Adsorptive heat

I. Adsorption Heats and Entropies. It is not necessary, phenomenologically, to state whether the process is adsorption, absorption, or solution, and for the adsorbent-adsorbate complex formal equations can be written, such as... 

XVIII-11 (the paradox of desorption heat exceeding adsorption heat is explainable in terms of a partial irreversibility of the adsorption-desorption process). 

Whilst the above analysis is detailed and quite complex, there are general trends that become apparent relating to how both the carbon properties and the operating conditions affect the OOP s of adsorption heat pumps and refrigerators. The cooling available from the cycle is approximately proportional to the difference between the high and low concentrations and to the latent heat of the refrigerant. The heat input to the cycle has three components the sensible... 

Meunier, F., Second law analysis of a solid adsorption heat pump operating on reversible cascade cycles application to the zeolite-water pair. Heat Recovery Systems, 1985, 5, 133 141. 

Turner, L., Improvement of activated charcoal-ammonia adsorption heat pumping/refrigeration cycles. Investigation of porosity and heat/mass transfer characteristics. Ph.D. Thesis, University of Warwick, UK, 1992. 

The applieation of aetivated earbons in adsorption heat pumps and refrigerators is diseussed in Chapter 10. Sueh arrangements offer the potential for inereased efficiency because they utilize a primary fuel source for heat, rather than use electrieity, which must first be generated and transmitted to a device to provide mechanical energy. The basic adsorption cycle is analyzed and reviewed, and the ehoiee of refrigerant-adsorbent pairs discussed. Potential improvements in eost effeetiveness are detailed, including the use of improved adsorbent carbons, advanced cycles, and improved heat transfer in the granular adsorbent earbon beds. 

Distinguishing between physical and chemical adsorption using the value of adsorption heat cannot lead to unambiguous results, too. An arbitrary classification of physical adsorption as having small heats Q - 0.01 + 0.2 eV typical for and attributing - 1 eV to diemisorption is often violated. A typical example can be provided by a dissipative chemisorption diaracterized by small total heat effect. 

Q is the adsorption heat for the simplest case 6o 4 105(MyiD Ma is the molecular weight of an absorbed particle. 

In case when the notion of inhomogeneous surface is introduced it is assumed that all surface can be represented by areas characterized by various adsorption heats Qi or, in a more general case, by various inverse adsorption coefficients 6,- = boi exp -Qi/kT. Introducing the distribution function of adsorption heats or inverse adsorption coefficients one obtains the following expression for a surface occupation degree... 

Introduction of a inhomogeneous (with respect to the adsorption heat) surface makes it easy to explain the dependence of differential adsorption heat on occupation degree of the surface observed in experiments [36]. 

Q being the adsorption heat. For small times (f expression... 

Consequently, for high concentration of adsorption particles (which is directly linked either with their high partial pressure in gaseous phase or with high value of their adsorption heat) all kinetic curves ait) whose shape (as it will be showed below) is notably dependent on concentration of adsorption particles Nt tend (at long times) to a specific value of (Tp dependent on the nature and history of adsorbent and independent on the value of Nt-... 

It should be noted that dissociation of surface complexes of oxygen in polar solvents on semireduced ZnO films is presumably justified from the thermodynamic point of view as oxygen adsorption heat on ZnO and electron work function are [58] 1 and approximately 5 eV respectively while the energies of affinity of oxygen molecules to electron, to solvation of superoxide ion and surface unit charge zinc dope ions are 0.87, 3.5, and higher than 3 eV, respectively [43]. 

Using the experimentally obtained coefficient of heterogeneous deactivation of 02( Aj) on glass (lO" ) and the adsorption heat... 

Commission. The work in this project will start in April 2003 and will continue for three years. One of the objectives is the development of a gas fired adsorption heat pump coupled to an underground storage system. That means the size of the sorption storage system is reduced significantly, a large number of cycles is possible and the actual seasonal storage is now realized by the UTES system. The economics of this concept are expected to be much better. 

The cooling cycle starts when all parts of the refrigerator are at about 1.3K. At this temperature, the 3He is completely adsorbed by the pump. The pump temperature is now raised to about 25 K by means of an heater. At 25 K, the 3He is desorbed, and its pressure increases over the saturation pressure at 1.3 K. Consequently, 3He condenses in the part of the tube T internal to the copper support C and drops down into the evaporator E. In this phase, the latent heat of condensation and the enthalpy variation are delivered to the 4He bath. The cooling phase starts when all the 3He is condensed in E and the power on the pump heater is switched off. The pump starts cooling towards the bath temperature, reducing the pressure on liquid 3He in E. The adsorption heat of the 3He vapour is delivered to the 4He bath by L. 

Adsorptions are exothermic, releasing more than the latent heat of the adsorbate gas, which quasi liquefies and solidifies besides being attracted. The first traces of adsorbed oxygen release even more heat than the combustion of carbon to carbon dioxide but after the first adsorption sites have been occupied, the adsorptive heat — based on unit mass of adsorbed oxygen — gradually drops to the level of condensation heat, far below the heat of combustion. 

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