## 1 Introduction
The hydrodynamic effects of gas extraction/addition on flow dynamics in a FBMR are closely related to operational factors, such as fluidization velocity, permeation flux, bed materials and operating temperature/pressure. Moreover, the change of configuration, location and arrangements of the membranes makes these interactions more complex. For example, formation of a densified layer, variation of bubble properties and gas back-mixing have preliminarily been investigated, but the experimental results reported by different researchers are inconsistent. [PERSON] et al. [16] and [PERSON] et al. [17] found that stagnant/densified zones readily formed near the membranes during gas extraction with flat membranes, which usually also acted as reactor walls, and would be improved by increasing the extraction fraction. On the other hand, [PERSON] et al. [18; 19] experimentally demonstrated that the formation of a stable densified zone in the bed while extracting gas via flat vertical membranes only occurred with smaller particles, especially at high extraction rates. No densified layers were observed during gas extraction when Geldart B type particles (400-600 um) were used as bed material in experiments conducted by [PERSON] et al. [20]. In addition, with flat membranes installed, both a reduction [20; 21; 22] and a slight increase [16] in the average bubble size were observed during gas extraction from the fluidized bed. Also, [PERSON] et al. [11] experimentally demonstrated that the distributive feeding of oxygen via porous membranes decreased axial gas back-mixing, while the addition of gas through flat membranes was found to increase the gas back-mixing [23]. As mentioned above, the dynamics of gas-solid flow in a membrane-assisted fluidized bed reactor varies with reactor scale, membrane configuration, membrane location, and bed material. A fundamental understanding of the hydrodynamic interaction of the membrane-fluidized bed is still lacking. Accordingly, a deeper insight into the hydrodynamic interaction between gas extraction and gas-solid flow in a FBMR is greatly needed. Different experimental techniques, such as particle image velocimetry, digital image analysis, electrical capacity, magnetic resonance tomography and X-ray analysis [19], have been successfully applied to investigate the solid hold-up or bubble properties in lab-scale FBMRs. [PERSON] et al. [24] and [PERSON] et al. [25] found that gas permeation exerted extensive influence on the flow dynamics in a FBMR, rather than only in the zones nearby the activated membrane. Moreover, the effects of gas permeation on the hydrodynamics in different regions of a FBMR were also different [18; 26]. As a result, more measurement points or larger detection regions are required, which would greatly increase the measurement cost, in order to properly determine the hydrodynamic effects of gas permeation in a scaled-up FBMR. In this case, pressure measurements would be particularly preferred for analyzing the hydrodynamic effects of gas permeation in a scaled-up FBMR with multiple membranes installed at different locations. Pressure measurement could successfully keep its spatial and temporal resolution by increasing the measurement points and sampling frequency, when applied to a scaled-up FBMR. Besides this benefit, pressure measurement not only can provide information about the flow dynamics in a local bed, but also in the other regions at a specified instant [27; 28]. Accordingly, it is a powerful tool to detect the synergistic effects of gas permeation via different membranes installed in a FBMR. Differential pressure fluctuations are closely correlated with the bubble dynamics between measurement intervals. Thus, they can be favorably applied to characterize the hydrodynamics in different parts of a pilot-scale FBMR [29; 30; 31]. With the proper resolution of the differential pressure signals by wavelet analysis or other multi-resolution methods, the multiscale dynamics of gas-solid flows could be adequately investigated [29; 30].