## 4 Results and Discussion
### Effects of Gas Extraction
[25] reported that the average bubble diameter increased, while the bubble number decreased in the area where active membrane plates were installed, in particular when a high extraction fraction of 40% was employed. Glass beads with a density of 2500 kg/m\({}^{3}\) and mean particle size of 190 \(\upmu\)m were used as the bed medium. Figure 3: Effects of gas extraction on the \(\sigma_{r}\) of the differential pressure signals with the individual activation of membranes #1–3. The other operation conditions were: \(T\) = 550 \({}^{\circ}\)C, \(P\) = 0.1 MPa, \(H_{ini}\) = 1.28 m, \(U\) = 0.09–0.14 m/s, extraction rate = 0.5–1.5 L/min, calcined dolomite. According to the results shown in Figure 3, not only were the differential pressure fluctuations from the local bed section affected by gas extraction, but also other regions in the bed. Figure 5 presents a comparison of the power spectra of the differential pressure measured during the gas extraction tests and normal operation, from the upward and downward bed sections rather than the local bed section. When membrane #2 (in the DP2 section) was activated for gas extraction, the main frequency of the power spectrum for the signal from DP3 (up bed section) was shifted from 1.3 to 1.1 Hz, and the peak intensity was significantly increased. However, the power spectrum of the differential pressure from DP1 (downward bed section) did not vary greatly compared with the normal operation, and only the main peak intensity increased slightly. Similar results were also observed for the gas extraction with the activated membrane #3, as shown in Figure 5. Therefore, it is reasonable to conclude that gas extraction in a FBMR not only greatly affected the hydrodynamics in the local bed section, but also in the deeper bed section, especially in regions above the extraction location. Figure 4: Comparison of the power spectra of the differential pressure in the local bed section measured at the gas extraction and normal operational conditions with the individual activation of membrane #1–3 for gas extraction. The other operational conditions were: \(T=550\) \({}^{\circ}\)C, \(P=0.1\) MPa, \(H_{ini}=1.28\) m, \(U=0.09\) m/s, extraction rate = 1.5 L/min, calcined dolomite. Figure 5: Comparison of the power spectra of the differential pressure in (**a**) the upward bed section and (**b**) the downward bed section measured at gas extraction and normal operational conditions. The other operational conditions were: \(T=550\) \({}^{\circ}\)C, \(P=0.1\) MPa, \(H_{ini}=1.28\) m, \(U=0.09\) m/s, extraction rate = 1.5 L/min, calcined dolomite. In fact, these effects of gas extraction exerted on the flow dynamics were closely related to the distance between the extraction location and the investigated zones. The power spectra of the differential pressure measured in the bed section DP5, with activated membranes #1, #2 and #3, are compared in Figure 6. As illustrated in Figure 1, membrane #1 is mostly far away from DP5, and so is membrane #2, while membrane #3 is just adjacent to DP5. Compared with the normal operation, the increase in the main peak intensity of the power spectra was the largest while extracting gas with activated membrane #3, but such an increase was not so evident with activated membrane #1, in which case only a minor shift of the peak frequency is observed in the power spectrum. This finding confirmed the proposed conjecture that the effects of gas extraction by the membrane unit on the hydrodynamics in FBMR decreased with the distance from the extracting location. According to the above analysis, when arranging multiple membrane units in a large FBMR, the membrane units should not be installed in or below fast-reacting zones, otherwise large bubbles induced by gas extraction would greatly decrease the overall reaction efficiency.