## 5 Conclusions  
This work presented experimental studies on the effects of gas extraction/addition on the hydrodynamics in a pilot-scale FBMR, with vertical porous membrane panels installed in an orderly way along the height of the reactor. All permeation tests were mainly conducted at fluidization velocities of 0.09 and 0.14 m/s at 550 \({}^{\circ}\)C, with calcined dolomite and silica sand particles as bed medium, respectively. The differential pressure signals were measured at five different intervals, and then subjected to analysis, such as statistical analysis, power spectral and wavelet analysis. The effects of gas permeation on the gas-solid flow dynamics in different regions of the FBMR were thoroughly investigated.  
The experimental results revealed that gas extraction via porous membrane panels caused the number of small- and medium-sized bubbles to decrease, and the average bubble size to increase in the local bed section, both of which were mainly attributed to the enhancement of the bubble coalescence. In bed sections above the activated membrane panel, the variation of the bubble properties during gas extraction was synergistically affected by the decrease of the effective fluidizing velocity and the drag force exerted by gas extraction on the gas movement. This drag force originated from the lower pressure head associated with gas extraction, and penetrated the bed through a gas continuum. As a result, it was attenuated with the increase of the distance from the extraction location, and quickly decreased in the fluidized bed with lower voidage. Accordingly, since calcined dolomite (light particles, larger bed voidage) was used as the bed material, the enhancement effect of gas extraction on bubble coalescence decreased gradually along the bed height. Thus, although the incrementation of the average bubble size was reduced, an average bubble size larger than that in normal operations was still observed. However, with silica sand particles (heavier particles, smaller bed voidage) as bed material, the attenuation of the enhancement effect from gas extraction was so fast that the average bubble size in the upper bed section was even smaller than that in normal operations. According to our experimental results, to avoid the occurrence of large bubbles the particles that readily achieve fluidization with lower voidage are preferred in a FBMR.  
Irrespective of whether calcined dolomite or silica sand were employed as bed material, consistent results were obtained during the gas addition tests. Small bubbles would initially form in the local bed section and then merged with up-rising bubbles, so that a slight increase in the average bubble size was observed. Due to the increase of the superficial gas velocity in the upper bed sections, the growth of bubbles was effectively enhanced, and led to larger incrementation in the average bubble size at higher bed sections. Therefore, this suggests that the increase of the total fluidizing gas velocity should be taken into consideration during the design and operation of a gas fluidized bed reactor used in volume-increased processes. In this study, limited effects of the gas extraction/addition on the bubble properties were observed in bed sections below the activated membrane panels. This finding suggested that it was better not to position the membrane units in or below the fast-reacting zone when arranging multiple membranes in a large FBMR.  
Conceptualization, Y.C.; supervision, J.R.G.; data curation, Y.X. and R.P.; writing-original draft, C.B. This work was supported by the Basic Research Programs of Science and Technology Commission Foundation of Jiangsu Province (BK20150196) and the Fundamental Research Funds for the Central Universities (2015 XKMS058). The authors declare no conflict of interest.