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Simple design of a crossflow moving bed heat exchanger-filter (MHEF)
@T EXCHANGER FI-LTER
Simple D e s i g n of a Crossflow Moving Bed Heat Exchanger-Filter (MHEF) By A . M a c i a s - M a c h i n
Escuela de Ingenieros Industriales de Las Palmas, Tafira Baja, s/n, 35017 Las Palmas de Gran Canaria, Spain a n d J. Cu611ar, A . Est~vez a n d E. Jaraiz
Departamento de Ingenieria Quimica. Facultad Quimicas. Plaza Caidos 1-5, Universidad de Salamanca, 37008 Salamanca, Spain In industrial applications hot combustion gases are often used in direct-contact gas-solid heat exchangers, such as fluidized bed, fixed bed and moving bed exchangersO). Granular beds are also often used for filtering dust particles carried by fluids (2). However, hot gas filters and gas-solid heat exchangers have been studied and designed separately and little consideration has been given to devices where heat exchange and filtration take place simultaneously. This paper describes the design of a crossflow moving bed heat exchanger-filter (MHEF) (3),a device which uses a moving granular bed for simultaneous dust filtering and heat recovery from hot gases. Charts are presented for determining the dust concentration leaving the device, contact area needed for a required dust concentration or temperature in the effluent gas, and other parameters of practical interest.
Coldparticles
The upper part of a moving bed heat exchanger-filter (MHEF), where cold granular particles enter and fall is shown in Fig. 1. A dust-laden hot gas is filtered and cooled when it passes laterally through the layer of downward-moving particles. The clean, cooled gas exits the M H E F and the hot particles fall into the lower part of the unit (not shown in Fig. 1 but with the same design as the upper part). There the particles contact a crossflowing co][dgas which is consequently heated. The particles, now cold, finally exit the lower end of the device and are carried upwards to be recirculated. The lifting operation can be performed by pneumatic, mechanical or other means. In one of several different designs (3) coils are located around a vertical conveying pipe. The coils are t u r n e d on and off sequentially by pulses of electric current. The moving magnetic fields produced in this way are able to carry the particles m which need to be of a magnetizable n a t u r e m to the top of the MHEF. Then the cold particles fall down and the heat exchange-filtering process goes on. A device for collecting the dust carried by the particles leaving the upper part of the a p p a r a t u s is placed between the upper and lower parts of the MHEF. Model In this design we assumed: a relatively short relaxation timelY); t h a t is (hg_ s a) -. a thin stream of solids being contacted by a crossflowing gas [] the solids are mixed laterally but are unmixed along the flow path [] plug flow of gas [] stationary filtration, with no effects caused by structural changes resulting from dust accumulation within l~he filter [] small concentration of dust in the gas This cross-contacting p a t t e r n is the simplest design of this MHEF, but was useful for m a k i n g preliminary estimates, since no published literature on simultaneous Filtration& Separation March/April1992
"
X
~
Dirty
Hotparticles
Fig. 1. Schematic view of the upper part of a Moving Heat Exchanger-Filter.
T EXCHANGER-FILTER filtration and heat exchange designs has been found. Once these assumptions have been made, the contacting pattern of the MHEF is similar to the one described in Ref. 1, which models a crossflow fluidized bed heat exchanger with an infinite number of stages.
(12) ER=- 2da dc
Theoretical background For the model described above the heat transfer efficiencies of the operations are expressed as? AT ng=-=-ATmax
T,-- 7’2
- ?- [L-e-+] T,- t, @
(1)
and At
qs=-=-=
tz-t1
l-e-k+qg
Tl-t,
WI-,,
.
(j)=.=--
l
f%%
-=XYZ
X~/U~ 1-E psc,
mScS
(3)
Also, from Eqs. (1) & (2)
tz- t1
Ils 4 =-=-
T,- T,
‘Ig
(4)
Defining dimensionless parameters X, Y and 2 as
x=-
W&S
(51
%xl~uo
y=-
1 (6)
1-E z-
wg --PS
(n
On the other hand, the rate at which the dust concentration in the gas decreases as the gas flows through the medium in the direction x, is obtained from Ref. 4. The rate at which the concentration of solids C decreases is given by: 3E(1-E) dC -_=dx 2dc 1 After solving this equation we get
where
In -=-$ cf Co F_
1
EF
(l-dXm
C
(9)
Equations (11 to (14) can be combined in different charts - related to the dimensionless parametersx, Y 2, E, F and Cf IC,, - that can be used to calculate the thickness x, of the MHEF and the dust concentration of the filtered gas.
Using the charts Figures 2 to 7 show the set of charts that aid the design of crossflow moving bed heat exchanger-filters. These charts simplify the determination of the thickness x, required of a particular exchanger-filter to give a defined dust concentration in the gas outlet or, alternatively, let us design the MHEF when we know the inlet and outlet temperatures of the gas and solids. Figures 2 to 6 show curves for selected values of 2 and represent the dimensionless parameters that let us design the MHEF and evaluate the performance of a given heat exchanger. Given a value of 2, and specifying any two of the three other parameters I$, X, E, the unknown can be read from the appropriate chart. For a design problem, if we know I) and E, we can find X and then evaluate the thickness of the MHEF. From Fig.7, knowing the thickness, x,, we can find the dust concentration in the filtered gas once we know the collection efficiency E, from Eqs. (11) to (141, and the diameter of the collector particle d,. Figure 7 represents a typical filter operation. This graph represents the variation of dust concentration in the filtered gas, in the direction x, and allows us to determine the thickness x, of our filter, In this figure we can observe that specifying any two of the three parameters E, F and Cf IC,, the unknown can be read from the chart, For the calculation of performance, we can know the dust concentration of the filtered gas and read from the chart the parameter F that will give us the thickness x, of the MHEF. Now, knowing X, E and 2 values we are able to calculate $Iand the final temperatures. The following examples illustrate the use of these charts for some different cases.
Numerical example 1 Required: To determine the thickness of the moving heat exchanger-filter (MHEF), x,, the gas flow rate and dust concentration in the filtered gas, given: T,= 500°C T, = 184 “C
(10)
(11)
where the suffixes refer respectively to inertia(I), interception(R), and diffusion(D) mechanisms. These terms are given@) as: 156
(141
(8)
and E is the collection efficiency of a single sphere@). This can be adequately expressed as the sum of the efficiencies for the individual mechanisms: E=EI+ER+ED
-2,‘3
ED=4Pe
(2)
where $Iis given by
rngcg WI?&
(13)
t,=40°c t, = 293 “C
and E = 0.5 pg = 0.456kg/m3 ps = 7200 kg/m3 w = 1152 kg C, = 346 J/kygK Cg= 1093 J/kgK
z”, E C, d,
March/April
= 0.5 m/s = 0.90 kg/s = 0.1 = 0.2 g/m” = 10 mm 1992
Filtration &Separetion
TEXCHANGER-FiLTER
0
2000
3000
4000
5000
6000
7000
SOtlO
9000
‘lOdO
l
X Fig. 2. The performance
1000
2000
3000
4000
Fig. 3. The performance
Filtration & Sepwation
March/April
1992
=-w/m, curves corresponding
5000
6000
curves corresponding
toZ=
l/3000.
7000
toZ=
8000
0.50
;I””
0.45 0.40 0.35
/
7
9000
10000
l/5000.
157
TEXCHANGER-FILTER 3.0
i.
i.
1.
L / ‘.
+
90.00
lob00
2.5
v
0.5
A
1000
/ATI
2000
3000
4600
I
I
I
I
5dOO
6000
7000
8000
WI& X=------&A Fig. 4. The performance curves corresponding tOZ=
1000
2000
3000
4000
5000 X
6000
Ir?OoO.
7000
8000
9000
10000
WI& =--&T&O
Fig. 5. The performance curves corresponding to Z = 1/9ooo. 158
March/April
1992
Filtration&Separation
T EXCHANGER-FILTER
0.50 0.45 0.40 0.35
Fig. 6. The performance
0
2.5
5
7.5
curves corresponding
Filtration & Separation
March/April
1992
to Z = l/l 1!300.
I
10
Fig. 7. Performance
_-. q
1’5
- 17.5
20
curves of filtration.
159
T EXCHANGER-FILTER Solution: First determine the value of the parameters I$and.Z. From Eq. (4): ~_ tz-tl
_ 293-40
T,-T, From Eq. (7) pgcg Z=-----=
WithX, Z and Ewe use Fig. 4 to get I$= 0.6
1 =2x1o-4=-
7200 x 346
5000
As 2 = l/5000, we use Fig. 3 (relating the parameters +, E,2 andX) to get: WIni, X=---Grl&
7000
= 0.8
500- 184
0.456 x 1093
P&s
9372 x 420
P&s
= 2000
From Eq. (3) . mg=
discs
=
0.6 x 0.8 x 420
cg * Then, from Eqs. (2) & (1)
1069
qs = 1 - e-4 = l-
= 0.188 kg/s
e-O.6= 0.451
From Eq. (51, the thickness required is: x,=-=
wuo
1152 x 0.5
??i,X
0.90 x 2000
0.451= t,T,-
= 0.32 m
=
400 - 41
0.451
(1 - 0.5) x 0.32
(I-E)+-, d,
=
10 x 10-s
Now, from Fig. 7, using the parameters E and F Cf = Co
x
Cf Co
= 0.0907
0.0907 => Cf = 0.01814 g/m3
Thus, having a thickness of xm= 0.32 m, the MHEF reduces the dust concentration m the gas from 0.2 g/m3 down to 0.01814 g/m3 which is a reduction of approximately 91% of the initial dust concentration. Numerical example 2 Required: To determine the outlet temperature of the solid, t 2,the exit temperature of the filtered gas, T,, and the gas flow rate, given: pg = 0.525 kg/m3 Ps = 9372 kg/m3 tis=0.8kg/s u. = 0.36 m/s w = 224Okg
cg = 1069 JfkgK es=420 JlkgK T, = 400 “C! t1=41V E = 0.2
d, = 18.5 mm &= 0.4 g f f ;.;z g/m3 m3 o*
13 x 1.85 x 1O-2
F-d,
0.6
m=(l--E)=
=
From Eqs. (5) & (7), the dimensionless and Z are x=
WIni, Xrn/uo
160
0.752 = T, T,-
0.40 m parameters X
= 2240i0.S = 2520 0.410.36
= t,
4oo - T2 400 - 41
0.752
; T2 ti 130°C
Conclusions A simple model has been established which allows us to calculate the performance of a moving heat exhangerfilter (MHEF). From charts obtained from this model it is possible to find the filter thickness for a given percentage of dust recovery from the gas. Also, the design of an MHEF is possible when the gas and solid inlet and outlet temperatures are known.
References
1. Levensoiel. 0. Enkneerin~ Flow and Heat Exchange. Plenum Press, New York, 1384. 2. Mizukami, S., Wakabayashi, M. and Murata, H. Inter-
action between pressure drop of gas and flow of medium in a moving granular bed filter. Particulate Science and Technology, 5,198’7, pp. 131-142. 3. Jaraiz, E., CuBllar, J., Est&ez, A. and Macias, A. Device and Procedure for Simultaneous Heat Exchange and Filtration of Fluids. (Spanish Patent Applied, 1991).
4. Clift, R., Ghaidiri, M. and Thambimuthy, K. J. Filtration of Gases in Fluidized Beds, in Progress in Filtration 2, (Edited R. J. Wakeman) Elsevier, Amsterdam, 1981, p. 84.
a
Solution: From Fig. 7, knowing E and Cf/C, we find the value of F to be approximately equal to 13. Then, from Eq. (10) we get: X
0.6
= 16
; tz = 203°C
=- =
t,
To calculate the dust concentration in the filtered gas, we use Eq. (10): F=
t,-41
5 cs
C
Nomenclature surface of solid per unit volume of bed, m-l specific heat capacity of gas, J/kg K specific heat capacity of solid, J/kg K dust concentration in gas, kg/m3 outlet dust concentration in gas, kg/m3 inlet dust concentration in gas, kg/m3 collector particle diameter, m dust particle diameter, m diffusion coefficient of particle, s.m-2 collection efficiency of a single sphere dimensionless parameter defined in Eq. 12 heat transfer coefficient between gas and solid, J/s.m2.K mass flow rate of gas, kg/s March/April
1992
Filtration&Separation
TEXCHANGER-FILTER mass flow rate of solid, kg/s
ZHEFMoving Heat Exchanger-Filter Pe
Sk T
t
Xlll
X
zy W
Peclet number ( = u&&J Stokes number gas temperature, K solid temperature, KU, superficial gas velocity, m/s thickness of the Moving Heat Exchanger Filter, m dilmensionless parameter defined in Eq. 5 dilmensionless parameter defined in Eq. 6 dilmensionless parameter defined in Eq. 7 mass of solid material, kg
Filtration 81Separation
March/April
1992
Subscripts Inlet ; Outlet Greek symbols E voidage gas efficiency % solid efficiency % gas viscosity, Pa.s dust density, kg/m3 Ld gas density, kg/m3 PP solid density, kg/m3 heat flow ratio of two contacting streams given : by Eq. 2
161
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El147
Das AusmaB der Feststoff-Verweilzeit in emer JaumelzentrffugentrommeI stellt ein bedeutendespraktisches Problamdar. EinerAnalysederKennwerteder Jrommelundder Partikelbewegung sowohl rn Jaumelzentrifugen als such in Slebzentrifugen mrt konrscher Jrommel schlief3t sich eine Berechnung der Feststoffverweilzeften ftir beide Zentrifugenarten m,t Stufentrommeln an. Es wird dargelegt, da8 bei Verwendung einer Stufentrommelm einer Taumelzentrifuged!e Verweilzeit /anger a/s be! Verwendungeiner Slebzentnfuge mit komscher Jrommelist. Der PartikelfluB durch Taumelzentrifugen und Maschinen m!t Stufentrommeln find& auf lmpulsbasis start: Wobbeiimpulse bewfrken
Partikelbewegung Ghrend emes begrenzten Zeitraums der Trommelrotation be! Taumelzentrifugen, wtihrend durch die Stufen strukturgebundene Impulse erzeugt werden, d!e die Partikelbewegung kurz unterbrechen. Bei guter Auslegung kann d,e Feststoffverweilzeitm TaumeizentrifugenmittelsStufentrommelverdoppelt werden. Esist damit zu rechnen, daR sich dadurch dieAnwendungsm6glichkeften der Taumelzentrifugen erweitern lassen Und d/e resultierenden Feststoffprodukte einen hdheren Jrockenheitsgrad errerchem(8 sn., 10 abb., 6 tab., 4 ref.)
Effets des tambours itag6s sur le temps de sijour des solides dans les centrifuges P panier conique et ir culbuteur par Fan Deshun, R J Wakeman et Huang Zhong L’accrolssement du temps culbuteur est un probl$me du tambour et le mouvement panier conique, on calcule tambours&ages. Onmontre p/us grande du temps de machine $ paniercomque.
de r&vdence des solides dans un tambour de centrifuge I pratiqua important. A&s avolr analyst? /es caract6ristique.s des particules d la fois dans /es centrifuges dculbuteur et d /es temps de &idence pour chaque type de centrifuge B quef’emploid’un tambour&ag&produit uneaugmentatfon r&dence dans une centrifuge B cuibuteur que dans une Le debit departicles dans les centnfuges B culbuteur et dans
Efectos de tambores
con grada en tiempos de permanencia con centrifugas por Fan Deshun, R J Wakernan y Huang Zhong
La extensidn
de/ tiempo de permanencia de sdlidos en el tambor de una centrifuga voltereta es problema Importante en la pr8ctica. Despues de anallzar /as caracterisbcas de/ tambor y ei movimvanto de part&/as en centrifugas voltereta y a casta cdnica. se ha calculado tiempos depermanencia con ambos tipos con tambores con grada. Se ha demostrado qua a/ ample0 da un tambor con grada produce mas aumento de permanencia en una centrifuga voltereta queen la mtiquina cesta cdmca. El flujo de partfcuias por /as con tambores con grada es impuisivo; impulses
Izl
/es machmes B tambour &age est pulsL des oscifiatrons cr&nt un mouvement particules pendant une p&/ode I/mit& de la rotation du tambour dans /es centrifuges cu@teur tandis qua les tambours &tag& causent des publications structur&s arretent momentan6ment le movement des particuies. Avec un bon design. ie temps s~jourdessolidesdansiescentrifugesdculbuteurspeuventbtredoubl~sparierecoursd un tambour @tag&. On &tend done d pouvoir Blargir fe champ des applications centrifuges d culbuteur et obtenir des produits plus sets. (8 p., 10 hg., 6 tab., 4 r6f.j
bamboleantes producen rotacidn de/ tambor estructurales /OS wales dwiro se puede doblar al empleo de un tambor voltereta para obtener
a cesta chica
des B qui de des
y a voltereta
movimiento de particulas durante un period0 limitado de /a con mlquinas voltereta y /as gradas mducen impulses paran momentaneamente movimfento particula. Con buen el tiempo que 10s sdlidos estan en una centn’fuga voltereta con con grada. Se espera extender /as aplicaoones de centrifugas productos s6lidos mas sacos. (8 pdgs., 10 figs., 6 tabs., 4 refs.)
Simple Design of a Crossflow Moving Bed Heat Exchanger-Filter (MHEF) Einfache Konstruktion eines Ouerstrom-Wirbelbett-W$irmeaustauschfilters von A Mcwias-Mach@ J CuWZar,A h’stiez und E Jara@
155
Beilndustrieanwendungen
werden oft hei& Verorennungsgase in Dlrektkontakt-GasFeststoff- WPrmeaustauschern, wie z.B. Flijssigbett-, Festbettund Wirbelbett-Austauscher, eingesetzt. Kornstrukturbetten werden ebenfalls oft zum Ausfiltern der won den Flijssigkeiten mitgefijhrten Staubpartikel benutzt. Heiflgasfilter und Gas-FeststoffWBrmeaustauscher sind war separat untersucht und konstruiert worden: /eider sind dabeijedoch Ger&e vernach&sigt worden, in denen W&meaustausch und Filtration gleichzeitfg stattfinden. In diesem Referat ist dre Konstruktion eines Querstrom-
Wirbelbett-
WPrmeaustauschfilters (MHEF) beschneben-ein Gerst, bei dem ein kbrniges Wirbelbett zur gieichzeitigen Staubfiltration und WBrmerDckgewinnung van heiRen Gasen eingesetzt wird. Das Referat enthblt Diagramme zur Bestimmung der Staubkonzentration an der Ablaffseite des Gerdts, Angaben iiber die erforderliche Kontaktfllche ftir eine bendtigte Staubkonzentration bzw. Temperatur rm Abgas sow/e andere Parameter YOR praktischem Interesse. (7sn., 7 abb., 4 ref.)
Un design simple pour un Cchangeur de chaleurlfiltre I lit mobile et courant crois& par A Maxias-Mach%%, J Cuillar, A Estiez et E Jara& Dans /es applications industrielfes, des gaz chauds de combustion sent souvent uti/i&s dans des &hangeurs de chaleur gazisolide B contact direct, comme /es bts fluidis&, /es fixes ou /es Bchangeurs d mobile. On utilise aussl souvent des /its granulaires pour filtrer des fluides. &pendant, /es filtres B gar chauds at /es echangeurs gazlsolide ont 61.6 Btudi& s6par6ment at i’on a peu cons!d&& les dispositifs oti &change de chaleur et filtration ont lieu simultan8ment. Cet article d&it le calcul d’un filtrel6changeur de chaleur d fit mobile flux crois& (MHEFj, un design
bts
lit
qui implique un lit granulaire mob& pour /a filtration des particules et i’6change simultan6 de chaleur des gaz chauds. On pr6santa des graphiques pour determiner /a concentration en poussi&es d /a sortie de i’appareil, /a surface de contact requise pour une concentration don& en pouss&?s ou pour une temp&ature donn& dans I’efffuent gazeux, ainsr que d’autres parametres flint&t pratique. (7p., 7fig., 4ref.l
et
Diseiio sencillo de un filtro intercambiador de calor a flujo cruzado con lecho mdvil por A Ma&x-MachGz, J Cu&ar, A Estiez y E Jara& En aplfcaciones industriales se emplean gases calientes de combustidn en intercambiadores de calor contacto direct0 gasls6lido, tales coma mtercambiadores /echo fiirido, /echo ffjo ye lecho mdvil. Lechos de grenulos se emplean muchas veces en /a filtrackjn de particulas de polvo transportadas por fliridos. Sin embargo, filtros de gases calientes y intercambiadores de calor Qaslsdhdo se han estudiado, y desifiado separadamente. y se ha dado poca relexi6n a aparatos en 10s wales tlenen iugar intercambio de caior y filtracidn a/ mfsmo tiempo. Este articulo describe ei diseito de un
intercambiador de caior a flujo cruzado con iecho mdni, que emplea un /echo de gr~nulos para /a filtracidn de poivos y ei recobro de caior de gases calientes. Se presentan tab/as para determmar la concentraci6n de polvo saliendo de/ aparato, area de contact0 necesarfa para /a concentracidn de polvo o temperatura de/ gas efluente y otros pardmetros de inter& prBctico. (7p8gs., 7figs., 4refs.) _,
Testing the Performance of Spool-Wound Cartridge Filters Leistungstest fiir Kerzenfilter mit Spulenwicklung von C J Williams Wegwerf-Kerzenfilter mit Spuienwicklung dnd mit Hilfe emer Abwandiung des in 1504572 beschriebenen, mehrfachen Durchiauftestsystems standardmHi3lgen dynamischen Leistungstests unterzogen worden. Es wurden konstante FiuBbedmgungen beibehalten, urn w%hrend der Blockierungs.Dhase die Druckprofile bestimmen zu kiinnen. In den Testergebnissen 1st die Filterlerstung in Form des Verhdltmsses zwischen der vom Filter abgefangenen, gegebenen PartikelgrbBe. dem Festsfoffaufnahmevermtigen und der Gesamtstandzeit definiert. Durch den Emsatz emes groben
standardm&?igen Jeststaubs, einer verminderten DurchfluBrate und den Einsatz van Baumwolie anstelle van Polypropylenmaterial ergab sich ungeachtet der theoretfschen Abfanglwstung bzw. der Filterherkunft eine erhBhte Filterleistung. Die Ergebnfsse werden in Form einer kritischen Begutachtung des Testsystems, der Methodik und des Versuchsmateriaals (Jeststaubs) erbrtert. (issn., 1 I ebb.. 2 tab., 12 ref.)
Test de performance des filtres & cartouche B fil bobin par C J Williams Des fiftres perdus B cartouche et B fil bobin6 ont BtB soumis a des tests d’efflcacit6 dynamiques standards par I’emploi d’un syst&me modifib du test multi-pass d&It dans f’fSO4572. On a maintenu des condmons de d&bit constant, ce qui a permis /a d&?rmination des profils de pressIon pendant le processus de colmatage. Les r&ultats du test definissent I’efficaot6 du fjltre en fonchon du rapport des partlcules d’une tai/le don&e retenues par le filtre, sa capacite de r&enbon en soiides et sa d&e de vie. L’emploi d’une poussiere grossi& et le racours au coton plutdt qu’8 un mat&au
filtrant en polypropyi&ne ont conduit d une augmentation de i’efficaot8 du filtre quelles que soient /es caract&isbques nominales du filtre ou son engine de fabncabon. On disc&e /es r&u/tats B /a lumrere d’une &valuation critique du systeme d’essai, de /a m&hodologie et du mat&au poussi&eux utilis6 dans ie test. (7p., 11 hg., 2 tab., 72&f,)
Probar el rendimiento de filtros de cartucho bobinados en carrete por C J Williams Filtros cartucho con cartuchos bobinados en carrete, de usar y tirar. han Bdo probados en pruebas standard de efioencia din&mica empleando una modificacidn de/ sistema de prueba multipaso de 1504572. Se han manrenldo estados de flu/o constante, permitiendo la determinacldn de parhlas de presidn durante el proceso de obturacidn. Los resultados de /a prueba determman rendrmrento an t&minos de /a proporcidn de particulas de un tamaiio determinado retemda por el filtro, /a capaciuad
de soportar s6lidos, y /a vida total. El us0 de un polvo grueso de prueba. una disminucidn de razin de flujo, y el empleo corn0 medio de algoddn en vaz de polipropileno producer? un aumento de rendlmrento de filtro a pesar de la clasificacfdn nominal de retencidn o la fuente de fabncaodn. Se dlscuten 10s resultados a /a luz de una vaforacidn critica del &sterna de prueba, /a metodologfa y el polvo de prueba. (7 pigs., 11 figs., 2 tabs., 72 ref.%)
March/April
1992
Filtration & Separation
Simple D e s i g n of a Crossflow Moving Bed Heat Exchanger-Filter (MHEF) By A . M a c i a s - M a c h i n
Escuela de Ingenieros Industriales de Las Palmas, Tafira Baja, s/n, 35017 Las Palmas de Gran Canaria, Spain a n d J. Cu611ar, A . Est~vez a n d E. Jaraiz
Departamento de Ingenieria Quimica. Facultad Quimicas. Plaza Caidos 1-5, Universidad de Salamanca, 37008 Salamanca, Spain In industrial applications hot combustion gases are often used in direct-contact gas-solid heat exchangers, such as fluidized bed, fixed bed and moving bed exchangersO). Granular beds are also often used for filtering dust particles carried by fluids (2). However, hot gas filters and gas-solid heat exchangers have been studied and designed separately and little consideration has been given to devices where heat exchange and filtration take place simultaneously. This paper describes the design of a crossflow moving bed heat exchanger-filter (MHEF) (3),a device which uses a moving granular bed for simultaneous dust filtering and heat recovery from hot gases. Charts are presented for determining the dust concentration leaving the device, contact area needed for a required dust concentration or temperature in the effluent gas, and other parameters of practical interest.
Coldparticles
The upper part of a moving bed heat exchanger-filter (MHEF), where cold granular particles enter and fall is shown in Fig. 1. A dust-laden hot gas is filtered and cooled when it passes laterally through the layer of downward-moving particles. The clean, cooled gas exits the M H E F and the hot particles fall into the lower part of the unit (not shown in Fig. 1 but with the same design as the upper part). There the particles contact a crossflowing co][dgas which is consequently heated. The particles, now cold, finally exit the lower end of the device and are carried upwards to be recirculated. The lifting operation can be performed by pneumatic, mechanical or other means. In one of several different designs (3) coils are located around a vertical conveying pipe. The coils are t u r n e d on and off sequentially by pulses of electric current. The moving magnetic fields produced in this way are able to carry the particles m which need to be of a magnetizable n a t u r e m to the top of the MHEF. Then the cold particles fall down and the heat exchange-filtering process goes on. A device for collecting the dust carried by the particles leaving the upper part of the a p p a r a t u s is placed between the upper and lower parts of the MHEF. Model In this design we assumed: a relatively short relaxation timelY); t h a t is (hg_ s a) -. a thin stream of solids being contacted by a crossflowing gas [] the solids are mixed laterally but are unmixed along the flow path [] plug flow of gas [] stationary filtration, with no effects caused by structural changes resulting from dust accumulation within l~he filter [] small concentration of dust in the gas This cross-contacting p a t t e r n is the simplest design of this MHEF, but was useful for m a k i n g preliminary estimates, since no published literature on simultaneous Filtration& Separation March/April1992
"
X
~
Dirty
Hotparticles
Fig. 1. Schematic view of the upper part of a Moving Heat Exchanger-Filter.
T EXCHANGER-FILTER filtration and heat exchange designs has been found. Once these assumptions have been made, the contacting pattern of the MHEF is similar to the one described in Ref. 1, which models a crossflow fluidized bed heat exchanger with an infinite number of stages.
(12) ER=- 2da dc
Theoretical background For the model described above the heat transfer efficiencies of the operations are expressed as? AT ng=-=-ATmax
T,-- 7’2
- ?- [L-e-+] T,- t, @
(1)
and At
qs=-=-=
tz-t1
l-e-k+qg
Tl-t,
WI-,,
.
(j)=.=--
l
f%%
-=XYZ
X~/U~ 1-E psc,
mScS
(3)
Also, from Eqs. (1) & (2)
tz- t1
Ils 4 =-=-
T,- T,
‘Ig
(4)
Defining dimensionless parameters X, Y and 2 as
x=-
W&S
(51
%xl~uo
y=-
1 (6)
1-E z-
wg --PS
(n
On the other hand, the rate at which the dust concentration in the gas decreases as the gas flows through the medium in the direction x, is obtained from Ref. 4. The rate at which the concentration of solids C decreases is given by: 3E(1-E) dC -_=dx 2dc 1 After solving this equation we get
where
In -=-$ cf Co F_
1
EF
(l-dXm
C
(9)
Equations (11 to (14) can be combined in different charts - related to the dimensionless parametersx, Y 2, E, F and Cf IC,, - that can be used to calculate the thickness x, of the MHEF and the dust concentration of the filtered gas.
Using the charts Figures 2 to 7 show the set of charts that aid the design of crossflow moving bed heat exchanger-filters. These charts simplify the determination of the thickness x, required of a particular exchanger-filter to give a defined dust concentration in the gas outlet or, alternatively, let us design the MHEF when we know the inlet and outlet temperatures of the gas and solids. Figures 2 to 6 show curves for selected values of 2 and represent the dimensionless parameters that let us design the MHEF and evaluate the performance of a given heat exchanger. Given a value of 2, and specifying any two of the three other parameters I$, X, E, the unknown can be read from the appropriate chart. For a design problem, if we know I) and E, we can find X and then evaluate the thickness of the MHEF. From Fig.7, knowing the thickness, x,, we can find the dust concentration in the filtered gas once we know the collection efficiency E, from Eqs. (11) to (141, and the diameter of the collector particle d,. Figure 7 represents a typical filter operation. This graph represents the variation of dust concentration in the filtered gas, in the direction x, and allows us to determine the thickness x, of our filter, In this figure we can observe that specifying any two of the three parameters E, F and Cf IC,, the unknown can be read from the chart, For the calculation of performance, we can know the dust concentration of the filtered gas and read from the chart the parameter F that will give us the thickness x, of the MHEF. Now, knowing X, E and 2 values we are able to calculate $Iand the final temperatures. The following examples illustrate the use of these charts for some different cases.
Numerical example 1 Required: To determine the thickness of the moving heat exchanger-filter (MHEF), x,, the gas flow rate and dust concentration in the filtered gas, given: T,= 500°C T, = 184 “C
(10)
(11)
where the suffixes refer respectively to inertia(I), interception(R), and diffusion(D) mechanisms. These terms are given@) as: 156
(141
(8)
and E is the collection efficiency of a single sphere@). This can be adequately expressed as the sum of the efficiencies for the individual mechanisms: E=EI+ER+ED
-2,‘3
ED=4Pe
(2)
where $Iis given by
rngcg WI?&
(13)
t,=40°c t, = 293 “C
and E = 0.5 pg = 0.456kg/m3 ps = 7200 kg/m3 w = 1152 kg C, = 346 J/kygK Cg= 1093 J/kgK
z”, E C, d,
March/April
= 0.5 m/s = 0.90 kg/s = 0.1 = 0.2 g/m” = 10 mm 1992
Filtration &Separetion
TEXCHANGER-FiLTER
0
2000
3000
4000
5000
6000
7000
SOtlO
9000
‘lOdO
l
X Fig. 2. The performance
1000
2000
3000
4000
Fig. 3. The performance
Filtration & Sepwation
March/April
1992
=-w/m, curves corresponding
5000
6000
curves corresponding
toZ=
l/3000.
7000
toZ=
8000
0.50
;I””
0.45 0.40 0.35
/
7
9000
10000
l/5000.
157
TEXCHANGER-FILTER 3.0
i.
i.
1.
L / ‘.
+
90.00
lob00
2.5
v
0.5
A
1000
/ATI
2000
3000
4600
I
I
I
I
5dOO
6000
7000
8000
WI& X=------&A Fig. 4. The performance curves corresponding tOZ=
1000
2000
3000
4000
5000 X
6000
Ir?OoO.
7000
8000
9000
10000
WI& =--&T&O
Fig. 5. The performance curves corresponding to Z = 1/9ooo. 158
March/April
1992
Filtration&Separation
T EXCHANGER-FILTER
0.50 0.45 0.40 0.35
Fig. 6. The performance
0
2.5
5
7.5
curves corresponding
Filtration & Separation
March/April
1992
to Z = l/l 1!300.
I
10
Fig. 7. Performance
_-. q
1’5
- 17.5
20
curves of filtration.
159
T EXCHANGER-FILTER Solution: First determine the value of the parameters I$and.Z. From Eq. (4): ~_ tz-tl
_ 293-40
T,-T, From Eq. (7) pgcg Z=-----=
WithX, Z and Ewe use Fig. 4 to get I$= 0.6
1 =2x1o-4=-
7200 x 346
5000
As 2 = l/5000, we use Fig. 3 (relating the parameters +, E,2 andX) to get: WIni, X=---Grl&
7000
= 0.8
500- 184
0.456 x 1093
P&s
9372 x 420
P&s
= 2000
From Eq. (3) . mg=
discs
=
0.6 x 0.8 x 420
cg * Then, from Eqs. (2) & (1)
1069
qs = 1 - e-4 = l-
= 0.188 kg/s
e-O.6= 0.451
From Eq. (51, the thickness required is: x,=-=
wuo
1152 x 0.5
??i,X
0.90 x 2000
0.451= t,T,-
= 0.32 m
=
400 - 41
0.451
(1 - 0.5) x 0.32
(I-E)+-, d,
=
10 x 10-s
Now, from Fig. 7, using the parameters E and F Cf = Co
x
Cf Co
= 0.0907
0.0907 => Cf = 0.01814 g/m3
Thus, having a thickness of xm= 0.32 m, the MHEF reduces the dust concentration m the gas from 0.2 g/m3 down to 0.01814 g/m3 which is a reduction of approximately 91% of the initial dust concentration. Numerical example 2 Required: To determine the outlet temperature of the solid, t 2,the exit temperature of the filtered gas, T,, and the gas flow rate, given: pg = 0.525 kg/m3 Ps = 9372 kg/m3 tis=0.8kg/s u. = 0.36 m/s w = 224Okg
cg = 1069 JfkgK es=420 JlkgK T, = 400 “C! t1=41V E = 0.2
d, = 18.5 mm &= 0.4 g f f ;.;z g/m3 m3 o*
13 x 1.85 x 1O-2
F-d,
0.6
m=(l--E)=
=
From Eqs. (5) & (7), the dimensionless and Z are x=
WIni, Xrn/uo
160
0.752 = T, T,-
0.40 m parameters X
= 2240i0.S = 2520 0.410.36
= t,
4oo - T2 400 - 41
0.752
; T2 ti 130°C
Conclusions A simple model has been established which allows us to calculate the performance of a moving heat exhangerfilter (MHEF). From charts obtained from this model it is possible to find the filter thickness for a given percentage of dust recovery from the gas. Also, the design of an MHEF is possible when the gas and solid inlet and outlet temperatures are known.
References
1. Levensoiel. 0. Enkneerin~ Flow and Heat Exchange. Plenum Press, New York, 1384. 2. Mizukami, S., Wakabayashi, M. and Murata, H. Inter-
action between pressure drop of gas and flow of medium in a moving granular bed filter. Particulate Science and Technology, 5,198’7, pp. 131-142. 3. Jaraiz, E., CuBllar, J., Est&ez, A. and Macias, A. Device and Procedure for Simultaneous Heat Exchange and Filtration of Fluids. (Spanish Patent Applied, 1991).
4. Clift, R., Ghaidiri, M. and Thambimuthy, K. J. Filtration of Gases in Fluidized Beds, in Progress in Filtration 2, (Edited R. J. Wakeman) Elsevier, Amsterdam, 1981, p. 84.
a
Solution: From Fig. 7, knowing E and Cf/C, we find the value of F to be approximately equal to 13. Then, from Eq. (10) we get: X
0.6
= 16
; tz = 203°C
=- =
t,
To calculate the dust concentration in the filtered gas, we use Eq. (10): F=
t,-41
5 cs
C
Nomenclature surface of solid per unit volume of bed, m-l specific heat capacity of gas, J/kg K specific heat capacity of solid, J/kg K dust concentration in gas, kg/m3 outlet dust concentration in gas, kg/m3 inlet dust concentration in gas, kg/m3 collector particle diameter, m dust particle diameter, m diffusion coefficient of particle, s.m-2 collection efficiency of a single sphere dimensionless parameter defined in Eq. 12 heat transfer coefficient between gas and solid, J/s.m2.K mass flow rate of gas, kg/s March/April
1992
Filtration&Separation
TEXCHANGER-FILTER mass flow rate of solid, kg/s
ZHEFMoving Heat Exchanger-Filter Pe
Sk T
t
Xlll
X
zy W
Peclet number ( = u&&J Stokes number gas temperature, K solid temperature, KU, superficial gas velocity, m/s thickness of the Moving Heat Exchanger Filter, m dilmensionless parameter defined in Eq. 5 dilmensionless parameter defined in Eq. 6 dilmensionless parameter defined in Eq. 7 mass of solid material, kg
Filtration 81Separation
March/April
1992
Subscripts Inlet ; Outlet Greek symbols E voidage gas efficiency % solid efficiency % gas viscosity, Pa.s dust density, kg/m3 Ld gas density, kg/m3 PP solid density, kg/m3 heat flow ratio of two contacting streams given : by Eq. 2
161
OFREFEREED PAPERS RACTS Effects of Step Drums on Solids Residence limes in Conical Basket and Tumbler Centrifuges < Auswirkungen von Stufentrommeln auf Feststoff-Verweilzeiten bei Siebzentrifugen mit konischer Trommel und bei Taumelzentrifugen von Fan Deshun, R J Wakeman und Huang Zhong
El147
Das AusmaB der Feststoff-Verweilzeit in emer JaumelzentrffugentrommeI stellt ein bedeutendespraktisches Problamdar. EinerAnalysederKennwerteder Jrommelundder Partikelbewegung sowohl rn Jaumelzentrifugen als such in Slebzentrifugen mrt konrscher Jrommel schlief3t sich eine Berechnung der Feststoffverweilzeften ftir beide Zentrifugenarten m,t Stufentrommeln an. Es wird dargelegt, da8 bei Verwendung einer Stufentrommelm einer Taumelzentrifuged!e Verweilzeit /anger a/s be! Verwendungeiner Slebzentnfuge mit komscher Jrommelist. Der PartikelfluB durch Taumelzentrifugen und Maschinen m!t Stufentrommeln find& auf lmpulsbasis start: Wobbeiimpulse bewfrken
Partikelbewegung Ghrend emes begrenzten Zeitraums der Trommelrotation be! Taumelzentrifugen, wtihrend durch die Stufen strukturgebundene Impulse erzeugt werden, d!e die Partikelbewegung kurz unterbrechen. Bei guter Auslegung kann d,e Feststoffverweilzeitm TaumeizentrifugenmittelsStufentrommelverdoppelt werden. Esist damit zu rechnen, daR sich dadurch dieAnwendungsm6glichkeften der Taumelzentrifugen erweitern lassen Und d/e resultierenden Feststoffprodukte einen hdheren Jrockenheitsgrad errerchem(8 sn., 10 abb., 6 tab., 4 ref.)
Effets des tambours itag6s sur le temps de sijour des solides dans les centrifuges P panier conique et ir culbuteur par Fan Deshun, R J Wakeman et Huang Zhong L’accrolssement du temps culbuteur est un probl$me du tambour et le mouvement panier conique, on calcule tambours&ages. Onmontre p/us grande du temps de machine $ paniercomque.
de r&vdence des solides dans un tambour de centrifuge I pratiqua important. A&s avolr analyst? /es caract6ristique.s des particules d la fois dans /es centrifuges dculbuteur et d /es temps de &idence pour chaque type de centrifuge B quef’emploid’un tambour&ag&produit uneaugmentatfon r&dence dans une centrifuge B cuibuteur que dans une Le debit departicles dans les centnfuges B culbuteur et dans
Efectos de tambores
con grada en tiempos de permanencia con centrifugas por Fan Deshun, R J Wakernan y Huang Zhong
La extensidn
de/ tiempo de permanencia de sdlidos en el tambor de una centrifuga voltereta es problema Importante en la pr8ctica. Despues de anallzar /as caracterisbcas de/ tambor y ei movimvanto de part&/as en centrifugas voltereta y a casta cdnica. se ha calculado tiempos depermanencia con ambos tipos con tambores con grada. Se ha demostrado qua a/ ample0 da un tambor con grada produce mas aumento de permanencia en una centrifuga voltereta queen la mtiquina cesta cdmca. El flujo de partfcuias por /as con tambores con grada es impuisivo; impulses
Izl
/es machmes B tambour &age est pulsL des oscifiatrons cr&nt un mouvement particules pendant une p&/ode I/mit& de la rotation du tambour dans /es centrifuges cu@teur tandis qua les tambours &tag& causent des publications structur&s arretent momentan6ment le movement des particuies. Avec un bon design. ie temps s~jourdessolidesdansiescentrifugesdculbuteurspeuventbtredoubl~sparierecoursd un tambour @tag&. On &tend done d pouvoir Blargir fe champ des applications centrifuges d culbuteur et obtenir des produits plus sets. (8 p., 10 hg., 6 tab., 4 r6f.j
bamboleantes producen rotacidn de/ tambor estructurales /OS wales dwiro se puede doblar al empleo de un tambor voltereta para obtener
a cesta chica
des B qui de des
y a voltereta
movimiento de particulas durante un period0 limitado de /a con mlquinas voltereta y /as gradas mducen impulses paran momentaneamente movimfento particula. Con buen el tiempo que 10s sdlidos estan en una centn’fuga voltereta con con grada. Se espera extender /as aplicaoones de centrifugas productos s6lidos mas sacos. (8 pdgs., 10 figs., 6 tabs., 4 refs.)
Simple Design of a Crossflow Moving Bed Heat Exchanger-Filter (MHEF) Einfache Konstruktion eines Ouerstrom-Wirbelbett-W$irmeaustauschfilters von A Mcwias-Mach@ J CuWZar,A h’stiez und E Jara@
155
Beilndustrieanwendungen
werden oft hei& Verorennungsgase in Dlrektkontakt-GasFeststoff- WPrmeaustauschern, wie z.B. Flijssigbett-, Festbettund Wirbelbett-Austauscher, eingesetzt. Kornstrukturbetten werden ebenfalls oft zum Ausfiltern der won den Flijssigkeiten mitgefijhrten Staubpartikel benutzt. Heiflgasfilter und Gas-FeststoffWBrmeaustauscher sind war separat untersucht und konstruiert worden: /eider sind dabeijedoch Ger&e vernach&sigt worden, in denen W&meaustausch und Filtration gleichzeitfg stattfinden. In diesem Referat ist dre Konstruktion eines Querstrom-
Wirbelbett-
WPrmeaustauschfilters (MHEF) beschneben-ein Gerst, bei dem ein kbrniges Wirbelbett zur gieichzeitigen Staubfiltration und WBrmerDckgewinnung van heiRen Gasen eingesetzt wird. Das Referat enthblt Diagramme zur Bestimmung der Staubkonzentration an der Ablaffseite des Gerdts, Angaben iiber die erforderliche Kontaktfllche ftir eine bendtigte Staubkonzentration bzw. Temperatur rm Abgas sow/e andere Parameter YOR praktischem Interesse. (7sn., 7 abb., 4 ref.)
Un design simple pour un Cchangeur de chaleurlfiltre I lit mobile et courant crois& par A Maxias-Mach%%, J Cuillar, A Estiez et E Jara& Dans /es applications industrielfes, des gaz chauds de combustion sent souvent uti/i&s dans des &hangeurs de chaleur gazisolide B contact direct, comme /es bts fluidis&, /es fixes ou /es Bchangeurs d mobile. On utilise aussl souvent des /its granulaires pour filtrer des fluides. &pendant, /es filtres B gar chauds at /es echangeurs gazlsolide ont 61.6 Btudi& s6par6ment at i’on a peu cons!d&& les dispositifs oti &change de chaleur et filtration ont lieu simultan8ment. Cet article d&it le calcul d’un filtrel6changeur de chaleur d fit mobile flux crois& (MHEFj, un design
bts
lit
qui implique un lit granulaire mob& pour /a filtration des particules et i’6change simultan6 de chaleur des gaz chauds. On pr6santa des graphiques pour determiner /a concentration en poussi&es d /a sortie de i’appareil, /a surface de contact requise pour une concentration don& en pouss&?s ou pour une temp&ature donn& dans I’efffuent gazeux, ainsr que d’autres parametres flint&t pratique. (7p., 7fig., 4ref.l
et
Diseiio sencillo de un filtro intercambiador de calor a flujo cruzado con lecho mdvil por A Ma&x-MachGz, J Cu&ar, A Estiez y E Jara& En aplfcaciones industriales se emplean gases calientes de combustidn en intercambiadores de calor contacto direct0 gasls6lido, tales coma mtercambiadores /echo fiirido, /echo ffjo ye lecho mdvil. Lechos de grenulos se emplean muchas veces en /a filtrackjn de particulas de polvo transportadas por fliridos. Sin embargo, filtros de gases calientes y intercambiadores de calor Qaslsdhdo se han estudiado, y desifiado separadamente. y se ha dado poca relexi6n a aparatos en 10s wales tlenen iugar intercambio de caior y filtracidn a/ mfsmo tiempo. Este articulo describe ei diseito de un
intercambiador de caior a flujo cruzado con iecho mdni, que emplea un /echo de gr~nulos para /a filtracidn de poivos y ei recobro de caior de gases calientes. Se presentan tab/as para determmar la concentraci6n de polvo saliendo de/ aparato, area de contact0 necesarfa para /a concentracidn de polvo o temperatura de/ gas efluente y otros pardmetros de inter& prBctico. (7p8gs., 7figs., 4refs.) _,
Testing the Performance of Spool-Wound Cartridge Filters Leistungstest fiir Kerzenfilter mit Spulenwicklung von C J Williams Wegwerf-Kerzenfilter mit Spuienwicklung dnd mit Hilfe emer Abwandiung des in 1504572 beschriebenen, mehrfachen Durchiauftestsystems standardmHi3lgen dynamischen Leistungstests unterzogen worden. Es wurden konstante FiuBbedmgungen beibehalten, urn w%hrend der Blockierungs.Dhase die Druckprofile bestimmen zu kiinnen. In den Testergebnissen 1st die Filterlerstung in Form des Verhdltmsses zwischen der vom Filter abgefangenen, gegebenen PartikelgrbBe. dem Festsfoffaufnahmevermtigen und der Gesamtstandzeit definiert. Durch den Emsatz emes groben
standardm&?igen Jeststaubs, einer verminderten DurchfluBrate und den Einsatz van Baumwolie anstelle van Polypropylenmaterial ergab sich ungeachtet der theoretfschen Abfanglwstung bzw. der Filterherkunft eine erhBhte Filterleistung. Die Ergebnfsse werden in Form einer kritischen Begutachtung des Testsystems, der Methodik und des Versuchsmateriaals (Jeststaubs) erbrtert. (issn., 1 I ebb.. 2 tab., 12 ref.)
Test de performance des filtres & cartouche B fil bobin par C J Williams Des fiftres perdus B cartouche et B fil bobin6 ont BtB soumis a des tests d’efflcacit6 dynamiques standards par I’emploi d’un syst&me modifib du test multi-pass d&It dans f’fSO4572. On a maintenu des condmons de d&bit constant, ce qui a permis /a d&?rmination des profils de pressIon pendant le processus de colmatage. Les r&ultats du test definissent I’efficaot6 du fjltre en fonchon du rapport des partlcules d’une tai/le don&e retenues par le filtre, sa capacite de r&enbon en soiides et sa d&e de vie. L’emploi d’une poussiere grossi& et le racours au coton plutdt qu’8 un mat&au
filtrant en polypropyi&ne ont conduit d une augmentation de i’efficaot8 du filtre quelles que soient /es caract&isbques nominales du filtre ou son engine de fabncabon. On disc&e /es r&u/tats B /a lumrere d’une &valuation critique du systeme d’essai, de /a m&hodologie et du mat&au poussi&eux utilis6 dans ie test. (7p., 11 hg., 2 tab., 72&f,)
Probar el rendimiento de filtros de cartucho bobinados en carrete por C J Williams Filtros cartucho con cartuchos bobinados en carrete, de usar y tirar. han Bdo probados en pruebas standard de efioencia din&mica empleando una modificacidn de/ sistema de prueba multipaso de 1504572. Se han manrenldo estados de flu/o constante, permitiendo la determinacldn de parhlas de presidn durante el proceso de obturacidn. Los resultados de /a prueba determman rendrmrento an t&minos de /a proporcidn de particulas de un tamaiio determinado retemda por el filtro, /a capaciuad
de soportar s6lidos, y /a vida total. El us0 de un polvo grueso de prueba. una disminucidn de razin de flujo, y el empleo corn0 medio de algoddn en vaz de polipropileno producer? un aumento de rendlmrento de filtro a pesar de la clasificacfdn nominal de retencidn o la fuente de fabncaodn. Se dlscuten 10s resultados a /a luz de una vaforacidn critica del &sterna de prueba, /a metodologfa y el polvo de prueba. (7 pigs., 11 figs., 2 tabs., 72 ref.%)
March/April
1992
Filtration & Separation