## 4 Results and Discussion
### Mechanism Discussion  
Gas extraction/addition through porous membrane panels mainly affected the hydrodynamics in the bed via two distinct ways as follows: (1) by causing changes in the superficial gas velocity in the fluidized bed; (2) by exerting external forces on the particles/gas. The increase of the average bubble size caused by gas addition, especially in the bed section above the addition locations, was mainly attributed to the increase of the superficial gas velocity in the bed. Gas injected into the bed first formed small bubbles, and then merged with the bubbles passing by, so the increase of the average bubble size in the local bed section would not be so obvious due to the concurrent occurrence of small-bubble formation and bubble integration, as illustrated in Figure 16c. Gas extraction through the porous membranes initially exerted external forces on the particles, which effectively balanced the gravity force with the assistance of the friction force of particles-particles and particles-membrane, so densified zones formed in the vicinity of the membranes [19]. The densified zones then reduced the space for bubbles to rise, thus enhancing bubble coalescence [16], which led to increase in the average bubble size. Besides, due to the narrowed cross-sectional area towards the upper bed section, the reduction of the effective fluidizing velocity caused by gas extraction would also be neutralized [19]. During the gas extraction experiment performed by [PERSON] et al. [16], the effects of gas extraction on particles movement were also found to disappear in the region above the extraction zones with a distance of 30 cm [16], but no more explanation of this phenomenon was provided. As an additional reason, here we propose another explanation for the effects of gas extraction on the bubble properties in different regions of a FBMR. Generally, in a gas fluidized bed with a stable bubble phase, gas from the emulsion would enter into the \"bubble\" from the bottom and then escape from the top, to maintain a constant pressure in the \"bubble\", and also increase the rise of \"bubbles\". In the case of gas extraction, a lower pressure head is provided by the active membrane panels, which enhances the movement of the in-bed gas towards the active membranes. Consequently, the rise of the bubbles (especially for large bubbles because they usually have faster rising velocity) is slowed down, and the routes of bubble movements in the bed were \"dragged\" towards a similar  
Figure 16: Illustration of the gas movement in the bed during (**a**) gas extraction; (**b**) normal operation; (**c**) gas addition. direction, so the mergence of bubbles was greatly enhanced, which caused the average bubble size to increase, as shown in Figure 16a. In support of this explanation, a remarkable decrease of the bubble rise velocity was also observed at the middle and upper part of the local bed section during the gas extraction experiments performed by [PERSON] et al. [25]. This drag force exerted on gas essentially originated from the pressure gradients between the specified regions and the extraction location, and was maintained by a continuum of gas. Therefore, as the distance between the measurement location and extraction location increased, the enhancement of the effect of gas extraction on bubble coalescence is correspondingly reduced due to the increase of the flow resistance. When the effects from the decrease of the effective fluidizing velocity outperformed the enhancement from the \"drag force\", the average bubble size decreased continually, and eventually would be smaller than that in normal operations. The penetration length of this drag force can be estimated by the Ergun pressure drop equation [38], which can be expressed as:  
\[\frac{dP}{dL}=\frac{150\mu}{d_{p}^{2}}\frac{(1-\varepsilon)^{2}}{\varepsilon^ {3}}U+\frac{1.75\rho}{d_{p}}\frac{(1-\varepsilon)}{\varepsilon^{3}}U|U| \tag{16}\]  
Here, \(dP/dL\) represents the pressure drop per unit length, and \(\varepsilon\) is the bed voidage.