## 1 Introduction  
Fluidized bed membrane reactors (FBMRs) have gained increasing interest as one of the promising technologies to facilitate process intensification, by combining the selective removal of reaction products or the addition of reactant via membranes with the advantages of fluidized bed reactors. With the assistance of membranes, higher product yields and selectivity are possible under safer operation conditions. Nevertheless, inefficiency issues associated with hot spot formation, large pressure drops, and concentration polarization could be avoided or improved [1; 2; 3]. Dense metal membranes have been widely used for product in situ separation, especially, for ultra-pure hydrogen production by hydrocarbon reforming in fluidized bed reactors. [PERSON] et al. [4], [PERSON] et al. [5], [PERSON] et al. [3], [PERSON] et al. [1] and [PERSON] et al. [6] experimentally investigated the subject of hydrogen production in a FBMR by steam or autothermal reforming of methane, with in situ removal of hydrogen via Pd-based dense membranes. H\({}_{2}\) permselective membranes also have been used in a fluidized bed reactor to promote the hydrogen yield in the water-gas shift reaction [2], steam reforming of higher hydrocarbons [7] and dehydrogenation reaction [8]. By integrating hydrogen separation with reforming reactions, higher reactant conversions were achieved at lower temperatures, and ultra-pure hydrogen flow with purity \(>\)99.995% were obtained in the permeate lines [2; 3; 4; 5; 6; 7]. Another interesting application of membranes is the selective dosing of reactant--mostly oxygen--into a chemical reactor. Similar operations, such as distribution of secondary air can also be facilitated by using porous ceramic membranes or side ports instead of selective membranes. The distribution of the oxidative reactant on the membrane was experimentally found to reduce the unwanted side reactions [9], improve yields [10], and benefit the safe operation with a wide range of reactant compositions, mainly due to the possible proper alteration of the concentration profile along the reactor by this method. Consequently, axial gas back-mixing was efficiently decreased [11; 12; 13], and the interphase mass transfer could be enhanced [14]. Despite the demonstrated advantages of integrating the membrane processes with a fluidized bed reactor, there are still several issues hindering the industrial application of FBMRs. For example, the efficiency of membrane units usually suffers serious deterioration in practical operations [5]. Also, the stability of the membrane in long-term operation at high temperature is far from satisfactory [2]. The fundamental principles for reasonable arrangements of membranes according to the reactor configuration, scale, and process design are not yet clear [10]. All the aforementioned challenges greatly depend on understanding the hydrodynamics in FBMRs. For example, the overall permeation rate of membrane units is mainly determined by the contacting efficiency of the membrane with target gases [3; 15], which is significantly related to the dynamic distribution of the emulsion phase in the bed. The interaction between particles and membranes, which is closely associated with the dynamic movements of the solids, is one of major factors affecting the thermal and chemical stability of membranes [2]. Additionally, the bubble-to-emulsion phase mass transfer limitation also tends to become more pronounced in a FBMR [1; 10]. Variation in the volumetric flowrate caused by gas extraction or addition could affect the pressure drop in the bed [7]. Therefore, research on the hydrodynamic effects of the membrane processes, especially the interaction of gas extraction/addition with in-bed hydrodynamics, is required to improve the performance of FBMRs. Although correlative studies focusing on this subject are still few, it has been attracting more and more attention as an emerging issue.