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This article appeared in a journal published by Elsevier. The attachedcopy is furnished to the author for internal non-commercial researchand education use, including for instruction at the authors institution
and sharing with colleagues.
Other uses, including reproduction and distribution, or selling orlicensing copies, or posting to personal, institutional or third party
websites are prohibited.
In most cases authors are permitted to post their version of thearticle (e.g. in Word or Tex form) to their personal website orinstitutional repository. Authors requiring further information
regarding Elsevier’s archiving and manuscript policies areencouraged to visit:
An L9 orthogonal design methodology to study the impact of operating parameterson particulate emission and related characteristics during pulse-jet filtration process
Arunangshu Mukhopadhyay ⁎, Kamal DhawanDepartment of Textile Technology, National Institute of Technology, Jalandhar - 144011, India
a b s t r a c ta r t i c l e i n f o
Article history:Received 23 December 2008Received in revised form 29 April 2009Accepted 29 May 2009Available online 10 June 2009
Keywords:Air to cloth ratioEmissionBaffle plateCycle timePM2.5
Punch density
This study embodies experimental characterization of emitted particulate and filtration performance undervaried situation in a pulse-jet cleaning process. Tests were conducted under simulated condition in afiltration apparatus consisting four bags. The effect of four different factors such as fabric punch density,baffle plate height, air to cloth ratio and cycle time have been investigated on the key parameters; emission,pressure drop along with PM2.5 and average particle diameter of emitted particulate matter in a pulse-jetfiltration process. Experimental investigation based on L9-orthogonal design shows that emission is reducedwith the increases in punch density and pulse cycle time; but it increases up to a certain extent with theincrease in air to cloth ratio. However baffle plate height has no effect on the emission. On the other handpressure drop across the tube sheet increases with the material consolidation, air to cloth ratio and pulsecycle time; but the above parameter first decrease with the increase in baffle plate height. PM2.5 (based onthe number distribution) is found to be mainly affected by the baffle plate height and cycle time; as it firstincreases and then decrease with the increase in baffle plate height but it shows reverse trend with theincrease in cycle time. Average particle diameter based on number volume is found to be mainly affected bythe baffle plate height and cycle time. With the increase in time of filtration, both emission and pressure droptend to increase without affecting PM2.5 and average particle diameter based on number volume.
Gas cleaning is of prime importance in many process industrieslike cement industries, coal-fired power plants, municipal wastecombustion systems (MWCS) andmany industrial processes [1,2]. Theemissions of particulate matter are variable with particle concentra-tion range from b1 g/m3 to more than 250 g/m3 and size of particlesare predominantly very fine (0.1–25 µm). These particulate emissionstandards differ for different countries [3]. It is reported that particlesless than 2.5 µm in diameter (PM 2.5) cause the greatest health risks[4]. Particular attention is therefore needed to study the controlmechanism of very fine particulate matter. In past decade, use ofindustrial bag filters operated in the principle of pulse-jet filtrationhas got rapid surge as it prove to be most efficient and versatile. In thefiltration process, pulse jet cleaning is a technique whereby a short,periodic, high pressure burst of air is fired into the clean side of thefabric. The particles are dislodged and the pressure drop falls to anacceptable level [3,5–8]. Pulse-jet cleaning is also called ‘on-linecleaning’ because the back pulse is of very short duration and airfiltration is continuously maintained [3,5]. Pulse-jet filtration can
meet the stringent particulate emission limits regardless of variationin the operating conditions.
The penetration or accumulation of particles in the fabric materialis assumed to take placewhen the fabric is least protected, i.e. exposedto high impact velocity which takes place just after the cleaning. Muchearlier Leith and First [9] and Leith and Ellenbecker [10] reported thatthe gradual seepage of collected dust through the fabric, into thecleaned gas stream, is more important than straight-throughpenetration in a pulse-jet filter well conditioned with dust. Seepageis a failure of the fabric to retain collected dust rather than a failure tocollect that dust in the first place. Seepage occurs when the dust-conditioned bag strikes its supporting cage at the end of a cleaning
Table 1Actual value of variables corresponding to coded levels.
Levels
Factor Code −1 0 1
Fabric (Punches/cm2) A 100 150 200Baffle plate height (mm) B 420 840 1260Air to cloth ratio (m/h) C 77 87 97Cycle time interval (s)⁎ D 18 36 54[Time between two rows of bags (s)] [2] [4] [6]
⁎Considering 10 rows of industrial bag house, a time interval of t s between two rows ofbag will result in cycle time of 9t s for each row of bags.
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pulse, driven by the pressure drop across the fabric during normalfiltration. The impact of the fabric on its cage drives dust loosened bythe cleaning pulse through the fabric and into the cleaned gas stream.Just after the cleaning the penetration is high because particlessmaller than the fabric pore size escapes till the time pore bridgingstarts. This is termed as straight through penetration which decreasesgradually as dust cake thickness increases. However, earlier studiesproposed seepage as the more prominent mechanismwhich is higherwith dust cake thickness. In a very recent study [11] based onexperiments performed with calendered PPS needle felts at differentstages of aging, showed that ~96–99% of recorded emissions werecaused by direct particle penetration, which is by far the dominantemission mechanism compared to re-entrainment.
There are some useful studies on the filtration performance ofpulse jet bag house with the change of operating parameters andmachine design such as impact of dust feed entry (top, bottom,tangential) [12], filtration velocity, effect of venturi [13,14], pulseinjecting nozzle height [15,16], cycle time, electrostatic pre-charger[17,18] etc., fabric construction [19], air to cloth ratio [20] on filtrationperformance have been investigated. However, the effect of baffleplate in the filtration process is still not clear. Since, one of the majorthrust of recent development is to control PM2.5, no such study on
pulse-jet system along with overall emitted particle characteristicshave been reported. In the present work overall performance of pulse-jet filtration has been investigated thoroughly in terms of air to clothratio, fabric construction, cycle timing and baffle plate. The emittedparticulate characteristics and filtration performance have beenstudied for better understanding about the filtration process.
2. Experimental
2.1. Materials
Three varieties of polyester needle felt nonwoven fabrics deferringin needle density are used to study the effect of different varieties onthe filtration performance. Three different nonwoven fabrics areproduced with the variation of punch density at three different levels(Table 1). All the nonwoven fabric samples of nominal areal density350 gsm are prepared from 100% polyester fiber using 1.4 denier. Thefabrics are pre-needled with 20 punches/cm2 and the final needlingwas done alternatively on each side of the fabric (first needling fromtop then from bottom and finally from top). After final needling,calendering of all the fabric was done. The machine speed, needledensity on the board and the strokes per minutewere chosen in such a
Fig. 1. The schematic diagram of filtration apparatus.
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way so as to obtain required punches/cm2. Each of the samples wastested for filtration efficiency, pressure drop and emitted particlecharacteristics. Length and diameter of filter bags used were 0.8 m and0.12 m respectively; and total surface area of single bag was 0.313 m2.
2.2. Experimental design and conduct of experiment
The validity of the conclusions that are drawn from an experimentdepends to a large extent on how the experiment was conducted.Therefore, the design of the experiment plays amajor role in the eventualsolution of the problem that initially motivated the experiment.
In this work L9 (34) factorial design with four experimental factorsand three levels of each factor is used to investigate the influence ofpunches/cm2, baffle plate height, dust rate feed and pulse timing in-terval (Table 1) on the filtration performance. The schematic diagram ofdesigned filtration apparatus is shown in Fig. 1. The said apparatus isdesigned based on simulated condition as operation of industrial baghouse. It consists of aerosol feeding zone, draft zone, filter unit zone andpulsing zone. This apparatus operates under the principle of ‘negativepressure’. Inside the filtration unit, 4 bags are placed in two rows (twobags in each row). During pulse jet cleaning, onset of pulse cleaning andthe time interval between two rows is adjustable. Based on L9 designmethodology, nine different combinations with four factors are given inTable 2. The other equipment related parameters such as tank pressure(200 kPA), nozzle diameter (20 mm), distance between filter openingand nozzle (30 mm), and inlet dust quantity (3500 g/h) are kept ascontrol factors. However, inlet dust concentration is changed even at afixed inlet dust quantity depending on air to cloth ratio.
The experiments are performed in a random order as the serialnumber in the order of 1, 6, 3, 8, 5, 4, 9, 7 and 2. Reason for ran-domization is for effective statistical analysis through unbiased es-timation of the impact of factors and for validity of inference drawn.The actual values of variables corresponding to the coded levels areshown in Table 1. Further to this normal probability plot is used toevaluate the normality of the distribution of the output parameters.From the normal probability plots it is found that the residuals for eachoutput parameters are fitted close to the normal distribution whichvalidates the design of experiment followed by statistical analysis.
The emission and tube sheet pressure drop for each sample ismonitored over a span of 12 h divided equally in three blocks. Towardsthe end of each block of 4 h time duration, emission value is measuredbased on last 1 h collection of particulate matter by stack sampler;whereas, pressure drop is monitored throughout the process.
However, residual pressure drop value at the end of each block isused for present analysis. Considering 10 rows of industrial bag house,a pulse time interval of t s between two rows of bag will result in acycle time interval of 9t s for each row of bag. In the present case,corresponding to maximum and minimum pulse timing interval foreach row, number of pulse cycle in first 3 h time (i.e., before themeasurement of emission at the first block) will be 200 and 600respectively. Over 200 filtration cycles, pressure drop is found to bestable for successive cycles. However, for greater number of cycles,there is still a slow increase. It may be added that in an earlierexperiment by Simon et al. [16], 200 or more number of clogging andcleaning cycles was considered for attaining a stable structure.
As mentioned, the emission is monitored by the use of stacksampler [21] which provides the assessment about the amount ofemitted particles per unit volume. From the above information onecan calculate the filtration efficiency of the bag house system. Furtherto this, the collected particles at the stack can be investigated (usingparticle size analyzer) for the assessment of their characteristics.
The dust particles collected in the thimble are fed to the laser basedCIS-50-Particle Size and Shape Analyzer [22] for the determination ofparticle size and particle size distribution based on Time of TransitionTheory. The average diameter based on number volume and PM2.5 areevaluated. Average diameter based on number volume [D (3,0)] can beexpressed as follows:
where, × indicates individual particle diameter.PM2.5 which is defined as the particles less than 2.5 µm diameter, is
obtained by geometrical construction (Fig. 2) of percentage cumula-tive number distribution against the particle size inmicrons. The point
Fig. 2. Geometrical analysis of particle size distribution.
Table 3ANOVA results and % contribution of different factors.
Fcalculated % Contribution
Parameters Punches/cm2 Baffle plate height ACR Cycle time Block Punches/cm2 Baffle plate height ACR Cycle time Block
Emission (mg/Nm3) 9.46 No effect 9.9 137.3 8.7 4.8 – 5.1 78.17 4.4Pressure drop (mm of water) 15.04 31.2 17.06 15.05 46.11 11.34 23.54 12.89 11.36 34.81PM2.5 No effect 16.01 No effect 8.27 No effect – 36.46 – 17.65 –
Diameter based on number volume No effect 4.19 No effect 5.11 No effect – 15.68 – 20.27 –
Fig. 3. Effect of punch density and air to cloth ratio on emission.
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E indicates the size of particles (2.5 µm). The measure of ‘AD’ is takenas PM2.5 on the basis of number distribution of particles.
The ANOVA techniquewas conducted to see the effect of individualfactors on the filtration performance and emitted particulate matter.Since three blocks (at different period of time frame) was taken, totaldegree of freedomwas 26 (total combination 9×3=27, hence degreeof freedom is 27−1=26). F ratio and percentage contribution ofdifferent factors were studied. From the ANOVA analysis, % contribu-tion of different factors was evaluated based on the followingexpression.
% contribution=(SSf−dff .Ve)/SSTwhere SSf = Sum of square of the factor,dff = Degree of freedom of the factor,Ve = Mean square of pooled error, andSST = Total sum of squares.
Finally regression equations relating the main factors withemission, pressure drop PM2.5, and average diameter based onnumber cumulative have been determined.
3. Results and discussions
3.1. Emission study
It is observed from the Table 2 that with the passage of time (i.e., theeffect of block) emissions are increasing for all experimental runs asconfirmed by paired t-test (calculated t values for Blocks 1 and 2, and forBlocks 2 and 3 are 2.49 and 2.53 respectively). It is also observed that themain factors affecting the emission are punch density (A), air to clothratio (C) and cycle time (D) (Table 3). Baffle plate (B) has a very littleeffect on the emission, and therefore neglected during forward feedregression equation. Out of three factors asmentioned above, cycle time(D) has a greater impact over the emission followed by air to cloth ratio(C) and punch density (A). Following ANOVA three different regressionequations have been derived for all three blocks (I, II and III).
All the regression equations possess high levels of R2 (0.96–0.98).It is observed from the Fig. 3 that with the increase in punches/cm2 ofthe fabric, initially there is a little decrease in emissions, but it reducesit to a larger extent at higher level of punch density (200 punches/cm2) of the fabric. The decreasing trend of emissions with the increasein punches/cm2 is expected due to the smaller size pore at higher
punch density which results in greater retention of the dust particleson the surface.
It is also observed from the Fig. 3 that with the increase in air tocloth ratio, initially there is an increase in emission up to a certainlimit and finally the emissions are reduced at higher air to cloth ratio.This is contrary to the earlier findings [20], where the emission isreported to have increased with the increase in air to cloth ratio. Thiscan be explained on the basis of decrease in dust concentration levelwith the increase in air to cloth ratio [Fig. 4]. Decrease in dustconcentration usually results in a lower value of emission; but in thepresent case it is counter-balanced by the effect of the higher value ofair to cloth ratio. Initially with the increase in the air to cloth ratio, theextent of emission is predominately affected by air to cloth ratio, butbeyond a certain level as the dust concentration decreases substan-tially, emission value decreases despite the increase of air to clothratio.
It is observed from the Fig. 5 that the emissions are higher at lowerlevel of cycle time and there is drastic decrease in emission at middlelevel of cycle time and finally a small change in emissions are observedat higher cycle time. This may be due to number of pulsing in a specifictime period. At lower cycle time number of pulsing will be more ascompare to higher cycle time in a specific time period. Duringexperimental run it was found that the emissions was prominent atthe time of pulsing only (through visual observation as the outer caseof filter unit is made out of acrylic sheet) and reason for the same maybe due to sudden opening of fabric pores at the time of pulsingfollowed by the direct penetration of dust particles as proposed byBinnig et al. [11].
As the cycle time increases, dust layer thickness over the fabricfurther becomes higher which also filter out the incoming dust overthe fabric, thus decreasing emission. On the other hand as thethickness of the dust layer increases with the increase in cycle time;the additional dust can penetrate during pulsing time followingseepage theory. The latter effect may counterbalance the earlier effectresulting in no change in emission beyond a certain limit of cycle time.
3.2. Pressure drop study
It is observed from the Table 2 thatwith the passage of time (Block-Ito Block-III), pressure drop increases for all experimental runs. It is alsoobserved from Table 3 that the factors affecting the pressure drop inorder of decreasing severity are baffle plate height (B), air to cloth ratio
Fig. 4. Effect of air to cloth ratio on dust concentration.
Fig. 5. Effect of air to cloth ratio and cycle time on emission.
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(C), cycle timing (D) and fabric consolidation (A). Three differentregression equations have been derived for all three blocks [I, II and III].
Multiple regression equations are derived without consideringquadratic function as the degree of freedom available for all blockswas eight. Since quadratic function has not been considered (becauseof non-availability of degree of freedom), R2 values are not at a higherlevel (0.62–0.72). However, present R2 values can be considered to beadequate since more than 60% data can be explained based on theregression equations.
It is observed from the Fig. 6 that the pressure drop follows anincreasing trend when punches/cm2 of the fabric is increased. Thisincreasing trend of pressure drop is expected due to decrease in thefabric compactness. As punches/cm2 is increased, the structure of the
fabric became more compact thus reducing the porosity level whichresults in higher pressure drop.
It is observed that with the increase in time (experiment at differentblocks), pressure drop increases. Although industrial filters arepredominately surface filters but depth filtration is inevitable. Smallparticles canpenetrate up to a certain depth of fabricwhich is difficult toclean. With the progress of filtration time [Block-I to Block-III], fabricpores gradually blocked by the dust particles thus reducing the overallporosity of the fabric which in turn results in increased pressure drop.
It is observed from the Fig. 6 that the pressure drop follows adecreasing trend when baffle plate height is increased. Baffle platehelps in removing the heavier dust particles through impingementand velocity reduction, followed by gravitational effect. It can reducethe load at the filter bag by removing heavier dust particles; thereforethe pressure drop across the tube sheet decrease as the baffle plateheight becomes higher. However, it has no effect on emission, sinceemission predominately depends on small size particles penetratingthrough the fabric.
It is observed from the Fig. 7 that the pressure drop follows anincreasing trend when air to cloth ratio is increased; this is expected
Fig. 8. Effect of baffle plate height and cycle time on PM 2.5.
Fig. 9. Effect of baffle plate height and cycle time on average particle diameter.Fig. 7. Effect of air to cloth ratio and cycle time on pressure drop.
Fig. 6. Effect of punch density and baffle plate height on pressure drop.
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due to greater air drag. Further it is noticed that pressure drop followsan increasing trend when cycle time is increased. This is expected dueto the greater time availability for the deposition of dust which offeredgreater drag on the incoming aerosol.
3.3. PM2.5 study
It is observed from Table 3 that PM2.5 is not affected by filtrationtime as confirmed by paired t-test. It is also observed that the mainfactors affecting the PM2.5 are baffle plate height (B) and cycle timing(D) (Table 3). Out of two main factors which are mentioned above,factor B has a greater impact than factor D over the PM2.5.
Increase in punches/cm2 although reduce the size of pores; but itmay not be sufficiently small to restrict particle size below 2.5 µm. Itmay be added that in recent practices through the incorporation ofmembrane at the upstream side of filter can restrict particle size below2.5 µm [23]. The effect of baffle plate (factor B) and cycle time (factorD)on particle number based on PM2.5 (N) can be seen in the followingregression equation.
N = 86:47 + 3:21B − 5:37B2 + 1:66D + 4:75D2:
Since R2 value is relatively small (0.59), the significance of theequation is judged by the F value (7.903) which is much higher thanthe table value (Ftable, 0.05,4,22=2.82, Ftable, 0.01,4, 22=4.31).
It is observed from the Fig. 8 that with the increase in baffle plateheight there is an increase in PM2.5 up to a certain extent and beyond acertain level, PM2.5 tend to decrease. It was observed earlier that baffleplate do not have influence over total emission. Since the larger sizeparticles are removed through baffle plate, proportion of small particlebecome larger in dust layer formed over the fabric which in turnresults in higher emission of smaller particle. However, beyond acertain limit the proportion of small particles in emission reduceswhich may be attributed to the decrease in pressure drop across thefilter at larger baffle plate. We envisage that pressure drop across thefilter during filtration can influence force applied on the particle overthe fabric and therefore less number of small particles can seepedthrough the filter media at lower pressure drop.
It is observed from the Fig. 8 that PM2.5 first decreases with theincrease in cycle time. The initial trendmay be due to the restriction ofsmall particles (PM2.5) by additional dust layer over the fabric. Greaterthe thickness of layer, greater will be the capturing of PM2.5. However,beyond a certain level of cycle time, increase in PM2.5 may be due tosubsequent increase in pressure drop across the filter, the effect ofwhich may be predominating over the effect of layer thickness.
3.4. Average diameter based on number volume
It is observed from the Table 2 that average diameter numbervolume is not affected by time. It is also observed from Table 3 that themain factors that affecting the number volume are baffle plate height(B) and cycle timing (D). Fabric consolidation (A) and air to cloth ratio(C) has a very little affect on the above parameter (Table 3). Out of twomain factors which are mentioned above factor D has a greater impactover the number volume than factor B. The following regression
equation shows the relationship between the operating parametersand average particle diameter (D).
D = 3:81− 0:74B + 0:68B2 − 0:16D − 1:58D2:
Although R2 value of the above equation is relatively small(0.46), but the calculated F value (4.65) is higher than the tablevalue (Ftable, 0.05,4,22=2.82, Ftable, 0.01,4, 22=4.31). It is observed fromthe Fig. 9 that at lower baffle plate height, average particle diameter islargest followed by little decrease and finally almost constant. It is alsoobserved from the figure that the average particle diameter increaseswith the increase in cycle time up to certain level and then decreases forhigher cycle time. It has been noted that the trend of average particlediameter with the change of process parameter is opposite to that ofPM2.5. This canbe justified sincewith either increase ordecreaseof smallparticles in emission, average particle diameter will be affectedinversely.
The overall summary of the findings of the preceding sections canbe presented in Table 4.
4. Conclusions
In this experimental study, the effect of different operating parametersand material consolidation of nonwoven fabric on the filtrationparameters and emitted particulate characteristics have been investigatedunder real operating situation in a pulse-jet filter unit. Based on L9orthogonal design of experiment for carrying out the test followed bystatistical analysis, the following conclusions have been drawn:
▪ With the increase in punch density of nonwoven fabric, emissiondecreases and pressure drop increases; however, PM2.5 andaverage particle diameter remains unaffected.
▪ Emission is not affected by the baffle plate height. With theincreases in baffle plate height, pressure drop decreases, whereasPM2.5 first increases and then decreases, and average particlediameter in the emitted dust shows reveres trend to that of PM2.5.
▪ With the increase in air to cloth ratio, emission first increases up toa certain level then decreases, whereas pressure drop increasessteadily. PM2.5 and average particle diameter based on numbervolume is not affected by air to cloth ratio.
▪ With the increase in pulse cycle time, emission decreases up to acertain level but pressure drop increases steadily; whereas PM2.5
first decreases then increases with the increase in pulse cycle time,and average particle diameter based on number volume firstincreases then decreases.
▪ With the increase in time of filtration, both emission and pressuredrop tend to increase without affecting PM2.5 and average particlediameter based on number volume.
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of air pollution control equipments, Tech Books International, New Delhi, India,2004.
Table 4Summary of impacts of studied parameters on filtration efficiency, pressure drop, PM2.5 and diameter based on number volume.
Parameters
Factors Emission (mg/Nm3) Pressure drop (mm of water) PM2.5 Average size of particles (µm)
Punches/cm2 Decreases Increases No effect No effectBaffle plate height No effect Decreases First increases then decreases First decreases then increasesAir to cloth ratio First increases then decreases Increases No effect No effectCycle time Decreases up to a certain limit Increases First decreases then increases First increases then decreasesEffect of time Increases Increases No effect No effect
133A. Mukhopadhyay, K. Dhawan / Powder Technology 195 (2009) 128–134
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