The identification of mass transfer coefficients was carried out at the end of the synthesis, i. e. X = 1, and at the end of the decomposition, i. e. X = 0. A parametric analysis shows the influence of k and b on the pressure evolution, figure 4. To clearly quantify the influence of k and b, in the working pressure range, a reduced pressure has been defined, equation. 4.

= (p — Pec )

Peg — Pec

The time to reach pr = 0,52 varies from 0 to 46 s while permeability varies from 5.10-15 to 3.10-12 m2. The permeability evolution versus t(pr) is non linear (in the permeability range of the investigated composite blocks). According to the figure 4a, the lower the permeability, the higher its influence on t(pr). On the opposite, the evolution of Klinkenberg’s coefficient with t(pr) is linear in the range of the composite block investigated. This coefficient varies from 0 to 3000 Pa when t(pr) varies from 2,7 to 4,5 s, figure 4b.

Figure 1: Influence of — a — permeability and — b — Klinkenberg’s coefficient versus time to obtain pr = 0,52.

The parametric analysis has demonstrated the influence of the two mass transfer coefficients. Thus, the most important parameter is the permeability. Therefore, the identification process will begin by the identification of the permeability (assuming b=0) and then, the Klinkenberg’s coefficient will be identified.

The figure 5 shows the experimental evolution of pressure versus time for a reactive bed (at X=0, end

of decomposition). The initial pressures in each volume and the evolution of the equalized pressure, from peg to pec, are shown on figure 5 a. The pressure difference between peg and pec is strongly affected by the ratio between the gas tank volume and the reactor dead volume. The ideal ratio is 1 but in the experimental apparatus, this ratio is close to 5, that explain why the different pressure is so small. Figure 5b shows the simulated pressure evolution compared to experimental pressure evolution. As a first step, the final pressure pec allows to determine the bed porosity. The permeability was identified by comparing the simulated and experimental pressure evolutions. Then, the Klinkenberg’s coefficient was identified. The resulting values of b and k are 500 Pa and 2,0.10-13 m2. They lead to a good agreement between experimental and simulated pressure evolutions. These coefficients were compared to a previous paper [2]. The value given by the author is slightly lower than the present value.

Figure 5: Identification of the permeability and of the Klinkenberg’s coefficient of the reactive bed.

The same method was applied to identify the permeability and the Klinkenberg’s coefficient of the reactive salt after synthesis, X = 1. The synthesis reaction was very difficult to complete. Therefore, the pressure evolution after the connection of the two volumes was affected by the remaining chemical reaction between steam and salt. This effect could be avoided using nitrogen instead of reactive gas in the gas tank. Unfortunately, the pressure sensor output was too small and, up to now, the parameter identification wasn’t possible by this way.