# Fluidization Dynamics of Hydrophobic Nanosilica with Velocity Step Changes
## 2 Experimental  
The schematic of the experimental setup used in the present investigation is shown in Figure 1. A 1.5 m-long 70 mm internal diameter Perspex column was employed as the test section. Below the test section, a 500 mm long column was used as the calming section. A perforated plate distributor covered with fine polymeric mesh separated the two sections. The distributor perforations' diameter was 2 mm, with a fractional open area of less than 4% [21]. At the top of the test-section, a disengagement section with an internal diameter of 140 mm helped to suppress the solid particle entrainment with the fluidizing gas. A stable supply of compressed air at ambient conditions was used as the fluidizing gas. For measuring the pressure transients in the fluidized bed, eight sensitive bidirectional pressure transducers were found along with the fluidized bed's height (Figure 1). The configuration is similar to the one employed before for the pulsed bed of hydrophilic nanosilica [11; 12]. The upper region dynamics were recorded using \(\Delta P_{1}\) and \(\Delta P_{2}\), located at the height of 230 mm from the bottom of the test section. The middle region of the bed (i.e., from 110 mm to 230 mm) was monitored using \(\Delta P_{3}\) and \(\Delta P_{4}\). The dynamics of the lower region of the bed were monitored using \(\Delta P_{5}\) and \(\Delta P_{6}\). The lower tap of these transducers, i.e., \(\Delta P_{5}\) and \(\Delta P_{6}\), was at a distance of 11 mm from the distributor plate, while the upper tap was located at a height of 110 mm. Besides, two transducers, \(\Delta P_{7}\) and \(\Delta P_{8}\), with their lower port connected with the lowest pressure tap, monitored the overall pressure transients. All the pressure taps were covered with a fine polymeric mesh to prevent the bed material from clogging the transducer's lines. The pressure transducers were calibrated using a Fluke 718 Pressure Calibrator (Everett, WA, USA). An 18-bit, 32-analog input (AI, 625 kS/s), 4-analog output (2.86 MS/s) data acquisition system (USB-6289, National Instruments Corporation, Austin, TX, USA) was used for signal acquisition at a sampling rate of 100 Hz. An electronic mass controller was used to control the fluidizing gas' flow rate to the test section of the fluidized bed. The velocities were gradually increased in regular steps up to a maximum of 10 mm/s, then decreased similarly to investigate both fluidization and defluidization cycles of the experiment. An analog output (AO) signal from the data acquisition system (DAQ) was used to control  
Figure 1: Experimental setup. (**1**) Calming section; (**2**) Fluidized bed test section; (**3**) Disengagement section; (**4**) Data acquisition system; (**5**) Analog input signals (red broken lines); (**6**) Compressed air supply; (**7**) Mass flow controller; (**8**) Analog output signals (blue broken lines); (**9**) Laptop with LabVIEW software. the mass controllers, as shown with a broken blue line in Figure 1, while AI signals from pressure transducers and the flow controller are shown with broken red lines. A sieved sample of ultrafine fumed nanosilica (Aerosil R812, Evonik GmbH, Wolfgang, Germany) of 7 nm primary size was used as the bed material. It is strongly hydrophobic due to the vapor phase treatment with hexamethyldisilazane (HMDS). Introducing hydrophobicity to the catalytic surface, for example, can significantly alter their surface dynamics [22]. Figure 2a shows the particle size distribution of the sample. Because of agglomeration, the size was found to vary in the range of 0.5 \(\upmu\)m to 500 \(\upmu\)m, with a median diameter of approximately 8.0 \(\upmu\)m. The scanning electron micrograph of the sample, shown in Figure 2b, confirms the particle size distribution analysis results. Overall, 83.2 g of the sieved sample was loaded into the test section. After settling, the bed height was 370 mm, with a bulk density of 58.4 kg/m\({}^{3}\).