Alternating Field Electronanofluidization

AIP Con e ence P oceedings 1145, 97 (2009); h ps://doi.o g/10.1063/1.3180095 1145, 97 © 2009 Ame ican Ins i u e o Physics. Al e na ing Field Elec onano luidiza ion Ci e as: AIP Con e ence P oceedings 1145, 97 (2009); h ps://doi.o g/10.1063/1.3180095 Published Online: 01 July 2009 M. J. Espin, J. M. Val e de, M. A. S. Quin anilla, and A. Cas ellanos ARTICLES YOU MAY BE INTERESTED IN Quasi-s a ic Compac ion o Polyhed a by he Disc e e Elemen Me hod AIP Con e ence P oceedings 1145, 90 (2009); h ps://doi.o g/10.1063/1.3180078 The Quan i a i e Mine alogy o G anula Ma e ials AIP Con e ence P oceedings 1145, 87 (2009); h ps://doi.o g/10.1063/1.3180070 Al e na ing Field Elec onano luidiza ion M. J. Espin , J. M. Val e de†, M. A. S. Quin anilla† and A. Cas ellanos† Depa men o Applied Physics II. Uni e si y o Se ille. A enida Reina Me cedes s/n, 41012 Se ille, Spain. †Facul y o Physics. Uni e si y o Se ille. A enida Reina Me cedes s/n, 41012 Se ille, Spain. Abs ac . The use o luidized beds o emo e submic on pa icles om gases has been in es iga ed since 1949. High e iciency emo al was achie ed in he 1970’s by imposing an elec ic ield on a luidized bed o semi-insula ing g anules ha we e able o collec he cha ged pollu an en ained in he luidizing gas. In spi e o hei ex ended use nowadays, he collec ion e iciency o elec o luidized beds (EFB) is s ill hinde ed by gas bypassing associa ed o gas bubbling and he consequen equi emen o oo high gas low and p essu e d op. In his pape we epo on he elec omechanical beha io o an EFB o insula ing nanopa icles. When luidized by gas, hese nanopa icles o m ex emely po ous ligh agglome a es o size o he o de o hund eds o mic ons ha allow o a highly expanded nonbubbling luidized s a e a educed gas low. I is ound ha luidiza ion uni o mi y and bed expansion a e addi ionally enhanced by an imposed AC elec ic ield o ield oscilla ion equencies o se e al ens o he zs and ield s eng hs o he o de o 1 kV/cm. Fo oscilla ion equencies o he o de o he zs, o smalle , bed expansion is hinde ed due o elec opho e ic deposi ion o he agglome a es on o he essel walls, whe eas o oscilla ion equencies o he o de o kilohe zs, o la ge , elec opho esis is nulli ied and bed expansion is no a ec ed. Acco ding o a p oposed model, he size o nanopa icle agglome a es s ems om he balance be ween shea , which depends on ield s eng h, and an de Waals o ces. The op imum ield s eng h o enhancing bed expansion p oduces an elec ic o ce on he agglome a es simila o hei weigh o ce, while he oscilla ion eloci y o he agglome a es is simila o he gas eloci y. Keywo ds: Fluidized beds, Nanopowde s, Fil e s PACS: 47.55.Lm, 81.07.Wx, 84.30.Vn INTRODUCTION E icien emo al o dus and mis om gases by pass- ing hese h ough a bed o luidized solids was i s claimed by Meisne and Mickley in 1949 [1]. Almos wo decades la e , p ac ical p oblems such as educing ai pollu ion om diesel exhaus and il a ion o smoke emissions om asphal ic pa emen ecycling p ocess, mo i a ed Melche o de elop elec o luidized beds o collec ion o submic on pa icles [2]. Melche and co- wo ke s s essed a luidized bed by imposing an elec- ic ield ha e ec i ely pola ized he millime e sized pa icles. These semi-insula ing pa icles hen se ed he unc ion o con en ional elec os a ic p ecipi a o s, ac - ing as collec ion si es o he cha ged pollu an s en ained in he luidizing gas. In compa ison o elec os a ic p e- cipi a o s, he collec ion su ace a ea pe uni olume o elec o luidized beds is g ea ly inc eased, making i pos- sible o educe he gas esidence ime and olume o he il e [2]. A majo conce n ha bese s classical lu- idized bed il e s lies in he bypassing o he con ami- nan s h ough gas bubbles. Gas bubbling is he ypical beha io ound in luidized beds o millime e sized pa - icles [3], p o iding li le gas-solid con ac and hampe - ing eac ion e iciency. Powde s o mode a e densi y nanopa icles (such as silica) can be uni o mly luidized and expe ience a an- si ion o elu ia ion a high gas eloci ies wi h ull sup- p ession o isible bubbles [4]. In his pape we s udy he beha io o an elec o luidized bed o d y silica nanopa - icles, which shows agglome a e pa icula e luidiza ion in he absence o ex e nally applied elec ic ield. Sam- ples we e subjec ed o DC and AC elec ic ields in o - de o check he in luence o he equency o he ex- e nal ield and o de e mine he size and cha ge o he powde . The supp ession o gas bubbles in luidized beds o nanopa icles has been causally ela ed o he o ma- ion o po ous ligh agglome a es. A phenomenological app oach o p edic he beha io o gas- luidized beds o nanopa icles is o conside agglome a es as e ec i e low-densi y sphe es which may exhibi nonbubbling gas- luidiza ion simila ly o coa se pa icles luidized by liq- uids. In his way, he Richa dson-Zaki (RZ) equa ion, o iginally in ended o desc ibe he expansion o uni- o m liquid- luidized beds [6], can be modi ied o con- side uni o m gas- luidiza ion o nanopa icle agglome - a es [4] p (1) whe e g is he supe icial gas eloci y, p is he e minal se ling eloci y o a single pa icle, φ is he pa icle olume ac ion, ka = d /dp is he a io o agglome a e size o pa icle size, and D = lnNa/ka, whe e Na is he numbe o pa icles in he agglome a e. CPl 145, Powde s and G ains 2009, P oceedings o he 6 In e na ional Con e ence on Mic omechanics o G anula Media edi ed by M. Nakagawa and S. Luding © 2009 Ame ican Ins i u e o Physics 978-0-7354-0682-7/09/S25.00 97 0.005 0.004 0.003 0.002 0.001 Solidlike - ... , T ansi ional egime egl„c Uni o m luidlike egime 0 0.5 1 1.5 2 2.5 3 , (cm/s) FIGURE 1. Pa icle olume ac ion as a unc ion o supe - icial gas eloci y o luidiza ion wi hou ex e nal elec ic ield applied. The solid line ep esen s he modi ied RZ equa ion i o he da a in he uni o m luidlike egime. The pho og aphs in he inse illus a e bed expansion. EXPERIMENTAL SETUP AND RESULTS The luidized bed appa a us consis ed o a e ical 2.54 cm in e nal dia. polyca bona e essel, 16.2 cm in heigh , which is i ed a he bo om wi h a sin e ed s ainless s eel pla e, ha ing a po e size o 5 jim, ha ac s as gas dis ibu- o . The luidiza ion cell is is placed be ween wo pa allel squa e elec odes o 14 cm o side leng h. The dis ance be ween he elec odes is ixed o I = 8 cm. One o he elec odes is g ounded and high ol age V is applied o he opposi e elec ode om an oscilla o /ampli ie ne - wo k. The elec ic ield s eng h wi hin he bed can be aken as app oxima ely cons an (E ~ V/l). The powe supply sys em allowed us o p o ide peak ield s eng hs up o E0 ~ 2 kV/cm o e a equency ange up o 10 kHz. Di e en ypes o wa e o m, sinusoidal, squa e and i- angula shape, we e applied. The low o gas (d y ai ) o he column was con olled by means o a MKS low con olle wi h a ull ange om 0 o 2000 cm3/min. The ma e ial es ed in he expe imen s was p e-sie ed (using a sie e opening o 500 jim) Ae osil©R974 , which is a hyd ophobic SiO2 nanopowde wi h a pa icle densi y o pp =2250 kg/m3, and pa icle size dp = 12 nm. In Figu e 1 we ha e plo ed expe imen al da a o </> as a unc ion o g in he absence o ex e nal ield applied. The bed expands mono onously as g is inc eased. By i ing he da a o g s. </> o he modi ied Richa dson- Zaki equa ion (Eq. 1), i is ob ained d** = 226jUm and D = 2.588, which is in good ag eemen wi h p e ious measu emen s [4]. Figu e 2 shows expe imen al da a on </> as a unc ion o g and o di e en alues o he s eng h o he c oss- low elec os a ic ield applied. The main e ec o he elec ic ield is an inc ease o F om local obse a- ions [7], i is seen ha he dynamics is uled by elec- 0.012 4 0.01 0.008 0.006 0.004 0.002 0 A 4 kV/cm o 2.6kV/cm • 1.3 kV/cm - 0 kV/cm 2 3 (cm/s) FIGURE 2. . A e age pa icle olume ac ion o he lu- idized bed φ as a unc ion o he supe icial gas eloci y g o di e en s eng hs o he elec os a ic ield applied. (Hz) FIGURE 3. Rela i e a ia ion o he pa icle olume ac- ion as a unc ion o he oscilla ion equency o he al e na ing elec ic ield. Peak ield s eng h is ixed o 1.25 kV/cm. Da a is shown o h ee di e en alues o he supe icial gas eloc- i y g. The inse shows elec opho e ic deposi ion (1Hz) and enhanced bed expansion (20 Hz). opho e ic deposi ion a he wall. In he s a iona y s a e, luidiza ion quali y is hampe ed and bed expansion is de- c eased. Insula ing d y pa icles in a luidized bed accumu- la e a signi ican amoun o cha ge. Mos in es iga ions sugges ha he main cause o he cha ge build-up on g anula ma e ials du ing indus ial handling and p o- cessing is con ac cha ging [5]. Cha ges a e exchanged whene e any wo su aces come in o con ac wi h each o he e en o cases whe e he bulk ma e ials a e he same due o su ace impu i ies and impe ec ions. The elec opho e ic o ce on ou agglome a es is Fe ~ Q**E, whe e Q** is hei cha ge. Using he S okes’ law o an isola ed agglome a e, i can be es ima ed Q** = 3nnd** *h*/E, whe e n is he gas iscosi y and *h* is he e minal eloci y o he agglome a e. We ha e ana- lyzed he ajec o ies o some agglome a es a he ini ial s age o applica ion o he elec ic ield by means o a 98 E=1.25kV/cm, =1Hz £=125 kV/cm, = 20 Hz E=1.25kV/cm, =1 kHz FIGURE 4. Pho og aphs o he elec onano luidized bed il- lus a ing bed expansion o h ee di e en equencies o he al e na ing elec ic ield (indica ed) as compa ed o bed expan- sion in he absence o ex e nally applied elec ic ield. Supe i- cial gas eloci y is ixed o g = 2.7 cm/s. -0.15 E0 (kV/cm) •Squa e —•— Sinusoidal —±— T iangula 0.2 0.4 Ems (kV/cm) 0.1 0.01 0. A+% • / 1 -0.15 -0.2 FIGURE 5. Rela i e a ia ion o he pa icle olume ac- ion o he elec onano luidized bed as a unc ion o he ield s eng h o h ee di e en wa e o m ypes. Top: Da a is shown as a unc ion o he peak ield s eng h E0. Bo om: Da a is shown as a unc ion o he oo -mean-squa ed ield s eng h E ms. elec ic ield oscilla ion equency and supe icial gas e- loci y a e ixed o 20 Hz and g = 2.7 cm/s, espec i ely. CMOS high-speed came a. The agglome a e cha ge o he agglome a es manually acked was ound o be o he o de o 10-14C, gi ing a cha ge o mass a io Qm in he ange 10-5 -10-4C/kg. In he case o an al e na ing ield, i is expec ed ha he cha ged agglome a es will be o ced o oscilla e a he ield equency. Figu e 3 shows he ela i e a ia ion A*/*, Uni o m luidlike egime FIGURE 6. Rela i e a ia ion o he pa icle olume ac ion o he elec onano luidized bed as a unc ion o he supe icial gas eloci y. Elec ic ield oscilla ion equency and s eng h a e ixed o 500 Hz and E0 = 1.25 kV/m (squa e wa e), espec- i ely. The lines a e p edic ed cu es by he model. Solid line: complex-agglome a e cha ge Q^* = 1.9 x 10~14 C. Do ed line: Q** = 1 x 10-14 C. Dashed line: Q** = 3 x 10"14 C o he pa icle olume ac ion ( φ/φ0, whe e φo is he pa icle olume ac ion in he absence o ex e nal elec- ic ield) o he elec onano luidized bed as a unc ion o he oscilla ion equency o a ixed peak ield s eng h (E0 = 1.25 kV/cm). Fo equencies o he o de o he zs he main mechanism is s ill elec opho e ic deposi ion o he agglome a es on he walls, which gi es ise o bed channeling and collapse. In he ange o in e media e e- quencies, be ween ens and hund eds o he zs, he op- posi e beha io is obse ed. Bed expansion is g ea ly en- hanced by he al e na ing ield. Finally, o equencies o he o de o kilohe zs and la ge , he al e na ing ield has no app eciable e ec on bed expansion (see Fig. 4). The e ec o he s eng h o he ield in he ange o in- e media e oscilla ion equencies and wa e o m ype is shown in Fig. 5. Bed expansion is u he enhanced as he s eng h o he ield is inc eased. The da a ma ches o a single end when i is plo ed agains he oo -mean- squa ed ield E ms, which indica es ha enhanced bed ex- pansion is he esul o a ime a e aged p ocess. F om a log-log plo (see inse ) i is obse ed ha φ/φ0 scales as he ield s eng h squa ed, which is p opo ional o he kine ic ene gy gained by he agglome a es du ing one semipe iod. This sugges s a possible ole o collisions be ween agglome a es o di e en cha ge on deagglom- e a ion. DISCUSSION A change o he pa icle olume ac ion when he ield is u ned on could be a ibu able o a a ia ion o he complex-agglome a e size. In o de o es ima e 99 heo e ically he complex-agglome a e size, simple- agglome a es, which exis be o e luidiza ion [4], will be conside ed as e ec i e pa icles unde going agglom- e a ion due o a ac i e o ces be ween each o he [8]. This a ac i e o ce is coun e balanced by he gas low shea o suppo he weigh o he complex-agglome a e in he g a i y ield plus he shea due o hei o ced oscilla ions in he elec ic ield. The balance be ween he o e all ime-a e aged shea and he a ac i e o ce de e mines he size o he complex-agglome a es. The complex-agglome a e weigh o ce W** is balanced by he hyd odynamic ic ion om he su ounding gas, which ac s mainly a i s su ace due o he low sc eening e ec . Using a sp ing model [8], he ypical s ain on he su ace o he complex-agglome a e can be es ima ed as jg ~ w**/K**R**. He e W** = N*W*, whe e N* is he numbe o simple-agglome a es in he complex-agglome a e and W* is he simple-agglome a e weigh o ce. K** is he complex-agglome a e sp ing cons an , which, using a heo y on he elas ic p ope ies o andom pe cola ing sys ems, can be ob ained om he simple-agglome a e sp ing cons an K* and he elas ici y exponen J3 (K** ~ K*/{k* , whe e k* is he a io o complex-agglome a e adius R** o simple-agglome a e adius R* and J3 = 3 in 3D). When he al e na ing elec ic ield is u ned on, he o ced complex-agglome a e oscil- la ions gi e ise o an addi ional shea o ce o balance he elec ic o ce Q**E. The oo -mean-squa ed shea s ain is hus inc eased up o j ms = V yg)2 + (YE)2, whe e JE ~ Q**E ms/K**R**. Thus, he shea o ce, which ac s on he simple-agglome a es a ached o he ou e laye o he complex-agglome a e, is Fs ~ K*Y msR* ~ (k*)D+W(W*)2 + (Q*E ms)2. He e i is assumed N* = (k*)D and ha he complex- agglome a e cha ge is equally dis ibu ed among he simple-agglome a es (Q** = N*Q*, whe e Q* is he simple-agglome a e cha ge). Simple-agglome a es a ach o he complex-agglome a e as long as he shea o ce is smalle han he a ac i e o ce. Thus, he limi condi ion Fs = F dW leads o he equa ion k* ~ 5o1/(D+2) o es ima ing he complex-agglome a e size d**, whe e he agglome a e Bond numbe Bo is de ined as he a io o he a ac i e o ce F dW o V(w*)2 + (Q*E ms)2. The simple-agglome a e weigh is W* = NPWP, whe e Wp is he nanopa icle weigh and Np is he numbe o p ima y nanopa icles in he simple- agglome a e, which is assumed o be Np = (d*/dp)D. The simple-agglome a e cha ge Q* is needed o es ima e he complex-agglome a e size. P o ided ha simple- agglome a es a e unal e ed by he elec ic ield, i is Q* = Q*0*/N%, whe e Q*0* and N% a e he complex- agglome a e cha ge and numbe o simple-agglome a es in he complex-agglome a e o E = 0, espec i ely. Thus k*=k*01+ Q 0* ™ (2) whe e k*0 = d^*/d* is he ela i e complex-agglome a e size in he absence o elec ic ield. Using a ypical alue o F dW = 10 nN, d* = 30,11m, and D = 2.5 - 2.6 [4], i is p edic ed d*0* ~ 150^m in he absence o applied elec- ic ield. Eq. 2 can be used in he modi ied RZ equa- ion (Eq. 1). Figu e 6 shows he ela i e a ia ion o he pa icle olume ac ion as a unc ion o he supe - icial gas eloci y o an imposed ex e nal ield o peak s eng h E0 = 1.25 kV/cm. The lines ep esen he p e- dic ions by he model o di e en alues o he complex- agglome a e cha ge. Complex-agglome a e size in he absence o elec ic ield and ac al dimension a e aken om ou p e ious de i a ion based on bed expansion ex- pe imen al da a (d** = 226 /im, D = 2.588). Rema k- ably, he p edic ion i s o he da a in he luidlike egime o a alue o he complex-agglome a e cha ge (Q*0* = 1.9 x 10-14 C) simila o he o de o magni ude es i- ma ed by acking agglome a e ajec o ies. The model p edic ion is howe e qui e sensi i e o he agglome a e cha ge. Conside able de ia ion om he da a is ob ained o o he cha ge alues o same o de o magni ude. This p e en s us om a conclusi e s a emen on he model alidi y. Mo eo e , he ole o collisions on deagglome - a ion is no conside ed by he model. ACKNOWLEDGMENTS We acknowledge Spanish Go e nmen Agency Minis e- io de Ciencia y Tecnologia (con ac FIS2006-03645) and Jun a de Andalucia (con ac FQM 421). REFERENCES 1. H. P. Meissne , H. S. Mickley, Ind. Eng. Chem. 47 (1949) 1238–1242. 2. J R. Melche , P oceedings o he IEEE 65 (1977) 1659–1672. 3. D. Gelda , Powde Technol. 7 (1973) 285–292. 4. J. M. Val e de, A. Cas ellanos, Chem. Eng. Sci. 62 (2007) 6947–6956. 5. M. Glo , John Wiley & Sons, New Yo k, 1988. 6. J.F. Richa dson, W.N. Zaki, T ans. Ins . Chem. Eng s. 32 (1954) 35–53. 7. J. M. Val e de, M.A.S. Quin anilla, M.J. Espin, A. Cas ellanos, Phys. Re . E 77 (2008) 031301. 8. A. Cas ellanos, J.M. Val e de, M.A.S. Quin anilla, Phys. Re . Le . 94 (2005) 075501(1)–075501(4). 100