Figure 2 represents the studied reactors in this work. They consist of 1 m long and 2 mm thickness steel tubes resisting to the maximum pressure reached by the fluid. The total surface covered is 1 m2 which is composed by a number of cylindrical tubes with an external diameter De, figure 2-a. This number is inferior when finned tubes are used considering a De/2 width fins, figure 2-b. The rectangular reactor has a width of 1 m and a height of H, figure 2-c. All these reactors are isolated from the lateral wall and the back. The tubes are filled by an amount of activated carbon reacting by adsorption with ammonia constituting a porous medium. This medium can be heated or cooled through the metallic wall along a daily cycle.

Figure 2-a. Reactor without fins

Figure 2-. Solar reactor

The thermal and mass transfer calculation in the reactor been made by taking account the next hypotheses; characteristics of the porous medium:

— the porous medium pressure is uniform and to be constant when the reactor is opened.

— the distribution of temperature is bi-dimensional in the rectangular reactor and radial in the case of cylindrical tubes.

— the three phases of the porous medium, in elementary elements of volumes, solid phases made up of coal particle of activated carbon, liquid phases (ammonia adsorbed) and gas phases (ammonia gas) are in thermal, mechanics and chemical local equilibrium.

— the thermal transfer by convection is neglected

— the exchange between the metal wall and the porous medium and the thermal transfer by conduction are characterized respectively by a coefficient of exchange hi and an apparent thermal conductivity Xe [16].

— the physical properties of metal and the activated carbon are constant.

— the condenser and the evaporator are considered to be idea.

ma = PL Vo Exp[-D (T Log ( fs / f ) )n] (1)

where; ma is the total mass of the adsorbed fluid at the pressure P and the temperature T; The product pL Vo is the maximum volume of the pores accessible to the adsorbate. D and n represents the parameters of Dubinin, which depend on the adsorbent / adsorbate couple and which are given experimentally for the activated carbon and ammonia pair [17].