RELATIONSHIP BETWEEN POWDER COMPOSITION AND SINGLE SCREW EXTRUSION FOR MANUFACTURING RELIABLE RANDOM CERAMIC PACKED BED ACCORDING STATISTICAL APPROACH

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1 journal webpage: Ceramcs-Slkáty 60 (2), (206) do: 0.368/cs RELTIOSHIP ETWEE POWDER COMPOSITIO D SIGLE SCREW EXTRUSIO FOR MUFCTURIG RELILE RDOM CERMIC PCKED ED CCORDIG STTISTICL PPROCH # MI SLEM Mneral Processng Research Center, Chemcal Engneerng Department, Sahand Unversty of Technology, Tabrz, Iran Center of Excellence for Color Scence and Technology, Tehran, Iran # E-mal: salem@sut.ac.r Submtted October 24, 205; accepted January 20, 206 Keywords: Random ceramc bed, Relablty, Compressve strength, Falure probablty, Mcrostructure The present nvestgaton provdes a detaled relatonshp between the powder composton and relablty of random ceramc beds. Ths evaluaton s mportant due to standng n the lqud-gas contactors as well as predctng lfetme. It s stll unclear whether the normal dstrbuton s the most sutable functon for estmaton of falure. y developng the applcaton of ceramc beds n the chemcal plants, a specal attenton has been pad n screenng strength dstrbutons. To acheve ths goal, an expermental theoretcal study was presented on compressve strength dstrbuton. The powder compostons were prepared accordng to the statstcal response surface methodology and then were formed by a sngle screw extruson as Raschg rngs. The compressve strength of specmens was measured to evaluate the strength data sets by normal and Webull dstrbutons. The results were analyzed by the kake nformaton crteron and the nderson Darlng test. The accuracy of dstrbutons n predcton fracture was dscussed. Introducton Commercally, many random norganc beds are avalable for chemcal engneerng process such as adsorpton [-3], dstllaton, humdfcaton and absorpton [4, 5]. Despte the progress n manufacturng dfferent knds of beds n the last decades, the brttle nature of ceramcs has stll made the lmted lfetme [6]. The lmtatons of metal beds due to corroson n acdc envronments have trggered the development of ceramc materals. The fracture of ceramcs n servce takes place wth lttle or no plastc deformaton when cracks propagate under appled compressve stress [7]. If the stress ntensty reaches a crtcal level at the crack tp, fracture occurs mmedately [8]. Ceramc bodes ndcate consderable varaton n strength, due to ther extreme senstvty to the presence of defects wth dfferent szes lke pores [9]. The fracture of body starts from crtcal flaws and always propagates from the largest flaw, favorably orented n the tensle stress drecton [0, ]. The defect sze, shape, dstrbuton and orentaton statstcally affect the compressve strength. Raschg rng s a hallow cylnder whch s the smplest shape of random beds and frequently s used n separaton processes [6]. Lessng rng and others wth nternal parttons are less appled n comparson to Raschg rng n the lqud-gas contactors. The saddle, erl, Intalox and Pall rng are the other types of random packngs. Unfortunately, the breakage of bed ncreases the process problems such as pressure drop. The ceramc bed compostons at least consst of three components that play the fundamental roles n processng and performance of chemcal unts. Kaoln, for plastcty [9, 2], nephelne syente [3, 4] or talc [5] wth lmted content as fluxng agents and alumna and zrconum slcate [6] as fllers were appled. fter sutable processng, these formulatons should lead to a matrx wth hgh strength and approprate chemcal resstance [7, 6]. The typcal mcrostructure of ceramc body conssts of a glassy matrx that contans mullte and/or partally undssolved quartz partcles. Mullte, 3l 2 O 3 2SO 2, s the prncpal crystallne phase formed durng the snterng process. The shape and content of ths crystal nfluence the mechancal propertes [7, 8]. ccordng to glass/crystallne rato and snterng temperature, C, ths type of ceramcs can be categorzed nto porcelan stoneware group. In order to acheve the relable methodology n the fabrcaton of ceramc beds, the strength data should be analyzed by statstcal approaches, because the probablty of falure potentally depends on a dangerous crack, greater than a characterstc crtcal flaw. lso, the fracture depends on volume of materal under the stress because the larger volume ncreases the probablty of crtcal flaw content. One of the commonly used methods for the evaluaton of strength dstrbuton and structural relablty of ceramc materals s the Webull statstcal theory, whch descrbes the strength of a brttle materal based on the falure probablty []. The stressed volume s evaluated by a normalzng parameter, correspondng 48 Ceramcs Slkáty 60 () (206)

2 Relatonshp between powder composton and sngle screw extruson for manufacturng relable random ceramc packed... to the strength n whch 63.2 % of the samples are faled and the Webull modulus whch ndcates the nature and dsperson of cracks. Hgher Webull modulus s related to materals wth homogenously dstrbuton of cracks n the narrow strength range whlst the lower values ndcate heterogeneously dstrbuton of flaws. The slow and stable crack growth s observed n ceramc materals when they are subjected to stress below the crtcal value, especally n the presence of chemcal agents. The strength degradaton over tme and reducng the lfetme of materal are result of crack growth. If chemcal agents are presented at crack tp under stress, the chemcal reacton between the agent and ceramc materal may be lead to break of bonds. Consequently, the crack grows slowly untl reachng the crtcal sze for fracture. Lteratures report the Webull modulus values for ceramc Raschg rngs between 3 and 5, [3-6]. Ths ntroduces the necessty to nvestgate the relablty of ceramc beds, whch are normally polycrystallne solds, exstng n abundant glassy phase. Generally, the man challenge n the relable desgn of ceramc beds s the exact estmaton of falure probablty under prescrbed load. Due to the scatter of poston, length and orentaton of crtcal cracks, the strength of ceramcs vary unpredctably from one composton to another composton, even f dentcal specmens are tested. Thus, n the desgn of engneerng ceramcs, a probablstc technque s recommended. In ths study, the author ntends to present a detaled mcrostructural characterzaton on dfferent ceramc compostons fabrcated for lqud-gas contactors. These observatons show that the well understandng fracture behavor and ts relaton to mcrostructure are mportant from capablty of standng n aggressve envronment. It also can be helpful n developng new materals wth suffcent mechancal propertes. In order to acheve ths goal, ten strength data sets of ceramc beds n the form of Raschg rngs are ftted by normal and Webull dstrbutons. ased on the mnmum nformaton crteron and the nderson Darlng methods, the most sutable dstrbuton s determned and the devatons are dscussed n detals. Materals and methods Raw materals Kaoln, nephelne syente and talc collected from a ceramc ndustry of Iran were selected as raw materals. These materals were procured n ground form wth gran sze of 63 mm, maxmum. The chemcal analyss was performed by X-ray fluorescence technque (Model S4 Explorer 7KP03, rucker, Karlsruhe, Germany). Mneralogcal analyses were carred out wth an X-ray powder dffractometer (Cu-Ka, -fltered radaton, (XRD; Model D8-dvance, rucker, Karlsruhe, Germany). Expermental desgn The conventonal technques for optmzaton nvolve changng one varable at a tme and keepng the others n constant level. The nteracton effects of varables cannot be consdered n these methods. However, the response surface methodology s a useful technque for studyng the effect of several parameters by changng the varables, smultaneously. lso, the optmzaton of mechancal characterstcs by the classcal methods s extremely expensve when more than two varables are evaluated. In order to overcome ths problem, the response surface methodology was selected to optmze the mechancal propertes. The man use of mxture desgn n ths study s to relably guarantee the dependency between composton and the outcome of experments, wth the mnmal number of experments. In the expermental desgn, the followng effects were consdered. () The effect of kaoln on mechancal propertes. () The bnary mxng of materals whch provdes the second order nteractons. () The expermenters whch represent the ternary nfluence of materals. The response surface algorthm, whch s consdered n ths nvestgaton for mxng of raw materals, s based on statstcal and mathematcal technque for determnng optmum composton. In ths experment approach, the total amount of materals s kept constant and measured propertes can be evaluated as a functon of component fractons. The best composton can be determned by usng the composton property n a traxal dagram. polynomal equaton, whch descrbes the property, can be correlated as a functon of materal fractons, x. In a system wth n components, there are n- ndependent compostons. The polynomal functon, f(x,x j,x k ), can be expressed by the followng equaton [9]: f ( x, x, x ) = β 0 + β x + βj x xj + < j Ceramcs Slkáty 60 () (206) 49 + j k < j j< k β x x x jk j k +... () where b s the constant of equaton. Ths equaton should be evaluated over M > n number of ponts (n ths study, M was consdered to be 0). Fgure shows the locaton of ponts n the mxture space and the component fractons are gven n Table. The maxmum amount of kaoln was consdered to be wt. % and at the most 5.00 wt. % of nephelne syente and/or talc was appled. The compostons were denoted by captal letters, -J, as reported n Fgure. The trangle desgn wth axal and central ponts s especally useful for ternary studes. In order to evaluate the effect of maxmum content of materals on mechancal propertes, the ponts at the vertexes of trangle were consdered. The ponts centered on each leg of trangle are related to the bnary mxtures. The pont n the center of trangle represents a ternary mxture

3 Salem. of materals. The nteror ponts between the centrod and vertexes represent the axal mxes. These mxtures contan a 2/3 porton of one and /6 portons of the other materals. The ndvdual proportons cover from zero to 5.00 wt.% for fluxng agent content, from base to vertex, n each of three axes. It s possble to ntroduce constrants on ndvdual postons and ncrease the number of expermental ponts. In ths case 0.00 D F 0.00 H Kaoln ephelne syente Talc Fgure. Expermental desgn and the locaton of selected ponts n the mxture space. Table. Proportons (wt. %) of raw materals n ceramc compostons. Composton Kaoln ephelne syente Talc C D E F G H I J G J E I C the expermental desgn wll be complcated and t s out of the artcle scope. The Mntab 5 software package (Iran) can set up the ndvdual constrans on the trangle desgn. The chemcal analyses of the compostons are lsted n Table 2. The clay s consttuted by kaolnte, llte and pyrophlte. lbte, orthoclase and olgoclase are the man crystallne phases of nephelne syente. The frequently phase, exstng n the second fluxng materal s talc. The content of other phases are neglgble. Ceramc Raschg rng preparaton Varous batches were wetted wth 30.0 wt. %, based dry weght of powder, of de-onzed water and mxed for 2 h. The samples were mantaned n closed plastc bags for 48 h to homogenze the paste humdty and then de-ared by a laboratory pug-mll. sngle screw extruder, prototype, were used n formng green tubes and the shaped samples were dred at 90 C durng 48 h. The obtaned specmens were cut n the shape of Raschg rng (Fgure 2), wth the same external dameter Lqud-n Lqud dstrbutors Support grds Gas-out Gas-n Compressve strength (MPa) eds flled by ceramc Raschg rng Lqud-out Fgure 2. Lqud-gas contactor and schematc of manufactured ceramc Raschg rngs. Falure probablty ormal Webull Falure probablty Table 2. Chemcal analyses (wt. %) of selected compostons. Oxde C D E F G H I J SO l 2 O K 2 O a 2 O CaO MgO Fe 2 O TO L.O.I* * Loss on gnton 50 Ceramcs Slkáty 60 () (206)

4 Relatonshp between powder composton and sngle screw extruson for manufacturng relable random ceramc packed... and length, 25 mm. The test specmens were heated n a laboratory electrcal kln (Model EX.500-6L, Excton, Iran) to 573 C at heatng rate of 5.0 C mn - and held n ths temperature for h to complete the quartz nverson. Fnally, the temperature of the furnace was ncreased at a rate of 0.0 C mn - to reach 90 C and held at ths peak temperature for 2 h n the ar atmosphere. Raschg rng characterzatons In order to characterze the fred samples, the total porosty was measured by rchmedes technque [20, 2]. The dameter and length of the samples were determned usng a mcrometer. The dametrcal compresson strength was measured by the unversal test machne (Model DY-26, damel Lhomargy, Rossy en re, France). The Raschg rngs were loaded usng the steel compresson plates at a head speed of.0 mm mn -. t least 20 samples were tested to measure the dametrcal compresson strength of Raschg rngs. The mcrostructure of the fred specmens was studed by scannng electron mcroscopy (SEM, Model 440I EOL, Oxford, UK) on freshly fracture and gold coated secton. Statstcal theores for ceramc bed evaluaton In relable desgn of engneerng ceramcs, the falure probablty should be consdered. It s dffcult and even mpossble to determne a falure probablty functon drectly by expermental tests. Therefore, the statstcal technques are frequently used for the determnaton probablty functon, ndrectly. The selecton a correct or best-fttng dstrbuton for a gven set of compressve strength data s an mportant step, especally when the probablty functon s senstve to the assumed model. In presented nvestgaton, normal and Webull dstrbutons are used n the predcton of falure. The normal dstrbuton s wdely appled to analyze lfetme of samples n desgn of ceramc devces. It s a dstrbuton that can be taken to account n the characterzaton the other types of dstrbutons. The form of normal functon can be presented as [22]: P( σ) 2 ( σ σ ) a exp α 2π 2α = 2 (2) where P(s) s the falure probablty of a ceramc rng due to flaws, σ s the unaxal appled stress, σ a and α 2 are the mean and varance of compressve strength, respectvely whch are calculated accordng to followng equatons: α = σ a 2 = σ ( σ σa) 2 (3) (4) where σ s the compressve strength of the th sample and s the number of data. The Webull dstrbuton for the brttle materals such as ceramcs was developed as a follows []: (5) where σ 0 s Webull characterstc strength corresponded to the stress at whch the probablty of falure s 63.2 %. Webull modulus, m, controls the shape of curve, beng larger as scatterng degree of dametrcal compresson strength decreases. Therefore, the hgh value of Webull modulus descrbes the relable materal. y maxmzng ts log-lkelhood functon, the two parameters n the Webull dstrbuton are determned [23]: and (6) (7) The kake nformaton crteron, IC, method s one of the strongest technques for determnaton approprate strength dstrbuton, promsng to obtan the confdence bounds. The IC determnes the agreement of ft for an estmated statstcal model by lnkng the lkelhood to a dstance between expermental and assumed dstrbutons [24] and s defned as: IC = 2 ln Y + 2k (8) where k s the number of parameters to be ftted and s consdered to be 2 for Webull dstrbuton. ln Y s the maxmzed log-lkelhood for a gven model and can be calculated by: lny = f ( ) (9) where f(y ) s the probablty densty functon of an estmated dstrbuton. The IC values can be drectly compared, preferrng the dstrbuton whch gves the smallest value. In typcal cases, the dfference n IC values (at least.5 2) corresponds to a relable ndcaton that one dstrbuton s superor to another [24]. The nderson Darlng test, DT, s used to select the best ft for a specfc populaton. It s one of the most powerful statstcal tools for detectng departures from normalty [25]. The equaton for the DT of ordered data can be presented as: DT 2 = S (0) where 2 S = [ ln F( Y) + ln( F( Y + ))] m σ P( σ) = exp σ 0 ( σ) ln ( σ) m ( σ ) m σ 0 = ( σ ) ln ( σ ) = 0 m m Y m () Ceramcs Slkáty 60 () (206) 5

5 Salem. where F s the cumulatve dstrbuton functon. ormally, the DT s compared to a crtcal value and then t s determned whether the data confrm specfed dstrbuton. In ths work, the dstrbuton s desrable that presents the small DT. Results and dscusson Mechancal behavors The parameters of normal and Webull dstrbutons are reported for data sets n Table 3. Ceramc Raschg rngs exhbt a rather wde range of mean strength. The addton of nephelne syente to the composton causes an enhanced strength, nferred by the abundant glassy phase n samples C, E and H respect to, D and G. Talc-bearng rngs are also characterzed by mproved compressve strength. Ths aspect was confrmed by the body C and I whch show strength near to 50.0 MPa. The other talc-rch rngs, F and J, dd not demonstrate the same effect on strength. However, the mxture prepared wth 2.50 wt. % of nephelne syente, D, ndcates a lower strength n comparson to F. The results of varance analyss for the nvestgated compostons are gven n Table 3. The α-value ndcates whch of the studed compostons are statstcally sgnfcant, based on examnaton of expermental data. If the α-value s less than or equal to.5 MPa, the effect s consdered sgnfcant. The man effect of a factor must be nterpreted ndvdually f there s no nteracton wth other factors. When one or more nteracton effects are sgnfcant, the nteractng factors must only be consdered jontly. ll α-values less than or equal to.5 are related to rngs wth lower strength, < 6.0 MPa. The compostons contanng lower amounts of nephelne syente and talc, both less than 2.5 wt. %, causes the lmted varance. α-values can be useful n comparng the strength dstrbuton as a functon of fluxng agent content. The sgnfcant change s observed wth addton of fluxng agents. The product prepared wth 3.33 wt. % nephelne syente and 0.83 wt. % talc, H, shows the Table 3. Parameters of normal and Webull dstrbutons. Composton ormal Webull σ a (MPa) α (MPa) m σ 0 (MPa) C D E F G H I J maxmum value but the mnmum α s related to sample G. ephelne syente s more effcent between the selected materals when more than 3.33 wt. % s added to composton. The ncrement n talc content also rases the α-value. Table 3 reports the Webull parameters. Sample ndcates the lowest m value among all materals and other ceramcs show smlar Webull modulus, rangng from 3.6 to 2.8. unform varaton s observable by changng kaoln and talc content. The hghest value s related to sample C n whch 5.00 wt. % talc s used. The nverse behavor s observed n Webull modulus wth content of nephelne syente and ths factor s very senstve to fluxng agent composton. Therefore, for manufacturng the satsfactory ceramc bed, the evaluaton of fluxng agent composton by response surface methodology s requred. The m-values of a smplex centrod desgn wth axal ponts are analyzed wth emphass on choosng and valdatng the correct model. polynomal model, Equaton 2, was adjusted to the expermental data and the standard non-lnear regresson coeffcent was calculated from the expermental and estmated values. The frst three terms of Equaton 2 represent the lnear model. The next three terms represent synergc or antagonstc bnary nteracton effects for the all possble pars of components and, along wth the lnear terms, forms the quadratc model. The last term represents the ternary nteracton effect and s usually mportant for systems havng maxmum or mnmum values n the nteror of trangle. Ths model was statstcally found to be most adequate wth a regresson coeffcent of R 2 = 98.2 %. m = 0x 375x x x x 2 (x x 2 ) 2 356x x 3 (x x 3 ) x 2 x 3 (x 2 x 3 ) x x 2 x 3 (2) Equaton 2 allows rapd predcton for the entre compostons under nvestgaton. The coeffcent of talc s hgher than the coeffcent of nephelne syente. On the contrary, the coeffcent related to the ternary combnaton of raw materals s postve. It can be concluded that the mxng of kaoln, nephelne syente and talc for manufacturng ceramc Raschg rng s effcent to acheve relable matrx and the only use of nephelne syente as a fluxng agent cannot play an effectve role n the mprovement of Webull modulus. Regard to the characterstc strength, the samples prepared wth low content of fluxng agents present the lowest value, < 5 MPa. The strength was found to decrease wth content of kaoln, ndcatng the week bondng between the partcles. The sgnfcant ncrease s observed n the rngs prepared wth maxmum amounts of talc, 5.0 wt. %. The opposte trend s observable n body contanng maxmum amount of nephelne syente. Regardless sample, when the summaton of fluxng agent content s consdered to be 5.00 wt. %, 52 Ceramcs Slkáty 60 () (206)

6 Relatonshp between powder composton and sngle screw extruson for manufacturng relable random ceramc packed... the maxmum characterstc strength s obtanable. The suffcent ncrement n strength s observed n samples C, E and I. The rng prepared wth 97.5 wt. % kaoln and 2.50 wt. % nephelne syente, D, shows the mnmum strength, 2.3 MPa, between ten studed compostons. The combnaton of nephelne syente and talc also slghtly affects the strength of rng. The total porosty of Raschg rngs are presented n Fgure 3 as a functon of startng materal proportons. The fluxng agent type and content nfluence the porosty, causng a sharp change n bulk densty due to dffuson of lqud phase nto the open pores n hgh snterng temperature. y applyng an approprate combnaton of raw materals t s possble to control the ceramc porosty. It s evdent that the low porosty s related to the rng prepared by hgh content of fluxng agents and s due to densfcaton of sample durng the snterng process. The hghest porosty corresponds to the composton prepared by low content of nephelne syente and talc. Total porosty effectvely controls the Webull modulus and characterstc strength. The strength was nversely proportonal to total porosty and ncreases exponentally wth reduce n porosty. The hghest characterstc strength that corresponds to the lowest porosty, < 3.0 %, was found to be 58.0 MPa, sample C. Generally, n the ceramc compostons contanng clays and fluxng agents, the mullte formaton mproves the strength [7]. The smultaneously mprovement n Webull modulus and characterstcs strength s the most pronounced f the amount of talc s hgher than 3.33 wt.%. 3.6 < m < < σ 0 < 50.0 MPa ephelne syente 9 Kaoln < m < < σ 0 < 60.0 MPa Fgure 3. Total porosty of Raschg rngs as a functon of materal proportons. Low Δ IC area 9.0 < porosty< 2.0 σ 0 < 20.0 MPa Kaoln.00 6 Talc Hgh Δ IC area porosty< < σ 0 < 50.0 MPa ephelne syente Talc Fgure 4. Varaton of ΔIC as a functon of materal proportons. 60 Statstcal analyses In order to compare the adjustment of falure probabltes, the IC ndex was calculated and the results are reported n Table 4. In the evaluaton of IC, the normal dstrbuton s used as a reference and ΔIC whch s the dfference between IC ndexes of normal and Webull dstrbutons are used. ΔIC > 0.0 means that the Webull dstrbuton fts strength data better than the normal dstrbuton. The varaton of ΔIC s llustrated as a functon of composton n Fgure 4. ased on the IC ndexes, when ΔIC > 2.0, an optmal dstrbuton can be dentfed. ccordng to the data presented n Table 4, there s bg dfference between the IC values of normal and Webull dstrbutons. For data set G, both of normal and Webull dstrbutons ft the strength data. Regardless the G composton, Webull dstrbuton gves the lowest IC values and s more sutable to ft those strength data. For data sets of, E and H whch contan the predomnately nephelne syente, Webull dstrbuton s an approprate functon for fttng the strength data and the normal dstrbuton cannot provde the better ft. For data set of G, there are two dfferent results: Webull s the best dstrbuton and normal functon can be used wth lmted error. Table 4. IC values of Raschg rng compostons calculated by normal (IC ) and Webull (IC W ) dstrbutons. Composton IC IC W C D E F G H I J Ceramcs Slkáty 60 () (206) 53

7 Salem. When the porosty of rng s less than 3.0 % normal dstrbuton fts strength data worse than Webull functon. The notceable success of Webull dstrbuton n fttng the strength data of dense rngs s substantated. Fgure 4 ndcates that the most favorable dstrbuton for rngs contanng hgh content of fluxng agents s Webull dstrbuton. Especally, for rngs prepared wth hgh amount of nephelne syente, a hgh devaton s observed from the normal dstrbuton. s llustrated n Fgures 3 and 4, the devaton from normal dstrbuton s drectly related to porosty. ased on the nderson Darlng test, the dstrbuton whch provdes the smallest DT value s more sutable functon for the nvestgated strength data. The comparson of results n Fgure 5 shows that the Webull dstrbuton gves the best fts wth all obtaned data. y consderng the content of raw materals n the compostons, t s possble to determne the desred area n whch the normal dstrbuton ndcates the better adjustment, Fgure 5a. The DT value ncreases as a functon of nephelne syente content n whch the lower porosty s obtaned. The lower value of DT s related to rngs manufactured wth lower content of fluxng agents. The hgh value of porosty s responsble for reducng DT. The adjustment of normal dstrbuton n dense rngs s mproved wth ncrement n talc content. From ths pont of vew, talc s more effcent materal compared to nephelne syente. The change n DT W of ceramc Raschg rng sets s llustrated n Fgure 5b. The samples prepared wth hgher content of nephelne syente ndcate the well ft wth Webull dstrbuton. DT W gradually ncreases wth addton n content of talc and kaoln. The DT W values of samples prepared by hgh amount of talc are consderably lower than that for the rngs contanng hgher content of kaoln. However, the composton contanng 2.50 wt. % nephelne syente and 97.5 wt. % kaoln, sample D, ndcates the maxmum devaton from Webull dstrbuton. The combnaton of materals sgnfcantly affects the adjustment of dstrbuton. The synergstc nteracton s observed between the porosty and DT W. The devaton ncreases wth porosty and the lower DT W value s obtanable for dense rngs. ccordng to the nderson Darlng technque, t s possble to fabrcate the rngs wth sutable relablty whch satsfes the Webull dstrbuton. In other word, mxng of talc and kaoln s more effcent wth the best performance due to an effectve ncrement n Webull modulus. Though, the DT W value ncreases drectly wth content of kaoln, the meanngful relatonshp s observable between DT W value and total porosty, Fgure 3. Mcrostructural evaluatons The morphology of crack n sample s presented n Fgure 6. The elongated cracks wth the sze of 500 mm are dspersed n some regons of ceramc matrx. The fne pores were more homogeneously dstrbuted n cross secton of rng. These nterconnected pores were presented n a wde sze dstrbuton and dfferent morphologes. There are two mportant prerequstes for the occurrence of a fracture n rngs: () the structure fals f one sngle flaw becomes crtcal and () nteracton dangerous flaws. neglgble nteracton between flaws s only possble f the defect densty s low. Sample s porous materals, n whch the grans are covered by small pores. In Fgure 6, the clusters of pores were observed around the crack. group of pores as well as Low DT area 9.0 < porosty< 2.0 σ 0 < 20.0 MPa Kaoln.00 Hgh DT area porosty< < σ 0 < 50.0 MPa Low DT W area 9.0 < porosty< 2.0 σ 0 < 20.0 MPa Kaoln.00.5 Hgh DT W area ephelne syente Talc ephelne syente a) Fgure 5. Varatons of: a) DT and b) DT W as a functon of materal proportons. b) 0.95 Talc 54 Ceramcs Slkáty 60 () (206)

8 Relatonshp between powder composton and sngle screw extruson for manufacturng relable random ceramc packed... ther nteracton cannot affect the fnal fracture because only the largest one s postulated the weakest-lnk hypothess. The defect sze s a natural consequence of Webull dstrbuton and by checkng the crack effect, t s possble to determne whether a data set obeys the Webull dstrbuton or not. If a pore dameter s large than the gran sze, the role of pore exponentally ncreases respect to fracture otherwse, t has no nfluence on strength. Under these condtons, strength depends on the szes of crack. The defect wth large dmenson n the structure of rng s responsble for the low values of characterstc strength. Hence, t s mpossble to elmnate the nherent defects n the structure of rng by addton of 5.00 wt. % nephelne syente. sgnfcant dfference between the morphology of pores was found n comparson to sample. The sphercal pores wth small sze are dstrbuted n dense mcrostructure. The potental reason that causes the devaton from the normal dstrbuton s the substantally decrease n the number of nterconnected pores. The SEM mcrograph taken from sample C, Fgure 8, represents some defects throughout the ceramc matrx. The lmted number of defects wth sze less than 250 μm embedded nto the structure. In comparson to sample, reduce n the number of cracks s the man reason for the ncrement n strength. The SEM mcrograph reveals the homogenzed dstrbuton of defects that causes the mprovement of Webull modulus. In the studed cases, the amount of talc s hghly mportant for the mechancal behavor, snce talc acts as an nterfacal phase that reduces the resstance to sldng of partcles [26]. The fne partcles of talc wth a lamnar structure and smooth surface provde an effectve lubrcaton on rough surface of clay mnerals therefore, the Webull modulus of sample s less than that for C. Fgure 6. SEM mcrograph of sample. The SEM mcrograph of sample s shown n Fgure 7. ccordng to the mcrostructural observaton, f 5.00 wt. % nephelne syente s partally replaced wth kaoln, the defect dmensons decrease consderably. These changes sgnfcantly promote the characterstc strength up to 32.0 MPa. However, reduce n Webull modulus s due to decrease n homogenety of ceramc matrx and randomly dstrbuton of cracks durng the extruson. Fgure 8. SEM mcrograph of sample C. Fgure 7. SEM mcrograph of sample. The SEM mages of sample E, F and I are represented n Fgure 9 to evaluate the effect of talc content on mcrostructure. Fgure 9a shows a relatvely heterogeneous structure wth some mcro-pores wth mean sze of 50 μm n dameter, estmated from SEM mcrographs. The presence of nephelne syente n composton makes the lqud phase formaton wth low vscosty as a result a dense structure was obtaned due to vscous flow snterng. The mcrostructure of rng prepared by 2.50 wt. % talc, sample F, s shown n Fgure 9b. The pore clusters were observed around elongated cracks. The mcrostructure exhbts homogeneous matrx wth relatvely low porosty, ndcatng that 90 C was enough to yeld dense ceramc materal. The large Ceramcs Slkáty 60 () (206) 55

9 Salem. cracks are seen to be dspersed n the body, generatng some stress that results n lower characterstc strength. Fgure 9c shows SEM observaton on fracture surface of the rng prepared by 3.33 wt. % talc and 0.83 wt. % nephelne syente, sample I. The exstence of small cracks wth rregular shape, 250 μm, homogenously dstrbuted n a ceramc matrx affects Webull modulus. The relablty s drectly related to crack dstrbuton n the ceramc structures. The materal partcles are rearranged by plastc flow due to lubrcatng effect of talc, producng relatvely unform mcrostructure. The rearrangement of partcles occurs steadly untl the talc content reaches the maxmum value, 5.00 wt. %. The lubrcatng effect becomes less marked when talc content s consdered to be less than 2.50 wt. %. Thus, greater talc content can facltate plastc flow durng the extruson as a result, the Webull modulus ncreases, consderably. However, some porous materals lke sample G may be faled due to surface or volume flaws. Each ndvdual flaw populaton has typcal sze and lead to dfferent dstrbutons. s a result, the strength dstrbuton resultng from dfferent flaw populatons wll overlap and t cannot be supported by an ndvdual dstrbuton. In the case the falure probablty can be defned by both of normal and Webull dstrbutons. Conclusons a) b) In the current nvestgaton t was demonstrated that the mcrostructural dfferences between the ceramc beds, result n dfferent behavors n terms of compressve strength and relablty. The procedure has been proposed to ascertan a more sutable falure probablty. The effect of mcrostructure defect on devaton from the normal dstrbuton was dscussed n detal by the expermental strength data. There s suffcent evdence that the Webull dstrbuton s frequently preferable respect to normal functon n the adjustment of strength data. The fndngs of the present study based on kake nformaton crteron and the nderson Darlng test also ndcate the Webull dstrbuton can provde an approprate fts for fully vtrfed rngs respect to porous beds. The nephelne syente-rch beds were more susceptble to crack propagaton whlst the talc-based rngs show the hghest Webull modulus and have the least susceptblty to fracture. Ths s of great mportance because the mcrostructure of ceramc materals can be modfed to ncrease ther lfetme. Though, the use of Webull dstrbuton for the analyss of strength data can be relable method n the desgn of random beds by modern technology, further processng factors such as materal preparaton, formng and snterng condtons should be consdered to apply the results n commercal stage. Therefore, the study remans open from the mentoned vew ponts. Fgure 9. SEM mcrographs of samples: a) E, b) F and c) I. c) References. Salem., kbar Sene. (202). Optmzaton of zeoltebased adsorbent composton for fabrcatng relable Raschg rng shaped by extruson usng Webull statstcal theory. Mcroporous and Mesoporous Materals, 63, do:0.06/j.mcromeso Ceramcs Slkáty 60 () (206)

10 Relatonshp between powder composton and sngle screw extruson for manufacturng relable random ceramc packed Salem., fshn H., ehsaz H. (202): Removal of lead by usng Raschg rngs manufactured wth mxture of cement kln dust, zeolte and bentonte. Journal of hazardous materals, 223, do:0.06/j.jhazmat Salem., kbar Sene. (20): Removal of lead from soluton by combnaton of natural zeolte kaoln bentonte as a new low-cost adsorbent. Chemcal Engneerng Journal, 74(2), do:0.06/j.cej Treybal R.E. (990). Mass Transfer Operaton. 3 rd ed. McGraw-Hll Publshng Inc, Tokyo. 5. McCabe W.L., Smth J.C., Harrott P. (200). Unt Operaton of Chemcal Engneerng. 6 th ed. McGrow-Hll Publshng Inc, ew York. 6. Strgle R.F. Jr. (987). Random Packngs and Packed Towers. Gulf Publshng Co, Houston Texas. 7. Saleh M., Salem. (2009): Influence of snterng temperature on mcrostructural changes of ceramc Raschg rng. Korean Journal of Chemcal Engneerng, 26(2), do:0.007/s y 8. Vllora J.M., Callejas P., arba M.F., audın, C. (2004): Statstcal analyss of the fracture behavour of porous ceramc Raschg rngs. Journal of the European Ceramc Socety, 24(3), do:0.06/s (03) Saleh M., Salem. (200). Porosty Strength Correlatons n Ceramc Raschg Rng: Effects of Snterng Temperature and Water Content. Internatonal Journal of ppled Ceramc Technology, 7(6), do:0./j x 0. Wachtman Jr. J.. (996). Mechancal Propertes of Ceramcs. John Wley and Sons, ew York.. Qunn G.D., Morrell R. (99): Desgn data for engneerng ceramcs: a revew of the flexure test. Journal of the mercan Ceramc Socety, 74(9), do: 0./ j tb08259.x 2. eyg Khosrowshah Y., Salem. (200): vew on organc bnder effects on techncal propertes of ceramc Raschg rngs ol. Soc. Esp. Cerám. Vdr, 49(5), Rostam., Salem., Paknjhad. (20): Influence of nephelne syente on mechancal relablty of ceramc Raschg rngs. Internatonal Journal of ppled Ceramc Technology, 8(2), do: 0./j x 4. Salem., Rostam,. (203): Effect of nephelne syente partcle sze on dametrcal compresson strength and relablty of extruded ceramc Raschg rngs used n packed towers. ol. Soc. Esp. Cerám. Vdr, 52(2), Salem., ghahossen S. (202): Determnaton of fluxng agents mxng rato for enhancng thermal shock resstance of ceramc Raschg rng va a systematc approach. Thermochmca cta, 545, do:0.06/j. tca Salem., Shrch S. (20): Renforcng of extruded ceramc Raschg rngs by dspersve partcles: the effects of alumna and zrconum slcate on the relablty of a ceramc body. Journal of Ceramc Processng Research, 2(6), Carty W.M., Senapat U. (998): Porcelan raw materals, processng, phase evoluton, and mechancal behavor. Journal of the mercan Ceramc Socety, 8(), do: 0./j tb02290.x 8. Lee W.E., Iqbal Y. (200): Influence of mxng on mullte formaton n porcelan. Journal of the European Ceramc Socety, 2(4), do:0.06/s (0) St John R.C. (984): Experments wth mxtures, ll-condtonng, and rdge regresson. Journal of Qualty Technology, 6(2), UI E ISO : Determnaton of Water bsorpton, pparent Porosty, pparent Relatve Densty and ulk Densty, Internatonal Organzaton for Standardzaton, Geneva, Swtzerland, STM Standard C : Standard Test Method for Specfc Gravty of Fred Ceramc Whteware Materals. nnual ook of STM Standards, West Conshohocken, P, Doremus R.H. (983): Fracture statstcs: comparson of the normal, Webull, and Type I extreme value dstrbutons. Journal of appled physcs, 54(), do: 0.063/ STM C239.: Standard Practce for Reportng Unaxal Strength Data Estmatng Webull Dstrbuton Parameters for dvance Ceramcs. nnual ook of STM Standards, West Conshohocken, P, kake H. (974): new look at the statstcal model dentfcaton. utomatc Control, IEEE Transactons on, 9(6), do: 0.09/TC Stephens M.. (974): EDF statstcs for goodness of ft and some comparsons. Journal of the mercan statstcal ssocaton, 69(347), do: 0.080/ Reed J.S. (995). Introducton to Prncples of Ceramc Processng, 2 nd ed. John Wley and Sons, ew York. Ceramcs Slkáty 60 () (206) 57