Model of Flow Calorimetry Experiment

Liquid-flow microcalorimefry is a reliable method to measure simultaneously the enthalpy changes and amounts of adsorption under dynamic conditions. Calorimetry experiments may be carried out in two different ways by following a pulse or saturation operating mode [64, 78-83]. In the pulse mode, small aliquots of a stock solution at a known concentration are injected into the carrier liquid (pure solvent) flowing through the adsorbent bed placed inside the calorimetric cell. In this case, the calorimetric system contains an additional loop injection facility (a manual injection valve with appropriate injection loops). The interpretation of the enthalpy data obtained is straightforward only when the whole amount of the solute injected is irreversibly adsorbed on the solid surface. 

The saturation mode (continuous-flow method) is more frequently used. Here changes in enthalpy and amount adsorbed of the solute correspond to the formation of a Solid-Liquid interface being in thermal and material equilibrium with the percolating stock solution of a given composition. Repeated adsorption and desorption cycles with the liquid phase in contact with the solid surface for a time required to reach equilibrium can be used to assess reversibility of the phenomenon, and quantify the reversible and irreversible adsorption components [79, 80]. In addition, the same equipment allows probing for some active sites in the solid surface. 

The physical meaning of thermodynamic quantities measured in the flow calorimetry experiment, may be discussed based on a simplified model of the system (Fig. 6.17). The model system is composed of three parts (i) a reservoir R containing a given volume of the stock solution of molality m , (ii) a cell C with the solid sample of mass ms, in contact with the solvent or the stock solution, (iii) a trap T for the effluent [65, 78], 

Initially, the outgassed solid sample is immersed in grams of pure solvent and the total quantity of stock solution in the reservoir is given by the mass of solvent the trap is empty. Then the flow of stock solution from the reservoir is directed to the cell under constant liquid-flow conditions. The temperature and pressure are assumed to be uniform throughout the system and there is neither loss of energy nor loss of matter between the reservoir, the cell and the trap. As the stock solution 

For the initial state, the amount of the solute in the model system and the total enthalpy of the system given by (cf. Sect. 6.4.1)  

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