Numerical simulation and experimental study of gas cyclone-liquid jet separator for fine particle separation

Liwang Wang ,Erwen Chen ,Liang Ma,,*,Zhanghuang Yang ,Zongzhe Li ,Weihui Yang ,Hualin Wang,Yulong Chang

1 School of Mechanical and Power Engineering,East China University of Science and Technology,Shanghai 200237,China

2 College of Architecture and Environment,Sichuan University,Chengdu 610065,China

Keywords:Gas cyclone-liquid jet Dust removal Fine particles Numerical simulation

ABSTRACT To address the shortcomings of existing particulate matter trapping technology,especially the low separation efficiency of fine particles,herein,a novel gas cyclone-liquid jet separator was developed to research fine particle trapping.First,numerical simulation methods were used to investigate the flow field characteristics and dust removal efficiency of the separator under different working conditions,and to determined suitable experimental conditions for subsequent dust removal experiments.Afterward,the separation efficiency of the separator against five kinds of common particles,including g-C3N4,TiO2,SiC,talc,and SiO2,was experimentally studied.A maximum separation efficiency of 99.48% was achieved for particles larger than 13.1 μm,and 96.55% efficiency was achieved for particles larger than 2 μm.The best crushing atomization effect was achieved for the separator when uG was 10 m·s-1 and uL was 3 m·s-1,while the best separation effect was achieved when uG was 10 m·s-1 and uL was 3.75 m·s-1.Studies have shown that the gas cyclone-liquid jet separator has excellent applicability in the separation of fine particles.

1.Introduction

Dust treatment is required in many industrial scenarios,including oil and gas well dust removal[1],mine dust treatment[2],fine aluminum dust treatment during polishing[3],and pulverized coal treatment [4],etc.

Traditional dust removal equipment can be divided into four main categories: gravity dust collectors,electrostatic dust collectors,bag filters and cyclone separators.The gravity dust removal method is one of the earliest developed dust removal methods and has maintained its popularity for its simple structure,convenient maintenance,and wide application range.The maximum efficiency of gravity precipitator in the general operating range is 68%,with an average of 46% [5].The advantage of an electrostatic precipitator is that its purification efficiency is high,and the resistance loss is very low.The reason for the low market share of electrostatic precipitator is that the input cost and working condition requirements are too high[6,7].Bag dust collectors area commonly used type of dry dust collector,with the advantages of high separation efficiency and simple structure [8].Due to the potential to clog and large pressure drop fluctuations,its use is declining [9].

Cyclone separators are widely used in gas-solid separation due to their simple structure,high separation accuracy,and strong impact resistance.Research on cyclone separators focused on the influence of operating and geometric parameters on the performance of cyclone separators,so as to optimize their geometry.Elsayed and Lacor [10] used the Reynolds stress model (RSM) to determine the optimal ratio of the entrance width to the entrance height of the cyclone separator to be 0.5 to 0.7.Mothilal and Pitchandi [11] deduced the empirical relationship between infinite retained mass and the Nusselt number by the regression method.A relationship between the Nusselt and Reynolds numbers,gassolid mass flow,particle size,and cyclone diameter was also proposed.Zhouet al.[12]studied the separation performance of ultralight particle cyclones at different inlet velocities based on the computational fluid dynamics and discrete elements method(CFD-DEM) coupling approach.Caliskanet al.[13] designed a multi-stage trap cyclone with a modular structure that can divide particles entering the cyclone into 2-4 levels according to their particle size.Although the cyclone separator had a higher treatment efficiency for particles larger than 1 μm,it was difficult to treat solid particles with a particle size smaller than 0.5 μm [14].

In addition to the study of cyclone separator itself,many researchers improved its dust removal efficiency by other ways.Fuet al.[15]and Wanget al.[16]significantly increased the separation efficiency of PM2.5to more than 90%,up to 96.7%,by adding a particle sorting classifier in front of the ordinary cyclone separator.Zhanget al.[17]greatly improved the effect of cyclone separator on 1-3 μm particle treatment effect by adding supersaturated steam.Caliskanet al.[13]proposed a new type of cyclone classifier,which achieved a maximum separation efficiency of 96.1%for particles with an average particle size of 29.7 μm.It is difficult to solve this defect by only optimizing the structure of the cyclone separator.Therefore,a new method is needed to effectively separate PM2.5.

The gas cyclone-liquid jet separator is a new type of gas purification equipment based on the traditional cyclone separator design.The equipment can utilize the atomization of the liquid jet in the three-dimensional swirling field,to mix the gas and liquid phases to a high degree and achieve high-efficiency purification.Due to the dense distribution of fine droplets in the atomization section of the jet,solid particles with a diameter greater than 2.5 μm would be captured and separated by the droplets.Haidlet al.[18] studied the liquid-gas ejector and believed that the recommended range of the diameter ratio of the mixing tube to the nozzle isdM/dN≈1.5-2.5,and the sufficient length of the mixing tube is within the range ofLM/dM≈20-30(dMwas core diameter;dNwas orifice diameter;LMwas core tube length).The gas cyclone-liquid jet separator has achieved good results in the field of gas treatment.Quanet al.[19] studied the gas phase pressure drop and mass transfer performance of the water-sparged aerocyclone(WSA).Quanet al.[20,21]optimized the porous structure of column WSA and carried out more effective ammonia blowing.Chenget al.[4] researched using WSA to treat fly ash from waste coal-fired power plants.Wanget al.[22] used gas cycloneliquid jet separator to conduct wet flue gas desulfurization experiments.Maet al.[23,24] studied the deamination performance of the gas cyclone-liquid jet separator and the influence of the overflow port position).Wanget al.[25]studied the performance of CS2purification and HCl purification by gas cyclone-liquid jet separator.

Compared with traditional methods,the gas cyclone-liquid jet separator has great advantages in dust removal.By studying the ability and parameters of the separator in dealing with fine particles,its applicability can be extended to the collection of fine particles.First,the flow field and separation characteristics of the separator under different conditions were studied by numerical simulation,and the optimal dust removal conditions were selected.After that,the dust removal performance and mechanism of the separator under different working conditions were explored through dust removal experimentation using different particle sizes.Through the combination of numerical simulation and experimental research,this paper provides a practical and effective treatment scheme for the low-cost and high-efficiency treatment of fine particles.

2.Experimental

2.1.Working principle

There are two main methods for the separation of solid particles in the gas cyclone-liquid jet separator: centrifugal separation and droplet trapping.Its working principle is shown in Fig.1.The dustcontaining gas enters the separator tangentially from the air inlet at the top of the separator,forming a strong gas-phase swirl flow field,and the heavy components are thrown to the side wall by supergravity.The absorption liquid enters the jacket from the liquid inlet of the separator,and sprays into the separator from the small holes in the side wall.The sprayed absorption liquid is cut and broken by the cyclone field,producing a large number of droplets with a diameter of about 500 μm.The droplets scattered in the atomization section of the jet continued to move with the high-speed turbulent flow field to capture fine dust in the gas.After trapping the fine dust,the droplets are thrown to the side wall by the strong cyclone field,and are discharged from the bottom of the separator together with the separated particles.The purified gas is then discharged from the top of the separator.

Due to high-gravity separation and the droplet capture effect,the separator can effectively separate particles of total dust with a diameter less than 2.5 μm.Jafaret al.[26] greatly reduced the generation of dust by adding liquid film in the process of abrasive jet micro-machining,which proved that the liquid had an effective trapping effect on dust.Han and Liu [27] adopted the arc fan nozzle,which obviously improves the dust removal efficiency.These studies have proved the trapping effect of droplets on dust.Compared with traditional cyclone separators,the gas cyclone-liquid jet separator not only greatly improves separation accuracy but also maintains the advantages of the cyclone separator,such as wide adaptability,high separation efficiency,and small floor area.

2.2.Experimental device and process

A variety of different solid particles were used to simulate dust in this experiment,and different concentrations of dust were obtained by controlling the talcum powder feed and air intake rates.The experimental system was mainly composed of a screw feeder,a gas cyclone-liquid jet separator,a fan,a water pump,a dehumidifier,and some supporting instruments.The specific experimental process and experimental device are shown in Fig.2.

This experiment used the dust detector to measure the dust concentration of the outlet gas.The measurement range was 0-2000 μg·m-3,and the measurement accuracy was 1% of the range.Specific information about the equipment of the experimental system is shown in Table 1.

The diameter of the cylinder of the separator used in this experiment was 40 mm,and the surrounding liquid jet holes were evenly arranged on the side wall of the upper part of the cylinder.The diameter of the jet holes was 1 mm,and each layer had a circular distribution of eight equally spaced holes.The layer spacing was 10 mm,with a total of 10 layers.The wall thickness of the equipment was 3 mm,which was made of transparent plexiglass to ensure the strength of the equipment and to observe changes in the flow field during the experiment.The specific structural dimensions of the experimental equipment are shown in,and structural dimensions of the gas cyclone-liquid jet seperator are shown in Fig.3 and Table 2.

The purpose of this experiment was to determine the influence of different concentrations of dusty gas,gas flow and liquid flow on the dust removal efficiency of the separator,and to analyze differences in the separation results caused by changes in the working conditions.At the same time,this experiment investigated the separation effect of the separator on different particle sizes of dust.

Fig.1.Principle of gas cyclone-liquid jet dust removal.

Fig.2.Experimental procedure and device (1.fan;2.screw feeder;3.gas mixing tank;4.gas flow meter;5.U-type differential pressure gauge;6.gas cyclone-liquid jet separator;7.jacket;8.gas-liquid separator;9.liquid flow meter;10.circulating liquid pump;11.circulating water tank).

In this paper,through numerical simulation analysis,changes in the internal flow field of the separator under different working conditions were explored,and the most suitable gas and liquid velocities were determined.At the same time,the overall dust removal performance of the separator under different working conditions was explored.After the numerical simulation,the dust removal experiment was carried out.Taking SiO2particles as an example,before the experiment,a certain amount of SiO2particles were weighed and added to the screw feeder.The circulating water tank was first filled to the scale mark,the circulating water pump was turned on,and the control valve on the liquid phase circuit was adjusted by observing the indicator of the rotameter to make the water flow reach the preset flow parameters.After 2 min,the fan was activated,and the electromagnetic flowmeter was observed while the valve switch on the gas path was adjusted to the preset gas flow parameters.After about 3 min,when the separator had worked stably,the screw feeder was turned on and the dust removal experiment was begun.After waiting for about 3 min,the dust content of the treated gas was measured with a dust detector at the sampling port of the outlet.Each group was measured in triplicate,and the average value of the measurement results was taken.If any values were outliers,the three groups of data were measured again,and the average separation efficiency was taken after eliminating values with large errors.

The dust removal efficiency (η) of the separator is shown as follows:

where,coutis the outlet dust concentration andcinis the inlet dust concentration.

The conversion formula of flow and speed is shown in Eqs.(2)and(3).Where,uGis gas phase velocity,uLis liquid phase velocity,VGis gas phase volume velocity andVLis liquid phase volume velocity.

The error range of the detection device was ± 0.5%,and the absolute error is shown in Eq.(4):

Fig.3.Structure of the gas cyclone-liquid jet separator.

The particle size distribution is shown in Fig.4.It can be seen from Fig.4 that the particle size distributions of the five kinds of particles were relatively concentrated.As a result,the average particle size of the particles can be well described by D50and D90.The average sizes of the particles used are shown in Table 3.

Table 1Experimental equipment information

Table 2Structural dimensions (mm) of the gas cyclone-liquid jet separator

Table 3Average particle size

Fig.4.Particle size distribution.

3.Simulation

3.1.Selection of a calculation model

The internal flow of the separator was an anisotropic,strong,three-dimensional spiral turbulent flow field [28].Many studies have shown that both the Reynolds stress model (RSM) and the large eddy model (LEM) can better predict the strong turbulence model [10,29].Considering the calculation cost and accuracy,the RSM model was used to study the influence of the structural parameters on turbulent flow inside the separator [30].The VOF model is applicable to stratified flow,free surface flow,filling,shaking,the flow of large bubbles in liquids,water flow when dams break,and calculation of steady-tate or instantaneous liquid-gas interfaces.The mixture model can be used for two-phase or multiphase flow and is suitable for low load particle flow,bubble flow,sedimentation,and cyclone separators.This model can also be used for homogeneous multiphase flow without discrete phase relative velocity.The Euler model is the most complex multiphase flow model,which is applicable to bubble columns,upward flotation,particle suspensions,and fluidized beds.The simulation in this paper includes three phases(gas phase,liquid phase,and solid particles).Thus,the mixture model is the most suitable [31].The gas phase is a continuous phase with no diameter,its viscosity is 1.7894 × 10-5kg·m-2·s.The diameter of water takes the system default value 1 × 10-5m,its viscosity is 1.003 × 10-3kg·m-2·s.The amount of water and particles in the air are very small and will not affect the viscosity of the mixed phase [32].

In fluent multiphase flow simulations,the discrete phase is very thin (10%-12%),while the interaction between particles and the influence of particle volume fraction on the flow field are not considered.However,the influence of particles on the flow field is significant in this experiment.As a result,the DPM model was selected for this paper.With the use of the DPM model,the function of the discrete phase relative to the continuous phase can be considered,and the governing equations of both phases can be solved alternately to form a coupling calculation.The existenceof the discrete phase affects the flow field of the continuous phase,which in turn affects the distribution of the discrete phase.In other words,the gas-liquid-solid three-phase flow was considered.

In the numerical simulation,the inlet particle concentration was kept at 20 mg·m-3,and the corresponding particle volume fraction was far less than 1%.It was assumed that the particle velocity at the inlet was equal to the gas velocity.Without considering the interaction between particles,the DPM model can be used to calculate the solid concentration distribution in the separator,and the discrete random walk model can be used to consider the turbulent dispersion of particles.The flow field in the separator was a gas-liquid two-phase flow field.In this simulation,the particle mass did not change,and there was no heat transfer.Only the momentum changed,and the disturbance of the gas flow field by the particles was considered.

3.2.Physical model and mesh generation

In this paper,Solid works was used to build the flow field model of the gas cyclone-liquid jet separator,as shown in Fig.5.In order to reduce the calculation cost,the cylindrical jet hole was approximated to a cuboid with the same cross-sectional area,which was not only conducive to the generation of structured mesh,but also made the iterative calculation not easy to diverge.

Fig.5.Flow field model and grid model of the gas cyclone-liquid jet separator.

According to the concept of topology,only quadrilateral and hexahedral meshes have the function of geometric mapping.The mesh model was generated in ICEM 19.0.Compared with other types of grids,e.g.,tetrahedral,the diffusion of the hexahedron grid was smaller,and a boundary layer grid was generated near the outer wall of the cyclone.In the tangential entrance and jet hole areas,mesh refinement was adopted to improve the mesh quality and reduce the error.Besides,the boundary layer mesh was added near the separator wall to better simulate the complex boundary layer flow fields,where fluid and particle movements are more complex [33].

3.3.Boundary condition

The operating temperature and ambient pressure were set at 20℃and 0.101 MPa,respectively.Considering air as the gas phase and water as the liquid phase,the size of the solid particles ranged from 0.6 to 20 μm.The density was 2719 kg·m-3,the inlet air velocities was 5,10,15 and 20 m·s-1,while the liquid velocities of the jet hole was 0.5,1,3 and 5 m·s-1,respectively.The‘‘velocity inlet”boundary condition was adopted for the gas inlet and the jet orifice,assuming that the velocity distribution at the inlet was uniform and the ‘‘outflow” boundary condition was adopted for the outlet.The DPM model was set in FLUENT,and the particles were released from the air inlet.The Rosin-Rammler model was selected for the diameter distribution.The type of gas overflow and underflow sports used were ‘‘escape” and ‘‘trap”,respectively.When simulating the convection field,the particle type used was ‘‘SiC”;when simulating dust removal efficiency,the selected particle type was ‘‘g-C3N4,SiC,SiO2”.‘‘Interaction with Continuous Phase” was activated in order to consider the disturbance of the gas flow field by the particles.The calculation conditions are shown in Table 4,and the particle sizes and density setting are shown in Table 5 below.The total flow rate was calculated using Eq.(5).

Table 4Calculating condition setting

Table 5Particle data for simulation

3.4.Grid independence verification and reliability verification

To consider the influence of different grid numbers on the simulation results,the grid numbers were divided into 1735218,2364852 and 3153096.The minimum orthogonal quality of the grid was greater than 0.3.

When the air inlet and liquid phase velocities were respectively 15 and 3 m·s-1,the tangential velocity distribution of a straight line at 3 mm above the bottom of the overflow pipe was obtained after 600 iterations.The simulation results are shown in Fig.6.The three tangential velocity distribution curves along the y axis were almost the same therefore,the number of grids used in this paper was 1735218.

Pressure drop was the parameter used to verify reliability.The reliability of the numerical simulation was obtained by comparing the experimental and numerical simulation of the pressure drop of the separator when the gas velocity was between 5 and 20 m·s-1.

Fig.6.Influence of different grid numbers on axial velocity distribution.

Fig.7.Numerical simulation and experimental results of pressure drop under different gas flow rates.

Fig.8.Gas-liquid two-phase distributions at different jet velocities: (a) 0.5 m·s-1,(b) 1 m·s-1,(c) 3 m·s-1,(d) 5 m·s-1.

Fig.9.Distributions of the radial velocity field in the separator under at different liquid velocities: (a) 0.5 m·s-1,(b) 1 m·s-1,(c) 3 m·s-1,(d) 5 m·s-1.

It can be seen in Fig.7 that the experimental values of the pressure drop of the separator were in good agreement with the numerical simulation values under different inlet rates.

3.5.Numerical simulation results

Setting the gas velocity as 10 m·s-1and the liquid velocity as 0.5,1,3 and 5 m·s-1,the flow field characteristics of the separator under different jet velocity conditions were studied.The gas-liquid two-phase distribution is shown in Fig.8.

When the liquid phase velocity was 0.5 m·s-1,because the liquid jet velocity was too low,the mixture was dumped into the separator by the strong rotating flow field to the side wall,and the effect of crushing atomization was slight.When the liquid phase velocity reached 1 m·s-1,fragmentation atomization began to occur,but the droplets were dumped to the side wall of the separator just after appearing.This prevented the droplets from completely mixing with the gas phase.When the liquid velocity was 3 m·s-1,the effect of crushing atomization was very significant.The liquid phase was spread throughout the atomization section,and its mixture with the gas phase was quite sufficient,which was suitable for dust removal.However,when the liquid velocity was further increased to 5 m·s-1,the liquid would break after directly impacting the overflow pipe.Different from the lowvelocity liquid,a large number of liquid droplets moved down the overflow pipe wall and,directly led to serious liquid phase entrainment,which was not conducive to the application of the separator.

Fig.10.Gas-liquid two-phase distributions under different gas velocity conditions:(a) 5 m·s-1,(b) 10 m·s-1,(c) 15 m·s-1,(d) 20 m·s-1.

It can be seen from Fig.9 that there was a large radial velocity distribution caused by the liquid jet near the nozzle hole,but the radial velosity distribution in the seperator was still symmetrical.The greater the liquid velocity,the greater the radial velocity.When it was far away from the orifiec,the liquid jet was cut and broken by the strong swirl field,the small droplets were dissipated in deifferent directions,resulting in a decreasse in the radial velocity.

Since the liquid jet had little effect on the tangential velocity and axial velocity of the flow field,the distribution of the tangential velocity and axial velocity of the flow field in the separator was similar to that of the cyclone separator [34].

With the liquid velocity as 3 m·s-1,and the gas velocities of 5,10,15 and 20 m·s-1,the field characteristics of the separator were studied at different gas velocities.The gas-liquid two-phase distribution is shown in Fig.10.

It can be seen from Fig.10 that the higher the gas velocity,the closer the liquid jet breaking point was to the nozzle hole and the fewer the droplets that were generated.The continuous increase of the gas velocity increased the intensity of the strong swirling turbulent flow field,and greatly improved the breaking and atomization effect of the gas phase to the liquid phase jet.When the gas velocity was 5 m·s-1,the liquid jet directly hit the wall of the overflow pipe,and the gas-liquid distribution was similar to Fig.8(d).When the gas velocity was 10 and 15 m·s-1,the liquid phase was broken at the middle of the atomization section,and the droplets could be densely distributed between the atomization sections to achieve a good gas-liquid mixing effect.When the gas velocity was 20 m·s-1,it was difficult to obtain a suitable gas-liquid mixing effect,as the liquid jet was broken prematurely and caused the droplets to be carried away from the atomization section by the strong swirling field.

The radial velocity field distribution in the separator at different gas velocities is shown in Fig.11.The radial velocity distribution law was similar to the gas-liquid two-phase distribution.As the gas velocity increased,the end point of the liquid radial velocity gradient was closer to the nozzle hole,which reduced the degree of gas-liquid mixing.

In summary,when the gas velocity was between 5 and 15 m·s-1and the liquid velocity was between 2 and 4 m·s-1,a good atomization effect could be obtained,which is suitable for dust removal.

Fig.11.Distributions of the radial velocity field in the separator at different gas velocities: (a) 5 m·s-1,(b) 10 m·s-1,(c) 15 m·s-1,(d) 20 m·s-1.

Fig.12.Influence of gas flow on dust removal efficiency.

4.Results and Discussion

4.1.Influence of gas flow on dust removal efficiency

Fig.12 shows the dust removal efficiency of the separator at different intake air flow rates.The separation effect of particulate matter at different intake air flow rates between 5 and 15 m·s-1was compared.The dust concentration was controlled at 25 mg·m-3by the screw feeder,and the particle selection was shown in Table 3.The velocity of the liquid phase was controlled at 3 m·s-1,and the temperature of the water was 24 ℃.

The dust removal efficiency was 92.65%-96.06%,94.44%-97.77%,97.51%-98.39%,98.06%-99.05%,and 98.51%-99.34% for g-C3N4,TiO2,SiC,talc,and SiO2particulate matter,respectively.

It can be seen from Fig.12 that at the same dust concentrations,the trends of different particulate dust removal efficiency curves were similar.The dust removal efficiency of the separator first increased and then decreased rapidly as the intake air flow increased.When the air intake rate was 10 m·s-1,the dust removal efficiency reached a maximum of 99.34%,which was the same as the result of the numerical simulation.When the intake rate was small,the rotating centrifugal force in the separator increased with increasing intake flow.In this case,dust moved more easily to the side wall of the separator,so it was separated.In addition,as the air intake rate increased,more air entered the separator,which increased the intensity of the strong swirling mixed air flow field in the equipment and made the jet of the small hole water column more effective.Fragmentation and recombination increased,the better coupling occurred in the gas-liquid two-phase system,and some dust agglomerated and flowed out on the water droplets,which improved the dust removal efficiency.However,upon further increasing the intake air flow,more dust-containing gas to be processed brought a greater purification load to the equipment and caused the resistance in the separator to increase sharply,which decreased dust removal efficiency.In the experiment,the overall dust removal efficiency was between 92.65% and 99.05%,which essentially met industrial needs.When it comes to conventional cyclones,the overall separation efficiency could only reach 90.5%.This efficiency decreases rapidly to less than 50% with decreasing particle size[15,35].In contrast,this separator has great advantages.

4.2.Influence of liquid flow rate on dust removal efficiency

The dust removal efficiency of the separator at different liquid flow rates is shown in Fig.13.By adjusting the liquid flow velocity between 2 and 4 m·s-1,the absorption effect of particles at different liquid flow rates was compared.The dust concentration was 25 mg·m-3,the control gas rate was 10 m·s-1,and the temperature of the system was 24 °C.

Fig.13.Influence of liquid flow rate on dust removal efficiency.

The dust removal efficiency was 94.28%-96.39%,96.81%-97.95%,97.76%-98.93%,98.06%-99.15%,98.84%-99.43% for g-C3N4,TiO2,SiC,talc,and SiO2respectively.

It can be seen from Fig.13 that as the liquid phase velocity increased,the dust removal efficiency showed a trend of initial rapid growth and then tended to be stable.Notably,when the liquid phase velocity exceeded 3 m·s-1,a decreasing growth of dust removal efficiency was observed.Due to the small average particle size of the g-C3N4catalyst,the dust removal efficiency could only reach 96.39%.However,compared with the 90.5% removal efficiency of traditional cyclone separator[14],the removal efficiency of particles below 2 μm had been significantly improved.For particles with an average particle size great than 4.5 μm (TiO2),the removal efficiency reached over 96.81%.Different from the numerical simulation results,the separation efficiency did not reach a maximum when the liquid velocity was 3 m·s-1,but increased with increasing liquid velocity.When the gas velocity was between 2 and 4 m·s-1,the effect of droplet breaking and recombination became more obvious and separation efficiency of the particles improved as the liquid jet velocity in the separator increased.

4.3.Influence of dust concentration on dust removal efficiency

Fig.14 shows the dust removal efficiency of the separator at different dust-containing gas concentrations.By adjusting the screw feeder,the dust concentration was controlled to be between 10 and 40 mg·m-3,and the absorption effect of particulate matter at different dust-containing gas concentrations was compared.The gas inlet velocity was 10 m·s-1,the liquid phase velocity was 3 m·s-1,and the temperature of the system was 24 °C.

Fig.14.Influence of dust concentration on dust removal efficiency.

The dust removal efficiency was 94.33%-96.55%,97.34%-97.77%,97.16%-98.59%,98.22%-99.23%,and 98.03%-99.48% for g-C3N4,TiO2,SiC,talc,and SiO2particulate matter,respectively.

It can be seen from Fig.14 that as the dust concentration in the dust-containing gas continued to increase,the dust removal efficiency first increased and then stabilized.When the dust concentration in the gas phase was low,the trapping and entrainment probability of fine particles by the droplets was also low,meaning that an optimal separation effect could not be achieved.With the increase of dust concentration,the probability of being trapped and entrained also increased,increasing dust removal efficiency.But when the dust concentration was greater than 30 mg·m-3,the increase of dust concentration could not strengthen the swirl flow field and the effect of recombination and fragmentation,which limited further improvement of dust removal efficiency.

5.Efficiency Comparison between Simulation and Experiment

5.1.Efficiency comparison between simulation and experiment under different gas flow rate

Fig.15 shows the comparison of dust removal efficiency between the numerical simulation and the actual test at different inlet flow between 5 and 15 m·s-1.The dust concentration was controlled at 25 mg·m-3by the screw feeder,the velocity of the liquid phase was controlled at 3 m·s-1,and the temperature of the gas and water at this time was 24 °C.

For dust with different particle sizes,the larger the particle size,the greater the probability of being captured by droplets.Therefore,in the process of experiment and simulation,the larger the particle size is,the better the separation efficiency is.With the increase of gas velocity,the initial velocity of particles entering the cyclone field increased,as well as the centrifugal sedimentation efficiency.However,when the gas velocity increased to the critical value,the suspension force generated by the gas velocity was the main guiding force,the separation and sedimentation effect of particles was weakened,and the separation efficiency also decreased.In addition,too high speed reduced the probability of particles being captured by droplets,and affected the separation efficiency to a certain extent.Therefore,with the increase of gas velocity,the separation efficiency of dust increased first and then decreased.The forces on particles in the flow field were complex,including centrifugal force,Magnus force,fluid medium resistance,additional mass force,pressure gradient force,Bassett force and the force generated by particle collision.In addition,the quality of dust will also be affected to varying degrees in the process of liquid phase injection.In the process of CFD simulation,the calculation model cannot perfectly fit the actual working conditions,the overall efficiency was slightly lower than the actual efficiency,the state of particles in the flow field was stable and constant,and the change of efficiency was gentler.On the contrary,the dust quality will change under the influence of jet liquid in the actual experiment,and its sedimentation efficiency will be higher than that of numerical simulation.Particles and gas entered the cyclone at the same time,the internal flow field of the cyclone was disordered,the pressure gradient was slightly stable,and the separation of particles in the initial time period will also be affected,especially the particle sedimentation at low gas velocity.Therefore,the efficiency of the experiment was higher than that of the simulation.

Fig.15.Comparison of dust removal efficiency between the numerical simulation and the actual test at different gas flow rate.

5.2.Efficiency comparison between simulation and experiment under different liquid flow rate

Fig.16 shows the comparison of dust removal efficiency between the numerical simulation and the actual test at different liquid flow rate between 2 and 4 m·s-1.At this time,the dust concentration was 25 mg·m-3,the control gas rate was 10 m·s-1,and the temperature of gas and water was 24 ℃.

With the increase of liquid jet velocity,the effect of droplet atomization and fragmentation was more obvious.The increase of flow velocity will increase the number of droplets,as a result,the dust was easier to be captured and the separation efficiency was improved.The change of liquid phase velocity also changed the swirl velocity of local dust and strengthened centrifugal sedimentation.Therefore,in the experiment and numerical simulation,the separation efficiency increased with the increase of liquid velocity.In the actual experiment,the liquid-solid two-phase interaction was more complex,including collision,polymerization,and other phenomena,which changed the physical properties of particles(particle size,mass,etc.)to a certain extent,making them easier to settle.Therefore,the separation efficiency was higher than that of numerical simulation.

In this experiment,the cost of gas cyclone liquid jet separator is about 300 USD,which can treat 12.6 m3·h-1dusty gas.Through the parallel arrangement of multiple pipes and the cost of shell and support,it is estimated that the cost of treating 1000 m3·h-1dusty gas is about 15000 USD.To achieve the same effect,we need to build a machine with the size of Φ 1600×7500 mm packed tower,the cost is up to 75000 USD.When considering the operating cost,the separator needs 6 m3·h-1circulating water,which is very advantageous compared to the 15 m3·h-1required by the spray tower.Due to the lower height of the equipment,the pressure head of the water pump is smaller,reducing its power requirements.In addition,as shown in Fig.7,when the gas velocity was 10 m3·s-1,the pressure drop of the equipment was only 800 Pa,while the pressure drop of the traditional packed tower/spray tower is as high as about 5 kPa.In a word,the gas cyclone-liquid jet separator has the advantages of small floor area,low equipment pressure and high purification efficiency,which leads to its high application potential.

Fig.16.Comparison of dust removal efficiency between the numerical simulation and the actual test at different liquid flow rate.

6.Conclusions

The gas cyclone-liquid jet separator could effectively remove fine particles in dust-containing gas,with an absorption efficiency between 92.65%and 99.48%.When the inlet flow rate was 10 m·s-1,the liquid phase flow rate was 4 m·s-1and the dust removal efficiency can reach a maximum of 99.48%.

When the gas flow rate was 10 m·s-1and the liquid inlet flow rate was 3 m·s-1,the optimal crushing atomization effect could be achieved in the separator,which was consistent with the experimental results.

Compared with the traditional cyclone separator,the removal efficiency of this separator for particles below 2 μm was significantly improved,and the maximum removal efficiency could reach 96.55%.

The gas cyclone-liquid jet separator has the advantages of large operation flexibility and good treatment effect.It can meet various industrial requirements and has good industrial applicability.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This research was supported by the National Natural Science Foundation of China (21878099),and the Science and Technology Commission of Shanghai Municipality (19DZ1208000).

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2021.12.015.