Improving Fluidization Hydrodynamics of Group C Particles by Mixing with Group B Particles

applied sciences

Article Improving Fluidization Hydrodynamics of Group C Particles by Mixing with Group B Particles

Ebrahim H. Al-Ghurabi, Abdelhamid Ajbar and Mohammad Asif * ID Department of Chemical Engineering, King Saud University, P.O. 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: 8 August 2018; Accepted: 22 August 2018; Published: 27 August 2018

Featured Application: As a first step towards developing a simple yet cost-effective technique based on the fluidized bed technology for the treatment of carbon dioxide present in post-combustion flue gases from power plants, we have rigorously investigated the improvement in the fluidized bed hydrodynamics of fine adsorbent particles by mixing with group B particles. Any improvement in the fluidization of fine particles will ultimately translate into higher capture efficiency of the carbon dioxide by the fluidized bed containing fine adsorbent powders.

Abstract: We have developed a new particle-mixing strategy for improving the fluidization hydrodynamics of Geldart group C powders by mixing with small proportions of group B particles. Two different group C particles with widely different physical properties, i.e., 1 µm calcium hydroxide powder and 27 µm porous activated carbon, were selected for investigation in the present work. A carefully sieved sample of inert sand was used as external group B particles for mixing. Fluidization experiments were carried out, and the quality of the fluidization was assessed using the fluidization index. For the monocomponent fluidization of fine calcium hydroxide powder, pressure drop was sometimes as much as 250% higher than the effective weight of the bed. The proposed strategy of particle mixing substantially improved its fluidization hydrodynamics. On the other hand, the development of channels and cracks during the monocomponent fluidization of the activated carbon led to gas bypassing, resulting in low pressure drop and poor contact of phases. Particle mixing was found to improve fluidization behavior, and the chi-squared test showed that the best results were obtained with 13 wt% particle mixing.

Keywords: group C particles; assisted-fluidization; particle mixing; fluidization index; chi-squared test

1. Introduction For efficient and effective utilization of the solid phase in process industries, the use of small particles is often recommended because of their high surface areas. A large surface area invariably ensures high heat and mass transfer rates and better utilization of catalyst particles where the internal mass transport resistance controls the overall mass transport process. However, gas-solid contacting involving small particles in process industries poses a major challenge irrespective of whether it is implemented in a fixed or a fluidized mode of operation. When a fluid flows through a fixed bed of solid particles in the laminar flow region, the pressure drop inversely varies with the square of the particle diameter. Thus, the high pressure drop associated with small particles would eventually translate into a high energy cost, thus compromising the cost effectiveness of the process. An otherwise preferred mode of gas–solid contacting for small particles is fluidization. However, fine powders consisting of small particles (less than 30 µm in size), often cohesive in nature, are difficult to fluidize as they develop channels and cracks through which the fluidizing gas tend to bypass the solid materials

Appl. Sci. 2018, 8, 1469; doi:10.3390/app8091469 www.mdpi.com/journal/applsci Appl. Sci. 2018, 8, 1469 2 of 13 present in the bed, thus resulting in poor contact and inefficient mixing. Geldart classified such powders as group C [1]. Similarly, powders typically in the range of hundreds of micrometers, classified as group B, exhibit bubbling behavior, in which a substantial fraction of the fluidizing gas tend to escape as bubbles without coming into contact with the resident solid phase of the fluidized bed. By contrast, powders in the intermediate size range, classified as group A, show smooth and particulate fluidization behavior, resulting in intimate contact and efficient mixing of the gas and solid phases. To improve the fluidization behavior of solids with poor fluidization hydrodynamics, various techniques and strategies have been proposed in the literature [2–10]. The main objective of these techniques, commonly known as assisted fluidization techniques, is to provide additional energy to the fluidized bed to overcome interparticle forces that are responsible for the cohesive nature and other non-homogeneities during fluidization. Of particular interest in this connection is the fluidization of fine cohesive powders due to their intrinsically high surface area to volume ratios. An extensive review on this topic has recently been presented by Raganati et al. [11]. They pointed out the dominance of interparticle forces as being primarily responsible for the non-homogeneities observed during fluidization. Several studies highlighted the effect of the interparticle force on the fluidization hydrodynamics [12–16] and proposed techniques for their measurement [17]. Among the assisted fluidization technique is the pulsation of the inlet flow to the fluidized bed, which has been shown to improve the fluidization hydrodynamics of ultrafine powders [18–21]. Another extensively employed technique is the introduction of acoustic vibrations during fluidization [22–24]. This technique helps to lower the minimum fluidization velocity and eliminate non-homogeneities in a bed of ultrafine powder [22]. Si et al. [24] used this technique for drying lignite by varying the sound pressure level and frequency and reported the fluidization quality. The rate of drying and drying completion time improved at high sound pressure levels. Despite being effective in improving the quality of the fluidization, this technique is energy intensive and requires modification in the fluidization setup for introducing the acoustic vibration in the bed. Another simple and cost-effective assisted fluidization technique that does not require input of energy is the addition of external particles. A careful selection of external particles even in small proportions can substantially improve the fluidization hydrodynamics [7,25,26]. The addition of external particles of Group A to the bed of nanoparticles suppresses hysteresis and substantially reduces the size of agglomerates [7,26]. The objective of the present work is to improve the fluidization hydrodynamics of Group C particles by using the assisted fluidization technique of particle mixing. However, instead of group A particles, we used large group B particles in the present study. This strategy can potentially offer several advantages as compared with group A particles. First, it can help in shifting the properties of the resulting mixture in the intermediate range of group A classification. Second, the milling action of the large and heavy group B particles during fluidization can suppress agglomeration that is often noticed with group C particles, thus improving the fluidization hydrodynamics of the mixture. Third, it can lower the amount of external particles required for achieving comparable efficacy of group A particle mixing. We carried out a detailed study by choosing two different kinds of group C particles such that one of the samples comprised 1 µm calcium hydroxide, whereas the other was a high-surface-area porous 27 µm activated carbon, which lies almost on the boundary of group C. For the addition of the group B particle, we used a carefully sieved sample of sand with 256 µm in size. We carried out fluidization studies in the presence and absence of external particles and evaluated the fluidization quality by computing the fluidization index. The data were then processed using the chi-squared test to characterize the fluidization hydrodynamics.

2. Experimental A schematic of the experiment setup is shown in Figure1. A vertical Perspex column of 70 mm internal diameter and 1000 mm length was used as a test section for ﬂuidization experiments. Appl. Sci. 2018, 8, 1469 3 of 13

A perforated plate distributor was located at the bottom of the test section to support the solids present in the bed and to ensure uniform distribution of the flow over the entire cross-section of the column. It comprised of a 10 mm-thick Perspex plate containing 1.5 mm holes on a square pitch such that 4% of the open area of the distributor area is open. It was covered by a nylon mesh to avoid the raining down of solid particles through the distributor’s holes. As shown in Figure1, a 250 mm-long calming section precededAppl. Sci. 2018 the, 8, testx FOR sectionPEER REVIEW of the column for eliminating entry effects. The top of3 of the13 column was connected with a flexible housing to release the outlet gases coming out of the test section to the environmentin the outside bed and the to ensure lab and uniform to prevent distribution the buildupof the flow of over nitrogen the entire inside cross-section the lab. of Openingsthe column. along the It comprised of a 10 mm-thick Perspex plate containing 1.5 mm holes on a square pitch such that 4% height of theof the column open area were of the provided distributor for area the is open. measurement It was covered of theby a pressurenylon mesh drop. to avoid The the lowestraining port was located at adown distance of solid of particles 50 mm through from the distributor’s distributor ho toles. avoid As shown distributor in Figure 1, non-uniformities a 250 mm-long calming affecting the measurements.section Otherpreceded openings the test section were of placed the column along forthe eliminating height entry of the effects. column The top at aof multiple the column of 250 mm from the distributor.was connected with a flexible housing to release the outlet gases coming out of the test section to the environment outside the lab and to prevent the buildup of nitrogen inside the lab. Openings along The inletthe height flow of assembly the column comprised were provided a for compressed the measurement nitrogen of the cylinderpressure drop. that The was lowest connected port to an electronicwas mass located flow at controllera distance of 50 for mm monitoring from the distribu andtor controlling to avoid distributor the flow non-uniformities of the inlet affecting gas to the test section of thethe column.measurements. We usedOther aopenings sensitive, were fast-response, placed along the differential, height of the bidirectionalcolumn at a multiple pressure of 250 transducer with a responsemm from time the distributor. of 1 ms to measure the global differential pressure drop across the fluidized The inlet flow assembly comprised a compressed nitrogen cylinder that was connected to an ± bed of adsorbent.electronic Itsmass range flow controller was 5 for inches monitoring of water and controlling (Omega modelthe flow 163PC01D36-124, of the inlet gas to the Norwalk, test CT, USA). Thesection pressure of the transducers column. We were used carefully a sensitive, calibrated fast-response, from differential, volts to Pa bidirectional using a pressure pressure calibrator (Fluke modeltransducer 718, Calgary, with a response Canada). time Theof 1 ms voltage to measur signalse the global from differential the pressure pressure transducer drop across were the captured by data acquisitionfluidized bed system of adsorbent. (DAQ) Its running range was LabVIEW ±5 inches softwareof water (Omega and then model transferred 163PC01D36-124, to acomputer Norwalk, CT, USA). The pressure transducers were carefully calibrated from volts to Pa using a for furtherpressure processing. calibrator As (Fluke shown model in Figure 718, Calgary,1, the Canada). lower pressure The voltage port signals was from located the pressure at a distance of 50 mm fromtransducer the distributor, were captured whereas by data theacquisition upper system port was(DAQ) at running a height LabVIEW of 500 software mm, thereby and then providing crucial informationtransferred aboutto a computer the overall for further bed processing. dynamics. AsThe shown lower in Figure pressure 1, the lower port pressure was deliberately port was located away fromlocated the bottom at a distance of the of bed 50 mm to avoidfrom the distributor-level distributor, whereas disturbances the upper port influencing was at a height the of pressure 500 data mm, thereby providing crucial information about the overall bed dynamics. The lower pressure port recorded bywas the deliberately transducer located [27 away,28]. from On thethe bottom other hand,of the bed the to location avoid distribu of thetor-level upper disturbances pressure port was always substantiallyinfluencing the higher pressure than datathe recorded total by height the transd of theucer bed[27,28]. even On the when other the hand, bed the was location fully of fluidized. The pressurethe upper tap, which pressure was port flush was always with substantially the column higher wall, than was the covered total height with of the anylon bed even mesh. when The mass flow controllerthe bed was was also fully controlled fluidized. The using pressure the analogtap, which output was flush of thewith DAQ, the column whereas wall, thewas voltagecovered signal of with a nylon mesh. The mass flow controller was also controlled using the analog output of the DAQ, the controllerwhereas was the recorded voltage signal throughout of the controller the experiments. was recorded throughout the experiments.

Figure 1. FigureSchematic 1. Schematic of the of experimental the experimental setup setup usedused fo forr fluidization ﬂuidization at ambi atent ambient conditions conditions using using compressed nitrogen as the fluidizing gas (broken lines represent data communication lines). compressed nitrogen as the ﬂuidizing gas (broken lines represent data communication lines). Appl. Sci. 2018, 8, 1469 4 of 13

Appl. Sci. 2018, 8, x FOR PEER REVIEW 4 of 13 CharacterizationAppl. Sci. 2018, of8, x Solid FOR PEER Particles REVIEW 4 of 13 Characterization of Solid Particles TwoCharacterization different of particle Solid Particles samples belonging to the Geldart group C classiﬁcation were used in Two different particle samples belonging to the Geldart group C classification were used in the the presentTwo work. different One particle was a high-surface-areasamples belonging to porous the Geldart activated group carbon C classification (Avonchem, were used Cheshire, in the UK), present work. One was a high-surface-area porous activated carbon (Avonchem, Cheshire, UK), whichpresent is commonly work. One used was as ana high-surface-area adsorbent for a wide porous variety activated of environmental carbon (Avonchem, applications. Cheshire, As UK), shown in which is commonly used as an adsorbent for a wide variety of environmental applications. As shown which is commonly used as an adsorbent for a wide variety of environmental applications. As shown µ Figurein2 ,Figure its particle 2, its particle size analysis size analysis reveals reveals a wide a wide size size distribution, distribution, varying varying in in the the range range of of 0.2 0.2 toto 200 m. in Figure 2, its particle size analysis reveals a wide size distribution, varying in the range of 0.2 to 200 The volume-meanμm. The volume-mean diameters diameters for three for differently-sizedthree differently-si analysiszed analysis runs runs were were 26.14, 26.14, 25.41, 25.41, andand 27.55 µm. μm. The volume-mean diameters for three differently-sized analysis runs were 26.14, 25.41, and 27.55 The SEMμm. The image SEM of image the activated of the activated carbon carbon is shown is shown in Figure in Figure3. 3. μm. The SEM image of the activated carbon is shown in Figure 3.

FigureFigure 2. Particle 2. Particle size size analysis analysis ofof thethe powderedpowdered activated activated carbon carbon sample. sample. Figure 2. Particle size analysis of the powdered activated carbon sample.

Figure 3. SEM image of activated carbon at a magnification of 20,000×. Figure 3. SEM image of activated carbon at a magnification of 20,000×. Figure 3. SEM image of activated carbon at a magniﬁcation of 20,000×. Another group C particle sample used was much smaller than calcium hydroxide with a mean Another group C particle sample used was much smaller than calcium hydroxide with a mean particle size of 1.08 ± 0.23 μm. Its SEM image, shown in Figure 4, reveals a wide size distribution Anotherparticle size group of 1.08 C particle± 0.23 μm. sample Its SEM used image, was shown much smallerin Figure than 4, reveals calcium a wide hydroxide size distribution with a mean owing to agglomeration. particleowing size to of agglomeration. 1.08 ± 0.23 µm. Its SEM image, shown in Figure4, reveals a wide size distribution owing to agglomeration. Appl. Sci. 2018, 8, 1469 5 of 13

Appl. Sci. 2018, 8, x FOR PEER REVIEW 5 of 13 TheAppl. sample Sci. 2018 of, 8, x external FOR PEER REVIEW inert particles, i.e., sand, was obtained by carefully sieving ﬁeld5 of 13 sand in the lab and collectingThe sample theof external sample inert retained particles, between i.e., sand, 300 wasµm obtained and 212 by µcarefullym sieves, sieving thus field obtaining sand in sample the labThe and sample collecting of external the sample inert retainedparticles, between i.e., sand, 300 was μm obtained and 212 μbym carefully sieves, thus sieving obtaining field sandsample in µ with a meanthewith lab a size meanand ofcollecting size 256 of 256m. the μIts samplem. SEMIts SEM retained image image between is is shown shown 300 in μ Figurem Figure and 5.2125 . μm sieves, thus obtaining sample with a mean size of 256 μm. Its SEM image is shown in Figure 5.

Figure 4. SEM images of calcium hydroxide at a magnification of 1000×. Figure 4. SEM images of calcium hydroxide at a magniﬁcation of 1000×. Figure 4. SEM images of calcium hydroxide at a magnification of 1000×.

Figure 5. SEM images of inert sand samples used at magnification of 100×. Figure 5. SEM images of inert sand samples used at magnification of 100×. × The physicalFigure 5.propertiesSEM images and the of inert theoretical sand samples minimu usedm fluidization at magnification velocities of of 100 all the. particle samplesThe arephysical presented properties in Table and 1. the To theoreticalcompute th minimue theoreticalm fluidization values of velocities the minimum of all fluidizationthe particle Thesamplesvelocities, physical are the presented properties pressure dropin andTable evaluated the 1. To theoretical compute using the th Ergun minimume theoretical equation fluidization values was equate of thedvelocities minimumwith the fluidized fluidization of all the bed particle samplesvelocities,pressure are presented drop the pressure as follows: in Table drop 1evaluated. To compute using the the Ergun theoretical equation was values equate ofd the with minimum the fluidized fluidization bed pressure drop as follows: velocities, the pressure drop evaluated using the Ergun equation was equated with the fluidized bed pressure drop as follows: 2 150µU 1 − εm f 1.75ρ f U m f + m f = − 2 3 3 g ρp ρ f (1) dp εm f dpεm f Appl. Sci. 2018, 8, 1469 6 of 13 Appl. Sci. 2018, 8, x FOR PEER REVIEW 6 of 13 where subscript ‘mf ’ signifies the incipient fluidization conditions, d is the particle diameter, µ is 150 1 − 1.75 p + = − the gas viscosity, ε the bed void fraction, U is the minimum fluidization velocity, g is gravitational(1) mf acceleration, and ρP, ρ f are particle and fluid densities, respectively. The Geldart classification of where subscript ‘mf’ signifies the incipient fluidization conditions, is the particle diameter, μ is different particle samples used is presented in Table1 and Figure6. The resident solid particles the gas viscosity, ɛ is the bed void fraction, Umf is the minimum fluidization velocity, g is gravitational of the fluidizedacceleration, bed and belong , are to groupparticle C,and whereas fluid densities, the external respectively. inert The sand Geldart particles classification belong of to group B. As mentioneddifferent particle earlier, samples the used selection is presented of group in Table B particles1 and Figure for 6. mixingThe resident with solid group particles C particles of the will force thefluidized mixture bed fluidization belong to group behavior C, whereas like group the extern A, whichal inert shows sand particles a particulate belong fluidizationto group B. As behavior. The selectionmentioned of groupearlier, Cthe particle selection samples of group B greatly particles differed for mixing from with one group another C particles as the will larger force the activated mixture fluidization behavior like group A, which shows a particulate fluidization behavior. The carbon lies at the A-C boundary, whereas the smaller calcium hydroxide was as much as 25 times selection of group C particle samples greatly differed from one another as the larger activated carbon smallerlies as comparedat the A-C boundary, with its whereas larger counterpart. the smaller calcium hydroxide was as much as 25 times smaller as compared with its larger counterpart. Table 1. Main characteristics of the particle samples used. Table 1. Main characteristics of the particle samples used. Size Density Group Theoretical Particle Sample Size Density Group Theoretical 3 Particle Sample (µm) (kg/m ) Remf (-/-) Umf (mm/s) (μm) (kg/m3) Remf (-/-) Umf (mm/s) −4 −1 ActivatedActivated carbon carbon 26.4 ± 26.41.1 ± 1.1 917 917 C C 3.23.2 × 10× −104 1.86 × 1.8610−1 × 10 −8 −4 CalciumCalcium hydroxide hydroxide 1.08 ± 1.080.23 ± 0.23 2211 2211 C 5.65.6 × 10× −108 7.80 × 7.8010−4 × 10 Inert sand 256 2664 B 2.9 × 10−1 1.17 × 101 Inert sand 256 2664 B 2.9 × 10−1 1.17 × 101

Figure 6. Particle mixing strategy based on the Geldart classification. Figure 6. Particle mixing strategy based on the Geldart classification. The fluidized bed was first loaded with 500 g group C particles. The initial static bed height was The300 fluidized ± 5 mm for bed the was case first of calcium loaded hydroxide. with 500 On g groupthe other C particles.hand, it was The 225 initial ± 3 mm static for the bed bed height of was 300 ± 5activated mm for carbon. the case of calcium hydroxide. On the other hand, it was 225 ± 3 mm for the bed of activated carbon. 3. Results and Discussion 3. Results andFirst, Discussion we considered the fluidization of the calcium hydroxide by carrying out two runs, as shown in Figure 7. A linear increase in the pressure drop at small velocities was initially observed, First,owing we to considered the fixed bed the mode fluidization of contacting of the between calcium the hydroxide two phases. by However, carrying as out the twovelocity runs, was as shown in Figure7. A linear increase in the pressure drop at small velocities was initially observed, owing to the fixed bed mode of contacting between the two phases. However, as the velocity was increased further, a prominent initial depression in the pressure drop profile before an increase sometimes existed. However, above 3 mm/s, the pressure drop tended to decrease with increased velocity. Moreover, the pressure drop profiles of the two runs had a significant difference, which can be attributed to bed non-homogeneities. Evidently, Run 2 showed better fluidization hydrodynamics as compared Appl. Sci. 2018, 8, 1469 7 of 13 with Run 1 because the fluidization carried out during Run 1 tended to partially eliminate the bed non-homogeneities. Therefore, Run 2 revealed improved fluidization behavior. Appl. Sci. 2018, 8, x FOR PEER REVIEW 7 of 13 The pressure drop in the initial portion of the pressure drop profile can be correlated with the Ergunincreased equation further, to compute a prominent the initial effective depression hydrodynamic in the pressure diameter drop profile of the before resident an solidincrease particles. The Ergunsometimes equation existed. is given However, as: above 3 mm/s, the pressure drop tended to decrease with increased velocity. Moreover, the pressure drop profiles of the two runs had a significant difference, which can be attributed to bed non-homogeneities. Evidently,2 Run 2 showed2 better fluidization hydrodynamics 150µUo∆L (1 − ε) 1.75ρ f Uo ∆L (1 − ε) as compared with Run 1= because the fluidization +carried out during Run 1 tended to partially ∆p 2 3 3 (2) eliminate the bed non-homogeneities.dp Therefore,ε Run 2 revealeddp improvedε fluidization behavior. The pressure drop in the initial portion of the pressure drop profile can be correlated with the where ∆Ergunp is the equation pressure to compute drop acrossthe effective the bed hydrodynamic length ∆L diameter, and Uo ofis the the reside superficialnt solid particles. velocity The of the gas throughErgun the bed. equation For smallis given velocities as: (laminar regime), the above equation yields a linear relationship between the pressure drop and the velocity. Therefore: 150∆ 1− 1.75 ∆ 1− ∆ = + (2) 150µ∆L (1 − ε)2 where ∆ is the pressure drop across∆p the= bed length ∆, and Uo is the superficial velocity of the gas (3) d2 ε3 through the bed. For small velocities (laminar regimep ), the above equation yields a linear relationship between the pressure drop and the velocity. Therefore: The initial linear part of the pressure-drop profile in Figure7 was used to compute the effective 150∆ 1− hydrodynamic diameter by evaluating the∆ mean= of both runs and with fitting Equation (3).(3) It is clear from Figure7 that the following equation: The initial linear part of the pressure-drop profile in Figure 7 was used to compute the effective hydrodynamic diameter by evaluating the mean of both runs3 and with fitting Equation (3). It is clear ∆p = 3670.4 × 10 Uo (4) from Figure 7 that the following equation:

yields a linear relationship between the pressure∆ = 3670.4 drop × 10 and the velocity such that the value(4) of the 2 coefﬁcientyields of a determination linear relationship (R between) is 0.9979. the pressure Note that drop the and location the velocity of lower such portthat the was value accounted of the for in the evaluationcoefficient of of the determination∆L in Equation (R2) is (3). 0.9979. Substituting Note that the location void fraction of lower obtained port was fromaccounted the experimental for in bed heightthe evaluation data in Equation of the ∆ (3), in andEquation equating (3). Substituting the slopes oftheEquations void fraction (3) obtained and (4), from it was the possible to computeexperimental the effective bed height average data in diameter Equation (3), of and the equating particles the present slopes of in Equations the bed. (3) Its and value (4), it was found to be 3.72possibleµm. Theto compute computed the effective particle average size isdiameter almost of as the much particles as fourpresent times in the higher bed. Its than value the was average found to be 3.72 μm. The computed particle size is almost as much as four times higher than the particleaverage size obtained particle usingsize obtained the particle using size the analysis.particle size This analysis. result This can result be attributed can be attributed to the agglomeration to the phenomenonagglomeration due to phenomenon interparticle due surface to interparticle forces. surface forces.

Figure 7.FigureEffect 7. Effect of velocity of velocity on on the the pressure pressure dropdrop pr profileofile of of the the bed bed of calcium of calcium hydroxide hydroxide during during fluidization at ambient conditions using nitrogen gas (the fit is for the initial linear portion of the fluidization at ambient conditions using nitrogen gas (the fit is for the initial linear portion of the pressure-drop profile). pressure-drop profile). Appl. Sci. 2018, 8, 1469 8 of 13

Appl. Sci. 2018, 8, x FOR PEER REVIEW 8 of 13 Next, we considered the case of particle mixing by adding 150 g of group B particles and carrying out the fluidizationNext, we considered of the mixed the case bed. of The particle results mixing are presentedby adding 150 in Figure g of group8. At B lowparticles velocities, and carrying Run 2 and out the fluidization of the mixed bed. The results are presented in Figure 8. At low velocities, Run 2 the mixed bed due to partial elimination of bed non-homogeneity during the first fluidization run have and the mixed bed due to partial elimination of bed non-homogeneity during the first fluidization not much difference. However, as the bed was fluidized, a substantial difference was observed between run have not much difference. However, as the bed was fluidized, a substantial difference was the pressureobserved drop between profiles. the pressure The mixed drop profiles. bed revealed The mi axed smooth bed revealed pressure a smooth drop pressure profile. Thisdrop findingprofile. is a clear indicationThis finding that is a clear the additionindication of that external the addition particles of external helped particles improve helped the fluidizationimprove the fluidization behavior of the mixedbehavior bed. of the mixed bed.

FigureFigure 8. Effect 8. Effect of theof the velocity velocity and and the the particle-mixing particle-mixing on on the the pressure pressure drop drop profiles proﬁles of the of bed the bedof of calciumcalcium hydroxide hydroxide powder powder using using nitrogen nitrogen gas gas ambient ambient conditions.

Next, we evaluated the quality of fluidization in terms of a dimensionless parameter, termed as Next,the fluidization we evaluated index the(FI) quality[29]. For ofthe fluidization case of the ga ins-solid terms fluidization, of a dimensionless this is defined parameter, as the ratio termed of as the fluidizationthe pressure index drop across (FI) [29 the]. Forbed to the the case effective of the bed gas-solid weight fluidization,per unit cross-sectional this is defined area: as the ratio of the pressure drop across the bed to the effective bed∆ weight per unit cross-sectional area: = (5) ∆pA FI = (5) where is the amount of solid particles in the bed,mg ∆ is the pressure drop across the bed, A is the area of the bed, and g is the gravitational acceleration. Once the incipient fluidization conditions are whereachieved,m is the the amount FI should of solid be unity particles as the in pressure the bed, drop∆p isduring the pressure fluidization drop should across be theequal bed, to Atheis the area ofeffective the bed, weight and gperis unit the gravitationalarea of the bed. acceleration. A low value Onceof FI would the incipient indicate fluidizationthe gas channeling conditions and are achieved,bypassing the FI without should besufficiently unity as thecoming pressure into dropcontact during with fluidizationsolid particle should present be in equal the tobed. the This effective phenomenon is highly undesirable because it will lead to poor mixing. By contrast, a high value of FI weight per unit area of the bed. A low value of FI would indicate the gas channeling and bypassing would indicate hindrance to the gas flow aside from the usual drag due to the powder cohesiveness. withoutSuch sufficiently kinds of bed coming non-homogeneities into contact are with also solid unde particlesirable owing present to the in high the bed. energy This cost phenomenon associated is highlywith undesirable a high pressure because drop. it willMoreover, lead to FI, poor as a measur mixing.e of By the contrast, quality of a highthe fluidization, value of FI is would applicable indicate hindranceonly when to the the gas bed flow is fully aside fluidized. from the usual drag due to the powder cohesiveness. Such kinds of bed non-homogeneitiesFigure 9 shows the are fluidization also undesirable indices for owing the monocomponent to the high energy bed of cost calcium associated hydroxide with and a high pressurethe binary-solid drop. Moreover, bed of FI,calcium as a measurehydroxide of and the sa qualitynd. We ofmade the necessary fluidization, corrections is applicable to account only for when the bedthe is location fully fluidized. of the lower port in our experimental setup. For the monocomponent bed, FI was Figuresometimes9 shows as high the as fluidization 2.5, which means indices that for the the pres monocomponentsure drop was as bed much of calciumas 250% hydroxidehigher than andthe the effective weight of the bed. By contrast, the mixed bed showed much better fluidization behavior as binary-solid bed of calcium hydroxide and sand. We made necessary corrections to account for the the FI is near unity once the bed becomes fully fluidized. location of the lower port in our experimental setup. For the monocomponent bed, FI was sometimes as high as 2.5, which means that the pressure drop was as much as 250% higher than the effective weight of the bed. By contrast, the mixed bed showed much better fluidization behavior as the FI is near unity once the bed becomes fully fluidized. Appl. Sci. 2018, 8, 1469 9 of 13 Appl. Sci. 2018, 8, x FOR PEER REVIEW 9 of 13

FigureFigure 9. Effect9. Effect of of the the addition addition ofof externalexternal particlesparticles on on the the fluidization ﬂuidization hydrodynamics hydrodynamics of the of the monocomponent bed and mixed bed of calcium hydroxide and sand. monocomponent bed and mixed bed of calcium hydroxide and sand.

To make a precise comparison, we have carried out chi-squared (χ2) test, which is a statistical Tomethod make of a assessing precise comparison, the goodness we of havefit between carried a outset chi-squaredof observed values (χ2) test, and which those isexpected a statistical methodtheoretically. of assessing More theprecisely, goodness it is essentially of fit between the sum a of set the of squares observed of the values differences and between those expected the theoretically.actual data More and its precisely, expected it value is essentially with each the squared sum ofdifference the squares divided of by the the differences corresponding between the actualexpected data value. and The its expectedvalue can be value written with as: each squared difference divided by the corresponding expected value. The value can be written as: − = (6) 2 FI − FI where the is taken as unity becauseactual the pressureexpected drop during fluidization should ideally χ2 = ∑ (6) equal the effective weight of the bed. Considering FItheexpected FI value for each individual velocity when the full fluidization was achieved ( > 2.0 mm/s), we computed the sum required in Equation (6) for whereeach the run.FIexpected We obtainedis taken as values unity to be because 10.65, 3.86, the and pressure 0.27 for drop Run during1, Run 2, fluidization and the mixed should bed case, ideally equalrespectively. the effective A weightcomparison of the clearly bed. indicates Considering a difference the FI valueof more for than each 10 individual times between velocity the mixed when the full fluidizationbed and the wasRun 2 achieved while the ( Usameo > 2.0is almost mm/s), 40 time we computeds when the Run the sum1 is considered. required in This Equation is a clear (6) for eachindication run. We obtained of the improvementχ2 values to in be the 10.65, hydrodynam 3.86, andics 0.27 of for the Run fluidization 1, Run 2, of and cohesive the mixed calcium bed case, respectively.hydroxide A powder comparison as a result clearly of particle indicates mixing. a difference of more than 10 times between the mixed We also considered the fluidization of the monocomponent bed of activated carbon, as shown bed and the Run 2 while the same is almost 40 times when the Run 1 is considered. This is a clear in Figure 10. When the gas velocity was very small, the pressure drop linearly increased with the indicationvelocity of due the improvementto the fixed bed in mode the hydrodynamics of contact of the ofgas the with fluidization the resident of solid cohesive particles. calcium Increasing hydroxide powderthe asvelocity a result further of particle causes particles mixing. to move and rearrange, which is reflected in the depression seen Wein the also pressure considered drop profile. the fluidization Finally, the pressure of the monocomponent drop attains a steady bed value of activated at high flow carbon, rates as as the shown in Figurebed of 10 activated. When thecarbon gas becomes velocity fully was fluidized. very small, The the pressure pressure drop drop in the linearly initial increasedportion of withthe the velocitypressure due todrop the was fixed correlated bed mode with of the contact Ergun equa of thetion, gas which with computes the resident the effective solid particles. hydrodynamic Increasing the velocitydiameter further of the resident causes solid particles particles. to move Its value and was rearrange, found to whichbe 27.47 is μ reflectedm, which inis very the depressionclose to 26.4 seen in the± pressure1.1 μm obtained drop profile.from the Finally,particle thesize pressureanalysis (Table drop 1). attains Although a steady the early value onset at highof fluidization flow rates as the bedwas of introduced activated in carbon the bed becomes at approximately fully fluidized. 0.2 mm/s, The the pressurebed appeared drop to in be the fully initial fluidized portion only of the when the gas velocity is 1 mm/s. pressure drop was correlated with the Ergun equation, which computes the effective hydrodynamic diameter of the resident solid particles. Its value was found to be 27.47 µm, which is very close to 26.4 ± 1.1 µm obtained from the particle size analysis (Table1). Although the early onset of fluidization was introduced in the bed at approximately 0.2 mm/s, the bed appeared to be fully fluidized only when the gas velocity is 1 mm/s. Appl. Sci. 2018, 8, 1469 10 of 13 Appl. Sci. 2018, 8, x FOR PEER REVIEW 10 of 13

Figure 10. Fluidization behavior of the monocomponent bed of activated carbon at ambient Figure 10. Fluidization behavior of the monocomponent bed of activated carbon at ambient conditions conditions using nitrogen gas. using nitrogen gas. The effect of the mixing of external group B particles is shown in Figure 11. Here, we broadened Thethe scope effect of theour mixingwork by of varying external the group fraction B particles of external is shown particles. in Figure Four different11. Here, cases we broadened were the scopeconsidered of our by work gradually by varying increasing the fraction the amount of external of external particles. particles Four by differentadding 25, cases 50, 75, were and considered 150 g by graduallyof sand in increasinga bed containing the amount 500 g of ofactivated external carbon. particles Evidently, by adding the initial 25, slope 50, 75, of the and pressure 150 g of drop sand in profile remained largely unaffected by the particle mixing. However, a substantial change in the a bed containing 500 g of activated carbon. Evidently, the initial slope of the pressure drop profile pressure drop profile for velocities higher than 0.2 mm/s was observed. The depression seen in the remained largely unaffected by the particle mixing. However, a substantial change in the pressure pressure drop profile was substantially attenuated even with the addition of 25 g of external particles. dropThe profile trend for is velocities similar when higher a high than proportion 0.2 mm/s of was external observed. particles The is depression added. This seen finding in the can pressure be dropattributed profile was to substantiallythe elimination attenuated of non-homogeneitie even with thes from addition the bed of 25due g ofto externalthe addition particles. of external The trend is similarparticles. when However, a high proportion the pressure of drop external at low particles velocities is added. was substantially This finding higher can bewhen attributed 150 g of to the eliminationexternal ofparticles non-homogeneities were added. The from result the is beddue dueto the to bed the contraction addition of leading external to a particles. decrease in However, the the pressurebed void dropfraction, at lowwhich velocities is reflected was in the substantially high pressure higher drop whenowing 150to a strong g of external dependence particles of the were added.pressure The result drop ison due the to bed the void bed contractionfraction [7]. leadingHowever, to as a decreasethe velocity in thewas bed increased void fraction, further, whichthe is reflectedpressure in the drop high decreased pressure and drop became owing constant to a strong due dependenceto the complete of fluidization. the pressure At drop high on velocities, the bed void the low fraction mixed bed (25 g sand) did not show any improvement as the pressure drop fell as fraction [7]. However, as the velocity was increased further, the pressure drop decreased and became low as 600 Pa due to the gas bypassing caused by the formation of channels in the fluidized bed. constant due to the complete fluidization. At high velocities, the low fraction mixed bed (25 g sand) Further addition of 25 g external particles nonetheless significantly improved the fluidization did notbehavior show at any low improvement and high velocities. as the The pressure behavior drop at high fell asvelocities low as 600became Pa duecomparable to the gaswhen bypassing the causedamount by the of formationexternal particles of channels was increased in the fluidizedfrom 75 g bed.to 150 Further g. At low addition velocity, ofthe 25 pressure g external drop particles for nonetheless150 g of significantlysand was high improved due to the the bed fluidization contraction behaviorphenomenon at low [30,31 and]. The high high velocities. pressure The drop behavior at at highhigh velocities velocity, becamewhen the comparable bed was fully when fluidized, the amount was due of external to the addition particles of wassand, increased leading to from an 75 g to 150increase g. At lowin the velocity, effective theweight pressure of the bed. drop for 150 g of sand was high due to the bed contraction phenomenon [30,31]. The high pressure drop at high velocity, when the bed was fully fluidized, was due to the addition of sand, leading to an increase in the effective weight of the bed. Appl. Sci. 2018, 8, 1469 11 of 13 Appl. Sci. 2018, 8, x FOR PEER REVIEW 11 of 13

Appl. Sci. 2018, 8, x FOR PEER REVIEW 11 of 13

FigureFigure 11. Effect11. Effect of velocityof velocity and and particle particle mixingmixing on th thee pressure pressure drop drop profile profile of a of bed abed of activated of activated carbon during fluidization at ambient conditions using nitrogen gas. carbon during fluidization at ambient conditions using nitrogen gas. Figure 11. Effect of velocity and particle mixing on the pressure drop profile of a bed of activated carbonFigure during12 shows fluidization the fluidization at ambien indext conditions for the using bed nitrogenof activated gas. carbon. Evidently, the FI is below Figureunity when 12 shows the bed the of fluidization activated carbon index contains for the eith beder of no activated external particles carbon. or Evidently, very small the fraction. FI is below unityThe when additionFigure the 12 bed of shows 50 of g activatedof the eternal fluidization particles carbon index containssignifican for thetly eitherbed enhanced of noactivated external the fluidizationcarbon. particles Evidently, hydrodynamics or very the FI small is below as fraction. FI The additionunityis very when close of 50 theto gunity. bed of eternal of Further activated particles addition carbon significantly of contains25 g help seith to enhanced erimprove no external the fluidization fluidization particles or behavior hydrodynamicsvery small of the fraction. mixed as FI is The addition of 50 g of eternal particles significantly enhanced the fluidization hydrodynamics as FI very closebed. However, to unity. not Further much additionimprovement of 25 was g helpsobserved to improve when a total the of fluidization 150 g of external behavior particles of the were mixed isadded very toclose the to bed. unity. The Further values addition were of 0.1512, 25 g help 0.6788,s to improve0.0393, 0.0172, the fluidization and 0.0572 behavior for 0, 25, of 50, the 75, mixed and bed. However, not much improvement was observed when a total of 150 g of external particles were bed. However, not much improvement was observed when a total of 150 g of external particles were 150 g of sand, respectively.2 In terms of the weight percentages, the corresponding values are 0, 5, 9, added to the bed. The χ values were 0.1512, 0.6788, 0.0393, 0.0172, and 0.0572 for 0, 25, 50, 75, and added13, and to 23. the Furthermore, bed. The the values optimum were fraction0.1512, 0.6788, of external 0.0393, particles 0.0172, in and our 0.0572 case was for 130, 25,wt%. 50, 75, and 150 g150 of sand,g of sand, respectively. respectively. In termsIn terms of theof the weight weight percentages, percentages, thethe correspondingcorresponding values values are are 0, 0,5, 5,9, 9, 13, and 23.13, Furthermore,and 23. Furthermore, the optimum the optimum fraction fraction of external of external particles particles in in our our case case was was 1313 wt%.

Figure 12. Effect of the addition of external particles on the fluidization hydrodynamics of the monocomponent bed and mixed bed of activated carbon and sand. Figure 12. Effect of the addition of external particles on the fluidization hydrodynamics of the Figure 12. Effect of the addition of external particles on the ﬂuidization hydrodynamics of the monocomponent bed and mixed bed of activated carbon and sand. monocomponent bed and mixed bed of activated carbon and sand. Appl. Sci. 2018, 8, 1469 12 of 13

4. Conclusions In our detailed experimental investigation, the choice of two different kinds of group C particles revealed a substantial difference in their fluidization behaviors. The monocomponent fluidization of the small calcium hydroxide particles led to a pressure drop that was sometimes several times higher than the effective weight of the bed. This result can be attributed to the cohesiveness of the powder, which caused a reduction in the bed void fraction, leading to an increase in the pressure drop. The initial portion of the pressure drop profile yielded a hydrodynamic diameter, which was as much as four times larger than the size obtained from the particle size analysis, mainly owing to the particle agglomeration. In fact, the SEM image of the sample also revealed the presence of the agglomeration phenomenon. The chi-squared test proves that the addition of group B particles helped to significantly enhance the fluidization behavior. The fluidization of activated carbon was substantially different from that of the calcium hydroxide. First, the hydrodynamic diameter obtained from the pressure drop data was very close to the actual values obtained from the particle size analysis. Second, unlike the high pressure drop exhibited by the calcium hydroxide, a significant gas channeling and bypassing were observed in the bed of activated carbon, which led to a low pressure drop. Thus, the addition of group B particles improved the fluidization hydrodynamics. In these experiments, the fraction of external particle also varied. The best results were obtained for 13% external particles.

Author Contributions: Experiments were carried out by E.H.A.-G.; Conceptualization of project was contributed by A.A.; Methodology, validation, resources, project administration and original draft was contributed by M.A.; funding acquisition and writing—review and editing was contributed by 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.

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