Yang Lü,Kai Lu,Youxiang Bai,Yulong Ma,Yongsheng Ren*

State Key Laboratory of High-efficiency Utilization of Coal and Green Chemical Engineering,Department of Chemistry &Chemical Engineering,National Demonstration Center for Experimental Chemistry Education,Ningxia University,Yinchuan 750021,China

Keywords:Fouling Waste water CFD Inlet velocity

ABSTRACT Due to the high salt content of coal chemical wastewater,pipeline fouling often occurs during wastewater treatment.Fouling will cause the diameter of the pipe to shrink or even block,which is not conducive to the safe and stable operation of the wastewater treatment process.In this paper,the experimental device was designed by using FLUENT software and the fouling deposition mechanisms at different flow velocities and different positions in a 90 deg bend were studied.The experimental results show that when the flow velocity is between 0.2 m·s-1 and 0.3 m·s-1,the thickness of fouling layer was positively correlated with the flow velocity;when the flow velocity is equal to 0.4 m·s-1,the formation of fouling is the most serious;when the flow velocity is between 0.4 m·s-1 and 0.7 m·s-1,the thickness of fouling layer was negative correlation with the flow velocity;with the increase of inlet velocity,the time for sediment point to develop into sediment surface is shortened.The fouling layer is easy to fall off because of the large shear force on the wall surface of the inner bend of the 90°elbow,so the density of sediment at this position is high.

1.Introduction

Zero liquid discharge (ZLD) of high salt wastewater not only reduces water pollution,but also plays an important role in saving water and recycling water resources.However,there are still many problems in the process,in which the fouling problem of pipes and equipment is the most serious.There are various types of blockage problems of the zero-discharge devices due to the high salt content in treated water.The blockage problem not only affects the longterm and stable operation of the device,but also causes damage to critical and membrane components in the processing facility[1,2].Therefore,it is imperative to study the fouling of pipelines in the wastewater treatment process.

According to the different fouling formation mechanisms,Norman [3] divides it into five categories such as crystallization fouling,particulate fouling,chemical reaction fouling,corrosion fouling and biological fouling.The fouling formation generally goes through five stages:initiation,transport,attachment,removal and aging[4].The flow velocity is an important factor of fouling[5–7].When the fluid passes through the heat transfer surface,the flow velocity has a direct effect on the fouling deposition rate and removal rate,and has an indirect effect on the heat transfer coefficient and adhesion strength of the fouling layer [8].The positions and amounts of fouling deposition are different under different flow conditions,such as two-phase flow and non-ideal flow [9–12].There are many types of fouling and are susceptible to process factors,such as flow velocity,solution temperature and scaleforming ion concentration,most of the current research focuses on the research of crystallization fouling and particulate fouling[13–15].At present,there are two very different research conclusions about the influence of flow velocity on fouling.One of them is that Hassonet al.[16] and Steinhagenet al.[17] believe that the flow velocity can inhibit the fouling process by increasing the removal rate.P??kk?nenet al.[18] discusses the relationship between the flow velocity and the fouling deposition rate of CaCO3,and the results show that with the increase of the flow velocity,the shear force at the phase interface increases and the fluid residence time decreases,which makes the fouling deposition more difficult,thus reducing the fouling deposition rate.Bansal investigated the fouling deposition rate of CaSO4in plate heat exchanger [19] and found that when the flow velocity is between 0.35 m·s-1to 0.67 m·s-1,the fouling process is controlled by the surface reaction and the fouling thermal resistance and fouling deposition rate gradually decrease with increasing flow velocity.Songet al.[20]studied the deposition process of CaCO3and CaSO4mixed fouling on the heat exchange surface of plate heat exchanger.The research results show that the progressive value of fouling thermal resistance of mixed fouling gradually decreases with the increase of Reynolds number,which is mainly due to the significant increase of fouling erosion rate with the increase of flow rate.The other is that Helalizadehet al.[21] and Najibiet al.[22] consider that the deposition of crystallization fouling is controlled by ion diffusion and the fouling formation is promoted by increasing the flow velocity [23,24].

During the treatment of high-salt wastewater,the main types of fouling generated in the pipeline are crystallization fouling and particulate fouling.Crystallization fouling is formed by the heterogeneous nucleation process on the surface,which has high adhesion and difficult to remove.Particulate fouling refers to a type of sediment that is formed by collision and adhesion between particles and the wall,which has low adhesion and is easy to be removed.The particles in the solution come from a wide range of sources,such as:particles produced by homogeneous or heterogeneous nucleation in solution,insoluble particles contained in the bulk solution,and particles that fall off the surface of the dirt layer.Flow velocity has a great influence on both types of fouling.

The research on the influence of fluid dynamics on fouling has not been concluded and the current research focuses on the fouling process of heat exchangers or straight pipes,while the fouling process of curved pipes has not been studied.Therefore,based on the above research gaps,the research contents of this paper mainly include the following two points:

(a) Configure artificial solutions according to the main inorganic ions contained in the actual wastewater and a pipe device with a 90 ° elbow is designed.After modeling the actual model,FLUENT was used to simulate the fluid flow in the experimental device,and the sampling point was determined according to the turbulent energy and the wall shear force at different positions.

(b) The fouling degree of each sampling position at different velocity was studied.The optimal velocity range and the fouling conditions at different locations were determined.The research results provide a theoretical basis for the anti-fouling of the pipeline and provide a safety guarantee for the treatment process of high salinity wastewater.

2.Experiments

2.1.Establishment of experimental device

2.1.1.Sampling point selection

Fig.1.Cloud picture of turbulent energy and wall shear stress distribution.(a) Turbulence energy;(b) Wall shear force.

In order to facilitate the sampling and test analysis of the fouling samples,we established the physical model according to the experimental device and meshed it (Fig.S1,Supplementary Material)Subsequently,FLUENT was used to simulate the water flow in the experimental device,and the three-dimensional turbulent energy cloud image and a wall shear stress cloud image were obtained,as shown in Fig.1.

In order to facilitate the analysis and design of the sampling point location,we use different angles instead of different section positions,as shown in Fig.2.Turbulent energy and wall shear force are obtained at different sections by using CFD-POST,as shown in Fig.3.The simulation results show that the turbulent energy and wall shear force have the same change trend at different velocity.We selected five representative locations as sampling points,which are 14.1°,26.8°,28.7°,34.6°,and 81.8°,respectively.

2.1.2.Experimental device

The schematic diagram of the experimental apparatus is shown in Fig.4.The experimental process consists of the thermostat water bath,centrifugal pump,test section and connecting pipelines.In order to ensure that the concentration of the solution is constant during the experiment,a standby thermostat water bath was set up and switched every 12 hours.The connecting pipeline adopts socket connection to ensure convenient installation and disassembly of the experimental device.A section of rubber pipe is installed between the outlet of centrifugal pump and the connecting pipe to reduce the removal of fouling layer caused by the vibration of the pump,which leads to unnecessary experimental error.In order to expand the experimental phenomenon at the sampling point and facilitate data analysis,the experimental pipelines and connecting pipelines are made of PVC pipes that are not easy to deposit and are not covered with functional materials.

Fig.2.Angle diagram corresponding to the position of different sample points.

Fig.3.Average turbulence energy and average wall shear force at different measuring points at different velocities.(a) Average turbulence energy;(b) Average wall shear force.★:0.3 m·s-1,▼:0.4 m·s-1,▲:0.5 m·s-1,●:0.6 m·s-1.

Fig.4.Schematic diagram of experimental apparatus.1–5:Fouling test sampling point.6:No.1 thermostat water bath.7:No.2 thermostat water bath.8:Outlet stop valve of No.1 thermostat water bath.9:Outlet stop valve of No.2 thermostat water bath.10:Centrifugal pump.11:Flow regulating stop valve.12:No.1 thermostat water bath flow adjustment backup valve.13:No.2 thermostat water bath flow adjustment backup valve.14:No.1 thermostat water bath backflow valve.15:No.2 thermostat water bath backflow valve.

The sampling location diagram is shown in Fig.5.First of all,according to the simulation results,punch holes at sampling positions No.1 to No.5,and use equal-diameter tees connections;Secondly,insert Q235 steel sheet with a size of 12 mm×12 mm×2 mm into the bottom of the 3#rubber stopper,as shown in Fig.5(b);Finally,insert the rubber stopper into the top of the equal-diameter tee and adjust the plane of the steel sheet to be flush with the plane of the test section channel.

2.2.Experimental method

Fig.5.Schematic diagram of sampling location.(a) Sampling position assembly drawing.1:Rubber stopper,2:Equal-diameter tees,3:Experimental pipeline;(b)Schematic diagram of inserting steel sheet into rubber stopper.

Fig.6.Variation of fouling layer thickness with time at different velocity and position.(a)v=0.2 m·s-1,(b)v=0.3 m·s-1,(c)v=0.4 m·s-1,(d)v=0.5 m·s-1,(e)v=0.6 m·s-1,(f)v=0.7 m·s-1.

Firstly,the content of each component in the artificial solution(Fig.S3)was calculated according to the actual wastewater composition (Fig.S2),and then added to the thermostat water bath in turn,and fully dissolved at the experimental temperature;Secondly,fully open No.8,No.11 and No.14 valves and make the other valves in the fully closed state,and turn on the power to make the centrifugal pump in the working state;Thirdly,when the device runs smoothly,gradually adjust the opening of No.11 valve to experimental flow velocity,and then start the experiment;Finally,when the experiment was carried out for 12 hours,No.8 and No.14 valves were gradually closed while No.9 and No.15 valves were gradually opened,complete the switch of standby pipeline to ensure the constant composition of solution.It is worth noting that when using flow adjustment backup valve(No.12 and No.13 valves),it is necessary to adjust the valve opening before the start of the experiment.At the end of the experiment,take out the steel sheet embedded in the rubber stopper and dry it at normal atmospheric temperature,and observe with an optical digital microscope.Ethanol and deionized water are used to thoroughly clean the experimental device at the beginning and after the end of each experiment.

Fig.7.The relationship between inlet velocity and Reynolds number,time and fouling layer thickness at position 1.(a) Two dimensional plan;(b) Three dimensional cloud picture.

Fig.8.Relationship between turbulent kinetic energy,wall shear stress and scale thickness.

3.Results and Discussion

3.1.Fouling layer thickness at different velocities,positions and times

Selecting Q235 steel as the test point material,polishing the surface of the test steel with 800 mesh metallographic sandpaper(19 μm),and putting it into the test pipeline after cleaning the steel surface with ethanol cotton ball.The volume of fouling on the steel surface was measured with an optical digital microscope.The fouling thickness is calculated according to the formulaL=V/S(L:average fouling thickness,V:fouling volume on the steel surface,S:steel surface area).

The fouling formation is the result of the competition between growth and removal of fouling layer [25].As can be seen from Fig.6,whenv=0.2–0.3 m·s-1,the relationship of the fouling layer thickness is 2 >1 >5 >4.The main reason is that within this flow velocity range,the fouling layer is greatly affected by wall shear force.The larger position of the wall shear force will cause the looser sediment to fall off the fouling layer surface and return to the bulk fluid.Whenv=0.4–0.7 m·s-1,the relationship of the fouling layer thickness is 4>5 >1 >2 and the fluid is in the turbulent region.At the same flow rate,the collision probability between insoluble particles and the surface and the reaction rate between fouling forming ions on the steel sheet surface will be increased in the region with larger turbulence energy,resulting in the increase of the thickness of the fouling layer.The removal process of the fouling layer is more random,so the above change law is not obvious in some places.

Fig.9.Surface morphology of fouling layer at different locations when v=0.2 m·s-1 and t=24 h.(a) position 1 (83.18 μm);(b) position 2 (87.15 μm);(c) position 4(68.56 μm);(d) position 5 (76.36 μm).

Fig.10.Three-dimensional morphology of fouling layer at different locations when v=0.2 m·s-1 and t=24 h.(a)position 1(83.18 μm);(b)position 2(87.15 μm);(c)position 4 (68.56 μm);(d) position 5 (76.36 μm).

Fig.11.Surface fouling morphology of position 4 at different times when v=0.2 m·s-1.(a) 24 h (68.56 μm);(b) 96 h (116.30 μm).

In this experiment,the relationship between the velocity and Reynolds number is shown in Fig.S4.With the increase of Reynolds number,the fouling layer thickness at the same position increases first and then decreases,and this rule is maintained at different times,as shown in Fig.7.In the transition flow region,with the increase of the flow velocity,the mass transfer rate of the solution increases [7],the contact probability of insoluble particles and fouling ions with the surface is increased,and the thickness of viscous bottom layer is reduced [26],which leads to the gradual increase of the fouling layer thickness and the degree of fouling becomes more severe,indicating that the fouling process is controlled by the mass transfer in this region.In the turbulent region,with the increase of flow velocity,the wall shear force on the surface of the fouling layer increases gradually,which makes it difficult for the loose sediment to adhere [27].Although the mass transfer rate still increases,the trend of promoting the fouling deposition process is far less than that of the wall shear force inhibiting the fouling deposition process.Therefore,the thickness of the fouling layer gradually decreases,indicating that in this region,the fouling process is affected by the surface integration control [28].

In addition,Fig.7 also shows that during the initial fouling process,the fouling deposition rate is higher and then gradually tends to be flat.The reason for this phenomenon is that with the progress of the fouling process,the necking of the pipeline caused by the accumulation of the fouling layer increases the wall shear force,making the sediment easier to be removed.When the deposition rate is equal to the removal rate,that is to say,when the net deposition rate is 0,it can be considered that the fouling process has reached a dynamic equilibrium.

In Fig.8,the red circle represents the region with high wall shear force and high turbulence intensity;the blue circle represents the region with medium wall shear force and medium turbulence intensity;the orange circles represent region with low wall shear and low turbulence intensity.It can be seen that the fouling layer in the blue circle is the thinnest,such as position 4 whenv=0.3 m·s-1.The fouling layer in the red and green circles is relatively thick,such as position 5 whenv=0.7 m·s-1(in the red circle) and position 2 whenv=0.2 m·s-1(in the green circle).

3.2.Fouling layer topography

The following conclusions were obtained by characterizing the sediment under different operating conditions through an optical digital microscope (OLYMPUS DSX510,Japan).It can be seen from Figs.9–12 that corrosion has occurred on the steel sheets surface.In our previous research[29],we conducted XRD and EDS analysis on fouling samples and found that:(a) the corrosion process of steel sheets mainly occurs in the early stage of fouling;(b) as the experiment progresses,the adhesion of sediment to the steel sheets surface prevents the corrosion process and the degree of fouling is gradually greater than that of corrosion;(c) the mass percentage of calcium carbonate in the components gradually increases and becomes the most important sediment;(d) as the experiment time increases,the crystal form of calcium carbonate changes from aragonite and vaterite to more stable calcite.

Fig.12.Surface fouling morphology of position 5 at different velocities when t=96 h.(a) 0.2 m·s-1 (118.65 μm);(b) 0.3 m·s-1 (186.97 μm);(c) 0.4 m·s-1(316.61 μm);(d)0.5 m·s-1 (186.71 μm);(e) 0.6 m·s-1 (139.45 μm);(f) 0.7 m·s-1 (181.79 μm).

Fig.13.Density change map at different position at different velocity and topography of position 3 at different time.:Position 3,:Position 4.

From Figs.9 and 10,it can be clearly observed that the relationship between the coverage and thickness of the fouling layer is:2 >1 >5 >4.Due to the higher wall shear force at position 4 than at the rest,which cause sediment is difficult to deposit and easily to remove.Moreover,at position 4 it can be clearly observed that the fouling deposition process first appears in the form of local sediment spots and then it is gradually connected to form the sediment surface,as shown in Fig.11.In the place where the shear force is large,the sediment is difficult to deposit.Therefore,the process of growing from sediment spot to sediment surface and finally covering the entire surface is slow.The initial fouling formation will lead to a low velocity area near the sediment point,which will reduce the removal rate and lead to the development from the sediment point to the sediment surface.

The mass transfer rate in the solution is strengthened with the increase of the flow velocity,and the time for the sediment point to develop to the sediment surface is reduced.In addition,the fouling layer becomes relatively smooth due to the increase of the wall shear force,as shown in Fig.12.

3.3.Fouling at position 3 of 90° bend

Position 3 is at the 90° bend,which is a special case.According to the density method,we analyzed the fouling samples at this position,and the results are as follows.

It can be seen from Fig.13 that the density of the fouling layer at position 3 is generally higher than that at position 4.The main reason is that the large wall shear force influenced by the solution flow direction while the position 3 is the inner bend of the pipeline,as shown in Fig.1.Loose sediment is easy to fall off,while the sediment with higher adhesion still remains on the sample.The fouling produced at No.3 is hard,adhesive and difficult to clean,which is harmful to the actual industrial production process.

4.Conclusions

In this paper,the mechanism of fouling deposition at different velocity and position in 90 ° bend was studied.The conclusions are as follows:(1) at the same position,the fouling formation increases first and then decreases with the increase of inlet velocity;it can be seen that whenv=0.4 m·s-1,the fouling thickness formation is the most serious.In the initial fouling process,the deposition rate is higher,and then gradually tends to be flat;with the increase of inlet velocity,the time for sediment point to develop into sediment surface is shortened and the fouling layer surface becomes smooth;(2) When the fluid state is in the transition region,the fouling process at different positions at the same flow rate is mainly affected by the wall shear stress.The sediment at the position with higher wall shear force is easier to remove from the surface of the fouling layer and return to the bulk fluid,resulting in the decrease of the thickness of the fouling layer;(3)When the fluid state is turbulent,the fouling process at different positions at the same flow rate is mainly affected by the mass transfer rate.At the same flow rate,the collision probability between insoluble particles and the surface and the reaction rate between fouling forming ions on the steel sheet surface will be increased in the region with larger turbulence energy,resulting in the increase of the thickness of the fouling layer;(4) The No.3 position is at the inner bend of the 90-degree elbow,the wall shear force is large,and loose sediment is difficult to deposit and easy to removal,so the density of the fouling layer at this position is large.

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 work was financially supported by the Natural Science Foundation of Ningxia Hui Autonomous Region (2020AAC03025),Undergraduate Training Programs for Innovation(2019107490001),East-West Cooperation Project of Ningxia Key R &D Plan(2017BY064) and National First-rate Discipline Construction Project of Ningxia (NXYLXK2017A04).

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2020.09.034.