THEORETICAL AND EXPERIMENTAL STUDIES ON ULTRAFILTRATION (UF) PROCESS FOR MILK CONCENTRATION

8/11/2019 THEORETICAL AND EXPERIMENTAL STUDIES ON ULTRAFILTRATION (UF) PROCESS FOR MILK CONCENTRATION

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THEORETICAL AND EXPERIMENTAL STUDIES ONULTRAFILTRATION (UF) PROCESS FOR MILK

CONCENTRATION

A. H. BAHNASAWY* and M. E. SHENANA**

* Agricultural Engineering Department

e-mail: [email protected]

** Food Science Department,

oshtohor, Faculty o! Agriculture,

"enha #ni$ersity, Egypt

".%. "%& '()(

ABSTRACT

Ultrafiltration (UF) is an important process in the food industry, particularlyfor dairy applications such as concentration of milk. The major problem in membraneseparation process is decline in flux over time of operation. This flux decline isattributed to the fouling of membrane. embrane fouling is affected by three major factors, namely, the membrane material properties, the feed characteristics and theoperating pressure. T!o models of UF process (resistance and diffusion) !eredeveloped to predict the permeate and retentate fluxes decline during concentration of milk as a function of time under different transmembrane pressures. "xperiments!ere carried out to study the effect of different transmembrane pressures on the permeate and retentate fluxes. #lso, milk concentration !as determined !ith time atdifferent pressures. Flux recovery during cleaning cycle !as also recorded. Theexperimental results !ere used to validate the model results. The experimental results!ere compared !ith the predicted data and they !ere in a good agreement !ith eachother.Keywords: Ultrafiltration, permeate, retentate, milk, pressure, model, flux decline,fouling, flux recovery.

INTRODUCTION

embrane separation processes for li$uid systems are conventionally classified interms of the si%e ranges of materials separated ( microfiltration, &' m '.& m*

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ultrafiltration, '.& m + nm* nanofiltration, + nm '.+ nm* reverse osmosis '.+ nm).embrane ultrafiltration is a pressure driven process !hich is capable of separating

macrosolutes or colloidal particles from a solvent or smaller solutes. These particlesmay be inorganic (metal oxides), polymeric (lattices) or biological (proteins). Theultrafiltration process has become particularly important for concentrating proteinaceous solutions. "xamples of commercial membrane processes involving thefiltration of protein solutions in the presence of electrolytes include the concentrationof !hey proteins in the dairy industry, protein recovery from blood plasma and protein concentration in do!nstream processing. Ultrafiltration performance islimited, ho!ever, due to the build up of the solutes at the membrane surface. This isthe so called concentration polari%ation effect(Bowen nd !"##" $s% &'' )

R " e* # (&'' ) reported that the major problem in membrane separation process is decline in flux over time of operation. This flux decline is attributed to thefouling of membrane. embrane fouling is affected by three major factors, namely,the membrane material properties, the feed characteristics and the operating

parameters. embrane fouling is of t!o types, namely, reversible and irreversiblefouling. -eversible fouling is mainly caused by a phenomenon, kno!n asconcentration polari%ation. oncentration polari%ation is the accumulation of solute particles over the membrane surface. This phenomenon is predominantly a function of membrane channel hydrodynamics. /n case of reversible fouling, the membrane permeability is recovered significantly after a proper !ashing protocol.

There have been some theoretical approaches to predict the ultrafiltration performance of colloidal solutions (e.g., milk). These are based on some models suchas mass transfer model (film theory), gel polari%ation model, osmotic pressure model, boundary layer adsorption model, 0ro!nian diffusion model, shear induced diffusionmodel, inertial lift model and surface transport model(A"$ r nd F"e#d% +,,&) and(B -er e* # % +,./) /n addition to the complexity of mathematical e$uationsinvolved, each of these models has a number of limitations1 (i) they demand someexperimental data for determining the input parameters. 2erhaps this is al!ays possible in practice, but the e$uipment re$uired are especially sensitive instruments,!hich might not be readily available. (ii) 3one of the methods can describe the fullflux time behavior of process* they often predict the steady or pseudo steady stateflux. (iii) "ach one has been sho!n to be valid for certain feeds under specialconditions. 4ence, modeling methods based on direct analysis of experimental dataappear to be good alternative to the models based on phenomenological hypotheses./t seems that the most important problem of using UF e$uipment is the flux decline

!hich is mainly affected the operational conditions especially the operational pressure. /n the present study, it is aimed to develop a model to study the effect of theoperational pressure on the flux decline. Filter medium resistance during bothconcentration and cleaning processes at different pressures !ith time !ill beestimated. Total soluble solids !ill be predicted and compared !ith the experimentalvalues. #lso, flux recovery during cleaning in place ( /2) !ill be predicted !ith timeat different operational pressures.

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MODEL DE0ELOPMENT

/n this study t!o different models !ere proposed. These models are1 first is theresistance model to study the medium resistance and their effect on the flux decline,second is the diffusion model to study the T55 change and flux decline.

Res"s* n1e $ode#:

/n UF of milk concentration, permeate flux declines !ith time due tomembrane fouling, !hich is very complicated phenomenon caused by many chemicaland physical properties interactions. /t is believed that membrane fouling is adynamic process starting !ith pore blocking follo!ed by continuous cake formationon the membrane surface. 2ore blocking is a fast process observed at the beginning of UF for a clean membrane due to its high initial permeate flux. #s UF process goeson, accumulation and deposition of particles on the membrane surface begin and gellayer is formed. The layer resistance becomes dominant after the initial stage. # littlereduction in permeate flux is observed in experimental results, and assuming very fast pore blocking, it could be supposed that the layer resistance is dominant resistance./gnoring the pore blocking resistance, the !hole resistance can be considered asmembrane resistance, deposited solute resistance and boundary layer resistance(M rs2 ## e* # % +,,3) Therefore, for UF of milk concentration using 6arcy7s la!,

the follo!ing e$uation can be !ritten1

)(&

bd mm + + + ,

dt d-

A .

++==

(&)

"$uation & could be re!ritten as1

)(&

pmm + + ,

dt d-

A +=

(8)

9here, - p is polari%ed solute resistance, using conventional filtration theory, thefollo!ing e$uation can be derived1

=

m

p p p A

-/ +

(:)

ombining e$uations 8 and : and integrating gives the follo!ing filtration e$uation1

, A +

-

, A

/

- t

m

m

m

p

+

= 88 (;)

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5olution flo! -etentate

g 2ermeate

C4

embrane

0oundary layer )0ayindirli et al., &?@?, tested e$uation + and discovered that a common n in e$uation> could not be found to describe all data at different body feed concentration.

onse$uently, as alternative e$uation !as proposed !hich combines the specific cakeresistance and solids concentration into a parameter, k.

[ ] A0- m e + , Ad- dt =

=

(A)

/ntegration of e$uation A gives1

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[ ] A0- m e + ,0 t =

=

(@)

Taking a natural logarithm of e$uation @1

- A

0

,0

+t m +

= ln)ln(

(?)

D"997s"on Mode#:

/n cross flo!, fluid flo!s over membrane at a fast rate. # laminar boundary layer of thickness (B) exists at the membrane surface. /f is the solid concentration in the boundary layer and is the concentration in the li$uid bulk, a mass balance of solidsentering the boundary layer !ith the solvent and those leaving the boundary layer bydiffusion !ill be

(To#edo% +,,,) 1

=

1/

D/ t -

(&')

6 is the mass diffusivity of solids. /ntegrating !ith respect to x, designating the

transmembrane flux t -

CD, using the boundary conditions, C at xC B1

)(ln 1 D .

/ / =

(&&)

#t the surface, xC', and Cs

D .

/ / s =

ln

(&8)

5ince flux must be maximi%ed in any filtration process, reduced polari%ationconcentration can be achieved at maximum flux if high li$uid velocities can bemaintained on the membrane surface to decrease the boundary layer thickness, B."$uation can be arranged to 1

=

/ / D

. sln (&:)

The ratio of 6= B may be represented by a mass transfer coefficient for the solids, k s.Thus 1

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This e$uation sho!s that a semilogarithmic plot of the bulk concentration, ,against the transmembrane flux under conditions !here s is constant is linear, !ith anegative slope of &=k s. Thus, transmembrane flux decreases !ith increasing bulksolids concentration, and the rate of flux decrease is inversely proportional to the mass

transfer coefficient for solids transport bet!een the surface and the fluid bulk.ass transfer coefficient can be determined using 6ittus 0oelter e$uation for

turbulent flo! as follo!s1

::.'@.'-e'8:.' ScSh = (&;)

,

(&+)

hd =-e

, (&>)

DSc =

(&A)

::.'::.'

::.'

@.'

@.'@.'

'8:.'

D

d d D

0 hh

s =(&@)

;A.'8.'

;A.'@.'>A.'

'8:.'

hd

D=

dh is the hydraulic radius C ;(cross sectional area)=!etted perimeter, m

ass diffusivity (6) !as estimated byPor*er (+, ,) using the follo!ing e$uation1

r

2 0

>

6 0

= (&?)

To make the UF operation more clear and understandable to those !ho run or redesign such e$uipment, a process calculation using "xcel as a program tool is used.Using "xcel as a calculation tool, it is easy to calculate any UF process kno!ing onlya fe! basic entering data.

6

Dd 0

Sh h s=


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Table (&) 12arameters that are used in the models1

P r $e*ers #7e 7n"*

9hole milk density8

&':' kg=m:

9hole milk Eiscosity: '.''@ 2a.s

9ater viscosity& '.''& 2a.s

2ermeate viscosity& '.'': 2a.s

k0 is 0olt%mann constant8 &.:@& &'G8: m8 kg sG8 H G&

ilk initial concentration '.&: decimal

the molecular radius&(r) &.8 I &' ? m

embrane features1

total area '.@; m8

membrane length &8' cm

3umber of modules :A

external diameter &.' cm

internal diameter '.> cm

edium resistance (-m)

For retentate ?.&"J'? &=m

For permeate +.A"J'@ &=m

leaning solution :"J'A &=m

ombined cake layer resistance, k

For retentate '.8AA &=m

For permeate '.&&A &=m

leaning solution 8.@&A &=m

6iffusivity, 6 '.AI&' &' m8=s

ass transfer coefficient in the diffusionmodel, k s

'.+;?@@ K=m8.min

Temperature :': H &-enner and #bd "l 5alam, &??&.,8Toledo, &??&, :2rokkopek et al., &?A+.

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MATERIALS AND METHODSM *er" #s:

Fresh !hole o!7s and 0uffalo7s milk !as obtained from the herds of theFaculty of #griculture, oshtohor, 0enha University, "gypt. T55, fat, protein

percentages and p4 !ere &:.', +.:, :.;+, and >.A8 , respectively.UF e;7" cm, respectively and empty !eight is@.@ kg, The details of schematic diagram of the experimental set up are presented infig. 8.

Bypass

Permeate

Modules

Heat exchanger

Coolingwater

P TH

Retentate

p

Fig. 8. schematic of the experimental setup.

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CP

FPFeedtank


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Me*2ods:-etentate and permeate fluxes !ere measured by dividing the volume by

product of the effective membrane and sampling time. Flo! velocity !as calculated

by dividing the flo! rate of both retentate and permeate by the cross sectional area of the UF module. -eadings !ere taken every &+ min. uring !F e"periments t#e e$e%ts o& t#e operating pressure

on t#e permeate 'u" and retentate %on%entration steady state werestudied( )#e operating transmem*rane pressures were sele%ted as3+ 4+ 5 and 6 *ars( ,a%# treatment needed 15- . o& milk and wasrepli%ated 3 times+ and measurements a/erages were taken(

C#e n"n= nd s n"*"6"n= *2e $e$4r ne:

embranes !ere cleaned and saniti%ed at different pressures as follo!s1 flushing!ater for &+ min, alkaline solution (3aL4, '.+M) for 8' min at A'o , then flushing!ater for &+ min, nitric acid ('.:M) for 8' min at +' , and finally, !ater !ashing for &+ min. 6uring cleaning, the flux should be monitored continuously until it reachesthe maximum at the time proceeds. The cleaning efficiency !as evaluated by theratio of flux during cleaning to the pure !ater flux measured under the sameconditions for each cleaning procedure(B"rd nd B r*#e**% &''&)

RESULTS AND DISCUSSION

F#7> 4e2 "or:Re*en* *e 9#7>

Figure : sho!s the experimental and predicted retentate fluxes at differentoperational pressures (:, ;, + and > bars) !ith time. "xperimentally, itcould be seen that the retentate flux declines rapidly during the first ;'min, slo!s do!n gradually during the period of ;' to &8' min, after this period, flux seems to be constant until it reaches the end of concentration process. -etentate flux increases !ith increasing the operational pressure.This increment !as clear during the first +' min of process, it seemed tohave little difference by the end of process as affected by pressure. #fter

&+ min, the flux !as as high as 8.&A K=m8

.s at > bars operational pressure,!hile it !as as lo! of &.:+ K=m8.s at : bars. 0y the end of process (after &@' min), flux decreased to reach '.&& K=m8.s at : bars and '.;& K=m8.s at > bars. -etentate flux !as predicted as a function of time (at the range of &+to &@' min) at different operational pressures (:, ;, +, and > bars). The predicted retentate flux !as in a reasonable agreement !ith theexperimental ones. These results are in agreement !ith that has beenreported byTon= e* # (+,..) /n early stage, of milk concentrating usingUF, adsorption fouling is probably the primary mechanism of flux decline(M **2" sson% +,./) /n the second stage, flux decline is probably due tothe concentration polari%ation. 0ut the majority of declination !as due to

the adsorption fouling.

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Figure ; sho!s the relationship bet!een the average experimental and predicted retentate fluxes (K=m8.s), !hich !as linear !ith coefficient of determination of '.??.

Per$e *e 9#7>Figure + sho!s the experimental and predicted permeate fluxes at different

operational pressures (:, ;, + and > bars) !ith time. /t indicate that the permeate flux declines rapidly during the first ;' min, slo!s do!n

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gradually during the period of ;' to &8' min, after this period, flux seemsto be constant until it reaches the end of concentration process. permeateflux increases !ith increasing the operational pressure. #t : bars,operational pressure, the flux decreased from &.A8 K=m8.s after &+ min to'.&;: K=m8.s by the end of process (after &@' min), #t : bars, operational pressure, the flux decreased from &.A8 K=m8.s after &+ min to '.&;: K=m8.s by the end of process (after &@' min). #t ; bars, operational pressure, theflux decreased from 8.8? K=m8.s after &+ min to '.&?& K=m8.s by the end of process (after &@' min). #t + bars, operational pressure, the flux decreasedfrom 8.@A K=m8.s after &+ min to '.8:? K=m8.s by the end of process (after &@' min). #t > bars, operational pressure, the flux decreased from :.;;K=m8.s after &+ min to '.8@A K=m8.s by the end of process (after &@' min).The predicted permeate flux !as in a reasonable agreement !ith theexperimental one.Figure > sho!s the relationship bet!een the experimental and predicted

values, !hich !as linear !ith coefficient of determination of '.??.

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To* # so#74#e so#"d "n $"#- * d"99eren* o


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The predicted values !ere in a reasonable agreement !ith the experimental valuesduring the first A' min of process and it !as in a good agreement after that time untilthe end of process. The relationship bet!een the predicted and experimental T55 of milk !as sho!n in Figure @. This relationship !as linear !ith '.?&: coefficient of determination, !hich may be attributed to effect of operational pressures in causingmembrane fouling !hich happens $uickly during the first +' min even before !hichmade variations bet!een both the predicted and experimental values of T55 duringthe first +' min of process.F#7> re1o ery d7r"n= 1#e n"n= "n .8M) at different pressures under study. #t both : and ; bars, flux recovery M !ere the same by the end of cleaning process (?>.++M), !hile it reach &''M flux recovery after ?' and @' min !hen it!orks at + and > bar operational pressure

F"#*er $ed"7$ res"s* n1e:F"#*er $ed"7$ res"s* n1e d7r"n= 1on1en*r *"on o9 $"#- 4y UFFigure &' sho!s the effect of operational pressure of UF during the concentration of milk on the predicted medium resistance (- m) !ith the process time. /t indicated thatthe - m increased linearly !ith the time at different operational pressures. -mincreases !ith increasing the pressure. #lso, it is !orthy to notice that at the higher pressures (+ and > bars), -m seems to have no big difference as affected by those t!o pressures, !hile there !ere big differences bet!een - m values !hen it !orks at : and; bars. These results are in agreement !ith those obtained by -ai et al., (8''+a).

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F"#*er $ed"7$ res"s* n1e d7r"n= 1#e n"n= "n x&'&' &=m at the lo!er pressure (: bars) atthe same time.

CONCLUSIONT!o models of UF process !ere developed to predict the permeate and retentatefluxes decline during concentration of milk as a function of time taking into accountmembrane resistance (resistance model) and concentration polari%ation (diffusionmodel). Fouling status during cleaning se$uence cycle under different transmembrane pressures !as also studied. The model has the capability to predict the follo!ing 1

-ententate and permeate fluxes at different operational pressures !ith areasonable accuracy.

Total solids in milk at different operational pressures.Flux recovery of the cleaning solution !ith time at different UF operational

pressures.Filter medium resistance during concentration of milk and during the cleaning

process.This model can be used to optimi%e the operating transmembrane pressure and

to identify the trade off bet!een productivity and !orking life of anultrafiltration membrane.

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NOMENCLATURE# surface area of the membrane (m8)#m membrane area (m8)

solid concentration, kg=m:N concentration in the li$uid bulk, kg=m:s concentration at xC'

6 effective diffusivity, m8=sk 0 0olt%mann constant, m8 kg sG8 H G&k s mass transfer coefficient, m=s-e -eynolds number, dimensionless5c 5chmidt number, dimensionless5h 5her!ood number, dimensionlesst filtration time (h)E filtration volume (m:)D permeate flux, m: =m8 s

cross flo! velocity, m=s2 transmembrane pressure, 2ar the molecular radius, m- b 0oundary layer resistance (m&)- d deposited solute resistance (m&)- m membrane medium resistance (m&)

filtrate viscosity (2a.s) 6ensity (kg=m:)O specific cake resistance (m &)

0oundary layer thickness, mdh hydraulic radius, m

T temperature, H

REFERENCES#imar, 2. and -. Field .&??8. Kimiting flux in membrane separations1 # model based

on the viscosity dependency of the mass transfer coefficient,/hemical Engineering Science 8 (:) (&??8), pp. +A? +@>.

0aker, -.D, #..0ird, .-. and . 0artlett. 8''8. easuring and modeling flux recovery during the

chemical cleaning of F membranes for the processing of !hey proteinconcentrate. ournal o! Food Engineering /3 , pp. &;: &+8.

0o!en 9., and . 9illiams. 8''A. Quantitative predictive modelling ofultrafiltration processes1 olloidal science approaches. #dvances in olloidand /nterface 5cience &:; &:+ (8''A) : &;

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arshall, #.6., 2. #. unro, and &8.

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! (UF) # " % $ &

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~~ . ]~~ _ Y _ }[ _ Z " ! $ # % ! ! ' )'& * ( .-, *+

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IH 6ABCDCE FG *@ 5 341 ; 7 6 $ J L 3 : K M + NO 123 4 P6 6 3 L K.')* 9 : IJ M 1 Q 4< $ Q5 R 'J ,5 $ 5 ? K.' )* 9 , 5 $ 5 ? > L : @ Q C

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THEORETICAL AND EXPERIMENTAL STUDIES ON ULTRAFILTRATION (UF) PROCESS FOR MILK CONCENTRATION

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