Enhancement of CO2 Removal Efficacy of Fluidized Bed Using

applied sciences

Article Enhancement of CO2 Removal Efﬁcacy of Fluidized Bed Using Particle Mixing

Ebrahim H. Al-Ghurabi, Abdelhamid Ajbar and Mohammad Asif * ID

Department of Chemical Engineering, King Saud University, PO Box 800, Riyadh 11421, Saudi Arabia; [email protected] (E.H.A.-G.); [email protected] (A.A.) * Correspondence: [email protected]; Tel.: +966-56-981-7045

Received: 28 July 2018; Accepted: 24 August 2018; Published: 27 August 2018

Featured Application: Towards developing a cost-effective technique based on the fluidized bed technology for the treatment of carbon dioxide in flue gases. We have clearly shown that assisted fluidization technique of particle mixing proposed here can improve the CO2 capture efficacy of fluidized bed containing fine adsorbent.

Abstract: The present study proposes a cost-effective assisted fluidization technique of particle mixing to improve the carbon capture effectiveness of a fluidized bed containing fine adsorbent powder. Using activated carbon as the adsorbent, we mixed external particle of Geldart group B classification in different fractions to examine the effectiveness of the proposed strategy of particle mixing. Four different particle-mixing cases were considered by varying the amount of added particle—0, 5, 10, and 30 wt %—on external particle-free basis. The inlet flow of the nitrogen was fixed, while two different flows of carbon dioxide were used. The adsorption experiment consisted of a three step procedure comprising purging using pure nitrogen, followed by adsorption with fixed inlet CO2 concentration, and finally the desorption step. Inlet flows of both nitrogen and CO2 were separately controlled using electronic mass flow controllers with the help of data acquisition system (DAQ). The CO2 breakthrough was carefully monitored using the CO2/O2 analyzer, whose analog output was recorded using the DAQ. Best results were obtained with 10% external particles. This is in conformity with the results of our previous study of bed hydrodynamics, which pointed to clear improvement in the fluidization behavior with particle mixing.

Keywords: CO2 removal; assisted ﬂuidization; particle mixing; ﬂuidized bed; Geldart group B; activated carbon

1. Introduction

Although carbon dioxide (CO2) constitutes only 0.035% of the atmosphere, it is one of the most abundant greenhouse gases (GHG). The widespread use of fossil fuel combustion in energy-intensive industrial activities accounts for 90% of total CO2 emissions [1,2]. The transition from carbon-based energy sources to cleaner ones would be an ideal solution; however, the proposed technologies are still not mature enough to allow for their large-scale implementation. Therefore, technologies for carbon capture and sequestration (CCS) will continue to play an important role until significant changes in the energy infrastructure can be achieved. The CO2 capture process constitutes the major component of the total CCS cost [3,4]. Research efforts have therefore focused on developing new techniques to reduce the economic cost while achieving significant levels of CO2 capture. Several technologies are available for this purpose. These include absorption, membrane separation, biofixation, and adsorption [5–7]. Commonly used in the chemical process industries, the use of absorption using solvents has also

Appl. Sci. 2018, 8, 1467; doi:10.3390/app8091467 www.mdpi.com/journal/applsci Appl. Sci. 2018, 8, 1467 2 of 12 been applied to postcombustion flue gases, e.g., Boundary Dam power station in Saskatchewan, Canada [8]. This technology, however, requires expensive pretreatment of the flue gases and is moreover energy-intensive. In addition, the solvent evaporation and equipment corrosion are two other important issues that also require careful consideration for the long-term viability of this technology. The use of the membrane separation technology has recently attracted considerable interest. However, some recent studies have suggested that this technology does not offer any notable advantage over chemical absorption, in terms of both energy consumption and cost [9], unless fresh approach towards the development of novel types of membranes are adopted. For example, mixed-matrix membranes (MMMs) composed of a polymer and activated carbon showed good performance in the separation of gases [10,11]. Another environment-friendly approach proposed in the recent literature is microbial CO2 biosequestration using microorganisms [12]. However, its application still requires considerable further basic research and applied development work. Among these technologies, adsorption is attractive in the long term owing to its operational flexibility, low energy requirements, and generally low maintenance costs [4,13–16]. Adsorption consists of an uptake of CO2 molecules onto the solid surface, which can occur via either weak van der Waals forces (physisorption) or stronger covalent bonding (chemisorption) [15]. However, adsorption also has potential disadvantages, including handling large volumes of adsorbent, particles attrition, and thermal control of large-scale adsorber vessels. The efficiency of adsorption depends naturally on the adsorbents used. In this regard, there are a number of efficient solid adsorbents for CO2 that include activated carbons, activated alumina, zeolites, ion-exchange resins, silica gel, metal oxides, mesoporous silicates, carbon fibers and their composites, and metal-organic frameworks (MOF) [17]. For the adsorption to become economically viable, the selection of a suitable adsorbent material is important. An ideal CO2 adsorbent should have high adsorption capacity, high hydrophobicity and tolerance to feed impurities, stability, low cost, and ease of regeneration [17]. Activated carbon powders, when used as adsorbents, possess a number of such characteristics. For example, they are insensitive to moisture, have low cost, and require low energy for regeneration [18] despite having relatively lower adsorption capacity [19]. For gas–solid contacting during the adsorption, either fixed bed or fluidized bed mode can be implemented. The application of fluidized bed instead of fixed bed yields lower pressure drop, better phase contact, and intimate mixing that ultimately translates into greater efficiency and lower costs [20,21]. Moreover, fluidization is particularly suitable compared to other available techniques for handling and processing large quantities of powders. While fine powders can theoretically provide a high surface area for the adsorption, the efficacy of gas–solid contact during fluidization is often severely compromised by the agglomeration of the fine particles as a result of interparticle van der Waals forces. For example, the fluidization of fine cohesive powders of Geldart group C is rather heterogeneous and is characterized by the formation of cracks and channels through which the gas phase tends to bypass solids present in the bed, resulting in poor contact and inefficient mixing [12–24]. This phenomenon can significantly lower the CO2 adsorption capacity of the fine adsorbent. In order to improve the fluidization hydrodynamics of solids that exhibit poor fluidization behavior, various assisted fluidization techniques have been proposed in the literature [24–34]. These techniques are generally implemented in conjunction with conventional fluidization to provide additional energy to the fluidized bed for overcoming interparticle forces that cause powder cohesiveness and other nonhomogeneities observed during fluidization. A recent interesting example of such assisted fluidization techniques is the continuous pulsation of inlet flow at regular time intervals instead of a continuous steady inlet flow of the gas to the fluidized bed. This technique greatly improves the fluidization hydrodynamics of ultrafine powders [26,27,35–38]. Similarly, the use of acoustic vibrations has also been reported in the literature [28–30,39–41]. These vibrations promote deagglomeration and eliminate bed nonhomogeneities, thus improving the gas–liquid phase contact. The CO2 removal efficacy of sound-assisted fluidized bed significantly improves as a result of acoustic Appl. Sci. 2018, 8, 1467 3 of 12 perturbations [41–45]. However, although effective, this technique is inherently energy intensive and moreover requires modification in the fluidization setup for the installation of sound system. Another simple yet cost-effective assisted fluidization technique is the addition of external inert particles. A carefully chosen sample of external particles with appropriate physical properties, when mixed with resident solid phase of the fluidized bed, has been found to significantly enhance fluidization hydrodynamics [23,30,31,46]. For example, the addition of Group A particles to a bed of nanopowder was found to promote deagglomeration and suppress the phenomenon of hysteresis [31,46]. In the present work, we applied this technique to improve the CO2 capture efficacy of a fluidized bed containing fine adsorbent particles. This was achieved by adding inert external particles of Geldart group B classification. This choice was dictated by the encouraging results of a recent study that clearly demonstrated the effectiveness of the particle-mixing strategy utilizing Group B particles in improving the hydrodynamics of the fluidized bed of Group C particles [47]. Two different Group C particles—activated carbon and calcium hydroxide—were considered, while the fraction of external particle used for mixing was varied. The bed hydrodynamics was characterized using the fluidization index. A statistical evaluation of results clearly indicated that the best fluidization is achieved when the proportion of inert particle is 10–15 wt % on the external particle free basis. We therefore extended the same strategy of particle mixing to improve the CO2 removal efficacy of fluidized bed containing fine adsorbentAppl. Sci. 2018 particles., 8, x FOR PEER Needless REVIEW to mention, the basic fluidization setup remains unchanged4 of 12 for the implementation of the proposed particle-mixing strategy; moreover, there was no extra consumption Omega, Stamford, CT, USA) with a response time of 1 millisecond. As shown in Figure 1, the lower of energy besides the one associated with the pressure drop. The use of inert sand as the external pressure port was located at a distance of 50 mm from the distributor, while the upper port was groupplaced B particles 250 mm further above minimized the distributor, the cost thus component providing ofcrucial the proposed information strategy. about the local bed dynamics. The pressure tap, flush with the column wall, was covered with a fine nylon mesh. The 2. Experimental pressure transducers were carefully calibrated from volts to Pa using a pressure calibrator (Fluke Model 718). The calibration showed excellent linearity of voltage–Pa profiles. Experimental Setup Five hundred grams of the activated carbon was loaded in the bed before starting the Figureexperiment.1 shows The thestatic schematic height of diagram the bed ofwas the 225 experimental ± 3 mm. First, setup. a number The testof fluidization section comprised and a 1-m-longdefluidization vertical experiments Perspex column were of carried 70-mm out internal using diameter.only nitrogen A perforated gas to study plate the distributor, fluidization located at itshydrodynamics bottom, ensured of the a uniformbed. There distribution were initially of some inlet losses gases. (<1% The of distributor the total solid was mass) a 10-mm-thick of the Plexiglasactivated plate carbon with 1.5-mm due to elutriation, holes and which 4% open were area. replenished To eliminate to keep entry bed effects,height constant a 250-mm-long throughout calming the experiment. These results, which are related to bed hydrodynamics, are reported elsewhere [47]. section was provided below the distributor, as shown in the figure. The outlet gases from the column The adsorption experiments were carried out at a fixed velocity, while two different inlet CO2 wereconcentrations released outside were the considered lab to prevent as the composition nitrogen andof post CO combustion2 build-up. flue Openings gases varies along depending the column heightupon provided the fuel ports feedstock for the used measurement for the power of generati the pressureon. As dropdetailed and later, the carbonthe four dioxide different concentration. cases of Beforeparticle the start mixing of the were experiment, carefully investigated. the setup was The tested opening, for located any leaks at a by distance ensuring of 500 inlet mm and above outlet the flows weredistributor, equal. was used as a port for monitoring the CO2 concentration.

Figure 1. Schematic of the experimental setup for the CO2 adsorption from the mixture of N2–CO2 Figure 1. Schematic of the experimental setup for the CO2 adsorption from the mixture of N2–CO2 gases.gases. Single Single black black broken broken lines lines represent represent voltage voltag signale signal connections connections of of different different equipmentequipment withwith data data acquisition system (DAQ); double black broken lines represent communication of DAQ with acquisition system (DAQ); double black broken lines represent communication of DAQ with computer. computer.

3. Results and Discussion In this study, we considered four cases of particle mixing by varying the fraction of external Group B particles in the fluidized bed containing 500 g of activated carbon. In the first case, adsorption studies were carried out in the absence of the external particles. In the second case, 25 g Group B particles were added, and adsorption experiments were carried out. In the third case, the amount of external particles was increased to 50 g by further adding 25 g particles. In the fourth case, the total amount of external particles was 150 g; thus, the amount of external particles defined on the basis of the external particle free weight basis (X1) were 0, 5, 10, and 30 wt %. All four cases will be individually discussed in this section before comparing the efficacy of the proposed strategy of particle mixing on the removal of CO2 using the fluidized bed technology. We first present adsorbent characterization data, followed by the selection of the external particles for our proposed particle-mixing strategy. Next, we describe our experimental strategy before presenting the results of our investigation.

3.1. Adsorbent Characterization Appl. Sci. 2018, 8, 1467 4 of 12

As shown in Figure1, the inlet gas flow assembly comprised two separate inlet lines from compressed cylinders of nitrogen and carbon dioxide. To ensure accurate measurement of the flow rates of gases, two different electronic flow controllers of different ranges were used. For nitrogen, the range was 0–2 SLPM (FMA-2605A-V2P), while 0–200 CCPM (FMA-2618A-V2P) was used for the carbon dioxide. Excellent linearity between the voltage output and the flow rate was noticed. The analog voltage inputs and outputs of the mass flow controllers were connected to the data acquisition system (DAQ) for controlling and monitoring the flow of gases to the column. Both gases were mixed in fixed proportion before entering the test section of the fluidized bed. For the measurement of the carbon dioxide concentration, we used an oxygen/carbon dioxide analyzer (Model 902P, Quantek Instruments, Grafton, MA, USA) in view of its high accuracy and measurement stability. The analog output options for the carbon dioxide and the oxygen were configured and connected to the data acquisition system. The minimum CO2 detection limit was 0.01%, while the upper concentration range was 20%. National Institute of Standards and Technology (NIST) traceable calibration certificate, supplied by the manufacturer, was used for converting the voltage output to percentage volumetric concentration. The voltage signals from the analyzer were captured by DAQ running LabView software and then transferred to a computer for further processing. We employed a fast response pressure transducer for monitoring the local transients of the fluidized bed. It was a sensitive, differential, bidirectional pressure transducer (PX163-005BD5V, Omega, Stamford, CT, USA) with a response time of 1 millisecond. As shown in Figure1, the lower pressure port was located at a distance of 50 mm from the distributor, while the upper port was placed 250 mm above the distributor, thus providing crucial information about the local bed dynamics. The pressure tap, flush with the column wall, was covered with a fine nylon mesh. The pressure transducers were carefully calibrated from volts to Pa using a pressure calibrator (Fluke Model 718). The calibration showed excellent linearity of voltage–Pa profiles. Five hundred grams of the activated carbon was loaded in the bed before starting the experiment. The static height of the bed was 225 ± 3 mm. First, a number of fluidization and defluidization experiments were carried out using only nitrogen gas to study the fluidization hydrodynamics of the bed. There were initially some losses (<1% of the total solid mass) of the activated carbon due to elutriation, which were replenished to keep bed height constant throughout the experiment. These results, which are related to bed hydrodynamics, are reported elsewhere [47]. The adsorption experiments were carried out at a fixed velocity, while two different inlet CO2 concentrations were considered as the composition of post combustion flue gases varies depending upon the fuel feedstock used for the power generation. As detailed later, the four different cases of particle mixing were carefully investigated. The opening, located at a distance of 500 mm above the distributor, was used as a port for monitoring the CO2 concentration.

3. Results and Discussion In this study, we considered four cases of particle mixing by varying the fraction of external Group B particles in the fluidized bed containing 500 g of activated carbon. In the first case, adsorption studies were carried out in the absence of the external particles. In the second case, 25 g Group B particles were added, and adsorption experiments were carried out. In the third case, the amount of external particles was increased to 50 g by further adding 25 g particles. In the fourth case, the total amount of external particles was 150 g; thus, the amount of external particles defined on the basis of the external particle free weight basis (X1) were 0, 5, 10, and 30 wt %. All four cases will be individually discussed in this section before comparing the efficacy of the proposed strategy of particle mixing on the removal of CO2 using the fluidized bed technology. We first present adsorbent characterization data, followed by the selection of the external particles for our proposed particle-mixing strategy. Next, we describe our experimental strategy before presenting the results of our investigation. Appl. Sci. 2018, 8, 1467 5 of 12

3.1. Adsorbent Characterization We chose commercial reagent-grade activated carbon powder (Avonchem, Macclesﬁeld, UK) as the adsorbent. The choice of activated carbons stemmed from extensive work reported in the literature, which have conﬁrmed that despite their relatively low CO2 adsorption capacity, activated carbons exhibit better stability, a very low sensitivity to moisture, and a large resistance to poisoning [48,49] and are therefore better suited for industrial applications. The adsorbent was characterized using surface area and porosityAppl. Sci. 2018 instrument, 8, x FOR PEER (MicromeriticsREVIEW Tristar II, Norcross, GA, USA). Relevant characterization5 of 12 parameters areWe reported chose commercial in Tables reagent-grade1 and2. activated carbon powder (Avonchem, Macclesfield, UK) as the adsorbent. The choice of activated carbons stemmed from extensive work reported in the 2 literature, whichTable have 1. confirmedSurface area that (mdespite/g) their characterization relatively lowof CO activated2 adsorption carbon. capacity, activated carbons exhibit better stability, a very low sensitivity to moisture, and a large resistance to poisoning [48,49] and areSingle therefore point surfacebetter suited area at for P/Po industrial = 0.303993275: applications. The adsorbent was 953.84 characterized using surface area and porosityBET Surfaceinstrument Area: (Micromeritics Tristar II, Norcross, GA, 941.51USA). Relevant characterization parametersLangmuir are reported Surface in Area:Tables 1 and 2. 1437.38 t-Plot Micro-pore Area: 431.81 Tablet-Plot 1. ExternalSurface area Surface (m2/g) Area: characterization of activated carbon. 509.70 BJH Adsorption cumulative surface area of pores 1.7–300.0 nm diameter 313.02 Single point surface area at P/Po = 0.303993275: 953.84 BJH Desorption cumulative surface area of pores 1.7–300.0 nm diameter 289.05 BET Surface Area: 941.51 Langmuir Surface Area: 1437.38 3 Table 2. Pore volumet-Plot (cm Micro-pore/g) characterization Area: of activated carbon. 431.81 t-Plot External Surface Area: 509.70 BJH Adsorptiont-Plot cumulative micropore surface volume area of pores 1.7–300.0 nm diameter 313.02 0.22973 BJH AdsorptionBJH Desorption cumulative cumulative volume ofsurface pores area 1.7–300.0 of pores nm 1.7–300.0 diameter nm diameter 289.05 0.29053 BJH Desorption cumulative volume of pores 1.7–300.0 nm diameter 0.28122 Table 2. Pore volume (cm3/g) characterization of activated carbon.

t-Plot micropore volume 0.22973 An important aspectBJH Adsorption of the adsorbent cumulative characteristicsvolume of pores 1.7–300.0 is its size nm distribution.diameter 0.29053 Therefore, three runs were carried out usingBJH MalvernDesorption particle cumulative size volume analyzer of pores to 1.7–300.0 determine nm diameter its size 0.28122 distribution. The average volume mean diameter was found to be 26.4 ± 1.1 µm [47]. The SEM micrograph of the sample is shown in FigureAn 2important. Using experimentalaspect of the adsorbent pressure characteristics drop versus is its superficialsize distribution. velocity Therefore, data, three the effective runs were carried out using Malvern particle size analyzer to determine its size distribution. The hydrodynamicaverage diameter volume mean of thediameter activated was found carbon to be 26.4 was ± also1.1 µm evaluated [47]. The SEM with micrograph the help of ofthe the Ergun equation [47sample]. Its is value shown was in Figure 27.5 2.µ Usingm, which experimental is close pressure to the sizedrop obtainedversus superficial from thevelocity particle data, the size analysis. Such a closeeffective agreement hydrodynamic between diameter the two of the is aactivated clear indication carbon was thatalso evaluated there was with almost the help no of agglomeration the of the adsorbentErgun equation particles [47]. during Its value the was gas–solid 27.5 µm, which contact is close in to the the bed. size obtained As part from of a the detailed particle size investigation analysis. Such a close agreement between the two is a clear indication that there was almost no of the fluidizationagglomeration hydrodynamics, of the adsorbent we particles also evaluatedduring the thegas–solid fluidization contact in index the bed. of theAs part monocomponent of a bed of thedetailed activated investigation carbon. of the Its fluidization value varied hydrodynamics, in the range we also of evaluated 0.76–0.80, the indicatingfluidization index the presenceof of nonhomogeneitiesthe monocomponent during fluidizationbed of the activated [47]. Note carbon. that Its the value bed varied nonhomogeneities in the range of 0.76–0.80, always affect the effectivenessindicating of the the adsorbent presence particlesof nonhomogeneities present in du thering bed. fluidization In an ideal [47]. case,Note thethat fluidizationthe bed index nonhomogeneities always affect the effectiveness of the adsorbent particles present in the bed. In an should beideal unity. case, the fluidization index should be unity.

Figure 2. SEM images of adsorbent at a magnification of 500×. Figure 2. SEM images of adsorbent at a magniﬁcation of 500×. Appl. Sci. 2018, 8, x FOR PEER REVIEW 6 of 12

3.2. Particle Mixing Scheme Appl. Sci.The2018 selection, 8, 1467 of external particles is crucial in the present particle-mixing strategy because6 of the 12 particle characteristics critically affect the fluidization hydrodynamics [22]. The group B particles used for mixing was inert sand of density 2664 kg/m3. It was obtained by sieving the field sand in the 3.2. Particle Mixing Scheme lab and collecting the sample retained between 300 µm and 212 µm sieves, thus obtaining sample withThe a mean selection size ofof external256 µm. particles As shown is crucialin the inFigure the present 3, addition particle-mixing of group B strategyparticles because shifted thethe particleproperties characteristics of the resulting critically mixture affect of the adsorbent fluidization and hydrodynamics external particles [22 ].to The group group A, Bwhich particles generally used foryields mixing particulate was inert fluidiza sandtion, of density thereby 2664 resulting kg/m3 .in It better was obtained mixing byand sieving higher the mass field transfer sand in rates. the labFurthermore, and collecting inert the nonporous sample retained external betweenparticles 300like µsandm and does 212 notµ mpose sieves, any contamination thus obtaining risk. sample with aFor mean all four size cases of 256 ofµ m.particle As shown mixing, in the the fluidization Figure3, addition behavior of groupwas carefully B particles investigated shifted the by propertiesmonitoring of the the pressure resulting drop mixture versus of adsorbentvelocity characteristics and external of particles the fluidized to group bed A, and which evaluating generally the yieldsfluidization particulate index fluidization,[47]. It was obvious thereby that resulting particlein mixing better improved mixing and the higherfluidization mass hydrodynamics transfer rates. Furthermore,such that fluidization inert nonporous index as external high as particles0.95 was likeachieved sand doeswith notthe poseaddition any of contamination group B particles. risk.

FigureFigure 3.3. Geldart’sGeldart’s classiﬁcation-basedclassification-based particle-mixingparticle-mixing strategystrategy usedused forfor thethe proposedproposed assistedassisted ﬂuidizationfluidization technique. technique.

3.3. Experimental Strategy and Data Acquisition For all four cases of particle mixing, the fluidization behavior was carefully investigated by monitoringThe experimental the pressure setup drop was versus first velocity tested characteristicsfor any leakage of by the ensuring fluidized the bed input and and evaluating output thegas fluidizationflows were indexequal. [ 47Next,]. It wasa three-step obvious thatadsorption particle ex mixingperiment improved was carried the fluidization out. The first hydrodynamics step was to suchstart thata controlled fluidization flow index of nitrogen as high to as test 0.95 column was achieved containing with the the adsorbent addition of for group two Bhours particles. (7200 s) to remove any traces of carbon dioxide and oxygen. During this purging step, the concentrations of 3.3.both Experimental gases were Strategy monitored and Dataand recorded Acquisition using the O2/CO2 analyzer. Next, the controlled flow of carbonThe dioxide experimental was started setup for was another first tested two hours for any to allow leakage its byadsorption ensuring by the the input adsorbent and output present gas in flowsthe test were section equal. of Next,the column. a three-step Finally, adsorption the CO2 was experiment stoppedwas to let carried the desorption out. The firststep stepproceed was for to startanother a controlled two hours flow before of nitrogen discontinuing to test columnthe flow containing of nitrogen, the as adsorbent shown in for Figure two hours 4. The (7200 flow s) and to removeduration any of tracesboth ofnitrogen carbon and dioxide CO2 and gas oxygen. flows were During always this purging controlled step, using the concentrations electronic mass of bothflow controllers that were connected to the data acquisition system. By monitoring the gas concentration gases were monitored and recorded using the O2/CO2 analyzer. Next, the controlled flow of carbon dioxidedata, the was duration started of for two another hours two (7200 hours s) was to allow found its to adsorption be sufficiently by the long adsorbent to ensure present completion in the test of each of the three steps of the adsorption studies, i.e., purging, adsorption, and desorption. Thus, a section of the column. Finally, the CO2 was stopped to let the desorption step proceed for another two hourstotal of before six hours discontinuing (21,600 s) thewere flow required of nitrogen, for each as showncomplete in Figureadsorption4. The experiment. flow and duration of both The data sampling frequency was kept at 10 Hz, thus recording 10 data samples every second. nitrogen and CO2 gas flows were always controlled using electronic mass flow controllers that were connectedEach data tosample the data involved acquisition recording system. the Byfollowing: monitoring (1) time, the gas (2) concentration pressure drop, data, (3) N the2 flow duration rate, of(4) twoCO2 hours flow (7200rate, (5) s) wasCO2 foundconcentration, to be sufficiently and (6) longoxygen to ensureconcentration. completion The of O each2/CO of2 analyzer the three provided steps of theboth adsorption CO2 and studies,O2 concentrations. i.e., purging, Note adsorption, that all and outputs desorption. were Thus,in volts a totaland ofwere six hoursconverted (21,600 into s) wereappropriate required units for each with complete the help adsorptionof calibration experiment. data. The responses of O2/CO2 analyzer are presented in Figure 5a,b, while the response of the CO2 and N2 mass flow controllers are shown in Figure 4. Note that the sharp decline in the oxygen concentration occurred (Figure 5b) due to purging of the experimental setup using nitrogen. Appl.Appl. Sci. Sci.2018 2018, 8, ,8 1467, x FOR PEER REVIEW 77 of of 12 12

Appl. Sci. 2018, 8, x FOR PEER REVIEW 7 of 12

FigureFigure 4. 4.Flow Flow rates rates of of nitrogen nitrogen and and carbon carbon dioxide dioxide during during the the experiment experiment comprising comprising consecutive consecutive purging,purging, adsorption, adsorption, and and desorption desorption steps. steps.

The data sampling frequency was kept at 10 Hz, thus recording 10 data samples every second. Each data sample involved recording the following: (1) time, (2) pressure drop, (3) N2 flow rate, (4) CO2 flow rate, (5) CO2 concentration, and (6) oxygen concentration. The O2/CO2 analyzer provided both CO2 and O2 concentrations. Note that all outputs were in volts and were converted into appropriate units with the help of calibration data. The responses of O2/CO2 analyzer are presented in Figure5a,b, while the response of the CO2 and N2 mass flow controllers are shown in Figure4. Note that the sharp Figure 4. Flow rates of nitrogen and carbon dioxide during the experiment comprising consecutive decline in the oxygen concentration occurred (Figure5b) due to purging of the experimental setup purging, adsorption, and desorption steps. using nitrogen.

(a) (b)

Figure 5. Response of O2/CO2 analyzer during the experiment comprising 2 h of purging, 2 h of

adsorption, and 2 h of desorption steps over a total period of 6 h (a) CO2 concentration; (b) O2 concentration.

3.4. Experimental Results

First, we discuss the case of the CO2 adsorption without the particles mixing, i.e., the fluidized bed containing only the activated carbon powder. Two different sets of experiments—Experiment 1 and Experiment 2—were(a )carried out by keeping the inlet nitrogen flow(b) fixed at 0.5 LPM and changing the CO2 flow as shown in Figure 6. The concentration profiles are shown for both FigureFigure 5. 5.Response Response ofof OO2/CO/CO2 analyzeranalyzer during during the the experiment experiment comprising comprising 2 2h hof of purging, purging, 2 2h hof adsorption and desorption 2processes.2 During the adsorption, the CO2 concentration asymptotically adsorption, and 2 h of desorption steps over a total period of 6 h (a) CO2 concentration; (b) O2 reachedof adsorption, a constant and value, 2 h which of desorption was expected steps overas the a bed total of period activated of 6carbon h (a) COgradually2 concentration; got saturated concentration. with(b the)O2 adsorbate.concentration. Similarly, a gradual decrease in the concentration was evident once the process of desorption was started by discontinuing the flow of the inlet CO2 and leaving the nitrogen flow 3.4.3.4. Experimental Experimental Results Results unchanged. The data recording in this particular experiment was stopped after 5000 s of desorption First, we discuss the case of the CO2 adsorption without the particles mixing, i.e., the fluidized unlikeFirst, other we discussexperiments the case where of the CO CO2 concentration2 adsorption without was monitored the particles for 7200 mixing, s (2 i.e., h) theas mentioned fluidized bedbefore.bed containing containing Note that only only 2 theh thepurging activated activated was carbon carbonalways powder. powder. carried Two outTwo differentto different ensure setsremoval sets of of experiments—Experiment experiments—Experiment of any remaining traces 1of 1 and Experiment 2—were carried out by keeping the inlet nitrogen flow fixed at 0.5 LPM and andthe ExperimentCO2 in the fluidized 2—were carriedbed before out bystar keepingting the the process inlet nitrogenof adsorption. flow fixed at 0.5 LPM and changing changing the CO2 flow as shown in Figure 6. The concentration profiles are shown for both the COWe2 flow have as presented shown in the Figure experimental6. The concentration data in Figure profiles 7, where are the shown ordinate for both is the adsorption normalized and CO2 adsorption and desorption processes. During the adsorption, the CO2 concentration asymptotically desorptionconcentration processes. obtained During by dividing the adsorption, the instantaneous the CO2 concentration concentration asymptotically by the inlet reachedCO2 concentration; a constant reached a constant value, which was expected as the bed of activated carbon gradually got saturated value,the abscissa which was is time expected plotted as theon bedthe oflogarithmic activated carbon scale. graduallyClearly, the got CO saturated2 profile with for theExperiment adsorbate. 1 Similarly,initiallywith the showed aadsorbate. gradual higher decrease Similarly, concentration in a the gradual concentration compared decrease to wasin the the evident one concentration for onceExperiment the was process evident2. This of can desorptiononce be the attributed process was of desorption was started by discontinuing the flow of the inlet CO2 and leaving the nitrogen flow startedto faster by saturation discontinuing of the the absorbent flow of the pres inletent COin the2 and fluidized leaving bed the nitrogendue the higher flow unchanged. CO2 concentration The data in recordingtheunchanged. inlet ingas. this TheIn particularFigure data recording 7b, experiment the case in ofthis wasparticle particular stopped premixing afterexperiment 5000 is presented s ofwas desorption stopped when unlikeaftersmall 5000 amount other s experimentsof ofdesorption external unlike other experiments where CO2 concentration was monitored for 7200 s (2 h) as mentioned whereparticle CO was2 concentration added to the was bed monitored (X1 = 0.05). for Its 7200 breakthrough s (2 h) as mentioned curves show before. similar Note trend that 2observed h purging for before. Note that 2 h purging was always carried out to ensure removal of any remaining traces of the CO2 in the fluidized bed before starting the process of adsorption. We have presented the experimental data in Figure 7, where the ordinate is the normalized CO2 concentration obtained by dividing the instantaneous concentration by the inlet CO2 concentration; the abscissa is time plotted on the logarithmic scale. Clearly, the CO2 profile for Experiment 1 initially showed higher concentration compared to the one for Experiment 2. This can be attributed to faster saturation of the absorbent present in the fluidized bed due the higher CO2 concentration in the inlet gas. In Figure 7b, the case of particle premixing is presented when small amount of external particle was added to the bed (X1 = 0.05). Its breakthrough curves show similar trend observed for Appl. Sci. 2018, 8, 1467 8 of 12 Appl. Sci. 2018, 8, x FOR PEER REVIEW 8 of 12

the monocomponent bed of activated carbon. The adsorption behavior of the bed for X1 = 0.10, 0.30 is was always carried out to ensure removal of any remaining traces of the CO2 in the ﬂuidized bed shown in Figure 7c,d, respectively, for both experiments. before starting the process of adsorption.

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the monocomponent bed of activated carbon. The adsorption behavior of the bed for X1 = 0.10, 0.30 is shown in Figure 7c,d, respectively, for both experiments.

(a) (b)

Figure 6. Variation in CO2 concentration with time during the adsorption and desorption processes Figure 6. Variation in CO2 concentration with time during the adsorption and desorption processes for for the monocomponent fluidized bed of activated carbon (a) Experiment 1; (b) Experiment 2. the monocomponent ﬂuidized bed of activated carbon (a) Experiment 1; (b) Experiment 2.

We have presented the experimental data in Figure7, where the ordinate is the normalized CO 2 concentration obtained by dividing the instantaneous concentration by the inlet CO2 concentration; the abscissa is time plotted on the logarithmic scale. Clearly, the CO2 proﬁle for Experiment 1 initially showed higher concentration compared to the one for Experiment 2. This can be attributed to faster saturation of the absorbent present in the ﬂuidized bed due the higher CO2 concentration in the inlet gas. In Figure7b, the case of particle premixing is presented when small amount of external (a) (b) particle was added to the bed (X1 = 0.05). Its breakthrough curves show similar trend observed for the Figure 6. Variation in CO2 concentration with time during the adsorption and desorption processes monocomponent bed of activated carbon. The adsorption behavior of the bed for X1 = 0.10, 0.30 is for the monocomponent fluidized bed of activated carbon (a) Experiment 1; (b) Experiment 2. shown in Figure7c,d, respectively, for both experiments. (a) (b)

(a) (b) (c) (d)

Figure 7. Response of the fluidized bed with particle mixing in different proportions of external

group B particles (a) X1 = 0.00; (b) X1 = 0.05; (c) X1 = 0.10; (d) X1 = 0.30.

Our main objective was to examine the effect of particle mixing on the adsorption characteristics of the fluidized bed containing activated carbon. This is shown in Figure 8 for both high and low concentrations of the CO2 in the inlet gas. Clearly, particle mixing made a significant difference on the CO2 removal. As seen in Figure 8a, the addition of 25 g Group B particle surprisingly led to a concentration profile that exhibited higher CO2 concentration than the monocomponent-activated carbon bed. This was a clear indication of lower CO2 removal efficacy of the bed in this case. However, when the amount of external particles was increased to 50 g (X 1 = (c) (d)

Figure 7. Response of the fluidized bed with particle mixing in different proportions of external Figure 7. Response of the ﬂuidized bed with particle mixing in different proportions of external group group B particles (a) X1 = 0.00; (b) X1 = 0.05; (c) X1 = 0.10; (d) X1 = 0.30. B particles (a)X1 = 0.00; (b)X1 = 0.05; (c)X1 = 0.10; (d)X1 = 0.30. Our main objective was to examine the effect of particle mixing on the adsorption characteristics of the fluidized bed containing activated carbon. This is shown in Figure 8 for both high and low concentrations of the CO2 in the inlet gas. Clearly, particle mixing made a significant difference on the CO2 removal. As seen in Figure 8a, the addition of 25 g Group B particle surprisingly led to a concentration profile that exhibited higher CO2 concentration than the monocomponent-activated carbon bed. This was a clear indication of lower CO2 removal efficacy of the bed in this case. However, when the amount of external particles was increased to 50 g (X1 = Appl. Sci. 2018, 8, 1467 9 of 12

Appl.Our Sci. 2018 main, 8, x objective FOR PEER was REVIEW to examine the effect of particle mixing on the adsorption characteristics9 of 12 of the fluidized bed containing activated carbon. This is shown in Figure8 for both high and low 0.10), the breakthrough curve showed a significant shift right, towards much lower CO2 concentrations of the CO in the inlet gas. Clearly, particle mixing made a significant difference on concentration compared2 to the previous two cases. Moreover, the time of breakthrough was also the CO removal. As seen in Figure8a, the addition of 25 g Group B particle surprisingly led to much 2higher. Increasing the fraction of particles to 150 g did not help as the breakthrough curve a concentration profile that exhibited higher CO2 concentration than the monocomponent-activated again shifted left, indicating lower CO2 removal efficacy. carbon bed. This was a clear indication of lower CO2 removal efficacy of the bed in this case. However, Next, we consider the case of lower CO2 concentration in the inlet gas (Figure 8b). when the amount of external particles was increased to 50 g (X1 = 0.10), the breakthrough curve showed Notwithstanding the change in the inlet concentration of CO2, the breakthrough curves for different a significant shift right, towards much lower CO concentration compared to the previous two cases. fractions of the external particles nonetheless showed2 the same trend. Adding 25 g sand decreased Moreover, the time of breakthrough was also much higher. Increasing the fraction of particles to 150 g the CO2 removal efficacy of the bed, while the best results were obtained with 50 g sand. did not help as the breakthrough curve again shifted left, indicating lower CO2 removal efficacy.

(a) (b)

FigureFigure 8. 8.Effect Effect of particleof particle mixing mixing on the on response the response of the fluidized of the bedfluidized (a) Experiment bed (a) 1;Experiment (b) Experiment 1; ( 2.b) Experiment 2.

4. ConclusionsNext, we consider the case of lower CO2 concentration in the inlet gas (Figure8b). Notwithstanding the change in the inlet concentration of CO2, the breakthrough curves for different fractionsResults of the presented external particles here nonethelessclearly highlight showed the the sameimportance trend. Addingof the 25 role g sand of decreased fluidization the hydrodynamics. The addition of external particles improved the fluidization hydrodynamics, CO2 removal efficacy of the bed, while the best results were obtained with 50 g sand. resulting in better quality of fluidization. This led to higher CO2 removal efficiency of the fluidized 4.bed. Conclusions In this context, it is worthwhile to point out the results of our earlier investigation of the hydrodynamics of the same system comprising activated carbon and group B particles. A statistical Results presented here clearly highlight the importance of the role of fluidization hydrodynamics. analysis of the fluidization indices for different levels of particle mixing showed a similar trend. The The addition of external particles improved the fluidization hydrodynamics, resulting in better quality hydrodynamics of mixed bed with 50 g group B particles was better than ones with 25 g and 150 g of fluidization. This led to higher CO2 removal efficiency of the fluidized bed. In this context, it is external particles. On the other hand, mixed fluidized bed of 25 g sand showed poor quality worthwhile to point out the results of our earlier investigation of the hydrodynamics of the same fluidization than its monocomponent counterpart [47]. We found the same effect of particle mixing system comprising activated carbon and group B particles. A statistical analysis of the fluidization in the present adsorption studies. indices for different levels of particle mixing showed a similar trend. The hydrodynamics of mixed bed An important consideration in any assisted fluidization technique is the cost involved with its with 50 g group B particles was better than ones with 25 g and 150 g external particles. On the other implementation and operation. Note that the main fluidization equipment did not require any hand, mixed fluidized bed of 25 g sand showed poor quality fluidization than its monocomponent modification or installation of any additional equipment for the implementation of the proposed counterpart [47]. We found the same effect of particle mixing in the present adsorption studies. technique of particle mixing. In addition, unlike other assisted fluidization techniques, there was no An important consideration in any assisted fluidization technique is the cost involved with need for extra energy. The cost of the procurement of group B particles was almost negligible when its implementation and operation. Note that the main fluidization equipment did not require using inert sand. Moreover, there was no contamination risk due to inert external particles. any modification or installation of any additional equipment for the implementation of the Combined together, these factors clearly highlight the potential of particle mixing as an effective strategy proposed technique of particle mixing. In addition, unlike other assisted fluidization techniques, to improve the carbon dioxide capture efficacy of a fluidized bed containing fine adsorbent powder. there was no need for extra energy. The cost of the procurement of group B particles was almost negligible when using inert sand. Moreover, there was no contamination risk due to inert external Author Contributions: Experiments, E.H.A.-G.; Conceptualization, A.A.; Methodology, M.A.; Validation, particles.M.A.; Resources, Combined M.A.; together, Original these Draft factors Preparation, clearly M.A.; highlight Writing—Review the potential & of Editing, particle A.A. mixing and asM.A.; an effectiveSupervision, strategy M.A.; toProject improve Administration, the carbon M.A.; dioxide Funding capture Acquisition, efficacy A.A. of a and fluidized M.A. bed containing fine adsorbent powder. Funding: This research was funded by the King Abdulaziz City of Science and Technology (KACST), Saudi Arabia, through the research grant number AT 35-125.

Conflicts of Interest: The authors declare no conflict of interest. Appl. Sci. 2018, 8, 1467 10 of 12

Author Contributions: Experiments, E.H.A.-G.; Conceptualization, A.A.; Methodology, M.A.; Validation, M.A.; Resources, M.A.; Original Draft Preparation, M.A.; Writing—Review & Editing, A.A. and M.A.; Supervision, M.A.; Project Administration, M.A.; Funding Acquisition, A.A. and M.A. Funding: This research was funded by the King Abdulaziz City of Science and Technology (KACST), Saudi Arabia, through the research grant number AT 35-125. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

1. Trends in Global CO2 and Total Greenhouse Gas Emissions: 2017 Report. Available online: http://www.pbl.nl/sites/default/files/cms/publicaties/pbl-2017-trends-in-global-co2-and-total- greenhouse-gas-emissons-2017-report_2674.pdf (accessed on 26 July 2018).

2. CO2 Emissions by Countries. 2017. Available online: https://knoema.com/atlas/ranks/CO2-emissions? baseRegion=SA (accessed on 26 July 2018). 3. Lackner, K.; Gibbins, J.; Roddy, D.; Pocklington, D.; Logue, P.; Riley, N.; Chapman, S.J.; Nijnik, M.; Kitidis, V. Issues in Environmental Science and Technology; Hester, R.E., Harrison, R.M., Eds.; RCS Publishing: London, UK, 2009; pp. 1–308. 4. Creamer, A.E.; Gao, B. Carbon Dioxide Capture: An Effective Way to Combat Global Warming; Springer: Berlin, Germany, 2015; pp. 25–49. ISBN 978-3-319-17010-7.

5. CO2 Capture Project, CCP Annual Report 2017. June 2018. Available online: http://www.co2captureproject. org (accessed on 26 July 2018). 6. MacDowell, N.; Florin, N.; Buchard, A.; Hallett, J.; Galindo, A.; Jackson, G.; Adjiman, C.; Williams, C.K.;

Shah, N.; Fennel, P. An overview of CO2 capture technologies. Energy Environ. Sci. 2010, 3, 1645–1669. [CrossRef] 7. Kargari, A.; Ravanchi, M.T. Carbon Dioxide: Capturing and Utilization, Greenhouse Gases-Capturing, Utilization and Reduction. ISBN 978-953-51-0192-5. Liu, G. InTech. 2012. Available online: http://www.intechopen. com/books/greenhouse-gases-capturing-utilization-and-reduction/carbon-dioxidecapturing-and-utilization/ (accessed on 26 July 2018). 8. Boundary Dam Carbon Capture Project. Available online: https://www.saskpower.com/our-power-future/ infrastructure-projects/carbon-capture-and-storage/boundary-dam-carbon-capture-project (accessed on 13 August 2018).

9. Wang, Y.; Zhao, L.; Otto, A.; Robinius, M.; Stolten, D. A Review of Post-combustion CO2 Capture Technologies from Coal-ﬁred Power Plants. Energy Procedia 2017, 4, 650–665. [CrossRef] 10. Mushardt, H.; Müller, M.; Shishatskiy, S.; Wind, J.; Brinkmann, T. Detailed investigation of separation performance of a mmm for removal of higher hydrocarbons under varying operating conditions. Membranes 2016, 6, 16. [CrossRef][PubMed] 11. Weigelt, F.; Georgopanos, P.; Shishatskiy, S.; Filiz, V.; Brinkmann, T.; Abetz, V. Development and characterization of defect-free matrimid® mixed-matrix membranes containing activated carbon particles for gas separation. Polymers 2018, 10, 51. [CrossRef]

12. Kassim, M.A.; Meng, T.K. Carbon dioxide (CO2) bioﬁxation by microalgae and its potential for bioreﬁnery and biofuel production. Sci. Total Environ. 2017, 584–585, 1121–1129. [CrossRef][PubMed] 13. Garcia, S.; Gil, M.V.; Martin, C.F.; Pis, J.J.; Rubiera, F.; Pevida, C. Breakthrough adsorption study of

a commercial activated carbon for pre-combustion CO2 capture. Chem. Eng. J. 2011, 171, 549–556. [CrossRef] 14. Valverde, J.M.; Pontiga, F.; Soria-Hoyo, C.; Quintanilla, M.A.S.; Moreno, H.; Duran, F.J.; Espin, M.J.

Improving the gas solids contact efficiency in a fluidized bed of CO2 adsorbent fine particles. Chem. Phys. 2011, 13, 14906–14909. [CrossRef][PubMed] 15. Zukowski, W.; Englot, S.; Baron, J.; Kandefer, S.; Olek, M. Reduction of carbon dioxide emission using adsorption-desorption cycles in a fluidized bed reactor. Environ. Prot. Eng. 2010, 36, 47–56. 16. Gil, M.V.; Martinez, M.; Garcia, S.; Rubiera, F.; Pis, J.J.; Pevida, C. Response surface methodology as an

efﬁcient tool for optimizing carbon adsorbents for CO2 capture, Fuel. Process. Technol. 2013, 106, 55–61. [CrossRef] 17. D’Alessandro, D.M.; Smit, B.; Long, J.R. Carbon dioxide capture: Prospects for new materials. Angew. Chem. Int. Ed. 2010, 49, 6058–6082. [CrossRef][PubMed] Appl. Sci. 2018, 8, 1467 11 of 12

18. Plaza, M.G.; García, S.; Rubiera, F.; Pis, J.J.; Pevida, C. Post-combustion CO2 capture with a commercial activated carbon: Comparison of different regeneration strategies. Chem. Eng. J. 2010, 163, 41–47. [CrossRef]

19. Ruiz, E.; Sánchez, J.M.; Maroño, M.; Otero, J. CO2 capture from PCC power plants using solid sorbents: Bench scale study on synthetic gas. Fuel 2013, 114, 143–152. [CrossRef]

20. Abanades, J.C.; Anthony, E.J.; Lu, D.Y.; Salvador, C.; Alvarez, D. Capture of CO2 from combustion gases in a ﬂuidized bed of CaO. AIChE J. 2004, 50, 1614–1622. [CrossRef]

21. Valverde, M.; Duran, F.J.; Pontiga, F.; Moreno, H. CO2 capture enhancement in a fluidized bed of a modified Geldart C powder. Powder Technol. 2012, 224, 247. [CrossRef] 22. Geldart, D. Types of Fluidization. Powder Technol. 1973, 7, 285–292. [CrossRef] 23. Ajbar, A.; Alhumaizi, K.; Asif, M. Improvement of the fluidizability of cohesive powders through mixing with small proportions of group a particles. Can. J. Chem. Eng. 2005, 83, 930–943. [CrossRef] 24. Zhu, C.; Liu, G.; Yu, Q.; Pfeffer, R.; Dave, R.; Nam, C. Sound assisted fluidization of nanoparticles agglomerates. Powder Technol. 2004, 141, 119–123. [CrossRef] 25. Ammendola, P.; Chirone, R.; Raganati, F. Fluidization of binary mixtures of nanoparticles under the effect of acoustic fields. Adv. Powder Technol. 2011, 22, 174–183. [CrossRef] 26. Ireland, E.; Pitt, K.; Smith, R. A review of pulsed flow fluidisation; the effects of intermittent gas flow on fluidised gas-solid bed behavior. Powder Technol. 2016, 292, 108–121. [CrossRef] 27. Ali, S.S.; Asif, M. Fluidization of nano-powders: Effect of flow pulsation. Powder Technol. 2012, 225, 86–92. [CrossRef] 28. Kaliyaperumal, S.; Barghi, S.; Zhu, J.; Briens, L.; Rohani, S. Effects of acoustic vibration on nano and sub-micron powders fluidization. Powder Technol. 2011, 210, 143–149. [CrossRef] 29. Valverde, J.M. Acoustic streaming in gas-fluidized beds of small particles. Soft Matter 2013, 9, 8792–8814. [CrossRef] 30. Ajbar, A.; Bakhbakhi, Y.; Ali, S.; Asif, M. Fluidization of nano-powders: Effect of sound vibration and pre-mixing with group A particles. Powder Technol. 2011, 206, 327–337. [CrossRef] 31. Ali, S.S.; Asif, M. Effect of particle mixing on the hydrodynamics of fluidized bed of nanoparticles. Powder Technol. 2017, 310, 234–240. [CrossRef] 32. Quintanilla, M.A.S.; Valverde, J.M.; Castellanos, A.; Lepek, D.; Pfeffer, R.; Dave, R.N. Nanofluidization as affected by vibration and electrostatic fields. Chem. Eng. Sci. 2008, 63, 5559–5569. [CrossRef] 33. Hristov, J. Magnetically assisted gas-solid fluidization in a tapered vessel: Part I. Magnetization-LAST mode. Particuology 2009, 7, 26–34. [CrossRef] 34. Ding, P.; Orwa, M.G.; Pacek, A.W. De-agglomeration of hydrophobic and hydrophilic silica nano-powders in a high shear mixer. Powder Technol. 2009, 195, 221–226. [CrossRef] 35. Bizhaem, K.H.; Tabrizi, H.B. Experimental study on hydrodynamic characteristics of gas-solid pulsed fluidized bed. Powder Technol. 2013, 237, 14–23. [CrossRef] 36. Ali, S.S.; Asif, M.; Ajbar, A. Bed collapse behavior of pulsed fluidized beds of nano-powder. Adv. Powder Technol. 2014, 25, 331–337. [CrossRef] 37. Akhavan, A.; Rahman, F.; Wang, S.; Rhodes, M. Enhanced fluidization of nanoparticles with gas phase pulsation assistance. Powder Technol. 2015, 284, 521–529. [CrossRef] 38. Ali, S.S.; Al-Ghurabi, E.H.; Ajbar, A.; Mohammed, Y.A.; Boumaza, M.; Asif, M. Effect of Frequency on Pulsed Fluidized Beds of Ultrafine Powders. J. Nanomater. 2016, 4592501, 1–12. [CrossRef] 39. Ammendola, P.; Chirone, R. Aeration and mixing behaviors of nano-sized powders under sound vibration. Powder Technol. 2010, 201, 49–56. [CrossRef] 40. Si, C.; Wu, J.; Wang, Y.; Zhang, Y.; Liu, G. Effect of acoustic field on minimum fluidization velocity and drying characteristics of lignite in a fluidized bed. Fuel Process. Technol. 2015, 135, 112–118. [CrossRef]

41. Raganati, F.; Ammendola, P.; Chirone, R. CO2 capture by adsorption on ﬁne activated carbon in a sound assisted ﬂuidized bed. Chem. Eng. Trans. 2015, 43, 1033–1038.

42. Raganati, F.; Ammendola, P.; Chirone, R. CO2 Adsorption on ﬁne powders in a sound-assisted ﬂuidized bed. In Proceedings of the XXXV Meeting of the Italian Section of the Combustion Institute, Milan, Italy, 10–12 October 2012; ISBN 978-88-88104-14-0.

43. Raganati, F.; Ammendola, P.; Chirone, R. CO2 adsorption on fine activated carbon in a sound assisted fluidized bed: Effect of sound intensity and frequency, CO2 partial pressure and fluidization velocity. Appl. Energy 2014, 113, 1269–1282. [CrossRef] Appl. Sci. 2018, 8, 1467 12 of 12

44. Raganati, F.; Gargiulo, V.; Ammendola, P.; Alfe, M.; Chirone, R. CO2 capture performance of HKUST-1 in a sound assisted ﬂuidized bed. Chem. Eng. J. 2014, 239, 75–86. [CrossRef] 45. Valverde, J.M.; Raganati, F.; Quintanilla, M.A.S.; Ebri, J.M.P.; Ammendola, P.; Chirone, R. Enhancement of

CO2 capture at Ca-looping conditions by high-intensity acoustic fields. Appl. Energy 2013, 111, 538–549. [CrossRef] 46. Ali, S.S.; Al-Ghurabi, E.H.; Ibrahim, A.A.; Asif, M. Effect of adding Geldart group A particles on the collapse of fluidized bed of hydrophilic nanoparticles. Powder Technol. 2018, 330, 50–57. [CrossRef] 47. Al-Ghurabi, E.H.; Ajbar, A.; Asif, M. Improving fluidization hydrodynamics of Group C particles by mixing with Group B particles. Appl. Sci. 2018. under review.

48. Sjostrom, S.; Krutka, H. Evaluation of solid sorbents as a retroﬁt technology for CO2 capture. Fuel 2010, 89, 1298–1306. [CrossRef]

49. Sjostrom, S.; Krutka, H.; Starns, T.; Campbell, T. Pilot test results of post combustion CO2 capture using solid sorbents. Energy Procedia 2011, 4, 1584–1592. [CrossRef]

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