Hongbing Mo,Bao Xu,Chuanbao Luo,Tao Zhou*,Jiangrong Kong

Hunan Provincial Key Laboratory of Efficient and Clean Utilization of Manganese Resources,College of Chemistry and Chemical Engineering,Central South University,Changsha 410083,China

Keywords:Nanoparticles Fluidization Agglomerate Breakup

A B S T R A C T In this study,the influence of fluid cracking catalyst(FCC)on the fluidization behavior of ZnO-CuO binary nanoparticles was systematically investigated by varying FCC size.High-speed camera was employed to analyze the collision and fragmentation process of agglomerates with adding FCC coarse particles.It can be found from photographs by the camera that fluidization performance improved by the agglomerate variation that is bound to be shaped a compact and spherical structure.Furthermore,the result of agglomeration composition analysis showed that uniform mixing of nanoparticles remarkably affected the fluidization behavior of ZnO-CuO binary system.Finally,the improvement of fluidization performance can be justified by the analysis of inter-cohesive force between the two agglomerates with sharp reduction of the newly-formed agglomerates.

1.Introduction

Nanoparticles have attracted an increasing attention in such industrial fields as foods,plastics,pigments,cosmetics,and catalysts,etc[1,2]for its small particle size yet large surface area per unit mass.Mixed nanoparticles used as catalysts can improve the selectivity over certain substances.For instance,CeO-doped SnO2possesses a higher selectivity over CO while ZrO-doped SnO2over H2S.Strong inter-particle force and electrostatic force[3,4]make it more possible for nanoparticles to agglomerate than to disperse homogenously,causing plugging or channeling and consequently,impeding the application of nanoparticles as catalysts.

Currently, fluidization performance of nanoparticles is enhanced either through crashing or friction of agglomerates as shown by magnetically-driven fluidization[5–7],sound-assisted fluidization[8,9],vibrated fluidized bed[10–13]or through transformation of agglomerate structure as shown by adding coarse particles[14,15].Compared with other methods,adding coarse particles is promising in a series of previous studies for its easy operation in an existing fluidized bed reactor with no extra equipment involved[16–18].Duan et al.[19]studied the fluidization behavior of SiO2-ZnO binary nanoparticles by adding fluid cracking catalyst(FCC)coarse particles in a three-dimensional fluidized bed and calculated the Richardson-Zakiexponents of the fluidization behavior ofmixed nanoparticles with a linear regression.The result showed that the fluidization behavior improved significantly with increasing content of coarse particles in smaller size.The core–shell structure of agglomerates revealed that the formation of multi-component had been observed in our previous studies,but the chemical composition of agglomerates and the process as well as the mechanism of agglomeration collision and fragmentation remained unclear.

Herein,the fluidization behavior of ZnO-CuO binary nanoparticles with FCC coarse particles added was studied in a two-dimension fluidized bed.Agglomeration composition was detected and analyzed to show that nanoparticles combined with FCC particles formed new agglomerates during fluidization process.The improved fluidization performance of binary nanoparticles by adding FCC coarse particles was reasonable explained based on analysis of cohesive forces,and agglomerate collision and fragmentation process was imaged by the highspeed camera.

2.Experimental

All the tests were conducted under the condition of room temperature and ambient pressure.The experimental apparatus is the same as the previous work[13]and the only difference is that the switch of vibration is off.It was worth noticing that the fluidized process of agglomerates was captured by high-speed camera(KC-4787-11/07,EYENCE)for the intense and instantaneous interaction corresponding to collision and breakup between agglomerate.The measuring location is in the upperboundary of emulsion phase and focusing is at the centre of cylindrical bed.The distinct images of agglomerates can be obtained only for passing through the centre.Then the images of the entire process of fluidization were analyzed by Image Software to understand the characteristic motive behavior of agglomerates in the bed.

Considering the striking difference in color between ZnO and CuO,their corresponded nanoparticles were employed as solid phase of fluidized bed in this work,which is beneficial to observe the mixing degree correlated to the quality of fluidization.The related physical properties were listed in Table 1.The FCC coarse particles were acted as auxiliary powder improving the fluidization quality and categorized into FFC1,FCC2 and FCC3 by using criterion sieves with different specifications.The corresponding physical properties were listed in Table 2 and the chemical compositions of FCC coarse particles were listed in Table 3.In the experiments,ZnO and CuO were mixed uniformly in different mass ratios R1,R2 and R3(as listed in Table 4)before adding FCC coarse particles to systematically investigate the effect of its content on fluidization quality.Subsequently,materials(i.e.CuO,ZnO and FCC)were dried by silica gel about 2 h before the experiments to minimize the effect of humidity on fluidization of nanoparticles.Finally,ZnO-CuO binary particles mixed by FCC were loaded to the bed in a fixed height of about 40 mm.

Table 1The physical properties of nanoparticles

Table 2The physical properties of FCC coarse particles

Table 3The content of SiO2 Al2O3,Fe2O3,Na2O,K2O in FCC coarse particles

Table 4The mass ratio of ZnO and CuO

Upon stable fluidization,the samples were taken out from a 5 mm circular hole at the upper and middle part of the bed to measure the agglomerate size.0.1 g samples were dissolved in a 5 mol·L-1sulfuric acid(H2SO4)solution under vigorous stirring and the solution was then diluted for composition analysis by inductively coupled plasma atomic emission spectrometer(ICP-AES(Optima5300DV,Perkin EImer))emission spectrum.

For each investigation,the average agglomerate size of the nanoparticle mixture is mean value of equivalent diameters of 50 measurements.The error limit of the experimental average agglomerate diameter of different binary mixed nanoparticles under the same fluidized conditions was within the range of±(0–15)%,giving acceptable reproducibility for this technique.

3.Results and Discussion

The internal flowing of bed showed that the fluidization performance of ZnO-CuO binary nanoparticles is similar to that of SiO2-ZnO or SiO2-TiO2binary nanoparticles[19].Specifically,agglomerate bubbling fluidization(ABF)and deposition are frequently exhibited at the upper and bottom of the bed,respectively.Moreover,channeling and slugging are also observed even at a higher gas velocity,revealing an inferior fluidization performance of the studied system.

3.1.Process of aggregation and fragmentation of mixing nanoparticle agglomerates

Three characteristic behaviors of agglomerates during collision is separation,aggregation and fragmentation according to previous studies[14].Based on the videos and graphs documented by the camera,typical collision process of agglomerates can be exhibited in Fig.1.As shown in Fig.1(a),two agglomerates with the similar size collided towards oblique course and then separated in opposite direction with one of them moving upwards while the other downwards.In addition,the newly-formed agglomerate becomes compact spherical structure by the combined interaction of gravity,cohesive force and the solid stress.However,it can be concluded from Fig.1(b)that a large agglomeration with a smaller one is adhered together in the same direction for the strong interactive force.Fig.1(c)even reveals the fragmentation of a newly-combined agglomerate into three smaller ones because the increased applied stress force works on the weakest points of the longchained agglomerate.As indicated in Fig.1(d),with its porosity reduced,agglomerates with large size are also observed to transfer from a loose structure to a compact and spherical one without hitting other agglomerates or/and breaking up during the fluidization.

These four variations extend the most common and typical in the fluidization process of nanoparticles.Besides,though nanoparticles are aggregated into much larger agglomerates during the fluidization process,separating from agglomerates is not as easily as that of agglomerates themselves.However,primary nanoparticles and smaller agglomerates tend to rearrange or form a long chain structure during collision with each other.In a word,agglomerates are bound to be shaped a compact and spherical structure with decreasingly size whether exhibits aggregation,separation or fragmentation after the agglomerate collision.Furthermore, fluidization performance is improved by the increased agglomerate size and spherical structure.

3.2.Analysis of agglomerate composition

Fig.2 shows the composition of the ZnO-CuObinary nanoparticle agglomerates for R1,R2 and R3,respectively.In the upper part of bed,the content of ZnO is 90.52%,82.76%and 80.45%respectively and CuO is 9.48%,17.24%and 19.55%,respectively.While in the middle part,the content of ZnO is 90.93%,83.31%and 81.85%and the CuO is 9.07%,16.69%and 18.15%,respectively.As is shown in Fig.2(a),the content of ZnO or CuO in the upper part is almost the same as that in the middle part,which shows that the circulation of agglomerates is relative competence and the bed attains statistically stable.The influence of CuO content on fluidization performance in the system can be clearly shown in Fig.2(b).With increasing CuO content in the system,the relative content of CuO(CuO content in the agglomerate/CuO content in the system)in the agglomerate decreases on the contrary whether in the middle or upper of the bed.It means the higher CuO content in the system leads to the lower relative content of CuO in the agglomerate,thus demonstrates inferior fluidization performance.It is worth noticing that the density of CuO nanoparticles is larger than that of ZnO nanoparticles and the smooth and tight structure of CuO nanoparticles(Fig.3)rather than porous structure of ZnO(Fig.4).At the same equivalent agglomerate size,the agglomerate with more CuO nanoparticles(R3 compare to R1)would be heavier.As a result,the larger drag force exerted by fluid is needed for the agglomerates of higher CuO nanoparticle content compared to ZnO agglomerates.

Fig.1.Agglomeration collision process variation with the time.

Fig.2.Agglomerate composition analysis of binary mixed ZnO and CuO nanoparticles.

Fig.3.SEM image of primary CuO nanoparticles.

Fig.4.SEM image of primary ZnO nanoparticles.

To improve the fluidization performance of mixed nanoparticles,FCC coarse particles were added into the fluidized bed.The FCC coarse particles were in collision with the mixed nanoparticles and eventually the component content in agglomerates is changed.As an example,the content of different components in agglomerates is shown here only for the mixed nanoparticles(ZnO:CuO=9:1)by adding FCC coarse particles.As shown in Fig.5(a),in the upper part of bed,the contents of ZnO decrease to 86.58%,80.89%,74.54%,and 68.06%corresponding to FCC1 addition of 10%,20%,30%and 40%,respectively and the contents ofFCC1 increase from7.86%to 13.17%,19.43%,25.50%.Butin the middle part of bed(Fig.5(b)),the contents of ZnO are 87.35%,82.21%,76.24%and 71.31%and contents of FCC1 increase from 7.17%to 12.28%,17.89%and 22.44%with FCC1 amount rising from 10%,20%30%to 40%.However,the content of CuO curve tends to be a horizontal line.The content of ZnO in the upper part is smaller than that of the middle part with same amount of FCC1 because the circulations of particles in the upper become more intense than those in the middle.The content curve of ZnO,CuO and FCC for FCC2 or FCC3 is similar to that of FCC1.

As shown in Figs.6(a)–7(a),the contents of ZnO in the upper part of bed are 85.77%,79.56%,74.41%and 67.11%(adding FCC2)and 84.56%,79.00%,72.90%and 66.38%(adding FCC3),the contents of FCC2 are 8.33%,14.10%,18.96%and 25.85%and FCC3 are 9.14%,14.46%,20.33%and 26.42%,respectively.As for the middle part(Figs.6(b)–7(b)),the contents of ZnO(adding FCC2)are 87.03%,81.17%,75.99%,and 69.21%,and 86.34%,80.6%,75.33%,and 68.08%(adding FCC3)and the contents of FCC2 are 7.18%,12.35%,17.55%,23.88%and FCC3 are 7.42%,12.88%,17.95%,and 24.93%,respectively.However,the content of CuO at both the upper and the middle parts of the bed changes slightly.This is because the experimental procedure is as follows:ZnO and CuO nanoparticles were mixed firstly and then FCC coarse particles were added.Therefore,agglomerates of ZnO and CuO nanoparticles were formed firstly.Those agglomerates collided with FCC,resulting in breakup and re-aggregate of agglomerates.But a majority of agglomerates did not completely break up and only outer ZnO or CuO nanoparticles exchanged with FCC.

Fig.8 shows that CuO nanoparticles collided with ZnO nanoparticles and eventually adhered on the surface of ZnO agglomerates during fluidization process to form a new agglomerate structure.But only a part of small-sized CuO nanoparticle agglomerates could be fluidized,so the collision probability of CuO and ZnO nanoparticle agglomerates is almost the same as that in the upper and middle part of bed.

FCC coarse particles can break up nanoparticle agglomerates and have coated on the surface of them,resulting in the drastic reduction in the cohesive force[20].Therefore,when fixing the size of FCC coarse particles,the coverage of FCC coarse particles increases with increasing the content of FCC coarse particles.Fixing the added content,smaller size of FCC coarse particle(i.e.FCC3)is more effective than the larger-sized ones(FCC1 or FCC2).Furthermore, fixing both the size and added amount,the coverage of FCC in the upper part is bigger than that of the middle part under the same conditions.

Fig.5.Agglomerate composition analysis by adding FCC1 while ZnO:CuO=9:1.(a)the upper part of bed;(b)middle part of bed.

Fig.6.Agglomerate composition analysis by adding FCC2 while ZnO:CuO=9:1.(a)the upper part of bed;(b)middle part of bed.

3.3.Analysis of cohesive force between the two agglomerates

Addition of FCC coarse particles is believed to improve the fluidization performance of mixed nanoparticles and it can be justified by analyzing the cohesive force of agglomerates.With no FCC particles added,van der Waals force plays a crucialpartin the interaction of nanoparticle agglomerate,which can be calculated by following formula:

where H stands for the Hamaker constant,da1and da2are diameters of two nanoparticle agglomerate,respectively,δ is the distance of van der Waals force.Average agglomerate diameter dais used here because the values of da1and da2are notavailable.Thus,Eq.(1)can be simplified as following:

With FCC coarse particles added,FCC coarse particles are adhered on the surface of agglomerates and the related cohesive force can be calculated by Eq.(3):

Fig.8.SEM image of binary nanoparticle agglomerate after adding FCC coarse particles.

where H1.fand δ1.fstand for the Hamaker constant and van der Waals force distance between nanoparticles and FCC coarse particles,respectively.And d1is nanoparticle diameter while dfis diameter ofFCC coarse particles.

Fig.7.Agglomerate composition analysis by adding FCC3 while ZnO:CuO=9:1.(a)the upper part of bed;(b)middle part of bed.

H is assumed to approximate H1.fdue to the same order of magnitude and the approximation applies to δ and δ1.f.According to Eqs.(2)and(3),Ff/F is expressed by Eq.(4):

Nano-sized d1and micro-sized damake 2d1/dafar less than 1.So,cohesive force Ff(with FCC particles adhered)is much smaller than the cohesive force F(without FCC particles adhered).For example,for ZnO nanoparticles with the size of 30 nm(d1)and the agglomerate size of 50–400 μm(da),Ff/F≈1.2×10-3-1.5 ×10-4,suggesting that the cohesive force reduces 3–4 orders of magnitude.Therefore,the fluidization performance of mixture nanoparticle can be improved significantly.

The coverage of FCC coarse particles is defined as FCC/ZnO.Tables 5 and 6 list the coverage of the FCC coarse particles in upper and middle parts of the bed,respectively.It can be seen that the number of the FCC coarse particles adhered by nanoparticles increased with increasing amount of FCC coarse particles added.This is attributed to the fact that adhesion of FCC to nanoparticle coarse particles hinders the nanoparticles from contacting each other and then decreases the Van der Waals force.Therefore,the more FCC coarse particles present in the system,the better fluidization performance could be achieved for the less probability of direct contact between agglomerates[21].

Table 5The adhered coverage of the FCC coarse particles in upper part of bed

Table 6The adhered coverage of the FCC coarse particles in middle part of bed

4.Conclusions

The fluidization behavior of ZnO-CuObinary nanoparticles can be improved by adding the amount of FCC coarse particles.The nanoparticles are adhered by FCC coarse particles,resulting in a sharp reduction of cohesive force.Itcan be observed from the process of agglomerate collision and breakup in the fluidized bed that agglomerate collision and fragmentation can reduce the size of agglomerates and lead to a compact agglomerate structure.