Dongyan Li *,Xi Tang ,Shasha Feng

1 Chemical Engineering Department,Nanjing Polytechnic Institute,Nanjing 210048,China

2 State Key Laboratory of Materials-Oriented Chemical Engineering,National Engineering Research Center for Special Separation Membrane,Nanjing Tech University,Nanjing 210009,China

Keywords:Relative humidity Collapse angle Pressure drop Poly tetra fluoroethylene (PTFE) membrane Fly ash

ABSTRACT In the present study,the effects of relative humidity on filtrating coal-fired fly ash with hydrophobic poly tetra fluoroethylene (PTFE) membranes were investigated.The intergranular force of particulate matter at different RH conditions was measured by analyzing the critical angle between particles.Effects of humidity (from 30% to 70%) on filtration pressure drop and membrane fouling conditions were characterized.It was found the membrane showed optimal filtration resistance of 530 Pa at RH of 60% and the gas permeance can be maintained at 1440 m3·m-2·h-1·kPa-1.Moreover,to optimize the operation parameters for this filtration system,effects of fly ash concentration,diameter,membrane pore size,and gas velocities were systematically investigated.

1.Introduction

Particulate matter(PM)in air affects vast majority of living species on the earth and poses a serious effect to climate and ecosystems [1-6].Fossil fuel combustion is the main source of PM,thus retention of PM from industrial processes is the most critical strategy to ameliorate air pollution[7,8].Currently,filtration is still the most preferred and effective method for ultra-low-emission of PM,especially that of membrane filtration technology [9-11].However,the complex mixture of the inlet flue gas and the variety of operation conditions may synergistically decrease the filtration performance of the system.In specific,humidity is the most important parameter that closely affect the filtration efficiency,resistance and even the service life of filter media [12,13].

Researches have demonstrated that relative humidity(RH),dust attributes (hygroscopic or non-hygroscopic) and filter media surface hydrophobicity are interrelated closely to each other in air filtration process.NaCl (hygroscopic) and Al2O3(non-hygroscopic)aerosols were typically used to evaluate the filtration performance of the filter at varies of humidity [14,15].Commonly,when the humidity below the deliquescent point of hygroscopic aerosols,or for the non-hygroscopic aerosols,the filtration resistance caused by particulate cake decreases along with the increase of the RH[16,17].When the RH above the deliquescent points of the hygroscopic aerosols,the resistance of the membrane caused by particulate cake increases more significant.In addition,the increase tendency of filtration resistance was more like to that of liquid aerosol,other than a liner relation to filtration time[18].In practical filtration processes,the relationship of filtration efficiency and resistance along with filtration time were more complicate than that in simulation processes.

In addition,in the humid filtration system,the hydrophobicity of membrane materials also have great influence on filtration resistance [19].Wanget al.[15] discussed the influence of filter hydrophilicity on filtration performance.As for the RH higher than the deliquescent of the hygroscopic aerosol,hydrophilic filter shows significant increase of resistance in the initial stage,while hydrophobic filter preserves higher increment of pressure drop as time pass by.When filtrating non-hygroscopic particulates at high RH,hydrophobic filters show lower pressure drop than that of hydrophilic filters.

Particulate matter produced in varies industrial processes usually has different properties.Using NaCl,Al2O3,or some other onecomponent PM as the simulation dust cannot exactly reflect the practical filtration process,especially for that with complex humidity atmosphere [20-22].Further,strong hydrophobic poly tetra fluoroethylene(PTFE)membrane composite filter,as the most widely used filtration media has demonstrated excellent application performance for flue gas ultra-low emission,especially for the filtration systems with high RH and hygroscopicity powders.However,the investigation on the interaction relationship between hydrophobic filter media and the humidity dust is still insufficient.

Here,basing on the practical industrial filtration system,we selected coal-fired fly ash as the filtration object,hydrophobic PTFE as the filter media to investigate the influence of RH on the filtration efficiency.The humidity conditions react on fly ash were analyzed to optimize the reunite condition of the fly ash.By investigating the influence of fly ash concentration,particles size,membrane properties,and operation parameters on filtration efficiency,the fly ash filtration mechanism at different RH was revealed,and the best humidity condition for this filtration system was confirmed.This work will provide a comprehensive study of hydrophobic filter media and their application on humid filtration system.Such work will be of potential benefit to optimize the operation parameters for humid filtration systems.

2.Experimental

2.1.Basic parameters of PTFE membrane and fly ash

Fly ash was obtained from the coal-fired flue gas to perform the filtration experiments.Five kinds of fly ash with different particle size distribution were named with FA-1 to FA-5.Filter media(PTFE membrane) used in the experiment was supplied by Jiangsu Jiulang High-tech Co.,Ltd.Different pore size of PTFE membranes were donated with PTFE-1 (1 μm),PTFE-2 (2 μm),PTFE-3(3 μm),and PTFE-4(4 μm).Detail information of PTFE membranes and fly ash are summarized in Table 1,Table 2 and Table 3,respectively.

The pore size distribution and fly ash size distribution were as shown in Fig.1.The average pore size of PTFE-1,PTFE-2,PTFE-3,and PTFE-4 are 1 μm,2 μm,3 μm,and 4 μm,respectively.Fly ash with their average particle size of 5 μm,3 μm,1.0 μm,0.5 μm,and 0.3 μm are correspond to FA-1 to FA-5,respectively.

2.2.Filtration tests

A filtration device (Fig.2) was used to determine the filtration performance of PTFE membranes.An air compressor (SLWY-300W,ShouLi,China) was employed to provide driving force for generating aerosolsviathe device of SAG 410,Bruker,Germany.The filtration experiment was conducted under a subatmospheric pressure controlled by a vacuum pump (VT4.10-40,Becker,Germany).Aerosols with controllable concentration can be completely mixed in the vertical tube and then filtrated by the membrane.Aerosol concentration before and after the membrane module were monitored continuously using a laser particulate detector(JCZXLD-5,Juchuang Environmental Products Co.Ltd.China).The pressure drop across the membrane was measured using a differential pressure transmitter (EJA110A,Yokogawa China Co.Ltd.).The gas velocity was controlled by a mass flow controller (D07-27F,Sevenstar Electronics Co.Ltd.,China).All the parameters were recorded continuously by an automatic monitoring and control system (610H,Yanhua Technologies Co.Ltd.,China).The steam humidification method was used to control the environmental humidity.A humidity generator,that can generate moisture with controllable content was connected to the inlet port of dust at the top of the vertical tube.The relative humidity was first measured and recorded at the position of that of dust detector before the membrane module.

Table 1 Main characteristics of the PTFE membranes

Table 2 Main characteristics of the fly ash

Table 3 Components of fly ash

The overall filtration performance of the filter considering both efficiency and pressure drop is assessed by quality factor(QF).The equation as shown in Eq.(1)whereRis the filtration efficiency,%,ΔPis the pressure drop,Pa.

2.3.Air permeance test

The air permeance of the membrane was tested at different humidity,the gas was used directly or connected to a humidifier before entering the filter medium during tests.The gas velocity was controlled by a mass flow controller.The air permeability was calculated by the following Eq.(2):

whereJis the air permeance,m3·m-2·h-1·Pa-1;Qis the permeate gas flow rate,m3·h-1;Ais the effective area,m2;and ΔPis the trans-membrane pressure,Pa.

2.4.Critical angle tests

The interaction force between particles can be described by critical angle [23].Fly ash was first put in a certain humidity atmosphere for 30 min before filling in the circular tube.The filling amount was controlled at 30% to 50% of the container.Gradually rotate the circular tube to make fly ash collapse,and then record the rotate degree as the critical angle [24].Each experiment was performed three times to get the average value.

Fig.1. (a) Pore size distribution of PTFE membranes;(b) Particle size distribution of different fly ash.

Fig.2. Schematic diagram of the gas filtration test apparatus.

3.Results and Discussion

3.1.Fly ash at different RH

Fig.3(a,b) shows the appearance difference of fly ash after exposing at different humidity atmosphere.There is hardly any aggregation of the fly ash at 30% RH,particles can still maintain well with a comparative dense structure in the glass bottle.With the increase of RH,fly ash begin to aggregate to large particles with a comparative loose structure in the bottle.And the higher RH resulted larger aggregated particles.Fig.3(c) gives the fly ash critical angle relationship with humidity,representing the interaction force degree of fly ash particles.The critical angle of the fly ash at 30% RH is less than 35°,while at 70% RH,the fly ash critical angle improved to almost 85°.The increased critical angle of fly ash at higher RH indicates that humidity may form bridges through‘‘capillary condensation”effect[25,26],leading to increased interaction force between particles.The evolution process of secondary particles was represented in Fig.4.The increasing humidity is benefit to enlarge the filling stage of water and particles.In the initial stage,the water combined particles like a pendulum.When the number of liquid bridges is basically stable,as the ambient humidity increases,the liquid bridges increase and the filter cake compresses.As water occupied the void space between the particles,the filter cake porosity decreased and the filtration pressure drop increased [27-29].It indicates that the filling state of water between particles changes from a pendulum to a ribbon.In this case,the porosity between particles is reduced by overfilling of water,filtration resistance rises even faster than that at room temperature.At this point,the effect of environmental humidity on dust removal performance has been changed to deterioration through strengthening [28].

3.2.Effect of humidity on pressure drop and gas permeance

Fig.3. (a,b) Photos of fly ash after exposing at different RH,(c) Critical angle of fly ash after exposing at different RH.

Fig.4. A schematic representation of particle interaction with the increase of relative humidity.

Fig.5. Gas permeance and pressure drop changing tendency at different RH for PTFE-2 membrane.The gas velocity was set at 2 m·min-1,fly ash average size is 1 μm,inlet fly ash concentration was 600 mg·m-3.

The influence of relative humidity on pressure drop and gas permeance were investigated under RH of 30%,40%,50%,60%and 70%respectively,the results are as shown in Fig.5.It was observed that with increasing of humidity from 30% to 60%,gas permeance has an improvement from 1200 m3·m-2·h-1·kPa-1to 1510 m3·m-2·h-1-·kPa-1,then decreases significantly (490 m3·m-2·h-1·kPa-1) when the humidity greater than 60%.While pressure drop show the opposite trend.The main reason is that with the increase of humidity,agglomeration occurs between particles,resulting in an increase of particle size and relatively loose secondary particles.The schematic of membrane filtration mechanism at different humidity system is as shown in Fig.6.At a comparative dry atmosphere,the interaction force between particles is too weak(Fig.3c)to combine the individual particles to secondary particle.Particles with the most penetration particle size(MPPS,～300 nm)may penetrate the membrane and resulting in a decrease of filtration efficiency (Fig.6a).When humidity increases to a certain degree(usually decided by temperature and particle property),particles can aggregate to secondary particle with large size and loose structure.It is beneficial to decrease the amount of the MPPS particles,and leading to optimal filtration resistance and efficiency(Fig.6b).However,humidity is not the higher the better for any system,particles may form a pasty layer under the effect of excessive humidity(Fig.6c).The strong interaction force between particles and the liquid constructed continuous film may extremely increase the filtration resistance of the system [30-32].Therefore,there is an upper limit for the enhancement effect of humidity on dust removal performance.

3.3.Filtration parameters optimization

3.3.1.Effect of dust concentration on filtration performance

PTFE-2,with its thickness of 5 μm and porosity of 80% was selected to evaluate the influence of fly ash concentration to the pressure drop and rejection at 60% RH.The results as shown in Fig.7,the initial pressure drop of the membrane is around 140 Pa,when the fly ash concentration is 600 mg·m-3,the membrane shows a gentle increase on pressure drop in 3600 s,and finally tend to stable at 180 Pa.While for that with high inlet concentration of 700 mg·m-3and 800 mg·m-3,the membranes existed higher pressure drop of 220 Pa and 240 Pa,respectively,and still on rising after 3600 s of filtration.Therefore,the formation time of stable filter cake is extended when the inlet concentration increases.Fig.7 (b) revealed that inlet concentration of fly ash has few effects on rejection,fly ash rejection maintained well with the value above 99.99%.

Fig.6. Schematic of membrane filtration mechanism at different humidity system.

Fig.7. Effect of dust concentration on (a) pressure drop and (b) fly ash rejection at 60% RH.PTFE-2,fly ash size is 1 μm,filtration velocity is 2 m·min-1,RH=60%.

Fig.8. Effect of membrane pore size on(a)pressure drop and(b)fly ash removal efficiency.Fly ash size is 1 μm,filtration velocity is 2 m·min-1,inlet fly ash concentration is 600 mg·m-3,RH=60%.

3.3.2.Effect of membrane pore size on filtration performance

The influence of pore size on pressure drop and fly ash retention rate was investigated (Fig.8).It differs from the results shown above,the initial pressure drop for varies PTFE membrane are different.PTFE-1 with smaller pore size exist higher pressure drop of～180 Pa.For PTFE-3 and PTFE-4 membranes,the initial pressure drop are 160 Pa and 150 Pa,respectively.In general,large pore size is positive for decreasing filtration resistance,while there is a special case that PTFE-2 membrane shows the lowest initial pressure drop (140 Pa) than that of other membranes.This result reveals that the blocking coefficient[33]of 2 μm membrane is the lowest.According to the pressure drop increase tendency,PTFE-2 also shows better performance than that of others.The final pressure drop of PTFE-2 is 160 Pa (only～14% higher than initial pressure drop) after 3600 s of filtration,while that for PTFE-1,PTFE-3,and PTFE-4 are 230 Pa,210 Pa,and 195 Pa,respectively (～28%,～31%,and～30%higher than that of initial pressure drop).All membranes show excellent fly ash rejection greater than 99.99%,while smaller membrane pore size exhibit higher fly ash retention rate.

3.3.3.Effect of particle size on filtration performance

To investigate the effect of fly ash size on filtration performance at 60% RH,fly ash with different average particle size were conducted to do the experiment.The results as shown in Fig.9.Fly ash with their average particles size much smaller than the membrane pore size is easy to penetrate in the filter,resulting in a severe pore blocking phenomenon.Thus,pressure drop of FA-1 and FA-2 was rapidly increased in 600 s.The resistance may attribute to the deep pore blocking.Subsequently,the growing trend of pressure drop begin to slow down,the resistance may assign to the formation of filter cake on membrane surface.While for that of FA-3(～1 μm),since parts of fly ash may aggregate to secondary particles with comparative large diameter.Therefore,two kinds of fouling conditions may simultaneously exist in the filtration process,leading to a quick pressure drop increase in 600 s,and then gradually increased in the following filtration process.As for FA-4 and FA-5,the pressure drop is much lower than that of others in the whole filtration process.This result can be explained as follows:the original particle size is much large than that of PTFE-2 membrane,combing the aggregation effect of fly ash under the humidity atmosphere,there may few of pore blocking in the filtration process.The fly ash rejection is similar to the above result(greater than 99.99%),membrane showed higher rejection to larger particles.

3.3.4.Effect of filtration velocity on filtration performance

Fig.9. Effect of particle size on (a) pressure drop and (b) fly ash removal efficiency.PTFE-2,filtration velocity was 2 m·min-1,inlet fly ash concentration was 600 mg·m-3,RH=60%.

Fig.10. Effect of filtration velocity on (a) pressure drop and (b) fly ash removal efficiency.PTFE-2,FA-3,inlet fly ash concentration was 600 mg·m-3,RH=60%.

Table 4 Quality factors under different operating conditions

The influence of filtration velocity on filtration pressure drop and removal efficiency was investigated at the filtration speed of 1,2,3,4 and 5 m·min-1.As shown in Fig.10,When the filtration velocity was below 2 m·min-1,the pressure drop only has a slight increase in 3600 s from 80 Pa to 90 Pa,and 140 Pa to 190 Pa,respectively,indicating a quick stability and slight increase of filtration resistance.When filtration velocity up to 3 m·min-1and even higher,there is an obvious increase of pressure drop in the filtration process.This phenomenon proved that higher filtration velocity is negative for maintain low filtration resistance,and more time will be spent to form a stable filter cake.The fly ash rejection of the membrane at higher filtration speed is lower than that at slow filtration velocity (Fig.10b).That is because the diffusion effect may be weakened with the increase of filtration velocity.More MMPS may pass through the membrane and leading to a decrease of filtration efficiency.

The Qf of each filtration conditions were calculated as shown in Table.4.When the pore size of PTFE membrane is 2 μm,fly ash average particle size is 1 μm,the inlet concentration is 600 mg·m-3,filtration velocity is 1 m·min-1,the system has the optimal Qf of 0.104.

4.Conclusions

Increasing humidity has a great influence on improving the interaction force of fly ash.The filtration efficiency and resistance can be optimized by controlling the humidity of the system.60% RH is the optimal condition to efficient filtrate coal fly ash with PTFE membrane.The filtration system can maintain well at a comparative low pressure drop of 530 Pa and high gas permeance of 1440 m3·m-2·h-1·kPa-1.By systematically investigate the effects of membrane and fly ash parameters and operating conditions,the optimal filtration conditions were determined.Controlling the humidity of 60%,PTFE membrane of 2 μm,inlet fly ash concentration of 600 mg·m-3,fly ash average particle of 1 μm,the system can maintain the minimum filtration resistance and comparative high filtration efficiency.This work will be of potential benefit to instruct the following research to optimize the filtration parameters for complex filtration system.

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 is supported by the National Key Research and Development Project of China (2018YFE0203500),the High-end Research and Training Project for Specialty Leading Person of Jiangsu Higher Vocational Colleges (2020GRGDYX039),and the Qing Lan Project of Jiangsu Colleges.