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Filtration
The separation of solids from a suspension in a liquid by means of a porous medium or screen which retains the solids and allows the liquid to pass.
Separation of solids from liquid i.e. when the amount of solid is relatively small as compared to the liquid Most common application of the flow of fluids through packed bed.
Removal of solid particles from a fluid by passing the fluid through a filtering medium or septum on which the solids are deposited.
Operation wherein heterogeneous miture of a fluid and particles of solids are separated by a filter medium by permitting the flow of fluid and retaining the particles of solids. !ssentially a mechanical operation and is less demanding in energy than evaporation or drying where the high latent heat of liquid" which usually water" has to be provided. #n chemical laboratory" filtration filtration is carried out using $%uchner funnel&" and the liquid is sucked through the thin layer of particles using a source of vacuum.
Three (3) major groups of filters:
• Cake Filters 'separate relatively large amounts of solids as a cake of crystals or sludge and often provide provisions for washing the cake and removing some of the solids before discharge. !ampl !am ple( e( )il )ilte terr pre press ss"" be belt lt fi filt lter er"" di disc scon onti tinu nuou ouss va vacu cuum um fi filt lter erss or nu nuts tsch che" e" continuous vacuum or rotary'drum filter" centrifugal filters *batch or continuous centrifuges+.
• Clarifying Filters - remove small amounts of solids to produce a clean gas or sparkling clear liquid such as beverages" pharmaceutical products" fuel oil and electroplating solution. The solid particles are trapped inside the filter medium or on it itss e ete tern rnal al su surfa rface ce.. Th Thee pa parti rticl clee are ca caug ught ht by su surf rfac acee fo force rcess an and d immobili,ed immobi li,ed on the surfa surface ce or with within in the flow channels channels where they reduce the effective diameter of the channels but usually do not block them completely. #n cleaning gases" the separation is by impingement of of the particles against a solid surface place in the flowing stream. The particles" because of their inertia" are epected to cross the streamline of the fluid and strike and adhere to the solid" from which they can subsequently be removed.
-
!am ! ampl ple( e(
lar l arif ifyi ying ng dis disk k filt filter er"" gas gas or or dust dust fil filte ter r
Cross ossflo flow w Fi Filte lters rs (Me (Memb mbran ranee fi filte lters) rs) / the fee feed d sus suspen pensio sion n flo flows ws und under er • Cr
pressure at a fairly high velocity across the filter medium. 0 thin layer of solids may form on the surface of the medium" but the high liquid velocity keeps the layer from buil building ding up. This can be appli applied ed to conce concentrat ntratee suspe suspension nsionss of fine particles or colloidal material material or to fractionate solutions solutions of macromolecules. !amp !a mple( le(
Microfi Mic rofiltr ltrati ation on *.2 *.2 to to 2 3m+ 3m+ and 4lt 4ltrafi rafiltr ltrati ation on *.2 *.23m 3m to 1
/ 5 / 5
3m+
Cake Filtration Priniples !oabularies: • Filter ' piece of unit operation equipment by which filtration is perfor med. • Filter me"ium or septum ' barrier that lets the liquid pass while retaining most of the solids. #t may be a screen" cloth" paper or bed of solids. / the the liquid that passes through the filter medium • Filtrate / #lurry rry / heterogeneous miture of solids and fluid which will be fed to the • #lu filter. / wet wet accumulated solids that piled up or trapped on the • #lu"ge or filter ake / filter medium. Four ($) groups of filters "epen"ing on ser%ie : &' #trainers ' little more than a metal screen set across a flow channel to remove dirt or rust from a flowing liquid. forr re remo mova vall of sm smal alll qu quan anti tity ty of so soli lids ds us usua uall lly y to pr prod oduc ucee ' Clarifiers ' fo sparkling clear liquid like in the ink or beverage industries. 3' Ca Cak ke fi filt lteers / for separation of large amount of solids from a liquid as a cake of crystals of crystals or sludge. 6. Fil Filter ter th thik ikene eners rs / gives partial separation of a thin slurry" discharging some clear liquid and thickened but still flowable suspension of solids. Classifiation of filtration: &' y "r "ri% i%in ing g for fore e The filtrate is induced to flow through the filter medium by hydrostatic head *gravity+" pressure is applied upstream of the filter medium" vacuum or reduced
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pressure applied downstream of the the filter medium" or centrifugal force across the medium. ' y filt filtrat ration ion me mehan hanism ism )iltration process can be accomplished by ( ake filtration / when solids are stopped at the surface of the filter medium and pile upon one another another to form a cake of increasing thickness. thickness. 7epth" filter'medium or clarifying filtration / when solids are trapped within the pores or body of the medium. 3' y *b *bje jet ti% i%ee The process goal of filtration may be dry solids *the cake is the product of value+" clarified liquid *the filtrate is the product of value+" or both. 8ood solids recovery is best obtained by cake filtration" while clarification of the liquid is accomplish by depth and also cake filtration. $' y ope opera rati ting ng y yle le )iltra )il tratio tion n may be int interm ermitt ittent ent *ba *batch tch++ or con contin tinuou uous. s. %at %atch ch fil filter terss may be operated with constant'pressure driving force" at constant rate" or in cycles that are variable with respect to both pressure and rate. %atch cycle can vary greatly" depending on filter area and solid loading. +' y natu nature re of the sol soli"s i"s'' ake filtration may involve an accumulation of solids that is compressible or substantially incompressible. Cake filtration ,uipment:
1. ath Cake filters a. 9u 9uts tsch chee fi filt lter erss /s /sim impl ples estt ba batc tch h fi filt lter er.. #t is a ta tank nk wi with th fa fals lsee bo bott ttom om"" perforated or porous" which may either supports a filter medium or act as the filterr medi filte medium. um. The slurry is fed into the filter vessel" and separation separation occu occurs rs by gravity flow" flow" gas pressure pressure " vacuum or combination of of theses forces. b. Rosemund filter /eample of commercially available large'scale nutsche filter. #t is totally closed operation which allows the handling of ha,ardous solvents. c. 9ut 9utre re / a nov novel el variatio variation n of nut nutsch schee filters filters which combin combines es a rea reacto ctor" r" a filter and a dryer. ' Filt Filter er Pr Pres esses ses a. :late and and )rame / an altern alternate ate assembly assembly of plates plates covered covered on on both sides sides with with filter medium" usually a cloth" and hollow frames that provide space for cake accumulation during filtration
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b. Recessed'plate / similar to plate and frame press in appearance but consist only of plates. %oth faces of each plate are hollowed to form a chamber for cake accumulation between ad;acent plates. 5. Pressure leaf filters / / sometimes sometimes called tank filters" they consist of flat filtering elements *leaves+ supported in a pressure shell. The leaves are circular" arc' sided or rectangular and they have filtering surfaces on both faces and are operated batch'wise. 6. .otary "rum Filter / is the most widely used in continuous filters. Most are fed by operating the drum with about 52< of its circumference submerge in a slurry trough" although submergence can be set as to any desired amount. +' 0' 2' 5' 6'
/n"ustria /n"ust riall tubula tubularr filter filterss 1isk 1i sk Fi Filt lter erss ori4o or i4onta ntall belt belt fil filter terss Filt Fi lter er thi thik ken ener erss 1eep 1e ep be be" " fil filte ters rs
*:lease Refer to any 4nit Operation %ooks and h.! =% for illustrative eamples of the !quipment + Fators to onsi"er for the seletion of filters
1. The properti properties es of the fluid" fluid" par partic ticula ularly rly its vis viscos cosity ity"" density density and corrosiv corrosivee properties. -. The nature nature of the solid such such as its particle particle si,e and and shape" si,e si,e distributi distribution on and packing characteristics. 5. The conce concentrat ntration ion of solids solids suspe suspension nsion.. 6. The quantit quantity y of material material to to be handled" handled" and its its value 2. >heth >hether er the valuable valuable produc productt is the solid" solid" the fluid fluid or both. both. ?. >heth >hether er it is necessa necessary ry to wash wash the filtered filtered solid solids. s. @. >heth >hether er the the feed feed liquor liquor may be heated heated.. A. >heth >hether er any form of pretre pretreatmen atmentt would would be helpfu helpfull
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Material Balance on Filtration:
Slurry
Wet cake Filter medium Solid balance: Mass Mas s of soli solid d in slurry slurry fed fed = Mas Mass s of soli solid d in cake cake (Mass of slurry) (Mass fraction of solids in slurry) = (Mass of wet cake) (Mass fraction of solid in wet cake) (VF + ε L ) ρL (!"#) w$ere: #
ε
=
%orosity
= L
= =
ρL ρS
= = =
[ [
(
(
)(
)(
)
)
][
][ ]
]
?
Filtration &'uation:
Linear elocity of t$e fluid at any instant ( ν ) : (
w$ere:
•
*urin filtration, t$e only ariables sub-ect to t$e control of t$e o%erator are t$e %ressure dro% ( . ∆ % ), filtrate olume (V F), and time (t F ) w$ile usually constant durin filtration are: µ ρL ρS # ε
•
/ombinin t$ese terms to a sinle term,
[
@
Rate of Filtration (
)
(
)
0f we consider
in terms of cake t$ickness:
Filter Operation and application: /ase !: 0ncom%ressible /ake
ε = constant / = constant 1$e 1$ e fl flow ow ra rate te is di dire rect ctly ly %r %ro% o%or orti tion onal al to to t$e t$e ar area ea an and d %ressure and inersely %ro%ortional to iscosity, to t$e amount of cake (or filtrate) and to s%ecific cake resistance, ∝ Constant Pressure Filtration (CPF):
• %%ly
full %ressure at t$e start and maintain %ressure differential constant t$rou$out t$e run2 1$e first %article cau$t will be com%acted into a ti$t mass2 1$is will fill t$e %ores of filter medium and will result in a low rate of filtration2
A
• 0f
filtration %ressure is constant, t$e rate of flow %roressiely diminis$
Analytical equation : (
if (
)
(
) is constant ,
)
∫(
(
) (
(
)
)
(
)
dt dV
b VF 1$e aboe ra%$ is a%%licable to constant %ressure differential, laminar flow and incom%ressible cake w$ere Ve and / V are constant
will
B
Constant Rate Filtration (CRF): • %%ly initial %ressure at low %ressure and maintain t$e flow rate constant t$rou$out t$e run2 First collected filtrate will not be so clear because t$e initial layer of t$e cake will crowded into t$e fibers of t$e filter medium and will not ad$ere more firmly to t$e filter medium2 1$is could be easily se%arated2 But t$e filtrate to be collected at a ien time would be $i$er t$an t$e /3F • 0f filtration flow rate is to be maintained, t$e %ressure must be radually increased2 Analytical equation : dV
dt • If (d V / d t )
V
4
t
4
t
4
Graphical equation : ( . ∆ % )
( . ∆ % ) 5
( . ∆ % )
b V
4
1
Constant Rate folloed !y Constant Pressure (CRPF): 1$e use of a centrifual %um%s result in t$is %attern2 1$e disc$are rate decreases wit$ increasin back %ressure Analytical equation :
dV dt VF
∫ 6( 6( VF + V e ) d V V4
( VF + V e )
6
6
.
( VF + V e )
( . ∆ %)ma7
/4F
( . ∆ % ) V
V
4
F
V Cyclic Operation : "ashin# of Ca$e : %& For throu#h throu#h ashi ashin# n# (Plate (Plate and Fra'e Fra'e filter filter press) • 1$e was$ li'uid and t$e filtrate $as similar %$ysical %ro%erties • Was$in %ressure is t$e same as t$e final filtration %ressure • 8o cake rearranement
4ate of was$in
dV / dt
ashin#
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*& Fo Forr llea eaff filt filter ers s: • 1$e was$ li'uid and t$e filtrate $as similar %$ysical %ro%erties • Was$in %ressure is t$e same as t$e final filtration %ressure • 8o cake rearranement • Flow %attern and route of was$ water is same as for t$e filtrate
4ate of was$in
+otal +o tal cyclic ti'e: ( t
+
+
Filtration Capacity : (,)
,
Continuous Filtration For continuous rotary filters: constant rate but for any %articular • 1$e feed, filtrate and cake moe at steady constant element of t$e filter surface, conditions are not steady but transient2 • :rocess consists of series of steps such as cake formation" washing" drying" and discharging which involves progressive and continual change in conditions • The pressure "rop aross the filter "uring the ake formation7 is always hel" onstant' Thus7 CPF euation is applie" to ontinuous filters' (
(
)
)
(
)
(
( )
)
where (
tc n
-fraction of drum submered cycle time drum s%eed
lso, -AF / A +
w$ere: AF
-
submered area of filter or area of filtration
A+
-
total area of t$e drum filter
1-
3roblems: !2 Small leaf filter filter is run at constant constant rate2 rate2 0t is found t$at t$at initial %ressure %ressure differentia differentiall is 9 3a and t$e %ressure differential after 6; minutes of o%eration is 9< 3a durin w$ic$ ;2! cu2 meter of filtrate is collected2 0f t$is filter were used wit$ t$e same slurry in a constant %ressure filtration at 9< 3a, $ow muc$ filtrate is collected in 6; minutes >ien: /4F .?3! = 9 k3a 'C:- D 562 k:a tR D D - minutes E FR D D .1 cu. m. Required( F) D G if :) with same slurry at /C: D 562 k:a and t) D - mins. Solution( 0ssume incompressible cake
0t R)" 562 'C:
9
@sin /4F e'An2, ( . ∆ % ) 5
6
0nterce%t : (6/ ) (dVdt) (Ve) = 9 3a Slo%e : t$erefore, 6 lso, /
(6 0nt =
For /3F,
6; min
= (92!; 7 !;
"
9 6
%a " min (m ) )
VF
6
9
+ 6(;2;!! 6(;2 ;!!6Cm 6Cm (VF)D
9< %a by Euadratic e'uation, VF - .&%.01 '
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62 %late and frame filter %ress contains 6< frames, eac$ 62 cm t$ick wit$ inside dimensions ;2 m 7 ;2 m2 Filterin time is 6 $ours2 Was$ water olume is !; G of t$e filtrate olume, %er cycle2 Filterin and was$in are done at t$e same %ressure2 Final 9 9 cake is ;2;; m m of filtrate2 a2 W$at is t$e t$e time of was$in was$in b2 W$at is t$e daily ca%acity ca%acity if dum%in, cleanin cleanin and assemblin assemblin is 9; minutes %er cycle >ien: 3HF Filter 6< frames (;2m 7 ;2 m 7 62 cm t$ick) tF = 6 $ours V W = !;G VF %er cycle 9 9 Final cake = ;2; m m of filtrate 4e'uired: a2 t W =
Solution: dV dt ssume 4m = ; , Ve Ve =; For /3F,
VF
=( ;2m 7;2m 7;2;6m 76
t$erefore, dVdt = ! (6)(;2!;I!I)(<296) For 3HF, JdV BdtD W tW= Vw B JdV BdtD W = ;2!; (<296) (!<) (!<) (!2;K) (!2;K) = !2 !2 $ou $ours rs 5 = VF t1 9 = J <296 m (6 +!2+;2) $rs D J 6< $rs day D = 9 6 26C m %er day
16
9
92 slurry is filtered to %roduce m %er day of filtrate2 Filtration is conducted at a rate 9 6 of ;26< m m .$r in a %late and frame %ress until t$e %ressure is <; k3a2 1$e filtration is $e $eld ld co cons nstan tantt at co const nstant ant %r %ress essur ure e unt untilil t$ t$e e rat rate e dr dro%s o%s to 9; G2 1$ 1$e e co const nstan antt 9 6 %ressur %res sure e star starts ts w$e w$en n ;2I ;2I9 9 m %er m of filtrate $as been %roduced2 1$e time for cleanin cle anin, , dum% dum%in in and reassembli reassemblin n t$e %ress is < min2 /alculate /alculate t$e filt filter er are area a re'uired, nelectin t$e resistance offered by t$e filter clot$2
>ien: Slurry 5= m9 %er day /43F: dVdt = ;29 (dVdt) <;
V4 = ;2I9 m
9
t4 4m = ; Ve = ; tdcr = = < minutes
9
6
/4F: dVdt = ;26< m $.m 4e'uired: = Solution: 6 Basis : ! m filter area
CRPF eq2n&: 6
6
6
VF " V4 =
3urin# CRF4 t4 = V4 (dVdt) also,
/4F
9
9
= ;2I9 m ;26< m $r =
dV dt
/4F
12
At CPF4 dV dt ;29; (dVBdt) /4F t$erefore, ;29; (;26<) =9 ;29;< 6 (V F) VF = 62<9 m Substitute alues to /43F e'An2 , 6 6 (62<9) . (;2I;) = ;29;< ( tF " 92;
6
9
6
5
= J62<9 m !C2!69 $rs.m D J 6< $rsdayD = 92;
A
- 05 ' / day / &.678 ' /day9' * - %1 %1& &5 5'
*
<2 disk.ty disk.ty%e %e filter $andles $andles %etroleu %etroleum m residue residue settlins2 settlins2 serie series s of test run at a constant rate of ;2! %m yielded t$e followin data: 3ressure differential (%si) 96 Filtrate Volume (al) a2 0t is %ro%osed %ro%osed t$at t$at t$is filter filter be o%erate o%erated d at a constant constant %ressure %ressure diffe different rential ial of ; %si2 0f t$e slude is $omoeneous, calculate t$e olume of oil filtered in 9; minutes under suc$ conditions if t$e c$ane in %ress resistance wit$ rate is neliible2 b2 /a /alc lcul ulat ate e t$ t$e e fi filt lter erin in ti time me if t$ t$e e %r %res ess s wer ere e o% o%er erat ated ed at co cons nsta tant nt ra rate te un unti till %ressure differential became ; %si, and t$en at constant %ressure differential until 6; al of oil were forced t$rou$ t$e %ress2 Gien: *isk "ty%e filter
/4F at ;2! %m 3ro%osal: /3F at ( . ∆ % / ) = :; %si tF = 9; mins2 and
;olution: at CRF: Ve-. dV dt /V
6
1?
/ (/ V& V &
= a
mins 2 V =
!
!; !CC2!I = = !CC2!I %si 62I<< al
;
V4 t4 6
( . ∆ 3 )
dV
dt
6 /V ( V4 ) 6
V4 = :; %si B 6 (!CC2!I %si.minB al ) ( ;2! %m) V4 t4
6
6
VF " V4 =
= !26 al = !62 min
( . ∆ % ) ma7 J tF . t4 D
CV
6
6
(6;al) " (!26 al) =(; %si !CC2!I) J tF " !62 minD
tF - %077&56 'ins&
