Fluidization Dynamics of Hydrophobic Nanosilica with Velocity Step Changes

applied sciences

Article Fluidization Dynamics of Hydrophobic Nanosilica with Velocity Step Changes

Ebrahim H. Al-Ghurabi 1, Mohammad Asif 1,* , Nadavala Siva Kumar 1 and Sher Afghan Khan 2 1 Department of Chemical Engineering, King Saud University, P.O. Box 800, Riyadh 11421, Saudi Arabia; [email protected] (E.H.A.-G.); [email protected] (N.S.K.) 2 Department of Mechanical Engineering, Faculty of Engineering, IIUM Malaysia, Kuala Lumpur 50728, Malaysia; [email protected] * Correspondence: [email protected]; Tel.: +966-56-981-7045

Received: 17 October 2020; Accepted: 11 November 2020; Published: 17 November 2020

Abstract: Nanosilica is widely used in various applications, with its market expected to grow over USD 5 billion by 2025. The fluidized bed technology, owing to its intimate contact and efficient mixing of phases, is ideally suited for the large scale processing of powders. However, the bulk processing and dispersion of ultrafine nanosilica using the fluidized bed technology are critically affected by the interparticle forces, such that the hydrophilic nanosilica shows agglomerate bubbling fluidization (ABF), while the hydrophobic nanosilica undergoes agglomerate particulate fluidization (APF). This study carried out a detailed investigation into the fluidization hydrodynamic of the hydrophobic nanosilica by monitoring the region-wise dynamics of the fluidized bed subjected to a regular step change of fixed duration in the gas velocity. The gas flow was controlled using a mass controller operated with an analog output signal from a data acquisition system. The analog input data were acquired at the sampling rate of 100 Hz and analyzed in both time and temporal frequency domains. The effect of velocity transients on the bed dynamics was quickly mitigated and appeared as lower frequency events, especially in regions away from the distributor. Despite the apparent particulate nature of the fluidization, strong hysteresis was observed in both pressure drop and bed expansion. Moreover, the fully fluidized bed’s pressure drop was less than 75% of the theoretical value even though the bed appeared to free from non-homogeneities. Key fluidization parameters, e.g., minimum fluidization velocity (Umf) and the agglomerate size, were evaluated, which can be readily used in the large scale processing of nanosilica powders using fluidized bed technology.

Keywords: ﬂuidization dynamics; hydrophobic nanosilica; hysteresis; hydrodynamics; a velocity step change

1. Introduction Intimate fluid-solid contact and good interphase mixing are essential to ensure the efficiency of processes that require the interaction of the solid and fluid phases. While small particles inevitably provide high interfacial area, thereby enhancing the surface-based rate processes, their effectiveness could however be compromised due to strong interparticle forces. The role of the interparticle forces has long been recognized in the context of the fluidization of powders [1]. Fine powders, composed of small particles (<20 µm), are mostly cohesive due to interparticle forces, which make their fluidization challenging as well as unpredictable [2,3]. Strong hysteresis effect and severe bed non-homogeneities are observed in their fluidization behavior. Interestingly, nanopowders, despite being composed of ultrafine particles, mainly exhibit two distinct kinds of fluidization behavior, viz.

Appl. Sci. 2020, 10, 8127; doi:10.3390/app10228127 www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 8127 2 of 12 agglomerate bubbling fluidization (ABF) and agglomerate particulate fluidization (APF) [4,5]. As the name suggests, the constituent nanoparticles of ultrafine powders exist as large agglomerates, which are sometimes as much as a thousandfold larger than the primary size of the individual nanoparticle. Therefore, the fluidization hydrodynamics of ultrafine powders, whether bubbling or particulate, are characterized by the fluidization behavior of their constituent agglomerates. Nano-silica is extensively used in various applications, e.g., concrete, agriculture, rubber, plastics, electronics, cosmetics, gypsum, battery, pharmaceuticals, and food [6–9]. The estimated global market of nanosilica in 2015 was 3.3 million tons, which is projected to reach over USD 5 billion by 2025 [10]. Depending upon the application requirement, silica nanoparticles’ surface treatment can significantly alter the interparticle force equilibrium, thereby affecting their mutual interaction. For example, hydrophilic nanosilica shows ABF, while its hydrophobic counterpart mostly undergoes APF, even if their primary dimension and density are comparable [5]. The bubbling in ABF limits the bed’s expansion and leads to a size-based stratification of constituent agglomerates along the bed height [11,12]. On the contrary, APF shows a smooth and homogeneous expansion of the bed [13]. The height of the expanded bed can reach several times the height of the original bed. The agglomerates of the hydrophobic nanosilica are morphologically soft and fluffy. The Umf in APF could sometimes be as much as an order of magnitude lower than the one in ABF. As a result, the fluidization hydrodynamics of the ABF have attracted a lot of attention. Efforts were mainly oriented towards understanding ABF hydrodynamics and using assisted fluidization techniques to improve their fluidization behavior [14–20]. On the other hand, any rigorous study on the hydrodynamics of the APF appears to be still lacking the literature. Therefore, a detailed investigation of the hydrophobic nanosilica undergoing APF has been carried out to thoroughly characterize its hydrodynamics. The information obtained from this study can be readily used during the processing of such nanopowders in large-scale industrial applications, where critical process conditions, such as velocity, could change during the operation. In this study, the nanosilica bed was subjected to velocity step changes (of fixed duration), while the transients in various bed regions were monitored using sensitive pressure transducers located at different heights in the fluidized bed. Analog voltage signals were acquired using an AD converter (18-bit) of a data acquisition system (DAQ). We used the sampling frequency of 100 Hz to ensure accurate monitoring of different events occurring in the fluidized bed. The velocity was first gradually increased during the fluidization cycle, followed by a gradual reduction during the defluidization cycle to delineate the effect of the hysteresis on the APF hydrodynamics. The electronic flow controllers were controlled using the AO signal of the DAQ. The bed transients were analyzed in both time as well as frequency domain. In addition, the experimental data were processed to examine the influence of the velocity on the mean pressure drop values in the upper, middle, and lower regions of the bed. The overall pressure drop data were used to evaluate the Umf, while the overall bed expansion was used to assess the terminal velocity and the agglomerate’s size.

2. Experimental The schematic of the experimental setup used in the present investigation is shown in Figure1. 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 (Figure1). 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 ∆P1 and ∆P2, located at the height of 230 mm from the bottom of Appl. Sci. 2020, 10, 8127 3 of 12 the test section. The middle region of the bed (i.e., from 110 mm to 230 mm) was monitored using Appl. Sci. 2020, 10, x FOR PEER REVIEW 3 of 12 ∆P3 and ∆P4. The dynamics of the lower region of the bed were monitored using ∆P5 and ∆P6. The lower tap of these transducers, i.e., ∆P5 and ∆P6, was at a distance of 11 mm from the the upper tap was located at a height of 110 mm. Besides, two transducers, ∆P7 and ∆P8, with their distributor plate, while the upper tap was located at a height of 110 mm. Besides, two transducers, lower port connected with the lowest pressure tap, monitored the overall pressure transients. All the ∆P7 andpressure∆P8, with taps were their covered lower portwith connecteda fine polymeric with me thesh lowestto prevent pressure the bed tap,material monitored from clogging the overall the pressuretransducer’s transients. lines. Allthe The pressure pressure tapstransducers were covered were calibrated with a fine using polymeric a Fluke 718 mesh Pressure to prevent Calibrator the bed material(Everett, from clogging WA, USA). the An transducer’s 18-bit, 32-analog lines. input The (AI, pressure 625 kS/s), transducers 4-analog output were (2.86 calibrated MS/s) data using acquisition a Fluke 718 Pressuresystem Calibrator (USB-6289, (Everett, National WA, Instruments USA). An Corpor 18-bit,ation, 32-analog Austin, input TX, (AI, USA) 625 was kS/ s),used 4-analog for signal output (2.86 MSacquisition/s) data acquisition at a sampling system rate of (USB-6289, 100 Hz. National Instruments Corporation, Austin, TX, USA) was used for signal acquisition at a sampling rate of 100 Hz.

FigureFigure 1. 1.Experimental Experimental setup. setup. (1) Calming (1) Calming section; (2) section;Fluidized bed (2) test Fluidized section; (3 bed) Disengagement test section; (3) Disengagementsection; (4) Data section; acquisition (4) Data system; acquisition (5) Analog system; input signals (5) Analog (red broken input signalslines); (6 (red) Compressed broken lines); air (6) Compressedsupply; (7 air) Mass supply; flow (controller;7) Mass ﬂow (8) Analog controller; output (8 )signals Analog (blue output broken signals lines); (blue (9) Laptop broken with lines); (9) LaptopLabVIEW with LabVIEW software. software.

An electronicAn electronic mass mass controller controller was was used used to to control control thethe fluidizingfluidizing gas’ gas’ flow flow rate rate to the to thetest section test section of of the fluidizedthe fluidized bed. bed. The The velocitiesvelocities we werere gradually gradually increased increased in regular in regular steps up steps to a maximum up to a maximumof 10 mm/s, of 10 mm/thens, then decreased decreased similarly similarly to investigate to investigate both fluidi bothzation fluidization and defluidization and defluidization cycles of the experiment. cycles of the experiment.An analog An analog output output(AO) signal (AO) from signal the from data theacquisition data acquisition system (DAQ) system was (DAQ) used to was control used the to mass control Appl. Sci. 2020, 10, 8127 4 of 12

Appl.Appl. Sci. Sci. 2020 2020, 10,, 10x FOR, x FOR PEER PEER REVIEW REVIEW 4 of4 12 of 12 the mass controllers, as shown with a broken blue line in Figure1, while AI signals from pressure transducerscontrollers,controllers, and as asshown the shown flow with with controller a broken a broken areblue blue shown line line in with Figuin Figu brokenre 1,re while1, while red AI lines. AIsignals signals from from pressure pressure transducers transducers andandA the sieved the flow flow samplecontroller controller of ultrafineare are shown shown fumed with with broken nanosilica broken red red (Aerosillines. lines. R812, Evonik GmbH, Wolfgang, Germany) of 7 nmA primarysievedA sieved sample size sample was of ultrafineof used ultrafine as fumed the fumed bed nanosilica material.nanosilica (Aerosil It (Aerosil is strongly R812, R812, Evonik hydrophobic Evonik GmbH, GmbH, dueWolfgang, Wolfgang, to the Germany) vapor Germany) phase treatmentof of7 nm 7 nm primary with primary hexamethyldisilazane size size was was used used as asthe the bed (HMDS). bed material. material. Introducing It isIt stronglyis strongly hydrophobicity hydrophobic hydrophobic due todue to the tothe catalytic the vapor vapor phase surface, phase fortreatment example,treatment with canwith hexamethyldisilazane significantly hexamethyldisilazane alter their(HMDS). (HMDS). surface Introducing Introducing dynamics hydrophobicity hydrophobicity [22]. Figure to2a tothe shows the catalytic catalytic the surface, particle surface, for size for distributionexample,example, can of can significantly the significantly sample. alter Because alter their their ofsurface agglomeration,surface dynamics dynamics [22]. the [22]. Figure size Figure was 2a found2ashows shows tothe varythe particle particle in the size size range distribution distribution of 0.5 µ m of the sample. Because of agglomeration, the size was found to vary in the range of 0.5 μm to 500 μm, with toof 500 theµ sample.m, with Because a median of agglomeration, diameter of approximatelythe size was found 8.0 toµ varym. Thein the scanning range of 0.5 electron μm to 500 micrograph μm, with of a mediana median diameter diameter of approximatelyof approximately 8.0 8.0 μm. μ m.The The scanning scanning electron electron micrograph micrograph of theof the sample, sample, shown shown in in the sample, shown in Figure2b, confirms the particle size distribution analysis results. Overall, 83.2 g FigureFigure 2b, 2b, confirms confirms the the particle particle size size distribution distribution anal analysisysis results. results. Overall, Overall, 83.2 83.2 g of g theof the sieved sieved sample sample was was of the sieved sample was loaded into the test section. After settling, the bed height was 370 mm, with a loadedloaded into into the the test test section. section. After After settling, settling, the the be dbe heightd height was was 370 370 mm, mm, with with a bulk a bulk density density of 58.4of 58.4 kg/m kg/m3. 3. bulk density of 58.4 kg/m3.

(a)( a) (b)( b)

FigureFigureFigure 2. 2. Size2. Size characterization characterization characterization of ofofthe thethe sieved sieved sample sample sample of ofhydrophobic of hydrophobic hydrophobic nanopowder; nanopowder; nanopowder; (a) (Particlea) Particle (a) Particlesize size sizedistribution; distribution; (b) ( (Agglomeratebb)) Agglomerate Agglomerate morphology morphology morphology in SEMin inSEM SEMmicrographs. micrographs. micrographs.

TheTheThe present present present experimental experimental experimental strategystrategy strategy involved involved a a regu regulara regularlar stepwise stepwise stepwise increase increase increase in inthein thethe velocity, velocity, followed followed byby a by similara similara similar stepwise stepwise stepwise decreasedecrease decrease toto investigatetoinvestigate investigate the the bed bed bed hydrodynamics hydrodynamics hydrodynamics during duringduring the thethe fluidization fluidizationfluidization and and and defluidizationdefluidizationdefluidization cycles cycles cycles (Figure(Figure (Figure3 3).). Each3).Each Each stepstep step was was 150 150 s long. s long. In In Inaddition addition addition to to tothe the the velocity, velocity, velocity, the thethe overall overalloverall and andand locallocallocal bed bed bed dynamics dynamics dynamics in in di differentinff differenterent bedbed bed regions regions were were were mo monitored monitorednitored using using using eight eight eight different didifferentfferent pressure pressure transducers, transducers, transducers, asas shown asshown shown in in Figure Figurein Figure1 ,1, with with1, with the the the data data data acquisitionacquisition acquisition frequencyfrequency frequency of of 100of 100 100 Hz. Hz. Hz. Thus, Thus, Thus, there there there were were were approximately approximatelyapproximately 15,000 data points for each analog voltage signal at a given velocity. The mean velocity was computed 15,00015,000 data data points points for for each each analoganalog voltagevoltage signal at at a a given given velocity. velocity. The The mean mean velocity velocity was was computed computed by carefully selecting a data window, as shown in Figure 3, and computing the average value. The byby carefully carefully selectingselecting a data data window, window, as asshown shown in Figure in Figure 3, and3, andcomputing computing the average the average value. value.The meanmean values values of ofthe the local local and and overall overall pr essurepressure drops drops were were similarly similarly computed. computed. The mean values of the local and overall pressure drops were similarly computed.

FigureFigureFigure 3. 3. Velocity Velocity3. Velocity variationvariation variation strategy strategy used used used an and dan evaluationd evaluation evaluation of of meanof mean mean velocities. velocities. velocities. Appl. Sci. 2020, 10, 8127 5 of 12

Appl. Sci. 2020, 10, x FOR PEER REVIEW 5 of 12 Two experimental runs were carried out to verify the data reproducibility. Since the pressure Two experimental runs were carried out to verify the data reproducibility. Since the pressure transients were used to characterize the bed dynamics, it was therefore essential to verify the pressure transients were used to characterize the bed dynamics, it was therefore essential to verify the pressure measurement system’s accuracy in this investigation. To this end, the local pressure drop values were measurement system’s accuracy in this investigation. To this end, the local pressure drop values were added together to obtain the overall pressure drop from the right-hand side of the following equation, added together to obtain the overall pressure drop from the right-hand side of the following equation, which was then compared with the direct monitoring of the overall pressure drop. which was then compared with the direct monitoring of the overall pressure drop. " # " # " # " # ∆(∆+∆P7 +∆ P8) ∆(∆+∆P1 +∆P2) ∆(∆+∆P3 +∆P4) ∆(∆+∆P5 +∆P6) == ++ ++ , , (1) 2 2 2 2 2 2 2 2 (1) overall upper middle lower TheThe results of of the the comparison comparison are are shown shown in Figure in Figure 4. Clearly,4. Clearly, the actual the actual values values of the overall of the overallpressure pressuredrop, i.e., drop, the left-hand i.e., the side left-hand of Equation side of (1), Equation offer excellent (1), oﬀ eragreement excellent with agreement the sum with of the the local sum pressure of the localdrops pressure (i.e., the drops above (i.e., equation’s the above right-hand equation’s side). right-hand side).

FigureFigure 4.4. AA comparisoncomparison ofof thethe sumsum ofof thethe locallocal pressurepressure dropsdrops withwith thethe actualactual overalloverall pressurepressure drop,drop, asas representedrepresented byby EquationEquation (1). (1). 3. Results and Discussion 3. Results and Discussion 3.1. Overall Bed Transients 3.1. Overall Bed Transients Figure5 shows the overall pressure drop and velocity transients in di fferent fluidized bed regions. The velocityFigure 5 is shows depicted the overall on the pressure right side drop while and the veloci pressurety transients is represented in different on fluidized the figure’s bed left regions. side. Initially,The velocity the velocity is depicted changes on the were right instantaneously side while the reflected pressure on is therepresented bed dynamics. on the The figure’s upper left region side. Initially, the velocity changes were instantaneously reflected on the bed dynamics. The upper region of of the bed, represented by ∆P1, was more sensitive to changes to the velocity than the middle the bed, represented by ΔP1, was more sensitive to changes to the velocity than the middle and lower and lower regions, i.e., ∆P3 and ∆P5. The step changes in the velocity were initially reflected in theregions, pressure i.e., Δ drops.P3 and Δ AtP5 600. The s, step however, changes when in the the velocity velocity were was initially increased reflected to 3 mmin the/s, pressure the dynamics drops. completelyAt 600 s, however, changed. when There the velocity was a steep was increased decline in to the3 mm/s, pressure the dynamics drop, indicating completely a transition changed. There from thewas fixed a steep to fluidizeddecline in bedthe pressure contact mode.drop, indicating Further increase a transition in the from gas the flow fixed above to fluidized 3 mm/s thenbed contact led to amode. different Further transients’ increase evolution in the gas in flow different above fluidized 3 mm/s bedthen regions. led to a Whiledifferent the transients’ upper region evolution showed in adifferent relatively fluidized consistent bed increase regions. inWhile the pressure the upper drop, region its showed magnitude a relatively in the lower consistent and middle increase regions in the hardlypressure increased, drop, its despite magnitude the velocity in the lower increase. and This middle behavior regions was hardly caused increased, by the velocity-induced despite the velocity bed expansionincrease. This after behavior the onset was of caused fluidization. by the velocity-induced Higher velocities bed caused expansion more after significant the onset bed of fluidization. expansion, leadingHigher tovelocities the migration caused ofmore bed significant material from bed theexpansion, middle leading and lower to the regions migration to the of bed’s bed uppermaterial region. from Giventhe middle that theand fluidized lower regions bed’s pressureto the bed’s drop upper corresponds region. Given to the that effective the fluidiz bed weight,ed bed’s the pressure increase drop in thecorresponds solid mass to inthe the effective upper regionbed weight, was thusthe increase reflected in in the the solid pressure mass drop in the rise. upper The region defluidization was thus experimentreflected in startedthe pressure at 1800 drop s with rise. a stepwise The defluidizat velocityion decrease. experiment The upper started region at 1800 transients s with a were stepwise slow tovelocity respond decrease. to the velocity The upper change region up transients to 5.6 mm were/s at 2560slow s.to Furtherrespond velocity to the velocity reduction change led toup a to consistent 5.6 mm/s butat 2560 sharper s. Further decrease velocity in the pressurereduction drop led into thea consistent bed’s upper but region. sharper On decrease the contrary, in the the pressure middle drop region in the bed’s upper region. On the contrary, the middle region pressure drop (ΔP3) showed a consistent increase, albeit small, during the defluidization cycle due to lower bed expansion, which caused the Appl. Sci. 2020, 10, 8127 6 of 12

pressure drop (∆P3) showed a consistent increase, albeit small, during the deﬂuidization cycle due to lowerAppl. bed Sci. expansion, 2020, 10, x FOR which PEER REVIEW caused the reverse migration of the solids from the upper to the6 of middle12 Appl. Sci. 2020, 10, x FOR PEER REVIEW 6 of 12 regionreverse of the migration bed. The of pressure the solids drop from in the the upper lower to regionthe middle of the region bed of remained the bed. mostlyThe pressure unaﬀ ecteddrop in by the velocity changes. reversethe lower migration region of of the the bed solids remained from the mostly upper unaffected to the middle by the region velocity of the changes. bed. The pressure drop in the lower region of the bed remained mostly unaffected by the velocity changes.

FigureFigure 5. Velocity5. Velocity and and pressure pressure transientstransients in in different different region region of ofthe the fluidized fluidized bed. bed. Figure 5. Velocity and pressure transients in different region of the fluidized bed. FigureFigure6 highlights 6 highlights the the overall overall pressure pressure drop drop dynamics dynamics (i.e., (i.e., Δ∆P7P) 7and) and the the upper upper region region dynamics dynamics monitored by diametrically opposite pressure transducers, ΔP1, and ΔP2. At the onset of the fluidization, monitoredFigure by diametrically6 highlights the overall opposite pressure pressure drop transducers,dynamics (i.e., Δ∆PP71) ,and and the∆ upperP2. Atregion the dynamics onset of the the overall pressure drop at 600 s showed a steep decline, followed by an increase with the velocity fluidization,monitored the by overalldiametrically pressure oppo dropsite pressure at 600 transducers, s showed a Δ steepP1, and decline, ΔP2. At the followed onset of bythe anfluidization, increase with increase. Its value remained relatively steady when the velocity was increased to 8.2 mm/s and then the velocitythe overall increase. pressure Its drop value at remained600 s showed relatively a steep steadydecline, when followed the by velocity an increase was increasedwith the velocity to 8.2 mm /s decreased to 4.7 mm/s during the defluidization cycle. A further decrease in the velocity below 4.7 mm/s and thenincrease. decreased Its value to remained 4.7 mm relatively/s during steady the defluidization when the velocity cycle. was Aincreased further to decrease 8.2 mm/s in and the then velocity was clearly reflected in the overall pressure drop profile. On the other hand, the bed dynamics monitored belowdecreased 4.7 mm to/s 4.7 was mm/s clearly during reflected the defluidization in the overall cycle. pressure A furtherdrop decrease profile. in the Onvelocity the otherbelow hand,4.7 mm/s the bed wasby Δ clearlyP1 and reflected ΔP2 showed in the excellent overall pressure agreement, drop indicating profile. On the the absenceother hand, of radial the bed non-homogeneities dynamics monitored in dynamicsthe upper monitored region of by the∆ Pfluidi1 andzed∆ Pbed.2 showed Similarly, excellent overall agreement,pressure drop indicating transients, thei.e., absenceΔP7 and ofΔP radial8 by ΔP1 and ΔP2 showed excellent agreement, indicating the absence of radial non-homogeneities in non-homogeneities(not reported here), in the showed upper excellent region ofagreement the fluidized that can bed. be Similarly,attributed overallto the absence pressure of radial drop transients,non- the upper region of the fluidized bed. Similarly, overall pressure drop transients, i.e., ΔP7 and ΔP8 i.e., ∆(nothomogeneitiesP7 and reported∆P8 (nothere), in the reported showed APF fluidizati here),excellent showedon agreement of the excellenthydrophobic that can agreement benanosilica. attributed that to can the be absence attributed of radial to the non- absence of radialhomogeneities non-homogeneities in the APF fluidizati in the APFon of fluidization the hydrophobic of the nanosilica. hydrophobic nanosilica.

Figure 6. Velocity, overall pressure drop (ΔP7), and upper region transients (i.e., ΔP1 and ΔP2) for the fluidization and defluidization cycles. FigureFigure 6. Velocity, 6. Velocity, overall overall pressure pressure drop drop (∆ (ΔPP77),), andand upper region region transients transients (i.e., (i.e., ΔP1∆ andP1 and ΔP2)∆ forP2 the) for the fluidization and defluidization cycles. fluidization3.2. Frequency and Response defluidization of Bed Transients cycles. 3.2. Frequency3.2. FrequencyIn this Response study, Response bed of transients Bedof Bed Transients Transients were also analyzed in the frequency domain. In particular, the effect of theIn velocity this study, changes bed transientson the region-wise were also bed analyzed dynamics in the was frequency examined. domain. Figure In7a particular,considers dynamicsthe effect Inin the this lower study, bed bed region. transients Both x- were and y-axes also analyzed present normalized in the frequency values. While domain. the amplitude In particular, was the effectof of the the velocity velocity changes changes on the on region-wise the region-wise bed dynamics bed dynamics was examined. was Figure examined. 7a considers Figure dynamics7a considers in the lower bed region. Both x- and y-axes present normalized values. While the amplitude was Appl. Sci. 2020, 10, 8127 7 of 12 dynamics in the lower bed region. Both x- and y-axes present normalized values. While the amplitude was normalized with the maximum value of the amplitude in the range of interest, the frequency Appl. Sci. 2020, 10, x FOR PEER REVIEW 7 of 12 axis was normalized with the frequency of the velocity step changes, which regularly took place 3 at an intervalnormalizedAppl. Sci. 2020 of , withevery10, x FORthe 150 PEERmaximum s. REVIEW Thus, value velocity of the amplitude changes in the occurred range of atinterest, a frequency the frequency of 6.67 axis7 wasof 1210 Hz. × − Therefore,normalized the event with occurring the frequency at the of normalized the velocity step frequency changes, of which unity regularly represents took the place velocity at an interval step changes. normalized with the maximum value of the amplitude in the range.67×10 of interest, the frequency axis was Higher frequencyof every 150 peaks s. Thus, on velocity the velocity changes signal occurred appearing at a frequency at the of integral 6 multiple Hz. Therefore, of unityare the higher-orderevent occurringnormalized at withthe normalized the frequency frequency of the velocityof unity steprepres changes,ents the which velocity regularly step changes. took place Higher at an frequency interval sinusoidpeaksof approximating every on 150 the s. velocityThus, the velocity signal velocity chanappearing stepges occurred changes. at the atinte a However, frequencygral multiple theof 6 .67×10of eff unityect of ar velocity e Hz. higher-order Therefore, step changes thesinusoid event was not stronglyapproximatingoccurring felt on ∆ atP the1, the since normalized velocity the peak step frequency changes. at 1 (Hz of However,unity/Hz) repres was th relativelyentse effect the of velocity velocity subdued step step changes. comparedchanges Higher was to not lowerfrequency strongly frequency peaks occurringfeltpeaks on onΔP 1the at, since 0.7velocity andthe peak 0.25,signal atas 1appearing (Hz/Hz) shown was inat the relatively inte figure.gral subdued multiple A typical compared of caseunity of arto hydrophilice lowerhigher-order frequency nanosilicasinusoid peaks with similar experimentaloccurringapproximating at 0.7 the conditionsand velocity 0.25, as step shown is presentedchanges. in the However, figure. in Figure A thtypicale 7effectbfor case of comparison. velocityof hydrophilic step changesThere nanosilica was are withnot prominent strongly similar peaks at 1 (Hzexperimentalfelt/Hz) onin ΔP the1, since lowerconditions the regionpeak is atpresented pressure1 (Hz/Hz) in dropwasFigure relatively signal. 7b for subduedcomparison. Unlike hydrophobiccompared There areto lower prominent nanosilica frequency peaks case, peaks at however,1 (Hz/Hz)occurring in at the 0.7 lowerand 0.25, region as shown pressure in thedrop figure. signal A. Unliketypical hydrophobiccase of hydrophilic nanosilica nanosilica case, however,with similar no no low-frequency event was visible in the pressure drop signal. This means that the bed dynamics are low-frequencyexperimental conditions event was is visible presented in the in pressure Figure 7bdr opfor signal.comparison. This means There thatare theprominent bed dynamics peaks atare 1 more sensitivemore(Hz/Hz) sensitive toin velocitythe tolower velocity transientsregion transients pressure in in ABF ABFdrop thanthan signal those those. Unlike in inAPF. hydrophobic APF. nanosilica case, however, no low-frequency event was visible in the pressure drop signal. This means that the bed dynamics are more sensitive to velocity transients in ABF than those in APF.

(a) (b)

Figure 7.Figure(a) 7. Frequency (a) Frequency response response of of the the velocity velocity and and lo lowerwer region region pressure pressure drop dropsignals signals for the for the (a) (b) hydrophobichydrophobic nanosilica. nanosilica. (b) Frequency (b) Frequency response response of theof the velocity velocity and and lower lower region region pressure pressure dropdrop signals signals for the hydrophilic nanosilica. for the hydrophilicFigure 7. (a) nanosilica.Frequency response of the velocity and lower region pressure drop signals for the hydrophobic nanosilica. (b) Frequency response of the velocity and lower region pressure drop Figuresignals for8 shows the hydrophilic the dynamics nanosilica. of the upper region. Surprisingly, the 150 s event associated with Figurethe velocity8 shows change the was dynamics hardly visible of the on upperthe frequency region. response Surprisingly, of the pressure the drop 150 ssignal. event Rather, associated with thetwo velocity lowFigure frequency change8 shows (i.e., the was 0.46 dynamics hardly and 0.25 of visible Hz/Hz)the upper on events theregion were frequency. Surprisingly, more prominent. response the 150 This ofs event themeanspressure associated that the effect dropwith signal. Rather, twoofthe the velocity low velocity frequency change step waschanges (i.e., hardly 0.46 was visible andmitigated 0.25 on the Hz by frequency/ Hz)the time events responseit reached were of morethe the fluidized pressure prominent. bed’sdrop signal.upper This means Rather,region. that the eﬀect ofInstead,two the low velocity itfrequency appeared step (i.e.,changes as a0.46 low-frequency and was 0.25 mitigated Hz/Hz) event events bysinc thee wereno time other more it event reached prominent. except the theThis ﬂuidized velocity means thatchanges bed’s the upper effect took region. Instead,placeof it the appeared in velocity the system step as a inchanges low-frequency the given was frequency mitigated event region. by since the time no it other reached event the exceptfluidized the bed’s velocity upper region. changes took Instead, it appeared as a low-frequency event since no other event except the velocity changes took place in the system in the given frequency region. place in the system in the given frequency region.

Figure 8. Frequency response of the upper region pressure drop and velocity signals for the fluidized bed of the hydrophobic nanosilica. Figure 8.FigureFrequency 8. Frequency response response of theof the upper upper region region pressure drop drop and and velocity velocity signals signals for the for fluidized the ﬂuidized bed of thebed hydrophobic of the hydrophobic nanosilica. nanosilica. Appl. Sci. 2020, 10, 8127 8 of 12 Appl. Sci. 2020, 10, x FOR PEER REVIEW 8 of 12

3.3.3.3. Region-WiseRegion-Wise PressurePressure DropDrop BehaviorBehavior TheThe dependence of of the the pressure pressure drop drop on on the the veloci velocityty is shown is shown in Figure in Figure 9a for9a the for fluidized the fluidized bed’s bed’supper upper region. region. Results Results of both of cycles, both cycles,i.e., gradual i.e., gradual velocity velocity increase increase (fluidization) (fluidization) and gradual and gradualvelocity velocitydecrease decrease (defluidization), (defluidization), are presented are presented together together in the figure. in the At figure. low velocities, At low velocities, the velocity the velocityincrease increaseled to the led pressure to the pressure drop increase drop increase due to the due fixed to the bed fixed mode bed of mode gas-solid of gas-solid contact. contact.When the When velocity the velocitywas increased was increased above 2 abovemm/s, 2there mm /wass, there a significant was a significant decline in decline the pressure in the pressure drop due drop to the due onset to the of onsetfluidization of fluidization that caused that motion caused motionof solids of in solids the bed. in the Further bed. Furtherincrease increase in the velocity in the velocity beyond beyond4 mm/s 4then mm led/s then to a rise led in to the a rise pressure in the drop pressure due to drop the bed’s due toexpansion. the bed’s As expansion. mentioned As earlier, mentioned bed expansion earlier, bedcaused expansion the mass caused (solid the materials) mass (solid upward materials) movement upward from movement lower bed from sections lower to bed the sections upper region. to the upperThis phenomenon region. This phenomenonwas reflected was in the reflected pressure in the dr pressureop increase. drop Unlike increase. the Unlikefluidization the fluidization cycle, the cycle,velocity the decrease velocity decreaseled to a ledconsistent to a consistent pressure pressure drop reduction drop reduction during during the defluidization the defluidization cycle. cycle. As a Asresult, a result, there there was wasa pronounced a pronounced hysteresis hysteresis effect eff observedect observed in the in upper the upper region region of the of thebed. bed.

(a) (b)

FigureFigure 9.9. EEffectffect ofof velocityvelocity onon thethe pressurepressure dropdrop inin didifffferenterent regionsregions ofof thethe fluidizedfluidized bedbed duringduring thethe fluidizationfluidization andanddefluidization defluidizationcycles cycles ( a(a)) Upper Upper ( b(b)) Middle Middle ( c(c)) Lower Lower ( d(d)) Overall. Overall.

FigureFigure9 9bb showsshows the the velocity velocity dependence dependence of the of thepressure pressure drop drop in the in middle the middle region regionof the fluidized of the fluidizedbed. The bed.behavior The behaviorat low velocities, at low velocities, in this case, in this was case, similar was to similar the one to theobserved one observed for the forupper the upperregion regionowing owingto the tofixed the bed fixed mode bed modeof gas-solid of gas-solid contact. contact. At the At onset the onsetof the of fluidization, the fluidization, the pressure the pressure drop dropdropped dropped significantly significantly due dueto the to so thelid solid motion. motion. As the As velocity the velocity was was raised raised above above 4 mm/s,4 mm there/s, there was was no nonoticeable noticeable change change in the in the pressure pressure drop drop profile. profile. OnOn the the other other hand, hand, decreasing decreasing the thevelocity velocity during during the thedefluidization defluidization cycle cycle led led to toa steady a steady increase, increase, albeit albeit small, small, in in the the pressure pressure drop drop behavior, behavior, due toto thethe decreasedecrease inin bedbed expansion,expansion, whichwhich causedcaused thethe downwarddownward migrationmigration ofof thethe bedbed materialmaterial upperupper regionregion ofof thethe bed.bed. AA significantsignificant eeffectffect ofof hysteresishysteresis atat lowlow velocitiesvelocities waswas seenseen inin thisthis region.region. TheThe pressurepressure dropdrop profilesprofiles inin thethe lowerlower regionregion areare shownshown inin FigureFigure9 9c.c. The The hysteresis hysteresis eeffect,ffect, isis atat leastleast inin thisthis case,case, comparedcompared toto thethe middlemiddle andand upperupper regionsregions of of the the fluidized fluidized bed. bed. TheThe overalloverall pressurepressure dropdrop profilesprofiles areare presentedpresented inin FigureFigure9 d.9d. These These profiles profiles represent represent the the combinedcombined behaviorbehavior of all all the the three three regions regions shown shown before. before. The The upper upper region region behavior behavior dominated dominated the overall behavior of the fluidized bed. Therefore, the hysteresis phenomenon was prominent in the Appl. Sci. 2020, 10, 8127 9 of 12 theAppl. overall Sci. 2020, behavior 10, x FOR PEER of the REVIEW fluidized bed. Therefore, the hysteresis phenomenon was prominent9 of in12 the overall pressure drop profiles as well. Another noteworthy feature in this figure was the overall pressureoverall pressure drop for drop the fully profiles fluidized as well. bed. Another In this case, noteworthy the theoretical feature pressure in this dropfigure can was be the defined overall as pressure drop for the fully fluidized bed. In this case, the theoretical pressure drop can be defined as

(∆P) = (ρb ρ)gL, (2) ∆theoretical = −−, (2)

3−3 wherewhere ρand andρ are are the the bulk bulk and and fluid fluid densities densities in in kg kgm∙m ,, respectively.respectively. Notably,Notably, gg isis thethe accelerationacceleration b · − due to gravity, andand LL isis thethe heightheight ofof thethe bed.bed. AccountingAccounting forfor 83.283.2 gg ofof thethe bed materialmaterial and correcting ∆ = 201 Pa for the height height of of the the lowest lowest pressure pressure ta tapp from from the the distributor distributor plate, plate, one one obtains obtains (∆P)theoretical = 201 Pa.. However, thethe experimentalexperimental overalloverall pressurepressure dropdrop forfor thethe fullyfully fluidizedfluidized bedbed obtained in this study was approximately 145 145 Pa, Pa, which which was was substantially substantially lower lower th thanan the the actual actual value value of of201 201 Pa, Pa, even even though though no noinstance instance of dead of dead region region or gas or gas bypassing bypassing was was obse observed.rved. This This reduction reduction in the in thepressure pressure drop drop can can be beattributed attributed to tothe the upward upward motion motion of of solids, solids, ther therebyeby lowering lowering the the upward upward moving fluidizing fluidizing gas’s relative velocity.velocity. ThisThis phenomenonphenomenon reducedreduced thethe gas-solidgas-solid drag,drag, resultingresulting inin aa lowerlower pressurepressure drop. drop.

3.4. Bed Height and Bulk Density The dependence dependence of of the the bed bed height height on the on velocity the velocity is shown is shown in Figure in Figure10a. The 10 beda. height The bed remained height remainedstagnant till stagnant the velocity till the was velocity raised wasto 6 raisedmm/s. toThen 6 mm, there/s. Then,was a theresteep wasrise ain steep height rise as inthe height velocity as thewas velocity increased. was On increased. the other Onhand, the the other defluidization hand, the defluidization cycle showed cyclea gradual showed decrease a gradual in height decrease with ina reduction height with in the a reduction velocity. inThus, the the velocity. hysteresis Thus, phenomenon the hysteresis was phenomenon once again very was prominent once again in very the prominentexpansion of in the the hydrophobic expansion of nanosilica. the hydrophobic The overal nanosilica.l bed expansion The overall affects bed the expansion overall bulk aff density,ects the overallwhich has bulk been density, lately which utilized has to been develop lately dry utilized beneficiation to develop technologi dry beneficiationes [23,24]. The technologies bed bulk density [23,24]. Theprofiles bed bulkare shown density in profiles Figure are 10 shownb for both in Figure fluidization 10b for bothand fluidizationdefluidization and cycles. defluidization The hysteresis cycles. Thephenomenon hysteresis is phenomenon once again very is onceobvious again from very the obvious figure. However, from the figure.it is interesting However, that it the is interestingbed shows thata several-fold the bed shows increase a several-fold in the bed height, increase resulting in the bed in height,the bulk resulting density, inwhich the bulk is extremely density, whichlow and is extremelyhas not been low reported and has in not the been fluidi reportedzation of in micron-sized the fluidization particles. of micron-sized particles.

(a) (b)

Figure 10. EffectEﬀect of velocity on the bed expansion (a) Bed height; (b) Bulk density.

3.5. Incipient Fluidization and Terminal VelocityVelocity Figure 11 showsshows the overalloverall pressurepressure drop,drop, whichwhich waswas normalizednormalized byby dividingdividing thethe actualactual pressure dropdrop byby itsits theoreticaltheoretical valuevalue obtainedobtained usingusing EquationEquation (2).(2). The experimental data for the defluidization cycle were used for evaluating the U , which is approximately 4 mm/s here for the defluidization cycle were used for evaluating the Umfmf, which is approximately 4 mm/s here for the hydrophobic nanosilica, as seen in the figure. On the other hand, the U of hydrophilic nanosilica hydrophobic nanosilica, as seen in the figure. On the other hand, the Umfmf of hydrophilic nanosilica waswas approximatelyapproximately 2222 mmmm/s,/s, almostalmost fivefive timestimes higherhigher thanthan thethe valuevalue obtainedobtained herehere [[12].12]. This shows that thethe surfacesurface treatment of the nanosilica can significantlysignificantly affect affect its dry dispersion characteristics. Moreover, given that the U depends on the particle size, a lower U indicated smaller agglomerates Moreover, given that the Umfmf depends on the particle size, a lower Umfmf indicated smaller agglomerates inin thethe presentpresent case.case. Besides Besides the the minimum minimum fluidization fluidization velocity, velocity, there was another aspect of the fluidizationfluidization hydrodynamic, which was noteworthy as well. The normalized pressure drop for the fully fluidized bed was 0.72. This means that the actual pressure drop was significantly lower than the theoretical pressure drop, despite the particulate nature of the fluidization, which rules out the occurrence of gas by-passing and other similar non-homogeneities in the bed. Appl. Sci. 2020, 10, 8127 10 of 12 fully fluidized bed was 0.72. This means that the actual pressure drop was significantly lower than the theoretical pressure drop, despite the particulate nature of the fluidization, which rules out the occurrenceAppl. Sci. 2020 of, 10 gas, x FOR by-passing PEER REVIEW and other similar non-homogeneities in the bed. 10 of 12 Appl. Sci. 2020, 10, x FOR PEER REVIEW 10 of 12

Figure 11. 11. MinimumMinimum fluidization fluidization velocity from velocity the overall from pressure the overall drop profile pressure of the drop defluidization profile of cycle. the defluidizationFigure 11. Minimum cycle. fluidization velocity from the overall pressure drop profile of the defluidization cycle. Next, the dependence of the bed expansion on the velocity was investigated here by using the Next, the dependence of the bed expansion on the velocity was investigated here by using the Richardson–ZakiNext, the dependence correlation of in the conjunction bed expansion with theon thtotale velocity mass balance was investigated considerations here as by follows using [5]:the Richardson–Zaki correlation in conjunction with the total mass balance considerations as follows [5]: Richardson–Zaki correlation in conjunction with the total mass balance considerations as follows [5]: ⁄ ⁄ ⁄ = − 1 − (3) 1/⁄n 1/⁄n 1/⁄n H0 U ==U −U 11 −ε g0 (3)(3) 0 t − t − H where Ut is the terminal velocity of the agglomerate, H0 and H are initial bed height and the bed whereheightwhere UUattt velocityis thethe terminalterminal U0, respectively. velocityvelocity ofof thethe is agglomerate,agglomerate, the inter-agglomerate H0 and H bedare void initial fraction bed height at the and initial the bed height atconditions, velocity Uand0,, respectively. respectively.n is the Richardson–Zaki εg0 is thethe inter-agglomerateinter-agglomerate exponent. The above bed void equation fractionfraction describes at thethe initialinitialthe linear bedbed ⁄ ⁄ heightrelationship conditions, between and the n isis thethe Richardson–ZakiRichardson–Zaki (y-axis) and the exponent.exponent. ⁄ (x -axis), The above thereby equation yielding describes theas the linear y- ⁄ 1/n⁄ ⁄ 1/n ⁄ relationshipintercept and betweenbetween 1 the −the U 0 as(y the -axis)(y-axis) slope. and and theThe H theresults0/ H (x are-axis), (shownx-axis), thereby inthereby Figure yielding yielding12U usingt as n the = 5. y-intercept asPrevious the y- 1/n ⁄ andstudiesinterceptU pointed and1 ε g outas1 that the − slope. the above as The the resultsequation’s slope. are The shownparameters results in Figureare were shown 12 relatively using in Figuren = independent5. 12 Previous using studiesn of = the5. Previous value pointed of t − 0 outnstudies [5]. that From pointed the the above figure, out equation’s that the the terminal above parameters equation’svelocity were of parametersth relativelye hydrophobic independentwere nanosilicarelatively of independentthewas value found of ton of be[5 the]. 21.6 From value mm/s. the of figure,Usingn [5]. From the Stokes terminal the figure, law velocity and the terminalthe of terminal the hydrophobic velocity velocity of th nanosilica datae hydrophobic, the values was found nanosilica of the to beagglomerate 21.6was mmfound/s. diameter Usingto be 21.6 the and Stokesmm/s. the lawterminalUsing and the theReynolds Stokes terminal law number velocityand the was data,terminal found the valuestovelocity be 87 of μdata them, and agglomerate, the 0.12, values respectively. of diameter the agglomerate and the terminal diameter Reynolds and the numberterminalwas Reynolds found number to be 87 wasµm, found and 0.12, to be respectively. 87 μm, and 0.12, respectively.

Figure 12. Evaluation of the terminal velocity from the dependence of the bed expansion on the gas Figure 12. Evaluation of the terminal velocity from the dependence of the bed expansion on the velocity.Figure 12. Evaluation of the terminal velocity from the dependence of the bed expansion on the gas gas velocity. velocity. 4. Conclusions 4. Conclusions The fluidized bed dynamics of agglomerated hydrophobic nanosilica with APF behavior were investigatedThe fluidized here by bed monitoring dynamics theof agglomeratedpressure tran sientshydrophobic in different nanosilica regions with of theAPF bed, behavior which were was subjectedinvestigated to regularhere by step monitoring changes inthe the pressure velocity tranof thesients fluidizing in different gas. The regions radial of non-uniformities the bed, which were was mostlysubjected absent to regular in the bedstep during changes both in the the velocity fluidization of the and fluidizing defluidization gas. The cycles. radial The non-uniformities effect of the velocity were changesmostly absent was observed in the bed on during the lower both region the fluidization dynamics. and In the defluidization upper region cycles. of the Thebed, effect however, of the the velocity effect changes was observed on the lower region dynamics. In the upper region of the bed, however, the effect Appl. Sci. 2020, 10, 8127 11 of 12

4. Conclusions The fluidized bed dynamics of agglomerated hydrophobic nanosilica with APF behavior were investigated here by monitoring the pressure transients in different regions of the bed, which was subjected to regular step changes in the velocity of the fluidizing gas. The radial non-uniformities were mostly absent in the bed during both the fluidization and defluidization cycles. The effect of the velocity changes was observed on the lower region dynamics. In the upper region of the bed, however, the effect of the velocity changes was substantially mitigated and mostly appeared as low-frequency events. Strong hysteresis effects on the pressure drop profiles were noted in the upper region, which was also clearly reflected in the bed’s overall behavior. The hysteresis phenomenon was also found in the overall bed expansion and the bulk density profiles. Although neither gas by-passing nor other non-homogeneities were observed in the bed, the actual pressure drop in the fully fluidized bed was significantly lower than the theoretical pressure drop obtained from the bed’s total mass. The minimum fluidization velocity was substantially lower than the ones usually obtained for the hydrophilic nanosilica. Similarly, the agglomerate diameter evaluated from the bed expansion data, when fitted with the Richardson–Zaki correlation, predicted the agglomerate size to be 86 mm while the actual primary nanoparticle size was reportedly 7 nm only.

Author Contributions: M.A. conceptualization; M.A. and N.S.K. methodology; M.A. and E.H.A.-G. software; M.A. and E.H.A.-G. validation; M.A. and S.A.K. formal analysis; E.H.A.-G., M.A. investigation; M.A. and N.S.K. resources; E.H.A.-G. data curation; M.A., N.S.K. and S.A.K. writing—original draft preparation; M.A., N.S.K. and S.A.K. writing—review and editing; M.A. supervision; M.A. project administration; M.A. funding acquisition. All authors have read and agreed to the published version of the manuscript. Funding: The Researchers Supporting Project at King Saud University Project No. (RSP-2020/42). Acknowledgments: The authors would like to thank the Researchers Supporting Project, RSP-2020/42, King Saud University, Riyadh, Saudi Arabia, for financial support. Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

References

1. Geldart, D. Types of gas ﬂuidization. Powder Technol. 1973, 7, 285–292. [CrossRef] 2. Raganati, F.; Chirone, R.; Ammendola, P. Gas-solid ﬂuidization of cohesive powders. Chem. Eng. Res. Des. 2018, 133, 347–387. [CrossRef] 3. Al-Ghurabi, E.H.; Ajbar, A.; Asif, M. Improving Fluidization Hydrodynamics of Group C Particles by Mixing with Group B Particles. Appl. Sci. 2018, 8, 1469. [CrossRef]

4. Yao, W.; Guangsheng, G.; Fei, W.; Jun, W. Fluidization and agglomerate structure of SiO2 nanoparticles. Powder Technol. 2002, 124, 152–159. [CrossRef] 5. Zhu, C.; Yu, Q.; Dave, R.N.; Pfeffer, R. Gas fluidization characteristics of nanoparticle agglomerates. AIChE J. 2005, 51, 426–439. [CrossRef] 6. Barik, T.K.; Sahu, B.; Swain, V. Nanosilica—From medicine to pest control. Parasitol. Res. 2008, 103, 253. [CrossRef] 7. Zhang, M.-H.; Islam, J. Use of nano-silica to reduce setting time and increase early strength of concretes with high volumes of fly ash or slag. Constr. Build. Mater. 2012, 29, 573–580. [CrossRef] 8. Guo, W.; Guo, X.; Chang, M.; Dai, W. Evaluating the Effect of Hydrophobic Nanosilica on the Viscoelasticity Property of Asphalt and Asphalt Mixture. Materials 2018, 11, 2328. [CrossRef]

9. Mallakpour, S.; Naghdi, M. Polymer/SiO2 nanocomposites: Production and applications. Prog. Mater. Sci. 2018, 97, 409–447. [CrossRef] 10. NanoSilica Market Size & Share, Global Industry Report, 2018–2025. Available online: https://www. grandviewresearch.com/industry-analysis/nanosilica-market (accessed on 1 July 2020). 11. Al-Ghurabi, E.H.; Shahabuddin, M.; Kumar, N.S.; Asif, M. Deagglomeration of Ultraﬁne Hydrophilic Nanopowder Using Low-Frequency Pulsed Fluidization. Nanomaterials 2020, 10, 388. [CrossRef] Appl. Sci. 2020, 10, 8127 12 of 12

12. Asif, M.; Al-Ghurabi, E.H.; Ajbar, A.; Kumar, N.S. Hydrodynamics of Pulsed Fluidized Bed of Ultrafine Powder: Fully Collapsing Fluidized Bed. Processes 2020, 8, 807. [CrossRef] 13. Shabanian, J.; Chaouki, J. Influence of interparticle forces on solids motion in a bubbling gas-solid fluidized bed. Powder Technol. 2016, 299, 98–106. [CrossRef] 14. Lee, J.R.; Lee, K.S.; Park, Y.O.; Lee, K.Y. Fluidization characteristics of fine cohesive particles assisted by vertical vibration in a fluidized bed reactor. Chem. Eng. J. 2020, 380, 122454. [CrossRef] 15. Lehmann, S.E.; Hartge, E.U.; Jongsma, A.; deLeeuw, I.M.; Innings, F.; Heinrich, S. Fluidization characteristics of cohesive powders in vibrated fluidized bed drying at low vibration frequencies. Powder Technol. 2019, 357, 54–63. [CrossRef] 16. Zhao, Z.; Liu, D.; Ma, J.; Chen, X. Fluidization of nanoparticle agglomerates assisted by combining vibration and stirring methods. Chem. Eng. J. 2020, 388, 124213. [CrossRef] 17. Saidi, M.; Tabrizi, H.B.; Grace, J.R. A review on pulsed flow in gas-solid fluidized beds and spouted beds: Recent work and future outlook. Adv. Powder Technol. 2019, 30, 1121–1130. [CrossRef] 18. Ali, S.S.; Asif, M. Fluidization of nano-powders: Effect of flow pulsation. Powder Technol. 2012, 225, 86–92. [CrossRef] 19. Al-Ghurabi, E.H.; Ali, S.S.; Alfadul, S.M.; Shahabuddin, M.; Asif, M. Experimental investigation of fluidized bed dynamics under resonant frequency of sound waves. Adv. Powder Technol. 2019, 30, 2812–2822. [CrossRef] 20. Ali, S.S.; Asif, M. Effect of particle mixing on the hydrodynamics of fluidized bed of nanoparticles. Powder Technol. 2017, 310, 234–240. [CrossRef] 21. Asif, M.; Kalogerakis, N.; Behie, L.A. Hydrodynamics of liquid fluidized-beds including the distributor region. Chem. Eng. Sci. 1992, 47, 4155–4166. [CrossRef] 22. Robinson, N.; Gladden, L.F.; D’Agostino, C. Exploring catalyst passivation with NMR relaxation. Faraday Discuss 2017, 204, 439–452. [CrossRef][PubMed] 23. Zhou, E.; Zhang, Y.; Zhao, Y.; Luo, Z.; Yang, X.; Duan, C.; Dong, L.; Fu, Z. Effect of vibration energy on fluidization and 1–6 mm coal separation in a vibrated dense medium fluidized bed. Sep. Sci. Technol. 2018, 53, 2297–2313. [CrossRef] 24. Fu, Z.; Zhu, J.; Barghi, S.; Zhao, Y.; Luo, Z.; Duan, C. Mixing and segregation behavior in an air dense medium fluidized bed with binary mixtures for dry coal beneficiation. Powder Technol. 2020, 371, 161–169. [CrossRef]

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