https://doi.org/10.1016/j.cesx.2020.100071
“The three different particle models, namely the RPM (Eq. (1)), the CGM (Eq. (8)) and the UCM (Eq. (11)), are assessed on how well they can describe the calcination reaction of limestone in the fixed-bed reactor. For each model, the fitting parameter was the reaction front velocity r. Fig. 3a shows the conversion versus time curve of the experimental data along with the fitting curves of each of the aforementioned particle models.”
“The conversion-time curve is fitted well by the CGM and UCM with CGM being slightly better, while the RPM produces significant discrepancies. This is also evident when plotting the conversion change rate (dX/dt) vs time in Fig. 3b, where although the experimental curve as well as the RPM exhibit a maximum of the rate, the CGM and UCM fit better the data during almost the whole reaction time. According to the RPM, the reaction surface increases as the pores grow during the first moments of the decomposition until the pore overlap starts to dominate decreasing the available reaction surface area. The latter two models (CGM, RPM) predict that the surface area is decreasing during the whole reaction time. The experimental data exhibit a maximum of the rate, but this increase happens for only few seconds in the beginning of reaction, and subsequently the rate follows a monotonic decrease, which CGM and UCM fit better. None of the assessed particle models describe perfectly the conversion-time experimental curve of limestone. This is probably produced from the assumptions made by each model. The RPM supposes cylindrical pores, while the Grain Model spherical grains. The UCM implies that the surface area of the reactant is proportional to the conversion. In reality the grains or the pores do not follow these ideal shapes. A more sophisticated model taking into account the pores or grains shape would possibly have better fitting. Nevertheless, the development of such a model is not in the scope of the present work.
Each of the three models uses a different value of reaction front velocity r for the same experimental data. The values of r obtained from the fitting procedure of each model are summarized in Table 3. “
“Fig. 3. Comparison of RPM, CGM and UCM on describing the decomposition of raw limestone at T = 885 °C and PCO2 = 30.4 kPa. Conversion (X) vs time (a) and conversion change rate (dX/dt) vs time (b).”
“These values indicate that for the same experimental data the obtained value of r can significantly vary depending on the model used. The lower value of r derived by the RPM is due to that the surface area of the initial material was calculated using the BJH data, while in the CGM and UCM the BET value was introduced. In the next sections, both CGM and UCM are used for the kinetic modeling of conversion-time data.”
