Yu. I. Aristov

A family of new working materials for solid sorption air conditioning systems

Abstract In this communication a family of new working materials, so called selective water sorbents, is presented for sorption air conditioning. These materials are composites “hygroscopic salt inside porous matrix with open pores” and possess intermediate behaviour between solid adsorbents, salt hydrates and liquid absorbents, so that their water sorption properties can be controllably modified by varying (a) porous structure of the host matrix, (b) chemical nature of the impregnated salt and (c) the amount of the salt inside the pores. For these materials the water sorption equilibrium and specific heat have been measured in a wide temperature and uptake range. This study shows that composites based on CaCl2 and LiBr as impregnated salts and different micro- and mesoporous silica gels as host matrices are able to absorb up to 0.75 g H2O per 1 g of the dry sorbent. After presentation of sorption equilibrium curves, the thermodynamic performance for their utilisation in heat pump and refrigeration systems, is calculated. The results show that these new materials can be operated with cycles whose maximum temperature is about 95 °C (for cooling) and 140 °C (for heating) that are lower than those for other pairs known so far. The values of coefficient of performance are considerably higher than those reported for silica gel/water system at the same temperature of the heat source.

SELECTIVE WATER SORBENTS FOR MULTIPLE APPLICATIONS, 2. CACL2 CONFINED IN MICROPORES OF SILICA GEL : SORPTION PROPERTIES

This paper presents sorption properties of a selective water sorbent based on mesoporous KSKG silica gel as a host matrix and calcium chloride as a hygroscopic salt. Sorption isobars, isochores and isotherms at T=20–150°C and vapor partial pressures of 8–133 mbar clearly showed two types of water sorption: 1) the formation of solid crystal hydrates at low amounts N of sorbed water, and 2) vapor absorption mainly by the salt solution at higher N. Sorption properties of CaCl2 crystal hydrates were found to change strongly due to their impregnation into mesoporous silica gel, whereas the solution confinement to the mesopores did not change its water sorption properties with respect to the bulk solution. Isosteric sorption heat was measured to depend on water sorption and to change from 62.5 kJ/mol for solid hydrates to 42.2–45.6 kJ/mol for solution.

New family of solid sorbents for adsorptive cooling: Material scientist approach

This paper summarizes literature data concerning a new family of materials for adsorptive cooling. They are composites of a type of “salt confined to a porous host matrix” (so called selective water sorbents or SWSs). These materials demonstrate an intermediate behavior between solid adsorbents, salt hydrates, and liquid absorbents. The thermodynamic equilibrium with water vapor has been measured for more than thirty SWSs based on halides, sulphates, and nitrates of alkaline and alkaline earth metals that are confined to various matrices (silica, alumina, porous carbons, clays, MCM-41, etc.). The important advantage of the SWSs is the possibility to controllably modify their water sorption properties in a wide range by varying (a) the chemical nature of the impregnated salt, (b) the porous structure of the host matrix, (c) the amount of the confined salt, and (d) the preparation conditions. This, in principle, allows purposeful synthesis of new solid sorbents with predetermined properties that fit the demands of particular adsorptive cycles. Appropriate examples are presented in the paper. A recent study of adsorption chillers with granulated and compact layers of SWS-1L (CaCl2 in mesoporous silica) showed an experimental COP as high as 0.6, which is larger than that reported for silica gel/water and zeolite/water systems under the same cycle conditions.The concept of matrix modification with an active salt can also be used for synthesis of efficient sorbents of carbon dioxide, methanol, and ammonia. Other practical applications of SWSs are briefly discussed, such as heat protection, gas drying, and fresh water production from the atmosphere.

Sorption of Carbon Dioxide from Wet Gases by K2CO3-in-Porous Matrix: Influence of the Matrix Nature

In a fixed-bed absorber at 40°C, the dynamics of carbon dioxide sorption over composite sorbents prepared by impregnation of potassium carbonate in various porous matrixes is studied. The dynamic capacity of the synthesized sorbents is shown to reach 0.12 g CO2 per 1 g of the sorbent. The composite dynamic capacity depends on the nature of the host matrix and decreases in the sequence alumina > activated carbon > vermiculite > silica gel. For K2CO3-on-alumina, the sorption capacity decreases considerably after the first cycle of «absorption and regeneration under 200–350°C», whereas the sorbents based on active carbons could be reversibly restored. The findings are discussed within the idea on a chemical interaction between the host matrix and the impregnated salt.

Thermal conductivity of selective water sorbents under the working conditions of a sorption chiller

Abstract This paper presents the experimental data on the thermal conductivity of the composite sorbents, CaCl2/SiO2 and LiBr/SiO2, measured by the “hot wire method” under various conditions of vapour pressure PH2O, temperature T and water uptake w. The measurement ranges were chosen according to the operating conditions of a typical sorption cooling cycle ( 10 H 2 O mbar , 40 ° C ). The obtained results show that the sorbent thermal conductivity λ(T,PH2O,w) increases considerably as the uptake of sorbate rises, while its dependence on pressure and temperature is, in the studied range, almost negligible. Finally, the influence of thermal conductivity on the specific power of the sorption chiller is discussed.

Selective Water Sorbents for Multiple Applications. 11. CaCl2 Confined to Expanded Vermiculite

This paper presents the water sorption properties of a new selective water sorbent based on expanded vermiculite as a host matrix and calcium chloride as a hygroscopic salt. Sorption isobars, isosters and isotherms at T = 30–150°C and vapor partial pressure 8.2–42.0 mbar clearly show that at low water contents crystalline hydrates with 0.33, 1 and 2 molecules of water per 1 molecule of CaCl2 are formed in the pores. These hydrates are stable over a temperature change of 20–30°C and exhibit kinetically slow transformations. At higher water uptake, the vapor absorption leads to the formation of a CaCl2 aqueous solution inside the pores, which properties are close to those in the bulk. Isosteric sorption heat was found to depend on water sorption and change from 76.3 kJ/mol for solid hydrates to 39.1–46.6 kJ/mol.

Sorption of carbon dioxide by the composite sorbent "potassium carbonate in porous matrix"

Sorption of CO2 in the presence of water vapor by the K2CO3—γ-Al2O3 composite sorbent was studied by IR spectroscopy in situ, X-ray diffraction analysis, and the differentiating dissolution method and reasons for a decrease in its dynamic capacity are given. The samples containing K2CO3·1.5H2O in pores are characterized by the maximal dynamic capacity. A mechanism for CO2 sorption was proposed, which qualitatively explains the obtained dependence of the capacity on the water content in the composite sorbent. A high dynamic capacity can be maintained by regeneration of the sorbents by water vapor at 170 °N. The capacity of the sorbents decreases during the first 10 sorption—regeneration cycles due to the formation of an inactive phase of potassium aluminum carbonate.

Selective water sorbents for multiple applications, 3. CaCl2 solution confined in micro- and mesoporous silica gels: Pore size effect on the “solidification-melting” diagram

In this communication we present a low-temperature “solidification-melting” phase diagram for CaCl2/H2O solutions confined in KSK and KSM silica gels. At salt concentrations of 0–48 wt. %, the diagram has been found to lie below the diagram reported for the bulk system by 15–30°C. It shows a depression of the solution melting point due to its confinment to the pores. Several other peculiarities of melting and solidification in this system are also reported and discussed. Beside fundamental interest, the data obtained could be of importance in many commercial areas such as refrigeration, accumulation of low temperature heat, frost prevention in building materials,etc.

Selective Water Sorbents for Multiple Applications, 10. Energy Storage Ability

The application of new selective water sorbents for storage of low temperature heat is analyzed. Values of energy storage ability E are measured by a DSC technique for more than fifteen samples of selective water sorbents based on silica gels, aluminas, carbon Sibunit and aerogels as host matrices and CaCl2, LiBr, MgCl2 as impregnated salts. E-values up to 8.4 kJ/g and 4.0 kJ/g are found for forced and naturally saturated sorbents, which are much higher than for common sorbents, like zeolites and unimpregnated silica gels. The temperature dependence of E gives an estimation of sensible and latent heat contributions as well as the average heat of water desorption and average specific heat of dry sorbents. A comparison between the SWSs and others materials proposed in literature is done.

Selective water sorbents for multiple applications, 4. CaCl2 confined in silica gel pores: Sorption/desorption kinetics

This paper presents experimental data on water sorption/desorption kinetics on composite SWS-1L and SWS-1S materials, bulk aqueous CaCl2 solutions and pure KSKG silica gel. Desorption kinetic curves are measured in the temperature range of 328–363 K at different vapor pressures. First order kinetics is found for both SWS materials and the bulk aqueous solution with the apparent activation energies of the water desorption equal to 23.5 kJ/mol and 48.6 kJ/mol, respectively. The difference in the kinetic behavior of the bulk and disperse systems is discussed.