Investigating Agglomeration Behaviors in High Temperature Gas

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General Research

Investigating agglomeration behaviors in high temperature gas-solid fluidized beds with liquid injection Qiang Shi, Shaoshuo Li, Sihang Tian, Zhengliang Huang, Yao Yang, Zuwei Liao, Jingyuan Sun, Jingdai Wang, and Yongrong Yang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00311 • Publication Date (Web): 29 Mar 2018 Downloaded from http://pubs.acs.org on March 29, 2018

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Industrial & Engineering Chemistry Research

Investigating agglomeration behaviors in high temperature gas-solid fluidized beds with liquid injection Qiang Shi, Shaoshuo Li, Sihang Tian, Zhengliang Huang, Yao Yang, Zuwei Liao, Jingyuan Sun*, Jingdai Wang, Yongrong Yang State Key Laboratory of Chemical Engineering and Department of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, P. R. China * To whom correspondence should be addressed. Tel.: +86-571-87951227. Fax: +86-571-87951227. Corresponding Author: Dr. Jingyuan Sun; E-mail address: [email protected]

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract An induction heating fluidized bed with liquid injection was designed to mimic the exothermic process of particles in industrial olefin polymerization fluidized beds. The effects of liquid injection flowrate on agglomeration behaviors were investigated through the characterization of agglomerate size and mass. Agglomeration mechanisms were proposed based on the experimental data of bed pressure drop, axial temperature distribution and micro-scale visualization. Results have revealed that, with the increase of liquid injection flowrate, the agglomeration mechanism was controlled by solid bridge force, liquid bridge force and the cooperative action of them, respectively. Wet agglomerates caused by liquid bridge force induced local dead zones and provided a good environment for the occurrence of hot spot and growth of solid bridge, and thus the formation of solid agglomerates. Besides, elevating liquid injection position and increasing induction heating power can widen the operation range of liquid injection flowrate for the fluidized bed reactors. Keywords: induction heating fluidized bed; liquid bridge force; solid bridge force; cooperative action; local dead zone


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1. Introduction In many industrial processes, liquid is injected into the fluidized beds.1 Some liquid acts as a binder to form solid bridges between particles after its evaporation, as in the spraying fluidized beds granulation[2-4]. Some is used as a condensing agent to remove the reaction heat in the reactor, as in the gas phase olefin polymerization operated in condensed mode. Some also acts as a reactant to produce distillate products, as in the fluid coking. Liquid plays an important role in these processes, but meanwhile the complexity of the system is increased.5 Liquid adhering to the surface of particles results in the formation of liquid bridge, which further promotes the adhesion of particles. With regard to the gas phase olefin polymerization reactor, as polymerization is a strong exothermic reaction, particle surface temperature will quickly rise to its softening temperature if the heat is not removed in time, thereby leading to the sintering of particles. As to the fluid coking, particles easily adhere to each other due to the poor liquid-solid contacting and high operation temperature of the reactor.6 With regard to the reactors operated under high temperature, liquid bridge force and solid bridge force both exist in the reactor after the injection of liquid. These forces should both influence the bed hydrodynamics and agglomeration behaviors in the reactor. A comprehensive understanding of the agglomeration mechanism is critical to avoid agglomeration and guarantee the safe operation of the reactor. Studies on the agglomeration behaviors in the fluidized beds with liquid injection can be divided into two aspects: (1) the effects of wet particles and agglomerates on the hydrodynamics of the fluidized bed; (2) the mechanism of wet agglomerate growth and the quantitative characterization of its properties. With regard to the first aspect, Mikami et al took the liquid bridge force between particles and between a particle and the wall into account and then used discrete element method (DEM) to simulate the fluid properties of wet particles.7 The fluidization with agglomerate formation in wet particle fluidization was successfully simulated. Wright et al experimentally investigated the role of liquid bridge force on the fluidization behavior of cohesive particles. They found that the volume of liquid on the particle surface and liquid-solid contact angle affected the adhesion of particles and further the minimum fluidization velocity.8 Passos et al quantitatively characterized the fluidization behaviors of dry particles and wet particles based on the relationship between bed pressure drop and superficial gas velocity, solid circulation rate



and the variation of bed voidage.9 They concluded that the state of wet bed failure depended on the strength of the liquid bridge bindings formed on the particle contact points and on the particle shape. McLaughlin et al studied the effects of liquid bridge force on the fluidization behavior of particles through the addition of non-volatile liquids with different viscosities and surface tension to a cold mode gas-solid fluidized bed. The incremental addition of liquid to a fluidized bed of Geldart group B particles was observed to result in a transition to group A behavior and eventually to group C behavior. The AC and BA transition boundary was dependent on the relative ratio of liquid bridge force and the fluid drag force.10 Nagahashi et al found that the fluidization of particles can be promoted by the addition of small quantities of liquid when the particles are larger than about 3 mm in diameter, the liquid and particles have similar densities, and the particles are poorly wetted by the liquid.11 They also proposed a “modified liquid-perturbed gas model” to predict the minimum fluidization velocity and bed pressure drop for low liquid loadings. McDougall et al quantified the bed fluidity and the formation of large wet agglomerates through the W statistic of dynamic pressure.12 However, this method, while adequate for controlled, laboratory measurements, cannot be implemented in pilot or commercial plants. Zhou et al studied the effects of liquid injection flowrate and superficial gas velocity on particle circulation pattern through the measurement and characterization of the axial distribution of bed temperature. They found that liquid evaporation and liquid bridging greatly influenced the dominant mechanisms of particle circulation. With increasing liquid flowrate or decreasing superficial gas velocity, wet agglomerates caused by liquid bridge force dropped onto the distributor and resulted in the deterioration of bed fluidity even bed defluidization.13 The above researchers have carried out extensive research on the effects of liquid bridge force on the bed hydrodynamics and particle fluidization behavior, whereas the agglomeration mechanism and quantification of wet agglomerates were rarely mentioned or investigated. As to the agglomeration mechanism and quantification of wet agglomerates, Ennis et al firstly analyzed the collision process of particles and proposed the Stokes number and critical Stoke number to judge whether particles adhere to each other after collision.14 The Stokes criterion was widely used in the granulation process. Later, Liu et al modified the model by taking particle deformation into account and proposed the deformation Stokes number and the critical deformation Stokes number as the criterion.15 But the complexity of deformation Stokes number


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made it hard to couple with population balance modeling. Cryer et al integrated the Stokes number proposed by Ennis et al into the discrete population balance equation and numerically modeling the variation of particle size distribution16, whereas Terrazas and Hussian et al integrated it into the Monte Carlo simulation and quantified the variation of particle size distribution.17-22 These researchers have done in-depth studies about the agglomeration mechanism and quantification of agglomerate growth or breakage in granulation process. However, compared with the liquid in granulation process, condensed liquid used in olefin polymerization process is low viscosity and volatile. As liquid properties greatly affected the coalescence behavior of particles, the agglomeration mechanisms in olefin polymerization reactor are supposed to be dramatically different. In summary, previous research mainly focused on the effects of liquid bridge force and wet agglomerates on the fluidization behavior of wet particles. In spraying fluidized bed granulation, the agglomeration mechanism has been studied extensively. Unlike the spraying fluidized bed granulation, olefin polymerization is a high exothermic reaction. To avoid the temperature runaway and subsequent polymer melting and agglomeration, condensed agents are added to intensify the heat removal and broaden the operating window. However, in industrial practice, we found that agglomeration still occurred in the olefin polymerization fluidized bed operated in condensed mode, as shown in Figure 1. Exploring the agglomeration mechanisms in the reactor is crucial to avoid agglomeration and maintain safe operation. But very limited work has been conducted on this issue. In this work, we aimed to investigate the agglomeration mechanism in a high temperature fluidized bed reactor with liquid injection. Electromagnetic induction heating was used to mimic the exothermic process of particles and liquid was injected into the bed through a nozzle. Through the measurements of pressure drop, temperature distribution and further characterization of agglomerates, the agglomeration mechanisms were proposed.



Figure 1. Agglomerates in the olefin polymerization fluidized bed reactor operated in condensed mode

2. Experiment 2.1 Apparatus and materials The experimental apparatus consists of a fluidized bed, an electromagnetic induction heating system and a measurement system, as shown in Figure 2. The fluidized bed is made of glass with an inner diameter of 109 mm and a height of 800 mm. A perforated Teflon plate with an open area ratio of 2.5% was used as air distributor. Graphite particles (0.3-1.0 mm) with polyethylene wax coated on the surface were used as bed materials. The mass ratio of polyethylene wax to graphite is 0.1. The density of the composite material is 1980 kg/m3. The particle size distribution of the bed materials is shown in Figure 3. The minimum fluidization velocity of the materials is 0.328 m/s at room temperature, which was determined through the ∆P~ug curve. The preparation method of the materials has been introduced in our previous work.23 Nitrogen with a superficial velocity of 1.6 umf was used as fluidization gas. The liquid was petroleum ether and its properties are listed in Table 1. The liquid was injected into the bed through a spray nozzle amounted on the side wall with a distance of 150 mm from the distributor and an insertion depth of 10 mm. The nozzle inlet diameter, tip diameter and spraying angle are 10 mm, 1 mm and 120 degrees, respectively. The upstream liquid pressure was in the range of 0.14-0.55 MPa. The liquid flowrate was adjusted by changing the stroke of reciprocation pump.

Figure 2. Schematic diagram of the experimental apparatus


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1 Nitrogen; 2 Heat exchanger; 3 Rotor flow meter; 4 Fluidized bed; 5 Copper coils; 6 Impendence matching device; 7 Radio frequency supply; 8 Pressure probe; 9 Pressure transducer; 10 Data acquisition card; 11 computer; 12 Plastic sector nozzle; 13 Reciprocation pump; 14 Liquid tank;

The induction system consists of copper coils wrapped around the side wall of the bed, a radio frequency supply and an impedance matching device. The number of turns of the copper coils is 3, and the frequency and maximum power of the radio frequency supply is 13.56 MHz and 1000 W, respectively. Alternating magnetic field induces eddy currents on the surface of electrically conductive material such as graphite particle.24 Given graphite has resistivity, r, against eddy currents, heat is generated, rI2. Heat produced by the graphite transferred from the internal to the external and is transferred to the fluidization gas through convection. In this way, the bed was heated after the radio frequency supply was turned on. 1.0

0.8

m0 (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.6

0.4

0.2

0.0 0.0

0.2

0.4

0.6

0.8

1.0

1.2

dp (mm)

Figure 3. Particle size distribution of the bed materials

The bed pressure drop was measured by the pressure probe amounted on the side wall of the bed with a distance of 2.5 mm from the distributor. The measuring range of the pressure sensor (CYG1219, China) is 0-5 kPa, with a relative accuracy of ± 0.25% in the full scale. Besides, for the measurement of the variations of bed axial temperature distribution, six red water thermometers were amounted at the side wall with heights of 30 mm, 70 mm, 110 mm, 150 mm, 190 mm and 230 mm, respectively. The radial insertion depth was 4 cm for each thermometer. Table 1. Properties of the petroleum ether Parameter

Value



a

b

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Composition (wt%)

1-hexane (25.7)a，iso-pentane (74.3)a

Boiling range (℃)

60-90

Density (kg/m3)

653

Temperature (℃)

25

Surface tension (N/m)

0.0497b

Contact angle on bed material (°)

18.2b

The composition of petroleum ether was measured by gas chromatography. The surface tension and contact angle on bed material were measured by video-based, contact angle

measuring device (OCA 20, Germany, Dataphysics)

2.2 Experimental procedure In each experiment, polyethylene wax/graphite particles with a total mass of 1460 g were firstly fed into the bed and fluidized with atmospheric air. Heat exchanger was used to heat up the inlet fluidizing gas and preheat the bed to a set temperature, 25 ℃. The fluidizing gas was then switched to nitrogen for the sake of safety because the liquid injected is explosive and flammable. The bed pressure drop was simultaneously recorded. 100 s later, the electromagnetic system was turned on and the bed temperature distribution was recorded every 30 s. Particles inside the bed quickly released heat, leading to the increase of bed temperature. The bed was preheated for 180 s before liquid injection so as to avoid the quick defluidization of bed caused by instant injection of liquid especially when the liquid flowrate is very high. The liquid flowrate varied in the range of 0-110 ml/min and the induction heating power changed in the range of 400-500 W. The liquid injecting and electromagnetic induction durations were 720 s and 900 s, respectively. After the electromagnetic induction heating system was turned off and liquid injection simultaneously stopped, the fluidizing gas was quickly shut down and plastic wrap was used to seal the entrance and exit point of the reactor (reducing the interference caused by liquid evaporation). The whole bed was weighed and materials inside were subsequently taken out to desiccate. After desiccation, the bed and materials were weighed again to calculate the mass fraction of liquid accumulated in the bed. As wet agglomerates easily broke up after the evaporation of liquid within them, only solid agglomerates were sampled after each experiment. As the particle size distribution of bed materials was in the range of 0.3-1.2 mm, we took particles larger than 1.2 mm as agglomerates. The large and obvious agglomerates were firstly picked up from the materials. The small


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agglomerates were gently classified by a sieve plate with a mesh of 1.2 mm. The sampled agglomerates were then weighed and their sizes and morphologies were characterized by Image analysis method.23 Table 2. Experimental procedure Time

Events

0s

Switching the gas to nitrogen and recording bed pressure drop

100 s

Turning on the induction heating system and recording bed axial temperature distribution

280 s

Injecting liquid

1000 s

Turning off the induction heating system and taking out the bed material for further characterization

2.3 Evaluation of induction heating capacity Figure 4 presents the variations of axial average temperature of bed during induction heating without liquid injection. The bed temperature reached 57.5 ℃, 62 ℃ and 68.5 ℃ after 1000 s of heating when the induction heating power was set as 400 W, 450 W and 500 W, respectively. The final bed temperature increased with the increase of induction heating power. The standard deviation of axially-averaged temperature was no more than 0.7 ℃ for a given induction heating power which demonstrated the uniformity of the axial temperature distribution of bed. Figure 5 shows the variations of the mass fraction of solid agglomerates formed with different heating durations. It can be found that no agglomerates were formed when the induction heating time was shorter than 3 minutes. However, with the increase of induction heating time, the amount of solid agglomerates increased. The higher the induction heating power, the larger the amount of solid agglomerates and the quicker it increased. Combined with Figure 4 and Figure 5, it can be concluded that the solid bridge force did work under the experimental conditions.



70

Ps=400 W

65

Ps=450 W

60

Ps=500 W

Tb (℃）

55 50 45 40 35 30 25 20 0

200

400

600

800

1000

1200

1400

t (s)

Figure 4. Variations of the axial average temperature of bed under different induction heating power 0.05

Ps=400 W Ps=450 W

0.04

Ps=500 W

0.03

χagg-s

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0.02

0.01

0.00 0

200

400

600

800

1000

1200

1400

t (s)

Figure 5 Variations of the mass fraction of solid agglomerates with heating duration under different induction heating power

In this work, we designed an induction heating fluidized bed and fabricated polyethylene wax/graphite composites to mimic the exothermic process and sticky characteristic of particles in industrial fluidized beds. Polyethylene wax on the surface of graphite particles became sticky when heated, thereby formed solid bridge between particles during contact. Liquid injected into the fluidized bed adhered to the particles and induced liquid bridge between particles after collision. Thus, two different interparticle forces simultaneously existed in the fluidized bed. By changing the liquid content in the bed through the adjustment of liquid flowrate, the liquid bridge force and solid bridge force were correspondingly changed. The sampled solid agglomerates and wet agglomerates deduced from the bed pressure drop were used to verify the proposed agglomeration mechanism. Moreover, the effects of induction heating power and location of liquid injection were also investigated. .

3. Results and discussion ACS Paragon Plus Environment

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3.1 Characterization of agglomeration behavior As mentioned in the introduction, the liquid bridge force and solid bridge force both control the evolution of particle to agglomerate in the fluidized beds with liquid injection. But there are obvious differences between the agglomeration mechanisms and agglomerate properties. Liquid bridge is the basis of liquid bridge force. The liquid volume between particles determines the strength of liquid bridge force. With the decrease of liquid volume, the liquid bridge force gradually weakens till zero and the formed wet agglomerates break up into particles. Therefore, the wet agglomerates are reversible. As the liquid used in the industrial olefin polymerization process or cold mode experiment is volatile, agglomerate easily breaks up with the evaporation of liquid within it. It is difficult to sample the wet agglomerates in the experiments. With regard to the solid bridge force, the formation of solid bridge is the basis of its action. If the particle surface temperature exceeds the softening temperature of polyethylene wax, particles will bond to each other due to the “viscous flow” of polymer chains.25 Once formed, the solid bridge is hard to break unless exerted on strong shear forces. The longer it exists in the bed, the stronger its mechanical strength. Moreover, the fusion and nucleation of particles inside the agglomerates further intensifies the structural strength of agglomerates. Therefore, the solid agglomerates can be sampled by sieving the bed materials after each experiment. In the following, we classified the agglomerates caused by solid bridge force and liquid bridge force with “solid agglomerates” and “wet agglomerates”, respectively.

3.1.1 Solid agglomerates Figure 6 presents the variation of the mass fraction of sampled solid agglomerates χagg-s with increasing liquid injection flowrate. The mass fraction of solid agglomerates decreased from 1.380% to 0.0376% with liquid injection flowrate increasing from 0 ml/min to 33 ml/min. Further increasing the liquid injection rate to 88 ml/min, the mass fraction of solid agglomerates fluctuated in the range of 0.0616%-0.23%. As the liquid injection rate exceeded 88 ml/min, the mass fraction of solid agglomerates sharply increased. Figure 7 presents the snapshots of solid agglomerates sampled after the experiments operating under different liquid injection flowrates. Without liquid injection, lots of solid agglomerates were formed. But as the liquid injection flowrate was set as 44 ml/min, few small solid agglomerates were formed in the reactor, whereas as the liquid injection flowrate increased to 110 ml/min, lots of large agglomerates were formed



0.014 0.012 0.010

χagg-s

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.008 0.006 0.004 0.002 0.000 0

20

40

60

80

100

ql (ml/min) Figure 6. Variation of the mass fraction of solid agglomerates with liquid injection flowrate (Ps=500 W)

(a) ql =0 ml/min

(b) ql =44 ml/min

(c) ql =110 ml/min

Figure 7. Snapshots of the solid agglomerates sampled in the experiments (Ps=500 W)

The formation of the solid agglomerates is directly dependent on the particle surface temperature. Figure 8 presents the variation of the axial average temperature of bed with liquid injection flowrate. Without liquid injection, the bed temperature rose to 63.4 ℃during induction heating as shown in Figure 3. Solid bridge was easily formed during particle collision, thereby the mass fraction of solid agglomerates was high. When the liquid injection rate was set as 44 ml/min, the bed temperature decreased to 32 ℃ because of the heat removal caused by liquid evaporation. As a result, the solid bridge force was weakened significantly and the agglomeration process was inhibited. Therefore, the mass fraction of solid agglomerates decreased obviously. Further increasing the liquid injection rate to 110 ml/min, the bed temperature decreased to 5 ℃ . Unexpectedly, the mass fraction of solid agglomerates increased significantly, which triggers us to think about the agglomeration mechanisms.


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70

60

50

Tb-avg (℃)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


40

30

20

10

0 0

20

40

60

80

100

ql (ml/min)

Figure 8. Variation of the axial average temperature of bed with liquid injection flowrate (Ps=500 W)

3.1.2 Wet agglomerates Correspondingly, the mass fraction of wet agglomerates formed in the bed under different liquid injection flowrates was also studied. As wet agglomerates are easily broken and loose textured, it was hard to sample and weight them. But bed pressure drop can be quantitatively correlated with the mass of bed materials, thereby calculating the amount of agglomerates formed in the bed.26 In stable fluidization, the relationship between the measured value of bed pressure drop ∆Pm and the theoretical value ∆Pc can be expressed by equation (1)

∆Pm = κ∆Pc = κ

mp g

(1)

A

Where, κ is the ratio of ∆Pm to ∆Pc, mp is the mass of bed material, g is the acceleration of gravity,

A is the cross-sectional area of the fluidized bed. Base on the assumption that agglomerates do not fluidize, the measured pressure drop ∆Pm,L and the theoretical value ∆Pc,L with liquid injection can be correlated by equation (2)

∆Pm, L = κ∆Pc, L = κ

mp (1 + χ L ) − magg

A

g

(2)

Where, χL is the mass fraction of liquid, χL=ml/mp; ml is the mass of liquid in the bed, magg is the mass of agglomerates in the bed. At the end of liquid injection, the amount of agglomerates in the bed can be calculated by equation (3)



magg =m p (1 + χ L )-

m p ∆Pm, L , f

(3)

∆Pm

Where, ∆Pm,L,f is the final bed pressure drop. As both solid agglomerates and wet agglomerates contributed to the decrease of bed pressure drop, the amount of wet agglomerates can be described by Equation (4)

magg =magg -w − magg -s

(4)

χ agg -w =magg -w / m p 1400 1200 1000

ql=110 ml/min

Injecting liquid

ql=88 ml/min

Injecting liquid

ql=66 ml/min

Injecting liquid

ql=44 ml/min

Injecting liquid

1400 1300 1200 1100 1400

∆Pt (Pa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1300 1200 1400 1300 1200 1400 1300

ql=22 ml/min

1200

Injecting liquid

1400 1300

ql=0 ml/min

1200 0

200

400

t (s)

600

800

1000

Figure 9. Variations of bed pressure drop with time under different liquid injection flowrates (Ps=500 W)

Figure 9 presents the variation of bed pressure drop with liquid injection flowrate. It can be seen that the decrease of bed pressure drop firstly decreased and then increased with the increase of liquid injection flowrate. Figure 10 presents the variation of the amount of liquid in the bed with liquid injection flowrate. No liquid accumulated in the bed when the liquid injection flowrate was below 22 ml/min. With increasing liquid injection flowrate, the amount of liquid accumulated in the bed quickly increased. Combined the variations of bed pressure drop and the amount of liquid in the bed, the mass of agglomerates formed can be calculated by Equation (3) and Equation (4)


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0.06

0.05

χL

0.04

0.03

0.02

0.01

0.00 0

20

40

60

80

100

ql (ml/min)

Figure 10. Variation of the mass fraction of liquid in the bed with liquid injection flowrate (Ps=500 W)

Figure 11 presents the variation of the mass fraction of wet agglomerates with liquid injection flowrate. It can be seen that no wet agglomerates were formed when the liquid injection flowrate was below 22 ml/min. With the increase of liquid injection flowrates, the mass fraction of wet significantly increased. The accumulation of liquid in the bed promoted the formation of wet agglomerates. As shown in Figure 10, the liquid bridge force became stronger with more and more liquid accumulated in the bed, thereby the mass fraction of wet agglomerates formed quickly increased. 0.16

0.12

χagg-w

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.08

0.04

0.00 0

20

40

60

80

100

ql (ml/min) Figure 11. Variation of the mass fraction of wet agglomerates with liquid injection flowrate (Ps=500 W)

From the results shown in Figure 6 and Figure 11, it can be concluded that the variation tendencies of the mass fraction of solid agglomerates and wet agglomerates with increasing liquid injection flowrate were significantly different. With the increase of liquid injection flowrate, the amount of liquid accumulated in the bed increased which enhanced the liquid bridge force between particles, thereby the amount of wet agglomerates formed increased. As shown in Figure 8, the bed temperature decreased with increasing liquid injection flowrate. The solid bridge force



between particles was supposed to be weakened and the amount of solid agglomerates formed decreased. However, the amount of solid agglomerates fluctuated at a lower level with the liquid injection flowrate ranging from 44 ml/min to 88 ml/min. The mass fraction of solid agglomerates sharply increased when the liquid injection flowrate was higher than 88 ml/min. This phenomenon aroused us to think about the reasons why so many solid agglomerates formed when the bed temperature was much lower than the softening temperature of particles. Based on the formation mechanism of solid bridge, it can be concluded that particle surface temperature inside the local areas especially “local dead zones” in the bed must reach or exceed the softening temperature. As the temperature probes used in the experiments can only measure the overall temperature of particles and gas phase. It was hard to measure the surface temperature of particles within the “local dead zones” unless the temperature probes were coincidentally inside them. Therefore, we proposed that with more and more wet agglomerates formed and segregated on the distributor with increasing liquid injection flowrate, the formation of local dead zones occurred. Heat quickly accumulated in the local dead zone and led to the occurrence of “hot spot”. As particles inside the local dead zone were almost stagnant and very close to each other, the solid bridge between particles quickly grew, leading to the formation of solid agglomerates. In the following, the overall bed hydrodynamics was firstly investigated to prove the occurrence of unstable fluidization under high liquid injection flowrate. Then temperature oscillation reflected from the bed axial temperature distribution was used to validate the occurrence of “hot spot” in the local dead zone.

3.2 Overall bed hydrodynamics 3.2.1 Bed pressure drop The bed pressure drops under different liquid injection flowrates were shown in Figure 9. Corresponding to the experimental procedures described in Table 2, the bed pressure drop fluctuated around 1360 Pa in the beginning. After the electromagnetic induction heating started, it gradually decreased. As bed temperature increased with the quick release of heat from particles, polyethylene wax coated on the surface of graphite particles became softening which resulted in the adhesion of particles. The minimum fluidization velocity increased. On the other hand, solid bridge formed during the contact of particle collision. Particles bonded to each other and agglomerates were formed. As the sizes of agglomerates were larger, they were hard to be


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fluidized. After generated, they fell onto the distributor and segregated. Therefore, the bed pressure drop decreased during the process of induction heating. As the liquid injection flowrate varied in the range of 22-44 ml/min, the bed pressure drop first decreased slowly and fluctuated around the mean value. Previous studies show that agglomerates were easily formed near the nozzle due to the direct contact of injected liquid with fluidized particles.27 With the evaporation of liquid, the weak wet agglomerates may be broken by the fluid drag force or bubble expansion force28-29, whereas the larger and stronger agglomerates accumulated on the distributor. When the break-up of wet agglomerate came to equilibrium with its formation, the amount of wet agglomerates in bed no longer increased. The fluidization behavior of particles was considered as agglomerating fluidization. Further increasing the liquid injection flowrate (ranging from 66 ml/min to 110 ml/min), the amplitude of bed pressure fluctuation increased and the bed pressure drop decreased, which were indicative of a portion of particles being unsupported and thus the formation of deadzones. Referring to the results shown in Figure 10 and Figure 11, the increase of the amount of liquid accumulated in the bed led to the increase of liquid volume between particles and the enhancement of liquid bridge force. The equilibrium of agglomerate formation and breakage was broken. Wet agglomerates quickly accumulated on the distributor, further resulting in the formation of “local dead zone” and the blocking of distributor. Therefore, the bed quickly defluidized when the liquid injection flowrate reached 110 ml/min,

3.2.2 Axial temperature distribution 50

50

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h=3 cm h=7cm h=11 cm h=15 cm h=19 cm h=23 cm

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20 10

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-10 -20

20 0

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400

600

800

0

200

400

600

800

t (s)

t (s)

(c) ql =66 ml/min

(d) ql =110 ml/min

Figure 12. Axial temperature distributions of bed under different liquid injection flowrates (Ps=500 W)

Figure 12 shows the axial temperature distributions of bed under different liquid injection flowrates. With liquid injection flowrate no more than 22 ml/min, the bed temperature firstly kept increasing and then remained stable with liquid injected into the bed, as shown in Figure 12 (a). As the heat releasing rate of particles was higher than the heat removal rate caused by liquid evaporation, the bed temperature firstly kept increasing and remained stable when the reactor achieved heat balance. Further increasing the liquid injection flowrate, the bed temperature continuously decreased and oscillated, as shown in Figure 12 (b) and 12 (c). Heat produced by particles were not enough for the evaporation of liquid, thereby the particle temperature decreased. As bed temperature distribution can reflect the mixing behavior of particles, the increase of axial temperature difference and temperature oscillation demonstrated that the fluidization homogeneity of bed greatly reduced. Liquid accumulation in the bed led to the action of liquid bridge force and the formation of wet agglomerates, which further resulted in the occurrence of local dead zones. In extreme case, as the liquid injection rate was too high, liquid quickly accumulated in the reactor and resulted in the formation of massive agglomerates and deterioration of fluidization quality, thereby the axial distribution of bed temperature oscillated significantly, as shown in Figure 12 (d). The occurrence of local dead zone reduced the heat transfer efficiency between particles and fluidization gas, thereby the surface temperature of particles within them quickly increased. With the breakage of agglomerates, the local dead zone gradually disappeared and particle surface temperature quickly decreased owing to the heat removal caused by convection with fluidization gas and liquid evaporation. Temperature oscillation provided evidence of the occurrence of “hot spot”.


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3.3 The action order of liquid bridge force and solid bridge force Through the investigation of overall bed hydrodynamics, the agglomeration mechanisms were investigated from a macro-scale perspective. In this section, we studied the action order of liquid bridge force and solid bridge force through micro-scale experiments. Polarizing microscope with hot stage was used to directly observe the liquid bridge and solid bridge between particles during heating. In the experiments, two polyethylene wax particles were firstly placed on the hot stage with their relative position adjusted. Then the liquid was injected into the gap between particles through syringe. The liquid used was the mixture of water and red ink. Red ink could change the luminousness of water, which is beneficial to distinguish the particles and liquid. During heating, the snapshots of liquid bridge and solid bridge were taken, as shown in Figure 13. The black particle was polyethylene wax while the light red background was liquid. When the temperature of hot stage reached 30 ℃, it is clear to find that particles infiltrated in the liquid, as shown in Figure 13 (a). With increasing the temperature of hot stage, liquid evaporated and the remaining liquid gathered around the particles and the size of liquid bridge got larger, as shown in Figure 13 (b). When the temperature of hot stage reached 77 ℃, the liquid bridge shrank and the gap between particles got smaller. Particles became softening and the solid bridge formed, as shown in Figure 13 (c). Further increasing the temperature, the liquid bridge disappeared and the size of solid bridge got larger, as shown in Figure 13 (d).

(a) Ts=30 ℃

(b) Ts=70 ℃



(c) Ts=77 ℃

(d) Ts=90 ℃

Figure 13. Snapshots of the liquid bridge and solid bridge between particles under different temperatures of hot stage

From the results present in Figure 13, it can be concluded that liquid bridge were formed when the wet particles contacted with each other. If the particle was heated or continuously released heat, the liquid bridge shrank and the gap between particles got smaller due to the liquid evaporation. When the particle surface temperature reached its softening temperature, solid bridge was formed between particles under the action of viscous flow of polymer chains. With regard to the fluidized bed reactors, with liquid evaporating and liquid bridge shrinking, particles within wet agglomerates will bond to each other if their surface temperature reached the softening temperature. Thus, particles inside the “local dead zone” easily transformed into agglomerates. Liquid bridge force promoted the formation of wet agglomerates and “local dead zone”, while solid bridge force transferred particles into solid agglomerates. They cooperatively controlled the agglomeration behaviors in the fluidized beds with liquid injection. In summary, the agglomeration mechanisms in the fluidized beds with liquid injection can be divided into three stages, as shown in Figure 14. Without liquid injection, solid bridge force was dominant in the agglomeration process. A large amount of solid agglomerates were formed by the solid bridge force. With increasing liquid injection flowrate, the bed temperature decreased thus the solid bridge force was weakened and few solid agglomerates formed in the bed, which can be described as the ① stage. Further increasing the liquid injection rate, the solid bridge force kept decreasing and accumulation of liquid led to the intensification of liquid bridge force. The amount of wet agglomerates increased with liquid injection flowrate. However, as wet agglomerates are reversible, fluid drag force and bubble expansion force could break them into particles. The bed could be well fluidized when the breakage and formation rate of wet agglomerates reach equilibrium. Therefore, local dead zone seldom occurred in the bed, which can be described as the ② stage. Under even higher liquid injection flowrate, lots of wet agglomerates formed in the bed, leading to the formation of “local dead zone” and deterioration of fluidization quality. Hot spot occurred inside the local dead zone. Particle surface temperature exceeded its softening temperature, leading to the formation of solid agglomerates. Therefore, both the amount of wet agglomerates and solid agglomerates increased, which can be described as the ③ stage.


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Page 21 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 14. The schematic diagram of agglomeration mechanisms

3.4 The effects of operating variables 3.4.1 Effect of induction heating power. The induction heating power directly determined the heat releasing rate of particle, the particle surface temperature and the heat transfer efficiency between gas phase and emulsion phase, thereby influenced the evaporation rate of liquid, the mass fraction of liquid in the bed and heat accumulation in “local dead zone” which finally resulted in the differences in the evolution of particles to agglomerates. Figure 15 presents the variation of the mass fraction of solid agglomerates with different liquid injection flowrate. Similar to the result shown in Figure 6, the amount of solid agglomerates firstly decreased and then increased with increasing liquid injection flowrate. Under higher induction heating rate, the mass fraction of solid agglomerates was higher without liquid injection. As the evaporation capacity of the bed increased with induction heating power, less liquid accumulated in the bed under higher induction heating power. Fewer wet agglomerates formed under the same liquid injection flowrate. Therefore, the mass fraction of solid agglomerates induced by wet agglomerates was lower. In this work, we set the mass fraction of solid agglomerates lower than 0.3% as the standard of safe operation of the reactor. Compared with Figure 6, it can be demonstrated that the stable operating window (the length of line segment AB) was wider under higher induction heating



power. The match of liquid injection flowrate and heat releasing rate is critical to control the bed temperature and liquid accumulation. In addition, Figure 16 and Figure 17 show the snapshots of solid agglomerates sampled in the experiments. The variation tendency of agglomerate size was similar with the results shown in Figure 10, which verified the reproducibility of the experiments. 0.014

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0.012

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ql (ml/min)

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(a) Ps=400 W

(b) Ps=450 W

Figure 15. Variations of the mass fraction of solid agglomerates with liquid injection flowrate

(a) ql =0 ml/min

(b) ql =22 ml/min

(c) ql =88 ml/min


(a) ql =0 ml/min

(b) ql =44 ml/min

(c) ql =88 ml/min


3.4.2 Effect of the liquid injection position. Figure 18 presents the variations of the mass fraction of solid agglomerates with different liquid injection flowrates and positions. As the liquid


Page 23 of 30

injection position hnozzle decreased from 15 cm to 3 cm, the mass fraction of solid agglomerates was always above 0.3% with increasing liquid injection flowrate, as shown in Figure 18 (a). The addition of liquid could not reduce the formation of solid agglomerates. However, when the liquid injection position hnozzle increased to 23 cm, the stable operation window of liquid injection rate (the length of line segment AB) was much wider. Therefore, from the results presented in Figure 9 and Figure 18, it can be concluded that increasing liquid injection position was beneficial to avoid agglomeration and widen the stable operation window. 0.020 0.035 0.030

0.015 0.025 0.020

χagg-s

χagg-s

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0.015 0.010

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Figure 18. Variations of the mass fraction of solid agglomerates with liquid injection flowrate (Ps=500 W)

Previous studies revealed that most wet agglomerates formed upon the direct contact of liquid with particles closed to the nozzle.27 The formation of wet agglomerates is greatly influenced by the initial liquid-solid contact efficiency which is determined by the operation conditions, spray nozzle design and liquid injection position.[30-31] The liquid injection position influenced the distance and time of wet agglomerates falling to the distributor, thus affected the accumulation of them. In the process of falling, the wet agglomerates would be broken by the bubble expansion force, the drag force or liquid evaporation. As the liquid injection position was far away from the distributor, the breakage of wet agglomerates was significant and few agglomerates survived and accumulated on the distributor. Thus, “local dead zone” seldom occurred under the same liquid injection flowrate. If the liquid injection position was closed to the distributor, wet agglomerates quickly accumulated, leading to the formation of “local dead zone”. Therefore, the mass fraction of solid agglomerates was higher. Figure 19 and Figure 20 present the axial distributions of bed temperature under different liquid injection flowrates and different liquid injection positions. It can be seen that the bed temperature oscillated remarkably especially for the



zone near to the distributor when the liquid injection position hnozzle was 3 cm, whereas no obvious temperature fluctuation occurred under the same liquid injection rate when the liquid injection positions hnozzle were 15 cm and 23 cm respectively. Certainly, with increasing liquid injection flowrate, the fluctuation of bed temperature would occur. As the axial distribution of bed temperature could reflect the occurrence of “local dead zone, it can be used to verified the

50

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Tb（ ℃)

Tb（ ℃)

elucidation above.

30


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400

600

800

0

200

400

t (s)

600

800

t (s)

(a) ql =22 ml/min

(b) ql =33 ml/min

Figure 19. Axial temperature distributions of bed under different liquid injection flowrates (Ps=500 W, hnozzle=3 cm) h=3 cm h=7 cm h=11 cm h=15 cm h=19 cm h=23 cm

50 50

40


30

Tb（ ℃)

30 40

Tb（ ℃)

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20 10 0 -10

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800

-20 0

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t (s)

400

600

800

t (s)

(a) ql =22 ml/min

(b) ql =121 ml/min

Figure 20. Axial temperature distributions of bed temperature under different liquid injection flowrates (Ps=500 W, hnozzle=23 cm)

4. Conclusions A cold mode fluidized bed with side wall liquid injection which was designed based on the principle of electromagnetic induction heating can be used to mimic the exothermic process of particles in industrial fluidized beds and established the foundations of investigating the


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agglomeration behaviors controlled by the liquid bridge force and solid bridge force. The effects of liquid on agglomeration behaviors were investigated through the characterization of agglomerate size and weight. Bed pressure drop and axial temperature distribution were used to validate the occurrence of local dead zone and hot spot. Micro-scale experiments based on visual observation of the liquid bridge and solid bridge between two particles being heated illustrated the action order of the two interparticle forces. In the last, the agglomeration mechanisms were proposed. Results revealed that the agglomeration behaviors experienced three stages, which was dominated by solid bridge force, liquid bridge force and the cooperative action of liquid bridge force and solid bridge force, respectively. In the third stage, a large number of wet agglomerates caused by liquid bridge force induced the formation of “local dead zones” and provided a beneficial environment for the occurrence of hot spot and growth of solid bridge. Particle agglomerates were then formed by the solid bridge force. In addition, through the investigation of the effects of operation variables, it was found that increasing liquid injection position can inhibit the formation of agglomerates and widen the safe operation window of liquid injection flowrate. The operation range of liquid injection flowrates is wider under higher induction heating powers. As to the industrial operation, like olefin polymerization fluidized beds operated in condensed mode, solid agglomerates will be formed if the liquid injection flowrate is very high. In the product grade transition, especially for the transition of gas phase polymerization to condensed mode, momentary injection of liquid induces great disturbance of bed hydrodynamic, thereby results in the formation of solid agglomerates. Safe control of the liquid injection flowrate and improve the way of liquid injection are crucial to maintain the stable operation of industrial fluidized beds.

Acknowledgements The work was supported by the financial support provided by the Project of National Natural Science Foundation of China (91434205), the National Natural Science Foundation of China (21406194), the National Science Fund for Distinguished Young (21525627), the Natural Science Foundation of Zhejiang Province (Grant No. LR14B060001), the Specialized Research Fund for the Doctoral Program of Higher Education of China (20130101110063).



Notations umf = Minimum fluidization velocity (m/s) Ps = Induction heating power (W) ∆Pt = Bed pressure drop (Pa) magg-s = Mass of solid agglomerates (kg) magg-w = Mass of wet agglomerates (kg) mp = Mass of bed materials (kg)

χagg-s = Mass fraction of solid agglomerates χagg-w = Mass fraction of wet agglomerates χL = Mass fraction of liquid in the bed ql = Liquid injection flowrate (ml/min) hnozzle = Liquid injection position (cm) Tb-avg = Axial average temperature of bed (℃) Tb = Axial temperature of the bed (℃) Ts = Temperature of the hot-stage (℃) ∆Pc = Theoretical value of bed pressure drop (Pa) ∆Pm = Measured pressure drop (Pa) ∆Pm,L = Measured bed pressure drop with liquid injection (Pa) ∆Pc,L = Theoretical value of bed pressure drop with liquid injection (Pa) ∆Pm,L,f = Final bed pressure drop with liquid injection (Pa)

κ = Ratio of ∆Pm to ∆Pc g = Acceleration of gravity (m/s2) A = Cross-sectional area of the fluidized bed (m2)

References (1) McDougall, S.; Saberian, M.; Briens, C.; Berruti, F.; Chan, E. Effect of liquid properties on the agglomerating tendency of a wet gas-solid fluidized bed. Powder Technol. 2005, 149, 61-67. (2) Horio, M. Binderless granulation—its potential, achievements and future issues. Powder

Technol. 2003, 130, 1-7. (3) Roy, P.; Khanna, R.; Subbarao, D. Granulation time in fluidized bed granulators. Powder


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2014. (5) Girardi, M.; Radl, S.; Sundaresan, S. Simulating wet gas-solid fluidized beds using coarse-grid CFD-DEM. Chem. Eng. Sci. 2016, 144, 224-238. (6) Morales, C. B.; Jamaleddine, T. J.; Berruti, F.; McMillan, J.; Briens, C. Low-temperature experimental model of liquid injection and reaction in a fluidized bed. Can. J. Chem. Eng. 2016,

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Powder Technol. 1991, 65 (1-3), 257-272. (15) Liu, L. X.; Litster, J. D.; Iveson, S. M.; Ennis, B. J. Coalescence of deformable granules in wet granulation processes. AIChE J. 2000, 46, 529-539. (16) Cryer, S. A. Modeling agglomeration processes in fluid-bed granulation. AIChE J. 1999, 45,



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Eng. Chem. Res. 2015, 54, 12177-12186. (24) Latifi, M.; Chaouki, J. A novel induction heating fluidized bed reactor: Its design and applications in high temperature screening tests with solid feedstocks and prediction of defluidization state. AIChE J. 2015, 61, 1507-1523. (25) Chmelar, J.; Matuska, P.; Gregor, T.; Bobak, M.; Fantinel, F.; Kosek, J. Softening of polyethylene powders at reactor conditions. Chem Eng J. 2013, 228, 907-916. (26) Lettieri, P.; Newton, D.; Yates, J. G. High temperature effects on the dense phase properties of

gas fluidized beds. Powder Technol. 2001, 120, 34-40. (27) Leach, A.; Portoghese, F.; Briens, C.; Berruti, F. A new and rapid method for the evaluation of the liquid-solid contact resulting from liquid injection into a fluidized bed. Powder Technol. 2008,

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For Table of Content Only

0.014

Solid agglomerates 0.012

Hot spot

ef dg bri

0.010

orc

0.008

e

χ agg-s

Wet agglomerates

lid So

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0.006

uid Liq

bri

e dg

c for

e

0.004 0.002 0.000 0

20

40

60

80

100

q (ml/min) l


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Investigating Agglomeration Behaviors in High Temperature Gas

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