## 4 Results and Discussion
### Effects of Gas Extraction  
The changes of the \(\sigma_{r}\) while extracting gas out of the bed at three different locations are shown in Figure 3, with the extraction fraction varying from 5 to 15%. The inlet superficial gas velocity was 0.09 m/s, and calcined dolomite particles were used as bed material. A remarkable increase in the \(\sigma_{r}\) was observed for signals measured in the local bed section. Gas extraction also led to an increase of the \(\sigma_{r}\) of the pressure fluctuations measured in the bed sections above the active membrane panels. However, this effect generally decreased with the distance of the measurement location from the extracting locations. For example, while extracting gas out of the bed from membrane #2 at a rate of 1.5 L/min, the \(\sigma_{r}\) at DP2 increased from 0.11 to 0.33, but the incrementation of the \(\sigma_{r}\) at DP3 decreased to 0.14, and was almost reduced to zero at DP5, as can be seen in Figure 3. Compared to regions above the extraction locations, the bed section below the active membrane panel seemed hardly to be affected by gas extraction, as indicated by only a slight increase of the \(\sigma_{r}\) at DP1 and DP2 with the activation of membranes #2 and #3, respectively. According to studies by [PERSON] et al. [22], gas extraction through an immersed tubular membrane or a reactor-wall membrane were also found to exert minor influences on the bubble properties in the region below the permeation zone in a two-section two-zone fluidized bed membrane reactor (TS-TZFBMR). Figure 2: Effects of (**a**) gas extraction and (**b**) gas addition through porous membranes on the \(\sigma_{r}\) of the differential pressure signals. The other operation conditions: \(T=550\)\({}^{\circ}\)C, \(P=0.1\) MPa, \(H_{ini}=1.28\) m, calcined dolomite. The power spectra of the differential pressure signals measured at DP1, DP2 and DP3 (local bed sections), with gas extracted from membrane #1, #2 and #3, respectively, at a rate of 1.5 L/min, are illustrated in Figure 4. For comparison, the power spectra for the normal operations are also shown. The main peaks on the spectrum of the differential pressure measured in the gas extraction tests showed much higher power density compared with the signals measured under normal operational conditions, which meant that the size of large bubbles passing through the local bed sections was increased. This result was consistent with analysis based on the evolution of the \(\sigma_{r}\). Moreover, the main peaks of the power spectrum were shifted to lower frequency regions, and the power density of minor peaks at higher frequency regions was greatly reduced. As can be seen in Figure 4, for the differential pressure measured at DP2, the main peak of the power spectrum was localized in the frequency region from 0.9\(\sim\)1.7 Hz at normal operational conditions, while another major peak appeared at the frequency region from 0.1-0.4 Hz in the case of gas extraction. A similar change was also displayed in the power spectra of the differential pressure measured at DP3. The pressure fluctuations with low frequency but high magnitude were mainly excited by large bubbles (voids), while the contents with medium magnitude were related to bubble coalescence/integration and medium size bubbles [30; 33]. As can be seen in Figure 4, gas extraction was supposed to enhance the coalescence of bubbles in the local bed section, thus the main frequency of the main peak decreased (number of bubbles decreased due to bubble coalescence), but the peak intensity greatly increased (size of the bubbles increased due to bubble integration). [PERSON] et al. [16] also experimentally found that gas extraction through flat membranes on the side walls led to increased bubble size in the local section of a fluidized bed reactor. Indeed, as indicated by their experimental results, the incrementation of the equivalent bubble size due to gas extraction also decreased gradually at higher axial position, but not as remarkably as in this study. [PERSON] et al.