## 2 Experimental Setup and Procedure  
A schematic of the gas extracting/adding experiment system is shown in Figure 1. The main cross-section of the stainless steel column is a circle with an internal diameter of 48 mm. Five porous panels made from steel powders, with a dimension of 162 mm \(\times\) 36 mm \(\times\) 6 mm (height \(\times\) width \(\times\) thickness), were installed along the height of the reactor to act as membrane. The distributor was doughnut-shaped, with six equally-spaced holes drilled on the inner side. These holes pointed inward and downwards at an angle of 45\({}^{\circ}\) to the vertical, to reduce the back-pouring of particles into the plenum chamber. Using this type of distributor, stable fluidization has been experimentally demonstrated to be established in FBMRs [3; 5; 31]. The reactor was heated inside by 4 cable heaters and outside the bed by 10 band heaters and 6 strip heaters. The temperature was measured at 8 vertical positions inside the bed by K-type thermocouples. Signals from the thermocouples were sent to a control system to adjust and maintain the required temperature during experiments. Compressed air at room temperature, controlled by pressure regulators and mass flow controllers, was used as fluidizing gas, as well as permeation gas. During the air addition tests (campaign I), compressed air was distributed to different membrane panels at the set flowrates through the permeation lines. In cases of air extraction (campaign II), a vacuum connected to a buffer was started to maintain the driving forces for gas permeating out from the reactor, and the extraction rates in each permeation line were also controlled individually by a mass flow controller, as shown in Figure 1.  
Differential pressure fluctuations, in the form of voltage signals, at five vertical locations were recorded using rapid-response pressure transducers (142 PC01D5V, Omega, Surrey, UK). The arrangement of the probe ports for pressure transducers, as well as the locations of the installed porous membranes, are illustrated in Figure 1. Each pressure port was located in the center of the column between two membrane panels, and covered by a porous stainless steel screen which was welded onto the end of the probe to prevent blockage by fine particles. The porous screen was purged with compressed air before starting a new experiment to minimize the damping-effect. The extended steel tubes connecting the probe leg and pressure transducer were cooled by water jackets during the gas-extraction tests. Voltage signals from the pressure transducers were logged into a computer via an A/D converter with 12-bit resolution. A total of 10,000 data points were acquired at a sampling frequency of 100 Hz. Two different types of particles were tested in this work, and the properties of these particles are listed in Table 1. The minimum fluidization velocity (\(U_{m\!f}\)) was determined from the evolution of the pressure drop across the particles bed while decreasing the inlet gas velocity from 40 to 0.5 L/min. This method has also been used by other researchers to measure \(U_{m\!f}\). In all experiments, the initial bed height was kept at around 1.28 m. For the convenience of the analysis, we called the bed section, where the porous membrane panels were installed and activated, the local bed section. Membrane panels were denoted as membranes #1-5 according to their position. The permeation line of membrane #5 was shut off during all tests. Operations without gas extraction/addition here were referred as normal operations. The operation conditions of experiments performed in this study are listed in Table 2. The highest gas extracting velocity of 0.036 m/s has been reported during the hydrogen permeation test operated at a hydrogen feed pressure of 2.6 MPa and permeation temperature of 400 \({}^{\circ}\)C [32].