Comparative adsorption study on carbons from polymer precursors

Hysteresis Phenomena in the Study of Sorptive Deformation of Sorbents

Published models and options for predicting equilibrium adsorption capacities of multicomponent mixtures using single component Dubinin/Radushkevich isotherm equations and parameters were reviewed. They were then tested for abilities to predict total and component capacities reported for 93 binary adsorbed mixtures. The best model for calculating molar distributions was the Ideal Adsorbed Solution Theory (IAST), which balances spreading pressures. Combined with the IAST, total and component capacities were best calculated using either the Lewis or original Bering equation with the Ideal Adsorbed Solution (Raoult’s Law) assumption.

https://digital.library.unt.edu/ark:/67531/metadc724161/m2/1/high_res_d/767434.pdf

Review and comparisons of D/R models of equilibrium adsorption of binary mixtures of organic vapors on activated carbons

Since liquid hydrocarbon fuels dominate the transportation sector for the foreseeable future, methanol could potentially be used as a much cleaner fuel than conventional petroleum-based fuels. Thus, development of methanol synthesis process to improve the methanol productivity has attracted increasing attention. This paper describes a steady-state mathematical model of a gas-flowing solids-fixed bed reactor (GFSFBR) with in situ water adsorption for methanol synthesis. Simulation result demonstrates that selective adsorption of water from methanol synthesis in GFSFBR leads to a significant enhancement in methanol production compared to zero solids mass flux condition. The remarkable advantage of GFSFBR over the conventional sorption-enhanced reaction process is the continuous adsorbent regeneration in this system. In the next step, a multi-objective optimization of GFSFBR is performed in order to maximize the methanol production rate and selectivity. Consequently, non-dominated sorting genetic algorithm-II (NSGA-II) is applied as a powerful method to optimize the GFSFBR. Optimization result has shown that there are optimal values of inlet temperature of gas and flowing solid phases, mass flux of flowing solids, flowing solid diameter, and pressure under which the highest methanol production rate and selectivity can be achieved. This paper shows how the concept of in situ water adsorption could be feasible and beneficial for methanol synthesis.

Methanol synthesis via sorption-enhanced reaction process: Modeling and multi-objective optimization

We report the observation of an unusual hysteresis loop in the water adsorption−desorption isotherm in NaY in the very low pressure range. Vacuum equilibrium thermodesorption and n-pentane adsorption experiments combined with grand canonical Monte Carlo simulations show that this hysteresis phenomenon is consecutive to the trapping of water in sodalite cages. We propose that the adsorption−desorption of water in NaY occurs as follows. (i) Initial adsorption of a few water molecules on the most accessible cations located on sites II in the supercages. The solvation of these cations makes the sodalite cages accessible to water. (ii) Complete filling of supercages and sodalite cages associated with a migration of nonframework cations from D6R prisms to sodalite cages. (iii) Desorption of water from supercages with water still trapped in the sodalite cages. (iiii) Thermally activated desorption of water from sodalite cages. Thus, according to the hydration procedure used, the residual water can be located eit...

Unusual Hysteresis Loop in the Adsorption−Desorption of Water in NaY Zeolite at Very Low Pressure

1.

Adsorption isotherms of cyclopentane on commercial activated charcoal of the AG type obtained by vapor-gas activation of coal raw material were measured in the temperature interval from 298 to 343 K and equilibrium pressures from 10−1 to 1000 Pa.




2.

The appearance of low-pressure hysteresis was detected under the given conditions. The position of the joining point of the hysteresis branches depends on temperature and shifts with a temperature increase into the region of lower pressures.




3.

The phenomenon of low-pressure hysteresis in the given system is basically connected with reduced (slowed) penetration of the adsorbate into the pores of charcoal, which are close in size to the size of the adsorbate molecules.

Low-pressure hysteresis in the cyclopentane-activated charcoal system

1.

The kinetics of the desorption of water vapors from molded zeolites NaA and NaX was investigated under vacuum and from a flow of carrier gas. Under nonisothermal conditions, desorption in the initial stage is determined chiefly by the rate of heating of the sample. After equalization of the temperature of the grain and that of the surrounding medium, desorption proceeds slowly, and the rate of the process is determined by the rate of diffusion within the grain.




2.

The rate of desorption in a flow of carrier gas is lower than under vacuum. At high rates of flow, this difference, due to the difference in the diffusion coefficients in the secondary pores, decreases with increasing temperature and becomes inconsequential at a temperature of 300–350°.

Kinetics of the desorption of water vapors from molded zeolites, types A and X

1.

Certain methods of calculation of adsorption equilibria of binary mixtures of vapors according to the individual adsorption isotherms of the components, known from the literature, are discussed.




2.

A new method of experimental investigation of such equilibria, which does not require the analysis of the equilibrium adsorption or gas phase and consists of measurement of the total pressure and the values of the adsorption of the components and calculation of the partial equilibrium pressures of the components for microporous adsorbents, is proposed. The operating principle of the corresponding instrument is described.




3.

The application of the proposed method of calculation to previously studied adsorption systems shows the presence of good agreement between the measured and calculated values of the partial pressures of the components.

A new method of calculation of the adsorption of binary mixtures of vapors on microporous adsorbents

1.

The kinetics of the sorption of vapors on granular, synthetic type A zeolites from a nitrogen stream was investigated.




2.

The water vapor diffusion coefficients in zeolites with adsorption from a nitrogen gas stream is substantially less than the values obtained from vacuum experiments. This difference is explained by an increase in the diffusion resistance in the secondary pores of the particle due to a decrease in the length of the molecular free path in the presence of a gas-carrier (at atmospheric pressure).

Investigation of the kinetics of sorption of water vapor from a gas-carrier stream on type a zeolites

1.

A thermodynatnic analysis of the temperature dependence of adsorption of individual substances has been carried out.




2.

The idea of isoergs, i.e., lines of constant value of free enthalpy of adsorption, has been introduced.




3.

The ratio of the differential entropy of adsorption, reckoned from the level of entropy of the liquid, to the thermal coefficient of adsorption is greater than zero along any isoerg.




4.

In In p vsa coordinates, the abscissa of the inflection point αi of an absorption isotherm is independent of temperature, and the ordinate of the inflection point, In pi, is a linear function of reciprocal temperature. One isotherm is sufficient for finding the parameters of this function.




5.

A thermal adsorption equation is proposed, all parameters of which can be determined from one adsorption isotherm. The equation describes adsorption equilibria well in a very wide interval of pressures, temperatures, and surface converages.

O. N. Kabanova

Papers

Low-pressure hysteresis in the cyclopentane-activated charcoal system

Kinetics of the desorption of water vapors from molded zeolites, types A and X

A new method of calculation of the adsorption of binary mixtures of vapors on microporous adsorbents

Investigation of the kinetics of sorption of water vapor from a gas-carrier stream on type a zeolites

Temperature dependence of adsorption