systems work with pure fluids such as water, ethanol, methanol, and ammonia. Nevertheless, the operating conditions make their commercialization still unfeasible, especially owing to safety and cost issues as a consequence of the working pressures, which are higher or lower than 1 atm. The present work represents the first thermodynamic insight in the use of mixtures for adsorption refrigeration and aims to assess the performance of a binary system of ammonia and ethanol. According to the Gibbs’ phase rule, the addition of a component introduces an additional degree of freedom, which allows to adjust the pressure of the system varying the composition of the mixture. The refrigeration process was simulated with isothermal- isochoric flash calculations to solve the phase equilibria, described by the Peng-Robinson-Stryjek-Vera (PRSV) equation of state for the vapor and liquid phases and by the ideal adsorbed solution theory (IAST) and the multicomponent potential theory of adsorption (MPTA) for the adsorbed phase. In operating condenser and evaporator, pressure levels around atmospheric pressure can be achieved using an ammonia/ethanol mixture with a mole fraction of ethanol in the range of 0.70−0.75. A good agreement in the predictions of the adsorbed phase composition was also reported using the IAST and the MPTA methods.

Adsorption Thicknesses of Confined Pure and Mixing Fluids in Nanopores

In this paper, adsorption thicknesses of confined pure and mixing fluids in nanopores are quantitatively determined and their influential factors are specifically evaluated. First, a new analytical formulation is developed thermodynamically to calculate the adsorption thicknesses. Second, a new generalized equation of state (EOS), which considers the confinement effect-induced phenomena, is developed analytically for calculating the thermodynamic confined fluid phase behavior. Third, the modified model based on the generalized EOS and coupled with the parachor model is applied to calculate the vapor-liquid equilibrium (VLE) and fluid adsorptions for the pure CO₂, alkanes of C₁-C₁₀, and two mixtures of CO₂-C₁₀H₂₂ and CH₄-C₁₀H₂₂ in nanopores. Finally, the following five important factors are studied to evaluate their effects on the adsorption thickness: temperature, pressure, pore radius, wall-effect distance, and feed gas-to-liquid ratio (FGLR). The proposed modified EOS is found to be accurate for the VLE and adsorption isotherm calculations. The adsorption thicknesses of confined pure or mixing alkanes are increased with the increasing carbon number but decreased with the temperature increase. For the alkanes of C₁-C₁₀, the degree of temperature effect is strengthened with the carbon number increase. Moreover, the adsorption thicknesses are significantly decreased with the pore radius increase until r_p = 50 nm, after which they have slight changes or are even constant at any pore radii. On the other hand, the wall-effect distance (δ_p) increase causes the adsorption thickness to be linearly increased at δ_p/ r_p ≥ 0.02. In addition, the effects of the FGLR and pressure on the adsorption thicknesses at the nanoscale are found to be negligible.

Application of adsorption potential theory in prediction of CO2 and CH4 adsorption on carbon molecular sieves

The adsorption of CO2 and CH4 on carbon molecular sieves was studied based on the adsorption potential theory, which is widely used in gas adsorption on microporous adsorbents. The methods to obtain the adsorption density in the adsorbed phase, including Ozawa’s method and the empirical method, and the methods to calculate the virtual saturation vapor pressure, including Dubinin’s method and Amankwah’s method, were discussed. A functional exponential form proposed in this study could describe the adsorption characteristic curve better than the cubic polynomial and logarithmic function used in the previous literature. A new model, which expresses the correlation of adsorbed amount, temperature, and pressure, was proposed and verified experimentally. The model provides a reliable way to predict the adsorption isotherms of CO2 and CH4 on carbon molecular sieves at different temperatures according to the data measured at a certain temperature and to select a proper kind of carbon molecular sieve in CO2/CH4 separation.

Homogeneous and heterogeneous micropore structures in carbonaceous adsorbents--twenty years later

The major aim of this study is to show that, developed by Dubinin and Stoeckli, the relation between the parameters of DA (Dubinin-Astakhov) and DRS (Dubinin-Radushkevich-Stoeckli) equations is an approximation. It is shown that the parameter B(0) of DRS isotherm should be taken into account in such a relationship; strictly speaking, the parameter n(DA) of DA adsorption isotherm equation is not only the function of the dispersion of the heterogeneity of carbon, it also depends on the location of the maximum of the distribution of this heterogeneity. Moreover, taking into account that the DR isotherm rather does not describe the adsorption in a homogeneous micropore system, the analogous relationship, however, for DA adsorption isotherm as a local equation in GAI (General Adsorption Isotherm) is proposed.

Analysis of Multicomponent Adsorption Close to a Dew Point

We develop the potential theory of multicomponent adsorption close to a dew point. The approach is based on an asymptotic adsorption equation (AAE) which is valid in a vicinity of the dew point. By this equation the thickness of the liquid film is expressed through thermodynamic characteristics of the bulk phase. The AAE makes it possible to study adsorption in the regions of both the normal and the retrograde condensation. A simple correlation of the Kelvin radius for capillary condensation and the thickness of the adsorbed film is established. Numerical testing shows good agreement between the AAE and the direct calculations, even if the mixture is not close to a dew point. Copyright 1998 Academic Press.