On the separation performance of a novel liquid-liquid dynamic

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Separations

On the separation performance of a novel liquid-liquid dynamic hydrocyclone Long Huang, Songsheng Deng, Jinfa Guan, Weixing Hua, and Ming Chen Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00137 • Publication Date (Web): 14 May 2018 Downloaded from http://pubs.acs.org on May 15, 2018

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Industrial & Engineering Chemistry Research

On the separation performance of a novel liquidliquid dynamic hydrocyclone Long Huang*,&, Songsheng Deng&, Jinfa Guan, Weixing Hua, Ming Chen

Department of Fuel, Army Logistics University of PLA, Chongqing 401331, PR China.

* Corresponding author: E-mail address: [email protected] (L. Huang) &

These authors contributed equally to this paper.

KEYWORDS: Separation performance; Liquid-liquid dynamic hydrocyclone; Rotation speed; Flow rate; Flow split ratio.

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ABSTRACT: To investigate the separation performance of a novel liquid-liquid dynamic hydrocyclone (LLDH), a series of experiments and numerical simulations were conducted. Algebraic Slip Mixture model was used to simulate the multiphase flow in the LLDH, in which the turbulence was modeled using Reynolds Stress model and the rotation of the walls was modeled by Multiple Reference Frame model. The numerical results showed a good agreement with the observations and measurements in experiments. The results showed that, increase of rotation speed would strengthen the swirling intensity in LLDH and thereby separation efficiency was raised. However, increase of flow rate would decrease the residence time of oil droplets, causing reduction in efficiency. By comparison, flow split ratio had slight influence on the flow field, and the efficiency rose a little as flow split ratio increased. The efficiency of the LLDH could remain high when the non-dimensional rotation rate was large enough.


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1. Introduction Oily water is ubiquitous in industries and daily lives today (e.g., in petroleum refinery, offshore platform, and on board ship), which is hazardous to both nature and human health.1-2 Therefore the oily water should be treated before discharged. Among the methods to deal with oily water, hydrocyclones are more efficient and cost-saving.3 However, the use of common hydrocyclones for liquid-liquid separation was earliest recorded in 1940s, which was more than fifty years after the first hydrocyclone appeared,4 indicating the difficulty of liquid-liquid separation. And the liquid-liquid hydrocyclone (LLHC) was not widely applied until 1980s when Colman and Thew conducted the fundamental studies of LLHC.5-8 LLHCs were firstly used commercially in the petroleum industry,9-10 and gradually popularized in other fields such as chemical,11-12 environmental,13-14 food industries,15 and biological applications.16 However, separation in different working conditions calls for different hydrocyclones. Therefore some new hydrocyclones such as micro-hydrocyclone,17 filtering hydrocyclone,18 and liquid-liquid dynamic hydrocyclone (LLDH) were proposed.19 The main characteristic of the LLDH is that the swirling flow is maintained by rotating vanes and the body (or the shell of the separation chamber). The LLDH is flexible and adaptive to changing conditions, and provides higher efficiency than conventional hydrocyclones.20 The LLDH was proposed in 1980s, but it was not widely applied until recent years. Skiftesvik and Svaeren21 designed a new subsea separation system with a dynamic hydrocyclone, enabling continuously separation of the well stream into oil, water and gas in deep water. Lv et al.22 developed a special LLDH for the thin oil dewatering, and achieved good results. Ma et al.23 designed a ship-born bilge water treatment system with a LLDH, whose separation efficiency was proven to be much higher than the system with static hydrocyclone. Chen et al.24-26 developed a novel LLDH for oil-water separation in deep water. The impacts of


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operational parameters such as flow rate, rotation speed, pressure parameters and inlet oil concentration on separation performance of that novel LLDH were studied experimentally and numerically. The results showed that the LLDH was highly efficient and adaptable to environmental pressure changes. Cheng et al.27 designed a new LLDH for ship-born wastewater treatment, and the new separator efficiently separated the oil and water, enabling water discharge directly from ship with secondary filtration adsorption treatment. For decades, vast literature threw light on the separation performance of conventional hydrocyclones.28-32 However, studies about the LLDH were nowhere near enough compared with conventional hydrocyclone. In this paper, the performance of a novel LLDH is presented. The influences of inlet flow rate, the rotation speed, and the flow split ratio were studied by conducting a series of experiments. Numerical simulations were carried out using Computational Fluid Dynamics (CFD) techniques. The confidence of numerical results was attained by the comparison with oil distribution and separation efficiency which were observed in experiments. The details of the flow filed were presented, which helped to understand the LLDH's main response behavior to the changes of operational parameters.

2. Experimental 2.1. Liquid-liquid dynamic hydrocyclone and experimental setup The structure and main dimensions of the novel LLDH are shown in Figure 1. The main body of the LLDH consisted of a cylindrical shell and the guide vanes. The guide vanes with 4 mm thickness were mounted on a 30 mm diameter central body. As the shell and guide vanes should be rotatable, seals and bearings were connected to them. Therefore, the shell and the guide vanes can be rotated by pulley and belt driven by a motor. As shown in the magnification of the guide vanes part, the guide vanes were mounted on the flange plate which was connected to the pulley,


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enabling the guide vanes to rotate with motor. The feed passed through the guide vanes via the channels between central body and shell. Moreover, a hole was bored in the central body on which guide vanes were mounted, and a thin tube was stick into the hole, acting as a vortex finder to exhaust the separated oil. The main feature of the novel LLDH distinct from the conventional hydrocyclone is that the swirling flow in the LLDH is maintained by the rotating guide vanes and shell, and the swirling intensity can be changed via adjusting the rotating speed of the motor that drives the guide vanes. Thus, unlike the conventional hydrocyclones with tangential inlet, the centrifugal effect in LLDH is less related with feed flow rate, making it more flexible and adaptable.

Figure 1. Structure and internal flow of the novel LLDH: 1-overflow outlet; 2-inlet; 3-seals; 4bearings; 5- belt and pulley; 6-guide vanes; 7-rotating shell; 8- underflow outlet. As shown in Figure 1, the mixture of oil and water flows as follows in the novel LLDH. The feed is pumped into the LLDH, and is made to swirl by the rotating guide vanes and shell. Due to the density difference between water and oil, they are separated by the centrifugal effect. Oil droplets move towards the center of the separation chamber, while water moves towards the wall. As the overflow outlet is on the same side of the inlet, the oil droplets aggregate together forming the vortex core and move in a reverse direction to the main flow, and exits from the overflow outlet. The water and residual oil are discharged from the underflow outlet.


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Figure 2. Process flow diagram for the experimental setup Figure 2 shows the overview flow sheet of the experimental system. Tap water was fed into the hydrocyclone by a screw pump whose flow rate could be controlled by an inverter-motor system. The oil was stored in the oil tank which was connected to the inlet of the pump. The oil-water mixture was then separated in the LLDH. After separation, the purified water was sent back to the water tank while the rich oil was discharged to a bucket.

Figure 3. Photo of the experimental setup. Figure 3 shows a photo of the experiment setup. The shell of the LLDH was made by Plexiglas, making the flow field in the separation chamber visible. The pulley was driven by a motor whose rotation speed could be changed by an inverter. The inlet and underflow outlet each had a branch pipe with valve and the fluid could be discharged from the branch pipe for oil concentration measurement. The overflow outlet was connected with a 10 mm hose and a ball valve. The flow


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split ratio could be controlled by the valves at the overflow outlet and underflow outlet. Here the flow split ratio ( R f ) is the ratio of the flow rate of overflow outlet to the flow rate of inlet, i.e., Rf =

Qo Qi

(1)

where Qo and Qi are the flow rates of overflow outlet and inlet respectively. 2.2. Materials and methods

The oil used in the experiment was diesel oil bought from the market, and the density of the oil was about 830 kg/m3. The oil and water were mixed in the pump and oil volume fraction was about 1%. The exact oil concentration in water was measured by ultraviolet (UV) spectrophotometric method,33 which is one of the most accurate methods. The UV characteristic absorption of conjugated bond and aromatic compounds in the oil is the base of this method, and the concentration is obtained by measuring the ultraviolet absorbance. The mixtures of oil and water were collected from inlet and underflow outlet in the experiment. Then the oil was extracted by petroleum ether from the mixture whose volume was measured beforehand, and strained into volumetric flask. Since the measuring range of the UV spectrophotometric method is 0.05~50 mg/L, the oil solution had to be diluted before the absorbance test. Next, the absorbance tests were conducted by an UNICO UV-2000 spectrophotometer from UNICO (Shanghai) Instruments Co., Ltd. Thus the oil content in the solution was measured and the oil concentration in the water could be calculated. Once the oil concentrations were measured, the total separation efficiency ( η ) could be calculated as

η = 1−

Cu Ci


(2)

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where Cu is the oil concentration (mg/L) in the underflow and Ci is the oil concentration (mg/L) in feed inlet.

3. Numerical simulation 3.1. Model description To simulate the complex oil-water multiphase flow in the novel LLDH, a set of governing equations needed to be solved. Oil distribution in water was resolved by Algebraic Slip Mixture (ASM) model. Turbulence was modeled using Reynolds Stress Model (RSM). And the rotation of the guide vanes and shell was modeled with Multiple Reference Frame (MRF) model. 3.1.1. ASM model The ASM model is a simplified multiphase model of the Euler-Euler two-fluid model. The basic assumption of ASM model is that dispersed phases are in equilibrium with continuous phase, but the phases can move at different velocities.34 The ASM model is a good substitute for the full Eulerian multiphase model, and has a wide application in modeling of sedimentation, cyclone separators, particle-laden flows and bubbly flows. The continuity and momentum equations of the ASM model are formulated as follows. ∂ ( ρ m ) + ∇ ⋅ ( ρ mu m ) = 0 ∂t

(3)

∂ ( ρ mu m ) + ∇ ⋅ ( ρ mu m u m ) = ∇ ⋅  µ m ( ∇u m + ∇uTm )  − ∇p + ρ m g ∂t  n  + ∇ ⋅  ∑ (α k ρ k u dr , k u dr ,k   k =1 

(4)

where α k is the volume fraction of phase k , and can be calculated as

∂ (α k ρ k ) + ∇ ⋅ (α k ρ k u m ) = −∇ ⋅ (α k ρ k u dr ,k ) ∂t


(5)

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ρ m is the mixture density, µm is the mixture viscosity, and u m is mass averaged mixture velocity, and they are defined as follows: n

n

n

k =1

k =1

ρ m = ∑ α k ρ k , µm = ∑ α k µ k , u m =

∑α

k

ρk uk

k =1

(6)

ρm

where n is the number of phases. And u dr ,k is the drift velocity for secondary phase k , and its relationship with the slip velocity with respect to the continuous phase is as follows:

α i ρi ui ,c i =1 ρ m n

u dr ,k = u k − u m = u k ,c − ∑

(7)

where u k ,c is the slip velocity of phase k , which can be calculated from the following algebraic equation:

u k ,c

ρ k − ρ m ) d k2  ( = g− 18µc f drag

 

( um ⋅ ∇ ) um −

∂u m  ∂t 

(8)

d k is the diameter of the particles (or droplets or bubbles) of secondary phase, and f drag is the

drag coefficient which is calculated as

f drag

1 + 0.05 Re0.687 =  0.018 Re

Re < 1000, Re ≥ 1000.

(9)

3.1.2. RSM model Turbulence is modeled by solving Reynolds averaged Navier-Stokes (RANS) equations. There are many models for RANS equations such as mixing length model, k − ε model, k − ω model and RSM. The essential difference of these models is the way to model the Reynolds stresses ui′u ′j . Previous researches have showed the superiority of RSM in the simulation of swirling

flow,35-37 So RSM was adopted in this work. Transport equations of Reynolds stresses in RSM are given as below:


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∂ ∂ ρ ui′u ′j + ρ uk ui′u ′j = DT ,ij + DL,ij + Pij + φij + ε ij + Fij ∂t ∂xk

(

)

(

)

(10)

where DT ,ij is turbulent diffusion, and DL,ij is molecular diffusion, calculated as DT ,ij = −

∂ ∂  ′u ′j uk′ + p′ (δ kj ui′ + δ ik u ′j )  , DL,ij = − u ρ i  ∂xk  ∂xk

 ∂  ui′u ′j  µ  ∂xk 

( )

(11)

And Pij is the stress production, φij is the pressure strain, ε ij is the dissipation tensor, Fij is the production by system rotation, which are calculated as

∂u  ∂u ′ ∂u ′   ∂u  ∂u ′ ∂u ′ Pij = − ρ  ui′uk′ j + uk′ u ′j i  , φij = p′  i + j  , ε ij = −2µ i j , ∂xk ∂xk ∂xk ∂xk   ∂x j ∂xi  

(

Fij = −2 ρΩ k u ′j um′ eikm + ui′um′ e jkm

(12)

)

where Ωk is the rotation vector and eikm is the Levi-Civita symbol. DT ,ij , φij , and ε ij need to be modeled further to close the equations, and the details could be found in relevant literature about CFD.38 3.1.3. MRF model As the vanes and shell of the LLDH are rotating when working, the movement should be considered in the simulation. For the simulation of the flow field involve with rotating parts, the MRF model is a good option which has been widely used in the research of other rotating machineries such as stirred tanks,39 pumps40 and fans.41 In MRF model, the fluid domain is divided into multiple zones, and individual cell zones can be assigned different rotational speeds. MRF model is a steady-state approximation in which flow in each moving zone is solved using the moving reference frame equations. Consider a coordinate system that is moving with a linear velocity v m and an angular velocity ω m relative


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to a stationary (inertial) reference frame. Then the fluid velocity can be transformed from the stationary frame to the moving frame as below: v r = v − ur

(13)

ur = vm + ωm × r

(14)

where

Here v r is the relative velocity (the velocity viewed from the moving frame), v is the absolute velocity (the velocity viewed from the stationary frame), r is the position vector of an arbitrary point in the CFD domain from the origin of the moving frame. 3.2. Numerical conditions

The computational domain was built according to the geometry of the experimental setup, and consisted of 3 subdomains, as shown in Figure 4. Only the separation chamber zone was rotating, while the overflow and underflow zones were stationary. Grid independence was tested using computational grids with 162,901 nodes, 217,967 nodes and 283,007 nodes, and the results showed that grid with 217,967 nodes was optimal for accuracy and computing speed.

Figure 4. grid of the computational domain The inlet boundary condition was set up as a “velocity-inlet”, where a velocity normal to the inlet is specified, so that the flow rate and oil concentration were the same as in the experiment. The diameters of oil droplets in our experiment were measured by a microscope, and the mean diameter is about 80 µm. For convenience of simulation, the mean diameter was used in the numerical simulation substituting for various diameters of oil droplets, and the interaction of the oil droplets were ignored. The two outlets in the LLDH were both set up as “outflow” boundaries,


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with a specified flow split ratio. The no-slip velocity boundary conditions were applied for all the walls. The Coupled algorithm was used for pressure–velocity coupling. To reduce numerical diffusion, the QUICK scheme was used for spatial discretization. 4. Results and discussion

The main aim of the study is to investigate the separation performance of the novel LLDH. While the performance depends on certain working conditions or operational parameters such as flow rate, rotation speed and flow split ratio. Therefore the effects of these parameters were evaluated. And CFD simulations were conducted to analyze the flow fields which caused the changes of separation performance. 4.1. Effect of rotation speed

Rotation speed ( ω ) is one of the most important operational parameters for a LLDH, as the swirling strength is controlled by rotation speed. The oil distributions in the novel LLDH with different rotation speed are shown in Figure 5, as the feed flow rate and flow split ratio were controlled the same ( Qi =3 m3/h, R f =10%), both in experiments and CFD simulations. As the emulsified oil in water is creamy white and non-transparent, while the pure water is transparent, the volume fraction of the oil could be estimated by the transparency of the stream in the experiment. As clearly shown in Figure 5, the oil can be efficiently separated from water in the novel LLDH. A white ribbon appeared in the center of the separation chamber, indicating that the oil concentration was high in the center. Actually, the white ribbon was the oil core in LLHCs.42-43 The oil core became thinner and thinner from overflow outlet to underflow outlet and disappeared eventually, indicating that the oil core flowed in a reverse direction to the main stream. Another phenomenon should be noticed is that the stream became more and more


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transparent as flowed downstream, indicating that the oil droplets were separated from the bulk flow and gathered to the oil core.

Figure 5. Experimental and simulated oil distributions with different rotation speed. The detailed oil volume distribution could be found from the numerical results in Figure 5. As can be seen, the simulated oil distributions showed a great agreement with experimental ones. The length and shape of the oil cores in simulations were in accordance with experiments, and the oil volume fraction in the external main flow decreased as observed in the experiment. All the oil volume distribution in experiments and simulations revealed the movement of the oil


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droplets. The mixed oil droplets and water were fed into the separation chamber, so the color around the inlet was more adiaphanous. Then the droplets moved downstream and swirled with bulk flow. Under centrifugal effect, the oil droplets moved towards center. As a result, the flow near the wall became more and more transparent in experiments and the oil volume fraction became smaller in simulations. The gathered oil droplets formed the oil rich core and moved towards overflow outlet, consequently, the oil core became thicker and the oil volume fraction along the centerline got larger when approaching overflow outlet. Figure 5 also shows the effect of rotation speed on the separation. As can be seen from both experimental and numerical results, when the rotation speed increased, the oil core became shorter and the milky area became smaller, meaning that the oil was separated more quickly. Moreover, the fluid near the underflow outlet got more and more transparent, or in other words, the oil volume fraction was smaller, as shown in numerical results. Consequently, when ω > 800 rpm, the volume fraction at the underflow outlet was lower than 0.1%, indicating a high separation efficiency would be achieved.

Figure 6. Influence of rotation speed on the separation efficiency.


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The influence of rotation speed on the separation efficiency is shown in Figure 6. It can be seen that the simulation results are in a good agreement with the measured data, with the same trend and small errors. Generally, the separation efficiency increased with the rotation speed, which is consistent with the analysis above. But the relationship between them was not linear. The efficiency dropped fast with the decrease of rotation speed when ω < 800 , while increased slowly with the increase of rotation speed when ω > 800 . However, the novel LLDH was very efficient in general. As Figure 6 shows, the efficiency could keep more than 90% when ω > 700 under appropriate conditions. (a)

(b)

Figure 7. Tangential and axial velocities at section z = 0.4 m with different rotation speed. Figure 7 shows the velocity distribution of the mixture at the middle of the separation chamber (the section z = 0.4 m as shown in Figure 5). The velocity distribution can reveal the mechanism how the operational parameters affect the separation performance. The separation in a hydrocyclone is caused by the centrifugal effect which is closely related to the tangential velocity.28, 44-46 Moreover, the axial velocity determine the residence time of the droplets which also influence the separation performance of hydrocyclone. Whereas the radial velocity component is usually negligible compared to the other components.47 Thus the tangential and


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axial velocities were analyzed herein. As shown in Figure 7(a), the tangential velocity profile in the novel LLDH had two regions, i.e. a forced vortex in the center which was similar to the conventional static hydrocyclone,48 and a outer region in which the tangential velocity changed slowly. However, the velocity near the wall remained large rather than zero, as the wall of the separation chamber was rotating. When rotation speed increased, the tangential velocity increased almost linearly, but the range of the forced vortex changed little, which was from center to about one fifth of the radius. Figure 7(b) shows the axial velocities with different rotation speed. It can be seen that the axial velocity in the novel LLDH also had two regions, namely the outer main flow region and the reverse core, in which the velocity directions were reverse. And when rotation speed increased, the axial velocity changed slightly: the velocity around the wall increased and near the core decreased. However, these changes were very small and the mean velocities of the outer region were almost the same, which meant that the residence times of the droplets were similar. Hence the separation efficiency increased with the rotation speed due to the rapid increase of tangential velocity. 4.2. Effect of flow rate The effect of feed flow rate (Qi) on the oil distribution is shown in Figure 8. Visibly, the feed flow rate was another important operational parameter that influences the separation performance significantly. As the flow rate increased in the experiment ( ω =800 rpm, R f =10%), the oil core lengthened. When Qi = 2 m3 /h , the oil core disappeared at about three-quarters

length of the separation chamber. However, when Qi = 4 m3 /h , the oil core extended for almost all the separation chamber. And it is noticeable that when the flow rate increased, the main stream in the outer region of the core became milkier and milkier. Or in other word, the volume fraction in the outer region became larger, as the numerical results showed. Moreover, the


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numerical results were well consistent with the experimental results, indicating that the numerical method was feasible.

Figure 8. Experimental and simulated oil distributions under different feed flow rate. It can be inferred from Figure 8 that the separation efficiency should decrease with the increase of feed flow rate, and it was confirmed by measurement of efficiency, as shown in Figure 9. Furthermore, the efficiency decreased faster and faster as the flow rate increased. One reason may be that the efficiency was almost larger than 95% as Qi ≤ 3 m 3 /h , which was close to 1 and hard to improve largely. Another reason laid in the velocity distributions which are analyzed


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below. Again, the simulated efficiencies showed a good agreement with the experimental ones, and correctly predicted the change rules of the efficiency.

Figure 9. Experimental and simulated separation efficiency with different feed flow rate. The influences of flow rate on the velocity distributions of oil-water mixture are shown in Figure 10. Intuitively, the flow rate would change the axial velocity, as shown in Figure 10(b). As the flow rate increased, the axial velocities in the reverse core and outer region were both increased, making the curve steeper. Moreover, the tangential velocity was also influenced by flow rate. As shown in Figure 10(a), the tangential velocity increased with the flow rate, but the increase was less and less. When the flow rate increased to 4 m3/h, the tangential velocity was almost the same as that with flow rate of 3.5 m3/h. Thus if the flow was small, when the flow rate increased, the residence time decreased but the centrifugal force increased slightly, leading to a little decrease in efficiency. However, if the flow rate was large, when the flow rate increased, the residence time increased significantly while the centrifugal force remained almost the same, causing a marked drop in the separation efficiency.


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(a)

(b)

Figure 10. Tangential and axial velocities at section z = 0.4 m with different feed flow rate. 4.3. Effect of flow split ratio

The effect of flow split ratio (Rf) on the separation performance was not as significant as rotation speed and flow rate. As shown in Figure 11, the oil distribution with different flow split ratio were almost the same, except that the oil core became shorter and thinner when Rf increased ( Qi =3 m3/h, ω =800 rpm). And the numerical results showed the same trend. The oil volume fractions with different flow split ratio were similar, and only small differences were present around the outlet, showing a slightly less of oil volume fraction with larger flow split ratio. The efficiency varied with flow split ratio as shown in Figure 12. In line with the oil distribution shown in Figure 11, the efficiency increased slightly with the flow split ratio. The simulated efficiencies were a little larger than the measured ones, but the difference was very small and acceptable. And the errors may be caused by ignoring of droplets breakup in the numerical simulations.


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Figure 11. Experimental and simulated oil distributions with different flow split ratio. Changes of oil distribution and separation efficiency were caused by variations of the flow field. The mechanism of the phenomenon that oil core became shorter and thinner shown in Figure 11 could be found in the velocity distribution of the oil-water mixture which was shown in Figure 13. As flow split ratio increased, the axial velocity of the reverse core increased, the oil could be therefore discharged faster, and as a result the oil core got shorter and thinner. And reason for the minor increase of efficiency shown in Figure 12 could be as follows. When the flow split ratio increased, the tangential velocity of the mixture also increased slightly as shown


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in Figure 13(a). Furthermore, the axial velocity in the outer region were almost the same (actually, the mean axial velocity should decrease a little, as the flow rate in the outer region was decreased) as shown in Figure 13(b), indicating that the residence time of the oil droplets remained nearly invariant. As a result, the efficiency increased slightly when flow split ratio increased.

Figure 12. Experimental and simulated separation efficiency with different flow split ratio. (a)

(b)

Figure 13. Tangential and axial velocities at section z = 0.4 m with different flow split ratio. 4.4. Suggestion on operation

The experimental and numerical results have shown that the flow field and separation efficiency of the LLDH was significantly influenced by rotation speed and flow rate, while less


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affected by flow split ratio. So, when the working condition changes (mainly the feed flow rate or inlet pressure), it is preferred to adjust the rotation speed and keep the split ratio at appropriate range (about 10% in this study). Further, in LLDH the swirling intensity (or centrifugal effect) is mainly affected by the rotation speed while the residence time of the droplets mainly determined by the flow rate. By comparison, the conventional hydrocyclone’s swirling intensity is closely related with the feed flow rate (or velocity). As a result, if the flow rate is too small to establish a sufficient acceleration field, the efficiency shows a sharp decline.19, 47 However, in the LLDH, the swirling intensity is controlled by rotating speed, and the efficiency will not lose at low flow rate. As the validity of the CFD method has been proved above, the combined effect of rotation speed and flow rate were studied by numerical simulation which is more cost-saving and effective. Since the design capacity of the LLDH in this study was below 5 m3/h, the separation performance within that range of flow rate was investigated. As shown in Figure 14, the separation efficiency of the LLDH showed a growing tendency with increase of rotation speed and decrease of feed flow rate. When the flow rate was low (1 m3/h for example), the efficiency kept high even the rotation speed was small. However, when the flow rate was large, the separation may drop sharply when the rotation speed was small. For instance, as Q = 5 m3/h,

ω = 600 rpm, the efficiency was about 34%. Nevertheless, high separation efficiency could get when the flow rate was large, provided a large rotation speed. It can be seen that the separation was larger than 90% when Q = 5 m3/h, ω = 1000 rpm.


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Figure 14. Separation efficiency at different rotation speed and flow rate. As the swirling intensity and residence time of droplets in the LLDH are controlled respectively by rotation speed and feed flow rate, making the LLDH more controllable. When the flow rate is low, the rotation speed could be small to get high efficiency as well as reduce energy consuming of motor. While the flow rate is high, the rotation should be large to gain establish adequate acceleration field for high separation efficiency. By contrast, the separation performance is less controllable when the flow rate is constant in a conventional hydrocyclone. Then how to determine the relationship of the rotation speed and flow rate? A method is provided here. Firstly, introduce the non-dimensional rotation rate, N, as follows:49 N=

vw rω π r 3ω = = U Qi π r 2 Qi

(15)

where vw is tangential velocity at the wall, U is the mean axial velocity. According to the data in Figure 14, the relationship of the separation efficiency and N is obtained. As shown in Figure 15, generally speaking, the separation efficiency grows with N. It is because that N varies proportional to the rotation speed and inversely proportional to feed flow rate. Increasing N means increasing swirling intensity and/or decreasing residence time. In order to keep high


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efficiency, N should be large enough. For example, to keep efficiency larger than 90%, N should be more than 4 for the LLDH in our study.

Figure 15. Relationship of separation efficiency and non-dimensional rotation rate N. 5. Conclusions

To investigate the separation performance of a novel deoiling dynamic hydrocyclone, an experimental setup was built. Through the transparent separation chamber made by Plexiglas, the oil distributions with different operational parameters were observed. The separation efficiencies were measured by UV spectrophotometric method. A series of numerical simulations were also conducted, in which multiphase flow was modeled by ASM model, turbulence was modeled using RSM model, and the rotation of the guide vanes and shell was simulated with MRF model. The results showed that the novel LLDH separated oil from water efficiently. A noticeable oil core appeared in the center of the separation chamber and transparency of the stream showed the oil volume fraction in the underflow was diminished a lot. Moreover, the numerical results showed a fairly good agreement with the observations and measurements in experiments. The effects of operational parameters on the separation performance were also investigated. The results showed that when the rotation speed increased, the oil core became shorter and the downstream flow became cleaner. Further, the efficiency also increased significantly with


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rotation speed when the rotation speed was small. While as the rotation speed was high, the efficiency approached 100% and thereby increased slowly with the growing rotation speed. The numerical analyses of flow field showed that the increase of rotation speed enlarged the centrifugal force while changed the residence time little, leading to the increase of separation efficiency. Whereas increase of feed flow rate made the oil core longer and the downstream flow milkier in the experiment. And the separation efficiency decreased monotonically with the increase of flow rate, because the increased flow rate raised the downstream axial velocity and therefore decreased the residence time of oil droplets. The flow split ratio had less influence on separation performance. The oil core became shorter and thinner as the split ratio increased, because the axial velocity of the reverse core enlarged. And the separation efficiency increased slightly with the flow split ratio, as the increase of split ratio increased the tangential velocity a little as well as decreased the axial velocity very slightly. Above all, the novel LLDH is an efficient separator, in which the swirling intensity (or centrifugal force) and the residence time of oil droplets can be respectively controlled by rotation speed and flow rate. According to the operational experience in this study, the separation efficiency of the novel LLDH could remain high in the separation process if the non-dimensional rotation rate N is large enough, making it a flexible separator adaptable to changing conditions. Supporting Information.

Cross-sectional isometric schematic image of the separation chamber (Figure S1), and Diameter distribution of the oil phase. (Figure S2) (PDF). ACKNOWLEDGMENT

The authors acknowledge the financial support of the Chongqing Research Program of Basic Research and Frontier Technology (Grant Nos. cstc2016jcyjA0171 and cstc2017jcyjAX0166).


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On the separation performance of a novel liquid-liquid dynamic

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