A Novel Mixer with a Hollow Spiral Structure for Preparing Inorganic ...

Research Paper

Journal of Chemical Engineering of Japan, Vol. 49, No. 6, pp. 503–510, 2016

A Novel Mixer with a Hollow Spiral Structure for Preparing Inorganic Solidified Foam Chao Zhu1, Botao Qin1,2, Yi Lu1, Fanglei Li 1, Yuwei Jia1 and Quanlin Shi 1 Faculty of Safety Engineering, China University of Mining and Technology, Xuzhou, Jiangsu 221116, China 2 State Key Laboratory of Coal Resources and Mine Safety, China University of Mining and Technology, Xuzhou, Jiangsu 221008, China 1

Keywords: Spontaneous Coal Combustion, Mixer, Hollow Spiral Structure, Homogeneity, Expansion Ratio Plugging leaks and controlling winds are the most effective approaches for preventing coal/oxygen-compounded reactions and spontaneous combustion of coal. Inorganic solidified foam (ISF) is an effective material for preventing spontaneous coal combustion, and the mixer used in formulating ISF is important for improving the homogeneity and expansion ratio. In order to prepare an ISF with a good performance, a novel mixer was designed for mixing aqueous foam with composite slurry. The mixer had a hollow spiral structure that reduced the breakdown of the aqueous foam and increased the foam slurry contact area. We analyzed the mixing mechanism and the process by which the composite slurry particles combined with the aqueous foam to form ISF. We found that there was an optimal relationship between the rotational speed of the mixer, homogeneity, and expansion ratio. The performance of the mixer was investigated experimentally using various rotational speeds and aqueous foam flow ratios. Experimental performance testing and evaluation showed that for different aqueous foam flow ratios, and for the rotational speeds of 100 rpm or 150 rpm, the relative standard deviation of the pore area was minimal and exhibited good homogeneity. Simultaneously, the value of the expansion ratio reached a maximum, and the breakage of aqueous foam was minimal. The results of this experiment provide the basis for coal mine field applications for ISF.

Introduction The spontaneous combustion of coal is one of the major hazards encountered in the coal mines of China. It seriously threatens the safety of the coal industry, greatly threatens the life and property of workers, affects society and the economic benefits of coal mines, and leads to significant losses of life and property (Wang, 1990; Luo and Yuntao, 2003; Colaizzi, 2004; Liu and Zhou, 2010). Inorganic solidified foam (ISF) is an effective material used for preventing spontaneous coal combustion. ISF has desirable characteristics, such as a wide coverage area, easy accumulation, heat insulation, fire resistance, lightweight, superior fluidity, and others (Qin and Lu, 2013; Qin et al., 2015). Therefore, ISF has the potential for broad applications in underground coal mines. Because homogeneity and the expansion ratio are important characteristics in the coal mine field application of ISF, the mixer used in the formulation of ISF is crucial. The expansion ratio of ISF reflects its cost effectiveness, and the strength and durability properties of ISF are determined by its homogeneity. Because ISF is a porous material, its pore structure is characterized by its porosity, permeability, and pore size distribution, which are determined by the mixer Received on January 4, 2015; accepted on September 17, 2015 DOI: 10.1252/jcej.14we416 Correspondence concerning this article should be addressed to B. Qin (E-mail address: [email protected]). Vol. 49  No.©6 2016  Copyright 2016The Society of Chemical Engineers, Japan

(Narayanan and Ramamurthy, 2000; Kunhanandan Nambiar and Ramamurthy, 2007; Lian et al., 2011; Panesar, 2013). The ISF is produced either by using a prefoaming method or a mixed foaming method. Preformed foaming is preferred to the mix-forming technique. The prefoaming method produces composite slurry and stable preformed aqueous foam separately. It then thoroughly blends foam in a mixer into a composite slurry (Ramamurthy et al., 2009). Most common types of mixers, such as the tilt drum, pan mixer, or mortar, are used in formulating foam slurry (Tonyan and Gibson, 1992; Karl and Worner, 1993). They all have characteristics of high-mixing efficiency. However, aqueous foam is an unstable thermodynamic system, and it will gradually burst after preparation. These mixers combine the slurry and aqueous foam through one direct mixing, a process that is liable to the acceleration of the breakage of aqueous foam. According to published statistics (Lu, 2015), the broken rate of aqueous foam is more than 40%. Therefore, such technologies are not suitable for mixing aqueous foam with composite slurry. In order to reduce the broken rate of aqueous foam and produce ISF with a good performance, we changed the conventional one-time, direct-mixing approach, using a step-by-step mixing process in which the aqueous foam was added from an inner mixer to maximize the foam contact area and reduce the rate of broken aqueous foam. Based on this idea, a novel mixer with a hollow spiral structure was designed, and its mixing performance was investigated and 503

Fig. 1 Photograph of the novel mixer

Fig. 2 Schematic depicting the structure of the novel mixer

evaluated under different rotational mixer speeds and aqueous foam flow ratios.

1. Design of the Novel Mixer Figure 1 is a photograph of the prototype novel mixer fabricated in this study, and Figure 2 is a schematic of the mixer. The mixer consists of a chamber with flanges welded at both ends and a hollow spiral pipe inside it. There are two openings in the chamber: one is the composite slurry inlet, and the other is the foam fluid outlet. In order to reduce breakage of the aqueous foam and improve the expansion ratio, we used a hollow steel tube to transport the aqueous foam. Helical blades and mixing blades were welded to the body of the hollow steel tube. Their role was to stir, mix, and transport the foam fluid. There are five aqueous foam outlets at mid-intervals between successive helical blades. Both ends of the hollow spiral pipe are equipped with bearings and a bearing pedestal. The left end of the hollow spiral pipe is connected to the aqueous foam pipe, and round steel welding was used on the right side of the hollow spiral pipe. A frequency modulation motor provided the impetus of the novel mixer, and was connected to the right side of the hollow spiral pipe. The direction of rotation is shown in Figure 2, whereby foam fluid was transported to the foam fluid outlet. It can realize continuous speed regulation. In the mixing process, aqueous foam produced by the foam generator flows into the mixer. The frequency modulation motor drive induces the rotation of the hollow spiral pipe, transferring aqueous foam from the aqueous foam outlets. The composite slurry flows into the mixer through the composite slurry inlet, mixing it with the aqueous foam via vortices generated by the helical blades of the hollow spiral pipe. Aqueous foam is thus added to the composite slurry step-by-step, which reduces the breakage of the aque504

ous foam and increases the foam-slurry contact area. This kind of mixing chamber can reduce the impact caused by the high energy of the aqueous foam and generate ISF with a good performance.

2. Experimental Procedures In this study, we varied the rotational speed of the mixer from 50 to 250 revolutions per min (rpm) in increments of 50 rpm, and the aqueous foam flow ratio from 5 to 8 m3/h in increments of 1 m3/h. We investigated the impact of these variables on ISF homogeneity and expansion ratio. 2.1 Raw materials The constituent materials used in this study are listed below: (1) Portland cement with a compressive strength of 64.5 MPa at 28 d, conforming to BS EN197-1 type I cement. (2) Fly ash with a median particle size of 35 µm and loss on ignition of 5.0%, conforming to BS EN450. The fly ash content was 40 wt%. 2.2 Experimental procedure (1) Composite slurry preparation: we first mixed cement and fly ash together. Water was injected into the blend and the mixture was stirred for 30 min, forming well-blended, composite slurry. The water/cement ratio was 0.5. The average density of the composite slurry, ρc, was then calculated. The range of ρc was 1,700–1,900 kg/m3. The flow rate of the composite slurry is denoted by qc. (2) Aqueous foam preparation: a foaming agent solution and high-pressure air flow through the foam generator were used to prepare the aqueous foam. A stopwatch was used to measure the time taken to fill the foam tank, which was calibrated for 25 L using a graduated cylinder, and to Journal of Chemical Engineering of Japan

Fig. 3 Experimental setup of custom-made aqueous foam outlet distributions

calculate the aqueous foam flow rate (qf ) and density ρf. The range of ρf was 50–80 kg/m3. (3) Composite slurry and aqueous foam viscosity test: information about the composite slurry and aqueous foam viscosity are very important for mixing. We used an NDJ-5s digital viscometer to test the composite slurry we prepared and its aqueous foam viscosity. (4) Aqueous foam outlet distributions test: information about the initial aqueous foam outlet distributions is very important for mixing. The aqueous foam pipe is connected to the foam generator outlet and the aqueous foam inlet of the hollow spiral pipe. Aqueous foam was generated by the foam generator and was introduced into the rotating hollow spiral pipe through the aqueous foam pipe. By varying the flow rates of the foaming agent solution and the highpressure air, we obtained different values for qf. We tested the flow rate of each aqueous foam outlet by changing its value from 5 to 8 m3/h in increments of 1 m3/h, and obtained aqueous foam outlet distributions, as shown in Figure 3. We varied the aqueous foam flow rate to control the initial aqueous foam distribution. (5) ISF preparation: ISF was prepared from the composite slurry and aqueous foam in the mixer by maintaining qc at 1 m3/h. The rotational speed of the mixer ranged from 50 to 250 rpm in increments of 50 rpm. The aqueous foam flow rate, qf, ranged from 5 to 8 m3/h in increments of 1 m3/h. Samples were taken at every rotational speed of the mixer, and at all aqueous foam flow rates used in the study. The sample mold dimensions were 100×100×100 mm3. The mixing chamber pressure for experimental tests was 0.32 MPa, and the aqueous foam outlet pressure required to transfer the aqueous foam into the outer flow region was 0.35 MPa. (6) Calculating the density ρF. and expansion ratio φ of fresh foam fluid: the expansion ratio is given as ρ φ= c ρF

(1)

where ρc is the average density of the composite slurry, and ρF is the density of fresh foam fluid. (7) Quantitative characterization of ISF homogeneity: the prepared ISF samples were cut to obtain smooth surfaces. A digital camera was then used to acquire images of the ISF cross-sections. Using the image processing software Vol. 49  No. 6  2016

Fig. 4 Variation of the viscosity of the composite slurry and aqueous foam

Image-Pro Plus 6.0 (Media Cybernetics), each digital image was transformed into a gray-scale image and converted into a binary form. The entire space was thereafter divided into 3×3 sub-areas, and each sub-area of pore size (Ai) was quantified using statistical analyses. An average area, Ā [µm2] was then obtained (Blatt et al., 2004; Chen et al., 2004; Nie et al., 2004; Isaacs et al., 2014; Tan et al., 2014). We used the relative standard deviation of the pore area to quantitatively characterize the homogeneity of the pore size distribution. The relative standard deviation of the pore area equation can be expressed as n

 (A − A) i

i=1

a0 =

2

n −1 A

(2)

where Ai is the total pore area of each sub-area, Ā is the average of Ai, and n is the number of sub-areas.

3. Results and Discussion 3.1 Composite slurry and aqueous foam viscosity The composite slurry and aqueous foam viscosity are shown in Figure 4. As shown in Figure 4, the viscosity trends of the composite slurry and aqueous foam are the same, and the rotational 505

Fig. 6 Schematic of composite slurry particles colliding with aqueous foam

Fig. 5 Distributions of the aqueous foam outlet flow rates

speeds of the rotor were 6, 12, 30, and 60 rpm. At this speed range, the viscosity decreased monotonically, a pattern characterized by shear thinning that revealed the nature of a pseudoplastic fluid. 3.2 Aqueous foam outlet distributions The flow rate of each aqueous foam outlet distributions is shown in Figure 5. As shown in this figure, the trend of each aqueous foam outlet flow rate is similar. The aqueous foam outlet flow rate is diminished gradually, and obeys a linear relationship as that is observed from results of the first to the fifth aqueous foam outlets. 3.3 Theoretical analysis of the novel mixer As shown in Figure 2, the aqueous foam produced by the foam generator flows into the hollow spiral pipe. The frequency modulation motor drive induces the rotation of the hollow spiral pipe, transferring aqueous foam from the outlets. The composite slurry flows into the mixer through the composite slurry inlet, mixing it with the aqueous foam. The fluid-mixing equations are governed by Navier–Stokes and the convection–diffusion equations (Bertrand et al., 1999; Iranshahi et al., 2006). Specifically, the Navier–Stokes equation can be expressed as  ∂v  ρ + v ⋅ grad v  = div 2η (| γ |) γ − grad P (3) t ∂  

(

)

Using the Lagrangian framework in which the observer is assumed to move with the helical blades, Eq. (3) can be rewritten in accordance to the following form, given that the flow is now geostrophic:  ∂v  ρ + v ⋅ grad v + ω × (ω × r )+ 2ω × v  t ∂  

(

)

= div 2η ( γ ) γ − grad P

∂c + v ⋅∇c = D ⋅∇2c ∂t

(4)

(5)

where c is the concentration, D is the molecular diffusion coefficient, and ∇ is the Hamiltonian operator. A violent vortex is generated when the composite slurry mixes with the aqueous foam, subjecting the fluid to a very strong shear force. This divides the fluid into microparts, further mixing the fluid. The vortex kinetic equation (Zalc et al., 2003) is given as ∂Ω + v ⋅∇Ω − (Ω + 2ω) ⋅∇v = u∇ 2Ω ∂t

(6)

where Ω represents the vorticity, v is the velocity vector, u the kinematic viscosity coefficient, ∇ the Hamiltonian operator, and ω the angular velocity vector. The mixing process for forming a foam slurry is considered to consist of three major subprocesses, namely, collision, attachment, and detachment (Huang et al., 2012; Moreno-Atanasio, 2013; Ren et al., 2014). Collision involves the approach of a composite slurry particle to the aqueous foam in the field of flow. The collision subprocess is governed by the liquid flow and the relative motion between the aqueous foam and the composite slurry particles in the mixer. Figure 6 shows a schematic of the composite slurry particles colliding with the aqueous foam. The quantification of the particle-bubble collisions requires solving the equation of particle motion around the bubble surface in order to calculate the aqueous foam–particle collision angle and efficiency. Traditionally, it starts with the Basset–Boussinesq– Oseen (BBO) equation, described as (Firouzi et al., 2011)

mp

where ρ represents the fluid density, P the pressure, γ̇ the rate of strain tensor defined as γ̇ =1/2(∇V+(∇V)T), ηγ̇ the generalized Newtonian viscosity, v=(vx, vy, vz) the velocity 506

in the x-, y-, and z-directions, ω=(ωx, ωy, ωz) the angular velocity, and r the radial coordinate. The two additional terms in Eq. (4), ω×(ω×r) and 2ω×v, represent the centrifugal acceleration and the Coriolis effects, respectively. The convection–diffusion equation is

dV DW = mf − 6πμRp (V − W) dt Dt m d(V − W) − f +(mp − mf )g 2 dt

(7)

Where V and W respectively describe the composite slurry particle and aqueous foam velocities, mp is the composite slurry particle mass, mf is the mass of liquid occupied by the Journal of Chemical Engineering of Japan

the axial velocity of the composite slurry particles, Vcxi. That is, V = VA + Vcx i = VS + VR + Vcx i sin λ 4q + c i sin(α + λ) πd22x

= ωr + ωr

sin λ 4q + c i  −1 p  πd22x sin  tan +λ 2πr  

= ωr + ωr

(11)

Combining Eqs. (7), (8), and (11), leads to 

Fig. 7 Mechanics of composite slurry particle action in the mixer

composite slurry particle, Rp is the composite slurry particle radius, μ is the liquid viscosity, g is the acceleration due to gravity, t is the reference time, and DW/Dt and d(V−W)/dt respectively describe the derivative operators (i.e., the operator that describes the motion following the liquid streamline) and the particle derivative operator (i.e., the operator that describes the motion following the particle trajectory). Aqueous foam can be described as a centrifugal case in which w is the tangential velocity of aqueous foam, and va is the axial velocity. The resultant velocity W is the vector addition of w and va. That is, W = w + v a = ωr1 + v a = ωr1 +

4qf πd12

(8)

where qf is the flow of aqueous foam, d1 is the diameter of the hollow spiral pipe, ω is angular velocity of the hollow spiral pipe, and r1 is the radius. Figure 7 shows the mechanics of the composite slurry particle action in the mixer. When the composite slurry flows into the mixer through the composite slurry inlet, Vc is the initial velocity of the composite slurry particles, VS is the tangential velocity of the screw at the radius considered, VR is the relative velocity of the particle with respect to the screw surface, and VA is the absolute velocity of the particle. The angle λ defines the direction of the absolute velocity (Yu, 1980; Roberts, 1999; Kumar et al., 2014). 4q Vc = c2 = Vcx i + Vcy j+ Vcz k πd2 =

4q c 4q 4q i + 2c j+ 2c k πd22x πd2 y πd2 z

(9)

where qc is the flow of the composite slurry, d2 is the diameter of the composite slurry pipe, and Vcxi is the axial velocity of the composite slurry particles. The helix angle α of the screw at the radius r is given by

α = tan −1

p 2πr

(10)

where p is the pitch. The absolute velocity, VA, is the vector addition of VS and VR. The resultant velocity V is the vector addition of VA and Vol. 49  No. 6  2016

d  ωr + ωr

 

mp

sin λ

 sin  tan −1 

p 2πr

 +λ 

+

4qc 2

πd2 x



i

 

dt



− 6πμRp  ωr + ωr

 



d  ωr + ωr −

 

mf

sin λ

 sin  tan −1 

p 2πr

 +λ 

sin λ

 −1 sin  tan 

p 2πr

 +λ 

+

+

= mf

4q c πd22x 4qc πd

 

D  ωr1 +

2 2x

i − ωr1 −

i − ωr1 −

4q f 2

πd1

  

Dt 4q f πd12 4q f 2

πd1

       

dt

2

+ (mp − mf )g

(12)

The key issue in quantifying bubble-particle aggregate stability is determining whether or not the adhesive force acting on the attached particle is sufficiently large to prevent the particle from detaching from the bubble surface under the dynamic forces existing in the mixer. The strength of the particle-meniscus aggregate is termed the tenacity, T, of the particle attachment. The particle detaches from the meniscus only if the detaching forces exceed the tenacity (Phan et al., 2003). The tenacity of attachment can be approximately described by

R   T = πRpσ (1 − cos θ )  1+ 0.016 p  L  

(13)

where L = σ / (δg ) is the capillary length, σ is the gas– liquid interfacial tension, θ is the contact angle, and δ is the liquid density. Let us assume that for a mixer used to prepare an inorganic solidified foam, turbulence is the main mechanism causing the particle detachment. On this basis, the detaching force, Fde, is given by Fde =

4πRp3 ( ρp − δ )(g +bm ) 3

(14)

where ρp is the particle density, and g and bm are the accelerations of gravity and turbulent eddies, respectively. The stability of the bubble-particle aggregates now becomes T ≥ Fde

(15)

Through the analysis of the process of the combination of the composite slurry particles with aqueous foam, in 507

order to form foam fluid in the mixer when the structural parameters of mixer are fixed, we can see that the rotational speed of the mixer, ω, plays an important role in preparing a homogeneous ISF, as denoted by Eq. (12). In Eqs. (14) and (15), we can see that the detachment efficiency depends on turbulent eddies. When the rotational speed of the mixer is lower, it provides less energy, and thus generates a less violent vortex, which is not beneficial for mixing the composite slurry with aqueous foam. The collision and attachment efficiencies of composite slurry particles on aqueous foam are low. When the rotational speed of the mixer is higher, more energy is provided and a more violent vortex is generated, which is beneficial for mixing the composite slurry with aqueous foam. However, excessive energy led to composite slurry particle detachment, or to an aqueous foam breakage. Therefore, there is an optimum relationship between the rotational speed of the mixer, homogeneity, and expansion ratio. This has important implications in determining the best rotational speed of the mixer for preparation of ISF with a good performance. 3.4 Relationship between rotational speed of the mixer and relative standard deviation of the pore area The effect of the rotational speed on the relative standard deviation of the pore area of the ISF is shown in Figure 8. With increasing rotational speeds, the relative standard deviation of the pore area initially decreases, and then increases for all aqueous foam flow ratios. When the aqueous foam flow rates are 5 or 6 m3/h and the rotational speed of the mixer is 100 rpm, the relative standard deviation of pore area is at a minimum, and the homogeneity is the best. At aqueous foam flow rates 7 or 8 m3/h and a rotational speed of 150 rpm, the relative standard deviation of the pore area is at a minimum and the homogeneity is the best. At constant rotational speeds of 50 and 100 rpm at increasing aqueous foam flow ratios, the relative standard deviation of the pore area gradually increased and homogeneity gradually decreased. In contrast, at constant rotational speeds of 150, 200, and 250 rpm and with increasing aqueous foam flow ratio, the relative standard deviation of pore area gradually decreased and homogeneity gradually increased. Based on a theoretical analysis of the novel mixer, we can see that the reason for these results is that variation of the aqueous foam flow ratio leads to information changes regarding the fluid density and initial aqueous foam inlet distributions. Additionally, the energy of the homogeneous mixture varies with increasing rotational speeds of the mixer. From 50 to 100 rpm, T≥Fde, the bubble-particle is stable. Based on Eq. (6) and Figure 4, it can be seen that with increasing rotational speeds of the mixer, the mixer can provide more energy and generate a more violent vortex, thereby leading to decreases in the viscosity of the composite slurry and aqueous foam. This is beneficial for mixing the composite slurry with aqueous foam, so that homogeneity is improved. With further increases in the rotational speed of the mixer from 150 to 250 rpm, T